A critical review of thermal enhancement of packed beds for water vapour adsorption

Packed beds are the simplest fluid-solid contact systems for water vapour adsorption from humid air. They usually are free from mechanical problems due to their non-moving parts and provide a high volume of adsorbents for fluid-solid interaction. However, issues relating to heat effects, pressure drop and the residence time of the fluid in the bed present significant challenges to the efficient adsorption of water vapour within them. These issues related to the packing structure coupled with the difficulties involved in visualising fluid flow within non-transparent beds further creates barriers in understanding fluid behaviour in the bed. In this paper, a critical review of water vapour adsorption in solid desiccant packed beds has been carried out to provide insights into packed bed adsorption process and its associated thermal effects. Crucial to this, is the insight provided into the packing structure and its influence on the adsorption process, the transport phenomenon in the bed, the heat of adsorption generating the thermal effects and thermal enhancement strategies for packed beds. Various approaches and models for analysing the adsorption process mainly involving flow and pressure drop, heat and mass transfer within packed beds have been reviewed. The potential for the thermal enhancement strategies employed in various studies to achieve isothermal adsorption process in packed beds have also been reviewed.