Perspectives: Plant InSites

Distillation Guidelines: Draw Some Important Lessons

Measuring liquid level on a column draw tray demands care

By ANDREW SLOLEY, Contributing Editor

One way to measure liquid flow inside a distillation tower is to infer the flow rate from a liquid level. Figure 1 shows a typical installation. A partial side draw takes a product from a sump on a draw tray while the remaining liquid overflows a weir and continues down the tower. The level measured on the draw tray goes to the plant’s advanced control system, which uses the value in calculating the internal liquid flow rate. In this case, the purpose of obtaining the internal liquid rate was to ensure the trays below the liquid draw had sufficient liquid for proper operation. Trays have a mechanical limit on liquid rates. Below that limit, efficiency may drop dramatically. The tray type and geometry as well as the physical properties of the system determine the minimum acceptable liquid rate. The plant found controlling the distillation specifications of the side-product draw extremely difficult. Purity, especially of heavy components, would vary dramatically for the same calculated liquid internal rate. (Sudden spikes in heavy components in the side draw are strong signals that the internal liquid rate is too low.)

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