Analysis of the reasons for the polymerization of hydroxyacetonitrile and solutions

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Analysis of the reasons for the polymerization of hydroxyacetonitrile and solutions to address it. Hydroxyacetonitrile is an important organic intermediate for cyanohydrins, and it is widely used in the chemical industry. Glycine is one of the derivatives of hydroxyacetonitrile; it is widely used in the pharmaceutical and food industries, has high demand, and is also a basic raw material for the production of pesticides. Another derivative of hydroxyacetonitrile, glycolic acid, is a pollution-free and efficient cleaning agent. Additionally, hydroxyacetonitrile is also widely used in the fine chemicals industry; in the dye sector, it can be used to produce high-quality dyes that cause no pollution. It has broad development prospects in fields such as mineral processing, electroplating, and animal husbandry. Hydroxyacetonitrile is widely used in China’s chemical industry, with its consumption increasing year by year. The raw materials for its production are readily available, which makes research and production of this product increasingly important. Daqing Petrochemical Company uses multi-reactor continuous production to manufacture hydroxyacetonitrile. Since the plant began operation in May 2003, the hydroxyacetonitrile product has experienced several instances of polymerization, resulting in significant economic losses. Through research and analysis, this problem that has plagued actual production for many years was resolved. 1 Production process and reaction principle 1.1 Production process Hydrogen cyanide and formaldehyde undergo an addition reaction in a reactor under the action of a catalyst, in a certain ratio⋯. The reaction of hydroxyacetonitrile occurs very rapidly, with a large amount of heat being released simultaneously. By removing the excess reaction heat, the equilibrium can be shifted in favor of the products; therefore, low-temperature freezing is used in production to increase the conversion rate. The reaction products pass through pre-cooling tanks and cryogenic tanks before entering the semi-finished product tank. Once the semi-finished products pass the quality tests, sulfuric acid is added to adjust the pH value to 2-3, resulting in the hydroxyacetonitrile aqueous solution, which is the final product. 1.2 Reaction mechanism of 2-hydroxyacetonitrile: The addition reaction of the aldehyde group is a typical nucleophilic addition reaction, and its reaction equation is: HCN + H–C–N. A circulation pump is used to draw the reaction mixture to the circulating cooler; brine at –10°C is employed to remove the heat generated by the reactions in the condensation reactor. The reaction temperature in the condensation reactor is maintained between 17 and 22°C, with a pH value of 4.7 to 5.1. The residence time is approximately 1.5 hours. An agitator is installed in the reactor to ensure uniform reaction progression. Then the product overflows into the pre-cooling vessel for further reaction, and the reaction temperature is maintained between 17 and 22°C through the flow of 10°C saline in the coiled tubes and jacket℃ ； The product overflows into the cryogenic reactor to continue the reaction, and the reaction temperature is controlled in the same manner as that in the pre-cooling reactor. After emerging from the cryogenic tank, the product enters the tanks for finished and semi-finished products; when the liquid level reaches 70%, a tank transfer cycle is carried out followed by sampling for analysis. If it fails to meet the standards, an appropriate amount of formaldehyde or a solution with a high cyanide content can be added to continue the reaction; once it meets the requirements, sulfuric acid is then added to adjust the pH value to 2.3, after which it is transferred to the large tank for final product storage. 2 Phenomenon of polymerization 2.1 First polymerization At 18:00 on May 14, 2003, when the level in the semi-finished product tank V-4 reached 64%, the cooling coil for brine in V-4 was activated, but the temperature dropped slowly. At 19:00, the temperature of V-4 suddenly rose from 27°C to 30°C; the operator immediately brought in a rubber belt to add water to it. Due to the highly intense polymerization reaction and the lack of emergency cooling facilities, the temperature could no longer be controlled even when water was added. By 22:15, the temperature continued to rise rapidly, reaching 98.1°C at 22:35. At 23:10, the temperature of the tank dropped to 68°C, and the material had turned brown. 2.2 Second polymerization: At 18:00 on December 1, 2003, during normal operation of the hydroxyacetonitrile plant, the temperatures in tanks V-3 and V-4 rose suddenly, resulting in a polymerization reaction; the temperature increased from 24.7°C to 96°C, with polymerization occurring simultaneously in both semi-finished product tanks. 2.3 3rd Polymerization: At 22:21 on December 23, 2005, when the circulation pump P-7A stopped working, the temperature in reactor R-1 was 21.9 °C; at 22:25 it rose to 23.5 °C. By 23:03, the temperature had increased to 70.2 °C, and at 23:09 it rose sharply to 97.1 °C. Polymerization occurred within the circulation loop of reactor R-1, and the color of the material changed from colorless to brown. Analysis of the reasons for the polymerization of 3-hydroxyacetonitrile 3.1 Analysis of the reasons for the first polymerization (1) Excessive amount of catalyst was added, resulting in a pH value of 6.42 (the acceptable range is 4.7–5.1); since this was the first time the facility was operated, there was no on-line pH meter available. (2) The circulation lines for V-3 and V-4 tanks lack circulation coolers; when the material polymerizes, a large amount of reaction heat is generated that cannot be removed. 3.2 Analysis of the reasons for the second aggregation: (1) There may be errors in the data collected; the actual pH value of the system is higher than the values obtained from laboratory analysis, and the actual amount of catalyst added exceeds the normal level, resulting in an excessively high pH value. (2) The HCN feed rate under the production load is 275 kg/h, which is lower than the normal load, resulting in an excessively long residence time of the material within the system. (3) The system contains free radicals that can cause polymerization; since alkaline materials are always stored in tank V-3, prolonged storage may lead to polymerization. 3.3 Analysis of the reasons for the 3rd aggregation: The circulation pump does not display an operating status; when the pump shuts down, it is not detected on the screen, and no alarm is issued. The process lacks an interlock control system; when the temperature exceeds the specified limit, the feed cannot be stopped in a timely manner, resulting in a longer residence time for the reactants. At the same time, the pH level of the materials inside the reactor is uneven. These 3 instances of aggregation share a common feature: all were caused by sudden changes in temperature, which led to the aggregation of the material. Reaction temperature is a very crucial factor in controlling polymerization. When the temperature rises to a certain level, its change becomes rapid; once the temperature exceeds that threshold, the polymerization reaction proceeds swiftly and uncontrollably. These 3 cases show that whether it is the reactant of hydroxyacetonitrile or