Hysys dew point and bubble point results differ, databook vs. envelope
Hysys dew point and bubble point results differ, databook vs. envelope
(OP)
Good day, gentlemen and ladies,
I'm dealing with gas mixtures, involving light hidrocarbons, water and sulphur oxides. As an experiment i created a stream with a composition of methane 0.4%mol, ethane 0.3, water 0.2, and SO3 0.1. P=1 bar, Flow=1 kmole.
The property package PR seems gives very close results compared with PR-sour, PR-TWU, SRK and so on.
The envelope for this stream gives a dew point (no bubble point) that looks a bit weird. Reading about this i took Mr. Joerds' suggestion and calculated in a case study, setting the vapour phase=1 and pressure 1 bar as independent vars, and temp as a dependent var.
The results are puzzling, to say the least. Temp keeps rising with pressure indefinetly, and the graph looks nothing like the envelope (values differ a lot).Same pb for a bubble point calculation ((20 bar,60 dC)on envelope; (20 bar, -100 dC)on table).
Wich do i believe?
Thanks in advance, hope this issue isn't too silly to deal with.
I'm dealing with gas mixtures, involving light hidrocarbons, water and sulphur oxides. As an experiment i created a stream with a composition of methane 0.4%mol, ethane 0.3, water 0.2, and SO3 0.1. P=1 bar, Flow=1 kmole.
The property package PR seems gives very close results compared with PR-sour, PR-TWU, SRK and so on.
The envelope for this stream gives a dew point (no bubble point) that looks a bit weird. Reading about this i took Mr. Joerds' suggestion and calculated in a case study, setting the vapour phase=1 and pressure 1 bar as independent vars, and temp as a dependent var.
The results are puzzling, to say the least. Temp keeps rising with pressure indefinetly, and the graph looks nothing like the envelope (values differ a lot).Same pb for a bubble point calculation ((20 bar,60 dC)on envelope; (20 bar, -100 dC)on table).
Wich do i believe?
Thanks in advance, hope this issue isn't too silly to deal with.





RE: Hysys dew point and bubble point results differ, databook vs. envelope
With polar fluids it would be better to select for fugacities a model based on complex mixing rules, however these models are difficult to solve at high p, t and plotting the entire phase envelope may become difficult for very non ideal mixtures...
If you attempt to model with PR or SRK and convenctional mixing rules a simple binary as Water 0.9 and Methane 0.1 you'll find a unusual behaviour and possibly convergence problems,
there are many examples of unusual behaviour for binaries as CH4-H2S etc. see
http://www.prode.com/en/phaseenvelope.htm
RE: Hysys dew point and bubble point results differ, databook vs. envelope
Regards,
JW
RE: Hysys dew point and bubble point results differ, databook vs. envelope
As written in previous post I think this is not a honest test case since selected thermodynamics are not suitable for that mixture.
However you could verify if the problem appears at very low pressures (the beginning of the phase envelope), if that is TRUE possibly the difference depends from different VLE initializations, if not (it works at very low pressures) it could depend from the different ways the procedure adopts to calculate a dew point value (possibly initializing at specified p or t and then attempting to converge) and the phase envelope (starting from very low pressure and then proceeding from converged values) , the second case being certainly more stable.
RE: Hysys dew point and bubble point results differ, databook vs. envelope
There is, however, a conclusion to be drawn from this. After testing a lot of property packages and comparing the results, i found that PR, PR-sour give the same results for bubble points of such a mixture between a 0-5 bar range, and Kobodi-Danner gives simmilar results with these two. Another important thing to note is to not trust the envelope Hysys prints for the same mixture. Calculating it yourself seems to give more accurate result, since the envelope looks as if it ignores the presence of water and SO(therefore the results differed with +50 dC).
In conclusion: envelope bad, var calculation much better but not best. In need of accurate dewpoints and bubblepoints for a complex mixture, Hysys is not your tool.
Thanks again Paolo for your posts, they guided me in the right direction.