Page 50 - Separation process principles 2

1.5 Separation by Solid Agent 15

where the heat of vaporization must be supplied, is used to face of the solid adsorbent, unlike absorption, which occurs
separate azeotropic mixtures. throughout the bulk of the absorbent. In all cases, the active
me separation of gas mixtures by selective gas perme- separating agent eventually becomes saturated with solute and
ation, (7), through membranes, using pressure as the driving must be regenerated or replaced periodically. Such separations
force, is a relatively simple process, first used in the 1940s are often conducted batchwise or semicontinuously. However,
with porous fluorocarbon barriers to separate 235~~6 and equipment is available to simulate continuous operation.
at
238~F6 great expense because it required enormous amounts Adsorption, Separation Operation (1) in Table 1.3, is used
of power. More recently, nonporous polymer mem- to remove components present in low concentrations in non-
branes are used commercially to enrich gas mixtures contain- adsorbing solvents or gases and to separate the components
ing hydrogen, recover hydrocarbons from gas streams, and in gas or liquid mixtures by selective adsorption on solids,
produce nitrogen-enriched and oxygen-enriched air. followed by desorption to regenerate the adsorbents, which
Liquid membranes, (81, of only a few molecules in thick- include activated carbon, aluminum oxide, silica gel, and
ness can be formed from surfactant-containing mixtures that synthetic sodium or calcium aluminosilicate zeolite adsor-
locate at the interface between two fluid phases. With such a bents (molecular sieves). The sieves differ from the other ad-
membrane, aromatic hydrocarbons can be separated from sorbent~ in that they are crystalline and have pore openings
~araffinic hydrocarbons. Alternatively, the membrane can be of fixed dimensions, malung them very selective. A simple
formed by imbibing the micropores with liquids that are adsorption device consists of a cylindrical vessel packed
doped with additives to facilitate transport of certain solutes, with a bed of solid adsorbent particles through which the gas
such as COz and H2S. or liquid flows. Regeneration of the adsorbent is conducted
periodically, so two or more vessels are used, one vessel de-
sorbing while the other(s) adsorb(s). If the vessel is arranged
1.5 SEPARATION BY SOLID AGENT
vertically, it is usually advantageous to employ downward
Separation operations that use solid mass-separating agents flow of a gas. With upward flow, jiggling of the bed can cause
are listed in Table 1.3. The solid, usually in the form of a gran- particle attrition and a resulting increase in pressure drop and
ular material or packing, acts as an inert support for a thin layer loss of material. However, for liquid flow, better distribution
of absorbent or enters directly into the separation operation by is achieved by upward flow. Regeneration is accomplished
selective adsorption of, or chemical reaction with, certain by one of four methods: (I) vaporizing the adsorbate with a
species in the feed mixture. Adsorption is confined to the sur- hot purge gas (thermal-swing adsorption), (2) reducing the

Table 1.3 Separation Operations Based on a Solid Agent
Separation Initial or Separating Industrial
Operation S ymbola Feed Phase Agent ~xarn~le~
Adsorption* (1) Vapor or liquid Solid adsorbent Purification of p-xylene
(Vol. 24, pp. 723-725)

Chromatography* (2) Vapor or liquid Solid adsorbent or Separation of xylene iso-
liquid adsorbent on mers and ethylbenzene
a solid support (Vol. 24. pp. 726-727)

Ion exchange* (3) Liquid Resin with ion-active sites Demineralization of water
(Vol. 13, pp. 700-701)

'Design procedures are fairly well accepted.
aSingle units are shown. Multiple units can be cascaded.
b~itations refer to volume and page(s) of Kirk-Othrner Encyclopedia of Chenlical Technology, 3rd ed., John Wiley and Sons, New York (1978-1984).