The known approaches for the calculation of solid±liquid
extraction are based on the experimental kinetics of the
process. The use of the characteristic function in the general
case of a polydispersed anisotropic solid allows for the
integral description of the diffusional resistance and structural
changes without the knowledge of the respective
kinetic coef®cients. In this way, the required retention time
and size of the extractor can be determined precisely [1,2].
In a second approach, a mathematical model, together with
the experimental information, is used to obtain the values of
the kinetic coef®cients (De and kl), followed by the sizing up
of the extraction unit. In most cases of practical interest,
constant values of the kinetic parameters are assumed [3±5].
When the dependence of the effective diffusion coef®cient
on the concentration is important, the precision of such
calculations is no longer satisfactory. When the De(C)
function is included in the model, a possible relation to
structural changes in the solid should also be considered.
The variation in porosity with time is supported by experimental
observations.
The aim of the present study was to apply a mathematical
model of variable internal diffusivity and porosity to experimental
kinetic data, and to compare the resulting diffusion