• TABLE OF CONTENTS
HIDE
 Title Page
 In memoriam
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 Introduction
 Fundamental theory
 Project design
 Experimental results
 Conclusions
 Appendices
 References
 Biographical sketch














Title: Removal and recovery of sulfur dioxide in the pulp mill industry
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Permanent Link: http://ufdc.ufl.edu/UF00097855/00001
 Material Information
Title: Removal and recovery of sulfur dioxide in the pulp mill industry
Alternate Title: Sulfur dioxide in pulp mill industry
Physical Description: xiv, 238 leaves. : illus. ; 28 cm.
Language: English
Creator: Galeano, Sergio Francisco, 1934-
Publication Date: 1966
Copyright Date: 1966
 Subjects
Subject: Wood-pulp   ( lcsh )
Air -- Pollution   ( lcsh )
Air -- Purification   ( lcsh )
Sulfur dioxide   ( lcsh )
Environmental Engineering Sciences thesis Ph. D
Dissertations, Academic -- Environmental Engineering Sciences -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis -- University of Florida.
Bibliography: Bibliography: leaves 226-236.
Additional Physical Form: Also available on World Wide Web
General Note: Manuscript copy.
General Note: Vita.
 Record Information
Bibliographic ID: UF00097855
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 - 000558959
oclc - 13429005
notis - ACY4403

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Table of Contents
    Title Page
        Title Page 1
        Title Page 2
    In memoriam
        Page i
    Acknowledgement
        Page ii
    Table of Contents
        Page iii
        Page iv
    List of Tables
        Page v
        Page vi
        Page vii
        Page viii
    List of Figures
        Page ix
        Page x
        Page xi
    Abstract
        Page xii
        Page xiii
        Page xiv
    Introduction
        Page 1
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    Fundamental theory
        Page 33
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    Project design
        Page 67
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    Experimental results
        Page 77
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    Conclusions
        Page 99
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    Appendices
        Page 101
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    References
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    Biographical sketch
        Page 237
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Full Text















REMOVAL AND RECOVERY OF SULFUR

DIOXIDE IN THE PULP MILL INDUSTRY












By

SERGIO F. GALEANO


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY











UNIVERSITY OF FLORIDA
August, 1966















IN MEMORIAL

MARGARITA ANDREU SANTOS












ACKNOWLEDGMENTS


The author wishes to express his grateful appreciation to the

chairman of his supervisory committee, Dr. E. R. Hendrickson, for his

concern and guidance during this course of study. The supervision and

valuable suggestions of Dr. C. I. Harding, co-chairman of the

supervisory committee are greatly appreciated. To Dr. A. P. Black,

Professor T. deS. Furman, and Dr. M. Tyner, the author's gratitude

for their help and understanding as members of the committee.

To Professor B. Spangler, who approved and supervised structural

details of the pilot plant, the author is indebted.

Appreciation is expressed to Dr. R. S. Sholtes and Dr. J. J.

Morgan for their helpful suggestions in different phases of this work.

Appreciation is also extended to Mrs. J. Larson and Mrs. Marjorie

Turner for their typing of the dissertation.

Expressing gratitude to each one of the University of Florida

personnel who have contributed in one way or another in different aspects

of this work will be too long, thus it will suffice to mention Mr. H.

McGraw, Mr. W. Shumate, Mr. E. Warshyk, Mr. B. Crenshaw, Mr. R. Booth,

Mr. W. Murray, and Mr. J. Smith, among others.

The donation of the structural steel by the Aetna Steel Company

is duly appreciated.

This research was supported by a grant from Owens-Illinois, Forest

Product Division and Public Health Service Air Pollution Training Grant

T-l-AP-4. The pilot scrubber and blower were donated to the Air Pollution

Research Laboratory by Airetron Engineering Corporation, Ridgewood, N. J.

ii















TABLE OF CONTENTS


ACKNOWLEDGMENTS . . . . . . . . . . . . .

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

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

ABSTRACT . . . . . . . . . . . . . .

CHAPTER

I. INTRODUCTION . . . . . . . . . . .


Industrial Development . . . .
The Air Pollution Problem . . .
Process Development in Removing Sulfur
from Flue Gases . . . . .
Chemical Recovery . . . . .
Purpose of this Investigation . .


II. FUNDAMENTAL THEORY . . . . .

Gas Absorption in General .
Theories Explaining Absorption .
Absorption with Chemical Reaction
Rate of Absorption in the Venturi
Rate of Absorption in the Spray Cha
Chemistry of the Sulfite-Bisulfite
Foaming . . . . . .


9
. . . . 9
Dioxide
. . . . 13
. . . . 17
. . . . 26


. . . . 33

. . . . 33
. . . . 39
. . . . 43
. . . . 46
mber . . ... 52
System .... 53
. . . . 63


III. PROJECT DESIGN . . . . . . . . . .

The Experiment . . . . . . . . . .
The Design . . . . . . . . . . .
The Analysis . . . . . . . . . .

IV. EXPERIMENTAL RESULTS . . . . . . . . .

Scrubbing with Water . . . . . . . .
Experiments in the Spray Chamber . . . .
Experiments in the Venturi . . . . .
Scrubbing with Sodium Carbonate . . . . .
Experiments in the Spray Chamber . . . .
Experiments in the Venturi . . . . .


Page

ii

v

ix

xii




1











Page

Oxidation of the Weak Black Liquor . . . ... 91
Foaming . . . . . . . . . . 93
Scrubbing with the Weak Kraft Black Liquor .... 94
Economic Evaluation . . . . . . .... 97

V. CONCLUSIONS . . . . . . . . . . . 99

APPENDICES

A. EXPERIMENTAL PILOT PLANT . . . . . . ... 102

Heating Plant No. 1 . . . . . . ... 102
Structural Details . . . . . . . ... .104
Gas Flow System . . . . . . . .... 106
Liquid Scrubbing Flow System . . . . .... 112
Recording Systems . . . . . . . ... .120
Electrical Installation . . . . . .... 129
Cooling System . . . . . . . .... 131
Steam and Air Lines . . . . . . ... .131
Transferring the Weak Kraft Black Liquor . . .. .134

B. EXPERIMENTAL AND ANALYTICAL PROCEDURES . . . ... .137

Preparation of the Scrubbing Solutions . . ... .137
Performing the Experiment . . . . .... 139
Sampling and Analysis for Sulfur Dioxide . ... 140
Analysis for Sulfur Content as Sulfates . . .. .148
Sampling and Analysis of Hydrogen Sulfide .... .148
Sulfide Determinations in Kraft Weak Black Liquor 150
Total Solids of Black Liquor . . . . .... 153

C. DATA OF THE EXPERIMENTATION WITH THE CARBONATE SOLUTION. 154

D. EXPERIMENTAL DATA OF THE WEAK BLACK LIQUOR OXIDATION 207

E. DATA OF THE EXPERIMENTATION WITH THE WEAK BLACK LIQUOR 210

F. ECONOMIC EVALUATION . . . . . . . ... .216

G. RECOVERY IN THE BLACK LIQUOR SCRUBBING SYSTEM .... .224

LIST OF REFERENCES . . . . . . . . . . . 226

BIOGRAPHICAL SKETCH . . . . . . . . ... . .237














LIST OF TABLES


Table Page

1. CLASSIFICATION OF EFFORTS IN SULFUR DIOXIDE REDUCTION
FROM BOILER FLUE GASES20 . . . . . . . .... 14

2. EXPERIMENTAL FACTORS AND THEIR LEVELS . . . . .. 73

3. PAYOUT PERIOD CALCULATIONS . . . . . . .... 98

4. EXPERIMENTS IN THE VENTURI. WATER AS SCRUBBING LIQUOR.
FIELD DATA . . . . . . . . . . . . 155

5. EXPERIMENTS IN THE VENTURI. WATER AS SCRUBBING LIQUOR.
SO2 REMOVAL . . . . . . . . ... ..... 157

6. EXPERIMENTS IN THE VENTURI. WATER AS SCRUBBING LIQUOR.
BLOCK DESIGN RESULTS . . . . . . . .... .158

7. EXPERIMENTS IN THE VENTURI. WATER AS SCRUBBING LIQUOR.
BREAKDOWN ANOVA . . . . . . . .... ..... 159

8. ESTIMATES OF THE CONSTANTS AND REGRESSION ANALYSIS FOR
EQUATION (79) . . . . . . . . . . . 160

9. EXPERIMENTS IN THE VENTURI. WATER AS SCRUBBING LIQUOR.
ESTIMATES OF THE INTERFACIAL AREA . . . . .... 161

10. EXPERIMENTS IN THE VENTURI. WATER AS SCRUBBING LIQUOR.
ESTIMATES OF THE OVER-ALL MASS TRANSFER COEFFICIENTS . . 163

11. EXPERIMENTS IN THE SPRAY CHAMBER. CARBONATE AS SCRUBBING
LIQUOR AT HIGHER TEMPERATURE. FIELD DATA . . . ... 164

12. EXPERIMENTS IN THE SPRAY CHAMBER, CARBONATE SCRUBBING
LIQUOR AT HIGHER TEMPERATURE. SO2 REMOVAL . . . ... .166

13. EXPERIMENTS IN THE SPRAY CHAMBER. CARBONATE SCRUBBING
LIQUOR AT HIGHER TEMPERATURE. BLOCK DESIGN RESULTS . . 167

14. EXPERIMENTS IN THE SPRAY CHAMBER. CARBONATE SCRUBBING
LIQUOR AT HIGHER TEMPERATURE. BREAKDOWN ANOVA ...... 168

15. ESTIMATES OF THE CONSTANTS AND REGRESSION ANALYSIS FOR
EQUATION (81) . . . . . . . . . . . 169











Table Page

16. EXPERIMENTS IN THE SPRAY CHAMBER. CARBONATE SCRUBBING
LIQUOR AT LOWER TEMPERATURE. FIELD DATA . . . ... 170

17. EXPERIMENTS IN THE SPRAY CHAMBER. CARBONATE SCRUBBING
LIQUOR AT LOWER TEMPERATURE. SO2 REMOVAL . . . ... .172

18. EXPERIMENTS IN THE SPRAY CHAMBER. CARBONATE SCRUBBING
LIQUOR AT LOWER TEMPERATURE. BLOCK DESIGN RESULTS .... .173

19. EXPERIMENTS IN THE SPRAY CHAMBER. CARBONATE SCRUBBING
LIQUOR AT LOWER TEMPERATURE. BREAKDOWN ANOVA . . ... 174

20. ESTIMATES OF THE CONSTANTS AND REGRESSION ANALYSIS FOR
EQUATION (84) . . . . . . . . . . . 175

21. EXPERIMENTS IN THE SPRAY CHAMBER. CARBONATE SCRUBBING
LIQUOR. BREAKDOWN ANOVA FOR TOTAL EXPERIMENT ...... .176

22. EXPERIMENTS IN THE SPRAY CHAMBER. CARBONATE SCRUBBING
LIQUOR. ESTIMATES OF THE HEIGHT OF TRANSFER UNITS .... .177

23. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
LOW TEMPERATURE AND HIGH RATES OF FLOW. FIELD DATA . . 178

24. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
LOW TEMPERATURE AND HIGH RATES OF FLOW. SO2 REMOVAL . . 180

25. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
LOW TEMPERATURE AND HIGH RATES OF FLOW. BLOCK DESIGN
RESULTS . . . . . . . . . . . . . 181

26. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
LOW TEMPERATURE AND HIGH RATES OF FLOW. BREAKDOWN ANOVA .182

27. ESTIMATES OF THE CONSTANTS AND REGRESSION ANALYSIS FOR
EQUATION (86) . . . . . . . . . . . 183

28. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
LOW TEMPERATURE AND HIGH RATES OF FLOW. ESTIMATES OF THE
INTERFACIAL AREA . . . . . . . . ... . .184

29. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
LOW TEMPERATURE AND HIGH RATES OF FLOW. ESTIMATES OF THE
OVER-ALL MASS TRANSFER COEFFICIENT . . . . . . 186

30. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURE AND HIGH RATES OF FLOW. FIELD DATA . . 187











Table Page

31. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURE AND HIGH RATES OF FLOW . . . ... .189

32. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURE AND HIGH RATES OF FLOW. BLOCK DESIGN
RESULTS . . . . . . . . . . . . . 190

33. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURE AND HIGH RATES OF FLOW. BREAKDOWN ANOVA 191

34. ESTIMATES OF THE CONSTANTS AND REGRESSION ANALYSIS FOR
EQUATION (88) . . . . . . . . . . . 192

35. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURES AND HIGH RATES OF FLOW. ESTIMATES OF
THE INTERFACIAL AREA . . . . . . . .... .193

36. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURE AND HIGH RATES OF FLOW. ESTIMATES OF THE
OVER-ALL MASS TRANSFER COEFFICIENT . . . . . ... .195

37. EXPERIMENTATION IN THE VENTURI. COMPLETE BLOCK DESIGN
RESULTS (SO2 REMOVAL). .. . . . . . . . .196

38. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
BREAKDOWN ANOVA FOR TOTAL EXPERIMENT . . . . ... .197

39. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURE AND LOW RATES OF FLOW. FIELD DATA . . 198

40. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURE AND LOW RATES OF FLOW. SO2 REMOVAL . . 200

41. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURE AND LOW RATES OF FLOW. BLOCK DESIGN
RESULTS . . . . . . . . . . . . . 201

42. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURES AND LOW RATES OF FLOW. BREAKDOWN ANOVA 202

43. ESTIMATES OF THE CONSTANTS AND REGRESSION ANALYSIS FOR
EQUATION (90) . . . . . . . . . . . 203

44. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURE AND LOW RATES OF FLOW. ESTIMATES OF THE
INTERFACIAL AREA . . . . . . . . . . .. .204











Table Page

45. EXPERIMENTS IN THE VENTURI. CARBONATE SCRUBBING LIQUOR.
HIGH TEMPERATURE AND LOW RATES OF FLOW. ESTIMATES OF THE
OVER-ALL MASS TRANSFER COEFFICIENTS . . . . .... 206

46. OXIDATION OF THE WEAK BLACK LIQUOR . . . . . .. 208

47. EXPERIMENTS IN THE VENTURI. WEAK BLACK LIQUOR AS A
SCRUBBING SOLUTION. FIELD DATA . . . . . .... 211

48. EXPERIMENTS IN THE VENTURI. WEAK BLACK LIQUOR. BLOCK
DESIGN RESULTS . . . . . . . . . . . 212

49. EXPERIMENTS IN THE VENTURI. WEAK BLACK LIQUOR AS SCRUBBING
SOLUTION. BREAKDOWN ANOVA . . . . . . . .. 213

50. EXPERIMENTS IN THE VENTURI. WEAK BLACK LIQUOR AS SCRUBBING
SOLUTION. EFFECTS OF SULFATES ON SO2 ABSORPTION . . .. .214

51. SCRUBBING WITH WEAK BLACK LIQUOR. RECIRCULATION
EXPERIMENTS . . . . . . . . . . . . 215

52. GENERAL CONSIDERATIONS . . . . . . . ... 217

53. RECIRCULATION EXPERIMENTS. OPTIMUM CONDITIONS ...... 218

54. RECIRCULATION EXPERIMENTS. INDUSTRIAL CONDITIONS . .. .219

55. RECIRCULATION EXPERIMENTS AT LOW SO2 CONCENTRATION .... .220

56. EQUIPMENT AND INSTALLATION COSTS . . . . . ... .221

57. FIXED-CAPITAL INVESTMENT . . . . . . . .... 222

58. ESTIMATED MANUFACTURING COST TO PRODUCE (1000 GAL) OF
BY-PRODUCT LIQUOR 0.6 M IN Na2SO3 . . . . . .. 223


viii














LIST OF FIGURES


Figure Page

1. SCHEMATIC DIAGRAM OF A TYPICAL KRAFT PROCESS5 ...... 6

2. SCHEMATIC DIAGRAM OF A TYPICAL NSSC PROCESS . . . .. 10

3. SIMPLIFIED FLOW SHEET OF THE MEAD PROCESS . . . ... .22

4. THE INSTITUTE PROCESS OF S02 RECOVERY BY DIRECT
SULFITATION . . .. . . . . . . . .... . 25

5. EQUILIBRIUM DIAGRAM FOR A SULFITE-BISULFITE SYSTEM . . 27

6. PROPOSED SYSTEM FOR SO2 REMOVAL AND RECOVERY ...... .29

7. INDUSTRIAL APPLICATIONS OF THE PROPOSED SYSTEM . . .. .31

8. EQUILIBRIUM DIAGRAM FOR SO2 ABSORPTION . . . ... .33

9. OPERATING LINE AND EQUILIBRIUM CURVE IN A GAS ABSORPTION
PROCESS . . . . . . . . . . . . . 37

10. EQUILIBRIUM DIAGRAM FOR SULFITE SPECIES . . . ... .56

11. EQUILIBRIUM DIAGRAM FOR CARBONATE SPECIES . . . ... .58

12. TITRATION CURVES pH VS. TIME. . . . . . .. .59

13. EXPERIMENTAL SET-UP FOR THE EXPERIMENTATION OF SO2
ABSORPTION . . . . . . . . . . . . 61

14. TITRATION CURVES AT DIFFERENT RATES OF SO2 INJECTION . 62

15. SCRUBBING WITH WATER IN THE VENTURI. EXPERIMENTAL RESULTS. 78

16. RESPONSE SURFACE OF EQUATION (80) . . . . . ... 80

17. SCRUBBING WITH SODIUM CARBONATE IN THE SPRAY CHAMBER AT A
HIGH TEMPERATURE. EXPERIMENTAL RESULTS . . . ... .81

18. SCRUBBING WITH SODIUM CARBONATE AT A LOW TEMPERATURE.
EXPERIMENTAL RESULTS. . . . . . . . .. .83

19. RESPONSE SURFACE FOR EQUATION (85) . . . . ... 84











Figure Page

20. SCRUBBING WITH SODIUM CARBONATE IN THE VENTURI AT A LOW
TEMPERATURE. EXPERIMENTAL RESULTS . . . . . . 85

21. CANONICAL REPRESENTATION OF EQUATION (87) . . . . 87

22. SCRUBBING WITH SODIUM CARBONATE IN THE VENTURI AT A HIGH
TEMPERATURE. EXPERIMENTAL RESULTS . . . . ... .88

23. CANONICAL REPRESENTATION OF EQUATION (89) . . . ... .89

24. SCRUBBING WITH SODIUM CARBONATE IN THE VENTURI, AT A HIGH
TEMPERATURE AND LOW RATES OF FLOW. EXPERIMENTAL RESULTS 90

25. COMPARISON IN REMOVAL EFFICIENCY BETWEEN THE VENTURI AND
THE SPRAY CHAMBER ................... .. 92

26. OXIDATION OF THE WEAK BLACK LIQUOR UNDER DIFFERENT
CONDITIONS . . . . . . . . . . . . 93

27. COMPLETE OXIDATION OF THE WEAK BLACK LIQUOR . . ... .94

28. FOAMING OF THE WEAK BLACK LIQUOR UPON OXIDATION . . .. .95

29. DISAPPEARANCE OF FOAM UPON ADDITION OF KEROSENE DURING
OXIDATION . . . . . . . . . . . . . 95

30. SCRUBBING WITH WEAK BLACK LIQUOR IN THE VENTURI.
EXPERIMENTAL RESULTS . . . . . . . . .. 96

31. EFFECT OF SULFATES ON SO2 REMOVAL . . . . . ... .97

32. RECORDER CHART FOR BOILING PLANT OPERATION . . ... .103

33. STRUCTURAL DETAILS OF SUPPORTS ... . . . . ... 105

34. SCALE PLOT PLAN OF ROOF . . . . . . . ... .107

35. PRESSURE BLOWER AND CONNECTIONS . . . . . ... .108

36. DETAILS OF THE VENTURI SCRUBBER . . . . . . .. 110

37. DETAILS OF THE SPRAY CHAMBER . . . . . . .. 111

38. EXPERIMENTAL PILOT PLANT. GROUND LEVEL INSTALLATIONS . 113

39. SCHEMATIC DIAGRAM OF THE LIQUID FLOW SYSTEM . . ... .114

40. OXI-HEATER TANK . . . . . . . .... ..... 115











Figure Page

41. SEDIMENTATION TANK . . . . . . . . . 117

42. DETAILS OF THE PUMPS AND PIPE ARRANGEMENTS AT GROUND
LEVEL . . . . . . . . . . . . . 119

43. GAS PRESSURE RECORDING SYSTEM . . . . . . ... .121

44. CALIBRATION CURVES FOR PRESSURE TRANSDUCERS . . ... 122

45. HONEYWELL RECORDER AND CONNECTIONS . . . . ... .123

46. TEMPERATURE RECORDING SYSTEM . . . . . . ... .125

47. DETAILS OF THE COPPER-CONSTANTAN THERMOCOUPLES . . .. .126

48. CALIBRATION CURVE FOR THE HONEYWELL POTENTIOMETER .... .127

49. SAMPLING SYSTEM . . . . . . . . ... . .128

50. ELECTRICAL INSTALLATION DETAILS . . . . . ... .130

51. DETAIL OF THE ELECTRICAL CONTROL PANEL AT GROUND LEVEL . 132

52. COOLING SYSTEM . . . . . . . . ... . .133

53. AIR AND STEAM SYSTEMS . . . . . . . . ... 135

54. RAILROAD CAR FOR TRANSFERRING WEAK BLACK LIQUOR . . .. .136

55. CALIBRATION CURVE FOR ORIFICE METER . . . . ... .141

56. TYPICAL TITRATION CURVE FOR SULFIDE DETERMINATION .... .152











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

REMOVAL AND RECOVERY OF SULFUR
DIOXIDE IN THE PULP MILL INDUSTRY

By

Sergio F. Galeano

August, 1966

Chairman: Dr. E. R. Hendrickson
Major Department: Bioenvironmental Engineering


Although pollution seems inevitable in a technological society with

a high gross national product, it has been a neglect of remedies, which

has brought about the present deterioration in the quality of the

environment. The legislation on air pollution control and the activity

of the regulatory agencies have increased in the last years in an

effort to reduce the increasing presence of pollutants in the air.

Among these pollutants, sulfur dioxide is one of the most important

not only because of its abundance but because of its detrimental effects

in animals, plants, materials, and human beings.

In a pulp mill, sulfur dioxide is present in different places along

the chemical recovery system. Sulfur dioxide emissions will take place

at the power plants of any type of pulp mill, and at the recovery

furnace of those mills engaged in the semi-chemical process.

The peculiar existing characteristics of the chemical recovery

system in the pulp mill and the fact that sulfur compounds in different

states of oxidation are required in the wood digestion process indicates

the convenience of a liquid/gas contact method to remove sulfur dioxide

from the flue gas and to recover the chemical in a proper form.

xii











This research studies the technical and economic feasibility of

a purification system for the removal of sulfur dioxide in the pulp

mills. It comprises applications in two important types of pulping

systems. The sulfate or kraft system is the one most in use

throughout the world. The neutral semi-chemical system is gaining

popularity and improving rapidly because of wood scarcity and higher

yields.

An experimental pilot plant with a capacity of 2800 cfm consisting

of a venturi scrubber and a cyclone has been used in the different

experiments. The pilot plant was designed for its use as a purification

system for either the kraft or the semi-chemical systems.

Two different scrubbing solutions were used. For the semi-chemical

process, the use of a carbonate solution had proved technically feasible,

with sulfur dioxide removals higher than 90 per cent. For the kraft

process, the weak black liquor before entering the evaporators was used.

Removals in the same order to the ones previously mentioned has been

found.

Two different purification units were studied with the above

mentioned scrubbing liquor: a venturi-cyclone combination, and a spray

chamber with radial inlet and lateral sprayers.

An orthogonal factorial design was conducted and the relative

significance of the operating factors was analyzed. When more than two

factors were studied a surface response equation was developed to

represent the phenomenon.

In the case of the carbonate scrubbing solution for the semi-chemical

process, the proposed system is both technically and economically feasible.

xiii











An economic evaluation shows a very small fraction of pay-out time.

In the case of the weak black liquor used as a scrubbing solution

the method is technically feasible. Its economy is a matter of more

flexibility dictated by the particular conditions of each plant.














I. INTRODUCTION


Industrial Development


The pulp and paper industry has experienced phenomenal progress

in the last thirty-five years. It is now the fifth largest manufacturing

industry in the United States, and predictions for even greater

development have been formulated. The actual production of paper and

paperboard in this nation, nearly 38 million tons, is expected to

increase to 50 million tons in 1975.1 Even this figure, however, will

be insufficient to meet the nation's demands.

The raw material for nearly all of the paper produced in the

United States today is wood. The history of the process of obtaining

paper from wood is somewhat vague. A decisive step towards the

manufacture of paper from pulp, made from sawdust and shavings, was

achieved by Dr. Jacob C. Schaffer, in Bavaria, around 1760. In 1800,

Matthias Koop of England disclosed his experiments in papermaking, using

wood straw and other fibers.


Mechanical Pulping

In the Western Hemisphere the first groundwood paper was probably

made in 1844, at Nova Scotia, by Charles Fenerty. The manufacture of

mechanical pulp moved forward slowly, gaining the acceptance of the

printers and displacing the well-used rag pulp. At the beginning,

groundwood pulp accounted for only a 20-25 per cent of the paper; rag

pulp accounted for the rest. Only after a new major development took










place in 1866 was it possible for groundwood pulp to gain complete

acceptance. This major development was the production of commercial

groundwood pulp at the Buntin Mill, Valleyfield, Quebec.

The basic principles of manufacturing groundwood or mechanical

pulp are simple, although the actual practice is quite complicated. It

is more an art than a science when compared to other methods of pulping.

A block of wood is forced by pressure against a grindstone, with water

being supplied continuously to keep the stone cool and to remove the

pulp from it. The action of the abrasive surface of the revolving stone

is such that the wood is reduced to a fibrous condition.


Sulfite or Acid Pulping

Man's continual need for improved methods of production brought

about new achievements in this field. In 1867, Benjamin Tilghman was

granted the U. S. Patent 70485, "Treating Vegetable Substances for

Making Pulp Paper." Thus a new process, afterwards named the sulfite

pulp process, was originated, which quickly began to displace the still-

new mechanical pulp process. The next decade witnessed a profusion of

investigations and works which greatly improved the basic idea of

Tilghman, and the original works of a Swedish chemist, C. D. Ekman, who

worked out a similar process, independently from Tilghman, about the

same time.

Essentially, the sulfite or acid process consists of the digestion

of wood in the form of chips at temperatures from 130 C to 1500C in an

aqueous solution containing alkaline-earth bisulfites, usually calcium

bisulfite or a mixture of calcium and magnesium bisulfites, and an











excess of sulfur dioxide. During the course of the pulping reaction,

the quantitative relationship among these components depends on the

conditions of temperature and pressure, which influence the reactions

taking place and the products of such reactions. During digestion,

lignin combines with sulfur dioxide or bisulfites and is rendered

soluble. The less resistant hemicelluloses are hydrolyzed to simpler

compounds, and a portion of the wood cellulose is degraded.

Other variants have been introduced to the above mentioned method.

It is sufficient to describe the most successful. Sulfite or acid

pulping has been improved by the use of magnesium acid sulfite, as

cooking liquor, instead of the calcium-base liquor. In this way,

economical heat and chemical recovery is possible, together with a

reduction in stream pollution problems. Today, nearly 700,000 tons,

or about 25 per cent of the total sulfite pulp manufacture in this
1
country, are produced annually with this process.


Kraft or Sulfate Pulping

Commercial paper in the eighteenth century was the product of

chemical pulping. Linen or cotton rags, straw and other non-wood,

fibrous materials were used, and the digestion was carried out in open
4
boilers. Strackan has reported that both sodium hydroxide and sodium

sulfide were used. This practice was, in effect, the beginning of a

process later to be called alkaline pulping. Because wood was less

readily delignified than straw upon alkaline digestion, the acceptance

of the alkaline pulping process for wood was retarded. New methods

for putting alkaline pulp production on a commercial basis were needed.










These methods involved higher pressure and temperature, and a relatively

large quantity of the reagents. Two major developments helped to do

this. First, in 1879, Dahl, in Danzig, introduced a modification in

the process by using a combination of caustic soda and sodium sulfide

as reagents. The latter was obtained as a reduction product of sodium

sulfate, which was previously added as make-up in the process. The

addition of this chemical gave the name of sulfate to the process. It

is also known as the "kraft" process, meaning strong, because of the

high strength of the paper manufactured from this pulp. The use of

sodium sulfate reduced considerably the cost of pulping, making the

process commercially feasible.

The second major development which helped the alkaline pulping to

attain its position was a unique combination of technical achievement

and consumer acceptance. The quality of the paper converted from kraft

pulp lacked the high opacity, excellent formation, and bulk character-

istics of the paper converted from soda pulp. The development of the

electrolytic process for the simultaneous fabrication of sodium hydroxide

and chlorine from sodium chloride made possible the commercial use of

soda pulp. For many years, mills using the sulfate process were engaged

in manufacturing a coarser paper, and those using the soda process in

manufacturing a finer paper.

The differences in the quality of paper between the two alkaline

processes existed until 1930. At that time, successful investigations

on chemical recovery and bleaching methods helped to make these two

methods quite similar. At the present time, there are only a few soda

plants as such in existence since in the majority of the plants sodium










sulfide is added as part of the process. Fine paper is no longer

produced exclusively by the soda process, since the multiple-stage

bleaching demonstrated that alkaline pulps from softwood could be

bleached satisfactorily.

The importance of the kraft or sulfate process has been generally

recognized. Since the first kraft mill was built in 1891, the sulfate

process has gradually outgrown all the others. Since World War II,

this growth has been remarkable. Today, more than 17 million tons of

kraft pulp are produced annually in this country, which is by far more

than half of the total wood pulp in the United States.

A modern kraft pulping process could be visualized with the aid of

Figure 1. The liquor, consisting mainly of a solution of sodium sulfide

and sodium hydroxide in water is mixed with wood chips in a pressure

vessel, the digestor, and cooked for about three hours with steam at a

gauge pressure of about 110 psi. Upon completion of the cooking phase,

the bottom of the digester is opened and the digestor's contents are

forced by differential pressure into a blow tank. An important

separation then takes place. The pulp in the blow tank is diluted and

pumped to multi-stage drum filters. The spent liquor is separated from

the pulp by means of countercurrent washing with fresh water. The pulp

follows several steps which constitute the kraft pulping process, and

the spent liquor, called weak black liquor, enters the chemical recovery

process, along which several pollution sources are present.

The chemical recovery process starts when the weak black liquor is

concentrated in multi-effect evaporators to from about 15 per cent solids

to about 45 per cent solids. The make-up, sodium sulfate, and recovered



















































Ca(OH2 L MLUD(QCO


SLAKER LME KILN



Fig. i.- Schematic diagram of a typical kraft process.











salt-cake are added prior to the direct-heat evaporators. Further

concentration takes place in a direct contact evaporator where flue

gases from the recovery furnace are used to evaporate water. The

concentrated black liquor is forced through nozzles and sprayed into

the recovery furnace. Reducing conditions are maintained in the lower

part of the furnace to obtain a reduced form of sulfur compound. The

resulting smelt from the furnace consists essentially of sodium sulfide

and sodium carbonate. Upon dissolution in water, a green liquor results

in which the sodium sulfide is converted into sodium hydroxide and

sodium hydrosulfide. Sodium carbonate is causticized by addition of

line to form sodium hydroxide with the precipitation of calcium

carbonate. The causticized solution, called white liquor, is ready for

use in cooking.

The general practice today is the disposal of the spent liquor by

burning, thus recovering chemicals and heat. This practice transforms

the lignin content in the liquor into heat. Although it is an economical

method of disposal of the spent liquor, it is a most wasteful manner of

treating such potentially valuable chemical raw material. Alkali lignin,

very similar to the ligno-sulfates regarding chemical and physical

properties, has been studied recently by many researchers, who pointed

out.its potentiality for new products in a more scientific chemical

recovery.


Semi-Chemical Pulping

This process is a direct result of both the increasing demand of pulp

and the search for energy savings in pulping operation. While wood from










coniferous species is more widely used because of its long-fibered pulp

and good bleaching characteristics, the wood from broad-leaf trees is

being employed in ever increasing amounts. The production of high-yield

pulps from different types of deciduous woods has been extended to

non-wood fibrous materials such as sugar cane bagasse and cereal straws.

Further research on weeds and grasses is being conducted with a certain

degree of success.

The energy requirements for pulping have been a matter of serious

consideration for many years. From 1875 to 1890, many investigators,

such as George Marshall in the United States, Cross in England, and

Enge in Germany, made progress along similar lines to lower the energy

consumption of the conventional mechanical process by using a chemical

pretreatment. One of the greatest advantages of the semi-chemical

process is that a high-yield pulping is obtained in which much of the

hemicellulose lost in conventional chemical pulping is saved.

The semi-chemical process can be divided into two stages, chemical

and mechanical. In the first process, the fibrous raw material is

subjected to chemical action. This causes a chemical reaction with the

lignin-carbohydrate complex of the middle lamella which partially

weakens or destroys the fibrous bond. The second stage is the one

which differentiates this process from full chemical pulping. The

material from the chemical stage has been partially disintegrated and

defibered, to about 20 per cent of its original fiber bonds. During

the defibering-refining process, the combined action of friction forces

among fibers, and between them and the plates, plus the compression

created by the centrifugal forces of the discs, complete the process.












The heat generated as a result of the transformation of energy from the

action of frictional forces contributes to further weaken or partially

dissolve the fiber bond.

According to the specific composition and acid-base characteristics

of the digesting liquor, the semi-chemical process consists of various

processes: the acid sulfite semi-chemical (ASSC), the neutral sulfite

semi-chemical (NSSC), the soda semi-chemical (SSC), the cold soda

semi-chemical (CSSC), and the kraft semi-chemical (KSC) process. Each

system works differently in removing the basic constituents of the

natural fibrous materials, such as lignin, alpha cellulose, hemicellulose,

and extractive. For the different fibrous materials, the amounts

extracted vary with the specific process. It suffices to say that the

NSSC process yields fairly good extractions for all species in

comparison with the combined yields of all the other processes.

Figure 2 indicates the important features of a NSSC pulping plant.


The Air Pollution Problem

Since the beginning of the pulp and paper industry, the operators

of the mills have been aware of the air pollution problem involved.

In the last decade, vigorous efforts have been made to reduce the air

pollution emissions. These efforts are reflected by different

activities. Installation of control units at the exhaust sources,

ambient sampling programs, and process changes are the principal

activities in which the industry has engaged in order to cope with the

air pollution problem.







-10-


z




0

obia
>iQ0
u L

-100
co i- U











In a pulp mill both particulates and odorous gases are involved.

Many investigators have reported qualitatively and quantitatively the
7,8
different air-borne wastes. Kraft pulping could be considered a

typical offender due to tie amount and variety of pollutants emitted.
9
Hendrickson reported that the three major sources of pollutant emissions

in kraft pulping are from liquor preparation, cooking, and chemical

recovery. In the previously outlined processes, the major sources of

sulfur dioxide, an important pollutant, result from the recovery

furnace and the boiler plant.

Since the pulp mill uses large amounts of steam, and since generally

the combustion of the black liquor does not supply all of the steam

needed, any other amount of steam needed for the process is generated

in primary fuel boilers. During the generation of this needed steam,

sulfur dioxide is emitted in those plants using coal or fuel oil.

Sulfur dioxide is of importance because, at certain concentrations,

it corrodes building materials, destroys cloth and leather, and affects

vegetation, animals and human health. The detrimental effect of

pollutants on human health is well known. When sulfur dioxide is

present at high concentrations its principal effect is irritation of

the respiratory tract. At lower concentrations it increases the possi-

bility of chronic respiratory diseases such as asthma, bronchitis and

pulmonary emphysema. The total anatomical system and the total integrated

physiological system (TIPS) of man depend upon the totality of his

environment, mainly the atmosphere. There is an evident liaison between

two of the several subsystems which constitute the TIPS. The respiratory-

metabolic subsystem (R-MS) is in direct contact with the atmosphere.





-12-


Its performance will depend on the degree of pollution of the atmosphere.

The cardiovascular subsystem (CVS) is in direct contact with the R-MS

and, through this contact, comes into indirect contact with the

atmosphere. Of the two subsystems, the R-MS is the controlling one and

the CVS the controlled. Gaseous pollutants in the inspired air can

become trapped in the alveolar air and can impede the exchange of gases

across the alveolar membrane. In that case, less oxygen would diffuse,

and hypoxia and some human cardioaccelerative responses would be

intensified by the presence of gaseous pollutants detrimental to the
10
system. Literature on the effects of sulfur dioxide is available in

abundance. 11,12,13

An estimate of 25 million tons of sulfur dioxide are released
14
annually into the atmosphere in the United States.4 This problem will

be aggravated in the coming years. Public awareness of air pollution

has been intensified in the two decades since the episode at Donora,

Pennsylvania,15 and an increasing number of air pollution laws have

resulted. The passage of the Federal Clean Air Act of 1963 and the

broadening of the Act in 1965 indicate the national magnitude of the

problem. The presence of sulfur dioxide resulting from the combustion

of fuels was one of the two problems singled out by the Act of 1963 for

special emphasis. The determination of the regulatory agencies to solve

this problem has been stated by responsible officials.16

Coal and fuel oil No. 6 are the most common solid and liquid fuels

for high capacity boilers.17 Sulfur content in the fuel varies from

0.34 to 4.0 per cent by weight, depending on the origin of the fuel.

Ash content by weight ranges from 0.2 to 1.5 per cent. The increased






-13-


use of foreign fuels and new refinery practices have increased both the
1l
sulfur and ash content. During combustion, virtually all of the

sulfur in the fuel is oxidized to sulfur dioxide and sulfur trioxide.

It is estimated that about 98 per cent of the sulfur in the fuel is

emitted as sulfur dioxide, 1 per cent as sulfur trioxide, and the

remaining 1 per cent is contained in the ash.19

Pollution control in the pulp mill industry seems to be a

difficult and complex problem. Not only is the number and quantity of

pollutants emitted a factor, but the more promising industrial control

methods need to be conducted within narrow limits. Paradoxically, the

very essence of pulping economy lies in the recovery process, and it

is here that air pollution occurs.


Process Development in Removing Sulfur Dioxide from Flue Gases

The reduction of the concentration of sulfur oxides in the combustion

products from boiler plants has been the object of intensive research

from various angles in recent decades. Both technical and economic

feasibilities are necessary for acceptance of any system. It means

that the process applied will operate efficiently and continuously over

long periods of time without interfering with the operation of the plant.

Thus, investment and operating costs of control units must be below the

cost of burning premium low-sulfur fuels.

Different approaches have been undertaken to achieve the desired

result. Table 1 summarizes these efforts. Their degree of success

varies according to the over-all industrial process.











Table 1


Classification of Efforts in Sulfur Dioxide
Reduction from Boiler Flue Gases20


General


Subclassification


Prior to combustion Selection of fuels


Desulfurization



After combustion Liquid scrubbing







Solid phase reaction



Gas-phase reaction


National
Foreign

Mechanical cleaning
Gasification of coal
Hydrodesulfurization of oil

Lime
Ammonia
Sulfite-bisulfite
0 CaO
sorption with metallic
oxides

Molecular sieves
Activated carbon
Oxidation catalysts

Hydrogen
Hydrogen sulfide
Carbon monoxide


Gas-Liquid Contact Methods

Of the few commercial installations for removing sulfur dioxide

from power plant flue gases, gas-liquid contact is used almost exclusively.

The method is expensive and ground level pollution near the stack may be

increased because of the lower exit temperature of the flue gas if the

removal of the pollutant is not complete. Rees21 enumerates some of

the apparently insurmountable difficulties found in its application, the

most important two of which are the need for a high percentage of removal

of sulfur dioxide, and the recovery of sulfur in a soluble form.












Because of the availability of alkaline solutions in the pulp mill

process, gas-liquid contact removal methods might prove feasible. The

characteristics of the enriched scrubbing liquor dictate the use of a

cyclic process.

To describe the progress in the removal of sulfur dioxide from

flue gases, it will be desirable to review the available literature

beginning with the catastrophic episode in the Meuse Valley in France

in 1930.22 Sulfur gases resulting from fuel combustion were officially

identified as responsible for the damage to vegetation, and consequently

the allowable amount of sulfur dioxide emitted was reduced.

The early work consisted mainly in the production of sulfuric acid

from the absorption and oxidation of sulfur dioxide in the presence of

catalysts in aqueous solutions. Copson and Payne started investigations

on sulfur dioxide removal using waste gases from petroleum refineries

with sulfur dioxide concentrations from 0.6 to 12 per cent volume.
24
Absorption rates in water using bubble washers were reported. Johnstone,

at the University of Illinois, analyzed the importance of catalytic

oxidation of sulfur dioxide using manganese and iron as catalysts.
25
Inhibitions of the catalysts were reported. In 1935, Grodzovskii

pointed out the acceleration of sulfur dioxide oxidation when ozone is

used in combination with manganese. Along this line, in 1945, Walthall
26
and co-workers developed a process for recovering sulfur dioxide.
27
Later in 1957, Tarbutton, et al., at the Tennessee Valley Authority,

revised the experimental cyclic method of sulfur dioxide removal with

manganese oxide and ozone, reaching the conclusion that although the

method is feasible technically, it is not so economically.




-16-


A brief survey of the use of lime as scrubbing liquor may be

summarized by saying that the method has been proved to be uneconomical.
28,29
The only cyclic process in use was the Howden process,2 9 which used

5 to 10 per cent of lime as a scrubbing solution. Conversion of the

calcium sulfate to ammonium sulfate gave a salable product.

Since 1937, the use of ammonia solution instead of lime has been

thoroughly studied, both in Europe and America. From Europe two main
30 31
systems have been patented, the Katasulf method and the Simon-Carves3

method. Johnstone333 has published complete data on the NH3-SO2

system fully proving the greater solubility of sulfur dioxide in

ammonia solution. Contributions along this line have been furnished
I/, 35
by Craxford, et al.,3 and Katz and Cole, among others. Later on, in

1948, Hixson and Miller36 proposed a more economical way of ammonia

regeneration than formerly proposed. Newall has reported recently

his work in the ammonia process. He indicated that the method is

limited by the cost of ammonia liquor and the price of salable

ammonium sulfate.38 The same conclusion was reached more recently by

Field, et al., in his cost analysis of SO2 removal processes.

The sodium sulfite-bisulfite system represents another line of

experimentation using gas-liquid contact methods. The system allows

for a lower percentage of removal, but favors chemical regeneration.

Johnstone, at al.,40,41,42 employed a method of regeneration based upon

addition of zinc oxide.


Other Methods

The description of the process development on sulfur dioxide with

specific application to the pulp industry would not be complete without












mentioning future developments. The technology of pulp processing,

related to heat and chemical recovery, has been the same for the last

thirty years. This situation is becoming critical, and two factors

will inevitably bring about a new picture in the pulp industry. The

enforcement of air pollution control regulations and the improvement

in the recovery of available products in the final cooking liquor

together will change substantially the pulp process. Actual air

pollution problems are brought about in the chemical recovery process.

When the research on better utilization of the black liquor makes

possible direct recovery from it, then it will be possible to justify

new methods on sulfur dioxide removal from flue gases. Methods
43
suggested by the studies initiated by Kurtzrock, et al.,43 and
44
Bienstock, et al., among others, would then have better justification.


Chemical Recovery

Since the pulp industry utilizes sulfur compounds in the digesting

liquor, recovery of sulfur in a suitable form has been sought as a way

to reduce the cost of chemicals, and partially offset the expenditures

for any air pollution control system. When a satisfactory gas

scavenging system is developed, it might find acceptance in any of the

three major pulping systems.


Sulfite Pulping

In the sulfite pulping system, experimentation on the production

of cooking acid has been conducted primarily on calcium bisulfite

systems. A great amount of information on calcium oxide-sodium

dioxide-water systems has been made available by Maas and co-workers.45'46






-18-


Johnstone and Leppla47 have also given information on equilibrium of

sulfur dioxide. White, et al.,48 in 1948, gave more information on this

system. A slight variation on the sulfite method has been introduced

by which a magnesium or an ammonia base substitutes for the calcium

base. A careful study of the method brought about the conversion of

the Weyerhaeuser plant at Longview, Washington, from calcium to

magnesium base.49 Markaut, t al.,50 recently published their work on

sulfur dioxide absorption studies in a magnesium oxide-sulfur dioxide

system. A more recent work using a combined sodium-calcium base

process was reported by Schmied, et al.5

The numerous problems involved in scrubbing solutions with calcium

and magnesium bases justify experimentation with ammonia bases in

which the scaling problems could be minimized. Ammonia-base sulfite
52
cooking acid has been studied by Merrimer and Whitney. The use of

ammonia base introduces certain modifications in the process because

ammonia is gaseous at normal temperature and relatively volatile in

aqueous solutions.


Kraft or Sulfate Pulping

Not only in the sulfite pulping system does sulfur dioxide

absorption present possibilities but also the sulfate or kraft pulping

system can benefit from scavenging sulfur dioxide. Operationally, the

method has been used to increase the concentration of the black liquor

from the multiple effect evaporators, and to recover large percentages

of chemicals from the flue gases of the recovery furnace. For this

latter application a venturi scrubber, which removes not only gases but

fumes and particulates, has been utilized primarily.











In dealing with black liquor as the scrubbing solution, many

complexities are present. Because the rate of absorption and removal

of particles is inversely proportional to the viscosity of the liquid,

a weak black liquor achieves better removal than the concentrated black
53
liquor, as stated by West, et al. Paradoxically, the benefits of this fact

in the past have been hampered by the foaming properties of the black

liquor. The excessive foaming from southern pine black liquor is

produced in inverse proportion to the black liquor solids concentration.

Anti-foaming devices and chemicals reportedly are expensive and

complicated to operate.

Another major item in dealing with sulfate black liquor as a

scrubbing medium is the emission of sulfur compounds during the flow

of black liquor from the digester to the recovery furnace. In the

digesters, the sulfide ion from the sodium sulfide combines with various

organic compounds of cellulose and lignin to form methyl mercaptan

(CH3HS), dimethyl sulfide (CH3)2S, dimethyl disulfide (CII3-S-CH3-S)

and other organic sulfur compounds.

The resulting liquor contains these compounds plus the original

excess of sodium hydrosulfide and sodium sulfide. In contact with the

flue gases, rich in carbon dioxide, the following reaction takes place
54
as indicated by Wright:5

Na2S + CO2 + 1120 -- Na2C03 + 2S (1)

Hydrogen sulfide is not the only gas emitted. Methyl mercaptan

and methyl sulfide are also released, which constitute much of the air

pollution odor problem associated with kraft pulp mills. This problem

was studied twenty years ago by Trobeck55 in Sweden and by Tomlinson











56 57
and Fergurson and Wright in Canada. These studies led to the

development of the black liquor oxidation method by which the sodium

sulfide is converted to sodium thiosulfate, and methyl mercaptan is

oxidized to dimethyl disulfide by the following reactions:

2 Na2S + H20 + 2 02 Na2 S203 + 2 NaOH (2)

4 CH3 SH + 02 -- 2(CH3)2S2 + 2 H20 (3)

The very extensive literature on black liquor oxidation deals mainly

with the problem of foaming. A good survey of the literature on this

subject is found in the work of Landry,58 but still the foaming problem,

in this country and abroad, has not been solved successfully.

Black liquor as a scrubbing agent is being used mainly in venturi

scrubbers in which the removal of gaseous products is efficiently
59,60
accompanied by the removal of particulates. Collins, at al., has

reported the use of venturi systems twenty years ago. Hendrickson and
61
Harding reported good results in laboratory studies with black liquor

in removing sulfur dioxide and other gases.


Semi-Chemical Pulping

The application in semi-chemical pulping of gas-liquid contact

methods for SO2 scavenging looks promising. Neutral and acid sulfite

semi-chemical pulping can benefit from the chemical properties of the

resulting scrubbing liquor. This process utilizes a digesting solution

based on sulfites and bisulfites with buffering characteristics. If a

similar liquor could be produced by scrubbing sulfur dioxide with a

carbonate buffered solution, the economic feasibility of the system

will be guaranteed.












Chemical recovery in semi-chemical pulping is low. The small

amount of heat that can be recovered from the waste liquor is a result

of the high yield process. The heat in the waste liquor solids

resulting from tlle manufacture of one ton of NSSC pulp ranges between

6 and 12 million BTU, whereas in the draft process it averages nearly

20 million BTU per ton of pulp manufactured.

Recovery processes in the NSSC method are very similar to the kraft

process. In many cases the NSSC method has been put in an existing

kraft mill and the waste liquor has been used to supply the make-up

soda and sulfur to the kraft system. In each one of the three main

processes used in NSSC recovery, the waste liquor is evaporated and

burned in a kraft-type furnace. This furnace operates on a two-stage

combustion process. A primary zone or hearth operates in a reducing

atmosphere with a small portion of air admitted to it. The sulfur

compounds from this primary zone are in the sulfide form, the state of

maximum reduction. Gases leaving the primary zone are burned with

additional air. Theoretically, the furnace should be of the type used

by the acid pulping industry in which the sulfur is recovered as sulfite,

a higher level of oxidation. Nevertheless, these methods use the kraft-

type furnace.
62
The Mead Corporation Process.-6 The simplified flow sheet in Figure

3 will help to visualize the main characteristics of this process. Upon

leaving the furnace, two products are treated in different ways. The

flue gases from the furnace pass through a series of devices for purposes

of steam production. Afterwards the dust is removed, resulting in a

clean gas containing N,, C02, 02, and SO2. The gas enters a sulfiting





-22-


to stack


-Na2CO3


Fig. 3.- Simplified flow sheet of the Mead process.











tower in which SO2 is absorbed by a sodium carbonate solution. The

remaining gas is rich in CO2 and is used in the carbonation of the

green liquor in the carbonating tower, thus providing the carbonate

solution needed for scrubbing in the sulfiting tower. The role of the

carbonation tower can be understood by using some chemical equations.

A high concentration of CO2 shifts the equilibrium of equation (4) to

the left.

CO2 (aq) ; CO2 (gas) (4)

At the same time, due to this shift to the left, equations (5), (6),

and (7) also shift to the left with a resulting increase in the concen-

tration of CO3 ions. An ionic equilibrium is indicated by equations

(8) to (12).

H2CO3 (aq) H20 + CO2 (aq) (5)

2 HCO3 Z C03 + H2003 (aq) (6)

C03 + HO 2- OH + HCO3 (7)

Na2S 2 Na+ + S' (8)

Na2C03 2 Na + CO3 (9)

S + HO2 OH + HS (10)

2 HS S" S + H2S (aq) (11)

H2S (aq) H H2S (12)

An increase in CO3 ions means an increase in combination with Na

ions, thus a release in I2S will take place. The gases leaving the

tower are high in CO2 and low in H2S. This low concentration in H2S

presents difficulties for combustion, thus necessitating a precarbonation

tower in order to bring about an increase in H2S concentration. Shaffer63

gives a more detailed explanation of the chemistry of the process.






-24-


Institute Method.- Another interesting method for NSSC recovery is

the one developed by the Institute of Paper Chemistry, which is explained
64,65
in detail by Whitney. et al. This method is less complex than the Mead

method. Figure 4 illustrates the main steps. One of the main drawbacks

of the method is the production of some sodium thiosulfate, which in

spite of being inert in the cooking stage, results in greater chemical

losses and some difficulties in bleaching. The original conception by

Bradley and McKeefe introduced the idea of direct sulfitation

according to these equations.

Na2C03 + SO2 = Na2SO3 + C02 (13)

Na2S + 02 + H20 = Na2SO3 + H2S (14)

2 Na2S + 3 SO2 + Na2SO3 = 3 Na2S203 (15)

In an attempt to eliminate the formation of thiosulfate, some investi-

gators67686970 employed both carbonation and sulfitation. It seems

that this method has not been very successful. Common practice has been

the direct sulfitation method by stages, in which the amount of sulfur

dioxide is controlled to avoid excesses, thus avoiding the formation of

thiosulfate. This continuous sulfiting method called direct sulfitation

has two disadvantages: the inevitable formation of some thiosulfate,
71
and heavy losses of sulfur into the atmosphere. Whitney, et al., have come

with a modification of the direct sulfitation method called the bisulfite

method in which sulfur dioxide is replaced by aqueous sodium bisulfite

as the sulfiting agent. The main reactions in the system are as follows:

Na2S + NaHSO3 = Na2SO3 + NaHS (16)

NaHS + NaHSO3 = Na2SO3 + H2S (17)








-25-


to st lC


Water r
I


NER


Fig. 4.- The Institute process of SO2 recovery by direct sulfitation.






-26-


Na2CO3 + NaHSO3 Na2SO3 + NaHCO3 (18)

2 NaHSO3 = Na2SO3 + H20 + SO2 (19)

The evolution of SO2 can be reduced, according to the data of Morgan,7

maintaining the bisulfite strength under 75 mole percentage, as shown

in Figure 5.


Purpose of this Investigation

Sulfur dioxide removal is being encouraged by new legislation on

air pollution control and by general public awareness. Sulfur dioxide

emissions take place at boiler plants in any type of mill, and at the

recovery furnace of pulp mills engaged in a semi-chemical method of

pulping. These investigations have been conducted to explore the

technical and economic feasibility of the implementation of a

purification system for SO2 removal.


Feasibility

Both technical and economic feasibility are sought in any system

proposed for the removal of sulfur dioxide. Technically speaking, it

is necessary for a purification system to give a high percentage of

removal to compensate for the loss of buoyancy and dispersion through

the cooling of the flue gas.

A flexible pilot plant with two different purification devices has

been selected for this study. It can be operated either as a simple

spray chamber with a radial inlet, or as a venturi-cyclone combination

in which atomization is achieved at the throat of the venturi.

Separation of the liquid droplets and the gas occurs at the cyclonic

demisting chamber. These two systems give a broad range of required























1U







8







6



909C



4







2

--5 O- C
0 O'C


90 95


75 80

NaHSO3 MOLE


Fig. 5.- Equilibrium diagram for a sulfite-bisulfite system.


01
-t
M
T
E
E

N
0
09

LL
0

119

Un
Ln
w
a

a

-J






-28-


energy over which removal of sulfur dioxide in gas-liquid contact

methods might occur.

Economic feasibility was sought according to the following

reasoning. Gas-liquid contact methods for scrubbing sulfur dioxide are

improved when a fairly rapid chemical reaction takes place. Since

sulfur dioxide is an acid gas, an alkaline solution is needed for

improving removal. It also happens that alkaline solutions are used

in the digestion process of the mill. These alkaline solutions are

rich in sulfur compounds. Thus, in principle, an immediate use of the

recovered sulfur might be possible. This chemical recovery will be

applied in two main processes, the kraft or sulfate process and the

neutral semi-chemical process. The degree of chemical recovery and the

operational characteristics of the mill will influence the merits of

the system.

Heat recovery is also possible due to the relatively high tempera-

ture of the flue gases. Many aspects can be considered in this type of

recovery whether it would be used for heating liquids, as in the

production of hot water, or increasing the concentration of chemicals

in the scrubbing liquor by evaporation of the water.


Equipment

Figure 6 illustrates the proposed system. Flue gases coming from

either the boiler plant or the recovery furnace will be scrubbed in the

venturi-cyclone combination, in order to obtain a 90 per cent removal

of the original sulfur dioxide concentration. The scrubbing liquor will

be a carbonate solution in the neutral semi-chemical sulfite mill, and






-29-


Srpiant
FLUE GASESItu
furnace
-- water & NaCOC3
or
weak bl. liquor

mixer





U .>. MAKE- UP
--- TANK

pump







RECIRC.
TANK
to suiting



SENR. to digester
pump I TANK



enriched
concentrated NSSC
blacK i quorr ALTERNA TVES _

to
-- (Kratt
d rect con. ev ap.



Fig. 6.- Proposed system for SO2 removal and recovery.











the weak black liquor in the kraft or sulfate mill. In the first case,

a sodium carbonate solution is used to recover the sulfur as sulfite.

The corresponding stoichiometric amount of carbonate disappears as

carbon dioxide. In the second case, sulfidity is increased if the

weak black liquor is oxidized prior to the scrubbing. In this fashion,

hydrogen sulfide emissions are avoided.

The bleed-off from the purification unit is transferred by gravity

to a sedimentation tank and recirculated back to the purifying unit.

The make-up tank for the experimentation was designed to allow for the

oxidation and heating of the black liquor.


Advantages

The proposed system has the following advantages:

1. It will remove high percentages of sulfur dioxide from the

flue gas at moderate energy requirements.

2. It will allow the advantageous use of a fuel with a high sulfur

content and a lower price.

3. The proposed system would be applicable in the neutral semi-

chemical sulfite mill where a sulfite-carbonate solution is required as

the digesting liquor. The bleed-off from the purification unit will be

rich in sulfites. The original carbonate concentration will be sacrificed

to obtain the sulfite. Thus, there is need for an enrichment in

carbonates and sulfites to make up for the digesting solution. Figure

7 (a) illustrates the adaptation of the system to mill operation.

4. For mills using the bisulfite method of sulfitation, the

proposed system will fit as follows. The bleed-off after several stages






-31-

Sclean gas

Hlue gas --
PURIFICATION UNIT MAKI






RECIRCULAT ION


soot removc1

S DIRECT
E NRICHMENT0 -


(a) In the production of NSSC make-up.









K PURIFICATION
1_ MAKE
CONVERSION RECIRCULATION I

TOWER




Sn t t flue gas
tvent



STORAGE DILUTION RECOVERY
1 ;'ANK FURNACE

water

DIGESTOR '



D.C. EVAPORATOR


(b) Application in the Bisulfite Method of sulfitation.


Fig. 7.- Industrial applications of the proposed system.


'CO3


Na CO 3











of recirculation could be made almost neutral, pH 7-8, with a

corresponding sulfite-bisulfite ratio of about 1.0. Then it will be

possible to use this bleed-off as the sulfiting solution, which upon

contact with the smelt from the furnace will produce a digesting

liquor. Figure 7 (b) illustrates the system.

5. In mills using the kraft process and interested in new

products, the bleed-off will provide a suitable compound which upon

enrichment would be used as the digesting liquor. The resulting black

liquor will be incorporated into the recovery system of the kraft mill.

6. In mills using the kraft process, it will be possible to scrub

the flue gases with oxidized weak black liquor. An increase in

sulfidity is logically expected, along with the possibility of some

concentration with the consequent increase in the evaporator's

throughput.














II. FUNDAMENTAL THEORY


Gas Absorption in General


When a soluble gas such as sulfur dioxide contacts water, the

sulfur dioxide will dissolve until an equilibrium point is reached,

at which point the partial pressure of sulfur dioxide in the main gas

stream holds a certain relationship with the dissolved sulfur dioxide

in the water. Thus it is possible to state that a very soluble gas

will require a lower partial

pressure to obtain a given "
Ul I
concentration of dissolved c

gas in the water than a less

soluble gas. In most cases /

dealing with a pure physical 0
S/
absorption phenomenon, the /

solubility of the gas in the / ,/

liquid phase is inversely -

proportional to the temperature, c ,

Figure 8 gives a graphical Fig. 8.- Equilibrium diagram
for SO2 absorption.
idea of these concepts.

Absorption deals with the transfer of material between two phases.

In this case, the solute must diffuse from one fluid, the main gas stream,

into a second fluid phase, the liquid.





-34-


Mass Balance

A mass balance will give the broadest picture of the mechanism,

from which we shall proceed into further details. If a general counter

current absorption tower is considered for the case of sulfur dioxide

in air being absorbed by water, V is the flow rate of air, L the flow

rate of liquid. Let Ll be the amount of sulfur dioxide in the inlet

water, which usually is zero. Let L2 be the amount of sulfur dioxide

in the outlet water. V2 and V1 will be the amounts of sulfur dioxide

in the inlet and outlet air. Then

L1 + V2 = L2 + V1 (20)

If X is the concentration of dissolved sulfur dioxide in the water

and Y its concentration in the air,

Ll LX1 and V1 VY1

L2 LX2 V2 = 2

X1 + VY2 = LX2 + WV

Y2 Y1 = L/V (X2 X1) (21)

This is the equation of a straight line with slope L/V. Considering

an interface at which the mass transfer is taking place, it will be

evident that all solute diffusing from the bulk gas phase to interface

must diffuse at the same rate into the bulk liquid phase from the

interface. Material is transferred from the main gas stream to the liquid

at every point along the interface, at a rate depending upon the driving

force and the resistance at each point. Because the transfer mechanism

is a combination of molecular and eddy diffusion processes, whose

individual and relative effects cannot be predicted, it is necessary

to employ the concept of a transfer coefficient. If Pai and Pag are











the partial pressures of the solute a at the interface and the bulk of

the gas stream respectively, then the rate of solute transferred from

the bulk gas phase to the interface is

N Akg (Pai Pag) (22)

If Cal and Cai are the concentrations of the solute a in the liquid

phase at the bulk and interface, then the rate of transfer from the

interface to the bulk liquid is

N = -AkI (Cal Cai) (23)

k and ki being the mass transfer coefficients for the gas and liquid

phases, A being the interfacial area.

Equation (22) determines the rate of transfer of solute from the

bulk phase to the interface, and thus the driving force is expressed in

terms of gas phase concentration units. Equation (23) expresses the

driving force in terms of liquid-phase concentration units. A relation-

ship for solute concentration at the interface, where the concentrations

of both phases are common, is needed before combination of these equations

is possible. From a general phase-equilibrium equation, the composition

of solute in the gas phase in equilibrium with a liquid of solute

concentration Ca is

Pa f(Ca) (24)

For dilute systems this function is frequently linear and becomes

Pa maC (25)

where m is a distribution factor like pure component vapor pressure in

Raoult's law, or Henry's constant, or the slope of the equilibrium curve

at Ca. After some manipulation73 with the above mentioned equations, the

following expressions can be obtained:




-36-


(Pa Pa)
N (26)
__L + m_
Akg Ak1



and N = -( Ca) (27)
1 + 1
mAk Ak1


Over-All Mass Transfer Coefficient

At the same time the individual mass transfer coefficients can be

substituted by the over-all mass transfer coefficients, since


__= __+ m__
AK1 Ag Akl (28)


and 1 ,_ 1
Akg mAKg Ak1 (29)


By using the over-all mass transfer coefficients, N can be

calculated without knowing the values of Pai and C ai which are
*
difficult to measure, at the interface. To make it clearer, P is the

partial pressure of solute in equilibrium over a solution having the

solute concentration Ca in the bulk, and C is the concentration of

solute in the bulk solution in equilibrium with the solute partial

pressure Pa. Then Pa P is the over-all driving force in the gas

phase, and Ca C is the over-all driving force in the liquid phase.

In the same fashion, the ordinate Pa can be substituted by ya, which

is the concentration of solute in the gas stream. Consequently, y

will be the concentration of solute in the gas stream in equilibrium

with a solution of solute concentration Ca.









Referring to Figure 9, A represents the conditions at any

differential surface through which diffusion is taking place. The

ordinate Pa is the average partial pressure of the diffusing gas in

the main gas stream, and the abscissa C is the average concentration of

the solute in the bulk liquid. In this case, the driving forces are

represented by the ordinate AE for the gas phase, and by FA for the

liquid phase.


Operating Line

The same Figure 9 could be used to visualize the concept of the

operating line. Point C represents the conditions of the inlet gas

where both gas and liquid concentrations are high. Point D represents

the conditions of the outlet

gas, the partial pressure of

the solute in the outlet gas/ i/

having been reduced from P
F F '
to P2. For absorption to o

take place, the solute
S-- E
concentration in the gas I

must be greater than in the

liquid, i.e., the operating

line must be above the Fig. 9.- Operating line and
equilibrium curve in a gas absorption
equilibrium curve, process.

As has been discussed previously, the magnitude of the slope of

the operating line equals L/V, indicating that the driving force is

dependent on how much the operating line differs from the equilibrium










line. If L is diminished, the slope diminishes to a point at which it

touches the equilibrium line, and then the driving force is zero.


Height of a Transfer Unit

Chilton and Colburn74 introduced the concept of the "height of a

transfer unit" to simplify the procedure for designing absorption towers.

This concept is still of great usefulness as a parameter for removal

efficiency. In the estimation of the height of the absorption tower,

the value of the definite integral


71
Nog (30)


Y2


is always important, for it expresses the difficulty of a scrubbing

solution to absorb the solute from the gas. It is called the number

of transfer units, based on an over-all gas-phase driving force. The

equation to obtain Hog, the over-all height of a transfer unit, is

given by

Hog = Z/Nog (31)

in which Z is the height of the tower.

From their analysis, Chilton and Colburn developed an equation for

design calculations which applies when only two gas compositions and one

liquid composition are involved.

S mVm) (Yl mX2) m
In L 2 2 Lm
Nog = I (32)
1 (mV /Lm)





-39-


Assuming that the liquid is well mixed vertically, and that

through a fixed area the liquid has a constant composition, equation

(32) can be expressed as

Nog In (1 Eg) (33)
OS
E I eNog (34)

Eog being the removal efficiency expressed as a fraction.

The validity of this equation is based on the assumption that the

product of the transfer coefficient per unit area and the interfacial area

per unit of liquid volume is constant along the vertical path of

integration. Also, assuming mVm/Lm is equal to zero involves an

irreversible chemical reaction. According to the data on sulfur dioxide

in alkaline solutions, an equilibrium line with a slope close to zero

is acceptable within the range of actual operation. The expression for

the effective height of a transfer unit can be related to the over-all

mass transfer coefficient by the expression

Vm
Hog = V (35)
KgaP



Theories Explaining Absorption

So far, operating expressions for the absorption process have been

presented in a rather simplified way. However, the theoretical principles

involved in the absorption process are complex and controversial. New

theories are in the process of resolving the apparent discrepancies.


Kinetic Theory

The kinetic theory is the ultimate explanation of diffusion because

the assumed movement of the gas permits the solute to move into the bulk











of the liquid and achieve the condition of uniform distribution. Fick's

equation can be expressed as
dc
dm = AD- dt (36)


where A is the interfacial area, D is the diffusivity coefficient,

t is the time, dc/dx is the concentration gradient.

Upon integration of this equation it is possible to ascertain the

amount of matter diffused during a given time and through a certain area,

using a diffusion coefficient, D, which depends on the gas involved.

The oldest theory regarding absorption of a liquid in a gas dates

back to 1878 when Stefan75 worked out simplified relationships deduced

from Fick's law. Stefan's theory was based on these assumptions:

1. The composition of the monomolecular gas layer at the interface

is equal to the bulk of the gas, due to the high velocities of the gas

molecules.

2. The concentration of the gas in the monomolecular liquid layer

equals the concentration of the gas in the liquid, according to the

expression of Henry's law.

Stefan gave this expression for the amount of matter diffused at

any time.


N = 2 (Cgi Cl)\ / (37)

Differentiating with regard to time gives


Sd = (Cgi C1 (38)


These expressions indicate that the amount of solute diffused is

proportional to the square root of time, and the rate of diffusion is





-41-


inversely proportional to the square root of time. Cgi and C1 are the

concentrations of solute at the gas interphase and at the bulk liquid

respectively.


Penetration Theory

Higbie,76 in 1935, arrived at a similar expression to Stefan's,

stating that even with a constant concentration gradient, the rate of

absorption decreases with the time of exposure. In Higbie's statement

the condition of an unsteady state mechanism was assumed.


Two-Film Theory

A decade before the restatement of Stefan's law by Higbie, Lewis
77
and Whitman had postulated their two-film theory. This theory has

been the most widely used model for gas absorption, and perhaps the most

misunderstood. Their theory is based on a series of assumptions:

1. Steady state conditions exist in both phases.

2. The rate of transfer is proportional to the concentration

gradient.

3. An equilibrium exists between liquid and vapor at the interface.

4. Retention in the interface is nil.

5. A stationary film exists at the interface.

The equations which describe Lewis' and Whitman's theory are the

same equations (26) and (27) which were used earlier to describe the

absorption process. To illustrate how the misapplication of this model

affects some systems, it will be convenient to refer to its application

in systems where chemical reaction takes place. In such a system, where











the gas is highly soluble, the over-all mass transfer coefficient is

given by the expression


Kga = 1 (39)
kga kla

Since m approaches zero, Kga = kga, This analysis leads us to the

conclusion that the liquid phase resistance is negligible, which could

be wrong in many instances. Teller78 pointed out the necessity of

applying the model that fits the situation and not to restrict our

interpretation to a single model.


Surface Renewal Theory

In 1951, Danckwerts79 proposed his surface renewal theory. The

turbulence in the absorption tower creates numerous infinitesimal liquid

elements which are constantly brought to the interface. While these

elements are exposed to the opposite phase at the interface, the diffusing

solute is transported by unsteady molecular diffusion into these elements.

Danckwerts stated that the concentration gradient is a value that changes

with time. In other words, the rate of diffusion at the interface is

determined by the concentration gradient, the value of which depends upon

the time of existence of the interface and not merely on the difference

in concentration. Where there is any degree of turbulence in a liquid,

the existence of a continuous exposure of fresh surfaces to the gas is

assumed, sweeping away and mixing these elements with the surfaces which

have been in contact with the gas for a certain time. This theory assumes

an infinite depth of penetration.





-43-


Film-Penetration Theory

More recently, in 1958, Toor and Marcello80 proposed the film-

penetration model. They showed that the two-film and the penetration

theories were not separate concepts, but only limiting cases of their

new film-penetration theory. Unlike Danckwerts' theory, they

postulated a finite value for the thickness of the surface element.


Absorption with Chemical Reaction

Most of the industrial processes involving absorption are

accompanied by a certain degree of chemical reaction. Processes for

the absorption of sulfur dioxide in aqueous solutions, which are

important in the pulping industry, in the recovery of pickle liquor

from steel manufacturing, and in the purification of flue gases, are

dealt with from the over-all mass transfer viewpoint. So far, only
81
one serious attempt has been made to divorce the influence of

molecular and eddy diffusion of sulfur dioxide into and out of the

solution from the diffusion of the constituents in the solution, so

that true reaction rate constants could be computed from the

experimental data.

In general, chemical engineers like to obtain gas-phase controlling

systems, so that the major resistance will occur in the gas-phase. To

understand this better, refer to the expression of the over-all mass

transfer coefficient.


Kga I m
ga l
ga la





-44-


If m is large (low solubility), Kga approaches kl/m. If m is small

(high solubility), Kga equals kga. Since kg is larger than kl, the

process will be more rapid in the gas-phase controlling system, and

consequently the size of the absorber will be smaller. The relative

resistances of both phases are functions of the diffusivities of the

solute in the vapor and liquid phases, concentration of unreacted reagent,

rate of diffusion of the reagent, and rate of reaction. Consideration of

these factors eliminates the false assumption that the process has to

become gas-phase controlling.


Hatta's Model

The process of absorption accompanied with chemical reaction has

been subjected to many interpretations. In the pioneer work of
82,83 84 85
Hatta,8283 Hatta and Katou,84 and Hatta, et al., the two-film theory was

adopted with the assumption of a stagnant film. Sherwood and Pigford86

discussed in detail the complexities of Hatta's theory when an infinitely

rapid second-order chemical reaction, or a slow first-order irreversible

reaction takes place. In the same work they discussed the theory of

Higbie in different cases.


Analytical Models.

Because of the complexities involved in the mechanism of gas

absorption accompanied with a chemical reaction, some investigators

decided to use analytical models. In these cases, attempts were made

to predict the behavior of a system by comparison with the behavior

of another system that had been evaluated previously. The same hydro-

dynamic conditions for both systems were utilized, thus eliminating new





-45-


variations due to interfacial area, time of penetration, and degree of

surface renewal. Danckwertz and Kennedy87 arrived at some expressions

relating the value of mass transfer coefficients for physical and chemical

absorption. He assumed that no chemical reaction occurred in the

physical process, i.e., assuming absorption in water as a physical

process, which is not fully true.


Film-Penetration Model

In recent years many studies88 to 99 have been conducted in an

attempt to obtain a consistent physical picture of the process according

to the different systems tested. Recently, in 1963, a general mathematical

model for mass transfer accompanied by chemical reaction has been
100
elaborated by Huang and Kuo. In their theory, which is similar to

the renewal theory, they postulate the interface as being replaced by

infinitesimally small fresh liquid elements. The mass transfer mechanism

consists of two steps: a surface renewal by fresh liquid elements, and

a simultaneous molecular diffusion and chemical reaction within the

exposed liquid elements. When the renewal rate is slow, the surface

elements remain at the interface, and the extreme condition is the

creation of a stationary liquid layer. This is the case with the model

proposed by Hatta for chemical reaction using the two-film theory. If

the turbulence of the liquid is great, the residence time of the elements

at the interface would be small and the surface renewal theory would

prevail. Unlike Danckwert's theory, this model assumes a definite value

for the average thickness of the surface as proposed by Toor and Marcello.

Huang and Kuo prove the similarity of their equations by two different






-46-


mathematical approaches, trigonometric function series and error

function series. The proposed equations contain the dimensionless

groups C(, /9 in which

Da sr D< D
k L r sL

where sr is the surface renewal rate, L is the thickness of the surface

elements, and Da is the molecular diffusivity of the solute a.

Thus, if / approaches zero, the main equations expressed in

trigonometric or error function series can be reduced to Hatta's

equation. Furthermore, when the reaction velocity constant is zero,

an identical expression to the one derived by Toor and Marcello is

obtained from the trigonometric function series expression. Also, the

equation expressed by Danckwerts is obtained from the error function

series expression. When the dimensionless group c< approaches infinity,

the expression from the trigonometric function series is identical to

the one derived by Lewis and Whitman. In conclusion, for limiting

conditions the derived equations can be reduced to the simple

postulations already mentioned. For nonlimiting conditions, the

film-penetration theory of Toor and Marcello gives information not

attainable through any of the original theories.


Rate of Absorption in the Venturi

The rate of absorption in the venturi scrubber is determined by

the concept of transfer units. The number of transfer units can be
101
expressed, according to Kuznetsov and Oratovskii, as

KF
Nog = K (40)











in which K is the over-all mass transfer coefficient, F is the interfacial

area, and Q, is the gas flow rate.

In the venturi scrubber, the absorption process takes place both

in the throat and in the diffuser section which follows the throat.

The magnitude of N should be analyzed separately for both parts of

the apparatus. The over-all coefficient of absorption accompanied by

an irreversible chemical reaction is defined differently depending on

the region in which the absorption occurs. If the rate of the chemical

reaction is much lower than the rate of diffusion,

K = K (41)

If the rate of diffusion is controlling, then

K = KoVg (42)

If the rates of reaction and diffusion equally influence the

process, the over-all mass transfer coefficient is given by an expression

of resistances in series.


K 1 (42)
i+ I
1, I
KoV- -

The variation of the over-all mass transfer coefficient with gas

velocity has been studied by different authors.102,103 The coefficient

also varies throughout the distance from the injection point to the end

of the diffusion zone. K, and n can be determined by experimentation

from the plot of the curve of gas velocity versus over-all mass transfer

coefficient. The proportionality coefficient Ko derives from the

expression of the Nusselt number, Nnu = Kd/D, and n is a quantity which

depends upon the hydrodynamm of the process.04
depends upon the hydrodynamism of the process.





-48-


If R is the specific flow rate of the liquid or the liquid/gas

ratio, the fraction of liquid in the two-phase system is equal to

R
R (43)
1 +R

Since R amounts to about 0.001, the fraction of the apparatus

volume containing liquid may be equal to R, and the interfacial contact

surface Ft in the throat of volume Vt could be expressed as

6RVt
Ft Dp (44)

To determine Dp, the average diameter of the particles dispersed,

the empirical Nukuyama-Tanasawa0506 equation is employed. Lewis107

revised this equation and found it applicable for the throat velocity

range of 200 to 600 feet per second.

The original expression for the average diameter of the aerosol is

given by

585V' /+ 0.45 1000 Q1 1.5
Dp = + 597 ( ) Qg (45)


in which 7-, and ,4 are the density, surface tension, and viscosity

of the liquid. This expression is simplified in this form,


D A + BR1'5 (46)
p

A and B are values which depend on the scrubbing liquor and the gas.

For air and water, these values are 16,050 and 1.41, resulting in


16050 1.5
Dp = 7-- + 1.41 R
9g










In the Nukuyama equation, V is really the velocity of the gas

relative to the liquid. In the venturi used in these experiments, the

liquid overflows a weir before entering the apparatus. Since its

velocity is very small, it is therefore valid to consider the value of

the gas velocity for Vg.

Because of mathematical complications, a venturi theory that comprises

the whole tube, throat, and disperser has not been proposed. At the

present moment only a study of the separate parts is available.

Boyadzhiev108 discussed in detail the significance of Kuznetsov's

equations. Separate considerations were made for absorption taking

place in the throat and in the diffuser.

In the throat, the number of transfer units, when the absorption

is determined by the rate of chemical reactions, is given by the

expression
K Rh
Nog (48)
600 (A + BR1.5Vg)

in which ht is the length of the throat.

When diffusion is controlling, the over-all absorption coefficient

is determined by equation (42), and the number of transfer units is

given by

KVgn R ht
N0, = 15 (49)
Ng 600 (A + BR Vg)

When the two rates are commensurate, the following dependence

derived from equations (41) and (42) is valid.

K, Kr V n R ht
Ng 600 (A + BRI.5 Vg)(KoVgn + Kr) (50)





-50-


In the diffuser region, the gas velocity decreases from the entrance to

the discharge section. Since the diffuser is a truncated cone, the gas

velocity, Vg, and the cross section of the diffuser at any distance is

given by
gt
V gt (51)
g (1 + ph)

p dd dt/dthd (52)

in which dd and dt are the diameters at the diffuser and at the throat,

respectively.

S = St (1 + ph)2 (53)

St is the cross section at the throat, and h is the distance from the

throat.

The interfacial area could be expressed in a differential form by


d Fd= 6 R Sdh (54)
Vt
2 2
By substituting x2 for (1 + ph)2 and grouping constant values

under q, it will be possible to have

q x2dx
d Fd q x 2d (55)
d p (a + x2)

xd x2dx
and Fd = P a x2 (56)



This is a general integral of the form


2x da tan (x \a/c) (57)
a. Tc










Integrating over the entire length of the diffuser, that is, from

x = 1 to x = Xd, the following equation results.


Fd = (xd 1) -\ a (tan-1 a tan-1 ) (58)


The value of Nog can be ascertained by integration of the expression


Nog = d a x2dx (kinetic region) (59)
P a + x


and

q V d 2(1-n)
No q x 2(n dx (diffusional region) (60)
Ng P a + x
1

In general, the value Nog passes through a maximum as R increases

for all the cases, both in the throat and in the diffusion region. The

significance of the gas velocity, V is not discussed here. It suffices

to say that when the reaction rate is controlling, Ng decreases when

Vg increases. When either the diffusion or the intermediate state is

controlling, Nog increases with an increase of Vg.

From the field data, it will be possible to ascertain the value

of the over-all mass transfer coefficient K. Since this coefficient

is defined differently according to the controlling region or its

combination, there is need to examine carefully the gas absorption

system. By means of experimentation, it could be possible to obtain

the value of KV n. Kr could be found by rigorous experimentation and

by the analysis of the reaction kinetics of the particular system.











Rate of Absorption in the Spray Chamber

For absorption in the spray chamber, a concept similar to the one

employed in the venturi calculations is used. The concept of height

of transfer unit is normally regarded in the chemical engineering field

as the parameter to measure the behavior of the spray chamber. In the

spray chamber the flow is generally counter current. Top and lateral

spray nozzles break the liquid into small drops, thus providing the

interfacial surface across which diffusion takes place. The interfacial

surface in the spray chamber is not large. Replication of experiments

in a spray chamber seems to be difficult, probably because of the

adherence of small particles to the spray chamber walls, and some

liquid entrainment in the gas phase. This last factor is less significant.

The equation to obtain Hog, the over-all height of a transfer unit,

is

og = Z/Nog (31)

Since the concentration of sulfur dioxide in the gas stream is

relatively small, the following equation may be used.

Nog (Y1 Y2)/(y Y*) Im (61)


For the specific case under consideration, y is negligible in

comparison with y. Thus it is possible to write



Nog dy dl In (Y ) (62)
y y Y

J2 Y2










This expression can be related to K., the over-all mass transfer

coefficient, by the equation

Vm
HS M (35)


Recent studies in the operation of spray towers with lateral

spray nozzles have been based upon the characteristics of the apparatus
109
and the hydrodynamic conditions prevailing. Mada and co-workers1

experimentally determined an expression for Hog and also for Hg and H1.

For each particular system they obtained a response function. The

response equation for the absorption of ammonia in water, for example,

is
0.70 -0.30 -080 012 -0.20 -0.15
og/D = 12.7 Reg 7Rel- 30n-080(Z/d) 012(d/D) 20Gag (63)


In which Re is the Reynolds number of the gas and liquid streams, d and

D are the diameters of the nozzles and tower, respectively, Ga is the

Galilei number for the gas stream, and Z is the height of the tower.


Chemistry of the Sulfite-Bisulfite System

When an acid gas as sulfur dioxide dissolves in water, the gas is

in equilibrium with the ions formed. It is possible to establish three

main equations for the ionization constants of the different species.
110
Latimer, from well-known data on free energies of formation, gives

the values for K1 and K2 as follows:

H2S03 __ H + HSO3 K = 1.25 x 10-2


K2 = 5.6 x 10-8


HSO3 ;= H+ + S03





-54-


Johnstone and Leppla determined the free energy of formation for

SO2 (g) : S02 (aq) as Fo = -123 cal

With this free energy data it is possible to calculate the

equilibrium constant for the following reaction:

SO2 (g) + 20O = H2S03 (aq) K = 1.2

The above three relationships can be expressed as

(HSO3")() )
(H2S03)


([O)(S03=) (65)
K2 ([ISO3.)


Kp 12S3) (66)
PS02


For a complete definition of the system, more equations are needed.

The total amount of sulfur is given by the mass equation,

St = (H2SO3) + (HS03") + (SO3 ) + (S02) aq (67)


For a sodium sulfite-bisulfite system, the ionic equilibrium

expression is

(Na) = 2 (SO3') + (HS03") + (OH-l (H') (68)


The solution of this system of equations and unknowns could be

simplified by the following reasoning. To obtain a simple expression

for PSO2, it is possible to transform equation (66) into


P ( 2S03) (69)
S02 Kp










Substituting for the value of (H2S03) in equation (65) gives

(HSO3)(H) (70)
PS02 KKp (70)

Substituting (HSO3 ) for its value in equation (64) gives


S =(H+)2 (03) (71)
S2 KIK2Kp

By means of this reasoning, one can establish an important fact in

sulfur dioxide absorption. That is, vapor pressure or volatility of

sulfur dioxide in solution is directly proportional to the square of

the hydrogen ion concentration. This means that at lower pH values,

the vapor pressure will be greater than at higher pH values. Consequently,

the capacity of absorption for sulfur dioxide of a solution tends to

decrease with lower values of pH.

Data from Figure 10 give valuable information on the bisulfite-

sulfite system. From equation (65),


K2 =- )(SO) and K = (S03=) (72)
(HS03") (H') (HS03-)

The ratio sulfite-bisulfite can be predicted from the pH of the

solution. If pH pK2, the ratio of sulfite to bisulfite will be more

than two orders of magnitude. Thus, for practical purposes we can

eliminate (HS03") from the mass equation (67). Following the same

reasoning, from equation (64)

S(H+)(HSO3") and KI (HS ) (73)
(H2S03) (H1) (H2S03)






-56-


-i

I-y


-t


I


1<
/
/
/
/ -


/

>/
0/
/
/
/
/
/


XL ----


0 1 2 3 4 5 6 7 8 Q 1C 11 12 13

pH

Fig. 10.- Equilibrium diagram for sulfite species.











Then is pH pK the concentration of the SO 3" species is much

greater than H2SO3, and it is possible to eliminate the term H2SO3

from equation (67). It follows that the term SO2 (aq) will be small

enough to be eliminated, leaving St = SO3 Substituting in this

fashion in equation (67)

(S03 ) = (Na/2) (74)


Buffered Scrubbing Solution

The above discussion indicates the need to have a solution which

changes pH very slowly when the solute is absorbed into it. In other

words,a buffered solution is needed. In many of its processes, the

pulp industry uses highly alkaline solutions with buffer characteristics.

If a carbonate solution is used as the scrubbing liquor, two factors

that may affect its pH need to be studied. Sulfur dioxide and carbon

dioxide are both present in the flue gas. Carbon dioxide and sulfur

dioxide are both weak acids, but sulfur dioxide is the stronger of the

two. The buffer capacity of a carbonate solution differs, depending
111
upon titration with a weak or a stronger acid. Weber and Stum

observed that the shift in pH caused by the weaker acid is smaller than

the one produced by the stronger acid. In an attempt to look into the

complexities of the sulfite-carbonate system, a series of titrations

of carbonate solutions with pure sulfur dioxide and pure carbon dioxide

were conducted. The theoretical relationship among carbonate species is

shown in Figure 11.

The curves in Figure 12 illustrate the titration of a 0.45 M

Na2CO3 solution with sulfur dioxide (curve A), with carbon dioxide






















0/


/\


C /
0/


C)r


Fig. 11.- Equilibrium diagram for carbonate species.













































z

0 I
Z
0
u
Zd
7


0
-r
J >4






,c
H






-60-


(curve B) at the same rate, and with sulfur dioxide (curve C) at a

lower rate of flow. Carbon dioxide gives a typical titration curve

for a weal: acid. Sulfur dioxide seems to be a stronger acid.

The exact mechanism of the absorption of sulfur dioxide in a

carbonate solution is not clear. Apparently, when the scrubbing

solution is water, both film resistances influence the rate of

absorption. It would be necessary to go into a rigorous study of the

reaction kinetics of the system to ascertain individual contributions.

In an attempt to obtain more information on the mechanism of

sulfur dioxide absorption in a carbonate solution, a series of

experiments was conducted with the experimental set up shown in

Figure 13. Pure sulfur dioxide was bubbled into 175 ml of 0.44 M

carbonate solution.

The rates of flow for the sulfur dioxide were 0.25, 0.50, 0.75

and 1.0 SCF/H as air. The number of moles of sulfur dioxide reacting

was calculated from the variation in pH recorded during the time of

injection. The number of moles, Q, injected at any time t, is given

by the expression Q = qt, where q is the rate of injection.

Since the volume and concentration of the carbonate solution

remained the same for all the experiments, the same quantity of

reacting sulfur dioxide was needed for a certain change in pH. Thus,

Q = qltl = q2t2 (75)

This expression permits the correction for time necessary to obtain

the curves of Figure 14. Curves A, B, and C represent the conditions








1'














35


25 [ .t






05 -
C

95
ol
//









5 / i
-75 9
65 - i t -













55






35 .
75- *-




















5/



0 30 60 90 120 150 180 210 240 270 300 330 360 390
Time in seconds


Fig. 14.- Titration curves at different rates of SO2 injection.










for 0.25, 0.50, and 0.75 SCF/H as air. At 1.0 SCF/H as air, a similar

curve was obtained, but not plotted for the sake of clarity.

In the laboratory experiments, an instantaneous reaction occurred

within a wide range of available sulfur dioxide. The gas phase resistance

was controlling. During field conditions, the presence of carbon dioxide

will alter this picture.


Foaming

Oxidation of the kraft weak black liquor represents a milestone

in the efforts to control sulfur emissions in the pulp mill. However,

particularly in Southern mills, the foaming created in the oxidation

process is possibly the main reason for delaying the implementation of

the oxidation practice as a standard procedure in all mills.

Foam is a dispersion of a gas in a liquid. It consists of gas

bubbles very close together, separated by a thin liquid layer. Kraft

weak black liquor foam is composed of a wide range of bubble sizes and

is rather long-lived if compared with ginger ale foam.

There are many theories that attempt to explain the formation of

foams. The most widely accepted is the balance layer theory.

According to this theory, two rising bubbles are subjected to a system

of forces. If brought together, the two bubbles might disappear; if

they remain apart they create a stable film at the surface. The force

which tends to separate them arises from the difference in concentration

between the surface layer and the bulk solution.






-64-


Foam Stability
113,114
Foam stability has been the object of many studies. At the

present time, it is possible to say that it is the result of several

interacting factors. In summary, Gibbs elasticity, viscosity, and

surface area are among the most important. Kevorkian5 listed

several others.

For a solution to give rise to a stable foam, its surface tension

must always be acting to oppose any force which deforms the film.

This property of the surface tension to reduce strains is called Gibbs

elasticity. In that fashion, a mechanical or thermal shock might

decrease the concentration of solute in the film, thus increasing the

surface tension. The increased surface tension will tend to contract

the film, counteracting the extending action of the external forces on

the film. If at certain moments the increase in surface tension contracts

the film, there should be an increase in the concentration of the

surface active agent which decreases the surface tension, thus extending

the film. This "stretching" ability should be present in any local area

of the film.

Viscosity is an important factor in foam stability. If the viscosity

of the film is decreased, the attracting forces will overcome the

dispersing ones, and the bubbles will coalesce and disappear.

The third important factor in foam stability is the value of the

free surface energy. Free surface energy is the amount of work necessary

to create a fresh surface. It is equal to the product of surface tension

and surface area. Stable systems are those with a minimum free surface











energy. For the same value of surface tension, foam stability is

greater if the bubble is smaller.


Drainage

An important characteristic in the lifetime of foam is drainage.

Different mechanisms of drainage exist, depending on the thickness of

the film. However, a discussion of such mechanisms is beyond the

objectives of the present study. Let it suffice to point out the

relationship of drainage with viscosity of the liquid film. A decrease

in the viscosity of the film surrounding the bubble enhances drainage,

causing the bubble to become thinner and thinner until it ruptures.


Selection of the Antifoaming Substance

An important consideration in this selection should be the

spreading characteristics of the antifoaming substance. One must be

chosen which guarantees the formation of a monomolecular layer on top

of the vessel. In general, oils, with a negative spreading coefficient,

give rise to a monolayer and a surface excess composed of lenses.

Another feature of interest for the selection of the antifoaming

agent will be its viscosity. Of all the commercial oils, kerosene

has one of the lowest viscosity values.

Antifoaming agents in general act either to prevent foam formation

or to "kill" the foam once it is formed. The former type of antifoaming

agent seems to change the existing hydrogen bonding between the film

and the substrate. The latter type is more dependent on viscosity and

spreading characteristics.






-66-





Kerosene was selected as the antifoaming agent because of the

above mentioned considerations, and the fact that it was readily

available and rather inexpensive.















III. PROJECT DESIGN


The Experiment


It was the purpose of this experimentation to explore the

possibilities of sulfur dioxide removal from the flue gases of a

boiler plant and to adapt to the pulping industry, in the most efficient

way, any recovery process developed.

The previous discussion on the mechanism of absorption in general,

and the applicability of sulfur dioxide absorption in recovery processes,

formed a basis for selection of the variables used in this experiment.

The description of the experimental pilot plant contained in Appendix A

will help clarify many of the decisions regarding the variable levels

and experimental procedure.


The Response

The response selected to characterize the results of the experimenta-

tion is the percentage removal of sulfur dioxide. This response clearly

gives a measure of the efficiency obtained in SO2 removal plus provide

suitable figures for ascertaining recovery and air pollution reduction

potentialities. Other characteristics in the absorption process, such

as the number of transfer units for both the venturi and the spray

chamber units, can be readily obtained from the response selected.


-67-











Independent Variables

An absorption phenomenon is complex per se, even under rigidly

controlled experimentation in a laboratory. The process is even further

complicated on a plant scale. As a result, few of the factors influencing

the absorption mechanism are purely independent. It is within this

limitation that field experimentation was conducted, and only a few

variables that were actually made independent were studied. Temperature,

total pressure, partial pressure of the solute in the feed gas stream,

hydrogen-ion concentration, liquid-gas ratio, gas velocity, and

concentration of the scrubbing solution were among the most important

variables considered. The following careful look at the system permits

the elimination of some of these variables.

Total pressure in the scrubber is constrained within narrow limits,

its value being a few inches of water above normal atmospheric pressure.

This is true, in general, of all standard gas-liquid contact units.

Partial pressure of the solute in the gas stream was beyond control

in the experimental pilot plant because the boiler plant output oscillated

continuously, as shown in Figure 32, Appendix A. Preliminary experimenta-

tion showed that similar oscillations occurred in the sulfur dioxide

concentration of the flue gas. Since it could not be controlled, the

partial pressure of sulfur dioxide was not considered as an experimental

variable. Precautions were taken to conduct the experiments within as

narrow a range of SO2 concentration as possible.

Gas velocity through the scrubber was an important factor, especially

in the venturi scrubber. However, due to the influence of gas velocity

in heat transfer processes, which introduced modification in the temperature






-69-


levels, gas velocity was not considered a variable. The influence of gas

velocity in the absorption process was studied at only two levels of

velocity. A complete block design, at the new gas velocity level, was

tested to ascertain the influence of gas velocity in the absorption

process under all existing combinations of factors.

The importance of hydrogen-ion concentration was outlined in

Chapter II. Since it was desirable to have a measure of the economic

value of the scrubbing solution, concentration of the scrubbing solution,

which is related to pH, was selected as a variable instead of pH.

After this elimination of factors or variables, only temperature,

liquid-gas ratio, and concentration of the scrubbing solution were

considered for most of the experiments. When using water or weak kraft

black liquor, concentration ceased to be a variable.


Orthogonal Factorial Design

For the study of the variation brought about by deliberate changes

in the independent variables, a useful technique is provided by a

factorial experimental design. The advantages of a factorial design are:

1. It is an efficient method, that is, a method which obtains the

desired information with the required precision for the minimum

expenditure of time and effort.

2. If interactions do exist, they are detected, and misleading

conclusions can thus be avoided.

Whenever possible, the experiment should be made orthogonal.

Orthogonality implies making the experiment symmetrical in all the

independent variables by testing all treatment combinations the same











number of times. Orthogonality insures that the main effects and

interactions can be estimated independently without entanglement.


Preliminary Investigations

Because no information was available about the operational levels

of the above mentioned variables, preliminary testing was necessary

to estimate the operating range for each of the variables.

To achieve a net reduction in ground-level air pollution using a

wet sulfur dioxide scavenging process, it is necessary to obtain close

to 90 per cent removal. This high percentage of removal will compensate

for the adverse effect in the dispersion pattern introduced by the

cooling of the gases.

Whenever it was possible, the levels of each variable were selected

in such a way as to reduce the possibility of overlapping values due to

the experimental error, the magnitude of which was unknown at the

beginning of the study.

The concentration level for the scrubbing solution had an upper

limit set by the common concentration of carbonates in the digesting

liquor used in the mills. This was not detrimental since the carbonate

was sacrificed to absorb the sulfur dioxide. Evidently it represented

a good starting value for an upper limit.

Liquid-gas ratio levels were selected on the basis of preliminary

experiment, since the values appearing in available literature differ

drastically. In any case, it was desirable to utilize the maximum gas

flow, because the results would resemble more closely those expected in

industrial scale operation. In the case of the spray chamber experimentation,











the rates of flow were substantially reduced because of the scrubber

flooding problems. It seems the capacity of the spray chamber is

about two thirds that of the venturi.

Temperature levels were limited by field conditions. Variations

in the flue gas temperature due to boiler plant operation, plus absence

of a temperature regulator, limited the choice of temperature levels.

A cooling system, as explained in Appendix A, was able to operate only

within certain variability. Within these constraints, the levels were

spread as much as physically possible.


Levels of the Experimental Variables

Once an idea of the maximum ranges of the different variables was

obtained, a decision was made regarding the number of levels. Only

two levels are often used in semi-industrial experimentation for both

economy and simplicity. However, it is advantageous to use three levels

of the independent variables if it is possible to do so, because

information can be obtained on both the linear and quadratic components

of each effect. A quadratic component may imply a maximum or minimum

response at some intermediate factor combination, or at a point outside

the range examined. This consideration applies especially to quantitative

factors, such as the ones involved in this project.

The levels of the independent variables were equally spaced when

using three levels. In some experiments in which only two variables at

four levels were involved, the levels were not equally spaced. In

general, the criterion used for choosing the levels was restricted

minimization of the generalized variance of the estimates, E(Y), where











Y is the response value for each different experimental condition. The

generalized variance is the determinant of the second order central

moment matrix about the regression equation.16 Table 2 indicates the

levels for the variables used in the different experiments.


The Design


Observations

It was decided after preliminary tests to allow the scrubber to

operate for a thirty-minute period, in order to stabilize the system

before taking samples. This period of time was necessary to arrive at

a fairly steady temperature. It was assumed that hydrodynamic

conditions became stabilized at the end of this period of time.

For each experiment, two observations were made. The duration of

each experiment was around 45 to 60 minutes. The general procedure for

plant operation and sampling is discussed in Appendix B. The capacity

of the make-up tank was only enough to permit running a few experiments

under the conditions already stated. Randomization also precluded the

reuse of a given batch. Thus, numerous independent batches were prepared

during the experimentation. However, the differences introduced in the

preparation of the scrubbing solutions were assumed to be negligible,

because the same technical grade of chemical from the same manufacturer

was consistently used, and the same preparation technique was rigorously

followed. It should be kept in mind that the concentration levels in

the solutions were spaced adequately to minimize inconsistencies in

successive batches of scrubbing liquor.














Experimental


Table 2

Factors and their Levels


Levels
System Scrubbing Factor
Liquor -L 0 +1


Venturi






Spray Chamber






Venturi






Venturi


Water Ratio (gpm/1000 cfm) 1.40

Concentration --

Temperature ( F, inlet) 270

Carbonate Ratio 1.3

Concentration 0.05

Temperature 260

Carbonate Ratio 1.00

Concentration 0.05

Temperature 260

Weak Black Ratio 3.15
Liquor


Concentration

Temperature


2.70 4.00




345 420

2.3 3.3

0.20 0.35

-- 320

2.1 3.2

0.20 0.35

-- 360

-- 4.0











Randomization

In statistical analysis, randomization is not absolutely necessary

for drawing conclusions from the data gathered. Nevertheless, it is

difficult, especially in the early phases of a stochastic experiment,

to know how much control can be exerted over the experiment. Randomi-

zation thus becomes the safest way to draw reliable conclusions.

Unexpected systematic effects can be cancelled out, the observations

can be made independent of each other, and any effect of uncontrolled

variables can be averaged out. The random order for each experiment

was obtained from the table of random numbers.117


Replications

Experimental errors can be of importance, especially when a new

system is under operation in a newly designed plant. Replication is

necessary to obtain a measure of precision and experimental error. Two

replications were performed for each experimental condition.


Mathematical Models

The mathematical models which describe the different experiments

can be summarized as follows:

In a 3 x 3 x 3 factorial, three factors or independent variables

are treated at three levels. The model for such an experiment is

Xijkm + Ai + Bj + ABij + k + BCjk + ACik + ABCijk + Em(ijk) (76)

in which Xijkm represents the m observation on the i, j, k treatments.

For this case,

m = 1, 2 j = 1, 2, 3

i = 1, 2, 3 k = 1, 2, 3





-75-


X1232 represents the second observation using factor A at first

level, factor B at second level, and factor C at third level, repre-

sents the average Xijkm over all populations, or E (Xijkm). Ai, Bj, and

Ck are the different treatment effects. The combinations of capital

letters stand for two-way and three-way interactions among the factors.

The error term Em(ijk) is normally considered an independently

distributed random effect. This could be expressed as NID (O,v ).

A similar mathematical model is applied to a 3 x 3 x 2 factorial.

Xjkm = + Ai + Bj + ABi + Ck + BCk + ABCijk + E(ijk) (77)

For this case,

m = 1, 2 j = 1, 2, 3

i = 1, 2 k = 1, 2, 3


The Analysis


Response Surface Experimental Technique

If the dependent variable (Y) and several independent variables

(X1, X2, Xk) are measurable, it is possible to express a response

surface equation for (Y) as follows:

Y = f (X, X2 . . Xk) (78)

For two independent variables, the response surface can be

represented graphically as a contour map or as equal response curves.

For three variables, a space solid representation could be used to

visualize the results.
S 118
Stochastic approximation procedures8 involve two basic

considerations: first, choosing a direction in which to search, and






-76-


then selecting the distance to travel in that direction. The main

difference between deterministic and stochastic problems is the higher

speed in approximation to the optimum of the deterministic problem.

The experimental error, or "noise," clouds the perception of what is

really happening. The steepest ascent method was selected under the

prevailing experimental conditions. It can be used in moderately

unimodal functions, and it can also work in the presence of experimental

error. The use of this method reduces the possibility of obtaining a

saddle point.

In industrial scale experiments, the exploration of new areas to

obtain an optimum is troublesome and expensive. For this project, the

results of a preliminary experimentation indicated an area to be

explored. This area was close enough to the optimum to be truly

representative of the response surface area.

When this is not possible, the steepest ascent PARTAN techniquell9

is indicated as an elegant and powerful tool. The methodology carefully
120
followed in this experiment is the same indicated by Davies and by

Hicks.121














IV. EXPERIMENTAL RESULTS


Scrubbing with Water


The results of the experiments using water as the scrubbing solution

are in keeping with the general theory describing the process. The

experiments were conducted on both the venturi and the spray chamber.


Experiments in the Spray Chamber

Scrubbing with water in the spray chamber proved unsatisfactory.

The percentage removals were not over 5 per cent even when the liquid

rate of flow was increased to 12 gpm and the gas rate of flow was

diminished to 1800 cfm. At this same gas rate of flow, it was not

possible to increase the liquid rate of flow above 12 gpm, because

flooding conditions at the gas inlet occurred. Another unsatisfactory

condition, typical of the spray chamber, was the difficulty in obtaining

meaningful results from replications at the same conditions.


Experiments in the Venturi

Table 4, in Appendix C, indicates the field data recorded for the

different experiments. The percentages of sulfur dioxide removal from

the flue gas stream appears in Table 5. Figure 15 illustrates those

results with an average value for each treatment. A statistical

analysis was performed with the data as it appears on the block design

of Table 6. From the ANOVA of Table 7, it is possible to ascertain the

relative significance of the different factors.

-77-
















60
S> 'EMF: LEVELS
-, (0)






-^ -/







Fig. 15.- Scrubbing with water in the venturi.
Experimental results.




The liquid/gas ratio has the highest significance of all the factors

studied. The low point, or "sag," noticed in Figure 15 is only

justifiable by the high significance of the interaction sum of squares.

Seemingly, flow patterns along the apparatus reduced the transfer area

for the process, at certain rates of flow. In general, the experiments

in the extreme regions of experimentation were dependent on temperature,

as predicted by the traditional theory on physical absorption. If they

did not agree even more, it is because of the existence of a moderate

chemical reaction.

The breakdown of the analysis of variance shows the significance

of linear and quadratic effects. Since quadratic effects are highly

significant, it was advisable to use a second degree equation in order

to represent the experimental response. The experimental response is

the percentage of sulfur dioxide removed. Table 8 indicates the estimates






-79-


of the constants and the regression analysis for the best fitted

equation representing the experimental conditions. The best fitted

equation is expressed as:

Y 23.62 + 10.42 xl 9.92 x2 + 21.41 xl2 1.73 x22 5.06 x1x2 (79)


Differentiating with respect to xl, the ratio, and x2, temperature,

and solving simultaneously, a possible maximum value of 31.18 was

obtained. Upon a Hessian inspection of the best fitted equation, it was

determined that there was not such a maximum, but rather a saddle point.

The canonical representation of the best fitted equation, as it appears

in Figure 16, clearly indicates the saddle. The canonical expression

for the best fitted equation is

2 2
Y 31.18 = 21.74 X1 2.06 X2 (80)

where X1 and X2 are the new canonical axis.

Had the purification unit been able to perform other experiments

at higher liquid rate of flow, then it would have been advisable to

follow an optimum seeking method as described in Chapter III.

Estimates of the over-all mass transfer coefficients for the whole

purification unit appears on Table 10, based on the calculations of

Table 9. It was assumed that all the removal occurs at the venturi

in both the throat and the diffuser zone. The cyclone only provides

separation between the liquid and the gas. The throat interfacial area

on Table 9 was estimated by the same procedure as outlined in Chapter II.

The diffuser interfacial area was estimated averaging the velocities at

both end sections.























F-* F

-2362 4.,- 992.i "2' 21. -,.73 5.60 .6O

a: 3 18 2 2. X2


Fig. 16.- Response surface of equation (80).




Scrubbing with Sodium Carbonate


Experiments in the Spray Chamber

For these experiments a 3 x 3 x 2 factorial was used. Only two

levels of temperature were established. For a better understanding of

the results and their significance, two completely separate analyses

were performed at both the higher and lower levels of temperature.

High Temperature.- Table 11 indicates the field data gathered for

the different treatments at the higher level of temperature. Table 12

gives the percentages of sulfur dioxide removed. Figure 17 shows the

major features of the experiments. Removal increases with an increase

of the liquid/gas ratio, but this increase is greater at the lower

level of concentration of carbonate in the liquor than it is at a

higher level. The relatively low percentages were expected for the





















2r"'













Fig. 17.- Scrubbing with sodium carbonate in the
spray chamber at a high temperature. Experimental results.



spray chamber. Once more, flow pattern characteristics were the reason

for a "sag" at the intermediate levels of ratio and concentration.

Figure 17 indicates the presence of a chemical reaction depending on

the concentration of the scrubbing liquor.

The same statistical procedure as the one described previously

was adopted for this experiment. Tables 11 to 15 inclusive give the

results of the analysis of variance and the determination of the best

fitted regression equation. The strong significance of concentration

in the ANOVA of Table 14 is noticeable. Linear effects of the

concentration factor are more significant than the quadratic effects.

Table 15 reveals a linear interpretation of the experiment based on the

values of the variances for the last three constants.









The best fitted equation considering quadratic effects, is given by

2
Y = 20.29 + 2.87 x, + 5.33 x2 + 1.36 x, 2.61 x2 1.28 x1x2 (81)


The canonical expression for the above equation is given by

2 2
Y 22.81 = 1.48 X1 2.73 X2 (82)


An inspection of the signs of these coefficients indicates the

presence of a saddle type of response surface. It seems enough to

represent the experiment by the linear expression

Y = 20.29 + 2.87 xl + 5.33 x2 1.28 xlx2 (83)



Lower Temperature.- Tables 16 to 20 inclusive give the data for the

statistical analysis of the results obtained in the experiments at the

lower level of temperature.

As Figure 18 indicates, removal increases with an increase in the

concentration of the scrubbing solution at all levels. Regarding the

liquid/gas ratio factor, removal only increases with an increase in

the liquid/gas ratio at the two extreme concentration levels. Once

more, the behavior of the spray chambers in general, and the flow

patterns in particular, explain this situation.

Table 19 indicates the significance of both factors and their

interaction. All are significant at the 1 per cent level.

The best fitted equation for the experiment is given by

Y = 15.71 + 4.41 xI + 5.82 x2 1.66 x2 0.86 x22 + 1.06 xx2 (84)






-83-


Fig. 18.- Scrubbing with sodium carbonate at a low
temperature. Experimental results.



The canonical representation in Figure 19 is given by the

expression
2 2
Y 37.48 = -0.61 X 1.92 X2 (85)


In the same Figure 19, the relative position of the experimental

area, the response surface, and the optimum point S are shown.

Table 21 indicates the significance of the three factors and their

interactions for the total experiment. All three factors and the AB

interaction are significant at the 1 per cent level. Table 22 gives

the value of the estimates of the height of transfer units.


Experiments in the Venturi

The same procedure for the statistical analysis used in the

previous experiments was followed for the experiments in the venturi.
























I /

















Fig. 19.- Response surface for equation (85).



Two levels of temperature were established and the analyses were

conducted separately for each one.

In venturi operation, the velocity of the fluid through the throat

is an important factor. This importance and the limitation of the pilot

plant in this aspect have been outlined previously. In an effort to

obtain some idea of this factor, a whole block design was conducted at

the higher level of temperature and at a lower rate of gas flow.




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