The amount of expansion air entering the upper column increases, and the reflux ratio of the rectification section decreases correspondingly. In order to achieve the required purity of the nitrogen product, more trays need to be provided. When the reflux ratio is reduced to the point that countless trays need to be set to make the nitrogen purity reach the required value, the reflux ratio at this time is called the minimum reflux ratio. The amount of expansion air entering the tower is first limited by the minimum reflux ratio. When the reflux ratio is closer to the minimum reflux ratio, the faster the number of trays required to ensure product purity, which will increase investment, increase tray resistance, increase operating pressure, and increase energy consumption. Therefore, the reflux ratio of the expanded air after entering the tower should be greater than the minimum reflux ratio.
When the purity of nitrogen is required to be low, the minimum reflux ratio is also reduced, and the amount of expanded air allowed to enter the tower can be more, and the oxygen extraction rate is reduced accordingly. In addition, if the liquid-air purity is high, the amount of liquid-air will be small, and the corresponding amount of liquid nitrogen will increase. This will increase the reflux ratio of the rectification section, and the amount of expansion air allowed to be sent to the upper column can be appropriately increased. The purity of oxygen is lower, and the amount of expansion air allowed to be sent in can also be higher. In short, the amount of expansion air sent to the upper column should consider the effects of various factors such as the reflux ratio, the number of plates, oxygen, nitrogen, and liquid-air purity, so as to ensure the purity of the product and not reduce the oxygen extraction rate too much. Without excessively increasing the number of trays and energy consumption, send in an appropriate amount of expanded air. At present, the amount of expansion air that is generally allowed to be sent to the upper tower is from 0.15 to 0.25 of processing air. For large air separation equipment, the cold loss is small. The amount of expansion air required to compensate for cold losses will not exceed the above requirements, so the entire amount of expansion air can be sent to the upper tower. For small and medium-sized air separation equipment, the cold loss is large. If the amount of expansion gas is greater than the above requirements, if the entire expansion air is sent to the upper column, the number of trays will increase too much, and the energy consumption will also increase significantly. In the design, it is more economical and reasonable to prefer to send only part of the air to the upper tower and the other part of the expanded air to the heat exchanger to recover only its cooling capacity.
