Crosslinked Copolyimide Membranes for Phenol Recovery from Process Water by Pervaporation

The effectiveness of different copolyimide membranes in the process of recovering phenol from water by pervaporation has been investigated. The polyimides were obtained by the polycondensation of 6FDA (4,4′-hexafluoro-isopropylidene diphthalic anhydride) with different diamines. The diamines 4MPD (2,3,5,6-tetramethyl-1,4-phenylene diamine), 6FpDA (4,4′-hexafluoro-isopropylidene dianiline), 6FpODA (4,4′-bis-(4′-aminophenoxyphenyl)hexafluoropropane), and DABA (3,5-diaminobenzoic acid) as a monomer providing a crosslinkable group, were used. In order to reach chemical stability at high phenol concentrations, the polymer structures were crosslinked with 1,10-decanediol and OFHD (2,2,3,3,4,4,5,5-octafluorohexanediol). Pervaporation experiments were performed at 60 °C, covering a concentration range of phenol between 2 and 11 wt. %. The best separation characteristics were obtained with a 6FDA-6FpDA/DABA 2:1 membrane crosslinked with 1,10-decanediol. Using a 7.8 wt. % phenol feed mixture, a total flux of 14 kg μm m⁻² h⁻¹ was reached with an enrichment of 40 wt. % phenol in the permeate. It was found that conditioning the membrane using high phenol concentrations (between 8 and 11 wt. %) is a necessary pretreatment in order to enhance the flux and improve enrichment, especially if process water with low phenol concentrations is to be treated. In addition to the experimental results, a comparison with rubbery membrane materials is presented in the discussion.