Experimental Investigation and Prediction of Density and Viscosity of

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Experimental Investigation and Prediction of Density and Viscosity of GTL, GTL−Biodiesel, and GTL−Diesel Blends As a Function of Temperature Su Han Park,†,§ Ki Bong Choi,† Myung Yoon Kim,‡ and Chang Sik Lee*,† †

School of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul, 133-791, Republic of Korea Powertrain Control System Team, Automotive R&D Division, Hyundai Motor Group, 772-1, Jangduk-dong, Hwaseung-si, Gyeonggi-do, 445-706, Republic of Korea § Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: The purpose of this study is to experimentally investigate the densities and viscosities of gas-to-liquid (GTL)− biodiesel and GTL−diesel blends for various fuel temperatures and blending ratios. The biodiesel used in this study was derived from soybean oil, and was added to GTL from 20% to 100% by volumetric ratio. In the case of the GTL−diesel blend, diesel was added at 30% and 70% by volumetric ratio. Based on the experimental results, the empirical correlations for densities and viscosities were derived for variations in fuel temperatures and blending ratios. The densities of GTL−biodiesel and GTL−diesel blends decreased linearly with increasing fuel temperature and GTL is insensitive to temperature change compared to biodiesel and diesel. The dynamic and kinematic viscosities of GTL−biodiesel and GTL−diesel blends decreased exponentially with increasing fuel temperatures. As the fuel temperatures increased, the rate of change in viscosities for the temperature change significantly decreased. The increase of biodiesel and diesel in GTL blended fuels caused an increase in density. At a given temperature, the rates of density increase in the GTL−biodiesel and GTL−diesel blends showed similar values. The rates of density increase caused by biodiesel blending were higher than that caused by diesel blending due to the high density of biodiesel. With increased fuel temperature, the variations in viscosity from the blending of biodiesel or diesel with GTL decreased. In terms of the interdependence of density and kinematic viscosity, the density and the kinematic viscosity were positively correlated. At the same density conditions, an increase in biodiesel or diesel content in GTL blended fuels caused a decrease in the kinematic viscosity of the blended fuels. high cetane number.6,7 Therefore, investigations of the application of GTL to diesel engines have actively progressed. Paul et al.8 investigated the combustion and emissions characteristics of GTL and GTL blended diesel fuels through a vehicle emission test, an engine bench dynamometer test, and an optical engine test. These tests revealed that the use of GTL fuel in a diesel engine causes a considerable reduction of exhaust emissions, such as a reduction of HC and CO emissions greater than 90%, a PM reduction of about 30% in the vehicle emission test, and a 30−60% reduction of soot emission and approximately a 10% reduction of NOx emissions in the engine bench dynamometer test. These researchers also observed better vaporization characteristics and more uniform flame distribution of GTL fuel in the optical engine test. Moon et al.9 studied the emission characteristics of a GTL−biodiesel blend. In their investigation, they reported that the GTL− biodiesel blend induced the reduction of THC (22−56%) and CO (16−52%) emissions, while causing a slight increase in NOx emissions due to the increase in oxygen contents by the addition of biodiesel fuel. Also, they investigated the particle size distribution. As the GTL−biodiesel fuel blends were

1. INTRODUCTION During the past few decades, the depletion of fossil fuel by increased energy consumption has increasingly become an issue of significant concern. Currently, eco-friendly alternative fuels, such as biodiesel, bioethanol, and biobutanol, are being developed to address this increased energy consumption, and research related to their performance in engines is actively progressing. However, the development and use of biofuel sometimes cause side effects such as increases in grain price. To compensate for this weakness, interest in gas-to-liquid (GTL) fuel is increasing. GTL fuel can be obtained from natural gas, and it is easily stored and transported. In addition, interest and investigations related to natural gas are progressing due to a decrease in coal-energy use and an increase in natural gas use.1 GTL is produced through the methane reforming process, Fischer−Tropsch (FT) synthesis, and hydrocracking processes.2−5 The FT process involves the conversion of a mixture of carbon monoxide and hydrogen to various liquid hydrocarbons using a catalyst. GTL fuel has lower sulfur and poly aromatic hydrocarbon contents than conventional diesel fuel, thereby reducing particulate matter (PM) formation. In addition, the engine noise and production of nitrogen oxides (NOx) can be reduced using high exhaust gas recirculation (EGR) and a low compression ratio because GTL fuel has a © 2012 American Chemical Society

Received: July 9, 2012 Revised: December 4, 2012 Published: December 4, 2012 56

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applied to a diesel engine, the total particle number decreased by 46% through the process of a decrease in the accumulation mode and a slight increase in the nucleation mode. In addition, Hassaneen et al.10 investigated the effect of GTL and rapeseed methyl ester on the fuel economy and emission characteristics of a multicylinder diesel engine under various engine load conditions. In their study, the CO and HC emissions of biodiesel and GTL were found to be below that of diesel fuel for most loads. In the case of NOx and particulate emissions, diesel and GTL fuel showed very slight differences but the GTL fuel had lower emissions than the diesel fuel. In addition, the CO2 emission of the GTL was almost 5% lower than that of the diesel and biodiesel fuels. In addition, Azimov et al.11 investigated the spray and combustion characteristics of GTL and a GTL−biodiesel blend under quiescent ambient conditions. They revealed that the GTL with 20 vol % biodiesel resulted in increased spray tip penetration, decreased spray cone angle, and advanced ignition timing. Furthermore, they reported that the GTL−biodiesel blend had a slightly higher soot concentration than pure GTL because of the sulfur content of biodiesel fuel. In addition to the above studies, there are many other works relating to the effects of GTL fuel on emission reduction.12−14 Thus, the application of GTL to diesel engines is progressing in various aspects. However, there are very few studies about the fuel properties of GTL fuels. To analyze the combustion and emission characteristics including emission formation in diesel engines operating with new types of fuel, it is essential to analyze and investigate the fuel properties as well as fuel spray behavior and atomization characteristics. Fuel atomization and spray behavior in the combustion chamber play an important role on the combustion and emission characteristics, and they are affected by the injector nozzle type, injection conditions, and fuel properties.15 The fuel density mainly affects the spray momentum and the distribution of the equivalence ratio. The viscosity and surface tension influence droplet atomization and mixture formation. Therefore, the analysis and understanding of fuel properties are very important in the study of engine performance and emission characteristics. In this study, we report on the fuel properties of GTL− biodiesel and GTL−diesel blends. In particular, the densities and viscosities, which are the most important properties affecting the spray behavior and atomization characteristics, of GTL fuels blended with diesel or biodiesel were measured and analyzed for various fuel temperatures. In addition, empirical correlations for density and viscosity are suggested using measured properties.

Table 1. Physical and Chemical Properties of the Pure Fuels test method

test items cetane index density at 15 °C, kg/m3 viscosity at 40 °C, mm2/s total sulfur, mg/kg flash point, °C pour point, °C cloud point, °C ash content, wt % polycyclic aromatic hydrocarbons, wt % lower calorific value, MJ/kg CFPP, °C

GTL

diesel

biodiesel (soybean)

EN ISO 4264 EN 12185 EN ISO 3104 EN ISO 20846 EN ISO 2719 ASTM D 97 ASTM D 2500 EN ISO 6245 EN 12916

85.5

53.2

54.7

778.4 2.704

826.2 2.359

882.2 4.2

1.4

7.4

1

85.0

61.0

180

−24

−39

−5

−19

−10

4

0.001

0.001

0.002

0

1.6

-

ASTM D 240 ASTM D 6371

43.561

43.038

31.926

−20

−27

0

containers, and the temperature was maintained around 18−20 °C. The composition ratios and abbreviations of the test fuels are listed in Table 2.

Table 2. Test Fuels Used in This Study and Their Abbreviations fuel type

abbreviation

diesel (ULSD) 100% GTL 30% + diesel 70% (vol) GTL 70% + diesel 30% (vol) GTL 100% GTL 80% + biodiesel 20% (vol) GTL 60% + biodiesel 40% (vol) GTL 40% + biodiesel 60% (vol) GTL 20% + biodiesel 80% (vol) biodiesel 100%

D100 G30D70 G70D30 G100 G80B20 G60B40 G40B60 G20B80 B100

2.2. Experimental Setup and Procedure. In this study, the fuel density was calculated from the specific gravity of the test fuel and the well-known density of pure water. Figure 1a shows the schematic of the modified device for measuring specific gravity at various fuel temperatures, from 20 to 150 °C. To measure the specific gravity for the fuel temperature change, a temperature bath was installed around the graduated cylinder, and it was filled with diesel fuel with a high boiling point for safety and to prevent vaporization. The fuel temperature was controlled using a heating control system and an alcohol lamp. The fuel temperature was measured using two K-type thermocouples (Omega, KMTSS-020G-12, 0.8 mm diameter), which were installed at the upper and lower regions of the measuring cylinder. The measured temperature was displayed using a thermometer (Omega, 2168A). For experimental accuracy, the specific gravity was measured when the temperature difference between thermocouples was within ±0.1 °C. Furthermore, the top of the measuring cylinder was covered with a cap to prevent the evaporation of test fuels. Each test fuel sample (400 mL) was placed in a graduated cylinder, and then the temperature bath and graduated cylinder were heated to the specific temperature for the measurement of specific gravity. Under the constant temperature, the specific gravity was measured using a hydrometer (ASTM E 100). The measurement temperature varied from 20 to 150 °C, with measurements performed every 10 °C. At each condition, the measurements were conducted more than five

2. EXPERIMENTAL SECTION 2.1. Test Fuels. As a pure test fuel for blending, we selected GTL and diesel fuel, which were obtained by the standard methods, ASTM WK 23320 and ASTM D 975, respectively. The properties of biodiesel fuel derived from soybean oil used in this study met the specifications in ASTM D 6751. The detailed fuel properties for the pure materials are listed in Table 1. In this work, the blended fuels were divided into GTL−biodiesel blend and GTL−diesel blend categories. In the GTL− biodiesel blend, biodiesel fuel was added to GTL in increments of 20 vol % from 20 to 100 vol %. In the GTL−diesel blend, there were two blended fuels, GTL with 30 vol % diesel and with 70 vol % diesel. For the comparative and accurate analysis, 100% GTL, biodiesel, and diesel fuels were also measured and investigated. The blended fuels were made through sufficient stirring for over 5 min, and the volume of all blended fuels was the same, 500 mL. In addition, all fuels tested in this study were stored in fully sealed 57

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rotation of the spindle is stable at a given rotational speed. In the same way, the band heater and electric refrigeration system were installed to investigate the effect of the fuel temperature on the dynamic viscosity. The sample tube was covered with the heater band and was installed with an adiabatic insulator to maintain the temperature. A K-type thermocouple with a 0.8-mm diameter was installed inside of the sample tube for the precise measurement of the fuel temperature. Measurements were performed in the temperature range of 20−150 °C in steps of 10 °C. A digital thermometer (Omega, 2168A) indicated the fuel temperature from the thermocouple. The quantity of the test fuel in the sample tube was 16 mL, and the sample tube was washed for each experimental case. In addition, all tests were performed in the test chamber to minimize the outside factors such as temperature fluctuations and changes in humidity and dust. In this investigation, the dynamic viscosity (mPa·s) was calculated from the measured viscometer reading and a conversion factor. The conversion factor for the spindle speed is listed in Table 3. To improve the

Table 3. Setup for Viscosity Measurements spindle speed (rpm)

shear rate

conversion factor (K)

60.0 30.0 12.0 6.0

73.38 36.69 14.68 7.34

0.1 0.2 0.5 1.0

measurement accuracy, the sample tube and spindle were cleaned with industrial-grade ethanol before every change in test fuel, and the correction of the zero point was implemented by using a standard viscosity liquid. Viscosity measurements followed ASTM D 4307. In this investigation, the spindle speed and conversion factor were fixed at 30 rpm and 0.2, respectively.

3. RESULTS AND DISCUSSION Figure 2 shows the density variations of the GTL−biodiesel and GTL−diesel blends for a range of fuel temperatures from 20 to 150 °C. The figure shows the calculated density values derived from empirical correlation, as well as the measured density values. In the figure, the symbols indicate the experimentally measured densities, and the solid line indicates the calculated densities. As shown in Figure 2, the increase in fuel temperature induced density decreases in the pure (GTL, biodiesel, and diesel) and blended fuels. Increased temperatures caused more active molecular motion due to the increased kinetic energy and motion velocity. Therefore, the molecular weight per unit volume decreased, and the density decreased. In many studies of the dependence of density on temperature, it has been shown that the density and temperature have a linear correlation.16−18 Using a linear correlation equation, the empirical correlation for the density−temperature of GTL− biodiesel and GTL−diesel blends can be obtained, as listed in Table 4. The empirical equations were derived from first-order linear interpolation, and the R2 values of the derived equations were greater than 0.99. R2 is the regression coefficient and indicates the suitability of the regression model. In addition, the accuracy of the model is high when R2 approaches 1. As shown in Table 4, as biodiesel or diesel fuel was added to GTL, the yintercept (corresponding to A) increased because the density of pure GTL was the lowest among the biodiesel, diesel, and GTL fuels. The gradient in the equations showed little difference for each blended fuel, and it generally increased with the increase in biodiesel or diesel blending ratios. This gradient illustrates a decreasing rate of density in blended fuels for an increase in unit temperature. The high gradient value means that the density of the blended fuels is sensitive to temperature change.

Figure 1. Schematic diagram of devices for the measurement of gravity and dynamic viscosity with variations in fuel temperature. The fuel density and kinematic viscosity were calculated from gravity and dynamic viscosity. times. Among the measured values, we selected the values with differences within 1%, which were then averaged. The measurement method for test fuels followed ASTM 1298 for reproducibility and accuracy. The specific gravity for the calculation of density was defined as the ratio between the density of the test material and pure water at 1 atm pressure and 4 °C at the same volume. The density was calculated as follows: densitytest fuel = specific gravity × densitypure water. In this study, 999.972 kg/m3 was used as the density of pure water. The measured specific gravities of GTL blended biodiesel and GTL blended diesel fuels for various fuel temperatures and blending ratios are listed in Supporting Information Table A. Figure 1b shows the schematic of the viscosity measurement system. The dynamic viscosity of the test fuels was measured with a viscometer (Brookfield, LVT) and an ultralow viscosity (UL) adapter (Brookfield, ULA-36). The viscometer consisted of a main body with an indicator needle and stand, and the UL adapter consisted of a precision cylindrical spindle rotating inside an accurately machined tube, as shown in Figure 1b. With the viscometer used in this study, it was possible to measure from 1.0 to 2.0 × 106 cP at spindle speeds of 0.3 to 60 rpm. Viscosity is a measure of the internal friction of a fluid. This friction becomes apparent when a layer of fluid is made to move in relation to another layer. As greater friction occurs, a greater amount of force is required to cause the spindle rotation. Measurement of the dynamic viscosity is possible by reading the indicator when the 58

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Therefore, GTL fuels are relatively insensitive to temperature change compared to biodiesel and diesel fuels. In contrast, the small differences in the gradient for each test fuel can be explained by the different composition components of the blended fuels (test fuels). GTL mainly consists of iso-paraffin and normal-paraffin, and the main component of the diesel used in this test was aromatic compounds. When these test fuels were mixed, the impact of each component had an influence on fuel properties including density; consequently, a difference in gradient was observed. Figure 3 and Figure 4 show the dynamic and kinematic viscosity characteristics of GTL−biodiesel and GTL−diesel blends, respectively, for the temperature range from 20 to 150 °C. As shown in the figures, the viscosities exponentially decreased with an increase in temperature. Generally, in liquids, the cohesive forces between the molecules dominate the molecular momentum transfer between the molecules. In addition, the increased temperature causes an increase in the average kinetic energy of the molecules in a liquid, which easily overcomes the attractive forces that hold the molecules together. The symbols and the solid line indicate the measured and calculated viscosities, respectively, in the figures. Because the viscosity exponentially decreased with the increased temperature, an empirical correlation for viscosity−temperature correlation can be calculated as follows: Dynamic viscosity:

Kinematic viscosity:

⎛ T − δ⎞ ηempirical = α + β exp⎜ − ⎟ γ ⎠ ⎝

(1)

⎛ T − δ′ ⎞ νempirical = α′ + β′exp⎜ − ⎟ γ′ ⎠ ⎝ (2)

Figure 2. Fuel densities of GTL−biodiesel and GTL−diesel blends at various fuel temperatures. The symbols and line indicate the measurement and calculated values, respectively.

In addition, the offset delta (δ) was applied to the correlation equation in order to improve the accuracy of the equation because the viscosity was measured starting at 20 °C. The detailed constants and R2 values for the blended GTL− biodiesel and GTL−diesel fuels are represented in Tables 5 and 6. As listed in these tables, the derived empirical correlation for both viscosities reflected the measured viscosities well (i.e., R2 values were greater than 0.99). However, the constant alpha (α) generally showed an increasing trend with increased biodiesel blending ratio because the viscosity of biodiesel is significantly higher than that of GTL. In addition, the beta (β), indicating the sensitivity of viscosity with temperature change, increased with the increasing proportion of biodiesel fuel. This is the reason biodiesel fuel is more sensitive to temperature change compared to GTL fuels. With the increase in fuel temperature, the rate of change of the viscosity gradually decreased. The lower part of Figure 3a is the extended figure of the dynamic viscosities of GTL− biodiesel blends at 50 and 100 °C, respectively. As shown in the figure, the gradient at both indicated temperatures increased with increasing biodiesel fuel content. This phenomenon can be mathematically confirmed from the derivative of the empirical correlation for viscosity. At a 50 °C fuel temperature, the gradient of B100 was higher than that of G100 by about 104.8% (G100: −0.029 cP/°C, B100: −0.061 cP/°C). This can also be observed at a fuel temperature of 100 °C (e.g., at 100 °C, B100 has a higher gradient than G100 by about 101.4%). As fuel temperature increased from 50 to 100 °C, the gradient of each test fuel significantly decreased by about 70%. The decreasing rate for each fuel ranged from 68.1% to 73.3%

Table 4. Empirical Equations Developed by Linear Fitting for the Fuel Densities of GTL−Biodiesel or GTL−Diesel Blends ρemp , testfuel = A − BT (a) GTL blended biodiesel fuels fuel type

empirical equations

R2

G100

ρemp , G100 = 784.93 − 0.644T

0.997

G80B20

ρemp , G80B20 = 806.08 − 0.633T

0.995

G60B40

ρemp , G60B40 = 829.93 − 0.707T

0.999

G40B60

ρemp , G40B60 = 848.96 − 0.686T

0.999

G20B80

ρemp , G20B80 = 870.37 − 0.677T

0.999

B100

ρemp , B100 = 891.39 − 0.706T

0.998

(b) GTL blended diesel fuels fuel type

empirical equations

R2

G100

ρemp , G100 = 784.93 − 0.644T

0.997

G70D30

ρemp , G70D30 = 800.34 − 0.692T

0.998

G30D70

ρemp , G30D70 = 817.465 − 0.692T

0.998

D100

ρemp , D100 = 830.38 − 0.694T

0.995

59

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Figure 3. Dynamic and kinematic viscosities of GTL−biodiesel blends for various fuel temperatures. The kinematic viscosities with respect to fuel temperature were calculated from density and dynamic viscosity.

Figure 4. Dynamic and kinematic viscosities of GTL−diesel blends for various fuel temperatures. The symbols and line indicate the measurement and calculated values, respectively.

(average: about 70.2%). From this result, we can conclude that the viscosity change is insensitive to fuel composition. For convenience, the dynamic and kinematic viscosities of the GTL−biodiesel blends are listed in Supporting Information Tables B and C. Figure 4 shows the dynamic and kinematic viscosities of GTL−diesel blends versus fuel temperature. As the GTL− biodiesel blend, the dynamic and kinematic viscosities of GTL− diesel blend showed a decreasing trend with an increase in fuel temperature. Based on the viscosities at 50 and 100 °C, the increase in the diesel blending ratio caused a decrease in the gradient of viscosity change per temperature change. The

gradient of D100 decreased about 16% compared to that of G100 (gradients were G100: −0.0299 cP/°C; G70D30: −0.0291 cP/°C; G30D70: −0.0252 cP/°C; D100: −0.0251 cP/°C). In addition, the gradient change rate at 100 °C was lower than that at 50 °C. For convenience, the dynamic and kinematic viscosities of the GTL−biodiesel blends are listed in Supporting Information Tables B and C. In the analysis of the gradients from Figures 3 and 4, the gradient of viscosity with respect to temperature was obtained from the derivative of the empirical equation, as follows: 60

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Table 5. Empirical Equations Developed by Exponential Fitting for the Dynamic Viscosity of GTL−Biodiesel and GTL−Diesel Blends ⎛ T − δ⎞ ηempirical = α + β exp⎜ − ⎟ γ ⎠ ⎝ (a) GTL blended biodiesel fuels fuel type G100 G80B20 G60B40 G40B60 G20B80 B100

α

β

0.5809 2.59 0.5793 2.86 0.6798 3.33 0.6530 3.69 0.8080 4.32 0.7495 5.33 (b) GTL blended

γ 43.27 43.22 39.75 43.77 37.91 42.65 diesel fuels

δ

R2

20.06 19.30 19.08 19.41 19.00 19.67

0.999 0.998 0.998 0.999 0.998 0.999

fuel type

α

β

γ

δ

R2

G100 G70D30 G30D70 D100

0.5809 0.5557 0.5290 0.5398

2.59 2.55 2.28 2.16

43.27 41.48 44.71 40.09

20.06 18.95 18.65 19.35

0.999 0.997 0.997 0.998

Table 6. Empirical Equations Developed by Exponential Fitting for the Kinematic Viscosity of GTL−Biodiesel and GTL−Diesel Blends ⎛ T − δ′ ⎞ νempirical = α′ + β′exp⎜ − ⎟ γ′ ⎠ ⎝ (a) GTL blended biodiesel fuels fuel type G100 G80B20 G60B40 G40B60 G20B80 B100

α′

β′

0.8551 3.24 0.8266 3.49 0.9480 3.96 0.8919 4.31 1.0581 4.92 0.9641 5.96 (b) GTL blended

γ′ 43.96 44.07 40.07 44.32 38.13 43.58 diesel fuels

δ′

R2

20.21 19.31 19.05 19.39 19.00 19.68

0.999 0.998 0.998 0.998 0.998 0.999

fuel type

α′

β′

γ′

δ′

R2

G100 G70D30 G30D70 D100

0.8551 0.8021 0.7498 0.7504

3.24 3.13 2.73 2.55

43.96 41.94 45.41 40.23

20.21 18.96 18.61 19.37

0.999 0.996 0.996 0.998

β ⎛ T − δ⎞ dη = − exp⎜ − ⎟ γ γ ⎠ dT ⎝

(3)

As shown in eq 3, the constant alpha (α) did not affect the gradient of the viscosity. In addition, the gradient was affected by beta (β), gamma (γ), delta (δ), and the indicated temperature. However, γ and δ did not affect the relative values of the gradient because they showed similar values regardless of fuel type, as seen in Table 5. Therefore, under the same temperature conditions, the gradient of the viscosity for temperature change was mainly affected by β. From this analysis, it can be said that the value of β in the empirical correlation is the index that represents the properties characteristics of the test fuel. Figure 5 represents the difference between measured and calculated properties as an error percentage. The error percentage of each test fuel was calculated with the following equation:

Figure 5. Error analysis of calculated value (density, dynamic viscosity, and kinematic viscosity) for measured value in the range of fuel temperatures from 20 to 150 °C.

Ea =

measured − calculated × 100(%) measured

(4)

As shown in Figure 5a, the allowable error of the calculated value for density was about ±0.5%. From this, the empirical correlation representing the density variation with temperature change reflects the measured density values well. In Figure 5b and c, most viscosity values were within ±4%. Therefore, the empirical equations for viscosity are suitable for predicting the viscosities with changes in temperature. However, for the high fuel temperature region (e.g., 140 and 150 °C), the error percentage for calculated dynamic and kinematic viscosities was 61

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volume, and the mass was directly affected by the molecular weight. The densities of blended fuels can be expected by the following equations using the volume and mass fractions of the binary fuel blend:

about 8%, which was relatively high. In this region, the absolute difference between measured and calculated values was insignificant, about 0.06, but the error seems high because the viscosities at high fuel temperature are very low, and slight differences lead to high error. Therefore, a difference of about 0.06 in viscosity can be disregarded at these temperatures. Figure 6 shows the density variations for the blending ratios of GTL−biodiesel and GTL−diesel blends at the specific

ρI = r1ρ1 + r2ρ2

Volume fraction:

Mass fraction:

y1 =

ρII = y1ρ1 + y2 ρ2

ρ1V1 ρ1V1 + ρ2 V2

,

y2 =

(5) (6)

ρ2 V2 ρ1V1 + ρ2 V2

(7)

In eqs 5−7, r and y indicate the volume fraction and the mass fraction, respectively, and subscripts 1 and 2 indicate the GTL and biodiesel or diesel fuels, respectively. In Figure 6, the dotted and dashed lines indicate the calculated densities by volume fraction and mass fraction, respectively. As shown in Figure 6, the density calculated by the two equations corresponded to the measured densities of the GTL−biodiesel blends. The allowable error of the calculated density was ±0.3%. These results can also be seen in the density results for the GTL−diesel blend, as shown in Figure 6b. GTL consists mainly of molecular structures with heavy molecular weight, such as normal paraffin and iso-paraffin, while diesel fuel consists of aromatic compounds and some additives with relatively light molecular weight.11 The 10 vol % increase in the diesel blended fraction caused a 4.3 kg/m3 increase in density in whole GTL−diesel blend. In addition, the variation in density with changes in the fuel temperature was insignificant. The mean variation of density for a 10 vol % increase of diesel fuel at a specific temperature is as follows: 20 °C: 4.3 kg/m3; 60 °C: 4.5 kg/m3; 100 °C: 4.0 kg/m3; 140 °C: 4.0 kg/m3. The calculated densities for GTL−diesel blends reflected these measured densities well, and their allowable error was ±0.3%. Figure 7 shows the variations in dynamic viscosity with increased biodiesel blending ratios in GTL−biodiesel blends. Figure 7a depicts the dynamic viscosity calculated from the linear correlation equation using mass and volume fractions, along with the measured dynamic viscosity. As shown in Figure 7a, the increase in the contents of biodiesel with a relatively high viscosity induced an increase in viscosity in the GTL− biodiesel blend. At a 20 °C fuel temperature, a 10 vol % increase in biodiesel in the blended fuel caused an increase in dynamic viscosity of about 0.28 mPa·s on average. In addition, this increased rate of viscosity for a 10 vol % biodiesel increase became gradually higher. At the initial 20 vol % addition of biodiesel to pure GTL fuel, the increased density per 10 vol % biodiesel was 0.12 mPa·s, while the final 20 vol % addition (G20B80 → B100) showed an increasing rate of 0.48 mPa·s for 10 vol % biodiesel. This means that the viscosity is mainly determined by the cohesive force among molecules. The molecules in biodiesel fuel have a higher cohesive force than those in GTL fuel. Therefore, the increased biodiesel fraction in the GTL−biodiesel blends caused an increase in the viscosities of the blended fuels. In addition, as shown in Figure 7, there are some differences between the measured values and those calculated using the mass fractions and volume fractions of G100 and B100. Specifically, at the lower fuel temperature conditions, a greater difference was observed compared to those at the higher fuel temperatures. This is due to the weakening of interactions (cohesive force) among molecules. This result also indicated that the viscosity was determined by the interaction among the molecules, as well as the ratio of the molecular weights. Therefore, it is difficult to predict the

Figure 6. Density variation of GTL fuel by blending biodiesel or diesel at constant fuel temperatures.

temperature conditions. Some empirical equations for the density of mixed fuels have been derived, especially for biodiesel and diesel blended fuels.17,19−21 As shown in Figure 6a, the increase in blending ratio of the biodiesel fuels with high density caused a linear increase in density in the GTL− biodiesel fuels blends. The density of GTL−biodiesel blend increased by about 10.4 kg/m3 for every 10 vol % increase in biodiesel fuel. The increasing rate variation of density with increased fuel temperature was negligible (i.e., the increasing rate of density at specific temperature: 20 °C: 10.4 kg/m3; 60 °C:10.3 kg/m3; 100 °C: 10.1 kg/m3; 140 °C: 9.8 kg/m3, for 10 vol % increase in biodiesel). At the same fuel temperature, the density variation for the biodiesel blend from 0 to 20 vol % represented a value similar to the density variation when biodiesel fuel increased from 80 to 100 vol %. This is the reason the density was affected by the fuel temperature and composition of the blended fuels. In addition, this is because the density was calculated as the ratio of the mass per unit 62

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Figure 8. Dynamic viscosity variations of GTL−diesel blends versus the biodiesel blending ratio using the volume and mass fractions at the same fuel temperatures.

figure, it can be observed that the increase in diesel contents caused a decrease in viscosity in the blended fuels, because the GTL fuel has a constituent with a long chain structure, and, accordingly, the high interaction among molecules caused a high viscosity in the GTL fuel.11 In addition, the difference in dynamic viscosity among GTL, D100, and the GTL−diesel blend significantly decreased with increasing fuel temperatures. In the GTL−diesel blends, the linear correlation equations using volume fractions and mass fractions predicted the measured values well, and their maximum allowable error was 3%. This small difference in allowable error results from the small difference in viscosity between GTL and D100. At a fuel temperature of 20 °C, the dynamic viscosity of GTL was 3.17 mPa·s and that of D100 was 2.69 mPa·s. Figure 9 depicts the differences between the measured and calculated values for density and dynamic viscosity. Figure 9a shows the density variations of GTL−biodiesel and GTL− diesel blends as a function of the fuel temperatures and blending ratios. The diagonal in the figure shows that the measured value is the same as the calculated value. Therefore, both calculated and measured results indicated that the empirical correlation derived by volume and mass fraction estimated the measured values well. As shown in Figure 9a, the measured and calculated densities were well matched, and the error was very low. In addition, the difference between the measured densities and those calculated using mass fractions and volume fractions was insignificant. In contrast, there were some differences between measured and calculated dynamic viscosities, as shown in Figure 9b. At higher viscosities (greater than 3.0 mPa·s), the differences became large. In the figure, the high viscosity was in the region of relatively low fuel temperature (20 °C). At low fuel temperatures, the interaction among molecules increased, and the cohesive force also increased. Therefore, the linear correlation equation using the mass fraction and volume fraction overestimated the dynamic viscosity. Figure 10 shows the interdependence of density and kinematic viscosity for GTL−biodiesel and GTL−diesel blends at various fuel temperature conditions. As shown in Figure 10a and b, the kinematic viscosity increased with the increase in density. This can be explained in that an increase in density is an increase in the number of molecules per unit volume. Hence, the interaction (cohesive force) among many molecules per unit volume increased, and consequently, the viscosity

Figure 7. Dynamic viscosity variations of GTL−biodiesel blends versus the biodiesel blending ratios at constant fuel temperatures: (a) using the volume and mass fractions, and (b) using the quadratic correlation equation.

dynamic viscosity of the fuels blended of GTL and biodiesel (or diesel) accurately by the linear correlation equation using volume fractions and mass fractions. Figure 7b shows the calculated dynamic viscosity values of GTL−biodiesel blend, which were obtained from the correlation equations using a second-order polynomial equation. As shown in the figure, the calculated dynamic viscosities agreed well with the measured values, and the allowable error was 0.5%. The correlation equations of the GTL−biodiesel blends for each biodiesel blending ratio are listed in Table 7. Figure 8 shows the dynamic viscosity characteristics of GTL−diesel blends according to the blending ratios. From the Table 7. Empirical Equations for the Kinematic Viscosity of GTL−Biodiesel Blendsa y = C + D1x + D2x 2

a

temperature

C

D1

20 °C 60 °C 100 °C 140 °C

3.1914 1.6261 0.9568 0.7134

0.00784 5.518 × 10−4 0.00383 0.001

R2

D2 20.36 10.49 2.187 2.143

× × × ×

10−5 10−5 10−5 10−5

0.993 0.991 0.992 0.998

x: Biodiesel contents ratio; y: kinematic viscosity. 63

dx.doi.org/10.1021/ef301150k | Energy Fuels 2013, 27, 56−65

Energy & Fuels

Article

Figure 10. Correlation between density and kinematic viscosity versus the temperature change in GTL−biodiesel and GTL−diesel blends (temperature range: 20−150 °C).

and cohesive forces of the GTL molecules are larger than those of the diesel and biodiesel fuels. In other words, the length of the molecules in GTL is highest among the three fuels (GTL, biodiesel, and diesel).

4. CONCLUSIONS In this study, the densities and viscosities (dynamic and kinematic) of GTL−biodiesel and GTL−diesel blends were experimentally investigated for various fuel temperatures and blending ratios. Also, the empirical correlations for the density and the viscosities were derived. The conclusions are as follows: 1. The density of GTL−biodiesel and GTL−diesel blends linearly decreased with increasing fuel temperature. GTL fuel is insensitive to temperature change compared with biodiesel and diesel fuels. Due to the different composition components of blended fuels, the gradients (decreasing rate of density) for each test fuel were different. 2. The dynamic and kinematic viscosities of GTL−biodiesel and GTL−diesel blends exponentially decreased with increasing fuel temperature. As the fuel temperature increased, the instant gradient of each test fuel significantly decreased. 3. Increasing biodiesel and diesel contents in GTL blended fuels caused increases in density in the blended fuels. At a given fuel temperature, the increased rates of density change in GTL−biodiesel and GTL−diesel blends show

Figure 9. Plots of correlations between measured and calculated values.

increased by the increase of attractive force. In contrast, in a comparison of densities and viscosities in GTL, biodiesel, and diesel fuel, the density of biodiesel (B100) was the highest, followed by diesel (D100) and GTL. In viscosity, the biodiesel was also highest, followed by GTL and diesel. The density of biodiesel was high, indicating numerous molecules per unit volume, which consequently led to a high viscosity. However, in the comparison of diesel and GTL, the trend orders for density and viscosity were different (density: diesel > GTL, viscosity: diesel < GTL). This can be explained by the constituents of diesel and GTL fuels. Diesel fuel has molecules of various lengths, while GTL has relatively long molecules.11,22 Therefore, GTL has a slightly lower density and higher viscosity than diesel fuel. At the same density conditions (vertical line in figure), the kinematic viscosities of the blended fuels with biodiesel or diesel decreased as biodiesel or diesel fuel was added to GTL. From this result, it can be said that the attractive 64

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(5) Gao, Y.; Gao, Z.; Jin, J.; Zhou, H.; Cohron, M.; Zhao, H.; Liu, H.; Pan, W. Synthesis gas production with an adjustable H2/CO ratio through the coal gasfication process: Effects of coal ranks and methane addition. Energy Fuels 2008, 22, 1720−1730. (6) Zheng, M.; Reader, G. T.; Hawley, J. G. Diesel engine exhaust gas recirculation − A review on advanced and novel concepts. Energy Convers. Manage. 2004, 45, 883−900. (7) Allansson, R.; Blakeman, P.; Cooper, B.; Hess, H.; Silcock, P. J.; Walker, A. P. Optimising the low temperature performance and regeneration efficiency of the continuously regenerating diesel particulate filter (CR-DPF) system. SAE Tech. Paper, SAE 2002-010428, 2002. (8) Schaberg, P.; Botha, J.; Schnell, M.; Hermann, H.; Pelz, N.; Maly, R. Emissions performance of GTL diesel fuel and blends with optimized engine calibrations, SAE Tech. Paper, SAE 2005-01-2187, 2005. (9) Moon, G.; Lee, Y.; Choi, K.; Jeong, D. Emission characteristics of diesel, gas to liquid, and biodiesel-blended fuels in a diesel engine for passenger cars. Fuel 2010, 89, 3840−3846. (10) Hassaneen, A.; Munack, A.; Ruschel, Y.; Schroeder, O.; Krahl, J. Fuel economy and emission characteristics of gas-to-liquid (GTL) and rapeseed methyl ester (RME) as alternative fuels for diesel engines. Fuel 2012, 97, 125−130. (11) Azimov, U. B.; Kim, K. S.; Jeong, D. S.; Lee, Y. G. Instantaneous 2-D visualization of spray combustion and flame luminosity of GTL and GTL-biodiesel fuel blend under quiescent ambient conditions. Int. J. Auto. Tech.-Kor. 2011, 12, 159−171. (12) Soltic, P.; Edenhauser, D.; Thurnheer, T.; Schreiber, D.; Sankowski, A. Experimental investigation of mineral diesel fuel, GTL fuel, RME and neat soybean and rapeseed oil combustion in a heavy duty on-road engine with exhaust gas aftertreatment. Fuel 2009, 88, 1− 8. (13) Nishiumi, R.; Yasuda, A.; Tsukasaki, Y.; Tanaka, T. Effects of cetane number and distillation of paraffinic diesel fuels on PM emission from a DI diesel engine. SAE Tech. Paper, SAE 2004-01-2960, 2004. (14) Kitano, K.; Misawa, S.; Mori, M.; Sakata, I.; Clark, R. H. GTL fuel impact on DI diesel emissions. SAE Tech. Paper, SAE 2007-012004, 2007. (15) Pandey, R. K.; Rehman, A.; Sarviya, R. M. Impact of alternative fuel properties on fuel spray behavior and atomization. Renew. Sustain. Energy Rev. 2012, 16, 1762−1778. (16) Tesfa, B.; Mishra, R.; Gu, F.; Powles, N. Prediction models for density and viscosity of biodiesel and their effects on fuel supply system in CI engines. Renew. Energy 2010, 35, 2752−2760. (17) Ramirez-Verduzco, L. F.; Garcia-Flores, B. E.; RodriguezRodriguez, J. E.; Jaramillo-Jacob, A. R. Preduction of the density and viscosity in biodiesel blends at various temperatures. Fuel 2011, 90, 1751−1761. (18) Esteban, B.; Riba, J.; Baquero, G.; Rius, A.; Puig, R. Temperature dependence of density and viscosity of vegetable oils. Biomass Bioenergy 2012, DOI: 10.1016/j.biombioe.2012.03.007. (19) Saldana, D. A.; Starck, L.; Mougin, P.; Rousseau, B.; Ferrando, N.; Creton, B. Prediction of density and viscosity of biofuel compounds using machine learning methods. Energy Fuels 2012, 26, 2416−2426. (20) Park, S. H.; Yoon, S. H.; Suh, H. K.; Lee, C. S. Effect of the temperature variation on properties of biodiesel and biodiesel-ethanol blends fuels. Oil Gas Sci. Technol. 2008, 63, 737−745. (21) Yoon, S. H.; Park, S. H.; Lee, C. S. Experimental investigation on the fuel properties of biodiesel and its blends at various temperatures. Energy Fuels 2008, 22, 652−656. (22) Lim, Y.; Shin, S.; Kim, J.; Yim, E.; Song, H.; Kim, D. Characteristics analysis of GTL fuel as an automobile diesel. J. Korean Ind. Eng. Chem. 2008, 19, 617−623.

similar values. The increasing rate of density change due to biodiesel blending is greater than that due to diesel blending, due to the high density of biodiesel. 4. The variation in viscosity due to the blending of biodiesel or diesel with GTL decreased according to the increase in fuel temperature. 5. The interdependence of density and kinematic viscosity showed that an increase in density caused an increase in kinematic viscosity. At the same density conditions, an increase in biodiesel or diesel contents added to GTL caused a decrease in kinematic viscosity in the blended fuels. Therefore, it can be said that the attractive force and cohesive force of GTL molecules are larger than those of diesel and biodiesel fuel.

ASSOCIATED CONTENT

S Supporting Information *

TablesA−C as noted in the text. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-2-2220-0427; fax: +82-2-2281-5286; e-mail: cslee@ hanyang.ac.kr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Second Brain Korea 21 Project and was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2011-0025295).

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NOMENCLATURE A, B, C, D = constants Ea = allowable error (%) r = volume fraction R2 = regression coefficient T = fuel temperature (°C) V = volume (m3) y = mass fraction ρcalculated = calculated fuel density (kg/m3) ρmeasured = measured fuel density (kg/m3) ηcalculated = calculated dynamic viscosity (mPa·s) ηmeasured = measured dynamic viscosity (mPa·s) α, β, γ, δ = constants

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REFERENCES

(1) Kimura, S.; Aoki, O.; Kitahara, Y.; Aiyoshizawa, E. Ultra-clean combustion technology combining a low-temperature and premixed combustion concept for meeting future emission standards. SAE Tech. Paper, SAE 2001-01-0200, 2001. (2) Rakopouls, C. D.; Antonopoulos, K. A.; Rakopoulos, D. C.; Hountalas, D. T.; Giakoumis, E. G. Comparative performance and emissions study of a direct injection diesel engine using blends of diesel fuel with vegetable oils or bio-diesels of various origins. Energy Convers. Manage. 2006, 47, 3272−3287. (3) Wang, T.; Wang, J.; Jin, Y. Slurry reactors for gas-to-liquid process: A review. Ind. Eng. Chem. Res. 2007, 46, 5824−5847. (4) Aasberg-Petersen, K.; Christensen, T. S.; Nielse, C. S.; Dybkjaer, I. Recent developments in autothermal reforming and pre-reforming for synthesis gas production in GTL applications. Fuel Process. Technol. 2003, 83, 253−261. 65

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