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Vapor Pressure Calculation


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#1 Bodhisatya

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Posted 15 October 2013 - 11:17 AM

Hello All

 

Needed a help regarding Vapor Pressure Calculation of Nitrogen,Oxygen and Argon.

 

Can you people let me know what should be the methodology.

 

As per as the Antoine Equation is concerned ,it says the co-efficient values are valid only over a certain range of Temperature Tmin to Tmax. What if I want to calculate the vapor Pressure at a Temperature ,other than the prescribed Range.

 

Bodhisatya. 



#2 MrShorty

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Posted 15 October 2013 - 11:27 AM

Because it only uses 3 parameters, the Antoine equation cannot accurately represent the complete vapor pressure curve of a substance (from melting point to critical point). The published Tmin and Tmax associated with the parameters you are given likely reflect that consideration. Extrapolating beyond the published Tmin and Tmax compromises the accuracy of the equation. The calculated values may be accurate enough for what you are doing, but the publisher believes there would be better equations available.

 

The usual methodology for calculating vapor pressures outside of the range of the Antoine equation is to use a different vapor pressure equation and published parameters for that equation. Two common equations are the Riedel equation and the Wagner equation.

 

Recognize that, no matter what equation you use, vapor pressure officially applies only between the melting point and the critical point.



#3 PingPong

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Posted 15 October 2013 - 01:47 PM

What if I want to calculate the vapor Pressure at a Temperature ,other than the prescribed Range.
Depends on what range you have in mind. There are many equations floating around.

 

I suggest you use Appendix A of the famous book The Properties of GASES AND LIQUIDS



#4 Bodhisatya

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Posted 16 October 2013 - 10:13 AM

 

Recognize that, no matter what equation you use, vapor pressure officially applies only between the melting point and the critical point.

 

Dear Sir 

 

Here lies the confusion.

 

Lets us for instance consider Nitrogen

 

It's Melting Point is (-)146.96 DegC and Triple Point is (-)210 DegC. Does this mean that Vapor Pressure can be calculated within that mentioned Temperature range.

 

But I have came across datas where Nitrogen Vapor Pressure at ambient Temperatures and above have been calculated.How was it done.

 

Please help me in correcting my understandings.

 

Bodhisatya.

 

 

Bodhisatya.



#5 MrShorty

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Posted 16 October 2013 - 12:57 PM   Best Answer

I think you have mislabeled your "melting point". that should be the critical point.

 

Perhaps it is necessary to more carefully define terms. Unqualified, the term vapor pressure is usually taken to mean "the pressure exerted by a pure vapor/gas in equilibrium with its liquid phase." Sometimes this is extended to solids, though it is also common to use the term sublimation pressure to distinguish vapor-solid equilibrium from vapor-liquid equilibrium. Because the liquid phase only exists between the melting point and critical point, vapor pressure only has physical meaning between those two temperatures. Here's an introductory discussion of pure component phase diagrams. What we usually call "vapor pressure" is the curve connecting the triple point and the critical on these kind of phase diagrams. http://www.chemguide...phasediags.html

 

Someone who uses the term "vapor pressure" in reference to nitrogen at room temperature is playing rather loose and careless with the terminology. The first step in understanding what he means is to get him to either define what he means by "vapor pressure" or get him to use a better term that better describes his meaning.

 

Perhaps he really means "extrapolated" or "hypothetical" vapor pressure of pure nitrogen. What is the purpose for this extrapolation? What equation is wanted, because each vapor pressure equation is going to give a different extrapolation that far removed from the critical point?

 

Or perhaps he is really referring to "partial pressure" of nitrogen over some solution -- more of a "how soluble is nitrogen in solvent x at room temperature?". If this is what is meant, then we would be looking at Henry's law or other mixture VLE type calculation, which is much more complex than simply using the Antoine equation.

 

Until we know what is meant by "vapor pressure nitrogen at ambient temperatures and above," I'm not sure we can really explain how the calculations could have been performed.


Edited by MrShorty, 16 October 2013 - 12:58 PM.


#6 PingPong

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Posted 16 October 2013 - 12:58 PM

It's Melting Point is (-)146.96 DegC and Triple Point is (-)210 DegC
Critical point (not melting point) is -146.96 oC.

 

Nitrogen has a vapour pressure below  -146.96 oC.

 

At ambient temperature it does not have a vapour pressure because that is above the critical temperature and there can not be an equilibrium between liquid and vapour nitrogen.

It can always have a partial pressure but that is something else. For example: ambient air has a nitrogen partial pressure of about 79 kPa.



#7 Bodhisatya

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Posted 17 October 2013 - 10:08 AM

 

 

Critical point (not melting point) is -146.96 oC.

 

Nitrogen has a vapour pressure below  -146.96 oC.

Sorry for the mistake.U are right sir.

 

Thank you sir for Correcting me and explaining the basics.

 

I obtained an excel sheet from this forum posted by one of the member where he has calculated the Vapor pressure of wide range of Components including Nitrogen and the range of  Chosen Temperature  is 0 DegC to 250 DegC  that too using Antoine Co-efficient.

 

Can you please look into that and check where from those Co-efficient values were obtained as the specified Temperatures doesn't fall within the Tmin to Tmax range. 

 

As understood the calculated value can never be a Vapor Pressure.Is the term used a misnomer.

 

Bodhisatya.

Attached Files


Edited by Bodhisatya, 17 October 2013 - 10:10 AM.


#8 MrShorty

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Posted 17 October 2013 - 11:09 AM

Hopefully this won't be too harsh, but you are reading way too much into a free spreadsheet that someone here programmed.

 

I expect that what you are seeing is merely because the programmer of this spreadsheet assumed that any user of this spreadsheet would understand the concept of vapor pressure well enough to know what should be reasonable values for input temperature for the compound said user selects.

 

Again, I don't mean to offend, but I think this is a perfect example of something my mentor used to talk about. He was always concerned that engineers would become so "trusting" of computers that they would forget how to critically evaluate what the computer does. In your case, I'm guessing you assumed that the spreadsheet programmer would have prevented the spreadsheet from calculating unreasonable values, so you started looking for physical meaning in anything this spreadsheet calculates. I expect the spreadsheet programmer expected you, the user, to be able to identify when the values calculated by the spreadsheet would be unreasonable or meaningless.

 

As a bonus evaluation of the spreadsheet, I would also point out that this database appears to be taken from the 3rd edition of the aforementioned Properties of Gases and Liquids which was published, I believe, in the '70's. I don't know if it matters for your purposes, but there may be newer data/correlations for the vapor pressure of nitrogen than this one.



#9 curious_cat

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Posted 17 October 2013 - 01:07 PM

Again, I don't mean to offend, but I think this is a perfect example of something my mentor used to talk about. He was always concerned that engineers would become so "trusting" of computers that they would forget how to critically evaluate what the computer does. 

 

 

I don't think you are too harsh at all. This is a very relevant point. 

 

I keep bumping into such stuff every week. An engineer comes (not always young) and says "But Aspen said....." or some such. Other variants are "But API xxxxx says....."

 

So it's not just computers but blind adherence to an authority without questioning what the document intended or the domain to which it applies. 






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