Process for the low-temperature fractionation of air

The method and device are used for the cryogenic separation of air in a distillation column system having at least a high pressure column (11) and a low pressure column (12). Feed air (20, 32) is introduced into the high pressure column (11). A liquid product stream (42; 46) is removed from the distillation column system is brought into a liquid state to an elevated pressure (43; 47) and under this elevated pressure by indirect heat exchange (6) is vaporized or pseudo vaporized and finally as a gaseous product stream (45; 49) withdrawn. The entire feed air (1) is compressed in a main air compressor to a first pressure which is at least equal to the operating pressure of the high pressure column (11), and subsequently cleaned in a cleaning device. A first partial flow (2, 7) of the feed air is supplied at approximately the first pressure to a first expansion machine (8), there to perform work on approximately the operating pressure of the low pressure column (12) is relaxed and then into the low pressure column (12) is introduced (10). A second partial stream (3) of the feed air is compressed to a second pressure in a first booster (14), which is higher than the first pressure. At least a portion (17) of the second partial stream (16) downstream of the compression is in a second booster (18) further compressed to a third pressure which is higher than the second pressure, the indirect heat exchange (6) for evaporation or ...