Electrodialysis

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 ASTOM Corp.(a joint venture between TOKUYAMA Corp., the main shareholder of Eurodia, and Asahi Kasei Chemicals Corp) supplies the extensive line of NEOSEPTA® 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.
 

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 an effective cell area of 0.17 m2 (1.83 ft2).

The EUR40 has cells with an effective cell are of 0.4 m2 (4.3 ft2). For instance, a EUR40-800 stack would contain an effective cell area of 320 m2.

For pilot 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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