Study On Corrosion Risk Prediction And Corrosion Mechanism Of Low Temperature Dew Point At The Top Of The Atmospheric Tower

The petrochemical industry is the pillar industry of modern industry in our country.China has a shortage of crude oil resources and a large import of crude oil.The processing of low-quality crude oil with high sulfur,high nitrogen,high acid,and high chlorine has become the development trend of the industry.Atmospheric and vacuum distillation units,as the first production process in the process of petroleum refining,are affected by the processing of inferior crude oil,and corrosion failure cases occur frequently in the unit.The corrosion failure type in the atmospheric tower is complex,the corrosion mechanism is not clear,and there is a lack of scientific quantitative characterization parameters,so it is impossible to form an effective corrosion prediction method.Therefore,the risk prediction and corrosion mechanism of low-temperature dew point corrosion at the top of an atmospheric tower are studied in this paper.It provides theoretical guidance for the prediction of dew point corrosion and the study of low-temperature dew point corrosion mechanisms at the top of an atmospheric tower.In this paper,aiming at the problem of low temperature dew point corrosion at the top of an atmospheric tower,a process model is established based on the BK10 physical property method and the Peng-Robinson cubic equation of state,the distribution characteristics of logistics in the tower are obtained,and the risk area of dew point corrosion is defined.Based on Eulerian Wall Films and Species Transport model,combined with Realizable k-εturbulence model,the fluid simulation calculation at the top of the atmospheric tower is carried out,the influence of hydrodynamic parameters on dew point condensation and corrosion is analyzed,and the prediction method of low temperature dew point corrosion at the top of the atmospheric tower is formed.Based on the simulated dew point corrosion risk area logistics parameters set experimental conditions,an independently designed and built dew point corrosion experimental platform using weight loss test and electrochemical measurement methods,combined with macro and micro corrosion morphology observation results,was studied systematically for the low temperature dew point corrosion mechanism of tower top material（Q345R）.The main conclusions of this paper are as follows:（1）The flow temperature in the atmospheric tower decreases with the increase of the tray;the initial condensation temperature of the water dew point in the tower is99℃,and the condensation point begins to appear at the top of the tower.The concentration of HCl in tower increased with the increase of the tray,and the concentration of HCl in the liquid-phase water reached 2.8×10^-4mol/L at the position of the first tray.The content of corrosive medium in the condensation area of the dew point is higher,that is,there is a high risk of corrosion at the top of the tower.（2）The prediction method of dew point corrosion at the top of the atmospheric tower based on process calculation and fluid simulation has high accuracy and effectively predicts the risk of dew point corrosion on the right wall of the vertical pipe section of the volatilization line and the risk of dew point corrosion on the inner elbow of the volatilization line.The lower wall of the volatilization line changes from dew point corrosion to liquid phase erosion due to liquid phase accumulation,and there is a risk of flow corrosion.The above areas are consistent with the actual failure areas at the top of the atmospheric tower,and the feasibility of the prediction model is verified.（3）At the top of the atmospheric tower,the liquid water content is higher in the regions of low temperature,low velocity,and low turbulent kinetic energy,and the dew point corrosion rate is higher in the regions of high liquid film temperature,high mass transfer rate,and low liquid film velocity.Based on the flow characteristics of the flow field,the characterization parameters of dew point condensation and dew point corrosion are proposed.The condensation risk at the dew point was characterized by temperature,velocity,and turbulent kinetic energy,and the risk of dew point corrosion was characterized by liquid film temperature,mass transfer rate,and liquid film velocity.（4）The primary corrosion type at the top of the atmospheric tower is HCl-H₂O dew point corrosion.Uneven pitting corrosion develops on the surface of the corroded material as a result of Cl^-penetrating the surface and dew point condensation.Temperature and concentration increases will speed up the corrosion response and encourage the transition from pitting corrosion to uniform corrosion.（5）The process of dew point corrosion of Q345R is affected by temperature,concentration,and composition.The increase in temperature increases the percentage of activated molecules and speeds up the corrosion reaction rate.The increase in HCl concentration leads to a decrease in passive film resistance and charge transfer resistance on the surface of the material,which increases the possibility of corrosion.The addition of oil components reduces the coverage area of liquid water on the surface of the specimen,affects the heat dissipation capacity of the material surface,and restrains the occurrence of corrosion.The main contents are as follows:（1）Based on the phase equilibrium calculation and fluid simulation model of the atmospheric tower,a method for predicting the low temperature dew point corrosion at the top of the tower is established,and the hydrodynamic parameters characterizing the condensation characteristics and corrosion risk are proposed.（2）The electrochemical test of dew point corrosion and the simulation of water dew point condensation in a corrosive medium are realized.Combined with weight loss and electrochemical testing methods,the characteristics and rules of dew point corrosion were studied systematically,and the mechanism of HCl-H₂O dew point corrosion was revealed.