Composite membranes for hydrophobic pervaporation: study with the toluene–water system

Composite membranes were utilized for hydrophobic pervaporation. Experiments were carried out with the toluene–water system using composite membranes (PERVAP®1060 and PERVAP®1070) and the results were analysed. The chosen membranes were characterised using a positron annihilations technique to measure free volume. In order to study solute–membrane interactions, the diffusion coefficient of toluene with in the membrane and Flory–Huggins interaction parameters (surface thermodynamic approach through contact angle measurement) were estimated. Influences of operating conditions (downstream pressure, feed toluene concentration, feed temperature) were observed on pervaporate fluxes (toluene and water) and pervaporate concentration of toluene. The selectivities of the chosen composite membranes were observed to be lower compared to known values of selectivities for dense PDMS membrane, suggesting the role played by the support layer in this regard. Individual fluxes of toluene and water increase with increase in feed temperature; however, fluxes decrease with increase in downstream pressure. Further, pervaporate concentration of toluene increases with increase in feed concentration. Similar expected trends were observed; but in case of PERVAP®1070, the toluene flux attained a plateau with increase in feed toluene concentration. Such a trend confirmed the presence of an extra component (possibly zeolites) in the skin layer of PERVAP®1070. A simple resistance-in-series model, along with the solution-diffusion model were employed for mathematical analysis of the results. Model predictions with experimental values showed close agreements for the PERVAP®1060 membrane while deviations were observed for the PERVAP®1070 membrane