During the operation of the air separation plant, the expanded air sent to the upper tower must be strictly maintained in a slightly overheated state, which is the core operating standard for taking into account the operation safety of the expander and the stability of the upper tower distillation condition. The industry has formulated a unified micro superheat control range for the full low-pressure external compression air separation unit. The conventional controllable range is 3 ℃ -8 ℃, and the optimal control range for the stable operation of the unit is 4 ℃ -6 ℃.
Micro superheat temperature has clear working condition boundary requirements. When the superheat is lower than 3 ℃, the air working condition approaches to the saturated state. Affected by the pressure fluctuation of the device, the cold loss of the pipeline and other factors, it is very easy to precipitate liquid droplets. The high-speed air flow with liquid will impact the impeller of the expander, causing wear and vibration of the equipment, causing liquid hammer failure, which seriously threatens the operation safety of the equipment. When the superheat degree is higher than 8 ℃, excessive air superheat will bring a large amount of excess heat to the upper tower, increase the consumption of cooling capacity in the tower, reduce the reflux ratio in the distillation section, resulting in the decline of gas-liquid mass transfer efficiency, and ultimately cause the fluctuation of oxygen and nitrogen product purity, the reduction of oxygen extraction rate, and the increase of energy consumption of the device.
It is important to clarify that the superheat is the temperature difference, not the fixed temperature value, which is calculated from the actual temperature at the expander outlet and the air saturation temperature under the corresponding pressure. During the start-up and cooling stage of the unit, it can be temporarily relaxed to 5 ℃～ 10 ℃. Under normal and stable conditions, the optimal range should be strictly locked. At the same time, the DCS should be used to calculate the difference in real time to avoid the measurement error caused by pressure fluctuation and ensure the safe, efficient and economic operation of the unit.
