反应工程基础chpt13-internaldiffu
60页1、Summary of Chapter 121.The molar flux of A in a binary mixture of A and B is a)For EMCD or for dilute concentration of the solute,a)For diffusion through a stagnant gas,a)For negligible diffusion,1精选课件Summary(contd)2.The rate of mass transfer from the bulk fluid to a boundary at concentration CAs is3.The Sherwood and Schmidt numbers are,respectively,4.If a heat transfer correlation exists for a given system and geometry,the mass transfer correlation may be found by replacing the Nusselt number b
2、y the Sherwood number and the Prandtl number by the Schmidt number in the existing heat transfer correlation.2精选课件Summary(contd)5.Increasing the gas-phase velocity and decreasing the particle size will increase the overall rate of reaction for reactions that are externally mass transfer-limited.6.The conversion for externally mass transfer-limited reactions can be found from the equation7.The shrinking core model states that the time to regenerate a coked catalyst panicle is3精选课件Chapter 13Diffus
3、ion and ReactionsDepartment of Chemical EngineeringTiefeng Wangwangtftsinghua.edu 4精选课件ObjectiveslDescribe diffusion and reactions in porous catalyst and in Tissue Engineering.lDefine the Thiele modules and the effectiveness factor.lDescribe the regions of reaction limitations and internal diffusion limitations and the conditions that affect them.5精选课件6精选课件12.1 Diffusion and Reaction in Spherical Catalyst Pellets12.1.1 Effective DiffusivitylThe pores in the pellet are not straight and cylindrica
4、l;rather,they are a series of tortuous,interconnecting paths of pore bodies and pore throats with varying cross-sectional areas.lIt is fruitful to describe diffusion within each and every one of the tortuous pathways individually.lDefine an effective diffusion coefficient so as to describe the average diffusion taking place at any position r in the pellet.lThe radial flux WAr will be based on the total area(voids and soIid)normal to diffusion transport(i.e.,4r2)rather than void area alone.lThis
5、basis for WAr is made possible by proper definition of the effective diffusivity De.7精选课件Effective DiffusivityThe effective diffusivity accounts for the fact that:lNot all of the area normal to the direction of the flux is available(i.e.,the area occupied by solids)for the molecules to diffuse(p).lThe paths are tortuous().lThe pores are of varying cross-sectional areas(c).8精选课件Effective Diffusivitytortuosity9精选课件Diffusion Mechanism in Pore Channels10精选课件Molecular diffusionSimple diffusion in the
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