The Road to Advanced Process Control: From DDC to Real-Time Optimization and Beyond

Abstract

Abstract The progression of computer control in the petroleum refining industry from Direct Digital Control (DDC) in the 1960s to Model Predictive Control and Real-Time Optimization (RTO) in the 1990s is briefly reviewed. The authors’ personal experience is used to illustrate the rapid advances in application of this constantly changing technology. The closed-loop optimization of the catalytic reforming process at the Sunoco refinery in Sarnia, Ontario, Canada will be described in sufficient detail to illustrate the complexity, challenges and practical solutions encountered during the implementation of this unique real-time optimizer. The unit operations are described by steady-state, open equation-based, mechanistic models. Global spline collocation is used to solve the differential equations associated with the reactor models and the catalytic deactivation over time. The entire set of non-linear equations is solved simultaneously using a successive quadratic programming method designed for these large-scale optimization problems. Using the models of the plant which are appropriate at specified times during the time-to-shutdown, the optimization determines that trajectory of operating conditions over the catalyst life which maximizes the total profit over that period. At the same time, the optimization accounts for catalyst deactivation over this period, thus ensuring that the catalyst does not deactivate too fast or leave unused activity. The optimization honours all equipment constraints, product qualities, and production requirements. Model parameters are estimated continuously using on-line data, thus ensuring that the model matches the plant at all times. The optimizer sends the required setpoints for the current operation directly to the model predictive controller that controls the whole plant. The optimizer is activated when steady state operation is achieved, about seven or eight times daily. An in-plant experimental program is established in which the effluent from the four reactors is periodically sampled and analyzed. These analyses along with on-line operating data and the process model are used to estimate the reaction parameters and the catalyst activity in each reactor. After the formulation of the process model, the optimization system, and the actual operating experience of the reformer RTO have been discussed, the authors’ vision for the near future in advanced process control and process optimization will be elucidated, concluding with some challenges for both the academic and technology provider communities.