EFFECT OF OPERATING-CONDITIONS AND MEMBRANE QUALITY ON THE SEPARATIONPERFORMANCE OF COMPOSITE SILICALITE-1 MEMBRANES

The separation capacity of silicalite-l membranes for various hydrocar bon mixtures is determined as a function of membrane quality, operatin g conditions, and orientation of the composite membrane with respect t o the feed side. The quality of the membranes is judged on the basis o f the n-butane/i-butane permselectivity. Membranes with a different n- butane/i-butane permselectivity showed an identical separation capacit y for ethane/methane mixtures, but the quality difference was affectin g separation of hydrogen from the butane isomers. The selectivity of t he membrane is significantly affected by the operating conditions, suc h as mixture composition, temperature, and absolute pressure. These ef fects are shown for ethane/methane, propene/ethene, and n-butane/i-but ane mixtures. The selectivity for ethane in ethane/methane mixtures, f ound when the zeolite layer is facing the feed side, is completely los t when the orientation of the composite membrane is reversed, due to c oncentration polarization. Depending on the membrane orientation, the major resistance of the composite is in the support layer or in the ze olite layer.