Catalyst-free production of fatty acid ethyl esters (FAEE) from macauba pulp oil

1. INTRODUCTION

Macauba (Acrocomia aculeata) is a promising oleaginous plant for the production of biodiesel, mainly due to its high oil content, with 23-46% in the pulp (Evaristo et al., 2016Evaristo AB, Grossi JAS, Pimentel LD, Goulart SDM, Martins AD, Santos VL, Motoike S. 2016. Harvest and post-harvest conditions influencing macauba (Acrocomia aculeata) oil quality attributes. Ind. Crops. Prod. 85, 63-73. https://doi.org/10.1016/j.indcrop.2016.02.052 ; Lescano et al., 2015Lescano CH, Oliveira IP, Silva LR, Baldivia DS, Sanjinez-Argandonã, EJ, Arruda EJ, Lima FF. 2015. Nutrients content, characterization and oil extraction from Acrocomia aculeata (Jacq.) Lodd . fruits. Afr. J. Food Sci. 9, 113-119. https://doi.org/10.5897/AJFS2014.1212 ) and 43-64% in the kernel (Evaristo et al., 2016Evaristo AB, Grossi JAS, Pimentel LD, Goulart SDM, Martins AD, Santos VL, Motoike S. 2016. Harvest and post-harvest conditions influencing macauba (Acrocomia aculeata) oil quality attributes. Ind. Crops. Prod. 85, 63-73. https://doi.org/10.1016/j.indcrop.2016.02.052 ; Nunes et al., 2018Nunes ÂA, Buccini DF, Jaques JAS, Portugal LC, Guimarães RCA, Favaro SP, Carvalho CME. 2018. Effect of Acrocomia aculeata Kernel Oil on Adiposity in Type 2 Diabetic Rats. Plant. Foods Hum. Nutr. 73, 61-67. https://doi.org/10.1007/s11130-017-0648-8 ). Several reports indicate that the oil extracted from the pulp of macauba presents a high content in free fatty acids (FFA), as observed by Dona et al., (2013)Doná G, Cardozo-Filho L, Silva C, Castilhos F. 2013. Biodiesel production using supercritical methyl acetate in a tubular packed bed reactor. Fuel Process. Technol. 106, 605-610. https://doi.org/10.1016/j.fuproc.2012.09.047 and Visioli et al., (2018)Visioli LJ, Trentini CP, Castilhos F, Silva C. 2018. Esters production in continuous reactor from macauba pulp oil using methyl acetate in pressurized conditions. J. Supercrit. Fluids 140, 238-247. https://doi.org/10.1016/j.supflu.2018.06.018 , who reported 79.24 and 65.20% of free fatty acids (FFA) in macauba pulp oil, respectively. The oil is often obtained by pressing and filtration without the need for refining steps (Colonelli et al., 2017Colonelli TADS, Trentini CP, Santos KA, Oliveira JV, Cardozo-Filho L, Silva EA, Silva C. 2017. Assessment of process variables on the use of macauba pulp oil as feedstock for the continuous production of ethyl esters under pressurized conditions. Braz. J. Che. Eng. 34, 831-839. https://doi.org/10.1590/0104-6632.20170343s20150768 ). Thus, the conventional method involving the use of homogeneous alkaline catalysts is not applicable for the transformation of this oil into biodiesel, since the presence of FFA neutralizes the catalyst and also produces soap emulsions, which hinders the separation of glycerol (Patil and Deng, 2009Patil PD, Deng S. 2009. Optimization of biodiesel production from edible and non-edible vegetable oils. Fuel 88, 1302-1306. https://doi.org/10.1016/j.fuel.2009.01.016 ).

Thus, other strategies need to be adopted for the synthesis of biodiesel using low quality raw materials, particularly when high concentrations of free fatty acids are present (Mardhiah et al., 2017Mardhiah HH, Chyuan H, Masjuki HH, Lim S, Pang YL. 2017. Investigation of carbon-based solid acid catalyst from Jatropha curcas biomass in biodiesel production. Energ. Convers. and Manage. 144, 10-17. https://doi.org/10.1016/j.enconman.2017.04.038 ). Reactions using alcohol under pressurized conditions have been widely investigated for biodiesel production (Visioli et al., 2016Visioli LJ, Castilhos F, Cardozo-Filho L, Mello BTF, Silva C. 2016. Production of esters from soybean oil deodorizer distillate in pressurized ethanol. Fuel Process. Technol. 149, 326-331. https://doi.org/10.1016/j.fuproc.2016.04.038 ; Santos et al., 2017Santos PR, Pedersen FA, Ramos LP, Corazza ML. 2017. Esterification of fatty acids with supercritical ethanol in a continuous tubular reactor. J. Supercrit. Fluids 126, 25-36. https://doi.org/10.1016/j.supflu.2017.03.002 ; Trentini et al., 2018Trentini CP, Fonseca JM, Cardozo-Filho L, Reis R, Sampaio SC, Silva C. 2018. Assessment of continuous catalyst-free production of ethyl esters from grease trap waste. J. Supercrit. Fluids 136, 157-163. https://doi.org/10.1016/j.supflu.2018.02.018 ). These are associated with the same energy cost as reactions conducted at low pressures using a basic homogeneous catalyst and generate a product with higher purity (Demirbas, 2002Demirbas A. 2002. Biodiesel from vegetable oils via transesterification in supercritical methanol. Energ. Convers. and Manage. 43, 2349-2356. https://doi.org/10.1016/S0196-8904(01)00170-4 ), eliminating the need for separation from the catalyst. In this method, high yields of esters are obtained without the use of catalysts in a relatively short time (Nan et al., 2015Nan Y, Liu J, Lin R, Tavlarides LL. 2015. Production of biodiesel from microalgae oil (Chlorella protothecoides) by non-catalytic transesterification in supercritical methanol and ethanol: Process optimization. J. Supercrit. Fluids 97, 174-182. https://doi.org/10.1016/j.supflu.2014.08.025 ) since, under conditions of high temperature and pressure, the miscibility of the triglycerides with the alcohol increases due to the decreasing polarity of the alcohol (Farobie and Matsumura, 2017Farobie O, Matsumura Y. 2017. State of the art of biodiesel production under supercritical conditions. Prog. Energy. Combust. Sci. 63, 173-203. https://doi.org/10.1016/j.pecs.2017.08.001 ; Kusdiana and Saka, 2004Kusdiana D, Saka S. 2004. Effects of water on biodiesel fuel production by supercritical methanol treatment. Bioresour. Technol. 91, 289-295. https://doi.org/10.1016/S0960-8524(03)00201-3 ; Liu et al., 2018Liu J, Nan Y, Huang X, Bond JQ, Tavlarides LL. 2018. Applied Catalysis B: Environmental Continuous esteri fi cation of oleic acid to ethyl oleate under sub / supercritical conditions over γ-Al₂O₃. Appl. Catal. B 232, 155-163. https://doi.org/10.1016/j.apcatb.2018.03.050 ; Osmieri et al., 2017Osmieri L, Esfahani RAM, Recasens F. 2017. Continuous biodiesel production in supercritical two-step process: phase equilibrium and process design. J. Supercrit. Fluids 124, 57-71. https://doi.org/10.1016/j.supflu.2017.01.010 ), which also behaves as an acid catalyst in the reaction besides being a solvent (Kusdiana and Saka, 2004Kusdiana D, Saka S. 2004. Effects of water on biodiesel fuel production by supercritical methanol treatment. Bioresour. Technol. 91, 289-295. https://doi.org/10.1016/S0960-8524(03)00201-3 ). Another advantage to this method is the efficiency and good results which have been reported for reactions where the oil had a high content in free fatty acids (Postaue et al., 2019Postaue N, Trentini CP, Mello BTF, Cardozo-Filho L, Silva C. 2019. Continuous catalyst-free interesterification of crambe oil using methyl acetate under pressurized conditions. Energ. Convers. and Manage. 187, 398-406. https://doi.org/10.1016/j.enconman.2019.03.046 ; Trentini et al., 2018Trentini CP, Fonseca JM, Cardozo-Filho L, Reis R, Sampaio SC, Silva C. 2018. Assessment of continuous catalyst-free production of ethyl esters from grease trap waste. J. Supercrit. Fluids 136, 157-163. https://doi.org/10.1016/j.supflu.2018.02.018 ; Visioli et al., 2018Visioli LJ, Trentini CP, Castilhos F, Silva C. 2018. Esters production in continuous reactor from macauba pulp oil using methyl acetate in pressurized conditions. J. Supercrit. Fluids 140, 238-247. https://doi.org/10.1016/j.supflu.2018.06.018 ). Thus, this appears to be a viable alternative for studies on macauba pulp oil.

The efficiency of the reaction under pressurized conditions is dependent on the adjustment of the operating variables (pressure, temperature and time), the nature of the raw material (oil and alcohol), the oil-to-alcohol ratio used and the additives added to the reaction medium. The reaction is commonly conducted at pressures in the order of 20 MPa at temperatures between 275 and 350 °C (Silva and Oliveira, 2014Silva C, Oliveira JV. 2014. Biodiesel production through non-catalytic supercritical transesterification: Current state and perspectives. Braz. J. Chem. Eng. 31, 271-285. https://doi.org/10.1590/0104-6632.20140312s00002616 ).

The amount of alcohol used in the process is higher than that of the stoichiometric oil-to-alcohol molar ratio of 1:3, since an excess of alcohol will favor the formation of the esters, shifting the reaction toward the formation of products (Colonelli et al., 2017Colonelli TADS, Trentini CP, Santos KA, Oliveira JV, Cardozo-Filho L, Silva EA, Silva C. 2017. Assessment of process variables on the use of macauba pulp oil as feedstock for the continuous production of ethyl esters under pressurized conditions. Braz. J. Che. Eng. 34, 831-839. https://doi.org/10.1590/0104-6632.20170343s20150768 ). This is because there is an increase in the miscibility and the area of contact between the substrates, which benefits the production of esters (Musa, 2016Musa IA. 2016. The effects of alcohol to oil molar ratios and the type of alcohol on biodiesel production using transesterification process. Egypt. J. Pet. 25, 21-31. https://doi.org/10.1016/j.ejpe.2015.06.007 ). Some authors have reported that the use of an oil-to-alcohol molar ratio of 1:40 (which equates to a mass ratio of ∼1:2) gives the highest ester yield (Silva et al., 2007Silva C, Weschenfelder T, Rovani S, Corazza FC, Corazza ML, Dariva C, Oliveira JV. 2007. Continuous Production of Fatty Acid Ethyl Esters from Soybean Oil in Compressed Ethanol. Ind. Eng. Chem. Res. 46, 5304-5309. https://doi.org/10.1021/ie070310r ; Trentini et al., 2018Trentini CP, Fonseca JM, Cardozo-Filho L, Reis R, Sampaio SC, Silva C. 2018. Assessment of continuous catalyst-free production of ethyl esters from grease trap waste. J. Supercrit. Fluids 136, 157-163. https://doi.org/10.1016/j.supflu.2018.02.018 ). However, other authors have found that the formation of esters was favored up to an oil-to-alcohol molar ratio of 1:20 (mass ratio of ∼1:1) (Colonelli et al., 2017Colonelli TADS, Trentini CP, Santos KA, Oliveira JV, Cardozo-Filho L, Silva EA, Silva C. 2017. Assessment of process variables on the use of macauba pulp oil as feedstock for the continuous production of ethyl esters under pressurized conditions. Braz. J. Che. Eng. 34, 831-839. https://doi.org/10.1590/0104-6632.20170343s20150768 ; Silva et al., 2010Silva C, Castilhos F, Oliveira JV, Cardozo-Filho L, Trentin CM, Lima AP, Oliveira JV. 2010. Continuous production of soybean biodiesel with compressed ethanol in a microtube reactor. Fuel Process. Technol. 91, 1274-1281. https://doi.org/10.1016/j.fuproc.2010.12.016 ; Trentin et al., 2011Trentin CM, Lima AP, Alkimim IP, Silva C, Castilhos F, Mazutti MA, Oliveira JV. 2011. Continuous production of soybean biodiesel with compressed ethanol in a microtube reactor using carbon dioxide as co-solvent. Fuel Process. Technol. 92, 952-958. https://doi.org/10.1016/j.fuproc.2010.12.016 ).

Another aspect to be considered is the addition of a co-solvent to the reaction medium, which can act to improve the operating conditions of the technology using alcohol under sub- and supercritical conditions. This generally involves the reduction of some adjustable parameters of the reaction, such as temperature, pressure, residence time and/or oil-to-alcohol ratio (Akkarawatkhoosith et al., 2019cAkkarawatkhoosith N, Kaewchada A, Jaree A. 2019c. Simultaneous development of biodiesel synthesis and fuel quality via continuous supercritical process with reactive co-solvent. Fuel 237, 117-125. https://doi.org/10.1016/j.fuel.2018.09.077 , 2019aAkkarawatkhoosith N, Kaewchada A, Jaree A. 2019a. Enhancement of continuous supercritical biodiesel production : influence of co-solvent types. Enrgy Proced. 156, 48-52. https://doi.org/10.1016/j.egypro.2018.11.085 ; Osmieri et al., 2017Osmieri L, Esfahani RAM, Recasens F. 2017. Continuous biodiesel production in supercritical two-step process: phase equilibrium and process design. J. Supercrit. Fluids 124, 57-71. https://doi.org/10.1016/j.supflu.2017.01.010 ; Tobar and Núñez, 2018Tobar M, Núñez GA. 2018. Supercritical transeterification of microalgae triglycerides for biodiesel production: Effect of alcohol type and co-solvent. J. Supercrit. Fluids 137, 50-56. https://doi.org/10.1016/j.supflu.2018.03.008 ). In addition, the miscibility and solubility of the reaction mixture are also improved (Akkarawatkhoosith et al., 2019aAkkarawatkhoosith N, Kaewchada A, Jaree A. 2019a. Enhancement of continuous supercritical biodiesel production : influence of co-solvent types. Enrgy Proced. 156, 48-52. https://doi.org/10.1016/j.egypro.2018.11.085 , 2019bAkkarawatkhoosith N, Kaewchada A, Jaree A. 2019b. Production of Biodiesel from Palm Oil under Supercritical Ethanol in the Presence of Ethyl Acetate. Energ. Fuel 33, 5322-5331. research-article. https://doi.org/10.1021/acs.energyfuels.9b00641 ; Đokić-Stojanović1 et al., 2019Đokić-Stojanović1 DR, Todorović ZB, Troter DZ, Stamenković OS, Veselinović LM, Zdujić MV, Manojlović DD, Veljković VB. 2019. Influence of various co-solvents on the calcium oxide-catalyzed ethanolysis of sunflower oil. J. Serbian Chem. Soc. 84, 253-265. https://doi.org/10.2298/JSC180827007D ; Muppaneni et al., 2013Muppaneni T, Reddy HK, Ponnusamy S, Patil PD, Sun Y, Dailey P, Deng S. 2013. Optimization of biodiesel production from palm oil under supercritical ethanol conditions using hexane as co-solvent : A response surface methodology approach. Fuel 107, 633-640. https://doi.org/10.1016/j.fuel.2012.11.046 ), ensuring a high production of esters in a short period of time and without the use of a catalyst (Lim and Lee, 2013Lim S, Lee K. 2013. Influences of different co-solvents in simultaneous supercritical extraction and transesterification of Jatropha curcas L . seeds for the production of biodiesel. Chem. Eng. J. 221, 436-445. https://doi.org/10.1016/j.cej.2013.02.014 ). Several co-solvents are cited in the literature. Notably, the use of n-hexane has proved to be efficient for reactions under supercritical conditions without the use of a catalyst (Abdala et al., 2014aAbdala ACDA, Colonelli TADS, Trentini CP, Oliveira JV, Cardozo-Filho L, Silva EA, Silva C. 2014a. Effect of additives in the reaction medium on noncatalytic ester production from used frying oil with supercritical ethanol. Ener. Fuels 283122-3128. https://doi.org/10.1021/ef402253e ; Colonelli et al., 2017Colonelli TADS, Trentini CP, Santos KA, Oliveira JV, Cardozo-Filho L, Silva EA, Silva C. 2017. Assessment of process variables on the use of macauba pulp oil as feedstock for the continuous production of ethyl esters under pressurized conditions. Braz. J. Che. Eng. 34, 831-839. https://doi.org/10.1590/0104-6632.20170343s20150768 ; Lim and Lee, 2013Lim S, Lee K. 2013. Influences of different co-solvents in simultaneous supercritical extraction and transesterification of Jatropha curcas L . seeds for the production of biodiesel. Chem. Eng. J. 221, 436-445. https://doi.org/10.1016/j.cej.2013.02.014 ; Muppaneni et al., 2013Muppaneni T, Reddy HK, Ponnusamy S, Patil PD, Sun Y, Dailey P, Deng S. 2013. Optimization of biodiesel production from palm oil under supercritical ethanol conditions using hexane as co-solvent : A response surface methodology approach. Fuel 107, 633-640. https://doi.org/10.1016/j.fuel.2012.11.046 ; Silva and Oliveira, 2014Silva C, Oliveira JV. 2014. Biodiesel production through non-catalytic supercritical transesterification: Current state and perspectives. Braz. J. Chem. Eng. 31, 271-285. https://doi.org/10.1590/0104-6632.20140312s00002616 ), besides reducing the viscosity of the reaction mixture and allowing the production in a continuous process (Sawangkeaw et al., 2011Sawangkeaw R, Bunyakiat K, Ngamprasertsith S. 2011. Continuous production of biodiesel with supercritical methanol : Optimization of a scale-up plug fl ow reactor by response surface methodology. Fuel Process. Technol. 92, 2285-2292. https://doi.org/10.1016/j.fuproc.2011.07.014 ).

Recently, obtaining oil esters from macauba pulp in a catalyst-free medium using pressurized ethanol was reported by Colonelli et al., (2017)Colonelli TADS, Trentini CP, Santos KA, Oliveira JV, Cardozo-Filho L, Silva EA, Silva C. 2017. Assessment of process variables on the use of macauba pulp oil as feedstock for the continuous production of ethyl esters under pressurized conditions. Braz. J. Che. Eng. 34, 831-839. https://doi.org/10.1590/0104-6632.20170343s20150768 . These authors found that the addition of a co-solvent (n-hexane) and the use of higher amounts of alcohol in the reaction medium were the variables that had the greatest influence on the formation of esters. However, the effects of these variables were observed in a fixed residence time.

In this context, the objective of this study was to evaluate the kinetics of ester production from macauba pulp oil (MPO) using ethanol under pressurized conditions and without the use of a catalyst. Experiments were carried out at different temperatures (200 ºC to 300 ºC) and the influence of adding a greater amount of ethanol to the reaction medium was evaluated. The effect of the addition of a co-solvent (n-hexane) and the ester yield at different residence times were also investigated.

2. MATERIALS AND METHODS

3. RESULTS AND DISCUSSION

3.1. Ethyl ester content

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Figure 1 shows the results from the experiments conducted with a mass ratio of MPO-to-ethanol of 1:1, as well as the effect of increasing the ratio to 1:2 and adding 20 wt% of co-solvent (n-hexane) at a ratio of 1:1, at temperatures of 200 to 300 °C. The mass ratios of MPO-to-ethanol of 1:1 and 1:2 are equivalent to molar ratios of triglycerides-to-ethanol of ∼40 and 122 and of free fatty acids-to-ethanol of 9 and 18, respectively.

Figure 1.  Ethyl ester contents of macauba pulp oil obtained at different temperatures: (a) 200 °C, (b) 225 °C, (c) 250 °C, (c) 275 °C and (e) 300 °C at 20 MPa, MPO-to-ethanol mass ratio of 1:1 () and 1:2 (), and MPO-to-ethanol mass ratio of 1:1 with 20 wt% of co-solvent (in oil) (). The values in the graphs represent the means of two determinations, with SD < 2.0%. Means followed by the same lowercase letters (with the same residence time) did not differ statistically (p > 0.05) using ANOVA (Tukey’s test).

3.1.1. Effect of temperature

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In the overall analysis of the data shown in Figure 1 it can be seen that an increase in temperature led to higher ester yields. When the temperature was increased from 200 to 250 °C and from 250 to 300 °C, with a residence time of 10 min, it was possible to obtain an increase in the ester content in the ratios of ∼2.35 and ∼1.15, respectively, for the three reaction media evaluated. The temperature effect is significant (p < 0.05) up to the residence time of 45 min (analysis not shown in Figure 1).

In reactions at high temperatures, changes in the solubility, density, dielectric constant and solvation of the mixture in the reaction medium occur (Farobie and Matsumura, 2017Farobie O, Matsumura Y. 2017. State of the art of biodiesel production under supercritical conditions. Prog. Energy. Combust. Sci. 63, 173-203. https://doi.org/10.1016/j.pecs.2017.08.001 ). This favors a reduction in the mass transfer limitations and increases the reaction rate (Abbaszaadeh et al., 2012Abbaszaadeh A, Ghobadian B, Omidkhah MR, Najafi G. 2012. Current biodiesel production technologies : A comparative review. Energ. Convers. and Manage. 63, 138-148. https://doi.org/10.1016/j.enconman.2012.02.027 ; Pinnarat and Savage, 2010Pinnarat T, Savage PE. 2010. Noncatalytic esterification of oleic acid in ethanol. J. Supercrit. Fluids, 53, 53-59. https://doi.org/10.1016/j.supflu.2010.02.008 ). In addition, near the critical temperature, the polarity of the alcohol decreases, and the alcohol starts to solvate the non-polar triglycerides, forming a practically homogeneous mixture (Srivastava, Paul, and Goud, 2018Srivastava G, Paul AK, Goud VV. 2018. Optimization of non-catalytic transesterification of microalgae oil to biodiesel under supercritical methanol condition. Energy. Conver. Manag. 156, 269-278. https://doi.org/10.1016/j.enconman.2017.10.093 ), which allows higher ester yields to be obtained.

Tobar and Núñez (2018)Tobar M, Núñez GA. 2018. Supercritical transeterification of microalgae triglycerides for biodiesel production: Effect of alcohol type and co-solvent. J. Supercrit. Fluids 137, 50-56. https://doi.org/10.1016/j.supflu.2018.03.008 showed that the reaction kinetics are affected by increasing temperature due to an increase in the kinetic energy of the particles and the relative probability of collisions between them.

In a study conducted by Silva et al., (2014)Silva C, Colonelli TAS, Silva EA, Cabral VF, Oliveira JV, Cardozo-Filho L. 2014. Continuous catalyst-free production of esters from Jatropha curcas L. oil under supercritical ethanol. Braz. J. Chem. Eng. 31727-735. https://doi.org/10.1590/0104-6632.20140313s00002709 , increased ester yields were observed on increasing the temperature from 250 to 300 °C, with a difference of ∼42% in only 10 min of reaction. A similar result was reported by Santos et al., (2018)Santos KC, Hamerski F, Voll FAP, Corazza ML. 2018. Experimental and kinetic modeling of acid oil (trans)esterification in supercritical ethanol. Fuel 224, 489-498. https://doi.org/10.1016/j.fuel.2018.03.102 with a 30% increase in the ester yield for the same temperature range. In addition, Zhou et al., (2017)Zhou D, Qiao B, Li G, Xue S, Yin J. 2017. Continuous production of biodiesel from microalgae by extraction coupling with transesterification under supercritical conditions. Bioresour. Technol. 238, 609-615. https://doi.org/10.1016/j.biortech.2017.04.097 and Akkarawatkhoosith et al., (2019a)Akkarawatkhoosith N, Kaewchada A, Jaree A. 2019a. Enhancement of continuous supercritical biodiesel production : influence of co-solvent types. Enrgy Proced. 156, 48-52. https://doi.org/10.1016/j.egypro.2018.11.085 obtained an increased ester content of ∼60% on increasing the operating temperature from 250 to 300 ºC.

Under the conditions where ethanol was below its critical temperature (243.2 °C), the results demonstrated an FAEE content of 30 to 60%. This is due to the high concentration of FFA in the macauba oil used (70.26%), resulting in the esterification reaction predominating, since FFAs are more reactive than triglycerides (Go et al., 2014Go AW, Sutanto S, NguyenThi BT, Cabatingan LK, Ismadji S, Ju Y. 2014. Transesterification of soybean oil with methanol and acetic acid at lower reaction severity under subcritical conditions. Energ. Convers. Manag. 88, 1159-1166. https://doi.org/10.1016/j.enconman.2014.03.014 ; Vieitez et al., 2012Vieitez I, Irigaray B, Casullo P, Pardo MJ, Grompone MA, Jachmanián I. 2012. Effect of free fatty acids on the efficiency of the supercritical ethanolysis of vegetable oils from different origins. Energy. and Fuels 26, 1946-1951. https://doi.org/10.1021/ef201977s ).

Pinnarat and Savage (2010)Pinnarat T, Savage PE. 2010. Noncatalytic esterification of oleic acid in ethanol. J. Supercrit. Fluids, 53, 53-59. https://doi.org/10.1016/j.supflu.2010.02.008 studied non-catalytic esterification and reported that an ester content of 70% was obtained at 230 °C with a residence time of 80 min and pressure of 5.2 MPa. Go et al., (2014)Go AW, Sutanto S, NguyenThi BT, Cabatingan LK, Ismadji S, Ju Y. 2014. Transesterification of soybean oil with methanol and acetic acid at lower reaction severity under subcritical conditions. Energ. Convers. Manag. 88, 1159-1166. https://doi.org/10.1016/j.enconman.2014.03.014 reported an ester yield of ∼60% with the reaction conducted at 200 °C for 30 min at a pressure of 2.8 MPa. At the same temperature but with a pressure of 20 MPa, Abdala et al., (2014b)Abdala AC, Garcia VA, Trentini CP, Cardozo-Filho L, Silva EA, Silva C. 2014b. Continuous Catalyst-Free Esterification of Oleic Acid in Compressed Ethanol. Int. J. Chem. Eng. 2014, 1-5. https://doi.org/10.1155/2014/803783 achieved ∼70% conversion of oleic acids to esters in only 10 min of reaction. Santos et al., (2017)Santos PR, Pedersen FA, Ramos LP, Corazza ML. 2017. Esterification of fatty acids with supercritical ethanol in a continuous tubular reactor. J. Supercrit. Fluids 126, 25-36. https://doi.org/10.1016/j.supflu.2017.03.002 obtained an esters content of ∼75% at 220 °C and 10 MPa with 80 min of reaction. Jesus et al., (2018)Jesus AA, Souza DF, Oliveira JA, Deus MS, Silva MG, Franceschi E, Dariva C. 2018. Mathematical modeling and experimental esterification at supercritical conditions for biodiesel production in a tubular reactor. Energ. Convers. Manag. 171, 1697-1703. https://doi.org/10.1016/j.enconman.2018.06.108 observed that the reaction between oleic acid and ethanol yielded ∼66% esters at 200 °C with a reaction time of 30 min and pressure of 15 MPa.

3.1.2. Effect of oil to ethanol mass ratio

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Increasing the MPO-to-ethanol mass ratio from 1:1 to 1:2 had a more pronounced effect on the formation of esters for the reactions conducted at temperatures of 200 to 250 ºC, particularly at the shortest times evaluated (10 and 15 min). At the highest temperatures considered, that is 275 ºC (Figure 1d) and 300 ºC (Figure 1e), the increase in the amount of alcohol in the reaction medium did not influence the FAEE content.

A greater amount of alcohol in the reaction boosts the formation of the products, since increasing the volume of ethanol available in the medium results in a decrease in the critical temperature of the reaction mixture (Osmieri et al., 2017Osmieri L, Esfahani RAM, Recasens F. 2017. Continuous biodiesel production in supercritical two-step process: phase equilibrium and process design. J. Supercrit. Fluids 124, 57-71. https://doi.org/10.1016/j.supflu.2017.01.010 ), promoting the occurrence of the reaction occurring in a homogeneous phase region and increasing the reaction kinetics. MPO is mainly composed of FFA and esterification predominates in the reaction medium. As reported by Santos et al., (2017)Santos PR, Pedersen FA, Ramos LP, Corazza ML. 2017. Esterification of fatty acids with supercritical ethanol in a continuous tubular reactor. J. Supercrit. Fluids 126, 25-36. https://doi.org/10.1016/j.supflu.2017.03.002 , this reaction occurs in a single homogeneous phase at 10 MPa with an FFA-to-ethanol molar ratio of 1:1 to 30:1 from 220 to 280 °C.

The alcohol concentration has little effect at higher operating temperatures, as observed at 275 and 300 °C (Figure 1), this effect occurs due to the increase in the critical pressure in the mixture, which makes it necessary to use high pressure to obtain a homogeneous phase, as noted by Nan et al., (2015)Nan Y, Liu J, Lin R, Tavlarides LL. 2015. Production of biodiesel from microalgae oil (Chlorella protothecoides) by non-catalytic transesterification in supercritical methanol and ethanol: Process optimization. J. Supercrit. Fluids 97, 174-182. https://doi.org/10.1016/j.supflu.2014.08.025 who obtained similar ester yields at 310 °C, after 30 min at 14 MPa, using microalgae oil-to-ethanol molar ratios of 1:26 and 1:42. The experiments conducted by Bezerra et al., (2018)Bezerra FWF, Costa WA, Oliveira MS, Andrade HÁ, Carvalho JRN. 2018. Transesterification of palm pressed-fibers (Elaeis guineensis Jacq.) oil bysupercriticalfluid carbon dioxide with entrainer ethanol. J. Supercrit. Fluids 136, 136-143. https://doi.org/10.1016/j.supflu.2018.02.020 also provided similar conversion to ethyl esters for oil:ethanol molar ratios of 1:20 and 1:40 at 350 °C, after 40 min at 20 MPa. A similar result was obtained by Costa et al., (2019)Costa WA, Bezerra FWF, Oliveira MS, Andrade HA, Santos APM, Cunha VMB, Junior RN. 2019. Supercritical CO₂ extraction and transesterification of the residual oil from industrial palm kernel cake with supercritical methanol. J. Supercrit. Fluids, 147, 179-187. https://doi.org/10.1016/j.supflu.2018.10.012 on raising the oil:methanol molar ratio from 1:21 to 1:41, at 300 °C, for 10 min of reaction at 20 MPa.

At lower temperatures (200 to 250 ºC) the effect of the ratio between the lipids (triglycerides or FFA) and the alcohol in the matrix can be noted (Figure 1). Mello et al., (2017)Mello BTF, Gonçalves JE, Rodrigues GM, Cardozo-Filho L, Silva C. 2017. Hydroesterification of crambe oil (Crambe abyssinica H.) under pressurized conditions. Ind. Crops Prod. 97, 110-119. https://doi.org/10.1016/j.indcrop.2016.12.014 reported a 12% increase in the ethyl ester yield on increasing the ratio of alcohol to crambe oil hydrolyzate from 12:1 to 15:1 (molar basis) in a reaction conducted at 275 °C, for 10 min at 15 MPa. Jesus et al., (2018)Jesus AA, Souza DF, Oliveira JA, Deus MS, Silva MG, Franceschi E, Dariva C. 2018. Mathematical modeling and experimental esterification at supercritical conditions for biodiesel production in a tubular reactor. Energ. Convers. Manag. 171, 1697-1703. https://doi.org/10.1016/j.enconman.2018.06.108 conducted the reaction at 250 °C and 15 MPa and found that on increasing the ethanol:oleic acid molar ratio from 1:1 to 6:1 the conversion of FFA to esters increased by 22%.

3.1.3. Effect of co-solvent

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The addition of 20 wt% of co-solvent to the reaction mixture (with an oil-to- ethanol mass ratio of 1:1) promoted an improvement in ester production. Thus, for a residence time of 10 min it was possible to obtain increases of 30 to 36% and 75 to 87% in the ester content at 200 and 300 ºC, respectively. The use of a co-solvent improves the mutual solubility between the alcohol and the oil (Tobar and Núñez, 2018Tobar M, Núñez GA. 2018. Supercritical transeterification of microalgae triglycerides for biodiesel production: Effect of alcohol type and co-solvent. J. Supercrit. Fluids 137, 50-56. https://doi.org/10.1016/j.supflu.2018.03.008 ), allowing for the presence of a homogeneous phase (Osmieri et al., 2017Osmieri L, Esfahani RAM, Recasens F. 2017. Continuous biodiesel production in supercritical two-step process: phase equilibrium and process design. J. Supercrit. Fluids 124, 57-71. https://doi.org/10.1016/j.supflu.2017.01.010 ), increasing the reaction rate and making it possible to obtain high ester yields at moderate temperatures (Maçaira et al., 2014Maçaira J, Santana A, Costa A, Ramirez E, Larrayoz MA. 2014. Process Intensification Using CO2 As Cosolvent under Supercritical Conditions Applied to the Design of Biodiesel Production. Ind. Eng. Chem. Res. 5 3, 3985-3995. https://doi.org/10.1021/ie402657e ).

Zhou et al., (2017)Zhou D, Qiao B, Li G, Xue S, Yin J. 2017. Continuous production of biodiesel from microalgae by extraction coupling with transesterification under supercritical conditions. Bioresour. Technol. 238, 609-615. https://doi.org/10.1016/j.biortech.2017.04.097 performed a coupled extraction and reaction process. When the reaction was conducted at 340 °C, for 120 min, with an n-hexane flow of 0.2 mL·min^-1, there was an increase of 63% in ester yield compared to the reaction without the addition of the co-solvent. In research conducted by Tobar and Núñez (2018)Tobar M, Núñez GA. 2018. Supercritical transeterification of microalgae triglycerides for biodiesel production: Effect of alcohol type and co-solvent. J. Supercrit. Fluids 137, 50-56. https://doi.org/10.1016/j.supflu.2018.03.008 , the highest yield of ethyl esters (68%) was obtained with the addition of CO₂ (0.001 g CO₂ per g of ethanol) as a co-solvent at 300 °C and 20 MPa. Akkarawatkhoosith et al., (2019c)Akkarawatkhoosith N, Kaewchada A, Jaree A. 2019c. Simultaneous development of biodiesel synthesis and fuel quality via continuous supercritical process with reactive co-solvent. Fuel 237, 117-125. https://doi.org/10.1016/j.fuel.2018.09.077 reported that an iso-propanol:oil weight ratio of 0.1:1 for only 3 min at 350 °C gave a 37% increase in the ester yield. The same research group (Akkarawatkhoosith et al., 2019bAkkarawatkhoosith N, Kaewchada A, Jaree A. 2019b. Production of Biodiesel from Palm Oil under Supercritical Ethanol in the Presence of Ethyl Acetate. Energ. Fuel 33, 5322-5331. research-article. https://doi.org/10.1021/acs.energyfuels.9b00641 ) also found that in the reaction conducted at 300 °C for 4 min the addition of a co-solvent (55 wt% ethyl acetate) promoted an increase in the ethyl ester production of ∼68% compared to the reaction with no co-solvent added.

It should be noted that adding more alcohol to the process would significantly increase the production costs, as the ethanol (99.5%) used for this purpose is ∼21% more expensive than n-hexane (95%), according to the company Tedia Brazil®. Thus, the costs related to increasing the oil-to-ethanol mass ratio to 1:2 would be ∼85% higher than using the mass ratio of 1:1 with 20 wt% of n-hexane (basis of calculation: 1 L of reaction mixture). However, it needs to be considered that high ester content (∼89%) can be obtained in a relatively short time (15 min) at 300 °C, further enhancing the benefits of using the co-solvent. In addition, as noted by Sawangkeaw et al., (2011)Sawangkeaw R, Bunyakiat K, Ngamprasertsith S. 2011. Continuous production of biodiesel with supercritical methanol : Optimization of a scale-up plug fl ow reactor by response surface methodology. Fuel Process. Technol. 92, 2285-2292. https://doi.org/10.1016/j.fuproc.2011.07.014 , n-hexane, the solvent most commonly used in the extraction of vegetable oils, can be removed after the oil extraction and employed in the production of biodiesel.

3.2. Triglyceride, diglyceride and monoglyceride contents

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In the samples obtained at 300 ºC, the triglyceride (TG), diglyceride (TG), monoglyceride (MG) and free fatty acid (FFA) contents were determined, as shown in Figure 2, considering that the highest levels of FAEE were obtained at this temperature. The contents in these compounds in the MPO were also determined at: 18.42 ± 0.15 wt%, 15.2 ± 0.62 wt%, 3.85 ± 0.12 wt% and 70.26 ± 0.05 wt% of TG, DG, MG and FFA, respectively. In general, the increase in mass ratio and addition of co-solvent did not influence the conversion of TG or the formation of DG. The reaction conducted with a mass ratio of MPO-to-ethanol of 1:1, showed a higher concentration of MG when compared to the others. It can also be seen that there was a higher consumption of FFA for the reaction with a mass ratio MPO-to-ethanol of 1:2.

Figure 2.  Triglyceride (), diglyceride (), monoglyceride (), free fatty acids () and total contents obtained () at 300 ºC: MPO-to-ethanol mass ratio of (a) 1:1 and (b) 1:2, and (c) MPO-to-ethanol mass ratio of 1:1 with 20 wt% of co-solvent (in oil). The values in the graphs represent the means of two determinations, with SD < 1.0 wt%. Means followed by the same lowercase letters (for the same compound) did not differ statistically (p > 0.05) using ANOVA (Tukey’s test).

For all conditions evaluated, high TG conversions were achieved, with contents in these compounds in the samples obtained being below 0.14 wt%. In other studies on the transesterification reaction under pressurized conditions conducted at the same temperature, the TG contents of 0.2 to 22 wt% (Santos et al., 2018Santos KC, Hamerski F, Voll FAP, Corazza ML. 2018. Experimental and kinetic modeling of acid oil (trans)esterification in supercritical ethanol. Fuel 224, 489-498. https://doi.org/10.1016/j.fuel.2018.03.102 ; Trentin et al., 2011Trentin CM, Lima AP, Alkimim IP, Silva C, Castilhos F, Mazutti MA, Oliveira JV. 2011. Continuous production of soybean biodiesel with compressed ethanol in a microtube reactor using carbon dioxide as co-solvent. Fuel Process. Technol. 92, 952-958. https://doi.org/10.1016/j.fuproc.2010.12.016 ; Trentini et al., 2019Trentini CP, Postaue N, Cardozo-Filho L, Reis RR, Sampaio SC, Silva C. 2019. Production of esters from grease trap waste lipids under supercritical conditins: Effect of water addition on ethanol. J. Supercrit. Fluids 147, 16. https://doi.org/10.1016/j.supflu.2019.02.008 ; Vieitez et al., 2010Vieitez I, Silva C, Alckmin I, Borges GR, Corazza FC, Oliveira JV, Jachmanián, I. 2010. Continuous catalyst-free methanolysis and ethanolysis of soybean oil under supercritical alcohol/water mixtures. Renew. Energ. 35, 1976-1981. https://doi.org/10.1016/j.renene.2010.01.027 ) were detected under the conditions that provided maximum ester yields.

The remaining FFA contents in the samples ranged from 4.0 to 6.6 wt%, which correspond to an FFA conversion of above 90%. A similar result was obtained by Visioli et al., (2016)Visioli LJ, Castilhos F, Cardozo-Filho L, Mello BTF, Silva C. 2016. Production of esters from soybean oil deodorizer distillate in pressurized ethanol. Fuel Process. Technol. 149, 326-331. https://doi.org/10.1016/j.fuproc.2016.04.038 for conversion under the thermodynamic equilibrium in the esterification reaction of soybean oil deodorizer distillate with pressurized ethanol. Vieitez et al., (2012)Vieitez I, Irigaray B, Casullo P, Pardo MJ, Grompone MA, Jachmanián I. 2012. Effect of free fatty acids on the efficiency of the supercritical ethanolysis of vegetable oils from different origins. Energy. and Fuels 26, 1946-1951. https://doi.org/10.1021/ef201977s performed the supercritical alcoholysis of raw materials with 0 to 100 wt% of FFA and achieved a maximum conversion of FFA to ethyl esters of ∼ 90%.

The total amount of unreacted compounds detected was less than 8.0 wt%. Soto et al., (2014)Soto G, Hegel P, Pereda S. 2014. Supercritical production and fractionation of fatty acid esters and acylglycerols. J. Supercrit. Fluids 93, 74-81. https://doi.org/10.1016/j.supflu.2014.04.017 found DG, MG and FFA contents of 11, 3 and 2%, respectively, in the reaction between sunflower oil and methanol with 40 min of reaction at 18 MPa. Ortiz-Martínez et al., (2016)Ortiz-Martínez VM, Salar-García MJ, Palacios-Nereo FJ, Olivares-Carrillo P, Quesada-Medina J, Ríos APDL, Hernández-Fernández FJ. 2016. In-depth study of the transesterification reaction of Pongamia pinnata oil for biodiesel production using catalyst-free supercritical methanol process. J. Supercrit. Fluids 113, 23-30. https://doi.org/10.1016/j.supflu.2016.03.009 reported a higher value of unreacted compounds, with ∼18 and 27% of MG and DG, respectively, for the reaction between Pongamia pinnata oil and methanol, with a reaction time of 20 min at 18 MPa. Low contents in MG (2%), DG (4%) and TG (0.2%) were reported by Trentini et al., (2019)Trentini CP, Postaue N, Cardozo-Filho L, Reis RR, Sampaio SC, Silva C. 2019. Production of esters from grease trap waste lipids under supercritical conditins: Effect of water addition on ethanol. J. Supercrit. Fluids 147, 16. https://doi.org/10.1016/j.supflu.2019.02.008 for the reaction between grease trap waste lipids and ethanol, with the addition of 2.5 wt% water, with a residence time of 30 min at 20 MPa.

4. CONCLUSIONS

Increasing the operating temperature up to 300 °C for the reaction between macauba pulp oil (MPO) and ethanol led to high ester contents. Moreover, it was found that the reactions conducted at high temperatures (275 and 300 °C) required less alcohol in the reaction medium in shorter residence times, demonstrating that temperature is a key factor to be considered in studies on the production of esters. The addition of n-hexane to the reaction, which increased the diglyceride and monoglyceride contents by only 0.6 wt%, allowed for a reduction in the reaction time of ∼44% and increased the ester production by up to ∼25%. The reaction in a pressurized medium without catalyst was effective, and produced low contents in unreacted compounds (∼ 8.0%) and high consumption of triglycerides (∼ 99%) and free fatty acids (∼90%). The highest FAEE content (∼90%) was obtained with an MPO-to-ethanol mass ratio of 1:1 with 20 wt% of co-solvent (in the oil) at 300 °C after 15 min of reaction.