Most larger air-separation plants continue to use cryogenic distillation to separate air gases. Before pure gases can be isolated from air, unwanted components such as water vapor, dust, and carbon dioxide must be removed. First, the air is filtered to remove dust and other particles. Next, the air is compressed as the first step in liquefying the air. However, as the air is compressed, the molecules begin striking each other more frequently, raising the air temperature. To offset the higher temperatures, water heat exchangers cool the air both during and after compression. As the air cools, most of its water vapor content condenses into liquid and is removed.
After being compressed, the air passes through beds of adsorption beads that remove carbon dioxide, the remaining water vapor, and molecules of heavy hydrocarbons, such as acetylene, butane, and propylene. These compounds all freeze at a higher temperature than do the other air gases. They must be removed before the air is liquefied or they will freeze in the column where distillation occurs. 02 December 2010Comment