Assessment of ZIF materials for CO2 capture from high pressure natural gas streams

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Although a considerable amount of the research is focussed in carbon capture specifically towards flue gas separations, another area that is of interest is CO2/CH4 separation for the natural gas industries. It is believed that 40% of the world’s reserves of natural gas are sour, and these gas reserves are typically left unexploited due to their high CO2 content and the costs associated with separation and transport. One recent class of adsorbents, metal organic frameworks (MOFs) has been advocated as potential candidates for CO2 removal from natural gas at high pressure. This can be attributed to their high CO2 capacities, which could be exploited in high pressure separations. In this work we synthesised ZIFs -8, -14 and -71 and measured CO2 and CH4 isotherms over a range of temperatures and pressures. The CO2 capacity for these materials at 303K and 45bar(a) was in the order of ZIF-8 (9.1molkg−1)>ZIF-71 (8.1molkg−1)>ZIF-14 (5.0molkg−1). The CH4 loading at 303K and 100bar(a) was in the order of ZIF-8 (6.8molkg−1)>ZIF-14 (4.8molkg−1)>ZIF-71 (4.4molkg−1). The ideal selectivity of these materials for a 15%mol CO2, 85%mol CH4 feed mixture at 100bar(a) and 303K was found to be 5.6 for ZIF-8, 4.5 for ZIF-14 and 13 for ZIF-71. This isotherm data was then used to design and simulate a pressure swing adsorption process for CO2/CH4 separation.Feed CO2 concentrations between 15%mol and 35%mol were investigated at a condition of 100bar(a) and 303K. It was found that only ZIFs -8 and -71 could achieve the 98%mol CH4 product purity required. ZIF-8 and ZIF-71 were able to achieve CH4 recoveries of 46%mol and 48%mol respectively. Furthermore, it was also found that ZIFs -8 and -71 behaved very similarly when compared on a volume of adsorbent basis. The CH4 uptake of ZIF-14 was found to be abnormally high, which resulted in a very low CO2/CH4 selectivity. The loading of CH4 was higher than CO2 for both the 15%mol and 25%mol CO2 feed cases, with only the 35%mol CO2 feed resulting in a higher CO2 capacity, 4.6molkg−1 of CO2 in comparison to 4.1molkg−1 of CH4.Although their CO2 capacities at high pressures are high, there is little discrimination between the adsorption of small molecules. Consequently, their CH4 loadings also increase substantially at those increased pressures. This results in a poor separation with the CO2 product becoming diluted with the co-adsorbed CH4 gas.