Low-Temperature Properties of Renewable High-Density Fuel Blends

Dec 4, 2012 - Environmental, political, and economic drivers have greatly accelerated the development of renewable fuels over the past decade.(1) Full...
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Low-Temperature Properties of Renewable High-Density Fuel Blends Heather A. Meylemans,* Lawrence C. Baldwin, and Benjamin G. Harvey* Chemistry Division, Research Department, Weapons Division (NAWCWD), Naval Air Warfare Center, Naval Air Systems Command (NAVAIR), United States Navy, China Lake, California 93555, United States S Supporting Information *

ABSTRACT: The low-temperature properties of high-density terpene dimer fuels and fuel mixtures with JP-8, JP-10, and hydrogenated pinene have been studied by shear viscometry and thermomechanical analysis (TMA). Neat terpene dimers have a viscosity of 3.94 × 103 mPa·s at −10 °C, while 50:50 mixtures with JP-10, RJ-4, pinane, and JP-8 have viscosities 2−3 orders of magnitude lower at 23.9, 53.0, 24.9, and 3.7 mPa·s, respectively. Linear and branched alkanes in JP-8 disrupt glass formation of the dimers, explaining the significant difference between the viscosity afforded by bicyclic diluents and JP-8. To complement the viscosity data, TMA was used to observe low-temperature transitions (Tm and Tg) of the blended fuels. Mixtures of the terpene dimers with cyclic molecules show only glass transition temperatures with no observable melting points, while mixtures with JP-8 and decane show Tg values that transition to melting points at high concentrations of terpene dimers. The results suggest that blending conventional fuels with terpene dimers is an effective strategy for mitigating the high viscosity of the C20 molecules. In addition, blending these renewable fuels with conventional jet fuel (JP-8) imparts both a higher density as well as an improved volumetric net heat of combustion while maintaining an acceptable low-temperature viscosity when compared to JP-8 alone.

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INTRODUCTION Environmental, political, and economic drivers have greatly accelerated the development of renewable fuels over the past decade.1 Full-performance jet and diesel fuels, particularly for military applications, have demanding specifications that require saturated hydrocarbon fuels.2 The most widely studied renewable jet fuel, broadly defined as hydrotreated renewable jet (HRJ) fuel, is primarily composed of linear and branched chain alkanes that impart good low-temperature properties, efficient and clean combustion, sufficient gravimetric heat of combustion, and excellent stability.3 Despite these advantages, HRJ and most other renewable jet fuels have relatively low densities (∼0.76 g/mL) that require them to be blended with conventional jet fuel to meet specifications. Cycloalkanes (naphthenes) and aromatic compounds in petroleum-derived jet fuel increase the density of the blends, while the aromatics are vital for swelling elastomeric seals to maintain engine integrity. To improve the density of renewable jet fuels and boost the volumetric net heat of combustion (NHOC) of petroleum-derived jet fuels, our research group has recently investigated renewable terpene dimers as high-density fuel additives. Further, we have explored these hydrocarbons as a potential surrogate for the high-density missile fuel JP-10. In previous work, a set of terpene dimer fuels was synthesized from a series of monomeric terpenes, including β-pinene, αpinene, camphene, and limonene (Scheme 1), as well as from a crude turpentine feedstock, which is composed primarily of αpinene and camphene, along with other terpenoids.4,5 Initial NHOC and density measurements for this series of fuels showed that they had volumetric NHOCs approximately 14% higher than JP-5, with potential applications as high-density turbine fuels. Of particular interest, because of its lower cost and availability, the turpentine dimer fuel (TDF) was made from crude turpentine. The NHOC for TDF is 141 000 Btu/gallon, This article not subject to U.S. Copyright. Published 2012 by the American Chemical Society

Scheme 1. Structures of Monomeric Terpenes Used To Synthesize Various High-Density Terpene Dimer Fuels

which is similar to the NHOC of JP-10 at 141 500 Btu/ gallon.6,7 TDF also has a density of 0.93 g/mL, which is comparable to JP-10 (0.94 g/mL) and significantly higher than JP-5 (0.82 g/mL). Although TDF and related fuels have exceptional NHOCs, a key drawback is their high viscosities, in the range of 20−35 mPa·s (measured at 40 °C). In a previous paper,4 we proposed that the high viscosity of these fuels could be mitigated by blending with other low-viscosity components. This is a strategy that has been exploited for other fuels to improve the low-temperature properties of commercial8,9 and experimental10 jet fuels, as well as renewable vegetable-oil-based fuels.11,12 Received: October 1, 2012 Revised: November 21, 2012 Published: December 4, 2012 883

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performed in triplicate, and the data reported are the average of these three readings. The error in these readings is ±1% at the 99.7% confidence level (3σ). Samples were measured at a variety of temperatures that were reported on the basis of the internal temperature of the sample as read by the viscometer. The temperature of the sample was held to within ±0.05 °C with a constant temperature bath (Brookfield TC-502). Between measurements, the sample adapter and spindle were rinsed several times with hexanes, followed by a water wash and acetone rinse. The components were then dried under a stream of air.

To evaluate the viability of TDF as a fuel additive and potential surrogate for high-density missile fuels, the current work explores the low-temperature properties of neat TDF and several blends with conventional military fuels, surrogate hydrocarbons, and pinane. Specifically, the viscosity of the various blends is measured in a range from 40 to −20 °C. To further characterize the low-temperature properties of these fuels, the glass transition temperatures (Tg) and melting points (Tm) of the fuel blends have been determined using differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). Previous studies exploring the low-temperature melt transitions of fuels and oils have been reported using these13 and other techniques, including cold-stage microscopy,14 modulated differential scanning calorimetry (MDSC),15 and near-infrared (NIR) spectroscopy.16

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RESULTS AND DISCUSSION In an effort to better understand the low-temperature properties of terpene-based fuels, the TDF was studied both as a neat fuel and in blends with JP-10, JP-8, RJ-4, pinane, decalin, and decane. As expected, the high viscosity of neat TDF makes it unsuitable as a stand-alone high-density fuel. As shown in Figure 1, the viscosity of TDF at just −10 °C reaches

EXPERIMENTAL SECTION

Conventional fuels used in these measurements were obtained from the fuel supply depot at China Lake Naval Air Warfare Center and analyzed by gas chromatography−mass spectrometry (GC−MS) to ensure relative purity. (−)-α-Pinene was obtained from Sigma-Aldrich and used as received. The turpentine dimer fuel was prepared as previously described.4 All blends were prepared on the basis of volume percent. Pure TDF and other terpene dimer samples were routinely stored in a freezer at −30 °C. No precipitates or crystals were observed at this temperature. Pinane Synthesis. α-Pinene was hydrogenated using 100 mg of PtO2/100 mL of α-pinene. The hydrogenation was performed using a low-pressure Parr hydrogenation apparatus at room temperature with an overpressure of 30−40 psi of hydrogen. The bomb was shaken until colloidal platinum began to flocculate, leaving a clear reaction mixture, and typical reaction times were on the order of 2 h. The reaction mixture was then filtered through a plug of celite to remove the catalyst, and the filtrate was analyzed by nuclear magnetic resonance (NMR) spectroscopy to ensure that the reaction was complete. DSC. DSC was performed on a TA Instruments Q100 differential scanning calorimeter with a programmable RSC 90 chiller attached. An indium standard was used to calibrate the instrument before the runs, and a new standard aluminum pan was used for each sample. The samples were rapidly cooled to −90 °C, subjected to a heat/cool cycle from −90 to 10 °C and back to −90 °C at a rate of 5 °C/min, and finally heated at a rate of 5 °C/min to 10−20 °C. All experiments were conducted under a nitrogen atmosphere with a flow rate of 50 mL/ min. The standard error for this instrument with our experimental setup is ±0.3 °C. TMA. TMA was performed on a TA Instruments Q400 thermomechanical analyzer under a helium gas purge with a flow rate of 100 mL/min. Indium was used as a standard, and data are all calibrated to the indium standard. For each run, a standard aluminum DSC pan was filled to just below the top of the pan with the fuel. The pan was placed onto the TMA stage, and with the furnace down, cooled by the addition of liquid nitrogen to the upper reservoir. Roughly 8 min elapsed to complete the cooling process (equilibration). For each measurement, the starting temperature was at least 30 °C below the observed melting point (Tm) or glass transition temperature (Tg) of the fuel. The probe was then contacted with the sample with a force of 0.02 N, and the sample temperature was held at that temperature for 3−5 min. The sample was then heated at a rate of 5 °C/min until at least 30 °C above the Tm or Tg. Viscosity. The viscosity of each sample was measured using a Brookfield DV-II+ Pro viscometer equipped with the small sample adapter (SSA). The instrument was calibrated before use by Brookfield Laboratories, Inc., and ethylene glycol was used as a standard to ensure proper operation of the system. Each sample was placed in the SSA and allowed to equilibrate at the set temperature for a minimum of 20 min before the viscosity was measured. Once the temperature had equilibrated, the viscosity was measured as outlined in American Society for Testing and Materials (ASTM) D2983. Each sample was

Figure 1. Viscosity of 100% TDF over a range of temperatures.

3939 mPa·s and rises exponentially as the temperature decreases. A simple exponential fit of these data allows us to determine that TDF reaches 1000 mPa·s, the practical maximum pumpable viscosity for a fuel,10 at only −6 °C. This observation is not surprising given that this fuel mixture is made up of multicyclic C20 molecules, which are nearly double the molecular weight of the C10 component (exotetrahydrodicyclopentadiene) that makes up JP-10. To mitigate the high viscosity of TDF, 50:50 blends of TDF were prepared with a series of military jet fuels, including those shown in Scheme 2. JP-10 is the optimum blendstock because TDF was designed to have a NHOC and density comparable to JP-10. The viscosity of a 50:50 mixture of JP-10 and TDF was measured over a range of temperatures, and the result was a decrease in viscosity of 2 orders of magnitude over the range Scheme 2. Structures of High-Density Fuels

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decalin, had viscosities in line with the other two cyclic fuel blends (Figure 3). The decane blend viscosity was linear down

studied. The viscosity increased from 6 to 27 mPa·s over a temperature range from 27 to −14.5 °C, respectively. This result suggested that JP-10/TDF mixtures may be viable highdensity fuel blends. These data were fit to an exponential curve (see the Supporting Information), and the estimated temperature at which the fuel reaches the pumping limit of 1000 mPa·s was determined to be −44 °C. This result is in line with melting point specifications for other aviation fuels, such as Jet A, which has a maximum melting point of −40 °C, and JP-8, at a maximum of −47 °C.17 In addition to blends with JP-10, two other fuels were blended with TDF and the viscosity was measured. A 50:50 blend of TDF and pinane was measured over the same temperature range as the JP-10 blend, and nearly identical viscosities were found (Figure 2). Given the similar size and

Figure 3. Viscosity data for 50:50 fuel blends with turpentine dimers: (■) decalin, (+) decane, (×) JP-8, (○) pinane, and (●) JP-10.

to about 0 °C and then began to deviate because of the proximity of the melting point of decane at −30 °C. Although for the purposes of this experiment, decane is a suitable model compound, clearly the complex composition of JP-8, including the presence of branched chain alkanes, allows for increased disorder in the system, which reduces the low-temperature viscosity of the TDF/JP-8 blend. In these blends, it appears that glass formation and crystallization could be hindered by the presence of a variety of dissimilar hydrocarbons. Similar effects have been studied by Paso et al. during an investigation of model n-paraffin systems to determine the temperature dependence and kinetics of crystal formation.21 In addition to studying blends made with currently used military fuels, two additional blends were made with dimethylJP-10, also referred to as RJ-4, and RJ-5 (Scheme 2), an experimental high-density fuel that can be synthesized from norbornadiene (Table 1). The data for the 50:50 blend of RJ-4

Figure 2. Viscosity data for 50:50 fuel blends with turpentine dimers: (×) JP-8, (○) pinane, and (●) JP-10.

structure of pinane compared to JP-10, this was not surprising. However, a 50:50 blend of TDF with JP-8 (Figure 2) resulted in viscosities of less than 5 mPa·s over the entire temperature range that was studied. Not only is the viscosity of the JP-8 blend another order of magnitude lower than the viscosities of the other blends, but the response over the measured temperature range is linear. This would suggest that significant amounts of TDF can be blended into jet fuel to increase the density and volumetric NHOC without significantly changing the viscosity of the fuel sample.18 An important observation is the lack of correlation between the molecular weight of the blendstocks and the reduction in viscosity of the blends. Pinane and JP-10 are composed exclusively of C10 molecules, while JP-8 is composed of linear, branched, cyclic, and aromatic hydrocarbons, each with about 9−16 carbon atoms.19 Despite the higher average molecular weight of the hydrocarbons in JP-8, this fuel is more capable of reducing the high viscosity of the TDF fuel than the other two blends. This suggests that cyclic molecules may co-crystallize or are more effective at forming glasses with the cyclic molecules present in TDF than the complex hydrocarbon mixture present in JP-8.20 To further explore this phenomenon, 50:50 blends of TDF with two model compounds were prepared. Decalin was chosen as a multicyclic compound, while decane was chosen to mimic linear hydrocarbons in JP-8. The viscosities of these blends were then measured over the same temperature range as the previous blends. As expected, the blend with the linear molecule, decane, had viscosities nearly identical to those of the JP-8 blend, while the mixture with the bicyclic molecule,

Table 1. Melting Transitions and Viscosity Data Measured for Various Fuels and Blends fuel samples JP-8 JP-10 RJ-4 RJ-5 TDF pinane 50:50 JP-8/TDF 50:50 JP-10/TDF 50:50 RJ-4/TDF 50:50 RJ-5/TDF 50:50 pinane/TDF a

glass transition temperature (Tg) (°C) −124.9 −109.8 −81.1 −49.0

melting point (Tm) (°C) −66.3 −83.4 < −40a >0a −56.3 −71.0

−83.0 −84.3 −83.8

viscosity at −10.0 °C ( mPa·s) 3.09 3.78 10.8 97.2 3939 4.53 3.66 23.9 53.0 626.9 24.9

From ref 10.

is similar to that for the JP-10 blend, although the viscosity is almost double that of the JP-10 blend at −10 °C. Despite this, RJ-4 is a particularly interesting blendstock given recent research in our lab that has shown that RJ-4 can be synthesized from linalool, a terpene alcohol.22 TDF/renewable RJ-4 blends 885

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This same trend in the low-temperature transitions is also observed for the blends of TDF with pinane. These data nearly overlap those of the JP-10 blends, despite the fact that the Tm for 100% pinane and Tg for 100% TDF are both near −50 °C. The 10% TDF blend actually shows a Tg of −121 °C, which means that just 10% TDF is enough to significantly disrupt the crystallization of pinane and cause formation of a glass. Again, the similarity in the chemical structure of the fuel molecules and resulting solvation effects are likely responsible for this response.20 In contrast to the pure cyclic blends, an entirely different trend was observed for blends of TDF with JP-8. In this case, the transitions were dominated by the melting point of JP-8, which was observed at −66 °C for the sample used in these experiments. These blends show sharp melting points at approximately −70 °C from 0% TDF up to 60% TDF. Comparatively, at 70% TDF, these transitions become more broad and the temperature actually depresses to −76 °C (Figure 5). This change represents a switch from a pure melting

could then be the basis for a fully renewable, high-density surrogate fuel that could be produced solely from terpenes. While the low-temperature viscosity of these fuel blends is important for their utility, the melting points and glass transition temperatures of these fuels are also of significant interest. DSC was used to determine the melt transition of TDF. The glass transition temperature of TDF (−59.4 °C) was measured by observing the heating cycle in the DSC and was designated as the midpoint between the two inflections of the broad endothermic peak. This value is in good agreement with previously published DSC data for similar terpene dimer-based fuels.4 However, given the broad and subtle nature of these transitions in the DSC, we decided to explore the lowtemperature transitions of the fuel mixtures using TMA. In contrast to DSC, TMA gave a sharp response for each sample that was tested. The difference between a glass transition temperature and melting point could be qualitatively determined by observing the rate of change of the probe height as a function of the temperature. A pure TDF sample showed a glass transition temperature (Tg) of −49 °C using the TMA method. It is important to note that, because of the difference in the sample size between DSC and TMA, it is expected that the melt transitions may be observed at slightly higher temperatures in the TMA. To evaluate the effects of adding TDF to various fuels, samples were blended with TDF from 10 to 90% to monitor low-temperature transitions as a function of the TDF concentration. As with the viscosity measurements, the mixtures were made with pinane, JP-8, and JP-10. Figure 4 shows the transitions (Tg or Tm) observed for each of the mixtures.

Figure 5. TMA data for (top) 60% TDF in JP-8 and (bottom) 70% TDF in JP-8 showing a change in transition from Tm to Tg.

point to a glass transition, and here, significant TDF is needed to disrupt the crystallization of linear hydrocarbons in JP-8. To test this hypothesis, TDF was blended with both decane and decalin as model linear and cyclic fuels, respectively. The low-temperature transitions for these fuels were observed, and the data revealed the same trend as was seen with the blends of JP-10 and JP-8, respectively. The data for the decalin/TDF blends were almost identical to both of the other cyclic blends with a slope of 0.72 and a glass transition temperature of −109 °C for the 20% TDF sample (Figure 6). Examination of the TMA data shows broad melting transitions for these samples, which are indicative of glass transition temperatures. Not surprisingly, the blend of decane with TDF yielded very different results compared to its cyclic counterpart decalin. The decane/TDF blends exhibited transitions in which both the glass transition and melting point could be observed (Figure 7). However, the dominant component of the scans was the sharp melting transition seen near the melting point of decane

Figure 4. Melting transition data for various fuel blends with turpentine dimers: (×) JP-8, (○) pinane, and (●) JP-10. Boxes around data points indicate a Tm.

The trend for JP-10/TDF mixtures ranging from 10 to 90% TDF is linear with a slope of 0.76 and a transition of −117 °C with 10% TDF present. This result was unexpected because the temperatures measured for many of the blends with a low TDF concentration are significantly below the melting point of JP-10 alone (−83 °C, measured for the sample used in these experiments). However, after examination of these data more closely, it is possible to see that these temperatures correspond to glass transition temperatures and not the melting points of crystalline solids. These blends may favor the formation of a glass because of the structural similarity of TDF and JP-10, because all of the compounds present are multicyclic systems with no branched or linear alkanes in solution. 886

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with widely used conventional fuels are quite promising. Specifically, the 50:50 blend of TDF with JP-8 shows no change in the melting point when compared to that of pure JP-8, while the −10 °C viscosity is less than 3.8 mPa·s. These results suggest that terpene dimer fuels are good candidates for blending in high concentrations with both renewable and conventional jet fuels to improve NHOC and density. With regard to high-density fuels for missile propulsion, the 50:50 blend of JP-10 and TDF has a glass transition temperature of −83.0 °C and a viscosity of 23.9 mPa·s at −10 °C. The fully renewable blend with 50:50 TDF and pinane also had promising properties with a glass transition temperature of −83.8 °C and an estimated operational temperature (