METHOD FOR PRODUCING SODIUM HYDROXIDE AND/OR CHLORINE, AND TWO-CHAMBER TYPE ELECTROLYTIC CELL FOR SALTWATER

Abstract

A method for producing sodium hydroxide and/or chlorine by electrolyzing saltwater includes supplying saltwater to an anode chamber of a unit cell in a two-chamber type electrolytic cell, humidifying oxygen-containing gas in a humidifying chamber of the unit cell, and supplying humidified oxygen-containing gas generated in the humidifying chamber to a cathode chamber of the unit cell. The humidifying chamber is adjoined to and in heat exchange relation with the anode chamber or the cathode chamber in the unit cell, or is adjoined to and in heat exchange relation with an anode chamber or a cathode chamber in another unit cell adjacent to the unit cell. The oxygen-containing gas is humidified by generating water vapor with heat from the anode chamber or the cathode chamber adjoined to the humidifying chamber.

Claims

1. A method for producing sodium hydroxide and/or chlorine by electrolyzing saltwater, the method comprising: supplying saltwater to an anode chamber of a unit cell in a two-chamber type electrolytic cell; humidifying oxygen-containing gas in a humidifying chamber of the unit cell; and supplying humidified oxygen-containing gas generated in the humidifying chamber to a cathode chamber of the unit cell, wherein the unit cell comprises: the anode chamber including an anode; the cathode chamber including a gas-diffusion cathode; an ion exchange membrane sandwiched by the anode chamber and the cathode chamber; and the humidifying chamber, wherein the two-chamber type electrolytic cell comprises one or more unit cells, wherein the humidifying chamber is adjoined to and in heat exchange relation with the anode chamber or the cathode chamber in the unit cell, or is adjoined to and in heat exchange relation with an anode chamber or a cathode chamber in another unit cell adjacent to the unit cell, and wherein the oxygen-containing gas is humidified by generating water vapor with heat from the anode chamber or the cathode chamber adjoined to the humidifying chamber.

2. The method according to claim 1, wherein the humidifying chamber is adjoined to the cathode chamber, and wherein the humidified oxygen-containing gas generated in the humidifying chamber is supplied from the humidifying chamber to the cathode chamber through at least one opening located at a partition between the humidifying chamber and the cathode chamber.

3. The method according to claim 2, wherein the at least one opening located at the partition between the humidifying chamber and the cathode chamber comprises a single opening.

4. The method according to claim 2, wherein the at least one opening located at the partition between the humidifying chamber and the cathode chamber comprises a plurality of openings.

5. The method according to claim 1, wherein the humidified oxygen-containing gas generated in the humidifying chamber is supplied from the humidifying chamber to the cathode chamber through at least one flow path located outside the humidifying chamber and the cathode chamber.

6. The method according to claim 5, wherein the at least one flow path located outside the humidifying chamber and the cathode chamber comprises a single flow path.

7. The method according to claim 5, wherein the at least one flow path located outside the humidifying chamber and the cathode chamber comprises a plurality of flow paths.

8. The method according to claim 1, wherein the two-chamber type electrolytic cell comprises at least two unit cells, wherein the at least two unit cells are connected with each other, and wherein the at least two unit cells are arranged such that the sequence of the anode chamber, the cathode chamber, and the humidifying chamber is repeated.

9. A two-chamber type electrolytic cell for saltwater comprising one or more unit cells, each unit cell comprising: an anode chamber; a cathode chamber; an ion exchange membrane sandwiched by the anode chamber and the cathode chamber; and a humidifying chamber, wherein the anode chamber comprises an anode, and is equipped with an inlet for saltwater as a starting material, an outlet for electrolyzed saltwater, and an outlet for chlorine, wherein the cathode chamber comprises a gas-diffusion cathode, and is equipped with an inlet for humidified oxygen-containing gas and an outlet for electrolytic reactant, wherein the humidified oxygen-containing gas is generated in the humidifying chamber and supplied to the cathode chamber, wherein the humidifying chamber is adjoined to and in heat exchange relation with the anode chamber or the cathode chamber in the unit cell, or is adjoined to and in heat exchange relation with an anode chamber or a cathode chamber in another unit cell adjacent to the unit cell, and wherein the humidifying chamber is equipped with an inlet for the oxygen-containing gas.

10. The electrolytic cell according to claim 9, wherein the two-chamber type electrolytic cell comprises at least two unit cells, wherein the at least two unit cells are connected with each other, and wherein the at least two unit cells are arranged such that the sequence of the anode chamber, the cathode chamber, and the humidifying chamber is repeated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a schematic cross-section view of an example of one of unit cells according to one or more embodiments of the present invention.

[0022] FIGS. 2A and 2B are schematic cross-section views of examples of the shape of an opening for supplying humidified oxygen-containing gas according to one or more embodiments of the present invention.

[0023] FIG. 3 is a schematic cross-section view of an example of one of unit cells equipped with a connecting pipe supplying humidified oxygen-containing gas according to one or more embodiments of the present invention.

[0024] FIG. 4 is a schematic cross-section view of an example of a monopolar electrolytic cell according to one or more embodiments of the present invention.

[0025] FIG. 5 is a schematic cross-section view of an example of a bipolar electrolytic cell according to one or more embodiments of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] Hereinafter, a two-chamber type electrolytic cell for saltwater and a method for producing sodium hydroxide and/or chlorine using the electrolytic cell according to one or more embodiments of the present invention will be described with reference to the drawings. The present invention, however, is not limited by the following drawings and can be altered in design within a scope in compliance with the intent described above and below.

[0027] FIG. 1 shows an example of one of unit cells in an electrolytic cell according to one or more embodiments of the present invention. Unit cell 1 has an anode chamber 3, a cathode chamber 4, and an ion exchange membrane 2 sandwiched by the anode chamber 3 and the cathode chamber 4. The anode chamber 3 includes an anode 3a closely adjoining the anode side of the ion exchange membrane 2. The anode chamber 3 is also equipped with an inlet 3b for saltwater as a starting material at a lower part and equipped with an outlet 3c for electrolyzed saltwater and chlorine at an upper part. The cathode chamber 4 is equipped with a liquid retention layer 4b adjoining the cathode side of the ion exchange membrane 2, a gas-diffusion cathode 4a, and if necessary, a gas-diffusion cathode support 4c and a cushion material 4d in this order.

[0028] The unit cell 1 according to one or more embodiments of the present invention is equipped with a humidifying chamber 5 separated from the cathode chamber 4 by a partition 6, and the humidifying chamber 5 is in heat exchange relation with the cathode chamber 4. The partition 6 exemplified in the drawing has a planar shape as shown in FIG. 2A, and the space above the partition 6 in whole performs as an opening 7, which enable humidified oxygen-containing gas to be supplied from the humidifying chamber 5 to the cathode chamber 4. In addition, the cathode chamber 4 is also equipped with a pressure equalizing line 4e to keep water level in the humidifying chamber 5 constant. However, in one or more embodiments of the present invention, if another measure can keep the water level in the humidifying chamber 5 constant, the pressure equalizing line 4e is not required.

[0029] In one or more embodiments, water in the humidifying chamber 5 may be in communication with the outside, or may not be in communication with the outside. In cases where the water is in communication with the outside, a line can be further provided to introduce water from the outside to the humidifying chamber and to discharge heated water to the outside (not shown in the figures). In the case where the water is introduced from the outside, the flow rate and the temperature of the water can be accordingly determined such that the water temperature in the humidifying chamber satisfies the predetermined condition (for example, 80 C. or higher). However, in either case, whether the water is introduced or not introduced from the outside, the only heat produced by electrolytic reaction may be used to heat the water up to the predetermined temperature in view of energy efficiency.

[0030] In the above unit cell 1, saltwater as a starting material is supplied from the inlet 3h for saltwater to the anode chamber 3, and at the same time, oxygen-containing gas is bubbled from an inlet 5a for oxygen-containing gas into water stored in the humidifying chamber 5 to generate the humidified oxygen-containing gas (oxygen concentration may be, for example, 90% or more in some embodiments, and 93% or more in another embodiment). By supplying the humidified oxygen-containing gas to the cathode chamber 4 and applying an electrical current, chlorine is generated art the anode 3a and sodium hydroxide is generated at the gas-diffusion cathode 4a. With progression of the electrolytic reaction of saltwater, heat generated at the cathode is conducted to the humidifying chamber 5, which can heat the water stored in the humidifying chamber 5 and facilitate vaporization of the water in the humidifying chamber 5. The following supply of the oxygen-containing gas to the humidifying chamber 5 by means like bubbling can generate oxygen-containing gas including water vapor with an amount approximately equal to the saturation amount at the water temperature in the humidifying chamber. Therefore, without using extra energy other than the energy for electrolytic reaction, the efficiency of humidifying the oxygen-containing gas can be improved. Moreover, in the case where oxygen-containing gas that contains highly concentrated water vapor is supplied from a humidifier located outside the electrolytic cell, which is disclosed in the Patent Document 1, enough amount of water vapor cannot be supplied because of water condensation in a pipe during being supplied. Additionally, especially in an electrolytic cell having some unit cells, each of the unit cells may be supplied with a different amount of water because the degree of water condensation may vary in each of the unit cells. On the contrary, in one or more embodiments of the present invention, since each of the unit cells is equipped with the humidifying chamber, enough amount of water can be supplied to each of the unit cells without variation of the water amount. Furthermore, by conducting heat of the cathode chamber to the humidifying chamber, overheating of the unit cells, that is, overheating of the electrolytic cell, can be prevented without extra energy for cooling.

[0031] In the example of FIG. 1 described above, the humidifying chamber 5 is adjoined to the cathode chamber 4 in the unit cell 1. However, the humidifying chamber 5 may be adjoined to the anode chamber 3 (as described, the unit cell equipped with the chambers in the sequence of the humidifying chamber 5, the anode chamber 3, and the cathode chamber 4 is hereinafter referred to as a B-type unit cell, the unit cell equipped with the chambers in the sequence of the anode chamber 3, the cathode chamber 4, and the humidifying chamber 5 is hereinafter referred to as a A-type unit cell.). Even in the case of the B-type unit cell, since the electrolytic reaction at the anode chamber 3 is an exothermal reaction, heat generated by the exothermal reaction can be used to heat the humidifying chamber 5 to improve the efficiency of generating water vapor. In addition, as described later, more than one B-type unit cells can be placed next to each other, and in such a case, the humidifying chamber 5 may be adjoined to the cathode chamber 4 of the adjacent unit cell. In this case, the exothermal reaction in the cathode chamber 4 in the adjacent unit cell can improve the efficiency of generating water vapor in the humidifying chamber 5.

[0032] In one or more embodiments, chlorine generated in the anode chamber 3 is discharged from the outlet 3c along with saltwater after electrolyzation. Sodium hydroxide generated in the cathode chamber 4 is transformed into aqueous sodium hydroxide having a concentration of about 32.0% to 34.0% with electro-osmotic water from the anode chamber 3 or moisture in the oxygen-containing gas transferred to the cathode chamber, which runs down the cathode chamber under its weight and is discharged along with exhaust gas of the oxygen-containing gas from an outlet 4g for electrolytic reactant. As described above, since enough amount of water can be supplied to the cathode in one or more embodiments of the present invention, the concentration of the aqueous sodium hydroxide can be kept from being too high, and as a result, damage of the gas-diffusion cathode 4a and the ion exchange membrane 2 can be prevented.

[0033] In one or more embodiments, the partition 6 in the unit cell 1 may have at least one opening 7 having various shapes, as long as the partition 6 allows the humidified oxygen-containing gas to be communicated from the humidifying chamber 5 to the cathode chamber 4 through an upper side of the partition 6. For example, as shown in FIG. 2B, the partition 6 may have a plurality of openings 7 on its upper side. The at least one opening 7 located at the upper side of the partition 6 may occupy the entire upper side of the partition 6 as shown in FIG. 2A, or may occupy a part of the upper side of the partition 6 as shown in FIG. 29. Furthermore, the number of the at least one opening 7 is not particularly limited and may be one or more. Further, the shape of the at least one opening 7 is not particularly limited.

[0034] Moreover, in one or more embodiments, the partition 6 may not have the at least one opening 7, as long as the humidified oxygen-containing gas is allowed to be communicated from the humidifying chamber 5 to the cathode chamber 4. For example, the humidified oxygen-containing gas may be supplied to the cathode chamber 4 through an external flow path such as a connecting pipe 8 as shown in FIG. 3. The unit cell 1 in FIG. 3 is the same as the unit cell in FIG. 1 except that the unit cell 1 in FIG. 3 is equipped with the connecting pipe 8 instead of the opening 7 shown in FIG. 1.

[0035] In the case where the aforementioned B-type unit cell is used, connecting the humidifying chamber 5 and the cathode chamber 4 by the connecting pipe 8 as described above enables the humidified oxygen-containing gas to be supplied from the humidifying chamber 5 to the cathode chamber 4. In addition, in the case where more than one B-type cell is placed next to each other, to enable the oxygen-containing gas to be supplied from the humidifying chamber 5 to the cathode chamber 4 of the adjacent unit cell, the opening 7 may be formed at the boundary of the humidifying chamber 5 and the cathode chamber 4 of the adjacent unit cell, or the connecting pipe 8 may be connected to the humidifying chamber 5 and the cathode chamber 4 of the adjacent unit cell.

[0036] In the unit cell as described above (including both the A-type unit cell and the B-type unit cell; the same applies hereafter), the anode 3a is not particularly limited, as long as it is an insoluble anode used for electrolysis of saltwater. For example, the anode may be such that coating of metal oxide including ruthenium oxide, titanium oxide, iridium oxide, or platinum-group metal oxides is applied on a base substance having a mesh structure such as expanded metal or fine mesh composed of metal including titanium.

[0037] In one or more embodiments, the ion exchange membrane 2 is not particularly limited as long as it can be used for electrolysis of saltwater, and for example, it can be exemplified by a cation exchange membrane of perfluorocarbon-type, in which the ion exchange group is carboxyl acid and/or sulfonic acid.

[0038] In one or more embodiments, the gas-diffusion cathode 4a is not particularly limited as long as it can be used for electrolysis of saltwater by the oxygen cathode method, and it is exemplified by a sheet-like triple-layer electrode in which a base material such as metal mesh-like material, carbon cloth, and/or hydrophobic resin is used, a reactive layer supported by a hydrophile catalyst is jointed on one side of the base material, and a water-shedding gas-diffusion layer is jointed on the other side of the base material. The catalyst can be exemplified by silver, platinum, gold, metal oxides, and carbon. The gas-diffusion cathode may be permeable to liquid, or may not permeable to liquid.

[0039] In one or more embodiments, in the cathode chamber 4, absence of liquid between the ion exchange membrane 2 and the gas-diffusion cathode 4a makes it impossible for current to flow therebetween. While liquid can be retained between the ion exchange membrane 2 and the gas-diffusion cathode 4 by capillary action if the ion exchange membrane 2 and the gas-diffusion cathode 4 are closely attached to each other, the liquid retention layer 4b may be placed between the ion exchange membrane 2 and the gas-diffusion cathode 4a to retain liquid more certainly. The liquid retention layer 4b enables liquid such as aqueous sodium hydroxide to be uniformly retained between the ion exchange membrane 2 and the gas-diffusion cathode 4a to prevent an increase in current density and voltage. Hydrophilicity and corrosion resistance are required for the liquid retention layer because the liquid retention layer needs to retain aqueous sodium hydroxide (having a concentration of about 30% and temperature of about 80 C. to 90 C.) generated by electrolytic reaction. Therefore, carbon materials such as carbon fibers and a porous structure composed of resin may be used.

[0040] An advantage of the two-chamber type method according to one or more embodiments of the present invention is that voltage can be made small due to small electric resistance between the electrodes since the anode, the ion exchange membrane, and the cathode are adjoined to each other. To closely attach the gas-diffusion cathode 4a to the ion exchange membrane 2 (if necessary, through the liquid retention layer 4b), the cushion material 4d may be placed in a compressed state to generate reactive force, which is utilized to closely attach the gas-diffusion cathode 4a to the ion exchange membrane 2. In the two-chamber type method according to one or more embodiments of the present invention, separated by the ion exchange membrane, liquid, pressure exerted by saltwater is applied to the anode chamber, and gas pressure is applied to the cathode chamber. The reactive force of the cushion material 4d is designed in conformity with the difference between the liquid pressure and the gas pressure. Since the deeper the depth of the saltwater is, the larger the liquid pressure is, making the reactive force of the cushion material at the lower side larger than at the upper side of the cathode chamber enables pressure applied to the ion exchange membrane or the anode electrode to be uniform. As such a cushion material 4d, a coiled material or a waved mat material can be used. Since the coiled material has elasticity in the diametrical direction and generates the reactive force in the diametrical direction, the coil axis can be placed parallel to the back board of the cathode gas chamber, and the reactive force of the cushion material can be designed to be larger at the lower side than at the upper side by selecting the wire diameter of the coil, the diameter of the coil material, and the laying density of the coil. As to the waved mat material, waved demister mesh in which metal wires are stocking stitched can be used, and the reactive force of the cushion material can be designed to be larger at the lower side than at the upper side by selecting the diameter of the wires, the number of the wires, and the number of the lamination layers of the mat material.

[0041] In one or more embodiments, the gas-diffusion cathode support 4c can be placed between the cushion material 4d and the gas-diffusion cathode 4a, if necessary. The gas-diffusion cathode support 4c receives the reactive force of the cushion material 4d to allow the force to be uniformed and transmits the uniformed force to the gas-diffusion cathode 4a and the liquid retention layer 4b, and farther, the ion exchange membrane 2. As the gas-diffusion cathode support 4c, mesh materials such as a woven metal wire can be used.

[0042] Since both the cushion material 4d and the gas-diffusion cathode support 4c are placed in the cathode chamber, which is in a high corrosive environment because of high temperatures and the existence of highly concentrated oxygen and highly concentrated sodium hydroxide, nickel or nickel alloy whose nickel content is 20% by weight or more, and silver plating thereof may be used in one or more embodiments.

[0043] As the material for the walls constituting the anode chamber 3, titanium or titanium alloy whose titanium content is 20% by weight or more may be used in one or more embodiments. As the material for the walls constituting the cathode chamber 4 and the humidifying chamber 5, nickel or nickel alloy whose nickel content is 20% by weight or more, and silver plating thereof may be used.

[0044] In one or more embodiments of the present invention, more than one of the aforementioned unit cells (the A-type unit cells or the B-type unit cells) may be placed next to each other to compose the electrolytic cell. In this case, each of the unit cells may be connected in parallel electrically to compose a monopolar electrolytic cell, or each of the unit cells may be connected in series electrically to compose a bipolar electrolytic cell. Hereinafter, the monopolar electrolytic cell and the bipolar electrolytic cell will be explained referring to an example in which more than one of the A-type unit cells having the aforementioned opening 7 for the means of communication of the oxygen-containing gas between the humidifying chamber 5 and the cathode chamber 4 are placed next to each other. The following example may be applied to another example in which the connecting pipe 8 is used or in which more than one of the B-type cells are placed next to each other.

[0045] FIG. 4 is a schematic cross-section view of an example of a monopolar electrolytic cell according to one or more embodiments of the present invention, in which three of the A-type unit cells having the opening 7 are placed next to each other. In the monopolar electrolytic cell 10 shown in FIG. 4, more than one of the A-type unit cells (three of the A-type unit cells in the example of the figure), in which the aforementioned anode chamber 3, the cathode chamber 4, and the humidifying chamber 5 are placed in this sequence, can be arranged such that the sequence in each of the unit cells may be alternately reversed, such as the regular sequence (the anode chamber 3, the cathode chamber 4, and the humidifying chamber 5 are placed in this sequence), the reverse sequence (the humidifying chamber 5, the cathode chamber 4, and the anode chamber 3 are placed in this sequence), the regular sequence, and the reverse sequence. The anode electrodes of the unit cells are connected to an external electric source in parallel electrically respectively, and the cathode electrodes of the unit cells are also connected to the external electric source in parallel electrically. The reference sign 9 shows a water storage tank for adjusting water level in the humidifying chamber. This example also allows heat of the cathode chamber 4 to be transmitted to the humidifying chamber 5 to humidify the oxygen-containing gas efficiently. In such an aforementioned example, in which more than one of the unit cells are placed next to each other such that the sequence in each of the unit cells may be alternately reversed, the humidifying chamber 5 of one unit cell (1) may be adjoined to the humidifying chamber 5 of the next unit cell (2). In this case, one humidifying chamber 5 may be shared by both the unit cell (1) and the unit cell (2).

[0046] FIG. 5 is a schematic cross-section view of an example of a bipolar electrolytic cell according to one or more embodiments of the present invention, in which four of the A-type unit cells having the opening 7 are placed next to each other. In the bipolar electrolytic cell 20 shown in FIG. 5, more than one of the unit cells 1 (four of the unit cells 1 in the example of the figure), each of which has the anode chamber 3 and the cathode chamber 4 internally having the humidifying chamber 5, are arranged such that the sequence of the anode chamber 3, the cathode chamber 4, and the humidifying chamber 5 is repeated. In the bipolar electrolytic cell 20, the anode electrode 3a in one unit cell (1) is capable of being electrically conducted to the cathode electrode 4a in the next unit cell (2) (not shown in the figure), and the cathode electrode 4a at one end and the anode electrode 3a at the other end are connected to an external electric source respectively to connect each of the unit cells in series electrically. Each of the inlets 3h for saltwater of the unit cells, each of the outlets 3c for electrolyzed saltwater and chlorine of the unit cells, each of the inlets 5a for oxygen-containing gas of the unit cells, each of water inlets 5b of the unit cells, each of the outlets 4g for electrolytic reactant of the unit cells, and each of the pressure equalizing lines 4e of the unit cells are respectively connected to one another by pipes, and the water inlets 5b and the pressure equalizing lines 4e are connected to the water storage tank 9. The mechanism of the electrolytic reaction in each of the unit cells of the bipolar electrolytic cell 20 is the same as in the aforementioned unit cells. However, because of the arrangement of the unit cells, Each of the humidifying chambers 5 is adjoined not only to the cathode chamber 4 in the same unit cell but also to the anode chamber 3 in the next unit cell. Therefore, the humidifying chamber 5 can use heat generated by electrolytic reaction in both the anode chamber 3 and the cathode chamber 4 for humidification, which results in the increase of the heat efficiency in humidification. In addition, since the heat generated in the anode chamber 3 is transferred to the humidifying chamber 5, the anode chamber 3 can be cooled at the same time.

[0047] Moreover, in the case where more than one of the B-type unit cells (having the sequence of the humidifying chamber, the anode chamber, and the cathode chamber) are arranged regularly without alternately reversing the sequence of each of the unit cells to compose a bipolar electrolytic cell such as the example shown in FIG. 5, the humidifying chamber 5 is sandwiched by the anode chamber 3 and the cathode chamber 4. In this case, the humidifying chamber 5 can also use heat generated in both the anode chamber 3 and the cathode chamber 4 during humidification.

[0048] The present application claims priority based on Japanese Patent Application No. 2017-068057 filed on Mar. 30, 2017. All the contents described in Japanese Patent Application No. 2017-068057 filed on Mar. 30, 2017 are incorporated herein by reference.

EXAMPLES

[0049] Hereinafter, one or more embodiments of the present invention are more specifically described with reference to examples. The present invention, however, is not limited by the following examples but can also be absolutely carried out with appropriate changes to the examples within a scope in compliance with the intent described above and later, and all the changes are to be encompassed within a technical scope of the present invention.

Example 1

[0050] Five of the unit cells shown in FIG. 1 (except not having a gas-diffusion cathode support) were arranged such that the sequence of an anode chamber, a cathode chamber, and a humidifying chamber was repeated in this sequence, and were connected to each other in series electrically to compose a bipolar two-chamber type electrolytic cell for saltwater. DSE manufactured by De Nora Permelec Ltd. (insoluble metal in which metal base substance is coated with platinum-group metal or oxides thereof as a main component) was used as an anode electrode; gas-liquid transmissive carbon-silver electrode (GDE2013) manufactured by De Nora Permelec Ltd. was used as an cathode electrode; 4403D manufactured by Asahi Kasei Chemicals Corporation was used as an ion exchange membrane; carbon fiber woven fabric, the thickness of which is 0.45 mm, was used as a liquid retention layer; and silver-plated nickel wire in spiral shape was used as a cushion material.

[0051] Saltwater having a concentration of 218 g/L and a temperature of 53.8 C. was supplied to the anode chamber at the rate of 183 L/m.sup.2/h. Water was stored in the humidifying chamber, to which 1.5 times the theoretical requisite moles of oxygen-containing gas (corresponds to oxygen-containing gas supplied to electrolytic cell shown in the below table 1) having a temperature of 25 C. and a concentration of 93.0% was supplied by bubbling. The temperature of the humidifying chamber was 84.0 C., and therefore, at the time when being supplied to the cathode chamber, the temperature of the humidified oxygen-containing gas was about 84.0 C. The saltwater was electrolyzed at the current density of 5.65 kA/m.sup.2, and each value was measured after ten days of the electrolyzation.

Comparative Example 1

[0052] Except not having the humidifying chamber in a unit cell, five of the same unit cells as Example 1 having the same material of the anode electrode, the cathode electrode, the ion exchange membrane, etc. and the same size of the cathode chamber, the anode chamber, etc. were arranged such that the sequence of the anode chamber, and the cathode chamber is repeated in this sequence, and were connected to each other in series electrically to compose a conventional bipolar two-chamber type electrolytic cell for saltwater (not shown in the figures).

[0053] Saltwater having a concentration of 219 g/L, and a temperature of 51.4 C. was supplied to the anode chamber at the rate of 183 m.sup.2/h. The cathode chamber of each of the unit cells was connected to a humidifier prepared outside the electrolytic cell. In the humidifier, 1.5 times the theoretical requisite moles of oxygen-containing gas having a concentration of 93.0% was bubbled into the water (25 C.) in the humidifier to generate humidified oxygen-containing gas having a temperature of 25 C., which was supplied to the cathode chamber at the same temperature. The saltwater was electrolyzed at the current density of 5.65 kA/m.sup.2, and each value was measured after ten days of the electrolyzation. Being different from Example 1, Comparative Example 1 did not have the humidifying chamber in the unit cell and the humidified oxygen-containing gas was supplied from the external humidifier, and therefore, the meaning of oxygen-containing gas supplied to electrolytic cell and oxygen-containing gas supplied to cathode chamber shown in the below table 1 are identical in meaning (the same applies to the following Comparative Examples 2 to 4).

Comparative Example 2

[0054] In the same manner as in Comparative Example 1 except that the water temperature of the external humidifier in Comparative Example 1 was altered to 84 C., and that humidified oxygen-containing gas generated at 84 C. was supplied to the cathode chamber at the same temperature (heat input rate to the humidifier was about 5.2 MJ/m.sup.2/h), each value was measured after ten days of the electrolyzation.

Comparative Example 3

[0055] With the same composition as in Comparative Example 2, additionally, heat-retention around the pipe connecting the humidifier and the anode chamber of each of the unit cells was improved to prevent the temperature of the humidified oxygen-containing gas from decreasing, and each value was measured after ten days of the electrolyzation. The heat input rate to the humidifier was about 5.2 MJ/m.sup.2/h, which was the same as in Comparative Example 2.

Example 2

[0056] The same electrolytic cell as in Example 1 was operated for 300 days, after which each value was measured.

Comparative Example 4

[0057] The same electrolytic cell as in Comparative Example 1 was operated for 300 days, after which each value was measured.

[0058] The measurement results of the Examples and the Comparative Examples are shown in Table 1 and Table 2. Table 2 shows both the average value of five of the unit cells and the difference between the average value and each value of five of the unit cells in the concentration of generated sodium hydroxide and current efficiency.

TABLE-US-00001 TABLE 1 Oxygen-containing gas supllied to Liquid in anode electrolysis cell Current Supplied Saline water compartment Multiple of density Amount Concentration Temperature Concerntration Temperature Concentration theoretical Temperature (kA/m.sup.2) (L/m.sup.2/h) (g/L) ( C.) (g/L) ( C.) (%) amount ( C.) Example 1 5.65 183 218 53.8 173 84.0 93.0 1.5 25.0 Comparative 5.65 183 219 51.4 174 84.0 93.0 1.5 25.0 Example 1 Comparative 5.65 183 219 47.0 174 84.0 93.0 1.5 84.0 Example 2 Comparative 5.65 183 219 47.0 174 84.0 93.0 1.5 84.0 Example 3 Example 2 5.65 183 218 53.3 173 84.0 93.0 1.5 25.0 Comparative 5.65 183 219 51.0 174 84.0 93.0 1.5 25.0 Example 4 Oxygen-containing gas supplied to Average cathode Temperature Temperature concentration compartment Cell in humidifying Temperature of exhaust of generated Current Temperature voltage compartment of catode gas NaOH efficiency ( C.) (V) ( C.) ( C.) ( C.) (%) (%) Example 1 84.0 2.30 84.0 82.0 82.0 32.2 96.5 Comparative 25.0 2.32 75.0 75.0 34.6 96.5 Example 1 Comparative 84.0 2.30 82.0 82.0 32.2 96.4 Example 2 Comparative 84.0 2.30 82.0 82.0 32.2 96.5 Example 3 Example 2 84.0 2.35 84.0 82.0 82.0 32.2 96.3 Comparative 25.0 2.43 75.0 75.0 34.6 96.0 Example 4

TABLE-US-00002 TABLE 2 Concentration of generated NaOH Current efficiency Average Average (%) #1 #2 #3 #4 #5 (%) #1 #2 #3 #4 #5 Example 1 32.2 0.2% +0.1% +0.1% +0.2% 0.2% 96.5 0.2% +0.1% .sup.0% +0.2% 0.1% Comparative 34.6 0.2% +0.2% +0.1% +0.1% 0.2% 96.5 0.3% +0.1% +0.1% .sup.0% +0.1% Example 1 Comparative 32.2 +0.5% 0.2% 0.4% 0.2% +0.3% 96.4 0.4% +0.2% +0.3% +0.2% 0.3% Example 2 Comparative 32.2 0.1% +0.2% +0.2% .sup.0% 0.3% 96.5 0.2% +0.2% +0.1% .sup.0% 0.1% Example 3 Example 2 32.2 0.1% +0.1% .sup.0% +0.2% 0.2% 96.3 0.2% +0.1% .sup.0% +0.2% 0.1% Comparative 34.6 .sup.0% +0.2% +0.1% 0.1% 0.2% 96.0 0.3% .sup.+0% +0.2% +0.2% 0.1% Example 4

[0059] In Example 1, due to the reaction heat, the temperature of the humidifying chamber was equivalent to the temperature of the anode chamber, and the concentration of generated sodium hydroxide was 32.2%, not too high, which shows that enough amount of water vapor was supplied (Table 1). In addition, variation in the concentration of generated sodium hydroxide and in current efficiency in each of five unit cells could be made small, which shows that the variation in the amount of the water vapor supplied to the anode chamber of each of the unit cells was small (Table 2). Moreover, Example 2, in which the electrolyzation was operated longer than in Example 1, showed good current efficiency of 96.3% even after 300 days, as well as good results in other values almost the same as in Example 1. Since enough amount of water vapor could be supplied in Example 2, low concentration of generated sodium hydroxide could be maintained, and since damage of the gas-diffusion cathode was prevented in Example 2, the difference in voltage of the gas-diffusion cathode after 300 days of electryzation was as small as 45 mV on average. The differences in voltage of the gas-diffusion cathode in each of the unit sells were 78 mV, 15 mV, 45 mV, 33 mV, and 54 mV respectively.

[0060] On the other hand, in Comparative Example 1, in which humidified oxygen-containing gas was supplied from the external humidifier, since the temperature of the external humidifier was 25.0 C., the pressure of water vapor in the gas was low, and the concentration of generated sodium hydroxide was as high as 34.6% which shows that the supplied amount of water vapor was insufficient (Table 1).

[0061] Comparative Example 2 was an example in which the temperature of the external humidifier was altered to 84 C. to increase the water vapor pressure in the oxygen-containing gas, which required energy to raise water temperature in the external humidifier. The average concentration of generated sodium hydroxide was 32.2% which means that enough amount of water vapor could be supplied on the average, however, as shown in Table 2, the variation in both the concentration of sodium hydroxide and current efficiency of each unit cell became large. The reason of this is considered that water was condensed in the process where the oxygen-containing gas was supplied from the external humidifier to the cathode chamber of each of the unit cells, and the degree of the condensation was different in each of the unit cells. In addition, since the oxygen-containing gas of high temperature was supplied from outside the electrolytic cell, the saltwater supplied to the anode electrode was required to have a low temperature to prevent the electrolytic cell from overheating (while the temperature of the supplied saltwater in Example 1 was 0.53.8 C., which is within the usual range of the temperature in saltwater electrolysis plants, the temperature of the supplied saltwater in Comparative Example 2 was 47.0 C.), and thus, extra energy was required for cooling the supplied saltwater.

[0062] Comparative Example 3 was an example in which heat-retention of the pipe from the external humidifier in Comparative Example 2 was improved, and required extra energy for heating in the external humidifier, which is the same as in Comparative Example 2. In Comparative Example 3, water condensation was suppressed in the pipes, and as a result, the variations in the concentration of the generated sodium hydroxide and the current efficiency of each of the unit cells could be made small, however, extra energy for cooling the supplied saltwater was required in the same manner as in Comparative Example 2.

[0063] Comparative Example 4 was an example in which the electrolytic cell was operated for 300 days in the same condition as in Comparative Example 1, and the difference in the voltage of the gas-diffusion cathode after 300 days of electrolyzation was 108 mV on average of five of the unit cells, which was much higher than in Example 2, and current efficiency was 96.0%, which was lower than in Example 2. The reason of this is considered that as the same as in Comparative Example 1, the concentration of generated sodium hydroxide was 34.6% in Comparative Example 4, which was 2% or higher than the concentration of sodium hydroxide of 32.2% in Examples 1 and 2, and as a result, the gas-diffusion cathode was damaged. The differences in voltage of the gas-diffusion cathode in each of the unit sells were 123 mV, 66 mV, 114 mV, 108 mV, and 129 mV respectively.

[0064] Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims.

DESCRIPTION OF REFERENCE SIGNS

[0065] 1: unit cell [0066] 2: ion exchange membrane [0067] 3: anode chamber [0068] 3a: anode electrode [0069] 4: cathode chamber [0070] 4a: gas-diffusion cathode [0071] 5: humidifying chamber [0072] 6: partition [0073] 7: opening [0074] 8: connecting pipe [0075] 10: monopolar electrolytic cell [0076] 20: bipolar electrolytic cell