How to separate butane from air?
Butane has pretty high  boiling point (around $\ce{0 °C}$). This means, that just cooling air to $\ce{-30 °C}$ should condense a lot (if not all) of butane from it.

However, as I guess, most of butane used in foam production remains in the foam, otherwise the foam would collapse.  So, it is very possible that there is not much to recycle in air itself. The gas, of course, will be released eventually, but only when foam itself is destroyed.