Controllability of Semibatch Nonisothermal Antisolvent Crystallization Processes

In this article the problems of input multiplicities arising in the nonisothermal crystallization process is first considered. Using controllability tools it was found that the system, along a certain trajectory is ill conditioned. Furthermore, the singularity loci of the gain matrix, calculated at asymptotic condition, were obtained. The ill-conditioned nature of the corresponding control problem may imply limitations and affect the responsiveness of the system, and thus the design of a proper control strategy is also addressed in this paper. The proposed control strategy consists of two-stages. In the first stage a feedforward control is used, where the antisolvent flow rate and temperature trajectories are the results of a static optimization procedure. At the second stage, a feedback control action is introduced, and it is used to eliminate the possible offsets due to modeling errors. Results are illustrated for the simulated NaCl–water–ethanol antisolvent crystallization system.