Lecture 6 External Diffusion PDF

Catalytic Reaction Engineering Yongda ongdan n Li  Professor of Industrial Chem Chemistry istry  Department of Chemical Chemical and Metallurg Metallurgical ical Engineering  School of Chemical Technology Technology  Aalto University  Email: [email protected]  [email protected]   Kemistintie 1, E404

 Nov-Dec, 2017 

6. External diffusion

2

Part 6-1 Diffusion and Molar flux

Steps of heterogeneous reaction Reactant: external (film) diffusion  Active site

• internal In princi principle ple,, diffusion all all ste steps ps (pore) can ation ffecon t thactiv e reaecsite tion adsorption adsorp active rate chemic mical reacti rea •che The sal low owe ection st on step (RDS RDS, Product: rate ra te de dete term rmin inin ing g step step)) dete de term rmin ines es the the re reac acti tion on desorption

rate ra te al almo most st al alon onee

intern int ernal al dif diffuu fuusio sion n external extern al dif diffus fusion ion

3

Part 6-1 Diffusion and Molar flux

What is diffusion? diffusion? •

Driving force?

4

Part 6-1 Diffusion and Molar flux

Definition of diffusion and mass transfer  

Diffusion: Spontaneous mixing of atoms or molecules due to thermal motion Driving force of diffusion is the concentration gradient (movement from higher concentration to lower one) The gradient causes a molar flux W A in the direction of concentration gradient



Transfer of a mass species through the Mass transfer: Transfer  bulk of a fluid or a solid matrix matrix Diffusion is a kind of mass transfer  5