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### Diffusion mass transfer in a stationary medium

The governing equations for diffusion mass transfer are [29]

| (113)

and

| (114)

where

| (115)

and

| (116)

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:

| (117)

and

| (118)

where

| (119)

and

| (120)

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

| (121)

or

| (122)

If and are constant, these equations reduce to:

| (123)

or

| (124)

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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