The plate unit of plate fin condensing evaporator in large air separation plant is often composed of multiple units. The determination of unit height should consider both heat transfer and safety. From the point of view of heat transfer, according to the mechanism of boiling in tube, the ratio of height to diameter (for plate fin type, the ratio of unit height to equivalent diameter of channel) should be more than 300, and the heat transfer coefficient should be higher.DEAR Cryogenic air separation unit.The separation of oxygen and nitrogen is realized by distillation. There must be rising vapor and falling liquid in the distillation process. In order to obtain oxygen and nitrogen products, the two-stage distillation process is realized in the upper and lower columns. Condensing evaporator is the link between upper tower and lower tower. It is used for the heat exchange between the reflux liquid oxygen at the bottom of the upper tower and the rising gas nitrogen at the top of the lower tower.
DEAR Cryogenic air separation unit.It is likely that acetylene accumulates in the local dead corner and micro explosion occurs. In addition, the emission of liquid oxygen is not measured, which is difficult to master. Some of them are caused by the failure of liquid oxygen circulation adsorption system to operate normally, and some by the unqualified grounding device.The unit cooling loss decreases with the increase of unit capacity, while the temperature difference at the hot end of large air separation units is generally about 3 ℃, and the unit incomplete heat exchange loss of different units has little change. Therefore, with the increase of unit capacity, the proportion of unit heat exchange incomplete loss in total cooling loss increases.The automatic valve is actually a kind of one-way valve, which is installed at the cold end and central extraction of the switching heat exchanger. It is matched with the forced valve. When the positive flow air is used, the air (and intermediate air extraction) automatic valve will open and the sewage nitrogen (or oxygen, nitrogen) automatic valve will close; when the backflow air is used, the sewage nitrogen automatic valve will open and the air automatic valve will close.
DEAR Cryogenic air separation unit.The purpose of purging after heating the fractionator is to remove the mechanical impurities in pipes and vessels. The blowing out steps are as follows: the preparation before blowing out: close the main inlet valve of heating air and all heating valves, open the blowing out valve of oil-water separator of purifier at the same time, loosen the ejector rods of inlet and outlet valves of expander, close the on-6 valve, pull open the stem of joint-1 valve, open the blowing-1 valve, blowing-2 valve and high pressure gauge valve, and close the blowing out valve of oil-water separator of purifier;DEAR Cryogenic air separation unit.When the temperature of the air entering the unit increases, not only the heat load of the accumulator (or switching heat exchanger) increases, but also the amount of water brought in increases, which increases the burden of self cleaning. With the increase of water freezing, the heat transfer efficiency is reduced. When the heat transfer area of the heat exchanger is fixed, the increase of the heat load and the poor heat transfer effect will inevitably lead to the increase of the temperature difference between the cold end and the hot end.
DEAR Cryogenic air separation unit.The greater the pressure difference of molecular sieve switching, the greater the pressure on the sieve bed, which will be detrimental to the safety of molecular sieve bed. Therefore, we should pay close attention to the molecular sieve heating, cold blowing and other processes. It is disadvantageous to take too long and too short time to equalize and relieve pressure.In the cold blowing stage, the gas used is still dirty nitrogen, and the dirty nitrogen is no longer heated. Obviously, the temperature of the gas entering the molecular sieve bed will decrease rapidly, and the temperature of the bed near the inlet side will also decrease. As the heat moves to the outlet side of the waste nitrogen, the bed layer at the outlet side will continue to rise, and this part of the molecular sieve will continue to regenerate.Generally, rivers, lakes or groundwater are used as cooling water for air separation units. This kind of water usually contains suspended solids (sediment and other pollutants) and bicarbonates such as calcium and magnesium [- Ca (HCO3) 2 and Mg (HCO3) 2], which is called hard water. When there are many suspended solids, it is easy to block the channels, filters and valves of the cooler. Calcium, magnesium and other bicarbonates are easy to form calcium carbonate (CaCO3) and magnesium carbonate (MgCO3) precipitates when the water temperature rises, that is to say scale. Generally, water temperature above 45 ℃ will start to form scale, and the higher the water temperature is, the easier it is to scale.
