"Emptying the tank" is not as easy as you think ! Many a welder has gotton a surprise when he welded a "empty tank" Whether it be propane , gasoline etc the gas is usually mixed with some air and becomes very explosive !
Proper welding proceedures include filling the tank with water or inert gas before attempting to weld .
My local waste transfer station will not take any tanks originally containing propane etc !!!
And Watson , bring your revolver !