ModeHing methods, i.e., sturlies of model processes permitting prediction of their development in apparatus of any· size, have attained extreme importance in chemical technology. They are the main way ofsolving problems connected with transition to a !arger scale, rational choice of the reactor design, optimization, analysis of the steady state and parametric sensitivity of chemical processes in reactors.The present paper is concerned with problems of modeHing chemical reactors, namely apparatus wherein · the process rate is a function of the chemical conversion as such. I t is for this apparatus that transition to a !arger scale encounters diffi.culties which are specific to chemical technology.
FUNDAMENTAL PRINCIPLES OF MODELLINGThe rate of chemical conversion as such is independent of the reactor size; but the development of a chemical reaction induces temperature and composition variations. These result in mass and heat transfer processes of a rate essentially dependent on the scale of the system. The composition and temperature of the reacting mixture exert, in their turn, a considerable effect on the chemical reaction rate. There arises therefore a complex dependence of the overall chemical process on the reactor size. Variations in the latter may bring considerable changes in the overall reaction rate and the ratio of rates, i.e., in the selectivity of the process.The attempts to solve the problern of transition to a !arger scale on the basis of similitude relations by means of physical modeHing of processes possessing the samenature as those occurring in a large-scale reactor, failed due to the incompatibility ofsimilitude conditions for the chemical and physical components of the processl. The reason for incompatibility of similitude criteria is the impossibility of ensuring a similar effect of physical factors on the rate of chemical conversion in reactors of different sizes. In the limiting case only, when the chemical reaction is considerably faster than the transfer processes and, consequently, does not affect the overall process rate ( as, for instance, in absorption of gases reacting with the absorbent at a high rate, or in catalytic reactors in the external diffusion region), the criteria of chemical similitude become unimportant and application of these relationships becomes possible. However, for a general case of chemical reactions transition to a larger scale on the basis of similitude relations and physical modeHing is impossible.Consequently, an empirical method of gradual increase in the reactor sizes, from enlarged, to pilot, and then industrial reactors, has tobe used for transition to a larger scale. As this involves no scientific prediction of the 611 Brought to you by | California Institute of Technolog Authenticated Download Date | 10/11/15 11:12 PM
