Permeation and sorption of organic solvents and separation of their mixtures through an amorphous perfluoropolymer membrane in pervaporation

Abstract

The permeation and sorption of a few common organic solvents and separation of their mixtures through a polymeric membrane of perfluoro-2,2-dimethyl-1,1,3-dioxole copolymerized with tetrafluoroethylene (PDD–TFE) was investigated by vacuum-based pervaporation. Pure component permeations of common pharmaceutical solvents such as, toluene, methanol, ethyl acetate and tetrahydrofuran (THF), were carried out using a 25μm thick dense polymeric membrane supported by a porous polytetrafluoroethylene (PTFE) sheet. Overall permeability coefficients for these solvents along with those of aprotic solvents dimethylformamide (DMF) and dimethylacetamide (DMAc) determined earlier (Tang and Sirkar, 2012 [1]), plotted against their longest molecular diameters were correlated to yield a relationship between the molecular solvent size and their permeation through the membrane. The sorption of each pure solvent and a few of their mixtures were determined. Separation of a few organic–organic mixtures of these solvents was also implemented. These systems include: toluene–methanol, toluene–ethyl acetate and toluene–tetrahydrofuran. Modest separation factors were achieved using the PDD–TFE membrane unlike the very large values achieved earlier between water and aprotic solvents such as DMF, DMAc and DMSO (Tang and Sirkar, 2012 [1]). The maximum separation factor value for all systems was approximately 7.8 for a 75wt% toluene and 25wt% methanol feed at 30°C. The separation factor obtained using a 50wt% toluene and 50wt% THF feed at 50°C was close to 1. Maximum flux values through a 25μm thick PDD–TFE membrane were approximately 5g/(m2h) for toluene–ethyl acetate systems and 2.2g/(m2h) for toluene–methanol and toluene–THF systems. The membrane was typically selective for the organic solvent with the smaller molecular dimensions. The solubility of the individual components in a mixture provided unique insights into such behavior. The observed separation behavior provides a window into the complex phenomena of organic solvent transport through the PDD–TFE membrane.