Re: convert propane tank to air
Problem is that propane tanks don't have condensation drains so one would have to be added.
The tank would have to be emptied and purged by flowing at least 10 changes of air through it. The vent must be near the bottom (liquid out line would be fine).
If it's at least 1/4 inch thick, you can drill and tap a 1/8 in pipe thread hole in the bottom of it for a drain. If it's thinner, you'd need to weld a threaded fitting to it. They are readily available for manufacturers. I could get one for you if you need it. You could make one also, a disk of metal with a pipe thread hole through it.
The Butyl Mercaptan odorant is another issue. You should be able to wash it out with several rinses of soap and water.
What's the air for? I'm thinking "Train Horn".
hope it helps
John