Antiplasticization and plasticization of Matrimid^® asymmetric hollow fiber membranes—Part A. Experimental

Abstract

The complex effects of highly sorbing feed gas contaminants such as toluene and n-heptane on performance of both annealed and non-annealed Matrimid^® asymmetric fibers relevant to CO₂/CH₄ separation are reported. Membrane performance was quantified both during contaminant exposure and after removal of the contaminant from the feed stream. Exposure to either toluene or n-heptane during permeation reduces carbon dioxide permeance and the carbon dioxide/methane selectivity in non-annealed fibers. After exchange with a contaminant-free feed containing only CO₂ and CH₄ mixed gas, the carbon dioxide permeance and carbon dioxide/methane selectivity were affected, indicating a glassy state conditioning effect due to the prior contaminant exposure. Interestingly, the conditioning effect after simultaneous exposure to toluene and n-heptane (284 ppm toluene and 504 ppm n-heptane) was less than the conditioning observed for either toluene (293 ppm) or n-heptane (505 ppm) individually. Sub-T_g annealing reduced carbon dioxide permeance during actual contaminant exposure more severely than in non-annealed fibers. On the other hand, except for exposure to the highest n-heptane contaminant feed (2003 ppm), annealing significantly reduced the post-exposure conditioning observed in carbon dioxide permeance and carbon dioxide/methane selectivity. It appears that annealing allows the consolidation of segmental packing which stabilized the glassy matrix against swelling. At sufficiently high activities of even a relatively non-interacting penetrant like n-heptane, the annealing-induced stabilization can be reversed.

Introduction

Membrane-based separations of gaseous feed streams have been performed industrially for the past 25 years [1] and are expected to grow in the future. Especially, membrane-based CO₂/CH₄ separation has been explored by many researchers [2], [3], [4], [5]. It is well known that CO₂ behaves as a plasticizer in CO₂/CH₄ separations at elevated pressures resulting in loss in selectivity. Bos et al. [6] demonstrated that heat treatment suppresses the undesired acceleration in CH₄ permeability due to CO₂-induced plasticization for the mixed CO₂/CH₄ permeation in polyimide Matrimid^® 5218. In addition to CO₂-induced plasticization, Visser et al. [7] showed that a subtle balance exists between competitive sorption and plasticization effects for a binary mixture of CO₂/CH₄. Separation of feed streams containing condensable components presents a large challenge for current generation membrane materials, which often experience substantial performance reductions, even at relatively low concentrations. Few studies of membrane performance in such situations are available [8], [9], [10], [11], and the mechanism of membrane performance degradation in the presence of aggressive feed streams, which is termed antiplasticization, is not completely understood.

Our previous work [12] explored the sorption of highly sorbing vapors such as toluene and n-heptane as model contaminants found in natural gas in the absence of CO₂ and CH₄. This investigation clarifies the effects of such contaminants on the permeation performance of hollow fiber gas separation membranes used for the removal of CO₂ from CH₄ in natural gases. The CO₂ and CH₄ permeances of both non-annealed and annealed Matrimid^® hollow fiber membranes will be quantified during exposure to feeds containing toluene or n-heptane or their mixtures to assess effects on separation performance. Also, CO₂ and CH₄ permeances following removal of the contaminants from the feed stream were studied to evaluate longer terms conditioning effects from the exposures.