The formation and harm of ferrous sulfide
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When a refinery is refining sulfur-containing crude oil, a large amount of FeS will be produced during the production process of atmospheric and vacuum distillation, catalytic cracking, hydrogenation, coking, gas separation, desulfurization and other equipment, which will be adsorbed on the equipment and pipelines of the equipment. When the equipment is opened for maintenance, the oxidation reaction will release a large amount of heat when it comes into contact with oxygen in the air, resulting in spontaneous combustion, which will not only burn the equipment pipelines, but also easily cause fire and explosion accidents when other flammable and explosive media are present, which is extremely harmful. The multifunctional ferrous sulfide cleaning agent can not only effectively remove FeS accumulated on equipment and pipelines, preventing FeS from self-combustion and destroying packing or equipment, but also has multiple functions such as removing rust and other inorganic scales and removing COD in sewage. This series of products are all water-soluble, have the characteristics of no deposition on equipment, minimal corrosion to equipment, no special impact on environmental protection, stable properties, non-toxic and harmless, safe and convenient to use, etc.
Regarding refining, let me talk about the coal chemical industry. In the low-temperature methanol washing of the coal chemical industry, ferrous sulfide is often produced. The main reason is that methanol carries water, which is more serious. It is understandable that methanol brings water back to dissolve hydrogen sulfide in the water, so we should try to reduce the water content in methanol. But let’s take a look at NHD desulfurization. It requires a certain amount of water in the NHD solution. Why doesn’t it cause great corrosion to the equipment? Is it the catalytic effect of methanol or the effect of low temperature that is beneficial to the formation of ferrous sulfide? Please join the discussion.
The NHD solution itself will form a protective film when it comes into contact with the equipment. Even if air enters the equipment and is isolated by the protective film, hydrogen sulfide will not react with the iron! So it won’t corrode! The generation of ferrous sulfide in the equipment is mainly due to the intervention of air. For the low-temperature methanol cleaning and purification system, there is hydrogen sulfide in the system. Once the air enters the inside of the equipment, it will cause hydrogen sulfide to react with iron to generate ferrous sulfide! Judging from long-term operating experience, in the low-temperature methanol washing system, ferrous sulfide is more likely to be formed in the high-temperature area than in the low-temperature area, and the corrosion in the high-temperature area is more severe! However, air generally does not enter the system during normal operation. Corrosion and spontaneous combustion may occur only during major overhauls due to incomplete isolation or operating errors that allow air to intervene! I don’t know if the answer can satisfy the people above, please continue the discussion!
The hazards of ferrous sulfide are not small, especially after the maintenance waste is transported out, it can easily cause fires caused by other combustible materials. This happened twice in the waste storage yard of my unit. However, the treatment of ferrous sulfide is also relatively easy. Spraying water to lower the temperature can ensure that it does not spontaneously ignite. The removed ferrous sulfide can be dug into deep pits and landfilled.
In the processing industry that uses natural gas, petroleum, etc. as raw materials, packed towers are widely used. However, due to the increase in the proportion of high-sulfur raw materials, the problem of sulfur corrosion is becoming increasingly serious. Sulfur corrosion products mostly appear in the form of FeS. During the equipment shutdown and maintenance stage, if effective preventive measures are not taken, FeS will quickly oxidize and even burn when exposed to air. Therefore, eliminating FeS has become the first necessary safety procedure for shutdown and maintenance of high-sulfur raw material processing equipment. FeS is a dark brown or black solid, insoluble in water, with a density of 4.74g/cm3 and a melting point of 1193°C. Sulfur in oil products is roughly divided into two categories: active sulfur and inactive sulfur. Active sulfur includes elemental active stream (S), hydrogen sulfide (H2S), and mercaptans (RSH). Its characteristic is that it can react directly with metals to form metal sulfides. Above 200°C, dry flow hydrogen can react directly with iron to form FeS. The generation rate is maximum between 360 and 390°C, and slows down to become insignificant around 450°C. At 350~400°C, elemental sulfur can easily combine directly with iron to form FeS. At this temperature, H2S can decompose: The active sulfur and iron decomposed from H2S→S+H2 have extremely strong effects. Above 200°C, mercaptans can also react directly with iron: RCH2CH2SH+Fe==RCHCH2+FeS+H2 Inactive sulfur includes thioether, disulfide, cyclic sulfide, thiophene, polysulfide, etc. Its characteristic is that it cannot react directly with iron, but decomposes after being heated to generate active sulfur, which reacts with iron according to the above rules. The decomposition of different sulfides at different temperatures produces different degrees of sulfur corrosion. Complex sulfides begin to decompose at 115~120°C to generate H2, which is relatively intense at 120~210°C, reaches the strongest level at 350-400°C, and is basically completely decomposed at 480°C. In the process system, as long as sulfur exists, ferrous sulfide will inevitably be produced, which is affected by the temperature, flow rate, sulfur content of the medium, and the existence form of sulfur. The composition and properties of ferrous sulfide also have a great influence on the continuous production of ferrous sulfide. If the structure of the generated ferrous sulfide is loose and has no protection for steel, the formation of ferrous sulfide will be accelerated.