Air separation plant in the start-up phase before the emergence of liquid, the lowest temperature is generated by the expander, the temperature in the distillation tower can not be lower than the temperature after expansion.
But when the lower tower appears liquid, saturated liquid section of the subcooling flow to the upper tower, the pressure is reduced, part of the gasification, the temperature also fell to the upper tower pressure corresponding to the saturation temperature. In addition, the bottom of the tower on the liquid oxygen temperature (-1800C) or so in the gasification process, and the liquid on the tower plate heat, mass exchange, nitrogen component evaporation, gas temperature reduction, to be the gas through a number of sections to reach the top of the tower when the gas has reached the pure nitrogen, the temperature has fallen to the temperature with the liquid temperature at the same place (-193 ° C). 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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