Catalytic kinetics controlling resistance

The assumption of global kinetic control is probably valid for only a handful of catalytic reaction processes. Nevertheless, some typical simulation results of the model of catalyst deactivation under kinetic control are presented here in order to emphasize some of the unique percolation-type aspects of the problem. The overall plugging time 0p, i.e., the time at which the catalyst becomes completely deactivated is shown is Figure 1, where it is plotted versus Z, the average coordination number of the network of pores, (in industrial applications, of course, the useful lifetime of the catalyst is significantly smaller than 0p). Note that as Z increases, (higher values of Z mean a more interconnected catalyst pore structure) 0p increases, i.e., the catalyst becomes more resistant to deactivation. The dependence of normalized catalytic activity (r/rQ) ([Pg.176]

Sundmacher and Qi (Chapter 5) discuss the role of chemical reaction kinetics on steady-state process behavior. First, they illustrate the importance of reaction kinetics for RD design considering ideal binary reactive mixtures. Then the feasible products of kinetically controlled catalytic distillation processes are analyzed based on residue curve maps. Ideal ternary as well as non-ideal systems are investigated including recent results on reaction systems that exhibit liquid-phase splitting. Recent results on the role of interfadal mass-transfer resistances on the attainable top and bottom products of RD processes are discussed. The third section of this contribution is dedicated to the determination and analysis of chemical reaction rates obtained with heterogeneous catalysts used in RD processes. The use of activity-based rate expressions is recommended for adequate and consistent description of reaction microkinetics. Since particles on the millimeter scale are used as catalysts, internal mass-transport resistances can play an important role in catalytic distillation processes. This is illustrated using the syntheses of the fuel ethers MTBE, TAME, and ETBE as important industrial examples. 

Figure 10 shows that Tj is a unique function of the Thiele modulus. When the modulus ( ) is small (- SdSl), the effectiveness factor is unity, which means that there is no effect of mass transport on the rate of the catalytic reaction. When ( ) is greater than about 1, the effectiveness factor is less than unity and the reaction rate is influenced by mass transport in the pores. When the modulus is large (- 10), the effectiveness factor is inversely proportional to the modulus, and the reaction rate (eq. 19) is proportional to k ( ), which, from the definition of ( ), implies that the rate and the observed reaction rate constant are proportional to (1 /R)(f9This result shows that both the rate constant, ie, a measure of the intrinsic activity of the catalyst, and the effective diffusion coefficient, ie, a measure of the resistance to transport of the reactant offered by the pore stmcture, influence the rate. It is not appropriate to say that the reaction is diffusion controlled it depends on both the diffusion and the chemical kinetics. In contrast, as shown by equation 3, a reaction in solution can be diffusion controlled, depending on D but not on k. 

In a study of the kinetics of two different gas-solid catalytic reactions it is found that all diffusional resistances are negligible. Also, both reactions are irreversible. As an aid in establishing the mechanism of the reactions the rate is measured at a constant composition over a wide range of temperature. For the first reaction, (1), the rate increases exponentially over the complete temperature range. For the second reaction, (2), the rate first increases and then decreases as the temperature continues to rise. What does this information mean with regard to the controlling step in each of the reactions ... 

Table 4.3 lists some typical gas-liquid hydrogenation reactions investigated in order to explore the features of three-phase catalytic membrane reactors. An example of the application of three-phase catalytic membrane reactors to the hydrogenation of sunflower seed oil can be found in Veldsmk (2001), where it was shown that for this hydrogenation running under kinet-ically controlled conditions the interfacial transport resistances and intraparticle diffusion limitations did not have any effect. Unfortunately the catalyst underwent a serious deactivation process. 

Given their lack of robust huhhles, interphase transfer resistances are much less likely to he rate hmiting for either the turbulent fluidization or the DSU regimes. Hence chemical kinetics is usually rate controlling, at least for exothermic reactions, in these two flow regimes. For endothermic catalytic reactions, the supply of sufficient heat is likely to also play a significant role. 

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