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Ind. Eng. Chem. Res. 2000, 39, 3402
Reply to Comments on “Surface-Enhanced Light Olefin Yields during Steam Cracking” Mike Golombok*,† and Jan Dierickx Shell International Chemicals, Badhuisweg 3, 1031 CM Amsterdam, The Netherlands
Sir: The interesting comments on our paper concentrate almost entirely on one pagessection 3.4 dealing with catalytic effects. The author is confused by our first conclusion, and perhaps we have not expressed ourselves sufficiently clearly. There are two chemical reactions involved here, namely, (1) a global cracking reaction to make ethylene and (2) a reaction to make coke. Our contention is that KVO3 and other metallic oxides did not enhance the yield of ethylene over and above the substrates which are described earlier in our paper. Thus, there was no catalytic effect. However, the rate of the second reaction was affectedsso that there was a catalytic effect. Catalysts do not “increase the kinetics of specific reactions” as the comment alleges; they merely modify the rate for a particular process by changing its associated activation energy. It would perhaps have been better to state that when talking of a catalyst, there are two possible functions: if the rate for a process can be enhanced, the catalyst is then a “promoter”. If the rate for a reaction is decreased, one speaks of an “inhibitor”. Our contention is thus that the metal oxides had no effect on reaction (1) but acted as an inhibitor on reaction (2). We cannot comment on the form which potassium adopts on the surface. However, the conclusion of other workers is rather clear on this matter: that it is KVO3 which produces the desired effect.1 We confined ourselves to the experimental evidence shown2 in Figure 7 that increasing potassium loading suppresses coke as shown by (1) retarded pressure drop increase and (2) weight changes in the catalyst. We accept, however, that our statement “the route to coke runs via methane” is infelicitously expressed. This could have been better said as “coke formation correlates well with methane make”. In fact, it is well-known that there are several mechanisms for coke formations primarily the gas phase and involving free radicals. Trimm identifies seven characteristic pathways including the classic acetylene mechanism and C2 polymerization.3 Several of these mechanisms do not, of course, involve methane. We merely made the obvious point, that because increased severity of operation leads to more coking, then methane make (which is a good measure of severity4) would give a good indication of the level of coke to be expected in as much that increasing cracking severity results in more coking and greater levels of methane formation as the hydrocarbon chains are broken down. As a final comment on this issue, we note that methane has been shown to result in extensive coke formation on nickel.5 * To whom correspondence should be addressed. Currently with Shell International Exploration and Production, Volmerlaan 8, 2288 GD Rijswijk, The Netherlands. Telephone: 31 70 311 2597. Fax: 31 70 311 3366. E-mail: mike.m.golombok@ opc.shell.com. † Also at Department of Applied Physics, Delft University of Technology, Lorentzweg 1, 2600 GA Delft, The Netherlands.
The second half of the comments are taken up with suggestions on ways to take pictures of coke particles. These studies have not proven particularly helpful in the past to increasing the yields of desirable chemical products. There are indeed a lot of observations that could be made, but these do not produce testable hypotheses and have tended to be rather short on useful prediction for increasing yields of products which sell. Experiments on engines do indeed suggest that some metallic “surface may be developed in the future to which no coke collects or adheres”. However, because these observations have been known for some time, one is inevitably drawn to ask where these new miracle materials are, suggesting perhaps that this is not a line of research liable to pay off in the short or even medium term with which we are concerned. Pictorial studies have failed to generate the process improvements because they relate to lower temperature coke formation (e.g., 600 °C) as opposed to coke formation under the radiant conditions (800 °C) appropriate to steam cracking. What one would wish to see is more research on high-temperature coke formation continuing along the already established line of in situ electrobalance kinetic studies.6 Research on catalytic steam cracking has not been particularly fruitful to date in generating a commercially viable process for enhancing the economically desirable production of lower olefins (i.e., ethylene). Unless our academic colleagues can link catalytic effects to enhanced production, then work on the details of coke production and related issues, photographically interesting and visually aesthetically diverting as these may be, will remain a rather obscure byway of process research. Literature Cited (1) Chernykh, S. P.; Adel’son, S. V.; Rudyk, E. M.; Zhagfarov, F. G.; Motorina, I. A.; Nikonov, V. I.; Mukhina, T. N.; Barabanov, N. L.; Pyatiletov, V. I. Catalytic pyrolysis of straight-run gasoline on a promoted vanadium catalyst. Sov. Chem. Ind. 1983, 15 (4), 414. (2) Golombok, M.; Kornegoor, M.; van den Brink, P.; Dierickx, J.; Grotenbreg, R. Surface enhanced light olefins yields during steam cracking. Ind. Eng. Chem. Res. 2000, 39, 285. (3) Trimm, D. L. Fundamental aspects of the formation and gasification of coke. In Pyrolysis: theory and industrial practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983. (4) Van Camp, C. E.; van Damme, P. S.; Willems, P. A.; Clymans, P. J.; Froment, G. F. Severity in the pyrolysis of petroleum fractions: fundamentals and industrial application. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 561. (5) Bernardo, C. A.; Alstrup, I.; Rostrup-Nielsen, J. R.; Tavares, M. T. Behavior of bimetallic supported catalysts in the steam reforming of methane. I. Coke deposition. Actas Simp. Iberoam. Catal. 1984, 2, 1523. (6) Dumez, F. J.; Froment, G. F. Dehydrogenation of 1-butene into butadiene. Kinetics, catalyst coking, and reactor design. Ind. Eng. Chem. Process Des. Dev. 1976, 15 (2), 291.
IE000541X
10.1021/ie000541x CCC: $19.00 © 2000 American Chemical Society Published on Web 08/06/2000
