The process of using nitrogen as the working medium of expansion machine is called nitrogen expansion. Nitrogen expansion is divided into pure nitrogen expansion and dirty nitrogen expansion. 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, a part of which is reheated through the circulation channel of the switching heat exchanger and then converged into the turbine expansion machine, after expansion, nitrogen is led out by Plate-fin Heat Exchanger as product nitrogen.
The expansion process of sewage nitrogen is shown in Figure 13. It extracts sewage nitrogen containing about 1% oxygen from the middle of the lower tower, and part of it is reheated through the circulation channel of switching heat exchanger and then joins into the turbine expansion machine, after expansion, the sewage nitrogen is reheated through the cooling liquid purifier and switching heat exchanger to recover the cooling energy, and then it is emptied after absorbing the steam and heat of cooling water through the evaporation cooling tower.
As the nitrogen is extracted from the lower tower, the condensation of liquid nitrogen in the main condenser is reduced, so the reflux liquid nitrogen sent into the upper tower is reduced, which makes the reflux ratio of the distillation tower reach a relatively reasonable value, in this way, the rectification potential of the upper tower is fully utilized to improve the extraction rate of oxygen.
The advantages and disadvantages of nitrogen expansion are:
1) gas does not enter the upper tower after nitrogen expansion, so it will not directly affect the rectification working condition of the upper tower, and its superheat after expansion can be higher than that of air expansion, the pressure after expansion can also be lower than that of air expansion. Therefore, the unit refrigerating capacity is larger than the air expansion. Under the condition that the cooling loss of the equipment is certain, its expansion capacity can be less;
2) because the inflated gas does not enter the upper tower, and the heat load of the main cooling decreases, the rising air volume of the upper tower is less than that of the air, thus the diameter of the tower can be narrowed and the structure can be simplified;
3) the increase of the amount of nitrogen expansion has a certain influence on the temperature difference of main cooling, which will increase the oxygen content in the polluted nitrogen and reduce the extraction rate of oxygen, but the direct influence on the purity of oxygen is not very big, the influence on argon distillation is smaller, and nitrogen expansion has been adopted in large air separation units abroad;
4) the working medium of nitrogen expansion is cleaner than that of air expansion, so the expansion machine can operate under safer conditions. Because the working medium of nitrogen expansion is extracted from the upper part of the lower tower, it is impossible to carry impurities such as solid carbon dioxide. However, the working medium of air expansion is extracted from the first bubble tower plate below, and the carbon dioxide in the air may be brought into the expansion machine, causing the abrasion of the expansion machine. In addition, when the nitrogen expands, the pressure after expansion is 0.12MPa, and the temperature will not appear until 78.7 K, so it is not easy to produce liquid in the expansion machine;
5) because the minimum reflux ratio of the lower tower is greater than the minimum reflux ratio of the distillation section of the upper tower, that is to say, the reflux fluid of the lower tower is less than that of the upper tower, therefore, the amount of gas and nitrogen that can be extracted from the lower tower is less than the amount of air that can be sent to the upper tower. Generally, the expansion amount of nitrogen extraction should not exceed 16% of the processed air volume, while the expansion amount of air should not exceed 25% of the processed air volume.
