A process that uses nitrogen as the working medium of the expander is called nitrogen expansion. Nitrogen expansion is divided into two categories: pure nitrogen expansion and waste nitrogen expansion.
The pure nitrogen expansion process is shown in Figure 12. Pure nitrogen is extracted from the top of the lower column and the top of the nitrogen side of the main condenser. A portion of it is reheated through the regenerative passage of the reversing heat exchanger, then combined and fed into the turbo-expander. After expansion, the nitrogen is drawn off as product nitrogen through the plate-fin heat exchanger.
The waste nitrogen expansion process is shown in Figure 13. Waste nitrogen containing approximately 1% oxygen is extracted from the middle of the lower column. A portion of it is reheated through the regenerative passage of the reversing heat exchanger, then combined and fed into the turbo-expander. After expansion, the waste nitrogen is reheated through the subcooler-liquefier and the reversing heat exchanger to recover cold energy, and then vented to the atmosphere after absorbing the evaporative heat of cooling water in the evaporative cooling tower.
Since nitrogen is extracted from the lower column, the condensation of liquid nitrogen in the main condenser decreases. Consequently, the amount of reflux liquid nitrogen fed to the upper column is reduced, allowing the reflux ratio of the distillation column to reach a more reasonable value. This fully utilizes the distillation potential of the upper column and improves the oxygen extraction rate.
The advantages and disadvantages of nitrogen expansion are as follows:
The gas after nitrogen expansion does not enter the upper column; therefore, it does not directly affect the distillation conditions of the upper column. Its superheat after expansion can be higher than that of air expansion, and the pressure after expansion can also be slightly lower than that of air expansion. Thus, the unit refrigeration capacity is greater than that of air expansion. Under a certain equipment cold loss, the required expansion quantity can be reduced.
Since the expanded gas does not enter the upper column and the heat load of the main condenser is reduced, the ascending gas quantity in the upper column is less than that in air expansion. Consequently, the column diameter can be reduced, and the structure can be simplified.
An increase in nitrogen expansion quantity has a certain effect on the temperature difference of the main condenser, which will increase the oxygen content in the waste nitrogen and reduce the oxygen extraction rate. However, its direct impact on oxygen purity is not significant, and its effect on crude argon is even smaller. Nitrogen expansion has already been adopted in large-scale air separation units abroad.
The working medium of nitrogen expansion is cleaner than that of air expansion; therefore, the expander can operate under safer conditions. This is because the working medium of nitrogen expansion is extracted from the upper part of the lower column, and it cannot carry solid carbon dioxide or other impurities. In contrast, the working medium of air expansion is extracted from below the first bubble cap tray, and carbon dioxide in the air may be carried into the expander, causing wear to the expander. Moreover, during nitrogen expansion, the pressure after expansion is 0.12 MPa, and liquid droplets will not appear until the temperature reaches 78.7 K; therefore, liquid is not easily generated in the expander.
Since the minimum reflux ratio of the lower column is greater than that of the rectifying section of the upper column-that is, the reflux liquid in the lower column is less surplus than that in the upper column-the amount of gaseous nitrogen that can be extracted from the lower column is less than the amount of air that can be fed to the upper column. Typically, the nitrogen expansion quantity does not exceed 16% of the processed air quantity, while the air expansion quantity does not exceed 25% of the processed air quantity.
