Page 49 - Separation process principles 2

14 Chapter 1 Separation Processes

1.4 SEPARATION BY BARRIER osmotic pressure, on the feed side. Using a nonporous mem-
brane, reverse osmosis successfully desalts water. Dialysis,
The use of microporous and nonporous membranes as semi-
(3), is the transport, by a concentration gradient, of small
permeable barriers for difficult and highly selective separa-
solute molecules, sometimes called crystalloids, through a
tions is rapidly gaining adherents. Membranes are fabricated
porous membrane. The n~olecules unable to pass through the
from natural fibers, synthetic polymers, ceramics, or metals,
membrane are small, insoluble, nondiffusible particles,
but may also consist of liquid films. Solid membranes are
sometimes referred to as colloids.
fabricated into flat sheets, tubes, hollow fibers, or spiral-
Microporous membranes can be used in a manner similar
wound sheets, which are incorporated into commercial mod-
to reverse osmosis to selectively allow small solute mole-
ules or cartridges, generally available only in certain sizes.
cules andlor solvents to pass through the membrane and to
For microporous membranes, separation is effected by differ-
prevent large dissolved molecules and suspended solids
ing rates of diffusion through the pores; while for nonporous
from passing through. Microfiltration, (4), refers to the re-
membranes, separation occurs because of differences in both
tention of molecules typically in the size range from 0.02
solubility in the membrane and rate of diffusion through the
to 10 pm. Ultrafiltration, (5), refers to the range from 1 to
membrane. The most complex and selective membranes are
20 nm. To retain molecules down to 0.1 nm, reverse osmo-
found in the trillions of cells in the human body.
sis, sometimes called hyperjiltration, can be used.
Table 1.2 lists the more widely used membrane separa-
Although reverse osmosis can be used to separate organic
tion operations. Osmosis, Operation (1) in Table 1.2, in-
and aqueous-organic liquid mixtures, high pressures are re-
volves transfer, by a concentration gradient, of a solvent
quired. Alternatively, pervaporation, (6), in which the
through a membrane into a mixture of solute and solvent.
species being absorbed by and transported through the non-
The membrane is almost nonpermeable to the solute. In
porous membrane are evaporated, can be used. This method,
reverse osmosis, (2), transport of solvent in the opposite which uses much lower pressures than reverse osmosis, but
direction is effected by imposing a pressure, higher than the
Table 1.2 Separation Operations Based on a Barrier
Separation Initial or
Oueration Symbola Feed Phase Separating Agent Industrial ~xam~le~
Osmosis (I) Liquid Nonporous membrane

Reverse osmosis* (2) Liquid Nonporous membrane with Desalinization of sea water
pressure gradient (Vol. 24, pp. 349-353)

Dialysis (3) Liquid Porous membrane with Recovery of caustic
pressure gradient from hemicellulose
(Vol. 7, p. 572)
Liquid Microporous membrane with Removal of bacteria
pressure gradient from drinking water
(Vol. 15, p. 115)
Ultrafiltration (5) Liquid Microporous membrane with Separation of whey
pressure gradient from cheese
(Vol. 15, pp. 562-564)
Liquid Nonporous membrane with Separation of azeotropic
pressure gradient mixtures
(Vol. 15, pp. 116-117)
Gas (7) Vapor Nonporous membrane with Hydrogen enrichment
pressure gradient (Vol. 20, pp. 709-710)

Liquid membrane (8) Vapor and/or liquid Liquid membrane with Removal of hydrogen sulfide
pressure gradient (Vol. 15, p. 119)

'Design procedures are fairly well accepted.
"ingle units are shown. Multiple units can be cascaded.
bCitations refer to volume and page(s) of Kirk-Othmer Encycloped~a of Chemical Technology, 3rd ed., John Wiley and Sons, New York (1978-1984).