In order to reduce the moisture brought into the purifier (or dryer) by the heating gas and ensure the service life of the purifier (or dryer), the heating pressure of the purifier should be controlled to the normal working pressure (2MPa). In addition, the gas flow velocity in the purifier is inversely proportional to the pressure.DEAR Air separation equipment.When the expander expands, the decrease of gas temperature is not only caused by the decrease of pressure, resulting in the increase of molecular potential energy, but also the decrease of molecular kinetic energy. It is mainly caused by external work, so the effect of temperature drop is much greater than that of throttling.DEAR Air separation equipment.When the forced valve is not tight, the upper tower pressure is usually reduced. When the automatic valve is not tight, the temperature of the positive flow air channel (or the cold accumulator) increases with the increase of air volume, and the temperature of the middle part of the polluted nitrogen channel (or the cold accumulator with nitrogen and oxygen) decreases with the increase of return flow.
DEAR Air separation equipment.The evaporation heat transfer surface of membrane evaporator is not immersed in liquid oxygen, but spray liquid oxygen onto the heat transfer surface by liquid oxygen pump to form a thin liquid film, which directly evaporates into gas in the process of contacting and heating with the heat transfer surface. The heat transfer effect is enhanced, the heat transfer temperature difference of the condensing evaporator is reduced from 1.3 ~ 1.5 ℃ to 0.7 ~ 0.8 ℃, and the pressure of the lower tower is reduced by about 0.02 MPa.Because the heat transfer coefficient increases greatly, it is not necessary to increase the temperature difference by increasing the pressure of the lower tower, and the actual operating pressure of the lower tower also decreases slightly. This is not exactly the same as the heat transfer of long tube condensing evaporator, because the mechanism of boiling heat transfer in tube is different from that in tank. It is safe and reasonable to adopt full immersion operation for plate condensing evaporator.
The working pressure of the full low pressure air separation unit is about 0.6MPa, so the refrigeration capacity of throttling effect is very small. For each cubic meter of process air, it is only 1.36kj/m3. For each cubic meter of process air, the cooling loss of the unit is 4.2-7.5kj/m3, and the incomplete heat exchange loss is about 3.9kj/m3 when the temperature difference at the hot end is 3 ℃.DEAR Air separation equipment.In addition, the critical point pressure of air is about 3.8mpa, and the critical temperature is about 132.5k (- 140.6 ℃). At this point, there is no difference between gas and liquid. When the oxygen generator is started, the air pressure is higher than the critical pressure. If the temperature in the main heat exchanger drops below the critical temperature, the air will be liquefied directly, and some will be gasified after throttling.DEAR Air separation equipment.Close the regulating valves of liquid air, liquid nitrogen and sewage liquid nitrogen. Fully close the oxygen and nitrogen product delivery valve, open the vent valve, and place the waste nitrogen vent three-way valve on the vent side. Fully close the butterfly valve of the expansion air into the upper tower, and open the bypass valve.
DEAR Air separation equipment.When the resistance of each group is different, the gas will produce bias flow, resulting in uneven gas flow distribution. The group with small resistance has a large amount of backflow, which reduces the temperature difference at the cold end and expands the temperature difference at the hot end, which is conducive to self removal, but the cold loss at the hot end will increase; the group with large resistance has a small amount of backflow, which expands the temperature difference at the cold end and makes the self removal incomplete, which will increase the freezing amount of water and carbon dioxide.DEAR Air separation equipment.When the distillation condition of the lower column is normal, but the purity of waste nitrogen is still too low, the distillation efficiency of the upper column may be reduced (for example, the tray is blocked or liquid leakage); or the amount of expansion air is too large; or the amount of oxygen removed is too small and the purity is too high, which may increase the amount of rising steam and reduce the reflux ratio.
