1 1 to 1 5 times the minimum reflux ratio however an increase in the reflux ratio leads to an increase in the quantity of the fluid that passes through the compressor 5 and thus results in an increase in the power required to operate the compressor accordingly in order to limit the power required to operate the compressor it is necessary to minimize the reflux ratio as much as possible from the relation of the synergistic functions described above however the reduction of the reflux ratio means an increase in the number of theoretical plates an increase in the number of theoretical plates leads to an increase in the number of stages or the height of the packing inside the column and results in an increased pressure drop inside the column accordingly the pressure difference between the column top and the column bottom becomes greater and results in an increase of the compression ratio pd ps of the compressor described in paragraph i therefore to limit the power of the compressor while economically restricting the reboiler heat transfer area described in paragraph i the discharge pressure of the compressor is set to a pressure at which the discharged vapor has a saturation condensation temperature tdb which is from 3 u00b0 to 20 u00b0 c higher than the boiling point tbb of the column bottom liquid for the reasons.