Effect of fluid properties on ultrasound assisted liquid-liquid extraction in a microchannel

When immiscible liquids are subjected to an ultrasonic field, they form emulsions. This principle has been used to improve the mass transfer characteristics of a liquid-liquid extraction process in microreactor systems. The formation of emulsion and its characteristics are prominently dependent on the properties of the liquids used and this also holds true for emulsion brought about by ultrasound. This paper focuses on the properties of fluids that are reported to have an influence on the cavitation behaviour, namely viscosity, interfacial tension and vapour pressure. These properties were examined by changing the solvent of the organic phase in the hydrolysis of p-nitrophenyl acetate. The study is performed by comparing pairs of solvents that are different in one property but similar in the other two. The pairs selected are toluene - chlorobenzene for viscosity, toluene - methyl Isobutyl ketone for interfacial tension and methyl isobutyl ketone - 2-Methyl tetrahydrofuran for vapour pressure effects. A qualitative study was performed with a high-speed camera in flow to understand the emulsification initiation mechanisms and behaviours. These findings were further explored by performing the sonicated emulsion in a batch-sonicated reactor. The quantitative analysis of the fluid properties was evaluated and compared based on the relative percentage increase in yield upon sonication with respect to their individual silent conditions. The quantitative results were further supported by the quantification of the emulsion performed with an FBRM probe. The results indicate a two times improvement in yield with solvent of lower viscosity as 2 times more droplets were formed in the emulsion. Both the solvent systems with higher interfacial tension and vapour pressure had an improved yield of 1.4 times owing to larger number of droplets formed.