Answer: in the whole low-pressure air separation process, some are equipped with a liquid purifier, and some are equipped with two or even three liquidators, which make part of the air liquefied by the cold energy of polluted nitrogen, pure nitrogen or pure oxygen respectively. Some even combine the liquid purifier with liquid air and liquid nitrogen supercooler. The air flowing through the liquid purifier comes from the washing air of the lower tower or the low temperature air at the cold end of the switching heat exchanger.
The function of the liquid purifier is related to the air separation process and start-up mode. Generally, it has the following functions:
1) in the stage of starting the hydrops, the liquid purifier plays the role of liquefied air and accumulating liquid. In some processes, the liquid purifier is not set up separately. For example, the 6500 m 3/h air separation plant of the French liquefied air company makes the liquid air cooler, the sewage nitrogen cooler and the expansion heat exchanger into a whole to recover the cooling, when starting up, it is completely used by the supercooler as the liquid purifier to accumulate the liquid;
2) in the normal operation stage, between the switching heat exchanger (or refrigerator) and the distillation column, the liquidator can play the role of cooling distribution and adjustment. Part of the dirty nitrogen coming from the distillation tower and the cold energy of the product oxygen and nitrogen are recycled in the liquid purifier, and the cold energy is directly transferred to the lower tower by the liquefied air. In this way, the amount of cold allocated to the switching heat exchanger (or the cooler) will be reduced, that is, the heat load will be reduced, which can avoid the air liquefaction at the cold end, the air temperature in the tower is about 1 ~ higher than that in the dry saturation state ~ 1.5 ℃.
Especially when the operating condition of the distillation column fluctuates, due to the self-balance effect of the liquid purifier, the temperature of the sewage nitrogen outlet liquid purifier (into the switching heat exchanger) can be basically unchanged, keeping the temperature difference of the cold end within the allowable range of self-cleaning is beneficial to the stability of the switching heat exchanger and the working condition of the distillation column. In addition, the liquid purifier can also adjust the liquid level of the condensing evaporator.
The self-balance of the liquid purifier means that the liquid purifier can automatically keep its outlet temperature of cold air flow basically constant. This is because when the amount of cold air flow (such as sewage nitrogen) or its inlet temperature changes, that is, when the heat load of the liquid purifier changes, the liquefied air volume entering the liquid purifier will also change accordingly. For example, when the temperature of sewage nitrogen entering the liquid purifier decreases, the temperature difference between sewage nitrogen and air increases, which increases the heat load of the liquid purifier and the amount of air liquefaction. However, the larger the liquefaction volume is, the lower the pressure on the air side of the liqufier will be, and the higher the differential pressure between the liqufier and the lower tower (or the low temperature air pipeline) will be, and the air volume inhaled into the liqufier will increase automatically, the recycled cooling will also increase, so the temperature of the dirt nitrogen outlet liquid purifier can be basically kept unchanged. This meets the requirement of switching heat exchanger (or cooler) to keep the temperature of the return gas at the cold end constant.
