CO₂–CH₄ permeation in high zeolite 4A loading mixed matrix membranes

Abstract

Mixed matrix membranes (MMMs) with low particle loadings have been shown to improve the properties of pure polymers for many gas separations. Comparatively few reports have been made for high particle loading (≥50 vol.%) MMMs. In this work, CO₂–CH₄ feeds were used to study the potential of 50 vol.% zeolite 4A-poly(vinyl acetate) (PVAc) MMMs for natural gas separations. A low CO₂ partial pressure mixed feed probed MMM performance below the plasticization pressure of PVAc and a high CO₂ partial pressure mixed feed probed MMM performance at industrially relevant conditions above the plasticization pressure.

Under both mixed feed conditions at 35 °C, substantial improvements in overall separation performance were observed. At low CO₂ partial pressures, CO₂ permeability roughly doubled with a nearly 50% increase in selectivity versus pure PVAc under the same conditions. For the high CO₂ partial pressure feed, CO₂ permeability remained effectively unchanged with a 63% increase in selectivity versus pure PVAc. Surprisingly, the performance of these PVAc based MMMs approached the properties of current “upper bound” polymers. Overall, this work shows that significantly improved performance MMMs can be made with traditional techniques from a low cost, low performance polymer without costly adhesion promoters.

Introduction

Dispersing filler particles in polymeric matrices is an increasingly popular method for improving pure polymeric gas transport properties [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Mixed matrix membranes (MMMs) typically consist of zeolite particles dispersed in polymers with the goal of increasing permeabilities and selectivities of desired components over those of undesired components. MMMs combine the ease of processing polymer membranes with the superior transport properties of zeolitic (and other filler) materials, ideally resulting in membranes with properties that exceed gas pair “upper-bounds” [1], [20], [21], [22]. Models predict that transport enhancements increase rapidly at high filler loadings [23], [24]. While there have been many reports of low filler loading (≤40 vol.%) MMMs showing improved transport properties over their pure polymeric matrices [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [19], [22], [25], [26], [27], only a few reports of high particle loading (≥50 vol.%) MMMs have been published [2], [3], [5], [11], [12].

Most MMM studies report only low pressure, pure gas permeation data. While low pressure, pure gas permeation experiments can be useful for characterizing new membrane materials, real feeds are complex mixtures and are often at high pressures. Mixed feed permeation properties can substantially deviate from pure gas data due to plasticization, competitive sorption, and many other complicating effects [28], [29]. Natural gas sweetening (i.e. removal of acid gases like carbon dioxide and hydrogen sulfide from methane) is among the most important target feeds for membrane separations. Moreover, economic analyses show that natural gas wells with high carbon dioxide concentrations (>20% CO₂) are especially attractive targets for membrane based separations [28], [30], [31], [32], [33]. Such natural gas sources are processed at total feed pressures as high as 900 psi or more [33]. These considerations encourage testing MMMs under high pressure, simulated natural gas feeds.

In this work, high zeolite 4A loading (50 vol.%) MMMs were made using poly(vinyl acetate) (PVAc) as the matrix polymer. PVAc is well known to be a model polymer for MMM studies due to its flexibility and adhesive properties [1], [22], [34]. Also, PVAc is known to be sensitive to plasticization-induced loss in selectivity; however, stabilization of chain segments in the vicinity of solids may reduce this sensitivity. PVAc was, therefore, selected as the polymer matrix for this study. Two 50 vol.% zeolite 4A-PVAc MMMs were made from in-house synthesized zeolite 4A via solution processing. These MMMs were tested under two simulated natural gas feeds (dry CO₂–CH₄ at 35 °C): 1) a low case and 2) a high $p_{\text{C}{\text{O}}_2}$ case. The individual component permeabilities and CO₂–CH₄ selectivities are reported and compared to pure PVAc values under the same conditions.