Looking back at safety incidents in reaction vessels and distillation kettles, many lessons cannot be ignored

Thread Content

Reactor vessels and distillation vessels (including rectification vessels) are among the most commonly used equipment in the chemical industry; they are also devices that pose a high risk of leaks, as well as fires and explosions. 　　In recent years, leaks, fires, and explosions in reaction vessels and distillation tanks have occurred frequently. Since the reactor is often filled with toxic and hazardous **chemicals, the consequences of an accident are more severe than those of a regular explosion. Next, we will introduce the risk factors that lead to accidents in reaction vessels and distillation tanks, list relevant accident cases, and propose corresponding safety measures. Inherent hazards The inherent hazards of reaction vessels and distillation vessels mainly include the following aspects: 1. Materials The materials contained in reaction vessels and distillation vessels are mostly **chemicals**. If the material has a low auto-ignition point and flash point, it will form an explosive mixture with air in the event of a leak, and exposure to an ignition source (such as an open flame, spark, or static electricity) could lead to fire or explosion. If the material is toxic, a leak may cause poisoning and suffocation among people. 　　On March 27, 1994, an explosion occurred in a reactor in the antistatic agent workshop of a certain auxiliary chemicals factory in Shaoxing, resulting in 4 deaths and 8 serious injuries. Inside the reaction vessel, there is primarily ethylene oxide with an explosion limit of 3% to 100%. The main cause of the accident was that the air inside the vessel was not completely replaced by nitrogen, resulting in a mixture concentration with ethylene oxide that reached its explosion limit. This factory is a newly established township enterprise; the pressure vessels in use have never been inspected. The operators lack the necessary educational and technical skills, as they have not received any specialized training, and they are completely unaware of the risks associated with the production process or of how to handle faults. Before the project was put into operation, it was not subject to the \"three simultaneities\" review; there were no complete safety operating procedures or technical measures. It was impossible to determine through instruments whether the air in the reaction vessel had been completely replaced, and no testing procedures were established, so the workers relied on experience and intuition to carry out their operations. 　　2. Manufacturing issues with equipment and devices Improper design of reaction vessels and distillation tanks, discontinuous shapes in the equipment structure, and inadequate weld placement can all lead to stress concentration. Incorrect material selection, substandard welding quality during container fabrication, and improper heat treatment can reduce the toughness of the materials. Corrosion by corrosive substances, reduced strength of the container shell, or the absence of necessary safety accessories can all result in explosions during the use of the container. 　　On December 1, 2007, an explosion occurred in a pressure vessel (oxygen steaming tank) in the fly ash aerated concrete block production workshop of a building materials company in Baoding City, Hebei Province, resulting in 5 deaths and 1 person suffering minor injuries. According to the investigation, the company’s management altered the process parameters without authorization, reducing the total number of bolts connecting the body and cover of the oxygen vaporization tank from 60 to less than 30. Repairs were not carried out in a timely manner, and the safety valves of the tank were not calibrated on time. As a result, the tank operated under conditions of excessive pressure and temperature for an extended period, which led to the accident. 　　On September 4, 2000, during operation in a biochemical reagent factory in Yiyang City, Hunan Province, the lid of a jacketed glass-lined reactor suddenly came off, causing a large amount of propylene to be released. This propylene mixed with air to form an explosive gas, leading to a massive explosion that resulted in 2 deaths and 6 injuries. The main cause of the accident was the aging of the gasket at the sealing surface of the reactor, which led to leaks during operation. Workers attempted to tighten it while pressure was still present, causing the reactor lid to come off and resulting in an explosion. This reactor is an old pressure vessel that was not inspected before use, was installed illegally, and its operators were not trained. Dangers associated with the operation process: During production, reaction vessels and distillation tanks are subject to the following risks: 1. Fire and explosion caused by uncontrolled reactions. Many chemical reactions, such as oxidation, chlorination, nitration, and polymerization, are highly exothermic. If these reactions get out of control or if there is a power or water supply interruption, the heat generated by the reactions can accumulate, causing the temperature inside the vessel to rise sharply and the pressure to increase. When this exceeds the vessel’s pressure tolerance limit, the vessel may rupture. Material is ejected from the rupture point, which can lead to fire and explosion accidents; the rupture of the reactor disrupts the equilibrium of the material’s vapor pressure, and the unstable superheated liquid can cause a second explosion (steam explosion). The ejected material then spreads rapidly, filling the space around the reactor with mist or vapor of flammable liquids, and the presence of an ignition source can result in a third explosion (explosion of the mixed gas). 　　The main reasons for reaction runaway include the failure to remove reaction heat in a timely manner, uneven dispersion of the reaction materials, and operational errors. 　　On March 16, 2007, an chemical manufacturing company in Dongtai City, Jiangsu Province, experienced a sudden explosion in its distillation tower while illegally attempting to produce a new product, ethoxymethylenemalononitrile, using existing production equipment; this incident resulted in 4 deaths and 1 injury. The direct cause of this accident was the excessive distillation of the crude ethoxymethylenemalononitrile, which led to high-boiling substances clogging the packing layer. This increased the pressure inside the distillation vessel, resulting in a physical explosion that blew apart the sections of the tower below the packing tower; this in turn caused the material to catch fire and lead to a chemical explosion. 2. Explosion caused by high-pressure material leaking into the low-pressure system in the reaction vessel. Equipment at normal or low pressure connected to the reaction vessel experiences a physical explosion of the vessel when high-pressure material leaks in, exceeding the vessel’s pressure tolerance limit. 　　On August 22, 1991, a major fire accident occurred at the Resin Factory in Pingdingshan City, Henan Province. It was caused by overpressure in the reaction vessels in the polymerization section; when the workers on duty tried to handle the situation urgently, they failed to fully close the valves leading to the foam catcher (a device that operates at atmospheric pressure), which led to an explosion. This explosion in turn triggered an explosion of the flammable gas mixture throughout that entire section. 3 Accidents occur when steam or water leaks into the reaction vessel. If steam used for heating, heat transfer oil, or water used for cooling leaks into the reaction tank or distillation tank, it may react with the materials inside the tank, releasing heat and causing a sharp rise in temperature and pressure. This can lead to the materials being ejected out of the tank, resulting in a fire accident. 4. Explosion due to a lack of cooling water in the distillation condensation system. During the distillation process, if the cooling water supply to the top condenser is interrupted, while the material inside the reactor continues to undergo distillation, this can cause the system to shift from a normal or negative pressure state to a positive pressure state, exceeding the equipment’s capacity and leading to an explosion. 　　On July 28, 2006, the explosion at Yancheng Fluorine Source Chemical Co., Ltd. in Sheyang County, Yancheng City, Jiangsu Province, occurred because the plant failed to shut down immediately when there was no cooling water in the condenser of the chlorination reactor and no product flow at the top of the tower; instead, heating continued erroneously, keeping 2,4-difluorobenzene under high temperatures for an extended period, which ultimately led to its decomposition and explosion. 5 container explosions caused by heating Reaction containers can experience sudden increases in temperature and rising pressure, leading to material leakage or explosions, as a result of fires involving external flammable materials or heat radiation from high-temperature sources. At 6:50 on October 8, 1991, in the pilot plant of the Huaiyin Organic Chemical Factory in Jiangsu, a 100-liter high-pressure reactor used for producing polymer polyethers suddenly exploded. The bolts that held the reactor lid in place broke, and the lid, weighing about 80 kg, was flung more than 80 meters away from its original location. The safety valves and pressure gauges installed on the high-pressure reactor were also destroyed in the explosion. The shock wave generated by the explosion ripped off the roof; the 20 m³ pilot plant collapsed completely, and 3 operators were killed on the spot. 　　The explosion of the high-pressure reactor was caused by excessive temperature and pressure. The production process for manufacturing polyether polymers using ethylene oxide and propylene oxide as raw materials requires operators to follow the regulations strictly; addition should be done in a gradual manner to control the amount of material added, thereby preventing dangerous situations that could result from intense reactions getting out of control. However, the plant operators violated the operating procedures by using a \"batch\" addition method, adding too much material at once, which caused the reaction to proceed too rapidly. 6 Accidents caused by improper handling of materials entering and leaving containers Many flammable liquids of Category A with a low flash point are fed into reaction vessels and distillation tanks through pumps or by vacuum extraction. Most of these materials are insulating and have poor electrical conductivity; if the flow rate of these materials is too high, the accumulated static electricity cannot be discharged in time, leading to combustion and explosion accidents. 　　On March 30, 1993, a major explosion occurred in a reactor at Jingzhou Petrochemical Plant, resulting in 4 deaths. The direct cause of the explosion was an excessive feeding rate of ethylene oxide. In less than 2 hours, 500 kg of feed had been added to the reactor, causing ethylene oxide to accumulate there without having time to react with propargyl alcohol. The pressure inside the reactor rose rapidly, and high-pressure gas was ejected violently; this, combined with static electricity, led to an explosion. The reason for the too rapid feeding rate is that the reaction vessel relies solely on the operator to manually control the feeding speed using valves, with no flow meter installed. The workers do not know what the main ingredients to be fed into the reactor are, nor what standards should be followed for adding them. Yet the technology transferor refused to disclose to the Petrochemical Plant the names and quantities of the raw materials, citing technical confidentiality as a reason. 　　On April 22, 2002, an explosion occurred in a chemical plant in Yuanping City, Shanxi Province, resulting in 1 death and 1 serious injury. The blast shock wave lifted almost the entire prefabricated panel roof of the 200 m2 workshop, shattered all the glass in the south wall windows, with debris flying as far as about 50 meters away; moreover, all 40 M20 bolts securing the top cover of the reactor were broken or pulled out. The cause of the accident was that during the unloading process of the reaction vessel, the mixture of carbon disulfide, isopropanol, and oxygen inside the vessel was discharged under a gauge pressure of 0.2 MPa. When material leaked from the flange, a pressure difference existed between the inside and outside, causing the leaking material to flow out at a certain speed; static electricity was generated during this process. When the liquid material in the reactor is almost completely drained, static electricity accumulates at the edges of the flange to a certain level, creating a discharge gap that generates static sparks. These sparks ignite the mixture of carbon disulfide, isopropanol, and oxygen, leading to a rapid backflow of fire into the reactor and resulting in a chemical explosion. 7. The operators were careless and failed to detect early signs of accidents in a timely manner. Reactors generally carry out reactions under normal pressure or in an open environment, while distillation reactors are typically operated under normal pressure or negative pressure. Some people believe that operating under normal pressure, in an open environment, or under negative pressure poses little risk; as a result, they become complacent and fail to detect and address early signs of unexpected accidents in a timely manner, which ultimately leads to accidents. In fact, in reactors at atmospheric pressure or with open tops, the pressure acting on the reactor walls is greater than that inside the reactor, resulting in a higher level of risk. 　　In the case of distillation vessels, if operators make mistakes and the reaction gets out of control, resulting in blockages in the piping and valve systems, the normal atmospheric or vacuum conditions can turn into positive pressure. If this is not detected and addressed in a timely manner, and there are no emergency pressure relief devices available, fires and explosions can easily occur. On November 27, 2007, an explosion occurred in a diazotization reactor at Jiangsu Yancheng Lianhua Technology Co., Ltd. This was due to the steam valve of the diazotization reactor not being properly closed; as a result, large amounts of steam continued to enter the reactor’s jacket during the insulation phase, causing the temperature inside the reactor to rise rapidly. This led to the intense decomposition of the diazonium salt, which in turn triggered the explosion. The operator on duty failed to properly monitor the temperature of the reactor, failing to detect the abnormal temperature inside it in a timely manner and thus delaying the optimal opportunity to address the issue. Safety measures The main function of a reaction vessel is to carry out processes such as the polymerization of raw materials, enabling the raw material production process to proceed successfully and achieve optimal results. It is important to follow standard procedures; if the operating steps are incorrect when using the reactor, it can lead to damage and disrupt production. Next, let’s take a brief look at the precautions for using chemical reaction kettles. 　　I. Operate strictly in accordance with the system 　　Strictly following the established rules is the most basic requirement for operation. Before operating a chemical reactor, it is necessary to understand the standard operating procedures for the equipment. Although the specifications provided in the equipment manual are not very detailed, there are certain rules that must be followed to ensure safe operation of the equipment. 　　II. Pre-operation inspection 　　Before operating the chemical reactor, it is necessary to check for any abnormalities in the equipment. If the device is operating normally, the upper cover and the access panel must not be opened to avoid electric shock. It is strictly prohibited to operate under pressure; this not only damages the equipment but also poses a safety risk. During the nitrogen pressure testing, it is important to monitor its changes to prevent excessive pressure. 　　III. Pay attention to observation 　When operating a chemical reaction kettle, it is important to pay close attention and observe each step of the process. Especially when the reaction kettle is heated to a certain stable temperature, it must not come into contact with the kettle body to avoid burns. After the experiment is completed, cooling treatment must be carried out first, waiting for the temperature to drop to prevent equipment damage caused by high temperatures; in addition, the power supply should be disconnected promptly. 　　IV. Pay attention to maintenance Equipment operation requires attention not only to the process of operation but also to the maintenance of the equipment. Equipment maintenance is not an easy task; only by learning how to maintain it can the equipment perform at its best and its lifespan be extended. Otherwise, it will severely affect the equipment’s operational performance. Key points to note: To prevent fires and explosions in reaction vessels and distillation tanks, it is necessary not only to strengthen safety training and on-site safety management, as well as to maintain the equipment properly, prevent the formation of explosive mixtures, remove scale from the equipment’s pipelines in a timely manner, control the flow rates of materials in and out, and use explosion-proof electrical equipment with proper grounding, but also to follow safety operating procedures and job-specific safety protocols strictly. During distillation, process parameters such as temperature, pressure, feed rate, and reflux ratio must be strictly controlled. When using steam for heating, the opening degree of the valves should be appropriate to prevent excessive and rapid evaporation of the material, which could lead to a sharp increase in pressure within the system. It is essential to always ensure that the equipment and pipes in the distillation system remain unobstructed, to prevent blockages in the inlet and outlet pipes as well as valves, which could lead to increased pressure and pose a danger. To prevent low-boiling substances and water from entering the high-temperature distillation system, it is necessary to drain all the condensate water from the reactors, towers, and associated equipment before starting up the system, in order to avoid sudden contact with the high-temperature materials, which could lead to instantaneous vaporization and pressure buildup, resulting in material spraying or explosion. Summary: Reactors and distillation vessels should be equipped with complete instruments for monitoring temperature, pressure, flow rate, etc. The vacuum pump used for vacuum distillation should have a check valve to prevent air from entering the system in the event of a sudden shutdown. A one-way check valve should also be installed at the connection between the low-pressure system and the high-pressure system, to prevent the material from the high-pressure container from entering the low-pressure system and causing an explosion. Reactor and distillation vessels that are at risk of overpressure must be equipped with emergency pressure relief devices; safety valves are generally installed on such equipment. For devices where it is not suitable to install safety valves or those involved in highly dangerous processes, burst discs can be used.