Energy & Fuels 2001, 15, 23-37
23
Diesel Fuel Analysis by GC-FIMS: Aromatics, n-Paraffins, and Isoparaffins Y. Briker,* Z. Ring, A. Iacchelli, N. McLean, P. M. Rahimi, and C. Fairbridge National Centre for Upgrading Technology, 1 Oil Patch Drive, Devon, Alberta T9G 1A8, Canada
R. Malhotra, M. A. Coggiola, and S. E. Young SRI International, Menlo Park, California 94025 Received May 22, 2000. Revised Manuscript Received August 21, 2000
A reliable characterization method giving a detailed hydrocarbon composition profile for transportation fuels is an important part of any process that optimizes fuel reformulation and engine design to reduce regulated emissions. This study describes the development of a modified gas chromatography field ionization mass spectrometry (GC-FIMS) method for detailed hydrocarbon type determination of diesel fuel. Diesel fuels were analyzed by GC-FIMS, and the calculated hydrocarbon type composition profile was compared with that determined by other standard techniques. Results for total saturates, total aromatics, monoaromatics, and polyaromatics contents correlated well. Selected ion chromatograms demonstrated the separation of isoparaffins and normal paraffins in typical diesel fuels. The GC-FIMS method produced results for isoparaffin, normal paraffin, and cycloparaffin contents that did not require sample pretreatment to separate saturates and aromatics fractions. Analysis of a gasoline sample showed excellent agreement between GC-FIMS and detailed hydrocarbon analysis or PIONA for total cycloparaffin content, and reasonable agreement for iso- and normal paraffin contents. The normal paraffin contents of selected diesel blends measured by GC-FIMS correlated well with that measured by other methods. Experiments with internal standardization verified the accuracy of the GC-FIMS method for selected hydrocarbon isomers.
Introduction Emissions from compression ignition engines continue to be studied in several countries around the world. Ultimately, regulated emissions will depend on both the diesel engine technology and the chemical composition of the diesel fuel. The physical properties of diesel fuel may also play an important role. Previous studies of the effects of diesel fuel composition on heavy-duty engine emissions often identified aromatics content, sulfur content, density, volatility, and cetane number as important fuel parameters,1 and recently the impact of diesel fuel quality on engine emissions was reviewed.2 The possible importance of isoparaffin and cycloparaffin compounds to diesel engine emissions has also been described elsewhere.3,4 To determine the overall effects of the chemical composition of transportation fuel on its physical properties and on engine emissions, a reliable and convenient characterization method giving detailed hydrocarbon and heteroatom composition is essential. (1) Karonis, D.; Lois, E.; Stournas, S.; Zannikos, F., Energy Fuels 1998, 12, 230-238. (2) Lee, R.; Pedley, J.; Hobbs, C. SAE Technical Paper Series; Society of Automotive Engineers: Warrendale, PA, 1998; Paper No. 982649. (3) Nakakita, K.; Takasu, S.; Ban, H.; Ogawa, T.; Naruse, H.; Tsukasaki, Y.; Yeh, L. I.; SAE Technical Paper Series; Society of Automotive Engineers: Warrendale, PA, 1998; Paper No. 982494. (4) Takatori, Y.; Mandokoro, Y.; Akihama, K.; Nakakita, K.; Tsukasaki, Y.; Iguchi, S.; Yeh, L. I.; Dean, A. M.; SAE Technical Paper Series; Society of Automotive Engineers: Warrendale, PA, 1998; Paper No. 982495.
There are several chromatographic methods for analysis of relatively simple fuels such as gasolines. These methods use special equipment, such as multicolumn automated gas chromatographs, and may require long analysis time or extensive sample preparation. The methods are computerized and applied extensively to PONA analysis (paraffins, olefins, naphthenes, and aromatics), to PIONA analysis (paraffins, isoparaffins, olefins, naphthenes, and aromatics), and to DHA (detailed hydrocarbon analysis). They offer detailed characterization of light fuels by compound type and carbon number. The analysis of more complex mixtures such as diesel and jet fuels is not possible using these methods because the number of components is too large, and the severe chromatographic peak overlap would prevent meaningful analysis. Current technology for distillates analysis offers several well-established analytical methods based on the utilization of the electron impact (EI) gas chromatography-mass spectrometry (GC-MS) system in which a gas chromatograph is coupled with a mass selective detector (MSD). These methods are developed with a high degree of sophistication and yet are simple to use, allowing characterization of a variety of fuel types. Methods based on high ionization voltage mass spectrometry are standardized into ASTM procedures and widely used by analytical laboratories5 Because there is low or no sensitivity for the molecular ion, the
10.1021/ef000106f CCC: $20.00 © 2001 American Chemical Society Published on Web 11/10/2000
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Energy & Fuels, Vol. 15, No. 1, 2001
calculations are based on the intensities of the various fragments representative of each compound type, and precalibrated sensitivity matrices are used. The methods yield the total amount of hydrocarbons by type, but not by carbon number. These methods are applied extensively for the characterization of petroleum distillates as shown in the following examples. At the National Center for Upgrading Technology (NCUT) the Robinson method6 was used for characterizing the total liquid product from catalytic hydrogenation and selective ring opening processing,7 and the total liquid product from catalytic cracking micro activity testing (MAT).8 The ASTM 2786 method for gas oil saturates analysis5 and the ASTM 3239 method for gas oil aromatic analysis,5 preceded by liquid chromatographic (LC) separation using the ASTM 2007 M or ASTM 2549 method,9 were used to characterize various diesel blends in the diesel fuel composition and emissions study undertaken by the National Research Council of Canada.10,11,12 In this latter work, the LC/ GC-MS results were compared with the results obtained by supercritical fluid chromatography (SFC, CAN/ CGSB-3.0 No. 15.0-94), high-pressure liquid chromatography (HPLC, IP 391/95), and hot fluorescent indicator absorption (Hot FIA, UOP 501-83). The coefficient of determination R2 (the fraction of the variation in the dependent variable that is explained by the independent variable) between the methods for total aromatics and for polyaromatics (diaromatics plus higher aromatics) was very high (R2 from 0.99 to 1.00). This high coefficient indicates that fuel aromatic content was measured consistently by all methods and, as a corollary, that total saturate content was also measured consistently. For saturate hydrocarbon type determination (i.e., n-paraffins, isoparaffins, and cycloparaffins), there is no independent technique available to verify the accuracy of the analyses. There is a patented method that describes determination of normal paraffins, iso(5) Annual Book of ASTM Standards, Section 5 Petroleum Products, Lubricants, and Fossil Fuels; American Society for Testing and Materials: Easton, MD (Standard test method for hydrocarbon type analysis of gas-oil saturate fractions by high ionizing voltage mass spectrometry; Designation: D2786. Method for aromatic types analysis of gas-oil aromatic fractions by high ionizing voltage mass spectrometry; Designation: D3239.). (6) Robinson, C. J. Anal. Chem. 1971, 43, 1425-1434; Robinson, C. J.; Cook, G. L. Anal. Chem. 1969, 41, 1548-1554. (7) Kimbara, N.; Charland, J.-P.; Wilson, M. F., Ind. Eng. Chem. Res. 1996, 35, 3874-3883. (8) Ng, S.; Briker, Y.; Zhu, Y.; Gentzis, T.; Ring, Z.; Fairbridge, C.; Ding, F.; Yui, S. Energy Fuels. 2000. Submitted for publication. (9) Annual Book of ASTM Standards, Section 5 Petroleum Products, Lubricants, and Fossil Fuels; American Society for Testing and Materials: Easton, MD (Test method for characteristic groups in rubber extender and processing oils by the clay-gel adsorption chromatographic method; Designation: D2007. Method for Separation of Representative Aromatics and Nonaromatic Fractions of High Boiling Oils by Elution Chromatography; Designation: D2549.). (10) Li, X.; Chippior, W. L.; Gu¨lder, O ¨ .L.; Cooley, J.; Richardson, E. K.; Mitchell, K. SAE Technical Paper Series; Society of Automotive Engineers: Warrendale, PA, 1998; Paper No. 982487. (11) Li, X.; Gu¨lder, O ¨ .L. Effects of Fuel Cetane Number, Density and Aromatic Content on Diesel Engine NOx Emissions at Different Operating Conditions. Presented at Fourth International Symposium on Diagnostic and Modeling of Combustion in Internal Combustion Engines COMODIA 98, Kyoto, Japan, The Japanese Society of Mechanical Engineers. (12) Neill, W. S.; Li, X.; Chippior, W. L.; Gu¨lder, O ¨ .L. Canadian Diesel Fuel Composition and Emissions-II; Combustion Research Group, Institute for Chemical Process and Environmental Technology, National Research Council Canada: Montreal Road, Ottawa, Ontario, Canada K1A 0R6, 1999.
Briker et al.
paraffins and naphthenes in a saturated hydrocarbon mixture by field ionization mass spectrometry.13 However, it cannot be applied to an unseparated distillate fuel. Without a way to truly calibrate the analysis for saturates, one has to rely on reproducibility and relative comparison of the analyses. Field ionization mass spectrometry (FIMS) is a “soft” ionization technique in which a relatively small quantity of internal energy is supplied to the molecule, so that molecular ions are produced for most molecular species.14 Thus, field ionization affords substantially reduced fragmentation and much higher molecular ion intensities compared with electron impact ionization. This feature makes assignment of peaks to compound types straightforward. It allows one to perform quantitation based on the intensity of molecular ions only and obviates the need for complex matrix inversion routines. It also opens up the possibility of analyzing complex multicomponent hydrocarbon mixtures without prior LC separation by utilizing gas chromatography with FIMS (GC-FIMS). FIMS has been used by many researchers for analysis of various fossil fuels. It was extensively studied by Beckey15 on a wide variety of organic compounds and applied to the analysis of low molecular weight petroleum fractions using a single focusing instrument. The technique has been extended to higher molecular weight fractions by using a double-focusing mass spectrometer to increase resolution.16 The multipoint field ionizing device developed at SRI was extensively used to analyze coal liquefaction residues,17 nondistillable coal liquids,18 and coprocessing distillates.19 FIMS analysis was also applied to various fractions of Athabasca tar sand bitumen.20 Generally, FIMS applications yield information on hydrocarbon components in a complex mixture in terms of a z-series analysis. The z-number arises from the nominal hydrocarbon molecular formula CnH2n+z. The latest development of combining field ionization mass spectrometry with gas chromatographic separation is a promising method for the analysis of transportation fuels. Previous communications reported the development of the GC-FIMS method for the analysis of diesel fuels.7,8,21,22 In this method, the carbon number distribution is provided for various classes of compounds. The (13) Liang, Z.; Hsu, C. S. Energy Fuels 1998, 12, 637-643. (14) St. John, G. A.; Buttrill, S. E., Jr.; Anbar, M. Organic Chemistry of Coal; Larsen, J. W., Ed.; ACS Symposium Series 71; American Chemical Society: Washington, DC, 1978; pp 223-239. (15) Beckey, H. D. Angew. Chem., Int. Ed. 1969, 8, 623-639. (16) Mead, W.L. Anal. Chem. 1968, 40, 743-747. (17) Malhotra, R.; McMillen, D. F.; Watson, E. L.; Huestis, D. L. Energy Fuels 1993, 7, 1079-1087. (18) Boduszynsky, M. M.; Hurtubise, R. J.; Allen, T. W. Anal. Chem. 1983, 55, 225-231. (19) Rahimi, P. M.; Fouda, S. A.; Kelly, J. F.; Malhotra, R.; McMilaan, D. F. Process Evaluation and Characterization of Coprocessing Distillates using Field Ionization Mass Spectrometry. In Proceedings of the 7th International Conference on Coal Science, Banff, Alberta, Canada, Sept 12-17, 1993; Canadian National Organizing Committee, 1993; pp 524-527. (20) Paizant, J. D.; Hogg, A. M.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1985, 1, 175-182. (21) Malhotra, R.; Coggiola, M. J.; Young, S. E.; Hsu C. S.; Dechert, J.; Rahimi, P. M.; Briker, Y. Prepr. Am. Chem. Soc., Div. Petr. Chem. 1998, 43,, 507-509. (22) Briker, Y.; Rahimi, P. M.; Iacchelli, A.; Ring, Z.; Fairbridge, C.; Malhotra, R. Prepr. Am. Chem. Soc., Div. Fuel Chem. 1999, 44, 172-175.
Diesel Fuel Analysis
hydrocarbon species sharing the same nominal mass but belonging to different hydrocarbon types are separated by gas chromatography. The GC separation removes the need for high-resolution mass spectrometry. The typical FIMS report by z-mass now can be augmented with the more useful information giving the fuel composition by compound type. The novelty of the instrumentation lies in the adaptation of a standard commercial benchtop Hewlett Packard (HP) EI GC-MSD system with electron ionization to a field ionization source. To minimize the modifications and simplify the usage, SRI replaced the HP electron impact ionizer with an SRI volcano-type field ionizer, and also modified the HP Chemstation software to include a z-series analysis into the “Chromatogram” menu of the MSD software. By extracting an ion chromatogram for a selected mass, assigning an appropriate time window for peak area integration, and applying specific sensitivity factors to the integrated intensities, the quantification by z-series is achieved.23 This paper reports further development of the GCFIMS method for transportation fuels characterization, particularly diesel fuels. The improvements include addition of several compound types, refinement of the sensitivity factors, and development of software procedures to facilitate postacquisition data processing. The method was used to analyze about thirty samples and, thus, to test the robustness of the method as well as compare the results with those obtained by other methods. The main objective of this study is to develop a method that gives more detailed hydrocarbon type characterization than afforded by existing ASTM methods based on EI systems,5,6 and that does not need prior chromatographic separation while maintaining the userfriendly features of a commercial instrument. Experimental Section Two gasoline samples and three sets of diesel fuels derived from oil sands and conventional crude oil sources were selected. The selected diesel fuels were analyzed by the GCFIMS method and the results were compared with those obtained with LC/GC-MS and Robinson methods. The latter two methods along with SFC, HPLC, and Hot FIA provided a sufficient amount of data for comparison of the aromatic types. To evaluate the GC-FIMS performance in the saturate region, selected samples were also analyzed by high-resolution gas chromatography (HRGC) which allowed an independent calculation of n-paraffin content. Selected diesel samples were also analyzed for n-paraffins, isoparaffins, and cycloparaffins at Core Laboratories, Inc., Houston, Texas, using an LC/GCMS method. GC-FIMS. For most compounds, field ionization produces only the molecular ions. Therefore, an adequate separation can be achieved by FIMS within a homologous series of a particular hydrocarbon type.23 For example, normal paraffins (in z ) +2) can all be identified by carbon number. The alkylbenzenes (in z ) -6) can also be identified by carbon number, in which case each carbon number represents all isomers sharing the same molecular weight (C8 alkylbenzene with nominal mass of 106 would represent all substituted dimethylbenzenes and ethylbenzenes). The species with different carbon numbers have different nominal masses and would be easily resolved by the unit-mass resolution of FIMS alone. However, molecular weight alone is not sufficient to uniquely identify the class of (23) Malhotra, R.; Coggiola, M. J.; Young, S. E.; C. A. Spindt, C. A. Prepr. Am. Chem. Soc., Div. Petr. Chem. 1996, 41, 652-657.
Energy & Fuels, Vol. 15, No. 1, 2001 25 a given hydrocarbon. In some cases, the overlapping occurs when the compounds representing different hydrocarbon types share the same nominal masses (e.g., nonanesan acyclic saturate and naphthalenesa diaromatic share the same nominal mass of 128). FIMS would not distinguish between these two compound types. These cases can be resolved by high-resolution mass spectrometry, or some separation by gas chromatography. There is a sufficient difference between the boiling points of a paraffin (z ) +2) and an alkylnaphthalene (z ) -12) of the same nominal mass so that their elution time from the GC would be quite different. By extracting the ion chromatogram for the selected mass in the appropriate time window for peak area integration, and applying the appropriate correction for relative sensitivity to the integrated intensities, the sample composition can be presented in a percentage form. This procedure is repeated for every mass, and the results are tabulated and normalized. The original SRI method23 assigned masses for hydrocarbons from C5 to C20 within the z range from 2 to -14. This covered the range of hydrocarbons from paraffins to naphthocycloparaffins. There were also overlaps that did not allow the calculation of certain hydrocarbon types. Examples of some of the overlaps include those between n- and isoparaffins (in z ) +2), benzothiophenes and alkylbenzenes (in z ) -6), and naphthalenes and dibenzothiophenes (in z ) -12) i.e., there was no provision in the method for separating the hydrocarbons sharing an identical mass within the same z-series. The calculations of hydrocarbon types by original SRI method with the trial set of factors did not match the results for total saturates and aromatics obtained with LC separation of diesel fuels. An example of the original z-series analysis of diesel fuel is given in Table 1. To remove the overlaps and cover a full diesel range, the z-table was expanded from final z ) -14 to z ) -18. This expansion was necessary since LC/GC-MS analysis of typical diesel fuels showed the presence of fluorenes (z ) -16) and phenanthrenes (z ) -18) in the tested diesel fuels. The column representing z ) -6 (alkylbenzenes) in the z-table was split into three columnssalkylbenzenes (z ) -6), 4-ring cycloparaffins (z ) -6), and benzothiophenes (z ) -10S). Similarly, the column representing z ) -12 (naphthalenes) was split into two columnssnaphthalenes (z ) -12) and dibenzothiophenes (z ) -16S). It was particularly important to separate alkylbenzenes from benzothiophenes and naphthalenes from dibenzothiophenes since every hydrocarbon report usually includes aromatic sulfur. As shown in the selected ion chromatograms of sample HGO TK43 in Figure 1, the aromatic sulfurcontaining compounds elute after the aromatic hydrocarbon with the same nominal mass. By assigning separate time windows and sensitivities, these groups can be calculated and reported separately. In Figure 1, for example, the m/z 184 selected ion chromatogram shows only a C4 substituted naphthalene, C14H16, for the hydrocarbon species n ) 14, z ) -12 (butylnaphthalene and its isomers). The m/z 198 selected ion chromatogram shows the separation of the alkylnaphthalene, C15H18, for the hydrocarbon species n ) 15, z ) -12 (pentylnaphthalene and its isomers), and the alkyldibenzothiophene C13H10S for the species n ) 13, z ) -16 (methyldibenzothiophene). The m/z 212 selected ion chromatogram shows the separation of the alkylnaphthalene C16H20 for the hydrocarbon species n ) 15, z ) -12 (hexylnaphthalene and its isomers), and the alkyldibenzothophene, C14H12S, for the species n ) 14, z ) -16 (dimethyldibenzothiophene or ethyldibenzothiophene). A similar procedure was applied to the column representing hydrocarbon series with z ) +2 (paraffins) in the diesel fuel z-series analysis. This column was split into two z ) +2 columns, separately representing isoparaffins and normal paraffins. In the selected ion chromatograms of sample S20A in Figure 2, these z ) +2 paraffin groups are shown as
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Briker et al.
Figure 1. Selected ion chromatogram of m/z 184, 198, and 212 extracted from GC-FIMS analysis of sample HGO TK43. Table 1. z-Series Analysis of D46B Diesel Fuel by the Original GC-FIMS Method z-series, wt % carbon number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 total
+2
0
-2
-4
0.935 0.585 0.471 0.382 0.303 0.274 0.162 0.107 0.085 0.092 0.134 4.923
0.002 0.058 0.166 0.301 0.868 1.673 1.313 0.91 0.856 0.717 0.639 0.54 0.533 0.479 0.405 0.366 11.584
0.01 0.151 0.435 0.819 1.738 0.705 1.15 1.145 1.04 0.931 0.986 0.723 0.519 0.46 0.403 0.345 9.002
0.061 0.156 0.379 0.064 0.133 0.239 0.349 0.454 0.529 0.546 0.462 0.378 0.351 0.243 3.748
separated peaks that elute before the alkylnaphthalene of the same nominal mass. The n-paraffin peak is always the last peak in the series for every extracted paraffin mass and, therefore, can be calculated separately by extracting the selected mass in the appropriate time window for peak integration in the chromatogram. To calculate the sensitivity factors, a number of test mixtures composed of representatives from various compound classes were analyzed. Although the factors vary quite significantly with the compound class (i.e., between different z-numbers), the variation in sensitivity factors within the same class (i.e., between hydrocarbons in the same z-series) is typically only ( 15%. Table 2 presents the z-series analysis of diesel fuel calculated by the modified GC-FIMS method. To allow comparison with other methods, the columns in the z-table were translated into hydrocarbon types. Just as with the ASTM GC-MS methods for hydrocarbon types calcula-
-6
-8
-10
-12
-14
3.963 2.321 1.64 1.341 1.105 0.953 0.857 0.715 0.641 0.519 0.367 20.714
0.022 0.276 1.44 4.554 0.981 2.86 4.44 3.588 2.341 1.558 1.139 0.856 0.692 0.503 0.362 19.471
0.001 0.149 0.000 0.019 0.182 0.635 1.055 1.127 0.951 0.712 0.531 0.377 0.252 5.841
0.218 1.48 4.181 4.152 2.236 1.118 0.716 0.458 0.353 0.242 0.171 15.324
0.224 1.155 2.218 2.331 1.628 0.905 0.495 0.275 0.161 9.393
tion,5 the current development of the GC-FIMS method assumes very little or no olefins in the analyzed samples. For the GC-FIMS runs, a 30 m × 250 µm × 0.25 µm HP1MS nonbonded column was used. The injection (0.2µL; 19:1 split) was made with the oven at 45 °C. The oven was heated at 10 °C/min to 300 °C. Two diesel samples were also analyzed by GC-FIMS using an internal standardization procedure (ISTD). The reason for this exercise was to evaluate the accuracy of the method for a particular cell in a result table (i.e., for a particular hydrocarbon of a particular carbon number). A calibration mixture consisting of eight compounds found in the analyzed diesels (n-dodecane, tetralin, o-xylene, naphthalene, 2-ethylnaphthalene, acenaphthene, phenanthrene and 3,6-dimethylphenanthrene) was prepared. The compound 2-ethylamine (MW ) 101) was chosen as an internal standard because it had an odd molecular mass and it was not found in any analyzed diesel fuels.
Diesel Fuel Analysis
Energy & Fuels, Vol. 15, No. 1, 2001 27
Figure 2. Selected ion chromatogram of sample S20A showing the C13 through C15 saturate series (short retention times) and the corresponding isobaric alkylnaphthalenes (long retention times). Table 2. z-Series Analysis of D46B Diesel Fuel by a Modified GC-FIMS Methoda z-series, wt % carbon number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 total
2 iP
2 nP
0 1RcP
-2 2RcP
0.888 0.447 0.612 0.763 1.488 2.504 1.768 1.464 1.252 0.926 0.909 0.550 0.607 0.865 0.727 15.770
0.035 0.032 0.191 0.330 0.304 0.708 1.018 1.400 1.308 1.176 0.913 0.866 0.508 0.335 0.222 0.184 0.416 9.945
0.023 0.391 0.293 0.577 1.477 1.735 2.110 1.538 1.551 1.298 1.159 1.086 1.074 0.963 0.814 0.739 16.828
0.096 0.227 0.522 0.906 1.447 1.404 1.264 1.094 1.162 0.966 0.694 0.621 0.546 0.466 11.417
-4 3RcP
0.006 0.045 0.095 0.199 0.340 0.503 0.628 0.729 0.768 0.641 0.561 0.527 0.365 5.407
-6 4Rcp
-6 AlkylBz
-10S Bzth
0.001 0.002 0.000 0.000 0.001 0.000 0.001 0.000 0.004
0.036 0.335 1.013 2.690 2.310 1.291 0.906 0.706 0.579 0.488 0.425 0.334 0.301 0.245 0.172 11.831
0.002 0.040 0.017 0.032 0.019 0.017 0.020 0.038 0.019 0.011 0.005 0.220
-8 5RcP
-8 Bzcald
-10 Bzdicalk
-12 Naphth
0.002 0.001 0.002 0.000 0.001 0.000 0.000 0.006
0.086 0.547 1.550 2.218 1.767 1.137 0.745 0.536 0.398 0.316 0.223 0.162 9.687
0.009 0.086 0.288 0.487 0.488 0.427 0.317 0.233 0.164 0.108 2.608
0.124 0.756 2.137 2.120 1.052 0.503 0.329 0.026 0.172 0.120 0.083 7.423
-16S Dbzth
-14 Naphcalk
-16 Fluor
-18 Phenanth
0.009 0.024 0.032 0.178 0.007 0.003 0.002 0.254
0.118 0.607 1.164 1.225 0.855 0.476 0.260 0.145 0.085 4.934
0.066 0.304 0.562 0.520 0.313 0.201 0.137 0.088 2.192
0.164 0.509 0.428 0.170 0.094 0.065 0.045 1.475
a iP: isoparaffins. nP: normal paraffins. 1RcP: one-ring cycloparaffins. 2RcP: two-ring cycloparaffins. 3RcP: three-ring cycloparaffins. 4RcP: four-ring cycloparaffins. AlkylBz: alkylbenzenes. Bzth: Benzothiophenes. 5RcP: five-ring cycloparaffins. Bzcalk: benzocycloalkanes. Bzdicalk: benzodicycloalkanes. Naphth: naphthalenes. Dbzth: dibenzothiophenes. Naphcalk: naphthocycloalkanes. Fluor: fluorenes. Phenanth: phenanthrenes
LC/GC-MS. The samples were initially separated into saturate and aromatic fractions (ASTM 2549 and ASTM 2007 M for colored samples) to be further analyzed by the GC-MS. The instrument was a Hewlett Packard GC-MS system consisting of HP 7673 GC/SFC injector, HP 6890 GC, and HP 5973 MSD. The column was a High-Resolution Gas Chromatography Column J122-5532 DB-5MS, length, 30 m: i.d., 250 µm; film, 0.25 µm. An injection of 0.1 µL was made with a cool on-column injector heated from 60 to 285 °C at a rate of 20 °C /min. The GC oven was held at 35 °C for 3 min and heated at 10 °C/min to 280 °C. The MSD temperature was 285 °C. For hydrocarbon types, the ASTM 2786 method was used to calculate the saturates distribution and the ASTM 3239 method to calculate the aromatics distribution. The samples
were also analyzed by GC-MSD without prior separation into saturate and aromatic fractions. In this case, the calculation was performed by the Robinson method. The software for the calculation of hydrocarbon types by the Robinson method was developed and supplied by R. Teeter of PCMSPEC.24 High-Resolution Gas Chromatography Analysis (HRGC) of Saturate Fraction. To determine the n-paraffin content, saturate fractions of selected diesel fuels were analyzed by a high-resolution GC technique. The analysis was performed on a GC with a flame ionization detector (GC-FID). The gas chromatograph was fitted with a RESTEK RTX-1 (24) Teeter, R. M. Software for calculation of hydrocarbon types; PCMASPEC: 1925 Cactus Court, #2, Walnut Creek, CA 94595-2505.
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Table 3. Comparison of Hydrocarbon Type Composition Calculated by Different Methods for Gasoline 1 carbon number
DHA
1 2 3 4 5 6 7 8 9 10 11 12 total
0.00 0.00 0.00 2.88 17.22 11.67 3.42 1.08 0.18 0.01 0.02 0.00 36.46
a
isoparaffins, wt % PIONA GC-FIMS 0.00 0.00 0.00 2.69 17.78 11.78 3.75 1.09 0.21 0.00 0.00 0.00 37.30
0.00 0.00 0.00 0.00a 17.74 11.84 3.79 1.23 0.20 0.03 0.00 0.00 34.83
DHA 0.00 0.00 0.00 8.15 5.22 2.32 0.92 0.22 0.04 0.00 0.00 0.00 16.87
n-paraffins, wt % PIONA GC-FIMS 0.00 0.00 0.00 0.00b 5.31 2.46 0.97 0.21 0.00 0.00 0.00 0.00 8.95
0.00 0.00 0.00 8.09 5.18 2.31 0.92 0.22 0.02 0.00 0.00 0.00 16.74
cycloparaffins, wt % DHA PIONA GC-FIMS
DHA
0.00 0.00 0.00 0.00 0.84 3.46 0.27 0.03 0.00 0.00 0.00 0.00 4.60
0.00 0.00 0.00 0.00 0.00 0.81 10.07 16.80 11.24 2.04 0.30 0.02 41.28
0.00 0.00 0.00 0.00 0.93 3.76 0.33 0.00 0.00 0.00 0.00 0.00 5.02
0.00 0.00 0.00 0.00 0.87 3.48 0.43 0.19 0.07 0.02 0.01 0.00 5.02
aromatics, wt % PIONA GC-FIMS 0.00 0.00 0.00 0.00 0.00 0.90 11.93 20.32 13.94 1.64 0.00 0.00 48.73
0.00 0.00 0.00 0.00 0.00 0.84 11.24 16.41 10.90 2.22 0.12 0.01 41.73
C4 isoparaffin fragmented under run conditions. b C4 n-paraffin lost during handling. Table 4. Comparison of Hydrocarbon Type Composition by Different Methods for Gasoline 2
carbon number
DHA
1 2 3 4 5 6 7 8 9 10 11 12 total
0.00 0.00 0.00 3.10 11.87 10.12 4.17 1.23 0.27 0.09 0.02 0.00 30.60
Isoparaffins, wt % PIONA GC-FIMS 0.00 0.00 0.00 3.66 12.09 9.94 4.41 1.26 0.33 0.00 0.00 0.00 31.69
0.00 0.00 0.00 0.00 17.33 12.69 4.18 1.28 0.25 0.02 0.00 0.00 35.75
DHA 0.00 0.00 0.00 7.71 4.57 2.50 1.09 0.36 0.07 0.00 0.00 0.00 16.31
n-paraffins, wt % PIONA GC-FIMS 0.00 0.00 0.00 4.58 2.56 1.10 0.33 0.05 0.00 0.00 0.00 0.00 8.63
0.00 0.00 0.00 8.31 5.48 2.49 1.01 0.30 0.00 0.00 0.00 0.00 17.59
PONA column coated with 100% dimethylpolysiloxane (100 m × 250 µm × 0.50 µm). A constant flow of 0.5 mL/min was maintained during the analysis. The injection (0.1 µL, 100:1 split ratio) was made with the GC oven at 120 °C. The hold time at this temperature was 20 min and then oven temperature was raised to 280 °C at a rate of 2 °C/min. At these conditions, a good separation of n-paraffins was achieved. The n-paraffin content of the saturate fractions was calculated by using the calibration factors obtained from running normal paraffin standards. PIONA and Detailed Hydrocarbon Analysis (DHA) of Gasolines. Several gasoline samples were analyzed by PIONA and DHA methods for hydrocarbon type composition, and these results were compared with the results obtained by GC-FIMS. The AC PIONA analyzer based on HP GC 5890 instrument was used to perform the analysis. It was operated under the “mode 20” conditions (normal paraffins, isoparaffins, naphthenes, and aromatics).
Results and Discussion Response Factors. The determination of relative sensitivity factors for hydrocarbon types did not present any difficulties with the exception of isoparaffins. For each compound class, the molar sensitivities were calculated first from the intensities of the molecular ion and the number of moles present in a standard. Then, they were converted to the relative mass sensitivities. In cases when standards were not available, it was possible to predict the factor since its value within a compound class was reasonably constant.23 Isoparaffins tend to fragment even under the soft ionization conditions of GC-FIMS analysis. Some isoparaffin compounds, while fragmenting, still produced the molecular ion with measurable intensity, while others (especially those having quaternary carbon) did not show the molecular ion at all. In this latter case, this particular
cycloparaffins, wt % DHA PIONA GC-FIMS
DHA
0.00 0.00 0.00 0.00 0.77 3.19 0.13 0.06 0.00 0.00 0.00 0.00 4.15
0.00 0.00 0.00 0.00 0.00 0.68 4.58 23.34 16.25 2.85 0.40 0.03 48.13
0.00 0.00 0.00 0.00 0.79 3.25 0.25 0.00 0.00 0.00 0.00 0.00 4.29
0.00 0.00 0.00 0.00 0.74 2.97 0.38 0.19 0.06 0.00 0.00 0.00 4.34
aromatics, wt % PIONA GC-FIMS 0.00 0.00 0.00 0.00 0.00 0.75 5.15 27.84 19.63 2.00 0.00 0.00 55.37
0.00 0.00 0.00 0.00 0.00 0.65 5.37 18.62 13.15 2.68 0.14 0.01 40.62
compound would not be accounted for by GC-FIMS analysis. In the diesel range, where the number of isomers increases significantly, the error from losing some of the isoparaffins to fragmentation may not be significant. Analysis of Gasoline Samples. The results of analyses of two gasoline samples by three different methods are presented in Tables 3 and 4. Since PIONA and DHA analyses give the hydrocarbon type composition by carbon number, it was possible to check the accuracy of the GC-FIMS calculation for each carbon number in the particular z-series. These results are presented as hydrocarbon type composition (isoparaffins, normal paraffins, cycloparaffins, and aromatics) versus carbon number. The samples had very low olefins content (0.58% in “Gasoline1” and 0.43% in “Gasoline2”). A preliminary set of experimentally determined relative sensitivities was used to calculate the composition of Gasoline1. The GC-FIMS results showed good correlation with the DHA results for each carbon number of each reported hydrocarbon class in this sample. There were no data for C4 isoparaffins measured by the GCFIMS while they were reported by the other two methods in Table 3. There is a possibility that this isoparaffin was fragmented under the FIMS conditions, rendering the intensity of the molecular ion below the detection limit. There was also no data reported for C4 normal paraffin by PIONA. It could have been lost during handling. The sensitivity and the retention timetables covered the full range from z ) +2 to z ) -18 and carbon numbers from C4to C20. The sensitivity factors for isoparaffins were later readjusted for carbon numbers C4 to C9 based on the DHA analysis of Gasoline1. Subsequently, a readjusted set of factors was used to calculate the composition of the second gasoline
Diesel Fuel Analysis
Energy & Fuels, Vol. 15, No. 1, 2001 29
Figure 3. Correlation of hydrocarbon type contents in Gasoline2 measured by different methods (the dotted line represents perfect agreement between methods.)
sample, Gasoline2, and all the diesel fuels used in the present study. Figure 3 shows the correlations between GC-FIMS results and the results from the PIONA and DHA for the measured hydrocarbon types for Gasoline2. Each point on a curve corresponds to a weight percent of hydrocarbon with a specific carbon number. There was relatively good correlation between the three analytical methods (R2 from 0.99 to 1.00 for data from C5 to C9 for all hydrocarbon typessisoparaffins, n-paraffins, cycloparaffins, and aromatics). PIONA results for n-paraffins were lower for each carbon number while GC-FIMS results for isoparaffins were lower for each carbon number. The total aromatics measured by PIONA were the highest. Obviously, these particular gasoline samples were of similar origin and had similar isoparaffin makeup. For samples of different origin and with higher isoparaffin content, it is possible that GC-FIMS results would not correlate well with the DHA or PIONA results if the same set of relative sensitivities were used. Analysis of Diesel Fuel Samples. The main purpose of this work was to initiate the development of a reliable and simple method for analysis of diesel fuel, a method that requires less time than the existing methods using LC separation prior to GC-MS analysis and that is more informative than the Robinson method. To
evaluate the GC- FIMS method for calculating specific hydrocarbon types, the aromatics contents determined by mass spectrometry methods (LC/GC-MS, Robinson, and GC-FIMS) were compared with the aromatics contents determined by other methods independent of mass spectrometry. These included supercritical fluid chromatography (SFC, CAN/CGSB-3.0 no. 15.0-94) performed by Syncrude Canada Ltd., high-pressure liquid chromatography (HPLC, IP 391/95) performed by Shell Canada Ltd., and hot fluorescent indicator absorption (FIA, UOP 501-83) also performed by Shell Canada Ltd. The data are summarized in Table 5. These results suggest very good agreement between most of the methods. However, the Robinson results differ quite significantly in most cases. For example, the total aromatics for sample D37A were 38.7% as obtained by LC/GC-MS, 39.8% by GC-FIMS, 38.7% by SFC, 39.5% by HPLC and 35.2% (v/v) by FIA, while the Robinson result was 48.9%. As observed earlier, the Robinson results were closer to the other data when the diesel fuels had low aromatic content (as in samples C10A, C10B, S10A, and S10B). The linear correlations between SFC and GC-FIMS data for total aromatic (Figure 4, R2 ) 0.96), monoaromatic (Figure 5, R2 ) 0.95), and polyaromatic (Figure 6, R2 ) 0.96) content for the 16 diesel fuels in Table 5 were comparable to correlations
30
Energy & Fuels, Vol. 15, No. 1, 2001
Briker et al.
Table 5. Comparison of Aromatics Contents Measured by Different Methods hydrocarbon type total aromatics monoaromatics diaromatics triaromatics total aromaticsa monoaromatics diaromatics triaromatics
a
total aromatics monoaromatics diaromatics triaromaticsa total aromatics monoaromatics diaromatics triaromaticsa total aromatics monoaromatics diaromatics triaromaticsa total aromatics a
method
units C10A C10B C20A C20B C30A C30B S10A S10B S20A S20B S30A S30B D46B D56C D37A D57D
LC/GC- wt % MS LC/GC- wt % MS LC/GC- wt % MS LC/GC- wt % MS GCwt % FIMS GCwt % FIMS GCwt % FIMS GCwt % FIMS Robinwt % son Robinwt % son Robinwt % son Robinwt % son SFC wt % SFC wt % SFC wt % SFC wt % HPLC wt % HPLC wt % HPLC wt % HPLC wt % hot FIA vol %
10.2
10.2
19.9
19.4
28.8
28.8
11.1
12.4
19.7
22.4
30.6
30.2
39.5
26.4
38.7
24.4
9.1
7.3
15.4
15.8
21.5
24.5
9.8
9.4
17.6
19.3
26.3
26.2
24.4
19.2
31.7
20.2
1.1
2.8
4.3
3.6
6.9
4.2
1.2
2.7
2.0
2.8
3.8
3.9
13.4
5.9
6.4
3.9
0.0
0.1
0.1
0.0
0.3
0.1
0.0
0.2
0.0
0.1
0.3
0.0
1.5
0.9
0.3
0.1
12.7
14.2
22.4
20.1
31.1
27.3
12.6
13.3
19.4
26.4
33.3
34.5
40.9
29.9
39.8
29.0
11.6
9.6
17.4
16.3
22.9
24.6
11.5
10.4
17.7
23.5
30.0
30.8
24.4
23.0
32.9
24.3
1.1
4.3
4.8
3.6
7.7
3.5
1.1
2.8
1.7
2.7
3.1
3.5
14.7
6.3
6.4
4.5
0.0
0.2
0.1
0.1
0.3
0.1
0.0
0.1
0.0
0.1
0.1
0.1
1.5
0.4
0.2
0.1
13.1
11.2
27.0
26.0
40.9
38.6
13.7
13.6
23.4
31.1
38.5
40.2
49.8
35.7
48.9
19.4
11.3
7.2
21.3
20.6
30.7
33.5
10.8
9.0
20.8
26.2
34.8
36.3
29.7
25.8
40.7
18.5
1.6
3.7
5.5
5.2
9.5
5.0
1.6
2.7
2.4
3.3
3.4
3.5
17.2
7.5
7.6
0.9
0.0
0.0
0.1
0.1
0.6
0.1
0.8
1.3
0.2
1.5
0.3
0.4
2.4
2.3
0.6
0.0
10.8 9.6 1.1 0.1 10.4 9.7 0.7
