(55e) Phase Equilibrium and Transport Properties of Water and Ionic Liquids Used in Biomass Processing

Ionic liquids (ILs) have been proposed in many potential engineering applications, including for biomass processing. Certain ionic liquids are known to have some of the largest known biomass solubilities; e.g. cellulose. To recover the biomass, often water is used as an anti-solvent. The ionic liquid and water mixture then needs to be separated and the ionic liquid recycled. The recycled IL must be highly purified (~98+%), otherwise the residual water may significantly decrease biomass solubility. The combination of ionic liquids and water occurs in a number of other applications, such as absorption refrigeration, reaction systems, separations, etc. Many ionic liquids are hygroscopic and can absorb substantial amounts of water when exposed to humid atmospheric conditions; these ILs often need to be purified for whatever their intended use. Here, the vapor-liquid equilibrium of water and two hydrophilic ILs, 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIm][DEP]) and 1-hexyl-3-methylimidazolium chloride ([HMIm][Cl]), was measured at temperatures up to 75 °C and relative humidity up to 95%. The viscosity and density of the water/IL mixtures are measured at varying temperatures and water compositions. The viscosity of both mixture systems decreases with increasing water content. Water uptake as a function of time is utilized to calculate the Fickian Diffusion coefficient of water in the ILs. Additionally, the diffusion coefficient alongside the measured viscosity is used in a Stokes-Einstein correlation to obtain the hydrodynamic radius of the water in the IL. The NRTL model is used to model the VLE data and approximate the heat of absorption of water in the ILs and give insight of the potential solute-solvent and solute-solute interactions occurring.