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Encyclopedia of Physical Science and Technology EN001-13 May 7, 2001 12:29

252 Adsorption (Chemical Engineering)

Macropore diffusion Diffusion in “macropores”—pores Chemisorption systems are sometimes used for remov-
that are large compared with the molecular diameter. ing trace concentrations of contaminants, but the diffi-
Several different mechanisms contribute to macrop- culty of regeneration makes such systems unsuitable for
ore diffusion, notably ordinary molecular diffusion in most process applications so most adsorption processes
larger macropores at higher pressures or in liquids and depend on physical adsorption. The forces of physical ad-
Knudsen diffusion in smaller macropores at low pres- sorption are weaker than the forces of chemisorption so
sures. Also referred to as intraparticle diffusion. the heats of physical adsorption are lower and the adsor-
Mass transfer zone Region in an adsorption column bent is more easily regenerated. Several different types of
where, at a given time, the concentration of one of force are involved. For nonpolar systems the major con-
the adsorbable species varies with distance along the tribution is generally from dispersion–repulsion (van der
column. Waals) forces, which are a fundamental property of all
Micropore diffusion Diffusion within the small micro- matter. When the surface is polar, depending on the na-
pores of the adsorbent which are of a size comparable ture of the sorbate molecule, there may also be important
with the molecular diameter of the sorbate. Under these contributions from polarization, dipole, and quadrupole
conditions the diffusing molecule never escapes from interactions. Selective adsorption of a polar species such
the force field of the solid surface and steric hindrance as water or a quadrupolar species such as CO 2 from a mix-
is important. For zeolites the terms micropore diffusion ture with other nonpolar species can therefore be accom-
and intracrystalline diffusion are synonymous. plished by using a polar adsorbent. Indeed, adjustment
Raffinate Product stream containing the less strongly ad- of surface polarity is one of the main ways of tailoring
sorbed species. adsorbent selectivity.
Selectivity Difference in the affinity of the adsorbent for The strength of the van der Waals interaction is directly
two components. Measured quantitatively by the “sep- related to the polarizability of the sorbate which depends,
aration factor,” q.v. in turn, on the molecular weight. The affinity sequence for
Separation factor Defined according to Eq. (5) in anal- nonpolar sorbates therefore generally correlates approxi-
ogy with relative volatility; provides a quantitative mately with the sequence of molecular weights.
measure of selectivity. Water is a small and highly polar molecule. It is there-
Zeolite Microporous crystalline aluminosilicate. In this fore adsorbed strongly on a polar surface, and such ad-
article the term is used in its broad sense to include sorbents are therefore commonly called “hydrophilic.” By
microporous crystalline silica and aluminophosphates contrast, water is adsorbed only weakly on a nonpolar sur-
as well as true zeolites. face so such adsorbents are called “hydrophobic.” How-
ever, this is something of a misnomer since water is not
actually repelled by a nonpolar surface.
ADSORPTION is the adhesion or retention of a thin layer
of molecules of a gas or liquid mixture brought into con-
tact with a solid surface resulting from the force field at the II. GENERAL APPLICATIONS
surface. Because the surface may exhibit different affini-
ties for the various components of a fluid, the composition A wide range of adsorption processes have been devel-
of the adsorbed layer generally differs from that of the bulk oped, and such processes are in common industrial use,
fluid. This phenomenon offers a straightforward means of particularlyinthepetroleumandpetrochemicalindustries.
purification (removal of undesirable components from a The traditional application of adsorption in the process
fluid mixture) as well as a potentially useful method of industries has been as a means of removing trace impu-
bulk separation (separation of a mixture into two or more rities from gas or liquid streams. Examples include the
streams of enhanced value). removal of H 2 S from hydrocarbon streams before pro-
cessing, the drying and removal of CO 2 from natural gas,
and the removal of organic compounds from waste wa-
I. FORCES OF ADSORPTION ter. In these examples the adsorbed component has little
value and is generally not recovered. Such processes are
Adsorption is conveniently considered as either “physical generally referred to as purification processes, as distinct
adsorption” or “chemisorption,” depending on the nature from bulk separations, in which a mixture is separated
and strength of the surface forces. Chemisorption can be into two (or more) streams, each enriched in a valuable
considered as the formation of a chemical bond between component, which is recovered. The application of ad-
the sorbate and the solid surface. Such interactions are sorption to bulk separations is a more recent development
strong, highly specific, and often not easily reversible. that was stimulated to a significant extent by the rapid