Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 4, 1979
681
Pzs = vapor pressure of pure component 2 Qi = group surface-area parameter for group i R = gas constant Ri = group volume parameter for group i V , = specific retention volume
Combination of infinite-dilution data with activitycoefficient data a t finite concentrations provides the best method for obtaining UNIFAC parameters. Since the largest deviation from ideal-solution behavior frequently occurs at infinite dilution, inclusion of the infinite-dilution datum significantly increases the information content of the input data to the minimization routine, thereby raising the quality of the UNIFAC parameters obtained. In the absence of activity-coefficient data at finite concentrations, the Flory-Huggins equation provides a valuable tool for parameter estimation. Unfortunately, the interaction parameters are not unique. For a given group interaction, several sets of interaction parameters can produce activity coefficients of equivalent accuracy. Confidence ellipses surrounding the parameter values reported can be large. The problem of non-uniqueness of UNIFAC parameters becomes even more complex when, as is often the case, four parameters (two pairs) must be found at one time. In this work, however, it was never necessary to find more than four parameters a t a time. Conclusion Gas-liquid chromatographic data are useful for estimating UNIFAC group-interaction parameters. A number of such parameters are presented here. While the accuracy of these parameters is probably not as high as that obtained from reduction of conventional vapor-liquid equilibrium data, the new parameters nevertheless appreciably extend the range of applicability of UNIFAC. Nomenclature umn= UNIFAC binary interaction parameter between groups m and n m = number of segments of polymer molecule M I= molecular weight of component 1
Greek Letters
activity coefficient of component 2 = volume fraction of component 1 = empirical energy parameter for Flory-Huggins equation
yz =
x
Superscript 05 = infinite dilution Literature Cited
Ashworth, A. J., Everett, D. H., Trans. faraday SOC.,56, 1609 (1960). Copp, J. L., Everett, D. H., Discuss. faraday SOC.,No. 15, 174 (1953). Fredensiund, A,. Gmehling, J., Michelsen, M. L., Rasmussen, P., Prausnitz, J. M., Ind. Eng. Chem. Process Des. Dev., 16, 450 (1977a). Fredenslund, A., Gmehling, J., Rasmussen, P., "Vapor-Liquid Equilibria using UNIFAC", Elsevier, Amsterdam, 1977b. Kwantes, A., Rijnders, G. W. A., "Gas Chromatography 1958", Academic Press, New York, N.Y., 1958. Letcher, T. M.. Bayles, J. W., J. Chem. Eng. Data, 16, 266 (1971). Littlewood, A. B., Phillips, C. S. G., Price. D. T., J. Chem. SOC.,1480 (1955). Locke, D. C., "Advances in Chromatography", Vol. 14, Marcel Dekker, New York, N.Y., 1976. McReynokls, W. O.,"Gas Chromatographic Retention Data", Preston Technical Abstracts Co., Evanston, Ill., 1966. Nelder, J. A.. Mead, R., Comput. J., 7, 308 (1965). Newman. R. D., Prausnitz, J. M., J. Paint Techno/., 45, 33 (1973). Othmer, D. F., Yu, E., Ind. Eng. Chem., 60, 22 (1968). Porter, P. E., Deai. C. H., Stross, F. H., J. Am. Chem. SOC.,78, 2999 (1956). Prausnitz, J. M., "Molecular Thermodynamics of Fluid-Phase Equilibria", Prentice-Hall, Englewood Cliffs, N.J., 1969. Schaefer, K., Rall, W., Wirthlindemann, F. C., 2.Phys. Chem., 14, 197 (1956). Schreiber, L. B., Eckert, C. A., Ind. Eng. Chem. Process Des. Dev., I O , 572 (1971).
Receiued f o r reuieu September 5 , 1978 Accepted June 18, 1979
The authors are grateful to the National Science Foundation for financial support.
Oxidation Kinetics of Oil Shale Char Yogendra Soni and William J. Thomson" Department of Chemical Engineering, University of Idaho, Moscow, Idaho 83843
The kinetics of the oxidation of oil shale char was studied over a temperature range of 700-1000 K and for oxygen partial pressures between 3 and 21 kPa. It was found that the reaction rate was first order with respect to both oxygen and char and that the activation energy was 97.2 kJ/mol. At temperatures above 825 K the oxidation rate was found to exhibit a shifting reaction order with respect to the char which was attributed to mass transport rate limitations. Although the oxidation rates were unaffected by acid leaching, thermally decarbonating the shale prior to oxidation resulted in a tenfold increase in the oxidation rate. It is hypothesized that the oxides present in the thermally decarbonated shale, specifically CaO, serve to catalyze the char oxidation. In addition, it was found that COP supplied by the char oxidation recarbonated CaO at a rate which was approximately two orders of magnitude higher than that observed when COP was supplied from t h e bulk gas stream. This suggests that recarbonation proceeds via an activated surface step.
Introduction When oil shale is retorted, whether by above-ground or in situ processes, complete recovery of the organic carbon contained in the shale is not achieved. Rather, a carbonaceous char is left behind which is not only an'important environmental consideration for above-ground retorting, but which also contains enough energy to supply all of the heat required for the retorting itself for oil shales assayed a t 20 gallons per ton (GPT) or above. Utilization of this energy rich char has generally consisted of combustion 0019-7882/79/1118-0661$01.00/0
although some attempts have also been made to gasify the char to a medium BTU gas (Duir et al., 1977). Further incentive to utilize this char has been supplied by the development of the in situ oil shale recovery program where a flame front generally passes over retorted shale and consumes the char for its energy. Even in the in situ applications some development work has also been conducted to gasify the char and produce a medium BTU gas (Burwell and Jacobson, 1974,1975). Because of the fact that in situ field experiments are expensive, there is great 0 1979
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Ind. Eng. Chern. Process Des. Dev., Vol. 18, No. 4, 1979
interest in the development of a working mathematical model to simulate the process. In this case knowledge of the reaction rates for the various reactions which occur during in situ recovery are an absolute necessity for a realistic and usable model. For above ground retorting it is important to control the local temperatures in that region of the retort which is undergoing char combustion in order to prevent the formation of clinkers. In this case one approach could be to lower the oxygen concentration and thereby control the rate of reaction and the rate of heat liberation. For this reason it would also be important to have a knowledge of the applicable kinetics. It is common knowledge that chars or various cokes all have different reactivities, most likely due to the manner in which the char was formed and in some cases due to the catalytic effects of minerals which might be contained within the char matrix. Consequently, we have undertaken a kinetic study of the oxidation of oil shale char to not only quantify the reaction rates under various conditions but also to examine the effects of the retorting process itself and the contributions of the mineral content of the shale on the subsequent reactivity of the char. One of the first studies of this nature was conducted by Dockter (1976), who studied the oxidation of cylindrical core samples of shales which assayed at 10-40 gallons per ton (GPT). He found that for oxygen concentrations between 7 and 21% and for temperatures between 673 and 873 K the burn depth pattern followed a diffusion control shrinking core model. This was followed by a study conducted by Mallon and Braun (1976),who experimented with large blocks of oil shale (0.15-0.25 m) and studied the oxidation as well as the reactions of COz with the char. They also found that the oxidation rate was limited by diffusion and concluded that the reaction rate of the COz (released by carbonate decomposition) with the char was significantly higher than the diffusion limited oxidation rate. Due to the possibility that the reactivity of the char could be related to the manner in which the retorting was conducted, Campbell et al. (1978) investigated the effect of particle size, retorting rate, and assay on the quantity of char produced. They concluded that only very low retort heating rates (approximately 2 OC/h) had a significant effect. They hypothesized that the char was produced from a liquid phase degradation as opposed to cracking in the gas phase and that the higher quantities of char produced at the low retorting rates were due to low rates of self-generated gas flows within the shale sample itself. This was later verified by Soni and Thomson (1978), who found that the combination of low retorting and external purge gas rates produced a higher quantity of char. Since oil shales generally contain a large percentage of carbonate minerals which can decompose at the high temperatures of char combustion, it is also important to have a knowledge of the reaction rates of the C02produced from the combustion with the char itself. This subject has been addressed by Campbell and Burnham (1978) and Burnham (1978) who found that the presence of high COz concentrations in the gas phase had a strong influence on the character of the thermal decomposition of the carbonate minerals. In addition, they found that the removal of the carbonate minerals by acid leaching caused the reactivity of the char with respect to the C 0 2 reaction to drop by more than an order of magnitude. They suggested that this observation could be due to catalytic effects of the minerals within the shale itself. With the exception of the work cited above on the C02 plus char reaction, the remaining kinetic studies of the char reactions have been conducted with relatively large-sized
'1
To
PCULN
ROT414ETERS
Figure 1. Experimental apparatus.
shale pieces. Although this is the situation which would be encountered in a commercial process, the fact that both kinetics and diffusion must by accounted for makes it difficult to obtain accurate kinetic data. As a result, the work reported here was conducted in such a manner as to attempt to eliminate all transport effects so that we could study the kinetics of the char oxidation in an unambiguous manner. In addition, it is well known that the minerals contained in oil shale have been found to catalyze some of the reactions of coal (Lewis et al., 1953). In fact, in one study (Haynes et al., 1972) found that dolomite, a mineral present in oil shale, acted to catalyze a number of the coal gasification reactions. In view of this work and the observation made by Campbell and Burnham (1978) that the mineral content of the shale could have catalytic effects, we decided to examine the effects of mineral alterations on the oxidation reaction rate of the char. Specifically, kinetic data were obtained over a temperature range of 700 to 1000 K and for oxygen partial pressures from 3 to 21 kPa. Evaluation of the effects of the minerals contained in the shale was accomplished by studying the oxidation rates in the presence of undecomposed mineral carbonates as well as with acid leached (H2S04and HCl) shales and for shales which had been thermally decomposed to the oxides. Experimental Apparatus A schematic of the experimental equipment is shown in Figure 1and additional details have been previously given by Soni and Thomson (1978). For the oxidation studies discussed here, air could be diluted with helium and fed to a reactor which was inserted into a high-temperature furnace. Approximately 2 g of previously retorted oil shale was placed in a "basket" which was then suspended on a tungsten wire which was in turn attached to a Cahn electrobalance in order to provide for a continuous measurement of the sample weight as oxidation proceeded. In addition to this, a separate measurement of the reaction rate could be obtained by sampling the gases exiting the reactor by means of an on-line gas chromatograph. The temperature of the sample was monitored by means of two chromel-alumel thermocouples, one placed just above the basket and the other just below. The reactant gases were admitted to the bottom of the reactor through a perforated, coiled stainless steel tube which acted as a sparger. Separate residence time distribution measurements indicated that the mixing phenomena approximate an ideal back-mix reactor. The chromatographic technique consisted of a carbosieve-B column operated by means of temperature programming from 300 to 425 K with a helium carrier gas flow rate of 16 cm3/min. This method was capable of baseline separation of hydrogen, air, CO, and C 0 2 over a 6-min total retention time. However, in all of the experiments which were conducted, only CO2 was
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 4, 1979
noted as a product and in this case the GC analysis time could be shortened to approximately 4 min by running isothermally at 425 K. Prior to the start of an oxidation run, the reactor was initially flushed with helium until the exit oxygen concentration dropped to the value inherent in the helium purge stream. The temperature was then raised to the desired level at which point the preselected oxygen mixture was introduced to the reactor. It typically took about 1 min before the reactor attained the inlet composition. All of the kinetic data were corrected for the measured residence time distrib,ution and since only 2-g samples were used, the exit and inlet reactant compositions were essentially equal. The oil shale was obtained from the Anvil Points area of Colorado and the nominal assays were 15 and 50 GPT. Since our goal was to obtain kinetic data in the absence of transport limitations, preliminary experiments were conducted to determine the particle size and the gas flow rates at which gas-solid mass transfer and internal diffusion were negligible. All of the experimental results reported here were conducted with a powdered shale of approximately 70 and at gas flow rates of 650 cm3/min. The latter corresponded to the maximum flow without inducing unacceptable fluctuations in the gravimetric data but was sufficient to eliminate gas-solid transport for all but the very high temperature runs. As already mentioned, a number of experiments were also conducted by altering the mineral composition of the shale. The shale “as is” contains silica, dolomite, and calcite, the latter two of which are mineral carbonates (magnesium carbonate and calcium carbonate). In order to study the char oxidation kinetics with these minerals intact, it was necessary to restrict the temperatures to below 825 K, the temperature at which the mineral carbonates begin to decompose. Studies of the oxidation rate with thermally decomposed shale were conducted by first raising the shale temperature to 950 K in flowing helium and maintaining this temperature until all of the carbonates had decomposed to the oxides. This was verified by the continuous gravimetric readings. At 950 K there was little reaction between the char and the CO, liberated during carbonate decomposition. The temperatures were then lowered to the desired values and the predetermined oxygen mixture was admitted to the reactor. To avoid the complexity introduced by carbonate decomposition during high-temperature oxidation (>go0 K) the samples can be leached in sulfuric acid which results in the production of calcium and magnesium sulfates and liberates C 0 2 in the process. In addition, this is one way of evaluating the effects of mineral composition on the char oxidation rates. Leaching the sample with sulfuric acid produces insoluble sulfates which remain with the oil shale; however, leaching with hydrochloric acid produces water soluble chlorides which are then removed from the shale sample. The acid leached samples were produced by immersion in either hot sulfuric or hot hydrochloric acid and collecting the evolved C 0 2 in a volumetric apparatus. This provided assurance that all of the carbonates had been decomposed and in all cases a material balance was found to be within 5%.
Results Most of the experimental work to be discussed here was conducted with 50 GPT assay shale which had been retorted at a fast rate using a high gas purge rate. As reported earlier (Soni and Thomson, 1978), the retorting conditions do not affect the char activity as long as the heating rates during retorting are greater than 1 K/min and the velocity of the helium purge stream is higher than
683
F ( t )-min
Figure 2. First-order analysis-carbonates present (Po, = 7.5 kPa, 50 GPT shale).
about 0.05 m/min. Below these conditions, there is a larger quantity of char produced during retorting and it has a lower activity. Because of the small quantity of char resulting from the retorting of shale assayed as low as 15 GPT, data from these oxidation experiments at high temperatures were found to be unreliable. Nevertheless, our earlier work showed the oxidation kinetics a t low temperatures (700-800 K) to be independent of assay and to be first order with respect to oxygen. In every experimental run C 0 2was the only product gas noted in the exit stream, indicating complete combustion. The experimental gravimetric data were typically analyzed in terms of standard first-order plots with respect to the char remaining on the shale. Thus, if X is the fraction of char reacted, then the applicable first-order rate expression would be d_ X - hPo,(l - X) dt In all experiments a large excess of oxygen was employed so that Po, would remain a constant for a particular run. However, there was a definite time lag from when the preset oxygen concentration was admitted to the reactor until the entire reactor reached the inlet concentration. As we showed earlier (Soni and Thomson, 19781, because of the sparger design, the reactor behaved similar to an ideally mixed vessel and thus the oxygen concentration varied with time according to PO, = PO,)"[^ - exp[-t/fIl (2) When eq 2 is substituted into eq 1 and integrated, the following equation is obtained -In (1 - X) = h(Po,)oF(t) (3) where F ( t ) = t - E [ l - exp[-t/f]] (4) This correction was typically small but not insignificant in those runs where the oxidation rates were high (high temperatures and oxygen concentrations). Figure 2 shows the results plotted in accordance with eq 2 for retorted shale which still contains its mineral carbonates. In a commercial process this would correspond to conditions where retorted shale is first exposed to
664
Ind. Eng. Chem. Process Des. Dev., Vol. 18,No. 4, 1979 F (t bmin
F(t)-min
I O
7d
I
*\
. I
,
I
,
,
2
4
6
8
,
,
IO
12
,
14
F ( t )-min
Figure 4. Effect of retorting conditions on char activity (PO,= 7.5 kPa, 50 GPT shale).
TEMP-K I
”T
-
5
K ’ x lo”
Figure 5. Arrhenius plot.
maximum gas flow rate was not sufficient to eliminate gas-solid mass transfer resistance at temperatures above 825 K and thus true kinetic data were only obtained at temperatures less than 825 K. This will be discussed in more detail below. Because of the higher temperatures associated with proposed commercial oil shale processing, particularly for in situ processing, carbonate decomposition will take place and thus it is likely that some fraction of the char oxidation will occur in the presence of thermally decarbonated shale. That is, the oxides of calcium and magnesium will be present and consequently additional experiments were conducted on thermally decarbonated shale prepared by the procedures described earlier. Figure 6 shows the raw gravimetric data for two of these runs, one at 700 K and the other at 945 K. It is interesting
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865
”I i1
.-
I
2
4
6
,
8
IO
,
I2
,
I
/
14
16
18
,
20
,
22
TIME- m i n
Figure 6. Gravimetric results for thermally decarbonated shale (Po, = 7.5 kPa, 50 GPT shale).
that in both cases the sample weight initially increases as char oxidation begins. Even more interesting is the fact that the weight increase is higher at the lower temperature. Evidently the CaO present in the sample is recombining with the C 0 2 produced by char oxidation to reform the carbonate (CaO + C 0 2 CaCOJ. Not only does this account for the initial weight increase but also for the larger weight increase at the lower temperature. This latter effect is apparently due to equilibrium considerations since lower temperatures favor carbonate reformation. As far as the influence of the oxides on char oxidation is concerned, analysis indicates that the initial oxidation rates in the presence of CaO are much higher than in the presence of CaC03. For example, in both the runs shown in Figure 6, substantial quantities of COz were also measured in the exit stream. This indicates that the char oxidation is occurring at a rate which is even higher than that demonstrated by the rapid weight increase during the first two minutes of the run. This was verified by calculating the oxidation rate from the gravimetric measurements and the rate of production of COSmeasured in the exit gas stream. The results of these calculations are shown in Figure 7 for the run at 700 K. The oxidation rate at the same temperature but in the presence of CaC03 is also given for comparison purposes. It is interesting that not only is the initial rate an order of magnitude higher in the presence of CaO, but that it rapidly decreases to values less than those measured in the presence of CaC03. However it should be noted that after 5 min the thermally decarbonated shale had lost about 60% of its carbon char while the other had only lost 10%.
-
Discussion During the presentation of the results it was pointed out that the high-temperature rate constants did not follow the Arrhenius plot at low temperatures. Furthermore, the data a t high temperat,ures and high conversions consistently fell below the apparent first-order line with respect to char. That these are both manifestations of significant gas-solid mass transport resistances will now be discussed. First of all, it is well known that rate constants which are only slightly dependent on temperature are an indication of mass transport limited rates, particularly when this occurs a t high temperatures (Smith, 1970). To see that mass transport limitations can also cause deviations from first-order behavior we need only consider the steady-state equivalency of mass transport and chemical reaction rates, or (5)
5
10
15
EZJ
25
TI ME-min
Figure 7. Oxidation rates for carbonated and thermally decarbonated shale (Po, = 7.5 kPa, T = 700 K).
where k , is the gas-solid mass transport coefficient, k is the kinetic rate constant, C, is the char concentration and (P0,)S is the oxygen partial pressure a t the solid surface. If eq 5 is solved for (Po )s and then substituted into the far right-hand side of t i e equation, r becomes
At constant Po, and temperature we need only refer to the bracketed term in order to explain the observed data. First of all, at high temperatures k will be large, the bracketed term will become independent of C, (zero order), and the rate depends only on k,. However as the reaction proceeds, C, decreases and k C , / k ,
