Simulation And Structural Optimization Of Vertical Settling Tank For Oilfield Produced Water

In the process of oilfield water treatment,settling tanks are essential treatment equipment because of their low cost,high efficiency and oil recovery characteristics.The oil-water separation effect of the settling tank determines the stable operation of the subsequent process,which affects the safe production and ensures the normal work of the plant’s piping system and equipment.Currently,there are still some problems in the use of settling tanks,such as low oil removal efficiency,the possibility of secondary pollution,the bottom is easy to sludge a large amount of sludge and the recovery efficiency of floating oil is not high.Through the simulation study,the characteristics of the flow field inside the settling tank were analyzed and its internal structure was optimized,so as to improve the oil-water separation efficiency and crude oil recovery rate.In this study,the geometric model of the vertical settling tank in oilfield produced water treatment was constructed as the research object.The velocity and concentration fields inside the settling tank are analyzed by Fluent software simulation of the oilfield produced water treatment process.The separation efficiency of sedimentation tank under different hydraulic residence time,oil droplet size and oil phase viscosity is studied,and the internal structure of sedimentation tank is optimized based on this.By changing the layout,length and aperture of the inclined pipe in the settling tank,the optimum conditions for the structure optimization of the settling tank are obtained.The specific research contents are as follows:(1)Analyze the internal structure of the vertical settling tank,and the main factors affecting the oil-water separation of the settling tank,and draw a three-dimensional geometric model using Revit.According to the symmetry of the internal structure of the settling tank,the geometric model is reasonably simplified and cut,and a 1/2 space geometric model is constructed;the area is divided into four parts,mainly including water distribution system,water collection system,oil collection tank and tank body.The Poly-Hexcore meshing method is adopted to encrypt the mesh in its small size and dense structure to get a good quality body mesh model.(2)Selection of turbulence model and multiphase flow model and determination of initial conditions.The oil in the oil-water mixture inside the settling tank is the dispersed phase,and the mixture model in the multiphase flow model and the RNG k-ε model in the turbulent flow model are selected for the calculation of its internal flow field in this study.The discretization method used is the finite volume method,based on which the spatial discretization and diffusion terms are in second-order windward format,the convection terms are in Quick format,the time discretization is in first-order windward format,and the calculation algorithm is Simple algorithm for numerical simulation.(3)The internal oil phase distribution is studied in depth by numerical simulation of different hydraulic residence time,oil droplet particle size and crude oil viscosity.The oil-water separation efficiency in the settling tank increases significantly with the increase of hydraulic residence time,and after reaching 4h,the effect of the increase of hydraulic residence time on the oil-water separation efficiency is not obvious;the oil droplet particle size in the tank increases,the upper water content gradually decreases,and the oil-water separation efficiency increases with the increase of oil droplet particle size;the oil-water separation efficiency in the settling tank gradually increases with the decrease of crude oil viscosity.(4)By optimizing the structure of the original settling tank,the simulation analysis was carried out mainly on the slant pipe arrangement,slant pipe aperture and slant pipe length.The results show that the oil-water separation efficiency of the slant pipe arrangement in the inner tank is significantly higher than that of the outer tank,and the oil-water separation efficiency of the settling tank increases with the increase of the slant pipe aperture and then decreases,and increases with the increase of the slant pipe length.The best oil-water separation efficiency and the best economic efficiency in the settling tank are the conditions of the slant pipe arrangement in the inner tank,the slant pipe aperture 100 mm,the slant pipe length 1200 mm,and the oil content of the outlet is 6.27% at this time.