Distillation of Air to Recover Argon

After nitrogen and oxygen, argon is the most abundant component in air. Its inert property is quite attractive for metals and several other material-processing applications. Within a very short time of the discovery of the double-column system, argon was distilled from air in 1913. The distillation arrangement to produce argon in modern plants was generally described in a German patent by 1935.

The arrangement for argon production starts with an examination of the argon concentration profile in the low pressure column of a double-column process. From the normal boiling temperatures listed in Table 1, it is readily observed that the volatility of argon is between that of nitrogen and oxygen and, furthermore, it is closer to oxygen than nitrogen. As a result, the concentration of argon in the liquid nitrogen stream from the high pressure column (Figures 3 and 4) is at p.p.m. level and virtually all the argon is contained in the crude liquid oxygen. Therefore, the bulk of the argon in air enters at an intermediate point on the low pressure column. When oxygen containing less than 0.5% argon is produced, argon is forced to escape from the top of the low pressure column in the nitrogen-rich vapour stream. However, the liquid nitrogen reflux stream is virtually free of argon and tends to drive argon down the low pressure column. Consequently, the concentration of argon in the vapour phase at an intermediate location between the crude LOX feed and the oxygen product withdrawal point reaches levels approaching 20%. A typical vapour-phase argon concentration profile in the low pressure column is shown in Figure 5. The build-up of argon provides an opportunity to withdraw a side vapour stream from near the location where a peak in argon concentration occurs and to distil it further in a side distillation column to produce concentrated argon.

A typical argon recovery arrangement is shown in Figure 6. An argon-rich vapour stream containing between 10 and 25% argon, p.p.m. levels of nitrogen and the rest oxygen is withdrawn from an intermediate location of the bottom section of the low pressure column and is fed to the bottom of a side argon column. The flow of this vapour stream is about 20% of the feed air. As vapour ascends the side argon column, it is depleted in oxygen. The development of structured packing for cryogenic distillation columns has allowed the modern cryogenic plants to use in excess of 175 theoretical stages of separation in the side argon column. As a result, the vapour at the top of this column can contain only p.p.m. levels of oxygen. This vapour stream is condensed in a reboiler-condenser; most of it is returned back as reflux to the side argon column and a small portion is recovered as a crude argon stream. The liquid to vapour flow ratio in this column is in the neighbourhood of 0.97. The liquid from the bottom of the side argon column is

Figure 5 Vapour-phase composition in the low pressure column.
Figure 6 Distillation arrangement for argon separation from air for the process shown in Figure 4.

pumped back to the argon-rich vapour draw location of the low pressure column. Condensation at the top of the side argon column is provided by boiling a portion of the crude liquid oxygen at nearly the low pressure column pressure, as shown in Figure 6. The vaporized crude liquid oxygen stream is fed to the low pressure column a few stages of separation below the location where the unboiled crude liquid oxygen is fed. The recovery of argon from cryogenic air is easily in the range of 70-85% and occasionally, if needed, it can be as high as 95% of the total argon contained in the feed air.

While the concentration of oxygen in the crude argon recovered from the arrangement in Figure 6 is below 5-100 p.p.m., the nitrogen concentration is much higher, as nearly all the nitrogen contained in the argon-rich vapour stream shows up in the crude argon. Generally, the argon product specification requires that the nitrogen concentration also be below 5 p.p.m. Therefore, the crude argon stream is subsequently distilled in a pure argon column with separation stages both below and above the feed. Boil-up and condensing duties for this column are extremely low and are easily provided by using side streams that are withdrawn from one or more appropriate process streams, shown in Figure 6, such as crude liquid oxygen, high pressure liquid air or high pressure nitrogen vapour. A waste vapour stream containing all the nitrogen is withdrawn from the top of the pure argon column and pure liquid argon product is collected and sent to a storage tank from the bottom of this column.

Solar Panel Basics

Solar Panel Basics

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