Air separation unit (ASU) takes cheap air as raw material, and separates oxygen, nitrogen, argon and other gas products through a series of processes such as compression, purification, liquefaction and cryogenic distillation. Air is a complex mixture containing a variety of harmful impurities, such as moisture, carbon dioxide, nitrous oxide and solid dust, which, if not removed, will seriously threaten the safety of the air separation unit. The plant owner and operators must pay sufficient attention to all risk factors to avoid the occurrence of explosion accidents.
Let's look at the possibility and types of explosions. In recent years, the reported causes of ASU explosion accidents include: The pearlite gets into the system; The main condenser-evaporator is not fully immersed, causing hydrocarbon accumulation; The release of hydrocarbons into the atmosphere; Forest fires lead to excessive nitrous oxide, carbon dioxide in the source air; Liquid nitrogen is discharged into the ditch, and there is a lot of oil mist on site; The lubricating oil is discharged into the ditch, and cold box leaks liquid oxygen; Hydrogen gets insdie, causing deformation of the cold box; The cold box wall is corroded by hydrogen sulfide, resulting in heat exchanger damage; Perlite soaked in liquid oxygen, detonated during stripping (for inspection), resulting in tower tilt; Mechanical impurities left in oxygen compressor after maintenance, resulting in the secondary cooler burned.
The places where explosion can take place in an ASU: The primary condenser-evaporator, the auxiliary conderser-evaporator, argon condenser-evaporator, crude krypton xenon tower, where hydrocarbon and nitrous oxide are possibly gathering and blocking the channels. And also oxygen compressor and oxygen piping, where welding slags might be existing.
The following measures for safety and anti-explosion can be taken.
-
Location selection. ASU shall be located upwind of the factory/plant, where the air is relatively clean.
-
Improve the air purification system of ASU.
Air purification system is the barrier for harmful substances to enter the air separation system, and it is also the key to ensure the safety of ASU, so it is of great significance.
Use molecular sieve purification process. The early freezing method switching process has many defects, and has been basically unused.
Perfect molecular sieve bed. Generally the molecular sieve is composed of three layers, from bottom to top, inert alumina, activated alumina, 13A molecular sieve. Adding a layer of 5A molecular sieve or mixing 13A and 5A molecular sieve can improve the adsorption effect of nitrous oxide.
Ensure the regeneration of molecular sieve is complete. The regeneration process of molecular sieve is the reverse process of adsorption process, if the regeneration is not complete, the residual impurities will occupy the adsorption capacity of molecular sieve, affecting the normal operation of molecular sieve. Normally, the heating temperature should be controlled above 180℃, and the temperature at the end of cold blowing should be above 80℃ to ensure that the static adsorption capacity of the adsorbent reaches about 5%.
3. Strengthen instrument monitoring of ASU.
It is necessary to install high quality analytical instruments, such as gas chromatograph, hydrogen flame detector, helium mass spectrometer leak detectotr, thermal conductivity analyzer and temperature, pressure, level controllers, etc.
It is necessary to strengthen the monitoring of hydrocarbons, nitrous oxide and liquid level in locations with potential risks such as the main condenser-evaporator, as well as strengthen the monitoring of atmospheric quality. In case of abnormalities, effective measures shall be taken.
Strictly abide by the operation rules and eliminate illegal operation. With the update and improvement of air separation equipment and the exposure of hidden dangers, the procedures and rules must be improved and updated.