308
Energy & Fuels 1989,3, 308-312
Cetane Number Predictions of a Trial Index Based on Compositional Analysis Seetar G. Pande* Geo-Centers Inc., Fort Washington, Maryland 20744
Dennis R. Hardy Fuels Section, Code 6181, Naval Research Laboratory, Washington, D.C. 20375-5000 Received October 24, 1988. Revised Manuscript Received March 1, 1989
Continuing revision of the Calculated Cetane Index (ASTM D976), which is based on physical properties, focuses on the need for a Calculated Cetane Index based on fuel composition rather than physical properties. To address this need, the development of a new cetane index was ihvestigated by using multiple linear regression analysis. The trial index was based on percent straight chain and branched chain saturates, and percent monocyclic and dicyclic aromatics, as determined by a combination of HPLC and proton NMR analyses. The 53 fuels that comprised the fuel set were obtained from a worldwide survey, and their cetane numbers ranged from approximately 43 to 64. For the same fuel set, the trial index was evaluated on its percent predictability and R2value, relative to those of three published cetane indices. For the fuels examined, the preliminary cetane index based on compositional analysis appears promising: it exhibited similar percent predictions and lower percent overprediction than the recent Calculated Cetane Index, ASTM D4737-87.
Introduction An accurate and reliable method of measuring cetane number, i.e. the ignition quality of diesel fuels, is important since diesel fuel is the primary mobility fuel used in the U S . Navy's ships and boats as well as in the nation's trucks, buses, etc. The standard method of determining cetane number is the engine test, ASTM D613.l However, for convenience purposes, most refineries rely on the approved alternative method of determining cetane number, viz., the Calculated Cetane Index.2 This is a predictive equation of cetane number and is based on bulk properties, namely, distillation temperature(s) and density of the fuel.*6 Historically, the need for greater accuracy in predicting cetane number led to several revisions of the original cetane index, which was recommended in 1944.6 These revisions include ASTM D976-663and ASTM D976-80.4 Also, to resolve problems of biases observed with ASTM D976-80, improvement equations for ASTM D976-80 were f~rmulated.'?~However, the ASTM D976 method was recently replaced by a more complex equation, ASTM D4737-87.6 Like the indices preceding it, D4737-87 is based (1)ASTM Method D-613, Standard Test Method for Ignition Quality of Diesel Fuela bv the Cetane Method. In Annual Book of ASTM Standards, Part k7, Test Methods for Rating Motor, Diesel, Aviation Fuels; ASTM. Philadelphia, PA, 1980. (2) Military Specification MIL-F-l6884H, Fuel, Naval Distillate: Amendment 2, Section 4.5.1. (3) ASTM Method D-976-66, Standard Test Method for Calculated Cetane Index of Distillate Fuels. In Annual Book of ASTM Standards, Part 23, Petroleum Products and Lubricants; ASTM Philadelphia, PA, 1976. (4) ASTM Method D-976-80, Standard Test Method for Calculated Cetane Index of Dietillate Fuels. In Annual Book of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM Philadelphia, PA, 1988; Vol. 05.01. (5) ASTM Method D-4737-87, Standard Test Method for Calculated Cetane Index by Four Variable Equation. In Annual Book of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM Philadelphia, PA, 1988; Vol. 05.03. (6) J. Inst. Pet. 1944, 30, 193-197. (7) Collins, J. M.; Unzelman, G . H. Oil Gas J. 1982, June 7,148-160. (8) Unzelman, G. H. Oil Gas J. 1983, Nov 14, 178-201.
on similar bulk properties, viz., 10,50, and 90% distillation temperatures and density. Continuing revision of the Calculated Cetane Index is likely indicative of fuel compositional changes such as changes in the crudes' composition or refinery operations or both, for the ignition characteristics of a fuel are related to its chemical composition.+l2 Consequently, an index based on compositional analysis would be better predictive of cetane number than calculated cetane indices based on physical properties. The National Research Council of Canada (NRCC) has developed a cetane index based on parameters involving combinations of physical properties that, on modeling, relate to the chemical structure of diesel fuels.12 Although the NRCC index was found to be an improvement over existing indices and was proposed12as an alternative to ASTM D976-80 and the Canadian General Standards Board cetane index,13 it is lengthy and involves six measurements, one of which is aniline point. This also detracts from its merits since aniline is toxic. In formulating cetane indices based on compositional analysis, it is important to identify and quantify the various types of hydrocarbons present in diesel fuels. Several indices based on the different types of carbons, as well as on hydrogen type distribution, have been reported. Determination of different types of carbons was based on proton NMR"J2J4 or carbon-13 NMRI5J6 analysis; hy(9) Olson, D. R.; Meckel, N. T.; Quillian, R. D. SAE Paper 263A; Society of Automotive Engineers Inc.: New York, 1960. (IO) Indritz, D. Prepr-Am. Chem. SOC.,Diu. Pet Chem. 1985,30(2), 282-286. ~.~
(11) Gulder, 0. L.; Glavincevaki,B. Prepr.-Am. Chem. SOC.,Diu. Pet
Chem. 1985,30(2), 287-293.
(12) Gulder, 0 . L.; Burton, G. F.; Whyte, R. B. SAE Paper 861519; Society of Automotive Engineers Inc.: Warrendale, PA, 1986. (13) Steere, D. E. SAE Paper 841344; Society of Automotive Engineers Inc.: Warrendale, PA, 1984; (14) Glavincevski,B.; Gulder, 0. L.; Gardner, L. SAE Paper 841341; Society of Automotive Engineers Inc.: Warrendale, PA, 1984. (15) DeFries, T. H.; Indritz, D.; Kastrup, R. V. PREPR.-Am. Chem. SOC. Diu. Pet. Chem. 1985, 30(2), 294-302. (16) DeFries, T. H.; Indritz, D.; Kastrup, R. V. Ind. Eng. Chem. Res. 1987,26, 188-193.
0887-0624/89/2503-030~~0~.50~0 0 1989 American Chemical Society
Cetane Number Predictions
drogen type distribution was based on proton NMR ana1ysis.l7J8 A disadvantage of cetane indices based solely on conventional NMR analysis for identification and quantification of the different types of hydrocarbons is that overlap of several chemical structures in the same spectral region can cause anomalous results.18 Thus, there is need for a cetane index based on improved compositional analysis. In this paper, the development of a new cetane index was investigated to address this need. The trial index was based on percent straight-chain and branched-chain saturates and percent monocylic and dicyclic aromatics as determined by a combination of HPLC and conventional proton NMR analyses. Evaluation of the index was based on percent predictions, over/underpredictions, as defined by specific criteria and on regression analysis. The percent predictabilities of this index and its correlation coefficient with cetane number, R2,were compared with those of three other published indices including ASTM D4737-87.
Experimental Section Fuel Set. The fuel set comprised 53 fuels obtained from a worldwide survey involving 38 countrie~.'~Classification of the fuels as documentedm were as follows: (a) 27 samples of commercial marine gas oil of which 3 were duplicates for quality analysis/quality control purposes, with cetane numbers of these fuels ranging from approximately 45 to 64,(b) 22 samples of Navy distillate fuel (F-76)with cetane numbers ranging from approximately 48 to 56;(c) 4 samples of commercial heavy marine gas oil with cetane numbers ranging from 43 to 56;Note: in Tables I and 11,for the same class of fuels, fuels were ranked in descending order of cetane number. Fuel Composition. Two types of compositional analyses were performed, which were as follows. Proton NMR Analysis. Analysis based on proton NMR was perfomed by Southwest Research Institute (SwRI) using a JEOL F X 9OQ Fourier transform NMR spectrometer. Sample concentration was 33% in deuterated chloroform (v/v) with tetramethylsilane as the internal standard; the NMR tube was 5 mm in diameter. The NMR integrations for specific chemical shifts, as reported by Bailey et al.,'* were measured by Geo-Centers Inc. HPLC Analysis. The method employed was based on the liquid chromatography/differential refractive index detection method developed by Sink,Hardy, and Hazlett?l By this method, the fuels were separated into three compound classes, viz., saturates, monocyclic aromatics, and dicyclic aromatics (% v/v). Fuels were analyzed on a silica gel column (Whatman M9 10/25 pm Partisii PAC semiprep, 25 cm long, 9 mm i.d.) with polar amino cyano groups as the bonded stationary phase and Freon 113 (1,1,2-trichloro-1,2,2-trifluoroethane) as the mobile phase. Analysis of all fuels except one (C-M-88) was done at least in triplicate (see Table IA).The results given in Table I are average values of replicate analyses. Repeatability was good for most of the fuels including those fuels that were recoded for quality analysis/quality control purposes. Thus, the standard deviation for approximately 80% of the fuels was within &el%. This can be considered as within the experimental error of the method. For approximately 17% of the fuels, their standard deviations were within &1-2%, and for approximately 2% of the fuels, the standard deviation was 4%. Standard deviations of &2-4% may be attributed to the assigned w the true base line in the automatic integration of those peaks. However, such problems are readily
Energy & Fuels, Vol. 3, No. 3, 1989 309 Table I. HPLC Compositional Analysis of Worldwide Survey I1 Fuels % compn ( v / v ) ~ CH&HI monocetane (proton satucyclic dicyclic fuel ID' no.b NMR data)' rates aromatics aromatics A. 27 Samples of Commercial Marine Gaa Oil (Ranked in Order of Measured Cetane Number) C-M-21 64.4 2.02 92 5 3 C-M-45 59.5 2.05 84 13 3 C-M-26 59.0 1.77 81 15 4 C-M-37 (A) 56.7 1.82 88 10 2 C-M-30 (A) 56.6 1.77 88 10 2 C-M-28 55.9 1.73 85 13 2 C-M-18 55.1 1.56 84 14 3 C-M-11 53.3 1.87 79 14 8 C-M-42 53.0 1.83 81 11 8 C-M-19 52.3 1.67 81 15 4 C-M-34 52.0 1.74 72 21 7 C-M-24 51.8 (49.6) 1.89 75 21 4 C-M-33 51.8 1.47 81 15 4 C-M-20 (B) 51.0 1.53 80 17 3 C-M-17 (C) 50.9 1.41 80 17 3 C-M-48 50.8 1.60 82 15 3 C-M-39 (B) 50.4 1.62 81 17 3 C-M-43 49.8 (48.5) 1.24 85 12 3 C-M-29 49.7 1.51 85 12 3 C-M-49 49.7 1.51 78 20 2 C-M-38 (C) 49.3 (51.7) 1.51 78 19 3 C-M-23 48.6 1.58 78 19 2 C-M-14 47.5 1.39 84 14 2 C-M-88 47.4 1.60 77' 15' 8. C-M-13 47.1 1.54 73 24 4 C-M-36 45.5 1.22 84 13 3 C-M-25 45.3 1.65 76 16 8
B. 22 Samples of Naval Distillate Fuel w d 4 Samples of Commercial Heavy Marine Gas Oil (for the Same Fuel Class, Fuels Ranked in Order of Measured Cetane Number) N-F-72 56.3 1.67 81 i7 2 N-F-55 54.0 1.40 85 11 3 N-F-71 53.9 1.62 83 14 3 N-F-53 53.9 1.48 84 12 5 N-F-73 53.2 1.71 83 13 4 N-F-63 53.1 1.77 80 17 3 N-F-83 53.0 1.58 81 15 4 N-F-66 52.7 1.61 80 18 2 N-F-59 52.1 1.34 85 12 3 N-F-61 52.0 1.70 3 79 18 N-F-64 51.9 1.55 81 17 2 N-F-56 51.8 1.68 72 24 4 N-F-57 51.6 1.68 77 19 4 N-F-81 51.6 1.47 3 84 13 N-F-77 51.0 1.41 81 18 1 N-F-52 50.8 1.57 (1.48)f 81 16 3 N-F-80 50.6 (51.2) 1.52 80 17 3 N-F-82 50.3 1.36 83 14 3 N-F-79 50.0 1.58 81 15 4 N-F-74 49.6 1.52 81 14 5 N-F-54 49.5 (49.6) 1.49 80 16 4 N-F-78 48.6 (48.2) 1.41 83 14 3 C-H-23 55.6 1.65 81 17 2 C-H-30 49.6 1.75 83 14 3 C-H-21 48.7 (47.9) 1.69 75 18 7 C-H-36 43.1 1.27 79 19 2
'Fuels with the identical letter in parentheses are identical.
(17) Gulder, 0. L.; Glavincevski, B. Combust. Flame 1986,63,231-238. (18) Bailey, B. K.; Russell, J. A.; Wimer, W. W.; Buckingham, J. P. SAE Paper 861521;Society of Automotive Engineers Inc.: Warrendale, PA, 1986. (19) Shaver, B. D.; Rigstad, D. A,; Modetz, H. J.; Shay, J.;Woodward, P. SAE Paper 871392; Society of Automotive Engineers Inc.: Warrendale, PA, 1987. (20) Shaver, B., David Taylor Research and Development Center, Annapolis, MD. Private Communication. (21) Sink, C. W.; Hardy, D. R.; Hazlett, R. N. Naval Research Laboratory Memorandum Report 5497, Part 2; NRL: Washington, DC, Dec 1984.
Key: N
= Navy; F = F-76 (Naval distillate fuel); C = commercial marine gas oil; H = heavy marine gas oil. Cetane numbers in parentheses are the
values obtained on recoding the same fuel. 'This ratio is a measure of the straight- to branched-chain saturates. dAverage of analysis performed at least in triplicate unless otherwise noted. 'Average of anal'ysis performed in duplicate. 'Value obtained on repeat NMR analysis.
resolved with the improved automatic integrators that are commercially available today. Included in Table I are the CH2:CH8 proton ratios obtained from the proton NMR data. This is a semiempirical ratio and was included as a means of fractionating the percent total saturata
310 Energy & Fuels, Vol. 3, No. 3, 1989 Table 11. Cetane Indiees Values and Predictabilities"of Worldwide Survey I1 Fuels cetane index value compn anal. trial cetane cetane ASTM ASTM index fuel IDb no. D4737-87 D976-80 SwRI (R2= 0.692) A. 27 Samples of Commercial Marine Gas Oil (Ranked in Order of Measured Cetane Number) C-M-21 64.4 68.2 OP 62.9 P 69.2 OP 62.6 P C-M-45 59.5 59.3 P 56.3 UP 62.3 OP 59.0 P C-M-26 59.0 56.6 UP 54.1 UP 57.0 P 54.0 UP C-M-37 (A) 56.7 55.9 P 53.2 UP 60.3 OP 58.5 P C-M-30 (A) 56.6 56.2 P 53.4 UP 58.6 P 57.9 P C-M-28 55.9 55.8 P 54.8 P 57.3 P 56.0 P C-M-18 55.1 54.1 P 54.4 P 56.0 P 52.7 UP C-M-11 53.3 52.0 P 49.4 UP 55.5 OP 53.1 P C-M-42 53.0 54.3 P 52.5 P 57.3 OP 53.8 P C-M-19 52.3 55.4 OP 51.9 P 54.9 OP 52.6 P C-M-34 52.0 52.9 P 51.3 P 54.5 OP 48.6 UP C-M-24 51.8 52.1 P 51.7 P 53.7 P 52.5 P C-M-33 51.8 47.7 UP 46.8 UP 52.5 P 50.2 P C-M-20 (B) 61.0 52.6 P 52.2 P 53.4 OP 50.7 P C-M-17 (C) 50.9 51.0 P 51.0 P 50.2 P 49.2 P C-M-48 50.8 53.5 OP 53.5 OP 54.0 OP 52.4 P C-M-39 (B) 50.4 52.1 P 51.9 P 53.7 OP 52.4 P C-M-43 49.8 49.4 P 49.9 P 47.0 UP 47.8 P C-M-29 49.7 51.4 P 50.5 P 52.3 OP 52.4 OP C-M-49 49.7 54.8 OP 52.7 OP 51.9 OP 49.9 P C-M-38 (C) 49.3 51.0 P 50.9 P 50.6 P 49.4 P C-M-23 48.6 52.5 OP 52.9 OP 52.6 OP 50.9 OP C-M-14 47.5 47.4 P 48.4 P 48.5 P 50.1 OP C-M-88 47.4 48.9 P 47.1 P 50.1 OP 49.0 P C-M-13 47.1 50.1 OP 50.4 OP 48.9 P 47.6 P C-M-36 45.5 48.2 OP 47.2 P 44.4 P 47.3 P C-M-25 45.3 43.1 UP 44.2 P 45.0 P 48.9 OP
B. 22 Samples of Naval Distillate Fuel and 4 Samples of Commercial Heavy Marine Gas Oil (for Same Fuel Class, Fuels Ranked in Order of Measured Cetane Number) N-F-72 56.3 55.4 P 53.5 UP 55.3 P 53.3 UP N-F-55 54.0 53.7 P 52.3 P 51.9 UP 50.6 UP N-F-71 53.9 54.6 P 53.5 P 55.6 P 53.3 P N-F-53 53.9 52.8 P 50.8 UP 51.3 UP 51.0 UP N-F-73 53.2 53.8 P 52.2 P 55.5 OP 54.2 P N-F-63 53.1 52.2 P 52.3 P 55.1 P 53.8 P N-F-83 53.0 52.0 P 51.6 P 52.8 P 51.8 P N-F-66 52.7 55.1 OP 54.3 P 54.4 P 52.2 P N-F-59 52.1 53.3 P 53.0 P 51.7 P 49.5 UP N-F-61 52.0 53.4 P 52.8 P 53.5 P 52.6 P N-F-64 51.9 52.9 P 52.7 P 53.4 P 51.4 P N-F-56 51.8 53.7 P 53.5 P 53.2 P 48.9 UP N-F-57 51.6 51.8 P 52.0 P 53.2 P 51.1 P N-F-81 51.6 51.7 P 51.3 P 51.7 P 51.5 P N-F-77 61.0 51.4 P 51.7 P 50.8 P 49.8 P N-F-52 50.8 53.3 OP 53.0 OP 54.2 OP 51.7 P N-F-80 50.6 52.3 P 51.7 P 52.5 P 50.8 P N-F-82 50.3 51.2 P 50.3 P 48.8 P 49.6 P N-F-79 50.0 54.3 OP 53.3 OP 54.4 OP 51.7 P N-F-74 49.6 50.3 P 49.8 P 51.9 OP 50.4 P N-F-54 49.5 51.1 P 50.6 P 51.2 P 50.1 P N-F-78 48.6 52.3 OP 51.4 OP 50.7 OP 49.9 P C-H-23 55.6 59.2 OP 54.6 P 57.7 OP 53.0 UP C-H-30 49.6 53.8 OP 52.7 OP 57.7 OP 55.2 OP C-H-21 48.7 48.7 P 47.1 P 51.0 OP 49.4 P C-H-36 43.1 49.6 OP 44.8 P 45.0 P 46.6 OP 'Based on the criteria imposed: P = prediction; OP = overprediction; UP = underprediction. bSamples with identical letters in parentheses are identical. Key: N = Navy; F = F-76 (Naval distillate fuel); C = commercial marine gas oil; H = heavy marine gas oil.
into percent straight- and percent branched-chain saturates. Cetane Indices. The indices that were evaluated included three published cetane indices and the trial cetane index. Their formulations are given below. Lotus 1-2-3 was employed in development of the new/trial cetane index. Thus the trial cetane index was based on the equation obtained on multiple linear
Pande and Hardy regression analysis of cetane number vs the pertinent parameters examined. Published Cetane Indices. 1. Calculated Cetane Index (CCI) by a Four-Variable Equation. ASTM D4737-876 (replacement of D976-80): CCI = 45.2 + (0.0892(TlON)) + [(0.131 + 0.901B)(T50N)] + [(0.0523 - 0.420B)(T90N)] + [0.00049((T10N)2(TgON)')] + 107B 60.0B2
+
where D = density at 15 "C in g/mL, B = exp(-3.50(DN))-l, DN = D - 0.850, TlON = T10 - 215, T50N = T50 - 260, and T90N = T90 - 310. T10, T50, and T90 refer to the distillation temperatures in "C of 10,50, and 90% recovered distillate, respectively. 2. Calculated Cetane Index (CCI) of Distillate Fuels. ASTM D976-804 CCI = 454.74
- 1641.4160 + 774.740' - 0.554(T50) + 97.803(log T50)2
where D = density at 15 "C in g/mL and T50 = distillation temperature in OC of 50% recovered distillate. 3. Southwest Research Institute (SwRI) Cetane Index.l8 PCN = 9.49 - 0.0298DHHc~,+ 0.0896DHHc~~ + 0.000097DH9 - 0.038DHHa where PCN = predicted cetane number, H = wt % hydrogen content, D = density, HCH,= % methyl protons of the total number of protons, HCH~ = % methylene protons of the total number of protons, S = sum of % (methyl, methylene, and methine) protons, and Ha = % a protons (protons immediately adjacent to an aromatic ring). Because of its simplicity,the SwRI index was selected over other indices that are based on a similar carbon/hydrogen type distribution and also exhibited good correlations with cetane number. For example, the best equation proposed by Gulder and Glavincevski" involves 14 coefficients as opposed to 4 for the SwRI index. Parameters. Those parameters employed in the formulation of the published indices were determined by the National Institute of Petroleum and Energy (NIPER). These include 10,50, and 90% distillation temperatures, density, and wt % hydrogen. Cetane number measurements were determined by Phillips Petroleum Co. These data were obtained through private communication.20 A summary of the analytical results obtained on characterization of the worldwide survey fuels has been published by Shaver et al.ls Trial Correlation Based on CompositionalAnalysis. This involved HPLC analysis by which percent total saturates, monocyclic and dicyclic aromatics were determined. Also, proton NMR analysis from which the previously determined percent total saturates was subdivided into percent straight- and branchedchain saturates. This fractionation of the percent total Saturates was based on the ratio of CHz to CH3 proton types, since these proton types are representative of straight-chain and branchedchain saturates, respective1y.l' Formulation of this trial index for the 53 fuel set is as follows. Equation obtained on regression analysis (R2 = 0.692): CI = 49.321 + 0.568(% straight-chain saturates) 0.600(% branched-chain saturates) - 0.317(% monocyclic aromatics) - 0.582( % dicyclic aromatics) Standard error of the cetane number estimate was 2.11.
Results and Discussion Cetane index values of the three published indices and the trial index, based on their respective parameters, are given in Table 11. Included in this table are the predictabilities of the various cetane indices for the fuels examined. Predictability of the indices was based on the following adopted criteria: Cetane indices whose predictive ranges (i.e., cetane index minus cetane number) were within f2 cetane numbers were designated as being predictive of cetane number; those whose predictive ranges
Cetane Number Predictions
Energy & Fuels, Vol. 3, No. 3, 1989 311
Table 111. Determination of Percent Predictability of Cetane Indices of Worldwide Survey I1 Fuels
65
% predictability for specific predictive ranges
for 53 fuels predictive range (CI - CN)" predictions within 0 to +LO -0.1 to -1.0 +1.1 to +2.0 -1.1 to -2.0 tot. 0 to +2.0 overpredictions within +2.1 to +3.0 +3.1 to +4.0 +4.1 to +5.0
>+LO tot. +2.1 to >+5.0 underpredictions within -2.1 to -3.0 -3.1 to -4.0 -4.1 to -5.0
