The most common method for air separation is cryogenic distillation. Cryogenic air separation units (ASUs) are built to provide nitrogen or oxygen and often co-produce argon. These components of air must be removed to meet product quality specifications. In addition, they must be removed prior the air entering the distillation portion of the plant; because very low temperatures would cause the water and carbon dioxide to freeze and deposit on the surfaces within the process equipment.
Most new air separation plants employ a “molecular sieve” “pre-purification unit” (PPU) to remove carbon dioxide and water from the incoming air by adsorbing these molecules onto the surface of “molecular sieve” materials at near-ambient temperature. Molecular sieve 13X conbined with activated alumina is designed for cryogenic air separation to remove of moisture and CO2. Activated alumina removes most of the water and 13X removes most of the CO2.