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### Diffusion mass transfer in a stationary medium

The governing equations for diffusion mass transfer are [29]

| (80)

and

| (81)

where

| (82)

and

| (83)

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:

| (84)

and

| (85)

where

| (86)

and

| (87)

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

| (88)

or

| (89)

If and are constant, these equations reduce to:

| (90)

or

| (91)

Accordingly, by comparison with the heat equation, the correspondence in Table (15) arises.

Table 15: Correspondence between the heat equation and mass diffusion equation.
heat | mass diffusion |
T |  |

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