Controllability of semibatch nonisothermal antisolvent crystallization processes

In this article the problems of input multiplicities arising in non-isothermal 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 flowrate 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.