Creating a temperature differential on SF6 to evacuate a cylinder

AI Thread Summary
The discussion centers on transferring SF6 gas from one cylinder to another using temperature differential. The setup involves a 50KG cylinder containing 1-2LB of SF6 at 15PSI and a second cylinder cooled to -40C. The user attempts to calculate the flow rate by applying the ideal gas law but realizes that their approach is flawed due to the compressibility of the gas. They initially calculate the pressure differential using the formula PV=nRT but find that this does not yield the expected results, as it does not account for the density difference when both valves are open. The key takeaway is that the pressure difference cannot be determined accurately with open valves, and the calculation should consider the constant amount of SF6 in both cylinders when the valves are closed.
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I have a 50KG cylinder with 1-2LB of SF6 at 15PSI. I want to transfer it into another cylinder by temperature differential.

Keeping it simple. I try to model it with two cylinders that has a valve on each cylinder. Connecting the cylinder with a 1/4" hose. Both valves are closed. The cylinder with the SF6 is at ambient temperature (25C). I cool the other cylinder to -40C (~-50C is solidifying point).. Open both valves.

How would I go about calculating the flow rate?

I try to find the pressure differential using ideal gas law

PV=nRT; P1/T1 = P2/T2, 15 PSI / 298.15K = P2/233.15K; P2 = 11.73PSI.

This is the wrong way since I know it is a compressible gas and the pressure should be much lower. Any help or direction would greatly be appreciated.

Thanks!
 
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Weird unit system :(.

PV=nRT takes compression into account, but you won't get a pressure difference if your valves are open. You just get a density difference, while the total amount of SF6 is constant (and known).

Your calculation would require the same amount of SF6 in both cylinders and closed valves.
 

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