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Inspection and Measures for Causes of Membrane Electrolyzer Voltage Rise

2021/03/08 14:50
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[Abstract]:
The main factor affecting the voltage of Membrane Electrolyzer is the salt electrolysis reaction, followed by the cell structure, membrane, electrode type, temperature, lye and brine concentration and other operating conditions.

After the Membrane Electrolyzer has been in operation for several years, the voltage of some Membrane Electrolyzers has increased a lot, and some have reached 3.5 V or even more than 4 V. What is the reason for the dramatic increase in the voltage of Membrane Electrolyzer, a comprehensive inspection must be carried out and corresponding measures must be taken. The reasons for the high voltage of Membrane Electrolyzer and the corresponding measures to be taken are introduced as follows for your reference.

Membrane Electrolyzer

The main factor affecting the voltage of Membrane Electrolyzer is the salt electrolysis reaction, followed by the cell structure, membrane, electrode type, temperature, lye and brine concentration and other operating conditions.

1 anode

1.1 Reason

(1) When OH^-reversely penetrates into the anode chamber, the oxygen generated by the discharge reacts with the titanium substrate, forming a non-conductive TiO2 layer between the titanium substrate and the active coating, causing it to suffer physical damage and increase the overvoltage .

(2) The caustic soda passes through the membrane pinhole to the anode chamber, corrodes and dissolves the titanium substrate and its surface active coating, and the overvoltage increases.

(3) When the impurity content in the brine is too high, especially when metals and organics such as Sr^4+, Ba^2+, Fe^2+ are high, it is easy to deposit on the anode surface to form an insulating layer, covering part of the anode surface, so that The active surface decreases and the chlorine evolution potential increases.

(4) Although Membrane Electrolyzer has measures to prevent or reduce the damage to the cathode and anode coatings and membranes caused by the reverse current of parking (the Asahi Kasei bipolar tank is parked for more than 2 hours to drain, the Tokuyama Caoda tank is provided with anti-corrosion current, and the Asahi Glass tank is used. Electrolyte circulation, strictly control the electrolyte concentration and make the anolyte free of free chlorine), but the parking is too frequent, repeated galvanic action, anode reduction, cathodic oxidation, and the cathode and anode coatings are still corroded to a certain degree. The voltage rises.

(5) Uneven coating thickness or weak adhesion.

1.2 Inspection method

(1) Observed by naked eyes, the new coating is gray-black, it turns yellow and white after corrosion passivation, the yellow is the natural color of the titanium matrix, and the white is TiO2. If the large area discolors at 3.3 kA/m^2, it is estimated that the cell voltage is higher than 4 V; If the color changes locally, the cell voltage is estimated to be higher than 3.5 V.

(2) Using a magnifying glass to observe the discolored parts of the anode coating, there is not much coating left, which can help us analyze the problem.

(3) A small sample of the new coating can be used as the chlorine generation (or precipitation) potential to identify the quality of the coating. The old coating is inconsistent due to the inconsistency of corrosion and passivation on the entire mesh surface. The sample taken is not representative. It is necessary to analyze the ruthenium content of the entire anode mesh surface by fluorescence X-ray analysis. If the average ruthenium content is only 20%~ 40%, it's time to repaint.

(4) Measure the coating thickness of the entire anode mesh with fluorescent X-rays. The new coating thickness is 1.8 μm (Asahi Kasei Natural Circulation Bipolar Tank). If the old coating thickness is less than 0.7 μm, it is time to recoat.

(5) The method of replacing Membrane Electrolyzer is more simple and intuitive. That is, remove the unit cell with a cell voltage exceeding 4 V (3.3 kA/m^2) and replace it with a spare cell with a new anode coating without changing the film. If the cell voltage drops to normal, it means that the anode coating of the original cell Repainted.

(6) The coating falls off.

1.3 Measures taken

(1) Membrane leakage should be replaced in time, and the membrane leakage should never be allowed to continue running.

(2) Strengthen management, improve the overall quality of the whole plant, and reduce the number of shutdowns of ion-exchange membrane electrolyzers.

(3) Add acid to the electric tank to neutralize the OH^- that migrates back from the cathode chamber to the anode chamber, and reduce the oxygen content in the chlorine.

(4) Ensure that the secondary refining index of brine is qualified.

(5) If there is a problem with the quality of the coating (uneven thickness or falling off, etc.) or during the warranty period, due to the deterioration of the cathode itself, the anode voltage is higher than the initial installation voltage by more than 0.15 V, the foreign company shall claim compensation.

2 cathode

2.1 Reason

(1) The thickness of the coating is uneven and the adhesion is not strong.

(2) The reverse current at parking reduces the activity of the active cathode.

(3) Mercury and aluminum poison the stainless steel cathode and increase the overvoltage.

(4) Iron is deposited on the surface of the cathode, which increases the overvoltage of the active cathode.

2.2 Inspection method

(1) Use the naked eye to see if the coating falls off, and use a magnifying glass to see if the coating is uniform.

(2) Measure the coating thickness of the entire cathode mesh with fluorescent X-ray. If the thickness of the old coating is 20%-40% of the thickness of the original new coating, it is time to recoat.

(3) Test the H2 generation (or precipitation) potential. If the old cathode active coating is 0.15 V higher than the original new cathode active coating, it is time to recoat.

2.3 Measures taken

(1) Adopt the above-mentioned methods of discharging liquid from the parking tank and passing anti-corrosion current to try to avoid or alleviate the impact of the reverse current on the active cathode during parking.

(2) The magnet powder (Fe3O4) attached to the cathode surface must be cleaned every time the film is changed in the entire tank. When cleaning, use a high-pressure water gun to clean it. If it can't be hit, use a bamboo strip or brush to clean it. Never use a metal brush to prevent the active coating from being brushed off.

(3) Asahi Kasei’s modified tank is equipped with a PTFE insertion tube at the cathode inlet (inlet header 1~49, tank inlet 50~98), and the outlet is equipped with nickel wire (tube outlet 1~49, tank outlet 50~98), just to relieve iron Deposition on the surface of the cathode.

(4) Reduce the number of stops.

(5) Prevent metals such as mercury and aluminum from entering the cathode system.

(6) Catholyte system piping and equipment (including the cathode part of the cell) should be made of high-quality materials.

(7) During the warranty period, due to the deterioration of the active cathode itself, if the active cathode voltage is higher than the initial installation voltage by more than 0.15 V, the foreign company shall claim compensation.

3 Structure resistance

The IR drop depends on the current load of the cell structure and power transmission hardware (electrode connections and necessary inter-slot connections), and its value is usually 0.15~0.30 V (including busbar loss). This value increases with time (such as due to corrosion). The copper guide plate must be polished every year for power outages and maintenance. The cell voltage is high. When checking the pole net, check the open welding between the pole net and the rib, the rib and the pole plate, and the pole plate and the pole plate (through the composite strip). If the open welding is very serious, It will also seriously affect the cell voltage. In this case, re-soldering is necessary (the operation is more difficult).

4 Electrolysis operating conditions

Although electrolysis operating conditions affect the voltage of each cell of Membrane Electrolyzer, when analyzing the voltage of individual Membrane Electrolyzer cells, the influence of electrolytic operating conditions on the cell voltage should also be considered. Therefore, the cell voltage increases, especially the voltage of all cell cells. When it rises, check the control of the operating conditions, and immediately adjust if it does not meet the requirements.

4.1 Catholyte NaOH concentration

As the concentration of NaOH increases, the water content in the membrane gradually decreases, resulting in an increase in membrane voltage drop and an increase in cell voltage. The membrane resistivity increases with the increase of NaOH concentration, which increases the cell voltage. The catholyte resistance increases as its concentration increases, so the cell voltage also increases. Generally, the mass fraction of catholyte NaOH changes by 1%, which affects the cell voltage from 0.014 to 0.030 V. During the operation of the cell, the catholyte NaOH concentration should be strictly controlled within the specified range. If the concentration meter is broken, it should be repaired in time. At the same time, the number of analyses of the catholyte NaOH concentration should be increased.

4.2 Anolyte NaCl concentration

Under normal circumstances, a certain amount of water can easily penetrate the carboxylic acid layer, but when the anolyte NaCl mass concentration is lower than 100 g/L, the water penetration rate of the sulfonic acid layer will exceed that of the carboxylic acid layer, causing a portion of the water to be accumulated in the two layers At the junction, delamination occurs or blisters occur. The delamination between the layers increases the voltage between the layers. The blisters have different sizes. Small blisters (1~2 mm) will not increase the cell voltage, but large blisters (above 2 mm) will increase the cell voltage to a certain extent (0~0.05 V). When the anolyte NaCl mass concentration is lower than 100 g/L, every 1% change will affect the cell voltage 0.003 V. During the operation of the cell, strictly control the NaCl concentration of the anolyte to meet the specified requirements. If there is a problem with the NaCl concentration meter, it must be repaired in time. During the repair, the frequency of analysis of the NaCl concentration must be increased.

4.3 Current density

The membrane voltage drop is directly affected by the current density. Generally speaking, the membrane voltage drop should be proportional to the current density, and a linear relationship (in the range of 1.5 to 4 kA/m^2). The current density not only affects the voltage drop of the membrane, but also affects the bubble effect, the overvoltage of the anode and cathode, and the voltage drop of the solution and the conductor. The overall effect is that as the current density increases, the cell voltage gradually increases. In general, every time the current density changes by 0.1 kA/m^2, the cell voltage changes from 0.014 to 0.021 V. The slope (K value) of the current density-voltage curve is 0.14~0.21 V·m^2/kA. Check the accuracy of the measured current, not because the above calculated value affects the cell voltage, but take measures to make the current error meet the specified requirements. If there is no special requirement in production, the electric tank should be opened with normal current density.

4.4 pH value of anolyte

Almost all ionic membranes used in the chlor-alkali industry are perfluorosulfonic acid and carboxylic acid composite membranes. Perfluorocarboxylic acid has excellent performance in the presence of —COO—Na^+. If the carboxylic acid group becomes —COOH type, it cannot work as an ionic membrane. Therefore, the pH value of the anolyte must be higher than a certain level. Value, otherwise the inside of the membrane will be destroyed due to blisters, which will increase the membrane resistance and the voltage of the electrolytic cell will rise sharply. Therefore, the anolyte should not be added acid excessively and should be uniform. Strictly control the pH value of the anolyte to not less than 2. It is best to use an interlocking device. When the brine stops or the power supply is interrupted, the addition of hydrochloric acid will be automatically stopped immediately. The amount of acid added should be considered comprehensively according to the starting current, current efficiency, acidity of the anolyte inlet and outlet, and pH value, especially the acidity of the anolyte inlet should not be greater than the specified requirements.

4.5 The temperature of the electrolyte

A rise in temperature will increase the pores of the membrane, which will help improve the conductivity of the membrane and reduce the cell voltage. Of course, the rise in temperature will not only help improve the conductivity of the membrane, but will also increase the conductivity of the electrolyte, which will help reduce Solution voltage drop. Under normal circumstances, the cell voltage decreases by about 0.01 V for every 1 ℃ increase in temperature. The operating temperature of the commonly used electrolytic cell is 80-90 ℃, which often changes with the current density. Under normal current operation, the bath temperature is controlled at 85~88 ℃. The bath temperature cannot exceed 90 ℃. If it exceeds 90 ℃, the evaporation of water will increase, which will increase the vapor-liquid ratio and increase the voltage. At the same time, the electrolyte tends to boil, which accelerates the deterioration of the membrane, and also aggravates the corrosion of the electrode and the active coating. Passivation. When you find that the cell voltage rises, especially when all cell voltages in the cell rise, check the cell temperature in detail, keep away from the control point, calculate the influence of the temperature on the cell voltage based on the above empirical data, and then try to make the cell temperature under control Point requirements, and pay special attention to the accuracy of temperature measurement.

4.6 Flow rate of electrolyte

In general uncoated ion-exchange membrane electrolyzers (for example, Asahi Kasei uncoated membrane forced circulation bipolar electrolyzers), the bubble effect has a significant influence on the cell voltage. When the circulation volume of the electrolyte decreases, the gas content in the liquid in the cell Will increase, and the adhesion amount of bubbles on the membrane and on the electrode will also increase, causing the cell voltage to rise. In a zero-pole-pitch electrolytic cell assembled with modified membranes (such as the AZEC-M3 electrolytic cell of Asahi Glass Co., Ltd.), because the membrane is hydrophilic, the bubbles are difficult to adhere to and can escape quickly, reducing the impact of the bubble effect . When the salt water supply is stopped in the laboratory electrolyzer, the cell voltage rises rapidly, up to about 30 V. If the salt water is continued to be supplied after the salt water supply is stopped, the current efficiency will gradually recover after 4 days, and the cell voltage will quickly return to normal. The same is true for industrialized electric tanks. For example, due to a computer failure of a domestically produced electric tank of a domestic factory, the pure water and salt water of the electric tank are stopped for about 5 minutes, the voltage of the tank rises, and the current efficiency decreases. When the fault of the computer is eliminated and the normal supply of pure water and salt water is restored, the tank voltage will quickly recover. Normal, but the current efficiency returned to its original level after 1 day. During the electrolysis process, a large amount of hydrogen and chlorine are generated in the electrolytic cell. The high gas-to-liquid ratio will lead to an increase in the voltage of the uncoated membrane forced circulation electrolytic cell, although the design requires that the unit cell and electrode can quickly discharge all the cells from the surface of the electrode. The generated gas, but there still needs to be enough electrolyte to supply the electrolytic cell to run and cycle, and to take the gas away in time, which plays an effective role in preventing the retention of chlorine in the electrolytic chamber. Although the flow rate of the Asahi Kasei forced-circulation bipolar tank can be reduced from 94 m^3/h to 80 m^3/h, it must not be reduced to below 65 m^3/h, otherwise it will not only affect the tank voltage and pressure difference, if the flowmeter does not Quasi (false height), lower real flow rate, will cause physical damage to the membrane. The flow rate of the anode chamber of the Asahi Kasei natural circulation cell was increased from 14 m^3/h to 16 m^3/h (at 3.3 kA/m^2), which not only improved the circulation of the anolyte in the anode chamber, but also reduced the cell voltage. The inlet and outlet hoses of individual cell of the bipolar tank are blocked, the flow rate drops, and the tank pressure rises. If the unit tank voltage is increased, check whether the inlet and outlet hoses of the unit tank are blocked. If it is blocked (can be judged from the air bubbles in the inlet hose fluid and the outlet hose flow and flow conditions), stop immediately and clear the blockage. In addition, it is necessary to carefully check whether the electrolyte supply of the whole electric tank is sufficient and whether the flowmeter is accurate.

4.7 Tank pressure and pressure difference

Increase the pressure of the electrolytic cell, the volume of gas in the electrolyte is reduced, the resistance of the electrolyte decreases due to the occurrence of bubbles, and the voltage of the cell is reduced. The positive pressure difference of the electrolytic cell reduces the cell pressure more than the negative pressure difference, because the conductivity of the anolyte is much smaller than that of the catholyte. A large positive and negative pressure difference occurs due to misoperation, which deforms the pole network, increases the pole distance, and increases the voltage. When the catholyte is 0.025 V/mm and the anolyte is 0.075 V/mm, the accurate value of the influence of the electrolyte on the cell voltage is calculated according to the deformation of the cathode and anode nets. Some manufacturers using Asahi Kasei’s forced-circulation bipolar electrolyzer have reduced the pressure control of chlorine and hydrogen in the electrolyzer from 0.040 MPa and 0.055 MPa to 0.010 MPa and 0.025 MPa, respectively, which not only affects the cell voltage, but also affects the stability of the cell pressure difference. The original regulations are controlled.