Dry ice and ethanol will lower the temp of the tank way more than necessary to get the “air” out. It would be helpful during recovery, but is unnecessary and possibly even counter productive for purging the atmosphere.
Ideal would be about -20C. which is about 20C below the boiling point of butane. So ALL his solvent will be a liquid. the “air” on the other hand will all still be a gas. at that temp, is is safe to vent ALL the pressure in then tank. except of course the OP only has a dip tube, so if they just “burp the tank” like @Concentrated_humbold suggests, all the liquid (their butane) will come out first. @spitzenkorper has to turn their tank upside down to vent gas from that tank.
As far as trapping moisture: molsieves!
Then there is the pump. The CMEP-OL’s used to come with a forced air heat exchanger plumbed after the pump, it did a great job of pulling off most of the heat put into the solvent by the pump. I’ve been told that so many folks were deleting that heat exchanger and adding a larger liquid cooled one, that the manufacturer deleted it to save money. Problem is, that means folks like @spitzenkorper’s boss think they are designed to run without a heat exchanger…they’re not.
How do I know?
See 1k word substitute above!!
Because I’ve run both versions, and had to implement heat exchangers on two of the newer flavor in the last month. Otherwise the tank heats up, and you get increasingly high back pressures. Took me by surprise on the first one, because I’ve run three of the old style…
It doesn’t take much water to remove that heat. even the mt-69 in a 5gal bucket with some ice water will greatly speed OP’s recovery. using a heat exchanger designed to self drain once the pump has been turned off is ideal, but those are usually enclosed and cooled by a circulating chiller.
Both machines I was adding CMEP-OL’s to had those heat exchangers built in already…does that tell you how important they are?!?
Edit: venting the tank till it self chills and stops is another option. it’s faster, but distinctly less safe.
the tank mentioned below had a dip tube, and no condensed liquid. Making it a slightly different critter than the OP’s