WO2000064816A1

WO2000064816A1 - Apparatus and process for deoxygenation of water

Info

Publication number: WO2000064816A1
Application number: PCT/FI2000/000365
Authority: WO
Inventors: Kai Vuorilehto
Original assignee: Fortum Power And Heat Oy
Priority date: 1999-04-27
Filing date: 2000-04-27
Publication date: 2000-11-02
Also published as: AU4406500A; FI990954A; FI990954A0

Abstract

The present invention concerns a process and apparatus for the removal of oxygen from water. According to the process, the water is fed into an apparatus which comprises an electrochemical cell having at least one three-dimensional flow-by cathode (6), which comprises an electrically conductive particulate electrode material (13) formed into a particle bed having at least one flat surface. The counter-electrode for the cathode (6) is made up of at least one anode (9, 10), which is arranged at a distance from the cathode (6). Furthermore, the cell has at least one ion-exchange membrane (11, 12), which is arranged between the cathode (6) and the anode (9, 10). According to the invention, a cathode having a thickness of 0.1 - 0.5 cm is used, and conductivity-improving ions are not substantially added to the water.

Description

Apparatus and process for deoxygenation of water

The present invention relates to a process according to the preamble of Claim 1 for the removal of oxygen dissolved in water.

According to such a process, water is directed to an electrochemical cell having at least one three-dimensional flow-by cathode, which comprises an electrically conductive particulate electrode material formed into a particle bed. The cell further has at least one anode, which forms the counter-electrode for the cathode, and an ion-exchange membrane arranged between the cathode and the anode.

The invention also relates to an apparatus according to Claim 5, by means of which dissolved oxygen is removed from water by electrochemical reactions.

Oxygen dissolved in water and aqueous solutions causes corrosion in many technical systems, such as conventional power plants and nuclear power plants, HEP AC engineering, remote-heating plants and networks, and in heating and cooling systems. For this reason, efforts are made to remove oxygen from water as precisely as possible. In the production of soft drinks, an effort is also made to minimize the presence of oxygen.

Physical or chemical deoxygenation methods, or combinations of these, are most commonly used for the removal of oxygen from water or an aqueous solution. The prerequisite of physical deoxygenation is the disturbing of the solubility equilibrium. A gas mixture in which the partial pressure of the oxygen to be removed is much lower than the equilibrium pressure is brought into contact with an aqueous solution. Thereupon the solution-gas system begins to shift towards a new equilibrium state, i.e. oxygen passes from the aqueous solution into the gas phase. Usually the gas phase used is water vapor. Physical deoxygenation methods include overpressure and vacuum gas removal, by which methods it is pos- sible to remove also other gases dissolved in water.

The weakness of physical methods is their high investment cost, in particular in uses in which steam is not easily available. In addition, the heating of the water requires a large amount of energy.

By chemical deoxygenation is meant the removal of oxygen from a solution by causing the oxygen to react with some chemical. Hydrazine, NH₂ - NH₂ is the most used among the deoxygenation chemicals. Sodium sulfite is also much used. Other deoxygenation chemicals include erythorbic acid and tannins.

Chemical methods involve the problems of environmental and work safety risks. Hydrazine is a carcinogen. The use of sodium sulfite is also not desirable. Non-toxic oxygen re- moval chemicals, many of which are multiple-component products, are less effective than hydrazine.

In electrochemical deoxygenation, oxygen is removed from an aqueous solution by reduction. Electrochemical removal of oxygen dissolved in water is described, for example, in patent publication FI 100 520 and published patent application FI 922 485.

In the methods and apparatus described in the two publications mentioned above, the removal of oxygen is carried out according to more or less the same principle. Water or an aqueous solution is introduced via the liquid feed unit of the inlet conduit to a flow-by chamber formed by a three-dimensional cathode, in which case the oxygen present in the water or the aqueous solution is reduced to water on the surface of the cathode particles.

The cathode reaction in the oxygen removal is thus the reduction of dissolved oxygen:

O₂ + 4H⁺ + 4e^" - 2H₂O (I)

At the anode, in turn, there forms oxygen gas, which exits to the atmosphere:

2H₂O -» O₂ + 4H⁺ + 4C (II)

Electric current travels between the cathode and the anode so that the hydrogen ions pro- duced at the anode will pass through the membrane to the cathode compartment and will move to the electrode particle surfaces of the cathode. Electrochemical deoxygenation methods indeed involve the problem of an electric charge traveling in the water. In order for the deoxygenation cell to function, ions must be able to travel from one electrode to the other.

FI application 922485 describes an option in which the electrode material is also arranged as a particle bed. The problem of conduction of electricity has been solved in the said application by adding a large amount of Na^O_^ to improve conductivity.

A deoxygenation apparatus similar to that in FI application 922485 is also discussed in a journal article (Vuorilehto, K., Tamminen, A. and Ylasaari, S., Electrochemical removal of dissolved oxygen from water, J. Appl Electrochem., 25 (1995), pp. 973-977). In the article, the optimal thickness calculated for the cathode according to the deoxygenation apparatus of the application publication is 0.35 cm (provided that the conductivity of the water is 10 mS/cm, which is produced by adding sodium sulfate to the water). It is, however, to be noted that the value calculated for the optimal thickness of the bed depends on, for example, the square root of the conductivity of the electrolyte, and in the application publication ions promoting conductivity were added to the water, whereupon the conductivity value was 50- to 100-fold compared with ordinary tap water. If 0.13 mS/cm is placed in the equation as conductivity (the normal conductivity of tap water in Finland), the optimal thickness obtained for the cathode is 0.04 cm. The result does not make sense, as in the same calculation the assumed size even of individual particles in the cathode is 0.045 cm. The cell and equation presented in the article are suitable only for highly electrically conductive waters. Furthermore, the article does not give instructions for the preparation of a thin cathode but makes reference to problems associated with the use of thin cathodes.

FI patent publication 100 520 discloses an apparatus which comprises an electrochemical cell wherein a three-dimensional electrode is used as the cathode. The electrode comprises an electrically conductive particulate electrode material and an ion exchange electrolyte mixed therewith. The invention according to the publication is based on the idea that ions travel along the ion exchange resin from one electrode to the other. In this case free ions are not required, i.e. a salt need not be added to the solution. However, the serviceable life of the ion exchange resin limits the useful life of the electrode.

The object of the present invention is to eliminate the problems associated with prior art and to provide an apparatus of an entirely novel type for the removal of oxygen from water or an aqueous solution.

The invention is based on the observation that, when a thin particle bed composed of highly electrically conductive particles is used as the flow-by cathode, it is possible to treat in the cell also poorly conductive water, without adding a salt any more than an ion exchange resin. Thus, according to the invention the thickness of the cathode in the travel direction of ionic electricity is at maximum 0.5 cm. A functioning option presupposes, in ad- dition to the thinness of the cathode, also that the cathode material used consists of particles having a high electrical conductivity. Since the electrons have to travel over long distances along a very thin particle bed, moving from one particle to another, it is required of the cathode material, in addition to high conductivity, also that on the particle surface there forms no oxide layer which would weaken conductivity. Thus, the cathode material used is preferably granules or spheres of a noble metal or a noble-metal alloy. Silver and silver alloy are especially suitable materials.

The cathode must be clearly thicker than the "optimum thickness" stated above, but clearly thinner than the previously used 1 cm. Too thin a cathode does not have a sufficient amount of the active metal and, on the other hand, in too thick an electrode the distance traveled by the ions is too long, in which case a portion of the cathode remains passive. It has been observed in connection with the invention that making the cathode thinner is possible if the ion-exchange membrane is placed tightly enough against the flat surfaces of the cathode bed and if the anodes functioning as the counter-electrode for the cathode are respectively arranged against the ion-exchange membrane. As will be described below in greater detail, the thickness of the cathode can be reduced further by organizing in a new manner the feeding in of the solution to be treated. According to the invention, the length of the cathode is also preferably increased.

More precisely, the method according to the invention is characterized by what is stated in the characterizing part of Claim 1.

The apparatus according to the invention, for its part, is characterized by what is stated in the characterizing part of Claim 5.

Considerable advantages are attained through the invention. Thus, by means of the inven- tion it is possible effectively to remove dissolved oxygen from water or an aqueous solution without a need to add an ionizing salt to the water. Also, on the other hand, an ion- exchange resin is not used as an electrolyte in the cell, a factor which simplifies the cell structure and manufacture and increases the useful life of the cell, because ion-exchange resins become brittle when large volumes of liquid are treated with the deoxygenation cell.

The advantages of the present invention over conventional methods include effectiveness of the deoxygenation and a low consumption of energy. In terms of work safety and the environment, it is also advantageous to avoid the use of toxic deoxygenation chemicals. The apparatus according to the invention is pro-environmental and easy to use.

A further advantage is the good mechanical strength of the cathode; the bed used as the cathode material in the present invention, preferably made up of silver particles, withstands mechanically well the flow of water and various phenomena associated with the flow of water, such as turbulences and vibrations. However, the flow resistance of the particle bed is not significant.

The invention is examined below in greater detail with the help of a detailed description and two example embodiments.

Figure 1 depicts a preferred embodiment of the cell according to the invention, and Figure 2 depicts in greater detail a cross-section of a preferred structure of the upper portion of the cell as seen from the side.

In the context of the present invention, by the expression "ions promoting the conductivity of water are not substantially added to the water" is meant that there is not added to the water to be treated an ionizing salt so that the conductivity of the water introduced into the cell would exceed the typical conductivity of tap water. Typically the conductivity of the water treated in the cell is thus within the range of 0.05 - 1 mS/cm, most typically within the range of 0.05 - 0.5 mS/cm.

By "electron-conducting particle" is meant that, deviating from ion conductivity (as with ion-exchange resins), in the said particle, electrons move instead of ions.

The principle of structure of the apparatus according to the invention, i.e. the electrochemical cell, is as follows: the cell has at least one three-dimensional first electrode, a cathode through which the liquid flows. The said electrode comprises an electrically conductive particulate electrode material formed into a particle bed having at least one flat surface. The cathode bed is most suitably arranged inside a cathode frame made from metal. Typically the cathode bed is formed into a flat layer in such a manner that it has, side by side, two flat, for example parallel, exterior surfaces. In the apparatus there are also arranged feed units for feeding liquid into the cathode structure, for example, from the ends of the flat layer, and outlet units for removing liquid from the said cathode compartment. The outlet units are located on the opposite side of the bed relative to the feed units. The apparatus has, arranged at a distance from the cathode, at least one other electrode, a counter- electrode, which is called anode. Between the cathode and the anode there is arranged an ion-exchange membrane, the cathode bed and the anodes pressing against the ion-exchange membrane. To ensure the tightness of the structure, seals are arranged between the cathode frame and the ion-exchange membrane and the anodes. According to the invention, the cathode is very thin. The thickness of the cathode is determined in the travel direction of the ionic electric current, i.e. a direction substantially perpendicular to the surface of the ion-exchange membrane. The thickness does not at any point exceed 0.5 cm. The distance traveled by ionic electricity in the cathode is preferably at maximum 0.25 cm and most preferably 0.1 - 0.25 cm.

According to a preferred embodiment of the invention, the cathode comprises two flat surfaces, which are located at a distance of at maximum approximately 0.5 cm from each other.

According to another preferred embodiment of the invention, the cathode comprises one flat surface, and the distance traveled by electricity in the cathode is at maximum 0.25 cm, preferably 0.1 - 0.25 cm.

For the thickness of the cathode there can also be presented an equation

V

=0.5 cm, (III) A

Where V is the volume [cm³] filled with the cathode electrode material, and

A is the area [cm²] of the flat surface of the cathode, more precisely the area of the flat surface which is substantially perpendicular to the travel direction of ionic electricity.

Preferably the equation 0.1 cm = V/A = 0.5 cm, where V/A stands for the same as above, is valid for the thickness of the cathode.

As is seen from the equation, the ratio can be reduced by increasing the surface area of the cathode. This can be done, for example, by increasing the length of the cathode, which is preferable to increasing the width of the cathode, because the latter may decrease the ca- pacity of the cell.

According to a preferred embodiment, the total thickness of the cathode frame and the above-mentioned seals in the transverse direction of the plane of the cathode frame is at maximum 0.5 cm, preferably at maximum 0.45 cm. The cell according to the invention can be used for the removal of oxygen dissolved in water or an aqueous solution by feeding the water to be treated into the apparatus and by removing the oxygen by means of electrochemical reactions in the cathode compartment of the cell.

The structure of a cell according to a preferred embodiment is shown in greater detail in Figure 1.

Figure 1 shows an electrochemical cell comprising a housing 1, in which there are a feed pipe 2 for the liquid to be treated and an outlet pipe 3 for the treated liquid. The housing 1 is made of a strong material, for example, steel plates. Inside the housing 1 there are formed anode compartments 4, 5, which are on the side of the housing delimited by second end plates 7, 8, which may be, for example, plastic, and a cathode 6. On the other side of the anode compartments 4, 5 there are the actual anodes 9, 10, which are typically made up of an insoluble material. The cathode 6 and the anodes 9, 10 are separated from each other by ion-exchange membranes 11, 12, through which the ions can travel between the elec- trode and the counter-electrode. __

The cathode 6 comprises an electrically conductive particulate electrode material 13, which is formed into a bed having at least one, preferably two, flat surfaces. Typically the electrode also comprises a cathode frame 14, which, together with the ion-exchange membranes, delimits the interior electrode compartment. The ion-exchange membranes delimit the interior electrode compartment in the travel direction of ionic electricity, whereas the cathode frame delimits the interior electrode compartment in other directions. The cathode frame is preferably made of metal.

For the feeding of liquid into the cathode 6 and for getting the liquid out of it, the apparatus has to be equipped with feed units 15 connected to the feed pipe 2 and with outlet units 16 connected to the outlet pipe 3. The feed units 15 preferably comprise grooves formed in the cathode frame 14, the grooves opening to the surface of the cathode frame and being connected to the feed pipe for liquid to be treated. The outlet units 16 preferably comprise grooves formed in the cathode frame 14, the grooves opening to the surface of the cathode frame and being connected to the outlet pipe for the treated liquid. There is at minimum one groove, preferably at minimum three grooves, and their depth is approximately 1 - 1.5 mm and their width approximately 2 mm. The feed units 15, and preferably also the outlet units 16, in a cathode frame made of metal are preferably made by cutting. This op- tion is especially advantageous when the feed and outlet units are made in a very thin (typically approximately 2 - 4 mm thick) cathode frame.

Preferably the particles 13 are arranged as an even layer. The particle bed fills the cathode compartment substantially entirely in the direction perpendicular to the flow direction of the water, and a large portion, or preferably substantially the entire cathode compartment, in the flow direction of water. The particles 13 are arranged so that they are by mediation of one another in electric contact with the cathode frame.

The electrode material used in the invention is made up of electrically conductive, more precisely electron conductive, particles. Preferably, particles entirely consisting of an elec- trically conductive material are used. Especially preferably, noble metals such as platinum, gold and silver, or alloys of noble metals, are used. Of these, silver or a silver alloy is especially suitable for this option, since it has excellent electrical conductivity, it does not oxidize or crumble, and only small amounts of hydrogen and hydrogen peroxide are formed on its surface. Thus the electrically conductive particles are most preferably, for example, silver or silver alloy granules or crush. It is also possible to use particles coated with noble metals or their alloys, preferably with silver or with silver alloy, in which particles the carrier is a less expensive metal, for example graphite. The particle nucleus to be coated may also be, for example, a glass, plastic, ceramics or metal bead. However, it is preferable to use solid noble-metal particles, in particular solid silver particles.

The particle bed is made up of particles 13 of a rather similar size in each given case. The size of the particles 13 can be varied. As the particle size diminishes, the surface area of the cathode increases, whereby deoxygenation is at the same time made more effective. The electrically conductive electrode material is usually spherical or granular, and typically incoherent. Most preferably the electrically conductive particles are spherical, in which case their diameters are approximately 0.1 - 0.7 mm, most preferably 0.2 - 0.6 mm.

According to the invention, the thickness of the cathode 6 is less than 0.5 cm. Typically the thickness of the cathode 6 is 0.1 - 0.5 cm. The other dimensions of the cathode 6 can be selected freely. The smaller the cell, the lower its capacity, and thus, for practical reasons, the cathode should not be decreased too much. On the other hand, an even distribution of the water flow may be difficult if the cathode is very wide. Typically the length of the flow-by cathode in the water flow direction is greater than its width, which for its part is greater than the thickness of the cathode in the travel direction of ionic electricity. Thus, the other dimensions of the cathode are most suitably such that the length of the cathode 6 is more than 20 cm, preferably 20 - 50 cm and its width is less than 15 cm, preferably 10 - 15 cm.

Figure 2 shows in greater detail the structure of the upper portion of the cell of Figure 1. The anode 9, 10 is arranged between the end plate 7,8 and the frame 14, against the cathode bed 6, but so that there is between the anode 9, 10 and the cathode bed 6 a membrane 11, 12, which separates them from each other. The membrane 11, 12 is typically placed not only against the cathode 6 but also against the cathode frame 14 in those parts in which the membrane 11, 12 is larger than the flat surface of the cathode 6. Preferably there are ar- ranged first seals 17, 18 between the cathode frame 14 and the membrane 11, 12, second seals 19, 20 between the anode 9, 10 and the membranes 11,12, and third seals 21, 22 between the anode 9, 10 and the end plate 7, 8. Since the ion-exchange membrane 11, 12 is flexible, the loose particles in the cathode compartment press it against the anode 9, 10. The anode 9, 10 is supported on the other side by support structures 23 - 28 arranged to the end plate 7, 8, which structures prevent the anode from bending. Thus the thickness of the cathode bed in the transverse direction is equal to the total thickness of the first 17, 18 and second 19, 20 seals and the cathode frame 14 in the transverse direction of the plane. In a case in which there is only one anode, there is, of course, included in the calculation only one first seal 17, 18 and one second seal 19, 20. The thickness of the cathode frame 14 is 1 - 4 mm, preferably 2 - 2.5 mm. The thickness of the seals is typically 0.2 - 0.5 mm. Thus the thickness of the cathode bed 6 also varies, but so that it is at maximum 0.5 cm, preferably 0.1 - 0.5 cm.

There may be one or more of both anodes 9, 10 and membranes 11, 12. The anodes are typically made up of an insoluble material. An example of a suitable anode material is a titanium grid coated with iridium-based oxide. The anode 9, 10 may comprise a net-like, grid-like or planar structure; in the last-mentioned case there are preferably liquid flow channels arranged therein.

The membranes 11, 12 are preferably semi -permeable, in which case they allow the movement of ions between the electrodes. An example of such a membrane is Ionac MC- 3470 cation-exchange membrane. The anode or anodes 9, 10 and the cathode or cathodes 6 are connected with leads, not shown, to a current source from which current can be fed to the cell. The cell voltage is 1.5 - 2.5 volts, most suitably 1.9 - 2.0 volts.

In the method according to the invention, oxygen is removed from water or an aqueous solution by feeding the liquid to be treated into an electrochemical cell. The liquid flows through the cathode 6 of the cell. The flow rate of the water may vary. The flow is preferably at minimum approximately 120 cmVmin, preferably at minimum 150 cmVmin, and especially preferably approximately 170 - 300 cmVmin. The cathode 6 comprises an electrode material which is in the form of particles 13 and preferably incoherent, the particles being arranged as a bed. The use of very small particles 13 in the particle bed of the cathode 6 increases pressure losses, but, overall, flow resistance in the particle bed is insignificant.

The oxygen dissolved in the water is reduced on the surfaces of the particles 13, whereupon the product obtained is water having an oxygen content even as low as less than 3μg of oxygen / kg of water. When necessary, the water or aqueous solution to be treated can be directed through a plurality of successive and/or parallel cells.

It should further be emphasized that by the method according to the invention it is possible to remove oxygen from water even if its conductivity is 0.16 mS/cm or lower, and conductivity-promoting ions need not be added to the water to be treated.

The following examples illustrate the invention in greater detail.

Example 1

A flow-by cell was constructed for deoxygenation experiments. The cell was divided with a membrane into a cathode compartment and two anode compartments in such a manner that water flowed only through the cathode compartment.

The cathode used was a 120 cm³ packed bed composed of silver granules having a diameter of 0.21 - 0.59 mm. The pressure loss was only 0.1 bar.

The dimensions of the cathode were as follows: length 26 cm and width 12.5 cm. The thickness, calculated as a ratio of the volume and the surface area, was approximately 0.4 cm. The anode consisted of two 325 cm² titanium grids coated with an iridium-based oxide. The anodes were placed in two separate anode compartments against a membrane. The anode compartments were filled with a very dilute nitric acid.

Ionac MC-3470 cation-exchange membrane was used for separating the cathode and anode compartments.

Between the cathode frame and the membranes, the membranes and the anode plates, and the anode plates and the end plates there were arranged PTFE seals having a thickness of approximately 0.25 mm.

In all of the tests the cell was filled with water in such a manner that no air was left in it. The water flow was set at 240 ml/min (4 g/s) and the cell voltage at 2 V. The oxygen content of the oxygen-saturated water entering the cell was approximately 8500 μg of oxygen / kg of water. The conductivity of the water was approximately 0.13 mS/cm.

The electrodes of the cell were connected to a direct-current source equipped with a voltage regulator. -

On the basis of the tests, the oxygen content obtained for the exiting water was <3 μg/kg.

Example 2 (reference example)

Deoxygenation on a laboratory scale was carried out in accordance with application publication FI 922 485, the cathode dimensions being: length 9 cm, width 9 cm and thickness 1.2 cm. In this test, the cathode was a 100 cm³ packed bed of copper granules.

It was possible to treat water at a rate of 120 ml/min (2 g/s). The cell voltage was 2 V. The oxygen content of the oxygen-saturated water entering the cell was approximately 8500 μg/kg. In order for the water to conduct electricity and for the copper not to be spoiled by oxidation, it was necessary to add a salt (Na-jSO,,) in an amount of 7.1 kg/m³ of water. Thus the salt content of the water was 100 times that in Example 1. The conductiv- ity of the water was 10 mS/cm.

Table 1 shows the results of the measurements. It can be observed from them that deoxygenation with the option according to the invention is as efficient (the oxygen contents remain equally low) as with a prior-art cell. However, the water treatment capacity is dou- bled. The essential point is that in Example 1 it was not necessary to add a salt to the water in order to improve conductivity.

Claims

1. A process for the removal of oxygen from water or aqueous solutions, according to which process the water is fed into an apparatus which comprises an electrochemical cell containing - at least one three-dimensional flow-by cathode (6) comprising an electrically conductive particulate electrode material (13), which is a noble metal or a noble metal alloy and which is formed into a particle bed having at least one flat surface, characterized in that - the cell further contains at least one ion-exchange membrane (11, 12), which is arranged against the flat surface of the cathode (6), and at least one anode (9, 10), which constitutes the counter-electrode for the cathode and which is arranged against the ion-exchange membrane (11, 12), - conductivity-improving ions are not substantially added to the water, and a cathode having a thickness of 0.1 - 0.5 cm is used. ^_

2. A process according to Claim 1, characterized in that the particulate electrode material (13) is silver or a silver alloy.

3. A process according to Claim 1 or 2, characterized in that the conductivity of the water is 0.16 mS/cm or lower.

4. A process according to any of Claims 1 - 3, characterized in that water is fed into the cell at a volume flow which is at minimum 150 ml/min.

5. An apparatus for the removal of oxygen dissolved in water from water or aqueous solutions, the apparatus comprising an electrochemical cell containing - at least one three-dimensional flow-by cathode (6) comprising an electrically conductive particulate electrode material (13), which is a noble metal or a noble metal alloy and which is formed into a particle bed having at least one flat surface, characterized in that - the cell further contains

- at least one ion-exchange membrane (11, 12), which is arranged against the flat surface of the cathode (6), and - at least one anode (9, 10), which constitutes the counter-electrode for the cathode and which is arranged against the ion-exchange membrane (11, 12), and the thickness of the cathode is 0.1 - 0.5 cm.

6. An apparatus according to Claim 5, characterized in that the cathode (6) is made up substantially entirely of particles (13) having a size of 0.1 - 0.7 mm.

7. An apparatus according to Claim 5 or 6, characterized in that the particulate electrode material (13) is silver or a silver alloy.

8. An apparatus according to any of Claims 5 - 7, characterized in that the particle bed of the cathode (6) has two flat surfaces, and against each flat surface there is arranged one ion-exchange membrane (11, 12), and against each ion-exchange membrane (11, 12) there is arranged one anode (9, 10).

9. An apparatus according to any of Claims 5 - 8, characterized in that it comprises a cathode frame (14) which delimits an interior electrode compartment to- gether with two ion-exchange membranes (11, 12), which are arranged against the flat sides of the cathode frame (14), two anodes (9, 10), which are arranged against the ion-exchange membranes

(11, 12), and - first seals (17, 18) arranged between the cathode frame (14) and the ion- exchange membranes (11, 12) as well as second seals (19, 20) arranged between the anodes (9, 10) and the ion-exchange membranes (11, 12).

10. An apparatus according to Claim 9, characterized in that the total thickness of the first (17, 18) and the second seals (19, 20) and the cathode frame (14) in the transverse direction of the plane of the cathode frame (14) is at maximum 0.5 cm.

11. An apparatus according to any of Claims 5 - 10, characterized in that the length of the particle bed of the cathode (6) is greater than 20 cm.

12. An apparatus according to any of Claims 5 - 11, characterized in that the anode (9, 10) comprises a flat net or grid structure.