increased. For example, as pressure increased from 20 MPa to
40 MPa, the oil yield increased from 2.99% to 3.39% at 50 C and
from 2.51% to 4.66% at 70 C. Such increments in yields are due
to the interactions between the pressure and temperature. However,
there was a negative quadratic effect at high pressures as
shown in Table 2. This is probably a reflection of the increased
repulsive solute–solvent interactions resulted from the highly
compressed CO2 at high pressures [18]. Furthermore, although at
a constant temperature, the increase in pressure would cause an
increase in fluid density, the fluid diffusion coefficient would
decrease. These counteracting effects result in higher pressures
having little effect on the oil yield [19,20]. Thus, a high pressure
is not always recommended for SFE.