HC Gas Properties above critical point
HC Gas Properties above critical point
(OP)
Dear All,
can you please tell me
1) Why in Hysys, if we have HC gas well above its critical point (by temp) we still can turn it into the liquid phase by increasing presure?
2) How do you understand the critical point and especially the critical pressure/temperature?
These seem to be simple issues which are often difficult to understand/explain
I asked my colleague - the principal process engineer and he could not give me the clear answer
thank you,
Jeyx
can you please tell me
1) Why in Hysys, if we have HC gas well above its critical point (by temp) we still can turn it into the liquid phase by increasing presure?
2) How do you understand the critical point and especially the critical pressure/temperature?
These seem to be simple issues which are often difficult to understand/explain
I asked my colleague - the principal process engineer and he could not give me the clear answer
thank you,
Jeyx





RE: HC Gas Properties above critical point
If you are speaking of hydrocarbon mixtures visit thread798-70867.
RE: HC Gas Properties above critical point
Cheers,
Joerd
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: HC Gas Properties above critical point
A pure component above its critical temperature can NEVER be liquified, no matter what the pressure.
(2) MULTICOMPONENT MIXTURES
To understand the issue you have raised, it is instructive first to study the concept of the phase envelope. This is a curve showing the locus of points on the two phase boundary on a P-T plot.
For many wide-boiling mixtures, the critical point is located BELOW the so-called cricondenbar pressure. In fact, it is this phenomenon that gives rise to the problem of "retrograde condensation", where reducing pressure for a supercritical mixture at constant temperature (starting above the 2-phase boundary) can result in liquid separation at temperatures above the true critical temperature of the mixture. This can, for example, have important safety implications for gas pipeline networks.
For a decent explanation of the relevant theory, please consult Professor S. M. Walas’ excellent book, "Phase Equilibria in Chemical Engineering", (Butterworth, 1985, pp 94-96). Walas also has excellent discussions of the critical state and critical properties at pages 87-94. This book contains an encyclopedic coverage of the subject and has numerous diagrams to aid comprehension. Two other good books on this subject are:
Professor T.F. Daubert, “Chemical Engineering Thermodynamics” (McGraw-Hill, 1985), and
Professors J.M. Smith, H.C. van Ness, and M.M. Abbott, “Chemical Engineering Thermodynamics”, (6th Edition, McGraw-Hill, 2001).
By the way, joerd's post appears to me to refer only to how Hysys handles computational issues related to flash calculations for a supercritical fluid, and not to the underlying phenomena raised by your original question.
RE: HC Gas Properties above critical point
Samiran