As can be seen from Fig. 1, at constant temperature FOEE
solubility increases with pressure, whereas the solvent selectivity
decreases. Each curve stops at a pressure value (P*) that gives a
homogeneous phase, which is reached at a solubility value of about
16.7 g/kg, corresponding to the complete solubilization of the oil
in the solvent for the stated S/F. The separation process must be
carried out at a pressure lower than P*. Reducing the pressure from
P*, the solubility decreases while the selectivity increases; thus,
at each temperature, an optimal operating pressure must exist.
Furthermore, a vast region of pressure and temperature exists,
wherein solubility decreases as temperature increases. In the range of pressure–temperature values under investigation, this effect is
stronger at the lower temperatures, being extremely remarkable
between 42 ◦C and 70 ◦C. The same feature was also observed
for S/F = 100 and S/F = 140 (data not shown). This effect is related
to the variation of the solvent density with temperature, which
importantly affects the solvent power. As an example, around
10 MPa the greatest density reduction of pure CO2 is obtained
increasing temperature in the range (40–50) ◦C, whereas this effect
is slighter at higher temperatures [34].