Organic solvent mixture separation during reverse osmosis and nanofiltration by a perfluorodioxole copolymer membrane

Highly selective separation of one organic solvent from its mixture with another organic solvent by reverse osmosis will be very useful. Using a 1.67 μm thick film of a particular variety of a glassy amorphous copolymer, perfluoro-2,2-dimethyl-1,3-dioxole copolymerized with tetrafluoroethylene, (PDD-TFE), supported on an e-PTFE support, organic solvent reverse osmosis (OSRO) separations of a number of types of binary organic solvent mixtures were studied. A few studies employed a 0.6 μm thick film of the same copolymer variety designated CMS-7. The feed pressures were varied between 1000 and 5000 kPa; most of the data involved 2500–3500 kPa. Different types of solvent mixtures were studied (similar classes mentioned together): ethanol-NMP, ethanol-dimethylsulfoxide (DMSO); methanol-DMSO, methanol-NMP; toluene-methanol, toluene-N-Methyl-2-pyrrolidone (NMP), toluene-dimethylformamide (DMF); heptane-dodecane; heptane-ethanol. In quite a few cases, a pure solvent was obtained as permeate; the permeate flux could be as much as 3.5 L/m²-hr depending on the feed composition and pressure. Since this behavior is unusual in reverse osmosis, mixture sorption studies were implemented. Mixture sorption data of various mixtures in the copolymer show exclusive sorption by the species showing up as the pure permeate under certain conditions. These and other observed OSRO permeation rate and sorption behavior in the perfluoro-copolymers explain the basis for such extraordinary selectivity for quite a few systems. To illustrate OSRO-facilitated solvent-exchange in organic solvent nanofiltration, studies were carried out also to retain the dye Oil Blue N using the following binary solvent mixtures: toluene-NMP; methanol-NMP. These and other potential applications of highly selective OSRO have been discussed.