In the air separation two-stage distillation system, the subcooler is the key heat exchange equipment to ensure the distillation efficiency and reduce energy consumption. It is mainly used to cool the saturated liquid air and liquid nitrogen from the lower tower.
When the uncooled saturated liquid is depressurized and sent to the upper column through the throttle valve, a large amount of flash steam will be generated, which will reduce the effective liquid phase, directly weaken the amount of reflux liquid required for the upper column rectification, and affect the effect of oxygen and nitrogen separation. With the help of the supercooler, the low-temperature nitrogen and waste nitrogen at the top of the upper tower are used as cold sources to cool the liquid air and liquid nitrogen, so as to obtain supercooling, which can significantly reduce the gasification rate after throttling, retain more liquid phase into the upper tower, stabilize the reflux ratio in the tower, and improve the product extraction rate.
At the same time, the subcooler can recover the cooling capacity. The cooling capacity lost with the venting of waste nitrogen was reused to reduce the cooling load of the expander and the main heat exchanger and effectively reduce the energy consumption of the whole unit. The stable feed phase state can also reduce the gas-liquid two-phase impact in the pipeline, avoid the large fluctuation of the gas-liquid load in the upper tower, and ensure the continuous and stable operation of distillation.
It should be noted that supercooling can only reduce fluidization and gasification, but cannot completely eliminate flash. The undercooling degree should be reasonably controlled in actual operation. Insufficient undercooling will lead to less reflux liquid; Excessive supercooling will occupy too much cooling capacity of the upper tower and break the cold and heat balance in the tower. Scientific performance of the subcooler is an important condition for stable production of qualified oxygen and nitrogen products and energy-saving operation of the air separation unit.
