RESEARCH  REPORT
                   to
       NATIONAL AIR POLLUTION CONTROL ADMINISTRATION

             CONTRACT NO. PH 86-68-8U
              Task Order No. 17
               November 30, 1969
BATTELLE  MEMORIAL  INSTITUTE
         COLUMBUS LABORATORIES

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    THE COLUMBUS LABORATORIES  of Battelle  Memorial Institute comprise the original
research center of an international organization devoted to research.

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has performed  in  more  than  90 countries. As an  independent  research institute, it  conducts
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programs in fundamental research and education.

    Battelle-Columbus — with its  staff of 3,000 —  serves industry  and government through
contract research. It pursues:

    •   research embracing the physical and life sciences, engineering, and selected social
         sciences

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    •   information analysis, socioeconomic and technical economic  studies, and manage-
         ment  planning research.
                       505  KING  AVENUE* COLUMBUS,  OHIO 43201

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                      FINAL REPORT
                           on
INVESTIGATION OP THE LIMESTONE-SO  WET SCRUBBING PROCESS
                           to
     NATIONAL AIR POLLUTION CONTROL ADMINISTRATION

                CONTRACT NO. PH 86-68-8U
                   Task Order No. 17
                    November 30, 1969
       R, W. Coutant, R. H. Cherry, H. Rosenberg,
                 J. Genco,  and A. Levy
              BATTELLE MEMORIAL INSTITUTE
                 Columbus Laboratories
                     505 King Avenue
                 Columbus,  Ohio  43201

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                               TABLE OF CONTENTS






                                                                        Page



MANAGEMENT SUMMARY	    i




INTRODUCTION 	    1




SUMMARY	    2




EXPERIMENTAL WORK AND DISCUSSION	    5




     SO -Uptake	    6




          Results	   12




          Analytical Model for the Wet Lime-SO  Scrubbing Process. .   37




     Hydration of Burnt Lime	   kl




          Apparatus and Procedure. ..... 	 ...   kl




          Results	   kk




     Dissolution	   $6




          Procedure.	   56




          Results	   58




     Analysis of Liquors	   60




CONCLUSIONS	   63




RECOMMENDATIONS	   6k






                                    APPENDIX




IDENTIFICATION AND COMPOSITION OF SAMPLES	   A-l

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                                MANAGEMENT SUMMARY






          The wet lime-SO  scrubbing process was investigated on a laboratory




scale in support of full-scale prototype studies being undertaken by the National




Air Pollution Control Administration (NAPCA).




          This investigation consisted of laboratory scale experiments in the




following areas:




          1.  Measurement of the overall rate of uptake of SO  in a stirred-pot




              reactor,




          2.  Measurement of the relative rates of hydration of selected




              limestones and dolomites,




          3.  Measurement of the relative rates of dissolution of selected




              limestone and dolomite materials,




          k.  Chemical analysis of selected dolomite- and limestone-based




              liquors prepared at three temperatures.





          These experiments were designed to yield qualitative indications of the




importance of individual physical and chemical processes to the overall limestone-




SO  wet scrubbing process.




          The results of the current experiments indicate that lime in particulete




form reacts readily with various sulfur species or carbonate in solution to yield




a coating which inhibits utilization of the bulk of the lime.  Fine grinding of




the lime might alleviate this problem to some extent.  However, because of the




observed tendency for particles to become cemented together forming large clusters,




a high degree of utilization of the lime may not be possible as long as the lime is




admitted to the scrubber in particulate form.  It is therefore recommended that

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further consideration be given to the importance of particle size in the overall



scrubbing process.  It is further recommended that consideration be given to the



possibility of predissolving the lime, or limestone, in the feed water to the



scrubber through the use of excess CO  or other solubilizing agents.



          Further development is also needed in the area of modeling of the



overall reaction system.  The model given in this report is only a first attempt



at description of the scrubbing process, and as such does not give adequate re-



presentation of the mechanical and chemical factors Involved.  For instance, the



dependence of the equilibrium partial pressure of SO  on solution composition



is not explicit in the model given.  Also, a detailed analysis of these data will



yield only rudimentary information on the various mess transfer resistances in



this system, which may not be directly applicable to large-scale systems.
                                        ii

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         INVESTIGATION OF THE LIMESTONE- SO  WET SCRUBBING PROCESS
                 R. W. Coutant, R. H. Cherry, H. Rosenberg,
                           J. Genco, and A. Levy
                                INTRODUCTION


          One of the major air pollutants in the United States is sulfur dioxide

produced by burning fuels containing sulfur.  As part of a national program to

develop air pollution control processes, the National Air Pollution Control

Administration is undertaking prototype studies of the lime/limestone scrubbing

process.  These studies will be carried out in three scrubbing systems, each

capable of handling 30,000 acfm of flue gas.

          Aqueous lime scrubbing to control SO  from combustion flue gas dates

back to the early 1930fs when several pilot and large scale projects were con-

ducted in England to develop a cyclic process in which all of the scrubbing

liquor would be recycled and only a solid waste product would be formed.  This

work was never completed partly because of the interruption of World War II and

partly because of the development of the non-cyclic process still in use at

Battersea and Bankside near London.  These units require large quantities of

Thames River water for scrubbing on a once-through basis.  Little information

on the cyclic lime process is available as a basis for current studies.

          A major departure from the earlier English practice involves the use

of the power-plant boiler as a calciner to produce the lime.  In this procedure

limestone or dolomite is pulverized and injected into the furnace.  The calcined

lime is subsequently collected in the scrubber as the reactant for removing SO .

In this system several factors could be key to a successful process design.  For

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example, the availability of the lime for reaction in the scrubber could be




dependent on its rate of hydration or rate of dissolution.  Any tendency for




insoluble carbonates or other reaction products to form dense layers on the




otherwise active reactants as veil as on scrubber equipment could be important.




Little is known about these or other potentially important reactions end




reaction rates.




          This study was conducted under Contract No. PH 86-68-8^ (Task Order




No. IT) to provide preliminary information on the chemistry and relative re-




action rates of some of the more important reactions as a basis for estimating




their probable importance in the more complex large scale lime/limestone




scrubbing process.  Its purpose was to investigate several of the individual




chemical processes which occur during the overall limestone-SO  wet scrubbing




process:  takeup of SO  by lime solutions and slurries, hydration of burnt lime,




and dissolution of hydrated lime.  This Final Report covers work done during




the period of April 15 through July 15, 1969.








                                   SUMMARY






          The wet-lime-SO  scrubbing process was investigated on a laboratory




scale in support of full-scale prototype studies being undertaken by the National




Air Pollution Control Administration (NAPCA).




          This investigation consisted of laboratory scale experiments in the




following area:




          1.  Measurement of the overall rate of uptake of SO  in a




              stirred-pot reactor,




          2.  Measurement of the relative rates of hydretion of




              selected limestones and dolomites,

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          3.  Measurement of the relative rates of dissolution of



              selected limestone and dolomite materials,



          U.  Chemical analysis of selected dolomite- and limestone-based



              liquors prepared at three temperatures.
    »


          The SO  uptake experiments were performed with simulated flue gas in



 a  stirred-pot reactor at 125 F operated batchwise with respect to the liquid



 charge.  Preliminary runs were made to determine the effects of gas-bubble



 size, and gas-flow rate on the observed rate of uptake of SO  by the liquid.



 The experiments were limited to a brief exploration to assess the magnitude of



 the effects and did not involve a detailed study of these variables.  Within the



 range of operations undertaken the effects of variation of these experimental



 parameters were judged to be not significant.



          The two primary variables studied were the flue ges and the scrubber-



 liquor compositions.  The change of pH of the scrubber liquor during a run



 generally follows a trend which includes a sharp initial drop as CO  is sorbed.



 The pH remains reasonably constant at values in the range 6-7 until the re-



 act ants of the liquid phase become exhausted or are rendered unavailable.   At



 this point a sharp drop in liquor pH is accompanied by SO  breakthrough.  SO



breakthrough occurred at liquor pH values in the range of 3-U when no NO  was
                                                                        Jt


present in the simulated flue gas;  with NO ,  breakthrough occurred in the pH
                                          X


 range of If-5.



          Little or no SO  appears in the scrubber outlet gas until the reactant



 is either exhausted or rendered unavailable.   The stoichiometry at the point of



 SO  breakthrough generally corresponds to the formation of the sulfite from the



 available reactants.   After breakthrough,  the SO  concentration in the outlet



gas increases rapidly and asymptotically approaches the value of the inlet



concentration of SO .

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          The rates of hydretion of several samples of burnt limes supplied by



NAPCA were determined by a temperature-rise procedure similar to ASTM Test C-110.



The results indicate a strong dependence of hydretion rate on the conditions of



calcination of the stone and particle size.  As particle size is increased,



or as temperature of calcination is increased, the rate of hydration of the



resultant lime decrease's.  The results indicate that the rate of the hydration



process is limited by diffusion of water through the lime particle, and that over-



burning of the lime markedly decreases hydration rate.  Other experiments, using



partially sulfated limes or solutions containing S07, showed that hydretion rates



are severely limited by the presence or formation of a layer of sulfate on the



lime particles.



          Rates of dissolution of hydrated lime were determined in a well-stirred



system using a calcium ion-specific electrode and a pH electrode for monitoring


                        •H-       —
the concentrations of Ca   and OH  in solution.  The results indicate that dis-



solution in water is rapid, only a few seconds being required for saturation



with Ca(OH) .  However, when the solvent contains sulfite, sulfate, or carbonate,



the rate of dissolution of Ca(OH)  is greatly decreased.



          Analyses of solutions saturated with respect to CaSO , CaSO, , MgSO ,



and MgSOi  showed that these solutions contained primarily MgSO, .  These enalyses



also indicated a buffering effect of these salts on the pH of the solutions.

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                       EXPERIMENTAL WORK AND DISCUSSION


          The limestone-SO  wet-scrubbing process may be considered in terras

of a series of physical and chemical interactions involving:   (l)  transfer of

the SO  to the surface of the liquid sorbent, (2) dissolution of SO  and
                     *
reaction with dissolved components of the liquor, and (3) regeneration of

active liquor through hydration and dissolution of solid lime.   It is generally

expected that the homogeneous solution reactions will be relatively fast and

not important in determining the overall rate of the scrubbing process.

Hence, this program has been concerned with an exploratory examination of the

heterogeneous processes; SO  uptake, hydration of the lime, and dissolution

of the lime.  As will be seen from the results, the rates of these processes

will not "be independent of each other in any real scrubbing system.  These

results are therefore, at best, qualitative indications of the importance of

individual segments of the overall scrubbing process.

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                                     SO -Uptake
          The apparatus shown schematically In Figure 1 was developed to perform




laboratory-scale experiments to study the wet lime-SO  scrubbing process.




Simulated flue gas was obtained by mixing gases from two cylinders, one contain-




ing 90 mole percent nitrogen and 10 mole percent oxygen end the other containing




75.5 mole percent nitrogen, 2^ mole percent CO , end 0.5 mole percent SO .   Other




cylinders were available, so that SO  or CO  could be deleted from the mix, or




NO  could be added to the mix.  The gases from the cylinders were passed through




calibrated rotemeters equipped with dial thermometers and pressure gauges.   The




rotameters were operated at 15 psig, and flow controllers were used to reduce




the pressure to atmospheric conditions.  The line from the cylinder containing




SO  had a check valve after the flow controller to prevent backflow from the




Np/0  cylinder.  A nitrogen line (not shown) was connected to each of the gas-




cylinder lines immediately downstream from the shut-off valves and was separated




from the latter lines by check valves.  The N  was used for purging and standardizing




purposes.  The line from the N /O  cylinder was constructed of copper end the line




from the SQ^CQ^IX  cylinder was constructed of stainless steel.  However,  down-




stream from the flow controllers, the entire flow system, with the exception of




the SO  analyzer, was constructed of Pyrex glass.




          After leaving the flow controller, the Hp/°o stream passed through s




preheat loop in a water bath and then through a water-bubbler humidifier, where




the stream was saturated with HO.  The SO /CO_/N  stream also passed through

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      FIGURE 3.  SCHEMATIC DIAGRAM OF LABORATORY APPARATUS
      vent
                 FOR STUDY OF WET LIME-SO  SCRUBBING PROCESS.
                     CTC
                            LEGEND
A   - Analyzer, S02                      P
CTC - Constant temp, chamber, 130°F      R
CV  - Check vclve                        S
F   - Klowmeters                         SC
FC  - Flow controller                    SO
H   - Hunidifier                         T
M   - Mixing chamber                     WB
   Pressure gauges
-  Reactor
   HpOp scrubber
   Stopcocks, 3-woy
   Shut-off vnlves
   Thermometer
-  Water bath, 125°F

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                                       8
a mixing chamber which vas immersed in the water bath.   From the mixing chamber,



the gas stream flowed to a manifold containing 3 three-way stopcocks so that the



flow could be directed either to the reactor end then to the SO  analyzer or vice



verse.  The stopcock manifold permitted checking of the SO  content of the inlet



gas to the reactor periodically during the course of a run.   The SO  content



of the outlet gas from the reactor was monitored continuously except for short



periods when the inlet concentration was being checked.  After returning to the



stopcock manifold, the gas stream was routed to an HO  scrubber to remove residual



SO  before being vented to the hood.  The reactor was also immersed in the water



bath, which was maintained at 125°F, and the entire flow system, between the flow



controller and the HO  scrubber, was enclosed in a plastic cabinet maintained  et



130°F.




         A detailed sketch of the reactor is shown in Figure 2.  The gas entered



the reactor through a hollow glass stirring rod and was dispersed through a



coarse glass frit at the bottom of the rod.  Two sets of three glass propeller



blades were located just above the frit, to insure good mixing.  In several runs,



a six-bladed Teflon impeller, having small holes drilled through the bottom, was



substituted for the glass frit-propeller blade combination.  The gas left the



reactor through a small packed bed of glass beads in order to remove any en-



trained liquid.  The reactor was equipped with a thermocouple well and fittings



for sealing in pH and calcium-ion electrode systems.

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        Electrode
Thermocouple well
                                              Stirring rod,  hollow glass
                                                    Packing,  6 mm dia.
                                                    Pyrex beads on
                                                    Teflon support
                                                      Reaction vessel,
                                                      125 mm dia.  Pyrex
                                            Frit for gas dispersion
                 FIGURE
REACTION VESSEL FOR STATIC
CHARGE OF SCRUBBER LIQUOR.

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                                         10






          The SO  analyzer was a Model 315 A Beckman Infrared Analyzer which




was calibrated with known mixtures of SO -containing dry gases.  This calibration,




shown in Figure 3> vas double- checked by using gases directly from several cylinders




containing known amounts of SO .  It was found that CO  had a negligible effect on




the analyzer readings (10 mole percent CO  was equivalent to about 10 ppm of SO ),




but that HO had a considerable effect on the readings.  Figure 3 also shows a




curve for analyzer reading versus HO composition in mole percent.  The theoretical




calibration curve for SO  in gas saturated with HO at 125 P vas obtained by




adding the analyzer reading for 13-2 mole percent HO (gas saturated at 125°F)




to the analyzer readings for SO  in dry gas.  However, it was determined that the




effects of SO  and HO on the analyzer were not additive.  Therefore, calibration




curves had to be faired in for each run, as shown in Figure 3> based on analyzer




readings for the inlet gas to the reactor, with and without the
stream.  That is, a zero reading was obtained from the saturated N /O  streem,




and another point on the curve was obtained from the reading for the N /O  streem




and the SO /CO 2/W  stream mixed together after correcting this stream to saturated




conditions.  The above two points were then used to fair in the curve.




          The SO  analyzer readings, reactor pH, and HO  pH were recorded contin-




uously on strip-chart recorders.  An attempt was also made to monitor the calcium-




ion concentration; however, difficulty was encountered with the calcium-ion




electrodes because of the changing pH of the scrubbing liquor.  Use of these




electrodes was abandoned during the experiments.  However, the calcium ion elec-




trodes were used to determine the initial concentration of the scrubbing liquors,




and two batches of these liquors were analyzed by atomic absorption as a check.




Good agreement was obtained between the atomic absorption and calcium-ion




electrode analyses.

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                       FIGURE 3.  CALIBRATION OF INFRARED ANALYZER FOR SO .
    120




    100
     80
c
•H
T3
a
H


I
     60
     20
                       SO  in  saturated  gas  at

                         125 F (theoretical)
                                                                                 in dry  gas
                                     S  _   SO   in  BeturfjXages  et  125  F (typical)

                                                                      HO  in  SO   free  gas
      0
0       1000
                               2000



                                 u
3000        4ooo
    ppra
  6           8

mole percent HO
5000        6000      7000




 10           12         It

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                                        12






Results






          Sixteen experiments were performed to study the SO  uptake "by various




scrubbing liquors.  The simulated flue-gas compositions for the experiments ere




listed in Table 1, and the experimental parameters are summarized in Table 2.




Runs A, B, end 2A were performed in addition to the runs listed in the original




work statement and Runs 10 end 11 from this statement were not performed, in




accordance with verbal agreements made with Mr. J. Phillips of NAPCA.  Curves of




SO  concentration versus time for the inlet gas to the reactor end the outlet gas




from the reactor, and curves of pH versus time  ere shown for each run in Figures




k through 19.  All of the runs yielded essentially the same type of curve for the




SO  concentration in the gas leaving the reactor, i.e., little or no SO  in the




outlet gas until breakthrough time, at which time the SO  concentration began




increasing and asymptotically approached a value close to the inlet value.  Even




when the scrubber liquor becomes saturated with SO , the SO  concentration in the




outlet gas should be slightly lower than that in the inlet gas because of the added




moisture picked up by the gas in passing through the reactor.  It is fairly safe




to assume that the gas leaving the reactor is saturated with HJ) end, thus,




contains 13.2 mole percent HO.  In most of the runs, the gas entering the re-




actor contained 7.06 mole percent HO.  Calculated values for the SO  concentration




in the outlet gas at liquor saturation ere listed for each run in Table 2 on




line 3.  In the runs in which the outlet SO  concentration had leveled off, in-




dicating that liquor saturation had occurred, there is good agreement between




the experimental value and calculated value of SO  concentration in the outlet




gas at liquor saturation.  It is estimated that the SO  concentrations are accurate




to within ±1056 for all runs except A, B, 6, 7, 8, and 9 in which cases

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                          13
TABLE 1.   COMPOSITION OF SIMULATED FLUE GAS FOR SO,
          UPTAKE EXPERIMENTS (INLET GAS TO REACTORJ
Concentration, mole percent
Run No.
A
B
1
2
. 2A
3
4
5
6
7
8
9
12
13
14
15
so2
0.22
0.22
0.22
0.22
0.22
0.22
0.22
0.22
0.22
0.22
0.26
0.25
0.089
0.044
0.00
0.25
N2
75.76
. 75.76
75.76
75.76
75.76
75-76
75.76
75.76
75.76
75.76
73-31 .
73.31
77.09
77.62
77.00
88.04
°2
4.65
4.65
4.65
4.65
4.65
4.65
4.65
4.65
4.65
4.65
4.65
4.65
7.03
7-87
4.65
4.65
C°2
12.31
12.31
12.31
12.31
12.31
12.31
12.31
12.31
12.31
12.31
14.68
14.68
5.^08
2.49
11.29
0.00
H20
7.06
7.06
7.06
7.06
7.06
7.06
7.06
7.06
7.06
7.06
7-06
7.06
10.71
11.98
7.06
7.06
NO
X
0.00
0.00
0.00
o.oo
0.00
0.00
0.00
0.00
0.00
0.00
0.046
0.046
0.00
0.00
0.00
0.00

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                                                                TABLE 2.   SUMMARY CF S02 UPTAKE EXPERIMENTS AT 125°F AND 1 ATMOSPHERE
Run Number
A 6 1
Inlet gas flow 10,760 10,760 1,076
rate, cc/r.in
at STP
S02 in Inlet 2,200 2,200 2,200
gas, ppm
S02 in outlet 2,060 2,060 2,060
gas at liquor
saturation'3'.
ppm
Impeller TI TI TI
type!*)
Impeller 1,500 1,500 1,500
speed,
rpn
Scrubber li- 1,OOO 1,000 1,000
qucr volume,
cc
Scrubber li- 1.43xlO~3M Dls- LOOxlO"1!1*
quor con- NaOH tilled HaOH
position H,0




Experimental <1 0 >180 '
breakthrough
tine, ir.in
Calculated 0.81 0 566
breakthrough
tirce'^J. rain
Ratio: — — —
experimental/
calculated
r, moles of re- 2 — 2
acta.it con-
sumed per mole
of. S02 con-
Buned •
2 2A 3 4 5 6
1,076 1,076 1,076 1,076 1,076 1,076


2,700 2,700 2,200 2,200 2,200 2,200

2,050 2,060 2,060 2,060 2,060 2,060



TI GDT TI GDT GDT GDT
1,500 1,500 0 1,500 1,500 1,500


1,000 1,000 1,000 1,000 1,000 1,000


0.975xlO~3M 1.05xlO'3M 0.925xlO~3M 1.20xlO~2M 6.06xlO~3M 9.BOxlO~3M
NaOH NaOH NaOH calcium doloroitic calcium
hydrate hydrate hydrate +
4.00 gra
solid
calcium
hydrate
55 8 H9 70 480


5.51 5.94 5.24 136 63.6 722


0.9 0.85 1.5 O.S8 1.02 0.66


222 1 11




7 8 9 12
1,076 1,076 1,120 1,136


2,700 2,500 2,500 890
fc
2,060 2,340 2,340 865



TI GDT GDT GDT
1,500 1,500 1,500 1,500


1,000 1,000 1,000 1,000


<1.42xlO"3M 4.42xlO~3M 6.85xlO~3M 1.09xlO"2M
dolorrutic calcium dolorsitic calcium
hydrate + hydrat* hyiirate hydrate
4.00 gm
solid
dolomitic
hydrate
650 76 105 155


841 43,8 67.8 290


0.77 1.7 1.5 0.53


(e) 1 11




13 14 15
1,136 1,076 1,076


440 0 2.500

434 0 2,340



GDT GDT GDT
1,500 1,500 1,500


1,000 1,000 1,000


1.02xlO~2M 1.0
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                                       Ua
                              FOOTNOTES FOR TABLE 2


(a)  Assuming reactor outlet gas is saturated with H?0.


(b)  TI = Teflon impeller;  GDT = gas dispersion tube.


(c)  Run was terminated at  180 min and no breakthrough had occurred.


(d)  Based on simplified process model.

                        •
(e)  r = 1 for dolomitic "hydrate solution and r = 0.5  for solid dolomitic hydrate,

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 OJ
W

§
a.
                                                        Inlet ges flow rate:   10,760 cc/min STP (wet bpsis)
                                                        Stirrer speed:  1500 RPM (Teflon impeller)
                                                        Scrubbing liquor:   1.1*3 x 10"3 M NsOH (l liter)
                                                        Temperature:   125 F
                                                        Theoretical breakthrough:  0.8l min
                                                                                                      Rerctor pH     	
                                                           16    18    20    22    24    26    28    30    32    3^   36
0    2
                                                     FIGURE U.  S0g UPTAKE, RUN A.

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                                           16
    2400


    2200


    2000



    1800


    1600
 C\J
g   1200
    1000
Inlet gas flow rate:  10,j60 cc/min STP  (wet basis)
Stirrer speed:  1500 RPM (Teflon impeller)
Scrubbing liquor:  distilled HO (l liter)
Temperature:  125 P                         Inlet SO,
Theoretical breakthrough:  0 min                    e-
                          13


                          12


                          11


                          10


                           9


                           8


                           1


                           6


                           5


                           it


                           3
          0
                  8    10     12

                      t, minutes
Ik    16    18   20
22
                                 FIGURE 5.   SO  UPTAKE, RUN B.

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                                                                             Inlet SOp
                                                                              Outlet S02
                                        Inlet gas flow rete:   1,076 cc/min STP (vet besis)
                                        Stirrer speed:   1500 RPM (Teflon impeller)
                                        Scrubbing liquor:   0.975 x 10" 3 M NeOH (l liter)
                                        Temperature:   125
                                        Theoretical breakthrough:  5-51
                                                                                Reactor pH
20
60
 80        100        120

       t, minutes

FIGURE 7. S02 UPTAKE, RIM 2,
11*0
160
180
200

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                                                 Outlet SO
    Inlet gas flow rete:   1,076 cc/mln STP (wet basis)
    Stirrer speed:   1500 RPM (ges dispersion tube)
    Scrubbing liquor:   1.05 x 10~3 M NaOH (l liter)
    Temperature:   125
    Theoretical breakthrough:   5-9U- min
                                         Reactor pH
      iO         100
          t, minutes
                                                     iou
FIGURE 8.  SO  UPTAKE, RUN 2A.

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 C\l
(X,
                                                              Inlet gas  flow  rate:   1,0?6 cc/min STP (wet bpsis)
                                                              Stirrer  speed:   0 RPM (Teflon libeller)
                                                              Scrubbing  liquor:   0.925 x 10'3 M NeOH (l liter)
                                                              Temperature:  125 F
                                                              Theoretical breakthrough:   5-2^ min
           0
         100
t, minutes
120
140
lfc>0
100
                                                 FIGURE 9.   S02 UPTAKE,  RUN 3

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21*00


2200


2000


1800


1600


itoo


1200


1000


 800


 600


 Uoo


 200


   0
                                                                                   Inlet SO.
                               •M-
                             Ca   concentration
                               (uncorrected)
                                                   Reactor pH
                                                        Outlet SO,
Inlet gas flow rete:   1,076 cc/mln STP (wet basis)
Stirrer speed:   1500 RPM (ges dispersion tube)
Scrubber liquor:  1.20 x 10-2 M calcium hydrate (l liter)
Temperature:   125 F
Theoretical breakthrough:  136 min.
             Jr
60          80          100
               t, minutes

     FIGURE 10.  SO  UPTAKE,  RUN k.
                                                                             120
160
180
                                                                                                        ro

-------
o
co

I
 OJ
     2600

     2^00

     2200

     2000

     1800

     1600
1200

1000

 800

 600

 uoo

 200

   0
            Inlet gas  flow rate:   1,OJ6  cc/min  STP (vet basis)
            Stirrer  speed:   1500  RPM (gas  dispersion tube)
            Scrubber liquor:   6.06 x 10"3  M dolomitic
                               hydrate (l liter)
            Temperature:   125  F
            Theoretical breakthrough:  68.6 min
                                                                                        Inlet SO,
            0     20      40       60     80      100     120

                                                      t, minutes

                                             FIGURE 11
                                                             1UO
160
180
200
220
                                    13

                                    12

                                    11

                                    10

                                     9

                                     8

                                     1

                                     6

                                     5

                                     l*

                                     3

                                     2

                                     1

                                     0
                                                    SO  UPTAKE, RUN 5.
                                             ro
                                             ro

-------
21*00
                                                                                        Inlet SOp
                                               Inlet gas flow rete:  1,076  cc/min STP  (wet besis)
                                               Stlrrer speed:  1500 RPM  (gas dispersion tube)
                                               Scrubber liquor:  9.80 x  10~3 M celcium hydrete  (l  liter) +
                                                                 U.OO CMS solid calcium hydrate
                                               Tempereture:  125
                                               Theoretical breakthrough:  722 min
      0
                                                                                                      20
22
                                          FIGUBE 12.  SO  UPTAKE, RUN  6
                                                                                                                     ro

-------
o
CO
    2400-


    2200


    2000


    1800


    1600


    1400


 cvi 1200

    1000


     800


     600


     400


     200

       0
          0
T
                                                    Inlet S02
             Inlet gas  flow rate:   1,076 cc/min STP (wet bssis)
             Stirrer  speed:   1500  RPM (Teflon impeller)
             Scrubber liquor:   4.42 x 10~3  M dolomitic hydrate
                                (l  liter) +  4.00 CMS solid
                                doloroitic hydrate
             Temperature:   125  F
             Theoretical breakthrough:   84l min
                                                         Outlet SO,
12


11


10


 9


 8


 7


 6


 5
 1

 0
                                                                                                                 ex
                                                                                        ro
                                                    FIGURE 13.   S02 UPTAKE,  RtM 7.

-------
o
w
2600






2400






2200






2000






1800






1600






11*00






1200






1000






 800






 600






 1*00






 200
                                                                                                      Inlet SO,
                     Inlet  gas  flow rate:   1.0?6 cc/mln STP (vet basis;  U60 ppm NO )

                     Scrubber liquor:   ^.^2 x 10~3 M calcium hydrate (l liter)

                     Temperature:   125

                     Theoretical breakthrough:   1*3.8 min
                 20
                                             60          80           100


                                                         t, minutes



                                                FIGURE Ik.  SO UPTAKE, RUN 8
120
160
                                      13





                                      12






                                      11






                                      10






                                       9





                                       8
                                                                                                                         U





                                                                                                                         3
180

-------
     2600
     2200


     2000


     l800


     1600
g    1200
     1000


      800


      600


      400


      200


        0
Inlet gas flow rate:  I,0j6 cc/min STP (wet basis; lj-60 ppm NO, )
Stirrer speed:  1500 RPM (gas dispersion tube)
Scrubber liquor:  6.85 x 10"3 dolomitic hydrate (l liter)
Temperature:   125 F
Iheoreticel breakthrough:  67.8 min
                                                                                                    Inlet S02
                                                                                                   Outlet SO,
                                                               100

                                                           t,  minutes
                                                  FIGURE 15.   S02 UPTAKE, RUN 9.
                                                                                                      ro

-------
   1200
   1000
Inlet gas flow rete:  1,120 cc/mln (wet basis)
Stirrer speed:  1500 RPM (Ges dispersion tubes)
Scrubber liquor:  1.09 x 10~2 M calcium hydrate (l liter)
Temperature:  125 F
Theoretical "breakthrough:  290 ndn
                                                                                            Inlet SO.
    800
8   6oo
6
a
    UOO  —
    200
                                     100
                                                      200
1*00
                                                      t, minutes
                                          FIGURE 16.  SO  UPTAKE, RUN 12.
                                                                                                                     ro

-------
   TOO
   600
   500
  ,400
C/3

B
Pi
   300
   200
   100
         0
Inlet gas flow rate:  1,136 cc/mln STP (wet basis)
Stirrer speed:  1500 RPM (gas dispersion tube)
Scrubber liquor:  1.02 x 10~2 M calcium hydrate (l liter)
Temperature:  125 F
Theoretical breakthrough:  5^2 min
                                                                                                  Inlet SO
                                                                                                          2     _
            100
        200

         t, minutes

FIGURE 17.  S02 UPTAKE, RUN 13.
400

-------
0
          Inlet gas flov rate:   1,076 cc/min STP (wet basis)
          Stirrer speed:   1500 RPM (gas dispersion tube)
          Scrubber liquor:   I.Ok x 10~2 M calcium hydrate (l liter)
          Tempereture:   125 F
20
60
80
     100

t, minutes
120
140
                                                                                            Beactor pH
                                                                                            H2°2
                                                                                       Inlet  end  outlet  SOp —
                                                                                             zero
                                                                                       I           I	
                                                                                                    13


                                                                                                    12


                                                                                                    11


                                                                                                    10


                                                                                                     9

                                                                                                     8
                                                                                                     5

                                                                                                     U


                                                                                                     3


                                                                                                     2
160
180
200
                                    FIGURE 18. CO  UPTAKE, RUN lU.

-------
    2800
o
w
PL,
 OJ
                                                                    1,076  cc/mln  STP  (wet bests
Inlet gas flow rate:
Stirrer speed:  1500 RPM (gas dispersion
                tube)
Scrubber liquor:
                  calcium hydrate
                  (l liter)
Temperature:  125F
Theoretical breakthrough:  102 min
                                                                1.03 x Kf2 M
                                                                                                  Reactor pH
                                              100   120      lUO    160    180    200

                                                         t, minutes
                                            220
240
260   280
                                                FIGURE 19.  S02 UPTAKE, RUN 15.

-------
                                       31




 the SO  concentrations are estimated to be accurate to ±20 percent.   This



 latter error was caused by experimental difficulties which will be discussed



 later.



          The breakthrough time for SO  was also reflected by a sharp decrease



 in the reactor pH and by decreasing pH in the HO  scrubber.  *n the simulated



 flue gas not containing NO , breakthrough of SO  occurred at a liquor pH in
                           Jv                    £.
 the range 3-^j with NO , the range was
                       JL


          The scrubber liquors used in this study included distilled water,  NaOH



 solutions, and lime solutions.  The lime solutions were prepared by saturating


                         #                     *#
 HO with calcium hydrate  or dolomitic hydrate   in a well- stirred beaker for



 about 2k hours at 125 F and filtering out the excess solids.  In the case of



 Run 8, the liquor was stirred for 66 hours prior to use.  The lower calcium



 concentration of this liquor is probably due to reaction with CO  from the air,



 resulting in the precipitation of CaCO .  Chemical analysis of the dolomitic



 solutions by atomic absorption revealed that there was no magnesium in solution,



 indicating that MgO does not readily hydrate and dissolve.  Therefore, the



 dolomitic hydrate apparently was a mixture of Ca(OH)  and MgO in a 1:1 mole



 ratio; the mole ratio was calculated from the chemical analysis of the lime-



 stone material used to prepare the hydrate (see Appendix).  Thus, the only



 scrubbing liquor that contained magnesium was the one in Run 7> for which k



 grams of solid dolomite hydrate was added to the reactor Just before start



 of the run.



          Some of the chemical reactions that can occur between the flue gas



 and the various scrubber liquors can be represented as follows:
 *  Supplier's notation for hydrated high-calcium lime.



**  Supplier's notation for hydrated lime derived from dolomitic limestone.

-------
                                        32




           S02 + H20 « H2S03                                       (l)



           HaOH + H SO  = HaHSO  + HO                             (2)



           2NaOH + H SO  = Na SO  + 2H20                           (3)



           Ca(OH)2 + H2S0



           Ca(OH)2 + C02 =



           CaCO  + HSO  = CeSO  + HO + CO                        (6)
           MgO + H2S03 = MgS03 + H20                               (7)



           MgO + C02 = MgCO                                        (8)



                                 + H0 + C0                        (9)
          Reactions involving NO  complicated further an already complex
                                J\.


sitxiation in Runs 8 and 9> vhere this component was present.   In Run lU, the



absence of SO  eliminated reactions (l), (k) , and (6), and in Run 15, the



absence of CO  eliminated reactions (5) and (6).  Reactions (?)> (8), end (9)



are applicable only in Run 7-



          It is postulated that the time until breakthrough of SO  in the outlet



gas represents the time until exhaustion of reactant in the scrubber liquor, and



that the increasing segment of the SO -concentration curve represents the effect



of mass transfer on the dissolution of SO  in the liquor.  The theoretical break-



through time can be calculated front the following equation:
                                    *B '

-------
                                         33"

where

          t£ = theoretical "breakthrough time,  min

          if - initial amount of reactant in scrubber liquor,  moles
                                      •3
           q ss inlet gas flow rate, cm  per min
                                                              A
          C  s± concentration of SO  in inlet gas, moles per cm

          r  = moles of reactant consumed per mole of SO  consumed.

The term r can best be understood by comparing reactions (2) and (3).   In

reaction (2) r has a value of 1 and the reaction product is NaHSO , while in

reaction (3) r has a value of 2 end the reaction product in Na SO .  Theoretical

break-through times were calculated for each run by using the value for r

appropriate to the sulfite rather than the bisulfite reaction product (r = 1

and 2, respectively, for Ca(OH)2,end NaOH solutions).  It is recognized that

continued sorption after the breakthrough point probably involves formation

of bisulfite, but the effect of this reaction on the equilibrium partial pressure

of SO  was not explicitly considered.  In the case of Runs 6 and 7, where solid

lime was added to the scrubber liquor, the breakthrough times were calculated by

assuming that all of the solid material would eventually dissolve.  Experimental

breakthrough times were obtained from the curves of SO  concentration in the

outlet gas Versus time and are compared with the theoretical breakthrough

times in Table 2.  In some cases the agreement is within experimental error end

in other cases the agreement is poor; the breakthrough time comparison is dis-

cussed for each run in the following paragraphs.

          Runs A and B were performed at a gas flow rate of 10 liters per minute,
       _-3
with 10 J M NaOH and distilled water, respectively, as scrubbing liquors.  Diffi-

culties were encountered in interpreting the data from the SO  enelyzer because of

-------
excessive pressure drop needed for the high flow rate.   Consequently,  all other .




runs vere performed at a flow rate of 1 liter per minute.   In Run Af  it was




impossible to determine the experimental breakthrough time accurately, but it




was less than one minute.  This compered favorably with the theoretical break-




through time of 0.8l min.  Except for the slight break in the curve at the start




of Run A, Runs A and B exhibited the same shape of curve for SO  concentration,




indicating that, once the NaOH reactant is exhausted, the scrubber liquor behaves




the same as distilled water at this low concentration of reaction product.




          Runs 1, 2, 2A, and 3 were performed with NaOH solutions as the scrubbing




liquor.  The concentration in Run 1 was 10"  M while in the other runs it was




10   M.  The theoretical breakthrough time for Run 1 is 566 min and,  since the




run proceeded for only l8o min, no breakthrough would be expected to occur, as




indeed was the case.  In Runs 2 end 2A, the experimental end theoretical break-




through times agreed very well, indicating that the reaction product was indeed




NapSO .  In Run 2A, a gas dispersion tube, resulting in somewhat smaller gas




bubbles, was used for stirring, rather than the Teflon impeller.  The SO  con-




centration curves were about the same for both runs, indicating that the mass




transfer rate is not a strong function of bubble size.   In Run 3, the Teflon impeller




was used but the stirring motor was not turned on; however, the SO  concentration




curve was- still about the same as in Run 2, for which the stirring speed was 1500




rpm.  This observation was taken to indicate negligible resistance to mass transfer




in the liquid phase.  In Run 3> the experimental breakthrough time was greater than




the theoretical breakthrough time, possibly indicating the formation of some




WaHSO .

-------
                                         35





          Runs 4 end 5 vere performed with saturated solutions of calcium hydrate




and dolomitic hydrate, respectively, es the scrubbing liquors.  There was very




good agreement between the experimental and theoretical breakthrough times for




both of these runs.  The calcium ion electrode was used in Run h, in an attempt




to monitor the Ca   concentration continuously.  However, as the pH of the liquor




decreased, the calcium i-on electrode output became erratic and increased rapidly,




thus negating its usefulness.




          Runs 6 and 7 were duplicates of Runs h end 5, except that ^.00 gms of




calcium hydrate or dolomitic hydrate was added to the respective liquors immediately




prior to the start of each run.  Difficulty was encountered because of pressure




buildup caused by clogging of the gas dispersion tube in Run 6, so that the Teflon




impeller was used in .Run 7-  However, even the relatively large holes (l/l6-in




diameter) in the Teflon became clogged with solid material during the course of




the run, and the run had to be terminated because of excessive pressure buildup




in the system.  At the conclusion of Runs 6 and 7, both liquors contained a




solid crystalline material that was grossly different in appearance from the




solid hydrates that were added to the liquors.  The crystalline materials were




not analyzed, but it is assumed that they are CaSO  and MgSO .  The experi-




mental breakthrough times in both runs were significantly lower than the




theoretical times, indicating that the crystalline material may have deposited.




around some of the solid hydrate particles, thus preventing their further reaction




with SO .  Also, the presence of solid particles added another resistance to the




reaction mechanism in the form of finite rate of dissolution of the hydrates.




This added resistance was not accounted for theoretically.  However, the break-




through times were only about 30 percent lower than expected.

-------
                                        36




          Runs 8 end 9 were also duplicates of Runs U end 5> except that U50 ppm


of NO  vas present in the flue gas.  The presence of NO  caused the infrared-
     *v                                                 "X

analyzer readings to drift upward with time and, consequently, made it more diffi-



cult to interpret the results.  However, the experimental "breakthrough times were


50 to 70 percent longer than the theoretical times.


          Runs 12, 13> and lU were similar to Run 4, except that the SO  concen-


tration in the inlet gas was lowered to 890, MtO, and 0 ppm, respectively, from


the 2,200 ppm in Run 4.  The CO  concentration was decreased and the HO concen-


tration was increased in the simulated flue gas for Runs 12 and 13 because of


limitations in the cylinder gases available.  The experimental breakthrough times


were about kO percent lower than the theoretical times in Runs 12 and 13-


Probable experimental error can account for only about a 25 percent difference.


At first glance it appears that the lower CO  concentration might offer an ex-


planation for the discrepancy.  However, in Run 15> in which no CO  was present,


there was excellent agreement between the experimental and theoretical breakthrough


times.  Therefore, it is difficult to explain the discrepancy between the break-


through times for Runs 12 and 13, especially in light of the results from the


other runs.  The reactor pH-versus-time curve for Run 14, in which no SO  was


present, indicates rapid formation of CaCO , followed by attainment of the carbonate



equilibrium at a pH of about 6.6.


          The absence of CO  in Run 15 had a significant effect on the shape of


the outlet SO  concentration curve.  Soon after breakthrough, the curve tended


to level off at about 500 ppm of SO  at a pH of 3.8 and then began increasing and



asymptotically approached a value close to the inlet concentration.

-------
                                         3T



Analytical Model for the Wet Liir.e-S0p

Scrubbing Process




          An attempt has been made to fit the data to an analytical model based


on chemical reaction followed by dissolution controlled by gas-phase mass trans-


fer.  The model is only a first attempt at an analytical description of these


experiments.  For simplicity, it was assumed that the chemical-reaction end mass-


transfer aspects of this problem can be uncoupled and treated independently.  The


chemical-reaction portion of the model has already been discussed in terms of


experimental and theoretical breakthrough times for SO  concentration in the


gas leaving the scrubber. The increasing portion of the SO  concentration curve


has been treated in terms of a gas-phase resistance to mass transfer present in


a thin film at the surface of each gas bubble passing through the scrubber liquor.


A material balance for SO  in the scrubber liquor in the reactor leads to the


following equation:



                                dC^




where


     C  = unreacted SO  concentration in scrubber liquor, moles per liter


     C  = SO  concentration in outlet gas, moles per liter of dry gas
      O     e-

     H  = Henry's law partition coefficient at liquor saturation with SO ,
     C   = SO  concentration in outlet gas at liquor saturation with SO ,
      G      £-                                                         <~

           moles per liter of dry gas


     CL  = unreacted SO  concentration in scrubber liquor at liquor saturation


           with SO , moles per liter

-------
                                         38



     t = time, min


     K ±= system constant having units of min"  defined by


                                      6k  q L H

                                  * =    g                             (12)
                                        v d V^^


where


     k  = gas-phase mess transfer coefficient, cm per min
      o
                           3
     q  -gas flow rate, cnr of dry gas per min


     L  = average path length traveled by a gas bubble in the liquor,  cm


     v  = average velocity of gas bubble, cm per min


     d  = average diameter of a gas bubble, cm


     V  e volume of liquor, cm .


A material balance for SO  over the entire system yields
                                 cg - * cg + vi
where
     C  = SO  concentration in inlet gas (equal to the previous C )
      6     f-                                                    S

          moles per liter of dry gas.


From the definition of the breakthrough time,
so that, at t = t,,, Equation (13) becomes
                 a
                                  at

-------
                                     39
Equations (ll) and (13) can be solved simultaneously for C  and C  yielding
                                                          -*•      o
                                  q C°   <*(t-tj
                                                                 6

                      u	 .     V

and

                       C  = C° [1 - «-0f t.)               (17)
                        g    g                     ~  B
where

                                                                       (18)
                                           K V-j^ *
                                       J_ T 'r  TT
          From Equation (l?)
                                     C
                             In (l - -S- ) = - «(t-tB).                (19)
                             C
                                      «
Therefore, a plot of In (l -- |) versus (t-t~) should yield a straight line vith
                             C
a slope equal to - ce.  A comparison of the experimental data with the analytical

model is shown in Figure 20 for Runs 2A and 5«  Considering the assumptions

involved, such as constant bubble diameter end constant residence time for the

bubbles in the liquor, the agreement between experiment end theory is good.  It

is interesting to note the similarity between the date for Runs 2A and 5.  The

experimental parameters are the same for these runs, but different scrubbing

liquors were used (0.975 x 10~^ M KaOH in Run 2A and 6.06 x 10"^ M dolomitic

hydrate in Run 5), resulting in different breakthrough times.  However, as far

as the dissolution portion of the model is concerned, the only difference between
                                                             o
the runs would be different partition coefficient (H) for 10   M Ne SO  end
      _o
6 x 10   M CaSO .  Evidently, at these low concentrations, the partition coefficients

are about the same, and about equal to that for distilled water.
                                                                     —2    —1
          The value of ot for Runs 2A end 5 is approximately 2. 76 x 10   min

as obtained from the slope of the line in Figure 20.  This value of ff, together

-------
   1.00
1 -
     s
   0.10
   0.01
          0
                                                         ORun 2A

                                                         A Run 5
20
                             Slope = -2.76 x 10~2 mirf1
                                                  I
60
                                          t -
    80
t,,, mln
100
120
          FIGURE 20.   COMPARISON OF EXPERIMENTAL DATA WITH ANALYTICAL MODEL
                      FOR S02 UPTAKE BY SCRUBBING LIQUORS.
ifco

-------
                                     41



with the calculated theoretical breakthrough time of 68.6 min end C° of 2370
                                                                   g

ppm of SO  on a dry basis, was used to construct the curve of SO  concentration


in the outlet gas versus time for Run 5 in accordance with Equation (l?).   A


comparison of the experimental date with the calculated curve is shown in


Figure 21 j  the agreement is very good,  considering the simplified analytical


model used.


          It is possible to calculate partition coefficients (H) for each  run by


graphically integrating the SO  concentration curves.   Once values of a and H


ere known,  one can calculate K from Equation (18) and, ultimately, the gas-phase


mass-transfer coefficient (k ) from Equation (12) if the average bubble diameter
                            D

and average residence time for the bubbles in the scrubbing liquor are known.


This was not done insofar as such an analysis did not fall within the scope of


this work.   It must be realized that the parameters in the analytical model


are highly dependent on the scrubber design, and that values derived from  the


current vork are not necessarily applicable to other types of scrubbers.
                            Hydration of Burnt Lime




Apparatus and Procedure




           The rates of hydration of burnt limes were determined using apparatus


similar to that prescribed for ASTM Test C-110.  A sample charge containing one


mole of calcium oxide was added to one liter of water (or other appropriate


liquor as specified by the Contract) at 125 F in a well-stirred and well-insulated


system and the rate of temperature rise in the system was monitored with a thermo-


couple connected to a recorder.  During these runs, solution pH was also monitored.

-------
       2400
                  O   Experimental data

                  —  Calculated curve
       2000
       1600
en
M
to
<
PQ
       1200
O

CO
 CJ
        800
        400
             0
80
120         160


 t, minutes
200
240         280
                                                                                                           ru
                   FIGURE 21.  COMPARISON  OF EXPERIMENTAL DATA WITH CALCULATED CURVE

                               FOR  SO   CONCENTRATION IN THE OUTLET GAS VS.  TIME.
                                     2

-------
          The experimental temperature-rise data were corrected for  thermal loss



by the system to obtain the true temperature change due to hydretion.   It wes



assumed that the thermal loss rate of the system could be represented  as





                                £2 - -k (T  - T )
                                dt -  k { e   V


where T  is the experimentally observed system temperature,  T  is  the



temperature of an arbitrary heat sink in the system, end k is a constant which



includes the heat capacity of the solution, the thermal conductivity of the system



walls, geometry of the system, etc.  Values of k and T  vere found by  determining



the thermal loss rate in the absence of hydration reaction at two  different values



of T , and solving the above equation.  The corrected temperature  chenge in the system



was then calculated as the sum of the observed temperature rise and  a  correction



for thermal loss:                                              .                • ;





                   AT  = AT  + k I (T  - T )dt

                                 Jt  e    °
                       = AT  + k £ (T .  - T )  At.,
                           e         el     o    i


where the summation is taken over sufficiently smell time intervals to keep the



effects of local curvature in the temperature-rise curve to a negligible  level.



In practice, it was found that T  did not remain constant during e run.   The



error resulting from this procedure was  considered insignificant for runs requiring



less than 1 to 1-1/2 hours for completion, but for runs lasting for longer  periods



of time the error became comparable with observed  temperature changes.  Therefore,



an arbitrary time limit of 83 min (5000  sec) was set for duration of hydration



experiments.



          All of the samples used in the hydretion experiments were supplied by



NAPCA.  These included limes derived from calcitic and dolomitic limestones which

-------
were prepared with en array of particle sizes and calcination temperatures.   The


notation used to designate samples is that used in the work statement of the


contract:  L   is the calcitic lime sample calcined at the lowest temperature and


having the largest particle size, and D   is the dolomitic lime calcined at the highest


temperature and having the smallest particle size.  A listing of specific particle


sizes end calcination temperatures is given in Table A-l of the Appendix.  Analyses


of the original limestones are given in Table A-2 of the Appendix.
 tesults
          Curves showing the observed temperature rise versus time for the various
samples investigated ere shown in Figures 22 through 31-   For both L   and D
                                                                    J J      O-J

there was very little apparent difference between hydration in tap water end


hydration in saturated lime water.  However, with partially sulfated L _


(Fig. 31), hydration in lime water was severely limited;  giving a temperature


rise of only 0.7°F in the first 30° seconds.  Hydration runs were also performed


using L__ and D__ and a liquid phase consisting of a saturated Ca,l% - SO ,SO.


liquor (Fig. 30)«  With these runs there was a rapid initial temperature rise


of about 5 F followed by extremely slow increase over a period of several hours.


As with the partially sulfated lime, the rate was too slow to be defined.  Pre-


sumably the initial temperature rise was due to reaction of the sulfite-sulfete


liquor with the lime.  The reaction product probably coated the lime particles


and prevented hydration.  In fact, there was a strong tendency for agglomeration


of the reacted lime, with resultant formation of large clumps (up to 1 cm in


diameter) of solids.

-------
 CM
o
         0
                                                 t, seconds x 10
                                     FIGURE 22.  HYDRATION OF LIME IN WATER.

-------
'.  12
                          1000
2000
3000
4000
5000
                                                             t,sec
                               FIGURE 23.  HYDRATION OF LIMESTONE-BASED LIMES  IN  SATURATED LIME WATER.
                                                                                                                       ON

-------
600
700     800
   100      200      300       kOO      500
                           t, seconds
FIGURE 2U.  HYDRATION OF HIGH CALCIUM LIMES IN SATURATED LIME WATER.

-------
8
10   12   Ik  16   18  20   22   2k   26   28   30  32   3k    36 -  38   kO

                                     -2
                      t, seconds x 10



   FIGURE 25.  RERUN OF HYDRATION OF HIGH CALCIUM LIME IN LIME WATER.
kk  k6

-------
24

22

20

18

16
                                                                          D33
12

10

 8

 6

 4

 2

 0
                       I
I
I
1
I
I
I
I
I
I
                      8   10   12  14   16   18    20   22    24    26   28    30   32    31
                                                   t,  seconds x  10"

                                   FIGURE 26.  HYDRATION  OF DOLOMTTIC LIME IN  WATER.
I     I     I     I    I     I
36  3"8   40   42  44   46

-------
0
                       1000
2000
3000
4000
5000
                                                        t,sec
                         FIGURE 27-  HYDRATION OF DOLOMITE-BASED LIMES IN SATURATED LIME WATER.

-------
                                    51
28


26

2U

22

20

18

16
12

10

 8

 6

 k

 2

 0
                        Dll
     I	I     I    I     I     I    I
                       8   10
12   14  16

 t, seconds
18  20   22   2k   26   28  30
                FIGURE 28.   HYDRATION OF DOLOMITIC LIMES
                            IN SATURATED LIME WATER.

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pt,
                                                                 I    I    I         I    I
                                                                                      I     I    I    !    I
                             10   12  14  16  Id  20   22  24   26  2«  30   32  34  36   3o  40  42  44   46   48  50
0
                                                                                                                        v/i
                                                                                                                        ro
                                                      t,  seconds x 10


                              FIGURE 29.  RERUN OF HYDRAHON OF DOLOMITIC LIME IN LIME WATER.

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20

18

16
12   -
10   _
 8

 6

 U

 2

 0
    0
                                                                                                                   OJ
8  10   12
.16
                            FIGURE 30.
 18   20  22  2k  26   28  30  32   3k  36  38  UO   U2  kk  U6  U8  50
    t, seconds x 10"^
HYDRATION OF LIME IN SATURATED SOLUTION OF
       CsSO^, MgSO , CaSO  AT 125°F.

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   20



   18



   16







   12
fc


 -.  10

S3

   8



   6







   2



   0
                                                                                L   (Sulfeted)
0  2
                      8   10  12  14   16  18  20   22  2^  26  28  30   32  3^  36  38  ^0   k2  kk  k6  k8  50

                                                            «2
                                             t, seconds x 10
                      FIGURE 31.  HYDRATION OF SULFATED LIME IK LIME WATER.

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                                        55



          The data shown in Figures 23 end 27 illustrate distinct trends in the


dependence of hydration rate on particle size and calcination temperature.


With the dolomitic limes, increasing particle size or increasing calcination


temperature clearly decreases the rate of hydration.  With the calcitic-based


limes, a similar trend was observed.  However, the data for L   end L   are con-


sidered suspect since a considerable amount of uncslcined stone was found in these


samples.  This material was a different color from the bulk of the lime end


evolved gas when reacted with HC1.  With L  , it is estimated that 30 to hO


percent of the sample was uncelcined.  For the remaining calcite-based materials,


increased particle size or calcination temperature decreases the hydretion rate.



          It should be noted that, for the limestone series, with only one


sample, L  , did the total temperature change approach the theoretical value


for reaction of one mole of calcium oxide in one liter of water, 23.4 F.  This


may be due to incomplete calcination of the particles, or the rate of reaction


in the later stages of hydration may have been too slow to be detected.  With the


dolomitic limes, the temperature change for two samples, D   end D  , exceeded the


value of 23. k F, indicating that some of the magnesium oxide in the samples may


have been hydrated in these experiments.  Upon calcinetion of the carbonate,


magnesium oxide sinters quite rapidly and becomes resistant to hydration.  It


is therefore expected that contribution to the temperature rise by hydretion of


magnesium oxide would occur only with the relatively soft-burned dolomitic limes.

                 *
          Boynton  suggests that the rate of hydration of a lime is controlled


largely by the permeance of the lime to water and the rate of diffusion of water
* R. S. Boynton, "Chemistry and Technology of Lime and Limestone", Interscience

  Publishers, New York, (1966), p 297-

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                                      56



through the leyer of hydrated lime which is formed around the particles during


the initial stages of the reaction.  This type of mechanism is consistent with the


results of the current work.  Figure 32 shows a plot of 1 - (l - F) '  versus


time (F = fraction of lime reacted) for D  .  Such a plot should yield e

                                                       *
straight line if the reaction is chemically controlled;  a curve of the type


shown is indicative of control "by diffusion within the particles.  It is reason-


able, then, that formation of e sulfate coating, either by exposure of the lime


to  SO  prior to contact with the hydrating liquor, or "by reaction between the


lime end sulfate or sulfite in the liquor, might further inhibit permeation of


water into the lime.
                                    Dissolution



Procedure



          The rates of dissolution of hydrated calcite- and dolomite-based


hydrated limes were studied in water and in a liquor saturated with CeSO ,


CaSO, , MgSO , and MgSO, .  Certain of the solutions were also saturated with CO


at e partial pressure of 77 torr.  In these experiments, a two-gram charge of


hydrated lime was added to a well-stirred isothermal vessel containing one liter


of solvent (water or sulfite-sulfate liquor), and the pH and calcium-ion activity


were monitored continuously using appropriate electrodes.  Calcium ion was detected


using an Orion liquid-liquid Junction-type electrode which was specific to calcium.
*  0. Levenspiel, "Chemical Reaction Engineering", Chapter 12, John Wiley and Sons,
   Inc., New York, 1962.

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                                          57
1.2

1.1

1.0

 .9

 .8

 .7

 .6
 i
rH
    .3

    .2

    .1

    0
                     AT.
                 F =
= 10.5°C
                       oo
             I
I
   I
I
   o  100   200   300   iioo   500  600   700  800   900  1000  1100 1200
                                                                        1500
                                 t,  seconds  x 10
                                                -2
                          FIGURE 32.  HYDRATION OF D
                                                       .

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With most of the runs, dissolution ves essentially complete within a few tens



of seconds, and except for the case of dissolution in tap water, the celcium ion



activity in solution did not change appreciably during the experiments.





Results





          Dete summarizing the essential characteristics of the dissolution



experiments are shown in Table 3-  I*1 those experiments involving the saturated



sulfite-sulfate solutions es the solvent, only the OH-activity changed during the



experiment.  This is probably due to precipitation of sulfite or sulfate on the



surface of the lime, i.e.,





                          Ca(OH)  (s) + SO"  -*  CeSO  (s) + 20H~  .
                                C.         X         Jn.



As in the hydration experiments involving the use of the sulfite-sulfate liquor,



there was a strong tendency for agglomeration of the lime particles.  Even with



tap water as the solvent, the dissolution of celcium was inhibited significantly



by contaminants in the water; the calcium ion activity never became es high as



would be predicted on the basis of the observed OH" activity.



          Rates cited in Table 3 ere average initial rates of dissolution computed



by dividing the half-time for reaction into the OH~ activity change up to thet



point.  The rate of dissolution in tap water was significantly greater than that



in the liquor.  The rate of dissolution in the liquor was further suppressed by



the presence of ,CO .  The latter fact could be due to deposition of carbonate as



well as sulfate on the surface of the particles.  The rate of dissolution of the



dolomitic lime did increase by en order of magnitude when the temperature wes



increased from 55 to 125 F.  The presence of fly ash lowered the celcium ion



activity, but hed only a minor effect on the apparent rate of dissolution.

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                                   •TABLE 3.  DISSOLUTION OF HYDRATED LIMES
Hydrated Lime
Dolomite
Dolomite
Limestone
Dolomite
Dolomite
Limestone
Limestone
Solvent
Tap water
Liquor(d)
Liquor
Liquor
Liquor + CO
Liquor + CO
(f)
Liquor + fly aahv '
T,°F
55
55
55
125
125
125
125
pOH
Initial
6.72
6.45
6.58
6.85
7.78
7.84
7.01

Final
1.47
4.69
4.76
5.88
5.88
6.03
6.44
pCe
Initial
3-35
2.76
2.43
1.80
1.65
1.65
2.68

Final
2.47
2.76
2.43
1.80
1.68
1.65
2.68
Va- ™M
16
. 15.5
7.8
1.3
11.5
18.5
1.8
Rete(c)
1 x 10~3
4.2 x 10"8
1 x 10'6
4.5 x 10~7
5.6 x 10"8
2.5 x 10"8
4.6 x 10'8
(a)  One liter at 77 F.



(b)  Time required to reach one-half final OH" reactivity.



(c)  Average rate = net change in OH" activity/2 t.. / , moles/liter-sec, for 2 grams of



(d)  Saturated solution of CaSO^, CaSO , MgSO^, MgSO .




(e)  PCQ  = 77 torr.



(f)  5 grams fly ash per liter of solution.
                                                                                                                     \r\

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                                        60



          As in the case of hydration, the rate of dissolution of lime eppeers


to be severely limited when a layer of reaction product can form eround the lime


particles.  This inhibition of hydration and dissolution mey well be the major


factor in reducing the efficiency of utilization of lime in an SO  scrubber which


receives the lime in particulate form.





                                 Analysis of Liquors
                *


          The compositions of saturated sulfite-sulfate-cerbonate solutions were


determined at 55> 90, and 125 F»  These solutions were prepared by addition of


amounts of the components considerably in excess of the published solubilities in


water.  After thorough mixing for a period of several hours, the solutions were

                                                  4-4-    4J—4-    •.•.•.
filtered at temperature,  and were analyzed for Ca  , Mg  , SO", SOT, CO"  HCO"
                                                              j    *»    j     j>


and pH.  Standard procedures were used in the chemical analyses; calcium was


determined atomic absorption, magnesium by gravimetry as the pyrophosphete, sulfite


by iodimetry, sulfate by gravimetry as the barium salt, and carbonate end bi-


carbonate by double-endpoint titration using phenolphthalein end methylorange


indicators.  Results of these determinations ere shown in Table k.  As can be


seen from the Table, the solutions are essentially magnesium sulfate solutions.


          Analyses were also made of selected liquors recovered from the SO -


sorption experiments.  These analyses were carried out using procedures similar


to those listed above, with the exception that atomic absorption was employed to


verify the absence of magnesium in the spent liquors.  Results of these analyses


are shown in Table 5-

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                                    61
                     TABLE 4.   COMPOSITION* OF SULFITE-
                               SULFATS-CARBONATE LIQUORS
Lime
Additive
Limestone
Dolomite
Limestone
Dolomite
Limestone
Dolomite
T°F
55
55
90
90
125
125
_ ++
Ca
0.
0.
0.
0.
0.
0.
03
03
03
03
Ok
03
MB"
4.97
4.90
6.11
6.18
6.07
6.13
SV
0.
0.
0.
0.
0.
0.
4o
15
43
16
37
056
SV
19.
19.
23.
24.
24.
24.
3
1
5
1
0
1
cv
0.
0.
o.
o.
0.
0.
50
18
43
16
26
10
HCO " pH
0.25 8.90
0 8.75
o 8.30
0.16 8.10
0.11 7.85
0.04 7.65
* Weight percent.

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                            62-
           TABLE 5.  COMPOSITION* OF SPENT LIQUORS
Run Ko.
k
5
12
13
Ik
15
Ca"^ Mg"^
0.2VT 0
0.167 o
0.2^3 0
0.235 0
0.082 o
0.315 o
SO; SOJ CO; HCO-
0.079 1.34 0 0
0.073 0.92 0 0
0.076 1.29 0 0
0.063 1.09 o o
0 <0.1 0 0,23
1.00 0.69 0 0
*  grains per liter.

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                                       63
                                 CONCLUSIONS
                 \

          Although it is difficult to draw meaningful conclusions from

one-of-a-kind type experiments, the following facts have been brought to light

as a result of this program,

          1.  Under the experimental conditions employed, the wet lime-SO

scrubbing process can be represented by an analytical model based on chemical

reaction until exhaustion of reactant, followed by dissolution controlled by

gas-phase mass-transfer.  For the scrubber investigated experimentally in this

program, there is little or no SO  in the outlet gas from the scrubber until the

reactant is exhausted.  The SO  concentration in the outlet gas then increases

and approaches the inlet gas concentration asywptotically.

          2.  The reaction product between SO  and the scrubbing liquor at the

breakthrough point is approximately represented by the sulfite rather than the

bisulfite.

          3.  The presence of solid lime caused serious clogging of the gas inlet

openings to the experimental scrubber.  The SO  breakthrough time is slightly

reduced, presumably because of coating of the limestone particles by sulfite

(or sulfate) crystals.  This observation suggests that full-scale scrubber

operation might be expected to experience both scaling and reduced utilization

of any undissolved solids present in the scrubber liquor,

          U,  Uptake of CO  causes the rapid initial drop in the pH of the

scrubber liquor during a run (by formation of CaCO ), but appears to have little

or no effect on the SO  breakthrough time.

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          5.  For the SO  uptake runs with slurries of hydrated material,




"breakthrough occurred significantly before the reactant was exhausted, indicating




solid-liquid mass transfer as a limiting step under these conditions.




          6.  The presence of NO in the flue gas significantly increases the




ratio of observed to calculated breakthrough times.  It has been suggested that




oxidation of sulfite to sulfate is the cause of this effect.




       7. Rates of hydration of burnt limes are controlled by diffusion of




water into the particles and hence are decreased by everburning of the lime




or use of large particles.  The effect of particle size on hydration rate is




apparently much stronger for hard-burned than for soft-burned lime.




          8.  Deposition of sulfete on the lime hinders the hydration process.




          9«  Deposition of sulfate or carbonate on the lime hinders the dis-




solution of hydrated lime.




          10.  The process of dissolution can be accelerated by increasing the




solution temperature.







                               RECOMMENDATIONS






          The current experimental work is largely of an exploratory nature,




and, as such, suggests several areas of need for better understanding and develop-




ment of the limestone SO  vet-scrubbing process.




          The results of the current experiments indicate that lime in particu-




late form reacts readily with various sulfur species or carbonate in solution




to yield a coating which inhibits utilization of the bulk of the lime.  Fine




grinding of the lime might alleviate this problem to some extent.  However,




because of the observed tendency for particles to become cemented together

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                                       65






forming large clusters, a high degree of utilization of the lime may not be




possible as long as the lime is admitted to the scrubber in particulate form.




It is therefore recommended that further consideration be given to the importance




of particle size in the overall scrubbing process.   It is further recommended




that consideration be given to the possibility of predissolving the lime, or




limestone, in the feed water to the scrubber through the use of excess CO




or other solubilizing agents.




          Further development is also needed in the area of modeling of the




overall reaction system.  The model given in this report is only a first attempt




at description of the scrubbing process, and as such does not give adequate




representation of the mechanical and chemical factors involved.  For instance,




the dependence of the equilibrium partial pressure  of SO  on solution composi-




tion is not explicit in the model given.  Also, a detailed analysis of these




data will yield only rudimentary information on the various mass transfer re-




sistances in this system, which may not be directly applicable to large-scsle




systems.







                              *•****#*#*****






          The data on which this report is based are recorded in Battelle




Laboratory Record Books No. 2175k, pp 1-^2, and No. 26980, pp. 1-57.

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                 APPENDIX
IDENTIFICATION AND COMPOSITION OF SAMPLES

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             A-l
TABLE A-l.  IDENTIFICATION OF
            HYDRATION SAMPLES
Sample
°11
\3
D23
D31
D32
D33
Ln
*13
L23
L31
I|
^^^iO
L.0
Type
Dolomitic
Dolomitic
Dolomitic
Dolomitic
Dolomitic
Dolomitic
Calcitic
Calcitic
Calcitic
Calcitic
Calcitic
Celcitic
Particle Size
(mesh range)
-325
-100 +
-100 +
-325
-200 +
-100 +
-325
-100 +
-100 +
-325
-200 +
-100 +

200
200

325
200

200
200

325
200
Calcine tion
Temperature
°F
1700
1700
2200
2700
2700
2700
1700
1700
2200
2700
2700
2700

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                               A-2
                  TABLE A-2.  CHEMICAL ANALYSES
                              OF LIMESTONES*
Component
CaO
MgO
Si02
Fe2°3
A1203
Loss on ignition
(iooo°c)
Dolomitic
Limestone
#*
30.39
21.54
1.05
0.28
0.10
46.63
High Calcium
Limestone
54.40
0.48
1.22
0.19
0.44
43.13
 *  Supplied by G&WH Corson, Inc.

**  Weight percent.

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