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Fire hazard and prevention in the chemical reaction process(Ⅲ)

Date: Feb 10, 2023

8. Chlorination

    The process of replacing Hydrogen atoms in organic compounds with chlorine atoms is called chlorination. Such as using methane to make methane chloride, benzene chlorination to make chlorobenzene, etc. Commonly used chlorinating agents are: liquid or gaseous chlorine, gaseous hydrogen chloride and various concentrations of hydrochloric acid, phosphoric acid chlorine (phosphorus oxychloride), phosphorus trichloride (acyl chloride used to make organic acids), sulfuryl chloride (dichlorosulfur acyl), hypochlorite, etc.

Hazard analysis and fire prevention points in the chlorination process:

    (1) The fire hazard of the chlorination reaction is mainly determined by the nature of the chlorinated substance and the conditions of the reaction process. Most of the raw materials used in the reaction process are organic combustibles and strong oxidants, such as methane, ethane, benzene, alcohol, natural gas, toluene, liquid chlorine, etc. For example, 2006m3 methane and 6960kg liquid chlorine are required to produce 1t of methane chloride, and there is also a risk of fire and explosion during the production process. Therefore, various ignition sources should be strictly controlled, and electrical equipment should meet fire and explosion-proof requirements.

    (2) The most commonly used chlorinating agent in chlorination reaction is liquid or gaseous chlorine. Chlorine itself is highly toxic, highly oxidizing, and stored under high pressure, so it is very dangerous if it leaks. Therefore, the liquid chlorine in the storage tank must first enter the evaporator to be vaporized before entering the chlorinator for use. Under normal circumstances, it is not allowed to use cylinders or tank trucks storing chlorine gas as storage tanks, because this may cause the chlorinated organic substances to flow back into the cylinders or tank trucks and cause an explosion. For general chlorinators, a chlorine buffer tank should be installed to prevent backflow when the chlorine gas is cut off or the pressure is reduced.

    (3) Chlorination reaction is an exothermic process, especially when chlorination is carried out at a higher temperature, the reaction is more violent. For example, in the production of epichlorohydrin, propylene needs to be preheated to about 3000°C for chlorination, and the reaction temperature can be raised to 500°C. At such a high temperature, if the material leaks, it will cause fire or explosion. Therefore, general chlorination reaction equipment must have a good cooling system and strictly control the flow of chlorine gas to avoid accidents caused by excessive flow and sharp temperature rise.

    (4) Since hydrogen chloride gas is almost generated in the chlorination reaction, the equipment used must be anti-corrosion, and the equipment should be guaranteed to be tight.

Because hydrogen chloride gas is easily soluble in water, most of the hydrogen chloride in the tail gas can be removed by adding absorption and cooling devices.

9. Diazidation

    Diazotization is a reaction that converts primary arylamines into diazonium salts. Usually, the organic compound containing aromatic amine reacts with sodium nitrite in an acidic medium to convert the amine group (-NH2) into a diazo group (-N=N-) chemical reaction. Such as the preparation of dinitrodiazophenol.

Fire hazard analysis of diazoization:

    (1)The main fire hazard of the diazotization reaction lies in the diazonium salts produced by the reaction, such as diazonium(C6H5N2Cl) hydrochloride(C6H5N2H504) and diazosulfate, especially diazonium salts containing nitro groups, such as diazonium- dinitrophenol(C6H5N2H504), etc. They are easily decomposed under the action of slightly higher temperature or light, and some can even decompose at room temperature. Generally, for every 10°C increase, the decomposition rate doubles. In a dry state, some diazonium salts are unstable and vigorous, and can decompose and explode when heated, rubbed or impacted. If the solution containing diazonium salt is spilled on the ground or on the steam pipe, it can also cause fire or explosion after drying. In acidic medium, some metals such as iron, copper, zinc, etc. can promote the violent decomposition of diazo compounds, and even cause explosions.

    (2) Aromatic amine compounds used as diazo agents are all combustible organic substances, and there is a danger of fire and explosion under certain conditions.

    (3) Sodium nitrite used in the diazotization production process is an inorganic oxidizing agent, which can react with organic matter and cause fire or explosion when decomposed at 175

°C. Sodium nitrite is not an oxidizing agent, so when it encounters an oxidizing agent stronger than it, it will be endowed with reducing properties, so it may catch fire or explode when it reacts with strong oxidizing agents such as potassium chlorate, potassium permanganate, and ammonium nitrate.

    (4) In the production process of diazotization, if the reaction temperature is too high or the feeding of sodium nitrite is too fast or excessive, the concentration of nitrous acid will be increased, the decomposition of materials will be accelerated, and a large amount of nitrogen oxide gas will be produced, which may cause fire and explosion.

10. Alkylation

    Alkylation refers to the chemical reaction of introducing an alkyl group R—on atoms such as nitrogen, oxygen, and carbon in organic compounds. The introduced alkyl groups include methyl (-CH3), ethyl (-C2H5), propyl (-C3H7), butyl (-C4H9) and the like.

    Alkylation often uses olefins, halogenated hydrocarbons, alcohols and other substances that can introduce alkyl groups on carbon, oxygen, nitrogen and other atoms in organic compound molecules as alkylating agents. Such as the role of aniline and methanol to produce dimethylaniline.

Fire hazard of alkylation:

    (1) Most of the alkylated substances have the risk of fire and explosion. For example, the flash point of benzene is -11°C, and the explosion limit is 1.5% to 9.5%; the flash point of aniline is 71°C, and the explosion limit is 1.3% to 4.2%.

    (2) The fire risk of alkylating agents is generally greater than that of the alkylated substances. For example, propylene is a flammable gas with an explosion limit of 2% to 11%; methanol has a flash point of 7°C and an explosion limit of 6% to 36.5%; dodecene has a flash point of 35°C and a spontaneous ignition point of 220°C.

    (3) The catalyst used in the alkylation process has strong reactivity. For example, aluminum trichloride needs to be stored in a dry place. It is highly corrosive, and when it encounters water or water vapor, it decomposes and releases heat, releasing hydrogen chloride gas, which can sometimes cause an explosion. If it comes into contact with combustibles, it is easy to catch fire. Phosphorus trichloride is a corrosive, non-wet liquid. It decomposes violently when it encounters water or ethanol, releasing a large amount of heat and hydrogen chloride gas. It is extremely corrosive, irritating, and poisonous. In contact with water and acid (mainly nitric acid and acetic acid), it will generate heat and smoke, and there is a danger of fire and explosion.

    (4) The alkylation reaction is carried out under heating conditions. If the feeding order of raw materials, catalysts, alkylating agents, etc. is reversed, the speed is too fast, or the stirring is stopped, violent reactions will occur, causing material run-off, resulting in fire or explode.

    (5) Alkylated products also have a certain fire risk. For example, the flash point of cumene is 35.5°C, the autoignition point is 434°C, and the explosion limit is 0.68% to 4.2%; the flash point of dimethylaniline is 61°C, and the autoignition point is 371°C; the flash point of alkylbenzene is 127°C.

11. Sulphur

    Sulfonation is a reaction that introduces a sulfo group (-SO3H) into an organic compound molecule. Commonly used sulfonating agents are fuming sulfuric acid, sodium sulfite, potassium sulfite, sulfur trioxide, etc. For example, nitrobenzene and oleum are used to produce sodium m-aminobenzenesulfonate, and haloalkane and sodium sulfite are used to generate sulfonate under high temperature and pressure conditions, which are all sulfonation reactions.

Hazard analysis of sulfonation process:

    (1) Sulfur trioxide is an oxidizing agent, which will quickly cause a fire when it encounters a substance that is more flammable than nitrobenzene. In addition, the corrosivity of sulfur trioxide is very weak, but when it meets water, it will generate sulfuric acid, and at the same time, it will release a lot of heat, which will increase the reaction temperature. This situation will not only cause fire or explosion due to boiling over or combustion reaction caused by sulfonation reaction, but also increase the corrosion damage to equipment due to the strong corrosiveness of sulfuric acid.

    (2) Since benzene, nitrobenzene, and chlorobenzene are all combustibles, and the sulfonating agents concentrated sulfuric acid, fuming sulfuric acid (sulfur trioxide), and chlorosulfonic acid are all oxidizing substances, and some of them are strong oxidants, so it is very dangerous to carry out sulfonation reaction under the condition of the interaction between the two. If the feeding order of this sulfonation reaction is reversed, the feeding speed is too fast, the stirring is poor, the cooling effect is not good, etc., it may cause the reaction temperature to rise, and the sulfonation reaction will become a combustion reaction, causing fire or explosion accidents.

    (3) The sulfonation reaction is an exothermic reaction. If the reaction process is not effectively cooled and well stirred, the reaction temperature may be too high, resulting in explosion or fire accidents.

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