Answer: The air separation unit hopes that the temperature of compressed air entering the unit should be as low as possible to reduce the content of saturated water in the air and the heat load of the main heat exchanger, etc. However, the air compressor is actually impossible to realize the isotherm compression, and the air temperature after the last-stage compression can reach up to 80 ~ 90℃. Therefore, the air should be cooled after the air compressor and before entering the air separation unit. Especially for the adsorption purification process of molecular sieve, since the adsorption capacity of molecular sieve is related to temperature, the lower the temperature is, the larger the adsorption capacity is. Therefore, reducing the air temperature can reduce the design size of the adsorber. During operation, the purification effect can be guaranteed, or the switching time can be prolonged to reduce the switching loss. Therefore, after the compressor, the compressed air is required to be cooled to a temperature as low as possible before entering the adsorber.
Currently, the air precooling system adopted has the following types:
1) air cooling tower with low temperature water. Use spray water to directly contact with air to cool the compressed air. Cooling water comes from two parts: use sewage nitrogen to lower the temperature of cooling water in the water cooling tower, and then use the pump to supply it to the air cooling tower; the other part is 4 ~ Low temperature water of 6℃, further cool the air to 8 ~ Temperature of 10℃ (see Figure 9 ). This cooling method relies on direct contact between air and liquid for heat exchange. Its energy consumption is low, but improper operation may lead to water accident, which makes the purifier or main heat exchanger unable to work normally;
2) low temperature water indirect cooling system. After the air is cooled to 40℃ by the end cooler of the compressor, it is then indirectly cooled to 8℃ by low temperature water in the precooler. Low temperature water is provided by the cooler. This kind of system equipment is simple and compact, and it can also avoid water inflow from the adsorber, but the energy consumption is relatively high;
3) the system of direct heat exchange between air and the cooler. After the compressed air is cooled to 40℃ by the end cooler, it enters the evaporator of the cooler, and cools the air directly by the evaporation and heat absorption of the refrigerant. In this way, indirect cooling of low-temperature water is not needed, which can reduce heat transfer resistance and improve heat transfer effect. Meanwhile, it also saves the low temperature water circulating water pump. However, the evaporator of the refrigerator needs to be specially designed for cooling air, which has large equipment investment and high energy consumption;
4) nitrogen evaporation cooling system. For users who have no requirements on the quality of nitrogen products, they can increase the amount of sewage nitrogen, make full use of the heat absorption potential of the dry sewage nitrogen discharged from the cold box, and absorb the latent heat of evaporation of water in the water cooling tower, lower the cooling water temperature to 12 ~ 14℃, and then cool the air to 16℃ with water in the air cooling tower. It is similar to the original nitrogen water precooling system. Because the refrigerator is omitted, the investment in this aspect can be reduced, and the influence of the refrigerator accident on the operation of the device can also be avoided. But the investment of the purifier will increase;
5) directly use the rear cooler to cool down. When adopting the purification system of variable pressure adsorption regeneration, the requirement of air temperature is relatively tolerant, as long as it is less than 40℃. Therefore, as long as there is some room left for the design of the back cooler of the compressor, the air can be cooled to 37℃ to meet the requirements. In this way, the system is greatly simplified, which saves the water pump, and uses indirect cooling to avoid accidents with water.
