Small distillation column
The small distillation column in the PMC labs is a pilot scale separations unit. This unit has 10 bubble-cap trays.The column is highly non-linear and presents a formidable challenge to linear control systems.
Operating Region
The small distillation column has a very narrow operating region. The exact constraints will depend on the specific base-layer setup. During the 2010 CML732 project the maximum plate 10 temperature that could be maintained while keeping column stable, was 81 degree Celsius. The maximum reflux IP transducer signal that could be implemented while still maintaining column stability was 14 mA. The exact IP signal will however depend strongly on the calibration of the IP transducer. A feed mixture of 30% ethanol provided the most operational flexibility.
Available Instrumentation and Final Control Elements
Several measurements are available on the column. Temperature measurements are available on all the trays. Differential pressure cells on the reboiler and feed drum can be used for level calculations. There are three control valves. The control valves are situated on the reflux, bottoms and top product lines. The reboiler is equipped with a robust electrical heating element.
Recent Improvements
A pump has been installed in the bottoms product line. The increase in head has dramatically improved the dynamic performance of the bottoms product control valve. Temporary insulation of the column with toilet paper and tin foil has increased the temperature obtained throughout the column with 12 degrees Celsius. It also offers the advantage of being hazard free and easily removable.
Operating Problems
Flooding is the biggest problem encountered during the column operation. Flooding usually start on the bottom tray and then propagates up to stage 1. Rusted bubble cap trays and faulty downcomers may be the factors responsible for this tendency. At high vapour flow rates, reversal of flow in the feed section is sometimes encountered.
Advanced Process Control
Several advanced process control software packages have been implemented on the column: Honeywell RMPCT (2010), Generic Simulink Based MPC (2010), Aspentech DMC Plus (2008) and Aspentech SmartStep (2008).
