Understanding errors in gas adsorption at low pressures: the case of direct air capture

Abstract

Accurate experimental adsorption equilibrium measurements are necessary for benchmarking adsorbents, validating molecular simulations and setting up process simulations. Although many sources of errors in these measurements have been reported in the literature, the purity of the gas used is generally not considered a major problem as long as research grade gases are used. In this work, we propose that significant deviations in the measured isotherms can potentially arise due to the accumulation of impurities in the measurement cell, especially in the low-pressure region, which is important for systems dealing with low partial pressure, such as \(\hbox {CO}_2\) direct air capture (DAC). We conduct numerical studies to highlight this issue. The first part of our analysis uses the Langmuir isotherm equation to generate baseline isotherms representative of adsorbents with varying affinities for \(\hbox {CO}_2\), enabling a parametric assessment of impurity effects. This is followed by a material-specific study examining the influence of impurities on isotherms for several zeolites and metal-organic frameworks (MOFs).