VLE

What is equilibrium condition

At equilibrium no driving force for energy or mass transfer .

Raoult’s law

p1=x1.p10

Where,p1-partial pressure of component p1

0-vapour pressure of pure component x1 -mole fraction of component in liquid phase

(a) a solution that shows positive deviation from Raoult's law

(b) a solution that shows negative deviation fromRaoult's law.

Dalton’s law

Ptotal = p1+ p2

Where,Ptotal -Total pressure p1 ,p2 -Partial pressure of component.

Composition of liquid phase :x1= p1/ p1

0

= partial pressure of component /vapour pressure of pure component

Composition in gaseous phase :y 1= p1/ P

= partial pressure of component /total pressure

Raoult’s law becomes

yiP=xiPisat (i=1,2,3…..n)

Where: yi = Vapor-phase mole fraction. xi = Liquid-phase mole fraction. Pi = Vapor pressure of pure species i at the T of the system. yi P = Partial pressure of species i.

VLE by Modified Raoult’s Law

yi P = xi γi Pisat ( i= 1,2,…..,N)

Where γi = Activity coefficient.

T-x-y diagram/boiling point diagram

Effect of pressure on VLE The high pressure diagrams are above low pressure.

Equilibrium DiagramVapor composition is drawn against liquid comp at constant pressure. Vapour is always rich in more volatile component the curve lies above the diagonal line.

Effect of varying pressure on equilibrium diagram

P-x-y Diagram

Fugacity: It is derived from Latin, expressed as fleetness or escaping tendency. It is usedto study extensively phase and chemical reaction equilibrium.

G = RT ln f +ɵ

ɵ - Is an constant depends on temperature and nature of gas.

fugacity has same units as pressure for an ideal gas.

Fugacity coefficient (ɸ) : Fugacity coefficient is defined as the ratio of fugacity

of component to its pressure.

ɸ=f/P

ɸ- Is the measure of non ideal behavior of the gas.

Activity(a):It is defined as the fugacity of the existing condition to the standard state fugacity

a=f/ fo

Activity coefficient :It measure the extent to which real solution depart from

ideality.lnγ₁=bx₂²lnγ₂=bx₁²

Activity coefficient equations

Wohl’s three suffix equation Margules equation Van laar equation Wilson equation Non random two liquid (NRTL)equation Universal quasi –chemical (UNIQUAC)equation Universal functional activity coefficient (UNIFAC)

method

Consistency test for VLE data

Using slope of lnγ curves Using data at mid point Redlich –kister method Using the coexistence equation Using partial pressure data

Thank You