pH measurement in the ion-exchange membrane caustic soda process

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This post was last edited by sunjl1981 on 2013-1-6 20:04. I. Overview 　　 When chemical reactions involve substances such as acids and bases, the pH value is often a key process variable during the reaction. When measuring the pH value of a solution, the acid-base indicator method is generally used in laboratories, while potentiometric titration is employed in industrial processes. In the process of producing caustic soda using ion-exchange membrane electrolysis, particularly before seawater desalination, it is important to measure the pH value. 　　 　　1. Ion membrane electrolysis process flow: Using raw salt as the starting material, the brine that flows out of the ion membrane electrolyzer passes through a dechlorination tower to have chlorine removed, after which it enters a brine saturation tank to become saturated brine ； Then, NaOH, Na2CO3, and BaCl2 are added to the reactor ； The brine coming out of the reactor enters a clarification tank for clarification. However, the brine coming out of the clarification tank still contains some suspended solids, which can have an adverse effect on the chelation resin column used for the secondary purification of the brine (it is generally required that the suspended solids in the brine be less than 1 mg/L). Therefore, the brine needs to be filtered and then passed through a chelating resin column to remove metal ions such as calcium and magnesium, before being fed into the anode chamber of the ion-exchange membrane electrolyzer ； Meanwhile, pure water and liquid caustic enter the cathode chamber together. When direct current is applied, chlorine gas and brackish water are generated in the anode chamber, and they are separated by a separator. Chlorine is delivered to the chlorine main pipe ； The NaCI content in lightly saline water is generally 200-220 g/L, and it passes through a dechlorination tower and a desalinated water saturation tank. Hydrogen and 30-35% liquid caustic are produced in the cathode chamber of the electrolyzer, and they also pass through a separator. Hydrogen is delivered to the hydrogen main pipe ； 30-35% liquid caustic can be sold as a product, or it can be sent to an evaporation unit to be concentrated to 50%. 　　 　　a. Control of the pH value and ORP measurement of filtered brine To maintain stability in the pH value of the brine fed into the electrolyzer, some processes involve adding a certain amount of hydrochloric acid to the filtered brine, thereby keeping its pH at 100.5. 　　The ORP value of saltwater reflects the concentration of free chlorine in it. To prevent free chlorine from entering the chelating resin tower, an ORP detection device is installed in the salt water filtration pipeline. The amount of sodium sulfite to be added is determined based on the ORP level. 　　 　　b. Control of the pH value and ORP measurement of brackish water The brackish water generated during the electrolysis process contains a certain amount of free chlorine. This free chlorine causes severe corrosion to equipment and pipelines, and must be removed. Taking advantage of the property that hypochlorous acid tends to transform into chlorine gas at low pH values, a certain amount of hydrochloric acid is added to fresh brine to keep its pH between 1.25 and 1.5, thereby facilitating the removal of free chlorine. The brackish water is dechlorinated under vacuum or by air blowing, followed by the addition of sodium sulfite to further remove free chlorine. 　　 　　2. Importance of pH measurement 　　　The material analysis performed by this device primarily includes acidity (pH value), redox potential (ORP), concentration, as well as the levels of calcium and magnesium ions. These parameters play a very important role in the production process. For example, the pH of refined salt water. The concentrations of calcium and magnesium ions are directly related to the operating condition and lifespan of the ion membrane, and they also serve as an important basis for calculating production volume and product costs. To this end, it is essential to ensure the measurement accuracy and reliability of each analyzer. 　　 　　3. Application challenges: In the ion membrane electrolysis process for producing caustic soda at Workshop 4 of the Electrochemical Plant of Shanghai Chlor-Alkali Chemical Co., Ltd., sodium sulfite must be added to the brine based on its pH value; therefore, the measurement and control of this pH value are of great importance for improving both yield and quality. The device is equipped with two pH meters: the 301 signal before dechlorination, and the 303 signal after dechlorination. The original two sets of pH meters were products from a Japanese manufacturer when they were introduced. Since the performance of the pH electrodes did not meet the actual requirements of the production process, they could not be put into use from the start of operation; as a result, they were discarded, and data had to be obtained by taking samples continuously in the workshop’s analysis laboratory. Doing this results not only in poor timeliness but also in large analysis errors, making it impossible to achieve automatic control of sodium sulfite feeding. Manual feeding control not only requires high labor intensity but also makes it difficult to maintain stable pH levels, affecting the quality of the product. The reasons why pH electrodes were not used effectively in the past were: (1) The signal measurement point at 301 had a high concentration of ClO4- ions, ranging from 700 to 800 mg/L. Since CIO4- ions react with KCI to form insoluble compounds, silver chloride electrodes further react with bromides, iodides, cyanides, and especially sulfides to produce cystine or cysteine. Since silver sulfide forms black precipitates on the membrane, this leads to an increased response time and poorer reproducibility of the electrode potential (which directly results in pH measurement errors), while also increasing the membrane impedance by several times. 　　 　　(2) The pH value at the measurement point is very low, usually around 1.9. Since measurements are difficult to take when the pH is below 2 or above 12, the measurement error is large. In a strong acid medium, the acid molecules absorbed by the gel layer lead to an increase in the hydrogen ion activity within that gel layer. Therefore, at very low pH values, an acidity error occurs, resulting in a falsely high pH value. 　　 　　(3) The temperature at the measurement point is relatively high, generally around 90C. This poses a significant challenge not only to pH electrodes (as the high temperature of 90C **reduces the lifespan of pH electrodes**), but also, if the pH value is measured in the analysis room after sampling, the pH value of the same medium varies at different temperatures; therefore, using the pH value obtained in the analysis room to control the amount of sodium sulfite added during the process leads to significant errors. 　　 　　(4) The measurement point is at the pump outlet, where pressure fluctuations are significant; this accelerates electrode degradation, affects the proper establishment of the pH electrode potential, and shortens the lifespan of the electrode. 　　 　　(5) The medium under test has strong corrosivity. 　　 　　 　　II. Mettler-Toledo’s online pH monitoring system 　　　Due to the reasons mentioned above, Mettler-Toledo provided users with a measurement system consisting of the HA465-50-90-T-S7 electrode and the InFlow764-22/-56 enclosure, which successfully met the requirements for online monitoring. 　　 　　1. Electrode: This electrode features a double salt bridge structure, uses Viscolyt as the reference electrolyte (a reference solution with high molecular viscosity), and employs a triple ceramic diaphragm. The pH range is 0 to 14, and the temperature range is 0–130C. 　　This type of pH electrode has the following characteristics: (1) The Viscolyt reference electrolyte does not undergo any chemical reaction with the substance being measured, thereby keeping the reference solution separate from the sample solution ； The double salt-bridge structure can also prevent free chlorine from interfering with the potential of the reference electrode, thereby ensuring its stability. 　　(2) The three ceramic diaphragms appropriately increased the flow rate of the reference electrolyte, ensuring stable outward flow of the electrolyte and thus more accurate readings. 　　(3) Maintain a positive pressure of around 2 Bar during use; this prevents chlorine from affecting the electrode potential and also limits the reverse osmosis of the medium being tested. 　　 　　2. Sheath The sheath is used to protect the electrode from external impacts and to fix the electrode at the measurement point. Since the commonly used jacket material, stainless steel, cannot resist corrosion from substances such as chlorine, the InFlow764-22/-56 model was chosen, which uses soluble polytetrafluoroethylene to make the jacket for the measurement chamber. 　　 　　3. Transmitters 　　To meet the explosion-proof requirements for use in hazardous areas within the chemical/petroleum industries, intrinsically safe transmitters such as 2100/2XH or 2220X can be used. 　　 　　 　　III. Application Results 　　　 The electrodes used before had a lifespan of only one week; however, with the use of Mettler-Toledo’s HA465 double salt bridge electrodes, only three electrodes needed to be replaced over two years. The use of Mettler-Toledo products ensures measurement accuracy, enables automatic control of sodium sulfite feeding, improves labor productivity and product quality, and reduces the maintenance workload for workers; as a result, it has received positive feedback from users. . Note ← ) # ← , .