Design and Control of Distillation Columns with Intermediate Reboilers

Distillation columns in which the components being separated have widely different boiling points feature large temperature differences between the condenser and reboiler. If cooling water is used in the condenser, the base temperature of this type of column is often quite high, requiring the use of expensive high-pressure steam. Energy costs can sometimes be reduced by using two reboilers. One at the base of the column uses high-pressure steam. A second at an intermediate tray in the stripping section can use lower-pressure steam because the column temperature is lower at that location. This paper compares the steady-state design and the dynamic control of a conventional single-reboiler distillation column with a column having both intermediate and base reboilers. Design involves determining the optimum heat removal in the intermediate reboiler, which affects both the column diameter and the rates of consumption of the two types of steam, having different costs. Control involves handling the large temperature difference in the column by the use of "average temperature" control and developing a control scheme to effectively use the additional control degree of freedom.