Control scheme for Asahi Kasei’s ion-exchange membrane caustic soda process

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Explanation of the Control Scheme for Asahi Kasei’s Ion-Exchange Membrane Caustic Soda Production Process I. Process Principle of Caustic Soda Production Using Ion-Exchange Membranes In the ion-exchange membrane process for producing caustic soda, a selectively permeable cation exchange membrane is installed between the anode and the cathode. Electrolysis takes place as saltwater circulates in the anode chamber while alkaline solution passes through the cathode chamber: Anode: Cl－ → Cl2 + e‑ Cathode: H2O + e‑ → H2 + OH‑ Equation: NaCl + H2O → NaOH + Cl2 + H2 Sodium chloride is ionized into sodium ions and chloride ions. Chloride ions are discharged in the anode chamber to transform into chlorine gas. At the same time, sodium ions move to the cathode chamber through the ion exchange membrane. In the cathode chamber, it is converted into hydrogen and hydroxide. Sodium ions react with hydroxide ions to form sodium hydroxide. Its reaction is essentially the same as that of the traditional diaphragm process. However, in the ion-exchange membrane process, pure caustic soda can be obtained through the selective permeation of the electrolyte and sodium ions. Under the influence of an electric current, the ion exchange membrane acquires a negative charge. Sodium ions in the anode chamber, having different charge levels, are attracted to the membrane and can pass through it into the cathode chamber. Chloride ions carry a negative charge similar to that of the membrane, and the membrane repels them, so they cannot pass through it. When a potential is applied to the membrane, not only cations but also water can move from the anode chamber to the cathode chamber. When the current is 12.2 KA (4.5 KA/M2), it is approximately 3.5 m3/hr. During electrolysis, a certain amount of OH- ions can be present in the membrane. If metal cations such as Ca++, Mg++, Fe++, Al++, and Ni++ are present in the anode solution, they will deposit on or within the membrane in the form of hydroxides, resulting in an increase in operating voltage and a decrease in current efficiency. Therefore, it is necessary to remove these metal cations and always keep the anode solution free of these metal ions. II. Control of the resin tower: Brine is fed into the resin tower to remove divalent metal cations. Of the three ion exchange towers T-160A/B/C, two operate in series while one is taken offline for regeneration. After 24 hours, the first tower operating in series is taken offline for regeneration; the second tower takes over its role as the first one, while the tower that has completed regeneration becomes the second one and continues to operate in series. The resin in the offline tower has absorbed a large amount of polyvalent cations, and needs to be regenerated using hydrochloric acid and caustic soda. Typically, the switching and regeneration of ion exchange resin columns are controlled by a program to take place every 24 hours, with each column being taken out of service after operating for 48 hours. Regeneration process of the resin tower: Step 1: Water washing. The remaining brine in the offline tower is replaced with pure water. Pure water enters from the top of the tower, while waste brine is recovered in the waste brine tank D-165. Step 2: Backwashing. Water enters from the bottom of the tower. The resin particles become loose, and the smaller resin particles are carried away; the wastewater is discharged to the resin catcher Z-164. Step 3: Acid regeneration. The hydrochloric acid, after being diluted, is fed into the resin column to replace the divalent metal ions in the column with H+. Step 4: Wash. The remaining hydrochloric acid in the tower was replaced with pure water. The wastewater flows into wastewater tank D-166. Step 5: NaOH regeneration. Caustic soda is diluted with water and fed into the resin tower, where Na+ ions displace H+ ions. The wastewater is sent to D-166. Step 6: Rinse with water. The remaining caustic soda in the tower is replaced with pure water. Step 7: Permutation. The remaining water in the tower is replaced by brine supplied from KV-162 at the bottom of the tower, and the wastewater is discharged to the exchange resin catcher Z-164. The resin column operates in three modes: automatic, semi-automatic, and manual. In automatic mode, the switching and regeneration of the resin column are carried out automatically. In semi-automatic mode, the regeneration of the column that is taken out of service takes place automatically, but the switching of columns is done manually. In manual mode, both the switching and regeneration of the resin column are performed manually; however, after the desired operation step is selected, programmable valves open or close automatically. Once this step is completed, the DCS automatically closes all KV valves on the regeneration lines. During the regeneration process, when the flow meter detects abnormal flow, the DCS will issue an alarm; the timer at this stage will pause. Once the flow returns to normal, the timer will resume counting. Anomaly detection of flow rate is not performed within 1 minute after the completion of the regeneration and tower switching steps; this 1-minute period is set by the DCS using TIM011. Fault alarm for the control valve KV: whenever the DCS detects an abnormality in any KV valve, it will issue an alarm, and that KV valve will flash on the display panel. The above actions are not performed within 1 minute after the end of the regeneration step or the end of a tower switching step; this 1-minute period is set by the DCS using TIM-KV. Interlock of KV valves during regeneration: 1. All the KV valves below are closed during the regeneration of each tower. If any of the valves below is opened during the regeneration of each tower, all valves except KV-169-3 will close. To wait for the shutdown of KV-162, its status is ignored for 1 minute after the saline filling is completed; this 1-minute period is set by the DCS in timer TIM016. Tower A: KV-161A, KV-164A, KV-165A, KV-164B, KV-161C, KV-165C, KV-162. Tower B: KV-161B, KV-164B, KV-165B, KV-164C, KV-161A, KV-165A, KV-162. Tower C: KV-161C, KV-164C, KV-165C, KV-164A, KV-161B, KV-165B, KV-162. Note: During the saltwater filling step, the saltwater valve for KV-162 is not interlocked. 2. Interlock of the KV valve during hydrochloric acid injection: 1) One minute after KV-169-3 is closed, KV-169-1 and KV-169-2 are opened; this one minute is set by the DCS on timer TIM014. 2) One minute after KV-169-1 and KV-169-2 are turned off, KV-169-3 is turned on; this one minute is set by the DCS on timer TIM013. 3) During the hydrochloric acid regeneration process, close KV-169-2 when the flow rate of FICA-162 (pure water) is low. Open KV-169-2 when the flow returns to normal. 3. Interlock of KV valve during alkali regeneration: During the caustic soda regeneration process, KV-168 is closed when the flow rate of FICA-162 (pure water) is low. Open KV-168 when the flow returns to normal. 4. In the event of a power outage, all KV valves will close, the timer will stop counting time, and the system will remain in the regeneration mode. Once power supply is restored, press the start button to resume the regeneration process from where it left off due to the power outage. 5. To switch the resin tower, close the KV valve that puts it in offline mode for one minute, and then open the KV valve of the regeneration tower; this one-minute period is set in the software TIM010. Stop alarm: When the T-160 operation is stopped by the stop button or when there is a malfunction in the program, the DCS will generate an alarm. III. Control of Process Parameters 1. Control of Anode Solution Concentration In ion-exchange membrane electrolysis, the number of sodium ions migrating through the membrane is greater than that migrating in the anode solution; as a result, the concentration of sodium ions at and near the membrane surface is lower than that in the anode solution. Therefore, sodium ions must be supplied through diffusion. At a certain current level, when the concentration of the anode solution drops below a critical value, the operating voltage rises sharply and the purity of chlorine decreases; water undergoes self-decomposition, resulting in bubbles forming on the membrane. On the other hand, if the concentration of the anode solution increases to a certain level, the operating voltage will rise due to membrane contraction; therefore, it is necessary to maintain the anode solution concentration at a specified value. In the structure of the anode chamber of the electrolyzer, the flow guide plates installed there are used to accelerate the circulation of the anode solution inside. Maintaining the acidity at a specific value in the anolyte determines the flow rate of the anolyte. The brine discharged from the anode solution circulation tank passes through a concentration analyzer. When the concentration of the brine is below the set value, the flow rate of the refined brine fed into the electrolyzer, FICZA-231x, must be increased; whereas when the brine concentration is above the set value, this flow rate must be decreased. The flow rate of the brine supplied to the electrolyzer is automatically adjusted based on the current level. 2. Concentration of the catholyte: If the concentration of the catholyte exceeds 34%, the voltage in the operation cell will increase. This design of the membrane results in the best current efficiency at a concentration of 32%. The internal flow rate in the cathode chamber is increased by hydrogen gas. 3. Circulation rate: To prevent the formation of bubbles, it is necessary to maintain a uniform concentration of the electrolyte near the membrane. During electrolysis, hydrogen and chlorine are generated in the cell compartments; these compartments and electrodes are designed to allow for the effective release of the gases produced at the electrode and membrane surfaces. If the electrolyte flow rate decreases, the cell voltage will increase due to the gas-liquid ratio. Therefore, the electrolyte must have sufficient circulation volume. Additionally, the heat generated by electrolysis is transferred from the anode chamber to the cathode chamber through the migration of sodium ions and other particles. Heat must be removed through the cooling of the electrolyte. Therefore, an adequate flow rate of electrolyte is necessary to maintain uniform membrane temperature. 4. To counteract the migration of OH– from the cathode chamber to the anode chamber, an appropriate amount of hydrochloric acid must be added to the anode solution. The membranes produced by Asahi Kasei are of the perfluorocarboxylic acid type; in the -COONa+ state, these membranes exhibit their superior properties. If the carboxyl groups become of the -COOH type, they can no longer function as ion exchange membranes. The anodic reaction involves competition between Cl‑ and OH‑, which produce Cl2 and O2 respectively; if the acidity of the anolyte decreases, the proportion of oxygen increases relative to that of chlorine. If the inlet acidity is higher than 0.15N, the auxiliary anode at the inlet pipe of the anolyte solution will be damaged and dissolved, and bubbles will form on the membrane. Therefore, it is important to maintain the acidity of the anode solution at a certain value or to keep the pH of the anode solution at a certain level. At low currents, the number of hydroxide ions that migrate through the membrane is small; there is a certain relationship between the magnitude of the current and the number of hydroxide ions migrating through the membrane. Therefore, the amount of acid added to the electrolyzer is determined based on the current density. 5. Membrane pressure difference control: A positive cell voltage difference effectively reduces the cell voltage. Conversely, a negative slot voltage difference increases the slot voltage by 0.4 to 0.5 VDC. If the pressure difference is too large, the anode will deform. If the pressure difference is too low or negative, the membrane will develop pinholes due to intense friction between the cathode and anode during short periods of operation. In response, the DCS maintains the voltage difference between the anode and cathode at 0.4 mH2O. The pressure difference at the top of the tank depends on the pressures of hydrogen and chlorine in the outlet manifold. 6. Gas pressure: Increasing the pressure in the cell reduces the volume of gas in the electrolyte, which helps to lower the cell voltage. However, higher pressures require more stringent design standards for the individual cells and other related equipment. To meet these requirements, the gas pressure must be controlled at a specific value; the chlorine pressure is controlled at 4 mH2O. 7. Temperature of the electrolyte: As the temperature of the electrolyte increases, its resistance decreases. However, when the temperature of water is above 90°C, evaporation accelerates, resulting in a high gas-liquid ratio. In practical operation, the electrolyte has the lowest cell voltage at 90°C, and there is a linear relationship between cell voltage and temperature. Most of the heat from the electrolyte is transferred from the anode solution to the cathode chamber, and the temperature of the cathode solution must be cooled using a cathode cooler. IV. Safety interlock devices To ensure the safety of the electrolysis equipment, the following interlock devices have been installed: Automatic shutdown of rectification. The direct current power used for electrolysis in each cell is automatically shut off under the following conditions. 1. Abnormal potential difference in the electrolyzer (EdIZA-230); an abnormal potential difference indicates unstable operation of the individual cells, such as rising voltage, short circuits, pressure fluctuations, and membrane leaks. The system will automatically shut down when this value reaches its limit. During the startup of electrolysis, as the electrolytic operation is not yet stable, it is particularly necessary to turn off this interlock. After the vehicle has stabilized, adjust this potential to 0 using a variable resistor, and then engage this interlock. 2. The direct current in the electrolyzer exceeds the normal value (IIZA-230). 3. When the electrolyzer is powered (from a rectifier), the center of the electrolyzer and the ground have the same potential; if certain parts of the electrolyzer are grounded, the voltage meter showing the grounding voltage will fluctuate, and the rectification process stops when these fluctuations exceed the specified range. 4. Insufficient flow of saline or alkaline solution (FICA-231, FIZA-232). 5. The fresh saline delivery valve is open (ZV-241). The electrolysis power supply for all electrolyzers is automatically turned off under the following conditions. 1. The pressure difference between chlorine and hydrogen affects the pressure difference between the anode chamber and the cathode chamber; when the pressure of chlorine or hydrogen is too high (PICA-216, PIZA-217, PICA-226, PIZA-227), all electrolytic DC power supplies are automatically stopped through a interlock system. 2. Abnormal difference in gas pressure (PDIZA-200) 3. Abnormal liquid level in the anode fluid circulation tank or cathode fluid circulation tank (LICA-260, LICA-270) 4. Disruption of instrument power supply (YL-100) 5. Disruption of instrument gas supply (PIZA-520) 6. Emergency stop button (YL-103) 7. Faults in downstream processes. Other interlock systems: 1. When a rectifier stops operating, the mode of the corresponding hydrochloric acid supply valve (FCV-211) is changed to manual; this valve closes quickly via a solenoid valve, and the signal level remains below 1.25 mA to prevent the acid from corroding the membrane. 2. When a rectifier stops, the mode of the corresponding brine supply valve (FCV-231) changes from cascade to automatic, maintaining a normal flow rate of 15 m3/hr, and LICZA-260 shuts down when an HH alarm is triggered. 3. When a rectifier stops, the corresponding brine supply valve (ZV-231) is switched to the refined brine valve (ZV-241) to ensure the elimination of Cl2 in the anode chamber. 4. When all rectifiers stop, the following equipment operates simultaneously: the analog output signal from the brackish water supply valve (FCV-265) for the secondary saltwater is less than 1.25 mA, which causes the valve to close ； The main HCl valve (FCV-425) is quickly closed via a solenoid valve, with the signal remaining below 1.25 mA in the closed state ； Change the mode of the pure water valve (FCV-221) to manual, and close the valve when the analog output signal is less than 1.25 mA ； The N2 gas valve (ZV-279) is opened. Emergency power supply: hydraulic oil pump for the electrolyzer extruder, pure water pump, chlorine absorption blower, and alkali solution circulation pump for the chlorine absorber. When TICZA-153 and TIZA-161 are set to “HH”, the control valve of TCV-153 becomes manual, and this valve is fully closed by a solenoid valve, with the analog output signal being less than 1.25mA.