The hazards of FeS spontaneous combustion during the maintenance of packed towers and its prevention

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The hazards and prevention of FeS spontaneous combustion during the maintenance of packed towers. In processing industries that use natural gas, petroleum, and similar materials, packed towers are widely used; however, as the proportion of sulfur-rich raw materials increases, the problem of sulfur corrosion is becoming increasingly severe. Sulfur corrosion products mainly appear in the form of FeS; during the shutdown and maintenance period of equipment, if no effective preventive measures are taken, FeS will oxidize or even burn rapidly when exposed to air. Therefore, removing FeS has become the first essential safety step in shutting down and maintaining processing equipment for high-sulfur raw materials. 1. Formation mechanism of FeS in a packed tower 　FeS is a dark brown or black solid that is insoluble in water, with a density of 4.74 g/cm3 and a melting point of 1193°C. 　　Sulfur in petroleum is roughly divided into two categories: active sulfur and inactive sulfur. 　　Active sulfur includes elemental active sulfur (S), hydrogen sulfide (H2S), and thiol (RSH). Its characteristic is that it can react directly with metals to form metal sulfides. Above 200°C, dry hydrogen gas can react directly with iron to form FeS. The formation rate is highest between 360~390°C, and it decreases to become insignificant around 450°C. At 350–400°C, elemental sulfur readily combines directly with iron to form FeS. At this temperature, H2S can undergo decomposition: 　　H2S→S+H2 　　The active sulfur produced by this decomposition reacts very strongly with iron. 　　Above 200°C, thiol groups can also react directly with iron: RCH2CH2SH + Fe == RCHCH2 + FeS + H2. Inactive sulfur compounds include sulfides, disulfides, cyclic sulfides, thiophenes, polysulfides, and so on. Its characteristic is that it cannot react directly with iron; instead, it decomposes when heated to produce active sulfur, which then reacts with iron according to the rules mentioned above. The decomposition of different sulfides at various temperatures results in varying degrees of sulfur corrosion. 　　Complex sulfides begin to decompose at 115–120°C, producing H2; the decomposition is more intense between 120–210°C, reaches its strongest level at 350–400°C, and is essentially complete at 480°C. 　　Within the process system, as long as sulfur is present, ferrous sulfide will inevitably be formed, and its formation is influenced by the temperature and flow rate of the medium, as well as the sulfur content and its form of existence. The composition and properties of ferrous sulfide also have a significant impact on its continuous formation. If the resulting ferrous sulfide has a loose structure and provides no protection for steel, it accelerates the formation of ferrous sulfide. 　　2. The hazards of FeS in packed towers The distribution of FeS in process equipment generally follows this pattern: the higher the sulfur content in the medium, the more FeS corrosion products are formed. However, even in equipment where the sulfur content is only a few parts per million, spontaneous combustion of FeS can occur when the equipment is opened. The reason is not the high sulfur content in the medium, but rather the fine FeS corrosion products that are carried downstream from the upstream area along with the material, and accumulate and deposit in areas where the flow velocity is relatively low. In tower equipment, the flow rate of the material inside is low, and the corrosion products of ferrous sulfide tend to deposit in certain areas; this is especially true for packed towers, where the packing not only serves a distillation function but also has an efficient filtering effect, allowing ferrous sulfide carried in from upstream to be easily trapped. At the same time, metal fillers have a large specific surface area, resulting in a large contact area with the material; even if the sulfur content in the material is low, it can still cause corrosion of the fillers. Due to the low flow rate of the material inside the packed tower, the ferrous sulfide formed by corrosion on the surface of the packing is difficult to be carried away by the material. In this way, in a packed tower that operates under high load, for long periods of time, or with multiple consecutive operating cycles, a certain amount of ferrous sulfide will accumulate inside the tower. 　　Since the ferrous sulfide in tower equipment is not a pure substance, it mixes with coke powder, oil residues, etc. to form scale, and its structure is generally relatively loose. When ferrous sulfide is oxidized in humid air, ferrous ions are oxidized to ferric ions, and sulfur with a valence of -2 is oxidized to sulfur with a valence of +4, releasing a large amount of heat. Due to the local temperature increase, the oxidation of the surrounding ferrous sulfide is accelerated, triggering a chain reaction. If carbon and heavy oil are present in the fouling, they will burn rapidly under the action of ferrous sulfide, releasing more heat. This spontaneous combustion phenomenon can easily lead to fire and explosion accidents. For example, in September 2003, Workshop A of a certain alkylbenzene plant was preparing to replace the internal components and packing of the C-405 packed tower during a major maintenance project. After removing oil and installing blind flanges, steam purging was carried out from September 13 to September 16. At 15:00 on September 17, the manhole of the tower was opened for ventilation ahead of the handover of the work site; however, ferrous sulfide inside the tower caught fire spontaneously upon contact with air. Around 22:00, the on-site personnel noticed that the middle part of the tower had turned red, realized that there was a fire inside, and immediately called for help to put out the fire. During the firefighting efforts, at around 23:00, the middle part of the tower softened and broke, causing the upper section to tilt and fall onto the open area surrounding the structure. 　　3. Prevention of spontaneous combustion of FeS during the maintenance of packing towers The presence of FeS, contact with oxygen in the air, and a certain temperature are the three factors that lead to spontaneous combustion of FeS during equipment maintenance. To prevent spontaneous combustion of FeS during equipment maintenance, at least one of these factors must be eliminated. 　　a) Desulfurize and dehydrate the feedstock before it enters the tower, to prevent the formation of FeS. Current desulfurization processes can effectively remove sulfides from the feedstock, with advanced treatment enabling the removal of over 99% of sulfur. This prevents sulfur corrosion and the formation of FeS from occurring in the first place. Of course, the problem of sulfur corrosion still exists in the desulfurization process system. 　　Hydrogen sulfide formed inside the tower causes corrosion, damaging the tower structure, internal components, and packing. However, carbon steel is essentially unaffected by anhydrous hydrogen sulfide at temperatures below 250°C, but significant corrosion occurs in its presence of water. Before the feedstock enters the tower, a \"one-desalination and three-injections\" process centered on electrodesalination and dehydration is employed. The salt content, water content of the desalinated crude oil, as well as the pH, Cl- and Fe levels in the effluents from the initial distillation column, atmospheric distillation column, and vacuum distillation column are analyzed in a timely manner and strictly controlled. This reduces the corrosion of the tower equipment caused by the feedstock, thereby decreasing the amount of FeS formed in the equipment. As a result, the frequency of replacing the packing is reduced, and the risk of spontaneous combustion during maintenance is also diminished. 　　b) Control the oxygen content during maintenance to prevent spontaneous combustion of FeS. The oxidation of ferrous sulfide requires the presence of oxygen; therefore, before shutting down the plant for maintenance, inert gases are used to displace the combustible gases inside the tower, so that the ferrous sulfide powder does not come into contact with oxygen in the air and undergo an oxidation reaction. But this method causes inconvenience in operation. When working inside the tower, it is not allowed to open both the upper and lower manholes at the same time; only the manhole at the location where work is being carried out should be opened. Otherwise, convection of air will occur, resulting in an **increase in the oxygen level** inside the tower. 　　c) Strict temperature control during maintenance The main factor affecting the oxidation of ferrous sulfide is temperature; therefore, after shutting down the system and performing steam purging, it is absolutely not permissible to open the manholes immediately for work. The temperature inside the tower must first be reduced below room temperature, and necessary measures must be taken to prevent spontaneous combustion of ferrous sulfide before the manholes can be opened. In the case of packed towers, due to the large volume and high heat capacity of the packing, natural cooling is slow in the absence of ventilation. Cooling water can be injected from the top of the tower to increase the cooling rate, which makes the process safer. 　　d) Use of passivators to eliminate the activity of FeS. Ferrous sulfide passivators are high-efficiency chemical cleaning agents composed of chelating agents and corrosion inhibitors that have a strong chelating effect on FeS; they can effectively remove inorganic deposits such as FeS and Fe2O3 that accumulate on equipment, thereby preventing FeS from causing spontaneous combustion and damaging the packing or equipment. It features no deposition on equipment, low corrosivity to equipment, no special impact on the environment, stable properties, being non-toxic and harmless, as well as being safe and convenient to use. The safest method to prevent the spontaneous combustion of FeS is to carry out chemical passivation cleaning. Its activity is eliminated through passivation, thereby preventing spontaneous combustion. This post was last edited by Final Fantasy on 2007-12-22 14:03.]