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Solved On A Dry Basis But Stream Has Water


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#1 chemproc

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Posted 17 October 2019 - 08:27 AM

Has anyone seen the following error before in hysys "Solved on a dry Basis but stream has water". Question can I trust the curve for my analysis? Also is there a work around? The phase envelope curve is attached and the process condition is attached.

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#2 MrShorty

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Posted 17 October 2019 - 11:10 AM

My mentor (who did his programming with FORTRAN in DOS before Windows was really a thing) used to observe that too many of us engineers treat our computers/simulators as "black boxes" into which data goes in and stuff comes out, but we don't have the understanding of the underlying engineering principles to understand what the black box is telling us. As one who doesn't use Hysis myself, but who would like to think he understands something of the phase equilibria behind what the black box is doing, here's what I see:

 

Your stream is essentially a CO2 + H2O stream. Yes, there are traces of various hydrocarbons and gases and such, but they are in such low concentrations that they will not impact the properties of the system very much. How well do you know the behavior of the CO2 + H2O binary without referring to Hysis?

 

I know that the CO2 + H2O system is a 3 phase system. There's a CO2 rich vapor phase, a CO2 rich liquid phase, and a H2O rich liquid/solid phase.

 

The solubility of H2O in the CO2 rich liquid phase is low, so I would expect the properties of the CO2 rich phase to be very similar to pure CO2. The bubble point/dew point curve (relative to the CO2 rich liquid) is probably pretty good. I might plot it against the vapor pressure of CO2 to verify.

 

The main things that are missing from the calculation when treating it as if the water wasn't there will be properties related to that water rich liquid phase. The first one is noting the freezing point of water where that phase will transition from solid to liquid. There could also be some concern with hydrates forming.

 

The other thing that is missing might be a dew point curve relative to the water rich liquid phase -- essentially what does the equilibrium between a CO2 rich vapor phase and a water rich liquid phase ("below" the dew point curve of pure CO2 that is on your chart) look like. I would expect this curve to follow the vapor pressure of water until the vapor pressure of water is greater than the partial pressure of water in the stream.

 

Not sure how to coax any of that out of Hysis, but that's what I would expect to see if it had handled this 3 phase system correctly.






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