11 replies to this topic

[kiqbal786]

Can Someone please explain me the following.
I have developed a HYsys model for one of my projects. At one point, there is a two phase flow through a pipe line. When I check the process conditions and properties, I get the following results.
Operating Pressure : 23 Psig
Operating Temperature: 105 deg F
Trure Vapor Pressure @ 100F : 77.2 Psig

As there is not much difference in the Operating temperature (105 F) and the Vapor pressure temperature (100 deg F) then why there is too much differecne in the Operating and Vapour Pressures?

[latexman]

Is stream a pure component or multicomponent?

[Amit1]

hi Kiqbal

Please have a look at the attached image. It's typical PV diagram for the multicomponent system.

Based on your pressure condition, you are operating somewhere between point E and F. Point E designates vapor pressure. This explains your observation.  PV Diagram.png   376.05KB   38 downloads

[kiqbal786]

Thank you very much for the reply and assistance.

It is a multicomponent stream with vapour to liquid ratio of 0.06. What actually my problem is that in normal practice the operating pressure of a two phase stream is usually greater than the liquid vapour pressure otherwise the liquid would flash so that the vapour pressure of the new liquid shall be less than the operating pressure.
In this case, we are specifying the vapor pressure @ 100 deg F while the operating pressure is @ 105 deg F so as far as my understanding of basic chemistry the vapour pressure should not be greater than the operating pressure.

Is there any scenario possible in which the vapor pressure of the liquid in a two phase flowing stream through a closed pipe could be greater than the operating pressure of the stream?

Thanks

[PaoloPemi]

you may consider that as a VLE/LLE problem,
generally when simulating a two/multiphase flow the procedure solves the phase equilibria at each pipe segment and calculates the phase fractions of coexisting phases, then it calculates transport properties and estimates the pressure drop,
for a Vapor liquid equilibria at a specified pressure (below cricondembar) depending from temperature you can have all liquid (temperatures below bubble line) to mixed (vapor liquid) or all vapor (temperatures above dew line,
there is no vapor as separate phase below bubble line,
to avoid confusion (with VLE in mixtures) I prefer to speak in terms of bubble line, dew line and phase fraction,
with some software you may get the value of pressure on bubble line as (true) vapor pressure,
but the meaning doesn'r change.

[latexman]

Sounds to me the problem has been over specified. What do you mean "we are specifying the vapor pressure @ 100 deg F". Are you specifying the temperature at 100 F and the pressure is calculated? Or, are you indeed specifying the vapor pressure @ 100 deg F. Something is confusing. As you said, having vapor pressure > operating pressure means flashing or boiling.

[Gabe G.]

I suspect you are looking at the wrong column in your properties table in Hysys. The TVP is based on the composition and if you are looking at the overall composition in the stream you may be misled because it is still using the portion of you stream that is vapor phase to run the calculation. If you look at the liquid phase only properties Im going to bet you will find your TVP to be lower than the operating pressure of the stream. Hope this helps!

[kiqbal786]

Thanks for the replies.
Mr. latexman,
Here we means the software. I mean the software is specifying the vapor pressure @ 100 deg F and the operating pressure @ 105 deg F as mentioned below.
Operating Pressure : 23 Psig
Operating Temperature: 105 deg F
Trure Vapor Pressure @ 100F : 77.2 Psig
Now as the operating temperature is higher than the TVP temperature, So the Operaring Pressure should be higher than the TVP of the stream as per the basic chemical engineering but that is not the case.

[PaoloPemi]

if you suspect a problem in vle calc's one of the best diagnostic tools is the phase envelope (at feed's composition),
I have a different software (Prode Properties) and I am not familiar with the tabs and columns of your software,
according the comment given by Gaben G. the software calculates the true vapor pressure from liquid and gas compositions as feeds, that seems strange as bubble line is usually calculated considering the whole (gas + liquid) composition, if that (true vapor pressure calculated for gas or liquid compositions as feed) is confirmed it could possibly be a cause of confusion...
in all these cases the phase envelope should permit to evidence the limits of liquid-mixed-vapor areas.

[kiqbal786]

Mr. Gabe G.
Thanks for the reply. I have checked in the liquid phase column and the liquid TVP is really less than the operating pressure and so u won the bet.
But I m still surprising that why the TVP of overall stream is much less than the Operating pressure of the stream. Becuase Operating pressure is acullay the sum of all the pressures and so it must be greater than or at least equal to the TVP of the overall stream at the same temperature.

[PaoloPemi]

perhaps there is some confusions interpreting results from tabs and columns in your software...
you should consider the bubble pressure calculated from whole composition (vapor+liquid) and compare with operating pressure.
If you suspect a problem calculate the phase envelope for that mixture, that should give you the information to diagnose the problem.

[latexman]

Call Hysys tech support.