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

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           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.

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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).

-------
             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

-------
     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.

-------
Figure 1-2.  Electron Micrograph of Calcium Sulfite Particles
             Sulfite and Sulfate Particles
Figure 1-3.  Electron Micrograph of Calcium Sulfite Particle
             Individual Agglomerate

-------
            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

-------
          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

-------
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

-------
 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

-------
                                    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

-------
                                   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

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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

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     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

-------
                      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














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-------
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

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     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

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       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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