Water vapor permeation in polyimide membranes

Abstract

The study of the influence of water vapor in the feed stream of a mixed gas membrane separation system is of considerable practical importance. Water vapor may plasticize the membrane, it may undergo competitive sorption with other gas species and it can form clusters as it permeates. In this work, a modified mixed gas permeation system was employed to accurately measure the permeation properties of the polyimides 2,2′-bis(3,4′-dicarboxyphenyl) hexafluoropropane dianhydrid-2,3,5,6-tetramethyl-1,4-phenylenediamine (6FDA-TMPDA) and poly(3,3′-4,4′-benzophenone tetracarboxylic – dianhydride diaminophenylindane) (Matrimid^® 5218) under exposure to humidified methane at 35 °C. This approach was then applied to further evaluate the permeation properties of the polymers in humidified mixtures of carbon dioxide and methane at 35 °C at atmospheric and elevated feed pressures. Water vapor permeabilities obtained at 2 and 7.5 bar total feed pressure, increased from 3200 to 3900 Barrer and from 20000 to 27000 Barrer as the water level increased for Matrimid and 6FDA-TMPDA respectively, reflecting increases in water vapor solubility and possibly plasticization and clustering effects due to the presence of water vapor. Conversely, the permeabilities of CH₄ and CO₂ declined due to competitive sorption of water.

Introduction

Gas separation membranes have been used commercially to separate gas mixtures. One important application is the capture of CO₂ from methane in natural gas sweetening. Membranes with high CO₂ permeabilities and selectivities also have great potential to be adopted in carbon dioxide capture from both pre and post combustion flue gases to mitigate against climate change. The implementation of such gas separation systems poses significant challenges. One of them is the presence of water vapor, a known plasticizer in membrane separation [1], [2], [3]. Water vapor is usually considered as a minor component of the system in such industrial carbon capture applications [4], [5]. However, this species often exhibits very different permeation behavior compared to other gas species because of its very small size and its high hydrogen bonding affinity [6], [7], [8]. To synthesize efficient membranes that perform well over many years of service, it is necessary to study and understand the permeation behavior of water vapor in polymeric membranes.

The transport of water vapor through a polymeric material has been studied in a number of areas: membrane dehumidification and dehydration [9], [10], [11], [12], protective apparel [13], and packaging and clothing materials [14]. Water vapor permeabilities of various polymer materials have been measured using a variety of techniques, but only a few of these studies have been extended to determine the water vapor sorption and transport properties and its effects on the permeation of other gas species [15], [16], [17]. Further, very few results have been obtained under elevated gas pressure. It is particularly difficult to obtain reliable permeability values under such conditions [18]. The combined effects of a high feed pressure and the very high permeability of water causes concentration polarization at the membrane surface and this can introduce a significant error into the measured values of the membrane gas permeability [18], [19], [20]. However, the ability to acquire water vapor and gas permeabilities under mixed gas conditions at such high pressures is essential for improving the design and operation of commercial scale natural gas sweetening and carbon capture facilities.

This paper applies an approach for simultaneously determining water vapor and gas permeabilities under atmosphere and elevated feed pressures, based on a modified mixed gas permeation apparatus in addition to minimizing concentration polarization effects. In particular, it is the first time that these permeabilities have been evaluated at the same time under elevated feed pressures in a laboratory scale flat sheet membrane system. Two polyimides, typical of those used in natural gas separation [21], are used to understand the effect upon membrane performance of exposure to humidified methane and wetted carbon dioxide and methane mixtures.