1. Lecture # 26 Chapter 12 Gas-Liquid Equilibrium Non-ideal VLE
PETE 310
Lecture # 26
Chapter 12
Gas-Liquid Equilibrium
Non-ideal VLE

2. Gas-Liquid Equilibrium Non Ideal Behavior
Applications to low & high pressures
Use models for the equilibrium ratios (ki-values)
Simpler models ki = f (P,T)
More complex models
ki = f(P,T,compositions)
use equations of state EOS, convergence pressure

3. VLE general (same)
The equilibrium criteria between 2 phases a and b is,

4. Statement of Equilibrium
Most general case is
that Ki is a function of
P, T, and compositions 1 P 2 3 T

5. Mathematical Relationships
with
In general

6. Bubble Point Evaluation (Non-Ideal Behavior Model)
The bubble point pressure at a given T is

7. Bubble Point T s P 1
zi=xi s P 2 x ,y 1 1

8. Dew Point Calculation
At the dew point the overall fluid composition coincides with the gas composition. That is.

9. Dew Point Calculation (Non-Ideal Behavior Model)
Find DP pressure and equilibrium liquid compositions

10. Dew Point T s P 1 s P
zi=yi 2 x ,y 1 1

11. Flash Calculations In this type of calculations the objective is to:
find fraction of vapor vaporized (fv) and equilibrium gas and liquid compositions
given the overall mixture composition, P and T.

12. Flash Calculations (Non-Ideal Behavior Calculations)
Start with the equilibrium equation
Material balance

13. Flash Calculations
Now replace either liquid or gas compositions using Ki values (exercise derive the expression)

14. Flash Calculations
Objective function (flash function) is

15. Flash Calculations
There are several equivalent expressions for the flash function
(a)
(b)
(c)

16. One of the Models for Equilibrium Ratios
1. Require critical properties & acentric factors

17. Example calculations in excel file VLE_310