The self-polymerization problem of acrylonitrile

Moderator, please come and help! Material Safety Data Sheet for Acrylonitrile MSDS Section 1: Chemical Product and Company Identification Product Name: Acrylonitrile Catalog Codes: SLA2566 CAS#: 107-13-1 RTECS: Not available. TSCA: Listed in TSCA 8(b) inventory; Acrylonitrile CI#: Not available. Synonyms: Vinyl Cyanide; Propenitrile Chemical Name: Acrylonitrile Chemical Formula: C3H3N Contact Information: Sciencelab.com, Inc. 14025 Smith Rd., Houston, Texas 77396, USA Sales: 1-800-901-7247 International Sales: 1-281-441-4400 Order online at: ScienceLab.com CHEMTREC (24-hour emergency hotline): 1-800-424-9300 International CHEMTREC: 1-703-527-3887 For non-emergency inquiries, call: 1-281-441-4400 This is the English version of the information. Has anyone encountered this before? Last edited by vrain on 2008-11-17 at 23:07.]

　　Acrylonitrile, as an important chemical raw material, is used in the production of acrylic fibers, engineering plastics, and synthetic rubbers; it is the second most important product in the acrylate series. Currently, the acrylonitrile production process in our country generally employs the ammoxidation of propylene. This method has advantages such as a wide range of inexpensive raw materials, ease of one-step synthesis, and low production costs. However, it poses a significant fire hazard during the production process, making fire and explosion prevention extremely important. 　　1 Process Principle 　　1.1 Reaction Principle 　　3H6 + NH3 + 3/2O2 = CH2=CH-CN + 3H2O △H = -515 kJ/mol 　　Hydrogen cyanide, acetonitrile, acrolein, and carbon dioxide are produced as by-products. 　　1.2 Process Flow and Equipment The process for producing acrylonitrile through the ammoxidation of propylene can be simply represented as follows (see attached diagram): The entire production process is divided into a synthesis stage and a purification stage. In the synthesis process, before the reaction materials enter the reactor from the bottom, liquid propylene and ammonia are evaporated, and air is compressed; all of these are pre-heated before being fed into the mixer ； Then, it is fed into a reactor at a reaction temperature of 440°C and a pressure of 0.065 MPa, where the reaction takes place under the action of a catalyst ； The generated gas, after having its heat recovered in a waste heat boiler, enters an ammonia neutralization tower for ammonia removal. The ammonia-free gas is cooled and then introduced into an absorption tower, where it is absorbed by water to yield a mixture of acrylonitrile, hydrocyanic acid, acetonitrile, and other compounds. The purification process involves taking the aqueous solution supplied from the synthesis process, removing acetonitrile and hydrocyanic acid from it, and then feeding it into a distillation tower to produce acrylonitrile. 　　The main production process equipment includes a raw material mixer, an oxidation reactor, and a light components tower. Oxidation reactors are generally fluidized-bed type reaction towers, consisting of three sections: a cone section, a dense-phase section, and a dilute-phase section. The dense phase zone is where the ammoxidation of propylene takes place ； The dilute phase section is mainly used for catalyst recovery. The light component tower is used to remove hydrogen cyanide from the water absorption solution. 　　2 Process Fire Hazard Analysis 　　2.1 Flammability, Explosivity, and Toxicity of Raw Materials and Products 　　Propylene, the raw material, is a colorless, flammable gas at normal temperature and pressure; its flash point is -108°C and its auto-ignition temperature is 460°C. When mixed with air, it can form explosive mixtures, with an explosion limit of 2%-11.1%. Propylene is prone to combustion and explosion when exposed to sparks or high temperatures, and it is also low in toxicity. 　　Ammonia is a flammable gas with a auto-ignition temperature of 650°C. Its explosive limit in air is 15%–28%, and in oxygen it is 13.5%–79%. Ammonia can also explode when combined with chlorine and iodine. 　　The product acrylonitrile is a colorless liquid that is flammable and explosive, with a slightly irritating odor; its vapors are non-toxic. It has a flash point of 0°C, a auto-ignition temperature of 481°C, and an explosion limit of 3.05% to 17.0%. 　　2.2 High reaction temperature, large heat release, and a high risk of combustion. Ammoniation of propylene is a highly exothermic reaction, and the high reaction temperature makes it difficult to control. If the heat of reaction cannot be removed in a timely manner, combustion is highly likely to occur in the dilute phase section of the reactor. Because a portion of the propylene (10%–20%) remains unconverted in the dense phase, it undergoes further reaction and releases heat upon entering the dilute phase. When the temperature reaches the material’s auto-ignition point (above 470°C), combustion may occur. 　　2.3 The raw material mixture is explosive. In the propylene ammonia oxidation reaction, propylene and air account for 6.16% and 67.7% of the total volume of the raw materials respectively, meaning that the propylene concentration falls within the explosive range. This concentration ratio is relatively favorable for reaction selectivity, reaction rate, energy savings, and cost reduction, but it increases the risk of fire and explosion during the reaction process. 　　2.4 Side reactions release heat, increasing the fire risk. In the acrylonitrile ammoxidation reaction system, in addition to the formation of acrylonitrile, a series of side reactions may occur. Its by-products fall into three categories; one category is hydrides, such as hydrocyanic acid ； The second category is organic oxygen-containing compounds, such as acraldehyde ； The third category are the deep oxidation products, carbon dioxide and carbon monoxide. All side reactions are highly exothermic, increasing the overall heat effect of the reaction process. Among them, the reaction in which acrylonitrile is deeply oxidized to carbon dioxide and carbon monoxide is another major cause of the increase in temperature in the dilute-phase section of the reactor, leading to combustion. 　　The by-products hydrogen cyanide, acetonitrile, and acraldehyde are all flammable and toxic substances. 　　Impurities in the raw materials may be one of the reasons for an increase in side reactions. 　　2.5 The product is prone to polymerization, leading to blockages in the equipment pipelines. The acrylonitrile structure contains double bonds and conjugated systems, which makes it susceptible to polymerization; hydrogen cyanide, acraldehyde, and other by-products are generated as a result ； These by-products also tend to polymerize on their own; if unreacted ammonia is present in the product gas, they will polymerize at lower temperatures. Polymerized products can cause blockages in reboilers, towers, and pipes, affecting normal production. Furthermore, if the oxidation temperature exceeds 500, the reaction products will coking, which likewise causes blockages in the pipelines. 　　2.6 Materials are prone to generating static electricity, posing a potential source of static fire. The materials used in the ammoxidation of propylene are dielectrics. They generate static electricity when flowing at high speeds within pipes or being ejected through valves and nozzles; the maximum static voltage can exceed tens of thousands of volts, and there is a risk of fires caused by static discharge in such devices. 　　3 Fire and Explosion Prevention Technologies 　　3.1 Strengthening the Management of Hazardous Materials 　　Based on the physicochemical properties as well as the fire and explosion risks associated with raw materials, products, and by-products during the production process, it is necessary to implement appropriate fire safety measures in strict accordance with relevant regulations for the management of hazardous materials. For example, store them separately ； Keep away from sources of fire, heat, and electricity ； Avoid high temperatures and direct sunlight ； To prevent friction or impact, etc. 　　3.2 Controlling raw material purity It is necessary to check the purity of the raw materials in order to avoid side reactions caused by the presence of impurities, which can lead to adverse effects such as increased heat generation. Removal of sulfides that can poison the catalyst. Before entering the reactor, air should have impurities such as dust, moisture, and oil removed to reduce the risk of fire and explosion. 　　3.3 Suppression of the explosion hazard in the raw material mixture The mixer of air, propylene, and ammonia is where an explosive mixture is formed. To ensure safety, the velocity of the air exiting the nozzle must be much greater than the possible flame propagation speed of the material. Among the combustible gases useful in industry, hydrogen has the fastest flame propagation speed, but it is generally less than 10 m/s; therefore, an air outlet speed of 25–30 m/s at the nozzle is sufficient to ensure the safety of the mixer. The mixer is placed near the reactor inlet to ensure that the feed gas enters the reactor immediately after mixing, thereby reducing the space in which an explosion could occur. Furthermore, it is also possible to feed propylene, ammonia, and air separately in order to avoid the formation of explosive mixtures. 　　3.4 Controlling the reaction temperature The reaction temperature is the main parameter for controlling the propylene ammoxidation process. During the production process, temperature is controlled primarily by promptly removing the heat of reaction from within the reactor; for this reason, fluidized bed reactors equipped with U-shaped cooling tubes are commonly used in industry. Part of the heat released by the reaction is carried away by the reaction gases, while most of it is removed by the cooling system of the reaction bed. The reactor must have sufficient cooling area, and a coolant with stable parameters must be supplied continuously. At the same time, the reaction temperature is fine-tuned by adjusting the preheating temperature of the feed air, keeping it below 470°C. 　　3.5 Installation of safety protection facilities Nitrogen and steam pipelines should be installed in the equipment system; these are used for protection as well as for fire extinguishing. As the temperature in the dilute-phase section of the propylene amin oxidation reactor continues to rise, nitrogen or steam should be injected promptly. Among them, water vapor has a high heat capacity, which allows it to absorb a large amount of heat generated by the reaction, preventing overheating and facilitating control of the reaction temperature. At the same time, water vapor or nitrogen can also dilute the propylene in the feed gas to prevent explosions. However, an excessive amount of steam can cause excessive pressure inside the reaction vessel, leading to the rupture of its walls and the risk of massive leakage of the material. 　　3.6 Installation of explosion-proof pressure relief devices To prevent harm to personnel and equipment systems in the event of an explosion or combustion of flammable feed gases, flame arresters should be installed on the gas feed pipes ahead of the reactor as well as on the vent pipes, in order to stop backflow from spreading throughout the system. Explosion vents are installed in pressurized equipment such as propylene storage tanks and the drum of waste heat boilers; these vents are required to rupture when the pressure inside the equipment exceeds the allowable pressure by 25%, in order to reduce the damage caused by explosions. 　　3.7 Preventing blockages in equipment pipelines Reducing side reactions of polymerization of the product can effectively prevent blockages in the equipment pipelines. To avoid low-temperature polymerization in the presence of ammonia in the gas phase, the temperature of the reaction gas products after heat exchange should not be too low; it is generally necessary to maintain it at around 250°C. The top temperature of the light component column should be maintained at 25–26°C, under normal pressure or slightly negative pressure, to prevent acrylonitrile from being carried to the rear recovery system at the top of the column and undergoing polymerization. 　　To prevent polymerization during the refining process, a small amount of inhibitor must be added when handling the material. Hydrogen cyanide tends to polymerize in alkaline media, so acidic inhibitors must be added: Hydrogen cyanide can polymerize both in the gas phase and in the liquid phase; sulfur dioxide is generally used as an inhibitor in the gas phase, while acetic acid is used as an inhibitor in the liquid phase. Acrylonitrile is polymerized in the liquid phase, and hydroquinone can be used as a polymerization inhibitor. The presence of a small amount of water can also have an anti-polymerization effect on acrylonitrile. 　　Polymerization may still occur even with the use of a polymerization inhibitor. For example, in a hydrocyanic acid treatment tower, hydrocyanic acid polymerizes in the dead corners within the tower ； Polymers also tend to form at the bottom of the product column. Polymers and char should be removed regularly to prevent them from clogging equipment and pipes. The cleaning method involves flushing the wall surface and pipes with water, and cleaning them with steam. The product taco is equipped with two reboilers to allow for alternating cleaning. 　　3.8 Eliminating the hazards of static electricity Equipment grounding is the simplest, most common, and most fundamental measure for preventing static electricity. Anti-static grounding devices must be installed on oxidation reaction equipment and pipelines. The grounding wire must be connected firmly and have sufficient mechanical strength; it should be inspected regularly to prevent failures.

Great post, let’s give it support. Could you tell me what type of inhibitor is used to prevent the polymerization of acrylonitrile? Thank you

What’s the answer? I’m working on this too. Please explain the reasons for aggregation. Thank you

I also want to know the answer from upstairs. Here, at the beginning of this year, an explosion occurred at a factory where acrylonitrile was being distilled, resulting in 3 deaths

Acrylonitrile self-polymerizes under alkaline conditions. For this reason, strict control of the pH value is required during production. When PH is low, sodium carbonate is used for adjustment; when it is high, acetic acid is used. Acetic acid is generally used as a polymerization inhibitor during storage

6th floor, could you provide more detailed information on the explosion that occurred during acrylonitrile distillation? Our company has an acrylonitrile distillation and recovery unit, and I’m also very worried. Thanks

The inhibitors for acrylonitrile include hydroquinone, hydroquinone methyl ether, and a small amount of water； Acetic acid, on the other hand, is a polymerization inhibitor for hydrocyanic acid.