How to prevent ferrous sulfide from self-igniting when the plant is shut down
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How can spontaneous combustion of ferrous sulfide be prevented when the plant is shut down? What process measures can be taken during shutdown?
⒈After the unit is shut down, cooling should be done slowly to reduce the system temperature to room temperature； ⒉Where conditions permit and ferrous sulfide is likely to form, soak those areas in water. For specific reasons, you can refer to the following information: 1. The auto-ignition temperature of dry ferrous sulfide in dry air is generally 300–350°C. Before auto-ignition occurs, an oxidation reaction takes place between ferrous sulfide and air under certain conditions; the heat released from this reaction raises the temperature of the system. Although auto-ignition does not occur, this endothermic oxidation reaction creates the conditions necessary for ferrous sulfide to ignite on its own. As the particle size of ferrous sulfide becomes smaller, its onset auto-ignition temperature tends to decrease toward lower values. 2. The addition of a small amount of water (with less than 20% water content in ferrous sulfide) reduces the initial auto-ignition temperature of ferrous sulfide to room temperature, allowing it to undergo auto-thermal reaction and self-ignition at room temperature as well. However, a water content of over 60% can effectively suppress the autoheating and spontaneous combustion of ferrous sulfide. 3. During the shutdown and maintenance of the simulated distillation tower, when manholes are opened under different temperature conditions, ferrous sulfide present inside the tower, in the presence of saturated water vapor, exhibits an increasing tendency to self-heat and self-ignite as air humidity rises. 4. Under saturated water vapor conditions at 70°C, as the air flow rate increases, the auto-ignition property of ferrous sulfide gradually enhances. 5. Oil sludge exhibits self-heating even at room temperature; the heat of reaction causes the system temperature to rise slowly. Once the temperature exceeds 200°C, spontaneous combustion leads to a rapid increase in the system temperature. 6. At 70°C under saturated water vapor conditions, with an air flow rate of 0.48 m/s, ferrous sulfide contaminated with 10% dirt can undergo rapid oxidation reactions even at temperatures below 100°C; the heat generated by these oxidation reactions causes the temperature of the system to rise quickly ； Spontaneous combustion occurred when the temperature rose to 120°C.
We usually carry out a more comprehensive pickling process after stopping production for purging
It is best to carry out a neutralizing wash; if that is not possible, simple water washing, soaking, or nitrogen protection can be used. After washing and turning on the device, keep an eye on it for a day or two; there is still a possibility of natural overheating, so connect the water supply line to allow for cooling at any time. Experts also say that iron sulfide does not ignite on its own; rather, it undergoes an oxidation reaction that releases heat, which in turn ignites the surrounding materials.
Post about ferrous sulfide: http://bbs.hcbbs.com/viewthread.php?tid=439266
Rinse with water, then treat the main equipment with passivation
The system is thoroughly boiled and purged with steam, or it can also be cleaned using ferrous sulfide cleaning agents.
Steam purging is used during shutdowns; cleaning with solutions can also be employed, but I don’t think either method is very effective
1. The hydrogen sulfide removal tower and its top reflux system use alkaline solution for cleaning; it is estimated that most plants have an alkaline washing circuit, which is the pipeline running from the outlet at the bottom of the hydrogen sulfide removal tower to the inlet of the top reflux pump. The prepared alkaline solution is added to the reflux tank and then pumped into the tower via the reflux pump. Once an appropriate liquid level is achieved, this circuit is used for circulation. 2. The reactor system uses a film-forming agent developed by Jiangsu Tianpeng Company. During shutdown periods, the viscosity of the system’s circulating oil is adjusted to 15–17 cs/m2, and the film-forming agent is injected; this helps to effectively cover the catalyst, reducing dust contamination during catalyst removal. Moreover, operation in an oxygen-rich environment can effectively prevent the risk of spontaneous combustion of ferrous sulfide.
Could someone use low-pressure steam with oxygen for purging, thereby directly oxidizing the substance, with the steam carrying away the waste gases and heat? The iron oxide produced in this way has high stability; it’s just not known whether any problems will arise when oil is added again... There should be no more free oxygen generated!
Rinse the container thoroughly and purge it with nitrogen to dry it out. Then open the manhole. As long as the flammable medium has been completely replaced and there are no combustible materials left. There’s no need to worry about stopping work.