Modelling of simultaneous absorption of H2S and CO2 in alkanolamine solutions: the influence of parallel and consecutive reversible reactions and the coupled diffusion of ionic species

Numerical models, based on Higbie's penetration theory, were developed to study the effect of the coupled diffusion of ions and the effect of parallel and consecutive chemical reactions on the mass transfer rate for the simultaneous absorption of H2S and CO2 in aqueous solutions of (mixtures of) alkanolamines. Prior to this complicated system, gas absorption accompanied by a single reversible reaction and in the presence of an inert salt has been studied in order to determine clearly the effect of coupled ion diffusion on the mass transfer rate. From the latter model simulations it was concluded that ion diffusion and consequently ion decoupling can have significant effect on the mass transfer rate, although a rather special set of conditions is required. Model simulations for the simultaneous absorption of CO2 and H2S showed that incorporation in the flux model of all relevant reactions, instead of only the direct reactions between CO2 and H2S and alkanolamines, only leads to more realistic concentration profiles and not to changes in absorption rate predictions for practical conditions. Correct modelling of ion diffusion does give significant though minor changes in absorption rate predictions: CO2 absorption is reduced and H2S absorption increased.