Reaction selectivity in a porous catalyst pellet: analysis of a kinetic model of two parallel, first order, irreversible reactions with a second order inhibition kinetic term in one of them

Abstract

This work deals with the analysis of a kinetic model of two parallel, first order, irreversible reactions that include a second order inhibition term in one of them (i.e. A+b 1 B → k 1C A/(1+KC A) 2 C, A+b 2 B → k 2C A D) . The continuity differential equation taking account of isothermal diffusion and reaction of A in a spherical catalyst pellet, was formulated. The numerical solution of the latter equation yielded useful results related to the variation of the effectiveness factor ( η) and the selectivity (S) (S=[k 1C A/(1+KC A) 2]/[k 2C A+k 1C A/(1+KC A) 2]) for the desired reaction (i.e A+ b 1B→C) versus the Thiele Modulus ( ϕ). Parametric studies involved the investigation of the effects of k= k 2/ k 1, the ratio of the intrinsic specific reaction rate constants, and the inhibition strength factor (i.e. KC A ), upon the η vs. ϕ and the S vs. ϕ curves. The η vs. ϕ curve turns faster towards lower η values for high k values, especially at high inhibition KC A values. Intraparticle diffusion imparts a pronounced effect upon selectivity, a fact contrasting markedly from the standard case of two parallel reactions without inhibition, where selectivity is independent of diffusion resistance. The S vs. ϕ curves show a step increase occurring at specified values of ϕ that increase at high inhibition strengths. The relative selectivity S′(= S/ S 0) vs. ϕ curves (where S 0 is the selectivity for reactant concentration at catalyst surface) increases monotonically with k and KC A values and go through a maximum for high ϕ values.