Adsorptive separation of CO2/CH4/CO gas mixtures at high pressures

The major objective of this communication is to compare the performance of three metal–organic frameworks (MOFs): CuBTC, MIL-101, and Zn(bdc)dabco, with that of NaX zeolite for selective adsorption of CO2 from mixtures containing CH4 and CO in a pressure swing adsorption (PSA) unit operating at pressures ranging to 60 bar. Data on the pure component adsorption isotherms in the published literature are available in terms of excess loadings. For purposes of isotherm fitting with fundamental Langmuir-type models, these data need to be converted to absolute loadings. Calculations using the Ideal Adsorbed Solution Theory (IAST), using the fitted isotherm data on absolute loadings, show that the CO2/CH4 and CO2/CO selectivities are significantly higher with NaX than for the three MOFs. The working capacity for CO2 adsorption, on the other hand, is significantly higher for MOFs than for NaX zeolite as the pressures are increased significantly above 2 bar. For a realistic comparison of the separation characteristics in a fixed bed adsorber unit, transient breakthrough calculations were performed for an equilibrium packed bed adsorber. For a specified purity of CO2 exiting the packed bed adsorber, the best CO2 removal performance is obtained with CuBTC. Our studies highlight the relative importance of adsorption selectivities and capacities in the performance of fixed bed adsorbers, and underline the significant advantage of MOFs over traditionally used zeolites.

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► Isotherm fitting should be done with absolute, not excess, loadings.
► The selectivity and capacity metrics do not go hand-in-hand.
► Breakthrough times reflect a combination of selectivity and capacity metrics.
► CuBTC is the best adsorbent for CO2 capture from mixtures containing CH4 and CO.