Air separation oxygen production area: [Weekly Topic] Week 8 of 2011——What is the difference between extracting liquid nitrogen from the lower column and not doing so?

The original poster had better make it clear whether the liquid nitrogen for pumping out contaminants is drawn from the bottom or from the top of the tower, as the effects are different in each case. In the internal compression process, the air entering the lower column has a high moisture content. To ensure the purity of the oxygen-enriched liquid at the bottom, contaminated nitrogen liquid is drawn off at the fifth tray above the bottom of the lower column and sent to the upper part of the upper column ； If too much medium-pressure nitrogen and product liquid nitrogen are drawn from the lower column, in order to increase the reflux ratio of the upper column, contaminated liquid nitrogen is drawn from the fifth tray below the top of the lower column and fed into the top of the upper column. For the former case, Hangyang should not extract contaminated liquid nitrogen ； The latter one isn’t quite clear.

Reply to 2# davidcao: My knowledge is limited; can liquid nitrogen be extracted from the top of the tower? I’m referring to the lower part of the tower, that is, the fifth tray as you mentioned. I also know that in most cases internal compression is used for extraction, and at Hangyang Oxygen, external compression is generally not employed in the design ； ”But I encountered an external compression Hangyang Oxygen process in which the extraction of liquid nitrogen was also designed, so I really don’t understand why It seems there’s no one in the air separation unit!

In air separation units with high production volumes of pure gaseous nitrogen, the lower column generally does not extract liquid nitrogen waste. When pressurized expansion is used with a small degree of expansion, it becomes unnecessary to extract liquid nitrogen from the lower column. In the previous process, contaminated liquid nitrogen was drawn off because of the high demand for nitrogen; the amount of liquid nitrogen flowing from the lower column to the upper column was small. To ensure the efficiency of distillation in the upper column, it was necessary to draw off contaminated liquid nitrogen and feed it into that column. Due to the use of a molecular sieve adsorption process, the demand for nitrogen is reduced; the liquid nitrogen reaching the upper column is sufficient to meet this demand, so there is no need to extract additional liquid nitrogen for supplementation.

Reply to 3# Youyou: Drawing off liquid nitrogen from the bottom of the lower column is done to ensure the purity of the oxygen-enriched liquid air. When the moisture content of the air entering the lower column is high, whether it comes from air separation processes involving internal compression, processes with high liquid output, or processes involving self-pressurization of liquid oxygen, one principle holds: the greater the amount of liquid drawn off from the main column (considering the lower column, upper column, main cooler, and argon column as a whole), the greater the amount of liquid that needs to be added back (i.e., the higher the moisture content of the air entering the column). I’m not sure what process is used for your Hangyang air separation unit, but the design principles for air separation are the same. You might as well post the product parameters of the air separation unit so that we can discuss them together. Drawing contaminated liquid nitrogen from the top of the lower column is used when a large amount of medium-pressure nitrogen is extracted from the lower column and a large quantity of liquid nitrogen product is produced (or when compression occurs within the liquid nitrogen), in order to use this contaminated liquid nitrogen as the reflux fluid at the top of the upper column. With a constant amount of air entering the tower and the same nitrogen level in the product, more nitrogen-rich liquid is drawn out from the lower section of the tower, resulting in a larger reflux flow from the upper section.

Reply to 5# davidcao: First of all, thank you for your answer (especially the mention of oxygen self-pressurization)； I still have two questions that aren’t entirely clear: First, is the total amount of cooling capacity or liquid volume in the air separation unit constant, regardless of the moisture content entering the tower? ； To maintain the liquid level balance in the lower column, all liquid can enter the upper column through the liquid air throttle valve or the liquid nitrogen throttle valve; then why is it necessary to use dirty liquid nitrogen as well? Secondly, regarding the determination of the amount of contaminated nitrogen to be removed, many people say it should be based on the purity of the liquid oxygen. However, from my experience with several air separation units I have worked on, contaminated nitrogen does have an impact on the liquid oxygen, but not a very significant one. So, should the valve for contaminated nitrogen be kept open? Or should we keep it at a lower level as long as the purity of the liquid air meets the requirements? Give an example: What should be done if the liquid nitrogen valve is fully open yet the purity of the liquid air still does not reach 37-39%?

Air separation technology evolves rapidly, but the principle should remain the same. I hope we can discuss together the new issues brought about by new technologies. In the lower tower, if the opening degree of the liquid nitrogen throttle valve is too small, the reflux ratio increases, resulting in a decrease in the purity of the liquid oxygen; if this purity falls below 38%, it further leads to a reduction in the purity of oxygen. In such cases, the opening degree of the liquid nitrogen throttle valve must be increased. At this point, although the purity of liquid nitrogen decreases, the purity of gaseous nitrogen actually increases. In processes equipped with a contaminated liquid nitrogen throttle valve and a pure liquid nitrogen throttle valve, during operation, the contaminated liquid nitrogen throttle valve is generally used to control the purity of the liquid air, while the pure liquid nitrogen throttle valve is used to control the purity of liquid nitrogen.

Can’t fully opening the sewage nitrogen extraction valve still ensure that the liquid-air purity meets the requirements?

This post was last edited by davidcao on 2011-2-23 at 19:18. Reply to 6# Youyou: You’re too kind; it’s through mutual discussion that we can improve. As mentioned earlier, extracting waste nitrogen from the lower column is done to ensure the purity of the oxygen-enriched liquid air. In conventional external compression air separation processes, the purity of the liquid air in the lower column can reach 40%; however, in internal compression air separation processes, if waste nitrogen is not extracted, the purity of the liquid air may only be 30%, resulting in a lower oxygen extraction rate. Of course, the amount of liquid nitrogen isn’t unlimited either, as the distillation capacity of the tower is after all limited. When designing, we generally ensure that the purity of liquid air is around 35%. The amount of contaminated nitrogen does not exceed the oxygen flow rate of the product.

Reply to 9# davidcao: “The amount of liquid nitrogen should not exceed the oxygen flow rate of the product.” ”Where does it originate? I would be grateful for your advice! I have this issue: even when the liquid nitrogen throttle valve is opened wide enough >70%, the purity of the liquid air remains the same

Reply to 8# shuhuahuang: Yes! What do you say we should do then? And could you please tell me what your air separation grade is? How large should the slurry nitrogen valve be opened to? What’s the diameter? Thank you