What are the primary factors that contribute to catalyst deactivation in a Fluid Catalytic Cracking (FCC) unit?

Catalyst deactivation in a Fluid Catalytic Cracking (FCC) unit is primarily caused by several factors: coke deposition, metal contamination, hydrothermal deactivation (steaming), and attrition. Coke deposition is the most significant cause. Coke, a carbon-rich solid, forms on the catalyst surface during the cracking reactions. This coke physically blocks the active sites on the catalyst, preventing reactants from accessing them and thus reducing catalyst activity. The amount of coke formed depends on the feed composition, operating conditions (temperature, residence time), and catalyst properties. Metal contamination occurs when metals present in the feed, such as nickel, vanadium, and iron, deposit on the catalyst. These metals act as dehydrogenation catalysts, promoting undesirable reactions like coke and hydrogen formation, and also poisoning the active sites. Nickel and vanadium can also cause excessive cracking, leading to increased gas and coke yields, reducing the yield of valuable products like gasoline. Hydrothermal deactivation, also known as steaming, refers to the loss of catalyst activity due to exposure to high temperatures and steam during the regeneration process. Steam can cause the collapse of the catalyst’s pore structure, reducing its surface area and acidity, both essential for cracking reactions. This structural change decreases the catalyst's ability to effectively crack large hydrocarbon molecules. Attrition is the physical breakdown of the catalyst particles due to mechanical forces within the FCC unit. The continuous circulation of catalyst, along with its impact against vessel walls and other equipment, causes the catalyst particles to wear down, reducing their size and increasing the amount of fine particles. These fine particles can lead to increased catalyst losses and operational problems, such as plugging of downstream equipment. Regular catalyst replacement, optimization of operating conditions, feed pretreatment to remove metals, and the use of additives to passivate metals are common strategies to mitigate catalyst deactivation in FCC units. For example, using a catalyst with a higher resistance to attrition can reduce catalyst losses, and employing a demetallization unit upstream of the FCC can minimize metal contamination.