This method envisions cooling the supply flow in a first heat exchanger, separation in a first separation flask in order to produce a light upper flow and a heavy lower flow and dividing the light upper flow into a supply fraction of a dynamic pressure reduction turbine and a supply fraction of a first distillation column. A cooled reflux flow is formed from an effluent from a dynamic pressure reduction turbine, the portion of the effluent being cooled and at least partially liquefied in a heat exchanger. The cooled reflux flow is introduced from the heat exchanger into the first distillation column.