# Vapor Pressure and non ideal gases

Hi,

I understand that vapor pressure is independent of initial pressure, and depends only on temperature.

However, is this true of a non ideal gas at high pressures?

(I am specifically interested in the vapor pressure of a meg/water mixture at approx. 100 bar),

Thanks

What do you mean by initial pressure? Vapor pressure is a function of temperature and the concept is applicable to non-ideal gasses as well.

Chestermiller
Mentor
Hi,

I understand that vapor pressure is independent of initial pressure, and depends only on temperature.

However, is this true of a non ideal gas at high pressures?

(I am specifically interested in the vapor pressure of a meg/water mixture at approx. 100 bar),

Thanks

Hi Jamese. Welcome to Physics Forums!!!!!

What is meg, methylene glycol?

If you have a two component system, then the saturated vapor pressure of the gas phase is a function both of temperature and concentration of the liquid phase, even in the ideal gas region and even if the liquid is an ideal solution. Is this what you are dealing with here?

chet

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Thanks for the replies.

I will take a step further back...because I think I might understand even less than I originally thought!

I have a cylinder full of water and air at a particular temperature, if the air is saturated then the air has a particular water vapor pressure.

If I then compress the cylinder to a higher pressure whilst the temperature remains constant, is the vapor pressure of the water vapor still the same?

Thanks

Chestermiller
Mentor
Thanks for the replies.

I will take a step further back...because I think I might understand even less than I originally thought!

I have a cylinder full of water and air at a particular temperature, if the air is saturated then the air has a particular water vapor pressure.

If I then compress the cylinder to a higher pressure whilst the temperature remains constant, is the vapor pressure of the water vapor still the same?

Thanks
Yes.

OK thanks.

Is the vapor pressure directly related to a grammes/actual volume in the cylinder?
And is the value for grammes/actual volume the same for both cylinder situations?

Chestermiller
Mentor
OK thanks.

Is the vapor pressure directly related to a grammes/actual volume in the cylinder?

If you mean the partial pressure of the water vapor, then no, as long as there is any liquid water present in the cylinder.
And is the value for grammes/actual volume the same for both cylinder situations?
Yes.

I found a graph for air water mixtures.

At the lower end of the pressure curves the grams of vapor per actual volume remains constant, regardless of the pressure.

However, as pressure increase the lines start to curve, showing that this relationship is no longer constant and there is less grams of water vapor per actual volume,

What causes this?

Thanks

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Chestermiller
Mentor
I found a graph for air water mixtures.

At the lower end of the pressure curves the grams of vapor per actual volume remains constant, regardless of the pressure.

However, as pressure increase the lines start to curve, showing that this relationship is no longer constant and there is less grams of water vapor per actual volume,

What causes this?

Thanks
See if you can calculate the points on the graph using the ideal gas law and the vapor pressure of water at the different temperatures. This should be easy to do. I haven't gone over the graph in detail, but I do know that at pressures above ~10 atm for this system, you start to get deviations from the ideal gas law.

Chet

So for pressures less than 10 atm whilst this system behaves as an ideal gas, then the amount of water vapor that air can hold is constant (on an actual volume basis) and varies only as function of its temperature.

In the less than 10 atm situation is the partial water vapor pressure the same at all system pressures?

Thanks

I found a graph for air water mixtures.

At the lower end of the pressure curves the grams of vapor per actual volume remains constant, regardless of the pressure.

However, as pressure increase the lines start to curve, showing that this relationship is no longer constant and there is less grams of water vapor per actual volume,

What causes this?

Thanks

The graph you showed plots water mass per air mass (called the mixing ratio) . That's different than water mass per air volume (called absolute humidity). Also note that you shouldn't say "grams of vapor per actual volume". grams is a unit, not a physical quantity. The physical quantity is called mass.

OK thanks noted about the units.

I converted the mass per mass graph, to give the mass of vapor per actual volume.

This then showed that the mass of vapor per actual volume remains constant at varying system pressures less than 10 atm.
This is because the system is behaving as an ideal gas at pressures up to approx. 10 atm and at pressures above this real gas behavior comes into play (as Chester pointed out to me).

Forgive me for the probably repeating myself, but the baby steps approach really help with my understanding.

Next question; In the less than 10 atm situation is the partial water vapor pressure the same at all system pressures?

Thanks

Chestermiller
Mentor
Next question; In the less than 10 atm situation is the partial water vapor pressure the same at all system pressures?

Thanks
Yes. Roughly. But, beyond the ideal gas region, the concept of partial pressure loses some of its meaning. That's because, at higher total pressures, the mole fraction of water in the gas phase is no longer equal to the vapor pressure of pure water at the system temperature divided by the total pressure (except, of course, for single component water).

OK thanks.

In the low pressure ideal gas situation; does the partial pressure of the water vapor depend on the particular gas in the system?

Thanks

Chestermiller
Mentor
OK thanks.

In the low pressure ideal gas situation; does the partial pressure of the water vapor depend on the particular gas in the system?

Thanks
Well, air is essentially non-condensible. But, if you have a mixture of water and a soluble species
like ethanol, then there can be a substantial amount of ethanol in the liquid phase, and the partial pressure of water vapor will be less than its pure liquid vapor pressure.

Chet

OK thanks.

Am I correct in believing what you describe with respect a soluble species is described by Raoult's law?

Chestermiller
Mentor
Yes, for an ideal solution.

Chet

In two mails up you mentioned the solubility of something in the liquid phase affecting the partial pressure.

Will the solubility of a gas, for example carbon dioxide (in gaseous form) in water also effect the partial pressure of the water vapor?

Thanks

Chestermiller
Mentor
In two mails up you mentioned the solubility of something in the liquid phase affecting the partial pressure.

Will the solubility of a gas, for example carbon dioxide (in gaseous form) in water also effect the partial pressure of the water vapor?

Thanks
A little, but not much. Even air dissolving in water affects its vapor pressure slightly, but the solubility of air in water is very low (and thus air is usually considered a "non-condensible").

Chet

At higher pressures (100 bar plus) do we have to be more concerned with dissolved gasses, or is the effect still negligible?

If I have the vapor pressure of a vapor in a gas at atmospheric system pressure, or low system pressures where vapor pressure is a constant.
How do I then find the vapor pressures at higher system pressures, where the real gas effects come into effect?

Thanks for the continued assistance, I am finding this very informative!

Chestermiller
Mentor
You need to learn about the behavior of gases at pressures higher than the ideal gas region, and you need to learn about solution thermodynamics in this region. The concepts involved are the Gibbs free energy of a gas at higher pressure, the fugacity, the partial molar free energy (aka the chemical potential), the conditions of equilibrium for a multicomponent system, and the methods of estimating these quantities in a gas mixture.

Chet

OK it just got a lot more complicated!

The reason for this query is that I will be involved in the dewatering of a gas export pipeline, using a pig train propelled by export gas, operating at export pressure.
The water ahead of the pig train is separated from the export gas by several pigs, in between the pigs there will be slugs of neat MEG.

During the dewatering operation a trailing film of fluid is left behind the pigs, this means that the slugs of neat MEG dilute with water as they travel down the pipeline.
At the end of the operation there will be a film of MEG/water left on the pipewall, this film of MEG/water is required to not wet the export gas beyond its normal wetness.

(Simple dilution calculations will give the concentration of MEG/water left on the pipewall, dependent on the trailing film thickness and the sizing of the MEG slugs chosen) (residual MEG/water film better than 98% MEG is achievable)

I know the mass per volume of water normally in the gas under transport conditions, so I was hoping to be able to derive a relationship for the MEG/water concentration whereby I could say at a particular pressure in the pipeline the remaining MEG/water film would not wet the gas.

However, based on your good advice it looks as though this is not going to be easily calculable (by me) and I am going to have to seek assistance from someone specialized in this, that is unless I can make some conservative assumptions?

Thanks

Chestermiller
Mentor
OK it just got a lot more complicated!

The reason for this query is that I will be involved in the dewatering of a gas export pipeline, using a pig train propelled by export gas, operating at export pressure.
The water ahead of the pig train is separated from the export gas by several pigs, in between the pigs there will be slugs of neat MEG.

During the dewatering operation a trailing film of fluid is left behind the pigs, this means that the slugs of neat MEG dilute with water as they travel down the pipeline.
At the end of the operation there will be a film of MEG/water left on the pipewall, this film of MEG/water is required to not wet the export gas beyond its normal wetness.

(Simple dilution calculations will give the concentration of MEG/water left on the pipewall, dependent on the trailing film thickness and the sizing of the MEG slugs chosen) (residual MEG/water film better than 98% MEG is achievable)

I know the mass per volume of water normally in the gas under transport conditions, so I was hoping to be able to derive a relationship for the MEG/water concentration whereby I could say at a particular pressure in the pipeline the remaining MEG/water film would not wet the gas.

However, based on your good advice it looks as though this is not going to be easily calculable (by me) and I am going to have to seek assistance from someone specialized in this, that is unless I can make some conservative assumptions?

Thanks
I don't know. I personally would have to think about this for a while before I got a sense of how I would approach it. You might be able to bound the answer if the MEG is more volatile than water (I don't know what the properties of pure MEG are).

Chet