Flash distillation, also called "equilibrium distillation", is a single-stage separation technique. Simple flash separations are very common in the industry, particularly petroleum refining. Even when some other method of separation is to be used, it is not uncommon to use a "pre-flash" to reduce the load on the separation itself.
Principle of Flash Distillation
The separation using
flash distillation is based upon flash vaporization. In flash vaporization hot
liquid mixture when passes from a high-pressure zone to a low-pressure zone
suddenly get vaporized. Reduced pressure reduces the boiling point which leads
to the vaporization of the liquid. The energy for vaporization is taken up from
the liquid itself, which causes a decrease in temperature. The molecules in the
vapour phase with low boiling points get condensed while high boiling point
molecules remain as vapour. The vapour and condensed liquid fraction remain in
contact till saturation. The liquid falls at the bottom and is collected whereas
vapours are further allowed to condense.
Construction of Flash Distillation
Flash distillation unit
consists of a pump attached to feed tank from where it pumps feed at high
pressure, Fig.1. This unit has a heating chamber through which the liquid
mixture carrying pipe passes. The chamber is insulated to avoid heat loss
during operation and maintain the desired temperature. There is a pressure
control valve fitted between the heating chamber and the flash drum. The other
end of the pipe directly opens into the flash drum. In a single-stage flash
distillation unit, a liquid outlet is provided at the bottom. In the case of
multi-stage distillation, the liquid and vapours are taken to the next unit for
further distillation. When designing a flash system it is important to provide
enough disengaging space in the flash drum. Flash drums can also be designed as
cyclone separators.
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| Fig.1: Flash Distillation |
Working of Flash Distillation
A liquid mixture feed is
pumped through a heater to raise the temperature and enthalpy of the mixture.
It then flows through a valve and the pressure is reduced, causing the liquid
to partially vaporize. Once the mixture enters a big enough volume, the
"flash drum", liquid and vapours get separated. Because the vapour
and liquid are in such close contact the "flash" occurs and the
product liquid and vapour phases approach equilibrium. Flash distillation can
be operated in continuous mode as well as at a multi-level. The operational
setting is synchronized in such a way that input equals the output of the
process and thus the vapour and liquid proportions at any instance remain
constant in the flash drum.
Application of Flash Distillation
- Flash distillation is used in the petroleum industry for refining crude oil.
- It is used in the desalination of ocean water by multi-stage flash distillation.
- It can also be used for the separation of heptane from octane.
Advantages of Flash Distillation
- Flash distillation is a continuous process.
- The equipment is smaller than the multi-stage flash distillation.
- The operating costs are low compared to multi-stage flash distillation.
Disadvantages of Flash Distillation
- Flash distillation is not effective in separating components of comparable volatility.
- It is not suitable for two-component systems.
- It is not an efficient distillation when nearly pure components are required, because the condensed vapour and residual liquid contain both components to some extent.
Make sure you also check our other amazing Article on : Methodology of Simple Distillation