How to deal with self-aggregation of monomer pipelines

At 15:13 on February 6, 2007, a fire broke out in the heavy and light removal unit of the carbon four extraction unit of the 45,000 tons/year carbon four workshop of the synthetic rubber plant of Lanzhou Petrochemical Company. After subsequent investigation, it was determined that the peroxide self-polymer deflagrated during the emptying operation of butadiene in the device. Analysis of the reasons showed that the workshop operators did not have enough knowledge of the characteristics of butadiene peroxidation autopolymerization, lack of understanding of the formation mechanism and hazards, and insufficient preventive measures in the production process, which led to the occurrence of fire accidents. 1. The basic physical and chemical properties of butadiene are C4H6 and molecular weight 54. Butadiene is a colorless, aromatic, and toxic gas at room temperature. It is a colorless gas that is easily liquefied and can form an explosive mixed gas with air. Slightly soluble in water, easily soluble in organic solvents such as propylene and benzene, and easy to polymerize, especially in the presence of oxygen. Steam density is 1.9kg/m3; butadiene relative density: 0.6211 (liquid at 20℃) ； melting point: -139℃ ； boiling point: -4.41℃ ； autoignition point: 414℃ ； vapor pressure: 101.3Kpa (-4.5℃) ； Flash point: -76℃ ； freezing point: -108.9℃ ； explosion limit: 2.16～11.47℃(volume). The chemical structure of butadiene is different from other monoolefins. The carbon-carbon double bond has a conjugation effect, which determines that its chemical properties are different from other monoolefins and diolefins. Butadiene has strong polymerizability and can produce high molecular polymers, such as butadiene polymers, rubber strong polymers, butadiene peroxides and butadiene end-group polymers. These polymers expand in volume and generate a large amount of heat, which can cause equipment pipes to become clogged or explode. 2. The dangers and hazards of butadiene (1) Flammable and explosive. Butadiene can form an explosive mixture when mixed with air, and can cause combustion and explosion when exposed to open flames or high heat. The vapor is heavier than air and can spread to a considerable distance at a lower level. It will cause backfire when exposed to an open flame. If encountering high heat, polymerization reaction may occur and a large amount of heat will be released, causing container rupture and explosion accidents. (2) Easy to gather by itself. Butadiene is very prone to self-polymerization, and its self-polymers come in four forms: butadiene polymers, rubber-like autopolymers, butadiene peroxide self-polymers, and terminal polymers. Butadiene dimerizes when heated to produce 4-vinylcyclohexene. Meanwhile, butadiene dimer is a thermal polymer of butadiene. The dimer is liquid at room temperature, fluid, with a boiling point of 116°C, and can be dissolved with butadiene in any ratio. Generally, butadiene dimer is not flammable or explosive. In small amounts, it can be regarded as a butadiene impurity. In large amounts, it will affect the polymerization of butadiene during product production. Rubber strong autopolymer is a thermal polymer of butadiene. When entering the polymerization system, it will affect the physical and mechanical properties of the rubber and easily block equipment and pipelines. Butadiene is prone to peroxidation in the presence of oxygen to form peroxide. Butadiene peroxide autopolymer is a light yellow viscous liquid with a relative weight of about 800 and a specific gravity larger than butadiene. It is easy to deposit and accumulate in dead corners of the equipment. Its properties are very unstable and will quickly decompose and spontaneously ignite when it is impacted or heated rapidly, causing an explosion. At the same time, it can decompose to generate active free radicals, which can initiate end-group polymerization of butadiene to generate rice flower solid polymer - end-group polymer (terminal polymer). When the growth of terminal polymer particles exceeds a certain limit, the chain growth becomes a non-termination reaction, resulting in the rapid increase and sharp expansion of terminal polymers. When the terminal polymer particles grow beyond an irregular limit, the heat of butadiene polymerization is difficult to discharge, resulting in local overheating, causing the peroxide to explode rapidly. Butadiene peroxidative self-polymerization is an autocatalytic reaction. Butadiene-Terminated Polymer A highly cross-linked, resinous polymer. If the system is not clean, the polymer is often contaminated by rust and iron ions and turns into dark yellow, dark camellia or coffee color. The large butadiene endpolymers resemble popcorn, so they are called popcorn-polymers. The chemical properties of end-group polymers are very active, prone to free radical chain polymerization, flammable and explosive, and bring considerable potential dangers to production and storage. (3) Easy to spontaneously ignite. Under a certain pressure, liquid butadiene is easily adsorbed in the small pores of the terminal polymer, and the butadiene cannot be replaced with nitrogen. When the oxygen content in the equipment exceeds the standard and the self-polymer is heated or heated by low-pressure steam, the evaporated butadiene combines with the oxygen in the air in the equipment to form peroxide, which will decompose and self-ignite and quickly ignite the self-polymer. Autopolymers tend to swell and crack valves and pipes. Butadiene often leaks out in large quantities due to the accumulation of butadiene in valves and pipelines. At the same time, due to the characteristics of butadiene, there is a possibility of freezing and cracking valves and pipelines after leakage. (4) It can easily cause poisoning and frostbite. Butadiene has * * and stimulating effect. acute poisoning: Mild cases include headache, dizziness, nausea, sore throat, tinnitus, general weakness, drowsiness, etc. In severe cases, symptoms such as drunkenness, dyspnea, pulse rate, etc. may occur, and then transition to loss of consciousness and convulsions. Sometimes, mental symptoms such as restlessness and running around may also occur. Recover quickly after disengagement. Headaches and drowsiness can sometimes last for a while. Direct skin contact with butadiene can cause burns or frostbite. chronic effects: Long-term exposure to a certain concentration of butadiene can cause symptoms such as headache, dizziness, general weakness, insomnia, dreaminess, memory loss, nausea, and heart palpitations. Dermatitis and polyneuritis are occasionally seen. 3. Reasons for the production of butadiene autopolymer Oxygen, water and rust are necessary conditions for the production of butadiene peroxide. Except for butadiene dimer, the production of other butadiene autopolymers requires the presence of oxygen. There is a small amount of oxygen in the butadiene system, which is easily absorbed by butadiene to form butadiene peroxide. Therefore, under the catalytic action of water, rust and iron ions in the system, the generated peroxide is broken to generate free radicals, free radical chain polymerization occurs, the chain polymerization gradually grows, and a block polymer with a cross-linked degree is generated. Once the end-group polymer is generated in the equipment, it spreads quickly like a "seed" and can continue to grow even in the absence of oxygen. In addition, the peroxide radical can also react with the initial active site to cyclize, or cyclize away from the initial active site to make the structure more complex. The peroxides contained in the cyclic structure further polymerize and lead to branching and cross-linking and multifaceted reactions. These chains and networks limit mobility and are therefore more difficult to terminate. The storage and transportation temperature of butadiene should be lower than 30℃. The higher the storage pressure of butadiene, the easier it is to self-polymerize. Generally, the storage pressure is 0.2~0.3Mpa. Since the end-group polymer is solid and insoluble in butadiene, and the active center is shielded internally and expands outward, it is difficult to dissipate the heat of polymerization, which accelerates the reaction rate and the breakage of the peroxy bond. In this way, a vicious cycle leads to violent reactions, and may even explode to form large-area polymers, blocking pipelines and generating internal pressure on the equipment, causing it to deform and rupture, leading to accidents. Butadiene peroxide polymer is a light yellow syrupy viscous liquid with a relative density greater than butadiene. It is almost insoluble in butadiene and deposits at the bottom of the container. The cumulative increase of butadiene peroxide is potentially dangerous. A large number of research results show that when the oxygen content in the system is greater than 1% (volume), the generation rate of butadiene peroxide depends on the decomposition rate, and butadiene peroxide autopolymers appear. On the contrary, since the formation rate of butadiene perchloride is slower than the decomposition rate, butadiene peroxide autopolymer cannot be produced, but the butadiene peroxide free radicals generated by decomposition can initiate the polymerization of butadiene to produce butadiene terminal polymers. Butadiene peroxide autopolymer will explode if it is hit or decomposes rapidly due to the heat generated by the end-group polymerization of butadiene. 4. Preventive and disposal measures (1) The installation of process equipment must meet the design requirements, temperature and pressure, material flow rate, and flow rate to ensure production needs. The pipeline circulation must be smooth to prevent the generation of autopolymers caused by defects in the process and equipment installation. (2) Pre-job training must be conducted for operating personnel. Everyone must be familiar with the operating procedures and the physical and chemical properties of butadiene, master the equipment process technology, and be able to handle accidents in a timely and safe manner according to the accident handling plan. (3) Frequently inspect and maintain the equipment to prevent "disease" operation, and promptly clean up internal pipeline blockages and sediments at the bottom of the container. (4) Equipment process installation should be equipped with safety protection devices, fire extinguishing equipment, and anti-fouling pools for accident disposal. Early detection and early disposal can eliminate accidents in the initial stage in time. (5) After an accident occurs, the first step is to stop and cut off the material, cool the equipment in time, control the combustion, then purge the internal pipelines with nitrogen or steam, and finally control to burn out the remaining material, or promptly export and transport the material after the fire is extinguished, and collect the fire-extinguishing water in an anti-fouling pool. Cool and isolate surrounding equipment, and stop production and export materials when necessary.

The accident investigation and analysis are good and have good academic research value. Collection: lol

1. According to the solubility of the monomer polymer, select a suitable solvent or monomer for cleaning and dredging; 2. Generally, the monomer polymer mainly accumulates in dead corners such as corners and low points. The pipes can also be disassembled for mechanical dredging.； 3. After adding the monomer, prevent self-aggregation again through cleaning, temperature control and other means. ；

Provide a simple method for reference: After adding the monomer, use nitrogen or other suitable gas to blow out the monomer in the pipeline.

Add polymerization inhibitor during normal use, add nitrogen purge during temporary shutdown, and boil with alkali solution to destroy peroxide during long-term shutdown.

1. Add polymerization inhibitors to high-concentration parts such as distillation towers and storage tanks. 2. According to the characteristics of the materials, set up equipment such as rapid heat extraction or ice machines to control the temperature of the materials, quickly pass through the self-aggregation temperature section, and control storage and transportation within a reasonable temperature. 3. The pipeline design should avoid sharp bends, and the weld height should be strictly controlled within the allowable range. If necessary, the pipeline should be ground and polished.

Add polymerization inhibitor to the monomer tank, and absorb the polymerization inhibitor in the silica gel tower before entering the polymerization tank. In addition, the degree of polish of the pipeline is also important.

This post was finally edited by Xiao Linsi on 2012-2-14 22:54. I read the introduction on the second floor and benefited a lot. However, there is one thing that I don’t quite understand. Why is the “end-group polymer” called “end-group”? What is the group at its end? What special functional groups are there inside it? Does anyone know?

Please tell the author the name of the monomer. Different monomers have different self-polymerization processes. We have no way to provide suggestions!