A separation system has a distillation column, an overhead vapor supply pipe connected to the top of the distillation column, first and second branch pipes branching from the overhead vapor supply pipe and a superheater connected to the first branch pipe. A separator downstream of the superheater has a separation membrane for separating overhead vapor from the top of the distillation column into permeable vapor and nonpermeable vapor by allowing only a selected portion of the overhead vapor to permeate the separation membrane. A reflux unit has a condenser connected to the second branch pipe for cooling a portion of the overhead vapor into a liquid and a gas-liquid separator for separating gas from the liquid produced by cooling and returning the separated liquid into the top of the distillation column.