VOC - air separations using gas membranes

Volatile organic components (VOC) are commonly emitted from chemical processes, ranging from dilute to concentrated streams. Membrane separation techniques are gaining increasing interest and acceptance for the recovery of VOCs. Various thin film composite membranes were experimentally assessed for the separation of gases and vapours. The membranes were used in a flat sheet configuration with a dense elastomeric silicone as the top layer on a porous polyether-imide (PEI) or polyvinylidene-fluoride (PVDF) support. Experimental results for pure substances and for binary mixtures were used to determine the permeability as a function of temperature and to calculate the selectivity towards nitrogen. Different models are available in the literature to predict the permeability of hydrocarbons and in this paper the free-volume model is evaluated. The predictions from this model overestimate the experimental permeabilities, even after corrections through integrating the concentration polarisation and resistance of the porous support. A design method is developed to calculate the necessary surface area of the membrane separation module for a given flow rate, concentration, operating temperature and pressure, and the characteristic (experimental) permeability and selectivity of the membrane. This design is applied as an example to a butane-air system. Finally, current industrial applications are briefly reviewed. (C) 2003 Society of Chemical Industry.