Analysis of the hidden danger of the main cooling system and preventive measures

More than half of all air separation equipment accidents were the explosion caused by the main cooling system. When processing air, all the impurities which have not been completely removed will finally gather in the main cooling liquid oxygen. Due to the gasification of liquid oxygen, these impurities will form a high concentration accumulation in some areas, and precipitate after crystallization. Under the combustion support of oxygen, they will explode, which is very harmful to the production and operation of industry and personal safety of employees. Next, we’ll make a comprehensive analysis of the causes of accidents and find out the effective preventive measures and methods to prevent the main cooling explosion.

With the increase of unsaturation, the risk of hydrocarbons with the same number of carbon atoms will increase relatively; while with the increase of carbon atoms, the risk of hydrocarbons with the different number of carbon atoms will increase relatively. The accumulation of hydrocarbons in the main cooling liquid oxygen is directly proportional to their boiling point, inversely proportional to their solubility in the liquid oxygen and saturated vapor pressure.

There are two types of hydrocarbon accumulation in the main cooling liquid oxygen: one is the partial blockage caused by the main cooling structure, which causes the local liquid oxygen to evaporate to dry, then make the hydrocarbon precipitate; the other is that the air without thorough purification enters the cold box to participate in the distillation, and the trace hydrocarbon does not fully circulate and be adsorbed, so it exceeds the standard in the liquid oxygen as a whole. In addition, low liquid level operation or large fluctuation of liquid level can make harmful impurities in liquid oxygen precipitate and accumulate.

For the above situations, the following measures can be taken for effective prevention:

1. Ensure the quality of raw air, the air separation unit shall be set on the windward side of the plant, and the air shall be washed by nitrogen water precooling system, and the molecular sieve absorber of the purification system shall be regenerated thoroughly and regularly as required.

2. Strengthen the liquid oxygen discharge and hydrocarbon dilution, increase the flow section of the liquid oxygen channel during designing of the main cooling, and there shall be no blind area in the process structure.

3. Regularly monitor the content of liquid oxygen and hydrocarbon. In the process design, chromatograph shall be used to continuously analyze the content of acetylene and hydrocarbon, with alarm and interlock in time.

4. The main condensing evaporator should be fully submerged. The liquid level of the main condensing evaporator shall be increased firstly when draining, and the pressure shall be increased slowly to minimize the pressure pulse.

5. More than 2 liquid oxygen discharge nozzles should be arranged, and the diameter of the nozzle increases correspondinglywith the increase of the equipment diameter. When the impurity in liquid oxygen exceeds the warning point, the liquid oxygen emission should be increased.

To sum up: the safety precautions of the main condensing evaporator is the most important for the operation of the air separation unit. We must pay high attention to it. While taking technical measures to control the hydrocarbon content in the liquid oxygen, we should strengthen the control of the ignition source and increase the monitoring measures, strengthen the management of all parties, improve the safety awareness of the staff, and prevent the occurrence of the malignant accidents of the main cooling explosion.