Article pubs.acs.org/EF
Effect of Aromatics on the Thermal-Oxidative Stability of Synthetic Paraffinic Kerosene Matthew J. DeWitt,*,† Zachary West,† Steven Zabarnick,† Linda Shafer,† Richard Striebich,† Ashil Higgins,† and Tim Edwards‡ †
University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469-0043, United States Air Force Research Laboratory/Propulsion Directorate, 1790 Loop Road North, Building 490, Wright-Patterson Air Force Base (WPAFB), Ohio 45433-7103, United States
‡
ABSTRACT: The effect of aromatic type and concentration on the thermal-oxidative stability characteristics of a synthetic paraffinic kerosene (SPK) aviation fuel was performed using batch and flow reactor systems, in combination with detailed chemical fuel analyses. An improved understanding of the impact of aromatic addition will assist in optimizing beneficial operational characteristics of the SPK feedstocks and the development of fully synthetic jet fuels. A primary goal of this study was to elucidate the controlling reaction chemistry and identify the cause for differing stability characteristics for varying types of aromatics. Studies were performed using a SPK comprised primarily of mildly branched iso- and n-paraffins as the base feedstock; limited studies were performed using a highly branched SPK. Commercially available aromatic solvents were used to represent petroleum-derived jet fuel and potential synthetic aromatic blending streams. These solvents were composed of mono- and diaromatic compounds of varying average molecular weight and size. The resulting thermal-oxidative stability characteristics were highly sensitive to the blend composition, with both increasing aromatic size and concentration, resulting in a higher deposition propensity upon stressing. It was determined that oxidation and molecular growth of the aromatic compounds are the probable primary pathways of surface deposit formation for these blends. Larger aromatic compounds (e.g., diaromatic) require fewer successive growth steps to produce insoluble deposit precursors, resulting in significantly higher deposition propensity than lower molecular weight (e.g., monoaromatic) species. Limited testing showed that the impact of aromatic type on deposition is consistent for different SPK compositions, but the deposit magnitude may be affected.
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INTRODUCTION There has been significant interest during recent years in the development and approval of alternative (non-petroleum) aviation fuels. Alternative fuels have the potential to increase the supply and availability of reliable domestic sources while reducing associated cost volatility. Extensive laboratory and fullscale testing have resulted in the approval of synthetic paraffinic-type fuels for use as a blending feedstock (up to 50% by volume) in both commercial and military aviation fuels (per ASTM D1655-11 and MIL-DTL-83133H). This includes synthetic paraffinic kerosene (SPK) produced via Fischer− Tropsch synthesis and hydroprocessed esters and fatty acids (HEFA) derived from plant oils and animal fats. These SPK and HEFA blend stocks are predominantly paraffinic (normal and iso-) in composition and contain minimal aromatic and heteroatomic compounds. The neat paraffinic blend stocks exhibit favorable characteristics, such as excellent thermaloxidative stability and significantly reduced particulate matter (PM) propensity during combustion, which is attributable to the lack of the aromatics and heteroatomic compounds.1 However, the lack of these compound classes results in fuels with insufficient material compatibility/seal swell and lubricity characteristics. Therefore, the current fuel specifications have taken a conservative approach, requiring a minimum total aromatic concentration of 8% by volume (%v) for synthetic blends to ensure that fit-for-purpose (FFP) requirements are satisfied. This constraint reduces the beneficial aspects of the neat paraffinic fuels, resulting in behavior that is generally © 2014 American Chemical Society
consistent with petroleum-derived fuels. Studies have been performed that demonstrate that specific types of aromatics can be added to SPK-type fuels, which provide acceptable seal swell characteristics while maintaining reduced PM emission production relative to petroleum-derived fuels.2 Determination of favorable characteristics and required concentrations of desired aromatics would also allow for the formulation of fully synthetic jet fuels (FSJF), which would not require blending with petroleum-derived fuels and could maximize favorable operational characteristics of the fuel. However, an improved understanding of the effect of the aromatic type and concentration on other FFP characteristics is needed. Thermal-oxidative stability, which is the propensity of a fuel to produce undesirable deposits in fuel system passages, controls, and nozzles as it absorbs waste heat from aircraft components and other fluids because of reaction with dissolved oxygen, is an important operational characteristic of a fuel. Fuel specifications evaluate the relative thermal stability using ASTM D3241 with a jet fuel thermal oxidation tester (JFTOT). The thermal stability of the fuel is evaluated via the amount of deposit on the aluminum heater tube using visual tube rating (VTR) methods and the rate of plugging [e.g., pressure drop (ΔP)] on a filter downstream of the heater tube. A VTR ≥ 3, a ΔP > 25 mmHg, a “peacock” rating, or an “abnormal” tube Received: February 24, 2014 Revised: April 30, 2014 Published: May 1, 2014 3696
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color (defined in ASTM D3241 as “a tube deposit color that is neither peacock nor like those of the Color Standard”) result in a failure. Recent studies have investigated the effect of aromatic addition (e.g., aromatic solvents and synthetic fuel streams) on the thermal-oxidative stability of synthetic paraffinic fuels.3−7 These have included the use of both petroleum-derived and synthetically produced aromatic compounds and evaluated thermal stability by measuring the JFTOT breakpoint temperature (highest temperature at which fuel passes the rating criteria). Studies performed by Sasol using synthetically derived aromatics, primarily composed of alkylbenzenes, have shown that the base SPK thermal stability is not affected.4,5 However, studies using petroleum-derived aromatics (mono- and diaromatics) have shown that the addition at concentrations ≥10%v can decrease the fuel thermal stability.3,6 The failures reported were attributed to the corresponding VTR rating of the JFTOT tube, because there was a minimal increase in ΔP during the testing. A separate study performed to investigate the effect of blending similar petroleum-derived aromatics at varying concentrations into several SPK and HEFA fuels also showed a corresponding decrease in thermal stability with an increasing aromatic concentration.7 However, there was not a distinct trend in the specific breakpoint temperature with respect to base fuel composition, aromatic type, or total concentration. In addition, many of the failures were due to “abnormal” deposit color. For example, photographs of JFTOT tubes with the corresponding VTR from this testing are shown in Figure 1.
Figure 2. Comparison of JFTOT tube maximum average deposit thickness determined using ellipsometry to VTR from testing with varying concentrations and types of aromatics in several SPK and HEFA feedstocks.
each VTR; a thickness level of 85 nm is highlighted because this is the proposed value that ASTM is considering as a pass/fail criteria for use of an interferometer in the D3241 test method. As shown, a VTR of 3 or greater correlates well with a high maximum deposit level and may be able to discern unacceptable thermal stability characteristics. However, there is a wide range of deposit thickness levels for tubes with abnormal VTRs of 1 and 2, with many “failures” having low deposition levels. It may be necessary to determine appropriate test conditions and rating criteria for using the JFTOT to evaluate the effect of aromatics on synthetic fuel thermal stability characteristics. Because of the varying observations reported in previous studies, a detailed study was performed herein to provide an improved understanding of the effect of the aromatic type and concentration on the thermal stability of SPK-type fuels. Primary objectives were to supplement the previous JFTOT studies with a more quantitative basis for evaluation and improve the understanding between chemical composition and properties on resulting performance. This approach will provide improved ability to identify optimal aromatic compounds and concentrations, which can be added to paraffinic fuels to maximize desired operational characteristics and allow use without blending with petroleum-derived fuel.
Figure 1. Photographs of JFTOT tubes and reported VTR following JFTOT testing with blends of an aromatic solvent blend in various SPK and HEFA feedstocks.
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EXPERIMENTAL SECTION
Two reactor systems were used to evaluate the thermal stability of a SPK blended with varying concentrations of different aromatic solvents. This included a batch [quartz crystal microbalance (QCM)] and a flowing (single-tube reactor) system. These systems have been previously used to successfully evaluate the thermaloxidative stability characteristics of aviation-type fuels. The design and operation of these have previously been described in detail;1,8,9 a brief description of each is provided below. QCM. The QCM is a batch reactor system that provides isothermal stressing of a test fuel for an extended duration with an oxygen headspace. Headspace oxygen and mass deposition are monitored in situ to provide time-dependent information regarding the thermaloxidative stability characteristics of the sample. For this testing, 60 mL of the test fuel was air-saturated under ambient conditions. The reactor was closed, heated to 140 °C, and stressed isothermally for 15 h. The
The 1A and 3A failures were reported because of the deposit having a “whitish/tannish” color, which is not typically observed for petroleum-derived fuels. The deposits were analyzed using infrared spectroscopy and elemental analysis (not previously reported); it was determined that the deposits were predominantly composed of carbon and oxygen and the color was hypothesized to be related to the deposit thickness. To provide improved insight into the experimental results, the JFTOT tubes from this study were analyzed using ellipsometry (Falex model 430) to quantify the deposit profile and thickness. A comparison of the maximum average deposit thickness (nm) over a 2.5 mm2 area to the corresponding VTR is shown in Figure 2. Results are shown as either normal or abnormal for 3697
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time-dependent deposition and oxygen consumption characteristics were used to evaluate relative thermal stability characteristics. Single-Tube Flow Reactor System. Thermal stability was evaluated using a single-tube, single-pass flow reactor.1,9 The reaction system has a 91.4 cm (36 in.) long actively heated section, where the temperature is sufficient to promote the desired reaction chemistry. Studies were performed using 0.216 cm (0.085 in.) inner diameter and 0.318 cm (0.125 in.) outer diameter reaction tubes constructed of 316 stainless steel. The fuel was air-saturated, and thermal stressing was performed with an inlet volumetric flow rate of 10 mL/min, reaction pressure of 38.9 bar (550 psig), and test duration of 6 h. The external wall temperature was measured at various axial locations using strapwelded K-type thermocouples. The temperature profile [Twall,max ≈ 357 °C (675 °F), and Tbulk,out ≈ 316 °C (600 °F)] was sufficient to consume all dissolved oxygen in the heated reaction zone. For the reaction condition studies, the bulk fuel temperature is estimated to be approximately 75−100 °C higher than the measured wall temperature.9 Following testing, the reaction tube was removed and cut into 5.1 cm (2 in.) long sections. The total carbon deposit on each tube segment was determined via temperature-programmed oxidation using a LECO RC612 multiphase carbon analyzer. The stability characteristics were determined by comparing the total mass quantity and axial deposition profile. Fuels and Aromatic Solvents. Experimental testing and analysis was performed to investigate the effect of the aromatic type and concentration on the thermal-oxidative stability characteristics of a SPK. The primary SPK used in this study was produced from natural gas via indirect liquefaction by Syntroleum Corporation (also termed S-8). This fuel is composed of mildly branched (e.g., monomethylsubstituted) isoparaffins with ∼20%v n-paraffins and a distillation range similar to a typical JP-8 specification fuel. This SPK was used during the certification of alternative blends for the B-52 platform. Limited testing was performed using a SPK [also referred to as isoparaffinic kerosene (IPK)] produced by Sasol. This fuel is produced via oligimerization of C3 and C4 olefins, followed by hydrotreating and fractionation, to produce a fuel with the desired boiling range.4,5,10 This process results in a SPK with a very high degree of branching and low concentration (∼10%v) of cycloparaffins. The Sasol fuel was the first synthetic fuel to be approved as a blend feedstock with Jet A-1 fuel. Both neat SPKs exhibit excellent thermal-oxidative stability, with negligible deposition during stressing.1,11 Additional details on the chemical and physical properties of these SPKs are provided elsewhere.1,4,5,10−13 The thermal stability of the neat SPK and aromatic blends was compared to a specification JP-8 (designated POSF 6169). This JP-8 had a total aromatic concentration of ∼15.7% v, total naphthalenes of ∼1.1%v, and a total sulfur content of ∼540 ppm by mass. Commercially available aromatic solvents were used to represent petroleum-derived jet fuel and potential synthetic aromatic blending streams. Aromatic 100, 150, and 200 (designated A100, A150, and A200) solvents were obtained from ExxonMobil and used in this study. The solvents were analyzed for chemical composition via gas chromatography/mass spectrometry (GC/MS) and two-dimensional gas chromatography (GC × GC). These solvents are composed of a range of mono- and diaromatic compounds with varying average molecular weight ranges,2,6 which are listed in Table 1. A100 is comprised primarily of C3 (83.6%v) and C4 (12.8%) alkylated benzenes, with a low concentration of indane (1.1%). A150 is comprised primarily of C3 (2.6%v), C4 (62.2%), C5 (18.3%), and C6
(1.3%) alkylated benzenes, C10- and C11-alkylated indans and tetralins (7.3%), and naphthalene (6.7%). A200 is primarily composed of C5− C8-alkylated benzenes (5.3%), C11−C13-alkylated indans and tetralins (10.1%), naphthalene (6.7%), and C1−C4+-alkylated naphthalenes (76.3%). The chemical compositions of these solvents are slightly different but generally consistent with those previously reported when used during previous research efforts.6 Analysis of these solvents showed negligible heteroatomic or polar compounds present. Testing with multi-component solvents allows for the evaluation of the effect of the chemical class and structure on reactivity, without favoring a specific reaction path, which could occur with single-component testing. Studies were performed with each of these solvents at varying overall concentration in the SPK fuel. In addition to the individual solvents, a composite blend (referred to as “A-Blend”) was formulated (30%v A100/60%v A150/10% A200), which simulates the range of aromatic compounds in a “typical” aviation fuel. In comparison of the gas chromatography/flame ionization detection (GC/FID) chromatograms of the individual and composite aromatic solvents, S-8, neat JP8, and the aromatic fraction of the JP-8 extracted using high-pressure liquid chromatography14 are shown in Figure 3.
Figure 3. Comparison of chromatograms of aromatic solvents, aromatic solvent blend (30:60:10), S-8, JP-8, and aromatic fraction of JP-8.
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RESULTS AND DISCUSSION Effect of the Aromatic Concentration. The effect of the overall concentration of aromatics on the resulting thermal stability characteristics was investigated using the composite aromatic blend. Studies were performed varying the concentration of the blend at 5%v increments in the SPK (S-8 unless otherwise specified). Results from QCM testing are shown in Figure 4; mass accumulation (solid lines) and headspace oxygen (dashed lines) profiles are shown as a function of the test duration and aromatic concentration. The corresponding profiles are also shown for the neat SPK and JP-8. The neat SPK exhibited excellent thermal stability characteristics, with minimal deposition during stressing. This behavior is consistent with that observed for other neat SPK and HEFA fuels.1 The specification JP-8 had a deposition level and profile consistent with that typically observed for a petroleum-derived fuel with good thermal stability evaluated using the QCM. A higher total aromatic content in the SPK blends increased the total mass deposition while decreasing the onset time for oxidation. Testing with 5−15%v total aromatic content resulted in comparable deposition to that for the JP-8. It should be noted that the deposition for the petroleum-derived fuel is also affected by heteroatomic compounds within the fuel, which are
Table 1. Aromatic Solvent Composition concentration (%v) solvent
monoaromatic
diaromatic
MW range (g/mol)
MWave (g/mol)
aromatic 100 aromatic 150 aromatic 200
100 94 18
6 82
106−148 120−162 128−196
122 136 152 3698
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Figure 5. Carbon deposition and measured wall temperature profiles from flow reactor testing with a varying total concentration of the aromatic solvent blend (30:60:10) in SPK.
Effect of the Aromatic Type. The effect of the type of aromatic on the deposition propensity of the SPK was evaluated by performing studies with the individual aromatic solvents blended at a varying concentration in the SPK. These were performed to determine if the aromatic structure and molecular size (e.g., mono- versus diaromatic) have a significant impact on thermal stability characteristics. QCM experiments were performed by blending each of the individual solvents or composite blend at 20%v with SPK and using the same reaction conditions as previously reported. This relatively high concentration was used to intensify the effect of the molecular type on deposition. The results from this testing are shown in Figure 6. The type of aromatic solvent had a major impact on both the total mass accumulation and the corresponding rate of oxygen consumption during thermal stressing. The total mass deposition with A100, comprised solely of alkylated monoaromatics, was similar to that for the neat SPK, even though the
Figure 4. QCM mass accumulation (solid lines and open markers) and headspace oxygen profiles (dashed lines and closed markers) at 140 °C with a varying total concentration of the aromatic solvent blend (30:60:10) in SPK.
negligible in both the aromatic solvents and the neat SPK. The 20%v blend had a substantially higher quantity of deposition relative to the other concentrations and JP-8 fuel. However, the total magnitude is within the typical range observed for evaluation of petroleum-derived fuels on the QCM (≤8 μg/ cm2). The flow reactor system was used to evaluate the thermal stability of the solvent blends for concentrations of 5−15%v in a flowing environment with a total dissolved oxygen concentration representative of actual systems (e.g., ∼70 ppm by weight). The carbon deposition and measured wall temperature profiles as a function of the axial position and aromatic concentration from this testing are shown in Figure 5. The neat SPK results are not shown because negligible deposition was observed during stressing. Consistent with the QCM results, an increase in the total deposition quantity was observed with an increasing total aromatic content. The onset of the deposition was similar for each of the aromatic/SPK blends, indicating that the aromatics do not significantly affect the initial rate of fuel oxidation for the reaction conditions studied. The observation of a maximum in the deposition profile with a subsequent decrease is indicative of complete consumption of the dissolved oxygen within the heated zone. The significantly higher deposition characteristics for the JP-8 fuel are most likely due to heteroatomic (e.g., oxygen, sulfur, and nitrogen) components within the fuel, which are known to promote undesirable deposit formation under oxygen-limited conditions.14−16 It is possible that slight differences in the specific aromatic compounds in the petroleum-derived fuel and aromatic solvents also impact the deposition propensity, but this is believed to be minor compared to the role of heteroatoms in the deposition process. Overall, the observation that the magnitude of deposit formation correlates with the total aromatic content indicates that this compound class participates in and promotes deposit formation pathways.
Figure 6. QCM mass accumulation (solid lines and open markers) and headspace oxygen profiles (dashed lines and closed markers) at 140 °C with a varying aromatic solvent type in SPK at 20%v. 3699
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increase deposit formation via pathways that promote oxidation and molecular growth at elevated reaction temperatures. The presence of heteroatomic compounds in the neat fuels prior to stressing intensifies deposition propensity, with increased concentrations typically promoting higher deposition levels. The SPK and aromatic solvents used in this study contained negligible concentrations of these heteroatomic compounds. However, the observation of significant carbon deposition during oxidation, with sensitivity to the aromatic type and concentration, indicates that molecular growth pathways do occur within the synthetic fuel blends. The neat SPK readily undergoes oxidation during thermal stressing but primarily produces stable oxygenated compounds, which are not prone to significant molecular growth. This is most likely due to the paraffinic nature (mild branching and primarily linear structure) of the SPK. Aromatic compounds undergo oxidation under these reaction conditions, with rates competitive with those for the neat SPK. This is due to the presence of weak bonds within alkylated aromatics (e.g., benzylic hydrogens), which provide energetically favorable sites for hydrogen donation and subsequent oxidation. Alkylated aromatics can undergo oxidation, producing compounds, such as phenols, aldehydes, ketones, and carboxylic acids. Once formed, these species can subsequently undergo ring-closure reactions, producing species, such as benzofuranones.17 These products have the potential to undergo molecular growth via further oxidation or reaction with other aromatic compounds. It is likely that larger aromatic compounds, such as diaromatics, require fewer consecutive reaction steps to produce high-molecular-weight, low-solubility species, which are prone to surface deposition. This would increase the deposition propensity for blends, which contain high concentrations of diaromatics, especially under oxygenlimited conditions. Analysis was performed on neat and thermally stressed samples to determine if there was quantitative evidence of molecular growth products following oxidative stressing. Polar species/products were separated and concentrated using a solid-phase extraction (SPE) pre-separation technique with subsequent gas chromatographic (GC) analysis. A total volume of 5−20 mL of sample was passed through 1 g of silica gel, followed by a wash of three aliquots of hexanes and extraction with 1 mL of methanol. Each extract was analyzed using GC × GC, GC/MS, and/or high-temperature GC/FID. GC/FID chromatographs from the analysis of polars for a mixture of the SPK with 10%v of the A-Blend (both neat and following stressing in the flow reactor system) and the A-Blend following stressing on the QCM are shown in Figure 8. Similarly, GC × GC chromatograms (color intensity correlates with the mass concentration) from the analysis of polar products following QCM stressing of the individual aromatic solvents (nonblended with SPK to intensify product formation) are shown in Figure 9. The unstressed SPK/aromatic blend had very low concentrations of polar compounds (representative of all aromatic solvents/blends used in this study). The oxidation products from both the aromatic solvents and aromatic/SPK blends exhibited a distribution consistent with sequential molecular growth steps. The primary product region (1°) is attributed to initial oxidation of the parent aromatic or SPK compounds to produce oxygenated species. The secondary (2°), tertiary (3°), and subsequent product distributions (evident from high-temperature GC/FID analysis; not shown) had molecular weight ranges (elution times) and mass spectra ion fragmentation patterns [from mass spectro-
aromatic solvent was present at 20%v. A100 did increase the rate of fuel oxidation; however, this is believed to be primarily due to dilution of the synthetic antioxidant in the neat SPK. The addition of both A150 and A200 significantly increased deposition levels, while testing with the 30:60:10 composite blend resulted in deposition levels between the individual solvent results. This observation is reasonable because of the inclusion of both solvents within the blend. These results imply that the molecular type of aromatics can affect the thermal stability characteristics of synthetic fuels, with diaromatics having a higher impact on the resulting deposition propensity. The impact of the aromatic type on the SPK thermal stability was also evaluated using the flow reactor system. Testing was performed by blending each of the individual and composite solvents at 10%v in the SPK using reaction conditions consistent with the aforementioned studies. The carbon deposition and measured wall temperature profiles as a function of the axial position and aromatic type are shown in Figure 7.
Figure 7. Carbon deposition and measured wall temperature profiles from flow reactor testing with a varying aromatic solvent type in SPK at 10%v.
Similar to the QCM testing, the addition of A100 showed minimal effect on surface deposition levels, which were similar to that for the neat SPK. The A100 is comprised solely of alkylated monoaromatics, which are similar in composition to the synthetic aromatic streams produced by Sasol. As previously mentioned, JFTOT breakpoint studies showed that the thermal stability of the Sasol SPK was not significantly impacted by this type of aromatic stream.4,5 The A150 blend showed very similar deposition characteristics to the 30:60:10 composite blend, while the A200 promoted a significant increase in deposition, which was substantially higher than that for the JP-8 fuel. These results further indicate that aromatic compounds participate in deposit formation pathways, with diaromatics having an increased propensity to produce high-molecular-weight compounds. Pathways for Deposit Formation from Oxidation of Aromatics. The thermal-oxidative stability characteristics of petroleum-derived fuels have been related to the presence of trace heteroatomic compounds within the fuels. These deleterious compounds include oxygenates (e.g., phenols) and reactive sulfur/nitrogen species.8,14−16 These species can 3700
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provides further evidence of molecular growth under conditions more representative of actual fuel systems (e.g., oxygen-limited, flowing reaction environment). The lower absolute concentration of higher molecular weight products for the flow reactor sample is most likely due to the loss of these species to surface deposit formation within the heated or post-reactor zones and the reduced oxygen availability (i.e., lower concentration of the limiting reagent). The significantly higher propensity of diaromatics to promote surface deposition during flow reactor stressing occurs because these larger precursors require fewer oxidation and oligomerization steps to produce higher molecular weight products, which are more prone to producing surface deposits.15 For example, the A100 and A150 appear to require 2−3 molecular growth steps to form products with molecular weight ranges observed following a single growth step for the A200 solvent (see Figure 9). This result supports the assertion that molecular growth and oligimerization are the primary steps required for oxidative deposition, especially for fuels with a low heteroatomic content. In addition, molecular type/size can have a significant effect on the resulting thermal stability behavior, especially under oxygen-limited conditions, where rapid growth to deposit precursors is necessary for high deposition levels. Effect of the SPK Composition on Deposition Propensity. There is evidence that oxidation products of aromatics can produce surface deposits, with increasing propensity for higher concentration and larger molecular size. It is possible that the type and composition of the SPK can also affect the deposition propensity of the aromatic compounds. A limited flow reactor testing was performed with a highly branched SPK produced by Sasol. This type of SPK contains monocycloparaffinic compounds (∼10−12%v) with negligible normal paraffins and was used during previous aromatic blending JFTOT studies.4,5 Testing was performed blending the individual solvents and A-Blend at 10%v in this SPK; results are shown in Figure 10. Consistent deposition trends with those for S-8 testing were observed with respect to the effect of the aromatic type. However, the absolute magnitude and initial onset of the deposit were affected by the base SPK, with the Sasol blends having a higher deposition quantity. For example,
Figure 8. Chromatographic analysis of polar compounds in neat and stressed samples.
Figure 9. GC × GC analysis of polar compounds following stressing of varying aromatic solvents in QCM.
metric (MS) analysis] consistent with species formed via molecular growth and oligomerization of the oxidation products and/or parent fuel molecules (e.g., 1° + 1° or fuel → 2°, and 1° + 2° → 3°). For example, approximate carbon number ranges for the A100 product regions shown in Figure 9 were 1°, C9−C13; 2°, C16−C22; and 3°, C24−C30. It was not feasible to identify specific structures because of the various isomers and types of alkylated aromatics in the solvent and coelution during analysis. Similar to the aromatic solvent testing, the stressed flow reactor sample had products consistent with molecular growth, as shown in Figure 8. There is a more diverse range of the initial oxidation products for this sample, most likely because of products formed directly from the SPK. However, there is clear evidence of molecular growth products consistent with those observed during the QCM stressing. GC/ MS analysis also confirmed these species as having an aromatic base structure. This is an important observation because it
Figure 10. Carbon deposition and measured wall temperature profiles from flow reactor testing with a varying aromatic solvent type in Sasol SPK at 10%v. 3701
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manuscript for useful feedback. This material is based on research sponsored by the U.S. Air Force Research Laboratory under agreement number FA8650-10-2-2934.
the total heated zone deposit quantity for testing with the ABlend in S-8 and Sasol was approximately 580 and 910 μg, respectively. This is most likely due to competition for oxidation of the parent SPK species and the subsequent propensity of these to undergo further molecular growth steps. Therefore, it is most likely feasible to assume that the impact of aromatic addition will be consistent for different SPKs, but the magnitude of the deposit quantity may be affected. The impact of the base SPK composition and properties on the resulting deposition characteristics with the addition of aromatics merits further study.
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SUMMARY Improved insight into the effect of aromatic addition on the thermal-oxidative stability characteristics of synthetic paraffinic feedstocks can assist in maximizing desirable operational characteristics of paraffinic fuels (e.g., reduced emission propensity and improved thermal stability). This can also provide a basis for producing FSJF compatible with legacy and future aircraft. SPKs exhibit excellent oxidative thermal stability, which has been attributed to the lack of heteroatomic and aromatic compounds within these fuels. Previous studies have shown differing impacts of adding petroleum-derived or synthetically produced aromatics on the resulting thermal stability characteristics. An improved understanding of the effect of aromatic type and concentration will assist in identifying optimal aromatic types that will satisfy FFP requirements while maximizing beneficial aspects of the paraffinic base feedstocks. Batch and flowing thermal stability tests were performed to examine the effect of the aromatic type and concentration on oxidative deposition characteristics. It was found that, for a specific aromatic type, the deposition propensity is higher with an increasing concentration. This result indicates that oxidation and growth products of aromatic compounds can promote undesirable surface deposits. The molecular size of the aromatic compounds also affected deposition propensity, with larger species (e.g., diaromatics) producing significantly higher quantities of insoluble deposits because fewer successive molecular growth reaction steps are required for deposition. Analysis of the oxidative products following fuel stressing provided further evidence of molecular growth of aromatic oxidation products. Additional studies can provide improved insight and understanding into the effect of the aromatic type on thermal stability, guidance for selection of optimal aromatic compounds to satisfy FFP requirements, and assistance in the development of viable test methodologies for fuel implementation.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Rhonda Cook and Doug Wolf of the University of Dayton Research Institute (UDRI) for preparing the fuel blends and the infrared/elemental analyses of the JFTOT tubes, Ann Mescher and Nick Stelzenmuller of the University of Washington for collaborations on initial testing, and Mariam Ajam of Sasol for useful discussions on the research topic. The authors also thank the reviewers of this 3702
dx.doi.org/10.1021/ef500456e | Energy Fuels 2014, 28, 3696−3703
Energy & Fuels
Article
for the prediction of autoxidation and deposition of jet fuels. Energy Fuels 2007, 21, 530−544. (17) Webster, R. L.; Evans, D. J.; Rawson, P. M.; Mitrevski, B. S.; Marriott, P. J. Oxidation of neat synthetic paraffinic kerosene fuel and fuel surrogates: Quantitation of dihydrofuranones. Energy Fuels 2013, 27, 889−897.
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dx.doi.org/10.1021/ef500456e | Energy Fuels 2014, 28, 3696−3703
