Fugacity: It is derived from Latin, expressed as fleetness or escaping ...

For binary mixture
P x P
1 1 1 V1
P
x
P
2 2 2 V 2
P T
T
P P
1
P x P
2
x
P
1 1 V1
2 2 V 2
P1
y1
PT
1x1P
V1
1x1P
V1
2 x2
PV
2
1
2 x2
PV
2
1
1x1P
V1
The above equation relates x and y for non ideal solution
Activity coefficients are functions of liquid composition x, many equations are available
to estimate them. The important equations are
Vanlaar equation
Wilson equation
Margules Equation
Vanlaar Equation:
Estimation of activity coefficient
2
Ax2
ln
1
2
A
x1
x2
B
ln
2
2
1
Bx
B
x1
x2
A
2
Where A and b are known as vanlaar constants, if the constants are known the
activity coefficients can be estimated.
Estimation of Vanlaar constants
1 st Method: If the activity coefficients are known at any one composition, then vanlaar
constants A and B can be estimated by rearranging the equation
A
x
ln
2 2
ln 1 1
x1
ln
1
2