| Filtration is the last step of the oilfield sewage treatment process.Therefore,the treatment effect of filter tanks can directly determine the injection water quality.Polymer flooding technology has been applied on a large scale to guarantee the national petroleum strategic security.However,there are three problems with the produced sewage: containing high concentration of polymer,high water viscosity and difficulty in separation of oil and water.The above situations have caused a series of production problems such as substandard water quality after filter tank filtration,unstable operation,filter material compaction,pressure out in backwash,etc.Therefore,it is a major issue for oilfield water treatment to solve the problems of filtration and backwashing in the treatment of polymer-contained sewage.At present,the research of this problem mainly focuses on strengthening the effect of backwashing,which is mostly based on the subjective experience,and lacks the systematic research on the factors influencing the effectiveness of backwashing.There is insufficient research on the causes of how the polymer-contained sewage can cause poor quality of filtered water during filtration.For these reasons above,this paper studies the influencing factors of polymer-contained sewage filtration-backwashing process from the aspects of theoretical analysis,laboratory simulation experiments,numerical simulation and field test.Meanwhile,the paper achieves optimization of process operation parameters.The filtration experiments by indoor simulation and the study of microscopic morphology of the polymer and filter material show that the filter material have a strong adsorption effect on the polymer.Polymers with oil and suspended solid form competitive adsorption on the surface of the filter media.After filtering,the surface of the upper filter material is significantly more polyacrylamide and remains in the retained oil.The polymer contained in the sewage occupies the surface of the filter material,so that the adsorption of crude oil and suspended solid by the filter material is reduced;on the other hand,the viscosity of the water is increased,the emulsification of the oil is promoted,and the difficulty of separating the oil from the water is increased.Moreover,polymer-contained sewage has a "displacement" effect on oil pollution,which affects its adhesion on the surface of the filter material.In addition,the adsorption and carrying effect of biological suspension on crude oil is also the main factors affecting the oil content exceeding the standard.A porous media model of filtration process is established,and the effects of filtration rate,filter particle size,water viscosity,filter layer thickness,and filter gradation on the filtration effect were simulated and analyzed.The results show that maintaining the filtration rate at a lower level,decreasing the particle size of the filter material,decreasing the water viscosity,and appropriately increasing the thickness of the filter layer were conducive to the improvement of the filtering effect and ensuring the compliance of the water quality.The existing single-layer quartz sand filter and double-layer filter media can meet the water quality requirements under the condition that the filtration water was up to the standard.The filter height of the existing quartz sand/magnetite double layer filter was optimized,and the optimized heights were 200 mm and 600 mm.A two-fluid model of thickset liquid-solid turbulent flow is determined under the effect of mixing mechanism for fulfilling the bidirectional coupled calculation between water phase and filter-material particles.Collision and friction of the particles and the effect of turbulent flow for the particle motion were both considered in the model.Concentration distributing of the filter-material particles in the filter tank was calculated during the backwashing process.The effect of backwashing was evaluated quantitatively by introducing the suspension uniformity and filter expansion rate.From the microscopic point,the flow energy and effect for the filter material were both described during the backwashing process by turbulence kinetic energy and dissipation rate.Collision and friction of the particles with the effect of flow and axial flow impellers were both described by the granular temperature in the backwashing process.The result indicated that with the backwashing strength increasing,the filter expansion rate and turbulence kinetic energy were both increased,the shearing and washing action of flow were both strengthened,the suspension uniformity was improved and the secondary sludge that the substance is existed in the filter-material porosity was better to remove.But the greater backwashing strength would reduce the collision and friction of filter material,and the primary sludge that the substance is existed in the filter-material surface was hard to remove.So the strength of backwashing should be 8~13 L/(m2·s).In addition,the effects of paddle height,paddle length,paddle velocity,filtering material original height,filtering material particle size,filtering material density,double-layer filter media form on the backwashing effect were also obtained.In the field test,the micro-flocculation filtration treatment process improved the filtering effect of the filter tanks,and the optimal flocculant was PAC,with the concentration of 3 mg/L.Installation program of the backwashing paddle and process parameter optimization were both presented for the polymer-contained sewage treatment station and the depth sewage treatment station.For the high polymer-contained sewage treatment station,the optimization program of gas-water backwashing was presented,the backwashing effect was great and the water quality reached the standard after the filtering.The study result has important scientific significance and engineering application value for improving the polymer-water quality and assuring the stability operation of polymer-contained sewage filtration. |
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