Adsorption - Principles of Chemical Separations with Environmental Applications - page 182

Adsorption processes are used economically in a wide variety of separations in the
chemical process industries. Activated carbon is the most common adsorbent, with annual
worldwide sales estimated at $380 million [1]. One common adsorption process is dehy-
dration for the drying of gas streams.
Adsorption offers several advantages as a separation process. It can be used in situations
where distillation is difficult or impossible due to components with similar boiling points,
vapor-liquid azeotropes, or species with low relative volatilities. A high loading of solute
is possible in adsorption processes and it works well for dilute systems. In addition the
energy-separating agent needs are usually low. The disadvantages of adsorption processes
are due to the use of an adsorbent mass-separating agent. Each adsorbent bed must be
removed from the process before regeneration, so that more than one column in series
or parallel is usually required. The regeneration process can involve losses in the sorbent
amount and loading over time. Column operation affects solute breakthrough that limits
actual bed loading.

7.2.1 Definition of adsorption terms

Adsorption : the process where a substance is accumulated on an interface between
phases.
Adsorbent: the phase that collects the substance to be removed at its surface.
Adsorbate : the solute that is to be adsorbed (removed from the gas or liquid stream).
Isotherm : A relation between the equilibrium amount of a substance adsorbed per weight
of sorbent and its concentration in the liquid or gas stream at constant temperature.

7.2.2 Environmental applications

There are many environmental applications of adsorption in practice and many others are
being developed. Both zeolite and activated carbon adsorbents are used in VOC removal
from gas streams. Molecular sieves are used to remove water from organic solvents,
while other adsorbent materials remove organics from water. Taste and odor and other
contaminant removal in water treatment is performed with activated carbon and other
adsorbents. Silica gel, activated carbon, zeolites, activated alumina, and synthetic resins
have all been applied to removal of H 2 S from gas streams. Phillips Petroleum has patented
a z-sorb technology that removes H 2 S and other sulfur compounds from gas streams
above 600 ◦ F (316 ◦ C) [2]. Adsorption is used to eliminate purge streams to remove con-
taminants. Mercury can be removed from chlor-alkali-cell gas effluent via adsorption.
Other adsorbents, such as bentonite [3], are studied as heavy-metal adsorbents in clay
barriers. Water removal from gas streams containing acid gases, odor or contaminant re-
moval from air, and radon removal from gas streams are all achieved through adsorption.
In addition, adsorption can be used to eliminate solvent use as an alternative to extraction
or azeotropic distillation.