Gibbs phase rule use for wet vapour

In summary, the Gibbs phase rule states that the number of independent intensive properties required to specify the state of a system is equal to the number of components minus the number of phases plus two. For a water and water vapor system, there is one component (water) and two phases (liquid and vapor), resulting in one degree of freedom.However, to completely fix the state of the system, the dryness fraction (or the proportion of vapor in the mixture) must also be known. This may seem like an additional variable, but in the context of the phase rule, it is not considered as an independent property. This is because the dryness fraction is not a variable that can be arbitrarily chosen, but rather it is dependent on the temperature
  • #1
Ravi Singh choudhary
125
7
I have a doubt regarding gibbs phase rule in thermodynamics.. It says the number of independent intensive properties required to specify the state of a system is F=C-P+2 where C is number of components and P is the number of phase.. So for a water and water vapour system, C=1, P=2 . So F=1. If we specify the temperature,pressure is fixed. But inorder to completely fix the state of the system, the dryness fraction should also be known. As the dryness fraction determines specific volume of the system. So 2 variables are required, which is not correct according to Gibbs phase rule. Am i making any mistake in this? Please help
 
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  • #2
Ravi Singh choudhary said:
dryness fraction
Not a variable in the sense you are using it.
 
  • #3
Bystander said:
Not a variable in the sense you are using it.
How? I can have different specific volumes based on different dryness fraction.
 
  • #4
Ravi Singh choudhary said:
I can have different specific volumes based on different dryness fraction.
Can you?
 
  • #5
Bystander said:
Can you?
Yeah in in Pv diagram of water I have series of points on the horizontal line inside the vapor dome.
 
  • #6
For application of the phase rule vapor is saturated or it is not; you don't have the option of looking at the average density of "wet steam." That is not a variable you can arbitrarily include in the degrees of freedom.
 
  • #7
Bystander said:
For application of the phase rule vapor is saturated or it is not; you don't have the option of looking at the average density of "wet steam." That is not a variable you can arbitrarily include in the degrees of freedom.

I am getting your point but somehow I am not satisfied, can you please elaborate espicially your density logic.
 

1. What is the Gibbs phase rule used for?

The Gibbs phase rule is used to determine the number of degrees of freedom of a system in equilibrium. It helps determine the number of variables that can be independently varied while maintaining equilibrium.

2. How is the Gibbs phase rule applied to wet vapor systems?

The Gibbs phase rule can be applied to wet vapor systems by considering the number of components, phases, and degrees of freedom. The rule states that for a system with C components and P phases, the degrees of freedom (F) is given by F = C - P + 2. This means that in a wet vapor system with two components and one phase, the degrees of freedom will be equal to 3.

3. Can the Gibbs phase rule be used for both liquid and vapor phases?

Yes, the Gibbs phase rule can be used for both liquid and vapor phases. It is a general rule that applies to systems in equilibrium, regardless of the phase of the components.

4. Is the Gibbs phase rule applicable to non-ideal systems?

Yes, the Gibbs phase rule can be applied to non-ideal systems. However, in non-ideal systems, the number of degrees of freedom may not correspond to the actual number of variables that can be independently varied due to interactions between components.

5. How is the Gibbs phase rule used in practical applications?

The Gibbs phase rule is used in various practical applications, such as in chemical engineering, thermodynamics, and material science. It is used to determine the equilibrium conditions of a system and to design and optimize processes in various industries.

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