(82a) Extraction of Lcst-Type Hydrogen-Bonding Organics from Aqueous Solution

Compared to distillation, liquid-liquid extraction often is better suited to recovering hydrogen-bonding organic solutes from dilute aqueous solution. This presentation will focus on the class of hydrogen-bonding organics characterized by the presence of a lower critical solution temperature (LCST) in the organic + water phase diagram, including glycol ethers, aliphatic amines, polyglycols, and nonionic surfactants. Even when distillation is technically feasible, extraction can allow a significant reduction in energy consumption. General rules for the temperature-dependence of relative volatility and partition ratios will be developed from representative activity-coefficient data: 1) The value of relative volatility with respect to water at infinite dilution is likely to increase with increasing temperature for stripping volatile LCST-type hydrogen-bonding organics from dilute aqueous solution, provided the pure-component vapor pressure relative to water also increases or stays about the same. 2) For extraction of LCST-type compounds from aqueous solution, the partition ratio is likely to increase with increasing temperature provided mutual solubility between phases is low. The application of MOSCED and UNIFAC activity coefficient prediction methods for screening extraction solvents also will be described. Example calculations will be compared with K data for extraction of propylene glycol n-propyl ether (PnP), a model hydrogen-bonding compound.