Investigation On Influencing Factors Of Ammonium Salt Crystallization Corrosion In Atmospheric Distillation Overhead System

The under-deposit corrosion of ammonium is one of the various corrosion forms in atmospheric distillation tower overhead condensation system,which has complex mechanisms.There are many influencing factors of the under-deposit corrosion,and it is difficult to prevent and control the under-deposit corrosion,causing great concern.Based on the summary of the research progress of corrosion failure forms,corrosion prevention and control technology of atmospheric distillation overhead system and ammonium salt crystallization and deposition characteristics,the following works has been carried out in this thesis:First of all,combined with the process of atmospheric and vacuum distillation and atmospheric distillation overhead system,the thermodynamic calculation of ammonium salt crystallization process in atmospheric distillation overhead system was carried out,crystallization equilibrium partial pressure product curves of ammonium chloride and ammonium bisulfide were drawn,and the corrosion mechanism of under-deposit corrosion was analyzed.The crystallization trend of ammonium chloride is much higher than that of ammonium bisulfide,and the under-deposit corrosion often arises with the combination of local multiphase flow erosion corrosion,aggravating the corrosion failure of metal materials.Secondly,decomposition pressures of methylamine hydrochloride,ethylamine hydrochloride,dimethylamine hydrochloride,morpholine hydrochloride and 4-(2-chloroethyl)morpholine hydrochloride at different temperatures were measured.Standard equilibrium constants of decomposition reaction were calculated,and the correlation between the natural logarithm of standard equilibrium constants and thermodynamic temperature was fitted.The product curves of crystallization equilibrium partial pressures of amine salts were drawn.Thirdly,the process simulation of operating condition of atmospheric distillation overhead system was carried out through Aspen Plus.According to the simulation results and the product curves of crystallization equilibrium partial pressures of amine salts,it was determined that ammonium chloride induced the crystallization corrosion in the overhead system,and the calculation formula of ammonium salt crystallization rate was fitted.Results showed that the crystallization temperature and crystallization rate of ammonium chloride are related to the content of hydrogen chloride and ammonia,overhead pressure and water injection rate,and the overhead parameters not only affect crystallization characteristics of ammonium salt,but also affect the dew point corrosion behavior in the atmospheric distillation overhead system.Finally,the fitting formula of ammonium salt crystallization rate was written into userdefined function and loaded into ANSYS Fluent software.The geometric models of various pipeline structures were established for multiphase flow numerical simulation,and the effects of different water injection structures and downstream pipeline structures on ammonium salt crystal and deposit distribution and erosion shear of tube wall were studied.This thesis investigated the influencing factors of ammonium salt crystallization corrosion in atmospheric distillation overhead system,which provides theoretical support for process parameter optimization and pipeline structure improvement of the overhead system.