Reducing the working pressure of the air separation plant can reduce the unit energy consumption of the product. The working pressure of all low pressure air separation plant is close to that of the lower tower, while the working pressure of small air separation plant is much higher than that of the lower tower. The working pressure is low, and the unit refrigerating capacity generated by expansion is also small. In order to keep the cooling balance, the unit cooling loss is required to be small first. For large air separation plant, the loss of unit cooling decreases with the increase of unit capacity. At the same time, a smaller temperature difference at the hot end is also selected in the design, and the loss of unit cooling is relatively small due to incomplete heat exchange, this creates favorable conditions for reducing work pressure.
In addition, low working pressure requires high efficiency of the expansion machine, so as to produce larger refrigerating capacity under the same pressure difference. As the capacity of the turbine expander increases, the optimum rotating speed decreases and the efficiency increases. Therefore, it is the most suitable for large air separation plant, making it possible to reduce working pressure.
For small air separation plant, the relative cold loss is large, even if the turbine expander is adopted, the rotating speed is up to 105r/min, the efficiency is also low, and the maintenance and management requirements are very high. In addition, for the large air separation plant, the expansion volume is relatively small compared with the processing air volume, and the air after expansion and refrigeration can still participate in the rectification to extract oxygen from it. However, if the small-sized air separation plant adopts low-pressure flow, because of the large amount of expansion gas needed to produce refrigerating capacity, it cannot be fully involved in rectification, thus the extraction rate of oxygen is very low, and the energy consumption per unit product will still be very large. Therefore, the full low pressure flow is the most suitable for large and medium-sized air separation units.
At present, with the adoption of the process of molecular sieve adsorption purification and booster turbine, as well as the progress of the Plate-fin Heat Exchanger technology, the minimum capacity of low-pressure air separation plant has been designed to 340 m 3/h oxygen production, the output of 800 m 3/h nitrogen (KDON-340/800), and the exhaust pressure of the air compressor is 0.59MPa.
