Mass Transfer Characterization of Ionic Liquid Solvents for Extracting Phenol from Aqueous Phase in a Microscale Coiled Flow Inverter

Abstract

In this work, the extraction of phenol from aqueous solutions toward the ionic liquids 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [Bmim][NTf 2 ] and 1-hexyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide [Hmim]-[NTf 2 ] was studied. To this aim, a microfluidic coiled flow inverter (CFI) was utilized in which the ionic liquid phase is contacted with the aqueous phase containing 2 wt % phenol. Segmented flow is generated and is intensified by the coiling due to leveraging Dean forces. As most common parameters that optimize such flow mode, the effects of flow velocity and volumetric flow ratio on extraction performance were investigated. Mass transfer coefficients of 1.52 and 0.84 s −1 were determined, respectively, for [Bmim][NTf 2 ] and [Hmim][NTf 2 ] at a maximum flow velocity of 0.63 m/s. This coiled flow inverter-intensified extraction performance was much superior to that of the best equipment reported and to the common solvents used therein. By decreasing the aqueous to organic volumetric flow ratio, the mass transfer coefficient was increased since radial mixing along the tube became more efficient. As expected for a Dean-enforced segmented flow, [Bmim][NTf 2 ] with its lower viscosity showed better performance as a consequence of assessing relatively higher Dean and Reynolds numbers. A practical model that predicts the mass transfer coefficient as a function of the microfluidic flow regimes was developed with relative deviations of 17% compared to experimental data for both ionic liquids. Besides delivering such phenol-extraction process-related information, generic conclusions are depicted on the suitability of using ionic liquids in microfluidics (in view of their viscosity and recirculation capacity) and the enhancing role of the CFI.