Vapor Pressure 1

[npf]

All:

How to determine vapor pressure using HYSYS? Also it is always true whether Critical pressure is always greater than vapor pressure.

Regards,
sp77

 

[BigInch]

I don't use Hysim, so can't say how Hysim calculates vapor pressure of a substance at a given temperature.

The quick answer is YES, Critical Pressure is almost always higher than Vapor Pressure... almost always, because at the Critical Point, Vapor Pressure = Critical Pressure!

Practically speaking, Vapor pressures only exist between the Tripple Point and the Critical Point. The define the transition point between the liquid and gas phases for any temperature between the tripple point and critical point. Above the Critical Temperature, distinguishig between the liquid and gas is difficult, so vapor pressures (per say) cease to exist.

For a detailed discussion of how it applies to the state of water, see

http://www.lsbu.ac.uk/water/phase.html

http://virtualpipeline.spaces.msn.com

 

[25362]

In petroleum engineering the cricondenbar (not the critical point) is the maximum pressure at which liquid and vapor can coexist.

 

[BigInch]

Nice point 25362! Now we're talking. I did say "practically speaking", thinking that (esp. from my perspective) it is only on limited occassions where it is necessary to distinguish in detail exactly when and how a phase change occurs. In I've never had to distinguish between the bubble point and the dew point, especially as they approach the critical point, as long as the product was delivered on one side of it or another. When operating I prefer to stay well to one side or the other of the phase change too. That approach is usually sufficient for pipeline work since, in pipelines if a critical point exists its usually not going to exist for a very long time, or failing that for a very long length but, I can see where it may make a difference in complex process works.

http://virtualpipeline.spaces.msn.com

 

[25362]

The last (July) issue of Hydrocarbon Processing has an article titled Understanding LNG rapid phase transitions that may interest sp77.

 

[MortenA]

SP77

1) Create the stream with the composition required
2) set the vapour fraction to 0 and the temperature to the desired temperature where you want to know the VP. Then HYSYS will calculate the pressure=the VP (or more precisely the bouble point pressure=the vapout pressure) at the specified temperature - similar if you want to calculate the dew point set the vapour fraction to 1

Since by the nature of your question i guess that you are very unfamiliar with the use of HYSYS and perhaps also thermodynamics? i must warn you that your result will depend on many parameters e.g. selection of EOS, components in your mixture, definition of pseudocomponents (if any) and so on!


Is the e.g. about pure water then use a table (not HYSYS)!

If im correct in my assuption then i must encourage you to seek professional assistance either in-house or by a consultant if these results shall be used for anything important (IMHO).


Best regards

Morten

 

[UmeshMathur]

(1) For a pure component, the critical pressure always exceeds the vapor pressure at any other temperature.
(2) For a multicomponent mixture, the vapor pressure at the dew point temperature may exceed the "true" critical pressure. As noted by 25362, the cricondenbar is the point at which the saturation pressure is highest. The best way to see this is by making a "phase envelop" calculation. This is basically a P-T curve showing the entire locus of the equilibrium curve - saturated liquid and saturated vapor all the way to the critical point.
(3) For multicomponent systems, the thermodynamic option (equation of state) chosen can have a huge impact on the resulting calculation and one must use a great deal of experience and insight to make the proper choice. Therefore, I heartily endorse Morten's cautionary notes.

 

[joerd]

One more word of caution - if you just set the vapour fraction to 0 (or 1), and the temperature specified is outside the phase envelope, HYSYS will give an unpredictable result - or nothing at all. In that case, a phase envelope will quickly show you what is going on.

Cheers,
Joerd

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[npf]

All:

Thanks for all the comments.........

Regards,
sp77

 

[Radionise]

Just to follow up on joerd's post. Yes, be very careful about setting the vapour fraction to 0 (or 1). By doing so, you effectively set the process stream to be at its bubble (or dew point).

PhD - Permanent Head Damage

 

[MortenA]

Radionise

As i also said - but the bubblepoint will be equal to the vapour pressure (as long is the fluid is not critical as joerd pointed out)!

The man _wants_ to calculate the vapour pressure.

Best

 

[BigInch]

Agreed.
It does seem
to be hard to
remember the original
question around here.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Abiodun1]

how does a pig system design?

 

[BigInch]

Abiodun,

Start a new thread and I'll post there.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[MortenA]

abiodun1

usually we dont critisize language problems (I know about these since english is not my morther tounge) but your little typo had me lauging

Best regards

Morten

 

[Abiodun1]

thanks morten,

i noticed the statement you are pointing at.