Combined High Pressure and Low Temperature Viscosity

Energy Fuels 2010, 24, 1293–1297 Published on Web 11/16/2009

: DOI:10.1021/ef900976x

Combined High Pressure and Low Temperature Viscosity Measurement of Biodiesel L. X. Robertson and C. J. Schaschke* Department of Chemical and Process Engineering, University of Strathclyde, 75 Montrose Street, Glasgow, G1 1XJ, Scotland, U.K. Received September 3, 2009. Revised Manuscript Received October 12, 2009

In this work, we report the measurement of the viscosity of biodiesel derived from sunflower oil and biodiesel blend with mineral diesel under combined conditions of high pressure and subambient temperature. Using a thermostatically controlled falling sinker-type viscometer, dynamic viscosity measurements were made at pressures up to 153 MPa, which are typical of those found in common-rail automotive diesel engines, and temperatures down to 0 °C. Reproducible and reliable viscosity data was obtained from sinker fall times. Calibration of the viscometer was based on n-dodecane. Biodiesel viscosities were found to increase exponentially with both rising pressure and reducing temperature. When combined with mineral diesel as a B20 blend, the viscosity was found to be less than for B100 for all temperatures considered. Both pressure and thermal freezing were found to also occur with increasing pressure and lowering temperature.

use of diesel blends with mineral diesel. European engine manufacturers have limited the biodiesel content to 5% (w/w) (B5) as a general requirement to maintain warranties.2 The majority of diesel engine vehicles are, however, capable of operating with up to 20% (w/w) biodiesel fuel blend (B20), without the need for engine modification, and consequently, biodiesel has been integrated into the fuel market. Another available blend is B10. For these blends, a stringent maintenance program must, however, be followed between oil and filter changes. Renault currently permits the use of a B30.2 Biodiesel can be produced by a number of possible process routes involving the transesterification of triglycerides that are reduced to free fatty acids (FFAs) and reacted with alcohol to form glycerol and fatty acid methyl esters (FAME).4 The biodiesel is separated and purified for use. The raw source of triglycerides may include vegetable oil, waste cooking oil (WCO), animal fat (tallow), or possibly algae (biofuel). The reaction is usually catalyzed by sodium hydroxide, although other catalytic agents such as potassium hydroxide and sodium methoxide are used to promote the solubility between the otherwise poorly miscible phases of oil and alcohol. The transesterification process can exhaust either the reagent or the catalyst. Correct operating conditions are required to produce biodiesel. The reaction may not be complete, however, should the operating temperature fall below that required for the reaction or should the processing time be insufficient. Where the reaction has not gone to completion, significant concentrations of diglycerides and monoglycerides may be present in the products. Diglycerides are responsible for coking in engines whereas monoglycerides are responsible for the cause of corrosion. For optimum performance of the biofuel, their combined concentration should be maintained less than 0.1%. Several biodiesel fuels and blends are currently available worldwide. Many vehicles now operate with high pressure common-rail automotive diesel engines. Like their

Introduction The quest for energy and fuel sources as an alternative to fossil fuels is not new. Only in recent years has there been a surge in interest in biodiesel largely due to its reduced environmental impact in comparison with petrochemical diesel.1 Derived from renewable organic sources, biodiesel is increasingly being used to substitute petrochemical diesel. From negligible production levels in 1990, EU production capacity of biodiesel exceeded 28 800 million liters in 2008 and is forecast to reach 35 500 million liters by 2010.2 Made chemically by combining any natural oil or fat with an alcohol, it can be made from a variety of renewable domestic raw materials and provides a competitive calorific value in terms of fossil fuel resources (around 32.3 MJ/L) and is of great importance in the petrochemical industry.3 In comparison to nonrenewable petroleum diesel, biodiesel is a sustainable energy source that is free of sulphates and aromatic compounds. It is also biodegradable. As a fuel, biodiesel is regarded as being carbon neutral insofar as its consequent release of carbon through combustion into the atmosphere is balanced by the mass removed from the atmosphere by its source crop. The apparent sustainability of biodiesel is, however, contradicted by the amount of land space that is required to grow oil-bearing crops. The complete replacement of diesel with biodiesel is therefore currently deemed unfeasible. There are also additional costs needed to modify automotive engines to combust biodiesel although certain older car engine models are capable of running on biodiesel alone without the need for ancillary equipment. These challenges have been in part overcome through the *To whom all correspondence should be sent. Telephone: 0141 548 2371. Fax: 0141 552 2302. E-mail: [email protected]. (1) Srivastas, R.; Prasad, R. Renew. Sustain. Energy Rev. 2002, 4, 111–113. (2) A Biofuels Compendium; IChemE.: 2009;ISBN 978-0-85295533-8. (3) Elsayed, M. A.; Matthews, R.; Mortimer, N. D. Carbon and Energy Balances For a Range of Biofuels Options, DTI Sustainable Energy Programmes, URN 2003, 03/836. r 2009 American Chemical Society

(4) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Renew. Sustain. Energy Rev. 2007, 11, 1300–1311.

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: DOI:10.1021/ef900976x

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petrochemical counterpart, diesel and biodiesel blends are expected to function through compressive combustion in which high-pressure injection enables rapid atomization and combustion to provide higher efficiencies and reduction in emissions.5 The specification of biodiesel derived from rapeseed or sunflower oil is defined in British Standard EN14214:2003. Under these extreme high pressure conditions both density and viscosity vary significantly and at a certain point will pressure-freeze. At low ambient temperature, diesel and biodiesel are also known to wax and solidify, which is a major issue particularly in the colder climes.6 Although there are some reports on the measurement and prediction of biodiesel properties at ambient or high-temperature conditions, comparatively little work has been undertaken at high pressure and reduced temperature.7-9 While high-temperature effects and the increase in viscosity at high pressure are well-known as well as the phenomenon of pressure-freezing, these phenomena are unlikely to effect the performance of the engine due to the length time to reach equilibrium in running engines. However, low-temperature applications, particularly at the point of starting an engine from cold and where the temperature is below ambient, are of more interest. In this work, we have therefore examined the combined pressure (up to 153 MPa) and reduced temperature effect (273, 278, 283, 288, and 294 K) on biodiesel and a biodiesel blend (B20) in terms of viscosity measurement to the point of pressure and temperature freezing.

position between each measurement the entire pressure vessel is inverted. From the time taken for the sinker to pass the detection coils, the viscosity can be determined analytically from the free descent of the sinker under the influence of gravity based on the shear stress profile across the annular gap previously shown to be given by:10 ! Fliq t ð2Þ η¼ 1FS A where A is based on the physical dimensions of the sinker and tube given by A¼

2πL L   S T  r22 - r21 r2 mg ln r1 - r2 þ r2 2

ð3Þ

1

In practice, there is a discrepancy between the actual viscosity and determined viscosity due to vortex shedding from the tail of the sinker present even at Reynold’s numbers below 25. This has been confirmed by both experiment and CFD analysis.10 It is also known that fully developed laminar flow does not exist within the annulus. In this work, we use a calibration coefficient, k, to correct this discrepancy, which was determined using a calibration liquid of known pressure-viscosity dependence. In this case, n-dodecane was used since it has a comparable viscosity to that of diesel and biodiesel to provide comparable sinker fall times to those of the tested fuels. Equation 1 was therefore modified: ! Fliq kt η¼ 1ð4Þ A FS

Experimental Section The high-pressure viscometer used in this work is based on a falling sinker design in which gravity is used to provide the applied force. Viscosity measurements are determined from the time taken for a cylindrical sinker to descend down a vertical tube containing the sample liquid. The entire viscometer tube is contained within a high-pressure vessel rated to 1 GPa. Having a hemispherical nose, and being self-centering, the descent of the sinker is detected by way of electrical signal induced by a ferrite core embedded into the sinker as it passes copper coils surrounding the tube. A change in inductance as the ferrite core of the sinker passes each coil is transmitted through a bridge circuit and amplified for capture on a PC and recorded as peaks. The time taken to pass two coils a given distance apart and the dimensions of the viscometer and sinker are sufficient to determine the viscosity of the liquid with an appropriate calibration. The applied pressure within the viscometer is imposed by a pressure intensifier using compressed air at a pressure of 7 bar through to a maximum operating pressure of 200 MPa, and is transmitted using a paraffin/Shell Tellus oil mixture as the hydraulic medium. The hydraulic pressure is able to be transmitted to the sample biodiesel using a PTFE expansion sheath located at the bottom of the viscometer tube to allow for compression of the biodiesel. A calibrated Kistler piezo-resistive pressure gauge type 4618A0 is used to measure the high pressure within the viscometer tube for which the correlation between voltage (mV) and pressure (MPa) is:

The viscometer operates with the vertical descent of the sinker down the tube. To return the sinker to its original starting

For the low-temperature measurements, the temperature of the viscometer tube was thermostatically controlled. The entire pressure vessel containing the viscometer tube was therefore immersed in 240 L of water-glycol mixture (3:1 volume ratio) with external circulation using a 2 kW Fryka DLK 2002 chiller unit. The tank containing the pressure vessel was insulated to reduce heat gain and was allowed to reach equilibrium after no shorter than 1 h. Materials. Pure biodiesel (B100) and a 20% (w/w) blend (B20) with mineral diesel were used in this study. The experimental data for mineral diesel used in this study has been previously reported.11 A supply of biodiesel was derived from pure sunflower oil and produced in the laboratory. This involved transesterification using methanol with potassium hydroxide as the catalyst following the procedure of Darnoko and Cheryan.12 Sulphuric acid is recommended to be added to methylate any free fatty acids (FFAs) to reduce the likelihood of saponification.13 Gas chromatography was used to confirm the composition of FAME against methyl palmitate, methyl stearate, methyl oleate, and methyl linoleate standards.14 This used a Hewlett-Packard 5890 capillary GC with FID detector. A Zebron column was used of length 30 m and inner diameter of 0.25 mm and film thickness of 0.25 μm. A 40:1 injector was used at 220 °C using helium gas as the carrier at 2 mL min-1. The oven temperature was controlled at 190-250 °C at 2 °C per minute for 30 min. Anhydrous sodium sulfate was used to dehydrate the FAME.

(5) Lee, S.; Tanaka, D.; Kusaka, J.; Daisho, Y. JSAE Rev. 2002, 23, 407–414. (6) Dunn, R.; Bagby, M. JAOCS 2006, 73, p1719–1728. (7) Ferrari, R. A.; Oliveira, V. S.; Scabio, A. Sci. Agric. 2005, 62 (3), 291–295. (8) Imahara, H.; Minami, E.; Saka, S. Fuel 2006, 85, 1666–1670. (9) Shu, Q.; Yang, B.; Yang, J.; Qing, S. Fuel 2007, 86, 1849–1854.

(10) Schaschke, C. J.; Abid, S.; Fletcher, I.; Heslop, M. J. Food Eng. 2008, 87, 51–58. (11) Paton, J. M.; Schaschke, C. J. Chem. Eng. Res. Des. 2009, doi:10.1016/j.cherd.2009.04.007. (12) Darnoko, D.; Cheryan, M. JAOCS 2000, 77, p1263–1267. (13) Maceiras, R.; Vega, M.; Costa, C.; Ramos, P.; Marquez, M. C. Fuel 2009, 88, p2130–2134. (14) Eder, K. J. Chromatogr. 1995, p113–131.

p ¼ 132V0 - 133

ð1Þ

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Figure 2. Variation of viscosity for B100 at ambient temperature and 0.1 MPa using the high pressure falling sinker viscometer. Figure 1. High pressure falling sinker viscometer calibration obtained using n-dodecane. Table 1. Composition of FAME in Biodiesel Produced from Sunflower Oil methyl palmitate methyl stearate methyl oleate methyl linoleate methyl linolenate

C16:0 C18:0 C18:1 C18:2 C18:3

6.63 ( 0.08% 1.89 ( 0.72% 57.81 ( 0.75% 33.49 ( 0.43% 0.18 ( 0.00%

Results In advance of high pressure viscosity measurements, the rheological behavior of the produced biodiesel at ambient temperature (293 K) and pressure (0.1 MPa) was examined using an AR1000N rheometer (TA Instruments, USA). This was based on a double concentric cylinder arrangement with electronic data capture. Applied shear rates up to approximately 30 s-1 were used for which the viscosity of the biodiesel was found to be 5.2 mPa s. It is usual to calibrate the high pressure viscometer using liquids of known viscosity and density with pressure. In this case, n-dodecane was used as the calibration fluid since it provided similar fluid properties to the biodiesel. Calibration data were obtained in triplicate (Figure 1). The calibration was examined in relation to a modified Reynolds number (Rem) within the annulus between the sinker and tube of the form: Rem ¼

2Fliq vS r21 ηðr2 þ r1 Þ

Figure 3. Variation of viscosity with temperature and pressure for B100.

The vegetable triglycerides were also analyzed for free fatty acid content. None was found. Using the high pressure falling sinker viscometer, the viscosities of the diesel and biodiesel were able to be measured at pressures up to 153 MPa, corresponding to the maximum pressure at which the sinker was able to descend freely. The pressure vessel, hydraulic fluid, viscometer tube and biodiesel were maintained at a controlled temperature. The viscosities at elevated pressure were obtained from direct measurement of the sinker fall times. Initial experiments were carried out at ambient pressure to determine the range of temperatures over which the testing could take place. Temperature-freezing was found to occur below 270 K and signified the lower end of the range. Figure 2 presents the range of temperatures for which viscosity data was obtained. The density data of biodiesel used to calculate viscosity data in eq 4 was determined previously based on the PengRobinson equation of state.11 Figures 3 and 4 present viscosity data for combined temperature and pressure for B100 and B20 (Table 2).

ð5Þ

It is reasonable to relate the coefficient, k, with Rem in the annulus. The derivation of Rem has been shown previously.11 High-viscosity fluids provide longer sinker fall times in which the fluid exhibits a lower Reynolds number. From the physical dimensions of the sinker, the constant A was found to be 3645 mPa-1 and from the calibration data using the properties of n-dodecane from Caudwell et al.,15 the value of k was determined to be 0.72. Both the commercially obtained diesel and laboratoryproduced biodiesel from sunflower oil were tested for FAME composition by gas chromatography. The FAME from sunflower oil was found to consist of chains of about 16-18 carbon atoms with varying degrees of saturation (Table 1).

The viscosity of polymers, lubricants, and oils is wellknown to increase with molecular complexity.16 Viscosity is also known to increase considerably with pressure. There are many approaches to the evaluation of viscosity with temperature for these complex fluids, although comparatively few are associated with the effects of pressure. The so-called Barus

(15) Caudwell, D. R.; Trusler, J. P. M.; Vesovic, V.; Wakeman, W. A. Int. J. Thermophys. 2004, 25 (5), 1339–1352.

(16) Schaschke, C. J.; Allio, S.; Holmberg, E. Trans IChemE 2006, 84 (C), 173–178.

Discussion

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Table 2. Viscosity Correlations with Pressure for B100 and B20 B100 temp (K) 273 278 283 288 290 294

B20 R2

viscosity (mPa s)

R2

η =7.31e (0.1 < p < 68 MPa) η = 6.23e0.0185p 0.1 < p < 106 MPa) 0.0189p (0.1 < p < 142 MPa) η = 4.54e η = 4.52e0.0148p(0.1 < p < 152 MPa)

0.973 0.996 0.991 0.995

η = 4.22e0.0140p (0.1 < p < 152 MPa)

η = 8.75e (0.1 < p < 30 MPa) η = 6.84e0.0183p (0.1 < p < 59 MPa) 0.0140p η = 5.89e (0.1 < p < 94 MPa) η = 4.76e0.0146p (0.1 < p < 120 MPa) η = 4.74e0.0112p (0.1 < p < 153 MPa)

0.882 0.967 0.944 0.979 0.997

0.999

viscosity (mPa s) 0.0254p

0.0351p

viscosity than biodiesel. It is also interesting to note that the maximum pressures obtained for the temperatures were comparatively less for the pure biodiesel. Again, this is attributed to the combined effect of biodiesel with mineral diesel. The mineral diesel is composed of hydrocarbon molecules ranging typically from C8 to C20 with the majority being around C16 in length. The biodiesel, on the other hand, comprises longer fatty acid methyl esters around C18 with the majority having one or two double bonds (methyl oleate and methyl linoleate). With the longer structure and more complex stereochemical effects, it can be surmised that the viscosity is higher for B100. Isothermal conditions were maintained by immersion of the pressure vessel containing the viscometer tube in an insulated water/glycol bath. An external chiller unit was used to maintain isothermal conditions. A sufficient period of one hour was used to ensure thermal equilibrium of both the vessel and tube within. The effects of adiabatic heating can also be reasonably assumed to be minimal since the mass and heat capacity of the pressure vessel and tube and the time of pressurization far outweigh the thermal properties of the test fluid. Temperature readings were taken immediately on opening the vessel to confirm isothermal temperatures inside. While the viscosity of the biodiesel increases exponentially according to the Barus equation for the range of temperatures used, sharp increases in sinker fall time were also noted at both high pressures and low temperatures. The increase in difficulty in sinker descent observed is due to a phase transition within the liquid and occurs at both low temperature and high pressure. The pressure-freezing phase transition of the longer chained molecules is well-known, particularly in tribology. The thermal-freezing effect producing waxes is particularly well-known and is a concern for motorists in cold atmospheric conditions not only in terms of fuel flow but also in terms of effectiveness of brake fluids. In both cases, once the biodiesel is returned to ambient conditions of temperature and pressure, the metastable solid material reverts to its liquid state. The implication for high-pressure automotive engines may be fuel blockage as either a pure biofuel or as a blended fuel, particularly on cold starts. Fortunately, the rate of solidification due to pressurization alone is unlikely to be a cause for concern in operating engines due to their high temperature environment. While the pressure-freezing and thermal-freezing points were noted, it is not evident how both pressure and temperature combine to influence phase change in biodiesel. In this work, we found that the freezing point of biodiesel occurred according to:

Figure 4. Variation of viscosity with temperature and pressure for B20.

equation, based on the concept of free volume, relates viscosity to pressure of the form: ð6Þ η ¼ η0 expðRpÞ Highly branched molecules require a greater free volume void in which to move.17 In this study, the experimental results for both B100 and B20 from the falling sinker viscometer illustrate this isothermal relationship in which samples of both B100 and B20 tested were found to increase with pressure. The Barus equation was applied to the experimental data with a good degree of certainty as shown in Figures 3 and 4. Experimentally, this presents itself as an increase in measured sinker fall time, taking typically 3000 ms to 300 000 ms at evaluated pressure. The exponential increase is to be expected due to the increasing compression of the molecules with pressure, which inhibits their movement. This is also seen in the increase in liquid density at elevated pressure. For both samples tested, the correlation coefficient R2 was better than 0.882 in all cases and generally was found to be around 0.99. Each data point presented was the mean of data repeated in triplicate, for which the standard deviation was better than 2%. The data presented was therefore considered to be both reproducible and reliable. Where markedly spurious sinker times were found during testing, these were able to be discounted. These were generally faster than expected times due to a known stable sinker position close to the tube wall in preference to the sinker descending in a self-centring manner. Comparing both sets of data that were carried out at the same isothermal conditions of 273, 278, 283, 288, and 290 K for B100 and 294 K for B100, it is evident that although the viscosities of both fuel types are similar at low pressure (0.1 MPa) and temperature, the effect of pressure on viscosity is more marked with the B20 blend of biodiesel with mineral diesel across the temperature range used. This is due to the dilution effect with mineral diesel, which has a slightly lower

p ¼ 6T - 1600

ð7Þ

and was valid from 273 to 294 K. Thus, at lower temperatures, the maximum pressure at which the biodiesel remained liquid reduced. There was no reading at 269 K (-4 °C) due to blockage. Figure 2 presents the temperature range over which viscosity measurements could therefore be taken.

(17) Kioupis, K. I.; Maginn, E. J. J. Phys. Chem. B 2000, 104, 7774– 7783.

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The accuracy of the derived viscosity data is dependent on the accuracy of density data for both the sinker and test fluid. Sinker density was measured from its dimensions and materials of construction. An allowance to the calculations with coefficients of thermal expansion and pressure compression can also be made. The use of thermostatically controlled and isothermal conditions were used to overcome adiabatic heating effects. The vessel and viscometer tube were then allowed to reach equilibrium for up to 1 h prior to measurement readings being taken. In practice, the chilling rate was found to be approximately 1 °C per 15 min. Another significant influence on determined viscosity is due to the compressibility of the test fluid. There is a growing body of literature reporting testing and prediction of liquid compressibility.15,18-21 Where data is not available, various expensive and often rarely available experimental methods such as micro-pVT and oscillating tube densimeters can be used.22 In the absence of experimental methods, correlation and predictive methods including the use of the Tait equation and equations of state for the prediction of compressed liquid densities are often used.23

is that increased viscosity affects the flow of fuel to the cylinders and affects performance of the engine. The accuracy of the data requires careful selection of a self-centring sinker and need for its calibration prior to testing. With sinker descents taking up to 5 min, and subsequent restart by returning the sinker back down the tube by inverting the pressure vessel, this can prove to be a lengthy process. The demand for accurate liquid biofuel data with pressure and temperature is essential if determined viscosity data is to be of any value. Nomenclature A = viscometer constant, mPa-1 g = gravitational acceleration, m s-2 k = viscometer coefficient (-) LS = length of sinker wall, m LT = length of tube between detection coils, m m = sinker mass, kg p = pressure, MPa Re = Reynolds number, (-) Rem = modified Reynolds number r1 = radius of sinker, m r2 = inner radius of tube, m T = temperature, K t = sinker fall time, s vS = terminal velocity of sinker, m s-1 V0 = voltage, mV

Conclusions Using a high pressure falling sinker viscometer, the viscosity of biodiesel as both a pure liquid fuel and as a blend with mineral diesel were found to increase exponentially with both increasing pressure and decreasing temperature below ambient. These conditions of pressure are typically found in the new generation of common-rail diesel engines. The temperatures correspond to cold climatic conditions. The implication

Greek Symbols R = coefficient, MPa-1 η = viscosity η0 = viscosity at ambient pressure, mPa s Fliq = liquid density, kg m-3 FS = sinker density, kg m-3

(18) Kashiwagi, H.; Makita, T. Int. J. Thermophys. 1982, 3 (4), 289– 305. (19) Dymond, J. H.; Robertson, J.; Isdale, J. D. J. Chem. Thermodynamics 1982, 14, 51–59. (20) Kiran, E.; Sen, Y. L. Int. J. Thermophys. 1992, 13 (3), 411–442. (21) Belonenko, V. N.; Troitsky, V. M.; Belyaev, Y. E.; Dymond, J. H.; Glen, N. F. J. Chem. Thermodynamics 2000, 32, 1203–1219. (22) Chang, R. F.; Moldover, M. R. Rev. Sci. Instrum. 1996, 67 (1), 251–256. (23) Dymond, J. H.; Malhotra, R. Int. J. Thermophys. 1987, 8 (5), 541–555.

Acknowledgment. Financial support is gratefully acknowledged from the Nuffield Science Bursary scheme grant reference URB/36706. The technical support from Mr S. Adams and Mr J. Murphy is also gratefully acknowledged.

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