Screening metal–organic frameworks for mixture separations in fixed-bed adsorbers using a combined selectivity/capacity metric

Abstract

The separation performance of a fixed bed adsorption unit is dictated by a combination of two metrics: selectivity and uptake capacity. Most commonly, the screening of adsorbent materials on the basis of either of these metrics leads to contradicting hierarchies. To resolve this dilemma, this article defines a combined metric, termed the separation potential (ΔQ), that is calculable on the basis of the Ideal Adsorbed Solution Theory (IAST) for mixture adsorption equilibrium. For a binary mixture of A, and B in which B is more poorly adsorbed, ΔQ reflects the maximum productivity of pure B that can be recovered in the adsorption cycle of transient fixed bed operations; the same concept holds for recovery of pure A in the desorption cycle. For validation of the combined metric, transient breakthrough simulations were performed for separation of mixtures of Xe/Kr, C₂H₂/CO₂, C₂H₂/C₂H₄, C₂H₄/C₂H₆, C₃H₆/C₃H₈, CO₂/CH₄, CO₂/N₂, CO₂/H₂, CO₂/CO/CH₄/H₂, and hydrocarbon isomers in fixed beds packed with a wide variety of metal–organic frameworks (MOFs). In every case, the productivities determined from transient breakthrough simulations are determined to be linearly related to the values of ΔQ; the actual values are lower because of the distended nature of concentration breakthroughs in fixed beds. Indeed, if the “fronts” of the concentrations traverse the fixed bed in the form of “shock waves”, the productivity values for fixed beds coincide precisely with ΔQ. The important conclusion to be drawn is that MOFs can be compared and evaluated on the basis of IAST calculations of the combined metric, thus obviating the need for performing transient breakthrough calculations.