JPH0874082A - Operation of ion-exchange membrane electrtolytic cell - Google Patents

Abstract

PURPOSE: To efficiently operate an ion-exchange membrane alkali chloride electrolytic cell by establishing a method for detecting the abnormality of the cell and optimizing the renewal order of ion-exchange membranes. CONSTITUTION: When plural ion-exchange membrane alkali chloride electrolytic cells are operated, the oxygen concn. in gaseous chlorine and the supply of hydrochloric acid to each cell are measured for each cell, and hydrochloric acid is supplied separately to each cell based on the measured values so that the oxygen concn. in gaseous chlorine is kept at 0.1-0.5vol.%. The unit requirement of hydrochloric acid (hydrochloric acid supply/chlorine yield) in each cell is compared with a specified value or relatively compared with the average value of the cells to detect the abnormality of the cell. The renewal order of the ion-exchange membranes is determined by the total cost of the respective cells calculated by converting the manipulated variables on the unit requirement of hydrochloric acid, cell voltage and working days for each cell separately into cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an easy operation method for controlling an electrolytic cell in an alkali chloride electrolysis plant having a plurality of electrolytic cells. More specifically, the present invention relates to a method for producing caustic alkali such as caustic soda and caustic potash, chlorine and hydrogen by electrolysis of alkali chloride such as sodium chloride and potassium chloride by the ion exchange membrane method, wherein the physical and chemical properties of the ion exchange membrane are used. The present invention relates to an operating method for early detection of abnormalities in an electrolytic cell and an ion exchange membrane by monitoring a decrease in electrolytic performance due to deterioration and making a relative comparison, and optimizing the priority of ion exchange membrane renewal. .

[0002]

2. Description of the Related Art In the ion-exchange membrane method alkali chloride electrolysis, usually, an ion-exchange membrane made of an organic material is sandwiched between
Chlorine and a strong oxidizing atmosphere exist on one anode side, and a strong alkaline solution exists on the other cathode side, so the ion exchange membrane is exposed to extremely harsh conditions, and in the operation for a long time, It is unavoidable that a deterioration phenomenon of the exchange membrane with time occurs. Such a phenomenon is not limited to deterioration of the performance of the ion exchange membrane, and in the case where a membrane generally called a pinhole is damaged, chlorine on the anode side and hydrogen on the cathode side are mixed, In the worst case there is even a risk of explosion. Also, when pinholes are generated in the membrane, the caustic on the high pressure side flows into the anode chamber and reacts with chlorine to produce hypochlorous acid, which is a battery case gasket (made of EPDM rubber).
As it corrodes, it causes liquid leakage to the outside and shortens the life of the battery case. Therefore, in the electrolysis using the ion exchange membrane method, it is extremely important to detect an abnormal phenomenon such as deterioration of the ion exchange membrane used as early as possible from the viewpoint of preventing accidents and reducing costs.

Conventionally, as a method for detecting an abnormality in the ion exchange membrane, chlorine purity is measured by Olsat analysis, and anolyte pH is measured.
And the analysis of free chlorine and hypochlorous acid in the anolyte were common. However, the Olsat analysis method
Sufficient accuracy cannot be obtained in the high chlorine purity range.
A method for measuring the pH of the anolyte is disclosed in Japanese Examined Patent Publication No. 57-5931.
As described in Japanese Patent Laid-Open No. 2 (1994), the sensitivity of the pH value to the fluctuation of the current efficiency (the actual production amount divided by the theoretical production amount; hereinafter synonymously used) is extremely high in the region where the pH exceeds 4. It is very low and it is difficult to detect abnormalities due to changes in pH. In addition, there is a problem that the measured pH value causes an error due to variations in liquid temperature during measurement and the influence of gas emission during sampling. . Furthermore, the method for analyzing free chlorine and hypochlorous acid in the anolyte has many problems such as being too time-consuming and difficult to carry out at a sufficient frequency, and any of the above methods is sufficient as an abnormality detection method. It was not satisfactory, and problems such as oversight of abnormal tanks and overmaintenance of normal tanks could not be resolved.

Usually, the ion exchange membrane needs to be renewed every 2 to 4 years, but in order to determine this priority among many electrolytic cells, the performance (current efficiency, cell voltage) of each electrolytic cell is determined. It is difficult to optimize the priority depending on the above anomaly detection method because it is necessary to strictly compare the above. In the method of JP-B-57-59312, the anolyte p
By keeping H at about 1 to 3, changes in current efficiency are indirectly captured as changes in anolyte pH. However, current efficiency changes due to temperature, caustic alkali concentration, etc. in addition to deterioration of the membrane. However, due to these changes, the pH also greatly changes. Although the anolyte pH is an index of the balance between the amount of hydroxide ions flowing into the anode chamber and the amount of hydrochloric acid supplied, it cannot directly reflect the current efficiency quantitatively. Cannot be a reliable guide to

[0005]

In view of the above situation, the present inventor has diligently studied the anodic reaction of the electrolytic cell in order to find an effective method for finding an abnormality in the electrolytic cell and the ion exchange membrane, and as a result, The oxygen concentration is an index for the balance between the amount of inflow of hydroxide ions into the anode chamber and the amount of supplied hydrochloric acid, similar to the pH value of the anolyte solution, but at the same time, a part of the direct current efficiency deterioration is stoichiometrically Since it is reflected, it was found that current efficiency can be accurately grasped by combining this with the oxygen concentration and the supply hydrochloric acid flow rate. Furthermore, when the oxygen concentration in chlorine gas is set to 0.5 (volume)% or less, most of the hydroxide ions flowing into the anode chamber are spent on neutralizing the hydrochloric acid supplied to the battery case and generating oxygen gas. It has also been found that only 10 to 15% is spent on the production of free chlorine and hypochlorous acid / chloric acid.

Therefore, the oxygen concentration in chlorine gas should be 0.5.
If it is less than (capacity)%, generation of chloric acid, etc. can be almost ignored. Therefore, by grasping the supply amount of hydrochloric acid and oxygen concentration, trend management of current efficiency of each electrolytic cell and relative comparison of all cells can be performed. It is possible enough. The present inventor, based on the above-mentioned novel findings, the operating guideline for controlling both the supply amount of hydrochloric acid and the oxygen concentration within a predetermined range and the basic unit of hydrochloric acid (supply amount of hydrochloric acid / amount of chlorine generation, which will be used interchangeably below). We have achieved a method for early detection of abnormalities in the electrolytic cell by controlling the unit and comparing the basic unit of hydrochloric acid in each electrolytic cell with a predetermined value or by making a relative comparison of all electrolytic cells.

[0007]

The present invention provides an effective method for detecting an abnormality in an ion-exchange membrane method alkaline chloride electrolytic cell, and optimizes the renewal order of the ion-exchange membrane,
It is intended to provide a method for efficiently operating an electrolytic cell.
Then, the gist of the present invention is to operate the battery array consisting of a plurality of ion-exchange membrane method alkaline chloride electrolytic cells, to supply oxygen concentration in the chlorine gas from the anode chamber of each electrolytic cell and supply hydrochloric acid to the respective cells. The amount is measured for each tank, and hydrochloric acid is individually supplied to each electrolytic tank so that the oxygen concentration in chlorine gas is maintained at 0.1 to 0.5 (volume)% based on the measured value, and By calculating the basic unit of hydrochloric acid (hydrochloric acid supply amount / chlorine generation amount) in each tank and comparing this calculated value with a predetermined value, or by making a relative comparison with the average value of the basic hydrochloric acid values of all tanks, electrolysis Ion exchange membrane method of alkali chloride consisting of detecting abnormalities in the cell The operating method of the electrolysis cell and the operation variables of the hydrochloric acid unit, cell voltage and number of operating days for each cell are individually different from the specified values. Calculate the total cost of each tank by converting the cost and compare the total cost Te resides in method of operation consists in determining the update order of the ion exchange membrane electrolyzer.

The present invention will be described in detail below. In the method of the present invention, while controlling the oxygen concentration in the chlorine gas discharged from each electrolytic cell to 0.5 (volume)% or less, the electrolytic cell is controlled and operated by comparing the values of the basic unit of hydrochloric acid. The basic unit of hydrochloric acid is calculated by dividing the indicated value of the hydrochloric acid flow meter attached to each electrolytic cell by the amount of chlorine generated in the electrolytic cell. Then, the chlorine generation amount is calculated from the energization current, oxygen concentration, HClO generation amount and ClO ₃ generation amount,
At that time, the oxygen concentration in the chlorine gas is 0.5 as in the present invention.
In the case of (volume)% or less, the amount of generated HClO and the amount of generated ClO ₃ are involved in only a small part of at most ten and several percent of the hydroxide ions that have flowed into the anode chamber, that is, in the amount of deterioration of current efficiency. From the viewpoint of low contribution rate and simplification of calculation, a fixed value (actual value balance) is adopted as the amount of generation. Regarding the oxygen concentration, the effect of the battery temperature and the caustic alkali concentration is great, so the corrected oxygen concentration in the standard state in which the deviation from the reference value of the temperature and the caustic alkali concentration is corrected is used. By using the basic unit of hydrochloric acid calculated based on such values, the operating conditions of the electrolytic cell,
In particular, it enables a strict relative comparison of ion exchange membrane performance.

The present invention will be described below with reference to FIG. 1 using salt electrolysis as an example. FIG. 1 shows an example of a factory consisting of n electrolytic cells. The salt solution is supplied to the electrolytic cell 4 ₁ 4 ₂ 4 ₃ ... 4 _{n from} the supply salt water header 1 and the hydrochloric acid header 2 by a branch pipe provided with concentrated salt water and hydrochloric acid, and the overflow of the anolyte is caused by the fresh salt water header. Recovered by 3. The chlorine gas is collected and collected in the chlorine gas header 5 from the chlorine gas outlet of each tank. In addition, in this figure, in order to clarify the present invention and to prevent the explanatory diagram from becoming complicated,
The piping on the cathode side is omitted. In the present invention, the sampling automatic valve 8 ₁ 8 _{2 is provided at} the chlorine gas outlet of each tank.
8 ₃ ... 8 _n are provided and this is a sampling sequencer 1
By performing on / off control at 1, sampling is performed sequentially from each electrolytic cell. The sample gas is sent to the analyzer 10 via the pretreatment device 9 to measure the oxygen concentration in the chlorine gas. The pretreatment device 9 removes water and solid impurities in chlorine gas and protects the analyzer from these.

The oxygen concentration in chlorine gas is set to 0.1 to 0.5.
In order to maintain (volume)%, preferably 0.3 to 0.5 (volume)%, an appropriate amount of hydrochloric acid is supplied through a branch pipe branched from the hydrochloric acid header 2. Adjustment of the flow rate of hydrochloric acid supplied to each tank is 6
_It is performed by adjusting the opening and closing of the adjusting valve of ₁ 6 ₂ 6 ₃ ... 6 _n , and the flow rate is recorded by the instruction of the measuring instrument such as the rotor meter of 7 ₁ 7 ₂ 7 ₃ ... 7 _n. Record the value and monitor the variation in that value. The supply amount of hydrochloric acid is such that the oxygen concentration in chlorine gas is maintained in the above range, and generally, the oxygen concentration decreases as the supply amount of hydrochloric acid increases. Then, when the performance of the ion exchange membrane deteriorates (current efficiency decreases), the oxygen concentration increases if the supply amount of hydrochloric acid is constant. Therefore, in order to keep the oxygen concentration constant, it is necessary to increase the supply amount of hydrochloric acid.

In the present invention, the amount of chlorine generated calculated from the corrected oxygen concentration, the flow rate of hydrochloric acid, the data of the energizing current, the temperature of the cell, the caustic alkali concentration, etc., and the basic unit of hydrochloric acid of each electrolytic cell are calculated, and the calculated values are calculated. Monitor trends. Then, when the calculated value changes at a certain rate with respect to a preset initial unit of hydrochloric acid, for example, when the rate of change reaches 10% or doubles, the tank concerned at that time Can be determined as an abnormal tank. Also, compare the basic unit of hydrochloric acid of each tank with the basic unit of hydrochloric acid of all tanks, that is, if the average value and standard deviation (σ) of the basic unit of hydrochloric acid of all tanks are ± 3σ or more, the tank concerned Can also be determined as an abnormal tank. The rate of change or the degree of deviation that serves as a criterion for the abnormal cell is determined experimentally and empirically from the operating conditions of the electrolytic cell. By detecting an abnormal tank early by these methods, the abnormal portion of the tank can be identified, and the ion exchange membrane can be renewed or repaired before the liquid leakage trouble occurs.

Further, the ion exchange membrane needs to be renewed every 2 to 4 years. Regarding the method of determining the priority of the cell in which the ion exchange membrane should be renewed from a large number of electrolytic cells in the conventional electrolysis plant. There was nothing satisfactory. In the present invention, the oxygen concentration in chlorine gas is measured and the supply amount of hydrochloric acid is adjusted so that the oxygen concentration in chlorine gas is 0.1 to 0.5, preferably 0.3 to 0.5 (volume)%. In addition to the above, by calculating the basic unit of hydrochloric acid, it was possible to detect the abnormality of the electrolytic cell at the rate of change of the value with respect to a predetermined value.
Based on this method, as a result of diligent examination of an ideal ion exchange membrane updating method, further incorporating operating variables such as operating performance and number of operating days, setting reference values for each tank for these various variables, The difference from this reference value was converted into cost, and the total cost of each tank was compared to make it possible to efficiently select the order of renewal of the ion exchange membrane.

Specifically, in consideration of actual operating conditions, arbitrary reference values are set for each of the hydrochloric acid basic unit, the cell voltage, and the number of operating days for each tank. Then, the basic unit of hydrochloric acid in each tank is calculated by the above-described method, and the difference between the calculated value and the reference value is converted into the cost for each of the caustic production amount, the chlorine production amount, and the hydrochloric acid consumption amount. For the battery voltage, calculate the power consumption rate from the current supplied and the battery voltage of each battery, and convert the difference from the reference value into the cost for power consumption. It is possible to determine a generally economical ion-exchange membrane renewal order by comparing the sum of the above cost differences for all tanks for all tanks and ranking the ones with the largest cost difference. In addition to the factors causing deterioration of the ion-exchange membrane, there is a possibility that a voltage increase may occur due to a decrease in the electrode activity, so if these are clear, the item of battery voltage should be excluded from the cost calculation. Is preferred. Further, in the present invention, the processing of the analytical value, the calculation of the basic unit of hydrochloric acid, the calculation processing such as the cost calculation, the relative comparison means and the like can be efficiently and economically performed by the process computer.

[0014]

According to the method of the present invention, by grasping the supply amount of hydrochloric acid and the oxygen concentration in chlorine gas and controlling these values within a predetermined range, the trend of current efficiency of each electrolytic cell and the relative relationship of all the electrolytic cells are controlled. By making a comparison, it is possible to easily detect the abnormality of the electrolytic cell at an early stage and optimize the update order of the ion exchange membrane, so that the electrolytic cell can be used for a long time, efficiently and economically. It also enables stable operation.

[0015]

EXAMPLES Next, the present invention will be described more specifically by way of examples, but the present invention is not limited to the following examples unless it exceeds the gist.

Example 1 A monopolar type electrolytic unit 220 having an effective area of 20 dm ^2.
As shown in Fig. 1, a pair of electrolyzers 82 were connected by pipes in a cation exchange membrane (Asahi Glass Co., Ltd.).
Made: Brand name Flemion), using 20 salt concentration of anolyte
0 g / l or more and catholyte caustic soda concentration 32%, 82
The operation was carried out under the conditions of ° C and 30 A / dm ² . The chlorine gas outlet of each electrolysis tank is equipped with a sampling automatic valve, and by controlling this with a sampling sequencer, chlorine gas is sequentially sampled from each electrolysis tank, and the sample gas is an analyzer (Yokogawa Electric Co., Ltd .: (Magnetic oxygen meter)
And the oxygen concentration in the gas was measured. The measured oxygen concentration is automatically stored in the database of the process computer for each tank, and based on this, the oxygen concentration in chlorine gas in each tank is adjusted to 0.3-0.5 (volume)%. To keep
Hydrochloric acid was supplied to each tank while adjusting the flow rate of hydrochloric acid.

Then, the basic unit of hydrochloric acid is calculated from the oxygen concentration, other operating conditions (current flowing, etc.) and the amount of chlorine generated by a process computer according to a predetermined calculation formula, and the result is a trend control chart of the basic unit of hydrochloric acid and a relative comparison. Output as a table. The operation manager periodically checks this output result to determine whether there is any abnormality in the electrolytic cell. In addition, the processing unit processes the data on the basic unit of hydrochloric acid, the cell voltage, and the number of operating days of each tank, so that the relative comparison of the operating costs of each tank can be performed at any time, making it easy to update the ion-exchange membranes of the electrolytic cells. I was able to decide. After the above system started operation, it became possible to detect abnormalities in the ion exchange membrane early, which could not be detected until the occurrence of liquid leakage by the conventional method, the frequency of sudden stop troubles decreased, and the life of the electrolytic cell was reduced. I was able to extend it.

[Brief description of drawings]

FIG. 1 is a part of a piping system diagram of a factory in which the present invention is implemented.

[Explanation of symbols]

1 Supply salt water header 2 Hydrochloric acid header 3 Fresh salt water header 4 ₁ 4 ₂ 4 ₃・ ・ ・ ・ 4 _n Electrolyzer 5 Chlorine gas header 6 ₁ 6 ₂ 6 ₃・ ・ ・ ・ 6 _n Hydrochloric acid flow control valve 7 ₁ 7 ₂ 7 ₃・ ・ ・ ・ 7 _n Hydrochloric acid flow meter 8 ₁ 8 ₂ 8 ₃・ ・ ・ ・ 8 _n Sampling automatic valve 9 Pretreatment device 10 Analyzer 11 Sampling sequencer

Claims (4)

[Claims] 1. When operating a battery array consisting of a plurality of ion-exchange membrane method alkaline chloride electrolytic cells, the oxygen concentration in chlorine gas from the anode chamber of each electrolytic cell and the supply amount of hydrochloric acid to each cell are adjusted. It is measured for each tank, and hydrochloric acid is individually supplied to each electrolytic tank so that the oxygen concentration in chlorine gas is kept at 0.1 to 0.5 (volume)% based on the measured value. Calculate the basic unit of hydrochloric acid (hydrochloric acid supply amount / chlorine generation amount) and compare this calculated value with a predetermined value, or by comparing the calculated value with the average value of the hydrochloric acid basic unit of all tanks A method for operating an electrolytic cell of an ion exchange membrane method of alkali chloride characterized by detecting. 2. The basic unit of hydrochloric acid in each tank is calculated by dividing the amount of hydrochloric acid supplied to each tank by the chlorine generation amount calculated from the energization current value and the oxygen concentration in chlorine gas. The method according to claim 1, wherein the corrected oxygen concentration value corrected by the battery cell temperature and the caustic alkali concentration is used. 3. A chlorine gas discharge pipe of each electrolysis tank is provided with a sampling automatic valve, the valves are sequence-controlled to automatically sample from each electrolysis tank, and the oxygen concentration in the sample gas is measured by an automatic analyzer. By measuring the measured value, the current value to each tank, the tank temperature and the caustic soda concentration, the corrected oxygen concentration value and the basic unit of hydrochloric acid are calculated, and the calculated value is compared with a predetermined value. The method according to claim 1, wherein trend monitoring of operation performance of each electrolytic cell and abnormality detection are performed. 4. The oxygen concentration in the chlorine gas from the anode chamber of each electrolytic cell and the supply amount of hydrochloric acid to each cell are controlled when operating a battery cell array comprising a plurality of ion-exchange membrane method alkali chloride electrolytic cells. It is measured for each tank, and hydrochloric acid is individually supplied to each electrolytic tank so that the oxygen concentration in chlorine gas is kept at 0.1 to 0.5 (volume)% based on the measured value. Calculate the basic unit of hydrochloric acid (hydrochloric acid supply amount / chlorine generation amount), convert the difference between this calculated value, the battery cell voltage and the number of operating days from the standard prescribed value into a cost, and calculate the total cost of this converted value for each tank. A method for operating an ion-exchange membrane method electrolytic cell of alkali chloride, characterized in that the order of renewal of ion-exchange membranes in the electrolytic cell is determined by comparing the sizes relatively.