Homogeneous-heterogeneous reactions

YaB Gorokhovatsky, TP Kornienko, VVShalya. Heterogeneous-Homogeneous Reactions. Kiev Tekhnika, 1972, pp 1-22 [in Russian],... 

Ya.B. Gorokhovatskii, T.P. Kornienko and V.V. Shalya, Heterogeneous-Homogeneous Reactions, Tekhnika, Kiev, 1972 (in Russian). 

This section briefly discusses some aspects of catalytic combustion mechanisms, i.e., surface reaction kinetics and heterogeneous-homogeneous reactions. Based on this discussion and the previous section, the extreme demands on combustion catalysts are presented. Finally, the role of mathematical modeling of this complex catalytic system is examined. 

Reviewing the available data in the literature. Kiperman [31] has discussed (1) the simplest variants of mechanisms for heterogeneous-homogeneous reactions, (2) the main regularities of these reactions under steady-state and non-steady-state conditions, and (3) examples illustrating specific features of these reactions. 

I.. D. Pfefferle, Modeling heterogeneous-homogeneous reactions and transport coupling for catalytic combustion systems, Proc. 2nd Inl. Workshop Catalytic Combustion. Tokyo, 18-20 April, (H. Arai. ed.). Catalysis Society of Japan, Tokyo, 1994, p. 78. 

The various hydrocarbon oxidation schemes discussed above were believed to proceed at the catalyst surface only. The present concepts accept the occurrence of complex heterogeneous-homogeneous reactions proceeding in part at the solid surface and in part in the gas or liquid phase. Many catalytic oxidation processes considered recently as purely heterogeneous appeared to proceed by the heterogeneous-homogeneous mechanism. Such are the oxidations of hydrogen, methane, ethane, ethylene, propene, and ammonia over platinum at elevated temperatures, as studied by Polyakov et al. (131-136). When hydrocarbons are oxidized over platinum the reaction sets in on the catalyst surface and terminates in the gas phase. 

Let us consider a few mechanisms of heterogeneous-homogeneous reactions using an oxidation reaction as an example... 

The approach to treat the kinetics of heterogeneous- homogeneous reactions can be summarized as follows. The rates of heterogeneous catalytic reactions occuring on the surfaces of solid materials are derived in a similar fashion as for the reactions without the formation of radicals. This gives a possibility to calculate the formation rate of radicals. Using the terminology of chain reactions the initiation rate is then computed. 

In conclusion, a few comments about the practical importance of heterogeneous-homogeneous reactions from kinetic viewpoint. Usually industrial reactors have a relatively large free volume and the packing material is also present in addition to the catalyst. If homogeneous reactions are beneficial for the overall process, then they will be retarded due to deactivation of radicals and the rate will be lower compared to laboratory reactors with more advantageous reactor geometries. 

Special opportunities for the industrial implementation of structured catalysts are offered by the growing interest in millisecond contact time processes, in view of the associated requirements on pressure drop and flow distribution to be matched with strict size constraints. In this case, a better control of the complex interplay between heat and mass transfer and heterogeneous/homogeneous reactions granted by structured catalysts would provide guidelines for the design of reactors and processes with optimized performances in terms of selectivity, yield, fast transient response and operational flexibility. 

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