Applying the wave theory to fixed-bed dynamics of Metal-Organic Frameworks exhibiting stepped adsorption isotherms: Water/ethanol separation on ZIF-8

Abstract

Zeolitic Imidazolate Framework-8 (ZIF-8) is a promising material for the energy-efficient recovery of bio-alcohols from aqueous mixtures. Its adsorption isotherms have inflection points and breakthrough curves obtained in dynamic column separation experiments exhibit stepped profiles. With the aim to optimize this adsorptive alcohol recovery process using ZIF-8, the correlation between the shape of the adsorption isotherms and the breakthrough curves was further explored in this work by means of liquid phase batch measurements and breakthrough experiments, with diluted ethanol/water mixtures. The shape of the concentration profiles, in adsorption mode as well in desorption mode, have been carefully analyzed making use of simple theoretical tools, including an axial dispersion model and a wave theory based approach. It was shown that, as a direct expression of the adsorption isotherm and the feed concentration, the concentration profiles were composed of different “shock waves” and “dispersive waves” propagating at different velocities along the column length. The description of the breakthrough and desorption curves in terms of propagation velocities enabled to gain insights into the fundamental role of the adsorption isotherm into the characteristics and the formation of the concentration profiles. The concept of “fractional used bed capacity” (FUBC) was introduced to illustrate the effect of the adsorption isotherm shape on the dynamic capacity at breakthrough point. According to the shape of the adsorption isotherm, efficient desorption could be achieved with the ZIF-8 material. This work provides, with limited computational effort, new perspectives for the further development and optimization of adsorptive separation processes with MOF materials, such as the recovery of bio-alcohols from aqueous mixtures with the ZIF-8 adsorbent.