The Effect of Fatty Acid Ethyl Esters Concentration ... - ACS Publications

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The Effect of Fatty Acid Ethyl Esters Concentration on the Kinematic Viscosity of Biodiesel Fuel Rustam A. Usmanov,† Sergei V. Mazanov,† Asiya R. Gabitova,† Lina Kh. Miftakhova,† Farid M. Gumerov,† Rashid Z. Musin,§ and Ilmutdin M. Abdulagatov*,‡ †

Kazan National Research Technological University, Kazan, Russia Geothermal Research Institute, §A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, and N. M. Emanuel Institute of Biochemical Physics, IBHF, Russian Academy of Sciences, Moscow, Russia

‡

ABSTRACT: Several biodiesel fuel samples produced from rapeseed oil by the noncatalyzed transesterification reaction with supercritical ethanol at various temperatures from (593 to 653) K and molar ethanol to rapeseed oil ratios from (6:1 to 20:1) were used to measure kinematic viscosity. Measurements were made using the capillary viscometer (VPZ-2, Labtex Com., Moscow). The combined expanded uncertainty of the kinematic viscosity measurements at the 95 % confidence level with a coverage factor of k = 2 is estimated to be 0.35 %. All measurements were made at temperature of 313.5 K (ASTM D445) and at atmospheric pressure. The effect of fatty acid ethyl esters (FAEEs) content on the kinematic viscosity of biodiesel fuel samples at 313.15 K was studied. The correlation between the FAEEs concentration in biodiesel fuel samples and their measured kinematic viscosity at 313.15 K was found. The derived correlation between the kinematic viscosity and the FAEEs contents allows controlling the progress of the rapeseed oil transesterification process with supercritical ethanol. and Slavinskas21 studied the influence of fuel viscosity on the performance of diesel injection pumps of different design in order to clear up some of the changes in fuel feeding when running them in accordance with governor characteristics on the biofuel derived from vegetable oil. The increase of the fuel kinematic viscosity from 3.6 (diesel fuel) to 15.8 mm2/s (rapeseed methyl esters, RME) leads to an average increase of fuel delivery per stroke of 2.6 %. The high viscosity of the vegetable oils can cause of some operational problems such as engine deposits,6,22−25 and high viscosity is responsible for premature injector fouling leading to poorer atomization.24−28 A fuel of high viscosity tends to form larger droplets upon injection, leading to poorer atomization during the spray and creating operation problems, increasing carbon deposits, for example.29 On the other hand, a fuel with low viscosity may not provide sufficient lubrication for the precision fit of fuel injection pumps, resulting in leakage or increased wear.30 Therefore, the optimal values of the kinematic viscosity of biodiesel at 313.15 K must be in the range of (3.5 to 5.0) mm2/ s, according to EN-14214 specifications in Europe and from (1.9 to 6.0) mm2/s in accordance with ASTM D-6751 specification in the USA.18,31 In addition, kinematic viscosity is a very important biodiesel property and impacts on the fuel quality and strongly depends on its composition. Knowing the profile of fatty acid ethyl esters (FAEEs) in biodiesel is very important to control their thermphysical properties.32 The

1. INTRODUCTION Biodiesel can be used as biodiesel fuel or as an additive to diesel fuel in compression engines with little or no modification.1−4 Biodiesel fuel advantages and applications are very wellknown.5−7 Over the past few years, the use and production of renewable biodiesel in the United States has almost doubled.8−11 Biodiesel blended with diesel fuel is able to reduce CO2 emission by 78 %.12 The pollution of the atmosphere by numerous trace gases in the form of acid rain, photochemical smog, and global climatic changes produced by the “greenhouse effect” is important because of its serious consequences to human health and to the environment.13−15 The combustion of fossil fuels is the main source of many pollution gases16 (NOx, SO2, CO, and CO2). Environmental advantages of biodiesel use include the following: it is simple to use, biodegradable, nontoxic, essentially free of sulfur and aromatics, and its emissions cause 50 % less ozone to form than conventional diesel fuel.17 The analysis of thermophysical properties reveals important information about physicochemical processes in the material and certainly may aid in the characterization of biodiesel fuel quality. Viscosity is one of the key properties of fuel for diesel engine.18 The value of kinematic viscosity along with the values of speed of the fuel through the injector characterizes the flow of liquids through the Reynolds number. The time necessary for a liquid jet to disintegrate into microdrops is determined through the (σ/η) and (σ/ρ), where σ is the surface tension, ρ is the density, and η is the dynamic viscosity.19 Also the diameter of the drops after atomization can be estimated using the kinematic viscosity and surface tension data.19,20 Labeckas © XXXX American Chemical Society

Received: August 10, 2015 Accepted: October 9, 2015

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DOI: 10.1021/acs.jced.5b00683 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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of several biodiesel fuels obtained from soybean, canola, sunflower, and waste oils and their blends with diesel. The measurements were made at temperatures between (293 and 353) K. The uncertainty of the measurements is 0.2 %. Nita et al.45,46 measured the viscosity of biodiesel fuel from (293 to 333) K. The measurements were made using an Anton Paar instrument (SVM 3000 type). The uncertainty of the measurements is 0.7 %. Knothe et al.48 reported the kinematic viscosity of numerous fatty compounds at 313.15 K (ASTM D445) using a Cannon−Fenske-type glass capillary viscometer. Allen et al.49 measured the viscosity of biodiesel fuels produced from natural canola, coconut, palm, peanut, and soya oils. The measurements were made using a Paar model AMV 200 microviscometer based on the rolling−ball method. The viscosity of the various biodiesel fuel samples were studied by Kerschbaum and Rinke11 in the range of (258 to 303) K using a Thermo−Haake viscometer (VT500). Tat and Van Gerpen51 reported kinematic viscosity measurements of commercial biodiesel and its blends with diesel fuels at temperatures from melting point to 373 K using the Cannon−Fenske-type glass capillary viscometer. Ramakrishnan and Jash52 measured kinematic viscosity of 16 biodiesel samples produced by transesterification of waste cooking oil with alcohol at various reaction conditions (reaction temperature, oil to alcohol molar ratio, catalyst concentration, and reaction time). The measurements were performed using Ostwald’s viscometer at 303 K. Barbosa53 measured the viscosity of two biodiesel samples produced from soybean and sunflower with methanol transesterification at temperatures from (293 to 313) K. The measurements were performed with Ubbelohde viscometer. The uncertainty of the kinematic viscosity measurements is within from (0.0038 to 0.0082) mm2/s or about 0.12 %. Freitas et al.41 employed an Anton Paar rotational Stabinger viscometer (model SVM 3000) to measure the viscosity of seven biodiesel samples obtained from soy B, palm, rapeseed, and GP (mix of soy and rapeseed methyl esters a 50 wt %) using a catalyzed esterification reaction with methanol (oil-tomethanol molar ratio was 1:5 with 0.5 % sodium hydroxide at 328 K). The uncertainty in viscosity measurements was less than 0.5 %. Macedo et al.54 reported viscosity data for canola, sunflower, corn, and soybean oils as a function of temperature in the range of (297 to 358) K. The measurements were made with a Haake RS50 rheometer. Gutti et al.55 measured kinematic viscosity for quality test of the biodiesel fuels produced from Balanite aegyptiaca seed oil by catalyzed (1.5 % KOH) transesterification with methanol at 333 K. The kinematic viscosity was measured using the ASTM D445 method at temperature of 313.15 K. Maksimuka67 measured the viscosity of FAEEs of rapeseed oil in the temperature range from (293 to 353) K using commercial capillary viscometer with uncertainty of 0.01 mm2·s−1. The viscosity of vegetable oils and diesel oil blends have been measured in several publications (see, for example refs 56 and 57). 2.2. Prediction Models. Predictive models for the viscosity of biodiesel were developed by several authors.41,42,49,51,58−63 In general, the discrepancy between the prediction and measured values of viscosity is within (4.5 to 8.5) %. Pratas et al.58 proposed a model to estimate biodiesel fuel viscosity using the knowledge of the properties of the pure compounds. Pratas et al.61 studied the viscosity of minority (polysaturated compounds, in C18; monosaturated, in C16, C20, and C22; and the long-chain saturated esters) FAME and FAEE present in biodiesel. Such type of data is required to develop reliable

difference between the viscosity of vegetable oil and its methyl esters derived from transesterification is about 1 order of magnitude.6,22 The viscosity of crude vegetable oil (about 27 to 33 mm2/s for rapeseed oil, for example) is higher than the viscosity of biodiesel (about 4.4 to mm2/s for fatty acid methyl esters (FAME) and fatty acid ethyl esters (FAEE), see also below and ref 20). Characterization of the biofuel sample quality by physical properties is one of the methods to monitor the biodiesel fuel production processes. Thermophysical properties of the complex systems during the biodiesel fuel production process (transesterification reaction) are very important for modeling and optimization of the biodiesel production technology parameters.19,26,33−39,49,58,64−66 Viscosity is one of the very sensitive properties of biodiesel fuel for its quality analyses (for example, composition changes). Thus, viscosity measurements can be used to quantify biodiesel content.19,33−39 Viscosity of the reaction product can be used as one sensitive tool for determining the composition of the mixture (biodiesel fuel) using the correlation between the viscosity and concentration. The progress (dynamics) of the transesterification and quality of the reaction product can be followed by online (in dynamic regime) control (monitoring) of the viscosity of the reaction product (in the reactor) based on the compositional changes. Since biodiesel is a mixture of various FAEEs, in which each component is contributing to the total measured viscosity, it is apparent that there is strong correlation between the viscosity of the biodiesel product and the FAEEs content. The main goal of the present work is to study of the effect of FAEEs in biodiesel fuel samples, produced from rapeseed oil by the noncatalyzed transesterification reaction with supercritical ethanol at various reaction conditions, on their kinematic viscosity at 313.15 K (ASTM D-6751). We have measured the kinematic viscosity of 18 biodiesel fuel samples derived by esterification of the rapeseed oil with supercritical ethanol at various temperatures, from (593 to 653) K, and various ethanol to rapeseed oil molar ratios (from 6:1 to 18:1). The other purpose of the present work is to develop a new correlation between the measured kinematic viscosity of the biodiesel samples and their FAEEs content. The measurements were made at a temperature of 313.15 K in order to compare with International Standards (ASTM D675131 and EN 14214,40 see above).

2. REVIEW OF THE MEASUREMENTS AND PREDICTION MODELS FOR THE VISCOSITY OF BIODIESEL 2.1. Measurements. The viscosity of vegetable oils and biodiesel fuels were widely studied in the literature by many authors.64,68−78,81,83 A brief review will be given here. Benjumea et al.42 measured the viscosity for palm oil biodiesel and its blend with diesel fuel using a capillary tube viscometer. The authors studied the effect of biodiesel content on viscosity. The kinematic viscosity of three biodiesel fuels obtained from canola, soy, and fish oil were reported by Tate et al.43 in the temperature range from (293 to 573) K using a Saybolt viscometer. Yuan et al.44 measured kinematic viscosities of four biodiesel fuels obtained from natural and genetically modified soybean oils, yellow grease, and their blends with diesel fuel at temperatures from (293 to 373) K using a Cannon-Fenske-type glass capillary viscometer. The standards uncertainty of the measurements is 0.02 mm2·s−1. The same type of viscometer was used by Moradi et al.47 to measure the kinematic viscosities B



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and de Filippis et al.35 applied these correlations (1 to 3) for biodiesel fuel obtained using conventional technique at low transesterification reaction temperatures between (333 and 353) K. These correlations have not been previously used for the biodiesel fuel samples obtained in the supercritical transesterification reactions at high temperatures (above 573 K). For monitoring the progress of biodiesel production, (transesterification) reaction, Ellis et al.39 used in situ viscosity measurements with an acoustic wave solid-state viscometer. Borges et al.36 used a rotational viscometer to measure dynamic viscosity of the biodiesel samples for FAMEs and FAEEs quantification. Ramakrishnan and Jash52 used kinematic viscosity measurements of biodiesel to optimize the transesterification process (reaction temperature, oil to alcohol molar ratio, catalyst concentration, and reaction time). In the present work, the concentration of the FAEEs in the rapeseed oil supercritical transesterification reaction product (biodiesel samples) was analyzed by measuring the kinematic viscosity of the extracted sample (indirect method) and using Fourier transform infrared (FTIR) and gas chromatography− mass spectroscopy (GC−MS) analyses (direct method). For FAEEs quantification, we have measured the kinematic viscosity of the 18 biodiesel samples obtained from the transesterification reaction of the rapeseed oil in supercritical ethanol at various reaction conditions (reaction temperature, oil-to-ethanol molar ratio, reaction time). The correlation between the FAEEs content and the viscosity of the biodiesel fuel samples was obtained. The correlation was tested on a wide range of samples with various FAEEs contents (from 0 to maxima FAEEs concentration of 98 wt %).

predictive models for the viscosity of biodiesel. Some authors proposed a correlation between FAEEs content and other properties such as the density,26,49 surface tension,26,64 boiling temperature,65 and cetane number.66 Allen et al.49,62 developed a viscosity prediction model for biofuels based on their components values. The average prediction uncertainty is 3 % (the maximum uncertainty is 5 %). Yuan et al.50 developed a method for the predicted temperature-dependent viscosities of biodiesel based on fatty acid ester composition. The method was based on acombination of the Grunberg−Nissan equation combined with a group contribution method. The predictive capability of the method is within 2.5 % (for soybean and yellow grease oils 3 %) and was verified for 22 mixtures of FAEE. For coconut, palm, and canola oil methyl esters the maximum uncertainty was about 7 %. Benjumea et al.42 evaluate the mixing rules for calculating the viscosity of palm oil biodiesel. The prediction of the viscosity by using the mixing rule was within 0.64 % at 313.15 K. Krisnangkura et al.60 proposed predictive correlation equation for the kinematic viscosity of FAMs as a function of temperature and carbon atom number in the homologous series. The proposed predictive model agrees with the experimental kinematic viscosity data within (0.002 to 0.116) mm2/s depending on temperature and carbon atom number. Freitas et al.41 proposed a revised Yuan’s model50 which considerably improves the description of the experimental viscosity data of biodiesel. The revised Yuan’s model is the original Yuan’s model in which the VTF parameters were refitted to the new experimental viscosity data.58,61 The model proposed by Freitas et al.41 was compared with the models by Ceriani et al.,59 Krisnangkura et al.,60 and Yuan et al.50 The average deviations between the same experimental viscosity data and the predicted values from models41,50,59,60 are 4.65 %, 5.34 %, 8.07 %, and 7.25 %, respectively. Wagner et al.56 showed that the Arrhenius method using volume fractions most accurately (within 2.31%) predicts the viscosity of binary blends of WSO.

4. EXPERIMENTAL SECTION 4.1. Reagents and Material Description. Rapeseed oil (yellow color) 100 % purity from Company OAO “Aston”, Rostov-na-Donu, Russia (2014), was used in this study. The fatty-acid composition of the rapeseed sample is given in Table 1. The rapeseed sample was analyzed by GC−MS (GC Trace1310 and MS detector ISQ). The separation was performed in a quartz capillary column TR-5MS with 15 m in length and 0.32 mm in inner diameter. The stationary phase was 5 % phenylpolyphenylsilicone and 95 % siloxane with a thickness of 0.25 μm. The column temperature was heated from (313 to

3. CORRELATION BETWEEN THE VISCOSITY AND FAEES CONTENTS The physical properties-based techniques to estimate the FAEEs in biodiesel fuel samples are widely using by many authors.19,26,33−39,49,58,64−66 A few works were published33−39 on the viscometric technique for biodiesel production control. The viscometry analysis technique is fast, inexpensive, and accurate. de Filippis et al.35 used a Hoeppler microviscometer to measure the viscosity of reaction product samples. The measured values of the viscosity were used to determine the FAMEs concentrations in the biodiesel fuel samples. de Filippis et al.35 proposed a correlation between the concentration of the FAMEs content in the reaction product and viscosity as w = a ln(η) + b

Table 1. Fatty-Acid Compositions of the Rapeseed Oil Sample fatty acids tetradecanoic (myristic) hexadecanoic (palmitic) hexadecenoic (palmitoleic) octadecanoic (stearic) octadecenoic (oleic) octadecadienoic (linoleic) octadecatrienoic (linolenic) eicosanoic (arachidonic) eicosenoic (gondoic) docosanoic (behenic) docosenoic (erucic) tetracosanoic (lignoceric) tetracosanoic (nervonic) unidentified acids vitamins (beta-carotene, tocopherol, etc.)

(1)

where w is the FAMEs weight fraction; η is the dynamic viscosity; a and b are the fitting parameters. Sousa et al.34 proposed a correlation for the kinematic viscosity of the biodiesel fuel and concentration of the FAMEs and FAEEs as FAME (%) = 152 exp( −ν/9.8)

(2)

FAEE (%) = 160 exp( −ν /9.0)

(3)

These results have been derived for soy oil and cannot be used for the biodiesel fuel from other vegetable oils because biodiesel fuel composition depends on the initial feedstock. Sousa et al.34 C

mass % of fatty acids C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:0 C20:1 C22:0 C22:1 C24:0 C24:1

0.12 13.23 0.45 6.49 37.51 20.82 5.80 2.79 6.48 1.93 1.01 0.64 0.41 1.44 0.88



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transesterification. Biodiesel fuel samples from rapeseed oil were produced in a noncatalyzed transesterification process with a supercritical ethanol unit described in our previous publications.75,76 In our previous studies75,76 the different biodiesel fuel samples were obtained from rapeseed oil by noncatalyzed transesterification with supercritical ethanol at various temperatures from (593 to 653) K and molar ethanol to rapeseed oil ratios from (6:1 to 20:1) at a pressure of 30 MPa. These biodiesel samples (18) were used to measure the kinematic viscosity. The biodiesel samples were taken after stabilization of the process at each 6 min during 30 min of the production process (reaction time). The test measurements75,76 showed that the effect of pressure on the FAEEs content in the reaction product is small. Thus, all transesterification experiments were made at a constant pressure of 30 MPa. The details of the method, apparatus, and procedures for biodiesel production in the continuous regime with supercritical ethanol have been reported in our previous publications,75,76 During the extraction and further treatment the samples were protected from the air contact and vaporization. All biodiesel fuel samples (about 40 to 50 mL) were extracted from the reactor into a special glass vessel with a volume of 100 mL at temperatures between (303 and 313) K and were tightly closed. After extraction, the sample was kept in the refrigerator. The viscosity measurements usually were made in the same or next day. 4.4. Viscosity Measurements. 4.4.1. Experimental and Procedure. The kinematic viscosity of the extracted biodiesel fuel samples (after purification) was measured with commercial capillary viscometer VPZ-278 (Labtex Com., Moscow) (see Figure 1, left). The details of the viscometer construction used in the present measurements are schematically shown in Figure 1 right. The viscometer has a U-tube shape. The arm (1) is welded with capillary (7). The capillary diameters were 0.56 mm and 0.73 mm with length of 216 mm (ΓOCT 10028-8178). Vacuum was applied to drain the sample through bulb (4). The

563) K at rate of 15 K/min and thermostated for 5 min at the final temperature. The temperature of interface and injector was 523 K. The total injected sample was 1 μL. Helium was used as carrier gas at a flow rate of 3 mL·min−1 with a split ratio of 1:40. The sample preparation was as follows: 5 μL of sample was added into 300 μL of chloroform, shaken vigorously, then 1 μL of the solution was used for analysis. Ethanol was analyticalgrade and used without drying and purification (had more than 95 vol % purity, ΓOCT P 51723-200177). 4.2. Analysis. FTIR was used to determine the FAEEs content in the transesterification reaction products (biodiesel samples). FTIR is well suited to the measurement of FAEEs content in biodiesel fuel. FTIR study of the transesterification reaction products was performed using ERALYTICS (Austria) ERASPEC spectrometer type NIR/MID-FTIR interferometer, 16384 data points. The scanning spectral range was from (4000 to 630) cm−1, with a resolution of 4 cm−1 and 32 scans. Warmup time is less than 60 s. Measuring time is 60 s. The working temperature is from (288 to 313) K. FITR allows the use of various spectral regions to determine the percentage of fatty ester in the reaction mixture. Characteristic absorption frequencies and assignments of the spectral region used for the determination of FAEEs in the transesterification reaction products (biodiesel) are band position 1745 cm−1 and 1180 cm−1 to monitor the complex fatty acid esters peak; 1165 cm−1 for fatty acids peak; and 3676 cm−1 and 3464 cm−1 for monoand diglycerides peaks. FTIR (ERASPEC) uses the absorbing peak of 1745 cm−1 to determine the concentration of the esters in the low concentration range, from 0 % to 10 %, while the absorbing peak of 1180 cm−1 is used for higher concentrations of esters. To obtain additional confirmation of the accuracy and reliability of the FTIR analyses results, a gas chromatography− mass spectrometry (GC−MS) study of the transesterification reaction products (some selected samples) was performed using a DFS Thermo Electron Corporation, Germany, instrument. The capillary column ID-BP5X (SGE, Australia), 50 m in length and 0.32 mm in inner diameter was used. The composition of the capillary column stationary phase was 5 % diphenyl and 95 % dimethylpolysilicone with a thickness of 0.25 μm. Helium was used as carrier gas. The column carrier gas flow rate was 2 mL·min−1. The split ratio was 1:10. The area percentage method was used to estimate the amount of alkyl esters in the product. Xcalibur software was used for analysis of the mass-spectroscopic data. The extracted FAEE samples were dissolved in ethanol in a ratio of 1:100. The total volume of the injected sample was 0.1 μL. The temperature of the injector and ion’s source was set to 553 K. The column temperature was maintained at 393 K for the first minute and then heated to 553 K, at a rate of 293 K·min−1, for a total of 50 min. Biodiesel Fuel Samples. Before measurement of the viscosity of the biodiesel sample extracted from the reactor, the sample was cleaned. To remove the excess ethanol from the reaction product, a rotor-type thin-filmed evaporator (Pope Scientific, USA) was used (see details in our earlier publications75,76). Therefore, the derived product contains FAEEs only (no unreacted ethanol or oil in the samples). 4.3. Supercritical Ethanol Transesterification. Rapeseed oil can be used as fuel for combustion engines. However, its viscosity is much higher (almost 6 to 10 times, see above) than usual diesel fuel and requires modifications of the engines. Therefore, rapeseed oil was converted into biodiesel fuel by

Figure 1. Schematic diagram of the capillary viscometer VPZ-2: (1 and 2) arms; (3) outlet tube; (4) bulb; (5) extension; (7) capillary. M1, beginning mark; M2, ending mark (timing marks). D



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Table 2. Kinematic Viscosity of Biodiesel Fuel Samples Obtained at Various Experimental Transesterification Reaction Conditions (Reaction Temperatures and Ethanol to Rapeseed Oil Molar Ratios at P = 30 MPa) at 313.15 K and at Atmospheric Pressure Together with Their FAEEs Concentration Values Derived from FTIR and GC Analyses. The Samples Were Extracted after 30 min of the Reaction Timea ethanol to rapeseed oil molar ratio

reaction temp (K)

measured kinematic viscosity (mm2·s−1)

concentration of FAEEs from FTIR analyses (wt %)

6:1

623 638 653 623 638 653 638 653 608 623 638 653 653 623 638 653 608 623

14.41 12.44 12.31 9.44 8.84 6.82 8.39 6.63 9.88 8.80 7.57 6.03 5.18 5.24 4.98 4.83 8.02 6.05

42.41 49.87 50.26 63.21 65.12 80.11 68.34 80.54 60.30 66.50 72.77 81.92 92.08 92.25 93.66 94.56 71.12 85.41

8:1

10:1 12:1

16:1 18:1

20:1 a

concentration of FAEEs from GC analyses (wt %)

81.61

82.97 92.81 94.35

Standard uncertainties u are u(T) = 0.02 K; u(ν) = 0.18 %.

where K = 0.01 mm2·s−2 is the viscometer calibration constant (for VPZ-2 with diameter 0.56 mm); τ is the liquid flow time (s); ν is the kinematic viscosity of liquid (mm2·s−1); g1 = 9.816 is the acceleration of gravity at the measurement place (m·s−2); and g2 = 9.807 is the acceleration of gravity at the calibration place (m·s−2). 4.4.3. Test Measurements. Before measurements of the kinematic viscosity for the biodiesel samples were made, the viscometer (VPZ-2) was tested on well-studied liquids. To check and confirm the reliability and accuracy of the viscometer and procedures of the kinematic viscosity measurements for the biodiesel samples, the measurements were first carried out for well-studied liquids such as water, ethanol, and n-hexane at selected temperatures (313.15 K) and at atmospheric pressure. The measurements for each fluid were performed at least five times. The measured values of kinematic viscosity at 313.15 K and 0.101 MPa are for water (0.657 mm2·s−1), for ethanol (1.073 mm2·s−1), and for n-hexane (0.397 mm2·s−1). These data were compared with the NIST reference values of 0.658, 1.061, and 0.398 mm2·s−1 calculated from REFPROP80 for water, ethanol, and n-hexane, respectively. As one can see, the differences are 0.15 %, 1.09 %, and 0.25 %, for water, ethanol, and n-hexane, respectively. The uncertainties of the NIST reference data are 0.17 %, 3 %, and 2 %, for water, ethanol, and n-hexane, respectively. Thus, the comparison shows that the agreement between the test measurements of kinematic viscosity for pure water, ethanol, and n-hexane and NIST reference data is good (within their uncertainties). In addition, we have also compared measured viscosity for rapeseed oil with the available reported data by other authors. The present measured value of the kinematic viscosity for rapeseed oil (before transesterification reaction started) ν = 32.63 mm2·s−1 (at 313.15 K) is in good agreement with the measured (37 mm2·s−1) and estimated (35.94 mm2·s−1) values at 311 K reported by Anand et al.81 and Balat82 (37.3 mm2·s−1). Noureddini et al.86 also reported the viscosity of rapeseed oil

vacuum apparatus was then withdrawn from the viscometer. The sample flow in the viscometer then started by gravity. The flow time of the sample was measured observing its meniscus from timing mark M1 to timing mark M2 (see Figure 1, right). In the experimental temperature range the values of flow time τ varied from (200 to 1000) s. All values of flow time τ were an average of at least 5 to 10 measurements. The temperature in the measuring cell, where the capillary was located, was controlled using a thermostat (Huber, Germany) with an uncertainty of 10 mK and measured using the (ITS-90) PRT100 thermometer with an uncertainty of 0.03 K. Distilled water was used as a thermostatting fluid. The flow time and amount of liquid in the viscometer are independent of the hydrostatic pressure. To measure of the viscosity of biodiesel fuel samples, the commercial viscometers (VPZ-2) with various capillary diameters of 0.56 mm (for measurements from 2 to 10 mm2·s−1) and 0.73 mm (for measurements from 6 to 30 mm2· s−1) were used. This type of viscometer (VPZ-2) allows for highly precise viscosity measurements in the wide measuring range from (0.6 to 30000) mm2·s−1 and at temperatures from (293 to 343) K by using a capillary with various diameters. The estimated combined expanded uncertainty (standard uncertainty multiplied by the factor of k) of the kinematic viscosity measurements at the 95 % confidence level with a coverage factor of k = 2 is to be 0.35 % (the standards uncertainty is 0.18 %, stating the expanded uncertainty). The repeatability of viscosity measurements is 0.1 %. Further details about the equipment and the method can be found elsewhere.79 4.4.2. Working Equation. The measurements of the viscosity for each extracted biodiesel sample (reaction product) were made at least 5−10 times at a temperature of 313.15 K and at atmospheric pressure. The viscosity values were calculated using the working equation

ν = K (g1/g2)τ

(4) E



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of 44.9 mm2·s−1 at 310.8 K. This value of kinematic viscosity of rapeseed oil is higher than the present and reported results. The measured (4.66 mm2·s−1) and estimated (4.40 mm2·s−1) values of the kinematic viscosity of biodiesel obtained from rapeseed oil at 313.15 K in the work81 and the value of (4.7 mm2·s−1) reported by Balat82 are also in good agreement with the present results of 4.83 mm2·s−1. The value of biodiesel kinematic viscosity of 4.49 mm2·s−1 at 313.15 K reported by Maksimuk et al.67 is also in good agreement with the present result. This clearly demonstrates the good consistency between the present viscosity measurements for rapeseed oil and obtained biodiesel and the reliability and accuracy of the results for various reaction product samples. The reliability and accuracy of the viscometer (VPZ-2) were also previously tested by other authors (see, for example ref 83) for various well studied fluids and presented very good reproducibility. In addition, the same viscometer (VPZ-2) was successfully used for accurate measurements of the kinematic viscosity of n-heptane by other authors.84,85 For example, the measured value (0.382 mPa·s) of viscosity for n-heptane at 0.101 MPa and at 298.15 K deviates from the NIST reference value (REFPROP80) of 0.388 mPa·s (uncertainty is 2 %) within 1.6 %. Also, the measured with (VPZ-2) value of the viscosity (0.382 mPa·s) for nheptane was compared with the value reported by other authors measured using a falling body viscometer.84 The deviation is within 1.05 %. Comparisons with the reference values of viscometer calibrating liquids in our laboratory and by other authors confirmed the uncertainty of the viscosity measurements stated by the manufacturer for (VPZ-2).

Figure 2. Measured values of viscosity as a function of concentration of FAEEs in the reacting mixture at 313.15 K and at atmospheric pressure for various ethanol to rapeseed oil molar ratios. ▲, 6:1; □, 8:1; △, 10:1; ●, 12:1; ×, 20:1; ○, 18:1; ■, viscosity of the reaction product at the maximal values of FAEEs concentration of 98.31 wt %;32 and ▼, viscosity of rapeseed oil at 0 concentration of FAEEs. From GC−MS analyses: ◇, 8:1; ◆, 12:1; ▽, 16:1; +, 18:1. Solid line calculated from correlation eq 6.

5. RESULTS AND DISCUSSION A total of 18 measured biodiesel samples were used for FTIR and GC−MS analyses (see Table 2). The measured values of the kinematic viscosity of 18 biodiesel samples with various concentrations of FAEEs obtained at various experimental transesterification reaction conditions (reaction temperatures and ethanol to rapeseed oil molar ratios) at 313.15 K and at atmospheric pressure are given in Table 2 and Figures 2 and 3 together with the concentration values derived from FTIR and GC−MS analyses. As one can see from Table 2, increasing the transesterification temperature and ethanol to rapeseed oil molar ratio, therefore, with lowering the viscosity, the final product (biodiesel fuel) yield also is increasing. The maximal concentration of FAEEs 94.56 wt % in the reaction product was found at temperature of 653 K and ethanol to rapeseed oil molar ratio of 18:1. As was confirmed by previous studies of other authors (see, for example ref 34), the FAEEs concentration in the extracted biodiesel samples obtained at various reaction conditions is the linear function of the logarithm of the kinematic viscosity w = A ln(ν) + B

(5)

ν = exp(w/A − C)

(6)

Figure 3. Measured values of logarithm kinematic viscosity of biodiesel fuel samples as a function of concentration of FAEEs from FTIR and GC analyses at 313.15 K and at atmospheric pressure for various ethanol to rapeseed oil molar ratios. Symbols see Figure 2. Solid line calculated from correlation eq 5.

yield values of concentration of w = 94.56 wt %. The derived values of fitting parameters are A = −49.392473 and C = −3.513642. Since the FAEEs concentration w is a linear function of logarithm viscosity ln(ν) (see eq 5), then values of fitting parameters A and B can be estimated using minimum (two) well-known experimental viscosity data points, namely, (1) for the rapeseed oil sample at 313.15 K for which w = 0, that is, the concentration of FAEEs in rapeseed oil before transesterification reaction started, the viscosity was ν = 32.63 mm2·s−1; and (2) the value of the viscosity, ν = 4.83 mm2·s−1, of the reaction product (biodiesel fuel) at the maximal value of the FAEEs concentration of w = 94.56 wt % for a molar ratio of 18:1 and at a transesterification reaction temperature of 653 K. The value of the viscosity, ν = 4.49 mm2·s−1, of the reaction

or where w is the FAEEs concentration in the extracted biodiesel fuel in wt %; ν is the kinematic viscosity in mm2·s−1; C = B/A, A and B are the fitting parameters. As one can see from Figures 2 and 3, the present viscosity measurement results are in good agreement with relations 5 and 6. The present measured viscosity data for various biodiesel samples (obtained at various reaction conditions, Table 2) were fitted to eq 6 in the concentration range from w = 0 (rapeseed oil) to a maximal F



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Figure 4. Measured kinematic viscosity of biodiesel fuel samples at 313.15 K as a function of transesterification reaction time for various ethanol to rapeseed oil molar ratios at selected temperatures. (A) 6:1; (B) 8:1; (C)10:1; and (D) 12:1. △, 593 K; □, 608 K; ▲, 623 K;●, 638 K; ■, 653 K.

Figure 5. GC−MS chromatogram of biodiesel fuel samples after 30 min transesterification reaction with supercritical ethanol at rapeseed oil molar ratio of 12:1 and the reaction temperature of 653 K.

product at the maximal value of the FAEEs concentration of w = 98 wt % for molar ratio of 20:1 and at transesterification reaction temperature of 638 K was found by Maksimuk et al.67 As one can see from Figures 2 and 3, both the present and reported67 data points for maximal FAEEs concentrations are in good agreement with the measured values of viscosity for other biodiesel samples. Several selected biodiesel fuel samples obtained with fixed molar ratio of (12:1) for different temperatures (623 K, 638 K, and 653 K) were also analyzed with a GC−MS to confirm the FTIR analyses results. Last two columns of Table 2 contain the values of FAEEs concentration

obtained using FTIR and GC−MS analyses. The agreement between the FTIR and GC−MS analyses is good enough. Table 2 provides also temperature and ethanol to rapeseed oil molar ratio dependences of the FAEEs concentration in biodiesel fuel together with the values of kinematic viscosity for each sample at 313.15 K. Therefore, the correlation (5) for concentration of FAEEs and viscosity of the supercritical transesterification reaction product (biodiesel) can be used to estimate the quality (FAEEs content in the transesterification reaction product) of the biodiesel production. As can be noted from Table 2, an increase in the reaction temperature decreases the kinematic G



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Table 3. Concentration of FAEEs in the Biodiesel Fuel Sample (Result of the Chromatographic Analyses) Ethyl Esters (EE), wt % samples after 30 min reaction time 653 653 653 638

K K K K

(8:1) (12:1) (16:1) (18:1)

EE of myristic acid C16H32O2

EE of hypogeic acid C18H34O2

0.89 0.35 0.02 0.23

3.44 2.37 0.84 3.69

EE of palmitin acid C18H36O2

EE of oleate acid C20H38O2

EE of arachidonic acid C22H44O2

25.17 31.21 19.57 28.73

16.38 25.28 60.67 36.76

4.39 2.78 1.23 3.25

viscosity of the biodiesel fuel sample, while the FAEEs concentration increases. Thus, an increase in the reaction temperature favorably affects the conversion yield of the transesterification reaction product (FAEEs), that is, biodiesel fuel production. To study how the reaction product content changes with time, the reaction product sample was extracted every 6 min during 30 min. The viscosity of the selected samples was also measured. The results of measured reaction time dependences of the kinematic viscosity (consequently FAEEs concentration changes) of the reaction product samples for various ethanol to rapeseed molar ratios (6:1; 8:1; 10:1; and 12:1) at selected transesterification reaction temperatures are shown in Figure 4. This figure demonstrates how the concentration of the FAEEs in the reaction products changes with reaction time. As one can see from Figure 4, the viscosity of the extracted reaction product samples decreases with reaction time. Therefore, as can be readily estimated from eq 6 or Figures 2 and 3, the concentration of the FAEEs in the reaction product is increasing. After about 30 min of reaction time the concentration of the FAEEs reached maximal values. This means that the best value of the kinematic viscosity was reached after 30 min of the transesterification (i.e., the viscosity of the biodiesel is close to the ASTM D6751 standard). Therefore, increasing the reaction time has the same effect on the reaction products as increasing both the temperature and molar ratios (see above), and plays an important role for conversion yield. The relative weight compositions of FAEEs compounds present in the biodiesel fuel sample with a fixed molar ratio of 12:1 were analyzed using GC−MS for the selected temperature of 653 K. The results are shown in Figure 5 and Table 3. The principal peaks identification (main peaks of the chromatographic analyses Figure 5) is given in Table 4. These data are very useful to develop predictive techniques for the viscosity of biodiesel fuels based on fatty acid ester composition.

EE of linoleate acid C20H36O2

EE of behenic acid C24H48O2

EE of linolenate acid C20H34O2

total (wt %)

2.95 3.04 1.62 4.14

30.58 16.72 8.86 10.52

85.61 82.97 92.81 94.35

1.81 1.22 2.29

Table 4. Principal GC−MS Peaks Data (Main Peaks of the Chromatographic Analyses, Table 2) of Biodiesel Samples from Rapeseed Oil after Supercritical Transesterification retention time (min) 8.34 9.11 9.17 10.13 11.10 11.20

component EE of palmitin acid EE of oleate acid EE of linolenate acid EE of arachidonic acid EE of hypogeic acid EE of behenic acid

chemical formula

peak area (mm2)

concentration (wt %)

C18H36O2

2.32 × 108

31.21

C20H38O2

1.78 × 108

25.28

C20H34O2

1.20 × 107

16.72

C22H44O2

3.38 × 107

2.78

C18H34O2

2.06 × 107

2.37

C24H48O2

2.70 × 107

3.04

wt % has the viscosity of 4.49 mm2/s at 313.15 K (ASTM D6751 standard). As the transesterification reaction time of rapeseed oil in the supercritical ethanol increases, the resulting kinematic viscosity of the biodiesel product decreases, while the observed concentration of the FAEEs in the biodiesel product increases. Increasing the reaction time has the same effect on the reaction products as increasing both the temperature and molar ratios (see above), and plays an important role for conversion yield. The proposed in the present work correlation between the FAEEs concentration and the kinematic viscosity of the transesterification reaction products (biodiesel) can be used to control the biodiesel production (transesterification) process in dynamic regime, i.e., adjust the reaction conditions (temperature, pressure, ethanol to oil molar ratio, mixing rate) to increase or decrease the FAEEs concentration of the product (control of the biodiesel quality).

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6. CONCLUSIONS The kinematic viscosity of the 18 biodiesel fuel samples produced from rapeseed oil by the noncatalyzed transesterification reaction with supercritical ethanol at various temperatures from (593 to 653) K and at pressure of 30 MPa for ethanol to rapeseed oil molar ratios from 6:1 to 20:1 were measured at 313.15 K and atmospheric pressure using a commercial capillary viscometer (VPZ-2). We experimentally found correlation between the viscosities of the biodiesel fuel samples (obtained at various reaction temperatures and ethanol to oil molar ratios) at 313.15 K and the FAEEs content. The derived correlation for the concentration of FAEEs and viscosity of the supercritical transesterification reaction product is in good agreement with the ASTM D6751 standard (the viscosities of the final product less than 6 mm2·s−1). The biodiesel sample with maximum FAEEs concentration of 94.6

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1 (303) 497 4027. Fax: +1 (303) 497 5224. Present address: National Institute of Standards and Technology, Boulder, Colorado, United States. Funding

The authors thank Russian Foundation of Basic Research No. 13-03-12078-office and Russian Scientific Fund (No. 14-1900749) for the financial support. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I.M. Abdulagatov thanks the Applied Chemicals and Materials Division at the National Institute of Standards and Technology H



Article

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for the opportunity to work as a Guest Researcher at NIST during the course of this research.

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