Flash Distillation

Flash distillation is defined as a process in which the entire liquid mixture is suddenly vaporized (flash) by passing the feed from a high pressure zone to a low pressure zone. Flash distillation is also known as equilibrium distillation, i.e., separation is attempted when the liquid and vapour phases are in equilibrium. This method is frequently carried out as a continuous process.

Principle

• When a hot liquid mixture is allowed to enter from a high-pressure zone into a low-pressure zone, the entire liquid mixture is suddenly vaporised. This process is known as flash vaporisation. During this process the chamber gets cooled.

• The individual vapour phase molecules of high boiling fraction get condensed, while low boiling fraction remains as vapour.

Working

• The feed is pumped through a heater at a certain pressure. The liquid gets heated, which enters the vapour-liquid separator through a pressure-reducing valve. Due to the drop in pressure, the hot liquid flashes, which further enhances the vaporisation process.

• The sudden vaporisation induces cooling. The individual vapour phase molecules of high boiling fraction get condensed, while low boiling fraction remains as vapour. The mixture is allowed for a sufficient time, so that vapour and liquid portions separate and achieve equilibrium.

• The vapour is separated through a pipe from above and liquid is collected from the bottom of the separator. By continuously feeding into the still, it is possible to obtain continuous flash distillation.

• The operating conditions can be adjusted in such a way that the amount of feed exactly equals the amount of material removed. Therefore, vapour and liquid concentrations at any point remain constant in the unit.

Uses: Flash distillation is used for separating components, which boil at widely different temperatures. It is widely used in petroleum industry for refining crude oil.

Advantages: Flash distillation is a continuous process.

Disadvantages: Flash distillation is not effective in separating components of comparable volatility. It is not an efficient distillation when nearly pure components are required, because the condensed vapour and residual liquid are far from pure.