Question

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Fundamentals of Instrumentation & Controls

Distillation is a process of continuous boiling and condensing used to separate mixtures of different fluids. A practical example of distillation is the separation of crude oil into multiple gases and liquids (propane, butane, naphtha, hexane, etc.). Components with higher boiling points (typically denser fluids) collect at the bottom of the tower, while components with lower boiling points (typically lighter fluids) and gases collect at the top of the tower.

In this distillation tower, the levels of liquid products at the bottom and top are being controlled. Explain how the control loops work

Answer 1

There are many different ways of controlling the tops composition. Three methods are described below.

• Reflux Rate

  • In this reflux rate is adjusted to control the composition of the tops product. As the amount of reflux is changed so the temperature profile in the column changes and hence the composition.

• Reflux Ratio
  • The second example uses the reflux ratio as the control parameter. When designing a distillation column it is usually the reflux ratio that is determined. This can be kept constant throughout operation by using two flow indicators and a ratio controller.
• Distillate Rate
  • The third example is for high purity tops. It uses the distillate flowrate to control the distillate composition. It can be shown that for a high purity column i.e. one with a large reflux, that the composition of the distillate is sensitive to the distillate flow but insensitive to the reflux rate. Therefore for a high purity column the control scheme outlined above is used. It should be noted that tight control on the level in the reflux drum is required using the reflux rate.