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

Butane isomer permeation through two types of ZSM-5 zeolite membranes was s tudied 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 selectivit ies are higher than ideal selectivities, and are much higher when a pressur e 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 wi th the pressure drop method, apparently because the i-butane coverage gradi ent is larger than the n-butane gradient. When a sweep gas is used, n-butan e permeates faster. For the other type of membrane, which has permeation mo stly through zeolite pores, separation is controlled by differences in diff usion rates and adsorbed coverages. The single-gas and mixture permeances f or these membranes are similar for each gas for the pressure drop and sweep gas methods. Ideal selectivities increase in the order pressure drop < swe ep gas < vacuum (maximum of 120), whereas separation selectivities increase in the order vacuum < pressure drop < sweep gas (maximum of 30). These res ults indicate the difficulty of comparing zeolite membranes characterized b y different methods. (C) 2000 Elsevier Science B.V. All rights reserved.