Dew Point - Pressure Relationship

[AreJay]

Hi all,

Looking at some hydrate curves for gas and was wondering if there is a quick formula for calculating the dew point with pressure change.

Hydrocarbon gas in the line with dew point of -20DegC at 35bar.

What would be the dew point at 110bar?

Gas temperature is 40DegC.

Thanks,

Ryan

 

[PaoloPemi]

I assume it's VLE not hydrate saturation pressures (in that case see GPSA manual)
If you wish to calculate dew point temperatures (at different pressures) but about the same composition you may calculate the entire dew point line (phase envelope) then calculate the parameters for a curve, fitting the calculatecd data (quite easy with Excel or similar tools)
If composition changes are important then you need a more complex approach.

 

[AreJay]

Composition is constant, as is temperature...

only change is the pressure

 

[PaoloPemi]

if composition is constant then you can calculate the entire dew line (a series of t,p equilibrium points) , if you have not access to simulators you may use a library (see for example Prode Properties) then use a tool as Excel or similar to calculate parameters of a curve fitting the calculated points.

 

[AreJay]

Component Mol % Mol Weight Mol Fraction yi (Vapour Fraction) xi (Liquid Fraction) K
CO2 1.39 44.00 0.0139 0.0139 #DIV/0!
N2 1.78 28.00 0.0178 0.0178 #DIV/0!
C1 77.25 16.00 0.7725 0.7725 #DIV/0!
C2 8.47 30.00 0.0847 0.0847 #DIV/0!
C3 6.10 44.00 0.061 0.061 #DIV/0!
iC4 1.31 58.00 0.0131 0.0131 #DIV/0!
nC4 2.28 58.00 0.0228 0.0228 #DIV/0!
iC5 0.54 72.00 0.0054 0.0054 #DIV/0!
nC5 0.01 72.00 0.0001 0.0001 #DIV/0!
C6 0.40 86.00 0.004 0.004 #DIV/0!
C7 0.06 100.00 0.0006 0.0006 #DIV/0!
C8 0.02 114.00 0.0002 0.0002 #DIV/0!
C9 0.03 128.00 0.0003 0.0003 #DIV/0!
C10+ 0.02 142.00 0.0002 0.0002 #DIV/0!

Composition is shown above... does anyone have the links to the required graphs for the xi at the stated P&T?

 

[PaoloPemi]

to calculate the liquid fractions of different components you need to solve a vapor liquid flash operation, for K values with this mixture you may use SRK or PR EOS, in API TDB and other documents you may find graphs of K values at different temperatures and pressures but I would suggest to use a EOS.

 

[AreJay]

I know have my yi (vapour fraction), xi (liquid fraction) and corresponding Ki for my composition at normal operating conditions of 110 bar and 40 DegC.

What formula do I use to find the dew point of the gas?

Do I need to vary the temperature to find new Ki values?

Thanks

 

[PaoloPemi]

yes, you have to change the temperature (at constant pressure) to calculate the dew point temperature for some specified compsition and pressure,

1) calculate Gas & Liquid fugacities (at p,t)
2) Ki=Fli/Fvi
3) correct t to find (for the dew point) SUM(Wi/Ki) = 1

a simple procedure should work with p , t far from critical conditions.
To learn more on the subject I would recommend a good textbook.

 

[AreJay]

I understand that thanks for your help... what is this Wi?

 

[PaoloPemi]

it's the molar fraction of component i
you get

SUM(Wi/Ki) = 1

solving Rachford and Rice

SUM (Ki-1)*Wi/((Ki-1)*vf+1)

putting vf (vapor fraction) as 1.0 (dew point specification)

You may read a good textbook for additiona information.

 

[AreJay]

Thanks for your help but I am still struggling to determine what I am doing... I have confused myself as I have my xi = 1 with my normal composition which I know is incorrect.

I have assumed that the vapour fraction is equal to the compositonal anlaysis (which was taken at 35bar and 40degC)

using this composition I have looked up the charts and determined the xi based on the K value and now my xi is equal to one which means that my due point is 40degC which is wrong...

Can someone have a look at the attached and help please...

I have put a guess temperature section where I would move on but I dont know what to do...

Thanks again...