Single and multi-objective shutdown optimization of a multistage continuous crystallizer

This study presents the first model-based optimal shutdown procedure of a multistage continuous crystallization process which aims at the maximization of on-spec production and minimization of the shutdown time. The cooling antisolvent crystallization of Aspirin (acetylsalicylic acid) in a three-stage continuous crystallizer was used as a case study. To address the optimal shutdown problem, several single optimization scenarios were considered to assess the impact of the degrees of freedom, discretiza-tion schemes, and optimization settings such as the constraints. The proposed optimal shutdown procedures showed that significant amounts of on spec crystals can be produced both at fixed and variable shutdown times. Most importantly, the optimal shutdown procedures can match the steady state productivity, based on the shutdown to steady state productivity ratio (STSPR) which can easily reach 100%. Moreover, the residual shutdown material, considered as waste, can be dramatically reduced by >80% compared to the current standard shutdown procedures. Given the conflicting nature of the maximization of on spec production and minimization of the shutdown time, a multi-objective optimization of the shutdown operation was also addressed to identify the set of Pareto optimal solutions. Finally, a multicriteria decision aiding method, based of multi-attribute utility theory, was proposed to rank the Pareto optimal solutions to support the decision making and help identify a suitable and feasible single optimal shutdown solution.