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

Date: Jan 19, 2023

1. Oxidation

Such as ammonia oxidation to nitric acid, toluene oxidation to benzoic acid, ethylene oxidation to ethylene oxide, etc.

    (1) Fire risk of oxidation

    ①Oxidation reaction requires heating, but heat will also be released during the reaction, especially the catalytic gas phase reaction. Generally, the oxidation reaction will be carried out at a high temperature of 250-600 ℃. If the heat generated by the reaction is not removed in time, the temperature will rise rapidly or even explode.

    ② For some oxidations, such as the oxidation of ammonia, ethylene and methanol vapor in the air, the material ratio is close to the lower explosion limit. If the ratio is out of balance and the temperature is not properly controlled, it is very easy to explode and catch fire.

    ③ Most of the oxidized substances are flammable and explosive substances. For example, ethylene is a flammable gas with an explosion limit of 2.7% to 34% and a spontaneous ignition point of 450°C, when ethylene oxide is oxidized to produce ethylene oxide; toluene is a flammable liquid, and its vapor is easy to form an explosive mixture with air, and the explosion limit is 1.2% to 7% when toluene is oxidized to produce benzoic acid; methanol is a flammable liquid, and the explosion limit of its vapor and air is 6% to 36.5% when methanol is oxidized to produce formaldehyde.

    ④Oxidizing agent has great fire hazard. Potassium chlorate, potassium permanganate, chromic anhydride, etc. are all oxidants, which can cause fire and explosion in case of high temperature or impact, friction, or contact with organic matter and acids; Organic peroxides are not only highly oxidizing, but also mostly flammable substances, some of which are particularly sensitive to temperature, and will explode when exposed to high temperatures.

    ⑤Some of the oxidation products are also fire hazards. For example, ethylene oxide is a flammable gas; although nitric acid is a corrosive substance, it is also a strong oxidant; the aqueous solution containing 36.7% formaldehyde is a flammable liquid, and the explosion limit of its vapor is 7.7% to 73%. In addition, some dangerous peroxides may be generated during some oxidation processes. For example, peracetic acid is produced in the process of oxidizing acetaldehyde to produce acetic acid. Peracetic acid is an organic peroxide with extremely unstable properties. It will decompose or burn when exposed to high temperature, friction or impact.

    (2) Fire prevention measures in the oxidation process

    ①If air or oxygen is used as the oxidant in the oxidation process, the ratio of the reaction materials (mixing ratio of combustible gas and air) should be strictly controlled outside the explosion range. Before the air enters the reactor, it should pass through a gas purification device to eliminate dust, water vapor, oil pollution and impurities that can reduce or poison the catalyst activity in the air, so as to maintain the activity of the catalyst and reduce the risk of fire and explosion.

    ②There are two types of oxidation reaction contactors, horizontal and vertical, filled with catalysts. Generally use vertical, because it is more convenient and safe.During the catalytic oxidation process, the appropriate temperature and flow rate should be controlled to prevent over-temperature, over-pressure and mixed gas from being within the explosive range when the exothermic reaction is carried out.

    ③In order to prevent the contactor from endangering personal and equipment safety in case of explosion or fire, a flame arrester should be installed in front of the reactor and on the pipeline to prevent the flame from spreading, prevent backfire, and prevent the fire from affecting other systems. In order to prevent the contactor from exploding, the contactor should have a pressure relief device, and adopt automatic control or adjustment and alarm interlocking device as much as possible.

    ④When using nitric acid, potassium permanganate and other oxidizing agents, the feeding speed must be strictly controlled to prevent excessive or wrong addition. The solid oxidizing agent should be crushed before use,it is best to use the oxidizing agent in solution if possible. The reaction should be stirred continuously, the reaction temperature should be strictly controlled, and the spontaneous ignition point of the oxidized substance should never be exceeded.

    ⑤When using an oxidizing agent to oxidize inorganic substances, if potassium chlorate is used to oxidize to produce iron blue pigment, the drying temperature of the product should not exceed its ignition point. The product should be washed with clean water to completely remove the oxidant before drying, so as to prevent the incompletely reacted potassium chlorate from causing the dried material to catch fire. Oxidation of some organic compounds, especially at high temperature, may produce coke in equipment and pipelines, which should be removed in time to prevent spontaneous combustion.

    ⑥The raw materials and products used in the oxidation reaction should be in accordance with the relevant regulations on the management of dangerous goods, and corresponding fire prevention measures should be taken, such as isolated storage, away from fire sources, avoiding high temperature and sunlight, preventing friction and impact, etc. If it is a flammable liquid or gas that is a dielectric, a grounding device that conducts static electricity should be installed.

    ⑦Nitrogen and water vapor fire extinguishing devices should be installed in the equipment system so that the fire can be extinguished in time.

2. Restore

For example, nitrobenzene is reduced to aniline by iron powder in hydrochloric acid solution, o-nitroanisole is reduced to o-aminoanisole by zinc powder in alkaline solution, use reducing agents such as hydrosulfite, potassium borohydride, lithium aluminum hydride, etc. for reduction.

    Hazard analysis and fire prevention requirements for the reduction process:

    (1) Hydrogen is involved (the explosion limit of hydrogen is 4%-75%) no matter it is using primary ecological reduction, or using a catalyst to activate hydrogen for reduction, especially catalytic hydrogenation reduction, most of which are carried out under heating and pressurized conditions. If there is misoperation or hydrogen leakage due to equipment defects, it is very easy to form an explosive mixture with air, and it will explode if it encounters a fire source. Therefore, the temperature, pressure and flow must be strictly controlled during the operation; the electrical equipment in the workshop must meet the explosion-proof requirements. Wires and wire junction boxes should not be laid and installed on the top of the workshop; the workshop should be well ventilated, a light roof should be used, and skylights or hoods should be installed to allow hydrogen to escape in time. The hydrogen produced in the reaction can be exported to the roof of the workshop through the exhaust pipe, and it is more than 2m above the roof ridge, and is discharged outside through the flame arrester. The equipment for pressurized reaction should be equipped with a safety valve, and the equipment that generates pressure during the reaction should be equipped with a bursting disc; install hydrogen detection and alarm devices.

    (2)The catalyst used in the reduction reaction, Raney nickel, has the danger of spontaneous combustion in the air after absorbing moisture. Even if there is no ignition source, the mixture of hydrogen and air can be ignited to form a fire and explosion. Therefore, when using them to activate hydrogen for reduction reaction, it is necessary to replace all the air in the reactor with nitrogen, and after the measurement confirms that the oxygen content has dropped to the standard, the hydrogen can only be introduced. After the reaction is over, the hydrogen in the reactor should be replaced with nitrogen, and then the hole cover can be opened to discharge the material, so as to prevent the outside air from contacting the hydrogen in the reactor. Raine nickel should be stored in alcohol. Palladium carbon should be fully washed with alcohol and water when recovering. When filtering and vacuuming, it should not be too dry to avoid oxidation and fire.

    (3)Solid reducing agents such as hydrosulfite, potassium borohydride, lithium aluminum hydride, etc. are all dangerous products that are flammable when wet. Among them, sodium bicarbonate generates heat when it meets water, and can decompose and release sulfur in humid air. Sulfur vapor has the risk of spontaneous combustion when heated, and sodium bicarbonate is also in danger of decomposing and exploding when heated to 190°C; Potassium borohydride (sodium) can spontaneously ignite in humid air, and when it meets water or acid, it will decompose and release a large amount of hydrogen gas, and generate high heat, which can cause hydrogen gas to catch fire and cause an explosion accident; lithium aluminum hydride is a reducing agent that is dangerous when wet. So make sure to keep them safe from moisture.

    When the sodium hydrosulfite is used for dissolving, the temperature must be strictly controlled. It is recommended to add hydrosulfide into water in batches while stirring, and then react with organic matter after dissolving. When sodium borohydride (potassium) is used as the reducing agent, special attention should be paid when adjusting acid and alkalinity during the process, and do not add acid too fast or too much. When lithium aluminum hydride is used as a reducing agent, it must be used under nitrogen protection, which requires special attention, and it is usually stored submerged in kerosene. The reducing agent mentioned above will react violently with the oxidizing agent, generate a large amount of heat, and has the danger of fire and explosion, so it should not be mixed with the oxidizing agent.

    (4) The intermediates of the reduction reaction, especially the intermediates of the reduction reaction of nitro compounds, also have a certain fire hazard. For example, in the process of reducing o-nitroanisole to anthraniloanisole, azoanisole oxide is produced, and this intermediate can spontaneously ignite when heated to 150°C. In the production of aniline, if the reaction conditions are not well controlled, cyclohexylamine with a high risk of explosion can be produced. Therefore, various reaction parameters and reaction conditions must be strictly controlled during the reaction operation.

3. Nitrification

Nitrification usually refers to the reaction of introducing nitro (-NO2) into organic compound molecules to replace hydrogen atoms to generate nitro compounds. For example,use nitration of toluene to product TNT, use nitration of benzene to produce nitrobenzene, use nitration of glycerin to produce nitroglycerin, etc.

    (1)Nitrification is an exothermic reaction, and a nitro reaction needs to release heat of 152.2-153 kJ/mol, so the nitrification reaction needs to be carried out under cooling conditions. In the nitrification reaction, if there is a slight negligence, such as stopping the stirring midway, poor cooling water supply, too fast feeding speed, etc., the temperature will increase sharply, the oxidation ability of the mixed acid will be strengthened, and multiple nitro substances will be formed, which will easily cause fire and explosion.

    (2) Nitrifying agents are oxidizing, and commonly used nitrating agents such as concentrated nitric acid, nitric acid, concentrated sulfuric acid, oleum, and mixed acids have strong oxidizing, water-absorbing and corrosive properties. They can cause combustion when they come into contact with grease, organic matter, especially unsaturated organic compounds. When preparing the nitrating agent, if the temperature is too high or a small amount of water is dropped, it will cause a large amount of decomposition and evaporation of nitric acid, which will not only cause strong corrosion of the equipment, but also cause an explosion accident.

    (3)Most of the nitrated substances are flammable, such as benzene, toluene, glycerin (glycerol), absorbent cotton, etc., which are not only flammable, but also toxic. If used or stored improperly, it is easy to cause a fire.

    (4)Most nitration products have the risk of fire and explosion, especially polynitro compounds and nitrates, which are prone to explosion or fire when they are heated, rubbed, impacted or contacted with ignition sources.

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