Screening Metal-Organic Frameworks by Analysis of Transient Breakthrough of Gas Mixtures in a Fixed Bed Adsorber

Metal-organic frameworks (MOFs) offer considerable potential for separating a variety of mixtures that are important in applications such as CO2 capture and H-2 purification. In view of the vast number of MOFs that have been synthesized, there is a need for a reliable procedure for comparing screening and ranking MOFs with regard to their anticipated performance in pressure swing adsorption (PSA) units. For this purpose, the most commonly used metrics are the adsorption selectivity and the working capacity. Here, we suggest an additional metric for comparing MOFs that is based on the analysis of the transient response of an adsorber to a step input of a gaseous mixture. For a chosen purity of the gaseous mixture exiting from the adsorber, a dimensionless breakthrough time tau(break) can be defined and determined; this metric determines the frequency of required regeneration and influences the productivity of a PSA unit The values of tau(break) are dictated both by selectivity and by capacity metrics By performing transient adsorber calculations for separation of CO2/H-2, CO2/CH4, CH4/H-2, and CO2/CH4/H-2 mixtures, we compare the values of tau(break) to highlight some important advantages of MOFs over conventionally used adsorbents such as zeolite NaX. For a given separation duty, such comparisons provide a more realistic ranking of MOFs than afforded by either selectivity or capacity metrics alone. We conclude that breakthrough calculations can provide an essential tool for screening MOFs.