The refrigeration capacity of the air separation unit is mainly generated by the expander. However, the lowest temperature of the air separation unit is at the top of the upper tower, which is maintained at about - 193 ℃, which is lower than the outlet temperature of the expander (about - 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 in the distillation column cannot be lower than the temperature after expansion. However, when the liquid appears in the lower tower and the saturated liquid is throttled to the upper tower, the pressure decreases, and part of the gasification occurs, and 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 about - 180 ℃. During the gasification process, heat and mass exchange is carried out with the liquid on the tray, the nitrogen component evaporates, and the gas temperature decreases. When the gas passes through dozens of trays and rises to the top of the tower, the gas has reached pure nitrogen, and the temperature also decreases to be equal to the liquid temperature (- 193 ℃) there. Therefore, the formation of the lowest temperature in the tower is the result of liquid throttling expansion and gas-liquid heat and mass exchange.
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