Commercial Applications
Electrodialysis is a well proven technology with hundreds of AQUALYZERS ® operating
systems worldwide. Eurodia / Ameridia has designed, built, and started
more than forty ED plants in Europe, America, and the Far East.
The following table gives an overview of the applications of electrodialysis.
Some of these applications will be discussed in more detail.
Applications of Electrodialysis
Eurodia Experience: More Than 100 Plants
- Cheese whey demineralization
- Brackish water desalination
- Nitrate removal for drinking water
- Food/sugar products desalting
- Tartaric wine stabilization
- NaCl removal from amino acid salts
- Acid removal from organic product
- Conversion of organic salts into acid and base (bipolar
membrane ED)
- Desalting of amines.
- De-acidification of fruit juices
- Metals removal from ethylene glycol
Tokuyama Corporation Experience
- Edible salt production from seawater (large capacities:
200,000 T/year)
- Many of the same applications as Eurodia (whey, organic
salts splitting, food,…)
- Desalting of surfactants
- Regeneration of electroless nickel plating baths, etc.
Also
- Desalting of hydrolyzed vegetable proteins (i.e. soy sauce)
- Concentration of dilute salt, acid, or base streams
- Recovery of salts, acids, and alkali from industrial rinse
waters
- Purification of fermentation broths for the production of
organic and amino acids, etc.
Technical Considerations
As we have seen, Electrodialysis is a powerful separation technique
with applications in many industries. However, it is important to
understand the key parameters that determine the optimum range of
applicability: these are the current density, the cell voltage,
the current efficiency, the diluate and concentrate concentrations.
The current density is the driving force of the process since it
determines the quantity of equivalent grams of product that are
transported across the membranes. Running at a high current density
reduces the required surface of ED cells making the process more
attractive. However, this has to be balanced with a disproportionate
cell voltage increase resulting in a much higher power consumption.
As the current density increases, there can be polarization when
the ions are transported faster across the membranes than are transported
in the cell solutions to the membrane surface: this results in a
very quick cell voltage increase. The "limiting current" is the
maximum allowed current density to avoid this steep cell voltage
increase and, in ED, it is critical to remain safely below. This
limiting current depends on parameters such as stack geometry (cell
thickness, turbulence,...), solution concentrations, temperature,
etc. Thus, for a given application, it is important to first determine
this critical parameter by doing a polarization curve (current vs.
voltage).
For a given current density, the cell voltage increases with time
as the membranes are either chemically affected or physically fouled
by contaminants in the solutions. Even with "perfect" solutions,
the voltage will eventually increase as the active sites in the
polymeric structure of the membranes disappear with use. This determines
when it is time to replace the membranes; either the rectifiers
being too small, the current efficiency becoming too low, or the
power consumption becoming too high. Depending on the application,
mainly on the product conductivities, this maximum voltage varies
between 0.8 and 1.5 V/cell. In applications with clean feed and
low current densities, membrane life can reach several years and
can be as high as ten years for drinking water nitrate removal.
Current efficiency also determines the surface of membranes required
for a given application. This critical parameter takes into consideration
all the parasitic phenomena occurring in the stack, such as the
non-perfect permselectivity of membranes or physical leakage (leading
to impurities in the products), that can be reduced by optimized
stack design and membrane selection. The current efficency is also
lowered by "shunt" or "stray" currents running in the non-active
cell area (i.e. manifolds). These can be minimized by stack design
features, and by limiting the cell voltage, as well as the conductivity
ratio between the diluate and concentrate loops.
Other major parameters are the concentrations (conductivities) of
the two streams. As seen above, the ratio of conductivities affects
the current efficiency, limiting the maximum concentration for the
concentrate (brine) stream. In most cases, 20 is the maximum concentration
factor that can be obtained (provided that the solubility is high
enough), unless more than one stage is used. This concentration
factor is generally much higher than with reverse osmosis, explaining
why ED is used to concentrate salt and produce table salt from seawater
in Korea and Japan. On the other end, the minimum diluate concentration
is limited by conductivity considerations due to the ohmic resistance
of the diluate cells and the low limiting currents at low conductivities.
As a rule of thumb, the minimum conductivity that can be considered
is ~0.5 mS/cm (at a price).
Many other parameters influence the design of suitable ED processes,
such as temperatures, product purity, cleaning, pH, etc. The maximum
temperature in ED stacks used to be 40°C, but recently developed
membranes allow operation at temperatures up to 60°C, which is useful
for viscous and low conductivity products, such as sugars. Membrane
fouling and stack plugging can be caused by many impurities in the
feed products, either soluble and insoluble, such as organic matter,
colloidal substances, microorganisms (yeast, bacteria,...), insoluble
salts, etc. A good pretreatment step is often necessary using Microfiltration,
Nanofiltration, and or Ion Exchange resins. It is also possible
to clean the membranes in the stacks with dilute acids and caustic,
as well as enzyme solutions. In many cases, if chemical cleaning
is not enough, current reversal also has a cleaning effect to remove
contaminants from membranes. The pH of the products also a consideration
as some membranes cannot tolerate very caustic solutions. Also,
the membranes cannot tolerate many organic solvents and most oxidizing
chemicals.
Eurodia Industrie and Ameridia have developed an extensive expertise
to evaluate the suitability of ED applications. However, for new
applications, pilot units are available for feasibility tests and
the generation of design data.
As discussed for most items above, a good stack design is critical
for an effective use of electrodialysis. Eurodia / Ameridia have developed
a stack technology with a wide selection of spacers that makes our
overall ED technology very competitive in terms of cell thickness,
stack tightness, fluid distribution, electrode replacements, etc.
One example is the very low pressure drop through the Eurodia stacks,
allowing for operation with viscous feeds and feed pressures up to
4-5 bars (high compared to other suppliers).
Stacks Available:
Eurodia Industrie/Ameridia provides two commercial stack sizes : the EUR40 and the EUR20. Both sizes can accommodate up to 1000 cells using various intermediate plates with and without electrodes.
The EUR20 has cells with a potential of 175 m2.
The EUR40 has cells with a potential of 370 m2.
For pilots units, there are two sizes available for either lease or sale : the EUR2 and EUR6. The EUR2 contains 10 cells of 0.02 m2 each with a total eff.cell area 0.2 m2. The EUR6 contains up to 80 cells of 0.06 m2 each with a total eff.cell area of up to 4.8 m2. The EUR2 stacks are used for short-term feasibility tests. The EUR6 stacks are used for membrane life tests and to generate data that are directly scaleable to commercial size since similar gaskets are available.
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