Those phenomena take place when there is a different concentration of a chemical between the cells of the CA (see Fig. 2). The amount of chemical in the phase a, Q_C, that in one step moves due to the hydrodynamic dispersion and/or diffusion is given by:

where D_a is the diffusion coefficient ([l² t^-1]) in the phase a, l_a is the dispersion coefficient in the phase a, n_a is the flow rate of the phase a and C is the concentration of the chemical C in the cell ([m l^-3]).