The LOX of 99.8% O2 is drawn from oxygen side of the evaporator-condenser, some of them enters storage tank as product. The product nitrogen of 99.99%N2 from recycle N2 compression system is supplied to the user for use. The waste nitrogen from the top of the upper column is warmed up, one part goes to the regeneration molecular sieve adsorber, another goes to waste nitrogen tower to cool the water.