Investigation Of Multicomponents Interfacial Behavior In Oil-water Separation By Membrane Technology With Molecular Simulation

For oilfield emulsified oil wastewater,which has a high proportion and is difficult to treat,membrane oil-water separation technology has been successfully applied.However,in the process,oil droplets and surfactant colloids in emulsified oil wastewater will produce adsorption and deposition in the membrane surface and membrane pores,causing membrane contamination phenomenon,which directly affects the application of membrane oil-water separation technology.In addition,inorganic salts and other substances in emulsion components are also key influencing factors.When oil,surfactant and salt are present together,the oil/water interfacial stability and the degree of membrane contamination will be different.Therefore,it is necessary to investigate the intermolecular interactions in different systems and thus guide the industrial application of membrane oil-water separation.Based on the above background,this paper takes membrane separation of actual emulsified oil wastewater from a tertiary oil recovery in China as the research background,and uses a combination of molecular dynamics simulation and experiment to analyze the behavior of surfactants and inorganic salts at the oil-water interface in emulsions.The surfactants were selected from anionic sodium dodecyl sulfate（SDS）and nonionic Tween-80 for compounding,and the inorganic salts were selected from Na Cl,Na HCO₃,Ca Cl₂,Ca（HCO₃）₂.The main research contents and results are as follows:（1）A two-phase interface model of"oil-compounded surfactant/salt/water"was constructed to analyze the effects of changes in surfactant compounding ratio and inorganic salt type on the interfacial properties and molecular behavior at the interface from four aspects:overall interface morphology,molecular morphology of surfactant layer,water adhesion of surfactant and oil-water interface stability.The results showed that the surfactants were different from those containing single-component surfactants.The results showed that the surfactants in the complex surfactant system could exhibit better molecular carbon chain extension,stronger water-oil adsorption ability and interfacial stability compared with the oil-water system containing single-component surfactants;in addition,the addition of salt enhanced the synergistic adsorption ability of salt and surfactant on water molecules and interfacial stability.This confirmed the interfacial synergistic stabilization effect between the compounded surfactants and between the salt-compounded surfactants.In comparison,the best oil/water interfacial stability was found for the SDS/Tween 80 system with a compounding ratio of 12/12 and the presence of Ca（HCO₃）₂.（2）A three-phase interface model of"oil-combination surfactant/salt/water-Ti O₂ ceramic membrane"was constructed to explore the interaction between the components and the Ti O₂membrane surface in different emulsion systems in terms of density distribution curve,interaction between components at the interface,molecular distribution and motion state at the interface.The results showed that the charge nature and molecular structure of the compound surfactants and salt ions could affect the degree of membrane surface contamination to different degrees.Among them,the SDS/Tween 80 system with 12/12 compounding ratio and the presence of Ca Cl₂ and Ca（HCO₃）₂ inorganic salts,the interaction distance between the oil and surfactant molecules and the membrane layer increased and the adhesion strength of the molecules on the membrane surface decreased,which could alleviate the membrane contamination caused by the deposition of oil and surfactant components in the process of membrane separation of oil and water.In summary,the paper explains the effects of complex surfactants and inorganic salts at the molecular level on the stability of oil-water emulsions and the interaction between components and membrane layers from a microscopic perspective,and the results of the study can provide a basis for oil extraction and the determination of oil-water separation schemes by membrane methods.