Operando monitoring of gas drying by adsorption on supported ionic liquids: Determination of velocity of adsorption front by microwaves

Abstract

For adsorption-based gas drying to reach very low dew points, ionic liquids (ILs) immobilized on solid porous supports are promising materials. Prior investigations with the IL 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][MeSO3]) show high absorption capabilities for water (vapour) and low regeneration (desorption) temperatures (< 100 °C). Due to the negligible evaporation losses of this IL combined with a high thermal stability and low melting point, [EMIM][MeSO3] is ideal for immobilization on highly porous supports. In this work, the IL covers at least a monolayer of the substrate (here silica), thereby filling about 30 % of the pores with the IL. Water then mainly (ab)sorbs in the IL and practically not adsorbs on the surface of silica. In the presence of water, the dielectric losses of the supported IL increase strongly and can therefore be measured by microwave perturbation methods. For this purpose, a quartz tube containing the fixed-bed of the supported IL was placed in the radial center of a microwave resonator operating in a mode so that the electric field strength varies in axial direction along the bed. During sorption experiments, the electrical signals measured with a network analyzer allowed to track the sorption front, to calculate the velocity of the moving front, and to detect the breakthrough of water vapor. The validation of this method was accomplished by simultaneously measuring the humidity of the carrier gas at the in- and outlet of the packed bed by humidity sensors. The water partial pressure (carrier gas N2), the temperature, and the gas velocity were varied in a range relevant for gas drying (5–15 mbar; 40–60 °C; 0.05–0.30 m s−1). The results demonstrate that microwave-derived parameters, like resonant frequency or quality factor, are suitable to locate the sorption front along the fixed-bed and to calculate the front velocity and the breakthrough time.