a system for recovering vapor , such as residual isobutane , from powder , such as polyethylene powder , comprises surge vessel and a two - compartment purge column . the polymer passes through the surge vessel by means of valves such as ball valves on the inlet and outlet of the surge vessel . hot vapor , such as isobutane , is used to strip a large portion of the residual isobutane from the polyethylene in the top section of the purge column . part of the isobutane leaving the column goes to a hydrocarbon recovery section and part is heated and recycled as the stripping medium . in the bottom section , nitrogen is used to strip the remaining isobutane from the polymer . an analyzer controller sensing nitrogen in the isobutane stream to recovery controls pressure in the bottom section by manipulating a valve in the nitrogen off - gas line form the bottom section to control the nitrogen content of the isobutane stream being recycled as stripping medium .