The distillation column of air separation unit is divided into sieve plate column and packed column. Packed column is divided into two types: bulk packed and regular packed. Sieve tray column is widely used in air separation plant because of its simple structure, strong adaptability and suitable for scale-up. However, with the development of gas-liquid heat and mass transfer technology, the research on high efficiency structured packed, the development of some structured packed with high efficiency, low pressure drop and small liquid holding capacity, has the trend of gradually replacing sieve tray column.
The structured packed is composed of corrugated metal plates. The cryogenic liquid forms a layer of liquid film on the surface of each packed and contacts with the rising steam for heat and mass transfer. The metal specific surface area of the structured packed is about 30 times of that of the sieve plate, and the liquid oxygen retention is only 35% - 40% of that of the sieve plate. Moreover, because the sectional area of the distillation column is 1 / 3 smaller than that of the sieve plate column, and the packed is arranged vertically, there is no problem of concentration gradient in the horizontal direction. As long as the liquid distribution is uniform, the distillation efficiency is high, and the pressure drop is small, the pressure difference of the gas passing through the liquid membrane of the packed is much smaller than that passing through the liquid layer of the sieve plate, which is only about 50PA. The decrease of the bottom pressure of the upper column will inevitably lead to the decrease of the pressure of the lower column, and then the outlet pressure of the main air compressor will be reduced accordingly, so that the energy consumption of the whole air separation system will be reduced. At the same time, the liquid retention of structured packed is small, so the strain capacity of structured packed is strong. Compared with sieve tray column, structured packed column has the following advantages:
1) The pressure drop is very small. The gas phase is convective heat and mass transfer on the surface of liquid membrane in the packed, and there is no resistance of liquid layer and sieve hole on the tray. Under normal conditions, the resistance of structured packed is only 1 / 5 ~ 1 / 6 of that of sieve tray column;
2) The heat and mass exchange is sufficient and the separation efficiency is high, which improves the extraction rate of the product;
3) Operation flexibility, no flooding or leakage, so the load adjustment range is large, strong adaptability. The range of load regulation is 30% - 110%, and the range of sieve plate column is 70% - 100%。
