The determination of the running cycle of the air separation plant is mainly based on the gradual accumulation of trace carbon dioxide after it is brought into the air separation tower during the design, until the time interval when it can not continue to run because of blocking. Under normal circumstances, the continuous running time of all low pressure oxygen generator should be more than one year, and the continuous running time of new molecular sieve adsorption process can be more than two years. However, in actual operation, the situation is much more complicated than the design. The main factors affecting the running cycle include the continuous working ability of the oxygen generator equipment and the running machine, the quality of heating and blowing before starting, the level of operation during the starting stage and normal operation, the size of the air load, etc.
Most of the reasons that cause the oxygen generator to be shut down and overhauled before the specified period are the failure of running machinery and switching system. It is mainly the failure of air compressor, expansion machine and liquid oxygen pump. At the same time, the internal leakage of forced Valve and automatic valve of air separation, some heat exchanger and the internal damage and internal leakage of low temperature valve, pipeline Expansion joint fatigue fracture, etc., will make the oxygen generator need to stop in the Midway for overhaul.
The heating blow-off before the start-up of the oxygen generator and the operation in the start-up stage also directly affect the operation cycle. This kind of situation often happens: due to the eagerness to produce oxygen, the heating and blowing are not complete, and the remaining water in the tower causes the resistance of the cold accumulator or the reversible heat exchanger to be too large after starting, and sometimes the resistance of the distillation tower is too large, so that liquid flooding occurs frequently. In the start-up stage, the time spent on crossing the frozen zone of water and carbon dioxide is prolonged, and the temperature difference at the cold end of the switching heat exchanger is not controlled within the allowable range (in the start-up stage, this temperature range is gradually reduced as the temperature decreases), which will cause the increase of moisture and carbon dioxide impurities brought into the air separation tower. The interruption of the start-up process of the air separation plant or the start-up for many times will cause the rise of the temperature of the cooler or the switching heat exchanger, which will bring a large amount of carbon dioxide into the tower, thus shortening the operation cycle.
In normal operation, the temperature difference control at the cold end of the switching heat exchanger is the one that has the greatest influence on the operation cycle. If the temperature is not well controlled, on one hand, it will cause the self-cleaning effect of switching heat exchanger is not good, and the resistance of carbon dioxide will rise due to accumulation in the heat exchanger; on the other hand, it will bring a small amount of carbon dioxide into the tower. Due to the impact of airflow, the actual content of carbon dioxide in the cold end air of the cooler and the switching heat exchanger will exceed the saturation content, which will also affect the operation cycle. Especially, the cold section of the switching heat exchanger is too short, and the precipitation area of carbon dioxide is shortened, which makes it easier to bring part of carbon dioxide into the tower, causing the resistance of the distillation tower to increase, the main cold heat transfer to be weakened, and the supercooler to be blocked, when the pressure of the lower tower rises, the air volume and oxygen output of the inlet Tower decrease.
Switching heat exchanger with water will also shorten the operation cycle. It is usually caused by improper operation of nitrogen-water precooler. While slight water inflow is often due to the neglect of blowing off the water separator and the emission of condensate in the air main tube of the switching heat exchanger.
The Air processing status of the air inlet sub-equipment is also an important factor affecting the operation cycle. Because the increase of air volume or inlet air temperature will increase the burden of cooling accumulator or switching heat exchanger to clear water, and increase the temperature difference of heat exchange. In the cold end, it is manifested as self-cleaning failure, and the resistance increases faster. Therefore, the operation cycle will be shortened in high-load production, while it can be extended in low-load production.
It is of great economic significance to extend the operation cycle of the air separation plant. It can reduce the standby machine, reduce the overhaul time, save money and produce more oxygen and nitrogen products. Therefore, the operation should be managed and maintained elaborately.
