Effectiveness of dew point hand calculations?

[AreJay]

Hi all,

I have been posting through the week about dew point calculations etc and I thank everyone for their help.

What I would like to know is how effective are the hand calculation for a natural gas stream. Composition is as follows;

Component Mol %
CO2 1.39
N2 1.78
C1 77.25
C2 8.47
C3 6.10
iC4 1.31
nC4 2.28
iC5 0.54
nC5 0.01
C6 0.40
C7 0.06
C8 0.02
C9 0.03
C10+ 0.02

When I do the calculation at 35bar the hand calculation shows a dew point around 52oC but HySys shows a temperature of 26.65oC.

Are hand calculations effective with multicompent streams?

Thanks,

R

 

[PaoloPemi]

it depends,
you can get exactly the same accuracy if you repeat the same procedures.
With simplified procedures some errors should be expected, most important contribute is the way you estimate the fugacities.
Nowadays there are many tools which do these calc's so you have alternatives.

 

[dcasto]

WOW, you must be board trying to do a dew point flash by hand, where did you get your K values?

If you are using some simple EOS based on lab work based around typicals for 1 site, then expanding outward to the real world won't work. Most simple dew point calcs I've seen stop at C6.

If you modify your examples C10 plus to be 50% C10 and 50% C11, the dew point changes 6 degrees F. Based on your nC5 being wrong, it is most likely to be .0035 mole fraction.

Look through threads here about dew point of gas discussions and how the gas industry is in a quandry on how to quickly see dew points of gas streams in real time.

 

[AreJay]

K values were taken from Campbells gas processing volume 1 chapter 5 appendix a

 

[PaoloPemi]

I have not access to those manuals and I can't comment, however I know API TDB (chapter 8) and GPSA (chapter 25) where the limts of the method are explained, for example your kvalues are probably "independent" from compostion which may be not true at 35 Bar, also accuracy of graphical figures is low and this may introduce errors etc. etc. if you are interested GPSA (25.2) gives an example with four components.

 

[AreJay]

Thanks for this I have re-calculated with the GPSA but still I have a different figure from HySys - I think it must be the reading from the graph...

What does confuse me however is the offshore dew point analyser reads -20oC for that composition at 35 bar. HySys and my calculations would suggest that -20oC would be around 0 bar. Does anyone know if the analyser takes the inline pressure and produces the result at 0 bar?
I can send you the pdf versions of the campbells if you wish? I have all 4 volumes.

 

[PaoloPemi]

there are several types of dew point analyzers, if it's a chilled mirror you can compare with a laboratory instrument, if it is a GC you can get a sample and go to laboratory... in the first case the instrument measures directly the dew point temperature, in the second case there is a software for these calc's, perhaps you can contact the manufacturer and ask to verify...
finally, if you have doubts about the values calculated (given composition and pressure) by simulator use another tool (see my previous post) to verify.

 

[dcasto]

The fact remains, you can spend months trying to get all the interaction coefficients between methane and C2 to C23 and you wear out your PC (or whatever hand calculations you are using) trying to solve for dew point.

The industry has spent millions trying to do what you are trying, finding a cheap and simple dew point analysis, and we can not agree. If you really think you have stumpled onto something, why not present your solution to the industry at the GPA convention.