Single and Multiobjective Shutdown Optimization of a Multistage Continuous Crystallizer

Area of Science:

• Chemical Engineering
• Process Control
• Crystallization Technology

Background:

• Continuous crystallization processes require efficient shutdown procedures to maintain productivity and minimize waste.
• Existing shutdown methods often lead to significant product loss and waste generation.

Purpose of the Study:

• To develop the first model-based optimal shutdown procedure for multistage continuous crystallization.
• To maximize on-spec production and minimize shutdown time.
• To reduce waste generated during the shutdown phase.

Main Methods:

• Aspirin (acetylsalicylic acid) cooling antisolvent crystallization in a three-stage continuous crystallizer was used as a case study.
• Single optimization scenarios were employed to evaluate degrees of freedom, discretization schemes, and optimization settings.
• Multiobjective optimization and a multiattribute utility theory-based decision-aiding method were used to address conflicting objectives.

Main Results:

• Optimal shutdown procedures can produce significant amounts of on-spec crystals at both fixed and variable shutdown times.
• The shutdown-to-steady-state productivity ratio (STSPR) can reach 100%, matching steady-state productivity.
• Residual waste material was reduced by over 80% compared to standard shutdown procedures.

Conclusions:

• Model-based optimal shutdown procedures offer a significant improvement over standard methods for continuous crystallization.
• The developed method effectively balances on-spec production maximization and shutdown time minimization.
• The approach provides a robust framework for decision-making in optimizing crystallization shutdown operations.