Study On Crystallization Of Ammonium Salt In The High-pressure Heat Exchanger Of Hydrogenation Unit

With the heavy and inferior petroleum,the content of chlorine,nitrogen and sulfur is increasing day by day.Hydrogenation products of chlorine,nitrogen and sulfur are ubiquitous in hydrogenation unit,resulting in the problem of equipment corrosion failure in the actual production process.The high-pressure heat exchanger is a common heat exchange equipment in hydrogenation units.In the actual production survey,the heat exchange equipment of many oil refining and refining enterprises appeared the situation of ammonium salt blockage and tube bundle leakage,resulting in huge economic losses.Therefore,the crystallization of ammonium salt in high-pressure heat exchanger of hydrogenation unit needs to be solved urgently to prevent it from becoming a safety problem.In this paper,the crystallization corrosion failure of high-pressure heat exchanger in petrochemical hydrogenation unit was studied.According to the physical-chemical picture of the crystallization and deposition corrosion of ammonium salt,the mechanism of the formation of ammonium salt in the gas phase and the first appearance on the wall was described.The corrosion reason of heat exchanger was analyzed by using ANSYS software and user-defined function.The single-tube model was established and the boundary conditions were set which were the same as the actual operation of heat exchanger.The user-defined function was verified to conform to the crystallization reaction of NH₃+HCl→NH₄Cl.The main findings of the present study are as follows:There are two vortex regions in the velocity distribution of multiphase flow field in heat exchanger,and the velocity at the vortex core is large,located at the top corner of the first pipe side.The velocity distribution leads to the uneven distribution of the multiphase system.Under the action of gravity and centrifugal force,obvious stratification is formed through the gas phase and liquid phase flowing through the u-bend.The gas content of the first pipe is greater than that of the second pipe.The heat transfer calculation results show that the temperature of the tube bundle decreases after the inlet due to the heat transfer between the fluids,the outlet temperature of the pipe is around 146℃.The tube bundle at vortex core was selected.The analysis results showed that the wall temperature was lower than the central temperature of the tube bundle,so ammonium chloride was first generated on the wall of the tube bundle.In addition,the 170℃ isotherm of the bundle continues to shrink to the center of the bundle with the progress of heat transfer,indicating that the low-temperature area in the bundle keeps increasing,and the ammonium chloride crystallization area keeps spreading to the center of the bundle.The easy crystallization area is determined as the top corner of the heat exchanger and its symmetrical position,located on both sides of the center of the bundle,and the maximum mass fraction of ammonium chloride is1.16×10^-4.The distribution diagram of ammonium chloride in heat exchanger is consistent with the actual corrosion failure diagram.The results are as follows:the first crystallization occurs at the bottom corner of the heat exchanger tube bundle,which is consistent with the actual corrosion position.Moreover,ammonium chloride in the tube bundle is first generated around the tube wall and spreads to the center continuously.The speed at the elbow is large and there is a risk of erosion and corrosion,which is consistent with the crystallization diagram of the tube bundle observed in the factory.The positions with greater corrosion risk under scale are located between 0.2-0.3 m in the X direction,0.456 m in the Y direction and 5 m in the Z direction,the maximum corrosion rate is 0.008 mm/y.The area with serious erosion corrosion is located at the end of straight pipe,which is near the elbow,and the erosion corrosion rate under scale is negligible compared with the erosion corrosion rate.Water injection can dissolve the ammonium salt generated under standard operating conditions,the salt concentration can be reduced by 40%（195℃）and 80%（205℃）with increasing temperature,and the water injected can completely dissolve ammonium chloride.In addition,the maximum erosion rate is 1.5 mm/y at 205℃,which is about 45%lower than the standard condition.