It would seem that the PRC will only output Ethalene to the right hand side, even if the gas tab's config is set to output gas on the left. Is this intended, seeing as the Ethalene tank in the gui is shown on the right side?
Some images:
http://imgur.com/a/m9wjC

Also related to the PRC: It has no "dump tank" buttons on its gui. I currently have one that is stuck full of water and oxygen, and cannot be used for anything as a result.

In terms of the dump tank button, I recommend using a gauge dropper :)

Looking into the first issue

Gauge dropped did the trick, thanks (although it takes quite a bit to drain an entire tank xD)

EDIT: It might also be useful to have a dump/dump excess/idle button on the output tank of the PRC as well, in case you want to just throw away the excess oxygen for example. (created from making HDPE Pellets/substrate)

I'll consider it!