Page 31 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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

Adsorption (Chemical Engineering) 253

escalation of energy prices during the 1970s. The tradi- A diameter of 20 A represents approximately the limit-
˚
tional method of bulk separation is distillation, and al- ing pore size that can be measured by mercury intrusion.
though distillation has the advantages of wide applicabil- In pores smaller than this, transport becomes increasingly
ity and proven technology, it suffers from the disadvantage affected by molecule–pore wall interactions, and conven-
of very poor energy efficiency, particularly when the dif- tional theories based on molecular and Knudsen diffusion
ference in volatility of the components to be separated is breakdown.Theclassificationissomewhatarbitrary,how-
small. With increasing energy costs the balance of eco- ever, since the point at which such effects become impor-
nomic advantage for such separations has shifted toward tant also depends on the size of the diffusing molecule.
alternative technologies, such as adsorption, that gener- Adsorption equilibrium in microporous adsorbents also
ally involve a higher capital outlay but offer the advantage depends to some extent on the pore size as well as on the
of greater energy efficiency and therefore lower operating nature of the surface, so control of the pore size distribu-
costs. Examples of large-scale bulk separation processes tion is important in the manufacture of an adsorbent for a
that are commonly accomplished by adsorption include particular separation.
the separation of xylene isomers (liquid phase), the sepa- Activated carbon is by far the most widely used ad-
ration of linear and branched paraffins (gas phase or liq- sorbent. It is available in a wide range of different forms
uid phase), and the separation of olefins from paraffins that differ mainly in pore size and pore size distribution.
(gas phase or liquid phase). Similar adsorption separation The carbon surface is essentially nonpolar although some
processes have also been developed for a number of im- polarity can be imparted by surface oxidation or other
portant carbohydrate separations (e.g., fructose–glucose) pre-treatments. It is widely used for removal of low con-
that cannot easily be accomplished by more traditional centrations of organics, either from aqueous streams (for
methods. example, decolorization of sugar or water treatment) or
The primary requirement for an economic adsorption from vapor streams (for example, in range hoods and other
separation process is an adsorbent with sufficient selec- pollution-control devices). Crystalline silica adsorbents
tivity, capacity, and life. Adsorption selectivity may de- such as silicalite are also organophilic but are substantially
pend either on a difference in adsorption equilibrium or, more expensive than activated carbon so their application
less commonly, on a difference in kinetics. Kinetic selec- is generally limited to situations where, for some reason,
tivity is generally possible only with microporous adsor- the use of carbon is not appropriate.
bents such as zeolites or carbon molecular sieves. One In “molecular sieve” adsorbents, such as zeolites and
can consider processes such as the separation of linear carbon molecular sieves, the micropore size distribution
from branched hydrocarbons on a 5A zeolite sieve to be is extremely narrow, thus allowing the possibility of ki-
an extreme example of a kinetic separation. The critical netic separations based on differences in molecular size.
molecular diameter of a branched or cyclic hydrocarbon However, this feature is utilized in only a few commer-
is too large to allow penetration of the 5A zeolite crystal, cial adsorption separation processes, and in the majority
whereas the linear species are just small enough to en- of such processes the separation depends on differences
ter. The ratio of intracrystalline diffusivities is therefore in the adsorption equilibrium rather than on the kinetics,
effectively infinite, and a very clean separation is possible. even though a “molecular sieve” adsorbent may be used.
The Al-rich (cationic) zeolites have highly polar inter-
nal surfaces. The polarity increases with increasing cation
III. MICROPOROUS ADSORBENTS charge and decreasing cation size. However, the relation-
ship between the nature of the cation and the surface prop-
Since adsorption is essentially a surface phenomenon, a erties is complex because the differences in cation location
practical adsorbent must have a high specific surface area, (sites) must also be considered.
which means small diameter pores. Conventional adsor- The commercially available zeolite adsorbents consist
bents such as porous alumina, silica gel, and activated of small microporous zeolite crystals, aggregated with the
carbon have relatively wide pore size distributions, span- aid of a clay binder. The pore size distribution thus has a
ning the entire range from a few angstroms to perhaps well-defined bimodal character, with the diameter of the
1 µm. For convenience the pores are sometimes divided intracrystalline micropores being determined by the crys-
into three classes: tal structure and the macropore size being determined by
the crystal diameter and the method of pelletization. As
Micropores: <20 ˚ A diameter originally defined, the term zeolite was restricted to alu-
Mesopores: 20–500 ˚ A diameter minosilicate structures, which can be regarded as assem-
Macropores: >500 ˚ A diameter blages of SiO 2 and AlO 2 tetrahedra. However, essentially