Influence of operation parameters on the separation of mixtures by pervaporation and vapor permeation with inorganic membranes. Part 2: Purely organic systems

Abstract

The performance of commercially available tubular Y-type zeolite membranes was studied for organic separations by vapor permeation. The separation characteristics were determined as function of operating conditions such as concentration, temperature, permeate and feed pressure. The production quality of the tested membranes, in terms of flux and selectivity, was reproducible within a range of 10%. The partial flux of both the preferentially permeating and the retained components generally increase with partial pressure difference in feed and permeate. A constant permeance model may be used to estimate the flux of lower alcohols in mixtures with various non polar organics. Y-type zeolite membranes exhibit permeance values of 2.5 and 1.2 kg / m 2 h bar , respectively for methanol and ethanol and a separation factor over toluene of 100 for methanol and 70 for ethanol, both determined at moderate to high alcohol concentration. There are, however, considerable differences between ideal and mixture selectivities and the error for the flux of the retained component may become large at small concentrations of the preferentially permeating component. The most remarkable abnormality in flux behavior can be observed for a partially Ca-exchanged NaY-Zeolite that shows strongly preferential permeation for methanol at higher methanol concentrations and a marked selectivity in favor of toluene at low methanol concentration. The flux increases with temperature according to an Arrhenius-function, while selectivity does not change significantly. The selectivity can be changed by ion-exchange of the incorporated counter-ions in the zeolite lattice. The separation mechanism is mainly based on preferential adsorption and differences in mobility rather than on size or shape selectivity. With regard to the operation stability of the membranes when separating purely organic systems with the Y-type zeolite, pore blocking, adsorption and coking occurred and reduced the measured flux rates with operation time. The membrane could be partly regenerated by thermal treatment with a nitrogen sweep at 200 °C under vacuum. The installation of a wire mesh demister to reject droplets of heavy boilers in the feed vapor solved the problem.