U. S, OF HEALTH, AND WELFARE
Service
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AIR POLLUTION
AND THE
KRAFT PULPING INDUSTRY
An Annotated Bibliography
by
Paul A. Kenline
Jeremy M. Hales
Field Studies Branch
Robert A. Taft Sanitary Engineering Center
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Division of Air Pollution
November 1963
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The ENVIRONMENTAL HEALTH SERIES of reports was established
to report the results of scientific and engineering studies of man's en-
vironment: The community, whether urban, suburban, or rural, where
he lives, works, and plays; the air, water and earth he uses and reuses;
and the wastes he produces and must dispose of in a way that preserves
these natural resources. This SERIES of reports provides for profes-
sional users a central source of information on the intramural research
activities of the Centers in the Bureau of Disease Prevention and En-
vironmental Control, and on their cooperative activities with State and
local agencies, research institutions, and industrial organizations. The
general subject area of each report is indicated by the letters that
appear in the publication number; the indicators are
AP - Air Pollution
RH - Radiological Health
UIH - Urban and Industrial Health
Reports in the SERIES will be distributed to requesters, as supplies
permit. Requests for reports in the AP SERIES should be directed to
the Air Pollution Technical Information Center, National Center for Air
Pollution Control, Public Health Service, U. S. Department of Health,
Education, and Welfare, Washington, D. C. 20201.
Public Health Service Publication No. 999-AP-4
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CONTENTS
Page
ABSTRACT v
INTRODUCTION 1
DESCRIPTION OF THE KRAFT PULPING PROCESS 2
SURVEY OF PULP MILL PROCESS EMISSIONS 4
Digester Blow System Emissions 4
Smelt Tank Emissions 5
Lime Kiln Emissions 5
Recovery Furnace Emissions 5
Multiple Effect Evaporator Emissions 6
MEASURES FOR CONTROLLING EMISSIONS 7
Control of Particulate Emissions 7
Control of Gaseous Emissions 8
INDEX TO BIBLIOGRAPHY 9
ANNOTATED BIBLIOGRAPHY 15
SELECTED PAPERS 77
111
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ABSTRACT
Since the first kraft mill came into existence in 1891, the potential odor
problem has been well recognized. Accordingly, a great deal of literature
has been published describing practical and theoretical work in the field
of kraft mill odor control. Some progress has been made, but even today
most kraft mills are faced with serious problems of atmospheric pollution.
The following work provides an annotated bibliography of articles
concerning measurement and control of kraft mill air-borne pollution
currently available in the literature. It is the authors' intention that this
should provide a retrospective view of what has been accomplished and
should form a sound basis for further work in the field. In addition to 154
abstracts, this bibliography contains a description of the kraft pulping
process, a survey of mill emissions, and a consideration of control measures.
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AIR POLLUTION
AND THE
KRAFT PULPING INDUSTRY
INTRODUCTION
Cellulose, produced by pulping processes, is one of the most versatile
and useful products of modern technology.
The production of pulp in the United States amounted to almost 22
million tons in 1957. This total was produced by the following processes:
Sulfate (kraft), 12 million tons; mechanical, 3 million tons; sulfite, 2%
million tons; soda, % million tons; all others, 4 million tons.
Thus, over half of the pulp produced in the United States comes from
sulfate, or kraft, processing.
When the first kraft mill was built in 1891, the operators were well
aware of the air pollution problem involved. The first papers concerning
kraft mill odor problems appeared in the literature around 1900. Today,
despite the progress in recent years, most kraft pulp mills are still living
with this problem.
The locations of kraft mills in the United States, and their capacities
are shown in Figure 1 and Table 1, respectively.
Figure 1 . Location of sulfate (kraft) mills in the United States.
I
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AIR POLLUTION
Table 1. SULFATE PULP MILLS IN THE UNITED STATES
ALABAMA (7)
"Brew ton 300
Cooso Pines 300
Demopolis 350
Tusca loosa 500
Mobile 1,015
Mobile 550
Naheola 310
ARIZONA (1)
Snowfioke 150
ARKANSAS (5)
Crasseit 575
Crossett 80
Pine Bluff 170
P,ne Bluff 745
CALIFORNIA (1)
Antioch 215
FLORIDA (9)
Foley 800
Jacksonville 500
Jacksonville 1,370
Polatka 810
Panama City 1 620
Pensacola 580
Pensacola 450
Port St Joe 1,200
GEORGIA (8)
Augusta 350
Brunswick 525
Mocon 700
Rome 725
St Marys 800
Savannah 2,100
Voldosta 700
Port Wentworth 625
IDAHO (1)
Lewtslon 500
LOUISIANA (6)
Bastrop 580
Bogalusa 1,180
Elizabeth 240
Hodge 500
Sprmghill 150
West Monroe
MAINE (3)
Lincoln 180
Old Town 140
Rumfard 350
MARYLAND (1)
Luke 515
MICHIGAN (2)
Filar City 165
Muskegon 125
MINNESOTA (2)
Cloquet 130
International Falls 125
MISSISSIPPI (3)
Lumberton 100
Moss Point 650
Natchez 900
MONTANA (!)
Missoula
NEW HAMPSHIRE (]}
Berlin 450
NEW YORK (1)
Ticonderoga 150
NORTH CAROLINA (4)
Acme 700
Canton 930
Plymouth 950
Roonoke Rapids 650
OREGON (4)
Albany 250
St Helens 375
Springfield 400
Toledo 600
Roaring Spring 120
Spring Grove 190
Tyrone 140'
Williamsburg 85
SOUTH CAROLINA (3)
Catawba 400
Charleston 1,250
Georgetown 1,455
TENNESSEE (2)
Calhoun 450
Counce 500
TEXAS (3)
Evadale 360
Lufkin 300
Pasadena 670
VIRGINIA (4)
Covington 660
Franklin 690
Hopewell 800
West Point 600
WASHINGTON (8)
Camas 700
Everett 80
Everett 325
Longview 825
Longview 575
Port Townsend 420
Tocoma 400
Wallula 200
WISCONSIN (2)
Kaukouno 250
Mosmee 165
DESCRIPTION OF
THE KRAFT PULPING PROCESS
Pulp wood can be considered to have two basic components, cellulose
and lignin. The fibers of cellulose, from which the pulp is made, are bound
together in the wood owing to the presence of the lignin. To render cellulose
usable for pulp manufacture, any chemical pulping process must first re-
move the lignin.
The major discriminating factor of the kraft process (Figure 2) lies
in its utilization of a solution of sodium sulfide and sodium hydroxide in
water to dissolve lignin from wood. This liquor is mixed with wood chips
in a large, upright pressure vessel, known as a digester, and cooked for
about 3 hours with steam at a gauge pressure of approximately 110 pounds
per square inch.
During the cooking period the digester is relieved periodically to reduce
the pressure build-up of various gases within.
When cooking is completed, the bottom of the digester is suddenly
opened, and its contents forced into the blow tank. Here the major portion
of the spent cooking liquor containing the dissolved lignin is drained and
the pulp enters the initial stage of washing. From the blow tanklthe pulp
passes through the knotter, which removes the chunks of wood not broken
down during cooking. It then proceeds through various intermittent stages
of washing and bleaching, after which it is pressed and dried into the fin-
ished product.
A major reason for the economic success of this type of pulping oper-
ation lies in its ability to recover most of the chemicals from the sn
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Kraft Pulping Industry
PULP ••
WOOD CHIPS
MUD (CALCIUM CARBONATE)
KILN GASES
Figure 2. The kraft pulping process.
cooking liquor for re-use in subsequent cooks. The recovery process is in-
itiated by introducing the spent ("black") liquor from the blow tank into
a multiple effect evaporator where it is concentrated into a mixture with
a density of about 25° Baume! The spent (or black) liquor, is further concen-
trated in a direct contact evaporator, which, by bringing the liquor into di-
rect contact with recovery furnace flue gases, evaporates an additional
portion of water.
The combustible, concentrated, black liquor thus produced is then
forced through spray nozzles into the recovery furnace, where it is burned
to recover a portion of the heat by oxidation of the dissolved lignin and
to conserve the inorganic chemicals, which fall to the floor of the furnace
in a molten state.
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4 AIR POLLUTION
The resulting melt, which consists mainly of a mixture of sodium sul-
fide and sodium carbonate, is withdrawn from the furnace and dissoivea
with water and weak liquor from the causticizing plant. The "green liquor
thus produced is pumped into a causticizer, where the sodium carbonate
is converted to sodium hydroxide by the addition of calcium hydroxide. The
calcium carbonate resulting from the reaction precipitates from the solution
and is collected and introduced into a lime kiln, where it is converted to
calcium oxide. This is slaked to produce calcium hydroxide for further
use in the causticizer.
The effluent solution produced by the causticizing reaction with the
green liquor contains sodium hydroxide, sodium sulfide, and smaller quan-
tities of sodium sulfate and sodium carbonate. Known as "white liquor,"
this solution is withdrawn and re-used in the digestion process.
SURVEY OF PULP MILL
PROCESS EMISSIONS
Digester Blow System Emissions
The major pollutants from the blow system(Tables 2 and 3)are organic
mercaptans and sulfides with lesser amounts of hydrogen sulfide. Sulfur
dioxide and particulates are negligible.*
Toble 2. BLOW SYSTEM EMISSIONS PRIOR TO TREATMENT
Digester
cooking
condition s
Cook time 3.75 hr
Temp 172 °C
Sulfidity 22%
Unknown
Hydrogen
sulfide
Methyl
mercapton
Dimethyl
sulfide
Ib /ton dry pulp
0.45
0 to 0.10
2.50
0 to 0.88
1.37
0.01 to 0.94
Ref
66
18
Table 3. GAS CONCENTRATIONS IN BLOW SYSTEM EMISSIONS
Hydrogen sulfide
Methyl mercaptan
Dimethyl sulfide
Dimethyl di sulf ide
Su I fur dioxide
Concentration, mg per liter
OtoO.4
3to72
0,5to35
0.2
0
* The values in Tables 2 through 8 were calculated from data in the Bibliography and from the
forthcoming PHS Report, The Study of Air Pollution in the Interstate Region of Lewiston Idaho
and Clarkston, Washington. These are rough estimates and not typical of a particular mill '
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Kraft Pulping Industry
Smelt Tank Emissions
Emissions from the smelt tank (Table 4) are primarily participate.
Uncontrolled particulate emissions may run about 20 pounds per ton of
pulp. Use of a simple water spray may reduce this figure to 5 pounds per
ton of pulp, while a mesh demister may further reduce this to 1 or 2 pounds
per ton of pulp. Thus an average 600 tons per day mill may emit 6 tons of
particulate if uncontrolled.
Table 4. PARTICULATE EMISSIONS FROM SMELT DISSOLVER TANKS
Influent particulate
loadings, ton/day
5.6
Efficiency of
mesh demister
80%
Effluent particulate
emissions, tons/day
1.1
Lime Kiln Emissions
The lime kiln is a major source of particulate emissions (Table 5);
gaseous emissions are not so pronounced- Particulate emissions reportedly
range from 5 to over 15 grains per cubic foot. This may represent up to 10
or more tons emitted per day. Use of scrubbers reduces these emissions to
about 0.5 grain per cubic foot, roughly 1 ton per day.
Table 5. PARTICULATE EMISSIONS FROM LIME KILNS
Particle concentration,
grains/ft-^
Particulate emissions,
Ib /ton dry pulp
0.86
18.7
Recovery Furnace Emissions
The recovery furnace is a major source of particulate and gaseous
pollution (Tables 6 and 7).
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AIR POLLUTION
Table 6. HYDROGEN SULFIDE AND PARTICULATE EMISSIONS FROM RECOVERY
FURNACES
Pollutant generated by
furnace (with primary
stack gas scrubber),
Ib /ton dry pulp
Hydrogen sulfide:
3.59
Particulates:
150
Final emissions with
venturi scrubber,
Ib /ton dry pulp
0.72
12
Final emissions with
electric precipitator,
Ib /ton dry pulp
3.59
7
Electrostatic precipitators remove 85 to 97 percent of the particulate. Ven-
turi scrubbers remove 60 to 95 percent of the particulate and some of the
gases.
Table 7. RECOVERY FURNACE EMISSIONS
Pollutant:
Emission:
Particulate
Particulate
Hydrogen sulfide
Methyl mercaptan
Dimethyl sulfide
Sulfur dioxide
10 to 40 tons / day
100 to 400 Ib /ton of pulp
130 to 935 ppm
60 to 1,400 ppm
125 ppm
1 to 350 ppm
Multiple Effect Evaporator Emissions
Table 8. EMISSIONS FROM MULTIPLE EFFECT EVAPORATORS WITH AND WITHOUT
PREVIOUS BLACK LIQUOR OXIDATION
Pollutant
Hydrogen
sulfide
Methyl
mercaptan
Dimethyl
suifide
Potential
emissions,
Ib /ton dry pulp
1.2
0.04
Oxidation tower
efficiency
95 to 100%
95%
10%
Emissions from
evaporators,
Ib /ton dry pulp
0 to 0.06
0.003
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MEASURES FOR
CONTROLLING EMISSIONS
Control of Participate Emissions
Attempts to curb the smoke and odors issuing from pulp mills have
been nearly as long lived as the kraft process itself. This is exemplified
by an article in a 1922 journal describing a process wherein the smoke
from the mill is passed through a pile of wood chips or straw to remove
both the odors and the particulate matter.
Subsequent developments have included more refined techniques of
collection. One such technique, electrostatic precipitation, utilizes an elec-
tric field to effect the collection of charged, air-borne, particulate matter.
Particulates can be removed from gaseous streams with efficiencies of
greater than 95 percent with such equipment.
The cyclone is another unit of equipment successfully applied to re-
move particulate matter from effluent streams, though it is generally used
in conjunction with other removal equipment because of its low efficiency
for small particulates. Cyclones effect a centrifugal force on suspended
particles by causing the carrier gas to spin in a tight, helical path. The
particles move outward, hit the peripheral walls of the unit, and fall out
of the stream into a hopper below.
Various modifications of the cyclone have been put into successful
operation. One utilizes a continuous spray dispersed inside the unit to in-
crease efficiency by impinging air-borne particles on liquid droplets. These
units, known as wet cyclones, have frequently been used in kraft mills
as flue gas scrubbers. Here black liquor is dispersed as the scrubbing agent,
and simultaneously concentrated by evaporation. The efficiencies of cy-
clones vary over a wide range and are considerably lower than those norm-
ally expected from electrostatic precipitators.
Filtration provides a third effective means of particulate removal, but
has been used only to a limited extent in kraft mill air pollution control.
Particulate emissions from lime kiln and dissolver stacks have been ef-"
fectively reduced, however, by use of mesh demisters, which are screen
filters located inside the stacks.
Venturi scrubbers constitute a fourth type of removal equipment. These
are employed mainly in cleaning the voluminous gas streams issuing from
the furnaces and lime kiln. Scrubbers of this type force the gases at high
velocity through the venturi, simultaneously injecting a spray of scrubbing
liquor at the throat. Particles of suspended matter are impinged upon the
spray droplets, which are in turn collected by a cyclone at the outlet of
the unit. Standard venturi scrubbers may attain efficiencies of 85 percent,
7
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8 AIR POLLUTION
and recently developed, steam-atomized types have been observed capable
of efficiencies slightly higher.
The major disadvantage of the venturi scrubber is its high power re-
quirement. This is offset somewhat by its ability to recover a portion of
the heat and odorous gases from effluent streams as well as by its lower
maintenance requirements. Electrostatic precipitators, on the other hand,
require little power for their operation and have slightly higher efficiencies.
They are ineffective as gas removers, however, and require greater "down"
times for maintenance.
Installation of collection equipment is made attractive to pulp mill own-
ers from an economic as well as from an air pollution control standpoint.
It has been estimated that the amount of particulate salts issuing from the
stacks is about 150 pounds per ton of pulp produced. For a 100-ton-per-day
mill, a collection system operating with 90 percent efficiency will save
approximately $200 per day by recovering these chemicals—a substantial
amount, which should rapidly pay off any reasonable investment in such
equipment.
Control of Gaseous Emissions
Owing to the high virulence of trace quantities of pulp mill pollutant
gases as well as to their capability of issuing from many points within a
mill, the control of gaseous emissions has been recognized as a much greater
problem than that of air-borne particulate matter. Attempts to solve this
problem have generally followed one of two paths: (1) Chemical conversion
of odorous sulfur compounds into ones that are not so objectionable, and
(2) retention of sulfur compounds within the system.
Among the first developments to become popular in commercial ap-
plication was the black liquor oxidation process. This involves oxidation of
the sulfur compounds in weak black liquor by contacting it with air prior
to its admission to the multiple effect evaporators. Oxidation effects a con-
version to less volatile compounds, which are less odorous and have less
tendency to escape into the atmosphere.
It is generally accepted that the oxidation process reduces the odor of
evaporator gases and retains in the black liquor the sulfur compounds that
would otherwise be emitted to the atmosphere. This larger concentration
of sulfur compounds results, however, in a tendency for their emission from
the recovery furnace stacks upon combustion of the black liquor. As a
corollary of odor reduction by black liquor oxidation, it is important, there-
fore, to exercise rigid control over furnace operating conditions to minimize
emission of reduced sulfur compounds.
Oxidation of black liquor has several economic implications one of
which is the retention of sulfur in the recovery system and a resulting
reduction in the amount of elemental sulfur needed to maintain a constant
sulfidity in the cooking liquor. Of much greater importance, however,
are the markedly reduced corrosion rates found characteristic of evap-
oration equipment processing oxidized black liquor.
Another method of mitigating odors from sulfur gases is simply to col-
lect these gases and dispose of them by burning. The main problem with
this method arises from tremendous periodical surges of gas that occur
when the digesters are blown. One company has attempted to solve this
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Kraft Pulping Industry °
problem by collecting the gases in a large spherical surge tank that evens
the flow and supplies a continuous stream to the furnace for burning.
Absorption of sulfur gases in liquids has been of limited use, especially
for handling gases from the evaporators. One such unit, called an "Inka
Tower," utilizes a sodium hydroxide solution as an absorbent. This method
has been found effective in deodorizing noncondensable gases from the
multiple effect evaporators.
Oxidation of exhaust gases by catalytic methods has also been pro-
posed, but its use has been limited. Processes designed for catalytic oxida-
tion may be extensively used in the future.
More dubious methods of controlling pulp mill odors have been those
of odor masking and counteraction, which involve introduction of a specific
substance at a selected point within a mill for dispersal with the mill gases,
to render them less noticeable. These methods are attractive to mill owners
because they are simple and inexpensive to incorporate into any pulp mill,
and are not affected by any basic changes in the process.
Results of odor masking and counteraction have been varied and diffi-
cult to evaluate. These methods also have the disadvantage that, in most
cases, nothing is done to eliminate the noisome gases, and any adverse
effect, other than odor, will persist.
INDEX TO BIBLIOGRAPHY
The following index serves as a key to the Annotated Bibliography. The
numbers refer to the correspondingly numbered abstract in the Biblio-
graphy.
For example, for collection and combustion as corrective measures for
digester relief and blow gases, the reader is referred to abstract 35 (among
others), which is an article by A. A. Coleman on "The Combustion of
Noncondensable Blow and Relief Gases in the Lime Kiln."
Digester Relief and Blow Gases
Major pollutants: hydrogen sulfide, methyl mercaptan, dimethyl sulfide,
dimethyl disulfide.
Corrective measures:
Collection and combustion
8, 11, 12, 35, 52, 54, 66, 95
Catalytic oxidation
83, 105, 106
Oxidation in liquor oxidation tower
13, 26, 71, 103, 107, 124
Odor masking and counteraction
63, 64, 85, 89, 116, 12.7, 128, 130, 132, 138, 147, 154
Liquid absorption
64
Multiple Effect Evaporator Gases
Major pollutants: hydrogen sulfide, methyl mercaptan, dimethyl sulfide,
dimethyl disulfide.
Corrective measures:
Black liquor oxidation
5, 10, 11, 13, 16, 19, 22, 23, 26, 28, 31, 33, 37, 38, 39, 44, 46, 53, 54, 59, 71, 77,
79, 81, 95, 102, 105, 121, 130, 131, 141, 142, 144, 145, 146, 148, 153
Collection and combustion
8, 11, 12, 35, 52, 54, 66, 95
Liquid absorption
54
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10 AIR POLLUTION
Catalytic oxidation
54, 84, 105, 106
Odor masking and counteraction
86, 90, 116, 127, 128, 130, 132, 138, 147, 154
Integrated operations
129, 131
Recovery Furnace Stack Gases
Major pollutants:
I. Particulate: fly ash, sodium sulfide, sodium sulfate, sodium oxide, sodium
carbonate.
Corrective measures:
Electrostatic precipitators
6, 9, 10, 14, 24, 50, 61, 81, 103, 104
Cyclones
43, 81, 87, 114, 124
Venturi scrubbers
29, 41, 43, 45, 48, 81, 87, 88, 101, 114, 136, 137
Condensation methods
65
Integrated operations
5, 7, 40, 58, 101, 124
Spray scrubbers
66, 81, 83, 114
II- Gaseous: hydrogen sulfide, methyl mercaptan, dimethyl sulfide,
dimethyl disulfide.
Corrective measures:
Air oxidation
65, 109
Cyclone scrubbers
87
Venturi scrubbers
29, 87, 88, 136, 137
Integrated operations
58, 62, 124, 129
Odor masking and counteraction
62, 63, 86, 89, 114, 127, 128, 130, 132, 138, 147, 154
Spray scrubbers
66, 83
Catalytic oxidation
84, 105,
Lime Kiln Stack Gases
Major pollutants: calcium carbonate.
Corrective measures:
Electrostatic precipitators
81, 120,
Venturi scrubbers
47, 75, 81, 114,
Cyclones
14, 81, 90, 114,
Wet scrubbers
90, 114,
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Kraft Pulping Industry
Dissolver Vent Gases
Major pollutants: sodium carbonate, sodium sulfide.
Corrective measures:
Filtration
20.
Integrated operations
101,
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ANNOTATED
BIBLIOGRAPHY
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ANNOTATED
BIBLIOGRAPHY
1. Adams, D. F. and R. K. Koppe: Application of instrumentation to
pulp mill atmospheric discharges. TAPPI, 41: 366-377. July
1958. Many different types of instrumentation are currently avail-
able for the quantitative and qualitative study of atmospheric pollu-
tants. The importance of obtaining quantitative information con-
cerning atmospheric discharges from the pulping process is cur-
rently being realized. This report describes the available instrumen-
tation applicable to the study of pulping emissions, including
instruments for (1) collection of field and source samples, (2)
laboratory analysis of collected samples, and (3) automatic, con-
tinuous analysis and recording. Instruments having immediate or
potential application in pollution studies are critically reviewed.
A new, versatile, automatic analyzer with potential application in
the analysis of malodorous gases in parts per billion concentration
range is described. A preliminary report on the development of
a gas-liquid chromatographic procedure for the separation of mix-
tures of malodorous sulfur-containing compounds is included plus
62 references
2. Adams, D. F. and R. K. Koppe. Gas chromatographic analysis of
hydrogen sulfide, sulfur dioxide, mercaplans, and alkyl sulfides
and disulfides. TAPPI, 42; 601-605. (1959). In view of analytical
difficulties arising from chemical similarities among sulfur com-
pounds commonly found in pulp mill stream samples, a technique
based solely on physical properties appears desirable. This article
presents such a method, which employs a chromatographic de-
termination to analyze for volatile sulfur gases. This method, with
adequate column packings and carrier solvents, results in highly
specific analyses with little interference from other compounds.
3. Adams, D. F., R. K. Koppe, and D. M. Jungroth: Adsorption samp-
ling and gas chromatographic analysis of sulfur compounds in
waste process gases. TAPPI, 43:602-608. June 1960. A new procedure
for source sampling and analysis of malodorous, sulfur-containing
gases from the kraft pulping process has been developed. The
pollutants are adsorbed and/or condensed on activated silica gel
at minus 78.5 ° C, desorbed under conditions of heat and vacuum,
trapped at minus 195.8 °C, and transferred onto a gas-liquid chrom-
atographic column for analysis in a conventional manner by use
of a rising column temperature technique. Water vapor, more polar
than the malodorous vapors, must be preferentially removed from
the source sample prior to adsorption to prevent chromatographic
elution of less polar compounds. The major portion of the water is
15
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16
AIR POLLUTION
5.
eliminated by condensation at 0°C, and additional water vapor is
subsequently removed on a solid desiccant, such as anhydrous
calcium sulfate.
Anderson, E-: Some gas cleaning problems in pulp and paper mills.
Pacific Pulp and Paper Ind., 13:23-26. Jan. 1939- Problems and
methods of cleaning air-borne effluent streams are discussed. Em-
phasis is placed upon Cottrell precipitators for particulate collec-
tion. A method of destroying odorous gases by passing them through
an oxidation heat exchange tower is described.
Anon: The B & W Tomlinson chemical and heal recovery process.
Paper Mill News, 69:98-99. Nov. 16, 1946. A process is described
that oxidizes weak black liquor to eliminate odorous fumes. The
stabilized liquor is further utilized in a cyclonic evaporator to en-
train fumes of sodium salts from the recovery furnace. Flow chart
included.
GAS TO ATMOSPHERE
WATER AND
CHEMICAL MAKE-UP
Figure 3. Schematic flow diagram of chemical and heat recovery process.
7.
Anon:Five precipitators at Union Bag to eliminate odor and smoke.
Paper Mill News, 69:7. Aug. 10, 1946. An installation of the largest
series of electrostatic precipitator units in the history of the pulp
and paper industry is being completed at the Union Bag and Paper
Corporation's Savannah plant. These units are expected to elimi
nate practically all the smoke and reduce odors to a minimum Cost
of the installation was $500,000.
Anon: How Mead eliminated recovery stack snow at Kinasoorl
Paper Trade J., ,141:48-50. Oct. 7, 1957. Since install;™
r omce installing recovery
-------
Annotated Bibliography '7
furnaces, the Kingsport plant has been plagued with the problem
of soda ash "snow" rising from their stacks and falling on the near-
by business district. This fallout occurred in spite of the electro-
static precipitator, which was about 90 percent effective in recover-
ing ash particles. To combat this problem, a fog scrubber was
purchased that was highly successful in eliminating snow when
placed in series following the precipitator. With this unit, an 88
percent efficiency has been obtained at the expense of only a 0.75-
inch H^O pressure drop. This also enables the carbonate to be re-
claimed. This scrubber has been in operation since 1956, and no
snow problem has arisen since that time.
8. Anon: HOW Weyerhaeuser controls kraft odor with the yaporsphere.
Paper Trade J., 139:12-13. Jan. 3, 1955. Twenty-five-foot spheres
are being used in two of Weyerhaeuser's plants to catch noncon-
densable gases from the kraft process. These two units, located at
Springfield, Oregon, and Everett, Washington, are said to be at
least 90 percent effective in removing odorous gases. Their pur-
pose is to collect the gases and emit them continuously for burning
or chlorination.
9. Anon: Koppers precipitator installed to control air pollution.
Paper Mill News, 81:28. June 9, 1958. The St. Regis Paper
Company's Jacksonville Mill has added a third electrostatic pre-
cipitator to their recovery furnace system. The combination of the
new and older units will effect a 112-ton-per-day salt cake recovery.
10. Anon: A new board mill - Springfield sets new standards. Pulp &
Paper Mag., 23:42. Nov. 1949. A new 150-ton-per-day kraft mill con-
structed by Weyerhaeuser in Springfield, Oregon, has incorporated
several measures to utilize materials! further, and decrease pollution.
Among these is a Bergstrom oxidation tower, which removes a large
portion of the malodorous gases. Here 25,000 cfm of stack gases cir-
culate countercurrentily to 250 gallons per minute of condensate.
The tower is 10 feet in diameter and 28 feet high. It is packed with
4-inch cross-partition rings. The recovery stack gases are passed
through an electrostatic precipitator prior to expulsion into the at-
mosphere. The resulting ash is conveyed dry into the salt cake-
mixing tank. Combination of these with other operations helps the
new mill to utilize a higher percentage of the raw materials and
reduce the emission of pollutants to the atmosphere.
11. Anon: New system controls dust and odor. Pulp & Paper Mag., 33:77.
1959. A gas washer, a black liquor oxidation tower, a gas holder,
and a high temperature furnace have been added to the air pollu-
tion control equipment of the San Joaquin Pulp Mill. This equip-
ment is designed to process 14,000 cubic feet of gas per minute. Its
cost was $600,000.
12. Anon: A new way to reduce kraft odors. Pulp & Paper Mag., 29:122.
Mar. 1955. Weyerhaeuser has developed a new type of gas holder
that collects odorous vapors arising from the kraft process and emits
them in a continuous stream for disposal. Called the "vaporsphere,"
it consists of a 27-foot steel sphere, inside of which is mounted a
gas-tight diaphragm. The diaphragm rises and falls with the rate
of gas flow and thereby evens out the tremendous surges of gas
that arise from the blowing of digesters. From the vaporsphere, the
gases are transferred to a furnace and burned. Weyerhaeuser also
-------
18 AIR POLLUTION
operates a black liquor oxidation process that reduces odors from
the evaporation system. Besides being effective in odor control, this
system has economized the sulfur balance of the process, making
addition of elemental sulfur unnecessary.
13. Anon: Odor abatement in pulp mills. Pulp & Paper Mag. Can.,
53'108 Dec 1952. In 1951 a large-scale experiment was carried out
in the Bloedel, Stewart, and Welch pulp mill at Port Alberni, B. C.
A black liquor pilot oxidation unit was constructed and tested, while
15,000 gallons of oxidized black liquor were being continuously accum-
ulated. This liquor was fed into the recovery furnace, and the efflu-
ent gases were analyzed. Studies indicated that a substantial de-
crease in odor resulted from the substitution of the oxidized liquor.
Also encouraging were the results of studies indicating that digester
relief gases could be rendered innocuous by absorption in the oxida-
tion tower. Plans are currently under way for installation of a full-
scale oxidation unit.
14. Anon: We don't have all the answers. Air Repair, 2 :52-59. Nov.
1952. The over-all problem confronting a pulp mill consists of two
general parts: the fly ash carried in the flue gas from the chemical
recovery furnaces, and odors emanating principally from the wood
digestion system. In the cook, the primary reaction is between the
cooking chemicals and the lignin. The product is soluble in water,
and is washed from the fibers and sent to the recovery furnace.
Electrostatic precipitation was found to remove 90 to 95 percent of
recovery furnace fly ash. Another dust source is the lime-producing
system. Champion Paper and Fibre Company (Pasadena, Texas) has
installed a rotoclone scrubber on their lime kiln at a cost of $40,000.
To combat the odor problem, relief gases are vented and condensed.
The noncondensable gases are destroyed by burning. Reaction prod-
ucts of digestion contain some t^S, Na2S, and mercaptans. Na2S
will yield H^S in presence of an acid. If Na^S is converted to
Na2SC>4 by oxidation it does not form t^S. For this reason Champ-
ion oxidizes their spent liquor. Champion uses scrubbers on vapors
that distill off from the multiple effect evaporators. This article
contains general statements concerning air pollution by M. D. May
of tha Champion Paper and Fibre Company at a public hearing in
Houston, Texas.
15. Anon: West coast steps up water, air controls. Pulp & Paper Mag.,
35:9. May 1, 1961. Portland, Oregon — This session has produced
increased activity in the legislation of controls over air and water
pollution. It appears that the state sanitary authority will have more
power and will have the authority to specify to cities and industry
what must be done, but shall not specify the means to these ends.
The demands specified by these committees are believed by some
to be so stringent as to make the resulting costs exceedingly high.
16. Anon: Western kraft deodorizing mill. Paper Mill News 83-28 Sept
12, 1960. The Western Kraft Corporation has recently installed a
Collins black liquor oxidation unit at their kraft mill in Albany
Oregon. It is too early to estimate accurately the reduction in pollu-
tion potential resulting from the tower, but every indication is given
that a marked improvement has taken place. The Albany Mill is
the fifth establishment to employ this type of tower The others
are at Thilmany, Wisconsin; Bathurst, New Brunswick; Jacksonville,
-------
Annotated Bibliography
19
Florida; and Longview, Washington. The Albany tower is, however,
designed for greater oxidation capacity per ton of pulp than any
of its predecessors.
17. Bergstrom, H.: Pollution of water and air by sulfale mills. Pulp &
Paper Mag. Can., 54:135-140. Nov. 1953. A discussion is given con-
cerning the evolution of odorous substances and their control in
kraft pulp mill operations. Poisoning of mill personnel from efflu-
ent gases has been a problem so serious as to cause death in some
instances. The following table indicates the concentrations of various
compounds that could quickly cause death to humans.
Table 9. RELATIVE VIRULENCE OF PULP MILL GASES
Gas
Hydrogen sulfide
Dimethyl disulfide
Methyl mercaptan
Dimethyl sulfide
mg/ml
1.4
20
20
140
Volume, %
0.1
0.5
1
5.4
Relative
vi ru lence
1
1:14
1:14
1:100
The figures in this table can be better appreciated when it is noted
that hydrogen sulfide has about the same toxicity as hydrogen cyanide.
The most practicable way of neutralizing these substances is by burn-
ing them. This may be done by conveying the vapors into the furnace.
Examples of the quantity and concentration of volatile sulfur com-
pounds produced by a sulfate pulp mill are shown in the following table.
Table 10. QUANTITY AND CONCENTRATION OF VOLATILE SULFUR COMPOUNDS
PRODUCED BY A SULFATE MILL
Source
Digester
condensate
gases
Diffusers
condensate
gases
Evaporators
dry distillation
in smelter
Hydrogen
sulfide,
Mercaptans,
Sulfides,
mg per liter
131
0
230
11.3
76.4
1.3
1,136
71.7
333
405
3.5
2.65
2,288
34.1
0
503
0
0.32
-------
20
AIR POLLUTION
18. Bergstrom, H., and K. G. Trobeck: Sulfur losses in the production
of sulfate cellulose. Translated from Svensk Papper, 48(3):49-54.
Feb. 15, 1945, by the Institute of Paper Chemistry, Appleton, Wis.
The losses of sulfur in the manufacture of sulfate pulp occur mainly
in the condensate; in noncondensable gases from the digester, dif-
fusers, and multiple effect evaporators; and from the recovery fur-
nace gases. Several investigations to determine the sulfur losses at
a given moment in the manufacture of sulfate pulp are reported.
19. Bergstrom, H., and K. G. Trobeck: U. S. Pat. 1,406,581. Process of
utilizing black liquors. This process utilizes liquor oxidation and
subsequent evaporation to dryness by concentration to 80 or 90 per-
cent solids prior to ejection through a flasher for final drying.
STEAM
WASTE LIQUOR
o;
N
5
X
O
c£
0
<
OL
2
$
LLJ
«
1
FLASH
DRYER
SOLIDS ^
FURNACE
Figure 4. Schematic flow diagram of a process for utilizing black liquor.
20. Bernhardt, A. A., and J. S. Buchanan: Recovery of dissolve! vent
stack soda losses. TAPPI, 43:191-A. June 1960. The smelt shatter
system at the Nekoosa Mill was troubled with the entrainment of
small particles in the air issuing from the dissolver stack. For this
reason, the mill installed a collection system consisting of a wire
mesh filter situated so that the vent gases pass through the filter
medium. The droplets impinge on the mesh, grow, and fall back
against the airflow. Cost of the unit was $2,600. Its payback time,
based on salt cake recovery was about 6 weeks.
Table 11. COLLECTION OF SODIUM SULFATE BY MEANS OF A MESH FILTER
Before use of
mi st eh mmotor
After use of
mi st el i minator
Salt cake loss,
Ib per ton of pulp
17.1
0.5
Value,
dol lars per day
68 90
250
-------
Annotated Bibliography 21
21. Bialkowsky, H. W., and G. G. DeHaas: A catalytic oxidation pro-
cedure for determining sulfur compounds in kraft mill gases. Pulp
& Paper Mag. Can., 53:99-105. Oct. 1952. Sources of atmospheric
sulfur loss from kraft mills are reviewed together with various tech-
niques for analysis of these sources. Among these techniques are
the mercury and cadmium salt precipitation methods for differenti-
ating between hydrogen sulfide, methyl mercaptan, and dimethyl
sulfide; the bromine oxidation method of Siggia and Edsberg for
determining alkyl sulfides and disulfides; and the potentiometric
titration method for sulfides and methyl mercaptan. A method for
determination of total sulfur and sulfur dioxide is described in de-
tail. This technique employs a quartz tower operating at 1,000 °C
for the catalytic oxidation of the sulfur compounds to sulfur dioxide.
The sulfur dioxide is further oxidized to sulfur trioxide in a hydro-
gen peroxide solution. The amount of sulfur present is then deter-
mined by resulting changes in pH. A single test requires only about
1 minute, and the apparatus can also be used as a continuous mon-
itoring device.
22. Bialkowsky, H. W., and G. G. DeHaas: Stabilization of Douglas Fir
krafi black liquor. Paper Mill News, 74:14-22. Sept. 1, 1951. Work
on the development of equipment and procedures for stabilization
of black liquor in an oxidation tower is described. Laboratory data
are given describing the surface requirements for complete stabili-
zation at various liquor flow and airflow rates. A full-scale tower was
constructed capable of handling 300 gallons of black liquor per
minute. This installation has resulted in a marked decrease in hy-
drogen sulfide losses, which has effected a corresponding decrease
in air pollution.
23. Blackerby, L. H.: Improving liquor oxidation. Pulp & Paper Mag.,
34:84-85. June 1960. William B. West of the Crown Zellerbach Corp-
oration won the 1960 Shipley Award with his paper concerning
black liquor oxidation. The author reported increasing oxidation
of black liquor at the Elk Falls mill by changing the flow of the
two oxidation towers from a parallel to a series arrangement.
24. Blessing, W. H.: Three unit precipitalor saves $1000 daily at Long-
view. Paper Trade J., 142:22-23. Sept. 29, 1958. A three-stage, wet-
bottom, electric precipitator installed at the Longview, Washington
plant has been successfully operating at an efficiency Of 96.7 per-
cent. About 38 tons of salt cake per day are recovered, representing
a daily saving of over $1,000.
25. Borlew, P. B-, and T. A. Pascoe: Potentiometric determination of
sodium sulfide in sulfate pulp black liquor. Paper Trade J., 122.'
(10):31. 1946. A high-sodium, glass electrode was developed that
was successful in determining the concentrations of inorganic sul-
fide in black liquor. The half-cell was prepared by mixing sodium
hydroxide, ammonia, and water in such amounts that the final
solution would contain a volume of 500 milliliters, and concentra-
tions of NH4OH and NaOH of 0.05 and 1.0 N, respectively. The
sodium sulfide content of the black liquor was determined with
this apparatus by a potentiometric titration with silver nitrate. This
analysis is subject to very little interference from other compounds
normally found in black liquor. It exhibits a sharp endpoint, and
-------
22 AIR POLLUTION
represents a substantial improvement over the older, more involved
techniques-
26. British Columbia Research Council discovers new methods to re-
duce krafi mill odors. Paper Trade J., 134(2):14. 1952. An apparatus
is described that exposes black liquor to air in a concurrent absorp-
tion column. Some oi the odorous compounds are oxidized, which
reduces the air pollution problems arising from the evaporators
and the furnace. Gases from the digesters can also be added to the
air supply of the unit and be oxidized in the column.
27. British Columbia Research Council: A manual describing a method
for the collection and analysis of pulp mill effluents. Van-
couver 8, Canada. Apr. 1959. In December, 1957, the Mead Corpora-
tion of Chillicothe, Ohio, commissioned the British Columbia Re-
search Council to develop a field method for the identification and
measurement of the substances responsible for kraft mill odors. One
year later, this manual describes a method for the collection and
subsequent laboratory analysis of samples of effluents from kraft
pulp mills. The method should, with some changes, be applicable
also to some other kinds of effluents, such as those, perhaps, from
petroleum refineries. The manual was submitted as a Progress
Report—the seventh—to the Mead Corporation. They have agreed
to make it available to other laboratories. The British Columbia
Research Council was happy to arrange for the distribution, and
will welcome criticisms and suggestions about the manual. It is to
be emphasized that the manual is in fact a Progress Report. The
method is not represented as being perfect, but then few entirely
new methods are. For example, the behaviour of sulfur dioxide,
particularly its influence on the measurement of hydrogen sulfide,
has not yet been completely studied. Besides, a smaller field kit
would be advantageous; the one described in the manual is, in fact,
the first to be constructed. The methods prescribed are, however,
the result of extensive laboratory studies on the quantitative re-
covery of known kraft mill pollutants in the trap train that com-
prises the sample collection device. These studies will be described
in papers to be submitted for publication. The field kit and pro-
cedure have been used successfully to sample and analyze all im-
portant kraft mill effluents and process streams. Collection of a
sample from the duct leading to a lime kiln stack was found to
be the one unwieldy operation, because of the large amount of
rain continuously forming in the duct. It is believed that this appa-
ratus and method are good starting points for a smaller, modified
field kit that would collect a much smaller sample in applying
this new, supersensitive instrument to the analysis of malodorous
effluents.
The introduction contains two parts:
1. Synopsis. Procedures are described for:
A. Collection of pollutants from a gaseous stream by use of the
field kit,
B. laboratory concentration of the collected sample by use of
fractionation apparatus,
C. gas chromatographic analysis of the concentrated samole
2. Time required for collection and analysis of a sample- '
-------
23
Annotated Bibliography
It requires 1 full day for one operator to obtain a sample from an
effluent or process stream and to complete the analysis for hydrogen
sulfide in the field laboratory. This day includes the periods re-
quired to transport the field kit to the sampling site, to take the
sample, to clean the kit, and transport it back to its original lo-
cation. It is advisable to examine a new sampling site a day or
more before the actual sampling operation. The exact arrangement
of sampling tubes and means for their support can be devised. The
location of a 110V a-c outlet, the length of extension cord needed,
the existence of a suitable hole in the duct for insertion of a sampling
tube, and other such matters can be determined. The laboratory
concentration and gas chromatographic analysis of a single sample
requires about IVz days, but if several samples are to be analyzed,
the time required is about 1 day per sample.
28 British Columbia Research Council: A study of the kinetics of oxi-
dation of kraft black liquor. Vancouver 8, Canada. June 1959. The
work described in this report is part of a continuing study to acquire
knowledge for the curtailment of kraft pulp mill odors. It deals with
the effects of sulfide concentration, temperature, oxygen pressure,
and liquor turbulence on the rate of oxidation of black liquor.
29. Callaghan, C. V.: The vemuri scrubber at Balhurst. Pulp & Paper
Mag. Can., 54:106-107. Apr. 1953. A venturi scrubber device is de-
scribed that has been installed at the Bathurst Mill. The unit proc-
esses gases given off from the recovery furnace, issues them into
a cyclone, and then emits them into the stack. The scrubbing liquor
caught by the cyclone is collected in a slurry tank and mixed with
make-up water for re-use as a scrubbing agent. The cost of building
the total unit, including the building and chimney, was about
$125,000 (Table 12).
30. Gate, F. L.:Design of chimneys for odor dispersion. TAPPI, 36:225-
228. 1953. The design of a chimney is a definite factor in the dis-
persion of air pollutants, but does not provide a complete solution to
the problem. It has been suggested that the concentration of pollutant
should vary inversely as the square of distance for locations farther
than 50 chimney heights from the stack. It follows, therefore, that
distant places may be affected by pollution regardless of the height
of the chimney. In view of these facts it must be acknowledged that
various collection equipment may be required, in addition to a well-
designed stack, for successful abatement of air pollution.
31. Champion Paper, Inc.: Black liquor oxidation. Champion Paper, Inc.,
Texas Division. Although black liquor oxidation has become an in-
tegral operation of many pulp mills, its use has been limited in south-
ern pine mills owing to excessive foaming of black liquor in these
plants. Champion has solved this problem by placing the oxidation
tower after the multiple effect evaporators, thereby processing the
concentrated black liquor, which has less tendency to foam. The
evaporator gases are washed with a weak sodium hydroxide solu-
tion, which further reduces sulfur losses to the atmosphere. An addi-
tional advantage of oxidizing strong black liquor is that the heat of
oxidation serves to evaporate the mixture to a concentration of 2'3
Baume higher than would otherwise be obtained. Although it has
been claimed that oxidation prior to evaporation is the cause of re-
-------
Table 12. PERFORMANCE DATA FOR BATHURST GAS-SCRUBBING UNIT
Pufp production
Gas volume at stack
Gas temperature at stack
Gas temperature into venturi
Venturi fan motor current
Total slurry flow
Gas velocity at venturi throat
Slurry velocity at jets
Furnace draught,
before venturi tube
before throat
before cyclone
after cyclone
after fan
Fan power consumption
Pump power consumption
Slurry pH
Slurry temperature
Na20 in slurry
Slurry Baurfie, hot
Salt cake in stack gas
Salt cake recovered per day
Recovery
260 tons per day
58,000 cfm
165°F
290 °F
75 amps
53.5 cfm
275 fps
20 to 25 fps
-0.2 in. H20
+ 0.5 in. H20
-0.8 in. H20
-19 in. H20
-22 in. H20
+ 0.5 in. H20
325 hp
22.4 kw-hr/ton
20 hp
1.4 kw-hr/ton
8.5
160°F
9.9 Ib/ft3
27 to 29 Bourne
17,200 Ib/day
6.6 Ib/ton of pulp
15.480 Ib
89.6%
-------
Annotated Bibliography
duced corrosion of evaporator tubes, this was not believed to be
the case with Champion's mill. Foam in the oxidation tower is con-
trolled by spraying with heating oil. A foam breaker is being planned
to replace this method.
32. Chase, S.: Control of air pollution at the Champion Paper and Fibre
Company. Proceedings semi-annual technical meeting. Air Pollution
Control Association. Houston, Texas. Dec. 3, 1966. The Pasadena,
Texas, mill of Champion Paper and Fibre Company was started in
1937. The first electrostatic precipitators were made for eliminating
or collecting into manageable quantities the air pollutants present.
Around 1950 a special department was created solely for the pur-
pose of dealing with problems of stream and air pollution. This
paper is a report of Champion's progress in the field of air pollution.
Sources and control of solid pollutants and obnoxious odors are
discussed. Solids originate from the recovery furnace and the lime
kiln. Odors arise from digester relief gases, blow gases, and the re-
covery furnace. Control equipment and techniques include electro-
static precipitators, water sprays, dust collectors, condensers; com-
bustion, absorption, scrubbing, and odor masking.
33. Anon: Tower oxidizes odors away. Chem. Eng., 59(9):232-235. Sept.
1952. The Springfield, Oregon, mill of the Weyerhaeuser Timber
Company uses air oxidation of black liquor in a tower to reduce
odors and save sulfur. Some observations on the operations are:
Saves 1,000 tons of sulfur per year.
Black liquor: 300 gpm at a sulfur concentration of 3 grams per liter.
Tower: 10 feet in diameter, 30 feet high, packed with 22 feet of 4-
inch partition rings;effective surface of 50,000 square feet.
Air: 10 cubic feet per minute for each gpm of liquor.
No foaming problem.
No H2S found in air leaving the oxidation tower. Complex chem-
ical reaction, but chiefly Na2iS—*• Na2 82O 3 .
Generation of H^S in evaporators has virtually been eliminated;
H2S is generated in the recovery furnace, and volatile organic
sulfur compounds are released from digesters and evaporators.
34. Chipman, W. A.: Physiological effects of sulfate pulp mill wastes
on shellfish. Paper Trade J., 127:47-49. Sept. 16, 1948. Although it has
been demonstrated many times that pulp mill effluents have adverse
effects on aquatic life, no work prior to that of this paper had been
done on the effects of pulp mill wastes on shellfish. The experimen-
tal work reported here involved measurements of the effect of con-
centration of pulp mill waste on the various normal activities of
oysters. It was found that effluent concentrations of 50 ppm were
sufficient to depress activity in some cases. The waste material re-
duced the time that oysters kept their shells open, and decreased
their gill pumping and filtration rates. An unsuccessful attempt was
made to isolate and determine the toxic compound responsible for
the observed physiological effects. It was demonstrated, however,
that the toxic compound can be precipitated from the black liquor
by salting out .
35. Coleman, A. A.: The combustion of noncondensable blow and relief
gases in the lime kiln. TAPPI, 41(10):166A-168A. Oct. 1958. The
burning of digester relief and blow gases in the lime kiln has been
-------
26 AIR POLLUTION
found practical from an operational standpoint. Over-all odor re-
duction has improved because of reduced downtime of the gas burn-
ing system. Maintenance labor for experienced mechanics has been
reduced by at least 35 hours per month. A fuel savings of about
$450 per month or $0.04 per air-dry ton has been obtained by elim-
ination of the separate, gas-burning furnace. At Weyerhaeuser's
Springfield, Oregon, mill, a system for destroying odorous digester
relief and blow gases has been in operation for several years. This
system is described by DeHaas and Hansen (TAPPI 38:732-738, 1955).
The lime kilns are now successfully burning blow and relief gases.
The distance from scrubber to kiln is 350 feet. Rupture plates along
the gas line will release pressure in case of an explosion; water seals
are below the vertical runs of piping. These catch any condensation
in the gas line. The system was put into operation in July 1957.
Thirty cfm of gas flows from sphere to kiln; dilution to beyond in-
flammability is 20:1.
27-FOOT STEEL FLAME (FLOW KILN
SCRUBBER ARRESTER CONTROL)
FAN
Q
v*J/
GASES
ROC
COU
WAT
-^1—
K-PACKED,
NTERCURRENT
ER SCRUBBER
12! n —
FLAME
ARRESTER
Figure 5. Schematic flow diagram of a system for destroying odorous digester
relief and blow gases.
36. Collins, T. T.: Comparison of methods for determination of sulfide
in oxidized sulfate black liquor. Paper Trade J., 129:29-33. Aug. 25,
1949. Several analytical techniques for determination of sulfide in
black liquor were tested, and their relative merits are discussed
Experimental difficulties owing to oxidation of the black liquor
are bothersome to all techniques, and it is highly desirable to use
an analytical method that can be completed in a short period. For
this reason, the Borlew-Pascoe method (Abstract 25), which in-
volves a potentiometric nitration with silver nitrate, was considered
the most adequate for the studies to be conducted.
37 Collins, T. T., Jr.: The oxidation of sulfate black liquor. Paper Trade
J., 131 : 30-38. Oct. 12, 1950. As a result of extensive pilot plant
and laboratory investigations, a full-scale sulfate black liquor oxi-
dation system has been erected and operated at Thilmany so that
its effect upon the sulfur balance of the recovery system could be-
determined. The oxidation unit at Thilmany is not operating at pres-
ent because an excess of sulfuric acid in the chemical makeup main-
tains the cooking liquor sulfidity in the desired range Although a
high sulfidity level (33 percent, TAPPH) of white liquor was attained,
no improvements in pulp quality were observed A high degree of
oxidation of black liquor resulted in the reduction of sulfur loss by
more than 90 percent in the vacuum and direct contact evaporators.
-------
27
Annotated Bibliography
The recovery furnace itself, however, proved to be the source of
a large quantity of hydrogen sulfide when.it was not properly oper-
ated and lacked sufficient air. This was true even if the black liquor
had been oxidized. As a complement to black liquor oxidation and
to a properly designed and operated recovery furnace, the venturi
scrubber, with its remarkable ability for absorbing sulfur dioxide,
appears satisfactory for reducing both sulfur and sodium losses from
the kraft recovery system.
38. Collins, T. T., Jr.: The oxidalion of sulfale black liquor. A review
of Ihe literature in two parts. Paper Trade J., 136: 37-40. Mar. 20,
1953. 136: 19-22. Mar. 27, 1953. In his introduction the author
states: "Recently, however, considerable impetus has been given to
research in the field of black liquor oxidation and this may be at-
tributed to the pressing odor problem of sulfate mills in some lo-
calities. Not only is the odor problem of some consequence, but
there seems to be considerable complaint (and some suits) about
the effect of sulfate mill recovery fumes in staining house paint.
The use of black liquor oxidation systems to reduce the sulfide con-
tent of the waste gas is mentioned as a partial solution to the prob-
lem." This is a fine review of the literature and patents covering
the subject of black liquor oxidation; there are 67 references.
39. Collins, T. T., Jr.: The oxidation of sulfate black liquor and related
problems. TAPPI, 38(8):172A-175A. Aug. 1955. Within the last 5
years there has been considerable interest in the field of sulfate
black liquor oxidation. Most of the basic work that has been publi-
cized has been carried on by a few groups of investigators. This
work is listed. An article by the same author 2 years before,
in which the literature and patents covering the subject of black
liquor oxidation were reviewed, is continued. Collins reports that,
"the final requirements for a successful scrubber for the fume-heat-
sulfur recovery process have now been met for over a year by the use
of a black liquor venturi scrubber on a 250-ton furnace at the Thil-
many mill at Kaukauna, Wisconsin." Other items discussed in this
article include foaming, analysis of black liquor for its sulfide con-
tent, beneficial results to the kraft process from oxidation, savings
achieved by oxidation, and redesign of the kraft recovery furnace.
40. Collins, T. T., and R. H. Collins: Recovery of heal and chemicals
in sulfate recovery furnace stack gases. Part I. Paper Ind., 29:1608-
11. 1948. Part II. Ibid, 1766-71. An extensive literature review is
given concerning the quantities of heat and materials lost through
stacks of recovery furnaces. The review also considers the various
methods of recovery used at the time and appraises their value.
41. Collins, T. T., C- R. Seaborne, and A. W. Anthony: Removal of salt
cake fume from sulfate recovery furnace stack gases by scrubbing.
Tech. Assoc. Papers, 30:168-172. 1947. A new apparatus for scrubbing
the fume of sodium salts from sulfate or soda furnace stack gases
has been developed successfully on a pilot plant scale. The equip-
ment consists essentially of a venturi through which waste gases
going to the stack pass at high velocity. At the same time, recirculated
-water injected into the constriction is atomized by the energy
of the gases under extremely turbulent conditions existing in
the throat and is then removed in a dry cyclone following the
-------
28
AIR POLLUTION
venturi atomizer. Efficiencies of sodium removal as high as 99
percent have been attained by a combination consisting of the
venturi atomizer followed by the conventional Pease-Anthony
scrubber, instead of a cyclone. The combination of venturi atomizer
followed'by a dry cyclone, called the venturi scrubber, has given 85
to 94 percent sodium recovery on a pilot plant scale with an esti-
mated power consumption for large-scale operation of approximately
43 killowatt-hours per ton of pulp. Because of the easily attainable
high efficiency, low first cost, and simplicity of operation of the
venturi scrubber, the addition of any other equipment to the com-
bination to improve the efficiency seems economically unjustifiable.
In the literature, reference is made to four devices for dust collec-
tion: electrostatic precipitator, wet scrubber, dry cyclone, and bag
filter. Only electrostatic precipitation has had extensive commercial
use. Electrostatic precipitator: high initial cost, high operating cost
for labor and maintenance, 85 to 95 percent efficiency, very low
power costs. Wet scrubbers: used in Scandinavia, claimed efficient
in heat and chemical recovery. Dry cyclones: ineffective because
of small size of sulfate fume. Bag filters: have not been studied
widely. Dust from mill is principally soda ash, while sulfur gases
contain much H2S and little SO2-
Table.13. SALT CAKE FUME CAUGHT BY ELECTROSTATIC PRECIPITATOR:
SIZE DISTRIBUTION
Size in microns
<0.1
0.1-0.2
0.2-0.3
0.3-0.4
0.4-0.5
0.5-0.6
0.6-0.7
0.7-0.8
0.8-0.9
0.9-1.0
1.1
1.2
1.3
1.4
1.5
Count
861
82
101
85
83
50
31
21
12
22
0
4
1
1
7
Wt percent
0.7
0.5
2.3
4.6
8.7
9.2
9.0
9.1
7.3
18.6
0
5.8
1.9
2.3
20.0
Kleinschmidt and Anthony, at Thilmany Pulp and Paper Company,
used a pilot plant scrubbing unit on sulfate recovery furnace fume
with 65 percent fume recovery. The plant used a 140-ton B & W-
-------
29
Annotated Bibliography
Tomlinson unit with gas conditions of 60,000 cfm at 300 °F dry
bulb and 163°F wet bulb. Losses of salt cake were as low as 60
pounds per ton of pulp, but it is reasonable to estimate this at 150
pounds per ton. Tests at Thilmany averaged 183 pounds per ton
of pulp with use of a Pease-Anthony cyclonic-spray scrubber.
Theory of operation of the venturi atomizer is discussed. It has as
high as 99 percent efficiency. Maximum power requirement for
90 percent efficiency is 43 kilowatt - hours per ton of pulp. The
venturi scrubber consists of a venturi atomizer followed by a dry
cyclone and induced draft fan. Scrubbing liquor is recirculated
until almost saturated, then returned to the pulping system as
make-up.
Conclusions:
1. Venturi scrubber can recover 90 percent of salt cake fume
within reasonable cost limits.
2. There is economic value in low-level heat recoverable from
scrubbed gases.
3. Gases may become saturated and may rain in cold weather.
4. Low first cost makes scrubbing attractive.
42. Collins, T. T.: The sampling and analyzing of sulfale recovexy
furnace stack gases. Paper Ind., 29:1437-39. 1948. A literature sur-
vey is presented concerning stack sampling and analysis. The theory
and conditions for isokinetic stack sampling are discussed at length.
43. Collins, T. T.: The scrubbing of sulfate recovery furnace stack
gases. Part I. Paper Ind., 28:680-86. 1947. A pilot plant study of
the operating characteristics of cyclonic and venturi scrubbers for
removing effluent compounds from kraft mills is described. A gen-
eral description of sampling and analyzing techniques is also given.
Part II. Ibid, 28:830-34. 1947. Operating curves, based upon the per-
centage of sodium salt removal, are given for both cyclone and
venturi scrubbers. Emphasis is placed upon the venturi scrubber
owing to its high efficiency. The effects of various types of injection
patterns were also studied, and operating characteristics are given
for each pattern. It is stated that 43 kilowatt—hours per ton of pulp
would be required for 90 percent sodium salt removal by the
venturi scrubber.
Part III. Discussion of data. Ibid, 28:984-86. 1947. The characteristics
of the cyclone scrubber are discussed in terms of the equation:
Efficiency : 1
where:
D : diameter of scrubber
W : effective volume of sprayed water
d : diameter of water droplets
G : volume of gas scrubbed.
It is noted that the efficiency of the cyconic scrubber increased
as the temperature of the inlet water was lowered. The reverse
of this effect was observed for the venturi scrubber. The following
advantages of the venturi scrubber are noted:
1. The venturi scrubber has as high an efficiency for small par-
ticles as any device presently known. This is true from an economic
as well as from a percentage recovery standpoint.
-------
30
AIR POLLUTION
2. This type of scrubber uses less water to recover a given amount
of material than do other wet devices.
44. Collins, T. T-: Some aspects of oxidizing sulfate black liquor. Paper
Trade J., 130:37-40. Jan. 19, 1950. An outline of a study, conducted
at the Thilmany Mill, of the oxidation properties of black liquor
is given. The effects of air and peroxide oxidation were studied.
It was concluded that commercial-scale oxidation by peroxide is
economically unfeasible, especially since equivalent oxidation of
the sulfur can be accomplished by air.
45 Collins, T. T., and P. H. West:Some recent developments in kraft
recovery. Part I. Paper Trade J., 129:19-22. Sept. 1, 1949- With the
continued interest in recovering more completely the heat and
chemicals given off by the kraft process, the reclamation of chem-
icals other than those composing salt cake has become important.
Of these, sulfur and lime losses are of special interest. In illus-
tration, the following quantities characteristic of the Thilmany
Pulp and Paper Company's operation are tabulated.
The quantities of salt cake recovered by the new venturi
scrubber at the Thilmany mill were significant. An estimated 90
percent recovery has been attained, which effects a reclamation
of from 7 to 12 tons of salt cake per day. The present consumption
of salt cake is 128 pounds per ton of pulp, as compared with a
previous average of 291 pounds per ton. This increase in efficiency
has been attributed to the venturi scrubber. Although the new
equipment has been ineffective in lessening odors arising from
noncondensable gases, emissions of particulates from the stack
have been visibly reduced. Corrosion of the removal equipment
Table 14. QUANTITY AND VALUE OF HEAT AND CHEMICALS IN THILMANY
SULFATE RECOVERY FURNACE STACK GASES
Pulp produced
Volume of stack gases
Temperature of stack gases
Stack dust equivalent to
Value (@$25/ton)
Hydrogen sulfide + mercaptans from stack
Sulfur dioxide from stack
Stack gas heat above 140 F
Value (@$0.80/1000 Ib steam)
150 ton/day
50 ,000 cfm
300°F dry bulb
163°F wet bulb
20,000 Ib salt cake/day
$250/day
4,000 Ib/day
400 Ib/day
500,000,000 Btu/day
$383/day
-------
Annotated Bibliography 3'
Table 15. QUANTITY AND VALUE OF CHEMICALS IN THILMANY LIME KILN
STACK GASES
Pulp produced 150 tons/day
Volume of stack gases 12,000 cfm
Temperature 525 °F dry bulb
163 °F wet bulb
Dust in stack gases 10,000 Ib/day
Value (r$2/ton) $10/day
has been somewhat of a problem, especially when the gases were
acidic. Stainless steel and protective coatings have been employed
as preventative measures.
Figure 6. Schematic flow diagram for thilmany salt cake recovery process.
46. Collins, T. T., and P. H. West: Some recent developments in krafl
recovery. Part II. Paper Trade. J., 129:25-29. Sept. 8, 1949. Much of
the original sulfur of the kraft process is lost out the stacks in
gaseous form. With the advent of modern particulate collection
equipment, the problem of decreasing sulfidity of the liquor has
arisen. The oxidation of black liquor has been employed to reduce
this tendency. This measure has also proved effective in reducing
corrosion and bad odors. The article also gives a review of the opera-
ting experience with the venturi scrubber at Thilmany, Wisconsin.
47. Collins, T. T.: The venturi scrubber and lime kiln slack gases.
?V1PP/,42(1):9-13. Jan. 1959. The literature on dust losses and scrub-
bers for lime kilns is reviewed. Comparative test data are given on
a number of units, including the Pease-Anthony venturi scrubber and
a recent modification of that unit called the Chemico S-F venturi
scrubber. Efficiencies of approximately 99 percent for removal of lime
dust are achieved at relatively low pressure drops of 6 to 10 inches
water gage across the unit. Heavy slurries are used as the scrubbing
liquor in the Chemico unit. Tests showed 63 to 80 percent removal
of soda fume, which is volatilized at the hot end of the kiln, and
has a smaller particle size than the larger lime dust mechanically
-------
32
AIR POLLUTION
entrained in the kiln gases. Because of the high soda content of the
lime sludge burned in the kilns at Palatake, a further reduction of
soda losses appears possible by improved washing of the sludge.
48. Collins, T. T., C. R. Seaborne, and A. W. Anthony: Use of the venluri
scrubber on sulfate recovery furnace gases. Paper Trade J'., 126:45-49.
Jan. 15, 1948. A description of the venturi scrubber employed at the
Thilmany, Wisconsin, Pulp and Paper Company is given. This Scrub-
ber was built and designed on the basis of pilot plant data, and has
proved itself to be above 90 percent effective in salt recovery. Scrub-
bing liquor is injected at the throat of the venturi by nineteen %-inch
nozzles. The composition of the liquor is kept constant_ by continu-
ous bleeding of a stream of 30 to 32° Baume' solution from the
Table 16. PERFORMANCE OF VENTURI SCRUBBER
Velocity of
entering gos m
throat, fps
217
216
219
211
250
235
259
248
233
Gal Ions of
liquor per
1,000 ft3 of gas
entering
venturi atomizer
7.3
5.9
5.5
7.5
5.0
3.5
2.6
3.8
5.2
Liquid |et
velocity
fps
11.0
8.5
8.0
10.5
8.2
5.0
9.2
13.0
18.0
AP across
atomi zer,
w g
18.6
20.9
15.8
19.4
18.0
13.9
12.7
16.1
18.7
Scrubbing
efficiency,
%
88
86
85
87
90
84
81
88
90
scrubbing fluid. During the test runs the gas velocities at the throat
of the scrubber were from 210 to 260 feet per second. For an average
daily production of 150 tons of pulp, the power requirement of the
scrubber was 40 kilo watt-hours per ton.
„ TO FURNACE
• TO STACK
Figure 7. Schematic flow diagram of thilmany venturi scrubber process.
48.A Collins, T. T.: Oxidation of kraft black liquor-where does it stand?
Paper Trade J., July 23, 1962. This process may well play an even
more vital role in odor control as increasing attention is focused on
pollution. The author sets out to alleviate confusion in this field and
-------
Annotated Bibliography 33
point out the latest advances. He discusses early studies on oxidation,
control of the foam problem, economic return, the Bergstrom - Tro-
beck systems, British Columbia Research Council systems, Weyer-
haeuser system, and other aspects of black liquor oxidation. The
article cites 113 articles in the literature and lists 41 patents.
49. Colombo, P., D. Corbetta, A. Pirotta, and A. Sartori: Critical dis-
cussion on the analytical methods for mercaplan and sulfur com-
pounds. TAPP1, 40:490-498. 1957. A short discussion of previous analy-
tical methods is given as an introduction to the presentation of a new
technique for analysis of combustion gases from recovery furnaces.
With this method, the sample is absorbed in a sodium acetate solu-
tion, where the sulfur dioxide undergoes chemical reaction. The
hydrogen sulfide is then precipitated in a solution of 5 percent cad-
mium chloride. The final steps involve absorption of methyl mercap-
tan in a methanol trap at minus 75 °C. The sulfur dioxide is analyzed
by a gravimetric determination of sulfuric acid produced by oxidation
of the cadmium sulfide precipitate. Methyl mercaptan and hydrogen
sulfide are determined by the standard potentiometric titration with
silver nitrate. The precision of this determination is about 20 percent
for mercaptan concentrations of 20 milligrams per cubic meter. It
becomes better, however, for samples of larger concentration.
50. Copcutt, V. W.: A report on precipitator applications. Air Eng.,
1:44,46. Apr. 1959. A summary is given of recent industrial appli-
cations of precipitators for process improvement, material recovery,
and air pollution control in the chemical field. Paper mills (all
types) have 167 precipitators in use, with a total capacity of 18.9
million cubic feet per minute. A typical 1-day collection at an in-
stallation (one or more precipitators per installation) is 50 tons of
sodium sulfate per recovery furnace.
51. Dalla-Valle, J. M., and H. C. Dudley: Evaluation of odor nuisance
in the manufacture of krafl paper. Public Health Reports, 54(2):35-43.
Jan. 13, 1939. also Tech. Assoc. Papers. 22:312-315. 1939. This article
contains a large table of odor threshold levels, some of which are
as follows (in mg/liter) Hydrogen sulfide, .0011; dimethyl sulfide,
.0011; methyl mercaptan, .0011; ethyl mercaptan, .00019; diethyl
mercaptan, .00025.
52. DeHaas, G. G., and G. A. Hansen: The abatement of kraft pulp mill
odors by burning. TAPP1, 38(12):732-738. Dec. 1955. This paper dis-
cusses the methods used to improve the recovery of sulfur and to
reduce the odor nuisance, particularly the digester relief and blow
gases, associated with kraft mills. The highest losses occurred in the
recovery furnace stack gases. These losses were practically elimina-
ted by installing a black liquor oxidation tower and improving the
operation of the recovery furnace. The hydrogen sulfide. released
with the lime kiln gases is not significant when the kiln is operated
with a slight excess of air. An analytical method, based on a modified
Orsat-type apparatus for the volumetric gas analysis of digester relief
and blow gases, is described. This analysis is suitable to determine
oxygen, hydrogen sulfide, and the total amount of organic compounds.
The chemical tests made show the efficiencies of black liquor oxida-
tion. Use of combustion at different temperatures and of chlorina-
tion as means of rendering the digester and blow gases harmless is
discussed. A detailed description is given of pilot plant tests of col-
-------
34 AIR POLLUTION
lecting the blow gases and digester relief gases in barrage balloons.
The large-scale installations now in operation include a vaporsphere
and a combustion chamber in one mill. In the other mill a vapor-
sphere and an arrangement to mix the digester relief and blow gases
with chlorination effluent is used. Details of operation and equipment
are given. The results of chemical tests to eliminate possible causes
of explosions are indicated.
S3. DeHaas, G. G.: Stabilization of Douglas Fir krafi black liquor. Pulp
& Paper Mag., 25:70-72. Dec. 1951. For the purpose of studying the
oxidation characteristics of Douglas Fir kraft black liquor, the author
employed a series of laboratory steel towers 6 to 8 feet high, and
packed with V4-to %-inch raschig rings. Extreme foaming difficulties
were encountered with these towers, so a glass tower was constructed
for visual observation. Results of these tests showed that 1,000 square
feet of packing surface will handle 10 pounds of sulfide ion per hour.
A stainless steel pilot tower was then built for further investigation.
The packing consisted of 3-inch cross-partition rings having a total
effective surface area of 5,750 square feet. Observed efficiencies were
about 97 percent for flow rates of 29-9 gallons per minute. A full-
scale tower was then constructed, 10 feet in diameter. 30 feet high,
and packed to a height of 22 feet, with 4-inch cross-partition rings.
With this unit in operation, it became possible to eliminate the addi-
tion of sulfur and yet observe an increase of black liquor sulfidity
to 20 percent from the 16 percent normally found under previous
operating conditions.
54. Dexter, G. M.: Elimination of kraft mill odors. Paper Trade J., 129:78-
81. Nov. 17, 1949. Perhaps the greatest disadvantage of the kraft
process is the nauseating odor it produces. This has been stated by
some to be unavoidable, the cost of control equipment being too
great. The following analysis shows, however, that these waste odors
can be eliminated economically, particularly in new mills. The odor,
resulting mainly from hydrogen sulfide and mercaptans, varies sig-
nificantly with the type of wood used, and with the ratio of sodium
sulfate to caustic in the cooking liquor. Generally, the odor increas-
es with the concentration of sulfate in the liquor. Pulp quality
requires, however, that the sulfate concentration be kept about one-
third that of the caustic.
Analysis of Operation
1. Scrubbing
The incondensable gases from the various sources may be introduced
into a tower for scrubbing with a weak oxidizing agent, such as sodium
hypochlorite, or air. This method has proved successful in the partial elim-
ination of odors from several kraft mills.
2. Oxidation Towers
A process has been introduced in which black liquor is passed through
an air oxidation tower before it is taken into the evaporators This process
has been claimed to be 90 to 100 percent effective in eliminating mato
dorous gases issuing from evaporators.
3. Inka Tower
An "inka tower" may be used to treat stack
passed through the precipitator, to remove
-------
Annotated Bibliography 35
percent effectiveness. This tower operates by passing the gases upward
through a series of perforated plates containing a sodium hydroxide solution.
4. Recovery Furnace
The operating conditions of a furnace have been shown to have a
direct consequence on the amounts of sulfur compounds in the stack gases.
For this reason, it is recommended that the furnace be slightly larger than
mill capacity, to avoid overloading. New furnace designs have been capable
of reducing sulfur gases to a large extent.
5. Direct Contact Evaporators
The direct contact evaporators tend to vaporize the odorous gases
contained by the black liquor. They thereby serve as a pollution source,
thought by many to be much greater than is the furnace itself. For this
reason it is suggested that these evaporators be replaced with multiple
effect units.
6. Burning Incondensable Gases
The incondensable gases from the various sources may be collected
and burned in an oxidizing furnace. A burner located outside the immediate
plant is recommended, to avoid explosion hazards.
7. Catalyst Chambers
Catalytic conversion of sulfur compounds presents a definite possi-
bility for the abatement of pulp mill odors. Although this type of operation
can be highly effective, prohibitive costs make it currently unfeasible.
8. Chimneys
Increased chimney heights would reduce odors to some extent in
some cases.
9. Waste Effluents
Effluents pumped from the mill to settling ponds are also a source of
odor. A possible solution would be to pipe in the stack gases from the lime
kiln. This would cause rapid precipitation of the sludge and release the
sulfur gases, which could be collected and introduced into the furnace
for burning.
55. Dudley, H. C., and J. M. Dalle-Valle: A study of the odors generated
in the manufacture of kraft paper. Tech. Assoc. Papers, 22:312-315.
1939. This report deals with the results of a study made on obnoxious
odors generated in the manufacture of kraft paper. There are three
sources of obnoxious odors owing to processes in the plant studied:
(1) Production of hydrogen sulfide, (2) production of volatile organic
sulfur compounds, and (3) release of large quantities of chemical
smoke, which contains sodium sulfate, sodium sulfide, traces of hydro-
gen sulfide, and large quantities of carbon arid organic matter. In
evaluating the sources of odors and their contribution to the generally
disagreeable conditions that arise downwind from the plant, it may
be stated that: (1) The amount of hydrogen sulfide released from the
plant is relatively small and may be considered of no consequence
in causing any odor except at the point of origin in certain buildings.
(2) The odor produced by the release of organic sulfur vapors from the
pulp digesters and the vapors from the turpentine condensers is very
disagreeable at the point of release; the quantity of such vapors,
however, is probably not sufficient to cause marked odors under
normal operating conditions at any great distance from the plant.
(3) The greatest and, probably, the only major contributing factor
to the obnoxious conditions arising within a mile radius of the plant
-------
36 AIR POLLUTION
1S the vast quantity of material blown out the stacks. From a
plant manufacturing 300 tons per day of kraft PW the smoke a
estimated to contain, under normal operating conditions, 18,000
pounds of sodium sulfate per 24 hours. In addition, there is much
carbon and partially carbonized organic matter, as well as a mixture
of somewhat volatile oils. This smoke, of rather low temperature,
soon reaches the ground and spreads over an area of several square
miles. The peculiar sweetish and somewhat sickening odor seems to
arise from the organic constituents. Methods of study are recom-
mended that may lead to control of the odor-generating processes.
The possibilities that electrical precipitators offer to prevent an ex-
cess of chemical smoke are stressed.
The concentrations of materials causing odors, are: Ethyl mercap-
tan 0.00019 mg/liter—odor of decayed cabbage; hydrogen sulfide
0.0011 mg/liter—odor of rotten eggs; methyl sulfide 0.0011 mg/liter—
odor of decayed vegetables; propyl mercaptan 0.000075 mg/liter-
unpleasant odor; thiocresol (Cr^CgE^SH) 0.0001 mg/liter—rancid,
skunk-like odor. Hydrogen sulfide is formed and released (1) at the
smelt tanks where molten sodium carbonate and sodium sulfide are
dropped into water, and (2) in the separator building where this
solution is filtered. Digestion brings about the formation of many
unknown volatile organic sulfur compounds. At nearly all points
of the process where the pulp is handled after digestion, certain of
these ill-smelling compounds are released, mainly from the turpen-
tine condenser and as blow gas from the digester.
Table 17. ANALYSIS OF AIR-DRIED SAMPLE FROM RECOVERY
FURNACE STACK
Material
Moisture
Na2S04
Na2S
Water-soluble organic matter
Ether-soluble organic matter
Free carbon
7.8
75.1
(Trace)
9.8
1.8
5.5
Production of 300 tons of finished paper per 24 hours means a loss of
18,000 pounds of Na2SO4 per 24 hours. Stack gases amount to 88,000 cubic
feet per minute. Thus the total solid concentration is 0.003 ounce per cubic
foot (3 mg/1). In addition, great amounts of moisture, some traces of hydro-
gen sulfide, and large quantities of volatile organic matter are released.
Twelve tons of solids per 24 hours pour out the stacks.
56. Felicetta, V. F., Q. P. Peniston, and J. L. McCarthy: Determination
of hydrogen sulfide, methyl m-rcaptan, dimethyl sulfide and disul-
fid« in kraft pulp mill process streams. TAPPI, 36(9):425-432. Sept.
1953. Mass spectrometric examination of the volatile components in
the condensate from kraft pulp digester blow gas has confirmed the
-------
Annotated Bibliography 37
presence of hydrogen sulfide, methyl mercaptan, dimethyl sulfide,
and dimethyl disulfide, and has shown that the corresponding ethyl
compounds and other sulfur-containing derivatives are not present
in significant concentration. A relatively simple and rapid method
for quantitative determination of each of the four above-named com-
ponents in gas or condensate streams has been evolved and is set
forth. The two acidic substances are batch-absorbed into aqueous
sodium hydroxide solution while the two thio-ethers are simultan-
eously taken up in benzene. The liquid phases are then separated.
Quantitative determination of hydrogen sulfide and methyl mercap-
tan in the alkaline aqueous solution is carried out by potentiometric
titrations with silver nitrate. Quantitative determination of dimethyl
sulfide and disulfide in the benzene solution is carried out by means
of bromate-bromide titrations of the components after they have
been partly separated by a single-stage batch distillation. Results
obained on known mixtures are set forth. The method has given
reasonably satisfactory analyses of two gases and two condensate
process streams in a kraft pulp mill.
57. Felicetta, V. F. and J. L. McCarthy: The pulp mill research pro-
gram at Ihe University of Washington. TAPPI, 40:851-866. 1957. Es-
tablished on January 1, 1944, the pulp mills research program has
received grants to date of about $500,000. Chemical investigations
have been conducted dealing with problems arising from pulp mill
effluent streams. Processes for utilization, control, and analysis of
these streams are now being studied. It is believed that, owing to
the contributions of this program, the by-products of the pulp and
paper industries are presently being utilized to a greater extent. This
program is helping to bring nearer the day when complete use can
be made of all the materials consumed in the kraft process.
58. First, M. W., H. E. Friedrich, and R. P. Warren: Reduction of emis-
sions from a recovery boiler. TAPPI, 43(6):182A-185A. June 1960.
This paper describes recent pilot studies conducted at large paper
mills and reviews the performance of full-scale installations with
gas-handling capacities of up. to 160,000 cfm. The spent digestion
liquor contains about one-half the weight of the original wood plus
almost all the original pulping chemicals. From the boiler, particu-
Jate loading of the furnace gas runs 3 to 6 grains per cubic foot. The
electrostatic precipitator removes approximately 90 percent of this
loading. Soda Process Plant at Kingsport, Tennessee: Recovery by
an electrostatic precipitator averages 90 percent. A Buffalo Forge
Company TE Washer was installed downstream of the precipitators.
The gas flow to the washer is 57,000 cubic feet per minute with a
dust loading of 0.21 grains per standard cubic foot. The effluent dust
loading is 0.027 grains per standard cubic foot for a recovery of 88
percent. Soda ash recovered is 1.7 pounds per minute- An analysis
of the particle size efficiency curve shows that the washer does not
collect material that the precipitator is incapable of collecting, but
captures instead agglomerated particles blown off the precipitator
plates. Kraft Process Plant at Everett, Washington: Tests on a pilot
scale TE Washer gave efficiencies of up to 95 percent depending upon
furnace operation. Kraft Process Plant at San Joaquin, California: A
full-scale TE Washer, of 160,000 cubic foot per minute capacity, was
-------
38 AIR POLLUTION
installed downstream of an electrostatic precipitator. Dust loadings
to the washer normally ranged between 0.05 and 0.15 grams per
cubic foot, but rose to greater than 3 grains per cubic foot when
the precipitators were down. This washer met the guarantee to re-
move 88.5 percent by weight of dust discharged from the pre-
cipitators.
Table 18. RELATION OF PARTICLE SIZE TO RECOVERY EFFICIENCY
Pa
rticle size, fji
20
15
10
5
2
1
Efficiency, %
95
92
85
65
30
20
59. Fones, R. E., and J. E. Sapp: Oxidation of krafl black liquor with pure
oxygen. TAPPI, 43:369-373. 1960. Black liquor oxidation has many
attributes that make it an attractive measure to incorporate into a
kraft mill. The high degree of foaming of southern black liquor,
however, makes this process difficult to operate. For this reason, in-
vestigations were carried out to determine whether oxidation in a
pressurized vessel would be economically practical. Addition of oxy-
gen to the digester toward the end of a normal kraft cook was found
to reduce the amount of sodium sulfide present, but the resulting
pulp was of reduced strength and brightness. Furthermore, the cost
of adding the oxygen was found to be prohibitive. Reduction of the
price of oxygen, however, would make a process of this type worthy
of further investigation.
60. Freudenberg, K., and M. Reichert: Sulfate pulp mill odors. TAPPI,
38(8):165A-166A. Aug. 1955. This letter to the editor refers to per-
sonal experience concerning the existence of the well-known odor
around the Vita Mayer Mill near Tradate Varese, Italy (see article
by P. Ghisoni, TAPPI, 37:201. 1954. Abstract 62). It also contains
information on the threshold value of methyl mercaptan.
61. Gaudette, P. R.: Electrical precipilators in air pollution control. Proc.
llth Indus. Waste Conf. May 1956. Purdue Eng. Extension Dfpt. Eng.
Bull. Vol. XLI. June 1957. Precipitators are 90 to 98 percent efficient.
Recoveries of chemicals in kraft mills run 100 to 150 pounds per ton
of pulp.
62. Ghisoni, P.: Elimination of odors in a sulfate pulp mill. TAPPI,
37(5):201-205. May 1954. A description of a plant, at Cartiera Vita
Mayer & Company of Milan, Italy, for the elimination of the mercap-
tans from a sulfate pulp mill is given. The main operations are: (1)
Condensation of all relief and blow gases with indirect condensers;
(2) use of the condensed water as dilution water in a chlorination
tower of the bleaching plant; (3)burning of the ncncondensable gases
m one boiler together with natural gas; (4) cooling of the smoke from
the recovery boiler to the dew point, followed by oxidation with
chlorine and reheating to avoid corrosion in the chimney
-------
Annotated Bibliography 39
63. Gordon, W. O., and E. E. Creitz: Chlorine as a deodorant in sulfate
paper pulp manufacture. Paper Trade J., 106:81-85. Apr. 28, 1938. The
characteristic odor of kraft pulp manufacture has been attributed to
methyl and ethyl mercaptans and sulfides. These are produced (1)
during the digestion process, (2) from the discharging of the digesters,
(3) from the condensate from the evaporators, and (4) from the burn-
ing of black liquor in the furnace. To test the odor-reducing capabil-
ities of chlorine, a 2.5-cubic foot digester was constructed. Experi-
ments were conducted by charging the digester with 35 pounds of
chips, and adding cooking liquor consisting of 6.0 pounds of sodium
hydroxide and 3.0 pounds of sodium sulfate added to 120 pounds of
water. Cooks were allowed to proceed at 169 °- C for periods of 3
hours, during which the digesters were relieved continually. Relief
gases were led to a condenser and passed through a spray chamber
containing 20 liters of hypochlorite solution of known chlorine con-
tent. Two alcohol traps were added to the end of the train as a safety
measure. It was found that the average amount of chlorine consumed
per run was 1.29 grams. The odor-reducing capabilities of the chlorine
were highly significant. The effect of chlorine on the condensed re-
lief gases was also studied. It was observed that chlorine was fairly
effective in reducing odors of these liquids, although much of the
odor reappeared when the mixtures were allowed to stand over-
night. From this study it was concluded that approximately 25 pounds
of chlorine per ton of pulp is necessary for the elimination of odor.
The cost of such a measure is prohibitive and can be justified only
in special cases.
64. Gordon, W. O., and E. E. Creitz: Elimination of obnoxious gases in
the sulfaie pulping process. Ind. Eng. Chem., 26:565-567. May, 1934.
During the kraft cooking process, some of the sulfur combines
with part of the organic constituents of the work to form mercap-
tans, alkyl sulfides, and similar compounds. Mercaptans and alkyl
sulfides can be removed from noncondensable relief gases from the
digesters of the kraft pulping process by means of a solution of hypo-
chlorite sprayed into the gases. The aqueous layer of the conden-
sate from these relief gases, as well as that from the evaporation
of the black liquor, is also treated with hypochlorite. The odor
of the organic sulfur compounds in these distillates is re-
moved, but the liquors are not rendered entirely odorless by
this treatment. The condensate from the digester relief
gases consumed about 21 kilograms of active chlorine per
metric ton (4.2 pounds per ton) of pulp produced, while
the distillate from the black liquor consumed several times
this amount. It is believed impractical to destroy all the mercaptans
by the chlorine treatment, and other means are suggested for elim-
inating these obnoxious odors. This article gives the results obtained
from an experimental sulfate mill, with a capacity of 16 kilograms
(35 pounds) of chips per batch. The chips are cooked at 100 pounds
per square inch for 4 to 8 hours. Sources of gases were digester re-
lief, blow-off pit, wash water used to wash pulp (which in older mills
may be discharged into a nearby stream), black liquor evaporator,
and black liquor furnace (recovery furnace). Gases from cooking
(relief and blow) were collected by dissolution in alcohol, and titrated
with iodine solution. Results indicate 0.0603 grams of mercaptan pro-
-------
4Q AIR POLLUTION
duced per 15.96 kilograms (35 pounds) of chips. Later experiments
with hypochlorite spray showed about 15 times as much chlorine
was necessary to oxidize the gases as would be required by this
amount of mercaptan. Chlorine evidently oxidizes the mercaptan
further than iodine does, and also oxidizes other gas constituents,
such as methanol. The average amount of chlorine necessary to de-
odorize the gas was 1.3 grams per cook. Hence chlorine consumption
amounted to 0.25 kilograms per metric ton (0.5 pounds per ton) of
pulp (each cook of 15.96 kilograms of air-dried wood produced 4.67
kilograms flO.3 pounds] of pulp). The digester condensate also
contained mercaptan. After turpentine removal, 2.1 kilograms of
chlorine per metric ton (4.2 pounds per ton) of pulp were needed
to deodorize the condensate. Vapors from the evaporation of black
liquor require much larger quantities of chlorine. The mercaptan
odor can be removed, but other odors remain. It is recommended
that the liquors containing mercaptans be used for washing pulp
and for making up cooking liquor. They should not be discharged;
gases containing mercaptans should be burned in the recovery
furnace.
65. HaegglUnd, E.: The odor question in sulphate pulp manufacture. Paper
Trade J., 83:50. Dec. 16, 1926. A critical review of the theories and
proposed methods of control of kraft mill air pollution is given. Em-
phasis is placed upon the procedure of Wislicenus, which involves the
mixing of stack gases with air to promote oxidation on the way up
the flue, and Schwalb's process, which utilizes wood chips for gas
absorption.
66. Hansen, G. A.: Odor and fallout control in a kraft pulp mill. J of the
Air Pollution Control Assoc., 12(9):409-413. Sept. 1962. A description
is given of the air pollution control measures used at the Weyer-
hauser mills. Emphasis is placed upon the vaporsphere surge tank,
which holds digester gases prior to their combustion in the lime kiln,
and the spray scrubber, which collects the sodium salts that pass
through the electrostatic precipitators. These measures have caused
an estimated 90 percent reduction in odors and particulate fallout.
67. Hendrickson, E. R.: Air pollution sampling and analysis with special
reference to sulfate pulping operations. Report to the National Coun-
cil for Stream Improvement. Unpublished. 1958. This is a report on
techniques and procedures for sampling materials emanating from
kraft mills, which have been under laboratory investigation at the
University of Florida, and also investigated in the field. It describes
stack-sampling equipment, analytical procedures, areal sampling, and
simplified techniques. While it is believed that these techniques and
procedures will provide adequate information at minimum cost, they
are not yet ready for routine use.
68. Hendrickson, E. R.: Air pollution sampling and analysis with special
reference to sulfate pulping operations. TAPPI, 42:173-176A. May
1959. In view of the present public interest in air pollution, it may
be a sound idea for the individual pulp mill to initiate its own samp-
ling programs. Such programs must be well-designed, however, to
provide meaningful data at a reasonable cost. A summary is given
of some analytical techniques available for determination of various
gases emitted by kraft mills.
-------
Annotated Bibliography 41
69. Hendrickson, E. R.: Atmospheric improvement activities of the pulp
industry. TAPPI, 43:193A-195A. Mar. 1960. Because of increased pub-
lic concern, the pulp, industry has been conducting research on con-
trol of its air-borne 'emissions. This has been difficult because con-
trol measures and restrictions are demanded even before research
can be conducted to provide for them. Frequently research is hamp-
ered by the lack of reliable techniques for analytical determination
of pollutant compounds. Study is therefore in progress to determine
new ways of analysis. Control of particulate emissions has been ac-
complished by equipment such as venturi scrubbers, cyclones, and
electrostatic precipitators. Gaseous emissions are more difficult to
control. Oxidation of black liquor prior to evaporation has been used
to curb gases issuing from the evaporators. Partially successful at-
tempts to eliminate digester gases by burning or chlorinating have
also been made. A few mills have constructed gas holders to reduce
surges in the line from the blowing of the digesters.
70. Hendrickson, E. R.: Technical procedures for determining pulp mill
stack losses. Report to the National Council for Stream Improve-
ment. Unpublished. 1959. This report describes research work being
done at the University of Florida on sampling and analysis tech-
niques for the National Council for Stream Improvement. Initial work
was in the area of stack sampling, with the recovery furnace as the
first target. Sampling is done for particulates, sulfur dioxide, hydro-
gen sulfide, and total sulfur as sulfur dioxide. It is believed that this
program provides necessary information at the least cost consistent
with providing reliable data. The laboratory work is reported nearly
complete, and additional field evaluation is being conducted. Basic
techniques of sampling and analysis are reviewed. The work being
done at Washington State for the National Council is also briefly
reviewed.
71. Hendrickson, E. R-, C. G. Walker, and V. D. Chapnerkar: Identifica-
tion of the nonsulfur organic compounds in stack gases from pulp
mills. Paper presented at the annual meeting of A7HA, Washington,
D. C-, May 1962. In the manufacture of wood pulp by the sulfate
or kraft process, the noncellulosic portions of the wood are dissolved
away by the cooking liquor. The cooking liquor consists essentially
of sodium hydroxide and sodium sulfide. This liquor is added to
wood chips, and the combination is cooked at high temperature and
pressure for several hours. The result is a mixture of wood pulp and
black liquor. The black liquor is washed from the pulp before further
processing. For the process to be economically feasible the cooking
chemicals must be recovered from the black liquor. This is accom-
plished in a reducing atmosphere in the recovery furnace. Heat, as
well as chemicals, is recovered. Because of the nature of the fuel,
many and various combustion products are formed. Some of these are
odorous sulfur compounds that, together with some nonsulfur com-
pounds, result in the typical kraft odor. The sulfur-bearing com-
pounds have been reasonably well identified. The objective of this
study was to identify some of the nonsulfur organic compounds. By
means of mass spectrometry, ultraviolet absorption photometry, gas
chromatography, and more conventional analytical procedures, the
following compounds were identified: Pyrene, fluoranthene, benz(a)-
anthracene, chrysene, benzo(3)pyrene, benzo(a)pyrene, benzo(g,h,i,)-
-------
42
AIR POLLUTION
perylene, coronene, and vanillin. Phenanthrene, anthracene, and an-
thanthrene have been tentatively identified on the basis of compari-
son of their absorption spectra with those published in the literature.
None of the pure, polynuclear, aromatic hydrocarbons tested gave
an odor. Benzo(a)pyrene has been indentified as a strong carcinogen
and benz(a)anthracene has mild carcinogenic properties. It is to be
emphasized that the results presented are merely qualitative. No
quantitative data have yet been obtained.
71 A, Hendrickson, E. R.: Air pollution control activities associated with
alkaline pulping, ca 1960. Unpublished: 18pp. This is a good, general
paper on the emission of pollutants from kraft pulping, their effects,
and control. It describes alkaline (kraft) pulping, sources of air pollu-
tants, effects of pollutants, control of particulates, odor reduction,
and other controls. Thirteen articles in the literature are cited.
72. Hisey, W. O.: Abatement of sulfate pulp mill odor and effluent nui-
sance. TAPPI, 34:1-6. Jan. 1951. A description is given of the recovery
system in the South African Pulp and Paper Industries' mill. This
plant utilizes a black liquor oxidation tower as a sulfur trap, where
all odorous sulfur gases are diluted with air and passed through the
oxidation tower. Residual gases from the tower are burned in the
furnace. Digester condensates are blown with air into a packed
tower, and the effluent is used as make-up water in a spray cooling
pond- This creates a negligible odor. Fume recovery towers and a
cyclonic evaporator were installed in the recovery furnace. So far
it has been shown that 90 percent of the sulfur gases leaving the
furnace is recoverable. The low odor of the stack gases is reduced
even further by injection of small amounts of chlorine. These efforts
have resulted in the maintenance of an exceedingly low degree of
odor in the atmosphere, and in the requirement of relatively small
amounts of new chemicals to make up the cooking solution. Operation
data are presented in Tables 19 through 22.
Table 19. ANALYSIS OF SULFUR COMPOUNDS IN GAS STREAMS (Ib sulfur/million
Digester
towe
Nonconde
Gases lea
Estimatec
to fur
Ib Na
elief gases to oxidation
(after dilution)
nsoble gases from evaporators
ving oxidation tower
quantity of sulfur returned
nace from oxidation tower (as
2SC>4 per ton pulp)
H2S and
mercaptans
1.78
575.9
3.55
Organic
di sulfides
2.40
99.2
7.42
Inorganic
sulfur
Nil
Nil
Nil
Total
sulfur
4.18
675.1
10.79
83
73. Institute of Paper Chemistry. Bibliographic Series No. 178. Alkaline
processes. II. Black liquor. 1952. Unpublished. An annotated bibli-
ography, consisting of 108 references to foreign and domestic publica-
tions and patents concerning odors from the kraft process, covering
both relief gases and black liquors. This document makes a good
reference piece.
-------
Table 20. FUME RECOVERY SYSTEM OPERATING TESTS
Gas temp, °F
Dew point, °F
GAS ANALYSES (equivalent
Ib Na2S04/ton pulp)
Smelt sulfidity, %
HoS and mercaptans
Organic disulfides
COS and C$2
so2
so3
Na2S204
Na2S04
Na2C03
NaCI
Total sulfur compounds in gas
(Ib Na2S04/ton pulp)
Salt cake recovery in fume
system (Ib Na^O^j/ton pulp)
Entering Leaving
1
195
170
14
19.0
1.9
Nil
Nil
Nil
Nil
148.0
2.9
27.3
168.9
2
193
157
16.5
8.8
Present
2.7
8.9
Nil
14.1
124.0
Nil
14.7
158.5
1
157
156
2.8
2.0
Nil
Nil
Nil
Nil
12.5
2.2
8.5
17.3
151.6
2
154
153
...
0.5
Present
1.6
Nil
Nil
3.5
4.0
2.0
8.9
9.6
148.9
Table 21. BLACK LIQUOR OXIDATION TOWER DATA
Sulfidity of smelt, percent Na^S in black
liquor
Entering tower: grams of Nc^S per liter
of black liquor
Leaving tower: grams of NaoS per liter of
black liquor
Oxidation efficiency, %
Run no.
i
14
3.4
0.58
63
II
18.7
5.4
0.45
92
Ml
25.5
8.2
1.28
84
-------
44
AIR POLLUTION
TABLE 22. CYCLONIC EVAPORATOROPERATING TESTS
Gas temp, °F
Dew point, °F
Black liquor concentrations,
% solids
Smelt sulfidity, %
Gas Analyses (equivalent Ib
Na2S04 ton of pulp)
H2S and Mercaptans
Organic disulfides
COS and CS2
S02
S03
Na2S204
Na2SC4,
Na2C03
NaCI
Total sulfur compunds in gas
(Ib Na2SO^j/ton pulp)
Salt cake recovery in fume
system (Ib Na2S04/ton pulp)
Entering
1
420
51.5
19.5
17.4
Nd
1.2
58.0
Nil
Nil
153.0
34.8
38.1
229.6
2
440
54.0
25
2.6
Nd
0.8
10.0
Nil
Nil
136.0
Nil
30.0
149.4
3
400
54.7
14
52.5
3.2
Nil
5.3
Nil
Nil
234.5
5.4
74.3
295.5
Leavi
1
62.5
9.1
Nd
0.9
5.5
Nil
18.8
86.0
11.4
20.8
120.3
109.3
2
189
169
65.6
10.6
Nd
0.8
2.0
Nil
6.0
82.0
Nil
Nil
101.4
48.6
L
ng
3
195
170
64.1
19.0
1.9
Nil
Nil
Nil
Nil
148.0
2.9
27.3
168.9
126.6
i
74. Kenline, P. A.: In quest of clean air for Berlin. New Hampshire.
Sanitary Engineering Center Technical Report A61-9. U. S. Dept. of
Health, Education, and Welfare, Public Health Service, Cincinnati,
Ohio. 1962. An appraisal of the nature, sources, extent, and effects
of air pollution in Berlin, New Hampshire, concludes that the situa-
tion warrants corrective action. Pollutants of concern include par-
ticulates, sulfur dioxide, hydrogen sulfide, and malodorous organic
sulfur gases. The major source of these pollutants is a pulp mill.
On the basis of this survey corrective measures are recommended.
Table 23. AIR QUALITY MEASUREMENTS
Pollutant
Suspended particulate, ug/m
Hydrogen sulfide, ppb
Sulfur dioxide, pch
Dustfall, tons/m^ per mo
Soiling, Cohs/1000 ft
Average
183
16
35
0.5
Maximum
309
23
33
AR
1.9
-------
Annotated Bibliography
Toble 24. MEASUREMENTS OF POLLUTION FROM VARIOUS SOURCES IN THE KRAFT
PULP MILL
Source
Recovery furnace
Blow gases
Smelt tank
Lime kiln
Boiler plant (1 boiler only)
Emission of pollutants in Ib per hr
Hydrogen sulfide
332
3.
1.7
9.7
Sulfur dioxide
50
0.1
0.06
0.6
248
Participate
97
65
106
740
Some of these measures are attractive because of resulting eco-
nomic advantages. Community good will, too, must be weigh-
ed with the other advantages of the installation of additional
control equipment. Sampling procedures are included, and the
report gives an over-all status of air pollution from kraft mills.
7$. Klason,P-: Malodorous substances in sulfate. Paper Trade J., 79:30.
July 10, 1924. Investigations concerning the nature of malodorous
compounds formed during the kraft process are described. Methods
of analysis for these compounds and suggestions for preventing their
formation are given.
76. Kopita, R.: The use of an impingement baffle scrubber in gas cleaning
and absorption. Air Repair, 4:219-222. Feb. 1955. In many paper mills,
gases vented from rotary lime kilns contain up to 15 grains of dust
per cubic foot of gas. Combination venturi slot and impingement
plate scrubbers are operating in many mills. The scrubber achieves
over 97 percent recovery and reduces the amount of dust in the dis-
charge to less than 0.3 grains per cubic foot of gas.
77. Kraft, F.: Marathon in Canada. TAPPI, 32:14-A. Sept. 1949. The
chemical recovery system of the new Marathon pulp mill in Canada
employs an air oxidation system for the black liquor that has ob-
tained oxidation efficiencies of up to 80 percent. Upon leaving this
system, the liquor goes to the multiple effect evaporators, after which
it is further concentrated in furnace gas cyclone scrubbers, which
clean the stack gases as well as evaporate black liquor.
78. Kress, O., and J. W. Mclntyre: The distribution of sulfur during the
kraft pulping operation. Paper Trade J., 100:43-45. May 2, 1935. A
laboratory scale investigation was conducted to determine the
stoichiometry of the sulfur compounds throughout the stages of
the kraft process. Both stationary and rotary autoclaves were used.
The rotary units had a capacity of about 14 pounds air-dry wood
chip's, while the stationary units had capacities of about 625 grams.
Because preliminary experiments with the rotary autoclaves re-
sulted in high sulfur losses, it was decided that the smaller, sta-
tionary units were more adaptable to the requirements of this
project. The gas relief lines from the autoclaves were fitted with
three glass absorption towers, packed with glass wool and filled
with nitric acid. When the autoclaves were relieved through this
system, no odor characteristic of mercaptans could be detected. It
was therefore assumed that all the effluent sulfur compounds were
trapped out. Two types of experiments were made, those in which
the cooks were discontinued and analyzed after given time periods,
-------
46 AIR POLLUTION
and those in which the cooks were allowed to run continuously, the
liquor being sampled and analyzed at various times throughout
the run. Screened spruce chips and prescribed quantities of cooking
liquor were used in the experiments to simulate mill operations.
The sulfide and hydroxide in the cooking liquor, and the total sul-
fur in the white liquor were determined, as well as the total sulfur
in the black liquor. Analytical techniques are given in the text.
It was concluded from the data that the loss in sulfur arising from
the blowing of digesters is very small (about 0-05 percent). About
3 to 4 percent of the sulfur is retained by the cooked wood chips.
79. Kress, O.-.U.S. Patent 2,570,460. Oxidizer for black liquor. The tower
consists of an enclosed series of downward inclined shelves that
distribute the downward flow of black liquor in a continuous, even
pattern. Air is passed into the tower from below, which gives a
thin gas-liquid contact area for mass transfer.
80. Landsberg, H-, and E. E. Escher: Poteniiometric instrument for sul-
fur determination. Ind. Eng. Chem., 46(7):1422-1428. July 1954. An
automatic instrument continuously records trace quantities of oxi-
dizable sulfur compounds in the range of one-tenth to several hun-
dred parts per million by volume of gases in atmospheres. Colori-
metric titration with electrolytically generated bromine is used.
Titration is electronically controlled at a potentiometric balance
point. Various applications in the natural gas, process, and air
pollution fields are discussed.
81. Lardieri, N. J.: Present treatment practice of kraft mills on air-borne
effluents. Paper Trade J., 142:28-33. Apr. 14, 1958. A survey of 58
U. S. kraft mills was taken to determine the extent of air pollution
control being exercised. The results indicated the following equip-
ment was being used:
Table 25. CONTROL EQUIPMENT ON RECOVERY FURNACE AND LIME KILN EXHAUST
Recovery furnace exhaust
Electrostatic precfpitators a
Venturi scrubbers a
No equipment a
Lime ki In exhaust
Wet scrubbers
Dry dust collectors
No equipment
No lime kiln
50
9
2
54
1
1
2
Three mills use both precipltators and scrubbers.
The electrostatic precipitators were from 90 to 98 percent efficient
as compared with 85 to 90 percent for the venturi scrubbers In ad-
dition, the scrubbers required more power for their operation.
These disadvantages may be offset somewhat, however by the rel -
ative simplicity of the venturi and its ability to trap some of the o-
dorous noncondensable gases.
-------
Annotated Bibliography 47
Odorous gases occur primarily in the digestion and recovery sys-
tems. Digester gases are sometimes collected and burned or chlor-
inated. Recovery gases have been passed through oxidation towers
or wet scrubbers. Odor masking has also been employed. Money
spent by the mills in the survey was broken down as follows:
Electrostatic precipitators and venturi scrubbers for recovery
furnace stack gases $24,142,000
Lime kiln exhaust equipment $2,661,000
Black liquor oxidation towers, chlorination towers, and other control
methods for sulfur compounds $1,885,000
Stack gas cleaning equipment for fuel and bark burners
$1,973,000
Total $30,661,000.
82. Lardieri, N. J.:Pulp and paper industry progress in atmospheric
pollution control. Paper Trade J., 143:42-46. 1959. The significant
progress made by the pulp and paper industry in the control of air-
borne effluents has fortunately preceded the current rise of public
interest in the subject. Although the research program is proceed-
ing along several lines, it has two divisions: measurement and con-
trol. Recent studies of measurement techniques have involved work
with gas chromatography and various chemical methods, such as
the ADAK analyzer; and the device for sampling hydrogen sulfide,
sulfur dioxide, and total sulfur developed by the University of
Florida research project. Black liquor oxidation has become of con-
siderable interest as a means of reducing emissions. This method
has been applied mainly in the northern regions of the frorth
American continent, because foaming difficulties have been encoun-
tered in the south. Other important developments in emission control
are sulfur dioxide recovery systems, venturi scrubbers, and electro-
static precipitators. With these and other developments, the pulp
industry should be able to meet the requirements of the rapidly
increasing regulatory activity in the air pollution field.
83. Maksimov, V. F., et al.: Purification of kraft mill -waste gases.
Bumazh. Prom.. 34(5):14-16. 1959. Chem. Abs., 54:1778. Jan. 25, I960.
Waste gases from the recovery furnace of the Svetogorsk kraft
mill were treated in an experimental foam-type scrubber. Results
of this experiment are given in Table 26. Furnace gas velocity was
3.2 meters per second, and the scrubbing solution contained 20 to
30 grams of NaOH per liter. In a full-scale installation (4,700 mm
high, 2,200 mm diameter scrubber, with a capacity of 30 cubic
meters per hour of black liquor and 800 cubic meters per hour of
gas), using black liquor as the scrubbing medium, the content of
sulfur gases contained in mixed gases was reduced (T able 26),
84. Marsch, J. D. F., and W. B. S. Newling: British Patent 7M.996
(1957). Oxidation of hydrogen sulfide to sulfur dioxide. A process
is discussed for treating hydrogen sulfide-containing gases by cata-
lytic oxidation of the hydrogen sulfide to sulfur dioxide. This process
comprises heating the gases that contain at least sufficient oxygen
to oxidize the hydrogen sulfide to sulfur dioxide (180 to 450 ° C), and
bringing them into contact with a catalyst consisting of one or
more sulfates of heavy metals superficially converted to sulfates
of metal oxide cations.
-------
48
AIR POLLUTION
Table 26. PURIFICATION OF RECOVERY FURNACE GASES
Furnace qas alone:
Hydrogen sulfid.e
Methyl mercaptan
Sulfur dioxide
gases, mixed:
Hydrogen sulfide
Methyl mercaptan
Sulfur dioxide
Furnace, evaporator, and
digester gases, mixed:
Hydrogen sulfide
Methyl mercaptan
Sulfur dioxide
Dimethyl sulfide
Dimethyl disulfide
Hydrogen sulfide
Methyl mercaptan
Sulfur dioxide
T
Cone pretreat, mg/m
34 to 1,200
12 to 204
15 to 937
792
149
254
529
2,560
87.8
330
166
57.2 to 181
78. 8 to 111.5
3.9 to 12.1
Cone after treatment, mg/m^
10to40
8 to 46
2tol20
41
22
7.4
42
335
7.3
26
26
3.5
0.58 to 3.4
0.44 to 0.52
85. May, B. F.: Experiences in the abatement of kraft mill odors. Un-
published. Gulf States Paper Corporation. Tuscaloosa, Alabama. 16
pp. The experiences at the Braithwaite, Louisiana, mill of Gulf
States Paper Corporation with the odor problem since the mid-
1920's are presented. A historical presentation is given, in the hope
that the rest of the industry may learn from the experiences of
this mill. Use of a direct condenser for digester blow gases brought
the first success in odor abatement. Chlorine was tried for deodor-
izing the condenser effluent, but was found ineffective. Until 1951
all attempts at odor abatement were made with condensers, water
spray scrubbers and improved processing equipment. Trials of odor-
masking aromatic compounds began in the fall of 1951. Efforts to
effect a milder and less obnoxious odor by use of these compounds
has produced a slight trend toward abating odors. Positive con-
clusions, however, await extended trials. Trial of a scrubber system
employing bleach plant waste liquors is under consideration. Eval-
uation of results is the most difficult aspect of this field of study.
A practical yet accurate means of odor measurement is needed.
86. ?4ay, B. F.: Experiences in the abatement of krafi mill odors. TAPPI,
36:374-378. 1953. A retrospective account of experiences of the Gulf
States Paper Corporation's problems in air pollution control is given.
A digester blow gas condenser was the first successful piece of
equipment used for odor control. Chlorine gas was also used but
was ineffective. The use of odor-masking compounds has been of
hmited success; further experimentation is needed before it will
be possible to assess the real value of these compounds A process
-------
Annotated Bibliography
49
utilizing bleach plant waste liquors in a scrubber system is currently
under study.
87. Meinhold, T. F.: Thilmany reclaims 90% of chemicals from flue
gas. Chew. Proc., 19:14-15. Mar. 1956. Thilmany installed a 250-
ton B & W recovery unit equipped with a black liquor venturi
scrubber. It has been in operation over 2 years. A schematic dia-
gram and operating characteristics are given below. The one dis-
Table 27 OPERATING CHARACTERISTICS OF B & W RECOVERY UNIT
Black liquor flow
Collection efficiency
Saltcake loss
Draft loss across venturi
Draft loss across separator
Pulp production
Recovery furnace weight
venturi scrubber
Dust loading at standard conditions
dry gas, gr/ ft^
Dust loading, stack conditions,
gr/ft3
90 %
To scrubber
5.87
3.90
From scrubber
0.66
0.40
50,000 Ib/hr
89%
4,500 Ib/day
30 in. water
4 in. water
165 tons / day
120 tons
DIRTY FLUE GAS 485°F, 3.9 GRAINS
OF DUST PER CUBIC FOOT
LIQUOR TO VENTURI
THROAT, 62 PERCENT SOLIDS
LIQUOR TO
CYCLONE WALL
62 PERCENT SOLIDS
LIQUOR TO MIXING •*-
TANK AND FURNACE,
62 PERCENT SOLIDS
CLEAN FLUE GAS 180 F,
0.4 GRAINS OF DUST
PER CUBIC FOOT
CYCLONE
LIQUOR FROM EVAPORATORS,
42 PERCENT SOLIDS
Figure 8. Schematic flow diagrams of thilmany chemical recovery unit.
-------
50
AIR POLLUTION
advantage—high power requirements—is more than offset by good
heat recovery (saves 3,000 pounds of steam per hour), good chemical
recovery (90 percent), low cost, and low space requirements.
88 Mellor D., and P. G. Stevens: Use of the venturi scrubber on alkali
fume. Australian Pulp & Paper Ind.. 9:222-248- 1955. To reduce
economic losses and air pollution from effluent particulate matter,
the Burnie Pulp Mill has conducted laboratory tests on various types
of collection equipment to determine the one most suitable for in-
dustrial use. Pilot plant experiments with bubble cap towers, cy-
clones, packed towers, spray chambers, and venturi scrubbers were
conducted with varying degrees of success. On the basis of favor-
able pilot data and encouraging results published by earlier in-
vestigators, a full-scale venturi scrubber was selected.
The full-scale unit operated with a recovery efficiency of 85 per-
cent or more when the injected cleaning liquor flow rate was not
less than 3 gallons per 1,000 cubic feet of stack gas. The gas
Table 28. STRUCTURAL SPECIFICATIONS OF THE VENTURI SCRUBBER
Part
Venturi inlet diameter
Venturi throat diameter
Venturi outlet diameter
Cyclone diameter
Cyclone height
Fan power
Spec
4' 8"
26"
3' 10"
12'
35'
450 hp
FROM FURNACE
BLEED OFF<
GASES TO STACK
CY-
CLONE
\7
SCRUBBING LIQUOR (*)PUMP
Figure 9. Schematic flow diagram of venturi scrubber unit at Burnie
-------
Annotated Bibliography
51
flow rates through the venturi were from 53,000 to 64,000 cfm.
Approximately 15 to 17 inches pressure water gauge was lost
across the venturi, and 2.0 to 2.5 inches were lost across the cyclone.
89. Meuly, W. C-, and B. K. Tremaine: Abatement of sulfale pulp mill
malodors by odor masking agents. TAPPI, 36:154-161. 1953. The
results of an attempt to mask sulfate pulp mill odors are described.
Masking compounds were added directly to the digester charge, and
allowed to flow through the plant equipment and remain with the
source of odor at all times. Laboratory tests were made to deter-
mine the relative amounts of odor produced by the various opera
tions of the kraft process. The results are presented in terms of
"Gram Sense Values" per ton of p.ulp- These results are given in
Table 29. The cost of applying the masking agent was from 12 to 24
cents per ton pulp for the usual concentrations. Results of full-scale
tests indicated that this type of odor masking was definitely ef-
fective.
Table 29. RELATIVE AMOUNTS OF ODOR PRODUCED BY VARIOUS OPERATIONS
OF THE KRAFT PROCESS
Location
Digester relief
Digester
blowdown
Multiple effect
evaporators
Cascade
evaporators
Total
"CSV" /ton of pulp
Total liberated
20,000 x 106
Min 27,000 x 106
102, 000 x 106
38,000 x 106
187, 000 x 106
Air-borne portion
20,000 x 106
Min 27,000 x 106
10, 000 to 30,000 x 106
Max 38,000 x 106
50, 000 to 100, 000 x 106
90. Mockridge, P. C.: The use of Peabody scrubbers on lime kiln stack
gases. TAPPI, 38(4):255-256. Apr. 1955. A cyclone is unsuited for
control of lime kiln gases because there is a considerable quantity
of material below 10 microns in size. Fines not collected by cy-
clones may run 2 to 4 grains per standard (60°F) cubic foot. Bag-
houses are at a disadvantage because of high moisture and temper-
ature. A Peabody scrubber in use at the Chesapeake Corporation
is described. The kiln is 11 feet in diameter with a rating of 250
tons of calcine per day; gas low is 26,000 standard (eO^F) cubic
-------
52 AIR POLLUTION
feet per minute. Temperature of gases leaving scrubber is 150°F
and the pressure drop across the scrubber is 4% to 5>/2 inches of
water. Dust load from kiln is 16 grains per standard cubic foot;
from the scrubber is 0.3 to 0.4 grains per standard cubic foot. This
corresponds to an efficiency of 97 percent.
91. Moon, D. G.: Practical aspects of recent kraft mill developments.
Paper Trade J., 104:103-107. Feb. 25, 1937. Since the depression the
construction of new mills has created interest in the application
of some of the new improvements.
These have generally been found to apply to:
1. Improved cooking techniques,
2. recovery and utilization of heat from blow-off steam,
3. use of more efficient evaporation systems,
4. improved recovery furnace units,
5. continuous causticizing systems and lime recovery,
6. reduction of odors.
Condensing the blow-off steam has probably been the greatest
help in reducing offensive odors. The noncondensables can be
exhausted through a fan and readmitted to the furnace, which
reduces much of the odor that would otherwise be emitted-
92. Moore, H., H. L. Helwig, and R. J. Graul: A speclrophotomeiric
method for the determination of mercapians in air. Am. Ind. Hyg.
Assoc.J., 21:466. I960-. With this method, mercaptans are collected
in 5 percent mercuric acetate solution and reacted with N,N-
dimethyl-p-phenylenediamine to form a red complex that can be
quantitatively determined with a spectrophotometer. The sensitiv-
ity of this method is about 0.00023 micromole of mercaptan per
cubic centimeter. There is no interference from H2S. Interferences
from SO 2 and NO 2 are present, but are slight in the concentrations
normally encountered in air pollution studies.
93. Murray, F. E., and H. B. Rayner: A procedure for sampling and
analysis of hydrogen sulfide in kraft mill stack gases. TAPPI, 44:219-
220. 1961. A description is given of a method of determining hydro-
gen sulfide in stack gases. The hydrogen sulfide content of the
gas is analyzed by absorption in zinc acetate solution and subse-
quent colorimetric observation by Lauth's violet method. This pro-
cedure permits rapid sampling and analysis for hydrogen sulfide
concentrations of 20 to 800 micrograms per liter.
94. National Council for Stream Improvement: Air pollution Technical
Bulletins. Unpublished. A series of bulletins relates to air pollution
and the pulp and paper industry. To date there are 16:
94-1. Sampling and analysis of air-borne gaseous effluents resulting from
sulfale pulping. Sept. 1957. 28 pp. A literature review of available
methods of sampling for atmospheric effluents from the sulfate
pulping industry. 54 references.
94-2 Present treatment practice of air-borne effluents in the United
States kraft industry. Nov. 1957. 18 pp. Results are given of a
survey of the kraft pulping industry in the United States. It was
the purpose of this survey, with respect to which most mills were in-
dividually visited by Council engineers, to obtain information
concerning the disposal of air-borne effluents within the industry.
A total of 58 mills participated in the survey.
-------
Annotated Bibliography
94-3. Applied meteorology for the pulp and paper industry. Mar. 1958.
21 pp. General.
94-4. Application of instrumentation to pulp mill atmospheric discharges.
Mar. 1958. 17 pp. A literature review of instruments for the samp-
ling, analysis, and automatic sampling and recording of particulate
and gaseous atmospheric pollutants. 48 references.
94-5. Methods for the determination of sulfur compounds in pulp mill
effluent gases. Sept. 1958. 13 pp. Report of research at Washington
State University on a gas chromatographic procedure for the sep-
aration and identification of sulfur-containing malodorous com-
pounds. This procedure has potential application to the study of
gaseous pulp mill effluents at the sources and in the atmosphere
in the vicinity of the sources.
94-6. Gas chromaiographic analysis of hydrogen sulfide, sulfur dioxide,
mercaplans, and alkyl sulfides and disulfidss. Apr. 1959. 14 pp. A
gas chromatographic technique is presented for the separation
and identification of complex mixtures of volatile sulfur-containing
compounds of the type found in kraft pulp digester blow gas and
black liquor combustion products. Research at Washington State
for the NCSI.
94-7 Studies on the analysis of kraft mill gases. Apr. 1959. 9 pp. A con-
tinuation of Bulletin 5-
94-8. Manual of methods for the determination of atmospheric coniamina-
ination (tentative). May 1959. 18 pp. Intended for the use of local
mills this manual has sections on: meteorological instrumenta-
tion and observations, recommended units for reporting air pollu-
tion measurements, recommended standard method for particulate
fallout, and recommended standard method for the determination
of hydrogen sulfide.
94-9. Progress report on studies on the analysis of pulp mill gases. July
1959. 17 pp. A progress report on the gas chromatographic tech-
nique, and on research at Washington State.
94-10- Progress report on studies on the analysis of pulp mill data. Sept.
1959. This summarizes the research work at Washington State Uni-
versity on methods for collection and analysis of gaseous pulping
emissions.
94-11. Laboratory evaluation of gas sampling procedure for recovery
furnace slack gas. Dec. 1959. 25 pp. Research at the University
of Florida for the NCSI to determine the concentration of gases
of interest that result from sulfate pulping operations is described.
Stack sampling is done for particulates, sulfur dioxide, hydrogen
sulfide, and total sulfur as sulfur dioxide. This method is believed
to provide the necessary information at the least cost consistent
with providing reliable data. The analytical techniques are rea-
sonably standard, and in all cases a colorimetric procedure has
been adopted.
94-12. Manual: Air pollution control combustion processes. Apr. 1960. This
manual, prepared for the NCSI at New York University, deals
with the legislative aspects of air pollution (particularly solid fuel
combustion) and methods of complying with various control codes
that have been promulgated. A section on measurement of atmos-
pheric pollutants and incineration is also enclosed, together with
a state air pollution control agency directory.
-------
54 AIR POLLUTION
94-13. A manual for adsorption sampling and gas chromalographic analy-
sis of kraft mill source gases. Sept. 1960. This bulletin presents de-
tailed methods for sampling and chromatographic analysis of kraft
mill gases developed at Washington State University. The method
is based upon the condensation and/or adsorption of the gaseous
effluents on activated silica followed by desorption and subse-
quent gas liquid chromatcgraphic analysis.
94-14. Manual for the sampling and analysis of kraft mill recovery stack
gases. Oct. 1960. This bulletin describes a method for the sampling
and analysis of kraft mill recovery stack gases developed at the
University of Florida. It is intended as an alternate to the pre-
viously described chromatographic technique. This method samples
for particulates, hydrogen sulfide, sulfur dioxide, and total com-
bustible sulfur gases.
94-15. Survey of kraft mill emission using gas chromatography techniques.
Oct. 1962. 23 pp. This bulletin presents the results of the survey
of gaseous emissions conducted at nine specific northwest kraft
mills by the National Council Research Projects at Washington
State University. This study was designed to evaluate gas chrom-
atographic sampling and analytical techniques developed at Wash-
ington State University and also to attempt to define the ranges
of emissions from typical operations and various pollution abate-
ment processes used in the industries. This report illustrates the
variation in gases discharged and gives evidence to indicate the
efficiencies of various abatement procedures. This study is con-
cerned primarily with the concentration of hydrogen sulfide, meth-
yl mercaptan, dimethyl sulfide, and dimethyl disulfide. The pres-
ence of many other gaseous components is established and two
of these are identified. Tests were run on digester gases, oxidation
towers, recovery furnace gases, lime kilns, smelt tanks, evapor-
ators, and miscellaneous sources.
94-16. Identification of non-sulfur organic compounds in the stack gases
from pulp mills. Dec. 1962. 19 pp. (Qualitative but not quantita-
tive) identification of a number of polynuclear, aromatic hydro-
carbons is made, as well as a tentative identification of vanillin.
The following have been identified or tentatively identified in the
recovery furnace stack gas: benz(a)anthracene, benzo(a)pyrene,
benzo(e)pyrene, benzo(g,h,i,)perylene, chrysene, coronene, fluor-
anthene, pyrene, anthanthrene, and phenanthrene. None of the
compounds identified were found to have an odor, except vanillin.
Some of these compounds are considered carcinogens.
95- National Council for Stream Improvement: A critical review of the
literature on black liquor oxidation and the burning of kraft mill
noncondensabls gases. Unpublished, ca 1959. 19 pp. The literature
on the oxidation of sulfate black liquor and the burning of non-
condensable gases produced in the kraft process has been reviewed.
The theoretical aspects of oxidation, existing treatment systems,
and the patent aspects of black liquor oxiaation are discussed. The
published results on oxidation indicate that, from an air pollution
standpoint, a considerable reduction in the amounts of reduced sul-
fur compounds from evaporation and recovery furnace operation can
be obtained. Evidence is also presented suggesting that the emission
of odorous sulfur compounds from the digestion phases of the kraft
-------
Annotated Bibliography
process can be greatly curtailed if these noncondensable gases are
passed through the black liquor oxidation unit. Other benefits at-
tributed to black liquor oxidation include chemical savings in the
form of lessened sulfur make-up and decreased lime usage in the
causticizing stage. Black liquor oxidation also results in increased
sulfidity in the cooking liquor. The patent aspects of black liquor
oxidation are rather confused at the moment but it appears that
the Bergstrom-Trobeck patents have priority in the United States.
Successful burning of noncondensable gases from digesters and
multiple effect evaporators in units such as lime kilns, bark burners,
separate furnaces, and recovery furnaces has been reported. There
has been an apparent decrease in this practice in the United States
during the past 20 years, since only 2 of 60 kraft mills surveyed
in 1957 engaged routinely in burning as compared with 14 of 30
mills in 1939. This may be attributed to the difficulties experienced
in the combustion of these highly explosive mixtures. This review
concludes that both black liquor oxidation and the burning of non-
condensable gases offer possibilities for greatly reducing the amounts
of reduced sulfur compounds emitted to the atmosphere from kraft
operations. Capital expenditures required for both processes would
appear to be moderate and operating costs low. In burning of non-
condensable gases, the use of existing combustion equipment seems
not only economical but desirable. Operating costs for nonconden-
sable burning are negligible, and may be more than met by chem-
ical and heat savings in the case of black liquor oxidation. It does
not appear from the previous literature on this subject that the
possible techniques for oxidizing foamy-type black liquor have been
exhausted. Additional research on this problem seems fully war-
ranted. There are 27 references.
96. National Council for Stream Improvement. A review of ih« krafi
odor problem and control processes. Unpublished. Oct. 16, 1951. 8 pp.
This paper updates the Institute of Paper Chemistry's bibliography;
it contains 29 entries.
97. Organization for European Economic Co-operation (Paris): The po-
sition in Europe and in the United States. Air and Water Pollution.
132-137. Feb. 1957. Papermaking itself is not a serious cause of air
pollution. The production, by the sulfite and sulfate processes of the
pulp industry, of the cellulose it requires is the real source of the
trouble. Both processes are liable to emit salts in the form of fly
ash and sulfur dioxide, while from the sulfate process come the
obnoxious odors of hydrogen sulfide, mercaptans, and analogous
sulfide compounds. The operations that cause pollutants to escape are
classified for the sulfate and the sulfite processes. The chief methods
for reducing air pollution are listed. A flow diagram (schematic)
is included to show where air polluting substances arise from a
sulfate mill. A very brief description of the air pollution situation
is given for four mills—two Swedish and two German.
98. Pate, J.: Air pollution in area of Snohomish River Bridge. Everett,
Washington. Unpublished. Extreme corrosion observed on the bridge
across the Snohomish River at Everett, Washington, prompted a
study to determine its cause. Since corrosion rates were decidedly
higher on the western side of the bridge, a pulp mill situated
-------
56 AIR POLLUTION
immediately to the west was suspected to be the major factor con-
tributing to the problem. The ensuing study indicated that the
bridge was being exposed to large quantities of air-borne particu-
late matter issuing from the mill, and that this was the prime cause
of the high corrosion rates.
99. Pate, J.: Air-borne effluent sludy, Longview. Unpublished. An in-
vestigation was carried out to study the sources and characteristics
of air-borne particulates issuing from a pulp mill at Longview,
Washington. For this purpose, dustfall jars were placed at various
strategic locations surrounding the mill, and samples were obtained
from the outlets of the lime kiln and the recovery furnace. The
results were analyzed from air pollution and corrosion standpoints.
It was found that calcium salts did not present a serious corrosion
problem. The major source of calcium deposits was the causticizer-
slaker operation. It was concluded that the lime kiln and recovery
furnace stacks provided the majority of the corrosive compounds.
An unexpectedly high concentration of sulfur compounds was found
issuing from the lime kiln stack.
100. Pate J.: Dispersion and fallout project, Longview paint discoloia-
-tion - Episode II. Unpublished. A description is given of the air pollu-
tion outbreak in Longview, where the paint on several houses showed
severe discoloration. Upon consideration of the location of the dam-
age and the meteorological aspects, it was concluded that the pollu-
tion must have arisen from a pulp mill. Suggestions for preventive
action against further occurrences are given.
101. Perttula, A. E.: Novel ideas in krafl recovery reduce losses, odors for
Finnish mill.. Paper Trade. J., 145:20-28. June 26, 1961. The re-
covery system of the Rosenlew kraft pulp mill has incorporated some
unique ideas that significantly add to its effectiveness. This system
has reversed the position of the boiler, placed the electrostatic pre-
cipitator before the economizer, and added a double venturi scrubber
to the system to provide a final cleansing of the gases before they
are emitted to the stack. This arrangement necessitates operation of
the precipitator at the abnormally high temperature of 600 to 800 °F.
This temperature range, however, is above the "back corona" region,
and reasonably high efficiencies are obtained. Resulting high heat
transfer coefficients, owing to the lack of dirt deposition on the
economizer, demonstrate a marked advantage of this type of system.
The temperature of the gases issuing from the economizer is about
250° F. Green liquor is used as a cleansing agent in the venturi scrub-
bers, which is advantageous because the sodium carbonate in the
liquor induces the absorption of the sulfur compounds in the flue
gas. This system removes, on the average, 38 pounds of sulfur and
7.5 pounds of sodium per short ton of pulp; the efficiency is greater
than 97 percent.
102. Reid, H. A.: The odor problem at Mary vale. Proc. Australian Pulp
& Paper Ind., 3:479-500. 1949. The evolution and control of odorous
gases at the Maryvale pulp mill are discussed. Emissions from var-
ious locations in the plant were estimated; the results are presented
in Table 30 and Figure 11.
-------
Annotated Bibliography
57
TO ATMOSPHERE
FIRST VENTURI
STAGE
STAGE
HOT, CLEAN
WATER
HEAT
EXCHANGER
COLD, CLEAN
WATER
TO GREEN _
LIQUOR CAUSTICIZING
PLANT
FLUE GAS
ELECTROSTATIC
PRECIPITATOR I
-JTURI L_l
T
-_ P
i
OVERFLOW I
TANK |
75~
SMELT-
DISSOLVING
TANK
o
/ \ FORCED DRAFT FAN
STACK
Figure 10. Schematic flew diagram of recovery system at Rosen lew mill.
Measures taken to control these effluents at the Maryvale Mill are
as follows:
Digesters
Digester blow gases have been vented through an air oxidation
tower. Various methods have been suggested to "even out" the sporadic
flow of the blow gases.
-------
Table 30. NUISANCE VALUE OF MARYVALE EFFLUENTS
Source
Gases
Digester
Evaporator
Furnace flue
Liquids
Oily condensate from
digester gas
Aqueous condensate
from digester gas
Digester foul water
Evaporator
Volume,
ft3/hr
1,000
300
3,000,000
0.05
2.0
240
8,000
Threshold odor di-
lution, 1 part in
10,000
2,000
4
20,000,000
300,000
10,000
1,500
0%
20% 40%
60%
PULP
DIGESTER DRAIN
DECKER DRAIN
WASHING AND SCREENING DRAIN
CAUSTICIZING
RECOVERY FLOOR
CLEANING
DIGESTER FOUL WATER
DIGESTER GAS
EVAPORATOR FOUL GAS
EVAPORATOR GAS
STACK
LEGEND:
Figure 1 1 .
SODIUM I I
Relative sodium and sulfur losses at Maryvale,
-------
Annotated Bibliography
59
Evaporator Gas
A tower packed with blue metal has been utilized for oxidation of
black liquor. This has reduced the odor potential of the resulting evapor-
ator gas about 30 percent. A much larger tower for this purpose is pro-
posed.
103- Roberts, L. M., and C. E. Beaver: Application of electrical precipi-
tation equipment for the reduction of atmospheric pollution. Proc.
Air Pollution Control Assoc., 50-59. 1951. As of 1951, Research Corp-
oration had in operation 67 precipitators handling 6.5 million cubic
feet of gas per minute. The gas has acidic constituents; particles
are of extremely fine size. Collected dust is extremely light, weighing
9 pounds per cubic foot. A recent survey indicated recoveries of 88
to 145 pounds of salt cake per ton of pulp produced, the average
being 118 pounds. At the average precipitator availability of 95
percent, a 250-ton pulp mill recovers 4,900 tons of re-usable material
per year.
104. Roberts, L. M., C. E. Beaver, and W. H. Blessing: Operating experi-
ences with Cottrell precipitators on sulfate recovery furnace gases.
Paper Trade J., 127:45-49. Oct. 28, 1948. A general discussion is given
on the theory and construction of Cottrell electrostatic precipitators.
These precipitators are built in multiple units, and are rated on the
basis of 90 percent removal for a flow rate of 2,000 cfm. The pressure
drop between inlet and outlet is less than 1 inch of water. A summary
of operating experiences was prepared from questionnaires sent to var-
ious plants currently operating precipitators of the Cottrell type.
These results are summarized in Table 31.
Table 31. AMOUNT OF PRECIPITATE COLLECTED WITH COTTRELL PRECIPITATOR
Plant
1
2
3
4
5
6
7
8
9
Production,
tons/day
225
250 to 300
630
300
288
230
450
980 to 1,120
225
Collected
precipitate,
Ib/tonpulp
88
100
145
117
135
135
130
110tol20
94
Make-up salt cake, Ib/ton pulp
Prior to in-
stallation of
precipitator
246
307
316
300
300
225
350
After installa-
tion of pre-
cipatator
158
163
193
180
170
132
256
105. Sands, A. E., and L. D. Schmidt: Recovery of sulfur from synthesis
gas. Ind. Eng. Chem.. 42(ll):2277-2287. Nov. 1950. ^his article de-
scribes process for H2S removal and sulfur recovery using dry box
(iron oxide) and activated carbon catalyst.
-------
60
AIR POLLUTION
106. Schneider, F.: Deodorizalion of krafl mill exhaust TAPPI, 41:70/A.
Jan. 1958. The most offensive emissions from the kraft process are
in the form of terpenes, mercaptans, and sulfides. Furthermore, the
operation most responsible for air pollution is the blowing of the di-
gesters. A process for collection and elimination of terpenes, mer-
captans, and sulfides arising from digester blow gases is therefore
suggested. The process first condenses out high boiling terpenes from
the blow gases in a preliminary condenser. The gases then pass
through a continuous filter into an adsorbing unit where the sulfur
compounds are removed. From the adsorbing unit the gases are
issued into the stack. Since the digester blows are discontinuous
the absorber can be intermittently steam-cleaned, and the resulting
liquid can be separated and processed (Patent applied for).
STEAM
BLOW GAS
TERPENES
TO ATMOSPHERE
DIMETHYL
SULFIDE
WATER
Figure 12. Schematic flow diagram of deodorization process.
107. Schoening, M. A., L. W. Shemilt, and R. H. Wright: Black liquor
oxidation and the absorption of noncondensable digester gases. TAPPI,
36:176-179. 1953. A pilot scale oxidation tower was constructed to
study black liquor stabilization. The packing consisted of vertical
asbestos sheets, the surfaces of which were imprinted with a pattern
of raised diamonds. Black liquor was allowed to flow over the sheets
and was oxidized by concurrently flowing air. The spent air from
the column contained none of the odors characteristic of hydrogen
sulfide or mercaptans. In further studies, noncondensable digester
gases were allowed to flow through the column with the air supply.
With correct operating conditions, all the odor was again eliminated.
This type of tower provides a method of continuous black liquor
oxidation that practically eliminates problems with foaming. It is
also compact.
-------
Annotated Bibliography
61
Toble 32- ABSORPTION OF NONCONDENSABLE GASES IN BLACK LIQUOR OXIDATION UNIT
Chips
Hemlock
Cedar
J-ir
Hemlock
Fir
Hemlock
F,,
Fir
Averoje
Air
flow,
cfm
21.2
31
21.5
21
30.2
19.4
28.7
19.4
Non
conden-
sable
gos flow,
7.5
13.4
7.3
7.0
13.5
3-6
13.8
3-6
feed, Ib/Jir
6,100
6,020
5,860
5,350
Nil
5,360
5,360
5,290
Na?S content
of Slack liquor,
9/1
To
2.7
2.9
2.3
2.3
2.4
2.7
3.0
F-
0.7
0.6
0.4
0.3
0.4
0.6
0.6
Digester
11 -24 to 11:35
14: 34 to 14:44
20:53to21:07
9:33to 9:43
11:37 toll:49
14:25to 14:36
15:42 10 15:52
17:15 to 17:27
Sampled
11:1110 11:13
11:141011:16
l:18toll:20
1:221011:24
l:26toll.27
1:30 to 11:33
4:181ol4:19
4:28 to 14:29
4:33 to 14:34
4:41 to 14:42
20:50to20:51
20:55lo20:56
20:59to21:00
21:061o21:07
9:25to 9:27
9:32lo 9:33
9:36to 9:37
9:42to 943
ll:37toll:38
lt'42 to 11:43
14:26 to 14:27
14:29 to 14:31
14:33 to 14:35
15:41 to 15:42
15:44 to 15:45
15.48 to 15:49
17:08 to 17:10
17 15to 17:17
17:19lo 17:20
17:22lo 17:24
Go,
To lower
H2S,
12
3
'"
12
10
24
28
32
8
20
16
l«
96
140
52
52
56
216
300
16
8
16
32
39
MeSH,
4./I
6,240
6,000
6,560
1,280
320
3,480
3,520
4,960
1,940
9,440
9,920
15,760
9,840
12,800
12,600
6,400
3,680
7,840
4,480
5,200
4,240
240
9,920
8,800
4,320
2,820
2,160
5,040
5,693
From to,e,
H2S,
4B/1
96
92
10
in
2
2
MeSH,
, 16
24
1,840
1,690
192
152
60
48
448
108. Schoening, M. A., and R. H. Wright: Intensity of the odor release at
various points in the kraft pulping process. TAPPI, 35:564-569. 1952.
A study was carried out at the Bloedel, Stewart, and Welch pulp mill
to determine the concentrations of hydrogen sulfide and mercaptan
issuing from various sources. The sources investigated were:
1. Digester relief,
2. blow gases,
3. evaporator effluents,
4. furnace gases,
5. stack gases,
6. washer room ventilators,
7. green liquor tank vapors,
8. sewer outfall,
9. lime sludge filter.
Results are summarized in Tables 33 through 39. It was concluded
that the blow tank and stack gases were the major sources of odor,
while the other effluents were minor in comparison.
-------
Chips
Fir
Fir
Muni lock
D.gester
charged
10:5-1
11 52
10-46
Sample token
11:121011:18
ll.lBto 11:26
11. 26 to 11. 35
953io 9:58
9 58 10 10:03
12-10to 12.13
12 13)o 12 16
12.16to 12-21
12 21 to 12.27
1? 27io 12:34
12 34to 12:42
10-53 to 10: 58
10. 58io 11-04
11-04 rol 1-10
15 10)o 11-17
11-17)0 1 1.23
11 23)oll.29
H2?i
J>9/l
104
96
112
40
120
16
8
methyl mer-
coplan, ftg/\
13,600
14,600
6,300
9^800
7,300
920
280
1,240
1,040
120
1,000
440
400
1,230
Con
H2S,
na/m \
18
19
14
41
38
1
1
3
6
7
9
8
2
4
den sate
Methyl mer-
captan,/ig/ml
304
64
32
224
176
144
Remarks
Digester charge included 200 Ib sulfur
Digester charge included 200 Ib sulfur
No sulfur added to digester charge
No sulfur added to digester charge
Table 34. DIGESTER RELIEF
p
Fir
Hemlock
Fir
Hemlock
Dige.ler
7.50
7:45
11-42
6:41
D.gester
10.33
11:00
4-27
9.20
OigeS)er
own
10-50
n-i9
4.51
9:41
Samp e la on
0-27)olO-30
0:32 to 10:35
0.36to 10: 39
1:03 to 11 06
I 06 to 11 13
l:13toll 19
4.33lo 4-37
4-37(0 442
4-42)0 4-47
4-47ra 4-51
9:26lo 9:29
9:29to 9-32
9.32)o 9:35
9:35io 9:38
9:38to 9:41
Nonconde
",2/i
fi9/l
60
76
104
16
32
64
160
93
147
73
84
104
124
32
72
nsab|ego5
Methyl
mercop an,
8,300
9,800
0,400
3,400
4,700
3,500
7,300
7,500
0,800
9,500
5,200
4,600
3,500
2,000
4,000
Con
HjS
^9 ml
08
40
51
66
65
24
73
49
48
56
16
22
42
43
71
Methyl
Tg/mV'
1,308
8 2
8 6
1,1 0
9 6
7 0
178
136
128
140
456
444
436
244
312
Renarks
Digester charge included 200
Ib sulfur
Digester charge included
200 Ib sulfur
No sulfur added to digcsler
charge
No sulfur added to digester
charge
Table 35. DIGESTER BLOW GASES
Chips
Fir
Fir
Hemlock
Fir
Fit
Hemlock
Digger
blown
1 45
9:30
9:52
9:09
1:10
10:37
3.34
Somple taken
1:47 lo :49
1:49 to :50
1.50 lo :52
1 52 to .52!;
l:52hto :53b
1-53'jto .55
1 55 to .57
9:31 to 9:33
9:33 to 9-:>l
9:35 to 9 36! 3
9-36!3to 9 38
938 lo 9:40
9 40 lo 9:42
9:42 to 9 43! i
9.43'ito 9-45
9:53 lo 55
9.55 to 57
9:59 to 1 00
10-00 io 1 01
10-01 lo 1 02
10-02 to 1 03
10-03 tol 05
9:10 to 9:14
9:14 lo 9:18
9:18 lo 9.23
1-10 lo 1-14
1:14 to 1 18
1:18 to 1 22
10:37 Id 10:42
10-42 to 10:46
10:46 to 10 SO
2:35 lo 2:40
2:40 lo 2:44
2:44 to 247
Nancon
H2J|
Nd°
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Vo
Methyl
merca^an,
12,500
31,700
49,000
93,100
99,900
102,700
51,400
5,680
11,400
12,800
16,800
23,200
19,200
30,000
22,000
11,200
33,600
36,400
49,800
29,600
38,400
16,800
4,600
3,800
19,800
3,800
8,800
30,800
800
6,200
3,300
6,600
10,400
1 1,700
Conde
^m'l
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
13
28
20
16
34
29
2
12
21
22.
31
1
Methyl
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
188
356
344
228
428
524
120
188
356
112
212
8
Total vapor
Methyl mer-
capton, Jig/
ml at 10OC
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
198
281
578
20fi
424
892
85
228
272
191
322
227
Remarks
20DB|btTuifuTa0
Digester chorge included
200 Ib sulfur
Digester charge included
200 Ib sulfur
No sulfur added to di-
No sulfur odded to di-
gester charge (blown
From higher than uiJal)
No sulfur added to di-
gester chorge
No sulfur added to
digester charge
-------
Table 36. EVAPORATOR JET CONDENSER EFFLUENT
Dote, 1950
May 25
May 30
May 31
June 2
Liquid effluent
H2S,
Ag/ml
2
Tra
Tr
Tr
Methyl
mercaptan,
(tg/m 1
24
12
Tr
Tr
Air above liquid surface
H2S,
M/l
Tr
,...
Tr
Methyl mercaptan,
*g/l
120
28
20
16
a Tr means trace.
Table 37, FURNACE GAS
Dote, 1950
May 27
May 29
May 30
June 1
June 9
June 12
June 15
June 20
Nonconden sable gas
H25
A9'l
390
84
52
1
2
3
Methyl
mercaptan,
*g/i
7
Condensable gas
H2S,
£ g/m 1
2
49
52
10
3
2
9
27
Methyl mercaptan.
/ig/m 1
40
56
112
40
4
10
78
24
Table 38. STACK GAS
Date, 1950
June 9
June 9
June 12
June 15
June 20
Noncondensable gas,
depth 6 ft
H2S,
A9/I
3
326
119
370
460
Methyl
mercaptan,
A9/I
15
Noncondensable gas,
depth 12 ft
H2S,
A 9/1
42
195
204
536
420
Methyl
mercaptan,
A9/I
36
Condensate combined,
6 and 12 ft
H2S,
/ig/ml
1
7
5
15
46
Methyl
mercaptan,
/t g/m 1
30
10
25
28
109. Schwalbe, C. G.: Removal of the odors in sulfate pulp mills. Paper
Trade J., -75:51. Nov. 23, 1922. A process is described that utilizes a
tower, packed with wood chips or straw, to absorb the obnoxious
gases produced in the kraft process. Since wood has the property
of absorbing various organic sulfur compounds, it is hypothesized
that these products in their absorbed form react with oxygen to form
relatively less obnoxious compounds. This type of absorbent also
provides a surface for condensation of the sodium salts of the stack
gases. These solids can be washed off and re-used, and the wood chips,
-------
64
AIR POLLUTION
Table 39. MISCELLANEOUS OPERATION DATA
Sampling
Roof vent over knotter
liquor smelt tank
Hood over lime sludge
filter
Sewer outfall
Date, 1950
May 10,
May 24,
May 30,
May 18,
May 30,
June 1,
May 18,
May 26,
May 31,
Noncondensable gas
H2S,
A9/I
T?
7?
Methyl
mercaptan,
A9/I
32
20
4
16
Tr
Liquid
H2S,
ftg/m 1
2
Methyl
mercaptan,
/ig/m 1
12
10
a Tr means trace.
thereby regenerated, may be replaced in the tower for further use.
After use, the wood chips can be burned as boiler fuel. When the
chijps are used to purify digester gases only, they can be cooked
for pulp with no adverse effects.
110. Schwalbe, C. G.: Removal of offensive odors in sulfate pulp mills.
Paper Trade J., 74:52. Jan. 19, 1922. The amounts of mercaptans,
methyl sulfide, and methyl disulfide emitted per ton of pulp are
stated, and a discussion of the pollution problem, with various meth-
ods of contaminant treatment, is given.
111. Segal, W.: Quantitative determination of methyl mercaptan, dimethyl
disulfide.and dimethyl sulfide in a gas mixture. Anal. Chem., 25(11):
1645-1648. Nov. 1953. Studies were undertaken to determine the sulfur
products arising from microbial decomposition of organic sulfur com-
pounds. Evidence was obtained that volatile sulfur compounds such
as methyl mecaptan and alkyl sulfide were produced from methion-
ine. No satisfactory methods were available for the quantitative de-
termination of these products, formed slowly during the decomposi-
tion process. The method described provides a means of quantitatively
determining methyl mercaptan, dimethyl disulfide, and dimethyl
sulfide occurring as mixtures in gas. The method should find appli-
cation where quantitative determination of mercaptans and alkyl
sulfides is needed.
112. Segerfelt B. N.: The bad odor from sulphate pulp mills. Paper Trade
J., 74:95 June 8 1952. Sources of odor and operating conditions for
their minimization in the kraft process are discussed It is stated
that odors arising from the cooking process SHTSnL eSJ
ated by cooling the gases and diluting the condensate with water
It is recommended that the operating temperature of the furnace be
kept high to reduce odorous compounds further lurnace be
-------
Annotated Bibliography °^
113. Segerfelt, B. N.: Method of removing malodorous gases formed in the
sulfate and soda pulp manufacture. U.S. Patent l.S.'i'j.'rIft. Apr. 19,
1932. This method removes malodorous gases formed in pulp manu-
facture by bringing them to their dew points, and treats them with
hydrocarbons belonging to the terpene and camphor groups.
114. Semrau, K. T.,et al.: Influence of power input on eificiency of dust
scrubbers. Ind. Eng. Chem., 50:1615-1620. Nov. 1958. The performance
of venturi, cyclonic spray, and pipeline scrubbers on dust and fume
from a black liquor recovery furnace and a lime kiln was investi-
gated on a pilot plant scale. Cold water was used as scubbing liquid
in the first two devices, and hot recirculated solution in the third.
Efficiency data were correlated on the basis of total theoretical power
input per unit of gas flow rate.
115. Sensenbaugh, J. D., and W. C. L. Hemeon: A low cost sampler for
measurement of low concentration of hydrogen sulfide. Air Repair,
4:5-25. 1954. The AISI hydrogen sulfide tape sampler is described.
This unit operates by forcing the sample air by means of a vacuum
through a paper filter impregnated with lead acetate. Any hydrogen
sulfide in the sample reacts to form lead acetate, the amount of which
can be determined by photoelectric techniques.
116. Sherwood, P. K.: Odor removal improves public reactions. Paper Ind.,
42:784-787. 1961. There are several general methods of solving the
odor problems of kraft mills. Among these are the methods of cata-
lytic combustion, absorption, and odor masking and counteraction.
Odor masking and counteraction are attractive because of their rela-
tively low cost. A specific counteracting compound, however, may
be required for each different odor.
117. Siggia, S., and R. L. Endsberg: Determination of alkyl sulfides and
disulfides. Anal. Chem., 20:938-939. 1960. A method for analysis of
alkyl sulfides, disulfides, and mercaptans is described. The procedure
involves titration with bromine water, the endpoint being determ-
ined by the appearance of the characteristic bromine color. The meth-
od is precise to about 0.3 percent in the best cases.
118. Stevens, R. H.: Report of committee on sulfate pulp. Paper Trade J.,
84:142-145. Feb. 24, 1927. (Section on sulfate mill odors.) The com-
mittee has made no direct progress on the problem of odor abatement.
A process has recently been proposed to eliminate digester odors by
adding disodium phosphate. This has proved effective in experiments
but has yet to be proved on a full-scale basis. It was reported that
a Nordstrom drying tower constructed in the Crown-Williamette
Mill in Camas, Washington, was highly effective in reducing effluent,
malodorous gases.
119. Stone, J, E.: Progress in alkaline pulping—1953. Southern Pulp &
Paper Manufacturer. Oct. 1, 1954. This is a literature review on
progress in alkaline pulping for 1953. It covers the following sub-
jects: expansion, modernization, new mills, the cooking process, sul-
fate liquor cycle (including black liquor oxidation and odor control),
stream pollution, and digester corrosion. It has 200 references".
120. Sultzer, N- W., and C. E. Beaver: Alkali recovery by electrical pre-
cipitation. Paper Trade J., 102:33-35. Jan. 23, 1936. Losses of sodium
salts through the stacks of recovery furnaces of both soda and kraft
pulp mills may cause a serious economic loss. Tests for the magni-
tudes of these losses have resulted in the following data:
-------
66
AIR POLLUTION
1 UUIC *tw.
Plant
1
2
3
4
5
6
7
8
9
10
11
12
Process
Kraft
Kraft
Kraft
Kraft
Kraft
Kraft
Kraft
Kraft
Soda
Soda
Soda
Soda
Type of
recovery
furnace
Kiln
Kiln
Spray
Spray
Kiln
Kiln
Kiln
Spray
Spray
Kiln
Kiln
Kiln
Capacity,
tons/day
60
220
no
60
100
125
38
72
50
37.5
35
100
Na20 loss,
Ib /ton pulp
57
68
69
85
20
22
32
40
59
31
67
30
1 On the assumption that sa
mill would save $24,800 a
It cake costs $15 per ton, this indicates that a 100-ton kraft
nnually by collecting these compounds.
A variety of equipment is used for particulate collection in pulp
mill stack gases, but the combined advantages of the Cottrell electro-
static precipitation equipment seem to outweigh those of other types
Electrostatic precipitators can collect particulate salts either as dry
solids, as wet salts, or solution. Their efficiency is high over a wide
variety of operating conditions. The electric power consumption aver-
ages about 8 killowatt hours per million cubic feet of gas treated.
The pressure drop is only about % inch of water, and labor costs are
low. Corrosion can become a problem with steel precipitators handling
Na2SC>4, and it is suggested that all steel parts exposed to the stack
.gases be continuously wetted with an alkaline solution.
121- Sylwan, O.: Practical results obtained with black liquor oxidation.
Paper Trade J., 137:14. Sep,t. 4, 1953. A description is given of the
operation of a black liquor oxidation plant of the B-T type. It is
suggested that this type of unit should have many important ad-
vantages, even in older mills. With the oxidation unit, a 34 percent
increase in capacity of the recovery system was observed. Important
savings were also made because of reduced corrosion and fuel con-
sumption owing to the presence of the oxidation plant.
-------
Annotated Bibliography
67
122. Tomlinson, G. H., G. H. Tomlinson II, J. N. Swartz, H. D. Orloff, and
J. H. Robertson: Improved heat and chemical recovery in the alkaline
pulping processes. Pulp & Paper Mag. Can., 47:71-77. 1946. A progress
report is given on the development of additional heat and chemical
recovery equipment with descriptions of the various units involved.
These are as follows: cyclones, cyclonic evaporators, oxidation towers,
and packed columns. This material is presented with emphasis on
economic advantages. It is acknowledged, however, that the air
pollution aspect is of extreme importance.
123. Tomlinson, G. H., and J. M. Ferguson: Odor abatement in an akaline
pulp mill. Pulp & Paper Mag. Can., 57:119-122. Dec. 1956. The prin-
cipal measures employed to overcome the problem of odorous emission
at the Cornwall Mill have consisted of cooling and condensation of
the digester blow and relief steam, chlorination of the nonconden-
sable gases, and complete fixation of the black liquor. These mea-
sures, combined with control of the sulfidity of the black liquor,
have significantly reduced the "kraft odors" in the area surrounding
the plant.
124. Tomlinson, G. H., and H. R. Douglas: A progress report on the second -
dary recovery of heat and chemicals in the alkaline pulp mill. Pulp
& Paper Mag. Can., 53:96-104. Mar. 1952. The TBT recovery process
is described, which employs a conventional Bergstrom-Trobeck oxi-
dation system in the fixation of black liquor. Upon leaving the oxida-
tion unit, the liquor is passed through multiple effect and cyclonic
evaporators and then burned in the furnace. The resulting gases are
scrubbed in the cyclonic evaporator, two deluge towers, and a cooling
tower before being emitted to the atmosphere. Resulting hot water
is used for washing the brown stock. It is noted that in former appli-
cations of the B-T oxidation process the resulting minimization of
sulfur losses at the evaporators was offset by corresponding greater
losses (as SO 2 and SOg) in the furnace. With the present system,
however, such furnace losses are greatly eliminated.
TO STACK
HOT WATER
i
CONDENSING TOWER DELUGE
TOWER
GASES
O
DIGESTER
B-T
OXIDIZER
MULTIPLE
EFFECT |
EVAPORATC
Figure 13. Schematic flow diagram of TBT recovery process.
-------
68 AIR POLLUTION
125. Tomlinson, G. H., and G. H. Tomlinson II: Recovery of heat and
chemicals from black liquor. U.S. Patent 2,840,454. Paper Ind, 40:550.
1948. A new recovery cycle process has been developed capable of
recovering almost all the sodium sulfate fume in the stack gases
and eliminating the noxious odors emitting therefrom.
126- Thompson, C. J., H. J. Coleman, C." C. Ward, and H. T. Rail: Separa-
ation of organic sulfur compounds of liquid thermal diffusion. Anal.
Chem.. 29:1601-1611. 1957. The apparatus and procedure for thermal
separation of sulfur compounds from petroleum products is given.
Varying separation efficiencies were found for the different sulfur
compounds from other sulfur compounds and from hydrocarbons.
In some cases no separation could be obtained with the apparatus
described.
127. Tremaine, B. K.: Masking air-borne malodors; utilization of aromatic
chemicals in industrial odor abatement. TAPP1, 36:143A. Aug. 1953.
Recently, several products intended for the masking of industrial
odors have become available. These may be used in pulp mills in the
following ways: Spray them into effluent streams, add them directly
to the process, add them to scrubbing liquors, or spread them on
contaminated surfaces. The cost of this type of measure was said
to be about 10 to 25 cents per ton of pulp.
128. Trobeck, K. G., W. Lenz, and A. A. Tirado: Air pollution control.
Pulp & Paper Inter., Apr. 1959. This article is a report on the new
TLT method of control of kraft mill odors, used at the Loreto and
Pena Pobre mills near Mexico City. The method makes use of the
conversion of mercaptans into hydrogen sulfide, which is further
oxidized by means of air. From 1 to 3 kilograms of chlorine per ton
of pulp is added to supplement the chemical treatment. Odor-mask-
ing agents are reported only partially effective. Black liquor oxida-
tion eliminates several sources of noxious odors. The digester heat
recovery system is an effective control for offensive compounds.
The TLT system is described. Results of odor measurements are
given.
129. Trobeck, K. G.: The B-T system for soda and heat recovery in sulfale
pulp mills. Paper Trade J., 130:40. Apr. 20, 1950. The B-T process,
successfully used for over 6 years, is described. This system takes
liquor from the evaporators and concentrates it with superheated
steam at 400° C and 10 atmospheres pressure. The resulting sat-
urated steam is recirculated to a superheater and repeatedly used
as a drying agent. The concentrated liquor is then ejected into a
vessel at 1 atmosphere. The resultant "steaming off" creates a solid
containing only 15 percent water, which is then burned in the fur-
nace. Advantages of this method are:
1. Less stack loss from steam in flue gases,
2. reduction of heating surfaces in the boiler.
-------
Annotated Bibliography
69
PUMP
55
£5
LU *—
a:
t/1
LIQUOR FROM CONC
EVAPORATOR 1
i
0
LLJ
l!
"* h-
£«
to
:ENTRATOR
3 ATM I
,„. , ,l^~~— iF
T
VAPORATIONI
VESSEL 1
TO FURNACE
I
1 SUPERHEAT
ER|
Figure 14. Schematic flow diagram of soda and heat recovery process.
130. Trobeck, K. G., W. Lenz, and A. Tirade: Elimination of malodors in
a kraft pulp mill. TAPPI, 42(6): 425-432. June 1950. Experiences in the
tj-aft pulp mill of Loreto and Pena Pobre indicate that only three
sources of malodor wiU remain if the black liquor is properly oxi-
dized. These are: Gases produced by the digesters, condensates also
produced by the digesjers, and gases delivered by the stack of the
recovery boiler. An installation for the elimination of odors from
gases and condensates from digesters, and one similar installation
for handling gases going to the stack of the recovery boiler have been
successfully operated in Loreto and Pena Pobre for about 1 year.
Use is made of the reaction between chlorine and gaseous sulfur
compounds. A preliminary treatment of the sulfur compounds with
air in presence of water produced a definite saving of chlorine as
well as some additional benefits. Results of tests to improve and
clarify the different stages of the odor elimination system, including
black liquor oxidation, are discussed.
131. Trobeck, K. G.: Some data on the oxidation of black liquor. Paper
Trade J., 135:27-31. July 4, 1952. It has been found that the high
sulfur losses as hydrogen sulfide and mercaptans that occur in the
evaporators of a kraft mill can be reduced by oxidation of the black
liquor prior to its evaporation. For this purpose, several air oxida-
tion units were set up in various Swedish plants to combine black
liquor with air and produce a foam. This foam was then separated
with a cyclone and converted to its normal liquid form by means
of "foam-breaking" equipment. The black liquor, thereby oxidized,
was then sent on to the evaporation units. Besides being effective
in retaining sulfur, the oxidation systems have markedly reduced
corrosion in various parts of the operations, especially in the evap-
orators. The sulfidity of the resulting white liquor was raised to an
average of 28.74 percent. This is compared to a previous average of
26.16 percent. Trobeck believes that increased sulfur emissions from
other operations, owing to the presence of the oxidation equipment,
prevented the sulfidity from becoming even higher than the value
obtained.
-------
70 AIR POLLUTION
Table 41. AVERAGE TUBE LIFE IN EVAPORATOR (IN MONTHS)
Effect
1
2
3
4
Unoxidized
22.0
33.4
25.3
29.2
Oxidized
More than 1 15
More than 103
More than 55
More than 48
Table 42. EXPERIMENTAL DATA ON OXIDATION SYSTEM
Liquor temperature, before oxidation
after oxidation
Air temperature, before oxidation
after oxidation
Concentration, before oxidation (% solids)
after oxidation (% solids)
Sulfide content, before oxidation
after oxidation
Air used, per short ton
Black liquor solids per ton of pulp
Total black liquor per ton of pulp
190.4 °F
185 °F
68 °F
183.1 °F
16.1
17.6
0.35%
0.07%
l,520lb
2,800 Ib
17,400 Ib
AIR OUT
BLACK LIQUOR
CYCLONE
TO EVAPORATORS
Figure 15. Schematic flow diagram of black liquor oxidation unit.
-------
Annotated Bibliography
132. Von Bergen, J. M.: Odor counteraction in kraft mill operations. Paper
Mill News, 77:120-158. July 3, 1954. A general discussion of public
relations is given with special reference to the aspects of air pollu-
tion. The field of odor counteractant study is described, the present
state of the art is indicated, and an experiment with a full-scale
kraft mill is cited. The advantages and applicability of odor
counteraction systems are discussed briefly. The author notes that
a combination of two normally obnoxious odors may tend to counter-
act each other so that no strong smell is apparent. The chief advan-
tage of odor counteraction is that it requires little expense and is
relatively simple to apply.
133. Wagner, C. L.: Alkali recovery from pulp liquors. Ind. Eng. Chem.,
22(2):122-127. Feb. 1930. A new process for the recovery of alkali
from wood pulp manufacture is described. This unit (furnace, boiler,
induced draft fan, and scrubber) is almost identical in design to the
soda and sulfate processes. Its function is to burn the volatiles and
fixed carbons, secure absolute combustion, return the chemicals, re-use
them in the most available form, and eliminate wastes, labor, dirt,
smoke, and odors. The water scrubber was 87 percent efficient at
loadings less than 1 grain per cubic foot and 69 percent efficient at
loadings up to 2 grains per cubic foot. Emissions from furnace per ton
of pulp amounted to: 30,400 pounds of air, 1,560 pounds of water,
and 400 pounds of salt cake (sodium sulfate).
134. Wagner, C. L.: Modern recovery in sulfate and soda mills. Paper
Trade J., 89:82-83. Oct. 3, 1929. In the author's opinion, the Wagner
recovery furnace is one of the most outstanding developments in
pulp mill operations in the last decade. Developed by the J. O. Ross
Engineering Corporation, this furnace utilizes several principles, i.e.,
centrifugal force, countercurrent sprays, and wet contact surfaces to
remove and recover 97 to 98 percent of the effluent particulate mat-
ter. It is also successful in eliminating odor problems arising from
hydrogen sulfide and mercaptans, which are destroyed upon intro-
duction to the secondary combustion unit. The payback time of this
unit, based on recovery savings is stated to be about 2 years.
135. Wells, S. D., and K. A. Arnold: Kraft pulping with low sulfidity.
Tech. Assoc. Papers, 24:156-159. 1941. The benefits derived from the
buffering action of the sulfide in a normal kraft liquor may be ob-
tained by continuous injection of a liquor of low sulfidity during
cooking. A marked reduction in mercaptans and other substances
causing objectionable odors was noted during a cook with a liquor
of low sulfidity.
136. West, P. H.: Chemical and heat recovery with the venturi scrubber
at Thilmany. TAPPI, 38(7):399-402. July 1955. In 1946, a pilot plant
venturi scrubber, using water to clean the recovery furnace flue gases,
was tested at Thilmany; it showed chemical recovery efficiencies
of about 90 percent. A similar full-scale unit, installed in 1947 follow-
ing the then existing 120-ton recovery unit, substantiated these find-
ings. Further pilot studies, employing concentrated black liquor in-
stead of water, indicated improvement in heat recovery efficiency
over that of conventional kraft recovery systems and maintenance oi
-------
72 AIR POLLUTION
a high degree of chemical recovery. In mid-1951, because of greatly
increased pulp production and the demand for 600-psi steam, Thil-
many purchased a new 250-ton B & W recovery unit equipped with
a black liquor venturi scrubber. The decision to include the venturi
as an integral part of the unit was based on several years of highly
satisfactory performance from the water scrubber, the black liquor
pilot unit, and from other considerations such as capital investment,
space requirements, operation, and chemical and heat recovery. This
entire unit, in operation since May 1953, shows chemical collection
efficiencies of approximately 90 percent and heat recovery greater
than normal.
137. West, P. H., H. P. Markant, and J. H. Coulter: New veniuri scrubber
developments. TAPPI, 44:710-715. Oct. 1961. In the interest of im-
proving recovery of particulates from stack gases, the authors did
extensive pilot level investigation of the operating characteristics
of steam-atomized venturi scrubbers. This type of scrubber differs
from those of the conventional type in that it utilizes steam to
atomize the scrubbing liquor in the venturi throat. After the
pilot test program, a plant-scale scrubber of a similar type was in-
stalled at the Thilmany Plant. This unit gave a 90 percent scrubbing
efficiency for a conventional scrubber. This required 700 pounds of
steam per hour, at a pressure of 50 psi, for a 100-ton-per-day pulp
operation. Owing to the addition of this unit, the power require-
ments of the venturi fan and the scrubbing liquor pumps have drop-
ped. The thermal efficiency of the boiler-scrubber combination has
increased because of the reduction in temperature from 190 to 175° F
as the gases passed through the venturi. The unit has operated sat-
isfactorily since it was installed in 1960.
138. Woodward, E. R.: Chlorine dioxide for odor control. TAPPI, 36:216-
221. 1953. It has been well established by other odor-producing in-
dustries that chlorine dioxide is a much more effective odor counter -
actant than either chlorine or oxygen. This compound has been suc-
cessfully used to remove odorous compounds such as amines, mer-
captans, and hydrogen sulfide. In view of these properties, it is
suggested that chlorine dioxide be employed in the abatement of
kraft mill odors.
139. Wright, R. H., M. A. Schoening, and A. M. Hay ward: The colorimetric
determination of hydrogen sulfide and methyl mercapian in sulfate
pulp mill effluents and of sodium sulfide in black liquor. TAPPI,
34:289-294. July 1951. A quantitative determination of hydrogen sul-
fide and methyl mercaptan has been developed that makes use of
the reaction with Bindschedler's green, which forms methylene blue
and a pink substance, respectively. There is little interference caused
by the mercaptans in the determination of hydrogen sulfide. All hy-
drogen sulfide interference is eliminated in the methyl mercaptan
determination by addition of cadmium chloride and use of a filter be-
fore the color is developed. A colorimetric determination of sodium
sulfide in black liquor is also possible by this same general method.
Adsorption of methylene blue by black liquor solids must, however,
, *n ™ PyeVented by addition of a cationic wetting agent to the reagent.
140. Wright, R H.: The dissemination of odors from kraft mill smoke
stacks. Pulp & Paper Mag.. Can., 56:131-134. Apr. 1955 Marked im-
provements have been made in our capabilities to control gases
-------
Annotated Bibliography 73
emitting from digesters and evaporators. There still exists, however,
the problem of emissions from the recovery furnace, which becomes
largely one of atmospheric dispersion. Because of the logarithmic
relationship between concentration and the relative odor of pollu-
tants, the problem of odor reduction is difficult. It should be realized,
however, that the fewer the pollutants emitted, the smaller the area
that will be affected. The atmospheric dispersion equation of Button
was used to analyze the behavior of smoke issuing from chimneys.
From this study it was concluded that chimney design alone should
rarely be sufficient to eliminate odor problems. The relative effec-
tiveness of a stack is highly dependent upon surrounding terrain
and weather conditions.
141. Wright, R. H., M. A. Schoening, and L. W. Shemilt: The effect of
black liquoi fixation on the release of kraft odors. TAPPI, 36:180-183.
1953. An experiment was conducted in which the normal black liquor
supply of a kraft mill recovery system was replaced with oxidized
liquor. Samples were taken at various locations throughout the proc-
ess, and appropriate analyses of sulfur compounds were made. In
spite of various operational difficulties encountered during the ex-
periment, it was concluded that black liquor fixation is an effective
factor in odor control.
142. Wright, R. H.: The effect of packing type on the rate of black liquoi
oxidation. TAPPI, 36:85. 1953. Oxidation of black liquor flowing over
various types of packing was studied, and a method of scale-up to
large columns from these data was proposed. The packing consisted
of vertical sheets with various surface patterns designed to break up
the flow pattern and increase the residence time in the column. It
was stated that this type of packing possessed significant advan-
tages in that it decreased foaming and improved the oxidation effec-
tiveness.
143. Wright, R. H.: It is possible to build and operate a completely odor-
less kraft mill? Can. Pulp and Paper • Ind., Sept. 1957. The answer
to the question of whether building and operating an odorless kraft
mill is a realizable objective is only partly a technical one. Techni-
cally, a completely odorless mill is possible if management is pre-
pared to pay the price. In discussing the technical aspects of building
a completely odorless kraft mill, the author describes the kraft
operating sequence, sources of dust emission, sources of odor release,
and measures for the control of odors, and gives an outline of the need-
ed research. Sources of dust emission listed are recovery furnace, lime
kiln, and dissolving tank; control methods include electrostatic pre-
cipitators, bag filters, and scrubbers. Odor sources are digester gases,
evaporator gases, furnace gases, and miscellaneous gases (digester
loading, vents and pressure release valves, oxidation tower, brown
stock washers, and others); measures for odor control include scrub-
bing, black liquor oxidation, condensation, burning, and oxidation
with chlorine or ozone.
144. Wright, R. H.: Kinetics of the oxidation of sulfale black liquor. TAPPI,
35:276-280. June 1952. The oxidation rates of black liquor were
studied at various temperatures between 40 and 90° C, and the re-
action kinetics were analyzed on the assumption that oxidation occurs
through several first-order reactions. The data were explained by
the suggestion that one of these reactions has a relatively large rate
-------
74 AIR POLLUTION
constant and goes to completion rapidly. When this reaction is almost
completed, the slower reactions become more significant and dom-
inate the conversion process from, then on. Since reaction rates are
relatively slow, it was suggested that, for a system exhibiting a
moderate surface area, diffusion should become relatively unimport-
ant as a rate-controlling factor. It was therefore hypothesized that
any attempt to raise the surface area of contact equipment beyond
a certain value should not appreciably increase black liquor oxi-
dation rates.
145. Wright, R. H.: New work in kraft mill odor control. Paper 62-99.
Presented at the annual meeting of the Air Pollution Control Assoc-,
Chicago, 111. May 1962. The first significant progress in the odor
control field was made about 20 years ago when it was discovered
that the oxidation of black liquor rendered its volatile gases relatively
odorless. This operation has been studied extensively in recent years.
Recovery furnaces may also be a major emission source. This depends
largely upon the operating conditions of the furnace. The common
situation is the overloaded furnace that emits hydrogen sulfide in
high concentrations. Several other substantial advances have been
made in the field of pulp mill contaminant control, but these methods
are by no means fully effective. Difficulties, such as the foaming
tendency of black liquor, have reduced the applicability of some
processes to below the point that might otherwise be expected. One
encouragement is the increasing demand for the chemicals that are
the by-products of recovery processes. The profitable sale of these
chemicals may well give rise to a positive incentive for further
control of pulp mill emissions.
146. Wright, R. H.: Odor control in Canada. Pulp & Paper, 26:89. Jan.
1952. In 1948 the British Columbia Research Council undertook an
investigation of kraft mill odor control. Of special interest in this
investigation was the black liquor fixation process, discovered some
years earlier by Bergstrom and Trobeck. In kinetic studies of black
liquor oxidation, it has been shown that chemical reaction rate is
the controlling step of the process when the gas-liquid contact area
is large. Also, a new type of tower packing has been designed for
this process that is inexpensive and highly effective. A pilot-scale
tower has been built and operated successfully for liquor rates of
5,000 pounds per hour. Noncondensable gases from the digesters have
also been introduced to this tower, and have undergone a high de-
gree of oxidation. This process has been investigated mainly for odor
control purposes. But the increased sulfur retention of the black
liquor, owing to the presence of the fixation tower, presents another
attractive feature.
147. Wright, R. H.: Odor counteraction. Chemistry in Canada, Apr. 1958.
The purpose of this paper is to examine evidence for the phenomenon
of odor counteraction and to inquire whether, in the light of current
theories of olfaction, it is a real effect. It is concluded that no sig-
nificant evidence for the existence of a phenomenon of odor counter-
action exists, and that current theories of the olfactory process lead
one to expect that it would exist.
148. Wright, R. H., and R. W. Klinck: What Port Alberni has done to
control kraft mill odors. Paper Trade J. 139:22-24 Oct 10 1955
On the basis of prior fundamental work on black liquor oxidation!
-------
Annotated Bibliography
the British Columbia Research Council undertook the design of a
full-scale unit for installation at the Port Alberni kraft mill of Mac-
Millan and Bloedel Company, Ltd. This apparatus has been in oper-
ation for over a year, during which it has produced a marked re-
duction in the odor and, at the same time, has benefitted the mill
operation in other ways, for example, by improving the sulfur re-
covery. The purpose of this paper is to give a short account of the
equipment and the results it has secured. This is the first published
report on the operation of this new odor reduction system, which
not only controls recovery system odors but successfully absorbs
objectionable digester gases.
149. Wright, R. Ho Pulp mill odor control. Pulp & Paper Mag. Can., Dec.
1961. This short article reviews some of the more successful mea-
sures of odor control now in use. With black liquor oxidation, the
release of odor from the multiple effect evaporators is practically
eliminated. The first step toward control of digester gases is the
installation of a heat recovery system that reduces the volume of
uncondensed gas to manageable proportions. Several processes are
now in use for treating both digester and furnace gases. These are
discussed. It is clear from this discussion that no single, simple
method of controlling and eliminating emission of odors exists. It
is worth considering the cost of not containing the malodoous gases-
Flue gases from a recovery furnace may carry 500 ppm or more
of hydrogen sulfide. If 250 ppm is taken as a middle value, and
reasonable assumptions as to gas flow and cost of chemical make-
up are made, the odor from the flue gas alone represents a loss of
$0.57 per ton of pulp produced.
150. Wright, R. H.: Black liquor oxidation: View in '62. Paper Mill News,
Mar. 26, 1962. Black liquor contains alkali, lignin residues, sodium
sulfide, and various organic sulfur compounds. Black liquor oxidation
is a process in which the liquor is made to react with air to destroy
the sodium sulfide and methyl mercaptan. The process reduces odors
from the evaporators, conserves sulfur, and reduces corrosion in
the evaporators. It is not a cure-all, but it does offer advantages in
chemical economy and odor reduction that make it an integral part
of a modern kraft mill. At present, the main obstacle to the univer-
sal adoption of the process is the problem of foaming that arises in
mills cooking highly resinous woods such as pine. This problem is
being actively studied and it will not be long before practical methods
of dealing with it are found.
151. Wright, R. H.: The reduction of odors from krafl pulp mills. British
Columbia Research Council, Technical Bulletin No. 27. 12 pp. This
short report discusses making pulp by the kraft process, the malod-
orous substances and where they originate, the dispersal of odors
in the atmosphere, and the principles of odor control. It emphasizes
black liquor oxidation.
152. Young, D. J.: U. S. Patent 2,115,83:). Sulfate pulp process. This
process involves sweeping all air out of the digester prior to the
cook, and then bleeding the air-free gases fom the digester and
burning them while cooking is in progress.
153. Ziegelmeyer, F., and O. Feischl: Oxidation of sodium sulfide in black
liquor. Paper Ind., 40:324. 1958. The oxidation reaction of black liquor
was studied, and the results indicated that the hypothetical equation
-------
76 AIR POLLUTION
2Na2S + 20 2 + H2O = Na2S2O3 + NaOH (minus 215 K cal)
is valid. It was shown that the organic substances present in black
liquor increase the oxidation rate 20- to 30-fold.
154. Zimm, F. P.: Abatement of kraft mill odors. Paper Mill News, 77:20-
22. Aug. 14. 1954. The use of "reodorants'7 to counteract kraft mill
odors is discussed. For normal operation, a kraft mill emits from 4
to 6 pounds of foul vapors per ton of pulp produced. The DuPont
Alamasic line of reodorants has been about 70 percent effective in
reducing these odors. Meteorological factors play an important role
in the effectiveness of reodorants, and more or less of the deodorizing
compound would be expected to be effective, depending on weather
conditions and the time of day.
-------
SELECTED PAPERS
The following papers are reprinted with permission:
Odor and fallout control in a krafl pulp mill. G. A. Hansen. Jour.
Air Pollution Control Assn. 12:409-413. Sept. 1962.
Air pollution control activities associated with alkaline pulping.
E. R. Hendrikson. 18 pp. Unpublished, qa. 1960.
Is it possible to build and operate a completely odorless kraft mill?
R. H. Wright. Can. Pulp and Paper Ind., Sept. 1957.
New work in krafi mill odor control. R. H. Wright Paper 62-99.
Presented at the annual meeting of the Air Pollution Control Assn.,
Chicago, 111. May 1962.
-------
ODOR AND FALLOUT CONTROL
IN A KRAFT PULP MILL
G. A. Hansen *
ABSTRACT
A brief description of the source and nature of odors and fallout
released in the kraft pulping process is presented. Development and in-
stallation of operating equipment to reduce both odor and fallout around
a modern kraft pulp mill are described. Among the processes for control are
(1) collection and destruction of relief and blow gases, (2) black liquor
oxidation, and (3) recovery furnace and lime kiln control to suppress odor
release. Flyash control with an electrostatic precipitator followed by a
Venturi-type, water-spray scrubber is also described.
INTRODUCTION
If this paper on kraft pulp mill air pollution control had been written
10 years ago, the discussion would have been quite limited. As a matter
of fact, there would have been no paper because none of the techniques
and processes presented here were being used on a mill scale by the Weyer-
haeuser Company or any other company at that time; some were not even
in the laboratory stage yet.
Since the time the first kraft mill was built back in 1891, the men
who operated these mills were well aware of the fact that they had an
air pollution problem. The first papers concerning kraft mill odor prob-
lems appeared in the literature around 1900. Today, even with all of the
progress in recent years, most kraft pulp mills are still living with this
problem. We at the Weyerhaeuser Company are the first to admit that
our various systems will not completely eliminate the kraft mill odors
or fallout. However, they will reduce by up to 90 percent or more all odor"
and flyash released to the atmosphere. We feel that this is a significant
improvement. Through constant research, we are continuing our efforts
to further reduce that remaining 10 percent. This will not be easy, I
can assure you.
Before discussing the actual plant installations, a brief discussion
of the scope of the problem is in order as there are actually two phases
to our Kraft Mill air pollution problem: (1) odor and (2) flyash. In the
'Technical Director, Weyerhaeuser Company, Pulp and Paperboard Di-
vision, Kraft Mill, Everett, Wash.
79
-------
80
AIR POLLUTION
case of the Weyerhaeuser Kraft Mill at Everett, Washington, these prob-
lems were made doubly acute because, over the years, a residential area
encroached upon the industrial area where our sawmills are located. For
efficiency of operation, it is essential to locate a pulp mill adjacent to its
wood supply, which, in this case, was our two sawmills. This meant that
we had private homes within one block of our Kraft Mill building site.
Therefore, when the Kraft Mill was built in 1953, the latest and most
efficient air pollution control equipment was installed. Furthermore, as
new techniques and new equipment were developed they were put to
use immediately.
SOURCES OF ODOR
The kraft mill odor problem arises from the use of sodium sulfide
as one component of the pulping liquor. This normally amounts
to about one-third of the active pulping chemical charge; caustic soda
(NaOH) constitutes the other two-thirds. In the digesters, the sulfide ion
from the sodium sulfide combines with various organic side-chain radicals
from the cellulose and the lignin of wood chips to form such organic
sulfides as methyl mercaptan (CH3SH), dimethyl sulfide (CH3SCH3),
dimethyl disulfide (CH3SSCH3), and small amounts of similar ethyl
sulfide compounds . i In addition, hydrogen sulfide is formed in consider-
able amounts. To give you an idea of the quantities involved, Table 1
shows the pounds of the three major odorous compounds released per ton
of wood for various cooking conditions. Since the yield of unbleached pulp
is about 45 percent based on Cook No. 2 from Table 1, we can see that
there would be about 6.2 pounds of combined sulfur compounds released
per ton of pulp produced. Since all of these compounds are objectionable
in concentrations as low as 1 ppm, a 300-ton-per-day kraft mill pollutes
a lot of air if these are released to the atmosphere. In mills using black
liquor oxidation and with closely controlled recovery furnace operations,
these digester blow and relief gases are the major sources of odor release.
Toblc 1. VOLATILE SULFUR COMPOUNDS FROM PILOT PLANT COOKS
Cook
1
2
3
4
5
6
7
B
9
Mox
l.mp.
172
172
172
172
150
160
170
182
172
coding
time,
3:45
3:45
3:45
3:45
5.30
4'00
4:00
3:00
3:00
Active
olkoh,
18
16
u
19
20
20
16
16
Sulkily,
0
22.5
18
18.5
20
20
20
20
S^^oo^,..
H2S
0
0.29
0.25
0.18
Troccs
True..
CHsSH
0
1.60
1.29
1.25
0.29
0.35
1.15
1.49
0.15
(CH3)25
0
0.38
0.99
0.88
0.17
0.35
0.74
1.25
0.05
Tolol
0
2.77
2.53
2.31
0.46
0.70
1.89
2.84
0,2
Until the recent introduction of the continuous digester, wood pulp-
ing had been a batch process. Generally, all kraft pulping processes follow
-------
Selected Papers
31
essentially the same steps as shown in Figure 1, which is a flow sheet of
the batch process used at the Weyerhaeuser Company's Kraft Mill in Ev-
erett, Washington. As a result, release of blow and relief gases is inter-
mittent and somewhat erratic. Attempts to burn these gases directly from
their points of discharge have met with limited success. Relief gases are
released at irregular intervals throughout the 3-hour cooking cycle. Al-
though the amount of relief gas is relatively small, it contains most of the
odorous gases. In addition it normally carries with it a substantial quantity
of turpentine from the wood resins. All of these materials are extremely
flammable. Mixed with the proper ratio of air, they are explosive. Without
a proper collection system, burning of these organic-rich gases has proved
hazardous. The blow gases, on the other hand, contain less of the foul
gases but are released in large volumes. In our Everett plant, we blow
a digester approximately every 40 minutes, with a total release of about
8,000 cubic feet of blow gases, relief gases and steam. To successfully "de-
odorize" the kraft pulping process, these gas streams must be contained
and treated.
CHIP SUPPLY'
TO RETENTION
BLACK LIQUOR PONI>
STORAGE
TO RECOVERY * STAGE BLEACH «*!"
Figure 1. Everett kraft mill pulping and bleaching system.
As indicated above, early attempts at burning these gases met with
limited success to say the least. Many recovery furnaces and lime kilns
have been severely jolted with gas explosions, some resulting in serious
damage, when these gases were introduced directly to the firing zones of
these units.
In spite of such setbacks, Weyerhaeuser approached the odor problem
from the theory that the only logical answer was to destroy the odorous
materials, not to mask them with costly perfumes. After much experimen-
tation, including a full-scale pilot plant trial using World War II surplus
barrage balloons as gas collectors, a suitable gas container was selected.
-------
Q2 AIR POLLUTION
Chicago Bridge and Iron Works had available a low-pressure gas con-
tainer called a "Vaporsphere." This gas accumulator is a steel sphere with
a lightweight fabric diaphragm attached inside at the middle. The gas
enters through the bottom and is retained under the diaphragm which
"floats" up and down on the gas cushion. Both the blow gases and relief
gases are collected in this system. As the Vaporsphere fills, an automatic
valve opens to discharge just enough gas to equal the net flow generated
for each cooking cycle; it then closes again. The entire system is care-
fully sealed to keep out all air dilution. In other words, we eliminate
the explosion hazard by keeping oxygen out of the gas mix in the storage
system.
Our main problem has been to find a diaphragm fabric that would
resist the conglomeration of organic chemicals discharged into the Vapor-
sphere. Plastics, rubber, miracle fabrics—none of these would withstand
the attack, until Du Pont developed Mylar. With a film of Mylar sand-
wiched between two layers of cotton ticking, we have now had dia-
phragms which have lasted over 27 months. The cost on this basis is but
a fraction of a cent per ton of pulp produced. This low cost factor is im-
portant when comparing this system to the cost of some of the reodorants
and masking chemicals being offered which are only partially effective
in eliminating objectionable odors in the first place.
Containing these obnoxious gases is only the first step in solving
our problem. Next, we must dispose of the gases safely and economically.
While we were experimenting with the barrage balloons as gas containers,
a local airplane pilot volunteered to haul them up into the sky to dump
them. Although he was apparently quite serious in his offer, we felt there
was a better way.
Since the initial work on this problem was done at Springfield,
Oregon, where we operate an unbleached board mill, I will first describe
their system for burning the accumulated gases. Originally, the collected
relief and blow gases were piped to an Anthony furnace designed to with-
stand relatively severe explosions. Just before discharging the gases into
this furnace, they were diluted with air at a ratio of more than 50:1. This
transition from no air to an excess of air kept the gas-air mixture from
being in the explosive range except for just the instant when the air was
added. When the gas-air mixture entered the furnace, which was operated
at 1400 ° F., complete combustion of the organic sulfides and the hydrogen
sulfide to CO2, SO2 and H2O took place. Only when the furnace temp-
erature dropped below 900° F. did any odorous compounds pass through
unburned.
After several years' experience with the Anthony furnace, the sys-
tem was shown to be practical and safe. Today, these combined digester
gases are being piped to the combustion chamber of the lime kiln, where
they are burned (Figure 2). Since the firing end of the lime kiln operates
at a temperature exceeding 2000° F., it is more than adequate for incin-
erating the foul gases. Moreover, the heat value of these gases, although
small when compared to the total requirements of the lime kiln, is put
to a useful purpose. Coleman3 has described this system in detail in the
October 1958 issue of TAPPI magazine.
Where bleached kraft pulp is being produced, a simple, safe, wet
oxidation of the combined relief and blow gases can be practiced. Nor-
mally, the first step in bleaching wood pulp is a chlorination stage. With
-------
Selected Papers
83
SCRUBBER
FLAME
ARRESTER
FLOW
CONTROL
GAS
FAN
FLAME
ARRESTER
KILN
Figure 2. Burning of digester blow and relief gases.
kraft pulps it is customary to use 10 to 20 pounds excess chlorine per
ton of pulp in this stage. Being a powerful oxidizing agent, chlorine readily
oxidizes organic sulfides and hydrogen sulfide to free sulfur, sulfonyl or
sulfoxy compounds. 1 The threshold of detection for these compounds is
several hundred times as great as the original sulfides. Thus, by judi-
ciously bleeding our foul gases from the gas-accumulation system into
the dropleg of the chlorination washer we can utilize the excess chlorine
to destroy the obnoxious gases, as shown in Figure 1. This system has
been used with success in our Everett mill since we first went into opera-
tion in 1953.
RECOVERY FURNACE
Odor release is not confined to the pulping operation in a kraft mill.
The superior economy of the kraft process is based on recovery of the
pulping chemicals for re-use. During the cooking cycle virtually all of
the caustic is used up in the digesters, but only about two-thirds of the
sodium sulfide is used—with one-third remaining in the spent liquor.
When I say "used up," I should say that these chemicals have reacted with
acidic groups or phenolic radicals of lignin to make these lignins soluble.
To reclaim these chemical components, it is necessary to concentrate the
spent liquor and then burn the organics (lignin and wood sugars) for
fuel. Salt cake (Na2SC>4) is added to the concentrated black liquor be-
fore it is sprayed into the furnace. Carbon from the organics reduces the
sodium sulfate to sodium sulfide in the furnace bed. Thus, the "ash" from
the furnace is a molten mixture of sodium carbonate and sodium sulfide,
which is drained off and dissolved in water. Lime is added to the solution
to convert the sodium carbonate to sodium hydroxide. We then have our
-------
84
AIR POLLUTION
wfth the exception of the odor control systems
haeuser mills.
by our Weyer-
OXYGEN
METER \
Figure 3. Everett kraft mill recovery system.
In evaporating the spent liquor, multiple effect evaporators can
be used effectively to go from around 15 percent solids up to approxi-
mately 50 percent. For efficient furnace operation, it is necessary to fur-
ther evapprate the black liquor to 60 percent solids or more. Normally,
this is accomplished with some type of surface-contact evaporator using
hot flue gas for the evaporating medium. We use a cyclone-type evap-
orator at Everett, while a cascade-type evaporator is used at Springfield.
As you know, carbon dioxide is one of the principal products of
combustion. When this comes in contact with sodium sulfide in water-
such as happens in the contact evaporators described above—sodium car-
bonate is formed and hydrogen sulfide is released. This is precisely what
happens in any kraft mill recovery unit not using oxidized black liquor.
In a 300-ton-per-day kraft recovery furnace, this escapement of hydrogen
sulfide might approach 5,000 pounds per day.
Two techniques are practiced in Weyerhaeuser kraft mills to pre-
vent loss of sulfur from black liquor, both in the contact evaporators and
in the furnace itself. The principle of oxidizing residual sulfides in black
liquor through contact with oxygen from the air in a packed tower has
-------
Selected Papers 85
been known for some time. However, attempts to apply the conventional
countercurrent flow of air up, through a packed tower generated uncon-
trollable foam problems as the black liquor was discharged from the bot-
tom of the tower. Modifying the oxidation tower to pass both the black
liquor and the air concurrently down through the packed tower, as de-
scribed by Bialkowsky and DeHaas,2 reduces foam formation to the
point where a simple foam tank suffices to allow the foam to decay. Al-
though the oxidation product has not been identified to my knowledge,
the important thing is that the oxidized sulfur compound is retained in
the black liquor until it is again reduced to sodium sulfide in the reducing
zone of the recovery furnace.
In addition to black liquor oxidation, our recovery furnace is equip-
ped with a two-pen, Bailey recording instrument to monitor both oxygen
and combustibles continuously in the flue gas leaving the furnace. DeHaas
and Hansen 1 have shown that where furnace drafts are controlled to
maintain 2 to 3 percent oxygen and less than 0.5 percent combustibles in
the flue gas, malodorous sulfur compounds can almost be eliminated.
The combination of black liquor oxidation and controlled oxygen
in the flue gas has reduced sulfur emission from around 5,000 pounds per day
to less than 500 pounds. Under ideal conditions, we have been below 100
pounds per day.
LIME KILN
One other source of odor release in a kraft mill is the lime kiln.
When the recovery furnace "ash," or smelt, is dissolved in water, one
main product is sodium carbonate; the ether is sodium sulfide. The sodium
carbonate is converted to sodium hydroxide by adding quicklime (CaO)
to the dissolved smelt. In the resulting reaction, calcium carbonate forms
and precipitates. In some older mills this may be discarded; but, most
modern mills feed the wet lime mud into a rotary lime kiln and reburn
the calcium carbonate to calcium oxide and carbon dioxide. A small amount
of sodium sulfide from the smelt carries into the kiln in the lime mud.
Again, the carbon dioxide in the kiln gases (released from both the lime
mud and the kiln fuel) will release hydrogen sulfide from the sodium
sulfide unless care is taken to prevent it. The control technique is the
same as for the recovery furnace: keep an excess of oxygen in the flue
gas. Therefore, the kiln operator is also equipped with a recording oxygen
meter to monitor the oxygen level in the exit gas from the kiln.
FLY ASH FALLOUT PROBLEM
The second phase of a kraft mill air pollution problem concerns
particulated material—in other words, flyash which is discharged with the
flue gas. Even though the Everett Kraft Mill was built with the best com-
bination flyash removal system available in 1953, there was still a notice-
able emission of flyash in our flue gas. In 1955 we turned our attention
toward either adapting an existing system or developing a new system
to further reduce air-borne flyash from our recovery furnace stack.
-------
86
AIR POLLUTION
When the Everett Kraft Mill was built, it came equipped with a
cyclone-type, surface-contact evaporator. This was followed by a Cot-
trell precipitator manufactured by the Western Precipitation Corpora-
tion. As illustrated in Figure 4, this arrangement removed about 97.5 per-
cent of the flyash leaving the furnace. Under normal operating conditions
in a 300-ton-per-day recovery furnace, this would mean roughly 150,000
pounds of flyash entrained in the flue gas leaving the furnace unit. Of
this, about 75,000 pounds would be retained in the cyclone evaporator,
71,250 pounds would be removed by the Cottrell precipitator and, if no
other treatment were made, 3,750 pounds would be lost to the atmosphere.
Fortunately, kraft mill fallout consists primarily of sodium sulfate and
sodium bicarbonate, with minor amounts of sodium chloride and sodium
150
<
Q
100
50
z
o
o
<
o
u
Figure 4. Stepwise removal of flyash from recovery furnace flue gas.
-------
Selected Papers
87
carbonate, all of which are nontoxic in the amounts present. They did,
however, constitute a nuisance in the immediate area around the mill.
Even though the adjacent residential area had developed within a zoned
industrial area, the presence of our Kraft Mill flyash was generating some
loss of community good will. We decided to see what could reasonably
be done about the problem.
Other mills were facing the same problem. One kraft mill at An-
tioch, California, put in a second electrostatic precipitator in the gas stream
following the first unit. As far as I know, this has been effective; how-
ever, this unit is quite expensive. Other mills have added Venturi-type
mixers to bring black liquor in intimate contact with the flyash and, thus,
remove it. This is also effective but, in addition to being expensive to
install, this unit has a high power requirement for operation. After con-
sidering various existing techniques for the removal of flyash, we decided
to evaluate gas-scrubber systems on a pilot plant scale.
Ultimately, we developed our own version of a Venturi-throated
scrubber, which operates on water. Murray 4 described the evaluation and
selection of this unit in the November 1960 issue of TAPPI. This system
provides no chemical recovery and it requires about 2 million gallons of
water daily- It is, however, much less expensive to build and to operate
than systems designed to salvage the small amount of chemicals involved.
The value of the flyash could not justify the installation of the larger,
more complex recovery systems. Figure 4 illustrates flyash removal from
the furnace through the scrubber.
That our installation is effectively reducing flyash losses to a min-
imum has been shown in two. ways. As part of our program, we installed
a permanent sample line to the top of the recovery furnace stack. Dur-
ing each shift, recovery plant personnel test stack gases for sodium salt
losses. Since virtually all of our flyash components are sodium salts, we
have found it convenient to compare our losses in terms of pounds of
sodium per day. On this basis, sodium losses before the scrubber- installa-
tion are compared in Table 2 with sodium losses after the scrubber in-
stallation. Per cent removal is also indicated. Figure 5 shows graphically,
by year, the average sodium fallout at the filter plant station. As indi-
cated by the dotted line, there is a background fallout of about 1.6 x 10"^
ounces per square foot per day even when the Kraft Mill is not operating-
The addition of a supplementary ID. fan ahead of the scrubber accounts
for the improvement in 1958 over 1957.
Table 2. EFFECT OF RECOVERY STACK SCRUBBER ON FLYASH DISCHARGE
Recovery stock emission, Ib /day
Fallout ot filter plant, oz /ItVday °
Sodium loss
Before
3,000
44 x 10'5
After
300
3.6 x ID'5
Reduction, %
90
92
0 Includes a background fallout of 1 .6 X 10'5 01 /ft2 per day, which is present even when the Kraft Mill is not operating.
With the recovery stack scrubber effectively controlling flyash from
that source, there was another, less severe, problem at the lime kiln stack.
-------
88
AIR POLLUTION
AGE ANNUAL SODIUM FALLOU
OZ NA/FT2 /DAY X 10"5 )
*»•
Ui
CO
Oi
(S3
W
— '
^
\
\
~- SCRUBBER
INSTALLED
MILL DOWN AVERAGE
' ~ i"~ T ~
1956 1957 1958 1959 1960 1961
Figure 5. Effect of scrubber on sodium fallout, filter plant station.
Our lime kiln has been equipped with a Peabody scrubber from the very
beginning of operations. However, periodic tests showed that the Peabody
unit removed virtually 100 percent of the lime dust but there was an
appreciable escapement of sodium salts. Without too much effort, we
could add the lime kiln stack gas to the existing recovery stack scrubber.
It was felt that the extra 11,000 cubic feet per minute could be handled
without upsetting the effectiveness of the unit. This was done, and with
gratifying results. Normal sodium losses through the Peabody scrubber
had been in the neighborhood of 1,000 pounds per day. The addition of
these gases to the recovery stack only increased sodium losses from the
main stack from 300 to 400 pounds per day. This resulted in a further
net gain of 900 pounds of sodium removed from the atmosphere.
Prior to the scrubber installation, we were losing 3,000 pounds of
sodium per day from the recovery furnace system and about 1,000 pounds
per day from the lime kiln. The scrubber reduced these losses to a
total averaging about 400 pounds per day. This amounts to approximately
90 percent reduction in flyash. After we have completed installation of
a heat exchanger to recover waste heat from the scrubber recirculation
water, we hope to improve this efficiency somewhat. Tests have shown
that the colder the scrubber water is, the more effectively it operates. We
hope to squeeze our sodium emissions to less than 200 pounds per day.
-------
Selected Papers 89
SUMMARY
To summarize, I have described the general odor-control systems
currently being used in two Weyerhaeuser kraft mills. The systems are
practical from a cost standpoint and are reasonably effective, although
we do not claim them to be the ultimate answer to the problem. These
systems are based on collecting and destroying the malodorous compounds
by burning or by chlorine oxidation in the case of digester blow and relief
gases. In the recovery furnace and evaporators, release of odors is pre-
vented through black liquor oxidation and by carrying excess oxygen in
the recovery furnace flue gas.
Flyash at the Everett Kraft Mill has been reduced effectively by
combining lime kiln and recovery furnace flue gases before passing them
through a modified Venturi scrubber. Particulate matter discharged, as
measured by sodium concentration in the flue gas at the top of the stack,
has been reduced over 90 percent. This is confirmed by fallout samples
taken in the vicinity of the mill itself.
Perhaps the most effective evaluation of odor or fallout abatement
programs is the complaint department. Inasmuch as the installation of
the systems described has reduced complaints to the vanishing point, we
feel reasonably certain that they are effective.
REFERENCES
1. G. G. DeHaas and G. A. Hansen,"The Abatement of Kraft Pulp Mill
Odors," TAPPI, 38, 732-738 (December 1955).
2. H. W. Bialkowsky and G. G. De Haas, "Stabilization of Douglas-Fir
Kraft Black Liquor," Paver Mill News, 74, 14-22 (September 1, 1951).
3. A. A. Coleman, "The Combustion of Non-Condensable Blow and Relief
Gases in the Lime Kiln,",TAPP/, 41, 166-168A (October 1958).
4. J. .S. Murray, "Scrubbing Kraft Recovery Furnace Gases,"TAPP7,43,
899-902 (November 1960).
AIR POLLUTION CONTROL ACTIVITIES
ASSOCIATED WITH ALKALINE PULPING
E. R. Hendrickson, Ph.D.*
ALKALINE PULPING
Of the nearly 25 million tons of wood pulp produced in this country
each year, somewhat more than half is manufactured by an alkaline process.
The original alkaline process for producing pulp from wood was the so-called
soda process developed in England about 1854. Essentially this method
*Air Pollution Research Laboratory, University of Florida
-------
90
AIR POLLUTION
consisted of cooking wood chips in a solution of sodium hydroxide under
high pressure. A modification of the soda process is still practiced in this
country to a limited extent. Dahl, about 1890, began investigations to im-
prove on the basic soda process. His studies led to the development of
what is known as the sulfate process, in which a cooking liquor consisting
essentially of sodium hydrosulfide and sodium hydoxide reacts with the
wood chips in a high-pressure digester. At the present time by far the
majority of wood pulp produced in this country is manufactured by the
sulfate process. Both of these alkaline pulping methods have one thing
in common, in order to be economically feasible the cooking chemicals
must be recovered. Recovery of the spent cooking liquors, while reducing
those materials commonly discharged to a stream, may result in an in-
crease of those materials discharged to the air. Gases and vapors, as well
as solid and liquid particulates discharged into the air in the vicinity of
pulp mills, have resulted in numerous complaints from people living in
the vicinity. The industry, through its organization known as the National
Council for Stream Improvement, has undertaken support of an exten-
sive program of research into the air pollution problems which face the
industry. These research programs are presently underway at the Uni-
versity of Florida and at Washington State University. Because of the
relative importance of sulfate pulping and because of the peculiar prob-
lems involved, this portion of the industry is being studied first.
Briefly (Figure 1), the process of producing pulp by the sulfate
method consists of placing wood chips into large digesters along with
the cooking liquor and heating to about 350° F and 115 pounds pressure
CHIPS
DIGESTER
BLOW TANK
PULP WASHER
PULP
LIME
LN
A---'
'A**
tN
1
'" A '
,' ^Ifrvl
V f,ccOi.
PREC1PITATOR
^S_ NEW
SALT CAKE
Figure 1 . Odor release in the kraft process.
-------
Selected Papers 91
for up to 8 hours. Following the digestion period, the material is dumped
into a blow pit or blow tank, where the liquor is drained from the pulp.
This liquid, together with the first wash water is known as weak black
liquor and contains the spent reagent and about half of the wood consti-
tuents. As mentioned previously, an essential part of the sullate process
is recovery of chemicals from the cooking liquors. Modern day practice
calls for concentrating the weak black liquor in multi-effect evaporators
and direct-heat evaporators before burning in a recovery furnace to gen-
erate steam and to recover the cooking chemicals for re-use in the process.
Make-up and recovered saltcake is added prior to the direct-heat evap-
orators. Reducing conditions are maintained in the lower part of the fur-
nace and the carbonaceous materials are burned off. The resulting smelt
consists essentially of sodium sulfide and sodium carbonate. When this
smelt is dissolved in water it results in a green liquor in which the sodium
sulfide is converted to sodium hydroxide and sodium hydrosulfide. Since
the sodium carbonate is inert as far as the cooking liquor is 'concerned,
it is causticized by the addition of lime to convert the sodium carbonate
to sodium hydroxide and precipitate calcium carbonate. The calcium car-
bonate is recovered in a lime kiln for re-use in the causticizing process.
The causticized solution, called white liquor, is ready for use in cooking.
Before the digester is charged with the white liquor, a quantity of black
liquor is frequently added to it.
AIR POLLUTANTS
From this brief description of the process, it would appear that the
three major sources of air-borne emissions are from liquor preparation,
cooking, and chemical recovery, and that both particulates and Odorous
gases might be involved. Actually the air-borne wastes from suMate pulp-
ing consist mainly of:
1. Solids from the:
(a) Recovery furnace, composed of sodium sulfate and sodium
carbonate plus carbon particles.
(b) Lime kiln, composed essentially of lime dust.
(c) Power plant, composed of flyash, soot, or unburned bark, de-
pending on the fuel used.
2. Mists from the:
(a) Recovery furnace.
(b) Lime kiln.
(c) Dissolving tank.
(d) Causticizer.
(e) Digester.
(f) Blow tank.
3. Odorous and nonodorous gases from the:
(a) Recovery furnace, composed essentially of sulfur dioxide and
hydrogen sulfide.
(b) Lime kiln, containing smaller quantities of the same two
gases.
(c) Power plant, consisting of sulfur dioxide if the furnace is
being operated properly.
-------
92 AIR POLLUTION
(d) Digester relief, containing inorganic and organic sulfur com-
pounds, such as hydrogen sulfide, methyl mercaptan, dimethyl
sulfide, and dimethyl disulfide.
(e) Blow tank, containing the same compounds.
(f) Turpentine recovery.
(g) Evaporators, consisting of hydrogen sulfide, methyl mercap-
tan, dimethyl sulfide, and dimethyl disulfide.
Other sources are active, but those mentioned are believed to be the
major ones. In terms of volume, the off-gases from the recovery furnace
make this the major source. Digester relief gases, blow gases, and the
off-gases from the evaporators represent a considerable smaller volume,
but have a potentially higher nuisance value.
EFFECTS OF POLLUTION
The possible effects of air-borne emissions from sulfate pulping
are numerous (1). In the immediate vicinity of mills, objections are fre-
quently raised about the effects of fallout from recovery furnaces, lime
kilns, and bark burners. Vegetation damage has been attributed to fallout
and sulfur dioxide. Damage to structural materials and painted surfaces
has been reported from air-borne saltcake particles and various sulfur-
containing gases. Lead base paint may be blackened in the presence of a
sufficient concentration of hydrogen sulfide under specific conditions.
Fallout, of course, may be in the form of saltcake, flyash, soot, burned
bark and wood particles, and lime. As mentioned, some of these actually
do damage to materials, vegetation, and painted surfaces. Others merely
contribute to the general dirtiness of the surrounding area. The larger
particles are usually deposited close to the source, but smaller particles
may be carried several miles. Those which remain suspended in the air
contribute to visibility interference, sky darkening, interference with
transportation services, and may help to carry odors long distances.
Health hazards from concentrations of gases found in the area around
sulfate pulping operations are not considered to be of serious consequence.
Of the compounds which might be involved, the toxicity of sulfur dioxide
and hydrogen sulfide are well known, but little is known about the true
air pollution signatures of sulfate pulping: mercaptans, dimethyl sulfide,
and dimethyl disulfide. The limited data presently available indicate that
hydrogen sulfide is about ten times as toxic to rats as disulfide and mer-
captan. From observations on the concentration of hydrogen sulfide in
the vicinity of sulfate pulping operations, it has been tentatively con-
cluded that toxic gases are not at sufficient concentration in the vicinity
of such operations to justify air pollution control on the basis of health
hazards. Although no disease or infirmity has been proved to be directly
caused by odors per se, odors in the vicinity of pulping operations are
considered unhealthy by the uninformed. At least they are considered
obnoxious by many people, and as such, they might be held to constitute
a nuisance. If a recent court decision starts a trend in this direction, the
encroachment of odors on private property could be considered trespass-
It is known that under appropriate weather conditions, which fortunately
are not present all the time, the odor from sulfate pulping may be notice-
able many miles away.
-------
Selected Papers 93
Paint damage in the form of discoloration by hydrogen sulfide is
one of the most common complaints around sources of this gas. Recent
investigations by the paint industry have thrown some additional light
on the blackening effects of hydrogen sulfide. It might be well to point
out that many master painters, paint salesmen, and paint manufacturers
still hold that a lead-pigmented paint is necessary for the best outdoor
coating. Most fume-resistant paints contain titanium oxide as the main
pigment, but many of the best outdoor paints contain some quantity of
lead pigment to improve weathering. The major conclusion reached in
the study by the paint industry (2) can be summed up by saying that any
concentration of hydrogen sulfide present in the atmosphere will pro-
duce darkening of paint containing lead pigments in time if the paint sur-
face is actually wet with water. The researchers found, however, that
blackening is considerably delayed if the lead content of the paint is low
and if the paint film remains glossy and unweathered.
Solutions to some of the problems of air pollution facing the in-
dustry are not easily forthcoming. Research on projects at universities
and at individual mills is slowly producing results. The objectives of the
two university projects previously mentioned were originally aimed at
immediate investigation of control activities through a multiple approach.
It was soon found, however, that one of the first needs was adequate and
accurate sampling methods and the original objectives thus had to be
postponed while sampling was investigated. Work on the two projects
has been reported previously in the technical bulletins of the National
Council for Stream Improvement (3). Research activities in the individual
mills have been more directly aimed at control. Unfortunately, many of
these developments never appear in the literature.
CONTROL OF PARTICULATES
Many activities of the industry which result in a lessening of air
pollution potential, fortunately result also in economic recovery of raw
materials or product. In fact, the units most responsible for reducing
particulate air pollution from sulfate pulping were originally installed
and later improved to reclaim valuable saltcake. For some time, practi-
cally all sulfate mills have been designed with cyclones and high-efficiency
electrostatic precipitators or Venturi scrubbers on the recovery furnace.
Twenty years ago, the process required an average of about 300 pounds
of make-up, saltcake per ton of pulp. Today many mills operate on 80
pounds or less. At a mill producing 500 tons of pulp a day, this repre-
sents not only a savings of the cost of some 55 tons of saltcake daily, but
this amount is no longer spread around in the environment. For many
years, other important savings have been realized by appropriate scrubbers
on lime kilns, by condensing digester relief and blow gases from which
turpentine and other compounds can be reclaimed, and by converting from
disc-type evaporators to multiple-effect units. Discharges to the environ-
ment also have been reduced by improved combustion practices in power
boilers utilizing waste wood residues and in recovery furnaces.
More recently the industry, stimulated by an increase in air pollu-
tion legislation and encouraged by improved sampling techniques, has
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94 AIR POLLUTION
developed new ways of reducing air-borne emissions to an even lower
level. Some of the procedures are not yet in widespread use, but have
been quite successful in plants where they are practiced. At present, three
general approaches are being used for reducing odors and reducing fall-
out from recovery furnace particulates. To improve saltcake recovery,
secondary scrubbers have been installed in some instances following pri-
mary collection devices such as electrostatic precipitators. In general, it
is not the purpose of these secondary scrubbers to collect particles too
small to be collected by the primary devices, but rather to reduce losses
caused by carry-over. An added dividend is obtained since these scrubbers
also collect some gaseous material and thus reduce the amount of make-up
sulfur required. The secondary scrubbers may take any of several forms
although most of them are commercially available devices. Venturi scrub-
bers as well as spray-type washers are popular (4). Although the Ven-
turi unit is most effective in some respects, the pressure loss through the
device may be as high as 25 inches of water. Loss in the spray-type scrub-
ber, on the other hand, may be only 0.5 to 6 inches of water. Water, weak
black liquor, or both are used as scrubbing liquid. Generally, the scrubbing
solution is recirculated to a fixed salt content. Normal loads to the sup-
plemental scrubbing devices are said to be 0.05 to 0.15 grains per cubic
foot. This compares with 3 to 5 grains per cubic foot as the load on the
primary collectors. Since the secondary units are not intended for high
dust loading, they will clog readily if the primary device stops functioning.
The soda ash in the effluent is probably recovered as sodium carbonate
because of the carbon dioxide content of the flue gases. In order to function
properly and eliminate environmental problems rather than merely trans-
fer their location, it is essential that some effective type of mist elim-
inator be installed on the effluent line before discharge. A number of
commercial devices are available, but frequently a tall stack will serve
as a mist eliminator. In mills where secondary scrubbers are installed,
they reportedly are performing with satisfaction.
Mist losses from dissolving tanks have been a source of local an-
noyance in pulp mills. The most recent attempts to control such emissions
have made use of mesh demisters. The mesh fibers are usually of stain-
less steel or plastic and frequently are backwashed with water. At one
installation (5), it is reported that 99 percent of all droplets over 5ju di-
ameter are recovered.
ODOR REDUCTION
One recent approach to odor control is collection and oxidation of
blow gases and digester relief gases. The most satisfactory way of collec-
tion appears to be removal of the condensable fractions followed by stor-
age in a floating-cover or diaphragm-type gas holder. The gas holder floats
on the line to take care of surges that occur during a blow. Oxidation
of the collected gases has been accomplished by burning and by mixing
with bleach plant effluent. Several mills have installed equipment in-
tended to oxidize the sulfur compounds by burning. Pure digester relief
and blow gases are reportedly nearly free of oxygen. According to De
Haas and Hansen (5) the gases should not contain more than 5 percent
oxygen, or a mixture of gas and air should contain no more than 0.5 per-
-------
Selected Papers
95
cent total organic compounds- All organic sulfides are reported to be de-
stroyed at 1200° F. The flame propagation rate in mixtures of these gases
is less than 90 feet per second. In one early installation of this type, an
Anthony furnace was used as the combustion unit and was operated at
1400° F. This furnace was fueled with, turpentine, fuel oil, or gas. The
non-condensable fraction was stored in a gas holder and metered into
the furnace. Operating difficulties, as well as potential fuel savings, promp-
ted a change about 2 years ago. The gases are now burned in the lime
kiln. Coleman (7) reports that the gas. now flows through a rock scrubber
to remove turpentine mist, through a flame arrester, a flow controller,
and another flame arrester before being diluted in the primary air-duct
to the kiln. This arrangement is shown in Figure 2. The noncondensables
require dilution with 19 or more volumes of air so the flow controller is
set to provide a fixed rate. Both the primary air fan and the induced draft
MIXER
ADSORBER
Figure 2.
fan on the kiln are electrically interlocked with the flow controller to
shut off the gas if either fan stops. Rupture plates have been placed along
the gas lines in several locations. Noncondensable digester relief and blow
gases are readily oxidized with bleach plant effluent. For the usual con-
centration of chlorine in this waste, about four volumes of effluent are
required to each volume of gas. The simplest method of contact is to in-
troduce the gas flow into the dropleg of the bleaching unit. Combustion
of these gases has the advantage of possible recovery of heat and sulfur.
It has the disadvantage, however, of requiring elaborate precautions to
prevent explosion. When the gases are oxidized with bleach plant effluent,
neither heat nor sulfur can be recovered.
Another recent approach to assist in odor reduction has been under-
taken in a number of mills in the northwest where black liquor oxida-
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96 AIR POLLUTION
tion units are installed. In theory, oxidizing the sulfur compounds in the
liquor before evaporating and burning will prevent the formation of many
of the odorous compounds which are otherwise released. Actually, black
liquor oxidation is effective only on the release of odorous gases from
vacuum and direct-contact evaporators. Some improvement may be noted
in the recovery furnace where combustion may result in less hydrogen
sulfide production. It is reported that in some types of furnaces, oxidized
black liquor burns better than unoxidized. Black liquor oxidation also has
no appreciable effect on digester relief and blowgases. Collins (8) reports
that a high degree of black liquor oxidation results in a reduction of sul-
fur losses from evaporators of more than 90 percent. Sulfur losses
according to various investigators, may average from 1 to 15
pounds per ton of pulp where black liquor oxidation is not practiced.
Wright et al (9) pointed out, however, that black liquor oxidation is no
cure-all for odors since odor control must be judged on,an all or nothing
basis. The public generally is concerned more with the presence or ab-
sence of odors than their intensity.
Black liquor oxidation units take three general forms: packed towers,
thin-film oxidizers, and porous-plate aerators. These various types of units
have the advantages and disadvantages well-known to chemical engineers.
Air is used as the source of oxygen and except in those cases where pine
is pulped, no excessive foam is produced. As you progress eastward across
the country, more and more severe foam problems are found until they
reach their peak in the pine pulped in the southeast. Because of the ad-
vantages of black liquor oxidation, a special study is underway at the re-
gional laboratory of the National Council for Stream Improvement in
Baton Rouge. This study is aimed specifically at the application of black
liquor oxidation to southern pulping conditions. The thin-film unit devel-
oped by the British Columbia Research Council reportedly has been suc-
cessfully applied to southern operation. The use of tonnage oxygen has
been reported by several investigators {10,11), but under the conditions
of the studies pure oxygen was not found to be suitable. The Air Pollu-
tion Research Laboratory at the University of Florida is undertaking a
study of black liqour oxidation using tonnage oxygen which appears feas-
ible. In addition to no foam formation, potential heat recovery is promis-
ing. Several mills on the west coast which utilize packed towers for black
liquor oxidation have attempted the destruction of odorous gases from
the digester and blow tanks in the oxidizing unit (12). DeHaas (6) reports
that while nearly all of the hydrogen sulfide was destroyed, about 45
percent of the mercaptans and 90 percent of the dimethyl sulfide passed
through unchanged.
OTHER CONTROLS
The recent literature contains a proposal for treating blow and re-
lief gases to recover salable chemicals (13). It is not known whether the
complete process is in full-scale use in any mill in this country. Schneider
proposed the condensation of the condensable fractions of the gases to re-
cover terpenes. A wet filter would remove any fibers present in the offgas
following which outside air would be mixed with the stream to cool it to
about 100° F. Activated carbon is suggested as an adsorption medium from
which mercaptans and sulfides could be recovered. It is proposed that the
adsorbers be regenerated and stripped by steam with the steam condensed
-------
Selected Papers
97
to remove water and dimethyl sulfide. Any mercaptan and sulfides in
the water maybe decanted and fractionated. The gaseous mercaptan which
would pass through the condenser would be contaminated with dimethyl
sulfide, but could be liquified and fractionated. A scheme for accomplish-
ing this proposal is seen in Figure 3. The author reports that at present
prices, the cost of the proposed installation would be low enough to re-
cover the capital investment in two years. The value of recovering sul-
fide alone for a 300-ton-per-day plant would amount to tens of thousands
of dollars each year.
LIME KILN
Figure 3.
Several practices to remove odo.rs have been in vogue abroad for
a number of years but have not yet found favor in this country. Many
of them have been investigated here but they have not yet been put into
practice. For example, Collins (8) reports that oxidation of black liquor
by the addition of peroxides and other chemical oxidizers was studied
but was found to be impractical economically. Reduction of odorous com-
pounds in the gas stream has been attempted by using chlorine dioxide,
chlorine, ozone, catalyst, and adsorption of wood chips. Work was re-
cently completed at the University of Florida on catalytic oxidation of
some of the organic sulfur compounds found in pulping operations. With
further investigation, the results of this basic research may soon be ap-
plied to plant operation.
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98 AIR POLLUTION
In most sulfate mills, improved design and improved operation of
the recovery furnace can pay large dividends. The recovery furnace is
intended to serve as a reclaiming unit for cooking chemicals in a re-
ducing zone near the bottom and a heat-recovery unit in a secondary
combustion zone near the top. Adequate secondary air and sufficiently
high temperatures can result in greater heat recovery by oxidizing the
hydrogen sulfide to sulfur dioxide. The sulfur dioxide can then be ab-
sorbed from the gas stream in a wet scrubber and the sulfur losses re-
duced still further.
This paper has been able to cover only briefly, some of the steps
being taken to control air pollution from alkaline pulping operations.
Further research into many of the problems of air pollution which po-
tentially face the pulp industry is urgently needed. Some of the investi-
gative work can best be undertaken at individual mills. Other problems
should represent a group effort and be logically handled at existing proj-
ects established at universities and research institutes. This industry is
developing sound approaches to the solution of its air polution problems.
From my personal experience with air pollution, control activities, con-
trol agencies look with considerably more favor on industries having a
planned program of attack on their air pollution problems and a research
program in existence before they encounter serious trouble.
REFERENCES
1. Hendrickson, E. R., TAPPI, 43, 193A (March 1960).
2. Ward, G. B., Official Digest of the Federation of Paint and Varnish
Production Clubs, 28, 1089 (1956).
3. Atmospheric Pollution Technical Bulletin Series, Nos. ,1-14, National
Council for Stream Improvement, Inc., New York.
4. First, M. W., H. E. Friedrich, and R. P. Warren, TAPPI, 43, 182A
(June 1960).
5. Bernhardt, A. A., and J. S. Buchanan, TAPP/43, 191A (June 1960).
6. DeHaas, G. G., and G. A. Hansen, TAPPI, 38,732 (1955).
7. Coleman, A. A., TAPPI, ,41, 166A (October 1958).
8. Collins, T. T., Paper Trade J. 130, 37 (January 19, 1950).
9. Wright, R. H., M. A. Schoening, and L. W. Shemilt,TAPP7, 36, 180,
(1953).
10. Fones, R. E., and J. E. Sapp, TAPPI, 43, 369 (1960).
11. Murray, F. E. Project Report 59-36, British Columbia Research Council,
Vancouver (June 9, 1959).
12. Bialkowsky, H. W., and G. G. DeHaas, 'Paper Mill News, 74, 14 (Sep-
tember 1, 1951").
13. Schneider, F., TAPPI, 41, 70A, (January 1958).
-------
Selected Papers
99
IS IT POSSIBLE TO BUILD
AND OPERATE A
COMPLETELY ODORLESS KRAFT MILL?
R. H. Wright *
PART 1.
A COMPLETELY odorless mill is 100 percent free from odor, not
99 percent or even 99.9 percent free, but 100 percent odorless. The an-
swer to the question of whether this is a realizable objective is only partly
a technical one. Technically, the answer is that a completely odorless
mill is possible if management is prepared to pay the price. But the price
is made up of three parts and all three parts must be paid in full.
THE ECONOMIC APPROACH
TO BE FACED BY MANAGEMENT
The first part of the price consists of an initial down payment
for odor control equipment, including enclosures and collecting ducts and
decontaminating apparatus to treat all the effluents leaving the mill. Some
of these, like the smoke stack, will be needed anyway, others will not
* Head, Division of Chemistry, B C Research Council
-------
100 AIR POLLUTION
be normal adjuncts to mill operation, but will be especially designed for
the purpose of containing an odor which has heretofore been allowed to
escape uncontrolled.
After the down payment has been made there will be instalment
payments which will go on as long as the mill continues to operate. The
instalment payments are partly direct and partly indirect.
The direct instalment payments are the costs of maintaining all the
odor control equipment at top efficiency. This does not present much
difficulty in the cases where the equipment directly assists the process
of making pulp: it is usually not very difficult to convince the operators
of the importance of keeping a chemical and heat recovery system in first-
class condition. It may be more difficult to ensure the same degree of
maintenance on a duct and fan intended to collect a few cubic feet of
foul gas from a tailings chest or foam breaker But if a completely odor-
less operation is to be achieved, all the odor control equipment must be
kept at top efficiency. Several technically excellent odor control systems
which have been built at one time or another have failed primarily for
this reason. It is therefore most important that the odor control measures
shall interfere as little as possible with normal mill operation, and that
they shall give a minimum of trouble and require a minimum of main-
tenance. Unless this requirement is met they are not likely to be ade-
quately served and maintained by the operators whose primary respon-
sibility is, after all, production. Odor control equipment must be acceptable
to and accepted by the operators, and must be designed accordingly-
Even more important than the direct instalment payments are the
indirect instalment charges. These arise whenever it is necessary to limit
production to prevent odor emission.
DESIGNED CAPACITY AND ODOR-FREE OPERATION
For example a mill designed to produce 300 tons of pulp daily could
probably turn out 425 tons if everything were run at maximum overload,
but the operation might cease to be odorless at anything over 350 tons.
Under these conditions, odor-free operation would require that the mill
never be operated at more than 50 tons per day above its designed capac-
ity. This is a very real situation inasmuch as the most common limiting
factor in output is the amount of overload that can be heaped on the re-
covery furnace, and when overdriven the furnace may generate unmanage-
able amounts of odor. The difference in output between the odor-free level
and the absolute maximum level at which the plant can be driven is dif-
ficult to estimate in advance, but it is probably the largest charge against
the odor control system. Indirect payments must also be made when-
ever it is necessary to shut down the whole mill on account of some pos-
sibly quite minor breakdown in the odor control equipment (in an extreme
case, perhaps, nothing more than a broken window).
Breakdowns in odor'Controlling apparatus can be minimized (though
not wholly eliminated) by good design and in some case's duplication of
equipment, and it may be possible to over-design the odor control facilities
to a point where the limiting factor may be somewhere else, but the fact
that the odor control programme can impose an indirect cost by limiting
-------
Selected Papers 101
production musi be squarely faced and accepted at every level of the
organization.
If this is accepted, then an odorless mill can be realized.
If this is not accepted, then there is little point in making detailed
estimates of the initial down payment and direct instalment payments on a
"completely odorless mill" because the mill will not be completely odorless.
A preliminary consideration of the technical problems suggests that
the direct costs of a complete odor control system would not be excessive
nor would they all be pure outgo. The release of dusts and odors repre-
sents a loss of material whose recovery (possibly with the simultaneous
recovery of heat) would constitute an operating economy. Moreover, some
of the odor control measures may be directly beneficial to the process, as
for example, when black liquor oxidation gives smoother evaporator opera-
tion, less corrosion, and reduced carry-over of chemicals into the hot
condensate which can then be used on the brown stock washers with the
production of cleaner pulp.
PART 2.
THE TECHNICAL ASPECTS OF
A COMPLETELY ODORLESS KRAFT MILL
THE UNPLEASANT, cabbage-like odor, which is characteristic of
the kraft process and is commonly called the "kraft odor," is due mainly
to a group of organic sulphur compounds of which the most important are
methyl mercapian and its oxidation products, dimethyl disulfide and dime-
thyl sulfide. These and possibly other malodorous substances are formed in
the digester by chemical reactions between the cooking chemicals, es-
pecially sodium sulfide, and certain constituents of the wood. They are,
therefore, inseparable from the kraft process and its odor-free operation
will depend on methods for retaining them in the system or converting
them to innocuous substances before allowing them to escape.
Certain parts of the process may also give rise to an emission of dust
and it is generally desirable that this form of air pollution be minimized
if possible by the same equipment as that used to contain the odors.
KRAFT OPERATING SEQUENCE
Figure 1 shows the usual sequence of ^operations in producing kraft
pulp. Wood chips are cooked at about 175°*C with a solution containing
sodium sulfide and sodium hydroxide. When cooking is complete the digest-
ers are blown into a blow tank and the steam which flashes off may be
passed into some form of heat recovery system. The pulp is then separated
from knots and incompletely-cooked chips and washed more or less free
-------
102
AIR POLLUTION
I. DIGESTER
IA. DIGESTER RELIEF
IB. DIGESTER BLOW
2. BLOW TANK
3. ACCUMULATOR
4 OXIDATION TOWER
5. MULTI EFFECT
EVAPORATOR
6. DIRECT CONTACT
EVAPORATOR
7. ELECTRIC
PRECIPITATOR
8. MAKE - UP
9. FURNACE
10. CONTAMINATED
HOT WATER
II. SEWER
12. SEWER OUTFALL
13. DISSOLVING
14. CAUSTICIZING
15. MUD FILTER
16. LIME KILN
17. SLAKING
18. KNOTTER
19. WASHER
20. SCREENS
21. DRIER
22. TAILINGS
23. FOAM TANK
Figure
Odor release in the kraft process.
from the spent cooking liquor, or "black liquor," and the latter is then
concentrated by evaporation first in the multiple-effect evaporator and
then in a direct contact evaporator using hot flue gas from the recovery
furnace- The concentrated black liquor is burned in the recovery furnace,
after adding the necessary make-up, chemicals. In the furnace carbonaceous
matter is burned, sodium hydroxide is converted to sodium carbonate, and
sulphur compounds are converted as far as possible into sodium sulfide.
The molten "smelt" from the furnace consists mainly of sodium carbonate
and sodium sulfide and is dissolved in water to give "green liquor" which
is then recausticized with a soluvion of slaked lime to give white liquor,
which is then sent back into the digester. The calcium carbonate that
is formed in the recausticizing may be collected on a filter and reburned
to calcium oxide which is then slaked and used to recausticize a further
lot of green liquor. The furnace gas, besides containing the usual products
of combustion, also carries a substantial load of particulate matter, mainly
sodium sulfate and sodium carbonate which is usually collected by an elec-
trostatic precipitator and returned to the system.
Thus, two types of air pollution may result from this sequence of
operations: pollution by solid particles and pollution by objectionable odor.
DUST EMISSION
The following is a fairly representative analysis of the particulate
matter entrained in the flue gas:
-------
Selected Papers 103
NaCl 17.6%
NasCO» 36.4
Na2Sb, 33.6
Na2SO3 4.4
NaOH 2.0
Na-S 0.7
Carbon + undetermined 5.3
The sodium chloride in this sample was derived from the pulping of logs,
which had been floated in salt water, and it would be practically absent in
an inland mill. The rest of the components, with the exception of sodium
sulfate, tend to be markedly caustic and can damage or kill vegetation and
mar the paint on automobiles for distances of from 1 to 2 miles down-
wind from the mill. The control of this type of pollution is therefore
important, when a pulp mill is located near a built-up area or a farm-
ing region.
In addition to the fall-out of dust from the main stack, there may be
additional fall-out of particulate matter from two other sources. If the
lime kiln which produces quick-lime for the recausticizing process is op-
erating wholly or in part on the precipitated calcium carbonate filter cake,
there will generally be some entrainment of lime dust contaminated with
a small proportion of sodium salts unless the lime kiln stack is provided
with an efficient scrubber, in which case the dust load is usually negligible.
The other source of dust is the dissolving tank. In many mills the red-
hot smelt from the recovery furnace is allowed to flow in a relatively mas-
sive stream into the dissolving tank. This usually results in very noisy
operation with a copious liberation of steam and occasional explosions in
the dissolving tank, but there is usually very little loss of chemicals. In re-
cent years it has become a common practice to disintegrate the stream of
molten material with a high pressure steam jet so as to eliminate both the
noise and the danger of explosions in the dissolving tank. However, the
use of steam jets may increase the loss of chemicals by atomization and
entrainment in the steam leaving the dissolving tank.
ODOR RELEASE
In Figure 1 the places where odors may be released are shown by
clouds of vapor. Not all these odor emissions are equally important, how-
ever, some being much larger or more concentrated than others. Yet if a
substantially odor-free process is to be secured, all must be brought under
control. The places where the largest amounts of odor are released and
therefore those which have received the most attention up till now are
the following:
Digester Gases—The gases given off when the digesters are relieved
and blown contain particularly high concentrations of mercaptans along
with steam and possibly other substances such as methanol. (This last may
have been the substance responsible for occasional reports of eye damage
following exposure to heavy concentrations of digester gas. It is an unusual
hazard inasmuch as most mills today possess blow heat recovery systems
which condense the greater part of the steam in the digester gas and with
it the very soluble methanol.) It is a fairly common practice to discharge
all these gases into the blow tank from which they pass into the heat
recovery system. There they distribute themselves between the vapor phase
and the contaminated hot water in the accumulator tank. The uncon-
-------
|04 AIR POLLUTION
densed part constitutes a very serious source of odor because the volume is
substantial and the concentration of malodorous material is very high in-
deed. The contaminated water usually finds its way eventually to the sewer
where its ill-smelling constituents may be released at a greater or lesser rate
depending upon the temperature, the amount of exposure to the atmos-
phere, and the degree of dilution with other liquid effluents-
The Evaporator Gases—The weak black liquor containing about 15
percent solids, after separation from the pulp, is normally evaporated to
about 50 percent solids in a multiple-effect evaporator. In this process there
may be a substantial production of foul gases which commonly include
substantial amounts of hydrogen sulfide. These gases are carried out of the
evaporator in the barometric leg of the jet condenser. The black liquor
leaving the multiple-effect evaporator is usually further evaporated to about
65 percent solids in some form of direct contact evaporator where it is
exposed to hot flue gas from the recovery furnace. The amount of odor
picked up by the furnace gases may be quite small, or it may be very
great, depending upon the way the equipment is operated. If Ihe viscous
film of strong liquor builds up a cake which is subject to destructive distill-
ation, then a large quantity of foul gas may be generated and carried
into the atmosphere with the flue gas.
The Furnace Gases—The operation of the recovery furnace may be
a critical factor in the odor producing process. The primary function of
the recovery furnace is to generate heat by oxidizing the carbonaceous
material in the strong black liquor, and to regenerate the cooking chemi-
cals by reducing sulphur compounds to sodium sulfide which is a principal
constituent of the smelt. The problem of reducing one constituent of the
black liquor while oxidizing another is solved in part by correct design of
the furnace and in part by correct control of the combustion conditions, and
in particular the primary and secondary air supply. Inasmuch as the main
purpose of the furnace is to generate heat and chemicals, there may be cir-
cumstances when, for example, the demand for steam may require that
the furnace be operated in such a way that it generates substantial quan-
tities of foul-smelling substances (mainly partial oxidation products of
methyl mercaptan), usually because there is insufficient secondary air or
because the furnace is forced beyond its proper capacity. Generally speak-
ing, the furnace plus the direct contact evaporator constitute a significant
source of odor comparable in importance to the digester gases.
Miscellaneous Gases—In addition to the foregoing, Figure 1 shows
a number of other points of odor emission, which individually may be rela-
tively unimportant, but which in the aggregrate are sufficient to produce
a significant odor problem over distances of a mile or two even though
the major sources may be completely controlled.
For example Figure 1 shows an intermittent odor release from the
top of the digester itself when it is opened to admit a fresh charge of chips,
and occasional puffs of odor from the blow tank and accumulator vents.
The blow tank is not usually designed as a pressure vessel, and is therefore
fitted with pop valves, which allow steam to escape if for any reason the
condenser on the heat recovery apparatus fails or is slow to start up. The
prevention of this kind of emission is mainly an operating problem, but a
comprehensive odor control system would make provision for some ad-
ditional condensing measures to take care of abnormal operating condi-
tions. In the same way there is normally no emission of vapor from the
-------
Selected Papers 105
accumulator tank vent when connected to the oxidation system in the way
shown in Figure 1, but if the heat recovery system fails for any reason to
condense the greater part of the steam when a digester is blown, then the
volume may be greater than the oxidation tower can handle and the ex-
cess will escape via the vent pipe. Under these conditions large quanti-
ties of malodorous material may be released from both the accumulator
vent and the oxidation tower exhaust. This again is an abnormal but by no
means an unknown event and a complete odor control system would in-
clude additional condensing capacity on the lower end of the accumulator
vent.
Again, when the mixture of pulp and black liquor from the digester
is separated on the brown stock washer the hot mixture releases substantial
quantities of water vapor and malodorous material which are commonly
collected in a hood and discharged to the atmosphere. The concentration of
mercaptan is normally quite small but the total volume of exhausting air is
so large that the aggregate amount of odor is quite significant from this
source. In general there will be some odor release wherever black liquor of
incompletely washed pulp is exposed to the atmosphere.
MEASURES FOR THE CONTROL OF DUST EMISSION
The dust content of the furnace gas is fairly large, in the order of
about 100 pounds per ton of pulp produced, and it is normally made up of
very minute particles less than a micron in diameter. The installation of an
electrostatic precipitator serves to retain 85 to 97 percent of this material,
but the operation is not 100 percent and the balance escapes to the atmos-
phere. During its passage through the precipitator, however, the dust is
largely flocculated so that the solid material which escapes from the stack
consists partly of fluffy aggregates up to a millimeter in diameter which
fallout within a mile or two of the mill and partly of fine particles which
are comparable to those found in ordinary industrial smoke. It is mainly
the large aggregates which damage paint and vegetation.
To remove the dust which passes through the electrostatic precipi-
iator, bag filters have been recommended. They are reported to provide an
effective solution, though possibly a costly and troublesome one because
of short bag life. An alternative, and on the face of it, an attractive solution,
is the use of a system of scrubbers designed to recover both heal and
chemicals from the furnace gases. Of these the two best known are the
Inka System and more recently the Tomlinson-B-W System. In the latter
the electrostatic precipitator is eliminated completely and the furnace gases
are passed* through a system of deluge towers or more recently cyclonic
scrubbers. These are arranged so that the hot gases are first brought in con-
tact with black liquor from the multiple-effect evaporators whereby addi-
tional moisture is removed from the black liquor while the flue gases are
cooled nearly to the dew point. Later, scrubbers take the flue gases through
the dew point so as to recover the latent heat of the water vapor while
at the same time scrubbing out a large proportion of the remaining solid
material. The cooling and condensation of the moisture reduce the total
volume of the gas to a point where a smaller and lower stack can be used
to carry it off.
Earlier attempts to use processes of this sort were frustrated by the
fact that the flue gases normally contained both sulphur dioxide and hydro-
-------
|06 AIR POLLUTION
gen sulfide, and this combination in conjunction with water makes an ex-
cessively corrosive mixture. Hence it was necessary to make sure that the
electrostatic precipitator and ancillary ducting were always maintained at
a temperature above the dewpoint. The position was changed when it was
shown that the process of black liquor oxidation (to be more fully discussed
below) largely eliminates the hydrogen sulfide from the furnace gases and
so overcomes the greater part of the corrosion problem. It also enables the
sensible and latent heat of the stack to be recovered without excessive cor-
rosion of the equipment.
The initial scrubbing of the flue gas with strong black liquor must be
carefully controlled. The black liquor is strongly alkaline because of its
sodium hydroxide and sodium sulfide content, and in contact with flue gas
it absorbs carbon dioxide which neutralizes the alkali and displaces hydro-
gen sulfide from the sodium sulfide. The displacement of hydrogen sulfide
can be prevented by the black liquor oxidation process already mentioned,
but the carbonation of the alkali, if allowed to go too far, may lower the pH
to a point where the organic material (present as sodium salts of lignin
fragments) is precipitated with the formation of a solid cake in the scrubber.
The power required to operate this system is substantially greater
than that required to operate a disc evaporator and electrostatic precipitator
but it is claimed that the improved heat and chemical recovery justify the
cost and the process can be operated with little difficulty and with a major
reduction in air pollution by both particulate matter and odors.
At the present time there do not appear to be any standard or ready-
made solutions to the problem of solid material from the dissolving tank
though technically the matter is not difficult. The lime kiln dust is usually
well taken care of by a scrubber of standard design which is normally
integral with the burning equipment.
MEASURES FOR THE CONTROL OF ODORS
The emission of odors from the recovery plant is claimed to be sub-
stantially controlled by the Tomlinson-B-W System already described-
Short of this a substantial degree of reduction may be effected in plants
equipped with the more conventional electrostatic precipitators by the pro-
cess of black liquor oxidation. In this process, the weak black liquor after
separation from the pulp and before entering the multiple-effect evaporator
is exposed to an oxidizing gas, usually air, in such a way that the sodium
sulfide in the liquor is destroyed by conversion to other compounds of
sodium and sulphur. (The precise chemical details of what happens have
not been fully worked out. It is generally agreed that the sodium sulfide
and mercaptan content is reduced by oxidation, but whether the main pro-
duct is sodium sulfite or sodium thio-sulfate, or some other compound is
not yet known.) Experience in a number of mills in various parts of the
world has shown thai when black liquor is properly oxidized the release
of odors in the multiple-effect evaporator and furnace is very greatly
curtailed.
Several kinds of black liquor oxidizing apparatus have been reported.
Generally speaking, they fall into two types: those which secure contact
between air and liquor by intentionally making foam (and then breaking it),
-------
Selected Papers
107
and those which avoid foam production as far as possible. In view of the
powerful froth-forming characteristics of black liquor from resinous types
of wood, the non-foaming oxidation processes (if they can be made to work)
have much to recommend them on the score of simplicity and freedom from
operating troubles. Generally they consist of a tower of some sort con-
taining a refractory packing over which the liquor trickles in good con-
tact with air which is passed through the tower.
If ihe air is passed countercurrent io the black liquor, it tends to strip
malodorous material and carry it out into the atmosphere. If. however, the
air and black liquor flow concurrently, any fbul smelling substances which
may escape from the fresh black liquor that enters the tower are re-
absorbed by the oxidized liquor farther down. This principle can also be
used to absorb the non-condensable digester gases, which otherwise would
continue to be formed and escape during the relieving and blowing opera-
tions. If the oxidation equipment is built close to the equipment that is
used to condense the steam and vapors from the digesters, a very simple ar-
rangement serves to ensure that any uncondensed gases are mixed with air
and drawn into the towers where their foul smelling components are ab-
sorbed and oxidized in the black liquor. This procedure was developed
and patented by the British Columbia Research Council.
A typical installation embodying the process is shown in Figure 2.
Steam and vapors from the blow tank (A) pass via the duct (B) to the direct
contact condenser (C) and into the hot water accumulator tank (D). The
noncondensable (and very malodorous) gases would normally escape to
the atmosphere via the stack (E) but are, instead, drawn through the duct
(F) into the oxidation towers (G) by fans mounted on top of the towers.
The additional air required for the oxidation is drawn downward through
the stack (E). The success of this process depends on the fact that black
Figure 2. Showing the blow tank (a), the accumulator tank (d), and the
oxidation tower (g).
-------
I08 AIR POLLUTION
liquor contains substances which strongly catalyze the oxidation of mer-
captan and that the tower design permits a very high ratio of air to black
liquor to be passed through the towers. In this way the peak emissions of
digester gas which occur when digesters are blown can be taken into the
towers without impairing their oxidizing capacity. This makes it un-
necessary to provide storage in vapor spheres or otherwise of the ex-
tremely foul and, on occasion, explosive digester gases. They are handled
automatically as they come.
It has sometimes been suggested that sulphur compounds which have
been absorbed and oxidized in this way would later be stripped out again
in the recovery plant, and especially in the direct contact evaporator. Ex-
perience with several installations now in operation has not borne out
this prediction.
The measures thus far described, namely, black liquor oxidation
(with absorption of digester gases) plus scrubbing and condensation of stack
gases, are capable of bringing about a very great diminution of the amount
of malodorous and particulate material released into the atmosphere. In
order to understand what effect these measures will have on the mill odor
it is necessary to understand certain characteristics of the sense of smell
and of the way gases are diffused through the atmosphere.
Figure 3 shows the relation between the concentration of ethyl mer-
captan and the "strength" of the odor expressed on a zero to five scale esti-
mated subjectively by each of a number of observers whose separate assess-
ments were averaged and plotted as points on the graph. It is evident that
the strength of intensity of the odor sensation, insofar as it can be given a
numerical rating, is proportional to the logarithm of the concentration of
odorous material. This means that, for example, a 90 percent reduction
of malodorous material at any point, i.e. a reduction by a factor of 10,
may make a scarcely perceptible reduction in the strength of the odor
and a 99 percent reduction in concentration would be required to reduce
the odor from, for example, "faint" (strength 2) to "very faint" (strength
1). At first sight this would seem to preclude the possibility of really elim-
inating the odor, but the situation is redeemed by the fact that as soon as
the odor is released it begins to mix with air until it becomes diluted to a
concentration that is below the threshold of perception. Hence if the quan-
tity of malodorous material that is released at the source can be reduced
by, say, 90 percent, the quantity of air required to dilute it will also be
reduced by 90 percent, which in practice means a reduction in the radius
over which the smell will be noticed.
The dilution of gases released into the atmosphere has been analyzed
mathematically by Button, who has taken into account such factors as
wind strength, atmospheric turbulence, height and strength of the point of
emission, and other relevant factors. His formulae have been tested experi-
mentally in connection with the travel of war gas clouds and chimney stack
plumes and have been shown to afford a reasonably accurate account of
what actually happens over short distances. For greater distance the postu-
lates tend to become artificial: for example it may be calculated that for a
source of given strength the odor would be perceptible at distances in the
order of 50 miles with a wind of 2 miles per hour velocity. But this would
imply that the wind blew in a constant direction and with a constant ve-
locity for 25 hours, which is somewhat unlikely, at least in the temperate
zone. However, taking Button's formulae at their face value, the downwind
-------
Selected Papers
109
I*
0
z.
t*
iff
O.Z
O
0
0
i
0
1
x
jX
3
^
X
oc
0,
X*
1
i
'[
xj«
.«J"
* )
O
J^O
3.
> O
'*o
s
,>-
o
*
•^
"<
j
•)
>••
0"s KJ4 IO'3 I0'z 10"' 10° 10' 10' 10
ETHYL MERCAPTAN
MICROGRAMS PER LITER
Figure 3.
travel of odor from a source releasing five grams of odorous material per
second has been calculated for average conditions, over level ground cover-
ed with short grass, and for a wind velocity of five miles per hour. Figure
4 shows three curves, one for a point source of the specified type at ground
level and the other for point sources of equal strength at heights of 225
and 450 feet.
Even discounting any absolute numerical accuracy in these curves
they nevertheless afford some interesting qualitative information.
Thus the downwind range of an odor from a source at ground level
is relatively much less than that from an elevated source. This is because
there is a high rate of shear in the wind near the ground which induces
very rapid mixing. The effect of a tenfold reduction in the strength of the
source, say from five grams per second to 0.5 grams per second, is to dis-
place the horizontal axis (which represents the threshold of perception)
upward as shown by the dotted line. This reduces the downwind travel
by a factor of about 4. A further tenfold reduction in the strength of the
source would completely eliminate the odor from the 450-foot stack, would
almost eliminate it from the 225-foot stack except for a zone on the
threshold of perceptibility two miles downwind, but from the source at
ground level the odor would be appreciable for distances out to about
half a mile from the source.
Thus it may be concluded that the effect of every reduction in the
amount of odor released is to contract the area within which the odor will
be perceived. When it is necessary to prevent odors very near the mill, ex-
cessively high stacks are not as necessary as stringent measures to prevent
any odor from being released anywhere and especially to prevent it from
being released near the ground.
-------
110
AIR POLLUTION
DOWNWIND ODOR
WIND 5 M.P.H.
SMOOTH LEVEL GROUND
MINIMUM PERCEPTIBE ODOR I X IO'12 g/l
THRESHOLD WHEN SOURCE 0.05 g/sec.
SOURCE AT
GROUND LEVEL
SOURCE AT
225 FEET
SOURCE AT
450 FEET
THRESHOLD
WHEN SOURCE 0.5 g/sec.
THRESHOLD WHEN SOURCE 5 g/sec. i
20
25
Figure 4.
Figure 1 shows that there are a multiplicity of such odor release
points. Many of them are quite minor and by themselves would probably be
quite unimportant. Taken altogether however they account in the aggre-
gate for a substantial part of the whole mill odor problem and must there-
fore be suitably cared for if completely odor-free operation is to be secured'
The manner in which they can be cared for is not yet subject to a ready-
made solution. The points of odor release are widely distributed through the
plant so that some could be vented into a common duct and treated to-
gether, while others because of their location or nature might better be
treated by themselves. The nature of the deodorizing treatment will not al-
ways be the same everywhere as some will be more suitable for some
effluents than others, and the most practical treatment will have to be
worked out for each odor source.
The following are some of the possible procedures.
Burning—When malodorous sulphur compounds are completely
burned the sulphur is converted to sulphur dioxide which has a relatively
faint odor. A large volume, low. intensity, and fairly constant stream of
contaminated air can be passed into the recovery furnace, but only if it
does not create an explosion hazard or otherwise interfere with the estab-
lished pattern of furnace operation, and if the amount of ducting is not
excessive. The use of small furnaces especially for odor destruction has
been reported but they seem to involve considerable trouble and expense
and should be used only as a last resort.
-------
Selected Papers III
Scrubbing—Scrubbers can be set up to absorb and destroy malod-
orous materials by means of solutions of chlorine or hypochlorite. Such
solutions may be available as spent liquors from the bleach plant, or they
may be especially prepared using chlorine purchased for the purpose. If
the volume of gas to be treated is small the scrubbers will be small and in-
expensive. It may also be possible to operate existing scrubbers partly or
wholly on spent bleach or other oxidizing solutions. For example, if a
Peabody scrubber is used in the final stage of stack gas treatment, it may
be operated partly or wholly on an oxidizing solution.
Destruction in the Gas Phase—Large volume, low intensity sources
may be deodorized by adding small amounts of ozone, or possibly gaseous
chlorine or by the action of deodorizing lamps.
It is probable that many and perhaps all of these measures would
have to be invoked in one place or another before the release of odors and
particulate air contaminants could be brought down to an acceptable level.
What is not known at present is the volume of contaminated ah- to be
expected from each of the residual release points, nor the precise chem-
ical identity of the contaminants, nor their concentration. Further research
and study are therefore necessary to secure the data on which specific rec-
ommendations for odor-free operation can be based.
AN OUTLINE OF THE NEEDED RESEARCH
It is first of all necessary to make a detailed survey of several opera-
ting kraft mills so as to establish as exactly as possible the volumes of con-
taminated air which are released and where they are released and with
what frequency, and what concentration of malodorous material they con-
tain. The measurement of gas volumes is not usually a very difficult matter,
as it normally reduces to a flow measurement. Around sewer gratings and
similar low velocity emission points it may be necessary to build some
sort of confining box, but the measurement of gas volume will not in gen-
eral call for the development of any new technique. The measurement of
the nature and concentration of the malodorous contaminant may present
more difficulty. Chemical tests (usually colorimetric) have been devised for
methyl mercaptan, and a few of the other substances, but the sensitivity of
these tests falls far short of that of the nose. It is possible that the re-
cently developed process of vapor phase chromatography may be applied
to the problem so that the malodorous materials may be identified more pre-
cisely than ever before and also their quantities. The techniques necessary
for this work are, however, not available ready-made at this time, and
difficulty may be encountered as a result of the high content of moisture
in many of the air streams to be analyzed. Some work on the method
will therefore be necessary. An alternative method of analysis, which can
be used if the vapor phase chromatographic method cannot be developed
or is otherwise unsuitable, is a simple measurement of the total amount of
sulphur in the gas stream. This is a relatively simple test requiring merely
a filter to remove solid material, an oxidizing operation to convert the sul-
phur into sulphur dioxide, and an absorber of appropriate type. This has
the virtue of simplicity and the defect of being non-specific and of lumping
together substances with powerful and substances with quite weak odors.
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N2 AIR POLLUTION
Coupled with this identification and evaluation of the significant
points of odor release, experiments to test the efficacy of various methods of
odor destruction will have to be carried out. In a mill which already oper-
ates a bleach plant it would be a relatively simple matter to set up a small-
scale scrubber fed with actual spent bleach liquor and use it to treat actual
effluent air streams. By analysis of gases entering and leaving the scrub-
ber the quantitative data needed for the design of full-scale deodorizing ap-
paratus would be secured. In the same way, the usefulness of free chlorine
or ozone in destroying residual traces of malodorous material in flue gas
from the recovery plant or elsewhere could be established by actual trial
using part of all of the gas issuing from the mill.
These methods of securing the required design data and of testing
out the proposed odor absorption or destructive methods have obvious ad-
vantages over an exclusively laboratory study. It is hoped that such a
comprehensive study can be undertaken in the near future.
NEW WORK IN KRAFT MILL
ODOR CONTROL
R. H. Wright *
The problem of kraft smells is as old as the kraft pulping process.
The smells are mainly organic sulfur compounds, which are unavoidable by-
products of the pulping reaction, and which are released in varying but
substantial amounts at many widely separated parts of the process. For the
benefit of those not familiar with the pulp industry, Figure 1 shows the
bare outlines of the kraft process.
Wood and chemicals are put into the reaction vessel, or digester, D,
where a reaction takes place which dissolves the noncellulosic part of the
wood and leaves the cellulose fibers undissolved. From the digester, one
obtains pulp, spent chemical solution (called weak black liquor), and
smelly gases.
The weak black liquor is passed to a series of evaporators E j and E2
where water is progressively removed to a point where the residue can be
burned in a furnace F to generate steam and regenerate the cooking chem-
icals. The mill depicted in Figure 1 has no provision for odor control, so
that there are substantial amounts of odor released from the digesters, the
evaporators, and the chemical recovery furnace. Unless special precautions
are taken, a mill making some hundreds of tons of pulp daily will easily re-
lease some hundreds of pounds of smelly material in the same time, and
as the threshold of perceptibility is not far from 10 parts per billion, the
smell can travel a long way before it is dissipated.
*British Columbia Research Council, Vancouver Canada
Ho,e,,
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Selected Papers
113
WOOD
Figure 1 .
Any odor control system must meet certain stringent but common-
sense requirements, which are represented diagrammatically in Figure 2.
First, the control system must be simp.le and it must not interfere with the
basic process or operation of the mill. If it does, the operators—who are
human—will not make the best use of it, and under those conditions even
a technically sound system will soon become discredited.
BASIC
PROCESS
X
£
H
N
G
E
S \
s.
B
fi
E
F
t
Figure 2.
As an example, there have been various attempts to collect the
smelly gases by means of ducts and then burn them at some central place
such as the recovery furnace. Apart from the cost and inconvenience of the
ducts, the malodorous gases are often inflammable or explosive, and the
ducts tend to ice up in cold weather. Understandably, few such systems
have worked.
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,14 AIR POLLUTION
If the odor control system can contribute some additional fringe
benefits, then it will be welcomed rather than tolerated, and the operators
will be more inclined to maintain and even improve it.
The first substantial advance toward an acceptable system of smell
control was made about 20 years ago as a result of studies by Bergstrom
and Trobeck in Sweden and by G. H. Tomlinson in Canada. These led to
the black liquor oxidation process. The weak black liquor from the digesters
is an alkaline solution containing various organic and inorganic substances,
including sodium sulfide and methyl mercaptan. If the hot liquor is brought
in contact with air, the sodium sulfide is oxidized to sodium thiosulfate:
2Na2S + H20 + 202 = Na2S203 + 2NaOH
and the methyl mercaptan is oxidized to dimethyl di sulfide :
4CH3SH + 02 = 2(CH3)2S2 + 2H20
b.p. 7.6°C b.p. 117°C
These reactions have several beneficial effects. The chemical destruction of
the Na2S prevents H^S from being formed by hydrolysis in the evapor-
ators, the extra NaOH generated by the reaction helps to maintain a high
pH, and the methyl mercaptan is converted into the disulfide which is much
less volatile and also less objectionable. All this helps to prevent sulfur
being lost and makes for better chemical economy.
Figure 3 shows how black liquor oxidation affects the odor released
from the mill. The oxidation tower, O.T., is situated so that the black liquor
passes through it before going to the evaporator E ^.
The smell from the multiple-effect evaporator E'} is eliminated or
very greatly reduced, and, if the oxidation is sufficiently complete, there
will be very little pick-up of odor in the direct contact evaporator Ej- Also,
with properly designed apparatus it is possible to pass the smelly digester
gases into the towers where they are largely absorbed and deodorized.
Thus, in a mill using black liquor oxidation, the most serious re-
maining sources of odor are, in order of decreasing importance, the recovery
furnace, the direct contact evaporator, and the air exhausting from the oxi-
dation towers. The'problem of coping with these odors has been studied
intensively in recent years and is the main theme of this paper.
FURNACE ODORS
By itself, black liquor oxidation has no effect on the generation of
odors in the furnace. The prime function of the furnace is to oxidize and
burn away the organic matter in the black liquor and at the same time to
chemically reduce the sulfur compounds to Na2S. These seemingly incom-
patible functions are brought about by maintaining reducing conditions at
the bottom of the furnace and, by introducing enough secondary air higher
up to finish off the oxidation of the organic matter.
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Selected Papers
115
0-0
Figure 3.
With a properly designed and operated furnace these functions are
fulfilled and the flue gas should contain no significant amounts of H^S or
other reduced sulfur compounds. However, at high furnace loadings it is
often necessary to reduce the supply of secondary air in order to prevent
overheating of the furnace, and the immediate result of this is incomplete
oxidation and the release of H2 S and other malodors. Typical behaviour is
shown in Figure 4, which also indicates that at the other extreme, when the
furnace is underloaded, it runs cool and the liquor is destructively distilled
without being fully burnt.
Since the recovery furnace is the most expensive piece of equipment
in a pulp mill, there are obvious incentives to load it as fully as possible.
Figures 5 and 6 show an experimental H2 S meter which is now under
development at the British Columbia Research Council for the purpose
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116
AIR POLLUTION
Figure 5. Flue gas analysis .
H S analysis unit.
Figure 6.
Flue gas analysis.
H-S recorder unit.
of monitoring the flue gas so that maximum loading can be made consist-
ent with minimum production of smell.
THE DIRECT CONTACT EVAPORATOR
In principle, a sufficiently complete prior oxidation of the black
liquor will prevent the furnace gases from picking up any malodors in the
evaporator, apart from substances like dimethyl sulfide, which are not acidic
and therefore not retained by the alkali. In practice, however, the direct
contact evaporator does tend to contribute odors of which the most import-
ant is hydrogen sulfide. For one thing, if the black liquor oxidation is incom-
plete so as to leave a small residue of sodium sulfide in the weak liquor,
this becomes magnified as a result of the evaporation of water and may
amount to a significant quantity by the time the liquor reaches the re-
covery plant.
Again, if the oxidation of the weak liquor is done at too low a temp-
erature, then instead of the sodium sulfide being oxidized to sodium thio-
sulfate, it is oxidized only as far as elemental sulfur, by the reaction,
2Xa.,S + 2H,0
= 4NaOH + 2S
When free sulfur is heated with alkali it is unstable with respect to sodium
sulfide and may undergo an auto-oxidation of the type
6XaOH 4- 3S = 2Xa,S + Xa2SO3 + 3H?<)
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Selected Papers 117
The regenerated sodium sulfide can then liberate H2S both by hydrolysis
and by reaction with carbon dioxide in the flue gas.
Since it is not aways possible to maintain a sufficiently high temp-
erature in the oxidation towers to prevent this, one may be forced to carry
out the black liquor oxidation in two stages, using a partial oxidation of
the weak liquor and finishing off the reaction after the liquor has been
partially evaporated. This is likely to be complicated and inconvenient.
An alternative is to do away with the direct contact evaporator en-
tirely and to make a corresponding increase in the capacity of the multiple-
effect evaporators. This means an increase in complication, but can be
made to yield some useful fringe benefits also, over and above the im-
proved control of odors.
For example the direct contact evaporator involves only a single
effect, and so requires about 1,000 Btu per Ib of water evaporated. Mills us-
ing direct contact evaporators also use a substantial amount of steam to
pre-heat the furnace air supply. If this steam is diverted to the extra mul-
tiple-effect evaporators, the flue gas can be used to heat the air. Thus the
elimination of the direct contact evaporators can confer an over-all opera-
ting benefit in the form of heat economy, but at the cost of a substantial
increase in complexity. The balance of advantage from this has to be
worked out by each mill for itself.
THE OXIDATION TOWER EXHAUST GAS
The reaction taking place in the oxidation tower effectively prevents
the release of H2S in the discharge gases. Dimethyl sulfide (CH3>2S, is
chemically neutral and has a boiling point of only 38 °C so that it can be
partially stripped from the liquor, and its smell is objectionable. Dimethyl
disulfide, (CH^^ 82, which is formed by the oxidation of methyl mercap-
tan has a boiling point of 117°C and is much less volatile than the mono-
sulfide, but it, too, can be carried off to some extent by the exhaust gases,
and is somewhat objectionable.
It is known in a general way that these substances can be destroyed
by oxidation using solutions containing active chlorine, in forms which are
often available as wastes from a pulp bleaching plant. However, only re-
cently have the correct conditions for the effective utilization of these
solutions been found.
It turns out that the oxidation proceeds at a sufficiently fast rate
when the pH is low — in the range one to three — and is much slower at
higher pH values of six to eight. However, at the low pH values there is a
considerable liberation of chlorine gas from the solutions and this can mask
the smell of the sulfur compounds. Many past attempts to deodorize these
gases with bleach plant wastes or other chlorine solutions have turned out
to be unsatisfactory because the reaction was judged to be complete when
only chlorine could be smelled in the effluent gas. This was incorrect. The
fact was that sulfur compounds were still present and their odors became
noticeable at some distance down wind, when the chlorine was too dilute
to mask their odor.
A two-stage process has now been developed for the absorption and
oxidation of the malodorous substances in the oxidation tower exhaust gas,
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,I8 AIR POLLUTION
using a gas-liquid reactor of conventional design. In the first stage the gases
are exposed to chlorine solutions of low pH so as to absorb and oxidize the
sulfur compounds to innocuous products by such reactions as
(CH3)2S + 2C12 + 2H20 = (CHOzSOj + 4 HC1
and
(CH3)2S2 + 5C12 + 6H20 = 2CH3S03H + 10 HC1
The second stage of the scrubbing uses a solution of pH about eight to
absorb any residual chlorine or acid products of the oxidation.
The process is particularly useful to mills producing semi-bleached
or bleached kraft pulp because these normally have bleach plant effluents
containing residual chlorine, hypochlorous acid, and calcuim hypochlorite,
and also large quantities of spent caustic solutions with an appreciable
residue of sodium hydroxide. Naturally, the oxidants must be used in a
chemically adequate amount, and it bleach effluents with only a small
residue of chlorine are employed, the total volume will be large. Thus,
the scrubbing apparatus will be rather big, but it is inherently simple
and need not be overly expensive either to build or operate. Provided
the necessary ducts are not too troublesome, the scrubber can be used to
deodorize other gas streams as well.
SOME DIFFICULTIES
Despite what has been said, kraft mill odor control measures are still
by no means complete or fully effective. Black liquor oxidation is not a
complete answer, though it would be difficult to imagine an effective con-
trol system that did not include it. Figure 7 depicts a mill equipped with an
oxidation tower arranged to absorb digester gases and fitted with an ex-
haust gas scrubber, and with the direct contact evaporator eliminated. Such
a mill would be expected to give off a minimum of odor provided the load
on the furnace was not excessive. One or two new mills using this system
are now nearing completion.
The greatest single obstacle to the general introduction of oxidation
towers is the tendency of the black liquor derived from resinous species of
wood to foam enthusiastically and with the slightest encouragement—or
no encouragement at all. The foam handling problems are sufficiently
serious to create substantial operating difficulties.
The surface active substances responsible for the foam are largely
soapy or soap-like products formed by the action of the alkaline cooking
liquors on the resinous constituents of the wood. Neutral extractable sub-
stances, including turpentine, are also present and may contribute to the
problem.
As long as no substantial markets exist for these chemical by-products
of the pulping reaction, they are best disposed of by burning and so the
temptation is to process them through to the recovery plant with the rest
of the black liquor if this is at all possible, even though it means a con-
stant battle with the foam.
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Selected Papers
119
BLEACH
Figure 7.
Fortunately there is a growing demand for at least some of these
substances, so that a few mills are beginning to install equipment for re-
covering them from the weak black liquor. This is bound to improve the
situation with regard to foaming, and together with unproved methods of
foam handling, such as ultrasonic breakers, it is likely that black liquor
oxidation will soon find its way into mills where it has been heretofore
regarded as impractical.
ODOR ANALYSES
As long as the nose remains the only instrument for detecting the
odorous materials at the low concentrations at which they are found in
the open air at some distance from their origin, the objective evaluation
of control measures will be difficult.
Developments in analytical methods, and especially in gas chromato-
graphy, give some promise of replacing subjective appraisal by instru-
mented measurement.
Figure 8 shows a convenient type of stainless steel collecting tube,
which has been developed for collecting samples in the open air. The pro-
cedure is quite simple. In Figure 9 a small pump operated from a car
battery is used to draw the air first through a drying train and then
through the collecting tubes. The tubes can then be capped and packed
in dry ice for shipment to the laboratory, where they are connected to a
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Figure 8. Stainless steel collecting tube.
Figure 9. Collecting samples in the field. Apparatus consists of a drying tube,
three collectors in parallel, and a battery-operated pump, together
with a foamed plastic refrigerator for the samples.
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121
gas chromatograph and the sample transferred as shown in Figure 10.
Figure 11 shows the chromatogram of a sample collected about a quarter
of a mile down wind from a pulp mill at a point where the odor was
Figure 10. Apparatus for chromatographing the samples. The collecting tube K
is warmed by the heater N and the sample is swept into the top of the
chromatographic column through the tap L.
MejS
START
Figure 11 .
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122
noticeable but by no means strong. The pulp mill was located in the east-
ern United States and the sample was shipped by air 3,000 miles to British
Columbia to be analyzed in the chromatograph.
Unfortunately, the technique is still not fully perfected because
samples collected in urban areas or in the vicinity of major highways con-
tain so many extraneous substances derived mainly from motor vehicle
exhausts (though in at least one case a manure pile made its contribution),
that the chromatograms are too cluttered to be very useful. The problem
is not insurmountable, and could, for example, be nicely taken care of
by equipping the chromatograph with a sufficiently sensitive detector
that responded only to compounds containing sulfur.
ACKNOWLEDGMENTS
My name is given as author of this paper, but it should more properly
be given as compiler. To my colleagues, past and present at the British
Columbia Research Council and especially to F. E. Murray, G. C. B. Cave,
J. B. Risk, and I. H. Williams must go the credit for whatever is novel
in what I have said.
U. S. GOVERNMENT PRINTING OFFICE : 1968 O - 307-332
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