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### Diffusion mass transfer in a stationary medium

The governing equations for diffusion mass transfer are [28]

| (72)

and

| (73)

where

| (74)

and

| (75)

In these equations is the mass flux of species A, is the mass diffusivity, is the mass fraction of species A and is the density of species A. Furthermore, is the rate of increase of the mass of species A per unit volume of the mixture. Another way of formulating this is:

| (76)

and

| (77)

where

| (78)

and

| (79)

Here, is the molar flux of species A, is the mass diffusivity, is the mole fraction of species A and is the molar concentration of species A. Furthermore, is the rate of increase of the molar concentration of species A.

The resulting equation now reads

| (80)

or

| (81)

If and are constant, these equations reduce to:

| (82)

or

| (83)

Accordingly, by comparison with the heat equation, the correspondence in Table (9) arises.

Table 9: Correspondence between the heat equation and mass diffusion equation.
heat | mass diffusion |
T |  |

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