Model Analysis for Optimal Operation – A Heat Integrated Distillation Case Study

The motivation of this thesis is to ensure optimal operation while respecting nonlinear consequences of process integration and optimality. The thesis develops a model based methodology for investigation the dynamic behaviour, optimal operation, and control structure design for the energy integrated chemical processes.

The work has contributed to ensuring optimal operation of integrated process plants through development of a procedure for control structure synthesis for optimal plant operation. The theoretical and methodological work has focused on development of a systematic methodology for determining a control structure which can be used as a basic structure upon which model based control may be implemented to manipulate the basic control loop setpoints to achieve optimal operation. This procedure also has the ultimate aim to facilitate integration of process design and control structure design. The procedure encompasses the following main steps:

1. Theoretical analysis of how the operational degrees of freedom may be used
through control to cover the desired operating window such that it is revealed which underlying process connectivity can give rise to static and dynamic complications.
2. Determining an optimal plant operation mode and its relation to the accessible degrees of freedom provided by the basic control structures.
3. Control structuring at the basic levels to account for and possibly exploit the
underlying connectivity by replacing constrained degrees of operational freedom with more easily accessible degrees of freedom. Thus control is used to move the degrees of freedom in a most suitable manner for plant efficiency and operability.
4. Demonstration of model based control to exploit the accessible degrees of freedom through validation both by simulation and experiment

For optimal column operation it is demonstrated that it is important to consider the pressure sensitivity of the mixture to be separated. It is shown that the pressure should be maintained at the proper end of the column depending upon the pressure sensitivity of the mixture. Significantly higher separation efficiency and also higher capacity may be obtained through controlling the pressure at the suitable location.