Hydrophobic pervaporation: toward a shortcut method for the pervaporation-decanter system.

Abstract

Pervaporation is a relatively new technology. Although hydrophilic pervaporation has become established, hydrophobic pervaporation for recovery of organics from water has not been a commercial success. Technologic reasons for this are suggested. However, as the pressure to include waste minimization and to recycle, as well as pollution prevention, increases, there will be opportunity for the development of new wastewater treatment processes. This may lead to hybrid processes, including a coupling of pervaporation with conventional technology. The hybrid process examined herein is a simple pervaporation-decanter system that is applicable to organics with limited solubility in water. In this system, the PV unit produces permeate that after condensation gives two liquid phases. The organic phase is relative pure and concentrated (and available for reuse), whereas the aqueous phase can be recycled into the PV feed stream. For a given feed concentration and water purity target, there is a minimum membrane selectivity (alpha(min)) that yields a two-phase condensate. If the membrane has a selectivity that is just greater than the minimum, the recycle rate of the aqueous phase relative to the feed rate will be very large and likewise the membrane area. Also, for a membrane with known organic permeability, one can define a theoretical A(min), the minimum membrane area required when the separation factor is so high that the aqueous phase is negligible. For membranes with alpha > alpha(min), values of the required membrane area, A, have been obtained for various representative duties, and a correlation between alpha/alpha(min) and A/A(min) has been obtained (compare with the Gilliland correlation used in distillation). An approximate relationship is (A/A(min) (- 1))(alpha/alpha(min) (- 1)) = 1. Since alpha(min) and A(min) can be calculated readily, this relationship is a shortcut tool that permits estimation of A for any alpha and any duty. Finally, membrane areas for the above hybrid system and those for PV alone are compared.