化工傳遞過程原理課件：第十二Simultaneous-Transfer(I)_276408014

1、Simultaneous Transfer of Mass and HeatTypical temperature and composition profiles in some nonisothermal processesEnergy flux, mass fluxes, and some of the reactions and equilibriums in gasification of coalMass transfer affects heat transfer:1.An additional enthalpy transfer in addition to the heat

2、flux, due to the species fluxes (profound effects )2.Direct contribution to the heat flux induced by species diffusion (Dufour)3.Effects on the propertiesEffects of Temperature Gradient on Mass transfer:1. It is the direct cause of molecular motion (Soret)2. It may cause bulk motion of the fluid3. I

3、t influences phase equilibrium and reaction rate4. It influences the properties Energy and Species Diffusive Fluxes Resulting from Gradients in Temperature and PotentialsGradientEnergy fluxFlux of species iTemperature Potential of species iConduction (Fourier) qcDiffusion-thermo effect (Dufour ) qxT

4、hermal diffusion (Soret) Diffusion (M-S) JdiTiTiJConservation of Chemical SpeciesiiViCRt N()() iBiASiCCRiIiIINvNvndTiiiiiiCCNJvJJvTTTiiiCJvv1nTTiiivvdddiiiCJvv1nddiiivvdTiiivvv+dTvvvThermal diffusion (Soret effect) contribution to the flux of component i relative to the mass average velocity:lnTTiii

5、DTM J , thermal diffusion coefficient, kg m-1s-1. for a binary mixture (A+B)TiDlnTTiiDT jTTBADD 10nTiiDThermal diffusion (Soret) :The mass diffusive fluxes due to temperature gradient.Only important for special cases:1. steep temperature gradient, e.g. chemical vapor deposition processes2. transport

6、 and reaction of gaseous mixtures inside catalyst particles (especially, large difference in the molar masses of the components) with large reaction heat3. isotope separation Mass Transfer Constitutive Relations for Simultaneous Heat and Mass Transfer1111lnTTnnijjijiiijjjtijijijj ij ixxDDx xTCDDNNd1

7、1,2,1,2,TTjiijijijjiijDDijnDijn Defining multicomponent thermal diffusion factor as:,1nijjiT Piiiijtiijj ixxRTVPFZTC xD NN1nijijjj iRxModified thermal diffusion factor11nijjitotalijtijj ixxCDNNd,()totaliiT PiiiixVPFZTRT dtotaltotaliiiiiiiiixxxTTRTRTRT ddConservation of Energy1niiiVC HHtE( )( )11nncx

8、iiiiiiHHEqqNqN11()() nniiBiiASiiC HC HHIIIEvEvn( )( )111()()nnncxiiiiiiiiiC HHC HEevqqJvDiffusion-thermo effect ( Dufour):The contribution of the mass fluxes to the heat diffusive flux relative to the mass average velocity. 1TnxtotaliiiiDMq1TnxtotalitiiiDC RTqd1112nnxijjiijijj iRTxxqNNThe Dufour eff

9、ect is usually not important in chemical engineering.11Tnnjiijxiijiijj ixxDRTDN -NqFor a binary mixture (A+B), the contribution of the mass fluxes to the heat flux relative to the mass average velocity:xABBAABRTxxqNN11TxABAABABABRTDxxDqNNEnergy Transfer Constitutive Relations for Simultaneous Heat a

10、nd Mass Transfer1112nnijjiijijj ik TRTxx qNNThe Dufour effect is usually not important in chemical engineering.1niiiHEqNAnalysis of Simultaneous Heat and Mass Transfer ProblemsiViCRt iN,1nijjiT Piiiijtiijj ixxRTVPFZTC xD NN10niiiN1niiiVC HHt E11()() nniiBiiASiiC HC HHIIIEvEvn11112nnnijjiijiiijij ik

11、TRTxxH ENNN0tv()() iBiASiCCRiIiIINvNvnExample I, Thermal Diffusion in a Binary Gas MixtureA gaseous mixture of A and B confined between parallel surfaces. Constant temperatures are maintained at top and bottom, such as TL T0.lnAABABdxx xdT Example II, Dufour Flux in a Binary Gas MixtureA gaseous mixture of A and B in contact with a catalytic surface where is a fast, irreversible reaction.Heat transfer coefficien