Corrosion mechanism of air cooling in hydrocracking
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There are many discussions regarding corrosion in high-pressure air coolers used in hydrocracking. What are the crystallization temperature ranges of NH4Cl and NH4HS? Also, what are the corrosion mechanisms for these two substances? Thank you
The corrosion of high-pressure air coolers in hydrocracking is not entirely due to ammonium salts; the fundamental issue is CL ion corrosion.
The crystallization temperature of ammonium chloride is generally 200°C, while the crystallization temperature of ammonia hydrosulfide is generally 150°C℃
The causes of corrosion include: 1. High levels of S and N in the raw materials, coupled with an air cooling system whose capacity is insufficient to handle the volume of material； 2. The original design did not include a circulating desulfurization facility ； 3. It is difficult to increase the water injection volume as the raw material changes ； 4. Excessively high flow rate in the air cooler, uneven distribution of fluid flow ； 5. Corrosion caused by reasons such as unstable operation ； 6. Excessively high Cl^- levels in the reaction effluent; carbon steel or Incoloy 825 is used for the materials of high-pressure air coolers. Factors such as the upper and lower limits for the medium flow rate and the content of ammonium salts that lead to water separation need to be taken into account. Corrosion mechanism: Within the temperature ranges at the inlet and outlet of the high-pressure air coolers in typical hydrogenation units, scaling easily occurs due to the crystallization of NH4Cl and NH4HS. In areas of the air cooler where the flow rate is low, the accumulation of scaling caused by NH4Cl and NH4HS leads to corrosion beneath the electrochemical scale, resulting in pitting that eventually causes perforations. http://bbs.hcbbs.com/viewthread.php?tid=25797 http://bbs.hcbbs.com/viewthread.php?tid=205676 http://bbs.hcbbs.com/viewthread.php?tid=165193
Our air-cooled system leaks all the time; the design institute says it’s a world-class problem.
It should be under-scale corrosion; if there is constant corrosion in air cooling, the properties of the materials used in design are closely related to those actually used in operation. Is the water injection volume too low? Is the load too low?
The causes of corrosion include: 1. High levels of S and N in the raw materials, and insufficient water injection； The water injection amount must be determined based on the S and N contents of the raw material. 2. Excessively high flow rate in the air cooler, uneven distribution of fluid flow ； The flow rate in the air cooler tube bundle exceeds the design value, causing erosion corrosion. 4. Scaling is very likely to occur in the high-pressure air coolers of hydrogenation units due to the crystallization and deposition of NH4Cl and NH4HS. In areas of the air cooler where the flow rate is low, the accumulation of scaling caused by NH4Cl and NH4HS leads to corrosion beneath the electrochemical scale, resulting in pitting that eventually causes perforations. It can be said that the problem of short-term corrosion and perforation of air-cooled tube bundles can be solved.