Usually, large air separation equipment takes more than 36 hours from starting cooling, liquid accumulation, and purification to producing qualified products, of which cooling is about 10 hours, liquid accumulation is about 20 hours, and purification is about 6 hours. For the air separation equipment in the molecular sieve purification process, there were no issues with water precipitation, freezing, and carbon dioxide precipitation. However, the required cooling capacity for the entire start-up process is certain. To shorten the cold driving time, firstly, it is necessary to maximize the refrigeration capacity of the expander, and secondly, it is necessary to shorten the purification time. Based on experience, the specific operations are as follows:
1) Try to reduce the pressure behind the expander as much as possible and increase the unit cooling capacity of the expander. For example, in the cooling stage, opening all the discharge valves of the upper tower to reduce the pressure of the upper tower (i.e. the outlet pressure of the expander) from 0.065MPa to 0.04MPa can increase the refrigeration capacity by approximately 15%;
2) During the process of liquid accumulation, the upper tower pressure should be increased. When the equipment cools to the liquefaction temperature, liquid begins to appear. As the liquefaction temperature increases with increasing pressure, the corresponding latent heat of condensation decreases. Therefore, during the liquid accumulation stage, increasing the pressure of the upper tower from 0.04MPa to 0.065MPa can increase the liquefaction temperature of oxygen from -180 ℃ to -178.3 ℃. Can accelerate liquefaction speed;
3) Start the purification phase in advance. When the liquid level reaches 60% of the normal liquid level, there is actually already liquid on the tray, which is undergoing the distillation process of gas-liquid heat and mass exchange.
