Simulation Of Mass Transfer And Hydrodynamic Characteristics In Internal Loop Fluidized Bed Reactor By Computational Fluid Dynamics Method

Internal loop fluidized bed reactor (ILFBR) is a new type of multiphase reactor, whichhas a serial of advantages, such as simple structure, good mixing and mass transfer effects,easy operation and maintenance and low energy requirement per unit of volumn. It has beenwidely used in many engineering fields, for example, petroleum chemical industry, biologicalfermentation and wastewater treatment. To understand the flow and mass transfercharacteristics fully is the premise for design and development of reactors for specialapplication, design of novel internal and optimizing design aiming at the improvement ofreactor efficiency. However, because of the intrinsic complexities of multiphase flow, theflow and mass transfer processes, which is participated by two phases of gas and liquid, threephases of gas, liquid and solid, even more phases, is very complicated. Conventionally, flowand mass transfer characters were studied by experimental methods, but it is usual that onlyexperiential relationships can be acquired because of the confinements of technologies,accuracy and times of experiments. So it can not effectively direct the design of reactors. Onthe contrary, computational fluid dynamics (CFD) simulation method can overcome the abovedefects. The detailed local flow and mass transfer characters and practical reactorperformance prediction can be acquired by simulation by computers. Simulations onconventional ILFBR, ILFBR enhanced by funnel internal for flow guidance and ILFBRmodified by introducing cross internal in clearance at bottom were carried out by advancedCFD numerical simulation method in this research. Results from numerical simulations andexperimental measurements were compared to verify the reliability of the CFD models.Verified CFD models were used to analyze more and detailed flow and mass transfercharacters of reactors. The purpose of this study is to provide supports from a large quantityof data and theoretical directions for design of novel internals for reactor, improving theefficiency of the reactor and orientational optimization design for special destination. Thefollowing results can be arrived at after the research of this dissertation.(1) Conventional ILFBR can be modeled by two dimensional axial symmetry grid,Eulerian-Eulerian two fluids model for gas and liquid and time-dependent simulation. Thesimulation process has the numerical stability and the simulation results are grid-independent when grid size is small than2mm. All the numerical simulations can reproduce theexperimental results well.(2) The drag force formula from Tomiyama and Clift can well illustrate the drag forcebetween bubbles and liquid for gas-liquid two phases flow in ILFBR. The lift forcecalculation mostly influences the radial profile of gas holdup and it does not effect much onaxial average gas holdup. Dispersed turbulence model is well in numerical accuracy and itscomputational expense is low. The setting of bubble diameter will significantly influence theprediction results of the model.(3) Coventional ILFBR can be enhanced by introducing funnel internal into reactor.Enhanced reactor has some new characters, for example, the total gas holdup will increase,liquid phase velocity will decrease and local turbulence energy will decrease. All this featureswill make the enhanced reactor to be more preferable to be used in oxygen demanded andshear stress sensitive biological processes.(4) After local flow characteristic parameters are acquired by CFD simulation,volumetric mass transfer coefficient can be estimated in further and mass transferperformance of the reactor can be predicted. It can be demonstrated by simulation that themass transfer performance of the reactor can be increased by the introduction of the funnelinternal.(5) Conventional ILFBR can be enhanced by the method of introducing cross shapeinternal or any other multi-radial internal in the bottom clearance of the reactor. Theintroduction of the internal can cause the obvious raise in liquid velocities of the riser and thedowncomer, the slight reduction for the total gas holdup. Internal can impose a rectified floweffect for mixture which goes into the riser from the bottom clearance, and it can decrease theturbulence energy of the fluid in the riser as well.(6) The CFD model which is developed in this study can be used in numerical simulationfor two phases flow process of gas and liquid in ILFBR reactor. It can replace the experimentsin majority cases and acquire reliable parameters for global and local flow and mass transferperformance. Especially it is helpful to use in the circumstances which design parametersneed to be modified frequently, such as the optimization of the internals and the scaling up ofthe reactors.