Energy & Fuels 1987,1, 320-323
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Intrinsic Kinetics of Oxidation of Residual Organic Carbon in Rapidly Pyrolyzed Oil Shale F. D. Fujimoto, R. L. Braun," R. W. Taylor, and C. J. Morris Lawrence Livermore National Laboratory, Livermore, California 94550 Received November 18, 1986. Revised Manuscript Received April 3, 1987 The intrinsic kinetics of reaction of oxygen with carbonaceous residue in rapidly pyrolyzed Colorado oil shale were determined by a series of isothermal fluidized-bed experiments in the temperature range 384-474 "C. The char was produced from 100 L/Mg (24 gal/ton) oil shale by rapid heating to 530 "C and isothermal pyrolysis for 5 min. The oxidation rate, determined by mass spectrometer measurements of evolved CO and COz, has second-order dependence on the unreacted char. The activation energy is 81.2 kJ/mol and the preexponential factor is 1.33 s-l Pa-', assuming first-order dependence on oxygen. Rate measurements at temperatures higher than approximately 475 "C are precluded by interference of COz that is evolved from carbonate capture of SOz. A lower heating rate during pyrolysis (i.e., a lower effective pyrolysis temperature) significantly decreases subsequent oxidation reactivity of char. Introduction During retorting of oil shale, kerogen is pyrolyzed to produce oil vapor, hydrocarbon gases, and carbonaceous residue (char). In an efficient retorting process this char will be burned to provide energy for heating raw shale. For modified in situ retorting, the overall rate of char combustion k largely controlled by diffusion of oxygen into shale particles, because of the relatively large partice size and high combustion temperature. The effective oxygen diffusivity has been studied by Mallon and Braun' and Dockter and Turner.z Conversely, for aboveground oil shale retorting, chemical kinetics may control the rate of char combustion, because of the small particle size and relatively low combustion temperature. The intrinsic kinetics of char oxidation have been reported by Thompson and Son? and Sohn and Kim.4,5 However, in those studies char was produced through a slow heating cycle such as that experienced during in situ processing. In aboveground retorting processes that use hot solids as the heat-transfer medium, raw shale is heated very rapidly to pyrolysis temperature (500-550 "C), producing char of unknown reactivity. The objective of this paper is to report results of our investigation to determine intrinsic combustion kinetics of the organic carbon in char produced by rapid pyrolysis. Chemical Considerations Oil shale char is composed primarily of organic carbon and hydrogen,with minor amounts of nitrogen, sulfur, and oxygen. Char oxidation may be characterized by char O2 C02 CO H 2 0 SO2 NO + NH3
+
-
+
+
+
+
t1)
Thus, by following the rate of evolution of CO and C02, one can determine the rate of combustion of carbon in char. We nominally refer to this as the rate of char combustion, since the organic carbon constitutes the primary fuel component of char. This is not meant to imply that the other char components (H, S, and N) necessarily ox(1) Mallon, R. G.; Braun, R. L. Oil Shale Symp. R o c . 1976, 9th,
309-333.
(2) Dockter, L.; Turner, T. F. In Situ 1978,2, 197-215. (3) Thompson, W. J.; Soni, Y.In Situ 1980, 4 , 61-77. (4) Sohn, H. Y.; Kim, S. K. Ind. Eng. Chem. Process Des. Deu. 1980, 19,550-555. (5) Kim, S.K.; Sohn, H. Y. Ind. Eng. Chem. Process Des. Deu. 1985, 24, 506-507.
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idize at the same rate as the organic carbon. Colorado oil shale contains approximately 45 % carbonate minerals. Carbon dioxide from carbonate decomposition can interfere with char combustion rate measurements based on COz evolution. The most abundant carbonate minerals, dolomite, ankeritic dolomite, and calcite, decompose only slowly at temperatures below 600 "C, so it is unlikely that they will have any real effect on the experimental results other than to increase the level of background COz and CO. Other mineral carbonates, such as dawsonite and nahcolite, are only present in small quantities and decompose at low temperatures. There is not significant contribution from these species, since they are decomposed during the oil shale pyrolysis step. Retorted shale also contains pyrrhotite (FeS) and pyrite (FeSJ, which can react with oxygen to form SO, and Fe203: FeS + FeS2 + O2 Fe203 SO2 (2) The released SOzcan be captured on the surface of mineral carbonates where it undergoes further reaction with oxygen to release C02 and to form the respective mineral sulfate: SOz + O2 + MC03 MSOl + C02 (3) In agreement with eq 3 and previous work: oxidation of our shale at temperatures above 475 dC resulted in a decrease in the amount of evolved SO2and a corresponding increase (above background) in the amount of evolved COP Oxidation at 475 "C, however, resulted in evolution of SOz equal to the total sulfur content of the retorted shale. Therefore, we restricted our experiments to temperatures below 475 "C to avoid interference of C 0 2 evolution from carbonate capture of SO2. A third potential interference in basing the char combustion rate measurements on COP+ CO evolution is the chluC02reaction. At combustion temperatures below 475 "C, however, the reaction has a negligible rate compared with that of the char-0, reaction.
-
+
Experimental Section A schematic of the experimental apparatus is shown in Figure
1. The fluidized-bed reactor was built from 316 stainless steel (diameter 4.5 cm and length 37.5 cm). The bed was 20 g 40-70 mesh quartz sand, which provided a nearly constant temperature bath for raw-shale retorting and retorted-shale combustion. A (6) Taylor, R. W.; Burnham, A. K.; Mallon, R. G.; Morris, C . J. Fuel 1982, 61,781-782.
0 1987 American Chemical Society
Energy &Fuels, Vol. I , No. 4, 1987 321
Rapidly Pyrolyzed Oil Shale ,-Raw
shale hopper
Gas
outlet
Time (s)
Figure 2. Normalized reaction rates for oxidation of residual organic carbon produced by rapid pyrolysis. Conical gar.distributor plate
Figure 1. Schematic diagram of experimental apparatus. conical gas-distributor plate was very effective in eliminating stagnation regions between gas-injection ports. The fluidized bed was heated externally by an electric furnace. An Analog Technology Corporation Model 2001 mass spectrometer was used to measure the concentrations of Oz, CO, COz,and SOz in a portion of the gas taken approximately 7 cm above the sand bed. The gas compositionwas recorded by computer as frequently as every 0.2 s. Each experiment used 0.5 g of 40-70 mesh raw shale with a nominal yield of 100 L/Mg (24 gal/ton) from Tract Ca of the Green River Formation. Most of the experiments were carried out by using raw shale with no further treatment. Some experiments were performed with shale decarbonated by stirring it in 6 M HCl at ambient temperature for 12 h, a procedure taken from Goklen et al.' Char was produced by dropping raw shale into 530 "C sand fluidized with argon flowing at 6 L/min. Retorting was continued for 5 min, and the reactor was then cooled in an argon atmosphere to the desired oxidation temperature. The approximate composition of the char produced by this procedure is CH0.6N0.06Oo.&30,0t.After a background measurement of the reactor off-gas composition, a mixture of approximately 13 vol % oxygen and 87 vol % argon was introduced into the reactor. An overall gas flow rate of 10 L/min was used, the minimum with which flowindependent reaction rates could be obtained. Elutriation of the retorted shale at this flow rate was not significant. When oxygen was introduced into the reactor, a maximum COzconcentration at the detector of the mass spectrometer was seen within 1-2 s. Oxidation was continued for 10-15 min until the CO and C02were at essentially constant background levels. Then the mass spectrometer was switched to sample only the inlet fluidizing gas to establish a base line for CO and C02 levels and calibrate instrumental drift. Char produced by pyrolysis at a slow heating rate was also studied. In this case, the raw shale was heated in a modified Fischer assay retort at a rate of 0.09 "C/min from ambient temperature to 530 "C and held at that temperature for 5 min. This material was subsequently oxidized in a fluidized bed in the same fashion as described above. Mathematical Modeling The rate of oxidation of residual organic carbon is expressed as --dx - kx"P0 (4) dt where x is the fraction of residual organic carbon remaining at time t, k is the reaction rate coefficient, n is the reaction order with respect to organic carbon, and Pois the oxygen (7) Goklen, K. E.; S h c k e r , T. J.; Baddour, R. F. Ind. Eng. Chem. Process Des. Deu. 1984, 23, 308-311.
partial pressure. We assume that oxidation of char produced by rapid pyrolysis is first order with respect to oxygen, consistent with previous measurements3p4for oil shale char produced at lower heating rates. The rate coefficient is k = A exp(-E/RT) (5) where A is the Arrhenius frequency factor (s-l Pa-l), E is the activation energy (J/mol), and R = 8.3143 J mol-l K-l. Integrating eq 4 a t constant temperature gives x ( t ) = [l - (1 - n)kPot]l/(l-n) if n # 1 (6)
or x ( t ) = exp(-ltPot) if n = 1
(7) The calculated char reacted at time t (expressed as a fraction of the calculated char reacted at time tf) is then CC(t)= [ l - x(t)l/[l - X(tf)l (8) where tf is the final time at which data are measured in a given experiment. Similarly, the measured char reacted at time t (expressed as a fraction of the measured char reacted at time tf) is
where Re is the measured rate of organic carbon oxidation. The reaction rate parameters can then be determined by nonlinear regression in which the function minimized is
where N is the total number of data points. The computer algorithm E04FDF' was used. Results and Discussion Normalized reaction rates are shown in Figure 2 for oxidation of residual organic carbon in char produced by rapid pyrolysis. These rates are based on total concentration of CO + C 0 2 in the slipstream sampled by the mass spectrometer. The C O / C 0 2ratio for a given isothermal experiment was not constant. It began in the range 0.20-0.25 and steadily increased to the range 0.75-1.8 for the experimental temperature range 384-474 "C, respectively. It is clear from the nonlinear curves in Figure 2 that a single first-order reaction does not fit the data. This is in marked contrast with previous res~lts,3,~ which dem(8) Fox, L.; Wilkinson, J. H. NAG Library, Numerical Algorithm Group, Ltd, NAG Central Office, Mayfield House, 256 Banbury Road, Oxford OX2 7DE, U.K.
Fujimoto et al.
322 Energy & Fuels, Vol. 1, No. 4, 1987 1.0,
,
,
,
I
,
I
,
,
I
I
'
,
,
I
,
, 1
-
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2o.ol
Temperature ("C) 650 600 550 500 450 400
350
J
\\,
r
b. l0.OF ,"5.0 z r
I
.\\
\
I
A = 1.33 1-l Pa-'
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E = 81.2 kJ/mol
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I
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' 400
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" 600
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Time (s)
1
I
1.2
1.4
1.6
10OO/T (K-')
Figure 3. Comparison of measured (--) fraction of residual organic carbon oxidized with that calculated (-) by a single second-order reaction.
onstrated that oxidation of char produced by a slower heating pyrolysis is first order with respect to unreacted char. In principle, rate parameters A , E , and n can be determined by the procedure described above by simultaneous least-squares analysis of isothermal data from experiments conducted at several temperatures. In practice, however, analysis of all experiments simultaneously can easily lead to unequal weighting of the different experiments. Therefore, eq 10 was applied separately to each experiment, and k alone was determined at the given temperature. This was done for n = 1and was repeated for other selected values of n to determine a single value for the reaction order that fits all experiments. Values of A and E were then obtained from the customary Anhenius plot of log k vs. 1/T. A second-order reaction fit the data very well, as shown in Figure 3 for the two extreme temperatures studied. The second-order reaction rate coefficient, determined from the Arrhenius plot shown in Figure 4 for eight isothermal runs between 384 and 474 "C, is
k = 1.33 exp(-81200/RT)
0.1;
Sohn and Kim Re-evaluated for n = 2 A = 1.71 s'l Pa-' E = 88.7 kJ/mol
s-' Pa-'
(11)
Two simultaneous first-order reactions also fit the data quite well, but not as well as a single second-orderreaction. This empirical second-order fit probably results from a distribution of reactivities in the char. An nth-order reaction can often be used to closely approximate a reaction that involves a distribution of activation energie~.~ Comparison of the present resulb for combustion of char prepared by rapid heating with the results of Sohn and Kim4 for char prepared by a slower heating pyrolysis indicates a significant effect of heating rate (i.e., effective pyrolysis temperature). In order to make a direct comparison of the intrinsic kinetics for these two chars, we reinterpreted the data of Sohn and Kim in terms of second-order kinetics. Comparison of the two chars is shown in Figure 4. Char from rapid heating appears to be two to three times more reactive than char from slow heating. Additional experiments with char prepared at an even slower heating rate of 0.09 "C/min confirmed the decreased oxidation reactivity of the char, but reaction rates were too low to obtain an accurate determination of kinetic parameters. Burnhamla found comparable heating-rate (9)Braun, R. L.;Burnham, A. K. Energy Fuels 1987, 1, 153-161.
Figure 4. Arrhenius plot for char oxidation, for second-order reaction with respect to residual organic carbon. The original data of Sohn and Kim4were reevaluated in terms of a second-order reaction.
effects for the reaction of oil shale char with CO,. Oxidation reactivity of char produced from bituminous coal is also reported to be quite sensitive to heating rate."J2 In some anticipated commercial schemes for pi1 shale pr~cessing,'~ char from rapidly pyrolyzed oil shale may be burned at temperatures ranging from 500 to 700 O C . We attempted to measure the rate of release of COz at 550 "C. The results are difficult to interpret, however, because of the contributions to the COz release by SO2-carbonate reactions. The rate appeared to be more rapid than that extrapolated to 550 OC from our measurements of char combustion at temperatures below 475 OC. This suggests that the reaction rate between iron sulfides and oxygen is more rapid than that between char and oxygen. Determination of char oxidation kinetics at temperatures above 475 OC could be done by the present method, if the interference due to C 0 2 evolution from carbonate capture of SO2 could be avoided. Removal of the carbonate minerals by acid leaching, however, resulted in a substantially reduced char reactivity. The oxidation rates for the acid-leached samples were an order of magnitude lower than those obtained for the undecarbonated shale. This apparent catalytic activity of the carbonate minerals, or other acid-soluble minerals, was also found by Burnham14 for the reaction between C02 and oil shale char. Conversely, Thompson and S0ni3reported that oxidation rates of oil shale char are not affected by acid leaching. This discrepancy has not been resolved. The present results for oxidation of residual organic carbon f i i an important gap in our ability to simulate the combustion of rapidly retorted oil shale. These results, however, must be supplemented by rate measurements for the oxidation of residual organic hydrogen and other shale componentsbefore we are able to accurately model the rate of heat released during the combustion process. For lean shales or shales with high sulfur contents, the heat from oxidation of iron sulfides may even exceed that from carbon combustion. (10)Burnham, A. K.Fuel 1979,58,285-292. (11) Easenhigh, R.H.Ln Chemistry of Coal Utilization; Elliott, M. A., Ed.;Wiley: New York, 1981;pp 1153-1312. (12)Solomon,P.R.;Serio, M. A.; Heninger, S. G.Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1986,31(3),200-209. (13)Lewis, A. E.;Braun, €2. L.; D i u , J. C . Oil Shale Symp. Proc. 1984, 17th, 1-16. (14)Burnham, A. K.Fuel 1979,58,713-718.
Energy & Fuels 1987,1, 323-331 Our fluidized-bed procedure appears to be an excellent means of determining kinetics for char oxidation. The large mass of inert quartz sand, relative to the mass of shale, provides a suitable heat sink that enables highly exothermic reactions to be investigated under nearly isothermal conditions. In addition, the specially designed gas distributor gave a very well-stirred reaction medium, with uniform temperature and no stagnation regions. Attrition and elutriation of the shale are the principal potential disadvantages. These were held to an acceptable level by using the minimum gas flow rate that gave a flow-independent reaction rate. Less than 10% of the shale was
323
elutriated. Most of this occurred during the pyrolysis step of the process, which did not interfere with subsequent measurement of oxidation kinetics. It would be desirable to measure the rate of oxidation of identical samples of retorted shale by different methods in order to observe the role of the apparatus and procedures.
Acknowledgment. Helpful discussions with A. K. Burnham and H. Y. Sohn are gratefully acknowledged. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.
Catalytic Hydrodeoxygenation of Dibenzofuran Vito LaVopa and Charles N. Satterfield" Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received July 24, 1986. Revised Manuscript Received February 1 1 , 1987
The hydrodeoxygenation of dibenzofuran (DBF) on a sulfided NiMo/A1203catalyst was studied at 350-390 "C and 7.0 MPa. The major products isolated were single-ring hydrocarbons, cyclohexane predominating; the remainder were double-ring hydrocarbons, cyclohexylbenzene predominating. No oxygen-containing species other than water were isolated in any significant amount. The initial reactions in the hydrodeoxygenation of DBF are rate-limiting. The non-sulfided ("oxide") catalyst is much less active, and double-ring products predominate over single-ring products. From studies of possible intermediates it appears that on a sulfided catalyst two pathways operate in parallel for the hydrodeoxygenation of dibenzofuran (Figure 14): (1)hydrogenation of DBF to hexahydro DBF, which reacts via 2-cyclohexylphenolto form single-ring hydrocarbons; (2) direct hydrogenolysis via 2-phenylphenol, without prior ring hydrogenation, to form biphenyl and cyclohexylbenzene(a minor route). On this catalyst the overall reaction is first order with respect to hydrogen and to DBF and exhibits an apparent activation energy of 67 kJ/mol.
Introduction Liquids derived from coal and oil shale differ from petroleum in their substantially higher oxygen contents and types of oxygen compounds present. Oxygen removal from coal liquids is required to ensure fuel stability during storage1 and may be required before further processing. Important interactions may occur between hydrodeoxygenation (HDO) and other hydroprocessing reactions, which as yet are poorly understood. The oxygen-containingcompounds found in coal liquids are phenols, aryl ethers, and benzofurans, as shown by Petrakis et a1.,2 who analyzed a heavy distillate from an SRC-I1 liquid, and Furimsky? who analyzed a coal hydrogenation liquid. Even in a California petroleum distillate phenols and dibenzofuran accounted for 11% and 17% respectively of the total oxygen ~ o n t e n t . ~ A considerable literature on HDO has developed since the early 1970s. Shah and Cronauer5reviewed 0, N, and S removal reactions in donor solvent coal liquefaction, and (1) White,C. M.; Jones, L.; Li, N. C.Fuel 1983,62, 1397. (2) Petrakis, L.; Young, D. C.; Ruberto, R. G.; Gates, B. C. Znd. Eng. Chem. Process Des. Deu. 1983,22, 298. (3) Furimaky, E. Fuel Process. Technol. 1982, 6, 1. (4) Snyder, L. R. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1970, 15(2), C44. ( 5 ) Shah, Y. T.; Cronauer, D. C. Catal. Rev.-Sci. Eng. 1979,20,209.
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Furimsky6 reviewed the chemistry of catalytic HDO. Several model compound studies have investigated the HDO of specific furans and phenols over conventional hydroprocessing catalysts. The only previous detailed study of the products formed from dibenzofuran (DBF) and its reaction network is that of Krishnamurthy et al.7 done in a batch autoclave at 343-376 "C and hydrogen pressures of 6.9 to 13.8 MPa, the DBF being dissolved in n-dodecane. A presulfided NiMo/A120, catalyst was used, and CS2 was added to maintain a H2Spartial pressure during reaction. They also reported studies of 2-phenylphenol and 2-cyclohexylphenol. Lee and Ollis8reported on the catalytic HDO of benzofuran and 2-ethylphenolat 69 atm and temperatures up to 340 "C, but the reaction network of these compounds is significantly different from that of dibenzofuran. A study of furan by Pratt and Christover~on~ and of tetrahydrofuran by Furimsky'O were performed under atmospheric pressure and therefore may not be representative of expected behavior under industrial processing conditions. (6) Furimsky, E. Catal. Rev.-Sci. Eng. 1983, 25, 421. (7) Krishnamurthy, S.; Panvelker, S.; Shah,Y. T. AIChE J. 1981,27,
W-_A-.
(8) Lee, C.-L.; Ollis, D. F. J. Catal. 1984, 325, 87. (9) Pratt, K. C.;Christoverson, V. Fuel Process. Technol. 1983,8,43. (10)Furimsky, E. Ind. Eng. Chem. Prod. Res. Dew 1983, 22, 31.
0 1987 American Chemical Society
