Highlighting the technological consequences of thermodynamic non-idealities in mixture separations using microporous crystalline adsorbents

Abstract

Microporous crystalline adsorbents such as zeolites, and metal–organic frameworks (MOFs) have potential use in a wide variety of separations applications. Published data from experiments and molecular simulations reveal scenarios in which the Ideal Adsorbed Solution Theory (IAST) fails to provide a quantitative description of mixture adsorption equilibrium. The failure of the IAST is ascribable to non-compliance with one or more tenets mandated by the IAST such as (a) homogeneous distribution of adsorbates within the pore landscape, (b) no preferential location of guest species, and (c) absence of molecular clustering due to say hydrogen bonding. The primary aim of this article is to underscore the technological consequences of thermodynamic non-idealities in separations by undertaking three different case studies.For CO₂ capture from humid gases in fixed bed adsorbers packed with CALF-20, the capture efficiency is significantly influenced by the thermodynamic non-idealities, necessitating the use of the Real Adsorbed Solution Theory (RAST) in simulations of adsorption/desorption cycles. The RAST based simulations underscore the strong influence of relative humidity on capture efficiency.The recovery of C₂H₄ from the reactor products of oxidative coupling of methane (containing CO₂), the proper accounting of selectivity reversals induced by non-idealities is the key to the choice of LTA-5A zeolite as adsorbent in the Pressure Swing Adsorption (PSA) scheme. Ignoring thermodynamic non-idealities fails to anticipate selectivity reversals that are essential in the PSA scheme.For dehydration of water/ethanol mixtures in a membrane pervaporation unit, the strong hydrogen bonding between water and ethanol results in enhanced water permeation that is not anticipated by the IAST in zeolites and ZIF-8.