The air separation unit relies on the expander and throttling effect to generate cryogenic cooling capacity. All the cooling capacity is used to offset various cold losses of the system and maintain the low-temperature distillation condition, which is mainly consumed in four aspects.
First, make up for the loss of external cold running of the equipment. The air separation tower, low-temperature pipeline and heat exchanger are all in a deep cold condition of more than 100 degrees below zero, and the ambient heat will continue to be transferred into the equipment through the cold insulation layer. This part of natural cooling loss is the basic consumption of steady-state operation. The larger the equipment scale, the worse the cold insulation effect, and the higher the cold running loss.
Second, compensate the incomplete cooling loss of heat exchanger. The main heat exchanger and condensing evaporator have inherent temperature difference at the hot end. The forward flow low-temperature air and the return flow oxygen and nitrogen product gas cannot realize complete heat exchange. The irreversible heat exchange temperature difference will produce fixed cooling loss, which is the conventional cooling consumption with the highest proportion in the air separation unit.
Third, low temperature process materials bring out cooling capacity. When the device produces liquid oxygen, liquid nitrogen and other low-temperature liquid products, the low-temperature finished products will carry a large amount of cold energy out of the system; The rewarming and conveying of gaseous products will also consume part of the cooling capacity to complete the temperature gradient conversion.
Fourth, the additional cooling consumption of the device startup and working condition adjustment. At the start-up stage of the equipment, a large amount of refrigeration capacity is required to cool the normal temperature equipment, packing and medium to the working low temperature; System load adjustment, valve throttling and slight low-temperature leakage of equipment will also consume additional cooling capacity. During normal steady-state production, the start-up cooling consumption disappears, and the cooling capacity mainly compensates for the cold running, heat exchange loss and the cold capacity brought out by the product, so as to maintain the liquid level and low temperature balance of the distillation column.
