Study On Enhancement Of Gas-Liquid Mass Transfer By Fine Particles In Slurry System

The intensification of chemical engineering process is one of the importantdevelopments that go beyond traditional chemical engineering. Slurry system of fineparticles suspending in liquid can increase chemical reaction rate and/or mass transferrate remarkably. This paper deals with the gas-liquid mass transfer enhancement offine particles with adsorptive and reactive activities. In slurry bubble column, both the concentration distribution of particles near thegas-liquid interface and the adhesion efficiency of particles towards the bubblesurface have important influences in determining the enhancement of gas-liquid masstransfer. Quantitative prediction of distribution function and adhesion efficiency ofparticles around a rising bubble in slurry systems is presented in this work. By solvingthe convection-diffusion equation (Fokker-Planck equation), the influence ofBrownian diffusivity of fine particles on concentration distribution and adhesionefficiency are demonstrated. It is found that two kinds of mechanism dominate theadhesion process of particles towards bubble surface according to different Pécletnumber or size of particles and bubble, as well as other properties of slurry system. Inaddition, the viscosity ratio of bubble to the suspending fluid was found to haveobvious influence on particle adhesion. By analysis the enhancement mechanism of adsorptive particles in slurry system, aheterogeneous, instationary 3-D model, which can consider the interaction of particlesnear interface and the complex distribution in this zone, was developed to discribe theeffect of adsorptive particles in slurry bubble column and three-phase agitated cell. Itwas found that the numerical results of 3-D model have distinct difference ascompared with that of 1-D and 2-D model in solid volumetric hold up above 1-2%,and 3-D model should therefore be used to investigate the enhancement effect ofslurry system with solid hold-up above 2%. The complex geometry of heterogeneous media near interface is difficult todescribed with global grid, thus the composite overlapping meshes scheme isdeveloped, which simplified the setup and solution of 3-D partial differential equation.According this method, the composite grid, consisting of simpler component grids,suitably chosen to describe a particular subdomain and overlapping where they meet,is chosen to cover the diffusion field on which the PDF can be solved using a finite iiiABSTRACTdifference scheme, and different component grids and coordinate systems are chosenfor different subdomain. The algorithm and step for constructing a composite grid aredemonstrated. With 3-D model, the influence of several system parameters was studied. It wasfound that only particles located closely to the gas-liquid interface determine masstransfer, the solid-liquid distribution coefficient, the distance of the fist layer ofparticles turned out to be most important parameters, and the enhancement effect ofparticles almost lineally increased with the decrease of particle size. In addition, theinfluence of apparent viscosity on enhancement factor was considered, and the correctfactor is μ0 /μsus . The interaction of different particles in liquid film on enhancement effect isquantified by interaction factor and it was found that the particle relative position haslittle influence on interaction factor. The 2-D model, after taking the binaryparticle-particle average interaction into account, was found to describe reasonablywell the local enhancement factors, thus the 3-D model can be instead of byinteraction factor corrected 2-D model, which shortened the computation time greatlyand improved the flexibility in dealing with the more complex particle geometry neargas-liquid interface. An experimental study on the chemical absorption of SO2 into reactive slurry ofsparingly soluble Mg(OH)2 particles in stirred vessel has been performed. Theenhancement factor of...