The process of using nitrogen as the working medium of the expander is called nitrogen expansion. Nitrogen expansion is divided into two types: pure nitrogen expansion and polluted nitrogen expansion. The pure nitrogen expansion process is to extract pure nitrogen from the top of the lower tower and the top of the nitrogen side of the main condenser. Part of it is reheated through the switch-type heat exchanger circulation channel and then merged into the turboexpander. The expanded nitrogen is heat-exchanged through the plate-fin heat exchanger. The device is led out as product nitrogen.
Sewage nitrogen expansion process, which extracts about 1% of the nitrogen gas containing oxygen from the middle of the lower tower. Part of it is reheated by the switching heat exchanger circulation channel and then merged into the turbo expander. The expanded sewage nitrogen passes through the cold liquefier. And the switching heat exchanger reheats to recover the cold energy, and then evaporates the cooling water of the cooling water through the evaporative cooling tower and then vents it.
Since nitrogen is drawn from the lower column, the amount of liquid nitrogen condensed in the main condenser is reduced, so the reflux liquid nitrogen sent to the upper column is reduced, so that the reflux ratio of the rectification column reaches a reasonable value, so that the rectification of the upper column is fully utilized Potentiato increase oxygen extraction rate.
The advantages and disadvantages of nitrogen expansion are:
1) The gas does not enter the upper column after nitrogen expansion, so it will not directly affect the distillation conditions of the upper column. Its superheat degree after expansion can be higher than that of air expansion, and the pressure after expansion can also be lower than that of air expansion. Therefore, the unit cooling capacity is greater than that of air expansion. Under certain conditions of equipment cold loss, its expansion capacity can be less;
2) Because the expanded gas does not enter the upper tower and the heat load of the main cooling is reduced, the amount of rising gas on the upper tower is less than that of air expansion, so the diameter of the tower can be reduced and the structure can be simplified;
3) The increase in the amount of nitrogen expansion has a certain effect on the main cooling temperature difference, which will increase the oxygen content in the polluted nitrogen and reduce the oxygen extraction rate. However, the direct impact on the purity of oxygen is not large, and the effect on the argon in the fraction is Smaller, nitrogen expansion has been used on large air separation equipment abroad;
4) The nitrogen-expanded working fluid is cleaner than the air-expanded working fluid, so the expander can operate under safer conditions. Because the nitrogen-expanded working medium is extracted from the upper part of the lower tower, it cannot carry impurities such as solid carbon dioxide. The working medium for air expansion is extracted from the first blister tray below, and carbon dioxide in the air may be brought into the expander, causing the wear of the expander. In addition, when the nitrogen is expanded, the pressure after expansion is 0.12 MPa, and the droplets will not appear until the temperature is 78.7K, so it is not easy to produce liquid in the expander;
5) Because the minimum reflux ratio of the lower column is greater than the minimum reflux ratio of the upper column rectification section, that is to say, the lower column reflux liquid has less surplus than the upper column reflux liquid, so the amount of gas and nitrogen that can be extracted from the lower column can be greater than that The amount of air entering the upper tower is small. Generally, the amount of nitrogen extraction expansion does not exceed 16% of the process air volume, and the air expansion volume does not exceed 25% of the process air volume.
