misconception about the negative enthalpy

[knapee]

Dear Professional engineer in the fourm

Sorry to bother you this trivial.
I tried to use simulation software such as pro ii, aspen plus
to find the latent heat of the multicomponent (the mixture also contain H2, N2).

I used three different flush drums to complete this job.
The first flash drum is set for duty=0, the second drum is set the
vapor fraction = 5%, and the third flush drum is set for the bubbling point.
After the simulation, I got the negative enthalpy on the last stream from the 3rd flush
drum liquid stream, and I wonder is the H2, N2 cause this negative result.
Then, I use the enthalpy in the stream of vapor from the 2rd drum substract the
enthalpy in the stream of liquid from the 3rd drum and substract the cp times the temperature difference
to get the latent heat.


But, our leader told me this negative enthalpy is impossible and I should
not input the H2, N2 stream infromation in the inlet stream in the pro ii or aspen.



Thank you very much for your comment and help.

 

[MortenA]

Are you referring to the HYSYS streanm property "Molar ethanlphy" or "Heat flow" or what?

You should be aware that the ethalphy is specified with a reference condition. Therefore i think that the molar enthalphy and heat flow can bhave a negative value - depending on the reference condition. Check the HYSYS help text for the reference condition HYSYS uses.

However: If theres no dP over the flash drum - then in HYSYS you have an energy stream into the drum when changing temperature. The heat input in this stream is equal to the heat needed to evaporate/condense the volume evaporated/condensed. Then use molar/mass stream to convert to latent heat.

Best regards

Morten

 

[don1980]

As Morten says, enthalpy is a relative term. Therefore, when the enthalpy is less than the reference point, you see a negative value. The change in enthalpy is what matters. Your simulation results should make sense if you look at the changes in enthalpy, and disregard whether or not any particular value has a negative sign.

 

[duuude]

Do anybody know how to change the reference point in aspen? I want to do an exergy analyse and therefore I need the relative term h(t)-h(tu) ..I am searching now for at least two hours, but still without success...

 

[MortenA]

I think the help states that the reference point cant be adjusted - but im not sure. Check the documentation, or raise a support case.

Best regards Morten

 

[StoneCold]

If you did three separate flashes then maybe it is enthalpy related. If you took your stream and flowed through three flash drums it is because you did the 5% flash before the bubble point flash.
Change the order of the flashes and try it again.

Regards
StoneCold