-------
TABLE 5 . (Continued)
Source Category
cn
0)
3
4J
r-l
3
CJ
•H
\4
CO
C
o
•H
4J
(0
4J
J-
O
c.
CO
c
a ci
i^ p- ei
H =3 CU
ca
iH
ca
-H
u
(U
jj
eg
00
G •
•H
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G
~
cn
0)
•H
o.
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ration:
0)
o.
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c
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C
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JE
0
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u
cd
TH
U
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G
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o
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ij
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3
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u
cn
C
0
u
cn
3
O
CU
G
ca
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r*^
u
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G O
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3 >->
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C
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a. c
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tower
to
c
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rH
O
O
O
Control
Technology
Special
control device **•*
speed reduction Var
enclosure F
exhausting F,G G F,G
reduction of fall F-P
wind screen VP
F = fair G = good P = poor VP = very poor
- 50% " 85% * 20% < 20%
UPR = unpaved roads NR = not rated Var = variable
PR = caved roads
Conclusions
While fair control of some agriculture emissions may be obtained by
wetting, this source lacks good controls. Physical stabilization remains
practically untried, chemical stabilization is rated poor, and vegetative
stabilization very poor.
Physical stabilization of dirt roads (chat is, paving) is the only
good control. Paved roads benefit from vegetative stabilization of
shoulders. Materials handling operations can attain fair to good con-
trol by wetting when the techniques can be used. Other technologies
remain unexploited at this time.
Stockpiles benefit from physical and vegetative stabilization.
Mining operations benefit from exhausting techniques, while wetting
is very poor and the stabilization technologies have not been rated, and
may not work well.
79
-------
Exhausting has been fair to good for control of beneficiation emis-
sions, while wetting has been poor to fair, and stabilization unrated.
Physical and vegetative stabilization is fair to good in construc-
tion operations, while wetting is poor, and chemical stabilization is
not rated.
None of these technologies are effective in open burning or on
industrial cooling towers. Incinerators, if properly designed, can be
effective as control devices.
EFFECT OF FUGITIVE EMISSION REDUCTION ON AQCR'S
To examine the effects of fugitive dust emissions reduction on total
AQCR emissions, emissions from unpaved roads, agricultural tilling, and
construction were reduced by appropriate measures reported in the litera-
ture. The reductions used were 50 percent for unpaved roads and 40
percent for agricultural tilling (chemical stabilization effectiveness),
and 30 percent for construction (wetting).
The results of the emissions reduction are shown in the following
summary:
Before Emissions
Reduction
After Emissions
Reduction
Total number of AQCR's not
meeting TSP Standards 150 150
Point > Area 9 17
Area > Point 139 131
Area 5x > Point 97 68
Area lOx > Point 58 38
Data Missing 2 2
CONCLUSIONS
1. Fugitive dust sources are significant emitters of particulates
in a majority of the AQCR's. Of the 150 AQCR's that do not meet
the TSP standards, fugitive dust emissions exceed point source
emissions in 139 AQCR's, or 92 percent. In fact, fugitive emis-
sions are 10 times greater than point source emissions in 58,
or 39 percent, of the AQCR's.
80
-------
2. In most cases, unpaved roads provide the largest source of
particulate emissions in the AQCR's. Agricultural tilling and
construction sources are also very important and in some cases
are the largest emitters.
3. The reentrainiaent of particles from paved roads is a source of
large quantities of particulates in many AQCR's.
4. Industrial sources of fugitive emissions are plentiful and can
have a substantial impact on surrounding areas.
5. Fugitive dust sources can contribute significantly to the TSP
burden of an entire AQCR as well as have an impact in a
localized area:
6. The relationship between pollutant exposure and human health
has been demonstrated. Increased hospitalization rates have
been observed with increased particulate pollutant exposure.
7. More attention should be given to the control of fugitive dust
emissions because of their contribution to ambient dust
loadings.
8. .Control effectiveness for fugitive sources is highly variable
and depends on such things as type of control, characteristics'
of the source, local climatic conditions, and source activity.
9. Present control technology for unpaved roads, agricultural
tilling, and construction activity is inadequate. Reducing
the emissions from these activities by the amounts reported in
the literature has only a small influence on fugitive emissions
in most AQCR's.
REFERENCES
1. Office of Air and Waste Management, State Air poilution Imple_
mentation Plan Progress Report, Jaunary 1 to June 30, 1976 "
Office of Air Quality Planning and Standards, U.S. EPA
EPA-450/2-76-026, October 1976.
2. Personal communication, Mr. Chuck Mann, NADB, EPA, Durham
May 9, 1977. '
3. Ibid., March 30, 1977.
4. Carpenter, B. H., and G. E. Weant, III, "Particulate Control
for Fugitive Dust," EPA 600/7-78-071, April 1978.
81
-------
5. Roberts, J. W., H. A. Watters, C. A. Mangold, and A. T. Rossano,
"Cost and Benefits of Road Dust Control in Seattle's Industrial
Valley." J. APCA. 25(9), September 1975, pp. 948-952.
6. Haws, R. C. and H. L. Hamilton, Jr., "North Carolina Air Quality
Maintenance Area Analysis, Vol. Ill: TSP Dispersion Modeling
and Analysis for Charlotte, Winston-Salem, and Greensboro AQMA's
for 1973, 1975, 1980, 1985," RTI Final Report, EPA Contract
68-02-1385, Task 15, April 1976.
7. Pierson, W. R. and W. W. Brachaczek, "Note on In-Traffic
Measurement of Airborne Tire-Wear Particulate Debris," J. APCA,
25(4), April 1975.
8. McCutchen, G., "Regulatory Aspects of Fugitive Emissions," paper
in Symposium on Fugitive Emissions Measurement and Control,
May 1976, Hartford, CT, EPA 600/2-76-246, September 1976.
9. Air/Water Pollution Report. Business Publishers, Inc., Silver
. Spring, MD, May 2, 1977, p. 177.
10. Personal communication, Mr. Steve Dennis, Massachusetts Depart-
ment of Environmental Quality Engineering, Boston, May 27, 1977.
11, Metzger, C. L., "Dust Duppression and Drilling with Foaming
Agents," in Pit and Quarry Magazine. March 1976, pp. 132-133
and 138.
12. Seibel, R. J., "Dust Control at a Transfer Point Using Foam
and Water Sprays," U.S. Department of the Interior, Bureau of
Mines, TPR 97, May 1976.
13. Jutze, G. and K. Axetell, "Investigation of Fugitive Dust,
Vol. 1: Sources, Emissions, and Control," EPA 450/3-74-036a,
June 1974.
14. Weant, G. E., Ill, "Characterization of Particulate Emissions
for the Stone-Processing Industry," RTI Final Report, Contract
No. 68-02-02607, Task 10, U.S. EPA, Industrial Studies Branch,
May 1975.
15. Armbrust, D. V. and J. D. Dickerson, "Temporary Wind Erosion
Control: Cost and Effectiveness of 34 Commercial Materials,"
J. of Soil and Water Conservation. .July 1971, pp. 154-157.
16. Dean, K. C., R. Havens, and M. W. Glantz, "Methods and Costs
for Stabilizing Fine-Mineral Wastes," U.S. Department of the
Interior, Bureau of Mines, RI 7894, 1974.
82
-------
17. Donovan, R. P., R. M. Felder, and H. H. Rogers, Vegetative
Stabilization of Mineral Waste Heaps," EPA 600/2-76-087,
April 1976.
18. Axetell, K. and J. Zell, "Control of Re-entrainment Dust from
Paved Streets," EPA 907/9-77-007, August 1977.
19. Sartor, J. D., B. Boyd, and W. H. VanHorn, "How Effective is
Your Street Sweeping," APWA Reporter. 39(4), 1972, p. 18.
20. Nichols, A. G., "Fugitive Emission Control in the Steel
Industry," Iron and Steel Engineer, July 1976, pp. 25-30.
21. Committee on Industrial Ventilation, Industrial Ventilation,
A Manual of Recommended Practice. 14th ed., 2nd Printing,
American Conference of Governmental Industrial Hygienists,
Lansing, Michigan, 1977.
22. Environmental Control Division, Control of Internal Foundry
Environment, Vol. 1, American Foundrymen's Society, Des
Plaines, IL.
83
-------
EPA BUBBLE
STEEL INDUSTRY EXAMPLE OF BEST USE OF BUBBLE
Iron Oxide Sources
Storage- Pile
Wind Loss
Blast-Furnace
Cast House
BOF Charging and
Tapping
Open-Hearth Charging
and Tapping
Emission Possible
(ton/yr) Controls
473 Spray System
309 Hoods and
Baghouse
326 Hoods and
Baghouse
140 Hoods and
Baghouse
Reduction Cost Cost per
% (ton/yr) $ Millions Annual Ton
60 (284) 0.2 $ 704
95 (294) 2.5 8,520
90 (293) 4.0 13,630
80 (112) 4.0 35,710
SOURCE: Armco Steel Corp., Middletown, Ohio.
-------
REMOVAL OF PARTICULATE MATTER FROM
GASEOUS WASTES
WET COLLECTORS
1.00 INTRODUCTION
1.10 Definitions
Wet collectors comprise all units in which a
liquid, usually water, is employed to achieve or
assist in the removal of dispersoids from gases.
As used in this report, the term "dispersoid"
will include dust, spray, fume, and mist. Lap-
pie " places all dispersoids into two general
categories: "mechanical dispersoids" and "con-
densed dispersoids." Mechanical dispersoids are
formed by comminution, decrepitation, or dis-
integration of larger masses of material, or by
the grinding of solids or spraying of liquids. A
wide range of particle sizes usually is involved.
Mechanical dispersoids are further classified as
"dust" and "spray," referring to dispersed sol-
ids and liquids, respectively.
Condensed dispersoids are formed by conden-
sation of the vapor phase or as the product of a
vapor phase reaction. Particles thus obtained
usually are characterized by a relatively nar-
row size distribution. Further classification of
condensed dispersoids into "fume" and "mist"
refers to solid and liquid dispersed phases, re-
spectively.
The sizes of dispersed particles generally are
related to the manner in which they are formed.
The following tabulation summarizes the classi-
fication and presents representative size ranges
of gas dispersoids:
Approximate Particle
Diameter (Microns)
> 1
< 1
Dispersoids
Mechanical:
Dust
Spray
Condensed:
Fume
Mist
The size ranges indicated in this tabulation
are representative of most typical cases; they
are not intended to be mutually exclusive nor
are they intended to serve as rigid definitions of
dispersoids which may be encountered in prac-
tice. For example, condensed dispersoids gen-
erally tend to agglomerate into particles of large
size, which may actually exceed the size of many
industrial dusts. A fine atmospheric mist, com-
monly called "fog," agglomerates into rather
large rain drops.
The term "aerosol" frequently is used in tech-
nical literature to designate the dispersion of
solid or liquid particles in a gas. The properties
of an aerosol thus involve the properties of
dispersed particles or dispersoids as well as
those of the gas phase. A summary of proper-
ties of typical aerosols, which is widely quoted
and frequently reproduced by a number of au-
thors, was compiled by C. E. Miller4J and is
reproduced as Fig. 1. Further references to
Miller's chart will be made throughout this
report.
For reasons of simplicity and convenience
herein, the term "aerosol" will be used to indi-
cate the more or less stable dispersions of
solids and liquids in a gas; another term,
"particulates," will be used to denote the dis-
persed particles, both solid and liquid. In con-
sonance with this terminology, the present re-
port will attempt to present information on the
theory and practice of the separation of par-
ticulates from aerosols by methods which utilize
a liquid to achieve or assist in the separation.
1.20 Classification of Wet Collectors
The unit operation by which particulate mat-
ter is removed from a stream of gases may be
viewed as a sequence of three steps:
1. Conditioning of particulate matter.
2. Disengagement or separation of the particu-
late phase from the carrier gas.
3. Removal of the separated particulates from
the collector.
In the industrial equipment, step 1 may be
absent altogether; may be accomplished sep-
arately; or may be combined with disengage-
ment, step 2.
In wet collectors, as defined in Par. 1.10, the
particulate matter leaves the collector as a
SOURCE: Removal of Particulate Matter From Gaseous Uastes--Wet Collectors
Engineering Report Prepared
New York, NY (1961).
for American Petroleum Institute,
-------
AMERICAN PETROLEUM INSTITUTE
f MiC'I'l)
Ctwpmini ler
Coiiiciif* 9*
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Gravity S«H'lnq el Soh«rtl in
0«> of SvHIing
Critical Par lid* DiQm*r*r
ADO«I wnich Lo. will
Not Apply
100.000
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ilh
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fg* ••rlieltl il «l •»•••••
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C, > Oi»-i'«» •* ti"»»it«l MMrcU. M,
f l It.li ••mil,. l«. IB«t«/(V. II.
FIC. 1—Summary of Properties of Some Typical Aerosols.
-------
REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
slurry or in a dissolved state. It is either dis-
carded to waste or is subjected to a separation
procedure—intended to recover the particulates,
if economically justified, or to permit recircula-
tion of the scrubbing liquid.
Conditioning of the particulate matter, step 1,
and its disengagement from the carrier gas,
step 2, in wet collectors may involve one or more
of several mechanisms. These mechanisms, dis-
cussed at greater length in Sect. 2.00, may be
listed as follows:
1. Impingement (impaction) and interception.
2. Brownian motion diffusion.
3. Humidification of gas.
4. Condensation of liquid on particles.
5. Agglomeration of particles.
6. Electrostatic precipitation.
Although it would appear, at a first glance,
that a classification of wet collectors based upon
the preceding mechanisms might be feasible,
in almost all instances commercial equipment
utilizes more than one of the mechanisms listed
and thus precludes such a classification. The
wide variety of scrubbers and the many possible
combinations of functional mechanisms make
it difficult to develop a comprehensive and mu-
tually exclusive classification of types.
A classification used by Lapple "•4- enjoys
wide acceptance and is adopted in the present
report. A summary of this classification of wet
collectors, which is used as the basis for presen-
tation of material in Sect. 3.00, is presented as
follows:
1. Chamber scrubbers.
2. Cyclonic scrubbers.
3. Inertial scrubbers.
4. Mechanical scrubbers.
5. Packed scrubbers.
6. Film scrubbers.
7. Miscellaneous scrubbers.
The several categories used in this classifica-
tion are briefly described as follows:
1. Chamber scrubbers are devices in which
the dust-laden gas simply enters and leaves a
chamber where one or more spray nozzles are
mounted. The chamber may be large in order
to slow down the gas flow (conventional spray
washer), or it may be in the form of a venturi
to speed up the gas flow. The spray nozzles may
be operated at a high pressure to produce a fog
or the liquid may be introduced into the throat
of an ejector to provide the draft for the move-
ment of the gas.
2. Cyclonic scrubbers contain structural char-
acteristics, which impart centrifugal forces to
the aerosol. The centrifugal action may be in-
duced by a tangential entrance of the dust-laden
air, by forcing the gas through specially shaped
vanes, or by constraining the gas to a spiral-
shaped chamber. The scrubbing liquid is intro-
duced in various manners, and vanes and baffles
frequently are used to facilitate disengagement
of the collecting liquid from the gas stream
before the latter leaves the scrubber.
3. Inertial scrubber is the term applied to
those devices in which the energy of the dust-
laden gas stream primarily is used to expand
the surface area of the scrubbing liquid and
thus obtain liquid contact.
In a venturi scrubber, a popular and effective
scrubber in the inertial group, scrubbing liquor
enters the throat of a venturi at a relatively low
velocity but encounters a high-velocity flow of
gas in the throat. In another type of wet in-
ertial scrubber, gas jets produced by an orifice
plate entrain the scrubbing liquor, which flows
over the orifice plate, and the mixed spray im-
pinges upon properly located target plates. In
still another type of inertial collector, the scrub-
bing liquor is contacted with dust-laden gas
when the latter is forced through suitably-
restrictive passages submerged in the liquor.
Other specific examples of the inertial scrubber
are described in Sect. 3.00.
4. Mechanical scrubbers are characterized by
the presence of some mechanical devices for
producing the spray. Thus, in one model of a
mechanical scrubber, widely used in blast fur-
nace gas cleaning, the dust-laden gas and scrub-
bing liquid are passed outward through a series
of rotating and stationary arms. In another
group of devices of the same general type, dust-
laden gas is scrubbed with liquor dispersed by
means of various shaped rotors dipping into the
liquid.
5. Packed scrubbers are conventional towers
packed with raschig rings, berl saddles, grids,
and so forth operated with liquor loadings well
below flooding point. In these scrubbers the
liquor primarily serves to wash the dust off
the surfaces and to avoid re-entrainment of
the dust.
6. Film scrubbers comprise those units in
which the scrubbing liquid is present only as a
film on collecting surfaces. In such units, except
for collection resulting from humidification and
-------
AMERICAN PETROLEUM INSTITUTE
condensation, the liquid serves merely to keep
the collecting surface free of solids and to pre-
vent re-entrainment of collected dust.
7. Miscellaneous scrubbers include a number
of wet collectors which are not conveniently
classified under the preceding categories.
1.30 Range of Application
Wet collectors as a class provide a moderate
cost, high-efficiency method for removal of par-
ticulates. Their use is indicated as a potential
means for the collection of particulates4= -when:
1. Addition of liquid to the gas stream is not
objectionable.
2. Particulate matter is very fine—diameters
predominantly ranging between 10 and 0.1 mi-
crons.
3. A moderately high collection efficiency is
required.
4. Gas must be cooled.
5. Vapors or gaseous contaminants must also be
removed from the air stream.
The use of a wet collector is not advisable
when a less expensive device, such as a cyclonic
scrubber, will perform satisfactorily. Wet col-
lectors normally are competitive with cloth
collectors (bag filters), although they may be
used to handle dust-laden gases under condi-
tions where bag filters are not applicable be-
cause of temperature, moisture conditions, ex-
plosion hazards, and so forth.
The use of wet collectors presents a number
of problems under certain conditions. Silver-
man <4 lists these conditions and resultant prob-
lems as follows:
1. Soluble particulates for recovery must be
recrystallized and may become contaminated
during collection.
2. Collector requires means of disposal for the
sludges, e.g., sludge ponds, tailing piles, etc.,
some of which may create a source of secondary
contamination.
3. Recovery of insoluble product requires a de-
watering step.
4. Low removal efficiency for particles smaller
than 1 micron in diameter.
5. Removal of soluble contaminants by the
scrubbing liquor may introduce corrosion prob-
lems.
6. Liquid entrainment in the effluent gas stream
represents a source of contamination.
7. Freezing problems in cold weather.
Wet collectors are used in many chemical and
process industries.
1.40 Comparison \vith Other Types of
Collectors
Kane " qualitatively compares the character-
istics of five principal types of dust collectors:
cyclones, high-efficiency cyclones, wet collectors,
fabric collectors, and high-voltage electrostatic
precipitators. The usual relationships between
the basic groups of collector designs are shown
in Appendix A; exceptions for specialized de-
signs are permissible.
The use of the collectors, representative of
the five basic groups in a number of industrial
processes, is summarized in Appendix B. Al-
though the ratings—usual, frequent, consider-
able, occasional, etc.—are qualitative and may
be expected to reflect the personal opinions of
the compiler," the tabulation nevertheless is
believed to be useful as a check list against con-
clusions reached by analysis of several factors
influencing collector selection.
2.00 REMOVAL OF SMALL PARTICLES
FROM MOVING GAS STREAM
2.10 General Considerations
As stated in Par. 1.20, as a class, wet collec-
tors are distinguished by the employment of
liquid to achieve or assist in the removal of
participate matter from gases. In most of the
wet collectors described in this report the liquid
is dispered through the gas and the collection
of the particles is the result of their interaction
with dispersed droplets of the scrubbing liquid.
Thus, the effectiveness of the removal of par-
ticles is the sum of interactions of all the drop-
lets with the particles of the aerosol. These
interactions are involved in both particle con-
ditioning and particle precipitation.
The several mechanisms of particle-droplet
interaction will be examined quantitatively in
the following paragraphs.
2.20 Particle Flocculation
The increase in the average size of the par-
ticles as a result of flocculation makes the sub-
sequent collection of particles much easier.
Flocculation occurs when small particles en-
gaged in Brownian motion collide. The colli-
sions are inelastic and if the particles are solid
—e.g., fumes of lead, oxides of zinc, magnesium,
iron—chainlike aggregates are formed. Liquid
particles coalesce into a single drop.
-------
REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
Since Brownian motion is a result of the
impact of molecules upon the particles, the
extent of this motion and, therefore, the impor-
tance of flocculation in collection of participates
increases with the temperature of the aerosol.
However, this is significant only for those par-
ticles which are sufficiently small to be in-
fluenced by molecular impacts. Thus, the phe-
nomenon of flocculation is of importance with
submicron sizes only, 0.1 micron or less. A high
concentration of particles, by increasing the
likelihood of collisions, similarly increases the
extent of flocculation.
The flocculation rate is given quantitatively
by an expression for the rate of decrease of the
particle concentration with time. Junge30 pre-
sents the flocculation rate for a homogeneous
aerosol composed of spherical particles in still
air as follows:
dt
""
(1)
Where:
dc
dt:
R':
T =
Km,=
Dp =
c =
flocculation rate, in particles per unit
volume per unit time.
gas constant=8.3(10r), in dyne-centi-
meters per (gram-mole) (degrees Kel-
vin).
absolute temperature, in degrees Kelvin.
gas viscosity, in poises or dyne-seconds
per square centimeter.
Avogadro's number = 6.02 (10=3) mole-
cules per gram-mole.
correction factor (1.72 for air).
mean free path of gas molecules, in mi-
crons (0.1 for air).
diameter of particles, in microns.
concentration, in particles per cubic cen-
timeter.
Flocculation is of some importance in the
conditioning of metallic fume particles before
removal in settling chambers or by bag filters.
In the case of wet collectors, where the usual
range of applications is in the micron range
or higher, Brownian diffusion and flocculation
do not contribute significantly to the collection
efficiency. Moreover, the interaction of par-
ticulates with droplets of the liquid predomi-
nates to such an extent that contribution of
collisions between particles themselves to the
phenomenon of conditioning is reduced further.
The occurrence of flocculation, nevertheless,
must be kept in mind when describing the
average particle size entering any collector, if
the process generating the aerosol gives rise to
a high concentration of submicron particles of
appropriate nature.
A substantial amount of flocculation may be
produced if an aerosol is exposed to sound waves
of high frequency such as from a siren. The
process is not well understood nor is it possible
to calculate accurately the coagulation rate from
theory.30- *° In general, the particles must be
smaller than 10 microns and their concentration
greater than 1 grain per cubic foot.
The process has been tried on a fair-sized
scale in the collection of carbon black 80 and for
the collection of sulfuric acid,13 but has not as
yet been established on a firm design basis.
2.30 Impaction of Particles on Liquid Droplets
Fundamental analysis of wet collectors re-
quires an analysis of impaction of small par-
ticles upon droplets of the collecting liquid with
which the collector, or a portion of it, is filled.
Such an analysis must include consideration of
all of the forces that operate between the par-
ticles and the droplets. For particles of normal
density in the micron and submicron ranges of
size, the primary mechanisms by which collec-
tion may take place are:
1. Inertia.
2. Interception.
3. Settling.
4. Brownian motion.
5. Electrostatic attraction.
In the case of inertia, a particle carried along
by the gas stream, on approaching an obstruc-
tion (liquid droplet), tends to follow the stream
but may strike the obstruction because of its
inertia. In Fig. 2 solid lines represent the fluid
streamlines around a droplet of diameter Db
and the dotted lines represent the paths of par-
ticles which initially followed the fluid stream-
lines.
For a flow around a spherical collector the
V
quantity i-, where X is the distance between
•L>1>
limiting streamlines A and B, represents the
fraction of particles initially present in a vol-
ume swept by the droplet which will be removed
by inertial impaction.
-------
AMERICAN PETROLEUM INSTITUTE
In interception, the trajectory of the center
of a relatively large particle, although not inter-
secting the collecting surface, may pass close
enough for the surface of the particle to touch
the collecting drop and be arrested by it. Also,
the particle may be encouraged to impact upon
the collecting surface by an actual or induced
electrostatic force between the particle and
collector.
surface provided by liquid droplets, Ranz and
Wong 5S defined the following parameters:
1. Inertial parameter, ^=18D ^Q^Pp1
Where:
C = empirical correction factor for resist-
ance of a gas to movement of small
particles, dimensionless =
-0.44D
0]
for
In the case of settling, particles in a slowly
moving air stream may settle out on the collect-
ing surface under the influence of gravity. For
particles smaller than 0.1 micron, Brownian
diffusion becomes significant.
Impaction of aerosol particles on body col-
lectors, such as cylinders and spheres, was stud-
ied theoretically by Albrecht,2 Landahl and
Herrmann,39 Langmuir and Blodgett,40 and by
Sell," all of whom considered inertia as the only
mechanism of collection. Because of the com-
plicated nature of the problem, the results of
these authors were not in good agreement, par-
ticularly with regard to the question of whether
a minimum particle size exists below which
impaction cannot occur.
Ranz and Wong 50 developed a mathematical
statement of the problem of impaction without
restricting their consideration to a single mech-
anism. They carried out limiting solutions and
made order-of-magnitude calculations, which
establish the nature and the importance of the
several mechanisms of impaction.
Through evaluation of the forces which affect
the motion of the particles and cause them to
move across the streamlines to the collecting
FLUID STREAMLINE
PARTICLE PATH
p = particle density, in grams per cubic
centimeter.
v0= velocity of aerosol stream, in centi-
meters per second.
D,,=diameter of aerosol particle, in microns.
Db = diameter of droplet, in microns.
/i=viscosity of gas, in poises.
A = ^£, mean free path of gas molecules, in
centimeters.
v = average molecular velocity of gas mole-
cules,* in centimeters per second.
Parameter ^ may be considered to be the ratio
of the force necessary to stop a particle initially
traveling at velocity v0 in the distance -£, to
£i
the fluid resistance at a relative particle velocity
of v0. It is also the ratio of the stopping dis-
tance—i.e., the distance a particle will penetrate
into still gas when given an initial velocity of
v0—to the diameter of the liquid droplet.
2. Interception parameter, R= -=p
Parameter R is the ratio of the particle di-
ameter to the diameter of the liquid droplet.
3. Settling parameter,
Where:
<>,,= particle density, in gram mass per cubic
centimeter.
gL = absolute value of local acceleration of
gravity, in centimeters per second per
second.
Parameter F is the ratio of the force of
gravity to the fluid resistance at a relative par-
ticle velocity of v0. It is also the ratio of the
FIG. 2—Inrrlial Impaction upon Single Droplet.
* v is the square root of the mean square velocity of
the molecular, the expression for which is given in all
standard treatments of kinetic theory; see for example,
Samuel Glasstone, Textbook of Physical Chemistry,
D. Van Nostrand Co., Inc., New York, 247 (1940).
-------
REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
free-settling velocity of the particle to the
stream velocity.
4. Broivnian motion diffusion parameter,
g_3Dr.M__ CR'T
v0Db /tDbDpv0
Where:
DDM = diffusivity, in square centimeters per
second.
R' = gas constant.
Parameter 8 is the ratio of diffusive force
caused by random thermal motion to the fluid
resistance.
5. Electrostatic attraction.
Two parameters are defined describing: 1,
the interaction of a positively charged particle
and droplet, KE; and 2, the interaction between
a charged droplet and a dielectric particle on
which the droplet charge induces a charge, K,.
These parameters are:
TABLE 1—Collection Parameters ut Venluri Throat
(Gas velocity at throat = 500 fps)
Ammonium
Sulflte
Dlhntyl
Phchnlate
K,= =
Where:
qp= electric charge on particle, in coulombs.
qac=electric charge on droplet, in coulombs
per square centimeter.
«„= permittivity of free space.
nl«ss parameters are based on
the mean diameters of the particles and water droplets. The
parameters for the electrostatic fon:i»3 are based on a maximum
charge density of (2.65)110-*) coulombs per square centimeter
for charged surfaces in air. Higher charges will leak away
through air lonizatlon and corona discharge. The dielectric
constant" for the ammonium salts was taken as 6.S nnd for the
dlbutyl phthalute as 6.
mechanisms of collection. At lower velocities,
electrostatic forces may become important if
particles are charged artificially. Collection by
Brownian diffusion is negligible even for the
0.27-micron ammonium chloride aerosol. This
parameter becomes important when the parti-
cles are smaller than 0.1 micron and when the
relative velocity between the droplets and par-
ticles is small.
The inertial impaction of aerosol particles
upon droplets of scrubbing liquid is discussed in
the following paragraphs.
2.40 Efficiency of Collection by Impaction on
a Single Droplet
The removal of aerosol particles through im-
paction on droplets of collecting liquid has been
the subject of a number of theoretical and ex-
perimental studies.3- =l--*• "• *°-:8' " Efficiency of
impaction, also termed "target efficiency," TH,
may be considered as the ratio of the area of the
aerosol tube—from which all particles are re-
moved—to the projected frontal area of the
droplet. In terms of Fig. 2, if X is the limiting
width of the initial streamlines, in which all
particles collide with and are impacted upon the
droplet of diameter Dh, the efficiency of impac-
tion is:
The several studies (see references cited),
notably that by Langmuir and Blodgett,40 es-
tablished that the target efficiency should be a
function of the dimensionless group
utV0 (3)
-------
REMOVAL OF PARTICIPATE MATTER FROM GASEOUS WASTES
11
varies linearly with the specific area of the
droplets.
2.60 Grade-Efficiency Curves for Collectors
Although the material presented in the pre-
ceding paragraphs is indicative of the effect of
particle size, droplet size, specific surface area,
properties of the gas phase, etc. on the collec-
tion efficiency, more dependable methods of pre-
dicting collector efficiencies must be based on
actual performance data. Unfortunately, the
collection of accurate performance data is time-
consuming, expensive, and is subject to a num-
ber of uncertainties involved in accurate char-
acterization of the particulates in the inlet and
outlet of a collector. Moreover, isolated data
for plant performance on different duties are
difficult to compare.
Stairmand ar proposed a method for practical
evaluation of performance of collectors and in a
subsequent publication "8 cited a number of ex-
amples drawn from extensive tests conducted
on large-scale industrial dust collection equip-
ment.
Stairmand describes the performance of a
number of dust collectors in terms of a "grade-
efficiency" curve characteristic for each type of
equipment. In such curves, the collection effi-
ciency for particles having an "effective den-
sity" of 2.7 g per cu cm is plotted against the
particle size in microns. Fig. 6, reproduced
from Stairmand," is an example of such curves
and is based upon a large industrial spray tower
processing 70,000 cfm of gas.
In addition, Stairmand "3 describes a method
for direct comparison of different collector types
S IO 19 20
OJBTICLE SIZE (MICRONS)
Courtesy of C. J. Stairmand and The Institute of Fuel
(London).
FIG. 6—Craclc-EfHciency Curve for Spray Tower.
based on the use of their characteristic grade-
efficiency curves. This method consists in cal-
culating the overall collection efficiency of par-
ticulates from a standard-test aerosol. It is
exemplified in Table 2, in which the overall col-
lection efficiency of a spray tower, with a grade-
efficiency curve as shown in Fig. 6, is computed
for three dusts which have the same grading as
shown in the first two columns of the table.
Table 2 also includes a prediction of the grading
of dust in the outlet gas. It is assumed that the
spray tower operates under essentially the same
conditions as those for which the grade-effi-
ciency curve was established.
The calculations summarized in Table 2 illus-
trate the effect of particle-size distribution upon
the overall collection efficiency of the scrubber.
The presence of sizable particles results in a
large predicted overall removal efficiency (ap-
proximately 92 per cent for minus 150-micron
dust), whereas the fine dust can be removed
much less efficiently (77 per cent for minus 10-
micron dust).
The method described by Stairmand M and
illustrated in Table 2 can be used to compare
directly the performance of different types of
collectors by computing in a similar manner the
expected overall collection efficiencies of the
standard dust by the different collectors. It
should be noted that the comparison depends
upon the availability of experimentally deter-
mined relations of the grade efficiencies versus
particle size for conditions of gas flow, droplet
size, and so forth for which comparison is to be
made.
3.00 INDUSTRIAL WET COLLECTORS
3.10 Chamber Scrubbers
Chamber scrubbers represent a class of wet
collectors in which dust-laden gas simply enters
and leaves a chamber where one or more spray
nozzles are mounted. There are many industrial
variations of this class of collectors. These
range from a stack spray arrangement, in which
a single spray is installed in a stack carrying
exhaust gases from a furnace, to much more
complicated, specially designed towers, where a
series of baffles is used to impart to the gas a
tortuous path and provide for repeated contact
of the dust-laden gas with scrubbing liquor.
Several examples of chamber scrubbers are de-
scribed herein.
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12
AMERICAN PETROLEUM INSTITUTE
TABLE 2 — Prediction of Overall Efficiency of a Spray Tower and Grading
J 2 »' <" 5' ^
Efficiency
Per Cent by at Mean
Size of Weight In Size of
Oracle Grade nt Grade
(Microns) Inlet (Per Cent)
Dust A (Minus 150 Microns)
104 to 150
75 to 104
60 to 75
40 to 60
80 to 40
20 to 30
15 to 20
10 to 15
7.5 to 10.0
5.0 to 7.5
2.5 to 5.0
1.0 to 2.5
0.5 to 1.0
0.0 to 0.5
3
7
10
15
10
10
7
8
4
6
8
5
4
3
100
100
99.5
99.1
98.7
98.3
97.9
97.2
97.0
95.5
94.5
93.0
85.0
42.0
15.0
Overall
Collection
(Per Ceni)
3.00
6.97
9.91
14.81
9.83
9.79
6.80
7.76
3.82
5.67
7.44
4.25
1.68
0.45
92.18
of Exit Dust
C"
Grading of Exit Dust
As Per Cent
of Inlet
0.00
0.03
0.09
0.19
0.17
0.21
0.20
0.24
0.1S
0.33
0.56
0.75
2.32
2.55
7.82
Acrunl
I'erCent
0.0
0.4
1.1
2.4
2.2
2.7
2.6
3.1
2.3
4.2
7.2
9.6
29.6
32.6
100.0
Dust B (Minus 40 Microns)
30 to 40
20 to 30
15 to 20
10 to 15
7.5 to 10.0
5.0 to 7.5
2.5 to 5.0
1.0 to 2.5
0.5 to 1.0
0.0 to 0.5
15
15
11
12
6
10
12
&
6
5
100
9S.3
97.9
97.2
97.0
95.5
94.5
93.0
85.0
42.0
15.0
14.75
14.69
10.69
11.64
5.73
9.45
11.16
6.80
2.52
0.75
SS.18
0.25
0.31
0.31
0.36
0.27
0.55
0.84
1.20
3.48
4.25
11.82
2.1
2.6
2.6
3.0
2.3
4.7
7.1
10.2
29.4
36.0
100.0
Dust C (Minus 10 Microns)
7.5 to 10.0
5.0 to 7.5
2.5 to 5.0
1.0 to 2.5
0.5 to 1.0
0.0 to 0.5
• Olnmn •"•:
11 (.'(iliiinn 4 :
« Column 0:
J Column li:
13
21
25
17
13
11
100
Rp.-ul OH" frum Fig. G.
(ColnmnS) (Column 2)
100
Column 2-Coliiinn 4.
(Column S) (1001
Total oC Culiiinii u
95.5
94.5
93.0
85.0
42.0
15.0
12.42
19.85
23.25
14.45
5.46
1.65
77.07
0.58
1.15
1.75
2.55
7.54
9.35
22.92
2.5
5.0
7.6
11.1
33.0
40.8
100.0
3.11 Gravity Spray Towers
The use of sprays located on top of foundry
cupolas for control of cupola emission is de-
scribed by Brechtelsbauer.' The final arrange-
ment recommended as a result of operating ex-
perience is shown in Fig. 7. Here the flue gas is
deflected by means of a conical baffle and is
forced to flow through the annulus. A single
spray delivers water onto the top of the conical
baffle; the water flows across the top of the baffle
and contacts the gas in the annular space be-
tween the edge of the baffle and the enclosing
shroud. The flow of water over the conical baffle
also serves to cool the baffle and extends the life
of the cone. The tests indicate that the dust
content of the outlet gas can be reduced to ap-
proximately 0.2 grain per cubic foot of gas,
measured at standard conditions, with liquid
-------
REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
13
consumption of approximately 6 gpm per sq ft
of cross-sectional stack area. The installation *
is inexpensive and this degree of scrubbing is
stated to meet the code requirements in many
areas of the country.
Updegraff'- reports the results of a labora-
tory study on collection of fly ash and dust from
boiler stacks during soot-blowing and fire-clean-
ing periods by means of a single nozzle installed
centrally in the stack. An arrangement recom-
mended by Bituminous Coal Research, Inc./"'-
is illustrated in Fig. 8. The spray collection
efficiency was shown to be influenced primarily
by the water rate. A rate of 0.4 gpm per sq ft
of stack area appeared to be optimum, giving
a collection of 60 to 70 per cent of the dust. The
selection of a proper nozzle is based on water
pressure available; nozzles which deliver the
recommended volume of water are easily se-
lected from manufacturers' catalogs.
An example of a considerably more compli-
cated spray tower is described by Ashman,5 see
Fig. 9. The dust-laden gas is made to follow an
upward tortuous path during which phase it is
contacted by liquid introduced at the top of the
tower and is cascaded downward over the baf-
fles. Primary sprays located near the gas inlet
are used to reduce the temperature of gas and to
facilitate dust removal. The mechanism of dust
collection is principally the inertial impaction
of the dust on wetted surfaces, although some
WATER
SUPPLf PIPE.
SPRAY
CLEAN OUT
DOOR
Courtesy of Bituminous Coal Research, Inc.
FIC. 8—Furnace Slack Spray Scrubber.
i UPPER
SPRAYS
GAS
OUTLET
^BAFFLES
Courtesy of American Foundrymen's Association.
FIC. 7—Cupola Cas Scrubber.
PRIMARY
—I SPRAYS
I DRAIN
Courtesy of R. Ashman and Institution of Mechanical
Engineers (London).
FIG. 9—Gravitational Spruv Scrubber.
-------
14
AMERICAN PETROLEUM INSTITUTE
impaction on water droplets cascading from the
edges of baffles undoubtedly also occurs.
The only data of sufficiently general interest
on the collection efficiency of gravity spray
towers is that given by Stairmand;" see Par.
2.60 and Fig. 6. The data refer to performance
of a large industrial spray tower, handling ap-
proximately 70,000 cfm of gas. The tower is
22 ft in diameter by 66 ft high and uses
75,000 gph of water, about IS gal per 1,000 cu
ft of gas. The pressure drop is stated to be
negligible, less than 1 in. of water. The efficien-
cies of removal of particulates in such a tower
range from 99 + per cent for particles 60 mi-
crons or larger to 55 per cent for 1-micron
particles, see Fig. 6. An overall efficiency of re-
moval of particles in a test aerosol containing
solid particles of sizes up to 150 microns is ap-
proximately 92 per cent, decreasing to 77 per
cent for a test aerosol containing solid particles
of sizes up to 10 microns only, see Table 2.
Although the gravity spray tower is to some
extent tending to become obsolete because of its
relatively high cost, it is often used as a pre-
cooler where very large quantities of gas are
involved, e.g., in blast furnace operation. An-
other advantage is that no very fine clearances
are involved, thus it can handle relatively high
concentrations of dust without chokage. As in-
dicated in Par. 2.40, because very small spray
droplet sizes are not called for, the spray nozzles
need not produce fine jets and reliability is im-
proved, with the added advantage that the spray
water can be recirculated until it contains quite
a high concentration of suspended solids, which
simplifies the effluent treatment and disposal
problems.
The grade-efficiency curve for a spray tower,
see Fig. 6, shows that the collection efficiency of
a spray tower decreases rapidly for particles
smaller than approximately 2 microns. The con-
sideration of the effect of droplet size on target
efficiency also shows that there is little point in
decreasing the droplet size, even if it could be
accomplished economically. Thus, inherently,
the use of spray towers is limited to cases where
a high collection efficiency of particles in sizes
below 2 microns is not required.
3.12 Water-Jet Scrubbers
In an effort to increase the relative velocity
between the droplets of the scrubbing liquor
and the aerosol, the scrubbing liquor may be
supplied in the form of a high-velocity jet di-
rected along the axis of a venturi nozzle. In the
usual industrial construction the water-jet is
utilized both as a means for moving the dust- or
fume-laden air and for removal of particulates.
A typical water-jet scrubber is shown in Fig. 10.
Manufacturers' catalogs "• »• provide perti-
nent data on the sizes of units and nozzles, water
consumption as a function of the water pres-
sure available, and the amount of gas to be
passed through the unit. The literature M states
that standard units permit a reduction of 70 to
95 per cent of particulates with a single unit
and a reduction of 80 to 99 per cent in a bank
of four units placed in series for particles larger
than 5 microns. Where the particle size is less
than 5 microns, much higher velocities and tur-
bulences are required for adequate collection.
No other information on collection efficiencies
has been published.
As is shown in Fig. 10, in addition to the jet,
a water-gas separating device, which may be
a simple gas reversal chamber, sometimes
Courtesy of Schutte and Koevting Company.
FIG. 10—Tvpical Water-Jet Scrubber.
-------
REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
15
equipped with baffles, must be provided on the
outlet side. Sizes ranging from 3 in. to 72 in.
in inlet diameter are available with capacities
up to 110,000 cfm (at 1-in draft, water pres-
sure is 200 psig, and water flow through the
nozzle is 10,000 gpm).
3.13 Chamber Scrubbers with Mechanical
Spray Generators
Kristal, et al.,38 describe a wet chamber scrub-
ber consisting of single or multiple collection
stages, each containing a venturi tube and two
mechanical spray generators. An experimental
unit, tested by the authors, consisting of four
similar collection stages is diagramed in Fig. 11;
the individual stages are contained in quadrants
of a cylindrical housing, and not in a simplified
structure as is schematically indicated. In each
stage the aerosol encounters a spray in the
plenum chamber, passes through the venturi
section and again is contacted with a spray in
a plenum chamber following the venturi section.
In multistage units, the spray generators are
used as indicated in Fig. 11.
The spray generators are of mechanical de-
sign: water jets produced by relatively large
orifices (TVm- diameter) impinge upon a motor-
driven, rapidly rotating disk (3,300 rpm)
equipped with beveled vanes. The rotation of
the disk produces a fine spray of water drop-
lets with a linear velocity initially approaching
that of the vane tips (approximately 150 fps).
The droplet size is of the order of magnitude of
100 to 400 microns and depends upon the
amount of water supplied, speed of the disk
rotation, orifice size, and so forth.
In operation, dust-laden gas enters the ple-
STJOE OUTLETS
Q SPHAV oeNEftirOfl. 9 CAL./MIN. Q 7 V.S.I.
Courtesy of the Harvard Air Cleaning Laboratory and
U.S. Atomic Energy Commission.
FIG. 11—Experimental Four-Stage Wet Collector
Equipped with Meclianicul Spray Generators.
num chamber of the first stage where it en-
counters the first-spray generator. Collisions
between spray droplets and particles in this
zone, followed by inertial separation of large
drops (prior to entering the venturi tube), af-
ford a considerable degree of dust removal. The
aerosol then enters the first-stage venturi tube
in which additional collisions occur. Here, be-
cause of an increase in approach velocity be-
tween the relatively large water droplets and
the small dust particles in the accelerating air
stream, significant target efficiencies are ob-
tained. The velocity of small particles remains
essentially the same as that of the gas so that
they pass through a zone of relatively slow-
moving, large droplets, which increases the
probability of capture by impaction. A small
temperature drop, produced by rapid (adia-
batic) expansion, occurs in the water-saturated
gas stream as it traverses the expanding portion
of the venturi. Contrary to the expectation of
the designers, the condensation of water vapor
on dust particles serving as nuclei, which would
"condition" the particles, is not an important
mechanism of collection because of the very
brief retention time.
After leaving the venturi tube the aerosol en-
ters a second plenum where it passes over the
second-spray generator. For single-stage opera-
tion the aerosol is then withdrawn through a
droplet eliminator. For multistage operation the
cycle is repeated as shown in the schematic dia-
gram, Fig. 11.
The collection efficiency of the device depends
upon the aerosol size, the number of stages, and
the rate of gas flow. Significant improvement
was observed by using a second stage, but use
of additional stages produced only minor im-
provement. Efficiencies determined for a single
stage varied from 99 per cent by weight for
resuspended fly ash (median diameter—count
basis, 0.6 micron; mass basis, 14.3 microns) to
22 per cent for iron oxide fume (median di-
ameter—count basis, 0.03 micron; mass basis,
0.6 micron).
The device is characterized by rather high
pressure loss, approximately 7 in. of water per
stage, and large power and water requirements
compared to other wet collectors capable of
similar performance. The mechanical spray
generator, however, is nonclogging; its use may
eliminate the need for elaborate filtering ap-
paratus and thus permit the economic recycling
of the spent spray water in regions of sparse
water supply.
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16
AMERICAN PETROLEUM INSTITUTE
3.20 Cyclonic Scrubbers
This group of wet collectors includes several
types of units in which scrubbing is combined
with cyclonic action. In these units, the action
of water droplets conditions the aerosol parti-
cles by a combination of impaction, humidifica-
tion, and condensation. The centrifugal motion
of the aerosol is utilized to separate the condi-
tioned particles from the gas phase by an in-
ertial mechanism. The numerous industrial
modifications differ in the methods of introduc-
ing the scrubbing liquor, inducing centrifugal
motion of the aerosol, arrangements for reduc-
ing entrainment, and in details of removing the
sludge from the unit.
Several typical industrial modifications of cy-
clonic scrubbers are discussed in the following
sections and illustrated in schematic drawings.
All of these units can be used to clean hot gases;
scrubbing liquor usually is recirculated, pro-
vided appropriate arrangements for removing
the collected solids are made; and soluble con-
stituents of the gas phase are absorbed in the
scrubbing liquor.
3.21 Cyclonic Scruhhers Equipped with Vane
Baffles
These units are cylindrical towers with a
tangential entry of the aerosol. Several typical
units are illustrated in Fig. 12 through 17.
The cyclonic wash scrubber, illustrated in
Fig. 12, accomplishes scrubbing "• *; of an aero-
sol by a combination of inertial and impaction
mechanism. The unit is an arrangement of one
or more washing stages followed by an elimi-
nator stage, as shown in Fig. 12. Centrifugal
motion is imparted to the gas stream both by
the tangential inlet and by the vanes. Washing
liquid usually is introduced through nozzles
above the top washing stage. In a multistage ar-
rangement the lower stages may be supplied
with additional liquid, this arrangement is rec-
ommended for high dust loadings to maintain
desired slurry characteristics and also for pre-
humidification where elevated temperatures are
encountered.
In operation, the dust-laden air enters the
lower chamber. The centrifugal motion aided
by initial wetting removes the relatively large
particles from the air stream. The air stream
then is accelerated by passing through the vanes
of the washing stage where particulates impinge
upon wetted surfaces of the vanes. The ac-
celerated, centrifugally moving air stream then
contacts the washing liquid in the center spray
CLEAN AIR OUT
WATER
MIST.LADEN
AIR IN
WATER OUT
Courtesy of C. F. Montross and Chemical Engineering.
FIG. 12—Cyclonic \Fash Scrubber.
GAS OUT
OiRTY
GAS IN
WATER OUT
Courtesy of C. F. Montross and Chemical Engineering.
FIG. 13—Cyclonic Baffle Scrubber.
-------
REMOVAL OF PARTICIPATE MATTER FROM GASEOUS WASTES
17
CLEAN AIR OUT
GAS OUT
SEPARATOR
IMPINGEMENT
PLATES
Courtesy of C. F. Montross and Chemical Engineering.
FIG. 14—Cyclonic Multiple-Ruffle Scrubber.
chamber, where the particles are impacted upon
the liquid droplets and the latter are separated
from the gas stream by centrifugal motion. A
shielded-cone baffle prevents formation of a vor-
tex in the air stream, and directs the liquid onto
the vanes and into the centrifugal path of the
air stream.
A set of vane baffles is provided at the top of
the tower to reduce the entrainment of liquid.
To increase the efficiency of removal, two or
more washing stages of similar design may be
required. For removal of submicron particles,
multivane designs are used; in these, two rows
of vanes are mounted inversely against each
other and are spaced more closely to provide a
larger surface area for impingement.
Units are designed with capacities ranging
from 500 cfm to 40,000 cfm of gas. Normal
water requirements for units having a single
washing stage are approximately 2 gal per
1,000 cu.ft of gas. The pressure drop ranges
from 1£ in. to 3 in. of water. Units employing
more than one washing stage are characterized
by a higher water requirement and pressure
drop.
ANTI-SPIN
VANES
CORE
BUSTER
DISK
DAMPER
WATER
IN
GAS IN
Courtesy of C. F. Montross and Chemical Engineering.
FIC. 15—Cyclonic Spray Scrubber I.
CLEAN AIR
DISCHARGE
NOZZLE
ORIENTATION
CONTAMINATED
AIR INTAKE
BANKS OF
NOZZLES
PUMP
LIQUID EFFLUENT
Courtesy of D. G. Hudson and Heating and Ventilating.
FIG. 16—Cyclonic Spray Scrubber II.
-------
18
AMERICAN PETROLEUM INSTITUTE
CLEAN AIR
OUTLET
VERTICAL
SPRAY
RISERS
QUICK OPENING
NOZZLE
LATCHES
.TOWER NOZZLES,
DIRECTED
CROSS-FLOW
FLUSHING JETS,
DIRECTED DOWNWARD
RECTANGULAR
INLET
FRESH WATER
SUPPLY
WASTE OUTLET
Courtesy of Buffalo Forge Company.
FIG. 17—Cyclonic Spray Scrubber III.
One manufacturer " states that collection ef-
ficiencies of 98 to 99 per cent by weight can be
realized in most cases. Friedlander, et al.,=-
reports a weight collection efficiency of 74 per
cent, when the effluent from a dry-cyclone in-
stallation was washed in a hydraulic scrubbing
tower of the design described herein. The two
figures are not necessarily contradictory. The
higher efficiency refers to collection of particu-
lates from usual industrial effluents in which a
fairly wide range of particle sizes is encoun-
tered; the lower efficiency refers to secondary
treatment of an effluent with low loading
(5.8 grains per cubic foot) and particle size of
1.5 microns and less.
Performance efficiency data of wider applica-
bility which would relate the collection efficiency
with particle size, such as is illustrated in Fig. 6,
is not available. It would be expected, however,
that a properly designed unit of this type would
give a grade-efficiency curve intermediate be-
tween that of a spray tower and that of a wet-
impingement scrubber, approaching the latter.
As is shown in Fig. 6 and 23, a recovery of
98 per cent is not unlikely for the hydraulic
scrubbing tower combining the cyclonic and
impingement effects.
Essentially the same principles are utilized
and comparable effectiveness of operation is
realized in other multistage towers with a some-
what different design of vane baffles, see Fig. 13
and 14.
3.22 Cyclonic Scrubbers with Radial Spray
Injection
Another widely used wet collector, a cyclone
spray scrubber, based on the" scrubbing of a
centrifugally moving gas stream with a liquid
spray,-'3' "•30 is illustrated in Fig. 15. In this
unit dust-laden gas enters tangentially at the
bottom of a cylindrical tower and pursues an
upward spiral path. No baffles are used. Spray
is introduced into rotating gas from an axially
located manifold in the lower part of the unit.
A core-buster disk is located usually above the
sprays to prevent formation of a vortex. The
spray droplets sweep across the path of the gas
stream and, because of the centrifugal motion
imparted to them by the gas stream, impinge
upon the wall of the tower, run down and out
the bottom of the unit together with the col-
lected particulates.
The cyclone spray scrubber is an efficient de-
vice for removing small particles from gases
when the particles are larger than 2 microns.
For smaller particles the efficiency of the cy-
clone spray falls off rapidly as the particle size
decreases, unless extremely high ratios of the
scrubbing liquid to gas are used. This decrease
in efficiency of deposition of the small particles
has been ascribed to streamlining of particles
around the drops.30
Kleinschmidt" and Kleinschmidt and An-
thony 3C developed an equation for the overall
efficiency of removal of dust particles by a
scrubber of the type described as a function of:
D = diameter of tower, in inches.
W = effective volume of the scrubbing liquid,
in cubic feet per minute.
d = diameter of droplets formed, in inches.
G = volume of gas scrubbed, in cubic feet
per minute.
For example, for an S-ft-diameter cyclone in
which 25,000 cfm of gas tangentially enter
through a 4-in. by 2-in. inlet, the centrifugal
force is approximately 27.5 times gravity and
-------
REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
19
the target efficiencies of individual droplets of
100-micron diameter are estimated to be:
Particle Size
(Microns)
5
2
1
0.5
0.2
Target Efficiency
0.98
0.32
0.11
0.0
0.0
Equation (6), developed in Par. 2.50, relates
the overall effectiveness of particle removal to
the conditions of scrubbing by including the
target efficiency in the exponent of e:
P'W
Thus, the overall efficiency of a cyclonic scrub-
ber is affected by all of the factors which deter-
mine the value of 171 :
1. Particle size.
2. Relative velocity between particles and spray
droplets.
3. Density of particles.
4. Viscosity of gas.
5. Droplet size.
In a cyclonic scrubber the relative velocity
between particles and spray droplets varies con-
tinuously as the droplet traverses the path from
the nozzle to the wall ; the distribution of drop-
let diameters is determined by the nozzle design,
pressure of liquid, rate of liquid, and so forth.
A theoretical construction of a grade-efficiency
curve thus becomes very difficult; experimental
data on effectiveness of cyclonic scrubbers are
fragmentary and meager. An empirical method
of scrubber design based on performance of a
small scrubber, or a large scrubber operating
under different conditions, is described by
Kleinschmidt.3*
The overall efficiencies typical of a number
of industrial installations are assembled below ;
these efficiencies apply only to specific installa-
tions described in the references cited :
Particle
Size Range
Dust (Microns)
Lime 2.0 to 40.0
Lime 1.0 to 25.0
Lead compounds 0.5 to. 2.0
Iron ore, coke, etc 0.5 to 20.0
Ammonium nitrate (Unstable)
Chemical fume 0.5 to 3.5
Chemical fume 0.2 to 2.0
Boiler fly ash 2.0 to 5.0
Cyclonic spray scrubbers require from 2 to
10 gal of water per 1,000 cu ft of gas. The pres-
sure drop is usually in the range of 1 in. to 4 in.
of water. A variety of corrosion-resistant ma-
terials is used in cyclonic spray scrubber con-
struction to permit its use for the scrubbing of
gases containing soluble and corrosive com-
ponents.
3.23 Cyclonic Scrubbers with Circumferential
Spray Injection
In another version of a cyclonic spray tower,
described by Hudson =' and Thomas,70 the spray
nozzles are placed in several parallel banks
throughout the height of a vertical tower. The
nozzles are arranged to discharge at a common
angle and are directed nearly tangentially to aid
in imparting a centrifugal motion to the aerosol
traversing the tower. A spray tower of this
design is shown in Fig. 16. The dust-laden
gases enter at the top of the tower, spiral down-
ward through the tower, and exit through a
central duct, concentric with the tower.
A spray tower of a similar design " features
a construction with easily accessible nozzles,
permitting inspection and replacement of the
nozzles while the unit is in operation. In this
design, shown in Fig. 17, the aerosol enters at
the bottom of the tower through a rectangular
tangential port and exits from the top of the
tower.
In both designs the scrubbing liquid is intro-
duced through the nozzles at high pressures.
400 psig to 600 psig, in an effort to improve the
collection of small particles. Information on
droplet size in a spray thus produced is not
available, although Silverman and Davidson81
give indirect evidence that at 400 psig the drop-
let size is less than 50 microns. In a fog-like
spray the droplet size is well below the optimum
size needed for impaction. The droplets quickly
approach the same velocity as the suspended
aerosol particles and thus high impaction effi-
ciencies cannot be realized. The increased spe-
Dust Loading
(Grains per Cubic Foot)
Inlet
9.2
7.7
Exit
O.OS
0.25
3.0 to 24.0 0.03 to 0.08
17.0
2.25
0.25 to 2.5
1.02
0.79
0.045 to 0.125
Removal
Efficiency
(Per Cent)
99
97
97
98
99 +
94
65
82 to 95
Reference
No.
51
51
51
51
51
36
36
36
-------
20
AMERICAN PETROLEUM INSTITUTE
cific surface area of the fine droplets, however,
would tend to counteract this effect and facili-
tate impaction. In addition, Brownian motion
diffusion would become operative and submi-
cron particles would collide with and impact
upon the fog droplets. This consideration of
mechanisms of collection suggests that fog-
producing nozzles would add to the effectiveness
of collection of submicron particles. In the
units diagramed in Fig. 16 and 17, larger
particles would be collected by inertial forces
induced by the centrifugal motion of the gas,
and the submicron particles would be collected
by combination of diffusion and inertial im-
paction.
Collectors of the type described herein some-
times are referred to as "fog niters." =*•:o The
fog filter, because of its limitations and the
great diversity of engineering processes, must
be engineered to the job. It appears to be more
effective on aerosols containing particles less
than 10 microns in diameter; presence of a large
proportion of larger particles requires the use
of a companion unit such as a dry cyclone. Per-
formance data for the fog filter on some indus-
trial gases given by Thomas T0 indicate a 100-
per-cent removal of sulfuric acid mist, 90.0- to
99.4-per-cent removal of solids from fertilizer
mixing plants, and approximately 90-per-cent
removal of low-density organic solids, such as
phthalic and maleic anhydrides.
A composite efficiency tabulation for a similar
scrubber using 400 psig sprays, cited by a
manufacturer," is as follows:
Size Fraction Efficiency
(Microns) (Per Cent by Weight)
0.0 to 0.5 78.6
0.5 to 1.0 33.4
1.0 to 2.0 89.7
2.0 to 3.0 94.0
3.0 to 4.0 96.4
4.0 to 6.0 97.5
6.0 to 8.0 98.6
8.0 to 10.0 99-2
These data are presented also in the form of a
grade-efficiency curve in Fig. 18 by plotting the
efficiencies versus the average of size fractions.
For scrubbers of this type., collection efficien-
cies can be brought to almost any desired level
for many materials by the following means: :o
1. Increasing tower height.
2. Compounding stages.
3. Controlling gas volumes.
4. Regulating liquid flow.
5. Regulating spray pressures.
FIG. 18—Cra
-------
REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
41
APPENDIX A
DUST COLLECTOR CHARACTERISTICS (ACCORDING TO KANE M)
Hlsh-
EtBclency
Cyclones
Wet
Collectors
Item Cyclones
Effect of Dust Variations
Efficiency, particles:
< 1 micron Poor Poor Poor to fair
1 to 10 microns Poor Poor to fair Fair to good
10 to 20 microns Poor Good Good
> 20 microns Fair to good Good Good
Abrasion resistance Fair Fair Good
Ability to handle sticky, adhesive ma-
terials Fair Poor Poor to good
Bridging materials give trouble Slight Yes No
Fire or explosion hazard minimized... Fair Fair Good
Can handle hygroscopic materials Yes Fair Yes
Large foreign materials cause plug-
ging Seldom Yes Seldom to yes
Effect of Gas Stream Variations
Maximum temperature (degF),
standard construction 750 750 No limit
Troubles from condensed or entrained
mists or vapors Slight Considerable Slight
Corrosive gases attack standard con-
struction Slight Slight Severe
Collector
Space Large Modest Modest
Pressure drop 1 in. to 2 in. 3 in. to 5 in. 3 in. to 6 in.
Reduced volume adversely affects col-
lection efficiency Yes Yes with mosc Depends on
designs design
1 Fabric
Collectors
Good
Good
Good
Good
Good
Poor
Yes
Poor
With care
Seldom
180 to 275
Considerable
Slight
Modest to
large
2 in. to 6 in.
No
High-
Voltage
Electrostatic
Prccipitators
Good
Good
Good
Good
Good
Poor
Yes
Poor
With care
Yes
750
Some
Slight
Large
1 in. to 2 in.
No
-------
APPENDIX »
USUAL AIR CLEANER SELECTIONS FOR INDUSTRIAL PROCESSES (ACCORDING TO KANE")
f'ulluirliit
Usoil In Industry
C)|)i!rnlliiii
Ceramics
Haw product handling
Fettling
Refractory sizing
Glaze and vitreous enamel spray. . . .
Chemicals
Material handling
Crush ing, grinding
Pneumatic conveying
Roasters, kilns, coolers
Coal Mining uiid rower I'lunl
Material handling
Hunker ventilation
Ucdusling, air cleaning
Drying
l-'ly Ash
Coal -burn ing:
Chain grate
Stoker fired
I'ulvcri'/.cd fuel
Wood-burning
Foundry
Shake out
Sand handling
Tumbling mills
Abrasive cleaning
Ciincriitrnlliin
Light
Light
Heavy
Moderate
Light to
moderate
Moderate
to heavy
Very heavy
Heavy
Moderate
Moderate
Heavy
Moderate
Light
Moderate
Heavy
Varies
Light to
moderate
Moderate
Heavy
Moderate
to heavy
rnrtii-li!
Si/.fM
Fine
Fine to
medium
Coarse
Medium
Fine to
medium
Fine to
coarse
Fine to
coarse
Medium
to coarse
Medium
Fine
Medium
to coarse
Fine
Fine
Fine to
coarse
Fine
Ifeavy
Fine
Fine to
medium
Medium to
coarse
Fine to
medium
C'yclnniM
Rare
Rare
Seldom
No
Occasional
Often
Usual
Occasional
Rare
Occasional
Frequent
Hare
No
Rare
Rare
Occasional
Rare
Rare
No
No
ni^ii-
(•'.Illrli'licy
ryrlnlli-X
Seldom
Occasional
Occasional
No
Frequent
Frequent
Occasional
Usual
Occasional
Frequent
Frequent
Occasional
Ran:
Usual
Frequent
Occasional
Rare
Rare
No
Occasional
\Vrl
Cnllrrliir.s
Frequent
Frequent
Frequent
Usual
Frequent
Frequent
liarc
Usual
Frequent
Occasional
Occasional
Frequent
No
Nn
No
No
Usual
Usual
Frequent
Frequent
Knlirlc
Am-Mlcrs
Frequent
Frequent
Frequent
Occasional
Frequent
Frequent
Usual
Rare
Frequent
Frequent
Often
No
No
No
No
No
Rare
Rare
Frequent
Frequent
IliBh-
Kli-rlrtisliilta 1
rrri:l|>itiili>ra
No
No
No
No
Rare
No
No
Often
No
No
No
Nn
No
Rare
Frequent
No
No
No
No
No
inn'iar
NIL*
1
2
3
4
r.
G
7
8
il
10
11
12
33
31
10
1C
17
18
-------
Cruin Elevator, Flour ami Feed Mills
Grain handling
Grain dryers
Flour dust
Feed mill
Metal MMi-ng
Steel blast furnace.
Steel open hearth.
Stool olcctric furnace....
ferrous cupola
Nonfcrrous rcvcrhcralory
Nonferrous crucible
Metal Mining imdRuek I'rotlueta
Material handling
Dryers, kilns
Cement rock dryer.
Coment kiln
Cement grinding
Cement clinker cooler
Melul Working
Froduclion grinding, scratch brushing,
abrasive cutoff
Portable and swing frame
Tinning
Tool room
Cast iron machining
I'liiirmiicenticnl mid Food Products
Mixers, grinders, weighing, blending,
bagging, packaging
Coating pans
/'/((sties
Uaw material processing.
1'laslic finish ing
1'roilitcts
Mixers ...................
liatchout rolls ............
Talc dusting and deducting.
Grinding .................
\VooituMrrkhnj
Woodworking machines .
Sanding
Waste conveying, hogs..
• l>'or lU-iimrka act- p. 4-1.
Light
Light
Moderate
Moderate
Heavy
Moderate
Light
Moderate
Varied
Light
Moderate
Moderate
to heavy
Moderate
Heavy
Moderate
Moderate
Light
Light
Light
Light
Moderate
Light
Varied
Light to
moderate
Moderate
Light
Moderate
Moderate
Moderate
Moderate
Heavy
Medium
Coarse
Medium
Medium
Varied
Fine to
coarse
Fine
Varied
Fine
Fine
Fine to
medium
Medium to
coarse
Fine to
medium
Fine to
medium
Fine
Coarse
Coarse
Varied
Varied
Fine
Varied
Medium
Fine to
medium
Varied
Fine
Fine
Medium
Coarse
Varied
Fine
Varied
Usual Occasional
No No
Usual Often
Usual Often
Frequent
No
No
Rare
No
No
Rare
Frequent
Karc
Uarc
Occasional
Frequent
Frequent
Frequent
Frequent
Rare
Hare
Uarc
Hare
No
No
Karc
No
No
Occasional
Frequent
Frequent
Frequent
Karc
Occasional
Frequent
Rare
Rare
Frequent
Frequent
Frequent
Rare
Rare
No
Occasional
Occasional
Frequent
Doubtful
Considerable
Frequent
Hare
Rare
Usual
Frequent
Occasional
It a re
No
Considerable
Frequent
Frequent
Frequent
Considerable
Frequent
Frequent
Frequent
No
Frequent
Frequent
No
Possible
Frequent
Occasional
Occasional
Considerable
Rare
No
No
Frequent
Considerable
Rare
Rare
Frequent
Considerable
Frequent
Frequent
No
No
No
No
Frequent
Probable
Rare
Occasional
Occasional
Occasional
Considerable
Rare
No
No
No
No
No
No
(Sec commmls under cliemiciila)
Frequent Frequent Frequent
No
No
No
Often
Usual
Frequent
Usual
No
No
No
Often
Occasional
Occasional
Rare
Frequent
Usual
Frequent
Frequent
Rare
Occasional
Occasional
Frequent
Usual
Frequent
Usual
Often
Frequent
Frequent
Occasional
No
No
No
No
No
No
No
No
10
20
21
22
23
24
25
2C
27
28
20
30
31
32
33
34
35
3(J
37
38
30
40
41
42
43
44
45
40
47
48
40
-------
REMOVAL OF PARTICULATE MATTER
FROM GASEOUS WASTES
FILTRATION
1.00 INTRODUCTION
1.10 Definition
Filtration is an operation in which a stream
of gas carrying suspended particulate matter
(dust, fume, fog, mist, or aerosol) is passed
through a porous medium in such a way that
the particles impinge on, and adhere to, the
medium and are thereby removed from the gas
which freely passes on through the filter. In
many filters the deposit of dust so collected be-
comes, in turn, the filtering medium for suc-
ceeding particles.
As the deposit accumulates, inevitably the
porosity of the medium is reduced, eventually to
the point where the flow of gas becomes so re-
stricted as to require either: 1, removal of the
deposit in order that the medium can be re-used;
or, 2, discarding of the clogged filter and re-
placement with new medium. It is an inherent
feature of filtration that arrangements must be
provided to accomplish one of these alternatives
either periodically or continuously.
1.20 Types of Filters
A variety of filter arrangements and many
kinds of porous media may be used. Possible
arrangements include passing the dust-laden
gas through:
1. A flexible sheet, layer, tube, or bag, as of
woven fabric, felt, or paper.
2. A semirigid supported fabric or nonwoven
mat of fibrous material.
3. A rigid porous solid.
4. A fixed or packed bed of dry granular par-
ticles.*
5. A fluidized or moving bed of granules or
fibers.
Fibers used to make fabrics or mats may be
of wool, cotton, metal (e.g., steel wool), asbes-
* The usual wet-packed tower is considered a wet
collector rather than a filter.
tos, cellulose, fiberglas, Fiberfrax (ceramic),
Orion, Nylon, or other synthetic polymers. They
may be used in the natural (dry) condition or
treated—as with resins, in order to give them
an electrostatic charge; or with a viscous oil, in
order to make them sticky. Rigid solids include
metal screens, porous metals, or porous ceram-
ics. Fixed or moving beds may be composed of
granules of sand, coke, slag wool, crushed rock,
and other materials.
The material and type of filter to be used in
a given application must be selected with due
regard for the composition, temperature, and
moisture content of the gas stream; particle
size and nature of the dust; corrosion and abra-
sive effects which may be present; collection
efficiency desired; and economic aspects of the
operation, including possible recovery of the
dust, re-use of the filter, or both.
Filters may be classified in various ways ac-
cording to type of media, arrangement of sup-
port, cleaning mechanisms, etc. A full discus-
sion of filter media and filter arrangements is
given in Sect. 3.00. However, it should be
pointed out here that a very basic distinction
is made between two general modes of filter
collection:
1. The medium itself is the essential separation
mechanism.
2. The medium acts primarily as support for
the collected particles, which become the effec-
tive filter.
Auxiliary equipment, such as dust removal
and filter removal devices, blowers, ducts, and
stacks, is usually involved. There may also be
the need for coolers and other gas-conditioning
devices before the filtration proper. Sometimes
another type of particle-collecting equipment
may also be used in series with a filter.
1.30 Range of Application
Dalla Valle has stated: "It is possible to se-
cure almost any kind of filter for handling any
type of particle with any desired degree of effi-
SOURCE: Removal of Particulate Matter From Gaseous Wastes—Filtration,
Engineering Report Prepared for American Petroleum Institute,
New York, NY (1961).
-------
AMERICAN PETROLEUM INSTITUTE
ciency. This can be said of no other method of
particle collection known."""
This statement indicates the most important
advantage of filtration as a method of particle
collection—that it is capable of high-efficiency
collection of very small particles. Its use is to
be considered whenever a very low dust load-
ing of the effluent gas stream is required, and
especially when the particles to be removed are
in the range of from 10 microns down to less
than 1 micron in size.
On the other hand, filtration is a relatively
expensive gas-cleaning operation with several
important limitations. If the dust load of the
gas to be treated is high, it may be necessary
to use another type of collector as a preliminary
stage ahead of the filter. Certain filters require
a rather low lineal gas velocity for effective
operation, and this may, in turn, call for a very
large area of filter surface. A relatively large
pressure difference is required to force the gas
stream through the filter, and this involves
large energy consumption in the blower or com-
pressor. Cleaning and renewal devices involve
mechanical apparatus requiring careful main-
tenance. Most filter media must be used at
relatively low temperatures and on relatively
dry gas in order to avoid excessive deteriora-
tion. Many of them have rather limited re-
sistance to corrosion and rather low mechanical
strength, which limits the amount of handling
they can withstand. Special media have been
developed to overcome some of these disadvan-
tages, but their higher cost may not be war-
ranted in many applications.
The following are examples of situations in
which filters have been used with satisfactory
results:
1. Removing dust from atmospheric air (as
used in heating or ventilating systems) by
panel filters made up of mats of glass, metal,
vegetable fibers, animal hair, etc.; often coated
with some oily or adhesive substance or resin
(to impart electrostatic charge); usually dis-
carded and replaced when dirty; sometimes
washed, retreated, and re-used.tos
2. Collecting oxide, ash, and carbon fumes from
gray iron or nonferrous cupolas in Orion, wool,
or silicpne-treated glass \vool bag filters, cleaned
by periodic manual shaking.10'll1
3. Removing sand and stone dust from kiln
stack gas leaving a bituminous mix asphalt
plant, by filtration through dense wool felt bags
(following a preliminary cyclone collector),
cleaned continuously by a flow of air from a
1 Figures refer to REFERENCES on p. 54.
"reverse jet" traveling over the obverse surface
of the bag."
4. Cleaning blast furnace gas by bag filters
made of a specially woven fabric of asbestos
and glass fibers, mounted in a special way to
minimize the deterioration due to the shaking
necessary for cleaning.70
5. Removing fine radioactive or other particles
with very high efficiency (99.99 per cent) from
exhaust or supply ventilation air by a soft felt-
like asbestos-bearing cellulose paper formed in
thin pleated sheets, used until dirty, and then
discarded.91
6. Cleaning dust (including carbon particles
from generator brushes) from fresh and re-
circulated air supplied to "motor rooms" by a
vertical endless-belt type of continuously travel-
ing screen, self-cleaning and oiled as it dips into
a pan of oil at the bottom of the belt.1"
1. Removing air-borne bacteria in order to
produce sterile air in industrial fermentation
plants (e.g., in penicillin manufacture), by
filtration through thick beds of slag or glass
wool, "cleaned" in situ by sterilization with dry
heat a large number of times before replace-
ment becomes necessary.18
8. Carrying fine particles of valuable catalyst
from a fluidized bed and recovering them by
passing the exhaust gas through porous stain-
less steel filters made by sintering powdered
metal, cleaned by automatic blowback.92
9. Removing iron oxide dust in stack gases
from an open-hearth furnace by filtration
through a horizontally moving bed of fine-fiber
slag wool, continuously reclaimed by passing
through a washing, drying, and bed-reforming
system.110
10. Removing coal dust from synthesis gas
streams passed countercurrently upward to a
vertically moving bed of granular coke, peri-
odically removed from the bottom, washed,
drained, and returned to the top."
11. Removing sulfuric acid mist from air by
passing it through a fluidized bed of porous
silica gel or alumina particles revivified by
washing with water and drying for re-use.38
The foregoing examples were selected only
to give a general idea of the possible applica-
tions of filtration methods of particle collection;
they are not intended to form a comprehensive
survey of the field. They do serve to indicate
two broad classes of filter duty, which may be
defined roughly as:
1. The air-cleaning range, characterized by a
relatively low dust loading of the carrier gas,
e.g., less than 1 grain per 1,000 cu ft.
-------
REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
2. The dust-collecting range, with loading of
the carrier gas ranging upward from 1 grain
per 1,000 cu ft to as high as (5) (104) grains
per 1,000 cu ft.
In general, the nonrenewable mat, pad, paper
or bed type of filter will be used for air cleaning
(examples 1, 5, 7), with a cleanable type used
in some cases (example 6). The heavier load-
ings involved in dust collecting usually require
renewal of the filter by cleaning, either to re-
cover valuable material in the dust (example 8)
or to prevent excessive coat of filter replace-
ment where the main purpose of collection is
to reduce air pollution (examples 2, 3, 9, 11)
or process gas contamination (examples 4, 10).
1.40 The Role of Filters in Collection
The position of filtration in comparison with
other methods of particle collection is a ques-
tion of relative effectiveness and relative cost.
This is indicated in a general way by the classi-
fications given by Kane ••:
1. High-efficiency, high-cost collectors:
a. Electrostatic precipitators.
6. Sonic agglomerators.
2. High-efficiency, moderate-cost collectors:
a. Fabric or fibrous filters.
6. Wet collectors, packed towers, scrubbers,
and centrifugals.
3. Low-cost, lower efficiency designs:
a. Cyclones and dry centrifugals.
b. Dry dynamic.
c. Inertial.
A chart presented by Stairmand1!1 based
upon actual operating data generally supports
the foregoing classifications, although showing
venturi scrubbers as being most costly and elec-
trostatic precipitators as less efficient than fil-
ters (see Sect. 4.00, Fig. 11).
The important effect of particle size upon col-
lector performance is illustrated by the charts
presented by Kane OB and McCabe." These indi-
cate that filters retain their high-efficiency char-
acteristics down to finer particle sizes than any
other collectors except electrostatic precipita-
tors.
These general results are supported by many
tests reported in the literature. They serve to
substantiate the statement that filtration is a
moderate- to high-cost method of collection, ca-
pable of the highest efficiency, and particularly
effective in dealing with fine particles.
It should be noted that these statements refer
mainly to fabric and fibrous filters, inasmuch as
the position of porous solid, packed-bed, and
moving-bed filters is not indicated in these clas-
sifications. This is because these methods are
of rather recent development and there is insuf-
ficient experience available from commercial
installations to judge their ultimate role. Most
of the pilot plant or developmental type of
studies which have been reported, however,
indicate that such filters are all also capable of
high efficiency, but probably at higher cost—
which may be justifiable in special applications.
2.00 THEORY OF FILTRATION
A comprehensive theory of the operation of
filters should deal with the mechanism and effi-
ciency of particle collection, the life of the filter
medium, and the pressure drop through the
filter. Each of these questions will be considered
in turn. For the reader who does not wish to
consider theory in detail, the following para-
graphs will give the highlights of the theoreti-
cal approach and sources of the practical results
obtained: 2.11, 2.127 (p. 12, 15), 2.133, and
2.30.
Theoretical developments have for the most
part been limited either to the performance of
a "clean" filter, i.e., at the beginning of the col-
lection of a homogeneous dust, or to one on
which a homogeneous dust has collected in a
uniform manner. That actual filter behavior
may be more complex than is contemplated in
these^implified models is pointed out in several
places in the following discussion.
2.10 Particle Collection
In order for a particle to be removed from
the gas stream, it is first necessary that it col-
lide with the surface of an element (e.g., fiber
or granules) of the filter and then adhere to
this element, at least until it is desired to clean
the filter. The theory must therefore deal first
with the interaction between individual parti-
cles and individual filter elements, and then
with the composite effect of all the elements
making up the filter medium.
2.11 Basic Mechanisms for Collision
Stoppage of particles by direct sieving action
is seldom an important aspect of dust filtration.
as the spaces between the filter elements are
usually much larger than the particles collected.
If this were not the case, clogging would occur
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4
AMERICAN PETROLEUM INSTITUTE
rapidly, with an attendant severe rise in pres-
sure drop across the filter.
1 Other mechanisms must be relied upon to
cause the particles to collide with the obstacle,
which the filter element represents, in their
path. These have been identified as:
1. Direct interception or flow line interception.
2. Inertial deposition or impaction.
3. Diffusional deposition or Brownian move-
ment.
4. Gravity settling.
5. Electrostatic precipitation.
6. Thermal precipitation.
Direct interception, or flow line interception,
occurs whenever the fluid streamline along
which a particle approaches a filter element
passes within a distance from the element equal
to one-half the particle diameter. If the particle
has a very small mass, although of finite size,
it will not deviate from the streamline as the
latter curves around the obstacle and will there-
fore collide if the streamline passes sufficiently
close. This is illustrated by path A in Fig. 1.
Inertial impaction occurs when the mass of
the particle is great enough that it cannot fol-
low the streamline rapidly curving around the
obstacle but tends to continue along a path of
lesser curvature. This brings the particle closer
to the filter element than it would have ap-
proached along the streamline. Collisions may
therefore occur due to this inertial effect, even
when flow line interception would not take place
(see path B in Fig. 1).
Brownian movement will be superimposed
upon the flow motion of very small particles.
This may cause a particle to diffuse toward, and
contact the surface of, a filter element as it
flows by.- The particle must pass sufficiently
close to the obstacle for a long enough time in
order for the relatively slow diffusional velocity
to bring about a collision. The process is akin
to mass transfer by molecular diffusion.
Gravity settling onto the filter surface may
result from vertical motion of a particle due to
its weight as it passes through the filter.
Electrostatic precipitation will occur as a re-
sult of electrostatic forces drawing particle
and filter element together whenever either or
both possess a static charge. These forces may
be either direct attraction, where both particle
and filter are charged, or induced, if only one
of them is charged. Such charges are usually
not present unless deliberately introduced dur-
ing the manufacturing of the filter. They must
be strong enough to draw a particle out of its
flow path to the filter surface during the time
the particle passes nearby.
Thermal precipitation may occur whenever
there is a temperature gradient between the
gas stream and the filter surface. Particles may
thus be caused to migrate toward a cold surface
or away from a warm one. Thermal gradients
do not exist in normal practice, and this princi-
ple has not yet found industrial application. It
will not be discussed further in this report.
All of these mechanisms are not usually in
effect at the same time in a given filtration sit-
Electrostatic Attraction.
b (A) Direct -Interception
FIG. 1—Streamlines and Particle Trajectories Approaching
Filter Element.
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REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
uation. Only one, or a combination of just two adheres to it and therefore may be regarded as
or three of them, may be involved. It is neces- "collected," the single-element efficiency of col-
sary to analyze each filter application to deter- lection or "target" efficiency is defined as
_cross-sectional area of fluid stream from which particles are removed
17 cross-sectional area of filter element projected in direction of flow
mine the controlling mechanisms present, in or,
order that the filter may be operated to the best _ b
advantage. Which mechanisms are important 1?=D7 ^
in a given case will be determined by such fac-
tors as: size and density of particles, size and
nature of filter elements, velocity and pattern of
fluid flow, temperature, nature of gas, existence
of electric fields, etc.
Each mechanism may be characterized by a
basic dimensionless parameter which may be
calculated with reference to a single aerosol
particle and an individual filter element. From
the magnitude of these parameters, the impor-
tance of each mechanism may be judged in a
given case and its contribution to the efficiency
of the filter estimated. These parameters are
discussed in detail hereinafter.
2.12 Collection Efficiency of Individual Filter
Elements
There are principally two kinds of elements
used to make up aerosol filters: fibers and gran-
ules. The theoretical ideal model of a filter ele-
ment may therefore be regarded as either a
cylinder (representing a fibrous element) or a
sphere (representing a granular element).
Granular filters (e.g., deep-bed, fluidized-bed,
moving-bed) have not as yet come into wide-
spread use for various reasons; therefore, the
spherical model collecting element has been the
subject of little or no theoretical investigation
as applied to dry filters. For wet collectors of
the spray type, the spherical model represents
the individual drop of liquid as the collecting
element and, therefore, has been studied exten-
sively.
The pattern of theoretical investigation of
the various mechanisms is much the same for
cylindrical and spherical collectors, with due
allowance for the consequences of the geometri-
cal differences. For the reasons just mentioned,
only the theory of cylindrical elements will be
presented here. A review of the theory of
spherical elements may be found in the report
Wet Collectors by Gilbert, which is another in
this API series on dust collection.
In theory, the idealized aerosol particle is al-
ways regarded as a sphere. Assuming that each
such particle which collides with a filter element
In Fig. 1, — is shown as the initial distance
from the central streamline of particles which
just graze the surface of the element, and D, is
the diameter of that element. The value of b,
and the resulting target efficiency, must be con-
sidered for each mechanism separately and for
various combinations of mechanisms operating
simultaneously.
Since any particle passing by whatever
mechanism within distance =£• of the element
0=| will be collected, the efficiency can also be
calculated as a flow ratio:
(2)
Here Q represents the volumetric rate of flow
per unit length of cylinder in the space within
a distance of-^of the surface at 0=|. Q is
found from the velocity profile v£ at B = ~
~ &
according to
(3)
2.121 DESCRIPTION OF FLOW PATTERN: A
description of the pattern of streamlines around
a filter element is evidently needed in develop-
ing the theory of collection by any of the mecha-
nisms. For a cylinder, exact mathematical de-
scriptions taking into account the boundary
layer and separation lines are available but
very complicated. It has been assumed that the
equations of streamlines relatively close to, and
in front of, the cylinder are all that is needed.
On this basis, two simplified descriptions have
been used: 1, ideal fluid; and, 2, viscous fluid.
Ideal fluid: At high Reynolds numbers
(strictly, as /VA.9-»co) the flow in front of a
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24
AMERICAN PETROLEUM INSTITUTE
Df. At medium velocities, direct interception
is important; and the smaller Dt, the higher the
filtration criterion.
3. For the same Df, Df, and ft: The filtration
criterion decreases at first, remains fairly con-
stant, and then increases as V increases.
4. For the same Df, V, and ft: At the high values
of V, the filtration criterion increases with in-
creasing Dr At medium values of V, the filtra-
tion criterion remains essentially constant. At
high values of V, the filtration criterion in-
creases with decreasing Dr
There are really no quantitative theoretical
methods to predict filter life or the optimum
time of effective performance. The experi-
mental data of Rowley and Jordan,107 for ex-
ample, show that r;, may remain essentially
constant for some time after Ap begins to in-
crease appreciably. When Ap is increasing more
rapidly than the penetration, the filtration cri-
terion begins to decrease rapidly. At this point,
it is necessary to stop and either clean or renew
the filter. However, this point can be deter-
mined only by trial under operating conditions.
3.00 INDUSTRIAL DUST FILTERS
Industrial dust filters may be classified ac-
cording to their construction and mode of op-
eration by the following scheme:
Cloth or Fabric Collectors
A. Intermittent—operation interrupted by
cleaning:
1. Bags, bag houses, tubes, hoses.
a. Cleaned by shaking or rapping.
6. Cleaned by reverse flow of air.
2. Screen-supported envelopes.
a. Cleaned by shaking or rapping.
b. Cleaned by reverse flow of air.
B. Continuous—cleaning continuous during
operation:
1. Multiple sections of A-l and A-2.
2. Reverse-jet filters.
3. Nonshaking types.
Fixed Beds or Layers
A. Granular—deep beds of coke, sand, etc.
B. Fibrous:
1. Air filters—mats of fibers for air cleaning.
a. Viscous impingement—fibers coated by
fluid adhesive: 1, disposable; 2, renew-
able; 3, washable; 4, automatic self-
cleaning (see Moving Beds, B-l).
b. Dry: 1, disposable; 2, renewable; 3,
washable; 4, automatic self-cleaning (see
Moving Beds, B-l).
2. Mats or pads for dust recovery.
a. Dry, through flow.
b. Radial flow, variable compression.
c. Treated—electrostatic, etc.
3. Papers.
a. High efficiency: cellulose, asbestos, glass,
plastic.
b. Multiple plies.
C. Rigid porous:
1. Porous metal.
2. Plastic.
3. Porous ceramic.
Moving Beds
A. Granular:
1. Gravity flow of collector granules.
2. Fluidized bed of collector granules.
B. Fibrous:
1. Self-cleaning air filters.
2. Traveling mat of collector fibers.
The construction, operation, applications, and
performance of each of the foregoing types of
filters will be described briefly.
3.10 Cloth or Fabric Collectors
The collecting medium is a woven or felted
fabric which first collects particles by the vari-
ous mechanisms of impingement until a layer
or floe of dust is formed. This, in turn, acts as
the medium to collect additional particles by
sieving action. The collection becomes more effi-
cient, but the pressure drop increases ever more
rapidly as the operation proceeds. Cleaning
removes most of the floe but leaves a basic layer
in the fibers so that efficiency increases after
new cloth is installed, and the layer of dust is
an essential feature in obtaining high efficiency.
For this reason, best results are obtained on
gas streams carrying a high load of dust. There
are two general designs available: the tube or
bag type, and the cloth screen type.
3.11 Bags, Bag Houses, and Tube Filters
The filter fabric is in the form of a bag, tube,
or hose, either cylindrical or oblong, and sus-
pended vertically. Many such bags are operated
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REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
25
in parallel, all confined in one structure called
a bag: house. This is possibly the oldest type
of filter.
In Bennett's description7 of one installed in
1906, it is stated that there were 1,920 bags,
each 18 in. in diameter by 28 ft 8 in. in length,
housed in a reinforced concrete building ap-
proximately 90 ft by 126 ft by 60 ft high. These
bags were suspended from their closed top ends,
with the open bottom ends held in place by
rings set in a concrete floor. Dust-laden hot
gases from a lead smelter were blown, at the
rate of 55,000 cfm, by an enormous fan into a
chamber below the bag floor, where much of
the coarser and heavier particles settled out.
The remaining finer particles were carried up
inside the bags by the gas flow and trapped on
the inner surface, the clean gas passing through
the bags and out to a common flue and stack.
Periodically the bags were shaken—apparently
manually—to dislodge the collected dust, which
then fell into the chamber below. With reduc-
tion in size, and improvements in capacity and
cleaning methods, this type of filter is still in
use today.
Present-day versions of the bag house ar-
rangement include one or more of the following
modifications:
1. A mechanical shaking device to agitate the
top hanger.
2. Subdivision of the bag house into 'units
which may be closed off individually for clean-
ing while the remainder continues to operate,
thus giving essentially steady continuous op-
eration.
3. Increase of gas pressure on the external side
during cleaning in order to cause a reverse flow
which aids in removing the dust loosened by
shaking.
4. Induced flow of gas by exhaust fans on the
clean side, where the dust cannot affect the fan
operation.
5. Automatic timing of the filtering and shak-
ing periods; and automatic control of the
switchover from one to the other, which may
be activated pneumatically by the increase in
pressure drop across the fabric as the dust
collects.
6. A precoat of filter-aid on the inside of the
bag to promote the highest collection efficiency
from the very beginning of the collection period.
7. Completely enclosed units for indoor use.
8. Envelope- or oblong-shaped bags for more
compact utilization of space.
Standard sizes of bag units are available
from a number of manufacturers.48' ur Bags
are usually made with a length-to-diameter
ratio of less than 20 to 1, often in the neighbor-
hood of 16 to 1, a fairly common size being 6 in.
in diameter by 8 ft long. Standard units con-
tain anywhere from just a few bags up to as
many as 1,300 bags and are capable of filtering
up to 60,000 cfm of gas at standard conditions.
A variety of fabrics is used, each having certain
desirable properties for certain applications.
Fabrics are discussed in detail in Par. 3.14.
Operating conditions are fairly standardized
with regard to velocity and pressure drop. The
value of V, referred to as the air-to-cloth ratio
or filter ratio (cfm flow per square foot of filter
surface), usually ranges from 2.0 to 5.0 fpm
and occasionally as high as 10 or 12 fpm. Pres-
sure drop, \T>, is on the order of 0.2 to 0.5 in.
of water for a clean filter, and 2.0 to 5.0 in. of
water for a "dirty" one. Operation is always
at or near atmospheric pressure, with allowable
temperatures depending upon the fabric used.
Obviously, the chemical nature of the dust and
gas must also be considered in the choice of
fabric.
It is generally considered =0- "• "• ur-ll3 that
the most appropriate range of dust loading for
bag filters is for 0.1
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AMERICAN PETROLEUM INSTITUTE
.SOLENOID
VALVE
DIRTY
GAS
SOLIDS
Courtesy of Chemical Engineering.
FIG. 7 Bag Filters Cleaned by Air Jets.
the cleaning jets. Standard sizes handling up
to 5,000 cfm per unit are offered.
Bag houses are used in industry for a wide
variety of nitration purposes. Their application
is limited chiefly by the range of temperature
and moisture content of the gas and by the
degree of shaking which available fabrics can
withstand. For this reason, gas conditioning
(cooling, and reduction of moisture content)
prior to nitration is often necessary. Fabric
properties are discussed in Par. 3.14 and gas
conditioning in Par. 3.42.
With proper preceding of the gas, bag houses
have been very successfully used to clean the
hot gases arising out of a variety of metallur-
gical operations such as those performed
gray iron cupolas,80 electric steel foundries,"
open-hearth steel furnaces," lead blast fur-
naces9 copper-base alloy smelters,112 and the
like When strict air pollution control was put
in force in Los Angeles County in the early
1950's bag houses were the only answer to the
dust recovery problems of many foundries.20
Summaries of such applications are given in a
number of articles."- >'•u4 A typical example
is that of the Alhambra Foundry, described by
Siechert and Menardi,110 data for which are
tabulated as follows:
Particulate emission: inert ash, silica, and iron
oxide—0.8 to 1.6 grams
per cubic foot, 25 per
cent finer than 325-mesh.
Gas volume: 13,100 cfm at 400 F.
Bag house: four compartments, 112 bags each
—each bag 11 in. in diameter by
15 ft long.
Fabric: silicone-treated glass wool—total area
4,835 sq ft.
Filter ratio: 2.7 fpm.
Operating conditions: 400 F; Ap = 3 m. to 4 m.
of water.
Shaking: manual, by compartment, every
90 min..
Collection efficiency: 99 4- per cent.
Applications to nonmetallurgical operations,
usually but not always involving gases at ordi-
nary temperatures, are also common. Gold-
field" describes an enormous bag house
48 compartments, each containing 1,200 bags
5 in. in diameter by 14 ft long, for filtration
of 90 tons per minute of air in an asbestos
mill Many uses are found in connection with
crushing and grinding operations such as are
associated with mining, materials preparation,
and the various mineral industries."-
handling equipment in foundries, coal grind-
ing " and similar operations produce dust often
recovered by bag houses. A typical example of
a small-scale operation is cited by Silverman
Particulate emission: dust from hydrated lime
packing operation — 7
grains per cubic foot;
100 per cent finer than
37 microns, 43 per cent
finer than 13 microns.
Gas volume: approximately 1,200 cfm to
1,500 cfm.
Bags: cotton fabric, 340 sq ft total surface.
Filter ratio: 8 f pm to 5 f pm.
Operating conditions: room temperature; A}>
= 2 in. to 5.2 in. of
water.
Shaking: once every 4 hr.
Collection efficiency: 99+ per cent
Aside from the limitations imposed by fabric
properties, bag house filters also suffer from the
disadvantages of low filter ratio which requires
large filtering surface and a large amount of
space for installation. Various other cloth t
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REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
27
ter arrangements, which represent attempts to
overcome some of these problems, are discussed
in the succeeding sections.
3.12 Screen-Supported Envelopes
In this arrangement the filtering surface con-
sists of a number of bags, each shaped as a
rectangular envelope open on one end. Each
bag or envelope is slipped over a wire screen
or similar supporting framework so that the
fabric is stretched taut. That end of the screen
where the envelope is open is attached to a rack
which holds a number of similar screens in
parallel. The envelope is always mounted so as
to stand on edge, but its long dimension may be
either in a horizontal or a vertical direction.
Fig. 8 shows a typical arrangement.
Dust-laden gas flows inward around the out-
side of the envelopes, and the dust collects on
their outer surfaces. The clean gas passes
through the fabric, flows inside the screen sup-
port, then out the open end to a manifold and
gas discharge duct. Therefore, the rack which
holds the screens also serves as a connection to
the clean-gas discharge system.
Cleaning of the bags is accomplished by shak-
ing the screen supports, by a reverse flow of
clean air through them, or by a combination of
the two actions. If the bags are to be shaken, the
supporting rack is attached to a motor-driven
vibrating system or to a device which raps the
rack intermittently with a series of sharp
blows. If a reverse flow of air is to be used,
a valve system must shut off the flow of dirty
CLEAN AIR
TO FAN
CLEANING AIR FROM /
ATMOSPHERE
Courtesy of W. W. Sly Manufacturing Company.
FIG. 8—Screen Cloth Filter.
air to a group of bags and, simultaneously,
admit clean air into the open end of the enve-
lopes. Ingenious arrangements are available
for doing either of these things continuously
and automatically over all of the bags in the
filter in sequence, thus giving an essentially un-
interrupted steady filtering operation. In one
make of filter, where a suction fan is installed
in the clean-gas duct to induce the flow, the
same suction is used to create the flow of the
cleaning air, as shown in Fig. 8.
In this type of filter the volume of space
needed for a given area of filter surface is
much less than in a bag house, because the flat
envelopes can be arrayed more compactly than
bags or tubes. The filter unit thus requires less
room and less floor space, and may be installed
in locations where other filters would not fit.
Since the fabric remains taut and is not
subjected to flexing (except for a slight balloon-
ing action in reverse-flow cleaning), it need
not have as much mechanical strength as that
used in bags. Certain fibers may therefore be
used which have excellent heat- or corrosion-
resistant properties, but which could not with-
stand the shaking action in a bag house. The
shaking mechanism for the screen supports
would necessarily be heavier and more power-
ful than for flexible bags. For this reason, the
reverse-flow cleaning is preferable, provided it
can be made to dislodge the dust as effectively.
Operating conditions for bag houses and
screen filters are very similar. A series of
comparative tests conducted by Dennis, et al.,so
showed essentially the same ranges of filter
ratio, pressure drop, inlet dust loading, and
collection efficiencies for both types. Standard
screen filters are available in capacities ranging
from a few hundred cubic feet per minute for
self-contained unit filters of less than 10 bags
to upwards of 60,000 cfm for filters of 600 to
700 bags.
Despite the advantages cited in the forego-
ing, the cloth screen seems not to be as widely
used as the bag house. There are many more
articles in the literature describing specific bag
house installations than those describing cloth
screen filters. There also seem to be more
manufacturers of the bag type, according to
Silverman.117 The reason apparently is that
many types of dust particles are particularly
difficult to dislodge on cleaning, and the clean-
ing of the cloth screen is not effective enough
to prevent clogging in these cases.-
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42
AMERICAN PETROLEUM INSTITUTE
reverse-jet filter (see Par. 3.13) the traveling
jet mechanism can be set in operation when-
ever, and only so long as, the pressure differ-
ential is above this limit. Bag-shaking devices
may also be operated automatically in this way.
Such control is especially desirable whenever
the dust loading or volume of gas flowing fluctu-
ates very much. However, whenever a steady
condition of rate of flow and dust concentration
prevails, the cleaning cycle may be controlled
simply by a program timer. This will auto-
matically, and regularly, cut the filter to be
cleaned out of operation for a fixed period of
time, during which the cleaning mechanism
operates. In this way a large bag house, for
example, can give overall continuous operation
as different sections of it are cleaned in turn.
3.45 Safety Measures
The principal hazard in connection with dust
filters is that of explosion and fire whenever
the dust consists of a combustible material.
Other hazards may arise in special cases de-
pending upon the nature of the dust, e.g., health
hazards such as silicosis, radiation injury from
radioactive substances, infectious diseases from
air-borne bacteria, etc. In these cases, the filter
is itself the principal safety device acting to re-
move injurious substances from the air. There-
fore, the principal precaution to be taken is
the elimination of all leaks from the filtration
system to avoid recontamination of the air. An
electric-eye dust detector may be installed in
the clean-gas discharge ducts or stack to detect
bag failures or leaks automatically.
Mumford, et al.,53 in discussing the applica-
tion of cloth filters to the collection of coal dust,
give a good summary of safety measures to be
taken against explosion and fire. These include:
1. Flameproofing of filter fabric.
2. Installation of explosion vents on each filter
compartment.
3. Elimination of horizontal runs of duct work
wherever possible.
4. Provision of access doors and cleanout plugs
in all locations where dust might settle in the
system.
5. Elimination of all possible sources of sparks
or static electrical discharges.
Jameson" points out the fire hazard inher-
ent in the use of oil-film air cleaners and in-
dicates that an oil or adhesive substance of
fire-resistant properties should be used. The
automatic self-cleaning type of continuous air
filter should be equipped with its own automatic
fire-extinguishing system.
3.50 Operating and Maintenance Problems
Throughout the literature the theme ex-
pressed in the following quotation from Kane68
is found over and over again:
"Because dust collection equipment is not truly
production machinery, it has too often been
installed in a place that is inaccessible with the
unfounded hopes that once installed it can be
forgotten. Nothing is further from the truth
and the more effective the design, the more
complicated will be the collector construction
and the more frequent need for inspection,
servicing and preventative maintenance."
Accordingly, it is important to consider plans
by which dust filters may be kept in smooth,
trouble-free operation. Such planning should
be kept in mind from the very beginning of
the design of the installation and should be
based upon a careful study of the filter manu-
facturer's instructions for installation opera-
tion and maintenance. This should result in the
establishment of a servicing schedule setting
forth the operations to be performed and their
frequency.
Several lists of items to be included in such
a servicing schedule have been proposed in
connection with installations in different in-
dustries by Bolt,10 Kidder," Smith,1" Mumford,
et al.,80 Swift,"1 and Harris and Mason," in
addition to the general comments of Kane.84-66
The following list of problems and comments
represents a composite digest of these refer-
ences :
Leakage through tlie filter: This is perhaps
the most important service problem. Bag filters
must be regularly inspected for holes or tears
in the fabric and fiber filters for channeling
or leaks around the edges between the bed and
frames. Regular measurement of the down-
stream dust concentration serves as a check on
faulty filtration. An electronic-eye dust detec-
tor may be installed in the outlet duct to warn
of an increase in dust content of this stream.
A visual inspection on the clean-air side of the
filter may reveal staining of parts by leaked
dust.
Ordinarily, a set of bags should last several
years if the proper fabric has been selected for
the operating conditions. Labbe and Donoso T5
recommend a monthly test of the acid content
and tensile strength of a fabric sample from
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REMOVAL OF PAETICULATE MATTER FROM GASEOUS WASTES
43
the bags in service. Howat80 suggests that bags
be washed every three or four months. Care
should be taken that the thread used to sew the
seams in bags is also of the proper material;
otherwise, splitting of the seams may be a
source of trouble.
Lubrication of moving parts: Fan motors
and fan bearings, shaking mechanisms, reverse-
jet blow rings, valves, dampers, etc. must be
lubricated regularly and checked for wear. To
avoid serious delays and shutdowns, worn parts
should be replaced before they fail in service.
Plugging of filter: This will be indicated by
abnormal pressure drop across the filter, re-
duced rate of flow, or both. It may be due to
improper performance of the cleaning system,
too high humidity in the dust stream, foreign
substances carelessly added to the gas, or a
sudden increase in dust load beyond the design
capacity of the filter. Constant monitoring of
the pressure differential gage is important.
Improper accumulation of dust: This may
occur in ducts, on fan blades, in hoppers, etc.—
in many locations where dust is not supposed
to accumulate. In time, this results in malfunc-
tion of the system, excessive wear, and possibly
a fire hazard. There should be provision for
frequent inspection of such locations, and clean-
ing whenever indicated. Sudden increases in
gas velocity may stir up such dust and cause
overloading and plugging of the filter.
Wear of metal -parts: There should be regular
inspection of ducts, hoods, framework, hous-
ings, etc. for signs of wear due to corrosion,
erosion, excessive heat, excessive moisture, etc.
Leaks which may develop in this way may re-
lease hazardous dusts into the atmosphere and
defeat the purpose of the filtering system.
Regular painting of the metal surfaces is in-
surance against some of this sort of trouble.
Electrical overloading: The current through
motors should be checked regularly for indica-
tion of overloading. Temperature of motors
and bearings should likewise be checked fre-
quently.
Improper tension in belt devices: The tension
in belt devices should be measured regularly
and adjusted before belts fail in service. Im-
proper speed of fans may result from incorrect
belt tension and this, in turn, may cause an
incorrect velocity of gas through the filter.
Thus, the collection efficiency could be impaired
even though there was no obvious malfunction
of the equipment.
Convenience of inspection and servicing: To
insure that workmen assigned to maintenance
work will follow the schedule of inspections and
servicing, it is desirable to make it as conven-
ient as possible. Outdoor locations should be
sheltered, service points should be readily ac-
cessible, and necessary tools and testing equip-
ment should be kept in good repair.
Instrumentation checkup: Pressure gages,
thermocouples, flow meters, and all other in-
struments must be constantly checked to insure
that they are giving accurate readings. This
is basic to the use of instrumentation for the
control of any operation.
Precautions against fire: Fire-fighting ap-
paratus should always be kept in working order
and regularly inspected and tested. During
repair work particular care should be taken
against sources of ignition such as welding
torches. Routine care against improper accu-
mulation of dust will also remove a possible
source of fire by spontaneous combustion.
As an illustration of a service schedule, the
following abbreviated version of the preventive
maintenance program followed by Bolt" is
given:
At beginning of each 8-hr shift:
Empty collectors of accumulated dust.
Every 24 hr:
Check temperature of motors and bearings.
Check all machinery for proper lubrication.
Weekly:
Check current in motors, for overloading.
Check tension on belt drives.
Clean out underground tunnels.
Inspect fire control equipment.
Every three weeks:
Clean overhead duct work.
Every four weeks:
Scrape blades of fans, and fan housings.
Change lubricant in all bearings.
Every three months:
Complete inspection of entire system.
Every year (during summer vacation shut-
down) :
Overhaul entire system.
4.00 COST DATA
Specific and up-to-date cost data are, of
course, not obtainable from the general techni-
cal literature. The only way to find the exact
cost of a piece of equipment at any given time
is to solicit price quotations from the manu-
facturers. The best that can be done in a gen-
eral survey of this kind is to indicate general
trends in costs, relative costs in comparison
with other types of dust collectors, and those
-------
46
AMERICAN PETROLEUM INSTITUTE
Repair materials and replacement parts
(bags, filter panels, etc.)
Dust disposal
Depreciation of equipment
Other fixed charges—insurance, taxes, etc.
The United States Atomic Energy Commis-
sion's Handbook on Air Cleaning 48 states that
for metallurgical bag filters total operating
costs are divided roughly as follows: 38 per
cent for labor; 25 per cent for power; 12 per
cent for bag renewal; 7 per cent for supervi-
sion; and the remaining 18 per cent for fuel,
tools and supplies, laundry, power shovel and
engine, and car service. No mention is made of
fixed charges. This handbook also cites quite
a number of specific cost data for a variety of
collectors; however, inasmuch as it dates from
1946 to 1950, the figures are no longer valid.
A useful source of current information on
costs and chemical engineering economics in
general is the annual review, "Chemical Cost
and Profitability Estimation," published in In-
dustrial and Engineering Chemistry in the May
or June issue. This is a survey and bibliography
of the literature which has appeared during the
previous calendar year, with a subject index.
"Dust collectors" and "Filters, air" are the
appropriate listings in the index.
5.00 PRACTICAL CONSIDERATIONS IN
FILTER SELECTION
In the approach to any dust collection prob-
lem there are always two basic considerations
to be taken into account:
1. What is the character of the gas-dust stream
to be dealt with? This must include a complete
description of all its properties: dust size
distribution, dust concentration, temperature,
moisture content, chemical composition of dust
and gas, physical nature of dust, etc.
2. What is to be accomplished by the collection
operation? The requirements to be met by the
collection equipment must be specified in terms
of effluent dust loading; particle size, re-use, or
disposal of collected dust; and quantity to be
handled.
Answers to the two foregoing questions will
generally lead to a preliminary selection of the
method of dust collection to be employed. In
fact, consideration of only particle size, particle
concentration, and desired collection efficiency
is often sufficient to indicate the kind of equip-
ment to be used, or at least to narrow the choice
down to between two or three types. Attention
may then be given to detailed selection of a
specific unit.
5.10 Selection of Collectors in General
A number of aids are available in under-
taking such preliminary selection studies. One
of these is a chart prepared by Sylvan of the
American Air Filter Company, Inc., presented
by Kane,66 with a more detailed explanation of
its use given by Kayse.07 This chart shows the
collection efficiency to be expected from any
particular type of equipment operating upon a
stream of specified concentration (in grains per
cubic foot) of dust having a given mean par-
ticle size (in microns). Typical ranges of par-
ticle size and concentrations encountered in
various practical situations are also shown.
Stairmand"3 lists the calculated collection
efficiency of a number of devices on a standard
test dust, W.C.3 silica powder, which has about
the same size distribution as typical fly ash
from a pulverized fuel boiler. A few values
selected from his table are listed in Table 11.
Other useful aids are the check list and
tables of Kane,64 the tabulation of First and
Silverinan,43 and the general discussions by
Hedberg56 and Lapple.81 These indicate that
the following factors should all be considered
in selecting the method of dust collection and
specific equipment to be used:
Mean dust-particle size and range
Dust concentration or loading
Abrasive characteristics of dust
Adhesive characteristics of dust
Bridging characteristics of dust
Fire or explosion hazard
Corrosive nature of gases
Volume of gases to be handled
Gas temperature and pressure
Condensable vapors present in gas
Possible fluctuations in operating conditions
Collection efficiency required
Method of dust disposal or recovery
Collector size, location, and space required
Collector cost, initial and operating
Collector installation work required
Collector servicing and maintenance required
Need for make-up air supply
Kane's tables summarize the operating char-
acteristics of different types of collecting equip-
ment with regard to many of these points.
They also show the collector types commonly
used for a wide range of industrial processes.
-------
REMOVAL OF PARTICULATE MATTER FROM GASEOUS WASTES
47
TABLE 11—Collection Efficiency on W.C.3 Teat Dust l"
Overall Efficiency Efficiency
Efficiency at 3 Microns at 1 Micron
Collector (Per One) (Per Cent) (Per Cent)
Cyclone, medium
efficiency 65.3 27 8
Cyclone, low-pressure
drop 74.2 42 13
Electrostatic precipi-
tator 94.1 92 70
Fabric filter 99.9 99.9 99
Spray tower 96.3 94 55
"enturi scrubber 99.7 99.6 97
Sp
Ve
A discussion of all of the factors listed in
the foregoing paragraph with reference to all
kinds of collectors is, of course, not within the
scope of this report. Such discussions for indi-
vidual types will be found in each of the reports
in this series prepared for the American Petro-
leum Institute. The remainder of this section
will be devoted to the interpretation of such
problems in terms of the use of filters of vari-
ous kinds.
5.20 Selection of Filters
The general place of filters in the scheme
of dust collectors has been indicated in Sect.
1.00. They are characterized as high-efficiency,
moderate-cost devices capable of application to
a wide variety of problems by virtue of the
many different kinds of filter media available,
and especially useful on medium- to very small-
sized particles. Assuming that it has been de-
cided to use a filter, the next question to be
answered is: What kind of filter shall it be?
The check list given in Par. 5.10 should be
repeated, this time with reference to the various
possibilities of filters. The order of importance
of these items is not necessarily as they are
listed. Possibly, the first consideration should
be given to the range of dust concentration.
There are two broad classifications of filters
to be considered at the outset, based upon dust
concentration. If this is less than, roughly,
1 to 3 grains per cubic foot, an air filter such
as is discussed in Par. 3.221 (stationary) or
Par. 3.322 (movable) is indicated. If the con-
centration is higher, a "dust collector" filter
(industrial) is indicated. This may be of the
fabric type (Par. 3.10), the fibrous bed type
(Par. 3.223 or 3.321), or the granular bed type
(Par. 3.21 or 3.31).
The selection of air filters is discussed by
May" and Rowe.103 The small unit or panel
type, with either disposable or renewable media,
will be preferred for small volumes of gas
where a minimum of attention is desirable, for
the collection of a dust which is not to be re-
covered. For large volumes of air one of the
automatic cleaning types will be more suitable,
especially if the filter has to be located in a
relatively inaccessible place or it is desirable
to reduce maintenance requirements to a mini-
mum. Collection efficiency is moderate, dust-
holding capacity high, and pressure drop low.
The dust-holding capacity may be defined as
the weight of dust accumulated either before
the pressure drop across the filter reaches a
specified maximum or before its collection effi-
ciency drops below a specified minimum. Rowe
recommends a maximum pressure drop of 0.2
in., water gage, for small units (such as a
home furnace) and 0.5 in., water gage, for
heavy-duty installations (such as industrial fil-
ter banks). Table 5 summarizes many of the
selection criteria involved. As the name im-
plies, such filters are principally used in general
ventilation work for the protection of people
and machinery from air-borne contaminants.
In the realm of dust collectors general discus-
sions of cloth filters are given by Stern l=8 and
Ebeling," and of porous materials by Silver-
man UT. us The general range of performance
to be expected from each type has been cited
herein in the sections dealing with each one.
Extensive specific performance test data are
cited for cloth bags, screens, and reverse jets
by Dennis, et al.M; and for unit collectors of
the screen, bag, or air filter type by Stern,
et aI.13T For other types, less extensive data
are indicated in the references cited under each
type. Such performance data must be at hand,
either from the literature or from the manu-
facturer, before a filter selection can be made.
Often the choice between cloth filter and fiber
mat may be resolved by the ultimate destination
of the dust. It is almost impossible to recover
dust out of a fiber mat for some re-use purpose.
Dust which has a re-use value must be collected
as a cake which can readily be freed from the
collecting medium.
The next step in filter selection is to deter-.
mine the volume of gas to be handled. This
will, in turn, fix the total area of filter needed
on the basis of the value of lineal velocity V,
or filter ratio, appropriate to the type selected.
Typical values of V have been cited in the
description of each kind of filter given herein.
As consideration proceeds to the other items
on the check list, the size of filter and the space
it will require can be kept in mind.
-------
48
AMERICAN PETROLEUM INSTITUTE
Particle size range and collection efficiency
required to produce the desired effluent concen-
tration should be considered next. If legal fac-
tors such as air pollution code requirements are
involved, they must, of course, be investigated
and taken into account. If possible, an analysis
of the dust size distribution on a weight basis
should be obtained. Knowing the collection effi-
ciency as a function of particle size according
to the collection mechanisms involved, one can
then calculate the size distribution of the efflu-
ent dust (or of that collected) on a weight
basis. Stairmand l"'13S gives examples of such
calculations. To do this, one must also know
the size of fibers to be used in the filter con-
templated. The finer the dust, the finer the
fibers needed.
If the size range is broad and includes an
appreciable amount of particles over 10 microns
in diameter, the possibility of preceding the
filter by a primary collector of another type
should not be overlooked. Coarser particles are
easily and efficiently removed by less expensive
collectors, which will reduce the duty of the
filter. Two kinds of filters may also be used in
series where the second is of a higher efficiency
than the first and is to be used for the ultimate
collection of very fine particles (such as radio-
active dusts) which cannot, under any circum-
stances, be permitted to escape in the effluent
gas stream.
Next, all the properties of the gas and dust
which will have an influence upon the filter
medium must be taken into account. Foremost
among these is temperature. The allowable
range of temperature for each of the various
fabrics, fibers, and other media has been given
herein. The possible need for cooling apparatus
must be contemplated. Abrasive, corrosive, and
flammability characteristics will also influence
the choice of filter medium.
Flexibility of operation may be very impor-
tant in some cases. If a plant is to operate
intermittently, or if there are likely to be
surges in the dust or gas flow rates, provision
must be made to handle the worst set of oper-
ating conditions. Filters which are primarily
designed to operate continuously under steady
conditions on a regular cleaning cycle will ob-
viously not be appropriate.
Consideration of all of the aforementioned
factors will probably lead to a tentative selec-
tion of a filter. Before a final decision is
made, however, the warnings against overlook-
ing questions of location, accessibility, servicing
and maintenance needs, installation problems,
and disposal methods must be heeded in order
that costly mistakes be avoided and smooth
operation assured. The size of the proposed
filter installation, including auxiliaries, must
now be definite in order to plan on location and
layout.
Finally, the question of cost must be raised
and answered. The ratio of pressure drop to
efficiency will be a useful criterion here as it
represents a ratio of operating cost (in a gen-
eral sense) to unit of achievement.
There are many references in the literature
to individual problems connected with the selec-
tion and use of filters in specific industries, and
examples of how these problems have been met
in certain cases. Following is a representative
list:
Air-borne bacteria '"• "•8l
Coal dust °3
Fluid catalyst fines B2
Foundry and cupola fumes :o> 80
Hot gases "•"°
Metallurgical dusts *
Protection of electrical equipment •»•105'140'IM
Radioactive dusts •• «•ST
6.00 PROBLEMS IN PERFORMANCE
TESTING
In testing the performance of a filter, there
are essentially three quantities to be checked:
pressure drop, gas flow rate, and collection effi-
ciency. Pressure drop and flow rate are fairly
easy to measure by standard techniques. The
matter of collection efficiency, however, raises
certain problems.
A statement of the collection efficiency, or
per-cent penetration, is meaningless without
an accompanying description of the method
of determination employed. Different methods
have been used, and they give different results
upon the same filter. If, in the specifications
for the purchase of a filter, a statement or
guarantee of collection performance is to be
included, it will be very important to agree with
the manufacturer upon the method by which
this performance is to be rated. The statement
of the test method should include a complete
description of the particle size distribution and
chemical composition of the dust which will be
used for performance testing.
-------
IV. SO.
-------
Fluidized Bed Combustion Update
The most recent research on fluidized bed combustion has developed
several possiblities for improving both the efficiency and emission con-
trol of coal-fired power plants. These include primarily the method of
feeding the coal to the fluidized bed and the efficiency of the sulfate
sorbent.
Washed Coal Feed
The standard fluidized bed combustion system has a deep fluidized bed
and a tall combustion chamber to ensure efficient combustion of finer coal
particles present in the crushed coal feed. It has recently been shown
that the coal does not have to be crushed before introduction to the
fluidized bed combustion chamber.
The main reason for firing crushed coal is'the need to maintain an
effective fluidized bed in the combustion chamber. The burning coal, ash,
and SOp sorbent must remain suspended in the upward airflow forming a
violently churning layer of particles. However, during normal operation,
the fluidized bed contains only a very small amount of coal (1-5 percent)
and consists mostly of ash and sorbent particles.
To maintain effective fluidization, particles in the bed must be less
than approximately 5 mm in size. Coal, however, often contains rocks and
stones up to 2 in. in diameter. If these were introduced to the combustion
chamber they would sink through the fluidized bed and deposit on the bottom
of the chamber. The coal crushing process assures that the stone contained
in the coal remains fluidized.
-------
If the coal is washed, however, the more dense stones can be easily
separated from the coal. Coal washing also removes much of the inorganic
sulfur contained in the coal, and reduces the amount of ash. One-half to
1 in. size coal can be burned in a fluidized bed as small as 15 cm high to
achieve a combustion efficiency of 97 percent.
Fluidized bed combustion boilers using this method of fuel feed are
currently in use in both the United States and Great Britain.
Sorbents
Much current research is being devoted to the improvement of SOo
sorbent efficiencies and regeneration. Currently, limestone is being used
as the SCU sorbent without regeneration. In this process, the limestone is
fed to the fluidized bed along with the feed coal. The carbon dioxide is
quickly driven off or calcined, leaving a porous CaO structure (Figure 1).
The S02 -then deposits on the surface of the remaining stone as calcium
sulfate (CaSO^). Normally, from 20 to 25 percent of the calcium in the
limestone feed can be converted to calcium sulfate by this method.
It recently has been found that the limestone efficiency can be
improved either by adding a small amount of sodium chloride with the
limestone or by hydrating the calcium sulfate in a regeneration process.
Both of these processes increase the pore diameter and prevent sealing off
of the small micropores present in the nonmodified calcined limestone.
(See Figure 1.) Up to 50 percent conversion has been obtained by adding
2
sodium chloride to the limestone (see Figure 2), while 85 percent calcium
3
conversion has been obtained with successive cycles of hydration.
-------
-MACROPQHE3
[-MICBOPORSS
CALCIA GRAINS-:
f- CALCIUM
5ULFATE
-CALCIA GRAINS
-MACROPOfiES "-MLCIUM suLFATf
Figure 1 Magnified cross-section comparison of limestone calcinalio.
and sulfation, with and without salt (schematic)
100
u
o
h-
a
e
U
1
20
Q RAW STONE
Q 0.3 .1 V. NaCI AOOmON
D 2.0 «1% NaCI ADDITION
CiLCITE ASL-9601 _ ANL.-950I
•NL-9TOI
ANL-9ZO) -ANL-S9OI - ANL-SIOl "
LIMESTONE aCSICKATICN
Figure -7, The effect of 0.5 and 2.0 wt % -NaCI treatment on limestone
subsequently precalcined and sulfeied at. 550 °C for 5 h-in 0.3% S02,
5% O2, 20% CO2. balance N2
Q Bcriun Titanste
C Calcium Aluminate Cement
S Conventional Sorbent (Grove Limestone)
100
2 J
HYORATION CrCLC
Figure ? Calcium conversion of the steady-stale product ax
a function o! hydration cycle. (Hydralion cycle 0 represent the
Initial sulfation).
„ 0.6
0.5
0.3
0.2
After
Sulfotion
After •-
Regeneration
CYCLE
Figure '; Comparison of new regenerable sorbents and limestone
-------
A major problem with using limestone as a sorbent in a regeneration
cycle, however, is a rapid loss or attrition through erosion in the flu-
idized bed. Also, the regeneration process involves high temperatures
which rapidly deactivate the limestone after only a few cycles.
Two additional minerals have shown promise as possible SOo sorbents
4
in the fluidized bed process. They are barium titanate and calcium
aluminate cement. Both of these minerals have shown acceptable conversion
to sulfate as well as erosion resistance. Preliminary economic assessment
indicates that either of these may be feasible on a commercial basis.
Regeneration of the SC^ sorbent used in fluidized bed combustion is
not currently practiced. However, research indicates that regeneration
will greatly reduce the solid waste associated with SO^ removal as well as
possibly providing a commercially usable sulfuric acid by-product.
. Sci. & Tech.. Vol. 14, No. 3, pp. 270-288 (1980).
2Ibid., Vol. 14, No. 9, pp, 1113-1118 (1979).
3J.A.P.C.A.. Vol. 30, No. 6, pp. 684-688 (1980).
4Env. Sci. & Tech.. Vol. 13, No. 6, pp. 715-720 (1979).
-------
2. The FGD System
The R-C/Bahco facility operating
at RAFB is a calcium-based
throwaway FGD and paniculate
removal system. Either pebble lime
(CaO) or ground limestone (CaCOs)
can be used for S02 removal to
produce mixtures containing
calcium sulfite (CaSOa), gypsum
(CaS04), and fly ash. The overall
chemical reactions for the
respective reagents are shown in
Table 1.
The scrubbing system comprises
the following major components:
• Flue-gas-handling equipment
• R-C/Bahco scrubber
• Reagent-handling and -storage
equipment
• Sludge disposal equipment
The entire FGD system (Figure 1)
is served by a centrally located
control room and is operated,
part time, by heat plant personnel.
Flue-Gas-Handling Equipment
The flue-gas-handling equipment
includes a flue gas header, bypass
stack, mechanical collector, and
booster fan. Flue gas from as
many as eight stoker-fired hot
water generators—up to 108,000
actual ftVmin (51 mVs)—passes
into the header and mechanical
collector where coarse particulate
' matter is removed before it enters
the booster fan and scrubber.
Removing paniculate minimizes
erosion of the fan and other
scrubber components and reduces
the amount of wet solids handled
by the scrubbing system. The ash
is disposed of via the existing
ash-handling system. A bypass
stack in the carbon steel flue gas
header serves two purposes: it
serves as a fail-safe emergency
bypass, and it permits air to enter
the system at low loads to
maintain gas velocity through the
mechanical collector and scrubber
to maximize collection efficiency.
Gas Flow
As shown in Figure 2, the
R-C/Bahco scrubber, which is
fabricated from 316L stainless
steel, is a two-stage inverted
venturi unit specifically designed
to operate with slurries containing
calcium sulfite, calcium sulfate
(gypsum), calcium carbonate,
calcium hydroxide, and fly ash. All
of the internal gas flow passages
are large, unobstructed, and well
irrigated with circulating slurry or
makeup water to essentially
eliminate the possibility of serious
plugging problems.
Hot flue gas from the booster fan
enters the first stage, where it
impinges on the surface of the
slurry, creating a cascade of
droplets that it carries into the
throat of the lower venturi. The
droplets, containing SC>2
scrubbing reagent, cool the gas to
its saturation temperature, absorb
sulfur dioxide, and trap particulate
Table 1.
Chemical Reactions for Lime and Limestone in SC"2 Removal
Reagent
Reaction
Lime:
Limestone:
Ca(OH)j * S02 — —
CaSO3 * ViO-2
faCn, *. I/,O,
*- CaSOj f H20
SOURCE: "Capsule Report--Bahco Flue Gas Desulfurization and Particulate
Removal System," EPA 625/2-79-022 (July 1979).
-------
Thickener
Reagent system
module
Reagent
storage
Reagent
feeder and
slaker
Lime or
limestone
conveyor
Unloading
station —
Overflow
to lime-
dissolving
tank
Reagent-dissolving Second stage
tank PumP
Mill pump
Figure 1.
R-C/Bahco Scrubber System
matter. Above the first venturi, the
gas stream is turned downward
by the bottom of the pan in the
second stage venturi causing most
of the droplets to fall out. In the
second stage, or upper venturi, the
process of impinging the gas
stream on the surface of a slurry
is repeated. Here the gas/droplet
mixture passes up through the
throat of the upper venturi where
final SOi absorption and
paniculate removal are
accomplished. A cyclonic mist
eliminator above the upper venturi
imparts a spinning motion to the
gas stream, causing the droplets
to move toward the wall where
they coalesce and drain from the
scrubber. From the mist
eliminator, clean gas, which is not
reheated, enters the surrounding
atmosphere via the stack.
Slurry Flow
Two techniques of handling slurry
flow in the system are used to
eliminate or minimize the plugging
and erosion problems often
associated with calcium-based
FGD systems: maintaining
essentially constant slurry flow
rates through the scrubber, and
eliminating turndown in slurry
bleed streams by operating in an
-------
Schedule:
Gas flow
Slurry flow
Sludge
removal
Slowdown valve •* ^
Stack
Manhole
Platform
Man door
To reagent dissolver
Platform
Plarform
First stage drop collector
r
r
i
J «-r
Ground level
£
Mill
pump
Figure 2.
R-C/Bahco Scrubber
-------
on-and-off mode with water
flushing after slurry flow is
interrupted.
Slurry flows by gravity from top to
bottom in the scrubber, counter-
current to the gas flow. Slurry
from the reagent dissolver {which
is also of 316L stainless steel)
contains makeup reagent—either
lime or ground limestone. The
slurry enters the pan in the upper
venturi. Slurry level in the pan
determines the upper venturi
pressure drop; the level is set by
adjusting a weir in the level tank
located outside the scrubber.
Slurry streams from the mist
eliminator and the pan are
combined in the level tank before
flowing by gravity to the mill
under the lower venturi, where
another level tank is used to set
the pressure drop in the lower
venturi. Part of the slurry collected
in the area between the upper and
lower Venturis, the part that has
contacted the gas stream twice,
flows by gravity to the sludge
disposal system. In the first-stage
level tank this slurry is combined
with overflow from the mill and is
returned to the reagent dissolver.
More reagent is added in the
dissolver before the slurry is
recycled to the upper venturi. The
fluid mill is powered by an
external pump and is used to grind
coarse limestone or other large
particles in the system.
First-stage venturi, showing gas inlets and makeup water spray
manifolds
Reagent-Handling and -Storage
Equipment
The reagent system installed at
RAFB is capable of handling both
0.75-inch (1.9-cm) pebble lime
and 200-mesh ground limestone.
Primary components include
truck-unloading equipment, a steel
silo with 3 weeks' storage capacity
at winter load conditions, a weigh
belt feeder, and a lime slaker. The
silo, feeder, slaker, and reagent-
dissolving tank are integrated into
a single module to minimize
materials handling, supports, and
space requirements. Lime or
limestone drops directly out of the
silo into the feeder-slaker and
overflows into the reagent-dissolv-
ing tank directly under the slaker.
Sludge Disposal Equipment
Calcium sulfite, gypsum, and fly
ash collected in the scrubber are
concentrated from 10 percent to
approximately 40 percent solids
(by weight) in a thickener. The
overflow from the thickener is
returned by gravity to the
reagent-dissolving tank. The
underflow from the thickener is
pumped underground to a
hypalon-lined storage pond.
-------
3. The Test Program
The R-C/Bahco FGD system test
program, carried out at RAFB
between March 1976 and June
1977, incorporated the following
categories:
• Material balance
• Lime reagent process variable
• Lime reagent verification
• Particulate collection efficiency
• Limestone reagent process
variable
• Sludge characterization
• Scrubber reliability monitoring
Material balance tests were
conducted to establish the range
of operating conditions over which
the R-C/Bahco scrubber could be
operated and to verify performance
at design conditions by completing
material balances. Maximum and
minimum gas flow rates, pressure
drops, and slurry circulation rates
were determined and preliminary
SOz and the paniculate
performance data at the limits of
the system's capabilities were
obtained. The system was
operated at the design gas rate of
50,000 stdftVmin (25 normal
m3/s) and complete material
balances on calcium, sulfur, and
total solids were performed.
Statistically designed lime process
variable tests helped to establish
the quantitative effect of the
following process variables on SC>2
removal: gas flow rate, first- and
second-stage pressure drops, mill
and second-stage slurry rates,
lime:S02 stoichiometric ratio,
slurry inventory, and slurry solids
concentration.
Lime reagent verification tests
were undertaken to verify the
results obtained in the lime
process variable tests, and to
determine the effect of very dilute
scrubber slurry (2 percent solids) .
on system performance.
Particulate collection efficiency
tests were a continuation of the
paniculate tests initiated during
the earlier sampling phase.
Relationships were determined
between system variables,
including panicle size distribution
and particulate removal efficiency.
Limestone process variable tests
were completed using the same
statistically designed test plan
used for lime. The effect of system
variables on SOa removal
efficiency and reagent use was
determined.
Sludge samples generated at
RAFB were tested to determine
dewatering, transport, and
disposal characteristics (sludge
characterization). Samples of
sludge from lime as well as
limestone scrubbing were tested.
The R-C/Bahco system was
monitored from March 1976 to
June 1977, to document its
operating and maintenance history
and to obtain data for a cost
analysis. Data were gathered on
reagent, coal, water, and power
consumption as well as on
operating and maintenance labor
requirements.
Throughout the test program
samples were taken of slurry, flue
gas, lime, limestone, and coal,
often in duplicate, for chemical
analyses, particulate loading, and
particle size distribution. A
field analytical laboratory was
established, and especially
developed and highly efficient test
methods using thermogravimetric
analysis were employed
extensively.
-------
4. Test Results
Capacity/Material Balance Tests
Performance of the size 50
R-C/Bahco scrubber at RAFB is
measured by its ability to handle
variations in system operating
parameters while reducing S02
and particulate emissions to the
limits allowed by the applicable
regulations, without exceeding the
capacity of the system. Regulations
applicable to RAFB limit S02
emissions to 2.2 lb/106 Btu (3.96
g/106 cal) and particulate to
0.16 lb/106 Btu (0.29 g/106 cal).
Table 2 lists maximum, minimum,
and optimum operating levels
determined for the system at
RAFB. The cost of reducing
emissions to meet requirements
will be minimized at optimum
operating levels.
Lime Tests
The SO? removal capabilities of
the R-C/Bahco system using
pebble lime were characterized in
two steps. First, a series of
screening tests determined the
effects of slurry rates, gas rate,
venturi pressure drops, slurry
density, system volume, and lime
stoichiometry on S02 removal.
Tests results indicated that lime
stoichiometry—the ratio of lime
feed in the system to SOa in the
flue gas—was the only variable
controlling SC>2 removal as long
as the system was operated within
the limits outlined in Table 2.
A second group of tests, in which
the effects of the gas flow, slurry
rates and slurry density were
determined, confirmed the initial
findings that stoichiometry alone
controlled SC>2 removal.
Results of these verification tests
are shown in Figure 3. The figure
also shows that lime use is
essentially 100 percent—that is,
no excess lime is needed—up to
90 percent S02 removal. Figure 4
illustrates system performance
when S02 removal is above
90 percent—that is, when S02
emissions at RAFB were reduced
below 0.6 lb/106 Btu (1.08 g/106
cal). The figure indicates that over
98 percent of the S02
corresponding to 0.1 Ib S02/105
Btu (0.18 g S02/106 cal). can be
Table 2.
R-C/Bahco Scrubber Operating Levels
Variable
Gas rate (actual ft3/min|
Slurry circulation rate (gal/min) . . .
Veniuri pressure drop for each
stage (inches H20)
Slurry concentration (wt %
solids)
ReagentSOj stoichiometry
(moles reagent: moles SO}.
based on inlet S02 levels):
Lime
Limestone
S02 removal efficiency (percent):
Lime
Limestone
S02 emission (lb/106 Btu):
Lime
Limestone
Particulate emission (lb/106
Blu)
Minimum
35,000
1.500
6
2
0.45
0.55
45
40
3.7
4.0
0.2-0.3
Maximum
55.000
3.000
12
25
1.05
1.2
98*
85
0.1
1.0
0.14
Optimum
40,000-50.000
2.300
7-10
10
0.7
0.75
70
70
2.0
2.0
0.16
-------
achieved with a stoichiometry of
1.1—that is, 10 percent excess
lime. The SC>2 emission rates
shown in Figure 4 are well below
the required 2.2 lb/106 Btu
(3.96 g/106 cal) and the guarantee
level of 1.0 lb/106 Btu (1.8
g/106 cal).
From the lime tests it is concluded
that lime:SO2 stoichiometry is the
controlling factor in determining
S02 removal efficiency. Virtually
any desired S02 removal
efficiency can be achieved when
lime is used in the R-C/Bahco
scrubber, simply by adjusting the
lime:SC>2 stoichiometry. Lime use
approaches 100 percent at
stoichiometric ratios up to about
0.9. At stoichiometric ratios up
to 1.1, producing up to 99 percent
removal, lime use is above
90 percent. Because most S02
regulations for industrial boilers
permit emissions in the range of
1.0 to 2.0 lb/106 Btu (1.8 to
3.6 g/106 cal), lime, with its high
removal capabilities, can be used
to obtain offset credits in a
nonattainment area to apply
toward an expansion or new
facility. No further capital
expenditure need be made,
because the R-C/Bahco system
normally would be designed to
handle lime as well as limestone,
and switching from limestone to
lime will increase the annual
operating costs only by about
1 5 percent.
Limestone Tests
System performance with ground
limestone was determined in a
series of screening tests very
similar to those used for pebble
lime. These tests indicated that
slurry circulation in addition to
limestone stoichiometry controls
S02 removal efficiency.
100
o
LU
20 -
0.2 0.4 0.6 08 10 1.2
LIME STOICHIOMETRY (moles lime per mole (S02>
Figure 3.
SOj Removal Efficiency as a Function of Lime Stoichiometry
Hypalon-lined storage pond
-------
Figure 5 shows the results of the
tests and gives limestone use
data. At a stoichiometry of 1.0 and
slurry circulation of 2.300 gal/min
(0.14 mVs), slightly over
80 percent S02 removal is
possible with 80 percent limestone
use. A practical limit for limestone
is 80 percent S02 removal,
because higher removals result in
substantial reductions in limestone
use.
Operation with limestone at RAFB
produced sludge that contained
much more gypsum than did
operation with lime. That is, there
was more oxidation of CaSOs to
CaS04. Table 3 shows an average
gypsum (CaS04- 2H20) and
calcium sulfite (CaSOa- '/zH20)
content of 33 and 55 percent.
respectively, when lime was used.
The limestone slurry was almost
completely oxidized and contained
78 percent sulfate and less than
1 percent sulfite. The comparison
of average lime and limestone
slurry analyses during similar
boiler load periods listed in Table 3
indicates that the oxidation trend
is probably attributable to the
lower slurry pH encountered when
using limestone, because all other
operating conditions were
essentially the same.
Paniculate Removal Efficiency
Paniculate Removal Tests. Initial
paniculate removal tests on the
R-C/Bahco scrubber, performed in
March, April, and May of 1976,
revealed the presence of
substantial amounts of soot in the
stack gas. The average paniculate
emission rate for these tests was
0.23 lb/106 Btu (0.42 g/103 cal).
cr
2
v>
CO
5
2.5
I 2.0
§ 1.5
Ul
0.5
O
in
RAFB EPA limit
Guarantee emission rate
90% lime use
0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
LIME STOICHIOMETRY (molss lime per mole S02)-
Figure 4.
Relationship Between S02 Emission Rates and Lime:S02 Stoichiometry
Overall paniculate removal '
averaged 93 to 94 percent. Ohio
emission standards require an
overall removal efficiency of
96 percent at a paniculate inlet
loading of 1.5 gr/stdft 3 dry
(3.4 g/normal m3 dry) to achieve
an emission rate of 0.16 lb/106
Btu (0.29 g/106 cal). Venturi
pressure drops were increased to
nearly double the design value of
7 inches (18 cm) H20 to reduce
these emissions. Below
approximately 18 inches (46 cm)
H20 total pressure drop,
particulate emissions increased
rapidly. The amount of soot
present in the flue gas at RAFB is
higher than in other stoker-fired
generators similar to the
Rickenbacker boiler.
The Air Force has undertaken an
extensive program to upgrade the
heat plant at RAFB. Data obtained
during this test program
contributed substantially to
information used to plan the
upgrading program, and so far the
following modifications have been
completed:
. Installation of a new 60-Btu/h
(18-Watt) generator to replace
the two old units
• Replacement of hot water
distribution piping
• Installation of flue gas oxygen
monitoring equipment
• Repair of firing air distribution
equipment and fire box pressure
controls in the generators
-------
• Rebuilding mechanical collectors
and induced draft fans on the
generators
• Replacement of burned out
ledge plates, which regulate
combustion air flow around the
grates
• Repair of traveling grates
The problem with soot at RAFB
points up a critical aspect of a
successful emission control
project—namely, that proper
operation of all equipment, boilers
as well as the scrubber, is
essential to maintain satisfactory
emission levels. Inadequate
combustion or inadequate air can
be as detrimental to emission
control as improper scrubber
operation.
Slurry Entrainment and Gas
Bypassing. During the paniculate
tests, two phenomena were
observed when the system was
operated above its capacity limits.
The first, called entrainment,
occurs at very low venturi
pressure drops—that is, under
6 inches (15 cm) HjO involves
small droplets of slurry carrying
through the second-stage mist
eliminator and out the stack. The
second, called bypassing, is
characterized by pulsations in the
gas flow through the scrubber; the
result is low collection efficiency
in all particle size ranges. The
second phenomenon takes place
when relatively high pressure
drops—that is. 12 inches (30 cm)
H20 or more in either venturi—are
coupled with slurry flows under
150 gal/min (0.01 m3/s) to the
scrubber.
Conclusion. The particulate
removal efficiency of the
R-C/Bahco scrubber is comparable
to that of low energy venturi
scrubbers for particles larger than
1 u.m, and appears to be better for
particles smaller than 1pm. In an
R-C/Bahco scrubber, the second
stage is the primary collector of
fine particles. Slurry carryover and
c
3)
u
U
100
90
80
70
60
t 50
40
O
U)
30
20
10
0.2 0.4 0.6 0.8 1.0 1.2
STOICHIOMETRY RATIO
1.4
1.8
Figure 5.
862 Removal Efficiency as a Function of Limestone:S02 Stoichiometry
Table 3.
Lime and Limestone Slurry Analyses
Slurry solids
CaS04 2H,0
CaSOs-'/iHjO
CaCO3
MgCOi
Acid insolubles
Total
Lime
Slurry
(wt%)
33 4
54 5
3.7
4.6
96.2
Limestone
Slurry
(wt%)
77 5
1 0
17.3
0.8
3.4
100.0
-------
gas bypassing limit paniculate Table 4.
collection in an R-C/Bahco
scrubber operated outside the Dewatering Test Results
levels shown in Table 1
Particulate emissions from
stoker-fired coal-burning Tesl
equipment ran HP rorjurpd to
levels required by regulatory
formation is prevented.
Centrifuge
Sludge Characterization
Filter leaf
A series of scrubber sludge
Slurry
type
I Lime
\ Limestone9
I Lime
( Limestone
I Lime
I Limestone
Feed
solids
(wt%)
162
16 7
16.7
263
18 A
37.4
24.6
41 *i
37.4
Final
solids
(wt%)
44
CO
58
51
65
58
CQ
74
Rate at
35 percent
solids
22 Ib/d/ft2
578 Ib/d/ft2
70 lb/h/ft2
1 24 lb/h/ft2
64 Ib/h/ft2
out at the Research-Cottrell
laboratories to:
• Determine scrubber sludge
dewatering characteristics
• Evaluate transportability of
dewatered sludge
• Determine physical/structural
properties of dewatered sludge
• Measure sludge leachate
properties
Slurry Dewatering. A series of
settling, centrifuge, and filter leaf
tests was run on lime and
limestone slurry samples. The
results are summarized in Table 4.
The settling tests showed that
limestone slurries settle more
rapidly and produce denser settled
layer than lime slurries.
Flocculation improved the settling
of limestone slurries, but not
that of lime slurries.
aWith 5 ppm flocculant.
Table 5.
Sludge Leachate Analysis
Analysis
TDS(mg/l)
S0a[mg/l)
C00(mg/l)
Cl (mg/l)
Pb (ppb)
Cd (ppb)
Cr (ppb)
Hg (ppb)
Lime
leachate
9 Qfifi
i flin R
ft 4
^ino
-------
The centrifuge tests indicated that
final cake density increased as the
solids concentration in the feed
was increased, and that limestone
slurries produced higher cake
densities than did lime slurries.
Filter leaf tests showed that
limestone slurry filtration rates
were significantly lower than lime
slurry rates. However, limestone
again produced a denser cake.
Leachate Tests. Leachate tests
were performed on samples of
lime and limestone sludges. The
results are listed in Table 5.
Leachate compositions from lime
and limestone sludges are
essentially the same. Total
dissolved solids (TDS) in the range
of 2,500 to 3,000 mg/l and sulfate
levels of 1,600 to 1.800 mg/l
indicate that the leachates were
saturated with respect to CaSCU.
Both sulfites in the sludge and
organic matter in the fly ash
contribute to the chemical oxygen
demand (COD) levels observed.
Although the chloride level in the
lime leachate is somewhat higher
than the limestone leachate, the
other trace elements are present
in similar concentrations in both
leachates. The constituents found
in these leachates are similar in
type and concentration to those
reported in other studies. If a
disposal site is placed so as to
avoid infiltration of leachate into
ground water, and if sludge and
soil cover are placed properly to
avoid excessive contamination of
runoff, leachate from these
sludges will not present an
environmentally unacceptable
disposal problem.
First-stage level tank
-------
5. Operating
Experience
Since startup in March 1976, the
R-C/Bahco system has performed
well in all areas essential to
successful FGD, including:
• S02 removal
• Paniculate removal
• Scrubber reliability
• Minimal routine maintenance
• Moderate operating costs
• Ease of operation
During the test period of about
11,000 hours, the scrubbing
system operated for 6,194 hours.
The operation is summarized in
Figure 6. It is of interest that from
December 1976 to February 1977
(when severe winter weather was
encountered), no outages resulted
from failure of auxiliary equipment.
There were a few brief shutdowns
caused by frozen air and water
lines during this period, but
system availability was over
95 percent.
Downtime is summarized in
Table 6. This table shows the
amount of time required to obtain
parts as well as the actual time
for repair work. Spare parts were
not kept on hand during the test
period, and this resulted in
substantial unnecessary
downtime. Since completion of the
test program, a full supply of
spares has been procured. Table 6
also shows that booster fan repair
time accounted for 90 percent of
the downtime caused by repairs.
The booster fan operates on the
inlet side of the scrubber.
downstream from the mechanical
collector, and handles only hot dry
flue gas with moderate amounts
of fine fly ash. Modifications to the
fan wheel and bearings, completed
in May 1977, have eliminated the
recurring failures associated with
this piece of equipment.
Total downtime, exclusive of fan
repairs and procurement, was
1,845 hours, or 17 percent of the
test period. During routine
operation, the system availability
should be over 95 percent, based
on the factors observed during
the test program.
Scrubber inspections were an
integral part of the program to
monitor scrubber performance. A
thorough internal inspection was
made in April 1976, approximately
1 month after startup. A followup
inspection was made 2 months
later, with subsequent inspections
during outages up to the end of
the test program in June 1977.
These inspections confirmed the
effectiveness of the water makeup
system in keeping key areas of
the scrubber clean.
-------
Accumulations of solids were
detected at seven locations within
the scrubber (Figure 7).
Accumulations in four areas—1, 5,
6, and 7—had no impact on
scrubber performance. Problems
of solids buildup in Areas 2, 3,
and 4 were easily corrected, as
follows:
In the first few months after
startup, the first-stage venturi
overflowed into the inlet manifold,
Area 2, resulting in an
accumulation of dried slurry in the
bottom of that area. Subsequent
investigation revealed that
operation of the first-stage at
pressure drops above 12 inches
(30 cm) H20 coupled with a
second-stage slurry pumping rate
more than 50 percent higher than
the design rate of 2,600 gal/min
(0.16 mVs), caused flooding when
the gas flow was reduced below
35,000 stdftVmin (17 normal
m3/s) or the booster fan was
shut down. This problem was
eliminated by decreasing the
speed of the slurry pump to
reduce the flow to design levels,
and by adding an interlock to
stop the pump when the booster
fan is shut down. The accumulated
material was removed during
subsequent heat plant outages.
Areas 3 and 4 were affected twice
during the test by accumulations
of a coarse sandy material. The
first incident, which occurred
shortly after startup, was caused
by inadequate removal of grit from
the lime slaker. The material in
the pan was removed and the
slaker was readjusted to eliminate
the problem. The second
accumulation took place during
the winter of 1976-77 when the
air lines, which activated the
blowdown valves on the first- and
second-stage level tanks, froze and
rendered these valves inoperative.
Scrubber Replace slurry Scrubber Replace slurry
startup pump lining inspection pump lining
'i^^g&H&&fa:?& I ;r^May-,:4/-.11
1976
Install
sludge I
thickener
rake
* t
ReplaC8 Correct
'orque fan v,bratlon
hmiter
Repair
sludge and
slurry lines
ly^Uury;^!J'v^gAug. •»&&• { f
"*£'r-i.
m
1976
Install improved
booster fan bearing
Replace
blowdown
valves
1976/1977
Inspection Repair and
water booster and grit modification
pump bearing removal of fan wheel
Replace
slaker
motor
'* ••' X"M~V ' ' '-".^'^V^i-'
'•&%§!::Mtif •• •$•&?*•
Scrubber F 1
operabilitv &>$n
Boiler E^
shutdown I
1977
Figure 6.
Downtime Related to Auxiliary Equipment
Table 6.
Downtime Summary
Hours
Percent
Booster fan
Thickener
Slurry pump ....
Water booster pump
Lime slaker
Modifications
Routine maintenance
Loss of utilities
Miscellaneous
Total
Procurement
• 514
471
252
190
122
56
1.605
Repairs
2 252
8
18
16
1 1
53
2.363
Downtime
2 766
479
270
206
133
388
139
116
1 14
4.611
period
25 1
4 4
2 5
1 9
1 2
3 5
1 3
1 1
09
41 9
-------
The results of this pan of the test
program demonstrated that there
are situations that can result in
deterioration of scrubber
performance, including:
• Infiltration of grit into the system
through the lime slaker
• Inadequate operation of the
scrubber blowdown valves
• Slow accumulation of solids in
the straightening vanes in the
stack
The infiltration of grit can be kept
to a minimum by paying close
attention to the operation of the
lime slaker grit removal circuit.
The blowdown valves should be
operated two to four times a shift,
depending on scrubber load, to
avoid accumulations of solids in
the slurry outlets.
The straightening vanes at the
base of the stack, which serve
only to minimize spin in the gas
stream leaving the scrubber, may
accumulate some material and
should be checked twice a year.
The possibility of accumulations
taking place can be minimized by
operating the scrubber within the
limits outlined in Section 3 to
avoid slurry carryover. Obviously,
elimination of the vanes would
prevent the problem entirely, but
accurate outlet particulate
sampling would then be difficult.
The RAFB operating experience
indicates that there are no
significant problems related to
the accumulation of solids in the
R-C/Bahco system. The scrubber
can tolerate substantial
accumulations of solids resulting
from external operating problems
before performance is adversely
affected. Any deterioration in
performance that does occur is
gradual and can be rectified at a
convenient time.
Stack
Manhole
6
*-
J
Platform
Mist eliminator
Removable
cover
Man door
Platform
Platform
First stage drop collector
Figure 7.
R-C/Bahco Scrubber Module
-------
E. C. McKemie, WoodaU-Duckham Ltd., Babcock & Wilcox Croup
Q Today, many plant operators must specify new boil-
ers that will have a life expectancy of 30 years or more.
Which fuels will be available for these plants, particu-
larly in their later years of operation? What environ-
mental restrictions will be imposed on emissions?
The trend is toward the prohibition of oil and natural
gas as boiler feeds. This has come about via either
legislation or problems with fuel availability and cost.
Let us, therefore, assume that the fuel for these boilers
will be coal. We will first examine options on the design
of a boiler and its fuel-preparation and firing equip-
ment, using the technology available prior to fluid-
ized-bed combustion. Then we will compare this tech-
nology to that of a fluidized bed.
Assume that the boiler output is greater than 400,000
Ib/h of evaporation, so that we will be considering a
pulverized fuel-firing system.
For a conventional boiler, we must next decide what
type of coal will be used, since a boiler can handle only
a limited range of fuel.
Bituminous-coal-fired boilers
Fig. 1 shows a sectional elevation of a 300-MWe
(megawatts, electrical) boiler designed for firing bitu-
minous coal. The coal usually is ground into particles,
70% of which are less than 75 fim, typically in a vertical
spindle mill. The mill is swept with sufficient heated air
to dry the coal and convey the ground material directly
to the burners. Power consumed in this process is about
18 kW/long ton of coal fired, i.e., the mill power plus
the fan power.
Furnace volume and shape are determined by the
heat absorption required to meet the design-furnace
exit gas-temperature, if there is no concern about NOX
emissions. This temperature is set below the coal-ash
melting point to prevent slagging on subsequent con-
vection banks.
The size and number of burners are chosen to provide
© 1973 Woodall-Duckham L:d
complete combustion within the furnace. A typical such
pulverized-coal burner is shown in F!g. 2. Primary air
and pulverized coal enter a tube fitted concentrically in
a cylindrical secondary-air-rcgister. The amount of
swirl given to the secondary air is controlled by vanes
on the periphery of the register. A smaller tube, located
in the center of the coal/primary-air inlet, has an impel-
ler that diverts the coal into the secondary-air stream.
' Inside this small tube is an oil burner for lighting up the
coal flame, and a gas torch for igniting the oil burner.
Both are withdrawn to a cool zone after they have been
used. ;;
The impeller acts also as a bluff-body stabilizer (a
nonstreamlined object placed in the gas stream to in-
hibit flow) along with the flow-reversal mechanism set
up by the swirled secondary air. The burners are posi-
tioned on the front wall (or on the front and rear wails
for large boilers), so as to yield a near-uniform heat
release over the furnace plan-area and minimize ash
deposits on the walls.
The turndown on a mill and its associated burners is
limited to approximately 5Q"o by the need to (1) main-
tain velocity in the fuel pipes to prevent fallout of th;
coal, and hence plugging and (2) restrict maximum
velocity so as to control erosion. Hence, the boiler load
is controlled by varying the number of mills and burn-
ers in service. For safety, the oil burners arc lit usually
during load changes or when mills are put in or out of
service, and are maintained in operation when the
boiler is on low load.
The spacing of the platens is fixed to ensure that sb:
accumulations will not cause blockages. (If the platens
are too close together, slag will bridge the spaces be-
tween them.) The velocity through the convection
banks is limited by the abrasive properties of the 2$-"
particles, in order to prevent erosion of the cubes. Thi
degree of abrasion with any given ash is proportional :o
the cube of the particle velocity. Normally, the boils-'
can accept a wide range of ash properties in bituminous
116
CHEMICAL E.NCINEEKLSC ALT.L'ST l+. I9i«
-------
i Secondary superheater
Plaren
superheater
fter setting the stage with a review
'the operation of conventional
ial-fired boilers, this report discusses
e use of fluidized beds, including
pics such as coal preparation, load
ntrol and emissions control.
Conventional 300-MWe boiler that handles.,,
bituminous coal affords complete combustion • Fig. 1
s^ "-
PJL1
1
V
I
i-.^'r.iV.'-^v
_^"— ^ ^"^"
-• .-V
•..-"••-•^•x
-Secondary-air
**» -vanes-". •, . .'
. v. ^L. A, :«.•". ;
. »" J . ' *: •'. "."
-.. .*»-•.
••- "^ir"^::^-!*.".^'^ -V." /"- ^*' ^1?*'*-
1- .^•rj ^; "*^*"- ;"--AT>iT" -,?- • U-
^SSsgSK^SR^i ^ ;
I
I
V;
^
^•ner for firing pulverized bituminous coal can be withdrawn from furnace :' , - : - : *':?£&&£& 2
CHEMICAL ENGINEERING AUGUST 14, 1978
117
-------
FLU1DIZE BEDS
Secondary Secondary
superheater raheater
Primary
superheater
r£ -x.*:.i. Designed for low-volatility, pulverized coaly':'- 'f^v-''l?5r3:A;
V:.^'-JP',;-300-MWe boiler requires high'miIf-poweK'.rVv-^y,f j9-.?.-'
coals. If a much different rank of fuel is used, such a
boiler and firing system are no longer suitable.
Low-volatility coal boilers
Figs. 3 and 4 show the boiler (also 300 MWJ and
burner systems for low-volatility coal. Here, one can no
longer use a turbulent burner. To assist ignition, the
coal is ground typically into particles, 85% of which are
less than 75 pm. Compared with bituminous coal, the
mill power-requirement is increased by about 50%. The
burners are set in the roof of the combustion chamber.
Primary air and coal are fired through rectangular sloes,
and the secondary air through adjacent slots. The rela-
tive velocities are designed so that once ignition is es-
tablished, the secondary air mixes with the fuel at a rate
that supports combustion but does not quench it. The
absolute velocities are designed to yield long flames that
sweep down into the lower part of the combustion
chamber and then turn up to enter the center pass at
the convection banks.
Mill and burner turndown are the same as for bitu-
minous-coal systems, but due to the relatively poor
reactivity of low-volatility coals, oil support is needed
earlier than it is with bituminous coal as the boiler load
is reduced from its normal maximum rating. These
supporting burners are often placed in the side walls:
Fig. 5 shows a 200-MWe boiler designed for Austra-
lian lignite. This fuel has a moisture content of about
70%, but when dry is extremely reactive. Usually, 99%
of the particles must be less than 1 mm, reqmring a mill
power input of about 2kW/long ton. Also, the coal
leaving the mill must be dried to about 20% moisture to
ensure stable ignition in the boiler. To achieve this,
gases from the furnace at about 1,000°C pass through
the mill with'the raw-coal feed. -
A hammer mill with a water-cooled shaft is the only
type of mill suited for these conditions (see Fig. 6). Since
Air supply for
oil burners'^
Lighting up
and
stabilizing.*
oil burner
j
i
! Flame detector-"
,Pulverized fuel and-
primary air ports
^xSecondary-air
' connections
xSight door
Primary air
and coal
Sida elevation
Section A-A
Nonturbulent burners for low-volatility coal. Units are set in chamber's roof
Fig. 4
118
CHEMICAL ENGINEERING AUGUST 14, 1973
-------
Coal and heating
flue-gas inlet
m$£ m
&Mfti -..--.,
!i:;; •"!l-I, I'r't !'•!»'•':.
i,;'':y , ': !'• I !•!' -I'i •
BllSogi
ilijpV^.vvii1"
WV. boiler for Australian li
i *"5h is dried by flue gasMto.20%moistwir^V^5H^S
; Hammer mill with, water-cooled shaft is .>;v;v.i>^.r.
^usedto grind lignite for boiler in Fig. 5 . -S^^^FJg; 6
relatively coarse product is acceptable, a simple classi-
r can be used to return oversized panicles to the mill
ses leaving the mill are heavily laden with water
vapor. The fuel and gases are partially separated in the
ict carrying the coal to the burner, which is a series of
>nzontal slots alternately containing fuel-and-eas
mixture and air.
•A fuel-rich mixture is fired in the lower slot of the
irner, as shown on Fig. 7, to improve ignition stabil-
- i he combustion chamber may be in the form of an
octagon, with one burner, served by one mill, firing
» of the eight sides. The combustion chamber is
ge compared with chambers for higher-grade coals
Ihe above brief survey on pulverized-coaJ firing il-
lustrates the variety of boiler, fuel-preparation and
ng systems necessary to suit the grade of the coal.
nc of these systems can control SO, emissions and
"'is must be done separately.
Also, because of its highly turbulent burner and con-
aently high flame temperature, the bituminous-
1-nred boiler produces high NO, emissions. If NO is
» be controlled, whether by two-stage combustion
-gas regulation, or both, then the combustion-
mber volume becomes much greater and the boiler
-->.*.?%Wu.iL^l*^te(.j^ii1i<^.^>^i^XJi*j|?
Secondary-air
'nozzles
Coal/flue-gas
mixture from mill
Burner for separation firing of lignite -,
alternates layers of air with coal/flue-gas
Fig. 7
CHEMICAL ENGINEERING AUGUST 14. 1978
119
-------
FLUIDtZE BKDS
more expensive. Anthracite and lignite boilers are likely
to produce much less NOX because of their lower flame
temperatures and, in the case of the lignite boiler, be-
cause of the high percentage of water in the fuel.
Fluidized-bed combustors
With fluidized-bed combustion, on the other hand, a
boiler and its fuel-preparation and Bring equipment
can be designed to suit any type of fuel, whether solid,
liquid or gaseous. This is provided that the fuel has a
net calorific value greater than that required to heat
both it and the makeup air to the bed temperature.
Obviously, fuel-handling systems must suit the various
forms of fuel, as well as have large enough capacities for
the lowest-caloriftc-value fuels foreseeable.
The major advantage of fluidized beds, therefore, is
that they allow the development of standard-design
boilers and combustion systems to a degree not possible
before. Fluidized beds also permit the use of low-grade
coal, even at low loads, without the need for expensive
support fuels to ensure stable ignition. Another advan-
tage is that the relatively low combustion temperature
results in low NOZ emissions and provides optimum
conditions for the retention of sulfur by limestone or
dolomite in the bed.
Principles of fluidized-bed combustion
The bed, which may be sand, firebrick, limestone or
coal ash, is fluidized by blowing evenly distributed air
through it. Both bed material and the coal fired to it are
crushed to a size compatible with the chosen fluidizing
velocity, i.e., a size that minimizes loss by elutriation.
The fluidizing velocity is calculated from the quantity
of air at bed temperature and pressure, and from the
bed's superficial area:
W Tb + 273 J_
"' ~ Cpa X 273 X /F
where of = fluidizing velocity, ft/s
W = air quantity, Ib/s
7"6 = bed temperature, "C
P0 = air density at 0"C, lb/ft3
A = bed area, ft2
C = bed pressure, atm
At a given excess-air level, heat-release rate per unit
area of the bed is a linear function of the fluidizing
velocity. The heat-absorption surface is immersed in the
bed and is matched to the heat input, so that the bed
temperature can be controlled between chosen limits.
Since the heat input is proportional to the fluidizing
velociry, as the latter increases, the amount of installed
surface increases to keep within the design's maximum
bed-temperature.
Furthermore, McLaren and Williams [/] have shown
that as the bed's mean particle size becomes greater to
suit higher fluidizing velocities, the heat-transfer coef-
ficient between the bed and the immersed surface de-
creases. Hence, two factors determine the area of heat-
absorbing surface required as the fluidizing velocity
increases. Since, for a given tube arrangement, addi-
tional surface can only be provided by deepening the
bed, an optimization study has to be made between the
capital cost of the bed area and the operating cost of
higher fan power needed for the deeper bed. Although
the combustion-air fan power requirement is considera-
."'fe^S^H?^-.^?.fl^^ed-bedcombustion ... 'S';^
* ^. •.' -V'': •""£"""•.' ; ..• -.,. :, • -.: .- -
; Companies involved with'this knowhow must choose among a variety of options; such as: Should the bed's
operating pressure be atmospheric or-elevated? Technological controversies now exist, and are examined here in an
excerpt from-a new report, "Fluidized-Bed Energy Technology: Coming to a Boil," by Waller C. Patterson and
'Richard Griffin.. Issued by Inform, 25 Broad Street, New York, N. Y. 10004, a nonprofit research organization,
the report is available for $45. . . '.'•.•"".. ' ' •• - ••"' ~ . -• .' . =.. • : .-• _•- . ',... •.. .-
Q Energy planners all over the. world are looking to
coal as the one essential fuel.. • • '
Actually, all but the most single-minded coal enthu-
siasts will concede that, given the choice, oil and natural
gas are more-satisfactory fuels than coal, and that solar
energy is even better in some cases. Interest in advanced
coal-technologies stems from the premise that oil or
natural gas is no longer reliably available, that nuclear
power carries high environmental risks and provokes
strong public opposition, and that solar technology is
not available or economical for certain important ap-
plications, particularly those requiring intense heat.
Thus, an increasing use of coal, especially over the
next few years, seems inevitable. If so, the potential
advantages of fluidized-bed systems warrant serious
study by those involved in energy-use and -supply deci-
sion-making.
Opinions about fluidized-bed technology differ
widely among specialists but all agree that the basic
concept is sound. Three U.S., one Norwegian, and three
British companies are offering fluidized-bed combustors
for small-scale industrial use, and a number of test
systems are already functioning. A typical capacity is
100,000 Ib/h of steam. When it comes to developmen:
120
CHEMICAL ENGINEERING AUCCST U. 1
-------
ble, it is largely offset by savings in mill and primary-air
' n power, when compared with conventional pulver-
ed bituminous or anthracite units for beds under 3 ft.
Choosing the maximum, rated bed-temperature de-
pends on several factors. First, the maximum tempera-
,re at which the bed can function is limited by the ash
sion characteristics. Obviously, when firing coal, no
matter what the starting bed material is, the bed will
"Itimately consist only of coal ash, and this must not be
lowed to sinter. So far, it has been found that trouble
.11 not occur if the bed temperature is at least 200°C
below the ash's initial deformation-temperature. Oper-
ing at the maximum permissible bed temperature
hieves the highest combustion efficiency within the
_.d, the highest heat flux to the immersed surface, and
the widest boiler-load control that can be practiced by
d-temperature control.
Combustion efficiency in the bed and the freeboard is
iitten high enough that sufficient carbon is burned up so
*« any carbon in the precipitators can be ignored. In
ne cases, it may be necessary to refirc a selected part
the solids carried over to achieve an acceptable un-
burned loss. As fluidized-bed boilers become bigger, and
iltiple beds are employed, there will be increased
:dom in choosing bed temperature and fluidizing
..ocity for the refired grits.
Boiler load is controlled by bed temperature and the
•aber of bed zones in operation. To reduce the load,
bed temperature is lowered by reducing the amount
3i incoming fuel and air. This is limited by the onset of
' "igmficant drop in combustion -efficiency. At that
at, selected zones will have the fuel and air cut off so
: the zones slump.
When this happens, the heat-transfer coefficient from
bed to tubes virtually instantaneously falls to zero,
resulting in a step change in load. The bed adjacent to
the tubes cools to the temperature of the tubes, but only
a small part of the bed is cooled, because of its insulat-
ing properties. The heat loss from the zone is so small
that after several hours' idleness, the zone may be reac-
tivated simply by restoring the air and fuel supply.
The extent of the load change by temperature control
is a function of the temperature range through which
the bed operates and the surface temperature of the
tubes immersed in the bed.
For example, consider a bed with a maximum load
temperature of 950°C, a tube surface temperature of
400°C, and a minimum bed temperature of 750"C at
which combustion efficiency begins to deteriorate. As-
suming half the heat flux is through convection and
half through radiation, then reducing bed temperature
from 950°C to 750'C would achieve an about 50% cut
in the work done in the bed. If, however, for any reason,
such as sulfur retention, it would be necessary to limit
the maximum operating temperature to 850"C, lower-
ing the temperature to 750°C would reduce the work
done in the bed by only 30%. Thereafter, load reduction
would be by slumping selected zones. The number of
zones required is such that by a combination of temper-
ature control and zone slumping, all of the stipulated
load range could be covered.
The rate at which the load is changed by temperature
control is very fast, as shown in Fig. 8. It will be seen
that by turning off the fuel and leaving on the fluidiz-
ing air, the temperature drops from 850°C to 750°C in
100 s, even with a fluidizing velocity as low as 4 ft/s.
;or larger, more-efficient or more-sophisticated systems',
;. aions differ." " " -•"-•.
j JKe most fundamental difference is between those
i.whoi fed that others are proceeding too fast, and those
'. ).feel that others are doing the opposite. In general,
t :' companies feel that European firms are moving
f --.-quickly, while Europeans.believe die U.S compa-
;nie?arc lagging behind.." '
I nother disagreement, more technical In -nature, is"
i ther to use. atmospheric or. pressurized systems."
S*TO£ companies, such as Pope, Evans. & Robbins, Inc.'
;i-^..York Ciry), are concentrating entirely on'atmos- '
•j ic systems; others such as Curtiss-Wright Corp.'
,1 pd-Ridge, N.J.), are concentrating on pressurized-'
^•sterns:.Still others, such as Babcock & Wilcox Ltd.'
',( ;don), arc actively involved in both lines of dcvel-
,c v":. " '•'•.:• :.:•: - '• . • : .
fi • ,!mpanics Pureuing both lines feel that atmospheric
;_uidized-bed combustion will be preferred for indus-
^ sized boilers, and that utilities will prefer pressur-
° systems, which can be adapted to combined-cycle
^- for a large utility plant, about 200 MW, important
•^ D"al-cost savings will be realized by the pressurized
*ed-bed combustor's smaller size (for the same
amount of output)." Also, improved ' efficiency "is
.achieved through combined cycles. ' "••" -'.". ' '-.' •
." Much of the controversy on how fast to proceed stems
._from uncertainty as to scaleup and the usefulness of
pilot-plant data. Some U.S. research laboratories carry
•out exhaustive small-scale investigations into emissions-
-and their control, sorbcnt "behavior, and. other fluid-!
^-ized-bed phenomena. Others, however, especially those '
'in Europe, insist that small-scale results will not apply"
to scaled-up units. Such companies,. Including' Stal-
- Laval .iTurbln (Fir.spong, Sweden) =an'd Babcock &'
•Wilcox Ltd.",'fed that" enough data already'exist to"
permit construction of major prototype facilities. V.
'.. A different opinion is held by those involved iri a
pressurized demonstration plant to. be built at
Grimethorpe, England, by the British government's
National Coal Board. The 80-MW, (megawatts, ther-
mal) unit will be of prototype size but will be used
purely for research. It is expected to be more flexible
than a commercial unit and should better lend itself to
full-scale experimentation. •• • •
Researchers at Grimethorpe' doubt there are enough
relevant data available to successfully design and oper-
ate full-scale, pressurized, fluidized-bed-combuscor/
CHEMICAL ENGINEERING AUGUST U, 1973
121
-------
FLCIDIZE BEOS
900
850
800
750
CI
•a
700
650
600
Fluidizing velocity. 4 ft/i. ji SSO'C
'''?£:* \''-V.'. 1^." '""• '•'.".•'' "."7":-.T-rT:"'' '"-I*
I
I
J_
i 0 50 100 150 2GO 250 300
1 Time, s •
In fluidized-bed combustion, temperature
dips rapidly as fuel is cut off ' . „•• t
Fig. 8
Obviously, by modulating the fuel input rather than
cutting it ofT, the rate can be made slower.
Separately slumping each zone is equivalent to turn-
ing off a burner group served by one mill in a conven-
tional pulverized-coal-fired boiler. There is, therefore,
much similarity in the boiler load-control systems, in
that the same sort of functions have to be performed. In
a pulverized-coal boiler, the removal of a mill and
burner group is dictated by its turndown capability,
which is defined for the operator. In a fluidized-bed-
fired boiler, the removal of coal feed and air from a
zone is dictated by the lowest temperature that provides
good combustion and maintains sulfur retention. Flu-
idized-bed firing has the advantage that once a bed is at
its operating temperature, an expensive fuel-oil support
system is not needed during load changes or at low load
operation, even with very-low-grade coal.
The author has described [2] the first year's experi-
ence with a 40,000-Ib/h boiler that was converted to
fluidized-bed firing. In subsequent trials with a coal
containing 60% ash, the boiler load was controlled from
46,000 Ib/h of evaporation to 12,000- Ib/h, with 'no
support fuel required. This is readily understandable, as
the coal in the bed is less than 1% by weight of the bed,
even at maximum load.
Atmospheric-pollution control
The main pollutants in the stack gases from current
conventional high-temperature combustion systems are
ash, sulfur oxides and nitrogen oxides. There is. how-
ever, a growing concern about the emission of trace
elements such as arsenic, antimony and mercury.
Ash is trapped with acceptable efficiency by precipi-
'"gas-turbine systems. However, Stal-Laval Turbin and
Babcock &-Wilcox, Ltd., among others, disagree. The
Grtmethorpe team considers it premature to tackle both
;the problenvoEa pressurized fluidized-bed cocnbustor_
and that of coupling such, a combustor to a gas turbine. •
.On'the other hand, the.Stal-Laval/Babcock & Wil-
_cox team feels that the Grimethorpe unit, will be."rein-
venting the wheel," carrying out research on a nohcom- .
mercial.uhit that might perfectly well be done by
building a. commercial prototype and acquiring the
desired experience in. actual service. Some U.S.'experts
feel that the Stal-Laval/Babcock & Wilcox team is
overlooking the problems of gas-turbine-blade corrosion
due to sodium and potassium in the coal.
One project, considered already obsolete by some, is a
30-\IWe (megawatts, electrical) prototype atmospheric
unit, built in Rivesville, W. Va., by Pope, Evans &
Robbins for the former U.S. Office of Coal Research,
now part of the U.S. Dept. of Energy (DOE). The project
was an attempt to develop a cheap coal-fired boiler for
power generation. Contracted for in 1972, before the oil
price-rise, the system was supposed to compete with
then-cheap oil. ' • . .
The plant is a multi-cell fluidized-bed unit, consisting
of four separate beds. The first bed boils the water, the
second heats- the resulting steam, the third superheats
the steam, and the fourth burns up unburned carbon.
Problems have been encountered, particularly with
coal-handling and coal-feed systems. One persistent
' problem is clogging of coal in feed lines, blocking the •
fuel supply.
Critics feel these problems are partly due to tight'
specifications laid down when the objective was to build
a very inexpensive boiler.
Today, DOE is primarily concerned with sulfur-oxide
emissions control. Fluidized-bed combustors must com-
pete economically not with oil-fired boilers but with
conventional coal-fired boilers using scrubbers. Pope,
Evans &. Robbins concedes that times and priorities
have changed. However, the company questions a
wholesale switch to pressurized fluidized-bed combus-
tion. It believes that money saved by smaller-size units
for the same output will be offset by the additional cost
of coping with the higher pressures, especially for feed-
ing coal and removing solid waste. Atmospheric systems
are thought preferable for all sizes.
Stal-Laval believes that pressurized systems may be
preferable For msdium-to large-scale applications. Ths
firm cites the advantage of being able to double a
pressurized system's useful energy output by using com-
122
CHEMICAL ENCI.Sf.EKING AUGUST 11. 1973
-------
tators. Sulfur oxides can be controlled by burning low-
sulfur fuels, which command high prices, unless they
are coals of the sub-bituminous types, such as those
found in the western U.S. and in south Australia, that
have a high alkali content and give rise to severe slag-
ging and fouling.
For high-sulfur fuels, expensive scrubbing systems
may be used. A popular method is to scrub the gases
\rith a limestone slurry. But this has two disadvantages.
First, it creates a residual pollution problem in dispos-
ing of the spent limestone, unless an expensive recovery
plant is installed to regenerate the limestone and re-
cover the sulfur, which has a doubtful market value.
Second, scrubbing carries a penalty of at least 4% on
gross thermal efficiency because, after scrubbing, the
gases have to be reheated before entering the stack.
Nitrogen oxides are generally controlled by interfer-
ing with the combustion process, again with attendant
.penalties.
:. With fluidized-bed firing, when there is no restriction
-on sulfur oxides emission, the removal of paniculate
matter from the gases is somewhat easier than it is for
pulverized-coal firing. This is because, in a fluidized
bed, for a given coal less goes to the precipitators, and
the mean particle size is much greater. Where sulfur
retention is practiced, adding limestone (or dolomite)
makes the dust burden greater and changes the charac-
ter of the dust to be caught. Opinions among the mak-
en of electrostatic precipitators appear to be rather
mixed. Some claim that enough is known to permit the
design of satisfactory precipitators, while others regard
this area as still developmental.
Garner, Howe and Dzierlenga [3] list proposals of
100
80
* 60
I
3
840
20
_X '
lim»ston« A
^Predicted pomti from
'"laboratory SCale tttisX
'-'•* *> *• T"" •? •" . • »
40.000 Ib/h fluidiztd-bcd boiltr
Sulfur content of coal. 5.5%
8«d wmperaturt, 850'C. ;'r'
•''
234
Calcium: sulfur mol ratio
0.2 0.4 0.6 0.8 1.0
Ratio of limestone to coal by weight
1.2
Better than 90% retention of sulfur is '. '•..- /••
achieved even when using high-sulfur coal Fig. 9
v.' ---- •, ..»;• . : -•. -- "--••••;, Tll-*"j;-~
p'- .blried cycles or cogeneration. The company believes this
[ >T|n>e the most effective lever in persuading industries
t; JO convert their boilers to ones that use pressurized
K.fiuidized-bed coal firing. . -. '..;',';„.-;,::;
.Fraas, a consultant to the Oak Ridge National
(Oak Ridge, Tenn.), points out that using a
hot-air turbine system makes it possible to
atmospheric fluidizcd-bed combustion and
cycles or cogeneration.. -., '"-::""-..i.v:i'I'::;-.i;:"/
, Freedman, -director of. DOE'S ^fluidized-bed
^^ ^hat industrial-sized atmospheric fluid-
ar« closest to becoming commercial. These
eatest advantage, he believes, is their fuel-use"
.. Freedman sees utility systems as a longer-
t?7lPfoposition. DOE hopes to have both large atmos^.
(:.P ^ ^nd large pressurized fluidized-bed plants online.
^- By;1984, so utilities can choose between the two systems.
§• . ?** advantages in both approaches: atmospheric is
^ *cipler and more reliable, while pressurized has a
'5"Uy higher efficiency, and produces slightly fewer
lfur-dioxide emissions.
.Other differences of opinion exist. Battelle Memorial
•.Institute (Columbus, Ohio) and various U.S. research-
•"!* are concerned about possible corrosion of boiler
. Others, especially those in Europe, see nothing at
.all to suggest that such problems will arise! ;-
Coal feed has caused continuous trouble at the'
: -Rivesville unit, and several U.S. firms,are also con-
• ' cerned about feed problems, while the'Europeans see no
such troubles. The general European view seems to be
~ that coal can be fed from above the bed,, within it or
'under it; and that the easiest way, from above, is satis-
factory. : Coal fed from above remains 'within the bed
;r"long enough to burn completely withoutTundue carry,-"
?.--over of unburned particles into the flue"gases. Europe-
: . ans feel that if a company uses a special carbon bumup"
cell for unburned particles when moving to large-scale
"; ^.applications (as is the case at RivesviJle),--thJs indicates
-•.the design of the main bed is hot optimum:.' . "• - '.':
-, '.' Sdll another difference of technical opinion concerns
•unit size. Some favor cell and bed dimensions small
enough to permit prefabrication in the manufacturers
shop, while others favor larger units. The smaller-scale
(or modular) approach offers the advantages of repli-
cation, and of stable working conditions in an estab-
lished shop. However, Babcock & Wilcox, Ltd., and
others believe the modular design may not be as flexible
in actual operation as the custom-built large-scale bed,
which may be subdivided into separate cells with sepa-
rate air and fuel feed. -; : ••*.
CHEMICAL ENGINEERING AUGUST 14, 1978
123
-------
FLU1DIZE BEDS
Raw-coal
inlet
,Throat gap
Primary-
air inlet
Mill modified to operate as a crusher. •
Unit separates out pyrites as well Fig. 10
400
350
M
C
g
| 300
o
o
.H
I 250
§
ic
1200
o
I 150
a.
x
O
100
50
. .- ..-
. with 1.154 nitrog«r> _.'-.'• •'-••"-. -"^
i,40.000-lb/hboiler ; 3 •••..;. • I-.-.V'."- .--,'•.'•""
^ir/5ivj;lvT.r
•-':»rj «">-• ,; •.*•'- .
.' .. ---;j.-^v^" - ^ r.
^~'r-'..- . Vl _ .-. \
-*v-.;v. -"..v>-.—. .T
''.»""•'•''•>:?-'~ ' t' -" -.'- -=rl
sp^:.-:;
-',.v^v--;^
^.^^s®'
650 700 750 800 850 900 950
Bed temperature, °C
NO*, emissions level off above SOO°C to
value below current U.S. legal maximum Fig. IT
three U.S. boiler makers. Two propose to trap larger
particles in multitube cyclones and do the final cleanup
in a baghouse. The third proposes using two hot-side
electrostatic precipitators, one to trap the coarse mate-
rial for possible reinjection and the other for final
cleaning. There is no doubt that baghouses can now be
used to meet particulate-emission limits. Precipitators,
however, may turn out to be the most economical way
to control particle emissions.
Limestone efficiency
Fig. 9 shows the effect of the limestone/coal ratio on
sulfur retention for two different limestones. Although
these have the same chemical composition, their effi-
ciency in reacting with sulfur is different, probably due
to differences in their porous structures. These results
show two other features: (1) even with very-high-sulfur
coal, retentions better than 90% are achieved; and (2)
small-scale tests can accurately predict the Ca:S ratio
required to yield the desired performance in a boiler.
Ehrlich [4] shows the effect of bed temperature on
sulfur capture and concludes that the optimum tem-
perature at atmospheric pressure is near 800 °C. A flu-
idized-bed combustor operating between 850 ""C and
750°C would probably provide the optimum in sulfur
retention and boiler load-modulation by bed tempera-
ture control.
Sulfur occurs in coal in organic and inorganic form.
The inorganic is mainly pyrites, an iron sulfide having a
specific gravity of 5.0. Coal has a specific gravity of
about 1.3, while that for coal ash or limestone is around
2.7. In low-sulfur coals, the organic form predominates.
As the total sulfur increases, more of it becomes pyrites.
This is separated out in some types of mills.
Fig. 10 shows a sectional elevation of such a mill. The
air for drying and conveying the coal through the mill
enters a plenum chamber under the yoke and then
passes through slots in the throat ring into the mill. The.
velocity through these slots keeps coal particles in the •
mill, while heavier particles (pyrites) fall through the
slots. The rejected material is then swept by ploughs to
a reject box. The high-sulfur coal used in Fig. 9 may
have half of its sulfur as pyrites. The more discrete the
distribution of the pyrites, the greater the proportion
will be rejected and, accordingly, the greater the savings
in limestone needed to reach the acceptable SO, emis-
sion level.
'With coals having a modest sulfur content, the cal-
cium oxide in the coal ash may be sufficient to reduce
SO, emissions without requiring the addition of lime-
stone to the bed.
In Fig. 11, NOX emissions from a Babcock boiler that
is firing a coal containing 1.1% nitrogen are shown as a
function of bed temperature. The analyses were done by"
the chemiluminescence method. The maximum effluent
level is well below the current limit of 525 ppm at 3%
excess oxygen set by the U.S. Environmental Protection
Agency for new coal-fired plants. In fluidized-bed com-
bustion, it is now generally accepted that most of the
NOX formed comes from the nitrogen in the fuel and
not from the atmosphere. The maximum figure of 325
ppm shown on Fig. 11 corresponds to the conversion of
about 30% of the nitrogen in the coal
124
CHEMICAL ENGINEERING AUGUST U. 1973
-------
Saturated-steam pressure
150ib/inZ
3i-drum boiler that will be converted to fluidized-bedfiringVv/U'
Fluidized-bed advantages
^rther advantages are apparent when comparing
idized-bed combustors with other furnaces:
Because combustion is carried out at a relatively low
iperature, the coal-ash particles are kept below their
cmng temperature, preventing slagging and
---ided-deposit formation. Hence, the need for soqr-
blowing is reduced. On the 40,000-lb/h boiler, the soot-
hers that were in place before conversion have never
tied to be used in over 6,000 h of operation. This
snould allow for more-compact convective-heat-transfer
-ks, thereby increasing heat-transfer coefficients and
umizing the surface area.
cry-high heat-transfer coefficients are obtained be-
en the bed material and the immersed surface, so
although the temperature differential is lower than
pulverized-coal- or oil-fired boiler, the overall heat
l«4* is greater. Therefore, less surface is required. Fur-
ther, there is relatively little variation in the heat flux
all the immersed surface, so there is small risk of
: failure due to high local heat flux which can lead
o departure from nucleate boiling in the tubes.
The low combustion temperature leads to markedly
reduced vapor pressures of alkali metal sulfates and
chlorides. Thus, the greater part of these harmful con-
stituents remain in the bed. This is true also of vana-
dium and sodium salts when heavy fuel-oil is fired in a
fluidized bed. Cooke and Rogers [5] report the results of
corrosion trials on a number of low-chromium and
austenitic steels. They conclude that all the materials
would provide satisfactory service under normal fiuid-
ized-bed operating conditions, if used at the tempera-
tures employed in a conventional plant. Even with that
limitation, however, the fluidized-bed system has an
advantage. Because of the high rate of mixing in the
bed, all the tubes will be at the same temperature, so
there is no need to add temperature margins in design.
Current designs of fluidized boilers
In the U.K., fluidized-bed-fired boilers having an
evaporation rate to 500,000 Ib/h are being offered. The
upper limit of evaporation is fixed at a level that is
achieved economically in a single bed, at fluidizing
velocities up to 10 ft/s. Above that size, multiple beds
CHEMICAL ENGINEERING AUGUST 14, 1978
125
-------
FLUIDIZE BEDS
Evaporation rate:
355,000 Ib/h
jj; Primary superheater
Air heater
Mills
Primary-air
fan
Forced-draft
fan
Two^pass; radiant fluidized-bed boiler uses convection banks for superheating xV'^-.rig.-.T.
• ••.^^•ifS^uC^^^f-.ff^.f^.\&JX^£:±^Z3:^y?::.:^:-* ::&.&..:- •>;':•f.~/^.->',-'W.<•-~SA'.;V^j--'-.l'V?.'C^'.^-yy-.^--, .^,»=U.'av:f •^--.'a^
are used. Not enough time has yet been devoted to
seeking the optimum arrangements for multiple beds.
Garner, et d. [3] describe conceptual designs for 500-
MW boilers by three U.S. boiler makers in conjunction
with two architectural engineering firms, and sponsored
by the U.S. Dept. of Energy. In the U.K., an order has
been received for an 80,000-lb/h boiler for a chemical
process company. It is a standard bi-drum boiler with
conventional oil burners in its front wall. However,
provision has been made for conversion within five
years to coal firing with a fiuidized bed.
Two other projects are illustrated. Fig. 12 shows an-
other bi-drum boiler, which will be converted from
spreader/stoker firing to a fluidized bed. Conversion
must be looked at carefully, since it may be more eco-
nomical to install a new boiler, depending on the state
of the plant and the original fuel for which the plant
was designed. Conversion of spreader/stoker firing to a
fluidized bed can be attractive, since the combustion
rate per unit area and the freeboard shape and size can
be the same for the two systems. Only three separate
slumped zones are required to meet the load changes
demanded by this system. ;.,-
Fig. 13 shows a design for the 30- to 40-MW, range.
In this unit, all the superheating is done in convection.
banks, so that only generating tubes are immersed in
the bed. Also, the boiler plan area is reduced and di-
vided to provide the gas velocities required to reach
adequate convection-heat-transfer coefficients. When
boilers demanding multiple tiered beds are called for,
the plan area will not need as much, if any, reduction.
Fluidized beds are used in the marine field [6\. The
ability of an oil-fired fluidized bed to retain vanadium
and sodium is being applied to raise superheat and
reheat temperatures to 600°C in steam turbines for ship
propulsion. Hitherto, such temperatures have been for-
bidden with conventional oil firing by high-tempera-
ture corrosion of superheater and reheater tubes.
Pressurized fiuidized beds
So far, we have concentrated on atmospheric fluid-
ized-bed boilers, principally because their development
126
CHEMICAL ENG1NF.F.KINC AUGUST I ». 1
-------
is more advanced. There is, however, a great deal of
work going on in pressurized fluidized beds—which
promise these further advantages:
1. For a given duty, the bed area is inversely propor-
tional to the absolute pressure; thus the plant can
be compact and comparatively lightweight.
2. Combustion efficiency within the bed is such that
good design should eliminate the need for grit
recycling.
3. Using dolomite, sulfur retention is improved with
pressurization. (Unfortunately, limestone effec-
tiveness decreases at increased pressures.)
4. NOX emissions are reduced further by pressure.
Pressurized combustion should permit the develop-
ment of combined gas and steam-turbine cycles, which
hitherto have been limited by the necessity to employ
clean fuels for the gas turbine. A gas turbine should
operate for long periods without significant fouling,
corrosion or erosion, due to the non-erosive nature of
the coal ash, the ability of the bed to retain alkali
metals and sulfur, and the effectiveness of mechanical
•emoval of paniculate matter from the gases with an
acceptable pressure drop. Roberts, et at. [7] describe the
experimental work and the types of cycle that may be
employed. Thurlow [8] mentions some design studies
:urrcntly in progress, and describes the application of
fluidized-bed combustion to boilers smaller than those
discussed here.
Conclusions
It has been said that there would be no interest in
fluidized-bed combustion if it were not for the stringent
•egulations concerning the emission of sulfur oxides and
nitrogen oxides. This is extremely difficult to believe.
While atmospheric pollution is important—and it has
Decn amply demonstrated that these beds can control
t—surely an important factor is the ability to operate a
plant at its design capacity, no matter how the fuel
supply may change during its life.
Another important factor is energy conservation.
oince the Industrial Revolution, coal mining has re-
jected combustible material consumers would not take,
lither because consumers could not use it or because it
*as uneconomical to do so. For similar, reasons, there
are large quantities of high-ash coal deposits in the
world that have been left in the ground. In the U.K.,
argc amounts of the heat content of the total coal
nined has been lost to pit spoil-heaps. [9]. The world
can no longer afford to squander or neglect its energy
•esources that way.
Cost comparisons between ffuidized-bed combustion
tnd present conventional units are not made here be-
cause the costs of the latter vary so much with the type
>f coal being fired. Thurlow gives the results of some
tudies both for atmospheric and pressurized applica-
tions. Generally, these favor fluidized beds, particularly
•f flue-gas cleaning is taken into consideration.
There is continuing development in atmospheric and
pressurized fluidized-bed combustors. In atmospheric
units, the emphasis is on increasing combustion effi-
ciency in the bed to reduce the need for refiring car-
bon-loaded grits. In pressurized designs, emphasis is on
the development of power turbines and control systems
that can be successfully integrated with the combustor.
It is hoped that this report has shown that atmos-
pheric fluidized-bed boilers are gaining acceptance; any
operator with a new plant in mind should give the
system careful study.
Acknowledgment
The author acknowledges with thanks the permission of
the directors of Woodall-Duckham Ltd. to publish this
repon, and thanks his colleagues who have helped and
advised him in its preparation.
References
1. McLaren. J. and Williams. D. F., "Combustion Efficiency, Sulphur Reten-
tion and Heat Transfer in Pilot Plant Fluidised Bed Combuston," /• liut
Fuel, Aug. 1969.
2. McKeruie. E. C. Fluidised Bed Firing in Boilers, Sfoct Httlna mi Air
Condiiwunf Jumol, Mar. 1977.
3. Garner, D. N., Howe, W. C. and Dtierlenga, P. S.. A Companion of Selected
Deign Aspects of Three Atmospheric Fluidised Bed Combustion Concep-
tual Power Plant Designs, Fifth International Conference on FBC, Wash-
ington, Dec. 1977.
4.. Ehrlich, S., A Coal Tired Fluidised Bed Boiler and Fluidised Combustion
Conference organized by the Inst. Fuel, London, Sept. 1975.
S. Cooke, M. J. and Rogers. E. A., "Investigations of Fireside Corrosion in
Fluidised Combustion Systems," Inst. Fuel Conference, Sept. 1975.
8. Stal-Laval publication, "Very Advanced Propulsion."
7. Roberts, A. G.. Stanton, J. E., Wilkins. D. M., Beacham, B. and Hoy, H. R.,
"Fluidised Combustion of Coal Si Oil Under Pressure," Insi. Fuel Confer-
ence, Sept. 1975.
8. Thurlow, G. G., The Combustion of Coal in Fluidised Beds. Pro. of Inst.
Mich. Engri., Vol. 192, No. 15, pp. 145-156.
9. Down, W. S. and Brown, A., "Colliery Spoil Heaps as a Source of Energy,"
Conference on Energy Recovers- in Process Plants, L Mech. E., London, Jan.
For an introduction to coal, its selection,
preparation and combustion, see the Feb., Mar.
and April, 1974, issues of Power.
The author
E. C. McKenzie acts as a full-time
consultant in fuel technology to
Woodall-Duckhara Ltd., Babcock 4
Vv'ilotw Group, Woodall-Duckham
House, The Boulevard, Crawley, Sussex,
England RH10 1UX. He received his
training in fuel technology at the Fuel
Research Station, which was pan of Her
Majesty's Dept. of Scientific and
Industrial Research. In 1936, he joined
Babcock & VVilcox, and has since
worked on problems including fuel
preparation and heat transfer.
Reprints of this report will be available shortly. To order, check number 295 on the reprint order-form
in the back of this or any subsequent issue. Price: S3.00 per copy.
Please circle No. 305 on the Reader Service Card for a complete Catalog of Reprints.
CHKMICAL ENGINEERING AUGUST U, 1978
127
-------
V. NO
-------
5.2 COMBUSTION PROCESS MODIFICATION TECHNOLOGY
As a result of emission control regulations for new and existing
stationary sources, NOX control techniques have been developed and
implemented in the past 10 years. Nearly all current NOX control applications
use combustion process modifications. Other- approaches, such as modifying
or switching fuels, using alternate energy systems, and treating post-combustion
flue gas as well as more advanced combustion process modifications are being
evaluated for potential future use. Experience has shown that the applicability
and effectiveness of combustion process modifications depend on the specific
equipment/fuel combination to be controlled, and on whetner the control is
to be applied to existing field equipment or new units. Accordingly, control
development is focusing on specific equipment categories and fuel types.
In general, the following sequence of control development is oemg pursued
for each major equipment/fuel category:
• Minor operational adjustments
t Minor retrofit mod.if i cat ions
SOURCE: "Environmental Assessment of Stationary Source NOX Control
Technologies: First Annual Report," EPA-600/7-78-046, U.S. EPA,
Research Triangle Park, NC (March 1978).
-------
• Extensive hardware changes, either retrofit or factor y- MiU" i^J of
units, of conventional design
• Major redesign of new equipment
Progress made in this sequence varies with the importance of the source in
local and national NOX regulatory strategies.
Currently, modifying combustion process conditions is the most
effective and widely-used technique for achieving 20- to 70-percent reduction
in combustion-generated oxides of nitrogen. These modifications include:
• Low excess air firing
• Flue gas recirculation
• Off-stoicnicmetric combustion
o Load reduction
• Burner modifications
• Water injection
• Reduced air preheat
• Ammonia injection
The following paragraphs summarize the status of each of these controls.
Low Excess Air Firing
Changing the overall fuel-air ratio is a simple, feasible, and
effective technique for controlling NOX emissions from all stationary sources
of combustion except gas turbines. For some sources, such as utility
boilers, low excess air (LEA) firing is currently a routine operating
procedure and is incorporated in all new units. Since it is energy efficient
and easy to implement, LEA firing will be increasingly used in other sources.
However, most sources will have to use other control methods, in conjunction
with LEA, to meet NOX- emissions standards. In such cases, the extent to
which excess air can be"lowered will depend upon the other control techniques
employed. Virtually all programs for developing advanced NOX controls are
emphasizing operating at minimum levels of excess air. Thus, LEA will be an
integral part of nearly all combustion modification NOX controls, both
current and emerging, to be assessed in the NOX E/A.
Flue Gas Recirculation
The primary near-term application of flue gas recirculation (FGR) is
in gas- and oil-fired utility boilers. Future applications are limited. FGR
may be used in industrial boilers as a retrofit or in new designs, but
43
-------
x ~sT6Tch foraetrTc"
Lonoustion, also are being evaluated and may prove more attractive
Off-Stolchlometric Combustion
, ity boners ^"? ac°TinS""ed •Ul'-"t « "" « '
re aca*1<"'s "> "e considered Include
^^^
Load Reduction
used only as a last resort to acfneve compliance with standards.
Burner Hodffications
. O
Water Injection
«ter ejection «,„ be replaced b, advanced cc'Stor dL^'fn^T,^"
-------
Reduced Air Preheat
Reduced air preheat for gas turbines and for boilers is not a
practical way to control NOX unless the energy in the exhaust gases can b*
used effectively for other purposes, such as in combined gas-steam turbine"
cycles. Reduced air preheat will thus be accorded low priority in the NO,
E/A because of associated efficiency losses. x
Ammonia Injection
* !! ®C * ' °? d?e* not appear to hflve significant near-term
O v "°x 5°nti;?Vn the U'S- However' ]t shows Promise f°r far-
applications and will be given primary emphasis in the N0y E/A for
assessment of advanced concepts for the 1980 's and 1990 's.
5.3 ALTERNATE CONTROL TECHNIQUES
In addition to combustion modifications, NOX can be controlled by one
or more of the following techniques: flue gas treatment, fuel
denitrification, fuel additives, alternate or mixed fuels, or advanced, low-
NOX combustion concepts. Each of these is briefly discussed below.
Flue Gas Treatment
The dry flue gas treatment (FGT) techniques used in Japan - notably
selective catalytic reduction with ammonia - can probably be applied to
gas- and oil-fired sources in the U.S. However, more pilot and full scale
demonstration tests are needed before full application of dry processes is
f?rPrf nM™heSUhSOUrhCeS'-i ?"y p:°cesses have ** to be demonstrated on coal-
fired sources although pl lot-scale tests are currently planned. Wet processes
are less well developed and more costly than dry FGT processes- however wet
processes have the potential to remove NOX and SOX simultaneously Again
"
X
SJj^;5"16/"??:? and field tests are needed to" determine costs, secondary
effects, reliability, and waste disposal problems. Flue gas treatment holds
scrne promise as a control technique if very stringent emissions standards
win n nheMSSsry ° ,9re!tly reduce N°*- However' even in these instances FGT
will probably be employed to supplement combustion modification.
Fuel Denitrification
Fuel denitrification of coal or heavy oils could, in principl° be
used to control the component of NOX emissions produced by the conversion
of fuel-bound nitrogen The most likely use of fuel denitrification would
De to supplement combustion modifications that reduce thermal N0y. Currently
denitrification occurs only as a side effect of pretreating fuel to remove
™ T\n ' °P ?therl P°]lutant Precursors. Preliminary data indicate that
?r t° 4°-P^ce nt reductions In fuel nitrogen result from oil desulfurization
Deference 22). Since these processes produce low denitrification efficiencies
45
-------
SOURCE: Technical Assessment of Thermal DeNOx Process—Interagency
Energy/Environment R&D Program Report. EPA-600/7-79-117,
U.S. EPA, Research Triangle Park, NC (May 1979).
SECTION 2
PAST EXPERIENCE
The noncatalytic reduction of NO by the Thermal DeNO Process was
discovered in August 1972 by Exxon Research and Engineering Co. (ER&E).
Since then, numerous laboratory, pilot, and full-scale tests have further
investigated the Thermal DeNOx Process. These tests have been designed
to understand the critical process parameters and how they can be used to
control N0x emissions from both stationary and mobile sources.
ER&E, developer of the Process and patent holder, has done most of
the laboratory research and field studies. However, Exxon's tests were
limited to gas- and oil-fired facilities, except for full-scale tests on a
solid waste incinerator. KVB Inc., under contract to ER&E and EPRI has
recently studied the use of Thermal DeNO on a 3 x 106 Btu/hr
A
coal-fired test boiler. KVB has also conducted the only domestic
full-scale application of NH3 injection. This was on a thermal oil
recovery steam boiler.
Table 2-1 presents a summary of all commercial installations
utilizing noncatalytic decomposition of NO by ammonia. All of these
installations use Exxon's ammonia injection technology- except for one
Japanese source noted. Detailed information on all installations using
Exxon's Thermal DeNOx Process is not available. However, depending on
the source and its operation, NO reductions as high as 70 percent have
been achieved.
2-1
-------
TABLE 2-1. SUMMARY OF COMMERCIAL APPLICATIONS OF EXXON'S THERMAL DeNOx PROCESS
ro
i
ro
Source
Steam Boiler
45 MM heat Input
Incinerator
7 ton/hr
Crude heater
ISO x lO^bb I/day
Steam boiler
76 KU heat input
Utility boiler
275 HW heat input
Utility boiler
275 HW heat Input
Crude heater
150 x 103bbl/day
Thermal recovery
heater
Utility boiler
375 KW
• Fuel
Burned
Gas/oil
Waste
Gas/oil
Gas/oil
Gas/oil
Gas/oil
Gas/oil
Oil
Residual
oil
Location
Japan
Japan
Japan
Japan
Japan
Japan
Japan
USA-
California
Japan
Initial Nitric
Oxide Emissions
(ppn as measured)*
120-150
100-180
150
95-145
80-120
80-120
80-85
260
HA
OeNOx Rate
(Percent)
60
20-70
35-65
35-50
60
50-60
40-65
50-70
40
Addltlveb
Yes
MAC
Yes
HA
NA
NA
NA
NA
No
Comments
No reduction obtained at full load
Difficult source to retrofit because
of constant change In fuel
Best reductions achieved at high
Injrf
IU6O
No details of retrofit system are
aval lable
No details of retrofit system are
available
No details of retrofit system are
available
Best reductions achieved at high
loads
First commercial U.S.
Installation
Does not use Exxon N)l3 Injection
technology — NHj emission limited
to 10 ppm
•No' indicates that no additive was used
CNA • no data are available
«H3 to obt ..... ported NO. reduction performances
-------
Based on these full-scale results, Exxon has commercialized the
process and will license it upon request on gas- and oil-fired boilers.
Additionally, they are continuing to study the feasibility of full-scale
applications on coal-fired boilers. These studies are aimed at maximizing
N0x reduction and cost effectiveness, and exploring and defining
potential operating problems.
This section summarizes the status of Thermal DeNO Process
A
developments. Results from gas and oil combustion in laboratory and
pilot-scale studies are discussed in Section 2.1. Section 2.2 presents
results from ammonia injection in a coal-fired pilot-scale facility.
Results from ER&E's full-scale commercial demonstration of the process are
described in Section 2.3.
2.1 SUBSCALE TESTING — GAS AND OIL
ER&E discovered a new reaction which is the basis of the Process in
research done in a laboratory flow reactor. Based on the observed
kinetics of this reaction Lyon proposed the following mechanism
(Reference 2-1):
NH2 + NO - »- N2 + H + OH
NO
H + 02 - >• OH + 0
0 + NH3 - >- OH + NH
OH
H
To further explore this reaction mechanism, ER&E conducted tests on
a small 0.3 MW (10 Btu/hr commercial size boiler). These tests
investigated how key operating variables such as reaction temperature,
2-3
-------
M, injection rate and flue gas residence time influence the NO,
-------
Lyon reported that the reaction temperature of about 955°C
(1750°F) results in the largest NO reduction. At temperatures higher
than approximately 1100°C (2010°F), the oxygen present in the flue gas
oxidizes the injected ammonia producing a net increase in NO. Below
900°C (1650°F), the reaction of NH3 with NO is slowed considerably
causing less NO reduction and more NH3 emission from unreacted gases.
Figure 2-1 shows how the reaction temperature affects the performance of
the Thermal DeNO Process.
A
This strong dependence of Thermal DeNO performance on reaction
A
temperature can limit the use on some systems. For example, the required
temperature window in gas turbines and 1C engines is located where a very
short residence time is available for reaction with NH.J. Thus, Thermal
DeNO may not be suited to these sources. In large steam generators,
A
the optimum temperatures and residence times for noncatalytic reduction of
NO are usually accessible in the boiler convective section. However,
cross-sectional temperature gradients as high as 200°C (360°F) often
exist making ammonia injection considerably less effective for some areas
of the flue gas ducts. Load variations also shift the temperature profile
causing the temperature window to move in the convective section.
These problems could theoretically be solved by installing more
than one injection stage to account for the shift in the temperature
window. However, additives have also been evaluated as methods to
accommodate temperature variations and unfavorable location of the
temperature window. For example, hydrogen additive is a demonstrated
alternative for controlling or shifting the inj'ection points to the
optimum temperature location for the Thermal DeNOx Process. Additive
injection is discussed in Section 2.1.3.
2-5
-------
1.0
0.8
0.6
0.2
Excess oxygen: 4*
Initial NO: 300 ppm
(NH3)/(NO)
1.6
700 800 ' 900
Temperature, C
1000
1100
Figure 2-1. Effect of flue gas temperature on Thermal DeNOx
performance (Reference 2-2).
2.1.2 Ammonia Injection Rate
Ammonia efficiently reduces NO because of its ability to react
A
selectively with nitric oxide regardless of the amount of oxygen present
in the treated gas. Thus, the amount of ammonia required in the Thermal
DeNO Process is on the order of the initial NO concentration. Other
A
additives, such as methane and ethane also can be used to reduce NO in hot
flue gases. However, because these reagents are nonselective, they do not
react with nitric oxide alone. That is, all the free oxygen present in
the hot gases must first be consumed by the reagents before the NO can be
reduced. Therefore, more hydrocarbons are necessary, thus causing these
additives to be economically unattractive.
2-6
-------
Experiments conducted by ER&E and KVB show that for typical
conditions, an ammonia injection rate of 2.0 (molar ratio of NH_ to
initial NO concentration, (NH./NO)) achieves the optimum maximum process
efficiency. Figure 2-2 shows that minimal additional NO reduction is
obtained by increasing the ammonia injection rate beyond the NH,/NO
molar ratio of 2.0.
23 456
(NH3)/(NO)
Figure 2-2. Effect of NH3 injection rate on NO emissions
(Reference 2-2).
Ammonia injection rates depend on the initial concentration of
nitric oxide. Experimental data illustrated in Figure 2-3 show that lower
molar ratios of NH.,/NO are needed to achieve a given process efficiency
when the initial NO concentration is greater than 400 ppm. These
experimental data further indicate that the percent oxygen in the flue gas
may also have some effect on required NH^ injection rates. Minimum
amount of dilution with excess air decreases the volume of flue gas to be
2-7
-------
treated and increases initial NO concentration thus possibly reducing the
amount of NH., needed.
1.0 o-
0.8 -3
1 0.6 -
I I
EXCESS OXYGEN: 22
TEMPERATURE: 960°C (1760°F)
INITIAL NO LEVEL (PPM) ~~
D 100
A 200
O 100 _
O 680
O 1050
Figure 2-3.
1 . 2 3 1- 5
(NH3)/(NO)
Effect of initial nitric oxide concentrations on
reductions with ammonia injection (Reference 2-2).
The ammonia injection rate, the reaction temperature and the
residence time are critical in maintaining ammonia emissions at minimum
levels. Test data depicted in Figure 2-4 indicate that the level of
unreacted ammonia at the injection temperature of 965°C (1770°F)
increases significantly only at NH3/NO ratios greater than 2.0, as
expected. When the reaction temperature is lowered to 870°C (1600°F)
the level of ammonia carryover is substantially increased because of the
slower chemical reaction. Therefore, the ammonia emission level can be
controlled by allowing the reaction to occur at slightly higher
temperatures than the optimum 955°C (1750°F). In fact, NH3
2-8
-------
injection, at temperatures above 1000°C (1830°F) virtually all NH,
breakthrough is eliminated.
2400
2000 -
(NH3)/(NO)
Figure 2-4. Effect of NH3 injection rate on NH3 carryover
emissions (Reference 2-2).
Poor mixing of ammonia with the flue gas may cause ammonia
carryover to occur also at NH../NO molar ratios much lower than 2.0. In
fact, large scale applications of the Thermal OeNO Process have shown
that NH-/NO ratios in nonideal gas conditions generally should be lower
than 1.5 to maintain minimum NH_ emissions. High levels of ammonia
breakthrough were caused by high ammonia injection rates combined with
ineffective mixing or low flue gas temperatures.
In summary, NH_/NO molar ratios can vary from 1.0 to 2.0 in large
scale applications of the Thermal DeNO^ Process. The actual injection
rate used will depend on the desired NOX reduction and could be limited
by ammonia breakthrough. Therefore, the injection rate is the result of
2-9
-------
an optimization performance study taking into account flue gas conditions
and source configurations.
2.1.3 Hydrogen and Other Additives
The Thermal DeNO Process can be applied over a greatly widened
A
range of temperatures if certain additives are injected with the ammonia.
Of the many additives investigated, hydrogen is the most effective over
the temperature range from 700 to 1010°C (1290 to 1850°F). Figure 2-5
shows the shifting effect of hydrogen injection on optimum reaction
temperature measured in a commercial size firetube boiler. The magnitude
of this shift depends on the amount of H_ injected relative to the
NH.,. For example, at H_/NH, molar ratios on the order of 2:1 selective
O CO
noncatalytic reduction of N0x can be made to occur at 700°C (1290°F)-..
Thus, by carefully selecting the H. injection rate, flue gas treatment can
be controlled over a wide temperature range.
200
150
E
a
>
£ 100
e
M
V*
&
50
NO . w/0 H-
* , Z
Injection
NHJt with H- ^ NH3' w/0 H2
Injection N^nject1on
700
800 900
Flue Gas Temperature, °C
1000
Figure 2-5. Thermal DeNOx reaction products as functions of temperature
with and without hydrogen injection (Reference 2-3).
2-10
-------
Exxon also investigated the use of combined additives. A mixture
of 50 percent hydrogen and 50 percent methane was found to be more
effective than either hydrogen or methane alone. However, the
introduction of methane in combustion gases, especially at low excess air
levels, increased cyanide emissions by a few ppm.
Additive injection can also be used to control ammonia breakthrough
emissions to concentrations lower than 10 ppm. For example, small amounts
of \\2 injection with amnonia would lower the optimum reaction temperature
from 955°C (1750°F) to 945°C (1733°F). This 10°C (18°F) temperature
differential is sufficient to deplete some excess NH3 which would otherwise
exit from the stack.
Because \\2 can control the temperature of the NH.-KO-O- reaction,
Thermal DeNOx is technically feasible for most boilers provided that the
hardware can be installed within the boiler configuration. However, the
cost of the DeNOx Process is greatly increased because of the large
volumes of hydrogen needed. This is especially the case for very low
temperature such as 760°C (1400°F).
Until recently, the NH3 injection was limited to boiler
cavities. Thus if the optimum reaction temperature of about 955°C did
not occur in an isothermal cavity, NH^ was injected at temperatures
below this level. This situation warranted the use of an additive to
maximize the efficiency of the DeNO Process. The in-tube bank
A
injection of NH3, recently demonstrated by Exxon, has diminished the
dependence of the process efficiency on the use of an additive. DeNO
x
rates of 60 to 70 percent were achieved by injecting NH in tube banks
<3
without the use of an additive.
2-11
-------
2.1.4 Byproduct Emissions
The Exxon Thermal DeNOx Process may form byproduct pollutants
directly or indirectly from the presence of NH3 in the combustion gas.
Potential byproduct emissions suggested by ER&E are NH3> CO, HCN and
N20 and, when sulfur-bearing fuel is burned, NH3 and S03 combine to
form ammonium bisulfate, NH^HSO^.
Ammonium bisulfate is a viscous liquid from 147°C to about
450°C (300-840°F). It has been known to cause corrosion of metal
surfaces. Thus far, however, no increase in metal corrosion attributable
to anmonium bisulfate has been identified when Thermal DeNOx has been
used. The formation of NH4HS04 can be controlled by limiting the
amount of NH, carryover. This can be accomplished by NH3 injection at
a temperature slightly higher than optimum or by using an H? additive.
In general, ammonium bisulfate is considered the most serious byproduct
and one which could effect the use of the Thermal DeNOx Process.
Carbon monoxide emissions may also be promoted by ammonia injection
because the Thermal DeNO reaction inhibits the oxidation of CO to
A
CO . Thus, if there is unburned CO at the point of NH3 injection, the
CO may not be oxidized, but will be discharged to the atmosphere. Under
normal operating conditions, CO levels are not usually significant in
steam generators. Using hydrocarbons as additives to control the
NH3-NO-02 reaction increases the concentration of CO in the flue gas.
Exxon reported that as much as 50 percent of the hydrocarbons may be
oxidized to CO. This CO may then be emitted to the atmosphere because the
ammonia inhibits the 02+CO-*~C02 reaction.
HCN is formed only if hydrocarbons are present in the region in
which NH3 is injected. Under normal boiler operation, gaseous
2-12
-------
hydrocarbons are not present unless they are injected along with the
NH^. KVB reported that for gas, oil and coal firing, HCN was present in
the untreated flue gas at 3 to 10 ppm concentration, depending on excess
air level. Injection of NH. did not measurably affect the HCN level.
The reduction of NO by NH3 and 0- forms N-O as a minor byproduct.
However, less than 2 moles are generated for every 100 moles of NO
reduced, according to ER&E experimental data. All the available evidence
indicates that N?0 is relatively harmless at those levels, and does not
represent an environmental concern.
2.2 SUBSCALE TESTING -- COAL
Recently, KVB has conducted a pilot-scale investigation of the
Thermal DeNO Process to reduce NO levels from combustion of coal
A
(Reference 2-4). The work was sponsored by the Electric Power Research
Institute (EPRI) and Exxon Research and Engineering (ER&E).
' The major objective of this investigation was to determine the
level of NO reduction which is achievable in flue gas resulting from
A
coal combustion. The primary variables investigated were the injection
temperature, the NH.,/NO ratio, and the coal type. Additionally, a
hydrogen additive was used to lower the temperature range for NO removal.
Four different coals were investigated; three coals were bituminous and
one subbituminous. Byproduct emissions were also measured at different
NH3 injection rates.
The combustion facility consisted of a 0.9 MW (3 x 10 Btu/hr)
firetube boiler equipped with a ring-type natural gas burner and a scaled
down version of a commercial coal burner presently used in utility boilers
firing Western coal. The NH- injection system consisted of five
2-13
-------
injectors located at the end of the firetube section distributing the
ammonia and nitrogen (carrier gas) counter-flow to the flue gas stream.
The injectors were designed to be movable so that they could be positioned
axially along the length of the firebox thus providing for evaluation of
the effectiveness of different temperature profiles. The injection method
was a result of an optimization study in which the injection grid and
nozzles were designed to provide substantial NO reductions that allowed
A
a valid comparison between the various coal types and natural gas. Since
the injection method directly affects the efficiency of the Thermal
DeNO Process, the results achieved by KVB do not necessarily represent
^
the maximum NO reductions achievable with coal combustion.
This section discusses the results of this investigation. These
results can be used to compare noncatalytic NO reductions and byproduct
emissions between coal and the gaseous and liquid fuels previously
investigated. Key parameters considered here are again:
o Reaction temperature
• Ammonia injection rate
» Hydrogen and other additive injection
• Byproduct emissions
2.2.1 Reaction Temperature
The temperature at which ammonia is injected into the flue gas is
the primary variable which determines the amount of NO removed with the
Thermal DeNO Process. A major objective of the KVB study was to
determine whether the additional pollutants resulting from combustion of
coal, such as S02 and particulates, would influence the temperature
dependence of the process or reduce the process efficiency. Figure 2-6
shows the effect of reaction temperature on NO reduction for the four
2-14
-------
TVA
U.S. Environmental
Protection Agency
Office of Research
and Development
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Tennessee
Valley
Authority
National Fertilizer Development
Center
Muscle Shoals AL 35660
TVAY-134
Impact of Ammonia
Utilization by NOX Flue
Gas Treatment Processes
Interagency
Energy/Environment
R&D Program Report
«85Sa«aeaa&ffigBa8s
~j>ffXS£l££&'IS&9f3X
-------
INTRODUCTION
Of the five most common air pollutants released In the United States,
nitrogen oxides (NOX) are the only ones projected to increase significantly in
the future. The total emission of the other four pollutants [particulate
matter, sulfur oxides (SOX), hydrocarbons, and carbon monoxide (CO)] has
decreased during the past 6 yr and is expected to decline further in the future
as more point sources are equipped with more efficient control systems. With
most public attention focused on these other pollutants which are typically
emitted at higher rates than NOx, the development of equipment and techniques
for controlling NOX has lagged behind. However, recently more attention has
been given to the possible health effects of NOx emissions. Although the NOx
emission from each source appears to be small, the cumulative total is measured
in millions of tons annually and is increasing substantially from year to year
due to the increasing number of sources.
NOX emissions are divided into two classes, mobile or stationary, depending
on the source. The mobile class, as its name implies, includes sources such
as automobiles, trucks, buses, trains, and planes. Attempts at controlling
the NOx emissions from these sources have been delayed due to both technical
and economic considerations. Stationary sources, which release about 56% of
the total amount of NC^ emitted in the United States (an estimated 11:15 Mtons
in 1975), are split into three groups: combustion sources, industrial processes,
and other miscellaneous sources. In 1975 combustion sources contributed approxi-
mately 93% of all stationary source emission (an estimated 10.4 Mtons) of which
nearly two-thirds came from fossil fuel-fired boilers (approximately 7.0 Mtons).
If these boilers are further divided according to size (heat rate of the boiler),
large boilers [i.e., greater than 250 MBtu/hr (25 MW equiv)] release 45% of al
NOX emitted from stationary combustion sources. Figure 1 and Table 1 show the
relative contributions from mobile sources and each type of stationary source
to the total NOX emissions in the United States during 1975. Since the mobile
source standards are not expected to become more stringent in the near future,
restricting the amount of NOx emissions from stationary sources, particularly
large fossil fuel-fired boilers, may becone necessary since these boilers are
the second largest source of NOX emissions in the U.S. The reason for the
possibility of stricter NOX emission control on large boilers are threefold.
The emissions from these boilers are projected to increase at about 4.9% annually
over the next decade (doubling every 15 yr) assuming present control levels.
Secondly, assuming the application of combustion modification which is now
considered the bast available control technology (about 30% decrease in the
NOx formation), the amount of N0.{ emitted by large boilers will increase from
4.7 Mtons in 1975 to 6.6 Mtons in 1985 (50). And thirdly, although the total
amount of NOx emitted by these boilers is only approximately one-half that of
mobile sources, there are orders-of-magnitude fewer boilers and hence each
boiler source emits significantly more NO^ than each individual mobile source.
-------
Industrial processes (2.3%)
Gas turbines (2.8%)
Other (1.5%)
1C
engines
(14.3%)
Other
boilers
(11.2%)
Mobile
sources
(44.3%)
Large
boilers
(23.6%)
Figure 1. Breakdown of the total U.S. NOX emissions by original source (45)
-------
Therefore, it is expected that based on each ton of HO* removed it will be
"
TABLE 1. NOX EMISSIONS BY SOURCE IN 1975 (45)
Annual amount,
ktons
Mobile sources 3 850
Stationary sources
Large boilers 4,728
Other boilers 2*238
1C engines 2,'849
Gas turbines 555
Incinerators 39
Industrial processes 454
Field burning 290
Total 20,000
FORMATION OF NOX IN LARGE BOILERS
vS f?™cd during hiSh-temperature combustion operation and can be
t
hP.? ?? v 2 / S temperatures. The second method is the oxidation of
neaically bound nitrogen within the fuel and is called "fuel NO! " i^oli
y b°th — "-isms, but in
POTENTIAL NOX CONTROL METHODS FOR LARGE BOILERS
NO, e,isston regulation, the e^has is on fLe gas trteent
-------
Cotabus t ion jnodif icat ion techniques attempt to prevent the formation
of NOx by altering the reaction conditions inside the boiler. Since thermal
fixation is primarily a function of the temperature (i.e., the rate of fonaatic-
increases with temperature) and the concentration of 02 in the boiler, reducin-
either of these parameters will decrease the amount of thermal NOx formed. This;-
same boiler modifications which reduce the 02 concentration in the boiler can
also reduce the amount of fuel NOx formed since the conversion of fuel-bound
nitrogen is a strong function of the 02 concentration. Various combustion
modification techniques, which are currently undergoing development work in the
United States, are compared in Table 2 for the three types of fossil fuel-
fired boilers. Combustion modifications are shown to be the most effective on
gas- and oil-fired boilers where a major portion of the NOx is formed by the
thermal mechanism. For coal-fired boilers where much of the NOx comes from
fuel-bound nitrogen, the available combustion modification techniques are
limited to about 40% reduction in
Although the resulting reduction in the total NOx emissions from large
boilers would appear to be significant, a recent study (45) has determined
that, even with combustion modifications on all new large fossil-fueled boilers,
the expected growth rate for new large boilers will result in a significant
increase in the total NO^ emissions from these boilers, rising from 4.73 Mtons
in 1975 to 6.56 Mtons in 1985. Unfortunately, combustion modifications also
have undesirable side effects of reducing boiler efficiency, increasing the
potential for flame instability, and increasing the soot, CO, and particulate
loadings in the flue gas. For these reasons, an alternative method of NOx
control technology, FGT, has recently begun to receive more attention. Although
probably aore expensive than combustion modification, FGT allows normal operatic:.
of the boiler and can remove at least 90-95% of the NOx zrcm the flue gas. Many
different types of FGT systems are currently undergoing development work in
Japan and the United States, but the most technically advanced type for removing
NO^Jfrora power plant stack gas is now selective catalytic reduction (|SCR) (30).
In chis type, ammonia CNH3) is injected into the flue gas ducts and the mixture
of NH3 and flue gas passes through a catalytic reactor containing a base-metal
catalyst. The NOx *s selectively reduced to molecular N2 by the following
reactions:
4NH3(g) + 41IO(g) + °2 *
4NH3(g) + 2N02(8) + °2'*
Although these SCR processes are still in early stages of development
(most have not been tested either on coal-fired flue gas or on a large scale
unit), they are receiving considerable attention because of both the potentially
low capital investment and the fact that they generate molecular ^ directly
without further chemical processing.
-------
TABLE 2. EFFECTS OF BOILER MODIFICATIONS TO REDUCE NOX
EMISSIONS BY FOSSIL FUEL TYPE (30)
Percent decrease of NO^ formation
in boilers by fuel type
modification
Gas
Oil
Coal
Prevention of thermal NOX by:
FLue gas recirculation
Reduced air preheat
Steam or water injection
Prevention of both thermal and fuel
NOx by:
Staged combustion
Low excess air
Reduced heat release rate
Combination of stage combustion, low
excess air, and reduced heat release
rate
Prevention of fuel NOx by:
Change to fuel with lower percent
nitrogen
60
50
60
55
20
20
50
20 Not effective
40 Not competitive
40 Not competitive
40 40
20 20
20 20
35 40
Not applicable 40 20
-------
A primary consideration in the widespread application of these SCR
processes in the United States is the potential availability and the cost of
NH3 in the future. Since nearly all of the NH3 produced in the United States
is made from natural gas, there are serious concerns over the potential impact
of widespread use of NH3 in FGT applications. The availability and cost of
both natural gas and NH3 may be affected. Would this additional demand for
NH3 create shortages in NH3~based fertilizers and thus increase food costs?
Will there be sufficient natural gas available for meeting this demand for
NH3? If not, what other methods and feedstocks are available for generating
NH3? This report will attempt to provide insights .into these questions.
-------
NH3 REQUIREMENTS FOR NOx CONTROL
NH3 REQUIREMENTS FOR TYPICAL POWER PLANT APPLICATIONS
Any evaluation of the impacts of the additional NH3 demand for power
plant FGT systems must begin with a determination of the magnitude of the
additional demand. In order to calculate the NH3 requirements for typical
fossil fuel-fired boilers, it is necessary to presuppose future NO., standards
for large stationary source boilers.
Although the future NOX emission limits have not been published, for the
purposes of this study the following scenario has been hypothesized. Since
the SCR and other FGT processes are still in the early stages of development
in the United States, strict Federal NOX emission regulations of 90% removal
from large stationary sources will probably not be enforced until 1985. Since
Federal new-source performance standards (NSPS) apply only to new sources,
existing sources will not be required to meet these standards but existing
sources converted from gas or oil to oil or coal after 1985 will be. Between
the present and 1985, the NOX emission limits may be reduced but only to the
extent that combustion modifications can be used to meet the newer regulations.
Two possible methods would be available for meeting these eml&sinn regula-
tions of 90% NOX removal, (1) the installation of an 1CR system designed'for
90% NOX removal or (2) the use of combustion modi ft.c.a^ilQ.n.f* tn cut NOx emissions
from the boiler by 50% with the additional installation of an SCR system designed
for 80% NOX removal to give an overall NOX removal efficiency of "90%. The
impacts of each of these methods are considered in later sections.
The typical fossil fuel-fired'boilers included in this study were 500-MW
coal- and oil-fired boilers with the flue gas compositions shown in Table 3.
Gas-fired boilers were not considered since they are not expected to be built
after 1985. For the two alternative cases, the uncontrolled conventional 500-MW
coal-fired boiler would release about 600 ppm NOX in the flue gas (3009 Ib/hr),
would require an SCR system capable of a 90% NOx removal efficiency, and would'
release only approximately 60 ppm NOx (300 Ib/hr) of NOX after FGT control.
Through the use of combustion modification techniques in the second coal-fired
case, the boiler would emit 300 ppm NOX (1504 Ib/hr) and the additional SCR
system would remove 80% of the remaining NOX to obtain a total of 90% NO
removal. In a similar manner, the uncontrolled 500-MW oil-fired boiler releasing
200 ppm NOX in the flue gas (856 Ib/hr) would be required 'to have 90% NOX removal,
i.e., down to 20 ppm (86 Ib/hr) NOX. Again this could be obtained either by an
SCR system designed for 90% removal or through combustion modification to lower
boiler emissions to 100 ppm NOX and then an 80% efficient SCR system.
-------
TABLE 3. FLUE GAS COMPOSITIONS FROM 500-MW
COAL- AND OIL-FIRED BOILERS
Constituent^
N2
°2
C02
S02
303
Oil-fired
Vol, %
73.60 2
2.54
11.96
0.13
0.0013
0.02
^
11.75
boiler
Lb/hr
,929,000
115,400
747,900
12,060
151
856
-
300,800
Coal-fired
Vol, %
73.76 3
4.83
12.31
0.24
0.0024
0.06
0.01
8.79.
boiler
Lb/hr
,450,000
258,200
904,200
25 130
317
3,009
661
264,500
HC1
H20
100.00 4,106,000 100.00 4,906,000
Fly.ash, gr/sft3 (wet) 0.032 6i06
lllTl^r*?'™ inSldf thlS rea"°r «e * '-?•» "•» =" "bout 3£Z£c
the range of 1.0-1.1:1 for 90% NOX rental in a cLier^l sy«« "
NO r »ol ratios for the 90% and the 80%
p?an t" rk i^e "l 0^1 S^ll'? ^ 10% h±gher th" ^ USed in ^S '"«-
oil-fired boiler without combustion modification, and (4) 500-MW oil-fired boil!?
vith combustion modification. The estimated annual NH3 consumptio^ I for each of these
-------
JOO
0
Figure 2.
0.6 0.8 i.
MOL N1I3/MOL NOX
NOX removal efficiency as a function of
1.2
mol ratio.
1.4
-------
10
LeoencL
500-MW conventional coal
500-MW coal with CM
500-MW conventional
500-MM oil with CM
1.0
1.5 2.0
MOL NH3/MOL NOX
10
-------
four cases is listed in Table 5. The values range from a high of 6266 tons
of NH3/yr for the 500-MW conventional coal-fired boiler without combustion
modifications to a low of 772 tons of NH3 annually for the 500-MW oil-fired
boiler with combustion modifications.
TABLE 4. PREMISES FOR THE CALCULATION
OF ANNUAL NH3 CONSUMPTION FOR 500-MW
FOSSIL-FIRED BOILERS
Parameter
Heat rate, Btu/kWh
Heating value
Coal, Btu/lb
Oil, Btu/gal
Availability, hr/hr
Value
9,000
10 , 500
144,000
7,000
TABLE 5.' NH3 CONSUMPTION FOR TYPICAL
500-MW COAL- AND OIL-FIRED BOILERS
Fuel
Coal
Coal
Oil
Oil
NOx
treatment
scheme
SCRa
CM & SCRb
SCR
CM & SCR
NOX
concentration
in flue gas, oom
600
300c
200
10 Oc
Mols NH3
per mol
NOx
1.05
0.91
1.05
0.91
NH3 consumption,
tons/yr
6,266
2,715
1,781
772
a. Selective catalytic reduction.
b. Combustion modification followed by selective catalytic
reduction.
c. Combustion modification was assumed to be 50% efficient
in controlling NOX emissions.
The economics of present-day NH3 generating plants dictate a minimum capacity
of 1,000 tons of NH3/day (330,000 tons/yr) and thus local, small NH3 plants at
each boiler would be highly unlikely. Even with large 2,500-MW coal-fired power
plants, the annual NH3 requirement of 31,000 tons would not justify a captive
NH3 plant. Thus it is assumed that for all NOX FGT applications the NH3 would
be purchased from existing fertilizer plants already producing NH3 and shipped
to the power plant and stored in large tanks onsite.
11
-------
VI. HYDROCARBONS
-------
HYDROCARBONS
SOURCES
Hydrocarbons by definition contain the elements hydrogen and carbon. In
addition, naturally-occurring, processed, and synthetic hydrocarbon molecules
may contain other elements such as oxygen, nitrogen, sulfur, and halogens.
The health effects of hydrocarbon and organic solvent emissions are of two
types, direct and indirect. Direct effects are caused by the original, unal-
tered emissions, and indirect effects are caused by substances formed by photo-
chemical reactions of the original emissions with other substances in the
atmosphere.
Generally, hydrocarbon materials at levels encountered in the ambient air
have no direct health or welfare effects and the prime reason for controlling
hydrocarbon emissions is to prevent their participation in atmospheric photo-
chemical reactions. Olefins are regarded as being the most reactive of the
organic compounds in photochemical smog formation, although reactivity varies
widely with chemical structure.
As in the case of nitrogen oxides and carbon monoxide, the largest single
source of hydrocarbons is transportation. Stationary sources of hydrocarbon
emissions include petroleum refining, gasoline distribution and marketing,
chemical manufacturing, coal coking, fuel burning, waste disposal, and food
processing. Sources of organic solvent emissions include manufacture and appli-
cation of protective coatings, manufacture of rubber and plastic products,
degreasing and cleaning of metal parts, dry cleaning operations, printing, and
manufacture of chemicals. A summary of total nonmethane hydrocarbon emissions
by source is given in Table I.
From the production of crude oil to the marketing of finished products, the
petroleum industry has the potential for emitting significant quantities of
-------
- 2 -
hydrocarbon gases and vapors. These emissions are often undesirable since they
may be precursors of photochemical smog. Crude oil is first produced from the
ground. Then the liquid hydrocarbons are separated from the gases, light
hydrocarbon vapors, and water. Finally, the crude oil is stored until it is
removed to the refinery where it is converted to salable products. In crude
oil production, most of the emissions are due to evaporation of hydrocarbons
from storage tanks.
The design of a refinery depends on the kind of- crude oil it processes and
on the final products it manufactures. Refinery operations are most easily
discussed, therefore, in terms of their similar functions.
Since crude oil as it is produced has few uses, it is processed to obtain
salable products, such as gasoline, kerosene, fuel oil, petrochemical raw mate-
rials, waxes, lubricating oils, and asphalt. Processing involves four major
steps: separation, conversion, treattnent, and blending.
The first refining step, separation by distillation within a specific
temperature range, yields fractions, the relative volumes of which are deter-
mined by the nature of the crude oil. These fractions are usually further
refined to meet the demands for the various petroleum products. These processes
are outlined below.
Conversion by cracking is employed to convert high-molecular-weight hydro-
carbons into products of lower molecular weights. For example> cracking
partially converts heavy gas oil to gasoline. If a catalyst is used (the more
usual case), it is called catalytic cracking, if not, it is thermal cracking.
Thermal cracking requires higher temperatures and pressures than those required
to catalytic cracking.
Gasoline yield and quality can be improved by several other processes. In
catalytic reforming, the molecules of the gasoline feed stock are rearranged
and dehydrogenated to produce high-octane gasoline blending stocks.
-------
- 3 -
Isomerization rearranges molecules to increase the octane number; it also
increases molecular branching, but it does not add to or remove anything from
the original material. In still other conversion processes, liquid gasoline is
made from the hydrocarbon gases generated during cracking. Polymerization joins
an olefin with a branched chain paraffin to yield a saturated hydrocarbon.
Treatment steps are used to purify the material or to prevent an undesir-
able reaction vith an impurity. For example, during selective, hydrogenation,
sulfur and nitrogen as impurities in the feed stocks are converted to hydrogen
sulfide and ammonia, respectively. In. addition, olefins and aromatic compounds
may be hydrogenated to partial or complete saturation. Three types of treat-
ment employed are acid treatment, "sweetening," and solvent extraction. Petro-
leum fractions may be brought into contact with concentrated sulfuric acid to
remove sulfur, nitrogen, and undesirable unsaturated compounds and to improve
color and odor. Sweetening converts mercaptans to disulfides and this improves
odor. Sodium plumbite (doctor), lead sulfide, hyprochlorite, and copper chloride
are common sweetening agents. In solvent extraction, solvents are used to remove
undesired contaminants or to concentrate desired components.
Physical treatments such as absorption, air-bloving, electrical coalescence,
and filtration are used in intermediate refining processes to remove contaminants.
Another commonplace activity at refineries is the blending of base stocks to
produce a wide variety of finished products.
A list of equipment, facilities, and processes likely to produce organic
emissions in crude oil production and in refining includes:
1. Storage.
2. Pressure relief valves.
3- Slowdown systems.
U. Flares.
5- Catalytic cracking units.
6. Asphalt oxidation.
-------
- k -
7. Chemical treatment.
8. Loading facilities.
9- Oil-water separators.
10. Pumps.
11. Valves.
REGULATIONS
Hydrocarbon emission standards for refineries and related petroleum indus-
try 'facilities were not included in the performance standards for new stationary
sources which were issued in December, 1971> by the Federal Environmental Protec-
tion Agency. The EPA has indicated, however, that petroleum refineries will be
a source classification in the next group of standards issued and that hydrocar-
bon emission standards will be included for all types of producing and refining
activities. It is anticipated that proposed standards will be issued before
mid-1972.
To assist the state regulatory agencies in preparation of implementation
plans, the EPA included extensive model regulations for control of hydrocarbon
emissions in the proposed implementation plan guidelines which were issued in
April, 1971. The model regulations, which vere patterned quite closely after
the Los Angeles County hydrocarbon emission regulations, require that:
1. Storage facilities for volatile hydrocarbons (> 1-5 psia) larger than
Uo,000 gallons must be equipped with floating roofs, vapor recovery systems, or
other control devices to control evaporative emissions. Further, the use of
floating roofs is limited to hydrocarbons with a vapor pressure of less than
11.0 psia.
2. Storage tanks for volatile hydrocarbons larger than 250 gallons must
be equipped with a permanent submerged fill pipe or other evaporative control
device to reduce hydrocarbon emissions during filling operations.
3- Loading facilities for volatile hydrocarbons must be equipped with a
vapor collection and disposal or recovery system to control losses during
-------
- 5 -
loading operations.
U. Oil-water separators which receive more than 200 gallons per day of
volatile hydrocarbons must be equipped with a solid cover, floating roof, vapor
recovery system or other means to prevent hydrocarbon losses.
5- Pumps and compressors handling volatile organic materials must have
mechanical seals to keep leakage to a minimum.
6. Hydrocarbon gases from vapor blowdown systems must be burned in smoke-
less flares.
7. Emissions of organic solvents from numerous varied operations must be
controlled through the use of such methods as incineration, carbon adsorption,
and vapor recovery.
In the final implementation plan guidelines issued by EPA in August, 1971,
the model hydrocarbon emission regulations were not significantly modified. It
was emphasized, however, in the final guidelines that the major reason for con-
trolling hydrocarbons is to prevent photochemical oxidant production and that
control of stationary hydrocarbon sources would be necessary only in areas
\
where the anticipated reduction obtained from the motor vehicle standards would
not be sufficient to result in attainment of the oxidant ambient air quality
standard.
Virtually all states have proposed or already adopted some hydrocarbon
emission regulations primarily based on the EPA model standards. The petroleum
industry, through the state petroleum councils and individual company state-
ments, has presented extensive testimony at state hearings which have been held
on the proposed hydrocarbon regulations. Principle points covered by the indus-
try include requests that:
1. Applicability of any hydrocarbon regulations be limited to those areas
where the photochemical oxidant standard is met or exceeded. This would limit
-------
- 6 -
imposition of costly control requirements to those major metropolitan areas
where there currently is a photochemical smog problem and would be consistent
with latest EPA recommendations.
2. The definition of volatile hydrocarbon be changed to mean any material
with a vapor pressure of over 2.5 psia instead of 1.5 psia. This would except
JP-k jet fuel, heavy naphtha solvents and feed stocks and kerosene-type materi-
als from the requirement of being stored in floating roof tanks and being loaded
in facilities with vapor recovery systems. This change could also make a dif-
ference in the number of oil-water separators which need to b-s covered.
3. The use of floating roofs be allowed for hydrocarbons with a vapor
pressure of up to 12-5 psia instead of 11.0 psia. This would eliminate the
need to store materials such as LVN and LCN in pressure tanks during the summer.
k. The requirement for vapor recovery at loading facilities be limited to
larger facilities which load more than 20,000 gallons per day. This would
except bulk plants which typically load only several truckloads per day.
5- The requirement for mechanical seals be limited to rotary and centrifu-
gal pumps and compressors since mechanical seals are not applicable to recipro-
cal units.
6. A differentiation be made between vapor blowdown systems and emergency
relief devices and that the requirement for smokeless flares be limited to blow-
down systems. This change would preclude the need to connect emergency pressure
relief valves, which are infrequently, if ever, used, to the blowdown and flare
system, which is normally used during startups, shutdown, and minor upsets.
Some of the industry recommendations have been accepted by some of the
states. In others, compromise versions have been adopted or the application of
the most stringent regulations has been limited to new facilities. It is likely,
when all the states have completed adoption of regulations and implementation
-------
- 7 -
plans later this year, that the hydrocarbon emission regulations will vary con-
siderably from state to state and that these variations will be disconcerting
to multiple-facility companies attempting to comply with regulations in several
states.
STORAGE
Storage is potentially the most important source of hydrocarbon emissions
in the petroleum industry. Vapors can be emitted when storage tanks "breathe,"
when vapors are displaced during filling, and when liquids evaporate. Tanks
"breathe" due to the expansion and contraction of their contents with the heat.
of the day and the cool of the night. When the contents expand, air mixed with
hydrocarbon vapors is forced out of the tank. Methods have been developed to
estimate losses and to minimize these losses from storage tanks.
Even in the most modern petroleum refineries and petrochemical plants,
storage facilities must be provided for large volumes of liquids and gases.
These facilities can be classified as closed-storage or open-storage vessels.
Closed-storage vessels include fixed-roof tanks, pressure tanks, floating-roof
tanks, and conservation tanks. Open-storage vessels include open tanks,
reservoirs, pits, and ponds.
Closed-storage vessels are constructed in a variety of shapes, but most
commonly as cylinders, spheres, or spheroids. Steel plate is the usual material
of construction though concrete, wood, and other materials are sometimes used.
Before modern welding methods, the sections of the tank shell were joined by
rivets or bolts. Welded joints are now used almost universally except for the
small bolted tank found in production fields. Capacities of storage vessels
range from a few gallons up to 500,000 barrels, but tanks with capacities in
excess of 150,000 barrels are relatively rare.
Open-storage vessels are also found in a variety of shapes and materials
of construction. Open tanks generally have cylindrical or rectangular shells
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of steel, wood, or concrete. Reservoirs, pits, ponds, and s-imps are usually
oval, circular, or rectangular depressions in the ground. The sides and bottom
may be the earth itself or may be covered with an asphalt-like material or con-
crete. Any roofs or covers are usually of vood with asphalt or tar protection.
Capacities of the larger reservoirs may be as much as 3 million barrels.
Vapors, gases, aerosols, and odors are examples of air contaminants emitted
from storage facilities. In most cases, practical and feasible air pollution
control measures are available to reduce the emissions.
Pressure Tanks and Fixed-Roof Tanks
Pressure tanks and fixed-roof tanks are grouped together because, in a
sense, pressure tanks are special examples of fixed-roof tanks designed to
operate at greater than atmospheric pressure. Horizontal, cylindrical (bullet)
pressure tanks are the most common pressure tanks. Other types of pressure
tanks include spheres, plain and noded spheroids, and noded hemispheroids-
Maximum capacities of these pressure tanks are as much as 30,000 barrels for
spheres and hemispheroids, and 120,000 barrels for noded spheroids. Spheres
can-be operated at pressures up to 217 psi; spheroids, up to 50 psi; noded
spheroids, up to 20 psi; and plain or noded hemispheroids, up to 15 and 2-1/?
psi,respectively. Horizontal, cylindrical pressure tanks are constructed with
various capacities and pressures.
Typical storage tanks are vertical, cylindrical, fixed-roof tanks. This type
of storage facility operates at or within a few ounces of atmospheric pressure and
may have a flat, recessed flat, conical, or domed roof. The term gastight, often
applied to welded tanks, is misleading. Many of the roofs of the welded tanks
have free vents open to the atmosphere. Others are equipped with conservation
vents that open at very slight positive pressures. A tank also has many stan-
dard appurtenances including gaging hatches, sample hatches, relief vents, and
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- 9 -
foam mixers. Any of these accessories may fail in service and result in vapor
leaks.
The operating pressure of a tank is limited by the thickness (weight) of
the roof. A cone roof tank may be operated at higher pressures, if necessary,
by structural reinforcement or weighting of the roof. Safe operating pressures
up to U ounces can be realized by this added expense. Use of unsupported dome-
shaped roofs is another method of'increasing the allowable operating pressure
of the fixed-roof tank.
Floating-Roof Tanks
Floating-roof storage tanks are used for storing volatile material with
vapor pressures in the lower explosive range, to minimize potential fire or
explosion hazards. These vessels also economically store volatile products that
do not boil at atmospheric pressures or less and at storage temperatures or
below. These tanks are subclass ified by the type of floating-roof section as
pan, pontoon, or double-deck floating-roof tanks.
Pan-type floating-roof tanks were placed in service more than 50 years ago.
These roofs require considerable support or trussing to prevent the flat metal
plate used as the roof from buckling. These roofs are seldom used on new tanks
because extreme tilting and holes in the roof have caused more than one-fifth
of installed pan roofs to sink, and because their use results in high vaporiza-
tion losses. Solar heat falling on the metal roof in contact with the liquid
surface results in higher than normal liquid surface temperatures, hydrocarbons
boil away more rapidly at the higher temperatures and escape from the opening
around the periphery of the roof.
To overcome these disadvantages, pontoon sections were added to the top of
the exposed deck. Better stability of the roof was obtained, and a center drain
with hinged or flexible connections solved the drainage problem. -Center-weighted
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- 10 -
pontoons, double pontoons, and high- and low-deck pontoon floating-roof tanks
are available today. Current practice is to use the pontoon roof on tanks with
very large diameters. Included with some pontoon roof designs is a vapor trap
or dam installed on the underside of the roof. This trap helps retain any
vapors formed as a result of localized boiling and converts the dead vapor
space into an insulation medium. This dead vapor space tends to retard addi-
tional boiling.
The more expensive double-deck floating roof was eventually introduced to
reduce the effect of solar boiling and to gain roof rigidity. The final design
generally incorporates compartmented dead-air spaces more than 12 inches deep
over the entire liquid surface. The top deck is generally sloped toward the
center or to a drainage area. Any liquid forming or falling on the roof top is
drained away through a flexible roof drain to prevent the roof from sinking.
The bottom deck is normally coned upwards. This traps under the roof any vapors
entrained with incoming liquid or any vapors that might form in storage. A
vertical dam similar to those used on pan or pontoon floating roofs can also be
added to retain these vapors.
Conservation Tanks
Storage vessels classified as conservation tanks include lifter-roof tanks
and tanks with internal, flexible diaphragms or internal, plastic, floating
blankets. The lifter roof or, as more commonly known, gas holder, is used for
low-pressure gaseous products or for low-volatility liquids. This type of
vessel can be employed as a vapor surge tank when manifolded to vapor spaces of
fixed-roof tanks.
Two types of lifter-roof tanks are available. One type has a dry seal con-
sisting of a gastight, flexible fabric; the other type employs a liquid seal.
The sealing liquid can be fuel oil, kerosene, or water. Water should not be
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employed as a sealing liquid where there is danger of freezing.
The physical weight of the roof itself floating on vapor maintains e slight
positive pressure in the lifter-roof tank. When the roof has reached its maxi-
mum height, the vapor is vented to prevent overpressure and damage to tank.
The conservation tank classification also includes fixed-roof tanks with
an internal coated-fabric diaphragm. The diaphragm is flexible and rises and
falls to balance pressure changes.
Two basic types of diaphragm tanks are the integrated tank, which stores
both liquid and vapor, and the separate tank, which stores only vapor. Common
trade names for integrated tanks are "diaflote," "dialift," and "vapor-mizer"
tanks, or they may be referred to as vapor spheres or vapor tanks. The separate
type of tank offers more flexibility and does not require extensive alteration
of existing tanks.
Open-Top Tanks, Reservoirs, Pits, and Ponds
The open-top tank is not used as extensively as in the past. Safety, con-
servation, and housekeeping are factors affecting the elimination of open vessels.
Even tanks that require full access can and should be equipped with removable
covers. The open vessels generally have a cylindrical shell, but some have a
rectangular shell.
Reservoirs were devised to store the large quantities of residual oils,
fuel oils, and, sometimes,-crude oils resulting from petroleum production and
refining. Safety considerations, larger fixed-roof tanks, and controlled crude
oil production have reduced the number of reservoirs in use today. Even when
covered, reservoirs have open vents, which maintain atmospheric pressures in the
reservoir. Windbreaks divert the windflov pattern over a large roof area and
prevent the roof from raising and buckling.
Open ponds or earthen pits were created by diking low areas or by excava-
tion. These storage facilities served for holding waste products, refinery
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effluent water, or inexpensive oil products for considerable periods of time.
In these, oils "weathered" extensively, leaving viscous, tar-like materials, and
water seeped into the lower ground levels. As the pond filled with solids and
semisolids, the contents were removed by mechanical means, covered in place, or
the pond was simply abandoned. The use of these ponds has diminished, and the
remaining ponds are usually reserved for emergency service.
Smaller ponds or sumps were once used extensively in the crude oil produc-
tion fields. This use was primarily for drilling muds though oil-water emulsions
and crude oil were also stored by this method. Their use is gradually disappear-
ing because unattended or abandoned sumps cause nuisance problems to a community.
Control of air pollution originating from storage vessels serves a three--
fold purpose: (l) elimination or reduction of air contaminants, (2) elimination
or reduction of fire hazards, and (3) economic savings through recovery of valu-
able products. Methods of control include use of floating roofs, plastic
blankets, spheres, variable vapor space systems, various recovery systems, and
altered pumping and storage operations.
Seals for Floating-Roof Tanks
The principle by which a floating roof controls emissions from a volatile
liquid is that of eliminating the vapor space so that the liquid cannot evapo-
rate and later be vented, to be successful the floating roof must completely
seal off the liquid surface from the atmosphere. The seal for the floating
roof is therefore very important. The floating section is customarily con-
structed about 8 inches less in diameter than the tank shell. A sealing
mechanism must be provided for the remaining open annular gap. The seal also
helps keep the roof centered.
Conventional seals generally consist of vertical metal plates or shoes
connected by braces or pantograph devices to the floating roof- The shoes are
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suspended in such a way that they are forced outward against the inner tank wall.
An impervious fabric bridges the annular area between the tops of the shoes con-
tacting the tank wall and the circumference of the floating roof. To reduce
emissions, a secondary seal or wiper blade has been added to the floating-roof
design by extending the fabric seal or by adding a second section of fabric.
This seal remains in contact with the tank wall. Its flexibility allows it to
make contact even in rivet head areas of the inner shell or in places where the
shell might be slightly out of round. This improvement lowers hydrocarbon emis-
sions further by reducing the effect of wetting and wicking associated with
floating-roof tanks.
Recently, other types of sealing devices to close the annular gap which
have been marketed include filled tube seals. These devices consist of a fabric
tube that rests on the surface of liquid exposed in the annular space. The
fabric tube is filled with air, liquid or plastic material. The pneumatic,
inflated seal is provided with uniform air pressure by means of a small expan-
sion chamber and control valves. The sides of the tube remain in contact with
the roof and inner shell. The liquid-filled tube holds a ribbed scuff band
against the tank wall. The ribbed band acts as a series of wiper blades as well
as a closure. All tubes are protected by some type of weather covering.
A weather covering can also be added to protect the sealing fabric of the
conventional seals. The covering includes flat metal sections held in place by
a metal band. The metal protects the fabric seal from the elements. When
floating-roof sections are added to older tanks constructed of riveted sections,
better contact of the shoes with the shell can be ensured by guniting or plastic
coating the inner shell. The wetting condition of gunited walls may, however,
offset the gain of better contact.
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Floating Plastic Blankets
A floating plastic blanket, operates on the same principle of control as a
floating roof. The blanket is usually made of polyvinyl chloride but can be
made of other plastics such as polyvinyl alcohol, superpolyaraides, polyesters,
fluoride hydrocarbons, and so forth. The blanket's underside is constructed of
a large number of floats of the same plastic material. The blanket is custom
manufactured so that only a 1-inch gap remains around the periphery. A verti-
cal raised skirt is provided at the edge of the blanket to serve as a vapor
seal over the annular area. Once this area is saturated, further evaporation
diminishes. The only remaining loss is gaseous diffusion. The seal is made as
effective as possible by using an elastic, Z-shaped skirt.
Provisions are made in the blanket for openings fitted vith vertical sleeves
for measuring and sampling operations. These openings have a crosscut, flexible
inner diaphragm to minimize exposure of the liquid surface. Small holes vith
downspouts to effect a liquid seal are used to provide drainage of any conden-
sate from the top of the blanket. Another feature includes a stainless steel
cable grid to prevent a buildup of static charges. The grid is closely attached
Just under the blanket in parallel lines and connected to the tank shell by a
flexible conductor cable. Installation of a plastic blanket is convenient for
both new and existing tanks. The blanket is made in sections and can be intro-
duced into a tank through a manhole.
A rigid foam-plastic cover constructed of polyisocyanate foam is also
available to equip small fixed-roof tanks with a floating cover. The cover is
manufactured in radial sections, each equipped with a flexible neoprene seel
attached on the outer edge. The sections are easily installed through roof man-
holes and assembled with slip-fit Joints.
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Plastic Microspheres
An outgrowth of application of plastic material provides another type of
control mechanism. This type of control is also similar to the floating roof
and involves the use of a phenolic or urea resin in the shap-j of tiny, hollow,
spherical particles. This material has the physical properties necessary to
form a foam covering over the denser petroleum products. Tho fluidity of the
layer enables it to flow around any internal tank parts while keeping the liquid
surface sealed throughout any level changes. These plastic spheres are known
under their trademark names of microballoons or microspheres. These coverings
have proved to be effective controls for fixed-roof crude oil tanks. Excessive
amounts of condensation or high turbulence should be avoided. The plastic foam
has not proved as satisfactory for one-component liquid or gasoline products.
A 1/2-inch layer of foam has been found sufficient for crude oil where
pumping rates do not exceed U,000 barrels per hour. A layer 1 inch thick is
recommended for pumping rates up to 10,000 barrels per hour. In order to over-
come wall holdup in smaller tanks, it is suggested that a 1-inch layer be used
regardless of pumping rates. For tanks storing gasoline, the recommended foam
thickness is ? inches for tanks up to ^0 feet in diameter, and 1 inch for all
larger diameter vessels.
Various methods can be used to put the foam covering on the crude oil. One
method is to inject the plastic spheres with the crude oil as it is charged to
the tank. Spheres are added by means of an aspirator and hopper similar to
equipment used in fire-fighting foam systems. The spheres can also be added by
placing the desired quantity on the clean, dry floor of the tank just before the
crude oil is charged. A wetting agent must be used when the foaa covering is to
be used on gasoline products. This is accomplished by slurrying the plastic
spheres, wetting agent, and gasoline in a separate container. The slurry is
then injected into the tank. Changes in tank operation are not necessary except
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for gaging or sampling. A floating-type veil attached to a common-type gaging
tape allows accurate measurement of the tank's contents. A sample thief with
a piercing-type bottom is needed for sampling.
Protection against excessive loss of the plastic spheres is necessary
because of the relative value of the foam covering. Precaution must be taken
against overfilling and pumping the tank too low. Standard precautions against
air entrainment in pipelines normally safeguard against the latter. Overfilling
can be prevented by automatic shutoff valves or preset shutoff operations. Low-
level shutoff should prevent vortices created during tank emptying. Other than
loss of the foam, no trouble should be encountered if the spheres escape into
process lines. The plastic material is not as abrasive as the sand particles
normally found entrained in crude oil. Excessive pressures crush the spheres
and the plastic settles in the water or sediment. At high temperatures, the
thermo-setting resins soften, liquefy, and mix with the fuel oil, asphalt, or
coke.
Plastic microspheres have proved to be effective for control of evaporative
losses from fixed-roof crude oil storage tanks but do not reduce emissions from
gasoline storage tanks as effectively as'other devices.
Vapor Balances Systems
Variable vapor space or vapor balance systems are designed to contain the
vapors produced in storage. They do not achieve as great a reduction in emissions
as an appropriately designed vapor recovery system does. A well-planned unit
includes storage of similar or related products, and uses the advantage of in-
belance pumping situations. Only the vapor space of the tanks is manifolded
together in these systems. Other systems include a vapor reservoir tank that is
either a lifter-roof type or a vessel with an internal diaphragm. The latter
vessel can be an integrated vapor-liquid tank or a separate vaporsphere.
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The manifold system includes various sizes of lightweight lines installed to
effect a balanced pressure drop in all the branches while not exceeding allowable
pressure drops. Providing isolating valves for each tank so that each tank can
be removed from the vapor balance system during gaging or sampling operations is
also good practice. Excessive vapors that exceed the capacity of the balance
system should be incinerated in a smokeless flare or used as fuel.
Vapor Recovery Systems
The vapor recovery system is in many ways similar to and yet superior to a
vapor balance system in terms of emissions prevented. The service of this type
of vapor recovery system is more flexible as to the number of tanks and products
being stored. The recovery unit is designed to handle vapors originating from
filling operations as well as from breathing. The recovered vapors are com-
pressed and charged to an absorption unit for recovery of condensable hydrocar-
bons. Noncondensable vapors are piped to the fuel gas system or to a smokeless
flare. When absorption of the condensable vapors is not practical from an
economic standpoint, these vapors, too, are sent directly to the fuel system or
incinerated in a smokeless flare.
The recovery system, like the vapor balance system, includes vapor lines
interconnecting the vapor space of the tanks that the system serves. Each tank
should be capable of being isolated from the system. This enables the tanks to
be sampled or gaged without a resulting loss of vapors from the entire system.
The branches are usually isolated by providing a butterfly-type valve, a regula-
/
tor, or a check valve- Since the valves offer more line resistance, their use
is sometimes restricted. Small vessels or knockout pots should be installed at
low points on the vapor manifold lines to remove any condensate.
In some vapor recovery systems, certain tanks must be blanketed with an
inert atmosphere in order to prevent explosive mixtures and product contamina-
tion. In other, larger systems, the entire manifolded section is maintained
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under a vacuum. Each tank is isolated by a regulator-control valve. The valves
operate from pressure changes occuring in the tank vapor space.
Because the vapor-gathering system is based upon positive net vapor flow to
the terminus (suction of compressors), the proper size of the vapor lines is
important. Sizing of the line, as well as that of the compressors, absorption
unit, or flare, is based upon the anticipated amount of vapors. These vapors
are the result of filling operations and breathing. The distance through which
the vapors must be moved is also important.
Miscellaneous Control Measures
Recent tests have shown that breathing emissions from fixed-roof tanks can
be reduced by increasing the storage pressure. An increase of 1 ounce per
square inch was found to result in an 8 percent decrease in emissions due to
breathing. Tanks operated at 2-1/2 psig or higher were found to have little or
no breathing emissions. The pressure setting, however, should not exceed the
weight of the roof.
Another method of reducing breathing losses is based upon the degree of
saturation in the vapor space. A baffle located in a horizontal position
immediately below the vent directs entering atmospheric air into a stratified
layer next to the top of the tank. Since this air is lighter, it tends to
remain in the top area; thus, there is less mixing of the free air and any of
the rich vapor immediately above the liquid surface.. The top stratified layer
is first expelled during the outbreathing cycle. Test data indicate a reduced
surface evaporation of 25 to 50 percent.
Hydrocarbon emissions can be minimized further by the proper selection of
paint for the tank shell and roof. The protective coating applied to the out-
side of shell and roof influences the vapor space and liquid tetnperatures.
Reflectivity and glossiness of a paint determine the quantity of heat a vessel
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can receive via radiation. A cooler roof and shell also allows any heat retained
in the stored material to dissipate. Weathering of the paint also influences its
effectiveness. Vapor space temperature reductions of 60"F. have been reported
with proper selection of paint color. Similarly, liquid-surface temperature
reductions of 3 to 11 degrees have been achieved. Data gathered by the American •
Petroleum Institute on hydrocarbon emissions indicate breathing emission reduc-
tions of 25 percent for aluminum over black paint and 25 percent for white over
aluminum paint. All paints revert to "black body" heat absorption media in a
corrosive or dirt-laden atmosphere.
Insulation applied to the outside of the tank is one method of reducing the
heat energy normally conducted through the wall and roof of the vessel. Another
method of controlling tank temperatures is the use of water. The water can be
sprayed or retained on the roof surface. The evaporation of the water results
in cooling of the tank vapors. Increased maintenance and corrosion problems may,
however, be encountered.
Storage temperatures may be reduced by external refrigeration or autorefrig-
eration. External refrigeration units require the circulation of the refrigerant
or of the tank contents. Autorefrigeration is practical in one-component liquid
hydrocarbon storage where high vapor pressure material is involved. The pres-
sure in the tank is reduced by removing a portion of the vapor. Additional vapor
is immediately formed. This flash vaporization results in lowering the tempera-
ture of the main liquid body.
Routine operations can be conducted in such a manner as to minimize other
emissions associated with storage tanks. Use of remote-level reading gages and
sampling devices reduces emissions by eliminating the need to open tank gage
hatches. Emissions can be further reduced by proper production scheduling to
(l) maintain a minimum of vapor space, (2) pump liquid to the storage tank during
cool hours and withdraw during hotter periods, and (3) maintain short periods
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between pumping operations.
Using vet scrubbers as control equipment for certain stored materials that
are sufficiently soluble in the scrubbing media employed is both possible and
practical. The scrubbers can be located over the vent vhen the scrubbing medium,
for example, a water scrubber for aqua ammonia storage, can be tolerated in the
product. In other cases, the vent of one or more tanks can be manifolded so
that any displaced gas is passed through a scrubbing unit before being discharged
to the atmosphere. A typical example is a scrubber 'packed with plastic spirals
that serves ketone storage vessels. The scrubbing liquid is water, which is
drained to a closed waste effluent disposal system.
Properly designed condensers can be used to reduce the vapor load from tank
vents in order that smaller control devices can be employed.
Costs of Storage Vessels
The installed costs of various types of hydrocarbon storage tanks are sum-
marized in Table I. Included in these costs are standard tank accessories such
as manholes, vents, ladders, stairways, drains, gage hatches, and flanged connec-
tions.
WASTE-GAS DISPOSAL SYSTEMS
Large volumes of hydrocarbon gases are produced in modern refinery and
petrochemical plants. Generally, these gases are used as fuel or as raw material
for further processing. In the past, however, large quantities of these gases
were considered waste gases, and along with waste liquids, were dumped to open
pits and burned, producing large volumes of black smoke. With modernization of
processing units, this method of waste-gas disposal, even for emergency gas
releases, has become less acceptable to the industry. Moreover, many local
governments have adopted or are contemplating ordinances limiting the opacity of
smoke from combustion processes.
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Nevertheless, petroleum refineries are still faced with the problem of safe
disposal of volatile liquids and gases resulting from scheduled shutdowns and
sudden or unexpected upsets in process units. Emergencies that can cause the
sudden venting of excessive amounts of gases and vapors include:
1. Failure of the cooling water supply.
2. Failure of a reflux system.
3. Entrance of a more volatile fluid into the equipment.
U. Vapor generation due to fire exposure.
5. Excessive heat inputs other than from fire.
6. Accumulation of noncondensible gases.
7. Closed or plugged equipment outlets.
8. Failure of automatic flow, temperature, or pressure control equipment.
9. Internal explosions.
10. Uncontrolled chemical reactions.
11. Failure of heat exchanger internals.
12. Power failure.
13. Thermal expansion.
ll». Compressor failure.
A system for disposal of emergency and waste refinery gases consists of a
manifolded pressure-relieving or blowdown system, and a blowdown recovery system
or a system of flares for the combustion of the excess gases, or both. Many
refineries, however, do not operate blowdown recovery systems. In addition to
disposing of emergency and excess gas flows, these systems are used in the
evacuation of units during shutdowns and turarounds. Normally a unit is shut
down by depressuring into a fuel gas or vapor recovery system with further
depressuring to essentially atmospheric pressure by venting to a low-pressure
flare system. Thus, overall emissions of refinery hydrocarbons are substan-
tially reduced.
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Refinery pressure-relieving systems, commonly called blowdown systems, are
used primarily to ensure the safety of personnel and protect equipment in the
event of emergencies such as process upset, equipment failure, and fire. In
addition, a properly designed pressure relief system permits substantial reduc-
tion of hydrocarbon emissions to the atmosphere.
The equipment in a refinery can operate at pressures ranging from less than
atmospheric to 1,000 psig and higher. This equipment must be designed to permit
safe disposal of excess gases and liquids in case operational difficulties or
fires occur. These materials are usually removed from the process area by auto-
matic safety and relief valves, as well as by manually controlled valves, mani-
folded to a header that conducts the material avay from the unit involved. The
preferred method of disposing of the waste gases that cannot be recovered in a
blowdown recovery system is by burning in a smokeless flare. Liquid blowdowns
are usually conducted to appropriately designed holding vessels and reclaimed.
A blowdown or pressure-relieving system consists of relief valves, safety
valves, manual bypass valves, blovdown headers, knockout vessels, and holding
tanks. A blowdown recovery system also includes compressors and vapor surge
vessels such as gas holders or vapor spheres. Flares are usually considered as
part of the blowdown system in a modern refinery.
The pressure-relieving system can be used for liquids or vapors or both.
For reasons of economy and safety, vessels and equipment discharging to blowdown
systems are usually segregated according to their operating pressure. In other
words, there is a high-pressure blowdown system for equipment working, for
example, above 100 psig, and low-pressure systems for those vessels with working
pressures below 100 psig. Butane and propane are usually discharged to a sepa-
rate blowdown drum, which is operated above atmospheric pressure to increase
recovery of liquids. Usually a direct-contact type of condenser is used to
permit recovery of as much hydrocarbon liquid as possible from the blowdown vapors.
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The noncondensibles are burned in a flare.
Design of Pressure Refllef System
The design of e pressure relief system is one of the most important problems
in the planning of a refinery or petrochemical plant. The safety of personnel
and equipment depends upon the proper design and functioning of this type of
system. The consequences of poor design can be disastrous.
A pressure relief system can consist of one relief valve, safety valve, or
rupture disc, or of several relief devices manifolded to a common header.
Usually the systems are segregated according to the type of material handled,
that is, liquid or vapor, as well as to the operating pressures involved.
The several factors that must be considered in designing a pressure relief
system are (l) the governing code, such as that of ASME (American Society of
Mechanical Engineers, 19^2); (2) characteristics of the pressure relief devices;
(3) the design pressure of the equipment protected by the pressure relief
devices, (U) line sizes and lengths, and (5) physical properties of the material
to be relieved to the system.
Safety Valves
Nozzle-type safety valves are available in the conventional or balanced-
bellows configurations. Backpressure in the piping downstream of the standard-
type valve affects its set pressure, but theoretically, this backpressure does
not affect the set pressure of the balanced-type valve. Owing, however, to
imperfections in manufacture and limitations of practical design, the balanced
valves available vary in relieving pressure when the backpressure reaches
approximately ^0 percent of the set pressure. The actual accumulation depends
upon the manufacturer.
Until the advent of balanced valves, the general practice in the industry
was to select safety valves that start relieving at the design pressure of the
vessel and reach full capacity at 3 to 10 percent above the design pressure.
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- 2k -
This overpressure vas defined as accumulation. With the balanced safety valves,
the allowable accumulation can be retained with smaller pipe size.
Each safety valve installation is an individual problem. The required
capacity of the valve depends upon the condition producing the overpressure.
Rupture Discs
A rupture disc is an emergency relief device consisting of a thin metal
diaphragm carefully designed to rupture at a predetermined pressure.
The obvious difference between a relief or safety valve and a rupture disc
is that the valve reseats and the disc does not. Rupture discs may be installed
in parallel or series with a relief valve. To prevent an incorrect pressure
differential from existing, the space between the disc and the valve must be
maintained at atmospheric pressure. In some cases a rupture disc may be used
to supplement a relief or safety valve. -In an installation such as this, the
relief or safety valve is sized by conventional methods and the rupture disc is
usually designed to relieve at 1.5 times the maximum allowable working pressure
of the vessel.
In determining the size of a disc, three important effects that must be
evaluated are low rupture pressure, elevated temperatures, and corrosion. Manu-
facturers can supply discs that are guaranteed to burst at plus or minus 5 per-
cent of their rated pressures.
The corrosive effects of a system determine the type of material used in a
disc. Even a slight amount of corrosion can drastically shorten disc life.
Discs are available with plastic linings, or they can be made from pure carbon
materials.
FLARES
The air pollution problem associated with the uncontrolled disposal of waste
gases is the venting of large volumes of hydrocarbons and other odorous gases
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and aerosols. The preferred control method for excess gases and vapors is to
recover them in a blowdown recovery system and, failing that, to incinerate them
in an elevated-type flare. Such flares introduce the possibility of smoke and
other objectionable gases such as carbon monoxide, sulfur dir.xide, and nitrogen •
oxides. Flares have been further developed to ensure that this combustion is
smokeless and in some cast:s nonluminous. Luminosity, while r.ot an air pollution
problem, does attract attention to the refinery operation and in certain cases
can cause bad public relations.
Smoke from Flares
Smoke is the result of incomplete combustion. Smokeless combustion can be
achieved by: (l) adequate heat values to obtain the minimum Theoretical combus-
tion temperatures, (2) adequate combustion air, and (3) adequate mixing of the
air and fuel.
An insufficient supply of air results in a smoky flame. Combustion begins
around the periphery of the gas stream where the air and fuel mix, and within this
flame envelope the supply of air is limited. Hydrocarbon side reactions occur
with the production of smoke. In this reducing atmosphere, hydrocarbons crack to
elemental hydrogen and carbon, or polymerize to form heavier hydrocarbons.
Since the carbon particles are difficult to burn, large volumes of carbon parti-
cles appear as smoke upon cooling. Side reactions become nore pronounced as
molecular weight and unsaturation of the fuel gas increase. Olefins, diolefins,
and aromatics characteristically burn with smoky, sooty flames as compared with
paraffins and naphthenes .
A smokeless flame can be obtained when an adequate amount of combustion air
is mixed sufficiently with the fuel so that it burns completely and rapidly
before any side reactions can take place.
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Types of Flares
There are, in general, three types of flares for the disposal of waste gases:
elevated flares, ground-level flares, and burning pits.
The burning pits are reserved for extremely large gas flows caused by catas-
trophic emergencies in which the capacity of the primary smokeless.flares is
exceeded. Ordinarily, the main gas header to the flare system has a water seal
bypass to a burning pit. Excessive pressure in the header blows the water seal
and permits the vapors and gases to vent a burning pit where combustion occurs.
The essential parts of a flare are the burner, stack, seal, liquid trap,
I
controls, pilot burner, and ignition system. In some cases, vented gases flow
through chemical solutions to receive treatment before combustion. As an exam-
ple, gases vented from an isomerization unit that may contain small amounts of
hydrochloric acid are scrubbed with caustic before being vented to the flare.
Elevated Flares
Smokeless combustion can be obtained in an elevated flare by the injection
of an inert gas to the combustion zone to provide turbulence and inspirate air.
A mechanical air-mixing system would be ideal but is not economical in view of
the large volume of gases handled. The most commonly enountered air-inspirating
material for an elevated flare is steam. Three main types of steam-injected
elevated flares are in use. These types vary in the manner in which the steam
is injected into the combustion zone.
In the first type, there is a commercially available multiple nozzle which
consists of an alloy steel tip mounted on the top of an elevated stack. Steam
injection is accomplished by several small Jets placed concentrically around the
flare tip. These Jets are installed at an angle, causing the steam to discharge
in a converging pattern immediately above the flare'tip.
A second type of elevated flare has a flare tip with no obstruction to flov,
that is, the flare tip is the same diameter as the stack. The steam is injected
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by a single nozzle located concentrically within the burner tip. In this type
of flare, the steam is prefixed with the gas before ignition and discharge.
A third type of elevated flare is equipped with a flare tip constructed to
cause the gases to flow through several tangential openings -;o promote turbulence.
A steam ring at the top of the stack has numerous equally spaced holes about 1/B
inch in diameter for discharging steam into the gas stream.
The injection of steam in flares may be automatically or manually controlled.
Most flares are instrumented to the extent that steam is automatically supplied
when there is a measurable gas flow. In most cases, the steain is proportioned
automatically to the reate of gas flow; however, in some installations, the
steam is automatically supplied at maximum rates, and manual throttling of a
steam valve is required for adjusting the steam flow to the particular gas flow
rate. There are many variations of instrumentation among various flares, some
designs being more desirable than others. For economic reasons, all designs
attempt to proportion steam flow to the gas flow rate.
Steam injection is generally believed to result in the following benefits:
(l) energy available at relatively low cost can be used to inspirate air and pro-
vide turbulence within the flame, (2) steam reacts with the fuel to form oxygen-
ated compounds that burn readily at relatively low temperatures, (3) water-gas
reactions also occur with this same end result, and (^) steam reduces the partial
pressure of the fuel and retards polymerization, (inert gases such as nitrogen
have also been found effective for this purpose; however, the expense of provid-
ing a diluent such as this is prohibitive.)
Ground-Level Flares
Ground-level flares are of four principal types: horizontal venturi, water
injection, multijet, and vertical venturi.
A horizontal venturi-type flare system utilizes groups of standard venturi
burners. In this type of burner, the gas pressure inspirates combustion air for
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smokeless operation.
A water-injection flare consists of a single burner witli a water spray ring
around the burner nozzle. The water spray inspirates air am provides water
vapor for the smokeless combustion of gases. Water is not aj effective as steam
for controlling smoke with high gas-flow rates, unsaturated materials, or wet
gases.
A multijet ground flare uses two sets of burners, one for normal gas release
rates and both for higher flaring rates.
A vertical, venturi-type ground flare also uses commercial-type venturi
burners. This type of flare is suitable for relatively small flows of gas at a
constant rate-
Ground-level flares are seldom used today in refineries because of space
limitations, the inability to safely dissipate heat generated, and the difficulty
of diffusing any vapors that may be emitted.
Effect of Steam Injection
A flare installation that does not inspirate' an adequate amount of air or
does not mix the air and hydrocarbons properly emits dense, black clouds of
smoke that obscure the flame. The injection of steam into the zone of combus-
tion causes a gradual decrease in the amount of smoke, and the flame becomes
more visible. When trailing smoke has been eliminated, the flame is very
luminous and orange with a few wisps of black smoke around the periphery. The
minimum amount of steam required produces a yellowish-orange, luminous flame
with no smoke. Increasing the amount of steam injection further decreases the
luminosity of the flame. As the steam rate increases, the flame becomes color-
less and finally invisible during the day. At night this flame appears blue.
The injection of an excessive amount of steam causes the flame to disappear
completely and be replaced with a steam plume. An excessive amount of steam may
extinguish the burning gases and permit unburned hydrocarbons to discharge to
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. - 29 -
the atmosphere. When the flame is out, there is a change in the sound of the
flare because a steam hiss replaces the roar of combustion. The commercially
available pilot burners are usually not extinguished by excersive amounts of
steam, and the flame reappears as the steam injection rate it: reduced. As the
use of automatic instrumentation becomes more prevalent in fl.are installations,
the use of excessive amounts of steam and the emission of unturned hydrocarbons
decrease and greater steam economies can be achieved. In evaluating flare
installations from an air pollution standpoint, controlling the volume of steam
is important. Too little steam results in black smoke, which, obviously, is
objectionable. Conversely, excessive use of steam produces a white steam plume
and an invisible emission of unburned hydrocarbons. A condition such as this
can also be a serious air pollution problem.
Design of a Smokeless Flare
The choice of a flare is dictated by the particular requirements of the
installation. The usual flare system includes gas collection equipment, the
liquid knockout tank preceding the flare stack. A water seal tank is usually
located between the knockout pot and the flare stack to prevent flashbacks into
the system. Flame arrestors are sometimes used in place of or in conjunction
with a water seal pot. The flare stack should be continuously purged with steam
or refinery gas to prevent the formation of a combustible mixture that could
cause an explosion in the stack.
The preferred method of inspirating air is injecting stest?. either into the
stack or into the combustion zone. Water has sometimes been used in ground
flares where there is an abundant supply. There is, however, less assurance of
complete combustion when water is used, because the flare is limited in its
operation by the type and composition of gases it can handle efficiently.
The diameter of the flare stack depends upon the expected emergency gas
flow rate and the permissible backpressure in the vapor relief manifold system.
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The stack diameter is usually the same or greater than that ->f the vapor header
discharging to the stack and should be the same diameter as or greater than
that of the burner section. The velocity of the gas in the r tack should be as
high as possible to permit use of lower stack heights; promote turbulent flow
with resultant improved combustion, and prevent flashback.
Adequate stack heights must be provided to permit safe dispersion of toxic
or combustible material in the event of pilot burner failure.
The structural support of an elevated-flare stack over 1*0 to 50 feet high
requires the use of guy wires. A self-supporting stack over f>0 feet high
requires a large and expensive foundation. Stacks over 100 feet high are
usually supported by a steel structure.
The amount of steam required for smokeless combustion varies according to
the maximum expected gas flow, the molecular weight, and the percent of unsatu-
rated hydrocarbons in the gas. Actual tests should be run on the various mate-
rials to be flared in order to determine a suitable steam-to-hydrocarbon ratio.
In the typical refinery, the ratio of steam to hydrocarbon varies from 0.2 to
0-5 pound of steam per pound of hydrocarbon.
Pilot Ignition System
The ignition of flare gases is normally accomplished with one of three pilot
burners. A separate system must be provided for the ignition of the pilot
burner to safeguard against flame failure. In this system, an easily ignited
flame with stable combustion and low fuel usage must be provided. In addition,
the system must be protected from the weather.
On elevated flares, the pilot flame is usually not visible, and an alarm
system to indicate flame failure is desirable. This is usually accomplished by
installing thermocouples in the pilot burner flame. In the event of flame fail-
ure, the temperature drops to a preset level, and an alarm sounds.
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Instrumentation and Control of Steam and Gas
For adequate prevention of smoke emission and possible '-iolations of air
pollution regulations, an elevated, smokeless flare should bf-- equipped to pro-
vide steam automatically and in proportion to the emergency ^as flow.
Basically, the instrumentation required for a flare is i flow-sensing
element, such as a pitot tube, and a flow transmitter that seids e signal
(usually pneumatic) to a control valve in the steam line. Although the pitot
tube has been used extensively in flare systems, it Is limited by the minimum
linear velocity required to produce a measurable velocity head. Thus, small
gas flows will not actuate the steam control valves. This problem is usually
overcome by installing a small bypass valve to permit a constant flow of steam
to the flame burner. A more sensititve type of flow-measuring device is the
inverted weir. A variation of the inverted weir is the slotted orifice.
The hot-wire flow meter has also been used in flare systems. The sensing
element is basically a heat loss anemometer consisting of an electrically heated
wire exposed to the gas stream to measure the velocity. The gas flow is perpen-
dicular to the axis of the hot wire. A conventional recorder is used with this
probe, modified for the resistance bridge circuit of the gas flow meter. As
the flow of gas past the probe varies, the heat loss from the hot wire varies
and causes an imbalance of the bridge circuit. The recorder then adjusts for
the imbalance in the bridge and indicates the gas flow. This type of installa-
tion provides sensitivity at low velocities, and the gas flow measurement can be
made without causing an appreciable pressure drop. This is an important advan-
tage in a system using constant backpressure-type relief valves. The hot-wire
flow meter can be used as a primary flow-sensing element or as a leak detector
in laterals connected to the main flare header.
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Maintenance of Flares
Most refineries and petrochemical plants have a fixed schedule for inspec-
tion and maintenance of processing units and their auxiliaries. The flare
system should not be exempted from this practice. Removal ol' a flare from
service for maintenance requires some type of standby equipment to disperse
emergency gas vents during the shutdown. A simple stack with pilot burner should
suffice for a standby. Coordinating this inspection to take place at time when
the major processing units are also shut down is good practice.
Flare instrumentation requires scheduled maintenance to ensure proper opera-
tion. Most of the costs and problems of flare maintenance ar;'.se from the instru-
mentation.
Maintenance expenses for flare burners can be reduced by constructing them
of chrome-nickel alloy. Because of the inaccessibility of elevated flares, the
use of alloy construction is recommended.
PROCESS OPERATIONS
Catalytic Cracking Units
Petroleum fractions are cracked to produce compounds of lower molecular
weight. Catalysts in the form of powders or beads are utilized. The catalyst
particles become coated with carbon and high-molecular-weight compounds. These
materials must be burned off the catalyst in order to maintain its activity.
The catalyst continuously circulates from the reactor chamber zo the regenerator
chamber. In the regenerator, a controlled amount of air is admitted to burn off
the coatings. This causes the formation of CO and hydrocarbons. Typical hydro-
carbon emissions from the regenerators of catalytic cracking units are estimated
to be 220 lbs./l,000 barrels of fresh feed for fluidized units and 8? lbs./l,000
barrels of fresh feed for moving bed units.
Hydrocarbon emissions from regenerators of catalytic cracking units are
generally of secondary importance compared to the carbon monoxide emissions and
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- 33 -
their control is incidental to CO control. A vaste heat or CO boiler will pro-
vide essentially a 100 percent reduction in hydrocarbon emissions as veil as
controlling CO emissions. The internal CO combustion technique also results in
significant reduction in hydrocarbon emissions but data are incomplete as to
the exact percentage reduction which can be obtained with thio technique.
Asphalt Oxidation
Asphalt is a dark brcvn to black, solid or semisolid mat-.-rial found in
naturally occurring deposits or as a colloidal suspension in «.:rude oil. Analyti-
cal methods have been used to separate asphalt into three conr:.)0nent groups--
asphaltenes, resins, and oils. A particular grade of asphalt may be character-
ized by the amounts of each group it contains. The asphaltenf particle provides
a nucleus about which the resin forms a protective coating. The particles are
suspended in an oil that is usually paraffinic but can be naphthenic or naptheno-
aromatic.
Over 90 percent of all asphalt used in the United States is recovered from
crude oil. The method of recovery depends upon the type of crude oil being pro-
cessed. Practically all types of crudes are first distilled at atmospheric
pressure to remove the lower boiling materials such as gasoline, kerosene,
diesel oil, and others. Recovery of nondistillable asphalt from selected
topped crudes may then be accomplished by vacuum distillation, solvent extrac-
tion, or a combination of both.
A vacuum distillation unit uses a heater, preflash tower, vacuum vessel,
and appurtenances for processing topped crudes. Distillation o.f topped crude.
under a high vacuum removes oils and wax as distillate products, leaving the
asphalt as a residue. The amount of oil distilled from the residue asphalt
controls its properties; the more oil and resin or oily constituents removed by
distillation, the harder the residual asphalt. Residual asphalt can be used as
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paving material or it can be further refined "by airblowing.
Asphalt is also produced as a secondary product in solvent extraction pro-
cesses. This process separates the asphalt from remaining constituents of
topped crudes by differences in chemical types and molecular weights rather
than boiling points as in vacuum distillation processes. The solvent, usually
a light hydrocarbon such as propane or butane, is used to retmve selectively a
gas-oil fraction from the asphalt residue.
Economical removal of the gas-oil fraction from topped crude, leaving an
asphaltic product, is occasionally feasible only by airblowing the crude residue
at elevated temperatures. Excellent paving-grade asphalts are produced by this
method. Another important application of airblowing is in the production of
high-quality specialty asphalts for roofing, pipe coating, and similar uses.
These asphalts require certain plastic properties imparted by reacting with air.
Airblowing is mainly a dehydrogenation process. Oxygen in the air combines
with hydrogen in the oil molecules to form water vapor. The progressive loss of
hydrogen results in polymerization or condensation of .the asphalt to the desired"
consistency. Blowing is usually carried out batchwise in horizontal or vertical
stills equipped to blanket the charge with steam, but it may also be done con-
tinuously. Vertical stills are more efficient because of longer air-asphalt
contact time. The asphalt is heated by an internal fire-tube heater or by
circulating the charge material through a separate tubestill. A temperature of
300° to ^00°F. is reached before the airblowing cycle begins. Air quantities
used range from 5 to 20 cubic feet per minute per ton of charge. Little addi-
tional heat is then needed since the reaction becomes exothermic.
Effluents from the asphalt airblowing stills include oxygen, nitrogen and
its compounds, water vapor, sulfur compounds, and hydrocarbons as gases, odors,
and aerosols. Discharge of these vapors directly to the atmosphere is objectionable
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from an air pollution control standpoint. The disagreeable c-dors and airborne
oil particles entrained with the gases result in nuisance complaints. Disposal
methods are available that can satisfactorily eliminate the pollution potential
of the effluents.
Control of effluent vapors from asphalt airbloving stilLs has been accom-
plished by scrubbing and incineration, singly or in combination. Most installa-
tions use the combination. For scrubbing alone to be effective, a very high
water-to-gas ratio of about 100 gallons per 1,000 scf is necessary.
Where removal of most of the potential air pollutants is not feasible by
scrubbing alone, the noncondensibles must be incinerated. Essential to effec-
tive incineration is direct-flame contact with the vapors, a minimum retention
time of 0-3 second in the combustion zone, and maintenance of a minimum combus-
tion chamber temperature of 1,200°F. Other desirable features include turbulent
mixing of vapors in the combustion chamber, tangential flame entry, and adequate
instrumentation. Primary condensation of any steam or water vapor allows use
of smaller incinerators and results in fuel savings. Some of the heat released
by incineration of the waste gases may be recovered and used for generation of
steam.
Catalytic fume burners are not recommended for the disposal of vapors from
the airblowing of asphalt because the matter entrained in the vapors would
quickly clog the catalyst bed.
Chemical Treating Processes
In acid treatment, emissions can be reduced by substituting continuous
mechanical mixing for batch-type agitators that employ airblowing for mixing.
Acid regeneration can also be used instead of the hydrolysis-concentration
method of acid recovery. Gases emitted during acid-sludge recovery can be
vented to caustic scrubbers to remove sulfur dioxide and odorants. Gases from
scrubbers can then be vented to a firebox or flare. For new installations,
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acid treatment can also be replaced by catalytic hydrogenation or by other pro-
cessing techniques that may prove to be more effective.
In doctor treating, the doctor solution can be steam-stripped to recover
hydrocarbons prior to airblowing for regeneration. The effluent from airblowing
can then be incinerated to destroy hydrocarbon vapors.
In the disposal of spent caustic, entrained hydrocarbons are often removed
by stripping with inert gases. The vapors removed in this stripping operation
can be vented to a flare or to a furnace firebox.
Whenever hydrocarbons are removed in air or gasbloving operations, the
effluent hydrocarbons can be destroyed by incineration.
LOADING FACILITIES
Gasoline and other petroleum products are distributed from the manufacturing
facility to the consumer by a network of -pipelines, tank vehicle routes, railroad
tank cars, and oceangoing tankers.
As integral parts of the network, intermediate storage and loading stations
receive products from refineries by either pipelines or tank vehicles. If the
intermediate station is supplied by pipeline, it is called a bulk terminal, to
distinguish it from the station supplied by tank vehicle, which is called a bulk
plant. Retail service stations fueling motor vehicles for the public are, as a
general rule, supplied by tank vehicle from bulk terminals or bulk plants. Con-
sumer accounts, which are privately owned facilities operated, for example, to
fuel vehicles of a company fleet, are supplied by tank vehicles from intermediate
bulk installations or directly from refineries.
Gasoline and other petroleum products are loaded into tank trucks, trailers,
or tank cars at bulk installations and refineries by means of loading racks.
Bulk products are also delivered into tankers at bulk marine terminals.
When a compartment of a tank vehicle or tanker is filled through an open
overhead hatch or bottom connection, the incoming liquid displaces the vapors
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in the compartment to the atmosphere. Except in rare instances, vhere a tank
vehicle or tanker is free of hydrocarbon vapor, as when being used for the first
time, the displaced vapors consist of a mixture of air and hydrocarbon concen-
tration, depending upon the product being loaded, the temperature of the product
and of the tank compartment, and the type of loading. Ordinarily, but not
always, when gasoline is loaded, the hydrocarbon concentration of the vapors is
from 30 to 50 percent by volume and consists of gasoline fractions ranging from
propane through hexane.
The volume of vapors produced during the loading operation, as well as
their composition, is greatly influenced by the type of loading or filling
employed. The types in use throughout the industry may be classified under two
general headings, overhead loading and bottom loading.
Overhead loading, presently the most widely used method, may be further
divided into splash and submerged filling. In splash filling, the outlet of the
delivery tube is above the liquid surface during all or most of the loading.
In submerged filling the outlet of the delivery tube is extended to within 6
inches of the bottom and is submerged beneath the liquid during most of the
loading. Splash filling generates more turbulence and therefore more hydrocar-
bon vapors.
Bottom loading has been introduced by a number of oil companies for new
•facilities. The equipment required is simpler than that used for overhead load-.
ing. Loading by this method is accomplished by connecting a swing-type loading
arm or hose at ground level to a matching fitting on the underside of the tank
vehicles. Aircraft-type, quick-coupling valves are used to ensure a fast, posi-
tive shutoff and prevent liquid spills. Several companies experienced in aircraft-
fueling operations have developed fully automatic bottom-loading systems. All the
loading is submerged and under a slight pressure; thus, turbulence and resultant
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production of vapors are minimized.
Loading losses for splash filling typically range from 0.2 to 0-35 percent
depending on loading rate, RVP and temperature of product, ar-.cl degree of satura-
tion of the displaced vapors. Loading losses for "bottom loading and submerged
filling are roughly equivalent with values ranging from 0.06 to 0.23 percent
depending on conditions.
The method employed for loading marine tankers is essentially a bottom-.
loading operation. Liquid is delivered to the various compartments through
lines that discharge at the bottom of each compartment. The vapors displaced
during loading are vented through a manifold line to the top of the ship's mast
for discharge to the atmosphere.
In addition to the emissions resulting from the displacement of hydrocarbon
vapors from the tank vehicles, additional emissions during loading result from
evaporation of spillage, drainage, and leakage of product.
An effective system for control of vapor emissions from loading must include
a device to collect the vapors at the tank vehicle hatch' and a means for disposal
of these vapors.
Overhead Loading
Four types of vapor collectors have been developed for use during overhead
loading operations. All are essentially plug-shaped devices that are inserted
into a fitting for the hatch opening. Gasoline flows through a central channel
in the device into the tank vehicle compartment. This central channel is sur-
rounded by an annular space into which vapors enter through openings on the
bottom of the hatch fitting. The annular space is in turn connected to a hose
or pipe leading to a vapor disposal system.
The Mobil Oil Corporation device is connected to a vapor chamber with a
transparent section to allow the operator to see the calibrated capacity markers
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located within the tank compartment. This closure has adjustable height. It
requires a constant downward force to keep it firmly in place during filling.
It is built to fit only hatches 8 to 10 inches in diameter.
The Chiksan device incorporates the hatch closure, the vapor return line,
and the fill line into an assembled unit. This unit has features to prevent
overfills, topping off, or filling unless the assembly is properly seated in
the tank hatch.
The Greenwood vapor closure developed by the Vernon Tool Company, also
requires downward force during filling. It ordinarily does not have a trans-
parent vapor chamber. This closure has an adapter for hatches larger than 10
inches.
The fourth device was developed by Standard Oil Company of California and
has a positive clamp for the hatch opening, which, when closed automatically
actuates the vapor chamber. It also has a safety shutoff float that senses the
gas level and prevents overfilling. These SOCO devices can be used with adapters
for hatches larger than 8 inches in diameter.
The slide positioner of the Mobil Oil Corporation device can be a source of
vapor leaks and requires close attention by the operator during adjustments for
fitting and submerged loading. The inner valves of the SOCO devices make them
considerably heavier than other types. This device increases pressure drop and
slows the loading rates. Mobil and Greenwood devices both require check valves
in the vapor-gathering lines to prevent the vapor from discharging back to the
atmosphere when the assembly is withdrawn. In addition, these devices require
nearly vertical entry of the loading tube into the hatch opening in order to
provide a tight seal against vapor leaks. An assembly is available to assure
that the Greenwood device maintains this vertical position.
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Bottom Loading
Bottom loading permits easier collection of displaced vapors. Because the
filling line and the vapor collection line are independent of each other, col-
lection during bottom loading is relatively easy. The vapor collection line
consists of a flexible hose or swing-type arm connected to a quick-acting valve
fitting on the dome of the vehicle. A check valve must, of course, be installed-
on the vapor collection line to prevent backflow of vapors to the atmosphere
when the connection to the tank is broken.
In designing for complete vapor pickup at the tank vehicle hatch, several
factors, including tank settling, liquid drainage, and topping off must be con- .
sidered.
The settling of a tank vehicle due to the weight of product being added
requires that provision be made for vertical travel of the loading arm to follow .
the. motion of the vehicle so that the vapor collector remains sealed in the tank
hatch during the entire loading cycle. Two solutions to the problem of settling
have been used. The first, applicable to pneumatically-operated arms, includes
the continuous application of air pressure to the piston in the air cylinder
acting on the arm. The arm is thus forced to follow the motion of the vehicle
without need for clamping or fastening the vapor collector to the tank vehicle.
The second solution, employed on counterweighted and torsion spring loading
arms, provides for locking the vapor collector to the tank vehicle hatch. The
arm then necessarily follows the motion of the vehicle. The second solution is
also applicable to vapor collection arms or hoses that are connected to the top
of a tank vehicle during bottom loading.
The second problem, that of preventing considerable liquid drainage from a
loading arm as it is withdrawn after completion of filling operations, has been
adequately solved. The air valve that operates the air cylinder of pneumatically
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- in -
6t>erated loading arms may be modified by addition of an orifice on the discharge
side of the valve. The orifice allows 30 to k$ seconds to elapse before the
loading assembly clears the hatch compartment. This time interval is sufficient
to permit complete draining of liquid into tank compartments from arms fitted
with loading valves located in an outboard position. Loading arms with inboard
valves require additional drainage time and present the- problem of gasoline
retention in the horizontal section of the arm. To prevent drainage the SOCO
vapor collection closure is fitted with an internal shutoff valve that is closed
before the loading arm is withdrawn from the tank hatch. Providing for thermal
expansion has been found necessary when an inboard valve and a SOCO vapor closure
are used. This has been accomplished by installing a small expansion chamber at
the normal position of the loading arm's vacuum breaker. In bottom loading, the
valve coupling at the end of the loading arm or hose, as well as the mating por-
tion of the valve on the trucks, is self-sealing to prevent drainage of product
when the connection is made or broken.
The third factor to be considered in the design of an effective vapor col-
lection system is topping off. Topping off is the term applied to the loading
operation during which the liquid level is adjusted to the capacity marker
inside the tank vehicle compartment. Since the loading arm is out of the com-
partment hatch during the topping operation, vapor pickup by the collector is
nil. Metering the desired volumes during loading is one solution to the problem.
Metered loading must, however,-be restricted to empty trucks or to trucks pre-
checked for loading volume available. Accuracy of certain totalizing meters or
preset stop meters is satisfactory for loading without the need for subsequent
ooen topping. An interlock device for the pneumatic-type loading arms, consist-
ing of pneumatic control or mechanical linkage, prevents opening of the loading
valve unless the air cylinder valve is in the down position. Thus, open topping
is theoretically impossible.
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Topping off is not a problem vhen bottom loading is employed. Metered
loading, or installation of a sensing device in the vehicle compartments that
actuates a shutoff valve located either on the truck or the loading island,
eliminates the need for topping off.
Vapor Disposal
The methods of disposing of vapors collected during loading operations may
be considered under three headings: using the vapors as fuel, processing the
vapors for recovery of hydrocarbons, effecting a vapor balance system in conJune-'
tion with submerged loading, or simple incineration of generated vapors.
The first method of disposal, using the vapors directly as fuel, may be
employed when the loading facilities are located in or near a facility that
includes fired heaters or boilers. In a typical disposal system, the displaced
vapors flow through a drip pot to a small vapor holder that is gas blanketed to
prevent forming of explosive mixtures. The vapors are drawn from the holder by
a compressor and are discharged to the fuel gas system.
The second method of disposal uses equipment designed 'to recover the hydro-
carbon vapors. Vapors have been successfully absorbed in a liquid such as gaso-
line or kerosene. If the loading facility is located near a refinery or gas
absorption plant, the vapor line can be connected from the loading facility to
an existing vapor recovery system through a regulator valve.
Vapors are recovered from loading installations distant from existing pro-
cessing facilities by use of package units. One such unit that, absorbs hydro-
carbon vapors in gasoline has been developed by the Superior Tank and Construction
Company. This unit includes a vaporsphere or tank equipped with flexible membrane
diaphragm, saturator, absorber, compressor, pumps, and instrumentation. Units are
available to fit any size operation at any desired loading location since they use
the gasoline product as the absorbent.
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Explosive mixtures must be prevented from existing in this unit. This is
accomplished by passing the vapors displaced at the loading rack through a
saturator countercurrently to gasoline pumped from storage. The saturated
vapors then flow to the vaporsphere. Position of the diaphragm in the vapor-
sphere automatically actuates a compressor that draws the vapors from the sphere
and injects them at about 200 psig into the absorber. Countercurrent flow of
stripped gasoline from the saturator or of fresh gasoline fror. storage is used
to absorb the hydrocarbon 'vapors. Gasoline from the absorber bottoms is returned
to storage vhile the tail gases, essentially air, are released to the atmosphere
through a backpressure regulator. Some difficulty 'has been experienced with air
entrained or dissolved in the sponge gasoline returning to storage. Any air
released in the storage tank is discharged to the atmosphere saturated with
hydrocarbon vapors. A considerable portion of the air can be removed by flash-
ing the liquid gasoline from the absorber in one or more additional vessels
operating at successively .lower pressures.
Another type package unit is offered by Farker-Hannifin. This unit does
not utilize a vaporsaver to level off the vapor flow rate to the compressor and,
therefore, requires a larger compressor and higher power usage. The unit also
incorporates a refrigeration unit in conjunction with the absorber. Costs of
the two systems are comparable since the savings from eliminating the vaporsaver
is offset by the cost of th'e larger compressor and refrigeration unit.
A third type of package unit adsorbs the hydrocarbon vapors on activated
carbon. The application of this type of unit is presently restricted to loading
installations that have low throughputs of gasoline, since the adsorbing capacity
and the life of the carbon are limited. Units of this type find application in
control of vapors resulting from fueling of jet aircraft.
The vapors displaced during bottom filling are minimal. Data indicate a
volume displacement ratio of vapor to liquid of nearly 1:1. A closed system can
-------
- kk.
then be employed by returning all the displaced vapors to a storage tank. The
storage tank should be connected to a vapor recovery system.
For medium-sized terminals loading less than about 1.8 million barrels per
year of gasoline, extensive recovery facilities such as those described above
cannot be economically Justified and the preferred emission control method is
incineration. For small facilities such as bulk plants which handle only
several truck loads per day, even incineration cannot be justified and a require-
ment for emission controls would result in closing of the facility and reorgani-
zation of the gasoline distribution system.
OIL-WATER SEPARATORS
A typical waste-water gathering system for a modern refinery usually includes
gathering lines, drain seals, junction boxes, and pipes of vitrified clay or con-
crete for transmitting waste water from processing units to large basins or ponds
used as oil-water separators. These basins are sized to receive all effluent
water, sometimes even rain runoff; they are constructed as earthen pits, concrete-
lined basins, and steel tanks.
Liquid wastes discharging to these systems originate at a wide variety of
sources such as pump glands, accumulators, spills, cleanouts, sampling lines,
and relief valves.
Organic compounds can escape to the atmosphere from openings in the sewer
system, channels, vessels, and oil-water separators. The large exposed surface
area of these separators can result in large hydrocarbon emissions to the atmo-
sphere.
The most effective means of control of hydrocarbon emissions from oil-water
separators has been the covering of forebays or primary separator sections.
Either fixed roofs or floating roofs are acceptable covers. Separation and skim-
ming of over 80 percent of the floatable oil layer takes place in the covered
sections. Thus, only a small amount of oil is contained in the effluent water,
-------
which flows under concrete curtains to the open afterbays or secondary separator
sections.
Satisfactory fixed roofs have been constructed by using vooden beams for
structural support and asbestos paper as a cover. A mastic-type sealing compound
is then used to seal all joints and cracks. Although this form of roof is accept-
able for the control of pollutants, in practice a completely vaportight roof is
difficult to achieve. The resultant leakage of air into the vapor space, and
vapor leakage into the atmosphere are not desirable from standpoints of air pol-
lution or safety.
The explosion hazard associated with fixed roofs is not present in a float-
ing-roof installation. These roofs are similar to those developed for storage
tanks. The floating covers are built to fit into bays with about 1 inch of
clearance around the perimeter. Fabric or rubber may be used to seal the gap
between the roof edge and the separator wall. The roofs are fitted with access
manholes, skimmers, gage hatches, and supporting legs. In operation, skimmed
oil flows through lines from the skimmers to a covered tank or sump and then is
pumped to demulsifying processing facilities.
A simpler type of floating cover can be provided by Foamglas slabs which
can be used to cover the primary section of the oil-water separator. This mate-
rial has been used to cover the oil-water separators at Wood River where it is
estimated that evaporation losses have been by 100 to 600 B/D depending on the
season and slop oil composition. Estimated efficiency is 80 to 90 percent.
Any type of floating cover on an oil-water separator will interfere with
skimming operations if the separator is equipped with flight skimmers. This
problem can be overcome by increasing the weir level so that the flights are
completely submerged. This will result in constant retention of a layer of oil
several inches deep in the separator at all times.
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- U6 -
o
The installed cost of Foam glas slabs is roughly $-50/ft. compared to
$1.00-3.00/ft.^ for other types depending on the elaborateness of the^cover.
In addition to covering the separator open sewer lines that may carry
volatile products can be converted to closed, underground lines with water-seal-
type vents. Junction boxes can also be vented to vapor recovery facilities, and
steam can be used to blanket the sewer lines to inhibit formation of explosive
mixtures.
PUMPS
Pumps are used in every phase of the petroleum industry. Their applications
range from the lifting of crude oil from the depths of a well' to the dispensing
of fuel to automobile engines. Leakage from pumps can cause air pollution wher-
ever organic liquids are handled.
Pumps are available in a wide variety of models and sizes. Their capaci-
ties may range from several milliliters per hour, required for some laboratory
pumps, to 3A million gallons per minute, required of each of the new pumps at
Grand Coulee Dam.
Materials used for construction of pumps are also many and varied. All the
common machinable metals and alloys, as well as plastics, rubber, and ceramics,
are used. Pumps may be classified under two general headings, positive displace-
ment and centrifugal.
Positive-displacement pumps have as their principle of operation the dis-
placement of the liquid from the pump case by reciprocating action of a piston
or diaphragm, or rotating action of a gear, cam, vane, or screw. The type of
action may be used to classify positive-displacement pumps as reciprocating or
rotary. When a positive-displacement pump is stopped, it serves as a check valve
to prevent backflow.
Centrifugal pumps operate by the principle of converting velocity pressure
generated by centrifugal force to static pressure. Velocity is imparted to the
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fluid by an impeller that is rotated at high speeds. The fluid enters at.the
center of the impeller and is discharged from its periphery. Unlike positive-
displacement pumps, when the centrifugal type of pump is stopped there is a
tendency for the fluid to backflow.
Other specialized types of pumps are available, but, generally, the pumps
used by the petroleum industry fall into the two categories discussed.
Power for driving the various types of pumps is usually derived from electric
motors, internal combustion engines, or steam drives.. Any one of these sources
may be adapted for use with either reciprocating pumps or centrifugal pumps.
Most rotary pumps are driven by electric motor.
Operation of various pumps in the handling of fluids in petroleum process
units can result in the release of air contaminants. Volatile materials such as
hydrocarbons, and odorous substances such as hydrogen sulfide or mercaptans are
of particular concern because of the large volumes handled. Both reciprocating
and centrifugal pumps can be sources of emissions.
The opening in the cylinder or fluid end through which the connecting rod
actuates the piston is the major potenial source of contaminants from a recipro-
cating pump. In centrifugal pumps, normally the only potential source of leakage
occurs where the drive shaft passes through the impeller casing.
Several means have been devised for sealing the annular clearance between
pump shafts and fluid casings to retard leakage. For most refinery applications,
packed seals and mechanical seals are widely used.
Packed seals can be used on both positive displacement and centrifugal type
pumps. Typical packed seals consist of a stuffing box filled with sealing
material that encases the moving shaft. The stuffing box is fitted with a takeup
ring that is made to compress the packing and cause it to tighten around the shaft.
Materials used for packing vary with the product temperature, physical and chemi-
cal properties, pressure, and pump type. Some commonly used materials are metal,
-------
rubber, leather, wood, and plastics.
Lubrication of the contact surfaces of the packing and shaft is effected by
a controlled amount of product leakage to the atmosphere. This feature makes
packing seals undesirable in applications where the product can cause a pollu-
tion problem. The packing itself may also be saturated with some material such
as graphite or oil that acts as a lubricant. In some cases cooling or quench
water is used to cool the impeller shaft and,"the bearings.
The second commonly used means of sealing is the mechanical seal, which was
developed over a period of years as a means of reducing leakage from pump glands.
This type of seal can be used only in pumps that have a rotary shaft motion. A
simple mechanical seal consists of two rings with wearing surfaces at right
angles to the shaft. One ring is stationary while the other is attached to the
shaft and rotates with it. A spring and the action of fluid pressure keep the
two faces in contact. Lubrication of the wearing faces is effected by a thin
film of the material being pumped. The wearing faces are precisely finished to
ensure perfectly flat surfaces. Materials used in the manufacture of the sealing
rings are many and varied. Choice of materials depends primarily upon properties
of fluid being pumped, pressure, temperature, and speed of rotation. The vast
majority of rotating faces in commercial use are made of carbon.
Emissions to the atmosphere from centrifugal pumps may be controlled in
some cases by use of the described mechanical-type seals instead of packing
glands. For cases not feasible to control with mechanical seals, specialized
types of pumps, such as canned, diaphragm, or electromagnetic, are required.
The canned-type pump is totally enclosed, with its motor built as an integral
part of the pump. Seals and attendant leakage are eliminated. The diaphragm
pump is another type devoid of seals. A diaphragm is actuated hydraulically,
mechanically, or pneumatically to effect a pumping action. The electromagnetic
pumps use an electric current passed through the fluid, which is in the presence
-------
of a strong magnetic field, to cause motion.
A pressure-seal-type application can reduce packing gland leakage. A
liquid, less volatile or dangerous than the product being pumped, is introduced
between two sets of packing. This sealing liquid must also be compatible with
the product. Since this liquid is maintained at a higher pressure than the
product, some of it passes by the packing into the product, '.Che pressure
differential prevents the product from leaking outward, and the sealing liquid
provides the necessary lubricant for the packing gland. Some of the sealing
liquid passes the outer packing (hence the necessity of low volatility), and a
means should be provided for its disposal.
This application is also adaptable to pumps with mechanical seals. A dual
set of mechanical seals similar to the two sets of packing is used.
Volatile vapors that leak past a main seal may be vented to vapor recovery
by using dual seals and a shaft housing.- ,77
Other than the direct methods used to control leakage, operational changes
may minimize release of contaminants to the atmosphere. One desireable change
is to bleed off pump casings during shutdown to the fuel gas system, vapor
recovery facilities, or a flare instead of directly to the atmosphere.
VALVES
Valves are employed in every phase of the petroleum industry where petroleum
or petroleum product is transferred by piping from one point to another. There
is a great variety of valve designs, but, generally, valves may be classified by
their application as flow control or pressure relief.
Manual and automatic flow control valves are used to regulate the flow of
fluids through a system. Included under this classification ere the gate, globe,
angle, plug, and other common types of valves. These valves are subject to
product leakage from the valve stem as a result of the action of vibration, heat,
pressure, corrosion, or improper maintenance of valve stem packing.
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- 50 -
Pressure relief and safety valves are used to prevent excessive pressures
from developing in process vessels and lines. The relief valve designates liquid
flow while the safety valve designates vapor or gas flow. These valves may
develop leaks because of the corrosive action of the product or because of fail-
ure of the valve to reseat properly after blowoff. Rupture discs are sometimes
used in place of pressure reflief valves. Their use is restricted to equipment
in batch-type processes. The maintenance and operational difficulties caused
by the inaccessibility of many pressure relief valves may allow leakage to
become substantial.
Obviously, the controlling factor in preventing leakage from valves is
maintenance. An effective schedule of inspection and preventive maintenance can
keep leakage at a minimum. Minor leaks that might not be detected by casual
observation can be located and eliminated by thorough periodic inspections. New
blind designs are being incorporated in refinery pipeline systems in conjunction
with flow valves. This is done to ensure against normal leakage that can occur
through a closed valve.
Emissions from pressure relief valves are sometimes controlled by manifold-
ing to a vapor control device. Normally, these disposal systems are not designed
exclusively to collect vapors from relief valves. The primary function of the
system may be to collect off gases produced by a process unit, or vapors released
from storage facilities, or those released by depressurizing equipment during
shutdowns.
Another method of control to prevent excessive emissions from relief valve
leakage is the use of a dual valve with a shutoff interlock. A means of removing
and repairing a detected leaking valve without waiting until the equipment can
be'taken out of service is thus provided. The practice of allowing a valve with
a minor leak to continue in service without correction until the operating unit
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- 51 -
is shut down for general Inspection is common in mny refineries. This practice
should be kept at a minimum.
A rupture disc is sometimes used to protect against relief valve leakage
caused by excessive corrosion. The disc is installed on the pressure side of
the relief valve. The space between the rupture disc and relief valve seat
should be protected from pinhole leaks that could occur in the rupture disc.
Otherwise, an incorrect pressure differential could keep the rupture disc from
breaking at its specified pressure. This, in turn, could keep the relief valve
from opening, and excessive pressures could occur in the operating equipment.
One method of ensuring against these small leaks in rupture discs is to
install a pressure gage and a small manually operated purge valve in the system.
The pressure gage would easily detect any pressure increases from even small
leaks. In the event of leaks, the vessel would be removed from service, and the
faulty rupture disc would then be replaced. A second, but less satisfactory
method from an air pollution control standpoint, is to maintain the space at
atmospheric pressure by installing a small vent opening. Any minute leaks would
then be vented directly to the atmosphere, and a pressure increase could not
exist.
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TABLE I
SUMMARY OF HYDROCARBON EMISSIONS
Source Percent of Total
Transportat ion 51-9
Motor Vehicles W-T
Aircraft 1-0
Railroads 1-0
Vessels 0.2
Other 1-0
Fuel Combustion 2-2
Industrial Processes ^'^
Solid Waste Disposal 5-0
Miscellaneous 2o"-5
Forest Fires 6-9
Organic Solvent 9-7
Gasoline Marketing 3-8
Agricultural Burning 5-3
Other 0 -8 _
100.0
Total Emissions - 32.0 X 10 Tons /Year
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TABLE II
INSTALLED COSTS OF STORAGE TANKS (1972)
Cost ($/B)
Type 20,000 BU0,000 BBo~COO B
Cone Roof 2.20 1.85 1.60
Floating Roof
a) Pontoon-Type 2.90 2.20 1.75
b) Double.Deck 3-20 2.80 1-90
Hemispheroids (2.5psi) ^.00 2-90
Spheroids (5 psi) 5-60 ^.00
(15 psi) 6.kQ U.90
Spheres (30 psi) 7-20 6.00
(50 psi) 10.to 8.30
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EVAPORATION LOSS IN THE PETROLEUM INDUSTRY—CAUSES
AND CONTROL
CHAPTER 1—SOURCES OF EVAPORATION LOSS
Evaporation loss is the natural process whereby a
liquid is converted to a vapor which subsequently is
lost to the atmosphere. The liquid may be unconfirmed
or may be enclosed in a container such as an oil-storage
tank. By definition, evaporation loss occurs only when
the vapors reach the atmosphere.
Evaporation loss is common to all branches of the
petroleum industry. Because tanks are used similarly
throughout the industry, sources of loss from tanks are
discussed first. Other sources of evaporation loss asso-
ciated with operational features of each branch are then
considered. To further emphasize true sources of loss,
conditions which can falsely indicate loss are also dis-
cussed.
A. Loss in Storage
Six kinds of evaporation loss occur from petroleum
in storage: breathing loss, standing-storage loss, riling
loss, emptying loss, wetting loss, and boiling loss.
Breathing Loss: Vapors expelled from a tank be-
cause of the thermal expansion of existing vapors,
and/or expansion caused by barometric pressure
changes, and/or an increase in the amount of vapor
from added vaporization in the absence of liquid-level
change, except that which results from, boiling, is de-
fined as breathing loss. The term vapor is used herein
to denote any mixture of hydrocarbon vapor and air.
The term hydrocarbon vapor refers to hydrocarbons in
the gaseous state independent of the presence or ab-
sence of air.
Breathing loss takes place in most types of tanks and
occurs when limits of pressure or volume change are
exceeded. Fixed-roof tanks, herein denoting ordinary
storage tanks designed for only a few inches of water
pressure or vacuum, suffer relatively large breathing
losses. Tanks protected from a loss or gain in heat by
reflective coatings, burying, insulation, or shading ex-
perience less breathing loss. Pressure tanks which
operate at 2| psig or higher, normally experience rela-
tively little or no breathing loss. Variable-vapor-space-
tank systems also normally experience little or no
breathing loss. Floating-roof tanks almost eliminate
vapor spaces, and little or no breathing loss occurs past
the seals.
Standing-Storage Loss: Vapor from tanks, which re-
sults from causes other than breathing or change in
liquid level, is defined as standing-storage loss. For
floating-roof tanks, the largest potential source of stand-
ing-storage loss is attributed to an improper fit of the
seal and shoe to the shell. This condition exposes some
liquid surface to the atmosphere; wind affects this
source of loss. Also, a small amount of vapor may per-
meate through the flexible membrane that seals the
space between the shoes and the roof. The permeation
of flexible membranes, or absorption in liquid seals,
may also be a source of loss from variable-vapor-space
tanks. Other sources of standing-storage loss are vapor
escape from open hatches or other openings, glands,
valves, and fittings.
Filling Loss: Vapors expelled from a tank as a re-
sult of filling, irrespective of the exact mechanism by
which the vapors are produced, is defined as filling loss.
This loss is common to all types of tanks except the
floating-roof tank and closed-system pressure storage,
such as for liquefied petroleum gas (LPG). It occurs
when the pressure inside the tank exceeds the relief
pressure. For fixed-roof tanks, the relief pressure is
low, therefore the filling loss is relatively high. Filling
loss from pressure and variable-vapor-space tanks is
somewhat less because these tanks have added vapor-
storage-capacity. The pressure tank also promotes con-
densation of hydrocarbon vapors during filling.
Emptying Loss: Vapors expelled from a tank after
the liquid is removed is defined as emptying loss. Be-
cause vaporization lags behind the expansion of the
vapor space during such withdrawal, the partial pres-
sure of the hydrocarbon vapor drops. Enough air enters
during the withdrawal to maintain total pressure at at-
mospheric pressure. When vaporization into the new
air reaches equilibrium, the vapor volume exceeds the
capacity of. the vapor space. This increase in vapor
volume causes the expulsion.
Emptying loss is common to all types of tanks ex-
cept the floating-roof tank and closed-system pressure
storage. Fixed-roof tanks are most vulnerable to this
loss. Pressure tanks and variable-vapor-space tanks are
less subject to this loss but will encounter it if the vapor-
storage capacity is exceeded.
In the loading of transportation vessels the definition
of emptying loss is restricted: The transporter con-
siders emptying loss to be only that portion which
evaporates into the vapor space of the tank during the
actual withdrawal, that is, between the opening and
closing of the gages.
Wetting Loss: Vaporization of liquid from a wetted-
tank wall, exposed when a floating roof is lowered by
withdrawal of liquid, is defined as wetting loss. This
source of evaporation loss is small.
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10
EVAPORATION Loss—CAUSES AND CONTROL
Boiling Loss: Vapors expelled from a tank as a re-
sult of boiling of the liquid is arbitrarily defined as boil-
ing loss. Boiling loss may occur from any tank. The
fixed-roof tank is more subject to this loss than the
pressure tank. The earliest floating-roof tank, the pan
type, is especially vulnerable to boiling loss because
heat is readily conducted through the roof directly to
the liquid and no vapor-storage capacity exists under
the deck.
B. Loss in Production
Production entails three operations which contribute
to evaporation loss: gas-oil separation, emulsion treat*
ing, and lease-tank operation.
in gas-oil separation, the oil may be rich in light
components, which later are lost from the usual fixed-
roof lease tank. In a recovery system, butanes and
pentanes may not be completely extracted from the gas
and may be lost. A true evaporation loss occurs when
the gas is flared or vented. In addition to the loss in
crude-oil volume, the API gravity is decreased.
In emulsion treating, heat is applied and released
vapors may be vented. Also, the crude oil reaching the
lease tank at elevated temperature contributes to the
evaporation loss.
At the lease tank, splashing may occur as oil is in-
troduced; in such cases vaporization and evaporation
loss are accelerated. Dark-colored tanks contribute
further to evaporation loss.
C. Loss in Refining
Refining involves three operations which are sources
of evaporation loss: treating and blending in freely
vented vessels, such as an agitator; pressure systems
which may leak; and sewers, ponds, and open sepa-
rators.
Use of air and agitation can result in high-evapora-
tion loss from vessels which are not part of a closed
system. Sweetening naphtha in agitators and blending
volatile components in a semiopen vessel are potential
examples of this source of evaporation loss.
Pressure systems, common to refineries and natural-
gasoline extraction plants, may have sources of evapora-
tion loss from leaking exchangers, glands, valves, and
fittings. Hydrocarbon vapor may leak directly to the
atmosphere. Also, liquid may leak and evaporate rap-
idly if volatile at the operating temperature. Besides
outward leaks, inward leaks of air, such as at pump suc-
tions, are sources of loss because this air becomes at
least partially saturated before venting.
Sewers, ponds, and open separators are sources of
evaporation loss if volatile liquids are permitted to reach
them. Such liquids usually encounter high turbulence
in sewers and collect in thin layers offering large ex-
posures for evaporation. The recovered skimmings
from ponds and separators require demulsification in-
volving heat and constitute another source of loss from
the recovery equipment.
D. Loss in Transportation and Marketing
Transportation includes pipeline shipments and the
loading, transit, and unloading of transport vessels from
which evaporation loss can occur. Pipelines are subject
to loss from air eliminators used in metering systems
and from leaking glands, valves and fittings, and cor-
roded pipes.
Filling and emptying losses occur from nonpressure
tankers, barges, tank cars, and trucks in much the same
manner as from tanks. If tank cars and trucks are top-
loaded with a short filling pipe, the undue splashing not
only accelerates vaporization but also produces small
droplets which may be lost by entrainment. Air in-
spired during loading can be an added source of evap-
oration loss because such air becomes at least partially
saturated with hydrocarbons before it is vented; loosely
connected, submerged loading spouts are sources of
such inspiration.
In-transit losses from transport vessels are essentially
breathing losses. Excessive heating of crude oil in
marine vessels is a potential source of evaporation loss
during transit.
Marketing operations entail many of the previously
discussed sources of loss, particularly those discussed in
Par. A, "Loss in Storage."
E. False Indications of Loss
Under certain conditions a loss appears to have oc-
curred which actually did not. Being aware of these
conditions and being able to distinguish between an
"actual" and a "false" loss is important, otherwise cor-
rective efforts may be directed toward conditions where
no real improvement can result. Conversely, these con-
ditions may balance out and cover up an actual loss,
with the result that necessary corrective action is over-
looked. Five such conditions are: inaccurate measure-
ment, gravitation between tanks, Inaccurate volume of
supply lines, inaccurate calibration of meters, and physi-
cal changes in volume.
Inaccurate Measurement: Apparent gains or losses
can result from inaccurate measurement, either of aver-
age liquid temperature, height of liquid, or incorrect
calibration of containers. Other sources of error would
be the failure to correct all volumes to a common tem-
perature base by use of the unabridged Table 6, "Re-
duction of Volume to 60 F Against API Gravity at
60 F," of the ASTM-IP Petroleum Measurement Tables
(1953)'.
Gravitation Between Tanks: Any leakage past a
valve believed to close a line between two tanks results
in gravitation—a loss of product in one tank results in
a gain in the other tank which may not be observed.
Inaccurate Volume of Supply Lines: If the volumes
of the supply lines are not known accurately, or if lines
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SOURCES OF EVAPORATION Loss
11
are not in the full or empty condition which is assumed,
unrealistic losses or gains may be indicated.
Inaccurate Calibration of Meters: The inaccurate
calibration of meters can result in apparent losses,
which may be attributed incorrectly to evaporation—
or in apparent gains which may conceal actual evapora-
tion losses.
Physical Changes in Volume: Certain processing
operations, such as cracking, polymerization, and the
blending and separation of light and heavy stocks, re-
sult in physical changes in volume even when full cor-
rection is made for changes in temperature. For ex-
ample, in a cracking process, where small molecules are
produced from large ones, the products will occupy a
greater volume than the charge. In a polymerization
process, where large molecules are produced from small
ones, the product volume shrinks. With such volume
changes API gravity always changes but the total
weight, before and after the volume change, is the same.
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CHAPTER 2-FACTORS AFFECTING EVAPORATION LOSS FROM TANKS
The total amount of evaporation loss depends upon
the rate of loss and the period of time involved. Primary
factors affecting the rate of loss are: true vapor pres-
sure of the liquid, temperature changes in the tank tank
outage, tank diameter, schedule of tank filling and
emptyings, tank condition, and type of tank. Satura-
tion and diffusion effects are only a part of the mecha-
nism of the loss and are classed as dependent, or second-
ary, variables. Although quantitative-loss relationships
for the primary factors are not yet available, a fair un-
derstanding based on theory and practice can be gained
by considering the mechanism of loss from fixed-roof
tanks. With such understanding, the advantages of
floating-roof, variable-vapor-space, and pressure-tank
systems will readily be apparent.
A. True Vapor Pressure of the Liquid
True vapor pressure affects the rate of loss because it
is the basic force causing vaporization. It varies with
liquid composition and temperature. True vapor pres-
sure at storage temperature is all important. For hydro-
carbon mixtures, this pressure decreases with evapora-
tion because of the change in liquid composition True
vapor pressure usually is determined from correlations
relating it to Reid vapor pressure (RVP). Such rela-
tjpnships are illustrated in nomograph form in Appen-
The effect of true vapor pressure on rate of breathinc
loss from a fixed-roof tank involves at least two internal
considerations—the saturation concentration and the
diffusion and convection factor. The maximum con-
centration of hydrocarbons which can be present in ex-
pelled vapor, known as the saturation concentration
increases in direct proportion to true vapor pressure It
follows that if vented vapors were fully saturated
evaporation loss would increase rapidly as true vapor
pressure approaches the tank relieving pressure (a boil-
ing condition). However, another mechanism—the
diffusion and convection of hydrocarbon vapor from the
hquid surface through the vapor space is too slow to
fully saturate it. Experience shows that vapors vented
during normal breathing are usually only 80 per cent to
90 per cent saturated. Thus, the driving force to over-
come resistance to diffusion factors and convection
through the vapor space is one of the controlling factors
Such driving force can be looked upon as bein<» the
true vapor pressure of the liquid minus the partial pres-
sure of hydrocarbons in the vapor space. As true vapor
pressure rises, this driving force would rise in direct
proportion if percentage saturation in the vapor space
remains constant. Thus, both the saturation considera-
tion and the diffusion and convection consideration sug-
12
gest that actual loss is at least directly proportional to
rising true vapor pressure.
Filling or emptying losses from fixed-roof tanks are
directly proportional to increasing true vapor pressure
because of the relationship between true vapor pressure
ana saturation concentration.
This concept does not apply when true vapor pres-
sure exceeds the absolute tank pressure because boilin*
occurs and losses may be large. Then, the main con-
trolling factor is heat input.
In terms of total loss over a period of time, the effect
of true vapor pressure depends upon the composition
ot the stock. For example, two crude oils of identical
true vapor pressure may weather at different rates. One
crude oil may contain a relatively high per cent of
volatile propane and ethane; for a specific starting loss
rate, the vapor pressure will drop rapidly and the loss
rate will drop shortly thereafter. The other crude oil
may derive vapor pressure from relatively hi°h con-
centrations of less-volatile pentanes and butanes- for
the same starting loss rate, the vapor pressure will drop
less rapidly but the loss rate will remain higher for a
longer period. This consideration is particularly si°-
nificant for newly produced crude oils at leases an°d
pipeline storage terminals where true vapor pressures
may be close to atmospheric pressure.
B. Temperature Changes in the Tank
Internal temperature changes, brought about by at-
mospheric and solar heat, tend to cause the tank vapor
space to breathe. During the day, heat flowing through
the roof and upper walls raises the vapor temperature
and expands the volume. The pure thermal effect is
augmented by vaporization of hydrocarbons from the
tank contents during the same period. The heat input
also may increase the liquid-surface temperature and
accelerate vaporization. At night, reverse processes
shrink the vapor and cause an intake of air.
Atmospheric and solar heat also cause forced con-
vection in the vapor space which promotes evaporation
from the liquid surface and aids in the dispersion of the
hydrocarbon vapor.
Although efforts have been made to develop more
precise criteria, the average daily change in atmospheric
temperature is still the only accepted way to charac-
terize atmospheric and solar-heat effects. Monthly
meteorological data for various locations in the United
States and Canada are presented in Appendix VI.
Theoretical considerations do not permit good es-
timation of how much loss will increase with increas-
ing atmospheric temperature change; however, it prob-
-------
EVAPORATION-LOSS FACTORS
13
ably will be somewhat less than directly proportional to
the increase in atmospheric temperature change.
C Tank Outage
The volume of most vapor spaces is directly propor-
tional to outage—the height of the vapor space. For a
fixed-roof tank, higher outage means greater loss be-
cause the larger volume will breathe more. However,
when outage is increased heat input is not increased in
direct proportion. Heat enters the vapor space through
the tank wall, the area of which increases in direct
proportion, and through the tank roof, the area of which
remains unchanged. Furthermore, with added height
of vapor space, resistance to transfer of hydrocarbon
vapors from the Liquid surface to the vent increases.
Therefore, the average concentration of hydrocarbons
in vented vapor should fall. Experience has confirmed
that loss will increase less than directly proportional to
increasing outage.
D. Tank Diameter
Tank diameter influences the volume of the vapor
space and the liquid-surface conditions. Breathing is
less than directly proportional to increase in vapor vol-
ume because of the less than proportional increase in
area for heat transfer into the vapor space. Further-
more, increasing diameter should reduce the tempera-
ture rise of the liquid surface because the rising hot
stock, in contact with the tank wall, must spread in a
thinner film over the surface area. Assuming constant
tank height, total breathing loss, therefore, increases at
a rate less than directly proportional to tank volume.
E. Schedule of Tank Fillings and Emptyings
Over a period of time, the frequency of stock turn-
over and the average outage affect total loss. Opera-
tions that promote high outages may result in relatively
high breathing losses. Fillings and emptyings scheduled
to compensate the dally temperature changes may re-
duce breathing loss. The time interval between empty-
ing and filling may have a significant effect on loss. For
a system of tanks connected with vapor lines, simul-
taneously filling one tank while emptying another main-
tains the vapor-storage capacity relatively constant and
filling loss is reduced.
F. Tank Condition
Tank condition is another factor affecting loss rate;
however, quantitative effects cannot be predicted. Open
vents result in high loss when gusty or turbulent winds
cause rapid pressure changes In tanks in which volatile
Liquids are stored. Rapid breathing occurs as short
puffs. Any hole in a tank roof, diaphragm, seal, or ac-
cessory results in the same type of loss.
Where there are two or more openings in the tank,
loss is further increased. Pressure differences, which
result from wind or thermal effects, cause a constant
flow of air through some openings into the vapor space
and an outflow of vapor through other openings.
G. Type of Tank
The type of tank or storage system will affect the
evaporation loss experienced. The amount of loss de-
pends upon the volume of the vapor space available
and the pressure limitations of the equipment.
If tanks have their vapor spaces interconnected,
vapor-space volume can be controlled to a Limited ex-
tent by scheduling fillings and emptyings, where feasible.
If the vapor space is allowed to change volume at
constant pressure, breathing loss can be practically
eliminated and filling loss can be reduced. The extent
of the reduction in loss is dependent upon the amount
of variable vapor space provided.
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CHAPTER 3-TANKS AND EQUIPMENT TO CONTROL EVAPORATION LOSS
The industry may choose from four basic types of
tanks for storage of petroleum and its products: fixed-
roof tanks, floating-roof tanks, variable-vapor-space
tanks, and pressure tanks. Each type is designed for
specific storage requirements, the actual storage prob-
lem should determine the type selected. In many in-
stances, the most economical tank can be selected only
after a detailed study comparing loss from, and the cost
of different tanks. For stocks having a low true vapor
pressure, less than 2 psia, the fixed-roof tank generally
will be the most economical selection. For stocks of
motor-gasoline range of volatility at high throuohputs
the floating-roof tank generally will be the best choice'
but at lower throughputs the variable-vapor-space tank
generally will be better. For stocks which boil at at-
mospheric pressure and storage temperature, pressure
tanks are best generally; however, in some cases use of
the fixed-roof tanks in conjunction with a vapor-recov-
ery system may offer more advantages.
The tendency to boil in storage is a function of vapor
pressure, altitude, barometric pressure, and liquid-sur-
face temperature. Maximum liquid-surface tempera-
tures vary throughout the United States. For the
indicated temperatures, the maximum Reid vapor pres-
sures (RVP) of stocks which can be stored at atmos-
pheric pressure without general boiling (but at the ex-
pense of high-loss rates) are:
Maximum
Liquid-
Surface
Tempera- Maximum
ture • Reid Vapor
(Degrees Pressure •
A"3 Fahrenheit) (Pounds)
West Coast (tempered by Pacific Ocean) . 80 18
Gulf Coast, Atlantic Seaboard, and ° 15'5
northern Middle West .............. IQO 13 j
Mid-Continent area and arid Southwest. .115 n'
120 10
at
Effective loss-control operation of each tank is de-
pendent upon certain accessory items, such as breather
valves and automatic gages. Continued effective opera-
tion is dependent upon a program to maintain the tank
and accessories in a gaslight condition.
Choice of paint color may be an important factor in
reducing loss. In special instances, loss may be reduced
by use of a floating plastic blanket; or, by employin»
insulation, a shading device, water sprays, mechanical
cooling, or by burying the tank. In some cases, it may
be possible to reduce loss further by specially schedul-
ing fillings and emptyings.
A. Fixed-Roof Tanks
The minimum accepted standard for stora°e of
volatile oils is the fixed-roof tank. It can sustain an in-
ternal pressure, or vacuum, of only an ounce or two per
square inch. Being susceptible to sizable breathing and
tiling losses, this type of tank is used most frequently
for services which cannot economically justify a con-
servation tank. }
Design of Tank: The fixed-roof tank, the predecessor
of conservation tanks, came into being durino the early
days of the petroleum industry. Wooden barrels were
used at first, but they could not keep up with the rising
flood of oil that poured from the Pennsylvania wells
As production increased, open pits and diked areas were
used, but they were hazardous. Wooden tanks caulked
with oakum and held together with iron hoops, first ap-
peared in 1861. The capacity of these tanks ranged
between 500 bbl and 1,000 bbl. °
The first iron tank with a wooden, gravelled roof ap-
peared in 1864; it provided larger and safer storage
Shortly after the Civil War, bolted- and riveted-stlel
tanks came into use. They ranged in size up to
35,000 bbl. After 1915 capacities were increased and
in 1919 the first 80,000-bbl tank was erected The in-
troduction of electric welding, in 1923, made possible
the fabrication of welded roofs and bottoms. The
welded tank was introduced in 1927.
Fixed-roof tanks built today usually are welded
throughout, but many riveted tanks are still used and
bolted tanks are common in the smaller sizes. Whereas
the seams of welded tanks are almost inherently <»as-
tight, the seams of bolted and riveted tanks frequently
require additional maintenance. In some areas, particu-
larly on leases where corrosion is a problem, wooden
roofs, which are seldom of gastieht construction are
still in use. Loss from these tanks is much greater than
from steel-roof tanks.
If a fixed-roof tank is found to be the best type for
a particular storage problem, careful consideration
should be g1Vea to the size before the final selection is
made. Because the loss rate increases significantly with
outage and tank diameter, the use of the smallest tank
possible for the given storage requirement results in a
minimum loss. For further insight as to the outage and
diameter effects, refer to Chapter 2, "Factors Affectino
Evaporation Loss from Tanks."
Maintenance of Tank: To maintain a gaslight con-
dition, tanks should be inspected at regular intervals
and repaired as necessary. The frequency of inspections
usually is determined by experience. Riveted-roof
tanks, because of their greater tendency to develop
leaks, should be inspected more frequently than welded-
roof tanks.
14
-------
EVAPORATION-LOSS CONTROL
15
When the tank is under pressure, five ways in which
leaks may be detected are:
1. Observation of heat-wave-like trails of the escap-
ing vapors.
2. Hearing the hiss of escaping vapors.
3. Smelling the vapors.
4. Use of gas testers or "sniffers."
5. Applying soap solution or linseed oil to seams.
Also, stains on the painted surfaces frequently indicate
leaks.
Design of Accessory Equipment: The fixed-roof tank
has several openings in roof for venting, gaging, and
sampling. To maintain a gaslight roof, accessory equip-
ment of a gaslight design must be provided for these
openings.
The accessory for the vent opening is called a
breather valve, pressure-vacuum relief valve, or con-
servation vent. When operating properly, this device
prevents either the inflow of air or the escape of vapors
until some preset vacuum or pressure is developed.
Most breather valves, especially the metal-to-metal
types, allow some leakage below the pressure or vacuum
setting. A tight breather valve is important in reducing
evaporation loss. The actual magnitude of the savings
will depend upon such factors as vapor pressure of
stock stored, weather conditions, paint color, and al-
lowable working-pressure range. However, the savings
realized usually will pay for the installation. The
breather valve also contributes to safe operation by
keeping the tank vent closed to the atmosphere most of
the time.
The pressure and vacuum settings of a breather valve
are dictated by the structural characteristics of the tank
and should be within safe operating limits. A certain
amount of pressure and vacuum beyond these settings
is necessary to overcome pressure drop in order to ob-
tain required flow. Proper size and settings can best be
determined by reference to API Sid 2000: Venting
Atmospheric and Low-Pressure Storage Tanks (1968)
and to the manufacturer's tank data determined in ac-
cordance with this publication. The pressure setting
for vent valves to be installed on large tanks constructed
in accordance with API 12D: Specification for Large
Welded Production Tanks (1957) usually is limited to
+ oz because roof plates will start to shift when the
pressure rises much above 1 02. For small tanks, and all
tanks having special structural features, the pressure
range can be increased in accordance with the manu-
facturer's recommendations.
Breather valves should be designed to give:
1. High-flow capacity at relatively small pressure or
vacuum above the setting.
2. A gaslight seal.
3. Freedom from sticking or freezing.
4. Easy access to all parts for inspection and main-
tenance.
Diaphragm and liquid-seal valves have less leakage
than metal-to-metal types. For dependable service,
diaphragms should be resistant to tank vapors.
Open vents of the mushroom or return-bend type
should not be used on fixed-roof tanks storing volatile
oils as they permit high loss. These vents are merely
hooded openings equipped with protective screens. The
opening is turned down to prevent any blockage by ice
or snow.
Venting accessories sometimes used are: flame ar-
restors, flame snuffers, and flash screens. They usually
have little effect on vapor loss except when they are
installed between the tank and vent valve and must be
removed for cleaning.
Some vapor loss is inherent in manual gaging and
sampling methods which necessitate opening a lank lo
Ihe atmosphere each time these operations are per-
formed. This loss can be minimized through the use of
automatic gaging equipmenl, double-closure gaging
locks, and a syslem of thermometers and sample valves
in the tank shell.
Accessories which help to reduce evaporation loss
from lease tanks include:
1. Pressure-vacuum thief halch and venl-line valve.
2. Automatic-closing valve in the equalizer line which
closes when the gage hatch is opened.
3. A diagonal-slotled downcomer type of fill line to
minimize free fall and splashing.
Maintenance of Accessory Equipment: To maintain
accessories in a gastight condition they should be in-
spected and restored periodically. Pallels of the metal-
to-metal breather valves which become warped in serv-
ice must be machined to restore a gastight fit. Defective
diaphragms of diaphragm brealher valves should be
replaced.
Liquid-seal breather valves may be affected by dilu-
tion or loss of liquid and may have lo be inspected and
mainlained more frequently as determined by experi-
ence. A loose filling gage-hatch lid can be made nearly
gastight by replacing the gasket or by machining the
sealing surface.
Rame arreslors and flash screens can become clogged
with dust, rust, and ice. Such obslruclions in Ihe venl-
ing system can cause severe damage to the tank from
excessive internal pressure or vacuum. These acces-
sories should be inspected and cleaned frequently.
Choice of Paint: Tank painting is imporlant in re-
ducing evaporalion loss as well as in preserving the
lank. An adequate paint program, using reflective
paints, will minimize the heat inpul lo the lank by re-
ducing Ihe metal lemperalure of Ihe lank.
While painl is a simple and effeclive means for re-
ducing evaporalion loss. The addilional low cosl of
mainlaining a clean white surface on a tank frequently
has an attractive economic return. Recenl dala indicale
that painting the tank roof and shell white, rather lhan
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16
EVAPORATION Loss—CAUSES AND CONTROL
gray, reduces the evaporation-loss rate by at least
20 per cent.
Use of Insulation: Insulation on the roof and shell
of a storage tank tends to reduce heat input and heat
loss; this tendency toward constant temperature reduces
breathing loss. However, when stock at higher than
normal storage temperature is added to an insulated
tank, the average tank temperature increases, which in
turn increases true vapor pressure and promotes a
higher concentration of hydrocarbons in the vapor
space. This condition may increase filling loss some-
what and breathing loss to a lesser degree. Insulation
usually is expensive to install and may involve con-
siderable maintenance. Unless moisture is prevented
from entering the insulation, loss of insulation effect as
well as corrosion of the tank shell may result.
Use of Shade: Shading of a storage tank from direct
sunlight reduces heat input, but generally does not re-
duce normal heat loss. Hence, compared to a bare
tank, shading provides less variation in internal tem-
perature and, usually, results in a lower average stock
temperature. Although the installation of shades gen-
erally has not been considered economical, usually the
maintenance is not an expensive factor.
Use of Flexible Blanket on Liquid Surface: Evapora-
tion loss in fixed-roof tanks can be reduced with the use
of flexible blankets which float directly on the liquid
surface. The blanket acts in the same manner as a
floating roof. There are two types of flexible blankets:
one is a foam blanket made up of plastic spheres; the
other is a blanket or raft made from plastic sheeting.
The latter type has not been tested extensively in this
country.
The floating plastic-foam blanket consists of micro-
scopically small, hollow, plastic, gas-filled spheres. This
material has been used extensively on fixed-roof storage
tanks in crude-oil service. Tests made in this country
and Canada have indicated that under favorable con-
ditions evaporation loss on crude oil can be reduced
from 50 per cent to 70 per cent in working tanks and
from 70 per cent to 90 per cent in static tanks. A i-in.
thickness usually is used on static tanks. A 1-in. thick-
ness is used on working tanks to avoid breaking up the
complete foam layer during filling and emptying. Gag-
ing difficulties with the plastic foam have been mini-
mized by the use of a portable type of gage well carried
on a gage tape.
Loss of the plastic spheres can occur if tanks are
pumped to low levels at high pumpout rates. The
operation of mixers must be controlled to prevent the
plastic spheres from being dispersed in the oil and then
pumped out during emptyings.
Not enough information is available to estimate ac-
curately the service life of the spheres. However, re-
ports received indicate that the material has operated
satisfactorily for two and one half years. The spheres
should not be installed in tanks containing liquids sub-
ject to boiling; water also may damage the plastic
spheres, especially, if agitation promotes contact of
moisture with the spheres.
B. Floating-Roof Tanks
The floating-roof tank is an effective conservation
device for stocks of motor-gasoline volatility. The basic
design virtually eliminates the vapor space, which re-
sults in low losses both from breathing and filling. The
exceptionally low losses from filling brought this tank
into widespread use. Other advantages of this tank
are excellent fire protection and corrosion resistance.
Ignition may occur only in the seal area; being con-
fined to this localized area, the fire normally is easy to
extinguish. However, should the stock boil, the fire
may be difficult to extinguish. Excellent protection
from sour or corrosive stocks is afforded by floating
roofs which are in contact with the entire liquid surface.
Design of Tank: There are three basic designs of
floating-roof tanks in operation: the pan type, pontoon
type, and double-deck type.
The first successful floating-roof tank, the pan type,
was built in Gushing, Oklahoma, in 1922. A typical ex-
ample of a pan-type floating roof in operation today is
shown in Fig. 1. A single deck covers most of the
liquid surface and a seal is attached to the rim of this
deck. The deck slopes to the center for drainage.
This roof has three disadvantages which account
for its limited use. The single deck is exposed to the
sun during the middle of the day. Because the deck is
held forcibly in contact with the liquid, heat is trans-
ferred directly to the liquid surface. The liquid-surface
temperature rises appreciably. Sometimes the product
boils and losses of gasoline and similar stocks may re-
sult. The pan roof also may tip and sink under heavy
loads of water or snow, or from leaks which may de-
velop in the deck.
The pontoon roof was developed in 1928 by adding
pontoons to the pan-type roof to give it greater sta-
bility and bouyancy. Thus, the pontoon roof has a
single deck over only a part of the total area, closed
pontoons cover the remaining area. Pontoon-type roofs
are illustrated in Fig. 2 and Fig. 3. The pontoons are
arranged and compartmented to provide floating sta-
bility under heavy loads of water or snow. Enough
bouyancy normally is provided so that the roof will not
sink when the single-deck area leaks or the drain fails.
Properly compartmented, the pontoons can be par-
tially flooded without endangering the roof structurally.
A pontoon roof having a single deck, which can
rise -above the liquid surface when boiling starts, pro-
vides an insulating vapor space and reduces the heat
transfer from the sun to the liquid surface. Boiling
losses usually will not occur with stocks in the motor-
gasoline range o£ volatility.
In the mid 1940's the double-deck roof was offered,
which in effect made the entire roof a series of pon-
toons, see Fig. 4. Circular and radial bulkheads divide
-------
18
EVAPORATION Loss—CAUSES AND CONTROL
Courtesy: Graver Tank and Manufacturing Company, Inc.
FIG. 3—Pontoon-Type Floating Roof.
the space between the two decks into compartments;
this design can give good stability and load-carrying
capacity and provides an insulating air space over the
entire area. Boiling losses usually will not occur with
stocks in the motor-gasoline range of volatility.
The usual seal in a floating-roof tank consists of a
relatively thin-gage shoe or sealing ring supported
against the tank shell around the edge of the floating
roof. The bottom of the sealing ring is below the liquid
surface, and the top is a few inches above the top rim
of the roof. A piece of flame-retardant rubberized cloth
closes the space between the sealing ring and the roof.
Another type of seal consists of a flexible tube, fastened
to the roof and occupying the annular space between
the roof and shell. The tube, rilled with a nonfreezing
liquid, is held on the liquid surface and completely'1
eliminates the vapor space. In tanks with riveted shells,
abnormally high losses of the more volatile stocks oc-
cur because the rivet heads and overlapping steel plates
hold the sealing ring away from the tank shell. Thus,
it is advisable to use a secondary seal on riveted tanks.
This type of seal consists of a strip or loop of rubber
adapted to cover the slot at the top of the sealing
ring. The value of a secondary seal for welded tanks
is uncertain.
Maintenance of Tank: Efficient and safe operation
of any mechanical device which moves intermittently
requires inspection and maintenance at regular inter-
vals; the floating-roof tank is no exception. Shoes must
fit well, seals must be in good condition, the roof should
be level at all times, and the breather valve and bleeder
vent must operate satisfactorily.
Before the tank is put into service the opening be-
tween tank shell and shoe should be minimized by ad-
justing shoe springs or hangers. If the liquid surface
is plainly visible between shoes and shell, after all ad-
justments are made, the tank may be out-of-round
caused by faulty construction or uneven settling. Be-
cause this condition leads to large losses it should be
corrected. Shoe fit should be checked periodically; at
the same time, the above-deck hangers should be serv-
iced to keep them in an operating condition.
The primary and secondary seals should be inspected
periodically for tightness and general condition. Sec-
tions of the primary seal which have deteriorated and
have weakened should be replaced. Holes that appear
in sections of good material may be repaired by patch-
ing. The secondary seals are subject to considerable
abrasive wear and are not amenable to patching, such
worn-out seals should be replaced.
The floating position (the level) of a roof depends
upon the weight of the load supported and how easily
the roof can move up and down. With riveted tanks,
shoes occasionally bear unevenly on the shell of the
tank. Inspection for this condition should be made peri-
odically and shoes adjusted as necessary. After every
rain, drainage from the roof should be checked and
any debris clogging the screened roof drains should be
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EVAPORATION-LOSS CONTROL
19
^
• -/ • • - _^-T»^ ^-
'
Courtesy: Hammond Iron Works.
FIG. 4—Double-Deck Floating Roof.
removed. Although a snow load is not serious for the
pontoon or double-deck roofs it can be with the pan-
type roof. When more than a foot of snow accumu-
lates on a roof of any type, it should be removed, par-
ticularly if it drifts unevenly.
Design of Accessory Equipment: Two accessories
are necessary to the operation of a floating-roof tank:
a breather valve for the rim space and a bleeder vent for
the roof. One breather valve, sometimes two, is pro-
vided at the outer edges of all types of floating-roof
tanks; it is similar in design to that used on fixed-roof
tanks. The bleeder vent for the pontoon and double-
deck tanks allows air trapped under the roof to escape
before the roof floats and prevents a vacuum as the
roof comes to rest on its supports.
Maintenance of Accessory Equipment: The fol-
lowing accessories should be inspected regularly and
repaired as necessary: breather valves (rim vents),
bleeder vents, sample and gage hatches, and any other
openings from which vapors might escape.
Choice of Paint: The value of a highly reflective
paint in reducing evaporation loss from floating-roof
tanks is questionable. Although the floating-roof tank is
a very efficient conservation device, savings effected
through the use of a particular color of paint will be
less than for other types of tanks. Reflective paint on
the pan-type roof may be justified because it will re-
duce the chance for boiling. It is not so important on
the pontoon and double-deck roofs because these roofs
are designed to provide insulating barriers to heat trans-
fer. Reflective paint on the shell of these tanks may
be justified because it may reduce boiling in the seal
area and it will help maintain a lower liquid tempera-
ture throughout the tank. Such reductions are beneficial
particularly for the older riveted tanks where it is dif-
ficult to maintain a good fit between the shoes and
shell.
C. Variable-Vapor-Space Tanks
The variable-vapor-space tank is an effective con-
servation device particularly suited to reducing breath-
ing losses. Expanding vapors are stored temporarily
in a gasholder device and vented to the atmosphere only
-------
EVAPORATION-LOSS CONTROL
25
E. Vapor-Recovery Systems
Design of System: Vapor-recovery systems collect
vapor from storage tanks and send it to a gas-recovery
plant. These systems have sensitive pressure-vacuum
controls and remove vapor as pressures build up dur-
ing pumping into the tanks or during breathing. The
vapor is collected, compressed, and then recovered by
absorption or condensation; with lease tanks, how-
ever, the compressed gas normally is discharged into
an extraction-plant gathering system. A properly de-
signed system should eliminate most of the evaporation
loss, but because of control difficulties the efficiency
actually is somewhat lower.
Refinery or natural gas is sometimes used for re-
pressuring the vapor spaces of tanks when air or a
corrosive atmosphere is undesirable in the vapor-re-
covery system. The vapors are withdrawn from the
tank as the internal pressure increases, and the repres-
suring gas is admitted to the tank when air normally
would be drawn in. Some provision must be made to
prevent collapse of the tanks when insufficient gas
evolves to maintain pressure in the tanks.
Where it is uneconomical to design a tank or a stor-
age system to operate at pressures high enough to
make evaporation loss negligible, various vapor-re-
covery methods can be utilized. To recover vapor by
condensation, one or a combination of four methods
may be used:
1. Absorption may be accomplished in a suitable liquid
of higher molecular weight than that of the vapors be-
ing recovered. This rich oil must be reprocessed if it
is desired to separate the absorbed vapors. The liquid
from which the vapors originally escaped also can be
used as the absorption medium and then the enriched
liquid can be returned to the storage tank without
further processing. Vapor usually is absorbed under
pressure.
2. Compression of the vapors, under suitable tem-
perature conditions, will condense part or all of the
vapors.
3. Cooling, alone or in combination with compression,
can return vapors to the liquid state.
4. Adsorption in suitable material, such as activated
charcoal or silica gel, is a means of collecting the hydro-
carbon vapors if they have been mixed with noncon-
densables, such as air or other gases. Further process-
ing by heat will remove the hydrocarbons from the
adsorbent material; the vapors may then be condensed
to the liquid state by cooling, for return to the tank.
Maintenance oj System: Vapor-recovery systems re-
quire that all the tanks be kept gaslight and that the
instrumentation and fittings be adequately maintained.
Vapor lines should be sloped to a low spot to col-
lect condensate. Condensed vapors and moisture
should be drained periodically from each line. If vapor
lines are underground, the low spot should be in a pit
and it may be necessary to pump the condensate.
F. Other Ways to Control Loss
Special techniques that reduce heat input minimize
evaporation loss. Before one of the techniques is used,
it should be considered in relation to the specific prob-
lem; the reduction in breathing loss anticipated should
be related to the cost of the technique adopted, i.e., that
an economic payout be obtained.
Water Sprays: Water sprays cause cooling due to
absorption of heat to vaporize the water.
Mechanical Cooling: In mechanical cooling, cooling
coils or refrigeration units are used to reduce the ef-
fect of heat input. This technique is probably most
commonly used for condensing vapors.
Underground Storage: In underground storage, the
earth eliminates absorption or emission of radiant en-
ergy to or from the tank. Breathing effects are, there-
fore, greatly minimized. Accordingly, where under-
ground storage is used for any reason, evaporation loss
is minimized. Burying tanks near hot lines should be
avoided.
Schedule o] Tank Fillings and Emptyings: Breath-
ing loss sometimes can be minimized where conditions
permit coordinating tank filling and tank emptying with
the daily breathing cycle. This is accomplished by fill-
ing during a normal period of inbreathing and empty-
ing during a normal period of outbreaking. The com-
pensating effects of breathing will partially cancel out
filling losses. Inbreathing normally begins when the
tank starts to cool in the afternoon; outbreaking nor-
mally begins when the tank starts to warm up in the •
morning.
Pumpings to fixed-roof tanks should be scheduled to
maintain minimum average outage. If volatile stock
accumulates in a group of non-interconnected tanks,
only one of which is a conservation-type, this tank
might be used for daily accumulation; enough stock
periodically would be transferred from it to completely
fill one of the fixed-roof tanks. Another method to
minimize average outage is to refill a tank as soon as
possible after emptying. This procedure also tends to
reduce filling loss. Immediately after a tank is emptied
the vapor space is lean in hydrocarbon. Refilling within
a day expels vapor which is lean in hydrocarbon. A
delay oE three days may nullify most of the advantage.
Evaporation loss from a variable-vapor-space sys-
tem can be minimized by balancing fillings and empty-
ings. Filling into one tank should be scheduled when
vapor expansion is at a minimum or when another tank
is being emptied.
Filling loss from pressure tanks can be minimized by
controlling the fill rate in order to avoid pressure build
up; this allows time for condensation of vapor and
equilibrium is maintained. If heat of condensation
cannot be dissipated as fast as condensation occurs, a
rise in internal temperature will result in a higher in-
ternal pressure.
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SECTION 1.0
INTRODUCTION
The specific objectives of the study were to:
1) Assess the feasibility of applying vapor control
technology for benzene transfer operations includinj
tank:- cars, railcars, barges, tankers, stor-a-j-e • ta-nfcs-,
and pipeline operations.
2) Determine the achievable emission level and emission
reduction for each vapor control alternative.
3) Determine any secondary emissions that would result
from applying each vapor control alternative.
U) Quantify the capital and annualized costs cf the
control alternatives.
Visits were made to the plants of two benzene producers to gather
information on liquid benzene storage and transfer operations. A
literature search was conducted to obtain data on benzene
handling and storage, as well as to investigate technological
aTter"na~ti'v'es to control emissions. This activity was brief
because of the desire to evaluate technolojies that could readily
be applied to industry. Equipment manufacturers were consulted to
determine the state-of-art of commercially available equipment
and__ascertain the effectiveness, cost, and operating history of
their treatment units. Three technologies exhibited promise as
effective nethocs to reduce benzene emissions, and were selected
for further study. These were a refri?eration and lean oil
absorption unit, vacuum regenerated carbon adsorption, and
thermal incineration.
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Hypothetical models were prepared to represent a typical
current-day benzene producer, and two benzene consumers. These
models serve as base cases for the study. Six control schemes
were developed and applied to the base cases. Four were applied
to the producer, and two to the consumers. Each of the three
control technologies discussed above were applied utilizing their
respective achievable emission levels to the control schemes
resulting in 16 case studies. The cost effectiveness of each
case study was calculated, and the technologies rated.
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SECTION 2.0
SUMMARY
The three control technologies evaluated were:
1) Condensation of benzene vapors by refrigeration
followed by absorption of benzene vapors. .Ln-a-n -o-il
absorbing/stripping systea.
2) Carbon adsorption beds regenerated by vacuum.
3) Theraal incineration using supplemental fuel.
Other technologies were considered, but dropped because of lack
of design information and/or cocanercial availability.
The control technologies were evaluated by applying then in
various configrations to hypothetical models which were prepared
to represent facilities and operations typical of current-day
p-ro
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The claimed reaoval efficiencies of the three technologies
studied are all high. The predicted benzene emission concen-
tration levels that are practical to achieve are:
Refrigeration-absorption - 1000 ppm
Carbon adsorption - 10 ppm
Thermal incineration - 10 ppm
The technologies were evaluated using the above emission levels.
The economic penality for installing and operating a thermal
incinerator at 10 rather than 1000 ppm is small. This is not the
case with carbon adsorption and a meaningful economic comparison
of this technology can only be made when it and competing tech-
nologies are evaluated at the same emission concentration level.
Using the above emission levels, refrigeration-absorption has a
cost effectiveness very close to that of thermal .incineration.
Averag.e cost effectiveness of the refrigeration-absorption
systems is $3.83/lb reduction, while that of thermal incineration
is $3.78/lb reduction. (Note: Units used in this report are the
same as used by suppliers of raw data. A metric conversion chart
is contained in Appendix A.) This is a negligible difference. A
slight rise in the value of benzene and/or the cost of natural
gas relative to electricity would make refrigeration-absorption
the most cost effective. Although there is no single component
in_. the—system that is unique; i.e., closed loop refrigeration
vapor scrubbing tower, gas-oil separation by distillation; the
combination of these components into a single package for remote
automatic efficient operation is not yet demonstrated. This
system is thought to need more control and fine tuning than the
other technologies to achieve efficient operation. A great deal
more operating experience would likely be required to make this
technology widely accepted. What makes refrigeration-absorption
particularly attractive is its potential to be the most cost
effective and its conservation of benzene.
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Thermal incinerator technology has been used aore in the control
of storage and transfer emissions than the other two techno-
logies. The transfer of sasoline handling knowledge to benzene
handlinj is much sore direct than that of the other technologies.
The state of the art for thernal incineration is at a high level,
and potential iaprovenents are possible with energy recovery by
heat exchangers. Advantage was-not taken for heat recovery In
the case study models. Also the particular commercially
available thermal incinerators investigated did not offer heat
recovery as a regular option. If heat recovery is a possibility
for any particular plant, theraal incineration would be even more
cost effective. Standard theraal incineration units are
available as "off the shelf" items froa at least two
manufacturers.
Vacuum regenerated carbon adsorption with 10 ppa emissions was
calculated to be the least cost effective means of controlling
benzene emissions but at 1000 ppa emissions aay be coapetitive
with other technologies. On a functional basis, carbon adsorp-
tion stands out as the aost attractive technology. It has a very
high efficiency of benzene recovery and reaoval, relatively
sinple operation well suited for automation, and wide turndown
ranges. Experience with benzene is presently linited to extrapo-
lation_of results gained from gasoline service with gasoline con-
taining benzene. Substantial advanceaent in the state of the art
is expected as aore experience is obtained.
Steaa regenerated carbon systems have wide experience in the
treatment and recovery of solvents from solvent contaainated air
in extreaely dilute concentrations. These units are available
froa several manufacturers as standard package iteas. However,
no experience was found pertaining to benzene, gasoline, or high
concentration hydrocarbon usage. Ho pricing estimates for
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benzene applications of steam regenerated systems were available.
Sone means for disposal of benzene contaminated condensate is
necessary for this type system.
Calculations revealed that there is considerably more benzene
lost as a result of loading and storage (per unit of benzene
handled) by producers than for consumers. The emission factor
for the base case producer is 2.608 lb/10 gallons compared to
.468 for the consumer case. Floating roof tanks represent a high
level of control. (Texas state regulations require floating roof
tanks for the base case.) Conversely if a plant has cone roof
tanks, the first efforts should be directed to reducing storage
losses by conversion to either open floating roof or internal
floating cover depending on their relative cost effectiveness.
Either method is highly cost effective.
When the implementation of carbon adsorption technology is
desired, the most cost effective design will incorporate features
to reduce the capacity (in terms of benzene loading and volu-
metric flowrate) of the individual treatment units, permit higher
ppm emissions, and minimize the number of units required. Capa-
city reducing features might include vapor holders to act as flow
equalizers and displacenent of vapor from tank to tank or -carrier
to tank. The additional cost due to capacity reducing measures
will. b_e_jnore. .than offset by the savings in capital costs of the
carbon adsorption units. Capacity reducing measures do not
provide similar cost effectiveness gains for refrigeration-
absorption and thermal incineration technologies. The increased
cost of the capacity reduction measures outweighs the cost
savings obtained by reducing the size and number of treatment
units.
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SECTION 3.0
CONCLUSIONS AND RECOMMENDATIONS
Conclusions and recoraaendations are:
1) It is concluded that thermal incineration offers the
best aeans for control of benzene vapor to lev"! =_c.f.. 1_3
ppo benzene. The risk in applying this technology to
benzene service is considered to be low. Thermal
incineration systeas hava the distinct advantage of
being able to dispose of other pollutants.
2} Theraal incineration at the level of 10 ppa benzene
aaission and rsfr igeration-adsorpt ior. at 1000 pea are
equal in cose effectiveness.
3) Carbon adsorption is not as cost effective as cheraal
incineration when both are compared at 10 pen.
**) The ccst of carbon adsorption is sensitive to final
benzene eaission level and a true cost comparison to
other technologies can only be aade when all tech-
. .oologies are evaluated at the sane eaission level.
5) Benzene emission control efforts are more cost effective
in producer racher than consumer facilities. Plants
with cone roof storage tanks should receive attention
before those using floating roof tanks. When the
producer plant is equipped with floating roof tanks, the
priority shifts to controlling the loading losses.
6) Modifications to carriers to reduce transit lo'sses
(defined =s breathing losses during shipaer.t) should
receive the lowest priority. Modifications to carriers
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should be limited to those which are required to reduce
loading losses.
7) Secondary emissions for the control systems evaluated
were low, and do not present a significant problem.
8) Air-benzene mixtures in pipe lines to recovery systems
introduce significant explosion hazards, and designs
must incorporate equipment to avoid this hazard. (This
was done for designs evaluated in this report.)
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VII. MOBILE SOURCE/CO
(ORAL PRESENTATION)
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Oxidation of CO in the Exhaust Gas
Exhaust manifold air injection, thermal reactors, and catalytic con-
verters all control CO emissions by oxidizing CO in the exhaust to COp-
The gas temperature, oxygen concentration, catalyst parameters and CO con-
centration are the important operating variables. Secondary air injection
and temperature control are often required. Two kinds of thermal reactors
have been developed for automotive (gasoline soark ignition) engines: the
Rich Thermal Reactor (RTR) for fuel rich air/fuel ratios and the Lean
Thermal Reactor (LTR) for lean ratios. The thermal reactor is a container
which, by its size and configuration, increases the residence time and
turbulence of exhaust gases, thereby providing a chamber for the high-
temperature oxidation reaction. High temperatures are maintained by the
exothermic oxidation of CO and HC in the exhaust gas. The rich thermal
reactor operates at temperature from 870 to 1,040°C (1,600 to 1,900°F) and
is designed for fuel rich operation. At rich air/fuel ratios of 11-12 to
1, NO emissions are reduced to less than 6 g/kwhr (4.5 g/hphr), but fuel
A
consumption penalties are incurred. Secondary air injection is normally
injected into the thermal reactor for complete oxidation, and construction
materials such as Inconel 601 are needed for the inner core, bafflers and
port liners. Temperature control devices are required to protect the
reactor construction materials against overtemperature.
Reference: Control Techniques for Carbon Monoxide Emissions, EPA-450/3-79-006
(June 1979).
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The lean thermal reactor operates at higher air/fuel ratios (17-19
to 1) and lower operating temperatures, 760-870°C (1,400 to 1,600°F), than
the rich thermal reactor. Secondary air-injection is not usually required
and construction materials have less severe durability requirements than
do the materials for rich thermal reactors. Oxidation catalysts and 3-way
catalysts are being used extensively in the control of CO from automotive
engines. This CO control strategy can be equally effective in the control
of CO from stationary engine sources. Recent literature describes a
patented platinum catalyst on a ceramic honey comb support that has with-
stood 50,000 hours of stationary engine testing. The catalytic converter
has also been used for small Diesel, LP gas, and gasoline engines in sizes
up to 13.1 litres (800 cu in.) displacement and is applicable to 2- and 4-
cycle naturally aspirated or turbocharged engines. Applications include
Diesel powered mining and tunneling equipment, locomotives, loaders, fork-
lift trucks operated in enclosed spaces, and electric generators located
near airconditioning intakes. For oxidation catalysts to be an effective
means of controlliIng CO and HC emissions, the engine must be properly
tuned and unleaded fuel must be used. Also, the control system should
ideally be adjusted to preclude the formation of sulfate emissions which
can be formed in the catalyst due to excess oxygen in the exhaust gases and
sulfur content of the fuel. Alternatively, sulfur can be removed from the
fuel. In the case of 3-way catalysts, rich mixtures are conducive to the
formation of HCN and ammonia.
Air injection into the exhaust manifold can reduce CO emissions by a
factor of 55 percent from baseline emissions on some engines with modifica-
tions to the air/fuel ratio, compression ratio, and spark ignition timing
schedule.
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Design Changes and Operating Adjustments
The air/fuel ratio is the operating variable that determines CO emis-
sions, and it has a significant effect on NO emissions. Operation at
A
air/fuel ratios that produce low CO emissions can produce high or low NO
/\
emissions depending on the exact value of the air/fuel ratio used. Since
NO emissions from stationary reciprocating internal combustion engines
are considered more of a problem than CO emissions, design and operating
changes are expected to be made in these sources primarily for NO control.
J\
Care must be taken to ensure that the entire emission control system
provides adequate control of all emissions that need to be controlled.
This sometimes leads to more sophisticated systems.
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VIII. EMISSION CONTROL/ENVIRONMENTAL ANALYSIS
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SECTION 1
SUMMARY
The Industrial Environmental Research Laboratory is responsible for
performing the research and development required to assess the impacts of
pollution from a variety of industries and to evaluate the applicability
of various control technologies for these industries. Pollution control
options must be evaluated for efficiency, reliability, economics, and
energy consumption. If secondary pollutants are generated by the cleanup
of the original pollutant, their impact must also be assessed. This
report on acrylonitrile plants addresses these aspects of control
technology evaluation.
The purpose of the report is to provide data for making decisions
about control technology. Control technologies are identified and ranked
in terms of efficiency, cost, and energy requirements. Control technology
demonstration opportunities in the acrylonitrile industry are also
identified.
There are six operating acrylonitrile plants in the U.S. Each has
several air pollutant emission sources. The effluent streams addressed in
this report are:
e The absorber vent gas stream
• The liquid waste streams that go to the holding ponds and
deep-well ponds
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e The HCN and acetonitrile incinerators and their off-gas streams
0 The reactor startup emission streams
The absorber vent gas stream, when unabated, emits large quantities
of hydrocarbons. Thermal incineration is used for abatement of this
stream at one acrylonitrile plant, and catalytic incineration is used at
another plant. Data for these streams and their abatement by the
incineration processes were available from EPA contractors. Using these
data, the effectiveness of catalytic and thermal incineration was
evaluated. A quick review of the literature showed other methods to be
unsuitable: carbon adsorption because the pollutants are too low in
molecular weight, and hydrocarbon absorption because the stream.is too
dilute. It was concluded that thermal incineration with waste heat :
recovery is the best method for abatement of this stream; catalytic
incineration has a high unburned-hydrocarbon passthrough rate.
High levels of hydrocarbon emission occur from the holding ponds.
There are no reasonable pollution control technologies for open ponds, but
there are control technologies for hydrocarbon removal from waste water on
its way to the ponds. A review of studies and demonstration projects on
solvent extraction of organic nitrogen containing waste waters was made.
In addition, a patent for changing the acrylonitrile processing to
eliminate water scrubbing of the product was reviewed. This would also
eliminate most of the waste water production. These methods are still in
the research and development stage, and conclusions about their efficacy
cannot be drawn.
All acrylonitrile plants have HCN and acetonitrile thermal
incinerators; the emissions data available (from other EPA contractors)
for the exit streams from these incinerators showed 0.6 percent conversion
1-2
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of fuel nitrogen to NO . A review of the combustion literature revealed
A
that 20 to 80 percent of conversion of fuel nitrogen to NO could be
expected. (This discrepancy should be resolved by further study.)
Catalytic incinerators were evaluated as replacements for the existing
incinerators; a literature review shows that similar levels of NO
production could be expected.
When reactors at acrylonitrile plants are started up, the emissions
from these reactors are vented directly to the atmosphere. To control
this intermittent pollution stream, which contains up to 10,000 Ibs of
acrylonitrile per reactor per emission, flares and carbon adsorption were
evaluated. Flares (and other combustion methods) form unacceptable
amounts of NO . Carbon adsorption, and wet scrubbing followed by carbon
rt
adsorption, appear to be more effective.
This report presents the following conclusions:
• Absorber vent stream: Thermal incineration is an acceptable
and efficient control method. .Thermal incinerators are
currently in use, and no further development is required.
e . Holding pond: Extraction of hydrocarbons from the waste water
before it is sent to a holding pond is the most desirable
control method.. Bench and pilot-plant scale research on carbon
adsorption and hydrocarbon absorption (solvent extraction) is
recommended. A literature review of the waste water control
methods in use in Europe (e.g., the Montecatini plant) is also
recommended.
e Hydrogen cyam'de/acetonitrile incinerators: Investigation of
the NO production of the existing incinerators is
recommended. Threre is a potential for high levels of N0x
1-3
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emissions from these incinerators. A feasibility study of
advanced incineration techniques — two-stage (low N0x)
thermal and catalytic incinerators — is also recommended.
• Startup emissions: A study of the feasibility of routing
startup emissions to the absorber tower for scrubbing and a
demonstration of a combined wet-scrubber and'carbon adsorption
abatement technique are recommended.
. 1-4
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