Absorption in a Pseudo-First-order Regime with Two Irreversible Reactions: Influence of the Second Reaction on the Transfer Rate

Gas-absorbing liquids such as CO2 often undergo two reactions: a main reaction and a secondary reaction. The solute flux transferred is a result of both reactions. Kinetics is one of the most important properties in the design of an absorber. To access the rate constant of the main reaction, the assumption of a pseudo-first-order (PFO) regime is often made, but the criteria for the system to be in a pseudo-first-order regime change between researchers. Furthermore, the impact of the secondary reaction is either neglected or overestimated. Recently, some researchers carried out simulations of multiple parallel gas–liquid reactions in the instantaneous regime and developed generalized criteria for this regime. In this study, to estimate the impact of the secondary reaction in a PFO regime, hundreds of simulations were carried out with two irreversible reactions within the framework of the film theory. This work proves that for Ha1 > 10, Ha2 > 10, Ha1/E∞,1 < 0.5, Ha2/E∞,2 < 0.5, normalized concentration profiles depend only on three factors: Ha1/ E∞,1, Ha2/ E∞,2, and Ha1/Ha2. Curves and generalized equations are provided enabling the estimation of the concentrations of reactants at the interface, the enhancement factor, and the influence of each reaction on the gas transfer flux (average absolute relative deviation ≤ 0.50 %). New criteria for determining whether the system can be considered in a pseudo-first-order regime are defined, giving accurate predictions with a success rate of 96.2 %.