One question per week; answers win prizes. Question 6 from Chlor-alkali Chemistry (Basic Chemical Engineering Knowledge)

This post was last edited by Zhong Hengming on 2010-7-21 at 10:16. The purpose of adding acid to the electrolyte cell is to neutralize the OH- ions that migrate back from the cathode, thereby reducing side reactions at the anode, improving current efficiency, and lowering the cell voltage. After a new membrane is installed and the system starts operating, it needs to run at a temperature of 80 degrees Celsius for some time without adding any acid. Once operation stabilizes, the cell temperature should be gradually increased to 88 degrees Celsius, while acid is added slowly. Typically, 50–80 liters of acid are added first, after which the amount is increased gradually at a rate of 20 liters per day, until the acidity levels at the inlet and outlet are within the acceptable range. In the later stages of operation, when the acidity level at the outlet can no longer meet the requirements, the acidity level at the inlet should be maintained between 0.13 and 0.14; it must not exceed this range under any circumstances.

This post was last edited and replied to by Zhong Hengming on 2010-7-21 at 11:16. Reply 1# Zhong Hengming: According to the principle of cell voltage, the amount of acid added has an impact on the cell voltage. Adding acid: 1. It is done to neutralize the hydroxide ions that come from cathodic reverse osmosis, thereby reducing the amount of deposits on the membrane, and ultimately achieving a reduction in the cell voltage. 2. The chloride ion concentration in the anode area can be appropriately increased. 3. Reduce losses of chlorine due to dissolution and increase electrolysis efficiency. But there are also some negative effects: 1. The corrosion of the auxiliary anode at the inlet site will worsen ； 2. The membrane is acidified and loses its function ； 3. Calcium and magnesium impurities in the saline water form precipitates within the membrane, damaging its structure.

The main function of adding acid is to increase the purity of chlorine and reduce its solubility, which is beneficial for the subsequent dechlorination process. During the electrolysis process, a small amount of the alkaline solution at the cathode penetrates through the ion membrane to the anode, where it reacts with the acid added there. The main causes of this are excessive cathode pressure, unstable pressure between the anode and cathode, too high an amount of alkali added on the cathode side, or minor pinholes in the ion membrane. Control: Follow the acidity levels for import and export specified in the respective operation manuals. Do not exceed the prescribed values

During the electrolysis process, the equivalent amount of OH- that migrates back from the cathode chamber to the anode chamber is (1—CE), where CE is the current efficiency. To neutralize the OH- that migrates back, an appropriate amount of hydrochloric acid must be added to the anode solution. Moreover, during the electrolysis process, the anodic reaction is a competitive reaction between CL- and OH-, resulting in the production of CL2 and O2 respectively; if the acidity of the anode solution is too low, the amount of oxygen present in the chlorine gas increases ； Using no acid or too little acid increases side reactions, reduces the efficiency of the anode, raises the chloride content, and generates oxygen; it also causes the anode coating to become passivated, which not only affects the lifespan of the anode coating but also increases the cell voltage. However, the amount of acid added cannot be too much; it must be strictly controlled.

This post was last edited by Zhong Hengming on 2010-7-22 01:05. The hide code cannot be used; please reply directly. During use, the electrochemical properties of ion exchange membranes gradually deteriorate over time due to factors such as high oxygen content in chlorine and low concentration of brackish water, which also leads to a decrease in current efficiency. This, in turn, has a growing negative impact on the lifespan of the anode in the electrolyzer. Therefore, from the perspective of maintaining the long-term economic operation of ion membrane electrolyzer systems, it is essential to add acid to the brine fed into the system. An acid addition valve should be installed at the anode inlet, and pure water should be used to dilute the acid, thereby preventing crystallization of the brine at the acid addition point. However, excessive addition of acid can damage the ion membrane; it is best not to add acid to new membranes.

This post was last edited by Zhong Hengming on 2010-7-22 01:05. The purpose of adding acid to the electrolyzer is to neutralize the hydroxide ions that move from the cathode chamber to the anode chamber through the ion membrane, which also reflects the current efficiency of the ion membrane. When the current efficiency is high, few hydroxide ions migrate; as operation time progresses and the performance of the ion membrane declines, the current efficiency gradually decreases, and more and more hydroxide ions migrate from the cathode chamber.

During high-current operation, a larger number of hydroxide ions pass through the membrane into the anode chamber; as a result, the anode reaction is a competitive process between Cl- and OH-, resulting in the production of Cl2 and O2 respectively. If the acidity of the anode solution decreases, the oxygen content in chlorine will increase. To control the high chlorine purity, reduce the occurrence of anodic side reactions, and improve current efficiency, it is necessary to add hydrochloric acid to the anode to neutralize the hydroxide ions.

Adding acid increases the concentration of chloride ions, which can improve the efficiency of electrolysis while also suppressing the dissolution of the chlorine gas produced during electrolysis. The amount of acid to be added should be increased gradually; different membranes require different amounts of acid, and this is mainly related to the structural properties of the membranes. This phenomenon of an increased need for acid over time can also be explained in this way.

Reply to 1# Zhong Hengming: Adding acid is to neutralize the hydroxide ions that penetrate into the anode chamber, thereby suppressing side reactions at the anode and ensuring a certain level of chlorine purity； Reduce the amount of impurity deposition on the membrane anode side, thereby lowering the cell voltage. As the membrane is used for a longer period of time, its selective permeability gradually decreases; therefore, the amount of hydrochloric acid added must be increased gradually to maintain a certain level of anode efficiency.

Reply to 1# Zhong Hengming: During the electrolysis process, many side reactions occur at the anode, which can affect current efficiency and the purity of chlorine gas; adding acid is intended to suppress these side reactions. Adding an appropriate amount of hydrochloric acid before the saline enters the tank can neutralize excess NaOH and decompose Na2CO3; this is also the main reason for adding a small amount of acid during the operation of the new membrane. At this point, the cascade relationship between acid addition and current is generally discontinued, and acid is added manually in small amounts. In the later stages of membrane operation, as the exchange capacity of the membrane declines and the amount of OH- ions that permeate to the anode increases, increasing the amount of acid added can suppress to the greatest extent the occurrence of the following reactions: CL2 + 2NaOH --> 1/3NaCLO3 + 5/3NaCL + H2O; CL2 + 2NaOH --> 1/2 O2 + 2NaCL + H2O; HCLO + NaOH --> 1/2 O2 + NaCL + H2O; 40H- + O2 --> 2H2O + 4e. The OH- ions that migrate in the reverse direction are neutralized by H+ ions in the acidic saline solution, reducing the likelihood of OH- ions discharging at the anode to produce O2, thereby improving the purity of chlorine gas and the anode current efficiency. The addition is carried out via cascade control, and its parameters need to be adjusted appropriately over time, while ensuring that the outlet pH remains above 2. Furthermore, appropriate acid addition can also decompose some of the chlorates generated by electrolysis to maintain the sodium chloride concentration in the brine.