Liquid Vapour Equilibrium in Liquid Cylinder

[Sam Low]

Hi All,

There is something lingering in my head lately regarding the liquid vapour equilibrium.
To be honest, I am not too sure if the problem that I am going to ask here is about this topic.

There is a pressurized cryogenic liquid at 6 barg under operating in a Low Pressured Cylinder (LPC). When the tank pressure goes down to 5 barg, the cryogenic liquid will be passing the Pressure Building Coil (PBC) (actually it is a vaporizer) and transferring back to the tank as a gas to pressurize the tank. Basically, I believed the theory behind is to increase the number of molecules of the gas (under the same volume) as well as the tank pressure. The pressure regulator regulate and stop when the tank pressure reach at 7 barg.

Then this question comes to my mind that if the tank pressure is at 7 barg, does it mean only the gas at 7 barg or the system is going to reach liquid vapour equlibrium and achieved 7 barg eventually? If not, how does the cryogenic liquid achieve at 7 barg?

Hope I explain my question well and looking forward to your reply!
Many thanks in advance!

 

[csk62]

In cryogenic tanks saturated conditions (equilibrium) is practically only achieved at the liquid vapor interface. In reality there is thermal stratification in the liquid giving some degree of subcooled product at the bottom of the tank.

If you do a volume balance on the cryogenic tank you will understand that the PBC is able to maintain a certain pressure during product withdrawal as the effectiveness is governed by the ratio of liquid/vapor densities (a function of pressure)

Also, just as an FYI most modern cryogenic storage systems have an economizer circuit as well.

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Not making a decision is a decision in itself

 

[Compositepro]

The surface of the liquid will always be at close to equilibrium with the vapor. When the vaporizer increases the pressure in the tank by absorbing heat and boiling some liquid, the temperature of the vapor also increases. Some of the vapor will condense on the fluid surface and raise the temperature slightly so that its vapor pressure is 7 bar. The liquid below the surface will heat only very slowly because warm liquid is less dense than cold liquid, inhibiting mixing.

The same thing would happen if you used steam to pressurize an insulated tank of cold water. This means that presurization can occur almost instantaneously, and you do not need to heat the whole tank of liquid to do so.

 

[Sam Low]

Hi csk62 and Compositepro,

Meaning eventually the liquid will be at same pressure and temperature with the vapour since it achieved equilibrium (even if it at the surface of the liquid, but I believed it will turn out to be the whole liquid slowly).


Thank you!