Enhancing Natural Gas Purification With Advanced Polymer/Molecular Sieve Composite Membranes

Abstract

Abstract Membranes have been of continuing interest to the petroleum and chemical industries for gas separations. While glassy, polymeric membranes have provided efficient performance to date, significant improvements over current membrane technology will likely require novel materials. This paper will review the development and status of a new technology based on composite membranes, termed mixed matrix membranes, of polymer matrices in which molecular sieves are dispersed to give enhanced separation of natural gas from its impurities compared to membranes of the polymer alone. The technology is particularly of interest to the separation of natural gas from its impurities, such as CO2 and H2S. In laboratory testing, the composite membranes, composed of molecular sieves in commercial membrane polymer matrices, showed significant improvement in both selectivity and flux compared to membranes of the polymers alone for the separation of CO2 from natural gas. Enhancements were obtained using both carbon molecular sieves and small pore zeolites. While membranes have been of interest due to their compactness, light weight, and ease of operation, there has not been widespread application due to low selectivity and flux, and limited robustness. This has led researchers to study molecular sieve membranes, including carbon molecular sieve and zeolitic materials. While these membranes offer very attractive properties, their cost, difficulty of commercial scale manufacture, and brittleness remain major challenges. Mixed matrix membrane technology, which combines the benefits of molecular sieves with the ease and low cost of processing polymer membranes, offers a potential solution to these challenges. Introduction With worldwide natural gas production at around 50 trillion standard cubic feet per year, there is continuing interest in improved technology for natural gas purification. A large fraction of natural gas resources is high in impurities (N2, H2S, and especially CO2), making production difficult or uneconomic. In the lower 48 United States, for example, 550 trillion SCF cannot be processed due to high CO2 content [1]. Since much of worldwide gas resources are offshore or in remote locations, membranes are becoming attractive for gas separations due to their light weight, compactness, and ease of operation. This makes membranes potentially more attractive than much larger and heavier amine units, especially for gas resources high in CO2 (>10%), where regeneration of the spent amine solutions is energy intensive and disposal of spent amine has environmental issues. Nevertheless, of the $5 billion/year market for natural gas separation equipment, membrane processing represents only about 1% [2]. Affecting the ability of membranes to compete against amine treating for natural gas has been low selectivity for removing the impurity gases and the low flux of current commercial membranes. Also detrimental to the greater use of membranes is poor reliability due to fouling and loss of selectivity (e.g., through plasticization), and inability to operate effectively at high temperature and high hydrocarbon partial pressure. The sensitivity of polymer membranes to impurities and high temperature has necessitated the use of extensive pretreatment equipment, in part offsetting the weight and space advantages of compact membranes. While improvements have been made in polymer membranes, further advances are needed to successfully commercialize a number of high impurity offshore and remote natural gas fields.