Experimental Investigation And Numerical Simulation On Hydrodynamic And Mass Transfer Characteristics In Three-phase Inverse Fluidized Beds

Inverse fluidized beds reactor (IFBR) is a new type of multiphase reactor, whichhas a lot of advantages, such as simple structure, convenient operation andmaintenance, low energy consumption, immediate interphase contact, little solidparticle wearing and good mass and heat transfer performance. It has been widelyapplied to many engineering fields, for example, environmental engineering,petroleum chemical engineering and biochemical engineering. It is essential tounderstand the distribution of flow filed and mass transfer characteristics fully for thedesign and development of reactors for special application, and realizing processoperation optimally. However, because of the intrinsic complexities of multiphase flow,the flow and mass transfer processes between multiphase are very complicated.Conventionally, hydrodynamics and mass transfer properties of fluidized bed reactorwere investigated by experimental methods, but it is usual that just some empiricalcorrelations can be acquired in consideration of the restriction of technologies,precision and number of trials, and it can not offer effectively direct for thedevelopment and design of reactors or process operation. Computational fluiddynamics (CFD) method is an emerging issue on numerical simulation and analysis forhydromechanics. It can describe the local hydrodynamics and mass transfercharacteristics quantitatively in time and space in a reactor, which not only makes upfor the deficiency of the experimental method, but can provide reliable date support forthe design and development of reactors for special application, and the optimization ofprocess operation.Experimental investigation and numerical simulations on hydrodynamics andmass transfer characteristics in a gas-liquid-solid three-phase inverse fluidized bedwere carried out by advanced CFD numerical simulation method and hydrodynamicssimulation software FLIUENT6.3in this research. Results from numerical simulationsand experimental measurements were compared to verify the reliability of the CFD models. Reliable CFD numerical simulation combined with experimental method wereused to analyze the flow and mass transfer characteristics with different operatingconditions and physical conditions. The purpose of this study is to provide detaileddate supports and reliable theoretical directions for the design and development of thiskind of reactor, the optimization of process operation and improving the efficiency ofthe reactor. The main work and results of this dissertation can be arrived at asfollowing:(1) Research results on flow pattern, pressure drop, phase holdup, minimumfluidization velocity, friction factor, the residence time distribution, bubble behavior,mass and heat transfer in a three-phase inverse fluidized bed were summarized. Theimportance and correlations of some parameters and the key research areas in futurewere proposed. From all the summation, lots of research on hydrodynamics and masstransfer characteristics in IFCR were found. However, most of them were finishedthough experimental measurement but CFD simulation method. Furthermore, Most ofthe subjects of the experiment were confined to air-water-solid system, ignoring theviscous medium system encountered usually in practical application. In addition, thedistributions of some local flow characteristics were reported rarely, and investigationson flow characteristics and mass transfer characteristics under different operatingconditions and physical conditions were inadequate. Therefore, those problems are theoriginal reason for the work of this research.(2) Axial and radial distributions of particle velocity, solid holdup and turbulentkinetic energy, flow pattern characteristics and phase holdups with different liquidviscosity, solid holdup and adding surface active agent were studied throughthree-dimensional numerical simulation combined with experimental method. Theresult showed that the reliability of the CFD model could be verified by experimentaldates. Transitions of Initial fluidization, full fluidization and particle accumulationexisted during the development of flow pattern. Axial distribution of particle velocitypresented that the end and the head were small, but near the middle bigger. Axialdistribution of particle velocity and turbulent kinetic energy showed opposite trend.Solid holdup decreased with increasing axial distance. Both of the particle velocity andthe turbulent kinetic energy were higher near the center than that near the wall, butopposite to solid holdup. Radial distribution of turbulent kinetic energy tended to beparabola shape. Solid holdup decreased with increasing superficial gas velocity orliquid viscosity, and increased with an increase in solid loadings. The addition ofsurface active agent to water induced an increase in gas holdup and a decrease in liquidholdup.(3) Bubble characteristics at different axial and radial direction and underdifferent operating conditions and physical conditions were investigated by means of a developed micro-conductivity probe technology. The result showed that bubble sizeand frequency increased and the distribution of the former varied from narrow to widebut opposite to the latter, with increasing the axial distance from the gas distributor aswell as increasing the radial distance from the wall to the center of the bed,respectively. There has been a similarity between the distributions of bubble risingvelocity and bubble size in the bed. The bubble size and rising velocity decreased andthe bubble frequency increased with increasing solids loading, particle density or theaddition of ethanol to water. All three parameters increased with increasing superficialgas velocity. The bubble size and frequency increased and the bubble rising velocitydecreased with increasing superficial liquid velocity. The bubble size and risingvelocity increased and the bubble frequency decreased with higher liquid viscosity.The bubble properties have been well correlated with operating variables and fluidproperties.(4) CFD simulation of three-phase inverse fluidization simplified by liquid-solidpseudo-homogeneous processing was combined with experimental method to studymass transfer characteristics in three-phase inverse fluidized beds. The result showedthat the dissolved oxygen concentration distribution and gas-liquid mass transfercoefficient from CFD simulation were in good agreement with those from experiment,respectively, which verified the reliability of the CFD model. The effect of particleswas little on mass transfer coefficient though the study of the particle presence in bedon mass transfer coefficient, which verified the feasibility and rationality ofliquid-solid pseudo-homogeneous processing. Dissolved oxygen concentrationincreased and tended to be saturated along axial direction from up to down. Anddissolved oxygen concentration was lower near the center region than that near thewall region at the radial direction. The gas-liquid mass transfer coefficient increasedwith increasing gas or liquid superficial velocity, The effect of superficial gas velocityon the gas-liquid mass transfer coefficient was smaller than that of superficial liquidvelocity. The gas-liquid mass transfer coefficient decreased with increasing liquidviscosity or the addition of the surface active agent.