WO2000039361A2

WO2000039361A2 - Ion exchange membrane cell for high product capacities

Info

Publication number: WO2000039361A2
Application number: PCT/EP1999/010120
Authority: WO
Inventors: Wolfgang Strewe
Original assignee: Krupp Uhde Gmbh
Priority date: 1998-12-23
Filing date: 1999-12-20
Publication date: 2000-07-06
Also published as: DE19859882A1; WO2000039361A3

Abstract

The product capacity of an ion exchange membrane cell for the generation of chlorine and alkali caustic solutions is limited by mechanical design facts. A cell according to this invention allows a substantial increase of its product rates by assembling of several individual electrode plates (4A, 4B) in each single cell element, which are placed piece by piece in the halfshells of the cells, aligned, adjusted and fastened by adjustment members (16) piece by piece. Dependent on the capacity the cell elements can be equipped with different numbers of inlet and outlet pipes. With respect on the recoating procedures of the electrode surfaces outside of the cells the fastening means (16) of the electrode plates are removable in such a manner that the plates can be replaced multiple times without mechanical destruction of cell or plate parts.

Description

Ion exchange membrane cell for high product capacities

Description

For the industrial production of chlorine, alkaline caustic and hydrogen by electrolysis of aqueous alkaline solutions ion exchange membrane cells are applied more and more, because they have remarkable ecological as well as economical advantages compared to the well known mercury and diaphragm cells, preferred in former times in the chlorine industry. Such ion exchange membrane cells are described in detail in the patents DE 1 96 41 1 5 A, EP 0 579 910 B1 and DE 44 1 5 146 C 2 for instance. Such electrolysis plants consist - dependent on the plant capacity - of one or more electrolysers, each with a plurality of cell elements electrical connected in series.

As shown in Fig. 1 these cell elements consist mainly of an anode halfshell 1 and a cathode halfshell 2 with an ion exchange membrane 3 positioned in between. The membrane separates liquids and gases of the anode and cathode compartment , so that only Na + - respectively K + - Ions can pass from the anolyte to the catholyte compartment through the membrane. Both electrodes, anode plate 4 and cathode plate 5 normally consist of thin, plane sheets with an open structure, allowing the gases, generated at the electrode frontside, to escape to the backside of the electrodes.

Anode and cathode plate are positioned in the cell vertical and parallel to each other with the membrane spread out in between. The outer anodeshell wall 6 and cathodeshell wall 7 are screwed tight together by flanges 12, thus making the cell element to a gas- and liquidproof containment. Both walls 6, 7 are connected with the electrodes 4, 5 by a group of busbars 8. The busbars 8 have a double function: they ensure a parallel position of the electrode plates to the outer walls and on the other hand they provide for the current transport between the outer walls and the electrodes.

In case the cells are used for the chlorine/caustic electrolysis than, caused 5 by the higher hydraulic pressure of the catholyte 10 compared to the anolyte 9, the membrane 3 will touch the anode 4, providing an electrolytic gap between membrane and cathode surface. To minimize the power consumption of the cell element this electrolytic gap has to be as small as possible. Normal distances for the gap are 1 to 2 mm, thus restricting the ι o cell voltage to about 3.0 Volts and the power consumption to about 2300 kWh per metric ton of chlorine at a current density of 3 kA/m².

According to the manufacturing standards of the ion exchange membranes, the maximum height of the electrode plates is around 1 200 mm. On the i s other hand the membrane manufacturing procedure allows the fabrication of membrane sheets in various lengths. Thus in theory the horizontal dimension of the electrode plates with a membrane in between could be unlimited large. But normally the horizontal length of such cell elements is limited to 2 to 3 meter, because with rising length the precise fabrication

20 of the electrode surfaces with tight tolerances becomes more and more difficult. But problems during fabrication, erection and operation of such cells can exist even at such electrode sizes caused by too intensive membrane contact with single spots of the electrodes, so that a premature membrane exchange can be necessary. Furthermore with rising cell current

25 gas generation in the electrolyte compartments will rise too, thus developing higher gashold, more foam generation and higher pulsation of the liquid/gas mixture in the gas release ducts of the cell.

Therefore it is an aim of this invention, to find a cell construction, which

30 can be operated at cell loads of 6 kA/m² and more with pulsation free gas release, without premature wear of the membranes, and which can be fabricated with minimal measure tolerances in an economical manner-with cell sizes of more than 3 m² electrode surfaces.

A cell according to this invention used in large industrial plants has remarkable advantages as shown in following example :

A conventional cell element with 2.1 m² size at a current density of 3 kA/m² can produce in one hour 1 0 kg chlorine and corresponding amounts of caustic and hydrogen. A cell element according to the invention however with 5 m² size and at 6 kA/m² will produce 40 kg chlorine in one hour and the corresponding byproducts.

So for a chlorine/caustic electrolysis with a daily capacity of 100 t CI2 electrolysers with a total of 420 conventional cell elements are necassary, compared to just about 1 00 cell elements of the inventive construction. This is a remarkable advantage by lower investments as well by remarkable savings for the routine membrane renewal and the routine reactivation of anode and cathode surfaces.

To solve the problem of developing cell constructions for high product capacities several difficulties are to overcome. Primarely there is to consider, that corresponding to the enlargement of the electrode area the heat expansion of the electrode plates will enlarge accordingly. Due to this fact there is a growing risk that during cell operation at internal temperaturs of 90 to 1 00°C the edges of the electrode plates may collide with the edges of the halfshell walls, so causing a deformation of metallic cell parts and membranes.

For instance at a cell length of 4 meter the anode plate made of titanium would expand in horizontal direction to each side by about 1 .3 mm, a cathode plate made of nickel by about 2.1 mm. The edges of the halfshell walls however do not follow the plate edge movements, because they are tightly screwed with the colder flanges 12 outside the cells. To avoid collision between plate and wall edges in any case, the cell elements have to be designed for a theoretical edge gap of about 5 mm in the state of fabrication (at 20°C), which would be reduced to about 4 mm during operation (at max. 100°C) . But even by highly precise manufacturing procedures with an measure accuracy of 1 per mil tolerances of + /- 2mm at each electrode side would occur, so that during cell operation from element to element edge gaps between 2 mm and 6 mm would exist.

Experience shows, that the edge gaps should be not larger than about 3 mm, because otherwise the sensitive membranes would be pressed too deep into the gap by the catholyte overpressure and would be damaged by abrasion at the metallic edges. Consequently there is to say, that common cell elements should be not longer than about 2 meter, to avoid such tolerance problems in the edge gap areas of the cell.

For cell elements for high product capacities a further problem exists by the increasing amount of gas flows in the electrolyte compartments. To minimize the power consumption both halfshells have to be designed as narrow as possible, consequently with increasing gas production a bottleneck occurs for the release of chlorine and hydrogen gas from the halfshells. At conventional membranecells as described above in each halfshell a vertical stand pipe 13 is provided, that will be inserted from below after assembling of the halfshells.

The stand pipes 13 serve for the level control and overflow for the anolyte liquid 9 and catholyte liquid 10 as well as for the outlet of chlorine and hydrogen. As the stand pipe diameter are limited by the small width of the halfshells, the gas production of the cell elements is limited accordingly. For membranecells of the common art therefore the maximum chlorine production is about 1 0 to 1 2 kg/h per element with corresponding production rates for hydrogen and caustic.

So base of the invention is the idea to approach higher production rates by installing in the cell elements instead of one anode plate and one cathode plate a couple of anode and cathode plates in such a manner, that each plate can be separately aligned and adjusted and so each plate allowing free movement during operation under the influence of heat expansion of the plates without the risk of collisions between the plates or between plate and halfshell wall. Furthermore each halfshell according to the invention is designed in a manner, that instead of one stand pipe a couple of stand pipes can be installed and fixed in the half shell during assembling. By the multitude of pipes the cross section for the gas outlet can be increased without enlarging the width of the halfshells. This design of the stand pipe device of the invention allows a substantial increase of the product capacity of the cell element.

At conventional ion exchange membrane cells the backwalls of the halfshells 1 , 2 are welded to the busbars 8 and the busbars 8 are welded to the electrode plates 4, 5, each defining a plane P-P, so that the bus bars cause the correct sizes of the total halfshells. Furthermore by the welded connections they cause the electrical contact for the current transfer backwall - busbar - electrode plates.

Now this invention is based on the fact, that electrical contact between metallic parts inside the halfshells can be achieved beside by weldings also by pressure contacts, if the contact pressure between the contact faces is high enough. According to this invention for the contact pressure of the cell parts the same mechanical force will be used, which outside the cell elements is made by the pressure device of the electrolyser rack to cause the contact and current transfer from the first to the last cell element. Therefore the cell is constructed in a way, that the busbars 8 are positioned in alignment with the contact strips 14 outside the cell element and with the spacers 15, which control the electrode gap and provide a widespread power distribution over the membrane 3. To limit the electrical losses by pressure contact between metallic parts to some millivolts a contact pressure in the range of about 1 0 N/mm² is necessary. Such pressure rates can be easily approached by adequate design ot the outer pressure device and the size of the contact faces for the contact backwall/busbars as -well as busbars/electrode plates.

The fixing of the electrode plates 4, 5 is shown in Fig. 2. Underneath of each electrode plate there are placed adjustment clips 16, which are in a rigid material connection, for instance by welding of the body strip 17 A with the back walls 6, 7 of the halfshells. The top strips 17 B with the holes 18 however can be fixed to the electrode plates 4, 5 by form connection, for instance by screws or buttons, or by force connection, for instance by wedges or plugs. The distance of the holes is different to the pitch of the electrode structure. With this position of the holes 18 it is assured, that the electrodes in every random position can be fixed to the adjustment clips 16, because in any case at least one of the holes is aligned to an open part of the structure, so that at this point the fixture device can be installed and fixed easily. This kind of fixture allows to position each single electrode plate exactly to a predeterminated edge gap to the neighbour plates and to the halfshell wall.

A further advantage of this plate fixing is the simple assembling and the simple and destruction free dismantling of the electrode plates. Anode as well as cathode plates are equipped with an activation of the surfaces, which wears out after some years and have to be recoated. The destruction free dismantling as well as the simple handling of the single plates makes the working process and effort of the recoating procedure significantly easier. Some procedures are known to simplify the recoating by destruction free electrode connections. Thus in patent DE 37 26674 A 1 a construction is described in which the electrical connection of the current contacts is made by plug contact instead of weldings. The spring part of those contacts is made as a rigid connection with the halfshell, so that an exact adjustment of the electrodes is not possible. Furthermore there is the risk, that by wear of material the spring force will decrease during operation, so that the contacts will fail early. Another possibility for a destruction free electrode connection is described in patent DE 421 1 2678. There the cathode plate will be positioned to the current conductors by loose fixing pins. The pins maintain a form connection with the current conductors in the level of the cathode plate only, and only after screwing to the anode halfshell there occurs a force connection between cathode plate and current conductor by the forces of the outside flange connection. Because of the requirement to assemble cathode halfshell, membrane and anode halfshell in a horizontal position, it is not possible to use this kind of fixing for the anode plates too. Furthermore an exact positioning of the electrode plates to the wall edges of the halfshells is not possible.

In industrial electrolysis plants the membrane cell elements are combined to so-called electroiysers in common mode by electrical connection in series. For that the cell elements are installed back to back either by suspending on a rack or by erection on a base plate. By pressure devices at both ends of the electroiysers the cell elements are pressed together like in a filter press, so that through the metallic backwalls the electric current can be transferred from element to element. But by the friction forces in the supports of the cell elements a part of the pressure force get lossed from every element position to the next one. So towards the centre of an electrolyser with conventional supports the back-to-back contact pressure becomes smaller and smaller, and the same happens with the mechanical pressure between backwall/current conductor and current conductor/electrode plate in the cell interior. Because there is an increasing loss of electrical energy with increasing contact losses, a further object of the invention is a new type of supporting cell elements in an electrolyser with outside pressure device, which maintains an equal transfer of the mechanical pressure from element to element without friction losses by flexible support devices. Such a cell element support is shown in Fig. 3. Each cell element 19 is attached on both sides at the horizontal beams 20 of the electrolyser rack. The support means 22 either are flexible like chains or ropes or they are connected by flexible joints 23 with the cell elements 19 and the horizontal beams 20. Either they consist in total of an electric insulating material or they include an intermediate insulation piece 24. The support means 22 can just transmit tensile forces but no pressure forces and no momentum, therefore it is assured, that the pressure transferred through the electrolyser from element to element can not be diminished by friction losses at the support bearings of the horizontal beams. Also the distribution pipe 25 and the header 26 of the electrolyser are flexible connected to the cell element 19 by the hoses 25 A, 26 A, so that even by this lines no pressure force can get lost. The individual objects of a cell element according to this invention are explained by the following example and illustrated by Fig. 4, 5 and 6:

A cell element with an active anode area of 5.4 m² is equipped with six single plates for the anode and six same sized single plates for the cathode. Each halfshell of the cell element contains one distribution pipe 30 for the steady feed of electrolyte, three stand pipes 13 A, 13 B, 13 C in equal distribution along the length of the cell and one release duct 13 D for the outlet of liquid electrolyte and electrode gases and different clips for the fixing of electrode plates, pipes and ducts.

Fig. 4 shows the cross section of a single anode plate 4 A and the plate 4 B underneath. The plates are 1 500 mm long and 600 mm high. They are made of titanium, full of holes or punched, the surfaces are coated with an activation layer. On the backside there are 1 1 vertical busbars 8 A, 8 B in a distance of 1 50 mm, which are combined with there longside 27 A, 27 B to the plates 4 A, 4 B by material connection. The opposite longsides 28 A, 28 B are machined parallel and coated with a high electric conductive metal, for instance platinum, thus providing a contact to the halfshell backwalls 6 with low electrical losses.

Fig. 5 is a top view to the interior of the anode halfshell 1 before assembling of the anode plates. The inner surface have 33 contact faces 29, coated for instance with platinum, positioned in alignment to- the contact sides 28 of the busbars 8 A. Before assembling of the anode plates the distribution pipe 30, the three stand pipes 13 A, 13 B, 13 C and the release duct 13 D will be installed and sealed . For a better overview just 4 of the 33 contact faces are shown.

In Fig. 6 the upper left anode plate 4 A after installation in the halfshell 1 is shown. At first the plate will be set loose on the adjustment clips 16 and than manually adjusted so, that the vertical gap 31 and also the horizontal gap 32 has a width of 2 mm. In this position anode plate 4 A will be fastened tight to the adjustment clips 16 by the help of selfcutting screws. In the same way the two upper plates 4 A will be installed. For these two plates and also for the three plates below during adjustment it is to realize to maintain the correct distances of the intermediate gaps 33 too. Furthermore in case of size differences at the rims of halfshell 1 and anode plates 4 A, 4 B the gaps 31 , 32, 33 have to be equalized, so that at no point distances fall below 1 .5 mm or exceed 3 mm. The 1 .5 m long anode plates will expand during operation at 100°C by 0.5 mm in each direction. So a minimal gap of 1 .5 mm is sufficient for a safe cell operation. The contact faces 29 as well as the outside contact strips 14 are 6 mm wide, so it is assured, that by the adjustment of the anode plates 4 A, 4 B all busbars 8 A, 8 B will be aligned with the contact faces 29 and the contact strips 14. Similar to the anode halfshell 1 the cathode halfshell 2 is eqipped with adjustment clips, distribution pipe and standpipes and the installation and adjustment of the 6 cathode plates 5 is similar too. The cathode plates made of nickel expand by heating up to 100°C to a maximum of 0.8 mm at each side. Compared with titanium the expansion is somewhat larger and a somewhat smaller tolerance range of 2 to 3 mm is necessary. But even this narrow tolerance range allows a trouble free cell operation by the posibility of exact adjustment of each cathode plate. With a cell element of this type and size it is easy to reach product capacities, which were not feasible up to now. At an economic reasonable current density of 4 kA/m² and a current efficiency of 95% a chlorine production of 27. 1 kg/h can be reached. By the triple number of stand pipes the pressure loss of the gas release is reduced to about 20% . This allows to operate the cell element even at 6 kA/m², increasing the production rate to 40.7 kg/h.

It is evident, that by such high capacity increase rates the investment costs of electrolysis plants can be substantially reduced.

Furthermore the cell element construction according to the invention allows a simple, destruction free dismantling and exchange of electrode plates as well as an exchange of all internal parts without destruction of the electrode parts. Consequently efforts for maintenance and repair are reduced and the electrode plates can be reused for reactivation of anode and cathode surfaces.

Type and size of the cell element described above is just one of different possibilities of the cell design according to the invention. This concerns for instance number and size of the electrode plates and the kind of fastening the plates at the halfshells. Especially it is possible to provide a destruction free connection electrode plate/busbar instead busbar/cell backwall or to provide both connections as destruction free contacts.

At least it can be practical, to use different designs for anode and cathode halfshells.

The ion exchange membrane cell for the electrolytic dissolvation of aequeous solutions mainly consists of an anode part. A cathode part and an ion exchange membrane is positioned in between, whereas anode and cathode part are connected liquid- and gastight. Each of both cell parts mainly consists of an outer metallic halfshell with internal electrode plates. Electrical conductive connection means are provided between the wall of the halfshells and the electrode plates, of pipe connections for feed of- the electrolytic solutions and release of the liquid and gaseous electrolytic products. The electrodes of the cell consist of multiple plates, which are positioned in the cell in such a way, that the single electrode plates during cell operation in spite of all external and internal forces effecting on the cell parts can free expand without build up of material stresses. The single electrode plates may be connected to the halfshells by force connection or form connection. The contact for transfer of the electrical current from the wall of the halfshells 1 , 2 to the busbars 8 may be effected by outer mechanical forces. The contact for transfer of the electrical current from the busbars 8 to the electrode plates 4, 5 may be effected by outer mechanical forces. For the mechanical fastening of the electrode plates to the wall of the halfshells adjustment clips 16 are provided, which are connected to the wails 6, 7 of the halfshells by material connection and to the electrode plates 4, 5 by form connection or force connection. The cell elements, arranged one behind the other in a rack, may be suspended at the rack beams 20 by flexible support means 22. The release of the electrolytic products out of the halfshells may occur by multiple overflow pipes 13 which are connected inside the halfshells as communicating tubes.

The product capacity of an ion exchange membrane cell for the generation of chlorine and alkali caustic solutions is limited by mechanical design facts. A cell according to this invention allows a substantial increase of its product rates by assembling of several individual electrode plates in each single cell element, which are placed piece by piece in the halfshells of the cells, aligned, adjusted and fastened piece by piece. Dependent on the capacity the cell elements can be equipped with different numbers of inlet and outlet pipes. With respect on the recoating procedures of the electrode surfaces outside of the cells the fastening means of the electrode plates are removable in such a manner that the plates can be replaced multiple times without mechanical destruction of cell or plate parts.

Claims

1 . Ion exchange membrane cell for electrolytic decomposition of aqueous solutions, comprising anodic and cathodic electrode constructions (4, 5) each defining a plane (P-P) and sandwiching therebetween a membrane (3), each electrode construction being electrically connected to and supported by adjacent wall sections (6, 7) of said cell, wherein at least one of said electrode constructions (4, 5) is divided into a plurality of electrode sections (4 A, 4 B), said electrode sections (4 A, 4 B) being removably and, at least within said plane (P-P) defined by the corresponding electrode construction (4, 5), separately adjustably fixed at said adjacent wall section (6, 7) of said cell.

2. Ion exchange membrane cell according to claim 1 , wherein a longitudinal gap (33) is provided between adjacent electrode sections (4 A, 4 B).

3. Ion exchange membrane cell according to claim 1 or 2, wherein said electrode sections (4 A,

4 B) are removably and adjustably fixed at said adjacent wall sections (4,

5) by at least two adjustment members ( 16) distributed over the area of each said electrode sections (4 A, 4 B) .

Ion exchange membrane cell according to claim 3, wherein predetermined positions for said adjustment members ( 16) are arranged in at least one row and at least one column, preferably with equal distances within each row and column.

Ion exchange membrane cell according to claim 3 or 4, wherein each of said adjustment members ( 16) is (at 17 A) non-detachably integrally fixed at the adjacent wall section but removably- and adjustably connected with the corresponding electrode section (at 17 B) .

6. Ion exchange membrane cell according to claim 3, 4 or 5, wherein said adjustment members ( 16) are adjustably connected with the corresponding electrode section (4 A, 4 B) by non-positive force connection or positive form connection, preferably elastical clips, wedges, pins or bolts.

7. Ion exchange membrane cell according to one of claims 3 to 6, wherein said adjustment members (16) and/or said electrode sections (4 A, 4 B) comprise a row of holes ( 18) for inserting a bolt in one of said holes ( 18) for optional fixing said electrode sections (4 A, 4 B) at said adjustment members (16) in one of different positions defined by said holes (18) of said row.

8. Ion exchange membrane cell according to claim 7, wherein the adjustment members ( 16) and said electrode sections (4 A, 4 B) each comprise aligning rows of holes, wherein the distances between adjacent holes of each row are different for the holes in said electrode sections and for the holes in the adjustment members.

9. Ion exchange membrane cell according to one of claims 3 to 8, wherein said electrode sections (4 A, 4 B) of one electrode construction 4, 5 are commonly electrically connected with said adjacent wall section (6, 7) by a plurality of longitudinal busbars (8) extending between adjacent adjustment members ( 16) .

1 0. Ion exchange membrane cell according to claim 9, wherein said busbars (8) extend vertically between adjacent positioning columns for said adjustment members (16).

1 1 . Ion exchange membrane cell according to claim 9 or 10, wherein said busbars (8) are commonly non-detachably integrally connected with said plurality of electrode sections (4 A, 4 B) of a respective electrode construction (4, 5), and are removably abutted on inner faces of said adjacent wall sections (6, 7) for electrical connection between said wall sections (6, 7) and said electrode constructions (4, 5).

1 2. Ion exchange membrane cell according to claim 9 or 10, wherein said busbars (8) are removably abutted on said plurality of electrode sections (4 A, 4 B) of a respective electrode construction (4, 5), and are non-detachably integrally connected with inner faces of said adjacent wall sections (6, 7) for electrical connection between said wall sections (6, 7) and said electrode constructions (4, 5).

1 3. Ion exchange membrane cell according to claim 1 1 or 1 2, wherein an external pressing device is provided for pressing said inner wall face of said walls (6, 7) onto said busbars (8), wherein said contact pressure between said inner wall face and said busbars (8) amounts between at least 5- 1 5 N/mm², preferably at least 1 0 N/mm².

14. Ion exchange membrane cell according to one of the preceding claims, wherein a plurality of spaced gas discharge pipes (13 A, B, C), preferably overflow-gas discharge pipes ( 13 A, B, C) extending in a common plane are sandwiched between at least one of said electrode constructions (4, 5) and the adjacent wall sections (6, 7) .

1 5. Ion exchange membrane cell according to one of the preceding claims, wherein the cell is adapted for producing chlorine gas, hydrogen gas and alkaline caustic by electrolysis of alkaline solutions.

1 6. Ion exchange membrane cell according to one of claims 9 to- 1 5, wherein current conducting elements of at least the anodic electrode (4) are made of titanium and the contact faces (29) for the current transfer from the wall sections (6) to the busbars (8) are coated with a chlorine and alkalichloride resistant metal with high electric conductivity.

1 7. Ion exchange membrane cell according to one of claims 9 to 1 6, wherein current conducting elements of at least the anodic electrode (4) are made of titanium and the contact faces for the current transfer from the busbars (8) to the anodic electrodes (4) are coated with a chlorine and alkalichloride resistant metal with high electrical conductivity.

1 8. Combination of a plurality of ion exchange membrane cells ( 19) for electrolytic decomposition of aqueous solutions stacked side by side and pressed commonly together by a pressing device, preferably according to one of the preceding claims, wherein each cell( 19) is separately supported at a carrier construction (20, 21 ) common for all cells by at least one non-rigid connection means (22,

23) so as to be displaceable in the direction of the pressing force applied by the pressing device.

1 9. Combination according to claim 1 8, wherein said carrier construction (20, 21 ) comprises at least two spaced parallel carrier beams (20) carrying said cells between them through said non-rigid connection means (22, 23).

20. Combination according to claim 1 8 or 1 9, wherein said non-rigid connection means (22, 23) inhibit transmission of pressure forces and/or moments and/or torques from said eletrolyte cells ( 19) to said carrier construction (20) . Combination according to one of claims 1 8 to 20, wherein said-non- rigid connection means (22, 23) are connected to said cells ( 19) or said carrier construction (20) through insulating means (24) .