I will tell you why Air Separation Unit has square column.
If you see an air separation plant you may wonder why they have a square column in their process. In fact that is not a distillation column but a box to minimize heat leak to the environment. We call this equipment COLD BOX.
The cold box enclosures equipment at cryogenic temperature. Equipment like distillation column, condenser and plate heat exchanger (PHX) inside the cold box deals with operation temperature as low as -196C. To insulate equipment and instrumentation, cold box is filled with nonconductive material like perlite, one amorphous volcanic glass. Vacuum may be used also to prevent heat transfer.
The cold box can be as high as 70m, depending on plant capacity and licensor. Its square shape is to help in the equipment layout. As they are for enclosure, there is no need to be cylindrical.
Now that you already know an Air Separation Unit uses enclosures that insulate the process equipment and interconnecting piping from ambient air and temperature, although the chemical process engineer is not directly involved with the Cold Box design, this equipment can influence plant operation if excessive heat leak occurs into the process.
Besides insulation, it is necessary to prevent ambient air from entering the enclosure. It can be done by keeping vacuum or positive pressure.
Continuous purging with oil-free and moisture-free (-75C dew point) nitrogen gas is an alternative to guarantee positive pressure. Stay alert that purge gas velocity must be less than the perlite fluidization velocity. It is preferred to have multiple points within the enclosure to ensure that the purge gas pressure is maintained at all points.
Loss of purging can be identified using a flowrate or pressure measurement. If the digital control system (DCS) is used, consider having low and high flow/pressure alarms.
Potential consequences for loss of cold box purging is:
a) Liquefied air inside the enclosure
b) Ice forms within the enclosure
c) Equipment or piping damage
d) Perlite or cryogenic liquid release
