Page 391 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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

326 Membranes, Synthetic, Applications

FIGURE 40 (a) Electrodialysis and (b) electrodialysis reversal
(EDR). Cation exchange membrane indicated by C and anion ex-
change membrane by A (Ionics Inc.)

In electrodialysis, membrane fouling occurs after rel-
atively short periods of operation because scale-forming
ions migrate unidirectionally to the membrane surface.
The result is gradually increasing electrical resistance and
reducing desalting efficiency. Pretreating the feed water
withchemicalsdelaysfoulingbutdoesnotpreventit.Elec-
trodialysis Reversal (EDR) was a process improvement
introduced in the early 1970s that overcame the fouling
problem by reversing the polarity of the DC field at 15- to
20-min intervals, as shown in Fig. 40(b), and purging the
removed scale and foulants from the stack. Today EDR
is a proven process for brackish water desalination: one
of the largest commercial installations in Florida, United
States, produces 12 million gallons of drinking water per
day from 0.13% TDS feedwater at 85% recovery.

FIGURE 41 Membrane-based electrochemical syntheses: (a)
H. Electrochemical Synthesis and Bipolar electrolysis; (b) substitution; (c) double decomposition; and (d)
Membrane Technology bipolar membrane synthesis.
The unique capability of ion-exchange membranes to sep-
reaction schemes into innovative waste treatment and
arate chemical species according to ionic charge makes
resource reclamation systems.
it possible to conduct various electrochemical synthesis
reactions otherwise difficult to perform. A number of
1. Electrolysis
such synthetic mechanisms are shown in Fig. 41. Al-
though each may be applied individually, a recent trend The largest scale synthesis based on electrolysis is the
has emerged toward assembling several electrochemical chlor-alkali process. Sodium ions in a salt brine migrate