Short notes on DIFFUSIONAL MASS TRANSFER AND CONTINUITYEQUATION

DIFFUSIONAL MASS TRANSFER,
CONTINUITY EQUATION

,OUTLINE
 Definitions
 Concentration
 Average velocity
 Mass transfer flux

 Fick’s law of diffusion
 Diffusion coefficient or diffusivity
 Temperature and pressure dependence on diffusivity

 Analogy between momentum, heat and mass
transfer
 Convective mass transfer

 Continuity equation for mass transfer operations

,DEFINITIONS
Concentration:
As discussed earlier, transfer of mass is caused by
the transfer of molecules of a species from one
location to another from higher to lower
concentration. The concentration of a species in a
mixture is defined in various ways.
Mass concentration: It is defined as the mass of the
species per unit volume of mixture, represented by
ρ i.
Molar concentration: It is defined as the moles of
the species per unit volume of mixture, represented
by ci.
The two concentrations are related as ρi = ci M i

, DEFINITIONS
The total concentration of mixture is defined as the
total mass or moles of the mixture per units of its
volume. Thus
ρ = ∑ ρi c = ∑ ci ρ = cM
i i


Concentration can also be represented in terms of
composition (mass or mole fraction) defined as
Mass fraction: defined as mass of the species per
unit mass of the mixture, represented as ωi.
Mole fraction: defined as moles of the species per
unit mole of the mixture, represented as xi or yi.