Ways of Using Heterogeneous Catalysts

There are cases where it is actually desirable to operate under conditions of mass-transport control this is so for example where it is an intermediate product that is wanted fat-hardening is a case in point. More usually, however, one wishes to work under conditions where conversion is as close as possible to 100% and here it is inevitable that mass-transport control will apply, at least at the end of the catalyst bed. The physical structure of the catalyst then becomes of great importance, and much thought and skill is exercised in maximising access of reactants to the active centres. The form of reactor and the appropriate physical form of the catalyst have to be chosen with care. 

If the reactants are in the gas phase, the reactor may be either static or dynamic. Small static reactors are convenient for basic research where either the reactants are expensive (e.g. isotopically labelled molecules) or the reaction slow. Dynamic reactors, where reactants flow through the catalyst bed, provide a better simulation of practical use in a fixed bed reactor the catalyst remains in place, and it is in the form of large particles or pellets. 

Alternatively it may take the form of a ceramic or metallic monolith, of which a variety of physical shapes is available monoliths are now widely used as supports for the active catalyst, which lines the channels which permeate the structure. They find particular application for the control of exhaust from vehicles powered by internal combustion or diesel engines. If the catalyst particles are small enough, a fast flow of reactants causes the bed to expand and the particles to move about like molecules in a liquid. We then have a fluidised bed reactor, which affords a more uniform temperature profile than is possible in fixed bed reactors, and is therefore more apposite to strongly exothermic reactions. 

In three-phase systems, where the solid catalyst is in contact with a liquid reactant or its solution plus a gaseous reactant, efficient agitation is required to effect dissolution of the gaseous molecule into the liquid and its transport to the catalyst surface. Such systems easily become mass-transport limited, especially when a very active catalyst is used. In a batch reactor, rapid shaking or stirring is needed, and catalyst particles must be small it may operate at atmospheric pressure, or at superatmospheric pressure as an autoclave. Large catalyst particles can however be used with liquid reactants either in a trickle-column reactor or a spinning-basket reactor. 

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