Analysis of hazardous factors in formaldehyde production

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1 Fire and explosion hazards 1.1 Methanol is a volatile liquid that falls under the category of materials with high fire hazard; improper storage or leaks can lead to combustion and explosions. The raw material, liquid methanol, is heated and evaporated in an evaporator to turn into methanol vapor. The evaporation system must not leak; otherwise, under pressure, the methanol gas will be ejected at high speed, generating static electricity or, in the presence of an open flame, it can easily lead to fires and explosions. A mixture of gaseous methanol and air can form an explosive mixture, which is prone to explosion and combustion in the presence of an open flame, high temperature, or static electricity sparks. Gaseous methanol has an extremely high explosion velocity, with flame temperatures exceeding 1000°C. Under standard conditions, the complete combustion of 1 m3 of gaseous methanol releases heat in the tens of thousands of joules; the overpressure generated by the explosion is similar to that produced by a T*T explosion of the same energy level. Due to its high heat of combustion and rapid explosion speed, it undergoes chemical changes in an instant, resulting in extremely high destructive power. 1.2 Methanol vapor is mixed with air and fed into the oxidizer where catalytic oxidation and dehydrogenation reactions take place. The reaction temperature ranges from 620°C to 650°C, and the overall heat effect of these reactions is highly exothermic; temperature differences exist both radially and axially within the oxidizer. The carrier of a catalyst is often a material with poor thermal conductivity; if the catalyst has good thermal conductivity and the gas flow velocity is high, the radial temperature difference is small. Generally, the axial temperature distribution has a maximum temperature point, known as a hot spot. If the temperature at this hot spot is too high, it reduces the selectivity of the reaction, slows down the catalyst’s efficiency, and may even cause the reaction to lose stability or result in excessive temperature rises. The oxidizer used for producing formaldehyde is a fixed-bed reactor, and the temperature distribution within the bed is limited by the heat transfer rate; this can result in large temperature differences, and even lead to excessive temperatures that may cause fires and explosions. During the reaction process, the oxygen-to-alcohol ratio (i.e., the molar ratio of oxygen to methanol) as well as the water vapor ratio must be carefully controlled to prevent overheating. As the temperature rises, the reaction rate increases, the conversion rate goes up as well, and the heat released also increases. If this reaction heat is not removed in a timely manner, it becomes difficult to control the temperature, leading to overheating. 1.3 In formaldehyde production, over 90% of the methanol participates in oxidation and dehydrogenation reactions; the remaining portion undergoes side reactions such as combustion and further oxidation of formaldehyde, producing CO, CO2, H2O, CH4, and H2. These are all exothermic reactions that increase the total heat generated during the reaction process, which may lead to a rise in temperature. When the temperature reaches the auto-ignition point of methanol or formaldehyde, combustion and explosion can occur. 1.4 The vapors of methanol and formaldehyde can both form explosive mixtures with air, but temperature has a significant impact on the explosion limits; these limits at different temperatures can be adjusted based on the limit at 25°C. The corrected explosion limits for methanol and formaldehyde are shown in Appendix Table 1-4. Tables 1–4: Explosion limits after temperature correction
Material | Temperature (°C) | Lower explosion limit (%) | Upper explosion limit (%)
Methanol | 25 | 6.0 | 36.5 | 600 | 3.2 | 53.3 | 700 | 2.8 | 56.2
Formaldehyde | 25 | 7.0 | 73 | 600 | 3.8 | 106.6 | 700 | 3.2 | 112.4
Under normal conditions, the volume ratio of methanol to air should be maintained between 0.48 and 0.60. According to Table 2, this ratio is outside the explosive range; however, if proper precautions are not taken, such as if the oxygen-to-alcohol ratio is too low, the reaction could fall within the explosive limit range. At the inlet from the superheater to the oxidizer, there are methanol and air, which form an explosive mixture ； The oxidizer outlet contains six components: methanol, formaldehyde, H2, CO, CH4, and O2, which also forms an explosive mixture. Therefore, whether at the inlet or outlet of the oxidizer, any contact with a fire source will immediately lead to combustion and explosion. 1.5 The absorption process involves using water in an absorption tower to absorb the majority of the formaldehyde present in the reaction gas, while the unabsorbed exhaust gas is sent to an exhaust gas boiler for combustion treatment. The gas involved in this process is an explosive mixture; if there is a leak in the equipment, it may lead to fires and explosions. 1.6 During operations such as loading and unloading methanol and formaldehyde, as well as tank cleaning, if improper procedures are followed or if leaks occur in the storage tanks and pipelines due to equipment or pipeline corrosion, manufacturing defects, or loose flanges, methanol or formaldehyde can come into contact with air, forming explosive mixtures. When these mixtures reach their explosive limit, they can explode and catch fire upon encountering a source of ignition. (1) Fill the storage tank with methanol or formaldehyde. If the tank overflows, failure to monitor the liquid level properly during loading and unloading can lead to leaks of methanol or formaldehyde. Once these substances spill out of the tank, their concentration in the surrounding air rises rapidly, reaching or exceeding the explosive limit; in such cases, an explosion and combustion can occur upon contact with a spark ； When methanol overflows, using metal containers to scoop it up or turning on lights for illumination can inadvertently generate sparks, leading to deflagration. B Methanol leakage: During loading and unloading, methanol leaks onto the ground due to reasons such as cracked hoses, damaged seals, or loose connection bolts, and it catches fire immediately upon contact with sparks. C Static electricity ignition: Static electricity accumulates and discharges due to reasons such as the lack of static electricity connections in the pipelines, the use of splash loading/unloading methods, and the absence of static electricity grounding on tank trucks, which in turn ignites flammable vapors. Exposure to open flames during loading and unloading: During unenclosed loading and unloading processes, large amounts of flammable vapors escape from the openings used for such operations; when there are flames or sparks in the surrounding area, explosion and combustion can occur. (2) Leaks in storage tanks, pipelines, or flanges were not detected in time, resulting in methanol or formaldehyde being released into the air; the vapors of methanol or formaldehyde then caught fire and exploded when exposed to an open flame. 1.7 Insufficient safety fire separation distances: Buildings constructed within or outside the production area are places where open flames may occur. If the distance between such buildings and the hazardous facilities within the production area is insufficient, it can lead to the combination of a fire source with flammable gases at an appropriate concentration, thereby triggering accidents. On the other hand, when a fire breaks out in one facility or equipment, insufficient fire separation distances can easily lead to fires in other facilities or equipment as well ； Or in the event of a fire in the production area, if there is insufficient fire separation distance, it can easily lead to fires in buildings outside the production area, resulting in greater losses. 1.8 During the production and storage processes in this workshop, flammable and explosive substances such as methanol, formaldehyde, and hydrogen are present. The production area and the tank storage area are classified as environments prone to explosions and fires; if the electrical equipment in these areas does not meet the requirements for fire and explosion prevention, it may lead to fire or explosion accidents. 1.9 Electrical wiring aging, damaged insulation, short circuits, unauthorized wiring, overloading, excessive current, improper wiring practices, overheating, and improper management of electrical appliances can all lead to fires. 1.10 Fires caused by lightning strikes. Due to the lack of reliable lightning protection measures, lightning strikes can directly hit the storage tanks or loading/unloading facilities, or induced charges can accumulate on them and discharge, all of which can lead to the combustion of methanol and formaldehyde or explosions of mixtures of methanol and formaldehyde with air. 1.11 If the fire resistance rating of buildings and structures within the production area does not meet the required standards, improper management of open flames and loss of control over their use can easily lead to fires. 2 Container explosion: Pressure vessels are present in production facilities, and if there are defects in their design or installation, ； The safety accessories or protective devices are defective or incomplete ； In the course of use, if phenomena such as erosion, corrosion, fatigue, and creep occur ； It was not inspected by a qualified quality inspection unit as required, nor was a safety approval certificate obtained ； Container explosion accidents can occur due to reasons such as human error. 3 Poisoning 3.1 Methanol has an **effect on the central nervous system ； It has a specific selective effect on the optic nerve and retina, leading to lesions ； It can cause metabolic acidosis. It is highly irritating to mucous membranes, the upper respiratory tract, eyes, and skin. Exposure to its vapors can cause conjunctivitis, keratitis, rhinitis, and bronchitis ； In severe cases, laryngospasm, glottic edema, and pneumonia may occur. Pulmonary edema is less common. It causes primary irritation and sensitization of the skin, leading to dermatitis ； Concentrated solutions can cause coagulative necrosis of the skin. Oral ingestion causes burns to the mouth and digestive tract, and may lead to gastrointestinal perforation, shock, as well as damage to the kidneys and liver. Therefore, during the operation process, if protective measures are inadequate or absent, it is possible to cause methanol poisoning in humans. 3.2 Short-term high inhalation of formaldehyde can cause mild irritation of the eyes and upper respiratory tract (oral ingestion causes gastrointestinal irritation) ； After a period of incubation, symptoms such as headache, dizziness, fatigue, vertigo, a feeling of being drunk, confusion, delirium, and even coma may occur. Optic nerve and retinal lesions can cause blurred vision, double vision, and in severe cases, blindness. In metabolic acidosis, there is a decrease in carbon dioxide combining capacity and accelerated breathing, among other symptoms. Therefore, during the operation process, if protective measures are inadequate or absent, it is possible to cause formaldehyde poisoning in humans. 4 Falls from height: The production facility in this workshop is a three-story building, and work is carried out on the floors above the second floor. If the protective railings are not properly installed or are damaged due to corrosion, or if adequate safety measures are not in place when performing maintenance on storage tanks, these factors can all lead to falls from height. 5 Mechanical injuries: At the operating parts of various pumps, if protective covers or other safeguards are not in place, contact with these operating parts by human hands can lead to mechanical injury accidents. 6 Electric shock: If protective measures for various electrical devices are inadequate (such as unreliable electric shock protection, leakage protection, short-circuit protection, overload protection, insulation, electrical isolation, shielding, and proper electrical safety distances), it is possible for people to suffer electric shock. 7　Scalding: Damage to steam pipes or flange connections can cause medium-pressure steam to escape, which may reach human bodies and lead to scalding injuries. 8 Vehicle injuries: Vehicles moving around in a workshop can easily collide with the facilities there if the protective measures in such workshops are inadequate; on the other hand, they can also cause injury to people. 9　Noise: Equipment such as rotary blowers and pumps present in the facility generate noise. Noise is a physical hazard; working in an environment with high levels of noise for extended periods can damage the hearing of those exposed to it, leading to noise-induced deafness. It also impairs the normal sensory abilities of operators, causes irritability, affects communication, and can even be a cause of accidents. 10　Toxins: Prolonged exposure to low concentrations of formaldehyde can cause mild irritation of the eyes, nose, and throat, as well as dry skin, cracked skin, and softening of the nails. Chronic effects: Prolonged inhalation of low concentrations of methanol may lead to neurasthenic syndrome, autonomic nervous system dysfunction, mucosal irritation, vision impairment, as well as skin problems such as depigmentation and dermatitis.