Mechanisms of corrosion and accident analysis in catalytic reforming units

Thread Content

The corrosive agents present in the system are hydrogen sulfide, hydrogen, and hydrogen chloride, all of which exist in a dry state. Therefore, corrosion of the equipment is primarily caused by hydrogen sulfide; the presence of hydrogen and hydrogen chloride accelerates this corrosion. Hydrogen accelerates the corrosion by undergoing a reduction reaction with ferrous sulfide (resulting in the formation of iron powder) as well as by penetrating the protective layer formed by ferrous sulfide. Hydrogen chloride is quite stable in a dry state, and the mechanism by which it accelerates hydrogen sulfide corrosion remains unclear and requires further investigation. Furthermore, although dry hydrogen chloride is non-corrosive to equipment and pipelines, it is highly corrosive in the presence of water. On the one hand, the crude oil used in the Jinan branch has a high water content; even after passing through the pre-distillation tower, the feed for pre-hydrogenation still contains about 100 μg/g of trace water. Since hydrogen chloride is present in the pre-hydrogenation system, it dissolves easily in water and is highly acidic, posing a severe corrosive threat to the pipelines. On the other hand, when ammonium salt crystals are detected in the pre-hydrogenation system, reforming units often use water injection to address the blockage caused by ammonium chloride crystals in the equipment and pipelines; moreover, the hydrolysis of ammonium chloride generates acidity, which leads to chlorine-induced corrosion in the pre-hydrogenation system. Hydrogen sulfide (H2S) produced in the pre-hydrogenation reaction is corrosive in both dry and wet forms. The H2S content in the system downstream of the pre-hydrogenation reactor is generally above 100 μg/g. It reacts with the equipment or pipelines to form ferrous sulfide (FeS), which normally provides some protection for the surface of the vessel walls. However, due to the presence of HCl in the system, HCl reacts with FeS, leading to cyclic corrosion and thus accelerating the rate of corrosion. That is, corrosion of the type “HCl——H2S——H2O” occurs in the pre-hydrogenation low-temperature area.