Effects Of Particles And Internals On Hydrodynamics And Reaction Behaviors In Slurry Bubble Column Reactors

Because of efficient transfer performance and wide operating range,slurry bubble column reactors(SBCRs)have been widely applied in process industries containing solid-catalyzed multi-phase reaction,i.e.,Fischer-Tropsch(F-T)systhesis.There are couplings of multi-scale flow,mass transfer and reaction in the SBCRs,and they are significantly influenced by material property and internals in reactors.Building on previous studies,effects of particles and internals on hydrodynamics and reaction behaviors in SBCRs,as well as flow distribution in pipe distributors have been further studied in this dissertation.Multi-stage pipe gas distributors are generally used in industrial-scale SBCRs due to uniform gas distribution and flexible nozzle orientation.The flow velocity through nozzles and pressure profile of single-stage distributors can be predicted by one-dimensional analytical model.However,the predicted results are highly dependent on selected parameter correlations.Therefore,systematical evaluation of the reported parameter correlations in the literature is necessary.And predictions of flow distribution in multi-stage pipe distributors by analytical model have been rarely reported.In recent decades,CFD simulation has been used to investigate the flow in pipe distributors and illustrate the local flow structures,but CFD simulation for industrial-scale multi-stage pipe distributors is still intractable in view of computational cost and modeling complexity.Developing accurate and efficient method is the facing challenge for design and optimization of multi-stage distributors.The prediction of flow distribution in multi-stage distributors by analytical models was realized in this dissertation by choosing correlations with wide applicability and constructing a double-convergence solution strategy.CFD simulation of single-stage distributors was also conducted to analyze reason behind mal-distribution behaviors and search for rational optimization measures.Addition of hydrophilic particles into air-water systems usually decreases gas holdup and increases bubble size.However,understanding about physical mechanisms behind the changed hydrodynamics is still limited.Different mechanisms were coupled into the CFD-PBM model for gas-liquid flows to analyze its influencing degree prograssivley,and then CFD-PBM model for gas-slurry systems was developed by coupling important particle mechanisms.For F-T synthesis,dense heat-exchanging tubes are equipped in SBCRs to remove reaction heat timely,and internals were reported to decrease liquid fluctuation velocity and change gas radial distribution.However,there are fewer investigations about vertical tubes effects on reaction behavior in SBCRs.Complex couplings of multiphase fluid flow,interfacial transport and chemical reactions were simulated with regarding the liquid-solid mixture as a single slurry phase and the interfacial catalytic reaction as homogeneous reaction in slurry phase.Chapter 4 focused on effects of internals on reactor performance.Chapter 5 simulated a pilot-scale F-T synthesis SBCR(with diameter of 0.2 m,height of 23 m),and effects of bubble size distribution(BSD)on predicted results were also studied.The main conclusions are as follows.Through a systematical comparison of the calculated results by analytical model coupled with varied empirical correlations of pressure recovery coefficient and discharge coefficient in the literature,we found that the correlation involving local flow parameters and branch structures was able to predict accurately the flow distribution of different single stage distributors.CFD simulations of single-stage distributors revealed that the gradually decreasing vortex in branch pipes and resulting reducing discharge coefficient along the flow direction was the main reason for mal-distribution behaviors.With increasing ratio of branch length to diameter,the vortex evolved gradually,and the discharge coefficient of upstream branches first increased obviously,then decreased greatly,and finally grew slowly.This indicates that there are effective measures to adjust the discharge coefficient of branch pipes.Improved uniformity of flow distribution in multi-stage distributors by adjusting the branch strutures tendentiously was proved from prediction results by developed analytical model.Moreover,effects of stage number of distributors on flow distribution and pressure drop were also studied.Three particle mechanisms were evaluated in this work,i.e.,increased apparent density and viscosity,reduced film-drainage time during bubble coalescence,and attenuated liquid turbulent dissipation rate.The first mechanism has a marginal effect on simulated results,and the predicted fraction of small bubbles with consideration of the second mechanism was larger than the experimental results.With application of attenuated turbulent dissipation rate in PBM kernels of breakage and coalescence,the simulated BSD in SBCRs agreed well with experimental data in the literature.Treating the suspension of solids in liquid as a single pseudo-homogeneous slurry phase,the CFD-PBM model for gas-slurry flow was developed with consideration of changed slurry properties and attenuated turbulent dissipation rate.The addition of internals enhanced axial liquid velocity,improved large-scale circulation of liquid phase and reduced gas holdup.Under the reaction-limited operating conditions,as the presence of internals increased slurry holdup and improved ratio of liquid reactants,the simulated reaction rate,syngas conversion and reactor productivity grew with increasing number of internals.Model framework with pairwise coupling of flow,mass transfer and bubble size evolution was constructed.The simulated gas holdup grew to the pilot-process level with consideration of BSD.The simulated syngas conversion and outlet concentration distribution of various species were consistent with the pilot process data.But the simulated syngas conversion with and without consideration of bubble size distribution were similar,as the bubble size changed a little in the reaction-limited regime with contrary interphase mass transfer processes.To summarize,the developed analytical model can be used to design and optimize pipe distributors in industrial processes.This work proposed a CFD-PBM model which can simulate accurately the meso-scale bubble behavior in SBCRs,and then the multi-fluid models were thereby extended for description of heterogeneous gas-liquid-solid systems.Consideration of effects of internals and bubble size distribution made the simulations for F-T synthesis in SBCRs closer to the industrial processes.All these results would help us understand the complex multi-phase behavior in SBCRs and provide guidance for the regulation and optimization processes.