Title: Reactions of reduced sulfur compounds with ozone
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00097693/00001
 Material Information
Title: Reactions of reduced sulfur compounds with ozone
Physical Description: xiv, 190 leaves. : illus. ; 28 cm.
Language: English
Creator: Tuggle, Michael Larry, 1935-
Publication Date: 1971
Copyright Date: 1971
Subject: Odor control   ( lcsh )
Sulfur compounds   ( lcsh )
Ozone   ( lcsh )
Paper industry   ( lcsh )
Environmental Engineering Sciences thesis Ph. D
Dissertations, Academic -- Environmental Engineering Sciences -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
Thesis: Thesis--University of Florida, 1971.
Bibliography: Bibliography: leaves 173-190.
Additional Physical Form: Also available on World Wide Web
General Note: Manuscript copy.
General Note: Vita.
 Record Information
Bibliographic ID: UF00097693
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000953401
oclc - 16938317
notis - AER5857


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The author wishes to acknowledge and express his appreciation and

thanks to Dr. R. S. Sholtes, his committee chairman, and to the other

members of the committee -- Dr. C. C. Curtis, Prof. T. DeS. Furman, Dr.

J. E. Singley, and Mr. Herbert Berger -- for their assistance in many

areas associated with the project, and for their patience and understanding

throughout the course of the research program.

He is also indebted to Mr. C. H. Wilson of the National Council

for Air and Stream Improvement for his assistance in assembling the test

stand used in the study, to Mr. H. R. King and Mr. E. C. Logsdon for

fabricating the reaction tube components, to Mrs. Irene Lehman for typing

the correspondence and providing other secretarial services required

during the study, and to Mrs. A. L. Hammond for typing this manuscript.

The author wishes to particularly thank Mr. Thomas Tucker, formerly

with the National Council for Air and Stream Improvement, for his assistance

with the laboratory and field experimentation and analysis during the


The author expresses his appreciation to the National Council for

Air and Stream improvement for financing his graduate studies, and

providing the facilities and equipment needed for the research project.

He also wishes to thank the faculty1 and staff of the Environrnental

Engineering Department for their assistance in courses, laboratories,

and other related areas during the research period.

And finally, the author would like to again express his appreciation

to his wife for her continued understanding and assistance throughout

the graduate program covering the past three years.


ACKNOWLEDGEMENTS.............................................. ii

LIST OF TABLES.................................................. vii

LIST OF FIGURES....................................... ........ ... x

ABSTRACT........................................................ xii


I. SCOPE OF RESEARCH PROJECT ................................
Introduction.......................................... 1
Research Objective .................................... 2
Research Outline..................................... 3

!1. ODORS AND THE SULFATE PROCESS............................ 5
The Odor Problem........................................ 5
Basis of Problem................................ 5
Effects of Odorous Emissions.................... 8
Source and Quantity of Odorous Emissions........ 9
Industrial Odor Control................................ 11
Process Modifications or Substitution........... 11
Adsorption........................................ 12
Absorption................................... ... 14
Condensation.................................... 15
Oxidation Systems............................... 16
Combustion Oxidation........................... 16
Chemical Oxidation............................ .. 17
Dispersal....................................... 18
Odor Masking. ................................... 20
Odor Counteract on............ ..... ............... 21
Odor Control in the Kraft Pulping Industry............ 23
Black Liquor Oxidation............... ............ 23
Elimination of Direct Contact Evaporation...... 25
Proper Operation, Maintenance, and Housekeeping. 26
Combustion Oxidation............................ 27
Chemical Oxidation................................ 28
Absorption............................. ...... 30
Dispersal....................................... 31
Oder ;.asking Iand Countracion. .................. 32
Condensation .................................... 33



Ill. OZONE CHEMISTRY AND APPLICATIONS.......................... 36
Characteristics, Formation, and Toxicity.............. 36
Characteristics ................................ 36
Ozone Formation................................. 38
Ozone Toxicity.................................. 42
Ozone Appl ications .................................... 47
Waste Treatment ....................... ...... . 47
Odor Control.................................... 48
Water Treatment............................... 49
Miscellaneous................................... 50
Ozone for Kraft Odor Control............ ... .......... 51
Previous Applications and Investigations........ 51
Basis of Present Study .......................... 60

IV. EXPERIMENTAL APPARATUS.................................. 63
Description of Test Stand............................. 63
Basic Flow Pattern.............................. 63
System Details.................................. 66.
NCASI Mobile Laboratory Equipment ..................... 71
General Description of Mobile Laboratory........ 7!
Electrolytic Titrator and Combustion Furnace.... 72
Gas Conditioning Oven........................... 73
Gas-Liquid Chronatography System................ 76

Experimental Procedures............................... 78
Hydrogen Sulfide...................................... 80
Methyl ilercaptan......................... ............. 85
Dimethyl Sulfide...................................... 88
Dimethyl Disuifide.................................... 93

System Modifications.................................. 96
Efflucnt Cas Source................................... 98
Experimental Procedures............................... 98
Results of Ozcne Oxidation............................ 99
Th.-rmal Dcc:y of Ozcn .............................
Experimental Error.................................... 106

VII. SUI-iARY AND CONCLUSIONS................................... 10..
Summary................................................. 110
Conclusions............................................... 114


OPERATING PROCEDURES ............................. ....... 118


APPENDICES (Continued) Page

B. EXPERIMENTAL DATA.................................... .... 1:32

Bi BL I OGRAPHY ................................................... 173




1. THRESHOLD ODOR VALUES ........................................ 7

2.. SUMMARY OF KRAFT EMISSION DATA............................. 10

3. BIOLOGICAL EFFECTS OF OZONE................................. 45

4. 070NE CONCENTRATIONS FOR ODOR CONTROL....................... 49


6. GLC SAMPLING LOOPS ........ ......... .......... .... ....... ... 74

7. CAS CHROrMTOGRAPH COLUMN ELUTION TIMES...................... 76






13. SUMMARY OF OZONE-SMELT TANK VENT GAS DATA................... 101

GASES .................................................. 112

15. RtACTION TU8E SPACE TIME.................................... 126

16. REACTION TUBE TEMPERATURE CONTROL........................... 127

17. SULFUR GAS TANK ANALYSIS ............................ .............. .. 127




LIST OF TABLES (Continued)



REACT I ONS............................................











EQUATION 5.1..............

EQUATION 5.2..............

EQUATION 5.3..............

DATA AT 380 C.............

DATA AT 1250 C............


EQUATION 5.4..............

EQUATION 5.5..............

EQUATION 5.6..............

DATA AT 380 C.............



































LIST OF TABLES (Continued)



44. OZONE-SMELT TANK VENT GAS REACTION DATA.................... 170

45. THERMAL DECAY OF OZONE IN REACTION TUBE.................... 172




1. Conversion of Initial H2S to SO2 and H2SO4 (Gregor and
Martin) ............................................. 55

2. Conversion of Oxidized H2S to SO2 and H2SO4 (Gregor
and Martin) ......... ..... .......................... 56

3. Front View of the Test Panel.............................. 64

4. Rear View of Test Panel................................... 65

5. System Schematic for Oxidation of Bottled Gases............ 67

6. Injection of Sulfur Gases with Spinning Syringe............ 69

7. Diagram of Reaction Tube .................................. 70

8. Mobile Laboratory Flow Diagram............................ 75

9. Ratio and Time Effects on H2S Oxidation ................... 83

10. Temperature and Time Effects on H2S Oxidation............. 83

11. Ratio and Time Effects on RSH Oxidation.................. 89

12. Temperature and Time Effects on RSH Oxidation............. 89

13. Temperature and Time Effects on RSR Oxidation............. 92

14. Ratio and Temperature Effects on RSR Oxidation..,......... 92

15. Temperature and Time Effects on RSSR Oxidation ............ 95

16. Ratio and Temperature Effects on RSSR Oxidation............ 95

17. System Schematic for Oxidation of Stack Gases.............. 97

18. Ozone Oxidation of Smelt Tank Vent Gas................... 102

T9. Thermal Decay of Ozone in Reaction Tube ................... 105



20. Ratio Effects on Oxidation of Reduced Sulfur Compounds.....

21. Ozone Generator Calibration Curve .........................

22. Spinning Syringe Calibration Device .................. .....





Abstract of Dissertation Presented to the
Graduate Council of the University of Florida in Partial Fulfillment
of the Requirements for the Degree of Doctor of Philosophy



Michael Larry Tuggle

December, 1971

Chairman: Dr. Robert S. Sholtes
Major Department: Environmental Engineering

Due to increasing emphasis on the elimination of industrial odors,

odor control is a major concern of the kraft pulping industry, and

continuous research is needed to evaluate any feasible method that will

aid in eliminating this problem. Ozone is one of the most powerful

oxidants known, and has been suggested for kraft odor control by gas-

phase oxidation of the sulfur compounds. The application of ozone in

the kraft industry is near non-existent, and laboratory investigations

are very limited and concern primarily only oxidation of hydrogen


The purpose of the present study was to evaluate the effectiveness

of ozone ;n gas.-phase oxidation of the four malodorous sulfur compounds

primarily responsible for the kraft odor problem; namely, hydrogen

sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide.

The initial phase of the study involved the use of bottled gases, whereas

the last phase evaluated the ozone oxidation of an actual kraft mill

source -- smelt tank vent gas.

Experimental data were obtained with the use of a heated, stainless

steel reaction tube. Six sampling ports spaced dovn the length of the

tube provided reaction times ranging from approximately 5 to 60 seconds.


For the bottled gases, ozone to sulfur gas ratios of 0.5 to 8.0 were

evaluated at reaction tube temperatures of 380 C to 1250 C. The

effectiveness of ozone oxidation was noted by monitoring the decrease

in initial sulfur gas concentrations at various conditions of reaction

time, temperature, and concentration ratios. Multiple regression

analysis was used to evaluate the data points obtained for each gas,

and provide several regression equations representative of the oxidation

process for the conditions studied.

Results of the data on bottled gases indicated that concentration

ratio and reaction temperature had a much more significant effect on

oxidation than did increased reaction time, and to achieve complete

oxidation of the malodorous sulfur compounds, ozone concentrations of

more than twice the stoichiometric amount would be required for the

reaction times and temperatures studied. The percent oxidation

achieved in this study for hydrogen sulfide was significantly higher

than that reported by other authors, and attributed primarily to the

difference in experimental conditions and procedures between the


For experiments with actual kraft emissions, total reduced

sulfur (TRS) of the smelt tank vent gas stream was continuously

monitored with cn cloctrolytic Litrator. Ozone was effective in

eliminating all of the methyl mercaptan, dimethyl sulfide, and dimethyl

disulfide from this source at an ozone to TRS ratio of approximately

2.50. These three sulfur compounds accounted for about 52 percent

of the TRS as determined by gas-liquid chromatography. In view of the

oxidatio' results :.it[l, otil(ed gases, the concentration ratio of 2.5

for a stack source was considered acceptable, recognizing the vast

difference in a stack effluent as opposed to a bottled gas source.


Additional studies are needed to further evaluate the ozone

oxidation of various kraft mill sources, covering a wide range of stack

conditions and reaction times. The current study indicated that complete

oxidation of the malodorous compounds would only be achieved with

reaction times far exceeding those normally experienced in an emission

stack, thus requiring the possible need of holding chambers to achieve

adequate mixing. Costs are also of major concern, both for ozone

production and additional facilities for increased mixing time, but

were not evaluated as part of this study.





Ozone is one of the most powerful oxidants known, and is finding

increasing use in the field of odor control. Due to its high electro-

negative potential (second only to fluorine) the potential applications

for ozone as an oxidant are considerable, but have been somewhat limited

in the past due to the practical requirements that it be used in its

gaseous state, the low concentrations of ozone to parent gas (air or

oxygen) delivered by ozone generators, and the cost of ozone in pounds/

kilcowatthours. Improvements in ozone generator design and efficiency,

the availability of tonnage oxygen at decreased costs, and the emergence

of cylinder ozone have expanded the economical applications of this

oxidant for various commercial uses.

Gas-phase oxidation of malodorous compounds with ozone for industrial

odor control is mentioned by Yocoin and Duffee (1) and Turk (2: 3) and is

used today for controlling odors from sewage treatment plants, lift

stations, and various commercial operations (kitchens, rendering plants,

food processing, rubber compounding mill, chemical exhausts, etc.). Its

use specifically for controlling malodorous sulfur emissions from sulfate

pulping operations has been suggested by Sarkanen, et al. (4), Douglass (5),

and Cromwel (6) and a patent has been issued for the process- (7), but

application of this method has been very limited. The article by

Crormwell (6) describing the experimental use of ozone to treat effluent

gases from a sulfate recovery system at a pulp and paper mill in Canada

was the only reference found on actual mill effluent application.

Ricca (8) reported on the use of ozone to oxidize exhaust gases from a

bench-scale, black liquor oxidation system, and Akamatsu (9) studied the

use of ozone in deodorizing kraft mill blow gases produced from an

autoclave. Laboratory studies on the reaction of ozone with hydrogen

sulfide (from cylinders) have been reported by Gregor and Martin (10),

Cadle and Ledford (11), Hales (12) and Finlayson, et al. (13).

Due to the decreased costs in tonnage oxygen over the past fe, years,

improved ozone generation, the possibility of on-site production of

oxygen for waste treatment systems and oxygen bleaching, and the increasing

emphasis and regulations directed toward eliminating industrial odors,

additional dataare needed on the feasibility of using ozone for odor

control in the sulfate pulping industry.

Research Objective

The objective of this study was to evaluate the effectiveness of

ozone in gas-phase oxidation of the four major components of kraft mill

odors; namely, hydrogen sulfide (H2S), methyl mercaptan (RSH), dimethyl

sulfide (RSR), and dimethyl disulfide (RSSR). The study involved mixing

ozone with each of the individual gases (from cylinder sources) at a variety

of concentration ratios, temperatures, and reaction times, followed by

analysis of the reaction products. Tests were also performed to evaluate


the effectiveness of various ozone concentrations (and reaction times) on

actual emissions from a smelt dissolving tank by analyzing the effluent

gas prior to and follaoing ozone injection. The range of experimental

conditions (sulfur concentrations, reaction temperatures, etc) for the

study included those normally found in the kraft pulping industry.

Research Outli i

The research was accomplished according to the follaozing outline:

(1) Revie.- the air pollution problem facing the pulp and paper

industry in the elimination of odorous emissions from sulfate

pulping operations, the basic approaches to industrial odor control,

and the methods currently used by the industry for control of

odorous emissions.

(2) Review the general characteristics and applications of ozone,

such as its physical and chemical properties, methods of generation,

toxicity, reactions with various compounds, and uses in the treat-

ment of dcrnestic and industrial waste, drinking water purification,


(3) Pei form a literature survey For previous studies and applications

of ozone for odor control in the pulp and paper industry, and in

other industries or operations with odor problems (sewage treatment

plants, food processing, rendering plants, etc.).

(1i) Select and obtain ozone generating equipment and other experi-

mental apparatus required for studying ozone reactions with bottled

sulfur gases and kraft mill effluent gases, both at various experi-

mental conditions.


(5) Perform experiments designed to evaluate the effectiveness of

ozone in oxidizing each of the four odorous sulfur gases (from

cylinder sources) at various concentration ratios, reaction

temperatures, and reaction times.

(6) Perform experiments designed to evaluate the effectiveness of

ozone at various concentrations and reaction times in oxidizing sulfur

compounds in an actual kraft mill effluent gas.



The Odor Problem

Basis of Problem

Since the development of the sulfate process by Dahl in 1879 as a

method of making pulp and paper from wood, the operation of the first

sulfate (kraft) mill in 1891, and the appearance in the literature of

the first papers concerning kraft mill odor around 1900, there has been

little doubt that odor control is a major problem of the kraft pulping


Recent evidence of this continuing problem is the number of papers

and reviews published in the last few years concerning atmospheric

emissions from kraft pulping operations, with the major portion concerning

odorous emissions. Literature published prior to 1963 on air pollution

and the kraft pulping industry is reviewed in an annotated bibliography

by Kenline and Hales (14). Hendrickson (15) edited the proceedings of an

international conference on sulfate pulping emissions held in 1964 at

Sanibel Island, Florida. Environmental Engineering, Inc. and J. E.

Sirrine Co. published a three-volume study in 1970 for the National Air

Pollution Control Administration (16) on control of atmospheric emissions

in the wood pulping industry, and a 1970 conference in Stockholm (17) on

methods for measuring and evaluating odors had several presentations


on odors from the kraft pulping industry. Annual reviews (18-23) of pub-

lished literature relative to air pollution control in the pulp and paper

industry are prepared by the National Council of the Paper Industry for

Air and Stream Improvement. Other excellent reviews are by Sarkanen,

et al. (4), Adams (24), and Wright (25).

The major components responsible for this characteristic kraft odor

are all sulfur compounds (16, 26-28); namely, hydrogen sulfide (H S),

methyl mercaptan (RSH), dimethyl sulfide (RSR) and dimethyl disulfide

(RSSR), and are referred to as reduced sulfur compounds. These four

gases were first identified by Klason during a series of studies from

1908-1924 (4), which is another indication of hoq long the kraft odor

problem has been under investigation and hao difficult it is to eliminate.

Although some of the odor-producing components of the effluent may be

associated with non-sulfurous compounds (29), most attempts to control

odors are directed to the four sulfur gases mentioned above.

The elimination of these odors is difficult primarily because of

their extremely lo. odor threshold, or minimum detectable concentration.

Table I is a summary of odor threshold values reported in the literature

for these four gases of interest. The values are for the pure compound,

not for the gas in combination with other gases (sulfur and non-sulfur)

as would be under actual conditions. Odor thresholds for a combination

of these gases may be completely different than those reported, but would

not be a simple addition of the threshold values of each component (37).

Due to different tests used to determine threshold values, data treatment,

purity of the compounds studies, and interpretations of the word

"threshold," a range of values are reported for each compound, but



Compound Threshold Odor (ppb)a ... Reference

Hydrogen Sulfide 4.1 30
6.0-12.0 31
7.1 32
4.7 33
100.0 34
20.0 35
Methyl Mercaptan 1.0 30
2.1 33
10.0 34
40.0 36
Dimethyl Sulfide 2.0 30
1.0 33
50.0 34
Dimethyl Disulfide, 5.6 30
500.0 .. 34-

Parts per billion by volume.

serve to indicate that the odor threshold of each gas


is extremely

In addition to the problem of lo.w odor threshold values, the fact

that kraft odors are released from so many different points throughout a

mill is another obstacle in odor control for th' industry. To reduce

ambient concentrations of these odorous emissions to threshold values in

mill localities, each source will have to be investigated and controlled.

Although the same basic process is used in each mill and the emission

sources are similar, each mill presents its own particular set of problems,

and what may be effective at one installation may not be at another with-

out modifications.

Effects of Odorous Emissions

Why are odors objectionable, and why all the concern about them?

Odors are intangible, but may be generally described by their odor

threshold, intensity of levels above this threshold, acceptability, and

their characteristic properties which distinguish them from other odors

regardless of intensity or acceptability (38). For air pollution studies,

the most important dimension of an odor is its acceptability, or how

many people are inconvenienced by the smell and to what extent. Numerous

articles (17, 31, 32, 39-48) are found in the literature describing the

evaluation of odors for their objectionability, measurement by test

panels and community observer corps, description of nuisance effects,

differences in human response to odors, measurement by chemical analysis,

and needs for future research in the odor field. Since a qi.antative measure

of inconvenience, objectionability, or acceptability is sometimes hard to

determine, the odor threshold value is normally used for evaluation.

As shown previously, even this value is subject to large variations.

Odors in themselves have not been shown to be the direct cause of

any disease, but various ways in which they may affect a person's well-

being are discussed by McCord and Witheridge (49) and Stockman and

Anderson (48). These include reduced appetite, nausea, headaches,

interference with sleep, and mental stress. Economic effects of odors,

such as the lowering of property values, reduced sales and rentals in

resort areas, and reduced desirability of a community in which to live

are also mentioned. In sufficient concentrations, sulfur emissions

common with kraft mills (particularly hydrogen sulfide) are highly toxic

and can be fatal (50), but health hazards resulting from the presence of

these gases in the atmosphere around sulfate pulping operations are not


considered to be of any concern due to their diluted concentrations (51,

52). Even though not considered a health hazard, discharges from kraft

mills have reportedly caused nausea, headaches, eye irritation, and upper

respiratory irritation (48). Other detrimental effects attributed to

sulfur emissions are corrosion (35, 48, 53) and darkening of paint by

hydrogen sulfide (48, 52, 54).

Regardless of the various economic and health effects that have been

attributed to odorous sulfur compounds, their nuisance aspect is of primary

concern. This is evidenced by the volumes of literature previously

mentioned concerning odors and the sulfate pulping industry, and leaves

no doubt that the odors are an annoyance to the public, and therefore


Source and Quantity of Odorous Emissions

The various process reactions and factors responsible for kraft odor

generation are beyond the scope of this study, but are well described in

the literature (4, 16, 26, 27, 51, 55-59). As previously mentioned,

odorous gases are generated and released at several points in a kraft

mill complex, and at varying temperatures and concentrations. Table 2

is a summary of data from the study for NAPCA (16) listing major sources

of odor release in a kraft mill, and ranges of temperature, gas flows, and

odorous emissions at each source. Similar data are also listed by Hough

and Gross (60), Adams (61), and Sarkanen, et al. (4). Considering the

amount of total reduced sulfur cQopounds that may be generated at various

points in a 500-1000 ton per day kraft mill complex, the low odor

threshold for each of these compounds (Table 1), and increasing emphasis

and regulations (62) directed toward eliminating these emissions, the

magnitude of the kraft pulping industry's odor control problem becomes

readily apparent.



Temp. Gas Flow Range of Sulfur Gas Concentrations (Ib/ADT)b
Source (oC) (CF/ADT)a H2S RSH RSR RSSR

Digester Relief 49 35 0-.01 .01-1.00 .10-.40 .10-.20
Digester Blow 49 300 .10-.12 .40-.47 1.20-1.40 1.30-1.50
Brown Stock Washers 52 80,000 ..01-.12 .10-.25 .01-.02 .01-.02
Oxidation Towers 57 13,000 .01-.02 .05-.10 .02-.08 .05-.15
Multieffect Evap.-Ox. 49 35 .01-.02 .10-.30 .05-.15 .05-.15
Multieffect Evap.-Unox. 55 35 .10-3.00 .10-1.50 .05-.08 .01-.02
Recovery Furnace & DCE-Ox. 163 400,000 1.00-5.00 .01-.25 .01-.10 .01-.20
Recovery Furnace & DCE-Unox. 163 400,000 5.00-30.00 .50-2.50 .10-.30 .10-.40
Smelt Tank 88 44,500 .02-.05 .02-.05 .01-.02 0-.01
Lime Kiln 82 50,000 .20-.52 .10-.50 .01-.28 0-.08

Cubic feet per air dried ton of unbleached pulp.

Pounds per air dried ton of unbleached pulp.

Industrial Odor Control

The control of odorous gases from industrial sources can normally

be accomplished with one or more of the following methods (1-3, 63):

(1) Reducing or eliminating the initial formation of odorous

compounds through process modifications or use of a completely

new process,

(2) Use of various techniques, such as adsorption, absorption,

condensation, combustion and chemical oxidation, to remove or

reduce odorous gases in the effluent stream prior to their emmission

into the atmosphere,

(3) dispersion of the odorous gases (usually through tall stacks)

to a greater extent so that they are less concentrated upon

reaching any point where they may be detected,

(4) Addition of other odorant gases to the effluent stream so that

the resultant odor becomes less objectionable through either odor

masking or odor counteraction.

The use of any of the above methods, or their combinations, depends

upon several factors, such as volume of the effluent stream, characteristics

and concentrations of particulates and odorous gases in the effluent,

location of the particular industry, normal atmospheric conditions, and

the chemical reactions taking place in the process. The folloa.ing sections

are brief descriptions of each method mentioned above.

Process Modifications or Substitution

Reduction, or elimination, of the initial formation of odorous gases

through process modifications or the use of a completely different process

would be a major step in any odor control program, and,in many cases,

could be more effective and cheaper than complete abatement procedures

at the stack. Just by reducing the initial odor formation, less load

would then be imposed on any control equipment downstream.

Odor control through process changes may include an adjustment of

process temperature, residence time, or pressure in various operations,

substitution of lai odor (or odorless) solvents or reactants for highly

odorous ones, a change in flo.w diagrams to prevent contact of odor

producing gases and/or liquids, and proper maintenance and housekeeping



Odor control by adsorption results from an interaction between the

malodorous gas and a solid due to the phenomena of surface attractions

universal with all substances. The forces that hold atoms, molecules,

and ions in the solid state exist throughout the solid and also at its

surface, and thus are available for binding other molecules which contact

the surface or come in close proximity to it. As a result of these

forces, any gas, vapor, or liquid will adhere to any solid surface to some

degree (64). This phenomenon is called adsorption (or sorption), the

adsorbing solid is called the adsorbent (or sorbent), and the adsorbed

material is the adsorbate (or sorbate). This process is useful in air

pollution control since it provides a means of concentrating malodorous

emissions for ultimate disposal or recovery.

Depending upon the adsorbent, the gases of concern (or adsorbate),

temperature, etc., various adsorption processes are available. In

chemical adsorption chemisorptionn), chemical bonds are formed between the

gas and the adsorbent, such as the formation of CO and CO2 when oxygen is

adsorbed on activated carbon at ambient temperatures. In physical


adsorption, the adsorbate is chemically unaltered, and the gas-solid bond

may be broken by elevation of the temperature. In some cases, the adsorbates

are reactive with each other, and,by concentrating them, the adsorbent

acts as a catalyst by speeding up their reaction rate. Adsorbents may

also be treated with a specially selected catalyst or reactant prior to

use in order to increase its capacity, rate, or selectivity for odor

removal. Various factors influence the quanity of material that can be

adsorbed by a given weight of adsorbent, and are discussed in detail by

Turk (64) and Summer (65). For all practical purposes, the control of

atmospheric odors by adsorption is limited to the use of activated carbon

as the adsorbent (1, 3). It adsorbs all types of odors under almost any

condition, performance is not weakened by the presence of moisture, and it

can be used without making a careful analysis of odor content. Von

Bergen (63) and Lee (66, 67) discuss the applications of activated

charcoal in air pollution control and the design of adsorption systems.

Other adsorbents are available, but show greater selectivity than

activated carbon due to their polar nature. For this reason, they are

more useful than carbon when separations are to be made among different

pollutants in an effluent stream, but are not as good for overall odor

control applications. Typical of these adsorbents are silica gel, fuller's,

diatomaceous, and other siliceous earths, and synthetic zeolites. Metallic

oxides are also used, but are even more polar than the siliceous

adsorbents listed above. For this reason they are never used directly

for source control of airborne pollutants by physical adsorption, but are

used as desiccants, catalyst carriers, or catalysts. Activated alumina

(aluminum oxide) is typical of this group.


Once the adsorbent is saturated with the odorous material, several

means of disposal are available depending upon the various materials

involved. The adsorbate may be desorbed and either discarded or recovered

(if valuable), the adsorbent and the adsorbate may both be discarded, or

the adsorbate may be oxidized on the adsorbent surface.


Where odorous gases are soluble in a liquid, with or without chemical

reactions, absorption methods may prove useful for odor control. The

transfer of an odorous gas from the effluent stream into a scrubbing

liquid requires initial contact of the two mediums in some type of wet

scrubber, and then separation into a clean gas and contaminated liquid

stream. Gaseous transfer is basically through diffusion, moving from a

region of high concentration in the gas stream to one of low concentration

in the scrubbing medium. The odorous molecules may also be attached (or

adsorbed) to particulate matter in which case both would be removed from

the gas stream through various particulate collection mechanisms

(inertial, gravitational, etc.).

Various methods are used for gas-liquid contact in wet scrubbers,

but are divided into two basic categories -- lao energy types and high

energy type. Low energy scrubbers achieve absorption by floa-ing gas

through spray chambers, or through restricted passages in a plate or

packing on which a liquid head is maintained. Examples of this type

include open spray towers, packed towers, wet centrifigal or cyclonic,

flooded bed, orifice, and wet dynamic (68). The high energy types, or

venturi scrubbers, use a venturi for gas-liquid interaction by impacting

high velocity gas on the injected liquid streams, and include the ejector

venturi, flooded-disc scrubber, and the dry or wet venturi. Additional

details, design information, etc.,on liquid scrubbers are given by

Imperato (68), Sargent (69) Calvert (70), and Summer (65).

In all types of scrubbers, it is possible to use an absorbing medium

other than water to achieve better odor control through chemical reactions

between the gas and liquid rather than just physical solution. Of course,

this would depend upon the odorants to be removed, origin and disposition

of the scrubbing liquid, corrosive effects, etc. As with any odor

control system, basic procedures should be follc.ed when considering or

evaluating absorption for odorant removal, and are listed by Von Bergen

(63) in addition to the above mentioned references (65, 68-70).


Many odorous gases can exist as liquids under ambient conditions,

and through simple condensation, cooling such vapors can remove much of

the odor frnii an effluent stream. If the odorant gases have an appreciable

solubility in water and moisture is present in the exhaust stream,

condensation of the water can also reduce the concentration of malodors

released to the atmosphere (71, 72).

The two basic types of condensers are surface and contact. Surface

condensers rely upon cooled surfaces where the coolant does not come in

contact with the odorous vapors. Contact condensers use a mixture of

coolant, vapors, condensate, and non condensable gases, and are applicable

to a wider range of odor-control problems and less expensive than surface

condensers. However, they produce large volumes of contaminated water

which can create a water pollution problem, and care must be taken to

prevent re-release of the odorous gases further downstream (1).

Condensation is basically the transfer of an air pollution problem to

one of water pollution, but can be very beneficial if an efficient

utilization or treatment of the aqueous effluent is available.

Oxidation Systems

Complete oxidation of odorous gases is an effective means of odor

control since the final products are either odorless (H20, C02) or have

comparatively very high odor threshold values. If oxidation is incomplete,

however, the odors may increase as evidenced by incinerators, incomplete

combustion in diesel and internal combustion engines, conversion of

alcohols to carboxylic acids, etc. (3, 63). Oxidation systems may be

classified as combustion (high temperature) or chemical (loa temperature).

Combustion Oxidation

The three basic methods used to incinerate waste gases are flame,

thermal, and catalytic combustion. Each method requires that the odorous

effluent be heated to the point where the combustible contaminants will

burn. The only basic difference is the temperature range at which the

units operate.

In flame (or direct-combustion) incineration, if the emission

concentrations are in the flammable range, they are destroyed by burning

as in a flare. For fluctuating process conditions, or for concentrations

at the lo-er flammable limit, auxiliary fuel is used to maintain a burning

mixture. Although flame incineration can be used over a wide range of

concentrations, it is most economical at high concentrations since the

contaminants could be utilized as fuel. Operating temperature of these

units is approximately 25000 F.

Normally, the concentration of combustible contaminants in an

effluent stream will be considerably below the laoer limit of flammability,

and thermal incineration is much more economical than flame incineration.

In this system a residence chamber is used, and the contaminants are

destroyed by exposure to temperatures of 900-14000 F in the presence of

a flame. Although electric heat energy can be used, the presence of a

flame is an important factor for contaminant removal. With electric

heat, temperatures of 1500-18000 F are required to obtain the same

efficiency achieved with a flame system at 900-14000 F. Continuous

operation at efficiencies of 90 to 99 percent have been achieved with

flame systems (73).

In a catalytic system, the oxidation of odorous effluent occurs

directly on the surface of a catalyst, usually composed of a precious

metal (platinum), and in the absence of a flame. Temperatures out of

the catalyst bed range from 600-10000 F. The basic parts of this system

are a preheat burner, catalyst bed, heat exchanger (for temperatures of

800-10000 F), exhaust fan, and control and safety equipment. Even though

some oxidation may take place in the presence of the preheat burner

flame, the primary function of this component is only to raise the gas

stream to the desired operating temperature for entry into the catalyst

bed. Efficiencies in the range of 85-92 percent are reported for properly

maintained catalyst systems.

Additional details on each of the above combustion systems, such as

initial costs, opcrction and maintenance costs, design variables, operating

characteristics, and Factors influencing the selection of each method,

are reported by Brewer (74) and Pauletta (73).

Chemical Oxidation

Chemical conversion of many organi c gases and vapors to odorless

compounds is generally accomplished with oxidizing agents such as ozone,

chlorine, chlorine dioxide, and potassium permanganate. An important

factor in this method of odor control is the fact that these agents do


not always convert organic substances to their most highly oxidized products

(CO2 and H20), so the odor of the intermediates may be of concern.

Depending upon the emission process and the character of the odorous

effluent (moisture content, temperature, etc.) contact between the oxidizer

and odorant may be gas-gas, gas-liquid, or gas-solid.

The only significant examples of gas-phase oxidation have been the

use of chlorine and ozone. Due to its possible corrosive effects, residual

odor, and toxicity, chlorine is seldom used in this method. Although

also toxic, ozone is used in many odor control applications through

release into a stack or vent containing odorous effluent, and is

discussed in Chapter III.

For gas-liquid contact, chlorine, chlorine dioxide, and potassium

permanganate are the principal oxidants and are normally added either to

the scrubbing liquid or to the gases entering the scrubber.

Gas-solid contact is achieved in many applications with the use of

activated carbon impregnated with various compounds which react with the

odorous effluent after initial adsorption by the carbon. Activated

alumina-and an aqueous potassium permanganate solution are also used in

a similar manner (1).


Reducing the concentration of a malodorous effluent to belcw the

sensory threshold by dilution is still one of the most common methods

of odor control. Such dilution is normally achieved by collecting and

discharging all emissions through a tall stack and/or increasing the

temperature and velocity of the effluent, or by locating the source of

emissions (such as a plant) at increased distances from any receptor.


Formulas are available (75) to estimate the average downwind, ground-

level concentration of a plume discharged from an elevated continuous

source (such as a tall stack), and are a function of emission rates, stack

height, effluent temperature, and various atmospheric parameters that

describe the meteorological situation. Up to distances of a few miles,

and for durations up to ten hours, these formulas will predict concentrations

that are within a factor of two or three of the observed values most of

the time. At greater distances and longer time intervals, the predicted

concentration is much more subject to being in error, but generally

should not exceed a factor of ten within the plume trajectory (1, 2).

The formulas predict average concentrations over a period of time,

but most odor complaints usually result from peak levels. Since even a

brief exposure to a peak concentration may be unacceptable to the populace,

the degree of dilution needed for adequate deodorization is normally

much greater than that predicted by the calculations. Some studies (1, 76)

have sho.,in that the distance from a source at which an odor was detected

was not even approximately predicted by dilution and dispersion calculations.

Large discrepancies were found by Wohlers (76) in comparing odor travel

from different plants based on stack-gas dilution calculations. His

most extreme sample was from a kraft pulp and paper mill for which the

average dilution needed to reach threshold was 32:1, and the maximum

64:1. The minimum dilution of actual stack effluent was calculated to be

990:1; thus predicting that the odors would not be detected at ground

level. Field surveys over a six-month period showed that the kraft odor

could be detected at distances up to eight miles where the calculated

dilution of stack effluent was 840,000:1, and there were some reports of

the odor at a distance of 40 miles.

In addition to errors which can be due to various factors used in

meteorological dispersion formulas, the variability of odor threshold in

humans, and the possible role of cumulative secondary sources, Turk (2, 3)

suggests the role of particulate matter in the transfer and/or perception

of odors by humans and as a possible explanation to some of the large

discrepancies found in previous studies.

Various design factors, cost information, etc., on the construction

of tall chimneys are given in an article by Carlton-Jones and Schneider

(77). Meterological influences on mill effluent dispersion are discussed

by Wright (78) and Cramer (79).

Odor Masking

Odor masking is a process in which the perception of an offensive

odorous gas is obscured by the addition of other odorous material under

conditions that do not involve chemical change to either agent. Hopefully,

a more pleasant and acceptable odor sensation is created. The masking

agent does not affect the composition of the original malodorous gas, and

if the two odors are of near equal strength, a blend of the two is

observed and both can be identified. If one is considerably stronger than

the other, generally it alone is perceived (2, 63).

Various chemicals are used for odor masking, such as vanillin,

methyl ionones, eugenols, benzyl acetate, phenylethyl alcohol, and

heliotropin, and must be carefully selected for each particular odor

problem. Factors which must be investigated include an analysis for (or

a lack of) toxic materials in the effluent stream, addition of the masking

agent into the exhaust stream, surrounding area, or as an additive to the

process, odor compatibility between the masking agent and malodorous


compounds so a more offensive odor will not result, amount of malodorous

material in the exhaust stream, temperature and other conditions of the

process and stack effluent, and amount of masking agent required. Pilot

plant or laboratory experiments should be run to establish ratios that

produce a desirable result.

Advantages of using masking agents are minimum or no capital invest-

ment for equipment, ease of application, relatively loa operation and

maintenance costs as compared with other odor control equipment, and

immediate availability for common odor problems. The main disadvantage

of this method is that the original malodorous gases are not eliminated

from the atmosphere, and may be of such magnitude that odor masking is

not economically feasible or desirable. Also, additional odors are

being injected into the atmosphere, and,although may be initially more

acceptable than the original malodors, they are still subject to

calnplaints from the population since individual reactions to odors are

quite variable and change from time to time.

In addition to the two references (2, 63) previously cited, a

thorough review. of masking is also given by Summer (65).

Odor Counteraction

Odor counteraction (neutralization) is a reduction in the intensity

of an offensive odor due to the fact that certain pairs of odors in

appropriate concentrations are antagonistic, and when sniffed together,

both are diminished. This principle of odor control is distinct from

that of odor masking in which strong odors tend to mask weaker ones and

the perception of the offensive odor is obscured.

Various pairs of counteracting odorous substances were first published

in 1895, and,since that time, similar studies have been made where it was

possible to compensate the olfactory effects of various chemicals to a

point of total odor disappearance, or close to it. From those studies,

it is knacn that in the group benzene, toluene, xylene, pseudocumene

and durene, combinations from this group in the correct proportions can

be produced which are almost odorless. Thus, many materials are available

for odor counteraction, including the essential oils (63).

Odor counteraction is particularly effective against multiple odor

sources (typical of many industrial plants) which form an odorous "pool"

that emanates from a plant site through molecular dispersion and general

air movement. An atomizing nozzle is used to vaporize the counteractants

and inject them into the air stream where they are designed to mix with

and follow the odorous pool. The vaporizing points are located either

in the exhaust stacks or in the general vicinity of the major odorous

sources. Odor counteraction formulas are designed for specific odor

groups, and the installations should be made by a specialist who can

discriminate between various odors and estimate the intensity of

concentrations involved in the overall problem.

As with odor masking, various procedures must be followed to obtain

the best results, and are covered in detail by Von Bergen (63) and

Summer (65). Advantages and disadvantages of the two'techniques are

similar also. Effectiveness of the installation must be determined by

actual odor perception of the populace, which is actually the criteria for

evaluating the success of any odor control program.

Odor Control in the Kraft Pulping Industry

The major odor control methods used by the kraft pulping industry

include process modifications (black liquor oxidation, elimination of

direct contact evaporator, etc.), proper operation and maintenance of

equipment, combustion and chemical oxidation, absorption, and dispersion.

Odor masking is used to some extent. Condensation, primarily for heat

recovery, does remove malodorous sulfur compounds from the digester and

evaporator gas streams, but presents additional problems in odor control

and waste treatment.

Brief descriptions of each method and literature references are

presented in the following sections.

Dlack Liquor Oxidation

Typical of a process change developed to prevent initial formation

of malodorous sulfur gases is the practice of black liquor oxidation

which is extensively employed throughout the world as a partial solution

to the kraft odor problem. The main purpose of black liquor oxidation

is to oxidize sodium sulfide in the liquor to sodium thiosulfate to

prevent liberation of hydrogen sulfide in the direct contact evaporator.

In the direct contact evaporator, black liquor (at approximately 50

percent solids) is brought into direct contact with hot gases from the

recovery furnace to increase its solids content to a firing concentration

of 65-70 percent solids. With unoxidized black liquor, carbon dioxide

and sulfur dioxide in the furnace gases will react with sodium sulfide

in the liquor to generate hydrogen sulfide as follows (16):

NaS + CO + H20~ Na2CO3 + H2S

NaS + SO2 + H 0 ---- 14 Na SO0 + HS .
2 2 23 2


With oxidized black liquor, ha'ever, the sodium sulfide has been converted

to sodium thiosulfate which is relatively stable, and will not break dawn

in passing through the direct contact evaporator. Thus, neither of the

above reactions will take place and the sulfur will be retained in the


Black liquor oxidation can take place prior to the multiple effect

evaporators (weak black liquor oxidation) or immediately before the

direct contact evaporator (strong black liquor oxidation). Depending on

which system is used, oxidation is normally accomplished by reacting the

sodium sulfide with oxygen from the air in either a packed taoer, bubble

tray tp er, or air sparged reactor. Various factors affect the choice of

a weak or strong black liquor oxidation system and each has distinct

advantages and disadvantages. Thorough reviews of black liquor oxidation

are by Collins (80), Guest (81), Landry (82), Hendrickson and Harding (83),

and Blosser and Cooper (84). The kinetics of oxidation are discussed by

Wright (85) and Murray (86), and effects of operating variables on strong

black liquor oxidation by Morgan, et al. (87). The use of pure oxygen

instead of air for oxidation was studied by Ricca (8), Galeano and

Amsden (88) and Fones and Sapp (89). Significance of the direct contact

evaporator as a major source of odor and the importance of practically

complete black liquor oxidation to prevent these odors is reported by

Murray and Rayner (58) and Blosser, et al. (90).

With na% recovery furnaces and unit processes designed to eliminate

the direct contact evaporator (as discussed in the following section),

the requirement for black liquor oxidation would be virtually eliminated

in the construction of new mills since it was originally developed to

prevent odor generation in this unit. Although it has been shotn that

weak black liquor oxidation will reduce sulfur emissions from the

multiple-effect evaporators, the disadvantages and difficulties with the

system would outweigh this minor contribution. The small volume of the

evaporator effluent stream (generally less than 100 cfm) makes collection

and burning in the lime kiln a relatively simple matter, and in addition,

elimination of black liquor oxidation would also eliminate the need for

treating the oxidizer off-gas (91).

Elimination of Direct Contact Evaporation

The direct contact evaporator has been shown to be a major source

of malodorous sulfur emissions (51, 58, 90, 92), and elimination of this

unit from the kraft recovery process is a major contribution toward

odor control in the kraft pulping industry. Operation of recovery

furnaces without direct contact evaporators has been practiced in

Scandanavion countries for scme time (93, 94) and is showing increased

interest and application in the United States.

Two basic methods have been employed to increase the black liquor

concentration to firing conditions and avoid direct contact with recovery

flue gases. Hochmuth (95) describes the Air Contact Evaporator (ACE)

system by Combustion Engineering, Inc. (96) in which combustion air for

the furnace is preheated by flue gas to approximately 6000 F in an

air heater. The flue gas is thus cooled from approximately 8000 F to

3250 F, and discharged to the atmosphere without coming in contact with

the black liquor. The stream of preheated air then passes to a direct

contact evaporator where it loses sensible heat while evaporating water

from the black liquor. Air leaving the direct contact evaporator is


conveyed to the primary and secondary air admission ports of .the furnace

as combustion air. Any malodorous sulfur gases that may have been

formed during the evaporation process are destroyed by incineration in

the high temperature combustion zone of the Furnace.

Clement and Elliott (97) describe the development of a new recovery

furnace by The Babcock and 'Jilcox Co. (98) to operate directly an unoxidized

high-solids liquor from multiple-effect evaporators, thus eliminating

direct and indirect contact between the furnace flue gases and black

liquor for evaporation purposes. American Can Company's new mill at

Halsey, Oregon, utilizes a three-section, forced circulation concentrator

in addition to a sextuple effect evaporator system to obtain the desired

black liquor concentration for firing, and is discussed in detail by

Canovali and Suda (99).

Proper Operation, Maintenance, and Housekeeping

Regardless of what process changes or additions are made, adequate

odor control will not be achieved unless operations are performed within

design capabilities of the equipment, and continued efforts are made to

prevent leaks and the accumulation of odor-producing materials. Proper

maintenance of control equipment to prevent breakdowns is an absolute

necessity also.

Even though the recovery furnace has long been regarded as a major

source of kraft mill odor, studies (51, 90) have been made on the relation-

ship between furnace operation and odorous emissions which shao that

malodorous sulfur compounds at the furnace outlet can be reduced to

negligible amounts if the furnace is operated within design capability

with enough excess air to provide about 3 percent oxygen in the flue gas,

and if sufficient turbulence is obtained in the furnace oxidation zone.

Furnace overlodaing by excessive black liquor firing rates is the normal

cause of high malodorous emissions since the air handling capacity of

the furnace is limited by the installation (4).

Combustion Oxidation

Along with black liquor oxidation, combustion oxidation has proven

to be one of the most effective means of odor control used in the kraft

pulping industry today. Whereas black liquor oxidation prevents the

initial formation of odors, combustion oxidation is used for the

destruction of malodorous gases generated in various kraft processes.

At the present time, this method of odor control is used primarily for

the destruction of digester and evaporator noncondensable gases in the

lime kiln, and has shown to be virtually 100 percent effective (100).

These two gas streams provide a law volume, high concentration source

of malodorous sulfur gases, and can be collected and piped to the kiln

relatively easily. The lime kiln has proven particularly useful for

this method since the temperature (approximately 20000 F) is sufficient

for complete oxidation, air volume is sufficient for dilution, and sulfur

dioxide (formed during oxidation) emissions are negligible (91, 100, 101).

Early work on the development and successful operation of burning

these gases was done by DeHaas and Hansen (102) using an auxiliary

furnace. Adaptation of the system for burning in the lime kiln is

reported by Coleman (103). Numerous other articles (91, 99, 101, 104-106)

are found in the literature describing the successful use of the lime kiln

for destruction of nonccndensable gases. Reports on he use of specially

designed boilers and incinerators (107-109), recovery furnaces (99, 107, 110),

waste wood incineration units (24) and natural gas-fired power boilers (111)

for burning the noncondensable gases are also found in the literature.

The report by Canovali and Suda (99) describes the system at American

Can Company's nve mill in Halsey, Oregon, in which the noncondensable

gases can be alternatively admitted to the recovery furnace in case the

lime kiln is not in operation. Incineration of brown stock washer hood

gases in the recovery furnace is also discussed in this article (99) and.

by Hisey (110).

Catalytic oxidation of kraft mill odors has been considered and

used to some extent. The major problem appears to be related to the

particulate loading of the gases and subsequent poisoning of the catalyst

by these materials (61). Blosser and Cooper (100) mention the experience

of two mills using catalytic furnaces containing a series of porcelain

rods coated with a thin film of catalytic alumina and platinum alloy.

Complete removal of mercaptan and dimethyl sulfide and more than 85 percent

removal of dimethyl disulfide has been observed in one of these units.

Maintenance of automatic controls has been a problem, and frequent

replacement of catalyst cells may be encountered where removal of

condensables is poor. A description of this system is also reported by

Landry and Longwell (112) with a cost breakdown.

Catalytic oxidation has been suggested (113) for controlling

emissions of organic solvent vapors (from printing and coating operations)

but the presence of inorganic materials or catalytic poisons in the gas

stream would have to be investigated.

Chemical Oxidation

Gas-liquid chlorination is widely used throughout the kraft industry

for the chemical oxidation of malodorous sulfur gases, particularly the

digester and evaporator noncondensables. Common practice is to expose


these gases to chlorination stage washer effluent from the bleach plant

in either a scrubber or the washer dropleg. Supplemental chlorine may

be required if sufficient amount is not available in the washer effluent.

In an unbleached pulp mill, chlorine gas would have to be supplied From

an outside source. Typical reactions in this process are discussed by

Douglass (5) and DeHaas and Clark (34), and indicate that excessive

amounts of chlorine are consumed. Ruus (114) reports the chlorine

demand to be 2-9 pounds per pound of malodorous compound, and discusses

the subsequent high cost of elemental chlorine oxidation if the bleach

residues are insufficient.

Thomas, et al. (115) discuss chlorination at the S. D. Warren

KraFt Mill in Westbrook, Maine, in which digester blow gases are passed

through primary and secondary deodorizing scrubbers utilizing the

chlorination stage effluent. As a backup, weak hypo bleach can be added

to the secondary unit. Ghisoni (1ll) discusses chlorine oxidation of

recovery boiler effluent gases, and Morrison (106) reports on the use of

chlorination as a backup needed when the lime kiln is not in operation

for burning the noncondensables. Archibald and Von Donkelaar (116),

Lindberg (107), Adams (61), and Trobeck, et al. (117) also discuss

chlorination installations.

Gas-phase chlorination of kraft pulp mill gases was studied by Koppe

and Adams (118) in laboratory experiments. Gas samples from the recovery

furnace, batch digester, multiple-effect evaporators, and lime kiln of a

mill were reacted with known volumes of chlorine gas and the reactions

recorded by gas chromatography. Concentrations of hydrogen sulfide and

dimethyl sulfide did not change, and the methyl mercaptan was oxidized

to dimethyl disulfide. The process appears to be of limited value as a


means of kraft odor control since the overall odor reduction of the total

gaseous effluent would probably not be sufficient to justify its use on

a plant scale. Nevertheless, gas phase chlorination of noncondensables

has been used for several years at the Peia Pobre mill in Mexico City.

Excellent control of malodorous sulfur gases are claimed for the TLT (117)

system, and obviously at reasonable cost. Tirado and Gonzalez (119)

explain that the total results at the Pena Pobre mill are a combination

of partial results obtained in each of various stages or processes, and

that the experiments by Koppe and Adams (118) may have been conducted

under too mild conditions. Air oxidation of the noncondensables in

the presence of water is used prior to chlorination to reduce chlorine

consumption. Following chlorination, a final wash of the gases with

white water completes the TLT system.


Gas scrubbing is used throughout the industry for odor control, but,

in the interrelationship of air and water pollution, using scrubbing to

reduce the odor in one gas stream may simply be transferring the problem

to some other point in the mill. Unless an effective oxidant is used

in the scrubber to convert the malodorous compounds to innocuous products,

care must be taken to prevent re-release of the odors. Keeping the

absorbing liquid in the process stream is another solution, and most

widely used.

Ruus (114) discusses the use of white liquor for absorbing hydrogen

sulfide and methyl mercaptan from evaporator noncondensables which keeps

them in the liquor cycle and eliminates the need for aqueous effluent

disposal. Secondary scrubbing of recovery stack gas with weak wash is

reported by Buxton and La Pointe (120) to give hydrogen sulfide recovery

of 90 percent, and Hawkins (121) discusses the successful use of weak

wash for scrubbing evaporator gases. At U.S. Plywood-Champion Paper

Inc., Pasadena, Texas, scrubbers utilizing weak wash from the causticizing

department are used to absorb evaporator noncondensable gases, while

gases from the blow tank, tall oil vent, chlorination tacer vents, and

chlorination stage washer seal box vents are scrubbed with water (122).

Adams (24) discusses the use of an absorption-adsorption combination

at a mill in Sweden. The digester noncondensable gases are scrubbed

with warm (950 F) white liquor to absorb hydrogen sulfide and methyl

mercaptan. The unabsorbed gases then pass through activated charcoal to

remove methyl sulfides and methyl disulfides. The charcoal is then steam

stripped to recover the organic sulfides.

Pilot plant studies are reported by Oloman, et al. (123) for selective

absorption of hydrogen sulfide from kraft stack gases using a solution of

sodium carbonate--sodium bicarbonate.


As a sole means of odor control, dilution of malodorous sulfur

emissions by dispersion from tall stacks is not always effective for a

kraft mill due to the lowI odor threshold of the gases, changing meteorological

conditions, and the varying concentrations of effluent gases. Average

stack height for most mills is 200-300 feet, and their effect on odor

control is most noticeable in the vicinity of the installation. It is

believed that the tallest stack presently in the paper industry is 475

feet high.

Tall stacks are very effective for kraft odor control when used in

conjunction with other control methods. Ghisoni (111) reports on the use

of a 260-Foot stack located on the rim of a 140-foot-deep valley in Milan.


Remaining odorous gases from a pulp mill located in the valley are piped

to the stack for release. The mill uses chlorine oxidation and incineration

for primary odor control. ':alther and Amberg (101) discuss the use of a

310-foot recovery stack to discharge gases from the lime kiln, poaver

boiler, recovery furnace, and black liquor oxidizer at Crown Simpson

Pulp Co. in California. Malodorous gases from the continuous digester

and multiple-effect evaporators are burned in the lime kiln and scrubbed

in a Venturi scrubber prior to release through the recovery stack.

Odor Masking and Counteraction

Odor masking and counteraction are not used as extensively in the

kraft pulping industry for odor control as some of the methods previously

discussed, and reports concerning their effectiveness are quite varied

and limited.

Tremaine (124) describes the application of Alamask (125) for kraft

odor control by either (1) spraying, vaporizing, or atomizing it into

the odorous gas stream, (2) adding directly to the process, (3) adding

to scrubbing liquors, or (4) spreading or floating on contaminated

surfaces, and states that addition directly to the odor-producing process

is the best application for the pulp and paper industry. DeHaas and

Clark (34) report on the use of a masking agent added directly on the

chips in the digester (3-4 pints per digester) prior to closing the lid.

Experiences at the mill indicated that the masking compounds would control

digester odors under ideal conditions (warm temperatures, unlimited ceiling,

and strong winds), but effects were not so pronounced under cold, foggy

conditions. Costs were also relatively high.

Meuly and Tremaine (126) discuss a joint study between Gulf States

Paper Co., Tuscaloosa, Alabama, and DuPont on the use of Alamask as a

masking agent. Results were favorable, but greatly depended-upon

meterological conditions, type of wood being pulped, and liquor sulfidity.

The effect of meteorological factors on the amount of masking agent

required is covered by Zirm (127). Tremaine (128) suggests that a masking

agent should be applied to the blow-down heat recovery and concentrated

black liquor at the cascade evaporator in addition to the digesters, and

lists favorable comments made at five kraft mills using odor masking.

U.S. Plywood-Champion Paper, Inc., Pasadena, Texas, has added masking

agents since 1952, and presently uses them in the digester and recovery

areas at a cost of $75-$100 per day (122).

Von Bergen (129) reviews the phenomenon of odor counteraction, and

states that a majority of observers in an evaluation test, covering over

30 days at a paper mill, were enthusiastically impressed by the absence of

mal dors while ccunteractznts weir being used.


Although not usually considered as part of an odor control program

at a kraft mill, condensation has been practiced in the kraft industry

probably ever since the operation of the first mill in 1891, and is

actually the first step in removing malodorous sulfur compounds from digester

and multiple-effect evaporator gas streams.

Condensation of the digester relief (or steaming vessel relief For

continuous digesters) and blow gas is primarily for heat recovery, and is

accacplished with either surface or spray (barometric) condensers. Surface

condensers are normally used to reduce the volume of liquid to be handled.

Turpentine recovery is another advantage for condensation of the relief

gases. In the multiple-effect evaporators, condensation of the steam

from the black liquor in the last body is used to maintain a high vacuum

in the system for circulation.


For the purposes of odor control, the result of condensation is two

effluent streams --. a condensate stream and a stream of noncondensable

gases. Both contain malodorous sulfur caopounds, and must be controlled

for an effective odor program. Various methods are discussed in

previous sections of this paper for odor control of the noncondensable

gas stream, and include incineration, chemical oxidation, absorption,

and adsorption. For the condensate stream, in addition to possible release

of the malodorous sulfur gases, contamination of water courses may

present another problem since the organic load of the combined condensates

can amount to approximately 50 percent of the total waste load in a mill

effluent going to the river or waste treatment plant (34, 53).

Reuse of the condensate within the mill as water makeup, wai water, etc.

has proven to be an effective way of keeping the sulfur compounds in the

process and alleviating some of the load on liquid waste treatment

facilities, but can result in the release of malodorous gases in open

systems such as holding tanks and washers (4, 53). Studies by 1/alther

and Amberg (91) show that whenever digester and evaporator condensates

are reused for pulp washing or scrubbing, the odor problem is intensified,

and that these streams should be completely treated prior to reuse in

these applications.

':alther and Amberg (101) discuss the use of air oxidation to

reduce the sulfur content of the condensate stream, and also report a

20 percent reduction in BOD and 50 percent reduction of COD in the

effluent. Air oxidation is also used in the TLT (Trobeck--Lenz-Tirado)

system (117) at the Pena Pobre mill in Mexico City, where both contaminated

condensate and noncondensables are reacted with air and water prior to

chlorination. As reported by Tirado and Gonzalez (119) the condensates

lose nearly all their H S and mercaptan content due to air stripping.

Hisey (110) discusses air oxio'ation at a South African mill in which

digester condensates are blan with air into a packed tacer, and the

effluent is used as make-up water in a spray cooling pond. Odors created

are negligible. Stripping of condensate streams with steam (53, 130)

and recovery furnace flue gas (61) are also effective for removing

malodorous sulfur compounds and producing reusable water. Gaseous

effluents from stripping operations must be disposed of similar to other

noncondensable gascs for effective odor control.

Land disposal of condensates and other mill effluent through spray

irrigation has been investigated (131, 132), and could offer an effective

and economical method of disposal for some locations. Odor release

could still present a problem near populated areas.



Characteristics, Formation, and Toxicity


Ozone (03) is a triatomic allotrope of oxygen with a molecular

weight of 48, and exists as a pale blue gas under normal conditions.

The molecule is unstable, and slowly decomposes to ordinary oxygen at

normal temperatures. Decomposition is accelerated by heat and moisture,

and is almost instantaneous at 300-4000 F. Due to its high electro-

negative potential of -2.07 V, which is second only to fluorine at

-2.1 V (133), ozone is one of the most powerful oxidants known, and its

reaction is rapid with most materials..

The scientific discovery (in 1840) of ozone and scme of the earliest

investigations of its characteristics are attributed to Christian

Schonbien of Switzerland. However, ozone was first detected in 1785 by

the Dutch chemist Van Maroom who noticed a previously unknown odor

appearing in the air during an electrical discharge. He considered this

phenomenon to be inherent to electricity. In 1801, Cruikshank of England

detected the same odor in oxygen which was liberated duringthe electrolysis

of water, and in 1840, Schonbien reported that the odor obtained during

an electrical discharge in oxygen was caused by the appearance of a new

gas which he named ozone. During the period 1845-1860, a number of

investigators (Marinyak and Delariv, Fremi and Bekerel, Andrews)

attempted to convert pure oxygen into ozone, and made the first

characterization of ozone as an allotropic modification of oxygen. In

1867, Soret used measurements of density during diffusion of ozone to

introduce the formula of 0 (134). The word ozone is derived from the

Greek "ozon" (smelling), from "ozein" (to smell), and from the base

"od" (to smell) as in Latin "odor."

Gas density of ozone at 00 C and 1 atmosphere is 2.144 g/liter.

At a temperature of -111.90 C ozone thickens and is transformed into a

highly unstable fluid of dark blue color. Density of ozone in the

liquid state at -1830 C is 1.571 g/cc, while in the solid state at

-195.20 C is 1.738 g/cc. The melting temperature of ozone is -192.50 C.

Ozone is absorbed by many liquids (although unstable in most) and its

solubility is inversely proportional to temperature, Between 00 and

200 C ozone is absorbed in water some 35 times as much as is air. At

500 C, the absorption is about five times as much as air, and at 600 C,

air absorption is still approximately 16 mg/lO000 g H2/atom whereas

ozone absorption is zero (65).

Although there is some difference in the literature concerning bond

dimensions and angles, there is agreement that the ozone molecule is

triangular in form with an obtuse apex angle, and a resonance hybrid of

at least two structures. According to Mortimer (135), both oxygen-to-
oxygen bonds have the same length (1.26 A) which is intermediate between
o o
the double bond distance (1.10 A) and the single bond distance (1.48 A),

and the ozone molecule is a hybrid of the following two structures:

U.0- .- 0. .0..

A reference by Summer (65) indicates an interatomic distance of 1.2 A

with an obtuse apex angle of 110 degrees. Bailey, et al.(136) and

Kozhinov (134) suggest that ozone is a hybrid of the following four


** we ee CC 9 ** d Qe e
:o= -0- 0: -- :0o -- --- 0: o -- o -- 0:
+ -- + +
-0 --0 -- 0:
+ ** t

Either way, the ozone molecule is extremely unstable and spontaneously

decomposes into an atom and a molecule of oxygen. The reaction is strongly

exothermic (34.5 kcal per mole of ozone) which explains the explosive

nature of ozone under certain conditions. Chemically pure ozone (100%)

explodes violently at the very slightest impulse, and pure liquid ozone

at temperatures near its normal boiling point is particularly dangerous

(137). In practice, explosion of ozone will not occur if its concentration

in the ozone-oxygen mixture or ozone-air mixture does not exceed 10 percent.

Such mixtures are safe at pressures of several atmospheres and under any

conditions -- during heating or shock and in reactions with traces of

organic contaminants (134).

Ozone Formation

Ozone in the earth's atmosphere is generated by the interaction of

solar ultraviolet rays and free oxygen atoms. The wavelcengths capable of

producing ozone naturally fall within the absorption spectrum of oxygen,

or approximately 1100-2400 A. Ozone is also decomposed by solar radiation,
particularly in the Hartley Band (2200-3000 A), and even at longer wave

lengths since the dissociation energy is only 24.6 kcal. As a consequence

of this generation and decomposition due to solar radiation, an ozone

concentration equilibrium occurs (subject to various meteorological parameters)

which results in an ozone profile as a function of altitude (138-140).

Although the distribution of ozone in the atmosphere varies from one

author to the next, several research projects indicate maximum concen-

trations at altitudes of 13-16 miles (139, 141). Dependent upon the

season and latitude, concentrations of 5-10 ppm can occur in this upper

atmosphere. Ozone concentrations at or near sea level are normally in

the range of 0.01 to 0.04 ppm, although a concentration of 0.99 ppm

was detected during Los Angeles smog conditions in 1956 (142). Processes

involved in the generation of atmospheric ozone, both in the upper and

lower atmosphere (photochemical oxidation), are well covered in the

literature (65, 141, 143).

For laboratory, municipal, and industrial applications, ozone can

be produced by three different methods: electrolytic, silent arc discharge,

and ultraviolet radiation. Of these, only the latter two have proved

practical.since, in the electrolysis of water, ozone yield is severely

influenced by thermal destruction resulting from high curireii densities

required at the anode (138).

For high ozone output, silent arc discharge generators are normally

used, and was the type used for this study. The generating unit consists

df a pair of large area electrodes, either flat or cylindrical, separated

by an air gap and a dielectric layer. In many generators, the electrodes

are stainless steel and aluminum, and the dielectric is borasilicate

glass. One electrode is contiguous to the glass dielectric, and the

gas gap occurs between the dielectric and the other electrode. A

common gap distance for voltages belao 15000 volts is 1-3 mm (138).

In the presence of high, alternating voltage discharges, ozone is

generated from the oxygen in the gap between the two electrodes. Key to

the process is the presence (in the air gap) of stray electrons from a

previous discharge or background radiation. In a field of sufficient

intensity, the electrons move to the positive electrode, thus setting

up a series of molecular collisions. Some achieve sufficient velocity

to penetrate oxygen molecules, producing additional free electrons or

high energy unstable molecules which break daon into free radicals --

single oxygen atoms. Most of these single atoms combine with oxygen

molecules (02) to form ozone (03 ). The process is repeated when polarity

is reversed in the second half-cycle, and the electron's movement to

the opposite electrode creates new molecular collisions. Thus, there

are two productive instants in each cycle, occupying a relatively small

portion of the total cycle time. The production efficiency is therefore

increased when the cycle is shortened; i.e., when the frequency is

raised. Most generating units are designed to operate in the 5000-25,000

volt range (through suitable transformers)) nd at frequencies up to

1000 Hz, although some designers are exploring the possibilities of higher

frequencies for increased production efficiency (144). Normal generator

output is 1-3 percent ozone by weight.

The ozone yield of the silent arc discharge generator is a function

of the following factors: oxygen purity of the parent gas, oxygen temperature,

pressure, and flo'i, peak voltage, Frequency, capacitance of discharge gap,

and capacity of dielectric. For a given configuration generator, ozone

output is a function of current density with all other conditions held

constant, and the output is approximately doubled when the parent gas

is pure oxygen rather than air.

Using oxygen as a feed, various effects have been noted in generator

output with different gas diluents. As discussed by Cromwell and Manley

(145), the greatest loss in ozone yield is noticed with the presence of

less than 1 percent of hydrogen, water, or Freon 12. Up to 50 percent

of carbon dioxide or argon reduces the yield to about 85 percent of that

expected from pure oxygen. Carbon monoxide or nitrogen, in amounts of

less than 10 percent, apparently increase ozone output, but in greater

amounts a gradual decrease is noted. Inoue and Sugino (146) studied

the inhibiting action of several hydrocarbons on ozone yield by silent

arc discharge, and report that the rate of ozone formation varies

linearly with the hydrocarbon concentration, regardless of the type of

hydrocarbon. Water vapor remains the most common contaminant to be

guarded against, however, and can be eliminated by adequate drying of

the parent gas (usually specified to -600 C dcpoint) prior to ozone


Additional information concerning ozone generation by electrical

discharge is discussed by Lunt (14/), Summer (65), Suzuki, et al. (148),

Ogden (144), and Fuji and Takemura (149).

Ozone generation by ultraviolet radiation is most commonly used

where la ozone concentrations are required, such as laboratory work, air

purification, elimination of molds and bacteria, etc. As with the silent

arc discharge generator, air or oxygen can be used as the parent gas.

Ozone yield is a function of total effective radiation cmitted in the

range of 1100-2200 A, which is in turn dependent upon lamp design,

emission area, current density, and oxygen fla-, pressure, and temperature

(138). The reactions involved are similar to those causing the formation

of ozone in the upper atmosphere, and are discussed along with generator

design information in the literature (65).

Ozone Toxicity

In an excessive dosage, most any substance is toxic to the human

organism. Substances assume the title "toxic," however, when the

dosage determined to be excessive is a very lo. unit quantity. Within

the framework of this definition ozone is definitely toxic, since the

maximum average concentration for an eight-hour exposure has been set

at 0.10 ppm by the American Conference of Governmental Industrial

Hygienists (150). With the increasing use of ozone in municipal,

commercial, and industrial installations, the occurrence of ozone in

smog conditions, and the possible hazards of high altitude flight from

atmospheric ozone above 50,000 feet, the toxicity of this gas is an

important problem.

Ozone was first recognized as a toxic substance by Andrews in 1874

when he exposed small laboratory animals to high concentrations of ozone,

resulting in their death even after a short exposure (151). Since then,

numerous toxicity studies have been performed on both animal and human

subjects. Naturally, a majority of these investigations have dealt

with animals (rats, mice, rabbits, dogs, cats, guinea pigs), and cover a

multitude of conditions, ozone concentrations, and exposure times (152-170).

Most of these studies have established an LD50 rate for the various

animals, which is the dose of ozone that will, on the average, kill 50

percent of a significant number of animals exposed under specified

conditions. Since there are several experimentally simple and statistically

sound methods of obtaining LD50 values and various procedures for determining

ozone concentration, and due to the multitude of test conditions for the

above referenced studies, there is some confusion in the literature on

the actual toxicity of ozone. Regardless of the exact toxicity, the

studies have left no doubt that ozone in acute exposures is a highly

toxic and lethal substance, and that its impact is directed primarily

on the soft tissues of the respiratory tract.

Gilgen and Wanner (151) review many of the ozone toxicity studies

on animals, and noting the diversity of values, provide a brief summary

of the literature. In the case of exposures between 3 and 24 hours, the

LD50 for rats and mice varies between 3.8 and 22 ppm. Hamsters, rabbits,

and guinea pigs tolerate higher concentrations. Death is caused by

pulmonary edema. Small concentrations lead to usually reversible

changes of the respiratory organs, to disturbances of respiration, and to

changes in the activity of various enzymes. Exposure to low concentrations

can also develop a tolerance against subsequent exposure to lethal

concentrations. Chronic exposure to ozone causes damage to the

respiratory organs (bronchitis and lung emphysema), inhibition of the

weight increase of younger animals, and decreased acidity of the urine.

The article by Stokinger (165) is also an excellent review of ozone

toxicity to animals.

Numerous articles are found in the literature describing controlled

experiments on the toxicity of ozone to humans (171-176), and also

some effects of ozone from actual working conditions (177-182). A brief

review of some of this literature to illustrate the toxicity of ozone


includes a decrease in vital capacity of the lungs after a single exposure

to 1.5 ppm for a half hour and a subsequent exposure to 2 ppm for 1.5

hours (176), decrease of the CO diffusion capacity after exposures to

1.2 to 6.0 ppm for 1-2 hours (171), significant reduction in diffusion

capacity, vital capacity, and maximum flow in the middle of the expiration

phase after inhalation of 0.6 to 0.8 ppm for 2 hours (175), Impairment of

the upper respiratory passages in welders working with the shielded arc

welding process who were exposed to 0.8 1.7 ppm ozone every day (180),

accelerated pulse, sleepiness, lasting headaches following brief

inhalation of 5-10 ppm and impulse to cough and great lassitude after

1.5 hour exposure to 1.0 ppm (182), severe ozone intoxication (dry cough,

weak pulse, locered blood pressure, nearly unconscious) of a crane

operator who operated a crane approximately 14 feet above a tank

containing 1 percent ozone (181) and lung edema following exposure to

ozone concentrations above 4-5 ppm for an hour (171).

The study by Clamann and Bancroft (171) involved five subjects, ranging

in age from 19 to 54 years. The highest ozone concentration applied was

6 ppm for 1 hour, and the longest exposure time was 2.5 hours at 1.2 ppm.

They concluded that ozone acts only superficially on the wet, soft

tissues of the respiratory tract, and is not able to penetrate deeply.

A definite impairment on the sense of smell was found at exposures of

2.41 ppm for 1.5 hours and 4.16 ppm for 22 minutes. No effects on

blood pressure, pulse rate, and blood itself were observed. Long exposure

times and higher concentrations tend to carry injury to the respiratory

tract deeper toward the alveoli. in two of their experiments involving

inhalation of air with 4.8 ppm ozone, the exhaled air contained practically

no ozone, suggesting that ozone decomposes completely in the respiratory

tissues; however, studies by ballet: (174) showed that the expired air

of his experimental subjects contained 25-75 percent of the inhaled

ozone. The effects of ozone are not limited to the respiratory system

according to some authors. Brinkman and Lamberts (183) have described

a lowered rate in the release of oxygen from oxyhemoglobin in the skin

capillaries, and Lagerwerff (172) reported an impairment of the visual


A summary of the biological effects of ozone for concentrations

ranging from 0.01 10 ppm is shown in Table 3 (65). Similar values are

reported by Biget, et al. (141).



Concentration (ppm) Effects

0.01 Threshold odor of pure ozone

0.05 Irritation of the lungs and respiratory mucosae
after prolonged inhalation

0.10 Respiratory discomfort, headache

0.15 Eye irritation threshold

0.50 Considerable discomfort, disordered breathing

1-10 Coughing, fatigue, headache, possible coma after
prolonged inhalation.

5-10 Stupefaction, body pain, accelerated pulse,

Tolerance to ozone has been demonstrated in laboratory animals

(153, 155, 156, 165-168), but it is still unclear whether a tolerance

develops in humans. Gilgen and Wanner (151) and Stokinger (155) suggest

the possibility that the population of Los Angeles has developed an ozone

tolerance resulting from the often repeated exposures to low ozone

concentrations, and due to the fact that the inhabitants have reported

no acute pulmonary damages even when the ozone concentration exceeds 0.5

ppm. In the case of welders, ozone concentrations of more than 0.5 ppm

led to the development of acute lung damages in every case, but their

re-exposures occur after short intervals, thus they are not able to

develop a tolerance. Animals always shqoed the best tolerance when their

exposures were separated by an interval of several days.

Various explanations are found in the literature concerning the

mechanism of ozone's action on the body (156, 167, 183-189). Fetner

(188) and Brinkman and Lamberts (183) suggested the possibility of radio-

mimetic effects of ozone. Fetner (185) showed that human cell cultures,

which had been exposed for 5-10 minutes to 8 ppm of ozone, exhibited

chromosomal cleavages similar to those resulting from an irradiation with

200 roentgens. Zelac (187) reported on chromosome aberrations in

circulating blood lymphocytes of Chinese hamsters due to ozone inhalation,

and stated that if the results of the study were directly extended to

the human case, presently permitted human ozone exposures wbuld be

expected to result in break frequencies that are orders of magnitude

greater than those resulting from permitted human radiation exposures.

An excellent review of possible mechanisms of ozone action is given by

Fenn and Rahn (190), and also covers other areas of ozone toxicity (tolerance,

factors affecting toxicity, effects on man and animals, etc.).

As stated by Gilgen and Wanner (151) and Fenn and Rahn (190), so

Far it has not been possible to determine the meaning of the immuno-

chemical changes taking place during the appearance of the ozone effect;

therefore, it is impossible to give a meaningful explanation for the

pulmonary damages resulting after chronic exposure to ozone. Regardless

of its mode of action, controlled studies on animal and human subjects,

and actual cases of ozone exposure in working environments, have

definitely established that ozone is a highly toxic gas.

Ozone Applications

Waste Treatment

The market for ozone application in the waste treatment field is

normally as a polishing agent in conjunction with other accepted waste

treatment systems. Studies have shown ozone to be very effective in

treating industrial waste streams, particularly those containing cyanide

and phenols. The Boeing Company plant at Wichita, Kansas, uses more than

350 pounds of ozone per day as a secondary waste treatment process to

cope with cyanide, phenols, oils, detergents, sulphides, and sulphites.

The waste is ozonated at approximately 20 ppm and discharged into a

lagoon where fish have survived since the beginning of the project in

1957 (144, 191, 192). Reports on the kinetics of ozone-cyanide reactions

(193, 194), comparisons of chlorine and ozone treatment of cyanide wastes

(195, 196), laboratory and pilot plant studies of the ozone oxidation of

phenols (197-201) and others (202-205) describe the feasibility of using

ozone for treating industrial waste waters.

Air Reduction Co. (Airco) was awarded a contract by FWlPCA to construct

a pilot plant at the Washington, D.C. Blue Plains treatment facility to


study tertiary treatment of municipal waste with ozone. Previous work

by Airco has demonstrated that ozone treatment of sewage plant effluent

destroys organic contaminants, surfactants, and bacteria, and produces

a clear, odorless effluent that meets PHS standards for drinking water.

Economic evaluations indicate that the ozone treatment will be competitive

with activated carbon (206, 207). Similar results were obtained in

pilot plant work using effluent frcm trickling filter waste treatment plants

at Totawa and Berkeley Heights, New Jersey (144). Additional studies

on ozone treatment of municipal waste effluent are covered in the literature

(203, 208-212).

Odor Control

The most prominent use of ozone for odor control is in the treatment

of sewage exhaust gases, primarily hydrogen sulfide. Installations have

included odor control for sludge storage and supernatant tanks (213),

treatment of ventilating and exhaust gases from fully enclosed aeration and

settling tanks (6), lift station and wet well exhaust (214), complete

underground sewage treatment plant in Atami City, Japan (215), grit

removal, bar screens, settling and chlorination buildings in Mamaroneck,

New York (191), and treatment plants in Florida (216, 217) and Michigan

(218). One of the earliest installations was the Ward's Island Treatment

Plant in New York City, installed about 1930 (6). The plant treats an

exhaust gas stream of 30,000 cfm with an ozone concentration of 1 ppm,

and reports complete elimination of the odor problem.

Other installations using ozone for odor control include a pharmaceutical

plant (219), rendering plants (214), Fish-processing operations (6), rubber

compounding plant exhaust, phenolic odors from felts, and commercial

kitchens (215).


Table 4 lists typical ozone concentrations For various installations

established from experience by the Welsbach Corp. (220), although exact

dosages would depend upon retention times, temperature, humidity, and

concentration of the exhaust stream, and nature of the malodorous gases

being treated. These figures are based on a contact time of at least

15 seconds.



Appl ication Ozone Concentration (ppm)

Se.vage plants, general 1
Morgues 3
Phenol plants 3-10
Rubber plants 3-10
Fish-processing plants 10
Sludge storage and vacuum filter 10
Rendering plants 10
Paper mills 10-50

\later Treatment

The most widespread application of ozone is in the treatment of drinking

water, and the first recorded commercial installation for this purpose began

operation at Nice, France, in 1906. Over 20 million gallons of water are

steril ized daily with ozone at the water works in this city (221). There

are currently more than 500 municipalities using ozone for water treatment

in over 50 countries. Its use is definitely more widespread in European

countries who use ozone as the disinfectant of choice, while in the

United States, a chlorine residual is required in order to provide a

disinfectant at the consumer's tap. In many of these installations, ozone

is used in conjunction with other water treatment techniques (chlorination,

microstraining, filtration, flocculation, carbon filtration, etc.), and

depends upon the raw water and the water quality requirement (211, 222,


The use of ozone for water disinfection and the removal of tastes,

odors, and color is well covered in the literature. Nobert (224) discusses

the use of ozone at five water works plants in Quebec to produce good

quality water free from tastes and odors, which had not been previously

achieved with chlorination and activated carbon. Studies comparing the

effectiveness of ozone and chlorine (156, 212, 225-228) for water treatment

stress that ozone leaves no residual taste or odor, and reacts more

rapidly than chlorine in killing most bacteria. In addition to municipal

water treatment, ozone has been used to obtain high quality water for

breweries, distilleries, water bottle operations, food processors,

carbonated beverage producers, hospitals, laboratories, etc. (144, 222).

Additional reports on ozone installations (204, 205, 229), production

(230), and methods of mixing for more efficient disinfection (231) are

available in the literature.


The use of ozone for sterilization of food containers (232), and

control of mold and bacteria (65, 156, 233, 234) demonstrate other uses

of this oxidant, although in some cases the required concentrations

exceed the recommended limit for humans and would limit these ozone

applications to certain situations.

Ozone for Kraft Odor Control

Previous A ppi ications and Investigat ions

As a method of eliminating odors in kraft pulp mill operations, a

patent was issued to Limerick (7) concerning the procedure of conducting

ozone to a line discharging odorous stack gases, mixing the ozone with

such gases in a proportion from about 10 to 100 ppm of the total gas

volume, retaining the ozone and odorous gases in intimate mixture for

not less than about two seconds, and thereafter discharging the mixture

to the atmosphere.

As mentioned in the introduction of this paper (Chapter 1), literature

concerning the application of ozone for kraft odor control is very limited.

The only reference found that mentions an actual mill application was by

Cromviell (6) who briefly describes the experimental use of ozone to

treat effluent stack gases from the sulfate recovery system of a pulp

and paper mill in Canada. Ozone was applied to a 100-foot stack at a

point 10 feet above the ground, giving a retention time of approximately

two seconds for mixing with the effluent gases. Maximum ozone output

was 300 pounds per 24 hours, which provided a concentration of 80 ppm

in an effluent flow of approximately 50,000 cfm. Concentration of sulfur

compounds in the effluent stream was not given, and there was no indication

of how effective the ozone application was. A thorough literature

search and correspondence (235) failed to provide any additional

information on these tests.

The literature is also somewhat limited on laboratory studies

concerning ozonation of the individual sulfur compounds primarily responsible

for kraft mill odors; i.e., hydrogen sulfide, methyl mercaptan, dimethyl


sulfide, and dimethyl disulfide. Barnard (236), discussing the reaction

of ozone with organic sulfur compounds, showed that the oxidation of

monosulfides (R.S-R) occurs in two well-defined stages to give, first,

the sulfoxide (R-SO-R) and then the sulfone (R.S02*R), and require

somewhat less than the theoretical amount of ozone for their oxidation.

With this analogy, disulfides would be expected to yield successively

thiosulfinate (R-SO.S-R), thiosulphonate (R-SO2S.R) or disulfoxide

(R-SO.SO-R), and disulfone (R.SO2-SO2 R). Ozone-uptake curves did not

reveal such stelpise reaction and indicated, for "normal" disulfides, the

absorption of 2.5-3.0 moles of ozone. The major product of the reactions

was a sulfonic anhydride (R-SO2.0-SO 2R) with small amounts of thio-

sulfonate and disulfone, although ozonation of dimethyl disulfide

showed the major product to be the thiosulfonate (50 percent) rather than

the sulfonic anhydride (39 percent).

Similar results are reported by Douglass (5) using nuclear magnetic

resonance (NMR) analytical techniques. Ozone was found to react rapidly

with dimethyl sulfide to form the sulfoxide and then the sulfone without

evidence of other products being formed (Equation 3.1).

0 0 0 0
CH SCH -3-- CH SCH --3 CH SCH (3.1)

With methyl mercaptan, a stel:p.ise reaction was noted, first forming

the disulfide which is oxidized to the thicsulfinate, and then to the

thiosulfonate (Equation 3.2).
0 0 0 0 0
CH SH --- CH SSCH -- CH SSCH ----3 CH SSCH (3.2)

Bubbling ozone through liquid dimethyl disulfide appeared to convert it

entirely to the thiosulfonate (R-SO *S-R), and attempting to distill

the thiosulfonate thus obtained gave a mixture of thiosulfonate and

sulfonic anhydride (R-SO2-0SO2R).

Ozone oxidation of kraft pulp mill blow gases (from an autoclave)

was studied by Akamatsu (9) and indicated that most of the odor could be

eliminated; however, concentrations of sulfur compounds in the blaw gas

or ozone concentrations used for odor control were not mentioned.

Results showed that ozone converts the methyl disulfide and methyl

mercaptan of the blaw gas to a "mild odored" oxide. Akamatsu, et al.

(237) also studied the ozone oxidation of dimethyl sulfide, and showed

that 75 percent of this malodorous gas could be oxidized to dimethyl

sulfoxide and the remainder to dimethyl sulfone, similar to the studies

by Barnard (236) and Douglass (5).

During a study on the oxidation of kraft black liquor with oxygen,

Ricca (8) evaluated the effects of oxygen enriched with 1450 ppm ozone.

The reaction rate of dissolved hydrosulfide was not increased but the

mercaptan odor of the black liquor was eliminated, thus indicating

that ozone was more effective in oxidizing gaseous odors than black

liquor. Additional tests showed that the black liquor oxidation exhaust

gases could be effectively deodorized with 100-150 ppm ozone (at 780 C).

Reactions between ozone and hydrogen sulfide have been reported

on by several authors. Gregor and Martin (10) studied the reaction at'

ambient temperature and illumination by injecting known quantities of

each gas into glass reaction vessels, and allca.ing them to stand for

16 hours to insure complete reaction. Results of their work indicated

that when hydrogen sulfide and ozonized oxygen are mixed under initially

anhydrous conditions, reaction occurs with water, sulfur dioxide and

sulfuric acid being formed as end products.


Under their reaction conditions, the percentage of hydrogen sulfide

actually oxidized steadily increased with an increase in the ozone to

hydrogen sulfide ratio, and complete oxidation only occurred at ratios in

excess of 9 to 1 (Figure 1). At ozone to hydrogen sulfide ratios below

9 to 1, sulfur dioxide was the principal product, and at ratios above

this, sulfuric acid became the predominant product. At a ratio of 18 to

1, oxidation of all the initial hydrogen sulfide to sulfuric acid was

considered complete (Figure 2). The overall stoichiometry of the

reaction (under the test conditions) was represented by the equation

H2S -- (1 x) H2SO4 + x H20 + x SO2 (3.3)

where 0 x 1 and x --:- 0 as the initial ratio of 0 /H2S -- 18

or greater.

Cadle and Ledford (11) also studied the gas-phase reaction of ozone

with hydrogen sulfide, and when the reaction was allowed to go to completion

in the cell of an infrared spectrometer, the only products found were

sulfur dioxide and water. Sulfuric acid was not detected as reported by

Gregor and Martin (10). The stoichiometry corresponded approximately to

the equation

H S + 03 -- H 0 + SO2 (3.4)

A summary of Cadle and Ledford's data is given in Table 5, indicating a

maximum ozone to hydrogen sulfide ratio of approximately two, which is

somewhat less than the maximum studied by Gregor and Martin.

At the low concentration ratios, the data of Gregor and Martin

(Figures 1 & 2) also show the major reaction product to be sulfur

Initial H S Oxidi

o /






- 40

> 30

L 20



Initial 0 3/H2S Ratio

Figure 1. -- Conversion of Initial H2S to SO2 and H2SO, (Gregor
and Martin).

Initial H2S
0 Converted to SO2

0 2

ConeInitial H 2 S
Converted to H 2SO

0 0

!/ o
idized H2S Converted to SO

2 /

\ /


o 50



10 a
0 Oxidized H2S Converted
tLo H 2S 04

Initial 0 /H2S Ratio

Figure 2. -- Conversion of Oxidized H2S to SO2 and H (Gregor
and Martin).



Concentrations (moles x 10 /cm3
Initial H S Initial 03 Final SO

78 30 19
31 30 23
78 30 18
16 30 18

dioxide, with only small amounts of sulfuric acid. Also, since the

oxidation of sulfur dioxide to sulfur trioxide is knawn to be very slow

(238), it is possible that sulfuric acid did not have sufficient time

to form in the study by Cadle and Ledford since reaction time was not

mentioned, whereas 16 hours were allowed in the work by Gregor and


Considering data from both studies (10, 11), it would appear that

the stoichiometry represented by Equation 3.4 is valid for lao ozone to

hydrogen sulfide rations, and for all ratios at minimum reaction times.

Cadle and Ledford (11) also studied the kinetics of the ozone-

hydrogen sulfide reaction using a I-meter long reaction tube constructed

of 16 mm Pyrex tubing. Maximum residence time in the reactor was about

3 seconds. Tests were run at 260, 650, and 1000 C with ozone concentrations

ranging from about 245-7350 ppm and hydrogen sulfide concentrations of

245-19,600 ppm. It was determined that the reaction had orders near

zero and 1.5 in hydrogen sulfide and ozone respectively, and followed

the rate expression

c = 2.5 x 108 exp (-8300/RT) c 15 mls (3.5)
dt 0 ml sec

The homogeneity of the reaction was examined by placing several

pieces of Pyrex tubing in the reactor which increased the surface to

volume ratio four-fold. The overall reaction rate was about double

that of the original system, indicating significant wall effects. No

attempt was made to quantify these effects. It was concluded that the

reactions in the system were at least partially heterogeneous, and the

rate law (based on a purely homogeneous reaction) could be used to

estimate upper limits for the rate of reaction between ozone and hydrogen

sulfide in the atmosphere.

Hales (12), considering the significance of simultaneous bulk and

wall reactions, and based upon the information and experiences of Cadle

and Ledford (11), performed additional studies on ozone-hydrogen sulfide

reactions in laminar fl.w reactors. Measurements were made at 28.50

and 480 C in the absence of light, and involved a number of reactant

concentration ratios. The reaction was observed to be almost totally

homogeneous, and described by the expression

dc_2 0.5 1.5 micromoles
dt 22.8 exp (-6500/RT) c0.5 1.5 micromoles (3.6)
dH2S 03 liter min

for the range of variables studied. All evidence obtained during the

experiments indicated that Equation 3.4 represents the true stoichiometry

of the reaction for the conditions studied.

Although the rate equations (3.5 and 3.6) are similar in that they

indicate a reaction order of 1.5 in ozone, they differ in the reaction

order of hydrogen sulfide and in the activation energy. Also, reactions


observed by Hales were thought to be almost totally homogeneous, whereas

Cadle and Ledford observed significant wall effects in their system.

Various possibilities for these differences were investigated by Hales,

and it was concluded that the disagreements stem mainly from a nitric

oxide quench used by Cadle and Ledford which, among other things, could

have conditioned the reactor wall in such a way that it became catalytically

active. This would also offer some explanation to the fact that the

absolute rates predicted by Cadle and Ledford (Equation 3.5) are about

two orders of magnitude higher than those of Hales (Equation 3.6) over

the range of hydrogen sulfide concentrations studied (12).

Noting that Equation 3.6 applies to rather specialized conditions

and that care should be exercised in its application to practical

situations, Hales considers a hypothetical odor control situation

involving the emission of hydrogen sulfide at 500 ppm from a 150 C

stack. Assuming no spontaneous ozone decay and a stack residence time

of 10 seconds, Equation 3.6 predicts that only about 2 percent of the

hydrogen sulfide will be oxidized upon applying a stoichiometric

quantity of ozone. Although recognizing the limits of his rate equation,

and the complexity of the ozone-hydrogen sulfide reaction mechanism and

the stack environment itself, Hales concludes that the effect of ozone for

odor control is due more to its odor-masking properties than to actual

oxidation of the hydrogen sulfide.

Because ozone is a powerful oxidizing agent, its classification as

a masking agent is subject for argument (213), since a true masking

compound does not enter into chemical reactions with the malodorous

substance being masked. Naturally, if excessive amounts of ozone are

injected into an effluent gas stream, and/or adequate reaction time is

not provided, ozone could be emitted into the atmosphere and act as a

masking agent due to its o.'in "sweet" odor. The inhibitory effect of

ozone on the sense of smell would also tend to negate the effect of a

malodorous compound, but its use in this manner, or as a pure masking

agent, could result in serious problems due to the knaon toxic effects

of ozone.

Basis of Present Study

Various methods used for odor control in the kraft pulping industry

were reviewed in Chapter 2. The referenced literature indicated that

proper application of these methods would permit a mill to comply with

most any current or proposed emission standard. What the future regula-

tions will be concerning odor compounds. is not known. Additional odor

control steps may be required to completely eliminate the malodorous

emissions, for whether these gases are released in concentrations of

5, 10, or 100 ppm, if they can be detected by the public and create a

nuisance, they are still too great.

The application of ozone as a polishing agent following a major piece

of odor control equipment is a possibility. Older mills may not find

it feasible to make major process changes and/or additions (black liquor

oxidation, elimination of DCE, etc.) to prevent initial odor formation,

but find it easier to treat the malodorous compounds at the emission

points by injecting ozone into the gas stream. If a mill has effective

recovery furnace control and incineration of the noncondensables, some

of the emissions which were once considered unimportant now become major

odor sources and may be suitable for ozone oxidation, such as the brown-

stock washer hood vents, knotter hood vents, and washer seal tank


Is ozone effective for controlling odors in the kraft pulping

industry, for which effluent gases, and under what conditions? A

relatively few laboratory studies investigating a limited range of

conditions, no reported results of ozone installations in kraFt mills,

and conflicting reports on the effectiveness of ozone for odor control

have not tended to provide suitable answers for these questions, although

ozone has been shown to be effective in other applications (water and

wastewater treatment) and for other malodorous installations.

In almost every investigation regarding either the sole use of ozone,

or comparing it with other oxidizing agents (chlorine, chlorine dioxide,

etc.) for water treatment, wastewater treatment, or odor control, the

cost of ozone production was invariably mentioned as its main disadvantage.

In many instances, it has been sho.n to be more effective and faster

acting than chlorine, and without the residual effects sometimes

experienced with chlorine. With the anticipated decrease in cost of

ozone production (as discussed in Chapter 1), its effectiveness for

kraft odor control needs to be established.

The end products resulting from ozone oxidation of the four malodorous

sulfur compounds of interest have been established by previous investigators

(5, 9-12, 236) and shown to be relatively odorless. Kinetics of ozone-

hydrogen sulfide reactions were studied by Cadle and Ledford (11), resulting

in a rate expression based on the depletion of ozone. Hales (12) also

studied the ozone-hydrogen sulfide reaction kinetics to resolve some of

the difficulties experienced by Cadle and Ledford, and derived a rate

expression based on the generation of sulfur dioxide.

For the present study, ozone oxidation of all four of the major

malodorous sulfur compounds was studied, and the effectiveness of the


ozone treatment was based on the depletion of the malodorous compounds

themselves rather than measurement of the end products or ozone uptake.

Although partial oxidation of some compounds, such as butanol (mild odor)

to butyric acid (very bad odor), can result in an odor increase (63), this

does not pose a problem in ozone oxidation of these four sulfur gases of

interest since their reaction products have a much higher odor threshold

than the original compound. Of course, the first step in the ozone

oxidation of methyl mercaptan is dimethyl disulfide, but the formation

and subsequent depletion of this compound was monitored along with the

destruction of the original methyl mercaptan.



Description of Test Stand

Basic Flaw Pattern

As mentioned in Chapter I, the initial phase of this study was to

evaluate the effectiveness of ozone in oxidizing bottled sulfur gases at

various temperatures, reaction times, and concentration ratios. The

equipment, flow diagrams, and techniques for this part of the study were

slightly different than those required for oxidation of actual stack

effluent. Descriptions in this chapter are directed primarily toward the

initial phase of the study, and any variations required for the stack

gases will be described in Chapter VI. Both phases of the study were

conducted from the NCASI Mobile Laboratory since a majority of the analytical

equipment used for the experiments is permanently mounted in this trailer.

A brief description of these units is given in a later section of this


With the exception of a Few components (primarily the analytical

equipment mentioned above), all the experimental apparatus used for the

study was mounted on a test panel as shown by Figures 3 and 4. This panel

was designed and constructed to (1) allow for changes in reaction tube

temperature, sulfur gas concentrations, ozone concentrations, and

residence time with a minimum time delay, (2) provide ease of handling for











all the components with subsequent minimum set-up and take-do.-n time,

(3) provide ease of operation by concentrating all the necessary controls

in a relatively small area, and (4) require a minimum area for operation

due to the limited space in the NCASI Mobile Laboratory.

A schematic of the complete system used for the initial phase is

shawn by Figure 5 and indicates the components attached to the test

stand. As shoin by the schematic, pure oxygen was released from a

cylinder source, passed through a moisture trap of silica gel, through a

flacmeter, and into an Ozonator Corporation (239) Model 4000 Ozone

Generator where ozone was produced by silent arc discharge. The ozone-

oxygen flay from the ozone generator was then directed to the heated

reaction tube or wasted to the atmosphere as required.

Nitrogen was used as a carrier gas for the sulfur compounds and was

admitted to the system from a cylinder also. Sulfur compounds were

injected into the nitrogen stream using a spinning syringe, and the

nitrogen-sulfur gas mixture combined with the ozone-oxygen flcw in a

mixing chamber attached to the end of the heated reaction tube. The

extent of ozone oxidation was monitored with a gas chromatograph through

sampling ports spaced do.n the reaction tube to provide various mixing

times. Waste gases were vented to the outside of the mobile laboratory.

System Details

Ozone production was controlled by varying the voltage to the ozone

generator with a voltage regulator (Variac). As sho.-n by the schematic

(Figure 5), a constant voltage transformer was installed prior to the

variac to dampen out any line fluctuations, and provide a steady 118 volts

to the variac. By regulating vol tge with the variac, output of the 6000


Limits of Test Panel-- Orifice

-- -

Sampling Ports



Nitrogen Flocmeter Control.ler oZ .-
Source Controller Atmosphere
11O V '
Line Control Waste Control
_--- Va sWaste Controlo
""--- ....... V, lv 've Waste 03 & 0O
Constant Variac 6000 V To Atmosphere
I VC olst a g e v r i c T r a n s f o rm e r O z o n e
/ Vol age
Transformer Ozone
T o __ Generator Ozone Sampling Train I

i l .
Oxygen IMoisture Control Oxygen Midget Moisture Sampling Control Vacuum
Source ; Trap Valve Flowmeter Manometer Impinger Trap Flcwmeter Valve ; Pump
Sore:Ta migr Ta

Figure 5. -- System Schematic for Oxidation of Bottled Gases.

0 -0,


volt transformer (supplied with the Model 4000 Ozone Generator) was

controlled, thus controlling ozone production in the ozone generator.

Oxygen flow to the ozone generator, which can also affect ozone production,

was maintained at a constant 350 cc/min, and pressure in the generator was

held at 34.0 in Hg. Voltage settings for desired ozone production were

determined from a calibration curve as described in Appendix A.

From the ozone generator, 80 cc/min of ozone-oxygen mixture was

directed to the heated reaction tube. The remaining 270 cc/min was discharged

to the atmosphere through a waste line. An ozone sampling train, set to

pull 150 cc/min, was connected to the waste line. This allaoed the

ozone output of the generator to be checked simultaneously with an

experimental run to verify the actual ozone concentration entering the

reaction tube. The neutral potassium iodide method was used for ozone

sampling, and is described in Appendix A.

Nitrogen flow to the reaction tube was 320 cc/min, and was used as

a carrier gas for the sulfur compounds. The spinning syringe used for

injecting sulfur compounds into the nitrogen stream is shown in Figure 6,

and flow rates from the syringe could be varied by changing orifice sizes

at the syringe outlet. Additional details and calibration data for this

apparatus are given in Appendix A. Sulfur concentrations in the syringe

were varied by diluting known bottled gases to different sulfur/air ratios,

and were checked periodically with the electrolytic titrator and gas


The reaction tube and mixing chamber used for the study were

fabricated from 316 stainless steel. A diagram of this apparatus is

shown by Figure 7. The mixing chamber was designed to provide thorough

interaction between the ozone and sulfur gases prior to the first sampling












1 37"

1 1/1

3/4" 0

Sampling Ports
1 1/2" Projection of 1/4"
Tubing (Typical)

Note: All material 316 S.S.

Figure 7. -- Diagram of Reaction Tube.




port. Volume of the reactor to the first port was 39.4 cc which provided

a residence time of 5.9 seconds based on a total gas flow of 400 cc/min

(80 cc/min ozone-oxygen and 320 cc/min nitrogen) at 250 C. Residence

time to the last port was 61.5 seconds for the same conditions.

For heating purposes, the tube was wrapped with 1 x 1/8-inch

asbestos tape followed by evenly spaced (at 1/4 inch), 18 gauge ni-chrome

wire. This was covered with two more layers of asbestos tape. With a

voltage regulator, a temperature range of ambient to 1250 C was achieved,

and was monitored with a thermometer inserted through a glass fitting (with

a ground-glass joint) at the reactor outlet. The glass piece allowed

visual observation of any deposits formed during the ozone-sulfur gas


Oxidation of the sulfur compounds was monitored with a gas chromato-

graph. Samples were pulled through a heated 1/8-inch Teflon line from the

various sampling ports, depending upon the particular residence time

desired. Exhaust from the reactor was discharged to the atmosphere through

the previously mentioned glass fitting into 1/2-inch polyethylene tubing

which extended outside the mobile laboratory.

NCASI Mobile Laboratory Equipment

General Description of Mobile Laboratory

The NCASI Mobile Laboratory is a modified, 19-foot travel trailer

containing the necessary sampling and analytical equipment needed to

investigate the emission sources in a kraft mill complex. The trailer

facilitates the transportation of equipment from one mill to another and

the location of it in close proximity to the emission source under

investigation. It also provides a suitable working environment while

making a study. The basic equipment includes an all Teflon-stainless

steel heated gas sampling system, combustion furnace, electrolytic

titrators, and a gas-liquid chromatography unit (GLC). These components

are supplemented with gas-fla measuring equipment, calibration systems,

and miscellaneous laboratory supplies, chemicals, tools, etc.

All the equipment in the mobile laboratory was not utilized for the

initial phase of the study since sampling and oxidation of actual stack

gases were not required. As shown by the schematic (Figure 5), only the

electrolytic titrator, gas conditioning box, and GLC unit were used in

the initial phase. Extra bottles of nitrogen and oxygen were available

in the trailer for connection to the test panel also.

Electrolytic Titrator and Combustion Furnace

As mentioned in the previous section of this chapter, a Barton Model

286 Electrolytic Titrator (240) was used to check concentrations of the

sulfur gases prior to entering the reaction tube. The use of this

instrument in the kraft pulping industry is well documented. Thoen,

et al. (51) and Thoen and DeHaas (241) report on the use of a Barton

Titrator to monitor the operation of kraft recovery furnaces. Blosser

and Cooper (242) evaluate it as a method for the continuous monitoring of

various process emission sources in the sulfate pulping operations, and

Thoen, et al. (243) describe its use in the analysis of sulfur compounds

in samples drawn from various points in the kraft process.

The operation of the Barton Titrator, using bromide coulometry,

depends on reactions between oxidizable sulfur gases from the syringe (or

from an emission source) and brocine in the titrating cell. These reactions,

and additional details on the operation of a Barton Titrator, are thoroughly


described in the literature (51, 243). The major interfering compound in

the bromine titration is alpha-pinene, usually emitted at some points in

the sulfate process, but definitely not of concern using knaon bottled

gases. Haoever, interference has not been noticeable in the analysis of

kraft effluent gases (243).

Sensitivities of the Barton Titrator as reported by Thoen, et al.

(243) are 7-10 ppb for hydrogen sulfide, 10-12 ppb for methyl mercaptan,

and 40-50 ppb for dimethyl sulfide. Previous applications of this instrument,

and its use in NCASI investigations, have sho.n it to be dependable, simple

to operate, and requiring minimum maintenance.

A combustion furnace, operating at 17500 F, was used in conjunction

with the titrator for measuring the sulfur gas concentrations. The

purpose of the furnace was to oxidize all forms of reduced sulfur to sulfur

dioxide, and oxidize organic compounds (such as terpenes) that interfere

with the titrator's response to carbon dioxide and water. Although not

necessary while using bottled gases, the furnace was used since it is

included in the normal gas flow pattern of the mobile laboratory, and only

one set of calibration data (that of sulfur dioxide) was needed to determine

sulfur gas concentrations from the syringe.

Gas Conditioning Oven

The gas conditioning oven, or dilution box, is primarily for use in

the sampling and analysis of moisture laden, kraft mill effluent gases.

It is constructed of 3/4-inch asbestos board and houses an electric

heater, thermostat, and air circulation fan. In addition, it contains

valves, flcimeters, and tubing and fittings necessary to control, monitor,

dilute, and distribute the gas floa. For a clearer understanding of the

function and location of the gas conditioning oven in the mobile laboratory,


Figure 8 shcas a simplified flow diagram of the system while sampling stack

gases. By maintaining a temperature of 1000-1100 C in the box, condensation

is prevented in the lines, valves, flaimeters, etc., and by diluting the

flow to the electrolytic titrator, the dewpoint can be reduced below

ambient temperature and condensation prevented in those lines.

For the initial phase of this study with the bottled gases, the

conditioning oven was used primarily to transfer a sample from the reaction

tube, or spinning syringe, to the gas chromatograph through the GLC sampling

valve. The sampling valve has eight ports, and is pneumatically operated

with nitrogen by a solenoid valve. Sample gases from either the reaction

tube, or spinning syringe, continually pass through the valve as does

the GLC carrier gas (nitrogen). Also connected to the sampling valve are

two sectionsof Teflon tubing (sampling loops) having identical volumes.

At any given time, one sampling loop is in series with the sample gas

flow and the other with the carrier gas flcw. When the solenoid valve is

activated, the relative positions of the sampling loops are changed with

respect to the two gas flows. Thus, the GLC carrier gas sweeps the

sample trapped in one loop into the GLC system for analysis.

Sampling loops of three different volumes were used depending upon

concentrations in the sample gas and are listed in Table 6.


Loop Length (in) 0.0. (in) Range (ppm)'

Small 2 1/2 1/16 100-2000
Medium 3 1/4 1/8 50-500
Large 22 . .... .. 1/8 . .1-60


Heated Probe


Carrier Gas




Glass Wool

Gas Conditioning Oven

Figure 8. -- Mobile Laboratory Flao Diagram.


Gas-Liquid Chromatography System

The use of gas chromatography for the analysis of sulfur gases

from kraft mills is also covered extensively in the literature (12, 39,

244-251). The unit contained in the mobile laboratory is a F & M Model

720 Gas Chromatograph (252) with a Melpar Model 65-34A Flame Photometric

Detector (253). Two analytical columns are utilized in the separation and

analysis of gaseous sulfur compounds. One is a 33-feet x 1/8-inch Teflon

column packed with 30/60 mesh Haloport F (Teflon), and coated with 5

percent polyphenyl ether (5-ring). The other column is constructed of

identical materials, but is only five feet long. The longer column is for

analyzing hydrogen sulfide, sulfur dioxide, methyl mercaptan and dimethyl

sulfide, while the five-foot column is for dimethyl sulfide and disulfide.

A valve allows the selection of either column, and samples are introduced

to the column selected through the GLC sampling valve previously

described in the gas conditioning oven. Both columns operate at a

temperature of 500 C with the carrier gas (nitrogen) set at 50 cc/min.

Under these conditions the elution time of each column is as shown in

Table 7.



Sulfur Compound Long 'Cdl'umn Short Column

Hydrogen Sulfide 2 minutes
Sulfur Dioxide 3 minutes
Methyl Mercaptan 4-5 minutes
Dimethyl Sulfide 7-8 minutes 48 seconds
Dimethyl Dis.ulfi.de. -.- ..... .. 4-5 minutes.......


From either column, the sample enters the Melpar Flame Photometric

Detector where it is mixed with oxygen, hydrogen and nitrogen and burned.

The burning sulfur emits a blue light which is detected by a photomultiplier

tube. The current output is proportional to the light produced, and is

displayed on a strip chart recorder. With the use of calibration curves,

the sulfur gas concentration is determined. Additional details on the

flame photometric detector are covered by Brody and Chaney (254).

Calibration of the GLC unit is discussed in Appendix A.



Experimental Procedures

To study the effectiveness of ozone in oxidizing the malodorous

sulfur gases of interest, measured amounts of ozone and individual sulfur

gases were injected into the heated reaction tube and allowed to react

for a specified period of time. The products of the reaction were

analyzed by the GLC unit, and the percent reduction in the malodorous

sulfur gas noted. Specific details on operation of the test panel are

covered in Appendix A, and describe the procedures used to obtain.

specific ozone to sulfur gas ratios, to change operating conditions

(ratios, reaction times, temperature), and other necessary operations to

run the experiments.

As discussed in Chapter IV (and Appendix A), reaction times in the

heated reaction tube were controlled by using different sampling ports

along the tube to withdraw samples for the GLC unit. Space times, or

residence times, to each sampling port along the reactor are listed in

Table 15 (Appendix A) for the various temperatures used during the study.

Since a quench was not used at the sampling port to stop the oxidation

process, any residual ozone in the sample would continue to oxidize the

sulfur gas while in the transfer line to the GLC unit, thus resulting in

an actual reaction time longer than those listed in Table 15.

Due to the fact that the transfer line was maintained at the same

temperature as the reaction tube, the assumption was made that any reaction

in the line would proceed similar to that in the reactor, and that the

additional reaction time would be based only on flow and volume of the

transfer line. At a flow of 200 cc/min through the transfer line,

approximately four seconds of additional reaction time would have to be

included. Table 8 reflects this additional transfer time, and lists the

total reaction time associated with each sampling port.



Sampling Total Reaction Time (sec)
Port 250C 380C 55uC 76C 1 25TC

1 9.9 9.7 9.4 9.0 8.4
2 21.1 20.4 19.5 18.6 16.8
3 32.4 31.2 29.8 24.3 25.3
4 43.4 41.8 39.9 37.7 33.6
5 54.5 52.4 49.8 47.1 41.8
6 65.5 63.0 59.9 56.5 50.0

The experimental procedures used for each of the four gases were

basically the same, the only difference being the concentration ratios

evaluated for some of the experiments. As discussed in Chapter III, a

majority of the laboratory investigations concerning ozone oxidation of

malodorous sulfur compounds have dealt only with hydrogen sulfide. The

range of conditions studied has been somewhat limited, thus restricting

the practical application of the investigation's results. Based upon

the data of Table 2 (Chapter II), which summarize k-aft emission sources,

stack temperatures, and sulfur gas concentrations, a series of experi-

mental tests were established for this study to cover a wide range of

conditions representative of the kraft pulping industry. The following

sections describe the experimental conditions established for each of

the four malodorous sulfur gases, and the resulting data obtained at

these conditions.

Hydrogen Sulfide

Cadle and Ledford (11) and Hales (12) agreed that the true

stoichiometry of the ozone oxidation of hydrogen sulfide is represented

by Equation 3.4, shown here again for convenience.

H2S + 0 ---- H20 + SO2 (3.4)

Theoretically then, an ozone to hydrogen sulfide ratio of 1.0 would

achieve complete oxidation to sulfur dioxide. However, studies by Gregor

and Martin (10) indicated that a ratio of 9.0 was necessary for complete

oxidation of all the initial hydrogen sulfide, even for reaction times cf

16 hours. Regardless of this, experimental conditions were established

to study low concentration ratios and reduced reaction times since

extremely high ratios and large holding chambers would be impractical from

a cost viewpoint. Table 9 lists the conditions established to study the

ozone oxidation of hydrogen sulfide.

Experimental data obtained on hydrogen sulfide oxidation are listed

in Tables 18, 19, 20 and 21 (Appendix B), and show the remaining hydrogen

sulfide at each sampling port for specific concentration ratios and

temperatures. As noted in the data tables, the exact concentration ratios



Initial H2S 03/H2S Temperatures
Concentrations (ppm) Ratios (0C)

10 0.5, 1.0, 2.0, 5.0 38, 76, 125
50 11" "I1
250 11 n 11 II II II
500 ii i" "I
1000 I" "
2000 1" "I

varied slightly from those proposed in Table 9 due to variations in ozone

generator output and sulfur concentrations in the spinning syringe (as

discussed in Appendix A -- Test Stand Operating Procedures).

Percent reduction of the initial hydrogen sulfide concentration for

each temperature, sampling point, and concentration ratio (236 data points)

is listed in Table 22 (Appendix B). Data from this table were used for

a multiple regression analysis of the hydrogen sulfide oxidation. The

data were fit to six different models (equations), using combinations of

the variables ozone-hydrogen sulfide ratio (03), temperature (Te),

residence time (Time), ratio squared (03 2), ratio-Te (03 x Te), ratio-

time (0 x Time), and temperature-time (Te x Time). The object was to

find a relatively simple equation with a high coefficient of determination

(R2), which indicates the percent of data variance accounted for by the

particular equation, thus giving a high multiple correlation coefficient (R).

The three equations resulting in a high R2 value and with minimum

variables are as follows:


y = B0 + B103 + B203 (5.1)

y = BO + B 0 + B2Te + B303 (5.2)

and y = B0 + B 10 + B2Te + B Time + B4032 (5.3)

where y = percent reduction of initial hydrogen sulfide,
BO, B, B2, . o = regression coefficients
0 = ozone-hydrogen sulfide ratio,
Te = temperature (oK),
Time = reaction time (seconds).

The R2 values for the above equations are 0.7958, 0.8261, and 0.8422

respectively. As expected with these high coefficients, the "F"

statistic for each equation was highly significant (*.) indicating a

relatively small amount of variance in the data due to error.

Using each of the above equations, estimated values for percent

reduction of hydrogen sulfide were calculated for concentration ratios

of 0.5, 1.0, 2.0, and 3.0, reaction times of 10, 30, and 60 seconds, and

for a temperature of 820 C. The temperature of 820 C (1800 F) is

typical of various kraft emission sources, and residence times of 10, 30,

or 60 seconds could be obtained for many sources to insure good oxidation.

Estimates for the above conditions, their standard error, regression

coefficients, and other data on each equation are listed in Tables 23, 24,

and 25 of Appendix B.

Using the data of Table 25, which estimate percent reduction using

Equation 5.3, the effects of increased concentration ratios and reaction

times are shown by Figure 9 for 820 C. This equation estimates a reduction

of approximately 44 percent for 60 seconds'reaction at a concentration

ratio of 1.0, whereas the data of Gregor and Martin, (Figure 1, Chapter 3)

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