p-xylene separation with MFI-type zeolite membrane

Several authors reported the separation of xylene isomers through MFI-type zeolite membrane, however, the results were inconsistent. They used the alu mina support and the defects can be induced in the zeolite membrane during the calcination due to the thermal expansion mismatch between alumina suppo rt and zeolite membrane. In this study, the self-supporting MFI-type zeolit e membrane was used to avoid the defect formation. The self-supporting MFI- type zeolite was fabricated on Teflon plate in an autoclave at 353 K for 24 -96 h. The vapor permeation were carried out by Wicke-Kallenbach method in the nitrogen gas flow to examine the p-xylene separation from the ternary m ixture of xylene isomers as a function of time up to 72 h, temperature betw een 303 and 673 K, feed partial pressure between 0.3 and 5.1 kPa and membra ne thickness between 60 and 130 mum. The permeation flux of p-xylene showed the maximum at 473 K in the p-xylene partial pressure of 0.3 kPa. This cou ld be described by the competitive effects between the amount of equilibriu m adsorption and the diffusivity. The permeation of m- and o-xylene were sm all values and almost constant between 473 and 673 K. As a result, the sepa ration factors of p- to m-xylenes and p- to o-xylenes showed the maximum va lue of 250 at 473 K. From 473 to 673 K, the permeation flux was proportiona l to the partial pressure both in the single component feed and the ternary mixture of xylene isomers. The permeation flux of p-xylene depended little on the apparent membrane thickness from 130 to 60 mum. It is suggested fro m the microstructure of the membrane that the dense layer may not be the en tire thickness but part of the thickness. We concluded that the MFI-type ze olite membrane can separate the p-xylene selectively from the ternary mixtu re of xylene isomers above 473 K. (C) 2001 Elsevier Science B.V. All rights reserved.