(477f) Membrane Distillation for Separating Water from Ionic Liquid Solutions

Ionic liquids, salts molten at temperatures below 100 °C, have exciting potential as “green” recyclable solvents for industrial processes.  Ionic liquids have been found that can dissolve the lignin from biomass at relatively low temperatures.  The pretreated biomass is then ready for hydrolysis and fermentation to produce biofuels. Water is added to the ionic liquid solution to surround the ions so that the dissolved lignin precipitates out. This water must be removed prior to recycling the ionic liquid for pretreatment.  Two ionic liquids known to be effective in biomass pretreatment are 1-ethyl-3-methylimidazolium acetate and 1-ethyl-3-methylimidazolium formate. These were diluted with water and their conductivities and densities measured at varying concentrations.  Membrane distillation of the ionic liquid – water mixtures was performed using two different well-characterized PTFE membranes.  The feed was maintained at 55 °C and the distillate at 20 °C. The feed and distillate were circulated counter-currently on opposite sides of the membrane at 1.5 L/min. Water flux obtained was ~10 L/m²∙h.  Concentration of IL in the feed concentrate and distillate was determined by conductivity and density measurements. The membrane distillation process provided a concentration factor of 10 (from 5% IL to 50% IL) with near 100% rejection; as no ionic liquid was detected in the final distillate.  Membrane distillation is a promising technique for re-concentrating ionic liquids for recycling.