5 replies to this topic

[Tummeq]

I use  ChemCAD software. Recently I found out the following issue in thermodynamic simulation. The cyclohexane (like many other hydrocarbons) does not flash at 50 °C and atmospheric pressure. However, when we add a small amount of hydrogen it starts flashing significantly, which seems incorrect. Have you met any similar issue?

[breizh]

Hi ,

Using PR EOS I have similar result.

Is this the right equation to use ? 

Breizh 

[SilverShaded]

Hydrogen will strip out significant amounts of heavier hydrocarbons.  I tried it with PRSV and PR, PRSV strips out more than PR.  That said if i have 0.01 Mole fraction of H2 in the stream I get 0.011 of total vapour in the stream, which seems to be entirely as expected.

[PaoloPemi]

vapor pressure at 323.15 K for cyclohexane is 36.50 KPa...
flash below 36.50 KPa to see the vapor phase...
std. Peng Robinson does a good job with vapor pressure for most low/medium Mw HC,
for heavy HC or polar fluids better to use extended versions... 

[Tummeq]

I found out that API SRK (SRK modified by Graboski and Daubert to better reflect mixtures with nonhydrocarbon gases) reduces amount of flashed HC.

I needed it for simulation of existing oil hydrodesulfurization unit and thanks to this modified EoS i managed to construct simulation from PFD and material balance.

[PaoloPemi]

the usual code for API SRK includes a specific alpha for hydrogen,

however there are not specific correlations for polar fluids etc.

with these better to select a extended EOS with specific alpha functions....