1. THERMODYNAMICS OF SEPARATION OPERATIONS
Aseotropes
The increased repulsion between molecules can result in the formation of an azeotrope, which is a liquid mixture whose equilibrium vapor has the same composition as the liquid ( i.e. xi = yi for an azeotrope).
Minimum-Boiling Homogeneous Azeotropes:
This type of azeotropes occurs due to repulsion between the molecules
ChE 334: Separation Processes
Dr Saad Al-Shahrani

2. THERMODYNAMICS OF SEPARATION OPERATIONS
subcooled
vapor
Pxy diagram
(T constant)
> 1.0
(+) deviation
from ideality
Txy diagram
P constant
subcooled
vapor
ChE 334: Separation Processes
Dr Saad Al-Shahrani

3. THERMODYNAMICS OF SEPARATION OPERATIONS
xy diagram,
ether P or T= constant
X=y
45 o line
ChE 334: Separation Processes
Dr Saad Al-Shahrani

4. THERMODYNAMICS OF SEPARATION OPERATIONS
b) Maximum-Boiling azeotropes
This type of azeotropes occurs due to attraction between the molecules.
Txy diagram
Pxy diagram
< 1
(-) deviation
from ideality
ChE 334: Separation Processes
Dr Saad Al-Shahrani

5. THERMODYNAMICS OF SEPARATION OPERATIONS
xy diagram
x=y
ChE 334: Separation Processes
Dr Saad Al-Shahrani

6. THERMODYNAMICS OF SEPARATION OPERATIONS
Example:
Ethanol and n-hexane from a minimum boiling point azeotrope at 3.2 mole% ethanol at oC and 760 mmHg pressure. The vapor pressure of ethanol and n-hexane are 6 psia and 12 psia respectively, at oC, determine iL for ethanol and n-hexane at the azeotropic condition
solution
At azeotrope x=y
For methanol
ChE 334: Separation Processes
Dr Saad Al-Shahrani

7. THERMODYNAMICS OF SEPARATION OPERATIONS
For methanol
Note: foe ethanol and n-hexane L> 1.0, indicating repulsion (positive deviation from ideality
ChE 334: Separation Processes
Dr Saad Al-Shahrani

8. THERMODYNAMICS OF SEPARATION OPERATIONS
DePriester Charts For Light Hydrocarbons
Figures (a,b) give K-value charts for some Iight hydrocarbons. These arts do not assume ideal vapor-phase behavior. Some corrections for pressure effects are included.
Figure (a) is used for low temperatures and Figure (b) high temperatures.
To find the appropriate K-values, a straight line is drown on the diagram connecting the temperature and pressure of the system. intersection of this line with the K-value curve for each hydrocarbons its K-value at this temperature and pressure.
ChE 334: Separation Processes
Dr Saad Al-Shahrani

11. THERMODYNAMICS OF SEPARATION OPERATIONS
RELATIVE VOLATILITY
The relative volatility is the ratio of K-values
For two component j and k
If the system is ideal (i,e. obeys Raoult’s law, i.e. no attraction or repulsion between molecules or =1.0)
ChE 334: Separation Processes
Dr Saad Al-Shahrani

12. THERMODYNAMICS OF SEPARATION OPERATIONS
For component j
For component k
ChE 334: Separation Processes
Dr Saad Al-Shahrani

13. THERMODYNAMICS OF SEPARATION OPERATIONS
Relative volatility for binary system
For two components system under equilibrium conditions (j,k):
Solve for yj
This equation is very important in distillation operation
ChE 334: Separation Processes
Dr Saad Al-Shahrani

14. THERMODYNAMICS OF SEPARATION OPERATIONS
Relative volatilities (are essentially constant. In general, they are functions of temperature and composition.
jk= f ( T and composition)
In most systems, () decreases as temperature increases, which means that separation of components becomes more difficult.
Therefore, It is often desirable to keep temperatures as low as possible (use low pressure) to reduce energy consumption.
The following figure shows some VLE curves on an xy diagram for various values of .
The bigger the relative volatility, the fatter the VLE curve and the easier the separation (low number of stages required).
ChE 334: Separation Processes
Dr Saad Al-Shahrani

15. THERMODYNAMICS OF SEPARATION OPERATIONS
As  → 1.0, the VLE curve approaches the 45o line x = y.
It is impossible to separate components by distillation if the value of  is too close to unity. Distillation is seldom used if  < X
ChE 334: Separation Processes
Dr Saad Al-Shahrani

16. THERMODYNAMICS OF SEPARATION OPERATIONS
Relative volatility For a multicomponents system.
For a multi-component system, the relative volatilities are defined with respect to some component, typically the heaviest one.
If we have multi-components system containing components (1,2,3, H), H is the heaviest one and (1) is the lightest one.
ChE 334: Separation Processes
Dr Saad Al-Shahrani

17. THERMODYNAMICS OF SEPARATION OPERATIONS
By the same manner . .
ChE 334: Separation Processes
Dr Saad Al-Shahrani

18. THERMODYNAMICS OF SEPARATION OPERATIONS
ChE 334: Separation Processes
Dr Saad Al-Shahrani

19. THERMODYNAMICS OF SEPARATION OPERATIONS
Substitute (6) in (4)
ChE 334: Separation Processes
Dr Saad Al-Shahrani

20. THERMODYNAMICS OF SEPARATION OPERATIONS
Example:
A multi-component liquid mixture has the compositions and relative volatilities given in the table below. Calculate the composition of the vapor phase.
ChE 334: Separation Processes
Dr Saad Al-Shahrani

21. THERMODYNAMICS OF SEPARATION OPERATIONS
Vap. yi
V, mol/h
Liq. xi
L, mol/h
The lever rule
F = L + V F zi
zi F = xi L + yi V
ChE 334: Separation Processes
Dr Saad Al-Shahrani

22. THERMODYNAMICS OF SEPARATION OPERATIONS
The ratio of the product flows (L,V) is the inverse of the ratio of the lengths of the lines connecting the feed mole fraction of each of the products. This is known as ”Lever Rule”
T2sat y x T
Temperature
T1sat xi zi yi
1.0
Note: the two phases must be under equilibrium conditions
ChE 334: Separation Processes
Dr Saad Al-Shahrani