Chemistry Reference
In-Depth Information
Fig. 19.1
8
Electrochemical ion
exchange.
erally the process can be used to perform a number
of functions, such as:
anionic, which are permeable to anions; and
cationic, which are permeable to cations. The mem-
branes are effectively impermeable to the hydraulic
transport of water and other non-charged species. In
practical applications the membranes are arranged
alternately in a stack, as shown in Fig. 19.19,
between two electrodes. The membranes in the stack
are separated from one another by thin plastic
spacers 0.5-2.0 mm in thickness, which creates dis-
crete compartments known as cells. The application
of a potential difference (direct electrical current)
between the two electrodes induces the transport of
ionic species. Anions migrate towards the anode,
passing through the anion-exchange membrane, and
then are blocked by the cation-exchange membrane.
The cations behave in a similar manner but move in
the opposite direction and are retained by the anion-
exchange membrane. Thus one type of cell will
become ion enriched, forming the concentrate
stream, and the other cell will become ion depleted,
forming the diluate stream. The main variable for
controlling the quantities of ions transferred through
(1)
The separation of salts, acids and bases from
aqueous solutions.
(2)
The separation of ionic compounds from neutral
molecules.
(3)
The separation of monovalent ions from ions
with multiple charges.
(4)
The introduction of ionic moieties to generate
new species.
Electrodialysis competes with other separation
processes, such as reverse osmosis, ion exchange,
dialysis, etc., and can offer advantages of high selec-
tivity for charged components, lower energy costs
and investment costs, continuous operation, high
product recovery rates and minimal change of
feed water constituents due to chemical or thermal
degradation.
The process of electrodialysis uses a direct electri-
cal current to transport ions through ion-selective
membranes. There are two types of membranes:
