This is because the temperature in the air separation tower in the start-up phase is away from the normal working temperature (-172 ~ -194 ℃) is relatively large, and the rectification needs a large amount of liquid, therefore, a large amount of cooling is needed in the start-up stage. The cooling capacity of the production depends on the refrigerating capacity and high pressure of the expansion machine. The expansion refrigerating capacity depends on the expansion machine efficiency, the pressure difference before and after, and the gas flow through the expansion machine. When the efficiency of the expansion machine is fixed, expanding the pressure difference before and after the expansion machine and increasing the air inlet can increase the refrigerating capacity. The way to enlarge the pressure difference between the front and back of the expansion machine is: control the high pressure to the highest pressure allowed by the equipment, and reduce the medium pressure as much as possible. All blow-off valves and analysis valves should be opened. Low pressure has influence on medium pressure, and it should be reduced as much as possible, as long as it can meet the regeneration of the purifier.
Cooling stage, when the temperature T2 after the expansion machine reaches ~ Open the joint-1 valve at 140℃. Either too early or too late is not good for shortening the start-up cycle. Opening the joint-1 valve too early will reduce the refrigerating capacity of the expansion machine; if it is too late, the cold loss of the hot end will be too large and the temperature drop in front of the joint-1 valve will be affected. From-140℃ to T2, the temperature reaches the normal temperature (-155 ~ Between-165℃), there is about 1 H gas distribution and conversion process, that is the transition from cooling stage to hydrops stage.
Since the air volume through the expansion machine cannot produce liquid, and the liquid can only be produced through the joint-1 valve after further cooling in the second heat exchanger. Strictly speaking, liquefaction starts when the air reaches 3.65MPa (absolute pressure) and-140.68 ℃, and only when the throttle reaches 0.6MPa and-173 ℃ can Liquid be produced. In order to reduce throttling gasification, a certain degree of supercooling is needed. That is to say, the front temperature of section-1 valve should be controlled at-155 ~ Between-165℃. In this transitional stage, in order to ensure the refrigerating capacity of the expansion machine, the temperature of T3 should be decreased rapidly. To distribute gas reasonably, the temperature of T2 must be controlled at-140 ~ Between-155℃.
The stage of hydrops: in order to produce as much liquid as possible and accumulate as soon as possible, on the one hand, the high pressure and T3 temperature should be maintained; on the other hand, turn off the oxygen flow and control the low pressure and outlet temperature difference. Turn off the oxygen flow and increase the evaporation capacity at the bottom of the upper tower, which is beneficial to the improvement of oxygen purity; reduce the temperature difference of condensation evaporator, which is beneficial to the accumulation of liquid oxygen. In this sense, the oxygen outlet valve should be completely closed. But this will reduce the heat transfer area of the heat exchanger and enlarge the temperature difference of the Heat end. Therefore, it is better to control the oxygen flow at 1/3 of the normal flow. The purpose of increasing the low pressure is to reduce the pressure difference between the upper tower and the lower tower, so as to reduce the temperature difference of the condensing evaporator, which is beneficial to the accumulation of liquid oxygen.
A large amount of cooling is needed at the initial stage of purging. When the liquid air and liquid nitrogen throttle are closed, the rising steam volume of the lower tower and the upper tower will increase to prevent the leakage of liquid in the small hole, and the liquid will accumulate on the tower plate and start to be rectified. Therefore, the valve must be closed slowly to ensure the stability of the liquid oxygen level. Whether the opening degree of the two valves is reasonable indicates whether the liquid oxygen level is maintained at 430 ~ The range of 500mm, when the valve is closed, whether the purity of liquid air and liquid oxygen is within the designed range.
After closing the valve, the rectification working condition in the tower has been established, and the cooling capacity is only used to make up for the cooling loss of the insulation layer and the cooling loss of the temperature difference of the hot end. Therefore, the pressure must be reduced to reduce the production of cold volume. The sign of cold volume is liquid oxygen level. On the premise of maintaining the liquid oxygen level, the lower the high pressure, the better.
The purpose of production is to obtain finished products. The output of finished products is inversely proportional to the purity and must be adjusted reasonably. Due to the change of flow, the temperature difference at the hot end will change accordingly. Therefore, the distribution of air volume entering the tower should be adjusted timely.
