The expander is the machine for gas expansion to work externally, and the original process is mostly to use the expander to drive the motor and output electrical energy externally. For the low-pressure air separation unit, the inlet pressure of the expansion machine depends on the air compressor pressure required by the lower tower, and the adjustment of the refrigerating capacity of the expansion machine mainly depends on changing the expansion capacity. However, the inflated air directly enters into the tower to take part in the rectification. If the amount of inflated is too large, the separation effect will be affected and the extraction rate of oxygen will be reduced.
Booster turbine expansion machine uses the expansion machine to drive a booster compressor to further increase the compressed air pressure supplied to the expansion machine, and then supply it to the expansion machine after cooling in the heat exchanger. As shown in Figure 16. In this way, the inlet pressure of the expander can be increased from 0.55 MPA to 0.9 ~ 1.0MPa, when the same refrigerating capacity is produced, the required expansion air volume can be reduced by more than 20%. In addition, it has the following advantages:
1) reduce the superheat of the gas after expansion, reduce the influence on the rectification of the upper tower, and improve the extraction rate of oxygen. At the same inlet temperature, the superheat after the machine can be reduced by 9.5 K. In addition, with the reduction of the inflated air volume entering the tower, the extraction rate of oxygen can be increased by 6.5% ~ 9.4%, the corresponding energy consumption can be reduced by 5% ~ 10%;
2) the increase of main cold and hot load is beneficial to improve the efficiency of rectification. As the expansion decreases, the air volume entering the cold section of the main heat exchanger and the lower tower increases, the enthalpy value of the cold air of the outlet heat exchanger increases slightly (about 1.5%), and the heat load of the main cooling needs to increase by 4%;
3) high cooling capacity adjustment and good stability. Under normal circumstances, the expansion amount is equal to the circular flow, and the outlet temperature of circulation is equal to the inlet temperature of expansion machine. When the refrigerating capacity needs to be increased, the expansion capacity is formed by the combination of bypass pressurized gas and circulation gas, which accordingly increases the inlet temperature of the expansion machine, which is beneficial to increase the unit refrigerating capacity and reduce the increased share of the expansion capacity. Therefore, the pressurization process has strong adaptability, good stability and great adjustment ability.
