In the process of full low-pressure switching air separation plant, a bypass valve is usually installed on the pipeline of the air-out switching heat exchanger and the sewage nitrogen-in switching heat exchanger, which plays an important role in the start-up stage:
1) a part of the air after cooling can return to the switching heat exchanger to release the cooling volume through the bypass valve. This is equivalent to adding a full circular flow gas, which can reduce the temperature difference at the cold end and ensure self-cleaning;
2) the cooling capacity of the throttling effect of the air through bypass valve can be recovered through the dirt and nitrogen channels;
3) make the plate heat exchanger cool down, and expand the freezing area. But it has the side effect of enlarging the temperature difference of the hot end and increasing the cold loss.
At the beginning of the start-up stage, all the air in the unit needs to be refrigerated by the expansion machine, while the capacity of the expansion machine is limited. In addition, the air temperature in the start-up stage is much higher than the designed working temperature. Therefore, the actual amount of gas that can enter the expansion machine is limited. In this way, due to the lack of air volume in the switching type heat exchanger, it is easy to produce uneven distribution of air flow, that is to say, "Partial current" is generated ". Bias will worsen the heat transfer and self-cleaning conditions, especially the multi-unit Plate-fin Heat Exchanger, which is more likely to generate bias current. The bypass valve can increase the air volume entering the switching heat exchanger, which can effectively prevent the generation of partial current. Usually the bypass valve is used to adjust, which can make the air volume at start up reach about 80% of the normal production space-time air volume, and also determine the air volume as 80% of the pollution nitrogen volume ~ 100%. At the same time, the throttling effect refrigerating capacity of bypass air can be recycled in the switching heat exchanger.
In general, the bypass valve can be opened step by step when cooling the switching heat exchanger. However, in order to get through the water freezing area as soon as possible, some people are used to using bypass valve only when the temperature at the cold end drops below-45 ℃. For the process of starting the dryer, since starting the dryer absorbs the water in the air, there is no need to control the temperature difference at the cold end when crossing the phase of water freezing, so the bypass valve should not be used at first. Otherwise, the increase of air volume will increase the burden of starting the dryer.
In the second stage of starting, without the scruple of water, carbon dioxide precipitation and freezing, the function of bypass valve can be given full play, the bypass valve can be enlarged, the air intake can be increased, and the time of this stage can be extended, it makes the device get better precooling, which is beneficial to shorten the whole start-up time.
In the third stage of crossing the frozen area of carbon dioxide, when the outlet temperature of the expansion machine drops below-150℃ to-165℃, the amount of carbon dioxide is more. Therefore, when the temperature does not drop to this value, the bypass valve can be opened; when the temperature reaches-150℃, the bypass valve and circulation will be closed, so that it can quickly cross the frozen area of carbon dioxide and will not block the expansion machine.
When the switching heat exchanger cools down to the normal working condition, that is, the self-cleaning working condition has been basically established, the temperature difference of the cold end has been basically stable, and the bypass valve gradually loses its function. If it continues to open, on the contrary, it will cause excess cooling capacity of switching heat exchanger, expanding temperature difference at the hot end, increasing incomplete loss of heat exchange and failing to make full use of cooling capacity to accumulate liquid. Therefore, the bypass valve should be closed gradually at this time. When it turns into the stage of liquid accumulation and the air volume in the distillation column needs to be increased, the bypass valve should be completely closed.
