METHOD FOR OPERATING APPARATUS FOR PRODUCING ALKALI HYDROXIDE

Abstract

Apparatus for producing alkali hydroxide and method for operating apparatus for producing alkali hydroxide are provided. A cooling chamber through which a coolant can pass is constructed by placing a separation wall in a cathode chamber on a side opposite to an ion-exchange membrane, and a flow rate adjuster, such as manual valves, which can adjust the supply flow rate of the coolant is placed in each unit cell. The electrolytic temperature of each unit cell is regulated at an optimum operating temperature depending on the current density by adjusting the flow rate of the coolant without individually adjusting the flow rate of salt water supplied to the unit cell or the concentration of the salt water.

Claims

1. A method for operating an apparatus for producing an alkali hydroxide, the apparatus having electrolytic cells each constructed by separating an anode chamber and a cathode chamber with an ion-exchange membrane, providing an anode in the anode chamber and providing a gas diffusion electrode in the cathode chamber, and electrolysis being conducted while an aqueous alkali chloride solution is supplied to the anode chamber and while an oxygen-containing gas is supplied to the cathode chamber, the method using: a plurality of groups of the electrolytic cells, the plurality of groups being connected in series in a current path, the electrolytic cells within a same group being connected in parallel or series in the current path, flow passages provided to the electrolytic cells, a coolant passing through the flow passages, a flow rate adjuster provided to each electrolytic cell or each group of the electrolytic cells, the flow rate adjuster being able to adjust flow rates of the coolant, a coolant supply passage for supplying the coolant to the flow passages, a pressure-adjusting valve and a pressure detector placed at an upstream part of a branch part in the coolant supply passage, the branch part being a part at which the coolant supply passage is branched corresponding to the plurality of groups of the electrolytic cells, and a heat exchanger placed at the upstream part of the branch part, the method comprising: adjusting a temperature of the coolant to a set temperature by the heat exchanger, cooling the electrolytic cells by passing the coolant through the flow passages, adjusting flow rates of the coolant passing through the flow passages individually in each electrolytic cell or in each group of the electrolytic cells by the flow rate adjuster, providing a set pressure value of the coolant based on an electrolytic current density and a stored predetermined relation between the electrolytic current density and the set pressure value of the coolant, the electrolytic current density being provided based on a detected value of a current supplied to the plurality of groups of the electrolytic cells, and adjusting a degree of opening of the pressure-adjusting valve based on a difference between the set pressure value and a pressure value measured by the pressure gauge.

2. The method for operating an apparatus for producing an alkali hydroxide according to claim 1, wherein each of the flow passages through which the coolant passes is provided on a wall side facing the gas diffusion electrode across the cathode chamber.

3. The method for operating an apparatus for producing an alkali hydroxide according to claim 1, wherein the flow rate adjuster is provided to each group of the electrolytic cells connected in parallel in the current path, or each of the electrolytic cells connected in series.

4. The method for operating an apparatus for producing an alkali hydroxide according to claim 1, the method further comprising: recovering the coolant discharged from the flow passages of the electrolytic cells in a recovery tank for recovering the coolant, recooling the coolant recovered in the recovery tank to a set temperature, and supplying the coolant recooled by the cooling unit to the flow passages.

5. The method for operating an apparatus for producing an alkali hydroxide according to claim 1, wherein a temperature of the coolant supplied to the flow passages is regulated at 60° C. or higher in an operation of heating the electrolytic cells before an electrical current is applied.

6. The method for operating an apparatus for producing an alkali hydroxide according to claim 1, wherein when an operation of the electrolytic cells is stopped by shutting off the electrical current, a supply of the coolant is continued, and a supply temperature of the coolant is regulated at 60° C. or lower.

7. A method for operating an apparatus for producing an alkali hydroxide, the apparatus having electrolytic cells each constructed by separating an anode chamber and a cathode chamber with an ion-exchange membrane, providing an anode in the anode chamber and providing a gas diffusion electrode in the cathode chamber, and electrolysis being conducted while an aqueous alkali chloride solution is supplied to the anode chamber and while an oxygen-containing gas is supplied to the cathode chamber, the method using: a plurality of groups of the electrolytic cells, the plurality of groups being connected in series in a current path, the electrolytic cells within a same group being connected in parallel or series in the current path, flow passages configured such that a coolant passes therethrough and the coolant is placed on a back-surface side of a wall facing the ion-exchange membrane across the cathode chamber, a flow rate adjuster provided to each electrolytic cell or each group of the electrolytic cells, the flow rate adjuster being able to adjust flow rates of the coolant, a coolant supply passage for supplying the coolant to the flow passages, a pressure-adjusting valve and a pressure detector placed at an upstream part of a branch part in the coolant supply passage, the branch part being a part at which the coolant supply passage is branched corresponding to the plurality of groups of the electrolytic cells, and a heat exchanger placed at the upstream part of the branch part, the method comprising: adjusting a temperature of the coolant to a set temperature by the heat exchanger, cooling the electrolytic cells by passing the coolant through the flow passages, adjusting flow rates of the coolant passing through the flow passages individually in each electrolytic cell or in each group of the electrolytic cells by the flow rate adjuster, providing a set pressure value of the coolant based on an electrolytic current density and a stored predetermined relation between the electrolytic current density and the set pressure value of the coolant, the electrolytic current density being provided based on a detected value of a current supplied to the plurality of groups of the electrolytic cells, and adjusting a degree of opening of the pressure-adjusting valve based on a difference between the set pressure value and a pressure value measured by the pressure gauge.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0026] FIG. 1 is a schematic figure showing the structure of a unit cell which is one unit when the apparatus for producing an alkali hydroxide according to an embodiment of the invention is applied to a monopolar electrolytic bath.

[0027] FIG. 2 is a sectional figure showing the details of the structure of the unit cell shown in FIG. 1.

[0028] FIG. 3 is a schematic figure showing the structure of an apparatus for producing an alkali hydroxide including a monopolar electrolytic bath having unit cells of the type shown in FIG. 1.

[0029] FIG. 4 is a figure explaining the electric circuit of the monopolar electrolytic bath shown in FIG. 1.

[0030] FIG. 5 is a schematic figure showing the structure of a unit cell which is one unit when the apparatus for producing an alkali hydroxide according to an embodiment of the invention is applied to a bipolar electrolytic bath or to a single-element electrolytic bath.

[0031] FIG. 6 is a schematic figure of a bipolar electrolytic bath or a single-element electrolytic bath in which unit cells of the type shown in FIG. 5 are layered.

[0032] FIG. 7 is a schematic figure showing the structure of an apparatus for producing an alkali hydroxide composed of a plurality (two sets for example) of connected electrolytic baths of the type shown in FIG. 6.

[0033] FIG. 8 is a graph showing the relation between the electrolytic current density and the pressure of the cooling water of a test apparatus in which the electrolytic cells are cooled using the cooling system shown in FIG. 3 or FIG. 7.

[0034] FIG. 9 is a graph showing the relation between the electrolytic current density and the flow rate of the cooling water of a test apparatus in which the flow rate of the cooling water can be adjusted independently in each of electrolytic cells having the cooling system shown in FIG. 3 or FIG. 7.

[0035] FIG. 10 is a graph showing the results of a comparative test in which the relations between the current efficiency of the cathode of the electrolytic bath and the operation period (days) were compared between the case using cooling water and the case without using any cooling water.

DESCRIPTION OF EMBODIMENTS

[0036] The apparatus for producing an alkali hydroxide and the method for operating the apparatus according to embodiments of the invention described below are used for the purpose of generating an alkali hydroxide and chlorine through electrolysis and mainly used for the purpose of generating sodium hydroxide and chlorine by electrolyzing brine.

[0037] FIG. 1 is a schematic figure showing a unit cell which is a component (one unit) of a monopolar electrolytic bath which is a two-chamber electrolytic bath, and FIG. 2 is a sectional figure showing the details of the partial structure of the unit cell of FIG. 1. In the unit cell, six electrolytic cells each obtained by separating an anode chamber (a white region) 2 and a cathode chamber (a black region) 3 with an ion-exchange membrane 1 are layered, and two adjacent electrolytic cells share one anode chamber 2.

[0038] As shown in FIG. 2, an anode 11 is placed on the anode chamber 2 side of the ion-exchange membrane 1, and a liquid-retaining layer 12 and a gas diffusion electrode 13 serving as the cathode are layered in this order on the cathode chamber 3 side of the ion-exchange membrane 1. An inlet 21 for salt water (a sodium chloride solution) as the anolyte is formed in the bottom surface of the anode chamber 2, and an outlet 22 for discharging brine as the anolyte and chlorine gas generated through the electrolysis reaction is formed in the upper surface of the anode chamber 2. 21a is a supply passage for the brine, and 22a is an outlet passage for the brine and the chlorine gas. The passages are each composed of a pipe.

[0039] An inlet 31 for an oxygen-containing gas is formed on the upper side of the cathode chamber 3, and a supply passage for the oxygen-containing gas, which is not shown in the figure, is connected to the inlet 31. An outlet 32 for discharging an aqueous sodium hydroxide solution, which is an aqueous alkali hydroxide solution generated through the electrolysis reaction, and excess oxygen is formed on the bottom side of the cathode chamber 3, and an outlet passage for the aqueous sodium hydroxide solution and excess oxygen, which is not shown in the figure, is connected to the outlet 32.

[0040] A cooling chamber 4 (a region with slant lines in FIG. 1) which forms a flow passage through which cooling water as a coolant passes is placed on the back-surface side of the wall facing the ion-exchange membrane 1 across the cathode chamber 3. In other words, a separation wall 40 (see FIG. 2) is placed in the frame constituting the cathode chamber 3, which is conductive and in which the gas diffusion electrode 13, a current collector, an elastic material and the like are arranged, on the side opposite to the ion-exchange membrane 1 seen from the cathode chamber 3. The separation wall 40 constitutes a region separated from the cathode chamber 3 as the cooling chamber 4. The material of the separation wall 40 is preferably a high nickel alloy material in view of the resistance to corrosion, the conductivity and the costs, and SUS310S, pure nickel and the like are preferable materials. When an electrolytic bath equipped with hydrogen generation cathodes is converted into a gas diffusion two-chamber electrolytic bath, the rigid mesh material attached in parallel to the electrolysis surface as a material constituting a cathode of the hydrogen generation electrolytic bath can be used for reinforcing the bend of the separation wall 40. In this case, the structural strength is enhanced. Moreover, because the coolant on the back surface of the separation wall 40 directly touches the rigid mesh material, the effect of enlarging the effective heat transfer area is obtained, and the thermal conductivity can be increased.

[0041] A cooling water inlet 41 and a cooling water outlet 42 are formed at the bottom and on the upper surface of each cooling chamber 4, respectively.

[0042] FIG. 3 shows a structure in which the invention is applied to a monopolar electrolytic bath composed of a plurality of, for example four, unit cells of the type shown in FIG. 1. As shown in FIG. 4, the six electrolytic cells constituting a unit cell are connected in parallel to each other and to a direct current power source, and the four unit cells are connected in series. The symbols U's in FIG. 4 each indicate a unit cell of the type shown in FIG. 1, and the symbols “+” and “−” indicate the positive electrode and the negative electrode of the direct current power source, respectively.

[0043] When the part for supplying cooling water to the electrolytic cells is called a cooling system here, the cooling system has a cooling water tank 51, a circulation pump 52 and a cooling water supply passage 53 and a cooling water recovery passage 54 which are each composed of a pipe as shown in FIG. 3. The cooling water supply passage 53 is branched into four passages to distribute the cooling water sent from the cooling water tank 51 to the unit cells. Manual valves V1 to V4 which are flow rate-adjusting valves for adjusting the flow rates of the cooling water supplied to the four unit cells independently (individually) are placed in the four branch passages. The cooling water recovery passages 54 connected to the cooling water outlets 42 of the six electrolytic cells constituting a unit cell meet and form a combined passage, and the four combined passages of the unit cells meet and are connected to the cooling water tank 51.

[0044] In the cooling water supply passage 53, a cooling-water-pressure-adjusting valve (simply called a pressure-adjusting valve below) 61 and a cooling water pressure gauge (simply called a pressure gauge below) 62 are placed in this order from the upper stream at an upstream part of the part at which the cooling water supply passage 53 is branched corresponding to the unit cells. The degree of opening of the pressure-adjusting valve 61 is adjusted by a first controller 63, and the pressure of the cooling water is thus regulated.

[0045] As shown in FIG. 3, the first controller 63 has, for example, a function generator 63a which defines the relation between the set pressure value of the cooling water and the electrolytic current density and an adjuster 63b which outputs a controlled amount based on the difference between the set pressure value output from the function generator 63a and the pressure value measured by the pressure gauge 62, for example, through PID calculation. In other words, the function generator 63a is an output unit which outputs a set pressure value based on the electrolytic current density. The electrolytic current density which is input into the function generator 63a is a value obtained by dividing the value of current flowing in all the four unit cells (the unit cells indicated by the symbols U's in FIG. 4) described above, namely the detected value of the current supplied to the four unit cells from the direct current power source (the current detector is not shown in the figure), by the entire electrode area (the entire area of the anodes 11) of one unit cell. Here, the function generator 63a and the adjuster 63b of the first controller 63 may be hard components or software. When the function generator 63 is software, two or more sets of the set pressure value of the cooling water and the electrolytic current density are input into a memory, and a graph is drawn by interpolating the input data with a program. The relation between the set pressure value of the cooling water and the electrolytic current density will be described in detail in the section explaining the function.

[0046] A heat exchanger 64 is placed between the pressure-adjusting valve 61 and the pressure gauge 62 in the cooling water supply passage 53, and a cooling water thermometer 65 is placed in a downstream part of the heat exchanger 64. 66 is a second controller. By adjusting the supply amount of the primary cooling water to the heat exchanger 64 with a flow rate-adjusting valve 67 placed in the flow passage of the primary cooling water based on the temperature value detected by the cooling water thermometer 65 and the set temperature value (set temperature), the temperature of the cooling water supplied to the unit cells is adjusted to the set temperature.

[0047] A bypass passage 68 which is composed of a pipe and which makes a detour around the four unit cells and returns to the tank 51 is connected to the cooling water supply passage 53 at a downstream part of the pressure gauge 62. The bypass passage 68 also serves as a flow passage to let the cooling water out of the unit cells. 69 is a circulation passage of the cooling water tank 51, and 70 is a supply passage for supplemental cooling water for adding cooling water to the cooling water tank 51. 71 is an overflow, and V0, V5 and V6 are valves.

[0048] In some cases, depending on the flow rate of the cooling water, the pressures applied to the separation walls 40 and the like in the cathode chambers 3 change due to siphonage caused by the downflow of the cooling water, or the cooling water comes out. Thus, siphon breakers 55 are desirably attached to the cooling water recovery passages 54 at a part higher than the unit cells.

[0049] Next, the structure of an apparatus in which the invention is applied to a bipolar electrolytic bath or to a single-element electrolytic bath is described. FIG. 5 is a schematic figure showing a unit cell which is a component (one unit) of a bipolar electrolytic bath or a single-element electrolytic bath, and FIG. 6 shows a structure in which six unit cells of the type shown in FIG. 5 are layered. As described above, because the electrolytic cells are connected in parallel in the current circuit of a monopolar electrolytic bath, there is one manual valve for individually adjusting the flow rate of the cooling water sent to one unit cell (any of V1 to V4). On the other hand, because the electrolytic cells are connected in series in the current circuit of a bipolar electrolytic bath or a single-element electrolytic bath, the unit shown in FIG. 6, for example, has six unit cells. Thus, six manual valves each for adjusting the flow rate of cooling water individually are described. The respective manual valves which are the flow rate-adjusting valves provided for the six unit cells are each given a symbol V in order to avoid any complicated description.

[0050] The structure of the flow of the cooling water in a unit cell is similar to the structure shown in FIG. 2, and the cooling chamber 4 is placed on the back-surface side of the separation wall 40 which is a wall facing the ion-exchange membrane 1 across the cathode chamber 3. In FIG. 7, two layered structures each having six unit cells of the type shown in FIG. 6 are used, and a cooling system similar to that shown in FIG. 3 is combined. In FIG. 7, the parts corresponding to those of FIG. 3 are given the same symbols. In this regard, the two layered structures each having six unit cells are electrically connected in series.

[0051] Similar effects can be expected in the case of attaching one siphon breaker 55 to each unit cell (FIG. 5, for example) and in the case of attaching one siphon breaker 55 to each layered structure (FIG. 6, for example). The siphon breakers 55 are attached at required sites, but one siphon breaker 55 is preferably provided for each layered structure in view of the management.

[0052] Ion-exchanged water having electrical conductivity of 10 microsiemens or less is preferably used as the coolant, and the stray current from a unit cell can be prevented from leaking to the outside when such a coolant is used. It is preferable to provide a measuring unit for continuously measuring at least one of the pH and the electrical conductivity of the coolant circulating through the flow passages of the electrolytic cells. With the measuring unit, a decrease in the cleanliness of the coolant or the presence or absence of contamination of the coolant with the electrolyte due to the breakage of the separation wall in an electrolytic cell or the like can be monitored.

[0053] Next, a method for operating the apparatus for producing an alkali hydroxide shown in FIG. 3 or FIG. 7 is described. First, a brief description of electrolysis reaction is as follows. An electrical current is applied to each electrolytic cell. Then, brine is supplied to the anode chamber 2, and a gas containing oxygen is supplied to the cathode chamber 3 at the same time. Water containing sodium ions exudes from the liquid-retaining layer 12 retaining an aqueous sodium hydroxide solution to the gas diffusion electrode 13 and reacts with oxygen in the cathode chamber 3 to generate an aqueous sodium hydroxide solution. Also, chlorine ions in the brine turn into a chlorine gas in the anode chamber 2 and are discharged with the brine.

[0054] Cooling water is supplied to the electrolytic cell (unit cell) by the cooling system, and the electrolytic cell is thus cooled. It is preferable to supply the cooling water to the unit cell at a sufficient flow rate, make the difference in temperature between the cooling water inlet 41 and the cooling water outlet 42 small and remove heat evenly from the electrolysis surface. It is preferable to flood and pass water through the electrolytic cell from the bottom to the top because the cooling water can be supplied to the electrolytic cell at a high cooling water flow rate.

[0055] When the internal temperature of the electrolytic cell (the temperature of the anode chamber 2 or the surface temperature of the cathode) and the temperature of the cooling water are too close, the heat transfer efficiency decreases, and the evenness of the internal temperature of the electrolytic bath is enhanced. Thus, the temperature difference between the internal temperature of the electrolytic bath and the supply temperature of the cooling water is preferably 5° C. to 60° C., more preferably 10° C. to 40° C., further preferably 10° C. to 25° C. The temperature difference between the temperature of the anode chamber 2 and the temperature of the cooling water outlet 42 is preferably 1° C. or more, more preferably 3° C. or more.

[0056] The temperature of the cooling water is set in the temperature range for the purpose of making the temperature difference from the internal temperature of the electrolytic cell small to make the current distribution of the electrolytic cell excellent. For example, a preferable example of the temperature of the anode chamber 2 of the electrolytic cell is 70 to 90° C. In the case of 85° C. for example, because the most preferable range of the temperature difference from the supply temperature of the cooling water is 25 to 10° C., the supply temperature of the cooling water is set in the range of 60 to 75° C. When the temperature of the cooling water outlet 42 is around the temperature of the anode chamber 2, the cooling efficiency deteriorates, and thus the flow rate is determined in a manner that an adequate outlet temperature is obtained during high current density operation with a heavy heat load. The value of the high current density operation with a heavy heat load is the maximum value in the determined operating range, and examples of the maximum value of the operating range are values of 3 kA/m.sup.2, 7 kA/m.sup.2 and the like.

[0057] The supply temperature of the cooling water is adjusted at an appropriate temperature by setting the set temperature value of the second controller 66 at a value selected in the temperature range described above, for example, and adjusting the flow rate of the primary cooling water with the flow rate-adjusting valve 67 in a manner that the temperature value detected by the thermometer 65 becomes the set temperature value.

[0058] The flow rates of the cooling water to the unit cells are adjusted by an operator depending on the respective operating voltages of the unit cells using manual valves which are individual flow rate-adjusting valves. The manual valves are “V1 to V4” in the apparatus shown in FIG. 3 and are “V's” in the apparatus shown in FIG. 7. The timing of adjusting the manual valves is, for example, after starting the first operation, after starting operation after the maintenance or the change of the electrodes or the ion-exchange membranes in the electrolytic bath or the like.

[0059] Accordingly, the cooling water is supplied at a relatively high flow rate to a unit cell in which the operating voltage becomes high and in which the temperature(s) of the electrolytic cell(s) is increasing, while the cooling water is supplied at a relatively low flow rate to a unit cell in which the operating voltage becomes low and in which the temperature(s) of the electrolytic cell(s) is decreasing. Therefore, the temperature difference among the unit cells can be kept small.

[0060] Next, the regulation of the pressure of the cooling water by the first controller 63 is explained. FIG. 8 is a graph showing the relation between the electrolytic current density and the pressure of the cooling water in the case of conducting cooling regulation using a test apparatus having one electrolytic cell and the regulation system shown in FIG. 3. The relation between the electrolytic current density and the pressure of the cooling water (an example is shown in FIG. 8) is input in advance into the function generator 63a of the first controller 63. The relation is input in a manner that the minimum area of the operating range of the electrolytic current density is ignored and that the ratio of the flow rate of the cooling water to the electrolytic current density becomes constant or the ratio of the flow rate of the cooling water to the electrolytic current density gradually increases in the range from ⅓ or ½ of the maximum electrolytic current density to the maximum electrolytic current density. The relation between the electrolytic current density and the pressure of the cooling water is preferably determined by an experiment in advance, and the maximum pressure value of the cooling water should be the maximum pressure which can be applied to the electrolytic cell cooling water part or smaller. In the case of the example of FIG. 8, this is an example in which the set pressure value of the cooling water at 4.0 kA/m.sup.2 is approximately 56 kpa/G, which is almost the maximum pressure, when the maximum pressure that can be applied to the cooling water part is 60 kpa/G and the maximum value of the operating range of the electrolytic current density is 4.0 kA/m.sup.2, and this is an example in which the cooling water amount increases in the range from 1.3 kA/m.sup.2, which is ⅓ of the maximum electrolytic current density, or from 2 kA/m.sup.2, which is ½, to 4 kA/m.sup.2 (FIG. 9).

[0061] FIG. 9 is a graph showing the relation between the electrolytic current density and the flow rate of the cooling water of a test apparatus in which six electrolytic cells are used and in which the flow rate of the cooling water can be adjusted independently in each electrolytic cell, and the graph shows the data of an electrolytic cell with the maximum flow rate of the cooling water and of an electrolytic cell with the minimum flow rate. From FIG. 8 and FIG. 9, it can be seen that, since the temperature of an electrolytic cell increases as the electrolytic current density increases, the action of cooling is exerted to inhibit the temperature increase.

[0062] In a method for adjusting the supply flow rates of the cooling water individually to each the unit cell, the supply flow rates are determined, for example, based on the subject to be cooled in which the water amount should be the lowest (an electrolytic cell with the lowest operating electrolytic temperature or the like). In this case, the degree of opening of the flow rate adjuster (any of the manual valves indicated by V1 to V4 and V's in the above examples) for the subject to be cooled with the lowest flow rate of the cooling water is adjusted to a degree of opening resulting in the lowest target flow rate under the operating conditions under which the cooling load becomes the lightest. Regarding the other unit cells which are the subjects to be cooled in which the flow rates should be increased one by one, the degrees of opening are adjusted in a manner that the flow rates correspond to the respective operating temperatures. In this case, the point at which a degree of opening becomes the full opening is the limit of cooling under the operating electrolysis conditions.

[0063] On the contrary, in an example in which the flow rates of the cooling water to the subjects to be cooled (unit cells) are individually adjusted based on a unit cell which is the subject to be cooled which should be cooled the most, the degree of opening of the flow rate adjuster of the unit cell to which the highest amount of cooling water should be sent is made full opening under the operating conditions under which the cooling load becomes the heaviest, and the flow rates of the unit cells which are the subjects to be cooled in which the required cooling loads are light are adjusted one by one by the degrees of opening. When a degree of opening is made totally closed, this does not contribute to cooling. Thus, a degree of opening at which the flow rate reaches the minimum control value is the lower limit of the adjustment. The minimum control flow rate relates to the speed of response to the temperature change of the unit cell due to a change in the electrolytic current density. It is necessary to make the flow rate high when the speed of the change in the electrolytic current density is high, but the flow rate can be made almost zero when the speed is low. A flow rate at which the cooling water is replaced in approximately 10 minutes to two hours is desirably selected.

[0064] As described above, the resistances of the cooling water inlets 41 of the unit cells, which are the subjects to be cooled, are each adjusted in a manner that the difference in the calorific value due to the difference in the electrolytic voltage disappears, and the supply pressures of the cooling water are regulated in a manner that the change in the total flow rate of the cooling water is in proportional to the electrolytic current density.

[0065] When the temperature of the coolant supplied to the cooling chamber 4 is regulated, for example, at 60° C. or higher during the heating operation of the electrolytic bath (the term here is not used in the context of classification of electrolytic cells or unit cells but is used as a general term meaning a bath for conducting electrolysis) before applying an electrical current, the temperature of the electrolytic bath can be increased quickly to the temperature suitable for the current application, and thus the preparation period for the current application can be shortened.

[0066] When the operation of the electrolytic bath is stopped by shutting off the current, the supply of the coolant is continued, and at the same time the supply temperature of the coolant to the electrolytic bath is regulated at 60° C. or lower. In this manner, the temperature of the electrolytic bath can be decreased quickly, and the materials constituting the electrolytic bath can be prevented from deteriorating due to the electromotive force caused by the potential difference between the electrodes after the electrolytic bath stops.

[0067] In the above embodiments, the cooling water is supplied to each unit cell, and the flow rate of the cooling water to each unit cell is adjusted individually depending on the operating voltage. In an electrolytic bath using two-chamber gas diffusion electrodes operated in one current circuit, the electrolytic temperatures are not the same because of the difference in the voltage properties among the unit cells constituted with ion-exchange membranes or the like. In the above embodiments, supply of salt water to all the anodes of the electrolytic bath to which the salt water is supplied is regulated under the same concentration and temperature conditions, while selective cooling regulation is conducted. Thus, efficient operation which equalizes the electrolytic temperatures can be conducted.

[0068] By regulating the temperatures of the unit cells in the preferable temperature range, the current efficiency and the durability of the ion-exchange membranes can be enhanced, and the concentration of the chloride ions in the sodium hydroxide solution generated at the cathodes can be decreased.

[0069] Although a manual valve is used to individually adjust the flow rate of the cooling water in each unit cell in the above examples, an automatic flow rate-regulating valve may be used instead of the manual valve. For example, it is possible to detect the operating voltage or the temperature of the unit cell and conduct automatic regulation by the automatic flow rate-regulating valve based on the detected value. When the costs of the apparatus should be kept low, however, it is advantageous to adjust the flow rate manually. Thus, in the method for supplying the cooling water, the supply pressure of the cooling water is changed depending on the operating electrolytic current as shown in FIG. 3 and FIG. 7, and the flow rate of the cooling water to the cooling chamber 4 of each unit for regulation of the flow rate is controlled by adjusting the degree of opening of the manual valve or the like to adjust the distribution. By such a method, the temperature of the electrolytic bath can be adjusted inexpensively and highly precisely.

[0070] The unit for individual regulation of the flow rate of the cooling water is not limited to the unit of unit cell described above but may be an electrolytic cell or a group of electrolytic cells depending on the state of deterioration of the equipment or the like.

[0071] The invention can be applied not only to an apparatus in which all the unit cells are operated in one current circuit, namely an apparatus operated in a current circuit to which electricity is supplied from a common direct current power source, but also to an apparatus in which a direct current power source is provided for each unit cell or for each group of unit cells.

[0072] Examples in which the coolant sent to each cooling chamber is water or air include

[0073] a) an air cooling method through natural airing in which air comes in from the bottom and goes out from the top through top and bottom holes,

[0074] b) an air cooling method in which air is forcibly sent with a blower or the like,

[0075] c) a method in which water mist is added to a method in which air is forcibly sent,

[0076] d) a method in which water is sprayed and

[0077] e) a method in which cooling water is passed.

[0078] The quantity of removed heat increases in the above order. a) and b) have small effects, and c), d) and e) are preferable examples. In c) and d), water is preferably supplied from the top of the electrolytic bath and removed from the bottom for the purpose of making the discharge of the water easy. In c), however, it is difficult to make the supply amount of water high, and the effect of removing heat is limited. d) has an advantage in that water does not easily leak even when the sealing structure is simple because almost no water pressure is applied to the cooling chamber. However, when the amount of the cooling water is low, the quantity of removed heat is small, or a difference in the quantity of removed heat arises easily between the upper part and the lower part. Thus, a large amount of cooling water is used for evenly removing heat from the electrolysis surface, and it is required to make the sealing structure of the cooling chamber strong. Because the temperature difference between the inlet and the outlet of the cooling water can be made small with a sufficient flow rate of the cooling water, the method e) is preferable to evenly remove heat from the electrolysis surface, and it is preferable to flood and pass water through the electrolytic cell from the bottom to the top to increase the flow rate of the cooling water.

EXAMPLES

Example 1

[0079] The electrolytic cells used for the test were DCM-type electrolytic baths manufactured by Chlorine Engineers Ltd. which were converted into a gas diffusion electrode method type. The original electrolytic baths each used an electrode having active carbon supported on stainless mesh as the hydrogen generation electrode. When the electrolytic baths were each converted into the gas diffusion electrode method type, a separation wall for a gas chamber and a cooling water chamber was provided on the electrode by welding, and a cooling structure was formed in the cathode chamber. The ion-exchange membranes were Aciplex F-4403D manufactured by Asahi Kasei Chemicals Corporation. The gas diffusion electrodes as the cathodes were GDE-2008 manufactured by Permelec Electrode Ltd., and the anodes used were DSE manufactured by Permelec Electrode Ltd. The operating conditions of the salt water and the cooling water supplied to each electrolytic cell (unit) and the like are shown below, and the operating conditions described are per effective electrolysis area. From such electrolytic cells, six electrolytic cells having electrodes and ion-exchange membranes with different degrees of deterioration were prepared. The electrolytic cells were arranged in a manner that the electrodes of each electrolytic cell were connected in series and that the cooling water could be supplied independently to each electrolytic cell. Conditions under which a difference in the electrolytic voltage would arise among the electrolytic cells (unit cells) were thus set.

[0080] Two kinds of current density conditions were set. Cooling regulation was conducted in each case (current density), and the property of controlling the unit cells (electrolytic baths) was examined. Salt water and an oxygen gas were supplied to the six unit cells each at the same temperature and at the same flow rate. The temperature of a unit cell was represented by the temperature of the anode chamber.

[0081] The supply conditions of the salt water to the unit cells and the like are shown in Table 1 as the other conditions. The maximum temperature differences among the unit cells without cooling were each estimated by calculating the heat balance difference calculated from the difference in the electrolytic voltage (the difference between the unit cell with the highest voltage and the unit cell with the lowest voltage) as the temperature difference and ignoring the decrease in the voltage due to the increase in the temperature. The results of the calculation are shown in Table 1.

TABLE-US-00001 TABLE 1 Without cooling Conditions of unit Maximum Salt water supplied to unit Cooling water cells Temperature Maximum temperature cells supplied to unit cells Anode difference electrolytic difference Current Temper- Concen- Temper- chamber among unit bath among unit density Flow rate ature tration Flow rate ature Voltage temperature cells temperature cells [kA/m.sup.2] [L/(h-m.sup.2)] [° C.] [g/L] [L/(h-m.sup.2)] [° C.] [V] [° C.] [° C.] [° C.] [° C.] Example 1 1.92 42.2 56.0 244 8.3 65.0 1.94 82.6 0.6 91.5 6.2 19.3 65.0 2.02 83.0 8.3 65.0 1.93 82.8 8.3 65.0 1.93 82.9 19.3 65.0 2.01 83.1 13.8 65.0 1.97 82.9 2.58 50.4 47.3 244 12.4 65.0 2.07 85.2 0.5 86.9 4.1 27.5 65.0 2.18 85.8 12.4 65.0 2.06 85.3 12.4 65.0 2.06 85.3 27.5 65.0 2.17 85.8 19.3 65.0 2.11 85.5

Example 2

[0082] A test similar to that of Example 1 was conducted using the same apparatus as that of Example 1 except that the conditions of the supplied salt water such as the flow rate and the concentration were changed and two kinds of current density conditions were set. The results are shown in Table 2.

TABLE-US-00002 TABLE 2 Without cooling Conditions of unit Maximum Salt water supplied to unit Cooling water cells Temperature Maximum temperature cells supplied to unit cells Anode difference electrolytic difference Current Temper- Concen- Temper- chamber among unit bath among unit density Flow rate ature tration Flow rate ature Voltage temperature cells temperature cells [kA/m.sup.2] [L/(h-m.sup.2)] [° C.] [g/L] [L/(h-m.sup.2)] [° C.] [V] [° C.] [° C.] [° C.] [° C.] Example 2 1.71 23.8 59.0 261 6.9 64.8 1.89 82.0 0.3 89.6 7.6 15.1 64.8 1.97 81.9 6.9 64.8 1.88 82.0 6.9 64.8 1.88 82.0 15.1 64.8 1.96 81.8 8.3 64.8 1.92 82.0 2.75 39.7 45.4 261 12.4 64.8 2.09 85.7 0.6 95.8 10.1 35.8 64.8 2.22 86.3 12.4 64.8 2.09 85.8 12.4 64.8 2.09 85.7 35.8 64.8 2.20 86.3 22.0 64.8 2.15 86.0

[0083] As it is seen from Table 1, the differences in the quantity of heat which arose from the differences in the voltage were removed by the cooling regulation action of the cooling water, and the temperature differences could be controlled to be small values as shown in the column of the temperature difference among the unit cells. As it is seen from Table 2, this control can be applied even when the flow rate and the concentration of the supplied salt water change, and the temperature difference among the unit cells can be kept within 1° C. or less, for example. When the cooling regulation is not conducted, the temperature differences in the columns of the maximum temperature difference among the unit cells without cooling would arise.

[0084] As explained in the section of the background art, the electrolytic temperature relates to the voltage. Influence of approximately 10 mV/° C. (an increase in the temperature of 1° C. results in a decrease in the voltage of approximately 10 mV) is an example of the relation, and operation with lower voltage (less energy) can be achieved as the temperature becomes higher. As already described above, in the conventional techniques, the upper-control limit temperature is set based on the electrolytic bath with the highest operating temperature, and the other electrolytic baths are forced to be operated at a lower electrolytic temperature. Thus, the voltages become high, and the operation efficiency decreases. In the invention, because there is almost no temperature difference among the unit cells, all the electrolytic baths can be maintained under preferable operating conditions under which low electrolytic voltages are achieved.

[0085] In the comparative examples (examples without cooling), temperature differences of 3° C. or more arose when the cooling water was stopped, and the experiment itself was inadequate because of the large temperature differences. Thus, the values were determined by calculation. In an actual case, the increases in the temperature have the effect of decreasing the voltages, and the temperature differences would be slightly smaller.

Example 3

[0086] To determine what cooling structure would be more preferable as the cooling system, the cooling effects of different cooling methods were examined using an apparatus similar to that of Example 1 but using one unit cell. The conditions c), d) and e) below were under the conditions under which the temperature of the electrolytic bath was 80° C. during cooling. The temperatures of the conditions a) and b), which are Comparative Examples, were 85° C., and the other conditions and the results are described in Table 3.

[0087] The symbols a) to e) for the methods are as follows.

[0088] a) An air-cooling method through natural airing in which air came in from the bottom and went out from the top through top and bottom holes.

[0089] b) An air-cooling method in which air was forcibly sent with a blower or the like.

[0090] c) A method in which water mist was added to a method in which air was forcibly sent. The air and the water mist were introduced from the top.

[0091] d) A method in which water was sprayed. The water was sprayed from the top and brought into contact with the entire surface.

[0092] e) Cooling water was introduced from the bottom and removed from the top.

[0093] The flow rates of air, water and cooling water and the quantities of removed heat described in Table 3 are the values per effective electrolysis area.

TABLE-US-00003 TABLE 3 Quantity of Overall Heat Removed Transfer Heat Coefficient No. Method [kcal/(h .Math. m.sup.2)] [kcal/(h .Math. m.sup.2 .Math. ° C.)] Experimental Conditions Example 3-1 c 1020 23 Air 1.09 m3(N)/(h .Math. m.sup.2) Water 2 L/(h .Math. m.sup.2) Example 3-2 d 3000 58 Cooling water amount 78 L/(h .Math. m.sup.2) Cooling inlet temperature 28° C. Example 3-3 d 1600 40 Cooling water amount 40 L/(h .Math. m.sup.2) Cooling inlet temperature 28° C. Example 3-4 e 2610 115 Cooling water amount 64 L/(h .Math. m.sup.2) Cooling inlet temperature 28° C. Example 3-5 e 2360 126 Cooling water amount 100 L/(h .Math. m.sup.2) Cooling inlet temperature 51° C. Comparative a tiny value tiny value Example 1 Comparative b tiny value tiny value Air 1 m.sup.3(N)/(h .Math. m.sup.2) Example 2

[0094] As shown above, the methods c), d) and e) are appropriate as the cooling methods, and d) and e) are preferable. The cooling method d) did not require strict airtightness of the cooling chamber (water pressure did not apply in the cooling water chamber), and thus a large quantity of removed heat could be achieved even with a simple structure. The cooling method e) was a method in which the flow rate of the cooling water could be increased easily. Thus, even when the temperature at the cooling water inlet was made high and when the temperature difference from the internal temperature of the electrolytic bath was made small, by increasing the flow rate of the cooling water, the overall heat transfer coefficient could be maintained high, and the difference in the quantity of removed heat between the upper and lower parts of the electrolysis surface could be made small. Therefore, preferable results could be obtained. In Comparative Examples 1 and 2, the sensible heat of the air was small, and the quantities of removed heat were tiny values.

Example 4 and Comparative Example 3

[0095] The same apparatus as that of Example 1 was used, and the presence or absence of the flow of the cooling water was changed. In Example 4, the temperatures of the anode chambers were 78 to 89° C., and the set temperatures at the cooling water inlets were 60° C. In Comparative Example 3, the temperatures of the anode chambers were 77 to 89° C., and the apparatus was operated without any cooling water. The operation period (days) and the changes in the current efficiency are shown in FIG. 10.

[0096] The influence, namely the decrease in the current efficiency, was smaller in Example 4, which was cooled. Almost no decrease in the current efficiency was observed on the 400th day of operation and later, and high performance could be maintained.