AU2011333018C1

AU2011333018C1 - Seawater electrolysis system and seawater electrolysis method

Info

Publication number: AU2011333018C1
Application number: AU2011333018A
Authority: AU
Inventors: Takashi Ike; Tatsuya Matsumura; Hiroshi Mizutani; Kenji Nakamura; Hiroyuki Takanami
Original assignee: Mitsubishi Heavy Industries Environmental and Chemical Engineering Co Ltd
Current assignee: Mitsubishi Heavy Industries Environmental and Chemical Engineering Co Ltd
Priority date: 2010-11-22
Filing date: 2011-11-17
Publication date: 2014-09-25
Anticipated expiration: 2031-11-17
Also published as: KR20150116914A; TWI504784B; CN103201412A; CN103201412B; KR101585304B1; KR20130079569A; BR112013010763A2; AU2011333018B2; TW201235512A; KR101624095B1; CN105239090A; AU2011333018A1; WO2012070468A1; CL2013001175A1; MY164970A; CN105239090B; BR112013010763B1

Abstract

This seawater electrolysis system is provided with: an electrode (30) which comprises a negative electrode (C) and a positive electrode (A) that is formed of titanium coated with a coating material that contains iridium oxide; an electrolytic cell main body (20) that contains the positive electrode (A) and the negative electrode (C); and a power supply device (40) that passes current between the positive electrode (A) and the negative electrode (C) so that the current density on the electrode surfaces is 20 A/dm

Description

1 Specification SEAWATER ELECTROLYSIS SYSTEM AND SEAWATER ELECTROLYSIS METHOD [Technical Field] [0001] The present invention relates to a seawater electrolysis system which has a seawater electrolysis device for generating hypochlorous acid by carrying out electrolysis of seawater and also relates to a seawater electrolysis method. [Background Art] [0002] Thermal power plants, nuclear power plants, seawater desalination plants, chemical plants and others which use a great amount of seawater include parts in contact with seawater. Conventionally, deposition (adhesion) and proliferation of marine algae and seashells at these parts, for example, intakes of seawater, piping, condensers and various types of coolers have caused serious problems. In order to solve the above-described problems, there has been proposed a seawater electrolysis device which carries out electrolysis of natural seawater to generate hypochlorous acid and fills the thus generated hypochlorous acid into an intake of seawater, thereby suppressing deposition of marine growth (refer to Patent Document 1, for example). [0003] The seawater electrolysis device has such a structure that anodes and cathodes as electrodes are arranged inside a cabinet-shaped electrolysis vessel main body to distribute seawater inside the electrolysis vessel main body. Since seawater contains chloride ions and hydroxide ions, electric current passing between the anodes and the cathodes 2 produces chlorine at the anodes and sodium hydroxide at the cathodes, Then, chlorine reacts with sodium hydroxide to generate hypochlorous acid which is effective in suppressing adhesion of marine growth. [0004] In general, an electrode, particularly an anode, which is arranged inside an electrolysis vessel of the seawater electrolysis device are constituted using a titanium base plate which is coated with a platinum-dominant composite metal (a platinum-dominant coating material) (refer to Patent Document 2, for example). Although not yet practically available as a seawater electrolysis device, such a suggestion has been made that an iridium oxide-dominant composite metal, that is, an iridium oxide-dominant coating material be used as a coating material for an anode for electrolysis (refer to Patent Document 3, for example). There is also known a seawater electrolysis device in which concentrated water which is high in salinity concentration and discharged from seawater concentration equipment such as a seawater desalination plant is used as treated water. This seawater electrolysis device is that in which hypochlorous acid contained in electrolyzed water produced by carrying out electrolysis of the concentrated water is increased in concentration to decrease the consumption of electricity, thereby enhancing the efficiency of the seawater electrolysis device and also downsizing the seawater electrolysis device (refer to Patent Document 4, for example). [Prior Art Documents] [Patent Documents] [0005] [Patent Document 1] Japanese Patent No. 3389082 [Patent Document 2] Japanese Published Unexamined Patent Application No. 2001-262388 [Patent Document 3] Japanese Published Unexamined Patent Application No. H8-85894 [Patent Document 4] Japanese Published Unexamined Patent Application No. H9-294986 3 [0006] At an electrode coated with a platinum-dominant coating material, due to influences from oxygen generated in the vicinity of an anode and scales (calcium, magnesium, or the like) deposited in the vicinity of a cathode during electrolysis, early erosion of the electrode takes place. Therefore, it is necessary to wash and replace the electrode frequently, resulting in increased maintenance costs. Further, there is a tendency that chlorine is generated more efficiently with an increase in electric current density on the surface of the electrode. This tendency is also found in a case where concentrated seawater is introduced into the seawater electrolysis device to generate hypochlorous acid. However, oxygen generated in the vicinity of the anode and scales deposited in the vicinity of the cathode are also increased in amount with an increase in electric current density, resulting in rapid wastage of electrode. As a result, in the electrode coated with a platinum-dominant coating material, it is impossible to increase the electric current density on the surface of the electrode. Thus, it has been considered technical common sense that the maximum value of electric current density is kept at approximately 2 15A/dm , for example. As described above, since it is necessary to keep low the electric current density during electrolysis, generation of hypochlorous acid from seawater in a sufficient amount requires arrangement of many electrodes, resulting in increased manufacturing costs and an increased size of the device. [0007] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.

4 Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. [Summary of the Invention] [0008] The inventors have diligently conducted research on an electrode for the seawater electrolysis device and found that when an electric current passes through an anode coated with an iridium oxide-dominant coating material at an electric current density 2 exceeding 15A/dm , it is effective in enhancing the resistance of the electrode and also in suppressing the lowering in chlorine generating efficiency in contrast to the technical common sense of conventional electrodes coated with a platinum-dominant coating material. According to the present disclosure, there is provided a seawater electrolysis device comprising an anode made of titanium coated with an iridium oxide-containing coating material containing no platinum, a cathode, an electrolysis vessel main body which houses the anode and the cathode, and a power supply unit which passes an electric current between the anode and the cathode, wherein the seawater electrolysis device passes an electric current between the anode and the cathode in such a manner that an electric current density on the surface of the anode and that of the cathode is included in a range of 20A/dm2 or more and 40A/dm2 or less to electrolyze the seawater inside the electrolysis vessel main body. Also disclosed herein is a seawater electrolysis method in which the above mentioned seawater electrolysis device is used, wherein, in the method, seawater is introduced into the electrolysis vessel main body, an electric current is passed between the anode and the cathode in such a manner that an electric current density on the surface of the anode and that of the cathode is included in a range of 20A/dm2 or more and 40A/dm2 or less to electrolyze seawater inside the electrolysis vessel main body.

5 [0009] In the present invention, the electric current density on the surface of the electrode is 20A/dm2 or more which is greater than a conventional electric current density 2 of 15A/dm . Therefore, in an embodiment, hydrogen gas is generated at the cathode during electrolysis in a greater amount than a conventional case. An effect of washing the electrode is exerted due to the great amount of hydrogen gas, thereby making it possible to prevent deposition of manganese scales on the anode and deposition of scales of calcium, magnesium or the like, on the cathode. Although oxygen generated in the vicinity of the anode increases in amount, an iridium oxide is sufficiently resistant to oxygen, thus making it possible to prevent the electrode from eroding away by the oxygen. [0010] In an embodiment of the present invention, the electric current density on the surface of the anode and that of the cathode between which an electric current is passed by the power supply unit is included in a range of 20A/dm2 or more and 40A/dm2 or less. The electric current density may be preferably be included in a range of 20A/dm2 or more and 30A/dm2 or less. Where the electric current density is excessively great, for example, in excess of 2 40A/dm , amount of scales deposited on the anode and the cathode exceeds the amount where the washing effect by the hydrogen is effective. In contrast, in the present invention, an upper limit value of the electric current density is set at 40A/dm 2 and 2 preferably at 30A/dm . Therefore, in an embodiment, it is possible to effectively develop the washing effect by hydrogen and also effectively prevent the deposition of scales on the anode and the cathode. [0011] The seawater electrolysis device may be further (additionally) provided with one or a plurality of electrolysis vessel main bodies, one or a plurality of connecting pipes each connecting an outlet port of seawater of one of the electrolysis vessel main body and an inlet port of seawater of the other one of the electrolysis vessel main body, and one or 6 a plurality of degassing units for removing a gas inside each of the connecting pipes. With an increase in electric current density, a liquid-gas ratio lowers due to hydrogen generated at the cathode, thus resulting in a lowering in chlorine generating efficiency. In contrast, where the gas, in particular, hydrogen gas is removed by the degassing unit installed at the connecting pipe, it is possible to keep the interior of the electrolysis vessel at a predetermined liquid-gas ratio or lower and also prevent the lowering of efficiency. [0012] Disclosed herein is a seawater electrolysis system comprising: a seawater electrolysis device having an anode which is made of titanium coated with an iridium oxide-containing coating material containing no platinum, a cathode, an electrolysis vessel main body which houses the anode and the cathode, and a power supply unit which passes an electric current between the anode and the cathode, and a concentrating unit for increasing the concentration of chloride ions contained in seawater to be introduced into the electrolysis vessel main body, wherein an electric current is passed between the anode and the cathode to electrolyze seawater inside the electrolysis vessel main body. Disclosed herein is a seawater electrolysis method which increases the concentration of chloride ions contained in seawater which is subjected to electrolysis, distributes the seawater increased in concentration of chloride ions inside the electrolysis vessel main body, and generates an electric current between the anode and the cathode to electrolyze the seawater inside the electrolysis vessel main body. [0013] In an embodiment, concentrated water which is increased in concentration of chloride ions and electric conductivity is introduced into the seawater electrolysis device. Further, since a coating material of the anode contains an iridium oxide, it is possible to set a electric current density on the surface of the electrode at high value, thereby increasing the concentration of hypochlorous acid contained in the produced electrolyzed water. That is, by generation of hypochlorous acid in an increased amount per unit area of the electrode, the electrode can be decreased in area to downsize the device.

7 [0014] According to an embodiment disclosed herein, the electric current density on the surface of the anode and that of the cathode between which an electric current is passed by the power supply unit is included in a range of 20A/dm2 or more and 60A/dm2 or less. The electric current density may be preferably included in a range of 20A/dm2 or more and 50A/dm2 or less. Where the electric current density is excessively great, for example, in excess of 2 60A/dm , scales are generated on the anode and the cathode in such an amount that exceeds amount where the washing effect by hydrogen is effective. In contrast, in embodiments disclosed herein, an upper limit value of the electric current density is set at 2 2 60A/dm and preferably at 50A/dm . Thus, it is possible to effectively exert the washing effect by hydrogen and also effectively prevent the deposition of scales on the anode and the cathode. [0015] According to the present disclosure, the seawater electrolysis system may be additionally provided with a hydrogen separation unit which separates hydrogen gas generated at the cathode from the seawater after electrolysis. It is, thereby, possible to more efficiently exert the washing effect by hydrogen gas and also effectively prevent the deposition of scales on the anode and the cathode. [0016] In the seawater electrolysis device according to the present disclosure, a tantalum oxide may be added to the coating material. Tantalum which is highly resistant to oxygen is added to the coating material, thereby making it possible to improve resistance to oxygen generated at the anode and more effectively prevent abnormal erosion of the electrode. [0017] In the seawater electrolysis device according to the present disclosure, it is acceptable that an electrode includes a plurality of double-pole electrode plates in which one portion thereof in a distributing direction of the seawater is given as the anode and the 8 other portion thereof is given as the cathode, a plurality of electrode groups in which the double-pole electrode plates are arrayed with an interval in the distributing direction are arranged so as to be parallel to each other, and the double-pole electrode plates in each of the electrode groups adjacently parallel to each other are arranged in such a manner that the anode is opposed to the cathode. [0018] As described above, the double-pole electrode plates, each of which has the anode and the cathode, are arranged in a concentrated manner, thus making it possible to downsize the device itself. Further, since each of the double-pole electrode plates is arranged along the distributing direction of seawater, there is no chance that distribution of seawater is prevented. It is, therefore, possible to keep a high flow speed of the seawater and also effectively prevent the deposition of scales on an electrode by the seawater. Still further, since the anode is opposed to the cathode in each of the electrode groups which are adjacently parallel to each other, an electric current is passed between the anode and the cathode, thus making it possible to effectively electrolyze the seawater that is distributed between electrodes. [0019] In the seawater electrolysis device according to the present disclosure, an interval between the double-pole electrode plates adjacent to each other in the distributing direction in each of the electrode groups may be eight times or more than an interval between the electrode groups which are adjacently parallel to each other. Where the interval between the double-pole electrode plates adjacent to each other in the distributing direction is small, there is generated an electric current which is distributed between the double-pole electrode plates, that is, a stray current which contributes less to electrolysis. The stray current becomes more apparent with an increase in electric current density on the surface of an electrode. In contrast, as described above, by keeping an appropriate interval between the double-pole electrode plates adjacent to each other in the distributing direction, generation of the stray current 9 can be suppressed to prevent a lowering in electrolysis efficiency of seawater. [0020] In the present disclosure, the seawater electrolysis device may be provided with a circulation flow path which mixes the seawater after electrolysis flowing out from an outlet port of the electrolysis vessel main body with the seawater before flowing into the electrolysis vessel main body from an inlet port. There are concerns that scales may deposit on the surface of an electrode with an increase in electric current density. However, where the seawater after electrolysis is mixed through the circulation flow path with the seawater before electrolysis, it is possible to prevent the deposition of scales on the surface of the electrode since seed crystallization effects can be obtained by scale compositions contained in seawater which has passed through the electrolysis vessel of the seawater electrolysis device. [0021] According to the present disclosure, it is possible to enhance the resistance of an electrode and also suppress the lowering in chlorine generating efficiency by preventing the deposition of scales on the electrode. [Brief Description of the Drawings] [0022] FIG. 1 is a diagram which shows a first embodiment of a seawater electrolysis system according to the present invention FIG. 2 is a longitudinal cross sectional view which shows a seawater electrolysis device of the first embodiment. FIG. 3 is an enlarged view which shows major parts of the seawater electrolysis device. FIG. 4 is a graph which explains a constant current control curve of a constant current control circuit on a power supply unit. FIG. 5 is a schematic view which shows a second embodiment of a seawater electrolysis system according to the present invention.

10 FIG. 6 is a schematic view which shows a modified example of the second embodiment. FIG. 7 is a schematic view which shows a third embodiment of a seawater electrolysis system according to the present invention. FIG. 8 is a schematic view which shows an outline of a hydrogen separator in the third embodiment. FIG. 9 is a graph which shows results of a test on determination of chlorine generating efficiency. FIG. 10 is a graph which shows results of a test on determination of erosion of electrodes. [Mode for Carrying Out the Invention] [0023] Hereinafter, an explanation will be made for the first embodiment of the present invention by referring to FIG. I to FIG. 4. A seawater electrolysis system 1 OA of the first embodiment is such a system that seawater is taken from an intake channel 1 through which the seawater is distributed, electrolysis of the seawater is carried out by using a seawater electrolysis device 10 and, thereafter, the thus treated seawater is filled into the intake channel 1. The seawater electrolysis system 1 OOA is, as shown in FIG. 1, provided with the seawater electrolysis device 10, a storage tank 50, an intake portion 60, and a filling portion 70. Seawater W which has been subjected to electrolysis by the seawater electrolysis device 10 is stored in the storage tank 50. The intake portion 60 introduces the seawater W into the seawater electrolysis device 10 from the intake channel 1. Next, the filling portion 70 fills the seawater W in the storage tank 50 into the intake channel 1. [0024] 11 As shown in FIG. 2, the seawater electrolysis device 10 includes an electrolysis vessel main body 20, an electrode supporting box 26, terminal blocks 28, 29, and a plurality of electrodes 30. The electrolysis vessel main body 20 is provided with a substantially tubular outer casing 21, both ends of which are opened. There is installed at one end of the outer casing 21 an upstream lid portion 22 for blocking the opening at the one end. There is also installed at the other end of the outer casing 21 a downstream lid portion 24 for blocking the opening at the other end. The electrolysis vessel main body 20 is secured for predetermined pressure strength by the outer casing 21, the upstream lid portion 22 and the downstream lid portion 24. [0025] Further, the upstream lid portion 22 is provided with an inlet port 23 communicatively connecting through the inside and outside of the electrolysis vessel main body 20, and the downstream lid portion 24 is provided with an outlet port 25 communicatively connecting through the inside and outside of the electrolysis vessel main body 20. That is, in the electrolysis vessel main body 20, the seawater W is introduced from the inlet port 23 of the upstream lid portion 22 and distributed in one direction inside the outer casing 21 from the inlet port 23 to the outlet port 25. Thereafter, the seawater W flows from the outlet port 25 outside the electrolysis vessel main body 20. Hereinafter, the side of the inlet port 23 inside the electrolysis vessel main body 20 is referred to as upstream, while the side of the outlet port 25 is referred to as downstream. [0026] The electrode supporting box 26 is a tubular member constituted with an electricity insulating material such as plastic and housed inside the electrolysis vessel main body 20 so as to extend in the distributing direction of the seawater W. The electrode supporting box 26 is fixed to the upstream lid portion 22 and the downstream lid portion 24 by way of a plurality of fixing pieces 27. Further, a plurality of supporting 12 bars 26a for supporting the electrodes 30 are installed inside the electrode supporting box 26. The terminal blocks 28, 29 have functions to supply an electric current to the electrodes 30 supported inside the electrode supporting box 26 from outside the electrolysis vessel main body 20, and they are arranged in pairs on both sides of the electrode supporting box 26. [0027] The electrode 30 is formed in the shape of a plate, and a plurality of electrodes 30 are fixed and supported in array on the supporting bar 26a in the electrode supporting box 26. In the present embodiment, there are used as the electrodes 30 three types of plates, that is, a double-pole electrode plate 31, an anode plate 32 and a cathode plate 33. [0028] The double-pole electrode plate 31 is structured in such a manner that a titanium base plate as an electrode plate is made up of two portions, one of which is given as an anode A and the other of which is given as a cathode K. That is, a half portion on one side of the double-pole electrode plate 31 is constituted as the anode A coated with an iridium oxide-containing coating material (iridium oxide-dominant coating material), while a half portion on the other side of the electrode plate 31 is constituted as the cathode K not coated with the iridium oxide-dominant coating material. [0029] Further, the anode plate 32 is structured in such a manner that the iridium oxide-dominant coating material is coated all over on the titanium base plate, and the anode plate 32 as a whole acts as the anode A during electrolysis. On the other hand, a titanium base plate which is not coated is adopted as the cathode plate 33, and the cathode plate 33 as a whole acts as the cathode K during electrolysis. [0030] 13 The amount of iridium oxide in mass ratio in the iridium oxide-dominant coating material is set at 50% or more, and preferably set in a range of from 60% to 70%. Thereby, it is possible to obtain favorable coating effects of the iridium oxide. Further, it is preferable that tantalum is added to the iridium oxide-dominant coating material. It is more preferable that the iridium oxide-dominant coating material contains no platinum. [0031] Here, an explanation will be made for arrangement of three different types of electrodes 30 inside the electrode supporting box 26. The double-pole electrode plate 31, the anode plate 32 and the cathode plate 33 are respectively fixed and supported on the supporting bars 26a inside the electrode supporting box 26. Of the above-described electrodes 30, as shown in FIG. 2 and FIG. 3, the plurality of double-pole electrode plates 31 are arrayed in such a manner that they extend along the distributing direction of the seawater W, while each anode A faces the seawater inlet side and each cathode K faces the seawater outlet side. Further, the double-pole electrode plates 31 are arrayed in series with an interval in the distributing direction, thereby constituting an electrode group M. Next, the plurality of electrode groups M are installed so as to be parallel to each other with an interval, that is, they are installed in a plural number parallel to each other. [0032] The electrode groups M which are adjacently parallel to each other are arranged so as to deviate only by one-half pitch from each double-pole electrode plate 31 relative in the distributing direction. Thereby, the double-pole electrode plates 31 in each of the electrode groups M which are adjacently parallel to each other are in a state that an anode A is opposed to a cathode K. Further, in the present embodiment, as shown in FIG. 3, it is preferable that an interval dl between the double-pole electrode plates 31 which are adjacent in the distributing direction in each of the electrode groups M is set to be 8 times or more an interval between the electrode groups M which are adjacently parallel to each 14 other, that is, an interval d2 between the double-pole electrode plates 31 which are adjacently parallel to each other. [0033] On the other hand, the plurality of anode plates 32 are arrayed parallel to each other along the distributing direction of seawater W in downstream side of the double-pole electrode plates 31, and the plurality of cathode plates 33 are arrayed parallel to each other along the distributing direction of the seawater W in upstream side of the double-pole electrode plates 31. A downstream end of each of the anode plates 32 is connected to the downstream terminal block 29, of the pair of terminal blocks 28, 29, while an upstream end of each of the anode plates 32 is opposed to the cathode K of each of the double-pole electrode plates 31 in a direction orthogonal to the distributing direction. In other words, the upstream end of the anode plate 32 and the cathode K of the double-pole electrode plate 31 are alternately arranged so as to overlap when viewed in the direction orthogonal to the distributing direction. Further, the upstream end of the cathode plate 33 is connected to the terminal block 28, of the pair of terminal blocks 28, 29. In addition, the downstream end of each of the cathode plates 33 is opposed to the anode A of each of the double-pole electrode plates 31 in the direction orthogonal to the distributing direction. In other words, the downstream end of each of the cathode plate 33 and the anode A of the double-pole electrode plate 31 are alternately arranged so as to overlap when viewed from the direction orthogonal to the distributing direction. [0034] The power supply unit 40 is a device for supplying an electric current to be used during electrolysis of seawater W and provided with a direct current power source 41 and a constant current control circuit 42. The direct current power source 41 is a power source for outputting direct electric power. In addition, alternating electric power output from an alternating current power source, for example, may be rectified into direct current and then output.

15 [0035] The constant current control circuit 42 is a circuit for outputting a direct current supplied from the direct current power source 41 as a constant current. Regardless of a change in electrical resistance across electric current passing sections, the constant current control circuit 42 is able to output a predetermined constant current to the electric current passing section. That is, when direct electric power is input from the direct current power source 41, as shown in FIG. 4, the constant current control circuit 42 controls a voltage value of the direct electric power in a range of deflection width AV, by which a desired electric current value on the constant current control curve is output as a constant current. In the above-described constant current control circuit 42, the anodes A are connected to the downstream terminal block 29 and the cathodes K are connected to the upstream terminal block 28 via a pair of lead wires 43, 44. Thereby, a constant current generated at the constant current control circuit 42 is passed across the electrodes 30 via the terminal blocks 28, 29. [0036] In this instance, in the power supply unit 40 of the present embodiment, the constant current control circuit 42 generates a constant current in such a manner that the electric current density on the surface of the electrode 30 is included in a range of from 20A/dm 2 to 40A/dm 2 and preferably in a range of from 20A/dm 2 to 30A/dm 2 . That is, the constant current is generated depending on a surface area of the electrode 30 inside the electrolysis vessel main body 20, and the constant current is supplied to the electrode 30, by which the electric current density on the surface of the electrode 30 is included in a range of from 20A/dm 2 to 40A/dm 2 and preferably in a range of from 20A/dm 2 to 30A/dm 2 . [0037] In an electrode coated with a conventional platinum-dominant composite metal (platinum -dominant coating material), oxygen and scales which accelerate erosion of the 16 electrode increase in amount, with an increase in electric current density. Therefore, a maximum value of the electric current density is set approximately at 15A/dm 2 . In contrast, in the present embodiment, electrolysis is carried out at an electric current density higher than the conventional electric current density. That is, electrolysis is carried out at an electric current density in a range of from 20A/dm 2 to 40A/dm 2 , and preferably from 20A/dm 2 to 30A/dm 2 . [00381 The storage tank 50 is a tank which temporarily stores the seawater W flowing out from the outlet port 25 of the electrolysis vessel main body 20 in the above-described seawater electrolysis device 10, The seawater W is introduced into the tank via an intermediate flow path 51 connected to the outlet port 25 of the electrolysis vessel main body 20. [0039] The intake portion 60 includes an intake flow path 61, a first pump 62, a first flow meter 64 and a first opening/closing control valve 63. The intake flow path 61 is a flow path which is connected at one end thereof to the intake channel 1 and at the other end thereof to the inlet port 23 of the electrolysis vessel main body 20 in the seawater electrolysis device 10. The first pump 62 is installed halfway along the intake flow path 61. The first pump 62 pumps up the seawater W in the intake channel I at a constant output, by which the seawater W is introduced into the inlet port 23. [0040] The first flow meter 64 is installed downstream to the intake flow path 61, detecting a flow rate Qi of the seawater W passing through the intake flow path 61. Further, the first opening/closing control valve 63 is a valve which is installed upstream to the first flow meter 64 on the intake flow path 61 and controlled for opening and closing on the basis of the flow rate Qi of the seawater W detected by the first flow meter 64.

17 By this constitution, the flow rate of the seawater W which is distributed through the flow path is adjusted depending on an area ratio of a seawater distributing region of the intake flow path 61 to that of the electrolysis vessel main body 20. Thus, it is possible to adjust the flow speed of the seawater W which is distributed inside the electrolysis vessel main body 20 at an arbitrary speed. [0041] In the seawater electrolysis device 10 of the present embodiment, it is preferable that the first opening/closing control valve 63 is controlled in such a manner that the flow speed of the seawater W which is distributed inside the electrolysis vessel main body 20 is at least 0.7 m/s (meter/second) or more. It is acceptable that the flow speed of the seawater W inside the electrolysis vessel main body 20 is adjusted not only by controlling the opening/closing of the first opening/closing control valve 63 but also adjusted, for example, by controlling the output of the first pump 62. [0042] The filling portion 70 includes a filling flow path 71, a second pump 72, a second opening/closing control valve 73, and a second flow meter 74. The filling flow path 71 is a flow path which is connected at one end thereof to the storage tank 50 and connected at the other end thereof to the intake channel 1. The second pump 72 is installed halfway along the filling flow path 71. The second pump 72 pumps up the seawater W inside the storage tank 50 at a constant output, by which the seawater W is introduced into the intake channel 1. [0043] The second flow meter 74 is installed downstream to the filling flow path 71, detecting a flow rate Q2 of the seawater W which passes through the filling flow path 71.

18 The second opening/closing control valve 73 is a valve which is installed upstream of the second flow meter 74 on the filling flow path 71 and controlled for opening and closing on the basis of the flow rate Q 2 of the seawater W detected by the second flow meter 74. By thus constitution, flow rate of the seawater W to be filled into the intake channel I is adjusted. It is acceptable that not only the second opening/closing control valve 73 is controlled for the opening/closing to adjust an amount of the seawater W filled into the intake channel 1 but also, for example, the second pump 72 is controlled for the output to adjust an amount of the seawater W filled into the intake channel 1. [0044] Next, an explanation will be made for the operation of the seawater electrolysis device 10 of the present embodiment and a method for carrying out electrolysis of the seawater W by using the seawater electrolysis device 10. The seawater W which is distributed through the intake channel 1 is partially introduced by the intake portion 60 into the electrolysis vessel main body 20 from the inlet port 23 of the electrolysis vessel main body 20 of the seawater electrolysis device 10. That is, the seawater W inside the intake channel 1. is pumped up into the intake flow path 61 by the first pump 62, by which the seawater W is introduced into the electrolysis vessel main body 20 via the intake flow path 61. By this constitution, the electrodes 30 inside the electrolysis vessel main body 20 are immersed into the seawater W, At this time, the first opening/closing control valve 63 is opened and closed depending on a flow rate detected by the first flow meter 64. Thereby, the seawater W which is distributed inside the electrolysis vessel main body 20 in the distributing direction is adjusted so as to have a desired flow speed. [0045] As described above, the seawater W which is distributed inside the electrolysis vessel main body 20 is subjected to electrolysis by the electrodes 30. That is, a desired constant current is generated at the constant current control circuit 42 on the basis of 19 direct electric power of the direct current power source 41 in the power supply unit 40, and the constant current is supplied to the terminal blocks 28, 29 via the lead wires 43, 44. The electric current supplied via the terminal blocks 28, 29 is distributed in series inside the electrolysis vessel main body 20 sequentially through the anode plates 32, the double-pole electrode plates 31 and the cathode plates 33. [00461 To be more specific, when an electric current which is distributed to the anode plate 32 from the constant current control circuit 42 arrives at a cathode K of a double-pole electrode plate 31 via the seawater W, the electric current is distributed inside the double-pole electrode plate 31 and, thereby, arrives at the anode A of the above double-pole electrode plate 31. Thereafter, the electric current which is distributed through the seawater W arrives at the cathode K of another double-pole electrode plate 31 opposite to the above anode A. As described above, the electric current is distributed from the anode plate 32 to the plurality of double-pole electrode plates 31 sequentially and finally distributed up to the cathode plate 33. At this time, an electric current density of electric current on the surface of each of the electrodes 30 is controlled by the constant current control circuit 42 in a range from 20A/dm 2 to 40A/dm 2 and preferably in a range of from 20A/dm 2 to 30A/dm 2 . [0047] The electric current which passes through the seawater W, as described above, is constant in electric current density on the surface of the electrode 30 by the operation of the constant current control circuit 42 regardless of a change in electrical resistance of the seawater W. That is, the seawater W which is distributed inside the electrolysis vessel main body 20 changes in value of the electrical resistance from moment to moment. However, as shown in FIG. 4, the constant current control circuit 42 controls the voltage in a predetermined deflection width AV, by which the electric current density on the surface of the electrode 30 is kept constant. [0048] 20 As described above, an electric current is passed through the seawater W between the electrodes 30 to electrolyze the seawater W. That is, at the anode A, as shown in the following Formula (1), chlorine ions in the seawater W lose electrons e to cause oxidation, thus resulting in generation of chlorine. [Formula 1] 20K~ -+ 012 + 2e --- (1) On the other hand, at the cathode K, as shown in the following formula (2), electrons are imparted to water in the seawater W to cause a reduction, thus resulting in generation of hydroxide ions and hydrogen gas. [Formula 2] 2H20 + 2e - 20H- + H 2 t -- (2) [00491 Further, as shown in the following formula (3), the hydroxide ions generated at the cathode K react with sodium ions in the seawater W to generate sodium hydroxide. [Formula 3] 2Na* + 20H - 2NaOH --- (3) [0050] Still further, as shown in the formula (4), sodium hydroxide reacts with chlorine to generate hypochlorous acid, sodium chloride and water. [Formula 4] C12 + 2NaOH -+ NaCIO + NaCi + H20 --- (4) 21 As described above, electrolysis of seawater W generates hypochlorous acid which is effective in suppressing adhesion of marine growth. [0051] Next, the seawater W which has been subjected to electrolysis flows out from the outlet port 25 of the electrolysis vessel main body 20, passes through the intermediate flow path 51 and is temporarily stored in the storage tank 50. Thereafter, the seawater W in the storage tank 50 is filled into the intake channel 1 via the filling portion 70. That is, the seawater W containing hypochlorous acid in the storage tank 50 is filled into the intake channel 1 via the filling flow path 71 by actuation of the second pump 72. At this time, the second opening/closing control valve 73 is opened and closed depending on a flow rate detected by the second flow meter 74, thereby, adjusting a quantity of the hypochlorous acid-containing seawater W which flows into the intake channel 1. [0052] In this instance, in general, manganese scales resulting from manganese ions contained in the seawater W deposit on the anode A coated with an iridium oxide-dominant coating material during electrolysis. Since the anode A undergoes accelerated erosion due to deposition of manganese scales and also catalytic activities on the surface of the electrode 30 are decreased, there is found a disadvantage of a lowering in chlorine generating efficiency. Further, scales resulting from magnesium and calcium contained in the seawater W deposit on the cathode K to accelerate the erosion of the electrode 30. [0053] In contrast, according to the above embodiment, the electric current density on the surface of the electrode 30 is set at 20A/dm 2 or more which is greater than a conventional electric current density of 15A/dm 2 . Thus, hydrogen gas is generated in association with electrolysis at the cathode K in a greater amount than a conventional case. The washing effect on the electrode 30 can be developed due to a greater amount of the hydrogen gas generated, thus making it possible to prevent the deposition of manganese 22 scales on an anode A and deposition of scales such as calcium and magnesium on the cathode K. Further, oxygen generated in the vicinity of an anode A is also increased in amount with an increase in electric current density on the surface of the electrode 30. However, since an iridium oxide is sufficiently resistant to oxygen, it is possible to prevent oxygen-caused erosion of an anode A coated with the iridium oxide-containing coating material. [0054] Where the electric current density on the surface of the electrode 30 is excessively great, for example, in excess of 40A/dm 2 , scales deposit on the anode A and the cathode K in such an amount that exceeds the amount where the washing effect by hydrogen is effective. In contrast, in the present embodiment, an upper limit of the electric current density is set at 40A/dm 2 . Thus, it is possible to effectively develop the washing effect by hydrogen and effectively prevent the deposition of scales on the anode A and the cathode K. Further, where an upper limit of the electric current density is set at 30A/dm 2 , it is possible to effectively develop the washing effect due to hydrogen and also effectively prevent the deposition of scales. [0055] As described above, in the present embodiment, an iridium oxide is contained in the coating material of the anode A and the electric current density on the surface of the electrode 30 is set in a range of from 20A/dm 2 to 40A/dm 2 and preferably in a range of from 20A/dm 2 to 3 OA/dm 2 . Therefore, it is possible to effectively develop the washing effect by hydrogen gas. Thereby, it is also possible to enhance the resistance of the electrode 30 and suppress the lowering in chlorine generating efficiency by preventing the deposition of scales on the electrode 30. In addition to improved maintenance of the seawater electrolysis device 10, the number of electrodes 30 can be decreased to downsize the device due to a higher chlorine generating efficiency.

23 [0056] Further, where a tantalum oxide is added to the iridium oxide-dominant coating material which coats the anode A, the tantalum exhibits a great resistance to oxygen. As a result, it is possible to more effectively prevent abnormal erosion of the electrode 30 due to oxygen generated in the vicinity of the anode A. It is noted that no platinum is contained in the iridium oxide-dominant coating material, thus making it possible to lower the cost. [0057] Still further, in the present embodiment, the double-pole electrode plates 31 are arranged in series to constitute an electrode group M, and the plurality of electrode groups M are arrayed parallel to each other, by which many double-pole electrode plates 31 are arranged in a concentrated manner. It is, therefore, possible to downsize the device in itself, while a large total generation amount of chlorine is secured. In addition, since the double-pole electrode plates 31 are individually arranged along the distributing direction of seawater W, there is no chance that distribution of the seawater W is prevented. It is, thereby, possible to keep the seawater W at a high flow speed and also effectively develop the effect of preventing deposition of scales on the electrode 30. Next, the anode A is opposed to the cathode K between the electrode groups M which are adjacently parallel to each other, Therefore, an electric current is passed between the anode A and the cathode K, thus making it possible to effectively electrolyze the seawater W which is distributed between the electrodes 30. [0058] Where an interval is small between the double-pole electrode plates 31 adjacent to each other in the distributing direction of the seawater W, there is developed an electric current which is distributed between the double-pole electrode plates 31, that is, a stray current which contributes less to electrolysis. The stray current becomes more apparent 24 with an increase in electric current density on the surface of the electrode 30, thus resulting in a lowering in electrolysis efficiency of seawater. In contrast, in the present embodiment, an interval dl between the double-pole electrode plates 31 adjacent in the distributing direction in each of the electrode groups M is eight times or more than an interval d2 between the electrode groups M which are adjacently parallel to each other. That is, an interval between the double-pole electrode plates 31 adjacent to each other in the distributing direction is kept appropriately, thus making it possible to suppress the occurrence of stray current and prevent the lowering in electrolysis efficiency of seawater. [0059] Next, an explanation will be made for a seawater electrolysis system 10013 of the second embodiment according to the present invention by referring to FIG. 5. In the second embodiment, constituents similar to those of the first embodiment are given the same reference numerals, and a detailed explanation thereof is omitted here. [0060] As shown in FIG. 5, the seawater electrolysis system 1OOB of the second embodiment is provided with a circulation portion 80 between the intake flow path 61 of an intake portion 60 and the filling flow path 71 of a filling portion 70. The circulation portion 80 mixes the seawater W inside a filling flow path 71 with the seawater in an intake flow path 61. The circulation portion 80 includes a circulation flow path 81, a third flow meter 84, and a third opening/closing control valve 83. The circulation flow path 81 is a flow path which is connected at one end thereof to the filling flow path 71 and connected at the other end thereof to the intake flow path 61. In the present embodiment, one end of the circulation flow path 81 is connected between a second pump 72 and a second opening/closing control valve 73 on the filling flow path 71. The other end of the circulation flow path 81 is connected between a first pump 62 and a first opening/closing control valve 63 on the intake flow path 61. [0061} 25 The third flow meter 84 is installed halfway along the circulation flow path 81, detecting a flow rate Q3 of the seawater W passing through the circulation flow path 81. Further, the third opening/closing control valve 83 is a valve which is installed downstream to the third flow meter 84 on the circulation flow path 81, and controlled for opening and closing on the basis of the flow rate Q3 of the seawater Q detected by the third flow meter 84. Thereby, it is possible to control, whenever necessary, the flow rate of the seawater W which is circulated from the filling flow path 71 via the circulation flow path 81 to the intake flow path 61. [0062] In the above-described seawater electrolysis system 100B, when seawater W stored in a storage tank 50 after electrolysis is introduced by the second pump 72 into the filling flow path 71, the seawater W is separated into seawater W which is distributed through the filling flow path 71 and seawater W which is distributed through the circulation flow path 81 at a branching portion of the filling flow path 71 to which one end of the circulation flow path 81 is connected. [0063] The seawater W which has passed through the circulation flow path 81 is introduced into the intake flow path 61 at the other end of the circulation flow path 81. That is, the seawater W after electrolysis which has passed through the circulation flow path 81 flows together with seawater W before electrolysis which passes through the intake flow path 61 and is introduced again into the electrolysis vessel main body 20. At this time, the third opening/closing control valve 83 is opened and closed depending on a flow rate detected by the third flow meter 84, thus making it possible to adjust the flow rate of the seawater W after electrolysis which flows together with the seawater W which flows through the intake flow path 61. As described above, the seawater W after electrolysis which has flowed out from the outlet port 25 of the electrolysis vessel main body 20 is distributed through the 26 circulation flow path 81 and, thereby, flows again into the electrolysis vessel main body 20 from the inlet port 23 thereof [0064] In this instance, scale compositions such as manganese, magnesium and calcium deposited during electrolysis are found in the seawater W after electrolysis. The seawater W is again introduced into the electrolysis vessel main body 20, thus making it possible to prevent the deposition of scales on the surface of the electrode 30 due to seed crystallization effects of the scale compositions. That is, the scale compositions act as seed crystals and newly generated scales deposit on the seed crystals, thus making it possible to avoid precipitation of scales on the surface of the electrode 30. It is, thereby, possible to enhance the resistance of the electrode 30 and also suppress the lowering of chlorine generating efficiency. [0065] An explanation has been so far made in detail for the embodiments of the present invention. The present invention shall not be, however, restricted, and may be modified in various ways including some change in design without departing from the technical idea thereof. For example, in the seawater electrolysis system 100B, it is preferable that the seawater W which is filled from the filling portion 70 into the intake channel 1 is set approximately at 2500 ppm in terms of the concentration of hypochlorous acid. [0066] In this instance, a total amount of the thus generated hypochlorous acid is substantially proportional to a total amount of electric current supplied from the power supply unit 40 to the electrodes 30. Therefore, an amount of electric current supplied to the electrodes 30 can be recorded to figure out a total amount of the thus generated hypochlorous acid. Further, the concentration of hypochlorous acid in the seawater W which is filled into the intake channel 1 can be calculated by dividing the total amount of the thus generated hypochlorous acid by a flow rate Q2 of the seawater W which is filled 27 into the intake channel 1. Therefore, the second opening/closing control valve 73 is controlled depending on the total amount of hypochlorous acid to determine the flow rate Q2 of the seawater W which is filled into the intake channel 1. It is, thereby, possible to easily adjust the concentration of hypochlorous acid in the seawater W to 2500 ppm. [0067] Further, as a modified example shown in FIG. 6, it is acceptable that the seawater electrolysis device 10 is provided with a plurality of electrolysis vessel main bodies 20 and there is installed a connecting pipe 85 for connecting the outlet port 25 to the inlet port 23 of each of the electrolysis vessel main bodies 20 and a degassing valve 86 as a degassing unit for removing a gas inside the connecting pipe 85. The degassing valve 86 is a valve which can be controlled for opening and closing. Where a pressure inside the electrolysis vessel main body 20 increases up to a predetermined high pressure, the degassing valve 86 is opened to release the gas in the seawater W. [0068] A liquid-gas ratio lowers due to hydrogen generated at a cathode K with an increase in electric current density, thus resulting in a lowering in chlorine generating efficiency. However, in particular, hydrogen gas is removed by the degassing valve 86 installed on the connecting pipe 85, by which it is possible to suppress the liquid-gas ratio inside the electrolysis vessel main body 20 to a predetermined level and prevent the lowering in efficiency. [0069] In the above embodiment, an explanation has been made for the electrode 30 by referring to an example using a double-pole electrode plate 31. However, it is acceptable that, for example, an anode plate 32 and a cathode plate 33 are arranged so as to oppose each other, without using the double-pole electrode plate 31 and an electric current is passed through seawater W between the anode plate 32 and the cathode plate 33. It is also acceptable that the anode plate 32 and the cathode plate 33 are alternately 28 arranged, by which the electric current is passed through the seawater W between the anode plate 32 and the cathode plate 33 which are adjacent and opposed to each other. Further, in the above embodiment, the double-pole electrode plate 31 is arranged in such a manner that an anode A is turned to the seawater inlet side and a cathode K is turned to the seawater outlet side. However, the double-pole electrode plate 31 may be arranged in such a manner that the anode A is turned to the seawater outlet side and the cathode K is turned to the seawater inlet side. [0070] Next, an explanation will be made for a seawater electrolysis system 100C of the third embodiment according to the present invention by referring to FIG. 7 and FIG. 8. In the third embodiment as well, the constituents similar to those of the first embodiment will be given the same reference numerals, with a detailed explanation omitted here. As shown in FIG. 7, the seawater electrolysis system 10 C of the third embodiment is provided with a seawater electrolysis device 10, an intake portion 60, a hydrogen separator 90, a storage tank 50, a filling portion 70 and a circulation portion 80. The intake portion 60 introduces seawater W into the seawater electrolysis device 10 from an intake channel 1. The hydrogen separator 90 separates hydrogen in electrolyzed water E discharged from the seawater electrolysis device 10. The storage tank 50 stores the electrolyzed water E which has been subjected to electrolysis by the seawater electrolysis device 10. The filling portion 70 fills the electrolyzed water E of the storage tank 50 into the intake channel 1. The circulation portion 80 circulates the electrolyzed water E into the seawater electrolysis device 10. A seawater desalination device 65 is installed at the intake portion 60. [0071] In this instance, at the power supply unit 40 of the present embodiment, a constant current is generated by the constant current control circuit 42 in such a manner that an electric current density on the surface of an electrode 30 is in a range of from 20A/dm 2 to 60A/dm 2 and preferably in a range of from 20A/dm 2 to 50A/dm 2 . That is, 29 the constant current is generated depending on the surface area of the electrode 30 inside the electrolysis vessel main body 20 and supplied to the electrode 30. Thereby, the electric current density on the surface of the electrode 30 is in a range of from 20A/dm 2 to 60A/dm 2 and preferably in a range of from 20A/dm 2 to 50A/dm 2 . [0072] In an electrode coated with a conventional platinum-dominant composite metal (platinum-dominant coating material), oxygen and scales which accelerate the erosion of the electrode are also increased in amount with an increase in electric current density. Therefore, a maximum value of the electric current density has been set approximately at 15A/dm 2 . In contrast, in the present embodiment, electrolysis is carried out in a range of the electric current density of from 20A/dm 2 to 60A/dm 2 and preferably in a range of from 20A/dm 2 to 50A/dm 2 which is higher than a conventional case. [0073] The intake portion 60 includes an intake flow path 61, a first pump 62, a seawater desalination device 65, a first flow meter 64 and a first opening/closing control valve 63. [0074] The seawater desalination device 65 is a device for separating seawater into plain water (desalted water) and concentrated water C by utilizing a reverse osmosis membrane (RO membrane). The plain water separated by the seawater desalination device 65 is fed via a plain water line 66 into a plain water tank (not illustrated), while the concentrated water C is introduced into the seawater electrolysis device 10 via the first opening/closing control valve 63 of the intake flow path 61. [0075] In the seawater electrolysis device 10 of the present embodiment, it is preferable that the first opening/closing control valve 63 is controlled so that the concentrated water C which is distributed inside the electrolysis vessel main body 20 has a flow speed of at least 0.7 m/s or more, 30 It is acceptable that not only the first opening/closing control valve 63 is controlled for opening and closing to adjust the flow speed of the concentrated water C inside the electrolysis vessel main body 20 but also, for example, the first pump 62 is controlled for output to adjust the flow speed of the concentrated water C inside the electrolysis vessel main body 20. [0076] The hydrogen separator 90 is a device for separating hydrogen gas contained in the electrolyzed water E which flows out from the outlet port 25 of the electrolysis vessel main body 20 in the seawater electrolysis device 10. As shown in FIG. 8, the hydrogen separator 90 is provided with a water receiving tank 92 having an exhaust tube 91 at an upper part thereof, an introduction pipe 93 which is connected to the outlet port 25 of the electrolysis vessel main body 20 by way of the intermediate flow path 8 to draw electrolyzed water into a gas phase portion 92a upward inside the water receiving tank 92, a spray nozzle 94 installed halfway along the introduction pipe 93 and an agitator 95 installed at a liquid phase portion 92b downward inside the water receiving tank 92. [0077] The spray nozzle 94 ejects the electrolyzed water E which has been introduced into the introduction pipe 93 into the gas phase portion 92a upward inside the water receiving tank 92. The agitator 95 is constituted with a screw 96 and a motor 97 which rotates the screw 96, thereby agitating a liquid pooled at the liquid phase portion 92b of the water receiving tank 92. Further, there is installed at a lower part of the water receiving tank 92 a discharge port 98 through which the electrolyzed water is discharged. [0078] The storage tank 50 is a tank which temporarily stores the electrolyzed water E which is discharged from the discharge port 98 of the hydrogen separator 90. [0079] The circulation portion 80 is a portion which circulates the electrolyzed water E flowing through the filling flow path 71 into the intake flow path 61 of the intake portion 31 60. The circulation portion 80 includes a circulation flow path 81, a third flow meter 82 and a third opening/closing control valve 83. The circulation flow path 81 is a flow path which is connected at one end thereof to the filling flow path 71 and connected at the other end to the intake flow path 61. In the present embodiment, the one end of the circulation flow path 81 is connected between the second pump 72 and the second opening/closing control valve 73 on the filling flow path 71. The other end of the circulation flow path 81 is connected between the first opening/closing control valve 63 and the first flow meter 64 on the intake flow path 61. [0080] The third flow meter 82 is installed halfway along the circulation flow path 81, detecting a flow rate Q3 of the electrolyzed water E which passes through the circulation flow path 81. Further, the third opening/closing control valve 83 is a valve which is installed downstream of the third flow meter 82 on the circulation flow path 81 and controlled for opening and closing on the basis of the flow rate Q3 of the electrolyzed water E detected by the third flow meter 82. By this constitution, it is possible to control the flow rate of the electrolyzed water E which is circulated to the intake flow path 61 from the filling flow path 71 via the circulation flow path 81 at arbitrary rate. [0081] Next, an explanation will be made for the operation of the seawater electrolysis system 1 00C of the present embodiment and a method for carrying out electrolysis of seawater W by using the seawater electrolysis system IOC. The seawater W which is distributed through the intake channel I is partially introduced into the seawater desalination device 65 by the intake portion 60. That is, the seawater W inside the intake channel 1 is pumped up into the intake flow path 61 by the first pump 62, by which the seawater W is introduced into the seawater desalination device 65 by way of the intake flow path 61. Thereby, the seawater W is separated into plain water and concentrated water C.

32 [0082] The seawater desalination device 65 allows the seawater W to pass through the RO membrane by applying pressure to the seawater W, thereby concentrating salt content of the seawater W to filter out plain water, Thereby, the concentration of chloride ions in the seawater W is increased, for example, up to a range from 20,000 mg/L to 30,000 to 40,000 mg/L, resulting in production of concentrated water C. The plain water is fed by way of a plain water line 66 to a plain water tank (not illustrated) which stores the plain water, while the concentrated water C is introduced by way of the intake flow path 61 into the electrolysis vessel main body 20. Thereby, the electrodes 30 inside the electrolysis vessel main body 20 are immersed into the concentrated water C. At this time, the first opening/closing control valve 63 is opened and closed depending on a flow rate detected by the first flow meter 64, by which the concentrated water C which is distributed in the distributing direction inside the electrolysis vessel main body 20 is adjusted so as to give a value of a desired flow speed. [0083] Therefore, the concentrated water C which is distributed inside the electrolysis vessel main body 20 is subjected to electrolysis by the electrode 30. That is, a desired constant current is generated at the constant current control circuit 42 on the basis of direct electric power of the direct current power source 41 at the power supply unit 40, and the constant current is supplied via lead wires 43, 44 to terminal blocks 28, 29. An electric current supplied via the terminal blocks 28, 29 is distributed in series sequentially through an anode plate 32, a double-pole electrode plate 31 and a cathode plate 33 inside the electrolysis vessel main body 20. [00841 To be more specific, the electric current distributed from the constant current control circuit 42 to the anode plate 32 arrives at the cathode K of the double-pole electrode plate 31 by way of the concentrated water C. The electric current is distributed 33 inside the double-pole electrode plate 31 and thereby arrives at an anode A of the double-pole electrode plate 31. Thereafter, the electric current which passes through the concentrated water arrives at the cathode K of another double-pole electrode plate 31 opposite the anode A. As described above, the electric current is distributed from the anode plate 32 sequentially through the plurality of double-pole electrode plates 31 and finally distributed up to the cathode plate 33. An electric current density of the electric current at this time on the surface of each of the electrodes 30 is controlled by the constant current control circuit 42 so as to be in a range of from 20A/dm 2 to 60A/dm 2 and preferably in a range of from 20A/dm 2 to 50A/dm 2 [0085] As described above, the electric current which passes through the concentrated water C is to be constant in electric current density on the surface of each of the electrodes 30 due to the operation of the constant current control circuit 42, regardless of a change in electrical resistance of the concentrated water C. That is, the concentrated water C which is distributed inside the electrolysis vessel main body 20 undergoes a change in value of the electrical resistance from moment to moment. As shown in FIG. 4, the constant current control circuit 42 controls the voltage so as to give a predetermined deflection width AV, by which the electric current density on the surface of the electrode 30 is kept constant. [0086] As described above, an electric current is distributed inside concentrated water C between the electrodes 30, by which the concentrated water C is subjected to electrolysis. That is, at the anode A, as shown in the formula (1) of the first embodiment, chloride ions in the concentrated water C lose electrons e to cause oxidation, thereby generating chlorine. On the other hand, at the cathode K, as shown in the formula (2) of the first embodiment, electrons are imparted to water in the concentrated water C to cause a reduction, thus resulting in generation of hydroxide ions and hydrogen gas.

34 [0087) Further, as shown in the formula (3) of the first embodiment, the hydroxide ions generated at the cathode K react with sodium ions in concentrated water to generate sodium hydroxide. [0088] Further, as shown in the formula (4) of the first embodiment, sodium hydroxide reacts with chlorine to generate hypochlorous acid, sodium chloride and water. As described above, on the basis of electrolysis of the concentrated water C, there is generated hypochlorous acid which is effective in suppressing deposition of marine growth. Since the concentration of chloride ions in the concentrated water C is increased up to a value ranging from 30,000 mg/L to 40,000 mg/L, the concentration of hypochlorous acid is preferably set in a range of from 2,500 ppm to 5,000 ppm. [0089] Next, the concentrated water C which has been subjected to electrolysis flows out from the outlet port 25 of the electrolysis vessel main body 20 as electrolyzed water E, together with hydrogen gas, passing through the intermediate flow path 8 and flowing into a hydrogen separator 90. A gas-liquid mixture fluid composed of hydrogen gas and the electrolyzed water E is introduced into an introduction pipe 93 of the hydrogen separator 90 and ejected by a spray nozzle 94 into a gas phase portion 92a of the water receiving tank 92. Thereby, hydrogen gas mixed with the electrolyzed water E as bubbles is subjected to deacration and exhausted from the exhaust tube 91. [0090] On the other hand, the electrolyzed water E is stored at the liquid phase portion 92b of the water receiving tank 92. The stored electrolyzed water E is agitated by an agitator 95. That is, the electrolyzed water E is forcibly agitated by a spiral flow caused by a screw 96 rotated by a motor 97. Thereby, scales deposited in association with 35 electrolysis are prevented from flocculating on the bottom of the water receiving tank 92. The electrolyzed water E which has been temporarily stored in the water receiving tank 92 is discharged from the discharge port 98 installed on the bottom of the water receiving tank 92 and introduced into the storage tank 50. [0091] When the electrolyzed water E which has been temporarily stored in the storage tank 50 is introduced by the second pump 72 into the filling flow path 71, the electrolyzed water E is branched into electrolyzed water E which is distributed through the filling flow path 71 and electrolyzed water E which is distributed through the circulation flow path 81 at a branching portion of the filling flow path 71 to which one end of the circulation flow path 81 is connected. [0092] The electrolyzed water E which is distributed through the filling flow path 71 is filled into the intake channel 1. That is, the electrolyzed water E which contains hypochlorous acid in the storage tank 50 is filled into the intake channel I by way of the filling flow path 71 by actuation of the second pump 72. At this time, the second opening/closing control valve 73 is opened and closed depending on the flow rate detected by the second flow meter 74, thereby adjusting the flow rate of the electrolyzed water E which is filled into the intake channel 1 and contains hypochlorous acid. [0093] In this instance, a total amount of the generated hypochlorous acid is substantially proportional to a total amount of electric current supplied from the power supply unit 40 to the electrodes 30, Therefore, the amount of electric current supplied to the electrodes 30 is recorded to figure out the total amount of the generated hypochlorous acid. Further, the concentration of hypochlorous acid in the electrolyzed water E which is filled into the intake channel I can be calculated by dividing the total amount of the generated hypochlorous acid by a flow rate Q2 of the seawater W which is filled into the intake channel 1. Therefore, the second opening/closing control valve 73 is controlled 36 depending on the total amount of hypochlorous acid to determine the flow rate Q2 of the electrolyzed water E which is filled into the intake channel 1. It is, thereby, possible to adjust the concentration of hypochlorous acid in the electrolyzed water E. [0094] On the other hand, the electrolyzed water E which is distributed through the circulation flow path 81 is introduced into the intake flow path 61 at the other end of the circulation flow path 81. That is, the electrolyzed water E which has passed through the circulation flow path 81 flows together with seawater W passing through the intake flow path 61 and is again introduced into the electrolysis vessel main body 20. At this time, the third opening/closing control valve 83 is opened and closed depending on the flow rate detected by the third flow meter 82, thus making it possible to adjust the flow rate of the electrolyzed water E flowing together with the seawater W which is distributed through the intake flow path 61. As described above, the electrolyzed water E which has flowed out from the outlet port 25 of the electrolysis vessel main body 20 is distributed through the circulation flow path 81, thereby flows again into the electrolysis vessel main body 20 from the inlet port 23 thereof [0095] According to the above embodiment, the concentrated water C increased in concentration of chloride ions and electric conductivity is introduced into the seawater electrolysis device 10. Further, since a coating material of the anode A contains an iridium oxide, it is possible to set the electric current density on the surface of the electrode 30 in a range of from 20A/dm 2 to 60A/dm 2 and preferably in a range of from 20A/dm 2 to 50A/dm 2 . Thus, it is possible to increase the concentration of hypochlorous acid contained in the generated electrolyzed water E. That is, hypochlorous acid is produced in an increased amount per unit area of the electrode, thus making it possible to decrease the area of the electrode and downsize the device. [0096] 37 Seawater in the vicinity of the mouth of a river or inside a bay is lower in concentration of chloride ion than ordinary seawater and also lower in electric conductivity. Therefore, there may be posed a problem on stability of operation due to abnormal erosion of electrodes. However, the concentrated water C is subjected to treatment by the seawater electrolysis device 10 to increase the concentration of chlorine ions and the electric conductivity. Thus, it is possible to stabilize treatment performance. [0097] Further, the increased hydrogen gas is subjected to a degassing process by the hydrogen separator 90. Therefore, there is no chance that the hydrogen gas damages the second pump 72 and piping that are subsequent to the storage tank 50. [00981 Still further, since the circulation portion 80 is installed, scale compositions such as manganese, magnesium and calcium generated during electrolysis are introduced into the electrolysis vessel main body 20 together with the electrolyzed water E. Therefore, the electrolyzed water E containing scale compositions is again introduced into the electrolysis vessel main body 20, thus making it possible to prevent deposition of scales on the surface of the electrode 30 by seed crystallization effects of the scale compositions. That is, the scale compositions act as crystal seeds and newly generated scales deposit on the crystal seeds. Thus, it is possible to avoid precipitation of scales on the surface of the electrode 30. It is, thereby, possible to enhance the durability of the electrode 30 and also suppress the lowering of chlorine generating efficiency. [0099] Where the electric current density on the surface of the electrode 30 is excessively large, for example, in excess of 60A/dm 2 , scales are generated on an anode A and a cathode K in such an amount that exceeds an amount where washing effect by hydrogen is effective. In contrast, in the present embodiment, since an upper limit of the electric current density is set at 60A/dm 2 , it is possible to effectively exert the washing 38 effect due to hydrogen and effectively prevent the deposition of scales on the anode A and the cathode K. Further, where an upper limit of the electric current density is set at 50A/drn 2 , it is possible to effectively develop the washing effect by hydrogen and effectively prevent the deposition of scales. [0100] As described above, in the present embodiment, a coating material of the anode A contains an iridium oxide and the electric current density on the surface of the electrode 30 is set to be in a range from 20A/dm 2 to 60A/dm 2 and preferably in a range of from 20A/dm 2 to 50A/dm 2 . Thus, it is possible to effectively obtain the washing effect due to hydrogen gas. Thereby, it is possible to enhance the resistance of the electrode 30 and suppress the lowering in chlorine generating efficiency by preventing deposition of scales on the electrode 30. Therefore, in addition to improvements in maintenance of the seawater electrolysis device 10, the number of electrodes 30 can be decreased to downsize the device due to a higher chlorine generating efficiency. [0101] In the above embodiment, an explanation has been made for the electrode 30 by referring to an example using a double-pole electrode plate 31. However, it is acceptable that, for example, an anode plate 32 and a cathode plate 33 are arranged so as to oppose each other, without using the double-pole electrode plate 31 and an electric current is passed through seawater W between the anode plate 32 and the cathode plate 33. It is also acceptable that the anode plate 32 and the cathode plate 33 are alternately arranged, by which the electric current is passed through the seawater W between the anode plate 32 and the cathode plate 33 which are adjacent and opposed to each other. Further, in the above embodiment, the double-pole electrode plate 31 is arranged in such a manner that an anode A faces the seawater inlet side and a cathode K faces the seawater outlet side. However, the double-pole electrode plate 31 may be arranged in 39 such a manner that the anode A faces the seawater outlet side and the cathode K faces the seawater inlet side. [0102] Further, in the present embodiment, there is adopted the seawater desalination device 65 using a RO membrane as a device for concentrating the seawater W to generate the concentrated water C. However, the device for generating the concentrated water C is not restricted to the above device and, for example, there may be adopted a method for concentrating the seawater W using a distillation method. [0103] Still further, a method for separating hydrogen gas from the hydrogen gas-mixed electrolyzed water E is not restricted to the method of the present embodiment in which the hydrogen separator 90 with the spray nozzle 94 is used but may include a method in which a gas-liquid separator using a centrifugal machine, for example, is used as long as a gas-liquid mixture fluid can be separated into a gas and a liquid. It is also acceptable that hydrogen is separated not by installing separately the hydrogen separator 90 as a gas-liquid separator but by adding to the storage tank 50, for example, a gas-liquid separating function for diluting hydrogen gas by supplying air to a liquid phase. [0104] In addition, the electrolyzed water E may be all supplied to the intake channel 1, with no circulation portion 80 installed, if deposition of scales on the surface of the electrode 30 does not pose a problem. [Example] [0105] Hereinafter, an explanation will be made for an example. (Test on determination of chlorine generating efficiency) 40 A test was conducted for studying a relationship between an electric current density on the surface of an electrode and a chlorine generating efficiency during electrolysis of seawater W and concentrated water C. An anode plate and a cathode plate were provided, each of which was a plate with an electrode area of 50 x 50 mm and arranged so as to oppose each other, with an interval of 5 mm. As the anode plate, used was a titanium base plate coated with a coating material containing an iridium oxide (Ir0 2 ) at 50% or more in mass ratio. Further, a titanium base plate free of a coating material was used as the cathode plate. The concentration of chloride ions in the seawater W was 20,000 mg/L and the concentration of chloride ions in the concentrated water W was 30,000 to 40,000 mg/L. [0106] The anode plate and the cathode plate were immersed into the seawater W and the concentrated water C. The seawater W and the concentrated water C were distributed at a flow rate of 250 mL/min and an electric current was passed between the anode plate and the cathode plate for electrolysis. Next, a determination was made for chlorine generating efficiency at each electric current density. The chlorine generating efficiency means a ratio of an amount of chlorine which is actually generated to the amount of chlorine which can be theoretically generated on the basis of the electric current density of distributed electric current. FIG. 9 shows the result of determination of the chlorine generating efficiency. [0107] As shown in FIG. 9, where the electric current density is less than 20A/dm 2 , both the seawater W and the concentrated water C are increased in chlorine generating efficiency with an increase in electric current density. The seawater W without concentration is constant in chlorine generating efficiency where the electric current density is in a range of from 20A/dm 2 to 30A/dm 2 and gradually lowers in chlorine generating efficiency where the electric current density 41 exceeds 30A/dm 2 . Further, where the electric current density is 20A/dM2 or 30A/dm 2 , the chlorine efficiency is the highest value obtained, that is, 96%. It has been found that where the electric current density is 15A/dm 2 which is taken as technical common sense for an electrode coated with a platinum-containing coating material, the chlorine generating efficiency is 93%. From this fact, it is now understood that even for the seawater W, at an electrode coated with an iridium oxide-containing coating material, the electric current density is set to be in a range of from 20A/dm 2 to 30A/dm 2 , thereby making it possible to obtain a high chlorine generating efficiency. It is considered that the high chlorine generating efficiency is caused by increase in the amount of generated hydrogen gas resulting in the effect of washing the scales deposited on an anode plate and a cathode plate by the hydrogen gas. [0108] An amount of chlorine which can be theoretically generated increases with an increase in electric current density. Therefore, even where the chlorine generating efficiency shows the same value, a greater amount of chlorine is generated where the electric current density is greater. Thus, where the electric current density is set at 40A/dm 2 , the chlorine generating efficiency is 93%, which is equivalent to that at the electric current density of 15A/dm. However, an amount of generated chlorine is greater at the electric current density of 40A/dm 2 than at the electric current density of 15A/dm 2 . Therefore, it is effective to set the electric current density at 40A/dm 2 in view of an amount of generated chlorine. On the other hand, where the electric current density exceeds 40A/ dm 2 , it is out of a range where the washing effect can be effectively developed due to hydrogen gas. In addition, the chlorine generating efficiency lowers as compared with that at the electric current density of 15A/ dm 2 . Therefore, an upper limit of the electric current density is preferably set at 40A/ dm 2 . Thereby, it has been found that a greater amount of 42 generated chlorine can be secured while the chlorine generating efficiency is also kept high. [0109] Where the electric current density is in a range from 20A/dm 2 to 50A/dm 2 , the concentrated water C is constant in chlorine generating efficiency. Where the electric current density is set at 60A/dm 2 , the concentrated water C keeps a high chlorine generating efficiency, that is, 96%. As apparent from the above description, the concentrated water C is able to obtain a higher chlorine generating efficiency by setting the electric current density to be in a range of from 20A/dm 2 to 60A/dm 2 . It has been found that the electric current density can be made high as compared with the seawater W without being concentrated. [0110] As described above, the test on determination of chlorine generating efficiency has revealed that the concentrated water C is introduced into the seawater electrolysis device 10, the electric current density on the surface of the electrode during electrolysis is set in a range of from 20A/dm 2 to 60A/dm 2 and preferably in a range of from 20A/dm 2 to 50A/dm 2 ' thus making it possible to obtain a high chlorine generating efficiency. The electrode gradually erodes away when electrolysis is carried out for a prolonged period of time. Thus, the curve indicating the result on determination in FIG. 9 is considered to become steeper. In addition, it is assumed more effective that the electric current density is set in the above range, in particular, after erosion of the electrode. [0111] (Test results of life span of electrolysis) A test was conducted for studying a relationship between an electric current density during electrolysis of seawater W and a catalyst retaining amount. As with the test on determination of chlorine generating efficiency, an anode plate and a cathode plate were provided, each of which was a plate with an electrode area 43 of 50 x 50 mm and arranged so as to oppose each other, with an interval of 5 mm. Two types of electrode plates were prepared as the anode plate, that is, a titanium base plate coated with a coating material which contains an iridium oxide (IrO2) at 50% or more in mass ratio and a titanium base plate coated with a platinum (Pt)-containing coating material. A titanium base plate free of a coating material was used as the cathode plate. The anode plate and the cathode plate were respectively immersed into seawater W, and the seawater W was distributed at a flow rate of 250 mL/min. Additionally, an electric current was passed between the anode plate and the cathode plate for electrolysis. Next, determination was made for a catalyst retaining amount at each electric current density with time. The catalyst retaining amount means an amount of catalyst of the electrode retained after electrolysis. The catalyst retaining amount decreases with time, by which the electrode erodes away accordingly. FIG. 10 shows the result of determination of the catalyst retaining amount. [0112] As shown in FIG. 10, it has been found that where the titanium base plate coated with the platinum-containing coating material (Pt/Ti) is used as the anode plate, the catalyst retaining amount gradually lowers with the elapse of time and, in particular, the catalyst retaining amount lowers apparently with an increase in electric current density. On the other hand, where the titanium base plate coated with the iridium oxide-containing coating material (IrO2) is used as the anode plate, the catalyst retaining amount did not decrease with time. Thereby, it has been found that the anode plate coated with the iridium oxide-containing coating material is higher in resistance of the electrode than the anode plate coated with the platinum-containing coating material. [Industrial Applicability] [0113] 44 The present invention relates to a seawater electrolysis system which is provided with a seawater electrolysis device for generating hypochlorous acid by carrying out electrolysis of seawater and also relates to a seawater electrolysis method. According to embodiments of the present invention, it is possible to enhance the resistance of an electrode and suppress the decrease in chlorine generating efficiency by preventing deposition of scales on the electrode. [Description of Reference Numerals] [0114] A: anode K: cathode M: electrode group W: seawater C: concentrated water 10: seawater electrolysis device 20: electrolysis vessel main body 30: electrode 31: double-pole electrode plate 32: anode plate 33: cathode plate 40: power supply unit 60: intake portion 65: seawater desalination device (concentration means) 70: filling portion 80: circulation portion 81: circulation flow path 90: hydrogen separator (hydrogen separation means) 100A, 100B, 10OC: seawater electrolysis system

Claims

1. A seawater electrolysis device comprising: an anode which is made of titanium coated with an iridium oxide-containing coating material containing no platinum, a cathode, an electrolysis vessel main body which houses the anode and the cathode, and a power supply unit which passes an electric current between the anode and the cathode, wherein the seawater electrolysis device passes an electric current between the anode and the cathode in such a manner that an electric current density on the surface of the anode and that of the cathode is included in a range of 20A/dm2 or more and 40A/dm 2 or less to electrolyze seawater inside the electrolysis vessel main body.

2. The seawater electrolysis device according to claim 1, wherein the electric current density on the surface of the anode and that of the cathode between which the electric current is passed by the power supply unit is included in a range of 20A/dm2 or more and 30A/dm2 or less.

3. The seawater electrolysis device according to claim 1 or claim 2, wherein a tantalum oxide is added to the coating material.

4. The seawater electrolysis device according to any one of claims 1 to 3, wherein an electrode includes a plurality of double-pole electrode plates in each of which one portion thereof in a distributing direction of the seawater is given as the anode and the other portion thereof is given as the cathode, a plurality of electrode groups in which the double-pole electrode plates are arrayed with an interval in the distributing direction, are arranged so as to be parallel to each other, and the double-pole electrode plates in the electrode groups adjacently parallel to each other are arranged in such a manner that the anode is opposed to the cathode. 46

5. The seawater electrolysis device according to claim 4, wherein an interval between the double-pole electrode plates which are adjacent in the distributing direction in each of the electrode groups is set to be 8 times or more than an interval between the electrode groups which are adjacently parallel to each other.

6. The seawater electrolysis device according to any one of claims 1 to 5 further comprising one or a plurality of the electrolysis vessel main bodies, one or a plurality of connecting pipes each of which connects an outlet port of the seawater of one of the electrolysis vessel main bodies with an inlet port of the seawater of the other one of the electrolysis vessel main body, and one or a plurality of degassing units for removing a gas inside the connecting pipes.

7. A seawater electrolysis system comprising: the seawater electrolysis device described in any one of claim 1 to claim 6; and a circulation flow path which mixes the seawater after electrolysis flowing out from the outlet port of the electrolysis vessel main body with the seawater before flowing into the electrolysis vessel main body from the inlet port.

8. A seawater electrolysis method by using the seawater electrolysis device described in any one of claim 1 to claim 6, the seawater electrolysis method, wherein seawater is introduced into the electrolysis vessel main body, an electric current is passed between the anode and the cathode in such a manner that an electric current density on the surface of the anode and that of the cathode is included in a range of 20A/dm2 or more and 40A/dm2 or less to electrolyze seawater inside the electrolysis vessel main body. 47

9. A seawater electrolysis system comprising: a seawater electrolysis device having an anode which is made of titanium coated with an iridium oxide-containing coating material containing no platinum, a cathode, an electrolysis vessel main body which houses the anode and the cathode, and a power supply unit which passes an electric current between the anode and the cathode, and a concentrating unit for increasing the concentration of chloride ions contained in seawater to be introduced into the electrolysis vessel main body, wherein an electric current is passed between the anode and the cathode to electrolyze seawater inside the electrolysis vessel main body.

10. The seawater electrolysis system according to claim 9, wherein the electric current density on the surface of the anode and that of the cathode between which the electric current is passed by the power supply unit is included in a range of 20A/dm2 or more and 60A/dm2 or less.

11. The seawater electrolysis system according to claim 9, wherein the electric current density on the surface of the anode and that of the cathode between which the electric current is passed by the power supply unit is included in a range of 20A/dm2 or more and 50A/dm2 or less.

12. The seawater electrolysis system according to any one of claims 9 to 11 which is provided with a hydrogen separation means for separating hydrogen gas generated at the cathode from the seawater after electrolysis.

13. The seawater electrolysis system according to any one of claims 9 to 12 which is provided with a circulation flow path which mixes seawater after electrolysis which is discharged from the electrolysis vessel main body with seawater to be introduced into the electrolysis vessel main body. 48

14. The seawater electrolysis system according to any one of claims 9 to 13, wherein a tantalum oxide is added to the coating material.

15. The seawater electrolysis system according to any one of claims 9 to 14, wherein an electrode includes a plurality of double-pole electrode plates in which one portion thereof in a distributing direction of the seawater is given as the anode and the other portion thereof is given as the cathode, a plurality of electrode groups in which the double-pole electrode plates are arrayed with an interval in the distributing direction are arranged so as to be parallel to each other, and the double-pole electrode plates in each of the electrode groups adjacently parallel to each other are arranged in such a manner that the anode is opposed to the cathode.

16. The seawater electrolysis system according to claim 15, wherein an interval between the double-pole electrode plates which are adjacent in the distributing direction in each of the electrode groups is set to be 8 times or more than an interval between the electrode groups which are adjacently parallel to each other.

17. A seawater electrolysis method by using the seawater electrolysis system described in any one of claim 9 to claim 16, wherein the concentration of chloride ions contained in seawater which is subjected to electrolysis is increased, the seawater increased in concentration of chloride ions is introduced into the electrolysis vessel main body, and an electric current is passed between the anode and the cathode to electrolyze the seawater inside the electrolysis vessel main body. 49

18. The seawater electrolysis method according to claim 17, wherein the electric current density on the surface of the anode and that of the cathode between which the electric current is passed by the power supply unit is included in a range of 20A/dm2 or more and 60A/dm2 or less.