AIR SEPARATION CHARACTERISTICS WITH LIQUID-CRYSTALLINE ALKYL CELLULOSE BLEND THIN-FILM COMPOSITE MEMBRANES

Composite membranes were made of liquid crystalline triheptyl cellulose (THC)/ethyl cellulose (EC) blends as dense thin films and poly(ether sulfone) (PES) or polysulfone (PSF) as porous support layer. The effects of the composite membrane composition and operating conditions on the air separation characteristics of oxygen-enriched air (OEA) permeating through the membranes were studied using a constant pressure-variable volume method. The flux (OEA) through the membranes decreases slightly and the oxygen concentration in the OEA permeated increases with increasing THC content in the thin film from 4 to 15 wt.-%. The OEA flux increases significantly with decreasing thin-film thickness or increasing operating temperature and transmembrane pressure difference. The oxygen concentration in the OEA increases with increasing the thin-film thickness or the pressure difference but decreases slightly with increasing the operating temperature. There is no regular variation in the air separation properties by changing the support from PES to PSF. In long-term tests, the air separation properties remained almost constant for as long as 800 h. An OEA flux of 1.0-1.9.10(-3) cm3 (STP)/s.cm2 containing 34.8-39.4 vol.-% oxygen can be attained at 30-55-degrees-C and 0.41-0.49 MPa pressure difference in a single pass through the membranes. The OEA flux is much higher for the thin-film composite membranes than for the homogeneous dense membranes made of the same materials.