A brief analysis of spontaneous combustion accidents and countermeasures of ferrous sulfide during maintenance of petrochemical equipment

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I recently found it on the Internet and forwarded it (without the author's permission, please forgive me). I hope it will be helpful to everyone. At the same time, I would like to thank the original author! A brief analysis of spontaneous combustion accidents of ferrous sulfide during maintenance of petrochemical equipment and countermeasures author: Wang Jinhong 1 Introduction Petrochemical enterprises are flammable and explosive production enterprises, and fire and explosion accidents caused by spontaneous combustion of substances occur from time to time. The spontaneous combustion process of materials is generally hidden and often not easily noticed by people. Spontaneous combustion accidents are sometimes difficult to predict. The vast majority of spontaneous combustion accidents occur during the shutdown and maintenance of production equipment.: 　　Case 1: On April 25, 2004, a certain catalytic device was completely shut down due to a power supply system failure. The shutdown process was carried out according to emergency shutdown procedures. Due to the blower stalling, the incinerator of the acid water stripping system stalled. The operator cut off the gas in time, and the remaining gas in the stripping tower was discharged through the incinerator chimney. About 4-5 hours after the shutdown, it was found that there was thick smoke coming out of the incineration exhaust chimney. The monitoring instrument showed that the temperature of the flue rose sharply. The workshop operator promptly cut off the acid gas furnace mouth valve and introduced temporary steam into the furnace for purging. ； Due to the discovery and proper handling in time to avoid further development of the accident, subsequent inspection found that the carbon steel chimney was severely deformed 10 meters from the ground. 　　Case 2: On May 2, 2001, during the maintenance of the catalytic workshop of a petrochemical plant, the fractionation system was purged and the equipment was opened to vent. At 2 p.m. the next day, thick smoke was discovered coming from the manhole of the oil and gas separator at the top of the fractionation tower, followed by a flash explosion accident, accompanied by the release of a pungent smell. It was judged to be sulfur dioxide gas. The workshop personnel immediately pumped water into the tank to cool it, stopping the development of the situation and causing no major losses. 　　Table 1 Domestic and foreign crude oil sulfur content table Crude oil name Sulfur content % Time Daqing crude oil 0.0971996.8 Shengli crude oil 0.771996.11 Huizhou crude oil 0.031991.2 Liaohe crude oil 0.17361984.5 Lufeng crude oil 0.11271998.7 Oman crude oil 1.08 961997.10 Iranian light crude oil 1.631998.4 Iraqi Basra crude oil 2.111997.12 Saudi light crude oil 2.031998.3 Saudi medium crude oil 2.561994.5 Saudi heavy crude oil 3.091994.8 Dubai crude oil 1.661994.2 According to relevant statistics, the current annual growth rate of my country's oil consumption is 4%, while the annual growth rate of domestic crude oil production is less than 1%, and the shortfall must be supplemented by imported crude oil. It can be seen from Table 1 that the sulfur content of foreign imported crude oil is relatively high, especially crude oil from the Middle East, which is greater than 1% and can reach up to 3.09%. Processing high-sulfur crude oil not only brings new problems to product quality control and environmental protection, but also aggravates equipment corrosion and brings safety hazards to daily production and maintenance processes. In particular, spontaneous combustion accidents of ferrous sulfide during device maintenance are difficult to prevent and have a high incidence rate. How to avoid and correctly handle spontaneous combustion of ferrous sulfide is very important for safe production. 　　2 Causes of the production of ferrous sulfide and the mechanism of spontaneous combustion 2.1 Causes of the production of ferrous sulfide (1) Electrochemical corrosion reaction generates ferrous sulfide. More than 80% of the sulfur in crude oil is concentrated in the atmospheric residual oil. The structure of these sulfides is relatively complex. Under high temperature conditions, especially under the action of a catalyst, it is easy to decompose to generate hydrogen sulfide and smaller molecule mercaptans. When water is present, these hydrogen sulfide and mercaptans have obvious corrosive effects on iron equipment. The reaction process is: 　　H2S=H++HS- HS-=H++S2- This is an electrochemical corrosion process: 　　anodic reaction: Fe→Fe2++2e cathodic reaction: 2H++2e→H2 (in permeated steel) Fe2+ reacts with S2- and HS-: Fe2++S2-=FeS↓ Fe2++HS-=FeS↓+H+ In addition, sulfur and iron can directly interact to form ferrous sulfide: Fe+S=FeS↓ The ferrous sulfide produced has a relatively loose structure and is evenly attached to the inner walls of equipment and pipes. 　　(2) Due to the long-term shutdown of the device that generates ferrous sulfide due to atmospheric corrosion reaction, the internal components of the equipment are exposed to the air for a long time, which will cause atmospheric corrosion and produce rust. Since rust is not easy to be completely removed, it will react with hydrogen sulfide to form ferrous sulfide during the production process. 　　The reaction formula is as follows: Fe+O2+H2O→Fe2O3•H2O Fe2O3•H2O+H2S→FeS↓+H2O This reaction is easier to proceed. Due to long-term shutdown, equipment with poor anti-corrosion has a tendency to produce ferrous sulfide. 　　2.2 Mechanism and phenomenon of spontaneous combustion of ferrous sulfide (1) Mechanism of spontaneous combustion of ferrous sulfide When ferrous sulfide and other sulfides of iron are heated or illuminated in the air, the following reactions will occur: FeS+3/2O2=FeO+SO2+49KJ 2FeO+1/2O2=Fe2O3+271KJ FeS2+O2=FeS+SO2+222KJ Fe2S3+3/2O2=Fe2O3+3S+586KJ (2) The phenomenon of spontaneous combustion of ferrous sulfide. If there is no certain combustible support during the spontaneous combustion of ferrous sulfide, white SO2 gas will be produced, which is often mistaken for water vapor and is accompanied by a pungent smell. ； At the same time, a large amount of heat is released. When there are other combustible materials (such as oil) around, thick smoke will emit and cause fires and explosions. 　　3 Factors affecting the formation rate of ferrous sulfide It can be known from the formation mechanism of ferrous sulfide that in daily production, the formation process of ferrous sulfide is the chemical corrosion reaction process of iron under the action of active sulfide. Therefore, controlling chemical corrosion reactions is a key means to limit the formation of ferrous sulfide. As long as we identify the parts of the production equipment that are prone to sulfur corrosion and take effective measures based on the characteristics of each part, we can reduce the amount of ferrous sulfide produced and fundamentally avoid the occurrence of spontaneous combustion accidents of ferrous sulfide. Factors such as the sulfur content of the oil, temperature, the presence of water and Cl- are important factors that affect the speed of this electrochemical corrosion reaction. 　　3.1 Sulfur distribution rules during crude oil processing Sulfur chemical corrosion will only occur when sulfur exists ； Parts with high sulfur content in oil are the most prone to corrosion. Therefore, analyzing the sulfur distribution of crude oil during processing has guiding significance for controlling the generation of ferrous sulfide. 　　 　　 　　Table 2 Sulfur distribution of fractions after atmospheric distillation No. Crude oil name Crude oil type Crude oil sulfur content, % fraction range Fraction sulfur content, % accounted for crude oil sulfur distribution, % 1 Oman crude oil intermediate base 1.16HK~200℃0.030.3200~300℃0.1081.4300~350℃0.488.7>350℃1.978 9.62 Zhongyuan crude oil paraffin base 0.65HK~200℃0.040.9200~300℃0.133.6300~350℃0.296.3>350℃1.0989.13 Tarim crude oil paraffin base 0.71HK~200℃0.041.3200~300℃0.154.7300 ~350℃0.478.3>350℃1.4785.34 Shengli crude oil intermediate base 0.84HK~200℃0.020.1200~300℃0.081.2300~350℃0.323.5>350℃1.0895.25 Changqing crude oil paraffin base 0.14HK~200℃0.03 3.0200~300℃0.044.2300~350℃0.085.0>350℃0.2487.86 Tahe crude oil intermediate base 1.43HK~200℃0.010.1200~300℃0.081.0300~350℃0.664.9>350℃2.0794.0Table 3 Product sulfur distribution after catalytic cracking No. 12 Raw oil sulfur content 0.800.92 Product name Dry gas Liquefied gas gasoline Diesel slurry coke Dry gas Liquefied gas gasoline diesel slurry coke Product yield 3.2812.4333.2932.556.0612.393.7814.7235.1329 .464.8512.06 Product sulfur content 8.190.150.080.561.461.668.060.0540.110.791.542.21 Sulfur distribution of this process 33.22.35.722.510.925.432.50.85.624. 87.928.4 No. 34 Raw oil sulfur content 0.960.77 Product name Dry gas Liquefied gas gasoline Diesel oil slurry coke Dry gas Liquefied gas gasoline Diesel oil slurry coke Product yield 4.9311.834.436.24.18.573.04.2540.135.96.410.3 Product Sulfur content 3.980.440.10.791.713.756.740.630.070.792.060.92 Sulfur distribution in this process 20.45.413.5829.77.3433.4626.63.483.636.817.112.4 (1) It can be seen from Table 2 that after normal pressure distillation of crude oil, 85% of the sulfur is concentrated in the fraction above 350°C, that is, the normal pressure residual oil. Therefore, the equipment through which the normal pressure residual oil flows is more likely to be corroded by sulfur. ； In actual production, the internal components of the pressure reduction tower and the heat exchanger of the pressure reduction unit are the parts where ferrous sulfide is most likely to be generated. (2) It can be seen from Table 3 that about 70% of the sulfur enters the fractionation and absorption stabilization system with the reacted oil and gas. ； Nearly 30% of the sulfur exists in the coke and is discharged with the regeneration flue gas. Therefore, the condensation system at the top of the fractionation tower, the condensed oil filling and reboiler of the absorption stabilization system, and the diesel extraction system are the locations where ferrous sulfide is easily generated. (3) Acidic water treatment systems with high sulfur content and equipment through which acidic water flows are also locations prone to sulfur corrosion. 3.2 High-temperature sulfur corrosion The sulfur corrosion reaction is a chemical corrosion reaction, and an increase in temperature can speed up the reaction speed. Therefore, high-temperature sulfur corrosion is more likely to occur at the bottom of the normal pressure tower and the normal slag heat exchange unit, pressure reduction unit, and catalytic cracking diesel extraction system where the logistics temperature is relatively high. 3.3 The presence of water and Cl- can promote sulfur corrosion of equipment. From the reaction mechanism of ferrous sulfide generation, it can be known that the presence of water can promote the progress of chemical corrosion. When Cl- is present, the following reaction will occur even at low temperatures.: Fe+2HCl→FeCl2+H2↑ FeCl2+H2S→FeS↓+2HCl Fe+H2S→FeS↓+H2↑ FeS+2HCl→FeCl2+H2S For the atmospheric pressure tower top condensation system, that is, the tower top, oil and gas volatilization line, water cooler, reflux tank and other parts, low-temperature H2H-HCl-H2O corrosion is prone to occur. 4 Case Analysis Case 1 Accident Analysis: ○1 Due to a long-term power outage, the fan stopped, causing the incinerator to stall, and acidic gas with a high concentration of H2S was emitted through the chimney. ； Since the furnace temperature (around 900°C) is still high, the hydrogen sulfide in the sour gas reacts with carbon steel to form ferrous sulfide after preheating and heating up. ○2 Due to the strong wind at that time, the speed of air entering the furnace and chimney was accelerated. As the acidic gas decreased and the oxygen content increased, ferrous sulfide reacted with oxygen spontaneously, releasing a large amount of heat, and finally the iron chimney was severely deformed. ○3. During the maintenance, it was found that a large area around the deformed part of the chimney was thinned and severely corroded. After analysis, it may be that during the start-up, shutdown and normal operation fluctuations, the incineration flue gas contains hydrogen sulfide gas that has not been converted into SO2, which promotes the continuous generation of ferrous sulfide. In addition, if the deformed part is not insulated, dew point corrosion may occur, making the pipe thinner. ○4 After learning the lessons from this accident, the workshop changed the chimney from carbon steel to steel with good corrosion resistance, and strengthened the thermal insulation of the chimney. In addition, a steam line is added to the fan outlet pipeline. When the incinerator is extinguished, it can be used to purge unreacted gas and reduce the probability of formation of ferrous sulfide. Case 2 Accident Analysis: ○1 After entering the tank for inspection, it was found that a thick layer of rust-like material was deposited on the bottom of the tank. Laboratory tests found that the content of ferrous sulfide was very high. ②Due to long downtime, the internal components of the equipment are exposed to the air for a long time, which will cause atmospheric corrosion and generate rust. ； It is not easy to remove it during cleaning before starting work. During the production process, rust and hydrogen sulfide react to generate ferrous sulfide. During the next shutdown and purging, the ferrous sulfide layer will fall off due to purging, and enter the oil-water separation tank with the air flow, where it will deposit. ③Due to the hot weather, with temperatures reaching 30°C, the accumulation of heat causes the oil film and water on the surface of the "rust" to evaporate and come into direct contact with the air, eventually causing the dry ferrous sulfide to spontaneously ignite and ignite the oil and gas to cause a flash explosion. 5 Prevention and control measures for spontaneous combustion accidents of ferrous sulfide 5.1 Control the generation of ferrous sulfide from the root The production process of ferrous sulfide is a corrosion process of equipment. It is necessary to take measures from multiple aspects to reduce sulfur corrosion of equipment. (1) Start with the process to reduce sulfur corrosion in equipment and control the production of ferrous sulfide. ○1. Strengthen the "one removal and four injection" of normal pressure devices to inhibit corrosion. According to the actual conditions of crude oil, select an effective demulsifier, optimize the electric desalting process, and increase the removal rate of inorganic salts (such as MgCl2, CaCl2), thereby reducing the Cl- content at the top of the tower. Use corrosion inhibitors suitable for high-sulfur raw materials to reduce the corrosion rate. Appropriately increase the amount of ammonia injection to reduce sulfur corrosion. ○2. Use the residual oil hydroconversion process to reduce the sulfur content of atmospheric residual oil. The catalytic cracking unit has high requirements for the sulfur content of the slag. When processing high-sulfur crude oil, the residual oil hydrogenation conversion technology can be used to reduce the content of sulfur, colloid, nitrogen and other substances in the residual oil, which can reduce the corrosion of catalytic equipment and produce high-quality products at the same time. ○3. Add corrosion inhibitor to the top of the fractionation tower to form a protective film on the surface of the steel to inhibit corrosion. (2) Take measures from the equipment perspective to prevent the production of ferrous sulfide. ○1. Replace parts easily corroded by sulfur with corrosion-resistant steel. Taking into consideration the cost, choose corrosion-resistant steel with a higher cost-effectiveness, such as aluminized steel that is reasonably priced and has anti-corrosion performance equivalent to that of expensive 316L steel. ○2 Use spray isolation technology to achieve isolation and anti-corrosion purposes by spraying corrosion-resistant metal or coating corrosion-resistant materials on the inner surface of corrosion-prone equipment. However, during the production process, if the flow rate of the oil flowing through the equipment and pipelines is large or the parts of the equipment that are prone to wear are not suitable for spray isolation technology. ○3. Strengthen anti-corrosion protection during shutdown. For devices that have been shut down for a long time, measures such as adding blind plates to seal them and injecting nitrogen to replace the air should be used to prevent atmospheric corrosion. (3) Strengthen daily operation management and strengthen the operation management of relevant positions to prevent the continuous generation of ferrous sulfide due to improper operation. 5.2 Use chemical treatment methods to eliminate ferrous sulfide. For packings such as vacuum towers, acidic water stripping trays can easily produce ferrous sulfide parts, which can be treated chemically. (1) Pickling: Dilute hydrochloric acid cleaning can be used to eliminate the presence of ferrous sulfide, but hydrogen sulfide gas will be released, and additional hydrogen sulfide inhibitors need to be added to convert and eliminate hydrogen sulfide gas. (2) Chelate treatment: Specially prepared highly acidic chelates are very effective in dissolving sulfide precipitates without producing hydrogen sulfide gas, but are actually more expensive. (3) Oxidation treatment: The oxidant potassium permanganate can be used to oxidize sulfide, which has the advantages of safe use and easy implementation. 5.3 Things that should be paid attention to during shutdown and maintenance (1) Prepare a plan to prevent spontaneous combustion accidents of ferrous sulfide before shutdown. Before stopping, based on the characteristics of the device and past practical experience, make a plan for the spontaneous combustion of ferrous sulfide. Once a spontaneous combustion accident occurs, take immediate measures to prevent the scope of the accident from expanding and reduce economic losses. (2) When purging and cleaning equipment, special treatment should be given to dead areas such as elbows and corners, and attention should be paid to condensation discharge at low points to ensure purging quality and prevent the existence of residual oil and remaining oil and gas. This prevents the spontaneous combustion of ferrous sulfide from causing explosions and fire expansion. (3) The equipment can only be opened when it cools down to normal temperature. Rinse it with clean water before entering to ensure that the internal components are moist. The removed ferrous sulfide should be put into a bag, moistened and transported out of the equipment, and buried deeply as soon as possible. (4) Strengthen inspections. During maintenance, especially in environments with high temperatures, inspections must be strengthened to detect and handle problems in time. References 1 Yang Shuxian. Analysis and control of sulfur distribution during crude oil processing. China Petroleum and Chemical Industry Science and Technology Guide, 2003 Volume 1 2 Huang Jingguo et al. Corrosion and control of atmospheric and vacuum distillation units. Petrochemical Industry, 2002.3 (5) 3 Corrosion status and anti-corrosion countermeasures of Huanghu catalytic unit cold exchange equipment. Petroleum Safety, 2002.3 (6)