Catalyst, decaying diffusion

The effects of diffusion and catalyst decay cause yields from a continuous backmix reactor to be 25 to 30% lower than from a semibatch reactor at the same residence time. This yield penalty can be reduced by staging backmix reactors in series. 

Pore Diffusion. For a pellet, pore diffusion may strongly influence the progress of catalyst decay. First consider parallel deactivation. From Chapter 18 we know that reactant may either be evenly distributed throughout the pellet Mj < 0.4... 

Copper catalysts used in methanol synthesis and in the water gas shift reaction (WGSR) are irreversibly poisoned by small quantities of chlorine (in the feed) and by temperature (sintering). The effect of internal diffusion on catalyst decay by poisoning has been discussed in a previous paper [1]. 

M. Grzesik, J. Skrzypek and B. Wojciechowski, Modelling of intraparticle diffusion affected by the time-on-stream catalyst decay, Chem.Eng. Set, 47(1992)2805. 

M. Chocron, M.C. Raffo Calderon, N. Amadeo and M. Laborde, Effect on intraparticle diffusion on catalyst decay, Chem.Eng.Sci. 51(1996)683. 

For the reaction under consideration, values of the reaction rate as a function of concentration and catalyst decay time may be obtained by numerical differentiation of the experimental data obtained by Prasad and Doraiswamy (1974) at various space velocities and catalyst decay times in a fixed-bed reactor. The bed length was maintained small enough to give isothermal conditions to within 2°C. It was also ensured that the feed velocity was high enough, and the particle size small enough, to eliminate external and internal diffusion effects, respectively. The kinetic parameters, including decay time, will vary with the type of silica gel used. The data obtained for the silica gel used are summarized in Table CS3.1. 

The DVD-ROM contains PDF files of the last live chapters from the fourth edition of the Elements of Chemical Reaction Engineering, which is mostly graduate material. These chapters, which were omitted from this book but are included on the DVD-ROM are DVD Chapter 10. Catalyst Decay DVD Chapter 11, External Diffusion Effects on Heterogeneous Reactions DVD Chapter 12. Diffusion and Reaction DVD Chapter 13, Distribution of Residence Times for Reactors DVD Chapter 14. Models for Non Ideal Reactors and a new chapter. DVD Chapter 15, Radial and Axial Temperature Variations in a Tubular Reactor. 

Table 4-4 summarizes the ratings of the various reactors. The CFSTR and the recirculating transport reactor are the best choices because they are satisfactory in every category except for construction. The stirred batch and contained solid reactors are satisfactory if the catalyst under study does not decay. If the system is not limited by internal diffusion in the catalyst pellet, larger pellets could be used and the stirred-contained solids reactor is the better choice. However,... 

Swartz and Stenzel (1984) proposed an approach to widen the applicability of the cathode initiation of the nucleophilic substitution, by using a catalyst to facilitate one-electron transfer. Thus, in the presence of PhCN, the cathode-initiated reaction between PhBr and Bu4NSPh leads to diphe-nydisulfide in such a manner that the yield increases from 10 to 70%. Benzonitrile captures an electron and diffuses into the pool where it meets bromobenzene. The latter is converted into the anion-radical. The next reaction consists of the generation of the phenyl radical, with the elimination of the bromide ion. Since generation of the phenyl radical takes place far from the electrode, this radical is attacked with the anion of thiophenol faster than it is reduced to the phenyl anion. As a result, instead of debromination, substitution develops in its chain variant. In other words, the problem is to choose a catalyst such that it would be reduced more easily than a substrate. Of course, the catalyst anion-radical should not decay spontaneously in a solution. 

The observed deactivation of a porous catalyst pellet depends on a number of factors the actual decay reactions, the presence or absence of pore diffusion slowdown, the way poisons act on the surface, etc. We consider these in turn. 

Reaction-diffusion systems with decaying catalyst... 

In the various situations we have seen before, allowing a finite decay rate for the catalyst B has had significant results. The concentrations of A and B are then decoupled and this has allowed oscillations, isolas, and mushrooms. In the present case of reaction-diffusion waves, the uncoupling is again an important step upwards in complexity, sufficiently so as to prevent any completely general form of analysis. 

Vapor-phase alkylation of benzene by ethene and propene over HY, LaY, and REHY has been studied in a tubular flow reactor. Transient data were obtained. The observed rate of reaction passes through a maximum with time, which results from build-up of product concentration in the zeolite pores coupled with catalyst deactivation. The rate decay is related to aromatic olefin ratio temperature, and olefin type. The observed rate fits a model involving desorption of product from the zeolite crystallites into the gas phase as a rate-limiting step. The activation energy for the desorption term is 16.5 heal/mole, approximately equivalent to the heat of adsorption of ethylbenzene. For low molecular weight alkylates intracrystalline diffusion limitations do not exist. 

If the catalyst were not to decay but for some other reason, perhaps temperature control, the particles were taken out and recycled, then each might be supposed to be in pristine condition on entering the bed. Each particle would then undergo a transition during which the steady state profile of reactant within the particle would be built up. The analysis of Amundson and Aris (1962 this part is not tainted with the error mentioned above in fn. 13) may be used. We assume spherical particles of radius R, and call the profile of concentration at time a, c r, a). If D is the diffusivity of the reactant and k the rate constant per unit volume of catalyst,... 

For all four alcohols in the zeolitic catalysts with small enough crystallite sizes—when diffusion limitations also disappear—dehydration kinetics are well approximated by the exponental function, a fact that is explicable in terms of the unimolecular decay of molecules of butyl alcohol adsorbed on identical active sites. With isobutyl alcohol, for example, the rate coefficient k may be written... 

A refined model can be written to describe deactivation by diffusion and fouling within a catalyst pellet or crystal. Nevertheless, it cannot be used for modelling a whole reactor which demands in itself, a complex model to be solved. We propose a simple decay function which can be easily introduced in the kinetic equations of a reactor model. This function is experimentally determined. It has a physical meaning and it allows to describe different behaviours of feedstocks between pure site fouling and strong diffusional limitation by pore plugging. 

The yield of the catalyst, 0, was measured at various ethylene concentrations (see Fig. 10). According to the results, initiation is rapid and the catalytic system maintains full capacity for a long time, for at least 1 h. In this interval, the polymeric particles increase their size 5-10 fold. Thus the monomer supply into the pores of the particles by diffusion cannot be hindered. In the subsequent phase, activity already decreases. Either the conditions for monomer transport to the centres by diffusion are deteriorating, and/or the centres are slowly decaying. The polymerization rate, i>pol, can be determined from the slopes of the curves in Fig. 10. The determined values of the initial rates are directly proportional to monomer concentration (except for the lowest values of [M]), as shown in Fig. 11. 

Rate decay is mainly ascribed to a chemical deactivation of active centers. Nevertheless, in the case of ethylene, it appears that diffusive phenomena play also a certain role in the drop of the polymerization rate88 94. Moreover, diffusivity of monomer in the reaction medium may restrict polymerization rate, as can be concluded from the dependence of catalytic activity on catalyst concentration 95... 

The aim of this work is to analyze the effect of internal and interfacial diffusion on a catalyst pellet in the presence of an activity decay by simultaneous poisoning and sintering. The study has been applied to a copper based catalyst used in the WGSR CO + H2O = CO2 + H2, for which a Langmuir-Hinshelwood type kinetics has been considered [2]. 

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