Risk analysis of the hottest typical chemical reac

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Risk analysis of typical chemical reactions

① oxidation reaction needs heating, but the reaction process is exothermic reaction, especially catalytic gas-phase reaction, which is generally at 250 ~ 600 ℃ × 1、 × 2、 × 5、 × 104 gears; If these reaction heats are not removed in time, the temperature will rise rapidly and even explode

② for some oxidation, 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, in the oxidation of ethylene to ethylene oxide, ethylene is a flammable gas, the explosion limit is 2.7% - 34%, and the spontaneous combustion point is 450 ℃; In the oxidation of toluene to benzoic acid, toluene is a flammable liquid, its vapor is easy to form an explosive mixture with air, and the explosion limit is 1.2% - 7%; In the oxidation of methanol to formaldehyde, methanol is a flammable liquid, and the explosion limit of its vapor and air is 6% ~ 36.5%

④ oxidant has great fire risk. For example, potassium chlorate, potassium permanganate and chromic anhydride are all oxidants, which can cause fire and explosion in case of high temperature, impact, friction and contact with organic substances and acids; Organic peroxides are not only highly oxidizing, but also most of them are flammable substances. Some are particularly sensitive to temperature and explode in case of high temperature

⑤ some oxidation products also have fire hazards. For example, ethylene oxide is a combustible gas; Although nitric acid is corrosive, 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% - 73%. In addition, some oxidation processes may also generate dangerous peroxides. For example, peracetic acid is generated during the oxidation of acetaldehyde to acetic acid. Peracetic acid is an organic peroxide with extremely unstable properties, which will decompose or burn under high temperature, friction or impact

(2) fire prevention measures in the oxidation process

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

② there are two types of oxidation reaction contactors, horizontal and vertical, which are internally filled with catalyst. Generally, vertical type is used, because this type of catalyst is convenient and safe for loading and unloading. In the process of catalytic oxidation, for exothermic reaction, appropriate temperature and flow should be controlled to prevent overheating, overpressure and mixture within the explosion range

③ in order to prevent the contactor from endangering the safety of personnel and equipment in case of explosion or fire, flame arresters should be installed in front of the reactor and on the pipeline, which has the advantage of high measurement accuracy, so as to prevent the spread of flame and backfire, so that the fire will not affect other systems. In order to prevent the explosion of the contactor, the contactor shall be equipped with pressure relief device, and the relationship between fire prevention and thermal insulation shall be adopted as far as possible. The specific problems shall be analyzed with automatic control or adjustment and alarm interlocking device

④ when using nitric acid, potassium permanganate and other oxidants, the feeding speed should be strictly controlled to prevent excessive or wrong addition. Solid oxidants should be used after crushing, preferably in a solution state. They should be stirred continuously during the reaction, and the reaction temperature should be strictly controlled, and must not exceed the spontaneous combustion point of the oxidized substance

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

⑥ the raw materials and products used in oxidation reaction should be taken with corresponding fire prevention measures according to the management regulations on dangerous goods, 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 of dielectric, a grounding device for conducting and removing static electricity should be installed

⑦ nitrogen and steam fire extinguishing devices should be set in the equipment system to put out the fire in time

2 reduction

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, and reduction is carried out with reducing agents such as sodium silicate, potassium borohydride, lithium aluminum hydride, etc

hazard analysis and fire prevention requirements of the reduction process:

(1) whether it is the use of primary ecological reduction or the activation and reduction of hydrogen with catalyst, there is hydrogen (the explosion limit of hydrogen is 4% - 75%), especially the catalytic hydrogenation and reduction, which are mostly carried out under the conditions of heating and pressurization. If the operation is wrong or there is hydrogen leakage due to equipment defects, it is very easy to form explosive mixtures with air, It will explode in case of fire. Therefore, temperature, pressure and flow should be strictly controlled during operation; Electrical equipment in the workshop must meet explosion-proof requirements. Wires and wire junction boxes should not be laid and installed on the top of the workshop; The ventilation of the plant is good. Light roofs, skylights or hoods should be used to make hydrogen escape in time; The hydrogen produced in the reaction can be led out of the workshop roof through the exhaust pipe, which is more than 2m higher than the roof ridge, and then discharged outward through the flame arrester; The equipment for pressurized reaction shall be equipped with safety valves, and the equipment generating pressure in the reaction shall be equipped with bursting discs; Install hydrogen detection and alarm devices

(2) the catalyst used in the reduction reaction has the risk of spontaneous combustion in the air after moisture absorption. Even if there is no ignition source, it can ignite the mixture of hydrogen and air to form a fire explosion. Therefore, when they are used to activate hydrogen for reduction reaction, all the air in the reactor must be replaced with nitrogen first, and the oxygen content is confirmed to be reduced to the standard after measurement; After the reaction, the hydrogen in the reactor should be replaced with nitrogen before opening the hole cover to discharge, so as to prevent the outside air from meeting with the hydrogen in the reactor, and the ignition and explosion will occur in the case of spontaneous combustion of Rexroth nickel. Rexroth nickel should be stored in alcohol. When palladium carbon is recovered, it should be fully washed with alcohol and clean water. When filtering and vacuumizing, it should not be too dry to avoid oxidation and ignition

(3) solid reductant fuse powder, potassium borohydride, lithium aluminum hydride, etc. are all flammable dangerous goods when exposed to humidity. Fuse powder heats up when exposed to water and can separate sulfur in humid air. Sulfur vapor heating has the risk of spontaneous combustion, regularly coating a thin layer of molybdenum disulfide grease, and fuse powder itself heated to 190 ℃ also has the risk of decomposition and explosion; Potassium borohydride (sodium) can spontaneously ignite in humid air, decompose and release a large amount of hydrogen when it meets water or acid, and produce high heat at the same time, which can cause hydrogen to catch fire and cause explosion accidents; Lithium aluminum hydride is a reducing agent that is dangerous in case of moisture. It must be properly kept to prevent moisture. When the insurance powder is used for dissolution, the temperature should be strictly controlled. When stirring is started, the insurance powder can be added to the water in batches, and then react with organic matter after dissolution; When sodium (potassium) borohydride is used as reducing agent, special attention should be paid to the adjustment of acid and alkalinity in the process to prevent too fast and too much acid addition; When lithium aluminum hydride is used as reducing agent, special attention should be paid to it. It must be used under the protection of nitrogen and stored in kerosene at ordinary times. The reducing agent mentioned above will react violently with oxidant, generate a lot of heat, and have the risk of ignition and explosion, so it must not be mixed with oxidant

(4) the intermediates of reduction reaction, especially the intermediates of nitro compound reduction reaction, also have a certain fire risk. For example, in the process of reducing o-nitroanisole to o-aminoanisole, azoanisole is produced, and the intermediate can self ignite when heated to 150 ℃. If the reaction conditions are not well controlled in the production of aniline, cyclohexylamine with great explosion risk can be generated. Therefore, various reaction parameters and conditions must be strictly controlled in the reaction operation

(5) carry out technological innovation and study the use of new reducing agents with low risk and high reduction efficiency to replace reducing agents with high fire risk. For example, using sodium sulfide instead of iron powder for reduction can avoid the generation of hydrogen and eliminate the problem of iron sludge accumulation

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