Separation of liquid mixtures by membranes.

Abstract

The study focused on the interface, morphology and transport involved in pervaporation and reverse osmosis separation of liquid mixtures. The first part of this study concerns pervaporation separation of liquid mixtures. A fluorine-containing surface modifying macromolecule (SMM) was added into a polyethersulfone (PES) membrane casting solution. Because of its lower surface energy, SMM is expected to move up to the top surface of the membrane. The resulting membrane is to have a layer of SMM at the top surface and a PES rich bulk substrate underneath. Asymmetric PES/SMM membranes were fabricated by the phase inversion method under various conditions and tested in pervaporation separation of chloroform/water mixtures. The principal findings are: (i)The contact angles of water on the surface of PES/SMM membranes were significantly larger than those of PES membranes. (ii) The XPS results indicate a much higher fluorine concentration at the membrane surface than in the bulk and upwards orientation of SMM's fluorine tail at the membrane surface. It is concluded that SMM migrates to and accumulates at the membrane-air interface. The membrane surface can be covered by a layer of SMM. (iii) A PES/SMM membrane has a superior performance to a PES membrane for pervaporation separation of chloroform/water mixtures, since the former has a much higher water selectivity than the latter. (iv) A PES/SMM membrane has a higher permeation rate of n-heptane and a lower permeation rate of ethyl alcohol than a PES membrane in pervaporation separation of ethyl alcohol/n-heptane mixtures. The second part of this study concerns reverse osmosis of liquid mixtures. A new method to determine the preferential sorption in binary mixtures based on liquid chromatography is presented. Liquid chromatography and liquid sorption experiments were performed for cellulose acetate butyrate (CAB)/ethyl alcohol/n-heptane system. An interaction force constant characterizing the interaction between the feed species and the membrane was generated and incorporated in the transport equations based on a pore model. Transport simulation was performed for reverse osmosis separation of ethyl alcohol and n-heptane by the CAB membrane. It was found that (i) Both CAB powder and homogenous membrane preferentially sorbed ethyl alcohol from the binary liquid mixtures of ethyl alcohol and n-heptane; (ii) Sorption data can be calculated based on the liquid chromatography data. The good agreement between the two indicates the validity of the proposed approach; (iii) The binary liquid mixtures can be separated by reverse osmosis, and its performance can be calculated using the transport equations based on a pore model. In the last part of this study, reverse osmosis and pervaporation experiments were performed and their results were compared for two cases, namely, separation of ethyl alcohol/n-heptane mixtures by the CAB membrane, and separation of a vinyl acetate/hexane mixture by four different membranes. It was found that pervaporation had a higher separation than reverse osmosis in the separation of ethyl alcohol/n-heptane mixture and vinyl acetate/hexane mixture. (Abstract shortened by UMI.)