The cold box of an air separation plant is filled with cold-insulation (adiabatic) materials, and the gaps between the particles of these insulation materials (such as perlite) are originally filled with air. After the air separation unit starts operation, the internal equipment enters a low-temperature state, and the temperature of the insulation materials drops accordingly. As the internal gas contracts in volume, a negative pressure will form inside the cold box.
If the cold box is fully sealed, the pressure difference between internal and external will easily cause the box body to be sucked and deformed. If the cold box is not tightly sealed, external moist air will easily invade, making the insulation materials damp, which degrades the insulation effect and increases the cold loss of the air separation unit.For common insulation material perlite, its thermal conductivity is about 0.040 W/(m·℃); while the thermal conductivity of ice is 2.2 W/(m·℃), which is more than 50 times higher.
Therefore, to prevent moist air and the moisture in it from condensing on the pipelines and cold box walls and invading the box, dry inert gas (nitrogen or waste nitrogen) is filled into the cold box during the operation of the air separation unit, to maintain a slight positive pressure inside the cold box, which is about 200~500 Pa.
