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The role of liquid nitrogen throttle small air separation plant
Liquid nitrogen throttle (Section -4 valve) in the use of different occasions: air separation tower should be fully open early start (about turn 12 to 15). When the condensing evaporator liquid level of liquid oxygen close to or reach 430mm, with the -2 valve at the same time slowly closed small. Off the valve speed in the initial liquid and liquid pressure and pressure conditions; when the section -4 valve is closed to about 2 rpm, liquid level in the liquid under normal circumstances, the liquid should be analyzed, the purity of liquid nitrogen , Depending on the purity of the situation. Finally, the control valve-4 opening in the liquid air, liquid nitrogen purity of the best position. In normal production conditions, liquid nitrogen throttle does not need to change frequently. When the liquid nitrogen purity automatically rises, the liquid-air purity automatically decreases, the liquid level rises automatically when the valve head may be blocked by dry ice, emergency turn the valve scraping cream after reset. When intermittent oxygen or temporary parking, the use of section -4 valve to maintain medium pressure, in order to shorten the restart time. Complex drive start again, depending on the fluid empty, liquid oxygen levels to decide. When the equipment is ready to stop warming, large section -4 valve should be opened before parking, the liquid sent to the tower. Air separation tower full heating, section -4 valve should be fully open. Section -4 valve is the role of the liquid nitrogen from the liquid nitrogen tank under the tower through the liquid nitrogen subcooler sent to the throttle on the top of the tower. During normal production, under the premise of proper opening, it plays the role of controlling the liquid-air and liquid-nitrogen purity, and at the same time it also affects the change of liquid-liquid level and the liquid-gas ratio of the upper tower, thus affecting the nitrogen purity and oxygen Yield. After the valve-4 valve is closed, the purity of liquid nitrogen is increased, the liquid-air purity is decreased, and the liquid-level of liquid is increased when the valve-2 valve opening is not changed. Open the big four-valve is the opposite. Why section 4 valve can control the liquid nitrogen and liquid air purity? Because the air into the tower is saturated gas and liquid mixed state, most of the steam. The vapor rises along the small hole of the lower tray plate, and the oxygen molecules in the vapor are condensed by the liquid on the tray to become the liquid oxygen; the nitrogen molecules in the liquid on the tray are condensed by the condensation heat released when the oxygen molecules condense Gas phase. Each plate by a heat transfer, mass transfer, the liquid oxygen molecules increased, while the vapor content of nitrogen molecules increased. Vapor through the tower after repeated condensation evaporation, so to the top of the tower, the vapor of nitrogen molecular content to meet the design requirements, and then in the condensing evaporator, liquid nitrogen is condensed into liquid nitrogen, the vast majority of liquid accumulation in the liquid nitrogen tank Inside. If the section -4 valve opening is too large, into the liquid on the tower more, back into the tower to reduce the amount of liquid nitrogen. Under the tower tray backflow is too small, it means that the next tower cooling capacity of the vapor molecules of oxygen can not be fully condensed down to the bottom of the tower to increase the oxygen content of the vapor, the purity of liquid nitrogen decreased. Also due to the tower under the reflux liquid is too small, under the tower plate full of nitrogen molecules evaporation, the downstream liquid oxygen molecules increased, so the liquid-air purity. In addition, the lower tower into the top of the tower of liquid nitrogen in the oxygen molecules more liquid nitrogen itself higher oxygen content, so that the oxygen content in the tower of nitrogen increased. Therefore, the liquid nitrogen purity is too low will lead to lower nitrogen tower, oxygen production decreased. Off section -4 valve, the amount of liquid nitrogen to the tower to reduce the liquid nitrogen tank overflow, the amount of liquid nitrogen reflux into the tower increased the cooling capacity of the lower tower increased oxygen molecules in the vapor to be fully condensed , The vapor content of oxygen in the bottom of the tower decreases, and the purity of liquid nitrogen increases. However, due to the increase of the reflux liquid, the nitrogen molecules in the liquid on the tray are not sufficiently evaporated, the liquid-air purity at the bottom of the lower tower decreases, and the liquid-air volume increases. If the section -4 valve is too small, the liquid nitrogen purity is too high, will inevitably lead to liquid air purity is too low, too much liquid space into the tower on the amount of liquid nitrogen is too small. From the requirements of the tower distillation conditions, the stripping section reflux ratio is smaller, to facilitate the improvement of oxygen purity; rectification section reflux ratio is larger, is conducive to the improvement of nitrogen purity, if the rectification section reflux ratio Too small, although the purity of liquid nitrogen is very high, but due to the amount is too small, the nitrogen purity will decline, reducing the amount of oxygen. From the above analysis point of view, section -4 valve opening too large, too small are unfavorable. Therefore, the adjustment of section 4 valve should be slow, and sometimes only 1 ° ~ 2 °. According to experience, the purity of liquid nitrogen can be consistent with the purity of the gas nitrogen by using the high-purity equipment (gas purity of over 99.5% Section -4 valve control, liquid nitrogen purity (nitrogen) can be 1% to 2% lower than the purity of nitrogen gas.