Flash or Equilibrium Distillation

Flash distillation or equilibrium distillation is normally carried out continuously. In this method a liquid mixture is partially vaporised, the vapour and liquid are allowed to attain equilibrium and finally withdrawn separately. 

Feed is heated in tubular heat exchange. The hot liquid mixture is then fed to a separator via a pressure-reducing valve whereby pressure is reduced and the vapour is formed at the expense of liquid adiabatically. The liquid is withdrawn from the bottom of the separator and the equilibrium vapour leaves the separator from the top which is then liquified in a condenser. Flash distillation is commonly used in the petroleum industry, handling multi-component systems in the pipe stills.

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Flash distillation diagram

Material Balance of Flash distillation

Let us be the fraction of feed that is vapourised and is of composition y thein (1 - y) will be the mole of residual liquid obtained let x be the mole fraction of more volatile component in liquid. Then a material balance of the more volatile component is given.

Flash distillation calculations

xF = fy + (1 - f) x

fy =  (1 - f) x + xF

y =  (1 - f) x/f + xF/f

y = - (1 - f) x/f + xF/f

Put = x = y, nf = - x + xf + xF

x = xF and y = xF

For F = feed totally vapourised 

Slope = - (1 - f)/f  = 0

For f = 0 no feed is vapourised then slope = 00

Thus these valves may be determined graphically using an x and y diagram. 

Definition of Equilibrium

It is a static condition in which the net transfer of material between the phases ceases for a given set of operating conditions that exist for all combinations of phases. Equilibrium refers to the absence of any tendency for a change to take place. 

Equilibrium stage 

It is the one in which the two phases not at equilibrium are brought into contact, time is provided to attain equilibrium, the phases are separated and streams leave the stage in equilibrium. For a given set of operating conditions, an equilibrium stage gives the maximum possible composition change, so it is also known as the ideal or theoretical stage. In actual practice, equilibrium is not achieved and hence stage efficiencies are always less than 100%.

Vapour - Liquid Equilibrium

The basic data for distillation are the equilibria existing between the vapour and liquid phases of a system under consideration. The equilibrium in the vapour-liquid system is governed by the phases rule. In a binary mixture subjected to distillation

We have C (component) = 2, and P (phase) = 2. So according to the phase rule [F = C-P + 2], the degrees of freedom F Or the number of intensive variables: temperature, pressure, vapour phase composition and liquid phase composition can be changed independently and temperature and vapour phase composition follow. 

Constant-Pressure Vapour-Liquid Equilibrium

The composition of the vapour and liquid phases that are in equilibrium is usually expressed in terms of mole fraction of the more volatile component in the respective phases (x and y) - we use mole fractions as a measure of concentrations.

Read Other.....

> What is differential distillation? 

> Dalton Law | Raoult's Law for distillation

>Fractionating Column | Rectification Or Batch Distillation

>Packed Columns/Towers for Distillation and  Gas Absorption 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane