Simulation And Experimental Research On Flow Field Of Dissolved Air Flotation Deoiling Process Of Extracted Liquid

The technology of dissolved air flotation is commonly used to deep degrease from the extraction solution.A small amount of oil in the solution can be removed using the dissolved air flotation.It is still difficult to deep remove the sub-micron oil droplets.The key factors to remove oil are the collision and adhesion of microbubbles or oil droplets through the flow field disturbance and the diffusion distribution of the gas phase in the contact area.In this paper,numerical simulation technologies were used to design the hydraulic background of the two-phase flow dissolved air flotation tank.Based on the theory of air flotation separation,this is combined with the vertical flow cylindrical air flotation tank of a large state-owned enterprise.Using the Reynolds average method,the three parts(the flow field characteristics of different turbulence models in the contact area,the influence of different geometric model structure sizes and working parameters on the flow field characteristics)are discussed in detail.Then the optimization study of the whole flow field is carried out and discussed in detail from the three parts of the treatment capacity,the reflux amount and the gas-liquid ratio.The optimization of the ideal gas phase distribution is also studied to improve the clarification efficiency of the air flotation process.The results are verified by experimental research.The contents are listed as follows:Simulation of flow field characteristics and gas phase behavior in the contact zone:the two-phaseκ-ωturbulence model shows strong disturbance of the flow field in the contact area and stable shear rate change among the different turbulence models.Based on this model,the wall velocity of the flow field tends to decrease and the gas phase distribution spreads towards the center of the contact zone.The gas inlet volume fraction and mass transfer rate of theκ-ωmodel have the highest agreement with the predicted gas phase velocity.Therefore,theκ-ωturbulence model is more suitable for the simulation of the two-dimensional vertical flow circular air flotation tank.In the comparative analysis of the single-phase flow and the two-phase flow,it is found that the introduction of the gas phase increases the disturbance of the flow field in the contact area.The gas phase distribution of the two-phase flow changes instantaneously.The results of working condition parameters show that the increase of liquid inlet and air blowing volume will produce unfavorable factors of increasing wall velocity,even if the gas phase distribution is concentrated on the wall surface.According to the analysis of model parameters,the ideal conditions for gas-phase diffusion and distribution are listed as follows:the liquid-phase inlet position is higher than 0.56 h,the gas-phase inlet is 0.13h,the overflow baffle is higher than 0.58 h,and the contact area is larger than 4.5 m~2.Gas distribution and optimization simulation in full flow field:When the throughput is 50 m~3/h,the increasement of the reflux ratio to 30%will not improve the flow field characteristics.However,the decreasement of the gas-liquid ratio can change the flow field disturbance in the contact area.So the diffusion distribution of the gas phase spreads to the center of the contact area.With a constant reflux ratio,the same results can be obtained as the former when the hydraulic retention time is less than 0.4 h.It shows that the decrease of residence time and the increase of treatment capacity have obvious improvement effect on the flow field.The optimization results show that the left wall of the trough has a uniform gas phase distribution and the disturbance in the contact area is weak while the results of the left wall of the deflector are opposite.The flow and gas distribution could not change by increasing the reflux ratio to 80%and reducing the gas-liquid ratio to 3%.When the residence time is 0.3 h and the reflux ratio is 25%,the gas-liquid distribution in the contact zone becomes uniform as the gas-liquid ratio decreases but the flow field disturbance in the center of the contact zone is weakened.Therefore,the residence time in the 2D geometric simulation is a key factor to determine the ideal distribution of the gas phase but the flow field disturbance in the contact area is small.According to the above numerical simulation analysis and research,a vertical flow circular air flotation tank with a residence time of 0.084 h was designed.Its gas holdup and bubble diameter are in line with the expected results.It is verified that the experimental results are basically consistent with the two-dimensional geometric simulation of the gas distribution behavior.But the gas-phase diffusion simulation ignores radial diffusion.Therefore,the diffusion behavior in the upper part of the contact zone cannot be fully explained.There is a gap with the actual effect in three dimensions.The two-dimensional simulation analysis of different reflux ratios is similar to the simulation results of residence time 0.3 h and gas-liquid ratio 3%.But the gas phase distribution is more ideal.That is to say that the bottom of the contact zone also has a uniform gas phase distribution.This thesis provides a reliable gas phase control mechanism for thehydrometallurgical post-treatment that the pressured-dissolved air flotation process for deep oil removal.