The broader impact/commercial potential of this project is to strengthen the competitive position of membrane companies in the natural gas treatment market. This would also allow users to benefit from the ease of processing and environmental advantages offered by membranes at substantially reduced costs. Natural gas processing to remove carbon dioxide and other contaminants is the largest industrial gas separation application. At present, membrane processes have 10% of the U.S. market of $1.2 billion. Amine absorption technology is the current industry standard. But membranes provide both cost and environmental advantages over amine absorption. The U.S. Energy Information Administration projects a rapid rise in domestic natural gas production over the next two decades. The expanding natural gas market presents a timely opportunity for membrane companies to acquire a greater market share. Today's membranes lose too much methane with the removed carbon dioxide. If more selective membranes could be made, the process would be much more widely used.
This Small Business Innovation Research Phase I Project work is targeted at developing advanced membranes for natural gas purification. At the high pressures and high CO2 concentrations in natural gas processing, the CO2/CH4 selectivity of commercial cellulose acetate membranes is 12 to 15. This modest selectivity has limited the application of membranes. Better membranes with higher CO2/CH4 selectivities are required. The objective of the proposed project is to achieve a mixed-gas CO2/CH4 selectivity of 30 and CO2 permeance of 400 gpu, which would be more than twice the levels of separation currently available from cellulose acetate membranes. To achieve this goal, we propose to use polymer/metal-organic framework (MOF) hybrid materials to develop advanced membranes for CO2/CH4 separation. Molecularly tailored MOFs will be added into suitable polymer matrices, to enhance CO2/CH4 diffusivity selectivity or CO2/CH4 solubility selectivity. This project will make thin-film composite membranes using different polymer/MOF combinations and evaluate the CO2/CH4 separation properties. Successful development of polymer/MOF-membranes can significantly reduce operating cost by 30 percent and capital cost by 40 percent. This could be transformative in natural gas processing. The proposed technology can potentially be extended to other applications such as hydrogen/carbon dioxide and olefin/paraffin separations.
Project Objectives. The project objective was to develop mixed-matrix membranes (MMMs) containing metal-organic framework (MOF) crystallites for natural gas purification. The goal was to achieve a mixed-gas CO2/CH4 selectivity of 30 and CO2 permeance of 400 gpu, which would be more than twice the levels of separation currently available from commercial cellulose acetate membranes. Successful development of MOF-MMMs with the target separation performance will reduce the cost of gas separation in natural gas processing. Background. The aim of MMMs is to combine the useful properties of inorganic materials and polymers. Conventional MMMs were comprised of ceramic fillers such as zeolites dispersed within a continuous polymer phase. A key issue with these zeolite-MMMs is the incompatibility between the inorganic and organic domains, creating non-selective defects in the hybrid membranes. This issue is most severe for rigid glassy polymer matrices. Also, it is difficult to make useful membranes with the 0.1 to 0.5 micron-thick selective skins required for practical applications, because the inorganic particles are too large. In the last few years, new types of MMMs based on metal-organic frameworks (MOFs) have been produced, which have the potential to solve the problems of zeolite MMMs. MOFs are crystalline, highly porous materials built up of inorganic clusters connected by organic linkers. Polymer-MOF nanocomposites display excellent compatibility between polymer chains and the organic linkers of MOFs. MOF particles can also be made very small, enhancing their dispersion in the polymer matrix and allowing the formation of thin, defect-free membrane slective layers. The discovery of MOFs creates a new platform for developing high performance MMMs for gas separations. Indeed, several studies have shown that MOF addition to a polymer improves gas transport properties. But the majority of these studies were performed using thick dense films, and tested using pure gases at unrealistic operating conditions. The separation performance of polymer/MOF membranes at industrially relevant operating conditions has not been properly investigated prior to this study. Summary of Results. In this project, we screen various polymer/MOF nanocomposites for their gas transport properties. Size-sieving MOFs such as zeolitic imidazolate frameworks were dispersed into suitable polymer matrices to enhance diffusivity selectivity. Molecularly tailored MOFs with gas-adsorbing sites were used to improve solubility selectivity. A number of polymer/MOF systems showed improved pure-gas CO2/CH4 selectivities with the addition of MOFs. However, the MOF-MMMs had only marginally higher mixed-gas CO2/CH4 selectivities than neat polymer membranes. The improvement in pure-gas separation performance was not necessarily reflected in mixed-gas performance. Further investigation is required to understand pure- and mixed-gas transport of CO2/CH4 gases in MOF-MMMs. Although we did not find a suitable MOF-MMM for high-pressure CO2/CH4 separation, our results do provide guidelines for the future design of polymer/MOF nanocomposites for this application. One of the key challenges in using MOF-MMMs for CO2/CH4 separation is obtaining suitable mechanical stability at elevated pressures of 600 psig and above. Despite the better compatibility between MOFs and polymer compared to conventional zeolite-MMMs, mechanical instability was observed for glassy polymer membranes containing high MOF loadings. While evaluating these polymer/MOF nanocomposites for their CO2/CH4 separation properties, we also measured the olefin/paraffin separation properties. Some of the polymer/MOF nanocomposites showed promising olefin/paraffin separation performances. For example, using a 50/50 vol% propylene/propane feed at 100 psi, we obtained mixed-gas propylene/ propane selectivities of 6 to 10 for some polymer/MOF composite membranes. All other pure polymer membranes we have tested under similar mixture conditions gave propylene/propane selectivities of 3 or less. Membranes with propylene/propane selectivities 6 or greater and propylene permeances of 20 gpu or more would find immediate application in propylene recovery from purge gas streams. The operating pressure of 150 to 200 psig in olefin/paraffin separations is significantly (3-5 times) lower than in natural gas processing; MOF MMMs should be mecahnically robust under these operating conditions. Our results suggest greater potential for using MOF-MMMs in olefin/paraffin separations than for natural gas applications. Concluding Remarks. Research on MOF MMMs is at the very early stages, and is a promising path to new membrane materials that can be designed for use in current and future membrane separation applications. The successful development of these membranes could be game changing in important gas separation processes.
