Study On Anticorrosion And Antiscaling Technology For Plate Heat Exchanger Surface In Oilfield Geothermal Water

In the process of geothermal energy utilization,corrosion and scaling on the surfaces of heat exchangers and pipes in geothermal water are ubiquitous problems.In recent years,in order to make full use of the heat energy in the oil production process,oil field geothermal water generation has been paid attention to.However,in the process of power generation using oilfield geothermal water,scaling and corrosion may be more obvious due to the presence of a small amount of oil in geothermal water,especially on the heat transfer surface of commonly used plate heat exchangers.The corrosion and scaling of the heat exchanger will reduce the heat transfer efficiency,increase the pressure drop of the channel,and even block the fluid channel.Therefore,it is necessary to carry out the research on the technology of corrosion and scale prevention for the utilization of oilfield geothermal water.However,at present,the research in this field is still very scarce,which is the research content of this dissertation.In this dissertation,based on the existing work,taking simulated oilfield water as the experimental fluid,the topics of oil-water separation pretreatment,anticorrosion coating development,lubricant infused anti-soiling surface development,and corrosion and scaling inhibition effect assessment of combined methods including surface hydrophilicity,increasing the velocity of fluid and applying chemical addition etc.in the plate heat exchanger were mainly investigated,in order to provide useful basis for the anti-corrosion and anti-scaling technology of oilfield geothermal power generation.The concrete research contents and innovative results are summarized as follows.Before entering the plate heat exchanger and related equipment and pipelines of power generation,the oil and water separation pretreatment can be applied to the oilfield geothermal water to remove oil phase from the oil and water mixture in order to alleviate the oil deposit on the surface of the plate heat exchanger.The separation pretreatment of removing oil from oil-water mixture is achieved by preparing a stainless steel mesh with bipolar wettability to attempt the gravity-driven separation of various oil-water mixtures.The highly hydrophilic filtration mesh was prepared by liquid phase deposition of Ti O₂ coating,and the hydrophobic mesh was realized via the subsequent modification of fluorosilane on highly hydrophilic mesh deposited by Ti O₂.The microscopic morphologies,surface chemical compositions,contact angles et al.of the modified wire mesh were tested and analyzed.The theoretical calculation and experiments show that only the meshes with relatively small pore size,highly hydrophilic and hydrophobic are applicable for separating oil-water mixtures.Secondly,based on the commonly used carbon steel substrate that is easy to be corroded in the geothermal power generation system,various polysiloxane and Fe₃O₄composite anticorrosive coatings were successfully fabricated on carbon steel substrates（the fourth chapter）.The microstructure,chemical composition,thermal stability,wettability and equivalent thickness of the prepared coating were investigated.The thermal stability was tested by thermal gravity analysis（TGA）.Wettability was investigated by contact angle measurement.Equivalent thickness was measured by weight method.The coating samples were impregnated in 3.5 wt.%Na Cl solution and simulated geothermal water.The corrosion behavior of the composite coating was studied by potentiodynamic polarization curve and electrochemical impedance spectroscopy（EIS）.The results show that the corrosion current density of the coating sample is reduced by 1-2 orders of magnitude and the corrosion resistance is obviously improved.Through the fitting analysis of the equivalent circuit,the corrosion mechanism of the coating sample is discussed.Thirdly,the fluorinated Ti O₂ coating infused by liquid perfluoropolyether（PFPE）was successfully fabricated on the substrate of titanium plate that is a kind of metal material made of plate heat exchanger applied in the geothermal power system by using anodization and chemical grafting（the fifth chapter）.PFPE was infused in fluorinated Ti O₂ via capillary force action of the titanium dioxide nanostructure on the titanium substrate.The anti-oil soiling performance of the coating samples was studied in the crude oil.The coating samples were placed in the 3.5 wt.%Na Cl solution to evaluate their corrosion resistance.The results showed that compared with the untreated titanium samples,the corrosion resistance of the coated samples increased,and the coating samples showed lower contact angle hysteresis and sliding angle for various liquids.The necessity of immobilizing PFPE with surface roughening and fluorination was revealed.Finally,surface hydrophilization via anodization,increasing flow velocity and adding chemical inhibitors and their combination methods were applied to solve the problem of crude oil soiling in the process of geothermal water power generation in the oilfield（the sixth chapter）.Research results show that surface hydrophilicity prolongs the fouling induction period;increasing velocity of hot fluid enhances heat transfer and reduces fouling resistance;under given low flow velocity,adding a certain amount of chemical scale inhibitor can significantly reduce fouling heat resistance.There measures reduce the oil soiling mainly via weakening the adhesion force between crude oil and heat transfer surface as well as intensifying the interaction between geothermal water and crude oil.The mechanism of fouling mitigation with chemical inhibitors was also investigated.