A method for recovering high-pressure oxygen and high-pressure nitrogen by low-temperature separation of air in a distillation-column system comprising a high-pressure column (4) and a low-pressure column (5), which are in heat-exchange connection via a main condenser (6) designed as a condenser-evaporator, said method comprising:
- Cooling a first air feed stream (100, 101) at a first, subcritical, pressure, which is less than 1 bar above the operating pressure of said high-pressure column (4), in a main heat exchanger (2) to approximately its dew point, and introducing (3) the cooled first air feed stream, at least partially, into said high-pressure column (4),
- Cooling a second air feed stream (200) at a second, supercritical, pressure, and subsequently depressurizing and introducing the cooled second air feed stream, at least partially, into said distillation-column system,
- Pressurizing (17) a liquid oxygen stream (16) from said low-pressure column (5), in the liquid state, to a first product pressure which is higher than the operating pressure of said low-pressure column, heating said liquid oxygen stream (16) at the first product pressure to approximately ambient temperature in a high-pressure heat-exchanger system (11, 12) having at least one helically-wound heat exchanger, and ultimately recovering as a high-pressure oxygen product stream (18),
- Pressurizing (20) a liquid nitrogen stream (19) from said high-pressure column (4) or from said main condenser (6), in the liquid state, to a second product pressure which is higher than the operating pressure of said high-pressure column (4), heating said liquid nitrogen stream (19) at the second product pressure to approximately ambient temperature, and ultimately recovering as a high-pressure nitrogen product stream (21),
- Cooling a first partial stream (201) of said second air feed stream (200) by indirect heat exchange in said main heat exchanger (2),
- Cooling a second partial stream (202, 221) of said second air feed stream (200) in said high-pressure heat-exchanger system (11, 12),
- Merging said first partial stream (211) and said second partial stream (221) of the second air feed stream downstream from their cooling to form a merged second air feed stream,
- Wherein the heating of the liquid nitrogen stream (19) that is pressurized in liquid form is performed in said main heat exchanger (2),
- Wherein the cooling of said first partial stream (201) of the second air feed stream (200) is performed in the main heat exchanger (2),
- Wherein said merged second air feed stream is depressurized in a liquid turbine (13) before said merged second air feed stream is introduced (205, 3) into said distribution-column system,
- Wherein said high-pressure heat-exchanger system (11, 12) has two helically-wound heat exchangers connected in series,
- a third partial stream (230) of said second air feed stream (200) is branched off from the second partial stream (206) of said second air feed stream between the two helically-wound heat exchangers (11, 12) of said high-pressure heat-exchanger system, and introduced into the main heat exchanger (2) at an intermediate point and further cooled therein, and
- after said third partial stream (230) is branched off from said second partial stream, the remainder of the second partial stream (206) of the second charging air stream is further cooled in the high-pressure heat-exchanger system (12).