Study On Gas-liquid Two-phase Hydraulic Characteristics Of Surface-aerated Air Flotation Cell And Treatment Of Dust-containing Wastewater

Surface-aerated air flotation is an important separation unit operation in the field of environmental engineering to remove dust-containing and oil-bearing wastewater.In this paper,the hydrodynamics characteristics of gas-liquid two-phase flow in a flotation cell equipped with a new type of surface self-aspirating agitator（Long-Short Blades,LSB）and a draft tube were studied by combining experimental research and numerical simulation,and its application in the treatment of dust-containing wastewater was also studied.In this paper,the characteristics of bubble distribution in the new type of surface-aerated air flotation cell were studied by using high-speed photography（HUT）.At the same time,the superficial gas velocity in the flotation region was measured by using a gas mass flowmeter.On this basis,the distribution of bubble surface area flux was obtained.In addition,the flotation performances of LSB agitator were evaluated by comparing the gas-suction characteristics with those of HRT.The results show that the bubble diameter increases with the increase of rotational speed of the LSB agitator or the power input（P/V）,and reaches a limit size d₃₂=2.68 mm,without adding foaming agent.While the relationship between superficial gas velocity and power input（P/V）shows a linear increase,and the smaller the distance between the draft tube and the bottom of the cell(L_dc),the greater the linear increase rate.Meanwhile,at the same speed or P/V,LSB agitator produces smaller bubbles than HRT.Compared with the superficial gas velocity,it is found that when the cut-off point of power input is exceeded（N=390 rpm,P/V=357 W/m³）,the LSB agitator produces higher superficial gas velocity,whereas the HRT shows better gas-suction performance.Secondly,after the accuracy of simulation results was verified by Particle Image Velocimetry（PIV）technique,Large Eddy Simulation（LES）method was used.The velocity field,distribution of the vorticity and distribution of the turbulent kinetic energy in the flotation cell were studied.The influence of the draft tube structure on the flow characteristics was analyzed.According to the bubble distribution characteristics in the flotation cell,the influence mechanism of the draft tube structure on the superficial gas velocity was also explored.The results show that the draft tube can effectively prevent the fluid from diffusing from the inside to the outside,and the flotation cell is divided into surface-aerated region and flotation region.In the surface-aerated region,the fluid in the upper half moves downward,and the fluid in the lower half moves upward,which enhance mixing efficiency.In the flotation region,the distance between the draft tube and the bottom of the cell,L_dc,is an important factor affecting the number and distribution of turbulent vortices.When the bottom gap is large(L_dc/H=5/9-7/9),two vortices are formed in the flotation region.With the further decrease of L_dc(L_dc/H=3/9),only one large elliptical vortex exists in this region.According to the distribution characteristics of bubbles in the flotation region obtained from the experiment,it can be seen that the larger the volume of the vortex at the lower position is,the more bubbles are gathered,which leads to the decrease of the overall bubble rising probability and the decrease of the superficial gas velocity.However,when there is a vortex(L_dc/H=3/9),the direction of rotation of the vortex changes clockwise,which is conducive to the upward movement of bubbles in the flotation region,and the superficial gas velocity is the maximum at this time.Finally,the application of the new type of surface-aerated air flotation in the treatment process of wastewater containing kaolin particles was discussed through experimental research,and the flotation efficiency of the cell was compared with that of standard Rushton turbine（RT）.The results show that,in the rapid mixing stage of flotation reagent and dust-containing wastewater,the speed of agitator is 300 rpm and the mixing time is 90 s,and in the flocculation stage,the speed of agitator is 90 rpm and the flocculation time is 20 min,which make that the surface-aerated air flotation cell equipped with LSB agitator has the best removal ability for kaolin particles.At the same time,under the same conditions,the flotation performance of LSB agitator is better than that of RT.