Tank to Tank Propane Transfer

I have switched propane suppliers and the 2 new 100 gal tanks have been set up. the old 100 gal tanks (about 30% full) are sitting next to the new. I was expecting a credit for the remaining propane. The old company has told me that they charge a "restocking fee" of $120 to pump out the gas before figuring any credit.

I would like to connect the two vapor lines together, place an electric warming pad under the old tank and fill the new tank. I'm thinking that heating the old tank will pressurize the gas into flowing into the new, cooler tank where it will condense into liquid. I've read where creating this heat differential will be a slow method but will work. This sounds like it can be done safely as long as precautions about ignition sources and exposure of skin to cold propane are taken. Thoughts?

I've read about making such a transfer by turning the "supply" tank upside down and elevating it above the "receiving" tank so that liquid propane is transferred - which is quicker. Unfortunately, the size of my tanks make this approach impractical.

 

why not just leave the old tanks attached and run them dry?

 

DIY transfer of propane is not practical or safe.

It physically will not work to transfer vapor and liquid transfer requires a pump.
Turning one tank upside down and elevating will also not work.
You need a pressure differential and it takes a liquid pump to do that.

$120.00 does not sound like too bad of a price to remove the propane as 100 gallon tanks do not have a liquid tap and they would have to upend the tanks themselves.

 

"DIY transfer of propane is not practical or safe", "why not just leave the old tanks attached and run them dry?" I have decided to play it safe because I think I can work out a way to run the tanks dry before they are collected, and I don't need to run the risk of an explosion - which is just too much of a possibility with propane. I am, however, curious about the mechanics/science involved. When you connect two tanks the pressure inside will equalize, but won't creating the temperature differential I discussed cause the evaporation and condensation I was talking about? Why wouldn't that work? Also this link Viewing a thread - transfering propane from one tank to another demonstrates how tank to tank transfer of liquid propane will wok without a pump by using heat to create the pressure differential. Is this wrong?