vvEPA
United States Industrial Environmental Research EPA-600/7-80-083
Environmental Protection Laboratory April 1980
Agency Research Triangle Park NC 27711
Sulfur Dioxide
Oxidation in
Scrubber Systems
nteragency
Energy/Environment
R&D Program Report
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EPA-600/7-80-083
April 1980
Sulfur Dioxide Oxidation
in Scrubber Systems
by
J.L Hudson
University of (Virginia
Department of Chemical Engineering
Charlottesville, Virginia 22901
Grant No. R805227
Program Element No. EHE624
EPA Project Officer: Robert H. Borgwardt
Industrial Environmental Research Laboratory
Office of Environmental Engineering and Technology
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460 I'ml
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ABSTRACT
This study relates the liquid phase oxidation of bisulfite and sulfite
anions to sulfate to the conditions in lime/limestone scrubbing systems such
as those used for removal of sulfur dioxide from power plant stack gases,
Experiments were carried out for the oxidation in both calcium sulfite
and sodium sulfite clear solutions and in calcium sulfite slurries. In the
slurry studies, both the rate of chemical reaction and the rate of solid to
liquid mass transfer were investigated, but with both slurries and clear
solutions, the experiments were run so that gas to liquid transfer of oxygen
was not a limiting resistance. A mathematical model for the dissolution of
solid particles and liquid phase chemical reaction was developed in conjunc-
tion with these experimental results, The oxidation was carried out over a
pH range of 4.0 to 5.5 (although some experiments were at pH 6 and pH 11),
at temperatures of 25°C to 50°C, and in the presence of iron and manganese
catalysts and several organic acid inhibitors. Batch reactors were used for
the slurry studies and for the slower clear solution oxidations while a flow
reactor was employed for faster clear solution reactions.
Although under special combinations of catalyst, pH, and organic acid,
the order was as low as one or rose to two, the homogenous chemical oxidation
rate of calcium sulfite oxidation is 1.5 prder over most of the range of
conditions. The rate increases strongly with increasing pH. Of the organic
acids studied, glycolic is the strongest inhibitor, followed by adipic and
succinic. Citric and acetic acids are less inhibitory than the others. Both
manganese and iron catalyze the reaction even in the presence of the organic
acid inhibitors. Oxygen concentration varied over a large range; its order
is 0.5 in the rate expression for the manganese catalyzed oxidation at higher
pH.
In a three phase slurry oxidation the overall reaction of sulfite to
sulfate declines with increasing pH; this decrease is caused by the sharply
reduced calcium sulfite solubility with increasing pH. This calcium sulfite
solubility was determined in independent experiments.
The results of the mathematical model for the dissolution and reaction
of calcium sulfite particles compare well with results of the three phase
slurry oxidations.
ii
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CONTENTS
Abstract • i:L
Figures v
Tables • • • • viii
1. Introduction . . . 1
A. General Considerations * 1
B. Background Literature 3
2. Experiments • 32
A. Apparatus • 33
1. Batch Reactor 33
2. Flow Reactor , 37
B. Procedure 44
1. Rate from Concentration Measurements 44
2. Rate from pH Measurements 46
3. Rates from Temperature Measurements 46
C. Analysis of Calcium Sulfite 48
D. Equilibrium Relationship for S 44Species 50
E. lodometric Titration 50
F. Analysis of Data 52
1. Analysis of a Single Experiment. 52
2. Multiple Regression Analysis 53
3. Analysis of pH Method Data 61
4. Analysis of Flow-Thermal Method 64
3. Oxidation in Calcium Sulf ite Solutions 69
A. CaSO- Oxidation (No organic acids) 70
B. CaSO^ Oxidation in the Presence of Organic Acids.. 71
1. Succinic Acid 83
a. Effect of pH on the rate of oxidation.... 83
b. Effect of succinic acid concentration.... 85
2. Adipic Acid 90
3. Glycolic Acid 90
4. Comparison of Rates of Oxidation with Succinic
Acid, Adipic Acid, Glycolic Acid, Citric Acid,
and Acetic Acid 96
5. Effect of Catalysts—Manganese and Iron 107
a. Succinic Acid 107
b. Glycolic Acid Ill
C. Reaction in Liquor from Penberthy Oxidation Runs at
Shawnee 118
D. Dependence of Oxidation Rate on Stirring Speed and
Oxygen Flow Rate 122
E. Rate of CaSO^ Oxidation at High Catalyst Concentra-
tions by Flow-Thermal Method 133
iil
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4. Oxidation in Slurries 139
A. Experiment 140
1. Solubility Studies 140
a. Theoretical Analysis 140
b. Experimental Results 141
2. Oxidation in Calcium Sulfite Slurries 143
3. Oxygen Flow Rate Studies 143
4. pH and Temperature Studies 146
5. Slurry Density Studies 153
6. Catalyzed Slurry Oxidation Studies 153
7. Liquid Phase Slurry Behavior 161
8. pH Behavior During Reaction 161
9. Slurry Reactions with and without pH
Controller 161
10. Liquid phase Catalyst Studies 171
B. Mathematical Model 171
1. General Description of Model 171
2. Physical Description of Particles 174
a. Electron Micrograph 174
b. Particle Size Distribution 175
3. Derivation of the Spherical Model. 175
4. Solutions of the Spherical Model. 179
5. Derivation of the Flat Plate Model 192
5. Oxidation in Sodium Sulfite Solutions 194
A. Na-SO^ Oxidation . . . 195
1. Manganese Catalyst 195
B. ^280^ Oxidation with Succinic Acid Buffer 195
1. Iron Catalyst 195
2. Manganese Catalyst 208
3. Mixed Catalysts. . 217
C. Rate of Na.SO- Solution Oxidation by the Flow-Thermal
Method 221
6. Conclusion 227
References . . . 233
iv
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FIGURES
Number Page
1-1 Effect of pH on the Relative Concentrations of Sulfur (4+)
Species in Solution .4
1-2 Electron Micrograph of Calcium Sulfite Particles - Sulfite
and Sulfate Particles . 8
1-3 Electron Micrograph of Calcium Sulfite Particle Individual
Agglomerate 8
1-4 Comparison of Gladkii's Figure 6 with Figure 5 23
1-5 Comparison of Slurry Oxidations at 50°C. . . 24
1-6 Comparison of Gladkii's Figure 6 with Soviet Slurry
Oxidation Rate Expression 26
1-7 Determination of Soviet Reaction Order .......... 27
1-8 Comparison of Slurry Oxidations at 40°C 29
2-1 Experimental Apparatus 34
2-2 Reaction Vessel 35
2-3 Tubular Flow Reactor 38
2-4 REPORT and CURVIT Programs 54
2-5 Element of Reacting Fluid 66
2-6 Sample Output for Flow Reactor 68
3-1 Rate of Calcium Slufite Oxidation; no added catalyst ... 72
3-2 Rate of Calcium Sulfite Oxidation; [Mn] = 0.6 ppm 76
3-3 Rate of Calcium Sulfite Oxidation; [Mn] = 0.53 ppm,
[Fe] = 1.02 ppm . 77
3-4 Rate of Calcium Sulfite Oxidation; variable [Mn] 79
3-5 Rate of Calcium Sulfite Oxidation; variable [Mn],
([S ] = 0.0037 mol/£) 80
3-6 Effect of Temperature on the Rate Constant ........ 81
3-7 Effect of pH on Rate of Reaction 86
3-8 Concentration Sulfur (4+) Versus Time for Various
Concentrations of Succinic Acid ..... 87
3-9 Concentration Sulfur (4+) Versus Time, Run 566, 0.2M
Succinic Acid 88
3-10 The Inhibition of the Sulfite Oxidation Due to Succinic
Acid 89
3-11 Initial Rate Versus Succinic Acid Concentration 91
3-12 The Inhibition of the Sulfite Oxidation Due to Adipic
Acid. • • /-+ • (^
3-13 Concentration S vs Time for Various Concentrations of
Glycolic Acid, pH = 4.0 97
3-14 Inhibition of the Sulfite Oxidation due to Glycolic Acid . 98
3-15 The pH Effect on the Inhibition of the Sulfite Oxidation
Due to Glycolic Acid 99
3-16 Effect of Glycolic Acid on Rate of Oxidation of CaSO , at
T=50°C, pH=4.0, [S ]=0.01M ....... 102
3-17 Comparative Strength of Various Organic Acids as
Inhibitors at 0.2M 103
v
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FIGURES (continued)
3-18 Comparative Strength of Various Organic Acids as
Inhibitors at 0.01M 104
3-19 Sulfur (4+) Concentration vs Time for Various Mn
Concentrations HQ
3-20 Effect of Mn on Oxidation Rate+of Succinic Acid Buffered
CaSO Solutions for [S ]=0.01M, T=50°C, pH=5.0 ... 113
3-21 Concentration Sulfur (4+) vs Time, 1000 ppm Glycolic Acid,
Various Concentrations of Mn 114
3-22 Effect of Mn Addition to Glycolic Acid Inhibited CaSO
Oxidation; rate at [S ] = 0.01M 116
3-23 Effect of Mn Addition to Glycolic Acid Inhibited CaSO,
Oxidation; Rate at [S ] = 0.01 M 117
3-24 Stirring Speed = 215 rpm '.'.'. 123
3-25 Stirring Sp,eed = 1800 rpm *'-.'' 124
3-26 Rate at [S ] = 0.01 M vs Stirring Speed . .' .' .' .' . .' .' .' 128
3-27 1.4 Order Rate Constant vs Mn Concentration in CaSO.,
Filtrate Oxidations ....... 132
3-28 Representative T vs t Results for CaSO- Oxidation. .'.'.'.' 134
3-29 Effect of [Mn] on CaS03 Oxidation Rate 136
4-1 Solubility of Calcium Sulfite at 40°C. . 142
4-2 Effect of Slurry Density on Oxidation Rate ........ 144
4-3 Typical Slurry Reaction Curve 145
4-4 Effect of Mn Catalyst on Slurry Oxidation Rate ...... 147
4-5 Slurry Oxidation, pH = 4.5 ' 143
4-6 Slurry Oxidation, pH = 4.7 "'.'.'.'.'.'. 149
4-7 Slurry Oxidation, pH = 5.0 .............. . . 150
4-8 Effect of pH on Slurry Oxidation Rates • • • ^
4-9 Slurry Oxidation, pH = 4.5 ' 154
4-10 Slurry Oxidation, pH = 4.5 '.'.'.'.'.'.'. 155
4-11 Catalyzed and Uncatalyzed Oxidations at 50°C 158
4-12 Effect of Catalyst on pH Behavior (40°C) ......... 159
4-13 Effect of Catalyst on pH Behavior (50°C) ......... 160
4-14 Slurry Oxidation, 200 ppm Mn, pH = 5.0 '. 164
4-15 Slurry Oxidation, 200 ppm Mn, pH = 4.5 165
4-16 Slurry Oxidation, 200 ppm, pH = 5.0 166
4-17 Slurry Oxidation, pH = 5.0 . . '.'.'. 167
4-18 pH Behavior During Slurry Oxidations; Effect of Slurry
Density . . . \ _ 168
4-19 pH Behavior During Slurry Oxidations; Effect of Initial
. • PH 169
4-20 pH Behavior During Slurry Oxidations; Highly Catalyzed
Runs 170
4-21 Slurry Oxidation with No Mn Added 172
4-22 Liquid Phase Catalyst Behavior During Slurry Oxidations. . 173
4-23 B-30 Program and Sample Calculation 181
4-24 Determination of ke from Highly Catalyzed Slurry Data
(2000 ppm Mn add^d) 187
4-25 Comparison of Total S from Model to Experimental Slurry
Oxidation Data for No Added Mn 188
VI
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FIGURES (continued)
4-26 Comparison of Model to Experimental Data for 6.66 ppm
Mn Added. . 189
4-27 Comparison of Model to Experimental Data for 200 ppm Mn
Added 19°
4-28 Comparison of Liquid S from Model to Experimental Slurry
Oxidation Data for No M£+ Added 191
4-29 Computer Predicted Liquid [S ] Profiles 193
5-1 Dependence of Rate on Sulfite Concentration 197
5-2 Dependence of Rate on Manganese Concentration 198
5-3 Sulfur S Concentration vs Time for Iron Catalyzed Na2S03
Oxidations 20°
5-4 Rate at [S ] = 0.028 mol/£ v£ Concentration Iron Added
for Sodium Sulfite Oxidations 203
5-5 Sulfur (4+) Concentration vs Time for Iron Catalyzed
Oxidations at Various Temperatures Using Na^O- . . . 204
5-6 Activation Energy Determination for Fe Catalyzed Na2Su3
Oxidations ' • • 206
5-7 Rate at [S ] = 0.028 M vs Concentration Fe 210
5-8 Sulfur (4+) Concentration vs Time for Manganese Catalyzed
Oxidations of Na9SO« Solutions 213
5-9 Rate at [S ] = 0.02S M vs Concentration Mn 215
5-10 Reduced Sulfur (4+) Concentration vs Time for Comparison
of Oxidation Reactions 218
5-11 Comparison of Mn Added Only to Oxygen Side or Only to
Sulfur Side 226
vii
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LIST OF TABLES
Number
1-1 Fly Ash Analysis 6
1-2 Limestone Analysis 9
1-3 Comparison of Literature of Sulfite Dioxide Oxidation. . . 15
2-1 Analysis of Calcium Sulfite Solid 49
3-1 Rate of Calcium Sulfite Oxidation; No added catalyst
(PH - 4.6) 73
3-2 Rate of Calcium Sulfite Oxidation; [Mn] = 0.6 ppm 74
3-3 Rate of Calcium Sulfite Oxidation; Added Mn and Fe . . . . 75
3-4 Rate of Calcium Sulfite Oxidation; variable [Mn] 78
3-5 Effect of Temperature on Rate Constant 82
3-6 Effect of pH on Succinic Acid Buffered CaSO» Solutions . . 84
3-7 Effect of Succinic Acid on Oxidation Rate. 92
3-8 Results of Order and Rate Determination Analysis of Data
for Calcium Sulfite Oxidation with Varying Concentra-
tions of Adipic Acid 93
3-9 Results of Order and Rate Determination Analysis of Data
for Calcium Sulfite Oxidations with Varying Concen-
trations of Glycolic Acid Added 95
3-10 Effect of Glycolic Acid on Oxidation Rate of Calcium
Sulfite 100
3-11 Results of Order and Rate Determination Analysis of Data
for Calcium Sulfite Oxidations with Varying Concen-
trations of Glycolic Acid Added 101
3-12 Effect of Organic Acids on Oxidation Rate of Calcium
Sulfite 105
3-13 Catalyst Impurity Levels in CaSO., Solutions 108
3-14 Effect of Mn Impurity on Oxidation Rate 109
3-15 Effect of Mn Concentration on Oxidation Rate of Succinic
Acid Buffered CaS03 Solutions 112
3-16 Effect of Manganese Addition to Glycolic Acid Inhibited
Runs . 115
3-17 Oxidation of Sulfite Solutions 119
3-18 Supplemental Analysis of Shawnee Sample 121
3-19 Effect of Stirring Speed - No added catalyst 125
3-20 Effect of Stirring Speed on Reaction Rate Constant - 0.5
ppm Mn. . 127
3-21 Effect of Oxygen Flow Rate on Oxidation in Unsaturated
Solutions . 129
3-22 Results of Filtrate Oxidations; pH,- = 4.5, T = 40°C. . . . 131
3-23 Effect of High Mn Concentration on CaSO.. Oxidation Rate . . 137
3-24 Effect of pH on the Rate of Mn Catalyzed CaS03 Oxidation. . 138
4-1 Results of Studies Showing Effect of Initial pH on
Oxidation 152
viii
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LIST OF TABLES (continued)
4-2 Effect of Slurry Load on Oxidation Rate at 40°C 156
4-3 Effect of Catalyst on Slurry Oxidation Rate 157
4-4 Experiments Made in Mn Catalyst Study 162
4-5 Experimental Conditions Used in Slurry Oxidations Shown in
Figures 4-5 to 4-7, 4-9, 4-10, 4-14 to 4-17 163
4-6 Size Distribution of Calcium Sulfite Particles (Coulter
Method) 176
5-1 Na SO Oxidation • 196
5-2 Results of Sodium Sulfite Oxidation with Iron Catalyst
Added 202
5-3 Results of Iron Catalyzed Reactions at Various Temperatures
Using Sodium Sulfite 205
5-4 Results of Sodium Sulfite Oxidations with Varying Initial
Sulfite Concentration and 5 ppm Iron Added 207
5-5 MULTREG Results for Iron Catalyzed Na2S03 Oxidations. ... 209
5-6 Results of Sodium Sulfite Oxidations with Manganese Added . 211
5-7 Regression Analysis of Manganese Catalyzed Na^O^
Oxidations 2l6
5-8 Results of Sodium Sulfite Oxidations with Iron and
Manganese Added.
5-9 Multiple Regression Results for Mixed Catalyst Sodium
Sulfite Oxidations 22°
5-10 Effect of [Mn] on Na2SOa Oxidation Rate 223
5-11 Effect of pH on Na2S03 Dxidation Rate 224
ix
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SECTION 1
I. Introduction
A. General Considerations
B. Background Literature
INTRODUCTION
A. General Considerations
In the removal of sulfur oxides from stack gases by any of the scrubbing
methods, a fraction of the sulfur compounds is oxidized to sulfates. This
oxidation occurs whether the scrubbing agent is a slurry (lime/limestone
systems) or a clear solution (double alkali or other such systems). Further-
more, the oxidation occurs not only in the absorber, but also in other sec-
tions of the system, such as in the hold tanks.
In lime/limestone scrubbing systems S02 oxidation is important for sev-
eral reasons. (The species actually taking part in the reaction is either a
bisulfite or sulfite ion depending on the pH of the solution; however, it is
convenient and it is common practice to refer to sulfur dioxide or calcium
sulfite oxidation.) The oxidation in the scrubbing system can increase the
degree of supersaturation of calcium sulfate, leading to an increase in the
rate of gypsum scale formation (183, 30). Thus, it is important to limit
calcium sulfite oxidation in some systems. On the other hand, calcium sulfate
is preferable to calcium sulfite from the standpoint of solids disposal since
the sulfate has a higher settling velocity than the sulfite and the sulfate
also has a higher compaction and a lower chemical oxygen demand (31-36). The
latter is important from water pollution considerations. Therefore, in some
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limestone scrubbing systems it may be desirable to promote or inhibit oxi-
dation.
Oxidation in lime/limestone systems, both in the scrubber and in the hold
tanks, is a very complicated process. The rate of oxidation can depend on
chemical kinetics since the rates of liquid phase reactions of sulfite ion and
bisulfite ion vary with concentration, with the presence of catalysts or
inhibitors, and with the type of oxidizing agents, viz., oxygen or nitrogen
oxides. (It should be noted that very small concentrations of some catalysts
can influence the reaction greatly and that, therefore, many reactions which
are supposedly being run without catalysts are in fact being catalyzed by
impurities present.) The rate of oxidation is also influenced by phase and
chemical equilibria. There are three phases present in the scrubbing system,
solid, liquid, and gas. Both the phase equilibria and chemical equilibria are
strongly dependent on pH. For example, lowering the pH in the presence of
calicium sulfite increases the solubility and, therefore, increases the con-
centration of dissolved sulfur containing species. This can increase the rate
of oxidation under some conditions, particularly, say, in a hold tank. How-
ever, a lowering of the pH of scrubbing liquor in the absorber converts sul-
fite ion to bisulfite ion; this would lower the rate of oxidation since
sulfite ion is oxidized much more quickly than bisulfite ion in a clear liquor
(but not in a slurry where solubility effects are important). Finally, the
rate of oxidation can be influenced or controlled by mass transfer, viz., by
the rate of transfer of oxygen to the liquor and by the rate of dissolution of
solid particles.
The complex sulfur dioxide oxidation process was broken down into steps
and each step studied separately. The oxidation of calcium sulfite and sodium
sulfite clear solutions was inventigated as a function of manganese and iron
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catalysts, pH, temperature, and organic acid inhibitors. The rate of dissolu-
tion of calcium sulfite particles in a stirred liquid was determined and this
information, along with the kinetic rate results, were used as a basis for a
mathematical model for the dissolution and reaction of calcium sulfite parti-
cles . Measurements were then made on the rate of oxidation in calcium sulfite
slurries and the results compared to predictions of the model.
B. Background Literature
Atmospheric sulfur dioxide affects our lives in many ways. It is a
health hazard (9, 10), corroder of structures (9.4) and equipment, visibility
impairer (106), and habitat modifier (88, 166). Man's activities of smelting
and burning fossil fuels among others have significantly altered the world
sulfur budget (97), threatened his health (10, 57), and now require control
(60, 61).
Control methods include: wet scrubbing, dry sorbents (furnace injection)
(144, 161), catalytic oxidation (5, 27, 37, 38, 73, 121, 123, 146, 164), dry
filters (and baghouse add-ons) (1), and fuel precleaning (111).
Today, wet scrubbing is the most widespread method (59) for cleaning S02
from stationary sources. When the S02 enters any scrubbing liquor, the fol-
lowing equilibria are set up (154):
S02 + H20^=^S02-mH20^=^HSO; + H+^=^So|" + H+
low *-pH — —*" high
These relationships are shown graphically in Figure 1-1. Water-only
scrubbing is ineffective. Removing the H+ in the last two equilibria above
effectively shifts the concentrations to the right, thereby sequestering the
sulfur dioxide as bisulfite on sulfite anions. All of the scrubbing methods
-------
1.0.
0.0-
0.6—
Hole
Fraction
0.4-
0.2-H
0.0.
HS03
Conditions
40°C
0.02 M Na SO
•'""'3
1 T
pH
T
8
S03
2-
10
Figure 1-1. Effect of pH on the Relati.ve Concentrations of Sulfur (4+) Species in Solution.
-------
turn on this principle of tying up hydronium ion by the addition of many kinds
of alkalis (eg. CaC03, CaO, NaOH, NH3). The resulting sulfite/bisulfite solu-
tion can be discarded (these are "throwaway" processes) or regenerated (132,
177). Overall, the scrubbing process consists of: Gas-Liquid mass transfer
from the stack gas, equilibria and reaction in the scrubbing mixture, modula-
tion by Solid-Liquid transfer of absorbents (and possible Liquid-Solid trans-
fer of products).
The Gas-Liquid mass transfer is accomplished in several ways (90): spray
tower, venturi (89, 98, 175), packed bed (159) and turbulent contact (moving
ball) absorber (113). The efficiency of the G-L contacting step has been the
subject of much study to assess major resistances (120, 178) and control them
(142). Since S02 is chemically transformed as it enters (102) the liquid
surface (from bubbles (104) or surface films (11, 46, 58)), the rates are best
treated as absorption with reaction and described by parallel approaches viz.,
film theory (47, 85, 157), penetration theory (40, 41), or surface renewal
(51, 52, 143).
From coal fired boilers, stack gas entering the scrubber carries (53) '4
to 7% 02; various oxides of nitrogen; C02, CO, and organic residues; as well
as inorganic residues entrained as fly ash (including chloride (29, 114) and
metals (105) as well as inert particulates (64, 77). Some scrubbing methods
remove particulates (69) as well as sulfur oxides, but these particulates
(inorganic and other coal debris) influence the scrubbing process as catalysts
and inhibitors of sulfite oxidation. See Table 1-1.
The sulfur oxides rapidly (23) form a potentially reactive mixture des-
cribed by the S02 dissociation equilibrium (99). The first ionization con-
stant being 1.74 x 10"2 (167), the second 6.24 x 10~8 (189). The equilibria
have been studied for S02/water systems (137, 169, 170) and scrubbing systems
(124, 144).
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TABLE 1-1. FLY ASH ANALYSIS
Chemical Analysis
Component Weight Percent
A1203 20-30
Fe203 12-23
CaO 2-7
0.5-1.5
6
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The most common absorbents (solids) are lime, limestone and dolomite
found as crystalline aggregates (see Figures 1-3 and 1-2) and solid mixtures
(78). Their solubility has been determined (65, 82) and noted in scrubber
operation (29, 70). These soluble minerals provide the carbonates and hydrox-
ides to fix the S02 in solutions. An analysis (43, 79) is given in Table 1-2.
2-
The dissolved sulfur species HSCL and S03 are subject to oxidation in
2_
solution by dissolved oxygen producing SO^ . This gypsum is produced in
lime/ limestone throwaway processes and can lead to supersaturation in all
calcium-based processes causing scaling (29, 56, 77). Scale formation can be
controlled by additive controlled oxidation (below 20%) (29) and gas pre-
treatments. The oxidation occurs along the trajectory of the S02 as it enters
the absorbing solution (127). Hydrodynamic conditions (151, 168) and mass
transfer to the solution (169, 170) determine the region where reaction
occurs. The limit of this region, called the reaction plane, determines what
theory best describes the rate of absorption (80, 120). For instance, when
the film is large (equal or greater than the diameter of the small particles
of solid), dissolution can occur in the reacting zone, further increasing
reaction and enhancing absorption (138).
One approach to scaling control is limitation of oxidation by the addi-
tion of an inhibitor (50, 77, 147). Many organic materials (especially those
bearing hydroxyl groups) inhibit the oxidation. This action is not likely to
be true catalysis (20, 50, 140), hence the inhibitors may be subject to con-
sumption and would require replacement. Factors to consider with regard to
inhibitors are:
i
Inhibitor must be water-soluble.
Its actions may be modeled well enough to limit oxidation in part
of the system.
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Figure 1-2. Electron Micrograph of Calcium Sulfite Particles
Sulfite and Sulfate Particles
Figure 1-3. Electron Micrograph of Calcium Sulfite Particle
Individual Agglomerate
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TABLE 1-2. LIMESTONE ANALYSIS
Chemical Analysis
Component Weight Percent
A12°3
Fe203
MnO
CaO
Cl
Cu
Cr
As
Hg
6.01
0.19
0.06
55.5
0.004
0.00044
0.00011
0.0002
6xlO"6
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Products formed from inhibitors may also retard (or promote) the
oxidation.
The contacting method in the scrubber changes the effectiveness of
inhibitors (43).
Fly ash may contain phenolic organic residues active as inhibitors.
Recently organic acids used to enhance limestone solubility were identi-
fied as rate retarders. The most extensive set of experiments dealing with
the organic acid effect of both the sodium and the calcium sulfite oxidation
are reported by Hatfield, Kim and Mullins (81). It was reported that in the
sodium system, organic acids promoted the sulfite oxidation with the order
being: adipic > glycolic > no acid. However, in the calcium system organic
acids were found to reduce the oxidation with the order being: glycolic >
adipic > no acid. The results dealing with the calcium system are comparable
to the results of the present work, dealing with the comparative inhibitory
strength of the organic acids. The relative strength was: glycolic > adipic
> succinic > no acid.
There are large discrepancies between the current results and those of
Hatfield et al. for the rate of oxidation of the sulfite. Hatfield et al.
reported that the sulfite oxidation goes toward completion in several hours,
but the results of this work show that the sulfite goes to sulfate in a matter
of minutes (usually less than 20 minutes even with inhibitors added).
Hatfield et al. added CaCO (calcium carbonate) to the reaction mixture for pH
control and used an oxygen flow rate of only 20 ml/min. The present exper-
iments used the addition of NaOH for pH control and a flow rate of oxygen at 3
A/min to maintain 0^ saturation in the reactor. This suggests that while the
CaC03 or the NaOH may have an effect on the sulfite oxidation, the experiments
by Hatfield et al. were oxygen limited (controlled by the oxygen flow rate
10
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rather than the sulfite oxidation kinetics). These workers also determined
the rate of reaction to increase with pH, which is in agreement with the
current findings.
In a recent work Altwicker (8) found hydroquinone caused a changing
reduction in the oxidation rate which he attributed to inhibitor in the re-
action plane being consumed quicker than it could be replenished, casting
doubt on the sulfite method for measuring interfacial area in contactors.
The scaling problem is greatest in the scrubber and mist eliminator. By
2-
inducing oxidation elsewhere in the system, supersaturation in SO, will not
happen. For this reason various approaches to forced oxidation in the holding
tank beneath the scrubber have been studied (29, 71). For example: forced
oxidation of the calcium sulfite reaction product in both lime and limestone
FGD systems has been successfully demonstrated in 10 megawatt prototype units
at the Shawnee Test Facility. The oxidized gypsum product (74) results in
less disposal volume and settles by an order-of-magnitude faster than the
unoxidized material. It filters to better than 80% solids and handles like
moist soil compared with the unoxidized material which filters only to about
50 to 60% solids and is thixotropic (83).
Improved settling is treated elsewhere as well (110, 124, 132, 161), and
good sulfate removal has been discussed (29). The problem of scrubber scaling
is eliminated in sodium-based liquors (56, 77, 110, 132, 161), but the added
cost of sodium absorbents (49, 69, 110) necessitates regeneration of the
"alkali values" of the sodium absorbent usually with a calcium base
(lime/limestone) that is discarded.
Recovery oriented, sulfur-concentrating processes are usually gaseous
(27, 128, 165), but some processes (55, 92) use a liquor, operating by electro-
lytic regeneration (92). The most prominent (68, 77, 103) two loop process is
11
-------
the double alkali process (dual alkali), featuring (56, 96, 110) efficient,
higher pH scrubbing with sodium liquor at low L/G rates, reduced scaling, and
thorough limestone utilization, but may have disposal problems.
Other companies which have utilized liquid phase oxidation in wet scrub-
ber flue gas desulfurization processes include Babcock and Wilcox
(magnesia-base wet scrubbing) (13), Wellman-Lord, Stone and Webster/Ionics
(63), Aerojet-General Corporation (zinc oxide process) (6,92), and Monsanto
(CALSOX system) (87). There are many other systems including ones based on
magnesia (100, 158).
Many models for the physical and chemical events underlying these removal
processes have emerged beginning with the equilibria among the solution ionics
(133) and a generalization of the oxidation in solution (109, 153). The real
challenge is in formulating (47) the complex equilibria, dissolution rates,
absorption rates (40), and reaction rates in a way suitable for computing
concentration profiles, removal rates and efficiences, and absorbent effi-
ciencies in the scrubber (114) and holding tank (175).
Any description requires the reaction kinetics of sulfite oxidation in
solution. Indeed, this topic has been the subject of multifaceted research
for 132 years yielding sure findings only slowly.
The oxidation is a free radical chain process subject to photochemical
initiation, the quantum efficiency of which is 5 x 104 (16). The impurity
level of the work that produced this figure raises doubt about its validity.
Further work shows no influence of UV on the oxidation rate in aerosols of low
pH (95). The most careful photochemical study concluded that there is no
simple relation between the light absorbed and the rate (112).
Experiments using various surfaces and particulate additives (86) found
no variation with contact area of catalyst. This well establishes the reac-
12
-------
tion as a case of homogeneous catalysis. Other workers (20) who exceeded
solubility limits of catalyst suggested the subsequent particles might have
provided sites for heterogeneous catalysis, but this is not the case (18).
The effect of impurities is dramatic. The sensitivity of the reaction
rate to stray metal ions is the hallmark of the reaction, but inorganic anions
(188) as well as organic molecules affect the rate (although in the opposite
way). In fact, the bane of much experimental work was rubber in the apparatus
(48, 115, 140, 171). Pure gum rubber stops the reaction (112) giving evidence
that the sulfur vulcanizing chemicals are not the cause. The degree of inhi-
bition by hydrocarbons is so sensitive that oxidation has been suggested as a
semiquantitative analysis for their presence (152). This sensitivity is an
important consideration for operations in rubber-lined slurry handling equip-
ment, such as those at Shawnee Valley.
-12
The acceleration of the rate by metals is extreme: 10 M copper ion
added during sulfite oxidation increased the rate (171). Many metals have
been studied:
CQ 7, 21, 42, 44, 54, 95, 107, 108, 109, 130, 126, 149, 163, 185, 187
r 7, 17, 21, 66, 84, 95, 109, 150, 140, 172
L»U
Mn 21, 42, 48, 75, 86, 93, 95, 179
Fe 21, 39, 84, 86, 93, 95, 140
21, 42, 179
Mg
21, 39
Ni 21, 93, 140
Al
Zn 21' 179
Ca 179
xra + 21, 45, 95, 115, 140, 152
JNI14
13
-------
General comparisons have tested their relative effectiveness:
Co, Ni > Cu > Fe ref 140
Mn > Co, Ni > Fe, Cu > Mg, Zn, Na, NH4+ ref 21
Mn » Zn, Mg > Ca ref 179
Mn > Cu > Fe > Co, NH > Na > uncat. ref 95
329 199 167 49 49 4 3 = Relative strength
Mn -\. 7000 x Mg ref 42
The most effective catalysts are clearly manganese and cobalt with iron giving
a notable effect. The metal showing the least effect is sodium (21, 91, 95).
For this reason, the spectator ion is nearly always chosen as sodium. Am-
monium, a constituent in many natural systems, increases the rate of oxida-
tion. The probable cause is not catalysis, but rather its effect on pH may
merely shift the equilibrium toward sulfite which is most rapidly oxidized.
Several workers (20, 45, 136, 150, 152) have considered rionmetallic catalysis
and mechanistic interactions of sulfur compounds. The very few experiments
have no salient results.
One curious observation is that sulfate formed by the reaction has less
effect than initially added sulfate upon the reaction rate (111, 134). No
satisfactory answer exists (42, 66, 141), but control of the position of the
sulfur species equilibrium is one candidate. An early controversy concerned
bisulfite oxidation rates increasing with dilution (17, 115, 119). The spur-
ious effect was due to decreased oxygen mass transfer at higher concentrations
(140).
The oxidation in all pH ranges has been studied with wide concentration
changes of sulfur species and catalyst and some slight changes of oxygen
concentration. Many of the important results in the literature are compared
Table 1-3. Although no firm results stand out, the many contrasts serve to
14
on
-------
TABLE 1-3. COMPARISON OF THE LITERATURE OF SULFUR DIOXIDE OXIDATION
Reference
lloalhcr &
Toodeve8'1
Coughanowr &
Krause"8
Walter1"
Powell135
Catipovlc1''2
Chen 6.
Barion &
Llnek & 108
Alper7
Relnders &
Fc"et^
Phillips i
Johnson
DeWaal &
Ok on 54
Uessellngh &
LI nek &
Tvrdlk109
Tek^
Saulckl f.
Onda129
Vagi i
I not187
Year
1934
1965
1972
1973
1974
1972
1966
1970
1973
1925
1941
1959
1966
1970
1971
1973
1973
1972
1962
S
Species Cone, H Order
II SO 5-50xlO-4 1
3 2.5xlO"5 0
SO.'H.O 1.7xlO"3 0
1 1.9x10-3 o
S02'H20 0.009 0
HSOj" 0.03-.09 1
IISO." .001-. 01 0
SO 2' 0.009 1
S032" .009-. 03 3/2
S032" 0.04-0.4 3/2
S032" 0.25-0.8 0
SO 2" 0.8 „
0.5-1.0
S032" 0.06 1
SO.2" 0.01-0.05 1
SO ,2~ .001-1.0 1
3 0
S032" 0.8 1
S032" 0.8
SOj2" 0.3-0.8 0
S032" 0.8 0
S032" 0.1-0.3
SO 2" 0.3-1
S032" 0.008 1
°2
Cone, H Order
2xlO"3 I
0
3-6xlO"4 0
4-8x10-4 0
1.2xlO'3 0
1.2xlO"3
l.lxlO-3
1-4x10-4 0
0-0.003 0
2-llxlO"4 1-2
,3-2xlO"3 2
1-8x10-3 1
l.lxlO'3
l.lxlO"3
1-7,10-* \
l.lxlO"3 1
1-11x10"* 2
6.4xlO"4 1
SxlO-* 2
l.lxlO"3 2
.3-3xlO"3 1
2
2-llxlO"4 2— 1
l-15xlO"4 1
Cat Ident Cone, M Order
Mn 3xlO"6-8xlO-5 2
2.7x10-4 2
Mn 1.8xlO-3-0. 18 2
Mn 1.8-9x10-5 2
MB 0.036-0.82 0.7
-
Mn 6.6-19.9X10"6 l
Co 10"7-3xlO"6 1/2
Cu ID'4 i
Co 0-10° 1/2
C" 1°~3_5 .3 t
Co 4x10 -10
Cu 10"5-10"3 1
Cu 10-9xl0'4 1
Cu .5
Cu.Veraene 10
Co 10"3
Co 4x10 * 1
Co 5xlO"6-10"3 1
Co 8.8xlO"6-10"3 1
Co 5xlO"7-10 6 1/2
Co 10"9-5xl06
.7
Co 0.5-7.0x10 1
Contacting
Method
6 1 i r r 1 n«
stirring
II «. R "T"
stirrinB
stirring
stirring
H & R
II & R
stirring
stirring
Pack col
stirring
stirring
low turb
high turb
Wet wall
Met wall
Stirring
Wet Wall
Wet Wall
Wet Wall
Pack col
bubbling
Temp
40°c
l-s°r
25°C
25°C
25°C
25°C
25°C
25°C
25°C
35°C
25"c
10°C
25UC
25°C
30°C
30°C
30°C
25°C
^°r
30°C
25°C
30°C
20°C
pll
7
5
-
-
1-2
1-4
1.1
-10
-
8-9
9.2
B.I
10
R.7
9.2
7-9
8.5
7-9
7-9
10
8.5
8-9
k
-
m O7i in-7 ™ole
.03-22.1x10 j gec
1.3-16x10-1° f-i^
n i i it ,n-lmole
0.3-2.46x10 ^ se-
1.66xlO'5sec"1
,~-R m
0.053s*c-l l "C
•> 7 nr^in-5 mole
3.2-66x10 j"sec
1200 1/g mole sec
19.3 sec l
-
-
0.013 sec *
2.5x10° I/mole sec
-
50 — 104sec"1
-
1-5x10* sec"1
1.43x10 I/mole sec
2.3xl06 I/mole sec
-
-i.Ar52^— s o
l-sec
+.4 l+1.46xlO"7 Cs
S 0
Comment A
impurity likely
Ea 32^4^. obs. mass trans
ll.il"01*
inert apparat u«, Dl)l
gave great care to purity of
mat'ls. Ea 18.7 kcal/mole
Ea 17.5 kcal/mole
Ea 18.3 kcal/mole
no effect due to stirring st
hip.h speed
stirring changed 02 order
NH^ present
no effect of added acid
inh drops rate to 10"s
Versene likely effect on
mech
Cat solubility exceeded Ea
12 kcal/mole
Ea 10.53 kcal/mole absorp-
tion Indep of hydrodynamics
0 order change sharp
Ea 15 kcal/mole
added SO, : no effect
Ea 12.4 \cal/mole
0- order switches with
Cat Cone
0, order change pradual
-------
define the main kinetic questions. The only problem comes from the varying
conditions in the experiments which render some data uncomparable.
The order in sulfur is usually zero or one, but this depends on the pH
range. For low pH the HS03" order seems to be 0: at high pH the S032" order
may be one. There are also changing order results.
The oxygen order is the most controversial and has the most complicated
fluctuations. The order has been observed to shift from zero to two as the
sulfite concentration changes (12), switch between one and two as the catalyst
concentration increases (148), and to vary as its own concentration changes
(129). Whether the change is gradual (129) or abrupt (109) is a question.
Catalyst dependence is stronger than that of the reactants, but the value
is unsettled. Mn and Co produce orders of magnitude change in the reaction
rate by variations of only a few ppm in their concentrations (48). Mg, con-
versely, produces a sluggish, but steady, increase in the oxidation rate as
its concentration changes over orders of magnitude (42).
Experiments on the pH are scattered widely in the literature. The treat-
ment by Fuller and Crist is worthy of note (66). (Linek and Tvrdik also treat
the subject well). (109). Because the effect of pH is so closely linked with
the sulfurous solution equilibrium, no clear chemical trends emerge. At
extremely high pH ( >12) , however, the rate is depressed (19, 115, 190). This
behavior is sometimes due to reduction in oxygen solubility (118). Never-
theless there appears to be a genuine chemical effect, for the rate reduction
occurs as well for oxygen already in solution (101).
The energetics of the reaction are not as fully examined as the kinetics.
All the works report operating temperatures between 20°C and 40°C. The low pH
activation energy is around 20 kcal/mol, at high pH it is near 15 kcal/mol.
16
-------
The published rates vary widely in first and second order kinetic equa-
tions. The values appearing in Table 1-3 are for conditions of low pH where
half-lives of a day are common as well as instances of high PH reacting to
half concentration in 0.01 sec and less.
In the low pH reaction, the stirring also causes discrepancies. Schultz
and Gaden (155) report a decreasing reaction rate with increased stirrer
speed. This finding touched off studies (131) that have still not resolved
the ambiguity. Also at low pH no true (86) induction period has been ob-
served, although start up lags due to physical causes do occur (152). At high
pH an induction period up to 2 sec is possible (48).
The above questions all touch on the molecular activity making up the
reaction mechanism. And, just as these specific questions remain open, the
final elucidation of the oxidation mechanism remains to be done. It is gener-
ally accepted (66) that the oxidation proceeds by a free radical chain- with
or without metal participation. Many aqueous sulfur radicals possibly in-
volved are characterized (190). The early suggestions by Haber (76) and
Titoff (171) have given way to the proposed mechanism of Abel (2, 3, 4) and
Backstrom (14, 15, 16). A few points of interest to all mechanisms are: the
oxygen atom transferred does not come from the solvent (186), the uncatalyzed
reaction occurs by initiation other than by stray metal ions (66), the acti-
vation energies in the catalyzed reaction agree closely with those for some
metal-ligand substitutions of the catalyst (108). The results of some recent
oxidation studies in slurries have given better definition to the oxidation
problems.
Ramachandran and Sharma proposed a model for gas absorption in a three
phase slurry system. An instantaneous chemical reaction was assumed to occur
in the liquid boundary layer surrounding the solid particles. Two cases were
17
-------
examined in their model. The first case assumed that solid dissolution was
unimportant in the rate of gas absorption. The second case assumed that solid
dissolution into the liquid phase was an important factor. Results from the
second case showed the rate of gas absorption to be proportional to the square
root of the concentration of the solids present.
Bjerle, Bengtsson, and Farnkvist (28) conducted an experimental examina-
tion of CaC03 slurry oxidation in a laminar jet absorber at 25°C and 45°C and
pH 8.5. In their weak (2%) slurry they found the S02 mass transfer coeffi-
cient to be nearly that in an otherwise identical clear solution (clear
kS02 = 1<19 x 10~ Xs. slurry k: 1.38 x 1(T4 mol/(cm2sec atm).
Uchida, Wen and colleagues have conducted a series of studies on slurry
oxidation. In 1975 (184), using a continuous stirred tank reactor to study
the oxidation of sodium and calcium sulfite solutions by air, they found that
for sodium sulfite concentrations of 0.1 M, the reaction was mass-transfer
limited up to at least 700 rpm impeller speed. The absorption rate was a
function of both gas velocity and impeller speed. They also found that the
rate of oxidation showed a dependency which was slightly greater than first
order in impeller speed and oxygen concentration, slightly less than zero
order in hydronium ion concentration, and independent of sulfite concentra-
tion.
In oxidizing calcium sulfite solutions, it was determined that the
oxidation was first order in sulfite, and reacted faster at pH 4 than pH 7.
Addition of copper had no catalytic effect on the reaction. Finally, they
found that a 2 g/£ calcium sulfite slurry reacted much faster at pH 7 than the
saturated solution.
In 1977, (174) Uchida and Wen extended the model, applying it to six
cases: I) no solid dissolution in the liquid film near Gas-Liquid interface;
18
r
-------
(a) reaction plane near the gas surface, b) reaction plane near solid sur-
faces, (c) reaction plane near the G-L surface and the bulk is constant at
saturation. II) Solid dissolution considerable in the liquid film near the
G-L interface; (a) reaction plane near G-L interface and [S lbulk at satur-
ation value, (b) reaction plane is near G-L interface and is less than sa-
turation, (c) reaction plane surrounds the solid particles. Concentration
profiles and tests for each case are given and a numerical example is shown' by
Uchida and Wen.
In a closely related study, Sada, Kumazawa, and Butt (145) modeled sulfur
dioxide oxidation and reaction in a Mg(OH)2 (0.5 to 10 at wt%) slurry using
the film theory. They took exception to Uchida's driving force for dissolu-
tion and recast the equations in terms of ([S lsurface ~ ts D and use<^ a
different mass transfer coefficient for the liquid film and the bulk.
A recent work by Uchida (173) was a deeper experimental study of lime-
stone slurry absorption of S02. He found particle size of the solid to
influence the S02 absorption rate indicating that the solid dissolution rate
is important.
In a TVA report (134), slurry oxidation was studied in a spinning cup
oxidizer. The degree of oxidation was determined by infrared analysis of the
solids. Various salts were added to the solution and it was found that mang-
anese and iron compounds were strong catalysts, nickel and vanadium had no
effect, and cobalt, copper and thiosulfate ions suppressed the reaction.
Tests also confirmed the fact that oxidation was faster at the lower pH
values.
Representative (30) of the extensive (31-38) articles by Borgwardt, is an
2_
interesting study of unsaturated (with respect to SO^ ) closed loop scrubber
operation at the pilot plant scale. The key to stable no-scale (undersatur-
19
-------
ated in S04 ) operation is limiting the oxidation of S+4 to 20%. In this way
the sulfate that is formed is cleared from the system as a solid solution with
the precipitating sulfite.
In another case, enhanced oxidation was examined. Using a pilot scale
scrubbing system with a forced oxidation tank, Borgwardt (29) found the oxida-
tion of calcium sulfite was limited by the mass transfer of oxygen from the
injected air into the liquid. The pH in the oxidation tank was 4.5. The mass
transfer limitation was eliminated by increasing the residence time of oxygen
in the reactor. An air stoichiometry of 2.6 was used in this investigation.
The product solids have improved settling and dewatering characteristics.
In a work similar to the current one, Gladkii at the State Scientific
Research Institute of Industrial and Sanitary Gas Cleaning at Moscow (70)
found results of immediate application. For this reason the following de-
scription and comparison is more thorough. Slurries of calcium sulfite were
oxidized in an agitated bubbler reactor. The initial concentration varied
from 300-1800 ppm, and temperature ranged from 40°C to 62°C. lodometric
titrations were used to find the total sulfite concentration, and the pH was
monitored continuously. The authors found that the reaction was in the kine-
tic-limiting regime above 700 rpm, since agitator speed, size and concen-
tration of solids, and quantity of air did not effect the oxidation rate. The
reaction began with an initial drop in pH from 6.8 to about 5.8, followed by a
long stretch where pH remained constant, and finally a sharp drop to pH 3.2 at
the end of the reaction. During the first pH drop the reaction rate acceler-
ated due to the increased concentration of bisulfite in solution. In the
constant pH region the oxidation rate also remained constant. The final pH
drop reflected the depletion of the solid sulfite, and the rate order in
bisulfite.
20
-------
The authors then varied the initial pH of the reacting solution from 3.6
to 6.0. The induction period of accelerating rate was not observed at initial
pH values lower than 6.0. The rate during the constant pH period was used as
a measure of reaction velocity and it was found that this velocity reached a
maximum at pH 4.5. Both of these facts (the induction period and the rate
maximum) are used to support the contention that the dominant reacting species
is the bisulfite ion.
Another significant result of the Russian research is an empirical cor-
relation between pH, temperature, and the liquid phase concentration of a
saturated calcium sulfite solution. This equation is combined with the deter-
mined first-order rate in sulfite to give a rate constant equal to 0.85 £/min
at 40°C and an activation energy of 21.5± 0.5 kcal/mol.
The catalytic effect of manganese sulfate was then studied, and it was
found that a concentration of 100 ppm MnS04 greatly accelerated the oxidation.
Furthermore, it reduced the activation energy to 13.8 ± 0.5 kcal/mol and the
order in sulfite to \.
Some additional comments and comparisons with the current work at the
University of Virginia are in order:
Comparison of Figure 5 and Figure 6 of ref (70): The rates given in
Figure 5 were checked to determine if they agree with the rate vs. pH diagram
in Figure 6. The rates in Figure 5 were taken as the linear portions of the
C~ „__ vs time curves for the various pH's. (Note that the CaSO» was given
LabOJ — J
in grams of CaS03 per liter.) The results were as follows:
21
-------
@ pH = 6
Sl°pe = V-'sO = °'°95 8/<>min)
R = (0.095 g/£-min) (1/120 g/(g-mol)
= 0.791 x 103 g-mol/(£ min)
@ PH = 5 R = 3.84 x 103 g-mol/(£-min)
@ PH = 4.5 R = 4.58 x 10s g-mol/U-min)
@ PH = 3-6 R = 2.28 x 103 g-mol/(A-min)
The points were plotted on Figure 1-4 showing good agreement with the pub-
lished curve, with the exception of the point at pH = 4.5.
Comparison of Figure 6 and Figure 7 of ref (70): Using the line at 40°C,
the concentrations of S02 were determined from Figure 7. These concentrations
were then used to calculate the reaction velocity. The reaction velocity was
then plotted on Figure 1-5. The equation used for calculation of the reaction
velocity was:
W = kcso2
where w = reaction velocity, gmol/(j2.-min)
k = 0.85 min'1
Only concentrations taken between pH = 4.5 and pH = 6 were used for this
comparison. In this pH range the SO present will exist as HSO~ At pH
values of 4.5 and less, considerable SO -ELO will be present in the solution
which will tend to give a lower specific reaction rate. Results were as
follows:
@ pH = 6.0
Ccn = 7.94 x 10"4
b°2
R = 0.85 (7.94 x 10~4)
= 6.75 x 10"4
22
-------
FIGURE 6,
Dependence of Reaction Velocity of
Oxidation (V, mol/*. x min) on pH of
Suspension at a Temperature of AO°C
X = Calculated points
o = Original points in paper
Figure 1-4. Comparison of Gladkii's Figure 6 with Figure 5 (70)
23
-------
•H
a
o
35-
30
25«
20-
u
O
>
e 15
O
(9
10
3.1 4.0
4.5 5.8 5.5
pH
calculated Russian results
experimental data
University of Virginia
Figure 1-5. Comparison of Slurry Oxidations at 50°C
-------
(9 pH = 5.0 R=4.26x 10~3 g-mol/(£-min)
@ pH = 4.7 R = 0.85 x 10" g-mol/(£-min)
These results were then plotted in Figure 1-6 giving good agreement between
the two curves except at pH = 4.7.
Comparison of Figure 7 and Figure 8 of ref (70): Assuming that the re-
action being studied is first order, the rate expression can be given by
w = k • CGn g-mol/(£-min)
bU2
or
log w = log GSO + log k
This is in the form of y = mx + b with a slope (m) equal to 1. However, the
slope in Figure 8 is computed as 0.5.
A new graph of log w vs log CQ~ was generated from the data given
— bU2
in Figures 6 and 7. Reaction velocities were obtained from Figure 6 and
concentrations were obtained from Figure 7 at 40° C for various pH values.
pH w log w log C
-3 ... -3.1
-2.77 -2.8
-2.59 -2.6
-2.41 -2.2
-2.25 -2.0
A new plot was made of log w v£ log C . total on Figure 1-7. This plot shows
2
a measured slope of 1.01 which is indicative of the reported first order
reaction. The reaction rate was calculated from the first graph and found to
be 0.85 which is in agreement with the published report. Therefore, Figure 8
as published must be in error.
6
5.6
5.3
5
4.7
0
1
2
3
5
.791 x 10
.7 x
.6 x
.84 x
.5 x
io-3
io-3
io-3
io-3
25
-------
X
W-IO'
ID'
PH
FIGURE 6.
Dependence of Reaction Velocity of
Oxidation (V, mol/2. x min) on pH of
Suspension, at a Temperature of AO°C
X = Calculated points, section-B
o = Original points in paper
Figure 1-6. Comparison of Gladkii's Figure 6 with Soviet Slurry
Oxidation Rate Expression (70)
26
-------
^2.0
log w
2.5
3.0
-3.0
-2.5
-2.0
log C
SO,
Figure 1-7. Determination of Soviet Reaction Order
27
-------
Comparison of University of Virginia and Gladkii Data: Experimental data
obtained at the University of Virginia are shown in Figure 1-5 along with the
calculated Soviet results. It should be noted that since the calculated
Soviet results at pH 5 and 40 C did not correlate with experimental data, the
calculated results at 50 C and pH 5 are also suspect. The reaction rates at
the University of Virginia are about twice those obtained by Gladkii.
Experiments in the current work have determined the reaction velocity of
calcium sulfite oxidation in slurries at 40 C and various pH's. These results
are shown in Figure 1-8 along with the reported Soviet experimental data.
Again, the current reaction velocities exceed those reported by Gladkii.
4+
Even though there is an extensive literature concerning S oxidation, in
the clear solution that surrounds the particles in slurries no certain expla-
nation of the process has emerged. The following studies are representative.
Pritchett (136) measured temperature rise in a flow reactor carrying
mixed oxygen and manganese, catalyzed sulfur dioxide solution (pH not given).
He found no dependence on [S ] but half order dependence on [0?] and a first
2+
order effect of [Mn ]. He found MnSO, more active than MnCl? and applied his
results to 0^ absorption experiments. The most salient product of this work
may well be the approach Pritchett took in relating the chain reaction mecha-
nism of sulfite oxidation, borrowing methodology from biochemistry where
cycles (such as chain intermediate regeneration) are more common.
In another flow/thermal study, Srivastana (163) obtained a curious result
also reported elsewhere (7, 108, 129, 130, 131, 139). With no cobalt catalyst
and below [SO,, ] = 0.04 M the rate depends on [SO,, ] but n becomes zero
above the sulfite concentration of 0.04 M. With catalyst added the "switch
2- 2+
point" occurred when [S0« ] = 0.08 M. The rate increased linearly with [Co ]
up to 3.4 x 10 M at which point cobaltous hydroxide precipitates. They
28
-------
20.0-
*»— s
a
•H
6
v_^
rH
| 15.0-
•>
o
X
4J
| 10.0.
V
o
*4
w
u
ID
* 5.0-
0.0
3
•
•
• •
• •
• ' •
.5 4.0 4.5 5.0 5.5 6
.0 pH
.PH
•= Soviet data points
•= University of Virginia data points
Figure 1-8. Comparison of Slurry Oxidations at
29
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indicate (and use in further calculations) an oxygen dependence of one but do
not support this finding in the report. They finally give an involved rate
expression for the high pH oxidation.
In another work (67) in which the [S ] dependence varies from low to
high pH (3-8), Fuzzi examined stirred sulfur solutions open to the air. He
found the order in sulfur to be 1 below pH 4 growing to 2 above pH 5. The
3+
dependence on Fe was linear. Fuzzi found the overall rate to have a strong
maximum with respect to pH at pH 4 which he attributed to a growing ratio of
— O_L
HS03/S02'H20 and a declining [Fe ] as Fe(OH) precipitated.
•J
Another low pH study by Catipovic (42) assessed the effects of catalyst,
pH and added sulfate. In a vigorously stirred batch reactor under a few
millimeters 02 pressure with no outlet he found: both Mg2+ and Mn2+ catalyzed
oxidations are zero order in [S +] on the pH range 1 to 4; Mn2+ is much more
2+
potent a catalyst than Mg ; added sulfate inhibited the reaction signifi-
cantly. He noticed that the ratio of the rate constants with and without
added sulfate plotted against the ratio of [S02~]added/fs°4~]initial expressed
the results in dimensionless form. The activation energy with Mn catalyst was
18.7 kcal/mol.
In research which predated one of the experimental methods of current
study, Bengtsson (24) studied the oxidation at intermediate pH (6-7.5).
Bengtsson did experiments on the pH range 6 to 8.5 and [SO 2~] from 3 x 10~5M
-2
to 10 M by using a pH method as described in Section III. For the pH in-
terval from 6.0 to 7.5 in order to simplify his eq. (3-11), Bengtsson made the
following assumptions:
[H+] + [OH"] « [HSO^]
[H+]2 « K3[HSO~]
kw « k3[HSO~]
30
-------
But these three in equalities are not valid once the pH value is lower than
6.0.
In Bengtsson's experiment, distilled water was first brought to temper-
ature and loaded with 02- Then 500 ml of this water were mixed with a buf-
fered Na2SO- solution in the reactor vessel. Then, 0.2 M H2S04 brought the
•J
solution to the desired pH value. Finally, a certain amount of catalyst
solution was added rapidly. The pH displacement produced by the oxidation
registered on the recorder. It happens that by using the buffered NagSO^
solution, he could neglect the oxidation before the catalyst addition because
of the inhibiting effect of buffer. Note that the derivation of pH method
should have considered this addition of buffer acid which is designated as
[HA] in Section III. For the unbuffered condition, equation 3-10 could be
simplified to be equation 3-11. This equations has been used for all of our
unbuffered runs in which oxidation was started by supplying the oxygen instead
of adding catalyst. The rate of sulfite oxidation found by Bengtsson is zero
order with respect to oxygen and three-halves order with respect to sulfite.
2+
The dependence of the rate of oxidation on [CO ] was half order. The activa-
tion energy in his rate constant was 57 kJ/mol.
The pH method which Bengtsson and Bjerle (25) developed was based on the
fast equilibrium between sulfite and bisulfite, and only applies when bi-
sulfite concentration is practically constant with pH. They used a value of
AH „ = 2.9 kcal/mol to calculate equilibria relationships at a variety of
temperatures. At pH values above 7.5, they tried an absorption method which
depended on keeping the reaction plane within the diffusion film. This method
proved less successful than the pH method.
31
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SECTION 2
EXPERIMENTS
OUTLINE
II. Experiments
A. Apparatus
1. Batch Reactor
2. Flow Reactor
B. Procedure
1. Rate from Concentration Measurements
2. Rate from pH Measurements
3. Rate from Temperature Measurements
C. Analysis of Calcium Sulfite
D. Equilibrium Relationship for S Species
E. lodometric Titration
F. Analysis of Data
1. Analysis of a Single Experiment
2. Multiple Regression Analysis
3. Analysis of pH Method Data
4. Analysis of the Flow-Thermal Method
32
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SECTION 2
EXPERIMENTS
A. APPARATUS
1. Batch Reactor
The experimental apparatus was a batch-type stirred reactor with a con-
tinuous gas feed (Figure 2-1). A constant temperature was maintained by
immersing the reactor in a Plexiglas water bath. The bath temperature was
controlled by a Sargent-Welch Thermonitor, Model ST, employing two heating
paddles totalling 750 watts. An auxiliary 125 watt heater was used to assist
in obtaining the selected temperature. A stirring motor provided circulation
of the water in the tank. Temperature was monitored with a thermometer cali-
brated against an NBS-certified thermometer to 0.1 C.
Materials for the reactor were carefully chosen to avoid possible contam-
ination by metals and organic materials which might affect the reaction. A one
liter Pyrex flask was used as the reactor (Figure 2-2). The flask had been
fitted with five standard 24/40 necks to accommodate various equipment. The
bottom of the flask had been flattened in order to make it more stable. The
flask was securely fastened to a weighted base in order to reduce vibrations
and movement in the water bath. Four indentations were made in the flask to
act as baffles to increase the turbulence.
A Trubore glass bearing and stirring shaft was located in the center neck
of the reaction flask in order to provide agitation of the reacting solution
and allow the addition of gases to the reactor. All reactions were conducted
at stirring speeds greater than 1500 rpm, requiring the addition of a film of
33
-------
TEMPERATURE
CONTROLLERV
GLASS BEARING
VOLTAGE
FOLLOWER
pH METER
DUAL PEN
CHART RECORDER
AUXILLIARY HEATER
HEATER
I
MOTOR
BELT AND PULLEY DRIVE
ADDITION BURET
GAS INLET
THERMOMETER
SAMPLING CAPILLARY
GAS VENT
TEMPERATURE SENSOR
WATER LEVEL
TEMPERATURE BATH
REACTION FLASK
(SEE DETAIL)
"-BASE
Figure 2-1. Experimental Apparatus
-------
HOLLOW GLASS STIRRER
GAS INLET
GLASS BEARING
pH ELECTRODE
TEFLON STIRRING
BLADE AND BUBBLER
10ml. ADDITION
BURET
VENT
SAMPLING
'CAPILLARY
REACTION FLASK
\WITH BAFFLES
Figure 2-2. Reaction Vessel
35
-------
Dow-Corning High Vacuum Silicone Lubricant to the shaft to reduce friction.
The lubricant was applied at least once a day following the removal of old
material with acetone and a rinse with deionized water. Gases were introduced
into the flask by means of a glass tube in the bearing, flowing through the
hollow shaft, and were released into the reactor through a small hole located
near the end of the shaft. A 7.5 cm Teflon stirring blade was fitted to the
end of the shaft in order to agitate the reacting solution and improve gas
dispersion throughout the reactor. A 1/25 hp, single-phase Teletype Cor-
poration motor provided power to the shaft through a Plexiglas pulley and
rubber o-ring drive belt arrangement. A Variac was used to set the stirring
speed which was checked regularly with a strobe light tachometer. A short
length of rubber tubing connected the pulley shaft to the glass shaft. This
provided a means of facilitating the shaft alignment with respect to depth in
the reactor and gas inlets.
A second neck of the flask was fitted with a pyrex glass sampling arm and
gas vent. The sampling tube extended to within 0.7 cm of the reactor bottom,
and it was of sufficient diameter to provide rapid sample taking. The sampling
procedure consisted of closing off the gas vent stopcock and allowing the gas
pressure to force the sample up through.the sampling tube.
In the slurry oxidation reaction, this sampling tube was used for taking
slurry samples. Clear solution samples were taken by using another sampling
attachment which was composed of a glass tube, with a Micropore filter fitted
on the outside end, leading to an aspirator. Filtered liquid samples from the
slurry were thus drawn through the Micropore filter into a test tube under the
pressure difference caused by the vacuum.
A glass combination electrode fitted in a special male 24/40 Teflon
fitting occupied the third neck of the flask. The electrode was connected to
36
-------
a Radiometer Copenhagen PHM63 digital pH meter, which displayed the pH of the
reacting solution. The pH changes of the solution during the course of the
reactions were recorded on a Health-Schlumberger Model SR-206 dual pen strip
chart recorder, when necessary.
In the unbuffered runs where constant pH was necessary, a pH controller
operating base and acid pumps was added to the apparatus. Each pump was
attached by Silastic tubing to a glass capillary tube fit with a 24/40 ground
glass joint. This was placed in one of the necks of the reactor flask.
The sulfite concentration was measured by iodometric titration (See
section 2-E) using a buret accurate to 0.002 ml. Pipettes used to measure
iodine and sample aliquots wre calibrated to 0.001 ml gravimetrically. A
total error was calculated.
All chemicals used in the reactions, with the exception of the calcium
sulfite and gases, were reagent grade materials with catalytic impurities in
concentrations of less than 0.0008%. The oxygen and nitrogen were 99.5% pure.
The calcium sulf ite used was very impure, and an analysis is included. (See
Section C.) Water used in the reaction solution, either city water or dis-
tilled water, was passed through a purification system supplied by Hydro
Service and Supplies, Inc., who claim the conductivity to be 18 x 10 18Q-cm or
better. The gas flow into the reactor was maintained at a constant flow of 3
£/min. A rotameter was used to measure the flow and it had been calibrated
using a wet-test meter.
2. Flow Reactor
The tubular flow reactor (Figure 2-3) is a coiled, inert tube situated
between an efficient mixer at the inlet and throttling valve at the exit to
maintain pressure in the tube. It is fed by oxygen and sulfur solution reser-
37
-------
00
Figure 2-3. Tubular Flow Reactor
-------
voirs and ended by a second mixer where an iodine quench solution may be
added. In the description that follows the circled numbers refer to points on
Figure 2-3. The entire device is built in a water bath that maintains its
temperature (hence the initial temperature of the reactants) to 0.002 C re-
ferred to a Fisher Calorimeter Thermometer (NBS certificate 141368). The
water bath temperature controller is a Sargent-Welch "Thermonitor" of the
phase-firing variety that provides excellent proportional, integral temper-
ature control although it injects much electrical line noise. To assure
temperature control operation well within the proportional band of the
"Thermonitor," a copper cooling coil (T) constantly passes city water through
the vigorously stirred bath. The twin four-blade stirrer Q) is externally
mounted to separate the reactor from unnecessary vibration.
The flow circuit including the reactor tube begins with the gas cylinders
(3) outside the thermostat. Both oxygen and nitrogen cylinders carry
Matheson #3 regulators (4) (for high pressure delivery with fine—no
droop—control). While these regulators proved superior to others tried, the
combination of the low gas flowrate needed to displace the reactants from the
reservoirs and the hysteresis (lockup) in the regulators necessitated a con-
stant bleed stream (s) of each gas during experiments to maintain constant
pressure over the reactants. Although the bleed stream did not pass through
the reactant solutions, this constant expansion of compressed gases gave
considerable Joule-Thompson cooling. To avoid cooling the reactants, all the
inlet gas lines and valves are immersed in the thermostat and include helical
coils
(§) . The stainless steel gas line tubing was well cleaned of contaminants
before assembly. The oxygen-side pressure gauge (JJ is a 12 in. Heise brand
solid front (0-1500 PSI x 2 PSI). The nitrogen pressure gauge (T) is a 10 in.
39
-------
J. P. Marsh type 100 Mastergauge (0-2000 PSI x 20 PSI). (The higher reso-
lution oxygen gauge was indicated since [02] in this reactor is calculated
from the partial pressure of oxygen over the reactant solution.) By driving
the reactants with equal pressure, it is easier to maintain nearly equal
reactant flowrates resulting in less leftover reactant after an experimental
run.
The reactant reservoirs come next. Because high pressure is involved,
pressure vessel construction techniques were used for the outer stainless
steel containers holding the four litre polyethylene reservoir bottles. The
x I
covers of the cylinders are fitted with: outlets @ and inlets @ for
the gas supply; wire passage @ for the reactant thermistors; solution
filling @ and withdrawal @ tubes; overflow purge tubes © , and
for the oxygen cylinder, egress for the fiberoptic flowmeter cable @
The gas outlets all proceed to protective mufflers @ . The nitrogen
inlet tubes (lO) for the sulfur and iodine reservoirs terminate in the space
above the solution bottle while the oxygen inlet penetrates to the bottom of
its reservoir via a Teflon gas distributor tube which provides mixing by the
02 bubbles to facilitate oxygen saturation and thermal equilibrium.
Reactant temperatures are monitored near the center of each polyethylene
bottle by a thermistor epoxied into a thin Teflon tube carrying the lead
wires. The epoxy is covered with paraffin for inertness (20, 54). The leads
are interrupted by a segment of varnish coated wire where they leave the
cylinder to eliminate pressure leaks between the wire and insulation.
When reactant is added, it fills the reservoir through a Teflon tube
(12) mounted outside the waterbath and ending above the highest liquid level
of the internal polethylene bottle. This tube and all the Teflon tubes
40
-------
carrying high pressure fluid are wound with polyester fiberglass lagging, even
though the bare TFE tubing (3 mm bare, 2 mm wall) survived intact an overnight
pressurization to 1500 PSIG. The lagging provides a safety margin against
TFE's tendency to creep and a higher barrier to heat flux. The reactant
leaves the reservoir by a continuous TFE tube, (jj) except, on oxygen so-
lution which passes through an intervening TFE/PVG flowmeter inside the res-
ervoir cylinder. The flowmeter is a model E25 Bearingless Flowmeter. The
other two flowmeters (ij) are model E35 and of stainless steel construction.
All cylinders bear a purging tube and valve (U) . The purge allows any
overflowed reactant to be forced out of the reservoir under gas pressure.
This feature prevents contaminated liquid from reentering the reactant solu-
tion in the bottle.
The reactant solution travels from the reservoir vessels to metering
valves by TFE lines (13) . The valves @ are specially constructed to
meter the reactants without contamination and to withstand the operating
pressure. From the Teflon valves, the reactants go through in-line temper-
ature sensors (R3) and on to the mixer (19) at the beginning of the reac-
tor tube (2^) . The mixer design is an adaptation of Berger's scheme (26)
incorporating interdigitated, tangential jets at the base of a hemispherical
chamber. The form of the outlet chamber forces the fluid streamlines to
detach from the curved surface. The resulting wake possesses compact, intense
turbulence to mix the two streams quickly.
The first reaction temperature sensor (R4) immediately follows the
mixer. Each temperature sensor is a 2000Q semiconductor thermistor emplanted
one of two ways. In one method, the thermistor projects through the Teflon
wall of the tube into the bulk of the flowing fluid; in the other, the ther-
mistor is cast in an epoxy plug and held in the fluid path by a plastic "Tee"
41
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tube fitting. The inert, though mechanically disadvantageous, Teflon of which
the reactor is made requires special construction techniques.
Thermistors were chosen for temperature sensing due to their large temp-
erature coefficient of resistance and inert glass coating. (For comparison of
several temperature coefficients: thermistors, 10%/°C; thermocouples,
0.6%/°C; platinum RTD, 1%/°C; —these are also prohibitively expensive). The
thermometrics devices (P/N SP60 BA 252 MIB & A) chosen are well aged at eleva-
ted temperature imparting a stability of 0.01%/yr. Even in the good heat
transfer conditions around the thermistors, the heatup of reacting fluid due
to Joule heating of the thermistor by the measuring current is negligible
since the ohmmeter (Keathley model 172) uses only femtoamp currents for resis-
tance measurements. Having all thermistor connections soldered or by gold
plated connections and having 2000Q resistance value makes wiring resistance
constant (or varying by amounts below the resolution of the ohmmeter) and low
enough to allow use of the simpler two terminal methods of resistance measure-
ment. The flow passes next through a flow selector valve (2l) . The six-
position, all Teflon valve allows the flow to be diverted to various-length
reactor tubes or directly to the quencher @ . The selector has ringless
sliding seals to contain the pressure without contamination.
At last the reacting liquid enters the main reactor tube (2(5) . This
tube is 397 cm long making the total reactor length 405 cm (maximum residence
time around 6 sec.). To submerse this length tube in the thermostat for
uniform starting temperature, it was coiled. Unfortunately, curvilinear flow
tends to stablize laminar flowlines at higher Reynolds numbers than those
which give turbulence in straight pipe. To overcome this effect, the coil is
square and as large as possible (17 cm sides) so turbulence is fully developed
in the straight segments between corners; furthermore, the projecting thermis-
tors obstruct half the flow channel inducing form drag in their wake.
42
-------
The thermistor spacing is close together at the entrance end of the
reactor tube for catching temperature profiles which end in the tube. The
sensors are further apart in the remainder of the tube.
After transiting the reactor tube the liquid passes through the comple-
mentary flow selector (||) and through the second mixer (£2) . This mixer
is fed by an iodine reservoir identical to the sulfur solution reservoir and
connected through a flowmeter and metering valve. The mixed flow then passes
to a total flowmeter and metering valve @ whose purpose is maintaining a
total pressure drop high enough to keep the oxygen in solution throughout the
reaction zone. After the valve, the flow exits the thermostat and is acces-
sible (25) for sampling for titrations before going to the drain.
The automated temperature measurement system includes the various ther-
mistors, thermistor selection, digital ohmmeter, and microcomputer. The var-
ious thermistors are: reactant reservoirs (2), reactant lines (2), reactor
tube (11), and waterbath (1). The thermistor selector receives a thermistor
number (channel address) from the microprocessor under program control, de-
codes (actually demultiplexes) the number, and connects the specified ther-
mistor to the digital ohmmeter which reads with a sensitivity of 0.1Q/2000Q
and has a settling time of 2 sec (this factor being the limit on measuring
speed). The actual time the program waits for settling was determined exper-
imentally by finding when no increased wait affected the resistance reading.
-18
The measuring current is 10 A. The digital output consists of 5 x 4 lines
of BCD code + 4 lines for the exponent. The microcomputer program reads these
24 lines and reassembles the resistance reading.
For calculating residence times the program needs flowrates. The three
flowmeters produce an optical signal. In each flowmeter's individual con-
verter the signal becomes an electrical square wave whose frequency is pro-
43
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portional to flowrate. A separate three-channel frequency to voltage conver-
ter processes the square wave for the computer's A to D converters. This
circuitous route has the advantages of low pressure drop (an inert bearingless
turbine), electronically smoothed output (large capacitors in the F/V inte-
grate noise peaks in the square wave frequency), and rapid reading (by the
A/D). The flowrate is correlated with the resulting readings by volumetric
calibration.
The only other variables required for the calculations in the program are
oxygen pressure, absolute temperature, [S4+], and catalyst concentration. The
oxygen pressure is read (±1 PSIG) from the Heise gauge, then entered manually
for calculations. The thermostat temperature is related to thermistor read-
ings by calibration with a Beckman thermometer (to 0.002°C). The concen-
trations of S + and the catalyst are prepared by formulation with the S4+
concentration checked by iodometry previously described.
B. PROCEDURES
1. Rate from Concentration Measurement
The reaction can be influenced by inorganic and organic impurities. The
organic impurities were handled by the water purification system. The sili-
cone grease used to lubricate the shaft was regularly removed and replaced to
avoid any build-up of material due to adhesion to the grease. The electrode
and Teflon fittings were rinsed thoroughly with deionized water before each
run. Prior to each reaction, the reactor and associated glassware were rinsed
thorougly with deionized water. Periodic cleaning of the glass items with
chromic acid and cleaning solution or dilute sulfuric acid, followed by sev-
eral water rinses, was also performed.
44
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The liquid volume added to the reaction was 600 ml. Added to the reactor
initially was 500 ml of solution and the nitrogen flow was started in order to
purge oxygen from the reactor. The stirrer was also started. All other
materials were added to the reactor through the one open neck of the flask.
The calcium or sodium sulfite was then weighed, along with the buffer, if it
was to be used in the reaction. The dry materials were poured into the reac-
tor and rinsed down with 100 ml of water.
The initial pH of the reaction was adjusted using the addition of either
acid or base. For the buffered reactions employing succinic acid, pellets of
sodium hydroxide were added. In the case of unbuffered reactions or where
only minor adjustment of pH was required 1.0 M NaOH or 1.0 N H2S04 was used.
Following the pH adjustment, the reactor was sealed with a ground glass
stopper and preparations were made to start the reaction. Twenty-five
milliter flasks containing predetermined aliquots of 0.1 N iodine solutions
were readied to quench the reactions. Sodium bicarbonate required to buffer
the solution during the titration and a Teflon coated stirring bar were also
contained in the flask. The flasks were kept in an ice bath at all times
prior to the titration. The samples at time zero were withdrawn directly from
the reactor by pipette and then transferred directly to the iodine. All other
samples were obtained using the sampling arm. The gas vent was closed, and
after the sample tube had been purged for a few seconds, a clean test tube was
used to collect the sample. From the test tube, the 5 ml aliquot was trans-
ferred to the flask containing iodine by pipette.
The reaction was then ready to start. With the stirring speed at the
desired level, the nitrogen flow was cut off. A stopwatch timed the experi-
ments starting when the oxygen flow reached operating level. The reaction was
then followed by periodic sampling until sufficient data had been obtained.
45
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2. Rate from pH Measurements
The procedure for the pH method runs was somewhat different than the
procedure of the other experiments. In this method, no sampling, base, or
acid addition tubes were necessary. The pH electrode was connected to the
strip chart recorder, which was floated on an expanded scale to better observe
the pH change in a small range. The charter recorder was then turned on just
prior to starting the reaction. All other aspects of the experimental pro-
cedure were as previously described.
3. Rates From Temperature Measurement
After the reactant solutions of calcium or sodium sulfite/bisulfite and
water, both of the desired pH and catalyst strength, have been prepared by the
volumetric methods previously described, the following sequence of steps made
up the experimental procedure.
o Change reservoirs well in advance of experiment to allow for thermal equili-
brium:
Use N2 purge (conveyed by flexible hose to cap) for the sulfite
solution to prevent unwanted oxidation.
Replace plug in filler port and close off reservoir immediately when
filled to prevent unwanted oxidation.
Overpressure the oxygen reservoir (bring pressure up slowly) to
assure saturation.
46
-------
o Wait for thermal steady state:
Use "reservoir" section of data taking program to observe temper-
atures inside the reservoirs.
Initiate bubbling in the 0- reservoir or bubble periodically to aid
mixing and saturation.
o When the solutions are in readiness, assure the temperature baseline of the
reactor tube thermistor calibration curves is flat:
Use "baseline"section of data taking program—or--
Observe temperature profile of only one reactant flowing.
Correct intercepts of thermistor calibration curves to compensate
for any bumps.
o Collect initial sample of sulfur solution for titration.
o Begin cycling through thermistor readings a few minutes in advance of data
taking to stabilize the electronics.
o Set the oxygen and nitrogen lines:
Open the vent from both lines slightly to raise regulator valve off
its seat (eliminating the effect of lockup).
Stop the 02 bubbling.
47
-------
o From the predetermined set of observations, use:
"Flows" section of the data taking program to set the required
flowrate using both reactant valves and the total flow valve.
Examine temperature profiles and save on file all profiles having no
equipment-made flaws.
o Drain both reservoirs separately when the first one runs out:
Shut off either reservoir, open the other fully to drain (avoiding
contaminating of next change.
Repeat for remaining reservoir.
Leave total flow valve somewhat open.
C. ANALYSIS OF CALCIUM SULFITE
An analysis of the calcium sulfite solid used in the experiments is given
in Table 2-1. The calcium sulfite is a product of City Chemical Corporation.
There are few sources of calcium sulfite available and most of the sulfites
are relatively impure. It can be seen in Table 2-1 that the calcium sulfite
from City Chemical is very impure, consisting of only 50.6 mole% sulfite. The
majority of the remainder is calcium sulfate, with several metal impurities
also present.
Calcium sulfite was also prepared in the laboratory by bubbling S02
through calcium hydroxide. It was possible to obtain a much higher sulfite to
sulfate purity by drying the compound carefully under nitrogen (to prevent
48
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TABLE 2-1
ANALYSIS OF CALCIUM SULFITE SOLID
WEIGHT %
(SO,2 ) 50.6
Manganese 0.0062
Iron 0.0248
Copper 0.0004
Magnesium 0.0505
Cobolt 0.0008
Zinc 0.0009
Atomic Absorption Analysis by North American Exploration, Inc.,
Charlottesville, Virginia 22901
49
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oxidation). Since there were several metals present in the calcium hydroxide,
these were also present in the laboratory prepared sulfite; however, there
was some reduction in the metal impurity levels over the commercial grade
calcium sulfite. The main advantage to the laboratory prepared compound was
the fact it was 85.6% sulfite, much higher than the commercial grade compound.
D. EQUILIBRIUM RELATIONSHIP FOR S4+ SPECIES
As noted in the introduction the presence of the various S4+ species
depends on the pH of the solution. The mole fraction of each species is shown
as a function of pH at 40°C in Figure 1-1 (170, 171).
E. IODOMETRIC TITRATION
The sulfite concentration of sulfur in the +4 valence state was deter-
mined by a difference method. An excess of iodine of a known volume and
concentration was used to quench known volumes of the reacting solution. The
reactions occurring during the quenching are as follows:
S0-H0 + H0 + I - ^SO2" + 21" + 4H+
HS03 + H20 + I2 - *- SO2" + 21" + 3H+
S032" + H20 + I - >- SO2" + 21" + 2H+
Arsenious acid was used in the titration in order to determine the number
of moles of unreacted iodine:
H3As03 + I2 + H£0 - *-H3As04 + 2I~ + 2H+
A balance on the iodine can be written:
Total Iodine Added = Iodine reacting with S +
Iodine reacting with Arsenious Acid
or (NI)(VI)=NSVS+NAVA
50
-------
„ , . . ^equivalents^,
where N = normality ( ^ liter - )
V - volume (liters)
I, S, and A represent iodine, sulfur (4+) , and arsenious acid
respectively.
Solving for Ng = (NjVj - N
In order to convert from normality to molarity,
MS =
-------
NA = 0.100 N
Vg = 4.984 ml
Vj = 1.993 ml
V = 1.265 ml '
M _
_ (0.099)(1.993) -
S 2(4.984)
MS = 0.0071 M
F. ANALYSIS OF DATA
1. Analysis of a Single Experiment
The results obtained from each experiment were analyzed using a
Hewlett-Packard 2000C computer. A BASIC computer program, "REPORT", provided
an output of conditions, data, concentrations, calculations of the rate of the
reactions, fits to the integrated form of zero, first, and second order re-
actions, and plots of the sulfur (4+) concentration vs time.
The program required the input of the conditions for the experiment which
were printed in an appropriate format. In addition to the conditions, the
time, number of aliquots of iodine used, and the amount of arsenious acid used
in the titration were input. These data and the concentrations in g-mol/Ji of
the sulfur (4+) species were then printed in the output. The program would
then take the concentration information and determine the slope between each
two data points. The slope was then output as the rate, along with the
average concentration for the two points. The next part of the program used
the concentration and time data in a modified version of the Hewlett-Packard
library program, CURFIT called CURVIT, to provide information about the zero,
first, and second order fit of the data. The final part of the program
plotted the concentration vs time data and any additional comments.
52
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The CURVIT program gave a goodness-of-fit parameter, referred to here as
the index of determination. The parameter has a value of 1.0 for a perfect
linear fit, and 0.0 for random scatter. It is defined as
= sxsy
where n = number of points
X.,Y. = actual value of i point
X^ Y1 = average values
S , S = standard deviations
x' y
The CURVIT program was employed alone to perform linear regression anal-
ysis of the log rate vs log concentration data. A Hewlett-Packard HP 25
linear regression analysis program was also used to perform some of this data
reduction.
Figure 2-4 provides a listing of the REPORT, the modified CURVIT program,
and a sample output of the programs.
2. Multiple Regression Analysis
MULTREG, short for multiple regression, is a program which performs a
least squares curve fit of data pairs under a wide variety of I/O and pro-
cessing options. The program was obtained from the Virginia Highway Research
Council through Messrs. William Carpenter and Philip Harris. No listing of
the actual program is included here; it is a library program for the Virginia
Highway Research Council contained in the CDC CYBER 172 computer, located in
Gilmer Hall at the University of Virginia. The program is in FORTRAN IV, and
the data and program instructions must be prepared accordingly. The data for
the rate and average concentration were obtained directly from the REPORT
program described earlier. Other similar multiple regression programs are
available in the Hewlett-Packard program library. Criteria for excluding data
were that the time was less than 60 seconds or that the rate as recorded on
53
-------
REPORT
10
11
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
*170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
360
370
•:380
390
400
410
420
430
440
450
COM N y R2 y R4 y Q »KC6D y K*C70D y T y VC50 y 3.'] y MC50D
COM E
FILES SULF21»SULF22
DIM AC50D»RC50'D»TC50D»A*C70D
PRINT "TYP1 TAPO"r
F
"RUN * "y
G
'* POINTS "y
N
"SLURRY "y
INPUT
PRINT
INPUT
PRINT
INPUT
PRINT
INPUT
PRINT
INPUT
PRINT
INPUT
PRINT
INPUT
PRINT
INPUT
PRINT-
INPUT
PRINT-
INPUT
PRINT
PRINT-
MAT
KC1D
"TEMP
KC2:i
" RF'M
KC'3D
" PH
"X 02 " y
KC5D
"LPM "i
KC6D
"ENTERCOMMENT"
K*
*2fG»N»KClD»KC23»Kt3D»KC4D»KC5D»KC6DfK*
TAB as)y"DATA: TIME 'y' * i 'y' TITER 'RETURN-
INPUT Vi:Ny33
MAT PRINT tl?M
IF F=0 THEN 1530
IF END *2 THEN
ASSIGN "SULF21"i
300
1 y Z
330
IF END fl THEN
MAT READ tlf-U
ASSIGN "SUI...F21" y
FOR P=l TO N
VCPf 1D=60*VCP»1D
P R I N T * 1 5 V i: P y 1 D y
NEXT ,'
P R I N T I... I N < 4 ) y " ••- " y L I N ( 4 ) y S P A ( 2 6 ) y
P R I N T I... I N ( 2 ) v " C 0 N D I T I ON S " , L I N < 1 )
* DATA % 02 (<> LPM RPM PH
USING 420?N»KC5D»KC6DyKC3D»KC4D»KC2DfK.ClD
C P y 2 D y V C P y 3 D
R U N * " y G
PRINT
PRINT
TEMPyC
SLURRY
IMAGE3X2D y 9X3D y 4XD » D y 6X4D y 2X2D , D y 5X2D y 7XD * 3D
REM B=CIODIND C=CARSE*ACIDD D=VOL OF I ODIN PIE*
B=» 079844
C_ j
— » 1
Figure 2-4 REPORT and CURVIT Programs
OFSAMPL
54
-------
460 D=2.03094
470 F=4.98472
480 FOR 1=1 TO N
490 MI" I 3 = ( B*D*V C I r 2 3 -C*V CI f 3 3 > / < 2*F)
500 NEXT I
510 FOR J=l TO N-l'
520 ACJ3=(MCJ3+M[:j+13)/2
530 RCJ3= LIN < 1)
630 PRINT" TIME,SEC"," AV MOLARITY",• RATE»MOL/L/SEC"
640 PRINT " 0%" %"
650 FOR 1=1 TO N-l
655 IF RCI3>1*E-08 THEN 660
656 RCI3=0
660 PRINT USING 670 JVC 1+1 * 13 , AL" 13 * RC13
670 IMAGE5X4D»llXD*5Df8XD*7D
680 NEXT I
690 PRINT LI NCI.)
700 PRINT LIN<39-2*N)»TAB(62),"PAGE "JQ
710 PRINT LIN(2>?"••-"
720 PRINT LIN<2)f"DATA REDUCTION? INITIAL SLOPE"yLINCt)
722 IF KC13*0 THEN 730
724 R4=930
725 R2=N
726 E=l
727 T=l
728 CHAIN "CURVIT1
730 R4=820
738 E==l
740 R2=-l
750 IF MCR23 <= *005 THEN 790
760 IF R2--N THEN 790
770 R2=R2+1
780 GOTO 750
790 R2=R2-1
800 T=l
810 CHAIN "CURVIT"
820 R4=940
830 IF Km 3=0 THEN 930
840 ASSIGN 'SULF22'»3»Z
860 E=R2+3
880 R2=N
Figure 2-4. REPORT and CURVIT Programs
55
-------
890
900
910
920
930
940
990
991
992
1020
1030
1032
1033
1034
1040
1050
1051
1052
1053
1060
1070
1080
1090
1100
1110
1120
1130
1140
1150
1160
1
T==2
PRINT LIN(l)"
CHAIN "CURVIT1
GOTO 940
PRINT LIN(8)
PRINT LIN(2)t '
FOR 1=1 TO N
NEXT I
MAT T=CON
MAT T••=••( 100) *T
FOR 1=1 TO N
FINAL SLOPE"?LIN(1)
PLOT OF CONCENTRATION US TIME %• LIN(1)
1.70
1 180
1 1.90
1200
1202
1210
1220
1222
1223
1230
1240
1250
1 2 6 0
1270
1280
1290
1 300
13.10
NEXT I
MAT M=CON
MAT M=<100)*M
FOR 1=1 TO N
NEXT I
IF TC:N:I
IF TI:N:.I
IF TEN 3
Tl=120
T2=,5
GOTO 1200
Tl = 20
T 2 -- 3
GOTO 1.200
Tl==40
T2==l ,5
GOTO 1200
Tl==60
T2 = l
IF MI::I::I <
IF MI::I :i <
Ml ==,099
GOTO ;|.24()
Ml ==,022
GOTO 1240
Ml ==,00 99
J=l
20
40
60
THEN
THEN
THEN
1120
1150
1180
>0099 THEN 1230
,022 THEN 1222
i
i
P R I N T S P A ( 1 0 ) , A *
FOR 1=33 TO 1 STEP -1
IF I/ll-INTd/ll) >= ,08 THEN 1320
PRINT USING 1300? Ml* I 733
IMAGE* t 5X . 4D ? " - "
GOTO 1330 Figure 2-4, REPORT and CURVIT Programs
i
56
-------
1320
1330
1340
1350
1352
1353
1354
1355
1356
1357
1360
1367
1368
1370
1380
1390
1400
1410
1420
1430
1440
1450
1460
1490
1500
1510
1520
1530
PRINT SPA(10)?"1"J
IF TCJi=100 THEN 1390
I F M C J II Ml*I/33 THEN 1390
M™ n \b H
•™ /i\
IF KC13=0 THEN 1368
IF J= E THEN 1367
B$"-" * *
GOTO 1368
PRINT SPA»TAB<59> J
PRINT LIN(l)?"-"
END
t"?K*
"PAGE "JQ5"»1"
"
Figure 2-4. REPORT and CURVIT Programs
57
-------
GURUIT
8000 COM DyR2y R4 y Q y K C6 3y K *C70 3y T y UC50 y 3 3y MC503
8003 COM E
8006 FI L. E S S U L F 21 y S U I... F 2 2 y S U L F12 y S U L F11
8009 DIM XL2003t'YC2003yUC2003yVC2003yAC73yBC73yCC73ySC63yFC73
8012 DIM TC7»33
8013 N=R2-E+1
8014 DC13=E
8015 DC53=R2
8016 DC23=0
8018 FOR 1=1 TO N
8021 XC.T3=WCI+E-1»13
8024 YCI3 = MCI+E-13'
8027 NEXT I
8030 MAT F==CON
8033 PRINT '"CURVE TYPE"y" INDEX OF"y" A" y " B"y" PTt'S1
8 0 3 6 F1 R INT " " y " D E T E R MIN A T10 N "
8039 FOR 1=1 TO 7
8042 IF (I-3)*.(I-4)*
-------
Ul
H-
00
>"i
fD
CO
1
-P-
•
td
M
^
O
*l
i-3
CD
3
CL
<3
H
H
^
H
O
(TO
i-i
9)
3
01
ro ro ro ro ro ro ro ro ro ro ro ro ro ro ro ro ro ro ro ro ro ro ro ro
xi xi X" o-- o-- c^ en en en -»=• .£=• •£> •£> 04 w oj ro i-o ro *-*• !-* <-*• M. o
cs Cw o xi i. :-•• co en ro --o CN Cs o 'xi -t> i-^ co en ro •<) CN Csi o xi
~ T. p-t Ti p-S Ti — : T: 7-, Ti p"i Ti ~?\ ~ •—* •-= >-* r-' ' r-" M ~ ~ : — IH
rn 23 m 23 rn 2? rn 2? rn 25 rn 23 rn 2? ~n ~n ~n — — — rn rn rn ~
—i !-i — i i- ! — i VH 1 — i r~ — i r-! 1 — i -~ —i 1-i H — ! — I
cz2:c:2:c:2:c2:c2:c:2:c2:~. ~.~. ~,~-.~. c: M
^H —3 *Ti — ^ |^i —4 ™- —4 "71 — < "n —K >1 i ii :: ii ii |: i: ^%" >^ fp "-./
2C 2t 21 2^ 2C 21 21 CN CH -*> 04 ro s-1 II 21 < i r-1
» = = = = = = !— : _£>
i-* ?o 04 *. en cs xi— i— i— i— i— <— ; i_j-<
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rnrnrnrnrnrn corn
-< -< -< -< -< -< -<212121212121 n 2:
II ii H II H II II -fc.
3> j> J> S> -i-* X j>COCOCOCOC003 UJCO
55 X X 55 X> 2> * W -C CJi i^ Xi W ?ib
* t? 3 x, -t- •«- r~ c_
X — 55X5555Q LJ
s-'55~-' — •3£3£Q T^
* = = X X -C
<» x • <« •« ^ ^ x —
•^f — -^r f
S N=> = K L_j
••tl •«>-«»
ro
o
-^
_
rn
— i
CO
n
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! J
::
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—
ro
LJ
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n
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ro
O -O • ^> c« 04 oj
w o xi 4> *-•• co en ro ~-o cs 04 o xi ^> ^ co en ro
~~CD21CDr~ M"nG52lG-jr- SH ~n CD 21CDi~
23~=omom"^cornc3rn~noOi°:Orn
h-= s-i CO X CO — i 23 — IX CO H 23— IX CO H
:z2ic_-ic; x O-HC: x o-ic
— i — < W 55C:~e_ 55 C — C_ 55C
C_ n C_ i! 03 C_ n C_ ii CO C_ n
=»nco coe-Lj^M- coe-^i-1^ co c_
-^nro roLJii xi roLj xi roLj
-iMXi OIIO-iXIOIIA-HXi OH
-i-Lj^O r-^-H' O i-* P" i! O i-'X
TO x= X -i C rn
•— 2> XJZ21 CDO21 C_
MI. n — rn -"• LJ
r— i • _• { ~~y '--.S —Jt
V f *^ •—. *^. <^s t
— LJ LJ — X
r-i ••= co C. rn
LJ 55 i-1 LJ J?.
••= n ro -^
j> J-i O CO
r~j LJ j— *•
=-i M: ro
LJ 55 O
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n LJ
:— j
LJ
-------
S279 LET B=/)
8282 LET A =(SC33-B*SC11)/N
8285 LET S1=SC53-/N
8288 LET S2=
-------
the REPORT program was zero. The useable data was then run through MULTREG.
They were fit to the equations:
log Rate = a(log[S +]) + log C
where a = order of reaction with respect to sulfur (4+) concentration, (mol/£)
C = rate constant
The MULTREG program had the capability to handle more than one inde-
pendent variable. In a second run of the MULTREG program, two independent
variables were considered: the sulfur (4+) concentration and the catalyst
concentration. In order to assure compatability with the program, catalyst
concentrations had to be converted from ppm to g-mol/£. This second multiple
regression analysis was done for the iron catalyzed sodium sulfite oxidation,
manganese catalyzed sodium sulfite oxidations, and the manganese catalyzed
calcium sulfite oxidations. The same criteria were used to screen the data
employed in the MULTREG program for the second analysis. The MULTREG program
fit the data for each of the three cases to the following equations:
.(4+)
log Rate = a(log[S ]) + b([Catalyst]) + log C
where a = order of reaction with respect to the sulfur
concentration
b = order of reaction with respect to the catalyst
concentration
C = rate constant
3. ANALYSIS OF pH METHOD DATA
An analysis of the sulfite oxidation rate based upon pH change was de-
veloped (based on the ideas of S. Bengtsson [24]). During sulfite oxidation,
2- 2-
sulfite oxidized to sulfate, SO + \Q^ » SO, . Also, the equilibrium
+ 2-
HSO«,— H + S0~ is established very rapidly. Thus, the pH value decreases
accordingly during reaction at the pH range being studied. From the rate of
change of pH the rate of oxidation can be determined.
61
-------
In dilute solutions, the activities and concentrations of the ions can be
equated. The following equilibrium applies:
[HS03] "
At the beginning of the test, the solution contains a known buffer concen-
tration and CaS03 or Na2S03 concentration. The pH value is set by the addi-
tion of H2SO^. If C-j^ designates the initial concentration of (Na2S03) or
CaS03, and C2 the added amount of H2S04, the following concentration condi-
tions apply:
[SO2;] + [S02~] + [HSO~] = CL + C2 (2-2)
[Na+] = 2CX
The condition of electro-neutrality is:
2[S02~] + 2[S02"] + [HS03] + [OH'] + [A~] = 2^ + [H+] (2-3)
multiply (2-2) by 2, then subtract it by (2-3), gives
[HSO'] = [OH"] + [A"] - [H+] + 2C0
J /
The equilibrium
k =
B
[HA] [HA]Q - [A"]
is also established
KB[HA]Q
[A ] = k^Il-H] (2-4)
Substitute (2-4) into (2-3) results in
MHA] .
[HS03) = i°H") + k^rw+)" |H ' + 2Ci
The above equation is valid during oxidation. Differentiation with respect to
time gives :
62
-------
d[
-------
dS k k k k k [HA] k_S.
U_fbbWB,b O , St
rl t l + + + + 1 jT~ T J- T
[H ] [H ]3 [H {kR+[H ]}2 [H ]{kc+[H ] [H ]2
dt
If there is no acid (HA) in the solution, the above equations can
be simplified to be:
dst ksst ks[OH
InlO ^ (2-11)
This equation can then be used to determine the rate of reaction from the
measured rate of pH change.
4. Analysis of the Flow Thermal Data
The bisulfite and sulfite oxidation are mildly exothermic giving up 29.3
and 63.2 kcal/mol respectively. Monitoring the temperature rise, therefore,
is a feasible method for following the course of these reactions. The diffi-
culties arise from the small temperature increases encountered and the in-
ability of a method to distinguish one sulfur species from the other, a
problem also shared by titration. The first problem is solved by resorting to
high temperature coefficient thermistor sensors capable of distinguishing AT's
as low as 0.001°C. The latter difficulty can be avoided all-together if only
bisulfite or sulfite is present. Initial opeartion at pH 4.6 makes bisulfite
the net holder of S while pH above 9 causes sulfite to be the dominant
species.
64
-------
Consider a cylindrical slice of fluid Figure 2-5 traveling away from the
mixer with the conditions:
Adiabatic (qw=0)
Isobaric (Pi = Po)
Negligible viscous heating
Steady flow (v. = v )
Homogeneous reaction (q = R'H )
The energy balance on this volume will be:
Rate of
Accumulation
Rate of Input
by Convection
Rate of Output
by Convection
at i
Rate of
Generation
at o
Rate by
Coduction
Compression
Acceleartion
Dissipation
Specifically,
0 = pf C A T. - pf C A TQ + R • HR A1A
Divide by AA1 and take the limit as Al -» 0 and integrate
RAHR
AT _
Al ~
Evaluate:
pf C
A = 0.0792 cm2
C = 0.998 cal/(cal°C)
P
-3
p = 0.997 g cm
Sulfite = (pH ? 9)
HR = 63.2 kcal/mol
Bisulfite = (pH = 4.6)
HR = 29.3 kcal/mol
The rate in the case of bisulfite then is
Al
HR
65
-------
so,
Newton i an Fluid
Constant k, ThermaI Cond.
Figure 2-5. Element of Reacting Fluid
66
-------
AT
= 0.4288 f - mol/U sec)
for sulfite
AT
= 0.1988 f
For example Figure 2-6 in run #3-23 of the flow reactor results where the
conditions were:
[HSO~] = 0.0018 mol/A
[02] = 0.0144 mol/A
T ., = 25.014°C
Bath
fa. *. =6.05 cm3/sec
tot
[Mn] = 10 ppm
The slope of the T vs 1 plot is 0.0.00025 °C/cm which gives
R = 0 • 4288 • 6.05 • 0.00025
= 6.5 x 10"4 mol/Usec)
67
-------
IIU3-23 on record * 409 on 25AUD79
00
New series with 10 PPB, hn.
CONDITIONS:
CSlvJr= 4.3E-O3 H/l pH 4.6 Bath 825.014 dea Ptot- 299 for t02tr* .025217 H/l
CSiv3t= 0.0018 H/l and LnCSivU* -4.2929 C023t=0.0144 M/l
CMrO= 10
Total flowrate* 4.03 cc/s 02 flowrate=3.45 Slv flourate=2.40
O 0.038 O.041 0.044 0.041) 0.040 0.071 0.115 0.094 0.109 0,154 0.114
O 0.072 0.321 0.543 0.744 1.211 1.456 2.323 2.991 3.880 4.993 S.215
25.015 25.052 25.054 25.058 25.063 25.075 25.084 25.130 25.111 25.124 25.169 25.131
0,00185 0.001577 0.001550 0.001534 0.001500 0.001412 0.001333 0,001017 0.001153 0.001060 0.000732 0.001010
-6.36934 -6.44046 -4.47440 -4.49107 -6.S3200 -6.59118 -6.69045 -6.80653 -6.87347
-.00378 -.00011 -.00007 -.00015 -.00020 -.00018 -.00013 -.OO010 -.00004
-5.5781 -9.1207 -9.5194 -8.7847 -B.5329 -8.4270 -8,9134 -9.1691 X-10,1812
DATA I dT !
((evidence tiniei
Temperature
Concentration
l.nfAv Cone)
Rate
Ln(Rate)
RESULTS • I
Linear fit (T v t) of first 12 points
except point numbers } 11 8
T»O.01B99*t+25.0454 for a RATE= 0,00064 H/l/s and Ln(R)—7.3S69
with rr»
Extent of reaction
937 kl.S-
54.59 Z up to point 12
8.02
PLOT OF T vs t I
+25.163
+ 25
I
I
!
+25,
j
:
i
t25.
I
I
I
t25
!
I
t o
+ 25
I
!
I
625
15.143
117
091
045
O 0
039
014
0.9 1,
-2.7 3.5 4.4 5.2
Figure 2-6. Sample Output for Flow Reactor
-------
SECTION 3
OXIDATION IN CALCIUM SULFITE SOLUTIONS
OUTLINE
A. Calcium Sulfite Oxidation (no organic acids)
B. Calcium Sulfite Oxidation in the Presence of Organic Acids
1. Succinic Acid
a. Effect of pH on the Rate of Oxidation
b. Effect of Succinic Acid Concentration
2. Adipic Acid
3. Glycolic Acid
4. Comparison of Rates of Oxidation with Succinic Acid, Adipic Acid,
Glycolic Acid, Citric Acid, and Acetic Acid
5. Effect of Catalyst - Manganese and Iron
a. Succinic Acid
b. Glycolic Acid
C. Reaction in Liquor from Penberthy Oxidation Runs at Shawnee
D. Dependence of Oxidation Rate on Stirring Speed and Oxygen Flowrate
E. Rate of CaSO- Oxidation at High Catalyst Concentration by the Flow-
Thermal Method
69
-------
OXIDATION IN CALCIUM SULFITE SOLUTIONS
As a basis for understanding oxidation in calcium sulfite slurries, a
detailed investigation of the reaction in clear solutions was undertaken. The
rate was studied as a function of temperature, pH, catalyst type and concen-
tration, and type and concentration of organic acid inhibitors.
The pH has a strong influence on the reaction rate over the range inves-
tigated with the rate of oxidation increasing with increasing pH. For the
studies with organic acid inhibitor the pH was held constant during the course
of an experiment, either by the buffering action of the organic acid or by
means of a pH controller. Without the organic acid this method could not be
used except at the lowest pH studied (4.0) since the reaction became too fast.
Therefore, a method was used in which the pH was allowed to vary throughout
the experiment and the rate determined from the rate of change of pH.
The catalysts studied were manganese and iron since these are present in
calcium sulfite and in sulfur dioxide scrubbing loops and are active at in-
creasing the rate of oxidation.
The organic acids investigated all inhibited the reaction to some extent,
with glycolic acid being effectively able to stop the reaction, if present in
large enough quantities. These organic acids can be used in scrubbing systems
to increase the sulfur dioxide absorption efficiency, and could also be used
to influence the rate of oxidation.
A. CALCIUM SULFITE OXIDATION
The oxidation in clear calcium sulfite solutions was studied in a semi-
batch reactor by following the pH change as described above. This method was
used for intermediate reaction rates, viz., when the reaction was too fast to
be studied by taking samples from the semi-batch reactor, but when the rate
was somewhat slower than those investigated in the flow reactor. The studies
70
-------
reported in this section are thus on the oxidation of calcium sulfite without
inhibitors and with the addition of no or small amounts of catalyst.
The rate of oxidation as a function of [S4+] at T = 25°C, 30°C, 35°C, and
40°C is shown in Figure 3-1 and Table 3-1. The reaction is 1.5 order with
respect to sulfur under these conditions and values for k^ ,. are given in
Table 3-1. Similar results are presented in Tables 3-2 and 3-3 and Figures
3-2 and 3-3 for oxidation in the presence of catalyst. The catalyst levels
[Mn] = 0.53 ppm and [Fe] = 1.02 ppm for the experiments of Table 3-3 and
Figure 3-3 were chosen because these catalyst levels are the same as those
present in liquid which is in contact with solid calcium sulfite under the
conditions of the slurry oxidations described in Section IV. Thus, the rate
constants obtained from Figure 3-3 are useful in the mathematical model used
to interpret slurry results.
The effects of additional manganese catalyst can be seen from Table 3-4
and Figures 3-4 and 3-5.
Activation energies were obtained from the data for the three conditions
(1) no added catalyst, (2) [Mn] = 0.6 ppm, and (3) [Mn] = 0.53 ppm, [Fe] =
1.02 ppm, as shown in Figure 3-6. The rate constants (K^ ^) and the rate of
reaction for [S ] = 0.01M were then calculated over a range of temperatures
20*C to 80°C; these are presented in Table 3-5.
B. CALCIUM SULFITE OXIDATION IN THE PRESENCE OF ORGANIC ACIDS
A detailed investigation has been made on the effect of organic acids on
the rate of oxidation in calcium sulfite solutions. These acids can be added
to scrubbing loops to improve the efficiency of sulfur dioxide absorption and
may be used to inhibit undesired oxidation. The oxidation was studied in the
presence of succinic, adipic, glycolic, citric and acetic acids as a function
of pH, acid concentration and type and catalyst concentration.
71
-------
8x10
-5
6x10
-5
4x10
-5
3x10
-5
2x10
-5
Rate 1x10
-5
mol/(H sec)
8x10
6x10'
4x10
3x10~
2x10"
-6
,-6
-6
0.001
: 40°C
: 35°C
: 30°C
'. 25°C
0.003 0.0 0.008
.0.02 0.03
Figure 3-1. Rate of Calcium Sulfite Oxidation; No Added Catalyst
Initial pH = 4.6.
-------
TABLE 3-1. RATE OF CALCIUM SULFITE OXIDATION;
NO ADDED CATALYST; INITIAL pH = 4.6.
pH - 4.60
Standard
Run
No.
88
89
90
91
104
101
102
103
97
99
98
100
105
108
106
Initial
s+4
Concn.
. • —
0.00432
0.00364
0.00309
0.00240
0.00536
0.00422
0.00375
0.00295
0.00552
0.00444
0.00321
0.00243
0.00536
0.00443
0.00353
Rate Added Total Added
wlff9.**<>V* "" F*
_5
2.469x10
-5
1.95x10
_S
1.547x10
-5
1.058x10
_5
2.409x10
_5
1.635x10
_5
1.413x10
-6
9.905x10
-5
1.631x10
_5
1.109x10
-6
7.381x10
-6
4.897x10
-6
9.989x10
-6
7.008x10
-6
5.236x10
i — i
° i*
0 ' — '
« 11
0 "
rv 1 1
0 to
+
0 ' — '
0 *
h- '
0 C
0 *£>
Ul
„ T3
0 ^
0 ^
0 O
h-1
0 to
+
0
0
0
-
Q
n
\J
-
Total Order|-4 error of
Fe for S Order
— — :
i — i
to
i i
II
.1 — i
to
4>
I I
X
IJ1
OO
•d
B
1.
to
+
1.49
1.49
1.49
1.49
1.48
1.48
1.48
1.48
1.44
1.44
1.44
1.44
1.54
1.54
1.54
0.048
0.048
0.048
0.048
0.047
0.047
0.047
0.047
0.062
0.062
0.062
0.062
0.17
0.17
0.17
"C Rate Cons
Temp, (for 1.5 0
_
40
40
40
40
35
35
35
35
30
30
30
30
25
25
25
0.0885
0.0885
0.0385
0.0885
0.061
0.061
0.061
0.061
0.039
0.039
0.039
0.039
0.025
0.025
0.025
73
-------
TABLE 3-2.
RATE OF CALCIUM SULFITE OXIDATION;
[Mn] =0.6 ppm
pH = 4.60
Run
No.
141
142
143
146
145
147
150
148
149
151
Initial
Concn.
0.0055
0.00441
0.00358
0.00456
0.00360
0.00249
0.00542
0.00436
0.00341
0.00252
Rate Added
mol/ (H sec)Mn
3.5304xlO~5
2.658xlO~5
1.951xlO~5
1.769xlO~5
1.217xlO"5
7.402xlO~6
1.469xlO~5
1.057xlO"5
8.036xlO~6
4.712xlO~6
0.520
0.533
0.548
0.533
0.548
0.567
0.520
.0.533
0.548
0.567
Total Added Total Orde^+
Mn Fe Fe for S
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0
0
0
0
0
0
0
0
0
0
r— i
'n>
i — i
ii '
r— i
Cfl
i — i
X
t_n
Ui
00
1
(D
3"
5
1.55
1.55
1.55
1,43
1.43
1.43
1.46
1.46
1.46
1.46
Standard
error of
Order
0.
0.
0.
0.
0.
0.
0,
o.
0
0
058
058
058
10
10
10
,092
.092
.092
.092
°C Rate Constant
Temp. (for 1.5 Order)
30
30
30
25
25
25
20
20
20
20
0.09
0.09
0.09
0.059
0.059
0.059
0.038
0.038
0.038
0.038
74
-------
TABLE 3-3. RATE OF CALCIUM SULFITE OXIDATION;
ADDED Mn AND Fe.
pH = 4.60
Run
No.
156
153
155
154
157
159
158
160
161
163
162
164
Initial
S4+
Cone.
0.00275
0.00373
0.00523
0.00578
0.00276
0.00406
0.00526
0.00606
0.003033
0.004232
0.005154
0.00589
$ate 4dded
mol/(£ sec)""
1.2532xlO~5
1.9608xlO~5
3.1713xlO"5
3.6497xlO~5
7.4903xlO"6
1.3428xlO~5
2.0135xlO~5
2.5687xlO~5
6.623xlO~6
1.0537xlO~5
1.4583xlO~5
1.7809xlO~5
0.497
0.478
0.450
0.447
0.497
0.478
0.450
0.447
0.497
0.478
0.450
0.447
Total
Mn
0.53
0.53
0.53
0.53
0.53
0.53
0.53
0.53
0.53
0.53
0.53
0.53
Added
Fe
0.888
0.813
0.702
0.699
0.888
0.813
0.702
0.699
0.497
0.478
0.450
0.447
Total Ordec,
Fe for S
1.02
1.02
1.02
1.02
1.02
1.02
1.02
1.02
1.02
1.02
1.02
1.02
1.44
1.44
1.44
1.44
1.56
1.56
1.56
1.56
1.50
1.50
1.50
1.50
Standard
error of
Order
0.013
0.013
0.013
0.013
0.024
0.024
0.024
0.024
0.043
0.043
0.043
0.043
°C Rate
Temp. (for 1
30
30
30
30
25
25
25
25
20
20
20
20
0.
0.
0.
0.
0.
0.
0-
0.
Constant
.5 Order)
084
084
084
084
0534
0534
0534
0534
0.0388
0.0388
0.0388
0.0388
75
-------
8x10
-5
6x10
-5
4x10
-5
2x10
-5
-5
Rate lxl°
mol/(£ sec) 8xlo"6
6x10
-6
4x10
-6
2x10
-6
0*001
[s4+]
* : 25°C
0.003 0.006 0.02 0.4
Figure 3-2. Rate of Calcium Sulfite Oxidation; [Mn] =0.6 ppm
-------
Rate
mol/(£ sec)
.-.I
8x10 L
6xlO~5
AxlO
-5
3x10
-5
2x10
-5
1x10
-5
8x10
-6
6x10
-6
AxlO
-6
3x10
-6
2x10
-6
0.001
[s4+]
: 30°c
* : 25°c
0.003 0.006 0.008 0.02 0.04
Figure 3-3. Rate of Calcium Sulfite Oxidation; [Mn] = 0.53 ppm, [Fe] = 1.02 ppm
-------
TABLE 3-4 RATE OF CALCIUM SULFITE OXIDATION;
VARIABLE [Mn]
Initial
Run
No.
166
165
163
164
169
167
168
170
173
171
172
Concn.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
002507
003209
003808
005692
002623
003703
005374
006392
002282
003903
005463
Rate
mol/(£ sec)
5.8112xlO~6
8.4172xlO~6
1.0557xlO~5
1.9732xlO~5
4.2683xlO~6
6.9873xlO~6
1.2189xlO~5
1.4892xlO~5
9.2781xlO~6
2.0372xlO~5
3.1309xlO~5
Added
Mn
5
5
5
5
3
3
3
3
10
10
10
Total
Mn
5
5
5
5
3
3
3
3
10
10
10
Total °rdeL.
Fe for S
"
-------
s:
a
B
o.
a.
O.
a
S
ex
a
C
C
T3
X
C
CO
— e
c a
o
c
c
IT*
C
C M
C ^=-
in i
1
0
X
00
in
1
o
•A
\ —
m m m in m
I I 1 I I
O 0 O O O
X X X X X
VD m
-------
2x10
-5
Rate
mol/(£ sec)
00
o
10
-5
3 ppm 5 ppm
10 ppm
[Hn]
Figure 3-5. Rate of Calcium Sulfite Oxidation; Variable [Mn],
= 0.0037 Mol/1
-------
-Ink
No added catalyst AE = 15.37 Kcal
k = 4.706xl09
o
[Mn] =0.6 ppm
AE = 14.31 Kcal
k = 1.798xl09
o
[Mn] =0.53 ppm AE = 13.09 Kcal
[Fe] = 1.02 ppm k = 2.156x10
k = k Exp(-AE/RT)
40°C
35°C
30°C
l/T x 1000
Figure 3-6. Effect of Temperature on the Rate Constant
81
-------
TABLE 3-5. EFFECT OF TEMPERATURE ON THE RATE CONSTANT
Temperature ( C)
Rate Constant, k
1.5
Rate of Reaction
[S 4+| = 0.01M
mol/(£ sec)
(No Added Catalyst)
20
30
40
50
60
70
80
(With Total 0.53 ppm Mn &
20
30
40
50
60
70
80
0.0161
0.0385
0.0871
0.1871
0.3841
0.7560
1.4321
1.02 ppm Fe)
0.03707
0.07785
0.1559
0.2992
0.5520
0.9827
1.6933
1.6xlO"5
3.9xlO~5
8.7xlO~5
1.87xlO~4
3.84xlO~4
7.56xlO~4
1.43xlO~3
3.7xlO~5
7.8xlO~5
1.56xlO~4
2.99xlO~4
5.52xlO~4
9.83xlO~4
1.69xlO~3
82
-------
1. Succinic Acid
a. Effect of pH on the Reaction Rate
Experiments were carried out on the oxidation of calcium sulfite at
a temperature of 50°C in the presence of 0.2 M succinic acid. Concentration
of S was determined as a function of time by titration as described earlier.
The reaction was studied over a range of pH from 4.0 to 5.5. The pH was
constant to within 0.1 pH units over the course of the experiment due to the
succinic acid buffer. No other pH control was required. Experiments could
not be carried out by this method below pH = 4.0 because of stripping SCL and
above 5.5, under these conditions, because of the high speed of the reaction.
(Measurements at both higher and lower pH can be done in the flow reactor
described earlier in this- report.) In the semi-batch reactor, about 9% of the
S is vented as SCL in 30 minutes at a pH of 4.0 and about 60% at a pH of
3.0. Corrections to the data for SC- loss can be made; however, since the
correction is so large at lower pH, results for pH less than 4.0 are not
reported. ,,
The results of the experiments are given in Table 3-6. Runs were made
4+
with an initial S concentration of 0.01 M for 4.0 < pH < 5.0. AtpH=5.5
4+
the initial S concentration was reduced to 0.005 M to insure complete dis-
solution. To compare pH = 5.0 and 5.5 experiments were also carried out at pH
4+
= 5.0, (S ). = 0.005 M, and 0.0075 M. Care must be taken in interpreting
results of this type since there are impurities in the GaSCL and these im-
purities are the catalyst for the oxidation reaction. The catalyst strength
is listed in columns 3 and 4. No catalyst was added to these solutions in
4+
addition to that already present in the CaSCL. The order with respect to S
i j
and the rate constant for that order are shown in columns 5 and 7 respec-
tively. Within the accuracy of the results, these reactions are all 1.5
83
-------
TABLE 3-6
EFFECT OF pH ON SUCCINIC ACID BUFFERED CaS03 SOLUTIONS
4+
oo
Initial
pH
4.0
4.3
4.6
4.8
5.0
5.0
5.0
5.5
Initial S
Concentrat ion
mol/£
0.01
0.01
0.01
0.01
0.01
0.0075
0.005
0.005
Mn Impurity
Concentration
(ppm)
0.14
0.14
0.14
0.14
0.14
0.105
0.07
0.07
Fe Impurity
Concentrat ion
(ppm)
0.56
0.56
0.56
0.56
0.56
0.42
0.28
0.28
Order With
Respect to
s4+
1
1
1
1
1
1
1
1
.35
.59
.52
.63
.64
.92
.25
.64
Standard
Deviation
on Order
0.10
0.89
0.32
0.11
0.12
0.36
0.12
0.25
Rate
Constant
(k)
0.0040
0.012
0.015
0.037
0.082
0.010
0.0034
0.28
1.5 Order
Rate Constant
0.
0.
0.
0.
0.
0.
0.
0.
0096
0074
014
019
037
016
015
109
Run
Numbers
566,
510,
506,
511,
502,
587,
3.2
517,
515,
520,
567
512
507
513
514,
588
518
516
522
All runs were done at 50 C
-------
order and the rate constant for a 1.5 order reaction is shown in column 8 and
graphed in Figure 3-7. The units of h^5 are ^^mol^^sec1. It can be
clearly seen from Figure 3-7 that the rate of reaction increases with in-
creasing pH with the rate increasing more rapidly above pH - 5.0. The cata-
lyst concentration also proves to have a substantial effect.
It has been proposed by several investigators that only the sulfite ion
reacts in solution. Although no conclusions on species reactivity can be
drawn from this data, the results are consistent with the concept since the
4+
concentration of sulfite ion increases with pH at constant total S concen-
tration over the range of pH studied.
b. Effect of succinic acid concentration
The rate of oxidation was studied over a range of succinic acid
concentrations from 0 to 2.0 M (23,600 ppm) at pH 4.0 and from 0.01 M to 0.2 M
at pH 5.0. At pH 5.0 lower acid concentrations could not be used since the
reaction rate would become too large. The reaction must proceed slowly enough
so that a sufficient number of samples could be taken and the pH held approx-
imately constant over the course of a run. At succinic acid concentrations of
0.1 M and higher, there was sufficient buffer to hold the pH constant at 4.0
without outside means; at lower concentrations a pH controller was used. The
catalyst levels were constant in all cases since the initial sulfite concen-
tration was 0.01M for all experiments. All experiments were performed at
50°C.
Typical results are shown in Figures 3-8 and 3-9 for pH 4.0 and in Figure
3-10 for pH 5.0 with varying amounts of succinic acid. From these measure-
ments of [S4+] vs time, an initial reaction rate and also a rate expression
were determined. The latter was obtained by the best fit of the data through
the expression r = kc®. Here C is the S4+ concentration. The initial rate is
85
-------
0.12
0.10
0.08
0.06
1.5
0.04
0.02
4.0
.4+,
/ I
'
I
8
4.5
pH
5.0
5.5
(S ) = 0.01M; (Mn) = 0.14 ppm; (Fe) = 0.56 ppm
(S4+) = 0.0075 M; (Mn) = 0.105 ppm; (Fe) = 0.40 ppm
(S4+) = 0.005 M; (Mn) = 0.07; (Fe) = 0.28 ppm
T = 50°C
Figure 3-7. Effect of pH on Rate of Reaction
86
-------
0.010 •
Run 600, No succinic acid
Run 602, 0.014 M succiaic acid
Run 603, 0.042 M succinic acid
50°C, pH - 4.0
0.008
mol/3.
0.006
0.004
0.002
tine (ndn)
Figure 3-8. Concentration Sulfur f4+) vs. Time for
Various Concentrations of Succinic Acid
87
-------
0.010'
0.008
s4+
mol/Z
0.006
0.004
o.oo;
Conditions
T - 50°C, pH * 4.0
20
40
60
80 100
time (min)
120
140
160
Figure 3-9. Concentration Sulfur (4+) vs Time
Run 566, 0.2M Succinic Acid
88
-------
0.01
0.008
0.006
Sulfur Anlon
concentration
0.004
0.002
Conditions
PH = 5.0
50°C
1800 rpm
time(min)
8
10
symbol
A
•
O
3.2
24.2
25.2
26,2
composition
0.2 M succinic acid
0.1 M succinic acid
0.05M succinic acid
0.01M succinic acid
Figure 3-10.
The Inhibition of the Sulfite Oxidation Due
To Succinic Acid.
89
-------
shown as a function of succinic acid concentration in Figure 3-11 for pH 4.0.
The strong influence of the organic acid is shown.
Summaries of the results may be found in Tables 3-7 and 3-8 for pH 4.0
and 5.0 respectively. The results without succinic acid at pH 4.0 were ob-
tained from only 3 or 4 titration points per run and are, therefore, somewhat
less reliable. It might be noted that the rate of reaction given in Table 3-7
for no organic acid and pH 4.0 is somewhat higher than that reported in
Section IIA for pH 4.6. Since the rate always increases with increasing pH,
under these conditions when all other parameters are held constant, the
higher rate is probably caused by the higher catalyst levels present at an
initial S4+ concentration of 0.01 M compared to the lower levels of catalyst
in Section IIA. The effects of catalysts are discussed elsewhere in the
report.
The order of the reaction increases with increasing pH for the ranges
being studied. At pH 4.0 and low succinic acid concentrations, the reaction
can be adequately described by a first order expression; however, with in-
creasing levels of acid, the order rises above 1.0.
2. Adipic Acid
The effect of adipic acid on the rate of oxidation of calcium sulfite was
studied at T = 50°C and pH = 5.0. Typical results are presented in Figure
3-12 as concentration vs time for various adipic acid concentrations. The
results are summarized in Table 3-9. The reaction order is approximately 1.5
at low adipic acid concentration and rises with increasing acid concentration.
All experiments were carried out with an initial S4+ concentration of 0.01 M.
3. Glycolic Acid
The effect of glycolic acid on the rate of oxidation was studied at
T = 50°C and pH = 4.0 and pH = 5.0. Representative results are presented in
90
-------
3.0 x 10~4-
2.0 x 10
Rate
mol/£-sec
—4
1.0. x 10
-4
0
.02
•04 .06 .08 .1
Goncentxation of Succinic Acid
(mol/£)
Figure 3-11. Initial Rate vs Succinic Acid Condentration
(All runs were at 50°C and an initial pH of 4.0)
0.2
91
-------
TABLE 3-7
EFFECT OF SUCCINIC ACID ON OXIDATION RATE
Run #
598
599
600
605
606
601
602
603
604
585*
586*
566*
567*
Succinic Acid
Concentration
mol/£
0
0
0
0.00141
0.00141
0.01355
0.01355
0.04234
0.04234
0.10021
0.10021
0.20000
0.20000
Initial Rate
mol/U sec)
2.45 x 10"4
2.21 x 10~
2.49 x 10"4
1.58 x 10~4
1.44 x 10~4
7.99 x 10~5
7.02 x 10~5
2.64 x 10~5
3.31 x 10~5
1.07 x 10~5
1.56 x 10~5
3.30 x 10~6
5.40 x 10"6
Order Witn stanaara
Respect to Deviation Rate
Sulfur (4+) of Order Constant
1.21 0.07 0.13613
0.94 0.08 0.01885
1.19 0.08 0.02339
1.34 0.11 0.02088
1.33 0.16 0.00757
1.35 .10 .00397
All runs were done at 50°C and an initial pH of 4.0. The * indicates that in
these runs there was sufficient succinic acid present to buffer the experiment;
thus the computer was not used. (To convert mol/A of succinic acid to ppm
multiply by 118090.)
92
-------
TABLE 3-8
RESULTS OF ORDER AND RATE DETERMINATION ANALYSIS
OF DATA FOR CALCIUM SULFITE OXIDATIONS WITH
VARYING CONCENTRATIONS OF SUCCINIC ACID ADDED
Concentration
of succinic
acid added
-(nil/A)
0.2
0.1
0.05
0.01
Order of
reaction with
respect to
sulfite
1.64
1.84
1.82
1.45
Standard
Deviation
0.12
0.12
0.15
0.06
Conditions:
pH = 5.0
50°C
1800 rpm
Rate constant
Rate (=) mol/sec
0.08230
0.245
0.350
0.231
93
-------
0.01
0.008
0.006
Sulfur Anion
Concentration
0.004
raol/H
0.002
pH
50°C
1800 rpm
symbol
•
A
•
O
0
11.2
12.2
27.2
34.2
33.2
composition
0.2 M adipic acid
0.1 M adipic acid
0.05 M adipic acid
0.01 M adipic acid
0.005M adipic acid
Figure 3-12. The Inhibition of the Sulfite Oxidation Due
To Adipic Acid.
94
-------
TABLE 3-9
RESULTS OF ORDER AND RATE DETERMINATION ANALYSIS
OF DATA FOR CALCIUM SULFITE OXIDATIONS WITH
VARYING CONCENTRATIONS OF ADIPIC ACID (72)
Concentrat ion
Order of reaction
Rate
or adipic acid
(mol/£)
0.2
0.1
0.05
0.01
0.005
with respect to Standard Constant
sulfite Deviation Rate = mol/se,
1.94 0.21
1.84 0.12
1.49 0.07
1.42 0.09
1.53 0.10
Conditions:
pH = 5.0
50°C
1800 rpm
0.218
0.314
0.109
0.193
0.279
95
-------
Figures 3-13, 3-14, and 3-15; summaries of the results are presented in Tables
3-10 and 3-11 for pH = 4.0 and 5.0 respectively. The rate of reaction at
[S4+-| _ Q Q1 M as caicuiated from the rate expression is shown in Figure 3-16
for pH 4.0 as a function of glycolic acid concentration.
Glycolic acid is a very strong inhibitor for the oxidation, much stronger
than any of the other acids studied. At pH = 4.0 and no organic acid, the
oxidation was completed in about one and one-half to two minutes. With an
addition of 1000 ppm glycolic acid the experiment takes up to an hour or more.
With a concentration of 0.2 M glycolic acid, 50% of the sulfur (4+) had not
oxidized in four hours.
4. Comparison of Rates of Oxidation with Succinic Acid, Adipic
Acid, Glycolic Acid, Citric Acid, and Acetic Acid
In Figures 3-17 and 3-18 a comparison is made between the succinic,
adipic and glycolic acids to demonstrate which has the greatest inhibitory
effect. In Figure 3-17 an acid concentration of 0.2 M was added in each run.
As can be seen, glycolic acid is the greatest inhibitor followed by adipic
acid then by succinic acid. In Figure 3-18 an acid concentration of 0.01 M
was added. Here again the same trend of inhibitory strength (glycolic >
adipic > succinic) is observed. This same information can be obtained by
noting the rate constants and rate of reaction at [S4+] = 0.01 M listed in
Tables 3-12.
The structure of the organic acids may be looked at to aid in the under-
standing of the inhibition. It has been reported by Backstrom that alcohols
inhibit the sulfite oxidation. This inhibition is due to the alcohol group
reacting with free radicals in the sulfite solution (15). Several organic
acids have lone alcohol groups off of the carbon chain, and all organic acids
have carboxyl groups that have reactive sites similar to alcohols. The re-
lative inhibitory strength of organic acids can be attributed to this reaction
96
-------
0.01
0.008
0.006
Concentration
mol/l
S
0.004
0.002
6)0
Q
S
Q
S 500 ppm Glycolic Acid
D 1000 ppm Glycolic Acid
A 2000 ppm Glycolic Acid
X 4000 ppm Glycolic Acid
10 2° 30 40 50 60 70 80
time (min)
Figure 3-13. Concentration S * va_ Tine for»Varioua
Concentrations of Glycolic Acid
90
100
97
-------
0.01
0.008
0.006
Sulfur Anton
Concentration
0.004
mo
0.002
pH » 5.0
50°C
1800
0246
8 10
time(rain)
symbol
46.2
38.2
37.2
41.2
0.0263 M(2000 ppm) glycolic acid
0.0131 M(1000 ppm) glycolic acid
0.0066 M(500 ppm) glycolic acid
no glycolic acid
Figure 3-14.
Inhibition of the Sulfite Oxidation Due to Glycolic Acid.
98
-------
0.01
0.008
0.006
Sulfur Anion
Concentration
0.004
mol/JZ.
0.002
Conditions
50°c
1800 rpm
8
10
tin«(min)
symbol
A
O
609
38.2
61?
37.2
composition
0.013 MjlOOO ppra) glycolic acid, PH 4.0
0.013 M(t000 ppn) glycolic acid, PH 5.0.
0.0066 M(500 pp») glycolic acid, pH 4.0
0.0066 M(5QO ppm) glycolic acid, pH 5.0
Figure 3-15.
The pH Effect on the Inhibition of the Sulfite
Oxidation Due to Glycolic Acid.
99
-------
TABLE 3-10
EFFECT OF GLYCOLIC ACID ON OXIDATION RATE OF CALCIUM SULFITE
Run*
598
599
600
613
617
609
610
611
618
619
621
622
623
Concentration of
Glycolic Acid
ppm
*
0
0
0
500
500
1000
1000
1000
2000
2000
4000
4000
15200
Order With Standard ^^.^
Respect to Deviation Rate gl^+l = 0>01
Sulfur (4+) of Order Constant moi/sec
__ • —
-4
1>21 0.07 0.136 5.18 x 10
X.48 • 0.10 0.046 5.05 x 10"
!.40 0.09 0.0118 1.87 x 10
1>37 0.12 0.00336 3.31 x 10
0<96 0.44 0.00009 1.08 x 10~
2>03 0.88 0.0060 5.24 x 10
T = 50°C; pH
• oH - 4.0. Initial concentration of sulfur (4+) is 0.01 M.
100
-------
TABLE 3-11
RESULTS OF ORDER AND RATE DETERMINATION ANALYSIS
OF DATA FOR CALCIUM SULFITE OXIDATIONS WITH
VARYING CONCENTRATIONS OF GLYCOLIC ACID ADDED (72)
Concentration
of glycolic
acid added
Order of
reaction with
respect to
sulfite
Standard
Deviation
Rate constant
Rate = mol/sec
0.0263 M
(2000 ppm)
0.0131 M
(1000 ppm)
0.0066 M
(500 ppm)
1.72
1.83
1.72
0.30
0.10
0.08
0.028
0.270
0.516
Conditions:
pH = 5.0
50°C
1800 rpm
101
-------
6x10
-4
5x10
-4
5x10
-5
Rate
mol/(£ sec) c
4x10
3x10
-5
2x10
-5
1x10
-5
4000 8000 12000
Concentration Glycolic Acid (ppm)
16000
Figure 3-16. Effect of Glycolic Acid on Rate of Oxidation
of CaSO., at Temperature = 50 C, pH = 4.0
[S4+J = 0.01 M.
102
-------
0.01
0.008
0.006
Sulfur
Anion
Concentration
0.004
mol/Ji
0.002
Conditions
50°C
pH - 5.0
1800 rpm
8
tiae(min)
10
symbol
• 11.2
• 3.2
negligible reaction in 10 Minutes
corn-position
0.2 M adipic acid
0.2 M auccinic acid
0.2 M glycolic acid
Figure 3-17.
Comparative Strength of Various Organic Acids
as Inhibitors at 0.2 M.
103
-------
0.01
0.008
0.006
Sulfur
Anion
Concentration
0.004
mo
I/A
0.002
symbol
38.2
34.2
26.2
coraT>Q3ition
0.01 M glycolic acid
0.01 M adipic acid
0.01 M succinic acid
Figure 3
_18. Comparative Strength of Various Orgnanic Acids as
Inhibitors at 0.01 M.
104
-------
TABLE 3-12
EFFECT OF ORGANIC ACIDS ON OXIDATION RATE OF CALCIUM SULFITE
Run//
— — — .
502,514,589,588
552,554,595,596
562,563,591,592
564,565,593,594
623
Acid Used
Succinic
Adipic
Citric
Acetic
Glycolic
Order With
Respect to
Sulfur (4+)
1.64
1.9.4
2.31
2.20
2.03
Standard
Deviation
of Order
0.12
0.21
0.15
0.20
0.88
Rate
Constant
0.082
0.218
12.78
1.61896
.0.0060
Rate at
fS4+l = 0.01M,
mol/sec
4.3 x 10
-5
2.87 x 10
-5
30.65 x 10
-5
All runs were done at 50°C and an initial PH of 5.0. All acid concentrations are
0.2 mol/£. (23618 Ppm succinic, 29230 Ppm adipic, 38426 Ppm citric, 12000 ppm acetic,
15210 ppm glycolic.
105
-------
with free radical type inhibition. Experimental work gives evidence favoring
this idea. Acetic acid (CH^OOH) was added to the reaction mixture in a
concentration of 0.2 M; the reaction was completed in just less than a minute.
Glycolic acid (HOCH^OOH) was added to the reaction mixture in a concentration
of 0,2 M also; the reaction did not proceed in 20 minutes. The difference
between glycolic acid and acetic acid is simply an alcohol group (OHO in
place of a hydrogen on the first carbon. This suggests that the alcohol group
of the glycolic acid is reacting with the free radicals needed for the oxida-
tion, thus slowing down the oxidation.
Another organic acid, citric acid, which is slightly more complex than
glycolic or acetic acid was added to the reaction mixture in a concentration
of 0.2 M. The sulfite oxidation was completed in about 1 minute. Citric acid
does have an alcohol group extending from the carbon chain, but it also has
three carboxyl groups. One possible explanation suggests that steric hin-
derance due to these large carboxyl groups prevents the alcohol group from
reaction with the radicals and slows down the reaction. Backstrom observed a
similar effect with tertiary butyl alcohol, a sterically hindered alcohol
(14).
Not only do the lone alcohol groups react with the radicals in solutions,
but the OH' groups of the carboxyl group can react slightly with the radicals.
This is shown with adipic and succinic acids since both inhibit the reaction
to some extent. With concentrations of 0.2 M in solution the oxidation was
50% completed in four minutes with succinic acid and in five minutes with
adipic acid. There is less hindrance between the groups in adipic acid
because the carboxyl groups are further apart. This allows slightly more
reaction between the radicals and adipic acid than with succinic acid in
solution. This result is seen in the fact that adipic acid inhibits slightly
more than succinic acid.
106
-------
Although some inhibition is observed from organic acids without alcohol
groups, more is observed from those with alcohol groups. The OH* group of the
carboxyl group does not react as much as the lone OlT group due to the shield-
ing by the oxygen on the carboxyl group.
5. Effect of Catalysts - Manganese and Iron
a. Succinic acid
Manganese and iron catalyze the oxidation even in the presence of
the organic acids which inhibit the reaction. In discussing the effect of
catalysts on the oxidation, it is important to consider both the catalysts
present in the calcium sulfite as an impurity and also catalysts added to the
reaction mixture. Manganese is present as are impurities also in most lime-
stone.
A dry sample of calcium sulfite has impurity levels of 62 ppm man-
ganese and 248 ppm iron. From this, the amount of Mn and Fe present at a
given liquid sulfite concentration can be calculated. Table 3-13 shows the
iron and manganese impurity levels for some initial CaS03 concentrations.
Knowing the amount of Mn impurity present at 0.01 M CaS03 (for example) makes
it possible to add a known amount of Mn to 0.005 M CaS03 making the total Mn
concentration equal to that of the 0.01 M sulfite solution. In Table 3-14 a
comparison between catalyzed and uncatalyzed runs is made. In runs 527 and
528 0.1 ppm Mn was added to bring the Mn level to that of 0.015 M CaSO^ As
can be seen the 0.015 M rate constants compare well to the 0.0075 M rate
constants with added catalyst, while the 0.0075 M runs without catalyst have
rate constants which differ. From a plot of sulfur (4+) concentration vs time
(Figure 3-19) it can be seen that the 0.015 M and the catalyzed 0.0075 M have
s^ilar rates of oxidation while the 0.0075 uncatalyzed run is noticeably
slower. Similar results can be seen in Table 3-9 for 0.005 M and 0.01 M runs.
107
-------
TABLE 3-13
CATALYST IMPURITY LEVELS IN CaS0.3 SOLUTIONS
4+
Initial S
Concentration (mol/fc.)
0.005
0.0075
0.01
0.015
Fe
(ppta)
0.279
0.419
0.558
0.837
Mn
(ppm)
0.070
0.105
0.140
0.209
108
-------
TABLE 3-14
EFFECT OF Mn IMPURITY ON OXIDATION RATE
Run//
517,518
527,528
525,526
o 515,516
529,530
502,514
Initials4"1"
Concentration
0.0075
0.0075
0.015
0.005
0.005
0.01
Mn Impurity
Level (ppm)
0.105
0.105
0.209
0.070
0.07
0.140
Added Mn
(ppm)
0
0.1
0
0
0.07
0
Total Mn
(ppm)
0.105
0.205
0.209
0.07
0.140
0.140
Order With .,
Respect to S
1.42
1.66
1.74
1.25
1.22
1.64
Standard
Deviation
of Order
0.36
0.24
0.17
0.34
0.28
0.12
Rate
Constant
0.0101
0.251
0.346
0.0034
0.0076
0.082
Temperature = 50°C; pH = 5.0
-------
12 13 14- 15 16 17 13
78 9 10 11
time (min)
Run 525, .015 M CaSO3, No Mn A3ded
Run 527, .0075 M CaS03, 0.1 ppn Mi
Run 517, .0075 M CaS03, So Mn Added
figure 3-19. Sal-fur ( 4<> Concentratrlon .„_ Time for Various Mn Concentrations
.igure QOc and H of
110
-------
Experiments were also performed keeping the initial sulfur (4+)
concentration constant and varying the amount of manganese catalyst added.
These experiments along with the runs mentioned earlier were analyzed using
the MULTREG computer program described above. Each different set of condi-
tions was run individually on the program to determine the order with respect
to sulfur (4+). The conditions and results of these experiments are listed in
Table 3-15. In the more highly catalyzed runs, the experiment was often over
within the first few minutes. Thus, in these runs there were fewer points
with which to make the determinations listed. This caused some scatter in the
results.
The rate of reaction at [S +] = 0.01 M is shown in Figure 3-20 as a
function of manganese concentration.
b. Glycolic acid
Experiments were run at a constant concentration of glycolic acid
with varying amounts of manganese catalyst. This study was done to see if the
addition of catalyst could overcome the effects of a strong organic acid
inhibitor and to what degree the two additives, inhibitor and catalyst, af-
fected the oxidation rate. All experiments were run at 50°C, initial pH of
4.0 and initial sulfur (4+) concentration of 0.01 M. A concentration of 1000
ppm glycolic acid was chosen for the organic acid additive.
Representative results are presented in Figure 3-21, where concen-
tration vs time is plotted at various glycolic acid concentrations. The
results are summarized in Table 3-16. The reaction order is between 1.0 and
1.5 for much of the range and in order to compare the behavior as a function
of manganese concentration the rate of reaction at a concentration of [S4+] =
0.01 M is given and plotted in Figures 3-22 and 3-23 (expanded scale). From
the table and the plots it can be seen that the effects of both additives is
111
-------
TABLE 3-15
OF Mn CONCENTRATION ON OXIDATION RATE OF SUCCINIC ACID BUFFERED
CaS03 SOLUTIONS
Initial
S
Cone. molM
0.015
0.0075
0.005
0.005
0.005
0.01
0.01
0.01
0.01
0.01
0.01
pH
.!•" —
5.0
5.0
5.3
5.5
5.0
5.0
5.0
5.0
5.0
5.0
5.0
Added
Mn
Catalyst
(ppm)
0
0.07
0.07
0.07
0.1
0.1
0.2
0.3
0.4
0.5
1.0
Total
Mn
(ppm)
0.21
0.17
0.14
0.14
0.17
0.24
0.34
0.44
0.54
0.64
1.14
Order with
Respect to
4+
S
1.75
1.23
1.91
1.78
1.66
1.79
1.53
1.41
1.47
1.71
1.82
Standard
Deviation
of Order
0.17
0.28
0.23
0.22
0.24
0.08
0.15
0.27
0.23
0.14
0.07
Rate
Constant
0.346
0.0076
1.249
1.078
0.251
0.379
0.148
0.0912
0.173
0.818
2.0148
Rate
[S4"1"] = 0.01M,
mo I/sec
1.09 x 10~4
-5
2.65 x 10
-4
1.89 x 10
-4
2.97 x 10
-4
1.20 x 10
-5
9.97 x 10
-4
1.29 x 10
-4
1.38 x 10
-4
1.99 x 10
-4
3.11 x 10
-4
4.62 x 10
Temperature = 50°C
-------
5x10
4x10
-4
Rate
3x10
mol/(l sec)
,-4
2x10
1.0x10
0.2 0.4 0.6 0.8
Total Concentration Mh (ppm)
1.0
1.2
Figure 3-20. Effect of Mn on Oxidation Rate of Succinic Acid Buffered
CaS03 Solutions for [S4+] • 0.01M
113
-------
0.010
• Run 609, 0.0 ppm Mci
Q Run 627, 0.5 ppra On.
• Run 624, 1.0 ppnm
ARun 634, 2.0 ppn to
638, 3.0
50°C, pH 4.0, 1000 ppm Glycolic Acid
0.002
Figure 3
;-21. Concentration Sulfur (4+) vs Time, 1000 ppm Glycolic Acid,
Various Concentrations of Mn
-------
TABLE 3-16
EFFECT OF MANGANESE ADDITION TO GLYCOLIC ACID INHIBITED RUNS
Concentration Order With Standard
Rate at
Run*
609
610
611
629
630
627
628
635
636
624
626
631
632
633
634
638
639
640
641
nuucu mi respect to Deviation
(ppm) Sulfur 4+ of Order
P.
0 1.40 0.09
o
1.26 0.16
0.2
1.26 0.07
0.5
1.17 0.06
0.7
1.0
1.23 0.07
1.0
1.5
1.27 0.07
-L • J
2.0
1.13 0.06
2.0
3.0
0.19
3.5
1.60 0.29
O*O
Rate
Constant
0.01182
0.00706
0.01404
0.00908
0.14941
0.03089
0.02154
0.23892
0.463
4+
[S J = 0.01
mol/(£ sec)
1.87 x 10~5
2.13 x 10~5
4.24 x 10~5
4.15 x 10~5
5.18 x 10~5
8.91 x 10~5
11.8 x 10~5
22.8 x 10~5
29.2 x 10~5
All runs were done at 50°C, PH of 4.0, initial sulfur (4+) concentration of
0.01 molar and glycolic acid concentration of 1000 PPm.
115
-------
3.0x10
2.0x10
Rate
4+
(S )
0.01
mol/U sec)
1.0x10
A A
1 2 3
Added Mn Concentration (ppm)
Figure 3-22. Effect of Mn Addition to Glycolic Acid Inhibited
CaS03 Oxidation. Rate at [S4*] - O.OIM.
116
-------
5.0xlO~5
4.0xlO~5
**** 3-OxlO-5
ratal/ (4 sec)
2.0xlO~5
l.OxlO"5
-
A
A
' ' 1 ' t 1
0.2 0.4 0.6 0.8 1.0
Added Mn Concentration (ppm)
Figure 3-23. Effect of Mn Addition to Glycolic Acid Inhibited
CaS03 Oxidation; Rate at [S4+] - 0.01 M.
U7
-------
being observed. Adding manganese increases the oxidation rate, but the strong
inhibitory effect of the glycolic acid is also apparent. With 1000 ppm gly-
colic acid, it takes a manganese concentration of 3.5 ppm to approach the
oxidation rate of an uninhibited run. Earlier it was found with 23667 ppm
(0.02 M) succinic acid only 1 ppm manganese was required for this oxidation
rate. Thus the oxidation rate of an inhibited run can be increased by adding
catalyst but the results of the experiment depend on the relative strength and
concentrations of both the catalyst and the inhibitor added.
C. REACTION IN LIQUOR FROM PENBERTHY OXIDATION RUNS AT SHAWNEE
Oxidation studies were performed using liquor from slurry samples from
the Shawnee test facility. The slurry samples were from Penberthy oxidation
runs TSO-2D, TSP-2F, TSP-2G and TSP-2H. The experiments were performed to
determine the degree of activity of the samples in catalyzing the oxidation of
sulfite.
The experiments were carried out in the semi-batch reactor used in the
majority of the work. The Shawnee samples were filtered and 0.2 M succinic
acid buffer and S4+ as sodium sulfite were added. Even though sodium sulfite
was added there is calcium in the filtrate and, therefore, these results are
included in the calcium sulfite section.
Data for the oxidation of sodium sulfite in clear (filtered) Shawnee
samples are collected in Table 3-17. Also listed for comparison are catalyzed
and uncatalyzed sulfite oxidations in water. All reactions were performed at
50°C and buffered. The Shawnee samples can be divided into three general
categories. The first type is the AP-2816 filtrate. This reacted at a rate
that was convenient to follow by our procedures. The Shawnee sample has some
catalytic activity but the results indicate that the rate of oxidation is less
than that obtained in unsaturated clear solutions of calcium sulfite.
118
-------
TABLE 3-17
OXIDATION OF SULFITE SOLUTIONS
Initial Rate
Solute
Solvent ( 0.0 1M)
Water CaSO
Water CaSO
Water Na0SO
Ap-2816
Filtrate Na2S03
2830
TSP-2D Na SO
Start J
2830
TSP-2D Na-SO,
End L J
2830
TSP-2F Na SO
Start 2 3
2830
TSP-2F Na-SO,
End 2 J
2830
TSP-2G Na SO
Start J
2830
TSP-2G Na,SO,
End z J
2830
TSP-2H Na SO
Start 3
2830
TSP-2H Na,SO,
End i 3
(first minute)
(mol/£ sec)
5.0 x 10~5
8.0 x 10~5
1.6 x 10~5
2.1 x 10~5
No reaction
^15. x 10~5
No reaction
>.13. x 10~5
No reaction
>_13. x 10~5
No reaction
^13. x 10~5
Catalyst
(ppm
0
0
0
0
0
0
0
0
0
0
0
0
Added
Mn)
.5
119
-------
The second category of the Shawnee samples contains all of the "starts"
of the 2830 TSP pairs. Sulfite solutions of these samples did not oxidize at
all over a 25-35 minutes period of oxygen sparging.
This behavior has been termed inhibition. In general it is a chemical
effect due even to very low concentrations of certain substrates (inhibitors).
The operation of inhibition takes two general paths: denying the reaction
mixture of the catalyst's activity; or, interrupting the active species being
oxidized. The inhibitors are usually organic molecules (although Cu and CN
show inhibitive properties) especially those bearing prominent OH groups.
Rubber shows total inhibition in only small concentrations. Since slurry
handling equipment is frequently rubber lined there .are opportunities for
contamination. Samples from the start of a run may have only low catalyst
concentrations and ambient inhibitor levels. As the run progresses, catalyst
ions may collect from successive additions of carbonate until the slurry is
saturated in these constituents. This much augmented catalyst content may now
overwhelm the inhibitors capacity to stop the oxidation.
The third type of Shawnee sample encountered had such a high catalytic
activity that reliable rates are difficult to obtain by the methods used.
These samples include all the "ends" of the 2830 TSP pairs. In oxidizing sul-
fite solutions in these liquors, most of the slufite had reacted between the
introduction of oxygen and the first sampling one minute later. The results
are meaningful, however, in that they indicate a lower boundary to the initial
reaction rate. The reaction initially proceeds at least as fast as oxidation
of calcium sulfite in water which contains 0.5 ppm Mn catalyst. Supplemental
analysis indicated a manganese level of over 11 ppm in the liquid (see Table
3-18).
120
-------
TABLE 3-18
SUPPLEMENTAL ANALYSIS OF A SHAWNEE SAMPLE*
Ca Cu Co Fe Zn Mn
818 0.13 0.15 0.06 18.10 11.50
Solid 14.0% 42.02 6.67 1.28% 122.06 42.0
Combined liquid and
solid 16354 4.79 0.89 1425 31.64 16.16
Total suspended solids = 110.97 g/£
*
Shawnee Number
2830
TSP-2H (All values are ppm unless specified)
End
Analysis done by North American Exploration Company of Charlottesville,
Virginia.
12.1
-------
D. DEPENDENCE OF OXIDATION RATE ON STIRRING SPEED AND OXYGEN FLOW RATE
The stirring speed and the oxygen flow rate must be fast enough to insure
that only chemical kinetics were being studied. Therefore, each of these
parameters was varied over a wide range.
Replicate runs were made at various stirring rates in order to determine
the point at which mass transfer no longer limits the rate of reaction and in
order to ascertain the reproducibility of the data. Oxygen was bubbled
through 0.01 M (unsaturated) calcium sulfite solutions at stirring speeds
ranging from 200 to 3500 rmp. These stirring speeds gave an agitation varying
from a gentle swirl to a complete froth. Temperature was held constant at
50°C. The pH was maintained at 5.0 with the use of 0.2 M succinic acid
buffer. A pH of 5.0 was chosen so that HSO~ would be the predominant sulfite
species at this temperature. Data were taken as sulfite concentration vs time
of oxidation and computer analysis determined rate constants and orders.
Typical plots of concentration vs time are shown in Figs. 3-24 and 3-25. At
low stirring rates (Fig. 3-24, 215 rpm) the reaction is mass transfer con-
trolled and the concentration of S4+ decreases linearly with time. At higher
speeds (500-3500 rpm) the data can be fit by a second order curve. In addi-
tion, initial rates (the decrease in sulfite concentration occurring in the
first minute of reaction) were determined. (The actual order of the reaction
is below 2.0 but above 1.0 as discussed above. However, a second order can be
used here since it fits the data reasonably well - see Table 3-19 - and can be
safely used as an internal comparison which is the only purpose of this sec-
tion.) These initial rates are collected in Table 3-19. It can be concluded
that mass transfer effects are unimportant at these concentrations for stir-
ring speeds above 500 rpm.
122
-------
0.010
0.008-
0.006-
0.004-
0.002-
0.0
Conditions; Run 136
pH = 5.0
215 rpm
50°C
2.25 g/£ calcium sulfite
1.0 a tin oxygen
No MnSO. added
4
i I r
10 15 20
time (minutes)
—r-
25
30
Figure 3-24. Stirring Speed =215 rpm
123
-------
0.010
0.008
0.006-
0.004-
0.002-
0.0
Conditions; Run 125
pH = 5.0
1800 rpm
2.25 & calcium sulfite
1.0 atm oxygen
No MnSO, added
5 10
time (minutes)
Figure 3-25. Stirring Speed = 1800 rpm
124
-------
TABLE 3-19
EFFECT OF STIRRING SPEED - NO ADDED CATALYST
Run#
121
124
125
126
127
128
1 29
130
131
132
133
134
135
136
137
138
139
140
141
142
143
Stirring
Speed
(rpm)
1750
1750
1750
920
920
920
500
500
500
3200
3200
3200
215
215
215
2500
2500
2500
345
345
345
Initial
Rate
(mol/Jl-sec)
4.66xlO~5
3.82
3.45
3.28
4.55
5.11
4.28
4.44
4.51
4.98
4.57
5.04
0.58
0.38
0.41
4.94
5.66
5.06
0.75
0.48
0.72
2nd Order
Rate Constant
(£/mol-sec)
0.7337
0.6945
0.7139
0.7751
0.7256
0.7149
0.6834
0.7249
0.7115
0.7485
0.8258
0.7933
0.7083
0.6908
0.7435
Index of Deter-
mination for 2nd
Order Fit
0.9999
0.9999
0.9971
0.9964
0.9886
0.9955
0.9989
0.9990
0.9976
0.9784
0.9987
0.9978
0.9958
0.9930
0.9981
Temperature = 50 C, pH = 5.0
125
-------
Since an increase in agitation has no effect on the reaction rate, it
would appear that under these conditions the rate is not limited by the mass
transfer effects of the system. In order to further test the hypothesis that
mass transfer is not controlling, experiments were made in the same reactor
with manganese catalyst being added. 'The addition of catalyst can have a
significant effect on the rate of a kinetically controlled reactor but would
have no influence if mass transfer were completely limiting (i.e., fast reac-
tion in the bulk of the liquid but diffusion resistance across a "stagnant
film). As can be seen from Table 3-20 and Figure 3-26 the addition of cata-
lyst does increase the rate of reaction for sufficiently high stirring speeds.
To further check for any mass transfer effects in the oxidation of clear
solutions, studies were made on the flow rate of oxygen into the reactor. In
an experiment, water is purged of oxygen by bubbling with nitrogen, then CaS03
is dissolved and the reaction started (time = 0) by turning off the nitrogen
purge stream and immediately turning on the oxygen stream. Questions arose
about the time delay between the start of oxygen sparging and saturation of
the 600 ml solution volume. At the previous flow rate of 0.88 £/min., it was
estimated to take several minutes for the oxygen to displace most of the
nitrogen. Therefore, runs were made at higher oxygen flow rates (from 0.77 to
7.6 £/min) in order to ascertain that oxygen saturation of the liquid was
rapid and complete. Results are given in Table 3-21. At higher flow rates
(about 4.0 ml/min) results are somewhat less accurate since aliquots were
withdrawn from the vessel, stoppers popped off, and severe splashing occurred.
Also, certain runs seemed to foam for no understandable reason. All of these
conditions detracted from the quality of the experimental results. Never-
theless, despite the problems, rate constants were not being significantly
affected by the tenfold increase in flow rate. Therefore a flow rate of 3.0
£/min was settled upon as being a satisfactory compromise.
126
-------
TABLE 3-20
EFFECT OF STIRRING SPEED - 0.5 ppm Mn
Run*
144
145
146
147
148
149
150
165
166
167
168
170
171
172
173
174
175
176
177
Stirring
Speed
1750
1750
1750
1750
1750
1750
3300
920
920
500
500
500
380
250
380
2050
380
250
250
2nd Order
Rate Constant
(£/mol *sec)
3.141
2.582
4.055
3.557
2.517
2.673
3.754
2.711
2.674
2.915
2.695
2.585
1.829
0.138
1.206
0.427
1.399
0.176
0.178
Order
of
Reaction
2
2
2
2
2
2
2
2
2
1
1
1
0
1
1
2
1
0
0
Run#
178
179
180
181
182
183
184
187
188
189
190
Stirring
Speed
1750
1700
1700
2000
1700
1700
1700
820
820
2250
820
Te
2nd Order
Rate Constant
(4/mol -sec)
2.568
3.900
3.716
3.501
4.045
3.705
3.176
3.709
3.058
2.730
2.603
;mp = 50°C; pH = 5.0
Order
of
Reaction
2
2
2
2
2
2
2
2
2
2
2
-------
4x10
3x10
Rate
mol(& sec)
2x10
-4
1x10
-4
0.0
o
o
o
o
•
•o
CP
0
00
o
00
8
• uncatalyzed reaction
o reaction catalyzed with 0.5 ppm Mn
pH = 5.0
T = 50°C
1.0 atm oxygen
2.25 g/i calcium sulfite
1000
2000
3000
Stirring Speed (rpm)
4+
Figure 3-26. Rate at [S J = 0.01 M vs.
Stirring Speed
128
-------
TABLE 3-21
EFFECT OF OXYGEN FLOW RATE ON OXIDATION
IN UNSATURATED SOLUTIONS
pH = 5.0 T = 50°C
Run #
151
152
153
154
155
156
157
158
159
160
162
2nd Order
Rate Constant
0.65
0.61
0.59
0.75
0.59
0.75
0.62
0.68
0.59
0.73
0.72
Oxygen Flow
Rate (£ /rain)
4.2
5.3
7.6
6.5
6.5
0.77
6.3
6.3
6.3
6.3
2.1
Sudsing or
Not
Yes
Yes
No
No
No
No
Yes
No
No
No
No
129
-------
Filtrate Oxidations: Slurries at a pH of 4.5 and temperature of 40 C
and with various densities were filtered under nitrogen after chemical equi-
librium had been reached. The concentrations of manganese and iron were
measured in the various filtrates. This analysis was performed by North
American Exploration Company. It was felt these catalyst concentrations would
be indicative of the total catalytic concentration in the filtrates. These
clear liquid filtrates were then buffered to a pH of 4.5 with succinic acid
and then oxidized. These experiments were conducted to study the reaction
kinetics in the liquid phase of the slurry. The oxidation of these calcium
sulfite slurry filtrates was very similar to work conducted by Nurmi (126).
See Section V.
A linear regression analysis was performed on the concentration vs time
data obtained from each filtrate oxidation to determine the reaction order
with respect to the bisulfite species. Results of this analysis are given in
Table 3-22 and shown in Figure 3-27.
All of the experimental data obtained from the filtrate experiments were
further analyzed by a linear regression analysis. This regression analysis
was made to simultaneously determine the relationship of the bisulfite con-
centraion and the catalytic content to the rate of oxidation. The MULTREG
computer program was used to do this analysis (see Section II).
Results from this analysis showed that the iron and manganese concentra-
tions were not independent of each other. The rate of oxidation was found to
be 0.51 order with respect to the manganese concentration and 1.44 order with
respect to S4+ which is present as bisulfite. The standard deviations and
variances for S4+ and Mn respectively were 0.069, 0.0475, and 0.075, 0.056.
130
-------
TABLE 3-22
RESULTS OF FILTRATE OXIDATIONSj
pH_ « 4.5 T » 40°b
Slurry
Density
4.33
4.4
10
10
20
35
50
50
80
80
ppm Mn Run f
in Filtrate
331
0.2 345
341
0.66 348
0.91 342
354
1.71 332
343
2.66 344
351
Order
1.72
1.80
1.62
1.64
1.32
1.24
1.46
1.49
1.42
1.74
1.4 Order
Rate Constant
0.0192
0.020
0.0158
0.02361
0.0271
0.04265
0.02554
0.0370
0.0422
0.0426
131
-------
0.06 .
s
I 0.04
8
01
4J
l-l
0)
t-i
O
0.02 -
0.4
0.8
i
1.6
1.2 1.6 2.0
Mn Concentration (ppm)
2.4
2.8
Figure 3-2?
l.if Order Rate Constant vs Mn Concentration
in CaSO- Filtrate Oxidations
132
-------
Because of these results, the experimental data were forced through a 1.4
order reaction model to give 1.4 order rate constants. These calculated rate
constants, graphed against the manganese concentration found in the filtrates
are shown in Figure 3-27.
E. RATE OF CaS03 OXIDATION AT HIGH MANGANESE CONCENTRATIONS BY FLOW-
THERMAL METHOD
Clear bisulfite solutions having calcium as the spectator cation oxidize
too rapidly for the time scale of the batch method. In the flow ractor even
highly catalyzed calcium bisulfite solutions oxidize at a readily measured
rate. In this way the data needed to describe the oxidation in the clear
solution surrounding slurry particles where the manganese may be present at a
few ppm is obtained as well as rates in extremely catalyzed solutions. The
extremely catalyzed solutions reveal the rates of oxidation in the set of
experiments performed to observe overall limitation by rate of mass transfer
(to assess the mass transfer coefficient).
The results of a flow-thermal experiment is a temperature vs time graph.
The T vs t curve (eg. Figure 3-28) is determined from the concentrations of
reactants and catalyst, pH, and initial temperature. In this set of experi-
ments the pertinent conditions were:
pH: 3.8 - 4.6
[S +]: 0.00136 - 0.00456 mol/£
[02]: 0.00642-0.0172 mol/£
[Mn]: 10 - 2000 ppm (wt.)
Initial: 25 ± 0.02 °C
Most experiments were performed at pH 4.6 with an excursion to lower
values since in the flow reactor, stripping does not occur. The rates for
these experiments came from the method described in Section II. That method
of analysis is based on only one type of sulfur species reacting;
133
-------
25.3
25.2
25.1
T,°C
25.0
24.9
Run Symbol Flowrate, cc/sec
3-23 X 6
3-26
3-27
3-28
8
10
7
0
Conditions: pH
[Mn]
j-s4+-
[02]
T
| . i i i . i i i » f « » ' t t I I i i I
1 2
Residence time, s
= 4.6
= 10 ppm
= 0.002 M
= 0.012 M
= 25°C
Figure 3_28 Representative T vs t Results for CaS03 Oxidation
134
-------
and, as such only applies to bisulfite solutions initially at pH 4.6 where all
4+
the S is in the form of bisulfite and only then to their initial moments of
reaction before the pH drops. However, for rough comparisons with the lower
pH values tested, this method is also good enough. The rates encountered
under the conditions in the list above range from 0.0004 to 0.017 mol/(£ sec).
The results in Figure 3-29 represent the effect of Mn2+ concentrations at
10, 200, 1000, and 2000 ppm. The rates coming from these oxidation are listed
in Table 3-23.
Because very low pH oxidation sometimes is important and in the flow
reactor there is no loss of reactant at low PH as there is in open-system
batch reactors, the pH over the range 3.3 to 4.7 was examined. The results
are in Table 3-24.
Although these experiments were all performed at 25°C it is likely the
results here are similar enough to those from the batch pH method that the
activation energy (EA) may be accepted for converting the rates found here to
any temperature by using the Arrhenius relation
Rate(T) = Rate(25°) x exp(-E./RT).
135
-------
0.3
0.2
A T
°C
a a
a [Mn] = 2000 ppm
[Mn] = 1000 ppm
0.1
+ [Mn] = 200 ppm
[Mn] = 10
I I
4+T
Conditions: [S J
[02]
pH
T
Residence (sec)
= 0.002 M
= 0.013 M
= 4.6
= 25°C
Figure 3-29 Effect of [Mn] on Oxidation Rate
136
-------
TABLE 3-23
EFFECT OF HIGH Mn CONCENTRATIONS
ON CaS03 OXIDATION RATE
[Mn] Rate
mol/g sec) Conditions
10 0.006 pH = 4.6
200 0.007 T = 25°,
1000 0.012 fq4+l
= 0.003 M
2000 °-014 [02]AV - O.'Ol M
137
-------
TABLE 3-24
EFFECT OF pH ON THE RATE OF Mn
CATALYZED CaSO- OXIDATIONS
pH Rate, mol/fl, sec")
3.3 0.00048 Conditions
3,8 0.00051 [Mn] = 10 ppm
4.5 0.00059 [S +]AV = 0.003 M
4.6 0.00078 [ 02 ] - 0.01 M
4.7 0.00105 T = 25°C
138
-------
SECTION 4
OXIDATION IN SLURRIES
OUTLINE
IV. Oxidation in Slurries
A. Experiment
1. Solubility Studies
a. Theoretical Analysis
b. Experimental Results
2. Oxidation in Calcium Sulfite Slurries
3. Oxygen Flow Rate Studies
4. pH and Temperature Studies
5. Slurry Density Studies
6. Catalyzed Slurry Oxidation Studies
7. Liquid Phase Slurry Behavior
8. pH Behavior During Reaction
9. Slurry Reactions with and without pH Controller
10. Liquid Phase Catalyst Studies
B. Mathematical Model
1. General Description of Model
2. Physical Description of Particles
a. Electron Micrograph
b. Particle Size Distribution
3. Derivation of Spherical Model
4. Solutions of the Spherical Model
5. Derivation of the Flat Plate Model
139
-------
OXIDATION IN SLURRIES
i
A. EXPERIMENTAL
Experiments were conducted to determine the effect of several different
experimental parameters on the rate of oxidation in calcium sulfite slurries.
These experimental parameters included temperature, stirring speed, initial
pH, slurry density, and added manganese catalyst. Both total and liquid
sulfite concentrations were examined during these studies. Other experiments
were run to determine whether the reacting system was limited by the mass
transfer of oxygen or by reaction of sulfur (4+) . Those experimental results
have been simulated by a computer-programmed mathematical model.
1. Solubility Studies
The solubility of calcium sulfite in water is a function of temperature,
pH, and ionic strength. It is very important and necessary for the liquid
phase kinetic studies, solid-liquid mass transfer studies, and the mathema-
tical modeling of slurry oxidation reaction. Therefore it has been investi-
gated by both theoretical analyses and experiments.
a. Theoretical Analysis
The solids used, even before oxidation, are a mixture of calcium
sulfite and calcium sulfate. A detailed computer program is available for
calculating the solubility of these solids (22). Approximate values, however,
can be obtained with the following equations:
[Ca2+] '
(4.2)
[H+] [OH"] = Kw (4-3)
These equations ignore ion-pair formation, CaSO^0 and CaSO^°, which
are not significant. During the course of dissolution, the equilibrium is
established rapidly. Then we have
140
-------
~.
HSO "• »H+ + SO,
and 4 4
IHSO J
[S042"]
[HS04~] 2 (4-5)
The condition of electro-neutrality is:
2[Ca2+] + [H+] = [OH'] + 2[S02-] + 2IS02'] + [HSOJ + [HSOJ
(4.6)
At 40°C we have
_7 (156)
kspl = 2.76 x 10 ; (mol/j>)2
ksp2 = !-2 x ID'6
k _ lft
kw ~ 10
-13.5348
« (25)
ks = 4.94 x 10 8 (mol/£)
k2 = 6.4565 x 10"3 (mol/A)
^ At a fixed PH value, we thus have 6 equations and 6 unknowns (i e
fCa+], [OH'], [SO2"], [SO2'], [HSO-]) and HSO
SO]). In all cases,
concentrations are used in favor of activities since the activity coefficients
are very close to 1.0 (Handbook of CheMstry and Physics, e.g. section D-205)
at our low concentrations (less than 0.01M). Those equations can then be
solved to give the saturation concentration of sulfite which is the sum of
[S03 ] and [HSO-]. The result is shown in Figure 4-1. Although the equaUons
are approximate, the results fit experimental data reasonably well.
b. Experimental Results
Experiments to get the saturaUoa concentration of calcium sulfite
in »ater were performed at different pH values. The experimental conditions
were as follows:
141
-------
S4+ mol/Jl
0.04
0.035
0.03
0.025
0.02
0.015
0.01
0.005
DA
a
X
6
a : Calculated Value
A : Experimental Data
H : Prediction from Radian
program
2
JL
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Figure 4-1. Solubility of Calcium Sulfite at 40°C
142
-------
Temperature' = 40°C
Slurry density = 10 g/£ (and 20 g/£)
Stirring Speed = 1800 rpm
The reactor was nitrogen sparged so that the S would not be oxi-
dized (PQ2 = 0). Stripping of S02 is minimal at pH = 4.5 and above.
A pH controller kept the pH of the slurry at a fixed value by pumping in H SO,
as needed. The slurry stirred for more than three hours to make sure that
thermal and chemical equilibrium had been reached before making measurements.
Clear solution samples were thus taken by letting the slurry flow through a
Millipore filter. The saturation concentration of sulfite was then determined
by iodometric titration. The result is shown in Figure 4-1 in which values
from the BMREP (22) computer program predictions are also given for compari-
son. It can be seen that the experimental values are much lower than the
computer predictions. Further, the changes of slurry density have no in-
fluence on the saturation concentration (Figure 4.2).
i
A power function of Cs = 1676.84 (pH~7<791) was found to best fit these
data.
2. Oxidation in Calcium Sulfite Slurries
When the calcium sulfite slurries were oxidized, the total concentration
vs time data obtained were very similar regardless of the experimental con-
ditions used. An example of the data obtained from the slurry oxidations is
shown in Figure 4-3. The reaction curves consisted of two distinct sections.
The first section of the curve in Figure 4-3 shows a very rapid initial reac-
tion rate. At some time during the slurry oxidation, the rate of oxidation
suddenly becomes much slower as shown by the the difference of the slopes of
two line segments shown in Figure 4-3.
3. Oxygen Flow Rate Studies
In an experiment, water is purged of oxygen by bubbling with nitrogen,
then CaS03 is dissolved and the reaction is begun (time = 0) by switching the
143
-------
6.0 -
5,0-
4.0-
Lnitial Rate
ID
'4
3.
j.-sec
2.0-
1.0 -
0.0
l
10
20 30
r
40
50 60
Slurry Density
(g/J.)
Temo = 40°C
=4.8
= 4.5
Figure 4-2. Effect of Slurry Density on Oxidation Rate
144
-------
0.10
0.08-
0.06-
4+
S mol/i
0.04-
0.02-
0.0
10
time (min)
r
15
Run #221 pH = 5.1, 50°C, 1800 rpm, 3.0£/min 0,
Figure 4-3. Typical Slurry Reaction Curve
20
145
-------
nitrogen purge stream and the oxygen stream. Questions arose about the time
delay between the start of oxygen sparging and saturation of the 600 ml solu-
tion volume. At flow rate of 0.77 £/min, it was estimated to take several
minutes for the oxygen to displace most of the nitrogen. Therefore, runs were
made at higher oxygen flow rates (from 0.77 to 7.6 £/min) in order to test
that oxygen saturation of the liquid was rapid and also complete. At higher
flow rates (above 4.0 ml/min) results are somewhat less accurate since, when
aliquots were withdrawn from the vessel by momentarily closing off the gas
escape vent, problems were encountered (stoppers popping off and severe
splashing). Also, certain runs seemed to foam for no clear reason. Never-
theless, despite the problems, rate constants were not being significantly
affected by the tenfold increase in flow rate. Therefore a flow rate of 3.0
Ji/min was settled upon as being a satisfactory compromise.
Slurries with 2000 ppm added manganese catalyst were oxidized with dif-
ferent oxygen flow rates. If the oxidation was being hindered by gas to
liquid mass transfer, an increase in the oxygen flow rate should have in-
creased the rate of oxidation. However, as seen below in Figure 4-4, no
increase in the slurry oxidation rate was observed.
4. pH and Temperature Studies
Slurry oxidations were conducted over a range of initial pH's to deter-
mine the effect of pH on the initial reaction rate. The pH's used in this
study varied from 6.0 to 4.3. Experiments were conducted at both 40°C and
50°C. All experiments used a slurry density of 20 g/£, an oxygen flow rate of
3 £/min and a stirring speed of 1800 rpm. Examples of the experimental data
obtained in this study are given by the total concentration vs time data in
Figures 4-5 to 4-7. The initial reaction rate from these experiments as a
function of pH and temperature is shown in Figure 4-8. Table 4-1 provides a
list of the experiments used in this study.
146
-------
-10 -t
n
o
T
3 4
Time, (minutes)
Initial pH = 5.0
3 £
Temp = 40° C
•--6.66 ppm Mn added
•--200 ppm Mn added
*--2000 ppm Mn added
iV--2000 pom Mn added
(6 £/ Mn 02)
D--OppmMn added
Figure 4-4
Effect of Mn Catalyst on Slurry Oxidation Rate
147
-------
U. JLU-
[
1
0.08-
0.06-
Total
S4+
0.04-
0.02-
0.0
g-mol/i
0.04-
0.03'
Liquid'02
.4+
5 0.01
0.0
• "
D
•
D
•
D
1
n ' D
D
• n
1 1 1 1 1 i i"""1
0123 4567
Time (Minutes)
Temp = 40°C
PHI = 4.5
Slurry Density = 20 g/£
•--Run #441
D--Run #465
Figure 4-5. Slurry Oxidation, pH = 4.5 (182)
148
-------
0.10-.
0.08-
Total
0.06-
0.04-
D
0.02-
D
0.0 •*
g-mol/fc
0.04-
0.03
0.02^
Liquid
C4+ 0.01-I
o.o
1234
Time (Minutes)
Temp = 40°C
pHj = 4.7
Slurry Density =20
•--Run #440
n--Run #435
Figure 4-6 Slurry Oxidation, PH = 4.7 (182)
149
-------
Total
U . IU-
$
0.08-
0,06-
0.04-
O.OZ
*
*
*
g-mol/4
0.04 -
0.03 -
Liquid
$4+ 0.02 -
*
0.01 ^
o.o -
• • : « :
D 1 2 3 4 5 6 7
Time (Minutes)
Temp = 40°C
pHx = 5.0
Slurry Density = 20 gm/£.
*--Run #463
• --Run #464
Figure 4-7 Slurry Oxidation, pH = 5.0 (182)
150
-------
6.0
5.0
4.0
Initial Rate
,-4
(x 10 mol/£'sec)
3.0-
2.0-
1.0.
4.0
4.5 5.0
Initial pH
Figure 4-8 Effect of pH on Slurry Oxidation Rates
(All runs at 1800 rpm and 3.0^/min 0 )
151
-------
TABLE 4-1
RESULTS OF STUDIES SHOWISG EFFECT OF.INITIAL pH ON OXIDATION (182)
Temperature Initial Rate
Run # pH-r (°C) X 10 mol/U sec)
235 4.3 50 5.97
237 4.3 50 5.86
238 4.3 50 5.76
227 4.5 50 5.41
228 4.5 50 5.31
229 4.5 50 5.34
241 4.8 50 5.42
245 4.8 50 4.60
223 5.0 50 4.10
220 5.1 50 3.08
221 5.1 50 3.54
222 5.1 50 2.95
244 5.2 50 2.62
242 5.3 50 2.77
243 5.3 50 2.77
232 5.5 50 1.07
234 5.5 50 1.33
253 6.0 50 0.61
251 4.3 40 3.27
257 4.3 40 2.99
264 4.3 40 3.00
256 4.5 40 3.25
263 4.5 40 3.46
250 4.5 40 3.29
255 4.8 40 2.75
258 4.8 40 2.83
259 5.0 40 2.23
260 5.1 40 1.50
261 5.1 40 1.73
262 5.2 40 1.24
265 5.2 40 1.00
248
266 5.4 40 0.68
267 5.4 40 0.92
252 5.4 40 0.83
268 5.6 40 0.47
269 5.6 40 0.58
270 5.8 40 0.15
271 5.8 40 0.27
272 6.0 40 0.12
441 4.5 40 3.76*
465 4.5 40 3.33*
440 4.7 40 3.57*
435 4.7 40 3.00*
463 5.0 40 3.00*
464 5.0 40 2.63*
*not shown in Figure 4-3
-------
5. Slurry Density Studies
Slurries with densities of 8, 10, 20, 40, 50 g/A were oxidized to deter-
mine the effect of slurry density on the initial reaction rate. Experiments
were made with initial pH's of 4.8 and 4.5 and a temperature of 40°C. The
stirring speed used was 1750 rpm and the oxygen flow rate was 3 £/min. Ex-
amples of the experimental data obtained from these experiments are given
below by the total concentrations vs time data in Figures 4-9, 4-10, 4-5.
Table 4-2 gives a list of the experiments used in this study. A graph of the
initial reaction rate as a function of slurry density is shown in Figure 4-2.
6. Catalyzed Slurry Oxidation Studies
By increasing the rate of one step in an overall process such as the
oxidation of sulfur (4+), the relative importance of the several steps in-
volved, reaction, absorption of oxygen, and dissolution of solids can be
determined. This information is necessary in developing a model of the pro-
cess. The model is discussed in more detail later.
Initial experiments were conducted in which 8 ppm manganese catalyst
2+
(Mn ) was added. The slurry load in the experiments was 20 g/SL. Experiments
had an initial pH of 4.5 and 5.1 and two temperatures 40°C and 50°C. The
results are listed in Table 4-3. The results for some representative runs are
shown in Figure 4-11. As expected the addition of catalyst speeds up the
reaction. It should be noted that the shapes of the catalyzed and uncatalyzed
runs are similar, the difference being in the rate of the process. The effect
of catalyst on the pH of the slurry during the course of the reaction is shown
in Figures 4-12 and 4-13. At an initial pH of 4.5, Mn simply speeds up the pH
drop. However, at an initial pH of 5.1 the general shape of the pH curve is
altered.
153
-------
U . /.<4-U -
0.20 -
0.160 -
Total
5 0.120 -
0.08 -
0.04 -
0.0
g-mol/J,
0. 04 -
0.03 •
Liquid
5 * 0.02 -
0.01 -
0.0
H
m
*
m
•£
•
*
ir if
•
. *
• *
-| r~ i i i i i i
0 1 • ' 2 3 4 5.6 7
Time (Minutes)
Temp - 40°C
pHj = 4.5
Slurry Density = 50
• --Run #474
• --Run #477
*--Run #385
Figure 4-9. Slurry Oxidation, PH = 4.5 (182)
154
-------
Total
S*f K
0.16-
0.12. |
0.08-
0.04-
g-mol/t
0.04-
0.03-
Liquid 0.02-
0.01-
0.0
1 234
Time (Minutes)
Temp = 40°C
pH-j. = 4.5
Slurry Density = 30 g/£
D
D
•--Run #473
D--Run #475
Figure 4-10. Slurry Oxidation, PH = 4.5 (182)
155
-------
TABLE 4_2
EFFECT OF SLURRY LOAD ON OXIDATION RATE AT 40°C (182)
Initial Slurry
Initial
pH
4.8
4.8
4.8
4.8
4.8
4.8
4.8
4.8
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
Concentration
40
40
40
20
20
20
10
10
50
50
40
40
40
20
20
20
10
10
8
8
50
8
8
50
50
30
30
20
Initial Rate
mol/2. * sec
2.5 x 10~4
3.2
3.0
2.7
2.8
2.8
2.3
2.2
4.3
4.9
4.0
3.0
3.6
3.5
3.2
3.3
2.2
2.8
2.1
2.2
4.8
2.5
2.1
4.7
4.5
3.3
3.1
3.3
Stirring Speed
(rpm)
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
1750
2800
2800
2800
1800
1800
1800
1800
1800
1800
Run #
306
307
313
249
255
258
304
305
315
316
311
309
312
250
256
263
308
310
318
319
321
320
322
474*
385*
473*
475*
441*
465*
*Not shown in Figure 4-2
156
-------
TABLE 4-3
EFFECT OF CATALYST ON SLURRY OXIDATION RATE
40°C
Run # Initial pH cone. Mn Initial Rate
(ppm) mol/U sec)
0 3.4 X 10~4
0 3.2
8 6.6
8 6.2
0 1.83
0 2.22
0 2.55
8 2.79
8 2.90
0 1.5
0 1.8
8 3.0
8 3.0
8 2.8
50°C
0 5.4 X 10~4
0 5.31
0 5.34
8 6.5
8 6.9
0 3.0
0 2.8
0 3.0
0 3.0
8 5.0
8 5.2
8 4.8
250
256
299
300
275
259
282
274
276
247
261
301
302
303
4.5
4.5
4.5
4.5
5.0
5.0
5.0
5.0
5.0
5.1
5.1
5.1
5.1
5.1
233
228
229
290
291
233
296
297
298
288
293
294
4.5
4.5
4.5
4.5
4.5
5.1
5.1
5.1
5.1
5.1
5.1
5.1
157
-------
0.08
co
0.06-
0.04-
PH
4.0-
3.0-
2.0
0 2
Run 228
PHZ = 4.5
0 ppm Mn
O O O Q Q {
•I- — • I —-W I •
-i 1 1
0.08
T~
4
T—
6
-j—
8
—i—
10
—i—
12
time (minutes)
0.06-
[S4+] 0.04-
0.02-
0.00
5.0
4.0-
3.0-
2.0-
Run 290
pH - 4.5
8 ppm Mn
2 4 6 8 10 12
time (minutes)
Figure 4-11. Catalyzed & Uncatalyzed Oxidations in Slurries at 50 C
-------
6.
Ln
VO
5.0-
pH 4.0.
3.0-
2.0.
8 10
time (minutes)
12
Figure 4-12. Effect of Catalyst on pH Behavior
-------
6.0-
5.0-
pH 4.0-
3.0-
2.0
8 10
time (minutes)
12 14 16 18
Figure 4-13. Effect of Catalyst on pH Behavior (50°C)
-------
Experiments were also conducted in which • manganese concentrations of
6.66, 200, and 2000 ppm were used. The operating conditions used were a 20
g/S, slurry density, 1800 rpm stirring speed, initial pH of 5.0, and temper-
ature of 40 C. Table 4-4 lists the experiments used in this study. Examples
of the data obtained in this study are shown in Figure 4-4. Figure 4-4 shows
that the slurry reaction rate is. strongly influenced by added manganese cata-
lyst.
7. Liquid Phase Slurry Behavior
A series of slurry oxidations were run in which both the total and liquid
sulfite concentrations were measured as a function of time. Table 4-5 gives
the operating conditions used in these experiments. The results are shown in
Figures 4-5 to 4-7, 4-9, 4-10, 4-14 to 4-17.
The pH vs time response curves for these slurry oxidations are shown in
Figures 4-18 to 4-20.
8. pH Behavior During Reaction
In all cases, there is an abrupt drop in slurry pH upon the introduction
; »
of oxygen. This occurs within the first five seconds and is always between
0.3 and 0.5 pH units. From here, the pH will (a) continue dropping at a much
slower rate, or (b) flatten out and begin to rise, or (c) a combination of (a)
and (b), depending on the initial pH. At a pH below 5, curve type (a) is ob-
served while at a pH above 5.1 type (b) is observed. The changeover between
(a) and (b) (i.e. type c) occurs between 5.0 and 5.1. The exact reasons for
the differing behaviors are not fully understood; however, it is the result of
a complex relationship between CaS03 solubility, pH, and oxidation rate.
9. Slurry Reactions with and without pH Controller
As was mentioned before, there is an abrupt drop in the initial pH as
soon as the reaction begins; thereafter, the pH varies according to the ini-
161
-------
TABLE 4-4
EXPERIMENTS MADE IN Mn CATALYST STUDY (182)
Slurry Density = 20 §/& Stirring Speed = 1 800 rpm
02 Flow Rate
3 a 'min
PHT = 5
Run #
450
459*
468
470
471
486
485
400
401
Mn Added
200 ppm
200 ppm
200 ppm
2000 ppm
2000 ppm
2000 ppm
2000 ppm
6.66 ppm
6.66 ppm
* not shown in figure IV- 4
3 *> /min
162
-------
TABLE 4-5
EXPERIMENTAL CONDITIONS USED IN SLURRY OXIDATIONS SHOWN IN
FIGURE 4-5 to 4-7, 4-9, 4-10, 4-14 to 4-17
Figure
IV- 5
IV- 6
IV- 7
IV- 8
IV- 9
IV- 10
IV-11
IV- 12
IV- 13
Material
Oxidized
Calcium
Sulfite
11
"
"
"
M
II
Laboratory
Prepared
Calcium
Sulfite
11
Mn Slurry
Added Density
200 ppm 20 g/£
200 ppm 20 g/£
None 50 %/n
30 g/£
20 g/a
" in
20 g/4
M <*in
^0 g/£
200 ppm 12 g/jt
None 12 / g/£
pHI
5.0
4.5
4.5
4.5
4.5
4.7
5.0
5.0
5.0
Run ••
461
468
487
488
474
477
385
473
475
441
465
440
435
463
464
453
454
455
458
466
467
469
476
163
-------
0.10-
0.08
0.06
Total
0.04
0.02
0.0
g-mol/J,
0.04 -r
0.03 -
0.02 -
Li
-------
0.10-,
0.08Jr
Total
S
0.06-
0.04-
0.02-
0.0
•
-tr
g-mol/*
Liqu1d
12 34 5 6 7
Time, (min.)
Temp = 40°C
pH-j. = 4.5
Slurry Density = 20 g/Jj!,
200 ppm Mn Added
--Run #487
--Run. #488
Figure 4-15. Slurry Oxidation, 200 Ppm Mn, PH = 4.5 (182)
165
-------
o.io -i
0.08
,
0.06 -•
Total
&*
0.04-
0.02-
Time (Sees)
Temp = 40° C
pHz = 5.0
Slurry Density = 12
200 ppm MN Added
*--Run #453
• --Run #454
D--Run #455
o -"
g-mol/A
0.04-<
0.03-
0.02-
D
.
k
Liquid m
S4+ o.Ol* »
o.
D
r ^ n
i r" i i 1 j
^ 20 40 60 80 100 120
• --Run #458
*--Run #466
D--Run #467
Laboratory Prepared Calcium Sulfite
Figure 4-16. Slurry Oxidation, 200 ppm Mn, pH=5.0 (182)
166
-------
0.10-r
0.08-
0.06-
Total
S4*
0.04-
0.02-
0
g-mol/4
0.04-r
0.03-
Liquid
C4+ 0.02-
0.01-
0 -
•A-
•'
T 1 T
5 6 7
•--Run #467
#476
0 1 23 4
Time (Minutes)
Temp = 40°C
pH-j- = 5.0 -A--
Slurry Density = 12 g/a
Laboratory Prepared Calcium Sulfite
Figure 4-17. Slurry Oxidation, PH = 5.0 (182)
167
-------
5.0-t
4.0-
pH
3.0-
2.0
0 g/Jl
4 6
Time (Minutes)
Temp = 40°C
pHT =4.5
10
Figure 4-18.
pH Behavior During Slurry Oxidations;
Effect of Slurry Density (182)
168
-------
6.0
5.0
4.0 -
pH
3.0 -
2.0
0
Time (Minutes)
Temp = 40°C
Slurry Density = 20 g/£
a) pHj = 5.0
b) pHj =5.0 (Laboratory prepared calcium sulfite Slurry
Density = 12 g/£ y
c) pHj = 4.7
d) PH = 4.5
Figure 4-19. pH Behavior During Slurry Oxidations;
if>9 °f Initial PH
-------
6.0-
pH
5.0
4.0
3.0
2.0
Time (Minutes)
Temp = 40° C
a) 200 ppm Mn Added, Slurry Density = 20 g/£
b) Laboratory Prepared Calcium Sulfite,200 ppm Mn Added,
Slurry Density = 12 g/£
c) 200 ppm Mn Added, Slurry Density = 20 g/£
Figure 4-20.
pH Behavior During Slurry Oxidations;
Highly Catalyzed Runs (182)
170
-------
tial conditions. Experiments were run by using a pH controller (to control
the pumping of acid or base) to keep the pH of the solution at a fixed value
in order to obtain reaction data at a constant pH over a longer period. The
experimental conditions were:
Temperature = 40°C
Slurry Density = 20 g/£
Stirring Speed = 1800 rpm
Initial pH = 5.0
Controlled pH = 4.6
Flow Rate of 100% 02 = 3 £/min
Both total and liquid sulfite concentrations were measured. The results were
shown in Figure 4-21 which will be used for simulation by a mathematical model
described below. Results of experiments under the same conditions but without
using a pH controller were also shown in Fig. 4-21 for comparison. The reac-
tion rate of the pH uncontrolled case is a little bit faster than that of the
pH controlled case. But the difference is almost unnoticeable.
10. Liquid Phase Catalyst Study
A 20 g/S. slurry with an initial pH of 5.0 at 40°C was oxidized. Filtered
samples were periodically taken from the reactor and analyzed for manganese
catalyst by North American Exploration Company. The results shown in Figure
4-22 show that the manganese concentration in the liquid phase of the reactor
increases as the oxidation proceeds.
B. MATHEMATICAL MODEL
1. General Description of Model
A mathematical model was developed to predict both the liquid-phase and
total sulfite behavior during slurry oxidations. The model was developed in
general terms so that it could be used to simulate any experimental condi-
171
-------
Total
S4+
mol/l
Liquid
mol/l
0.10
0.09
0.08
0.07 '
0.06
0.05
0.04
0.03
0.02
0.01
0
(
0.012
0.010
0.008
0.006
0.004
0.002
0
$ : Reaction with pH controller
•* : Reaction with pH controller
A : Reaction without pH controller
.1
0
4 :
A 0
* *
A •
A *
D 5 10 15 20 25 30 35
•
•
*
_ A
1 t J * t* * ' -*
0 5 10 15 20 25 30 35
Time (minutes)
Temp .= 40°C Stirring speed = 1800 rpm
pH = 5.0 Slurry Density = 20 g/&_
Figure 4-21. Slurry Oxidation with No Mn Added
172
-------
2.0-
1.6-
1.2-
•
ppm Mh
in Liquid
Phase
0.8-
0.4-
0.0-
T"
4
8
Time (Minutes)
Temp = 40 C
pHj = 5.0
Slurry Density = 20 g/.
12
16
Figure 4-22.
Liquid Phase Catalyst Behavior
During Slurry Oxidations (182)
173
-------
tions. This model is based on three simplifying assumptions. These assump-
tions can be relaxed if the model is further developed at a later date.
The first assumption is that the only loss of sulfite from the slurry
occurs from the liquid phase oxidation of sulfite. It is also assumed that
the oxygen concentration is constant at saturation at all times during the
oxidation. Finally, the assumption is made that the hydrogen ion concentra-
tion is constant througout the liquid phase at all times during the oxidation
of the slurry.
2. Physical Description of Particles
a. Electron micrograph
Electron micrographs were made of the crystals in the calcium sul-
fite slurries. A slurry with a density of 20 g/£ was prepared and the pH of
the slurry was adjusted with sulfuric acid to a value of 4.5. A sample of
this slurry was taken, filtered under nitrogen, washed with acetone, and dried
in a vacuum oven. A JEOL JSM-35 scanning electron microscope set to magnifi-
cations of 3,000 X and 10,000 X was used to study the crystal structure of the
calcium sulfite in the slurries. Electron micrographs obtained from this
examination are presented in Figures 1-2 and 1-3. The general shape of the
particles can be seen in Figure 1-2 while the features of the surface are
shown in Figure 1-3. These solids, even before oxidation are a mixture of
CaSO (50% by weight) and CaSO,. The flat platelike crystals are calcium
sulfite and the large cylindrical crystals are calcium sulfate (162). The
average aspect ratio of flat plates was found to be 8.46 : 3.7 : 1 (182). It
can be seen from these figures that particles of calcium sulfite and calcium
sulfate aggregate together and exist in the solution as rough spheres.
174
-------
b. Particle size distribution
The particle size distribution, before oxidation, was determined by
means of a Coulter counter using a electrolyte of 5% LiCl in methanol. The
equivalent spherical diameter and the volume fraction are given in Table 4-6
for a typical sample. This initial size distribution was used in the mathe-
matical model for the case of spherical particles as will be stated in subsec-
tion (3) below. For flat plates an equivalent size distribution was calcu-
lated by using the aspect ratio of 8.46 : 3.7 : 1.
3. Derivation of the Spherical Model
In the instant case the solid particles are rough spherical aggregates
made up of tiny, flat platelike sulfite and cylindrical sulfate crystals. The
model was thus developed for the case of spherical particles. The analysis
for the case of all particles being flat plates is given in subsection (5).
The liquid phase concentration of sulfite in the slurry is governed by
two competing mechanisms. Sulfite in the liquid is lost due to the oxidation
reaction but is replenished by the dissolution of the solid calcium sulfite.
This system of competing mechanisms can be represented by a differential
equation of the form
d(VC )
where V = reactor volume
C = [S ] liquid
= reaction rate
R = disssolution rate
The kinetic rate expression has already been shown to be
RR = - k
The mass transfer of S ions from solid to liquid can generally be given by
Rm =
175
-------
TABLE 4-6
SIZE DISTRIBUTION OF CALCIUM SULFITE PARTICLES
(COULTER METHOD)
Equivalent
Spherical
Diameter (urn)
2,
3.
.52
.18
4.
5.04
6.35
8.
10.08
12.7
16.
20.16
25.4
32
40.32
50.8
64.
80.64
Volume
Fraction
1.5
2.3
4.6
9.6
17.
.3
.1
.5
.3
19.
20.
14,
7.
2.1
.7
.1
0.
0.
0.
Geometric Mean Diameter 11.62 ym
Median 11.94 ym
Mode 13.07 ym
Standard Deviation 1.59 ym
176
-------
Substitution of (4-8) and (4-9) into (4-7) gives the equation
where k, = mass transfer coefficient
a = interfacial area
C = saturation concentration
s
d(VC ) 1.5
The volume of the reactor can be cancelled out of this expression as it
remains constant
dC£ 1.5
— = - k CjL + k£ a(Cs - C£) (4.11)
where a, the interfacial area of particles per unit volume, changes with time
as the particles shrink and also as the smallest particles in the original
size distribution dissolve away. It can be represented as
n
h. f.
a _ (4_12)
where Yi = the radius of i type particles which is a function of time
Nt=0 = the initial total numbers of particles
IK = No. fraction of i type particles
f^ - fraction of original number of i type particles remaining
in reactor, fj. equals to 1 or 0
n = the number of particle size being considered. In this
work, n is equal to 13.
Therefore
13
I y| hi fi
a = = _ (4_13)
Substitution of (4-13) into (4-11) gives the equation for liquid phase
behavior in the slurry.
177
-------
dC0 1.5 4rtN._n kfl (C - CJ 13
ar i v =l i i
The rate of mass transfer from solid particle to liquid phase is
given by the expression
d(Total moles) _ _ TO (4.15)
dt m
or
d(Total moles) = _ ^ a(^ _ c^ (4.16)
Multiplying by the molecular weight of calcium sulfite yields
ft = - V. k, a(Cs - Cs) (4.17)
where
m = molecular weight of calcium sulfite
w = total mass of solid particles
= p V
= n " - Ttv3 N h f
P I ., 3 Yi t=0 i i
3
- I «p Nt--o l=l ^ "i fi (
Substitution of (4-13) and (4-18) into (4-17) gives (4-19)
I np Nt=o at 4' ^3 hi fi^ = - 47lmk£Nt=o(cs - V^!, ^? hi fi}
i=i i"-1-
Equation (4-19) can be split into 13 equations corresponding to 13 types of
solid particles
d\. m kn
-i = -- £ (C - C J (4-20) - (4-32)
dt p
i = 1, 2, 3, — , 13
These equations describe the time rate of change of particle sizes,
178
-------
The total concentration of sulfite at any time t is the sum of liquid
phase and solid phase concentrations remaining in the solution.
_ r , Total moles of solid sulfite
~ a - ~~ - -
Volume of Reactor
13 ,
2 ' | Tty? £ N n h. f.
• _n 3 si m t=0 i
4
o np N 13
= C* + - - Vm^ ^ihifiJ (
Equations (4-14), (4-20), - (4-32) can be solved simultaneously to give C and
Xs
•y±. Then from equation (4-33) the total concentration of sulfite, C? can also
be calculated.
4. Solutions of the Spherical Model
In solving the model, the loss of particles due to dissolution must be
taken into account. The total number of particles remains constant until the
smallest portion of particles in the original distribution dissolve away. The
new number of particles can then be calculated by subtracting the number of
particles in the lost portion from the original number of particles. The
initial particle size distribution and the total number of particles can be
found in Table 4-6 which is the result from Coulter counter analysis. The
solution of the model will continue in this stepwise manner until all solid
calcium sulfite is dissolved.
In the slurry oxidation reaction, as was noted before, there is a sharp
drop in the initial pH as soon as the oxidation begins. Because of this sharp
drop in the pH the initial liquid phase concentration of S4+ will no longer be
equal to the saturation concentration of S4+. Therefore, in the model, the
initial liquid phase concentration is taken to be equal to the saturation
179
-------
concentration at time equal zero and initial pH. For times greater than zero,
the saturation concentration in the model will be the saturation concentration
at the average pH of the slurry as it oxidizes. For reactions without adding
Mn catalysts, a pH controller was used to keep the pH at a fixed value during
the course of reaction. Thus the saturation concentration at this pH can be
used in the model.
A Runge-Kutta-Gill algorithm was chosen to solve this system of simul-
taneous differential equations. This model programmed in the BASIC language
with the Runge-Kutta-Gill algorithm (program B-30) is shown in Figure 4-23.
An example calculation of this program is shown in Figure 4-23.
The model was first used to determine the solid-liquid mass transfer
coefficient. Computer runs were made to model the 2000 ppm Mn catalyzed
slurry oxidation shown in Figure 4-4. From Section III we know that the
liquid phase oxidation is 1.5 order. Computer curves were generated using a
variety of mass transfer coefficients and a rate constant of 85 SL /g-mol
sec as shown in Figure 4-24. A mass transfer coefficient of 0.02 cm/sec was
seen to most closely fit the experimental data.
Using the mass transfer coefficient of 0.02 cm/sec, the model was then
tested against the slurry oxidations with 0, 6.66, and 200 ppm added Mn cata-
lysts. Results are presented in Figure 4-25, 4-26 and 4-27. The 1.5 order
rate constants for 0, 6.66 and 200 ppm Mn added reactions are 0.35, 2.0, and
4.5 £°'5/g-mol sec. The corresponding values of 1.5 order rate constants
from experiments (Section III) are 0.162, 0.35, and 0.8 ~ 5 H ' /g-mol ' sec.
The liquid phase S concentrations predicted by the model using k^ =
0.02 cm/sec for the 0 ppm Mn added case were also compared to the experimental
data as shown in Fig. 4-28. It can be seen that the predicted results don't
fit the experimental data well. The liquid S concentration profiles pre-
180
-------
BJO
10
20
30
40
<=:A
«J V
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
360
370
380
390
400
410
420
430
440
450
460
470
480
500
510
520
530
540
550
560
570
580
590
REM B3.0
REM 23MAY79
REM
REM THIS PROGRAM MODELS SISSOLUTION & f
H' F" M i i THn*IITCC7f"*TTn>
REM DENSITY
R7=2,29
REM CHE.'iICAL REACTION RATE CONSTANT<1.:
A=5
REM MASS TRANSFER COEFFICIENT
K=0
REM INITIAL NUMBER OF PARTICLES
M=1,34E+10
Al = 0
REM COEF JRHO*4/3*M*PI/MOL UT/RXR VOL
Z=R7*.0541*M
REM SATURATION CONCENTRATION OF THE LI
C=.01
REM INITIAL TIME
Tl=0
REM FINAL TIME
T2=420
REM STEP SIZE
CEACTION IN CAS04 SLURRIES
l
i ORDER)
QUID AT THE INITIAL PH
E= . 5 , .
REM NUMBER OF ITERATIONS PER STEP
N2=10 ' -
REM FRACTION OF EACH STEPSIZE ON THE HISTOGRAM
DIM WC153,P[:i43TXC153»aC153rDri43»BC143
WC13=,2018
XC13=1.26 . .
WC23=.1674
XC23=1.59
WC33=.129
XC33=2
WC43=.129
XC43=2,52
WC53=.1345
XC53=3.175
WC63=.1191
XC63=4
WC73=.0676
XC73=5.04
WC83=.0352
XC83=6.35
WC93=.0127
XC93=8
UC103=. 00319
XC103=10.08
WC11J=, 00046
XC113=12.7
UC123=. 000077
XC123=16
UC133=. 000005
Xtl33=20,16
XC143=,0055
DIM RC163
MAT READ R
DATA 0, .5,0, ,5f ,5» .292893, i. 70711, , 166667
Figure 4-23 B30 Program and Sample Calculation
181
-------
600 DATA 2,1,1,2,.5,.292893»1.70711,,5
610 REM
620 REM INITIAL VALUES NEEDED
630 C5=0
640 REM FLAG FOR FIRST PASS THROUGH PROGRAM
650 F7=0
660 REM Y IS A COUNTER FOR HISTOGRAM STEPS
670 Y=l
680 REM F IS THE FRACTION OF PARTICLES REMAINING
690 F=l
700 REM INITIAL SULFUR +4 CONCENTRATION TOTAL
710 C7=,0822
720 REM VARIOUS VARIABLES INITIALIZED
730 FOR 1=1 TO 14
750 XCI3--XII3*.0001
760 QCI.1=0
770 DCIJ=0
780 BCIJ-l
790 NEXT I
800 XC143=.0055
810 REM INITIAL TIME
820 T=T1
830 REM COUNTER VARIABLE
840 N3=N2
850 REM
860 REM HEADING —
870 PRINT
880 PRINT 'SOLUTION ON 'JTlf TO •JT2f'SEC BY'5E5"SEC PRINTED EA 'J£*N;
890 PRINT
. 900 PRINT 'RATE CONSTANT (L90) A='5A
910 PRINT "MASS TRANSFER COEF (L110) K='JK
920 PRINT "INITIAL * PARTICLES (L130) M='?M
930 PRINT "SATURATION CONC (L180) C="?C
935 PRINT "INITIAL TOTAL CONC (L710) C7="?C7
940 PRINT "INITIAL LIQUID CONC (L560) X<14> ="5XC143
945 PRINT
950 PRINT "TIME " ?TAB( 12) r " TOTAL S+4 " ', TAE«27> ', "S+4 IN SOLN'f
960 PRINT TABC41)j"BIGGEST XTAL "i TAB<55) ) 'FRACTION LEFT'
970 PRINT
980 REM
990 REM CALCULATIONS
1000 GOTO 1160
1010 FOR J=l TO 4
1020 T=T-fRCJ3*E
1030 REM SUBROUTINE CALL FOR EFFECT OF HISTOGRAM
1040 GOSUB 1290
1050 FOR I=Y TO 14
1060 S=RCJ+43*(DCI3-RC8+J3*QCI3)
1070 XCI]=XCI3+E*S
1080 OCI3=QCI3+3*S-RC12+J3*DCI3
1090 NEXT I
1100 C5=0
1110 NEXT J
1120 FOR I=Y TO 13
1130 C5=C5+XCI3"3*WCI3
1140 NEXT I
1150 C7=C5*Z+XC143
1160 IF N3-N2<0 THEN 1260
1170 N3=0
1180 REM
1190 REM OUTPUT — —
1200 REM
1210 PRINT INT(T)f
1220 PRINT TA,(12)rC7;TAE(27)rXC14];
1230 PRINT TAB(41)JXC133tTAB(55)JF
1240 PRINT
Figure 4-23 B30 Program and Sample Calculation
182
-------
1250
1260
1270
1280
1290
1300
1310
1320
1330
1340
1350
1360
1370
1380
1390
1400
1410
1420
1430
1440
1450
1460
1470
1480
1490
1500
1510
1520
1530
1540
REM --------- END OF LOOP -----
N3=N3+1
IP T-T2<0 THEN 1010
STOP
REM
REM -------------------- SUBROUTINE -----------------------
REM
REM DETERMINES K FROM HISTOGRAM
FOR I=Y TO 13
IF XCID>0 THEN 1400
BCID=0
Y=Y+1
XCI3=0
F=F-WCY-13
GOTO 1410
NEXT I
IF F<. 000005 THEN 1430
GOTO 1440
F=0
REM THIS IS THE SET OF FOURTEEN GOVERNING EQUATIONS
FOR I=Y TO 13
NEXT I
Al=0
FOR I=Y TO 13
A1=A1+WCI3*XCI3"2
NEXT I
RETURN
END
Figure 4-23 B30 Program and Sample Calculation
183
-------
Sample Calculation
90 A=20
110 K=0,008
130 M=1.16E10
180 C=0,012
710 C7=0.07
RUN
B30
SOLUTION ON
TO 420 SEC BY .5
SEC PRINTED EA 5
SEC
RATE CONSTANT
MASS TRANSFER COEF
INITIAL * PARTICLES
SATURATION CONC
INITIAL TOTAL CONC
INITIAL LIQUID CONC
TIME
0
5
10
15
20 •
25
30
35
40
45
50
55
60
65
70
75
80
85
90
TOTAL S+4
.07
S.78475E-02
4.97928E-02
.042669
3.64141E-02
3.09583E-02
.026223
2.21298E-02
1.86077E-02
1.55933E-02
1 .30272E-02
1.08547E-02
9.02356E-03
7.4S844E-03
6.20423E-03
5.13367F -03
4.24517E-03
3.50686E-03
2.89307E-03
(L90) A= 20
(L110) K= .008
(L130) M= 1.16000E+10
(L180) C= .012
(L710) C7= ,07
(L560) X(14> = .0055
S+4 IN SOLN
.0055
1.94055E-03
1.79136E-03
1.64645E-03
1.50611E-03
1.37228E-03
1.24701E-03
1.12986E-03
1.0202BE-03
9.18142E-04
8.22961E-04
7.35504E-04
6.54963E-04
5.81645E-04
5.16100E-04
4.56256E-04
4.02797E-04
3.56470E-04
3. 14528E-04
BIGGEST XTAL
.002016
1.99436E-03
1.97152E-03
1.94836E-03
1.92487E-03
1.90107E-03
1.8769SE-03
1.85262E-03
.001828
1 .80315E-03
1.77807E-03
1.75279E-03
1.72731E-03
1.70167E-03
1.67586E-03
1 .64992E-03
1 .62385E-03
1.59766E-03
1 .57138E-03
FRACTION
1
1
1
1
1
1
.7982
.6308
.6308
.5018
.5018
.3728
.3728
.3728
.2383
.2383
.2383
.1192
.1192
Figure 4-23 B30 Program and Sample Calculation
184
-------
95
100
105
110
115
120
125
130
135
140
145
150
155
160
.165
170
175
180
185
190
195
200
205
210
215
220
225
230
235
240
245
250
255
2.38594E-03
1 .96842E-03
1.62346E-03
1.33784E-03
1.10293E-03
9.10928E-04
7.54061E-04
6.24711E-04
5.17282E-04
4.28522E-04
3.55763E-04
2.96447E-04
2.48060E-04
2.08136E-04
1.74703E-04
1.46657E-04
1.23251E-04
1.03851E-04
8.78620E-05
7.47100E-05
6.38342E-05
5.46983E-05
4.69002E-05
4.02028E-05
3.44502E-05
2.95233E-05
2.53211E-05
2.17520E-05
1.87297E-05
1.61717E-05
1 .39981E-05
1 .21346E-05
1.05246E-05
2.76438E-04
2.42906E-04
2.14327E-04
1.88613E-04
1.65146E-04
1.44219E-04
1.26295E-04
1.11525E-04
9.84341E-05
8.64308E-05
7.55250E-05
6.58617E-05
5.76447E-05
5.10263E-05
4.54171E-05
4.03409E-05
3.56746E-05
3.14083E-05
2.75745E-05
2.42269E-05
2.14349E-05
1.92097E-05
1.73375E-05
1 .56727E-05
1.41508E-05
1.27446E-05
1.14448E-05
1.02517E-05
9. 17095E-06
8.21200E-06
7.38694E-06
6.68743E-06
6.07580E-06
.001545
1.51855E-03
1.49203E-03
1.46544E-03
1.43880E-03
1.41211E-03
1.38538E-03
1.3S861E-03
1.33181E-03
1.30498E-03
1.27812E-03
1.25124E-03
1.22434E-03
1.19743E-03
1.17050E-03
1.14356E-03
1.11660E-03
1.08964E-03
1.06267E-03
1.03569E-03
1.00870E-03
9.81709E-04
9.54712E-04
9.27711E-04
9.00706E-04
8.73698E-04
8.46687E-04
8.19673E-04
7.92656E-04
7.65638E-04
7.38617E-04
7.11594E-04
A.84570E--04
. 1192
.1192
5.16001E-02
5.16001E-02
5.16001E-02
5.16001E-02
5.16001E-02
1.64001E-02
1.64001E-02
1.64001E-02
1.64001E-02
1.64001E-02
1.64001E-02
3.70005E-03
3.70005E-03
3.70005E-03
3.70005E-03
3.70005E-03
3.70005E-03
3.70005E-03
3.70005E-03
5.10053E-04
5.10053E-04
5.10053E-04
5.10053E-04
5.10053E-04
5.10053E-04
5.10053E-04
5.10053E-04
5. 10053E-04
5.00532E-05
5.00532E-05
5.00532E-05
Figure 4-23 B30 Program and Sample Calculation
185
-------
260
265
270
275
280
285
290
295
300
305
310
33.5
320
325
330
335
340
345
350
355
360
365
370
375
380
385
390
395
400
405
410
415
420
9.12890E-06
7.91825E-06
6.86981E-06
5.96450E-06
5.18575E-06
4.51858E-06
3.94921E-06
3.46468E-06
3.05276E-06
2.70178E-06
2.40076E-06
2.14048E-Q6
1.91393E-06
1.71576E-06
1.54177E-06
1.38861E-06
1.25354E-06
1.13429E-06
1.02895E-06
9.35874E-07
8.53619E-07
7.80915E-07
7. 16623E-07
6.59712E-07
6.09237E-07
5.64329E-07
5.24210E-07
4.88221E-07
4.55816E-07
4.26532E-07
3.99983E-07
3.75838E-07
3.S3814E-07
5.52662E-06
5.02473E-06
4.56117E-06
4.13083E-06
3.73107E-06
3.36087E-06
3.02040E-06
2.71067E-06
2.43340E-06
2.19090E-06
1.98351E-06
1.80465E-06
1.64819E-06
1.50960E-06
1.38555E-06
1 .27353E-06
1 .17166E-06
1.07852E-06
9.93049E-07
9.14453E-07
8.42144E-07
7.75703E-07
7.14843E-07
6.59383E-07
6.09232E-07
5.64329E-07
5.24210E-07
4.8S221E-07
4.55816E-07
4.26532E-07
3.99983E-07
3.75838E-07
3. J3814E-07
6.57545E-04
6.30518E-04
6.03491E-04
5.76462E-04
5.49433E-04
5.22402E-04
4.95371E-04
4.68339E-04
4.41306E-04
4.14273E-04
3.87239E-04
3.60205E-04
3.33171E-04
3.06136E-04
2.79101E-04
2.52065E-04
2.25030E-04
1 .97994E-04
1.70958E-04
1.43922E-04
1.16885E-04
8.98487E-05
6.28120E-05
3.57751E-05
8.73S11E-06
0
0
0
0
0
0
0
0
5
5
5
5
5
5
5
5
5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.00532E-05
.00532E-05
.00532E-05
.00532E-05
.00532E-05
. 00532E-05
. 00532E-05
t00532E-05
. 00532E-05
Figure 4-23 B30 Program and Sample Calculation
186
-------
Total
S4+
mol/£
1.5 Order Rate Constant k = 85
•: Experimental Data Points
Experimental Conditions:
Temperature: 40 C
Initial pH; 5.0
Slurry Density: 20 g/£
Stirring Speed: 1800 rpm
0 1 2 3 4 56
TIME (minutes)
Figure 4-24. Determination of ke from Highly Catalyzed
Slurry Data (2000 ppm Mn Added)
187
-------
Mass Transfer Coefficient ke = 0.02
00
00
Total
S4+
mol/£
9 • Experimental Data Points
Experimental Conditions:
Temperature: 40°C
Initial pH; 5.0
Controlled pH: 4.6
Slurry Density: 20 g/2,
Stirring Speed: 1800 rpm
12 15 18
TIME (minutes)
,4+
21
24
27
30
4+
Figure 4-25. Comparison of Total S from Model to Experimental Slurry
Oxidation Data for No Added Mn.
-------
Total
S4+
mol/H
0.09
0.08
0.07
0.06 .
0.05 -
0.04 .
0.03 -
0.02 .
0.01 -
Mass Transfer Coefficient ke
• : Experimental Data Points
Experimental Conditions:
Temperature: 40 C
Initial pH: 5.0
Slurry Density: 20 g/£
Stirring Speed: 1800 rpm
0.0
1234 5 6
TIME (minutes)
Figure 4-26. Comparison of Model to Experimental Slurry Oxidation
Data for 6.66 ppm Mn Added
189
-------
Total
S4+
mol/fc
0.05 -
0.04
0.03 .
0.02
0.01
Mass Transfer Coefficient ke = 0.02
• : Experimental Data Points
Experimental Conditions:
Temperature: 40 C
Initial pH: 5.0
Slurry Density: 20 g/j,
Stirring Speed: 1800 rpm
k1.5=1-°
0123
TIME (minutes)
Figure 4-27. Comparison of Model to Experimental Slurry
Oxidation Data for 200 ppm Mn Added.
190
-------
0.012
Liquid
S4+
mol/£
0.010
0.008
0.006
0.004
0.002
Mass Transfer Coefficient ke = 0.02
• : Experimental Data Points
Experimental Conditions:
Temperature: 40 C
Initial pH: 5.0
Slurry Density: 20 g/ SL
Stirring Speed: 1800 rpm
=0.16
15 18
TIME (minutes)
21
24
27
30
Figure 4-28
Comparison of Liquid S from the Model to Experimental Slurry
Oxidation Data for No Mn Added.
-------
dieted by the model for 0, 6.66, 200, and 2000 ppm Mn added cases, using the
best 1.5 order rate constants obtained from earlier graphs, are also shown in
Fig. 4-29 for reference.
5. Derivation of the Flat Plate Model
The model was also derived for the solid particles being flat plates.
The following assumptions were made in developing this model. The particles
were all flat plates with a mean aspect ratio of 8.46 : 3.7 : 1. By neglect-
ing the surface areas offered by four edges, the total surface area, Ap, of
each plate was defined to be that of the two large surfaces. It was also
assumed that there was negligible mass transfer from the edges of the crys-
tals. Therefore Ap remained constant as the particle dissolves. The particle
size distributions (including surface area and thickness) were calculated from
the Coulter counter's data (spheres) by using the aspect ratio of
8.46 : 3.7 : 1.
Following these assumptions and also the assumptions made in subsection
(3) above, the model for the flat plate crystals is
dC0 1.5 N._ft k0 (C - CJ 13
j =i i ht f, (4-34)
H. 2mk0
at = - T^
-------
Liquid
S4+
mol/£
0.012
0.010
0.008
0.006
0.004
0.002
(D k = 0.35
Mass Transfer Coefficient ke = 0.02
(J> k = 0.35,0 ppm Mn added
(?) k * 2.0, 6.66 ppm Mn added
(3) k » 4.5,200 ppm Mn added
<4) k » 85, 2000 ppm Mn added
33 6 9 12 15 18 21
TIME (minutes)
Figure 4-29. Computer Predicted Liquid [S ] Profiles
24
27 30
-------
SECTION 5
OXIDATION IN SODIUM SULFITE SOLUTIONS
OUTLINE
V. Oxidation in Sodium Sulfite Solutions
A. Na-SO,, oxidation
1. Manganese Catalyst
B. Na_SO« Oxidation with Succinic Acid Buffer
1. Iron Catalyst
2. Manganese Catalyst
3. Mixed Catalysts
C. Rate of Na^SO- Solution Oxidation by the Flow-Thermal Method
194
-------
OXIDATION IN SODIUM SULFITE SOLUTIONS
The oxidation of sodium sulfite was studied in the presence of either
iron or manganese catalysts and with a mixture of the two. It was of interest
to supplement the calcium sulfite studies described above with sodium sulfite
work, since the calcium sulfite (commercial and laboratory produced) contains
numerous impurities while the sodium sulfite (reagent grade) is much purer.
Therefore, using sodium sulfite, the effects of the added catalyst can be
determined more precisely.
A. SODIUM SULFITE OXIDATION
The rate of sodium sulfite oxidation without organic acid buffer was
determined using the pH method described above.
1. Manganese Catalyst
The rate of reaction at T = 40°C and pH = 4.6 is given in Table 5-1
for various concentrations of [S +] and manganese. A rate expression was
determined for each manganese concentration by means of a regression analysis,
and the order is given in Table 5-1. Results for [Mn] =2.67 ppm are also
presented in Figure 5-1. The reaction is very close to second order for all
manganese concentrations and the second order rate constants are also pre-
sented in Table 5-1. Note that this is a different order than observed for
CaS03. Under the conditions of these experiments the rate is only weakly
influenced by the manganese concentration as can be seen in Figure 5-2.
B. SODIUM SULFITE OXIDATION WITH SUCCINIC ACID BUFFER
1. Iron Catalyst
Since the calcium sulfite was found to have significant concentrations of
iron, sodium sulfite was used to examine the catalytic effect of iron on the
oxidation of the sulfite. Ferric sulfate was selected to be the catalyst.
195
-------
TABLE 5-1
Na2S03 OXIDATION
Run No.
49
52
50
51
46
63
47
48
57
64
60
59
70
71
72
74
65
75
76
67
68
66
69
53
54
73
55
56
Iniital [S*
Constant of
Sulfite
0.092
0.140
0.210
0.354
0.092
0.129
0.202
0.354
0.082
0.117
0.180
0.425
0.060
0.190
0.354
0.075
0.125
0.211
0.392
0.087
0.184
0.243
0.326
0.095
0.221
0.251
0.333
0.383
Rate
[Mn] [Fe]
2.928x10 "
— n
6.621x10 "
1.630x10";?
4.360x10
-6
2.826x10 "
— n
5.730x10 I
— s
1.480x10
4.972x10
-6
2.761x10 ,
—n
5.810x10
1.433x10 I
_ s
9.093x10
-6
1.191x10 "
1.185x10 ;!
5.685x10
-6
1.976x10 °
— n
5.283x10
1.409x10
6.626x10
-6
2.641x10 "
w
1.115x10 ;:
— S
2.326x10 -1
4.650x10
-6
2.348x10 °
1.866x10 ,
2.247x10 I
— S
4.667xlO_5
5.906x10
0.5
0.5
0.5
0.5
1.33
1.33
1.33
1.33
2.67
2.67
2.67
2.67
4.0
4.0
4.0
6.0
6.0
6.0
6.0
8.0
8.0
8.0
8.0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Index for Rate Constant
Order Order of 2.0 Order
2.09
II
II
II
2.05
2.05
2.16
2.15
2.34
0.999
0.999
n
n
0.999
0.996
2.10 0.0995
0.994
0.998
0.000376
0.000384
!!
II
II
0.000436
n
0.000434
0.000410
0.000415
0.000375
T= 40°c, pH = 4.6
196
-------
10
2x10
Rate
mol/(£ sec)
10
-5
5x10
-6
2x10
-6
10
-6
40°C
pH = 4.60
[Mn] =2.67 ppm
(Run No. 57,64,60,51)
0.06 0.08 0.1 0.2 0.3 0.4 0.5 0.6 0.8
i- 4""f~T
|_S j Concentration, mol/£
Figure 5-1. Dependence of Rate on Sulfite Concentration
197
-------
00
6x10
_c
5x10
-5
Rate
4x10
mol/(& sec)
3x10
-5
-5
2x10
-5
10
-5
0.3 0:4 0.4 0.8 1
[S4+] = 0.30
40°C
pH = 4.60
456 8 10
[Mn] Concentration, ppm
Figure 5-2. Dependence of Rate on Manganese Concentration
20 30
-------
Ferric sulfate was found to be soluble in the reacting solution at concentra-
tions less than 6.0 ppm at a temperature of 40°C and a pH of 4.6. A stock
slurry was employed throughout the experiments. It was kept well mixed in
order to assure that the concentrations of iron added to the reacting solution
were uniform.
The initial sulfite concentration in the reactor ranged from 0.0045 to
0.0483 M. Oxygen was maintained at a flow rate of 3.0 £/min. The concen-
tration of iron added to the reacting solution ranged from 0 to 10 ppm. The
temperature of the reactions was either 30, 40, or 50°C. Stirring speed was
1620 rpm. A buffer to maintain the pH within 0.25 pH units of the initial pH,
4.6, throughout the course of the reaction was prepared using 14.2 g of suc-
cinic acid per 600 ml of water or catalyst solution. NaOH pellets were used
to adjust the initial pH of the reacting solution.
The iron catalyzed sodium sulfite oxidations can be better understood if
they are broken down into three sets. The first set of experiments consisted
of varying only the concentration of ferric sulfate added to the reactor. The
temperature of the water bath was varied in the second set of experiments.
Iron concentration in the reactor was maintained at 5 ppm. In the third set
of experiments the bath temperature was maintained at 40°C, the added iron
concentration was 5 ppm, and the initial sulfite concentration added to the
reactor was varied.
Typical data from experiments in which the concentration of ferric sul-
fate was varied are presented in Figure 5-3 where the total concentration of
the sulfur (4+) species vs time is plotted. It can be seen that 1 ppm Fe
increases the rate of oxidation significantly, but increasing concentrations
of iron up to 10 ppm does not have such a dramatic effect upon the rate of
oxidation. An initial analysis of the data fit the data to zero, first, and
199
-------
0.03
S *
•
a _
0.02-
[s4+]
mol/£
0.01
0.00
a
*
Conditions
pHi= 4.6
1620 rpm
40°C
* = Run 412, 0 ppm Fe
• = Run 426, 1 ppm Fe
• = Run 429, 3 ppm Fe
o = Run 421, 5 ppm Fe
*r= Run 423,10 ppm Fe
—r-
10
—T-
15
—i—
20
25
30
Time (min)
Figure 5-3. Sulfur (>+) Concentration vs Time for Iron Catalyzed
Na2S03 Oxidations (126)
200
-------
second order models. Of the integer values the iron catalyzed oxidations fit
a second order model best although a non-integer order is superior as noted
below. Table 5-2 presents a listing of the second order rate constants, the
index of determination used to provide an indication of the theoretical
curve's goodness-of-fit, and the calculated order of the reaction with respect
to the sulfite concentration.
Figure 5-4 is a plot of the reaction rate at [S ] = 0.028 M calculated
from the second order rate expression.
The data obtained from experiments performed at various temperatures are
shown in Figure 5-5 where the concentration of sulfur (4+) vs time is plotted
for typical reactions. Analysis by the REPORT program indicated that the
second order model fit the data better than zero or first order at the three
different temperatures. These data are given in Table 5-3, An activation
energy of 18.8+0.8 kcal/(g mol) was calculated by the linear regression analy-
sis of a plot of ln(£/sec order rate constant) vs (temperature)"1 presented in
Figure 5-6.
The third set of experiments was performed at 40°C,5 ppm iron were added
to the reactor. The initial concentration of the sulfite was varied in order
to make a second determination of the order of the reaction with respect to
the sulfite. Table 5-4 contains data obtained from the REPORT program. A
second order model fit the data better than zero or first. This information
indicated that the reactions maintained consistent behavior over the range of
sulfite concentrations considered.
The lower values for the second order rate constants calculated for both
the lowest and higher concentrations of sulfite used are probably due to
measurement limitations at low concentrations, and the inability of the buffer
to control the pH of the reacting solution at high concentrations.
201
-------
TABLE 5-2
RESULTS OF SODIUM SULFITE OXIDATION WITH
IRON CATALYST ADDED (126)
Initial S
+4
Iron
Concentration Second Order
Added Rate Constant
(ppm) (5,/mol sec)
Rate at
T J n* [S4+] = °'028
Index Of
Determination mol/fiL sec')
412
413
425
426
427
429
421
422
442
£t *•"*-* •*- 1
0.
0.
0.
0.
0.
0.
0.
0.
0.
n/
0280
0266
0290
0283
0278
0286
0278
0277
,0281
0
0
1
1
3
3
5
5
5
0.
0.
0.
0.
0.
.0..
0.
0.
0119
0133
0750
0805
128
147
202
,181
0.166
0.
0.
0.
0.
0.
0.
0.
0.
982
991
995
998
956
995
977
,990
0.988
9.
1,
5,
6
1
1
1
1
1
,33
.04
.88
.31
.00
.15
.58
.42
.30
-6
x 10
_5
x 10
_5
x 10
-5
x 10
-4
x 10
-4
x 10
-4
x 10
-4
x 10
-4
x 10
202
-------
0.25
0.20
Rate 0.15
mol/(S, sec)
0.10
0.05
0.00
0 2 4 68
ppm Fe added
Figure 5-4. Rate at [S4+] = 0.028M ys Concentration
Iron Added for Sodium Sulfite Oxidations (126)
203
-------
2.0-
1.5-
1.0-
Cs4+]
mol/X,
0.5 -
Conditions
« 4.6
1620 rpm
5 ppm Fe2(S04)3 Added
•=• Run 445, 30°C
* = Run 432, 40°C
a= Run 444, 50°C
0.0
a a
10 15
Time
1
20
r-
25
30
Figure 5-5. Sulfur (4+) Concentration vs Time ior Iron Catalyzed
Oxidations at Various Temperatures Using Na2SO~ (126)
-------
TABLE 5-3
RESULTS OF IRON CATALYZED REACTIONS AT VARIOUS
TEMPERATURES USING SODIUM SULFITE (126)
Second-Order
Rate Constant Index of
Run
445
446
421
422
442
443
444
Temp °C
30
30
40
40
40
50
50
£/(mol sec)
0.0675
0.0605
0.202
0.181
0.166
0.511
0.426
Determination
0.998
0.997
0.977
0.990
0.988
0.973
0.988
Conditions: Stirring Speed 1620 rpm
Initial pH 4.6
5 ppm Fe Added
205
-------
3.0-
-In
2.0-
-1
1.0
3:0
—i—
3.1
second order constant
AQ = 2.46 X 10 £/(g-mol-sec)
E » 18.8 kcal/(g-moi)
3.2
1/T x 1000 (T-°K)
3.3
Figure 5-6. Activation Energy Determination for Pe Catalyzed
Na2SO., Oxidations (126)
206
-------
TABLE 5-4
RESULTS OF SODIUM SULFITE OXIDATIONS WITH VARYING
INITIAL SULFITE CONCENTRATION AND 5 ppm IRON ADDED (126)
, / Second Order
Run
451
452
434
436
432
433
421
422
442
437
439
448
449
Initial S
g-mol/£
0.0045
0.0046
0.0094
0.0096
0.0186
0.0181
0.0278 '
0.0277
0.0281
0.0370
0.0379
0.0483
0.0473
Rate Constant
£/(mol sec)
0.114
0.062
0.142
0.160
0.147
0.188
0.202
0.181
0.166
0.165
0.158
0.067
0.059
Index of
Determination
0.902
0.922
0.996
0.990
0.991
0.987
0.977
0.990
0.988
0.990
0.994
0.997
0.997
Conditions:
Temperature = 40°C
Initial pH =4.6
Stirring Speed = 1620 rpm.
5 ppm Fe Added as Fe^(SO.)
2. 43
207
-------
The use of a constant succinic acid concentration over a wide sulfite
concentration was observed to cause problems in pH control. At an initial
sulfite concentration of 0.0438 M the pH dropped 0.27 units, but at 0.0277 M
the pH drop was only 0.14 units during the reaction.
Regression analysis of the data for Fe concentrations of 1.0, 3.0, and
5.0 ppm was done to determine the dependence of the rate of reaction at each
of these iron concentrations. The results are presented in Table 5-5.
A multiple regression analysis of all iron catalyzed experiments per-
formed at 40°C was done in order to determine the order of the reaction with
respect to iron and sulfur(4+) concentrations simultaneously. Data at time
greater than sixty seconds were used in the analysis. (See section on MULREG
Analysis). Based on 184 data points, the order of the sodium sulfite oxida-
tion with respect to the sulfur(4+) concentration was 1.69 with a standard
deviation of 0.06. The order with respect to the iron concentration was 0.18
with a standard deviation of 0.08. The rate of reaction of sodium sulfite
with iron catalyst at T=40°C and pH=4.6 is therefore given by
rate=0.155 [Fe] °'18 [S4Y'69
where [Fe] has units of moles per liter. Figure 5-7 is a plot of rate vs iron
concentration at [S ] = 0.028 M and pH 4.6.
2. Manganese Catalyst
Reactions using buffer, sodium sulfite solutions of approximately
the same initial concentration were performed under the conditions described
in Table 5-6. A solution of manganese sulfate was added to the water in the
reactor in concentrations ranging from 0 to 4.00 ppm Mn. The reactions had an
induction period of about four minutes. During this period, the concentration
of sulfite decreased only slightly. After the induction period, the reaction
proceeded significantly faster. The sulfur(+4) species were consumed more
208
-------
TABLE 5-5
MULTREG RESULTS FOR IRON CATALYZED Na^S00 OXIDATIONS
Fe Concentration
Added (ppm)
1.00
3.00
5.00
Number Of
Data Points
21
19
123
Order With ,
Respect to [S ]
1.78
1.78
1.68
Standard
Deviation
0.10
0.10
0.08
Conditions:
pH =4.6
Temperature = 40°C
Stirring Speed = 1620 rpm
Fe added as Fe (SO.)
209
-------
1.75x10
-4
1.25x10
-4
Rate
mol/(£ sec)
0.75x10
-4
0.25x10
-4
4+
3 4
[Fe], ppm
Figure 5-7. Rate at [S ] = 0.028m_v§ Concentration Fe
(Temp = 40°C; pH = 4.6)
210
-------
TABLE 5-6
RESULTS OF SODIUM SULFITE OXIDATIONS
WITH MANGANESE ADDED (126)
Rate at
Run #
412
413
405
407
415
417
419
418
404
406
408
409
410
411
Initial
Sulfite
g-mol/Jl
0.0280
0.0366
0.0262
0.0264
0.0279
0.0283
0.0281
0.0281
0.0.266
0.0259
0.0268
0.0266
0.0264
0.0268
Cone . Mn
Added
(ppm)
0.00
0.00
0.08
0.08
0.50
0.50
0.83
0.83
1.33
1.33
2.67
2.67
4.00
4.00
First Order Index of [S4 J - 0.028
Rate Constant* Determination mol/(£ sec)
2.40 x 10~4(sec)~1
1.81 x 10~4
4.18 x 10~4
4.68 x 10~4
8.66 x 10~4
8.81 x 10~4
9.25 x 10~4
1.06 x 10~3
2.51 x 10~3
2.22 x 10~3
5.23 x 10~3
4.45 x 10~3
1.05 x 10~2
1.05 x 10~2
0.985
0.966
0.914
0.987
0.990
0.989
0.989
0.983
0.969
0.994
0.973
0.987
0.838
0.898
6.72 x
5.07 x
1.17 x
1.31 x
2.42 x
2.47 x
2.59 x
2.968 x
7.03 x
6.22 x
1.46 x
1.246 x
2.94 x
2.94 x
io-6
io-6
io-5
io-5
io-5
io-5
io-5
io-5
io-5
io-5
io-4
io-4
io-4
io-4
The rate constant is based on curve fitting of data for times
greater than three minutes.
Conditions:
Temperature = 40°C
Initial pH =4.6
Stirring Speed = 1620 rpm.
Mn Added as MnSO, Solution
4
211
-------
rapidly as the manganese concentration was increased throughout the range
investigated. Figure 5-8 is a plot of the concentration of the sulfur(4+)
species y_s_ time for typical reactions performed.
The induction period was neglected in the analysis. Data at times
greater than four minutes were analyzed using an HP 2000C library program,
CURFIT, in order to check the fit of the data to a zero, first and second
order model. In all cases where manganese was added to the solution, the data
fit the first order case best. The rate of reaction at,[S ] = 0.028 M is
shown in Table 5-6 for both catalyzed and uncatalyzed runs and the results are
plotted in Figure. 5-9.
A linear regression analysis was done to determine the order with respect
to sulfur (4+) for each concentration of manganese. These are presented in
Table 5-7. With the manganese runs there was an induction period as reported
earlier. In the analysis of this data the induction period was neglected and
only points after the induction period used in the analysis.
A multiple regression analysis of all manganese catalyzed experiments,
performed at 40°C, was done in order to determine the order of the reaction
with respect to the manganese and sulfur(4+) concentrations simultaneously.
Results from the time of the maximum rate until the end of the reaction were
used in the analysis. Seventy-eight data points were used in the analysis and
the order of the reaction with respect to sulfur(4+) concentration was 0.94
with a standard deviation of 0.04. The order with respect to manganese was
found to be 0.67 with a standard deviation of 0.04. The rate of oxidation for
sodium sulfite with manganese catalyst at 40°C and pH = 46 is the given by
Rate =2.141 [Mn]0'67 [S4+]°'94
where [Mn] has units of moles per liter.
212
-------
0.03
0.02
* * *
[s4+]
mol/5,
0.01-
0.0
To"
15
Time (nrin)
20
25
30
Figure 5-8.
Sulfur (4+) Concentration vs_ Time for Manganese Catalyzed
Oxidations of Na2S03 Solutions.
(Conditions and Key for Symbols on Next Page) (126)
213
-------
Conditions
4.6
1620 rpm
40° C
solution used as catalyst
Key ^ of SymbolJL-
^JJ 1 ! IW W 1
*
.
a
-fr
a
412
407
415
404
410
Figure 5-8 (continued)
0.08
0.08
0.50
1.33
4.00
214
-------
3x10
-3
•JA. JL\J
2xlO~3
1.0xlO~3
Rate
mol(£sec)
8x10 ~5 ^
6xlO~5
4xlO~5
2xlO~5
(
0
c
i
m
,
> *
•
•
A
A A
12 3
Concentration Mn (ppm)
4+
Figure 5-9. Rate at S = 0.028M vs Mn Concentration
Temp - ^0°C{ pH = 4.6
215
-------
TABLE 5-7
REGRESSION ANALYSIS OF MANGANESE
CATALYZED Na0S00 OXIDATIONS (126)
Mn Concentration
Added (ppm)
0.08
0.50
0.83
1.33
2.67
4.00
Number Of Order With
Data Points Respect to [S* ]
12 2.31
10 1.64
21 0.91
14 0.90
11 1.03
10 0.96
Conditions:
pH]. = 4.6
Standard
Deviation
0.30
0.07
0.14
0.06
0.08
0.09
Temperature = 40°C
Stirring Speed = 1620 rmp
Mn added as MnSO.
216
-------
3. Mixed Catalyst
An inspection of the data obtained from the iron and manganese catalyzed
sodium sulfite oxidations led to speculation concerning the effect of com-
bining the two catalyst materials in the same reacting solution. Various
combinations of the catalysts were added to solutions 0.03 M Na^SO . At an
£* -D
initial pH of 4.6 and temperature of 40°C the reactions showed an induction
period. Concentrations of manganese and iron similar to those found in the
buffered calcium sulfite solutions were used in the experiments. Figure 5-10
is a plot of the reduced sulfur (4+) concentration, [S4+]/[S4+]o, vs time for
some typical experiments. There is no real similarity to the reactions.
In order to investigate the observed results further, a comparison of the
mixed catalyst oxidations for two different concentrations of catalysts was
made with the manganese catalyzed Na2SC>3 oxidations. Table 5-8 contains a
comparison of first order rate constants determined by the REPORT program for
the mixed catalyst oxidations.
All of the experiments done with mixed catalysts were found to have an
induction period. The rate and average concentration data from the REPORT
program were used to determine the order of the sulfur (4+) concentration.
Data were taken from the point where the rate increased indicating the end of
the induction period.
A multiple regression analysis was performed on the mixed catalyst data
to determine the order with respect to sulfur (4+), manganese, and iron. The
results are presented in Table 5-9. As can be seen the order with respect to
iron can be considered to be 0. Thus, a second multiple regression was done
where the mixed catalyst experimental data were used but orders were only
determined for S and Mn. These are also reported in Table 5-9. These
results show little change in the orders with respect to S4+ and Mn.
217
-------
1.0 4
0.9 •
0.8
0.7
0.6
0.5 .
[s4+]
0.3
0.2
0.1
O.Q
§ a
Conditions
• pHj » 4.6
1620 rpm
* 40°C
*
a
a
A a
d
^
6 8
Time (min)
Symbol
Run #
91.1
65.1
93.1
Composition
, 2 ppm Mn & 2 ppm Fe Added
CaS03 , No Catalyst Added
Na2S03, 0.5 ppm Mn 4 3 ppm Fe Added
Figure 5-10.
Reduced Sulfur (4+) Concentration vs. Time for
Comparison of Oxidation Reactions
218
-------
TABLE 5-8
RESULTS OF SODIUM SULFITE OXIDATION WITH IRON
AND MANGANESE ADDED
Run #
101.1
102.1
90.1
91.1
93.1
94.1
97.1
98.1
ppm Fe
added
1.0
1.0
2.0
2.0
3.0
3.0
6.0
6.0
ppm Mn
added
0.50
0.50
2.0
2.0
0.50
0.50
3.0
3.0
First Order
Rate Constant
8.01 x 10~4
8.60 x 10~4
4.31 x 10~3
4.82 x 10~3
1.11 x 10~3
1.34 x 10~3
5.88 x 10~3
5.78 x 10~3
Index of
Determination
0.982
0.989
0.977
0.970
0.988
0.988
0.938
0.929
219
-------
Order with
Respect to S'
0.96
0.96
0.94
TABLE 5-9
MULTIPLE REGRESSION RESULTS FOR MIXED
CATALYST SODIUM SULFITE OXIDATIONS
Standard
Deviation
0.03
0.03
0.05
Order with
Respect to Fe
0.04
Standard
Deviation
0.07
Order with
Respect to Mn
0.94
0.06
0.67
Standard
Deviation
0.06
0.05
0.04
Rate
Constant
71.6589
60.2560
2.1406
Temp = 40°C, PH = 4.6
* In these runs both catalyst were present, but in the analysis of the data it is
assumed that the reaction is zero order in iron.
220
-------
However, from the results of the manganese only catalyzed runs, iron does seem
to have an effect on the influence of manganese concentration on the rate.
The rate expression obtained from the manganese only catalyzed runs was: Rate
=2.141 [Mn]0'67 [S4+]0'94.
The results obtained from the experiments wth manganese only (no iron)
are also presented in Table 5-9 for comparison. It is noted that although the
order of the reaction with respect to iron is zero, the results obtained with
and without iron are different. Therefore, iron does influence the rate of
reaction, but that rate does not depend on the amount of iron for Fe > 1 ppm.
C. RATE OF Na2S03 SOLUTION OXIDATION BY THE FLOW-THERMAL METHOD
Sodium sulfite/bisulfite solutions have long been employed to study the
oxidation kinetics of aqueous S ions. These sodium salts are much more
soluble, less expensive, and available in purer lots than the corresponding
calcium salts. Several studies have shown the sodium cation to have no effect
on the oxidation rate (21, 91, 95). No mechanism proposed to date involves
the spectator cation (Na or Ca) in a kinetically important step. So the impor-
tant effects of catalysts, [S ], or inhibitors may profitably be studied in
the experimentally more advantageous sodium solutions. Furthermore, the
results of sodium sulfite oxidations may have direct application in sodium-
based wet scrubbing processes for sulfur dioxide removal from stationary
sources.
The conditions selected for the sodium bisulfite/sulfite oxidations were
therefore in two parts: low pH and high pH.
pH: 4.5, 5, 6 11
[S ]: 0.05 - 0.14 0.009 - 0.09 mol/£
221
-------
[OJ : 0.004 - 0.015 mol/SL
[Mn]: 10, 100, 1000 \, 0.4-0.6, 1, 3 ppm
T: 25±0.02 °C
The rates encountered for sodium sulfite oxidations span a broader range
than the range for calcium sulfite oxidation. The greater solubility of
sodium salts especially at higher pH values, results in much faster oxida-
tions. In the current work rates ranged from 0.0002 to 0.7 mol/(£ sec).
Table 5-10 contains the results of varying the Mn concentration over a
small, but likely range. In this range, tripling the catalyst concentration,
doubles the reaction rate. This range of \ to 3 ppm is a likely range for
solutions made from commercial salts.
The effect of a rise in pH is to increase the homogeneous reaction rate.
For the NaHSO oxidation the results of varying pH from 4.5 to 5 to 6 are in
Table 5-11. At pH 4.5 the rates for [Mn] at all concentrations were too low
to measure. The increase of rate with [Mn] is greater than linear for both pH
5 and 6.
The reactant orders for the high pH experiments were examined by Multiple
Regression Analysis for the conditions:
[S4+] = 0.0086-0.0781 M, [02] = 0.0043-0.0118 M, [Mn] = 0.5 ppm
The results were:
Sulfite Order = 1.4910.17 Oxygen Order = 0.49±0.19 So the order
is about 3/2 in sulfur and 1/2 in oxygen. The figure for oxygen varied in
several multiple regressions varying as far as 0.43±0.6.
2+ +
The oxidation state of the catalyst, Mn or Mn , is important in deter-
mining the rate of the reaction. To see what effect the two oxidation states
222
-------
TABLE 5-10
EFFECT OF [Mn] on Na,
^
OXIDATION RATE
[Mn], ppm
0.5
Rate, mol/(£ sec)
0.404
0.2004
0.148
Conditions;
pH: 11
[S ]: 0.01 mol/£
[02J: 0.015 mol/£
T: 25°C
223
-------
TABLE 5-11
EFFECT OF pH
NaSO OXIDATION RATES
[Mn]
ppm
PH
10
100
1000
4.5
too
low
too
low
too
low
0.00022
0.0206
0.298
0.0112
These rates are in mol/(£ sec)
Dashes indicate conditions not tested
Conditions;
[S4+]: 0.05 - 0.14 mol/£
[0 ]: 0.008 - 0.013 mol/£
T:
25°C
224
-------
might have, the Mn catalyst at 1 ppm was"put alternately in the sulfite reser-
~f~ 9+
voir (Mn ) and the oxygen reservoir (Mn ). The difference is notable and
shown in Figure 5-11. Faster initial temperature rise is observed when Mn is
added to the sulfur side, indicating a greater rate.
Although these experiments were done at 25°C, the similarity with the pH
method results suggests the rates may be calculated for higher temperatures
with the Arrhenius relation using the pH method activation energy.
225
-------
Conditions: [S ] 0.05 M
[02] 0.008 M
[Mn] 1 ppm
pH 11
T 25°C
1.0
0.9
0.8
0.7
0.6
AT 0.5
°C
0.4
0.3
0.2
0.1
A
•fc
* fe
.
fc
fc
*
4 X Run 1-186
Mn in
S _. , ,„-, Sulfur reservoir
* D Run 1-187
S
B * A Run 1-206
A Mn in
* * Run 1-209 °Xygen Reservoir
o o
3 A 5
Residence Time, sec
Figure 5-11. Comparison of Mn Added only to Oxygen Side
or Only to Sulfur Side
226
-------
SECTION 6
CONCLUSION
For convenience and rapid comparison, this section is a synopsis of this
study of aqueous sulfur dioxide oxidation. It is organized by the sections of
the report, and in each section, the findings of the subsections are given in
order.
Section 1: Introduction
o The conditions surrounding aqueous sulfur dioxide oxidation: inci-
dence, physical considerations, and kinetics are treated in an extensive
literature review having 190 references.
o Some individual articles were commented upon or amplified; most nota-
bly, the work by Gladkii in Russia. For this work there are detailed
comparisons with the instant work.
Section 2: Experiments
o The batch reactor and experiments included many details to ensure
kinetically-limited oxidation free from uncontrolled catalysis and physi-
cal variations.
o The flow reactor represents an advance in the application of automated
flow calorimetry to chemical kinetics by incorporating the capacity for
high pressures with a completely inert flow circuit.
o The materials used and reacting solutions which resulted were all
examined for composition. The calcium sulfites contained much sulfate as
well as catalytic impurities.
o Analyses of experimental results were by several means: a comprehen-
sive computer program was prepared to format all data reduction to aid
227
-------
in comparison of 0, 1st, and 2nd order rate expressions as well as vary-
ing experimental parameters; for specific experimental sequences, a
multiple regression program gave results for reactant orders and rate
constants.
For the pH method considerable extensions and corrections of Beng-
tsson's ingenious scheme were employed. The flow thermal experiments
gave rates via an enthalpy balance on the reacting fluid.
Section 3: Oxidation in Calcium Sulfite Solutions
o The reaction order of the oxidation of clear calcium sulfite solutions
is 1.5 by the pH method. Tables 3-1 to 3-5 give the 1.5 order rate con-
stants under different reaction conditions.
o The reaction order of oxidation of calcium sulfite with the presence
of succinic acid is also 1.5 and the k 5 are shown in Table 3-6. It can
be seen from Figure 3-7 that the rate of reaction increases with increa-
sing pH with the rate increasing more rapidly above pH = 5.0.
o At pH = 4.0 and low succinic acid concentrations, the reaction is
adequately described by a first order expression; however, with increas-
ing levels of acid, the order rises above 1.0.
o The effects of adipic acid on calcium sulfite oxidation are summarized
in Table 3-9. The reaction order is approximately 1.5 at low adipic acid
concentration and rises with increasing organic acid concentration.
o Glycolic acid is a very strong inhibitor for the oxidation, much
stronger than any of the other acids studied. Summaries of the results
are presented in Tables 3-10 and 3-11.
o The trend of inhibitory strength is:
glycolic acid>adipic acid>succinic acid
228
-------
o The oxidation rate of an inhibited run is increased by adding catalyst
(i.e. manganese and iron) but the results of the experiment will depend
on the relative strength and concentrations of both the catalyst and the
inhibitor added.
o Oxidation studies were performed using liquor from slurry samples from
Penberthy Oxidation Runs at the Shawnee Test Facility. The experiments
were to determine the degree of activity of the samples in catalyzing the
oxidation of sulfite.
o Experiments were made to find the safe (kinetics limited not mass
transfer limited) range of the operating parameters. At low stirring
speeds (Figure 3-24, 215 rpm) the reaction is mass transfer controlled
and the concentration of S decreases linearly with time. At higher
stirring speeds (500-3500 rpm) mass transfer effects are unimportant.
o Experiments of CaS03 oxidation at high manganese concentrations (10,
200, 1000, and 2000 ppm) were conducted by flow-thermal method. The
results are listed in Table 3-23 and 3-24, with a range of PH from 3.3 to
4.7.
Section 4: Oxidation in Slurries
o Solubilities of CaS03 under different pH were obtained by both theore-
tical calculation and experimentation. The data agree with each other
well. A power function of Cs = 1676.84 (pH~7'791) fits the experimental
data well. There is notable difference between these results and the
predictions of the Radian equilibrium program.
o An oxygen flow rate of 3 H/min was used in the oxidation reaction
after showing that there is not gas-liquid mass transfer control during
the reaction at these conditions.
229
-------
o The initial rate of reaction increases with decreasing initial pH or
'increasing slurry density.
o Figure 4-4 shows that the slurry reaction rate is strongly influenced
by added manganese catalyst.
o In all cases there is an abrupt drop in slurry pH upon the intro-
duction of 02. From this lowered value, the pH will (a) continue drop-
ping at a much slower rate, or (b) flatten out and begin to rise, or (c)
a combination of (a) and (b), depending on the initial pH. At a pH below
5, curve type (a) is observed while at a pH above 5.1 type (b) is ob-
served. The changeover between (a) and (b) occurs between 5.0 and 5.1.
o Figure 4-22 shows that the manganese concentration, in the liquid
phase of the reactor increases, as the oxidation proceeds and the solids
dissolve. These changes in catalyst concentration have a strong effect on
the rate of reaction.
o A mathematical model for the slurry reaction predicts the concen-
trations of both total sulfite and liquid phase sulfite as a function of
time as the reaction proceeds.
o By modeling the slurry oxidation with 2000 ppm Mn added, a mass trans-
fer coefficient of 0.02 cm/sec was found.
o From this mathematical model, the k^ for 0, 6.66, and 200 ppm Mn
added reactions are 0.35, 2.0, and 4.5 £°"5/g mol0'5 sec. The correspond-
ing experimental values are 0.162, 0.35, and 0.8-5.0 2°-5/gmol°- sec.
o The mathematical model predictions for total sulfite concentration
agree very well with experimental observation, but the prediction of
liquid phase sulfite concentration differ significantly from experimental
results.
I
230
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Section 5: Oxidation in Sodium Sulfite Solution
o For examining bisulfite/sulfite kinetics, sodium salts may be used as
well as calcium, especially because of their higher solubility and lower
impurity level
o With manganese catalyst, T = 40°C, PH = 4.6, [Mn] =2.67 ppm the rate
expression is about second order in sulfur with a rate constant of 0.0004
£/(mol/sec). Under these conditions, these results of the pH method show
only a weak dependence of rate on [Mn] on the range 0.3-30 ppm.
o Various organic acids have been used as buffers for this reaction
system. One of these same acids has been suggested to improve limestone
solubility. The kinetics with these acids present was studied:
o Ferric sulfate solubility in the reacting solution is less than 6 ppm
at 40°C and pH 4.6. On the range T = 30° to 50°C, the activation energy
is 18.8 ± 0.8 kcal/mol. The best integer order for [S4+] is two with an
average rate constant of 0.14 £/(molsec). On the range 0-5 ppm, the
slope of the Rate vs [Fe] line is 0.025--not a strong dependence. By
multiple regression analysis, the rate law is:
Rate = 0.155 [Fe]°'18 [S^]1'69
o Buffered manganese experiments on the range 0-4 ppm[Mn] showed a
4-minute "induction period." After the induction period the reaction is
nearly first order in sulfur (pH: 4.6, T = 40°fc) with a rate constant
depending on [Mn]. Multiple regression analysis showed
Rate =2.141 [Mn]°-67[S4+]°-94
o Commercial sulfites and limestone contain mixed profiles of iron and
manganese. The experiments showed an induction period followed by rates
similar to manganese-only catalyzed reactions. Multiple regression
231
-------
analysis confirmed the absence of an iron order; however, the presence of
iron changes the dependence on [Mn] to 0.67 though not affecting the
sulfur order.
o The effect of oxygen concentration and higher pH may not be studied in
the batch reactors. The flow-thermal method is suited for these studies.
At pH 5 and 6 the increase of rate with [Mn] is greater than linear. At
pH 11 the.slope of rate vs [Mn] was 0.67. Multiple regression analysis
show the order in sulfur, to be approximately 1.5 (1.49 ± 0.17) and the
oxygen order to be about 0.5 (0.49 ± 0.19). When manganese is added to
the reaction mixture in the +1 oxidation state, the rate is half again
faster than it is for manganese in the +2 state.
232
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TECHNICAL REPORT DATA .
(Please read Instructions on the reverse before completing/
|i REPORT NO.
EPA-600/7-80-083
I. TITLE AND SUBTITLE
Sulfur Dioxide Oxidation in Scrubber Systems
7. AUTHOR(S)
J.L. Hudson
9. PERFORMING ORGANIZATION NAME AND ADDRESS
University of Virginia
Department of Chemical Engineering
Charlottesville, Virginia 22901
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
RECIPIENT'S ACCESSION-NO.
REPORT DATE
April 1980
. PERFORMING ORGANIZATION CODE
. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
Grant No. R805227
13. TYPE OF REPORT AND
Final; 7/77-6/79
,ND PERIOD COVERED
14. SPONSORING AGENCY CODE
lesearch Triangle parK, JNU arm | EPA/600/13 _
.SUPPLEMENTARYNOTES jjERL-RTP project officer is Robert H. Borgwardt, Mail Drop
15, 919/541-2336.
65,
is. ABSTRACT The repOrt relates the liquid-phase oxidation kinetics of bisulfite and sul-
fite anions (determined in bench scale experiments) to conditions representative of
limestone scrubbers used for flue gas desulfurization. The chemical reaction rates
were determined for clear solutions and slurries of calcium sulfite when gas-to-
liquid transfer of oxygen was not a limiting resistance. From th« e.?*™^*1 .
results, a mathematical model was developed for the overall oxidation rate of cal-
cium sulfite slurries, including solids dissolution and chemical reaction The over-
all rate is shown to decline with increasing pH due to the reduced solubility of cal-
cium sulfite; the solid dissolution rate is thus the limiting factor at high pH. The
homogeneous chemical oxidation rate is 1. 5 order and increases with pH. Organic
acids inhibit the oxidation reaction, especially glycolic acid. Manganese and iron
catalyze the oxidation reaction even in the presence of organic inhibitors.
17.
KEY WO'RDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
Pollution
Sulfur Dioxide
Oxidation
Scrubbers
Calcium Carbonates
Flue Gases
Desulfurization
Mathematical Models
Sulfites
Calcium Inorganic
Compounds
Organic Acids
13
I. DISTRIBUTION STATEMENT
Release to Public
EPA Form 2220-1 (9-73)
Pollution Control
Stationary Sources
Calcium Sulfite
Glycolic Acid
19. SECURITY CLASS (This Report)
Unclassified
20. SECURITY CLASS (Thispage)
Unclassified
COS AT I Field/Group
13B
07B
07A,13I
2 IB
07D
12A
21. NO. OF PAGES
254
22. PRICE
245
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