"WATER SPLITTING"
Bipolar membrane electrodialysis or "Water Splitting" efficiently
converts aqueous salt solutions into acids and bases without chemical
addition. It is an electrodialysis process since ion exchange membranes
are used to separate ionic species in solution with the driving
force of an electrical field, but it is different by the unique
water splitting capability of the bipolar membrane. In addition,
the process offers unique opportunities to directly acidify or basify
process streams without adding chemicals, avoiding by-product or
waste streams and costly downstream purification steps.The Bipolar
Membrane Under the driving force of an electrical field, a bipolar
membrane can efficiently dissociate water into hydrogen (H+, in
fact "hydronium" H3O+) and hydroxyl (OH-) ions. It is formed of
an anion- and a cation-exchange layer that are bound together, either
physically or chemically, and a very thin interface where the water
diffuses from the outside aqueous salt solutions. The transport
out of the membrane of the H+ and OH- ions obtained from the water
splitting reaction is possible if the bipolar membrane is oriented
correctly (there is no current reversal in water splitting). With
the anion-exchange side facing the anode and the cation-exchange
side facing the cathode, the hydroxyl anions will be transported
across the anion-exchange layer and the hydrogen cations across
the cation-exchange layer. Therefore, a bipolar membrane allows
the efficient generation and concentration of hydroxyl and hydrogen
ions at its surface (up to 10N). These ions are used in an electrodialysis
stack to combine with the cations and anions of the salt to produce
acids and bases (see below).
A good bipolar membrane has a strong,
permanent bond between the two layers and a thin interface to reduce
the voltage drop. It also allows the water to easily diffuse inside
to the interface and feed the water splitting reaction so that a
high current density can be applied to minimize the required membrane
area. Tokuyama Corporation, the main shareholder of Eurodia, has
developed such an effective membrane in its Neosepta family of ion
exchange membranes: the BP-1 membranes have been successfully used
in several commercial applications. They are currently developing
other bipolar membranes for different applications. The Three-Compartment
Cell
A three-compartment cell is obtained by adding the bipolar membrane
in a conventional electrodialysis cell. The bipolar membrane is
flanked on either side by the anion- and cation-exchange membranes
(see electrodialysis section) to form three compartments (see schematic
below): acid between the bipolar and the anion-exchange membranes,
base between the bipolar and the cation- exchange membranes, and
salt between the cation- and anion-exchange membranes. As in ED
stacks, many cells can be installed in one stack (up to 200 for
bipolar membrane ED) and a system of manifolds feeds all the corresponding
compartments in parallel, creating three circuits across the stack:
acid, base, and salt.
SCHEMATIC OF THREE-COMPARTMENT SYSTEM
It becomes easy to see how, by feeding the salt solution to the
salt compartments, water to the acid and base compartments, and
by supplying a DC current across the electrodes, it is possible
to convert an aqueous salt solution such as NaCl into the base NaOH
and the acid HCl. Similarly, other salts such KF, Na2SO4, NH4Cl,
KCl, etc., as well as the salts of organic acids and bases can be
converted. It is important to note that, in this process, the electrodes
are only used to supply the current and that the electrode reactions
are basically negligible: only a small percentage (1-2 %) of the
power is consumed at the electrodes where a small amount of hydrogen
and oxygen is generated.
For a complete installation, there are three loops with circulation
tanks, pumps, valves & piping for the three loops through the stack,
plus one double (or two) loop(s) for the electrode rinse solution.
The loops can either operate in a feed & bleed mode or in a batch
mode. Instrumentation can be added to control or monitor flows,
pressures, conductivities, temperatures, pH's, voltage and current
according to the process requirements.
Other Configurations
There are two other main configurations that can be commonly
considered: two-compartment cells with bipolar and cation-exchange
membranes (only) or with bipolar and anion-exchange membranes. Using
either two-compartment configuration might be only feasible in some
cases and bring economic benefits such as lower investment costs
(one less loop, fewer membranes) and a lower operating cost (lower
power, fewer membrane to replace).
SCHEMATIC OF TWO-COMPARTMENT CELL WITH CATION-EXCHANGE MEMBRANES
SCHEMATIC OF TWO-COMPARTMENT CELL WITH ANION-EXCHANGE MEMBRANES
The two-compartment cells with bipolar and cation-exchange membranes
only are useful to convert the salts of weak acids and strong bases,
such as sodium acetate, lactate, formate, glycinate, etc. and of
other organic and amino acids. Since the conductivity of these weakly
dissociated acids is very low, it is not possible to use the three-compartment
cell with pure acid. In the two-compartment configuration (see schematic),
aqueous base (e.g. up to 10 w% NaOH) is obtained in the base loop
and the other product is a mixture of acid and a residual of salt
(e.g.1-2 w%) to give sufficient conductivity: the conversion rate
that can be achieved depends on the salt concentration since most
water remains with the acid/salt loop. A conversion of up to 95
% can be reached with feed at 30-35 w% and the acid is slightly
concentrated as some water is transported in the base loop. There
is little loss of H+ across the cation-exchange membranes since
the acid is weakly dissociated.
Similarly, the two-compartment cells with bipolar and anion-exchange
membranes only are useful to convert the salts of weak bases (ammonia)
and strong acids, such as ammonium chloride, ammonium sulfate, and
ammonium lactate. |
