critical phase behaviour of hydrocarbon and water mixtures

[frogcurry]

If I have a vessel with a mixture of hydrocarbon (2 phase) and water (liquid), then raise pressure and temperature till they are higher than the critical points of the hydrocarbons alone, what happens? Does the water affect the critical point (assuming that effectively all the water is in its own phase) of the hydrocarbons? Will all the contents become part of a single critical phase fluid or is it practical to have a critical phase and a liquid phase in the same vessel?

Also what happens in the same situation in a case where the fluid is flowing in a line and is turbulently mixed (i.e. effectively the two liquid phases are combined, and at critical point conditions or higher for the hydrocarbons)?

Grateful thanks to anyone who is able to answer these questions, as I can't grasp the system behaviour in my mind.

 

[EricKvaalen]

If the water stays essentially in its own phase, then the hydrocarbon phases will act pretty much as though the water weren't there. So if the temperature is sufficiently high, the hydrocarbon phases will combine into one supercritical phase. (The minimum temperature for this to occur depends on the composition and is not simply the highest of all the critical temperatures of the individual components.) The water would remain as a separate phase.

It doesn't matter if there is turbulent mixing. There will still be the number of phases dictated by the thermodynamics.

Actually, the supercritical "hydrocarbon" phase will have a considerable amount of water in it, because whatever water vapor was in the vapor phase will now be in this supercritical phase. (In fact, even the hydrocarbon liquid phase will start to have water in it as it approaches criticality, because at criticality the two hydrocarbon phases meet at some common composition.)

If the temperature is raised sufficiently (without raising the pressure), the water will all evaporate and you will be down to one phase. Even if you raise the pressure at the same time as the temperature, you will eventually (above the critical temperature of water) get to the point where everything becomes miscible in one phase. You can't have immiscibility if the temperature is high enough.

Eric Kvaalen
consultant
La Courneuve, France

http://Eric.Kvaalen.com