The air separation unit adopts a two-stage distillation structure. The lower tower is a pre distillation system, and the upper tower is a final distillation system. There is no direct air feed to the upper tower, and all distillation raw materials come from the lower tower. Therefore, the stability of the purity of the lower tower is the core foundation to ensure the purity of oxygen and nitrogen products in the upper tower is qualified.
After the air is preliminarily separated in the lower tower, liquid nitrogen is produced at the top of the tower and oxygen enriched liquid air is produced at the bottom of the tower. These two streams of fluid are throttled and sent to the upper tower, which is the only material source for the upper tower rectification. The composition of the materials in the lower column directly determines the initial working condition of the upper column distillation. If the purity of liquid nitrogen and liquid air in the lower column is unbalanced, even if the valve parameters in the upper column are fine tuned, the product purity problem cannot be fundamentally corrected. When the purity of liquid nitrogen in the lower tower is low and the oxygen content exceeds the standard, the nitrogen products in the upper tower will be directly polluted, resulting in the nitrogen purity not reaching the standard; However, the abnormal oxygen content in liquid air will lead to the imbalance of oxygen source ratio in the upper tower, and the oxygen purity is difficult to stabilize.
At the same time, the upper and lower columns are coupled by condensing evaporator. The fluctuation of the purity, pressure and liquid level of the lower column will directly change the heat exchange load and the gas-liquid reflux ratio of the upper column, disrupt the distillation balance of the upper column, and cause repeated fluctuation of product purity. Therefore, the air separation operation should follow the principle of stabilizing the lower column first and then adjusting the upper column. Only by controlling the purity of liquid nitrogen and liquid air in the lower column within a reasonable range and stabilizing the raw material composition and distillation conditions, can conditions be created for the deep distillation of the upper column and finally produce qualified high-purity oxygen and nitrogen products.
