What measures are taken for corrosion protection against high-temperature sulfur and sulfides?

Corrosion caused by high-temperature sulfur and sulfides refers to the corrosion resulting from sulfur, hydrogen sulfide, and thiols at temperatures above 240°C, such as the corrosion in the lower section of the main fractionation tower in delayed coking units. Under high-temperature conditions, active sulfur reacts directly with metals, resulting in uniform corrosion, among which hydrogen sulfide exhibits the strongest corrosive effect. The factors affecting high-temperature sulfur corrosion include temperature, hydrogen sulfide concentration, fluid flow rate, material properties, and the flow state of the medium. 　　a) Temperature: Since coking takes place at high temperatures of nearly 500°C, the inactive sulfides in the feed materials are fully decomposed to produce hydrogen sulfide, which is then further decomposed into elemental sulfur and thiol compounds. As a result, the content of active sulfur increases significantly, leading to increased corrosivity; the higher the temperature, the greater the corrosion rate. 　　b) Hydrogen sulfide concentration: Hydrogen sulfide is the most corrosive among all active sulfides; the higher the concentration of hydrogen sulfide, the more severe the corrosion. c) Medium flow rate: The greater the flow rate, the easier it is for the protective film formed by iron sulfide corrosion products on the metal surface to come off, resulting in increased corrosion. 　　d) Material: Carbon steel has a high corrosion rate. 　　e) Medium flow conditions: Parts in the pipeline such as elbows, tees, inlet and outlet connections of equipment, or orifice plates, which alter the flow pattern of the medium. These areas are prone to turbulence, vortices, and chaotic flow, which can erode the metal surface and increase the corrosion rate. In areas where the medium does not flow for extended periods, the corrosion rate is higher. Preventive measures against sulfur corrosion 1. Material upgrading Studies have shown that the sulfide film formed on the surface of Fe—Cr alloys has a three-layer structure: Fe—S, FeCr2S4, and iron-chromium sulfides. Due to the effect of the high Cr content in the matrix, spinel sulfide FeCr2S4 is formed, creating a denser film that can prevent further corrosion. Therefore, using Cr5Mo steel and stainless steels with more than 13% chromium in high-temperature areas, especially those with solid particles present, is an effective corrosion prevention measure. 2. Controlling flow velocity and flow pattern: Turbulence and eddies easily occur at bends, tees, elbows, and the inlet and outlet connections of equipment, resulting in a high corrosion rate. Therefore, the equipment design and pipeline layout should be rational. Heat stress, liquid stagnation, or localized overheating should be avoided, as well as eddies and dead zones; sudden changes in flow direction and the formation of low-pressure areas should be reduced to prevent erosion. 3. Adjust the process and optimize operations by utilizing advanced process technologies to control the sulfur content in the raw materials. Currently, the method of mixing crude oils is commonly used, with a mixture of high-sulfur crude oil and low-sulfur crude oil in a 1:1 ratio being considered appropriate. Adhere to strict procedural guidelines; prevent the equipment from reaching excessive temperatures and pressures, and minimize fluctuations as much as possible. 4. Stress relief: Perform stress-relief heat treatment on pipelines and equipment that are operated at high temperatures to prevent stress corrosion.

The vacuum system of our atmospheric and vacuum distillation unit operates in an environment subject to high-temperature sulfur corrosion; originally carbon steel was used, which led to leaks, but it was later replaced with Cr5Mo steel, and the results have been good

Our unit uses lead lining, and it has worked fine for over a year