Measurement of Physicochemical Properties during Microwave

Mar 7, 2016 - Departamento de Física, Universidade Federal de Santa Catarina, Campus Universitário Trindade Florianópolis, Santa Catarina, Brasil...
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Measurement of Physicochemical Properties during MicrowaveAssisted Acid-Catalyzed Transesterification Reactions Jonas Miguel Priebe,† Evandro L. Dall’Oglio,*,† Paulo T. de Sousa, Jr.,† Leonardo Gomes de Vasconcelos,† and Carlos Alberto Kuhnen‡ †

Departamento de Química Universidade Federal do Mato Grosso Av. Fernando Corrêa da Costa s/n, Cidade Universitária, Cuiabá, Mato Grosso Brasil ‡ Departamento de Física, Universidade Federal de Santa Catarina, Campus Universitário Trindade Florianópolis, Santa Catarina, Brasil S Supporting Information *

ABSTRACT: In this study the physicochemical properties governing ethylic biodiesel production reactions via homogeneous acid catalysis and waste Brazil nut oil were measured. The behavior of the catalyst, water, and ethanol concentrations in the liquid phase as well as the temperature in both the liquid and the vapor phases and the volumetric flow of the condensed volatile components were monitored during the reaction together with the conversion to biodiesel. The transesterification reactions were induced by microwave heating in a monomode reactor varying the input power to obtain power to sample mass ratios of 1.0− 2.25 (W/g) in reactions with 1:6, 1:12, and 1:30 oil to alcohol molar ratios. It was verified that the 1:6 stoichiometric ratio led to higher values for the volumetric flow, the temperatures in the liquid and vapor phases, and the water concentration, along with a greater degradation of the catalyst and reduction in the alcoholic component in the liquid phase. The results indicate that 1:30 is the most favorable molar ratio for the transesterification of waste oils with high FFAs content, providing lower water content and acid degradation. For waste oils, removal of the water is a precondition for the completion of the transesterification process. reactions.12−26 The high potential of microwave heating in organic synthesis is well-known, and it has been successfully employed in an array of different reactions, as demonstrated by the large number of studies recently reported.1−3 Microwave heating is a fast and easy method to obtain biodiesel through homogeneous acid-catalyzed reactions,13−16 homogeneous alkaline-catalyzed reactions,12,17−21 and heterogeneous catalyzed reactions.22−26 Microwave-assisted reactions are greatly accelerated in relation to conventional heating, and a significant reduction in the reaction time and an increase in product yield on applying microwave-assisted transesterification reactions have been reported.12−26 Microwave heating is a macroscopic effect involving the interaction of electromagnetic fields with the reaction media,2 and accordingly the characterization of continuous media is carried out based on their intrinsic dielectric properties, which involves the empirical measurement of both simple and complex materials, mainly mixtures such as reaction solutions. To optimize the reaction and to achieve a low-waste process, an understanding of the dielectric properties of the reagents is mandatory, since it aids the process design calculations and compliance with the green chemistry principles and it is also crucial for the identification of the most suitable catalysts.2,16 Recently, we reported the dielectric properties of biodiesel precursors and of the reaction mixtures during the acidcatalyzed transesterification (conventional heating) employing Brazil nut oil and sulfuric acid.27 A knowledge of the dielectric

1. INTRODUCTION Transesterification reactions are currently widely used to produce biofuel as an alternative to fossil fuels, providing solutions to address the environmental and economic issues related to the high demand for energy in the industrialized world. Biodiesel is a mixture of methyl or ethyl esters, and it can be produced from renewable biological sources such as animal fats, refined or waste vegetable oils, and algae oil using two main technologies, that is, chemical synthesis and biological production.1−5 Chemical technologies are the mainstream methods used for the industrial production of biodiesel, and in general, methanol is used in transesterification reactions to promote alcoholysis with the reaction being catalyzed by a basic catalyst.1,6,7 The products of the reaction are methyl esters (biodiesel) and glycerin, which have numerous industrial applications in the pharmaceutical, cosmetics, and food sectors. However, depending on the feedstock used to obtain biodiesel, appreciable amounts of fatty acids, water, sterols, phospholipids, odorants, and other compounds can be present which are undesirable since they can have significant negative effects on transesterification reactions employing either alkaline or acid catalysts. These compounds can result in the formation of emulsions inhibiting the separation of the biodiesel and glycerol.1,6 In the case of highly acidic waste frying oils, acid catalysts must be used, and sulfuric and hydrochloric acids are the most commonly employed catalysts in the transesterification process.1,8−11 However, acid catalysis exhibits drawbacks such as the need for higher temperatures and pressures.8,9 To overcome these drawbacks, microwave heating has been employed to induce homogeneous or heterogeneous catalyzed © XXXX American Chemical Society

Received: January 4, 2016 Revised: March 3, 2016

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Energy & Fuels properties of the components and of the reaction mixtures involved in biodiesel production contributes to the numerical solution of the electromagnetic fields and of the heat transfer during microwave heating, enabling the optimization of the process to be simplified. In most studies reported in the literature methanol was used as the solvent for biodiesel production, but ethanol also can be used, and since it is produced from renewable agricultural resources, this adds a renewable aspect to the overall transesterification process. In this regard, Brazil is the largest producer of sugar cane bioethanol in the world, and therefore, the ethanolysis of triglycerides induced by microwaves is potentially a viable alternative technology for worldwide biodiesel production. In this study, crucial issues concerning the microwaveassisted ethanolysis of waste Brazil nut oil catalyzed by sulfuric acid are addressed. Due to the appreciable loss tangents exhibited by the reaction mixtures when employing acid catalysts,27 leading to strong interaction with the microwaves, high temperatures are expected in the liquid phase, which raises questions concerning the true concentration of ethanol in the liquid and vapor phases during the reaction. In addition, the presence of water and its formation in the parallel reaction of esterification, due to the presence of free fatty acids, and the degradation of the catalyst during the reaction are factors that prevent the effective completion of the transesterification reaction.28,29 To address issues such as determining the amount of ethanol and water content in the liquid and vapor phases and the concentration of the catalyst required during the microwave-assisted ethanolysis of waste Brazil nut oil, in this study certain physicochemical properties of interest were measured considering various oil-to-ethanol molar ratios and different applied microwave power to sample mass ratios.

2. EXPERIMENTAL SECTION

Figure 1. A schematic view of the microwave heating system.

2.1. Materials. The anhydrous ethanol used in this study was purchased in a local store and H2SO4 (95−99%, SPECTRUM) was employed as the acid catalyst. The waste Brazil nut oils had water and free fatty acids (FFAs) contents of around 0.44 and 23 wt %, respectively. 2.2. Equipment. The transesterification reactions induced by microwave heating were performed in a monomode reactor specifically developed for this purpose (Figure 1). The 0.9 L nominal capacity, thermally isolated microwave reactor was machined in stainless steel (ref 304) and consists of a cylindrical resonant cavity with a height of 11.5 cm and internal diameter of 10.20 cm. The cavity is supplied with electromagnetic irradiation by an Alter SM 1150 microwave power supply (variable power 0.3−3.0 kW) and Richardson Electronics TMO 3.0 microwave magnetron head emitting at 2.45 GHz, and the magnetron valve is water cooled with a thermostatically controlled bath (DIST DI-980). The system has a directional coupler/water load (National Electronics D0937), power sensor (Alter RD8400), and waveguide tuner (Alter AG340M3), and the entire system is controlled by Front Panel 500 software. In the reacting cavity a mechanical stirrer was adapted with a Teflon paddle, and there is a connector to remove samples in the liquid phase and a condenser for the vapor phase, which makes use of a thermostatically controlled bath (Tecnal TE 184 model). The system is built to remove samples and to return the condensate to the reacting cavity. The reacting cavity and condenser were adapted with connectors to insert thermal sensors comprised of optic fibers and k-type thermocouples, respectively. The temperature in the liquid phase is measured by a Neoptix optic fiber thermometer (model Reflex RFX 378A), at five different heights starting at 5 cm from the bottom flange with intervals of 1 cm and these temperature measurements were taken at time intervals of 1 min. In the vapor phase, the temperature measurements

were carried out with a Minipa digital thermometer (MT-405) equipped with an Itest thermocouple. 2.3. Methods. 2.3.1. Volumetric Flow. In the first step in the monitoring of the composition of the reacting system, the volumetric flows of the condensed vapor of the mixtures consisting of waste Brazil nut oil and ethanol at molar ratios of 1:3, 1:6, 1:9, 1:18, 1:30, and 1:82 and the pure ethanol were measured at time intervals of 1 min. Two sets of experiments, with and without returning the volume collected to the reactor, were performed using a volume sample of 620 mL and heating with 300 W of electromagnetic power without mechanical stirrer. Measurement of the volumetric flows was performed for the transesterification reactions employing molar ratios of 1:6, 1:12, and 1:30 and 3% (V/VT) of H2SO4 under constant mechanical stirring (700 rpm) and six different power/mass ratios (see below). In this set of experiments, the volume collected was measured and then returned to the reactor. 2.3.2. Transesterification Process. The reactions were induced by microwave dielectric heating using 3% (V/VT) of H2SO4, with various oil−alcohol molar ratios and a fixed irradiation time (80 min). To perform the transesterification process, the reactor was loaded with the oil, and the catalyst was added separately to the anhydrous ethanol, which in turn was added to the reactor under mechanical stirring (700 rpm) for 3 min followed by microwave emission for 80 min. During the reaction, samples of the liquid and vapor phases were collected at time intervals of 10 min for subsequent analysis to determine the composition profiles. The reactions were performed with 1:6, 1:12, and 1:30 oil-to-ethanol molar ratios and employing six different power to sample mass ratios, that is, 1.0, 1.25, 1.5, 1.75, 2.0, and 2.25 (W/g). Data on the mass and volume of the components for each reaction and for each power/mass ratio are given in Table S1 of the Supporting B

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Figure 2. Measured volumetric flow for oil/ethanol mixtures and pure ethanol under microwave heating: (a) returning the collected volume to the reactor; and (b) without returning the collected volume to the reactor. Information. Analysis of the mixture components was performed with a CG-FID automatic injector (AOC-5000 and headspace), and water content were analyzed with a Karl Fischer Coulometric Tritino 831KF (Metrohm); the acid value was determined by potentiometric titration with a Tritino 848-Plus (Metrohm). The ethanol and ethyl esters were determined based on the European Standard BS EN 14110:2003 and the European Standard BS EN 14103:2003 and the water content and the acid value according to the standards DIN EN ISO 8534 and EN 14104, respectively. Measurements of water content were taken in triplicate, whereas acid value and ethyl esters and ethanol content, temperature in both vapor and liquid phase were taken in single measurements. The measurements of the ethanol content in the liquid phase were carried out with a precision of ±0.01%, whereas temperature measurements were performed with precision of ±0.1 °C (optic fiber thermometer) and (±)1 °C (digital thermometer).

collected in the vapor phase ranged from 32 to 35 mL, except for the lower molar ratios (1:3 and 1:6). The data for the vapor phase revealed a correlation of the temperature and the ethanol volume in vapor phase with the molar ratios of the mixtures in the liquid phase. For high molar ratios, the temperature in the vapor phase at equilibrium reaches 77 °C, while for pure ethanol the equilibrium is established at 76 °C. In the case of a low ethanol concentration in the solution, with molar ratios of 1:3 and 1:6, despite the slow temperature growth, the temperature in the vapor phase quickly reaches equilibrium values of 87 and 81 °C. Initially, it seems that the temperature in the vapor phase increases as the molar ratio decreases, while the volume of ethanol in the vapor phase decreases with a reduction in the molar ratio. However, on taking into account the volume collected in relation to the total sample volume, i.e., the corresponding volume fraction, the opposite trend is observed. The volume fraction in the vapor phase is inversely proportional to the amount of ethanol present in the medium, since this is the component with the lowest boiling point contained in this system. This occurs regardless of the power−mass ratio applied to the mixtures. The same trends are observed during the transesterification reactions (see below). The behavior of the volume of ethanol collected in the vapor phase was evaluated considering the continuous change in the composition of the oil/ethanol mixtures, and a set of experiments was performed without returning the volume collected in the vapor phase to the reactor. Figure 2b show that the profiles of the volume of each of the proportions collected exhibit similarities between them, with a maximum volumetric flow, which is followed by a gradual decrease due to the continuous decrease in the ethanol concentration of the mixture. The maximum volumetric flow of each mixture increases with ethanol content, reaching its highest value for a molar ratio of 1:82, while for pure ethanol the maximum value for the volumetric flow is equivalent to that of molar ratio of 1:9. It is clear from Figure 2b that the amount of ethanol in the vapor phase is dependent on the intermolecular forces between ethanol/ethanol molecules and the oil/ethanol molecules. The above results reveal that it is important to understand the volumetric flow during microwave-assisted transesterification reactions. In the following, the

3. RESULTS AND DISCUSSION 3.1. Volumetric Flow. The measurement of the volumetric flow of the condensed vapor can help to clarify the actual proportion of ethanol contained in the vapor phase during the synthetic process. In this context, a larger volume of sample contained in the vapor phase during the reaction corresponds to a lower alcoholic fraction present in the liquid phase, thereby making it less available to contribute to the conversion of oil into biodiesel. Thus, the volumetric flow of the vapor phase was determined by considering different stoichiometric ratios between the waste Brazil nut oil and ethanol, under microwave heating with the power maintained at 300 W. The following relationships between the emitted power and the sample mass were obtained: 0.54, 0.55, 0.56, 0.57, 0.58, and 0.60 (W/g), and the respective molar ratios were: 1:3, 1:6, 1:9, 1:18, 1:30, and 1:82. The oil/ethanol mixtures were initially homogenized (to the extent possible) and then submitted to microwave heating without mechanical stirring. Figure 2a shows the volume collected for each oil/ethanol mixture and for pure ethanol. In these experiments, the volume collected was measured and then returned to the mixture. The mixtures quickly reached equilibrium with times ranging from 5 min for pure ethanol and high molar ratios to around 15 min for small molar ratios. At equilibrium, the volumes of the mixtures and pure ethanol C

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Figure 3. (a) Volumetric flow measured during the transesterification reaction using 3% H2SO4 and oil-to-ethanol molar ratios of 1:6, applying various power to mass ratios (W/g). (b) Relative volume collected in vapor phase (see text).

Figure 4. Profiles for ethanol (measured mass/initial mass) in the liquid phase during the transesterification reaction using 3% H2SO4 and oil-toethanol molar ratios of (a) 1:6 and (b) 1:30, applying various power to mass ratios (W/g).

volumetric flow is considered on applying different emitted power-to-sample mass ratios, that is, 1.0, 1.25, 1.5, 1.75, 2.0, and 2.25 (W/g). Figure 3a show the behavior of the volumetric flow for a molar ratio of 1:6 for each power/mass ratio. For the other two molar ratios (1:12 and 1:30) the results are shown in Figures S1 and S2 of Supporting Information. As expected, for all concentrations of ethanol, the volume in the vapor phase increases as the power/mass ratio increases. Figure 3a shows that for a molar ratio of 1:6 the volume collected gradually increases, reaching a plateau, and then decreases continuously. This behavior reveals that during the initial stages of the process the concentration of volatile compound is large enough to increase until stability is reached. Nevertheless, due to the constant consumption of ethanol necessary for the reaction to proceed, which minimizes its availability in the liquid phase, a consequent decrease in the volumetric flow is observed. It can also be noted that the equilibrium stage of the collected volume is more quickly achieved for the higher power/mass ratios applied (2.0 and 2.25 W/g).

Different behavior is observed with increasing ethanol content in the liquid phase, as shown in Figures S1 and S2, where the volumetric flow increases and reaches a plateau. However, the continuous consumption of ethanol as the reaction proceeds does not show a strong influence on the quantity of volatiles in the vapor phase. Therefore, the volume collected decreases slightly after achieving the maximum plateau (Figure S1). For a higher ethanol content in the liquid phase, Figure S2 shows that the volume in the vapor phase increases until reaching its equilibrium value, which is dependent on the applied power/mass ratio. For a molar ratio of 1:30 (Figure S2) the applied power/mass ratios are restricted to the lowest values (1.0, 1.25, 1.50 W/g) due to the appearance of instability in the reaction system. As the energy is supplied to the system, an increasing amount of ethanol molecules passes into the vapor phase causing an upward movement in the medium, and these molecules carry with them other nonvolatile components. This promotes the contamination of the condensation system, and accurate measurements can no longer be obtained for these systems. D

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inversely proportional to the initial ethanol content in the reaction mixture regardless of the applied power/mass ratio, and this was more accentuated for lower molar ratios. 3.3. Temperature Profiles for Vapor and Liquid Phases. Data on the temperature of both the vapor and liquid phases during microwave heating allows a posteriori control of the process temperature to ensure, as far as possible, a uniform distribution of the temperature of the reaction mixture, aiming to avoid superheating, which can lead to undesirable side reactions. Figure 5 shows the vapor temperature profile

As observed in Figure 3a and Figures S1 and S2, the volume of volatiles collected in the vapor phase increases with the ethanol content initially present in the reaction mixture. However, the opposite behavior is observed for the relative collected volume, that is, the volume collected as a fraction of the total volume corresponding to the molar ratio employed. Figure 3b shows the relative volume for molar ratios of 1:12 and 1:30 under 1.0 W/g of applied power, where it is evident that lower molar ratios lead to a higher relative volume in the vapor phase. For other molar ratios and applied power/mass ratios the same behavior was verified, that is, the volumetric fraction of volatiles in vapor phase exhibits an inverse profile relative to the absolute volume collected (data are available in the Supporting Information, Figures S3−S14). This means that during the transesterification reactions the volumetric fraction is inversely proportionally to the ethanol content initially present in the reaction mixture and is independent of the power/mass ratio employed. It should be noted that for a molar ratio of 1:6 the volumetric fraction can become lower than those observed for the 1:12 and 1:30 molar ratios, in the final stages of the reaction, due to the consumption of ethanol during the transesterification process (see Figures S3−S14). 3.2. Ethanol Profile in Liquid Phase. The procedure to determine the ethanol content in the liquid phase consists of the addition of around 3 drops of the liquid sample to previously weighed vials containing 2 mL of biodiesel (95%). This biodiesel was produced previous by conventional heating, purified to extract the residual ethanol, and used as a standard. The results are given as the ratio between the measured mass during transesterification and the initially ethanol mass present in the reacting mixtures. For a molar ratio of 1:6, in the first 20 min a rapid decrease in the ethanol content in the liquid phase was observed, which increases with the applied power/mass ratio, as shown in Figure 4a. The decrease in the ethanol content is mainly associated with the volumetric flow, which reaches a maximum after around 20 min (Figure 3a), and the consumption of ethanol is the second contribution to this decrease, as the measured conversion to ethyl esters is of the order of 38%. As observed in Figure 4b, for higher molar ratios the decrease in ethanol, within the same time interval, is less accentuated, which is related to the reduced volumetric flow (Figure 3b) and the conversion into ethyl esters, the values for which are 28% and 38% for molar ratios of 1:12 and 1:30, respectively. In fact, even for higher applied power/mass ratios, the reduction in the ethanol content of liquid phase for a molar ratio of 1:30 is observed only in the first 20 min, as shown in Figure 4b and in Figure S15 for an oil-to-ethanol molar ratio of 1:12. After this time, the ethanol mass remains almost constant together with the maximum volumetric flow achieved, as shown in Figure S2, indicating that the consumption of ethanol during the reaction has little effect on the total mass of ethanol, since for this stoichiometry there is a large excess of alcohol. Furthermore, it was verified that in the final stages of the reaction (between 60 and 80 min) there is a very slight increase in the ethanol mass, which is more accentuated for molar ratios of 1:6 and 1:12. This slight increase in ethanol is related to the observed decrease in the conversion, which is clearly pronounced for the molar ratio of 1:6 at a power/mass ratio of 2.25 W/g, which is mainly due to the hydrolysis of ethyl esters resulting in ethanol and fatty acids, as will be discussed below. Overall, the results shown in Figure 4a and b indicate that the reduction in the ethanol mass in liquid phase is

Figure 5. Temperature profile in the vapor phase during transesterification reaction using 3% H2SO4 and oil-to-ethanol molar ratios of 1:6, 1:12, and 1:30 applying various power-to-mass ratios (W/g).

obtained in the stoichiometry studies applying various power/ mass ratios. The temperature increases very fast between 0 and 10 min, with an almost vertical ramp rate and hence the temperatures are shown in Figure 5 for times greater than 10 min to obtain a better resolution of the results. A gradual increase in the temperature was observed during the reaction, mainly for a lower molar ratio (1:6), whereas for a higher ethanol content there was less temperature variation. This behavior diverges from that observed during the boiling of ethanol using conventional heating where the temperature in the vapor phase remains constant. The fact that the temperature in the vapor phase does not reach equilibrium suggests that a continuous change in the volatile compounds composition occurs, due to an increase in the water content in this phase, since the esterification reaction in liquid phase increases the amount water in the reaction medium, as will be discussed in section 3.4 below. This behavior is corroborated by the phase diagram of the ethanol/ water binary mixture,30 where an increase in the vapor phase temperature is observed with an increase in the fraction of the less volatile component of the mixture. In fact, for a molar ratio of 1:6 the temperature changes markedly as the applied power/ mass ratio is increased, due to a pronounced reduction in the ethanol content in both the vapor and liquid phases (Figure 3a, Figure 4a). For molar ratios of 1:12 and 1:30 the change in the temperature in the vapor phase is not as great, due to the high ethanol content, and therefore the effect of increasing the water content on the vapor temperature is less accentuated. Clearly, the most important question here relates to the expected temperature profile for the liquid phase during the reaction. The change in temperature influences the progress of the E

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Figure 6. (a) Temperature profile for the liquid phase during the transesterification reaction using 3% H2SO4 and oil-to-ethanol molar ratios of 1:6, 1:12, and 1:30 applying various power to mass ratios (W/g). (b) Temperature in the liquid phase at different depths of the reaction mixture. (c) Fitting of the temperature profile for a molar ratio of 1:6 (see text).

Figure 7. (a) Water content and conversion during transesterification reaction using 3% H2SO4 and oil-to-ethanol molar ratios of (a) 1:6 and (b) 1:30 applying various power to mass ratios (W/g).

reaction, not only due to a change in the kinetics but also it promotes a considerable decrease in the oil viscosity, promoting its solubility in ethanol. Further temperature

changes in the liquid phase alter its dielectric properties, which affects the interaction of the reaction mixture with the electromagnetic field at microwave frequencies, since the F

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chemical transformation occurring in the mixture. An exponential function was selected for the fitting procedure since constant, linear, parabolic, and cubic behaviors can be taken as limiting cases of an exponential function of small values of the argument. Equation 1 establishes an implicit relation between the temperature of the mixture and its dielectric properties, which are defined primarily by the alcohol and oil content, via the molar ratio, at a given sulfuric acid concentration.27 This implicit relation can also be observed from the electromagnetic power absorbed31 by the sample P, that is, ⟨P⟩ = ωε0εef″ E2rmsV where ε0 is the vacuum permittivity (= 8.85 × 10−12 F/m), εef″ is the loss factor, and Erms is the rootmean-square (rms) of the applied oscillating electrical field (Volt/m) with frequency f (ω = 2πf) over the volume V (m3) of the sample. The volume V of the sample effectively under the action of the microwave field can be approximated by the volume defined by the penetration depth, the distance at which the amplitude of the electrical field is damped to 1/e = 0.369 of its initial value at the surface of the material. The penetration depth is related to the loss tangent by31−34 Dp = (c/ω)(2/ ε′)1/2[(1 + tan2δ)1/2 − 1]−1/2, and therefore, it is frequency and temperature-dependent. The loss tangent which is defined by tan δ = εef″ /ε′ is an important parameter in the description of the dielectric response of materials.31−34 Therefore, it is clear from eq 1 that the temperature of the reaction mixture is dependent on the dielectric properties and on the applied electrical field via the rms value. The local electrical field is dependent on the dielectric properties of the mixture, which are frequency- and temperature-dependent. This means that as the temperature increases the dielectric properties change, affecting the value of the local field and altering the temperature which, in turn, affects the dielectric properties and so on. Hence during microwave heating in the transesterification reaction it is very difficult, or even impossible, for the system to achieve an equilibrium temperature, since it is undergoing physicochemical transformations which continuously alter the dielectric properties in the continuous presence of an electromagnetic energy density. Figure 6c shows the temperature profile with the fitting curves for the lowest molar ratio used, where it can be observed that the increase in the temperature during the reaction is large. For all molar ratios employed, eq 1 provides a good fit with the temperature data, as shown in Figure 6c and in Figures S16− S18 of the Supporting Information. The α coefficients calculated from the fitting of eq 1 for the different molar ratios and for each applied power to mass ratio are given in Table 1. The values found for α, and the respective standard deviations, as well as the R2 values, demonstrate a good fitting regardless of the molar ratio or applied power, with the exception of the data for a 1:6 stoichiometry at 1.50 W/g (where the temperature profile is more disperse, Figure 6c). Hence, for the entire transesterification process, eq 1 quantifies the complex relationship between the temperature of the liquid and its temperature-dependent dielectric properties when an average electromagnetic energy density, specified by the rms value of the electrical field, is defined for the medium. Therefore, if the electrical field is not turn off during the experiment, the temperature cannot reach equilibrium, unless the heat is continuously removed. On the basis of Figure 6c, it is clear that the deviation in the temperature profile from a linear behavior is greater for a higher applied power, which is confirmed by the values in Table 1 (for the other molar ratios see Figures S16−S18). It is clear that for a higher molar ratio and applied power the argument in the

dielectric constant and loss factor are frequency- and temperature-dependent. The temperature profiles for the liquid phase can be seen in Figure 6, where the data are shown for times greater than 10 min. With regard to the temperatures in the vapor phase, in the liquid phase the growth of the temperature in the first few minutes is almost vertical. During this time interval, the conversions to ethyl esters is low, being around 15%, as shown in Figure 7a. The data in Figure 6a were obtained from measurements performed at 5 cm depth in the liquid, whereas Figure 6b shows the temperature profile at different depths of the reaction mixture. The temperature in the liquid phase is dependent on the molar ratio and applied power/mass ratio, and this dependence is more prominent at lower ethanol content where a temperature increase during the reaction of around 60 °C was observed (Figure 6a,c). In fact, for a molar ratio of 1:6 the liquid is superheated by microwave absorption after 10 min, with temperatures well above the ethanol boiling point, reaching values of around 147 °C at the end of process when applying 2.25 W/g. The temperature increase during the reaction for a molar ratio of 1:12 is around 16 °C, and the liquid maintains temperatures above 84 °C, reaching almost 100 °C at the end of reaction. For a molar ratio of 1:30 the temperature increased by around 5 °C, ranging from 80 to 85 °C. A large amount of ethanol in the mixture brings the liquid temperature close to the boiling point of ethanol. It was noted that, although the temperature can vary markedly during the reaction (Figure 6a), the temperature profiles at different points within the mixture are almost the same (Figure 6b), verifying good uniformity in terms of temperature of the liquid, which is achieved by mechanical stirring. Nevertheless, hot spots can still appear in the rotating liquid, although this is unlikely since the appearance of hot spots is expected in stationary liquids heated by a nonuniform spatial field distribution. The temperature profile in Figure 6a demonstrates the dependence of the temperature on the intrinsic properties of the mixture, such as the dielectric properties and specific heat as well as with the applied power/mass ratio. The dielectric properties of reaction mixtures are strongly dependent on the oil-to-ethanol molar ratio and the acid concentration, and they can vary appreciable during the reaction, even when the temperature is maintained constant.27 Although there are no data concerning the specific heat of the reaction mixtures investigated in this study, the temperature profiles in Figure 6a indicate nonlinear behavior, although for a high ethanol content the behavior becomes almost linear. Considering the lack of data related to the specific heat and observing the strong dependence of the liquid phase temperature on the molar ratio and power-to-mass ratio (Figure 6a), a fitting procedure was used to relate the temperature profile to the liquid properties. Therefore, the increase in the temperature during the reaction is fitting by means of the exponential function given in eq 1: T (t ) =

⎡ ⎤⎫ T0 ⎧ P ⎨1 + exp⎢αroil/eth (t − t0)⎥⎬ 2⎩ Ms ⎣ ⎦⎭ ⎪

⎪

⎪

⎪

(1)

where P is the applied power (in W), Ms is the sample mass (in g), roil/eth is the oil to ethanol molar ratio (roil/eth = 1/6, 1/12, 1/ 30), and T0 is the temperature at t = t0 = 10 min. The constant α is determined by the fitting procedure and is related to the thermal properties of the solution hence incorporating its specific heat, which changes during the reaction, due to the G

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reported for pure substances such as ethanol (2.43 J/g °C), glycerol (2.40 J/g °C), olive oil (1.88 J/g °C), and sulfur acid (1.37 J/g °C). The low values found of the estimate specific heat at a molar ratio of 1:6 and at high power to mass ratio are related to the low ethanol content in the liquid phase as the reaction proceeds, as verified in section 3.2 (Figure 4a). Clearly, under continuous microwave heating, the temperature of the reaction mixtures cannot reach equilibrium, and their specific heat varies continuously since their composition varies with the consumption of ethanol to give ethyl esters and the production of glycerol and with the formation of water due to the esterification of FFAs and finally through acid degradation, as we shall see below in sections 3.4 and 3.5. Temperature monitoring under continuous microwave heating is crucial to gaining an understand of the key role of temperature when explaining the effectiveness of microwave heating, which exhibits a considerable reduction in the reaction times compared with those of conventional heating.13−26 It should be noted that during these experiments the microwave irradiation was not turned off, since the aim was to monitor and not control the temperature. Controlling the temperature by continuously switching on/off the electrical field in the solution is not an appropriate approach to investigating the effects of microwave heating during the reaction, which is characterized by continuous changes in the composition as the reaction proceeds. The higher temperatures reached for the mixture with low ethanol content directly affects the conversion to biodiesel, as demonstrated in Figure S19, where it can be observed that for a molar ratio of 1:6 the superheated liquid exhibits a conversion equivalent to that using a ratio of 1:30. In the first system (1:6) the small number of ethanol molecules is compensated by the higher temperatures, and in the second (1:30) the relatively lower temperature is compensated by the large number of ethanol molecules. Nevertheless, it can be observed in Figure S19 that for a molar ratio of 1:6 the conversion decreases after 70 min, which is caused by the presence of water in the system (as discussed in the next section). The distinct temperature profiles in Figure 6a reflect different dielectric properties of the mixtures, which affect the penetration depth of the electric field in the medium, among other parameters. To exemplify this, for mixtures with a molar ratio of 1:90, the penetration depth is around 1 cm, whereas for a molar ratio of 1:6 the penetration depth of the mixture at 2.45 GHz during the reaction varies from 5.6 to 11.6 cm.27 Therefore, the volume of the sample under effective microwave heating can differ considerably depending on the ethanol content, which in turns leads to different temperature profiles being observed for the liquid. The higher liquid temperatures noted during the reaction can partially explain the large differences between the reaction times when conventional and microwave heating are used. Acidcatalyzed transesterification reactions employing conventional heating are very slow, requiring reaction times of the order of 30−60 h.8,9,27,35 The ethanolysis of refined Brazil nut oil (1:6 molar ratio) using 3% (V/VT) of H2SO4 under conventional heating,27 for example, requires 14 h to achieve near 100% of conversion, while the ethanolysis of refined maize oil using the same molar ratio and catalyst concentration under microwave heating affords around 90% of conversion in less than 60 min.16 In this study, the same conditions applied to waste Brazil nut oil afforded around 80% of conversion in 80 min employing molar ratios of 1:6 or 1:30 using 2.25 W/g as the power/mass ratio. It is not clear whether the results for the measured temperatures

Table 1. Calculated Parameters for the Fitting of the Temperature Profile for the Reaction Mixturesa molar ratio 1:6 P/Ms (W/g) 1.50 1.75 2.00 2.25

−1

−3

α(min ) × 10 7.45 (10.8) 10.97 (13.9) 13.31 (17.69) 25.61 (40.30)

P/Ms (W/g)

α(min−1) × 10−3

1.00

24.8 (23.3) 24.0 (27.0) 22.6 (24.0) 16.0 (17.8) 22.6 (24.2) 22.3 (26.4)

1.25 1.50 1.75 2.00 2.25

SD × 10−3

R2

1.00 0.777 (1.76) (0.836) 0.50 0.957 (1.00) (0.965) 1.0 0.879 (2.64) (0.862) 1.10 0.945 (4.47) (0.918) molar ratio 1:12 SD × 10−3

R2

1.6 0.875 (2.85) (0.903) 0.53 0.987 (1.16) (0.987) 1.20 0.926 (2.48) (0.929) 0.48 0.978 (1.20) (0.968) 0.81 0.961 (1.45) (0.975) 0.62 0.979 (1.47) (0.978) molar ratio 1:30

Cp (J/g °C) 1.92 (1.32) 1.30 (1.03) 1.01 (0.76) 0.55 (0.35) Cp (J/g °C) 1.05 (1.12) 1.07 (0.95) 1.14 (1.07) 1.63 (1.46) 1.12 (1.05) 1.16 (0.98)

P/Ms (W/g)

α (min−1) × 10−3

SD × 10−3

R2

Cp (J/g °C)

1.00

7.60 (7.28) 8.73 (7.86) 8.47 (6.47) 11.97 (9.39) 8.88 (6.47) 8.54 (6.17)

0.2 (0.65) 0.45 (1.6) 0.36 (1.17) 0.26 (0.84) 0.47 (1.39) 0.45 (1.30)

0.982 (0.986) 0.937 (0.930) 0.947 (0.961) 0.987 (0.989) 0.928 (0.923) 0.932 (0.924)

5.67 (5.92) 4.98 (5.53) 5.07 (6.64) 3.57 (4.55) 4.82 (6.64) 5.02 (6.96)

1.25 1.50 1.75 2.00 2.25 a

SD: standard deviation; R2: Adj. R-Squared. The values obtained with a linear fitting are shown in parentheses (see text).

exponential becomes increasingly smaller and the exponential can be expanded as a linear function, T(t) − T0 = αT0roil/eth(P/ 2Ms)(t − t0), where the reciprocal of αT0roil/eth plays the role of the specific heat multiplied by the mass of the sample, with α divided by 60 to convert from minutes to seconds. The results obtained with a linear fitting are given in parentheses in Table 1. Table S1 shows that the sample masses for molar ratios of 1:6, 1:12, and 1:30 are 263.97 g, 328.52 g, and 512.46 g, respectively. From the results obtained for the fitting, it is possible to infer an estimation of the specific heat, which is in fact an estimate of the average value, since the composition of the system varies continuously during the transesterification. The estimated values in Table 1 indicate that the average specific heat does not differ appreciable for molar ratios of 1:6 and 1:12, whereas increasing the ethanol content increases the estimated specific heat to values higher than that for pure ethanol. The estimate values obtained for the average specific heat of the reaction mixture can be compared with the values H

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during the esterification of FFAs, which are present in high concentrations in waste frying oils. In the present case, the initial water content was 0.44 wt %, and the oil had 23 wt % of FFAs in its composition. Considering the various power values applied, the conversions obtained for the different stoichiometries (Figure 7) show a greater similar between molar ratios of 1:6 and 1:30, with the intermediate 1:12 stoichiometry (Figure S20) affording a lower degree of conversion. For instance, Figure 7b exemplifies the similarity between the conversions for molar ratios of 1:6 and 1:30. Thus, equivalent amounts of water are formed by esterification; however, on comparing Figures 7a and b, it can be noted that the amounts of water in the vapor and liquid phase differ for these cases. For high ethanol content, the amounts of water in the two phases are almost equal, and the temperature of the liquid phase is lower (80−85 °C); however, for low ethanol content, the temperature of the liquid phase is higher (90−150 °C), and the amount of water in vapor phase is four to six times that found in the liquid phase. The behavior observed for the water concentration in Figure 7a and b denotes clearly the diluting effect as the ethanol content in the reaction mixture increases. It is known that another source of water is alcohol dehydration due to the activity of the acid catalyst under microwave irradiation, but this aspect was not investigated herein.9,15,16 Figure 7a and b reveal that the formation of water contributes considerably to inhibiting the completion of the transesterification reaction as its presence causes the hydrolysis of ethyl esters28,29 resulting in ethanol and fatty acids and it also contributes to catalytic deactivation. Therefore, a first step in the optimization of the entire process is to control the water content in the mixture through its removal during the process. Due to the strong affinity between the catalyst and water, the behavior of the acid value during the transesterification is addressed in the next section. 3.5. Acid Value. By definition, the catalyst concentration must not change during the reaction, and its availability must be the same at the beginning and at end of the reaction. Figure 8 shows the behavior of the catalyst concentration, relative to its initial concentration, during the transesterification reaction for the considered stoichiometry and different applied powers. For the lower stoichiometry, it was verified that a high power/mass ratio leads to a strong decrease in the acid number as the

of the reaction mixtures (Figure 6a), i.e., the fact that the liquid is superheated during the reaction under continuous microwave irradiation, can alone explain the large differences between the reaction times when compared with conventional heating. Thus, in an investigation on the effectiveness of microwave heating aimed at verifying whether or not there are athermal effects, experiments need to be carried out at a fixed temperature, but the heat must be removed rather than applying the intermittent exposure of the reaction mixture to microwave irradiation to control the temperature, as reported recently.35 In this regard, simultaneous microwave heating/ cooling technology is currently available, where microwave power applied to the reaction mixture is accompanied by cooling from the outside with compressed air or nitrogen, thereby preventing overheating by continuously removing latent heat. Published reports on studies applying this technology have contributed to the development of the concept that simultaneous cooling and dielectric heating of reaction mixtures leads to an enhancement of the overall process36−40 although some controversy still remains.41−45 Applying an intermittent emission of microwave irradiation to control the temperature leads to an equilibrium configuration that mainly comes from heat transfer and not electromagnetic energy absorption, and therefore no appreciable differences will be verified between microwave and conventional heating, as in fact has been reported.35 Not only the type of exposure to microwave irradiation (continuous or intermittent) but also the total irradiation time is an important and limiting factor in microwave-assisted biodiesel production, both in acid and alkaline-catalyzed transesterification reactions. In a study by Refaat and El Sheltawy,19 for example, on the alkaline transesterification of refined and waste vegetable oils (6:1 methanol−oil molar ratio and 1% of KOH) yields of 99.63% were obtained in a system submitted to microwave irradiation for 120 s; however, on exceeding this time a considerable decrease in the biodiesel yield was observed. 3.4. Water Content in Vapor and Liquid Phases. A limiting factor of the transesterification process used in biodiesel production using a feedstock with a high FFAs content is the amount of water present in the reaction mixture. It is important to monitor this parameter since its increase, due to the esterification process, amplifies the possibility of hydrolysis occurring, reducing the conversion to ethyl esters. Figure 7 shows the water content in the vapor and liquid phases during transesterification for the molar ratios studied together with the conversion for each case. Throughout all of the reactions, the water content in the vapor phase was greater than that in the liquid phase regardless the applied power. On employing a molar ratio of 1:6, a marked increase in the water content of the vapor phase during the reaction was observed, while the water content of the liquid phase continuously decreased due to the high temperatures observed for the liquid during the transesterification, mainly when a high level of power was applied (Figure 6a). The same trends were observed for a molar ratio of 1:12, where an increase in the water content of the vapor phase was also noted, while for a molar ratio of 1:30, the changes in the water content are more subtle in both the vapor and liquid phases. With a low ethanol content a marked increase in the water content in the vapor phase was observed, which explains the temperature increase in this phase for a 1:6 stoichiometry, as shown in Figure 5. Clearly, the total amount of water in the vapor and liquid phases originates from that initially present in the reaction mixture and that formed

Figure 8. Acid value during transesterification reaction using 3% H2SO4 and for oil-to-ethanol molar ratios of 1:6, 1:12, and 1:30 applying various power-to-mass ratios (W/g). I

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the system with an increase in the applied power, although there were lower amounts of water in the liquid phase compared with the other stoichiometries. It was also noted that, for all applied power/mass combinations, with a molar ratio of 1:6 there was a decrease in the conversion after 70 min. This indicates that after this time interval the system had reached equilibrium and the decrease in the conversion was the result of the hydrolysis of biodiesel promoted by the presence of high concentrations of water in the acidic medium. The decomposition of the acid and decrease in the ethanol content of the liquid phase may also help to decrease significantly the conversion rate. The lowest conversion rates, considering the three stoichiometries, were obtained for the molar ratio of 1:12. For this stoichiometry the water content and acid values exhibited intermediate values, however, this intermediate behavior was not desirable for the process optimization, since poor conversions were obtained under these conditions. In summary, considering the various parameters involved in the optimization, the results presented here demonstrate that the molar ratio of 1:30 leads to higher conversions for all power/ mass ratios employed. This suggests that the process is governed by the greater availability of the alcohol molecules in the medium, as well as the presence of active catalytic ions in higher concentrations, since the deactivation promoted by the solvation of the catalyst is minimized by the relatively lower water concentration in the system. It is clear that more data on the parameters that may affect the conversion via the acid route need to be acquired. Information concerning the contamination of biodiesel with components resulting from glycerin degradation in the presence of sulfuric acid, or even due to parallel addition, etherification, sulfonation, and sulfation reactions or the formation of anhydrides, will contribute to the search for a complete optimization of the transesterification process.

reaction proceeds and at the end of the process the catalyst concentration was reduced to a few percent of its initial value. The catalytic deactivation was less accentuated for the molar ratio of 1:12 with a slight decrease in the catalyst concentration when applying 1.00 to 1.75 W/g and a rapid decrease in the first 10 min followed by stabilization when the reaction was submitted to 2.0 and 2.25 W/g. For the molar ratio of 1:30 there was a fast decrease in the concentration of the catalyst in the first 10 min, reaching around half of its initial value, followed by an approximate stabilization of its value until around 80 min of reaction, when a further decrease was observed. Clearly, the marked reduction in the catalyst concentration represents a severe restriction regarding the completion of the transesterification process and conversions greater than 80% cannot be achieved, as shown in Figure 7a and b. The issue of a decrease in the amount of catalyst in the reaction mixture can be addressed by considering the sulfuric acid degradation in the presence of water and glycerol induced by microwave irradiation. Glycerol is denser than the triglycerides/biodiesel and does not mix with the ethyl esters formed during the reaction, and although the mixtures are submitted to mechanical stirring to obtain better miscibility of their components, glycerol deposition can be observed at the end of the process. During the deposition it is possible that glycerol drags the sulfuric acid with it, and in the presence of microwave heating, a side-reaction could occur leading to SO2, acrolein, and other undesirable byproducts, contributing to decreasing the efficiency of the process and adding impurities to the biodiesel. The results obtained from the measurement of the various parameters that govern the transesterification process, such as the amount of ethanol in the liquid and vapor phases and the temperature of both phases, as well as the water and acid content, contribute considerably to the optimization of the biodiesel production process via the acid route employing microwave heating. Initially, we considered that the higher the proportion of alcohol the higher the conversion into biodiesel will be, since the excess alcohol displaces the equilibrium to the products of this reaction. However, as can be seen from the results obtained in this study, other factors play an important role in overcoming the effect promoted by a high alcohol content, as observed using a lower molar ratio (1:6) where the conversions were similar to those obtained with a greater excess (1:30) of ethanol. While in the case of the 1:6 ratio the reaction was largely driven by the high temperatures of the reaction medium, the 1:30 stoichiometry offers a high concentration of ethyl alcohol as well as a reduced contents of agents which inhibit the achievement of higher reaction rates through, for instance, a reduction in the catalytic activity and an increase in the amount of water in the system. The water, in turn, presents an even lower deactivation effect by enhancing the interaction of the acid with ethanol molecules, which are present in greater proportions, hindering the interaction of the aqueous fraction with the acid and causing its concentration to remain almost constant during the reaction (Figure 8). The decrease in protonic species in free form in solution does not seem to represent a sufficiently negative contribution in terms of overcoming the kinetic effects caused by the high temperature (80−85 °C) of the 1:30 mixture. Despite the high conversions obtained employing a molar ratio of 1:6, for these mixtures the greatest decreases in the concentrations of acid and ethanol were obtained and also the highest concentrations of water in

4. CONCLUSIONS Parameters such as the ethanol volume in the vapor phase, mass of ethanol in the liquid phase, temperature of both the liquid and vapor phases, water content, and acid value during the microwave-assisted acid-catalyzed transesterification reaction of waste Brazil nut oil were measured. It was verified that the volumetric fraction of volatiles in vapor phase exhibits an inverse profile relative to the absolute volume collected meaning that during the transesterification reactions the volumetric fraction is inversely proportionally to the ethanol content initially present in the reaction mixture and is independent of the power/mass ratio employed. The decrease in the ethanol content in the liquid phase was measured, and it increases with the applied power/mass ratio, mainly for low ethanol content. The decrease in the ethanol content is mainly associated with the volumetric flow, and the consumption of ethanol is the second contribution to this decrease. The temperature in the liquid phase is dependent on the molar ratio and applied power/mass ratio, and this dependence is more prominent at lower ethanol content where a temperature increase during the reaction of around 60 °C was observed (ranging from 90 to 147 °C). The temperature increase during the reaction for a molar ratio of 1:12 is around 16 °C, and the liquid maintains temperatures above 84 °C, reaching almost 100 °C at the end of reaction. For a molar ratio of 1:30 the temperature increased by around 5 °C, ranging from 80 to 85 °C. Due to the high content of FFAs, the measurements revealed that water content increases during the reaction due to J

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(2) Motasemi, F.; Ani, F. N. A review on microwave-assisted production of biodiesel. Renewable Sustainable Energy Rev. 2012, 16, 4719−4733. (3) Gude, V. G.; Patil, P.; Martinez-Guerra, E.; Deng, S.; Nirmalakhandan, N. Microwave energy potential for biodiesel production. Sustainable Chem. Processes 2013, 1, 5. (4) Ebenezer, A. V.; Arulazhagan, P.; Kumar, S. A.; Yeom, I. T.; Banu, J. R. Effect of deflocculation on the efficiency of low-energy microwave pretreatment and anaerobic biodegradation of waste activated sludge. Appl. Energy 2015, 145, 104−110. (5) Martinez-Guerra, E.; Gude, V. G.; Mondala, A.; Holmes, W.; Hernandez, R. Microwave and ultrasound enhanced extractivetransesterification of algal lipids. Appl. Energy 2014, 129, 354−363. (6) Rashid, U.; Anwar, F. Production of biodiesel through optimized alkaline-catalyzed transesterification of rapeseed oil. Fuel 2008, 87 (3), 265−273. (7) Singh, A. K.; Fernando, S. D. Transesterification of soybean oil using heterogeneous catalysts. Energy Fuels 2008, 22 (3), 2067−269. (8) Freedman, B.; Butterfield, R. O.; Pryde, E. H. Transesterification kinetics of soybean oil. J. Am. Oil Chem. Soc. 1986, 63 (10), 1375− 1380. (9) Canakci, M.; Van Gerpen, J. Biodiesel production via acid catalysis. Trans. ASAE 1999, 42 (5), 1203−1210. (10) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Biodiesel production from high FFA rubber seed oil. Fuel 2005, 84 (4), 335− 340. (11) Zheng, S.; Kates, M.; Dubé, M. A.; Mclean, D. D. Acid-catalyzed production of biodiesel from waste frying oil. Biomass Bioenergy 2006, 30 (3), 267−272. (12) Azcan, N.; Danisman, A. Microwave assisted transesterification of rapeseed oil. Fuel 2008, 87, 1781−1788. (13) Patil, P.; Gude, V. G.; Reddy, H. K.; Muppaneni, T.; Deng, S. Biodiesel production from waste cooking oil using sulfuric acid and microwave irradiation processes. J. Environ. Prot. 2012, 3 (1), 107− 113. (14) Lin, C.-C.; Hsiao, M.-C. Optimization of biodiesel production from waste vegetable oil assisted by co-Solvent and microwave using a two-step process. J. Sustainable Bioenergy Syst. 2013, 3 (1), 1−6. (15) Dall’Oglio, E. L.; de Souza, P. T., Jr.; Gomes de Vasconcelos, L.; Parizotto, C.; Barros, E. F.; Kuhnen, C. A. Monitoring of microwaveassisted acid-catalyzed transesterification for biodiesel production using FT-NIR spectroscopy with continuous-flow cell. Current Microwave Chemistry 2016, 3 (1), 1−10. (16) Dall’Oglio, E. L.; de Sousa, P. T., Jr.; Campos, D. C.; Gomes de Vasconcelos, L.; da Silva, A. C.; Ribeiro, F.; Rodrigues, V.; Kuhnen, C. A. Measurement of dielectric properties and microwave-assisted homogeneous acid-catalyzed transesterification in a monomode reactor. J. Phys. Chem. A 2015, 119 (34), 8971−8980. (17) Chee Loong, T.; Idris, A. Rapid alkali catalyzed transesterification of microalgae lipids to biodiesel using simultaneous cooling and microwave heating and its optimization. Bioresour. Technol. 2014, 174, 311−315. (18) Groisman, Y.; Gedanken, A. Continuous flow, circulating microwave system and its application in nanoparticle fabrication and biodiesel synthesis. J. Phys. Chem. C 2008, 112 (24), 8802−8808. (19) Refaat, A. A.; El Sheltawy, S. T. Time factor in microwaveenhanced biodiesel production. WSEAS Transact. Environ. Develop. 2008, 4, 279−288. (20) Barnard, T. M.; Leadbeater, N. E.; Boucher, M. B.; Stencel, L. M.; Wilhite, B. A. Continuous-flow preparation of biodiesel using microwave heating. Energy Fuels 2007, 21 (3), 1777−1781. (21) Kumar, R.; Kumar, G. R.; Chandrashekar, N. Microwave assisted alkali-catalyzed transesterification of Pongamia pinnata seed oil for biodiesel production. Bioresour. Technol. 2011, 102, 6617−6620. (22) Dall’Oglio, E. L.; de Souza, P. T., Jr.; de Jesus, P. T.; Gomes de Vasconcelos, L.; Parizotto, C.; Kuhnen, C. A. Use of heterogeneous catalysts in methylic biodiesel production induced by microwave irradiation. Quim. Nova 2014, 37 (3), 411−417.

the esterification process. It was observed that the formation of water contributes considerably to inhibiting the completion of the transesterification reaction as its presence causes the hydrolysis of ethyl-esters. In this work, conversions greater than 80% cannot be achieved due to formation of water and due to the reduction in the catalyst concentration, which represents a severe restriction regarding the completion of the transesterification process. It was verified that for the molar ratio 1:6 a high power/mass ratio leads to a strong decrease in the acid number as the reaction proceeds. In this case, at the end of the process, the catalyst concentration was reduced to a few percent of its initial value. The catalytic deactivation was less accentuated for the molar ratio of 1:12, whereas for the molar ratio of 1:30, there was a fast decrease in the concentration of the catalyst in the first 10 min, reaching around half of its initial value followed by an approximate stabilization of its value. The results reveal that 1:30 is the most favorable molar ratio for the transesterification of waste oils with high FFAs content under microwave heating, affording high conversions with lower water content and acid degradation. For these waste oils, it is demonstrated herein that water removal is a precondition for the completion the transesterification process.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b03024. Table S1: Data concerning reaction mixtures. Figures S1−S14: Measured volumetric flow during transesterification reaction. Figure S15: Profile for ethanol in the liquid phase during the transesterification reaction. Figures S16−S18: Fitting of the temperature profile in the liquid phase during transesterification reaction. Figure S19: Temperature profile and conversion to biodiesel. Figure S20: Water content and conversion during transesterification reaction. Tables S2−S4: Ethanol (measured mass/initial mass) in the liquid phase during Transesterification reaction. Tables S5−S7: Water content in vapor and liquid phase during transesterification reaction. Tables S8−S10: Acid value during transesterification reaction (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (55) 65 36158798. E-mail address: dalloglio.evandro@ gmail.com (E. L. Dall’Oglio). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank CNPq (National Council for Scientific and Technological Development − Brazil), FINEP (Funding Agency for Studies and Projects- Brazil), CPP (Pantanal Research Centre) and CAPES (Coordination for the Improvement of Higher Education - Brazil) for financial support.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b03024 Energy Fuels XXXX, XXX, XXX−XXX