# Gas liquefaction through compression

by henrymanu2005
Tags: compressed gas, compression, gas cylinder, liquefaction, phase change, pressure vessel
 P: 1 This is more a thought experiment that I'm not completely sure I'm getting correctly... So here we go: Imagine we have a tank that we are filling up with a substance in gaseous form. Imagine also that we have a good pump that will allow us to fill this tank with this gas up to very high pressures, and that the tank will not collapse or burst or leak. The tank starts empty and its pressure increases as gas comes in. There comes a point in which the pressure inside the tank reaches the saturation pressure of the substance at whatever temperature the system is in. Upon reaching Psat, some of the gas will condense into a liquid and the pressure inside the tank will fall. While this happens, more gas is still being pumped into the tank, so the tank pressure is brought up to Psat again and more gas condenses into liquid. The filling process continues, and suddenly there is more liquid than gas inside the tank. The tank pressure during the phase change process is Psat, even though more mass is coming in. Theoretically, the maximum pressure of the tank is Psat, and the maximum mass that can be pumped into it is equal to the mass that fills the constant volume of the tank at the liquid density of the substance (assuming an incompressible liquid.) Is this thought process correct? Specifically, can the pressure inside the tank ever be any higher than Psat or does it effectively plateau until all gas has become liquid? Thank you for your help
PF Gold
P: 1,113
 Quote by henrymanu2005 Upon reaching Psat, some of the gas will condense into a liquid and the pressure inside the tank will fall. While this happens, more gas is still being pumped into the tank, so the tank pressure is brought up to Psat again and more gas condenses into liquid.
This part is not correct. As the liquid forms, the density increases (as the liquid is more dense), but the pressure never falls. It stays at Psat. New gas comes in because the liquid takes less volume.

 Quote by henrymanu2005 Theoretically, the maximum pressure of the tank is Psat, and the maximum mass that can be pumped into it is equal to the mass that fills the constant volume of the tank at the liquid density of the substance (assuming an incompressible liquid.)
That sounds ok, provided that the temperature is maintained constant during the entire process.

 Quote by henrymanu2005 Is this thought process correct? Specifically, can the pressure inside the tank ever be any higher than Psat or does it effectively plateau until all gas has become liquid?
I've never worked with gases like that, so someone with practicle experience may prove me wrong. But from a theoretical, thermodynamics point of view, P plateaus at Psat and everything stops when the tank is filled with liquid.

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