Hello, I have a liquid hat I want t 2

[Smith]

Hello, I have a liquid hat I want to pump. The composition of the liquid varies, so the phase envelope also chanches. Some times, with the differential pressure I want to use, the "liquid" goes over the critical pressure.

What are the consequence of this? Can I use a normal pump?

If I vaporize at the high presure the liquid, I supose that the temperature will be constant if I work in the cases that the presure is below the critical presure (and latent heat will exist). If I vaporice over the critical presure I supose that although I have a vaporizer, really there is no more than heating up and no change of phase occurs, in that cases it is really a heater.

Is this correct?

Thank you very much in advance.

 

[MortenA]

Assuming that you _mean_ critical pressure:

If you are pumping a liquid it should have no effect on the phase that you go above Pc. Increasing the pressure (while keeping temperature close to constant) could never cause a liquid to start evaporating.

If your temperature is below Tc then the liquid will remain a liquid even though you go above Pc. If you then (P>Pc) start to heat the liquid you could move into the "fluid zone" Here gas/liquid terms does not apply and the cossing is not a phase change as such with no associated change of enthanphy.

E.g: Water has a Pc of 220.5 bara and Tc=647.3 K =374 deg C.
But water still remains a liquid even at depth exceeding 2500 m where the pressure is above 220.5 bara. The remperature is below 647 K What happen close to volcanos etc. is another story.

Best Regards

Morten

 

[Smith]

Thanks a lot MortenA, the example of the water is really clear.

There is still something about this that I am not sure. When do you go to the "fluid zone" if your liquid is over the critical pressure & you heat up? I suppose when you cross the isothermal of the Tc. Is it ok?

If you have for example ethylene in a pipe at 298 K
& 100 bar how shall I call it, gas?, (it is in the "fluid zone".)

Thank you for your helps.

 

[TD2K]

I'm not sure what you mean by 'go up to the fluid zone and you heat it up, is it ok?'.

For ethylene at the conditions you have, it's a dense phase gas. As you are above the critical temperature (48.5 deg F/9.2 deg C), you can not liquify the material purely by pressure, you need to cool it below the critical temperature to be in the liquid phase. Best way to see this is via a Mollier chart, the GPSA data books have them for ethylene as does Perry and lots of other references.

 

[MortenA]

TD2K

I disagree when in the critical zone (above Pc and Tc) you cannot really talk about liquids and gas. For convenience we often label it as such anyway.

Chemshare calls it a liquid (ethylene at 298 K/100 barg). Most likely because of the density (319 kg/m3) or because you are above Pc. Smith & Van Ness, Introduction to Chemical Engineer Thermodynamics Calls the region right of the Tc and above Pc for the "fluid region" and the region right of Tc and below Pc for the "gas region" (left of Tc its a vapour). (T on the x-axis P on the Y)

Best Regards

Morten