I n d . Eng. Chem. Res. 1988,27,2123-2126 ture; Wexler, A., Ed.; Reinhold: New York, 1963; Vol. 3. Miura, T.; Ohtani, S.; Maeda, S. “Heat and Mass Transfer to and from Spray“. In Drying ’80, Mujumdar, A. S., Ed.; Wiley: New York. 1980; Vol 1, pp 351-356. Papadakis, S. E. “Air Temperatures and Humidities in Spray Drying”. Ph.D. Dissertation, University of California, Berkeley, 1987. Papadakis, S. E.;King, C. J. “Air Temperature and Humidity Profiles in Spray Drying. 1. Features Predicted by the Particle
2123
Source in Cell Model”. Znd. Eng. Chem. Res. 1988, preceding paper in this issue. Wexler, A.; Brombacher, W. G. “Methods and Measuring Humidity and Testing Hygrometers”. Natl. Bur. Stand. Circ. (US‘.) 1951, 512.
Received for review December 28, 1987 Revised manuscript received July 13, 1988 Accepted August 3, 1988
Coprecipitation of Organic Acids with Calcium Sulfite Solids Rosa Ruiz-Alsop and Gary Rochelle* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712
Coprecipitation of dicarboxylic acids with calcium sulfite was studied a t conditions typical of limestone slurry scrubbing for flue gas desulfurization. The effects of solution composition were modeled by equilibrium using the calculated activity of the ion pair of the calcium salt of the organic acid. The ratio of the organic acid concentration in (CaS03)o.e(CaS04)o.2-1/2H20 solids to ion pair activity a t 55 “Cwas 6.9,13.2, and 194 for adipic, glutaric, and succinic acids, respectively. These relationships are reflected in reduced coprecipitation a t lower dissolved calcium and pH. Addition of carboxylic acids to limestone slurry scrubbers for flue gas desulfurization (FGD) enhances SOz removal and limestone utilization (Rochelle, 1983; Cmiel and Seeman, 1987; Chang and Brna, 1987). The organic acids buffer the scrubber solution, enhancing liquid-phase mass transfer. The use of adipic acid was first proposed by Rochelle (1977), and since early 1981, more than six different organic acids have been tested in full-scale scrubbers. Adipic acid and dibasic acid are now used in a number of commercial facilities. Dibasic acid (DBA) consists of a mixture of dicarboxylic acids, mainly adipic (C&,glutaric (C&,and succinic acids (C4),and is a waste from adipic acid production. Since DBA is equivalent in effectiveness, but costs about 50% less than adipic acid, it is themost widely used organic acid and is currently being used in about 10 plants (Jarvis et al., 1987). The cost of using organic acid additives in a limestone slurry scrubber results from losses by three mechanisms: chemical degradation, liquid entrainment with waste solids, and coprecipitation with waste solids. Coprecipitation can be the dominant loss mechanism in systems without forced oxidation. This paper is an experimental study of organic acid coprecipitation with calcium sulfite. Jarvis et al. (1982) studied the coprecipitation of adipic acid with CaS03/CaS04solids and showed that adipic acid precipitates as a solid solution with calcium sulfite hemihydrate and that gypsum (CaS04.2H20) solids have a negligible content of adipic acid. They also found that the ratio of adipic acid in the CaS03/CaS04 solids to adipic acid in solution varied with the sulfate content of the solids (oxidation) and the pH of the solution. The only data available for coprecipitation of other organic acids were obtained by Radian Corporation (Jarvis et al., 1987). They measured the organic acid content of the solids by measuring the total organic carbon content of the solids. Their data suggest that glutaric and succinic acids coprecipitate to a greater extent than adipic acid. This study investigated the coprecipitation of organic acids as a function of pH, calcium chloride concentration, and organic acid concentration. The study was carried out in a semibatch bench-scale reactor. The organic acids tested were DBA, adipic acid, and glutaric acid. Six s a m ples from industrial scrubbers were also analyzed and the 0888-5885188/2627-2l23$01.50 J O
results compared with the bench-scale data.
Theory At typical wet scrubbing conditions, organic acids should not precipitate with the waste solids as a pure calcium salt. Calcium adipate has a solubility of 0.11 M at 55 OC (Hatfield and Potts, 1972), and typical concentrations of adipic acid in a scrubber system are in the range 0.01-0.02 M. Nevertheless, adipic acid coprecipitates as a solid solution with calcium sulfite (Jarvis et al., 1982, 1987). Previous work with solid solutions of calcium sulfate or calcium adipate in calcium sulfite hemihydrate have suggested that the composition of the solid phase is determined by thermodynamic equilibriumrather than kinetics. Jones et al. (1976) found that the sulfate content of calcium sullite hemihydrate solids was a function of CaS04 activity in the solution, but not of precipitation rate or other variables that would suggest any effect of kinetics. In previous work with organic acids, Jarvis et al. (1982,1987) found that the organic acid content of the solids was generally a function of solution composition, but not of precipitation rate. We propose to show that the effects of solution composition on coprecipitation of organic acids into calcium sulfite hemihydrate are modeled by thermodynamic equilibrium. With this hypothesis, the total organic acid content of calcium sulfite hemihydrate solids should be a function only of the activities in the solution of calcium sulfite and of the calcium salt of the organic acid. Because calcium sulfite activity will always be near its saturation value, the solid composition may be a function only of the activity of the calcium salt of the organic acid. In this work, we successfully correlated the solids composition as a function of solution composition by the simple relationship csolids
= Kaliquid
where Csolidsis in millimoles of total organic acid per kilogram of (CaSO~)o.s(CaSO~)o.z~’/zHzO, aliquid is the calculated activity of the calcium adipate ion pair, and K is the distribution coefficient in millimoles/kilogram of solid. Changes were made to the Bechtel-modified Radian equilibrium program (Lowell et al., 1970; Epstein, 1975) to permit calculation of the appropriate activity. The 0 1988 American Chemical Society
2124 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
same ratios between activity of the calcium organic acid salt and the organic acid concentration calculated for adipic acid were used for succinic and glutaric acids.
0.8
0.7 0.6 Ca Adipate
0.5
&i!%$iE
0.4
Acid Conc
0.3
0.2 0.1 0 5
6
PH
Figure 1. Calculated activity of calcium adipate in solution, y of calcium adipate = 1, gypsum saturation = 1,calcium sulfite saturation = 1.
distribution of the different adipic acid species in solution is determined by the equilibrium reactions
CaAd
KO
Ca2++ Ad2-
KO=
aCaz+aAd-
(3)
~
aCaAd
and the mass balance equation
If eq 1-4 are combined, the following expression can be obtained for the activity of the calcium adipate ion pair:
aH+ ?'HAd-Ka2
+
aH+2 ?'H2AdKazKal
)
(5)
From eq 5, it is clear that the activity of the calcium adipate ion pair will be a function of the pH, the Ca2+ content of the solution, and the ionic strength of the solution. The activity and activity coefficients of the different species in solution were calculated by using the Bechtelmodified Radian equilibrium program (Lowell et al., 1970; Epstein, 1975). The program was run using an activity coefficient of 1.0 for the calcium adipate ion pair and assuming that the solution was saturated to gypsum and calcium sulfite hemihydrate. The values used for the solubility products of gypsum and calcium sulfite hemihydrate a t 55 "C were 2.1 X and 2.67 X lo-' M2, respectively. The values used for Kal,Kat)and KOwere 6.35 X 1.16 X and 3.18 X M, respectively. The rest of the parameters were the same as those used by Epstein (1975). Figure 1can be used to estimate the calculated ratio of the activity of the calcium adipate ion pair and the total concentration of adipic acid in solution as a function of the solution pH at three levels of chloride (0, 0.3, and 1 M) and for a magnesium concentration of 0.1 M. The activity of the calcium adipate in solution decreases with pH at all chloride levels and is a strong function of the calcium chloride content of the solution. The presence of 0.1 M Mg (Le., a very low calcium concentration) also decreases the activity of the calcium adipate ion pair. The
Experimental Apparatus and Procedure A simple batch crystallizer was used to study coprecipitation. The reaction consisted of a 800-mL flask with a water jacket. The temperature of the reactor was controlled within 2 "C by circulating hot water from the water bath using a peristaltic pump. The reactor was equipped with a mercury thermometer and a pH probe and was blanketed with nitrogen to prevent oxidation. Magnetic stirrers provided agitation in the reactor and water bath. In a typical experimental run (55 "C, pH 5.7), 400 mL of distilled water containing 10 mM adipic acid, 10 mM glutaric acid, 10 mM Na&30r, 3 mM Na2S03,and 280 mM C1- (added as CaC12.2H20)was placed in the reactor. The pH was adjusted to 5.7 by adding 1N HC1. The reactants, 25 mL of 1 M CaC1, and 25 mL of 0.3 M Na2S04/0.7 M Na2S03,were fed to the reactor with a double-head peristaltic pump, at a flow rate of about 1mL/min (*lo%). During the course of the reaction, the pH was controlled within 0.1 unit by adding small amounts of 1 N HC1 or 2 N NaOH as needed. This experimental procedure is only an approximate simulation of calcium sulfite crystallization in limestone slurry scrubbing. The use of Na2S02and CaC12results in accumulation of about 0.125 M NaC1. The calcium sulfite precipitation rate (0.0025 mol/(L.min)) is comparable to commercial practice. However, the solids concentration (0.06 mol/L) is about 10 times less than commercial practice and could result in somewhat higher calcium sulfite supersaturation. Liquid samples were taken from the reactor before starting to add the reactants and at the end of the reaction. At the end of the experimental run, the solution was vacuum filtered and the solids collected were washed with 300 mL of distilled water and dried in a vacuum oven at 150 O F for about 2 h. Experiments were performed with equimolar mixtures of adipic acid and glutaric acid made from pure acids and with DBA obtained from Du Pont. Five slurry samples from four industrial scrubbers (Southwest, Seminole, San Miguel, and Indianapolis) were also studied. Analysis The liquid and solid samples were analyzed for the organic acids by ion chromatography exclusion (Dionex 2000i/SP system). A HPICE-AS1 separator column with a Ag+-form packed bed suppressor was used. The eluent was 1 mM HC1 at a flow rate of 1.1cm3/min. The solid samples were dissolved by titration to the colorimetric end point with 1 N iodine. The resulting slurry was fiitered and the filtrate was diluted and analyzed by HPICE. Analysis of samples of CaS03.1/2Hz0 with known amounts of organic acids added gave accuracy of better than 2.5% when the concentration of organic acids was of the same order as the concentrations encountered in most of the experimentalruns (60 mmol/kg for glutaric acid and 15 mmol/kg for adipic acid). When low levels of acid were added (comparable to the levels encountered in some of the industrial samples), the accuracy of the method decreased to about 11%. This reduced accuracy resulted from tailing of the sulfate peak into the organic acid peaks. This effect was estimated and subtracted from the organic acid peaks. The liquid samples were also analyzed for Ca2+content by atomic absorption spectrophotometry.
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2125 Table I. Effect of Acid Concentration on Coprecipitation" adipic acid sample
pH C1-, mM Ca2+,mM 4 5.7 282 144 10 5.7 282 144 11 5.7 282 144 13 5.7 282 144 DuPont DBA 5.7 282 144 282 144 Du Pont DBA 5.7 Southwest 5.8 32 Seminole 5.4 245 San Miguel Ab 5.8 19 San Miguel Qu 5.4 72 Indianapolis 30 32.5 6 5.0 282 144 9 4.5 282 144 14b 4.5 282 144 5 4.0 282 144 16 5.7 282 144 15 5.7 282 144 3 5.7 1408 877 7 5.7 99 66 8 5.7 62 37
oxidation, % 32 32 34 33 31 31 48.3 24.8 57.7 56.1 5.6 33.0 40.6 34.1 56.1 22.3 59.0 35.7 32.0 33.1
Cliquid,
glutaric acid C,tid,
mM 9.2 3.3 1.1 0.3 0.5 0.9 0.3 0.20
mM 5.4 1.9 0.67 0.15 0.32 0.65 0.12 0.13
mmol/kg 39.0 12.3 4.7 1.0 2.9 6.2 15.1 3.0
2.03 9.4 9.6 10.0 9.5 9.9 9.4 8.2 10.2 8.8
7.3 4.6 2.9 3.0 0.86 5.9 5.6 6.9 5.1 3.5
3.78 25.2 9.8 7.6 4.3 43.6 40.5 47.9 34.5 21.9
K
Cliquid,
%quid*
CmW,
mM 9.2 3.4 1.3 0.2 2.51 4.72 1.55 0.36 4.9 5.5
mM 5.4 2.0 0.8 0.1 1.5 2.9 0.63 0.23 1.17 2.74
mmol/kg 72.5 41.0 16.5 2.7 20.2 42.0 24.5 21.2 17.3 23.0
13.4 20.5 21.2 19.3 13.6 14.5 38.9 92.2 14.8 8.4
9.7 10.8 11.3 9.1 9.6 10.8 7.8 9.4 8.7
4.8 3.2 3.4 0.83 5.7 6.4 6.6 4.7 3.4
65.4 28.6 28.7 17.4 77.4 96.2 74.7 65.8 53.8
13.6 8.9 8.4 21.0 13.6 15.1 11.4 14.0 15.8
7.2 6.5 7.0 6.7 9.1 9.5 125.8 23.1
5.5 3.4 2.5 5.0 7.4 7.2 6.9 6.8 6.3
K
"55 OC; 23 min of reaction time; K = Clolib/atiquid, Caolida= mmol of organic acid/kg of (CaS03)o~8(CaS04)o,z~*/zHz0, aliquid = calculated activity of calcium adipate pair. 100 min of reaction time.
Table 11. CoDreciDitation of Succinic Acid from DBA' sample Du Pont DBA Du Pont DBA Southwest Seminole San Miguel Ab San Miguel Qu
PH 5.7 5.7 5.8 5.4 5.8 5.4
Ca2+,mM 144 144 32 245 19 72
oxidation, % 31.3 31.0 48.3 24.8 57.7 56.1
mM 0.97 1.82 0.042 0.17 0.35 0.21
mM 0.57 1.1 0.014 0.12 0.08 0.10
cliquid,
aiiquid,b
Csotid.,
mmol/kg 108 219 6.3 4.5 11.7 15.3
K 189 199 423 38 140 146
" K = Cdolid./aljquid, aliquid = calculated activity of calcium adipate pair CSolib= mmol organic acid/kg (CaS03)o,s(CaS04)o,z~1/zHz0 *Using same ratio between atiquid and Cfiquid as for adipic acid. 50
Total Adipic Acid in Solids (mmoi/kg)
100
A
40t
i
80 Oxldatlon Chlorlde A Southwest A Semlnole X lndlanapolls 0
0
0
1
2 3 4 5 6 Ca Adipate Activity (mM)
Total Glutaric Acid in Solids ("OW)
Coprecipitation of adipic acid with (CaS03)o,s(CaS04)o.z~*/zHz0 solids.
Results The effects of pH, chloride content, organic acid concentration, and solids oxidation were studied. The pH was varied from 4.0 to 5.7, the chloride concentration from 54 to 1410 mM, the organic acid concentration from 0.1 to 10 mM, and the solids oxidation from 22 to 59 mol 9%. All the experimental runs were made at 55 OC. Most of the experimental runs were made using adipic and glutaric acids as the organic acid additives, but two runs were made with Du Pont DBA. The organic acid content of the solids was calculated based on (CaS03)o.8(CaS04)o.2.1/2Hz0 solid solution, assuming negligible coprecipitation with gypsum. The effect of solids oxidation reported by Jarvis et al. (1982) was not observed in this study because all of the experiments included excess gYpsum. The experimental results obtained for adipic and glutaric acids are presented in Table I. The values of the activities of the calcium organic acid ion pair calculated by wing the
Chlorlde
6o
0
4o
A A
X
20
7
Figure 2.
Concentratlon
0
1 2 3 4 5 6 Ca Giularate Activity (mM)
Oxldatlon Semlnole
Southwest San Mlguel
7
Figure 3.
Coprecipitation of glutaric acid with (CaS03)o,8(CaS04)o.z~1/zHz0 solids.
200
. Thls work
Total Succinlc Acid in Solids (mmoi/kg)
150
0
100
.
50
.
Jarvls, (987
San Mlguel Southwest A Semlnola 0
0 0
0.2 0.4 0.6 0.8 1 Ca Succinate Activity (mM)
1.2
Figure 4.
Coprecipitation of succinic acid with (CaSOs)o,8(CaS04)o,zJ/zHz0 solids.
Radian equilibrium program also appear in Table I. As shown in Figures 2 and 3, the coprecipitation data for adipic acid and glutaric acid are correlated quite well by the simple equilibrium model. Data corresponding to examination of the four important variables are indicated
2126 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
as such in Figures 2 and 3 as “concentration” of total organic acid, “pH”, solids “oxidation”, and “chloride” concentration. The model correctly predicts higher solids concentrations as pH varies from 4.0 to 5.7, chloride varies from 0.06 to 1.4 mol/L, and total organic acid varies from 0.3 to 10 mmol/L. We would not expect an effect of solids oxidation (fraction sulfate) over the range studied here (22-59%), but the data of Jarvis et al. (1982) would suggest higher solids concentrations of organic acids at solids oxidation of less than 20%. The coprecipitation of adipic acid is correlated quite well, with a distribution coefficient of 6.9 corresponding to the ratio of solids composition to calculated ion pair activity (Figure 2). Also shown in Figure 2 are analytical results with three of the industrial samples and the data reported by Jarvis et al. (1987). The Jarvis data indicate somewhat less coprecipitation than measured in this study. The industrial samples deviated somewhat from the experimental runs; the Southwest and Seminole samples indicated higher coprecipitation, while the Indianapolis sample gave lower coprecipitation. It must be noted that some of the industrial samples contained extremely low concentrations of adipic acid in solution, nearly the limit of detection of the ICE, so the error in these measurements was higher than in the experimental runs (of the order of 11%). Also, the ionic strength of the industrial samples solutions was not known exactly, so the activity calculated by using the Radian equilibrium program is only approximate. Coprecipitation of glutaric acid was correlated with a solids concentration to ion pair activity ratio of 13.2 (Figure 3). Data from Jarvis et al. (1987) and results with the industrial samples, with the exception of the Seminole sample, seem to agree with the pure acid results. Finally the experimental results obtained for succinic acid (when DBA was used) are presented in Table I1 and plotted in Figure 4. The K value for succinic acid is 194. Also shown in Figure 4 are the data from Jarvis et al. (1987) and analytical results with three industrial samples. The Jarvis data indicate somewhat lower coprecipitation for succinic acid. These results can be used to predict losses of organic acid additives from limestone slurry scrubbing systems. For example, a t the conditions of sample 4 (pH 5.7, 144 mM Ca”, 9.2 mM total adipic acid), the measured and predicted concentration of adipic acid in the waste solids is about 40 mmol/kg of solid solution. If the waste was 50% solids, the loss of adipic acid in the solution would be 9.2 mmol/kg of solids. Therefore, loss of adipic acid by coprecipitation is 4 times greater than loss as solution entrained with the waste solids. In general, the expected losses of organic acid by coprecipitation will be directly proportional to total organic acid in the solution and will increase with increased pH, increased chloride concentration, and reduced magnesium concentration. The primary component of DBA (dibasic acid) is glutaric acid, which coprecipitates almost twice as much as adipic acid. The small portion of DBA which is succinic acid will be very quickly lost by coprecipitation. Therefore, DBA losses will be about double those of pure adipic acid. Even though adipic acid is about double the cost of glutaric acid, it may be economically competitive in systems where losses are dominated by coprecipitation. Conclusions The effect of solution composition on the coprecipitation of organic acids with calcium sulfite/sulfate solids can be modeled as an equilibrium process. The total concentration of organic acid in the solids is proportional to the
activity of the organic acid calcium salt in solution. Reduced pH decreases the coprecipitation of the organic acids because it decreases the activity of the organic acid calcium salt in solution. Increased calcium chloride concentration increases coprecipitation by increasing the activity of the organic acid salt in solution. High levels of magnesium in solution reduce coprecipitation by reducing the activity of the organic acid salt in solution. This reduction of the activity is due to the lower concentration of calcium ion in solution. In the range studied (22-59%) oxidation does not affect coprecipitation of organic acids. In this study, the organic acid concentration in the solids was calculated based on the weight of (CaS03)o.s(CaS04)a2.1/2H20 in the solids and not based on the total solids. In other words, it assumed a negligible coprecipitation with gypsum. The ratio between the organic acid concentration in the solids and the activity of the organic acid salt in solution was 6.9 for adipic acid, 13.2 for glutaric acid, and 194 for succinic acid. Acknowledgment
This work was sponsored in part by Peabody Process Systems, now a subsidiary of Flakt, Inc. Registry No. Adipic acid, 124-04-9; glutaric acid, 110-94-1; succinic acid, 110-15-6; calcium sulfite, 10257-55-3; calcium chloride, 10043-52-4; calcium sulfate, 7778-18-9; magnesium, 7439-95-4; sulfur dioxide, 7446-09-5.
Literature Cited Chang, J. C. S.; Brna, T. G. “Enhancement of Wet Limestone Flue Gas Desulfurization by Organic Acid/Salt Additives”. In Proceedings: Tenth Symposium on Flue Gas Desulfurization;EPA: Washington, DC, Feb 1987; EPA-600/9-87-004a, pp 6-10-6-24. Cmiel, R.; Seeman, D. “Three Years of Organic Acid Use at San Miguel”. In Proceedings: T e n t h Symposium on Flue Gas Desulfurization; E P A Washington, DC, Feb 1987; EPA-600/9-87004a, pp 6-1-6-9. Epstein, M. “EPA Alkaline Scrubbing Test Facility: Summary of Testing through October 1974”. Report EPA-650-2-75-047,1975; EPA, Washington, DC. Hatfield, J. D.; Potts, J. M. “Removal of Sulfur Dioxide from Stack Gases by Scrubbing with Limestone Slurry: Use of Organic Acids”. In Proceedings of Second International LimelLimestone Wet-Scrubbing Symposium; EPA: Washington, DC, 1972; APTD-1161, p 63. Jarvis, J. B.; Owens, D. R.; Stewart, D. A. “Comparison of the Effectiveness of FGD Additives for SOz Removal Enhancement and Additive Consumption”. In Proceedings: T e n t h Symposium on Flue Gas Desulfurization; EPA: Washington, DC, Feb 1987; EPA-600/9-87-004a, pp 6-25-6-44. Jarvis, J. B.; Terry, J. C.; Schubert, S.A.; Utley, B. L. “Effect of Trace Metals and Sulfite Oxidation on Adipic Acid Degradation in FGD Systems”. Report EPA-600/7-82-067, Dec 1982; EPA, Washington, DC. Jones. B. F.: Lowell. P. S.: Meserole. F. B. “Exaerimental and Theoretical Studies of Solid Solution’ Formation in Lime and Limestone SCIl Scrubbers, Vol. I.”. Report EPA-600/2-76-273a (NTIS No. PB264-953), Oct 1976; EPA, Washington, DC. Lowell, P. S.; Ottmers, D. M.; Schwitzgebel, K.; Strange, T. I.; Deberry, D. w. “A Theoretical Description of the Limestone Injection-Wet Scrubbing Process”. Report APTD-1287 PB 1931-029, 1970; EPA, Washington, DC. Rochelle, G. T. “The Effect of Additives on Mass Transfer in CaC03 and CaO Slurry Scrubbing of SOz from Waste Gases”. Znd. Eng. Chem. Fundam. 1977, 16,67-75. Rochelle, G . T.
