Main fractionator crude switch control

Abstract

The Kalundborg Refinery is operating 10–12 different crudes on a regular basis and as a consequence of limited storage capacity the refinery is switching crude every 3–4 days. Normally the feed to the crude fractionator is a mixture of different crudes and therefore prediction of distillation curves and specific gravities is difficult resulting in big offsets in sidestream qualities during crude switches. Also stratification caused by poor mixing in the crude tanks will affect tower controls. In order to cope with differences in feed quality and keep product streams on specifications during crude switches the refinery has implemented an online advanced control scheme automatically adjusting product flow rates and pumparound duties on the the main crude fractionator. The control scheme is based on an on-line estimation of the feed TBP(True Boiling Point) curve as changes in feed volatility is detected by a heat balance around the tower. Changes in feed qualities are then fed forward to the sidestream quality controllers significantly improving the qualities during crude switches. In addition the fractionator is supplied with the conventional control schemes, i.e. sidestream quality controls, downwards decoupling for interactions between quality controllers and feed rate feedforward. © 1992, Pergamon Press Ltd. All rights reserved.