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.