The OECs of caffeine static-SFE are depicted in Fig. 1 for the
case of pure CO2 (Fig. 1a), CO2 + EAC (Fig. 1b), CO2 + ETOH (Fig. 1c)
and CO2 + EL (Fig. 1d). The OECs were adjusted using the model of
Sovová [33] which is based on assumption that Xp mass of solute
is easy accessible to the supercritical solvent (due to cell wall disruption)
while the rest (Xk) remains inside cell walls. Thus, three
steps are considered in the SFE process: (i)the constant rate period,
where only the easily accessible solute is removed and thus, is controlled
by convection in the fluid phase; (ii) the falling rate period,
where both convection and internal mass transfer are important;
(iii)the internal diffusion controlled rate period, where the remaining
solute is only inside the cell walls. The corresponding model
equations describing each extraction step are given in theAppendix
A.