MASS TRANSFER ADSORPTION


Adsorption:
Adsorption is a separation process in which certain components of a fluid phase are transferred to the surface of a solid adsorbent. This phenomenon is now widely used in designing of the water treatment systems and pollution control system for liquid effluents. The phenomena of adsorption may occur gas-solid or solid-liquid contacting. Adsorption of a gas onto a solid is an exothermic process. Adsorption of a gas onto a solid can be conducted most efficiently at high pressure and low temperature for bulk separations adsorption becomes competitive with distillation when the relative volatility is less than about 1.25. A good adsorbent should have a high selectivity for the solute, sufficiently high capacity and long life. 
The separation factor in adsorption varies with temperature and for an ideal Langmuir system the separation factor is independent of composition. In adsorptive separation processes, physical adsorption will occur. Physical adsorption is nonspecific and significant at relatively low temperatures. In certain adsorptive separation processes differences in diffusion rates between molecules of comparable molecular weight are exploited to achieve useful separation between molecules. This type of separation is known as kinetic separation. Kinetic separation is generally accomplished withzeolites and carbon sieves.
A toxic solute present in an industrial wastewater is to be removed by its adsorption onto adsorbents in a single-stage operation. The equilibrium relation is given by Freundlich equation: Y* = mXn, where Y* = mass solute / mass solvent and X = mass solute/ mass adsorbent.
Now keeping m constant, as n is varied from n > 1 to n < 1, the adsorbent requirement for the same degree of separation will increase. For crosscurrent two-stage treatment of liquid solutions by contact filtration, when the adsorption isotherm is linear, the least total adsorbent results if the amounts used in each stage are equal The forces involved in physical adsorption are van-der-waals forces electrostatic interactions and chemical interactions.

The hypersorption process is a countercurrent adsorption process regeneration of adsorbent by purge gas stripping is possible when the adsorbed species are weakly held. Regeneration of adsorbents containing several adsorbates of widely different adsorption affinities adsorbed on them is preferably done by
thermal swing