FCC BOTTOMS CRACKING MECHANISMS AND IMPLICATIONS FOR CATALYST DESIGN FOR RESID APPLICATIONS

摘要

It is very tempting to simplify the whole bottoms conversion process to one simplistic measurement or test, but it is in reality a very complex process and no one simple measurement or single catalyst property can guarantee good bottoms cracking performance for resid applications. One has to carefully take into consideration the feed types, unit constraints and unit operating philosophy. There are three types of bottoms conversion reactions. Type I bottoms conversion requires an optimal mesopore design to facilitate the diffusion and precracking of heavy molecules. The Type II reaction improves the conversion of alkylaromatics with mostly zeolite activity, while the Type III reaction requires a maximum matrix activity for cracking naphthenoaromatics. Each one of the three reactions can dominate an operation for a specific application. In addition, a particular unit's need for coke selectivity plays a critical role as any of these reaction types can lead to the conversion of bottoms into coke. For most applications, a balanced approach is needed to achieve the best bottoms conversion performance within the context of unit constraints. An understanding of the types of reactions a unit can exploit to improve its bottoms conversion is the first step towards an optimal catalyst design and application. The extensive understanding of the reaction mechanisms has led to the design of Davison's latest generation of bottoms conversion catalysts, the Midas family.