Microchemical Journal 109 (2013) 46–50

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Microchemical Journal

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Monitoring the conversion of soybean oil to methyl or ethyl esters using therefractive index with correlation gas chromatography

Regina. C. R. Santos, Rômulo B. Vieira, Antoninho Valentini ⁎Langmuir — Laboratório de Adsorção e catálise, Departamento de Química Analítica e Físico-Química, Universidade Federal do Ceará, Fortaleza, Ceará, Brazil CEP: 60.451.970

⁎ Corresponding author. Tel.: +55 85 3366 9951; faxE-mail address: valent@ufc.br (A. Valentini).

0026-265X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.microc.2012.05.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 November 2011Received in revised form 16 April 2012Accepted 2 May 2012Available online 9 May 2012

Keywords:BiodieselTransesterificationSoybean oilRefractive indexGas chromatography

A simple method (refractive index) was applied to monitor the progress and the end point of the trans-esterification reaction of soybean oil to biodiesel (methyl or ethyl esters). Additionally, the same methodmay be used to determine the methyl or ethyl ester content during the transesterification reaction progress.To do so, blends of biodiesel and soybean oil were prepared at different wt.% to obtain a simple linear corre-lation with the refractive index and a correlation coefficient (R2) of 0.9997 and 0.9996 for the FAMEs andFAEEs, respectively. The transesterification process of soybean oil with methanol and ethanol was performedto determine how the refractive index properties change due to the ratio of conversion. It was concluded thatin the reaction kinetics of the methanolysis reaction, the efficiency was over 90% in 8 h. Compared with exis-ting chromatographic techniques, the refractive index method for monitoring the transesterification of veg-etable oils presented good results. Additionally, the method was rapid, inexpensive and especially suitable forprocess control applications.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

With the increasing energy crisis due to fossil fuel depletion andenvironmental degradation, considerable effort has been devoted tothe development of alternative fuels [1]. One of the most promisingalternative diesel fuels is biodiesel, which is derived from the trans-esterification reaction of vegetable oils or animal fats with alcohol oflower molecular weights, such as methanol or ethanol, using homo-geneous or heterogeneous catalysts (acid, base, or enzyme) [2–4].The primary advantages of using biodiesel are that it is biodegradableand nontoxic, can be used without modifying existing engines, andproduces less harmful gas emissions such as SOx, CO, unburned hy-drocarbons, and particulate material [5,6].

However, incomplete transesterification and/or insufficient purifi-cation that results in even a small amount of the original unconvertedoil compounds getting into the final methyl or ethyl ester product cancause severe operational problems, such as engine deposits, whichcan increase the production of hazardous emissions [1–3,7]. Hence,monitoring the conversion of vegetable oils to methyl or ethyl estersis necessary to assess the quality of biofuels.

Different methods have been used for determining or verifying theconcentration of biodiesel obtained by the transesterification of vege-table oils. These include techniques such as 1H NMR spectroscopy[8–10], infrared (IR) spectroscopy [10,11], high-performance liquid

: +55 85 3366 9982.

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chromatography (HPLC) [10,12] and gas chromatography [13,14].However, gas chromatography (GC) [14,15] is the analytical methodthat is typically used for evaluating FAME conversion and qualityaccording to European standard specifications based on the EN in14103.

Gas chromatography (GC) is a very sensitive analytical method thatmeasures by-products such as monoglyceride (MG), diglyceride (DG)and unreacted triglyceride (TG), in addition to fatty acid methyl ester(FAME). Nevertheless, this method has some drawbacks, such as requir-ing very accurate sample preparation. For example, prior to analysis bytrimethylsilylation of the free hydroxyl groups inMGs andDGs, the sam-ple must be derivatised. Furthermore, the analysis is time-consuming,and the method requires expensive instruments with highly trainedpersonnel. Because of these requirements, “on-line” application in atransesterification factory would be very difficult [13–15]. Thus, lowcost and rapid analytical methods are desirable.

An alternative non-destructive, rapid analytical technique thatdoes not require sample pre-treatment involves measuring refractiveindices; this method has received little attention for use in monitor-ing the quality of biodiesel [16,17]. Because the possible componentsof the reaction mixture, i.e., MG, DG, TG, and methyl or ethyl esters,have different refractive indices, which change from base oil to bio-diesel properties, they can be correlated to the mixture compositionto give an indication of the conversion rate of the transesterificationreaction.

In the present study, we investigated the use of refractive indexmeasurements for monitoring a transesterification reaction process.This method is simple, rapid, and inexpensive. Thus, it is especially

47R.C.R. Santos et al. / Microchemical Journal 109 (2013) 46–50

suitable for process control applications. As a means of correlatingand cross-checking the results with another analytical method, gaschromatography (GC) was selected. Thus, we performed a series oftransesterification reactions using refined soybean oil with methanoland ethanol, and the methyl or ethyl ester content of the sampleswere determined by the refractive index and gas chromatography.

2. Experimental

2.1. Materials

Refined bleached deodorised soybean oil was purchased fromlocal stores, and its physicochemical properties are illustrated inTable 1. The reagents used during synthesis and purification proce-dures were methanol (99.9%), anhydrous ethanol (99.8%), sodiumhydroxide (97%) pellets, and hydrochloric acid (37%). Methyl andethyl laurate (methyl or ethyl dodecanoate, 98.0% and 95.5%, respec-tively) were used as an internal standard for GC analysis.

2.2. Synthesis of the methylic and ethylic biodiesel and soybean oil blendsfor the standard correlation

To obtain a correlative equation among the refractive indices andthe ester content, mixtures of soybean oil with methylic or ethylicbiodiesel at different proportions were prepared, and the refractiveindex was measured in each sample. The biodiesel used in these mix-tures was synthesised by repeating an alkaline transesterification re-action twice, using 0.6% (w/w) or 0.8% (w/w) sodium hydroxide as acatalyst for methanol (FAME — fatty acid methyl ester) or ethanol(FAEE — fatty acid ethyl ester), respectively. The alcohols were usedin excess at 100% vol. The reaction temperatures were 25 °C and60 °C for the methylic and ethylic routes, respectively, with a reactiontime of 1 h. The transesterification reaction was performed in a 250-mL three-neck flat-bottom flask, equipped with a reflux condenser,a thermometer and magnetic stirring. The reactor was initiallycharged using 50 g of soybean oil and heated to the reaction temper-ature. Subsequently, the above mentioned amount of catalyst wasdissolved in alcohol and added to the reactor, marking the beginningof the reaction. After the end of the reaction period (1 h), the glycerolphase was discarded, and the same amount of alcohol and catalystused in the previous step was added, and the procedure was repeat-ed. Finally, the resulting biodiesel sample was washed with distilledwater at least three times to remove the remaining catalyst and glyc-erol; the water volume corresponded to 10 wt.% of the biodiesel.However, for the first washing, an HCl (0.1 N) solution was used toneutralise the alkali catalyst. The product was heated to 105 °Cunder vacuum for the subsequent analyses. The main properties ofthe biodiesel were evaluated and are shown in Table 1.

In the next step, soybean oil and biodiesel mixtures were preparedin different weight proportions, as follows: B0: 100% soybean oil, B15:15% biodiesel+75% oil, B30: 30% biodiesel+60% oil, B45: 45% biodie-sel+55% oil, B60: 60% biodiesel+40% oil, B70: 70% biodiesel+30%

Table 1Soybean oil and biodiesel properties.

Property Units Soybeanoil

FAME FAEE Europeanspec.

Acid value mg KOH/g 0.099 0.48 0.29 0.5 max.Kinematic viscosityat 40 °C

mm2/s 30.1 4.2 4.5 3.5–5.5

Density at 20 °C g/cm3 0.9237 0.881 0.876 0.86–0.9Refractive indexat 40 °C

1.4680 1.4498 1.4480 –

Fatty acid esterscontent

% mass – 96.54 98.2 96.5

oil, B85: 85% biodiesel+15% oil and B100: 100% biodiesel. The refrac-tive index of these mixtures was measured, and the correlative equa-tion for the refractive index with the ester content was fitted andevaluated. The blends were prepared by weight because the weightfraction does not change with temperature.

2.3. Product analysis (GC analysis for fatty acid esters)

Biodiesel analyses (fatty acid methyl esters, FAME and fatty acidethyl esters, FAEE) were performed using a gas chromatographequipped with a flame-ionisation detector and a capillary non-polarcolumn measuring 30 m in length, 0.25 mm in internal diameter,and 0.25 μm in film thicknesses. The column temperature programwas as follows: initial temperature of 135 °C, hold for 3 min, andramp at 15 °C/min up to 230 °C, hold at 20 min. Nitrogen was usedas the carrier gas at 1.0 mL/min. The temperatures of the injectorand detector were 250 °C.

The preparation of the sample for analysis and quantification wasperformed following standard method EN 14103 [18] using methyl orethyl laurate as an internal standard. The injection was performed insplit mode with a split ratio of 80:1 and sample size of 1.0 μL. The bio-diesel yield (per cent FAMEs or per cent FAEEs) was calculated usingEq. (1):

FAMEs or FAEEs ¼ AesterCEIVEI f ester laurate=AEImð Þx PEI ð1Þ

where, Aester is the peak area of FAMEs or FAEEs, CEI and VEI are theconcentration and volume of the methyl or ethyl laurate solution, re-spectively, f methyl or ethyl laurate is the response factor, AEI and PEI arethe peak area and purity (wt.%), respectively, of the internal standardand m is the mass of the sample.

2.4. Physicochemical properties' measurement of soybean oil andbiodiesel

The refractive index was measured to an accuracy of 10−4 with anABBE refractometer at 40 °C, which was thermostatically controlledby bath to maintain the temperature, in keeping with ISO 6320. Themeasurements were conducted three times for each sample, and theresults were averaged. Physicochemical analyses of the soybean oiland biodiesel were performed according to ASTM methods D4052and D445 for density and kinematic viscosity, respectively, andAOCS Cd 3d-63 for the acid value [19–21]. The values obtained forthese properties were compared to the European specifications (EN‐14214), for which the accepted values fall between 0.860 and0.900 g/cm3 for density, 3.5 and 5.5 mm2/s for viscosity, and ≤0.5 mgKOH/g for the maximum acid value.

2.5. Synthesis of the methylic and ethylic biodiesel for the evaluation ofthe method

To obtain real data from the soybean oil transesterification reac-tion progression, a new process of transesterification to methyl estersand ethyl ester was performed; however, a heterogeneous catalyst(dolomite) was employed [22]. It is known that a homogeneous pro-cess is faster than a heterogeneous process [23]; thus, this methodmakes it possible to obtain mixtures with compositions that arevery different from that of the standard in Section 2.2.

Methyl esters were produced at different reaction times (1, 2, 4, 8,16 and 24 h), at 50 °C and with a catalyst amount of 0.6 wt.% and amethanol/oil molar ratio of 6. Ethyl ester synthesis was performedat different temperatures (50, 55, 60, 70 and 80 °C), maintaining thereaction time at 24 h, with a catalyst amount of 0.8 wt.% and an etha-nol/oil molar ratio of 6. These new samples were monitored by gaschromatography according to Eq. (1) and compared to the calculated

Fig. 2. Refractive index of the biodiesel blends at 40 °C versus wt.% total methyl ester

48 R.C.R. Santos et al. / Microchemical Journal 109 (2013) 46–50

data using the correlations from measurements of the refractiveindices.

3. Results and discussion

3.1. Physicochemical characterisation of pure soybean oil and biodiesel

Some of the main properties of the soybean oil and the biodieselsynthesised by the homogeneous method are listed in Table 1, whichalso lists the comparisons of the obtained soybean FAME and FAEEwith the European specification limits. The properties of the obtainedbiodiesel, in general, are similar. However, the triacylglyceride conver-sion into methyl and ethyl esters through the transesterification pro-cess, as evaluated by gas chromatography, revealed a higher estercontent than soybean FAEE. The acid numbers for both samples meetthe standard limit, indicating that the free fatty acid content will notcause operational problems, such as corrosion and/or pump pluggingfrom corrosion-induced deposit formation. With regard to the densityand viscosity, both are within the specification limits, indicating thatthe sample can be used directly in a diesel engine. A chromatogram ofthe FAMEs and FAEE obtained from the soybean oil transesterificationreaction is shown in Fig. 1. The analysis of the main compounds in thebiodiesel was identified by CG/MS. It is clear that the biodiesel obtainedin the experiment primarily contains palmitate, oleate, linoleate, andlinolenate fatty acid esters (Fig. 1).

3.2. Refractive index and ester content correlation

The refractive index of a liquid is easily measured and does not re-quire expensive and complicated equipment. To correlate the wt.% ofthe total esters of soybean oil and biodiesel mixtures studied in thiswork to their refractive indices, regression analysis based on the ex-perimental results was performed. Fig. 2 (a and b) shows the varia-tion in the physical properties as a function of weight per cent oftotal methyl and ethyl esters in the blends. As can be observedthere was a linear correlation between the FAME and FAEE ratiosand the refractive index, which decreased with the increase in the

Fig. 1. The chromatographic profile of the soybean oil fatty acid methyl and ethylesters. 1=Internal standard (Lauric acid ester); 2=Hexadecanoic acid ester;3=9,12-Octadecadienoic acid ester; 4=9-Octadecenoic acid ester; 5=Stearicacid ester.

(a) and (b) ethyl ester.

ester content of the samples. Such a linear correlation further con-firms the reliability of the method. For the correlations with wt.% ofthe total ester–refractive index, Eqs. (2) and (3) for the methylester+soybean oil and ethyl ester+soybean oil mixtures, respec-tively, and their R-squared value were obtained.

Y ¼ −1:90735�10−4Xþ 1:4679 R2 ¼ 0:9997 ð2Þ

Y ¼ −1:99958�10−4Xþ 1:4675 R2 ¼ 0:9996 ð3Þ

In the equation, ‘Y’ is the refractive index as a function of ‘X’, whichis the weight per cent total acid fatty ester (FAME or FAEE) in thesample.

Thus, due to the good R-squared value, the suggested correlationcould be used to determine the conversion to methyl or ethyl estersin the transesterification process by measuring the refractive index.However, some real data points are needed to evaluate predictionsusing the above correlations.

Therefore, as described in Section 2.5, a new series of methyl andethyl esters were synthesised.

Fig. 3 shows the methyl ester production from soybean oil, whichwas determined by gas chromatography (CG) and the refractiveindex (RI). It is important to note the excellent correlation obtainedfor the different methods. Fig. 3 shows that both techniques reveal asoybean oil conversion of approximately 90% in the first 8 h of reac-tion time. In fact, after this point, the reactions continue, but the con-version rate is very low, indicating that reaction equilibrium was

Fig. 3. The conversion of soybean oil to methyl esters at various reaction times deter-mined by gas chromatography (CG) and the refractive index (RI). Reaction conditions:catalyst amount, 0.6 wt.%; methanol/oil molar ratio, 6; temperature, 50 °C.

Fig. 4. The conversion of soybean oil to ethyl esters at various reaction temperatures, asdetermined by gas chromatography (GC) and the refractive index. Reaction conditions:catalyst amount, 0.8 wt.%; ethanol/oil molar ratio, 6; reaction time, 24 h.

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almost reached. In the reaction time from 1 to 8 h, a modest diver-gence in the methyl ester content is calculated by the differentmethods. However, during this reaction time (from 1 to 8 h), the re-fractive index undergoes a significant change, primarily due to themonoglyceride and diglyceride content. While the standard mixture(Fig. 2), is composed primarily of methyl ester and triglycerides, thereaction mixture (Fig. 3) is composed of methyl ester, triglyceridesand a substantial ratio of monoglycerides and diglycerides, whichhave different refractive indices [16,24]. The presence of mono- anddiglycerides is indicated by the per cent relative error for each datapoint reported in Table 2.

The higher relative error observed for the samples from the first8 h of reaction time is due to the difficulties in producing a standardmixture with a composition that is very close to the real mixture,which contains mono and diglycerides in addition to triglyceridesand the esters.

A very low relative error was observed for the sample, whichreached a good conversion.

Fig. 4 presents the effect of reaction temperature (50, 55, 60, 70and 80 °C) on the soybean oil conversion for FAEE, for a fixed reactiontime of 24 h. As expected, an increase in temperature promoteshigher conversion. Thus, the equilibrium position shifted toward the

Table 2FAME or FAEE soybean oil conversions measured by gas chromatographya and calculat-ed brefractive index properties and their relative per cent error.

FAMEs

Reaction time (h) 1 2 4 8 16 24 MeanR.E. %

Calc. Eq. (1) GC a 13.50 28.50 75.50 92.80 93.70 94.10Calc. Eq. (2) RI b 14.73 31.05 78.17 89.18 93.89 94.42Relative error % 9.111 8.947 3.536 3.901 0.203 0.340 4.339

FAEEs

Temperature(°C)

50 55 60 70 80 MeanR.E. %

Calc. Eq. (1)GC a

13.30 16.50 91.53 95.70 97.10

Calc. Eq. (3)RI b

13.20 16.70 91.22 94.22 96.72

Relativeerror %

0.752 1.212 0.338 1.546 0.391 0.847

a From Eq. (1) according to EN 14103 based on the different reaction conditions.b From Eqs. (2) and (3) based on blend data.

ester as the temperature increased, in agreement with data fromthe literature [25–27]. Although the reaction kinetics of ethanolysisare slower, if they are compared to methanolysis, there is no signifi-cant difference regarding the accuracy of the FAEEs calculated bythe refractive index using Eq. (3) and gas chromatography usingEq. (1); their values change very little (Fig. 4).

The low relative error observed for FAEE production may suggestthat the ratio of mono and diglycerides is lower than that of FAMEproduction. In other words, the ethanolysis of mono and diglyceridesis much faster than the ethanolysis of triglycerides.

Aside from the kinetics steps and each rate value, according toTable 2, the FAME and FAEE relative errors are less than 10% and 2%,respectively, and the maximum mean relative errors were 4.339%and 0.847% for FAME and FAEE, respectively. Although there aresome deviations in these data and correlations, the differences mayoccur primarily when the equilibrium of the reaction has not yetbeen achieved, at which point the refractive index changes signifi-cantly during a short time as the composition of the reaction mixturechanges; however, in the second part of the reaction, at equilibrium,the composition is nearly constant and very similar to the standardmixtures [24].

Therefore, the relative errors indicate that the equations obtainedfrom the proposed analytical method, which is rapid and low-cost,are adequate for monitoring the progress of the soybean oil trans-esterification reaction. These methods were developed using soy-bean oil instead of other vegetable oils. Specific calibration curvesfor each vegetable oil should be prepared because of their differentcompositions.

4. Conclusions

Equations to correlate the wt.% total methyl or ethyl ester with therefractive index for biodiesel plus soybean oil mixtures were proposed.As observed in this study, the refractive index decreased linearly withthe increase of methyl or ethyl ester wt.% from soybean oil. Thus, avery good linear relationship was obtained with an R2 of 0.9997(FAMEs) and 0.9996 (FAEEs) for the blends. Comparisons between theexperimental data obtained by gas chromatography and the valuespredicted by the proposed correlations showed that the per centmean relative error observed was higher for FAME than FAEE. Such rel-ative error may occur because the state of reaction equilibrium is notreached. However, in general, there is a good agreement between thegas chromatography data and the values predicted by the proposedequations, suggesting that this methodology can be very useful for the

50 R.C.R. Santos et al. / Microchemical Journal 109 (2013) 46–50

rapid and low-costmonitoring of the conversion of soybean oil tomethylor ethyl ester in the biodiesel production process.

Acknowledgements

The authors acknowledge the “Universidade Federal do Ceará”(UFC) and the Brazilian research-funding support agencies CNPq.

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