The refrigeration capacity of the air separation unit is mainly generated by the expander, but the minimum temperature of the air separation unit is at the top of the upper tower, maintained at around -193 ℃, which is lower than the outlet temperature of the expander (around -180 ℃). How is this formed?
Before the liquid appears in the start-up stage of the air separation unit, the lowest temperature is generated by the expander, and the temperature inside the distillation tower cannot be lower than the temperature after expansion. However, when liquid appears in the current tower and the saturated liquid is throttled to the upper tower, the pressure decreases, and some gasification occurs. The temperature also decreases to the saturation temperature corresponding to the upper tower pressure. For example, when the liquid nitrogen at the top of the lower tower -177 ℃ is throttled to the upper tower, the temperature can be reduced to -193 ℃. In addition, the temperature of liquid oxygen at the bottom of the upper tower is around -180 ℃. During the gasification process, it exchanges heat and mass with the liquid on the tray, evaporates the nitrogen component, and the gas temperature decreases. When the gas passes through dozens of trays and rises to the top of the tower, it has reached pure nitrogen, and the temperature also drops to the same level as the liquid temperature at that point (-193 ℃). Therefore, the formation of the lowest temperature inside the tower is the result of liquid throttling expansion and gas-liquid heat and mass exchange.
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