Air Separation Low Purity Oxygen Production

Air Separation Low Purity Oxygen Production

There are many process cycles designed to meet the wide variety of different product requirements of cryogenic air separation plants. One of the most economic arrangements for the production of low purity (95%) gaseous oxygen, at reasonable pressures (say, 6 bar), is known as the “mixing column cycle.” This requires a separate column to produce the product oxygen. This research note describes how this column can be fitted inside an existing column at an overall reduced cost.

Basic Cycle

In the straightforward form of the cycle shown in Fig. 1, air is purified by refluxing it with fairly pure oxygen pumped, after having been purified, from the Low Pressure (LP) column.

Thus, the resultant gaseous oxygen product is available at about the same pressure as the feed air. The column here is a separate pressure vessel handling about a third of the flow through the conventional high pressure (HP) column.

Higher Pressure Version

A variant of the cycle as shown in Fig. 2 can deliver gaseous oxygen at a somewhat higher pressure than the feed air. This is done by installing a booster compressor in the air supply to the mixing column and running the reflux pump at a higher (matching) pressure. Other modifications shown in Fig. 2 improve the system.

Since the mixing column and the HP column operate at about the same pressure, with the same feed, about the same number of theoretical stages, and with a favorable flow ratio, it is feasible to put the mixing column inside the HP column. In the basic cycle, then, the mixing column will no longer be a pressure vessel, there will be no requirement for a lower hea...