Ethanol recovery from dilute aqueous solution by perstraction using supported ionic liquid membrane (SILM)

Abstract

The recovery of ethanol from dilute aqueous solution by supported ionic liquid membrane (SILM) perstraction was investigated using three potential extracting solvent of trihexyl(tetradecyl)phosphonium ionic liquids (THTDP IL): [THTDP][Br], [THTDP][N(CN)2] and [THTDP][Tf2N], and three potential polymeric microporous membranes: polyvinylidene fluoride (PVDF) (hydrophilic Durapore® GVWP and hydrophobic Durapore® GVHP) and polypropylene (PP). The solvent was chosen based on hydrophobicity, thermal stability and negligible vapor pressure; while the membrane was chosen based on physicochemical properties, commercial availability and affordability. The optimized combination of solvent and membrane was assessed in a perstraction system. The [THTDP][N(CN)2]-GVHP SILM gave the best overall performance in terms of extraction and stability. At a feed concentration of 2 wt% ethanol, the selected SILM was able to maintain its functionality for ∼240 h without observable phase intermixing. Despite being subjected to constant lateral shear on the aqueous side, the SILM retained its integrity by maintaining a high ethanol flux of > 2.2 kg/m2·h (vs. ∼10-4 – 10-1 kg/m2·h in pervaporation) and selectivity of > 320 (vs. ∼20 in liquid-liquid extraction). Subsequently, the extracted ethanol was recovered from IL by single-stage vacuum-distillation with a final purity of 80% (vs. ∼20% in single-stage vacuum-distillation alone) and an overall selectivity of 200. The long-term stability, high flux and superior selectivity of the SILM perstraction system makes it a promising alternative to other energy intensive purification techniques such as distillation for the extraction of dilute ethanol as low as 2 wt% from fermentation broths.