If the amount of expanded air entering the upper column is reduced, will the oxygen content in the crude argon increase?
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Recently, our plant (which produces argon without using hydrogen, with compressed air being fed into the upper column) has operated at reduced load levels by decreasing the amount of expanded air fed into that column (by about 2000 NM3/H, from 27,000 to 25,000 NM3/H). This was done to prevent excessive nitrogen content in the crude argon, which could lead to nitrogen clogging. The goal was achieved, but the oxygen content in the crude argon increased, which is different from what I expected. I thought that reducing the amount of expanded air and increasing the reflux ratio in the upper column would lower the oxygen purity; therefore, the oxygen content in the crude argon should have decreased. What are everyone’s expert opinions on this?
Your understanding is correct; it’s right to change the approach. The question is that you are operating at reduced load; as oxygen production decreases, does the air volume decrease as well? In other words, has the oxygen purity increased?
Go to the 2nd floor first; the air volume, the amount of oxygen removed, and the lean liquid returned to the upper column remain unchanged – only the expanded air fed into the upper column is reduced
To reduce the amount of expanded air entering the upper tower, one should first decrease the amount of air supplied to the air-cooled tower, thereby reducing the overall load on the air separation unit and making adjustments accordingly
The argon extraction port is generally located slightly below the argon-rich zone; since the argon-rich zone has a high argon content but also a high nitrogen content, which makes nitrogen clogging likely, the argon extraction port is placed slightly below that zone. The argon extraction ports are mostly located below the feed inlet, and the position of the tray plates is fixed. When you reduce the degree of expansion, you are also reducing the amount of feed entering the tower; oxygen and nitrogen separate on these trays. The further up one goes in the retention section, the higher the oxygen content becomes, while the nitrogen content decreases accordingly. So one of the two values increases, while the other definitely decreases; the reduction in expansion is equivalent to moving the argon extraction port downward. As a result, the oxygen content is high, while the nitrogen content is low. As for the effect of a reduced reflux ratio on oxygen purity, this only occurs when the ratio is reduced to a certain extent
Thank you all for your valuable suggestions. After numerous tests, it has indeed been confirmed that the oxygen content in crude argon is on the rise.
With the processing air volume remaining constant, reducing the amount of expanded air fed to the upper column increases the amount of air fed to the lower column. As a result, more nitrogen condenses on the nitrogen side of the main cooler, and the volume of gas rising in the distillation section at the lower part of the upper column (below the expanded air inlet) increases. With the amounts of gaseous oxygen and liquid oxygen removed remaining unchanged, the reflux ratio in that distillation section decreases, the nitrogen content in the argon fraction drops, while the oxygen content in the argon fraction rises；
The last edit to this post was made by sds666 on 2010-8-7 at 12:04. Reply: 7# BLUEPrince – There seems to be an issue with the analysis in 7#: Reducing the amount of expanded air entering the upper tower does not mean that the total volume of expanded air is reduced (remember that it’s possible to reduce the amount of air entering the Rahman inlet through the expanded air bypass)； What you mean is that reducing the expansion amount (by narrowing the expander nozzle) can increase the amount of air entering the lower tower; analysis of #7 follows thereafter ； In short, the expanded air fed into the upper column (Rahman inlet) contains a high level of nitrogen. Increasing the amount of expanded air supplied to the upper column aims to improve the oxygen extraction rate; this, however, leads to an increase in nitrogen content and a decrease in oxygen content in the argon fraction ； Conversely, reducing the amount of expanded air fed into the tower will lead to a decrease in nitrogen content and an increase in oxygen content in the argon fraction. I believe that the effect of introducing expanded air into the upper column on the distillation section can be analyzed through the reflux ratio: that is, as the amount of expanded air fed into the upper column increases, the reflux ratio decreases, the nitrogen purity rises, and the argon enrichment zone shifts downward. It can also be determined that this leads to an increase in nitrogen content and a decrease in oxygen content in the argon fraction ； The stripping section cannot be analyzed using the reflux ratio, as it is quite complex and includes an oxygen-enriched liquid nitrogen inlet, an argon fraction extraction port, a cold source of liquid nitrogen for the purified argon condenser as well as liquid nitrogen and liquid argon steam inlets, and a liquid argon reflux inlet. The more textbook-style explanatory analysis is for 5#, as clearly stated in the questions and answers for oxygen producers. Those with different opinions are welcome to add corrections.