EPA-650/2-73-047
December 1973
Environmental  Protection Technology Series


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                                        EPA-650/2-73-047
          ABSORPTION  OF  S02
          INTO  LIME  SLURRIES:
ABSORPTION  RATES  AND  KINETICS
                         by
                   J.  Edward Vivian

            Massachusetts Institute of Technology
             Department of Chemical Engineering
              Cambridge, Massachusetts  02139
             Contract No.  68-02-0018 (Task 4)
                  ROAP No.  21ADE-21
               Program Element No.  1AB013

         EPA Project Officer:  Charles J. Chatlynne

               Control Systems Laboratory
           National Environmental Research Center
         Research Triangle Park, North Carolina  27711
                     Prepared for

            OFFICE OF RESEARCH AND DEVELOPMENT
            U.S. ENVIRONMENTAL PROTECTION AGENCY
                 WASHINGTON, D.C. 20460

                     December 1973

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This report has been reviewed by the Environmental Protection Agency and
approved for publication.  Approval does not signify that the contents
necessarily reflect the views and policies of the Agency, nor does
mention of trade names or commercial products constitute endorsement
or recommendation for use.
                                   ii

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                                                                             iii
                           ABSTRACT








     The absorption of sulfur dioxide from sulfur dioxide-nitrogen gas



mixture into water and lime solutions and slurries was studied in a short



wetted wall column at approximately one atmosphere and 25 deg. C.  The



mole per cent of sulfur dioxide in the gas was varied from 0.03 to 0.37.



In solutions of calcium hydroxide greater than 0.01 g.mole/liter and in



slurries the system appeared to be gas absorption accompanied by an



"infinitely" rapid irreversible second-order reaction in the liquid phase.



The only effect of the solid in the slurries was to increase the absorption



capacity.  In water the system appeared to be gas absorption accompanied



by a fast reversible hydrolysis reaction in the liquid phase.  The major



resistance to gas absorption at these concentration levels was found to



be in the gas phase and at the lowest concentrations studied the system



was gas phase controlled.

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                                                                             iv
                              FOREWARD






     The work reported here was carried out in accordance with Task Order




No. 4 under HEW Contract CPA-68-02-0018«  The principal investigator was




J. Edward Vivian, Professor of Chemical Engineering.  Research assistants




who carried out the laboratory work were W. D. Franklin, R. N. Hindy and




H. F. Jackson.

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                               TABLE OF CONTENTS









                                                                 Page




SUMMARY                                                            1






INTRODUCTION                                                       3







THEORETICAL CONSIDERATIONS                                         5






APPARATUS and PROCEDURE                                           14







RESULTS and DISCUSSION                                            20







CONCLUSIONS and RECOMMENDATIONS                                   31







APPENDICES                                                        33






      A.  DETAILS OF PROCEDURE                                    34






      B.  SAMPLE CALCULATIONS                                     38






      C.  SUMMARIZED DATA                                         50






      D.  NOMENCLATURE                                            55






      E.  LITERATURE CITED                                        57

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                              SUMMARY








     Restrictions on the emission to the atmosphere of sulfur dioxide have




generated the need for effective methods for its removal from various waste




gas streams such as stack gas from power plants burning sulfur-bearing fuels.




One of the techniques under investigation is the absorption of the sulfur




dioxide into lime slurries in water.  This absorption system involves gas




absorption accompanied by simultaneous chemical reactions in the liquid




phase, and because of the low solubility of calcium hydroxide in water




slurries are used in order to provide sufficient absorption capacity.  This



study was concerned with the effect of the reactions in the liquid phase upon




the rate of absorption of sulfur dioxide by the slurries.




     The experimental data were obtained with a short wetted wall column in




which sulfur dioxide in nitrogen flowed counter to a falling film of liquid.




Rates of absorption of sulfur dioxide into calcium hydroxide solutions and




slurries and into water were measured at approximately 25 deg. C and one



atmosphere total pressure.  The concentration of sulfur dioxide in the gas




was varied from 0.03 to 0.37 mole percent.  Liquid phase absorption coeffi-




cients were obtained by deducting the gas phase resistance from the overall




resistance derived from the experimental data.  Comparison of these liquid



phase coefficients with data on liquid phase controlled physical absorption




permitted the calculation of the enhancement factor for the effect of liquid




phase reactions on the absorption rate.



     At the concentration levels used in this study the major part of the




resistance to absorption both into slurries and into water was in the gas




phase, and at the lowest concentrations the process was entirely gas phase




controlled.



     The absorption of sulfur dioxide into calcium hydroxide solution and

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slurries appeared to be a system of gas absorption accompanied by an "in-




finitely" rapid second-order irreversible chemical reaction involving




molecular (unhydrolysed) sulfur dioxide and hydroxide in solution.  The rate




of solution of solid calcium hydroxide did not appear to influence the




absorption process at the interface.  The effect of the solid in suspension




was to increase the capacity of the liquid phase to absorb sulfur dioxide.




This mechanism indicates the need in continuous contacting to provide hold-




up to allow for the dissolution of the solid in the slurry to maintain the




concentration of hydroxide near saturation.




     The absorption of sulfur dioxide into water is a system of gas absorp-



tion accompanied by a fast reversible hydrolysis reaction.  The reaction




is sufficiently rapid to maintain hydrolysis equilibrium in the liquid as




sulfur dioxide diffuses away from the interface.  The enhancement factor




for this case becomes very large at low concentrations of sulfur dioxide




in the gas,  and consequently, the system becomes gas phase controlled at



these concentration levels.  At calcium hydroxide concentrations greater




than about 0.01 g.mole/liter the rate of absorption of sulfur dioxide into




these hydroxide solutions was somewhat greater than that observed for the




absorption into water under similar conditions.



     Experimental work on the absorption of sulfur dioxide into slurries in




a continuous absorber such as a long wetted wall column or a packed bed of




spheres in which the liquid holdup undergoes repeated exposure to the gas




followed by bulk mixing is needed to supplement these short wetted wall data




to show the effect of the rate of dissolution of the solid on the absorption



process.

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                           INTRODUCTION








     The absorption of sulfur dioxide into alkaline media is an important



process which is receiving considerable attention as a prime means of con-




trolling sulfur dioxide emission in stack gas from the combustion of sulfur-




bearing fossil fuels.  The process has been widely used in the process




industries and in particular in chemical pulping, and the technology to




meet the requirements of these industries is available.  However, the scale




of operations and particularly the concentration ranges of sulfur dioxide




in feed gas and exhaust gas encountered in emission control problems impose




new and more stringent requirements on this technology.  For example, process



exhaust gas may be the feed gas for emission control processes, and these




processes are required to reduce sulfur dioxide levels to a few hundred




parts per million.




     The absorption of sulfur dioxide from gaseous mixture into alkaline



media is a process involving gas absorption accompanied by simultaneous




chemical reactions in the liquid phase.  Depending on the relative rates of




diffusion and reaction one or the other phenomenon may be the controlling



factor.  An understanding of the interaction of diffusion and reaction is




necessary for design of equipment and also for operation of such processes




since operating conditions may affect the relative rates significantly.




Fundamental kinetic data are often not available, and furthermore since




such reactions fall in the category of "fast reactions" fundamental kinetic




data are difficult to obtain independently without the complicating factors




involved in the combined diffusion and reaction processes.  Consequently




these kinetic data are generally determined in the laboratory by comparing




absorption rates with and without reaction under carefully controlled fluid




dynamic conditions.  Laminar jets and short wetted wall columns have been

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useful for this work.




     Several alkaline reactants can be used to absorb sulfur dioxide.  Sodium




hydroxide, sodium carbonate, sodium sulfite and magnesium hydroxide are



effective reactants of commercial importance for emission control, providing




the process includes a chemical regeneration system.  The only reactants for




sulfur dioxide absorption which appear to be economically justifiable in a




once-through process are calcium hydroxide and calcium carbonate.  Since




plant investment and technological development will be considerably less for




chemical processing which does not include recovery operations, substantial




emphasis is being directed to the development of lime/limestone absorbers




for first generation plants for sulfur dioxide emission control.  In view




of the low solubility in water of these reactants aqueous slurries must be




used to provide adequate absorption capacity.  Thus in developing an absorp-




tion model for absorption of sulfur dioxide into these slurries not only is




consideration of diffusion and reaction in the liquid phase important but




also the rate of dissolution of the solid reactant.




     In this investigation an attempt was made to determine the effect of




the liquid phase reactions of sulfur dioxide with lime slurries upon the rate




of absorption of sulfur dioxide under known fluid dynamic and diffusion con-




ditions with a view to determining the rate limiting steps.  A short wetted




wall column was used and the laboratory work was limited to reagent chemicals




and to concentrations of sulfur dioxide in the gas phase of interest in the




emission control program.

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                    THEORETICAL CONSIDERATIONS







     Sulfur dioxide dissolves in water and undergoes hydrolysis to form a



solution of molecular unhydrolysed SO. and hydrogen, bisulfite and sulfite



ions.  Sulfurous acid (H SO.) is believed not to exist in significant amount.



Equilibrium exists between the various species in solution and between the



unhydrolysed molecular SO  and the partial pressure of SO  in the gas phase.



The equilibrium between SO. in the gas and the unhydrolysed molecular SO.



follows Henry's law and can be written in the form:



            CA  -  HP



where C  is the unhydrolysed molecular SO- concentration, g.mole/liter, and



p is the partial pressure in atmospheres in the gas phase.  The equilibrium



established in the liquid phase for the hydrolysis of the dissolved SO. to



form bisulfite can be written in the form:
                   [HSO "][H+]
     The dissociation of HSO ~ is very small unless the hydrogen ion



concentration is suppressed by the presence of alkali.  Consequently the



total concentration C  of sulfur dioxide in solution can be represented by



the sum of the concentrations of the unhydrolysed molecular SO., C , and
                                                              t+   A


the bisulfite ions, C_:
                     E




            CT  -  CA*CE


Combining this with the expression for Henry's law to substitute for the



molecular SO  concentration and the expression for the dissociation con-



stant to substitute for the bisulfite concentration and noting that the



concentration of hydrogen and bisulfite ion concentrations are essentially



equal an expression for the gas-liquid equilibrium for sulfur dioxide in

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water is obtained:




            CT  -  Hp +




The importance of this expression lies in the fact that it permits the




evaluation of the Henry's law constant.  This constant is required to




predict the interfacial concentration of unhydrolysed molecular S0_ in




an absorption process.




     Several investigators have studied the equilibrium solubility of




sulfur dioxide in water.  The work of Johnston and Leppla (2) is repre-




sentative of these studies and  has been used in this work to derive




the Henry's law constant.  The data are shown in Figure 1 where C




has been plotted against ^p so that the slope of the isotherm is equal




to the Henry's law constant, H, and the intercept is equal to /K H.




The constants are given as a function of temperature in Figure 2.  Equa-




tions for these functions are:






            In H  »  317T5'2  -  10.4523






            In K  -  2-3%2'6  -  11.929




where H is given in g.moles/liter-atm, K  in g.moles/liter and T in degrees



Kelvin.  It is to be noted that the values of K  derived from these gas-




liquid equilibrium data agree closely with values reported in the literature.




     The model assumed in this investigation for the absorption of sulfur




dioxide into water is based on diffusion of SO  from the bulk of the gas




phase at a partial pressure p across a gas phase resistance to the gas-




liquid interface where the partial pressure p. establishes an equilibrium




concentration CA. of unhydrolysed molecular SO. in the liquid interface.




Molecular SO  is removed from the interface by diffusion as molecular SO..




and by reaction with water.  Depending on the relative rates of diffusion




and reaction, liquid phase equilibrium between the species in solution at

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                    1000 / T
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                   3-3      3-
                    1WO/T   frft. A)"

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any given distance from the interface may or may not be maintained.




Eigen, Kustin and Maass (1_) report data on the hydrolysis reaction:




          S02 + H20  t  H+ + HS03-




which indicate the forward rate as written to have a pseudo-first order




rate constant in excess of 10  sec  .  Relative to ordinary diffusion




rates this is a very fast reaction, and it is expected that the various




species will be approximately in equilibrium during diffusion away froin




the interface.




     In cases involving absorption with simultaneous chemical reaction it




has been convenient to retain the usual forms of the absorption rate



equation and to allow for the effects of the associated chemical reactions




by an enhancement factor.  Thus the rate equation becomes:
                                           2

where N  is the solute flux  (g.moles/sec-cm ) and fe * is the physical
       A                                           I»



absorption coefficient dependent on physical properties and fluid dynamics




of the system.  The driving-force  (CAi~ Cft) is the difference between the-




concentration at the interface and in the bulk liquid of the specie enter-



ing the liquid phase, in this case unhydrolysed molecular SO , and  is




the enhancement factor dependent on the relative rates of diffusion and




reaction.  For a limited number of absorption models  can be estimated



from properties of the system.




     Applying the concept of equilibrium among all species during the




diffusion of SO_ away from the interface in the absorption of SO. into




water the enhancement factor was found to be:
where D  and D  are the diffusivities of molecular SO. and bisulfite ion
       A      £                                      2

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                                                                             10
respectively, and K. is the equilibrium constant for the reaction of SO.



with water.  The model used here to derive this expression for  was based



on film theory.  The corresponding expression for penetration theory was



not available, but the two theories are not expected to differ signifi-



cantly in this case.



     To model the absorption of SO  into lime slurry the reaction of SO-
                                  A                                    A


with hydroxyl ion and the dissolution of the lime particles must be



included.  At very low concentrations hydroxyl ions will have negligible



effect on increasing the rate of absorption over the rate into water.



However, as the hydroxyl ion concentration is increased eventually the



rate of absorption will be governed by the reaction of SO  with hydroxyl



ion to produce bisulfite and sulfite ions.  These reactions are expected



to be very rapid.  For the case where there is no sulfite in the initial



absorbent liquid, hydroxide will diffuse from the bulk toward the inter-



face as SO. in one or more forms diffuses away from the interface.  This



is a situation typical of a fast irreversible (homogeneous) second-order



reaction in the liquid phase.  The overall effect is:



          S02(aq) + Ca(OH)2  •+  CaS03 + H2O



The fact that CaSO, has very limited solubility and will precipitate will



have only a secondary effect on the rate at which SO.(aq) is removed from



the interface.  The presence of solid Ca(OH). in the slurry however may



affect the rate depending on its rate of dissolution.



     If the rate of dissolution is relatively slow, the absorption process



during a single exposure will be similar to the infinitely rapid irrever-



sible  reaction case involving reacting species in solution.  The 
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                                                                            11
where C_ is the saturated calcium hydroxide concentration in the bulk
       15



liquid.




     On the other hand if the rate of dissolution of the calcium hydroxide




particles is sufficiently rapid relative to the rate at which hydroxide




in solution is consumed by the absorbed SO. so that a concentration of




hydroxide close to saturation is maintained, the reaction accompanying




the absorption will appear to be pseudo-first order.  The <|>-values will




lie along the 45 degree line on the Peacemen-van Krevelen plot, or in




other words the  fe * product will be a constant independent of flow rate
and equal to /fe D  C   where feds the second-orderireaofcion rate constant.




     Thus experiments measuring the rate of absorption of SO. into lime




slurries at different flow rates (which varies fe *) should give an indic-
                                                ii



ation of whether the rate of dissolution of the solid is rapid enough to




increase the overall rate of absorption above that expected for absorp-



tion into the homogeneous liquid phase or whether the advantage in the




presence of the solid is only to increase the capacity of the liquid




phase to dissolve and react with SO..  In the latter case, holding time



between surface exposures ,to complete reaction and saturate the liquid



will be required to use the slurry reactant effectively.




     Since sulfur dioxide has appreciable solubility in water and under-




goes rapid reactions in the liquid phase, there will be significant gas




phase resistance to absorption.  To reduce the data on SO. absorption to




obtain data on liquid phase phenomena an estimate of the gas phase resis-




tance must be made and deducted from the overall resistance.  The short




wetted wall column was used to study gas phase controlled absorption by




Behrmann  (8) and by Schoenburg (9).  Liquid flow rate had negligible




effect on the gas phase coefficient, and the results for the gas phase




were correlated by reporting the Sherwood number, N_. , as a function of

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                                                                            12
the Schmidt number, N  , and Reynolds number, N   as shown in Figure 3.



These data were used to estimate the gas phase resistance in this work.



Additivity of resistances as demonstrated by Mayr (2/4) was assumed in



computing the liquid phase resistance.  Thus the additivity equation



appropriate for this work can be written:
          (KC'G
where (K )_ and (fe )_ are the overall and individual gas phase coeffic-
        C G       C G


ients, k * is the liquid phase physical absorption coefficient, P and



p  are the total gas pressure and molal gas density and H is Henry's



law constant.  It should be noted that as  increases the controlling



resistance will shift to the gas phase, and at low liquid phase resis-



tance calculation of the small difference between the overall and the



individual gas phase resistance will be subject to difficulties due



to precision of measurements.

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NSh


Ni/3
 Sc
ou


30


20

10



o WATER VAPORIZATION
• METHANOL DESORPTION
x AMMONIA ABSORPTION












•



0

i V*"
X*x
o



/
o
0
o
yv
^
0





'/
so
o
ft







/










     1000
2000
3000
5000
7000
                          FIGURE 3

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                                                                            14
                     APPARATUS AND PROCEDURE








     In the study of the relatively rapid processes at the gas-liquid




interface which occur in gas absorption the development of suitable




experimental techniques to obtain reproducible data under known experi-




mental conditions was an important problem.  Several laboratory devices



have been used with varying success each with its particular advantages




and disadvantages.




     Two of these devices, the laminar jet developed by Pigford and




Scriven and the short wetted wall column developed by Vivian and Peace-



man perform closely according to theoretical predictions.  The laminar



jet gives highly reproducible data and is relatively easy to operate.



However, it is limited to the use of a pure gas phase (i.e., no gas




phase resistance), and therefore, it could not be applied for this



study involving absorption of SOQ at very low partial pressures.




     The short wetted wall column has been used to study both liquid



and gas phase controlled absorption.  It provides surface exposures




of the same order of magnitude as many commercial types of absorbers,




and penetration theory can be used to analyse data obtained with it.



As originally designed, it was not suitable to operate with a slurry




system.  However, a special feed system was designed and constructed




which allowed its operation with slurries, and all the data reported




here were obtained with a short wetted wall column.



     Operating a relatively small flow system with slurries presents



considerable difficulty, and in this case particularly with maintenance




of uniform slurry concentration at the column.  By use of a circulating



system with a short downward-sloping drawn-off through a small-bore



flow meter directly to the head of the short wetted wall column data with

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                                                                            15
a fair degree of reproducibility could be obtained.



     The overall apparatus is shown schematically in Figure 4.  Slurry



was prepared by mixing distilled water and reagent grade calcium hydroxide




in the feed tank in which an agitator maintained the hydroxide in suspen-



sion.  A pump circulated the slurry through a small diameter line at high




velocity to an open surge tank mounted above the column.  From the surge



tank the slurry flowed directly back to the feed tank.  Flow of slurry



to the column was drawn from the line to the surge tank through the cali-




brated small-bore flow meter.  Slurry discharged from the column was




discharged to waste except when liquid samples were being collected.



Liquid samples were collected under N_ to reduce possible oxidation of SO..



     The gas phase was prepared by mixing in a baffled section of the gas




line metered flows of N. and SO- supplied by cylinders.  The N2 flow meter




was calibrated by use of a calibrated dry test meter and the SO. flow meter



was calibrated by analysis of the mixed gas streams.  Essentially all of



the data were taken at a Reynolds number of about  3000.  This was chosen



as a balance between the need to minimize gas phase resistance and the



need to maintain an undisturbed liquid layer on the column wall.



     The short wetted wall column is shown in outline in Figure 5, and is



the same construction as described elsewhere  (8).




     A special head for the column was constructed to avoid dead spots




where slurry  solids could collect and to give a uniform liquid layer on



the wall.   After considerable experimentation the  design shown in Figure 6



was developed.  All of the data reported here were obtained with the use




of this head.



     Run procedure consisted of continuous operation of the flow systems



to achieve  steady-state after which the liquid phase to and from the column



was  sampled and pertinent data on operating conditions were read.   Only

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                                                          Flowmeter
     Flowmeter
  N.
 A    A
N.
                          Gas
                          Saturator
                                      Flowmeter
                        A
                                   SO,
                                                                  sww
                                                                  Column
Gas
Heater
                                                    Gas
                                                    Mixer
                                                        Liquid
                                                        Sampler
ABSORPTION
                                                      Sampler
                      FIGURE   4

          SCHEMATIC   DIAGRAM   OF   APPARATUS

     OF  SO2  IN   SLURRIES  IN  SHORT   WETTED  WALL
                                                                        Liquid
                                                                        Sampler
                                                                             Liquid
                                                                             Supply
                                                                             Tank
                                                                          Pump
                COLUM N
                                                
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                                                         17
                                TEFLON
    TEFLON
    0-RING
                                                STAINLESS
                                                STEEL
MOOD:
           SHORT WETTED WALL COLUMN ASSEMBLY

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                           18
                   Tef Ion
                   Head - Piece
                            7
             Glass Column
FIGURE

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                                                                            19
the outlet liquid analyses have been reported here since, in general,




the total alkali content of both samples agreed within operating pre-




cision.



     The procedure used in runs with calcium hydroxide solutions and




with water was essentially the same.



     In spite of the high velocity circulating liquid system and short



draw off to the column, entirely satisfactory operation with a calcium




carbonate slurry was not obtained before the laboratory work was termin-




ated.  Further modification of the liquid feed system would have been



required to obtain satisfactory data.



     Details of the analytical methods and equipment calibration are




given in Appendex A.

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                                                                            20
                      RESULTS AND DISCUSSION
     The rate of absorption of S0_ into lime slurry was calculated from



the volumetric flow rate of the slurry and the concentration of total SO




(i.e., unhydrolysed SO , bisulfite and sulfite ion) in the slurry leaving



the short wetted wall column.  An overall coefficient,  (K )  expressed as
                                                         C G


cm/sec, was obtained by dividing the rate of absorption by the interfacial



area of the column, the znolal density of the gas and the mole fraction of




S02 in the entering gas.  The data are summarized in Appendix C.  In order



to interpret these data it was necessary to distinguish between the com-



ponents of the overall resistance to absorption.



     The gas phase resistance was estimated from previous work on the




short wetted wall column (£,9.) , and assuming additivity of resistances



as demonstrated by Mayr (2^,4) the liquid phase resistance was calculated



by deducting the gas phase resistance from the overall resistance.  This



liquid phase resistance was assumed to be the result of kinetic and




diffusional effects, and the liquid phase coefficient is reported in



Appendix C as the product <(> fe * where fe* is the absorption coefficient
                             l<         L


without chemical reaction and <|> is the enhancement factor to account for



the effect of chemical reaction in the liquid phase.  The dependence of




the absorption coefficient without chemical reaction, k,*» on liquid rate



and fluid properties is well understood, and by estimating fe   from



previous work on the short wetted wall column such as that of Mayr (4_)




the value of  was obtained.



     It should be noted that for the absorption conditions encountered



in this work, the liquid phase resistance accounted for only a small



fraction of the total resistance with the result that ordinary precision



of measurements introduced considerable uncertainty in the small differences

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                                                                            21
representing the liquid phase resistances.  This uncertainty directly



affected the calculated values of the enhancement factor.  Consequently



the value of these data lie not in their absolute value but in their



order of magnitude and directional trends in supporting theoretical



considerations.  In the runs with the lowest SO  concentration the cal-
                                               £


culation of a liquid phase resistance had no significance within the



precision of the experimental data, and consequently/ no conclusion can



be drawn from those runs except that in fact the absorption process



becomes gas phase controlled as the concentration of SO. in the gas



decreases.



     The absorption of SO  into lime slurries and solutions appears to



be a case of absorption accompanied by very fast chemical reactions.



Furthermore in the presence of excess hydroxyl ion the reactions appear



irreversible.  Theory indicates that under these conditions an approxi-



mation for the enhancement factor is given by:
where C  is the concentration of reactant B in the bulk of the liquid,
       B


CA.  is the concentration of solute A in the liquid at the interface and



v  is a stoichiometric factor giving the number of molecules of B which



react with A.  D. and D  are the respective diffusivities.  In the system
                A      D


under study here, C_ is the calcium hydroxide in solution and CA. is  the
                   B                                            J-


molecular SO   in solution at the interface.  Cft. is assumed to be in



equilibrium with the gas phase concentration at the interface.  The



stoichiometric factor is 1, assuming that the reactions of importance in



the presence of relatively high hydroxyl ion concentration are:



          SO.(aq)  +  OH"  -»•  HSO,"
            *                    J

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                                                                            22
and       HS03~  +  OH~  "*"  S03~  + H2°



which can be written as an overall reaction to indicate the relative




quantities of diffusing species:



          S02(aq)  +  Ca(OH)2  -*•  CaS03 + H2O




     The data obtained for the absorption into lime slurries and solutions




are plotted in Figure 7 to show the relationship of -1 to the ratio




CR/C ..  In the runs with the slurries C  was assumed to be saturation
 *5  /\1                                  B



at the liquid temperature.  The line through the data is drawn with a



slope of 1 as suggested by theory.  While the data are somewhat scattered



due to difficulties with precision of data they tend to substantiate the




assumption for these conditions of absorption accompanied by very rapid




second-order irreversible chemical reaction.



     The ratio C_/CA. is based only on a small range of C  from half




saturation to saturation.  The value of CA. however, varies widely due



to changes in gas phase concentration and gas phase driving force.  The



CaSO, formed had very low solubility and precipitated.  In the runs with



clear Ca(OH). solutions this precipitate was readily visible in the column.




     These experiments indicate that the presence of solid calcium



hydroxide had little effect on the absorption rate which depended only




on the calcium hydroxide concentration in solution and not on slurry




concentration.  Even though a highly reactive analytical grade of calcium




hydroxide was used to prepare the slurries, no effect of solid concentra-



tion is evident.  Apparently the rate of solution of the solid was not




sufficiently rapid to influence the processes occurring at the interface




during the 0.1 sec (approx) exposure time.



     These rate measurements were made for a single exposure of the



surface of the liquid layer, initially of uniform equilibrium composition.




Repeated exposure would, of course, deplete the reactant in solution

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fooo

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                                                                            24
unless it were replenished by dissolution of the solid in the slurry.




Thus the rate of dissolution of the solid assumed importance in main-




taining the capacity of the liquid to absorb S0_ rather than partici-




pating in reactions at or near the interface.  In view of this mech-



anism it should be possible to rank the behavior of various lime slurries




by observing their rates of solution in simple liquid phase measurements




without the necessity of characterizing them in actual absorption mea-



surements which are difficult to make.  The mechanism also indicates the



desirability of providing adequate liquid holdup per unit interfacial




area to allow dissolution of solid and re-saturation of the liquid phase




between surface exposures.  To perform satisfactorily, the rate of




dissolution per unit volume of slurry multiplied by the volume of holdup




per unit interfacial area must be equal to or greater than the rate of




absorption per unit interfacial area.




     The short wetted wall column is well suited for investigating




absorption phenomena which occur at or near the interface and which



conform approximately to the surface renewal or penetration theory model




of gas absorption.  Exposure times are relatively short, ranging from




about 0.05 to 0.2 seconds.  Penetration depths are relatively small




compared to the thickness of the liquid layer.  Consequently bulk pheno-




mena cannot interact to any great extent with what occurs in the liquid




at the interface until mixing occurs following the surface exposure.




     In the system of interest here the absorbed SO  reacted with




hydroxide in solution near the interface and depleted the hydroxide




concentration in that region.  Solution of solid hydroxide proceeded in




the bulk liquid after mixing with the depleted surface liquid.  To observe



the overall effect of interfacial and bulk processes an experimental




system which provides for repeated surface exposure and associated bulk

-------
                                                                            25
reaction volume is required in addition to the short wetted wall column.




For example, rate data obtained with turbulent slurry film flow in a




long wetted wall column or with slurry flow over a packed bed of spheres




would probably be more representative of the overall process of absorp-




tion into calcium hydroxide slurry than the data obtained with the




short wetted wall, and such data combined with the fundamental homo-




geneous rate data obtained with the short wetted wall column would



allow analysis of the effect of the processes in the bulk liquid on the




absorption rate.



     Data were obtained for the absorption of SO. into water in the




short wetted wall column over the same concentration range as used in




the lime slurry studies.  These data were taken to indicate the perfor-




mance of the column and establish the experimental procedure without the




complicating factors due to the calcium hydroxide.  In addition they




provide an indication of the effect of hydrolysis of SO  in water on the




rate of absorption.



     The rate of the hydrolysis of SO. in water has been studied  (1.*6),




and the data obtained indicate only that it  is a very fast reaction.   The




reaction is:




          S02(aq)  +  H20  t  H*  +  HS03~




The few qualitative data available give rates so high that the maintenance




of equilibrium  in water during diffusion of  SO. away from the interface



would be expected.  On this assumption mass  balances on the rate of removal




of various  forms of S0_ from the  interface into the bulk of the liquid




allow the calculation of the enhancement of  the absorption rate due to the




hydrolysis  reaction.  The  absorption rate can be written as:





          NA '  *  feL*  

-------
                                                                             26
where, as before, CA>, C  is the unhydrolysed molecular SO   in solution.
The effect of the hydrolysis reaction was shown to be:
          <)>  =  1 +
                     D
                      A
/C/C
  AAi
where K  is the equilibrium constant for the hydrolysis reaction, and D
and D  are the diffusivities of unhydrolysed and hydrolysed SO_ respec-
     Jo                                                        Z
tively.
     When the diffusivities of species A and B are nearly equal as in
the SO -water system  (5) , and the concentration of SO. in the bulk, C ,
      «               ""^                             £•               A
is negligible relative to the concentration at the interface CA., the
expression for the enhancement factor reduces to:
          4>  *  i -f
     The data for the absorption of SO. into water are plotted in Figure 8
to show the relationship of <|>-1 to /K^/C^..  The line as indicated repre-
sents the theoretically derived relation for an assumed infinitely rapid
hydrolysis reaction.  Although the data are somewhat scattered due to the
effect of precision of measurements in deriving the liquid phase resistance,
the trend of the data substantiates the theory based on a very rapid
hydrolysis reaction.
     It is instructive to compare the hydrolysis reaction of SO. and water
with that of Cl. and water.  The two systems have many similarities, and
the Cl_ system has been extensively studied.  Both solutes react directly
with water to form hydrolysis products and establish a reaction equilibrium.
The evidence seems to indicate that dissolved SO  reacts directly with
water to form an essentially completely  dissociated product consisting of
hydrogen and bisulfite ions.  Dissolved Cl_ reacts directly with water to
form hypochlorous acid and hydrogen and chloride ions.  The rate of the

-------
FIGURE 3
                                      ro

-------
                                                                            28
chlorine hydrolysis has been measured by at least three techniques which



agree surprisingly closely.  The rate is rapid but not as rapid as SO



hydrolysis appears to be.



     The presence of hydrogen ions inhibits the hydrolysis of both Gl-



and SO .  Under such conditions, absorption follows the laws of physical



absorption, and the driving-force for mass transfer is the unhydrolysed



molecular solute.  At low hydrogen and hydroxyl ion concentrations data



on the absorption of Cl_ follow a pseudo-first order rate equation.  At



hydroxyl ion concentrations above 0.01 N Cl  hydrolysis involves hydroxyl



ions and appears to be second-order and irreversible.  It is likely that



SO  follows a similar pattern in its reactions with water and alkali



solutions, although apparently the hydrolysis reaction at medium and low



pH appears to be very much more rapid than is the case with Cl_.



     At high concentrations of SO. in the gas and liquid the reaction
                                 &


appears reversible and the rates of absorption approach physical absorp-



tion.  As the concentration of SO_ in solution and in the gas is decreased,



the enhancement factor increases as the interfacial concentration CA.



decreases with the result that at very low concentrations the absorption



process will become controlled entirely by the gas phase resistance even



for absorption into water.



     A number of S02-water runs (No. 52-57) were made with very low S0_



flow rates (the lowest used in these experiments), and it is suspected that



the flow meter did not operate reliably at this end of the scale.  Consider-



able difficulty was experienced in this range during the calibration runs.



It is not known whether this was due to the float sticking or to analytical



procedures.  However, the absorption data indicated that the SO. concentra-



tion in the gas calculated from the flow rates is too low since the overall



gas phase coefficient cannot be greater than the individual gas phase co-

-------
                                                                            29
efficient.  If it is assumed that the liquid phase offered negligible resis-



tance in runs 52-57 so that the overall (K )_ would be equal to (fe )_, the
                                          C G                     C G


flowrates of SO  would be approximately 1.5 times the rates indicated in



these runs.  This appears to be within the precision of the calibration



in this range of the flowmeter.  Consequently these data serve only to



substantiate the prediction that SO  absorption at very dilute gas concen-



trations becomes essentially gas phase controlled.



     A few runs were made using a CaCO, slurry.  These are reported in



Table 3, Appendix C.  Great difficulty was encountered in maintaining a



stable slurry flow due to the settling and plugging characteristics of



the CaCO. particles.  Unfortunately, time did not permit resolution of



these operating problems.  Definitive conclusions cannot be drawn from



this phase of the work.



     Comparison of the magnitude of the enhancement factors for absorption



into water and into alkaline solution reveals that for the lower hydroxyl



concentration range  (below 0.01 g.mole per liter) the enhancement is about



the same as for water.  Further work is required to indicate the actual



effect of hydroxyl ion concentration/ although these data indicate that



above 0.01 g.mole Ca(OH)  per liter the enhancement is expected to increase



with alkaline concentration.  In this behavior, SO. absorption into water



and alkaline solutions appears to be similar to the Cl.-water system.

-------
                                                                            30
                  CONCLUSIONS AND RECOMMENDATIONS








     The distribution of resistance to the absorption of SO  into lime



slurry in the short wetted wall column depended on the gas phase concen-



tration.  At low concentrations  (0.3 mole per cent SO } the major resis-



tance was found to be in the gas phase, and at concentrations below 0.1



mole per cent the absorption process was essentially gas phase controlled.



     The absorption of SO2 into lime slurry appeared to be a system of



gas absorption accompanied by an "infinitely" rapid second-order irreversible



chemical reaction involving unhydrolysed molecular SO  and hydroxide in
                                                     £


solution.  The rate of solution of solid calcium hydroxide did not influ-



ence the absorption process at the interface during the short exposure time



characteristic of the short wetted wall column.  The effect of the solid



calcium hydroxide was to increase the capacity of the liquid phase to ab-



sorb SO..  Adequate holdup of liquid would be required in a continuous



absorber to allow sufficient dissolution of calcium hydroxide to maintain



the hydroxide in solution near saturation.



     The absorption of SO  into water is a system of gas absorption accom-



panied by a fast reversible hydrolysis reaction.  The reaction is suffi-



ciently rapid to maintain hydrolysis equilibrium in the liquid as SO



diffuses away from the interface.  In agreement with theory the enhancement



factor varied approximately inversely with the square root of the gas phase



interfacial concentration,  and the system became gas phase controlled at



concentrations of SO. in the gas phase below 0.1 mole per cent.



     The rates of absorption of SO  into water and into calcium hydroxide



solutions of about 0.01 g.mole per liter were about the same, and the rate



into hydroxide solutions increased slightly with increase in hydroxide



concentration in solution.

-------
                                                                            31
     Significant data on the absorption of S0» into calcium carbonate




slurries could not be obtained with the equipment available.




     Experimental work on the absorption of SO  in slurries in a long



column or packed bed is needed in addition to studies on equipment such as




the short wetted wall column in order to investigate the overall effect




of interfacial and bulk liquid reactions since the dissolution process does




not appear to affect the processes in the region of the interface.

-------
                                                32
APPENDIX

-------
                                                                            33
                            APPENDIX A



                       DETAILS OF PROCEDURE







1. Gas Flow Rate and Composition



     The nitrogen-sulfur dioxide gas mixture feed to the column was pre-



pared by mixing metered streams of nitrogen and sulfur dioxide.  The



rotameter used to meter the nitrogen gas from the supply cylinder was



calibrated on air using a calibrated gas test meter.  The small correction



for variable conditions and from air to nitrogen was made as shown on



Figure 9.



     The sulfur dioxide rotameter was calibrated by determining analytic-



ally the dilution of the sulfur dioxide in a measured stream of nitrogen.



The sulfur dioxide was feed into the nitrogen gas stream in the 1-inch



diameter glass line to the short wetted wall column.  The gas mixture was



sampled at a point about six  feet downstream from the mixing-point and a



series of baffles to promote  mixing.  The gas sample was taken in an evacu-



ated Gaillard flask of calibrated volume  (approximate volume was 300 cm ).



The vacuum system was connected directly to the sample point and the Gaillard



flask.  A mercury menometer was used to measure the pressure in the Gaillard



flask before and after admitting the gas sample to the flask.



     The flask with the sample was  cooled in an ice bath to reduce the pressure



sufficiently to allow introduction  of 10 cm  of 0.01 N KIO^ solution, 0.2 cm



of 20% KI solution and 2 cm3  of 0.1 N HC1.  The KIO3 oxidized  the SC»2 to



SO, or SO,	 to SO.— and  the remainder reacted with hydrogen  ion to  form



I_ which was titrated with standard thiosulfate solution.  The m. equivalents



of iodate less the m. equivalents of thiosulfate required  for  the-I   gave



the m. equivalents of SO.  in  the  sample.  The ratio of the m.  moles of  SO_
                        M                                                 ft


to the m. moles of gas in  the Gaillard flask gave  the mole fraction of  SO. in

-------
                                                                            34
the gas sample.  By material balance the sulfur dioxide flow rate was




calculated from the metered nitrogen flow rate to calibrate the sulfur




dioxide rotameter.  When the gas phase was not analyzed during the absorp-




tion runs, the gas composition was calculated from the metered flow rates




of the nitrogen and sulfur dioxide.




2. Liquid Flow System




     The liquid system consisted of a polyethylene feed storage container




(approx. 30 gallon capacity) in which the slurries of hydrated lime and




calcium carbonate were prepared by mixing the dry powder with distilled




water.  A centrifugal pump circulated the slurry through a loop of 12-mm




glass tubing which extended about four feet above the column with the return




leg discharging into the feed storage container.  A stirrer in the feed




storage container was used in addition to the recirculation to maintain




the slurry solids in suspension.  A side stream through a small glass line




was taken from the upper part of the recirculating loop through a capillary




flowmeter and fed directly to the head-piece of the short wetted wall column.




The flowmeter was calibrated by direct measurement of the flow.




     Liquid effluent from the column which was not needed for sampling for




analysis was discharged to the sewer.




3. Liquid Sampling and Analysis




     In runs in which a slurry was used as absorbent samples of the effluent




liquid were taken from a sampling point immediately below the wetted wall




column in flasks purge with N_.  Operating difficulties in handling the




slurry absorbent often interferred with steady operation and to check these




conditions duplicate samples were taken ten to twenty minutes after the




first set of samples.




     In the runs using water as absorbent/  the effluent liquid sample stream




flowed continuously through a twenty-five ml pipette.  After steady state

-------
                                                                            35
had been established the pipette was removed from the line, drained to



the mark and the measured sample volume introduced under the surface of



a known amount of KIO  solution.



     The effluent liquid was analyzed for calcium hydroxide, carbonate



and sulfite.  To determine the hydroxide and sum of the carbonate and



sulfite concentrations, a 25 ml sample of the slurry was titrated with



0.2 N HC1 solution to the phenolphthalein (PH) and methylorange  (MO)



end points.



     The hydroxide concentration in g.mole/liter is given by:
             /ran      m. eg. HC1  (to PH) - m. eg.  (PH to MO)
             (On) „  »       'm "     .          •     *     "•'- -'-—
                2             2  (ml sample)
     The carbonate plus sulfite concentration in g.mole/liter  is given:
where PH refers to the phenolphlthalein endpoint and MO refers to  the



methylorange endpoint.  To determine the sulfite and dissolved sulfur



dioxide concentration, an effluent liquid sample  (25 ml) was added to  a



titration flask containing five or ten ml of  0.01 N KIO- to which  had  been



added 0.2 to 0.4 ml of 20% KI and two to four ml of 1.0 N  HC1,   The HC1



added in excess dissolved the insolubles in the slurry and released I



from the excess KIO_ not required to oxidize  SO  and SO,.  The I  was



titrated with standardized sodium thiosulfate solution to  the starch endpoint.



     The oxidizable SO  and SO— in m. moles/liter is given by:
                      JL       3






           ten  a. cr. —\  =   (m. eg. KIO3 - m. eg. thiosulfate) 10
           (S02 + S03  )               2  (ml sample)	

-------
                                                                             36
                            APPENDIX B
                        SAMPLE CALCULATIONS
     Sample calculations are given for  the data  recorded for tabulated
Run No. 1, Appendix C except as noted otherwise.


Nitrogen Flowrate
Rotameter reading        70.5
Gas temperature          14.1 °C  (287.2 °K)
Gas pressure  (abs)       770 mm Hg
From Figure 9, Q/erpn\ =  2.37 cu.ft./min.
                  QN  »   (0.638)(2.37)/(770/287.2)
                      -   2.48 SCFM
Sulfur dioxide Flowrate
Rotameter reading        4.0
Gas temperature          24.5  °C  (297.6  °K)
Gas pressure  (abs)       759.1 mm Hg
                                     cu.fl
                                          s-3,
From Figure 10, Q/ornr..sa  0.78 x 10   cu.ft./min.
                  (5TD)
                  Qso -   (0.626)(0.78 x  10  J)/(759.1/297.6)
                         0.78 x 10~3 SCFM
Gas Analysis  (inlet)
Mole fr. S02  =  QSo2/<°-N + °-S02>
              •  0.78 x 10~3/2.48
              -  0.315 x 10~3
Liquid Flowrate
Flowmeter reading        201 mm HO
Liquid temperature       23.2  °C
From Figure 11,Flowrate =325 cm /min.
Diameter of column      =3.18  cm
Flowrate -   ., ,Q.      -32.5  cm /min-cm
           71 (j . La )

-------
                                         37
$  2
S

-------
U)
00

-------
                               //
to
                             J£>t>
260
foo

-------
                                                                             40
Liquid analysis  (outlet)

Sample (A) volume         25oQ cm
Titration with 0=2 N HC1 solution
     to phenolphthalein endpoint     6.54  m.eq.
     to methyl orange endpoint       0,175 m.eq.
     total                           6,715 m.eq.

          ca,OH,2 cone  .   ''

                        =  0.1273 g.mole/liter
    CaCO  4- CaSO  cone  «  0.175/25.0
                        «  Oo0070 g0mole/liter
Sample (B) volume       25 oO cm
KIO, solution added      0»1987 m.eq0
Titration with Na S_O  solution     0.1882 m.eq.
KIO. solution used for SO  oxidation
     -  0 = 1987 - 0«,1882
     =  0.0105 m.eq,
     „ „„           0.0105   __3
     CaS03 cone  =  {       x 10
                    0=210 m. mole/liter
     CaCO  cone  =  0.00679 gdnole/liter
Kate of Absorption of SO2
            (Liquid flowrate)(Outlet CaSOg cone)
                        (Surface area)
             (32o5) (0,210)
            (60) (4.5) (1000)
                     — fi
            25.3 x 10   numole SO /sec-cm

-------
                                                                             41


Overall Gas Phase Coefficients

By definition:


               NA                          2
     K   «•  	j     g.mole/sec-cm -atm
            (p - p*) 10

               Na
     Ky
                        —     g.mole/sec-cm
             (y - y*) 10
                NA
  (K }     »  - — -   cm/sec
             PG(y - y*) i
-------
                                                                             42
Gas phase mass transfer  studies  in  short wetted  wall  column  (9)  indicate



that:





     -^-  .   =  18.5  at  Nn    =   2980

     (N0  )1/3                RS
      sc




The N    for SO  in N  was calculated  from data given  by  Sherwood and  Reid,



"Properties of Gases and Liquid" McGraw-Hill Book Co., Inc.





     Diffusivity, DP  =  0.122 cm -atm/sec at 273 °K



     At  298 °K and 1 atm, D  -   0.139 cm2/sec



     Gas density, p  =   0.00114  g./cm



     Gas viscosity, y =»  0.018 x  10    g./sec-cm



         .". N    »  1.136
            Sc


                   k d

     and   N    -  --  -  18.5 (1.136)1/3 - 19.3
          ih *       (19.3) (0.139)
          (fec,G  .  - __ -  =  o.84 cm/sec





     Gas phase resistance =  .\ . —  =  1.190  at  N    •  2980
In this range of N  ,  (k )  is approximately proportional to the N   .
Liquid Phase Coefficients



     By definition the liquid phase coefficient for physical absorption



is defined by:




     NA  "  feL*  (Ci - C)


where C is the concentration of the absorbed specie at the interface and in



the bulk liquid.  When a chemical reaction occurs in the liquid phase in-



volving the absorbed specie, the rate of absorption is also governed by the



unreacted specie driving force.  The enhancement of the rate due to the

-------
                                                                             43
 reaction is allowed for by a factor ,  must be defined for the various kinetic schemes
 encountered.   The  hydrolysis of SO  in water can be written as:
      S0_(aq)   J H+  +  HSO ~
        £                 *3
 where SO_(aq)  refers to unhydrolysed molecular SO  in solution in water.
 This  reaction is reversible and is believed to be very rapid (I)  so that
 during a diffusion process,  equilibrium is  maintained between reactants
 and products.
      If C.  is the  molecular SO  concentration and C_ is the bisulfite
         A                     £•                    E
 ion concentration, the liquid phase equilibrium  for SO^ absorbed  into water
 is given by :
Theoretical analysis assuming film theory for simplicity gives  the  enhance
ment factor, , in terms of the equilibrium concentrations:
and the rate of absorption is given by:

     N   -   fe_*  (C .  - C )
      A        L    Al    A

If D  - D , substituting the expression for  in the rate equation gives:
    Ct    A

-------
                                                                             44
where CTi and CT refer to the total SO  in solution  (i.e., SO  (aq) plus



HSO ~) .  Furthermore if the concentration o



zero, the enhancement factor, , reduces to
HSO ~) .  Furthermore if the concentration of  total  SO   in  the  bulk is
     4>  =   (1
Thus  becomes very large at low partial pressures of  SO,,  since CA.



This of course shifts the controlling resistance to the gas phase.



     Estimation of the value of the enhancement factor, cj>, for the absorp-



tion of SO  into calcium hydroxide and calcium carbonate solutions and



slurries is complicated by the fact that the H+ ion concentration is not



coupled directly with the bisulfite ion concentration.  Consequently,



calculation of the <£ factor is dependent on specification  of the H+ or OH~



ion concentration.  These cases have not yet been worked out, and to analyse



the experimental data for absorption into calcium hydroxide slurries it will



be assumed that the molecular SO  concentration at the interface was in



equilibrium with the SO  concentration in the gas phase at the interface.



On this basis the product,  k *, can be calculated from the data.   can
                              L


then be estimated using correlations for physical absorption such as Mayr's



equation (4) for k *.
          ~™       L


     The equilibrium relation connecting gas phase concentration and



molecular SO. in the liquid is Henry's law:
Combining this with the liquid phase rate equation:





     NA  '  * kL   

-------
                                                                             45
we have



     NA  -   feL* HP  (y± - y*)



Applying additivity of resistances, we get:
      (KC>G PG       fe *  »  0.334 cm/sec





     The physical absorption correlating equation due to Mayr is:
          fe */-*-= 1.759
           L  V/DA

-------
                                                                             46
Data for Run 3:



     z  -  4=5 cm



     D  -  1,753 x 10~5 cm/sec at 25  °C
      A


     T  =  32c5 g./min-cm
          feT* .If-  '-  1.759(32.5)°'354

                 A



                    =  6.03   (cm/sec)z




     ~-  -  0.505 x 103

      A



          fe *  «  0.0119 cm/sec



Then


          A  =  (4> ki.*)  „  0.334
          

0119 " 28.1 Enhancement factor for absorption into water (Data for Run 33) (fe ) = 0.84 cm/sec (K )„ - 0.504 cm/sec C G P « 1.0 atm p_ = 40.5 x 10 g.mole/cm G T - 23.4 °C (296.5 °K) F » 25.0 g./rain-cm Henry's law constant - io-4523 H = 1.302 g. mole/1 iter-atm = 1.302 x 10 g. mole/cm -atm


-------
                                                                              47
                                      [   i     JL_1
                                       0.504   0.84 J
1    m  (1.302 x 10~3)(1.0)
*L          40.5 x 10"6}
        fe *  BE  0.0392 cm/sec
         L
        ~   -  0.532 x 103  (sec/cm)'
         A
                1.759(25.0)°*354  -  5.48  (cm/sec)^
        fe *  -  0.0103 cm/sec

                0.0392
                           3*81
Theoretical  (j>
                 K
      (J)  «  1 +
      H  **  1.302 x 10   g. mole/cm -atm
      y  m  3.16 x 10"3
      K  -  0.019 g. mole/liter
Applying additivity,
      y±  -  (3.11 x 10"3) (1 -  ^    )  -   (1.53 x
     CAi  -  (1.302 x 10"3) (1.0) (1.53 x lo"3)
          -  2.00 x 10~  g. mole/can
                  0.019 x IP"3
                  2.00 x 10

-------
                                                  48
  APPENDIX C
SUMMARIZED DATA

-------
              TABLE I
Absorption of SO2 into Lime Slurries

Run
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Flowrate
SCFM
CM
2
2.48
2.48
2.43
2.49
2.49
2.48
2.43
2.45
3.37
2.48
2.42
2.50
2.43
2.43
2.44
2 Flowrate
SCFM x 10 3
o
W
0.78
0.78
9.02
8.82
8.69
9.02
9.30
8.82
8.92
9.02
9.02
8.82
2.58
2.49
2.40
1
b g
n 75
(0
13
2980
2980
2950
3030
3030
3020
2960
2980
4080
3010
2940
3040
2930
2930
2950
n N
n to
H •«"!
id M o
0> X
« l-l
id g
CD g
0.315
0.315
3.71
3.54
3.48
3.63
3.83
3.60
2.64
3.64
3.73
3.51
1.06
1.025
0.982
tal Pressure
ironPg
£
759.1
759.1
759.1
756.8
756.8
756.8
758.8
758.8
765.2
765.2
765.2
765.2
760.0
760.0
760.0
quid Temperature
°C
3
25.6
26.3
26.4
25.5
26.9
26.4
32.3
31.4
27.9
29.0
28.7
28.5
28.4
28.3
28.4
quid Flowrate
cra^/min-can
•H
32.5
18.5
32.5
24.0
11.0
35.0
21.5
7.65
22.0
31.5
8.35
21.7
33.0
22.8
10.8
OUTLET
LIQUID ANALYSIS
O^X X V.
(00) 0) (MO)
H H ~ H
3 i 8na si
o *
0.210
0.486
0.988
1.337
2.854
0.730
1.390
3.748
1.524
1.006
2.742
1.294
0.298
0.474
0.890
3 »
0.0068
0.0054
0.0346
0.0383
0.0399
0.0385
0.0354
0.0323
0.0191
0.0074
0.0071
0.0079
0.0081
0.0071
0.0047
id •
o o>
0.1273
0.1263
0.0716
0.0772
0.0644
0.1924
0.1944
0.1664
0.0984
0.0510
0.0416
0.0495
0.1222
0.1194
0.1098
ABE.
RATE
\o
o
X
a M
25.3
33.3
118.9
118.8
116.3
94.6
110.7
106.2
124.2
117.4
84.8
104.0
36.4
40.0
35.6
0
04)
^^ n
OX

1.97
2.60
0.787
0.825
0.821
0.637
0.705
0.719
1.139
0.781
0.550
0.717
0.831
0.945
0.878
*
-S-
-
-
0.661
1.054
0.986
0.115
0.169
0.265
0.007
0.485
0.080
0.213
-
-
—

^QL
-
-
5.5
79.8
117.4
9.2
15.1
34.4
0.7
38.8
9.9
19.7
-
-
—

-------
               TABLE II
Absorption of SO- into Lime Solutions


Run
16
17
18
19
20
21
22
23
24
25
26
27


MS
W «C|
H CO
sT
2.40
2.46
2.43
2.39
2.40
2.40
2.43
2.42
2.41
2.43
2.43
2.40


•* x

•H a
• rica
o
en
8.95
6.01
8.55
5.97
3.33
0.80
8.61
6.87
5.12
3.37
8.58
6.83


•k
5
rH
fa 8)
M
U Z
id
u
2920
2980
2950
2900
290.0
2900
2950
2940
2920
2940
2950
2910


(0 Oi
•HO
n to
f*4 *f^
tA ^4 ^^
2«MH
ex
01 •-«
 o
•H
33.0
33.0
33.0
32.8
33.0
32.8
26.8
26.8
26.8
26.8
20.0
20.0


ABE.
RATE
OUTLET *
LIQUID ANALYSIS °
OM * *3 X
o \ \ x. .
M «
H
1!
0.828
0.652
0.734
0.524
0.238
0.048
0.734
0.414
0.518
0.352
1.096
1.068


0)
H
8wa
3 CT>
0.0014
0.0013
0.0013
0.0013
0.0006
0.0005
0.0011
0.0009
0.0011
0.0009
0.0016
0.0017


NO) O
§1 i
Si a
N
B U
Q) W S
0.0138 101.2 0.666
0.0139 80.4 0.806
0.0091 89.7 0.626
0.0093 64.0 0.629
0.0090 29.1 0.514
0.0090
5.8 0.428
0.0118 72.8 0.505
0.0114 41.1 0.356
0.0115 51,4 0.560
0.0112 34.9 0.617
0.0055 81.2 0.565
0.0048 79.1 0.682





V
0.150
0.870
0.112
0.112
0.059
0.040
0.055
0.026
0.075
0.104
0.078
0.183



"Si
12.8
73.1
9.4
10.2
5.0
3.4
5.0
2.5
6.9
9.5
7.9
17.9
t/i
o

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                   TABLE III
Absorption of SO. into Calcium Carbonate Slurries



Run
28
29
30
31
5
2s
gs
H W
fa
IN
Z
2.38
2.43
2.44
2.43
din
4* 0
<-t a
01 en
o
01
8.95
9.00
2.71
0.80.

H
fa 01
M
0) Z
BJ
O
2890
2950
2950
2930
n n
•rtO
n ca
1-1 •«»>
* fc o
0 X
0)f-l
m g
o 6
3.76
3.70
1.11
0.33
res sure
Hg
1
a
g
(760)
767.2
767.2
767.2
Temperature
C
TJ
I
•H
26.2
28.9
28.7
28.6
Plowrate
dLn-cm
•o x
&1
•rt
32.5
22.9
22.9
23.0
OUTLET
LIQUID ANALYSIS
IN J J J
OX. X X

31
O '
EH a
0.714
0.340
0.200
0.200

§1
nj .
U Of
—
-
-
_
                                                     ABS.
                                                     RATE
§
U
fl)
n
                                                      85.9
                                                      28.9
                                                      17.0
                                                      17.0
                                                             «-« n
                                                              o x.
     0.561
     0.192
     0.376
     1.265
0.082
0.012
0.034
7.6
1.1
3.1

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TABLE IV
Run
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
VI
N2 Flowrate
SCFM
2.41
2.41
2.39
2.39
2.39
2.41
2.35
2.39
2.44
2.44
2.43
2.43
2.43
2.43
2.44
2.44
2.40
2.47
2.47
2.47
2.41
2.39
2.41
2.39
2.32
7..^%
S(>2 Flowrate
SCFM x 10 3
7.60
7.50
7.45
7.60
7.50
7.45
2.70
2.70
2.70
2.70
2.70
2.70
2.65
2.65
8.85
8.70
8.80
8.80
9.20
9.05
0.80
0.70
0.90
0.90
0.95
^.^s
|
D^ flj
M
10 S3
n)
O
2910
2910
2880
2880
2880
2910
2840
2880
2950
2950
2930
2930
2930
2930
2950
2950
2900
2980
2980
2980
2910
2880
2910
2880
2800
7.ft%fc
Absorption of
Gas Analysis
mole f r . SO0
x 103 2
Total Pressure
mmHg
3.16 765
3.11 765
3.11 765
3.18 765
3.14 765
3.10 765
1.15 765
1.13 765
1.11
1.11
1.11
1.11
1.09 760
1.09 760
3.63 760
3.56 760
3.66 760
3.56 760
3.73 760
3.66 760
0.331 766
0.293 766
0.373 766
0.376 766
0.410 766
a.^% 16*
S02 into
£-•
•O
F
•H
23.2
23.4
23.6
23.6
24.3
24.3
25.4
25.5
25.5
24.8
24.8
24.8
26.2
26.1
28.5
27.6
28.2
28.0
26.3
26.1
25.0
25.0
25.0
25.0
25.3
T.S.I
Water
01
%
u
f-t §
0^0
38
25.0
25.0
13.0
13.0
41.2
41.0
25.6
25.6
32.4
32.4
20.2
20.2
10.6
10.6
33.8
33.5
20.2
20.2
9.5
9.5
33.0
33.0
20.2
20.2
10.2
•va.a
OUTLET
LIQUID
ANALYSIS
rn
ui O
•H h
3 gs
g «P
0.599
0.690
1.174
1.195
0.615
0.517
0.365
0.343
0.278
0.286
0.444
0.415
0.705
0.705
0.608
0.602
0.860
0.874
1.818
1.850
0.150
0.153
0.312
0.308
0.570
a.&tfi
ABS.
RATE
vo
o
•-I
X
55.5
63.8
56.6
57.5
93.8
78.5
34.6
32.5
33.4
34.3
33.2
31.1
27.7
27.7
76.4
74.5
64.1
65.7
64.2
65.3
18.3
18.7
23.4
23.1
21.3
«.-T
O 0)
~» n
n
0.425
0.504
0.440
0.438
0.718
0.610
0.725
0.693
0.725
0.745
0.720
0.675
0.613
0.613
0.507
0.504
0.433
0.433
0.415
0.430
1.33
1.54
1.51
1.49
1.25
V--i»
V
0.0267
0.0392
0.0287
0.0285
0.1570
0.0708
0.1795
0.1340
0.1795
0.2231
0.1704
0.1166
0.0772
0.0772
0.0472
0.0461
0.0330
0.0330
0.0285
0.0300
-
-
-
_
_
_

2.59
3.81
3.43
3.41
11.89
5.40
16.03
11.96
14.71
18.59
16.87
11.54
9.27
9.27
3.66
3.63
3.08
3.08
3.56
3.75
_
_
_
_
_
_
                                                                        in

-------
                                                                            53
                            APPENDIX D
                           NOMENCLATURE
C         Concentration in liquid phase, g.mole/cm ; subscript A refers to


          molecular  (unhydrolysed) so., B to calcium hydroxide, E to bisul-


          fite ion, i to interfacial concentration.


                         2
D         Diffusivity, cm /sec; subscript A refers to molecular  (unhydrolysed)


          SO , B to calcium hydroxide, E to bisulfite ion.



d         Column diameter, cm



G         gas rate, g.mole/sec-cm




H         Henry's law constant, g.mole/cm -atm



K         Hydrolysis constant for SO  in water, g.mole/cm




(K )      Overall coefficient based on gas phase concentration units, cm/sec.
  C G


(fe )      Gas phase coefficient, cm/sec.
  c G


k *       Liquid phase (physical absorption) coefficient/ cm/sec.
 L


L         Liquid rate, cm /sec-cm




N         Schmidt number, y/pD
 Sc


N .        Sherwood number, k d/D
 Sn                         c


N         Reynolds number, 4F/y   or   4Qp/irdu
 Re


P         Total pressure, atm



p         Partial pressure, atm; superscript * refers to partial pressure


          in equilibrium with bulk liquid phase.



Q         Gas flow rate,  cm /sec or cu.ft./min.



R         Gas constant, 82.1 cm -atm/g.mole - degree K.

-------
                                                                            54
T         Temperaturep degree K«

y         mole fraction in gas phase

Z         Column length? cm
Greek Symbols
F         Liquid rate; g°/sec-cm

y         Viscosity,, g/sec-cm

p         Gas density, go/cm ; subscript G refers to molal gas density,
          g0mole/cm

4>         Enhancement factor (dimensionless)

-------
                                                                            55
                            APPENDIX E
                         LITERATURE CITED


1.   Eigen, M., K. Kustin and G. Maass, Z. physich. Chem. N.F. 30^130  (1961)

2.   Brian, P. L= T., J. E» Vivian and So T. Mayr, Ind. Eng. Chem.
     Fundamentals, 10, 75  (1971).

3.   Johnstone, H. F. and P. W. Leppla, J. Amer= Chem. Soc., 56, 2233  (1934)

4.   Mayr, S. T., Sc.D» Thesis, Dept. of Chem. Eng., M.I.T.  (1970).

5o   Peaceman, D. W., Sc.D. Thesis, Dept. of Chem. Eng., M.I.T.   (1951).

6,   Phipps, R. L., S.B. Thesis, Depto of Chem. Eng., M.I.T.  (1947).

7.   Spalding, C. W., A.I.Ch.E., Jl, _8_, 685  (1962).

80   Vivian, J. E. and Behrmann, W. C,, A.I.Ch.E., Jl, 11,  656  (1965).

9.   Vivian, J. E. and T. Schoenberg, A.I.Ch.E., Jl, 14, 986  (1968).

-------
 [BIBLIOGRAPHIC
  SHEET
              DATA
1. Report No.
  EPA-650/2-73-047
 "*• Title and Subtitle
 Absorption of SO2 into Lime Slurries: Absorption
    Rat es and Ki net i cs
S.^ecipient's Accession No.
                                                                "5. Report Date
                                                                December 1973
                                                                6.
   Author(s)
 J. Edward Vivian
                                                                8. Performing Organization Kept.
                                                                  No.
  • Performing Organization Name and Address
  Massachusetts Institute of Technology
  Department of Chemi cal Engineering
 Cambridge, Massachusetts 02139
                                                                10. Ptoject/Task/Work Unit No.
                                                                ROAP 21ADE-21
                                                                11. Contract/Grant No.
                                                                68-02-0018 (Task 4)
 12. Sponsoring Organization Name and Address
 EPA, Office of Research and Development
 NERC-RTP, Control  Systems Laboratory
 Research Triangle Park, North Carolina 27711
                                                                13. Type of Report & Period
                                                                  Covered
                                                                Final
                                                                14.
 15. Supplementary Notes
     stractsThe report gives results of a study of the absorption of SO2 from an SO2/N2
 gas mixture into water and lime solutions and slurries (in a short wetted wall
 column at approximately 1 atmosphere and 25°C). The mole per cent of SO2  in the gas
 Was varied from 0.03 to 0. 37.  In solutions of calcium hydroxide greater than 0. 01 g
 mole/liter and in slurries, the system appeared to be gas absorption accompanied by
 an "infinitely" rapid irreversible second-order reaction in the  liquid phase.  The only
 effect of the solid in the slurries was to increase the absorption capacity.  In water,
 the system appeared to be gas absorption accompanied by a fast reversible hydrolysis
 reaction in the liquid phase.   The major resistance to gas absorption at these concen-
 tration levels was  found to be in the gas phase.  At the lowest concentrations  studied,
 the system was gas-phase controlled.
   Keywords and Document Analysis.  17o. Descriptors
                     Hydrolysis
                     Kinetics
•'• Key Words and L
Air Pollution
Flue  Gases
Des ulfur ization
Sulfur Dioxide
Absorption
Calcium Oxides
Slurries
Solutions
17b. Identifiers/Open-Ended Terms

Air Pollution Control
Flue Gas  Cleaning
I7c. COSATI Field/Group 7D, 13B
'8. Availability Statement
Unlimited
19.. Security Class (This
Report)
UNCLASSIFIED
20. Security Class (This
Page
UNCLASSIFIED
21. No. of Pages
61
22. Price
   NTIS-35 (REV. 3-72)
                              THIS FORM MAY BE REPRODUCED
                                                                       U5COMM-DC M852-P72

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