Argon production from air distillation: Use of a heat pump in a ternary distillation with a side rectifier

Abstract

A ternary feed mixture ABC can be separated into individual components through the use of a main distillation column with a thermally linked side rectifier. To enhance such a separation, a heat pump can be implemented to transfer heat from the condenser at the top of the side rectifier to the reboiler at the bottom of the main column. In this paper, one such heat pump is described and applied to an air distillation system separating the ternary mixture containing nitrogen, oxygen and argon. The separation is performed by a conventional double column with a crude argon side column. When this system is operated at an elevated pressure to obtain higher product pressures, the separation of oxygen and argon becomes very difficult and leads to reduced argon recovery. The proposed heat pump enhances the separation by providing a supplementary crude argon condensing duty through the vaporization of a liquid oxygen stream from the bottom of the low pressure (LP) column. This scheme improves the liquid/vapour ratio (L/V) in the bottom section of the LP column and, more importantly, increases the vapour feed to the crude argon column. This increased feed rate leads to a substantial increase in argon recovery for the elevated pressure air distillation process.