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?

Many thanks in advance.