Model IV Fluid Catalytic Cracking Units (FCCUs) differ from other cracking units in that model IV FCCUs do not have slide valves in the catalyst circulation lines to enable direct control of catalyst circulation rate through the unit. Reducing fluctuations in catalyst circulation rate is found to significantly improve closed loop performance of the FCCU. Some design and operational modifications that can be made to model IV FCCUs to improve closed loop performance at the regulatory level based on this insight are modeled and compared. Closed loop performance of a model IV FCCU operated with the weir and standpipe always flooded is examined. The achievable performance is significantly better than that of the standard model IV FCCU. The closed loop performance of the model IV FCCU modified to incorporate slide valves in the catalyst circulation lines is also examined. The performance of the FCCU with slide valves is better than the performance achievable by the FCCU with the weir flooded. It is found that model IV FCCUs are ill-conditioned owing to the use of the weir and standpipe arrangement in the regenerator section. Both the operational and design modifications studied reduce plant ill-conditioning appreciably
