[T6022]

Hi everyone,

I have some question for you. I'm currently studying the application of Shrinking Core Model to the combustion of char with oxigen (air). I have some doubts about External Diffusion of oxigen through gaseous film, which surrounding a spherical particle of pure and porous carbon (particle of radius R).

Mass balance (steady state. Limitating step: External Diffusion. Stokes flow): 

- δnox/δt = k_m(C_ox,g-0)4πR²

C_ox,g is the concentration of oxigen in the gas phase (constant). δnox/δt is the number of moles per unit of time consumed by combustion reaction. k_m is the (average) mass transfer coefficient.

Questions:

1) Is it correct to say that the transport in the film (stagnant film?) is due to molecular and convective phenomena?
2) Why the considered area is 4πR²? Doesn't the area change from 4π(R+Δ)² to 4πR² during the diffusion (where Δ is the thickness of the layer)?
3) What assumption was made for this mass balance to be correct?
4) Can you suggest me a good book or good scientific paper to study this kind of topic?

Sorry for my poor English. I'll appreciate all your help.