Appliance of a high pressure semi-batch reactor: supercritical transesterification of soybean oil using methanol

Nowadays, the need for renewable energy sources increases everyday due to the greater demand for petroleum products, which are not renewable and tends to be more expensive in the future. Another aspect of these products is that they produce a lot of harmful composites, such as carbon monoxide, nitrogen and sulfur oxides. Therefore, attention has been focused on developing renewable, biodegradable or alternative fuels, such as biodiesel to replace petroleum based fuels (Fukuda et al., 2001; Gopinath et al., 2009; Javidialesaadi & Raeissi, 2013).


Introduction
Nowadays, the need for renewable energy sources increases everyday due to the greater demand for petroleum products, which are not renewable and tends to be more expensive in the future.Another aspect of these products is that they produce a lot of harmful composites, such as carbon monoxide, nitrogen and sulfur oxides.Therefore, attention has been focused on developing renewable, biodegradable or alternative fuels, such as biodiesel to replace petroleum based fuels (Fukuda et al., 2001;Gopinath et al., 2009;Javidialesaadi & Raeissi, 2013).
Biodiesel is the result of the transesterification of triacylglycerols in vegetable oils or animal fats.Ot works as an alternative fuel for compression-ignition engines.Since the transesterification reaction needs a short-chain alcohol, methanol and ethanol are the most used (Bunyakiat et al., 2006;Ali et al., 2013).
At first, the transesterification is a reversible reaction (Figure 1).However, the glycerol formed is immiscible in biodiesel, greatly reducing the extent of the reverse reaction.
The miscibility of glycerol in the biodiesel is a factor favoring the yield of reaction; however, the possibility of forming stable emulsions, in certain cases, may require a long period of rest for phase separation of biodiesel and glycerol (Atadashi et al., 2011;Lourenço & Stradiotto, 2009).
On biodiesel production with base catalysts, besides the transesterification reactions, saponification of alkyl esters or of triacylglycerols themselves may also occur.Saponification is the formation of soap, i.e., salts of fatty acids, which can also occur due to a high content of free fatty acids in oils or fats.The soap formation involves three problems: it consumes the catalyst used in the reaction; it causes an increase in viscosity or formation of gels that interferes in the reaction.Also, the presence of soap in the reaction mixture stabilizes the biodiesel/glycerol emulsion.A technical problem is also related to the use of biodiesel with high soap content: engine damage due to fouling and corrosion (Van Gerpen, 2005;Ma & Hanna, 1999;Meher et al., 2006).
The biodiesel production may involve the use of catalysts: acids but mostly bases (hydroxide or sodium alkoxide or potassium).Ot has the advantage of being carried out at room temperature and still be faster than the transesterification catalyzed by acids, which needs to be performed at elevated temperatures (170 °C) (Rinaldi et al., 2007;Vicente et al., 2004).
Many types of vegetable oils can be used to produce biodiesel.Sunflower, corn, olive, palm and soybean oils are good examples of raw materials that are widely used in this process.Since Brazil is a great producer of soybean, its oil is one of the most used as cooking oil, in the food industry and in the transesterification technique (Geris et al., 2007).On addition, its composition of fatty acids is well known, consisting mostly of linoleic, oleic and linolenic acids (Kawazoe, 2014).
Several authors, such as Bernardes et al. (2007), Antunes et al. (2008) and Li et al. (2012) have worked with transesterification of soybean oil, and obtained high conversion of fatty acid esters.On these works, methanol and/or ethanol as the reaction alcohol, as well as different kinds of catalysts were used, such as potassium hidroxyde, zinc oxide, aluminium oxide and even enzymes.Dealing exactly with transesterification of soybean oil with supercritical alcohol and carbon dioxide, the papers of Bertoldi et al. (2009) and Cheng et al. (2010) can be highlighted.
On addition to the conventional method of producing biodiesel, another technique has been investigated over the past few years: the supercritical transesterification process.This method has been studied by many authors such as Demirbas (2005), Varma et al. (2010), Sawangkeaw et al. (2011), Anikeev & Yakovleva (2013), Tsai et al. (2013) and Ngamprasertsith et al. (2014).According to Marulanda (2012), this process has the advantage of promoting fast transesterification reactions of triacylglycerols and immediate esterification of free fatty acids without the excessive formation of glycerol and no need of a conventional catalyst.Ot also forms a homogeneous phase of the alcohol and triacylglycerols mixture.
The use of supercritical CD 2 can affect the kinetic behavior of the transesterification reaction.This may occur due to the increase of alcohol solubility in the oil phase (Galia et al., 2011).Ot is reported that the methylation of fixed oils using supercritical fluids may be a viable alternative for the production of biodiesel from different plant matrices (Chen et al., 2010;Soh & Zimmerman, 2011).
The type of reactor used in transesterification processes is also a very important factor to consider.Authors such as Spallina et al. (2017) andGianotti Pret et al. (2015) worked with the design, modeling and validation of reactors.Mjalli et al. (2009) dealt with the developing and validation of a transesterification reactor.According to Kern & Shastri (2015), this process can be carried out in a continuous or batch reactor, but the batch process is often preferred because of its flexibility and accommodation of various types of raw materials, composition, and quantity, thus making possible to satisfy specific product requirements.Lim & Lee (2011) and Ghoreishi & Moein (2013) also used batch reactors to transesterificate vegetable oils using methanol in supercritical conditions, obtaining high yields of fatty acid methyl esters.
On this context, the objective of this work was to evaluate the process of supercritical transesterification of soybean oil with methanol in the presence of carbon dioxide, and provide comparisons to the conventional technique.

Equipment description
The supercritical transesterification reaction of soybean oil with methanol and CD 2 was performed in a stainless-steel autoclave (model 2777 1000, Top Ondustrie, France) with internal volume of 0.095 L (0.128 x 0.152 m).To the autoclave were attached a heating collar (model 2777 1021, Top Ondustrie, France), a magnetic stirrer (model 616 0100, Top Ondustrie, FR), a cooling bath (model Q214U2, Quimis, Brazil), a thermocouple TKA 15 x 20 HMP, Erciat, France), a safety head (model 728 0350, Top Ondustrie, France), a rupture disk (model 728 0400, Sitec, Switzerland), a pressure transducer (model PA-Keller, Switzerland), a CD 2 supply system, a sample collection system with approximately 8 mL, and a control panel (model TS1070, Monitouch Technoshot, The Netherlands).More details are in the Appendix A at the end of the references section.

Materials
For the transesterification process, commercial refined soybean oil (LOSA, Brazil), obtained in a supermarket in Belem/Para/Brazil was used as the vegetable oil.The alcohol used was Methanol (99.9% purity, TEDOA, USA).The cosolvent used was carbon dioxide (99.9% purity, White Martins, Brazil).

Transesterification method
The transesterification reaction of soybean oil with methanol and supercritical CD 2 is represented in the system shown in Figure 2.For the study, the operating conditions were those used in the paper of Han et al. (2005), who also dealt with the supercritical transesterification of soybean oil using methanol.Molar ratios of 1:24 (oil:alcohol) and 0.1 (CD 2 to methanol), rotation of 700 rpm, pressure of 130 bar, temperature of 280 °C and heating rate of approximately 4 °C/min were used.After stabilization of the selected temperature and pressure, the CD 2 inlet valve was opened so that there was no influence of temperature on the system pressure (autogenous pressure).After the reaction time (10 minutes), the sample (Figure 3) was collected and centrifuged (Multifuge X1R-N/S 41464410, ThermoElectron Led GMBH, USA) at 10,000 rpm for 15 minutes to separate the water and glycerin formed.

Analysis of the methyl esters obtained
The fatty acid methyl esters were analyzed as described in the literature (Xavier et al., 2011) by gas chromatography-mass spectrometry (GC-MS),using a QP 2010 Shimadzu system equipped with an ADC-20i auto injector, Rtx-5MS silica capillary column (30 m x 0.25 mm, film thickness of 0.25 μm) at temperatures ranging from 100 °C to 280 °C, with a gradient of 4 °C/min and injector temperature of 100 °C; helium as carrier gas at a flow rate of 1.2 mL/min (measured at 100 °C), The injector was of splitless injection, with 0.1 µL of a transesterified solution; programmed temperature of 100 °C-280 °C with a gradient of 5 °C/min.The quadrupole filter scanned from 39 to 550 Daltons once every second.The ionization was acquired by the electron impact technique with energy of 70 eV.Each chemical component was identified by comparing their mass spectra with those in the literature (Adams, 2007) and on the NOST database (Stein, 2011).

Yield of biodiesel production
The yield of biodiesel production was calculated (Equation 1) based on the fatty acid methyl esters (FAME) analyzed by GC (Baroutian et al., 2010).
Where: A is mass of biodiesel produced; B is total mass % of FAME; and C is mass of oil used in the reaction.

Comparison to other studies on supercritical transesterification
Table 1 presents some published works on transesterification by supercritical method.
The novelty of this work is in the equipment assembly.On fact, it was specifically made with certain specifications required by the authors, for example, inlet and outlet valves for supercritical CD 2 , a sampling valve, volume of the autoclave and the maximum pressure and temperature conditions that the reactor would withstand.None of the studies mentioned in Table 1 dealt with the assembly of the reactor used in their respective works.Therefore, the apparatus employed in our research is not a serial device but an exclusive equipment tailored to the authors's needs.The best conditions of pressure, temperature and molar ratio and the advantages of using methanol were considered from the study of Cao et al. (2005), which was the main paper we made comparisons with.

Influence of molar ratio
Molar ratio of alcohol to oil is one of the most significant factors that affect the conversion efficiency, yield of biodiesel and production cost (Musa, 2016).Since transesterification is an equilibrium reaction, the excess of methanol was used in order to shift the equilibrium to the right and produce more methyl esters as result of the reaction.Higher molar ratios of methanol to oil also result in a more efficient transesterification reaction, due to the increased contact area between methanol and triacylglycerols (Cao et al., 2005).Some authors such as Van Kasteren & Nisworo (2007), Baroutian et al. (2010) and Wang et al. (2012) conducted the transesterification reactions using excess alcohol and molar ratio of methanol to oil of 24:1, which was the same proportion used in this work.At this ratio, all of them presented high yields of biodiesel produced (over 90%).

Yield of fatty acids methyl esters
On Figure 3, it's possible to see the result of the transesterification process, with a low amount of wastewater and glycerin.The biodiesel yield obtained (Equation 1) was high and equal to 95.19%, which  was close to the results found in the literature.Han et al. (2005) obtained a biodiesel yield of 98%, using the same raw material and operating conditions of this study.Palacios-Nereo et al. ( 2016) also performed the supercritical transesterification of soybean oil using methanol.They reached yield of 99.9%, at molar ratio 1:42 (oil:methanol), applying gradual heating (0.51 °C/min), starting from 150 °C up to 320 °C and maintaining this temperature for 10 minutes.They found out that this hybrid process prevented the reactions of thermal decomposition while transforming all triacylglycerols into biodiesel.
According to Shin et al. (2013), in the supercritical transesterification reaction, the purification of products is much simpler, since low amount of unwanted compounds is generated, and high biodiesel yield is reached in a very short time.Thus, the technique with supercritical methanol is considered more economically viable than the use of conventional catalytic reactions.
Regarding the rate of methyl ester conversion, in the sample of oil transesterified with methanol, it is observed (Table 2) that the highest concentration is of linoleate, followed by methyl hexadecanoate, oleic acid methyl ester and methyl stearate with 41.92%, 18.07%, 7.67% and 7.2%, respectively.On general, 10 compounds were identified; together they represent 81.41% of the components present after the transesterification of the soybean oil.
On recent article, regarding production of biodiesel from vegetable matrix, Xavier et al. (2011) obtained the compounds methyl hexadecanoate (32%), methyl octadecanoate (29%) and methyl (E)-octadecanoate (29%) of Caryocar villosum (Aubl.)Pers, in higher concentration after the synthesis with methanol, under normal conditions of temperature and pressure.Df these components, only methyl hexadecanoate was identified in the present work in a lower concentration.Wu et al. (2013) performed the conventional transesterification process of soybean oil using methanol and CaD/NaY catalyst.They obtained biodiesel yield of 95% with 30 wt.% of CaD loaded on NaY zeolite, molar ratio 1:9 (soybean oil:methanol), temperature of 65 °C, time of 3 h and catalyst/oil mass ratio of 3%.Liu et al. (2008) also dealt with biodiesel production from soybean oil and methanol, using CaD as a solid base catalyst.Assays at molar ratio 12:1 (methanol to oil), addition of 8% CaD, and 65 °C gave the best results (over 95%), in 3 h.

Comparison to the conventional transesterification method
On both studies, the biodiesel yield obtained was close to that reached in this work.Despite the fact that lower reaction temperatures and molar ratios were necessary, with no need of increasing pressure, they had to use base catalysts and reaction time was quite superior to that applied in this study.Also, fewer residues are generated, and the product purification is performed in fewer steps.However, it cannot be said that the supercritical transesterification is economically viable compared to the conventional technique, due to energy and equipment costs are higher than those observed in a conventional process.Thus, a comparative economic analysis is necessary.Silva & Dliveira (2014) state that the biodiesel production under supercritical conditions requires high processing costs (high molar ratios, pressures and temperatures) and some strategies to reduce the expected high operating costs usually involve the addition of cosolvents and the use of different reactor configurations with increased mass transfer, operating  in continuous mode (microreactor or packed bed reactor).Many authors such as Pohar & Plazl (2008), Hessel et al. (2005), Wen et al. (2009) recommend to conduct transesterification reactions in microreactors in order to reduce the limitations of mass and heat transfer, since such parameters tend to increase due to the small size and large contact area (Pohar & Plazl, 2008), and the small internal diameters (typically 10-300 μm) promote interaction with the reagents at molecular level.On the case of the reactor used in this study, its internal diameter (51 mm) and volume (0.095 L) represent the most suitable configuration, which was verified by the high biodiesel yield obtained (95.19%).

Influence of the reactor type used in the transesterification process
Comparing to other studies, Wang et al. (2008), for example, used a continuous tubular reactor with no mechanical blending to perform the transesterification reaction of soybean oil with methanol under supercritical conditions.Thus, in order to increase the biodiesel yield, they had to acidify the soybean oil by adding a certain amount of organic into the reaction system.At the molar ratio of 1:20 (oil:methanol), the biodiesel yield of acidified oil, and non-acidified oil were 82.3 and 60.0%, respectively.

Conclusion
The reactor used in this study proved to be efficient for the transesterification process.The reaction time was only 10 minutes, under experimental conditions of 130 bar, 280 ºC and molar ratio of raw material to methanol of 1:24, and CD 2 to methanol of 0.1.The yield of biodiesel was 95.19% and the yield of methyl esters was 81.41%, with linoleate being the compound found in the highest concentration.The results indicate that the undesirable compounds glycerin and wastewater were obtained in low concentrations, which means that this method can be applied to obtain biodiesel with no need of base catalysts and fewer steps of purification compared to the conventional method.761/773 761 Apenddix A. Equipment description 1 Autoclave characteristics

Technical datasheet and marking
Table A1 shows specifications such as brand, model, size and operating range of pressure and temperature.

General characteristics
Figure A1 shows the autoclave in its structural frame coupled to the gas supply system and stirrer with water for its cooling system.Table A2 shows the dimensions of the structure with the autoclave.

Control cabinet
From the control cabinet (Figure A2), it's possible to do: i. reading and regulation of the stirring speed; ii.temperature reading and regulation; iii.pressure reading; iv.flow measurement; v. emergency stop button.The control cabinet has the following features: • Electrical supply: 230 VAC / 50-60 Hz / 16 A • Maximum power: 3000 W

Assembly of the autoclave
For correct assembly of the autoclave it is necessary to perform the following procedures: • Check the property of the surfaces receiving the seal; • Pull of the seal and clean it; • Check the property of the threads on the body and the ring; • Put if possible high temperature grease on the threads; • The liquid should never completely fill the autoclave; • Close the autoclave and tighten with the torque wrench showed in Figure A3 (Facom/nS.306A100+ extension S.210 + socket Hex 24mm S.24H), in cross with tightening torque 20 N.m, then tightening torque 50 N.m (see Figure A4 for the detail of tightening in cross); • Make if possible a tightness test at room temperature; • Mount the insulating mantle on the top of the autoclave.
Food Sci.Technol, Campinas, 39(3): 754-773, July-Sept.2019 763/773 763 Dbservation: Never scratch contact surfaces from body or plate with a metal object, above all the effective cover area of gaskets.Never loosen or tighten up connections under pressure.

Detailsofthe autoclave
The Figure A5 shows the location of the pressure transmitter, the safety head and disc, the manual valves, the mechanical boy and the floor fixing squares.

Use of the control cabinet
To use the control cabinet is required:i.powering of box by the master switch on the rear side (Figure A6

Touchscreen general screen
The screen presents two main boxes for controlling of temperature and speed (Figure A7): • Upper-grey box: measured temperature and speed.Ot's necessary to press on the wanted box no change the settings ("A.1"); • Lower grey box: temperature and speed setting ("A.2"). Notes: • To save the data, insert the USB key before the beginning of test to begin the recording of the test data (USB connection beside the box) -No data storage without USB key.
• Press on "B" (Figure A8) to start the recording of the temperature value; • Press on "C" (Figure A8) to go to the "Warring screen"; • Press on "D" (Figure A8) to go to the "Curves screen".

Setting screen
Press on the "Settings" (Figure A8 -"E") on the general screen: • Press on the wanted box to have access to the HDUR screen (Figure A8 -"A"); • Modify the values with the boxes "+" and "-", then validate on "B" and go back to the general screen by pressing on "C" (Figure A8).

Alarm threshold
To access this function press on the settings screen (Figure A8 -"D") to reach this screen (Figure A11).
On this screen you can put alarm on different temperatures (right box in green) and measured temperatures (left box in blue).

Principles for high pressure parts
• Pipe connections: -Make sure that the connection is equivalent to the desired pressure; -Oncrease pressure by stages and check the mechanical resistance; -Never look for a leak using the hands; -Never stand in front of a connection which has just been made; -All connections shall be tested at maximum operating pressure before normal operation.
• Put a plug on all openings not used (circuit cleanliness).
• All used connections shall be referenced showing the operating pressure.
• List the internal connections; • All connecting capillary tubes shall be fitted with a whiplash prevention system.

Autoclave 2777 1000
Before carrying out maintenance, each time check that appliance is at ambient temperature and zero pressure using the pressure and temperature sensors located on the appliance.

Annual preventive maintenance
• Check the inside and outside parts under pressure to look for any damage or signs of corrosion.
• Check the safety organ (we recommend to change the safety disc).
• Check the pressure transmitter and the temperature measure loop.

Corrective maintenance
• Check the gasket faces in the autoclave in the event of a leak, and stripe impose a machining repairing.

Maintenance expected for spare parts:
The designation, size, supplier and use of spare parts are shown in the Table A3.

Bearings
• Of the stirrer becomes noisy and/or vibrates, the bearings must be changed; • Of the noise and/or the vibrations persist, the stirrer must be returned to our after-sales service; • The transversal groove on the bearing must not be in contact with the encapsulation.

Monthly preventive tightness maintenance
• D-ring seal marker 8: if the 'D' ring seal marker 8 is faulty, the gas leak will be canalized into the leak hole of the distance sleeve marker 2 (Figure A12).Use a bubbling product (water and soap) to control whether the D-ring is perfectly thight/fitting; • Seal (between part marker 2 and flat bottom): use a bubbling product (water and soap) directly on the connection (M20 x 150) between the stirrer and the autoclave head (Figure A12); • Motor, ball bearing, magnetic coupling: no maintenance.Of a problem appears, the complete stirrer must be returned to our after-sales service.

Annual preventive maintenance
• Change the bearing and the D-ring; • Check the magnetic coupling efficiency; • For maximum protection, the stirrers are designed so that the compressed air or electric motor blocks and not the magnetic coupling.Corrective maintenance • Of the stirrer becomes too noisy or vibrate, change the bearings; • Of the noise still exists, the stirrer must be returned to the SAV service for repairing.
Corrosion: The parts that are in contact with the fluid are made of stainless steel 1.4404 and 1.4980.No traces of corrosion should appear.Of this were to happen, it would be because the equipment was not suitable for application.Ot is dangerous and forbidden to use a stirrer under such conditions.

Dismounting
• Disconnect the stirrer from its power or energy supply; • Disconnect the stirrer from the autoclave; • Unscrew and remove the screw marker 6 (Figure A12); • Unscrew the motor-distance sleeve (marker 12) while holding the body (marker 2) and the ring marker 9; apply pressure slightly in order to defeat the attraction of the magnetic field (Figure A12); • Unscrew the ring number 9 while holding the body marker 12 (Figure A12); • Pull the upper body marker 11 towards the top.Remove the stirring module; • The central axis marker 9 (Figure A12) may now be removed from its housing; • The bearings (marker 1) and the D-ring (marker 8) are now accessible for checking or replacement (Figure A12).

Dry contact
On the event of an inversion of the rotation direction with a non-zero speed, the servo amplifier waits until the current is cancelled in the motor before reversing the rotation direction.

Figure 1 .
Figure 1.Dverall scheme of a generic triacylglycerol transesterification with methanol.

Figure 3 .
Figure 3. Sample obtained by supercritical transesterification of soybean oil with methanol and CD 2 .

Figure A3 .
Figure A3.Torque wrench to close the autoclave.

Figure A4 .
Figure A4.Detail of tightening in cross to correctly close the autoclave.

Figure A5 .
Figure A5.Details of the autoclave.
.a); ii.start of the general by the green button "O" (Figure A6.b) without pushing the green button, the warm up can´t start.

Table 1 .
Studies on the transesterification by supercritical method.

Table 2 .
Fatty acid methyl esters identified in the transesterified product.Composition obtained by gas chromatography coupled to mass spectrometry.

Table A2 .
Measures of the frame with autoclave.