Response to Professor Yablonsky

Octave Levenspiel

Chemical Engineering Department, Oregon State University, Gleeson Hall 103, Corvallis, Oregon 97331-2702

Sir: Prof. Yablonsky raises two objections to mypresentation. First, let me explain my reluctance to usethe Langmuir-Hinshelwood-Hougen-Watson (LHHW)equations because “they are too complicated”. If, forexample, a researcher comes up with a LHHW equationto represent his porous catalyst data, then I probablycould come up with five different other LHHW equationswhich could fit the data equally well.1,2 Because theseequations extrapolate differently, which should I use forextrapolation? I do not know.

Even if I cannot find any other LHHW equation tofit his data, let me ask whether he can use his equationwith confidence to extrapolate for scale-up? Does hisequation account for film diffusion or how about diffu-sional resistance within the particles? What about filmheat transfer at the surface of the particles? Finally,how about nonisothermal gradients within the particles?The LHHW equations completely ignore these physicalfactors; hence, they are not useful for extrapolation.They simply fit the data. On the other hand, thechemical reaction engineering (CRE) approach accountsfor all of the pertinent factors, and that is why in themarketplace of ideas it won out over the “petrotech”approach, which uses the LHHW kinetics.

The second objection concerns my ignoring extremelyfast reactions. Researchers of such systems use basket-fuls of elementary reactions, hundreds and hundredsof them,3 to study free-radical reactions, flames, burn-ers, and other such devices, with computational fluiddynamics (CFD) to represent the flow and mixing. This

type of problem is dealt with primarily by combustionspecialists. I see their goals as differing from CRE,whereas Prof. Yablonsky does not. I suppose that thisis a matter of opinion.

Prof. Yablonsky also says that I do not account forthe correspondence between model and goal. I think thatthe goal of CRE is to come up with a good reactor designto produce efficiently what you want to produce. Kineticmodels which use basketfuls of elementary reactions orLHHW mechanisms, but then ignore pertinent factorsand at the same time are unnecessarily complicated, arereally unsuited to the above-mentioned goal.

In conclusion, Prof. Yablonsky says that “we have tolive with unavoidable complexity of reactions” “...com-plexity of equations”. I am uncomfortable with suchstatements. I feel that we should try to come up withsimple models which do account for all of the pertinentfactors. As Denbigh4 so wisely put it, “It is alwaysnecessary to abstract from the complexity of the realworld...and in its place...to substitute a more or lessidealized situation that is more amenable to analysis.”

Literature Cited(1) Chou, C. H. Ind. Eng. Chem. 1958, 50, 789.(2) Hougen, O. A.; Watson, K. M. Chemical Process Principles;

Wiley: New York, 1947; part III, pp 943-958.(3) Rojnuckarin, A.; et al. Ind. Eng. Chem. 1996, 35, 683-696.(4) Denbigh, K. G. The Thermodynamics of the Steady State;

Methuen’s Monographs on Chemical Subjects; London, 1951.

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