Separation of Olefin/Paraffin Gas Mixtures Using Ceramic Hollow Fiber Membrane Contactors

Abstract

Previous studies on gas/vapor separations using polymeric membrane contactors have always been accompanied with a membrane stability issue: either a wetting phenomenon or a membrane degradation concern because of extensive contact with the solvent. To possibly overcome these conventional challenges and provide extended stability, the use of a ceramic hollow fiber membrane contactor is proposed in this study where propylene/propane separation using silver nitrate as a carrier was investigated for a continuous period of 6 months. Alumina hollow fibers with asymmetric structures consisting of a spongelike outer layer and a fingerlike inner layer were successfully fabricated, modified, characterized, and finally examined for propylene/propane separation. The modification step using silane solutions was critical in improving the membrane hydrophobicity to ensure that no wetting occurred during contact with the solvent. Initial studies on the separation performance confirmed that the membrane module was able to operate in a nonwetting mode, where the observed overall mass-transfer coefficient was the highest. Moreover, the ceramic membrane module proved to be stable throughout continuous experiments up to a period of 2 months, where no decline in the performance was observed. However, beyond this investigation period, deposition of silver on the membrane surface started to appear significantly where the membrane's appearance becomes dark. This silver deposition seemed to diminish the membrane's hydrophobicity as it continued to accumulate on the membrane surface, where a slight decline in the separation performance was finally observed by the end of the 6-month period. By exposing the advantages of ceramic membranes over their polymeric counterparts, a novel regeneration method was demonstrated where the membrane module undergoes treatment with strong nitric acid to remove silver deposits, followed by remodification of the membranes with silane solutions to restore its hydrophobicity. The performance of the membrane modules was regained completely after regeneration. Thus, this technology can be performed for an extended period without the need to replace the membranes whenever a drop in the performance is observed due to deformation of the membranes, an issue commonly found with polymeric materials.