Molecular sieve membranes for N₂/CH₄separation

Abstract

Natural gas consumption has grown from 5.0 trillion cubic feet (TCF) in 1949 to 27.0 TCF in 2014 and is expected to be ;31.6 TCF in 2040. This large demand requires an effective technology to purify natural gas. Nitrogen is a significant impurity in natural gas and has to be removed since it decreases the natural gas energy content. The benchmark technology to remove nitrogen from natural gas is cryogenic distillation, which is costly and energy intensive. Membrane technology could play a key role in making this separation less energy intensive and therefore economically feasible. Molecular sieve membranes are ideal candidates to remove natural gas impurities because of their exceptional size-exclusion properties, high thermal and chemical resistance. In this review, the state of the art of molecular sieve membranes for N 2 /CH 4 separation, separation mechanisms involved, and future directions of these emerging membranes for natural gas purification are critically discussed. I. NATURAL GAS GENERALITIESThe United States is the world's largest producer of natural gas.1 As of 2015, the United States produces 28.8 trillion cubic feet (TCF) of natural gas per year.2 The United States natural gas production has been increasing every year since 2010, 3 and this increase in production can, in part, be contributed to the increasing use of fracking and other technologies that release previously untapped natural gases in shale.3 According to US Energy Information Administration, the natural gas consumption is expected to rise to 31.6 TCF in 2040. 4 Processing of natural gas is by far the largest industrial gas separation application. 5 Every year close to 100 trillion standard cubic feet of natural gas are used worldwide. This large demand requires an effective technology to process and purify natural gas.Natural gas consists primarily of methane, higher alkanes, carbon dioxide, nitrogen, and hydrogen sulfide as shown in Table I. In particular CO 2 and N 2 decrease the heat value of the natural gas. Therefore, it is highly desirable to remove CO 2 and N 2 from natural gas to improve its heat content, and to avoid the erosion of pipelines from CO 2 in the presence of moisture.Approximately 14% of US natural gas contains .4% N 2 .5 However, most pipeline standards require that natural gas contains less than 4% inert gases. N 2 dilutes the heating value of the gas, resulting in lower British thermal units. In the process of purification, it is highly desirable to maintain the gas at high pressure when explored to save recompression costs. In this respect, it is necessary to develop efficient processes to remove N 2 from natural gas. II. NATURAL GAS NITROGEN REJECTION TECHNOLOGIESNitrogen rejection is a challenging technical separation because of the similar molecular size of N 2 (;0.36 nm) His research focuses on molecular gas separations, heterogeneous catalysis, and gas storage, and aims at tackling relevant societal issues related to energy and environment, including carbon dioxide capture and utilization, biomass convers...

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