Describe the impact of feed metals (Ni, V) on catalyst activity and product quality in a hydrotreating unit.

Feed metals, primarily nickel (Ni) and vanadium (V), have a detrimental impact on catalyst activity and product quality in a hydrotreating unit. Hydrotreating is a process used to remove impurities like sulfur, nitrogen, and metals from petroleum feedstocks by reacting them with hydrogen over a catalyst. The presence of Ni and V in the feed leads to catalyst deactivation and affects the desired chemical reactions. When feed containing Ni and V enters the hydrotreating unit, these metals deposit on the catalyst surface. The deposition of Ni and V physically blocks the active sites on the catalyst, reducing the surface area available for the desired hydrotreating reactions like hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). This physical blockage decreases the catalyst's ability to remove sulfur and nitrogen, leading to products that do not meet required specifications. Ni and V also promote undesirable side reactions. They act as dehydrogenation catalysts, accelerating the formation of coke, a carbon-rich solid that further deactivates the catalyst by covering its active sites. The increased coke formation reduces the catalyst's lifespan and requires more frequent regeneration or replacement. Furthermore, Ni and V can alter the selectivity of the catalyst. They can suppress the hydrogenation activity of the catalyst, leading to increased formation of olefins and aromatics, which may be undesirable in certain products like diesel fuel. The deposition of these metals also affects the pore structure of the catalyst. As Ni and V accumulate within the pores, they can block access to the active sites located deep within the catalyst structure, further reducing its effectiveness. This pore blockage limits the catalyst's ability to process larger molecules in the feed. The impact of Ni and V depends on their concentration in the feed and the type of catalyst used. Certain catalysts are more tolerant to metals than others. Demetallization units are often used upstream of hydrotreating units to remove Ni and V from the feed, protecting the hydrotreating catalyst and ensuring optimal performance. These units use adsorbents or other separation techniques to reduce the metals concentration in the feed to acceptable levels. For example, processing a vacuum gas oil with a high concentration of Ni and V without pretreatment would quickly deactivate the hydrotreating catalyst, leading to increased sulfur content in the diesel product and reduced catalyst life. Therefore, managing feed metals is crucial for maintaining catalyst activity, achieving desired product quality, and ensuring the economic viability of the hydrotreating process.