The expansion air volume entering the upper tower increases, and the reflux ratio of the distillation section decreases correspondingly. In order to achieve the required purity of nitrogen products, more tower plates need to be set. When the reflux ratio is reduced to countless Tower plates needed to be set to make the purity of nitrogen reach the required value, then the reflux ratio is called the minimum reflux ratio. The expansion air volume entering the upper tower is first limited by the minimum reflux ratio. The closer the reflux ratio is to the minimum reflux ratio, the faster the number of tower plates needed to ensure the product purity increases, which will lead to the increase of investment, the increase of tower plate resistance, the increase of Operation pressure and the increase of energy consumption. Therefore, the reflux ratio after the inflated air enters the tower should be larger than the minimum reflux ratio.
When the purity of nitrogen is required to be low, the minimum reflux ratio will also decrease, and the expansion air volume allowed to enter the upper tower will be more, then the extraction rate of oxygen will accordingly decrease. In addition, if the purity of liquid space is high, the amount of liquid space will be less, and the corresponding amount of liquid nitrogen will increase. This will increase the reflux ratio of the distillation section and allow the expansion air volume to be sent into the upper tower to increase appropriately. The purity of oxygen is lower, and the amount of inflated air that is allowed to be delivered can be larger. In a word, the inflated air volume sent to the upper tower should comprehensively consider the influence of factors such as reflux ratio, Tower plate number, oxygen, nitrogen and liquid-air purity, so as to ensure the product purity and not to reduce the oxygen extraction rate too much, under the condition of not increasing the number of tower plates and energy consumption too much, send proper amount of air expansion. At present, generally, the expansion air volume allowed to be sent to the upper tower is 0.15 ~ 0.25 processing air volume. For large equipment, the cold loss is relatively small. The inflated air needed to make up for the cold loss will not exceed the above requirements, so all the inflated air can be sent to the upper tower. For small and medium-sized equipment, the cold loss is relatively large. If the expansion volume is greater than the above requirements, if all the expansion air is sent to the upper tower, the number of tower plates will increase too much, and the energy consumption will also increase significantly at the same time. When designing, we would rather only send part of the air to the upper tower, and the other part of the air will pass through to the heat exchanger and only recover its cooling capacity, which is more economical and reasonable.
