3.2.1. Effect of BO Deacidification on the Quality of Raffinates
To evaluate the effect of BO
3 deacidification by LLE using aqueous methanol as extraction solvent on the quality of raffinate streams (deacidified BO
3), the effect of water content in methanol, carboxylic acid content in feed (BO
3) and mainly, temperature on the specific gravity at 20 °C, kinematic viscosity at 40 °C, saponification value, ester value, refractive index and corrosiveness to copper was investigated. The results of this sequence of experiments are presented in
Figure 12,
Figure 13,
Figure 14,
Figure 15 and
Figure 16. Results for corrosiveness to copper using different water contents in methanol at 25 °C and 35 °C are available in
Table S3 of Supplementary Materials.
According to
Figure 12,
Figure 13,
Figure 14,
Figure 15 and
Figure 16, a behavior similar to that observed for
Experiment Group I was observed, in which there is a significant effect of BO
3 deacidification by LLE using aqueous methanol on the quality of the deacidified bio-oil. Therefore,
Figure 12,
Figure 13,
Figure 14,
Figure 15 and
Figure 16 show that the water content present in the methanol, the content of carboxylic acids present in the original bio-oil and the operating temperature of the LLE significantly modify most of the physical-chemical properties, indicating that the LLE process changed the chemical composition of the bio-oils. As reported in
Section 3.1.1, this result was expected, as the main objective of this study is to remove carboxylic acids (FFA). According to Santos et al. [
54] and Buzetzki et al. [
55], the physical-chemical properties of bio-oils are profoundly dependent on their composition, justifying the results obtained in the present study.
Another point that should be highlighted is that the results shown in
Figure 12,
Figure 13,
Figure 14,
Figure 15 and
Figure 16 show that there is a non-linear trend, either of decrease or increase, of the physical-chemical properties of the deacidified bio-oils as there is an increase in the water content in the methanol, for both operating temperatures. This result is due to the effect of deacidification, in particular the solubility of the solvent used, on the levels of hydrocarbons (reduction or increase of light or heavy fractions) and oxygenated compounds, which can be confirmed by GC-MS analysis (
Section 2.3.2).
Figure 12 also shows a significant increase in the specific gravity values of the raffinate streams when LLE is performed at 25 °C and with methanol containing 10, 15 and 20% water. This behavior is repeated for kinematic viscosity and refractive index. Therefore, there was a similarity regarding the behavior of the results obtained for the properties of specific gravity, kinematic viscosity, and refractive index, as illustrated in
Figure 12,
Figure 13 and
Figure 16, indicating that there is a direct correlation between these properties, as reported by Santos et al. [
54].
Figure 14 shows that an increase in the extraction temperature causes a reduction in the values of the BO saponification value. This result is consistent with the literature because, according to Haas [
56] and Gunstone [
57], the lower the saponification value, the longer the carboxylic acid chain. In this study, it was found that the carboxylic acids (palmitic acid and oleic acid) that have a relatively long chain are the acids that are extracted from BO
3 (see Figure 25), mainly at 35 °C, resulting in lower levels of this chemical group when compared to those obtained at 25 °C. Therefore, this fact results in a reduction in the saponification values when there is an increase in the extraction temperature.
Figure 15 shows that adding water to methanol and the extraction temperature change the ester values. In addition, it was found that the ester value when 5% of water was added to methanol was higher than that obtained for the original bio-oil. This is due to the conversion of a portion of carboxylic acids into methyl esters, mainly when 5% water is used in the solvent; the highest deacidification efficiency is obtained under the conditions used in the present study. Then, it was found that there is a tendency of the ester values to reduce from 10% of water and 15% of water to the extraction temperatures of 25 °C and 35 °C, respectively, due to the reduction of the solubility of the binary solvent before the esters as there is an increase of water to the solvent, as shown in Figure 25.
3.2.3. Chemical Composition
In the
Supplementary Materials, Figures S4–S7 present the chromatograms and
Tables S5 to S25 present the retention times, relative contents and identification (Molecular Formula and Compound name) of the prominent peaks obtained by GC-MS analysis of BO
3, which was taken as feed, and from the streams of raffinate and extract obtained after deacidification by liquid-liquid extraction of BO
3 at 25 °C and 35 °C, using methanol with different water contents as solvent.
The results in
Figures S4–S7 and Tables S5 to S25 of the Supplementary Materials show that 30 to 70 components with a high similarity index were detected in BO3 and the extract and raffinate streams. These components were classified into two major groups: hydrocarbons (normal paraffinic, branched paraffinic, olefinic, naphthenic and aromatic) and oxygenated compounds (carboxylic acids, alcohols, aldehydes, ketones, esters, and others). Therefore, the results presented in
Figure 19,
Figure 20,
Figure 21,
Figure 22,
Figure 23,
Figure 24,
Figure 25 and
Figure 26 represent the sum of the areas of GC-MS peaks of the total number of compounds of various chemical classes detected in the respective BO
3 and extract and raffinate streams from the LLE. The values are presented as percentages and show the relative content of aqueous methanol with these classes.
Based on the analysis of
Figure 19, it was found that BO
3 consists of 50.55% hydrocarbons and 49.45% oxygenated compounds. In the hydrocarbon group, it was possible to identify three distinct classes: normal paraffinic (19.15%), olefinic (25.58%) and naphthenic (5.82%), as shown in
Table S5 and shown in
Figure 21. The oxygenated compounds consisted of carboxylic acids (42.35%), alcohols (2.21%), ketones (3.73%) and esters (1.16%). The analysis of the chromatograms referring to the five raffinate streams obtained after the LLE of BO
3 showed the presence of the same substances, although with different contents concerning hydrocarbons and oxygenated compounds. In such a way that the levels of oxygenated compounds of all raffinate streams, obtained from the LLE process at 25 °C and 35 °C, are lower than the content of oxygenated compounds of BO
3, indicating that the aqueous methanol can remove the oxygenated compounds present in the original bio-oil, reflecting in the reduction of TAN and, consequently, in the efficiency of deacidification, as noted in
Section 3.2.2.1.
Therefore, it was confirmed that deacidification by liquid-liquid extraction with aqueous methanol, in addition to extracting oxygenated compounds, especially carboxylic acids, also promotes an increase in the concentration of hydrocarbons, mainly when extraction occurs at 35 °C, as shown in
Figure 19. Thus, some water contents had a relatively small loss of hydrocarbons (basically normal paraffin) to the extract streams. In others, the absence of hydrocarbons was verified, indicating no losses to the extract stream, as illustrated in
Figure 20 and
Figure 22. The absence or low content of hydrocarbons in the extract indicates its slight solubility in aqueous methanol, probably due to hydrophobic interactions, especially at low temperatures. Kanaujia et al. [
34] describe similar behavior when reporting a low concentration of hydrocarbons in the aqueous phase compared to the content of hydrocarbons found in the organic phase (bio-oil).
Figure 19 also shows that the water content hurt the deacidification process by LLE with aqueous methanol because while the hydrocarbon content decreases as there is an increase in the water content, the content of oxygenated compounds increases, confirming the findings made about the efficiency of deacidification from the results of TAN values presented in the previous sections. According to Kanaujia et al. [
34], most of the solvent-analyte interactions in LLE are based mainly on polar-polar and hydrophobic interactions, which justifies the results obtained in the present study as there is an increase in water content.
In this context, the results presented in
Figure 19,
Figure 20,
Figure 21 and
Figure 22 demonstrate that an increase in the temperature of the extraction process allows a more significant removal of oxygenated compounds when compared to the results obtained for the same water content. This makes it clear that while the temperature favors reducing the oxygenated compound content, water content is a detrimental factor.
To evaluate the effect of water content on the distribution of oxygen classes, we plotted the graph presented in
Figure 23, which shows that as the water content increases, there is a tendency to increase the concentration of carboxylic acids both at 25 °C and 35 °C. Therefore, although the contents of oxygenated compounds as a whole and, specifically, of carboxylic acids are lower than those of BO
3 for all water contents and for both extraction temperatures, it was observed that an increase in the water content causes a reduction in the ability of the solvent to extract carboxylic acids. In addition, it was possible to observe that the concentration of oxygenated compounds such as alcohols and ketones changes very little with the increase of water content, indicating that the binary solvent presents more excellent selectivity for the compounds of interest in the present work, which are carboxylic acids. Oliveira et al. [
60] investigated the liquid-liquid equilibrium of systems composed of rice bran oil, free fatty acids, ethanol, and water at temperatures ranging from 10 to 60 °C. The results of the study conducted by Oliveira et al. [
60] indicated that the mutual solubility of compounds, including carboxylic acids (FFA), decreased with an increase in the water content of the solvent and a decrease in the extraction temperature. Therefore, the results obtained in the present study are consistent with those reported in the literature.
Figure 23 also shows that the ester content in the raffinate streams is higher than that found in BO
3 for all water contents and both extraction temperatures, indicating that part of the carboxylic acids present in the original bio-oil was esterified when subjected to deacidification by LLE. This fact, in principle, is not a problem since it can be seen from
Figure 24 that esters are the class of oxygenated compounds that are in the highest concentration in the extract streams, followed by carboxylic acids, indicating that they are extracted by aqueous methanol. However, not in its entirety. This suggests that more than one extraction step is required to remove oxygenated compounds, especially carboxylic acids and esters formed during deacidification.
Carboxylic acids and esters are the two main chemical groups extracted from BO
3 by aqueous methanol. Palmitic acid, oleic acid and decanoic acid were detected in higher relative content, totaling 8-24% and 5-13% when extraction is performed at 25 °C and 35 °C, respectively, as shown in
Figure 25. In addition, the analysis of
Figure 25 also indicates that carboxylic acids such as hexadecanoic acid (palmitic acid), oleic acid, and decanoic acid are the ones that are in higher concentration in BO
3 and that, after the LLE process, these acids are the ones that are extracted due to a significant reduction in their contents, mainly when methanol is used with 5% water, promoting the reduction of TAN values for the raffinate streams, as observed in
Section 3.2.2.1.
Figure 25 also shows that the water content and extraction temperature have a significant effect on the contents of hexadecanoic acid (palmitic acid), oleic acid, and decanoic acid in such a way that there was little change in the levels of the other FFAs present in the original bio-oil, remaining practically constant.
It was expected that hexadecanoic acid, oleic acid, and decanoic acid would present relatively high levels in the extract streams compared to the other FFAs. However, this result was obtained only for decanoic acid, as illustrated in
Figure 26. The explanation for this result is that most of the hexadecanoic acid and oleic acid are esterified during the LLE process, being removed in the form of esters such as hexadecanoic acid, methyl ester and 9-octadecenoic acid (Z)-, methyl ester, respectively. This result can be seen in
Tables S11–S15 and S21–S25 of the Supplementary Materials, confirming what was previously reported regarding converting a large part of the carboxylic acids into fatty acid methyl esters during the deacidification process by LLE. According to Lee et al. [
64], adding methanol increases the selectivity of esters because the acidic compounds in the bio-oil, such as carboxylic acids, can engage in an esterification reaction with methanol. Therefore, the results obtained are consistent with the literature.
Bio-oil produced from the thermochemical route of triglyceride-based biomass is increasingly recognized as a potentially abundant source of renewable fuels and chemicals. Carboxylic acids, mainly free fatty acids, are significant constituent groups in bio-oil and end products or intermediate substances. Therefore, upgrading bio-oil through deacidification by liquid-liquid extraction is a relatively new proposition that can be employed to provide renewable chemistries.