Air separation by cryogenic distillation using medium and low pressure columns, for production of oxygen and/or nitrogen, with residual stream extracted from low pressure column to maintain product purity

摘要

In the separation of air by cryogenic distillation, by supplying cooled, purified, compressed air to medium pressure column(s) (1) for separation into oxygen-enriched fluid (5) and nitrogen-enriched fluid (6, 7), then supplying at least part of the O2-enriched fluid (in)directly to a low pressure column (2) (operating at at least 2 bars) from which N2- and O2-enriched streams (9) and (10) are extracted at first and second levels respectively, the purity of the N2- and/or O2-enriched stream(s) is/are maintained by extracting a residual stream (11) between the first and second levels, this stream optionally then being depressurized. An independent claim is included for a corresponding plant, including the medium pressure column(s) (1); the low pressure column (2), supplied (in)directly with O2-enriched fluid from the medium pressure column(s) via line (5); a heater for a sump reboiler for the low pressure column; lines for extracting the N2-enriched stream (9), O2-enriched stream (10) and residual stream (11) from the low pressure column at the first, second and intermediate levels respectively; and a monitor for the purity of the N2- and/or O2-enriched stream(s), the values from which are used to regulate the flow rate of the residual stream.