The Electrodialysis Stack
The heart of the process is the electrodialysis stack (see
schematic) consisting of alternating anion- and cation-exchange
membranes separated by proprietary spacers (or gaskets). There
are several circuits in the stack, such as feed (diluate) and
brine (concentrate), thanks to channels formed by manifolds in
membranes and spacers. The spacers direct the feed and brine solutions
into the corresponding chambers and promote flow distribution.
A set of two membranes and two spacers forms a cell or cell pair
and hundreds of cells can be installed in one stack. A clamping
system keeps the assembly together under a uniform closing pressure.
The driving force is a direct current between anodes (positive
electrodes) and cathodes housed at the two ends of the stack inside
electrode plates.
SCHEMATIC ED STACK
Careful design of the various components of the electrodialysis
stacks can make a large difference in the overall performance
of the system. For instance, good sealing is key to avoid physical
leaks between the feed and product solutions. This results in
higher product purities and less wastes. An optimized netting
geometry will insure that the solutions are well distributed in
the entire cell and increase turbulence to reduce membrane surface
phenomena and improve ion transport. The thickness of the spacers
will affect the power consumption. The ion exchange membrane selection
is also critical since selectivity will affect the purity of the
product and the efficiency of the separation, their electric resistance
will impact the power consumption, their chemical resistance will
determine the compatibility and the feasibility of a given separation,
etc. Eurodia has developed a family of proprietary spacers that
are tailored to various applications and TOKUYAMA CORPORATION,
Eurodia and Ameridia's main shareholder, supplies the extensive
line of NEOSEPTAŽ ion exchange membranes that are used in the
stacks (see Table of Available Membranes).
How Does the Separation Work?
Ion exchange membranes are thin films of polymeric chains containing
electrically charged functional sites. These selectively charged
membranes can separate ions: if the membrane is positively charged
(e.g. with quartenary ammonium groups) only anions will be allowed
through it and it is called an anion-exchange membrane. Similarly,
negatively charged membranes (i.e. with sulfonate groups) are
called cation-exchange membranes. This membrane property is named
permselectivity and can be customized to meet specific requirements.
There are also membranes that only allow monovalent cations or
anions through them and reject multivalent ions: these are called
monovalent-selective membranes. Such selectivity is typically
obtained by adding a thin ion-exchange layer of opposite sign
at the surface of the membrane. Note that all ion-exchange membranes
are not 100 % permselective: most Neosepta membranes have a permselectivity
of 98 % or higher.
The salt solution is fed into the electrodialysis stack through
the diluate circuit (and possibly through the concentrate circuit
as well). When the solution arrives in the active area of the
cells, the DC voltage causes the positively charged cations to
migrate toward the cathode and the negatively charged anions to
migrate toward the anode. When the ion reaches an ion exchange
membrane, the membrane properties determine whether the ion is
rejected or allowed to pass through. The ions that can pass though
the membranes are retained in the next compartment since the next
membrane in its path will be of the opposite charge. Therefore,
there are compartments from where the ions are removed and some
where they are concentrated: if the solutions are circulated rapidly
through the stack, a diluate and a concentrate stream are obtained.
Depending on the objectives, the product can be either the desalted
stream (i.e. wine, drinking water or demineralized cheese whey)
or the concentrate stream (i.e. salt brine), or both.
The low amount of water transported with the salt across the membranes,
or "concentration transport", enables the brine stream to have
a higher concentration than the feed stream. Therefore, it is
possible not only to remove salts from a solution, but also to
concentrate it by electrodialysis, as does evaporation. As for
most processes, there are practical limits to the desalting and
concentration rates: these will be discussed later.
As seen on the simplified flow sheet below, a complete system
includes circulation pumps, tanks, piping & valves for the concentrate
and diluate loops, plus for the electrode rinse solution(s). The
process can operate either in the feed and bleed, batch, or even
the single pass modes. Instrumentation (flowmeters, pressure gauges,
temperatures, pH's, conductivities, cell voltage, current) are
added depending on requirements.
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