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

Abstract

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.