Rare gases refer to argon, neon, helium, krypton, and xenon. Due to their different boiling points and significant differences in their content in the air, each component gathers at different parts of the distillation tower. Krypton and xenon have the highest boiling points (at standard atmospheric pressure, the boiling points of krypton are -152.9 ℃ and xenon are -108.1 ℃). As the processing air enters the lower tower, both krypton and xenon condense in the liquid air of the lower tower. Then, the liquid air enters the upper tower through the throttle valve, and flows down plate by plate to collect in the liquid oxygen and gas oxygen at the bottom of the upper tower. Therefore, if you want to extract krypton and xenon from an air separation unit, you usually introduce product oxygen into the krypton tower and use distillation to obtain lean krypton and xenon feed gas.
The boiling point of neon (-245.9 ℃) and helium (-268.9 ℃) are much lower than the nitrogen component. So, the neon and helium components in the processing air are combined with the low boiling nitrogen components. After the processing air enters the lower tower, the neon and helium components rise together with the nitrogen components to the nitrogen side of the main condenser evaporator, and the gas nitrogen is condensed. However, neon and helium, due to their low boiling point, cannot be condensed yet and become "non condensable gases" in the main cooling. Therefore, it can be extracted from the top of the main cold nitrogen side as the raw gas for extracting neon and helium.
The boiling point of argon is -185.7 ℃, between the boiling points of oxygen and nitrogen, and close to oxygen. Most of the argon in the air entering the lower tower enters the upper tower with the liquid air, while a small portion enters the upper tower with liquid nitrogen. There are enrichment zones for argon components in the distillation and extraction sections of the upper tower. The upper part of the distillation section mainly separates nitrogen and argon. The lower part of the distillation section mainly separates oxygen and argon.
