Fluid Catalytic Cracking (FCC) is a major secondary conversion process in a refinery for upgrading low value heavy hydrocarbons into high value light distillates and LPG. The process works in the presence of fine zeolitic catalyst particles in a fluid bed reactor/regenerator system. FCC is a flexible process in terms of loading/unloading and switch over of catalysts/additives. In FCCUs the spent catalyst can be withdrawn from the system and fresh catalyst can be loaded to maintain the catalytic activity and get the desired yield pattern.