The liquefaction unit is divided into low pressure liquefaction cycle and medium pressure liquefaction cycle, and the circulation medium can be air or nitrogen. A certain amount of liquid oxygen and liquid nitrogen, or all liquid nitrogen can be produced at the same time as required. Here, we mainly take the cycle process of medium pressure liquefaction (see Figure 20) as an example to illustrate the principle of oxygen and nitrogen liquefaction.
13 low pressure nitrogen is extracted from the air separation unit, the pressure is about 50kPa, compressed to 500kPa by primary nitrogen compressor 1, pressurized to 2.46MPa by nitrogen circulation compressor 2, one part is pressurized to 5.28MPa at the second-stage booster 3 and 4, and the other part is precooled to 270 K directly through the heat exchanger 7, then enters the expansion machine 5. The pressure of 13 is 616kPa, and the temperature is 189 K, 5.28MPa high pressure nitrogen, 75% through heat exchanger cooling to 176 K, enter expansion machine 6, outlet pressure is 583kPa, temperature is 96 K. The low-temperature nitrogen after two-branch expansion returns to the inlet of the circulating nitrogen compressor after reheating; the other 25% of the nitrogen with 5.28 MPa pressure exchanges and throttles with 139 K low-pressure liquid nitrogen in the liquid purifier, generating 827kPa liquid nitrogen, part of the heat exchange with low-pressure oxygen turns into liquid oxygen, and part of the flow-saving turns into liquid nitrogen.
The liquefaction unit can adjust the quantity of liquid oxygen and liquid nitrogen production according to the amount of oxygen and nitrogen emission. Therefore, it can be used as auxiliary equipment to adjust the air separation load and reduce the amount of oxygen emission in the oxygen plant. If large liquid storage tank and liquid evaporation system are configured, the peak regulation system of oxygen and nitrogen pipe network can be formed. When the pressure of the pipe network is low, take out liquid oxygen and liquid nitrogen from the liquid tank, and after pressure gasification, it will be converted into oxygen and nitrogen and sent to the pipe network to regulate the peak.
