This is because during the startup phase, the temperature inside the air separation column is still far from the normal operating temperature (–172 to –194°C), and rectification requires a large amount of liquid; therefore, a substantial quantity of cooling capacity is needed during startup. The amount of cooling capacity produced depends on the refrigerating capacity of the expander and the high pressure. The refrigerating capacity of expansion depends on the expander efficiency, the pressure differential across it, and the gas flow rate passing through it. When the expander efficiency is fixed, increasing the pressure differential before and after the expander and increasing the inlet gas flow rate can increase the cooling capacity. The method to enlarge the pressure differential before and after the expander is: control the high pressure at the maximum pressure allowed by the equipment, reduce the medium pressure as much as possible, and open all blow-off valves and analysis valves. The low pressure affects the medium pressure and should also be reduced as much as possible, provided that it can still meet the regeneration requirements of the purifier.
During the cooling phase, when the expander outlet temperature T₂ reaches approximately –140°C, open throttle valve No. 1 (J-1). Opening it too early or too late is unfavorable for shortening the startup cycle. Opening J-1 too early will reduce the expander's refrigerating capacity; opening it too late will cause excessive cold loss at the hot end and affect the temperature drop before valve J-1. From –140°C until T₂ reaches the normal temperature (–155 to –165°C), there is approximately a one-hour process of gas distribution and switching-that is, the transition from the cooling phase to the liquid accumulation phase.
Since the gas passing through the expander cannot produce liquid, liquid can only be generated after the air is further cooled in the second heat exchanger and then passes through throttle valve J-1. Strictly speaking, air begins to liquefy when it reaches 3.65 MPa (absolute) and –140.68°C, and liquid can only be produced after the throttle valve when it reaches 0.6 MPa and –173°C. To reduce throttling vaporization, a certain degree of subcooling is required. That is, the temperature before valve J-1 should be controlled between –155 and –165°C. During this transition stage, to ensure the expander's refrigerating capacity while also allowing T₃ to drop rapidly, the gas flow must be reasonably distributed, and T₂ must be controlled between –140 and –155°C.
Liquid accumulation phase: To produce as much liquid as possible and accumulate it as quickly as possible, on the one hand, maintain the high pressure and T₃ temperature; on the other hand, reduce the oxygen flow rate and control the low pressure and the outlet temperature difference. Reducing the oxygen flow rate increases the evaporation rate at the bottom of the upper column, which is beneficial for improving oxygen purity; reducing the temperature difference in the condenser-evaporator is conducive to the accumulation of liquid oxygen. In this sense, the oxygen outlet valve should be fully closed. However, doing so will reduce the heat transfer area of the heat exchanger and increase the temperature difference at the hot end. Therefore, controlling the oxygen flow rate at one-third of the normal flow rate is optimal. The purpose of increasing the low pressure is to reduce the pressure differential between the upper and lower columns, thereby decreasing the temperature difference in the condenser-evaporator and facilitating the accumulation of liquid oxygen.
Purity adjustment phase: A large amount of cooling capacity is required in the early stage of purity adjustment. When the liquid air and liquid nitrogen throttle valves are closed slightly, the ascending vapor in the lower and upper columns increases, preventing leakage through the small orifices; liquid accumulates on the trays and rectification begins. Therefore, valve closing must be carried out slowly to ensure the stability of the liquid oxygen level. The criterion for whether the opening of these two valves is reasonable is whether the liquid oxygen level is maintained within the range of 430–500 mm, and whether the liquid air and liquid oxygen purity are within the design range when valve closing is basically completed.
When valve closing is basically finished and the rectification conditions inside the column have been established, the cooling capacity is only used to compensate for the cold loss due to heat ingress through the insulation layer and the cold loss from the temperature difference at the hot end. Therefore, pressure must be reduced to decrease cooling capacity production. The indicator of the amount of cooling capacity is the liquid oxygen level. Provided that the liquid oxygen level is maintained, the lower the high pressure, the better.
The purpose of production is to obtain the finished product. The output of the finished product is inversely proportional to its purity, so reasonable adjustment is necessary. Since flow rate changes alter the temperature difference at the hot end, the distribution of the inlet air flow into the column should be adjusted in a timely manner.
