PPT: Vapour - Liquid Equilibria Mechanical Engineering Notes | EduRev

Thermodynamics

Mechanical Engineering : PPT: Vapour - Liquid Equilibria Mechanical Engineering Notes | EduRev

 Page 1


OUTLINE 
1. The Nature of Equilibrium 
2. Duhem’s Theorem 
3. Simple Models for VLE 
4. VLE by Modified Raoult’s Law 
5. VLE from K-value Correlations 
Page 2


OUTLINE 
1. The Nature of Equilibrium 
2. Duhem’s Theorem 
3. Simple Models for VLE 
4. VLE by Modified Raoult’s Law 
5. VLE from K-value Correlations 
1. The Nature of Equilibrium 
• Equilibrium is a static condition in which no 
changes occur in the macroscopic properties 
of a system with time. 
– Eg: An isolated system consisting of liquid & vapor 
phase reaches a final state wherein no tendency 
exists for change to occur within the system. The 
temperature, pressure and phase compositions 
reach final values which thereafter remain fixed. 
 
Page 3


OUTLINE 
1. The Nature of Equilibrium 
2. Duhem’s Theorem 
3. Simple Models for VLE 
4. VLE by Modified Raoult’s Law 
5. VLE from K-value Correlations 
1. The Nature of Equilibrium 
• Equilibrium is a static condition in which no 
changes occur in the macroscopic properties 
of a system with time. 
– Eg: An isolated system consisting of liquid & vapor 
phase reaches a final state wherein no tendency 
exists for change to occur within the system. The 
temperature, pressure and phase compositions 
reach final values which thereafter remain fixed. 
 
The Nature of Equilibrium 
• At microscopic level, conditions are not static. 
– Molecules with high velocities near the interface 
overcome surface forces and pass into the other 
phase. 
– But the average rate of passage of molecules is 
the same in both directions & no net interphase 
transfer of material occurs. 
 
Page 4


OUTLINE 
1. The Nature of Equilibrium 
2. Duhem’s Theorem 
3. Simple Models for VLE 
4. VLE by Modified Raoult’s Law 
5. VLE from K-value Correlations 
1. The Nature of Equilibrium 
• Equilibrium is a static condition in which no 
changes occur in the macroscopic properties 
of a system with time. 
– Eg: An isolated system consisting of liquid & vapor 
phase reaches a final state wherein no tendency 
exists for change to occur within the system. The 
temperature, pressure and phase compositions 
reach final values which thereafter remain fixed. 
 
The Nature of Equilibrium 
• At microscopic level, conditions are not static. 
– Molecules with high velocities near the interface 
overcome surface forces and pass into the other 
phase. 
– But the average rate of passage of molecules is 
the same in both directions & no net interphase 
transfer of material occurs. 
 
Measures of Composition 
1. Mass fraction: the ratio of the mass of a particular chemical 
species in a mixture or solution to the total mass of mixture or 
solution. 
 
 
 
2. Mole fraction: the ratio of the number of moles of a 
particular chemical species in a mixture or solution to the number 
of  moles of mixture or solution. 
m
m
m
m
x
i i
i
?
?
? ?
n
n
n
n
x
i i
i
?
?
? ?
Page 5


OUTLINE 
1. The Nature of Equilibrium 
2. Duhem’s Theorem 
3. Simple Models for VLE 
4. VLE by Modified Raoult’s Law 
5. VLE from K-value Correlations 
1. The Nature of Equilibrium 
• Equilibrium is a static condition in which no 
changes occur in the macroscopic properties 
of a system with time. 
– Eg: An isolated system consisting of liquid & vapor 
phase reaches a final state wherein no tendency 
exists for change to occur within the system. The 
temperature, pressure and phase compositions 
reach final values which thereafter remain fixed. 
 
The Nature of Equilibrium 
• At microscopic level, conditions are not static. 
– Molecules with high velocities near the interface 
overcome surface forces and pass into the other 
phase. 
– But the average rate of passage of molecules is 
the same in both directions & no net interphase 
transfer of material occurs. 
 
Measures of Composition 
1. Mass fraction: the ratio of the mass of a particular chemical 
species in a mixture or solution to the total mass of mixture or 
solution. 
 
 
 
2. Mole fraction: the ratio of the number of moles of a 
particular chemical species in a mixture or solution to the number 
of  moles of mixture or solution. 
m
m
m
m
x
i i
i
?
?
? ?
n
n
n
n
x
i i
i
?
?
? ?
Measures of Composition 
3. Molar concentration: the ratio of the mole fraction of a 
particular chemical species in a mixture or solution to the molar 
volume of mixture or solution. 
 
 
 
4. Molar mass of mixture/solution: mole-fraction-
weighted sum of the molar masses of all species present. 
q
n
V
x
C
i i
i
?
? ?
i
i
i
M x M
?
?
?
i
n
?
? q
Molar  flow rate 
Volumetric flow rate 
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