Failure analysis of ammonium chloride salt coagulation corrosion of U-tube heat exchanger in diesel hydrogenation unit

Abstract

Diesel hydrogenation unit is an important equipment for processing inferior oil in petroleum refining enterprises. Among them, the U-tube heat exchanger has been in high temperature and corrosive environment containing chlorine for a long time. Serious problem of tube bundle corrosion failure caused by ammonium chloride salt deposit. Anatomy of the diesel hydrogenation heat exchanger showed that the leakage part mainly appeared in the outermost bundle of the heat exchanger tube near the exit section, and the local area showed a large area of gully damage. Metallographic and XRD results show that the failure tube bundle material does not have manufacturing defects, and the corrosion location scale contains NH4Cl and iron oxides. The process flow analysis based on PR-NRTL model showed that the heat exchanger was in the NH4Cl crystallization temperature range, and the crystallization temperature increased with the increase of N and Cl contents in raw materials. Fluid simulation analysis of inlet tube box and single U-tube is carried out by using component transport equation and condensation model. Computational fluid dynamics (CFD) results indicate that there is a low velocity zone with high volume fraction of gas phase in the tube box. The higher gas volume fraction results in the formation of more NH4Cl particles into the U-tube bundle during heat transfer cooling. With the decrease of temperature, liquid water content in U-tube increases gradually and mainly distributes at both sides and bottom of tube bundle. The gas phase gradually fills the top of the tube bundle, and the oil and gas stratification becomes more obvious. The mass transfer rate and corrosion rate were the highest at the bottom and both sides of the tube bundle. With the temperature decreases, the NH4Cl particles generated in the tube continue to increase. The particles were deposited at 2000–3000 mm near the outlet section of the U-tube tube. NH4Cl particles absorbed liquid water, resulting in under-deposit corrosion of the tube bundle, and eventually led to the thinning and leakage of the heat exchanger tube bundle.