Diffusion Flashcards
(16 cards)
Recipe card
Diffusion for stagnant B
A + B binary mixture
- start with eq for Ficks + absolute
- N of B = 0
- assume N of A = constant (steady state)
- solve for Na as a constant for profile
Recipe card
Equimolar counter diffusion
A+B binary mixture
- start with Fick for absolute
- Na = -Nb
- solve and integrate
[should give linear profile]
How to make use of partial pressure boundary condition
- conc. = n / v
- Ct = P/RT
Recipe card
How to solve a bulk diffusion + bulk diffusion
Oxygen diffuse into layer
For reaction:
1. Mass balance
2. assume steady state
For bulk conc. + dilute:
1. Xa = 0
2. simplify Fick’s law
3. sub into balance
should obtain a SOLDES - test function as a exp(mx)
What happens when diffusivity is high in a material
reaction vs diffusion
- diffusion is no longer a limiting factor
- rate constant is only limiting factor
Recipe card
steady state diffusion through a cylindrical film
binary (B = stagnant )
- use cylinderical laplacian operator
- steady state
(mass in = mass out) - Fick’s law
Ficks law
(diffusive, 1D)
Fick’s law
(Diffusive, radial)
Fick’s law
(absolute)
Ficks law
(absolute + dilute)
mass balance operator
for 3D cartesian
change of flux in each direction + generation
mass balance for radial 2D
-1/r and r
+ generation term
mass balance
for spherical
-1/r2 and r2
+ generation term
total diffusive flux of system
0
Expression of diffusive flux
J = Ci (Vi-Va)
where
Vi = velocity of i
Va = avg. velocity
What is kept constant in steady state
radial/spherical diffusion
Rate of moles entering and leaving the surface
NOT flux - moles movement per surface decreases with area increase
