A heat exchanger is used between the stripper section and rectifier section. The heat exchanger (10) has channels closed on all sides with an end inlet and end outlet for the related section. A compressor is fitted between the sections. According to the invention the heat exchanger is a so-called compact heat exchanger, that is to say a heat exchanger made up of a series of adjacent plates (15, 16) between which channels (11, 12) closed on all sides for the related section are delimited. These channels (11, 12) extend essentially vertically and are so constructed that all liquid contained therein is able to move towards the bottom of the section concerned. The flow cross-section of the channels of the stripper/rectifier section changes depending on the process carried out therein.