WO2005009596A1

WO2005009596A1 - Membrane assembly, electrodialysis device and method for continuous electrodialytic desalination

Info

Publication number: WO2005009596A1
Application number: PCT/EP2004/007961
Authority: WO
Inventors: Gerhard Eigenberger; Heiner Strathmann; Andrej Grabovskiy
Original assignee: Universität Stuttgart
Priority date: 2003-07-18
Filing date: 2004-07-16
Publication date: 2005-02-03
Also published as: DE10332789A1

Abstract

The present invention relates to a membrane assembly for continuous electrodialytic desalination, comprising at least one cation- and anion-exchange membranes, parallel to each other. The membrane surfaces comprise a regular pattern of equal or different projections and/or recesses, at least partially on the mutually facing sides or on both sides thereof in the area of the fluid flow, so that the recesses form channels between the projections, the distance between said projections ranging from 0.01 mm to 10 mm and the heights thereof ranging from 0.005 mm to 5 mm. The membranes are partially in contact with each other, by means of the projections on the surfaces thereof, forming corresponding contact points, so that a continuously-branching channel system is formed between the contact points of the membranes, through which a diluate or concentrate flows, said diluate or concentrate being regularly distributed perpendicular to the flow direction and continuously mixed. Said invention also relates to an electrodialysis device provided with said membrane assembly, as well as to a method for continuous electrodialytic desalination or complete desalination.

Description

Membrane assembly, electrodialysis device and method for continuous electrodialytic desalination

The present invention relates to a membrane arrangement for continuous electrodialytic desalination, comprising at least one cation and anion exchange membrane in a parallel arrangement, the surfaces of the membranes in each case at least in regions on the mutually facing surface side or on both surface sides of a regular pattern of identically or differently designed elevations and / or have depressions in the area of the fluid flow, so that the depressions form channels between the elevations, the distance between the elevations being in the range from 0.01 mm to 10 mm and the height of the elevations being in the range from 0.005 mm to 5 mm, and the membranes are in contact with one another in regions via the elevations arranged on their surfaces with the formation of corresponding contact points, so that between these contact points of the membranes a continuously branching channel system is formed, which is diluted or diluted The concentrate is flowed through, the diluate or concentrate being uniformly distributed transversely to the direction of flow and mixed continuously. Furthermore, the present invention relates to an electrodialysis device comprising the membrane arrangement and a method for continuous electrodialytic desalination or full desalination.

In many areas of industry, considerable amounts of partially or fully demineralized water are required. The requirements placed on the degree of purity of this water can be quite different. They range from high-purity water with a conductivity of approx. 0.06 μS cm ^"1 for rinsing and production processes in the semiconductor industry and analytical laboratories to boiler feed water, where a residual conductivity of up to 0.2 μS cm ^{" 1} can be tolerated , A technically simple and reliable procedure for The production of demineralized water for industrial applications is therefore of considerable economic importance.

Processes such as reverse osmosis, electrodialysis or ion exchange processes are usually used to produce demineralized water. Since it is generally not possible to economically produce water of the desired quality with one of the aforementioned processes alone, a combination of different processes is often used, e.g. a process based on reverse osmosis or electrodialysis with a mixed bed ion exchanger. Since the necessary regeneration of the mixed bed ion exchanger is labor-intensive and cost-intensive, such a method is increasingly being replaced by a method which is referred to as CEDI ("continuous electrodeionization") (cf. Ganzi, GC; 1988, "Electrodeionization for high purity water production "in: New Membrane Materials and Processes for Separation, Edts: KK Sirkar, DR Lloyd, AlChE Symposium Series 84, 73-83 and Thate, S., 2002, investigation of electrochemical deionization for ultrapure water production, dissertation University of Stuttgart, ISBN 3-89722-911-0). In principle, this process is an electrodialysis in which the diluate chambers are filled with a mixed bed ion exchange resin.

FIG. 1 shows a basic unit of an electrodialysis device used in the CEDI process, consisting of a diluate chamber 10 and a concentrate chamber 11, which are repeatedly arranged in a membrane stack between two electrodes 2, 3. The diluate chamber 10 is filled with a mixed bed ion exchanger 1. If an ion-containing raw solution (feed) 4 flows through the diluate chamber, the ions are transferred into the so-called concentrate by the applied voltage across the adjacent ion exchange membranes 8, 9, and the solution is thus desalinated. The mixed bed ion exchanger 1 in the diluate chamber 10 serves to increase the conductivity in the diluate chamber 10.

However, the CEDI process described above also has considerable disadvantages. They lie in the complex production of electrodialysis chambers filled with ion exchange fillings of constant thickness, in the risk of settling Bulk due to swelling and shrinking of the ion exchange grains and in an uneven flow through the ion exchange bed.

A conventional electrodialysis is constructed analogously to FIG. 1. Instead of the mixed bed ion exchanger, the membranes are separated from one another by so-called spacers. These are spacers made of a plastic that is usually not ion-conductive. They have the task of guiding the flow and distributing it evenly across the depth of the membrane, ensuring a good mass transfer to the membrane surface by swirling the flow and thereby reducing the so-called limit current density. Because of their special shape, the costs for spacers are a significant factor today, which often exceeds the pure membrane costs in electrodialysis systems.

Conventional electrodialysis is used technically for a large number of ion separation tasks from aqueous and non-aqueous solutions. However, it cannot be used for ultrapure water production, since the electrical resistance of the desalinated water in the diluate chamber (without ion exchange resin filling) is so high that the process can only be carried out with an extremely low current density and is therefore uneconomical if the so-called Limit current density should not be exceeded. There have also been attempts to produce the spacers from ion-conducting material and thus to achieve a similar effect to that achieved with the ion exchange filling (cf. DE 43 33 020 A1 and DE 43 33 019 A1). However, these approaches have so far hardly been implemented technically for reasons of cost.

It is therefore an object of the present invention to provide a membrane arrangement which makes it possible to produce demineralized water of high quality directly from a raw water, the above-mentioned problems to be avoided.

This object is achieved by the embodiments characterized in the claims. In particular, a membrane arrangement for continuous electrodialytic desalination or full desalination is provided, comprising at least one cation and anion exchange membrane in a parallel arrangement, the surface of the membranes in each case at least in regions on the facing surface side or on both surface sides of a regular pattern of identically or differently designed surveys and / or has depressions in the area of the fluid flow, so that the depressions form channels between the elevations, the distance between the elevations being in the range from 0.01 mm to 10 mm and the height of the elevations being in the range from 0.005 mm to 5 mm, and the membranes are in contact with one another in regions via the elevations arranged on their surfaces with the formation of corresponding contact points or support points, so that a continuous distortion between these contact points or support points of the membranes is formed ^' channel system which is flowed through with diluate or concentrate, the diluate or concentrate is uniformly distributed transversely to the direction of flow and continuously mixed.

According to one embodiment of the present invention, the distance between the elevations is preferably in the range from 0.5 mm to 4 mm and the height of the elevations is preferably in the range from 0.1 mm to 2 mm.

A membrane arrangement according to the invention is usually constructed from a multiplicity of alternatingly arranged, structured in this way cation and anion exchange membranes. The membrane arrangement according to the invention can then be constructed in particular from membranes modified on both sides of the surface.

An important basis of the present invention lies in the knowledge that in the case of electrodeionization with diluate chambers filled with ion exchange resin, raw water with low electrical conductivity can still be completely desalinated at still acceptable current densities if the cation and anion exchange membranes are in direct contact with one another via an ion exchange resin so that the transport of the ions from the water to be desalinated via the ion exchange resin to the surface of the membranes. The same effect can also be achieved if the surfaces of the ion exchange membranes in the diluate chamber are designed in such a way that as many contact points as possible exist between the membranes and the ratio of free volume to membrane surface is as low as possible. At the same time, it is ensured according to the invention that the raw water or fluid flow flows through the diluate chamber with optimal distribution and mixing and with little pressure loss.

According to one embodiment of the present invention, the elevations are in the form of webs, so that the depressions between the webs form channels or grooves. The shape of the ridges is not specifically limited. For example, the webs can be designed in a substantially rectangular shape, prismatic shape, sinusoidal or conical shape. The channels can, for example, also be conical or tapered. The structuring or arrangement of the webs or channels is not subject to any specific restriction. The webs can be continuously, i.e. run without interruptions, so that the depressions between the webs form, for example, channels or grooves running parallel to one another. Alternatively, the arrangement can be such that the webs are interrupted in some areas. Furthermore, the surface of a membrane of the membrane arrangement according to the invention can also contain knobs or a pattern of offset webs, for example, so that it is in contact or is supported with the adjacent membrane in such a way that the contact is punctiform or linear.

The webs can be arranged at an angle β to the main flow direction, the angle β being able to assume values in the range from 0 ° to + 90 °, preferably in the range from + 30 ° to + 60 ° or -30 ° to -60 °, and the membranes are each arranged in such a way that a flow channel system with a crossed channel structure which is open to one another is formed. In addition, the elevations and / or depressions in the membrane arrangement according to the present invention can also be in the form of columns, cylinders or truncated cones with any cross section, the height and extent of the elevations or depressions in the range from 0.005 to 5 mm, preferably from 0 , 1 to 2 mm, and the membranes are each arranged in such a way that a flow system with flow obstacles arranged regularly and in rows is formed. This creates, for example, an arrangement of continuously intersecting flow paths (Dalton's board).

The membrane profiling according to the invention ensures optimal flow guidance and mixing of the fluids in the membrane arrangement. This minimizes the mass transfer between fluid and membranes and increases the limit current density. At the same time, the profiling increases the effective membrane surface. In addition, the electrical resistance is reduced by the contact points between adjacent membranes. This means that only a fraction of the otherwise usual membrane area and the electrical voltage is necessary for the electrodialytic ion separation.

The figures show:

FIG. 1 schematically shows a conventional electrodialysis device used in the CEDI process, in which an ion-containing feed stream 4 is divided into diluate chambers 10 and concentrate chambers 11 and leaves the device as an (almost) ion-free diluate 5 and ion-rich concentrate 6. The diluate chamber 10 is filled with a mixed bed ion exchange resin 1, wherein the base unit 7 is often arranged between the electrodes 2, 3 (8 - cation exchange membrane, 9 - anion exchange membrane).

FIG. 2 shows the current density as a function of the applied voltage, measured in a conventional electrodialysis stack with a 0.5 N NaCl solution. FIG. 3 shows a schematic representation of a membrane structured on one side according to the invention, with a) the cross section perpendicular to the membrane channels, b) the surface of the membrane with a web or groove structure and c) the assembly of two surface structured membranes to form a diluate chamber. The arrow 12 marks the main direction of flow of the fluid between two membranes.

FIG. 4 schematically shows the arrangement of membranes 13 with a surface structure on both sides in a cell stack with conventional flow guidance.

FIG. 5 schematically shows the flow guidance according to the invention in the stack with membranes 8, 9 structured on both sides for the electrodialytic splitting of a feed 4 into a fully desalinated diluate 5 and an ion-rich concentrate 6, with a) showing an overall view and b) a detail of the ion transport.

FIG. 6 schematically shows the exemplary construction of seals 18 and membranes 13 with surface structure according to the invention for the construction of a basic unit 7 of a corresponding electrodialysis device shown on the right.

Since the conductivity of the diluate is orders of magnitude lower than that of an ion exchange membrane, it was previously assumed that there is a limiting current density in electrodialysis in which the ion concentration on the membrane surface in the diluate chamber goes to zero and the resistance in the diluate solution is very high increases rapidly. Since a water dissociation and thus a shift in the pH value in the chambers separated by the membrane can occur when this limit current density is exceeded, it has previously been assumed that the limit current density may not be exceeded if effective and economical water desalination is to be ensured , However, since the limiting current density in demineralized water in a conventional electrodialysis device has a very low value, correspondingly large membrane areas are required if a product water low conductivity should be maintained. However, it has been known for some years that when the limiting current density in the electrodialysis is exceeded, there is only a limited increase in the resistance. This is followed by an area that is far above the limiting current density limit, in which the resistance only behaves according to the concentration of the diluate and hardly any water dissociation occurs. This area is referred to as "overlimiting current density". Extensive experimental studies have shown that the limit current density can be exceeded many times over without a significant pH shift and a reduction in the current efficiency (see Krol, JJ, Wessling, M., Strathmann, H., 1999, "Concentration polarization with monopolar ion-exchange membranes", J. Membrane Sei., 1999, 162, pages 145-154; Rubinstein, I., Warshawsky, A., Schechtman, L., Kedem, O., "Elimination of aeid-base generation in electrodialysis ", Desalination, 1984, 51, pages 55-60). As an explanation for this, it was shown in detailed experiments and in theoretical considerations that a certain stack construction and the nature of the membranes result in so-called electro-convection, which increases the mass transfer between the fluid and the membrane and largely negates the consequences of exceeding the limiting current density can be excluded.

FIG. 2 shows the current density, measured in an electrodialysis cell with a 0.1 N NaCl solution, as a function of the applied voltage. There are three characteristic ranges: a range (I) in which the current density increases linearly with the applied voltage in accordance with Ohm's law, a range (II) in which the current density hardly increases with the applied voltage, and one Range (III) in which the current density increases linearly with the applied voltage. The area (I) is called the work area with subcritical current density. In this area, the electricity is based exclusively on the transport of the salt ions in the solution. At the transition from area (I) to area (II) there is a depletion of the salt ions on the membrane surface in the solution in the diluate chamber. As a result, the resistance increases sharply and there is hardly any increase in the current density despite an increase in the applied voltage. The critical current density, at which area (I) merges into area (II) has so far been interpreted as the limiting current density at which the salt concentration in the boundary layer on the membrane surface goes towards zero. In area (III) there is again a linear increase in current density with increasing voltage. This increase is attributed to water dissociation and other effects such as electro convection and thermal convection.

Although this means that conventional electrodialysis can also be operated above the limit current density, this operating range is not technically attractive. The cause is the sharp increase in the resistance of the diluate chamber as soon as very low diluate concentrations are sought, and the resulting drop in voltage or electrical energy requirement.

Instead, in the CEDI process, the diluate chamber is filled with mixed bed ion exchangers. On the one hand, the ions are taken up by the exchange grains as in the classic mixed bed exchange, and on the other hand the exchanger is continuously regenerated in the electric field. The regeneration takes place through the water dissociation at the contact points of the cation and anion exchange grains, the H ⁺ and OH ^' ions formed displacing the other ions taken up into the concentrate chambers. Although this method is increasingly being used technically, its disadvantages lie in the complex structure with the risk of an uneven distribution of the bed and an overall less efficient regeneration of the mixed bed. FIG. 1 shows a basic unit of an electrodialysis device used in the CEDI process, consisting of a diluate chamber 10 and a concentrate chamber 11, which are repeatedly arranged in a membrane stack between two electrodes 2, 3. The diluate chamber 10 is filled with a mixed bed ion exchanger 1.

In contrast, the membrane arrangement according to the invention represents a new, efficient combination of the two approaches above. On the one hand, the available membrane surface is significantly enlarged by the surface design of the membranes according to the invention, and by the forced flow. guidance of mass transfer between fluid and membrane significantly improved. This increases the electrodialytic ion deposition and significantly increases the limit current density. On the other hand, continuous water dissociation takes place at the contact points of the anion and cation exchange membranes, as a result of which the membrane surface is continuously regenerated into the H ⁺ or OH ^" form, so that it absorbs other ions like an ion exchanger.

The present invention further relates to an electrodialysis device which comprises at least one membrane arrangement 7 according to the invention. An electrodialysis device according to the invention usually comprises a large number of such membrane arrangements 7 with the formation of corresponding channel systems for diluate or concentrate flows.

For example, the electrodialysis device according to the invention can be constructed in such a way that a membrane arrangement composed of a plurality of membranes 8, 9 is arranged between two electrodes 2, 3 in such a way that a space, i.e. a corresponding channel system is present, through which the diluate 5 and the concentrate 6 flow alternately.

FIG. 3 shows schematically and by way of example the structure of a membrane arrangement according to the invention for a corresponding electrodialysis device. The individual membranes 13 have a thickness of, for example, d + h = 0.7 mm and contain parallel channels on the surface with a depth of, for example, h = 0.5 mm. The channels are arranged such that they run at an angle β of, for example, 45 ° to the main flow direction. The width of the ridges a and that of the valleys b are, for example, 0.4 mm and 0.32 mm, the slope angle α is, for example, 60 °.

FIG. 4 shows an electrodialysis device unit constructed from four membranes, which is arranged between two electrodes 2, 3 in such a way that there is a free space between the membranes, which alternates with the diluate 5 and the

Concentrate 6 is flowed through. Due to the mutual arrangement of catalytic Applying an electrical potential difference leads to the transport of ions, which takes place in such a way that depletion takes place in the diluate stream 5 and enrichment takes place in the concentrate stream 6. If the solutions remain in the membrane unit for a sufficiently long time, the diluate stream 5 is desalted.

A certain disadvantage of a conventional flow guidance, as shown in FIG. 4, is that the diluate 5 emerging can be contaminated by the concentrate 6 emerging on the same side of the stack or the membrane arrangement. On the one hand, this can be due to the inadequate barrier effect of the membranes used against coions and, on the other hand, they can be due to sealing leaks between the channels for diluate and concentrate and / or electrode rinsing solution. However, these disadvantages can be prevented by branching off part of the generated diluate and returning it in countercurrent to the diluate in the adjacent concentrate chambers. In conventional electrodialysis, the concentrate conductivity would be too low with this flow and would cause excessive voltage drops. Due to the membrane arrangement according to the invention, preferably with surface-structured, touching membranes, the majority of the current conduction takes place via the highly conductive membrane phase, so that a very low concentrate conductivity can be tolerated.

Another object of the present invention therefore relates to a method for continuous electrodialytic desalination or full demineralization, in which diluate or concentrate chambers are formed by an arrangement of membranes and the corresponding diluate streams are conducted in countercurrent to the concentrate streams, with a partial stream of the diluate drain which is between 2% and 70% of the solution to be deionized, is used as an inlet for the concentrate chambers and the electrode rinsing is carried out separately from the concentrate outside the membrane arrangement. In one embodiment of this method according to the invention, the diluate and concentrate chambers formed by the membranes according to the invention can additionally be filled with ion-conducting material.

In a further embodiment of the method according to the invention, membranes with a smooth surface are used, the diluate and concentrate chambers formed by the membranes being filled with ion-conducting material.

In all of the cases mentioned, the flow guidance according to the invention in the stack with partial return of the diluate in the concentrate chambers ensures that the area of desalinated solution is completely spatially separated from the area of more concentrated solution, so that contamination of the desalinated solution by leakage currents in the membranes or leaks in the feeds / Drainage channels can be largely excluded.

In a preferred embodiment of the method according to the invention, an electrodialysis device according to the invention described above, comprising at least one membrane arrangement according to the invention, as described above, is used. For this case, in the process according to the invention, water can be desalinated at a current density which is about a factor of 30 higher than that in a conventional device, without a change in the pH value in the diluate and without the diluate chamber being filled with a mixed bed ion exchanger is. This means that correspondingly smaller membrane areas are required for a device of a given capacity and that the investment costs are accordingly lower than in conventional electrodialysis of the same capacity.

FIG. 5 shows the flow guidance according to the invention in the overall stack of an electrodialysis device according to the invention, comprising a multiplicity of repeating basic units 7 between two electrode chambers 19, 20. The membranes 8, 9 used are profiled or surface-structured in the area through which they flow so that they are both in the diluate as in the concentrate lean on the neighboring membrane or be in contact with it and that the flow takes place in the spaces between the supports or contact surfaces. The solution 4 to be desalinated (feed) enters through supply channels on one side of the stack and is distributed to the diluate chambers. At the end of the flow path through the diluate chambers, a partial flow of the diluate is diverted and flows back through the concentrate chambers as "concentrate inlet 21 in counterflow to the diluate. It takes up the anions X ^" and cations Me ⁺ transferred into the concentrate chamber, while OH ^" and H ⁺ At the diluate end of the stack, the concentrate 6 thus formed is collected in drain channels and leaves the stack.

According to the invention, the stack is delimited by two chambers 22, 23 on the electrode sides. They are both backwashed with the branched diluate in countercurrent to the diluate. The drain from the chambers 22, 23 is then led into the concentrate manifold 6. Contamination of the diluate 5 by ions of the electrode rinse 14, 15 is thus avoided according to the invention, since all ions which pass into the chamber next to the electrode are removed again by the flow and the electric field. Therefore, the chamber 22 closest to the electrode is separated from the cathode 2 by a cation exchanger membrane 8 and from the anode 3 by an anion exchanger membrane 9. If the same rinsing solution 14, 15 is used for both electrodes, the connecting lines must be routed outside the stack.

5, the stack shown has a feed / concentrate side 24 with a comparatively high ion concentration and a diluate side 25 with a very low ion concentration at the top. The structure of the membrane arrangement or the electrodialysis device shown in FIG. This means that contamination on the diluate side is not possible due to liquid or ion leakage.

The membrane arrangement and flow guide shown in FIG. 5 can also be used according to the invention in electrodialysis with chambers filled with ion exchangers according to FIG. 1, if both the diluate and the concentrate chambers are made of a porous ion-conducting material or an ion-extracting material. exchange exchange are provided. The ion exchange bed can either consist of a mixture of cation and anion exchangers or of a succession of cation and anion exchanger sections. Instead of a bed, correspondingly ion-conducting spacers or ion-conducting fiber or fabric structures can also be used.

The concentrate feed 21 branched off from the diluate is preferably between 2% and 70%, more preferably between 5% and 50% of the feed stream 4. The profiles or surface structures on the concentrate side can therefore be chosen according to the invention in such a way that, in comparison with the di luatseite gives a correspondingly smaller free flow cross-section.

Due to the decreasing ion concentration in the diluate and concentrate from the feed / concentrate side 24 to the diluate side 25, the electrical resistance between the anode and cathode also increases from the feed / concentrate side 24 to the diluate side 25. It can therefore be expedient according to the invention to separate the electrodes 2, 3 on one side or on both sides such that the voltage can be adapted to the requirements of the separation at high and at low ion concentration. Figure 5 shows an example of a separation on three pairs of electrodes to be controlled separately. In the membrane arrangement according to the invention, a profiling or surface structuring of the membrane surface can be present in the entire flow area. According to the invention, the profiling of the membranes can be aligned in one direction and on the back in the other direction, so that a system of crossed flow channels is established in the membrane stack, as has proven itself, for example, for plate heat exchangers. At the edges, however, and in the area of the feed / concentrate channels, the membranes are usually smooth and thin. Here, the sealing to the outside and the sealing between the feed and concentrate channels is usually carried out by means of flat gaskets 18 of sufficient elasticity. For example, inlet and outlet openings 26 to the feed channels 4 or concentrate channels 6 can be arranged in these seals. On the other hand, the transition to the diluate collecting channels through simple openings in the profiled part. Alternatively, the seals can also be constructed according to the invention from membrane material and integrated directly into the membranes.

Figure 6 shows schematically the structure of the seals 18 and the profiled membranes 13 for the structure of a basic unit 7 shown on the right. For use in a stack according to Figure 5, the individual membranes 13 can e.g. as shown in Figure 6. There is a profiling of the membrane surface in the entire flow area, e.g. as already shown in FIG. 3.

The surface-structured membranes thus lie directly on top of one another without spacers, the sealing to the outside and to the flow channels not affected being effected by means of flat seals arranged therebetween, by stacking correspondingly profiled membranes or by gluing or welding successive membranes.

All of the disadvantages of the CEDI method practiced hitherto are avoided with the membrane arrangement according to the invention, which makes mixed bed ion exchangers or ion-conducting spacers in the diluate chambers of an electrodialysis stack superfluous. At the core of the invention are thus novel shaped membranes with a structured surface, the membranes being arranged in such a way that cation and anion exchange membranes in the diluate chamber and possibly also in the concentrate chamber touch directly at a large number of points.

There are a variety of options for manufacturing the structured surface ion exchange membranes that are well known to those skilled in the art. Profiling can preferably be achieved by embossing, pressing or rolling smooth membrane blanks, which are produced as so-called heterogeneous membranes by mixing ion-exchange powder into a matrix of a thermoplastic polymer. The membrane arrangement according to the invention with a corresponding configuration of the membrane surfaces advantageously ensures a uniform distribution of the supplied flow over the entire membrane width, so that electrodialysis takes place evenly over the flow path. In addition, the membrane arrangement according to the invention with a corresponding configuration of the membrane surfaces ensures a pronounced, regularly repeated cross-mixing and (turbulent) swirling of the fluid flow, so that differences in concentration are eliminated, the mass transfer to the membrane surface is intensified and a possible concentration polarization is minimized.

Furthermore, the membrane arrangement according to the invention with the corresponding surface structuring advantageously enables a significant increase in the outer membrane surface, which is decisive for the mass transfer, so that overall less membrane surface has to be used. The successive membranes support each other at the contact or contact points, which eliminates the use of spacers.

By providing contact points, lines or areas between successive membranes of different polarity in the arrangement according to the invention, a water splitting localized there is initiated, whereby a constant regeneration of the membrane surface into the H ⁺ or OH ^" parallel to the ion separation. A significantly increased ion separation is obtained due to the lower mean membrane spacing and a lower concentration polarization (mass transfer inhibition) as a result of better mixing, which results in a higher separation performance and low electrical energy consumption.

The design of sealing surfaces at the edge of the membrane and around openings for fluid flows, as provided in accordance with a preferred embodiment, advantageously ensures that a simple seal between successive membranes can be achieved by means of flat seals, gluing or welding. With the electrodialysis device according to the invention, demineralized water of high quality can be produced directly from raw water or in combination with reverse osmosis or nanofiltration considerably more cost-effectively than with the previously known methods.

LIST OF REFERENCE NUMBERS

Mixed bed exchanger 25 diluate side

Cathode 26 inlet and outlet openings for

Anode feed / concentrate Ionic raw solution

(Feed)

Diluate ion-rich concentrate

basic unit

cation

anion exchange membrane

diluate

concentrate chamber

Main flow direction of the

Fluids between two membranes

Surface-structured ion exchange membrane

cathode flushing

anode purge

poetry

cathode chamber

anode chamber

Diluate return for the concentrate chamber

Diluate return for the chamber closest to the electrode

Diluate return for the concentrate chamber next to the electrode

Feed / concentrate side

Claims

Expectations

1. Membrane arrangement for continuous electrodialytic desalination, comprising at least one cation and anion exchange membrane in a parallel arrangement, the surfaces of the membranes in each case at least in regions on the facing surface side or on both surface sides, a regular pattern of identically or differently shaped elevations and / or depressions in the area of the Have fluid flow so that the depressions between the elevations form channels, the distance between the elevations in the range from 0.01 mm to 10 mm and the height of the elevations in the range from 0.005 mm to 5 mm, and the membranes over the their elevations arranged on their surfaces are in contact with one another in some areas, so that a continuously branching channel system is formed between these contact points of the membranes, through which diluate or concentrate flows, the D iluat or concentrate is evenly distributed transversely to the direction of flow and mixed continuously.

2. Membrane arrangement according to claim 1, wherein the elevations are in the form of parallel webs, so that the depressions between the webs form parallel grooves, the webs and grooves being arranged at an angle β to the main flow direction, the angle β values in the range can assume from 0 ° to ± 90 °, and wherein the membranes are each arranged in such a way that a flow channel system with a crossed channel structure is formed.

3. Membrane arrangement according to claim 1, wherein the elevations and / or depressions are in the form of webs, knobs, columns, cylinders or truncated cones and wherein the membranes are each arranged in this way are that a flow system is formed with flow obstacles arranged regularly and in rows.

4. Membrane arrangement according to one of claims 1 to 3, wherein the sealing of the diluate and concentrate chambers formed by the membrane arrangement to the outside and to the non-connected supply and discharge channels through flat seals arranged between the membranes, through sealing surfaces incorporated into the membranes and / or by welding or gluing successive membranes.

5. Electrodialysis device comprising at least one membrane arrangement according to one of claims 1 to 4.

6. Process for continuous electrodialytic desalination, in which the corresponding diluate streams are conducted in countercurrent to the concentrate streams, a partial stream of the diluate outlet, which is between 2% and 70% of the solution to be deionized, being used as feed for the concentrate chambers and the electrode rinsing being separated is carried out from the concentrate outside the membrane arrangement.

7. The method according to claim 6, wherein the branched diluate is returned through the two intermediate membrane membranes closest to the electrodes in countercurrent to the diluate into the concentrate outlet and the electrode closest chamber is closed on the cathode side with a cation exchange membrane and on the anode side with an anion exchange membrane.

8. The method according to claim 6 or 7, wherein an electrodialysis device according to claim 5 is used.

9. The method according to claim 6, 7 or 8, wherein the diluate and concentrate chambers formed by the membranes are filled with ion-conducting material.

10. The method according to claim 6 or 7, wherein membranes with a smooth surface are used and the diluate and concentrate chambers formed by the membranes are filled with ion-conducting material.