Cutting into an LP/Propane tank has HUGE RISK! There is a thread from RCWelding (07-10-2010) with much discussion on this subject.
I did not find any of the following important information in that discussion. Using Dawn soap is discussed in his thread & is warranted.
The question is: What will work best to displace Gasified Propane?
Something to consider in trying to displace Gasified Propane.
Molecular Weights of the fuels/gasses in RCWelding's discussion by lightest to heaviest.
CO (Pure Carbon Monoxide) 28.011
CO (Exhaust/Impure CO) 28.011+ = (unknown wt.)
N3 (Nitrogen) 28.013
Air (common blend of gasses) 28.966
AR (Argon) 39.948
CO2 (Carbon Dioxide) 44.010
C3H8 (Gasified Propane) 44.097
As you can see the Gasified (Vaporized) Propane is the heaviest of all of them with CO being the lightest.
If you are trying to displace the propane vapor, the tank openings MUST BE ON THE BOTTOM to allow the volatile gasses to escape.
If you are purging the tank and the openings are anywhere other than on the bottom you are asking for serious trouble!
None of the lighter gasses can blend with or move the very HEAVY Vaporized Propane uphill to an opening above the bottom.
YES - CO2 will mix fairly well with the vaporized Propane but only to an UNKNOWN dilution at best. Don't chance it!
CO & N3 will "push" the vaporized Propane out a tank bottom better than any of the others as they are the lightest.
However CO (Carbon Monoxide) is very flammable in air mixture concentrations of 12.5% to 74%...
A much wider explosivity range than air mixtures of Propane at 2.15% - 9.6% - a lower explosive limit than CO requiring less material (propane) to achieve said concentration.
Using "Exhaust" (impure) CO is even more hazardous as it has additional unburned oxygen/fuel components from incomplete combustion within the "Impure" Carbon Monoxide, producing a truly unknown explosivity range.
Only INERT Gasses shall be utilized. Anything with an oxidizing component shall be avoided.
Accordingly - N3 (Nitrogen) is the best purging gas as it is very light and has NO OXYGEN or Fuel component! BUT, the tank openings MUST be facing down!
Sources:
http://www.stadealer.com/uploads/1/8/4/1/18412059/coalarminstallationheight.pdf Propane Research and Education Council – www.propane.com