Article pubs.acs.org/EF
Hydrocarbon Group-Type Analysis of Petroleum-Derived and Synthetic Fuels Using Two-Dimensional Gas Chromatography Richard C. Striebich,* Linda M. Shafer, Ryan K. Adams, Zachary J. West, Matthew J. DeWitt, and Steven Zabarnick University of Dayton Research Institute (UDRI), 300 College Park, Dayton, Ohio 45469-0043, United States ABSTRACT: A group-type analysis of hydrocarbons in a complex jet fuel may be more useful than attempting to analyze every component because the latter inevitably leaves a large portion of the fuel unidentified. While it may be difficult to accurately determine the identity of a particular compound, that compound can often be classified as belonging to a group or compound class because of its chromatographic retention and mass spectral properties. Compound class quantitation is often capable of relating compositional information to fuel properties. Two-dimensional gas chromatography (GC × GC) is a technique capable of providing this group-type separation and quantitation in jet fuels. This technique was used to examine a large set of fuels (Jet A, Jet A-1, JP-5, and JP-8, primarily) from petroleum sources and non-petroleum alternative sources, such as synthetic paraffinic kerosene (SPK). By comparing results from GC × GC analysis to established techniques and model compound studies, we have found that the accuracy of GC × GC for group-type analysis is excellent. Quantitation of group types for alternative fuel sources were also investigated and compared to conventional techniques. The possible uses and applications of group-type measurements using GC × GC for fuels and fuel-related materials are discussed.
1. INTRODUCTION 1.1. Petroleum-Derived and Synthetic Jet Fuels. The use of synthetic fuels is becoming more widespread with the desire for energy independence and the threat of unstable costs. The U.S. Air Force has been investigating the use of emerging synthetic fuels since the early 2000s.1 Fischer−Tropsch (FT) and other hydrocarbon processes can be used to create synthesized paraffinic kerosenes (SPKs) from sources such as coal, biomass, and natural gas, which can be blended to a maximum of 50% with conventional petroleum-derived sources. These mixtures can then be used as replacements for the current military jet fuels, such as JP-8. Other sources of hydrocarbons have also been produced for blending into current fuels to create a semi-synthetic hydrocarbon, which could meet the specification established for SPKs, ASTM D7566.2 These other sources have recently included animal fats, vegetable oils, and algae, all processed and classified as hydroprocessed esters and fatty acids (HEFAs).3 FT-SPK and HEFA-SPK products consist mainly of paraffins (normal and/or iso-) and cycloparaffins and generally lack a significant concentration of aromatics. ASTM D7566 describes the specification of these fluids before being blended into conventional fuels. Currently, the D7566 specification calls for limited aromatic levels (≤0.5% by mass) and low cycloparaffin content (≤15 mass %). To characterize these fuels as well as the mixtures that are produced when the SPKs are blended with petroleum fuels, a dependable group-type analysis technique is needed, which quantifies the major hydrocarbon types. Particularly, it is important to be able to separate the compound classes accurately at low levels of aromatics and cycloparaffins. 1.2. Hydrocarbon Types and Their Properties in Jet Fuel. Jet fuel is mainly composed of normal paraffins, isoparaffins, cycloparaffins (primarily single and double ring), alkylbenzenes, indans and tetralins, naphthalene, and alkylnaph© 2014 American Chemical Society
thalenes. All of these hydrocarbon group types provide particular properties to a fuel. For example, normal paraffins are important because of their energy content and their effect on the fuel freezing point. Isoparaffins have the same chemical formulas as their n-paraffin counterparts, but because of shape irregularities of branching, they have much lower freezing points. The branching provided by isoparaffins may be an important factor in cetane number, a measure of combustion stability, similar to knocking tendency in gasoline engines. Cycloparaffins have lower freezing points and higher densities than normal paraffins and isoparaffins of the same carbon number. Substituted, singlering aromatics (alkylbenzenes) impart energy density per unit volume and affect elastomer (O-ring) swelling, both of which are necessary for fuel system integrity. Naphthalene, alkylnaphthalenes, and indans and tetralins are multi-ring aromatics, which are generally thought to have an adverse effect on combustion by contributing to soot formation. Each hydrocarbon class, containing potentially tens or hundreds of individual chemical compounds, provides some contribution to the properties of a fuel. 1.3. Hydrocarbon Group-Type Separations. It is tedious to conduct complete analyses of aviation fuel because petroleumbased aviation fuels may contain 1000 or more components (Figure 1a).4 Identification of each individual component is difficult and often unnecessary, because fuel component classes (not individual compounds) tend to impart chemical or physical properties. The use of “group-type” separations gives a more complete picture of the chemical makeup of the fuel and is often preferred in the sense that it is easier to describe and document from a chemical perspective. Received: June 17, 2014 Revised: August 18, 2014 Published: August 21, 2014 5696
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Figure 1. (a) Jet-A sample analyzed by GC−MS. Each peak represents at least one compound. (b) Group-type HPLC separation of saturates, aromatics, and diaromatics for a similar fuel with RI detector.
Figure 1b4 shows an example chromatogram of a simple group-type separation of fuel components by high-performance liquid chromatography (HPLC), containing three major peaks eluting from a polarity separation of the whole fuel: saturates, single-ring aromatics, and diaromatics. This normal-phase separation is the basis for one of the class separations discussed here: hydrocarbon type by mass spectrometry. Hydrocarbon type by mass spectrometry, as practiced in our laboratory, is a modified version of ASTM D2425.5 The basis of this technique is to conduct the same separation shown in Figure 1b, physically collect the saturates and total aromatics in separate fractions, and then separate each fraction based on volatility using gas chromatography−mass spectrometry (GC−MS). The second technique is GC × GC, which is a two-dimensional gas chromatographic separation (the first separation based on volatility and the second separation based on polarity).6−9 1.3.1. Hydrocarbon Type by Mass Spectrometry (ASTM D2425). ASTM D24255 was developed for middle distillates having a boiling range of 204−343 °C (5−95% recovered). In this technique, hydrocarbon samples are pre-separated as saturate and aromatic fractions by ASTM method D2549,10 which is a glass-column separation that also measures the percentages of saturates and aromatics. In the modified version of the method employed here, ASTM D6379,11 a HPLC technique was used to quantify the saturate and aromatic fractions. These fractions were also separated by HPLC and collected for mass spectral analysis by GC−MS. While no GC separation is required by the ASTM D2425 method, it was used here for convenience of sample introduction into the mass spectrometer and increased confidence in the accuracy of the HPLC separation. The mass spectral information obtained for each fraction was summed over the length of the chromatographic analysis. Because compounds in the same class fragment similarly, ion responses tend to be similar in a compound class. These summed ion responses determine the concentration of hydrocarbon types. Interferences exist, however, and empirical relationships provided in method D2425 adjust for many of the major interferences. These calculated adjustments are based on analyses of petroleum-based fuels, which have ion responses based on higher aromatic and (perhaps) higher cycloparaffin levels, and may be less valid for alternative fuels, which may contain highly branched isoparaffins. The classified ions are summed, and their responses are normalized to the aromatic and saturate fraction percentages obtained from the saturate/ aromatic group separation (D6379 in our laboratory). Results for ASTM D2425 are reported for the hydrocarbon type categories listed in Table 1. It is unclear whether the empirical
Table 1. Categories of Hydrocarbon Type Analysis by ASTM D2425 compound classes paraffins monocycloparaffins (non-condensed) dicycloparaffins (condensed) tricycloparaffins (condensed) alkylbenzenes indans and tetralins
indenes (CnH2n−10) naphthalenes acenaphthenes acenaphthylenes tricyclic aromatics
relationships used in this method are appropriate for new, alternative fuels (non-petroleum), even though they may be of similar boiling range. In addition, this technique can be tedious, labor-intensive, and equipment-prohibitive, using four analytical separations (two HPLC and two GC) with three different detectors (refractive index detector, ultraviolet detector, and mass selective detector) and a fraction collector. 1.3.2. Two-Dimensional Gas Chromatography (GC × GC). GC × GC is well-described previously.6−9,12 This technique is performed using either (1) thermal modulation (cooling to trap and then heating to release) or (2) flow modulation.13 In either case, a primary column separation occurs, followed by a repeated modulation of a narrow zone onto a secondary column. The separation from this shorter column occurs fast enough to keep up with the primary column separation. The result is a twodimensional separation with zones containing a retention time having both primary column and secondary column coordinates and response in the z axis. This response is measured with a traditional chromatographic detector, such as a flame ionization detector (FID), a mass selective detector (MSD), or both. A popular arrangement of GC × GC is to have the primary column as nonpolar and the secondary column as polar; the resulting separation is mostly a function of volatility on the primary (x) axis and a function of polarity on the secondary (y) axis.7 Thus, hydrocarbon group types are separated as shown in the example of a GC × GC output (Figure 2). Each of the group types are contained inside the regions shown. By summing the responses in these regions and normalizing based on carbon number to the total response, hydrocarbon type concentrations (weight percent) can be calculated. The objective of the work described in this contribution is to examine, evaluate, and compare the use of ASTM D2425 and GC × GC for performing accurate analyses of hydrocarbon grouptype separations of jet fuels, from both petroleum and synthetic sources. 5697
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Figure 2. Example GC × GC chromatogram (FID detection) of a jet fuel with the primary axis separation by a nonpolar column and the secondary axis separation by a polar column. Section identifications: (a) n- and isoparaffins, (b) monocycloparaffins, (c) dicycloparaffins, (d) alkylbenzenes, (e) indans and tetralins, (f) naphthalene, and (g) alkylnaphthalenes. Response is in arbitrary volume units. These fractions were examined for hydrocarbon type analysis using an Agilent 6890-5973 GC−MS system. Mass spectrometry results were then processed using routines that summed ion responses14 and reported results according to the D2425 method. 2.2. GC × GC. This technique was conducted using an Agilent 5975 GC−MS system equipped with capillary flow technology (CFT) flow modulation. A 20 m, DB-5MS, 0.18 mm inner diameter primary column and a 5 m, DB-17MS, 0.25 mm inner diameter secondary column were used. A programming rate of 1.5 °C/min was used to obtain the primary separation, and a 6 s modulation time was chosen. Data were evaluated using GC Image software (Zoex, version 2.2b0). Both FID and MS data were taken simultaneously, using post-column splitting and short transfer lines to each detector. The primary column flow rate used was 0.4 mL/min, and the secondary column flow was 36 mL/min. This high flow through the secondary column allowed peaks from the polar column to be relatively narrow compared to other flows examined. A template was developed for sample evaluation by close examination of MS results and subsequent translation to the co-generated FID file. These templates or two-dimensional boundaries (Figure 2) included the following hydrocarbon classes: paraffins (isoparaffins and normal paraffins), monocycloparaffins, dicycloparaffins, alkylbenzenes, indans and tetralins, naphthalene, and alkylnaphthalenes. Quantitation of classes was performed by the total FID response of the compounds in each hydrocarbon class, because FID has been shown to respond
2. EXPERIMENTAL SECTION 2.1. ASTM D2425. As mentioned previously, quantitation of saturate and aromatic fractions was performed using a HPLC technique (ASTM D6379) with conditions described in Table 2. Saturate and
Table 2. Conditions for HPLC and GC−MS Separations in ASTM D2425 HPLC column mobile phase injection saturate/aromatic quantitation detector cut point detector fraction collector GC−MS GC column program
Agilent 1100 cyano column (4.6 × 150 mm) hexane (normal phase) 1:50 dilution, 20 μL injection (50 μL for fraction collection) refractive index detection UV−vis diode array detector Gilson model FC203B Agilent 6890-5973, splitless injection, 1 μL DB5-MS, 0.25 mm inner diameter, 0.25 μm 40 °C (3) to 280 °C (5) at 10 °C/min
aromatic fractions were separated via HPLC with ultraviolet−visible (UV−vis) detection and were collected with a Gilson fraction collector.
Table 3. Mixtures and Fuel Samples Evaluated by Hydrocarbon Type Analysis Model Mixtures mixture class
components
description
7 20 21 38 3 3
n-C9−n-C15 C8−C11 isoparaffins C9−C10 cycloparaffins benzene−C6-alkylbenzene tetralin, indan, and indene naphthalene and C1- and C2-alkylnaphthalenes Petroleum-Derived Fuel Sets
paraffins isoparaffins cycloparaffins alkylbenzenes indans/tetralins naphthalenes
a
range (wt %) a
13−32 13−32a 6−16 0−27 0−13 0−6
source Restek PN30713 Restek PN30715 Restek PN30719 Restek PN30717 Sigma-Aldrich Sigma-Aldrich
name
number of fuels
description
source
world survey specialty set
54 8
JP-8 (10), Jet A (13), Jet A-1 (27), JP-5 (2), synthetic Jet A-1 (1), semi-synthetic Jet A-1 (1) JP-8, Jet A, Jet A-1, JP-5, JP-7, F-76 (diesel), RP-1, RP-2 Non-petroleum (Synthetic) Fuel Sets
Air Force, Navy, airports Air Force
name
number of fuels
description
source
USAF samples
8
FT-SPK, HEFA-SPK, others
Air Force, alternate fuel producers
Isoparaffins + n-paraffins. 5698
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consistently by the number of carbons for a wide range of hydrocarbons.15 2.3. Mixtures Investigated. Model compound mixtures were purchased for their complexity and documented concentrations (Table 3). Together, they represent a mixture with 118 unique, quantitative components, similar to JP-8 or Jet-A. Military fuels (JP-8, JP-5, etc.), commercial fuels (Jet-A and Jet A-1), and alternative (non-petroleum) jet fuels were also analyzed using both ASTM D2425 and GC × GC. These mixtures are also provided in Table 3.
3. RESULTS AND DISCUSSION 3.1. Model Mixture Studies. The accuracy of both ASTM D2425 and GC × GC is most readily evaluated using model mixtures. ASTM D2425 had been evaluated in previous work using hydrocarbons with 1−5 compounds per group type.6 This work showed that ASTM D2425 successfully predicted the concentration of hydrocarbon group types in model mixtures. GC × GC analysis of model mixtures was evaluated in this current work, using complex mixtures available from Restek. Table 3 shows the large number of chemicals in these mixtures from 6 different hydrocarbon groups. The 6 hydrocarbon groups were blended together in 12 different mixtures to provide a wide range of concentrations. These available mixtures simulated gasoline components and contained some components in the C5−C7 region that are not typically in middle distillate jet fuels in any significant concentrations. The GC × GC configuration used here (without cryogenic oven temperatures) could not separate paraffins from cycloparaffins in the C5−C7 range; therefore, these components were not included in the calculations. Figure 3
Figure 4. Relative error versus compound class for the 12 model mixtures examined using GC × GC.
about +5%. These two classes are adjacent to each other in the chromatographic separation; therefore, a slight consistent deviation may represent an impurity or a slight misidentification of a particular compound. Cycloparaffins were slightly overpredicted (less than 10% relative to known values), probably because of normal paraffinic and isoparaffinic peaks, which may have been misidentified. Overall, however, the agreement between predicted and actual levels of compound classes is excellent. Both ASTM D2425 and GC × GC successfully predict the concentration of group types from known mixtures. The linearity of the plot in Figure 3 shows that the FID is responding quantitatively to the mass of the carbon in the compounds being detected without regard to compound class. This observation is consistent with other studies, which have examined the use of FIDs for different compound types.15 These studies show that FID response factors (in the great majority of hydrocarbon compounds) are directly related to the weight percentage of carbon in each compound in a mixture and not directly related to the weight percentage of each compound in the mixture. Consequently, the concentration of each compound must be calculated from the weight percentage of carbon that is obtained from the FID response. Precision for the two methods was measured through replicate analyses of jet fuel samples. The measurements indicate that statistical deviation because of the sampling, injection, GC × GC separation, detection, and quantitation is ≤2.1% for the general types of hydrocarbons in jet fuel (Table 4). This table shows the mean and standard deviation values (expressed in volume percentage) for an n = 4 sampling of a JP-8 fuel analyzed by both GC × GC and D2425 at four different points in time that were a minimum of 6 months apart. Assuming that real world, complex jet fuels would show higher standard deviations from replicate measurements than standard mixtures, any error bars for the data in Figure 3 would generally be smaller than the symbols used in the figure. 3.2. GC × GC Results for Middle Distillate Jet Fuels. The modified ASTM D2425 method was used as a reference technique for comparison to GC × GC measurements for
Figure 3. GC × GC results for 92 different chemical components blended into 12 different mixtures reflecting jet fuel ranges of hydrocarbon group types.
shows the comparison of actual versus measured results using GC × GC. There were some slight biases between GC × GC predictions and known levels of some group types. Figure 4 shows the relative errors of the compound classes as a function of the mean. Generally, the deviation from known results was less than 10% relative to the known concentration. Indans and tetralins, as a class, showed a slight negative bias of −5%, while the alkylnaphthalenes showed a corresponding positive bias of 5699
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Table 4. Replicate Analysis of Fuel for GC × GC and D2425 Hydrocarbon Class Determination
percent to volume percent. These class densities were estimated from the available densities of one or more compounds from the class. Figure 6 also shows good agreement between the two techniques for the more difficult separations within the saturate or aromatic compound classes. Paraffins and cycloparaffins share similar fragmentation ions by mass spectrometry; therefore, they are more reliant on the empirical corrections in ASTM D2425, which were developed for middle distillate fuels. In the situation where highly branched isoparaffins are present (e.g., the synthetic and semi-synthetic fuels), ion ratios that are used to calculate concentrations by D2425 are very similar to those for higher concentrations of cycloparaffins. Consequently, there may be some misclassification of these two groups, especially if highly branched isoparaffins are present in greater amounts. We have observed some instances where, for fuels containing highly branched isoparaffins, the concentration of cycloparaffins by D2425 may be overestimated in comparison to GC × GC cycloparaffin content. This situation may exist for two of the fuels in Figure 6, marked “A” and “B” on the “paraffins” and “cycloparaffins” plots, where the cycloparaffins are significantly higher by D2425 than those by GC × GC. More work is anticipated to verify these initial observations, and more discussion follows for synthetic fuel measurements. As for the GC × GC separation between the chromatographically adjacent monocycloparaffins and isoparaffins, it is not ideal, because the polarity differences between these two groups are small. Despite these difficulties, both techniques are in good agreement with each other for a wide range of petroleum-derived jet fuel samples. Results initially reported for indans and tetralins using GC × GC6 were re-examined for these fuels. New identifications of indans and tetralins were made, which affected the results for both indans/tetralins and alkylbenzenes, improving both. These updated results are shown in Figure 6. Table 5 summarizes the hydrocarbon type results of the world survey fuels by fuel type for both GC × GC and ASTM D2425. This table is not a direct comparison of GC × GC to ASTM D2425, because it compares averages of different JP-8, Jet-A, Jet A-1, and JP-5 fuels. The data give a snapshot view of the general hydrocarbon composition of an average JP-8 or other petroleum fuel, as measured by both techniques. It is important to note that the GC × GC technique presented here does not adequately measure indenes (CnH2n−10) or some of the aromatics that are not typically present in jet fuel (acenaphthenes, acenaphthalenes,
GC × GC (vol %; n = 4) mean alkylbenzenes naphthalene and alkylnaphthalenes indans and tetralins cycloparaffins paraffins and isoparaffins
alkylbenzenes naphthalene and alkylnaphthalenes indans and tetralins cycloparaffins paraffins and isoparaffins
standard deviation
RSD (%)
12.9 1.23 5.37 25.1 55.3
0.20 0.02 0.07 0.67 0.53 D2425 (vol %; n = 4)5
1.6 2.0 1.3 1.2 2.1
mean
standard deviation
RSD (%)
11.9 1.3 5.6 27 54
0.2 0.1 0.6 1.3 1.3
1.7 8.8 10.1 4.7 2.4
hydrocarbon type analysis applied to a variety of jet fuel petroleum samples (along with one synthetic and one semisynthetic fuel) collected during a worldwide survey.16 Because the first part of the D2425 analysis employed here was to measure aromatic content using ASTM D6379, results from GC × GC and D6379 are compared in Figure 5 for the 54 fuels available. The methods are in excellent agreement for nearly every fuel tested. Plots for the more specific hydrocarbon classes (paraffins, alkylbenzenes, cycloparaffins, and indans/tetralins) are shown in Figure 6. The agreement between ASTM D6379 and GC × GC for saturate and aromatic contents is very acceptable. The complete baseline resolution of saturates and aromatics in D6379 using HPLC (see Figure 1b) makes quantitation for this measurement very accurate. The refractive index (RI) detector used for D6379 is very linear for measuring aromatic compounds. GC × GC also shows reasonable chromatographic separation between these two major classes (Figure 2), which is reflected in the ability to accurately quantify the responses for each class. GC × GC is typically quantified in weight percent, which is calculated using FID response for mixtures with low heteroatomic content, whereas ASTM D6379 is often quantified in volume percent to evaluate against fuel specifications. To compare the two techniques, class densities were calculated from individual compound densities to convert GC × GC results from weight
Figure 5. Comparison of saturates and aromatics by GC × GC to measurements made by ASTM D6379, a HPLC-based method. 5700
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Figure 6. JP-8, Jet A-1, JP-5, and Jet A sample hydrocarbon type analysis conducted by GC × GC and compared to ASTM D2425. Data points labeled “A” and “B” in the paraffins and cycloparaffins plot represent a synthetic fuel (A) and a 50:50 synthetic fuel/petroleum fuel blend (B).
Table 5. Average Measured Hydrocarbon Type Compositions of Samples by Fuel Type6 a JP-8 (n = 10)
Jet A (n = 13)
Jet A-1 (n = 28)
JP-5 (n = 2)
JP-8 (n = 10)
D2425 (vol %) paraffins 52 total cycloparaffins 29 alkylbenzenes 12.6 indans and tetralins 4.3 indenes CnH2n−10
