Study On Separation Of Microplastics In Water By Cyclonic Air Flotation

Microplastics have attracted great attention as an emerging pollutant,which is very easy to pollute water bodies due to its small size effect,large specific surface area and easy adsorption of other pollutants.In recent years,studies on the removal of microplastics from water have gradually increased.However,there are few reports on the separation of microplastics from water by cyclonic air flotation.In this study,microplastics in water were separated in a hydrocyclone enhanced with air flotation.The influences of operation parameters on the separation efficiency in the cyclonic air flotation process were investigated through a combination of theoretical analysis,experiments,and numerical simulations.The spatial distribution of microplastics was studied and analyzed.The main work and conclusions obtained are as follows:(1)The multiphase flow model,turbulence model,and the population balance model involved in this study were presented.The dynamics mechanism for the separation of the bubble-microplastic by the cyclonic air flotation was stated.(2)The cylinder diameter,vortex finder diameter,underflow diameter,and cone angle of the two cone sections of the hydrocyclone were optimized by orthogonal experiments and numerical simulations with the goal of high separation efficiency and a low split ratio.Experiments on the separation of microplastics were conducted on the hydrocyclone with the selected of structural parameters to study the effects of feed rate and split ratio on the separation efficiency without microbubble injection and the effects of gas holdup and split ratio on the separation efficiency when microbubbles were injected.The experimental results showed that the separation efficiency of microplastics with the increase of feed rate without microbubbles injection.The separation efficiency increased first and then decreased with the increase of split ratio,and the optimal split ratio is about 0.1.The injection of microbubbles could significantly improve the separation efficiency of microplastics,and the separation efficiency increased and then slightly decreased with the increase of gas holdup,and the optimal gas holdup was 5.4%.The separation efficiency was 97%at split ratio of 0.1 with a gas holdup of 5.4%and a feed rate of 0.45 m~3/h,which was 19%higher than the separation efficiency without microbubble injection.(3)Numerical simulations of solid-liquid two phase flow were conducted to study the cyclonic separation process of microplastics,and the characteristics of the velocity field in the liquid phase and the effects of feed microplastic volume fraction,feed velocity,and microplastic density on microplastic distribution were summarized.The research demonstrated that the separation effect of microplastics gradually decreased as the volume fraction of feed microplastics increased,and the volume fraction of microplastics in the radial distribution exhibited a tendency of decrease with the increase of radial distance.The high feed rate and low particle concentration both contribute to the enrichment of microplastics in the cyclone axis,and the area near the axis of the large cone section and the axis of the upper end of the small cone section are microplastic enrichment areas.(4)Numerical simulation of gas-liquid-solid flow was applied to research the cyclonic air flotation separation process of microplastics.The velocity field characteristics of the microplastic phase and the effects of the volume fraction and gas holdup of the feed microplastic on the radial distribution of microplastics were investigated.It is shown that the injected microbubbles coalescence and become large bubbles near the axis of the hydrocyclone,which behaves as an air column with a large air volume fraction.As the gas holdup rises,the extension length and diameter of the air column gradually increases,and the microplastic enrichment area on the outside of the air column migrates towards the vortex finder thus increases the probability of microplastics being captured by the vortex finder.As the volume fraction of feed microplastics increases,the movement of microplastic enriched at the vortex finder is restrained.The rate at which microbubbles at the axis of the cyclone coalesce into large bubbles is higher than the rate at which large bubbles break up to form small bubbles.The aim of this study is microplastic-separation and wastewater purification,intending to separate microplastics in water in an environmentally friendly and energy-efficient way,and to provide a theoretical basis and practical reference value for the application of microplastic separation technology in water.