(172e) Distillation Control: Practical Aspects

One of the most important aspects in implementing, commissioning or troubleshooting distillation control strategies resides in the recognition that modifications to existing equipment are improbable. This paper illustrates several cases where changes to original designs were made in order to be able to complete a commissioning or resolve long standing issues related to distillation control in industrial columns and fractionators.

Some of the cases, including the proposed and implemented solutions to be presented are:

• Benzene tower: First, the measurement of the temperature difference by using the difference calculation of two precision RTD’s is not a substitution of a straight differential temperature transmission. And second, the direct relationship between the differential temperature intended to measure impurities at the bottom of the tower became indirect upon increasing of the energy input to the column, making impossible to use it as a reliable composition surrogate.
• Toluene tower: The bottom control tray Temperature vs. Composition relationship intended for the control of toluene impurities at the bottom of column turned out to be parabolic instead of logarithmic, preventing its use in any type of control strategy.
• Deisobutanizer: This bottom sidestream drawoff tower was designed with an overhead accumulator level control done through the sequencing of three valves, which could not work and had the operators continuously changing the valves opening to keep the drum from flooding or running empty. The selected flowmeters are not reliable for this type of application. The tower base level control was attempted with the bottom product that lacks the rangeability required to close the tower material balance.
• Atmospheric crude tower: This tower turned out to be the cause of the frequent oscillatory behaviour in the crude unit, all caused by the pressure control paired to the wrong manipulated variable.
• Reformate Splitter: This tower pressure control was initially designed to manipulate a nitrogen purge/vent to flare split range controller in the overhead accumulator, however the tower was subject to frequent disturbances caused by ambient conditions variations.