5 - Processes and cycles

Abstract

Adsorbent particles have a finite capacity for fluid phase molecules. Extended contact of adsorbent particles with a feedstock ultimately leads to the formation of a thermodynamic equilibrium between the solid and fluid phases. At this equilibrium condition, the rates of adsorption and desorption are equal and the net loading on the solid cannot alleviate further. Thus, it becomes mandatory either to regenerate the adsorbent or to dispose of it. For those applications in which it is economically favorable to regenerate the adsorbent, it is necessary to devise processes in which the regeneration method can be incorporated. This chapter discusses the various bed processes. In batch processes, the adsorbent moves relative to the walls of the containment vessel. In fixed bed processes, adsorption therefore occurs only in a particular region of the bed, called mass transfer zone, which moves through the bed with time. In moving bed processes, adsorbent particles move relative to the walls of the containing vessel or the particles remain in a fixed position relative to the walls of the vessel. Disposal of the adsorbent as a waste is not an economic option and therefore, regeneration is carried out either in situ or external to the adsorption vessel to a sufficient extent that the adsorbent can be reused. Adsorbates can be removed from the adsorbent surface by replacing them with a more adsorbed species. This displacement fluid, which can be a gas, vapor, or liquid, should adsorb about as strongly as the species that are to be desorbed.