Systematic Tuning of PI Averaging Level Control for Recycle Systems

This work examines the use of P-only and PI averaging level control (ALC) of a tanks-in-series recycle process, the most basic model for understanding material balance dynamics in non-reactive recycle systems. We show that P-only ALC tuning for a standalone tank Effectively extends to the recycle process. However, directly applying isolated tank PI ALC tuning is not possible due to the complex dynamics and flow amplification that occur along the tank cascade. A systematic procedure for plantwide PI ALC (de)tuning is developed, ensuring acceptable flow amplification while fully utilizing the available surge capacity for the worst-case disturbance. The application of the systematic tuning method to a three-column azeotropic separation process shows that PI ALC achieves significantly higher high-frequency variability attenuation compared to P-only ALC at the expense of mild flow amplification around a small low-frequency resonance peak. These findings suggest that PI level control, which eliminates level offsets, a feature favored by operators, may be applied in plantwide non-reactive systems with systematic detuning.