Chapter 6 – Air Distillation

The distillation of air was the first distillation carried out under cryogenic conditions. The temperature in this process ranges from 80 to 100 K. Through describing a typical air separation process, special constraints that influenced the development of the air distillation columns and needs are explained. The use of an insulating cold box requires a low height of the columns. Pressure drop greatly influences the operation costs. To meet these constraints, special sieve trays and structured packing with high specific surface areas are used. The development of the sieve trays in air separation unit (ASU) columns over the last 100 years is described. Their design is compared to the design of traditional trays. A characteristic of ASU sieve trays is the use of small sieve holes of around 1-mm diameter. This is one method to achieve a high specific mass transfer area and high capacity for these trays considering the low tray spacing, typically ranging from 80 to 250 mm. The application of structured packing is demonstrated in a crude argon column. This difficult separation of argon and oxygen requires about 200 theoretical trays and is done in five to eight packed beds. The detrimental effect of an initial maldistribution caused by liquid distributors on separation efficiency is demonstrated. It is also shown that the development of liquid maldistribution within the bed height is strongly influenced by gaps and other disturbances in the homogeneity of the packing layers. By improving the packing uniformity, the efficiency can be strongly improved.