Nearly irreversible, fast heterogeneous reactions in premixed flow

Abstract

When heterogeneous chemical reaction is sufficiently fast, transport of reactants becomes limiting. In a fixed bed reactor, macroscopic concentration gradients cannot be eliminated as a factor limiting the rate of reaction, possibility coupling to the mesoscopic mass transfer of reactants to the surface of the catalyst as limiting, if the reaction does not occur inside a porous support. A theory for strictly irreversible binary reaction is developed that shows the possibility of regimes of kinetic asymmetry in which a crossover point occurs internally in the reactor, demarking a region of high supersaturation in which the surface is effectively depleted of one reagent, followed by a region of rapidly decaying supersaturation, where the surface is effectively depleted of the other reagent. The parametric dependence of this crossover point is given in terms of a transcendental equation which depends on operating parameters (superficial velocity and inlet concentrations) and ratios of transport properties of the reagents. These solutions are corroborated by full nonlinear numerical computations of the boundary value problem, for the case when asymmetric mass transfer coefficients admit the possibility that the mode of operation switches from relative surface depletion of one reactant to depletion of the other in a binary reaction. It is shown that X indicates the region of greatest molecular efficiency in the reactor, and if operating parameters are so chosen such that X is large, high conversion is achieved in the precrossover region. The modified Thiele modulus analysis of the numerical solutions identifies the crossover point and the region of greatest product formation.