Experimental configuration and adsorption effects on the permeation of C 4 isomers through ZSM-5 zeolite membranes

Abstract

Butane isomer permeation through two types of ZSM-5 zeolite membranes was studied as a function of temperature using three experimental configurations: pressure drop, sweep gas on the permeate side, and vacuum on the permeate side. For one type of membrane, which has a significant permeation through small non-zeolite pores, separation occurs by preferential adsorption and pore blocking. For these membranes, n-butane/i-butane separation selectivities are higher than ideal selectivities, and are much higher when a pressure drop is used (maximum selectivity of 140). For single gases, the larger i-butane molecule permeates faster than n-butane in this type of membrane with the pressure drop method, apparently because the i-butane coverage gradient is larger than the n-butane gradient. When a sweep gas is used, n-butane permeates faster. For the other type of membrane, which has permeation mostly through zeolite pores, separation is controlled by differences in diffusion rates and adsorbed coverages. The single-gas and mixture permeances for these membranes are similar for each gas for the pressure drop and sweep gas methods. Ideal selectivities increase in the order pressure drop<sweep gas<vacuum (maximum of 120), whereas separation selectivities increase in the order vacuum<pressure drop<sweep gas (maximum of 30). These results indicate the difficulty of comparing zeolite membranes characterized by different methods.