In the operation of the air separation unit, the superheat of the expanded air sent to the upper column is a key parameter affecting the stability of distillation conditions and product purity. The fluctuation of superheat will directly change the gas-liquid load and mass transfer efficiency of the upper column, and then affect the distillation effect and energy consumption of the unit.
If the superheat of the expanded air is too high, the gas entering the upper tower will carry a large amount of excess heat, which will significantly increase the gas phase temperature in the tower and aggravate the consumption of cooling capacity in the tower. This will lead to the decrease of reflux ratio in the distillation section of the upper column, insufficient gas-liquid two-phase contact, and poor mass transfer and heat transfer efficiency. At the same time, the increase of superheated gas will raise the rising gas flow in the tower and destroy the balance of distillation conditions, resulting in the decline of nitrogen and oxygen purity, the reduction of oxygen extraction rate and the increase of energy consumption in the operation of the unit.
If the superheat of the expanded air is small, and the gas temperature is close to the saturated state, it is conducive to the mass transfer of distillation, but it is very easy to separate out droplets. When liquid gas enters the expander and the upper tower, it will impact the impeller running at high speed, causing equipment wear and vibration, causing equipment failure in serious cases, and there are great potential safety hazards.
In actual production, it is necessary to strictly control the superheat degree of the expanded air and maintain the micro superheat state, which can not only eliminate the risk of liquid carrying and ensure the safety of the equipment, but also ensure the reasonable reflux ratio and mass transfer efficiency of the upper tower, stabilize the purity of the product, and realize the efficient, safe and economic operation of the air separation unit.
