Thermodynamic properties of a liquid-gas mixture

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Discussion Overview

The discussion revolves around calculating the thermodynamic properties, specifically entropy and enthalpy, of a mixture consisting of 70% steam and 30% air at approximately 90 degrees Celsius and 10 MPa. The participants explore the implications of high pressure on the state of water and the mixture's behavior.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant inquires about calculating the thermodynamic properties of a steam and air mixture at specified conditions.
  • Another participant notes that the boiling point of water at 10 MPa is 311°C, suggesting that water will likely be in a liquid state at 90°C under these conditions.
  • There is a clarification regarding whether the 30% air refers to mole or mass fraction, with one participant confirming it is by mass fraction.
  • Concerns are raised about treating the mixture as an ideal gas mixture given the high pressure, with a focus on the need to consider it as a liquid-gas mixture instead.
  • Participants discuss the equilibrium vapor pressure of water vapor at 90°C, with one stating it is around 70,000 Pa (0.07 MPa).
  • Questions are posed about the mole fraction of water vapor in the gas phase and the mass fractions of air and water based on a 1 kg sample.
  • There is uncertainty expressed about the presence of water vapor at 10 MPa, with one participant questioning if all water vapor would condense into liquid.
  • A later reply suggests that while not all water vapor will condense, most of it will.

Areas of Agreement / Disagreement

Participants express differing views on the state of water at high pressure and whether it can be treated as a gas mixture or a liquid-gas mixture. The discussion remains unresolved regarding the exact behavior of the mixture under the specified conditions.

Contextual Notes

Participants highlight the importance of reference states for air and water in determining thermodynamic properties, as well as the implications of high pressure on phase states. There is an acknowledgment of the need for further clarification on the assumptions made regarding the mixture's behavior.

Rafe Zayed
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How can I calculate thermodynamic property like entropy and enthalpy of 70% steam and 30% air mixture ?( at about 90 degree Celsius and 10 MPa)
 
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The boiling point of water at 10 Mpa is 311°C
 
Rafe Zayed said:
How can I calculate thermodynamic property like entropy and enthalpy of 70% steam and 30% air mixture ?( at about 90 degree Celsius and 10 MPa)
Welcome to the PF.

This is a pretty useful website that you may find helpful in your learning: http://www.thermopedia.com/content/1150/

:smile:
 
Do you mean 30 mole %? Are you willing to assume that the mixture is an ideal gas mixture? What are the reference states of the air and water relative to which you desire to determine the entropy and enthalpy?
 
Chestermiller said:
Do you mean 30 mole %? Are you willing to assume that the mixture is an ideal gas mixture? What are the reference states of the air and water relative to which you desire to determine the entropy and enthalpy?
No I mean 30% by mass fraction.
And at this high pressure ,water will be liquid. So I am not sure whether I can treat them as gas mixture or liquid gas mixture.
Any suitable reference point will do.Basically I want to find the property of a liquid -gas mixture
 
What is the equilibrium vapor pressure of water vapor at 90 C?
 
Chestermiller said:
What is the equilibrium vapor pressure of water vapor at 90 C?
According to internet I found it to be around 70000 Pa
 
Rafe Zayed said:
According to internet I found it to be around 70000 Pa
Good. So that's about 0.07 MPa. If you're willing to tentatively approximate the gas phase as an ideal gas mixture, then that would be the partial pressure of water vapor in the gas phase. And at a total pressure of 10 MPa, what would be the mole fraction of water vapor in the gas phase? From these results, what would be the mass fractions of air and water in the gas phase? On the basis of 1 kg of water and air, what would be the mass of air? What would be the mass of water vapor in the gas phase? How much water would that leave for the liquid phase?
 
Chestermiller said:
Good. So that's about 0.07 MPa. If you're willing to tentatively approximate the gas phase as an ideal gas mixture, then that would be the partial pressure of water vapor in the gas phase. And at a total pressure of 10 MPa, what would be the mole fraction of water vapor in the gas phase? From these results, what would be the mass fractions of air and water in the gas phase? On the basis of 1 kg of water and air, what would be the mass of air? What would be the mass of water vapor in the gas phase? How much water would that leave for the liquid phase?
Sorry ,I didn't get that.At this pressure will there be any water vapour?Won't all the water vapour turn into liquid?
 
  • #10
Rafe Zayed said:
Sorry ,I didn't get that.At this pressure will there be any water vapour?Won't all the water vapour turn into liquid?
Not all, but most.
 

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