Oxygen is produced commercially in large quantities by the cryogenic rectification of feed air, generally employing the well known double column system, wherein product oxygen is taken from the lower pressure column.  At times it may be desirable to produce oxygen at a pressure which exceeds its pressure when taken from the lower pressure column.  In such instances, gaseous oxygen may be compressed to the desired pressure.  However, it is generally preferable for capital cost purposes to remove oxygen as liquid from the lower pressure column, pump it to a higher pressure, and then vaporize the pressurized liquid oxygen to produce the desired elevated pressure product oxygen gas. 
The pressurized liquid oxygen is vaporized against a pressurized working fluid which is then introduced into the cryogenic rectification plant.  The working fluid is throttled from the pressure required for the heat exchange to the pressure required by the plant.  This results in an energy loss due to the thermodynamic irreversibility of the throttling step.  It would be desirable to recover at least some of the lost work associated with the throttling of the pressurized working fluid to the pressure needed by the cryogenic rectification plant. 
Accordingly, it is an object of this invention to provide a cryogenic rectification system which can produce elevated pressure gaseous oxygen by the vaporization of pressurized liquid oxygen against a pressurized working fluid while recovering at least some of the work lost when the pressurized working fluid is expanded to a pressure suitable for the cryogenic rectification plant.