Page 349 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

CHAPTER

12
Adsorption

12.1 Introduction
Adsorption is one of the unique mass transfer processes in which component(s) from a fluid, either gas
or liquid adheres to the surface of solid without intimate admixture with the solid atoms. As mentioned
in Chapter 9, the solid on which adsorption occurs, often a porous material of a high specific surface
2
(m /g), is referred to as adsorbent while the substance that is adsorbed on the solid surface is termed as
adsorbate or solute. All solids exhibit adsorption to some extent but certain substances exhibit pref-
erential affinity towards specific solutes and can retain higher concentrations of those on their surface.
This results in selective mass transfer and enrichment/separation/fractionation of components from a
liquid solution or a gaseous mixture. Typical applications of liquid adsorption include adsorption of
coloured matter from sugar solutions, petroleum, and vegetable oil; removal of objectionable taste and
odour from potable water; removal of grease from dry cleaning liquids; removal of moisture dissolved
in gasoline and fractionation of mixtures of paraffinic and aromatic hydrocarbons. Gas adsorption is
widely used in industrial drying/dehumidifying air and other gases, removal of objectionable odours
and impurities from industrial gases, recovery of valuable solvent vapour from dilute gaseous mixtures
and fractionation of light hydrocarbons.
In all these operations, the mixture to be separated is brought in contact with an insoluble solid, and
the preferential distribution of the adsorbate on the (solid) adsorbent leads to the desired separation.
Like other mass transfer operations, the contacting arrangement can be stagewise or continuous and
the stagewise operation can be single-stage or multistage in crossflow or countercurrent mode. Since
countercurrent operations are more efficient from the viewpoint of mass transfer, an approach to the
same is achieved in some commercial adsorption processes that comprise an arrangement called
simulated moving bed. MOLEX and PAREX are two such patented processes. Semi continuous mode
of operation with the fluid flowing through a stationary bed of solid is also possible.

12.1.1 Modes of operation
The modes of contact are presented as a classification tree in Fig. 12.1. For the purpose of calculation,
the operation within each category except fixed-bed adsorption can be considered analogous to other
mass transfer operations discussed in this book. For example, single-component adsorption from a
gaseous mixture can be treated analogous to gas absorption where the added insoluble phase is
adsorbent in the present case and liquid solvent in absorption. Single-component adsorption from

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