The first commercial oxygen generator appeared in 1903; in 1908, Camerin Onnes of the Netherlands pre-cooled helium with liquid hydrogen and expanded it isenthalpic under adiabatic conditions, reducing the temperature to below 4.2K. Obtain liquid helium; in 1965, the Soviet Union’s Neganov and others invented a dilution refrigerator to make the temperature reach 0.025K; since the 1970s, people have applied demagnetization refrigeration technology to further reduce the equipment's refrigeration temperature.
Gas liquefaction Gas liquefaction is realized by tissue liquefaction equipment based on the liquefaction cycle. The main liquefaction cycles are the Linde liquefaction cycle and the Claude liquefaction cycle.
① Linde Liquefaction Cycle: A cycle that uses the throttling effect of a throttle valve to liquefy the raw material gas (Figure 1). The raw material gas of normal pressure p1 and normal temperature T1 is compressed in the compressor from state 1 to state 2, and the corresponding pressure is p2. The temperature is reduced to state 3 by the heat exchanger, and then the pressure is reduced by the throttle valve, and the isoenthalpy expansion is performed to the state. 4. At this time, part of the gas is converted into liquid and discharged from the liquid reservoir; part of the gas that has not been liquefied is reheated to state 1 in the heat exchanger, thus forming a thermal cycle.
② Claude Liquefaction Cycle: A cycle that uses isentropic expansion and isenthalpic expansion combined with refrigeration to liquefy the raw material gas (Figure 2). The raw material gas of normal pressure p1 and normal temperature T1 is compressed from state 1 to state 2 at the intermediate temperature in the compressor, the corresponding pressure is p2, and the temperature is reduced to state 3 by the heat exchanger E1. After that, the gas is divided into two parts, a part of the gas continues to pass through the heat exchangers E2 and E3, and is cooled to states 4 and 5, and then is enthalpy expanded to state 6 through the throttle valve. At this time, part of the gas turns into liquid and is discharged from the liquid reservoir; the unliquefied part of the gas is reheated to state 8 in the heat exchanger E3, and then merges with another part of the gas that is expanded to state 8 in the expander with medium entropy, and finally is exchanged The heaters E2 and E1 are reheated to state 1, thereby forming a thermodynamic cycle. Other liquefaction cycles developed on this basis, such as throttling liquefaction cycles with additional refrigeration cycles (such as pre-cooling cycles with ammonia or liquid nitrogen or other cold sources) or isentropic expansion liquefaction cycles, with external refrigeration cycles ( Such as external nitrogen refrigeration cycle) isentropic expansion liquefaction cycle, regenerative gas refrigeration cycle (see refrigerator cycle) and multi-stage isentropic expansion liquefaction cycle.
The above various cycles are ideal cycles. However, in practical applications, the compression process of the compressor is not an isothermal process, the heat exchanger has insufficient reheating and cold capacity loss due to external heat intrusion, and the expander has adiabatic loss and mechanical loss, so compensation needs to be taken in the actual refrigeration process. Measures to achieve the heat balance of the process.
Gas separation The commonly used raw gas separation principles include deep cryogenic rectification, deep cryogenic fractional condensation and deep cryogenic adsorption. ①Deep and low temperature distillation: first liquefy the raw material gas, and then separate the components according to the different condensation (evaporation) temperature of each component, using the principle of rectification. The separation process is realized in a deep cryogenic rectification tower. This method is suitable for the raw gas with similar condensation temperature of the separated components, such as the separation of oxygen and nitrogen from the air. ②Deep low-temperature segregation: use the difference in the condensation temperature of each component in the raw gas to reduce the temperature of the raw gas in the heat exchanger, liquefy the components one by one from high to low, and separate the liquid in the separator. This method is suitable for the separation of raw gas such as coke oven gas where the condensation temperature of the separated components is far away. ③Deep and low temperature adsorption: The use of porous solid adsorbents has the characteristics of selective adsorption to adsorb certain impurity components at deep and low temperatures to obtain pure products. For example, a molecular sieve adsorber is used to adsorb oxygen and nitrogen from crude argon at the temperature of liquid air to obtain refined argon.
According to the needs of the process, sometimes one principle is used alone, and sometimes several principles are used simultaneously.