Time Optimal Control of the Molecular Weight in a Semi Batch Emulsion Polymerization

In this contribution a hierarchical approach to the control of the molecular weight in a semi-batch emulsion polymerization is derived, which ensures minimum batch time. Rather than performing a numerical optimization to determine an optimal trajectory for the process at any point in time, the existing boundary conditions are examined and used to determine the time optimal operation at each time step. This procedure provides the set points for the molar holdups of monomer and chain transfer agent (CTA) in the system. Either a decentralized PI controller or a nonlinear model predictive controller (NMPC) can be applied to track the calculated trajectory. As the set point calculation is a solution of a system of algebraic equations it can easily be performed on-line. It is shown that this concept can be used to produce multi modal molecular weight distributions and can also handle disturbances in the process, such as an unexpected nucleation of particles.