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Ind. Eng. Chem. Res. 2000, 39, 3120
CORRESPONDENCE Comments on a Commentary by Professor Levenspiel Gregory Yablonsky Department of Chemical Engineering, Washington University, 1 Brookings Drive, Campus Box 1198, St. Louis, Missouri 63130
Sir: Recently, Ind. Eng. Chem. Res. inaugurated a new kind of paper called a “Commentary”. The concept is excellent. The paper must be written by individuals highly respected in the area of chemical engineering. A “Commentary is meant to be critical and selective because it is based on a broad perspective that is both historical and cautiously prophetic... A Commentary is not meant to be exhaustive, on the contrary, it is highly selective, a product of the author’s judgment” (ref 1, p 4139). The first “Commentary” appeared and was written by Prof. Octave Levenspiel, who is a worldwide wellknown author of Chemical Reaction Engineering and The Chemical Reactor Omnibook2 and who is superbly familiar with the history of chemical engineering. In our times of information overloading, one cannot emphasize enough the importance of initiatives such as commentaries. The message of Prof. Levenspiel’s commentary is the following: “Don’t complicate a kinetic model! Pay more attention to the flow model!” He is somewhat sarcastically describing the contemporary practice of reactor modeling with “some researchers have used as many as 60 individual elementary reaction steps, while at the same time completely ignoring the question of looking for the best contacting or flow model” (ref 3, p 4142). Certainly, Prof. Levenspiel is right. We have to analyze the simple models first, using even semiempirical kinetic equations for an nth-order reaction, and results of analyses of overcomplicated models should be viewed with caution. However, in chemical reaction engineering there are many problems in which the complexity of the chemical reaction seems to be unavoidable. It is obvious that, analyzing selective oxidation reactions over oxide catalysts, pyrolysis, and polymerization reactions, we have to take into account the change of complex chemical composition in time and with the length of the reactor. To calculate these profiles, we have to estimate the parameters of the different steps of complex reactions, particularly for steps with participation of the intermediates (radicals or surface substances). Unfortunately, Prof. Levenspiel does not pay attention to these problems. Moreover, he criticizes an application of Langmuir-Hinshelwood and Hougen-Watson (LHHW) equations for describing heterogeneous reactions (Prof. Levenspiel calls this the petrotech approach). In his opinion, the LHHW equation (“numerator over denominator” equation) is too complicated. However, the complexity of this equation is unavoidable too. Its denominator reflects the fact that some
intermediates occupy the surface of the catalyst. Even Prof. Levenspiel will apply the simple nth-order kinetic equation; the apparent order will be a function of the chemical mixture composition and temperature. Another important thing which is not taken into account in Prof. Levenspiel’s Commentary is the special correspondence between the model and its goal. There is no model “for all times”. For some purposes the simple model is the best one, and this simplicity relates to the level of physicochemical understanding. However, for other purposes we really need a complex model. Typical strategies of mathematical modeling, in chemical engineering as multiscale hierarchical modeling4 or a “multiscale approach to a multi-objective task”,5 are wellknown. In our understanding, simplicity has to be emphasized, and in that Prof. Levenspiel is completely right. However, we also have to learn how to live with the unavoidable complexity of chemical reactions which is a really new trend in chemical engineering. It is always difficult to distinguish the “useless fashion” and the “new trend”. It is also difficult even for Prof. Levenspiel.
Acknowledgment I am very thankful to Prof. Guy Marin, who brought to my attention the new type of “Commentary” papers in Ind. Eng. Chem. Res., and Prof. Mike Dudukovic for editing my comments.
Literature Cited (1) Paul, D. R. Commentary: A New Type of Paper. Ind. Eng Chem. Res. 1999, 38, 4139. (2) Levenspiel, O. The Chemical Reactor Omnibook; OSU Bookstores: Corvallis, OR, 1996; Chapter 31. (3) Levenspiel, O. Commentaries: Chemical Reaction Engineering. Ind. Eng. Chem. Res. 1999, 38, 4140-4143. (4) Dudukovic, M. P.; Larachi, F.; Mills, P. Multiphase Reactors Revisited. Chem. Eng. Sci. 1999, 54, 1975-1995. (5) Lerou, J.; Ng. K. Chemical Reaction Engineering: A MultiScale Approach to a Multi-Objective Task. Chem. Eng. Sci. 1996, 51, 1595-1614.
IE001098T
10.1021/ie001098t CCC: $19.00 © 2000 American Chemical Society Published on Web 05/27/2000
