Chueh, P. L., Prausnitx, J. Ll., ‘[Computer Calculations for High-pressure VaDor-Liauid Eauilibria,” Prentice-Hall, EnglewoGd Cliffs, N.J.,-1968. Cohen, A. E., Hipkin, H. G., Koppany, C. R., Paper presented a t AIChE National Meeting,, Salt Lake City, Utah, May 1967. Corey, E. J., Wipke, W. T., Sczence, 166, 178 (1969). Edmister. W. C.. Hudrocarbon Process.. 47 (9). 239 11968a). Edmister; W. C., zb:d. (lo), 145 (196813). Fair, J. R., Bolles, UT.L., Nisbet, W. R., Chem. Eng. Progr., 54 (12), 39 (1958). Feigenbauni, E. A., Feldman, J., Eds., “Computers and Thought,’’l\IcGraw-HillJ Kew York, N.Y., 1963. Frank, S. hl., In “Ethylene and I t i Industrial Derivatives,” Miller, S. A., Ed., p 103, Ernest Benn, Ltd., London, England, 1968. Hanson, G. H., Hogan, R. J., Ruehlen, F. K.,Cines, M. R., Chem. Eng. Progr., 54 ( l a ) ,37 (1958). Kesler, 11. G., Parker, R. O., Chem. Eng. Progr. Symp. Sa?., 65 (92), 111 (1969). King, C. C., Trans. Inst. Chem. Eng., 36,162 (19%). King, C. J., “Separation Processes,” Chap. 13, IlcGraw-Hill, New York, N.Y., 1971. King, C. J., B,zrnBs, F. J., U.S.Patent applied for, assigned to Regents of the University of California.
Kuehn, A. A., Hamburger, ?* J.,IManagement . Sci., 9, 643 (1963). Lee, K. F., Nlasso, A. H., Rudd, D. F., Ind. Eng. Chem. Fundam., 9, 48 (1970). hlasso, A. H., Rudd, D. F., AIChEJ., 15, 10 (1969). Redlich, O., Kwong, J. K. S., Chem;lRev.,44, 233 (1949). Robinson, C. S., Gilliland, E. R., Elements of Fractional Dist,illation,” 4th ed., AIcGraw-Hill, New York, X.Y., 1950. Sherwood, T. K., “-4 Course in Process Design,” Chap. 2, MIT Press, Cambridge, Mass., 1963. Siirola, J. J., Powers, G. J., Rudd, D. F., AIChE J . , 17, 677 (1971). Tully, P. C., Edmister, W. C., ibid., 13, 1% (1967). Wilson, G. M., Advan. Cryog. Eng., 11, 392 (1966). RECEIVED for review July 2, 1971 ACCEPTED October 15, 1971 Presented at the Division of Industrial and Engineering Chemistry, 161st hleet,ing, ACS, Los Angeles, Calif., March 1971. Financial support was supplied to one of the authors (D. W. G.) by a Sational Science Foundation Traineeship and to another of the authors (F. J. B.) by Consejo Nacional de Ciencia y Tecnologia of Mexico.
Catalytic Reduction of Calcium Sulfate to Calcium Sulfide with Carbon Monoxide Thomas W. Zadick,’ Ronanth Zavaleta, and F. P. McCandless2 Department of Chemical Engineering, Montana State University, Boreman, Mont. 69’715
The reduction of calcium sulfate to calcium sulfide with carbon monoxide was studied using various catalysts. Ferric oxide, stannous sulfate, and vanadium pentoxide were found to have a pronounced catalytic effect on the reduction reaction. The ferric oxide was the most active catalyst and resulted in about 9770 reduction of the calcium sulfate in 45 min a t 680°C when a t the optimum concentration of about 9 wt Yo.The system a t 660°C showed reproducible oscillations of SO?content with time. In addition, calcium sulfide was found to autocatalytically favor its own rate of formation. A mechanism involving the formation of active carbon monoxide is postulated.
T h e reduction of calcium sulfate has been extensively studied as a first step in various processes for the winning of elemental sulfur from gypsum (George et al., 1968). A number of ieducing agents such as coal, coke, CH,, CO, and H2 can be used, but temperatures of 900’ to 1000°C with reaction times of 1 hr or more are reported as requirements to obtain near stoichiometric conversions. Apparently catalysts to promote the reduction reaction have not been investigated in the past because of the extreme conditions required to make the unpromoted reaction proceed. However, the reduction is thermodynamically feasible even a t quite lo^ temperatures, and this fact promoted this study on the use of catalysts to promote the reduction reaction. Thermodynamic Study
A brief thermodynamic study was made of various possible reducing agents using existing data. Table I summarizes this study. Several interesting conclusions can be drawn Present address, Shell Oil Co., Midland, Tex. 79701. To whom correspondence should be addressed.
from this table. First, t’he reduction reactions are highly endothermic except for CO and HI which are exothermic. Also, a favorable free energy change, A F Z , was calculated for all reducing agents above about 200”C, but a t the lower temperatures, CO is best from a standpoint of equilibrium and free energy driving force. Finally, Reactions 5 and 6 in which SO2 is liberated become t,hermodynamically feasible only a t temperatures above 1000°C. Therefore, high temperatures must be avoided if CaS is the desired product’. Experimental
All experimental runs were made in a semibatch fluidized bed reactor, the details of which are shown in Figure 1. The reactor was constructed from a 12-in. length of I-in. schedule 40 stainless steel pipe. The bottom 8 in. of the pipe were packed with small stainless steel wire rings (Fenske rings) to increase the heat transfer area, The last 4 in. constituted the fluidized bed reaction chamber, and this was contained between two porous stainless steel plates. In operation, the reactor was mounted in a tubular electric furnace capable of Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 2, 1972
283
~
Table I.
---
Reaction
+ + +
1. Cas04 2C Cas 2C02 2. Cas04 4CO C a s f 4C02 CH4 CaS COB 2H20 3. Cas04 4. Cas04 $- 4H2 Cas 4H20 5. CaS 3CaS04 -+ 4Ca0 SOB CO CaO SOZ Coz 6. Cas04
+
+
+
+
+
~
AHR, kcal/mol
AFR, kcal/mol
+ + +
~~
Calculated Free Energy Changes and Heats of Reaction
+
298'K
500'K
1 OOO'K
1500'K
500'K
1 OOO'K
f12.9 -44.3 +6.6 -17.1 +195.8 +37.9
-4.7 -44.9 -12.7 -25.9 f158.3
-48.4 -46,4 -61.2 -47.7 f65.5
-92.1 -47.9 -109.4 -69.5 -27.3 -14.1
+38.8 -44.1 +35.7 -6.0 f250.6 f51.6
+35.4 -46.2 +33.0 -12.6 f242.3 +49.0
+28.5
+6.1
again purged with nitrogen, and a slow purge continued as the reactor cooled. The product was quantitatively analyzed for sulfate using a barium precipitation method and for sulfide using an iodimetric analysis. The final form of the catalyst after reaction was not determined, although in the case of Fe2O3,the product was magnetic indicating that some Fe304 or elemental iron was formed. Results and Discussion POROUS STAINLE STEEL PLATE
PSUM BED
STAINLESS STEEL SHAVINGS
w REACTANT GAS INLET
Comparison of Different Catalysts. The catalytic effect of the compounds investigated is shown in Figure 3, which shows the conversion of Cas04 as a function of temperature for the various catalysts. Also shown is the conversion obtained using no catalyst. As can be seen, ferric oxide, stannous sulfate, and vanadium pentoxide all had a pronounced catalytic effect, while sodium carbonate, chromic oxide, nickel oxide, ferrous sulfate, and ferric sulfide exhibited a lesser catalytic activity. Also shown is the conversion for one run in which 10.5% calcium sulfide was added to the calcium sulfate charge. This shows that the reaction is autocatalytic, although the effect is less than for the other good catalysts. I n addition, a series of runs was made in which about 2 moles of water per mole of CO was also added to the reacting system. This was done t o see if an increased rate of reduction would result from "nascent" hydrogen formed from the reaction H2O CO @ HB COS. This system has been reported as being very effective for the hydrogenation of coal (Xppel and Wender, 1968). However, as can be seen from Figure 3, the water effectively increased the temperature required to obtain a given conversion. I n addition to decreasing the conversion, the addition of water also changed the appearance of the product from a gray color to light yellow. It also resulted in strong H2S odor. This indicates the partial hydrolysis of CaS to CaO and H2S in the presence of steam and the formation of some elemental sulfur. This was not investigated further since the presence of water retarded the desired reaction. Effect of Catalyst Concentration. A brief study was also made for the effect of initial catalyst concentration on the reduction reaction (Figure 4). As can be seen, there is an optimum catalyst concentration of about 9 wt % for both Fe203 and SnS04. However, about 17.5y0 VPOSis required for maximum conversion. Effect of Time. Figure 5 shows the conversion as a function of time using Fez03 as the catalyst a t temperatures of 660' and 68OoC, and for V20, a t 695OC. The run a t 66OOC using the Fe203catalyst showed a definite reproducible OScillation in SO4 content with time although none was observed a t 68OOC. This has not been explained, but it should be pointed out that all points on the curve were checked two or three times, and so it appears that the oscillation is real and not due to experimental error.
+
THERMOWELL TUBE
Figure 1. Details of reactor
heating the reactor to 1000°C. A schematic flow diagram of the apparatus is shown in Figure 2. The small metering pump was used to add water to the system in only one series of runs where a mixture of CO and water vapor was used as the reducing medium. Reagent grade (Baker, 98.5% CaSO4) anhydrous calcium sulfate powder (-100 mesh) was used for all tests, while the CO was a commercial grade (Matheson Co.). The catalysts were all reagent grade (Fisher Chemical Co.). To make a run, 3.5 grams of Cas04 were thoroughly mixed with the proper amount of catalyst using a mortar and pestle and placed in the reactor. Once the system was fitted together , nitrogen was introduced to purge the system, and this purge was continued throughout the preheat period. After the system had reached the desired temperature, CO was continuously introduced through a calibrated rotameter a t a rate of 0.022 g-mol/min. This gas rate gave a superficial gas velocity of about 4 ft/min and had previously been determined to be adequate for fluidization. After the reaction had proceeded the desired length of time, the system was 284 Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 2, 1 9 7 2
+
THERMOWELL TUBE WATER IN
VENT TO ATMOSPHERE
FURNACE
L REACTOR
MIX LINE
*
CO FLOW METER WATER
FLOW
METER
Figure 2. Flow diagram of experimental apparatus
REACTION TIME 45MINFOR Fe203 50 MIN. FOR OTHERS
, e 0 4 5 MOLE / MIN, Y O
m+-105 ,
e/o
Cas
NO CATALYST
/ 12.5"ioC5O3~./>
600
I
I
640
680
120 MIN.
CATALYST, 51 MIN. I
I
I
720 760 aoo REACTOR TEMPERATURE, "C
I
8 40
I
080
9 0
Figure 3. Comparison of various catalysts Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 2, 1972
285
autocatalytic reactions, and it may be due to increasing C a s content with time coupled with the catalytic mechanism of the Fez03 catalyst. As noted earlier, C a s was shown to autocatalytically favor its own rate of formation. The scope of the present study was exploratory in nature and limited to the investigation of several possible catalysts for the reduction of Cas04 with CO. However, the system represents a class of catalytic reactions (gas-solid reactions catalyzed by solids) which have not been extensively studied, and it is interesting to conjecture on a mechanism, -4similar study was recently conducted on the catalytic reduction of SazSOd with Hz using an Fez03 catalyst (Puttagunta et al., 1970), and the following mechanism was proposed as being feasible because the SazSOd-KanS system was in the liquid phase, and hence the reactant was mobile and readily available to the solid catalyst:
100
2t80 8
b
8 60 3 w
U
>
Z
8 40 I-
Z
w
b!
kt 20
Fez03
+ HS
(Fe and/or FeO) 2
4 6 8 10 12 14 16 WEIGHT PERCENT CATALYST
18
-
(Fe and/or FeO)
+ KazSO4
20
+ HzO
(Iron oxides)
+ NazS
At lower temperatures where the system was solid, the reaction was very slow. However, in the CaS04-Cas system the reactants and prod-
Figure 4. Effect of catalyst concentration on conversion
k?
+
I
20-
I
'I I
1
I
I
I
1
I
Figure 5. Conversion of calcium sulfate as a function of time Discussion
This research has shown that the reduction of Cas04 to C a s can indeed be catalytically promoted. This results in a very significant reduction in the time and temperature required to obtain a high conversion. A temperature of about 950°C for 1 hr is required to obtain a comparable conversion using no catalyst. Fe203was the best catalyst investigated, resulting in 9701, reduction in 45 min a t 680°C a t the optimum concentration of about 9 wt yo. The apparent oscillation noted a t 660°C using the Fe203 catalyst is intriguing although the cause is not understood a t this time; however, it probably indicates that the catalytic mechanism is quite complex. Theoretical studies of oscillatory reaction systems have been made (Lotka, 1910; Higgins, 1964) that indicate that this type of behavior is possible with 286 Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 2, 1972
uct remain in a solid phase. Hence, a reaction mechanism analogous to that proposed by Puttagunta et al. is not probable because it is inconceivable that a mechanism with a solid-solid reaction as one of the steps in series would result in an increased rate of reaction since the catalyst and reactant were only physically mixed. Rather, it seems more likely that the function of the catalyst is to produce a form of active reducing agent which, in the gaseous phase, is very efficient for carrying out the reduction reaction. A mechanism involving this active gaseous intermediate would be consistent with the principle of Sabatier and with modern theory of chemical transport reactions (Schafer, 1964). With this in mind, one experiment was run in which the Fe203catalyst was physically separated from the Cas04 by the addition of another porous stainless steel plate to the
reactor below the fluidized bed chamber, and in this test conversion was increased t o 12.5% vs. 4.1% with no catalyst under the same conditions. Although the conversion was not as high as when the Fe20s was thoroughly mixed with the CaS04, it does lend credence to the theory of a reactive reducing intermediate. One explanation for the decreased conversion when the catalyst is separated from the reactant would be that the active intermediate was largely decomposed on the porous stainless steel plate, or another equally likely explanation would be that the active species is short-lived and that the Cas04 must be in the immediate vicinity of the catalyst for the reaction to take place before the intermediate decomposes. If the active CO intermediate complex theory mere correct, there would be several plausible mechanisms involving carbonyl type compounds or mechanisms involving nascent CO.
This, hopefully, will be the subject of a future study together with a detailed investigation of the oscillation phenomena observed a t 660°C. Literature Cited
.4ppel, H. R., Wender, Irving, 156th Sational Meeting, ACS, Fuel Divibion Preprints, Vol. 12, KO.3, 220-4 (1965). George, D'Arcy R., Crocker, L., Rosenbaum, J. B., "Current Research on the Production O f Sulfur from Gy hum at the Salt U.S. Bureau Lake City 1Letallurgy Research Center of of Mines," 1larch 1968. Higgina, J., Proc. S a t . A c a d . Sci., 51, 989 (1964). Lotka, A. J., J . Phys. Chem., 14,271-4 (1910). Puttagunta, V. R., DeCoursey, W. J., Bakhshi, N. N., Can. J . Chem. Eng., 48,73-9 (1970). Schafer, H., "Chemical Transport Reactions," Academic Press, Yew York, N.Y., 1964. RECEIVED for reviex July 15, 1971 -4CCEPTED Iiovember 29, 1971
Transient Behavior of a Solar Pond and Prediction of Evaporation Rates Su bra m a nia n Pa ne harat na m Department of Chemical Engineering, Stanford rniversity, Stanford, Calif. 94305
A model has been presented for correlating and predicting the rates o f evaporation from solar ponds. The nonlinear equations governing the transient energy balance of a solar pond and conduction of heat in the ground have been solved numerically. The effect of heat transfer with the pond bottom has been found to b e significant for pond depths less than 2 ft. The model requires determination of three transport coefficients.
T h e various phenoniena associated with solar evaporat'iori from a shallow (0-5 ft' deep) pond constitut'e a comples system of heat, mass, and momentum t'ransfer. Empirical and semiempirical methods have been proposed for the prediction of rates of evaporation from solar ponds (Bowen, 1926; Penman, 1948; Bonython, 1958). Good bibliographies on this subject have been given by Hickos (1946) and Chow (1964). The number of variables affecting the evaporation rate is so large t'hat no single relationship could be found to describe all the effects. I t has been shown by Fergusoii (1952) that by t'he application of transient energy balance, it is possible to predict the diurnal variat'ioii of brine temperature and hence the evaporation rate with good accuracy in spite of many simplifying assumptions. This work is based on the same principle but provides a more general treatment by avoiding many of the assumptions made by Ferguson. Energy Balance for Solar Pond
Energy i b received by the pond as direct radiatlon from the sun and indirectly from the atmosphere. Part of this energy is reflected, part absorbed, aiid the rest is transniitted to the pond bottom. The radiatioii received by the pond bottom i> partly reflected, the remainder being absorbed by a very thin layer of soil. An appreciable fraction of the energy reflected by the pond bottom gets trapped by internal re-
flect,ioii a t the brine surface and is absorbed by the brine. The brine loses energy by reradiat'ion into space, which is supposed to be a t 0'1. Some heat is lost during the sunlight hours by conduction into t,he ground, but all of this heat is regained at night when the brine and soil surface become colder t'lian the soil sublayer. Heat t'raiisfer also takes place betFveeii the brine and surrouiiding air hy coilduction and convection; it may represent, gain or loss depeiiding on whether t,lie air or the brine is a t a higher temperature. The difference between the energy gain aiid loss is accounted for by change in the brine t'emperature and by evaporation of water. Assumptions of the Model
The brine is assumed t,o be well-mixed with respect to temperature and coniposkion (vert,ical and horizontal mixing). I3rine depth aiid composition are assumed to remain coiistant over the time period under consideration. The extent of evaporation must t'hen be small compared to the brine depth. The rate of solar evaporat,ion is typically 0.1-0.4 in. HzO,'day and for brine depths exceeding 8 in., this assumption is quite acceptable. -15y0 decrease in brine depth would increase t'he concentration of solids by 57, (if no solids precipitate) which would decrease the vapor pressure of the brine by less than 1% and have a negligible effect on the Ind. Eng. Chem. Process Des. Develop., Vol. 11, No. 2, 1972
287
