Distillation Column Control Design

The distillation column takes advantage of a separation process that converts a mixture of substances into two or more distinct products mixtures (fractions)

In general, the separation process is based on differences in chemical properties or physical properties between the constituents of a mixture.

In the distillation column the separation process is based on the different boiling points of the mixtures.

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Related concepts:

Boiling point:  When a liquid boils, its vapor pressure equals the surrounding pressure and changes into vapor.

Vapor pressure: Is the equilibrium of the pressure exerted by molecules leaving and entering the liquid surface, at a given temperature.

Relative volatility: The measure of the differences in volatility between components. Compares vapor pressures of the components in a mixture.

Heat exchange: this method transfers the heat from a fluid to another fluid without having to mix together or having a direct contact.

Separation by flashing: Is the separation of fluids that uses a flash drum under certain temperature and pressure.

Distillation: The objective of this method is to separate or purify the liquid, it consists on heating the liquid to vapor (boiling point) and condensing the vapor back to liquid. This separation can be done by the volatility differences between the components of the mixture. To condense the vapor to liquid normally is used a series of trays, the liquid from each tray reach the equilibrium with the vapor, then the vapor rises up to the next tray and the liquid falls to the tray bellow it. The components of the mixture with lower boiling point will exit at the top of the tower and the components with the higher boiling point will fall to the bottoms.

The vapor leaving the distillation tower is sent to a condenser or heat exchanger, part of the liquid on the condenser returns to the column as a reflux and the other part is the product or distillate. The vapor is also a distillate product.

The reboiler is a heat exchanger connected to the bottom of the tower, its function is to heat the liquids leaving the tower and the vapor generated of this heating is send it back to the tower. The rest of the liquid is a product called residue.

Trays of distillation column

Distillation Column Control Design

The typical control strategy for distillation column consists on the master level controller on the tower that regulates the level of the product on the tray. This level controller sends the output to the set point of the secondary flow controller that regulates the fluid leaving the tower. If the level increases, then the flow control opens the valve reducing the liquid on the tower in order to reduce the level. 

Basic Distillation column controller

But what happen when the flow valve is completely open and the level continues increasing? This scenario could happen. To avoid a high level, we can send the extra product to storage tanks. To implement this solution, it is required another level controller that sends the output as a set point to other slave controller in order to open the valve to the storage tank and reduce the level in the tower. This level controller is going to open the storage valve just above certain limit or set point. We can add as many storage tanks we want and split the output.

Basic Distillation Column Controller with storage lines