Real-time optimization of a cat cracker unit

Abstract This paper describes the real-time optimizer (RTO) for the Residual Catalytic Cracker plant at the Statoil Mongstad refinery. The RTO completes a project for control and optimization of this unit. The RTO models are basically regressed on data from a plant test sequence of 60 planned experiments, along with data from ordinary operation. The optimization problem has about 20 independent variables and 40 process model equations. Process models are continuously compared to actual measurements and biases are updated at process steady-state conditions. The predicted economics (objective function) is calculated under the restriction of entire plant mass balance satisfaction, while the constraint predictions are based on delta values from the current operating point. The products are priced in two or tree stages depending on their use, for instance naphtha used as blending component has a higher value than naphtha sold as a product solely. A commercially available sequential quadratic programming (SQP) routine solves the optimization problem. The optimization system is coded in FORTRAN and implemented in a plant computer along with model-predictive control (MPC) applications, both interacting with the real-time process database. Optimized independent RTO-variables are implemented on the plant as set-points for the MPC controlled variables or as desired values of the MPC manipulated variables. Based on preliminary estimates and optimization runs during commissioning the expected project pay back time is two months.