Study of NH ₄HS concentrating mechanism in air cooled heat exchanger tubes

Highly concentrated NH ₄HS causes significant corrosion. In high pressure plants such as hydrodesulfurization units operated under NH ₄HS environments, leakage caused by corrosion could result in catastrophic disaster. Therefore, in the design phase, corrosion prevention techniques should be considered. In this study, based on the severe NH ₄HS corrosion observed selectively in the vapor phase inside air cooled heat exchanger tubes, the NH ₄HS concentrating model was developed to discern the difference in NH ₄HS concentration between the vapor phase and the liquid phase. We elucidated that NH ₄HS concentration in the condensed area in the vapor phase can be much higher than the liquid phase due to the difference of the heat transfer coefficient. This result quantitatively demonstrated the cause of corrosion observed in the air cooled heat exchanger tubes. In addition, this study indicated that the design of the air cooled heat exchanger tube where NH ₄HS corrosion shall be concerned should consider a flow regime, air temperature or predicted NH ₄HS concentration in the condensed area in the vapor phase.