Electrode assembly, electrolysers and processes for electrolysis

Abstract

The present invention relates to an electrode assembly and an electrolyser using said assemblies/structures, wherein the electrode assembly comprises an anode structure and a cathode structure, each of said anode structure and cathode structure comprising an outlet header for evolved gas and spent liquid, wherein each of said anode structure and cathode structure comprising an outlet header for evolved gas and spent liquid, wherein the outlet header on the anode structure has a total internal volume of V.sub.A cm.sup.3 and the outlet header on the cathode structure has a total volume of V.sub.C cm.sup.3 wherein V.sub.A is less than V.sub.C, and/or i) the outlet header on the anode structure has an internal volume, V.sub.A cm.sup.3, an internal cross sectional area at the exit end of the header of A.sub.A cm.sup.2 and an internal length L.sub.A cm, and ii) the outlet header on the cathode structure has an internal volume, V.sub.C cm.sup.3, an internal cross sectional area at the exit end of the header of A.sub.C cm.sup.2 and an internal length L.sub.C cm, and one or both of the ratios V.sub.A/(A.sub.AL.sub.A) and V.sub.C/(A.sub.CL.sub.C) are less than 1.

Claims

1. An electrode assembly comprising an anode structure and a cathode structure, each of said anode structure and cathode structure comprising an outlet header for evolved gas and spent liquid, wherein i) the outlet header on the anode structure has an internal volume, V.sub.A cm.sup.3, an internal cross sectional area at the exit end of the header of A.sub.A cm.sup.2 and an internal length L.sub.A cm, and ii) the outlet header on the cathode structure has an internal volume, V.sub.C cm.sup.3, an internal cross sectional area at the exit end of the header of A.sub.C cm.sup.2 and an internal length L.sub.C cm, And wherein one or both of the following apply: a) the outlet header on the anode structure is an external header and the ratio V.sub.A/(A.sub.AL.sub.A) is less than 1, and b) the outlet header on the cathode structure is an external header and the ratio V.sub.C/(A.sub.CL.sub.C) is less than 1.

2. An electrode assembly comprising an anode structure and a cathode structure, each of said anode structure and cathode structure comprising an outlet header for evolved gas and spent liquid, wherein the outlet header on the anode structure has a total internal volume of V.sub.A cm.sup.3 and the outlet header on the cathode structure has a total volume of V.sub.C cm.sup.3 wherein V.sub.A is less than V.sub.C.

3. An electrode assembly according to claim 2 wherein i) the outlet header on the anode structure has an internal cross sectional area at the exit end of the header of A.sub.A cm.sup.2 and an internal length L.sub.A cm, and ii) the outlet header on the cathode structure has an internal cross sectional area at the exit end of the header of A.sub.C cm.sup.2 and an internal length L.sub.C cm, and wherein one or both of the following apply: a) the outlet header on the anode structure is an external header and the ratio V.sub.A/(A.sub.AL.sub.A), and b) the outlet header on the cathode structure is an external header and the ratio V.sub.C/(A.sub.CL.sub.C) is less than 1.

4. An electrode assembly according to claim 1 wherein the outlet header on the anode structure is an external header and has V.sub.A/(A.sub.AL.sub.A) of less than 1, more preferably less than 0.95, such as less than 0.7, and preferably wherein the outlet header on the anode structure is tapered such that its cross-sectional area increases along its length.

5. An electrode assembly according to claim 1 wherein A.sub.A is at least 7 cm.sup.2 and preferably at least 15 cm.sup.2.

6. An electrode assembly according to claim 1 wherein A.sub.C is less than A.sub.A, and preferably at least 5 cm.sup.2 less than A.sub.A.

7. An electrode assembly according to claim 1 wherein V.sub.A is less than 3100 cm.sup.3 and/or wherein V.sub.A is 100 cm.sup.3, more preferably 250 cm.sup.3 less than V.sub.C.

8. An electrode assembly according to claim 1 having an external anode outlet header and an internal cathode outlet header, or vice versa.

9. An electrode assembly as claimed in claim 8 wherein each outlet header is an outlet volume which is provided on the individual anode or cathode structure and by which evolved gas exits the anode or cathode structure to an electrolyser collection header, and preferably is an extended volume aligned parallel with the long horizontal axis of the electrode structure.

10. An electrode assembly according to claim 1 wherein the external outlet header or headers comprises one or more internal cross members located along part of or all of the horizontal length of and attached internally to the sides of the header.

11. An electrode structure comprising: i) a pan with a dished recess and a flange which can interact with a flange on a second electrode structure to hold a separator in between the two and the dished recess further having a plurality of inwardly or outwardly projecting projections which can mate with corresponding projections on a third electrode structure in an electrode unit or in a modular electrolyser, ii) an inlet for liquid to be electrolysed and iii) an outlet header for evolved gas and spent liquid, wherein the outlet header is an external outlet header in which V.sub.E/(A.sub.EL.sub.E) is less than 1, where V.sub.E is the internal volume of the external outlet header in cm.sup.3, A.sub.E is the internal cross sectional area at the exit end of the header L.sub.E is the internal length, and preferably is a tapered external outlet header which increases in cross-section area in the direction of gas/liquid flow towards the exit ports.

12. An electrode structure according to claim 11 wherein the external outlet header comprises one or more internal cross members located along part of or all of the horizontal length of and attached internally to the sides of the header.

13. An electrode structure according to claim 11 which is an anode structure.

14. A modular or filter press electrolyser comprising a plurality of electrode assemblies according to claim 1, and preferably which comprises 5-300 electrode assemblies.

15. A process for the electrolysis of an alkali metal halide which comprises subjecting an alkali metal halide to electrolysis in a modular or filter press electrolyser according to claim 14, and in particular wherein the process is operated at a production rate per anode electrode assembly of W.sub.A, kg Cl.sub.2/hr, wherein W.sub.A/V.sub.A is greater than 0.006 kg Cl.sub.2/hr cm.sup.3.

16. A modular or filter press electrolyser comprising a plurality of electrode assemblies according to claim 2, and preferably which comprises 5-300 electrode assemblies.

17. A process for the electrolysis of an alkali metal halide which comprises subjecting an alkali metal halide to electrolysis in a modular or filter press electrolyser according to claim 16, and in particular wherein the process is operated at a production rate per anode electrode assembly of W.sub.A, kg Cl.sub.2/hr, wherein W.sub.A/V.sub.A is greater than 0.006 kg Cl.sub.2/hr cm.sup.3.

Description

(1) The present invention is further illustrated by reference to, but is in no way limited by, the following drawings, in which:

(2) FIG. 1 is a cross-section of the top part of a preferred bipolar electrode unit showing an example with a combination of internal and external headers;

(3) FIG. 2 is a cross-section of the top part of a preferred electrode module showing an example with a combination of internal and external headers;

(4) FIGS. 3A and 3B show, respectively, examples of spiders suitable for use in the anode and cathode structures;

(5) FIGS. 4A and 4B show close-ups of examples of preferred structures of cross-members in external and internal outlet headers;

(6) FIG. 5 is an isometric view looking at an anode structure showing an example of a preferred external header design according to the present invention; and

(7) FIG. 6 is a cross-section of the bottom part of a bipolar electrode unit.

(8) In FIG. 1 there is shown a bipolar electrode unit comprising an anode structure (10) and a cathode structure (30).

(9) The anode structure (10) comprises a flange (11), and a dished recess (12) with an inwardly projecting projection (13), which forms an electrolysis compartment (14) containing an anode (15). The anode structure has an external outlet header (16). The anode (15) is typically in the form of a perforated plate.

(10) The cathode structure (30) comprises a flange (31), and a dished recess (32) with an outwardly projecting projection (33), which forms an electrolysis compartment (34) containing a cathode (35). The cathode structure internal outlet header (36). The cathode (35) typically in the form or a perforated plate.

(11) The anode structure (10) is electrically connected to the cathode structure (30) via a conductivity enhancing device (50) disposed between the inwardly projecting projection (13) on the anode structure (10) and the outwardly projecting projection (33) on the cathode structure (30).

(12) In practise there are multiple inwardly outwardly projecting projections on each electrode structure, and multiple conductivity enhancing devices such that when the two electrode structures are urged together, the conductivity enhancing devices afford good electrical continuity between the peaks of the cathode structure projections (33) and the anode structure projections (13). The conductivity enhancing device may be in the form of an abrasion device (more preferably) a bimetallic disc. When the bipolar electrode unit is supplied pre-assembled for use in a filter press bipolar electrolyser, it is possible for the conductivity enhancing device (50) to be omitted completely and instead for the anode and cathode structure to be electrically and mechanically connected together by welding, explosion bonding or a screw connection.

(13) The anode and cathode structures further comprise electrically conductive posts (17, 37), which connect to the respective projections (13, 33), electrically insulating cushions (18, 38) and current carriers which are each in a form having a central portion from which two or more legs radiate (hereinafter referred to as spiders)(19, 39). The spiders (19, 39) are mounted between the respective posts (17, 37) and the respective electrodes (15, 35). At the location of the respective posts (17, 37), the electrodes (15, 35) are apertured and the cushions (18, 38) are received within the holes and rest on the central base of the spiders (19, 39).

(14) Flow of liquor from the anode electrolysis compartment (14) to the external outlet header (16) takes place via a slot at the upper end of the anode structure (10), the slot being located immediately above the anode (15).

(15) Flow of liquor from the cathode electrolysis compartment (34) to the internal outlet header (36) takes place via a slot in the internal outlet header in the upper region of the cathode structure (30).

(16) In FIG. 2 there is shown an electrode module comprising an anode structure (10) and a cathode structure (30). The anode and cathode structures are broadly as defined for FIG. 1 and the same numbering is used as for the corresponding features already described for FIG. 1. However, the respective electrode structures are in this Figure joined with the anode (15) and cathode (35) facing each other with a membrane (51) in between. In particular, the flanges (11, 31) are provided with backing flanges (20, 40) with holes to accept bolts (not shown) for bolting be anode structure (10) and the cathode structure (30) with two gaskets (52) and the membrane (51) to form a module. The membrane (51) passes down through the electrode module between the anode (15) and cathode (35), providing fluid separation between the respective electrolysis compartments (14, 34) of said anode and cathode structures (10, 30).

(17) The spider (19) in the anode electrolysis compartment (14) comprises a disc-shaped central section (21) which can be connected to the end of the post (17), e.g. by welding, screw-fixing or push-fit connectors, and a number of legs (22) which radiate from the central section (21) and are connected at their free ends, e.g. by welding, to the anode (15). Usually the legs (22) are arranged so that the current supply via the post (17) is distributed to a number of equispaced points surrounding the post (17).

(18) The spider (39) in the cathode electrolysis compartment (34) comprises a disc-shaped central section (41) which can be connected to the end of the post (37), e.g. by welding, screw-fixing or push-fit connectors, and a number of legs (42) which radiate from the central section (41) and are connected at their free ends, e.g. by welding, to the cathode (35). Usually the legs (42) are arranged so that the current supply via the post (37) is distributed to a number of equispaced points surrounding the post (37).

(19) In practice, during the production of the electrode structures (10, 30), the spiders (19, 39) may be welded or otherwise connected to the electrodes (15, 35) and the spiders may then be subsequently welded or otherwise secured to the posts (17, 37). This arrangement facilitates replacement or repair of the anode/cathode plates or renewal/replacement of any electrocatalytically-active coating thereon.

(20) Also shown in FIG. 2 are baffles (23, 43) which may serve to partition, respectively, each anode compartment and each cathode compartment into two communicating zones to provide liquor recirculation as discussed further below. The provision of baffles in either compartment is optional, but it is particularly preferred that baffles are provided in the anode compartment. Without wishing to be bound by theory it is believed that recirculation in the anode compartment is useful in providing increased rates of electrolysis, for example by facilitating operation at higher current density.

(21) The baffles (23, 43) may be mounted on the electrically conductive posts (17, 37). Each of the posts may be provided with a shoulder (24, 44) to facilitate installation and accurate location of the baffles.

(22) Also shown in FIG. 2 are a cross-member (25) in the external outlet header (16) of the anode and a cross-member (45) in the internal outlet header (36) of the cathode.

(23) FIGS. 3A and 3B show, respectively, examples of suitable spiders for use in the anode and cathode structures.

(24) With respect to FIG. 3A the spider comprises a disc-shaped central section (21) and 4 legs (22) which radiate from the central section (21). The legs (22) radiate symmetrically so that in use the current supply is distributed to a number of equispaced points.

(25) Especially when intended for use in the electrolysis of alkali metal halides, the anode spiders are fabricated from a valve metal or alloy thereof.

(26) With respect to FIG. 3B the spider comprises a disc-shaped central section (41) and 4 legs (42) which radiate from the central section (41). The legs (42) radiate symmetrically so that in use the current supply is distributed to a number of equispaced points.

(27) Especially when intended for use in the electrolysis of alkali metal halides, the cathode spiders may be may be fabricated from materials such as stainless steel, nickel or copper.

(28) As shown, the legs (42) of the cathode spider are longer and configured to be relatively springy, whilst the legs (22) of the anode spider are shorter and more rigid.

(29) FIGS. 4A and 4B show respectively close-ups of the preferred structures of the cross-members (25) and (45). The preferred structure of the cross-member (25) in the external outlet header (16) is in the form of a ladder type arrangement, whilst the preferred structure of the cross-members (45) in the internal outlet header (36) is in the form of plates with round holes. As shown in FIG. 4B, there may be more than one cross-member (45) in the outlet header (36). Although only a single cross-member (25) is shown in FIGS. 1 and 2 there may also be more than one cross-member in the outlet header (16)

(30) FIG. 5 shows an anode structure (10) more detail, showing inwardly projecting frusto-spherical projections (13) and a tapered external outlet header (16). FIG. 5 also exemplifies the locations for the measurements of the A.sub.A and L.sub.A.

(31) FIG. 6 shows a cross-section of the bottom part of a bipolar electrode unit. As with the Figures above the same numbering is used as for the corresponding features already described. In this Figure the anode structure is provided with an anode inlet tube (26) whilst the cathode structure is provided with a cathode inlet tube (46). Ports (not shown) are provided in the respective inlet tubes for discharge of liquor into the respective electrolysis compartments, and are preferably formed such that liquor discharged therefrom is directed towards the back of the pans behind the baffles (23, 43) to aid mixing. The baffles (23, 43) extend vertically within the respective anode and cathode compartments from the lower end of the electrode structure to the upper ends thereof and form two channels within each electrode structure which communicate at least adjacent the top and bottom of the structure.

(32) In a third aspect the present invention provides a modular or filter press electrolyser comprising a plurality of electrode assemblies according to the first and/or second aspects.

(33) For example, the third aspect of the present invention may provide a filter press electrolyser comprising a plurality of connected bipolar electrode units, adjacent bipolar electrode units being connected via a separator and sealing means between flanges on the adjacent units. The separator and sealing means are preferably as described between electrode structures when configured as an electrode module in the first aspect.

(34) A bipolar electrode unit comprises an anode structure and a cathode structure which are electrically connected to each other. Preferably, in particular using the preferred electrode structures comprising a pan with a dished recess, the recessed dish of the anode pan and the recessed dish of the cathode pan are electrically joined, preferably at the apices of the projections.

(35) Electrical conductivity may be achieved by the use of interconnectors or by close contact between the electrode structures. Electrical conductivity may be enhanced by the provision of conductivity-enhancing materials or conductivity-enhancing devices on the outer surface of the pans. As examples of conductivity-enhancing materials may be mentioned inter alia conductive carbon foams, conductive greases and coatings of a high-conductivity metal, e.g. silver or gold.

(36) Preferably the anode structure and cathode structure in a bipolar electrode unit are electrically connected via welding, explosion bonding or a screw connection.

(37) Alternatively, the third aspect of the present invention may provide a modular electrolyser. A modular electrolyser comprises a plurality of connected electrode modules. In this case the electrode modules may be connected to each other by providing suitable electrical connections between adjacent modules.

(38) For example, the recessed dish of the anode pan and the recessed dish of the cathode pan in adjacent modules are electrically joined, preferably at the apices of the projections.

(39) Electrical conductivity may be achieved by the use of interconnectors or by close contact between the electrode structures. Electrical conductivity may be enhanced by the provision of conductivity-enhancing materials or conductivity-enhancing devices on the outer surface of the pans. As examples of conductivity-enhancing materials may be mentioned inter alia conductive carbon foams, conductive greases and coatings of a high-conductivity metal. e.g. silver or gold.

(40) When connecting adjacent electrode modules together connections via welding, explosion bonding or a screw connection are not preferred. Instead connections are preferred which are formed by close physical contact between the adjacent electrode structures.

(41) Electroconductivity-enhancing devices which can enhance the contact include electroconductive bimetallic contact strips, discs or plates, electroconductive metal devices, such as washers, or electroconductive metal devices adapted to (a) abrade or pierce the surface of the pans by cutting or biting through any electrically-insulating coating thereon. e.g. an oxide layer, and (b) at least inhibit formation of an insulating layer between the device and the surface of the pan (which may be referred to as an abrasion device).

(42) Such devices are described further in U.S. Pat. No. 6,761,808.

(43) The number of anodes and cathodes (or modules or bipolar units) may be chosen by the skilled man in the light of inter alia the required total production, available power and voltage and certain constraints known to the skilled man. Typically, however, a modular or filter press electrolyser according to the third aspect of the present invention comprises 5-300 assemblies i.e. 5 to 300 anode electrode structures and the same number of cathode electrode structures.

(44) In a fourth aspect there is provided a process for the electrolysis of an alkali metal halide which comprises subjecting an alkali metal halide to electrolysis in a modular or filter press electrolyser according to the third aspect.

(45) The modular or filter press electrolyser according to the fourth aspect of the present invention may generally be operated according to known methods. For example, it is typically operated at pressures between 50 and 600 kPa (0.5 and 6 bar) absolute pressure, preferably between 50 and 180 kPa (500 and 1800 mbar).

(46) Liquid to be electrolysed is fed to the inlet-tubes in each electrode structure. For example, the inlet-tubes allow caustic to be charged to the cathode structure and brine to be charged to the anode structure. Products, namely chlorine and depleted brine solution from the anode structure and hydrogen and caustic from the cathode structure, are recovered from the respective headers.

(47) The electrolysis may be operated at high current density, i.e. >6 kA/m.sup.2.

(48) The preferred features of the electrode assemblies/electrolyser used for the fourth aspect are generally as described above.

(49) A particular advantage of an electrode assembly where the outlet header on the anode structure has a reduced volume, V.sub.C, and/or a V.sub.A/(A.sub.AL.sub.A) of less than 1 is that higher chlorine production can be obtained per unit volume of outlet header on the anode structure in an electrolyser.

(50) Thus, in a fifth aspect, the present invention provides a process for the electrolysis of an alkali metal halide which comprises subjecting an alkali metal halide to electrolysis in a modular or filter press electrolyser which electrolyser comprises i) a plurality of anode electrode structures having anode outlet headers, the anode outlet headers having an internal volume, V.sub.A cm.sup.3, ii) a plurality of cathode electrode structures, having cathode outlet headers, the cathode outlet headers having an internal volume, V.sub.C, cm.sup.3,
wherein the process is operated at an production rate per anode electrode assembly of W.sub.A, kg Cl.sub.2/hr, wherein W.sub.A/V.sub.A is greater than 0.006 kg Cl.sub.2/hr cm.sup.3.

(51) It is particularly preferred in this fifth aspect that i) the anode outlet headers have an internal volume, V.sub.A cm.sup.3, an internal cross sectional area at the exit end of the header of A.sub.A cm.sup.2 and an internal length L.sub.A cm, and ii) the cathode outlet headers have an internal volume. V.sub.C cm.sup.3, an internal cross sectional area at the exit end of the header of A.sub.C cm.sup.2 and an internal length L.sub.C cm.
and wherein one or both of the ratios V.sub.A/(A.sub.AL.sub.A) and V.sub.C/(A.sub.CL.sub.C) are less than 1, and most preferably that at least the anode outlet headers have ratios V.sub.A/(A.sub.AL.sub.A) less than 1.

(52) In relation to the fifth aspect of the present invention it should be noted that all anode electrode structures in an electrolyser are usually identical and all cathode electrode structures in an electrolyser are usually identical.

(53) In such a scenario V.sub.A, A.sub.A and L.sub.A are the same for all anode electrode structures and V.sub.C.

(54) A.sub.C and L.sub.C are the same for all cathode electrode structures. The requirement for W.sub.A/V.sub.A and V.sub.A/(A.sub.AL.sub.A) should be met by all anodes and/or the requirement for V.sub.C/(A.sub.CL.sub.C) should be met by all cathodes.

(55) However, if it were the case that one or more anode electrode structures are provided which have different outlet header dimensions than others present then V.sub.A, L.sub.A, A.sub.A and W.sub.A should be taken for the anode outlet headers with the lowest volume among those present, and W.sub.A/V.sub.A greater than 0.006 kg Cl.sub.2/hr cm.sup.3 and V.sub.A//(A.sub.AL.sub.A) less than 1 need be met by these anodes only.

(56) Preferably at least 80% by number of the anode electrode structures have the same V.sub.A, L.sub.A, A.sub.A, and most preferably all anode outlet headers have the same V.sub.A, L.sub.A, A.sub.A.

(57) Similarly, if it were the case that one or more cathode electrode structures are provided which have different outlet header dimensions than others present then V.sub.C, L.sub.C and A.sub.C should be taken as required for the cathode outlet headers with the lowest volume among those present.

(58) In those cases where the electrolyser contains cathodes with V.sub.C/(A.sub.CL.sub.C) less than 1, preferably at least 80% by number of the cathode electrode structures have the same V.sub.C, L.sub.C and A.sub.C, and V.sub.C/(A.sub.CL.sub.C) less than 1 need be met by these cathodes only. Most preferably all anode outlet headers have the same V.sub.C, L.sub.C and A.sub.C.

(59) In this fifth aspect preferably W.sub.A/V.sub.A at least 0.008 kg Cl.sub.2/hr cm.sup.3, such as at least 0.010 kg Cl.sub.2/hr cm.sup.3. There is no specific upper limit but W.sub.A/V.sub.A may be generally up to 0.020 kg Cl.sub.2/hr cm.sup.3, such as up to 0.015 kg Cl.sub.2/hr cm.sup.3.

(60) It should be noted that, once an electrolyser is built, the value of V.sub.A is fixed. However, electrolysers can be operated at varying production rates, and hence W.sub.A/V.sub.A can vary during operation depending on the total production rate.

(61) Typically production rate increases with increased current density. However, electrolysers and their membrane separators are designed to operate at a particular maximum current density and significantly increasing production rate by increasing current density is not possible above a certain limit. Thus, the values of W.sub.A/V.sub.A provided by the present invention are considered to be higher than those obtainable whilst operating stably in current commercial electrolysers.

(62) The typical current density at which modern electrolysers are routinely operated is 4 to 7 kA/m.sup.2.

(63) The current density when operating the process according to the present invention is typically similar to this range, and hence is preferably at least 4 kA/m.sup.2, especially at least 6 kA/m.sup.2, The current density is preferably less than 7 kA/m.sup.2.

(64) W.sub.A in the fifth aspect of the present invention is the production rate from the individual anode under consideration. W.sub.A is typically 4 to 40 kg Cl.sub.2/hr, and preferably 20 to 40 kg Cl.sub.2/hr. Alternatively, or additionally, W.sub.A is above 12 kg Cl.sub.2/hr at a current density of 4 KA/m.sup.2 and above 21 kg Cl.sub.2/hr at a current density of 7 KA/m.sup.2. W.sub.A may be determined by methods known to those skilled in the art, for example by measuring the current flow through the electrolyser over a given time period and the current efficiency of the electrolyser over the same period, for example using the sulphate key technique, using these numbers to calculate the mass of chlorine in kg produced in the entire electrolyser over that time period, dividing the number obtained by the number of electrode assemblies in the electrolyser and then dividing by the length of the measurement period in hours to produce the measured chlorine production per electrode assembly in kg Cl.sub.2/hr.

(65) In one embodiment the electrolyser of the third to fifth aspects of the present invention may also be characterised that it has W.sub.A/V.sub.A of at least 0.006, preferably of at least 0.010, when operated at a current density of 7 kA/m.sup.2 and W.sub.A/V.sub.A of at least 0.003, preferably of at least 0.005, when operated at a current density of 4 kA/m.sup.2. For avoidance of doubt, this does not mean that the electrolyser must be operated at all times at one of these current densities, but simply that such minimum values of W.sub.A/V.sub.A are obtained if it is operated at these current densities.

(66) The combination of high current density and high anode production rate per unit volume of outlet header on the anode structure is typically achieved by reducing the total volume, V.sub.A, of the outlet header compared to current commercial electrolysers.

(67) In a preferred embodiment of the third to fifth aspects of the present invention the modular or filter press electrolyser comprises a plurality of anode electrode structures having external anode outlet headers, and a plurality of cathode electrode structures, having internal cathode outlet headers or vice versa.

(68) Particularly preferred however, is a modular or filter press electrolyser which electrolyser comprises a plurality of anode electrode structures having external anode outlet headers, and a plurality of cathode electrode structures having internal cathode outlet headers.

(69) In a yet further aspect the present invention provides an electrode structure comprising: i) a pan with a dished recess and a flange which can interact with a flange on a second electrode structure to hold a separator in between the two and the dished recess further having a plurality of inwardly or outwardly projecting projections which can mate with corresponding projections on a third electrode structure in an electrode unit or in a modular electrolyser, ii) an inlet for liquid to be electrolysed and iii) an outlet header for evolved gas and spent liquid,
wherein the outlet header is an external outlet header in which V.sub.E/(A.sub.EL.sub.E) is less than 1, where V.sub.E is the internal volume of the external outlet header in cm.sup.3, A.sub.E is the internal cross sectional area at the exit end of the header L.sub.E is the internal length, and preferably wherein the outlet header is a tapered external outlet header which increases in cross-section area in the direction of gas/liquid flow towards the exit ports.

(70) The features of the electrode structure in this aspect may be generally as described for the corresponding individual electrode structure with external header in the first aspect.

(71) For example, the preferred electrode structure comprises a dished recess which is provided with a plurality of inwardly projecting projections.

(72) Similarly, the external outlet header in this aspect preferably comprises one or more internal cross members located along part of or all of the horizontal length of and attached internally to the sides of the header.

(73) As a further example, the depth of the external outlet header may exceed the depth of the claimed electrode structure. In particular, when connected to said second and/or third electrode structure in an electrode module, electrode unit or modular electrolyser, the external outlet header of the claimed electrode structure can occupy space which is vertically above the second and/or third electrode structures.

(74) In a most preferred embodiment of this aspect the flange is around the periphery of the dished recess and being for supporting a gasket capable of sealing the separator between the electrode surface of the claimed electrode structure and the electrode surface of the second electrode structure such that the electrode surfaces are substantially parallel to and face each other, but are spaced apart from each other by the separator and are hermetically sealed to the separator. Further, the electrode structure comprises an electrode spaced from the pan hut connected to the pan by electrically conductive pathways between the pan and the electrode with the proviso that where the claimed electrode structure is provided with a plurality of inwardly projecting projections the electrode may be directly electrically connected to the pan.

(75) The electrode structure in this aspect is preferably an anode structure. In particular, as already described the separator is most prone to damage caused by the formation of a gas space adjacent the separator on the anode side in the upper region of an electrolysis compartment, and also because the separation of formed chlorine from spent brine is the most problematic. The external outlet header located above the electrolysis compartment allows to minimise these problems because its location moves the gas disengagement area away from the separator and also provides increased flexibility to design its shape and size to improve the separation.

EXAMPLE 1

(76) A bipolar electrolyser was formed of 5 modules of the general structure shown in FIG. 2, with an external anode outlet header and an internal cathode outlet header. The anode structures were themselves as shown in FIG. 5 with a tapered external outlet header. The anode outlet header extends across the full width of the anode to have a length, L.sub.A of 244 cm, and depth of 1.9 cm, but with an increasing height, thereby leading to an increased cross-sectional area, A.sub.A, at the end of 18.8 cm.sup.2. The anode outlet header had a volume, V.sub.A, of 2294 cm.sup.3.

(77) The cathode structure has an internal outlet header which also extends across the full width of the cathode to have a length (L.sub.C of 244 cm), but has a constant rectangular cross-sectional area A.sub.C of 11.6 cm.sup.2 and a volume, V.sub.C of 2030 cm.sup.3. The ratio V.sub.A/(A.sub.AL.sub.A) in this electrolyser was 0.5 and V.sub.A was 264 cm.sup.3 lower than V.sub.C

(78) Electrolysis was performed over an operating life of 4 years using Nafion 2030 membrane from The Chemours Company LLC (a subsidiary of E. I. DuPont de Nemours & Company) at an inlet sodium hydroxide concentration of 30%, and exit sodium hydroxide concentration of 32%, an inlet brine concentration of 300 g NaCl/litre and an exit brine concentration of 220 g/NaCl/litre, an average sodium hydroxide exit temperature of 87 C. and an operating pressure of 250 mbarg hydrogen and 235 mbarg chlorine. Current efficiency over the 4 year period ranged from 97% at first start-up to 95.5% after 4 years with an average of 96.5%. The average operating current density over the 4 year period was approximately 5 kA/m.sup.2 with the maximum 6 kA/m.sup.2. The average rate of evolution of chlorine gas from each anode over the entire 4 year period of operation was 18.4 kg/hr with the maximum rate being 22.3 kg/hr.

(79) Operation was performed without any problems of separation in either the anode or cathode outlet headers as indicated by the stability of the operating voltage and current efficiency of the electrolyser, which was identical to a comparison electrolyser with external, non tapered, anode and cathode headers (see below). Electrodes and membranes were removed from the test electrolyser for examination after 4 years on load and showed no signs membrane blistering or electrode coating damage which might have otherwise been indicative of inadequate internal circulation caused by poor gas separation in the headers.

COMPARATIVE EXAMPLE

(80) An electrolyser was formed of 138 modules of the general structure shown in U.S. Pat. No. 6,761,808, having both an external anode outlet header and an external cathode outlet header, and in which neither was tapered.

(81) The cathode structure had an external outlet header which also extended across the full width of the cathode (L.sub.C=244 cm), but had a constant rectangular cross-sectional area, A.sub.C of 18.8 cm.sup.2 and a volume, V.sub.C of 4587 cm.sup.3.

(82) The anode structure also had an external outlet header which also extended across the full width of the anode (L.sub.A=244 cm) and had a constant rectangular cross-sectional area, A.sub.A of 18.8 cm.sup.2 and a volume, V.sub.A of 4587 cm.sup.3, the ratio V.sub.A/(A.sub.AL.sub.A) in this electrolyser was 1.0 and V.sub.A was identical to V.sub.C.

(83) Electrolysis was performed over an operating life of 4 years using Nation 2030 membrane from The Chemours Company LLC (a subsidiary of E. I. DuPont de Nemours & Company) at an inlet sodium hydroxide concentration of 30%, and exit sodium hydroxide concentration of 32%, an inlet brine concentration of 300 g NaCl/litre and an exit brine concentration of 220 g/NaCl/litre, an average sodium hydroxide exit temperature of 87 C. and an operating pressure of 250 mbarg hydrogen and 235 mbarg chlorine. Current efficiency over the 4 year period ranged from 97% at first start-up to 95.5% after 4 years with an average of 96.5%. The average operating current density over the 4 year period was approximately 5 kA/m.sup.2 with the maximum 6 kA/m.sup.2. The average rate of evolution of chlorine gas from each anode over the entire 4 year period of operation was 18.4 kg/hr with the maximum rate being 22.3 kg/hr.

(84) Operation was performed without any problems of separation in either the anode or cathode outlet headers. As indicated by the stability of the operating voltage and current efficiency of the electrolyser. The values for the operating voltage and current efficiency of the electrolyser measured over time over me were virtually identical to those measured in example 1 above. Electrodes and membranes were removed from the test electrolyser for examination after 4 years on load and showed no signs of membrane blistering or electrode coating damage which might have otherwise been indicative of inadequate internal circulation caused by poor gas separation in the headers.