You need a compressor to suction the propane out into a fresh cylinder. You can't "pump" a vapour. Need to re-compress it. The liquid in the bottom will flash in the tank as the volume is being drawn out. This is not worth it. Just vent it to atmosphere, or burn it.
"The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents... some day the piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the light into the peace and safety of a new Dark Age."
-H.P. Lovecraft