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Electromembrane processes in technology

A general introduction into the various membrane processes

 

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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].

Fig. 1: Schematic view of a membrane between two electrolyte solutions of different concentration. Indicated are the concentration of the counter ion i (upper part) and the electrical potential (lower part) [3, S. 336].

The electrochemical potentials μi of ion species i in the single phases are

.

(Quantities for outer solutions are indicated by a prime and the standard state is indicated by a circle). When μioi’o holds, we get for the distribution of the concentrations at the interface, where ED = ψ - ψ‘ is the Donnan potential.

Gl. 1

Writing equation 1 for the counter ions with index g and for the co-ions with index c, gives

and

by solving for ED and equating we get the Donnan distribution [3, S. 134]


M: molal concentration of the solid ions in the membrane
vc,vg : Numer of ions per formula unit
v = vc + vg
m': Molality of the elektrolyte in the solution
γ‘± : mean activity coefficients
.

 

Gl. 2

For a 1:1 valent electrolyte the Donnan equilibrium simplifies to:

mc x (M + mc) = (m')2

Gl. 3

Calculation example: An ion exchange membrane with a capacity of 1, 2 or 3 meq/g thus contains in equilibrium with a 1:1 electrolyte the amounts of absorbed electrolyte as shown in the following figure.


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

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    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

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