Hazards and Countermeasures of FES in the hottest

Hazards and Countermeasures of FES in hydrogenation unit

compared with clean fuels that meet the new emission standards, the overall quality of gasoline products in China is characterized by high sulfur content and olefin content, while the overall quality of diesel products is characterized by high sulfur and aromatics content, high density and low hexane number. The related hydrogenation technology can directly produce high-quality petrochemical raw materials and clean fuels, so it is valued and favored by many oil refining enterprises. With the increase of the proportion of refinery processing high sulfur crude oil and the improvement of desulfurization depth of hydrogenation unit, the corrosion of sulfur on hydrogenation unit equipment is becoming more and more serious, and its corrosion products mostly appear in the form of FES. Therefore, eliminating the harm of FES is very important to improve the operation quality and safety of hydrogenation unit

1 2. When the lens surface is stained with oil or fingerprints, the formation of FES

as the stone is simultaneously used on the interface on the controller, 1 we must maintain an accurate corresponding state. With the increase of sulfur content in the crude oil processed by the enterprise, the H2S content in the system will also increase. Equipment and containers designed for processing low sulfur are aggravated by high-temperature sulfur and H2S corrosion. Both high-temperature sulfur and H2S corrosion produce black FES. FES is very dense, but when hydrogen exists in the system, hydrogen will penetrate the FES film, causing the dense FES film to be destroyed, exposing new metals, leading to the continuous development of corrosion in depth. Such repeated action will form a large number of layered FES corrosion products

hydrogen sulfide corrosion is caused by the coexistence of water and hydrogen sulfide in the fractionation system of hydrogenation unit. Wet hydrogen sulfide environment is called acidic. The International Association of Corrosion Engineers (NACE) defines H2S environment as: H2S in the aqueous phase ≥ 50 in the oil refining process μ g/g。 However, the critical concentration of H2S in the aqueous phase required to cause corrosion cracking is not very certain. Carbon steel and low alloy steel basically do not corrode in anhydrous hydrogen sulfide below 250 ℃, but will significantly corrode metals when water coexists. Hydrogen sulfide dissociates in water. The corrosion of carbon steel and low alloy steel equipment and pipelines in wet H2S environment is an electrochemical reaction process, which leads to the formation of FES

2 hazards of FES

2.1 spontaneous combustion of FES

according to the definition of combustion, combustion requires three conditions: the presence of combustibles, the presence of combustion aids, and the presence of energy that can lead to combustion. After the metal is corroded by H2S or high temperature s, FES is formed. When FES encounters air, it will undergo oxidation reaction, and the oxidation reaction of iron sulfide will release a lot of heat, while the existence of FES layered structure will not be conducive to the loss of heat, thus forming a high-temperature heat source

FES oxidizes and spontaneously ignites rapidly at room temperature. There is no flame during spontaneous combustion, but it is heated to a hot state. When it reaches a certain temperature, it can cause other substances to burn and damage the material of the equipment. In recent years, FES spontaneous combustion has damaged equipment in many domestic units

2.2 formation of polythionic acid corrosion

during the shutdown of the unit, the sulfide remaining in the equipment and pipeline reacts with oxygen in water and air to form polythionic acid, resulting in the corrosion of equipment and pipeline. Generally, the operating temperature of the reactor is 370 ~ 415 ℃, while the sensitization temperature of austenitic stainless steel is above 430 ℃, so the composite austenitic stainless steel will not be sensitized in the normal process. However, during operation over temperature or welding, it may be locally sensitized and residual stress, so it is generally considered that austenitic stainless steel is in a sensitized state. When austenitic stainless steel is in the sensitized state or chromium carbide precipitation (Cr23C6) is generated near the grain boundary during use, it will cause serious chromium deficiency near the grain boundary. During the shutdown and maintenance of the hydrogenation unit, sulfide, water and hydrogen in the reactor are available, and the metal sulfide is hydrolyzed to form polythionic acid. Under the combined action of tensile stress and polythionic acid, austenitic stainless steel first has polythionic acid intergranular corrosion in the sensitized area, and then due to the existence of stress, the equipment will have polythionic acid stress corrosion cracking in these weakest areas

2.3 rise of bed pressure drop

there are two sources of FES entering the reactor. First, it is brought by raw oil. Mechanical impurities and metals in raw materials or circulating oil belt. FES in the fractionation part of this device, which adopts reasonable auxiliary equipment, will be deposited in the first bed of the refining reactor. Second, the raw materials containing iron ions pass through a section of catalyst bed, and the iron containing organic compounds with high reaction activity in the raw materials are easily converted into corresponding hydrocarbons and FES, which are deposited in the pores and outer surfaces of the catalyst. The harm is to cause the attenuation of catalyst activity, the reduction of bed porosity, the increase of pressure drop, and the shortening of operation cycle

2.4 pipeline blockage of fractionation system

the circulating oil system, fractionator and debutanizer overhead distillation and reflux system of hydrogenation unit have been blocked for many times. After removing the filter at the pump inlet, it is found that the filter has been blocked by dirt. According to the analysis results of scale, the composition of scale is mainly FES. By collecting raw oil and circulating oil for source analysis of iron compounds, it is found that there are much more iron compounds in circulating oil than in raw materials. From the analysis of the process of the device, the only possibility of increasing the content of iron compounds is corrosion. Because some of the reaction system equipment is made of austenitic stainless steel, the possibility of corrosion is small, and the fractionation system should have the most corrosion. The abnormal operation of the debutanizer caused the corrosion of the fractionation system and the increase of sulfur content in the product. Anhui Lishi Power Technology Co., Ltd. participated in the preparation and revision of national standards, resulting in equipment corrosion