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Energy & Fuels 1996,9, 384-385
Communications Kinetics of Coal Liquefaction: Interpretation of Data Sol W. Weller Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260 Received September 9,1994 This Communication addresses the following questions: How should one interpret experimental data on the kinetics of coal liquefaction? What theoretical validity resides in a rate equation for liquefaction? What deductions about chemical mechanism can be deduced from the form of the rate equation? The progress of coal liquefaction is typically characterized by quantitative measurement of masses of products-insolubles, preasphaltene, asphaltene, oil, and gas-as a function of coal, catalyst, temperature, and time. The measured product fractions are lumped species; the solid and liquid fractions are differentiated only by their common solubility behavior in arbitrarily chosen solvents. Historically, data analysis in liquefaction kinetics has had diverse goals. One method of analysis ends with a reaction pathway (model or reaction scheme) that is compatible with the observed kinetic data. Deduction of the reaction pathway from inspection of the data is essentially qualitative and intuitive, in the sense that it does not depend on data fitting through the formalism of a mathematical rate expression. For the sake of illustration only, two specific examples, one old and the other recent, will be mentioned. In a study of the conversion of coal and of isolated asphaltene, Weller et al.lw3were led to a simplistic, essentially linear reaction pathway: coal to asphaltene, followed by asphaltene to oil. Reactive fragments were proposed as intermediates. Side reactions were the production of gas, and of new insolubles by polymerization of reactive fragments if these were not stabilized in a timely fashion. Very recently, Keogh and Davis4 studied the production of individual fractions (insolubles, preasphaltene, etc.) and three lumped parameters (oils plus gases, preasphaltenes plus asphaltenes, and insoluble organic matter) as a function of a severity index, defined in terms of reaction temperature and time. From their innovative introduction of ternary plots of the three lumped parameters, along with plots of individual fractions vs severity index, Keogh and Davis deduced reaction pathways for a number of coals during either thermal or catalyzed liquefaction. From inspection of the data they were able to show, for (1)Weller, S.; Clark, E. L.; Pelipetz, M. G. Ind. Eng. Chem. 1950, 42, 334. (2) Weller, S.;Pelipetz, M. G.;Friedman, S. Ind. Eng. Chem. 1951, 43, 1572. (3) Weller, S.;Pelipetz, M. G.;Friedman, S. Ind. Eng. Chem. 1951, 43, 1575. (4) Keogh, R.A,; Davis, B. H.Energy Fuels 1994, 8 , 289.
example, that addition of a catalyst affects reaction rates but does not alter the reaction pathway. A different method that has been used to analyze liquefaction kinetics is quantitative. From the experimental data one derives a mathematical, overall rate expression (“rate law”) that fits the data. Such an empirical rate law has been used in two ways. In one of these, the rate expression is treated as purely empirical, not necessarily implying anything about detailed chemical mechanism. The engineering application of this procedure for scale up and reactor design is clearly useful and not open to question. A second approach to using the quantitative rate expression has been based on the assumption that the form of the expression reflects rate-limiting elementary reactions actually occurring in liquefaction. In this interpretation of the data, one proceeds to deduce chemical mechanism from the mathematical form of the rate expression. An interesting reversal of this approach, in which the argument was from assumed mechanism to empirical rate equation, was presented in two papers by Gun et al.596 In the first paper the authors pointed out the complexity of fitting their own experimental data for coal conversion: First-order plots indicated the occurrence of four steps, differing in reaction order and activation energy. Gun et al. were led t o the following empirical rate equation: d[P,l/dt = b,[Cll.O
+ b,[C11.5-2.0+ b3[cIo=O
(1)
where [P,] denotes gas and benzene soluble products, [Cl the percentages of organic matter in coal and benzene insoluble intermediates, and bl, b2, and b3 are constants. In the second paper, the authors noted that free radicals had been found in such end products from coal as chars and low-temperature carbons. From this qualitative observation, Gun et al. concluded that the course of liquefaction itself proceeds via free-radical intermediates. From this premise, an eight-step freeradical mechanism (initiation-propagation-termination) was proposed for coal conversion. Deduced from this proposed mechanism was a rate equation whose form was roughly consistent with eq 1. However, the constants bl, b2, and b3 in eq 1 were found to involve complex combinations of eight rate constants, and the varying orders of reaction were not accounted for. (5) Gun, S. R.; Sama, J. K.; Chowdhury, P. B.; Mukherjee, S. K.; Mukherjee, D. K.Fuel 1979,58, 171. ( 6 ) Gun, S. R.; Sama, J. K.; Chowdhury, P. B.; Mukherjee, S. K.; Mukherjee, D. K. Fuel 1979, 58, 176.
0887-0624/95/2509-0384$09.00/0 0 1995 American Chemical Society
Communications
Is there justification for deducing chemical mechanism from the form of an empirical rate equation for coal liquefaction, or, conversely, for justifying an assumed mechanism on the grounds that it leads to compatibility with the empirical rate expression? Some assumptions are implicit when mechanistic meaning is read into the mathematical form of an experimental rate equation for liquefaction. Foremost is the assumption that the law of mass action can be validly applied in this special case. The mass action principle was proposed (independently by investigators in three countries during 186267)to quantify the rate of an elementary reaction in a single-phase system between identifiable chemical species. Mass action rate equations are properly expressed in terms of concentrations of individual species, and the expressions in general do not describe the rates of nonelementary reactions. The complex reactions occurring in coal liquefaction are not elementary, and the system is not single phase. The quantities of coal (or insolubles) and of reaction products are expressed in terms of mass and not concentration. The measured fractions, coal, preas-
Energy & Fuels, Vol. 9,No. 2, 1995 385 phaltenes, etc., are lumped quantities, differentiated only on the basis of solubility. They are not individual species, and their composition and reactivity change as the reaction proceeds. In short, the assumptions which are necessary for mechanistic interpretation of the mathematical form of the empirical rate expression are not fulfilled. What conclusions may be drawn? First, temporal studies of changing product distribution during coal liquefaction give useful insight into the reaction pathways occurring during these complex processes, even without deduction of an empirical, mathematical rate expression which fits the experimental data. Second, if an empirical rate expression which fits the data with satisfactory accuracy can be deduced, such an expression is useful, within the regimes which have been studied experimentally, for the engineering functions of scale up and reactor design. Finally, attempts to read chemical mechanism into the empirical rate law are without theoretical validity and should be avoided. EF940175S
