Selecting Appropriate Control Variables for a Heat- Integrated Distillation System with Prefractionator

A heat-integrated prefractionator arrangement is studied for a ternary separation of a propane-butanepentane mixture. These types of integrated prefractionator arrangements have large energy savings compared with the best of the direct or indirect sequence with no heat integration. However, a heatintegrated distillation system can be more difficult to control than a non-integrated arrangement, so good control systems are essential. In this work the focus is on selecting control variables that will ensure that the promised energy savings are achieved. The method of self-optimizing control (Skogestad, 2000) has been used to provide a systematic procedure for the selection of controlled variables, based on steady state economics. The results show that the integrated prefractionator arrangement can have large energy savings compared with non-integrated arrangements and that controlling the ratio of the distillate flow to the feed flow has good self-optimizing properties.