Failure analysis of a reactor effluent air cooler

In the present research, root causes of the tube explosion of a refinery hydrocracking reactor effluent air cooler (REAC) were studied. For this purpose, anatomical analysis was firstly used and ammonium salt crystals were found. Based on the actual operating conditions, ionic equilibrium model was established by Aspen software to calculate the crystal temperature of the NH4Cl and NH4HS. The results indicate that NH4Cl crystal temperature ranged from 175 °C to 210 °C with different chlorine and nitrogen content of the feedstock, which is higher than the operating temperature of REAC. To analyze the impact of the air cooler structure on the ammonium salt deposition location, numerical simulation including the air cooler header box and bundles was conducted by the Fluent software. The results show that a bias flow regime exists because of the large vortex formed in both sides of the header box inlet. The velocity and aqueous phase fraction of the multiphase were low in the tubes of the header box inlet both sides, which result in the NH4Cl deposition and under-deposit corrosion. In order to verify the failure analysis results, other in service air coolers were inspected using the infrared camera, and the results show that other air coolers had the plugging features at the same position. Finally, some recommendations were given to mitigate the NH4Cl deposition.

► The self-accelerating mechanism of ammonium salt deposition is proposed.
► The conditions for crystal and deposition of NH4Cl and NH4HS are different.
► The NH4Cl is prone to deposition in the inlet tubes of the REAC header box.
► The simulated result was in good agreement with the REAC failure observed.