Mass transfer enhancement of gas absorption in oil-in-water systems: a review
Résumé
A review is presented on the gas–liquid mass transfer enhancement due to the presence of a second dispersed liquid phase. An attempt has been made to describe the mass transfer characteristics in a gas–liquid–liquid system. The ability of an immiscible oil phase to influence the possible pathway for gas transfer from the gas phase to the aqueous phase and to affect the gas–liquid interface and the volumetric mass transfer coefficient kLa is considered. Though the mass transfer in series looks the most logical explanation, there are many gaps and contradictions in the reported results of kLa, preventing any definite conclusion being reached. An enhancement factor (E), which quantifies the effect of the oil addition on the gas–liquid mass transfer, is defined. Experimental enhancement factors are reported and compared to the theoretical maximum attainable enhancement factor (Emax). Possible mechanisms (“bubble covering”, “shuttle effect” and “permeability effect”) involved in mass transfer enhancement are assessed in detail. The commonly used “shuttle effect” mechanism, whose model proposes a direct usable expression of the enhancement factor, underestimates the reported experimental enhancement factors by about 20%. However, to date, it is not possible to satisfactorily propose a unique theory explaining the influence of the presence of an immiscible oil on mass transfer enhancement. Moreover, the development of sophisticated models has not yet reached satisfactory levels. Recommendations have been made for future research.