(102e) Improved Debutanizer Quality Control Using Weighed Average Temperature

Karl-Martin Hansson¹, Nick Alsop, Linda Fredriksson

PREEM AB, Preemraff Lysekil SE-453 81, SWEDEN

Abstract

This article describes a successful implementation of a control scheme based on weighted average temperature for improving the stability of product qualities in a debutanizer at Preem Refinery in Lysekil Sweden.

The original control scheme for the 30 tray debutanizer tower was based on a temperature controller using the tray 8 temperature to control the split between C_4- and naphtha. The control never worked and for many years the top LPG-draw flow rate was controlled manually by the control room operators. The poor control of the cut-point thus achieved, resulted in on average, fairly high concentrations of pentane in the LPG and of butane in the naphtha, even though separation in the tower was good at all times.

The reason the original control scheme failed to control the cut-point was due to the highly non-linear relationship between tray 8 temperature and the flow used to control the temperature. Tray 8 temperature is highly sensitive to pentane in the top, but insensitive to if the LPG-draw is much too low at which times significant amounts of butane end up in the bottom of the tower.

It was noted by control room operators that the temperature on tray # 24 is very sensitive to the concentration of butane in the bottom. Even so, the temperature on tray # 24 is not suited for cut-point control since it is not very sensitive to if the LPG-draw is too large. Therefore neither temperature is suitable for cut-point control.

The solution to the control problem was to use a calculated control temperature that weighted the temperatures on tray # 8 and tray # 24. The temperature on tray # 8 picks up if the LPG-draw is too large whereas the temperature on tray # 24 picks up if the LPG-draw is too small. Using the calculated control temperature, it is possible to attain concentrations of pentane in the LPG and of butane in the naphtha that are lower than before, using a lower reboiler duty than was used before the control system was taken in service.