Air separation oxygen production area: [Weekly Topic] Week 13 of 2011——The differences between nitrogen expansion and air expansion

Nitrogen expansion media are clean and offer a large specific enthalpy drop, but the nitrogen required for nitrogen expansion is not readily available! I previously worked on a liquid process using nitrogen expansion; the nitrogen used for expansion was taken from the top of the lower column, and liquid nitrogen was returned to that column. Since the nitrogen at the top of the lower column was impure at the beginning, the nitrogen circulating in the compressor and expander was also impure, which resulted in a slow improvement in the purity of the resulting liquid nitrogen, and it took a long time before the liquid nitrogen could be used again!

Nitrogen expansion is generally used in the air separation units of ammonia synthesis plants; due to the large number of nitrogen users and various pressure levels, air separation expansion makes use of nitrogen expansion more often. The working fluid used in nitrogen expansion turbines, which do not involve liquid nitrogen, is cleaner than the working fluid used in air expansion turbines

This post was last edited by Fengxing Tianxia on 2011-3-30 22:14. Reply 1# gader: 1. Nitrogen circulation involves one additional cycle compared to air circulation, which means there is one more irreversible loss. 2. Compared to nitrogen, air has a higher liquefaction temperature； s 3: Compared to nitrogen, air has a greater enthalpy drop; that is, the cooling capacity per unit volume of air is higher than that of nitrogen ； 4. The air circulation results in a large amount of liquefaction; therefore, a stream of lean liquid air must be drawn into the upper column, which reduces the extraction efficiency of the system.

I have previously used nitrogen expanders in ammonia synthesis air separation processes; the nitrogen is compressed by a booster pump, then sent to the heat exchanger before being compressed again by the expander, and finally expanded so that the output is in a near-liquid state.

Reply to 1# gader: 1. The nitrogen expansion process is mainly used when there are multiple pressure levels for nitrogen products, as it allows for the reduction in the number of machines required. 2. Compared to the equivalent air processing process, the nitrogen expansion process has higher energy consumption

Reply to 3# koreaboy: It will also produce liquid; I used to need to control the exit temperature of the dry product

Summary: Advantages and disadvantages of nitrogen expansion versus air expansion. 1. After expansion, the gas does not enter the upper column, so it does not affect the distillation conditions in that column; as a result, the superheat after expansion can be higher than that in the case of air expansion, the pressure after expansion can be lower, and the cooling capacity per unit volume is greater compared to air expansion； 2. After expansion, the gas does not enter the upper tower, resulting in less expansion of the rising vapor in that tower compared to air; thus, it is possible to reduce the tower diameter and simplify its structure ； 3. The expansion medium is clean, which facilitates the safe operation of the expander, reduces wear, and prevents the formation of liquid within the machine ； 4. The air expansion process has a high argon extraction capacity ； 5. The nitrogen expansion process tends to result in an insufficient lower-stage reflux ratio ；