Removal and deposition of Fe from the feedstock of hydrogenation units

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In the feed of the hydrogenation unit, Fe mainly deposits in the protectant and demetallization catalyst beds, with little or no deposition in the subsequent beds. The highest amount of Fe deposition occurs on the catalyst support, as the Fe compounds that are easy to remove (such as iron naphthenate) have reactions that are primarily temperature-dependent and do not require strong hydrogenation centers; therefore, most of them deposit in the catalyst support bed at the front of the reactor. Metal Fe is mainly deposited between the catalyst particles (in the bed gaps). This is also because most Fe compounds (such as iron naphthenate) are reactive and do not require strong hydrogenation centers; at a certain temperature, they can react within the gaps in the bed to form FeS deposits between the catalyst particles. Therefore, when developing catalysts, the porosity of the protective agent bed and the demetallization bed should be appropriately increased to provide sufficient space for metal impurities such as Fe to deposit between the particles. Radial temperature differences and the formation of hot spots can cause overheating and caking of the catalyst, with local temperatures often exceeding 430°C. A catalyst powder sample from the compacted area was taken, and the contents of Fe and C in this powder sample were much higher than those in the corresponding catalyst sample. A large amount of iron sulfide can arise from catalytic reactions; at high temperatures, this iron sulfide facilitates the coking process in the materials that give rise to coke, resulting in the formation of large quantities of coke. Therefore, hot spots in the catalyst bed should be avoided as much as possible. Otherwise, a vicious cycle is likely to arise: hot spots cause massive deposition of iron sulfide, and the deposition of iron further promotes coking reactions, leading to agglomeration that expands the hot spot area. This prevents the plant from increasing the temperature, causes the bed pressure to rise rapidly, and forces it to shut down. Some Fe is present in the resins and asphalts of residue and is difficult to remove.