Scaling analysis of large pressure jump driven adsorption of water vapour in columnar porous silica gel bed

Abstract

Adsorption process can be initiated either by Large Temperature Jump (LTJ) or Large Pressure Jump (LPJ). In this study, a theoretical analysis of coupled heat and mass transfer process during LPJ driven adsorption has been carried out using scaling principles of the governing conservation equations. A columnar domain consisting of silica gel (adsorbent) and water vapour (adsorbate) pair has been considered with heat and mass transfer directions orthogonal to each other. This domain is subjected to a sudden LPJ with isothermal boundary condition. Adsorption process has been divided into three phases for scaling analysis viz. i) pressure equalization ii) adsorption accompanied by heat generation and iii) heat removal phase. From order of magnitude arguments, various important physical scales for each phase have been derived and validated with a 2-dimensional computational fluid dynamics (CFD) study. The temperature rise in the adsorber bed is found to be 14.5 K, and the time scale to attain peak temperature is ~100 s from the initiation of adsorption. This justifies the short cycle time deployed in practical PSA systems to operate near isothermal condition. The outcomes of this investigation serve as a fundamental insight into LPJ driven adsorption process and help identify the various factors which practically effect this mode of adsorption.