Fractionation efficiency and operating pressure

[luca]

An effective way to improve a distillation tower fractionation efficiency is to reduce the tower’s operating pressure.
But what that reduction in operating pressure would cause? What are the other factors to be considered?

[tuan]

The statement "reduce pressure to improve fractionation efficiency" is not universally true.
What improves is primarily the thermodynamic ease of separation, because relative volatility often increases. But actual column efficiency also depends on tray/packing hydraulics, mass-transfer efficiency, flooding, weeping, entrainment, reflux ratio, feed condition, and pressure drop.

[jeem]

Yes. Reducing distillation-column operating pressure can improve fractionation, but it also changes several important operating parameters. The key reason is that lower pressure increases the relative volatility of many components, making separation easier.

Tray hydraulics can actually become more challenging. This is a very important distinction.
You can think of the effect as:
Lower pressure → better thermodynamic separation

but simultaneously:

Lower pressure → larger vapor volume → higher superficial vapor velocity → greater hydraulic limitations

So reducing pressure isn't automatically beneficial without checking the column hydraulics.

[ivani1]

Lower pressure leads to lower boiling temperature.
For example, a component that boils at 180°C at one pressure might boil considerably lower at reduced pressure.
That can be very beneficial when:

• Components are thermally sensitive

• Polymerization/decomposition is possible

• Heavy hydrocarbons are being separated

• You want to avoid excessive reboiler temperature

So vacuum distillation is essentially exploiting this principle.