Numerical Analysis Of Dense Liquid-solid Two-phase Turbulent Mixing And Its Application

Liquid-solid mixing is the key operation in a variety of industries, such as petroleum, chemical industry, biochemical, metallurgical, pharmaceutical, food processing, wastewater treatment, marine engineering. In engineering practice, the liquid-solid system usually contains a large number of particles, and it is a dense liquid-solid two-phase flow.Although the study on liquid-solid suspension mixing began in the sixties of last century, for a long time, at home and abroad the financial and human resources invested for basic research is not enough, the progress is slow. Especially in the limits of the experimental measurement and numerical calculation techniques, most studies have focused on thin liquid-solid two-phase system,'deep and systematic understanding of dense liquid-solid mixing mechanism is lack, which results in operating cost increasing, product quality not meeting the requirements, improper selection of mixing equipments, as well as over-mixing. Therefore, it is an important research direction of hybrid technology to accurately understand and clearly describe the mixing mechanism of dense liquid-solid two-phase flow, achieve numerical analysis to the two-phase flow in a variety of mixing equipment, and provide the necessary theoretical guidance for the design, optimization and industrial scale of mixing equipments.Liquid-solid circulating fluidized bed, as a new type of two-phase mixed reaction equipment, has begun to attract more and more widespread concerns because it has the advantages of good mixing effect, effective mass and heat exchange, and convenient operation etc.. In liquid-solid circulating fluidized bed used by practical industry, particle volume concentration is generally higher than 15%, in-depth understanding of the flow characteristics of the particles is very necessary for the design, analysis and improvement of the reactor. Research is currently conducted mainly through experimental measurements. However, it is difficult to obtain comprehensive information on the macro-and micro-flow field only by experiments, and the particle concentration in the system is limited by existing measurement techniques. Therefore, it is very important to establish computing fluid dynamics (CFD) model, and based on it numerically analyzed the dense liquid-solid two-phase mixing process in circulating fluidized bedStirring, as the hybrid technology commonly used has been concerned by many researchers, but so far theoretical and experimental research aimed at the critical state of particles just completely suspending in the mixing tank. Despite the critical suspension can meet the requirement of many industrial processes, under this state distribution of solid concentration is uneven in stirred tank, especially with the higher ratio of the height and width. For the industries processing in which equal reaction rate, uniform properties of the product or a mixture (such as the slurry for oil well cementing operations) are concerned, higher suspension quality, even uniform suspension is demanded. However, the research on high-quality suspension mixing is very rarely carried.This work was undertaken around numerical analysis approach for the dense liquid-solid two-phase turbulent mixing and its specific application in circulating fluidized bed and the mixing tank.First, based on theoretical research results of thin liquid-solid flow, according to the characteristics of dense liquid-solid system-particle collisions and two-way coupling, in the Euler coordinate two-fluid model was derived and established in order to describe dense liquid-solid two-phase turbulent flow. In solid-phase momentum equation, the viscous drag, which expressions takes into account the impacts of particle concentration and inertia, represents the role of fluid drag on the particles; solid-phase pressure and shear stress, which expressions is established according to granular kinetic theory, represent the effect of particle collisions; turbulence dispersion force represents the transport role of fluid turbulence on particle. Three facets fully show particles dispersion mechanism in the dense liquid-solid system. In addition, mutual force in the liquid-phase momentum equation and the source term in turbulence kc-εc equations separately reflect the effect of particles on the liquid-phase average motion and turbulent fluctuation.Then, on the basis of the fluid dynamics model, two-dimensional numerical calculation for two-phase turbulence in the dense liquid-solid circulating fluidized bed was completed by using Fluent software. Predicted results of particle velocity, concentration distribution and turbulence characteristics are in good agreement with the literature experimental data, which verifies the validity of the theoretical model, also shows that:Syamlal and O'Brien drag coefficient model suites to be applied into the dense liquid-solid system; the values of collision restitution coefficient are close to 1 due to liquid lubricating; even in dense liquid-solid two-phase flow, the role of liquid turbulent diffusion can not be ignored; and with the riser size increasing, the diffusion becomes increasingly clear, so turbulent dispersion force should be added into the theory model; with the riser size increasing, the uneven distribution of flow field will escalate and mixing effect will deteriorate, which need to be suppressed in industrial process.To study stirring process with uniform suspension for the head, this paper first analyzed particles suspension mechanism in different regions of stirring tank:In mainstream area, particle suspension mainly depends on liquid drag and particle turbulence fluctuation driven by large-scale liquid turbulence eddies; near tank bottom and wall area, particle turbulence motion driven by small scale turbulence eddies make particle suspend up; in tank bottom and wall film, particle suspension depends on non-turbulence particles fluctuation caused by colliding with other particles or tank wall.In this work, the concept of suspension uniformity was introduced, and the evaluation system with three quantitative indicators (suspension uniformity, mixed time and stirring power) was first established in order to assess performance of mixing systems.Based on the above-mentioned study, three-dimensional numerical calculation for dense liquid-solid two-phase turbulent flow in several mixing systems were carried, and mixing effects prediction, stirring performance evaluation and impact analysis of operating conditions were done. The results show that:impeller type have a decisive influence on mixing effects; according to three evaluation indexes, surface pitched blade turbine is optimal choice; when single pitched blade turbine pumps up, two flow vortex will arise from bottom to top in one side of the axis; when the blade pumps down only one vortex appears, but pumping direction has little effect on mixing effect, and it is a little better when pumping down; the fitting relationship between suspension uniformity and stirred speed is approximatelyσ(?)N-1.12; with the speed increasing, suspension uniformity will decease, but there is limits for a specific system; then even if speed is higher, suspension effect become no more better, but energy consumption will significantly increase in accordance with the relationship P (?) N3; in dense system, the distribution of particles concentration along the radial position is uneven, and there is a gradient, which is different from experimental results; particles loaded holdup has little effect on the radial distribution of solid-phase concentration, but, with particles loaded holdup increasing, axial concentration has more deviations from the average one, which makes suspension effect bad.This work has laid a foundation for theoretical research of dense liquid-solid two-phase turbulent flow, provided application examples for CFD analysis of two-phase flow in the circulating fluidized bed and stirred tank, and provided theoretical guidance for design and optimization of mixing devices.