Page 390 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GLQ Final Pages
Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

Membranes, Synthetic, Applications 325

reasons. The first relates to critical applications—e.g.,
medical/immunological separations and salt removal from
solutions of genetically engineered proteins—where leak-
age of undesirable species from the feed stream into the
permeate cannot be tolerated. (Also see Sections VI and
VII.) The second aspect is the absence of concentration
polarization arising from convective flow through an ul-
trafilter, for example, and the consequent accumulation of
rejected species in the boundary layer.

2. Donnan Dialysis
Ion exchange membranes contain high concentrations of
fixed charges. They are permeable to ions of opposite
charge (counterions) but repel ions of the same charge
(coions). Protons are the only exception; they can perme-
atefreelythroughhydrationpassagesinananionexchange
membrane. The functions of anion- and cation-exchange
FIGURE 39 Donnan dialysis application to the separation of sul-
membranes are illustrated in Fig. 38.
furic acid from aluminum sulfate. Al 2 (SO 4 ) 3 designated by ❡ and
Donnan dialysis functions through the interaction be-
H 2 SO 4 by (HPD, Inc.).
tween ions and ion-exchange membranes in the absence
of an externally applied electrical field. When an ion
the ion exchange membrane into a water stream, form-
exchange membrane separates two electrolyte solutions,
ing sulfuric acid. Aluminum cations are rejected by the
and a second electrolyte with the same counterion but a
fixed positive charges on the membrane and exit as a less
nonpermeating coion is added to one side of the mem-
acidic aluminum sulfate stream for recovery or disposal.
brane, counterions migrate across the membrane until the
Similar applications include the recovery of sulfuric acid
charge separation stops further flow and electroneutrality
from nickel sulfate steel pickling waste, and the recovery
is established on both sides of the membrane. This phe-
of nitric and hydrofluoric acids produced during stainless
nomenon is known as Donnan equilibrium. Donnan dialy-
steel etching. Donnan dialysis is effective because high
sis refers to the process of separating ionic components in
concentration gradients yield concentrated products, and
a feed stream according to their tendency to migrate across
because direct input of electrical energy is not required to
ion-exchange membranes to achieve equilibrium.
achieve separation.
The example shown in Fig. 39 illustrates the treatment
of an aluminum anodizing bath waste stream by Donnan
dialysis. Sulfate ions and protons freely permeate from a 3. Electrodialysis
feed stream of aluminum sulfate and sulfuric acid across
Although the development of electrodialysis desalination
technology predated that of reverse osmosis (q.v.), at
present both processes compete favorably with distillation
for potable water production. In electrodialysis, salts are
removed from a feed solution by using an electric current
(DC) to transport ions across anion-exchange and cation-
exchange membrane pairs. By restricting the migration
of ions to no more than one adjacent solution compart-
ment, as shown in Fig. 40(a), alternate streams become
enriched and depleted of electrolytes. Electrodialysis op-
erates most economically when the feed water contains
less than 0.5% TDS, but medium-salinity seawaters (up
to about 1.2% TDS) can also be desalted. Product wa-
ter containing less than 0.01% TDS can be obtained. The
FIGURE 38 Selective diffusion across ion-exchange mem-
branes. (a) Anion exchange, and (b) cation exchange. Metal capability of electrodialysis to remove salts from neutral
cations are designated by M , anion A , proton H , and the fixed solutes is also exploited in other applications, e.g., desalt-
+
+
−
charges in the membrane by + and −. ing proteins.