Mass Transfer Flashcards
What is mass transfer?
Linear relation between the rate of diffusion of chemical species and the concentration gradient of that species
What is a concentration gradient?
The change of concentration over distance.
What is thermal diffusion?
Diffusion due to a temperature gradient. Usually negligible unless the temperature gradient is very large.
What is pressure diffusion?
Diffusion due to a pressure gradient. Usually negligible unless the pressure gradient is very large.
What are the 2 modes of mass transfer?
- Molecular diffusion
- Convective mass transfer
What is molecular diffusion?
Movement of a chemical species from an area of high concentration to an area of low concentration.
What is convective mass transfer?
Mass transfer due to movement of a fluid. Mass transfer that is not due to molecular diffusion. Can only occur in fluids.
What is Fick’s Law of diffusion?
Molecular diffusion. The transfer or movement of individual molecules through a fluid by means of each individual movement.
What is concentration?
The number of molecules of a species present per unit volume. Unit = Molecules/m^3
What is molar concentration?
Number of moles of a species per unit volume. Unit = mol/m^3
What is mass concentration?
Mass of a species per unit volume. Unit = kg/m^3
What is the molecular diffusion equation?
J*(Az) = -D(AB) dc(A)/dz
J*(Az) = the molar flux of species A in the Z direction (kg mol / s m^2). D(AB) = The molecular diffusivity of the molecule A in system B (m^2/s). c(A) = the concentration of A (kg mol/ m^3) z = the distance of diffusion in m
What is the equimolar counter diffusion in gases equation? What is it for an ideal gas?
J*(Az) = D(AB) x (C(A1) - C(A2)) / (z2 - z1)
For an ideal gas:
J*(Az) = D(AB) x (p(A1) - p(A2)) / (RT(z2 - z1))
What are the properties of equimolar counter diffusion in gases (4)?
J*(Az) = J*(Bz) c = c(A) + c(B) dc(A) = -dc(B) D(AB) = D(BA)
How do you work out convective mass transfer?
N(A) = k(c) (c(L1) - c(Li))
k(c) = mass transfer coefficient (m/s) c(L1) = bulk fluid concentration c(Li) = fluid concentration near interface
