Transfer Dynamics And Reaction Mechanism Over A Single Micrometer-scale Catalyst Particle For Syngas Methanation

Methanation is an important unit in the coal-to-SNG(synthetic natural gas)process.As a highly exothermic reaction,one of the major challenges for methanation technology is the efficient removal of the excessive reaction heat.Taking the full advantages of fluidized bed,mainly the high heat and mass transfer efficiency,a novel methanation process combined a transport bed reactor with a clean-up fixed bed reactor has been proposed by institute of Process Engineering,Chinese Academy of Science,.This process utilizes the catalyst particles rather than flowing gas as the main heat transfer medium.As methanation occurs over a catalyst particle,the generated reaction heat could transfer both inside of the catalyst itself and also the gas around the catalyst particles.However,there is deficient knowledge about how the balance between these two transfer pathways is achieved,i.e.whether the reaction heat generated will prefer transferring to particle center or directly to the surrounding gas.Aimed to solve this question,the following research contents were covered in this thesis.(1)Dynamic characteristics for methanation over a single millimeter-scale catalyst particle under atmospheric pressure.Both experiments and 2D time dependent numerical simulations have been performed on catalyst particles with diameters ranging from 4-6 mm to investigate the reaction and heat transfer behavior under laboratory conditions.The experimental and calculated results fit well to each other verifying the applicability of the mathematical model.The results clarified that:1)The competition between diffusion and reaction inside the catalyst particle.Under high operation temperature,the reactions mainly take place on the surface of the catalyst particle since the reaction rate is quick enough,on the contrary the reaction region would extend further inside the catalyst particle.2)The bi-directional transfer behavior of the reaction heat.In the unsteady state,the generated reaction heat would transfer both outwards to the atmosphere and inwards to the inside of the catalyst particle,this effect increases the temperature inside the catalyst particle.3)The competition between reaction heat generation and its transport.Both experimental and simulated results show that the temperature inside the catalyst particle decreased gradually from center to surface when the steady state is achieved.In particular,the location of reaction divided the temperature distribution profile into two parts,temperature inner than which remains nearly constant while temperature in the outer part decreases sharply.In steady state,the reaction heat generated in the near-surface region inside the catalyst particle would transfer more into the surrounding gas owing the intensive effect of convection,while that formed in the core region transfer less in this direction,thus leading the temperature distribution that the temperature decreased gradually from center to surface.4)The influence of heat transfer resistance.Through the calculation of Biot number which serves as the criterion of the heat transfer resistances on the surface and at the inside of the body,it is suggested that interphase heat transfer resistance is relatively larger than its intra-phase counterpart so that the reaction heat tends to transfer more to the inside of catalyst itself.5)The effects of operating conditions on heat transfer behaviors.Specifically,the gas-solid heat transfer coefficient would decrease when catalyst particle grows larger,and it would increase as the temperature or gas flowrate increases.(2)The reaction mechanism and dynamic heat transfer over a single micrometer scale catalyst particle for transport bed methanation.The heat and mass transfer behaviors as methanation occurs under the conditions in transport bed was predicted by the numerical simulations verified in the first part.The results revealed that:1)The dynamic period over a single catalyst particle is short,on 100 μm catalyst particle it takes only 0.1s to get through this period.2)The temperature profile still characterizes decreasing gradually from center to surface in spite of the little temperature difference(0.02 K)between particle center and surface,suggesting that methanation reactions occur inside the catalyst particle.3)The reaction distribution inside the catalyst particle is affected by two opposite factors.On one hand,as mentioned previously,the temperature increased from surface to center and it accelerates reaction at the center.On the other hand,however,the diffusion of reactant inside the catalyst particle would be hindered by the internal diffusion resistance thus resulting in lower reactant concentration in core area,which is unfavorable for reaction.Therefore,under conditions in which the reactants diffuse quick enough,methanation inside the particle is controlled by the kinetics,and the reaction rate would decrease from particle center to its surface.On the contrast,the reactions rate distribution would be reverse and the reaction is subject to mass transfer.