for the first and second F-HPE steps for the same solvent flow rate
considering the process error for total yield (Table 3). Therefore, the
solvent mixture CO2:EtOH (90:10) used in the NF-HPE experiments
possibly promoted the solubilization of most compounds that were
also soluble in pure scCO2 (and that were extracted during the first
F-HPE step), due to the low proportion of EtOH applied.
Considering the solvent composition effect on MCER and YCER
obtained for NF-HPE using 7.6
×
10−5 kg/s of solvent flow, the
increment in the EtOH percentage from 10 to 70% had a positive
influence on these kinetic parameters. YCER varied in the range
4.1
×
10−4 to 22.1
±
5.2
×
10−4. The higher solubility achieved with
the higher EtOH amount in the solvent mixture should be related
with the existence of more compounds soluble in the employed
solvent mixture which were readily available for extraction (and
where phenolic compounds may be included). RCER values and
global yields followed the same behavior, while tCER values followed
approximately the opposite behavior. Therefore, the choice
of CO2:EtOH (30:70) as solvent mixture seems to be the most appropriate
one to obtain the highest extract amount in the shortest time
from pine bark at 303 K and
∼25.1 MPa at the employed flow rate.
Extract yield retained in the recovering flask, trap and lines
cleaning process represented in average 88.3, 2.0 and 9.7% of the
total obtained extract, in that order.
Comparing the F-HPE first step (scCO2) yield with that of HD
(0.010
±
0.005% yield), it can be concluded that supercritical fluid
extraction was an efficient process for pine bark low polarity compounds
extraction. Soxhlet extract yield was 6.85
±
0.4%, close to
the one achieved with NF-HPE using 70% of EtOH (6.51
±
1.20%).
The work developed in this paper was based on a previous
work performed by the authors [23]. In that work pine bark was
subjected to fractionated supercritical fluid extractions where pressure
and temperature were varied over the ranges 303–323 K and
11–30 MPa, using consecutive extraction steps with scCO2 and with
CO2:EtOH (90:10), at a low mass flow rate (∼7
×
10−5 kg/s). In that
study the total achieved yield in the first step, at 323 K and 20 MPa
(the same conditions of the first extraction step presented in this
paper) was 12.2%, which was considerable higher than the one
obtained in this work (which was 1.37%). Pine barks used in the
two studies came from distinct geographical origins with diverse
edaphoclimatic conditions (though both from Beira Litoral, in the
center of Portugal). Such high dissimilarities in extraction yields
were certainly related to the dissimilarities in their original bark
compositions which were in turn related to the tree age, radial location
of the collected bark (i.e., phloem, newly formed outer bark or