Ammonium Bisulfide Deposition Mechanism And Numerical Analysis Of Multi-physics Coupling In Reactor Effluent Air Cooler

In a petroleum refinery, Reactor Effluent Air Cooler (REAC) and its related piping system are key systems in a hydrocracking process. However, as the content of sulfur, nitrogen and chloride in crude oil increases, corrosion failure caused by ammonium salt deposition has occurred frequently in REAC tubes. Because ammonium salt deposition failure mechanism is complex and relates to many fields, domestic and foreign studies have not been able to propose a scientific and effective prediction method. Therefore, the failure problem of REAC system has not been solved fundamentally. On the basis of mechanism analysis and numerical computation of ammonium bisulfide deposition in REAC tubes, the failure position of ammonium salt deposition was predicted, which is conducive to the optimization design and safety operation of REAC system.In this paper, the physical parameters of multiphase fluid in REAC system were obtained by using a petrochemical process simulation software HYSYS. By analyzing the crystallization and deposition of ammonium bisulfide in tubes, the initial temperature of crystallization reaction was determined to be 44℃. A deposition mechanism of multi-physics coupling was proposed and the mathematical models of multi-physics coupling and ammonium salt deposition were further established. The heat exchanger analysis of this air cooler was calculated with the help of a heat exchanger simulation software HTRI. Then, the initial position of ammonium salt crystallization reaction was determined, which is 5.9m away from the entrance. The computational fluid dynamics software FLUENT and multi-physics coupling software were used for numerical simulation. The field distribution of fluid flow, multiphase temperature, ammonium bisulfide concentration and wall deposition of ammonium salt were analyzed. As a result, the specific position with highest susceptibility was predicted. Combined with on-site bundle thickness data, the accuracy of simulation result and the reliability of numerical method were verified. According to the influence factors analysis of ammonium bisulfide deposition failure, some valuable conclusions were found. The initial temperature of ammonium bisulfide crystallization reaction increases with the increasing of N or S content, which would enhance the failure risk of ammonium salt deposition. On the premise of meeting technological requirement and ensuring no erosion failure in REAC tubes, the failure of ammonium bisulfide deposition could be effectively prevent by adjusting the efficiency of blower cooling, improving multiphase flow velocity, or increasing water injection. Many measures could be done to reduce the imbalanced degree such as improving the device structure, setting appropriate injection mode, adjusting the fan rotational speed and so on, which will helpful to increasing the safety margin.The main innovation in this paper is as following. Combined with complex processes such as chemical reaction, fluid flow, heat transfer, mass transfer in practical engineering system, the multi-physics coupling mechanism of ammonium bisulfide crystallization and deposition is established. The numerical simulation of ammonium bisulfide flow deposition under the coupling effect of flow field, temperature field and concentration field is carried out. Based on these mechanism and numerical results, a reliable prediction method for ammonium bisulfide deposition failure is proposed.