Page 38 - Pressure Swing Adsorption

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12 PRESSURE SWING ADSORPTION FUNDAMENTALS OF ADSORPTION 13
solid. The result 1s that gas molecules tend to concentrate m this region so Table 2.1. Limiting Heats of Sorplion tor CH and H 0 (kcal; mole)
4
2
that the molecular density m the vicinity of the surface is substantially greater
Aci. carbon (nonpolar) 4A Zeoiite (polar/
than m the free-gas phase. The strength of the surface forces depends on the
nature of both the solid and the sorbate. If the forces are relatively weak, CB (noopolar) 4.3 4.5
4
involving only van der Waals interactions supplemented in the case of polar H 0 (polar) 6.0 18.0
2
or oual1rupolar species by electrostatic forces (dipole or auadruoole mterac-
t,or;s), we have what is called "phys1cai adsorotion" or "phys1sorpt1on." By
contrast, if the mteraction forces arc strong, mvolvmg a significant degree of
(a small nonpoiar molecule of similar molecular we,gllt an<l therefore with
electron transfer, we have "chenusorption." Chem1sorotton 1s limited to a comparable van der Waals mteract10n energy) 1s ,only weakly adsorbed. In
monolayer, whereas, m pJiys1cal adsorptmn, mult10le rnoiecular layers can contrast, on a clean activated carbon (a nonpoiar surface) both these com-
form. Most nractical adsorption separation processes (including PSA) depend
pounds are adsorbed to a comparable extent. Furthermore, while the affimty
on physical ·adsorption rather than on chcmisorption, since, except for a few
of the zeolite surface for water 1s much higher than that of the carbon
rather specialized applicatmns, the capacities achievable m chemisorption
surface methane 1s retained with comparable affinity on both these adsor¥
systems are too small for an econom1c process. Since the adsorotton forces
bents (;ee Table 2.1). Clearly the oolar zeolite surface JS "hydrophilic'· and,
cteoend on the nature of the adsorbing molecule as well as on the nature of
by companson, the nonoolar carbon surface 1s "hydrophobic." ·
the surface, different substances are adsorbed with different affinities. It 1s Jome adsorbents such as the zeolites owe the1r' hydrophilic nature to the
this i•selectiv1ty" that orovides the basis for adsorption separation processes.
polarity of the heterogeneous surface. However. when the surface contains
The role of the adsorbent 1s to provide the surface area required for
hydroxyl groups (e.g., silica gel, aiumma, or some o:olymenc resms) rnolecuies
selective sorpt1on of the preferentially adsorbed species. A high seiect1vity is
such as water can also interact strongly by Jwdrogen bond formation. As with
the pnmary requ1rement, but a high capacity ts aiso desirable since the
polar adsorbents, water 1s therefore oreferent,ally :adsorbed, but m this case
capacity determines the size and tl1erefore the cost of the adsorbent beds. To the hydroohiiic select1v1ty 1s attributable mainly to the hvdrogen bond energy
achieve a high capacity commercial adsorbents are made from m1croporous
rather than to surface oolantv.
materials. As u. result the rate of adsorption or desorotwn 1s generally
It should be notcci that hydrophobic surfaces do not actually rcpci water.
controlled by diffusion through the pore network, and such factors must be
In general water will be adsorbed on any surface with at ieast the affirntv
considered in the selection of an adsorbent and the choice of operating
dictated by the van cter Waals forces. The pomt 1s that on a hydrophilic
conditions. Certam materials (zeolites and carbon molecular sieves) that have
surface water (and other polar molecules) will be adsorbed much more
very fine and uniformly sized m1cropores show significant differences m strongly than would be expected simply from the van der Waals forces alone.
sorot1on rates as a result of steric hindrance to diffus10n within the m1cro-
Furthermore, while hydrophilic adsorbents generaHy also show select1v1ty for
oores. Such adsorbents offer the possibility of achieving an efficient kinetic
other polar molecules relat1Ve to similar nonoolar ,species, this 1s not aiways
separation basect on differences m sorotion rate rather than on differences in true. Where the hydrophilic selectivity comes from hydrogen bonding, polar
sorpuon equilibnum.
molecules with no "active" hydrogens will be held only with an affirnty
comparable to nonpolar sotbates.
2.1.2 Hydrophilic and Hydrophobic Behavior The possibility of crcatmg polar select1v1ty by oretrcatment of the surface
is well illustrated by activated carbon adsorbents (see Figure 2.1). On a ciean
For equilibrium-controlled adsorbents, the primary ciassificat10n is between carbon surface n-hexane 1s adsorbed much more strongiy than sulfur dioxide
"hydrophilic" and "hydrophobic" surfaces. If the surface 1s polar, generally
(a polar sorbate), but on an oxidized surface this selecnv1ty is reversed.
as a result of the presence of ions in the structure but possibly also as a result
Control and modificat10n of surface poiarity is mdeed the most important
of the presence of ions or polar molecules strongiy bound to the solid practical tool m the tailoring of equilibnum seiectiV1ty.
surface, tt will preferentially attract polar molecuies-in oart1cular water.
This 1s because the field-dipole and/or field gradient-Quadrupoie interac-
tions provide additional contributions to the energy of adsorption. This 2.1.3 Pore Size Distribution
additil;nai energy will anse onlv when both conditions are fulfilled (i.e., a
According to the IUPAC ciassificat1on, pores are divided mto three cate-
polar or auadrupoiar molecule and a polar adsorbent). If either of these is
gones by size:
lacking there can be no significant clectrostatrc contribution to the energy of
sorotion. Thus, on highly poiar actsorbents such as zeolites or activated
Microoores < 20A; Mescooores 20-500 A; Macrooores > 500 A
alumina, water (a small polar molecule) 1s strongly adsorbed while methane