1. Introduction

Electromembrane processes are based on the transport of ions in the electric field, where the transport of the ions is meaningfully guided by ion exchange membranes. One utilizes the characteristic of ion exchange membranes that they are permeable for certain ions and block others. The membrane enables either the passage of anions (anion membrane, AAM) or of cations (KAM), while the other ions with opposite polarity are nearly totally blocked.

Through this, various opportunities result, when the space between two electrodes is divided by membranes into separated compartments. The classical electrodialysis, e.g., involves the alternating arrangement of cation- and anion-exchange membranes between the electrodes, while membrane electrolysis utilizes a single membrane as separator between cathode and anode compartments.

2. Ion exchange membranes and their properties

Ion exchange membranes are about 20 - 200 µm thick films of an ion exchanger (IAT), mostly consisting of organic polymers, whose characteristic feature is their content of polymer-bound charge carriers (solid ions), which retain mobile ions (counter ions) in the IAT for charge neutralization.

In contact with water or salt solutions, IATs swell until a certain water content. The first water molecules entering the membrane hydratize the solid ions. This water uptake proceeds until an equilibrium value, which depends on the composition of the contacting solution. This results in a mobility of the counter ions. The equilibrium between electrolyte and membrane determines the distribution of the counter ions and the uptake of further electrolyte, i.e., including ions of the same charge as the solid ions (co-ions). The resulting equilibrium of water in the membrane is determined by the osmotic pressure of the outer solution. The water content for the same outer solution is commonly proportional to the content of solid ions [1], especially when the matrix is very hydrophobic or contains a certain crystalline or quasi-crystalline portion.

Cross-linking limits the sorption capacity for the solvent, because internal stress develops in the polymeric micro structure when the excess osmotic pressure is taken up by the covalent polymer network.

The selectivity of the ion exchange membrane is caused by the exclusion of the blocked ions upon swelling. This phenomenon was described by Teorell, Meyer and Sievers for biological membranes (TMS-theory, [2]). The theory is mainly based on the assumption that only processes in the interior of the membrane determine the fluxes between the solutions at both sides. A thermodynamic equilibrium at all interfaces between solutions and membrane is assumed, what however is not always fulfilled at high flux densities.

Taking as a model (fig. 1, upper part) a membrane M, which separates two solutions of a salt with concentrations c'₁ and c'₂, we can describe the stationary equilibrium state by the Donnan distribution [3, S. 336].

3. Transport through ion exchange membranes

Due to the ion exclusion, the ion exchange membrane contains only few co-ions, so that they contribute only marginally to the current. Thus, the current across the membrane consists almost exclusively of counter ions.

This makes possible to prevent that, in a two-compartmet arrangement, no cations are transferred from the anode compartment into the cathode compartment, while anions move from te cathode compartment to the anode compartment.

4. Electrodialysis

When cation- and anion-membranes are arranged alternately between the electrodes, alternating compartments result, in which all ions are depleted, and other compartments, where the ions are enriched.

Each pair of membranes or compartments represents a repeat unit, which can be stacked as often as desired. This theoretically allows to transport with one Faraday of charge as many moles of salt from one compartment into another as the number of repeat units in the stack. The energy consumption is given by the ohmic resistance and the current efficiency of the repeat unit, while the contribution of the electrode processes is only marginally.

This is a standard configuration for electrodialysis, which is applied since more than 50 years for the production of drinking water and brine from sea- and brackish water (sea water desalination). Electrodialysis also has proven useful for the industrial desalination of foodstuffs: Soy extracts are desalinated by electrodialysis, so that their quality can be standardized, and the industrial production of lactose from whey became competitive through the economical whey desalination by electrodialysis.

Is the cation M⁺ a proton, acids become diluted or concentrated. This ?acid electrodialyis? (A-ED) however, results in additional technical problems (proton leakage). This opens new fields of application, especially at the end of certain industrial process chains, where acids are so diluted that they can no longer be used:

• The pickle solution recovery in the hot-dip galvanizing industry

• The recycling of HNO₃ / HF - mixed acid, which is used for pickling and passivation of stainless steels

5. The electromembrane process construction kit

Besides the spectrum of the various process variants, that results from different feed-compositions, also the membrane arrangement can be varied, or electrode reactions can be employed, as in membrane electrolysis.
Regarding a single diluate compartment, the ions are transported to the anode or to the cathode from that compartment into the adjacent compartments.

At electrodialysis, these elementary units are stacked in series and the ions removed from the diluate are merged into one common output flow. Optionally, arrangements with two different diluate compartments with different salt solutions are also possible.

 

From the different salt solutions, each one cation and one anion are combined, resulting in two new salt solutions, in which the ions are exchanged [4]. For example, a salt can be converted into the corresponding acid by feeding the corresponding amount of another acid, resulting in the salt of this acid as the byproduct (" salt-metathesis ").

A further possibility to utilize the separated ions from an elementary unit is the application of a bipolar membrane.

6. Literature

• [1] P. Maeres, NATO ASI Ser., Ser. C (1986) 169

• [2] T. Teorell, Proc. Sci. Exp. Bio. Med. 33 (1935) 282

• [3] F Helfferich, "Ionenaustauscher" (ion exchanger), Verlag Chemie Weinheim 1959

• [4] R. Audinos, Chem. Eng. Tech. 20 (1997) 247

Principle of Electrodialysis

Inside an electrodialysis unit, the solutions are separated by alternately arranged anion exchange membranes, permeable only for anions, and cation exchange membranes, permeable only for cations.
By this, the two kinds of compartments are formed, distinguished in the membrane type that faces the direction of the cathode. Applying a current causes cations in the diluate (blue compartment set) to move towards the cathode, passing the cation exchange membrane, and anions move towards the anode, passing the anion exchange membrane. The ions have now reached the concentrate chambers (red compartments), where a further transport is stopped by the respective next membranes:
 

Electrodialysis Stack Construction

An electrodialysis cell (left: an ED 1000H with 5 m² active membrane area) consists of two electrode-end blocks (PP, grey) and the membranes stacked between them. The end blocks contain the in- and outlet adapters and the electrical connections. They are pressed together by a steel frame. The membrane stack, consisting of alternately arranged membranes and spacers, is located between the plastic end plates. In this picture, the membranes are dark and the spacers white, resulting in the lamellar appearance.

The general construction principle of an electrodialysis cell is shown in the following sketch:

The membranes are separated by spacers (5), consisting of a fabric in the active area, which is filled with the electrolyte, sourrounded by sealing area. The spacer net prevents the membranes from touching each other. The holes in the stacked spacers are arranged in a way form tubes, which build two different channel systems. By this way, the concentrate and diluate circuits are established.

1. Polypropylene end plate
2. Electrode
3. Electrode chamber
4. Spacer-sealing area
5. Spacer fabric
6. Screws
7. Steel frame
8. Inlet of anode cell
9. Inlet of concentrate cell
10. Cation exchange membrane
11. Anion exchange membrane
12. Inlet of diluate cell
13. Inlet of cathode chamber

The cells differ in the size of used membranes, the shape and the thickness of the used membranes. For example, an ED 64 cell has a square basis of 11 cm x 11 cm with an active membrane window of 8 cm x 8 cm (the rest is covered by sealing (4) and inlet/outlet areas). Other common sizes are summarised in our datasheet .

Shape and length of the cells are process-determining as shown below. Thin spacers are good for desalination applications (low energy consumption for low target conductivities) while thick spacers are well suited for applications with higher turbidity and higher concentration of the feed solution.

Applications of Electrodialysis

Electrodialysis makes it possible to transport ionic compounds from one solution to another. Therefore, its application covers the transfer of salts and acids between solutions. A common example is sea water desalination.

Not only salt solutions can be desalted and concentrated, but also acids. Examples illustrating this important application field are the recovery of pickling acid and the recycling of rinsing solution from hot dip galvanizing.

An important feature of electrodialysis is the capability to remove salt from non-ionic solutions, e.g. sugar solutions. As non-ionic molecules are not transported, salt can be selectively removed. Examples are the removal of NaI byproduct from a reactand solution or the desalination of polyalcohol-water mixtures.

Finally, membrane properties determine the process results: Besides the permselectivity (current efficiency), mainly water transfer (EOP, electroosmotic co-transfer) and ionic selectivities (preference of e.g. monovalent ions against divalent ions) determine the results. Considering these effects, even for example lime containing water can be concentrated without relevant scaling problems.

Different Types of ED Processes

It is also possible to run an ED process continuously or in the feed and bleed mode. Both process schemes are shown above. To decide whether a batch or a continuous process should be performed, the stack design has to be taken into account. To run in a continuous mode, the module has to treat the solution in one pass. This requires a certain residence time in the cell, i.e. with a given flow velocity of the solution, this corresponds to a certain process length.

Process Conditions of Electrodialysis

A running ED process involves that the ions in the cell are through the membranes, which determines which type of ion is blocked and which is transferred (see for more details: Transport in Ionenaustauschermembranen (in German) ). This key process governs all other process parameters and conditions around the membrane: The stack, the feed flow, the current and the temperature.

An important effect is the polarization of the ions at the membrane surface: All ions in the solution move to the extend of their concentration and mobility. On the membrane surface, both mobiltiy and concentration may change dramatically. This is due to a boundary layer of ions depletion or accumulation. An important point is that ionic depletion has to be avoided, because it leads to a high ohmic resistance and to water splitting, which may lead to burning of the membranes.

Application Examples

This Figure shows an example of a batch desalination (conductivity of diluate against time). The effect of a single pass desalination results in conductivity jumps at the start and stop times. It depends on the current, flow rate and other factors. The slope of the plot is, - more ore less - proportional to the current.

The next diagram shows the salt removal (calculated as NaCl) in dependence of the current at theoretical current efficiency (ce) and at 85% ce. With the PCCell ED 200 you can expect for sodium chloride ce's in the range between 90 and 95 %. It depends on current density, concentration and other factors. The amount of transported salt is given per cell pair. A 25 cell pair-unit will make 25 times of this.

The Principle

An electrodialysis cell can also be used for splitting of a salt into an acid and a base corresponding to the following reaction:

4 NaCl + 6 H₂O => 4 NaOH + 4 HCl + 2 H₂ + O₂

The reaction can be performed in various types of electro-electrodialysis cells. A typical setup is shown in the following picture:

The picture shows a four-chamber cell, where the main reaction occurs in the salt chamber (salt NaX) with the adjacent anion and cation exchange membranes.

In this setup, the cations move towards the cathode and enter the catholyte solution, where the cation hydroxide is formed. The anions move towards the anode and enter the acid chamber, which is separated from the anolyte solution by a cation exchange membrane. Protons from the anolyte solutions move into the acid chamber to produce the acid.

Applications

The four chamber cell process is a general process covering many applications. For example

Fermentation processes producing organic acids such as gluconic-, lactobionic-, lactic- or acetic acid and many others from fermentation broth.

Production of mineral acids from salts such as nitric acid from nitrate solutions, hydrochloric acid from chlorides or sulphuric acid from sulphates.

Variations

The next image shows variations of this setup: In certain cases, the membrane between anolyte and acid is not necessary, in other cases, an separating membrane towards the catholyte is helpful.

Various possible electromembrane processes for splitting of a salts.

The Principle

Conventional electrodialysis changes concentrations of salt solution. Within this transport process, a separate movement of the cations is achieved towards the anions. To get the higher concentrated salt, both ions are collected together in one solution. However, it is also possible change the setup by using two different salts and collecting the anions and cations crosswise into two new salts, in which the ion pairs have changed.

MA + ma -> Ma + mA

How is this done?

As within a conventional electrodialysis  unit, the solutions are separated by alternatively arranged anion and cation exchange membranes. The two kinds of compartments formed, are now driven as four compartments: Conc1, Dil1, Conc2, Dil2. The repeated unit into the cell now contains four units. Applying a current, cations within the first diluate (grey compartment set) move into a different concentrate compartment than the anions from this cell. And the cations in this cell, wherein the said anions arrive, are the cations from the second diluate compartment. These anions have changed their partner. Putting together all elementary processes, simply the salt pairs are changed.

Salt Metathesis Systems:

Running a Salt Metathesis Process

Salt metathesis is usually performed as a batch process.

An important point in running a salt metathesis process is the mass balance concerning the diluates. Application is very interesting in cases, a diluate stream is available with a high concentration and which has not to be completely desalted. This is the case e.g. for an fermentation, where the product should be removed by a certain extent. After ED-treatment the fermentation solution is recycled into the fermenter. The result is a continuos removal of product at a continuous high concentration level of the diluate. In such application, high current densities may be achieved.

Application examples of the salt metathesis

Typical metathesis applications are conversions of Calcium salts of organic acids into their acidic form by metathesis with hydrochloric acid. Often, calcium salts are obtained in fermentation processes or by the tartaric stabilization of wine. If the calcium salts are poorly soluble, they may also be processed into the sodium salts by processing with hot sodium carbonate solution. In each case, a metal free acid of a relatively high concentration is obtained. In cases of soluble calcium salts, a calcium chloride solution is obtained, which may be recycled into the process of obtaining the calcium salt.

SPE Electrolysis

SPE Electrolysis processes use a membrane as Solid Polymer Electrolyte between electrodes. Typically both electrodes are pressed on the membranes to get an optimum contact.

An example is modern brine electrolysis. But the application is not limited on this process. An other process is the Kolbe Synthesis.

Kolbe Synthesis

The Kolbe Synthesis is an anodic oxidation process of a carboxylate anion. A radical is formed which decarboxylates. The resulting radical combinates with another to form a dimer.
For example, acetate will react to ethane and propionate will react to butane.
However, in practise, this reaction work in the first case but does not take place in the second case because there are many side processes: the ethyl radical may not only dimerize, it also may eliminate a hydrogen radical to form ethylene. The hydrogen radical may react with a second ethyl radical to form ethane. In the case of propionate, kolbe electrolysis efficiency is sensitive to water. It is essential therefore, to run the reaction in (almost) water free conditions. The process is descibed below:

Advantages

Running a Kolbe Synthesis as a SPE reaction with anion exchangers has the advantage of solvent and product cotransport into the reaction zone (see picture above). PCA developed an optimized SPE matrix with low resistance in nonaqueous media. Results for propionate are given below: