Microsoft PowerPoint - Introduction to Diffusion Mass Transfer2

Microsoft PowerPoint - Introduction to Diffusion Mass Transfer2

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INTRODUCTION TO DIFFUSION MASS TRANSFER prepared by: S.M. rafigh 2011
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General Considerations General Considerations Mass transfer refers to mass in transit due to a species concentration gradient in a mixture . h Must have a mixture of two or more species for mass transfer to occur. h The species concentration gradient is the driving potential for transfer. h Mass transfer by diffusion is analogous to heat transfer by conduction. • Physical Origins of Diffusion: h Transfer is due to random molecular motion. h Consider two species A and B at the same T and p , but initially separated by a partition. Diffusion in the direction of decreasing concentration dictates net transport of A molecules to the right and B molecules to the left. In time, uniform concentrations of A and B are achieved.
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Definitions Definitions : i C Molar concentration of species i. ( 29 3 kmol/m : i ρ Mass density (kg/m 3 ) of species i. : i M Molecular weight (kg/kmol) of species i. i i i C = M * : i J Molar flux of species i due to diffusion . ( 29 2 kmol/s m h Transport of i relative to molar average velocity (v*) of mixture. : i N ′′ Absolute molar flux of species i. ( 29 2 kmol/s m h Transport of i relative to a fixed reference frame. : i j Mass flux of species i due to diffusion . ( 29 2 kg/s m h Transport of i relative to mass-average velocity (v) of mixture. h Transport of i relative to a fixed reference frame. : i x Mole fraction of species i ( 29 / . i i x C C = : i m Mass fraction of species i ( 29 / . i i m = Absolute mass flux of species i. ( 29 2 kg/s m : i n ′ ′
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Property Relations Property Relations Mixture Concentration : i i C C = 1 i i x = Mixture Density : i i ρ = 1 i i m = Mixture of Ideal Gases : i i i p C T = i i i p RT = i i p p = i i i C p x C p = =
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Diffusion Fluxes Molar and Mass Fluxes of Species A due to Diffusion in a Binary Mixture of Species A and B • Molar Flux of Species A:
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Microsoft PowerPoint - Introduction to Diffusion Mass Transfer2

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