| According to the production experience of Yangzi Petrochemical olefin plant, the limited stability of dilution steam generator is one of the main blockages for their practical applications since leakage and corrosion of pipelines often occurs due to the high temperature and high pressure environment. In this dissertation, the mechanism of corrosion of dilution steam generator, corrosion monitoring method and anticorrosive process optimization were investigated in detail. The main parts of the results are summarized below:Study on the corrosion mechanism of dilution steam generatorIn order to analyze the corrosion mechanism of dilution steam generator, the corrosive medium and corrosion object of dilution steam generator were characterized by Scanning electron microscope (SEM), Optical microscope (OM), Inductively coupled plasma emission spectrometer (ICP-ES), and X-ray fluorescence spectrometry (XRF). Based on the working principle of dilution steam generator, the corrosion apparatus was designed to test the electrochemical impedance spectroscopy (EIS) under different conditions such as reaction time, temperature, pH, and petroleum concentration. The corrosion model and actual situation corrosion equivalent circuit of dilution steam generator were proposed by EIS. Experimental results indicated that the pH and components of petroleum may play the crucial role in the corrosion of dilution steam generator.The mechanism of corrosion may involve the acid corrosion, alkali corrosion and corrosion produced by the precipitation of organics. In order to investigate the influence of measurement conditions and the parameters of equivalent circuits, the equivalent circuit L (QR) R (QR) was proposed by based on corrosion model and actual situation corrosion. The results revealed that the simulation values resulted from the equivalent circuit were well-fitted to the values of actual measured.Study on corrosion monitoring methodsA three electrodes corrosion measurement system with same material by two frequency method of AC impedance was proposed to test the corrosion rate based on technology characteristics of dilution steam generator. The corrosion measurement system by the same material three electrodes compared with the Ag/AgCl referenceelectrode working in high temperature and high pressure environment has potential applications in corrosion test. Experimental monitoring results demonstrated that percentage of difference between the two corrosion measurement could be less than 10%. In order to realize online monitoring of corrosion rate a lateral line installation with corrosive probes made from 10# carbon steel moreover having identical operating conditions with the dilution steam generator process was designed and manufactured. A on-line corrosion monitoring system for the dilution steam generator composed of corrosion electrode, electrochemical workstation, computer was set up. The electrochemical signal was transmitted to the central control room by shielding wire. The influence of offset resulted from measuring signal due to transmission lines on the corrosion rate measurements could be calibrated by impedance of conductor under different frequency. The results of the site monitoring is in close agreement with results of laboratory test. With the help of online monitoring trends charts, the variation tendency of process water and corrosion could be monitored contributing to regulate timely operating conditions to reduce corrosion.Study on anti-corrosion optimization technologyUtilizing the homemade corrosion apparatus, the corrosion rate was tested under different pH, Cl-, SO42-,carbonate and petroleum by EIS. The corresponding database for monitoring the corrosion was established based on experimental values and monitoring results. Meanwhile, the model of corrosion prediction was established by artificial neural network optimized by genetic algorithm based on experimental values and monitoring results. The influence of the neuron function, transfer function, the number of hidden layer, the training function and genetic algorithm on predicting accuracy was discussed. The results showed that the prediction accuracy of corrosion rate for different operating condition had greatly improved after the neural network optimized by genetic algorithm.Based on the analysis results of prediction model, we got the optimum conditions of anti-corrosion optimization: Cl-<10mg/L, pH,6～9, petroleum,0.8～1.4g/L, SO42-,10～30 mg/L, HCO3-,200～300 mg/L.In this paper the online corrosion monitoring method and optimization technology help to keep the dilution steam generating system well working in safe, stable, long-term, which can effectively extend the service life of dilution steam generator. It would be of great significance for energy saving and raising efficiency of ethylene production... |
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