Study On The Mechanism Of Heat And Mass Transfer Coupling With Chemical Reaction In Porous Media

Catalytic reaction,such as catalytic combustion,catalytic reforming,catalytic cracking,catalytic hydrogenation,catalytic partial oxidation and so on,is one of the most important reaction types in energy and chemical engineering fields.It has shown wide applications in industrial.The conversion of reactants is accompanied with a complex heat mass transfer process in most of the catalytic reactions.Currently,the catalytic en/exo-thermic reactions are mainly achieved using the fixed bed reactors.This type of reactor is involved in heat and mass transfer in porous media.There exist flow,heat and mass transfer in different scales and chemical reaction.Moreover,these physical processes complexly couple with each other.The flow behavior,the heat and mass transfer properties and their coupling and synergistic characteristic with chemical reaction in porous media are dependent on the morphology structure of porous media,the material and the chemical reaction type.These factors have a decisive effect on the pump work required for the reactor running and the reaction and heat exchange efficiencies of the reactors.It is therefore of great importance to obtain the flow characteristic and the heat and mass transfer properties of different porous media and to determine the best porous structure suitable for specific chemical reaction,in order to achieve the good synergy between heat and mass transfer and chemical reaction and furtherly the optimal conversion and energy utilization efficiencies using the pump work as little as possible.This dissertation makes a systematic study on the mechanism of heat and mass transfer coupling with chemical reaction in porous media,closely surrounding the complex heat and mass transfer processes in the thermochemical utilization of energy.The purpose of this disse-rtation is to detailly describe the law of heat and mass transfer in porous media respectively with micro-nano scale and big pore scale,to analyze the synergy characteristic between heat and mass transfer and chemical reaction,to obtain the characteristics of flow and heat/mass transfer in different porous media,to comprehensively compare the flow and heat/mass transfer performances of different porous media and to clearly point out the application scope of every porous medium,hoping to make a certain contribution to the developments of the heat and mass transfer in porous media and the thermochemical utilization of energy.First of all,a detailed 3D study on flow,heat and mass transfer and their coupling and synergy with chemical reaction in traditional particles-packed beds is carried out based on discrete element method(DEM)-computational fluid dynamics(CFD).Two synergies are emphatically analyzed:one is the synergy analysis between flow fleid among particles' void and temperature fleid;another is the synergy analysis between heat and mass transfer in intra-particles and chemical reaction rate.In this study,reveal the oscillating behaviors of porosity,local particle-to-fluid heat transfer coefficient and local wall-to-fluid heat transfer coefficient along the bed with small tube-to-particle diameter ratio,and explain the reason based on the field synergy principle,simultaneously proposing the packing structures making the wall-to-fluid heat transfer enhanced.In addition,the heat and mass transfer processes coupling chemical reaction in particles-packed beds are numerically simulated by CFD with acetone hydrogenation used for chemical heat pump as an example.The heat and mass transfer resistances at the intra-particles and in the bed are analyzed and the maximum resistances for heat and mass transfer are obtained,providing the guidance for the enhancement of heat and mass transfer in particles-packed bed reactors.The influence of the structural parameters,namely particle size,porosity and pore size,of catalyst particles on acetone conversion rate and isopropanol selectivity are studied,establishing a fundament for the design of catalyst particles used for acetone hydrogenation and building a general method to achieve CFD simulation on gas-solid catalytic reaction.Secondly,the heat transfer characteristics of coated metal foams in metal foam reactors are systematically studied by the numerical method and the experimental method,including the effective thermal conductivity,the gas-solid heat transfer and the radiation heat transfer.In this study,successfully build an unit cell geometrical model of coated metal foams,describe its structure parameters representing its structural properties,simulate the effective thermal conductivity due to pure conduction of coated metal foams with different structural parameters and different material,obtain the influence law of washcoat on thermal conductivity of metal foams,and propose a correlation of effective thermal conductivity for coated metal foams.Then,the flow and gas-solid heat transfer in coated metal foams are simulated by CFD.Obtain the influence law of washcoat on the flow pressure drop and the gas-solid heat transfer and validate the enhancement of the Knudsen flow appearing within the washcoat on gas-solid heat transfer.At last,make an experimental study on the characteristics of radiation heat transfer in metal foams and coated metal foams,obtain the radiation heat transfer characteristic in metal foams and the influence law of washcoat on the radiation heat transfer characteristic of metal foams,and get a conceptual correlation of radiation effective heat transfer coefficient for coated metal foams.Thirdly,the structure properties,the flow characteristic and the heat transfer performance are comprehensively evaluated with the oscillation flow regenerator as an example.In this study,build a general flow and heat transfer model of oscillation flow regenerators suitable for different porous structure such as particles,wire screens and foams,and the regenerative processes of different regenerators are simulated using the finite difference method by C language after introducing the specific characteristic parameters of every porous structure into the general model.Compare the structure properties of different porous structures,obtain the pump work,the heat exchange efficiency and the comprehensive performance of the regenerators with different porous structures,and clearly point out respective advantages and defects of every porous medium in flow and heat transfer.Finally,the one-dimensional two-phase reactor model is established for the traditional packed-bed reactors and the new metal foam reactors.The simulations are achieved using the finite volume method by C language.The reactor model of metal foam reactors takes the washcoat thickness and the effect of washcoat on heat transfer characteristics in reactors into consideration.The particles-packed bed reactors and the metal foam reactors are made a comparison in performance,with acetone hydrogenation,a slow reaction,as an example.The applicability of these two reactors to slow reactions is evaluated in this study.What's more,a further comprehensive study on structure,flow,heat and mass transfer and the synergy of them with chemical reactions are carried out for the particles-packed structure and metal foams.