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Development of ‘cycle to cycle’ control of simulated moving bed process with flow rates and switching time as manipulated variables


C. Grossmann

International Symposium, Exhibit & Workshops on Preparative / Process Chromatography, (PREP 20). Baltimore, MD, USA.

Simulated Moving Bed (SMB) chromatography is a well established technology for continuous chromatographic separation especially for enantiomers. However, optimum SMB operation is a challenge and the current practice is to operate SMB units sub-optimally to guarantee robustness. As a result, control and automation of SMB is receiving increasing interest to exploit the full economic potential of this process. Nevertheless, an automatic control algorithm for SMB units that guarantees an optimal, robust operation is still an open and challenging problem because of the complex dynamics involved in this process, e.g. its cyclic and hybrid nature of inlet/outlet port switching, with nonlinearities and delays in the feedback information. We have developed a control scheme that combines the optimization and control of the SMB unit. The control problem is formulated as a constrained dynamic optimization problem within the repetitive model predictive control (RMPC) framework. The controller predicts and optimizes the future inputs over a given horizon of time by making use of a linear time-invariant ‘cycle to cycle’ SMB model derived from the system of PDEs describing the process. This scheme is implemented on the SMB unit according to a receding horizon strategy, i.e. a new optimization problem based on the latest estimate of the plant state is solved as new measurements are available from the plant. The state estimate is computed using a linear Kalman filter. This approach provides the flexibility to apply the controller to a wide range of separation tasks, regardless of the type of isotherm characterizing the mixture to be separated in an SMB unit. This work presents development of the ‘cycle to cycle’ control scheme where the feedback information to the controller is the average purity of the product streams over a cycle. The 'cycle to cycle' formulation of the control scheme facilitates using switching time, in addition to the four flow rates in the four sections of the SMB unit, as manipulated variables. In this way, a complete optimal performance and control of the SMB process is achieved. The performance of the controller is illustrated through simulation studies that demonstrate how the controller fulfills the product and process specifications while operating the SMB unit optimally, regardless of the uncertainties in the isotherm parameters and disturbances in the system for a chiral separation.


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M. Morari

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