An apparatus comprising first to third columns, wherein the outlet of a first column reboiler and the inlet of a second column condenser are connected by a first introduction conduit, and the outlet of a second distillation column reboiler and the inlet of a third column condenser are connected by a second introduction conduit, and additionally the outlet of the second column condenser and the inlet of the first column reboiler are connected by a first return conduit, and the outlet of the third column condenser and the inlet of the second column reboiler are connected by a second return conduit.