Simultaneous absorption with reversible instantaneous chemical reaction

Abstract

The approach presented recently [1] for analyzing absorption and desorption mass transfer problems with instantaneous chemical reaction is extended to the case of simultaneous absorption of two gases, A and A′. The analysis is developed for arbitrary stoichiometry. The following simple case is discussed in detail: A + B 1⇌ B 2 A′ + B 1⇌ B 3 where B 1 is the liquid phase reactant and B 2, B 3 the reaction products. The analysis takes into account the “shift” reaction, which for the simple case above is: A + B 3⇌ A′ + B 2 This reaction takes place in the region near the interface. The analysis differs from previous work which, with one exception, ignored the “shift” reaction and restricted attention to zero values of the concentrations of B 2 and B 3 in the bulk liquid. The analysis shows that the conditions where the physical driving forces ( a i- a o) for absorption of both gases are large and positive does not imply that the chemical driving forces (α i -α o ) are both positive. In fact, it is shown that cases arise where one component may desorb even though its physical driving force is positive. A simplified thermodynamic model useful for extrapolation of mixed CO 2 and H 2S equilibrium data in amine solutions to very low values of acid gas loading in solution is developed. Tower profiles for simultaneous absorption of CO 2 and H 2S in monoethanolmine solution are considered in light of the new analysis. The good kinetic selectivity measured for H 2S at the absorber lean end is due to the fact that carbon dioxide is not absorbed in the instantaneous reaction regime. At the absorber rich end, where a temperature bulge develops, CO 2 is absorbed in the instantaneous regime, causing H 2S to be desorbed even though its physical driving force favors absorption.