(215a) Distillation – Energy Conservation and Process Control, a 35 Year Perspective

It is commonly estimated that separation processes account for 40-70% of both capital and operating costs in petrochemical processing and that distillation is used to make 90-95% of all separations in the chemical process industry. The preference for distillation is easy to understand considering its high level of technological and use maturity, its simple flowsheet and favorable scalability. These attributes outweigh the fact that distillation is not an energy efficient process, at least not from a First Law viewpoint (heat balance). From a Second Law perspective (Exergy balance) distillation can be made to look more efficient on the surface but the gains might not be so easy to realize in practice. Furthermore, there is a direct connection between our ability to change the Exergy dissipation rate in a device and its controllability.

My personal interest in distillation, energy conservation and process control started over 35 years ago as I did my dissertation under Professor Luyben on the topic of "Control of multi-effect, energy conserving distillation systems". The topic of energy conservation was as relevant then, after the energy crunch of '73, as it is today with increasing fuel prices. In this presentation I review some of the energy conservation ideas proposed in the literature for distillation columns and reflect on how these improvements might affect controllability. I also look at some of the most popular control structures for distillation columns in general and seek a thermodynamic interpretation for their functionality.