Multiscale Modeling Of Liquid-solid Systems And Slurry Bubble Column

Liquid-solid and gas-liquid-solid systems are ubiquitous in nature and industrial processes.Generally,the flow structure in liquid-solid systems is well recognized as particulate fluidization,characterized by homogeneous bed expansion and phase distribution.However,the structure becomes heterogeneous in practice,e.g.,the slurry loop reactors of swollen particles.Swollen particles may aggregate due to adhesive forces,which then causes operation instability and even reactor blockage.More complex mesoscale structure exists in gas-liquid-solid systems.The rise of bubble swarms is accompanied by bubble breakage,coalescence,deformation and bubble wakes.The effects of particles on fluid dynamics,bubble swarms,gas-liquid interaction and bubble breakage and coalescence are important to understanding and modeling,but remain a challenge.The Energy-Minimization Multiscale(EMMS)models have been developed for the study of heterogeneous structures of gas-solid and gas-liquid systems in recent two decades.The hallmark of this approach is to analyze the different dominating mechanisms in non-linear non-equilibrium multiphase systems with the so-called stability conditions.Currently,the effect of swollen particles have not been considered in modeling liquid-solid systems and the solid effect on bubble behaviors have not been incorporated into models of gas-liquid-solid systems.Multiscale simulations or EMMS models provide potential methods to account for above effects.This work then focused on applying the multiscale method in liquid-solid and gas-liquid-solid systems.Chapter 2 firstly evaluates the traditional two-fluid model in description of homogeneous liquid-solid flows.The hydrodynamics in a novel double-partition and double impeller stirred tank for crystallization process are investigated.Simulations show that differences of the agitation speeds between two impellers are relevant to the total power consumption and solid distribution.On the other hand,the physical time for crystallization process costs several hours,which consumes huge computational cost.A simplified model strategy is thereby proposed in this section to improve the efficiency for engineering application.Chapter 3 develops a swelling-dependent two-fluid model(STFM)for the liquid-solid flows of swelling particles in polyethylene reactors.The microscale mass transfer and mesoscale particle aggregation process are considered into the multi-fluid model through the species transport equation and the population balance equation.Simulations show that only the TFM fails to capture the main features of swelling systems.By contrast,the STFM captures the gradual increase of power consumption due to particle swelling and aggregation,which agrees with the experiments in astirred tank.The STFM predicts also the slug formation and a sharp increase of power consumption in a slurry loop reactor as well as the solid accumulation behind pump.The difference of model prediction for stirred tanks and loop reactors suggests the potential of reactor optimization by enhancing local mixing while still keeping high solid concentration for productivity.Chapters 4 deals with low-solid-concentration slurry bubble columns.we demonstrate that the Dual-Bubble-Size(DBS)drag model based on the Energy-Minimization Multiscale(EMMS)concept,is capable of effectively predicting the distribution of gas holdup in slurry bubble columns.For solid-free or low solid loading systems,a thorough comparison of the DBS-Global and other drag models indicates that,without any fitting parameters,the DBS-Global drag model prediction on the radial distribution of gas holdup and solid hydrodynamics is in conformity with experiments over a wide range of superficial gas velocities.Chapter 5 further explores the multiscale models for high-solid-concentration slurry bubble column,where the solid effect must be considered and gas-liquid models thus cannot be directly used.We propose a particle-dependent new model in this part.The model well captures the main feature in high solid concentration systems and predicts well of gas distribution when coupled with the three-fluid model.To summarize,this work proposes new models that account for mesoscale structures in heterogeneous liquid-solid and gas-liquid-solid systems.The new model captures typical flow structures in above systems.On the other hand,the new models propose modifications in previous multi-fluid models and the multi-fluid models are thereby extended for description of heterogeneous liquid-solid and gas-liquid-solid systems.