Phase cooling method is one of the start-up operating force methods of the air separation plant which uses switching heat exchanger to self-remove moisture and carbon dioxide. In the full low pressure air separation plant, under the operating pressure of 0.5MPa (surface pressure), the moisture content is-40 ~ -60℃ has been basically frozen. However, carbon dioxide will not be precipitated and frozen until it is below-133 ℃ until-165 ~ -170℃ frozen. Thus, the range from-60℃ to-133 ℃ is dry zone. Using this principle, the cooling process of the equipment during the start-up operation is carried out in stages, which is the stage cooling method. The whole cooling process is divided into four stages: the first stage is the stage of moisture precipitation and freezing. In this stage, only the switching heat exchanger is first cooled until the temperature at the cold end reaches-60℃; the second stage is drying stage, and the available water has been cleared, while for the machine which has not been eluated out of carbon dioxide, the initial cooling of other equipment will end when the temperature of expansion machine reaches-130℃; the third stage is the stage of eluating out carbon dioxide. In this stage, only the switching type heat exchanger is cooled down to let the carbon dioxide freeze in the heat exchanger and be carried out by the low pressure return gas until the temperature at the cold end reaches-170℃; the fourth stage is to use the purified low temperature gas to further cool other equipments until the liquid is accumulated in the tower, and then further adjust the rectification working condition.
The cooling of each equipment in this method is carried out in stages, so the cooling is uniform and the thermal stress is small. In addition, due to the large amount of cooling required by each equipment in the cooling tower in the second cooling stage, the expansion machine runs for a longer time at a higher inlet temperature, thus, it can give play to the feature of large refrigerating capacity of the turbine expander under high temperature and shorten the start-up time. However, due to the need of repeated cooling of the equipment, the valve opens and closes many times, and the operation is relatively complex.
