The system is freezing up from the cooling effects of evaporation as the butane is recovered by boiling under vacuum. The conversion from a liquid to a gas, releases the heat of vaporization.
The vapor ends up close to the vaporization temperature, or about 31.5F, until it is compressed again, in which case the heat of compression raises its temperature, as does mechanical friction in the system.
To cool back to a liquid, requires losing those BTU's, or high pressure to turn it to a liquid at higher temperatures.
