Page 51 - Separation process principles 2

16 Chapter 1 Separation Processes

pressure to vaporize the adsorbate (pressure-swing adsorp- Centrifugation, Operation (1) in Table 1.4, establishes a
tion), (3) inert purge stripping without change in temperature pressure field that separates fluid mixtures according to mol-
or pressure, and (4) displacement desorption by a fluid con- ecular weight. This technique is used to separate 235~~6
and
taining a more strongly adsorbed species. from 238~~6, large polymer molecules according to mol-
Chromatography, Separation Operation (2) in Table 1.3, is ecular weight.
a method for separating the components of a feed gas or liq- If a rather large temperature gradient is applied to a ho- 1
uid mixture by passing the feed through a bed of packing. The mogeneous solution, concentration gradients can be estab-
lished and thermal diffusion, (2), is induced. It has been used
feed may be volatilized into a carrier gas, and the bed may be to enhance the separation of uranium isotopes in gas perme- i
a solid adsorbent (gas-solid chromatography) or a solid-inert
support that is coated with a very viscous liquid that acts as an ation processes.
absorbent (gas-liquid chromatography). Because of selective Natural water contains 0.000149 atom fraction of deu-
adsorption on the solid adsorbent surface or absorption into terium. When water is decomposed by electrolysis, (3), into
liquid absorbent, followed by desorption, different compo- hydrogen at the cathode and oxygen at the anode, the deu-
nents of the feed mixture move through the bed at different terium concentration in the hydrogen produced is lower than
rates, thus effecting the separation. In afinity chromatogra- that in the water. Until 1953, this process was the only com-
phy, a macromolecule (called a ligate) is selectively adsorbed mercial source of heavy water (D20). In electrodialysis,
by a ligand (e.g., an ammonia molecule in a coordination (4), cation- and anion-permeable membranes carry a fixed
compound) that is covalently bonded to a solid-support charge, preventing the migration of species of like charge.
particle. Ligand--ligate pairs include inhibitors-enzymes, This operation can be used to desalinize (remove salts from)
antigens-antibodies, and antibodies-proteins. Chromatogra- sea water. A somewhat related process is electrophoresis,
phy in its various forms is finding use in bioseparations. (5), which exploits the different migration velocities of
Ion exchange, (3), resembles adsorption in that solid par- charged colloidal or suspended species in an electric field.
ticles are used and regeneration is necessary. However, a Positively charged species, such as dyes, hydroxide sols,
chemical reaction is involved. In water softening, a typical and colloids, migrate to the cathode; while most small, sus-
ion-exchange application, an organic or inorganic polymer pended, negatively charged particles are attracted to the
in its sodium form removes calcium ions by exchanging cal- anode. By changing the solvent from an acidic to a basic
cium for sodium. After prolonged use, the (spent) polymer, condition, migration direction can sometimes be changed,
which becomes saturated with calcium, is regenerated by particularly for proteins. Electrophoresis is a highly versatile
contact with a concentrated salt solution. method for separating biochemicals.
Another separation technique for biochemicals and
difficult-to-separate heterogeneous mixtures of micromolec-
1.6 SEPARATION BY EXTERNAL
ular and colloidal materials is field-jow fractionation, (6).
FIELD OR GRADIENT For the mixture to be separated, an electrical field, magnetic !
1
External fields can be used to take advantage of differing field, or thermal gradient is established in a direction 4
degrees of response of molecules and ions to forces and perpendicular to a laminar-flow field. Components of the ;
gradients. Table 1.4 lists common techniques, with combina- mixture are driven to different locations in the stream; thus,
!
tions of these techniques with each other and with previ- they travel in the flow direction at different velocities, so a
1
ously described separation methods also being possible. separation is achieved.

Table 1.4 Separation Operations by Applied Field or Gradient
Separation Initial or Force Field or Industrial
Operation Feed Phase Gradient Examplea
Centrif~rgation (1) Vapor Centrifugal force field Separation of uranium isotopes
(Vol. 23, pp. 531-532)
Thermal diffusion (2) Vapor or liquid Thermal gradient Separation of chlorine isotopes
(Vol. 7, p. 684)
Electrolysis (3) Liquid Electrical force field Concentration of heavy water
(Vol. 7, p. 550)
Electrodialysis (4) Liquid Electrical force field and Desalinization of sea water
membrane (Vol. 24, pp. 353-359)
Electrophoresis (5) Liquid Electrical force field Recovery of hemicelliiloses
(Vol. 4, p. 551)
Field-flow fractionation (6) Liquid Laminar flow in force field

Titations refer to volunie and page(s) of Kirk-Otltmer Encyclopedia of Chemical Tech~lology, 3rd ed., John Wiley and Sons, New York (1978-1984).