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Phase envelope

Phase envelope

Phase envelope

(OP)
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.

RE: Phase envelope

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

RE: Phase envelope

(OP)
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

RE: Phase envelope

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  

RE: Phase envelope

(OP)
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...

RE: Phase envelope

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.

RE: Phase envelope

(OP)
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.

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