Overview of fire and explosion hazard analysis and

2022-06-22
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Overview of the production process (synthetic resin) fire and explosion risk analysis and preventive measures

Author: wangwenji

the main raw material of plastics is synthetic resin. However, the monomers of synthetic resin, such as acetylene, ethylene, propylene, formaldehyde, benzene, etc., are flammable and explosive substances, and the production process is often carried out under high temperature and high pressure, so there is a certain risk of fire and explosion. In the production process, there will be static sparks, equipment friction ignition and other faults, and even a little carelessness will lead to a huge fire. The explosion of chemical production equipment is often accompanied by heat, light and pressure rise, which has a great destructive effect. The main forms of explosion damage include shock wave, shock wave and fire

common fire and explosion in synthetic resin plant are as follows:

1 Fire and explosion of mixed gas

flammable gas and liquid vapor form explosion with air 2 The explosion caused by the propagation of combustion wave or explosion wave at a certain concentration is called the explosion of mixed gas

in the synthetic resin plant, the leakage of combustible gas or steam from the process pipeline, equipment or container, or the leakage of air into the container containing combustible gas will produce mixed gas. In case of fire source, explosion accident will occur

2. Dust explosion

dust of combustible resin solids, such as high-pressure polyethylene, polypropylene, nylon resin and polystyrene, will generate dust clouds during granulation, drying and transportation, or in bag filters. When the dust cloud reaches a certain concentration, once it encounters a fire source, it will produce violent combustion and explosion, which is called dust explosion

generally speaking, dust that is easily charged is more likely to cause explosion. In the production process, various resin dusts always carry a certain charge due to mutual collision, friction, corona discharge and other reasons. Most of the dust of synthetic resin may be suspended in the air as very fine solid particles to adsorb oxygen in the air. The finer the dust particles are, the more oxygen is adsorbed, the more prone to explosion. At the same time, the finer the dust is, the lower the ignition point is, and the lower the lower limit of dust explosion is

the explosion pressure of dust is related to the chemical composition of dust. For example, the maximum dust explosion pressure of polystyrene and polyethylene are 0.293MPa and 0.588mpa respectively. In contrast, the maximum dust explosion pressure of polyethylene is higher than that of polyphenylene

3. Steam fire explosion

the low boiling point liquid in the chemical storage tank is heated or exposed to the sun, which increases the steam pressure in the tank and will explode at the moment of vaporization

prevention of fire and explosion (2)

prevention of combustion and explosion is the main content of safety production technology. In order to ensure safe, stable and long-term production, the prevention of combustion and explosion should be done carefully to eliminate the risk factors causing combustion and explosion

1. Control of ignition sources

the ignition sources of fire and explosion caused by synthetic resin plant include open fire, high heat, electric spark, static electricity, friction, impact, adiabatic compression, spontaneous combustion, chemical reaction heat, etc

(1) no open fire

open fire in synthetic resin plant mainly refers to heating fire, maintenance fire and other fire sources during resin production. For example, the heating furnace for ethylbenzene dehydrogenation in polystyrene production unit is an example of using open fire

try to avoid using open fire for heating flammable liquid, and use steam, pressurized superheated water and hot air for heating; If the heating furnace for ethylbenzene dehydrogenation must be heated by open fire, the equipment used shall be fully sealed and separated from the production unit with fire and explosion hazards to prevent fire caused by leakage

(2) prevention of friction and impact

friction and impact are often one of the causes of fire and explosion in synthetic resin plants, such as friction and heating of transmission machinery bearings, rupture of pipes or containers, friction and fire during internal flammable solution or flammable gas injection, or the generation of decomposition explosives under certain specific conditions, such as acetylene copper, etc

the methods to prevent friction and impact are as follows:

① the mechanical transmission parts shall be lubricated, lubricating oil (grease) shall be added frequently, and the attached combustible dirt shall be removed frequently

② collision and friction between iron containers are strictly prohibited

③ when handling metal containers containing flammable gas or flammable liquid, do not drag, vibrate or collide with each other

④ it is forbidden to wear shoes with nails to enter the explosion-proof plant

(3) prevention of electrical sparks

explosion accidents caused by electrical equipment in synthetic resin plant are mostly caused by electric arc, electric spark or electric leakage. Therefore, the required explosion-proof electrical equipment shall be correctly selected according to the nature of the explosive medium

2. Fire and explosion-proof safety facilities

fire and explosion-proof safety facilities include flame arrester, explosion-proof membrane, check valve, safety valve, vent pipe, alarm safety interlock device, etc

3. Static electricity prevention

strictly control the flow rate of materials to prevent static electricity. Reaction equipment has been paid more and more attention by countries all over the world. It is equipped with good electrostatic discharge devices

4. Control of process parameters

(1) strictly control the feeding of

synthetic resin production unit. During the polymerization reaction, the catalyst (or initiator) is mostly used, and the catalyst (or initiator) has a great impact on the polymerization reaction speed. Excessive catalyst will make the polymerization reaction intense, which may lead to the risk of fire and explosion. For the polymerization of flammable and combustible materials with peroxides, the feeding speed and amount shall be strictly controlled

(2) strictly control the polymerization temperature

the polymerization of synthetic resin monomers can only take place at a certain polymerization temperature. In case of overtemperature, the reactants may decompose and ignite, resulting in pressure rise and explosion

strictly control the polymerization reaction temperature. In addition to controlling the normal feeding of reactants, the removal of reaction heat should be strictly controlled to prevent mixing interruption, and appropriate heat transfer medium should be used

① removal of reaction heat

polymerization of ethylene, propylene, vinyl chloride, styrene and other monomers will be accompanied by the generation of reaction heat. In order to remove the reaction heat, jacket cooling, internal cooling, slurry circulation, liquefied propylene circulation, diluent recovery cooling and other methods are generally used. In the production process, the polymerization temperature can be controlled as long as the temperature and flow of cooling medium, the amount of slurry and the amount of liquefied propylene are controlled

② prevent mixing interruption

in the polymerization reactor, mixing by agitator can accelerate heat transfer. Once mixing is interrupted, poor heat dissipation or local overheating may occur. Therefore, safety measures such as two-way power supply should be adopted in the design to prevent fire and explosion accidents caused by local overheating due to interruption of mixing

(3) strictly control the polymerization reaction pressure

the polymerization reaction is generally carried out in a reactor with a certain pressure. If the pressure control is abnormal, the polymerization reactor may cause overpressure explosion

5. Inert gas protection

according to the explosion limits of flammable gas, flammable liquid vapor and dust, it can be judged in advance under what conditions it is easy to enter the explosion range. For example, the lower explosion limit of hydrogen, ethylene, propane, 1-butene, ethylene oxide, styrene, hexane and other monomers is very low. If there is a little leakage in equipment, vessels and pipelines, it is easy to enter the explosion range and cause fire and explosion. On the contrary, for flammable gas, flammable liquid vapor and dust with high explosion limit, if air enters its containers, equipment and pipelines, even a small amount of air will enter the explosion range, causing great risk of fire and explosion

if the percentage of inert gas in the mixture increases, the range of explosion limit will be reduced. After the concentration of inert gas increases to a certain value, the mixture will not explode. Therefore, when combustible gas leaks, N2 can be introduced to prevent explosion

fire protection requirements for several common synthetic resin production processes (3)

1 When PVC

is used to prepare vinyl chloride monomer, the temperature and the ratio of hydrogen chloride to acetylene should be strictly controlled in the initial mixing stage. Prevent decomposition and poisoning of catalyst caused by excessive acetylene. During synthesis, the hydrogen chloride content shall be sampled and analyzed regularly to prevent the formation of explosive mixture with acetylene due to excessive oxygen content. The content of free chlorine in hydrogen chloride should be strictly controlled. If it is too high, it will react with acetylene to generate chloroacetylene, which will cause combustion and explosion

in order to prevent PVC from sudden polymerization, strict emergency cooling treatment measures must be specified, and sufficient terminator must be prepared so that the polymerization reaction can be quickly stopped in the kettle in case of sudden power failure

2. Low pressure polyethylene

the catalyst produced by low pressure polyethylene, i.e. the mixture of triethyl aluminum (or magnesium) and titanium tetrachloride, will burn when exposed to air and explode when exposed to water. Therefore, it should be nitrogen sealed or immersed in pure gasoline. It is strictly prohibited to contact air and water, and the sealing condition should be checked frequently

before feeding ethylene purification, the tower must be cleaned with high-purity nitrogen to remove the oxygen in the tower and prevent the formation of explosive mixture

during packaging, the falling height of powder shall not be too high, the scale shall be well grounded, and the relative humidity of the packaging workshop shall be properly increased

3. When high pressure polyethylene

is added to the catalyst, the temperature must be strictly controlled. In case of overtemperature and overpressure, the power supply shall be cut off immediately according to the maintenance and inspection methods of the pendulum impact tester, and the ethylene in the kettle shall be vented. At the same time, the water vapor with higher pressure shall be ejected from the vent pipe. The compressor room shall be equipped with an automatic measurement and alarm device for ethylene gas concentration. During secondary compression, pay special attention to strictly control the temperature

in the heat exchange and polymerization stages, such as after heat exchange, the temperature may be adjusted by adding the amount of catalyst. When the temperature is high, the addition amount is less, otherwise it is more. The polymerizer shall be equipped with an explosion-proof sheet, and a 2.5MPa steam nozzle or sodium bicarbonate shall be added in the pipe; The polymerizer is equipped with jackets, and it is often measured whether there is combustible gas penetrating into the jackets; The polymerization room shall be equipped with an automatic combustible gas concentration detection and alarm system; Blast proof wall shall be set

in the high-pressure separation process, a safety valve shall be set on the separator, and an automatic combustible gas concentration detection and alarm system shall be set nearby; The pressure, temperature, flow and other parameters are programmed into the "operation procedure" connected with the interlocking system; During low-pressure separation, except that the pressure shall be higher than 0.5MPa, the others are the same as those for high-pressure separation

in the post-treatment section, the storage tank and air duct shall be protected with nitrogen during mixing and packaging. The storage container shall be replaced with nitrogen before use, and air extraction shall be carried out during use; All kinds of machines are explosion-proof, and pipes are made of non flammable or flame retardant materials and grounded

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