Interactive porous particles

If we consider the chemical interactions within a catalyst grain, the number of elementary processes involved can become significant since, in the case of a catalytic reaction where only reactant A reacts at the surface of a catalyst grain to give a product P, we already observe seven elementary processes (figure 1 ), i.e. :

• a transfer by external diffusion of the reagent A through the boundary layer located in the vicinity of the grain ;

• transfer by internal diffusion of reactant A into the porous catalyst structure. This transfer can take place solely by diffusion, but convective flow can also occur in certain cases;

• chemisorption of reagent A on an active site on the inner surface of the grain;

• a reaction between adsorbed species to transform adsorbed reagent A into adsorbed product P ;

• desorption of adsorbed product P (inverse of A adsorption);

• internal diffusion transfer of product P into the porous catalyst structure (inverse of internal diffusion transfer of reactant A);

• external diffusion transfer of product P into the boundary layer near the grain (inverse of external diffusion transfer of reactant A).

The three steps - adsorption of A, reaction between adsorbed species and desorption of P - constitute the reaction process and are integrated into the expression of the rate of catalytic chemical transformation. This rate is generally expressed per unit mass or volume of catalyst (r _M or r _V ).

In the transfer processes, we have neglected diffusional transfer in the adsorbed phase. We shall see that, under certain conditions, taking these transfers into account can significantly modify the performance observed at the catalyst grain level.

For chemical transformations involving several reactants and/or products, or several independent reaction steps (chemical transformations with multiple stoichiometries), as well as for non-isothermal and/or non-isobaric chemical transformations, the number of elementary processes is higher, and energy and/or momentum balances need to be associated with material balances.

Analysis of the interaction at the level of a catalyst particle is necessary to achieve one of two objectives:

• specify the flow rate of reactant consumed for a given chemical...