Dynamic rate analysis for low frequency operation of chemical reactors

Abstract

Periodic operation of chemical reactors can enhance reactant conversion and product selectivity. It is desirable to identify reaction kinetics and networks that may benefit from non-steady state operation and to quantify the extent of that enhancement without extensive computations. In this study, we describe a simple method to establish approximate functional dependencies of isothermal point reaction enhancement on dynamic and kinetic parameters during low frequency operation. Low frequency operation permits the use of steady state as opposed to transient conservation equations to identify limits of fractional rate enhancement. By approximating local dynamic rates as square waves modulating between two steady states, an analytical function can be derived from global rate expressions. The function relates fractional rate enhancement to concentration wave amplitudes, phase shifts, cyclic averages, duty cycles and reaction orders for one or more dynamically-fed reactants. We show that the method may be applied to predict integral conversion enhancement for ideal reactors. During integral operation, nonstoichiometric concentration waves may change phase through disproportionate consumption of reactants at wave maxima and minima, hence inverting waves out-of-phase (180°) from their initial waveform. This may cause local rate enhancement to transition to rate diminishment. Three experimental examples involving catalytic oxidations are described that illustrate selected theoretical aspects of the study.