Q0O
Control of Gaseous Emissions
ENVIRONMENTAL PROTECTION AGENCY
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Control of Gaseous Emissions
Training manuals are especially prepared for
trainees attending a Gaseous Emissions course.
The manual should not be included in reading
lists or periodicals as generally available.
Conducted by
INSTITUTE FOR AIR POLLUTION TRAINING
ENVIRONMENTAL PROTECTION AGENCY
National Air Pollution Control Administration
Office of Manpower Development
Post Office Box 12055
Research Triangle Park, North Carolina 27709
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INSTITUTE FOR AIR POLLUTION TRAINING
OFFICE OF MANPOWER DEVELOPMENT
The Insititue for Air Pollution Training (1) conducts training for the
development and improvement of State, regional, and local governmental
air pollution control programs, (2) provides consultation and other training
assistance to governmental agencies, educational institutions, industrial
organizations, and others engaged in air pollution training activities and,
(3) promotes the development and improvement of air pollution training
programs in educational institutions and State, regional, and local go\crn-
mental air pollution control agencies
One of the principle mechanisms utilized to meet the Institute's goals is the j
intensive short term technical training course A ful time professional staff j
is responsible for the design, development and presentation of these courses '
In addition the services of scientists, engineers and specialists from other
NAPCA programs, governmental agencies, industry and universities are '
used to augment and reinforce the Institute staff in the dcM-lopment and I
presentations of technical material j
Individual course objectives and desired learning outcomes are delineated
to meet specific training needs Subicct matter areas covered include
process evaluation and control, atmospheric sampling and analysis, field
studies and air quality management These courses arc presented in the
Institute's resident classrooms and laboratories and at various field locations
Harry P. Kramer, Sc. D.
Director, Office of Manpower Development
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CONTENTS
SECTION I
BASIC INFORMATION
Classification and Sources of Gaseous
" Pollutants
Physical Properties of Gases
SECTION II
AIR POLLUTION STANDARDS
Air Pollution Standards: Gases
SECTION III
CONTROL BY MODIFI CATION OF PROCESSES
Control by Modification of Processes
SECTION IV
CONTROL BY COMBUSTION
Control by Combustion
Fundamentals in Combustion Calculations
Disposal of Gaseous Wastes - Flame and
Catalytic Incineration
SECTION V
CONTROL BY ABSORPTION
Absorption in Control of Gaseous Pollutants
Gas Absorption Equipment
Absorption Nomenclature and Definitions
Absorption Equipment Selected Applications
A Guide to Scrubber Selection
Performance of Commercially Available
Equipment in Scrubbing Hydrogen Chloride Gas
SECTION VI
CONTROL BY ADSORPTION
Principles of Adsorption
Adsorption Systems and Their Application to
Air Pollution Control
SECTION VII
CONTROL OF ODORS
Measurement and Control of Community Malodors
Controlling Industrial Odors
Industrial Odor Control and its Problems
SECTION VIII
DESIGN OF LOCAL EXHAUST SYSTEMS
Fluid Flow Fundamentals
Hood Design
Fan Design
SECTION IX
MISCELLANEOUS
Economics of Pollution Control Systems
Control Methods for the Removal of Sulfur
Oxides from Stack Gases
Techniques for Controlling the Oxides of Nitrogen
SECTION X
APPENDIX
Air Pollution Control Equipment Buyer's
Guide
Conversion Factors
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SECTION I
BASIC INFORMATION
Classification and Sources of Gaseous Pollutants
Physical Properties of Gases
-------
CLASSIFICATION AND SOURCES
OF GASEOUS POLLUTANTS
I CLASSIFICATION
A The gaseous pollutants in a community atmos-
phere may be separated into two classes:
1 Inorganic gases, and
2 Organic vapors.
B Inorganic gases of significance as air pol-
lutants include
1 Sulfur dioxide
2 Hydrogen sulfide
3 Nitrogen oxides
4 Hydrogen chloride
5 Silicon tetrafluoride
6 Hydrogen floride
7 Carbon monoxide
8 Ammonia
C Organic vapors of significance as air pol-
lutants include.
1 Hydrocarbons
2 Mercaptans
3 Alcohols
4 Ketones
5 Esters
II SULFUR-CONTAINING INORGANIC GASEOUS POL-
LUTANTS
The principal sulfur-containing inorganic
gaseous pollutants are; sulfur dioxide and
hydrogen sulfide. The principal sources of
these pollutants in order of decreasing quan-
tities of sulfur emitted to the atmosphere
are:*
1 The combustion of coal.
2 The production, refining, and utiliza-
tion of petroleum and natural gas.
3 Industries manufacturing and using
sulfuric acid and sulfur.
4 The smelting and refining of ores,
principally copper, lead, zinc, and
nickel.
A The Combustion of Coal
Table 1 shows the sulfur content of various
solid fuels. In a few cases the maximum
sulfur contents in solid fuels is limited
by law.
Table l.(l) SULFUR CONTENT OF SOME SOLID
FUELS
Type of fuel
Bituminous coal
Common range
Anthracite
Metallurgical coke
Wood
Sulfur, % by ut.
0.3 - 6
0.7 - 2.0
0.6 - 1.0
1.0
Negligible
The sulfur in coal does not occur in the
free state, but is present in combined
form as "organic sulfur" and "inorganic
sulfur".
The proportion of the total sulfur existing
in the organic form varies widely among
the different coals. This organic sulfur
cannot be separated by mechanical clean-
ing processes.
The inorganic portion of sulfur in coal
includes pyritic and sulfate forms. The
sulfate sulfur may be present as calcium
sulfate, or in weathered coals as ferrous
sulfate formed as a result of the oxida-
tion of the pyrite. Pyritic sulfur is pre-
sent as pyrite (FeS2) or as marcasite
which has the same chemical composition
but differs in crystalline form. Pyritic
sulfur can, at times, be removed by
*In general, those operations emitting the least sulfur to the atmosphere are those in which the
sulfur dioxide concentration in the waste gases is also highest. Thus, the combustion of coal
is responsible for the heaviest contamination of the atmosphere by sulfur dioxide although the
sulfur dioxide concentration in the effluent gases from such combustion is lowest (less than 1°).
PA.C.ge.7a.5.65
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Classification and Sources of Gaseous Pollutants
cleaning operations. Thus, the sulfur con-
tent of coal can be reduced 2S-35%(1) by
economical and practical cleaning procedures.
During the combustion process, a portion
of the sulfur is converted to sulfur dioxide
which escapes in the stack gases. The con-
version to S02 varies from source to source,
with conversions of 95% or higher for pul-
verized coal. The actual concentrations
in the effluent gases depends on the method
of firing, the percentage of excess air
and the sulfur content in the coal.
When coal is carbonized to form metallurgi-
cal coke, approximately 60%^ •* or more of
the sulfur in the coal remains with the
coke which is then charged to a blast fur-
nace or used in other operations. During
combustion of the coke, a major portion
of the sulfur is converted to S02 which
escapes with stack gases.
B The Production, Refining, and Utilization
of Petroleum and Natural Gas
The sulfur content of various liquid and
gaseous fuels is shown in Table 2. The
maximum allowable concentrations of sulfur
in light refined fuel oils, gasoline,
diesel oil, liquified petroleum gases and
natural gas distributed by utilities is
defined by standards set by government and
industry. As v>ith the solid fuels, the
sulfur is converted to SO. during combust-
In the neuer refining processes, almost
40% of the sulfur in the feed stock is
converted to H,S gas and to low boiling
sulfur compounds. Such large quantities of
H2S do not permit combustion by flares.
But, fortunately the concentration of F^S
in the refinery gases is sufficiently high
(3.5 to 7?o) so that the HzS can be convert-
ed economically to elemental sulfur.
C Industries Manufacturing and Using Sulfuric
Acid and Sulfur
The problem faced by this segment of in-
dustry is similar to that of the burning
of coal, that is, the concentration of SO2
in the waste gases is so low as to be un-
economical to recover. In some of these
industries the atmospheric emission problem
is primarily one of sulfuric acid mist
emissions rather than SO,.
D The Smelting and Refining of Ores
The sulfur compounds from non-ferrous
smelters result principally from sulfide
contained ores of lead, zinc, copper and
nickel. Since the smelter effluent gases
contain relatively high concentrations of
S02 (1 - 6% and higher) economical recovery
of S02, normally as sulfuric acid or sulfur,
has been accomplished. l1'
III NITROGEN-CONTAINING INORGANIC GASEOUS
POLLUTANTS
The two principal nitrogen compounds present
in the air are: (1) oxides of nitrogen and
(2) ammonia. The oxides of nitrogen are part-
icularly important since such compounds react
photochemical ly with some organics to from
smog.
A Oxides of Nitrogen
The following processes emit oxides of
nitrogen to the atmosphere-
1 The manufacture of nitric acid
2 The manufacture of sulfuric acid by
the chamber process.
3 The manufacture of paint, roofing,
rubber, and soap.
4 The manufacture of nylon intermediates.
5 The refining of petroleum (the regenera-
tion of cracking catalysts)
6 The nitration of organic compounds
7 Pickling of stainless steel
8 The exhaust gases of trucks and pass-
enger vehicles
9 The combustion of natural gas, fuel
oil and coal
10 The incineration of organic wastes
The removal of the oxides of nitrogen from
waste gases is not easy. Nitric oxide,
(NO), is only slightly soluble in water;
hence, it cannot be easily scrubbed out.
NO reacts with the oxygen of the air to
form the (brown) gas, N02- Although NC^
is readily soluble and can be removed,
the reaction does not go to completion,
and there is always some NO present with
B Ammonia
The following processes emit ammonia to
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Classification and Sources of Caseous Pollutants
Table 2.(1) SULFUR CONTENT OF SOME LIQUID AND GASEOUS FUELS
Type of fuel
Liquid
Crude petroleum
Fuel oil, No. 6
Fuel oil, No. 2
Diesel oil (1-D)
Gasoline
Gaseous
Liquified petroleum gas
Manufactured gas
Natural gas
Sulfur, % by wt.
0.2 - 1.7
0.3 - 3.0
1.0 (Max. spec.]
O.S (Max. spec.)
0.01 - O.I
2-10 gr/100 ft3
2-10 gr/100 ft3
0-10 gr/100 ft3
the atmosphere-
1 Fertilizer and organic - chemical in-
dustries
2 The manufacture of nitric acid by the
oxidation process
3 The exhaust of automobiles
4 The incineration of organic wastes
5 Refineries
6 Stock yards
The principal effect of ammonia in the air
is that of neutralization. Thus, it off-
sets the acid condition resulting from
the oxides of sulfur and nitrogen. Never-
theless, it is seldom present in an equi-
valent amount, and its action is not en-
tirely beneficial since when it reacts
with acidic gases, there results an aerosol
of fine ammonium salt fume or droplets
of solution. Thus, there is usually a
characteristic haze when aanonia or ammonia
salts are released into the air. The gas
is very soluble in water and can be removed
from effluent gas streams by water scrub-
bing.
IV INORGANIC GASEOUS HALOGEN COMPOUNDS
Two important types of halogen compounds
from the standpoint of atmospheric pollution
are fluorine and chlorine compounds.
A Fluorine Compounds
Fluorine occurs in varying amounts in
most ores, coals, clays, and soils. It is
present in the form of apatite in phosphate
rock. The main use of phosphate rock is
as a raw material for the manufacture of
phosphoric acid and superphosphate. Super-
phosphate is used in the manufacturing of
fertilizer. Fluoride mineral such as cry-
olite, fluorspar, and sodium fluoride are
used as fluxes in some metallurgical pro-
cesses.
The major air pollution from fluorides
result from gaseous fluorides and fluoride
(water) mists. Gaseous fluorides formed
in manufacturing processes are generally
silicon tetrafluonde (SiF4) or hydrogen
fluoride (HF). Participate fluorides in
process effluents may include sodium alum-
inum fluoride, apatite, calcium fluoride,
iron fluoride or sodium fluoride. The
activity of elemental fluorine is so great
that there is little likelihood that it
will exist in the elemental F_ form.
Hydrogen fluoride and silicon tetrafluonde
are effectively removed by adsorption
processes. Lime or water scrubbers have
been used to control these pollutants.
Processes fron which fluorine compounds
may be emitted include the following-
I Aluminu* industry
2 Phosphate fertilizer manufacture
3 Brick plants
4 Pottery and ferroenamel works
5 Steel plants
6 Refineries
B Chlorine Compounds
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Classification and Sources of Gaseous Pollutants
Hydrogen chloride and chlorine are evolved
in numerous industrial processes. However,
it is easily recovered by adsorption tech-
niques and little reaches the atmosphere.
Hydrogen chloride is emitted in small
quantities from refineries, especially in
the regeneration of cracking catalysts.
(The gas appears to originate in the salt
which contaminates the oil and is hydroll zed
by moisture at the high temperature in the
presence of the catalyst)•
V CARBON MONOXIDE
Carbon monoxide (CO] is a gas that has a
strong affinity for the hemoglobin in the blood.
When CO combines with hemoglobin it reduces
the bloods capacity to carry oxygen to the
body tissue. Under ordinary conditions the
breathing of 30 ppm for 8 hours reduces by
5% the oxygen capacity of the blood.[4)
The major source of carbon monoxide results
from the combustion of motor fuels used in
trucks and passenger vehicles. Carbon monoxide
concentrations have been measured in the pass-
enger compartment of an automobile during both
moving and stationary traffic conditions.(5)
The highest value recorded was 370 ppm. This
occurred during stationary traffic conditions.
Various methods of controlling carbon monoxide
from motor vehicles have been investigated
including
1 r.ngine modification
2 Afterburner type muffler
3 Catalytic muffler
4 Positive crankcase ventilation system
VI ORGANIC VAPORS
Organic vapors emanate from various sources
and process operations including the follow-
ing
1 Operation of motor vehicles (gasoline,
diesel)
2 Petroleum refining (includes evapora-
tion losses from storage, skimming
tanks, sewers, etc.).
3 Petroleum production (field operations
for crude oil and natural gas, in-
cludes natural gas released directly
into the atmosphere].
4 Petroleum marketing (filling tank
trucks, railroad cars, sea-going
vessels, retail stations, bulk ter-
minals, etc.)
5 Chemical, paint, roofing, rubber,
and soap industries
6 Fuel oil and coal burning
7 Incineration of refuse
Organic vapors are a major source of odors.
Various types of odors and the sources per
se are listed below(°).
1 Animal odors:
a Meat packing and rendering plants
b Fish oil
c Poultry ranches and poultry pro-
cessing
2 Combustion odors.
a Gasoline and diesel engine ex-
hausts
b Coke-oven and coal-gas odors
(steel mills)
3 Food processing odors:
a Coffee roasting
b Restaurant odors
4 Paint and related industrial odors:
a Paint, lacquer, and varnish
manufacturing
b Paint spraying
c Commercial solvents
5 General industrial odors'
a Oil refining (mercaptans)
b Sulfate pulping operation (mer-
captans)
c Dry-cleaning shops
d Fertilizer plants
e Asphalt odors (roofing, street
paving)
f Asphalt odors (manufacture)
g Plastics manufacture
!i Amines (usage or manufacturing)
6 Foundry odors
a Heat treating, oil quenching (and
pickling)
b Smelter operations
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Classification and Sources of Gaseous Pollutants
7 Combustible wastes odors.
a Single chamber incinerators and
backyard trash fires
b Municipal incinerators
c Open dumps
d Demolition disposal
8 Waste decomposition odors:
a Putrefaction and oxidation (organic
acids)
b Decomposition of protein (organic
nitrogen compounds)
c Decomposition of lignite (plant
cells)
9 Sewage odors-
a City sewers
b Sewage treatment plants
VII SUMMARY
A Sources of the Principal Gaseous Pollut-
ant's Pollutants^6)
Source
Pollutant
Combustion processes..NO,, SO,,CO, organics,
acids i *
Automotive engines N02, CO, acids, organics
Petroleum operations..502, H2S, NH3, CO, N02
hydrocarbons, mercaptans
Chemical processes S02, CO, NH3, acids,
N02, organics, solvents,
odors, sulfides
Pyro- and electro-
metallurgical processes.S02, CO, fluorides,
organics
Mineral processing S02, CO, fluorides,
organics
Food and feed
operations Odorous substances
REFERENCES
1 Nelson, H.W., and Lyons, C.J. Sources and
Control of Sulfur-Bearing Pollutants.
JAPCA 7, No. 3 Nov. 1957.
2 McCabe, L. Air Pollution. McGraw-Hill Book
Co., Inc. N.Y. 1952.
3 Semrau, K.T. Emission of Fluorides from
Industrial Processes. JAPCA 7, No. 2
Aug. 1957.
4 Haagen-Smit, A.J. Time. Feb. 19, 1965. p.
70.
5 Rose, A.M. etal. Exhaust Contamination in
Passenger Cars. Tech. Report A61-2
U.S. Public Health Service. Robt. A.
Taft Sanitary Engineering Center, Cin-
cinnati, Ohio. 1961.
6 Rose, A.M. etal. Prevention and Control of
Air Pollution by Process Changes or
Equipment. U.S. Public Health Service,
Sanitary Engineering Center, Cincinnati,
Ohio. 1957.
7 Kerka, W.F., and Kaiser, E.A. An Evaluation
of Environmental Odors. JAPCA 7, No. 4.
Feb. 1958.
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PHYSICAL PROPERTIES OF GASES
C.A. Lindstrom
I NOMENCLATURE
B = proportion by volume of a gas-
component of a gas mixture
°C = degrees Centigrade
°F = degrees Fahrenheit
g = local acceleration due to gravity
g = dimensional constant
h . = absolute pressure-head
h = atmospheric pressure-head
h = gage pressure-head
°K = degrees Kelvin
M = molecular weight of a gas (mass
per mole)
M = molecular weight of a gas-compon-
x ent of a gas-mixture (mass per
mole)
M = apparent molecular weight of a
gas-mixture (mass per mole)
m = mass of a gas
m = mass of a gas-component of a gas
mixture
m = mass of a gas-mixture
P = absolute pressure
P = partial pressure of a gas-compon-
ent of a gas-mixture
P = absolute pressure of a gas-mixture
P = atmospheric pressure
Pgage = gage pressure
R = universal gas constant
°R = degrees Rankine
T = absolute temperature
T = absolute temperature of a gas-
mlx mixture
V = volume of a gas
V = volume of a gas-component of a
gas-mixture
V = volume of a gas-mixture
mix
p = density (mass) of a gas
p_ = density (mass) of a fluid
II EXPRESSION OF GAS TEMPERATURE
A The Fahrenheit and Centigrade Scales
The range of units on the Fahrenheit scale
between freezing and boiling is 180; on
the Centigrade scale, the range is 100.
Therefore, each Centigrade-degree is equal
to 9/5 Fahrenheit-degree. The following
relationships convert one scale to the
other
°F = 9 CC * 32
•CD
•(2)
C.A. Lindstrom is presently Chief, Field
Studies Section, State of Arizona
°C = ^ (°F - 32)--
9
B Absolute Temperature
Experiments with perfect gas have shown
that, under constant pressure, for each
change in Fahrenheit-degrees belov* 32°F
the volume of a gas changes (1/491.6).
Similarly, for each Centigrade degree,
the volume change is (1/273). Therefore,
if this change in volume per temperature
degree is constant, the volume of gas
would, theoretically, become zero at 491.6
Fahrenheit-degrees below 32°F, or at a
reading of -459.6 °F. On the Centigrade
scale, this condition would occur at 273 Cent-
igrade-degrees below 0 °C, or at a temper-
ature of -273 °C.
Absolute temperatures determined by using
Fahrenheit units are expressed as degrees
Rankine (°R); those determined by using
Centigrade units are expressed as degrees
Kelvin (°K). The following relationships
convert one scale to the other1
°R = °F + 459.6 - (3)
"K = °C + 273 (4)
Relationship of the various temperature
1
PA.FA.gc.28.2.62
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Physical Properties of Gases
Fahrenheit
Scale
Centigrade
Scale
212
100 c r\
.32°F.
491.6° R
I ' Fahrenheit-
Absolute - 459.6° F il -degrees -273 ° C
Zero
Absolute
Scale
671.6° R Q 373° K
,0°C
491.6°R.
273
h Centi grade
273 °K
Absolute
Zero
Figure 1
systems are shown graphically in Figure.1.
Ill EXPRESSION OF GAS PRESSURE
A Definition of Pressure
A body may be subjected to three kinds
of stress: shear, compression, and ten-
sion. Fluids are unable to withstand
tensile stress; hence, they are subject
to shear and compression only. Unit com-
pressive stress in a fluid (e.g., pounds
per square inch] is termed pressure.
Pressure is equal in all directions at a
point within a volume of fluid, and acts
perpendicular to a surface.
B Barometric Pressure
Barometric pressure and atmospheric
pressure are synonymous. Such pressure
is measured with a barometer and is
usually expressed as inches, or milli-
meters, or mercury. Standard barometeric
pressure is the average atmospheric
pressure at sea level, 45° .north latitude
at 35°F. It is equivalent to a pressure
of 14.696 pounds per square inch exerted
at the base of a column of mercury 29.921
inches high. Barometric pressure varies
with weather and altitude.
C Gage Pressure
Measurements of pressure by ordinary
gages are indications of the difference
in pressure above, or below, that of the
atmosphere surrounding the gage. Gage
pressure, then, is ordinarily, the pres-
sure indicated by the gage itself. If the
pressure in the system is greater than
. the pressure prevailing in the atmosphere,
the gage pressure is expressed positive;
if smaller, the gage pressure is expres-
sed negative. The term, "vacuum", desig-
nates a negative gage pressure.
The abbreviation, "g", is used to specify
a gage pressure. For example, psig means
pounds per square inch gage pressure.
D Absolute Pressure
. Since gage pressure (which may be either
positive or negative) is the pressure
relative to the prevailing atmospheric
pressure, the gage pressure, added alge-
braically to the prevailing atmospheric
pressure (which is always positive)
provides a value that has a datum of
"absolute zero pressure". A pressure
calculated in this manner is called,
absolute pressure. The mathematical, ex-
pression is;
P = T)
*
where:
gage
P = absolute pressure
•(S)
-------
Physical Properties of Gases
P = atmospheric pressure
gage = gage-pressure
The abbreviation, "a", is used to indi-
cate that the pressure is absolute. For
example, psia means pounds per square
inch absolute pressure.
Equation (5) allows conversion of one
pressure system to the other. Relation-
ship of the pressure systems are shown
graphically in Figure 2 using two typical
•»•
{G«p
PMMUIV Datum
t B
a?
Absolute PraMura Datum
Figure 2
gage pressures, A and B. Gage pressure
A is above the datum from which gage
pressures are measured, and, hence
is expressed as positive value; gage
pressure B is below the gage pressure
datum, and therefore is expressed as
a negative value.
E The Conception of Pressure-Head
Pressure-head is the height of a column
of fluid required to produce a given
pressure at its base. Pressure-head may
be calculated by use of the following
expressions:
abs
= P
•(6)
•Pfg
gage
rgage
-Pfg
atm
atm
-(8)
Pfg
where:
g
abs
atm
= local acceleration due to
gravity
= dimensional constant
= absolute pressure-head
= atmospheric pressure-head
= gage pressure-head
gage
P = absolute pressure
P = atmospheric pressure
P = gage pressure
gage B B r
p. = density (mass) of a fluid
Pressure-head may be expressed in terms
of any fluid that is convenient.
IV THE LAW FOR IDEAL GASES
A The Laws of Boyle and Charles
1 Boyle's Law
Boyle's Law states that, when the
temperature is held constant, the
volume of a given mass of a perfect
gas of a given composition varies in-
versely as the absolute pressure.
2 Charles' Law .
Charles' Law states that, when the
volume is held constant, the absolute
pressure of a given mass of a perfect
gas of a given composition varies
directly as the absolute temperature.
B The Law for Ideal Gases
Both Boyle's and Charles' Law are satis-
fied in the following equation:
•(9)
M = molecular weight of a gas (mass
per mole)
m = mass of a gas
P = absolute pressure
R = universal gas constant
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Physical Properties of Gases
T = absolute temperature
V = volume of a gas
The law states that, if a perfect gas has
mass (m) and molecular weight (MJ it
occupies volume (V) at absolute pressure
(P) and absolute temperature (T). It
should be noted that (=—) is constant only
if (Jl) is constant.
M
Values of the universal gas constant for
several dimensional systems are shown in
Table 1.
temperature is the same as it would
exert if it filled the whole space alone
B Definition of Partial Pressure
The pressure that one component of a
gas-mixture exerts is called its partial
pressure. The total pressure of the
gas-mixture is the sum of the partial
pressures.
C The Mathematical Expression
Referring to equation (9), the mathe-
Table 1
Values of the Universal Gas Constant for Various Dimensional Systems
Dimensional System
cm - gm - sec - "K
8.31UO>Z-S==£S-
sec -mole-°K
ft -ft -sec - "R
1544 ft-*f
mole - "R
ft -tf, H» - sec
1544 ft - «f
•ole - °R
- °R
V Till; DINSITY OF A PERFECT GAS
Rearranging equation (9)
m nki
•(9A)
RT
Substituting (o) for (r-), equation (9A) be-
comes
PM
=RT
•(10)
uhere
p = density (mass per unit volume) of
a gas
P = absolute pressure
M = molecular weight of a gas (mass
per mole)
R = universal gas constant
T = absolute pressure
VI DALTON'S LAW OF PARTIAL PRESSURE
A Statement of the Law
When gases or vapors having no chemical
interaction are present as a mixture in
a given space, the pressure exerted by
a component of the gas-mixture at a given
matical expression of Dalton's Law is-
p,v»,, " "x^""
x mix ..
where-
P = partial pressure of a gas-
x component
V = volume of a gas-mixture
m = mass of a gas-component
R = universal gas constant
T = absolute temperature of a
mi x
gas-mixture
M = molecular weight of a gas-
component (mass per mole)
It should be noted that
constant only if
is constant.
P V
mix
is
D The Density of a Gas-Component
-------
Physical Properties of Gases
Rearranging equation (11):
6 6 ^
m P M
X = X X
Vx RTnux
•(HA)
Substituting (p ) f or
becomes .
where:
Jjc
u 1
v .'
mi
equation (11A)
= P M
X X
D T
= density of a gas-component
(mass of gas-component per unit
volume gas mixture)
P = partial pressure of a gas-com-
ponent
M
= molecular weight of a gas -com-
ponent
R = universal gas constant
T = absolute temperature of a gas-
mixture
VII PROPORTION BY VOLUME OF A COMPONENT IN A
GAS-MIXTURE
m PV .
mix = mix
Mmx RT
By substitution, equation (14) becomes.
i V
x = «
p~ —
mix mix
, equation (17) becomes-
•(18)
Letting B
„ „
B = P
x x
mix
where:
B = proportion by volume of a gas-
component
PX = partial pressure of a gas-component
Pmix = aosolute pressure of a gas-mixture
VIII THE APPARENT MOLECULAR WEIGHT OF A GAS-
MIXTURE
Rearranging equation (11):
P V = m R T
mix nix mix mix
PxVmixMx ' raxRTn,ix-
•dlA)
mix
Repeating equation (11), for a gas-corn-
P V = m R T
x mix x mix
•(11)
Therefore:
V IP M = RT . Em
mix x x mix x
But:
Since
mix
is common to both of the
mx = "mix
and
equations above-
Px fy
r =LMxJ
mix
mix
mix ~ mix mix mix (See equation 13)
RT
mix
Substituting for Z m in equation (19):
IP M
x x
P M
mix mix
-(20)
At an arbitrary T and P:
Solvin* for M
mix
RT
-(15)
EPxMx
Pmix
-------
Physical Properties of Gases
Since -P
=— = B (see equation 18), by
mix substitution,
equation (21) becomes
M = ZB M
mix x x
•(22)
hhere
M = apparent molecular weight of a
gas-mixture
B = proportion b> volume of a gas-
x component
M = molecular weight of a gas-compon-
x ent
-------
SECTION II
AIR POLLUTION STANDARDS
Air Pollution Standards: Gases
-------
AIR POLLUTION STANDARDS: GASES
(Compiled January 1972)
J.E. Sickles, II
I THRESHOLD LIMITS AND AIR QUALITY CRITERIA
In order to give a basis for comparison,
the global background concentrations of
selected air pollutants are listed in
Table I.
Air quality criteria are the bases for air
quality standards. Table II is a listing
of workroom air threshold limits and air
quality criteria for the U.S.S.R.
II AIR QUALITY STANDARDS
A list of ambient air quality'standards for
various pollutants in various political
jurisdictions is given in Table III. The
national primary and secondary ambient air
quality standards are listed in Table IV.
Ill EMISSION STANDARDS
Emission standards have been adopted in
order to maintain the air quality within
specified air quality standards. Selected
emission standards for several gaseous
pollutants are shown in Table V. A listing
of the U.S. performance standards for five
stationary sources categories are included
in Table VI.
REFERENCES
1. A.C. Stern, "Air Pollution Standards,"
Air Pollution. Vol. III. 2nd Ed.,
A.C. Stern, ed., Academic Press, New
York, 1968.
2. E. Robinson and R.C. Robblns, "Sources,
Abundance, and Fate of Gaseous Atmos-
pheric Pollutants," Project PR-6755,
Stanford Research Institute, Melno Park,
California, 1968.
3. Federal Register. 36_, No. 84, April 30,
1971, 8186-8201.
4. U.S. Dept. H.E.W., Control Techniques for
Carbon Monoxide. Nitrogen Oxide, and
Hydrocarbon Emissions from Mobile
Sources. NAPCA Publication No. AP-66.
Washington, D.C.: Government Printing
Office, 1969.
5. Federal Register. 36, No. 84, December 23,
1971, 24876-24895.
J.E. Sickles, II is a Chemical Engineer,
NAPCA, Air Pollution Training
PA.C.ge.l8a.l2.70
-------
Air Pollution Standards Gases
TABLE I GLOBAL BACKGROLND CONCCMTRATIOXS (1,2)
POLLUTANT
CONCENTRATION'
Sulfur Dioxide
Hydrogen Sulfide
Nitrogen Oxide
Oxidant or Ozone
Methane
Non-Methane Hydrocarbons
Carbon Monoxide
Carbon Dioxide
0.2 ppb
0.2 ppb
1 ppb
0.015 - 0.03 ppm
1.5 ppm
< 1 ppb
0.1 ppm
320 ppm
TABLE II THRESHOLD LIMIT VALUES AND USSR AIR QUALITY CRITERIA (1)
POLLUTANT
Ammonia
Benzene
Carbon Dioxide
Chlorine
Ethyl Alcohol
Formaldehyde
Hydrogen Cyanide
Hydrogen Chloride
Hydrogen Fluoride
Hydrogen Sulfide
Me c ha no 1
Nitrogen Dioxide
Ozone
Sulfur Dioxide
CONCENTRATION FOR HUMAN ODOR PERCEPTION (mg/m3) TVL (PPM)
0.5
3.0
1.0
0.07
0.1
0.03
0.01
4.3
0.87
50
25
5000
___
1000
5
10
___
3
10
200
5
0.1
5
-------
Air Pollution Standards: Cases
TABLE III AMBIENT AIR QUALITY STANDARDS (1)
POLLUTANT
Ammonia
Benzene
Carbon Monoxide
Chlorine
Ethanol
Formaldehyde
Hydrogen Chloride
Hydrogen Fluoride
Hydrogen Sulfide
Methanol
Nitrogen Dioxide
Nitrogen Oxides
Sulfur Dioxide
POLITICAL JURISDICTION
Czech.
Ontario
U.S.S.R.
Czech.
U.S.S.R.
Czech.
U.S.S.R.
Czech.
U.S.S.R.
Ontario
U.S.S.R.
Czech.
U.S.S.R.
Ontario
Netherlands
U.S.S.R.
Czech.
U.S.S.R.
U.S.S.R.
U.S.S.R.
Czech.
Ontario
Czech.
mg/m
0.1
3.5
0.2
0.8
0.8
1
1
0.03
0.03
0.3
5.
0.015
0.012
0.05
0.01
0.005
0.008
0.008
0.5
0.085
0.1
0.18
0.15
PPM
0.14
5.
0.28
0.25
0.25
0.9
0.9
0.01
0.01
0.1
2.5
0.01
0.01
0.04
0.008
0.005
0.005
0.38
0.045
0.06
0.1
0.06
AVERAGING TIME
24 hr.
30 min.
24 hr.
24 hr.
24 hr.
24 hr.
24 hr.
24 hr.
24 hr.
30 min.
24 hr.
24 hr.
24 hr.
30 min.
24 hr.
24 hr.
24 hr.
24 hr.
24 hr.
24 hr.
24 hr.
24 hr.
24 hr.
-------
13
o
o
3
TABLE IV NATIONAL AIR QUALITY STANDARDS (3) APRIL 30, 1971
Primary standards protect the public health, secondary standards
protect the public welfare.
O.
Hi
rt
a.
in
ro
in
POLLUTANT
sox
PARTICULATE
CO
PHOTOCHEMICAL
OXIDANTS
NOX
HC
PRIMARY
CONCENTRATION
Ug/M3; P. P.M.
80; 0.03
365; 0.14
75; -
260; —
10,000; 9
40,000; 35
160; 0.08
100; 0.05
160; 0.24
SECONDARY
CONCENTRATION
yg/M3; P. P.M.
60; 0.02
260; 0.1
1300; 0.5
60; -
150; --
SAME AS PRIMARY
SAME AS PRIMARY
SAME AS PRIMARY
SAME AS PRIMARY
TYPE OF MEASUREMENT
ANNUAL ARITHMETIC MEAN
MAXIMUM 24 HR. CONCENTRATION
MAXIMUM 3 HR. CONCENTRATION
ANNUAL GEOMETRIC MEAN
MAXIMUM 24 HR. CONCENTRATION
MAXIMUM 8 HR. CONCENTRATION
MAXIMUM 1 HR. CONCENTRATION
MAXIMUM 1 HR. CONCENTRATION
ANNUAL ARITHMETIC MEAN
MAXIMUM 3 HR. CONCENTRATION
(6 to 9 A.M.)
-------
Air Pollution Standards. Cases
TABLE 5 EMISSION STANDARDS
POLLUTANT
Ammonia
Benzene
Carbon Monoxide
Chlorine
Formaldehyde
Hydrocarbons
Hydrogen Chloride
Hydrogen Fluoride
Hydrogen Sulfide
Nitrogen Oxides
(as NO.)
£
POLITICAL JURISDICTION
Czech.
Czech.
Paris, France
United States (1970)
Czech.
Great Britain
Czech.
U.S.A. (1968)
U.S.A. (1971)
U.S.A. (1970)
Great Britain
Great Britain
Czech.
Great Britain
Czech.
Great Britain
SOURCE
Combustion
Auto. Exhaust
Auto, crankcase
Auto, evaporation
Auto, exhaust
Acid plant
STANDARD
3kg/hr.
24 kg/hr.
llg/m3; 104 PPM
23g. /vehicle mile
1 kg/hr..
230 mg/m ; 77PPM
0.5 kg/hr.
0
0.49 g. /vehicle mile
2.2 g. /vehicle mile
460 mg/m3; 328 PPM
230 mg/m3
0.08 kg/hr.
7.5 mg/m ; 5 PPM
3. kg/hr.
2300 mg/m ; 1280 PPM
REFERENCE
(1)
(1)
(1)
(4)
(1)
(1)
(1)
(4)
(4)
(4)
(1)
(1)
(1)
(1)
(1)
(1)
-------
-o
o
TAHL1. VI U.S. PLKKORMANCI. SIANDARDS (5)
INDUSTRY
I'OI LUTANT
STANDARD
Fossil fuel-fired steam generators
with heat input greater than 250
ill 11 ion B'lU per hour)
Incinerators
(with charging rate greater than
50 tons per day)
Portland Cement Plants
lltric Acid Planta
(producing weak acid)
Sulfuric Acid Plants
(except scavenger acid plants)
Visible I'mission
('articulate
SO_ (liquid fossil fuel)
(solid fossil fuel)
NO (gas fossil fuel)
X (liquid fossil fuel)
(solid fossil fuel,
except lignite)
Particulate
Visible Lmission
Particulate (kiln)
(clinker cooler)
Visible Emission
NO
X
Visible Emission
Acid Mist
so2
20% Opacity
0.10 lb/106 BTU
0.80 lb/106 BTU
1.20 lb/106 BTU
0.20 lb/106 IHU
0.30 lb/106 HTU
0.70 lb/106 UTU
0.08 gr./scf
(corrected to 122
coz)
10 Z Opacity
0.30 Ib/ton feed to kiln
0.10 Ib/ton feed to kiln
10Z Opacity
3 Ib/ton acid
10% Opacity
0.15 Ib/ton acid
it Ib/ton acid
e>
3
O.
CI
•-I
o.
Ul
o
01
(D
Ul
-------
SECTION III
CONTROL BY
MODIFICATION OF PROCESSES
-------
CONTROL BY MODIFICATION OF PROCESSES
An industrial process may be an unnecessary
contributor to air pollution in the sense that
an equally effective procedure is available,
or can be developed, which yields the same
desired product but which does not give off
the atmospheric contaminant in troublesome
quantities, or which produces a less object-
ionable waste. This suggests two approaches
to reducing air pollution- (1) Process modifi-
cation and (2) Change of process materials.
I PROCESS MODIFICATION
The process may be selected or the equipment
modified so that no contaminant, or a minimum
quantity of contaminant, is released.
A Process Change
At one time, coke was manufactured primarily
by bee-hive coke ovens. Coke manufacturing
by this method emits organic vapors to the
atmosphere as a result of the destructive
distillation process involved in coke man-
ufacturing.
In most areas of the United States the bee-
hive coke ovens have been replaced with
by-product (slot-type) coke ovens. By-
product coke ovens are normally equipped
with a chemical recovery system to collect
organic (coal-tar) vapors. These coal-tar
products are sold as by-products in the
manufacture of coke. In 1960U) about 94
percent of the coke manufactured in the
United States resulted from the by-product
manufacturing method.
B Process Control
Through improved control of the rate of
material feed, reaction temperature and
pressure, uniformity of mixing, and other
factors influencing reaction rate in chem-
ical processes, the generation of contam-
inant may be kept under better control and
especially, accidental periods of high
discharge may be avoided. In one situation
the release of H2S to the atmosphere was
caused principally by a failure of controls
to keep the reaction rate within proper
limits.I2)
C Equipment Modification
Several major automobile manufacturers plan
to modify automobile engines so that the
1966 models and later models sold in
California will meet California emission
standards for gaseous hydrocarbons and
carbon monoxide in exhaust gases.
The General Motors and Ford Systems work on
the principle of forcing air into the ex-
haust manifold to complete combustion of
unburned gasoline and carbon monoxide. The
Chrysler "Cleaner Air Package" modifies
engine design to prevent formation of
excessive hydrocarbons and carbon monoxide.(3)
II CHANGE OF PROCESS MATERIALS
By the substitution of materials used in the
process a less troublesome contaminant may be
produced.
In certain metal-cleaning operations, organic
solvents may be replaced with less volatile
agents. In one case an organic chemical plant
replaced benzol for process equipment clean-
ing with a less volatile and less toxic sol-
vent; a major reduction in airborne solvent
loss was the result.(2)
Rules and Regulations of the Los Angeles Air
Pollution Control District limit or specify
the content of several process materials in-
cluding the following:
A Suflur Content of Fuels
Rule 62 bans the use of high sulfur fuel
oils. The rule bans gaseous fuels contain-
ing sulfur compounds in excess of 50 grains
per 100 cubic fee of gaseous fuel (cal-
culated as H2§) or any liquid or solid
fuel having a sulfur content in excess of
0.5 percent by weight.
PA.C.ge.17.4.65
-------
Control by Modification of Processes
B Gasoline Specifications
Rule 63 prohibits the sale and use of fuel
for motor vehicles having a degree of un-
saturation exceeding a bromine number of
20. This rule limits the olefin content of
gasoline. Oelfin are very photochemically
reactive
REFERENCES
1 DeCarlo, J.A. et al. Coke Plants in the
United States on December 31, 1960.
U.S. Dept. of the Interior, Bureau
of Mines, Information Circular 8061.
1960.
2 Stern, A.C. Air Pollution II, Chapter 26.
Academic Press. New York. 1962.
3 Anon. Air/Water Pollution Report.August
24, 1964. pp 135.
4 Griswold, S.S. Air Pollution Control Field
Operation Manual. (Los Angeles Air Pol-
lution Control District). U.S. Public
Health Service Publication No. 937. 1962.
-------
SECTION IV
CONTROL BY COMBUSTION
Control by Combustion
Fundamentals in Combustion Calculations
Disposal of Gaseous Wastes - Flame and Catalytic Incineration
-------
CONTROL BY COMBUSTION
Darryl J. von Lehmder
I INTRODUCTION
Many organic compounds released from manu-
facturing operations can be converted to
innocuous carbon dioxide and water by rapid
oxidation—combustion. Ihree rapid oxidation
methods are used to destroy combustible con-
taminates: 1) furnaces, 2) flares and 3)
catalytic combustion. The furnace and flare
methods are characterized by Che presence of
a flame during combustion. Whereas, catalytic
combustion utilizes a metallic catalyst to
promote rapid oxidation. Catalytic combustion
is a flameless-type combustion.
II PRINCIPLES OF COMBUSTION
To achieve complete combustion, i.e., the
combination of the combustible elements and
compounds of a fuel with all the oxygen which
they can utilize, sufficient space, time,
turbulence, and a temperature high enough to
Ignite the constituents must be provided.
The "three T's" of combustion—time, tempera-
ture, turbulence—govern Che speed and com-
pleteness of the combustion reaction. For
complete combustion, the oxygen must come
into Intimate contact with the combustible
molecule at sufficient temperature, and for a
sufficient length of time, in order that the
reaction be completed. Incomplete reactions
may result in the generation of aldehydes,
organic acids, carbon and carbon monoxide.
The factors influencing completeness of com-
bustion are evaluated in more detail below:
A Temperature
Every combustible substance has a minimum
ignition temperature, which must be at-
tained, or exceeded, in the presence of
oxygen, if combustion is to ensue under
the given conditions. This ignition temp-
erature may be defined as the temperature
at which more heat is generated by the
reaction than is lost to the surroundings.
The ignition temperature for flow combust-
ion of combustible substances cover a large
range, as indicated in Table 1. (1)
D.J. von Lehmden
Chemical Engineer,
Office of Manpower Development.
National Air Pollution Control Administration
The ignition temperature of the gases vol-
atlzed from coal vary considerably, and are
appreciably higher then the Ignition temp-
eratures of the fixed carbon in the coal.
The gaseous constituents in the coal are
usually distilled off, but not ignited, be-
fore the ignition temperature of the fixed
carbon is attained. Therefore, if complete
combustion of the gases is to be achieved,
it is necessary that the temperature of the
effluent gases be raised to the ignition
temperature of the gases.
The same principle applies to the complete
combustion of any mixture of combustible
substances. A sufficiently high temperature
must be achieved which will burn all the
combustible compounds. To achieve such a
temperature it may be necessary to add
auxilllary heat to the combustible-laden
gas stream (e.g., via a gas fired burner).
Since the reaction rate increases with
temperature, temperatures considerably
above the ignition temperatures of the
combustible may be necessary to accomplish
complete combustion in a reasonable amount
of time.
B Oxygen
Oxygen is necessary for combustion to
occur. The end products of combustion de-
pend on the supply of oxygen. When methane,
for instance, is burned with too little
oxygen, solid carbon results thus:
Ch^ + 02 = C + 2H20 + Q (heat of reaction)
The solid carbon agglomerates forming part-
icles of soot and smoke. If enough oxygen
is supplied, the carbon is burned to carbon
dioxide, thus:
Here, then, It is completely burned, no
solid is set free, and hence, there is no
smoke .
When carbon is burned with an insufficient
supply of oxygen, carbon monoxide results:
PA.C.50a.4.65
-------
Control by Combustion
Table 1. FLAME IGNITION TEMPERATURE IN AIR*
(AC Pressure of One Atmosphere)
Combustible
Formula
Temperature"?
Sulfur
Charcoal
Fixed carbon
(bituminous coal)
Fixed carbon
(semibituminous coal)
Fixed carbon
(anthracite)
Acetylene
Ethane
Ethylene
Hydrogen
Methane
Carbon Monoxide
Kerosene
Gasoline
s
C
C
C
C
C2H2
470
650
765
870
840 - 1115
H2
CM*
CO
580
880
900
1065
1170
1130
490
500
825
1165
1020
1095
1380
1215
560
800
*Rounded-out values and ranges from various sources; a guide only.
2C + 02 = 2CO + Q
If enough oxygen is available, then carbon
dioxide results:
C + 02 = C02 + Q
The chemical reactions which occur during
the combustion of many gaseous compounds
are shown in Table 2. (1)
To achieve complete combustion of a com-
bustible compound with air, a Stoichiomet-
ric (theoretical) quantity of oxygen must
be available. The quantity of air which
must be furnished to obtain theoretical
complete combustion for many combustible
compounds is shown in Table 3. '''
It is necessary, however, to use more than
the theoretical air required to assure suf-
ficient oxygen for complete combustion.
Excess air would not be required if it were
possible to have every oxygen molecule
combine with the combustibles. The amount
of excess air added to insure complete
combustion must be held at a practical
minimum to reduce the stack heat losses.
Realistic values of excess air necessary
to burn various fuel are given in Table
4. (!)
C Time
A fundamental factor in the design and per-
formance of combustion equipment is the
time required for combustion of a particle
in relation to the residence time in the
equipment at combustion conditions. The
residence tine (at conditions conducive
to complete combustion) should be greater
than the time required for combustion of
the particle.
The time of residence depends primarily on
aerodynamic factors. Including size, which
are arbitrarily set in the design of the
unit. The time of combustion is controlled
by the temperature and aerodynamic factors.
The time of residence, then, becomes a
question of economy; namely size versus
temperature. The smaller the unit, the
higher the temperature must be to oxidize
the material ac the time of contact.
D Turbulence
Not only must the oxygen be supplied, but
it must be intimately mixed with the mater-
ial being burned so that it is available to
the combustion substance at all times. When
burning solids without turbulence, the init-
ial products of combustion act as a screen
for the incoming oxygen, and thereby alow
down the rate of surface reaction. The
burning of gases requires a thorough mix-
ing of them with air; otherwise separate
zones between the gases and air will form
and they will escape unchanged or incom-
-------
Control by Combustion
pletely burned.
Through Che proper regulation and control
of these four factors, complete combustion
can be attained.
Table 2. COMMON CHEMICAL REACTIONS OF COMBUSTION
Combustion
Reaction
Carbon (to CO)
Carbon (to CO-)
Carbon monoxide
Hydrogen
Sulfur (to SO.)
Sulfur (to SO..)
Methane
Acetylene
Ethylene
Ethane
Hydrogen sulfide
where Q = the heat of reaction
2C +
C +
2CO +
2H2+
S +
2S +
c*V
°2 =
°2 =
°2-
O.o
°2 =
302-
202=
2CO +
CO,
CO,
2H20
SO,
2S03
CO 2
Q
+ Q
+ Q
+ Q
+ Q
+ Q
+ 2H20
50,
2C0
2S0
Q
Q
Q
E Heat of Combustion
The rapid oxidation of combustible com-
pounds results in the exothermic reaction
(evolution of heat). The heat evolved (Q)
is known as the "heat of reaction" or more
specifically the "heat of combustion."
The principles involved in the development
of heat by combustion, generally accepted
as authoritative, were propounded by
Berthelot. His "second law," as applied
to combustion in furnace practice, is of
particular interest and may be stated as
follows ( ): In a furnace (where no
mechanical work is done) the heat energy
evolved from the union of combustible
elements with oxygen depends upon the
ultimate products of combustion and not
upon any intermediate combinations that
may occur in reaching the final result.
The heat of combustion for a number of
substances is shown in Table 3.
Ill TYPES OF COMBUSTION
A Principles of Flame Combustion
1 Yellow flame
A luminous (yellow) flame results
when air and fuel flowing through
separate ports are ignited at the
burner nozzle. Combustion occurs over
an extended area in the combustion
chamber, producing a highly radiant
flame. The expansion of the gases as
the flame progresses provides the
necessary turbulence, while a large
combustion chamber assures the neces-
sary time at the combustion tempera-
ture to complete the reaction.
2 Blue flame
A burner utilizing the same fuel, but
arranged to premix the air and fuel
-------
Combustion Constants
No Substance
1 Carbon*
2 Hydrogen
3 Ovtcten
4 Nitrogen (aim)
5 Carbon monoxide
6 Carbon dioxide
Para Km scnts
7 Methane
8 Eihane
9 Propane
10 n-Bulanc
1 1 Isobutane
12 n-Pemane
\ 1 Isopemane
1 4 Neopeniane
15 n-Hctarte
Olcfin series
16 Elh>lene
17 Prop>lcne
IS n-Bulenc
19 Isobuiene
20 n-Peniene
Aromauc vena
21 Benrcne
22 Toluene
23 X>lene
Miscellaneous gases
24 Acei>lene
25 Naphthalene
26 Meihyl alcohol
27 tthvl alcohol
28 Ammonia
29 Sulfur'
30 H>drogen sulndc
31 Sulfur dioxide
32 Wjter vapor
33 Air
'
Formula
C
H:
O?
N:
CO
CO:
CH.
C-H,
C.H.
C.Hi,
C.Hn
C,H,:
C,Hi:
C.Hi:
C.H..
C-H.
C.Hi
C.H.
C.H.
C.H,,
C.H.
C-H.
C-Hn
C=H5
d H.
CH.OH
C:H>OH
NH>
S
H-S
SO
H O
Molecu-
lar
Weigh!
1201
2016
32 000
2SOI6
2801
4401
16041
30067
44092
$8 118
58 118
72 144
72 144
72 144
86 169
28 Oil
42077
56102
56 102
70 128
78 107
92132
106158
26036
128 162
32041
46067
17031
3206
34076
6406
18016
2K9
Lbper
Cu II
00033
00846
00744
00740
01170
00424
00803
0 1196
01582
01S82
01904
0 1904
0 1904
02274
00746
OHIO
0 1480
01480
0 1852
02060
02431
02803
00697
03384
00846
01216
00456
00911
0 1733
00476
00766
Cu r\
per Lb
187 723
II 819
13443
13506
8548
23565
12455
8365
6321
6321
5252
5252
5252
4398
[3412
9007
6756
6756
5400
•4852
4 113
3567
14344
2955
II 820
8221
21914
10979
5 770
21 017
13063
Sp Gr
Air
10000
00696
1 1053
09718
09672
1 5282
05543
10488
1 5617
20665
•20665
24872
24872
24872
29704
09740
1 45O4
19336
1 9336
24190
26920
3 1760
36618
09107
44208
1 1052
1 5890
05961
1 1898
22640
06215
1 0000
Blu pe
Grow
(High)
325
322
1013
1792
2590
3370
3363
4016
4008
3993
4762
1614
2336
3084
3068
3836
3751
4484
5230
1499
5854
868
1600
441
647
Heal of C
r Cu H
Net
(Loo)
275
322
. .
913
1641
2385
3113
3105
3709
3716
3693
4412
1513
2186
2885
2869
3586
3601
4284
4980
1448
5654
768
1451
365
596
'omhusiion
Blu per
Gross
(High)
14.093
61,100
'.
4.347
.
2J.879
22.320
21,661
21,308
21,257
21,091
21,052
20.970
20,940
21,644
21.041
20,840
20.730
20.712
18.210
18.440 '
18,650
21,500
17.298
10.259
13.161
9,668
3.983
7,100
For 100'; Total Air
Mules per mule of Combustible
Lb
or Fur 100', Total Air
Cu Fl per Cu Ft of Combustible Lh per Lb of Combustible
Nrl Required for Combustion Flue Products Required for Combustion 1 luc Produc
(Low)
14.O93
51.623
.
4.347
21.520
20,432
19,944
19.680
19,629
19,517
19,478
19,396
19,403
20,295
19,691
19.496
19.182
19,363
17,480
17.620
17.760
20.776
16.708
9.078
11.929
8.001
3,983
6.545
O, N, An CO, HjO N, O. N. An CO, II,O
10 3 76 4 76 10 3 76 2 66 8 86 1 1
HX6
2A4I
MX)
i<:x
i: »
1:07
H »i
H 11
11 XI
II 11
II Kl
11 14
II 3sl
II 39
II 39
II il
II 39
1022
1040
10 SJ
1022
9
-------
Control by Combustion
Table 4. USUAL
Fuel
Pulverized coal
Crushed coal
Coal
Fuel-oil
Acid sludge
AMOUNT EXCESS AIR SUPPLIED TO FUEL-BURNING EQUIPMENT
Type of furnace or burners
Completely water-cooled furnace for slag-tap or
dry-ash-removal
Partially water-cooled furnace for dry-ash-removal
Cyclone furnace - pressure or suction
Stoker-fired, forced-draft, B&W chain-grate
Stoker-fired, forced-draft, underfeed
Stoker-fired, natural-draft
Oil-burners, register-type
Multlfuel burners and flat-flame
Cone-and flate-flame-type burners, steam-atomized
Excess Air
% by wt.
15-20
15-40
10-15
15-50
20-50
50-65
5-10
10-20
10-15
Natural, Coke-oven,
& Refinery gas
Blast-furnace gas
Wood
Bagasse
Black Liquor
Register-type burners
Multifuel burners
Intertube nozzle-type burners
Dutch-oven (10-23X through grates) and Hofft-type
All furnaces
Recovery furnaces for kraft and soda-pulping processes
5-10
7-12
15-18
20-25
25-35
5-7
prior to delivery to the burner nozzle,
will produce a short, intense, blue
flame, permitting complete oxidation
within a confined space.
In any fuel-fired burner, whether it
Is of the luminous (yellow flame) or
permixed (blue flame) type, substalned
combustion depends upon maintaining
the air-gas supply to the burner within
the flammable range.
B Flare Combustion
All process plants which handle hydro-
carbons, hydrogen, ammonia, hydrogen
cyanide, or other toxic or dangerous
gases are subject to emergency conditions
which occasionally require immediate
release of large volumes of such gases
for protection of plant and personnel.
In any petrochemical process, hydro-
carbons present with inert gases, such
as nitrogen and carbon dioxide, must
be continuously released in variable
volume and concentration. Where these
gases are released at energy concentra-
tions constantly within or above the
flammable range, their disposal can be
handled most economically and safely by
application of flares. However, smoke-
leas burning of huge quantities of gases
by flares presents soae serious design
problems. First, the flare must be suff-
iciently elevated above ground level for
heat and flame protection of adjacent
buildings and personnel. Flame must be
sustained at varying rates exceeding by
many times the operating range of in-
dustrial burners. These physical demands
prevent the employment of combustion
chambers.
Flare combustion Is often characterized
by a luminous (yellow) flame. The lumin-
ous flame results when oxygen in the air
surrounding the flame comes in contact
with the hydrocarbons by diffusion only.
..The luminous color results from Incandes-
cent carbons which result from the
cracking of the hydrocarbon molecules.
-------
Blue-flame flare combustion can be ac-
complished by adding water vapor, under
proper temperature conditions, as the
gas is burned. A water-gas reaction is
set up, generating carbon monoxide and
hydrogen which assists in the production
of blue flame burning by removing the
unburned carbon. Combustion of carbon
monoxide and hydrogen results in carbon
dioxide and water. (2)
C + H20 = CO + H. (water-gas reaction)
CO 4- 1/202 = C02
H + 1/20, = H,0
One design of a steam injected flare is
illustrated in Figure 1. (2)
steam |et'
steam manifold
flare tip
pilot tip
V- flame front ignitor tip
steom —
supply line
/
flame front ignitor
~ tube
-— pilot mixer
- pilot gos connection
*—to pilot 2
to pilot 3
olves
C Furnace Combustion
Whereas flares are effective in destroy-
ing waste gases which are released
continuously or periodically at concentra-
tions above the lower limit of flammability,
gases vented from industrial processes are
generally exhausted at concentrations far
below the lower flammable limit. At these
concentrations of gases, combustion in
an enclosed chamber is necessary.
Furnace combustion is commonly called
''direct flame incineration", since a
separately fired burner is normally em-
ployed to sustain rapid oxidation. The
flame, per se. has no influence on the
reaction except as it provides the time-
temperature - turbulence factors.
Since "the three T's" allow considerable
latitude in design, numerous combinations
of the "three T's" will result in com-
plete combustion. Generally, however,
furnace construction costs required a
practical limit on holding time of less
than 0.5 to 0.75 seconds. Residence
temperature may vary from 950°F ^or
naphtha vapors to 1600°F for methane and
somewhat higher for some aromatic hydro-
carbons.^) Higher percentages of inerts
in the gas stream which act as oxidation
depressants will demand higher tempera-
tures. (-
A diagram of a waste gas and odor in-
cinerator is shown in Figure 2.
from
process
exhauster
burner air
scroll heater
END ELEVATION
SECTIONAL SIDE ELEVATION
Figure 1. STEAM INJECTION TYPE FLARE
Figure 2. DIAGRAM OF A WASTE GAS
AND ODOR INCINERATOR
-------
Table 5. INDUSTRIAL APPLICATIONS OF CATALYTIC COMBUSTION
Industrial process
Contaminating agents
in waste gases
Approximate tempera-
ture required for
catalytic oxidation
Asphalt Oxidizing
Aldehydes, Anthracenes,
Oil Vapors, Hydrocarbons
600° - 700"F
Cabon Slack "fg.
Catalytic Cracking
Units
Core Ovens
Formaldehyde Mfg.
HN03 Mfg.
Metal Lithography
Ovens
Octyl-phenol Mfg.
Phthalic Anhydride
Mfg.
Polyethylene Mfg.
Printing Presses
Varnish Cooking
Wire Coating and
Enameling Ovens
H2, Cf\ CH4, Carbon
CO, Hydrocarbons
Wax, Oil Vapors
H2, CHA, CO, HCHO
NO, N02
Solvents, Resins
C, H.OH
0 J
Maleic Acid, Phthalic Acid,
Naphthaquinones , Carbon
Monoxide, Formaldehyde
Hydrocarbons
Solvents
Hydrocarbon Vapors
Solvents, Varnish
*1200° -
650° -
600° -
6SO
**500" -
500° -
600° -
600" -
500° -
600°
600° -
600° -
1800'F
800° F
700" F
°F
1200" F
750" F
800" F
650" F
1200eF
F
700° F
700°F
* Temperatures in excess of 1200°F required to oxidize carbon.
** Reducing atmosphere required.
D Catalytic Combustion
Catalytic combustion is the lowest
temperature method of rapidly oxidizing
combustible gases and vapors. Many
substances exhibit catalytic properties,
but metals in the platinum family are
recognized for their ability to produce
the lowest catalytic ignition temperatures
and are therefore conventionally used.
Since catalytic oxidation is a surface
reaction, relatively small amounts of
platinum are used in a way which exposes
the maximum surface area to the gas stream•
This is accomplished by coating a high
surface area substance with the catalyst
and arranging the catalyst coated sub-
stances in catalyst beds.
Sufficient surface area must be supplied
to permit the oxidation reaction to be
completed within the bed, since "the
three TV still apply if "exposed catalyst
surface" is substituted for "time."
Turbulence is achieved in the passage of
the contaminated gases through the bed.
Catalyst temperature results from the
oxidation reaction itself, plus burner
or electric preheating where necessary.
With platinum alloy catalysts, oxidation of
hydrogen will be initiated at ambient
temperature, naphtha at 450°F, and methane
at 750°F.<12)
The temperatures required to catalytically
oxidize many organic compounds are given
in Table 5.^
-------
preheat
burner
inlet
exhaust fan
inner metal liner
mineral wool
insulation
outer metal jacket
catalyst elements
*-access door
stock outlet
connection
heot
exchanger
exhaust
from process
Figure 3. CATALYTIC COMBUSTION SYSTEM INCLUD-
ING PREHEAT BURNER AND EXHAUST FAN
Catalytic combustion is generally appli-
cable where the following conditions
apply: 1) where the gas stream to be
handled contains vaporized or gaseous
combustible materials, and 2) where there
is no large amount of dust, fly ash, or
other solid inorganic material in the gas
stream.
Catalytic systems are designed to prevent
condensate formation in exhaust equip-
ment. The exhaust fan in a catalytic
system is located on the hot side of the
system so that all vapors passing through
it are above the condensation temperature.
A typical catalytic combustion system
employing a preheat burner is shown in
Figure 3. (jL>
Heat evolved by the catalytic oxidation
can also be used to preheat the gas
stream. Figure 4(4) shows a heat ex-
changer and preheat burner arrangement to
heat the gas stream to the catalytic
ignition temperature.
Figure 4. CATALYTIC COMBUSTION SYSTEM INCLUD-
ING HEAT EXCHANGER, PREHEAT BURNER
AND HEAT FAN
IV
SELECTED APPLICATION OF COMBUSTION TO
AIR POLLUTION CONTROL
A Flare Combustion
1 Dimethylamine odor control during
the manufacture of soaps and
detergentsO)
Providing new and improved products for
the consumer requires new processes and
new chemical raw materials. The manu-
facture of new products by a soap and
detergent company required the use of
dimethylamine as a raw material.
Dimethylamine NH(CH3>2 is a
gaseous material at 40°F and atmospheric
pressure. The material is a first cousin
of ammonia (NH ) and at concentrations
in excess of 100 ppm the odor of this
amine is nearly identical to that of
ammonia. As the amine concentration
becomes diluted it takes on an odor
resembling fish which has been in the
sun too long. As the concentration
falls below 100 ppm the fish odor be-
comes predominant over the ammonia odor.
-------
discharge to stack
blower
' open kettles
closed kettle
Figure 5. LAYOUT OF FUME COMBUSTION SYSTEM FOR PAINT AND VARNISH COOKING
To provide relief from this fishy odor
for residential areas 2500 feet from the
plant, a method was needed to destroy the
dimethylamine emissions. The solution
to this odor problem was obtained through
the use of a 100-foot flare stack. The
amine-laden waster gases are vented to a
holding tank for storage. A continuous
flow of the amine-laden gases enters the
base of the flare stack; passes through
a flame arrestor and are incinerated at
the top of the flare stack. The stack
is equipped with four natural gas pilot
lights to assure ignition of all combus-
tibles released from the stack. The
result has been a smokeless flare which
reduces the odor problem several orders
of magnitude.
B Furnace Combustion
1 Methyl mercaptan, hydrogen sulfide
and methyl sulfides odor control from
the Kraft Pulping Process
Odors resulting from the Kraft (sulfate)
pulping process have been reduced by
rapid oxidation of the non-condensable
gases emitted from the black liquor.
Gaseous emissions from the black liquor
contain the odorous compounds, methyl
mercaptan, hydrogen sulfide and methyl
sulfide. The oxidation of these compounds
results in sulfur compounds which are
less volatile and therefore less odor-
ous than the original contaminants.
Several west coasts pulp and paper
mills have installed equipment to
incinerate these odorous sulfur gases.
One mill utilizes an integral part of
the Kraft process, the lime kiln, to
incinerate the odors. Another method
of reducing the odors has been the
oxidation of the black liquor itself.
Emissions of gaseous sulfur compounds
may be as high as 100 pounds per ton of
pulp from an uncontrolled Kraft process.
Vapor control from paint and varnish
cookers (?)
In the burning of combustible vapors
from paint and varnish cookers, adequate
consideration must be given to prevent
fire or explosion in the kettle as a
9
-------
result of flashback through the vapor.
Safeguards can be achieved by diluting
the vapor concentration to less than 25
percent of the lower explosive limit
and by maintaining gas velocities in the
ducts well in excess of 20 feet per
second, the rate at which flame could
propagate along the duct.
A fume (vapor) combustion system for
incinerating vapors from paint and
varnish cookers is shown in Figure 5.
A correctly proportioned and well-In-
sulated furnace requires a fuel input
between 600 and 1200 BTU per hour per
gallon of processed batch. In some
processes, enough vapors are produced
to appreciably supplement the regular
fuel.
3 Odor control from coffee roasters (°)
The combustibles in effluent gases from
coffee roasting may be present in con-
centration ranging from 0.17 to 0.27
grains per cubic foot, depending upon
the type of roaster and the rate of
exhaust flow.
The roaster exhaust gases include the
following compounds: formic acid, high
fatty acids, furfural, methylamine,
pyrole, acetic acid, acetone, ammonia
pyridine, and hydrogquinone.
Furnace combustion is presently being
employed to incinerate the combustible
gases from coffee roasters and to re-
duce the odor per se.
Vapor and particulate control from a flue-
fed (apartment house size) incinerator (9)
Flue-fed incineration is a batch-type
operation. To effect complete combustion
requires that the correct amount of com-
bustion air (Stoichiometric air plus some
excess air) be supplied during each seg-
ment in the combustion cycle. Normally,
however, batch-type incineration results
in a deficiency of air after the refuse
is charged and too much excess air during
the burn-down portion of the cycle. The
result is incomplete combustion of com-
bustible gases and particulates (solids) .
Afterburners are used to effect complete
combustion of the incinerator effluent
gases. Normally an afterburner built
into the incinerator or a roof-type
afterburner is used to incinerate the
effluent gases. A diagram of a roof
afterburner is shown in Figure 6.
Burner
Settling Chamber
Roof
Figure 6. ROOF AFTERBURNER
10
-------
return duct
stack damper *
exhaust fan
catalyst bed
LITHO-OVEN
Figure 7. CATALYTIC OXIDATION OF SOLVENT FROM METAL LITHOGRAPHIC OVENS
C Catalytic Combustion
1 Catalytic oxidation of lithographic oven
vapors (4)
Catalytic combustion is used in many
industrial processes to destroy odors
and contaminant gases. Among the processes
in which catalytic oxidation is used in-
cludes varnish cooking, carbon black
manufacture and metal lithographic ovens.
Figure 7 shows a catalytic oxidation
system used on a metal lithographic oven.
Here a portion of the process effluent,
after having been catalyzed and cleaned,
is channeled back to a lithographying
oven. The returned exhaust, split among
several zones in the oven, provides all
or a part of the oven heat requirements.
Excess exhaust over that needed to heat
the oven is vented directly to the at-
mosphere.
2 Catalytic reduction (deoxldation) of
nitrogen oxides in waste gases from
nitric acid manufacture
Catalytic combustion is also used to reduce
contaminants to lower oxidation-state com-
pounds. Waste gases (tailgas) from the
manufacture of nitric acid contain NO.N02
and nitric acid vapors. Concentrations of
nitrogen oxides in these waste gases range
from less than 0.1 per cent to almost 50
per cent by volume. By mixing a hydrocarbon
(e.g., methane) or reactive fuel (e.g.,
carbon monoxide) with the waste gases and
passing the gases through a catalyst, the
following reaction takes place if the
reaction goes to completion.
(n2)HC
(n3)H2)
This reaction can be made to proceed at
comparatively low temperature (500-1200"F).*
The amount of free oxygen contained in the
waste gas stream presumably influences the
ease with which the reaction can be com-
pleted. Obviously, when the waste gas
stream is entirely devoid of free oxygen,
then oxidation of the hydrocarbons can occur
only through simultaneous reduction of the
nitrogen oxides to a lower oxidation state
or free nitrogen.
A schematic of a catalytic reduction sys-
tem for nitric acid waste gases is shown
in Figure 8.
exhaust to atmosphere
exhaust fan
catalyst bed
recycling
gases
preheat burner ^
reducing fuel process waste gas
Figure 8. SCHEMATIC OF CATALYTICAL REDUCTION
SYSTEM FOR NO
* Table 5
11
-------
REFERENCES
1 Babcock and Wilcox Co. Steam-Its Genera-
tion and Use. 37th Edition, Chapter 4
1963.
2 Stern, A.C. Air Pollution. Academic Press
New York City, Vol. II, Chapter 32.
3 Coward, H.F. et al. U.S. Bureau of Mines-
Bulletin 503. Vol. 4, 1952.
4 Oxy-Catalyst, Inc. Basic Engineering
Principles of the Oxycat. Berwyn, Penn.
5 Byrd, J.F. et al. Solving a Major Odor
Problem in a Chemical Process. JAPCA,
Vol. 14, pp 509-516. December 1964.
6 Hendrickson, E.R. et al. Black Liquor
Oxidation as a Method for Reducing Air
Pollution from Sulfate Pulping. JAPCA,
Vol. 14, pp 487-490. December 19bi.
7 Stenburg, R.L. Control of Atmospheric
Emissions from Paint and Varnish
Manufacturing Operation. U.S. Public
Health Service. "..». Taft Sanitary
Engineering Center. Technical Report
A58-4.
8 Anon. Discussion of Coffee Roasting Pro-
cess. LAAPCD.
9 MacKnight, R.J. et al. Controlling the
Flue-Fed Incinerator. JAPCA, Vol. 10.
April 1960.
10 Donahue, J.L. System Designs for the
Catalytic Decomposition of Nitrogen
Oxides. JAPCA, Vol. 8, pp 209-212, 222.
November 1958.
12
-------
FUNDAMENTALS IN COMBUSTION CALCULATIONS
Darryl J. von Lehmden
I COMBUSTION TERMINOLOGY
A Numerous terms are used in combustion
calculations. Occasionally more than one
term will have generally the same mean-
ing. In order to avoid ambiguities in
terms used in combustion calculations,
the terms commonly used must be under-
stood. Therefore, definitions of terms
commonly used in combustion calculations
are listed below.
B Definitions
1 Heat of Combustion: The heat released
by the complete combustion of a speci-
fic quantity of fuel, etc. * with molecu-
lar oxygen. Heats of combustion are
normally reported in BTU per cubic
foot or BTU per pound of fuel.
2 Gross Heating Value: The total heat ob-
tained from the complete combustion of
a fuel which is at 60°F when combustion
starts, and the combustion products of
which are cooled to 60°F before the quan-
tity of heat released is measured. Con-
stant pressure, normally 1 atmosphere
(29.92 m. Hg), is maintained throughout
the entire combustion process. Gross
heating values are also referred to as
total or higher heating values.
3 Net Heating Value: The gross heating
value minus the latent heat of vaporiza-
tion of the water formed by the com-
bustion of the hydrogen in the fuel. For
a fuel containing no hydrogen, the net
and gross heating values are the same.
4 Latent Heat of Vaporization: Heat given
off by a vapor condensing to a liquid or
gained by a liquid evaporating to a
vapor, without a change in temperature.
The latent heat of vaporization of water
at 212°F is 970. 3 BTU per pound.
D.J. von Lehmden, Chemical Engineer
Office of Manpower Development
National Air Pollution Control Administration
5 Sensible Heat: Heat, the addition or
removal of which results in a change
in temperature, as opposed to latent
heat of vaporization.
6 Available Heat: The gross quantity of
heat released within a combustion
chamber minus (1) the sensible heat
carried away by the dry flue gases and
(2) the latent heat and sensible heat
carried away in water vapor contained
in the flue gases. The available heat
represents the net quantity of heat
remaining for useful heating. Figure
2 and 3 shows the available heat from
the complete combustion (No excess
air) of various fuels at various flue
gas temperatures.
Figure 4 is a generalization for all
fuels giving percent available heat
with various flue gas temperatures
and various amounts of excess air.
7 Heat Content; The sum total of the
latent and sensible heat present in a
substance (gas, liquid, or solid)
minus that contained at an arbitrary
set of conditions chosen as the base
or zero point. Heat content is usually
expressed in units of BTU per pound.
For gases, the heat content may be
expressed in BTU per cubic foot if the
conditions of pressure and temperature
under which these volumes are
measured are specified.
The heat content of various gases in
BTU per pound is given in Table 1.
Figure 1 is a graphical representation
showing the inter-relationship of the
terms previously defined.
*Includes combustible waste solids, waste
liquids and waste gases resulting from man's
activities. Hereafter referred to in the
definition of combustion terms under the
title; fuel(s).
PA.C.ge.10.12.05
-------
TAIILK 1
Combustion Constants
No. Substance
I Carbon*
2 Hydrogen
3 O>% gen
4 Nitrogen (aim)
5 Carbon monoxide
6 Carbon dioxide
PanHin scries
7 Methane
8 Ethane
9 Propane
10 n-Butane
1 1 Iiobmanc
12 n-Pentane
1 * Isopeniane
14 Neopcntanc
15 n-Hexane
Olcfin series
16 hthyltne
17 Prop>lene
IK n-Rutene
19 Ivobutene
2O n-Pentene
Aromatic verjes
21 Ben/enc
22 Itiluenc
2* \tleiie
Misiclljneiiui gases
24 Aicl)lcne
2^ Naphthalene
26 Mnh>l alcohol
27 1 th*l alcohol
28 Ammonia
29 Sulfur*
30 ll>drogen sulfide
31 Sulfur diondc
^2 Wjier vapor
33 Air
Formula
C
Hi
Oi
N.
CO
COi
CH,
C-H,
CiH.
C.H,.
CiH.o
C.H,:
C.H,:
CtHi:
C.H,,
C:H.
CiHi
C.H-
CiH.
C.H,.
C.H.
C H.
C,H,0
C-H,
C,.H,
CHiOH
NHi
S
H:S
SO-
H-0
Molecu-
lar
Weight
1201
2016
32000
28016
2801
4401
16041
30067
44O92
38 118
58 118
72 144
72 144
72 144
86 169
28051
42077
56102
56 102
70 128
78 107
92 132
106 158
26036
128 162
32041
46067
17031
3206
34076
6406
III 016
:s.
Lbper
CuFt
00053
00846
00744
00740
01170
00424
00803
01196
0 1582
01582
019O4
01904
01904
02274
00746
0 II 10
0 1480
0 1480
0 1852
02060
02431
02803
00697
03384
00846
0 1216
00456
00911
01733
00476
00766
Cu Ft
per Lb
187 723
11819
13443
13.506
8.548
23565
12 455
8365
6.321
6321
5252
5252
5252
4398
13412
9007
6756
6756
5400
4852
4 113
3567
14344
2955
11 820
8221
21 914
10979
5770
21 017
I3O63
SpGr
Air ---
10000
00696
1 1053
09718
09672
1.5282
05543
10488
1 5617
20665
70665
24872
24872
24872
29704
09740
1 4504
1 9336
19336
24190
26920
3 1760
36618
09107
44208
1 1052
1 5890
05961
1 1898
22640
06215
10000
Heat of (
Btu per Cu Ft
Gross Net
(High) (Low)
325
322
_
1013
1792
2590
3370
3363
4016
4008
3993
4762
1614
2336
3084
3068
3836
3751
4484
5230
1499
5854
868
1600
441
647
275
.
322
_
913
1641
2385
3113
3105
3709
3716
3693
4412
1513
2186
2885
2869
3586
3601
4284
4980
1448
5654
768
1451
365
S96
Combustion
Btu per Lb
Gross Net
(High) (Low)
14.093
61,100
4.347
23,879
22.320
21,661
21.308
21.257
21,091
21,052
20.970
20,940
21,644
21,041
20.840
20,730
20,712
18,210
18.440
18,650
21,500
17,298
10.259
13.161
9,668
3.983
7,100
14.093
51.623
4,347
21,520
20.432
19.944
19.680
19,629
19.517
19.478
19.396
19,403
20.295
19.691
19.496
19,382
19,363
17,480
17.620
17,760
20.776
16,708
9,078
11.929
8,001
3.983
6.545
For 100% Total Air
Moles per mole of Combustible
or for 100**; Tuial Air
Cu Ft per Cu Ft of Combustible Lb per Lb of Combustible
Required for Combustion Flue Products Required for Combustion Klut Products
O, N, Air CO, H,O N, O. N. Air CO- H.O N..
10 376 476 10 . 376 266 886 1133 366 1 X6
OS 1.88 238 10 188 794 2641 3434 8 SI4 :s4l
0.5 1 88 2 38 10 . . 1 88 0 57 1 90 2 47 1 57 1 -10
20 753 953 10 20 7 53 3 99 1328 1727 274 223 \\ M
35 1318 1668 20 30 1318 373 1239 1612 293 1 HI) 12 ">
SO 1882 2382 30 40 1882 363 1207 1570 299 163 12 O7
65 2447 3097 40 50 2447 358 1191 1549 303 155 11*41
65 2447 3097 40 50 2447 358 1191 1549 303 155 11 tl
80 3011 3811 SO 60 3011 355 1181 1533 305 MO 11 XI
8.0 3011 3811 SO 60 3011 355 1181 1535 305 150 11X1
80 3011 3811 SO 60 3011 355 1181 1535 305 150 1181
95 35.76 4526 60 70 35 76 3 53 1174 1527 306 146 1174
30 1129 1429 20 20 1129 342 1139 1481 314 129 1134
45 1694 2144 30 30 1694 342 1139 1481 314 129 1139
60 2259 2859 40 40 2239 342 1139 1481 314 129 1139
60 2259 2859 40 40 2239 342 1139 1481 314 129 1139
73 2823 3373 50 50 28 23 3 42 1139 1481 314 129 1139
75 2823 3573 60 30 2823 307 1022 1330 338 069 1022
90 3388 4288 70 40 33 88 3 13 1040 1353 334 078 1040
10S 39S2 5002 80 50 3952 317 1053 1370 332 083 1053
25 941 1191 20 10 941 307 1022 1330 338 069 1022
120 4517 57.17 100 40 4517 300 997 1296 343 056 997
'3 565 715 10 20 565 150 4 98 648 137 113 498
30 1129 1429 20 30 1129 208 693 902 192 117 693
075 282 357 15 332 141 469 610 . 139 < si
SO: SO.
10 3 76 4 76 10 3 76 1 00 3 29 4 29 2 00 3 >>>
15 365 7I5 10 10 565 141 469 610 188 051 46V
'Curtail and sulfur art- fon-.idt.Tcd as gases for molal calculations only
Note: This talilc is rupnnU-d from Fuel Fine Cases, 19-11 Edition,
courtesy of American Cai Aibocution.
All g.ii \olunics corrected to 60 F and 30 in Hg dry.
-------
Fundamentals in Combustion Calculations
HEAT
COMB
OP F
OF
USTION
UEL
No H,
Present
GROSS HEATING VALUE (H.V.G)
AVAILABLE HEAT (HA)
NET HEATING VALUE (H.V.jj) LAT1
NET HEATING VALUE
HEAT LOSS IN
EXIT FLUE GAS
ENT HEAT OF VAPOR. OF WATER
HEAT PRESENT PRIOR
TO COMBUSTION
H.C
H.C
- Hg + Hy
Hg + Heat
- present prior
to combustion
FIGURE 1. INTER-RELATIONSHIP OF TERMS USED IN COMBUSTION CALCULATIONS
C Tables Useful in Combustion Calculations
1 Gross heating value, net heating value,
theoretical combustion air and specific
gas volumes.
Heating values for the complete com-
bustion of various fuels arc shown in
Table 1. Theoretical air require-
ments, for complete combustion are
shown also in Table 1.
The volume occupied by one pound of
gas under any specified condition of
temperature and pressure is called
the specific volume. The specific
volume for various gases (including
fuels) at 60°F and 1 atmosphere are
also given in Table 1.
Since most combustion processes take
place at constant pressure (normally 1
atmosphere) the specific volume of
gases at elevated temperatures may
be calculated from Charles' Law.
Charles Law states that the volume
varies in direct proportion to the
absolute temperature under constant
pressure.
V V
1 2
Charles Law: — = —
1 2
2 Available heat (complete combustion,
no excess air)
a General type fuels
The curves in Figure 2 show the
available heat when the hydrogen
to carbon ratio in the fuel is known.
The available heat is for complete
combustion of the fuel using the
theoretical amount of air required
for combustion.
The curves serve as a generalized
comparison of the available heat
from pure hydrocarbon fuels. In
order to use the curves properly a
fuel must be chemically analyzed
to determine the carbon/hydrogen
ratio.
When available heat curves for
specific fuels are accessible (e.g.
Figure 3), such curves should be
used in preference to the curves
in Figure 2.
-------
f-
1
Fundamentals in Combustion Calculations
in
3
U
fi
i-
Ol
0.
I/I
0)
c
o
,
£
3
o.
1.
to
g-
o
(J
T3
01
TJ
i.
a>
c
01
The available heat for fuels not shown
in Figure 3 may be calculated from the
following ratio (See Part B of the ex-
ample calculation for more detail).
(Specific fuel) =
HA
H.V.,
(Fig. 3)
0.75 080 0.85 090 0.95 100
Pounds Carbon / Pound combustible
0 25 0 20 015 0 10 0.05 0.00
Pounds Hydrogen / Pound combustible
3 ' 4 5 4 7 8910 00
Carbon / Hydrogen ratio
Figure 2. GENERALIZED COMPARISON OF
AVAILABLE HEATS FOR PURE
HYDROCARBON FUELSd) (Refer to 60°F)
b Typical fuels
Figure 3 shows the available heats
for some typical fuels. All available
heal values are based upon complete
combustion (theoretical air) and a
fuel input temperature of 60°F.
The gross heating values for the
fuels arc indicated next to the name
of the fuel. The scale on the left
side of Figure 3 is for the solid
curves. The scale on the right side
is for propane and butane (dashed
curves).
Table 2 shows a number of gross
heating values for some specific
gaseous fuels.
Available Heat (Complete Combustion;
Excess Air Present)
Figure 4 is a generalization for all
fuels giving the available heat as a
percent of the gross heating value at
various flue gas temperatures and
various amounts of excess combustion
300 600 9001200 15001800 2100 2400 2700 3000
Flue gas exit temperature °F
Figure 3. AVAILABLE HEATS FOR SOME
TYPICAL FUELS* 1) (Refer to 60°F)
-------
Fundamentals in Combustion Calculations
air. This chart is approximate since
it is based upon the assumption that
the combustion air required per gross
BTU is the same for all fuels.
The curves in the chart assumes that
combustion of the fuel is complete.
4 Heat Content of Gases
The heat contents for several gases at
various temperatures are given in
Table 3.
The heat contents in Table 3 are ex-
pressed in units of BTU per pound of
gas. To convert the heat contents in
Table 3 from BTU per pound to BTU
per cubic foot, multiply by the specific
volume of the gas at the temperature
desired.
Table 2. GROSS HEATING VALUES FOR
SPECIFIC GASEOUS FUELS*1*
Type of fuel
Natural Gas:
Birmingham, Ala.
Cleveland, Ohio
Kansas City, Mo.
Pittsburgh, Pa.
Commercial Propane:
Natural Gas
Refinery Gas
Commercial Butane:
Natural Gas
Refinery Gas
Gross heating value
BTU per cu.ft(60°F)
1002
1059 (See Fig. 3)
974
1129
2558 (See Fig. 3)
2504
3210 (See Fig. 3)
3184
<0
ss
en wi
(Refer to 60°F)
-------
Fundamentals in Combustion Calculations
Table 3. HEAT CONTENTS OF VARIOUS GASES(1)
Relative heac content in Btu per pound (it atmospheric r>rct%tire)
icmp
't
oo
1(10
;oo
MO
400
>oo
(00
700
HOO
900
1000
1200
MOO
itoo
1800
2000
2200
2400
2(00
2800
3000
3200
3400
3600
0,
0
88
309
533
762
994
1231
1472
171.7
196.6
221 7
272.5
324.3
377.3
430.7
4840
5393
594.4
649.0
702.8
7)8.6
816.4
873.4
931.0
N,
0
9.9
34.8
59.9
850
1103
1361
161.7
187.7
2139
240.7
294.7
3508
407.3
4650
523.8
583.2
642.3
7028
763.1
824.1
885.8
947.6
1010.3
Air
0
9.6
336
577
81 8
106.0
1302
1545
1789
203.4
2350
288.5
3430
3980
4550
513.0
570.7
6285
687.3
7466
806.3
866.0
925.9
986.1
CO
0
100
349
59.9
85.0
1106
1363
1624
1887
215.6
242.7
297.8
3543
407.5
465.3
523.8
5833
6430
7032
771.3
832.6
894.0
936.0
1018.3
CO,
0
80
29.3
520
753
99.8
125 1
1496
177.8
205.6
233.6
2909
349.7
4163
470.9
5328
596.1
6592
723.2
787.4
8520
916.7
981.6
1047.3
SO,
0
5.9
21.4
375
544
718
898
108 2
1270
146.1
165.5
205 1
245.4
286.4
327.8
369.1
411.1
4527
495.2
537.5
580.0
622.5
665.0
707.5
H2
0
M7
484
832
1182
1532
1882
2233
2584
2935
3291
4007
4729
5460
6198
6952
7717
8490
9272
10060
10870
11680
12)10
13330
CH4
0
21 0
761
1364
202 1
2726
3478
427.4
511.2
5992
691 1
886.2
1094.1
1313.0
1)42.6
.
H,O
0
iioy
1212
1259
1307
1355
1404
1454
1505
1609
1717
1829
II EXAMPLE CALCULATION ILLUSTRATING
THE USE OF THE COMBUSTION TABLES
(Based on Reference 1, Example 15, Page
54)
A boiler burns 10,000 cubic feet per hour of
Pittsburgh natural gas.
DETERMINE:
A The gross heating value per hour for
complete combustion.
B The available heat if the flue gases leave
the heat exchanger at 500°F and complete
combustion is achieved with theoretical
combustion air.
SOLUTION:
A The gross heating value for Pittsburgh
natural gas is 1129 BTU per cubic foot
(Table 2). The gross heating value for
the boiler equals 11, 290,000 BTU per
hour.
B The available heat for Pittsburgh natural
gas at 500°F and theoretical combustion
is calculated from the following ratio:
-------
Fundamentals m Combustion Calculations
H
:—
H.V.
(specific fuel) =
H
..G
(Fig. 3)
HA (specific fuel) =
__ __
H. (Pitt, natural gas) =
H,
G
H.
(Fig. 3) X H.V.., (specific fuel)
gas in JX H.V. (natural
Fig. 3/
gas/
1129} BTU per cubic foot
The available heat (HA) for Pittsburgh
natural gas is 915 BTU per cubic foot.
The available heat for the boiler at 500°F
and theoretical combustion equals
9, 150,000 BTU per hour.
If 20 percent excess combustion air was
used, the available heat at 500°F would
be decreased to about 79 percent of the
gross heating value for natural gas
(calculated from Figure 4).
REFERENCES
1 Anon. North American Combustion Hand-
book, 1st Ed., Copywrite 1952 by
North American Manufacturing Company,
Cleveland, Ohio.
2 Hougen, O.A. Chemical Process Prin-
ciples Part I, 2nd Ed., Copywnte 1954
by John Wiley and Sons, Inc.
-------
DISPOSAL OF GASEOUS WASTES
FLAME AND CATALYTIC
INCINERATION
*
L. C. Hardlson
Introduction
Gaseous wastes from industrial processes
contribute to the rising level of air pollution.
The most frequently used method of treating
waste gases is incineration. Catalytic, ther-
mal and direct flame incineration methods are
used to dispose of organic materials vented
from chemical manufacturing processes, the
application of coatings, etc. Alternative
methods of disposing of gaseous wastes involve
such processes as wet scrubbing, adsorption
and mechanical separation. Because of the
wide variety of materials that constitute air
contaminants, and the variations in local
requirements, sophisticated analysis of the
source of the gaseous wastes is often required
to select the proper control method. Gaseous
waste disposal is a subject about which much
has been written recently. The Questions people
ask about it which have broad interest have
been selected for discussion in this presen-
tation, commencing with incineration of gaseous
wastes, and then the less widely used alter-
native methods.
1-fhat is Incineration of Gaseous Wastes?
Gaseous waste incineration is a method
of controlling the contaminant level in a gas
stream being discharged into the atmosphere
by oxidation of combustible materials in the
stream. It is called incineration because it
is used for the disposal of undesirable com-
bustible material. Burning natural gas to
produce steam in a boiler, for example, ^s_
not incineration because the purpose of the
process is not the destruction of the natural
gas, but the production of usable heat. Oxi-
dizing the fumes evolved from an asphalt manu-
facturing operation in a gas-fired thermal in-
cinerator to control the odor emission ^.s_
incineration, because the purpose is the des-
truction of the unwanted air contaminants.
What are the purposes of incinerating gaseous
wastes?
Incineration of gaseous wastes has two
fundamental purposes: to mitigate local nui-
sances, and to control the quality of ambient
air. Local nuisances include odors, eye-nose-
throat irritation, visible emissions, fallout
of soot or aerosols, and other obnoxious phe-
nomena which may be traced directly to a sin-
gle source or sources. Some examples of local
nuisances are:
1) The characteristic odor of in-
sulating enamel near a plant manufacturing
magnet wire.
2) The dust fallout downwind from
coal-fired power generating plants.
3) The reddish brown fume of nit-
rogen peroxide emitted from a tungsten lamp
filament manufacturing operation.
4) The strong odor of roasting
coffee near a coffee processing plant.
Local nuisances have been regulated for
many years on the basis that a source of air
contamination specific enough to produce local
complaints constituted a nuisance, whereas
lack of complaints implied the absence of a
nuisance. In many cases, physical design cri-
teria have been established to prevent or re-
duce the local effects of a nuisance. For
example, minimum stack heights have been spec-
ified for boiler plants in many areas. Not
much has been done towards the specification
of equipment to prevent (rather than to miti-
gate) local nuisances.
The recent emphasis on air pollution con-
trol has been primarily related to control of
the quality of ambient air. This is a term
coming into wide use to describe the concen-
Technical Director, UOP
Greenwich, Connecticut
PA.C.ce.42.8.69
Air Correction Division
-------
Disposal Of Gaseous Wastes
tration of contaminants which prevail in a
geographic area, and which cannot be related
to a single source. Concentrations of sulfur
dioxide, nitrogen oxides, ozone, partially
oxidized hydrocarbons and other pollutants
are reaching disagreeable and sometimes dan-
gerous levels in many cities without there
being any single identifiable source to which
a local nuisance ordinance can be applied.
Most major cities and many of moderate size
have been involved recently in studies of
ambient air quality for the purposes of es-
tablishing standards to be used by various
agencies in regulating the emission of air
pollutants. Ambient air quality standards
are not regulations set by control agencies,
but are goals used as a basis for preparing
regulations. The regulations invariably ap-
ply to emission sources.
The best examples of such regulations are
those currently in effect in Los Angeles County
and in the San Francisco Bay area. Both of
these apply to the emission of solvents from
industrial operations, and limit the solvent
emission to 10 or 15% of the total consumption
of the solvent. Rule 66 in Los Angeles County,
and Regulation 3 in the San Francisco Bay area,
are both intended to reduce the incidence of
photochemical smog by eliminating the emission
of reactive hydrocarbons which contribute to
smog production. These rules are the fore-
runners of regulations which we may expect to
see in every major city in the United States.
"rfhile the regulations do not specify inciner-
ation, this is the principal method being used
to comply with them.
What are the methods of gas incineration?
Table 1 lists the three basic methods
employed to incinerate waste gases. All
three are oxidation processes. Ordinarily,
each of the three methods requires that a
gaseous effluent from an industrial process
be heated to the point where oxidation of the
combustible contaminants will take place. The
three methods differ basically in the tem-
perature to which the gas stream must be
heated.
Flame incineration is the easiest of the
three to understand, as it comes closest to
everyday experience. When a gas stream is
contaminated with combustibles to a concen-
tration approaching the lower flammable limit,
it is frequently practical to add a small
amount of natural gas as an auxiliary fuel,
sufficient air for combustion when necessary,
and then pass the entire contaminated stream
through a burner. The contaminants serve as
a part of the fuel. Flame incinerators of
this sort are most often used for closed chem-
ical reactors. Figure 1 is a schematic il-
TABLE 1
METHODS FOR INCINERATING WASTE GASES
OPERATING EQUIPMENT ANNUAL FUEL
TEMPERATURE COST COST
($/SCFM) <$/IOOOSCFM)
FLAME
THERMAL
CATALYTIC
2500 +
1000-1500
600-900
5-50
175-10
1 75- 5
0 -20
0 - 750
0-450
LADEN PROCESS
STREAM
COMBUSTION
AIR
FIGURE 1
SCHEMATIC DIAGRAM OF A UOP FLAME INCINERATION UNIT
lustration of a flame incineration unit, which
resembles a flare operated within a combustion
chamber where the combustion conditions may be
controlled carefully.
It is fnr porn H'-oJv that the concen-
tration of combustible contaminants in an
air stream will be well below the lower limit
of flammability. When this is the case, di-
rect thermal incineration is considerably
more economical than flame incineration.
Direct thermal incineration is carried out
by equipment such as that illustrated sche-
matically in Figure 2. Figure 3 is a photo-
graph of a typical thermal incineration unit.
In this equipment, a gas burner is used to
raise the temperature of the flowing stream
sufficiently to cause a slow thermal reaction
to occur in a residence chamber.
-------
FIGURE 2
SCHEMATIC DIAGRAM OF A THERMAL INCINERATION
FOR VARNISH KETTLES SYSTEM
1
•*—
\J
\
TABLE 2
COST COMPARISONS
( 10,000 SCFM EXHAUST AT 550 °F )
FLAME
THERMAL
(NO HEAT EXCHANGER)
CATALYTIC
CAPITAL
EQUIPMENT
$35,000
$20,000
$23,000
FUEL
COST
$173,000
(40.3MMB/HR)
$31,600
(7.35MMB/HR )
$4,850
( I.I3MMB/HR )
TABLE 3
COST COMPARISON FOR TYPICAL APPLICATION
THERMAL
CATALYTIC
350°F
BASIC UNIT
MU EXCHANGER
MU EXCHANGER-I-SR
550 °F
BASIC UNIT
MU EXCHANGER
MU EXCHANGER+ SR
EQUIP ANNUAL EQUIP ANNUAL
COST FUEL COST COST FUEL COST
$20,000 $48,000 $23,000 $14,600
24,000 33,500 28,500 7,100
33,200 17,500 — —
19,500
23,900
32,900
30,700
12,200
4,950
23,000 4,800
33,900 (14,600)
-------
FIGURE 3
UOP DIRECT THERMAL INSTALLATION
Whereas flame temperatures bring about
oxidation by free radical mechanisms at tem-
peratures of 2500°F and higher, thermal in-
cineration of ordinary hydrocarbon compounds
begins to take place at temperatures as low
as 900 to 1000°F. Good conversion efficien-
cies are produced at temperatures on the or-
der of 1300°F with one second residence time.
The optimum combination of temperature and
residence time is selected to keep the capi-
tal and operating cost of thermal incinera-
tors to a minimum.
Frequently, it is possible to identify
specific materials which must be oxidized in
order to conform to the prevailing air cor-
rection ordinances, or to good practice. For
example, a strip-coating operation, using a
mixture of toluene and Cellosolve acetate,
may be incinerated satisfactorily by choosing
the Cellosolve acetate residence time-temper-
ature curve and selecting conditions which
will give a 90% conversion of this material.
As the toluene is less refractory than the
Cellosolve, this selection is "safe".
Often, however, it is not possible to
chemically identify the specific compounds.
In these cases, the emission must be treated
on the basis of past experience or by using
pilot incineration equipment for field test-
ing. Because the incineration rates given
are influenced by burner design, distribution
in the combustion chamber, etc., it is wise
to follow the recommendations of the manu-
facturer supplying the equipment rather than
transfer data of this sort from one design
to another.
Catalytic incineration is carried out
by bringing the gas stream into intimate
contact with a bed of catalyst. In this
system, the reaction takes place directlv
upon the surface of the catalyst, which is
usually composed of precious metals, such as
platinum and palladium. Catalytic incineration
has been widely used for the oxidation of
paint solvents, odors arising from chemical
manufacture and food preparation, and for
accomplishing many other functions which
help to offset the cost of the air pollution
control equipment. For example, catalysts
may be used to reduce the partial pressure
of condensable hydrocarbons in an oven to
keep it clean, to return the heat of com-
bustion of the solvent to the process, or
to increase the flexibility of an oven. While
thermal incineration equipment brings about
oxidation at concentrations below the limits
of flame combustion, catalytic incineration
operates below the limits of flammability
and below the normal oxidation temperatures
of the contaminants.
How much do these systems cost?
Table 2 indicates some approximate eco-
nomic comparisons of the three types of in-
cineration equipment. Generally speaking,
the catalytic incineration systems are the
least costly when comparisons are made at
the optimum level of heat recovery. Very
small systems of any design tend to run at a
high cost per CFM, largely because these systems
are custom-designed, complex pieces of mach-
inery. If the market for this equipment in-
creases over the next few years, it should
be possible to substantially reduce the first
cost of relatively standardized pieces of
equipment for the smaller systems, while the
larger ones will continue to be designed and
built to custom specification. The factors
influencing the cost of equipment at a given
size level are:
1) the difficulty of the reaction;
2) the inclusion of auxiliary
equipment;
3) the materials of construction;
4) the extent of heat recovery.
-------
Disposal Of Gaseous Wastes
For example, a 10,000 SCFI1 oven exhaust
containing solvent equivalent to a BTU/SCF
may be incinerated by any one of the three
techniques described. Comparative costs for
this application are listed in Table 3. If
one assumes that the oven is operated at
500°F, the top line lists the fuel consump-
tion and capital cost on the assumption that
no heat exchange equipment is supplied. On
the second line, heat exchangers have been
added to heat the make-up air to the oven at
550 F. The addition of heat exchangers to
the direct thermal incineration equipment
shows a much more significant advantage than
for the catalytic equipment. To complete
the picture, the third line shows the com-
parison with self-recuperative equipment added
to the thermal incinerator to minimize the
total of capital-plus-operating cost.
Several comments are in order with re-
gard to the operating cost of incineration
equipment in general. For catalytic systems,
the equipment must be designed to maintain a
minimum entry temperature to the catalyst
bed. This varies from compound to compound,
but is generally in the neighborhood of 600
to 800°F. It is ordinarily feasible to
operate with a preheat burner system set to
produce this temperature at the inlet to the
catalyst bed whether or not there is solvent
to be incinerated.
When small quantities of solvent are
present, they will be consumed at a lower
level of combustion efficiency than when
high levels are present, because of the
lower average temperature in the catalyst
bed. It is generally satisfactory to accept
the lower conversion level, because the to-
tal content of contaminant in the emission
is lower than for the high solvent emission
case.
In some instances, it is necessary to
maintain a high level of conversion even at
very low concentrations. In such cases, a
"split temperature control system" is utilized.
Here the discharge temperature of the catalyst
is maintained, for instance, at 900°F. If
the solvent load is low, the entry temperature
to the catalyst bed also approximates 900°F.
However, as the solvent content increases,
the inlet temperature is decreased to hold
the constant discharge temperature until a
preset minimum inlet temperature is reached.
At this point, for instance at 650°F inlet
temperature, the burner control is switched
to the inlet thermocouple, and the discharge
temperature is allowed to rise as the solvent
content increases. Temperature limit controls,
of course, are provided to prevent damage to
the equipment should the solvent content
exceed the design rate for the oven.
Heat exchangers for catalytic systems
handling high-temperature oven exhausts are
usually limited to preheat of make-up air
to an oven or the building. When oven tem-
peratures are in the 200 to 300°F range, the
addition of self-recuperative exchangers also
becomes profitable.
As the oven temperature increases, the
economics of catalytic incineration of the
solvent with heat exchange to preheat the
make-up air to the oven becomes increasingly
attractive. At oven temperatures on the or-
der of 650°F, it may be possible to supply
the entire heat load of the oven by inciner-
ation of the solvent.
While catalytic incineration is attract-
ive from an economic standpoint, several fac-
tors must be considered in selecting this form
of waste gas treatment. Catalysts require some
maintenance in the form of periodic washing to
remove atmospheric dust and dirt, and, in the
case of higher temperature ovens, to remove
traces of paint pigments and other ash-like
residues originating in coating materials. In
addition, it is necessary to reactivate the
catalysts periodically. There are a number of
materials, such as phosphorus, silicon, and
lead, known to shorten the active life of
these catalysts. However, when the character-
istics of the gas to be treated are suitable,
catalytic incineration is a highly satisfac-
tory method for air pollution control.
Thermal incinerators have inherently
higher operating costs than do catalytic units,
and generally speaking require more expensive
materials of construction. For example, the
catalytic system operating with a 1200°F max-
imum temperature may be designed with inner
walls of aluminized steel. A thermal incin-
eration unit operating at 1350°F will require
the use of a stainless steel or, alternatively,
carbon steel with internal insulation, or a
refractory lining. Heat exchangers for use
with catalytic incinerators may be designed
of carbon steel or aluminized carbon steel
throughout. While aluminized steel tubes are
frequently satisfactory for thermal inciner-
ators, many cases require heat exchanger tubes
fabricated from a stainless steel, or some in-
termediate alloy to withstand the higher
operating temperatures.
The expansion problems encountered in
thermal incineration units are generally
much more severe than those for equivalent
catalytic units. Careful mechanical design
-------
Disposal Of Gaseous Wastes
is required to nrevent stressing the struc-
tural parts of the units during start-up
and shutdown, and to avoid failure of these
parts by buckling, twisting or tearing. The
most common heat exchanger used with thermal
incineration units is the self-recuperative
type of exchanger which allows part of the
heat in the effluent gas to be used for pre-
heating the gas before it reaches the com-
bustion chamber. Gas-to-gas heat exchangers
of this kind are generally supplied as a
part of the thermal incineration unit and are
ordinarily available only from the manufac-
turers of thermal incineration equipment.
Inherently, these exchangers are costly, and
have a very low heat transfer coefficient.
Whenever thermal incineration systems are
used at nominal oven temperatures (that is,
below 500°F) self-recuperative heat exchangers
are justified and generally have a payout
time in the order of two years.
Flame incineration systems are generally
difficult to handle with regard to either self-
recuperative or make-up types of heat exchangers
because of the extremely high temperature of
the effluent from the combustion chamber. For
this reason, it is usually most economical to
provide a steam generator to recover the waste
heat rather than a gas-to-gas heat exchanger.
With water as the cooling fluid, the hot side
of the heat exchanger may be exposed to the
temperatures generated in the combustion zone.
What are the applications of waste gas
incinerators?
Gas incineration is coming into use in
operations which were previously considered
insufficiently objectionable on a local nui-
sance basis to warrant the installation of
air pollution control facilities. However,
many applications over a wide span of years
provide a broad background of experience in
the use of gaseous waste incineration equip-
ment. A feu of the more common applications
are described in the next f>\7
Wire enameling is the term used to des-
cribe the application of insulating varnishes
and lacquers to copper wire, primarily used
for the manufacture of magnets. The ap-
plication of Formvar , a polyvinyl formal
resin, was one of the first applications to
which catalytic incineration was applied.
Wire enameling ovens provide an ideal appli-
cation, in that the solvent loading is high
and consists primarily of xylenes and mixed
cresols, all of which are very easv to burn
cataly tically . The oven temperatures are
FIGURE 4
TYPICAL WIRE ENAMELING UNIT
generally high and large economics in fuel
consumption can be achieved by recirculating
a portion of the hot effluent gas from the
catalyst to the oven to preheat incoming
make-up air. In many cases the catalyst
elements are incorporated directly into the
oven, serving to keep the partial pressure
of resinous materials low and prevent con-
densation in the oven.
Frequently the catalytic system can be
conveniently incorporated into the design
of the oven, whereas a thermal system with
its larger residence-time requirement cannot.
Wire enameling oven effluents have been
treated by catalytic incineration almost to
the exclusion of other methods. Figure 4 is
a photograph of a typical wire enameling
aoplicat ion.
Kettle cooking refers to the batch
preparation of chemical formulations in
general, but is most often involved in the
manufacture of paints, varnishes, etc. Here,
again, the use of catalytic systems has
predominated; but thermal incineration has
also found broad application, and other
Trademark - Monsanto Co.
-------
Disposal Of Gaseous Wastes
INSULATED 8 JACKETED
HOUSINGy
/ INSULATED DOUBLE
CATALYST / MANIFOLD-f
ELEMENTS-;
STACK
DISCHARGE
t
EXHAUSTER
RECYCLING
DAMPER
PREHEAT BURNER
FIGURE 5 KETTLE
COOLING
STATION
COOKING
STATIONS
COOKING APPLICATION
svstems, such as scrubbing, have been used
with limited success. Like wire enameling,
kettle cooking involves a condensation problem
which tends to make the cost of maintaining
ductwork, ventilating fans, etc., high. In
addition, there is a safety hazard involved with
the ignition of accumulated deposits of resin-
ous material in the ducts. The use of either
thermal or catalytic incineration for air
pollution control makes possible the use of
double-manifolded ventilation system. This,
for all practical purposes, eliminates the
problem of condensation in ductwork and air-
moving equipment. Numerous such installations
have been used with success over a period of
many years. A typical kettle cooking schema-
tic is shown in Figure 5, and a photograph of
a similar application in Figure 6.
is a straightforward oven application wherein
the effluent heat from either the catalytic
or thermal unit may be used to preheat oven
make-up air. Frequently the control system
Tor the thermal or catalytic incinerator is
coupled with the sheet feed mechanism so
that the air pollution equipment is auto-
matically operated while sheets are being
fed into the drying oven. Figure 7 is a
photograph of a typical thermal incinerator
installation for a metal lithographing ap-
plication.
FIGURE 6
KETTLE COOKING INSTALLATION
FIGURE 7 THERMAL INCINERATION UNIT FOR METAL LITHOGRAPHING
Continuous strip-coating of aluminum
and steel to provide stock for the manufacture
of pre-painted metal goods, such as al Linnnum
siding and prefabricated steel building
materials, may be handled by thermal or
catalytic incineration. The modern high-
speed ovens operate at temperatures from
500 F to 750 F, and evaporate the solvents
from the coating materials much fnstor tbnn
do any of the other applications mt-nt. ioiu'i!
previously.
However, silicone additives in the paints
used for high-speed strip-coating, phosphorus
paint additives used for plasticizing, as flow
agents, as combustion retardants, etc., mav
cause some trouble in this sort of application,
which is not experienced in similar drying sit-
uations where the temperatures and evaporation
rates are lower. While the economics of cata-
lytic incineration of the waste gases f-om a
coil-coating operation are most favorable, they
are recommended only for those applications
where poisoning agents are absent, or where tlip
strip speeds nnd clrvint' tPi"piT;itii"er: •"""<• rr'1;i
tivelv low.
Metal lithographing is used to prepare
metal sheets for can manufacture and similar
applications. Both thermal and catalytic
incineration have been used successfully to
handle the air pollution problem arising
from the drying of the inks and solvent-con-
taining coatings. Generally speaking, this
What are the alternatives to gas incineration?
Several situations exist in which gas
incineration is not the only solution to a
gaseous waste disposal problem, or not the
correct solution. For example, halogenated
solvents may be turned into even more serious
-------
Disposal Of Gaseous Wastes
contamination by incineration. Two acceptable
solutions to the correction of chlorinated
solvent emissions exist. When the solvent
is being emitted from a solvent manufacturing
process, or when a single-component solvent
is being used, it may be economical to re-
cover the solvent for sale or for re-use.
For small applications, or where the solvent
is such that recovery and subsequent separa-
tion would be unattractive, incineration in a
thermal incinerator, followed by quenching and
wet scrubbing for removal of the hydrogen
chloride, is the preferred treatment.
For very large applications, where the
concentration of contaminant is low, carbon
adsorption may be used to concentrate the
combustible materials into a much smaller
stream, which is then handled by thermal or
catalytic incineration. An example of such
i situation exists in large paint snrav npu-
lications.
Some odors that are biologicallv derived
can be oxidized chemically in a wet scrubbing
system. For instance, the effluent from cof-
fee roasting can be handled with a wet scrub-
ber which collects the solid coffee chaff,
and in addition, destroys the odor bodies by
means of an oxidizing solution recirculated
within the scrubbing system. The combination
of wet oxidation, plus scrubbing of particu-
lates in a non-plugging, mobile bed absorber
offers many attractive possibilities. An in-
stallation uf this type is currently being
applied on a coffee roaster in Chicago.
Wet scrubbing also finds application
when the contaminants are water-soluble, or
consist mainly of particulate matter. Exam-
ples of these situations are emission of
ethyl alcohol-ether from a chemical process,
fumes, or aluminum chloride particulates
from a secondary aluminum smelting process.
How can one select the correct waste gas
disposal method7
The selection of the most economical of
the effective methods for control of a par-
ticular waste gas emission may require soph-
isticated analysis of the emission source
and the prevailing air pollution ordinances.
In general the following steps must be in-
volved in the selection.
1) Define the source with respect to
a) bulk gas stream flow rate,
temperature and overall
composition;
b) identity of air contaminants,
such as hydrocarbons, for-
maldehyde, sulfur dioxide,
etc.;
c) measurement of concentration
or quantity of significant
gas contaminants.
2) Establish the acceptable con-
centration of contaminants in
the effluent after treatment.
This may be done on the basis
of local ordinances, published
standards for air quality con-
trol, or by following trade as-
sociation or technical society
guide lines, etc.
3) Establish the degree of removal
from the information in Items
1 and 2.
4) Establish which processes are
capable of producing the desired
conversion level.
5) Compare the economics of the
effective processes, carefully
evaluating any "offsetting
advantage", such as heat re-
covery, product recovery, re-
duction in maintenance costs,
etc., which may be credited to
the gas waste disposal process.
In many cases these steps have been
clearly defined by experience within a par-
ticular industry. This is certainly true in
the case of wire enameling, phthalic anhydride
manufacturing, etc. In others, relatively new
ground is being broken. In such cases sophis-
ticated source testing, economic studies and
product application may be required. Frequent-
ly the decision to install a system for dis-
posal of the gaseous waste product is made under
a great deal of "pressure" to comply with a
new ordinance, or with a new attitude toward
pollutants of long standing. The best advice
is to recognize as early as possible that the
need for correction exists in a given situation,
and provide adequate time for a realistic selec-
tion of the process or equipment to correct the
problem. Apply the same considered judgement
to the air correction problems which are applied
to production problems with a particular sphere
of interest. The air pollution control equip-
ment selected may have to protect the contin-
ued operation of a facility from interference
by regulatory agencies for many years to come.
-------
SECTION V
CONTROL BY ABSORPTION
Absorption in Control of Gaseous Pollutants
Gas Absorption Equipment
Absorption Nomenclature and Definitions
Absorption Equipment: Selected Applications
A Guide to Scrubber Selection
Performance of Commercially Available Equipment
in Scrubbing Hydrogen Chloride Gas
-------
ABSORPTION IN CONTROL
OF CASEOUS POLLUTANTS
William F. Todd
INTRODUCTION
Absorption is a gas-liquid contacting
process for gas separation which utilizes
the preferential solubility of the pollutant
gas in the liquid phase. The prime objective
in pollutant removal is to obtain a removal
efficiency high enough to reduce the outlet
concentration of the pollutant to the
acceptable level. A difficult separation
may result in equipment and operating
costs that significantly affect process
economics. This unwelcome situation should
rightfully be considered as a routine part
of process design.
Corrosion in absorbers is frequently
a problem where aqueous solutions are used.
For this reason resistant materials such
as glass fiber reinforced resins are be-
coming popular in the pollution control
field.
BASIC PRINCIPLES
The absorption process has been "des-
cribed in the introduction, but the use
of the term "solubility" doesn't define the
phenomenon in vivid terms. The mechanism
is illustrated in Figure 1. Molecular dif-
Gas phase
Liquid phase
0. 0
Transfer
zone
OCarrier gas .^Solute OLlcluid "'
FIGURE 1. Gas to Liquid Transfer Phenomenon
William F. Todd is Chief (Acting)
Engineering and Enforcement Section,
OMD.NAPCA.
fusion in a gas is rapid compared to molec-
ular diffusion in a liquid, because of the
greater space between molecules in the gas.
The more closely spaced molecules of a liq-
uid inhibit this free motion, and the move-
ment is predominantly by convective
transport (mechanical mixing), which sweeps
the absorbed molecules away from the gas-
liquid interface.
It is possible that the absorption rate
(mass transfer of the solute from the gas
to the liquid phase) can be controlled by
either of the two mechanisms just described.
If a gas is very soluble in the liquid phase
it Is absorbed so rapidly that the rate
of pick-up is limited mainly by its rate
of diffusion through the gas. If we
picture the transfer zone as a "gas film"
and a "liquid film", we can say that this
mass transfer rate is gas film controlled.
To the contrary, if a gas is sparsely
soluble in the liquid, the movement of the
solute gas through the gas film is re-
latively fast compared to the rate
through the liquid film, and the mass
transfer rate ia referred to as liquid film
controlled. As might be suspected, certain
situations occur between these two extremes
and are sometimes not clearly defined.
An understanding of the theories of
diffusion in gases is basic to the calcu-
lation of diffusion coefficients used in
absorption calculations. However, it is
not essential to understanding how the
process variables influence the operation
and design of absorption devices. Diffus-
ion coefficients are used for calculating
mass transfer rates. These coefficients
are obtained largely from the literature,
but are frequently estimated, or are cal-
culated, using one of several empirical
formulas. A good reference for this is
Absorption and Extraction. Sherwood and
Pigford, McGraw - Hill, 1952.
PA.C.ge.27a.5.70
-------
DATA FROM TABL
PERRY'S CHEMICAL ENGINEERS
HANDBOOK 4TH EDITION
3
O
O
rt
O
I—
o
-------
Absorption in Control of Caseous Pollutants
To describe the absorption process, we
shall use the ammonia-air-water system.
Ammonia is very soluble in water and
results in the mass transfer rate being
"gas film" controlled. This mass transfer
rate has been.shown to be related to the
concentration of the solute .(ammonia) in the
gas and liquid. Equilibrium conditions,
determined experimentally in the laboratory,
are shown for the ammonia-air-water
concentrations at various temperatures in
Figure 2.
Several equilibrium lines at selected
temperatures are shown. These were
calculated from equilibrium vapor pressure
data in Perry's Chemical Engineers Handbook.
Absorption systems do not operate exactly
at equilibrium conditions but do approach
this state, and the equilibrium curves shown
in Figure 2 can be used to relate the
liquid .capacity and the gas concentration in
the system. These curves provide a basis
for the design and operation of the ab-
sorption system.
If the curves in Figure 2 are plotted
on arithmetic coordinates so that the
curves will extend to the origin, they
each will resemble the curve shown in
Figure 3.
ra
oo
c
-------
Absorption in Concrol of Gaseous Pollutants
Inspection of Figure 4 suggests a higher
concentration, at the interface, of the
pollutant in the liquid phase than in the
gas phase. Thus it appears unlikely that
the pollutant will pass the interface at
all. This illusion is due to the use of
different concentration units for the
gas and the liquid phases •
MASS TRANSFER COEFFICIENTS
Mass transfer represents a flow of
material from one phase to another across
the interface. This flow encounters resis-
tance, and a force is required to initiate
and maintain the flow. An analogy to the
better known Ohm's Law is helpful to
illustrate this concept.
ti E = I R
~ Potential = current x resistance.
Here current represents electron flow and
voltage difference represents a driving
force.
In mass transfer "current" is re-
presented by mass flow rate (N ) , "Driving
force" by concentration difference, and
"resistance" by the reciprical of the mass
transfer coefficient.
A Concentration = mass transfer rate
x resistance of liquid film
c '
This expression represents a situation
where the resistance to mass flow is
mainly in the liquid film, a condition that
exists when a gas is not very soluble in
the liquid.
It can also be expressed in terms of
the gas phase concentration.
A Partial pressure of solute gas = mass
transfer rate x resistance of gas film.
This situation exists where a gas Is very
soluble in the liquid and the resistance
is mainly in the gas film.
The mass transfer coefficients used
here are overall values related to the
concentrations of solute in the bulk of
the gas and liquid; and includes resistance
to mass flow existing in the bulk of the
fluids in addition to that existing in the
gas-liquid films.
The film coefficients are expressed by
the symbols k_ and k , for gas and liquid,
\j L
respectively. Values for these film co-
efficients are not available for many gas-
liquid systems, and cannot be accurately
estimated. The overall mass transfer co-
efficients, K, and K , are however, related
to the film coefficients by the following
equations:
m
"L
The equations for film coefficients, are
mainly of academic interest since the over-
all mass transfer coefficients are generally
used in design and process control engineering.
The equations do however, help us understand
the absorption process.
These equations can be derived using Figure
5.
slope = m
Concentration In Liquid
FIGURE 5 Mass Transfer Example
f' P " k
L AC. which is the slope
tangent ru
Se.
** Reference: Treybal,
Hill, 1955 Page 84.
lass
fer. McCraw -
-------
Absorption in Control
Pollutaut!>
The driving force A P - A P. is
OA A
equal to the slope of the equilibrium
curve m times the distance A C . Then
the total distance A P
is:
A P
OA
A C.
m A C =
+ m AC.
Ue also know now that Che mass flow rate
is expressed by:
(overall) (film)
Let's substitute for A P
A C.
Now
which can be easily rearranged Co
Page 6 is a worksheet If you care to try
1C.
A simlliar analysis of figure 5 can be
used to derive Che following equation:
1
K
These equations just derived can be
used to evaluate che absorption process and
to estimate values for film, coefficients.
For example, when the slope of the
equilibrium line is small ( gas A is very
soluble in the liquid ) the term m/k will
11
become very small and T: — = T — which
*-G KG
we can interpret to mean that the gas-film
nrovides the major resistance to mass
flow. This also means that a change in k.
will not effect a significant change in
.
G
Vigorous agitation of the liquid will
alter K. (remember that the solute diffuses
in. the liquid by turbulent mixing) but
in this case little advantage will be
gained. To the contrary, in the case of a
slightly soluble gas the term 1
ink
will be small and IL will be significant.
Agitation of Che liquid, in this case proves
to be beneficial.
The design of air pollution control
systems, unlike those for some industrial
systems, involves a free choice of the
absorbing liquid. With this in mind, it
is always best to select the most reactive
or absorbent liquid that your budget will
allow. This will reduce equipment size
and capital Investment. Reduced equipment
size may also be reflected in reduced
installation costs.
REGENERATION OF ABSORBENTS
Regeneration of an absorbing solution
is usually desirable from an economic view-
point buC is not always possible. If raw
water is used as Che absorbent one might
be tempted Co dispose of it with no
attempt Co recover the solute or to restore
Che water co ics original state. Reactive
absorbents on Che other hand contain
chemicals or catalysts that are so expensive
that some recovery through absorbent re-
generation is essendal Co economical
operation.
The objecCive of desorpclon, in pollution
control systems, is co regenerate Che
absorbent as economically as possible and
Co concentrate the pollutant gas for further
processing; or Co chemically neutralize the
pollutant for disposal as a solid or liquid.
Some spent absorbents can be regenera-
ted by heating. This will drive the solute
out of the liquid by (1) lowering the solu-
bility, (2) by dissociating a chemical
compound in the liquid. The liquid, when
cooled, is again ready for absorption.
Alternate methods for regeneracing the
absorbent are (1) lowering che total pres-
sure to shift the equilibrium, (2) precipi-
tation of Che solute without destroying the
absorption capacity of Che liquid.
Both approaches are used; the former would
be preferred for its simplicity and econo-
my.
In some cases a once-through passage
of water has proven effective and economical.
-------
Absorption in Control of Gaseous Pollutants
WORK SHEET
( There is additional working space on page 8. )
-------
Absorption in Control of Caseous Pollutants
o
0)
o
0!
a
u
Desorbed gas
Desorption line
Desorption equilibrium line
(high temperature)
Inlet gas
Absorption equilibrium line
(low temperature)
X, Liquid composition
Figure 6.
This approach is not too practical due to
the water pollution aspects of disposal.
The London Power Company used water scrubbing
to remove SO. from the Battersea station
flue gas but this scrubber unit was shut
down during World War II and never restarted.
The scrubber was not restarted because
occasional "looping" of the cool flue gases
into the air Inlet ports occurred, and
caused operational problems. It was also
observed that the cooled flue gases lacked
buoyancy and contacted the ground in the
surrounding area more intimately than be-
fore. The residual SC>2 in the scrubbed
flue gas then caused as many problems as
had the SC>2 in the more buoyant unscrubbed
gas.
The conditions for thermal desorptlon
will change the equilibrium diagram as
shown in Figure 5. This will result in a
diagram as shown in Figure 6.
Although It would seem that pure (100Z)
solute could be recovered by desorptlon in
the absence of a purge gas, this isn't so
because some mechanism must be introduced
which will dilute the desorbed solute gas to
some extent for carrying the solute gas
away from the Interface.
Advantage can be made of the effect of
reduced pressure to aid desorption of the
solute. This, however, involves additional
pump capacity and increases the possibility
of contamination with ambient air.
(1)
(2)
FOOTNOTES
Lewis, U.K. & W. G. whitman: Ind. Eng.
Chem. 16, 1215 (1924).
Whitman, W.G., Chem. Met. Eng. 29, 147
(1923).
(3)
Perry's Chemical Engineer's Handbook,
4th Edition, Page 14-4.
-------
GAS ABSORPTION EQUIPMENT
Harry E. Chatfield
Ray M. Ingels
Gas absorption is the mechanism whereby one
or more constituents are removed from a gas
stream by dissolving them in a selective
liquid solvent. This is one of the major
chemical engineering unit operations and is
treated extensively in all basic chemical
engineering textbooks. These texts deal with
gas absorption as a method of recovering
valuable products from gas streams, for ex-
ample, in petroleum production, natural gas-
oline is removed from wellhead gas streams
by absorption in a special hydrocarbon oil.
Absorption is also practiced in industrial
chemical manufacturing as an Important oper-
ation in the production of a chemical com-
pound. For example, in the manufacture of
hydrochloric acid, one step in the process
involves the absorption of hydrogen chloride
gas in water.
From an air pollution standpoint, absorption
is useful as a method of reducing or elim-
inating the discharge of air contaminants to
the atmosphere. Even in this application,
absorption can yield profits to the user.
For example, it can be employed to remove
hydrogen sulfide from process gas streams in
a petroleum refinery to meet air pollution
regulations. With further processing, this
hydrogen sulfide can be converted to elemental
sulfur, a valuable product.
The gaseous air contaminants most commonly
controlled by absorption include sulfur
dioxide, hydrogen sulfide, hydrogen chloride,
chlorine, ammonia, oxides of nitrogen, and
light hydrocarbons.
In other examples, such as solvent recovery,
desorption or stripping may be practiced
after absorption not only to recover val-
uable absorbed constituent, but also to
recover valuable solvent for reuse. Some-
times, after absorption, solute and solvent
are not separated but are used as a prod-
uct or intermediate compound in chemical
manufacture.
Treybal (1955) lists some important aspects
that should be considered in selecting absorp-
tion solvents.
1. The gas solubility should be relatively
high so as to enhance the rate of ab- '
sorption and decrease the quantity of
solvent required. Solvents similar chem-
ically to the solute generally provide,
good solubility.
2. The solvents should have relatively low
volatilities so as to reduce solvent
losses.
3. If possible, the solvents should be non-
corrosive so as to reduce construction
costs of the equipment.
4. The solvents should be inexpensive and
readily available.
5. The solvents should have relatively low
viscosities so as to increase absorption
and reduce flooding.
6. If possible, the solvents should be non-
toxic, nonflammable, chemically stable,
and have low freezing points.
GENERAL TYPES OF ABSORBERS
Gas absorption equipment is designed to pro-
vide thorough contact between the gas and
liquid solvent in order to permit interphase
diffusion of the materials. The rate of mass
transfer between the two phases is largely
dependent upon the surface exposed. Other
factors governing the absorption rate, such
as solubility of the gas in the particular
solvent and degree of chemical reaction,
are characteristic of the constituents in-
volved and are more or less Independent of
the equipment used. This contact between
gas and liquid can be accomplished by dis-
persing gas in liquid or vice versa.
Absorbers that disperse liquid include packed
towers, spray towers or spray chambers, and
venturl absorbers. Equipment that uses gas
dispersion includes tray towers and vessels
with sparging equipment.
PACKED TOWER DESIGN
A packed tower is a tower that is filled w^th
one of the many available packing materials,
as shown in Figure 1. The packing is de-
signed so as to expose a large surface area.
When this packing surface is wetted by the >
solvent, it presents a large area of liquid
film for contacting the solute gas.
Usually the flow through a packed column is
countercurrent, with the liquid introduced
at the top to trickle down through the packing
while gas is introduced at the bottom to pass
"Gas Absorption Equipment" was taken from the
Air Pollution Engineering Manual
Public Health Service Publication No. 999-AF-40
PA.C.ge.25.8.69
-------
Gas Absorption Equipment
G»S OUT
LIQUID DISTRIBUTOR
LIQUI 0
RE-DISTRIBUTOR
POCK ING SUPPORT
GtS IK
Figure 1. Schematic diagram of a packed tower
upward through the packing. This results
in highest possible efficiency, since, as the
solute concentration in the gas stream de-
creases as it rises through the tower, there
is constantly fresher solvent available for
contact. This gives maximum average driving
force for the diffusion process throughout
the entire column.
In concurrent flow, where the gas stream
and solvent enter at the top of the column,
there is initially a very high rate of ab-
sorption that constantly decreases until,
with an infinitely tall tower, the gas and
liquid would leave in equilibrium. Con-
current flow is not often used except in
the case of a very tall column built in
two sections, both located on the ground,
the second section using concurrent flow
merely as an economy measure to obviate the
need for constructing the large gas pipe from
the top of the first section to the bottom
of the second. Moreover, for an operation
requiring an exceptionally high solvent flow
rate, concurrent flow might be used to pre-
vent flooding that could occur in counter-
current flow.
Packing Materials
The packing should provide a large surface'
area and, for good fluid flow character-
istics, should be shaped to give large void
space when packed. It should likewise be
strong enough to handle and install without
excessive breakage, be chemically inert, and
be inexpensive.
Rock and gravel have been used but have
disadvantages of being too heavy, having small
surface areas, giving poor fluid flow and,
at times, not being chemically inert. Coke
lumps are also used sometimes and here the
weight disadvantage is not present. Owing
to its porosity, coke has a large surface
area per unit volume. The exposed surface
is not, however, as large as might be ex-
pected since the pores are so small that
they become filled or filmed over by the
solvent, which considerably reduces the
effective surface.
Generally, packing in practice consists of
various manufactured shapes. Raschig
rings are the most common type, consisting
of hollow cylinders having an external di-
ameter equal to the length. Other shapes
include Berl saddles, Intalox saddles,
Lessing rings, cross-partition rings,
spiral-type rings, and drip-point grid
tiles. Figure 2 shows several common
shapes. Physical characteristics of these
various types of packings have been de-
termined experimentally and compiled in
tables by Leva (1953).
BESl S«DOLE
RISCKIG RING
«m01 SADDLE
Figure 2. Common tower packing materials
-------
Gas Absorption Equipment
Table 1. COSTS OF REPRESENTATIVE
TOWER PACKINGS (Teller, 1960)
Packing
Raschig rings, ceramic
Raschig rings, carbon
Berl saddles, ceramic
Intalox saddles, ceramic
Intalox saddles, carbon
Tellerettes, polyethylene
Low density
High density
Pall rings, ceramic (BASF)
Pall rings, polypropylene
Pall rings, stainless steel
Cost of packing,
1/2-in.
11.70
16.90
24.80
23.55
—
—
—
41.00
186.50
1-in.
6.50
9.60
9.90
9.40
18.60
16.00
23.00
5.00
26.00
96.00
$/ft3 (1959 basis)
1-1/2-in.
5.05
8.00
7.50
7.15
18.40
--
—
20.75
83.00
2- in.
4.85
6.60
7.70
7.30
—
—
—
18.50
69.00
Packing may be dumped into the column ran-
domly, or regularly shaped packing may be
manually stacked in an orderly fashion. Ran-
domly dumped packing has a higher specific
surface contact area and a higher gas pressure
drop across the bed. The stacked packings
have an advantage of lower pressure drop and
higher possible liquid throughout, but the
installation cost is obviously higher. Table
1 and Figure 3 list typical packing costs
and packed-tower installed prices for 1959.
Liquid Distribution
Since the effectiveness of a packed tower de-
pends on the availability of a large, exposed,
liquid film, then obviously, if poor liquid
distribution prevents a portion of the packing
from being irrigated, that portion of the
tower is ineffective. Poor distribution can
be due to improper introduction of the •
liquid at the top of the tower and to chan-
neling within the tower.
At least five points of introduction of
liquid per sauare foot of tower cross-
section must generally be provided to ensure
complete wetting. The liquid rate must be
sufficient to wet the packing but not to
flood the tower. Treybal (1955) states
that a superficial liquid velocity of at
least 800 pounds of liquid per hour per
square foot of tower cross-section is
desirable.
Solid-cone spray nozzles make excellent dis-
tributors but may plug if solid particles
are present in the solvent. In randomly
packed towers, the liquid tends to channel
toward the walls, because of the usuallv
lower packing densltv near the walls. In
tall towers this channeling is controlled
by liquid redistributors at intervals of
10 to 15 feet. Moreover, this effect is
minimized if the packing pieces are less
than one-eighth the diameter of the tower.
Tower Capacity
The terns used to indicate capacity of a packed
column or Cover are load point and flood point.
For a given packing and liquid rate, if gas
pressure drop is plotted against gas velocity
on a logarithmic scale, there are two distinct
breakpoints where the slope of the curve in-
creases. AC low gas velocities the curve is
almost parallel to that obtained with dry pack-
ing, but above Che breakpoints, the pressure
drop increases more rapidly with increased gas
velocity. The lower of these two breaks is the
load point and Che higher one the flood point.
As gas velocity increases above the load
point, the liquid holdup in the bed Increases
until, at the second breakpoint, the flood
point, most of the void space in the tower
is filled with liquid and there is liquid
entrainment in the gas stream. Of course,
-------
43*
A
3D
::
145 10 20 30 »0 50 I0g
DIUEIER -ft!
Figure 3. Packed-tower costs, 1959, with
Raschig rings as packing
Figure 4. Correlation for flooding rate in
randomly packed towers
-------
Gas Absorption Equipment
at this point there is an excessive pressure
drop. Columns should seldom be operated above
the load point, but since the load point is
sometimes more difficult to establish than
the flood point, it is common practice to de-
sign for 40 to 70 percent of the flood point.
In general, flooding velocities are con-
siderably higher for stacked packing than
for dumped packing. The plot of Lobo (Figure
4 ) can be used to determine flow rates that
will cause flooding. This curve is based
on measurements with several liquids and
gases on a variety of packings.
For many years packed towers were designed
on the same basis as plate or tray towers.
The number of theoretical plates or trays
required for a given degree of separation
,was calculated and this quantity multiplied t
by a figure call height equivalent to a
theoretical plate (HETP). This HETP was an
experimentally determined figure varying
widely with packing, flow rates of each
fluid used, and concentration of solute for
any specific system. Experimental evaluation
of these variables made use of this system
too cumbersome and it is now rarely used.
Design procedures now employ the concept of
the transfer unit. The major design items
to be calculated are the column diameter,
number of transfer units, the height of a
transfer unit, and the system pressure drop.
These will be discussed individually.
Tower Diameter
As mentioned previously, gas velocity is
limited by flooding conditions in the tower,
By use of the design gas volume, design
solvent flow rate, and type of packing, the
tower diameter can be computed by using
Lobo's correlation in Figure 4. Packing
factors are obtained from Figure 5. The
procedure is as follows:
s
SJ
PICKING FICTDB FOR I • ' INTlLOl
STOLES .1 . 90
15 ! 0 25
NOVINIl PICKING SIZE HCI'fS
Figure 5. Packing factors for Raschig rings
and saddles
-------
Gas Absorption Equipment
iO.5
1. Calculate the factor L' ( PC
V ^
where
L1 = liquid flow rate, Ib/hr
V1 = gas flow rate, Ib/hr
PG = gas density, lb/ft3
PL = liquid density, lb/ft3
2. Using the calculated value in (1), obtain
from Figure 4 the value of
where
8c "G "L
G' = gas flow rate, Ib/sec-ft of tower
cross-section
a = packing factor from Figure 5
PG = gas density, lb/ft3
" L = liquid density, lb/ft3
M' = liquid viscosity, centipoises
f
gc = gravitational constant, 32.2 ft/sec^
3. Solve for G', the superficial mass gas
velocity at flood point from the factor
determined in (2).
4. Calculate S, the tower cross-section area
in ft for fraction of flooding velocity
selected, f, by the equation
V
(1)
(G')(f)(3,600)
5. Calculate the tower's inside diameter,
DC, by the equation
DC = S(4)
T
0.5
(2)
Tower diameter should be calculated for
conditions at both top and bottom of the
tower. The tower is designed to the
larger diameter.
Number of Transfer Units (NTU)
A transfer unit is a measure of the diffi-
culty of the mass transfer operation, and is
a function of the solubility and concentra-
tions of the solute gas in the gas and liquid
streams. It is expressed as NQG or NQL, de-
pending upon whether the gas film or liquid
film resistance controls the absorption rate.
The gas film resistance usually controls
when solubility of solute in solvent is high
and conversely, the liquid film controls
when the solubility is low.
In air pollution control work where, in
general, a relatively small concentration
of solute is to be removed from an airstream,
a solvent in which the solute gas is highly
soluble is usually selected in order to
obtain the highest possible economic sep-
aration. Thus, for the majority of cases
encountered, the gas film resistance will
be controlling.
One of the most widely used methods of
determining the number of transfer units is
that proposed by Baker (1935), which is
based upon an operating diagram consisting
of an equilibrium curve and an operating
line. For a given gas-liquid system, if
the temperature is constant and the gas
partial pressure is varied, the gas con-
centration in the liquid changes to an
equilibrium concentration at each partial
pressure. If the system consists of a
soluble gas to be removed, an insoluble
carrier gas, and a solvent, then, as the
amount of soluble gas in the system increases,
the equilibrium concentration of the soluble
gas in the liquid increases but not pro-
portionally.
These equilibrium conditions can exist for
an infinite number of concentration states
and, when plotted on X - Y coordinates,
becomes the equilibrium curve. The oper-
ating line represents the concentrations of
solute in the gas stream and in the liquid
phase at various points in the tower. When,
plotted as moles solute per mole solvent
versus moles solute per mole gas on X - Y
coordinates, the result is a straight line.
Thus, when the composition of the inlet gas
and the desired or required degree of ab-
-------
Gas Absorption Equipment
sorpcion are known, the points on the oper-
ating line for each end of. the column can
be calculated. The operating line is the
straight line connecting the two points. For
absorption to occur, the operating line
must lie above the equilibrium curve on the
diagram. The relative position of the
operating line and equilibrium curve indicates
how far the tower conditions are from equil-
ibrium. The more widely separated the lines,
the further the tower conditions are from
equilibrium and the greater is the driving
force for the absorption operation.
Figure 6. illustrates the graphical method
of determining the number of transfer units
for a countercurrent packed tower with the
gas film controlling the absorption rate.
The equilibrium curve (line AB) for the
particular gas-liquid system is plotted from
experimental data, which, for most common
systems, has been determined. Much of these
data can be located in the International
Critical Tables and in Perry (1950). The
operating line is a straight line drawn
between points D and C. D is the point
representing the concentrations of solute
in the gas stream and in the liquid stream
at the gas inlet and liquid outlet (bottom
of the tower). Point C corresponds to these
concentrations at the top of the column.
Line EF is drawn so that all points on the
line are located midway on a vertical line
between the operating line and equilibrium
curve. Starting at point C on the operating
line (conditions at the top of the column),
draw a horizontal line CH so that CG = GH.
Then draw a vertical line HJ back to the
operating line.
> = SOLUTE moles SOLVENT mole
Figure 6. Graphical determination of the
number of transfer units
The step CHJ represents one gas transfer
unit. This stepwise procedure is continued
to the end of the operating line (conditions
at the bottom of the column). Two gas
transfer units (N--) are shown in Figure 6.
If the liquid film resistance is the controlling
factor in the transfer of solute to solvent,
draw the line EF so that all points on the
line are located midway on the horizontal
/axis between the operating line and equilib-
rium curve. Then, starting at point D on
the operating line, draw a vertical line
DK so that DL = LK. The step is completed
by drawing a line KJ back to the operating
line. This procedure is then continued to
point C on the operating line. Figure 6
does not accurately indicate the number of
liquid transfer units since the line EF was
drawn for the case where the gas film resis-
tance controls.
Height of a Transfer Unit
Generalized correlations are available for
computing the height of a transfer unit and
are expressed as H. and H, for heights of
gas and liquid transfer units respectively.
These use experimentally derived factors
based on the type of packing and the gas
and liquid flow rates as shown in equations
(3) and (4).
\
D-D
0.5
(3)
where
H • height of a gas transfer unit, ft
2
G • superficial gas rate, Ib/hr-ft
L » superficial liquid rate, Ib/hr-ft2
a - a packing constant from Table 2
6 • a packing constant from Table 2
V • a packing constant from Table 2
'G • gas viscosity, Ib/hr-ft
j 3
G • gas density, Ib/ft
D - gas diffusivity, ft /hr.
' »G ^
The group
is known as the
Schmidt number as shown in Table 3.
-------
Gas Absorption Equipment
Table 2. CONSTANTS FOR USE IN DETERMINING GAS FILM'S
HEIGHT OF TRANSFER UNITS (Treybal, 1955, p. 239)
Packing
a
Raschig rings
3/8 in. ' 2.32
1 in. ', 7.00
j 6.41
1-1/2 in. | 17.30
2.58
2 in. 3.82
fieri saddles
1/2 in.
1 in.
32.40
0.81
1.97
1-1/2 in. 5.05
3- in. partition rings
Spiral rings (stacked
staggered)
3- in. single spiral
650
2.38
3- in. triple spiral! 15.60
Drip-point grids
»
0.45
0.39
0.32
0.38
0.38
0.41
0.30
0.30
0.36
0.32
0.58
0.35
0.38
i
j No. 6146 • 3.91 j 0.37
No. 6295 i 4.56
0.17
Y
0.47
0.58
0.51
0.66
0.40
0.45
0.74
0.24
0.40
0.45
1.06
0.29
0.60
0.39
0.27
Range of
G1
200 to 500
200 to 800
200 to 600
200 to 700
200 to 700
200 to 800
200 to 700
200 to 700
200 to 800
200 to 1,000
150 to 900
130 to 700
200 to 1,000
130 to 1,000
100 to 1,000
L'
500 to 1,500
400 to 500
500 to 4,500
500 to 1,500
1,500 to 4,500
500 to 4,500
500 to 1,500 .
1,500 to 4,500
400 to 4,500 |
400 to 4,500
1
1
3,000 tolO.OOO
i
3,000 to 10,000
500 to 3,000
3,000 to 6,500
2,000 to 11,500
where
(4)
IL = height of a liquid transfer unit, ft
2
L = superficial liquid rate, Ib/hr-ft
M = liquid viscosity, Ib/hr-ft
^ = a packing constant, Table 4.
iJ = a packing constant, Table 4
P = liquid density, lb/ft3
L
D. = liquid diffuslvity, ft2/hr.
The group
PLDL,
is the Schmidt number
as shown in Table 5. Each of these empir-
ical equations neglects the effect of the
other film's resistance. Actually, however,
even in the case of absorbing highly soluble
ammonia in water, experimental results have
t shown that the liquid film resistance is
! significant. The height of an overall gas
I transfer unit, H__, is determined by the
following equation, which takes into
account the liquid film resistance.
-------
r,as \hserntior:
j pnent
Table 3. DIFFUSION COEFFICIENTS OF
GASES AND VAPORS IN AIR AT 25°C AND
1 ATM (Perry, 1950)
Substance
Ammonia
Carbon dioxide
Hydrogen
Oxygen
Water
Carbon disulfide
Ethyl ether
Methanol
Ethyl alcohol
Propyl alcohol
Butyl alcohol
Amyl alcohol
Hexyl alcohol
Formic acid
Acetic acid
Propionic acid
i-Butyric acid
Valeric acid
i-Caproic acid
Diethyl amine
Butyl amine
Aniline
Chloro benzene
Chloro touene
Propyl bromide
Propyl Iodide
Benzene
Toluene
Ethyl benzene
Propyl benzene
Diphenyl
n-Octane
Mesitvlene
D, cm /sec
0.236
0.164
0.410
0.206
0.256
0.107
0.093
0.159
0.119
0.100
0.090
0.070
0.059
0.159
0.133
0.099
p
PD
0.66
0.94
0.22
0.75
0.60
1.45
1.66
0.97
1.30
1.55
1.72
2.21
2.60
0.97
1.16
1.56
0.081 1.91
0.067
0.060
0.105
2.31 •
2.58
1.47
0.101 ! 1.53
0.072
0.073
0.065
2.14 :
2.12
2.38
0.105 ' 1.47
0.096 , 1.61
0.088 1.76
0.084 1.84
0.077 ; 2.01
0.059 . 2.62 .
0.068 2.28
0.060 2.58
0.067 , 2.31
Hoc = HG + (mlPM (HL) ,5,
where
m «= slope of the equilibrium curve
G = gas rate, Ib-moles/hr
L = liquid rate, Ib-moles/hr
These equations are widely accepted for
design purposes. Another group of equations
listed by Leva (1953) include overall gas
and liquid transfer coefficients,K a and
K^. Still more recently, Cornellpet al.,
(1960) published correlations based on a
mass of experimental data reported up to 1957.
Table 4. CONSTANTS FOR USE IN
DETERMINING LIQUID FILM'S HEIGHT OF
TRANSFER UNITS (Treybal, 1955, p. 237)
Packing
Raschig rings
3/3
III
1 in
in
in
\-\li in
i in
Bcrl saddles
Ml
\ in
in
.
0 00191
0 00 3°- 7
0 0100
0 ON 1
0 OU=.
0 OOdMj
(0 OO'-'M
l-l/i in
J-,n
Spiral
slag
i-in
)-m
partition rings
rings (stai ki 'J
gired)
single spiral
triple spiral
0 OObi-
0 Obi *
0 OQ101'
0 01 h,
! n 1
j
0 4b!
'0 I'1
o a
'0 It
0 i.'
o in
0 iS
o is
1 0 U"
0 it
U is
Rangi>
100 IM
10') ii-
mn t,.
HI" l.i
100 in
400 1..
IOO l.>
tun i,,
! 1)00 l.i
Illll t»
t 00') in
tf L
I'.OOO
1 - onn
1 - iino
1 =. (Illll
1 - linn
1 - U'»O
1- ODD
i - . i)'>n
1 1 Hill)
1 s (HID
1 1 llOW
Drip-point grids
No 6146
' 0 01M 0 -M ) '0(1 l.) 10 000
. o ot)7i'iO u.i *on t« ji.ono
Pressure Drop Through Packing
Treybal (1955) states that pressure drop
data of various investigators varies widely
even for the same packing and flow rates.
These discrepancies were probably due to
differences in packing density. Moreover,
j not enough work has been done on liquids of
high viscosity for proper evaluation, though
• it is recognized that, at equal mass flow
rates, high-viscosity liquids cause greater
gas pressure drop than those of low viscosity
do.
Leva's empirical relation applies below the
load point. This is as follows:
(10"8)(10nL'/pL) G'2
(6)
where
Z "1
f - pressure drop, Ib/ft
Z - packed height of tower, ft
aij • pressure drop constant from Table 6
n • pressure drop constant from Table 6
L' • superficial mass liquid velocity,
lb/hr-ft2
G* • superficial mass gas velocity,
lb/hr-ft2
PL • liquid density, Ib/ft3
Pr - gas density, Ib/ft3.
-------
Gas Absorption Equipment
Table 5. DIFFUSION COEFFICIENTS IN
LIQUIDS AT 20°C (Perry, 1950)
Solute3
°2
co2
N20
ci2
Br2
H2
N2
HC1
H2S
'H2S04
lHN03
Acetylene
jAcetic acid
•Methanol
Ethanol
JPropanol
IButanol
iAllyl alcohol
[Phenol
iGlycerol
•Pyrogallol
D x 105 M
(cm2/sec) x 105 pD
1.80 . 558
1.50 570
1.51 665
1.76
1.22
5.13
1.64
2.64
1.41
1.73
2.60
1.56
0.88
1.28
1.00
0.87
0.77
0.93
0.84
0.72
0.70
Hydroquinone ' 0.77
Urea ' 1.06
'Resorcinol . 0.80
Urethane . 0.92
Lactose 1 0.43
Maltose : 0.43
Glucose ' 0.60
Mannitol 0.58
Raffinose 0.37
Sucrose ', 0.45
Sodium chloride 1.35
Sodium hydroxide ; 1.51
co2b '
Phenol b
Chloroform
3.40
0.80
1.23
Phenol0 ' 1.54
Chloroform 2.11
Acetic acid° 1.92
Ethylene dichloridec ' 2.45
i
570
824
196
613
381
712
580
390
645
1,140
785
1,005
1,150
1,310
1,080
1,200
1,400
1,440
1,300
946
1,260
1,090
2,340
2,340
—
1,730
2,720
2,230
745
665
445
1,900
1,230
479
350
384
301
Solvent is water except where Indicated.
Solvent is ethanol.
Solvent is benzene.
10
-------
Gas Absorption Equipment
Table 6. PRESSURE DROP CONSTANTS FOR TOWER
PACKING (Treybal, 1955)
Packing
Raschig rings
Berl saddles
Intalox saddles
Drip-point grid
tiles
Nominal
size,
in.
1/2
3/4
1
1-1/2
2
1/2
3/4
1
1-1/2
1
1-1/2
No. 6146
Continuous
flue
Cross flue
No. 6295
Continuous
flue
Cross flue
m
139
32.90
32.10
12.08
11.13
60.40
24.10
16.01
8.01
12.44
5.66
1.045
1.218
1.088
1.435
n
0.00720
0.00450
0.00434
0.00398
0.00295
0.00340
0.00295
0.00295
0.00225
0.00277
0.00225
0.00214
0.00227
0.00224
0.00167
Range of L1,
lb/hr-ft2
300 to 8,600
1,800 to 10.800
360 to 27,000
720 to 18,000
720 to 21,000
300 to 14,100
360 to 14,400
720 to 78,800
720 to 21,600
2,520 to 14,400
2,520 to 14,400
3,000 to 17,000
300 to 17,500
850 to 12,500
900 to 12,500
Range
of P/Z,
Ib/ft2-ft
0 to 2.6
0 to 2.6
0 to 2.6
0 to 2.6
0 to 2.6
0 to 2.6
0 to 2.6
0 to 2.6
0 to 2.6
0 to 2.6
0 to 2.6
0 to 0.5
0 to 0.5
0 to 0.5
0 to 0.5
Illustrative Problem
The following example illustrates the pre-
ceding principles of packed tower design.
Knowing the amount of solute in the gas
stream, the total flow rate of the gas
stream, the most suitable solvent, an ac-
ceptable packing, and the desired degree of
absorption, calculate the tower dimensions.
Given:
Design a packed tower to remove 95Z of the
ammonia from a gaseous mixture of 10Z by
volume of ammonia and 90Z by volume of air.
The gas mixture consists of 80 Ib-moles/hr
at 68°F and 1 atm. Water containing no
ammonia is to be used as solvent and the
packing will be 1-inch Raschig rings. The
tower will be designed to operate at 60Z
of the flood point, and Isothermal con-
ditiona at 68°F will be assumed. The water
will not be recirculated.
Problem:
Determine water (low rate, tower diameter,
packed height, and tower pressure drop.
Solution:
1. Calculate the water rate:
a. Equilibrium data for the system ammonia-
water are aa follows:
X 0.0206 0.0310 0.0407 0.0502 0.0735 0.0962
Y 0.0158 0.0240 0.0329 0.0418 0.0660 0.0920
11
-------
Gas Absorption Equipment
Plot the equilibrium curve as shown in
Figure 7:
0 0 01 0 02 0 03 0 04 0 05 0 06 0 0? 0 01 0 09 0 10
X moles NH] mole H70 • 6B °F
Figure 1. Equilibrium curve for ammonia-water
system.
The curve is straight approximately to
the point P, with a slope of about 0.75.
Above point P, the slope is variable and
higher than 0.75. Use 0.75 as the slope,
m, of the equilibrium curve.
When the temperature rise of the solvent
is negligible, apply the relation
Gm (m) = 0.70
where
m = slope of equilibrium curve = 0.75
G = gas rate = 80 Ib-moles/hr
L = liquid rate, Ib-moles/hr
(80)(0.75)
0.70
= 85.8 Ib-moles/hr
2. From the given gas flow rate, the cal-
culated liquid rate, and the degree of
absorption desired (95% of ammonia),
tabulate gas and liquid flow rates at
both ends of the tower:
Density,
Ib-moles/hr Ib/hr lb/ft3
Inlet gas(bottom) 80 2,221 0.0720
Outlet gas (top) 72.4 2,092 0.0750
Inlet water (top) 85.8 1,542 62.4
Outlet liquor(bottom) 93.4 1,671 62.4
3. Calculate the tower diameter:
a. Use conditions at top of tower:
/1,542\ [0.0750
2,092 62.4
,0.5
0.0256
(2) From Figure 4 , the relationship
(G' } (.Tr) (M2} - 0.19
<8C)(PG>
(3) From Figure 5, the packing factor
(4) From the relationship in (2), calculate
the superficial mass gas velocity (G1)
at flooding:
G'(0.19)Q2.2)(0.0720)(62.A)V'30-424 Ib/sec_ft2
(5) At 60% of flooding:
Gf = (G1) (0.60) - (0.424) (0.60) =0.254 lb/sec-ft2
(6) Tower cross section:
S = 2,092 ib/hr
2.29
(0.254 lb/sec-ft )(3,600 sec/hr)
U2
-------
i,.is b..cjr|)LLon l.aui,>ni_nL
b. Use conditions at bottom of tower.
0.5
(1) /!/
(2) From Figure 4 , the relationship
0.2
(G")
(M2)'
<3
• 0 19
(3) From Figure 5 , tlie packing factor
- 160
c, = (0.19) (32.2) (O
(160) (1) °'
(5) Gf' = 0.415 (0.60) = 0.249 lb/sec-ft2
(6) S =
(0.249 Lb/sec-ft2) (3,600 sec/nr)
c. Select tower with uniform cross-section
of 2.48 ft2
d. Tower diameter:
DC = f(2.48)(4)\ °'5 = 1.78 ft or 21.4 in.
3.14 J
It. Determine the number of transfer units:
a. Calculate mole fractions of solute in
gas liquid streams at both ends of the
cower.
13
-------
Gas Absorption Equipment
(1) Bottom of tower:
vl = JL = °-]
72
NH,/mole air
3
X, = 7.6 = 0.088 mole NH,/ mole/air
85.8 3
HG = (7.00)(896)°-39 (0.66)0-5 - 1.92 ft
(622)0'58
b. Liquid transfer unit:
/ \>»
ML \ 0.5
(2) Top of tower:
V2 = °'4 = °-0056 n>°le NH3/mole air
X2 = 0 (entering water is NH. free)'
b. Plot the operating line from the data in
(a) on the same graph used for the equi-
librium curve.
c. By the method of Baker (described pre-
viously) graphically determine the num-
ber of transfer units:
MTU = 6
5. Calculate the height of a transfer unit:
a. Gas transfer unit:
.where
* = 0.01 from Table 4
' • 0.22 from Table 4
L = 622 Ib/hr-ft2
! ML = 1 centipoise = 2.42 Ib/hr-ft
tl "L \ » 570 from Table 5
< H. - 0.01 /622
x
l0.22
(570)0'5 = 0.79 ft
,c. Overall gas transfer unit
' HOG ' H^-nffn,
L
m
Hg
where
G
PG
'
2.221 Ib/hr
2.48 ft2
L =
1.542
2.48
622 Ib/hr-ft
896 Ib/hr-ft
2
MG
prDr
\G G/
7.00 from Table 1
0.39 from Table 1
0.58 from Table 1
0.66 from Table 2
• where
slope of equilibrium curve = 0.75
gas rate = 80 Ib-moles/hr
liquid rate = 85.8 Ib-moles/hr i
1.92 + (0.75)(80)(0.79) = 2.47 ft
(85.8)
6. Calculate the packed tower height (Z):
NTU x H
OG
6 x 2.47 = 14.8 ft
14 _
-------
Gas Absorption Equipment
7. Calculate the tower pressure drop
R / nL/e\/ ,N
A£ = m. (10~8) 10 LY G2\
rj 1 I — f
\ h)
where
2
Ap = pressure drop, Ib/ft
• Z = packed height = 14.8 ft
m = 32.10 from Table 6
; n = 0.00434 from Table 6
; L = 622 lb/hr-ft2
! PL = 62.4 Ib/ft3
; G = 896 lb/hr-ft2
I PG = Avg gas density = 0.036 lb/ftJ
i •
• (32.1 x 10-8mo)(°-00434><622>'62-W2(14.8)
0.0736}
Ap = 57.2 Ib/ft2
Ap = 57.2 Ib/ft2 (1 in. WC)
5.197 Ib/ft2
is equipped with openings (vapor riser)
surmounted with bubble caps. Typical bubble
caps are illustrated in Figure 9. The gas
rises through the tower and passes through
the openings in the plate and through slots
INTEMEQIITE
FEED
Figure 8. Schematic diagram of a bubble-cap
tray tower
11.0 in. WC
3 CM SUPPORTS
»T UO°F
HOLD-DOWN BJR
SLOTS
PLATE OR TRAY TOWERS
In contrast to packed towers, where gas and
solvent are in continuous contact throughout
the packed bed, plate towers employ stepwise
contact by means of a number of trays or
plates that are arranged so that the gas is
dispersed through a layer of liquid on each
plate. Each plate is more or less a separate
stage, and the number of plates required is
dependent upon the difficulty of the mass
transfer operation and the degree of separ-
ation desired.
Types of Plates
The bubble cap plate or tray is most common,
and most general references deal primarily
with it when discussing plate towers. Other
types of plates include perforated trays,
Turbogrid trays, and Flexitrays.
A schematic section of a bubble cap tray
tower is shown in Figure 8. Each plate
unit RISER
SHEET «ET»L TRAY
Figure 9. Illustration of some typical bubble
caps,
15
-------
Gas Absorption Equipment
in the periphery of the bubble caps, .which
; are submerged in liquid. The liquid enters
at the top of the tower, flows across each
plate and downward from plate to plate
through downspouts. The depth of liquid on
the plate, and liquid flow patterns across
the plate are controlled by various weir
arrangements, which will be discussed in
greater detail.
In perforated plates or sieve trays, the gas
passes upward through a pattern of holes
drilled or punched in the trays. Three-
sixteenth-inch-diameter holes spaced on a
3/4-inch triangular pitch are commonly used
A disadvantage of this type is the tendency
of liquid to "weep" or leak down through the
holes instead of through the downspouts at
low gas velocities. Moreover, the trays
must be installed perfectly level, or chan-
neling, with resultant loss of efficiency, ;
will occur. On the other hand, a perforated
tray costs only 60 to 70 percent as much as
a bubble cap plate designed for the same
throughput. With towers of the same diam-
eter, perforated trays supposedly have a
capacity 10 to 40 percent greater than that
of bubble cap plate towers.
With Turbogrid trays, licensed by Shell
Development Company, the vapor passes up
through the spaces between parallel rods or
bars, and the liquid level on the tray is
maintained by the gas pressure beneath the '
tray. There are no down spouts, and liquidi
flows downward through the same openings ' ;
used by the upward flowing gases. A
Turbogrid tray is shown in Figure 10.
These are reputed to have high absorption
efficiencies even at high capacities with
liquids containing a small amount of suspended
,' solids. For example, a 50 percent increase!
, in capacity has been reported where bubble ;
. cap plates have been replaced by Turbogrid
trays in an existing tower.
Flexitrays, licensed by the Koch Engineering
Company, have floating caps that allow vari4~
tions in the vapor openings with varying ga$
flow. Different weights can be put on the :
caps so that the slots will be only partially
open at low gas flow rates. This tray also^
has relatively low resistance to liquid ',
crossflow and supposedly has advantages ovef
bubble cap trays in large columns or opera-7
tions that require high liquid rates. Flexi-
trays are claimed to have a capacity 12 to >
50 percent higher than that of bubble cap :
plates and cost only 60 to 80 percent as
much.
Although the proponents of the various trays
: make each sound attractive, it should be re-
membered that the bubble cap plate is still!
! the standard of the industry and presently i
outnumbers all the other types. Thus further
discussion of plate towers will be devoted :
exclusively to the design of bubble cap
plates.
BUBBLE CAP PLATE TOWER DESIGN
Liquid Flow
Common variations in liquid flow across a
bubble cap plate include: (1) Crossflow in
opposite directions on alternate plates, (2)
crossflow in the same direction on all plates,
and (3) split-flow arrangements. There are
also variations in weir and downspout design.
Several liquid flow patterns are diagrammed
in Figure 11, and typical bubble cap tray
arrangements for different liquid flow paths
are shown in Figure 12.
TOP VIEI
COLUMN
SHELL
SIDE VIE*
TRAr SUPPORT KING
Figure 10. Illustration of a typical Turbogrid
tray
16
-------
Gas Absorption Equipment
LIQUID
00 IN
1
1
1
I
1
J
1
VAPOR UP
LIQUID
00»N
u.
VAPOR UP
INLET
IE IB
OVERFLOI
IE Id
Figure II. Vapor and liquid flow patterns for
bubble cap tray towers: (a) One-pass tray bub-
ble plate column, liquid crossflow, opposite di-
rection on alternate plates; (b) one-pass tray
bubble plate column, liquid crossflow, same di-
rection all plates; (c) two-pass tray bubble
plate column, split liquid crossflow, opposite
directions on alternate plates; (d) one-pass
cascade tray bubble plate column, liquid cross-
flow, opposite direction on alternate plates.
Figure 12. Typical bubble cap tray arrangements:
la; Single crossflow, rectangular weirs; (b)
split crossflow, rectangular weirs; (c) cascade
crossflow, rectangular weirs; (d) reverse flow
rectangular weir and dividing dam; (e) crossflow
circular weirs; (f) radial flow, circular weirs
The single-pass plate with a rectangular weir
shown in Figure 12a is the most common.
Much of its cross-sectional area Is devoted
to vapor flow, whereas, a split crossflow
plate, shown in Figure 12b, has more of its
cross-sectional area devoted to liquid flow.
The split-flow tray also has greater down-
spout area, and the liquid flows a shorter
distance from the tray inlet to the overflow
weir. Thus split-flow trays handle higher
liquid flow rates and are suitable for large-
diameter towers.
Cascade tray arrangements, shown in Fig-
ures lid and 12c, are used to keep the
liquid level at a more constant depth over
the entire tray area despite considerable
liquid head differential across the tray.
These are used for exceptionally large-
diameter towers. Radial flow, Figure 12f,
is also a common arrangement in large-
diameter towers. The liquid flow may be to
and from the center on alternate trays,
or it may be in the same direction on all
trays.
Plate Design and Efficiency
For the most efficient operations bubble
cap tray towers must be designed to com-
promise opposing tendencies. High liquid I
levels on the trays tend to give compar-
atively high tray efficiency through long
contact time but also give high pressure
drop per tray. High gas velocity, within
limits, gives efficient vapor-liquid con-
tact by creating turbulent conditions but
also leads to high pressure drop as well
as high liquid entrainment. Treybal (1955)
lists recommended conditions and dimensions
for bubble cap trays that have been found
to be a useful compromise; these are listed
in Table 7. In this table, the liquid seal
(ha) is the depth of clear liquid over the
top of the bubble cap slots.
As stated before, each tray or plate is a
separate stage and, for ultimate efficiency,
the gas and liquid would leave each tray
in equilibrium with each other at tray
conditions. This would be a theoretical
17
-------
Gas Absorption Equipment
Table 7. RECOMMENDED CONDITIONS AND DIMENSIONS FOR
BUBBLE CAP TRAYS (Treybal, 1955)
Iray spacing
Liquid seal
Liquid flow
!
'Superficial slot velocity
1
Skirt clearance
i
jCap spacing
i
l
i
iDownspout holdup
(Downspout seal
t
[Weir length
i
i
•
Liquid gradient
Pressure drop per tray
Tower diameter, ft
4 or less
4 or less
4 to 10
10 to 12
12 to 24
Pressure
Vacuum
Atm - — -
500 lb/in
a. Not over 0.22 ft /sec-ft
diameter for single-pass
crossflow trays
3
b. Not over 0.35 ft /sec-ft
weir length for others
3.4/pg 0.5 ft/sec minimum
12/pg 0.5 ft/sec maximum
0.5 in. minimum; 1.5 in. for
dirty liquids
1 in. minimum (low slot ve-
locities); 3 in. maximum
(high slot velocities)
Minimum of 0.5 sec
0.5 in. minimum at no liquid
flow
Straight rectangular weirs for
crossflow trays, 0.6 to 0.8 of
tower diameter
0.5 in. (1 in. maximum)
Pressure
Atm _
300 lb/in'
Tray spacing, in.
6 minimum
18 to 20
24
30
36
Liquid seal,
h , in.
s
0.5
1
3
Pressure drop
0.07 to 0.12 lb/in2
0.15 lb/in
I
plate. This theoretical condition does
not normally exist in practice and this
the actual number of trays required to
accomplish a specified degree of absorption
usually exceeds the number of theoretical
units required. The overall plate efficiency
of a tower is defined as the number of
theoretical equilibrium stages required for
a given degree of removal of solute from
the gas stream, or concentration of solute
in solvent, divided by the actual number
of trays required for this same operation.
, According to Clarke (1947) an overall plate
efficiency of 25 percent Is a conservative
estimate for hydrocarbon absorbers.
O'Connell (1946) correlates plate efficiency
with gas solubility and liquid viscosity.
This correlation is shown in Figure 13.
All such correlations are empirically
derived, and attempted theoretical methods
based on mass-transfer principles do not
I successfully predict overall plate efficiency.
18
-------
Gas Absorption Equipment
Figure 13. Correlation of plate efficiencies of gas absorbers
with gas solubility and liquid viscosity according
to method of O'Connell (Sherwood and Pigford, 1952
p. 301).
Flooding
When the liquid capacity of a plate absorber
is exceeded, the downspouts become filled.
Then, any slight increase in liquid or gas ,
flow increases the liquid level on the trays.
A further increase in pressure across the
trays causes more liquid to back up through
the downspouts, resulting in still higher
liquid levels on the trays until, eventually,
the tower fills with liquid. This is known
as flooding, and at this point, the tray
efficiency falls to a very low value, the
gas flow is erratic, and liquid may be forced
out the gas exit pipe at the top of the
tower. Flooding occurs more rapidly with
liquids thaftend to froth.
Tower design should allow sufficient down-
spout area and tray spacing to prevent flood-
ing under anticipated operation variations
in both gas and liquid flow. If there is any
question, it is better to over-design down-
spouts since they represent a relatively
small-cost item but are important from the
standpoint of potential flooding.
Liquid Gradient on Plate
The liquid gradient on a plate is the decreas-
' ing liquid depth from the liquid inlet to out-
let of the plate due to resistance to fluid
flow by the bubble caps and risers. If this
gradient is appreciable, more vapor flows
through the bubble caps where the liquid depth
is least. In extreme conditions the caps near
the liquid inlet may become completely inoper-
ative and liquid may flow down through the
risers. This is called an unstable plate.
Liquid gradient problems would naturally be
more likely in large towers, and in these cases,
the vapor distribution is controlled by two-
pass, split-flow, cascade- or radial-type trays.
Plate Spacing
Operationally, the main consideration re-
garding tray spacing is to allow sufficient
space for the desired liquid level plus
space above the liquid for disengagement
of the gas and liquid phases without en-
trainment. Thus, in this respect, tray
spacing is closely related to gas velocity
19
-------
Gas Absorption Equipment
through the tower. Spacing should also be
sufficient to provide insurance against
flooding. If flooding conditions exist even
for a short time, a tower with closely
spaced trays could become flooded. In
actual practice, however, trays are normal-
ly spaced for ease in cleaning and mainte-
nance and are not placed close together un-
less headroom limits the overall tower height.
Six inches is usually a minimum, even for
very small-diameter towers, and 18 to 24
inches is normally used for towers up to 4
feet in diameter.
Tower Diameter
The superficial linear gas velocity that will
usually insure against excessive entrainment
is chosen by the equation
0.5
(7)
jrate. Treybal (1955) states that a well-
tiesigned single-pass crossflow tray usually
handles up to 100 gpm per foot of diameter
without excessive liquid gradient.
Number of Theoretical Plates
The number of theoretical plates or trays is
usually determined graphically from an oper-
ating diagram composed of an operating line
and equilibruum curve constructed as previ-
ously described in the discussion of packed
towers. The actual procedure will be de-
scribed in the example problem that follows.
If the solute concentrations in the gas and
liquid phases are low, as is frequently the
case in air pollution control, both the equi-
librium and operating curves can be con-
sidered as straight lines, and an analytical
solution may be used. The relationship as
taken from Sherwood and Pigford (1952) is:
where
liquid density, Ib/ft
gas density Ib/ft
an empirical constant.
m G
The constant K can be determined by Figure
14, which is based on results of experimen-
tal study and good commercial practice. The
velocity calculated in equation 98 is valid
except for hydrocarbon absorbers, which, ac-
cording to Perry (1950), should be designed
for vapor velocities 65 to 80 percent that
of the calculated values. From this cal-
culated velocity, if the volumetric gas flow
•rate is known, the diameter can easily be
determined. In most cases the diameter
fchosen in this manner is also adequate to
Candle the normally expected liquid flow
BIT SMCINC It) inchll
Figure 14. Tray-spacing constants to estimate
bubble cap tray tower's superficial vapor vel-
ocity.
(8)
mG
where
N » number of theoretical plates
m - slope of equilibrium curve
G - superficial molar mass flow of gas,
2
Ib-moles/hr-ft column cross-sec-
tion
L^ « liquor rate, Ib-moles/hr-ft column
cross-section
Y = mole fraction of solute in gas
stream at concentrated end of
countercurrent tower
¥„ = mole fraction of solute in gas
stream at dilute end of countercurrent
tower
X = mole fraction of solute in liquid at
concentrated end of counter-current
tower.
X_ = mole fraction of solute in liquid
stream at dilute end of counter-
current tower.
20
-------
Gas Absorption Equipment
Illustrative Problem
The following example illustrates a method
of determining the number of plates or trays
required and estimated diameter for a tray
tower. No attempt is made to design the
bubble cap plate itself for characteristics
such as number of caps, cap spacing, slot
dimensions, and so forth.
Problem:
Determine the number of actual plates and the
diameter of a bubble cap plate tower for re-
moving 90% of the ammonia from a gas stream
containing 600 Ib-moles/hr of gas at 68°F and
1 atm composed of 10% by volume of ammonia
and 90% by volume of air.
Solvent rate expressed as moles solute/mole
solvent is obtained from an operating line
displaced substantially from the equilibrium
curve (Treybal, 1955) as shown in the il-
lustration that follows.
Solvent rate selected is 900 Ib-moles/hr of
water at 68°F. The tower contains 24-inch
tray spacing and 1-inch liquid seal and oper-
ates at isothermal conditions.
0 12
0 04 0 06 0 0) 010
I . Mill NH, Mil H,0 it II >l
Figure 15. Plot of operating line from the
conditions at top and bottom of bubble cap
plate tower.
546 Ib-moles/hr
residue gas >
900 Ib-moles/hr
' fresh solvent
60
540
600 Ib-moles/hr
feed gas
Bubble cap
Tray
Tower
/ \
900
54
954 Ib-moles/hr
rich liquid
Y X
Ai
Flow,
Ib-moles/hr
Flow,
Ib/hr
Density,
lb/ft3
Solution:
1. Calculate the mole ratios of solute in
gas and liquid streams at both ends of
the tower:
(a) Mole ratios at bottom of tower:
Yl " 3?0 " 0'1U raole N
Xl " ~900 " °-06 mole NH3/mole
(b) Mole ratios at top of tower:
6
Y2 " 540
- o.o
0.0111 mole NH /mole air
Feed gas 600 16,680 0.0722;
Residue gas 546 15,762 0.0750
Absorbent liquid 900 16,200 62.3
Rich liquid 954 17,118 62.3
2. The operating line is plotted as shown
in Figure 15 from the conditions at
top and bottom of the column as deter-
mined in step 1. A straight line is
drawn between points Xj, Yj and X,, Y,.
21
-------
Gas Absorption Equipment
Clean gas
Liquid out
Liquid in
Figure 16. Venturi scrubber or absorber with,
cyclone-type liquid separator.
3. The curve of ammonia-water equilibrium is
plotted on the same graph from data
taken from Leva (1953) in terms of mole
ratios.
4. Number of theoretical plates or trays:
A horizontal line AB is drawn from the
operating line at the conditions at the
top of the column to the equilibrium
curve. Line BC is then drawn vertically
from the equilibrium line back to the
operating line. The step ABC is a
theoretical plate. The stepwise procedure
is repeated to the end of the operating
line. The solution shows 2.45 theoretical
plates.
5. Number of actual plates or trays:
With a viscosity, ML, of 1 centipoise for
water and a slope of the equilibrium
curve, m, of 0.83, (this assumes the
equilibrium curve to be straight over
the area covered by the operating line),
the value m^ is (1) (0.83) = 0.83 From
Figure 14 the overall plate efficiency
is 72%.
Actual plates required :
2.45
0.72
3.4 - use 4 bubble cap trays.
6. Tower diameter:
From Figure 16 , with a 24-inch tray
spacing and 1-inch liquid seal, K-0.17.
(a) Superficial linear gas velocity at
bottom of tower:
.. . 17 (62.3 - 0.0722}"-"- - 5.00 ft/sec
V 08C ft
(d) Tower diameter:
(4)(12.80)\j
!\ 3.14
\0.5
4.04 ft.
The principles Just discussed are for ab-
sorption of a single component. Multicom-
ponent absorption is of great industrial
importance in the natural gasoline, petro-
leum, and petro-chemical industries. When
emissions consist of mixed-solvent vapors,
control by adsorption of Incineration would
probably be more economical than absorption.
Design procedures for multicomponent absorp-
tion are more complicated than those des-
cribed previbusly and will not be attempted
here. Sherwood and Pigford (1952) devote an
entire chapter to these procedures.
COMPARISON OF PACKED AND PLATE TOWERS
i
While devices such as agitated vessels, spray
chambers, and venturi absorbers have lim-
ited application for gas absorption, the
choice of equipment is usually between a
packed tower and a plate tower. Both devices
have advantages and disadvantages for a
given operation, depending upon many factors,
such as flow rates for both gas and liquid,
and degree of corrosiveness of the streams.
Final selection should be based upon the
following comparative information:
22
-------
Gas Absorption Equipment
1. Packed towers are less expensive than
plate towers where materials of con-
struction must be corrosion resistant.
This is generally true for towers less
than 2 feet in diameter.
2. Packed towers have smaller pressure
drops than plate towers designed for the
same throughput and, thus, are more
suitable for vacuum operation.
3. Packed towers are preferred for foamy
liquids.
4. The liquid holdup is usually less in a
packed tower.
5. Plate towers are-preferable where the
liquid contains suspended solids since
they can be more easily cleaned. Packed
towers tend to plug more readily.
6. Plate towers are selected in larger sizes,
to minimize channeling and reduce weight,
Channeling is corrected in the larger di-
ameter and tall packed towers by instal-
lation of redistributor trays at given in-
tervals.
7. Plate towers are more suitable where the
operation involves appreciable tempera-
ture variation since expansion and con-
traction due to temperature change may
crush the packing in the tower.
8. In operations where there is heat of solu-
tion that must be removed, plate towers
are superior in performance since cool-
ing coils can be easily installed on the
plates.
9. Most conditions being equal, economic con-
siderations favor packed towers for sizes
up to 2 feet in diameter.
the necessary small bubbles, power requirements
to force the gas through the small openings
are high.
Increased dispersion may also be achieved
by injecting the gas Just below a rotating
propeller, where the shearing action of the
blade breaks up the large bubbles. With a
single vessel, the advantage of true counter-
current flow cannot be fully realized since,
if there is good agitation, the concentration
of absorbed gas in the liquid is uniform
throughout the vessel. Thus, absorption
equivalent to only one theoretical plate can
be achieved per vessel. Although absorption
with this equipment is usually batchwise,
continuous operation can be obtained with a
series of vessels wherein the gas and liquid
pass from vessel to vessel in opposite dir-
ections.
Vessels such as these have been used to re-
move highly odorous gaseous products from the
reaction of sulfur and sperm oil in the manu-
facture of specialty lubricants. Here the ef-
fluent gases, containing a considerable per-
centage of hydrogen sulfide, are forced by
their own pressure from the closed reactor,
through a vent pipe fitted with a sparger,
into a tank filled with caustic soda. This
arrangement, without auxiliary mechanical
agitation of the liquid, reduces the odor of
the effluent gas to an innocuous level.
Control, however, is effected primarily by
chemical reaction rather than by true ab-
sorption.
Small tanks containing water or caustic soda
are used to eliminate visible emissions from
vents of hydrochloric acid storage tanks
during tank loading. Without any control
device, these emissions of hydrogen chloride
vapor are dense enough to violate most air
pollution ordinances regarding opacity. The
opacity can be reduced to a negligible amount
by bubbling the displaced tank vapors through
a simple perforated pipe into the water or
caustic soda.
VESSELS FOR DISPERSION OF GAS IN LIQUID
Probably the simplest method of dispersing a
gas in a liquid for absorption is by Injecting
the gas through a perforated pipe or sparger
of some type into a vessel filled with the liq-
uid. Unless the sparger has minute perfora-
tions, the gas bubbles formed tend to be too
large and thus present a relatively small
interfacial surface for the absorption oper-
ation. If the sparger is designed to create
SPRAY TOWERS AND SPRAY CHAMBERS
Interphase contact in spray-type absorbers is
achieved by dispensing the liquid in the form
of a spray and ppssing the gas through this
spray. In order to present a large liquid
surface available for contact, sprays of
droplets ranging in size from 500 to 1,000
microns are necessary. Fine droplets require,
however, high pressure drop across the spray
nozzles, and there is danger of liquid entrain-
ment at all except very low gas velocities.
23
-------
Gas Absorption Equipment
In a simple countercurrent spray tower where
the liquid is sprayed down from the top and
the gas passes upward through the spray, ab-
sorption equivalent to one transfer unit is
about all that can be expected. Unless the
•diameter-to-length ratio is very small, the
gas will be well mixed with the spray, and
true countercurrent flow will not be realized. ';
• Higher gas velocities without excessive en-^
trainment can be obtained with a centrifugal-
type spray chamber, whereby the spray droplets !
are forced to the chamber walls by the centrif-i
ugal action of tangentially entering gas before!
they can be carried out the top of the chamber,]
With this arrangement, there is a crossflow
type of contact, and the degree of contact is
limited to about one theoretical plate or ttans-*
fer unit. I. <
:. Spray chambers or towers have .been used
extensively for control of particulate matter
but, according to Sherwood and Pigford (1952),
their use for pure gas absorption seems to be
limited to air conditioning or deaeration 6f
water where very few transfer units are re-;
quired. These chambers may also be used for
some highly soluble gases when the degree of
required removal is small, but, in air pollution
control work, this type of operation is not'
common. They have been used as precleaners
for particulate removal from gas streams where :
other devices are used for ultimate control of
air pollution.
PRESSURE
DISCHIRGE
Figure 17. Venturi liquid-jet eductor-type
absorber.
VENTURI ABSORBERS
Like spray towers and spray chamber, equip-
ment using the venturi principle is primarily
used for removing particulates from gas streams,
; though it has some application to gas absorption.
In gas absorbers, the necessary interphase con-
tact is obtained by differences between the ve-
locity of gas and liquid particles, and by
turbulence created in the venturi throat. Dis-
persion in venturi devices is achieved in two
.ways: By injecting the liquid into the gas
•stream as it passes through the venturi, as
'shown in Figure 16, or by admitting the gas
to the liquid stream as it passes through the
venturi, as shown in Figure 17. In the latter
case, the venturi is also a vacuum-producing
device and insplrates the gas into the venturi
throat. With both types, a gas-liquid separa-
,| tion chamber is necessary to prevent entrain-
iment. This can be a simple tank, the stream
;from the venturi tube impinging on the liquid
.surface, or, more efficiently, a cyclone-type
/separator.
'i
For the unit shown in Figure 16, the gas
•.velocities in the venturi throat range from i
_200 to 300 feet per second, and the liquid
,;is injected into the stream at a rate of
about 3 gpm per 1,000 cfm of gas handled.
These units are designed specifically for
collection of submicron particulate matter,
and utilize high horsepower. For the liquid-
jet eductor types, the liquid consumption is
j 50 to 100 gpm per 1,000 cfm of gas handled at
a draft of 1 inch of water. The liquid-jet
eductor types are capable of developing drafts
up to 8 inches of water at higher liquid flow
rates. They find application principally for
the absorption of soluble gases, but are also
used for collection of particulate matter
larger than 1 or 2 microns in diameter.
Venturi units obtain a high degree of liquid-
gas mixing but have a disadvantage of a rel-
atively short contact time. Various literature
sources have indicated a high efficiency of
: absorption for very soluble gases such as
sulfur dioxide and ammonia; however, for oxides
of nitrogen where contact time is of utmost
importance, Peters (1955) reports -efficiencies
of absorption of from 1 to 3 percent. Because
of the high degree of efficiency of venturi
scrubbers for particulate removal, they seem
desirable for use with a dirty gas stream that
also contains a highly soluble gas that must
be removed. A major disadvantage of venturi
units is the high pressure drop (often as high
as 30 inches of water) with attendant high
power requirements for operation.
24
-------
Gas Absorption Equipment
NOTATIONS
A = surface separating hot and cold media,
ft2
C = specific heat, Btu/lb-°F
D = outside diameter tube, ft
D. = inside diameter tube, ft
2
D = diffusion coefficient, ft /hr
f = friction factor
g = acceleration of gravity, 64.4ft/sec-sec
2
G = mass velocity of flow, Ib/hr-ft
G. = mass velocity through baffle opening,K
b Ib/hr-ft2
2
G = maximum cross flow velocity, Ib/hr-ft
c <•
G = weighted mass velocity, G x G. , I
6 Ib/hr-ft2
2
G. = mass velocity inside the tube, Ib/hr-ft
G = gas rate Ib-moles/hr
ill
2
h = coefficient of heat transfer, Btu/hr-ft
°F
k = thermal conductivity, Btu/hr-ft -°F
K = coefficient of mass transfer, Ib moles/
hr-ft2 atmospheres
L = tube length, ft
L = liquid rate Ib-moles/hr
CBM
molecular weight of mixture (vapor
plus inert gas)
molecular weight of vapor
partial pressure of vapors at t , atm
partial pressure of vapor at t , atm
logarithmic mean of the vapor pressures
at the interface and at the vapor stream,
atm.
F = total pressure on system, atm
q - quantity of heat, Btu/hr
t = condensate temperature, °F
t = vapor temperature, °F
t = water temperature, F
U = condensing coefficient for pure vapor
c between tc and tw> Btu/hr-ft2-°F
w = rate of flow, Ib/hr
A = latent heat, Btu/lb
\t= viscosity at average temperature,
Ib/hr-ft
H, = viscosity at average film temperature,
Ib/hr-ft
11 = viscosity at tube wall temperature,
w Ib/hr-ft
P= density at average fluid temperature,
lb/ft3
25
-------
ABSORPTION NOMENCLATURE AND DEFINITIONS
A = Cower cross-sectional area (ft )
2
a = specific packing surface area .
a" = interfacial surface area (ft )
C = concentration of solute in gas phase
(any units but usually will be in terms
of partial pressure, P. or mole fraction,
Y).
concentratlon °f solute in gas phase
at gas-liquid interface (any units
but usually will be in terms of
partial pressure, P, or mole fraction,
Y).
C * = the equilibrium concentration of solute
in gas phase that is associated with
the solute concentration (C,) in the
liquid phase (any units but usually will
be in terms of partial pressure, P, or
mole fraction, Y).
AC_ = change in concentration of solution in
gas phase (any units but will usually
be in terms of partial pressure, F, or
mole fraction, Y).
C, = concentration of solute in liquid phase
(any units but usually will be in terms
of mole fraction, X).
C ..v = concentration of solute in liquid
phase at gas-liquid interface (any
units but usually will be in terms of
mole fraction, X).
C * = the equilibrium concentration of solute
in liquid phase that is associated with
the solute concentration (C ) in the
gas phase (any units but will usually
be in terms of mole fraction, X).
AC. = change in concentration of solute in
liquid phase (any units but will usually
be in terms of mole fraction, X.)
dc = concentration gradient across the film
dz thicknesses (any units depending upon
the units used to express concentration).
D =• inside diameter of tower (ft)
E = plate efficiency (dimensionless)
F = a, = packing factor
G = gas mass velocity
gas mass velocity / Ib
f Ib m ., \
\hr - ft /
M
(for use in
pressure drop correlation curves only) .
G • gas molar mass
in
TOTAL
g = gravitational conversion facto
velocity /(Ib m - moles )\
\ hr - ft*—;
/Ib m ^
\-hT-j
r/32.2 Ib m-ft\
\j..b f- sec2" /
total gas mass velocity
H " Henry's law constant (any units depending
upon units used to express concentrations
but will usually be Jib m - mole]
_J
pr
Ji
(_f
- atm
H_
height of overall transfer unit, gas
phase (ft)
height of overall transfer unit, liquid
phase (ft)
HTU - Height of a Transfer Unit (ft)
Ah •= liquid head in a plate tower (ft)
X* » mass transfer coefficient (any units de-
pending upon units used to express con-
centration)
K_ • overall capacity coefficient, gas phase
G [ (Ib m - mole) 1
L hr - ft* - atmj
JC - overall capacity coefficient, liquid
L phase [ (Ib m - mole) T
I hr - ftz -(Ib m - mole)
verall capacity^coefficient, gas phase
(Ib B - mole)
•overall capacity c
I (;ib a - mole) "I
[hr - ft-* - atm J
Ka • overall capacity coefficient, liquid
phase f (Ib m -mole) "*
hr - ftj -(Ib m - mole)
ft3
k « individual gas film coefficient /(Ib m -mole)\
atir )
/(Ib m -n
(hr-ft2-a
= individual liquid film coefficient
[(Ib m-mole) T
ir - ft^ - (Ib m-mole) |
ft3 J
PA.C.ge.26.8.69
-------
Absorption Nomenclature and Definicions
L = liquid mass velocity f Ibra
L = liquid molar mass velocity f(lb m - moles))
L i«-"2"j
f lb m "1
Ulb m -moles)!
M = apparent molecular weight
apparent molecular weight of gas stream
m
tlb m 1
lb m - moles)]
apparent molecular weight of liquid stream
lb m
f
|(
[(lb m - moles)!
m = slope of equilibrium line (dimensionless)
N = number of moles of solute transferred per
unit time [(lb m - mole)1
hr
J
rate of mass transfer per unit area
f (lb m - moles)]
L ft^ - hrJ
N-_ = number of overall gas-film transfer
units (dimensionless)
NQL = number of overall liquid-film transfer
units (dimensionless)
N
number of plates required to obtain a
desired removal of solute (dimensionless)
NTU = number of transfer units (dimensionless)
dn
rate of mass transfer
*.. ~ Lobe wi. uiaaa LLcaussier I (lb m — moles) I
dt I—hr J
P = partial pressure of a gas component (atm)
PTOTAL = average pressure in column (atm)
AP = pressure drop per unit height of tower
(in. H20/ft of packing)
QTOTAL = total volumetric gas flow
R = universal gas constant
S = solubility factor
T = absolute temperature (°R)
V - tower volume (ft )
X = concentration of solute in liquid phase
in mole fraction |*(lb m - mole) solute "I
(lb m - mole) solution
, c d
' '
• concentration of solute in liquid
phase leaving a plate of a plate
tower in mole fraction
[(lb m -mole) solute ]
|Ub m - mole) solution)
Xj^ = concentration of solute in liquid phase
at concentrated end of tower (liquid exit)
in mole fraction f(lb m - mole) solute "1
Klb m - mole) solution
X2 = concentration of solute in liquid phase
at dilute end of tower (liquid entrance)
in mole fraction [" (lb m - mole) solute "1
L(lb m - mole) solution]
X* = solute concentration in liquid phase in
equilibrium with gas phase in mole frac-
tion [ (lb m - mole) solute ]
[(lb m - mole) solution
AX = change in solute concentration in liquid
phase between any two points in tower in
mole fraction (lb m - mole) solute
(lb m - mole) solution
Y = concentration of solute in gas phase in
mole fraction f(lb m - mole) solute ] ]
[(lb m - mole) effluent gas
b.c.d.
concentrati°n of solute in gas
phase leaving a plate of a
plate tower in mole fraction
(lb m - mole) solute "I
lb m - mole) effluent gasl
Y^ = concentration of solute in gas phase at
concentrated end of tower ( gas entrance)
in mole fraction ["(lb m - mole) solute ~]
[(lb m - mole) effluent gasl
YZ = concentration of solute in gas phase at
dilute end of tower (gas exit) in mole
fraction |"(lb m -mole) solute ~|
|{lb m - mole) effluent gas J
Y* = solute concentration in gas phase in
equilibrium with liquid phase in mole
fraction Rib m - mole) solute
L(lb m - mole) effluent gas
-------
Absorption Nomenclature and Definitions
AY = change in solute concentration in gas
phase between any two points in tower
in mole fractionRlb m - mole) solute
(Ib m - mole) effluent gas
Z = total depth of packing in tower (ft)
z = thickness of gas film (ft)
z, = thickness of liquid film (ft)
e = percent of free gas space (dimensionless)
U = viscosity of liquid (centipoises)
I = density of gas stream
HjO = density of water fib m
= density of liquid
= °H20
(dimensionless)
DEFINITIONS
Height of a Transfer Unit (HTU) - height of
a section of packing producing a change
in vapor compositions equal to the log
mean difference between the actual and
equilibrium vapor compositions over the
section.
Theoretical Plate - a plate of sufficient
size to yield the same change in composi-
tion as that brought about by one equili-
brium stage.
Carrier Gas - the inert and disposable portion
of the inlet gas mixture.
Solute - the removable or recoverable portion
of the inlet gas mixture.
Absorbent - the liquid removing the solute
from the inlet gas stream.
Mass Transfer - the process of moving a material
from one phase to another, e.g., a gas
transferred from a gas mixture into a
liquid.
Mass Transfer Coefficient, N - the material
transferred per unit time per unit area.
Subscript indicates the basis of the
coefficient, i.e. gas phase C overall gas
phase OG, liquid phase L, etc.
Equilibrium - a state of the absorption pro-
cess in which the mass transfer rates into
and out of absorption liquid are equal.
Partial Pressure - the pressure of an ideal
gas in a mixture of ideal gases proport-
ional to its molal concentration or its
volume percent. This is a statement of
Dalton's Law.
Pound Molecular Weight - quantity of a sub-
stance in pounds mass equal to its mole-
cular weight.
Mole Fraction - the concentration of a sub-
stance expressed as the fraction of the
total number of molecular weights present
in the mixture for all substances.
Diffusion Coefficient - a constant which in-
dicates the rate of random movement of one
material through another.
Molecular Diffusion (gases) - average uni-
directional movement of molecules toward
an area of lower concentration and due to
their constant motion.
Eddy Diffusion - turbulence or currents in
the body of the fluid; the major mechan-
ism for mixing in liquids.
-------
ABSORPTION EQUIPMENT: SELECTED APPLICATIONS
I INTRODUCTION: LIMITATION OF APPLICATIONS
Absorption of gases is one of the oldest and
most common methods of preventing the discharge
of gas and vapor pollutants into the atmosphere.
Various means to achieve maximum absorption
are designed into such control equipment, but
most types are either packed, grid or plate
towers, spray towers or chambers, or wet type
inertial scrubbers. While this equipment is
similar to those in the process industries,
absorbing systems employed in air pollution
control have certain limitations.
A Excessive Temperature
Pollutant gases, especially from combustion
processes, are emitted at elevated tempera-
tures, requiring cooling before gas ab-
sorption becomes practical.
B Large Volumes, Small Amounts of Pollutants
Gas volumes are often very large in com-
parison to the amount of pollutant to be
removed, therefore costly to move and cool.
C Choice of Absorbing Liquid
1 water
If water is used in great quantities,
this may be expensive; if waste water
is used, it may transform an air pollu-
tion problem into a water pollution
problem. Sludge and liquid wastes are
often real disposal problems.
2 Liquids Other Than Water
Other absorbing liquids usually require
stripping for reuse, an additional cost.
Also provisions often have to be made
to prevent absorbing solution spray
carryover.
D Economic Recovery Considerations
Most processes vent to the atmosphere only
gases which are beyond the point of econo-
mic recovery. Unless the residual gases
received can be turned into a saleable
or usable product, most industries consider
further scrubbing an added cost to their
product and do so only because legal or
public opinion requires them to control
their vent gases.
E Loss of Effluent Buoyancy
Liquid scrubbed gases exit at lower tempera-
tures and with a higher water vapor content.
This is a disadvantage for good meteorolog-
ical dispersion of the effluent.
These problems then make most gas absorption
control methods rather specific to meet the
circumstances of the application, such that
the same pollutant may be economically
scrubbed from the atmospheric discharge of
one industrial process and not from some
other.
II APPLICATIONS
Briefly described below are several applica-
tions of absorption methods employed (or
proposed upon the basis of laboratory and pilot
plant studies) for the reduction of gaseous
pollutants.
A Sulfur Dioxide In Flue Gas
Emission of sulfur dioxide has become an
increasing problem in the very large coal
burning steam boiler plants. For economic
reasons, coal of relatively high sulfur
content is being used. No method, absorp-
tion or any other, is presently completely
satisfactory for use with large steam
boiler plants, but several wet scrubber
methods have been devloped and have shown
promise. Three such methods are considered
here:
1 Limestone or Dolomite Injection with Wet
Scrubbing
Sulfur oxides produced by burning coal
or oil can be reacted with the calcined
products of limestone of dolomite to
form removable calcium - sulfur salts.
Iron oxide, which is present in most
dolomites, acts as a catalyst by speed-
ing the reaction of CaO and MgO and S02
to CaS04 and MgSO^. The injection pro-
cess can take two forms: (1) Dry removal
of SOX by injection of powdered limestone
or dolomite into the gas steam, or (2)
wet removal by following the dry inject-
ion by wet scrubbing.
The wet absorption process was developed
by Combustion Engineering in cooperation
with the Detroit Edison Company. There
are two commercial installations using
this process at present and a third is
PA.C.ge.l2a.l2.70
-------
Absorption Equipment- Selected Applications
slated to be operational by 1971. In this
process the pulverized additive Is in-
jected Into the furnace where 20 to 30%
of the SOX is removed. The remaining 502
and calcined additives then pass through
a preheater and into a wet scrubber. The
calcined additives react with wash water
in the scrubber to form hydroxides. These
hydroxides react with the SO2 and any
unreacted 803 to form slightly soluble
sulfites and sulfates. These salts, along
with the fly ash which is 99% removed by
the scrubber, are sent to a clarifier and
to disposal. A flow diagram is seen in
Figure 1.
2 Wellman-Lord Process
A wet absorption method using potassium
or sodium sulfite scrubbing solution has
been developed by Wellman-Lord, Inc., a
subsidiary of Bechtel Corporation. A
pilot plant unit has been operated at
the Gannon station of Tampa Electric
Company. A demonstration unit at the
Crane station of Baltimore Gas and
Electric was operated until September
1969. The process has been licensed to
two Japanese firms for Asian sales and
promotion. The first full-scale commerical
application will be for the Olin Corp.
TO SUCK
r
COAL
SUPPLY
I „.
LIMESTONE
SUPPLY
MILL
AND
MAKE-UP
WMER
TO DISPOSAL
FIGURE 1. WET COMBUSTION ENGINEERING
Although, 98Z removal of SO2 is possible
under ideal conditions, 60-75% removal
has been reported for the commerical
installations (using less than stoichio-
metric additive) along with 99% removal
of fly ash and 100% elimination of 803.
There are also indications that this
process can remove 20 to 30% of the
nitrogen oxide emissions.
at a 700 ton per day sulfuric acid plant
scheduled for completion late in 1970.
The original design called for a potassium
sulfite scrubbing solution. The absorbed
S02 formed potassium bisulfite which on
cooling precipitated out as potassium
pyrosulfite. Steam stripping the pyro-
sulfite produced anhydrous SO2 for re-
-------
Absorption Equipment Selected Applications
covery and regenerated potassium sulfite
for recycle. Pilot plant work ac the
Gannon and Crane stations indicated
high energy requirements for the potassium
solution system. Parallel work was done
on a sodium solution system to reduce
utility requirements and capital costs.
This sodium system will be used in
Wellman-Lord's first commercial instal-
lation for Olin. Detailed technical
information for the process has not been
released and only the most general type
of flow diagram is available (See Figure
2). Wellman-Lord has predicted performance
figures for their process using the
sodium system. Better than 90% S02 re-
moval, 97% 803 removal, and 90% fly ash
removal (with a loading of 0.5 grains per
cubic foot) are claimed.
SODIUM SYSTEM
TO STACK
passes on to the reducer. At 1100°F in
the reducer the sulfates and sulfites in
the solution react with producer gas
(CO and H2) to form a sulfide solution
which is sent to the regenerator. In the
regenerator at 800°F the sulfide solution
is reacted with C02 and water (from the
reducer) yielding a regenerated molten
carbonate mixture to the absorber and
a H2S stream to a Claus unit. A flow
diagram is seen in Figure 3.
Although only bench scale work has been
completed, 99% removal of SO. in gas
streams containing 0.3 to 3% SO, has
been reported. There are also indications
that the molten salt technique can control
nitrogen oxides.
H20
? — n
H-D
—1
r
i
i
i
i
i
T
i
SULFURC
ACID PLANT
LIQUEFACTION
PLANT
TION
NT
- ABSORBER AREA
•CHEMICAL AJEA--
FIGURE 2 WELLMAN-LORD PROCESS
3 Molten Carbonate Process
A process for scrubbing hot flue gases
with a mixture of molten salts has been
developed by Atomics International, a
division of North American Rockwell Corp.
In this process, the flue gas passes
first through a high temperature electro-
static precipitator and is then contacted
in an absorber with an eutectic mixture
of lithium, sodium, and potassium carbon-
ate at about 800'F. The resulting solution
of carbonates, sulfates, and sulfites
B Sulfur Dioxide in Smelter Gases
Sulfur dioxide is a waste gas product in
the smelting of many metal sulfide ores.
Many years ago, these waste gases were
emitted to the atmosphere, but today,
both to prevent air pollution, and for
economic reasons sulfur dioxide is re-
covered from smelter waste gases. It
should be noted the sulfur dioxide content
of smelter gases is much higher than for
flue gases. The recovered sulfur ends up
as liquid sulfur dioxide, sulfate fertilizer,
-------
Absorption Equipment- Selected Applications
TO STACK
RECOVERED H S
2
HOT RUE
GASES
CARBONATES
SULFATES
SULFITES
•PRODUCER
GAS CO, H
ASH
TOVASTE
FIGURE 3 MOLTEN CARBONATE PROCESS
or sulfuric acid. Two basic methods used
in this country are:
1 ASARCO Reduction Process
The American Smelting and Refining Co.
(ASARCO) first used a similar but now
obsolete process in the early 1940's
on a semi-commercial scale. Although
this process does not involve gas ab-
sorption, it is included in order to
be complete. The new ASARCO process
uses sufficient natural gas both to
react with all the oxygen in the waste
gas and to reduce sulfur oxides to sulfur.
The hot off gas is first water-cooled
and then cleaned by an electrostatic
precipitator. After heat exchange,
the off gas is mixed with natural
gas and sent to a temperature controlled
combustion chamber. In this furnace the
sulfur dioxide is reduced to sulfur,
and carbonyl sulfide (COS) and hydrogen
sulfide (H2S) are formed by side reactions.
The gases are further cooled and then
enter a bauxite catalyzed reactor where
the carbonyl sulfide is converted to
sulfur. The sulfur in this stream is
then collected in the liquid form by
an electrostatic precipitator and sent
to storage. The gas stream is then heated
and passed through two catalytic reactors
which convert the remaining hydrogen
sulfide to liquid sulfur. This sulfur
is also sent to storage. See Fig. 4.
Economically the optimum range of off
gas concentrations is from 52 SO. and
122 02 to 71 SO. and 9% 0.. Sulfur re-
covery Is reported to be 95% of the
total sulfur in the off gas.
2 Ammonia-sulfurlc acid process
This process, sometimes known as the
ammonium sulfite-blsulfite system was
pioneered at Trail, British Columbia,
years ago, as a means to recover sulfur
dioxide and prevent air pollution. In
brief, an aqueous solution of an ammon-
ium sulfite and bisulfite is recirculated
through a scrubbing system. Continuous
portions of the scrubbing liquid are
drawn off, and also anhydrous ammonia
is continuously added to the scrubbing
liquid. A proper balance of addition
and bleeding for optimum scrubbing con-
trol is determined by continuous pH
control. A portion of scrubbing liquid
removed from the system is treated wiCh
sulfuric acid, forming ammonium sulfate
-------
HEAT EXCHANGER
* ""
COOLER
HEAT EXCHANGER^
REACTOR
HEAT EXCHANGER
Steam
HEAT EXCHANGER
Liquid sulfur
to storage
*j HEAT EXCHANGER h*"Sarl.
S§
«/> s
HEAT EXCHANGER
Condensate
Condensate
h
Condensate
FIGURE 4. ASARCO REDUCTION PROCESS
-------
Absorption Equipment1 Selected Applications
and evolving sulfur dioxide gas. One mole
of sulfuric acid produces between one
and two moles of sulfur dioxide gas,
the exact amount varying with the ratio
of ammonium sulfite to ammonium bisulfite
in the scrubbing liquid removed from the
scrubber. The pure sulfur dioxide obtained
is dried with sulfuric acid. The ammon-
ium sulfate obtained is stripped with
steam in order to eliminate any residual
SO. in the ammonium sulfate solution
which is crystallized, centrifuged and
finally dried as a marketable product.
See Figure 5.
Falk describes the control of sulfuric
acid fumes emitted in the manufacture of
a paint pigment. Many industrial sources
of sulfur acid mist and gases are emitted
continuously, but in the batch production
of titanium oxide, the emission of sulfuric
acid is very intense for a relatively short
time and very small the rest of the time
during production. Ilmenite ore, an Iron
titanium oxide, is added with sulfuric
acid to the reaction vessel vented to the
atmosphere. To initiate the acid-ore re-
action, oleum or dilution water Is added.
either of which produces heat. After "set-
FfGURE 5. AMMONIA- SULFURIC ACID PROCESS
More recently, this method has been
developed by the Olin-Mathieson Chemical
Corporation to control air pollution and
recover sulfur dioxide in tall gases
emitted in the manufacture of sulfuric
acid. This latter process reduces tail
gas content as high as .9% sulfur dioxide
to a value as low as .03 of 1% in a two
stage scrubbing process.
C Acid Fumes in a Pigment Manufacture (DuPont)
off" as this addition Is called, the re-
action proceeds slowly, until suddenly a
tremendous amount of steam and pollution
("attack gas") are emitted reaching a peak
of the order of 120,000 cfm; in a few
minutes the mist and gas release again
return to a low level.
Abatement measures In this process require
equipment capable of handling sudden peak
loads of sulfur dioxide, sulfur trioxide,
-------
Absorption Equipment Selected Applications
and sulfunc acid droplets along with much
steam and some metal ore dust in a manner
easily adapted to batch processing. If a
wet control process is to be used, it
should not be too costly or create a water
pollution problem.
Decided upon at this plant (DuFont) was the
use of a 72" steel plate rubber lined jet
scrubber system (Schutte-Koerting) employ-
ing a primary and a circulating secondary
jet scrubber system. To reduce equipment
corrosion, the pH of the scrubbing system
is controlled with the addition of lime
slurry to a collecting-holding tank. A 1000
gal. per minute, 100 HP pump continuously
supplies water to the 3" primary Jet through-
out the attack cycle. Surrounding the primary
jet are 72 secondary Jets, each 3/4" in
diameter, supplied by an 8000 gpm 300 HP
pump. Both jets discharge downward into a
15,000 gal. tank. The stack that vents the
reaction vessels is connected to the scrub-
ber through a tee and damper arrangement.
The system depends primarily on condensation
of steam through the jet, thus creating a
vacuum to draw the fumes out of the reaction
vessel stack. The holding tank is emptied
once a day or more frequently if necessary
in order to keep the scrubbing solution cold
and to prevent build-up of the lime salts
that would clog the system.
D Hydrogen Sulfide in Refinery Gases (Girbotol)
Sulfur impurities in natural gas and pet-
roleum appear as hydrogen sulfide and
mercaptans. In some refinery off gases,
hydrogen sulfide was burned in flares, but
the trend today Is to recover the sulfur
as a valuable by-product. A regenerative
system for the recovery of hydrogen sulfide
is the Girbotol process. By this process,
the gas to be scrubbed passes counter- cur-
rent in a plate tower to an absorbing solu-
tion of either monoethanolamine or die-
thanolamine. This absorbent solution is
regenerated in a second plate tower or
reactlvator (stripper) where the hydrogen
sulfide is stripped with steam.
E Chlorine - Carbon Tetrachlorlde Process
(Diamond Alkali)
Chlorine gas is a part of many chemical
reactions in the chemical industry not
all of which are closed systems. When
this gas is vented to the atmosphere,
even in small amounts, it is a potential
air pollution problem unless means
are taken to remove the chlorine from vent
gas.
There are various scrubbing methods to
prevent the emission of chlorine in vent
gases, but the method developed by Diamond
Alkali Company is somewhat unusual. It is
based upon the differential absorption
of chlorine in carbon tetrachloride in
a pressurized packed tower, followed by
recoverey of the chlorine in a pressurized
stripping tower.
Figure 6 shows a typical recovery unit where
a chlorine-air mixture is pressurized to
100 psi and at this pressure cooled succes-
sively in a water cooled and refrigerant
cooled heat exchanger. The cooled gas at
approximately 5°F enters the bottom of
a packed column where it is scrubbed counter-
current with carbon tetrachloride entering
the top at 0°F.
CONDENSER
FIGURE 6. RECOVERY OF CHLORINE OR'SNFT GAS*
The absorbing carbon tetrachloride enters
the stripping tower half-way up where chlorine
Is stripped In the lower half with a siphon
reboiler used at the bottom to provide
the heat necessary for stripping.
The chlorine free carbon tetrachloride
leaves the bottom of the stripping tower
at 260°F, is cooled to 0°F and returns to
the top of the packed tower absorber. The
upper section of the stripping tower Is the
rectifying section; here rising chlorine
vapor in contact with the refluxing liquid
chlorine and the resultant pure chlorine
gas, free of carbon tetrachloride, exits
from the top of the stripping tower. The
chlorine can be recovered as a gas or con-
densed and recovered as a liquid according
to the needs of plant operations.
F Laboratory Hood Exhaust: Spray Tower
Application (ANL)
-------
Absorption Equipment: Selected Applications
An interesting application of a horizontal
spray tower for the removal of halogens,
carbon dioxide and aerosols in laboratory
air is described by Liiraatainen and Mechara
of the Argonne National Laboratory. The
system was designed for removal of highly
reactive and toxic halogen compounds such
as bromine pentafluoride, bromine trl-
fluoride, etc., but was found also very
effective for removing carbon dioxide. A
spray tower with its relatively low pres-
sure drop was chosen instead of a packed or
plate tower, since low pressure blowers
and light duct work moved air through the
system at 6000 cfm.
The scrubber, A ft. x 17 ft. of welded
1/8" steel plate consisted of three succes-
sive sections, each having a group of four
hollow cone spray nozzles (Schutte-Koerting
662C) directed toward cast iron throat
pieces mounted upon the scrubber dividers.
The tower was horizontal, with gas and
spray liquid moving co-currently. A stag-
gered array of baffles followed by a 4"
filter of monel fibers to remove entrained
droplets was located inside the tower (or
chamber) at the exit end. Finned tube steam
heaters were used to prevent condensation
and give buoyancy to the effluent.
The scrubbing solution was between 6 and
10% by weight of potassium hydroxide and
was recirculated through the scrubber and
a recycle holding tank with a black iron
7.5 HP centrifugal pump. Piping to the spray
headers was 3" blackiron. Liquid flow was
of the order of 100 gal. per minute.
For a range of inlet concentrations up to
700 ppm of bromides, fluorides and iodine,
efficiencies of absorption range from 70
to 100% with the least efficiency applying
to the bromides. Aerosol removal efficiency
measured at 0.5 and 0.1 microns ranged
between 35 and 572.
G Oxides of Nitrogen - Ammonlacal Process
(DuPont)
Nitrogen dioxide is one pollutant which
reveals Itself as a characteristic reddish
brown plume when emission concentrations
are 50 ppm or greater. Waste oxides of
nitrogen are vented to the atmosphere in
several industrail processes, e.g., the
manufacture of nitric acid and the manu-
facture of sulfuric acid by the chamber
process. One company, DuPont of Canada,
recognized this potential air pollution
at their nylon intermediate plant at
Maitland, Ontario, and so applied controls
on tail gases of the nitric acid plant
and also on off gases from other processes.
The first step in their control of oxides of
nitrogen was to collect all vented gases
and deliver them to an absorber where waste
liquor containing dilute ammonia and caustic
scrubbed the gases in a packed tower. See
Figure 7. This system controlled oxides
of nitrogen, but created another air pollu-
tion problem in the form of a heavy white
plume which contained finely dispersed
ammonium salt particles. This secondary
air pollution problem was solved by a
venturi scrubber and mist collector.
TO ATMOSPHERE
VD4TUW _L
TO DRAM
FIGURE 7. NITROGEN OXIDES ABATEMENT PROCESS
The absorber was constructed of rubber lined
mild steel. Duet works and other equipment
coming in contact with the waste gases were
constructed of 304 type stainless. Piping
handling the alkali scrubbing solution was of
mild steel. The scrubbing liquor averaged
around one half of IX sodium hydroxide and
sodium carbonate with generally slightly
smaller amounts of ammonia. This scrubbing
liquor was recirculated until exhausted,
then it was dumped. It was found that the
degree of exhaustion could be measured by
pH. This presented an easy method of control
using a recording pH meter and an alarm.
The venturi scrubber and separator were de-
signed for a gas flow of 700 cfm and a
liquid flow of 42 gpra with a total pressure
drop of up to 22" water gauge. Recovery
efficiency was 99Z.
-------
Absorption Equipment Selected \pplications
REFERENCES
Maurin, P.O. and Jonakin, J., "Removing
Sulfur Oxides from Stacks," Chem.
Eng.. 21 (9): 173-180, (1970).
Martin, J.R., Taylor, W.C., and Plumley
A.L., "The C-E Air Pollution Control
System," Paper presented at 1970
Industrial Coal Conference, Lexington,
Kentucky, April 8-9, 1970.
Craig, T.L., "Recovery of S02 from Stack
Gases—The Wellman-Lord S02 Recovery
System, "Paper Presented at 1970
Industrial Coal Conference, Lexington,
Kentucky, April 8-9, 1970.
Oldenkamp, R.D., and Margolin, E.E.,
"The Molten Carbonate Process for
Sulfur Oxide Emissions."Chem. Eng.
Progress. 65 (11): 73-76, (1969).
Argenbright, L.P. and Preble, B., "S02
from Smelters: Three Processes Form
an Overview of Recovery Costs,"
Environmental Science and Technology.
4 (7): 554-561, (1970).
"Cost Estimates of Liquid Scrubbing Pro-
cesses for Removing Sulfur Dioxide
from Flue Gases," U.S. Bureau of
Mines Report of Investigation No.5469,
1959.
7 Mallette, F.S., (Ed.), Problems and
Control of Air Pollution, Reinhold
Pub. Corp., N.Y., 1955.
8 Kohl, A.F., and Riesenfeld, F.C., "Gas
Purification," Chem. Eng.. £6 (12),
1959.
9 Falk, L.L., "Reduction of Sulfuric Acid
Fumes," AIHA Quart.. _12 (4), 1951.
10 Sutter, R.C., "Recovery of Chlorine
from Air-Chlorine Mixtures," JAPCA,
2 (1), 1957.
11 Straight, H.R.L., "Reduction of Oxides of
Nitrogen in Vent Gases," Canadian ±.
Chem. Eng.. February 1958.
12 Liimatainen, R.C., and Mecham, W.J.,
"Removal of Halogens, Carbon Dioxide
and Aerosols from Air in a Spray
Tower." JAPCA 6 (1), 1956.
-------
Edward B. Hanf
Ceilcote Co., Berea, Ohio 44017
A guide to scrubber selection
pollutants, certainly a far-
ranging industrial problem, are perhaps
best classified by their physical charac-
teristics and not necessarily by individ-
ual source The following list of major
types of air pollutants is useful for
discussing control techniques
• Noxious gases—substances like hy-
drogen chloride or sulphur dioxide,
that normally are emitted in a vapor
state
• Liquid enlrainmenl—liquid par-
ticles 10 microns and over in size
created by sprays, dragout, agitation,
or bubbling, and picked up in exhaust
air streams
• Mists—liquid particles formed by
condensation of molecules from the
vapor state, particle size is usually
10 microns or lower
• Dusts—solid particles, usually five
microns and larger, formed by grind-
ing or disintegration of solids
• Fumes—solid particles smaller
than one micron, formed by condensa-
tion sublimation, or oxidation of me-
tallic vapors
• Entrained particles—particles of
mists, liquids, dust, or fumes, that are
collected and conveyed by an air
stream through an exhaust ventilation
system.
Once the type of pollutant is thus
broadly defined, the engineer respon-
sible for environmental quality faces
the question "What system can best
handle the problem'" Wet scrubbers
arc versatile, and are probably the
closest to a universal answer; if the
pollutant is corrosive or exists in a
corrosive environment, glass fiber
reinforced plastic (FRP) systems arc
most often called for Yet. confusion
does exist about wet scrubbers and
about PRP This confusion is under-
standable for two major reasons Wet
scrubbers abound in a wide variety of
designs sizes, efficiencies, and collec-
tion principles At the same time, en-
gineers arc hard-pressed to keep
abreast of advances in corrosion-re-
sistant materials development, es-
pecially plastics I propose to discuss
typical wet scrubber designs and the
advantages and limitations of rein-
forced plastics in designing for cor-
rosive environments.
Wet scrubbers are suitable for a
wide range of corrosive applications
for which other types of collectors
may require expensive modifications
or unusual designs The main advan-
tage of wet collectors are constant ex-
haust volume, elimination of secondary
dust problems during disposal, small
size, and ability to clean hot or mois-
ture-laden gases Possible trouble spots
for wet scrubbers are applications
where costly water clarification may
be necessary before disposal or reuse
Freezing of water lines is a potential
hazard, and vapor plumes may be
present during cold weather operation
Principles
Scrubbers can remove either solu-
ble gases, mists, or paniculate matter
in two basic ways By gas absorption
or by impingement or interception
Nuclcation—a third collection method
—is a patented process used for dust
and fume particles in the submicron
range.
Gas absorption involves the transfer
of noxious gas from an exhaust stream
into a liquid phase. Basic factors con-
trolling the gas absorption process arc
the degree of solubility or chemical re-
activity of the gas to be removed in
the scrubbing liquid, and the means
of obtaining intimate contact between
the gas and liquid streams Normally.
plant water is used to remove such
gases of high solubility as hydrogen
fluoride or hydrogen chloride In some
cases, caustic or salt solutions may be
used because they chemically react
with less soluble gaseous contaminants
For example, sodium hydroxide scrub-
bing liquid is used to react with
chlorine gases to produce sodium hypo-
chlonte
Impingement can be described as in-
terception of the contaminant and its
removal from the air stream Particles
impinge or impact upon targets—pack-
ing or other media—placed in then
path The contaminants then arc
washed away The nucleation process
employs a humidification and cooling
cycle to cause water condensation on
submicron particles The particle size
builds up to a level where particles can
be removed by impingement on the
packing
Selection criteria
Basically, there arc five major types
of wet scrubbers Cross-flow, counter-
current, wet cyclone, ventun, and ver-
tical air washer Packing is used in
the first two types. These packed
scrubbers—as well as spray towers
and baffles—are described below, along
with their most efficient pollution con-
trol use
In cross-flow packed scrubbers, the
air stream moves horizontally through
a packed bed and is irrigated by the
scrubbing liquid which flows vertically
down through the packing. Cross-flow
designs are characterized by low water
consumption and fairly high air flow
capacity at low pressure drop. Thcv
are commonly used for removing en-
trained particles from air streams, as
well as for eliminating gaseous pollu-
tants Packed beds will effectively re-
move mist and spray particles three
microns and larger by impingement
Particular matter in the air stream
strikes the wet packing, adheres to the
surface and is washed away by the
scrubbing liquid The packing—es-
pecially for removing particulars—
must be kept wet to prevent the par-
ticles from becoming rccntramcd in the
gas stream
A large amount of entrained liquid
particles themselves may provide irriga-
tion of the packed bed In most indus-
trial applications, however, the packing
usually is irrigated because of the na-
ture of particulars removed, the low
fluctuating concentration of particles
removed, or the possibility of reentram-
ment if not washed away For removal
of mists and entrained particles greater
than three microns in diameter, either
cross-flow or parallel-flow scrubbers
give satisfactory collection efficiency
and low operating costs.
The parallel-flow design is a modi-
fication of the cross-flow design in
which a front washing spray is used to
Reprinted from Environmental Science & Technology, Vol.
pages 110-115. Copyright 1970 by the American Chemical
Reprinted by permission of the copyright owner.
4, Feb. 1970,
Society.
-------
Wet scrubber
principles
give rise
to wide range
of designs
Spray towers
Packed bed
Cross-flow packed scrubber
Baffled spray towers
Liquid entrainment
separator
Packed bed
Counter-current-flow
packed scrubber
Combination units
Gas inlet
Spray inlet
Shallow packed
bed
Wet cyclone scrubber
Gas outlet -
to entrainment separator
Venturi scrubber
Vertical air washer
Volume 4, Number 2, February 1970 111
-------
achieve a parallel flow of both the gas
stream and scrubbing liquid. This ays-
tern is particularly effective for re-
moving entrained liquids where the
stream is loaded with solid dust par-
ticles. (The front spray prevents solid
buildup on the packing which causes
excessive pressure drop.) Because the
scrubbing liquid drops through the
packed bed, no more than one foot of
packing depth can be used. Often, the
cross-flow and parallel-flow designs are
combined when front-washing is re-
C|L roil for packing depths of more
than one foot. This combination is ef-
fective for control of air streams carry-
ing solid and liquid particulates and
trace amounts of noxious gases.
The cross-flow packed scrubber is
also used to remove submicron dust
and fume particles. In the patented
nucleation process, cross-flow packed
scrubbers can obtain high collection
efficiencies at particle sizes in the
submicron range. This involves pre-
treatment of the inlet air, before it
enters the scrubber, to achieve the
proper stream conditions. The stream
passes through a packed bed and rapid
condensation around the dust particl :s
takes place. This process enlarges the
particles to a size such that they can be
removed effectively by the packed bed.
With proper humidification and operat-
ing conditions available for nuclea-
tion. a one micron particle can be
built up to five or six microns. With
this increase in particle size, collec-
tion efficiencies of 90-99% can be
achieved on a four to five foot bed in
a cross-flow scrubber.
In the counter-current packed scrub-
ber, the gas stream moves upward in
112 Environmental Science & Technology
direct opposition to the scrubbing
liquid stream, moving downward
through the packed bed. With the use
of specific tower packings, liquid sur-
face regeneration is obtained with no
increase in energy consumption. Liquid
surface regeneration is more critical
to the efficiency of a packed column
than the surface area of the packing
used in the column.
The gas stream—rich in contami-
nants—comes into contact with spent
liquor at the bottom of the packed bed.
Fresh liquid coming in at the top of the
scrubber is in contact with the least
contaminated gas. This characteristic
provides a fairly constant force
throughout the packed bed for driving
the gaseous contaminant into the
scrubbing liquid. There also is less
chance that the dissolved gases will
be stripped out of the liquid.
A modification of the counter-cur-
rent design is the concurrent packed
scrubber. Here, the gas and liquid
streams move in the same direction—
usually down—through the packed
bed. These scrubbers can be operated
at high gas rates, and, in the case of
gas absorption, at high liquid rates.
They easily remove gases of high
water solubility and usually are ef-
ficient enough for removal of less solu-
ble gases.
This design is useful in limited
spaces, since it handles high gas veloci-
ties, and relatively low cross-sectional
areas can be utilized. High gas veloci-
ties in these designs do not cause flood-
ing, as they would in counter-current
design, because the gas stream helps
to push the liquid stream through the
packing.
Cross-flow and counter-current
scrubbers appear to perform the same
function, but where highly soluble
gases or mists are to be removed, the
cross-flow packed scrubber has several
advantages. The benefits are derived
from the ease with which gas and
water streams pass through the cross-
flow design, compared to the resis-
tance of the opposing streams in coun-
ter-current flow. Using the same gas
and liquid mass flow rates, a cross-
flow scrubber has lower total liquid rate
and pressure drop. Besides reducing
water consumption drastically, the
cross-flow principle also reduces pump
and fan motor sizes. Other advantages
include less plugging from solids
dropout at the packing support plate
and the possible use of higher gas and
liquid rates because of the extremely
low pressure drop.
The economical break-even point
between the two scrubber designs is
based on the packing depth required.
If packing requirements for cross-
flow exceed six feet, the counter-cur-
rent flow design usually has a lower
operational cost. As previously noted,
cross-flow and counter-current scrub-
bers use packings. Traditionally, ce-
ramic packings have been common;
plastic packings, however/ are coming
into wider use. Plastic pickings have
the advantage of beingjvirtually un-
breakable, and their lighAr weight also
permits smaller wall sdlubbers, pro-
viding equipment cost livings. One
example of the newer molyethylcne
packings (trademarked Tfllerette), has
the shape of a helix that^s formed
into a doughnut (toroid) jfcape. In
addition to a low pressure <
-------
Comparison of scrubber operating costs
Scrubber type
Cross flow
Tellerette packing
Berl saddle packing
Raschig ring packing
Counter-current
Tellerette packing
Berl saddle packing
Raschig ring packing
Wet cyclone
Spray tower
Jet
Venturi
Liquid
rate
(g p.m.)
50
60
60
120
140
140
100
600
80
Liquid
pressure
(P.s.1.8)
5
5
5
5
5
5
60
60
20
Pump h p
0.3
0.4
0.4
0.7
0.8
0.8
5.6
9.6
42.0
1.9
Scrubber
pressure
drop
(in water)
0.5
1.2
3.8
0.75
2.2
6.7
3.5
2.0
15.0
Total
pressure
drop
(in water)
1.5
2.2
4.8
Fan h p
4.3
6.3
13.8
Total h p
4.6
6.7
14.2
1.75
3.2
7.7
4.5
3.0
1.0
16.0
5.0
9.1
22.0
12.8
8.6
none
46.0
5.7
10.0
22.8
18.4
18.2
42.0
47.9
Annual
power
cost
$370
540
1140
460
810
1840
1490
1470
3380
3860
a superior ability to provide liquid
surface regeneration.
Absorption is relatively rapid when
gas first comes into contact with a
liquid. After the surface of the liquid
becomes saturated, however, the dis-
solved gas must diffuse from the sur-
face—a slow process. Turbulence will
bring fresh liquid to the surface and
hasten the absorption, but this requires
additional energy and higher operat-
ing costs
Tellerette packing was designed with
a large number of interstitial holdup
points at which the liquid conies to-
gether and disperses again, exposing
new surface for absorption without
adding energy to the system This
means that the liquid surface is con-
stantly changing and is renewed as the
liquid progresses through the packed
bed
Wet cyclone scrubbers are efficient
for removing entrained liquids and
dusts from air streams High gas
throughputs are achieved by forcing
the gas into a spiral flow pattern Cen-
trifugal force on the entrained par-
ticles because of this flow pattern
reaches a force of several hundred
times the force of gravity; larger en-
trained particles are forced to the wall
of the scrubber, since their mass is
much greater than that of the air
stream
High pressure spray nozzles generate
minute liquid spray droplets which in-
tercept smaller particles and further in-
crease collection efficiency The scrub-
bing liquid from the-spray systems also
is thrown to the wall by centrifugal
force and, along with the larger parti-
cles, drains down the wall to the sump
The wet cyclone design is effective
when the air stream contains dust
particles. These are removed along
with the liquid particles by centrifugal
force and interception. Limiting fea-
tures of this design are its higher
pressure drop, greater pumping costs
for the spray nozzles, and inability of
most wet cyclone designs to remove
particles smaller than five microns. A
patented liquid injection process, how.
ever, is available that reduces pressure
drop to two to four inches of water
compared to the six to eight inches
common to standard cyclone scrubbers.
This process, in turn, eliminates the
high pressure drop and reduces pump-
ing costs
Venturi scrubbers are best suited for
applications where solid or liquid par-
ticulates in the low and submicron
ranges—05-5 microns—must be re-
moved from exhaust air streams Such
participates are created by condensa-
tion of a liquid or metallic vapor or
when a chemical reaction causes the
formation of a mist or fume. Typical
examples arc ammonium chloride
fumes from steel galvanizing, phos-
phorus pentoxide fumes from phos-
phoric acid concentration, mists from
dry ice plants, and zinc oxide fumes
from rcverbcralory furnaces.
To effectively remove these par-
ticles, turbulent contact must occur
between the gas stream containing the
participates and the scrubbing liquid
The ventun scrubber does this by
passing the two streams concurrently
through the extremely small throat
section of the ventun. The velocity of
both streams is accelerated in the
ventun throat, causing the liquid to
break up into extremely fine drops.
These liquid drops collide with the
particles carried by the gas stream to
effect their removal. The drops then
agglomerate to form larger particles
again. The cleaned gas stream then is
passed through a separator unit to
eliminate relatively large entrained
liquid particles.
A dominant operating feature ot
Venturi scrubbers is their high pressure
drops, which can range from five
inches of water to as high as 100
inches. Venturi scrubber design dic-
tates that the pressure drop must be
increased as the particle size being re-
moved becomes smaller
Venturi scrubbers also arc used for
removing soluble gases from air
streams. Usually, such applications are
limited to those cases where small
particulars also are present, because
the high energy requirements for op-
erating ventun scrubbers do not nor-
mally make them economically feasi-
ble for controlling gaseous pollutants.
The vertical air washer is a low
cost design used primarily for remov-
ing liquid participates (entrapment,
carryover, and mists) from exhaust
air streams in metal finishing and plat
ing operations Using a concurrent up-
ward irrigation system with shallow
packed beds, the vertical air washer
is used to remove liquid particulates
from anodizing, pickling, etching,
cleaning, rinsing, and some plating op-
erations. This design is effective in re-
moving particle sizes down to eight mi-
crons at high collection efficiencies
Two important features of this de-
sign are its low irrigation rate (which
can be as low as 2 g p m. per 1000
Volume 4, Number 2, Februar> 1970 113
-------
cfm), nnd its low pressure drop
(which usually is less than I 0 inches
of water) The low presure drop re-
sults in extremely low operating costs,
bcc.iuse of the savings in required fan
horsepower However, this washer de-
sign should not be used where a rela-
tively high concentration of gaseous
contaminants is present in the air
stream
Spray chambers utilize the principles
of interception—contact between the
mist particle and spray droplet They
arc an economical solution for remov-
ing large liquid particles from air
streams where efficiencies below 90%
arc satisfactory
Baffle spray towers require a high
gas velocity, with a resultant high
pressure drop to force particles against
the baffles which are wetted continually
with spray droplets Well designed
spray lowers give collection efficiencies
to 90% on entrained liquid particles
greater than 10 microns They are ex-
pensive to operate because of high
pressure, high volume, and high fan
operating requirements
Several types of scrubbers arc some-
times in series A wet cyclone/con-
current packed scrubber combination,
for example, removes 'ughly soluble
noxious gas and dust present in the
same gas stream This combination
consists of two sections built essen-
tially into one housing or shell The
top section is a wet cyclone design
which removes the dust particles by
centrifugal force The dust is washed
down the shell of the scrubber into a
launder placed above the packed bed
After passing through the cyclone sec-
tion, the gas stream enters a packed
bed where the highly soluble gas is
absorbed in the irrigating liquid This
particular design permits a high face
velocity in both the cyclone section
and concurrent packed section This
reduces the space requirements for the
mam scrubber shell, but a separate
enlramment separator must be pro-
vided when this* combination is used
The wet cyclone does provide some
initial gas absorption while removing a
large percentage of the solid particu-
lates The concurrent section removes
the majority of the remaining soluble
gases and most of the remaining solid
participates down to five microns.
The disadvantages of this approach
(wet cyclone/concurrent combina-
tion) arc that the height requirements
arc excessive and may not be available
A top inlet/bottom outlet arrangement
may not surt the total system require-
Industrial wet scrubber applications
Steel
Metal finishing
Aluminum
Pulp & paper
Pharmaceutical1,.
Food processing
Textiles
Gases—HCI
Liquid entrainment—H.SO,
Gases—HF, NOZ
Liquid entrainment—NaOH,
HNO,, HjPCL cyanides
Mists-H2Cra
Dusts—AIF,, Al,0,
Gases—HF
Fumes—AlCli
Gases-a,CIO>,CS02
Liquid entrainment—CHjOH,
(CH,)SCHOH, amines DSMO
monochlorobenzene
Gases—HjS. CS=
Liquid entrainment—HjPCX,
acetic and maleic acids
Gases—H:S, CS>, HCI
Liquid entrainment—HjSO,
menls An entrainment separator must
be supplied.
Another typical combination is a
wet cyclone followed by a cross-flow
packed scrubber Similar to the con-
current combination, this combination
will handle dusts above five microns at
high loadings, and highly soluble gases.
The wet cyclone/cross-flow com-
bination provides economical recovery
of gases from an air stream loaded
with solid participates Again, the wet
cyclone acts as a pretrcatment or
initial scrubbing section while the
cross-flow acts as the tail gas scrubber
The heaviest portion of the solids is
knocked out in the wet cyclone. The
gases and smaller participates are re-
moved in the tail gas packed scrubber.
Elimination of the solid particulates
before the gases enter the packed
scrubber lessens the possibility of blind-
ing the packing support plate or build-
ing up heavy solid depositions in the
packed bed
The advantages of a wet cyclone/
cross-flow combination over the wet
cyclone/concurrent flow are-
• Greater flexibility in choice of
scrubbing liquors for each section
• Recovery in either stage without
contamination from previous stage
• More efficient gas absorption with
greater overall efficiency
• Separate housing permits separate
installation, allowing for spreading of
initial investment over a greater time
period
• A flexibility that permits addition
of more packing at later dale for in-
creased gas absorption.
• The cross-flow scrubber acts as a
built-in entrainment separator.
Other combinations, such as ven-
turfs followed by cross-flow scrubbers,
concurrent followed by counter-cur-
rent, ventun followed by counter-cur
rent, etc , can be adapted to handle a
specific requirement. The exact com-
bination best suited for the problem
would be based on performance re-
quirements, nature of contaminants.
and costs (initial and operating).
Reinforced plastic
Construction materials available for
scrubbers used in corrosive service
include glass fiber reinforced plastic.
lined steel, stainless steel, and costly
titanium and nickel alloys. Reinforced
plastic, for a number of reasons, is
probably better suited to more cor-
rosive services than any other ma-
terial Three major criteria—resistance
to temperature, corrosion, and abra-
sion—decide what FRP scrubbers can
handle.
• Temperature limitation of FRP is
usually in the range of 250° F -300° F
Quench chambers located before FRP
wet scrubbers, however, frequently can
solve a high temperature problem.
• Corrosives generally arc not a
problem; polyester, epoxy, or furan
resin systems handle most corrosive
materials. The exceptions arc a few
114 Environmental Science & Technology
-------
concentrated oxidizing agents, such
as sulfuric acid above 70% concen-
trations. Reinforced polyester offers
the broadest range of corrosion pro-
tection; furans usually are used where
solvents or organics are present in
quantity; and epoxies have the best
alkali resistance.
• Abrasion resistance usually does
not present a problem with FRP wet
scrubbers, because adding air and
water in the control process reduces
the abrasive action of the solids.
Generally, initial equipment costs
of FRP units are as low as three
quarters the covt of rubber-lined steel,
one third the cost of ceramic, and one
half the cost of acid brick-lined equip-
ment, and much less than units of
stainless steel and more exotic alloys.
On large equipment, reinforced plastic
may be competitive with such com-
mon materials as carbon steel because
the cost of handling and field-welding
large steel equipment is high. Installa-
tion cost generally is low, due to
lighter weight, less supports, flexibility
of location, reduced shipping cost, and
less field fabrication (equipment fre-
quently is shipped assembled).
Maintenance of FRP equipment is
minimal, as FRP resists corrosion and
weathering. Repairs, if necessary, can
be made by nonspecialized personnel.
Maintenance costs can be as low as
one half to one fifth that of compa-
rable lined steel equipment, since there
is no external corrosion. The equip-
ment can either be shipped in unitized
or packaged form, or fabricated at the
job site from subassemblies. The limi-
tation in equipment size for reinforced
plastic is the same as other construc-
tion materials—limitation of shipping
size of the equipment.
Due to the much lighter weight of
reinforced plastic, large equipment fre-
quently can be shipped in several
pieces for final field assembly with a
total overall savings over a com-
pletely field assembled steel construc-
tion. For example, in a recent installa-
tion, the FRP scrubber assemblies were
completely unitized, including a de-
sign for roof-mounting into a standard-
ized steel grid system. This concept in-
volved a wet scrubber/fan/pump as-
sembly mounted on a plastic coated
steel skid that tied directly into the
roof grid structure. The systems are
controlling air quality where acid etch-
ing and cleaning of integrated cir-
cuits produce a variety of corrosive
mists and gases.
Total weight of the packed wet
scrubber/fan/ pump on the skid was
10,000 pounds. Light weight of the
reinforced plastic system permitted
minimum steel reinforcing. An esti-
mated 15-20% cost savings resulted
by assembling the scrubber package on
skids at the factory and shipping it to
the customer as a unit.
Edward B. Hanf is field sales man-
ager of air pollution control equipment,
Ceilcote Co., a position he has held
since 1966. Hanf received his B.S.
from Case Institute of Technology
(1958), and has been active in the field
of wet scrubbers and air pollution con-
trol since that time. He is a member of
the National Association of Corrosion
Engineering, Air Pollution Control As-
sociation, and the Cleveland Engineer-
ing Society.
JUprinfed rrorn
ENVIRONMENTAL
Science & Technology
February 1970, Pages 110-115
COPYMCHT IY
THf AMfMCAN CHEMICAL SOCKTY
Volume 4, Number 2, February 1970 115
-------
performance of
commercially available equipment
in scrubbing hydrogen chloride gas
Reprinted by special permission from JOURNAL OF THE AIR POLLUTION
CONTROL ASSOCIATION, March 1970, Volume 20, Number 3, Pages 139-143.
Copyright 1970 by the Air Pollution Control Association.
Stanley K. Kempner and E. N. Seller, Western Electric Company
and
Donald H. Bowman, Purdue University
Mcsiia Kcmpnei and Scilct
:uo associated with Western
Klcctric Co , Inc , 222 Broadway,
New York, X Y 10038
Di IBoumiin is on the faculty
of I'uidue ITuucisity, liuli,ai:i]>-
olis, lad
March 1970 Volume 20, No 3
Six fume scrubbers wore tested to establish a compaiativc uiting for the
different types of equipment that are commercially available for removing
hydrogen chloride (HC1) gas from plating room exhaust air. Bids were
sent out to several manufacturers asking them to quote on a scrubber thnt
would remove 90% of the contaminant from a 2500 scfm gas stream con-
sisting of ambient air w ith a concentration of 20 mg/m' of HC1 gas Scrub-
bers were choben on the basis* that they v.ere representative of the different
types of equipment available for this application. The units were con-
nected to ductwork and piping simulating actual production conditions.
The contaminant was carefully metered into an accurately measured air
stream being drawn through the scrubber A continuous sample of the
scrubber discharge was recorded by a conductivity meter The results
have been published in a series of curves plotting Efficiency vs Water Kate.
Efficiencies o'f close to J 00% were obtained with the vertical packed sci ubbcr
and the extended surface scrubber. The horizontal packed scrubber and
the plate scrubber attained efficiencies of better than 95% The fan-type
scrubber was entirely unacceptable and is not recommended for use with a
gaseous contaminant. The tests also revealed that optimum efficiencies
can be obtained at much lower water rates than are generally recommended
by the manufacturers. This makes it practical to use non-recirculutcd
water in production scrubbers Variations in scrubber design and effec-
tiveness of mist eliminators are also discussed
139
-------
1. Two stage extended surface
2. Vertical packed tower
3. Deep bed horizontal
4. Two stage plate tower 1250scfm
5. Two stage horizontal
6a. Fan scrubber, modified
6b. Fan scrubber
50
0 2 4 6 8 10 12 14 16 18
Water rate, gpm
Figure 1. A comparison of scrubber effectiveness.
Roof
fan
Motor operated
"by-pass damper
Vacuum
pump
Recorder JU] |
W.B.&D.B.
thermometers.
HCI gas
cylinder
(D
Damper control
potentiometer
Manometer
Orifice
1 ma '
m
P
L
I I
ductivity
nete
JT
iq
tori
1
r
-
r—
f
5
c
s
o
t
1 1 r
«
^
JD
3
^
1214"
'"*
Sample
probe
a a
0 |
•a I
03 E
*!
II
1 1
1 — ^
'T
~~
2£
Figure 2. Schematic diagram of test set-up, (not to scale).
In early 1968, Western Electric Company authorized a test
program to investigate and compare efficiencies of scrubbers
used in cleaning fumes from their plating exhausts. The first
phase of this investigation involved the removal of hydrogen
chloride (HCI) gas from a 2500 cfm gas stream.
Invitations to bid were sent to several manufacturers of
scrubbing equipment. Each manufacturer was asked to
furnish a scrubbing device capable of developing 90% effi-
ciency. Prime consideration in evaluating the bids was the
procurement of a representative cross section of the scrubber
industry's equipment. Cost considerations were secondary
but important, and with the exception of one unit, the scrub-
bers purchased were economically competitive. The following
six scrubbers representing the different types of equipment
available for this application were selected for test:
1. Extended Surf ace Scrubber
2. Vertical Packed Scrubber
3. Horizontal Two Stage Scrubber
4. Horizontal Deep Bed Scrubber
5. Plate Scrubber
6. Fan-Spray Scrubber
In making a comparison of the six units we must keep in mind
the purpose of the tests and the particular application that
was being investigated. The purpose was to establish a com-
parative efficiency rating for the different types of scrubbers
that are commercially available for removing HCI gas from
the exhaust air at plating areas. The application anticipated
an installation without recirculation where water would be
passed through the scrubber on a once thru basis prior to
draining to the Waste Treatment Plant.
The above criteria dictated the representative contaminant
concentration of 20 mg/m3 and put a premium on the conser-
vation of water. Most of the presentation of the data
compares water rate versus efficiency because this was
Western Electric's area of interest. Information has been
recorded, however, that permits the comparison of other '
parameters if it is deemed desirable.
Figure 1 shows a plot Efficiency vs. Water Rate for the
six units. Note the range of water rates and efficiencies for
units all purported to do the same job.
140
Test Equipment
Figure 2 is a schematic diagram of the test setup. The
contaminant, technical grade HCI gas, was fed from a 20 Ib
cylinder through a carefully calibrated rotometer and inserted
into the air stream immediately downstream of the calibrated
orifice. Feed was through a % in. glass tube located in the
center of the duct and pointing in the direction of air flow.
The PVC inlet duct to the scrubber was 18 ft long with a
13.5 in. inside diameter. The outlet duct extended from the
scrubber to the underside of the 28 ft high roof deck, made a
U-bend and returned to floor level for accessibility of instru-
mentation, then made another U-bend and exhausted through
the roof. This duct was also PVC and had an inside diameter
of 12.25 In.
The roof-mounted coated fan was capable of drawing 16 in.
static pressure at 4000 cfm. Airflow was measured by read-
ing the pressure drop across a stainless steel calibrated orifice
located 6 ft downstream of the inlet or 12 ft ahead of the
scrubber. The orifice diameter was 9.52 in. A manually
controlled motorized by-pass damper located in the scrubber
exhaust duct on the roof permitted adjustment of the airflow.
The gas sample was withdrawn through a piece of l/i in.
glass tubing, also pointing in the direction of airflow, inserted
about half way into the exhaust duct in the downward pass
between the two U-bends. This was about 25 ft downstream
of the scrubber. The sample was drawn through a conduc-
tivity meter and a continuous reading was registered on a
strip chart recorder.
Figure 3 is a photograph of the test setup showing the
vertical packed tower.
Prior to running the test, extensive calibration runs were
made with the conductivity meter. Eeadouts were compared
with and verified by the standard wet sampling method.
Sampling traverses were made across the duct to establish the
validity of single point sampling. The details of the entire
calibration procedure which cover conductivity reagent flow,
sample flow, relative humidity of the sample, temperature of
the conductivity meter cabinet, back pressure in the exhaust
duct, etc. are too extensive to be included herein and are in
fact the subject of another paper. However, in spite of the
Journal of the Air Pollution Control Association
-------
> 100
ID
^ 90 -
80
° 70
1 stage
1500 scfm (350 fpm)
2000 (460)
2500 (580)
3000 (700)
4 6
Water rate, gpm
10
Figure 3. Photo of test set-up shows airflow orifice at left, water and
air control panel in center, and monitoring instrumentation at right.
Figure 4. Performance of extended surface scrubber at
20 mg/m1, HCI.
many variables, including the control of air flow anil rate of
contaminant feed, it was firmly established that results were
consistently reproducible to within a ±2J.& in. holes in the
bottom side of it. The separation between the two beds was
1 ft. A plate or baffle type eliminator was standard with this
March 1970 Volume 20, No. 3
141
-------
3000(600)
2500(500)
2000 (400)
ISOOclm (300 fpm)
4 6
Water rale, gpm
10
Figure S. Performance of vertical scrubber at 20 mg/m».
HCI
unit, At (he manufacturer's suggested water rate of 26 gpm,
thisiiiinngcmcnt of the horizontal scrubber gave an efficienc}
of 96 7% When the bottom row of frontal sprn> s on each
stage wa* •shut off (total 8 Rpm), there was no decrease in
efhciencj Fui Ihei, by shutting off both the top and bottom
frontal «|>r:n s on the second stage and also the bottom frontal
spravs on the fiont stage the efficiency was only ieduced to
96% A «alur reduction from 26 gpm to ]4 gpm was
effected with a minimal sacrifice of 7% efficiency
The effect ncnc*s of the baffle type eliminator was suspect
both from visual observation and experimental results
Close observation of the discharge end of the s
had little effect on the efficiency and the conclusion is drawn
that, they aic ncccssaiy only when there is a possibility of the
packing being clogged For expediency the same plate elim-
inator as described above was used although a preferential
nirnngement would have been a I ft bed of packing
Test lesults indicated that the modification of the horizontal
scrubber fiom a two-«tnge unit to u deep bed unit and improv-
142
ing the water distribution o\cr the packed bed significantly
impiovcd the performance.
Figuie 6 shows the improved cfRcicnc) that can be expected
w ith the deep bed scrubber This iniprot emciu is gamed by
a change in design with no additional cost to the unit
The pressure drop through this unit at 2500 scfm wu<
2 28 in of w ater with a 6 gpm water rote
Plate Tower Scrubber
The Plate Scrubber was the only piece of equipment, to be
tested that was not purchased in accordance with our speci-
fications For expediency, a test unit was renter! Tht«
was a two-stage stainless steel vertical scrubber that was
onginully built for use with an apartment house mctnetntor.
Its height, was 7 ft 6 in and the inside diameter was 33 in
The sepaiation between the two "flexi-tray" vuhc type tmvs
or stages was 1 ft The mist climmatoi was a 12 in packed
bed using 2 in "flexi-rmg" packing and located 1 ft Above the
second stuge Water was fed to the second stage ft om whence
it flowed over a weir, through a down-comer, and across the
first stage The gas discharge was through a 10 in hoi izontal
take-off. The rated capacity was 1250 scfm.
Wnh 11 water rate of 4 0 gpm, 96 7% of the HCI gas at 20
mg/m* concentration was lemoverl from the nitcd 1250
scfm an stieam At higher totes tin efficiency of 978%
was leached with the same air flow Increasing the cfm ua«
impiautical for two reasons Fust, when these tra\^ ait-
operated at a capacity above rated they develop a tremendous
pressure drop Second, because the unit was designed foi a
basement height incinerator installation, the vertical dis-
tance between the first and second stuge and between the
second stuge and the mist eliminator was compicsscd Thi*
resulted in excessive carry over from the first to the second
stage and fiom the second stage to the eliminator. The valve
cap design feature of the trays pcimittcd a reduction in an
flow thiough the sciubbcr without a detrimental effect, on the
efficiency In fact at the lowci water rates the cfh'cicucv
impiovcd with a decrease in cfm
It is possible to improve the pressure drop characteristics
of this unit by providing trays with shaped nozzles, howcvei,
Journal of the Air Pollution Control Association
-------
100
! 90
'80
70
;eo
so
2500 (325) deep bed
2500 (325) 2 stages—.
1500(200) 1 stage
2000 (260)
2500 (325)
3000dm (390fpm)
Gross face area—10 2 sq (t
Net face area-7 7 sq ft
6 8 10
Water rate, gpm
12
Figure 6. Performance curves of horizontal scrubber at
20 mg/m>. HCI
these clclails me not pcitmcnt to this test. This type of
scrubber is not rccomniciidcd foi use with HCI gas because it
is not economical!) practical to provide non-metallic or non-
con osive tra>s. However, it is our understanding that it has
shown excellent results when used on an incinerator to icmove
participate matter and should be considered for this applica-
tion 01 for use with non-corrosive gases
The pressure diop through the two stage unit at 1250
scfm iind 4 gpm was 358 in of water. At 2000 scfm and
4 gpm it was 7 00 in of watei and at 2000 cfm and 10 gpm
it was 8 001 n of w ater
Fan Spray Scrubber
The Fan Spray Scrubber was made of PVC. It consisted
of n PVC ccntnfugal fan immediately preceded by a spray
nozzle and cli»chniging into a plenum with a mist eliminator in
front of the outlet duct This unit has tremendous appeal to
the plant and facilities engineers because it is represented as
having extremely high efficiencies with very low water con-
sumption and it comcb as a pi c-paukagcd unit with the fan
built in
UnfoiUuiatch, its performance was very poor At the
niaiuifactuiei-. suggested water rate of IJ/g gpm, only 57%
of the HCI ga.s was removed from the 2500 scfm air stream
containing 20 ing/in' of contaminant Furthermore, the
factory installed 3 hp fun motor would only develop 2000
scfm and had to be replaced with a 5 hp motor to attain
the rated 2500 scfm It is not recommended for air pol-
lution control where the contaminant is a gas.
Where they exist, the efficiency of fan units can be improved
b\ installing three spray nozzles in the plenum chamber in
front of the mist eliminator With this modification an
efficiency of 84% was recorded with 11 5 gpm flowing This
modification is recommended for all units now being used for
the removal of a gaseous contaminant
The one redeeming feature of the Fan-Spray Scrubbci was
its mist eliminator This component did an excellent job in
pi eventing moisture cany o\ cr
Conclusion
My breaking with the traditional wet sampling methods it
has been possible to quickly and accuiately evaluate the pei-
formance of commercially available equipment for scrubbing
hydrogen chloride gas.
The gas lent itself to very accurate measurements when
laboratory conditions were imposed on the production sized
scrubbers being tested Furthei, the continuous monitoring
approach showed that on the spot evaluation of design changes
was possible and extremely desirable In fact, this method
now appears to be the preferable approach for equipment
monitoring whether in the laboratory or on the production
line
The increasing pressures being put on the plant engineer to
clean up low concentration exhaust gases have created a
demand for more detailed performance characteristics in the
manufacturer's equipment catalogue Todnj there is insuffi-
cient information available to the piacticmg engineer to
enable him to optimize his design selections even for a simple
contaminant such as hydrogen chloride gas It appears that
water rates and efficiencies that have been accepted for years
don't always apply In our tests a scrubber that apparently
was effective for hydrochloric acid mist was not neatly as
efficient when the mist became hydrogen chloride gas. A
scrubber operating at 20% of specified watei rate gave near
perfect performance. Unquestionably the information avail-
able was incomplete.
Rapid evaluation of scrubber performance with HCI as a
gaseous contaminant was made possible at a reasonable cost
by using the continuous monitoring method. It remains to
be seen whether or not these results can be applied to other
contaminants or if this method can develop the needed infor-
mation regarding scrubber efficiencies for other contaminants
both gaseous and aerosol. Hopefully it will, because the
plant engineer who is specifying a scrubber installation must
have this information if he is to do his job coircctly
Acknowledgment
This paper is based on work performed at the Indianapolis
Works of the Western Electric Company, Incorpoiatcd The
contributions made by the people of this plant particularly
C. J Kulczak, W. P. Russell and R H. Phillips sue gratefully
acknowledged.
March 1970 Volume 20, No. 3
143
-------
SECTION VI
CONTROL BY ADSORPTION
Principles of Adsorption
Adsorption Systems-and Their Application to Air Pollution Control
-------
PRINCIPLES OF ADSORPTION
J. Louis Kovach
I CONCEPT OF ADSORPTION
A Introduction
Molecules or atoms interacting individually
or in a group are considered a system. A
system which consists of physically homo-
geneous parts is referred to as a phase.
A system consisting of a single phase is
called homogeneous, while those consisting
of two or more phases are called hetero-
geneous systems. When we are dealing
with a heterogeneous system, the boundary
between the phases is termed as an inter-
face. These heterogeneous systems may exist
in both equilibrium and non-equilibrium
state. When the phase composition sta\s
constant with time, it is considered to be
at equilibrium If the phase composition
or its thermodynamic parameters change
with time, the system is non-equilibrium
Naturally, a spontaneous change in the state
of a non-equilibrium system goes toward
the establishment of the equilibrium state.
In any heterogeneous system consisting of
atoms, molecules or ions, the interaction
between the phases begins with chemical
or physical interaction at the phase inter-
face. When a molecule under kinetic motion
hits the surface (as an example of phase
boundary) from a random direction, it can
either bounce back from the surface elast-
ically with the angle of reflection equall-
ing the angle of incidence, or the molecule
may stay at the surface for a period of
time and come off in a direction unrelated
to that from which it came. Generally, the
latter is the case, the residence time
depending on the nature of the surface
and the molecule, the temperature of
the surface and the kinetic energy of the
molecule.
This phenomenon, molecular or atom inter-
action at the surface, can be observed at
interfaces between gas and solid, gas and
liquid, liquid and solid, between two liquids,
and in rare circumstances, between two
solid phases. In cases where the residence
time of a molecule at the surface is very
short, it is difficult to determine the
residence time by conventional physical or
J. Louis Kovach, Vice President
North American Carbon, Columbus, Ohio
chemical means. However, that the phenomenon
exists can be proven by the fact that, for
instance, gas can be heated by a hot surface
with which it is in contact. The interaction
at the interface involving the transition
of a molecule from one phase to another
can be considered the sorption of the mole-
cule by the given phase. The t>pe of inter-
action between the phases of a heterogeneous
system depends on the properties and com-
position of all the components of the system.
On the standpoint of the distribution of
the substances to be sorbed, two t>pes of
sorption can occur' adsorption and absorp-
tion. Adsorption takes place when the mole-
cules or atoms sorbed are concentrated only
at the interface, while absorption takes place
when the molecules or atoms are distributed
in the bulk of the interacting phases. As
a rule, adsorption takes place when one of
the phases consists of solid.
Adsorption of atoms or molecules on a solid
from the gas phase is, therefore, a spontan-
eous process, and as such it is accompanied
by a decrease in the free energy of the
system. The process involves loss of degrees
of freedom. Therefore, there is also a de-
crease in entropy.
Thus,
AF = AH - TAS
(1)
The adsorption process is always exothermic
regardless of the type of forces involved.
B Gas Phase Adsorption
Almost all air pollution problems where ad-
sorption is considered as a unit operation
involve gaseous contaminants. Gas (the term
gas will denote both gas and vapor) phase
adsorption is treated in detail.
As mentioned in the introduction, the number
of molecules which are present at the sur-
face and on the residence time of these
molecules on the surface. According to de
BoerC1), if n molecules strike a unit area
of a surface per unit time, and remain there
for an average time t, then o number of
molecules are present per unit area of sur-
PA.C.ge.21.1.66
-------
Principles of Adsorption
face.
a = nt
(2)
Using cm as unit surface and second as unit
time, n is number of molecules falling on 1
cm'/sec. The number n thus denotes the number
of molecules striking each cm2 of the surface
every second, and this number can be calculated
using Maxwell's and the Boyle-Gay Lussac equa-
tions. The number n is directly related to
the speed of the molecules within the system.
It is important to realize that the velocity
of the molecules is not dependent on the pres-
sure of the gas, but the mean free path is
inversely proportional to the pressure.
Thus,
n = 3.52 x
1022 x p
~
(3)
where
p = pressure in fig. mm.
M = molecular weight
T = absolute temp., °K.
From this equation at 20°C and 760 mm. Hg.
pressure, the following values can be obtained.
•53 2
HZ • n= 11.0x10" molecules/cm /sec.
N2 n= 2.94xl023 molecules/cm2/sec.
23 •>
Oj n= 2.75x10 molecules/cm"/sec.
The molecule residence time t on the surface
is not as easy to determine as the number n.
Reflection experiments can indicate the re-
sidence time on a smooth surface, because if
a molecule is retained on the surface for any
finite time, the angle of removal will be
random.
Early attempts to directly determine t by
Moist 6 Clausing(2)were not very successful.
Mowever, the exchange of thermal energy bet-
ween a molecule and the surface it strikes
is indicative of the residence time. In case
of clastic rebound from the surface, there is
hardly an^ degree of exchange of energy.
Clausing 1J) later used a method involving the
estimation of the velocity uith which molecules
of a gas pass through narrow capillaries when
the pressure is low enough that no mutual
collisions of the gas molecules take place.
Experimenting with argon in glass capillaries.
Clausing obtained the following results
at 90°k t= 3.1 x 10"S sec.
at 78°K t= 75 x 10'5 sec.
Same order of magnitude results were obtained
also for nitrogen on glass, but for neon the
value of
t = 2 x 10
was obtained.
-7
sec. at 90°K
It is evident from the argon residence time
results at two different temperatures, that t
is greatly dependent on temperature. According
to Frenkel(4)
toe
0/RT
(4)
where t is the time of oscillation of the
molecules in the adsorbed state, and Q is the
heat of adsorption. It can be seen from equa-
tion (4) that the adsorption energy (heat of
adsorption) Q is the all determining factor
for the magnitude of t.
C Adsorption Forces
It has to be strongly emphasized that there
are no special adsorption forces. Forces
causing adsorption are the sane ones which
cause cohesions in solids and liquids, and
uhich are responsible for the deviation of
real gases from the lav>s of ideal gases.
The basic forces causing adsorption can be
divided into two groups intermolecular or
van der Kaals, and chemical, which generally
involves electron transfer between the solid
and the gas. Depending on which of these
two force types plays the major role in the
adsorption process, we distinguish between
physical adsorption, where van der Waals
or molecular interaction forces are in pre-
valence, and chemisorption, where hetero-
polar or homopolar forces cause the surface
interaction. Thus, if in the process of ad-
sorption, the individuality of the adsorbed
molecule (adsorbate) and of the surface
(adsorbent) are preserved, we have physical
adsorption. If between the adsorbate and
the adsorbent any electron transfer or
sharing occurs, or if the adsorbate breaks
up into atoms or radicals which are bound
separately, then we are presented with chem-
isorption.
Although the theoretical difference between
physical and chemical adsorption is clear
in practice, the distinction is not as simple
as it may seem. The following parameters
can be used to evaluate an adsorbate-adsor-
bent system to establish the type of adsorp-
tion-
1 The heat of physical adsorption is in
-------
CL
Principles of Adsorption
the same order of magnitude as the heat
of liquefication, while the heat of
chemisorption is of the sane order as of
the corresponding chemical reaction. It
has to be pointed out here that the heat
of adsorption varies with surface cover-
age, because of lateral interaction
effects. Therefore, the heat of adsorp-
tion has to be compared on corresponding
levels.
2 Physical adsorption will occur under
suitable temperature, pressure conditions
in any gas-solid system, while chemisorp-
tion takes place only if the gas is cap-
able of forming a chemical bond with the
surface.
3 A physically adsorbed molecule can be
removed unchanged at a reduced pressure
at the sane temperature where the ad-
sorption took place. The removal of the
chenusorbed layer is far more difficult.
4 Physical adsorption can involve the form-
ation of multimolecular layers, while
chemisorption is always completed by the
formation of a nonolayer. (In some cases
physical adsorption may take place on
the top of a chenusorbed monolayer.)
5 Physical adsorption is instantaneous
(It is the diffusion into porous adsor-
bents which is time consuming)., while
chemisorption may be instantaneous, but
it generally requires activation energy.
D Adsorption Rate in Porous Adsorbents
As it is shown in the previous discussion,
the boundary layer is the most important
in the phase interaction. Therefore, to
achieve a high rate of adsorption, it is
expedient to create the maximum obtainable
surface area within the solid phase. High
surface area can be produced by creating
a large number of micro capillaries in the
solid. All commercial adsorbents, such as
activated carbon, silica gel, alumina, etc.
are prepared in this manner.
While, as it was described in the previous
paragraphs, adsorption is nearly instanta-
neous, the passage of molecules through
capillaries (pores) may involve some time.
There have been observations that in rare
cases it took several days to reach adsorp-
tion equilibrium.
The movement of molecules into the pores
is a diffusion process. Regardless of the
mechanism used, a correlation between o,
the number of molecules adsorbed per cm at
a given pressure and temperature, and time
tr required to complete the adsorption pro-
cess-
ot = (1 -
-k,t .
e d r)
(5)
where
o = value of o at the time t
00 = the equilibrium value of o
k. = a constant which is inversel> pro-
portional to the square of the
distance which the molecules have
to travel, and proportional to the
diffusion constant D, which can be
obtained from the following equa-
tion-
D = DQe-VRT
(6)
where
D = a constant
o
Q, = the energy of activation
The temperature dependence of the diffusion
constant and the temperature dependence of
constant k, in equation (5), and of the
whole phenomenon of the rate of reaching
adsorption equilibria depends on the activa-
tion energy Qj. The molecules may proceed in
three major manners within the pore structure.
They may collide with the wall of the capi-
llary and bounce off immediately according
to the cosine law, collide again, etc. In
this case, there is no activation energy.
Secondly, the molecules may collide with the
wall, stay there for time t, reevaporate,
collide again, etc. The energy of activation
will then be equal with the heat of adsorp-
tion. Thirdly, the molecules may migrate
along the wall of the capillary during a
sufficiently long time t, or they might
make hopping movements as described by de
Boer(5). The energy of activation is then
given by the fluctuations in the heat of
adsorption AQa. As an example, the rate of
adsorption of ethane on activated carbon gives
an energy of about 3 kcal/mole,which is
roughly half of the value of the heat of
adsorption.
II ADSORPTION EQUILIBRIUM
Both in nature and in technological processes,
the solid-gas phase interaction may occur
under two conditions. One involves the random
mixing of the phases, the other, their direct
relative motion. Thus, static adsorption occurs
-------
Principles of Adsorption
when the adsorption process takes place in
relative rest, or random mechanical mixing
of the phases of the solid-gas system takes
place, and ends in the establishment of an
adsorption equilibrium among the interacting
phases. Dynamic adsorption represents a sorp-
tion process accomplished under conditions of
direct relative motion of one or both phases.
Although in air pollution control most applica-
tions involve dynamic conditions, where adsorp-
tion equilibrium is not reached, it is essential
to survey the equilibrium conditions because
their modified effect is of major importance
of the dynamic non-equilibrium systems.
Adsorption equilibrium is defined when the
number of molecules arriving on the surface
is equal to the number of molecules leaving
the surface into the gas phase. As we have
seen in the previous chapter, the adsorbed
molecules exchange energy uith the structural
atoms of the surface, and provided that the
time of adsorption is long enough, they will
be in a thermal equilibrium with the surface
atoms, In order to leave the surface, the
adsorbed molecule has to take up sufficient
energy from the fluctuations of thermal energ>
at the surface so that the energy corresponding
to the vertical component of its vibrations
surpasses the holding limit.
A Adsorption Isotherms
Mien studying the adsorption of a particular
gas on the surface of a particular adsorbent,
we ma> start by inserting into equation (2)
the generalized values for n from equation
(3) and for t from equation (4), we get
o =
0/RT
because the adsorbent and the adsorbate are
defined M and t are constants thus
0/RT
in which
VT
(8)
(9)
If we consider the temperature constant,
we arrive at the simplest form of adsorp-
tion isotherm1
a =
(10)
which shows that the amount of gas adsorbed
will be directly proportional to the pres-
sure.
Considering equation (1) it can be seen
that isotherms shov. essentially the free
energy change as a function of the amount
adsorbed. Under real conditions where the
mutual interaction of the adsorbed molecules
takes place and at increasing coverage, the
adsorption energy Q changes, deviation from
equation (10) takes place.
Isotherms as measured under existing con-
ditions can yield qualitative information
about the adsorption process and also give
indication to the fraction of the surface
coverage, thus, with certain assumptions
to the surface area of the adsorbent.
In figure 1, the five basic types of adsorp-
Fype I T>pe II T\pe III
I
Sp Sp Sp
Increasing Adsorbate \dpor I'res.surc •
c t-
3 O
Type IV!
Type V i
Sp Sp
Increasing Adsorbace Vapor Pressure -*-
Sp = Saturation Pressure
Figure 1. Types of adsorption isotherms as
classified by Brunauer
tion isotherms are presented as classified
by Brunauer. et al.(6) The Type I isotherm
represents systems where adsorption docs
not proceed beyond the formation of a mono-
molecular layer. Such an isotherm is obtained
when adsorbing oxygen on carbon black at
-183eC. They Type II isotherm indicates an
indefinite multilayer formation after the
completion of monolayer. As an example, the
adsorption of water vapor on carbon black
at 30°C results in such a curve. Type III
isotherm is obtained when the amount of gas
adsorbed increases without limit as its
relative saturation approaches unity. The
convex structure is caused by heats of ad-
sorption of the first layer becoming less
than the heat of condensation due to mole-
cular interaction in the monolayer. This
-------
Principles of Adsorption
type of isotherm is obtained when adsorbing
bromine on silica gel at 20°C. The Type IV
isotherm is a variation of Type II, but
with a finite multilayer formation corres-
ponding to complete filling of the capill-
aries. This type of isotherm is obtained
by the adsorption of water vapor on active
carbon at 30°C. The Type V isotherm is a
similar variation of Type III obtained for
instance when adsorbing water vapor on
activated carbon at 100°C.
Although a large number of equations have
been developed to date based on theoretical
considerations, none of them can be general-
ized to describe all systems. A critical
evaluation of these equations can be found
elsewhere (7,8), therefore, only the two
most important methods will be presented
here.
The potential theory of adsorption was in-
troduced by Polanyi in 1914 (9), and be-
cause of its thermodynamic character, it
is still regarded as fundamentally sound.
The Polanyi concept for a typical cross-
section for a gas-solid phase boundary
system is shown on Figure 2. The forces
holding a molecule to the surface diminish
with distance and the multi-molecular ad-
sorbate film is lying in the intermolecular
potential field. The force of attraction
is conveniently measured by the adsorption
potential E, defined as the work done by
the adsorption forces in bringing a molecule
from the gas phase to that given point. The
adsorbate volumes included in these equipo-
tential fields E = EI, £2, £3 EX 0
are i, 2, *3 *j *max. the last
quantity denoting the volume of the entire
adsorption space. As ^increases from 0 to
$max, E decreases from its maximum value
E0 at the adsorbent surface to 0 to the
outermost layer. The process, therefore,
is represented by the curve
E = f(4>)
(11)
which in reality is a distribution function.
The adsorption potential is postulated to
be independent of the temperature, there-
fore, equation (11) is the same for a given
system at all temperatures. The usefulness
of the potential theory is that it is not
explicitly an isotherm equation. The character
istic curve replaces the normal p versus v
curve. The application of the potential
theory requires the empirical determination
of a single isotherm, the calculation of
the characteristic curve, and the prediction
of the isotherms at other temperatures from
this curve. As an example, adsorption iso-
Gas phase
Adsorbed layers
Solid phase
Figure 2. The Structure of the Adsorbed
Phase ^cording to the Polanyi
Potential Theory
therms of carbon dioxide determined at 0°C
were found to correspond empirically to the
calculated values from about -110°C to +120
"C. Polanyi (10) and Berenyi (11.12) examin-
ed the relationship between the character-
istic curves of different gases on the same
adsorbent. For activated carbon, they found
empirically that for any two gases
(12)
where a is the van der Waals constant.
The actual determination of the work done
when the adsorbed layers are assumed to
be liquid
E = RTln Po/Px
(13)
where
Px = the equilibrium vapor pressure
in the gas phase
Po = vapor pressure in the adsorbed
phase
The value Q^ corresponding to Ex is x/dT
where x is the weight of the adsorbed film
and dT is the density of the liquid at
temperature T. The Polanyi equation, be-
cause of its ability of predicting isotherms
for any gas, given a single isotherm for
one gas on the same solid, is very import-
ant in chemical engineering design calcul-
ations. Dubinin, et al. (13,14,15,16) pub-
lished a large number of adsorption correla-
ation results using the Polanyi equation.
Lewis, et al. (17) improved the Polanyi-
Dubinin method by substituting fugacity in
-------
Principles of Adsorption
place of pressure and V, the molar volume
of the liquid at temperature T is replaced
by V1, the molar volume of the liquid at
a temperature where its vapor pressure is
equal to the equilibrium pressure p.
presented an ideal monolayer
adsorption isotherm
(14]
kherc
V = volume of gas (0°C, 760 mm Hg)
adsorbed per unit mass of adsor-
bent
V = volume of gas (0°C, 760 mm Hg)
m adsorbed per unit of adsorbent
with a layer one molecule thick
b = empirical constant in reciprocal
pressure unit which has limited
practical application
Brunaucr, Emmett and Teller(19)expanded
the Langmuir isotherm to include multi-
layer adsorption
V Cx
m
(15)
where Vm and C are empirical constants
and x = p/ps. The constant C is derived
from the heat of adsorption and Vm repre-
sents the volume of gas required to cover
the surface v useful when a certain amount
of contaminant has to be adsorbed, however,
some degree of freedom is available in
selecting temperature and pressure condi-
tions of the adsorption The equation for
adsorption isostere is
-Q/RT
(17)
For approximation, the effect of ^T~"can be
neglected and
In
- JL +
RT
B
(18)
is obtained, where Ba is constant. The
plot of log p against 1/T for a constant
value of o gives a straight line arithmetic
coordinate paper. The other importance of
adsorption isosteres is that the slope of
this straight line corresponds to the heat
of adsorption Q.
-------
Principles of Adsorption
0 Entropy of Adsorption
It is preferable here to discuss briefly
the general entropy consideration of adsorp-
tion. In most gas-solid adsorption systems,
the heat of adsorption is greater than the
heat of evaporation or condensation of the
same substance. This means that the entropy
of the molecules when adsorbed on a parti-
cular surface will be greater than the en-
tropy of the same molecules in their liquid
or solid state. When studying gas phase
adsorption as Che van der Waal forces bet-
ween different molecules are approximately
the geometrical mean between the values for
each of the two molecules, when combined
with a molecule of its own kind, it is
evident that the van der Waals forces of a
gas molecule on the surface of a solid will
be generally greater than the van der Waals
forces holding it in liquid form. There are
some exceptions to the fact that the heat
of adsorption is higher than the heat of
liquefication. Such is the case, for instance,
when water is adsorbed on activated carbon,
the polar character of the water molecule
causing only weak bonds. In this case, the
heat of adsorption is indeed smaller than
the heat of liquefication. Adsorption, never-
theless, takes place because the influence
of the entropy difference is dominating.
The fact that the entropy in the adsorbed
state is higher than in a liquid state
points to the fact that the adsorbed mole-
cules have a greater degree of freedom than
the molecules in the liquid state.
Ill ADSORPTION DYNAMICS
The application of adsorbents in air pollution
control generally involves the use of a dy-
namic system. The adsorbent is generally used
in a fixed bed and the contaminated air is
passed through the adsorbent bed. Depending
on the concentration and market value, the
contaminant is either recovered or discarded
when the loading of the adsorbent requires
regeneration. Although isotherms are indicative
of the efficiency of an adsorbent for a part-
icular adsorbate removal, they do not supply
data to enable the calculation of contact
time or the amount of adsorbent required to
reduce the contaminant concentration below
the required limits.
At this point, it is necessary to evaluate the
dynamic capacity in a little more detail. When
a fluid is first passed through a bed of ad-
sorbers, most of the adsorbate is initially
adsorbed at the inlet part of the bed and the
fluid passes on with little further adsorption
taking place. Later, when the adsorber at the
inlet end becomes saturated, adsorption takes
Inlet
i.
4J
IO
.0
1_
o
in
•a
•=£
LMTZ
Breakthrough
Bedlength
T = MTZ concentration gradient at the formation of the zone
Th = MTZ concentration gradient at half life
T, = MTZ concentration gradient at breakthrough
Figure 3. Formation and Movement of the MTZ through an Adsorbent Bed
-------
Principles of Adsorption
place further along the bed. The situation in
the bed while it is in normal operation may
therefore, be represented by Figure 3, which
shows the building up of a saturated zone of
adsorbers from the inlet end of the bed.
As more gas is passed through, and adsorption
proceeds, the saturated zone moves forward
until the breakthrough point is reached, at
hhich time the exit concentration begins to
rise rapidly above whatever limit has been
fi.xed as the desirable maximum adsorbate level
of the fluid. If the passage of the fluid is
continued on still further, the exit concen-
tration continues to rise until it becomes
substantially the same as the inlet concen-
tration. At this point the bed is fully satu-
rated. While the concentration when saturated
is a function of the material used and the
temperature at which it is operated, the dyn-
amic capacity is also dependent on the operat-
ing conditions, such as inlet concentration,
fluid flow rate and bed depth. The dependence
of inlet concentration and fluid flow rate
arise from heat effect and mass transfer rates,
but the dependence on bed depth, as can be
seen from the above description is really
dependent on the relative sizes of unsaturated
and saturated zones. The zone of the bed where
the concentration gradient is present is often
called the mass transfer zone or NfTZ. In most
published data on dynamic adsorption, results
are expressed in terms of the dynamic capacity,
or breakthrough capacity at given inlet con-
centrations, temperatures and flow rate con-
ditions of the bed, together with the bed
dimensions. It is extremely important that
the adsorber bed should be at least as long
as the transfer zone length of the key com-
ponent to be adsorbed. Therefore, it is im-
portant to know the depth of this mass trans-
fer zone.
The following factors play the most important
role in dynamic adsorption and the length and
shape of the ^f^Z.
1 The type of adsorbent
2 The particle size of an adsorbent (may
depend on maximum allowable pressure
drop)
3 The depth of the adsorbent bed
4 The gas velocity
5 The temperature of the gas stream and
the adsorbent
6 The concentration of the contaminants
to be removed
7 The concentration of the contaminants
not to be removed, including moisture.
8 The pressure of the system
9 The removal efficiency required
10 Possible decomposition or polymerization
of contaminants on the adsorbent.
In the following, the individual and combined
effect of these factors is described in detail.
A Selection of Adsorbent
Most industrial adsorbents are capable of
adsorbing both organic and inorganic gases.
However, their preferential adsorption
characteristics and other physical proper-
ties make each one more or less specific
for a particular application. As an example,
activated alumina, silica gel and molecular
sieves will adsorb uater preferentially
from a gas phase mixture of water vapor
and an organic contaminant. This is a con-
siderable drawback in the application of
these adsorbents for organic contaminant
removal. Activated carbon preferentially
adsorbs nonpolar organic compounds. Recent-
ly, through evaluation of the type of sur-
face oxides present on activated carbon
surfaces, it was found that the preferential
adsorption properties of carbon can be
partially regulated by the type of surface
oxide induced on the carbon.
Silica gel and activated alumina are struc-
turally weakened by contact with liquid
droplets, therefore, direct steaming can-
not be used for regeneration.
In some cases, none of the adsorbents have
sufficient retaining adsorption capacity
for a particular contaminant. In these
applications, a large surface area adsor-
bent is impregnated with inorganic or, in
rare cases, with a high molecular weight
organic compound which can chemically re-
act with the particular contaminant. Iodine
impregnated carbons are used for removal
of mercury vapor, bromine impregnated
carbons for ethylene or propylene removal.
The action of these impregnants is either
catalytic conversion or reaction to a non-
objectionable compound, or to a more easily
adsorbed compound. It has to be noted
here that the general adsorption theory does
not apply anymore on the gross effects of
the process. For example, the mercury re-
-------
Principles of Adsorption
moval by an iodine impregnated carbon pro-
ceeds faster at a higher temperature and a
better overall efficiency can be obtained,
than at a low temperature contact. An im-
pregnated adsorbent is available for most
compounds which under the particular con-
ditions are not easily adsorbed by non-
impregnated commerical adsorbents.
As it was previously mentioned, adsorption
takes place at the interphase boundary,
therefore, the surface area of the adsorbent
is an important factor in the adsorption
process. Generally, the higher the surface
area of an adsorbent, the higher is its
adsorption capacity for all compounds. How-
ever, the surface area has to be available
in a particular pore size within the adsor-
bent. At low partial pressure (concentra-
tion) the surface area in the smallest pores
in which the adsorbate can enter is the
most efficient. At higher pressures the
larger pores are becoming more important,
while at very high concentrations, capillary
condensation will take place within the
pores, and the total micropore volume is
the limiting factor. Figure 4(20) ..hows the
relationship between maximum effective pore
size and concentration for the adsorption of
benzene vapor at 20°C. It is evident that
the most valuable information concerning the
adsorption capacity of a certain adsorbent
is its surface area and pore volume distri-
bution curve in different diameter pores.
Figure 5 shows the characteristic distri-
bution curves for several different adsor-
bent types. As Figure 4 indicates, the
relationship between adsorption capacity
and surface area in optimum pore sizes is
concentration dependent, thus, it is very
important that any evaluation of adsorption
capacity is performed under actual concen-
tration conditions. In Figure 6, benzene
adsorption isotherms are presented for sev-
eral carbon types. These lines cross at
different concentrations, depending on the
surface area distribution of the carbons.
The action of molecular sieves is slightly
different from those of other adsorbents
in that selectivity is more determined by
the pore size limitations of the molecular
sieve. In selecting molecular sieves, it
is important that the contaminant to be
removed is smaller than the available pore
size, while the carrier gas or the not to
be removed component is larger, thus, not
absorbed. Because the optimum pore size
varies with concentration, molecular sieves
are limited in their use by the applicable
concentration ranges. Figure 7 shows water
adsorption isotherms for type 5A molecular
sieve, silica gel and alumina(21).
B The Effect of Particle Size (d)
The dimension and shape of particle size
affects both the pressure drop through the
adsorbent bed and the diffusion rate into
the particles. The pressure drop is lowest
when the adsorbent particles are spherical
and uniform in size. However, the external
mass transfer increases inversely with the
d^/2 and the internal adsorption rate in-
versely as d^. The pressure drop will vary
with the Reynolds number, being roughly
proportional to velocity and inversely with
particle diameter. It is evident that
everything else being equal, adsorbent beds
consisting of smaller particles, although
causing a higher pressure drop, will be
more efficient. Therefore, sharper and
smaller mass transfer zone will be obtain-
ed.
C The Depth of the Adsorbent Bed
The effect of bed depth on adsorption mass
transfer is twofold. First, it is important
that the bed is deeper than the length of
the transfer zone which is unsaturated. The
second is that any multiply of the minimum
bed depth gives more than proportional
increased capacity. In general, it is ad-
vantageous to size and adsorbent bed to
the maximum length which is allowed by
pressure drop considerations. The determina-
tion of the depth of the transfer (MTZ)
zone or unsaturated depth may be determined
by experiment
Total bed depth (19)
-x
NTTZ
t2/ (t, -
where
t. = time required to reach breakpoint
t- = time required to saturation
x = the degree of saturation in the
MTZ
or,
NTTZ
1
1 - x 1
D, ( 1 > (20)
where
D. = bed depth
-------
Principles of Adsorption
Concentration g/m
Activated carbon (Small pore)
Figure 5
1000
Pore radius in A-
10
-------
Principles of Adsorption
50,
Large pores predominant
Medium pores predominant
Small pores predominant
1.0 10
Relative saturation
Figure 4 (above left)
Relationship between pore size and
vapor concentration
Figure 5 (below left)
Cumulative pore volume versus pore
size for different adsorbents
Figure 6 (above)
Benzene adsorption isotherms on
various pore structure activated
isotherms
Figure 7 (right)
Water adsorption isotherms
c
V
o
in
•
I/I
•o
3
a
o
o
o>
a.
•a
o
VI
•o
T>
in
•a
c
O
a.
Silica type desiccant
Alumina type
desiccant
20 40 60 80 100
Percent, Relative humidity
11
-------
Principles of Adsorption
C. = breakthrough capacity of bed D.
C = saturation capacity
x = the degree of saturation in the
Csof the above equation can be obtained by
measuring the breakthrough capacities of
two beds and using the following equation
C2D2 - C1D1
D2'D1
(21)
where
C. is breakthrough capacity for bed
1
"1
length of D
C2 is breakthrough capacity for bed
length of D-
Direct methods for the calculation ot the
NfTZ are also available using transfer units,
however, particularly for multi-components
systems, the calculation becomes very ted-
ious.
D Gas Velocity Effects
The velocity of the gas stream through
adsorbent beds is limited by the adsorbent
crushing velocity which varies with different
types of adsorbents. The data on crushing
\elocities can be obtained from manufact-
urers of adsorbents. As an example, the
crushing velocity for a 6x10 mesh* nutshell
carbon is
V(MW) (P) £ 50,000
(22)
where
V = superficial velocity, ft/mm.
MW » molecular weight of gas
P = system pressure in atm.
crushing velocity pressure ^ 50,000 deter-
mined experimentally. The length of the
MTZ is directly proportional with velocity,
thus, at high velocities, the unsaturated
none is elongated. An example is given in
Figure 8 where the effect of gas velocity
on the length of NfTZ is shown for the ad-
sorption of ethanol on activated carbon.
E Temperature Effects
As it was discussed in the basic adsorption
*6X10 mesh reters to adsorbent which passes
through a Number 6 mesh screen and is retain-
ed on a Number 10 mesh screen.
HZ
o<
c
0}
10 20 30 40 50
Superficial gas velocity, ft./min.
60
70
Figure 8. Effect of Gas Velocity on Length of KTZ
12
-------
Principles of Adsorption
theory, adsorption decreases with increas-
ing temperature. Because the equilibrium
capacity of adsorbents is lower at higher
temperatures, the dynamic or breakthough
capacity will also be lower and the MTZ
is proportionally changed with temperature.
In Figure 9, the variation of benzene vapor
adsorption on carbon with temperature is
shown. In some cases, refrigerated systems
the outlet gas stream is essentially the
same as that of the bed.
At =
6.1
(S /C) X ID* + 0.51 (S /W)
g A
(23)
where
At = temperature rise, °F
Benzene adsorbed, ml(STP)/g
t— • i— •
ro .& CTV on o ro
o o o o o o
Figure 9. Benzene Adsorption Isobar on Carbon
\
\
V
\
\
\
Benz
»v
\
ene at '.
•"-•^
LO.O mm h
^
N.
9
0 50 100 150 200 250 300 350 400
Temperature,°C
are used to enhance increased adsorption.
For example, the removal of radioactive
krypton and xenon is handled by carbon at
liquid nitrogen temperatures.
It was also shown that the adsorption pro-
cess is exothermic. As the adsorption front
moves through the bed, a temperature front
also proceeds in the same direction, and
some of the heat is imparted to the gas
stream. When the gas leaves the adsorption
front, the heat exchange will reverse and
the gas will impart heat to the bed. This
increase in temperature during the adia-
batic operation of the adsorber bed de-
creases the capacity of the adsorbent. The
adiabatic temperature rise in an adsorber
can be calculated by assuming that there
is a thermal equilibrium between the gas
and the bed, and that the temperature of
W = saturation capacity of bed at
t + At, "F
C = inlet concentration, p.p.m.
S = specific heat of gas,
8 B.T.U./ftVF
S = specific heat of adsorbent,
B.T.U./lb./'F
Value of SA for common adsorbents are under
ambient conditions-
Activated Carbon
Alumina
Molecular Sieve
0.25
0.21
0.25
F The Concentration of Adsorbate
-------
Principles of Adsorption
The adsorption capacity of adsorbents is
directly proportional to the concentration
of the adsorbate. The reasons for this were
discussed under the equilibrium adsorption
part. The concentration of the adsorbate
is inversely proportional to the length of
the MTZ. Thus, everything else being equal,
a deeper bed will be required to remove a
lower concentration contaminant with equal
efficiency than to remove the same contam-
inant at higher concentrations. It is very
important that for combustible gases, the
concentration entering the adsorbent is
kept below the lower explosive limit. The
concentration and value of the contaminants
also determines if recovery of the adsorb-
ate is warranted or not.
G The Presence and Concentration of Other
Contaminants
It is important to stress the fact that
some portion of all gases present will be
adsorbed on the adsorbent surface. Because
these gases compete for the available sur-
face area and/or pore volume, their effect
will be the lowering of the adsorption
capacity for the particular adsorbate,
which is to be removed. Under ambient con-
ditions, very little (10-20 ml STP/g) air
is adsorbed on commercial adsorbents, how-
ever, moisture or carbon dioxide has a
more significant effect. Although activated
carbon is less sensitive to moisture than
silica gel, and alumina, at high gas mois-
ture content, its adsorption capacity can
be considerably lower than adsorption from
dry air stream. It is preferred to adsorb
organic contaminants from the lowest R.ll.
gas stream when using unimpregnated adsor-
bents. The reverse is true for most impreg-
nated adsorbents, where the moisture en-
hances the reaction between the gaseous
contaminants and the impregnating agent.
H Pressure Effects
Generally, the adsorption capacity of an
adsorbent increases with increasing pres-
sure, if the partial pressure of the con-
taminant increases. However, at high
pressures (over 500 psig) a decrease in
capacity will be observed due to retrograde
condensation and a decrease in the fugacity
of the more easily adsorbed compound and
increased adsorption of the carrier gas.
I The Removal Efficiency of the System
At times, it is sufficient to lower the
adsorbate concentration only to a small
extent, while in other cases, total re-
moval is required. Naturally, deeper
adsorbent beds are required to achieve a
99.9+% single pass removal, than for a
partial 60-80% removal efficiency.
Decomposition and Polymerization of the
Adsorbate
Some solvents or compounds may decompose,
react or polymerize when in contact with
adsorbents. The decomposed product may be
adsorbed at a lower capacity than the
original substance or the decomposition
product may have different corrosion, etc.
properties. As an example, in an air
stream, NO is converted to N02 when in
contact with activated carbon. Polymeriza-
tion on the adsorbent surface will signifi-
cantly lower its adsorption capacity and
render it non-regenerable by conventional
methods. Such an example is the adsorption
of acetylene on activated carbon at higher
temperatures. Decomposition may also take
place in regenerative systems during direct
steam stripping of the adsorbent bed.
It is evident that most of the factors in-
fluencing adsorption which were treated
individually above have a combined or in-
terrelating effect on the adsorption system.
The effect of some of these factors is well
illustrated by Edeleanu(22) an
-------
Principles of Adsorption
achieved if the gas is slowly passed through
the bed until a uniform charge is obtained
in the entire carbon bed.
In practice, the gas is passed through at
a high rate and theoretically, the charge
of carbon decreases from 100 percent at the
inlet of the bed to 0% (breakthrough) at
the outlet. The height of the carbon bed
where saturated adsorption takes place is
considerably reduced. The length of the
zone is indicated by Curve B. As the adsorp-
tion progresses through the bed, the carbon
will be heated up by the heat of adsorption,
and the effect of the temperature increase
lowers the adsorption capacity of the car-
bon as shown by Curve C. Curve D represents
the lowering of the adsorption capacity by
a non-gasoline contaminant, such as moisture,
and Curve E shows the reduction of adsorp-
tion capacity due to the moisture, and Curve
E shows the reduction of adsorption capacity
due to the moisture content of the adsorbent.
The last Curve, E, is the resultant function
of these effects, and the area which is
under it divided by the height of the total
bed gives the dynamic capacity of the carbon
in the adsorber for gasoline under the above
described conditions.
It is evident now that dynamic adsorption
in practice is a rather complicated process
influenced by a large number of complex
factors. Although some attempts were made
to develop a strictly theoretical formula
for the design calculation of the adsorption
system in practice adsorptive capacity,
MTZ and several other factors are experiment-
ally determined in small scale equipment.
K Intermittent Operation of the Adsorber
Very often, the adsorbers are operated
periodically, or the concentration of the
contaminant greatly varies depending on
the periodic discharge of contaminants. The
preformance of the adsorption system is im-
paired under these conditions. This is caused
by the variation of adsorbate concentrations
with bed height. In Figure 11, a WTZ dia-
gram of a system is shown where under normal
operation a MTZ curve (A) is obtained. The
continued circulation of the carrier gas
in the absence of contaminant causes the
adsorbate to diffuse through the bed by the
process of desorption into the carrier and
readsorption until the low concentration
caused elongation of the MTZ takes place,
represented by Curve B (dashed line).
It may be concluded that short periods of
intermittent operation do not effect greatly
the overall capacity of and adsorption
system if the bed depth equals several
MTZ lengths, but long periods of intermit-
tent operation, particularly in an under-
sized system, will cause a serious capacity
drop.
L Regeneration Conditions
Several considerations have to be made
when establishing the conditions of re-
generation for an adsorber system. Very
often, the main factor is an economical
one to establish that an in-place regenera-
tion is or is not preferred to the replace-
ment of the entire adsorbent charge. Aside
from this factor, it is important to establish
that the recovery of the contaminant is
worthwhile, or only the regeneration of
the adsorbent is required. If recovery is
the main problem, the best design can be
based on a prior experimental test to
establish the ratio of the sorbent fluid
to the recoverable adsorbent at the dif-
ferent working capacities of the adsorbent.
A well designed plant, for example, will
have a steam consumption in the region of
1-4 Ibs. of steam per Ib. of recovered
solvent. Desorption of the contaminant
can be achieved by several different methods.
A comparative table is shown on Figure 12
(23). it is evident that under most condi-
tions, a direct steam regeneration is the
most efficient. The steam entering the ad-
sorbent bed not only introduces heat, but
adsorption and capillary condensation of
the water will take place, which supplies
additional heat and displacement for the
desorption process. The following factors
must be considered when designing the
stripping process:
1 The length of time required for the
regeneration should be as short as
possible. If continuous adsorption and
recovery are required, multiple systems
have to be installed.
2 The short regeneration time requires a
higher steaming rate, thus, increasing
the heat duty of the condenser system.
3 The steaming direction should be in the
opposite direction the adsorption to
prevent the possible accumulation of
polymerizable substances, and also to
permit the shortest route for the de-
sorbed contaminant.
4 To enable a fast stripping and efficient
15
-------
Principles of Adsorption
•o
Ol
" •,
CURVE A
MTZ after diffusion
CURVE B
MTZ under normal operation
TOP OF BED
Bed Height
Figure 11. Effect of Diffusion on MTZ
Percentage of charge expelled
Heating at 100 C (212 F) for 20 minutes
Vacuum 50 mm Hg at 20 C (68 F) for 20 minutes
Gas circulation at 130 C (266 F) for 20 minutes
Direct stream at 100 C (212 F) for 20 minutes
15
25
45
98
Figure 12. One Lb. Activated Carbon Loaded with 20 Per Cent Ethervx^ '/
16
-------
Principles of Adsorption
heat transfer, it is necessary to sweep
out the carrier gas from the adsorber
and condenser system as fast as possible.
5 A larger fraction of the heat content
of the steam is used up to heat the ad-
sorber vessel and the adsorbent, thus
it is essential that the steam condenses
quickly in the bed. The steam should
contain only a slight super heat to
allow condensation.
6 It is advantageous to use a low reten-
tivity carbon to enable the adsorbate to
be stripped out easily. When empirical
data is not available, the following
heat requirements have to be taken into
consideration:
a Heat to the adsorbent and vessel.
b Heat of adsorption and specific heat
of adsorbate leaving th«> adsorbent.
c Latent and specific heat of water
vapor accompanying the adsorbate.
d Heat in condensed, indirect steam.
e Radiation and convection heat loss.
Since the adsorbent bed must be heated in
a relatively short time to reactivation
temperature, it is necessary that the re-
activation steam rate calculation is in-
creased by some factor which will correct
for the nonsteady state heat transfer.
During the steaming period, condensation
and adsorption will take place in the ad-
sorbent bed, increasing the moisture con-
tent of the adsorbent. Also, a certain
portion of the adsorbate will remain on the
carbon. This fraction is generally referred
to as "heel". To achieve the minimum
efficiency drop for the successive adsorp-
tion cycles, it is important that the ad-
sorbent bed should be dried and cooled
before being returned to the adsorption
cycle. The desired state of dryness will
depend on the physical properties of the
adsorbate and on the concentration of the
adsorbate in the carrier stream. When using
high adsorbate concentrations, it may be
desirable to leave some moisture in the
adsorbent so that the heat of adsorption
may be used in evaporating the moisture
from the adsorbent, thus preventing any
undue temperature rise of the adsorbent
bed.
It is also necesary to establish the
materials of construction on the basis
that several compounds, particularly
chlorinated hydrocarbons, will undergo a
partial decomposition during regeneration,
forming hydrochloric acid.
Some safety considerations have to be
weighed also in designing a regeneration
system, assuring that the adsorber is not
being used at temperatures higher than the
self-ignition point of the contaminant.
Experiments(23jshow that carbon does not
lower the ignition temperature of solvents,
and as an example, solvent adsorbed on car-
bon ignites at the same temperature as the
solvent vapor alone.
In some cases, a "pressure swing" operation
of the adsorber is used when adsorption
takes place at high pressure and desorption
at low pressures. Such processes are in use
in the purification of inert gases.
LITERATURE
General
1 Young, D.M. and Crowell, A.D. Physical
Adsorption of Gases. Butterworths,
London. 1962.
2 Nonhebel, G. Gas Purification Processes.
G. Newnes Ltd.London. 1964.
3 de Boer, J.H. The Dynamical Character of
Adsorption. Clarendon, Oxford. 1953.
4 Drew, T.B. and Hoopers, J.W. Advances in
Chemical Engineering. Academic Press,
New York. 1958.
5 Kuczynski, W. Adsorpcja Gazow, Part I.
Poznan. 1963.
6 Brunauer, S. The Adsorption of Gases and
Vapors. Oxford, London. 1953.
TEXT
1 de Boer, J.H. (same as above Reference No.
3, p.5).
2 Hoist, G. and Clausing, P. Physic, 6, 48.
1926.
3 Clausing, P. Ann. d. Physic, 7, 489. 1930.
4 Frenkel, J. Z. Physic, 26, 117. 1924.
5 Same as Reference No. 3 (General) above.
17
-------
Principles of Adsorption
6 Same as Reference No. 6 above.
7 Adamson, A.IV. Physical Chemistry of Surfaces.
Interscience, New York. 1960.
8 Same as No. 1 (General) above.
9 Polanyi, M. Verh. dt sen. phys. Ges. 16,
1012. 1914.
10 Polanyi, M.Z. Elektrochem. 26, 371. 1920.
11 Berenyi, L. Z. Phys. Chem. 94. 628. 1920.
12 Berenyi, L. Z. Phys. Chem. 105, 55. 1923.
15 Dubinin, M.M. and Zaverina, E.D. Acta.
Physicochim. U.R.S.S. 4, 647. 1936.
14 Dubinin, M.M. and Timofeev, D.P. C.R. Acad.
Sci. U.R.S.S. 54, 701. 1946.
15 Dubinin, M.M. and Timofeev, D.P. C.R. Acad.
Sci. U.R.S.S. 55, 137. 1947.
16 Dubinin, M.M. and Timofeev, D.P. Zhur.
Fiz.-khim. 22, 133. 1948.
17 Lewis, W.K., et al. Ind. Eng. Chem. 42,
1362. 1950.
18 Langmuir, I.J. An. Chem. Soc. 40, 1361.
1918.
19 Burnauer, S., Enunett, P.II. and Teller,
E.J. Am. Chem. Soc. 60, 309. 1938
20 Griffiths, H. J. Inst. Fuel. 8, 277. 1935.
21 Gnesmer, G.J., et al. Chem. Eng. Prog.
S)mp. 55 (24) 47. 1959.
22 Edeleanu, I.J. Inst. Petrol. Tech. 14,
286. 1928.
23 Woods, F. J. and Johnson, J. E. N'RL
Report 6090. 1964.
18
-------
ADSORPTION SYSTEMS AND THEIR APPLICATION
TO AIR POLLUTION CONTROL
Don R. Lee
I ACTIVATED CHARCOAL IN AIR POLLUTION CONTROL
Adsorption deals In the realm of molecules
rather than solid materials. As such, it
should be considered wherever air contamin-
ants are gaseous in nature. An adsorbent
is a material which ucilizes the phenomen-
on of adsorption to collect, entrap, and
concentrate these molecules.
Activated charcoal has become almost synony-
mous with adsorption because it is the most
universal adsorbent known. It has long
been used in gas masks, respirators, atomic
submarines, CBR filters, space capsules,
and wherever a man's liie depends on the
removal of toxic gases from the air.
For those who wish to delve into the theory
of adsorption, please refer to the refer-
ences at the end of this article.
For our consideration it is important to
know the following facts about activated
charcoal and adsorption:
1. Activated charcoal and adsorption is
but one method among many to eliminate
gaseous pollutants from the air. Each
case has to be considered individually
to determine the best method or combin-
ation of methods to solve the problem.
As a rule of thumb, based on experience,
activated charcoal appears to have an ad-
vantage over other methods when:
A. Large amounts of air have to be
utilized to capture the pollutant.
B. Concentrations of contaminants are
exceptionally low, so as to make
other methods impractical but for
one reason or another must still
be removed.
C. In higher concentrations where the
adsorbed material has a recovery
value.
D. Smaller applications where space
and economy dictates the use of
activated charcoal filters over
bulkier systems.
2. Activated charcoal is not in itself a
method of disposal, but a means of
concentrating adsorbed materials so
that they, In turn, can be disposed of
or recovered. In a sense it can be
compared to a vacuum cleaner bag which
has a definite capacity for contaminants
and once filled, must either be emptied
or replaced.
3. There are other types of adsorbents,
such as activated silica or activated
alumina. Most of these adsorbents
are selective towards polar compounds,
such as water. Activated charcoal is
unique in that it is more selective
towards non-polar compounds, thus making
it particularly suitable for organic
contaminants. Activated charcoal
will adsorb water molecules as well as
any other type of molecule that comes
in contact with it, but as a rule will
be displaced by organic molecules. The
same applies to most true gases, such
as oxygen, nitrogen, argon, etc.
It. Among organic materials, activated char-
coal is more selective towards larger
molecules and those having higher boiling
points. At any given operating condition,
activated charcoal will have good cap-
acity for those organic materials which
would ordinarily be liquids or solids
at that condition. As an example, at
normal room pressure and temperature,
activated charcoal would have a good
capacity for ethyl alcohol but a poor
capacity for ethane. However, altering
these conditions by lowering the temp-
erature below the boiling point of ethane,
the capacity can be increased greatly
for this material. As such, activated
charcoal might be compared to a "super"
condenser.
5. Adsorption Is an extremely rapid pheno-
menon and can remove gaseous contaminants
from the air with an exceptionally short
contact time. This phenomenon can be
utilized in combination with chemical
reaction by Impregnating activated char-
coal with various materials to enhance
its' ability to pick up and retain
reactive inorganic gases. For instance,
Don R. Lee, Consultant, Columbus, Ohio
PA.C.ge.32.12.70
-------
Adsorption Systems and Their Application to Air Pollution Control
an activated charcoal impregnated with
a suitable alkaline material can do an
excellent Job of adsorbing and retaining
acid gases, such as, sulphur dioxide,
hydrochloric acid, various oxides of
nitrogen, etc. In this case, we combine
the speed of adsorption with the slower
and more positive chemical reaction.
6- Types of Activated Charcoal - There
are a wide variety of activated char-
coals or activated carbons (synonymous).
Some are more suitable for liquid puri-
fication, others more suitable for de-
colorization, others for air purifi-
cation, etc.
Activated charcoals vary widely in their
pore size, pore volume, total surface
area, pore distribution, hardness, im-
purities, granular particle size, etc.
An activated charcoal highly suitable
for decolonization of sugar would, more
than likely, be completely inadequate
for air purification and vice versa.
This wide variety in activated charcoal
leads to a lot of misunderstanding
about selecting this material. The
selection of a proper adsorbent can
be affected by: The type of contaminant
or contaminants, their concentration
in the air-stream, operating conditions,
such as: temperature, pressure, relative
humidity, regenerative or non-regenera-
tive systems, efficiency of removal
desired, whether or not the adsorbed
material has a tendency to break-down
on the charcoal, and contact time re-
quired.
ADSORPTION SYSTEMS - With the previous
facts in mind, adsorption systems might be
broken down into; thin bed or deep bed,
regenerative or non-regenerative systems.
To clarify terminology, "regeneration" usually
indicates the process by which the adsorbed
contaminants are removed "in place", usually
by a heat source, such as steam. "Reactiva-
tion" usually refers to the process by which
the adsorbed contaminants are burned off
at high temperatures in special furnaces.
This process is sometimes done at the job
site, but invariably involves the removal
of the adsorbent from the adsorber.
"Regeneration" is only used when the air-
stream is clean (devoid of participate
matter) and the organic contaminants are
well defined as to their nature and con-
centration. Since steam is invariably used
as the heat source, two further prerequisites
are required: We must be able to remove
these adsorbed organic materials at the
temperatures of steam or super-heated steam,
and these materials must not polymerize
on the carbon at these temperatures.
"Reactivation", on the other hand, does not
have such limitations inasmuch as all or-
ganic materials break down at the higher
reactivation temperatures. In some extreme
cases, the reactivator must give considera-
tion to the elution of other adsorbed mat-
erials so that he, in turn, does not be-
come a source of pollution himself. This
could apply in the case of adsorption of
various acid gases, sulphur dioxide, halo-
gens, poisonous metallic compounds, and
radio active materials.
If the concentration of a contaminant in
the air is low enough and the capacity of
activated charcoal for this contaminant
large enough, expensive regeneration systems
are not necessary. If the activated char-
coal is used at a rate which would necessi-
tate replacement every two or three days,
the chances are a regenerative system would
be desirable, if feasible. There are many
cases where the charcoal is replaced as
often as every week and it is still less
bothersome and expensive than trying to in-
stall a regenerative system.
There are many applications where activated
charcoal filters as thin as 1/2 inch to 2
inches give excellent results in both effic-
iency and longevity.
SOLVENT RECOVERY SYSTEMS - To date, most of
these systems have been installed by industry
to recover valuable solvents for re-use,
which would otherwise be lost. As such,
most of these systems pay for themselves
in a short period of time. For this reason,
they have been referred to as "solvent
vapor recovery systems."
At present, because of air pollution problems,
consideration must be given to these systems,
even though there's no economic gain to be
achieved by recovery. In some cases, the
recovered solvent can be re-diluted into
an air-stream in suitable concentration
for combustion or incineration, prior to
exhausting. Also, it may be feasible to use
the solvent as a fuel or utilize it in some
other process.
The following is a list of those air puri-
fication applications where non-regenerative,
inexpensive, thin-bed adsorbers can be used.
-------
Adsorption Systems and Their Application to Air Pollution Control
TABLE 1
*Acid Gases
Air Conditioning Systems
Allergy Patients (Air Purif.)
*Ammonia
*Amine Odors
Animal Shelters
Apple Storage
Archives
Art Galleries
Atomic Power Plants
ATomic Submarines
Auditoriums
Automobile Exhaust Fumes (organic)
Bacteria Removal
Brine Solution Odors
Burn Patients (Air Purif.)
*Carbon Dioxide
Cancer Patients (Air Purif.)
CBR Filters
Chemical Plants
Chloramine (odor)
^Chlorine (odor)
Chromate Baths
Churches
Cigarette Odor
Clean Rooms
Community Defense Shelters
Computer Rooms
Conference Rooms
*Corrosive Gases
Crane Cabs
Diethanolamine (DEA)
Display Cases (tarnishing)
Dry Cleaning Shops
Electrical Controls
Embalming Rooms
Ethylene (orchid growing)
Exposition Halls
^Fabrics (Permanent Press)
Fertilizer Plants
Flower Shops
Food Processing
Food Storage
*Formaldehyde
Funeral Homes
Garbage Storage
*Cas Masks
Gasoline Fumes
Greenhouses
Gymanasiums
*Halogens
High Rise Apartments
Homes
Hospitals
Hotel Rooms
*Hydrogen Cyanide
*Hydrogen Sulfide
Incinerators
Infirmeries
Instruments (Air Purif.)
Jet Aircraft Terminals
Jet Airplane Cabins
Jet Airport Field Busses
Jewelry Stores (tarnishing)
Kitchen Range Hoods
Laboratories
Laboratory Fume Hoods
Laundries
Lead Tetraethyl
Locker Rooms
Mercaptans
*Mercury Vapors
*Metal Pickling
Mildew Odors
Mold Odors
Monethanolamine (MEA)
Morgues
Moving Vans
Museums
*Nitrogen Oxides
Nursing Homes
Nurseries
Office Buildings
Ozone
*Pacivation Tanks
Pharmaceutical Odors
Photographic Dark Rooms
Pickle Mfg. (brine odors)
Pizza Ovens
Plastic Mfg.
Pulp & Paper Mills (Electronic Control)
Active Gases (Hot Cells)
Range Hoods
REfrigerators (Domestic)
REfrigerators (Commerical)
Refrigerator Cars
Rendering Odors
Resin Cooking (Hot Melt)
Respiratory Patients (Air Purif.)
Respirators
Rest Rooms
Restaurants
Rubber Mfg.
Schools
Scuba Diving (Compressed Air)
Septic TAnk Trucks (Vents)
Sewage Treatment Plants
Sewer Vents
Sick Rooms
Smog Irritants (Gases)
Solvents (Low Concentrations)
Space Capsules
Stadiums (Enclosed)
*Steel Plants
Submarines
*Sulphur Dioxide
Theaters
Toxic Gases
-------
Adsorption Systems and Their Application to \ir Pollution Control
TABLE 1 (Cont.)
Underground Parking Areas
Underwater Experimental Stns.
Universities
Veterinary Clinics
* May require impregnated charcoals
The most common types of regenerative systems
are utilized in handling high concentrations
of solvents or solvent-like vapors. The
following is a list of applications that
have lent themselves to this type of system.
Virus Removal
Warehouses
Waste Gases
Whiskey Warehouses
TABLE II
Acetone
Adhesive Solvents
Amyl Acetate
Benzene
Benzol
Brom-Chlor Methane (BCM)
Butyl Acetate
Butyl Alcohol
Carbon Bisulfide
Carbon Dioxide (Controlled Atmos.)
Carbon Tetrachloride
Coating Operations
Degreasing Solvents
Diethyl Ether
Distilleries
Dry Cleaning Solvents
Drying Ovens
Ethyl Acetate
Ethyl Alcohol
Ethylene Dichloride
Fabric Coaters
Film Cleaning
Fluoro Hydrocarbons
Freons (some)
Fuel Oil
Gasoline
Cenetrons (some)
Heptane
Hexane
Hydrocarbons-Aliphatic
Hydrocarbons-Aromatic
Isopropyl Alcohol
Isotrons (some)
Ketones
Methyl Alcohol
Methyl Chloroform
Methyl Ethyl Ketone (MEK)
Methylene Chloride
Mineral Spirits
Mixed Solvents
Monochloro Benzene
Naphthas
Paint Manufacturing
Paint Storage (Vents)
Pectin Extraction
Perchlorethylene
Pharmaceutical Encapsulation
Plastic Manufacturing
Rayon Fiber Manufacturing
Rotogravure Printing
Smokeless Powder Installations
Soya Bean Oil Extraction
Stoddard Solvent
Tetrahydrofuran (THF)
Toluene
Toluol
1.1,1, Trichlorethane
Trichloroethylene
Varnish Storage (Vents)
Xylene
Xylol
A simple regenerative system would consist
of the following:
(See Diagram 1)
A. Activated charcoal adsorber: With an
inlet and outlet, a steam inlet and
an outlet to exhaust the solvent vapor-
steam mixture, and suitable valves to
control the intermittent steam and
air-flow.
B. Steam or heat source: The flow of
steam is counter-current to the
orginal air-flow.
C. Condenser: To cool and liquify the
stripped solvent vapor and steam mix-
ture.
-------
Adsorption Systems and Their Application to Air Pollution Control
DIAGRAM I
SOLVENT
LADEN_
AIR
iu
CLEAN
DRY AIR
SOLVENT WOH- STEAM MIXTURE
PURIFIED AIR
m
STEAM
SOLVENT TO
STORAGE
VWK TO DRAIN
OR REUSE
Adsorption-Desorptlon Cycle
Adsorption - Valves 1 and 2 are open. Valves 3, 4 and 5 are
closed. Air containing vapor is passed through
"F" pre-filtration system for removal of solid
particles, then through activated charcoal bed
"A" for adsorption of the solvent vapors. The
purified air is then exhausted or returned for
re-use.
- After the activated charcoal has become saturated,
Valves 1 and 2 are closed and Valves 3 and A are
opened. Steam from boiler B heats the activated
charcoal bed and strips the solvent vapor. The
solvent vapor-steam mixture is passed through
Condenser C. The condensate then drains into the
decanter "D" for separation (immiscible solvent).
- Valves 1, 3 and 4 are closed and Valves 2 and 5
are opened. Clean, dry air is passed over the
activated charcoal bed to remove excess moisture.
Entire cycle is then repeated.
Drying
Decanter or Distillation system: If
the solvent is immiscible with water,
the separation of the condensate can
be simple by the use of a decanter. In
most cases this is sufficient to obtain
a pure re-usable solvent. In some cases,
it may be necessary to further purify
the solvent or the weak water solution
before use or disposal. If the solvent
is mlscible with respect to the water,
distillation may be required for
separation.
-------
Adsorption Systems and Their Application to Air Pollution Control
The solvent vapor recovery system will also
require equipment for purifying (removal of
particulate matter) and moving the contaminated
air through the bed of charcoal; a blower and
a source of clean, dry air for cooling and
drying the activated charcoal after the steam-
ing cycle; valves and other control equipment
for diverting the air and vapors in carrying
out the various adsorption-desorption cycles.
Additional equipment may be required to bring
the air, to be treated, to a suitable condition.
For instance, air coming off of drying ovens
will have to be cooled. Air coming off of
scrubbers may have to be treated with de-
misters or even raised slightly in tempera-
ture to lower the relative humidity below 1002.
Air containing high boilers, such as oil vapor,
may have to be passed through an activated
charcoal protector bed. Since all particulate
matter is harmful to a regenerative system,
high efficiency particulate filtration may be
required, as in the case of a spray paint op-
eration.
COST OF REGENERATIVE SYSTEMS
The following table may be used for approxi-
mate pricing of solvent recovery systems:
Since some solvents have a tendency to break
down into highly corrosive substances, due
consideration must be given to the materials
of construction of the system. Impurities
in the system and the activated charcoal may
catalyze the breakdown. The temperature and
pressure conditions during the desorption
cycle can also be critical.
As in the case of the handling of any flammable
mixture, due consideration must be given to
the lower explosive limit and the use of ex-
plosion proof electrical equipment and wiring.
In cases where the LEL is somewhat critical,
special care must be given to the heat develop-
ed during the adsorption cycle (heat of adsorp-
tion) and assurance that the bed is cooled
properly following the desorption and drying
cycle.
The solvent vapor recovery system shown in
Diagram I must have a shut-down during the
adsorption cycle to carry out the regenera-
tion process. Addition of more adsorbers with
a pre-determined cycle can eliminate shut-down
and will lend itself to automatic controls.
The following example will describe such a
system.
Table 3. INSTALLED EQUIPMENT COST PER SCFM
SCF.M
Air-rate
through system
1,000
2,000
4,000
8,000
15,000
30. 000
60, 000
100, 000
150, 000
Solvent concentration (GPH per 1000 SCFM)
0.02
$13.00
9 00
6.00
5.25
3.95
3. 15
2.35
2. 15
1.95
1
$14.00
10.00
7.75
6.25
4.35
3.40
2.50
2.20
2.00
2.5
$16.00
10.00
8.75
7.25
4.75
4.23
3.75
3.30
5
$19.00
12.50
10.05
8.45
7.05
6.65
6.25
6.00
10
$21.00
16.00
12.00
10.25
9.00
8.40
8.20
8.00
20
$22.00
19.50
17.50 :
i
15.50
13.00
11.35
-------
Adsorption Systems and Their Application to Air Pollution Control
These prices are for the solvent recovery
system only including adsorbers, cycling
valves and controls, charcoal beds, condenser,
decanter (if needed), blower(s) and motor(s)
of standard construction, interconnecting
ductwork-piping and sewers, all installed
with necessary foundation on ground level.
It is assumed that a cleared level site is
available - ductwork, piping and sewers lead-
ing to and from the system, insulation of
equipment, not included.
Add for additional costs (if needed):
A Explosion proof electrical equipment &
wiring - 10%.
B Corrosion resisting materials: Type 30A
Stainless Steel - 20 to 25%; for other
materials, extras will vary.
C Particulate pre-filter, aircooler, chemical
scrubber according to need.
D Distillation equipment as needed.
ACTUAL ADSORPTION SYSTEM
A photograph and process flow diagram for
an activated charcoal adsorption-regenerative
system is shown in Figure 1. Other applications
of adsorption-regenerative systems are shown
in Figures 2 and 3.
Figure 1. AUTOMATIC RECOVERY SYSTEM FOR
ISOPROPYL ALCOHOL FROM A CITRUS FRUIT
PROCESSING PLANT
-------
Adsorption Systems and Their Application to Air Pollution Control
Figure 2. AUTOMATIC RECOVERY SYSTEM FOR METHYL CHLOROFOHM
FROM ULTRASONIC FILM CLEANERS IN A MOVIE FILM PROCESSING PLANT
Figure 3. SEMI-CONTINUOUS RECOVERY SYSTEM TO RECOVER
ETFIYL ALCOHOL VAPORS FROM A WHISKEY WAREHOUSE
-------
Adsorption Systems and Their Application to Air Pollution Control
Additional Reading
1 ASHRAE Guide and Data Book (Handbook of
Fundamentals) 1967 Chpt. 12 "Odors"
2 ASHRAE Guide and Data Book (Systems and
Equipment) 1967 Chpt. 65 "Odor Control"
3 Brunauer, S. "The Adsorption of Gases and
Vapors" Oxford, London 1953
4 Hassler, John W., "Activated Carbon"
Chemical Publishing Company 1963
5 Lee, Don R-, "Using Impregnated Activated
Charcoal", Journal of the American
Association for Contamination Control, Dec.
1965
6 Steele, William A., "The Physical Adsorp-
tion of Gases on Solids", Advances in
Colloid and Interface Science, Elsevler
Publishing Company, Amsterdam, 1967.
7 Young, D.M. and Crowell, A.D. "Physical
Adsorption of Gases", Butterworths, London
1962
II ADSORPTION DESIGN PROBLEM
PROBLEM: - Recovery of acetone from air stream
under following conditions:
A. Air Flow - 32,000 cfn air has
been cleaned of all known part-
iculate natter.
B. Operating Conditions-Tempera-
ture is 20°C 1 atmosphere, RH
is 402 to 602. Continuous op-
eration 24 hrs/day, 7 days/wk,
yr. around.
C. Concentration of acetone is
.152 by vol. Lower Explosive
Limit of acetone is 2.62 by
vol.
DETERMINE: - I
II
Adsorption capacity to satura-
tion of selected activated
charcoal at given acetone
concentration.
Adsorption capacity to break
of various adsorber beds based
on MTZ information.
Ill Working charge.
IV Type of system and adsorber
design.
V Cycle time of system based on
acetone flow.
VI Pressure drop through adsorbers.
VII Steam requirements and factors
governing regeneration cycle.
Required information on activated charcoal
under consideration:
1. Adsorption capacity for ace-
tone at given concentration.
2. Information on Mass Transfer
Zone (MTZ) for acetone at
given concentration.
3. Working charge vs. steam re-
quirements for acetone.
4. Pressure drop vs. velocity.
Normally, given the problem, manufacturers
of activated charcoal can provide specific
information on their products for most common
solvents. However, for this example we intend
to show how specific information is obtained
from basic information.
I Adsorption capacity, at a given concentra-
tion, is obtained from an "adsorption
Isotherm".
Graph No. 1 shows the isotherm of three
different charcoals. Percent of relative
saturation is determined as follow:
Vapor pressure of acetone at 20°C =
170 mm Hg
Partial pressure of acetone in air
stream = 760 mm x .0015 = 1.14 mm
Relative Saturation % RS = (1.14/170)
x 100 = .672
Capacity of charcoal A (highest at
this concentration) is +262
II The 26Z to saturation should be considered
a maximum. However, in a dynamic system,
"saturation to break" is more importanr
since it is the point at which the bed
begins to lose solvent.
In a sufficiently deep bed there will be
two zones at break-through, one in which
the charcoal will have reached "capacity
to saturation", and the other, the
charcoal will be at different stages of
-------
Adsorption Systems and Their Application to Air Pollution Control
GRAPH NO.l
ACETONE ADSORPTION ISOTHERM - 20 C
100 %
50
40
30
20
10
s
>-
.1 .2 .3 .4 .5 .6 .7.8 1.0 2.0
5.0 10.
RELATIVE
SATURATION
10
-------
Adsorption Systems and Their Applicatior to Air Pollution Control
saturation. This latter is referred to
as the Mass Transfer Zone (MTZ). See
diagram Number 2.
charcoal as the "heel".
It will normally take several adsorption-
regeneration cycles to arrive at an equili-
DIAGRAM NO. 2
30%
CAJVCITY
WT.
20%.
10%
MTZ
KO LENGTH
I •••rtfWDUyl
The bed depth in the adsorber should al-
ways be at least as deep as the MTZ and
preferably much deeper. Since the MTZ
varies per air velocity, solvent concen-
tration and other operating conditions,
it should be measured under the actual
conditions or the application.
Experience has shown that It is desirable
to maintain the velocity through the char-
coal bed between 60-75 PPM. The above was
measured at 75 FPM and found to be approx-
imately 2 inches.
The average capacity in the MTZ is normally
1/2 "capacity to saturation". Therefore,
if the adsorber bed depth is 2", the
capacity to break is 13Z, If 12" it would
be 23.82, if 2V - 24.82.
Ill Once the bed is saturated to break the
solvent is removed by steaming. The more
steam that is used, the greater the
removal of the solvent. (See graph No. 2)
The function is not lineal and it is
not economical to remove all of the
solvent each cycle. The amount that is
removed is referred to as the "working
charge", and the amount remaining on the
brium. Repeated calculations in this case
showed the best ratio of pounds of steam
to pounds of solvent to be 2.5/1. This
does not include the amount of steam to
heat the vessel, valves, duct work, etc.
It does indicate a working charge of about
17Z, If capacity to break is kept fairly
close to capacity to saturation.
Although, most lab data of this type is
conducted under adlabatlc conditions, it
is almost impossible to duplicate the
actual conditions in the system prior to
building It. Therefore, we should allow
some margin of variation. We should also
make some allowance for a decreasing
working charge over a long period of time.
This can be caused by undetected impuri-
ties in the air stream, a build-up of high
boiling compounds in the charcoal, or
some polymerization of adsorbed materials.
A buyer of a solvent vapor recovery system
should be wary of a low bid of an under-
sized system. The dollars saved may prove
to be costly in the long run.
A good rule of thumb is to use a working
charge 1/2 the adsorptive capacity to
break. If we aimed at a bed depth of 12"
11
-------
Adsorption Systems and Their Application to Air Pollution Control
95%
20%
< 15%
% WORKING CH
t i
0
GRAPH NO 2
WORKING CHARGE VERSUS STEAM - SOLVENT RATIOS FOR
SOLVENT RECOVERY ACETONE
/
L
/
/
/
i
/
/
/
X
x"
***
^*>
~~~~
~* —
D 1 2 3 4 5 6
POUNDS OF STEAM /POUNDS OF SOLVENT
1
i
co 24" at 75 FPM velocity, we could use
a working charge of 12.5%.
IV In this case, we will assume a 3 adsorber
system, two adsorbing and one regenerat-
ing at any one time.
Each adsorber must handle 16,000 CFM.
At 75 FPM velocity, 213.3 sq. ft. face
area is required.
If 24" bed depth is used, adsorber would
hold 426.7 cu. ft. or 12,000(1* charcoal
(28///cu.fc.).
At 12.5% working charge, adsorber would
hold a minimum 1,5000 solvent.
Bed dimensions indicate that the adsorbers
should be of the horizontal tank variety.
8 ft. diameter and 26.5 ft. length would
give the desired bed face area. In ace-
tone recovery, stainless steel or stain-
less-clad should be used.
V Rate of Acetone Flow:
32,000 CFM x .0015 acetone (vol.)X
LB, mole acetone (58) x 273'K
359 cu. ft.
293'K
7.230 acetone/
mln
16,000 CFM = 7.23 acetone/minute
2
Adsorption time of adsorber = 1500 x 2 minutes
7.23
= 415 min. or 6 hrs. 55 min.
12
-------
Adsorption Systems and Their Application to Air Pollution Control
Since each adsorber Is adsorbing 2/3 of
the total cycle time, the total cycle of
system is 10 hrs., 22 min.
VI Charcoal "A" is a 4 x 6 mesh material.
Since the bed depth is 24 inches and the
velocity 75 FPM, the pressure drop is
9.6 inches w.g. (See graph No. 3). This
does not include air flow resistance of
ductwork nor valves.
VII We've already determined from graph No. 2
that the ratio of steam to solvent is 2.S//
steam per pound of solvent. To this we
would have to add the steam required to
heat the rest of the steam cycle system.
For preliminary consideration we could
use, as a rule of thumb, .59 steam per
pound of solvent for a total requirement
of 34 steam/0 solvent.
Steam requirement for each adsorber =
steam or approx. 4,500,000 B.T.U.
§
&
I
I
IU
GRAPH NO. 3
PRESSURE MOP CURVES
0.1
20 30 40 50 40 70 K> 90
VELOCITY (STANDARD AKJ-F.P.M.
13
-------
Adsorption Systems and Their Application to Air Pollution Control
The steaming-cooling cycle for each ad-
sorber is 3 hrs., 27 rain. Of this, 15-30
mm. will be required for cooling and
drying.
From this information it is possible to
calculate; the condenser size, the GPM
water-flow for cooling, the boiler size,
sizes of steam and vapor valves, and in
the case of acetone, the distillation
equipment required.
Alternate Design Consideration: The steaming
could actually be accomplished in 1/3 of the
time. Since the steaming-cooling cycle is a
function of the adsorption cycle and the
number of adsorbers, the system could be re-
calculated several ways to determine best
over all economics. For instance: 5 smaller
adsorbers, 4 adsorbing and 1 regenerating.
It is doubtful if this would result in any
advantages.
We purposely based our design on a 12.5% work-
ing charge even though we know it can .operate
at a 17% working charge. The system can be
operated within this range without ever chang-
ing the total cycle time. The additional re-
generation time gives us more flexibility
in altering the capacity of the system at some
future date, such as the addition of another
adsorber. In comparing costs of various system
designs, it Is important to consider the poten-
tial growth of the operation.
A more logical alteration in design would be
to use a 12" deep bed instead of the 24".
Based on our MTZ information, the difference
in the adsorptive capacity to break would be
about 15!, and the working charge about .5%.
Using a 12% working charge each adsorber would
hold 720// of solvent. The adsorption cycle
of each adsorber would be 3 hrs., 20 min. and
the regeneration cycle 1 hr., 40 min. The
total cycle would be 5 hrs.
The activated charcoal would be cut in half.
The bed face area requirements would be the
same. The resistance would be 4.8 inches w.g.
The total solvent adsorbed over any period of
time would remain the same, but the B.T.U.
requirements would be greater because the
heating and cooling of the regeneration equip-
ment would be twice as frequent. The distill-
ation equipment would remain the same.
Advantages of the smaller system:
1. Less total weight.
2. Slight cost savings on orginal system.
Disadvantages of the smaller system:
1. Greater operating cost.
2. Since only half as much activated
charcoal is used, decrease of working
charge over a period of time will oc-
cur twice as fast.
3. Greater difficulty in increasing
total capacity of system at some
future date.
The larger system would probably still be the
best overall recommendation. It should be kept
in mind that this is only the preliminary de-
sign. More detailed study of the specific
application, available standard equipment
(valves, blowers, etc.) and shop fabrication
problems, would have to be made before che
final design is resolved.
14
-------
Adsorption Systems and Their Application to Air Pollution Control
Clean dry air
Solvent vapor-Steam mixture
Stripped air for reuse
Water to drain
A Activated charcoal adsorbers
B Blowers
C Condenser
0 Distillation
R Storage tank for solvent-water mixure
S Steam source
T Storage tank for separated acetone
/ Valve
IS
-------
SECTION VII
CONTROL OF ODORS
Measurement and Control of Community Malodors
Controlling Industrial Odors
Industrial Odor Control and its Problems
-------
MEASUREMENT AND CONTROL
OF COMMUNITY MALODORS
Amos Turk, Ph.D.
Most gases and vapors that are not one
of the "normal" components of air (oxygen,
nitrogen, carbon dioxide, water vapor, and
the "inert" gases) are odorous in some ranges
of concentration. An Important exception is
carbon monoxide. Most odors that are by-
products of manufacturing operations, or of
the bacterial or thermal decomposition of
organic matter, are objectionable to the
vast majority of people. There are some
notable exceptions, especially chose In-
cidental to food preparation, like bakery
or coffee roasting exhausts, but even these
become unpleasant to people who are exposed
to them continually over a prolonged period
of time.
Odor problems arise when an affected
group of people complain about a smell and
demand that it be corrected. The complaint
may refer to the source, like a specific
process effluent or the exhaust from diesel-
powered vehicles, or may refer only to the
effect, like "this area has a foul odor
most of the time." The engineer who is
called upon to remedy the situation is faced
with a variety of problems: for appraising
the situation, he may wish to assess the
nature of the complaint to determine how
intense the odor is, what kind of odor it is,
how objectionable it is, and when it occurs.
He may also have to trace or establish the
source, if this is uncertain, and, if there
are multiple sources, he may have to make
some analysis of their contribution to the
overall problem. For remedying the nuisance,
he will have to Judge among a number of
alternatives available to him - increased
dispersal (as by a stack or ventilation),
activated carbon systems, catalytic or direct
flame after-burners, scrubbers, and masking
or counteracting agents, and, in some cases,
an adjustment of the process itself with a
view to minimizing odor release.
What is Odor?
Odor Is defined either as (a) the per-
ception of smell, referring to the experi-
ence, or (b) that which is smelled, referring
to the stimulus. The word odorant Is pre-
ferred for the latter meaning. The ex-
perience of smell, moreover, can be taken
either to mean any perception that results
from nasal inspiration, including common
chemical irritations such as those produced
by acid fumes, or to refer only to the
true odor sensations perceived via the
receptors of the olfactory epithelium, an
area of about 2.5 sq. cm. within the
upper nasal cavity.
In practical industrial hygiene and
community air pollution applications, irri-
tants and stenches are usually grouped
together as objectionable atmospheric
contaminants that ought to be removed even
though they may not be specifically toxic.
In establishing sensory measurement scales,
however, it is important to recognize that
"Irritating odor" is not necessarily an
extension In magnitude of "strong odor",
but refers to a different type of sensation.
Odor sources may be confined to a
specific emission, such as a vent, stack, or
exhaust duct. Under these conditions, the
location, composition, concentration and
volumetric discharge of the source can be
specified. Such definite characterizations
facilitate the establishment of relation'
ships between the odor source and the odor
measurements made in the community. In
general, an odor source may be said to be
confined when its rate of discharge to the
atmosphere can be measured, and when the
atmospheric discharge is amenable to repre-
sentative sampling and to physical or
chemical processing for purposes of odor
abatement. For meteorological diffusion
calculations, the location where the odor
Is discharged into the atmosphere is assumed
to be a point In space.
The characterization of an odor source
in terms of chemical composition will de-
pend on what relationships are to be estab-
lished between sensory and chemical
measurements. If there are no technical or
legal uncertainties concerning the source
of a given community odor, a detailed chem-
ical analysis of the source is not needed;
instead, a comparative method of appraising
the effect of control procedures will suf-
fice. On the other hand, chemical charac-
terization may be helpful in tracing a
community odor to one of several alleged
sources, in relating variations in odor
Amos Turk, Ph.D.
Professor, Department of Chemistry
College of the City University of New York
PA.C.ge.33.12.70
-------
Measurement and Control of Community Malodors
from a given source to changes in process
conditions, or in appraising the effective-
ness of chemical procedures that are designed
for odor abatement.
The direct sensory characterization of
a confined odor source is frequently imposs-
ible, because extremes of temperature or of
concentration of noxious components make the
source Intolerable for human exposure. Suit-
able methods of dilution and cooling for
sensory evaluation of odor sources are need-
ed.
Unconfirmed sources include drainage
ditches, garbage dumps, settling lagoons, and
chemical storage areas. An unconfined source
may be represented by an imaginary emission
point for the purpose of meteorological diff-
usion calculations. Such assignments may be
made on the basis that, if all the odor from
the unconfined area were being discharged
from the "emission point", the dispersion
pattern would just include the unconfined
source. Figure 1 is a schematic illustration
of this procedure.
Sampling for Odor Measurement
Dilution procedures for sampling odor
sources such as oven exhausts can reduce con-
centrations and temperatures to levels suit-
able for human exposure without permitting
condensation of the odorant material. The
dilution ratio must be specified; it can be
determined on the basis of either pressure
or volume:
Dilution
ratio
Volume of sample after dilution
Volume of sample before dilution
(Pressure is constant; temperature is Irrelevant)
Dilution
ratio
Total pressure of sample
in container after dilution
Partial pressure of odorant
in container
The container must be rigid and the temper-
ature must be constant, or appropriate corr-
ection of pressure must be made according to
the relationship that pressure is proportion-
al to the absolute temperature.
The material of choice for dilution
sampling of hot odor sources is stainless
steel. A suitable device is a 1000 cu. in.
stainless steel tank (Figure 2), fitted with
a flow control inlet valve (shown with handle
CALCULATED
DISPERSION
DIRECTION
Figure 1. ASSIGNMENT OF EMISSION POINT TO LNCONFINED ODOR SOURCE
STAINLESS STEEL TANK
11000 CU IN I
Figure 2. SAMPLING TANK WITH VALVES AND FLOW CONTROLLER
-------
Measurement and Control of Community Xalodors
removed), a constant differential type flow
controller (Moore Products Company, Model
63SU), and two brass or stainless steel
bellows seal valves, one between the flow
controller and the tank, and the other at the
opposite end of the tank. Before use in
sampling, the tank is evacuated, and the
inlet valve is adjusted to the desired flow
rate, as indicated on a flowmeter (not shown
in the figure). A convenient rate for many
applications is one that fills the tank to
within about 75% of its capacity over a 20
minute interval. Once the adjustment Is
made, the handle is removed from the inlet
valve and Its stem is sealed with wax so as
to insure against accidental tampering; the
tank is then reevacuated and the two seal
valves are closed. For sampling, it is
necessary to open only the seal valve between
the flow controller and the tank and to record
the time during which the odorant is being
sampled. The total volume of sample will be
equal to the time of sampling multiplied by
the flow rate through the inlet valve. The
flow controller assures that this rate will
be constant over the sampling period if the
tank is not filled beyond about 75X of its
capacity. As an alternate method the flow
control system is omitted and the end
opposite the sampling probe is fitted with
a vacuum gage; the partial pressure of the
odorant is then the difference between the
vacuum readings before and after sampling.
The method is further illustrated by the
following example.
Example 1. A direct-flame afterburner
is to be used to deodorize an oven exhaust
that contains malodorous lacquer solvent
fumes. The afterburner discharge is to be
evaluated to determine what flame temperature
and residence time are needed to effect com-
plete deodorization. The odor can be
adequately characterized even after a 100
to 500-fold dilution. Discharge temperatures
are around 1100° F (590°C). For sampling,
a stainless steel tank fitted with a length
of stainless steel tubing is evacuated
to a pressure of 100 torr. The steel probe
is Inserted in the burner discharge and a
sample is drawn into the tank until the
pressure reaches 110 torr. At this point
the temperature of the gas mixture in the
tank is 28°C. The tank is now pressurized
with odor-free air to 3.00 atmosphere (2.28
x 103 torr). The final temperature is 22°C.
The dilution ratio then is:
The diluted sample in the tank is then
removed to a test room where Judges can
draw samples for sniffing simply by opening
the valve. A sequence of samples taken from
afterburner discharges under different
temperatures and flame detention times can
be used for tests to ascertain the conditions
necessary for deodorixation.
A dilute odorant may have to be con-
centrated before it can be adequately
characterized by chemical analysis. Such
circumstances are likely to arise when a
community malodor is traced to one or more
possible sources. The ratio of concentrations
between the odor source and the odorant
outdoors where it constitutes a nuisance may
be in the range of 103 to 106. Under such
circumstances the comparison of the odorant
with the alleged source is greatly facili-
tated If the concentration is increased to
approximately that of the source. Comparative
examinations of infrared spectra or gas
chromatograms then become much more in-
formative. The most widely used methods of
concentration are freeze-out trapping and
adsorptive sampling, usually with activated
carbon or silica gel.
Odor Intensity Is the magnitude of the
stimulus produced when a person is exposed
to an odorant. The magnitude of an odor
stimulus Is best measured along an intensity
scale whose reference points consist of
standard odorants. The composition of the
odor reference standards should be known
and reproducible. Because odor, like other
sensations, is a logarithmic or exponential
function of the stimulus, it is appropriate
for the concentrations of the reference
odor standards to be distributed along some
exponential scale; a convenient scale is
based on exponents of 2. The procedure is
illustrated by the following example:
Example 2. The intensity of a series of
phenolic odors is to be measured. The re-
ference standards are a concentration series
of phenol solutions in water on an exponential
scale. The solutions are:
Diution No.
1
2
3
Concentration of phenol in water
5% by weight at 20° C =C,
1/2 C,
1/4 Ct
C, x2-<
2.28 x
103 torr
-233
(110 - 100) torr 1273+22)
273+28)
-------
Measurement and Control of Community Malodors
About 25-30 ml. (1 fl. oz.) of each solution
may be placed in a 4 oz. plastic squeeze
bottle. A judge sniffs the odor standard
when he squeezes the bottle to expel about
half (50 ml.) of the vapor phase in
equilibrium with any given solution. In-
structions are given to the judges as
follows: "The samples lined up in front of
you all contain solutions of phenol in water.
They differ from each other in odor strength.
the most intense odor is on the left, and the
intensity gradually gets less from sample to
sample towards the right. Be cautious in
sniffing the strong odors. The last bottle
on your right has so little phenol odor
that it may not be detectable at all. You
are to Judge the phenol odor intensity of the
unknown sample by picking the standard
solution that matches it most closely."
Judges may make subjective estimates of
magnitude between points that are represented
by reference standards. It is also possible
for a judge to estimate intensity ratios that
are based on only one reference point. Thus,
a judge may estimate that a given phenolic
odor is five times as strong as that of a
standard phenol odorant. Measurements set
up in this way are called ratio scales.
Some odor intensity scales are not
anchored in specific reference standards at
all, but Instead are defined by descriptions
like "slight", "moderate", "strong", and
"extreme". Such adjectives are actually
implied reference standards because an odor
judge, even without instructions, will refer
to his experience. Thus, for example, a
fragrant odor of "moderate" intensity may
be taken to mean the typical odor level
in a florist shop. Such scaling can be
grossly imprecise.
The quality of an odor is its character
described in terms of resemblance to some
other odor. Descriptions of quality include
words like "fragrant", "putrid", "musky", and
"phenolic". If a general description of
odor qualities could be set up, it would
be possible to describe any odor in terms of
a number of primary odor standards. Taking
the Crocker-Henderson system as an example
of this sort, odors are classified according
to four primary qualities: fragrant, acidic,
burnt, and caprylic (goaty or rancid). Each
quality is also rated on an odor intensity
scale from 0 (none) to 8 (strongest). Thus,
acetic acid is described by the number 3803,
which implies that the odor is moderately
fragrant, highly acidic, not burnt, and
mildly caprylic. Systems of this type,
though they date from 1895 or earlier and
are described in standard psychological texts,
have not become useful methods of odor
measurement in air pollution problems. The
reason is that no conceptual system has
yet been devised which accurately predicts
odor quality from the chemical composition
of the odorant, especially from such complex
mixtures as outdoor pollutants.
An empirical approach that does not
entail any assumptions concerning primary
odor types can be realized by using specific
odor quality descriptors that are re-
presented by odor quality reference
standards. In this method, the group of
odors to be judged is defined in terms of
a few (usually 3 to 8) qualities that seem
reasonable in the light of subjective
associations and chemical analysis. The
selections are made by people who are
familiar both with the odors in question
and with the analytical findings, even
though the latter may be incomplete. Then
an odor quality reference standard is made
up to represent each quality description.
The chemicals used in a reference standard
represent the best choice available on
the basis of odor, stability, lack of
toxicity, and correspondence with constituents
that are known or suspected to exist in the
odorant. Each reference standard may then
be expanded into a dilution scale using a
suitable odorless diluent.
For convenience, and in order not to
overload the Judges' capacity for yielding
informative responses, the number of points
on the dilution scale should correspond
to the following relationships: No. of
quality stds. x No. of intensity stds. per
quality = 12 to 36. Then an odor to be
appraised may be described in terms of
intensities of the various qualities by
proper matching with the reference standards.
Such a description is called a quality-
intensity profile. The procedure is
illustrated by the following example:
Example 3. Diesel exhaust is to be
appraised in terms of its quality intensity
profile. It is found that the odor can be
characterized in terms of four descriptors;
burnt/smoky, oily, pungent/acid, and
aldehydic/aroraatic. The odor quality re-
ference standards are made up of the
following components: (burnt): oil of cade
-------
Measurement and Control of Community Malodors
(Juniper wax), guaiacol, carvacrol, and
acetylenedicarboxylic acid; (oily) :
n-octyl-benzene; (pungent/acid): crotonic
acid and propiolic acid; (aromatic/aldehyde):
a mixture of aromatic hydrocarbons and
aromatic and aliphatic aldehydes. Each
reference standard is diluted in mineral
oil, with benzyl benzoate added if necessary
for solubilization, to four different
concentrations representing different
levels of intensity. The resulting kit of
sixteen reference standards is used as a
basis for quality-intensity profiling of
diesel exhaust odors.
Dilution and Threshold Measurement
The odor threshold is the minimum concen-
tration at which an odorous substance can
be distinguished from odor-free air
("detection threshold") or at which its
quality can be recognized ("recognition
threshold"). The latter is the higher value.
Odor threshold levels depend on the nature
of the substance and on the sensitivity of
the judge. The "50Z threshold" is the
concentration at which the odor can be
detected (or recognized) by 50X of the
population.
Threshold data can be used to predict
the conditions under which a given substance
will be odorous or odorless. Such predictions
provide a basis for calculating (a) the
degree of dilution, by ventilation or out-
door dispersal, that is needed to deodorize
a given odor source, (b) the proportion of
odorant that must be removed from a space,
by methods such as activated carbon
adsorption, to effect deodorization, (c)
the amount of a substance that must be
injected into a space to odorize it, or (d)
the volume of air that can be odorized by a
given amount of substance. Threshold con-
centrations are not, however, measures or
reliable predictors of odor intensity at
supra-threshold levels.
The threshold level, C is often measured
by quantitative dilution of a given volume,
V, of odorant of known concentration, C.
Then C = W/V, and Ct = W/Vt> where W is the
quantity of odorant and V is the volume of
sample after it is diluted to threshold
level.
Dividing and solving for C ,
C = CV/V
The same procedure can be applied to char-
acterize an odorant or odor source of
unknown concentration. Then the threshold
dilution ratio, V~/V = C/Ct. This ratio,
also called the threshold odor number, or
the odor pervasiveness, can be used as a
basis for the sane calculations as those
previously cited for the odor threshold con-
centration. Likewise, the threshold dilution
ratio is not a measure of supra-threshold
intensity.
A related unit, called the "odor unit",
Is defined as one cubic foot of air at the
odor threshold. A cubic foot of odorant that
must be diluted to n cubic feet to reach the
odor threshold level is said to contain n
odor units. The odor concentration of an
odorant is then expressed in terms of "odor
units per cubic foot". This nomenclature Is
unfortunate, in that the meaning of concen-
tration" is foreign to chemical usage, but
it will be recognized that It is Identical
to the threshold dilution ratio, with volumes
being expressed in cubic feet.
The acceptability of an odor can be
rated either on a hedonlc (like-dlsltke), or
an action basis. A typical hedonic odor
scale is verbally anchored with nine categor-
ies in a linear arrangement (Table IX For
appraisal of community maladors, only the
"dislike" half of the scale is appropriate.
Another method of rating the acceptabil-
ity of an odor is to score its value on an
"action" scale. Such a scale describes what
action a person would take In response to
being exposed to a certain odor. Scales of
this type have been used to evaluate foods,
but have not yet been applied to air pollution
problems.
Human Judges for Odor Measurements
Candidates for a sensory odor panel that
involve judgements other than acceptability
should demonstrate (a) that they can discrim-
inate among the different odors at low or
moderate levels of Intensity, and (b) that
they can focus their attention on more than
one odor quality or character in a given odor
sensation. Practically all programs that
involve discrimination among odorous materi-
als or atmospheres demand sensory abilities
of one or both of these types.
Three variables characterize the ef-
ficiency of a screening procedure: (a) the cost,
as determined by the number of sensory tests,
-------
Measurement and Control of Community Malodors
(b) the proportion of potentially suitable
candidates rejected by the screen, and (c)
the proportion of potentially unsuitable
candidates accepted by the screen. These
variables are functionally dependent - that
is, specification of any two determines the
value of the third. Thus, if the screening
procedure Is to be economical and is to ac-
cept a large majority of the potentially
able candidates, It will also be necessary
to accept some predetermined proportion of
the potentially unsuitable candidates. The
unsuitable candidates can be subsequently
eliminated during training sessions when
their poor performance becomes apparent.
Conversely, if an economical procedure Is
to screen out a large majority of the unsuit-
able candidates, it will also be necessary
to lose some of the potentially able ones.
Tasks based on measurements of odor in-
tensity and quality of standard samples can
be used to screen candidates for sensory
judgment panels. It is somewhat advantageous,
however, for a screening test to be compli-
cated enough to result in easily observable
differences in test behavior among competing
candidates. The attentive panel moderator
will gain rapid initial insight inco the po-
tential value of a candidate by careful ob-
servation of his performance. The specific
items to which the panel leader should be
attentive are: (a) Speed. The best behavior
is purposeful and deliberate, neither exces-
sively hasty nor too slow, (b) Interest le-
vel. The candidate should feel challenged
and motivated. Candidates who find the work
distasteful or uncomfortable should not be
selected, (c) Domination. In group testing,
the candidate should be helpful when asked,
but should not try to push his opinions on
others. (d) Independence. The candidate
should be willing to consider the suggestions
of others, but should not be influenced to
change score against his own judgment, (e)
Honesty. Candidates who try Csuccessfully
or not) to decode labels or peek under bot-
tles should be rejected.
When judges are to be assigned to accep-
tability measurements, discriminatory
abilities are not necessarily relevant. Ins-
tead, the judges should share the same likes
and dislikes as the general population of
interest. Panels of this type, therefore,
consist of a large number of judges, perhaps
several hundred, and they yield results that
are interpreted as "consumer surveys".
The candidates that have been selected
Table 1
ACCEPTABILITY SCALE
1. Like extremely
2. Like very much
3. Like moderately
4. Like slightly
5. Neither like nor dislike
6. Dislike slightly
7. Dislike moderately
8. Dislike very much
9. Dislike extremelv
should repeat the screening procedures in or-
der to become familiar with the conduct and
execution of sensory testing. During the
training, no errors are ignored. As soon as
a trainee or group of trainees scores a test,
the correct answers are disclosed, and the
trainee repeats the exercise with attention
focused on elimination of the error. If the
test program will involve the use of odor
reference standards, a set of standards should
be made available to each panel member, and
he should be trained in the recognition and
use of the standards until his performance
reaches a satisfactory level.
A space for sensory testing should be
free of competing distractions. In-and-out
visits and socialization bv non-participants
should be restricted, except in the case of
demonstration exercises. The area for prep-
aration and coding of test samples or make-up
and dilution of gas samples should be separat-
ed from the panel members and not visible to
them. There should be no extraneous sounds
related to sample preparation. Color schemes
should be neutral and not striking.
The test area should be provided with
some means for space odor control, especially
for control of the odors introduced from the
samples being tested. Such air cleaning may
be accomplished either by local exhaust of
vapors from the test samples, or by general
purification of the room air through a recir-
culating device (preferably an activated
carbon unit) or by some combination of both
methods. Transite wall panels and asphalt
tile flooring have been found to be low in
background odor. If it is feared that high-
ly odorous materials may contaminate the
test space, the walls may be sprayed with an
odorless, strippable, replaceable coating.
When the tests are to be conducted and
reported on an individual basis, without com-
-------
Measurement and Control of Community walodors
munlcation between judges, It is desirable to
have separate booths or enclosures for the
individual panel members. When the test is
to be cooperative, with discussion and com-
parisons among the judges, then a conference
table setup is convenient. The panel moder-
ator or chairman should be able to communi-
cate readily and conveniently with all of the
Judges. The general atmosphere should be
comfortable and relaxing, but attentive and
serious.
Several simple rules for conduct should
be imposed. No smoking, eating, or drinking
that is not associated with tests should be
allowed in the test area. Women should be
discouraged from the use of perfumes or per-
fumed cosmetics just prior to a test session.
A period of about one-half hour or more
should be interposed after smoking, eating a
meal, or drinking coffee, before participat-
ing in a test exercise.
There is no "magic number" of panel
members. Many panels number five to fifteen
judges, and a number close to ten Is probab-
ly suitable for most sensory work that in-
volves discrimination. A larger number of
panelists should be available for testing so
that no interruptions need be caused by tem-
porary absenteeism or personnel turnover.
The panel members should not be divided, how-
ever, into "regulars" and "standbys". Some
rotation scheme should be set up so that no
trained panel member is allowed to go stale.
Chemical Analysis of Odorous Substances
Odorants, like other substances, are
also amenable to chemical analysis. In many
cases, such analysis is difficult and time-
consuming, because an odorant may be a com-
plex mixture of many components in extremely
high dilution in air. Since the specific
chemical and physical determinants of odor
are not yet known, a chemical analysis, no
matter how complete, is not a measurement
of odor. To correlate the chemical with
the sensory properties of an odorant, it is
necessary to assign odor intensity and
quality attributes to the various components
that the analyst has Identified, and pos-
sibly to consider interaction effects.
Very few complex odorous substances have been
carefully investigated in this way.
Odor Control
There are two distinctly different app-
roaches to odor control: (a) the odorant
may be reduced in concentration, so that its
smell is less Intense, with the result that
any objectionable effects are diminished, or
(b) the odorant may be changed in quality so
that it becomes more pleasant or acceptable
to people. The first approach includes re-
ducing the source of the odor, diluting the
odor by ventilation or dispersal, or removing
the odorant from an atmosphere by adsorption,
scrubbing, or by chemical conversion to odor-
less, or more nearly odorless, products. The
second approach attempts to make an odor more
pleasant by counteraction or masking.
Ventilation is the most common method of
removing odorous air from enclosed spaces;
and dispersal, especially with the aid of
stacks, is still the most common method for
odor abatement outdoors. In enclosed spaces,
proper O>2 and 02 levels can be maintained
with ventilation rates as low as 4ft3/min per
person, but rates of 30-50 ft'/min per person
are often needed to control odors from sources
such as people, tobacco, cosmetics, and food.
Of course, ventilation control is inapprop-
riate when the quality of the outdoor air is
unsatisfactory. In any case, when odors are
continuously generated and ventilated, the
odor level approaches an equilibrium whose
value depends on the rates of odor input and
removal, but is Independent of the volume of
the space.
When odors are to be dispersed from an
elevated source such as a stack, the maximum
possible concentration at ground level can
be calculated as a function of the stack geom-
etry, the concentration of odorant in the
emitted gas stream, the effluent temperature,
and the atmospheric turbulence. Such calcu-
lations predict average concentrations over a
specified interval of time. Since even mom-
entary exposure to a foul odor may be
unacceptable to people, the degree of gas
dispersal needed to effect deodorization may
be considerably greater than is predicted by
conventional meteorological calculations.
The control of atmospheric odors by ad-
sorption is, for all practical purposes.
limited to the use of activated carbon as the
sorbent. Activated-carbon equipment will
serve to remove odors until the carbon is
saturated, at which time it must be reactiva-
ted. Carbon is also a catalyst for decompo-
sition and other reactions of organic matter,
and the hot carbon in a freshly reactivated
solvent recovery bed may actually produce
nuisance odors if it is not cooled before
being returned to its adsorption function.
-------
Measurement and Control of Community Malodors
Oxidation systems can be fully effective
methods of odor control if the oxidation is
complete The final products of oxidation
are either odorless (t^O, C02>, or much less
odorous than their precursors (sulfur oxides
are less odorous than tnercaptans or sulfides,
and nitrogen oxides less so than amines).
Partial oxidation, however, may increase
odor, as exemplified by the conversion of
alcohols to highly odorous carboxylic acids.
Equipment used for odor control by air
oxidation comprises (a) direct-flame incin-
erators, and (b) catalytic combustion systems.
In direct incineration, the odorous air is
mixed with combustion gases and thus heated
to the kindling temperature of the odorants,
generally in the range 1100 - 1500°F. Flame
detention times are usually 0.3 sec. or long-
er. Such processes can be reasonably
economical if they are used in conjunction
with heat-recovery systems.
Odorous air passed through catalytic un-
its may be deodorized by oxidation at temp-
eratures about 500 to 800°F lower than by
uncatalyzed incineration. The loss of cata-
lyst activity, which determines catalyst life
and, hence, equipment maintenance costs, is
related to three major factors: (a) the pres-
ence of catalyst poisons (such as metallic or
organo-metallic vapors) in the odorous air;
(b) the obstruction of catalyst surface,
either by deposit of carbonaceous material
from incomplete combustion, or by mechanical
adherence of particulate matter; and (c) the
mechanical loss of catalyst due to abrasion
by solid particles in the air stream.
Chemical conversion of odorants is prac-
tically always accomplished by oxidizing
agents. Except for air or oxygen, the oxi-
dants that are usually considered are ozone,
permanganates, hypochlorites, chlorine, and
chlorine dioxide. Since these agents do not
convert organic substances to their most high-
ly oxidized products (C02 and H20) and since
products of lower oxidation states are usual-
ly odorous, it is important to appraise each
agent strictly in terms of the odor abate-
ment it accomplishes.
Ozone converts organic matter by oxida-
tive degradation, usually to aldehydes, ke-
tones, and acids. Such conversion is
impractical in occupied spaces because ozone
concentrations required for effective action
would be far too odorous and toxic to be tol-
erable to man. Ozone treatment has been used,
however, to deodorize exhaust gases in stacks,
where no human exposure is involved.
Permanganate oxidants are used for odor
control by direct treatment of odor sources,
by scrubbing with aqueous permanganate solu-
tions, or by the use of granular beds or
adsorbent impregnated with permanganate. The
oxidizing effectiveness of permanganate de-
pends on the pH of the medium, and increases
in the order neutral < alkaline < acid.
Acidic permanganate is very corrosive and
therefore alkaline media are predominantly
used. Potassium permanganate solutions have
been shown to have deodorizing action in the
treatment of sulfur compounds, amines, phen-
ols, unsaturated compounds like styrene and
acrolein, and other organic substances. A
practical example would be the direct appli-
cation of a permanganate solution to the
ground in the deodorization of feed lots.
Pellets, with potassium permanganate ad-
sorbed on activated alumina, have been des-
cribed for use in air-conditioning systems.
Air scrubbers or washers may effectivelv
remove odorants by any of three mechanisms:
(a) When the concentration of an odorous va-
por in the air is sufficiently high, the
vapor may have an appreciable solubility in
the scrubbing liquid, (b) An odorous vapor
may condense from air into a cold liquid,
just as dehumidification is accomplished by
cold water sprays. (c) Odorant molecules may
be attached to particulate matter, that is,
adsorbed on dirt particles, and the dirt and
odor together can be mechanically scrubbed
from the air. The choice of scrubbing liquid
depends on the odorant to be removed. Chilled
water is effective for some odorants at low
concentration and is used extensivelv for
water-soluble vapors in high concentration.
Chemical reactants may be added to the water,
usually oxidizing agents. Clycol solutions
are effective for some odors and can be re-
generated.
It is possible to control odors bv
modifying their quality by mixing them
with more pleasant odors under conditions
that do not involve anv chemical changes.
Such processes are called counteraction, or
cancellation, and odor masking or reodoriza-
tion. Many common complex malodorants, such
as tobacco smoke, body sweat, and the like,
are sufficiently unique in odor qualities, so
that it is possible to formulate modifying
agents that can be used to improve the odor
quality in a way that is acceptable to many
people. Odor-modifying compositions are
usually formulated by experienced chemists or
perfumers. Materials used are selected from
8
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Measurement and Control of Community Malodors
industrially available high-intensity odor-
ants, often from by-product sources.
Alcohols, aldehydes, and esters are function-
al groups which are frequently present in
chemicals used for odor counteraction or
masking. For certain processes, such as
digester operations in kraft sulfate plants,
or the cooking of meat scraps and bones in
rendering plants, odor modifiers may be
added directly into the process, or metered
automatically by means of proportioning
pumps. Concentrations may range from 1
to 10 p.p.m., based on the weight of the pro-
cess charge under treatment. Odor modifiers
may also be applied by air or pressure atoa-
ization (through properly designed spray
nozzles) of a dilute dispersion of the mater-
ial into the duct from which malodors are
discharged. The odor modifier is first dil-
uted to a practical concentration, usually
about IX, either dissolved in oil or emulsi-
fied in water. Injection is usually made at
a point well below the top of the stack, to
assure good mixing with the effluent vapors.
As industrial activity increases, and
the public's tolerance for obnoxious odor
decreases, we do not yet seem to be on the
verge of completely eliminating community
malodors in problem areas. Diverse methods
of odor control, as outlined above, are avail-
able, but it ia often difficult to select the
best combination of them. Careful analytical
monitoring should accompany the implementation
of any odor control program. As the demands
for highly effective odor control continue to
mount, one likely result is that engineers
will more carefully re-examine manufacturing
processes with a view to modifications that
will reduce odorous emissions.
GENERAL REFERENCES
1. A. Turk, "Obnoxious Odors", Ind. Wastes
3, 9 (1958).
2. A. Turk, "Odor Measurement and Control",
in Air Pollution Abatement Manual,
Manufacturing Chemists' Association,
Washington, D.C., (I960) Chap. IX
3. A. Turk, "Odor Control Methods: A Criti-
cal Review", Symposium on Odor.
American Society for Testing Materials,
Special Tech. Fubl. No. 164. (1954)
p. 69.
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Reprinted by special permission from CHEMICAL ENGINEERING,
June 15, 1970. Copyright 1970 by McGraw-Hill, Inq., N.Y., N.Y.
Controlling Industrial Odors
Industrial odors can be handled as chemical engineering problems;
the key is to reduce the concentration of odorants at the receptor, i.e. the nose.
JOHN E. YOCOM and RICHARD A. DUFFEE. the Research Corp. of New England
Like all forms of pollution, odors are subject to
strong public criticism (see below).
While many people consider odor control a special-
ized application of the perfumer's art, it can and
should be approached through sound engineering
Odor Regulations
Regulation XV of the 51. Louis Air Pollution Control
Regulations is typical of ordinances adopteu by more
than a dozen communities; it specifies thoti*
A. No person shall emit odorous matter such as to
cause an objectionable odor . . .
1. on or adjacent to residential, recreational,
institutional, retail sales, hotel or educational
premises.
2. on or adjacent to industrial premises when
air containing such odorous matter is diluted
with twenty or more volumes of odorfrae air.
3 on or adjacent to premises other than those
in 1. or 2., when air containing such odorous
matter is diluted with four or more volumes
of odarfree air.
8. The above requirement shall apply only to ob-
jectionable odors. An odor will be deemed objection-
able when 30% or more of a sample of people ex-
posed to it believe it to be objectionable in usual
places of occupancy, the sample size to be at least 20
people or 75% of those exposed if fewer than 20
people are exposed.
As can be seen, the number of odor units allowed
in this ordinance ranges from 0 to 20, depending on
the location.
• tiaulotion XV, Control of Odon In The Ambl.nt Air, PHS
Publication 999 AP-43>.
principles. Control of odor is very much in the chemi-
cal engineering domain, and many chemical engineer-
ing unit operations can be used effectively to ac-
complish it.
Too Much Is a Very Small Number
Controlling odor is a complex and often frustrating
problem. With the exception of halogen compounds
and inorganic compounds of sulfur, phosphorus, selen-
ium, tellurium and nitrogen, most of the odorous gases
or vapors are organic compounds In all cases, the
odorant must be in tne .upor state to be detected,
and the olfactory detection threshold in humans is
very low. For example, it has been estimated that
10* or 10* molecules of odorant in the nose is suf-
ficient for detection.4 To illostrate-the minuteness
of this value, a microgram of ethyl mercaptan in
die vapor state contains approximately 1014 molecules
at standard conditions or 108 times the number of
molecules required for detection.
Odor thresholds fo. yore compounds vary widely
as can be seen from the following table showing
threshold values for a few familiar materials:11
Compound
Acetone
Diethylamine .. .
Trimethy lamine.
Ammonia
Butyric acid .
Methanol
Pyridine
Ethyl mercaptan.
Odor Threshold, Ppm.
100
05
0002
50
001
100
02
001
It is apparent that odorants require considerable
dilution to prevent their being detected. Also, odor
Reprinted from CHEMICAL ENGINEERING, June 15. 1970 Copyright © 1970 by McGraw-Hill Inc 330 West 42nd St. New Vork. N V 10036
-------
List of Odors Reported by City Bureaus—Table I
No. of Tlnut
Source of Odor Reported
l-Anlnul Odor*
Meat packing and rendering plants 12
Fish-oil odors from manufacturing plants 5
Poultry ranches and processing 4
II—Odors from Combustion Processes
Cosolme- and diesel-engine exhaust. 10
Coke-oven and coal-gas cdors (steel mills) 8
Maladjusted heating systems .. . 3
Ill-Odors from Food Processes
Coffee roasting . . ... . a
Restaurant odors.. — . 4
Bakeries . . 3
IV-Palnt and Related Industries
Mfg of paint, lacquer and varnish 8
Paint spraying 4
Commercial solvents 3
V-Ceneral Chemical Odors
Hydrogen sulfide 7
Sulfur dioxide 4
Ammonia 3
VI-General Industrial Odors
Burning rubber from smelling and debondmg 5
Odors from dry cleaning shops . 5
Fertilizer plants 4
Asphalt odors—roofing and street paving 4
Asphalt odors—manufacturing 3
Plastic manufacturing 3
intensity is proportional to the logarithm of the con-
centration of the odorant Thus, to reduce odor in-
tensity by a factor of 2 requires a reduction of odorant
concentration by a factor of 10 In general, therefore,
techniques for reducing odor levels at the receptor
must be highly effective
Odor problems are seldom created by single odorous
compounds Kraft pulping mills, for example, produce
several organic sulfur-containing compounds, plus
H,S, and rendering plants release an extremely com-
plex mixture of organic compounds, many of which
contain nitrogen in the molecule Thus, it is seldom
possible to use odor thresholds for pure compounds
in specifying odor control equipment Rnlher, a
method such as the modified ASTM procedure (see
box below) must be used to specify the control re-
quired for each problem
More than a dozen years ago, Kerka and Kaiser*
compiled a list of the most frequent odorant com-
plaints reported by municipal air-pollulion-control
bureaus The physical sources of odor were classified
according to 11 calegones of odor type, with the
most frequent sources identified in each category
(Table I)
Odors associated with animal or protein processing
are certainly among the most frequently encountered
In this case, the raw material (animal matter in
various states of decomposition) is a significant odor-
ant source Decomposition products are diammes
(such as cada\enne), various alkvlammes. mercap-
tans, organic sulfides, butyric, xalenc and caproic
acids and higher fatt\ acids, and the corresponding
aldehvdcs Ml of these are bad actors, olfaclonly
speaking. c\en at verv low concentrations For ex-
ample, the Los Angeles County Air Pollution Control
VII-Foundry Odors
Core-oven odors 4
Heat treating, oil quenching, and pickling 3
Smelting 2
Vlll-Odors from Combuitlbla Watte
Home incinerators and backyard trash fires 4
City incinerators burning garbage .. 3
Open-dump fires 2
IX-Roilnory Odors
Mercaptans 3
Crude oil and gasoline odors 3
Sulfur 1
X-Odors from Decomposition of Waste
Putrefaction and oxidation—organic acids 3
Organic nitrogen compounds—decomposition
of protein Above odors are probably re
lated to meat processing plants 2
Decomposition of lignite (plant cells) 1
XI—Sewage Odors
City sewers carrying industrial waste 3
Sewage treatment plants 2
Measuring the Amount of Odor
Most of the techniques to quantify odorous emis-
sions involve vapor dilution to an arbitrarily defined
threshold-sensing volue The most commonly used
method is ASTM procedure D1391 57. as modified by
Mills and hn coworkers of the las Angeles County
Air Pollution Control District' A panel of six to eight
members is used Successive dilutions of an odorous
sample are made with odarfree air, until 50% of
the panel members report no detectable odor
This concentration is called the "Minimum Identi-
fiable Odor" (MIO) The volume ratio of o4orfree air
to the sample at the MIO dilution » stated in "odor
units." which are the number of volumes of odorfree
air required to dilute one volume of odorous air to
the MIO level.
The product of the volume rate of emission of an
odorous gas multiplied by the number of odor units
in the exhaust stream is thus a measure of the release
rate of odarants (odor units per minute) and can be
used in much the same wa> as emission rates of other
pollutants (eg, Ib/mm of particulars matter or of
pure gases such as SOi)
CHEMICAL ENGINEERING/JUNE 13. 1970
161
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ODOR CONTROL . . .
District group7 reported the following odor concen-
trations (per std. cu. ft) from various rendering plant
operations:
1. Straight rendering cooker vapors: 50,000, with
a range of 5,000 to 1 million odor units;
2. Straight blood-drying cooker-vapor: 100,000,
with a range of 10,000 to 7 million odor units;
3. Feather dryer gases: 2,000 odor units.
No single method other than complete oxidation
appears feasible for such complex mixtures, which
include high-molecular-weight fatty acids that are
normally solids or liquids at ambient temperatures.
Adsorption might be applied for odorants other than
non-fatty acids, but the efficiency required, e.g., a
reduction from 7 million to say 20 odor units, or
99.9997%, seems to preclude use of this control
method. Since cooking and drying are basically de-
hydration processes, use of condensers should be of
value in reducing odorant emissions.
Odor Engineering
Choosing the precise operation for a specific prob-
lem involves consideration of the nature of odors, and
this can involve a significant departure from most
engineering principles. An odor is in itself an effect,
i.e., an individual's reaction to a stimulus of his olfac-
tory region. The materials producing the stimulus
are termed odorous substances or simply odorants.
Odor control, therefore, can be directed either at (a)
reducing the concentration of odorants at the re-
ceptor or (b) interfering with the receptor's olfaction
process. The latter alternative (including olfactory
anesthesia, masking and counteraction) is not yet
amenable to engineering principles, and thus is be-
yond the scope of this paper.
Odorants must be in the form of gases or vapor
in order to be sensed by the human olfactory system.
Although solid participate matter and liquid droplets
present in an odorous gas stream may have a .pro-
nounced effect on the persistence and other character-
istics of the odor, the wide range of methods used
for odor control apply to pollutants in the gaseous or
vapor state.
Accordingly, it is possible to classify the approach
to odor control according to the chemical engineering
operations that are effective. There are: (1) process
changes, (2) oxidation techniques, (3) condensation,
(4) absorption, (5) adsorption, and (8) source
modifications. Each of these methods can be evaluated
according to principles of chemical engineering cost
estimation.
Process Changes
There is nothing special about the concept of
piocess changes as a method of pollution control.
This is not a definable technology; it is more a
state of mind. Chemical engineers through training,
and application of this training, have been extremely
successful in implementing this state of mind, by de-
veloping improved processes when given the incentive
to do so.
Odor emissions present a sizeable challenge to
the process engineers involved with an odor-pro-
ducing process. Approaches to odor control by proc-
ess modification will vary, but a number of truisms
are perhaps worth mentioning with respect to tem-
perature, pressure, volume, maintenance and good
housekeeping.
Temperature: The temperature regime of a process
can have a pronounced effect on odorous emissions.
Simply chilling the water of a vapor condenser dur-
ing warm weather when water temperature is high-
est and windows are open may solve an odor problem.
Excessive temperatures during drying of a heat-sensi-
tive material may produce odorous decomposition
products, which could be avoided by adequate tem-
perature control. Insufficient temperature in a fur-
nace burning odorous waste gases on the other hand
may produce intermediates more odorous than the
original material.
Pressure: Converting a process environment from
slightly positive to slightly negative pressure by chang-
ing damper positions or the location' of the fan will
reduce the number of leak points for odorous mate-
rials and make the odor control job easier.
Volume. A large ventilation volume for an odor-
ous process will tend to dilute odorous emissions but
may actually increase the quantity of odorants emitted,
if the odorant is a vaponzed liquid. Odor intensities
downwind from a source are proportional principally
to emission rates, rather than the concentration
of odorants in the effluent. Reducing the ventilation
volume may decrease odor levels downwind and will
simplify and reduce costs for additional odor control
equipment required. However, such volume reduc-
tion can be achieved only if explosion hazards and
other constraints have been adequately considered.
Maintenance: In many odorous processes, the great-
est amount of odorant released is from leaks (flange,
pump seals, uncovered vessels, etc.). Controlling such
sources of odor is a continuing job for maintenance
people.
Housekeeping: Odor-producing materials allowed
to accumulate in the open (volatile wastes or pu-
trescible foods and food byproducts) are sometimes
the only significant sources of odorous emissions
from certain types of operations. Good housekeeping
can eliminate such problems.
Oxidation Techniques
Since die most important and the largest categories
of odorants are organic gases and vapors, it is clear
that complete oxidation of these materials will effec-
tively control odor. In die following sections, the dis-
tinction is made between combustion (high tempera-
ture) oxidation, and chemical (low temperature) oxi-
dation.
With many organic compounds, the degree of
oxidation is directly related to the strength of the
162
JUNE 15, 1970/CHEMICAL ENGINEERING
-------
odor or how objectionable it is. Thus, for example:
Bulanol mild odor
Butyraldehyde bad odor
Butyric acid very bad odor
Complete oxidation of such compounds yields
COj and water vapor, both odorless, but if oxidation
is not complete, an offensive odor may result.
Combustion: Organic odorous gases and vapors
can be destroyed by both flame (high temperature)
and catalytic (low temperature) combustion.
Flame Combustion: Destruction by flame is the
most certain and most flexible method of odor control
for organic odorous materials. Even when the material
contains sulfur, the resulting SO2 has an insignificant
odor compared to the parent material, although SO2
in sufficient concentrations should not be ignored as
an air pollutant. Flame combustion can be used over
a wide range of concentrations. At the rich end, there
may be sufficient material to support combustion
(flares); at the lean end, auxiliary-fuel firing will
have to provide the necessary combustion temper-
ature for complete destruction of odors.8-10- "•ia-18
Afterburners have been applied to a number of odor
producing processes, including: coffee roasting, meat
smoking, rubber curing, resin curing, rendering, var-
nish cooking, enamel baking, asphalt blowing, phthalic
anhydride production. In most applications, it is
necessary to achieve a temperature in the after-
burner of at least 1,200 F. in the presence of excess
air to completely destroy odors.
The Los Angeles County Air Pollution Control
District in its Rule 64, which applies to rendering-
plant odors, requires that cooker effluent must be
heated to at least 1,200 F. for a penod of at least 0.3
sec., or controlled by some other method that is
equally effective in controlling odorous emissions.14
The fire boxes of boilers and other direct-fired
processes such as kilns can be used as flame after-
burners in the destruction of odors. In Los Angeles,
considerable success has been achieved in consuming
smoke-house and rendering-plant effluents in this
way. In kraft-paper mills, odorous gases are commonly
consumed in the lime kiln. There are several require-
ments that must be met, however, before such a
method can be considered:
1. The furnace operating schedule must be com-
patible with the requirement for disposal.
2. If the odorous gas is basically air and is to be
used as combustion air, its volume must not exceed
the combustion air requirements of the furnace.
3. The odorous compounds and any other contam-
inants in the stream entering the furnace must be
completely combustible unless any noncombustible
residue can be tolerated along with fuel ash as ex-
pected deposit on heat-transfer surfaces.
4. If the heating value of the contaminated stream
is appreciable, it must be considered in selecting
proper air/fuel ratios for good combustion.
5. If the fuel content of the contaminated streams
is within the flammable or explosive range, safety
precautions must be taken, such as installation of
water seals, spray legs, flame arresters, etc.
Catalytic Combustion: Combustion with air on
catalyst surfaces reaches completion at lower tern-
peratures-at which point, combustion is Homeless.
The oxidation catalysts used in odor-control appli-
cations are platinum alloys and/or a combination
of platinum and alumina. A thin film of catalyst
is coated on a support material consisting of either
tin, crimped nickel-alloy ribbon, or porcelain rods.
The support material is so arranged as to provide
intimate contact between the catalyst and the gas
stream at a reasonably low pressure drop.
The basic flow system for catalytic combustion
consists essentially of a catalyst chamber, a blower,
and a preheat burner where auxiliary fuel and air
are supplied when needed. Preheat temperature of
about 500 F. is sufficient for the more stable vapors.
The heat generated by catalytic combustion raises
the temperature of the gas streams without the
catalyst element by about 55 F for each Btu./cu.ft.
of calorific value in the entering gas Thus, when the
odorous gas stream entering the system contains
combustible gases having a total calorific value of
10 Btu /cu.ft., the resulting temperature rise of 550
F is sufficient to sustain catalytic combustion of
many hydrocarbon and organic gases without the
use of auxiliary heat, once the process is started.
Abnormally high concentrations of water vapor can
of course impose an extra heat load.
With a gas stream containing relatively high con-
centrations of combustibles (within the flammability
limits), the stream must first be diluted with air to
less than one-fourth of the lower flammability limit
(LFL), as required for insurance approval. At a con-
centration of one-fourth of the LFL, many hydro-
carbon gases have a calorific value of about 12 Btu./
cu.ft., which is sufficient to sustain catalytic combus-
tion for those hydrocarbon gases having catalytic
ignition temperatures below 660 F. For example,
formaldehyde with a calorific value of 640 Btu./
cu.ft. and an LFL of 7% by volume gives 11.2
Btu /cu.ft.
Also, hot exhaust gas from the catalyst chamber
can be recycled for recovery of heat, to reduce the
cost of operation. Another alternative is to pass the
hot gas back through a heat exchanger to heat
the incoming odor stream. If the odorous material
may exit as liquid droplets or as a mist, it must be
evaporated before combustion to avoid condensation
on the surface of the catalyst elements. In this case,
hot exhaust gas can be recycled directly into the
flow system ahead of the blower, where sufficient
space can be provided for residence time to evap-
orate the droplets.
Because catalytic combustion takes place on the
exposed surface of the catalyst elements, it is im-
portant that these surfaces be kept clean and active
to maintain satisfactory performance. Gas streams
containing low concentrations of inorganic dust can
be handled if the elements are washed frequently,
CHEMICAL ENGINEERING/JUNE 15, 1970
163
-------
ODOR CONTROL . . .
but at high dust loadings washing becomes im-
practical. Another limitation is that catalyst surfaces
become poisoned by certain metallics such as arsenic,
lead, mercury or zinc when these are present in the
gas stream. Halogen gases and SOs act as suppressants
to catalytic combustion.
In view of these limitations, catalytic combustion
should be applied only after pilot tests, unless the
odorous gas stream consists solely of pure solvent
vapors that are free of solids, high-boiling organic
liquids, and materials capable of poisoning or sup-
pressing catalytic combustion.
Chemical Odixation: Many of the organic gases
and vapors responsible for odors are converted to
odorless compounds by means of relatively gentle
forms of chemical oxidation using oxidants such as
chlorine, chlorine dioxide, ozone or potassium per-
manganate. Depending upon the process emitting
the odorous materials and the character of the odor-
ous gas (wet, hot, etc.}, the contact between odorant
and oxidizer can be gas-gas, gas-liquid, or gas-solid.
Gas-Gas Contact: The only significant example
of this method is in the use of oxidizing gases such
as chlorine and ozone. Because of its toxicity, residual
odor and possible corrosive effects, chlorine is seldom
used in this way, except in combination with a
scrubber. Rather, the principal method involves the
release of ozone into stacks or vents exhausting odor-
ous materials. Ozone is sometimes released into a
plant atmosphere to control odors, but this is risky
because of its toxitity. Power requirements for gen-
erating ozone are in the neighborhood of 4.5 kv./lb.
of ozone.
Gas-Liquid Contact: In this method, chlorine, chlor-
ine dioxide and potassium permanganate are the
principal oxidants. Ck or C1O3 can be added either
to the gases entering the scrubber or to the scrubber
liquid. According to Mills,7 chlorine for controlling
the odor from fishmeal driers achieves best results
with a slight excess of chlorine in a large volume of
water. Insufficient chlorine permits odors to escape,
too great an excess causes free chlorine to evolve from
the scrubber. A relatively recent development in this
category of odor-control technology has been the
use of dilute alkaline KMnC>4 solution in the scrubber
water.7 A significant advantage to this method is
the lack of toxic, odorous or corrosive residual ma-
terial in the exhaust gases. The permanganate does not
leave the system and is reduced to solid MnO2,
which tends to settle out of the scrubber liquor.
Since the permanganate solution is colored (pink),
it can be used as an indicator of solution effective-
ness.
Gas-Solid Contact: A unique deodorant system
combining adsorption and oxidation has been devel-
oped by Borg-Warner scientists.1* It consists of a
combination of an aqueous potassium permanganate
solution (3%) and a mineral substrate of activated
alumina. The alumina adsorbs water and odorant
molecules from the air. SoHtbitized by die adsorbed
water, the permanganate oxidizes die odorant to
nonodorous end-products. Thus, the system avoids
the major disadvantages of adsorbents, namely po-
tential desorption of odorants, and the need for re-
generation (which creates another odor-control prob-
lem).
The deodorant is usually produced as porous,
essentially dry, spheres or pellets packed into rec-
tangular filters of various sizes, or into canisters
similar to those used with activated carbon. Bed
depths vary with application and have been re-
ported ranging from % in. to 16 in.1*
Activated carbon impregnated with various com-
pounds has been developed recently for application
to specific chemical systems. The mechanism is
similar to that involved in the alumina permanganate
system. The carbon adsorbs the odorant, which re-
acts with the impregnated material on the substrate
surface.
Condensation
Many odorants can exist as liquids under ambient
conditions; cooling such vapors can remove much of
the odor by simple condensation. If moisture is
present in an odorous stream, condensation of the
water may reduce the concentration of odorants
to some degree, particularly if they have an appre-
ciable solubility in water.8
Condensers are of two basic types: surface con-
densers and contact condensers. The former rely upon
cooled surfaces where the coolant does not come in
contact with the odorous vapor. The latter rely en
a mixture of coolant, vapors, condensate, and non-
condenable gases.
Chemical engineers should be intimately familiar
with condensation principles and condenser design,
thus no details need be presented here. However, it
should be pointed out dial contact condensers are
applicable to a wider range of odor-control appli-
cations and are less expensive than surface con-
densers. Furthermore, jet-type contact condensers
provide aspiration of vapors from their source.
Contact condensers operate like scrubbers, and
therefore achieve a higher degree of control than
surface condensers. However, they produce large
volumes of contaminated water, which can create a
water pollution problem. Furthermore, care must
be taken to prevent rerelease of odorous vapors con-
siderable distances downstream, as is fairly common
with pulp mills. Condensers are particularly useful
in odorous effluents containing a significant propor-
tion of water vapor, such as rendering-plant cookers.3
Absorption
Non-chemical absorption is widely used for re-
moval of soluble inorganic gases such as HC1 and
NH3. The removal of HzS by amine solution, the
formation of weak complexes between the gas and
liquid, and the subsequent desorption of W.J> for
later use might also be considered as non-chemical
164
JUNE IS, 1970/CHEMICAL ENGINEERING
-------
absorption. The jet condenser operates as a scrubber
or non-chemical absorber and although odor removal
is significant, such devices using only water are gen-
erally not effective enough to solve tough odor prob-
lems.
The addition of chlorine permanganate or caustic
can make absorbers highly effective in odor removal.
Such additions convert absorbers into chemical con-
tacting devices.
Adsorption
Odorous gases and vapors composed of organic
molecules can be removed from air streams by means
of adsorption. A number of adsorbents can be used,
but activated carbon is by far the most important.*' *
Granular activated carbon is used in a size small
enough for good contact between the granules and the
gas passing through the carbon bed, but not so
small as to create excessive pressure drop across the
bed (Fig. 1). Activated carbon can retain adsorbed
materials equivalent to a significant proportion of its
dry weight (Table II). Retentivity data show the
relative weight of material that the carbon can retain
when completely saturated, and after a stream of
pure air at constant temperature and pressure has
passed over it and come to equilibrium.
Activated-carbon adsorbers in their simplest form
are fixed beds in a cylindrical container, with a
suitable perforated support for the carbon. Contam-
inated gases are introduced at the top of the container
and flow down through the carbon. In an upflow ar-
rangement, velocities must be kept below the value
that will cause suspension of the granules.
When a carbon bed becomes exhausted, it can be
regenerated by steam stripping. The collected odorous
vapors together with waste steam are normally
passed through a condenser and sump, where the
organic fraction can be decanted from the water
fraction. To permit continuous operation, activated-
carbon adsorbers are arranged in multiple beds, so
that one can be stripped while the others are in
operation (Fig. 2).
Source Modifications
Unlike other common air pollutants, many odorants
are innocuous compounds having no known detri-
mental or lexicological effects at or near the threshold
of detection. Consequently, the old saw "the solution
to pollution is dilution" seems to be applicable for
solving some odor-control problems.
Such dilution can be achieved by:
• Collecting and discharging all process and
plant emissions through a tall stack and/or
Increasing the temperature and velocity of the
stack effluents,
• Relocating the plant at a greater distance from
any receptor.
Stack Design: Contaminants emitted into the at-
mosphere are diluted by turbulence and diffusion.
100
« 80
o 60
- 40
& 10
~° 8.0
"g 6.0
J 2.0
b
S. i.o
a 0.8
80.6
« 0.4
0.2
0.1
10
j 1—i—i i i
20 30 40 50 60
Linear velocity, ff./min.
80 100
PRESSURE DROP through carbon beds—Fig. 1
The dilution of a contaminant depends directly on
the wind speed—the mass emitted in unit time being
spread over the distance traveled in unit time by
the air blowing over the discharge point. In addition
to this thinning of the material in the direction of the
mean wind, there is also mixing along and across the
mean wind horizontally as well as in the vertical di-
rection, because of the natural turbulence resulting
from the wind.
A number of formulas have been developed to pre-
dict the downwind ground-level concentrations of a
plume discharged from an elevated continuous point-
source, such as a tall stack. In general, when used
under the proper conditions by a capable meteor-
ologist the formulas yield average concentrations,
which are within a factor of two of the measured
values in 75%, and within a factor of three in 90%,
of the time. These predictions have been made at
distances up to a few miles and for durations up to
two hours. At longer distances (up to tens of miles)
and longer periods, the prediction error increases
but, in general, it will not exceed a factor of 10.20
Wind
Ground |5?
H = effective
stock
Stock h = stock height
height
EFFECTIVE stack height—Fig. 2
CHEMICAL ENGINEERING/JUNE 15, 1970
165
-------
ODOR CONTROL . . .
i.rTii TCDLIZ s ILU! ILLUUL sou1 QXCTUJZILCO
Comparative Costs of Direct-Flame Afterburners on Rendering-Plant
Emissions With and Without Heat Recovery—Table II
System
Initial cost, $
Approximate erection, $
Hourly operating cost, $/hr
Annual operating cost, $/yr.,= fuel cost minus makeup-air savings. . .
Annual fuel savings for makeup air, S/yr
Total system cost, $
Annual cost, including initial cost plus operating cost, based on 15-yr.
life, $
Operating cost S/hr./l.OOO cfm
Incineration
Without
Heat
Recovery
16,750
3,800
4.33
25,900
None
20,550
27,270
.64
Incineration with
a Single- Pass-
Primary and a
Single-Pass-
Secondary Heat
Exchanger
42,300
4,600
2.58
13.510
1,960
46,900
16,630
.38
Incineration
with a
Double-Pass-
Primary and
a Single-Pass-
Secondary Heat
Exchanger
58,800
5,400
1.84
9.630
1,370
64,200
13,910
.27
Notei.
Fuel coil XM/10* Btu
Annual operating houri, 6,000
Ervcllon of Incinerator only
The formulas available have a Gaussian form such
as that developed by Pasquill 2l
Concentration
(1)
where
Q = emission rate, odor units/see.
fl = average wind speed, m./sec.
a,a, => lateral and vertical diffusion parameters.
H = stack height, m.
As can be seen from this equation, concentration
at the ground level downwind is directly proportional
to the rate of emission (rather than concentration)
and inversely proportional to the stack height.
The effective stack height is the plume rise plus
the actual height (Fig. 6). The dominant forces are
the inertia of the gases as they rise through the
stack, and the buoyancy force due to low density.
A number of formulas have been developed for
piedichng plume rise above the stack. They are
most applicable when the atmosphere is moder-
ately stable and wind speed is moderate (from 5 to
25 ft /sec ) .20 One frequently used equation, for ex-
ample, is that of Holland:
where
wd (
'~l
1.5 + 268 X 10-»p
d = dia = 2ft, m
•p = atm. press., millibar!)
2 68 X 10-* = a constant, mb ' X m1.
w = stack exit velocity, m /sec
a = wind velocity at stack height, m /sec.
T = atm temp , K.
T, = stack-gas temp, K.
AA = plume rise, m.
Relocation: The values of
-------
produced by the plant. Accordingly, the decision
was made to vent these odorous gases to the atmos-
phere through the stack. As a result, odor levels in
the neighboring community were reduced markedly,
but reliable reports of the odor started to occur in a
community 48 miles away. Turk has reportedi similar
findings.23
Costs
Flame Combustion: In flame combustion, the largest
portion of the total operating cost is invariably fuel,
except where the contaminated air stream can be
introduced into a boiler. However, this fuel cost can
be reduced substantially by heat exchangers for
recovering a portion of the waste heat. Gas require-
ments for operating a flame afterburner at 1,200 F.
without heat recovery—assuming no heating value
(either sensible or latent) from the contaminated
stream—will be on the order of 1.9 cu.ft./(hr.) (std.
cu.ft./mm ) of odorous gas Assuming a process emit-
ting 100,000 cu.ft./min. air with 5 ppm. butanol
and 1,000-hr, operation, operating costs for the 1,000
hr. would be $11,500, assuming natural gas at 6fl
1,000 cu.ft. Thus, for the situation of no sensible or
latent heat in the contaminated stream and 1,200 F.
operation, fuel costs are in the neighborhood of $1.15/
(hr) (1,000 cu.ft./min.) of capacity.
Capita] costs for flame afterburners cover the wide
range between $5 and $25/cfm. of odorous gas to
be treated, depending upon materials of construction,
controls, safety features, etc.
Table II presents data developed by Heilman and
Welling19 on the cost of direct-flame afterburners
with and without heat recovery. In this case, the
waste heat was used for heating plant makeup air.
It is clear that when there is a use for waste heat,
heat exchangers on flame afterburners can easily pay
for themselves.
Catalytic Combustion: As might be expected, first
costs for catalytic units are usually higher than flame-
fired units for the same application, but the lower
operating temperature and lower fuel costs for cat-
alytic units permit them to operate less expensively.
Hem18 made the comparison (Table HI) of the total
costs for installing and operating both catalytic and
direct-flame afterburners, assuming different heat-
ing values for the incoming gas stream and different
Comparative Costs for Catalytic and Flame Afterburner—Table III
Operating Condition
Extent of
Case Waste Heat
No. Recovered
Calorific
Value of Odor
Source Gas
Item of Cost
Unit Costs,
$/hr. per 1,000 elm.
Catalytic Direct-Flame Cost Ratio
Method Method Flame/Cat.
1 None Nil Fixed charges $0.15 $0.10
Burner fuel 0.51 1.06
Maintenance 0.09 0.06
Labor 0.03 0.03
TOTAL $0.78 $1.25 1.60
2 None 1/4 LFL Fixed charges $0.15 $0.10
Burner fuel 0.03 0.58
Maintenance 0.09 0.06
Labor 0.03 0.03
TOTAL $0.30 $0.77 2.45
3 1/2 of Input Nil Fixed charges $0.19 $0.17
Burner fuel 0.27 0.58
Maintenance 0.11 0.10
Labor 0.03 0.03
TOTAL $0.60 $0.88 1.33
4 1/2 of Input 1/4 LFL Fixed charges $0.19 $0.17
Burner fuel (-0.20) 0.10
Maintenance 0.11 0.10
Labor 0.03 0.03
TOTAL $0.13
$0.40
3.10
CHEMICAL ENGINEERING/JUNE 15, 1970
167
-------
ODOR CONTROL . . .
levels of heat recovery. In looking at the ratio of
costs for the two methods, it is clear that when the
incoming contaminated stream has some heating value
the catalytic method has even greater advantages over
flame units.
No cost data are available on the comparable costs
of chlorine and permanganate oxidation, since the
quantities of reducible organics, the method of scrub-
bing, and the volumes will influence these costs.
However, based on the cost per pound of Clj and
KMnO4 and the quantities required for their respec-
tive Redox reactions, it appears that chlorine is by
far the cheaper oxidizer. However, chlorine losses
inherent in such a system could change this relation-
ship.
Condensation: Because of the limited use of surface
condensers for odor control, no data could be found
on costs of the condensers alone in an odor-control
application. However, for a specific surface-condenser
type, it should be possible to develop costs utilizing
standard chemical engineering cost-estimating pro-
cedures. The generally higher costs for surface con-
densers must be weighed against the higher water
costs and possible costs for water-treatment facilities
for the contact condensers.
Contact (jet) condensers cost between 250 and
80t!/cfm. in the range of 40,000 down to 1,000 cfm.
(rated capacity basis).
Adsorption.- Because of the wide variety of ad-
sorption-system arrangements, it is difficult to gen-
eralize on adsorber costs. Activated carbon itself
varies widely in cost (350 to $4/lb.). Even the sim-
plest systems are expensive.
Assuming an average cost of $1 per pound of car-
bon and a retentivity of 15%, the cost for the carbon
alone for a nonregenerative system to handle 10,000
cfm. of air containing 5 ppm. butanol for a total of
1,000 hr. of operation would be on the order of
$4,000. If one assumes the cost for bed holders and
frames for the nonregenerative system to be of the
order of three times the carbon costs, total first costs
exclusive of auxiliaries (piping, ducting, etc.) will
be on the order of $16,000, or $1.60/cfm. of air
to be treated.
In sum, the complex, often frustrating problem of
odor control can be solved. There are a variety of
methods available, varying widely in applicability
and costs. Selection of the methods best suited to
a particular problem requires a thorough knowledge
of the process, the chemical and physical nature of
odorous materials, control technology, meteorology,
and odor measurement. Misapplication of control al-
ternatives can at best be costly and only partially
effective. At the worst, more-serious odor problems
can be created. The assistance of odor-control pro-
fessionals can help prevent such olfactory disasters. •
References
1. Duflee, R. A.. Appraisal of Odor Measuring Technique*.
J, Am. Pollvt. Contr. Assn., July, 1968.
2. Mills. J. L... Walsh, R. T.. Luedke. K. 1).. and Smith.
L. K., Quantitative Odor Measurements. J. Am. Pollut.
Contr., Ann., 13. 10, pp. 467-75, Oct., 1963.
3. A Compilation of Selected Air Pollution Emission Con-
trol Regulations and Ordinances, U.S. Dept. of Health.
Education, and Welfare. Public Health Service Publica-
tion Number 999-AP-43.
4. Moncrleff, A. L., "The Chemical Senses." Chap. 4, pp.
315-6, Wiley (1946).
5 Byrd, J. P., Phelps, A. H.. Jr., Odor and Its Measure-
ment. Chap. 23 In Stern, A. C. "Air Pollution," Viol. 2.
Academic Press (1968).
6. Kerka, W. F., and Kaiser, E. R.. An Evaluation of En-
vironmental Odors. Reprint 57-1, Fiftieth Amuial Meet-
ing (1957) of Am. Pollut. Contr. Assn.
7. Mills, J. L., Danlelson. J. A , and Smith, L. K., Control
of Odors from Inedible Rendering and Fish Meal Re-
duction In L. A. County. Annual Meeting (1967) In
Cleveland, O., of Am. Pollut. Contr. Assn.
8. Walsh. R. T., others, Oontrol Equipment for Oases and
Vapors, Chap. 5, In "Air Pollution Control Engineering
Manual." PHS Publication No. 999-AP-40. Cincinnati,
O. (19'67).
9. Turk. A.. Source Control by G-as-Solld Adsorption and
Related Processes. Chap. 47. In Stern, A. C. "Air
Pollution," Vol. 3, Academic Press (1968)
10. Tocom, J. E.. Incineration of Gases ond Vapora. Chap.
10, In "Air Pollution Manual. Part II. Control Equip-
ment," published by Amerloan Industrial Hygiene
Assn. ( 1968).
11. Heln, Q. M., and Duffee, R. A.. Odor Control -Combus-
tion Methods. Chap 63 A.SHTIAE Systems ami Equip-
ment Guide (1967).
12. Paulus, H. J.. Nuisance Abatement by Combustion,
Chap. 48, In Stern. A. C.. "Air Pollution." Vol. 3. Aca-
demic Press (1968).
13. Ingrels, R. M.. The Afterburner Route to Pollution Con-
trol, Air Engrg., «. «, pp. 39-42. June, 1964.
14. APCD Rules and Regulations. Los Angeles County Air
Pollution Control District, 434 South San Pedro Street,
Ixw Angeles. Calif.. 1968.
15. Hellman, W., and Welling. J. C.. Odor Control In
Rendering by Direct Flame Incineration, presented at
the 62nd Annual Meeting of Am. Pollut. Contr. Asusn.,
N«w York City, June. 1969.
16. Heln, O. M.. Odor Oontrol by Catalytic and High
Temperature Oildation, In Recent Advances In Odor
Theory. Measurement and Control. Annals of New York
Academy of Sciences, 116, 1964, pp. 656-62.
17. Faith, W. L., "Odor Control Manual for The Rendering
Industry," National Renderers Assn. (1969).
18. Harms, others. Annals of the New York Academy of
Sciences, 116, pp. 663-75.
19. Richardson, J., A Solution for Industry Odor Problems,
paper presented at ASHRAE Meeting. Jan.. 1969.
20. Anderson, O. E., Hippler. R. R.. and Robinson. O. D.,
An Evaluation of Dispersion Formulas. Travelers Re-
search Corp. Report for the API, Oct., 1969.
21. Paaqulll, P., "Atmospheric Diffusion," Van Nostrand
(1962).
22. Turk, A.. Reckner, D. R., and Robinson, O. D.. The
Mechanism of Odor Transport, paper 69-123 of Am.
Pollut. Oontr. Assn. Annual Meeting. New York City.
June, 1969.
Meet the Authors
4John E. Yocom is vice-president and director of engineering
and technical programs for The Research Corp. of New England.
P.O. Box 3335, Hartford, Conn. 06103. His past experience in
air pollution control includes direction of air pollution and solid
waste studies with The Travelers Research Corp., plus 20 yrs.
with Battelle Memorial Inst . Bay Area Pollution Control Dist.,
Arthur D. Little and Kaiser Engineers. He holds a BSChE from
M.I T., is a registered professional engineer and a Diplomat
of the American Academy of Environmental Engineers.
Richard A. Duffee is manager of regional program develop-*1
ment for The Research Corp. of New England. He has 15 yrs.
experience in a variety of odor measurement and odor-control
problems, obtained at Battelle Memorial Inst. and The Travelers
Research Corp. He is a member of the American Society of
Heating, Refrigerating and Air Conditioning Engineers Committee
TC 1.6 Odors: Causes and Control. He holds degrees in chemis-
try (Boston College), meteorology (The Penn. State University).
168
JUNE 15, 1970/CHEMICAL ENGINEERING
-------
Chemical
Engineering
Reprinted by special permission from CHEMICAL ENGINEERING,
November 3, 1969. Copyright by McGraw-Hill, Inc., N.Y., N.Y.
Industrial
Odor Control
lts
Problems
\ \_rVIV71
Ahd
AMOS TURK
City College of the City University of New York
Chemical plant odors are more likely than any other
form of pollution to cause community resentment.
Not because odorous materials are dangerous (though
some are) but because odors are so obvious that they
attract immediate public attention.
Elimination of odors is not nearly so straight-
forward as the elimination of particulates, acid fumes
and the like. For one thing, odors are particularly
difficult to measure; for another, it is extremely difficult
to decide what levels are acceptable.
Generally, the discharge of odorous matter to the
atmosphere from a chemical process can be controlled
by (a) modifying die process so as to reduce the pro-
duction of odorous matter, (b) using a device to
reduce the amount of odorant at the point of emission,
(c) dispersing the emitted matter to a greater extent
so that it is less concentrated by the time it reaches
any point where there are people to smell it, or (d)
adding other odorants to the discharge so that the
resultant odor becomes less objectionable.1' 2' 3
In practice, the goal of odor abatement is often
elusive, expensive, or both. The equipment may seem
to work, the calculations may seem rational, but the
-------
a -Average plume
b -Instantaneous plume
c-Puff
d -Single particle
ODOR CONCENTRATIONS along
line BC: a. Average plume, b. In-
stantaneous plume, c. Puff, d.
Single particle—Fig. 1
complaints continue. Occasionally, the complaints get
even worse after the abatement system is in operation.
In this discussion, therefore, I shall reexamine some
of the accepted methods of odor1 control, with special
emphasis on what can go wrong either in the methods
themselves or in our assumptions about how much
control is necessary.
Process Modification
Chemical processes are so diverse that it is difficult
to suggest specific remedies. Nonetheless, process
modification merits first consideration. On occasion,
slight changes in the process are more effective or
cheaper than abatement procedures at the stack.
Alternatively, they may constitute a first stage in odor
abatement that will reduce the load imposed on sub-
sequent stages. Examples are the substitution of low-
odor solvents or reactants for highly odorous ones, or
the adjustment of process temperatures, residence
times, or other conditions so as to minimize odor
production.
The relationship between process temperature and
odor, however, is not a simple one. In high-tempera-
ture ranges (from 950-1,250 F.) a higher temperature
means more complete oxidation, which is usually
helpful; at lower temperatures, an increase in tempera-
ture may mean more volatilization, cracking, or smok-
ing, which can be detrimental. These factors will be
discussed in more detail later, under "air oxidation."
Dispersal Methods
We assume that malodors become less objectionable
as they get more dilute, and that they are abated com-
pletely when their concentrations reach the threshold
level of perception. Since the dispersal of gas from a
stack can be monitored by tracers or calculated
theoretically, it should be possible to predict how
much odor can be emitted from a given stack without
causing a nuisance. If the actual rate is higher than
this calculated value, the dispersal can be increased
(by raising the stack), or the concentration of odorants
can be decreased (by some abatement device), or
both can be done, with the object of diluting tho
odorant at ground level to below the sensory threshold.
This approach has been generally used in the chemical
industry as a basis for design of odor abatement sys-
tems.4 In addition, dilution-to-threshold has become
the most prevalent method of measuring odors in air
pollution applications (see Fig. 2).
There have been very few reported experimental
appraisals of this concept. One investigation5 com-
pared odor travel from four different plants, with
stack-gas dilutions calculated according to Sutton's
equation. Large discrepancies were found. The most
CHEMICAL ENGINEERING/NOVEMBER 3, 1969
71
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ODOR CONTROL . . .
Measuring Odors by the Dilution-to-Threshold Technique
Principle:
Sample of odorous
air or process
exhaust
Same sample diluted
to odor threshold
Dilute with
odor-free air
to threshold
m = mass of odorant
V = volume of sample
C concentration of odorant
m'
V.
C.
: (no change)
: volume at threshold
= threshold concentration
C=m/V
Ct
^ >^0dor
Odor dilution ratio
I Threshold odor number.
-k. s II - J Odor pervasiveness
C, V I Odor units per cu. ft (ASTM
*s definition, V in cu. ft.).
Containers used:
Syringes
Plastic bags
Steel bombs (using partial pressure ratios,
V,/V=PfPt)
Flow systems (using flow rates, Vt/V= Qt/Q),
where Q is flow rate)
Odor-free diluent air
Usually ambient air purified through activated carbon.
Assumptions:
• Mass of odorant is constant (ignores effect of
adsorption on container walls at very low
concentrations).
• Odorant is gaseous (ignores possible effect of
aerosols or condensation nuclei)
• Odor threshold concentration is an intrinsic
property of the odorant (ignores variability of
response criterion of fudges)
The remit theoretically provides • baili tar calculetlnf.
• The degree of dilution needed to deodorize a
given odorous emission.
• The proportion of odorant that must be
removed from a sample of air to deodorize it
• The volume of air that can be odonzed by a
given volume of the odorous sample.
The rnutt don not provide • bull for ettimitlitf:
• The odor intensity of the sample at
concentrations above threshold
• The quality of the odor.
• The objectionability or acceptability
of the odor.
ODOR MEASUREMENTS are generally made by human noses that sniff carefully diluted samples—Fig. 2
extreme sample was odor from a kraft-paper mill, for
which the average dilution needed to reach threshold
was 32 1, and the maximum 64.1. The minimum cal-
culated dilution of stack effluent was 990 1, predicting
that the odors would not be detected in the field. Field
surveys over a six-month period showed that the
kraft odor could be detected at distances up to eight
miles, where the calculated dilution of the stack
effluent was 840,000 1 There were occasional reliable
reports of the odor at a distance of forty miles!
My own experience agrees with these findings that
the distance from a source at which odor may be
detected is not predicted even approximately from
dilution ratios and dispersion calculations. There are
many examples of odor sources whose effects can be
detected at distances of tens of miles, but for which
no threshold or dispersion data are reported m
-------
('=Time>
Let hourly average concentration of odorant
Then, maximum concentration of
odorant in 1 sec
MAXIMUM ODOR is many times average odor—Fig. 3
More serious, however, is the fact that odor com-
plaints are usually responses to peak levels-people do
not smell averages. The meteorological situation is
illustrated in Fig. 1 The shaded areas of the plume
(at left) represent a series of puffs of odorant matter
(•released from Point A; the dotted line indicates an
instantaneous outline of the plume from a continuous
release at A; and the full line shows the outline of the
long-term average plume from a continuous release at
A The right-hand part of Fig 1 shows cross-sections,
along the line BC, of the concentration in the three
cases. Meteorological dispersion formulas give little
help in estimating the ratio of peak to mean concen-
tration, since the meteorological parameters they
incorporate arc to a large extent empirical and are
well established only for prolonged release and
averaged plumes Recent analyses7-s have suggested
that at a considerable distance from the source, the
concentration in the puff is proportional to (-»,
where t is the time of release (see Fig 3)
What we do not know is how long or how fre-
quently a person must be exposed to an unpleasant
odor before he will consider it a nuisance. Thus there
are large uncertainties inherent in all dilution cal-
culations for odor
The Possible Role of Particulatc Matter-In the
widely scattered literature on the measurement, con-
trol and theory of odor, it is repeatedly asserted that a
compound must be volatile to be odorous It is some-
times also inferred lhat the transfer of odorous
material from a solid or liquid source to a human
sensor always occurs by vaporization of the source
material, and by subsequent diffusion or convection of
the gaseous odorant until it reaches the nose of the
subject. However, there is evidence that this picture
is incomplete, and that participate matter can play a
role in the transfer and/or perception of odorous
matter by humans. Various experiments showing that
odors may be reduced by filtration methods, for
example, support this possibility.
Particles may contribute to odor if they are volatile
(like particles of camphor), if they release adsorbed
odorous vapors, or if they are indeed odorous in them-
selves. Since no study has ever rigorously defined the
upper limit of parbcle size for odorous matter, the
latter possibility cannot be excluded.
The implications of particle-odor association may
be even more serious than the computational uncer-
tainties outlined in the previous section. Thus, if a
single particle (d in Fig. 1) carries odor, it consti-
tutes transport without dilution.
Variability of the Odor Threshold-trie odor thresh-
old is usually defined as the concentration at which
an odor can be detected (detection threihold) or
a particular odor actually recognized (recognition
threshold) by a given proportion (often 50%) of
the population. The recognition threshold is consid-
ered to be the more appropriate measure for air
pollution work. The appearance of tables of odor
thresholds implies that these values are properties
of substances, like densities or melting points This
means that each person has a barrier against odor
perception than can be overcome by so many mole-
cules of phenol, or so many molecules of ethyl mer-
captan, and so on, and that this quantity (or concen-
tration) of each substance is therefore its odor thresh-
old Variations in odor threshold can be attributed to
variations in human sensitivities, and can be smoothed
out by selection of representative samples of the
population.
These concepts are rejected by modem "detec-
tion theory,"•10 which assumes instead that when
an individual responds to the question, "Do you
detect odor X in this sample?" he is making a de-
cision between signal and noise, both of which are
present in his environment. This decision depends
not only on the sensitivity of the judge and the prop-
erties of the odorant, but also* on factors like his
expectations and his willingness to risk an error when
he answers "yes," or one when he says "no " Re-
cetly Engen" has iliown that simple reward and
punishment payoffs can manipulate the decision
criterion and, as a result, make the subject appear
"more sensitive" or "less sensitive" to a given odor—
which is the same as making the odor threshold
"lower" or "higher "
Applications of these procedures to studies of
odors in air pollution contexts have not yet been
reported. However, the concepts illuminate such
well-known phenomena as complaints of odor when
the presumed source is inoperative, or when the
wind is blowing the wrong way, or the fact that
phenol is sometimes detected more readily from a
source miles away than from its 1% concentration
in the phenolated calamme lotion one applies to
mosquito bites
Possible Bok of Secondary Sources-Attribution of
a community nuisance odor to a single source is
often an oversimplification The error may become
significant when control procedures are applied only
to the major source
It will be best to illustrate this point with a hypo-
thetical example A manufacturer produces a certain
chemical by the catalytic oxidation of raw material
with air. After the oxidation stage, the process air
stream is discharged to the atmosphere. A com-
munity malodor is attributed to unconverted starting
CHEMICAL ENGINEERING/NOVEMBER 3, 1969
73
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ODOR CONTROL . . .
oxictotlon
CHjSH-S^-SOj
, Aftar
oxidation
Approximate odor
threshold 1 ppb,
Odor level, Stench
assuming source (500 x
concentration is odor
0.5 ppm. threshold)
OXIDATION, If complete, removes odors—fig. 4
material in the exhaust air, which is subsequently
dispersed.
Calculations based on the odor threshold concen-
tration of the raw material and on the expected at-
mospheric dilution predict that a 90% reduction
of the odor's concentration in the exhaust stream,
plus a doubling of the stack height, will eliminate the
problem. Accordingly, the stack height is doubled
and a two-stage activated-carbon unit is installed
which, by material balance data, is shown to recover
90% of the raw material formerly discharged. How-
ever, community complaints about malodois persist
The question is this: Was the prediction concern-
ing the exhaust air stream incorrect because of un-
certainties described previously, or are the malodor
complaints now being generated by odors from
other sources?
It is recognized that the exhaust air stream was
originally the major source, but, after installation of
die abatement system, cumulative secondary sources
may become relatively significant. These would in-
clude vapor displacements from transfer operations,
vapor evaporation during catalyst bed cleanouts,
vapor losses of other materials that are produced as
byproduct streams, various spillages, leaks, etc.
The important point is that the qusstion cannot
be answered In the context of a chemical manufac-
turing operation. The secondary sources cannot be
eliminated for the purpose of conducting odor tests
without stopping the entire production. A further
abatement of the primary source may yield success, but
if it does not, the same question is simply displaced into
another range of concentrations. Furthermore, re-
peated redesign of full-scale abatement procedures is
prohibitively expensive.
ODOR CONTROL METHODS
We will now consider some of the generally recog-
nized odor control methods. Remember that all ex-
cept the sensory modification methods (odor masking,
etc.) reduce the concentration of discharged odorous
matter to the atmosphere. The uncertainties in deter-
mining just how much reduction is necessary are
the same as those discussed in the preceding section
on dispersal
Air Oxidation
Complete oxidation of odorous vapors eliminates
their odors. This statement includes sulfur and nitro-
gen compounds in air pollution contexts, because
their oxidation products, even though odorous, have
comparatively very high threshold values (Fig. 4).
When oxidation is incomplete, however, the odor
quality may actually get worse before it gets better
(see Fig. 5). The various components in a complex
mixture may undergo oxidation at different temper-
atures; it is therefore necessary to ensure adequate
conversion of the most "refractory" (difficult to oxi-
dize) component to ensure adequate odor abate-
ment.
There are three general methods of air oxidation:
flame, thermal and catalytic. The typical flame unit
is essentially a flare that is operated within a com-
RH
Saturated hydrocarbons
(almost odorless)
ROH
Alcohols (low odors)
ArH
Aromatics (moderate
odors)
PlitM
audition
CH20
Formaldehyde
(typical aliphatic end-
group product)
RCOOH
Organic acids (sour
odors)
ArCHO
Aromatic aldehydes
(fruity odors)
a, fl-unsarureted
aldehydes and acids
(irritating pungency)
burnt odors',
such
'-
Most components
mhnown.
C02
Complete t and
oxidation ri20
(no odor)
PARTIAL OXIDATION may make bad odors even worse, but complete oxidation removes them—Fig. 5
74 NOVEMBER 3, 1969/CHEMICAL ENGINEERING
-------
bustioh chamber under carefully controlled condi-
tions (Fig. 6), auxiliary fuel is usually added to
maintain flammability. Thermal incineration occurs
al hot surfaces in the absence of injected fuel (Fig.
7) Catalytic combustion usually utilizes a precious
metal catalyst to attain conversion at minimum tem-
peratures. These methods have been described in
previous reviews.12 The overall sequence of applica-
bility is.
Flame -» Thermal -» Catalytic
Decreasing concentrations of combustibles
»
Table I, taken from Hardison,12 gives ranges of oper-
ating temperatures and costs for the three methods.
Note that these numbers are grossly approximate,
with overlapping cost ranges, and cannot be used as
predictors in individual cases.
Listed below are some of the main problems in air
oxidation to which the engineer should be alert:
Catalyst Deterioration—Catalyst surfaces may ac-
cumulate removable dirt or ash, such as paint pigment
residues, that obstruct the access of fumes. Periodic
wash/ng may relieve the problem. More serious are
(rue catalyst poisons like lead, zinc, silicon and phos-
phorus that necessitate reactivation of the catalyst.
Selection of Temperature and Residence Time-
Fuel costs in air oxidation are so critical that it is
important to determine accurately the conditions
needed for optimum deodonzation Temperature-
rise indications or chemical analyses do not satisfac-
torily predict odor-abatement performance, especially
when intermediate oxidation products may be pro-
duced. It is best therefore to test a pilot unit by direct
Cooling
Laden process I I 1
stream ~~* I '—
Natural gas
Combustion air
FLAME OXIDATION system is an enclosed Hare—Fig. 6
,\ /t\/f
Odor sources
THERMAL OXIDATION system. Oxidation occurs on hot surfaces; no fuel is added—Fig. 7
CHEMICAL ENGINEERING/NOVEMBER 3, 1969
75
-------
ODOR CONTROL . . .
Methods for incinerating waste gases—Table I
Operating
Temperature,
Equipment Annual Fuel
Cost, Cost,
$/Scfm. $/l,000 Sefm.
Flame
Thermal
Catalytic
2,500+ 5-50 0-20
1,000-1,500 1.75-10 0- 7.50
600-900 1.75- 5 0- 4.50
sampling of the hot effluent into an evacuated stainless-
steel tank. The tank contents are then diluted by
pressurization and presented to judges for evalua-
tion (Fig. 8). In this way, a grid of oxidation tem-
peratures and residence times can be rapidly
explored. Each tank contains enough sample for evalu-
tion by a panel of judges.
Comparison With Solvent Recovery—Favorable
heat-recovery economics are no assurance that air
oxidation is the best choice for odor abatement. In
fact, the more attractive the oxidation, the more the
engineer should consider the possible alternative of
solvent recovery, because a high concentration of a
valuable component (for example, toluene) may
be worth more as a solvent than as a fuel.
Oxidation by Agents Other Than Air
Other oxidizing agents used for deodorization in-
clude ozone, chlorine, chlorine dioxide, hypochlorites
and permanganates. It is important to recognize that
these agents generally do not convert organic sub-
stances to their most highly oxidized products (CO:
and H2O), and therefore the question of odors of
intermediates may be critical. For example, ozone
converts a mixture of styrene and vinyl toluene mon-
omers to a mixture of aromatic aldehydes with al-
mond and cherry-like odors. This may not be bad,
but it isn't odorless Chlorine treatment sometimes
yields chlonnation products that are more offensive
than the original odorant. Obviously, each situation
requires separate evalaution.
Adsorption
Activated carbon is the only practical adsorbent that
can be used for odor control because its performance
is not weakened by the presence of moisture. How-
ever, there are several aspects of the behavior of acti-
vated carbon systems to which the engineer should
be alert
Catalytic Action-In an activated-carbon solvent-
recovery system, the adsorbate is stripped off with
steam, leaving a fresh, hot carbon bed. Such hot
carbon is an effective catalyst for various oxidation
and decomposition reactions; hence, if it is not al-
lowed to cool before being returned to the adsorp-
tion cycle, a malfunction of the system may result.
In one such case, methyl ethyl ketone (MEK) on
hot carbon was converted to a complex mixture of
contaminants, including acetic acid and some acrolein
derivatives, making the odor effluent more objection-
able even in the face of 90% material recovery of
the MEK. Proper cycling of the system solved the
problem.
Rapid Saturation From Spills, Vapor Surges, Etc.-
The service life, t, of an activated-carbon system is
given by:
SW
where S = proportionate maximum saturation of the
carbon (fractional); W = weight of carbon; Q =
rate of air flow through carbon (volume/time);
C = concentration of odorants by weight (weight/
volume, units consistent with those used for W and
Q); E = adsorption efficiency (fractional, usually
close to 1).
Usually the factor least subject to prediction or
control is the contaminant concentration, C. Further-
more, when a carbon system is designed for abate-
ment of offensive odors, concentrations on a mass
basis may be quite small, and the relative effect of
an accidental spill may therefore be great. In fact, a
carbon system is sometimes designed only for random
discharges, such as loss of radioiodine and radon from
a nuclear reactor, or spillage of mercaptan from a
gas odorizing site.
On some occasions, the source of the adsorbate
is mysterious, as was the case in a large hotel in-
stallation, in which the carbon bed became saturated
in one month instead of its predicted service
life of one year. The source was finally traced to the
xylene solvent used (during the nighttime, by an
outside contractor) to clean the bowling alley lanes
in the basementl
Some of these problems can be anticipated in
designing the system. In the scheme shown in Fig.
9, with a dampers 1,4 open, and 2,3 closed, the
downstream bed B serves as a standby to prevent
system malfunction if a random spill saturates the
upstream bed A. When A becomes saturated and is
replaced with a fresh bed, it is economical to reverse
the flow (dampers 2,3 open and 1,4 closed) to
utilize fully the capacity of bed B.
Special Reactivation Methods-The approximate
range of contaminant concentrations from 5 ppm
to 500 ppm. represents a special problem in odor
control because these values are high for activated-
carbon systems when the contaminant is not worth
recovering, and low for oxidative systems. Such
concentrations saturate an activated-carbon system
rapidly and are not high enough to make any appre-
ciable contribution to temperature rise in an oxi-
dation system: both effects are economically disad-
vantageous.
It seems sensible in this concentration range to
76
NOVEMBER 3, 1969/CHEMICAL ENGINEERING
-------
SMELL-TESTING effluent from
hot exhaust, a. Sample to 20
torr. b. Pressurize to 2 atm.
(approximate 75:1 dilution).
c. Make sensory judgments on
dilute sample—Fig. 8
DUAL-BED activated carbon >
system uses downstream bed
as standby if random spill
rapidly saturates the upstream
bed—Fig. 9
Out to
atmosphere
Z_~4~—Catalyst
---Carbon bed
„.— Resistance
heater
"•--Steam jet
x Damper
Out to
atmosphere
•* CATALYTIC OXIDATION and
adsorption are combined in
this dual bed, parallel arrange-
ment—Fig. 10
CHEMICAL ENGINEERING/NOVEMBER 3, 1969
77
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ODOR CONTROL . . .
combine the two approaches; that is, to concentrate
the contaminants by adsorption and then dispose of
them by oxidation. There are two approaches to this
combination. The first is illustrated in Fig. 10, which
shows two parallel arrangements of a carbon bed
and a catalytic oxidation unit. Bed A is first used
as an adsorbent. When it is saturated, the contam-
inated air stream is diverted to Bed B, but a small
flow of hot moist air is passed through A for reacti-
vation. The desorbed effluent is then catalytically oxi-
dized to prevent discharge of odors. The energy
requirement for incineration is thus drastically re-
duced by the concentrating action of the adsorbent.
When Bed B becomes saturated, the damper positions
are changed and the cycle is reversed.
A second approach to achieving the same objec-
tive combines the adsorbent and catalytic action in
the same bed by impregnating the carbon with an
oxidation catalyst. The catalyst becomes active only
when the air stream is heated, and the adsorbate
burns itself off. Some years ago, I reported13 that
this effect can be accomplished without burning the
carbon. Systems of this type are not yet in use
but they are being developed.
Washing and Scrubbing
The terms "air scrubber" and "air washer" de-
scribe various types of equipment that remove either
particles or vapors from the air by liquid washing.
Such equipment may therefore be useful for the
control of odors. The air conditioning industry uses air
washers to control temperature and humidity and,
as added benefit, to help control odors. The textile
and the chemical industries, among others, apply
scrubbers either to recover valuable vapors from an
air stream or to remove objectionable odors.
The effectiveness of scrubbers for odor abatement
is difficult to predict from chemical and physical
measurements. As an example, an offensive odorous
effluent from the dehydration of animal matter is
found to contain a significant concentration of am-
monia. Laboratory studies show that when the am-
monia is removed by water scrubbing, the total
nitrogen content of the effluent is reduced by 95%.
Nonetheless, water scrubbing may yield no meas-
urable benefit in the reduction of community mal-
odors. The odor of ammonia itself is readily reduced
to the threshold level by dilution, and therefore does
not account for complaints from distant locations.
Many organic nitrogen compounds of higher molec-
ular weight are extremely offensive even in parts-
per-billion concentrations, and are not removed by
water scrubbing. Thus, in this case, scrubbing has
created a water pollution problem without reducing
odor complaints.
Counteraction and Masking
The sensory quality of an odor may be modified by
the addition of other odorousjiatter under condi-
tions that do not involve chemical charges. When
such modification is used for odor control, the
process is called odor counteraction (implying an
overall reduction of intensity), or odor masking
(implying that the quality of the original malador
is obscured). The method is used in competition with
chemical, adsorptive, or scrubbing systems, and
for unconfined odor sources such as lagoons and
ditches, to which other systems cannot readily be
applied. The relative effectiveness of odor modi-
fication as compared with the physical/chemical sys-
tems is very difficult to evaluate. Scales of odor ac-
ceptability, long used in the food and beverage
industry, have not been developed for community
malodors. Furthermore, the installation of an odor
modification system is often accompanied by other
changes (such as improved housecleaning, disinfec-
tion, increased confinement of sources—all to the good)
so that strictly comparable before-and-after compar-
isons cannot be made. Finally, the odor preferences
of the populace are not stable, and their reactions
to any community odors may change significantly
from time to time.
The above delineation of odor control problems
implies that this area of air pollution technology does
not always lend itself to reliable predictions. Until
the odorant properties of substances are better under-
stood on a theoretical basis, this will probably con-
tinue to be the case. •
References
1. Von Bergen, J.. Chrm. Eng., Auc. l!>.r>7, p. 23!).
2. Turk, A., Odor Measurement and Control. Chap. 13 in
"Air Pollution Abatement Manual," iMfs. Chemists Assn.,
Washington, 1 >. C.. IHUO.
3. Turk, A., Odor Control, in "Ivirk-Othmer Encyclopedia
of Chemical Technology," 2nd ed., Vol. 14, p. 170. Inter-
science, New York. IHB7.
4. Walsh, W. H., Dispersion as a Tool for Odor Control,
presented at a meeting of Mid-Atlantic States Section
of Air Pollution Control Assn., Wilmington, Del.. Nov.
12, 1965.
5. Wohlers, H. C., Intern. J. Air Wut>-r 1'ollntion, 1. pp.
71-78 (1963).
6. Turner, D. B., "Workbook of Atmospheric Dispersion
Estimates," U.S. Public Health Service Pub. 999-AP-26.
1967.
7. Pasquill. F., Meteorol. .Mat/., 90, p. 33 (1!>61).
8. Hino, M., Atmos. Envirvn., 2, pp. 14!t-l(>5 (1968).
9. Green, D. M., Swets, H. A.. "Signal Detection Theory and
Psychophysics," Wiley, IS'ew York, 1966.
10. Swets. J. A., "Signal Detection and Recognition by
Human Observers," Wiley, Xe\v York 1964.
ll.Engen, T., Man's Ability to Perceive Odors, in "Commu-
nication by Chemical Signals," ed. by Johnston, J. W.,
others. Appleton-Century Crofts, Xew York (in press).
12. Hardison, L.. Disposal of Gaseous Wastes, presented at
Seminar on Waste Disposal, sponsored by Bast Ohio Gas
Co., Cleveland, Ohio, May 18, 1967.
13. Turk, A.. Ind. Eng. Vhem., 47, p. 966 (1955).
Meet the Author
Amoi Turk is a Professor,
Dept. of Chemistry, The City
College of the City University
of New York, New York, N. Y
10031. He received his B.S
(cum laude) in chemistry from
City College of New York, and
his M.A. (physical chemistry)
and Ph.D (organic chemistry)
from Ohio State University.
He is a special consultant on
odor and air pollution to Nnti.
Air Pollution Control Adminis-
tration, is or has been a mem-
ber of committees on odor
and air pollution of American
Soc. of Heating, Refrigerating
and Air-conditioning Engi-
neers, American Industrial
Hygiene Assn.. Air Pollution
Control Assn. and ASTM.
78
NOVEMBER 3. 1969/CHEMICAL ENGINEERING
-------
SECTION VIII
DESIGN OF LOCAL EXHAUST SYSTEMS
Fluid Flow Fundamentals
Hood Design
Fan Design
-------
FLUID FLOW FUNDAMENTALS
Herbert Simon
John L. McGinnity
John L. Spinks
Introduction
Local exhaust systems are devices used to
capture dusts and fumes or other contaminants
at their source and prevent the discharge of
these contaminants into the atmosphere. Close-
fitting hoods are used to capture the con-
taminants from one or more locations so that
the laden gases can be conveyed through a
system of ducts by one or more exhaust fans.
An air pollution control device can then be
used to collect the air contaminants and
discharge the cleansed air into the at-
mosphere.
In designing a local exhaust system, sufficient
air must be provided for essentially complete
pickup of the contaminants. Conversely, too
much air can result in excessive construction
and operation costs. It is, therefore,
necessary for the designer to understand
certain physical principles that are useful
in analyzing the ventilation needs and in
selecting the hooding devices.
The nature of flow of a real fluid is very
complex. The basic laws describing the
complete motion of a fluid are, in general,
unknown. Some simple cases of laminar flow,
however, may be computed analytically. For
turbulent flow, on the other hand, only a
partial analysis can be made, by using the
principles of mechanics. The flow in exhaust
systems is always turbulent; therefore, the
final solution to these problems depends
upon experimental data.
BERNOULLI'S EQUATION
The basic energy equation of a frictionless,
incompressible fluid for the case of steady
flow along a single streamline1 is given by
Bernoulli as
h + £+£•_• C (1)
2g
where
h = elevation above any arbitrary datum, ft
p = pressure, Ib/ft
Y = specific weight, Ib/ft
v = velocity, ft/sec
g = acceleration due to gravity, 32.17
ft/sec2
C = a constant, different for each
streamline.
Each term in Bernoulli's equation has the
units foot-pounds per pound of fluid or
feet of fluid. These terms are frequently
referred to as elevation head, pressure
head, and velocity head. They also represent
the potential energy, pressure energy, and
velocity energy, respectively.
When Bernoulli's equation is applied to
industrial exhaust systems, the elevation
term is usually omitted, since only re-
latively small changes in elevation are
involved. Since all streamlines originate
from a reservoir of constant energy (the at-
mosphere) , the constant is the same for all
streamlines, and the restriction of the
equation to a single streamline can be
removed. Furthermore, since the pressure
changes in nearly all exhaust systems are at
most only a few percent of the absolute
pressure, the assumption of incompressibility
may be made with negligible error. Although
steady-flow conditions do not always exist
in exhaust systems, it is safe to make the
assumption of steady flow if the worst
possible case is considered. Any error will
then be on the safe side.
All real fluids have a property called
viscosity. Viscosity accounts for energy
losses, which are the result of shear stresses
during flow. The magnitude of the losses
must be determined experimentally, but once
established, the values can be applied to
Herbert Simon, Senior Air Pollution Engineer
John L. Spinks, Air Pollution Engineer
L.A.C.A.P.C.D.
John L. McGinmty, Air Pollution Engineer
N.A.P.C.A.
PA.C.ge.34.12.70
-------
Fluid Flow Fundamentals
dynamically similar configurations.
Bernoulli's equation may be applied to a
real fluid by adding an energy loss term.
Letting 1_ be an upstream point and 2_ a
downstream point, the energy per unit
weight at 1_ is equal to the energy per unit
weight at 2_ plus all energy losses between
point _!_ and point 2_.
PITOT TUBE FOR FLOW MEASUREMENT
The velocity of a fluid (liquid) flowing
in an open channel may be measurpd by
means of a simple pitot tube, as shown in
Figure 2 (Streeter, 1951). Although this
instrument is simple, usually consisting
of a glass tube with a right-angle bend, it
is one of the most accurate means of
measuring velocity. When the tube opening
Flow
Figure 2. Simple pitot tube
(Streeter, 1951].
is directed upstream, the fluid flows into
the tube until the pressure intensity builds
up within the tube sufficiently to withstand
the impact of velocity against it. The
fluid at a point directly in front of the
tube (stagnation point") is then at rest.
The pressure at the stagnation point is
known from the height of the liquid column
in the tube. The velocity of the fluid in
the stream may be evaluated by writing
Bernoulli's equation between point ^up-
stream of the stagnation point and point 2
the stagnation point. Note that h = h, and
v, = 0. Therefore
Pi
Ah (2)
solving for the velocity,
1
A simple pitot tube measures the total head
or total pressure, which is composed of two
parts, as shown in Figure 2. These are the
static pressure h and the dynamic or
velocity pressureSh_. In open-channel flow,
h is measured from the free surface. '\Tien
trie fluid is in a pipe or conduit in which
it flows full, a simple pitot tube will again
indicate the total pressure; but now the
portion of the total head caused by velocity
cannot be distinguished. The static pressure
in this case can be measured by a piezometer
or static tube, as shown in Figure 3. The
r
hc
i
Figure 5. Static tube (Streeter, 19511.
total pressure H consists of the sum of the
static pressure h and the velocity pressure
h , or
(4)
The velocity can be determined, therefore,
from the difference between the total and
static heads.
In practice, measurement of total pressure
and static pressure is combined into a single
instrument (pitot-static tube, Figure 41,
which permits direct measurement of velocity
head since the static head is automatically
subtracted from the total head. An inclined
manometer (Ellison gauge] is particularly
useful when the heads are small as in
exhaust systems. Use of this device to
measure the flow of a gas introduces, how-
ever, an additional factor, which is the
conversion of readings in inches of
manometer fluid into meaningful velocity
terms. This relationship, when water is
used as the manometer fluid to measure the
velocity of air, is
-------
Fluid Flow Fundamentals
(5)
where
h = velocity pressure or head, inches
of water
4005 = 1096.2
V
Volume in ft of 1 lb of
air at 70°F and 14.7 psia
= velocity of air, fpm.
/
Flow
'Piezometer openings
CORRECTION FACTORS
The relationship expressed in equation 5 is
exact only for air at standard temperature and
pressure, 70°F and 14.7 psia, respectively.
A correction must be applied for other than
standard conditions. If the air in the duct
departs from 70°F by more than about 50°F,
a correction is required:
h =
/ Va \2 /
\400Sy ^
460 + 70
460 + t
(6)
where
the temperature of the air, °F.
For smaller temperature deviations, the error
is not significant and may be neglected. If
the gas is other than air, a correction for
the difference in density may be applied:
/ density of gas \ (7)
4005
density of air
where the density of the gas under conditions
actually existing at the time of the measure-
ment includes the effects of temperature,
pressure, and molecular weight.
Figure 4. Pitot-static tube CStreeter, 1951),
-------
HOOD DESIGN
Herbert Simon
Introduction
Hoods are devices used to ventilate process
equipment by capturing emissions of heat or
air contaminants, which are then conveyed
through exhaust system ductwork to a more
convenient discharge point or to air pollution
control equipment. The quantity of air
required to capture and convey the air con-
taminants depends upon, the size and shape of
the hood, its position relative to the points
of emission, and the nature and quantity of
the air contaminants.
Hoods can generally be classified into three
broad groups: Enclosures, receiving hoods,
and exterior hoods. Enclosures usually
surround the point of emission, though
sometimes one face may be partially or even
completely open. Examples of this type are
paint spray booths, abrasive blasting
cabinets, totally enclosed bucket elevators,
and enclosures for conveyor belt transfer
points, mullers, vibrating screens, crushers,
and so forth.
Receiving hoods are those wherein the air
contaminants are injected into the hoods.
For example, the hood for a grinder is
designed to be in the path of the high-
velocity dust particles. Inertial forces
carry the air contaminants into the hood.
Exterior hoods must capture air contaminants
that are being generated from a point outside
the hood itself, sometimes some distance
away. Exterior hoods are the most difficult
to design, require the most air to control
a given process, and are most sensitive to
external conditions. For example, a hood
that works well in a still atmosphere may
be rendered completely ineffectual by even
a slight draft through the area. The best
rule to follow in hood design is to place
the hood where the air contaminants are
generated. Since this is not always
physically possible, it is important to
consider the design criteria for external
hoods.
CONTINUITY EQUATION
The volume of air flow is dependent upon the
crossectional area and the average velocity
Mr. Simon is a Senior Air Pollution Engineer.
L.A.C.A.P.C.D.
of the air. The relationship may be re-
presented by the familiar equation.
= Av
(8)
where
V = total air volume, cfm
A = cross-sectional area,
v = velocity, fpm.
The continuity equation (equation 8) shows
that, for a given quantity of fluid, the
velocity must increase if the area decreases.
Imagine that air is being withdrawn from a
point at the center of a large room. Since
an imaginary point has no dimensions, there
will be no interference with the flow of air
toward the point. The air will, therefore,
approach this point radially and at a
uniform rate from all directions. The
velocity of the air must increase as it
passes through a succession of diminishing
areas represented by spherical surfaces in
its approach to the imaginary point,
according to the relationship.
(9)
where
r =
the distance from the imaginary point,
ft.
AIR FLOW INTO A DUCT
If a circular duct opening, representing
a simple hood, is substituted for the
imaginary point, the pattern of flow into
the end of the duct, or hood, will be
modified as shown in Figure 5 because of the
interference from the duct. The velocity of
the air approaching a plain, circular opening
along the axis of the duct is given by
Dalla Valle (195?) as approximately
100 - Y
0.1A
(10)
where
PA.C.ge.35.12.70
-------
Hood Design
1 234 56 789
Distance from opening, inches
Flange
Duct wall
Center-line
tJ
123456789
Distance from opening, inches
u
c
01
c
HI
u
ID
4J
\n
Figure 5. Actual flow contours and streamlines for flow into circular openings.
Contours are expressed as percentage of opening velocity. (Dalla Valle, 1952)
-------
Hood Design
(where)
Y = the percent of the velocity at
the opening found at a point x
on the axis
x = the distance outward along the
axis from the opening, ft.
A = the area of the opening, ft .
The velocity at the opening is computed from
the continuity equation.
The actual flow pattern is found to be as
shown in Figure 5 from studies by Dalla
Valle and others. The lines of constant
velocity are called contour lines, while
those perpendicular to them are streamlines,
which represent the direction of flow. The
addition of a flange improves the efficiency
of the duct as a hood for a distance of about
one diameter from the duct face. Beyond this
point, flanging the duct improves the
efficiency only slightly. Figure 6 illustrates
4000
3000
2000
c
o>
1000
0123456789 10
Distance outward from opening, inches
Figure 6. Actual velocities for square
openings of different sizes. Air flow
through each opening is 500 cfm (Dalla Valle,
1952) .
flow patterns for several sizes of square
hoods. Because there is little difference
in the center line velocity of hoods of equal
air volume at a distance of one or two hood
diameters from the hood face, Hemeon (1955)
recommends using one equation for all shapes--
square, circular, and rectangular up to about
3:1 length-to-width ratio. He also does not
distinguish between flanged and unflanged
hoods, which appears justified when these
hoods are used only at distances of one
diameter or more from the hood face. At
close distances, flanged hoods are far
superior at the same volume. By rearranging
terms in equation 10 and combining with
equation 8, the following is obtained:
(lOx
Af)
(11)
where
V = the volume of air entering the hood,
cfm
v = the velocity at point x, fpm
x = the distance to any point x on the
axis or center line of the hood
measured from the hood face, ft
A,. = the area of the hood face, ft .
Analysis of equation 11 shows that at the
hood face x = 0 and the equation becomes
identical to equation 8. For large values
of x, the Af term becomes less significant,
as the evidence shows it should. To use
equation 11, select a value of v that is
sufficient to assure complete capture of the
air contaminants at point x. From the
physical dimensions and location of the hood,
A,, and x are determined. The volume required
may then be calculated.
While equation 11 applies to a freestanding
or unobstructed hood, it can also be applied
to a rectangular hood bounded on one side by
a plane surface, as shown in Figure 7. The
Table top
(Bounding plane)"
Imaginary
i hood | / /
r
Figure 7. Rectangular hood bounded by a
plane surface (Hemeon, 1955).
hood is considered to be twice its actual
size, the additional portion being the mirror
image of the actual hood and the bounding plane
being the bisector. Equation (11) then becomes
-------
Hood Design
V = v
t x
(I0.v2 + 2H{\
2 /
(12)
where the terms have the sane meaning as
before.
NULL POINT
Air contaminants are often released into
the atmosphere with considerable velocity
at their point of generation Because
the mass is essentially small, however, the
momentum is soon spent and the particles are
then easily captured. Hemeon (1955) refers
to a null point, shown in Figure 8, as
Null
points
Null
points
Figure 8 Location of null point and x-
distance (Hemeon, 1955") .
the distance within which the initial
energy of an emitted air contaminant has
been dissipated or nullified in overcoming
air resistance. If an adequate velocity
toward the hood is provided at the
farthest null point from the hood, all the
air contaminants released from the process
will be captured. What constitutes an
adequate velocity towards the hood depends
upon drafts in the area and cannot, there-
fore, be determined precisely.
Establishing the null point in advance for
a new process is not always easy or even
possible. Tor existing equipment, however,
direct observation will usually establish
a locus of null points. Obviously, in the
absence of external disturbances, any
positive velocity toward the hood at the
farthest null point will give assurance of
complete capture. When this is put into
practice, however, the results are dis-
appointing. Even closed rooms have drafts
and thermal currents that destroy the hood's
effectiveness unless a substantial velocity
toward the hood is created at the farthest
null point. Experience has shown that a
velocity of less than 100 fpm at a null
point can seldom, if ever, be tolerated
without a loss in the hood's effectiveness.
Draft velocities in industrial situations may
almost always be expected to be 200 to 500 fpm
or more periodically, and draft velocities of
500 to 600 fpm are not unusual in many cases.
Prafts such as these may prevent capture of
air contaminants by exterior hoods, as
illustrated in Figure 9 for the case of a
Figure 9 Drafts divert the rising column
of air and prevent its capture by the hood
(Hemeon, 19S5) .
high-canopy hood, unless adequate baffling
is provided or hood volume is increased
to unreasonable values. Baffling provides,
in effect, an enclosure that is almost
always the most efficient hooding.
DESIGN OF HOODS FOR COLD PROCESSES
A large body of recommended ventilation
rates has been built up over the years by
various groups and organizations who are
concerned with the control of air con-
taminants. This type of data is illustrated
-------
Hood Design
Table 3, EXHAUST REQUIREMENTS FOR VARIOUS OPERATIONS
Operation
Abrasive blast
rooms
Abrasive blast
cabinets
Bagging machines
Belt conveyors
Bucket elevator
Foundry screens
Foundry shake out
Foundry shake out
Grinders, disc
and portable
Grinders and
crushers
Mixer
Packaging
machines
Paint spray
Rubber rolls
(calendars)
Welding (arc)
Exhaust arrangement
Tight enclosures with air
inlets (generally in roof)
Tight enclosure
Booth or enclosure
Hoods at transfer
points enclosed as
much as possible
Tight casing
Enclosure
Enclosure
Side hood (with side
shields when possible)
Downdraft grilles in
bench or floor
Enclosure
Enclosure
Booth
Downdraft
Enclosure
Booth
Enclosure
Booth
Remarks
For 60 to 100 fpm downdraft or
100 fpm crossdraft in room
For 500 fpm through all openings,
and a minimum of 20 air changes
per minute
For 100 fpm through all openings
for paper bags; 200 fpm for cloth
bags
For belt speeds less than 200 fpm,
V = 350 cfm/ft belt width with at
least 150 fpm through openings. For
belt speeds greater than 200 fpm, V =
500 cfm/ft belt width with at least
200 fpm through remaining openings
2
For 100 cfm/ft of elevator casing
cross-section (exhaust near elevator
top and also vent at bottom if over
35 ft high)
Cylindrical--400 fpm through openings,
and not less than 100 cfm/ft2 of cross-
section; flat deck--200 fpm through
openings, and not less than 25 cfm/ft
of screen area
For 200 fpm through all openings, and
not less than 200 cfm/ft2 of grate
area with hot castings and 150 cfm/
ft2 with cool castings
For 400 to 500 cfm/ft grate area
with hot castings and 350 to 400 cfm/
ft2 with cool castings
For 200 to 400 fpm through open
face, but at least 150 cfm/ft2 of
plan working area
For 200 fpm through openings
For 100 to 200 fpm through openings
For 50 to 100 fpm
For 75 to 150 fpm
For 100 to 400 fpm
For 100 to 200 fpm indraft, depending
upon size of work, depth of booth, etc.
For 75 to 100 fpm through openings
For 100 fpm through openings
-------
Hood Design
in Table 3. The use of these recommended
values greatly simplifies hooding design
for the control of many common air pollution
problems. Note that almost all published
recommendations have specified complete or
nearly complete enclosure. These published
data provide a reliable guide for the
design engineer. The recommended values
must, however, be adjusted to specific
applications that depart from the assumed
normal conditions.
Spray Booths
Spray booths of the open-face type are
generally designed to have a face indraft
velocity of 100 to 200 fpm. This is usually
adequate to assure complete capture of all
overspray, provided the spraying is done
within the confines of the booth, and the
spray gun is always directed towards the
interior. It is a common practice,
especially with large work pieces, to place
the work a short distance in front of the
booth face. The overspray deflected from
the work may easily escape capture,
particularly with a careless or inexperi-
enced operator. If this situation is
anticipated, the equipment designer can
provide a velocity of 100 fpm at the
farthest point to be controlled, as in the
following illustrative problem.
vf =
19,500
(7H10)
= 280 fpm
When the spraying area is completely
enclosed to form a paint spray room, the
ventilation requirements are not greatly
reduced over those for spraying inside an
open-face booth. The reason for this is
that a velocity of approximately 100 fpm
must be provided through the room for
the comfort and health of the operator.
Abrasive Blasting
Abrasive blasting booths are similar to
spray booths except that a complete en-
closure is always required. In addition,
particularly for small booths (bench typel.
the ventilation rate must sometimes be
increased to accommodate the air used for
blasting. The volume of blasting air can
be determined from the manufacturer's
specifications. For a small blasting booth,
this will usually be about 50 to 150 cfm.
The following illustrative problem shows
how the ventilation rate for this kind of
equipment is calculated.
Example 1
Given
A paint spray booth 10 feet wide by 7 feet
high Work may be 5 feet in front of the
booth face at times Nearly draft less area
requires 100 fpm at point of spraying
Prob 1 em
Determine the exhaust rate required.
Solution1
From equation 12, volume required =
Vt = 100 /(10K5)2 * (21(7) C 10
19,500 cfm
From equation 8, face velocity =
Example 2
Given
A small abrasive blasting enclosure 4 feet
wide by 3 feet high by 3 feet deep. Total
open area equals 1.3 ft-.
Problem
Determine the exhaust rate required.
Solution1
From Table 3, ventilation required = 500 fpm
through all openings but not less than 20
air changes per minute
Volume at 500 fpm through all openings
Vt = 500 x 1.3 =650 cfm
Volume required for 20 air changes per
minute
V = 20 x volume of booth
V't = 20 x 4 x 3 x 3
= 720 cfm
-------
Hood Design
Open-Surface Tanks
Open-surface tanks may be controlled by
canopy hoods or by slot hoods, as illustrated
in Figure 10. The latter are more commonly
Slot with b
Liquid 1eve1\
Width of plenum
Exhaust duct
Figure 10. Slot hood for control of
emissions from open-surface tanks (Adapted
from Industrial Ventilation 1960).
employed. The ventilation rates required
for open-surface tanks may be taken from
Table 4, which is a modification of the
American Standards Association code Z 9.1.
These values should be considered as min-
imum under conditions where no significant
drafts will interfere with the operation
of the hood. When slot hoods are employed
the usual practice is to provide a slot
along each long side of the tank. The
slots are designed for a velocity of 2,000
fpm through the slot face at the required
ventilation rate. For a tank with two
parallel slot hoods, the ventilation rate
required and the slot width b may be
taken directly from Figure 11^ which graphs
the American Standards Association code
Z 9.1.
Neither the code nor Figure 11 makes
allowance for drafts. The use of baffles
is strongly recommended wherever possible
to minimize the effect of drafts. If
baffles cannot be used or are not suf-
ficiently effective, the ventilation rate
must be increased. The slot width is also
increased to hold the slot face velocity in
the range of 1,800 to 2,000 fpm.
The use of Figure 11 is illustrated in the
following problem:
Example 3
Given•
A chrome plating tank, 2 feet wide by 3
feet long, to be controlled by parallel
slot hoods along each of the 3-foot-long
sides.
Problem-
Determine the total exhaust rate required
and the slot width.
Solution-
From Figure 11, the ventilation rate re-
quired is 390 cfm per foot of tank length.
V
390 x 3
= 1,170 cfm
1
From Figure 11, the slot width is 15 inches
o
If a slot hood is used on only one side of
a tank to capture emissions, and the
opposite side of the tank is bounded by a
vertical wall, Figure 11 can be used by
assuming the tank to be half of a tank
twice as wide having slot hoods on both
sides. This procedure is illustrated
below.
1000
qnn
ann
<" ?nn
5 600
2 £.500
c c
O O)
+j 400
ID -*
•— C
c^ 300
> o
§+j
s-
£^ 200
•r- U
—
—
—
—
1-
~
=
E
" 'I
^
~l 1
*
/
/
mi
/
Illl
/
Illl
/
Illl
/
Illl
/
HI
X
Illl
>
3
2-1/2
1-3/4
-1/2
1-1/4
7/8
1 IA
J/4
c yo
D/0
1/2
VI
Ol
cr
£
12
15 20 30 40 50 60
Width of tank (b), inches
Figure 11. Minimum ventilation rates
required for tanks.
Example 4
Given:
The same tank as in Example 3, but a slot
hood is to be installed along one side only.
The other side is flush with a vertical wall,
-------
Hood Design
Table -J. VENTILATION RATES FOR OPEN-SURFACE TANKS
(American Air Filter Company, Lie , 196-0
Process
{Minimum \ rntilauon rale,,
crm per ft' 01
hood opening
Mininiuiii \umilauon rate,3
cim per ft oi la.i't area
Lateral eshausi
Enclosing
hood
'One !
.open1
side'
75
6 =
75
75
75
65
75
7*
75
7 -
7*
7*
0
n5
75
7*
75
ds 75
1*0
Tf \
1C)
7 ^
'50
i 7;
— 1
long oni
ik and as
Two
open
sides
100
90
100
100
100
90
100
100
100
100
100
100
'10
100
100
100
100
7*
90
100
7*
100
51,1,.
1 Canop\
hood
Threi
open
sides
125
100
125
12*
12*
100
12*
125
125
12*
12*
125
100
12*
12*
12*
. 12*
7*
100
125
7 *
i 12*
|Four
op. n
sides
17-
1*0
17*
17*
175
1*0
17*
17*
175
175
175
17*
1*0
17*
175
17*
175
12*
1*0
17*
125
,175
iir i\\o paralli
.,» i.ink is u ul
\\/L
W/L
00 lo
\
125
°0
12*
12*
125
QO
12*
12*
125
125
12*
12*
•'0
12*
125
12*
12*
60
"0
12*
1)0
12*
0. 2
B
17*
130
17*
175
17*
130
75
1 ?
7*
7?
-=.
7 •*
130
17*
17*
175
17*
00
130
17*
QQ
17*
1 s i d o i \\ h
i . .ilso in
tank width
lank length
\\,'L
0 2*
A
1*0
110
1*0
150
150
110
1*0
1*0
150
1*0
150
1*0
1 10
1*0
150
1*0
1*0
7*
110
1*0
7*
1 *0
_
, n n '
tanks
to 0 -)'1
ij
200
150
200
200
200
150
200
200
200
200
200
20G
150
200
200
200
200
100
150
200
100
200
1 hone! 1
with . si
-ratio
tt/L
0 ^C
A
17*
MO
17*
17*
17*
1 VJ
17*
17*
17*
17;.
17*
17*
no
17*
17*
17*
175
on
130
17*
. oo
!l7*
I..
to 1
B
22*
170
2 ' *
> > ^
22*
170
22 =
22*
225
J25
22i
22*
170
225
225
22*
22*
1 10
170
22*
110
22*
11M Jl
Plating
Chromium (chromic .,cm mist
Arsenic (arsmr)
Hydrogen cyanide
Cadmium
Anodizing
Metal cleaning (pickling)
Cold acid
Hot acid
Nitric and sulfuric acids
Nitric and hvdrofluorir ai ids
Metal cleaning (degrcasini:)
Trichloroethjlsne
Ethylcne dichloride
Carbon tctrachloride
Metal dialling (cniistic or <_•!< . trohtic)
Not boiling
Boiling
Bright dip (nitric acid)
Stripping
Concentrati d nitric acid
Concentrated nitric ant! suliuric. acids 75
Sail baths (molten salt)
Sail solution (Parkcrise Rondrrisi
Nol boiling
Boiling
Hot water (if \«-nl desired)
Not boiling
Boiling
"Column A refers lo lank u nh hood along oni
uall or a baffle running lenuth 01 tank and as
fold along center line uilli \W2 l>< co-Mini: l.\.il, wi.llh in W'/I. ratio
Column B rclirs to freos.andinp tank \iith hond alonu one >ide or l\\o paralUI sirlts
Prob 1 em •
Determine the total exhaust rate and slot
width required.
Solution1
The ventilation rate in cfm per foot of tank
length is taken as half the rate for a tank
twice as wide from Figure 11. Use width of
4 feet.
t = 880 = 440 cfm per foot
L 2
V = 440 x 3
1,320 cfm
Total exhaust volume required
Slot width is read directly from Figure 11
for twice the width b = 2-5/8 inches.
DESIGN OF HOODS FOR HOT PROCESSES
Canopy Hoods
Circular high-canopy hoods
Hooding for hot processes requires the
application of different principles than
that for cold processes because of the
thermal effect. When significant quantities
of heat are transferred to the surround-
ing air by conduction and convection, a
-------
Hood Design
thermal draft is created that may cause a
rising air current with velocities sometimes
over 400 fpm. The design of the hood and
the ventilation rate provided must take this
thermal draft into consideration.
As the heated air stream rising from a hot
surface moves upward, it mixes turbulently
with the surrounding air. The higher the
air column rises the larger it becomes and
the more diluted with ambient air. Sutton
(1950) investigated the turbulent mixing
of a rising column of hot air above a heat
source. Using data from experiments by
Schmidt published in Germany, and his own
experiments with military smoke generators,
Sutton developed equations that describe the
velocity and diameter of a hot rising jet
at any height above a hypothetical point
source located a distance z below the actual
hot surface. Hemeon adapted Button's
equations to the design of high-canopy
hoods for the control of air contaminants
from hot sources. The rising air column
illustrated in Figure 12 expands approxi-
mately according to the empirical formula
the rate at which heat is trans-
ferred to the rising column of
air, Btu/min.
= 0.5
0.88
(13)
where
D = the diameter of the hot column
of air at the level of the hood
face, ft
x^ = the distance from the hypothetical
point source to the hood face, ft.
From Figure 12 it is apparent that xf is
the sum of y, the distance from hot source
to the hood face, and z the distance below
the hot source to the hypothetical point
source. Values of z may be taken from
Figure 13. According to Hemeon, the ve-
locity of the rising column of air into
the hood may be calculated from
1/3
(14)
where
the velocity of the hot air jet
at the level of the hood face, fpm
the height of the hood face above
the theoretical point source =
y + z, ft
Hypothetical \ /
point source~\A
Figure 12. Dimensions used to design
high-canopy hoods for hot sources (Hemeon,
1955) .
4->
01
Z 100
-M-—
M cn
fu^' 3U
u 40
I*'?
"izo
S"
-C J-J
E.S 10
Distance fro
hypothetical po
i— no to * 01
/
/
/
s
f
Ll
7
f
/]
= (
. , , Ji
J
/
/
j
j
20 J1"
58 "
1 2345 10 20 3
Diameter of hot source (D
0
, feet
Figure 13. Value of z for use with high-
canopy hood equations.
The rate at which heat is absorbed by the
rising column may be calculated from the
appropriate natural convection heat loss
-------
Hood Design
coefficient q listed in Table 5 and from
the relationship
60
As At
(15)
where
the total heat absorbed by the
rising air column, Btu/min
minimize drafts and other disturbances.
\evertheless, Sutton reports that there was
a considerable amount of waver and fluctua-
tion in the rising air column. In develop-
ing his equations, Sutton defined the
horizontal limits of the rising air column
as the locus of points having a temperature
difference relative to the ambient
atmosphere equal to 10 percent of that at
the center of the column.
the natural convection heat loss
coefficient listed in Table 5,
Btu/ft2 per hr per °F
S
At
In view of the facts that this arbitrary
definition does not truly define the outer
limits of the rising air columm and that
greater effects of waver and drafts may be
expected in an industrial environment, a
safety factor should be applied in calcu-
lating the size of the hood required and the
minimum ventilation rate to assure complete
capture of the emissions. Since high-
Table 5. COEFFICIENTS FOR CALCULATING SENSIBLE HEAT LOSS
BY NATURAL CONVECTION ( Hemeon, J955 )
the area of the hot source, ft
the temperature difference between
the hot source and the ambient air,
Shape or disposition of heat surface
Natural convection
heat loss (q ) coefficient
Vertical plates, over 2 ft high
Vertical plates, less than 2 ft
high (X = height in ft)
Horizontal plates, facing upward
Horizontal plates, facing downward
Single horizontal cvlinders (hhere
d is diameter in inches}
Vertical cylinders, oxer 2 ft high
(same as horizontal)
0.3 (At)
1"
0.28 ()
0.38 (At)174
0.2 (At) 1/4
0.42 (|p) 1/4
0.4 fj—)
Vertical cylinders less than 2 ft
high. Multiplv q from formula
above by appropriate factor.
Height, ft
0.1
0.2
0.3
0.4
0.5
1.0
Factor
3.5
2.5
2.0
1.7
J.5
1.1
Heat loss coefficient, q, is related to
q as follows:
Schmidt's experiments were conducted in a
closed laboratory environment designed to
10
canopy hoods usually control emissions
arising from horizontal-plane surfaces, a
-------
Hood Design
simplification can be derived by combining
equations 14 and 15 with the heat transfer
coefficient for horizontal-plane surfaces
and allowing a 15 percent safety factor.
v =
f
(16)
exceptional circumstances. The following
problem illustrates the use of this method
to design a high-canopy hood to control
the emissions from a metal-melting furnace.
Example 5
Given•
Although the mean diameter of the rising
air column in the plane of the hood face
is determined from equation 13, the hood
must be made somewhat larger in order to
assure complete capture of the rising
column of contaminated air as it wavers
back and forth and is deflected by drafts.
The exact amount of allowance cannot be
calculated precisely, but factors that must
be considered include the horizontal
velocity of the air currents in the area,
the size and velocity of the rising air-
jet, and the distance y of the hood above
the hot source. Other factors being equal,
it appears must likely that the additional
allowance for the hood size must be a
function of the distance y. Increasing the
diameter of the hood by a factor of 0.8 y
has been recommended (Industrial Ventilation,
1960). The total volume for the hood can be
calculated from
Vc + vr(Af - V
(17)
where
V = the total volume entering the
hood, cfm
vf = the velocity of the rising air
column at the hood face, fpm
A = the area of the rising column of
contaminated air at the hood face,
ft*
v = the required velocity through the
remaining area of the hood, A,. -
AC, fpm
Ar = the total area of the hood face,
f ft2.
The value of v selected will depend upon
the draftiness, height of the hood above
the source, and the seriousness of permitting
some of the contaminated air to escape
capture. The value of this velocity is
usually taken in the range of 100 to 200
fpm. It is recommended that a value less
than 100 fpm not be used except under
A zinc-melting pot 4 feet in diameter with
metal temperature 880°F. A high-canopy
hood is to be used to capture emissions.
Because of interference, the hood must be
located 10 feet above the pot. Ambient
air temperature is 80°F.
Problem:
Determine the size of hood and exhaust rate
required.
Solution*
Z = 11 feet from Figure 13 for 4-foot-
diameter source
x£ = z + y
xf = 11 + 10
=21 feet
Diameter of rising air stream at the hood
face from Equation 13:
0.5 x
0.88
D = 0.5 (21)
0.88
=7.3 feet
Area of rising air stream at the hood face
2
AC = (0. 7854) (7. 3)'
= 42 square feet
Hood size required- -including increase to
allow for waver of jet and effect of drafts-
D
0.8y
(0.8) (10) = 15.3
Use 15-foot-4-inch-diameter hood
Area of hood face-
Df = ', .3
11
-------
Hood Design
(Area of hood face:)
(Df)
Af = (0.7854)(15.33)2
= 185 square feet
Velocity of rising air jet at hood face1
1/4
v = (8)(12.57)L/3(800)5/12
(21)
1/4
143 fpm
Total volume required for hood
Vt = VfAc * 100(Af-Ac]
Vt = (143) (42) + (100) (185-42)
= 20,300 cfm
If the hood could be lowered, the volume
required to capture the emissions would be
reduced substantially as illustrated below:
Example 6
Given-
The same furnace as in example problem No.
5, but the hood is lowered to 6 feet above
the pot.
Problem
Petermine the size of hood and exhaust rate
required.
Solution
: " 11 feet from Figure 13 for 4-foot-
diameter source
= 17 ft
11 + 6
Diameter of rising air stream at the hood
face from equation 13.
Area of the rising air stream at the hood
face1
A = (0.7854)(6.ir = 29.2 square
C feet
Hood size required-
D, = D + 0.8 y
Df = (6.1) + (0.8) (6) = 10.9 feet
Use 10-foot-ll-inch-diameter hood
Area of the hood face:
Af - -j-tv2
Af = (0. 7854) (10. 92) 2 = 93.7 square feet
Velocity of rising air jet at hood face:
8(A)"3 cat)5"2
_ (8)(12.57)1/5 (800)5/12
(17)
1/4
149 fpm
Total volume required for hood'
0.5 (17)
°-88
= 6.1 feet
Vt = vfAc * 100 (Af _ACJ
Vt = (149) (29. 2) * (100)(93 7-29.2)
= 10,800 cfm
Rectangular High-Canopy Hoods
The control of emissions from sources with
other than circular shape may best be
handled by hoods of appropriate shape.
Thus, a rectangular source would require a
rectangular hood in order to minimize the
ventilation requirements. A circular hood
used to control a rectangular source of
emission would require an excessive volume.
The method used to design a hood for a
rectangular source is illustrated in example
7.
Example 7
Given•
12
-------
Hood Design
A rectangular lead-melting furnace 2 feet
6 inches wide by 4 feet long. Metal
temperature 700°F. A high-canopy hood is
to be used located 8 feet above furnace.
Assume 80°F ambient air.
Problem:
„ _(8)(10)1/3 (620)5/12
vf -- 174 -
(14-2)
Total volume required for hood:
= 130 fpm
Determine the dimensions of the hood and
the exhaust rate required.
Solution:
z = 6.2 from Figure 13 for 2.5-foot source
xf = z+y=6.2+8 =14.2 feet
The width of the rising air jet at the
hood may calculated from
0.5 x
0.88
D
ft QQ
0.5 (14.2) = 5.2 feet
The length of the rising air jet may be
assumed to be increased over that of the
source the same amount as the width
D = (4) + (5.2 - 2.5) = 6.7 feet
The area of the rising air jet is
AC = (5.2)(6.7) = 35 square feet
The hood must be larger than the rising air
stream to allow for waver and drafts. By
allowing 0.8 y for both width and length,
the hood size is
Width = (5.2) + (0.8)(8) = 11.6 feet
Length = (6.7) + (0.8)(8) = 13.1 feet
Use hood 11 feet 7 inches wide by 13 feet
1 inch long
Area of hood-
(11.58)(13.083) = 152 square feet
Velocity of rising air jet:
CIHA/'W"
1/4
Vt = (130) (35) * (200) (152-35) = 28,000 cfm
Note that in this problem a velocity of 200
fpm was used through the area of the hood
in excess of the area of the rising air column.
A larger value was selected for this case
because lead fumes must be captured com-
pletely to protect the health of the workers
in the area.
Circular low-canopy hoods
The design of low-canopy hoods is somewhat
different from that for high-canopy hoods.
A hood may be considered a low-canopy hood
when the distance between the hood and the
hot source does not exceed approximately
the diameter of the source, or 3 feet, which-
ever is smaller. A rigid distinction
between low-canopy hoods and high-canopy
hoods is not intended or necessary. The
important distinction is that the hood is
close enough to the source that very little
mixing between the rising air column and the
surrounding atmosphere occurs. The diameter
of the air column may, therefore, be considered
essentially equal to the diameter of the hot
source. The hood need be larger by only a
small amount than the hot source to provide
for the effects of waver and deflection due
to drafts. When drafts are not a serious
problem, extending the hood 6 inches on all
sides should be sufficient. This means
that the hood face diameter must be taken as
1 foot greater than the diameter of the
source. For rectangular sources, a rectangular
hood would be provided with dimensions 1 foot
wider and 1 foot longer than the source.
Under more severe conditions of draft or toxic
emissions, or both, a greater safety factor
is required, which ran be provided by
increasing the ST.ZC of the hood an additional
foot or more or by providing a complete
enclosure. A solution to the problem of
designing low-canopy hoods for hot sources
has been proposed by Hemeon (1955).
Although the hood is usually larger than
the source, little error occurs if they
are considered equal. The total volume for
13
-------
Hood Design
the hood may then be determined from the
following equation obtained by rearranging
terms in Hemeon's equation and applying a
IS percent safety factor.
Vt = 4 7 (Df)2'33(At)5/12 C18)
where
V
total volume for the hood, cfm
the diameter of the hood, ft
At = the difference between the
temperature of the hot source
and the ambient atmosphere, °F.
A graphical solution to equation 18 is
shown in Figure 14. To use this graph,
Example 8
Given-
A low-canopy hood is to be used to capture
the emissions during fluxing and slagging
of brass in a 20-inch-diameter ladle. The
metal temperature during this operation will
not exceed 2,3SO°F. The hood will be
located 24 inches above the metal surface.
Ambient temperature may be assumed to 80°F.
Problem:
Determine the size of hood and exhaust rate
required.
Solution-
Temperature difference between hot source
and ambient air
2,350
80 = 2,270 F
QJ
£ 5
T. 2
•o
o
100
200
300 400 500
350
500
1000 „
2000
3000
Vt = 4.7Df2'33At5/12
1,000
2,000 3,000 5,000
10,000
Total ventilation (V.), cfm
Figure 14 Minimum ventilation rates
required for circular low-canopy hoods.
select a hood size 1 or 2 feet larger than
the source. The total volume required for
a hood D- feet in diameter may then be read
directly from the graph for the actual
temperature difference At between the hot
source and the surrounding atmosphere
Use a hood diameter 1 foot larger than the
hot source
1.67 t 1.0
=2.67 feet
Total exhaust rate required from Figure 14.
V
1,150 cfm
-------
Hood Design
Rectangular low-canopy hoods
In a similar manner, Hemeon's equations for
low-canopy hoods may be modified and simplified
for application to rectangular hoods. With a
15% safety factor, the equation then becomes
Vt = 6.2 b4'3 At5/12
where
V = the total volume for a low-canopy
rectangular hood, cfm
L = the length of the rectangular hood
(usually 1 to 2 feet larger than
the source), ft
b = width of the rectangular hood
(usually 1 to 2 feet larger than the
source), ft
At = the temperature difference between
hot source and the surrounding atmos-
sphere, °F.
Figure 15 is a graphical solution of
equation 19. The use of this graph to
design a low-canopy rectangular hood for a
rectangular source is illustrated in
example 9.
Example 9
Given:
A zinc die-casting machine with a 2-foot-
wide by 3-foot-long holding pot for the
molten zinc. A low-canopy hood is to be
provided 30 inches above the pot. The
metal temperature is 820°F. Ambient air
temperature is 90 F.
Problem-
Determine hood size and exhaust rate
required.
Solution-
Use a hood 1 foot wider and 1 foot longer
than the source.
Hood size = 3 feet wide by 4 feet long.
60 80 100 150 200 300 400 500 800 1000 1500
Minimum ventilation rate (Vt/L), cfm/ft of hood length
Figure 15. Minimum ventilation rates for
rectangular low-canopy hoods.
15
-------
Hood Design
Temperature difference between the hot
source and ambient air-
290
,1/3
(20)
At = 820 - 90
730°F
Exhaust rate required per foot of hood
length from Figure IS.
t =
430 c fin/ft
Total exhaust rate required for hood-
\'t = 430 x 4 =1,720 cfm
where
V = the total hood exhaust rate, cfm
W = the rate at which steam is released,
Ib/min
A, = the area of the hood face, assumed
approximately equal to the tank
area, ft2
D = the diameter for circular tanks or
the width for rectangular tanks,
ft.
Enclosures
A low-canopy hood with baffles is essentially
the same as a complete enclosure. The
exhaust rate for an enclosure around a hot
source must, therefore, be based on the
same principles as that for a low-canopy
hood. Enclosures for hot processes cannot,
however, be designed in the same manner
as for cold processes. Here again, the
thermal draft must be accomodated b\ the
hood Failure to do so will certainly
result in emissions excaping from the hood
openings. After determining the exhaust
rate required to accommodate the thermal
draft, calculate the hood face velocity
or indraft through all openings. The
indraft through all openings in the hood
should not be less than 100 fpm under any
circumstances. When air contaminants are
released with considerable force, a min-
imum indraft velocity of 200 fpm should be
provided. When the air contaminants are
released with extremely great force as,
for example, in a direct-arc electric steel-
melting furnace, an indraft of 500 to 800
fpm through all openings in the hood is
required
Specific Problems
Steaming tanks
When the hot source is a steaming tank of
water, Hemeon (1955) develops a special
equation by assuming a latent heat of 1,000
Btu per pound of water evaporated. He
derives the following equation for the total
volume required for a low-canopy hood venting
a tank of steaming hot water.
Preventing leakage
Hoods for hot processes must be airtight.
When leaks or openings in the hood above
the level of the hood face occur, as
illustrated in Figure 16, they will be
a source of leakage owing to a chimney
effect, unless the volume vented from the
hood is substantially increased. Since
openings may sometimes be unavoidable in the
upper portions of an enclosure or canopy
11 feet
7 feet
Figure 16. Illustration of leakage from
top of hood (Hemeon, 1955).
hood, a means of determining the amount
of the leakage and the increase in the
volume required to eliminate the leakage
is necessary. Hemeon (1955) has devel-
oped an equation to determine the volume
of leakage from a sharp-edge orifice in
a hood at a point above the hood face.
16
-------
Hood Design
200
(21)
where
v = the velocity of escape through
orifices in the upper portions
of a hood, fpm
1 = the vertical distance above the
hood face to the location of the
orifice, ft
qc = the rate at which heat is trans-
ferred to the air in the hood
from the hot source, Btu/min
A = the area of the orifice, ft
tm = the average temperature of the
air inside the hood, °F.
A small amount of leakage can often be
tolerated; however, if the emissions are
toxic or malodorous, the leakage must be
prevented completely. If all the cracks
or openings in the upper portion of the
hood cannot be eliminated, the volume
vented from the hood must be increased so
that the minimum indraft velocity through
all openings including the hood face is in
excess of the escape velocity through the
orifice calculated by means of equation
21. The value of q may be determined by
using the appropriate heat transfer
coefficient from Table 5 together with
equation 15 or by any other appropriate
means. This method is illustrated in
example 10.
Example 10
Given:
contaminated air through the upper openings
by assuming all openings are sharp-edge
ori f i ces.
Solution1
The rate of heat generation:
. 30^1 x 140,000^ x _!_
= 70,000
Total open area:
\ = (20 x 7)
Btu
min
= 141 ft
The escape velocity through the leakage
ori f i ce :
1/3
/ (11) (70, OOP) V/3 =
^ 141 (460 + isoy
v = 200 (11) (70
141 (460
42° fPm
The required exhaust rate:
Vt ' veAo
(420) (141)
= 59,000 cfm
Check mean hood air temperature:
Since q =
Where
p c fit:
Several oil-fired crucible furnaces are
hooded and vented as illustrated in Figure
16. The enclosure is 20 feet long. It is
not possible to prevent leakage at the top
of the enclosure. Total area of the leakage
openings is 1 square foot. The fuel rate
is 30 gallons per hour and the heating
value is 140,000 Btu per gallon. Assume
80°F ambient air and 1SO°F average
temperature of gases in the hood.
Problem:
Determine the minimum face velocity and total
exhaust rate required to prevent leakage of
p = average density of mixture, 0.075
lb/ft3
c = average specific heat of mixture,
p 0.24 Btu/lb per °F.
At = average hood temperature minus
ambient air temperature.
70.000 o_
(59,000)(0.07S)(0.24) " °° f
= 80 + 66
146°F
17
-------
Hood Design
This adequately approximates the original
assumption.
HOOD CONSTRUCTION
If air temperature and corrosion problems
are not severe, hoods are usually con-
structed of galvanized sheet metal. As
with elbows and transitions, the metal
should be at least 2 gauges heavier than
the connecting duct. Reinforcement with
angle iron and other devices is required
except for very small hoods.
High-Temperature Materials
For elevated temperatures up to approxi-
mately 900°F, black iron may be employed,
the thickness of the metal being increased
in proportion to the temperature. For
temperatures in the range of 400 to 500°F,
10-gauge metal is most commonly employed.
When the temperature of the hood is as
high as 900°F, the thickness of the metal
may be increased up to 1/4 inch. Over
900°F, up to about 1,600 to 1,800°F,
stainless steel must be employed. If the
hood temperature periodically exceeds 1,800°F
or is in excess of 1,600°F for a substantial
amount of the time, refractory materials
are required.
Corrosion-Resistant Materials
A variety of materials are available for
corrosive conditions. Plywood is some-
times employed for relatively light duty
or for temporary installations. A rubber
or plastic coating may sometimes be applied
on steel. Some of these coatings can be
applied like ordinary paint. If severe
corrosion problems exist, hoods must be
constructed of sheets of PVC (polyvinyl
chloride), fiberglass, or transite.
Design Proportions
Although the items of primary importance in
designing hoods are the size, shape, and
location of the hood face, and the exhaust
rate, the depth of the hood and the
transition to the connecting duct must also
be considered. A hood that is too shallow is
nothing more than a flanged-duct opening.
On the other hand, excessive depth increases
the cost without serving a useful purpose.
Transition to Exhaust Duct
It is desirable to have a transition piece
between the hood and the exhaust system
ductwork that is cone shaped with an
included angle of 60° or less. This can
often be made a part of the hood itself.
The exact shape of the transition is the most
important factor in determining the hood
orifice losses. Examples of good practice
in this regard are illustrated in Figure 17
POURING STATION FOR SMALL MOLDS
Transition L,I
-^J^jfr
^~~
Top baffle
5/3 b
Mm
-i — u
b
Mold
'Conveyor
Clearance space
V = 200(10X£ + A)
where V = minimum ventilation rate,cfm
X = distance between hood and ladle, ft
A = face area of hood, ft*
ENCLOSURE FOR FOUNDRY SHAKEOUT
45° Mm
Mold
conveyor
Shakeout
Provide a minimum indraft of 200 cfm per square
foot of opening but not less than 200 cfm per
square foot of grate area for hot castings
Figure 17. Examples of good hood design.
Note use of enclosure, flanges, and transi-
tions (Industrial Ventilation, 1960)
18
-------
FAN DESIGN
Edwin J. Vincent
Lewis K. Smith
Introduction
Fans are used to move air from one point to
another. In the control of air pollution,
the fan, blower, or exhaust imparts move-
ment to an air mass and conveys the air
contaminants from the source of generation
to a control device in which the air con-
taminants are separated and collected,
allowing cleaned air to be exhausted into
the atmosphere.
Fans are divided into two main classifica-
tions: (1) radial-flow or centrifugal type,
in which the airflow is at right angles to
the axis of rotation of the rotor, and (2)
axial-flow or propellor type, in which the
airflow is parallel to the axis of rotation
of the rotor.
CENTRIFUGAL FANS
A centrifugal fan consists of a wheel or
rotor mounted on a shaft that rotates in a
scroll-shaped housing. Air enters at the
eye of the rotor, makes a right-angle turn,
and is forced through the blades of the
rotor by centrifugal force into the scroll-
shaped housing. The centrifugal force
imparts static pressure to the air. The
diverging shape of the scroll also converts
a portion of the velocity pressure into
static pressure.
Centrifugal fans may be divided into three
main classifications as follows:
1. Forward -curved blade
type.
, blad
The rotor
of the forward-curved blade fan is known
as the squirrel-cage rotor. A solid
steel backplate holds one end of the
blade, and a shroud ring supports the
other end. The blades are shallow with
the leading edge curved towards the
direction of rotation. The usual number
of blades is 20 to 64.
2. Backward-curved-blade type. In the back-
ward-curved-blade fan, the blades are
inclined in a direction opposite to the
direction of rotation, and the blades
are larger than those of the forward-
curved-blade fan. The usual number of
blades is 14 to 24, and they are support-
ed by a solid steel backplate and shroud
ring.
3. Straight blade type. The blades of the
straight-blade fan may be attached to
the rotor by a solid steel backplate or
a spider built up from the hub. The
rotors are of comparatively large diame-
ter. The usual number of blades is 5 to
12. This classification includes a num-
ber of modified designs whose character-
istics are, in part, similar to those of
the forward- and backward-curved blade
types.
AXIAL-FLOW FANS
Axial-flow fans include all those wherein
the air flows through the impellers subs-
tantially parallel with the shaft upon which
the impeller is mounted. Axial-flow fans
depend upon the action of the revolving air-
foil-type blades to pull the air in by the
leading edge and discharge it from the
trailing edge in a helical pattern of flow.
Stationary vanes may be installed on the
suction side or the discharge side of the
rotor, or both. These vanes convert the
centrifugal force and the helical-flow pat-
tern to static pressure.
Axial fans may be divided into three main
classifications:
1.
E.J. Vincent is an Intermediate Air Pollution Engi
L.A.C.A.P.C.D.
neer,
L.K. Smith is an Air Pollution Engineer with N.A.P.C.A.
Propellor type. Propellor fans have
large, disc-like blades or narrow,
airfoil-type blades. The number of
blades is 2 to 16. The propellor fan
blades may be mounted on a large or
small hub, depending upon the use of
the fan. The propellor fan is distin-
guished from the tube-axial and
vane-axial fans in that it is equipped
with a mounting ring only.
Tube-axial type. The tube-axial fan
is similar to the propeller fan except
it is mounted in a tube or cylinder.
It is more efficient than the propeller
fan and, depending upon the design of
the rotor and hub, may develop medium
pressures. A two-stage, tube-axial
fan, with one rotor revolving clock-
wise, will recover a large portion of
the centrifugal force as static
pressure, which would otherwise be
lost in turbulence. Two-stage, tube-
axial fans approach vane-axial fans in
efficiency.
PA.C.ge.36.12.70
-------
Fan Design
3. Vane-axial type. The vane-axial fan
is similar in design to a tube-axial
fan except that air-straightening
vanes are installed on the suction
side or discharge side of the rotor.
Vane-axial fans are readily adaptable
to multistaging, and fans have been
designed that will operate- at a
, pressure of 16 inches water column
at high volume and efficiency.
FAN CHARACTERISTICS
The performance of a fan is characterized
by the volume of gas flow, the pressure at
which this flow is produced, the'speed of
rotation, the power required, and the
efficiency. The relationships of these
quantities are measured by the fan manufac-
turer with .testing methods sponsored by
the National Association of Fan Manu-
facturers or the American Society of
Mechanical Engineers. Briefly, the method
consists of mounting a duct on the fan '
outlet, operating the fan with various
sized orifices in the duct, and measuring
the volume, pressure, velocity, and power
input. About 10 tests are run, with the
duct opening varied from wide open to
completely closed. The test results are
then plotted against volume on the abscissa
to provide the characteristic -curves of the
fan, such as those shown in'Figure 25.
20 40 60 80 100
Percent of wide open volume
Figure 25. Typical fan characteristic
curves (Air Moving and Conditioning Assn.,
Inc., 1963] . '
From the volume and pressure, the air
horsepower is computed, either the real
power based on total pressure or the
fictitious static power based on fan static
pressure. The efficiency based on total
pressure is called mechanical efficiency.
INFLUENCE OF BLADE SHAPE
The size, shape, and number of blades in a
centrifugal fan have a considerable in-
fluence on the operating characteristics
of the fan. The general effects are in-
dicated by the curves in Figure 26.
VOLUME
VOLUME
VOLUME
Figure 26. Centrifugal fan typical
.. characteristic curves (Hicks, 1951).
These curves are shown for comparison
purposes only; they are not applicable for
fan selection but do indicate variations
in the operating characteristics of a
specific type of fan.
1. Forward-Curved-Blade Fans. This type of
fan is normally referred to as a volume
fan. In this fan, the static pressure
rises sharply from free delivery to a
point at approximate maximum efficiency,
then drops to point a_ shown in Figure
26, before rising to static pressure at
no delivery. Horsepower input rises
-------
Fan Design
rapidly from no delivery to free
delivery. Sound level is least at
maximum efficiency and greatest at free
delivery. Forward-curved-blade fans
are designed to handle large volumes of
air at low pressures. They rotate at
relatively low speeds, which results in
quiet operation. Initial cost of such
a fan is low.
Resistance of a system to be served by
this type of fan must be constant and
must be determined accurately in ad-
vance because of sharply rising power
demand. This type of fan should not
be used for gases containing dusts or
fumes because deposits will accumulate
on the short curved blades resulting in
an unbalanced wheel and excessive
maintenance. The pressure produced by a
forward-curved-blade fan is not normally
sufficient to meet the pressure require-
ments for the majority of air pollution
control devices. They are, however,
used extensively in heating, ventilating,
and air conditioning work. Also, they
are commonly used for exhausting air from
one enclosed space to another without the
use of ductwork.
2. Backward-Curved Blade Fans. The static
pressure of this type fan rises sharply
from free delivery almost to the point of
no delivery. Maximum efficiencies occur
at maximum horsepower input. The
horsepower requirement is self-limiting;
it rises to a maximum as the capacity
increases and then decreases with
additional capacity. Thus, when the
resistance of a complex exhaust system
is frequently changed because of pro-
duction demands, the self-limiting
power requirements prevent overloading
the motor.
This type of fan develops higher
pressure than the forward-curved-blade
type. Sound level is least at maximum
efficiency and increases slightly at
free delivery. The physical sizes of
backward-curved-blade fans for given
duties are large, but for most in-
dustrial work this may be unimportant.
The operating efficiency is high, but
initial cost is also high. Blade shape
is conducive to buildup of material and
should not be used on gases containing
dusts or fumes.
The backward-curved-multiblade fan is
used extensively in heating, ventilating,
and air conditioning work and for
continuous service where a large volume
of air is to be handled. It is commonly
found on forced-draft combustion
processes. It may be used on some air
pollution control devices, but must be
installed on the clean air discharge as
an induced system.
3. Straight-Blade Fans. The static pressure
of this type fan rises sharply from free
delivery to a maximum point near no
delivery, where it falls off. Maximum
static efficiency occurs near maximum
pressure. Mechanical efficiency rises
rapidly from no delivery to a maximum
near maximum pressure, then drops slowly
as the fan capacity approaches free
delivery.
This type fan is utilized for exhaust
systems handling gas streams that are
contaminated with dusts and fumes.
Various blades and scroll designs have
been developed for specific dust-
handling and pneumatic-conveying
problems. This fan is too large for
some duties, but for most industrial
work this may be unimportant. Initial
cost of this type fan is less than that
of the backward-curved-blade type, but
efficiency is also less. Fan blades
may be made of an abrasive resistant
alloy or covered with rubber to prevent
high maintenance in systems handling
abrasive or corrosive materials.
A number of modified designs of straight-
blade fans have been specifically
developed for handling contaminated
air or gas streams.
4. Axial Fans. For this type fan, the
horsepower curve may be essentially
flat and self-limiting, depending
upon the design of the blades, or it
may fall from a maximum at no delivery
as capacity increases. The type of
vanes in the vane-axial fans measurably
affects the horsepower curve and
efficiency. Maximum efficiencies occur
at a higher percent delivery than with
the centrifugal-type fan.
Space requirements for a specific fan
duty are exceptionally low. Available
fans can be installed directly in
circular ducts (vane-axial or tube-
axial type). Initial cost of the fan
is low.
-------
Fan Design
The axial-type fan is best adapted for
handling large volumes of air against
low resistance. The propeller type,
which is equipped only with a mounting
ring, is commonly used for ventilation
and is mounted directly in a wall.
Although the vane-axial and tube-axial
fans can deliver large volumes of air
at relatively high resistances, they are
best suited for handling clean air only.
Any solid material in the air being
handled causes rapid erosion of
impellers, guide vanes, hubs, and the
inner wall of the cylindrical fan housing.
This results from the high tip speed of
the fan and the high air velocity through
the fan housing.
Geometrically Similar Fans
Fan manufacturers customarily produce a series
of fans characterized by constant ratios of
linear dimensions and constant angles
between various fan parts. These fans are
said to be geometrically similar or of a
homologous series The drawings of all
the fans in the series are identical in
all views except for scale.
It is usual for a manufacturer to produce
homologous series of fans with diameters
increasing by a factor of about 1.10. Each
is designated bv the impeller diameter or
by an arbitrary symbol, often a number
proportional to the diameter.
Multirating Tables
The performance of each fan in a homologous
series is usually given in a series of
tables called multirating tables. Values of
static pressure are usually arranged as
headings of columns, which contain the fan
speed and horsepower required to produce
various volume flows. The point of maxi-
mum efficiency at each static pressure is
usually indicated.
FAN LAWS
Certain relationships have been established
among the variables affecting the per-
formance of fans of a homologous series or a
single fan operating at varying speed in a
constant system. The quantity, V , and
the power, p, are controlled by four inde-
pendent variables (1) Fan size, wheel
diameter, D, (2) fan speed, N, (3) gas
density, 0, and (4) system resistance,h
Since all dimensions of homologous fansr
are proportional, any dimension could be
used to designate the size. The wheel
diameter is, however, nearly always
used.
In order to develop these relationships,
the effects of system resistance must
be fixed by limiting the comparisons
to the same points of rating. For two
fans of different size, the same point
of rating is obtained when the respective
volumes are the same percentage of wide
open volume, and the static pressure is
the same percentage of shut-off static
pressure. For the same fan, the same
point of rating is obtained when the
system is held constant and the fan
speed is varied.
For homologous fans (or the same fan)
operating at the same point of rating,
the quantity (Vt) and the power (P) will
depend upon the fan size (D), fan speed
(N) , and gas density (p). The flow
through a fan is always in the turbulent
region, and the effect of viscosity is
ignored. The form of dependence can be
derived from dimensional analysis by the
equation*
k Da Nb CC
(27)
By substituting fundamental dimensional
units,
L t ' = L" t ' ml "
equating exponents for like terms,
m 0 = c
L- 3 = a - 3c
f -1 = -b
and solving the equations simultaneously
a=3;b=l, c=0
hence•
k D3N
(28)
Repeating for the system resistance
developed and noting that h is fundamentally
force per unit area = mass x acceleration
per area.
-------
Fan Design
mL'V2
m 1 = c
L -1 = a - 3c
t -2 = -b
a = 2; b = 2; c = 1
h, = kD2N2 p
(29)
And repeating again for the power required:
m 1 = c
L 2 = a - 3c
t -3 = -b
a = 5, b = 3;c
Equations 28, 29, and 30 for V , h , and P
define the relationships among allrthe
variables, within the limitations originally
stated. The equations can be simplified,
combined, or modified to yield a large
number of relationships. The following
relationships derived from them are
usually referred to as the Fan Laws .
1. Change in Fan Speed.
Fan size, gas density, and system
constant.
a. V varies as fan speed.
b. hr varies as fan speed squared.
c. P varies as fan speed cubed.
2. Change in Fan Size.
Fan speed and gas density constant.
a.
V varies as cube of wheel diameter.
b. h varies as square of wheel
diameter.
c. P varies as fifth power of wheel
diameter
d. Tip speed varies as wheel diameter.
3. Change in Fan Size.
Tip speed and gas density constant.
a. V varies as square of wheel
diameter.
b. h remains constant.
r
c. P varies as square of wheel
diameter.
d. rpm varies inversely as wheel
diameter.
4. Change in Gas Density.
System, fan speed, and fan size constant.
a. V is constant.
b. h varies as density
c. P varies as density.
5. Change in Gas Density.
Constant pressure and system, fixed
fan size, and variable fan speed.
a. V varies inversely as square
root of density.
b. Fan speed varies inversely as
square root of density.
c. P varies inversely as square root
of density.
6. Change in Gas Density.
Constant weight of gas, constant system,
fixed fan size, and variable fan speed.
a. V varies inversely as gas density.
b. h varies inversely as gas density.
c. Fan speed varies inversely as gas
d. P varies inversely as square gas
density.
The fan laws enable a manufacturer to
calculate the operating characteristics for
-------
Fan Design
all the fans in a homologous series from
test data obtained from a single fan in the
series. The laws also enable users of fans
to make many needed computations. A few of
the more important cases are illustrated
as follows.
Example 1
A fan operating 830 rpm delivers 8,000 cfm
at 6 inches static pressure and requires
11.5 horsepower. It is desired to increase
the output to 10,000 cfm in the same
system. What should be the increased speed
and what will be the horsepower required and
the new static pressure7
Solution*
Use fan law la , b , c
N' = 830
10,000
8,000
11.5
1.037
830
1,037 rpm
= 9.35 in. WC
= 22.4 hp
11,000
4,600
1/3
(30) = 40 0 in.
Selecting a Fan From Multirating Tables
A typical multirating table is given in Table
20 The data in this table are for a paddle
wheel-type industrial exhauster. In using
multirating tables, use linear interpolation
to find values between those given in the
table. For instance, from Table 20 it is
desired to find the fan speed that will
deliver 6,300 cfm at 6-1/2 inches static
pressure. The nearest capacities are 6,040
and 6,550.
At 6 in. h the speed is
1,088
6,300-6.040 (1,095 - 1,088)
6,550-6,040
1,092 rpm
At 7 in. h the speed is
1,160
6,300-6,040 (1.171 - 1,160) =
6,550-6,040 . .,,
1,167 rpm.
Example 2
A fan is exhausting 12,000 cfm of air at
600°F. (density = 0 0375 pound per cubic
foot at 4 inches static pressure from a
drier) Speed is 630 rpm, and 13 horsepower
is required. What will be the required
horsepower if air at 70<>F (density 0.075
pound per cubic foot) is pulled through the
system '
Solution
Use fan law 4 c
P1
13
0.075
0.0375
26 hp
If a IS-horsepower motor were used in this
installation, it would be necessary to
use a damper when starting up cold to prevent
overloading the fan motor.
Example 3
A 30-inch-diameter fan operating at 1,050
rpm delivers 4,600 cfm at 5 inches static
pressure. What size fan of the same series
would deliver 11,000 cfm at the same static
pressure7
Solution:
The required speed at 6-1/2 inches static
pressure and 6,300 cfm is halfwa> between
1,092 and 1,167 or 1,129 rpm.
CONSTRUCTION PROPERTIES
Special materials of construction must be
used for fans handling corrosive gases.
Certain alloys that have been used have
proved very satisfactory. Bronze alloys are
used for handling suIfuric acid fumes and
other sulfates, halogen acids, various
organic gases, and mercury compounds. These
alloys are particularly applicable to low-
temperature installations Stainless steel
is the most commonly used metal for
corrosion-resistant impellers and fan
housings It has proved satisfactory for
exhausting the fumes of many acids. Pro-
tective coatings on standard fan housing
and impellers such as bisonite, cadmium
plating, hot galvanizing, and rubber
covering have proved satisfactory. Cadmium
plating and hot galvanizing are often used
in conjunction with a zinc chromate primer,
with which they form a chemical bond. The
zinc chromate primer may then be covered
with various types of paints. This
combination has proved favorable in
atmospheres near the ocean.
Use fan law 2 a
The increasing use of rubber for coating
-------
Table 20 TYPICAL FAN MULTIRATING TABLE (New York Blower Company, 1948)
Volume,
cfm
2, 520
3, 120
3, 530
4.030
4.530
5.040
5.540
6,040
6. 550
7.060
7, 560
8.060
8.560
9,070
9,570
10, 080
10, 580
11, 100
11,600
12, 100
12.600
15. 120
Outlet
velocity,
fpm
1, 000
1, 200
1,400
1.600
1.800
2.000
2,200
2. 400
2.600
2,800
3,000
3.200
3.400
3,600
3,800
4,000
4,200
4.400
4,600
4,800
5,000
6,000
Velocity
pressure,
in. WC
0.063
0.090
0. 122
0.160
0.202
0.250
0.302
0.360
0.422
0.489
0.560
0.638
0.721
0.808
0.900
0.998
. 100
.210
.310
.450
. 570
2.230
1 in. SP
rpm
437
459
483
513
532
572
603
637
670
708
746
bhp
0.63
0.85
1.05
1.33
1.61
2.00
2.36
2.79
3.27
3.81
4.42
2 in. SP
rpm
595
610
626
642
666
688
712
746
762
795
833
866
900
bhp
1.27
1. 55
1.87
2. 18
2.56
2.97
3.43
3.99
4.62
5.32
6.05
6.96
7.93
3 in. SP
rpm
728
735
746
759
774
797
816
840
866
892
920
943
964
1.010
1.038
1.162
bhp
2.00
2.30
2.72
3. 17
3.63
4. 12
4.66
5.33
6.05
6.72
7.70
8.71
9.80
11.00
12.25
13. 6C
4 in. SP
rpm
837
842
847
858
876
890
910
926
954
963
993
1.020
1,053
1,078
1,108
1,138
1, 168
1,198
1,232
1,270
1.301
bhp
2.66
3. 10
3.57
4. 12
4.63
5.30
5.93
6.73
7.83
8.78
9.32
10.40
11.48
12.70
14. 15
15.40
16.90
18.58
20.30
21.00
24.20
5 in. SP
rpm
943
950
964
976
999
1,017
1,032
1,050
1,068
1,097
1, 120
1, 148
1, 170
1,200
1,230
1.258
1.290
1.321
1.355
bhp
4.60
5.21
5.82
6.50
7.38
8. 17
9.08
9.97
11.00
12. 10
13.30
14.65
14.90
17.35
19.05
20.55
22.50
24.40
26.40
6 in. SP
rpm
1,030
1.040
1.052
1.068
1,088
1,095
1, 125
1. 142
1. 168
1, 188
1,213
1,240
1,270
1,283
1,322
1,355
1, 383
1,410
bhp
6.29
6.92
7.75
8.60
9.50
10.50
11.60
12.75
14.02
15.35
16.70
18.80
19.70
21.50
22.50
23.80
25.65
28.80
7 in. SP
rpm
1, 125
1, 134
1, 145
1, 160
1, 171
1, 188
1,210
1,228
1,248
1,270
1,292
1, 320
1, 348
1,373
1,405
1,432
1.462
1.622
bhp
8. 18
8.96
9.93
10.88
11.98
13.06
14.28
15. 50
16.93
18.42
19.46
21.70
23.50
25.40
27.40
29.60
31.80
45.90
8 in. SP
rpm
1,208
1,210
1,230
1,245
1.257
1,277
1.292
1, 310
1,335
1,355
1.380
1,405
1,430
1.450
1.482
1.513
1.670
bhp
10. 15
11. 18
12.25
13. 50
14. 70
15.98
17.36
19.00
20.75
22. 35
23. 15
26. 10
27.95
30. 15
32.40
34.60
49.00
9 in. SP
rpm
1.270
1.279
1,288
1.298
1,310
1,328
1,340
1, 360
1,380
1,405
1,430
1,450
1,478
1,500
1, 528
1, 555
1,702
bhp
11.67
12.82
13.92
15. 10
16.48
17.80
19. 15
20.90
22.60
24.40
26.40
28.45
30.60
32.90
35.20
37.80
51.50
p>
3
•s
-------
Fan Design
fan impellers and housings deserves special
mention. Rubber is one of the least porous
materials and, when vulcanized to the metal,
surrounds and protects the metal front
corrosive gases or fumes. Depending upon
the particular application, soft, medium,
OT firm rubber is bonded to the metal.
A good bond will yield an adhesive strength
of 700 pounds per square inch. When pure,
live rubber is so bonded, it is capable of
withstanding the high stresses set up in
the fan and is sufficiently flexible to
resist cracking. Rubber-covered fans
have proved exceptionally durable and
are found throughout the chemical industry.
Heat Resistance
Standard construction fans with ball
bearings can withstand temperatures up to
250°F. Water-cooled bearings, shaft
coolers, and heat gaps permit operation
up to 800°F. A shaft cooler is a separate,
small, centrifugal fan that is mounted
between the fan housing and the inner
bearing and that circulates cool air over
the bearing and shaft A heat gap, which
is merely a space of 1-1/2 to 2 inches
between the bearing pedestal and fan
housing, reduces heat transfer to the
bearings by conduction.
Certain types of stainless steel will with-
stand the high temperatures encountered in
the induced-draft fan from furnaces or
combustion processes. Stainless steel fans
have been known to withstand temperatures
as high as 1,100°F without excess warping.
Explosive-Proof Fans and Motors
When an exhaust system is handling an
explosive mixture of air and gas or power,
a material to be used in the construction
of the fan must generally be specified
to be one that will not produce a spark
if accidentally struck by another metal
Normally, the fan impeller and housing
are constructed of bronze or aluminum
alloys, which precludes spark formation.
Aluminum is frequently used on some of the
narrower or smaller fans, especially
those overhung on the motor shaft.
Aluminum reduces the weight and vibration
of Che motor shaft and protects the motor
bearing from excessive wear.
Explosive-proof motors and fan whee Is are
required by law for installation in places
where an explosive mixture may be en-
countered. Exhaust systems such as those
used in paint spray booths usually consist
of an aluminum or bronze tube-axial fan
and an explosive-proof motor that drives
the fan wheel by indirect drive.
Fan Drives
411 types of fans may be obtained with
either direct drive or belt drive.
Directly driven exhausters offer the
advantage of a more compact assembly
and ensure constant fan speed. They are
not troubled by the belt slippage that
occurs when belt-driven fan drives are not
properly maintained. Fan speeds are,
however, limited to the available motor
speeds, which results in inflexibility ex-
cept in direct-current application. \
quick change in fan speed, *hich is
possible wittx belt-driven fans, is a
definite advantage in many applications.
-------
SECTION IX
MISCELLANEOUS
Economics of Pollution Control Systems
Control Methods for the Removal of Sulfur Oxides from Stack Gases
Techniques for Controlling the Oxides of Nitrogen
-------
ECONOMICS OF POLLUTION CONTROL SYSTEMS
A. H. Phelps* and G. F. Gall**
SO 2cannot
I INTRODUCTION
It would be desirable for your purposes to
have a tabulation of costs of control per cfm
of gas controlled or per pound of material
collected. This would only be possible if all
the many variables in a control installation
could be held constant except cfm. There
are many obvious variations which prevent
this tabulation having any general application.
Some devices cannot be compared on a
dollars per cfm basis because some devices
will not apply for some problems.
be controlled by combustion, for example,
or air containing great quantities of particu-
late or moisture could not be used in an
absorption system without extensive pre-
cleamng. Geographical factors will cause
variation due to different degrees of control
required, availability of fuel, water cost,
etc.
These variables can be held fairly constant
for each of you in your own area with a given
industry. Thus, it should be possible over
the years for you to build up a tabulation of
dollars per cfm for specific control problems
most common to your area. Consideration
of the factors in the cost of control systems
follows for your use in developing such a
tabulation for your own problems.
II COST FACTORS
A Equipment
The particular problem faced and the
particular industry will often dictate the
specific method used. A cost tabulation
for comparison between several alternate
methods will not be necessary when the
control is determined by the problem.
B Size
Obviously the cost of equipment increases
with the size and, in general, increases
with the volume handled. One factor used
for a rough estimate of the cost of one
size unit if the cost of a different sized
unit is known is the so-called six tenths
rule.
cost,
cost.
/size2
ysize
f)
0.6
This rule may be applied to the entire
cost of a system or just to the cost of
individual elements of a system. It is
not safe to use when the range of the
equipment sizes exceeds ten to one.
Common sense will often dictate that the
rule be ignored. For example, the cost
of a smoke stack does not increase at the
. 6 power of its height but obviously at
some much higher rate.
C Location
This applies to geographical factors men-
tioned in the introduction such as varying
degrees of control, varying availability
of water, fuel, etc. For a given city it
should not be a major factor. Although
even here, devices using great quantities
of scrubbing water, for example, might
be more desirable close to a river than
at the other end of town.
D Auxiliaries
1 Air moving equipment such as fans and
their drives, blowers, hoods, and
dampers.
2 Liquid moving equipment such as pumps,
vessels, agitators, piping and valves.
3 Instruments for measurement and/or
control of gas or liquid flow, tempera-
ture or pressure, operation and capa-
city, effectiveness (smoke meters,
analyzing devices).
A.H. Phelps, Engineering Department, Proctor and Gamble Company
G.F. Gall, Cost Estimating Department
PA.C.ge.20.12.65
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Economics of Pollution Control Sj stems
4 Collection media such as chemical
solutions and their attendant mixing,
supply, storage, rccirculation, etc.
E Piping
Liquid: approximately $2/in. diameter/
ft. purchased cost for carbon steel lines
with relatively few complex connections
or controls and around 2 to 4 in. diameter.
Installation cost would run SI. 15/in.
diameter/ft. The latter figure is less
accurate than the purchase cost.
Gas- ductwork, hoods, cabinets and other
sheet metal for the control of gas flows,
approximately 60£ per pound purchase,
60£ per pound installed.
F Electrical
Cost of starters, switches, wire conduit,
motor control centers, total at $400 per
horsepower.
G Requirement for Larger Existing
Equipment
The addition of equipment adding pressure
drop to a system or the addition of a
system requiring additional horsepower,
may often require the replacement of
motors, fans, pumps, ductwork, stacks,
and so forth if they are already operating
at maximum capacity. This incremental
load can sometimes be the final straw
that breaks the electrical capacity and
requires an additional substation or major
power supply to the plant. The same is
true for water supplies, cooling or heating,
steam lines and even sewerage. The
addition of control equipment may cither
require replacement of these overloaded
facilities or loss in productive capacity.
H Building and Equipment Structure
Space must be found for the installation of
this equipment, the site cleared, possibly
improved, possibly sheltered. New
structures may be required for support
and the problem of the last straw just
discussed may occur here. Thus, footings
may at last be o\ erloaded, support
steel within the building be overloaded,
and a distant location required or exten-
sive revision of existing buildings and
structures.
I Special Considerations
1 Special materials because of corrosion,
for high temperature, erosion or pro-
duct contamination may be required.
2 Insulation of equipment to prevent
freeze-up, loss of BTU's, or injury
to employees.
3 Weather considerations: wind, wind
loading, bracing, flood, earthquake,
supports.
J Class of Equipment
A maior production facility operated at
full rate will require heavj duty equip-
ment that will not require continuous
attention. Temporary installations,
marginal installations, low profit instal-
lations at minimum cost may utilize an
entirely different class of material with
an entirely different price rating.
K Other
This heading includes the category of
items often included in what estimators
call "other costs. " This would be sales
tax, if applicable, at 17o to 3%, engineering
of the installation and design; contractor's
overhead and fee; escalation at 5% per
year, if these figures were used for com-
parison from year to year for repetitive
installations; and occasionally research
necessary to develop the solution. These
will total about 60% to 70%.
Ill OPERATION COSTS
A Utilities
1 Electric power assumed at 1-1/2 cents
per kilowatt hour. Horsepower - $60
per horsepower year for 3 shift opera-
tion. Pump horsepower approximately
$200 per year per 100 GPM per 100 ft
-------
Economics of Pollution Control Systems
of head - 3 shift operation. Fan horse-
power $12 per 1000 cfm per inch of
H2O static pressure - 3 shift operation.
2 Water varies between 10 to 20 cents
per 1000 gallons. In some areas sewer
surcharges are based on water usage
and should be included in our operating
costs. The sewer taxes are in the same
order as water costs - about 100/1000
gallons, depending on the locality and
type of sewage treatment plant.
3 Steam approximately 50£/1000 pounds
or approximately two million BTU's
for a dollar.
4 Fuel costs for direct heating, excluding
cost of operating the equipment or
capital costs, etc., would be 25
-------
Economics of Pollution Control Systems
WATER SCRUBBER FOR 50,000 CFIU
Carbon steel equipment used throughout this system was for two 12 ft. diameter by
24 ft. high carbon steel packed scrubbers using water once through for cleaning.
The cost estimate is for an actual installation.
Item
Scrubbers
Supports for scrubbers (13 tons)
Duct and elbows
Supply pump (7000 GPM, 100 ft.
head 30 HP), pump foundation,
flow meter, and back pressure
control valve
Construction Subtotal
Engineering
Subtotal
Plant supervision
Total
Composite index*1' for construction
cost when this unit was built was
156.1.
Present index is 174.0.
174.0
Material
Labor
96,000 X
156.1
(1) The composite index is a private construction index,
but one similar to commercially available indices.
Total
13,200
9,900
8,500
36,400
68,000
21.000
89,000
7,000
96,000
107,000
(May, 1065)
-------
Economics of Pollution Control Systems
ACID SCRUBBER FOR 5,000 CFM
An absorption system involving scrubbing of dimethylanune from 5,000 cfm of air
using glass lined 18" diameter glassed absorption column, packed. Equipment in-
cludes the acid scrubber recirculation pump, acid scrubber cooler, mixing tee for
scrubber neutralization, acid blow drum, caustic delivery pump, and caustic stor-
age tank. This cost estimate for an actual installation.
Item
Material
Labor
Total
Site clearance
Yard and underground
Building
Equipment Structures
Equipment
Piping and insulation
Electrical
Subtotal
Inspection and controlled items
Sales tax
Premium time
Maintaining production
Indirect personnel
Field office and fee
Subtotal
Engineering
Subtotal
Contingency
Total Cost
9,180
44,000
(May, 1965)
-------
Economics of Pollution Control Systems
CATALYTIC FURNACE FOR 50,000 CFM
An air preheater to bring exhaust gases up to about 600°F. Furnace size at about
15 million BTU/hour with not very much insulation or refractory, thus, lightweight.
Replace existing fans for increased pressure drop. This cost estimate based on
costs from similar equipment.
200
2,400
8,500
83,000
1,000
12,000
107,100
10,900
2,000
8,000
5,000
14,000
3,000
2,000
34,000
4,000
2,200
10,400
13,500
97,000
4,000
14,000
141, 100
14,900
Item Material Labor Total
Site clearance
Yard and underground
Building and equipment structures
Equipment
Piping
Electrical
Totals
Unlisted items
Subtotal 118,000 38,000 156,000
Inspection and controlled items 2,000
Sales tax 3-l/27o of material 4,000
Indirect personnel
Field office and fee 38,000
Subtotal 200,000
Engineering 38,000
Subtotal 2J8.000
Contingency plus escalation 24,000
Total Cost 262,000
(Maj, 1965)
-------
Economics of Pollution Control Systems
CATALYTIC FURNACE 5,000 CFM
Air preheater to 600°F. Furnace about 1.5 million BTU/hour. Not too much
insulation or refractory, new blower and ductwork. This cost estimate prepared
for management's consideration.
Item Material Labor Total
Site clearance 700
Building and equipment structures 2, 220 3, 560
Equipment 11,500 3,595
Electrical 250 750
Piping 500 1,400
Subtotal 14,470 10,005 24,475
Inspection and controlled items 500
Sales tax 350
Indirect personnel
Field office and fee 10,000
Subtotal
Engineering
Subtotal 42,325
Contingency at 5% 2,000
Total Cost 44, 325
(May, 1965)
-------
Economics of Pollution Control Systems
COMBUSTION OF MATERIAL IN A FURNACE FOR 50,000 CFiM
Heat 50,000 cfm to 1, 300°F for complete combustion of material. 'Use hcai ex-
changers to preheat incoming gas and discharge from stack at 600°F. High tem-
perature furnace will require much brick work about 105 ton installation requiring
new fans, motors and drives. This cost estimate prepared for management's
consideration.
Item Material Labor Total
Site clearance 200 2,000 2, 200
Yard and underground 2,400 8,000 10,400
Building and equipment structures 8,500 5,000 13,500
Equipment 239,000 42,000 281,000
Piping 1,000 3,000 4,000
Electrical 12,000 2, OOP 14,000
Subtotal 263,100 62,000 323,100
Unlisted items 26,300 6,200 32,500
Subtotal 289,400 68,200 357,600
Inspection and controlled items 2,000
Sales tax 3-l/2To of material 600
Indirect personnel
Field office and fee 68, 000
Subtotal 4^8,200
Engineering 87,000
Subtotal 513, 200
Contingency plus escalation 30,000
Total 545,200
(May, 1965)
-------
Economics of Pollution Control Systems
COMBUSTION 5, 000 CFM
ADSORPTION SYSTEM
Heat 5,000 cfm to 1300°F to destroy small
amounts of material. This requires about
5 million BTU/hour. For such a small fur-
nace no heat recovery would be likely. 5
million BTU/hour is approximately the size
of a small package boiler for 5,000 pounds
of steam an hour. Package furnaces can be
bought for something in the range of $10.00
per pound of steam per hour. Thus, no close
estimate was made of this particular process.
If we were to have such an application we
would buy a package boiler, vent the gas to
the boiler for combustion and use the steam
somewhere in the plant as a recovery method
for the cost of operating the pollution control
system.
Thus, the cost of this is about $50,000
(May, 1965).
These systems were not estimated in great
detail but costs of them were derived from
vendors charts. This is a basic system of
steel construction, air cooler, blower, ad-
sorber, recovery system with condenser and
decanting tank sized at the assumption of 5
gallons/hour or organic material per 1,000
SCFM of air.
The unit for 5, 000 cfm was estimated at
$70,000. If a pre-cleanmg scrubber were
required for participate or aerosols which
might plug the carbon beds then add $20, 000
for that, giving a total of $90, 000.
The unit for 50,000 cfm would cost approxi-
mately $330,000 and if a preabsorbing scrub
ber were needed on this one for the same
reasons, it would cost approximately $100,000
as per our initial estimate on the 50,000 cfm
water scrubber, thus bringing the total of
$430,000 (May, 1965).
-------
Economics of Pollution Control Systems
TYPICAL COST BUILD-UP OF AN AIR POLLUTION CONTROL DEVICE
50,000 CFM SIZE
Horizontal Array
Assume a package which sits on a foundation
independent of the process equipment and
supports and tied in with simple duct con-
nections and auxiliaries.
Vertical Array
Difficult job. Same equipment installed
among the process equipment. Requires
structural support above the building and
relocation of existing materials to fit into
the process.
Carbon Steel Equipment
100 Base
+ 10
2
5
2
1
11
20
10
16
2
4
0
0
11
22
40
10
100 + ICC = 2GG
-------
CONTROL METHODS FOR THE REMOVAL
OF SULFUR OXIDES FROM STACK GASES
J. E Sickles, II
J.lv. Sullivan
I INTRODUCTION
The control of oxides of sulfur is a problem
that is world-wide. Strict antipollution
laws are being considered in various nations
of Europe, in Japan, and in the United States.
It has been suggested by Japan's Living
Environments Council, an advisory body of the
Ministry of Health and Welfare, that the
maximum allowable concentration for S0_ in the
atmosphere be set at 0.05 ppm. Levels of
0.15 ppm. as a 24 hour average and 0.29 ppm.
as a half-hour average have been set in West
Germany. In France the Ministry of Industries
has fixed the allowable level of atmospheric
S02 at 0.39 ppm. (1) The U.S. Public Health
Service has recommended that the S0_ concentra-
tion for a 24 hour period not exceed 0.1 ppm
more than 1% of the time (2).
It has been estimated that 29 million tons
of SO. were emitted into the atmosphere
within the United States in 1967. If the
present growth rate continues and control of
S0_ is not enforced, between now and the year
1990, the U.S. will have emitted the
equivalent of 1/2 trillion tons of sulfur
in the form of S02 into the atmosphere (3).
For example, a 1000 mw. plant emits 600 tons
of S02 per day or the equivalent of 300 tons
per day of sulfur or enough sulfur for a
production of 900 tons per day of sulfuric
acid. If all the S02 emitted in the U.S.
were converted into sulfuric acid, the amount
produced (24 million tons) would be con-
siderably more than the present U.S. con-
sumption (22 million tons) (4). If only one
third of the total emission were recovered,
this would satisfy about two thirds of the
demand for sulfur by the fertilizer industry
(5).
The largest single source of S02 is the com-
bustion of fossil fuels for power generation
which contributes 46% of the total. Com-
bustion of fossil fuels for other uses con-
tributes another 32%, giving a total of 78%
of the emissions coming from fossil fuel
combustion. The remaining 22% of the emissions
J.E. Sickles, II is a Chemical Engineer,
Institute for Air Pollution Training,
National Air Pollution Control Administration
J.W. Sullivan is an Air Pollution Control
Engineer, Kentucky Air Pollution
Control Coramision
comes from ore smelting (12%) , petroleum
operations (5.5%), and other miscellaneous
sources (4.6%) such as coke processing,
sulfuric acid manufacturing, paper mills,
coal refuse banks, incinerators and others
(3).
There are three general methods of reducing
the amount of S02 released into the
atmosphere: desulfurization of fuels, process
modification, and flue gas desulfurization.
This paper will deal with the present
status of techniques for the removal of
sulfur oxides from flue gas. This is an
area that is receiving considerable
attention at the present time, particularly
from the power industry. Their problem
is unique because of the low S02 concentra-
tions in the tremendous volumes of their
flue gas emissions. A power plant using
a 3.5% sulfur coal will have a SO2
concentration in the stack gas of about
0.2 to 0.25% by volume (4).
Most industrial sources such as ore
smelting, refinery operations, and kraft
paper mills emit concentrations which
are relatively high when compared to
combustion processes and in many cases may
be treated using a standard sulfuric acid
plant. For example, in the ore smeltering
industry if the SO. concentration in the
gas stream is above 3% by volume then it
can usually be fed directly to an acid
manufacturing plant. In the United States,
17 (which handle 42% of the ore concentra-
tions) out of 35 sulfide ore smelters
are recovering SO- or sulfuric acid (6).
The scope of this paper will be further
limited to the area of methods for
removing the low concentrations of sulfur
oxides found in stack gases from fuel
combustion. Many of these methods could
be applied in other areas where very low
concentrations of sulfur oxides are emitted
and where there is a necessity for control
of these emissions Most of the current
work in this area of SO- control is aimed
at the power industry.
Nuclear power looms in the future as the
probable replacement for fossil fuel-
burning power plants. The National Coal
Association indicates that coal prices
PA.C.ge.31.12.70
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
can absorb pollution control costs of $1
per ton and still remain competitive with
nuclear power (5). The economics of
the two governs which will be used. Any
additional expense such as that for
pollution control levied on the fossil-fuel
utilities pushes them a step closer to
obsolescence in the shadow of nuclear
power.
Table 1 illustrates the projected increase
in fossil fuel consumption to the year
2000. If one considers fuel oil and
coal to have a 1 and 3% sulfur content,
respectively, then the burgeoning threat of
sulfur oxides to the atmosphere becomes
more evident.
the trade name of CAT-OX (8). The only
successful marketing venture to date has
announced in July 1970. A demonstration
unit is to be installed at the Illinois
Power Wood River generating station on a
100,000 kw. generating unit. The $6.8
million project is to be operational within
two years (9).
The process involves passing the hot
flue gas at 950 F from the boiler through
a high temperature electrostatic precipitator
to remove 99.5% of the particulates. The
gas is then taken through a catalytic
converter (using a catalyst such as vanadium
pentoxide) where the S02 is converted to
Table I—Anticipated Power Plant Coal and Oil Consumption (7)
1966
1970
1975
1980
1990
2000
coal xlO tons
267
375
550
750
875
800
oil barrels xlO
142
175
225
240
250
230
II CONTROL METHODS
Numerous methods for control of the oxides
of sulfur have been proposed and many have
reached the pilot plant stage or have shown
considerable promise. The majority are
expensive and few have been tried on large
plants. They generally use either an
absorption or adsorption technique followed
by some means of separating the sorbent
from the flue gas for regeneration. One
exception is the catalytic oxidation
technique.
A. Catalytic Oxidation
1. CAT-OX. The most prominent process in
the area of catalytic oxidation was
developed by the Monsanto Company
in conjunction with the Pennsylvania
Electric Company, Air Preheater Company,
and Research-Cottrell. A small pilot
plant was constructed at Pennsylvania
Electric's Seward generating station.
This pilot plant proved successful, and
Monsanto Company and Metropolitan
Edison Company constructed a prototype
plant at Portland, Pennsylvania. This
plant is presently in operation.
Monsanto is trying to market this process,
which is illustrated in Figure 1, under
SOs. The gas is then passed through the
boiler economizer and air preheater for heat
recovery. The S03 and water vapor present
in the gas stream are combined forming
sulfuric acid vapor at a temperature still
above the dew point for the acid. The gas
is then passed through an absorption tower
(using sulfuric acid as the coolant) where
the sulfuric acid vapor is condensed. Any
acid mist which escapes from the tower is
captured by a high efficiency mist eliminator
(8).
The process has been able to remove all the
fly ash, 90*. of the S02, and 99.5% of the
sulfuric acid produced It has produced
a sulfuric acid with an average concentra-
tion of 80% (8) .
Monsanto estimates a capital cost of $25 per
kw. above that for a new power station
for the unit. They estimate that the
breakeven point on a 1000 mw. unit operating
at 80% load and burning 3% sulfur coal
would be reached if they received a netback
price of $13.50 per ton for 100% acid. If
5% sulfur coal were used an acid netback
price of $8 per ton would have to be
realized (8).
The process has several advantages•
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
High Efficiency
Electrostatic
Preci pita tor
*• 900°-*
r
950°
t
Hot Flue Gas
from Boiler
Catalytic
Converter
Gas to
Stack
. 650° -*
Air
Preheater
U4500.*
Figure 1. Cat-Ox
1. It is relatively simple.
2. No recycling step is involved.
3. The flue gas leaves the stack at a
temperature high enough to maintain
buoyancy.
4. A useful by-product is produced.
5. All the necessary raw materials for the
by-product are available in the flue
gas.
6. Converter bed catalyst can be cleaned
without requiring converter shut down.
The process also has several disadvantages•
1. Corrosion-resistant materials are
required in all portions of the equipment
where the temperature falls below 500°F.
2. The flue gas entering the catalytic
convertor must be at a temperature near
8500F. This requires the insertion of
the unit ahead of the boiler economizer,
thus making the process very difficult to
apply to existing units.
3. The catalyst is very succeptible to
poisoning by particles which escape the
electrostatic precipitator.
4. Although no loss in catalyst activity
has been observed during catalyst
cleaning, a 2.5% mass loss of catalyst
is observed.
2. KIYOURA-TIT: A similar process, the
Kiyoura-TIT process, has been tested at
the Toyo Koatsu Fertilizer Plant in
Omuta, Japan. This project has been
installed since the two 0.15 mw. test
units were stopped in 1967 (1). The
process is illustrated in Figure 2. In
this process ammonia is injected into
the gas stream just before it enters
the air preheater. It reacts with the
SOj to form ammonium sulfate. The
ammonia must be injected at this point
to avoid condensation of sulfuric acid.
The solid paniculate, ammonium sulfate,
is then collected with an electrostatic
precipitator or with bag filters and is
ready for shipment as fertilizer (4) .
For an 800 mw. power plant the capital
cost is estimated to be $11.10 per kw.
A credit of $32 per ton of ammonium
sulfate gives an operating cost of $0.35
per ton of coal (10).
The process has most of the advantages
and disadvantages of the CAT-OX process
with the added advantage of eliminating
acid corrosion. The by-product, however,
does not have much of a demand in the
United States (4).
B. Char as an Adsorbent
There are three major processes which
have been developed using activated char
as an adsorbent. They all have the
Ammonia
Hot Flue Gas
from BoilerH
Dust Collector
Gas to Stack
Electrostatic
Precipitator
Ammonium
Sulfate
Figure 2. Kiyoura-Tit"
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
advantages of simple regeneration and
of sulfuric acid being obtained directly.
All three systems, however, require
large amounts of make-up carbon.
1. REINLUFT- The most prominent of these
processes is the Reinluft process
developed by Reinluft, Inc., Essen,
Germany. The process, for which
95% efficiency is claimed (6), is
presently being tested on two 10 mw.
units in the Ruhr Valley. Reinluft,
Inc. recently signed over its rights
to the process to Chemiebau-Zieren Co.
(1).
In the process, as illustrated in Figure
3, the flue gas is passed through an
The process is estimated to have a
capital cost of $17.80 per kw. for an
800 mw. power plant using 3% sulfur
coal, and an operating cost of S2.45
per ton of coal (6) .
The process has several advantages
(4,6):
1. A desirable by-product in the form
of concentrated sulfuric acid is
produced.
2. Cooling of the stack gas is not
excessive, thus it maintains
adequate buoyancy.
to Stack
Air t
4
r
lectrost
reel pita
LUQ.
210°
.
F
350° F
75i
i. u T 1 -
Coal-*.2S«— 1 I
o^ 99% of Flyash
T
Coal Ash
Figure
1 "
or
3
h
i-»
,
Adsorber
C02
210° F
Heat Exchanger!—
— * 50% SO-
f.
Activated Char
. Reinluft10
electrostatic precipitator for fly ash
removal and then into the adsorber. The
SC>2 is oxidized to 803 in the adsorber
and this 803 is then reacted with the
water vapor in the gas stream to form
sulfuric acid. The adsorbent is then
regenerated by passing an inert gas
stream such as nitrogen or carbon
dioxide through it at 750°F. The
sulfuric acid is reduced by the carbon
to S02- This reduction consumes part
of the carbon and renders the granules
porous, thus activating the carbon (4).
The concentrated 502 stream containing
up to 50% can then be passed on to a
standard sulfuric acid manufacturing plant
(10).
3. Carbon steel can be used in its
construction.
4. It is not necessary to incur the
high cost of activating the carbon.
The disadvantages are:
1. The system is prone to develop hot
spots or areas of uncontrollable
oxidation.
2. The recirculation of large amounts
of carbon is expensive.
3. The costs for make-up char are too
high. Peat coke, which is used as an
adsorbent, costs about $100 per
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
metric ton and 0.2 Ib. must be re-
placed for every pound of SO2 gas
obtained. Reinluft is trying a new
lignite coke which costs less ($25
per metric ton), has low attrition,
and maintains a SO- removal rate of
70 to 85% (1).
2. SULFACID: Another process which uses
char for an adsorbent is the Sulfacid
process developed by Lurgi Gesellschaft
ftlr Chemie und Huttenwesen mbH. in
Germany. A plant for treating 1.8
million cubic feet per hour of waste
gas from a sulfuric acid plant is
presently being built (1).
In the process, as illustrated in Figure
4, the dust free flue gas with S02
2. The acid concentration is low.
3. Acid resistant materials of con-
struction are required.
HITACHI: A process similar to the
Sulfacid process was developed by
Hitachi, Ltd., of Tokyo, Japan and is
presently being operated on a SO mw.
unit in Japan (1).
The process uses six gas-contacting towers
in a cyclic manner. A single tower goes
through a 30-hour cycle of adsorption with
uncooled gas, 10 hours of washing with no
gas, and 20 hours of drying. The stack
gas first passes through a wet tower and
then a dry one which removes the acid
mist. A weak sulfuric acid of 10 to 15%
Gas to Stack
Flue Gas from
Dust Collector
Sulfuric Acid
Figure 4. Sulfacid (Lurgi)'
concentrations from 0.5 to 1.5% is passed
through a cooling tower where weak sul-
furic acid acts as the coolant. The cool-
ed gas is then passed through an adsorp-
tion tower in which sulfuric acid is
formed in the pores of the carbon. The
acid is removed by water which is sprayed
in intermittently without interruption
of the gas flow. Although this acid is
weak, further concentration occurs in
the cooler (4), attaining up to 70% sul-
furic acid. When large volumes of gas
are handled or when dust is present in
large amounts, the maximum concentration
is 25 to 30%. The final concentration
depends on the inlet gas temperature (1).
The process has the advantage of being
simple. The disadvantages, however,
include (4):
1. The flue gas is cooled to between
140 and 160°F, thus limiting stack
gas buoyancy.
concentration is produced, while removing
90% of the sulfur oxides (11).
The process has the advantage of a tempera-
ture drop of only 55°F. The cost of the
elaborate damper system required to change
the flow from tower to tower and the cost
of concentrating the weak acid are, how-
ever, drawbacks (4).
C. Additive Injection
Sulfur oxides produced by burning coal
and oil can be reacted with the calcined
products of limestone or dolomite to
form removable calcium sulfur salts. The
principal chemical reaction is believed
to be:
CaCO.-MgC03 * S02 + 1/2 02 =
CaS04 + MgO + 2C02.
The effective reactions are MgCO, +
CaC03 • MgO + CaO + 2C02, 3
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
1.
MgO + CaO + 2S03 = MgS04 + CaSO^ and
MgO + CaO + 2SO,+ 02 = MgS04 * CaS04-
Iron oxide, which is present in most
dolomites, acts as a catalyst by speeding
the reaction of CaO and S02 to CaS04 and
by catalyzing the reaction of MgO and
S02 to MgS04 (12). Dolomite or limestone
injection can take two forms1 dry
removal of S02 by injection of the
additive into the gas stream, or wet
removal following the injection by
wet scrubbing (6).
TVA DRY LIMESTONE INJECTION- TVA has
been investigating a dry process and has
started an 18-month tune up campaign on
the process at its Shawnee Station in
Paducah, Kentucky. The goal of this study
will be to improve the removal of sulfur
oxides from the presently attainable 20-
35% to at least 50% (13). The flow diagram
for this process is illustrated in Figure
5.
For this process the limestone should be
ground as finely as is economically pos-
sible.
Preliminary work indicates that an econo-
mic optimum particle size may be as low
as 10 microns. These particles need to be
uniformly distributed in the 2200 to
2300°F temperature zone. The gas
temperature in the reaction zone is
between 1200 and 2300°F. The temperature
should be high enough to calcine the
limestone particles but not so high as
to cause them to "glass over" or sinter.
The concentration of S02 in the boiler-
reactor is about 0.3'o, and the contact
time here is from one to two seconds.
This short contact time in the temperature
zone is believed to have an adverse
effect on the process efficiency.
During this time the limestone must be
calcined, the S02 oxidized to SOj, the
SOj reacted with calcine, and this
diffused at least part of the way into
the solid to permit further reaction.
Sulfates, unreacted lime, and fly ash
Steam Superheaters
and Reheaters
Economizer
High
Efficiency
Precipitator
figure 5. TVA Dr> Injection Process"
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
are then removed by a mechanical dust
collector and an electrostatic pre-
cipitator. The remaining gases are emit-
ted via the stack (14).
Investment costs range from $3.95 per
kw. for a 1000 mw. unit to $7.75 per
kw. for a 200 mw. unit in existing plants.
Depending on the plant size, operating
costs are expected to vary from $1.03
to $1.39 per ton of coal burned (for
$2.05 per ton of limestone cost, 3.5%
sulfur in the coal, and 200% stoichio-
metric limestone addition) (14).
Some of the advantages of the dry process
include (14):
1. Simplicity of process.
2. Ease of operation.
3. Capability of responding to varying
load requirements.
4. Small capital investment and ease
of installation for existing plants.
5. Small direct effect on combustion
efficiency.
6. Reduction of boiler fireside tube
corrosion.
7. Control of sulfur oxides without
production of a saleable by-product.
Some of the disadvantages are-
1. Poor sulfur oxide removal even with
excess limestone.
2. Necessity to upgrade particle
collector for installation in
existing plants.
3 Reduction in boiler efficiency and
increase in operating and mainten-
ance costs due to increased slagging
and higher dust loading.
4. Sensitivity of operating and capital
costs to plant capacity.
5. Increase in the amount of solid
waste for disposal.
Although much work has been done by
TVA, NAPCA, ESSO, and many others on
the development of this system, much
more work is necessary to solve certain
engineering problems which would hinder
if not prevent the use of such a process
commercially.
Other dry injection processes have been
proposed. Descriptions of these techniques
will follow.
2. BABCOCK AND WILCOX - ESSO A new dry
injection flue gas desulfurization pro-
cess has been recently announced by
Babcock and Wilcox and Esso Research and
Engineering. Sixteen utility companies
will support a $7 million development
program. Pilot plant studies will be
conducted at a plant of Indiana Michigan
and Electric. The ultimate goal will be
a commercial system capable of removing
99% of the particulates and 90% of the
sulfur oxides from power plant flue gas
by 1973 (15).
In the process, as illustrated in Figure
6, SO2 is absorbed by a unique dry mat-
erial which is easily regenerated to
recover marketable suIfuric acid. This
process offers no stack gas buoyancy
problems and eliminates the waste disposal
problems that have beset other injection
processes. No cost data are currently
available (15).
3. DRY NAHCOLITfc INJECTION: Nahcolite is a
naturally occurring form of sodium bi-
carbonate. Precipitair Pollution Control,
Inc. has investigated and developed an
S02 removal process using Nahcolite in-
jection. The mineral is injected into
the flue gas stream at 300°F to 350°F
forming sodium sulfate which is then re-
moved along with the fly ash. Pilot plant
studies at Southern California Edison's
Alimitos plant have indicated 70% S02
removal and 100% S03 removal. Further
testing is planned at a U.S. Gypsum Plant
in Clark, N.J. Although the low-tempera-
ture reactivity is an asset enabling in-
jection outside the boiler, the main
problem for industrial application will
be the availability of Nahcolite at a
at a reasonable price (16).
4. MOLTEN IRON. Black, Sivalls, and Bryson,
a subsidiary of International Systems and
Controls, has announced a new S02 removal
process, as illustrated in Figure 7.
Pulverized coal, limestone, and air are
injected into the molten iron where the
coal is partially burned to carbon mon-
oxide. Elemental sulfur is released,
binds with the iron, and floats to the
surface forming a slag with limestone.
Elemental sulfur can be recovered from
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
Electrostatic
Precipitator
Regeneration
Gas Producer
to Stack
Figure 6. Babcock and Kilcox-Esso Process
15
Limestone-air
lance
Slag-iron
separator
Slag to
desulfurizer
Iron to
granulator
Metal /
shell
Refractory
Figure 7. Molten Iron Process
17
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
the slag. Combustion off gases are then
completely oxidized with secondary air
and proceed to a steam boiler (17). No
efficiency or cost data are presently
available.
5. FLUIDIZED BED: The Office of Coal Research
and NAPCA are sponsoring research on
fluidized bed, coal-fired boilers. Utili-
zation of this combustion technique with
the addition of crushed limestone into
the fluid bed has been proposed as another
route to controlling sulfur oxide emis-
sions. Before fluidized bed combustion
can become commercial, much basic re-
search is needed in the areas of additive
size, gas velocity, operating temperature,
and waste recycle and disposal (18).
6. COMBUSTION ENGINEERING WET PROCESS The
wet process combines the afore mentioned
dry injection process with wet scrubbing.
Combustion Engineering in cooperation with
Detroit Edison Company has developed
this process, which is illustrated in
Figure 8. There are two commercial instal-
lations using this process at present
and a third is slated to be operational
by 1971 (11).
In this process the pulverized additive
is injected into the furnace where 20 to
30% of the sulfur oxides is removed.
The remaining S02 and calcined additives
then pass through a preheater and into
a wet scrubber. The calcined additives
react with wash water in the scrubber
to form hydroxides. These hydroxides
react with the S02 and any unreacted SOj
to form slightly soluble sulfites and
sulfates. These salts, along with the
fly ash, which is 99% removed by the
scrubber, are sent to a clarifier and
to disposal (19, 20).
Although 98% removal of S02 is possible
under ideal conditions, 60 to 75%
removal has been reported for the two
commercial installations (using less
than stoichiometnc additive) along with
99% removal of fly ash and 100%
elimination of 503. There are also
indications that this process can remove
20-30% of the nitrogen oxide emissions
(19, 20).
Investment costs for an 800 mw. plant
are $2.22 per kw. (10). Gross operating
costs are $0.63 per ton of coal. When
total credits for reduced corrosion and
elimination of precipitator operating
costs are considered, a net operating
cost of $0.36 per ton of coal is realized
(5).
Many of the advantages and disadvantages
of the wet process are the same as have
been mentioned for the dry process.Some
additional advantages include (19)•
1. High S02-S03 removal.
2. Elimination of an electrostatic
precipitator.
3. Reduction of stack costs.
4. Indications of a 20% - 30% removal
of nitrogen oxides.
Some disadvantages are.
1. Increased solids for waste disposal.
2. Necessity to reheat stack gas to
maintain buoyancy.
7. CHEMICO-BASIC- The Chemico-Basic Corpor-
ation process uses a central recovery
plant approach, in which the central pro-
cessing plant recovers sulfur from the
spent absorbent of several power plant
boilers or other S02 emitting industrial
plants. The process is slated for full
commercial testing on a 150 mw. oil burn-
ing station of Boston Edison. A reduction
of S02 emissions of 90% is expected. The
spent absorbent is to be shipped to an
Essex Chemical Corp. plant to recover up
to 17 tons per day of sulfur as sulfuric
acid (21).
In the process, magnesium oxide absorbs
SO- in the flue gas forming magnesium
suffite, which is scrubbed from the gas
stream along with fly ash by a venturi
scrubber. The scrubbing medium is then
sent to the processing plant for removal
of fly ash, regeneration of the MgO for
recycle, and recovery of sulfur as ele-
mental sulfur, S0_, or sulfuric acid
(22). *
One advantage of the central processing
plant idea 13 that it removes the
utilities from the chemical business. A
potential problem arises, however, in
the form of complicated logistics.
D. Alkalized Alumina
-------
Control Methods for the Removal of Sulfur Oxides from Stack Cases,
COAL
SUPPLY
LIMESTONE
SUPPLY
MILL
FURNACE
TO STACK
250°F
f 600
AIR
HEATER
Op
AIR
DEMISTER
SCRUBBER
SETTLING
TANK
1 RECYCLE
AND
MAKE-UP
WATER
TO DISPOSAL
Figure 8. Combustion Engineering het Process5
BUREAU OF MINES The Bureau of Mines under
the sponsorship of the 0 S. Public Health
Service has selected and developed a can-
didate for large-scale sulfur oxide re-
moval Although developers claim 90°,
recovery of the S02 from the gas stream
(6), indications are that further work on
the process is being halted.
The absorbent medium is a co-precipitate
of sodium and aluminum oxides (with sod-
ium oxide making up 201, of the total) (4).
The absorbent is regenerated bj contacting
it with a reducing gas such as natural
gas, hydrogen, or producer gas. The fol-
lowing reactions are believed to occur
(23)
Na,0
Na_SO.
2 4
Na.SO,
£. 4
SO,
4H_
4CO + H20 = 4C02 +
II S
The flow diagram is illustrated in Figure
9. Flue gas from the boiler is cleaned
and passed at a temperature of about
625°F to the reactor where it flov.s
countercurrent to a stream of semi spherical
alkalized alumina pellets having a size
range of 10 to 14 mesh (23). This
material, essentially Na2Al2°4 °r
sodium aluminate, reacts with S02 and
oxygen to form a stable sulfate (10).
From the reactor the spent absorbent
passes into a regenerator where it is
contacted by a reducing gas at about
12000F. The alkalized alumina is re-
generated and hydrogen sulfide is formed.
The regenerated absorbent is recycled
back to the reactor and the hydrogen
sulfide is converted to elemental
sulfur by the Claus reaction (the burning
of one-third of the H2S to S02 followed
by the reaction of the two to produce
elemental sulfur) (4) . There is some
attrition of the alkalized alumina. This
plus its relatively high cost ($0.25
10
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
SORBENT
MAKE-UP
FLUE GAS
"
FROM BOILER
^
w u
3 UJ
O J
o
\
DUST
REMOVAL
I
I
SORBENT
STORAGE
HOPPER
FLUE
GAS
ts)
ui
a:
5
Q.
tn
1
et
^
UJ
et
\
PURIFIED FLUE GAS
TO AIR PREHEATER
AND STACK
f
|
UJ
UJ
£
IjAb IU SULhUK
RECOVERY PLANT
REDUCING GAS
Figure 9. Bureau of Mines"
per pound) necessitates a fines collector
and an absorbent make-up stream.
Costs for this process are difficult to
estimate. For an 800 mw. plant the
estimated capital investment is $10.64
per kw. and the operating cost is $1.54
per ton of coal. The operating cost is
in the range of $0.75 to $1.00 per ton
if the by-product is considered (6).
Some of the advantages of the alkalized
alumina process include (4,23):
1. Stack gases are released at high
enough temperatures to maintain
buoyancy.
2. The system has a low pressure drop
and is flexible in its range of
usable gas velocities.
3. Because the design is simple and
because carbon steel can be used,
the fabrication costs are low.
4. Elemental sulfur is a recoverable
by-product.
Some of the disadvantages are-
1. The process is difficult to apply
to existing plants.
2. High temperature and large tonnages
make circulation of the absorbent
difficult.
3. Loss of the absorbent by side react-
ions and attrition increases operat-
ing costs.
2. CEGB. The British CEGB (Central
Electricity Generating Board) has per-
formed bench-scale research and develop-
ment of a metal oxide process similar to
the Bureau of Mines process. The only
major difference between the two is that
the CEGB process used fluidized bed con-
tractors instead of the Bureau of Mines
entrained solids reactor. Cost data for
a 500 mw. power station using 2.3% sulfur
coal have been reported, capital costs
at $3.60 per kw. and a breakeven price
of about $30 per ton of recovered sulfur.
A 50 mw. prototype plant is now under
consideration (3,24).
11
-------
Control Methods for the Removal of Sulfur Oxides from Stack Cases
3. SRI- Variations on the alkalized alumina
process are being studied. A process
financed by Slick Industrial Corp. and
developed by the Southwest Research
Institute substitutes sodium aluminate
for alkalized alumina (5). According
to SRT the sodium aluminate absorbs
0.34 grams of S02 per gram of absorbent
or 85% of the SO2 for the stoichiometric
amount of aluminate. This compares to
0.25 grams of S02 or 651 of the S02 for
the stoichiometric amount of alkalized
alumina. Fly ash present in quantities
equal to that of the absorbent does not
hinder the absorption efficiency. SRI
states that it should be possible to
recover the sulfur directly in the
absorbent regeneration step (25).
The sodium aluminate process has several
obvious advantages over the Bureau of
Mines process (26)
1. Higher efficiency permitting the use
of smaller capacity equipment.
2. Insensitivity to fly ash permitting
a lower efficiency particle collector.
3. Elimination of the ClauS unit for
recovery of the sulfur.
More work must be done in the form of
pilot and prototype testing before sound
cost data are available.
E. Other Processes Using Metal Oxides (
1. DAP-Mn Probably the most advanced ab-
sorbent procesb is the DAP-Mn process
developed by Mitsubishi Heavy Industries,
Ltd., Tokyo A unit for a 50 mw. oil
burning plant has operated successfully
for almost a year (1) More than 90%
removal of the sulfur content is claimed
for a .11% S02 stream. The process, as
illustrated in Figure 10, is designed to
remove sulfur oxides from flue gas using
an activated manganese oxide absorbent and
to recover an ammonium sulfate by-product
through regenerating the spent absorbent.
The following reactions are involved (27)
MnSO,
+ 2NH4OH = Mn(OH) +
MnOx-YH20
MnOx'YH20
Yll 0
tti(OH), + 0,
H2°
MnOx-YH20
= MnS0
The powdery manganese oxide is fed into the
fluidized bed type absorber where it is
dispersed in the flue gas stream and reacts
with the sulfur oxides. The excess manganese
sulfate and unreacted absorbent are sent
to a cyclone where 90% of the solids are
collected and sent to the mixer. The re-
cycled solids and the regenerated manganese
oxide from the crystalliier are mixed in a
ratio of 6 or 7 to 1 depending on con-
centration of sulfur oxides in the flue
gas. This stream is now mixed with a flue
gas slip stream through the disintegrator
and accelerated by mixing with the main
flue gas stream at the bottom of the
absorber. Absorption occurs between 210
and 360°F (27).
The fines portion of the recycled solids
stream is collected by an electrostatic
precipitator and dissolved to form a 70%
water slurry. The slurry and an ammonium
hydroxide solution are fed into the
oxidizer where injected air regenerates the
absorbent forming manganese oxide and
ammonium sulfate. Any soot in the stream
is removed by kerosene flotation. This
would be unnecessary if the flue gas were
prccleaned (as uould be the case for a
coal-fired power plant). The solution
is filtered, the ammonium sulfate solution
is sent to a crystallizer, and the wet
cake of absorbent is sent to the mixer (27).
Estimated capital costs for the operation
of an oil-burning 500 mw plant are S13 per
kw. (28). For an oil-burning 1000 mw. plant
with $32 per ton credit for ammonium
sulfate, the net operating cost is SO.55
per ton of fuel oil (4).
The DAP-Mn process has many advantages (28)
1. Low pressure drop.
2. Low power costs.
3. Low maintenance costs.
4. Low absorbent regeneration costs.
5. Small gas cooling (20-60°F) .
6. Carbon steel construction in most of
the equipment.
Two drawbacks would be possible problems
in scale-up and the precleaning expense
12
-------
Vent
Oxidizer
LQJ
Filter
a
Disintegrator
from
Filter
I Soot separator *- —
• -p, Air
2—Q tz
Aqueous Compressor
ammonia
—•- to
Crystallizer
to
Mixer
§
3
»—>
s
3-
O
O.
Ul
3"
3-
m
73
n
3
O
o
i-n
en
S1
o
X
M
D.
TO
Ul
Figure 10. Mitsubishi (Dap-Mn)
26
CO
rt
O
-------
Control Methods for the Removal of Suflur Oxides from Stack Cases
for coal-burning power plants.
2. GRILLO: A.G. fllr Zinkindustrie vorm.
Wilhelm Grillo has developed a process
for removing sulfur oxides which uses a
mixture of metal oxides as an absorbent.
A test unit has operated successfully for
about 9 months (1). This unit has reduced
90% of the sulfur oxide emissions from
a sulfuric acid plant (11).
The absorbent, a slurry of manganese and
magnesium oxides, is deposited on a
carrier such as coke,and contacted with
flue gas from a dust collector in two
series reactors, the first at a temperature
between 250 and 300°F and the second
between 100 and 175"F. The absorbent is
then regenerated by heating with coke at
1470 to 1560"F (6). This reduces all the
sulfur compounds to S, HjS, or COS which
are subsequently burned to give a rich
stream of SC^. The calcined absorbent is
now slurried with water, make-up absorbent
is added, and the slurry is ready for re-
use (4).
Costs have been estimated at between SO.75
and SI.20 per ton of fuel for a 300 mw.
plant (6)
Some of the advantages are (1,6):
1. Attrition is not a problem.
2. Construction can be of carbon steel.
5 Absorption is rapid.
4. Absorption and regeneration steps may
take place in different locations. This
means a reduction in the space requir-
ed for power plant installation.
The disadvantages are (4)•
1. The untreated flue gas must be cooled
before treatment.
2. The pressure drop through the coke
bed is appreciable.
5. Power plant installation would require
a high efficiency particle collector.
3. FIRMA CARL STILL The Still process
using lignite ash as an absorbent is
said to attain up to 95% S02 removal
(1). This process is attaining 80%
sulfur oxides removal on a 10 mw.
power plant, although it is reported
that this pilot^plant is not currently
operating (11).
The lignite ash, which contains 40-50°
lime, is hydrated to form calcium hydro-
xide. Flue gas from the poi»er plant
dust collector is contacted with the
absorbent at about 300"F in three series
reactors. The resulting calcium sulfite
can be heated to liberate S02 for sul-
furic acid production.
The desulfunzed mass is recycled with
a recycle-to-feed ratio of 2 or 5 to 1
(6).
Although no cost data have been released,
investment costs are said to be lower
than for the Reinluft process (1)
The advantages for the Still process
are (4):
1. Low absorbent cost.
2. Elimination of corrosion.
3 Limited cooling of the flue gas.
The only drawback of the process is
that suitable lignite ash is not avail-
able
F. Chemical Scrubbing
1. BATTERSE-V Chemical scrubbing for re-
moval of SO? was first put into full
scale use in 1954 at the Battersea power
station in London, England. In this pro-
cess, as illustrated in Figure 11, chalk
is added to normally alkaline Thames River
water. The flue gas is passed through two
water-scrubbing towers in series, where
calcium sulfite and calcium sulfatc are
formed. An aeration tank is provided to
oxidize any calcium sulfite to the sulfate.
The total effluent is then returned to
the river. The process has been able to
remove 90 to 95% of the S02 from flue
gas. The now obsolete system had an
operating cost of SI.25 to $1.40 per ton
of coal burned (10).
Some major disadvantages of the system arc
1. The flue gas exits at a temperature
near 125°F and hence has almost no
buoyancy.
2. Air pollution is curbed at the expense
of water pollution.
14
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
TO STACK
RIVER WATER '
FLUE GAS
1
BOILER ^
FURNACE
1
COAL ASH
ATP
1
-
£i!LCD 35°°V SCRUBBING
HEATER — TOWFR
|
j
TO
•+- CHALK
SCRUBBING
TOWER
120"F
1
1
1
1
1 it o r '
AERATION
TANK
f
AIR
' i
11
1
1
|
RIVER
g
Figure 11. Battersea
SHOWA DENKO. A scabbing process using
an ammoniacal solution has been tried
on a large scale by the Showa Denko
Co., Tokyo, Japan (1). In the process,
as illustrated in Figure 12, the flue
resulting liquor containing both
ammonium sulfite and bisulfite is
treated with ammonia for complete
reduction to the sulfite. Air is in-
jected by a special method to convert
AMMONIA
WATER
,
.
GAS FROM
AIR HEATER
1 *
1
TO STACK
t
MIST
ELIMINATOR
SCRUBBER ^* WATER AMMONIA
1
1
L _!__
1
1
|
\ \
SURGE 1
TANK 1
1
I
]
m
-i
Figure 12. Showa Denko
AMMONIUM
SULFATE
SOLUTION TO
CRYSTALLIZER
gas stream from the air preheater is
injected with ammonia as early as
possible to react with any S03, thus
preventing corrosion. The water is
then injected to cool the stream
before scrubbing. After this stream
is scrubbed with more water, the
the sulfit', to a recoverable ammonium
sulfate (4).
The company operated a 25 mw. test
unit on an oil-fired process unit
but has now stopped operation because
the prices for ammonium sulfate were
15
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
too low to warrant operation on a
commercial basis (1) .
The process has the following
advantages (4).:
1. Fast reaction rate.
2. Simple process design.
3. Negligible absorbent loss.
The disadvantages are:
1. The gases are cooled to about
115°F, thus losing their buoyancy.
2. Corrosion resistant equipment is
required.
3. AMMONIA AND NEW LIME: Mitsubishi, in
addition to the DAP-Mn process, has
developed two wet absorption processes
which .are variations on older ammonia
injection and lime injection processes
(11). Although sulfur removal
efficiencies are claimed at 90% for
both processes, development work has
been reported to have been discontinued
(29).
4. WADE SCRUBBER: Another wet scrubbing
SO- removal technique has been de-
veloped by the Wade Co., a Division of
Ovitron Corp. A 3000 cfm. coal-fired
test unit at the Gilbert station of New
Jersey Power and Light Co. has de-
monstrated 98% removal of the S02 and
fly ash (30) .
The overall process is very similar to
the Mitsubishi Ammonia process mentioned
above. The scrubber design, however,
is unique: the lower portion consists of
a packed cooling tower and the upper
zone consists of the separator. In the
separator the gases are accelerated
and decelerated with venturi-type ele-
ments. This change in velocity allows
the particles to agglomerate, fall out,
and then be rinsed back to cooling tower
by the ammoniacal scrubbing water. A
flow diagram is seen in Figure 13 (30).
5. WELLMAN-LORD: A wet absorption method
using a potassium or sodium sulfite
scrubbing solution has been developed
by Wellman-Lord, Inc., a subsidiary of
Bechtel Corp. A pilot plant unit has
been operated at the Gannon Station of
Tampa Electric Co. A demonstration
Linac Separator
disks x
•\
COOLING
TOWER
Hot flue—j
gases
-»•
i
BBB| *• to Stack
™P?L
Ammonia—*
Water i
SETTLINGS
H2SOa
~1
f
1
to (NHJ2 SO^ ^ 1 t_
STRIPPING &
OXIDATION
TOWER
Air
recovery
1
Ammonia 1 \
MIKING
TAKK \ I TANK .
1
Settled
fly ash
/
fli v*
1
OXIDIZER
'
FILTER
J
• Flyash
30
Figure 13. Wade Scrubbing Process
16
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
unit at the Crane Station of Baltimore
Gas and Electric was operated until
September 1969. The process has been
licensed to two Japanese firms for
Asian sales and promotion. The first
full-scale commercial application will
be for the Olin Corp. at a 700-ton-per-
day sulfuric acid plant scheduled for
completion in late 1970 (31).
The original design called for a
potassium sulfite scrubbing solution.
The absorbed S02 formed potassium
bisulfite which on cooling precipitated
out as potassium pyrosulfite. Steam
stripping the pyrosulfite produced
anhydrous 502 for recovery and re-
generated potassium sulfite for recycle.
Pilot plant work at the Gannon and Crane
stations indicated high energy require-
ments for the potassium solution system.
Parallel work was done on a sodium
solution system to reduce utility re-
quirements and capital costs. This
sodium system will be used in Wellman-
Lord's first commercial installation
for Olin. Detailed technical information
for the process has not been released
and only the most general type of flow
diagram is available (see Figure 14),
(31).
Wellman-Lord has predicted performance
figures for their process using the
sodium system. Better than 90% S02
removal, 97% SOj removal, and 90% fly
ash removal (with a loading of 0.5 grains
per cubic foot) are claimed (31).
The capital cost of the unit for a 500
mw. coal-fired power plant using 3%
sulfur coal and recovering gaseous 502
is estimated at $7.8 million or about
$15.5 per kw. This figure drops to
$13.9 per kw. for a 1000 raw. plant (31).
MOLTEN CARBONATE. A process for
scrubbing hot flue gases with a mixture
of molten salts has been developed by
Atomics International, A Division of
North American Rockwell Corp.
In this process, the flue gas passes first
through a high temperature electrostatic
precipitator and is then contacted in
an absorber with an eutectic mixture of
lithium, sodium, and potassium carbonate
at about 800°F- The resulting solution
of carbonatesj sulfates, and sulfites
passes on to the reducer. At 1100°F
in the reducer the sulfates and sulfites
in the solution react with producer gas
(CO and H2) to form a sulfide solution
which is sent to the regenerator. In
the regenerator at 800OF the sulfide
solution is reacted with CO2 and water
SODIUM SYSTEM
Water-
to Stack
f
I
1
1
Fly ash,
Qn
MJ,
*1
steayi
_
f
P
i
I
1
_»r-
n^
w-
I
Mater
-1 _
J m
Water
— i
[*-
L L
P
i
r
T
u.
Sulfunc
acid plant
Liquefaction
plant
Sulfur
reduction plant
Figure 14. Wellman-Lord Process
32
17
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
(from the reducer! yielding a regenerated
molten carbonate mixture to the absorber
and a H.S stream to a Claus unit. A flow
diagram is seen in Figure 15, (32).
The disadvantages are
1. The system is corrosive.
RbCOURLU H,S
TO sna
~! °
HOT FLUE
GASES—•
I. LI U ROS FAT 1C
PRI ( IP11 VTOR
800 * "1
MOLTEN
CARBORATES
& SULFIDES
-PRODUUK
CAS CO.H,
HMT1R
Abll
10 hASTI
Figure IS. Molten Carbonate Process
0
Although only bench scale work has been
completed. 99% removal of S02 in gas
streams containing 0.3 to 3°o SO, has been
reported. There are also indications that
the molten salt technique can control
nitrogen oxides [32] .
Atomics International estimates that a
sulfur price of S48 per ton would com-
pletely offset the operating cost, and a
sulfur price of S22 a ton would make it
economically competitive with the
dolomite process. The estimated capital
cost is $10.25 per kw. with an operating
cost of SI.30 per ton before credits
for recovered sulfur are considered C25).
The process has the following
advantages (6)
1. The liquid requirement is
smaller than for aqueous systems.
2. A hot, undiluted gas stream is
emitted.
3 Nitrogen oxides may be removed.
2. The regeneration is difficult
because the rate of reaction of
sulfite and sulfate to sulfide
is slow until a temperature of
about 1150°F is reached.
3. The process is hard to apply to
existing plants because access
to an 800°F gas stream is com-
plicated.
POTASSIUM FORMATE Laboratory and
bench scale studies have been de-
veloped for removing better than
85% of the sulfur oxides from flue
gas by scrubbing with a concentrated
solution of potassium formate (KOOQO-
The SO2 and the formate react at
200°F to form potassium thiosulfate.
The thiosulfate is then reacted with
additional formate at S40°F to
produce potassium hydrosulfide.
The hydrosulfide is stripped with
carbon dioxide and steam, releasing
H.S to a Claus unit. The remaining
potassium carbonate and bicarbonate is
reduced at S40°F and LOOO PSI with
18
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
carbon monoxide and steam to form regen-
erated potassium formate for recycle
back to the scrubber (33).
A preliminary economic analysis has been
made for a 1300 mw. power plant at 57%
load factor using 3.5% sulfur coal. The
capital costs are estimated at $13.6 per
kw., and the operating costs are estimat-
ed at $2.08 per ton of coal before sulfur
credit, or $1.43 per ton coal with a
sulfur credit of $25 per long ton C33).
The process has the following advantages
(33):
1. No gas reheat is required.
2. Mild conditions prevail for all stages
of regeneration.
3. The system is all liquid and requires
no solids handling equipment.
The process has one drawback, the stack
gas must be precleaned before treatment.
G. Others
1. CATALYTIC REDUCTION: Catalytic reduction,
a process developed by Princeton Chemical
Research, Inc., is being studied under a
$84,000 HEW grant (26) and a pilot plant
operation is in the planning stage (5).
In the process, the S02 in the stack gas
is reacted with hydrogen sulfide in the
presence of a catalyst. Sulfur and water
are produced, part of the sulfur is re-
covered, and the remaining portion is
reacted with methane to produce hydrogen
sulfide for use in the process (26).
2. CHROMATOGRAPHIC: Chromatographic absorp-
tion, a process proposed by Dr. Aaron J.
Teller of Cooper Union, uses alumina pel-
lets or pellets of a similar material in
an open-tube tower. The pellets act as
carriers for either an organic (amine or
amide) or an inorganic (metallic sulfite)
absorption agent. The pellets are regen-
erated by heating. The S02 liberated from
the regeneration can be used to make
sulfuric acid. Although the process is
now under bench-scale investigation by
the Bureau of Mines (26), preliminary
cost estimates indicate that the capital
costs would be $13.80 per kw. for an 800
mw. plant and that a sulfuric acid price
of $18 per ton would have to be realized
for breakeven operation (34).
3. IONICS/STONE 6 WEBSTER: A demonstration
plant is currently being developed by
Stone 5 Nebster Engineering Corp. and
Ionics, Inc. This process will use
caustic soda to scrub SO2 from flue gas
and will convert it to sodium bisulfite.
This bisulfite will be fed to a stripping
column where SO2 will be recovered. An
electrolytic technique will be employed
to regenerate the caustic soda from the
sodium sulfate in the column bottoms.
Saleable by-products from this regenera-
tion include dilute sulfuric acid, oxygen,
and hydrogen (5).
4. SEAWATER SCRUBBING: Professor L.A.
Bromley of the University of California,
Berkeley has demonstrated that seawater
is an effective absorbent for removing
SO2 from flue gases. Efficiencies were
reported at 90% using spray chambers and
99% using packed columns with only a 10%
rise in sulfate content of the seawater
(35).
5. FIBER COMPLEXING: A Uniroyal research team
has demonstrated that specially developed
fibers can complex S02. The fibers consist
of styrene-dimethylaminopropylmaleimide
copolymer cospun with polypropylene. The
complexing is reported to occur up to
131"F and to disappear at 212CF. This
allows absorption and regeneration with
the fibers. A tendency of the fibers to
deactivate can be solved by treatment with
alkaline solutions (36).
Ill CONCLUSION
A summary of economics to compare the many
processes for the removal of sulfur oxides
would best be accomplished by a listing in
tabular form (see Table 2 and Table 3).
Numerous processes for the removal of S02
from flue gases have been proposed and
many are reaching the final stages of
development as can be seen from those
previously discussed. The selection of a
specific process as the ideal one would be
virtually impossible since so many factors
are involved. The engineer required to
select a process for a specific installa-
tion must not only study the actual cost of
construction and operation, but he must also
19
-------
Table 2--Capital Investment, Operating Costs (excluding by-product credits)
and Breakeven Prices for By-Products for S02 Removal Systems for an 800 mw. Power Plant
Process
CAT-OX
Kiyoura
Reinluft
Limestone Injection-Dry
Limestone Injection-Wet
Bureau of Mines
Battersea
Wei Iman-Lord
Molten Salts
Chroma tographic
Capital Investment
$ per kw.
21.25
11 10
17.80
4 33*
2.22
10.60
15.20*
10.25
13.80
Operating Cost
(Before By-Product Credit)
per ton of coal
1.75
1.65
2. 45
1.13*
0.36
1.54
1.25-1.40
1.55*
1.30
Breakeven Price
for By-Product
S13.5/T H2S04
S36.5/T (NH4)2S04
S30/T H2SC-4
$47/T Sulfur
$21/T H2S04
$48/T Sulfur
$18/T H2S04
• Calculated from referenced cost data by using the "Six tenths approximation".
References
37, 8
10, 34
37, 34
14
5
37,34
10
31
25
34
r>
o
a
a
Ul
o
•1
:r
n
<
01
c
"I
o
X
Table 3--Current Selling Prices for Various Chemicals (38)
Chemical
Ammonium Sulfate
Liquid SCL
Sulfur
SuIfuric Acid (100%)
Price
$25-$33 per ton (bulk) $39 per ton
(bagged)
$0.11-$0.15 per Ib. (cylinders)
$39 per long ton
$34.65-$35.80 per ton
o
in
rt
ta
n
r
-------
Control Methods for the Removal of Sulfur Oxides from Stack Gases
determine if a by-product is wanted, if
the company in question is in a position
to market such a product, if a market is
available for such a product, and what
effect the amount of by-product produced
would have on the local market.
Each process has some obvious advantage
over the others; the CAT-OX and the char
adsorbent processes produce a saleable
sulfuric acid; the Kiyoura and the
Mitsubishi processes produce an acceptable
ammonium sulfate fertilizer; the dry
limestone injection is applicable to
existing power plants; the limestone in-
jection wet scrubbing process has a high
efficiency and low costs; and the
alkalized alumina processes produce a
marketable sulfur. Economics will determine
the choice of one process over another,
but it is possible that depending on
location, advances in technology, the fuel
to be used, and the local market for a by-
product, many of the systems may be used
eventually.
REFERENCES
"Outside U.S., Tough Laws Spur S02 Re-
moval Efforts," Chemical Engineering,
75_ (24) • 84-88, (1968).
M.D. High and W.H. Megonnell, "Pollution
Control- Federal Leadership,"
Mechanical Engineering, 91^ (2) • 20-23,
(1969).
R.P. Hangebrauck and P.W. Spaite,
"Controlling Oxides of Sulfur," Air
Pollution Control Association Journal,
18 (1) : 5-8, (1968).
1.
2.
3.
4.
5.
A.V. Slack, "Air Pollution- The Control
of S02 from Power Stacks, Part III,"
Chemical Engineering, 7£ (25)' 188-196,
(1967).
"S02 Control Processes for Stack Gases
Reach Commercial Status," Environmental
Science and Technology, 2_ (11) :
944-997, (1968).
6. U.S. Department of Health, Education,
and We 1 fare , Control Techniques for
Sulfur Oxide Air Pollutants, National
Air Pollution Control Administration
Publication No. AP-52. Washington,
D.C • Government Printing Office,
1969.
7. F.A. Rohrman, B.J. Steigerwald, and
J. H. Ludwig, "S02 Pollution The
Next 30 Years," Power, 111 (5)' 82-83
(1967).
8. J.G. Stites, Jr., W.R. Horlacher, Jr.,
J.L.C. Bachofer, Jr., and J.S. Bartman,
"The Catalytic-Oxidation System for
Removing S02 from Flue Gas." Paper
presented at the 61st meeting of the
American Institute of Chemical Engineers,
Los Angeles, California, December 5,
1968.
9. "Illinois Power to Install Cat-Ox
S02 Removal Unit," Clean Air and Water
News. 2_ (31)- 9, (1970).
10. W.T. Reid, "Sulfur Oxides in Central
Station Power Plants," Heating, Piping,
and Air Conditioning. 40 (3): 148-154,
(1968). ~
11. P.G. Maurin and J. Jonakin," Removing
Sulfur Oxides from Stacks," Chemical
Engineering, 77 (9) 173-180, (1970).
12. A.M. Squires, "Air Pollution: The
Control of S02 from Power Stacks,
Part II," Chemical Engineering, 74,
(24) • 133-140, (1967).
13. "A Second Look at Dry Limestone
Injection," Environmental Science and
Technology, £ (2)- 95, (1970).
14. H.L. Falkenberry and A.V. Slack,
"Removal of S02 from Power Plant
Stack Gases by Limestone Injection."
Paper presented at the 61st meeting
of the American Institute of Chemical
Engineers, Los Angeles, California,
December 5, 1968.
15. "Utilities to Fund Study of New
S02 Process," Environmental Science
and Technology, £ (1) : 11, (1970).
16. "Sodium Bicarb Tested for S02 Control,"
Environmental Science and Technology,
3^ (9) : 797, (1969) .
17. "Coal Combustion in Molten Iron Avoids
S02 Formation," Environmental Science
and Technology. £ (8): 630, (1970).
18. "Fluidized - bed Combustion Should
Reduce Air Pollution," Power. 114
(7): 46, (1970).
19. A.L. PIurnley, J. Jonakin, J.R. Martin,
and J.G. Singer, "Removal of S02 and
Dust from Stack Gases," Combustion, 40_
(1) • 16-23, (1968) .
21
-------
Control Methods for the Removal of Sulfur Oxides from Stack Cases
20. J.R. Martin, W.C. Taylor, and A.L.
Plumley," The C-C Air Pollution
Control System," Paper presented at
1970 Industrial Coal Conference,
Lexington, Kentucky, April 8-9, 1970.
21. "Stack Gas Treatment Processes go
Commercial," Environmental Science and
Technology. £ (8) : 629, (1970).
22. "Joint Venture Formed to Market S02
Control Process," Environmental Science
and Technology. 3^ (8) • 703, (1969) .
23. J.H. Field, R.C. Kurtzrock, and D.H.
McCrea, "How to Prevent S02 Emission,"
Chemical Engineering, 74 (13)- 158-160,
(1967). —
24 J.E. Newell," Making Sulfur from Flue
Gas," Chemical Engineering Progress.
65 (8) 62-66, (1969).
25. "Tho Processes Offer Economic Recovery
of Stack Gas S02," Chemical and
Engineering News. 46_ (29)- 13, (1968).
26. "S02 Is It a Health Hazard under
Normal Conditions' What Are the
Practical Approaches to Control7",
Coal Age, 73_ (4) 72-81, (1968).
27. T. Uno, S Fukui, M. Atsukawa, M.
Higashi, H. Yamada, and K. Kamei,
"Scale-up of DAP-Mn Sulfur Oxide
Control Process," Paper presented at
the 61st meeting of the American
Institute of Chemical Engineers, Los
Angeles, California, December 5, 1968.
28. S. Ludwig, "Antipollution Process Uses
Absorbent to Remove S02 from Flue Gas,"
Chemical Engineering, 7_S (3) 70-72,
(1968).
29. "And in Japan, '^ei* Air Quality Criteria
are Imminent," Environmental Science
and Technology, 3_ (2) • 99, (1969).
30. "Scrubber System Aids Byproduct
Recovery," Environmental Science and
Technology, ^ (9)- 806-807, (1969).
31. T.L. Craig, "Recovery of Sulfur
Dioxide from Stack Gases," Paper
presented at 1970 Industrial Coal
Conference, Lexington, Kentucky, April
8-9, 1970.
32. R.D. Oldenkamp and E.D. Margolin, "The
Molten Carbonate Process, for Sulfur
Oxides Emissions," Chemical Engineering
Progress, 6S_ (11)- 73-76, (1969).
33. P.M. Yavorsky, N.J. Mazzocco, G.D.
Rutledge, and E. Gorin, "Potassium
Formate Process and Removing S02
from Stack Gas," Environmental Science
anil Technology, 4 (9) • 757-765, (1970).
34. "S02 Removal- Still an Industry
Challenge," Electric World, 166
(24): 75, (1966).
3S. "Seawater Seen Useful as S02 Absorbent,"
Fnvironmental Science and Technology,
4_(10)' 797, (1970).
36. "Attendees Hear About New S02 Removal
Process," Fnvironmental Science and
Technology, 4_(10): 799, (1970).
37. S. Katell and K.D. Plants, "Here's
What S02 Removal Costs," Hydrocarbon
Processing, 46 (7) • 161-164, (1967).
38. Oil Paint and Drug Reporter, 195 (9),
3, 1969.
-------
techniques
for controlling
the oxides of nitrogen
Reprinted by special permission from JOURNAL OF THE AIR POLLUTION
CONTROL ASSOCIATION, June 1970, Volume 20, Number 6, Pages 377-382.
Copyright 1970 by the Air Pollution Control Association.
Hardison, UOP Air Correction Division
Mr. Hardtson was formerly Tech-
nical Director of UOP Air'Correc-
tion Division He 15 now Vice
President of Air Resources, Inc.,
SOO E XoiihweM, Highway, Pala-
tine, III GOUG7
June 1970 Volume 20, No. 6
Oxides of nitrogen (NO,) constitute a family of air contaminants. Much of
this comes from automobiles and fuel-burning processes. However, chemical
and manufacturing processes, while a relatively small contributor to the
problem, also cause such nuisances. This paper nail touch briefly on the
broad aspects of the problem and will deal specifically with the major
techniques currently being applied. The paper first deals with the properties
of oxides of nitrogen, particularly as they pertain to air pollution. It then
describes the principal way in which nitric acid is produced in the U. S.,
since the manufacture of nitric acid is a prime source of nuisance problems.
The practical solutions now available for solving the NOZ emission problem
as it pertains to nitric acid plants are detailed. The principal system now
used, catalytic reduction, is fully discussed, including detailed sections on
the following design criteria: Color freedom, fuel conversion, and tempera-
ture rise. Two-stage reduction is outlined as the solution now available when
there is pressure to greatly limit emissions, e.g . to 200 ppm, without regard
to color. For those industries using nitric acid to earn- out a chemical reac-
tion of one sort or another, both catalytic and thermal reduction systems are
described, along with the situations under which each is most applicable.
377
-------
Oxides of nitrogen (NO,) constitute a family of serious air
contaminants The latest U. S. Department _of Health,
Education, and Welfare statistics,1 shown in Table I, indi-
cated a total of 12 million tn/yr of oxides of nitrogen emitted
in the U. S Much of this comes from automobiles, but fuel
bui nmg processes also constitute a major source. Chemical
and manufacturing processes, on the other hand, are rela-
tively small in total contribution to the problem, but cause
such concentrated emissions as to require control to prevent
their becoming intolerable local nuisances.
Because nuisance sources can be identified easily, and be-
cause chemical processes can more easily bear the cost of
control, there are several systems in wide use for control of
specific chemical sources These are limited to nitric acid
production and processes using nitric acid as a reagent.
None are used currently for NO. control from normal com-
bustion processes.
This paper will touch briefly on the broad aspects of the
problem and deal specifically with the major control
techniques currently being applied.
Properties of Oxides of Nitrogen
The important oxides of nitrogen are listed as follows:
Formula Chemical Name
NO
NO,
NiO,
N,0
Nitric oxide
Nitrogen dioxide
Dinitrogen tet-
roxide
Nitrous oxide
Form
Colorless, odorless gas
Brown pungent gas
Brown volatile liquid
Colorless, odorless gas
Of these, NjO can be eliminated from consideration as an air
contaminant. Emission sources are rare and it has no effect
whatever at Ion concentrations At concentrations of 30%
or so, it is useful as an anesthetic.
NjO, is a polvmer of NO:, which forms when the N0« is
compressed and liquefied. When NO* is diluted to any extent
\\ilh air, N»04 does not e\ist in significant amounts.
NO: is the pungent, dark brown vapor emitted from chemi-
cal processes which has required emission source control for a
number of years The gas is toxic and a level of 5 ppm is set
by the ACGIH* for continuous exposure. Some authors have
reported chronic lung changes in animals exposed to 2.8
ppm.1 Significant plant damage has been reported at con-
centrations of 25 ppm.4 In addition to the toxic effects, the
color limits visibility and contributes a brown cast to the air.1
Los Angeles sometimes has concentrations exceeding 3 ppm,
with poor visibility due to the very apparent brownish haze.
Table I. Sources of air pollution. (Units: tn/yr).
Motor
vehicles
SO,
CO
NO.
Hydrocarbons
Participates
1
66
6
12
1
86
Indus-
trial
plants
9
2
2
4
6
23
Power
genera-
tion
12
1
3
"3
19
Heat-
ins
3
2
1
1
1
8
Refuse
dis-
posal
i
Y
1
3
Total
25
72
12
18
12
139
Effluent
to stack
Ammonia to
process
f r
if
Air to
process
Absorber
column
with cooling
on each tray
58%
HN03(wt)
figure 1. now diagram of Dupont process.
NO is not ordinarily considered to ha\e any toxic effects
other than those caused by NO8) to which it converts quite
easily. The reaction
2NO + O, ** 2NO,
is something of a classic in that the equilibrium shift* very
easily at high concentrations and NO mn\ be o\idized to the
brown NO* very readily in the laboraton tiecause this is a
tn-molecular gas-phase reaction, the concentration of NO
and NOi (or NO.) tremendously affects the rate at \\hich the
oxidation takes place. At low concentrations, e.g., 1-5 ppm
in air, the reaction is so slow that it would be negligible ex-
cept for the photochemical reaction which takes place in
presence of sunlicht. In general. NO and NO: are lumped
together as oxio.es of nitrogen (NO,) and considered as a
single pollutant because of this tendency to react photo-
chemicaUy. This same type of reaction results in the forma-
tion of Oi (o2one) and PAN (pero.\\acet\l nitrates) which
are the constituents of the eye irritant smogs encountered in
Los Angeles County, and are just starting to be significant
air pollutants in other major cities.
Sources of NO,
Most of the oxides of nitrogen are produced by oxidizing
fuels, during which a part of the ox-ygen combines with atmo-
spheric nitrogen in the flame rather than with the fuel. This
process is called nitrogen fixation and occurs in flames, but
not ordinarily when fuels are burned catal} tically. When
fixation occurs, it results almost exclusively in NO formation.
Table II gives data on the equilibrium concentrations of NO
in air at various temperatures.
Emitted into the atmosphere hot, NO does not oxidize
rapidly enough to cause a visible brown plume in the stuck
even at concentrations as high as 1000-1500 ppm
Systems for limiting the production of oxides of nitrogen
from power generation include:
1. "Two-stage combustion" where on I) the theoretical an
or less is introduced with the pulverized coal or oil Ex-
cess air is added through secondary |X>rts to produce a
clean flame. This has resulted in reductions of a« much
as 50% in the emissions.'
378
Journal of the Air Pollution Control Association
-------
2. Scrubbing of effluent gases for simultaneous removal of
fly ash, SOj, and NO,. This has resulted in reductions of
15-30% as a byproduct of SO. removal.7
A system for limiting the NO, produced by automobiles
involving recycle of some of the combustion products through
the engine has been proposed. Catalytic treatment of the
exhaust gases may bring about significant decreases when
earned out with an excess of fuel.
Nitric Acid Plant Air Pollution Abatement
The Pressure Process
The pressure process for nitric acid manufacture produces
NO by combustion of ammonia in air. The process is carried
out under pressure with oxidation of the NO to N02 and ab-
sorption of the NOa (with additional oxygen usage) in water
to produce nitric acid. Not all of the NO is oxidized and ab-
sorbed. In a typical plant, some 3.5% of the NO remains in
the gas stream leaving the absorber. Without correction,
this stream creates a noxious and highly visible plume.
A flow sketch of the nitric acid process is shown in Figure 1.
The following reactions take place in the process:
1. Oxidation of ammonia
4NH, + 50, — 4X0 + 6H20
2. Oxidation of NO
2NO + Oj — 2N02
3. Absorption of NO.
4NOi + 2H20 + 0. — 4HN03
The operating conditions are t> pically set so as to give the
following effluent gas composition at the absorber outlet.
N.
0,
NO,
H20
Temperature,
Pressure, psig
96 5%
3 0
0 5
100 0% (dry gas basis)
saturated
100
85
The gas is frequently reheated for power recovery through
an expansion turbine. A typical plant without air correc-
tion reheats the effluent stream to 900°F before discharging
through the turbine.
Most modern plants and many older plants employ a
catalytic reduction technique to reduce the concentration of
oxides, reduce N0: to NO, and heat the effluent stream for
power recovery purposes. This process requires a substantial
fuel input (usually natural gas or hydrogen), and allows
significant emis«ions of colorless NO. In some areas the emis-
sion of NO will be controlled to an arbitrary low level of 100
or 200 ppm. This can be met onl\ by increasing the quan-
tity of catalyst and the amount ot fuel by a factor of about
two over the conventional partial reduction technique.
Conventional Catalytic Reduction
The conventional system for fume clean-up is shoun sche-
matically in Figuie 2,
Design criteria for these units are:
1. Color freedom—NO* below the visible threshold.
2. Conversion of fuel high enough—85% or better—for
economical operation.
3. Sufficient temperature rise to secure design conditions
at inlet to power recovery turbine.
Each of these criteria is discussed in some detail in the fol-
lowing paragraphs.
Color Freedom
Color freedom is achieved if the concentration of NO* in
the effluent is below the visible threshold. The visible thresh-
old, from our laboratory work, seems to be around 1-2%
absorption of white light. Beer's Law states that the at-
tenuation of light in an absorbing medium is proportional to
the concentration of the absorbent. Again, based on labora-
tory tests with some supporting observations in the field, the
visible threshold concentration is related to the depth of the
plume by
Co
2400
where:
d = stack diameter, inches
Co = NOj threshold concentration, ppm
Using this equation, the visible concentration in ppm, for
several stack sizes d, iu inches, are given.
0.4
4
40
Co
6000
600
60
This indicates clearly the need for better reduction on large
plants than small ones, and also the probability that data on
color clean-up from a small unit cannot be used directly for
design of a large one.
Measurement of NOs content of ox\ gen-containing streams
is difficult, due to rapid oxidation of NO to N02 at low tem-
peratures. For this reason, we prefer to represent the sub-
jective plume appearance rather than the N02 content to
customers.
Natural gas
fuel supply
Reheater
exchanger
Steam generator
Catalyst
Expansion turbine
Figure 2. Single stage partial reduction system.
June 1970 Volume 20. No. 6
379
-------
After the visible threshold has been established for a given
nitric acid plant, an npproMination to the minimum tempera-
ture for color freedom can be made on the basis of knowledge
of the way catalysts behave and tlic equilibrium between NO
and NO* as a function of temperature. For catalytic units
sparging methane, the following rules may be used.
1. Assume an evit temperature.
2 Calculate the disappearance of oxygen equivalents for
the gi\en jnlet temperature from
3.
4.
Equivalents 02 = —
Assume that 0: and N02 react with the sparge fuel in
proportion; that is, if a 20% reduction in 0: occurs,
theie will be a 20% reduction in NO,. (This ts slightly
conservative for methane and other hjdrocarbons.) For
H: little reduction in NO* can be counted upon (and
none should be used for conservative calculations).
With a known discharge temperature, NO, and Oz con-
tent, calculate N02 content from
k, = /(O
(PNO)'(pO.)
(pNO)'
5.
6.
n-here it, is given b\ the plot in Figure 3 or data in Table
III, and the partial pressures are in atmospheres.
Compare the calculated N02 concentration with the cal-
culated threshold of visibility If it is higher, choose a
higher temperature and repeat
Be siue that the total fuel being consumed is greater than
that required to reduce all the NO, (figured as NO») to
NO
The inlet temperature must be relatively high for initiat-
ing methane conversion, lower for other Indrocarbons
and loner yet for hvdrogen The following values may
be used for conversion over metal supported catalysts.
Lower values are attainable jn the laboratory and in
commercial applications over limited periods However,
sustained performance for a year or more dictate these
requirements.
Inlet Temp.,
*F minimum
300
700
850
950
Fuel
Hydrogen
Naphtha
Propane
Methane
Notice that the conditions existing at the catalyst outlet
were calculated to establish whether or not the plume would
be visible when the gas is exhausted into the atmosphere at a
lower temperature and pressuie. The equilibrium conditions
are quite different, but there JS not sufficient time to reach
the new equilibrium before thorough dilution with air reduces
the NO concentration 10 a low level. Equilibrium is reached
in the catalytic unit only because of the speed of the reaction
on the surface of the catalj st. The residence time m one mtnc
acid plant was 1 second from the catalyst discharge to the
outlet of the stack.
Similarly, it is surprising that recombination of the NO
with atmospheric oxygen does not occur as soon as the hot
gases are discharged into cold air. Figure 3 gives the results
of a calculation1 for an atmospheric system with a high con-
tent of oxides of nitrogen. These conditions encourage rapid
recombination as compared with those which evist in mtnc
acid plant«. Were the e\haii=t JM=P- di=ohaigefl into air
380
from a 24 in. pi|ic at 1000 ppm NOj (right at the visible
threshold), to reach equilibrium instantaneously the concen-
tration of NO; in the plume as it mixes with ambient air
would follow the upper (lotted line from "E," the exhaust
condition, to "A ," the atmospheric condition. The NOi con-
centration would be well above the visible level and a marked
plume would be visible
However, the reformation takes time. The rate is third
order, given by'
where all concentrations are in ppm.
If the plume is diluted instantaneously with an infinite
amount of cold air. the NOi concentration in the mixtures
would follow the lower solid line Somewhere between the
idealized cases of infinitely slow mixing and infinitely rapid
mixing is a realistic rate of dilution. One dilution of the
plume per second nas chosen as a reasonable rate for this
calculation, the resulting NO: values aie plotted as the inter-
mediate line Ab indicated, practical!) no reformation of
X02 occurs, even at this relatively slow rate of mixing.
Two-Stage Reduction
In some areas the pressure to limit emissions has reached
the point of setting an absolute limit on the total emissions
on the order of 200 ppm without regard to the color of the
emission. This is fairl) common now in Germany.
In these areas a two-stage svstem is usually required, be-
cause it is necessary to remove practical!) all of the oxygen
from the stream to secure catalvtic reduction of all of the
NO, to Ni. The operating limits for most catalysts are
about 900°F minimum inlet temperature and 1400°F maxi-
mum outlet temperature. If the Oi content is 4%, the theo-
retical temperature rise is on the order of 227°F X 4 - 908°F
which obviously exceeds the tolerances of a single reactor.
Table II. Equilibrium concentration of
NO in air.
Temperature
CO
64
802
982
2800
4000
NO concentration
(mol ppm)
0.001
0.3
2.0
3700
25.000
Table III. Tabulated values for NO/NOi
equilibrium.
Temperature
(•F) Log,okp- kp
200
400
600
800
1000
1100
1200
1300
1400
1500
1600
-4 80
-2 10
-0.70
40.54
+0 97
+1.37
+181
+198
+2.26
+2.67
0.000016
0.0079
0.199
3.47
9.33
23.4
64.6
95.5
180
468
•Kp = (pNO)»(pD,)
(pNO,)'
where partial pressures are in atmospheres.
Journal of the Air Pollution Control Association
-------
5000
4000
N 3000
2000
1000
\
\!
200 400 600 800 1000 1200
Ambient "A" Temperature°F
Figure 3. Calculated NO> reformation after discharge.
In the two-stage sj stern a steam generator is generally
placed between the two reactors to reduce the temperature,
so that each stage may be kept within the normal tempera-
ture limits. For some ca^es, the temperature may be reduced
by taking part of the t:nl gas into the second stage only, at a
very low temperature. This serve-* sis a sort of quench. The
first stage may u«iuillv be designed with less catalyst than a
single stage unit because it ia not necessary to run at high
fuel efficiency. The -second stage must use the fuel efficiently,
however, and mu«t have a lull complement of catalyst.
Miscellaneous Methods
Several method^ ha\e been |)io])o^ed for air pollution con-
trol in a nitric acid plant that have not found any substantial
•application. These are
a. NHj selective catalUic reaction, which works between
about 400° and 500°F. The NH3 reacts almo.-t exclu-
sively with NO,. However, it is limited to very lo\\ NO*
concentrations foi, if the temperature gets veiy high, the
ammonia will start to react with O^ to form even more
NO.
b. Expansion of absorption capacity to reduce NO, to low
levels. This has proved entirely unacceptable because of
the tremendous increase in size of the N0: absorber.
Atmospheric NO. Reduction Processes
A number of processes utilize nitric acid to carry out a
chemical reaction of one kind or another. One common
process is that used by lamp manufacturers to pioduce the
tiny tungsten filaments Used in all incandescent and fluores-
cent lamps and tclc\ ision and other electron tubes. The fila-
ments arc wound on molybdenum mandrels and annealed.
Then the molybdenum is. dissolved in a mixture of sulfunc
and nitric acids.
Mo + GHNO, — MoO, + 3H,O + 6NO,
Typically, this process is carried out by dipping thousands
of coils into n pan of heated acid under a fume-hood. The
fume-hood is ordm;mly ventilated to produce something in
excess of the 200 fpm cutty velocity u-oiallv recommended
for hood design. 'Apical application* involve 1000-3000
scfm ventilating rates and concentrations ot 1.5-2% N0» at
the peak emission rates. These ate not piocps>es where the
customer's product is the contaminant, <>o it K more economi-
cal to decompose the oxides than recover them.
Water scrubbing of the effluent wa-s universally used by
light-bulb manufacturers, but, while a good part of the N0:
was removed by
3NO, + HjO — 2HNO, + NO
the NO exiting the scrubber recombined quickly with Oj to
yield the obnoxious brown plume. Disposal of the weak acid
from the absorber was usually not a problem in plants
equipped to handle acids. Water scrubbing is applicable to
these processes but requires that the concentration be limited
to about three times the visible threshold concentration.
One such scrubber on a NO* emission source is in operation
and produces satisfactory results.
The equipment applied by I'OP Air Correction Division to
date has consisted principally of a catalytic partial reduction
system, which follows the same rules as does' the nitric acid
system.
There are some peculiarities of the application that require
comment.
I. NOz depresses the activity of the catalyst, apparently
by adsorbing strongly on the catalyst surface and ex-
cluding light hydtocarbon fuels. Generally, concen-
trations greater than 1.0% cause tiouble. This fre-
quently sets a dilution rate requirement.
2. The process relea^e-s NO- intermittently, and anj thing
that spreads the emi*>ion out with time can reduce the
required dilution rate and also reduce the temperature
requirement.
3. While any reducing fuel can be used as sparge, natural
gas and propane are the only convenient ones In an,
these are even harder to burn than in the nitric acid
plant applications. VOP has developed a system 101
generation ol hydrogen and CO JS >psirge fuels in the
preheat burner.
Typical opiating condition* for the atmospheric icduction
process are:
Flow, scfm
NO, Peak Flow, sclm
Carbon Bed Parameters
Depth, in.
Velocity, fpm
&P, in. w c.
Temperatures, °F
Process gas
Catalyst entry
Catalyst exit
Catalyst Parameters
Depth, in.
Velocity, fpm
Type
1000
10
2 5
150
1
100
750
1000
3.75
250
Metal-supjx>i ted
This system operates with the burner firing natural gas at
about a 7.2/1 ratio.
Table IV. Maximum and minimum NO* conditions °
Max. NO,
Mm. NO,
NO,
16
0
Air
224
240
Scfm
0,
47
50
"Total
0,"
63
50
Fuel
32
32
%
Theo-
retical
Fuel
1.02
1.28
%
On + NO.
4
23.2
18.4
• These design parameters are based an laboratory work performed
in (he LIOP laboratory in Bloomer, Wis.
June 1970 Volume 20, No. 6
381
-------
The secret of success here is in cooling the combustion
products to below about 950CF before mixing them with air.
This prevents ignition of the CO and Hz before it reaches the
catalyst. Commercial applications using both propane, hy-
drogen, and CO, produced by burner operation have been in
use for several years.
In general, this system is applicable for any NOj-contami-
nated stream in air.
Wet scrubbing may be used to decolorize NO? streams if thfe
concentration is below 600 or 700 ppm for small applications
and under about 200 ppm for large ones. These scrubbers
are limited by the rate of oxidation of the NO released as
X02 is absorbed.
Concentrated NO* Streams
For concentrated N0z-containing streams (with more than
10% N0r), the amount of dilution air required is so great
that the catalytic system indicated is not economical. In
these eases, the recommended treatment is a flame incinera-
tor where the NO, serves as the oxidizer or a part of the oxi-
dizer in a burner. The burner must be very carefully set up
to run with a strong reducing flame — at least 10% excess
fuel — and to stay within the combustible limits of the fuel-
air-XO., stream.
Flames using NOj as a part of the oxidizer have much in
common with flames in which the air and fuel are diluted
with an inert gas, such as nitrogen. A test program was run
in the UOP laboratory at Bloomer, Wis." to determine the
characteristic changes in flame length and flame stability
when inert diluents are added to flames, and when NO,, is
added to replace part of the oxidizer. Some of the conclusions
are listed here, as they apply to an ordinary nozzle-mix
burner with a normal ratio of ten volumes of air to one
volume of methane, a maximum ratio of 16/1 and a minimum
of 6/1.
Minimum flame stability conditions are given by—
1.
2.
The flame length increases greatly for "dilute" flames,
and may be as much as four times as long as for the same
rates without dilution.
The inclusion of an inert gas narrows the combustible
•limits of fuel and oxygen. Mixtures with less than 12.5%
0» were not capable of sustained combustion even at the
theoretical fuel/oxygen ratio.
Substitution of ICO? for 02 in the oxidizing stream narrows
the limits of combustibility even more. It was recom-
mended that the minimum concentration of oxidizer in
the burner feed be taken as:
%02
%N02
12.5%
4.
6.
A reduction of NO, by 90% or more is feasible at 10%
excess fuel when O|>erating with a rich flame.
The residual NO, consists almost entirely of NO, and
produces a colorless effluent.
Reductions to a low absolute level, say 50 tnol ppm, can
be obtained only with unusually uniform mixing with
the burner. Originally, it was expected that a catalytic
"finishing" of the reduction might be required to achieve
such a high degree of conversion. This consists of cool-
ing the burner effluent to 1000°F and passing it through a
2.5 in. deep catalyst bed at 250 fpm. However, field ex-
perience has shown that very low levels can be reached by
thermal conversion.
Based on these assumptions, burners have been sized to use
auxiliary fuel and air to stay within these limits.
0.6:
theoretical fuel
fuel
^ 0.9
"Equivalent = Os + (NOg/4)
oxidizer" 0, + XO,. + X2 + fuel
0.125
Secondary combustion
when running rich
332
f "-Natural gas
• fuel
Figure 4. Sketch of atmospheric reduction unit
This is the minimum operable limit. Air and fuel should
be added to give a minimum of 15% "oxidizer" in order to
insure stability.
By way of example, the design shown in Figure 4 was
based on a source test which indicated the emission to be
almost pure NO? which varied from 0 to 16 scfm. A system
was devised wherein the NOj and air would be drawn through
a combustion-air blower of 240 scfm capacity and burned
with 32 scfm natural gas. The maximum and minimum NO,
conditions are given in Table IV.
References
1. The Sources of Air Pollution and their Control, Public Health
Service Publication 1548, U. S. Department, of Health, Edu-
cation, and Welfare, 1967.
2. Threshold Limit Values, American Congress of Governmental
and Industrial Hygienists.
3. Stokinger, H. E., "Effects of Air Pollution on Animals," in
'Air Pollution, A. C. Stern, Editor: Vol. 1, Academic Press,
New York, 1962, pp. 282-334.
4. Thomas, M. D. and R. H. Hendricks, "Effect of Air Pollution
on Plants," Air Pollution Handbook, Section 9, pp. 1-27,
McGraw Hill, New York, 1956.
5. Faith, W. L., "Oxides of Nitrogen," Chem. Ena. Progr. 52,
• 342 (1956).
6. Barnhart, D. H. and Diehl, E. K., "Control of Nitrogen
Oxides in Boiler FJue Gases by Two-Stage Combustion,"
Air Pollution Control Assoc. Annual Meeting, June 21-26,
195!). lopp.
7. Tomany, J. P., Pollock, W. A., and Frieling, G., "Removal of
Sulfur Dioxide and Fly Ash from Coal Burning Power Plant
Flue Gases, Paper 66-WA/CD-4, American Society of
Mechanical Engineers, 1!)66.
8. Hardison, L. C., "Atmospheric NOj Reduction: NO-NO,
Equilibria," CCC Project APA-7-25, Report No. 3, Dec. 2,
1964.
9. Leighton, Phillip A., Photochemistry of Air Pollution, Aca-
demic Press, New York, 1961.
10. Hardison, L. C., "Direct Flame Reduction of NO,," CCC
Project APFt-5-43, Report 1, January lo, 1965.
Journal of the Air Pollution Control Association
-------
SECTION X
APPENDIX
Air Pollution Control Equipment Buyer's Guide
Conversion Factors
-------
CONVERSION FACTORS
Page
TEMPERATURE 9-24
PRESSURE 9-25
AREA 9-26
VOLUME 9-27
FLOW 9-28
WEIGHT 9-29
CONCENTRATION 9-30
LENGTH 9-31
EMISSION RATES 9-32
VELOCITY 9-33
9-23
-------
Conversion Factors
CONVERSION FACTORS - TEMPERATURE
Desired
Units
Given
Units
°C
Degrees
Fahr
.5555x
(°F - 32)
460
5555 x
(°F-3Z) + 273
Degrees
Centigrade
1 8°C 4 32
1. 8°C + 492
273
Degrees
Rankin
3R - 460
5555 x
(°R - 492)
5555 x
; (°R-492) + 273
Degrees
Kelvin
1 8(°K-273) +32 °K - 273
1.8(°K-273)+492
9-24
-------
CONVERSION FACTORS - PRESSURE
Desir
um
Given
units
8mm
cm-sec^
dynes
cm2
" m
ft- sec2
poundals
ft2
gmf
cm2
*£
ftZ
#f
uT2
ed
ts
Smm
cm-sec^
1
1
14. 882
14. 882
980.665
478.80
6 8948
x 104
"Atmospheres" 1. 0133
I x 106
dynes
cm2
1
1
14. 882
14. 882
980.665
478.80
6. 8948
-xlQ4
1.0133
x Ifl6
*m
ft-sec^
6.7197
x 10-2
6. 7197
x 10-2
1
1
65.898
32.174
4. 6331
x 103
6.8087
x 104
poundals
ft*
6.7197
x 10-2
6.7197
x 10-2
1
1
65.898
32.174
4.6331
x 103
6.8087
x 104
gmf
cm2
1. 0197
x 10-3
1.0197
x 10-3
1. 5175
x 10-2
1. 5175
x 10-2
1
4.8824
x 10-1
70. 307
1.0332
x 103
*f
rtz
2.0885
x 10-3
2. 0885
x 10-3
3.1081
x 10-2
3.1081
x lO-2
2.0482
1
144. 00
2. 1162
x 103
*f
ln
-------
O
o
3
CONVERSION FACTORS - AREA
0
J"
o
1
0)
Desired
Units Square Square
Given Inch Feet
Units
Square
Yard
Square
Mile
Acre
Square
Centimeter
square oquare
Decimeter Meter
Square
*" 'meter
Square
Inch
Square
Foot
Square
Yard
Square
Mile
Acre
Square
Centimeter
Square
Decimeter
Square
Meter
Square
Kilometer
1
144
1296
40. 144
x 108
62.73
x 107
15.5 x ID'2
15. 5
15. 5 x 10^
15. 5 x 10H
- - i
6. 9444
x 10'3
1
9
2.788
x 107
4. 3560
x 104
10.764
x 10'4
10 764
x 10'2
10 764
10.764
x 106
77 1605
x 10'5
0 1111
I
3.09.8
x 106
4840
1 1960
x 10'4
1 I960
x 10'2
1 1960
1.1960
x 106
2.49
xlO'10
3. 587
x 10~8
3 228
x 10'7
1
15. 625
x 1C'4
3.8610
x 10'11
3.8610
xlO-9
3.8610
x 10~7
3.8610
x lO'1
15. 94
x 10'6
2 296
x 10'5
2. 066
x 10
640
1
2. 471
x 10'8
2.471
x 10'6
2.471
x ID'4
2. 471
x 102
6.452
929.0341
83 61
x 102
2. 589998
x 10
4046.873
x 104
1
1 x ID2
1 x 104
1 x 1010
6. 452
x 10'2
929. 0341
x HT2
83.61
2. 589998
x 108
4046. 873
x 102
1 x 10'^
1
1 x 10^
1 x 10B
6.452
x LO'4
929 0341
x lO'4
83.61
x 10'2
2 58.9998
x 10*
4046.873
1 x 10-*
1 x 10'^
1
1 x 10°
6 452
x ID'10
929. 0341
x ID'10
83 61
x 10'8
2. 589998
4046 873
x 1C'6
1 x 10'1U
1 x 10~8
1 x 10'b
1
To convert a value from a given unit a desired unit, multiply the given value by the factor opposite the given units
and beneath the desired unit
-------
CONVERSION FACTORS - VOLUME
^X,. Desired
GivenV^Units
Units ^s.
Cubic
Yard
Cubic
Foot
Cubic
Inch
Cubic
Meter
Cubic
Decimeter
Cubic
Centimeter
Liter
Cubic
Yard
1
3.7037
x io~2
2. 143347
X 10~5
1.30794
1.3079
x io"3
1.3079
x io"6
1.3080
x io"3
Cubic
Foot
27
1
5.78704
x io"4
35.314445
3.5314
x io"2
3.5314
x io"5
3.5316
x io"2
Cubic
Inch
4.6656
x io4
1728
1
6.1023
x io4
61.023
6. 1023
x io"2
61.025
Cubic
Meter
0.764559
2.8317
x io"2
1.63872
x io"5
1
0.001
1 X IO"6
1.000027
x io"3
Cubic
Decimeter
764.559
28.317
1.63872
x io"2
1000
1
1 X IO"3
1.000027
Cubic
Centimeter
7.64559
x io5
2.8317
x io4
16. 3872
1 X106
1000
1
1000.027
Liter
764.54
28.316
1.63868
x io"2
999.973
.99997
9.99973
x io"4
1
To convert a value from a given unit to a desired unit, multiply the given value by the factor opposite the given units
and beneath the desired units.
VO
i
NJ
-------
o
I
00
n
§
CONVERSION FACTORS - FLOW
§
Desir
Un
Given
Units
M3
sec
M3
mm
M3
hour
ft3
sec
ft3
mm
ft3
hour
L_
sec
L
min
cm3
sec
cm3
mm
ed
its M3
sec
1
0. 0167
2.778
x 10-5
28. 317
x 10-3
4. 7195
x ID'4
7.8658
x 10'6
1. 000027
x ID'3
1. 6667
x lO'5
Ix 10'°
1.6667
xlO-8
M3
mm
60
1
16. 667
x 10-3
1.699
28.317
x 10-3
4. 7195
x ID'4
6. 00016
x 10'2
1. 000027
x 10-3
6 x 10"5
Ix ID"6
M3
hour
3600
60
1
101. 94
1.699
28.317
x 10-3
3.6
6. 0016
x 10 "2
3. 6 xlO"3
6 x 10-5
ft3
sec
35 3144
0. 5886
98.90
x ID'4
1
16. 667
x 10'3
2.778
x 10-4
35.316
x lO"3
5.886
x 10-4
3. 5314
x lO-5
5.886
x ID'7
ft3
mm
21. 1887
x 102
35. 3144
0.5886
60
1
16.667
xlO-3
2.11896
35.316
x ID'3
2.1189
x 10-3
0.3531
x 10-4
ft3
hour
12. 7132
x 104
21. 189
x 102
35. 3144
3600
60
1
127.138
2. 11896
1. 271
x 10-3
2. 11887
x 10-3
L
sec
999.973
16 667
27.777
x 10-2
28 316
47.193
x lO-2
7.866
x 10-3
1
1.6667
x lO-2
9. 99973
x 10-4
5. 9998
x 10-2
L
min
59.998
x 103
999. 973
16.667
16.9896
x 102
28.316
0.4719
60
1
5.9998
x lO-2
9.99973
x 10-4
cm3
sec
1 x 10b
16. 667
x 103
2.777
x 102
2 8317
x 104
4.7195
x 102
78.658
1000. 027
16. 667
1
60
cm3
min
6 x 107
1 x 106
1.666
x 104
1.699
x 106
2.8317
4. 7195
x 102
16.667
1000. 027
16.667
x ID'3
1
To convert a value from a given unit to a desired unit, multiply the given value by the factor opposite the given
units and beneath the desired unit.
-------
CONVERSION FACTORS WEIGHT
Desired
Units
Given
Micro-
gram
Milli-
gram
gram
Kilogram
grain
Ounce
(avdp)
Pcund
,'avsJp)
Tsn
(U.S. short)
Tonne
(metric)
Micro-
gram
1
1 x 103
1 x 106
1 x 109
64.799
x 103
28.349
x 106
453.59
x 106
905.185
x 109
1 x 1012
Multi-
gram gram
1 x 10~3 1 x 10"6
1 1 x 10"3
1 x 103 1
1 x 106 1 x 103
64.799 64.799
x ID'3
28.349 28.349
x 103
453.59 453. 59
x 103
907.185 907.185
x 106 x 103
1 x 109 1 x 106
Kile-
gram
1 x 10~9
1 x 10"6
1 x 10"3
1
64.799
x 10~6
28.349
x 10~3
453.59
x 10-3
907.185
1 x 103
grain
15.4124
x 10-6
15.4324
x 10-3
15.4324
15. 4324
x 103
1
437.5
7000
14 x 106
1.543 x 10?
Ounce
(avdp)
3.5274
x 10-8
3.5274
x lO-5
3.5274
x 10-2
35.274
22.857
x 10-4
1
16
3.2
x 104
3.5274
x 10A
Pound
(avdp)
2.2046
x 10-9
2.2046
x 10-6
2.2046
x 10-3
2.2046
1.4286
x 10-4
62.5
x lO-3
1
2000
2204.62
Ton
(U.S. short)
1.1023
x 10-12
1.1023
x 10-9
1.102:
x 10-6
1.:.023
x 10-3
7.143
x ID'8
3.125
x 10~5
5 x 10~A
1
1.10231
Tonne
(metric)
Ix 10-12
1 x 10~9
Ix 10~6
1 x 10-3
64.799
x 10-9
28.349
x 10-6
453.59
x 10-6
0.907185
1.
-------
Conversion factors
CONVERSION FACTORS - CONCENTRATION
\ Desirec
\Units
GivenV
Units \
La
-a.
M3
U&
L
02.
ft.3
Ibs.
ft.3
grams
3
ft.
Ibs.
1000 ft.3
grains
ft.3
H£
M3
1
1 x 10"3
.999973
1.00115
x 106
1.602
x 107
3.531
4
x 10
1.602
x 104
2.288
MO3
P£
M3
1000
1
9.99973
x 102
1.00115
xlO9
1.602
xlO10
3.531
7
x 10
1.602
xlO7
2.288
x 106
VS.
L
1.000027
1.000027
x 10"3
1
1.00118
MO6
1.602
x 107
3.531
4
x 10
1.602
x 104
2.288
x 103
oz
ft.3
9.989
x 10"7
9.989
x 10-10
9.988
x 10"7
1
16
3.5274
x 10
1.6
MO'2
2.2857
3
x 10
Ibs.
ft.3
6.243
x 10"8
6.243
x 10-11
6.242
x 10"8
62.5
MO'3
1
2.20462
x 10"
IxlO"3
1.4286
x 10"4
grams
ft.3
2.8317
' x 10-5
2.8317
MO"8
2.8316
x ID'5
28.349
453.59
1
453.59
..-3
\ 10
6.4799
HO-
Ibs.
1000 ft
6.243
MO'5
6.243
MO'8
6.242
x 10-3
62.5
K10J
.2046
1
14.286
grains
ft.3
4.37
MO'4
4.37
xlO-7
4.37
x 10'4
4.375
i
\ 10'
7.U03
15.43
7
1
To convert a value from a given unit to a desired unit, multiply .he given value by
the factor opposite the given units and beneath the desired ui it
9-30
-------
CONVERSION FACTORS - LENGTH
Desirec
Units
Given
Units
Inch
Foot
Yard
Mile
Micron
Millimeter
Centimeter
Meter
Kilometer
J
Inch
1
12
36
6.3360
x 104
3.937
x ID'5
3. 937
x ID"2
3. 937
x 10'1
39.37
3. 937
x 104
Foot
83.33
x 1(T3
1
3
5280
32.808
x 10"7
3 2 £08
x ID'4
32.808
x lO"3
32.808
x 10-1
32.808
x 102
Yard
27.778
x 10-3
3333
1
1760
10.94
x 10-7
10.94
x ID'4
10.94
x 10- 3
10.94
x 10'1
10.94
x 102
Mile
1.578
x 10'5
1.894
x 10~4
5.682
x lO'4
1
62. 137
x 10"11
62.137
x 10'8
62.137
x 10~7
62.137
x 10-5
62.137
x 10-2
Micron
2. 54
x 104
30.48
x 104
91.44
x 104
1.6094
xlO?
1
1 x 103
1 xlO4
1 x 106
1 xlfl9
Millimeter
25.4
304. 8
914.4
1. 6094
x 106
1 x lO'3
1
10
1 x 103
Ix 106
Centimete
2. 54
30.48
91.44
1. 6094
x 105
1 x lO"4
0.1
1
1 x 102
1 x 105
sr Meter
2. 54
x ID"2
30.48
x ID'2
91.44
x lO-2
1. 6094
x 103
1 x 1C'6
1 x 10-3
Ix ID'2
1
1 x 103
Kilometer
2. 54
x ID"5
30 48
x ID'5
91 44c
x lO'5
1. 6094
1 x 10-9
Ix 10'6
1 x 10-5
1 x 10'3
1
n
§
a
O
To convert a value from a given unit to a desired unit, multiply the given value by the factor opposite the given
units and beneath the desired units.
n
rr
O
-------
•J3
I
CVJ
CONVERSION FACTORS - EMISSION RATES
Desired
units
Given
units
gms /sec
gms /mm
kg/hr
kg/day
Ibs/min
Ibs/hr
Ibs/day
tons/hr
tons/day
tons /y r
grns/sec
1 0
1 6667
X 10-2
2 7778
X ID'1
1 1574
X ID"2
7 5598
1 2600
X 1C"1
5 2499
x io-3
2 5199
X 102
1 0500
X 10
2 8766
X ID'2
gms/min
60 0
1 0
16 667
6 9444
x 10-1
4 5359
X IO2
7 5598
3 1499
X lO'1
1 5120
X IO4
6 2999
X 102
1 7260
kg/hr
3 6
6 0
X 10-2
1 0
4.1667
X ID'2
2. 7215
X 10
4 5359
X 10"1
1 8900
x io-2
9 0718
X IO2
3 7799
X 10
1 0356
x io-1
kg /day
8 640
X 10
1 4400
2 4000
X 10
1 0
6 5317
X IO2
1 0886
X 10
4 5359
x io-1
2 1772
X104
9 0718
X IO2
2 4854
Ibs/min
1. 3228
x io-1
2 2046
X 10 '3
3.6744
X ID'2
1. 5310
x io-3
1 0
1 6667
X ID'2
6 9444
x io-4
3 3333
X 10
1.3889
3.805?
X 10-3
Ibs/hr
7 9367
1 3228
x io-1
2 2046
9 I860
x io-2
60 0
1 0
4.1667
x io-2
2 0
X IO3
8 3333
X 10
2.2831
x io-1
Ibs/day
1.9048
X 102
3 1747
5 2911
X 10
2 2046
1 44
X IO3
24 0
1 0
4 8000
X IO4
2.0
X IO3
5 4795
tons/hr
3 9683
X 10°
6 6139
x io-5
1 1023
X 10'3
4 5930
x io-5
3 000
X ID"2
5 0000
x io-4
2 0833
X 10-5
1 0
4.1667
X IO-2
1 1416
X ID'4
tons/day
9 5240
X ID'2
1. 5873
x io-3
2 6456
X ID"2
1 1023
x io-3
7. 2000
x io-1
1. 2000
X lO-2
5 0000
X ID'4
24.0
1.0
2. 7397
X 10"3
tons/yr
3 4763
X 10
5 7938
X lO'1
9 6563
4 0235
x io-1
2 6280
X IO2
4 3800
1. 8250
X 10'1
8 7600
X IO3
365.0
1 0
o
o
A
O
n
ff
To convert a value from a given unit to a desired unit, multiply the given value by the factor opposite the given units
and beneath the desired units
-------
CONVERSION FACTORS - VELOCITY
Des
u
Given
units
m/sec
ft/sec
ft/mm
km/hr
mi/hr
knots
mi/day
ired
nits
m/sec
1 0
3 0480
X 10"1
5 0080
X 10'3
2 7778
X lO"1
4 4707
X 10"1
5 1479
X ID'1
1.8627
X 10'2
ft /sec
3 2808
1.0
1. 6667
X 10"2
9 1134
X 10'1
1.4667
1 6890
6.1111
X 10'2
ft/mm
1.9685
X 102
60
1 0
5.4681
X 10
88 0
1 0134
X 102
3 6667
km/hr
3 6
1.0973
1.8288
X 10'2
1 0
1. 6093
1 8533
6 7056
X 10'2
mi/hr
2 2369
6 8182
X 10"1
1 1364
X ID'2
6.2137
X 10'1
1 0
1.1516
4 1667
X 10'2
knots
1 9425
5 9209
X 10-1
9.8681
x io-3
5 3959
X lO"1
8 6839
X lO'1
1.0
3 6183
x io-2
mi/day
5 3687
X 10
1.6364
X 10
2 7273
X 10'1
1.4913
X 10
24
2 7637
X 10
1. 0
To convert a value from a given unit to a desired unit, multiply the given value oy the factor opposite the
given units and beneath the desired umtQ
o
o
O
(U
o
-------
Conversion Factors
LOGARITHMS TO BASE 10
N
10
11
12
13
14
IS
ie
17
IS
19
20
21
22
23
24
25
20
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
4G
47
48
49
50
51
52
53
54
N
01234
0000 0043 0086 0128 0170
0414 0453 0492 0531 0569
0792 0828 0864 0899 0934
1139 1173 1206 1239 1271
1461 1492 1523 1553 1584
1761 1790 1818 1847 1875
2041 2068 2095 2122 2148
2304 2330 2355 2380 2405
2553 2577 2601 2625 2648
2788 2810 2833 2856 2878
3010 3032 3054 3075 309G
3222 3243 3203 3284 3304
3424 3444 3464 3483 35C2
3617 3G36 3653 3674 3G92
3802 3820 3538 3856 3874
3979 3997 4014 4031 4048
41oO 4 ICG 4183 4200 4216
4314 4330 4340 4362 4378
4472 4487 4J02 4518 4.133
4G24 4G39 4054 4609 4GS3
4771 4786 4800 4814 4829
4914 4928 4942 4955 4969
5051 5065 5079 5092 5105
5185 5198 3211 5224 5237
5315 5328 5340 5353 5366
5441 5453 5465 5478 5490
5563 5.375 5587 5599 5G11
5682 5094 5705 5717 5729
5798 5809 5821 5832 5843
5911 5922 5933 5944 5955
6021 G031 6042 6053 GOG4
6128 6138 0140 6IGO G170
6232 6243 62J3 62G3 0274
6335 G345 6355 6365 6375
6435 6444 6454 G4G4 6474
6532 6542 6551 6561 6571
6628 G637 6646 6656 6G65
6721 6730 6739 6749 6758
6812 8821 0830 0839 6848
6902 6911 6920 6C28 6937
6990 6998 7007 7016 7024
7076 7084 7003 7101 7110
7160 7168 7177 7185 7193
7243 7251 7259 7207 7275
7324 7332 7340 7348 7350
01234
56789
0212 0253 0294 0334 0374
0607 0645 0682 0719 0755
0969 1004 1038 1072 1106
1303 1335 1367 1399 1430
1014 1644 1673 1V03 1732
1903 1931 1969 1987 2014
2175 2201 2227 2253 2279
2430 2455 2480 2504 2529
2G72 2695 2718 2742 2765
2900 2923 2945 2967 2989
3118 3139 3160 3181 3201
3324 3345 3365 3385 3404
3522 3541 3560 3579 3598
3711 3729 3747 3766 3784
3892 3909 3927 3945 3962
400 j 4082 4099 41 1C 4133
4232 4249 4265 4281 4298
4J93 4409 4425 4440 4456
4548 4564 4579 4594 4609
4G98 4713 4728 4742 4757
4843 4857 4871 4886 4900
4983 4997 5011 5024 5038
5119 5132 5145 5159 5172
5250 5263 5276 5289 5302
5378 5391 5403 5416 5428
5502 5514 5527 5539 5551
5G23 5G35 5G47 5G58 5G70
5740 5752 57C3 5775 5780
5855 58G6 5877 5888 5899
5966 5977 5988 5999 6010
G075 6085 0096 6107 Gfl7
G180 6101 6201 6212 G222
6284 6294 6304 6314 6325
6385 6395 6405 6415 6425
G484 6493 6503 6513 6322
6580 0590 6599 6609 6618
G675 GG84 6693 6702 6712
6767 6776 6785 6794 6803
6857 6866 6875 GSS4 6893
6946 6955 6964 6972 6981
7033 7042 7050 7059 7067
7118 7126 7135 7143 7152
7202 7210 7218 7226 7235
7284 7292 7300 7308 7316
7304 7372 7380 7388 7396
56789
Proportional Parts
123456789
4 8 12 17 21 25 29 33 37
4 8 11 15 19 23 26 30 34
3 7 10 14 17 21 24 28 31
3 6 10 13 16 19 23 26 29
3 6 9 12 15 18 21 24 27
3 6 8 11 14 17 20 22 25
3 5 8 11 13 16 18 21 24
2 5 7 10 12 15 17 20 22
2 5 7 9 12 14 16 19 21
2 4 7 9 11 13 16 18 20
2 4 6 8 11 13 15 17 19
2 4 6 8 10 12 14 16 (8
2 4 6 8 10 12 14 16 17
2 4 8 7 9 11 13 15 17
2 4 5 7 9 11 12 14 10
2 3 5 7 9 10 12 14 15
2 3 5 7 8 10 11 13 18
2 3 5 8 8 9 11 13 14
2 3 5 6 8 9 11 12 14
1 3 4 6 7 9 10 12 13
3 4 6 7 9 10 11 13
3 4 6 7 8 10 11 12
34578911 12
3 4 6 8 8 9 10 12
3 4 5 6 8 9 10 11
2 4 5 6 7 9 10 11
2 4 5 6 7 8 10 11
2356789 10
2356789 10
2345789 10
23 5 6 8 9 10
23 56789
23 56789
23 56789
23 56789
23 56780
23 56778
23 55878
23 5678
23 6678
233 5678
233 5678
223 5677
223 5667
223 5607
123456789
The proportional parts are stated in full for every tenth at the right-hand side.
The logarithm of any number of four significant figures can be read directly by
adding the proportional port corresponding to the fourth figure to the tabular
number corresponding to the first three figures. There may be an error of 1 in
the last place.
9-34
-------
Conversion Factors
LOGARITHMS TO BASE 10
(continued)
N
5
50
5
58
5'
GO
G
62
63
64
65
66
67
68
6
------- |