Influence of intraparticle diffusion on selectivity

Wheeler s treatment of the intraparticle diffusion problem invokes reaction in single pores and may be applied to relatively simple porous structures (such as a straight non-intersecting cylindrical pore model) with moderate success. An alternative approach is to assume that the porous structure is characterised by means of the effective diffusivity. (referred to in Sect. 2.1) which can be measured for a given gaseous component. In order to develop the principles relating to the effects of diffusion on reaction selectivity, selectivity in isothermal catalyst pellets will be discussed. 

Attention will be focussed on three typical chemical reaction schemes. For the first illustration, two parallel competing reactions are considered. For instance, it may sometimes be necessaru to convert into a desired product only one component in a mixture. The dehydrogenation of six-membered cycloparaffins in the presence of five-membered cycloparaffins without affecting the latter is one such example of a selectivity problem in petroleum reforming reactions. In this case, it is desirable for the catalyst to favour a reaction depicted as 

however, both reactions were influenced by intraparticle diffusion effects, the rate of reaction of a particular component would be given by the product of the intrinsic reaction rate, fecg, and the effectiveness factor, Tj. Substituting eqn. (6) for the effectiveness factor gives (for a first-order isothermal reaction) the overall rate as 0tanh . As is often the case, the molecular weights of the diffusing reactants are similar and can be 

The conclusion, therefore, is that when parallel competing first-order reactions occur in isothermal pellets with large pores the intrinsic selectivity is unaffected. However, in Izurge pellets with small pores, the selectivity reduces to the square root of the value for the unimpeded reaction. Thus for large 0 

The corollary is that, when intraparticle diffusion impedes reaction, maximum selectivity is achieved by operating with small pellets and large diameter pores. 

---