Book Description
Pervaporation is a membrane based process which overcomes many of the deficiencies of current technologies for the removal/recovery of volatile organic chemicals (VOCs) from aqueous streams. In this process, VOCs from a liquid stream are driven across a permselective membrane and exit as a vapor due to the vacuum maintained in the downstream side of the membrane. Proven hydrophobic membranes used for pervaporation today suffer from limitations of mechanical stability, while membranes with superior mechanical characteristics do not possess sufficient selectivity to be useful for these applications. To overcome these limitations surface modifying macromolecules (SMMs) have been used as additives in the preparation of polyethersulfone (PES) membranes, which inherently have good mechanical characteristics but are intrinsically hydrophilic. This approach produced membranes with high hydrophobicity (based on contact angles), which were expected to be permselective to VOCs, like chloroform. This thesis investigates the impact of PES/SMM membrane preparation parameters on the reported separation of chloroform from aqueous solutions via pervaporation. These parameters include polyvinylpyrrolidone (PVP) addition levels, solution mixing period, solution age and membrane age. A chemical analysis of the permeate revealed that the permeate contained ethanol but virtually no chloroform. Permeate ethanol concentrations were higher with fresh membranes and decreased both with membrane age and period of use. This indicates that ethanol which was used during the solvent exchange drying step of membrane preparation, was being leached from the membrane. It was concluded that there was in fact no enrichment of chloroform in the permeate as reported earlier and that the surface hydrophobicity introduced was insufficient to dominate over the intrinsic bulk hydrophilic characteristics of the PES membranes. These findings indicate a need to reevaluate the levels of SMM addition and the process parameters to develop a sufficiently dominant hydrophobic surface layer.
