(329a) Integrating Mpc and Rto in the Process Industry by Economic Dynamic Lexicographic Optimization; An Open-Loop Exploration

The current state-of-the-art
for industrial process operation is a cascaded structure of Model Predictive
Control (MPC), Real Time Optimization (RTO) and Scheduling and Planning
(S&P). In this structure RTO represents the weakest link because it is
based on steady state models; infrequent model updates (only at steady state!)
result in slow ineffective feedback [1]. So there is an opportunity to extend
MPC and obtain economic dynamic integration with S&P. It is exactly this
integration that will allow MPC and especially NMPC to develop its full
economic potential. The research focuses on two questions: 1. How to formulate
MPC to achieve dynamic economic integration with S&P? 2. Does this
formulation fix all degrees of freedom? Both questions will be dealt with in an
open-loop setting.

The formulation boils down to
the following constrained optimization problem:

Minimize integrated operational costs from now to t_f.
Subjected to process behavior, required product quantity/quality at t_f
and operational limitations.

Here
t_f stands for the time horizon; t_f and the required
product quantity/quality at t_f are supplied by S&P (schedule!).
The objective (F₁) is solely based on economics. The last research
question was addressed in two numerical experiments [2]. Both experiments show
the existence of multiple (non-unique) solutions. So there are still degrees of
freedom left to optimize operation further. An attractive way to do this is by
introducing a 2nd non-economic objective (F₂) and lexicographic
ordering; F₁, F₂. Lexicographic optimization was applied
successfully to both experiments. It resulted in unique solutions and the
remaining degrees of freedom were used to achieve other operational objectives.

References

[1] D.C. White, Online
Optimization: What have we learned?, Hydrocarbon Processing, 77:55-69, 1998.

[2] A.E.M. Huesman, O.H.
Bosgra and P.M.J. Van den Hof, Degrees of Freedom Analysis Of Economic Dynamic
Optimal Plantwide Operation, Preprints 8th IFAC International Symposium on
Dynamics and Control of Process Systems (DYCOPS), Vol. 1, pp. 165-170, Mexico, 2007.