Chapter 7 - Cascading

Abstract

Cascading involves the use of spent catalysts in either more or less severe operation. In the latter case, the spent-regenerated catalyst from a severe service (e.g., gas oil) is used in less severe operation (e.g., naphtha or straight-run distillate). Guard bed as metal trap or guard reactor to protect catalysts in the downstream reactors of multistage process is another option for cascading spent catalysts. If silicon from the antifoaming agent is present in feeds, spent-decoked catalyst placed in front of a fixed bed can alleviate pressure drop build-up in the bed. In ebullated bed reactors, the spent catalyst comprises a portion of the fresh and/or very lightly deactivated catalyst that was admixed with spent catalyst during ebullition. If separated from the mixture, the least deactivated portion of the spent catalyst may be returned either directly to the same operation or cascaded to less severe service.