A method for the cryogenic fractionation of air, in which a liquid volume present in a vaporization chamber in a distillation column system of an air fractionation plant is fed by means of a cryogenic liquid and in which a proportion of the liquid volume is continuously transferred into the gas phase by vaporization, wherein, in addition to oxygen, the cryogenic liquid contains components, including xenon, which are higher-boiling than oxygen. The content of xenon in the cryogenic liquid is determined and used as a measure of any enrichment of the components which are higher-boiling than oxygen in the cryogenic liquid. A corresponding air fractionation plant is also described.

Recovering xenon and/or krypton from a feed gas can include utilization of a purge stream from a separation column positioned and configured to output at least one stream of fluid that is substantially nitrogen and at least one stream of fluid that is substantially oxygen. The purge stream can be split so that a first portion of the purge stream is fed as a liquid adjacent to a top of a purge treatment column and a second portion of the purge stream can be fed to a heat exchanger for superheating the second portion to feed a superheated vapor at or adjacent to a bottom of the purge treatment column. The purge treatment column can output a liquid stream that has a relatively high concentration of Xe and/or Kr therein as a feed stream for an Xe and/or Kr recovery system.

A method and apparatus for producing compressed nitrogen and liquid nitrogen. A separation system has a high-pressure column, a low-pressure column with a top condenser and a main condenser. Air is compressed in an air compressor, purified, cooled in a heat exchanger and introduced into the high-pressure column. A first part of the gaseous top nitrogen from the low-pressure column becomes compressed nitrogen product. A second part of the gaseous top nitrogen is condensed in the condensing space of the top condenser and vapour is drawn off as a residual gas stream. The vapor is expanded in a first expansion machine. A second compressed nitrogen stream from the top of the high-pressure column is expanded in a second expansion machine and then drawn off as compressed nitrogen product. A part of the nitrogen condensed in the top condenser is drawn off as liquid nitrogen product.