A method for cryogenic separation of air in an air separation plant which has a main air compressor, a main heat exchanger and a distillation column system with a high-pressure column and a low-pressure column, said method comprising:
- compressing feed air in the main air compressor to a first air pressure which is at least 3 bar higher than an operating pressure of the high-pressure column, in order to form a compressed feed air flow,
- splitting the compressed feed air flow into at least a first air flow and a second air flow,
- cooling and liquefying or pseudo-liquifying the first air flow at said first air pressure in the main heat exchanger, then expanding said first air flow and introducing said first air flow into the distillation column system,
- compressing the second air flow in an air post-compressor to a second air pressure which is higher than the first air pressure, to form a compressed second air flow,
- splitting the compressed second air flow into at least a third air flow and a fourth air flow,
- further compressing the compressed second air flow, before the compressed second air flow is split, or further compressing the third air flow or the fourth air flow in a post-compression system to a third air pressure which is higher than the second air pressure, wherein the post-compression system has at least one first turbine-driven post-compressor,
- introducing the third air flow at a first turbine inlet pressure into a first turbine, where the third air flow is expanded, performing work, and then introducing the third air flow into the distillation column system, wherein the first turbine drives the first turbine-driven post-compressor,
- cooling and liquefying or pseudo-liquifying the fourth air flow, in the main heat exchanger, then expanding said fourth air flow and introducing said fourth air flow into the distillation column system,
- withdrawing a first product stream from the distillation column system in liquid form, pressurizing the first product stream, in liquid form, to a first elevated product pressure, evaporating or pseudo-evaporating and further heating the first product stream at the first elevated product pressure in the main heat exchanger to form a first compressed gas product and withdrawing the first compressed gas product from the air separation plant,
- cooling a sixth air flow in the main heat exchanger to a first intermediate temperature, and compressing the sixth air flow in a cold compressor to a fourth air pressure which is higher than the third air pressure, and
- cooling and liquefying or pseudo-liquifying the sixth air flow at the fourth air pressure in the main heat exchanger, then expanding the sixth air flow and introducing the sixth air flow into the distillation column system,
- wherein said method has at least a first mode of operation and a second mode of operation, and in both said first mode of operation and said second mode of operation at least one liquid product is produced in the distillation column system and withdrawn in liquid form from the air separation plant,
- in the first mode of operation the at least one liquid product is withdrawn from the air separation plant in a first amount,
- in the second mode of operation the at least one liquid product is withdrawn in a second amount, wherein the second amount of the at least one liquid product is 15 to 50% of the first amount of the at least one liquid product, and
- in the first mode of operation a first amount of the sixth air flow flows through the cold compressor, in the second mode of operation a second amount of the sixth air flow flows through the cold compressor, and the first amount of the sixth air flow is less than the second amount of the sixth air flow.