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Simulations Involving Hydrocarbon "characterised Fractions"

modelling simulation gas hydrocarbons

5 replies to this topic
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#1 winchester427

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Posted 10 June 2018 - 03:05 PM

Hello

I have a (hopefully!) simple question on modelling a hydrocarbon gas mixture in process simulators...

Is there a generally accepted method when simulating characterised fractions e.g. C5+, C6+, C7+ etc in HYSYS/UniSim Design, when the physical properties e.g. molecular weight, density of the fraction etc are not known? i.e. when we do not have the required properties available to simulate it as a pseudo-component (e.g. feasibility studies)?

I have heard of several simplifying assumptions:

1) assume the composition of the characterised fraction is the same as the light end of the fraction e.g. assume all C5+ is C5 or that all C6+ is C6 etc

2) Assume the composition of the characterised fraction can be modelled as the next heaviest component e.g. assume all C5+ is equivalent to C6 or assume all C6+ is equivalent to C7 etc

Obviously this assumption can be very important with respect to simulating the hydrocarbon dewpoint curve.

My senior engineer is suggesting to just simply use option 1 but is there a best practice with respect to this?

#2 Pilesar

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Posted 11 June 2018 - 01:23 PM

I've used both of these simplification methods and also sometimes assuming even heavier components than '+1 C'. It depends on what you want to learn from the model and what is most conservative. One nice thing about computer simulation is that with very little effort you can try several characterizations to see how sensitive your model is to your assumptions.

#3 serra

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Posted 11 June 2018 - 01:55 PM

for specific applications (as natural gas dew points) there are well known methods, see for example thid thread

https://www.eng-tips....cfm?qid=290989

for general applications, as said by Pilesar, you can select yourself the method which best fits your experimental data

#4 winchester427

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Posted 11 June 2018 - 03:01 PM

Thanks for the replies.

Sounds like there is no one accepted method. GPA 2261 seems to suggest for C6+ fractions to assume a 60:30:10 split for n-C6, n-C7 & n-C8. Although still just an assumption at least it can be referenced

#5 serra

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Posted 12 June 2018 - 08:02 AM

to monitor natural gas dew points, cricondentherm etc. typical (realtime) applications include a (on line) gas analyzer  (for example a gas-chromatograph) and a thermodynamic server to calculate all the required properties,

nowadays models such as GERG 2008 are available which allows accuracy within 1 K but small differences in heavy components fractions have large impacts on results, then one should define a suitable method to characterize these fractions,

the thread illustrates several methods ((GPA, AGA, fractions based on laboratory analysis etc.) available in Prode Thermodynamic Server and compare results, however the methods to characterize C6+ and heavy components can be duplicated with a different software as well.

#6 Technical Bard

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Posted 20 June 2018 - 11:37 PM

The methods mentioned by others work, with the exception if the stream is primarily aromatic.  The boiling points of aromatics are significantly different than the alkanes and iso-alkanes of the same carbon number.