In general, single column distillation will be selected for the fractionator of air separation unit with nitrogen production capacity less than 20000 nm3 / h. When the unit scale is larger than 20000 nm3 / h, it is necessary to consider the application of double column distillation unit for investment recovery comparison. For the distillation column, the lower the working pressure, the higher the distillation efficiency, but the diameter of the column will increase.