In a method for separating air by cryogenic distillation using a column system consisting of a higher pressure column operating at a first pressure and a lower pressure column operating at a second pressure, a first air flow constituting between 75% and 98% of the air sent to the column system compressed to a third pressure above the first pressure, is sent to the higher pressure column, a second air flow constituting between 5% and 25% of the air sent to the column system is compressed to a fourth pressure above the second pressure but lower than the third pressure, is sent to the lower pressure column, a third column separates an argon-enriched flow and the air sent to the lower pressure column constitutes between 10% and 25% of the total air sent to the column system.

An apparatus for the separation of air by cryogenic distillation comprises three columns, including two concentric columns, the external diameter of the third column being at most equal to that of the second distillation column, a pipe for feeding the third column with air, a reflux pipe connected to an intermediate level of the upper section of the first distillation column in order to withdraw a liquid enriched in nitrogen, the pipe being connected to the head of the second distillation column and passing through a region of the third column devoid of heat exchange means and of mass exchange means, and an intermediate pipe for withdrawing a liquid at an intermediate level of the first distillation column.

In a method for separating air by cryogenic distillation using a column system consisting of a higher pressure column operating at a first pressure and a lower pressure column operating at a second pressure, a first air flow constituting between 75% and 98% of the air sent to the column system compressed to a third pressure above the first pressure, is sent to the higher pressure column, a second air flow constituting between 5% and 25% of the air sent to the column system is compressed to a fourth pressure above the second pressure but lower than the third pressure, is sent to the lower pressure column, a third column separates an argon-enriched flow and the air sent to the lower pressure column constitutes between 10% and 25% of the total air sent to the column system.

Plate-fin heat exchanger with mitered distributor design for improved fluid flow distribution through the plate-fin heat exchanger resulting in improved heat exchanger efficiency. Sections of the distributor have different fin types that provide improved distribution of the fluid through the heat exchanger. The fin types for the different sections of the distributor are selected based on a friction factor parameter ratio and a j-factor parameter ratio for the different fin types.

An object of the present invention is to provide a heat exchanger that can be used in an air separation unit including a low-pressure column, a high-pressure column, and a mixing column, and that can suppress increases in equipment costs. The present invention provides a heat exchanger used in an air separation unit (1100) including a low-pressure column (600), a high-pressure column (500), and a mixing column (400), wherein the heat exchanger is composed of plates and fins, and wherein a warm gas which is at least a part (at least one of W1, W2, W3) of a feed air and at least one warm liquid (at least one of W4 and W5) which is led out from the mixing column (400) are cooled by at least one cold gas (at least one of C2 and C3) which is led out from the low-pressure column (600) and a cold gas (C1) which is led out from the mixing column (400), and a cold liquid (C6) which is led out from the low-pressure column (600) through a pressure boosting pump (800) and supplied to the mixing column (400) is heated.