Oxygen Production via Cryogenic Distillation

This column is based on “Oxygen Production – Cost Analysis,” a report published by Intratec. It can be found at: www.intratec.us/analysis/oxygen-production-cost.

Oxygen is a nonmetallic element and one of the most abundant elements on Earth’s surface. Under standard pressure and temperature conditions, oxygen is a diatomic gas with the molecular formula O₂. The gas is used in several industries, including steel-making, chemicals, wastewater treatment, pulp and paper manufacturing, and others.

The process

The following paragraphs describe a process for oxygen production based on cryogenic distillation. Figure 1 presents a simplified flow diagram for the process.

Figure 1. The diagram shows a process for oxygen production via cryogenic distillation

Feed preparation. Initially, atmospheric air is passed through a mechanical air filter to remove dust particles, and compressed to a pressure of about 6 bars. The compressed air is fed to a direct-contact cooler, where it is cooled first with cooling water and then with chilled water. Subsequently, the cooled, compressed air is passed through an adsorbent bed of molecular sieves for the removal of water, carbon dioxide and other trace impurities.

Cryogenic separation. Purified air is then cooled down to nearly liquefaction temperature by means of expansion and heat exchange into a plate-fin heat exchanger (the main heat exchanger) in counterflow against cold nitrogen and oxygen product streams.

Gaseous and liquid air streams from the main heat exchanger are fed to the high-pressure (HP) column, the first of two distillation columns whose purposes are to separate an oxygen-enriched liquid stream from nitrogen. The product from the HP column overhead, consisting of pure nitrogen vapor, is condensed in the reboiler of a second distillation column downstream, operating at lower pressures (the low-pressure (LP) column), and used as reflux for both the HP column and LP columns.

The bottom product from the HP column, a liquid stream rich in oxygen, is split into two streams: part is fed into the LP column, while the remaining amount is routed to the combined evaporator-condenser of the argon column, where it is evaporated and subsequently sent to the LP column.

The LP column overhead, consisting primarily of nitrogen with traces of argon, is fed to the main heat exchanger, where it is warmed by heat exchange against purified air and subsequently compressed. The stream composed of high-purity oxygen, obtained as the LP column bottom, is compressed and vaporized by heat exchange against purified air in the main heat exchanger, and is then routed to nearby consumers via pipeline.

A side-stream rich in argon is withdrawn from the LP column and directed to a column where it is purified from most of the argon and returned to the LP column.

Production pathways

Oxygen can be commercially separated from atmospheric air, via cryogenic distillation and adsorptive separation (vacuum-swing adsorption processes) (Figure 2).

Figure 2. Several variations exist for oxygen production

Economic performance

The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce industrial oxygen was about $0.03 per normal cubic meter of oxygen in the first quarter of 2015, including credits associated with sale of nitrogen byproduct. The analysis was based on a large-scale plant constructed in the U.S. with the capacity to produce 3,000 tons per day of O ₂.

Edited by Scott Jenkins