Phase envelope 1

[Arcante]

I am getting strange results when printing a phase envelope of a natural gas mixture from our simulator (package Peng Robinson), when I add 6% H2S to a C1 60% C2 11% C3 C4 C6 C8 N2 CO2 the dew and bubble lines don't connect but go up and down in a strange fashion, also I am not able to get the dew point specification. The software house says it's the charateristic of the mixture but it's the first time that I observe this behaviour in a natural gas mixture and I would appreciate comments from experts here.

 

[PaoloPemi]

mixtures containing Hydrogen Sulfide are well known for the complex behaviour, you can find lots of references in works of Michelsen and other experts, as an example here there is a simple 2 components C1-H2S mixture where there is a critical point but bubble and dew point lines don't connect

http://www.prode.com/en/phaseenvelope.htm

If the software house says the procedure works well I would presume they have verified that, you shouldn't forget that in general it isn't simple to duplicate a result with a different simulator because of differences in databases, interaction parameters and models (for example I know at least 10 variants of Peng Robinson...),
Paolo

 

[Arcante]

Paolo,
I didn't suppose that the addition of 6% H2S could generate that complex behaviour, I found several papers of Michelsen discussing phase equilibria with several examples and many of these examples do include H2S,
a interesting reading,
Alfonso

 

[PaoloPemi]

yes, process simulation is not easy when a natural gas mixture includes Hydrogen Sulfide, and things are even more complex when there is water, in that case I have found that only rigorous multiphase flash with SRK (or PR) and Huron-Vidal or Wong-Sandler rules can produce decent results (see also the work of Michelsen and Pedersen in this area), simple phase diagrams which show a generic water dew point line are in general very unreliable.
Of course I am prepared to learn from others which use different methods,
Paolo

 

[Arcante]

fortunately in this project I have a very low (negligible) amount of water, with water I use the mixing rule by Kabadi and Danner which is avaiable in our simulator but usually I have no feedback confirming the accuracy, the point is that there is not much time for testing the real consistency of a thermodynamics model in a specific case, one selects simply the model suggested in the operating manual of the simulator for similar cases and must hope it works well...

 

[frankboyle]

Just a note, with water I would be prudent when selecting Kabadi Danner as mixing rule and verify the limits for this model, Huron Vidal, Cubic Plus Association or Wong Sandler when available should offer a more extendend range of application.

 

[Arcante]

thanks, I was aware of the limits of application for KD as they are reported in the operating manual of simulator, KD is the suggested model for hydrocarbons+water.