Fig. 2 shows that the extraction kinetics was characterized by
three periods. The first period (constant extraction rate) was characterized
by extraction of the solute more accessible to the solvent,
found in the superficial layer of the particles. In the intermediate
period, the amount of solute available on the surface decreased and
the solute inside the particle started diffusing to the surface. Finally,
in the third period, exhaustion of the solute on the surface limited
extraction, due to diffusion of solute from inside the particle, with
a decrease in the extraction rates. Almost 70% of the total oil was
recovered in the constant extraction rate phase (linear), employing
approximately 20% of the CO2, whilst in the subsequent stages, with
decreasing rates, the remaining 30% were recovered using 80% of
the total CO2. In the first period, the surface of the material was completely
covered by oil and there was no resistance to mass transfer,
the solubility of the oil being the limiting factor. In the second and
third periods, in tra-particle diffusion controlled oil transfer from
the interior to the surface of the particles.