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

Date: Feb 08, 2023

4. Electrolysis

    When electric current passes through electrolytic solution or molten electrolyte, the chemical change caused on two poles is called electrolysis. Electrolysis has a wide range of functions in industry. The smelting of many non-ferrous metals (sodium, potassium, magnesium, lead, etc.) and rare metals (zirconium, hafnium, etc.), the refining of metal copper, zinc, aluminum, etc., as well as the preparation and electroplating, electropolishing, anodic oxidation, etc. of many basic chemical industry products (Hydrogen, oxygen, chlorine, caustic soda, potassium chlorate, hydrogen peroxide, etc.) are all realized by electrolysis.

    Such as salt water electrolysis production sodium hydroxide, hydrogen, chlorine, electrolysis water production, etc. Risk analysis and fire prevention points in the process of salt water electrolysis:

    (1). If the brine contains iron impurities, it can produce a second cathode and release hydrogen from the brine. When ammonium salt is added to brine under suitable conditions (pH<4.5), the action of ammonium salt and chlorine can produce ammonium chloride, and the action of chlorine in concentrated ammonium chloride solution can also produce yellow oily nitrogen trichloride.

    Nitrogen trichloride is an explosive substance, which will violently decompose and explode when it comes into contact with many organic substances or is heated to above 90°C and hit.

    Therefore, the quality of brine preparation must be strictly controlled, especially the content of iron, calcium, magnesium and inorganic ammonium salts. General requirements: Mg2+<2 mg/L, Ca2+<6mg/L, SO42-<5mg/L. An automatic analysis device for brine purity should be adopted as much as possible, so that the change of brine composition can be observed, and the dosage of sodium carbonate, caustic soda, barium chloride or acrylic acid amine can be adjusted at any time.

    (2). The height of brine addition should be appropriate. When brine is added to the anode chamber of the electrolytic tank during operation, if the liquid level of the brine is too low, hydrogen may penetrate into the anode chamber through the cathode mesh and mix with chlorine gas; if the brine in the electrolytic cell is overfilled, the brine will rise due to pressure. Therefore, the addition of brine should not be too little or too much, and a certain safety height should be maintained. If the brine feeder is used, the brine should be supplied intermittently to avoid the loss of current and prevent the brine conduit from being corroded by the current.

    (3). Prevent hydrogen from mixing with chlorine. Hydrogen is an extremely flammable gas, and chlorine is a highly oxidizing poisonous gas. Once these two gases are mixed, it is easy to explode. When the hydrogen content in chlorine reaches more than 5%, it may explode under light or heat at any time. 

    The main reasons for the mixture of hydrogen gas and chlorine gas are: The brine level in the anode chamber is too low; the hydrogen outlet of the electrolyzer is blocked, causing the pressure in the cathode chamber to rise; The quality of asbestos wool is not good, and the diaphragm is damaged when installing the electrolyzer, causing the diaphragm to fall off partially or the amount of salt water injected before power transmission is too large to damage the diaphragm. In addition, when the pressure in the cathode chamber is equal to or exceeds the pressure in the anode chamber, hydrogen may also enter the anode chamber, which may cause the hydrogen content in the chlorine gas to increase.

    At this time, a comprehensive inspection of the electrolytic cell should be carried out, and the concentration of chlorine and hydrogen in a single tank should be controlled below 2%, and the concentration of chlorine and hydrogen in the main pipe should be controlled below 0.4%.

    (4). Strict installation requirements for electrolysis equipment. Due to the presence of hydrogen in the electrolysis process, there is a risk of fire and explosion, so the electrolytic cell should be installed in a single-story building with good natural ventilation, and the factory building should have sufficient explosion-proof pressure relief area.

    (5). Master the correct emergency handling methods. In the case of sudden power failure or other reasons, the high-pressure valve cannot be closed immediately to prevent the chlorine gas in the electrolytic cell from flowing back and causing an explosion. A vent pipe should be installed behind the electrolyzer to reduce pressure in time, and a one-way valve should be installed on the high-pressure valve to effectively prevent chlorine gas from escaping and avoid polluting the environment and causing fire hazards.

5. Agglomerative

    Polymerization is the process of combining several molecules into a larger compound with the same composition and higher molecular weight. Such as vinyl chloride polymerization to produce polyvinyl chloride plastic, butadiene polymerization to produce butadiene rubber and styrene-butadiene rubber.

According to the type of reaction, polymerization can be divided into two categories: addition polymerization and condensation polymerization; According to the polymerization method, it can be divided into five types: bulk polymerization, suspension polymerization, solution polymerization, emulsion polymerization, and condensation polymerization.

(1). Bulk polymerization

    Bulk polymerization is a polymerization method carried out with a tubular polymerization tank (or polymerization kettle) immersed in a coolant in the absence of other media. (Such as the high-pressure polymerization of ethylene, the polymerization of formaldehyde, etc.)

(2). Solution polymerization

    Solution polymerization is a polymerization method in which a solvent is selected to dissolve monomers into a homogeneous system, and a catalyst or initiator is added to form a polymer. During the polymerization and separation process of this polymerization method, the flammable solvent is easy to volatilize and generate static sparks.

(3). Suspension polymerization

    Suspension polymerization is a polymerization method in which water is used as a dispersion medium. It uses an organic dispersant or an inorganic dispersant to break the water-insoluble liquid monomer together with the initiator dissolved in the monomer into small beads after vigorous stirring, and disperse it in water to form a suspension. Polymerization is carried out in very fine unit beads (0.1um in diameter), so it is also called bead polymerization. In this polymerization method, if the process conditions are not strictly controlled during the entire polymerization process, resulting in abnormal operation of the equipment, material overflow is prone to occur. If the material is spilled, the unpolymerized monomer and initiator after the water evaporates will easily cause fire or explosion accidents when they encounter an ignition source.

(4) Emulsion polymerization

    Emulsion polymerization is a method of polymerizing by using an emulsifier to disperse liquid monomers in water (bead diameter 0.001-0.01um) under strong mechanical stirring or ultrasonic vibration. In this polymerization method, inorganic peroxides (such as hydrogen peroxide) are often used as initiators. If the proportion of peroxides in the medium (water) is improper, the temperature is too high, and the reaction speed is too fast, it will cause flushing. At the same time, flammable gases will be generated during the polymerization process.

(5). Condensation polymerization

    Condensation polymerization, also known as polycondensation reaction, is a polymerization reaction in which monomers with two or more functional groups condense with each other and precipitate small molecular by-products to form polymers. Condensation polymerization is an endothermic reaction, but because the temperature is too high, it will also increase the pressure of the system, and even cause a burst, leaking flammable and explosive monomers.

6. Catalytic

    A catalytic reaction is a chemical reaction carried out under the action of a catalyst. For example, the synthesis of ammonia from nitrogen and hydrogen, the synthesis of sulfur trioxide from sulfur dioxide and oxygen, and the synthesis of Ethylene Oxide from ethane and oxygen are all catalytic reactions.

    Catalytic Fire Hazards:

    (1). In the reaction operation, if the catalyst is not selected correctly or added in an appropriate amount during the catalytic process, it is easy to form a localized violent reaction;In addition, most of the catalysis needs to be carried out at a certain temperature, if the heat dissipation and temperature is not well controlled, it is easy to cause an over-temperature explosion or fire accident.

    (2). Some of the catalytic products produce hydrogen chloride during the catalytic process, and hydrogen chloride has the risk of corrosion and poisoning. Some produce hydrogen sulfide, the risk of poisoning is greater, and the explosion limit of hydrogen sulfide in the air is wide (4.3% to 45.5%), and there is still a danger of explosion during the production process. Some catalytic processes produce hydrogen, and the risk of fire and explosion is greater. Especially under high pressure, the corrosion of hydrogen can embrittle metal high-pressure vessels, resulting in destructive accidents.

    (3). The amount of impurities in the feed gas that can react with the catalyst increases and may become an explosive hazard, which is very dangerous. For example, in the catalytic oxidation of ethylene to synthesize acetaldehyde, because the catalyst system often contains a large amount of cuprous salt, if the acetylene content in the feed gas is too high, the acetylene will react with the cuprous salt to form copper acetylene. Copper acetylene is a red precipitate, which is an extremely sensitive explosive. Its spontaneous ignition point is between 260 and 270 ° C. It is very easy to explode in a dry state, and it is easy to oxidize into dark black and catch fire under the action of air.

7. Cracking

    Cracking refers to the reaction process in which molecules of organic compounds decompose at high temperatures. Cracking can be divided into three types: thermal cracking, catalytic cracking, and hydrocracking.

    (1) Thermal cracking

    Thermal cracking is carried out under high temperature and high pressure. The temperature of the oil in the device generally exceeds its spontaneous ignition point. If the oil leaks out, it will catch fire immediately. A large amount of gas fractionation equipment will be used in the thermal cracking process, and a large amount of cracked gas will be produced. If the gas leaks out, an explosive gas mixture will be formed; if it encounters an open flame in a heating furnace or other equipment, there will be a danger of explosion. Among the various units in the refinery, the number of fires in the thermal cracking unit is relatively high.

    (2) Catalytic cracking

    Catalytic cracking is generally carried out at a relatively high temperature (460-520 ° C) and a pressure of 0.1-0.2 MPa, and the risk of fire is relatively high. If the operation is not done properly, the air and flame in the regenerator will enter the reactor and cause a severe explosion. There are many small devices and small valves on the U-shaped pipe, which are prone to oil leakage and fire. In the catalytic cracking process, flammable cracked gas will also be produced, and flammable carbon monoxide gas may also appear when the burnt activated catalyst is abnormal.

    (3) Hydrocracking

    Since hydrocracking uses a large amount of hydrogen, and the reaction temperature and pressure are high, the carbon molecules in the steel are easily captured by hydrogen when the steel is in contact with hydrogen under high pressure, which increases the hardness of carbon steel and reduces its strength, resulting in hydrogen embrittlement. If the equipment or pipeline is not inspected or replaced in time, the equipment will explode under high pressure (10-15MPa). In addition, hydrogenation is a strong exothermic reaction, and the reactor must pass cold hydrogen to control the temperature. Therefore, it is necessary to strengthen the inspection of equipment, replace pipelines and equipment regularly, and prevent accidents caused by hydrogen embrittlement. The heating furnace should be operated smoothly to prevent partial overheating of the equipment, and prevent the furnace tube from burning through or high-temperature pipelines and reactors leaking to cause fire.

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