Energy & Fuels 1988,2, 854-860
854
Investigation of the Chemical Basis of Diesel Fuel Properties David J. Cookson, C. Paul Lloyd, and Brian E. Smith* Melbourne Research Laboratories, The Broken Hill Proprietary Co., Ltd., 245 Wellington Road, Mulgrave, Victoria 31 70, Australia Received April 4, 1988. Revised Manuscript Received August 17, 1988 Diesel fuel properties, a number of which are routinely used to define fuel quality, have been investigated with a view to relating property values to fuel composition in a quantitative manner. Fuel composition is expressed in terms of the abundances of hydrocarbon compound classes (n-alkanes, branched plus cyclic saturates, and aromatics) and is determined by using HPLC and either GC or 13CNMR procedures. A suite of 54 fuels has been included in the study. These derive from petroleum sources (a range of disparate, mainly Australian, oils) and from upgraded synfuel sources (originating from coal hydroliquefaction, coke and char manufacture, and oil shale pyrolysis). Properties considered include pour point, cloud point, elemental hydrogen content, 'H and 13CNMR aromaticities, specific gravity, aniline point, gross and net heats of combustion, diesel index, and cetane index. It has been found that for each of these properties a simple linear function is effective in relating property values to compound class abundances. From these relationships it is possible to translate fuel quality specifications from constraints on fuel properties to constraints on chemical composition. This leads to the identification of a compositional domain that, within the limitations discussed herein, represents the target which must be achieved in any viable refining strategy.
Introduction The primary purpose of the current work has been to better understand the range of chemical compositions that must be achieved in order to produce hydrocarbon diesel fuels which meet current fuel quality specifications. Fuel quality is usually defined in terms of a set of prerequisite responses to standard test procedures.'-* Such specifications are not usually expressed in a chemically explicit fashion. To achieve the desired chemical viewpoint, a range of petroleum and synthetic hydrocarbon fuels has been investigated. Fuel compositions have been determined in terms of the abundances of n-alkanes, branched plus cyclic saturates, and aromatics. Simple linear relationships between standard fuel properties (as listed in Table I) and fuel compositions have been sought and found. It will be shown that these relationships can be used to graphically describe compositional prerequisites for acceptable diesel fuels in a similar manner to that recently reported for jet fuels.5 Experimental Section Sample Origins a n d Characteristics. The 54 samples used in this study are divided into two groups. The f i n t group (group A) consists of 40 diesel fuels of which 36 are derived from the following Australian petroleum sources: Amadeus Basin (1); Bowen Basin (1);Browse Basin (1);Canning Basin (5); Camarvon Basin (3); Cooper Basin (9);Eromanga Basin (8);Gippsland Basin (4);Perth Basin (3);Surat Basin (1). One additional petroleum fuel in group A is derived from Daquing (China) crude oil. The three remaining group A samples are synfuels which were produced by the direct hydroliquefaction of black coal (1)and brown coal (2)followed by hydrotreatment of the resultant synthetic crude oils. Group B consists of 14 samples of which 10 are derived from the following petroleum sources: Amadeus Basin (2); Canning Basin (1); Carnarvon Basin (2); Cooper Basin (3); Gippsland/Minas (Indonesia) crude oil blend; Saudi Arabia export crude. The four remaining group B samples are hydrotreated synfuels derived from anthracene oil (l),brown coal tar (l), and
* Author to whom correspondence should be addressed. 0887-0624/88/2502-0854$01.50~0
Table I. Listing of Diesel Fuel ProDerties Investigated measurement method used oroDertv units svmbol ASTM D97 pour point "C PP ASTM D2500 cloud point "C CP ASTM D3701 hydrogen content wt% %H 'H NMR hydrogen aromaticity atom % H, 13CNMR carbon aromaticity atom % C, ASTM D4052 inverse specific gravity cm3/g SG-' ASTM D611 aniline point "C AP IP12 gross heat of combustion M J / k 8, IP12 net heat of combustion MJ/b 8, DI IP21 diesel index ref 6 CI cetane index shale oil (2)feedstocks. Samples are included in group B for two reasons. Their normal b o i i g range may differ from that for group A samples (230-320 "C). Also, unlike group A samples, GC data may not be available. As shown by Figure lA, compositions of the group A diesel samples vary considerably, with n-alkane contents ranging from ca. 0 to 60%, branched plus cyclic saturates contents from ca. 35 to 90% and aromatics contents from ca. 5 to 30%. The three coal-derived diesel fuels, designated by open circles in Figure lA, have low n-alkane contents ( u2 > u3 (Table 11). Diesel index is calculated from two observables (AP and SG-', eq 6 and 7), which have individually been described above by three-parameter models. Thus diesel index could be calculated using the values of AP and SG-' individually obtained from the coefficients in Table 11. Such an approach appears even more successful (R2 = 0.97, rmse = 2.6) than the direct three-parameter model (R2= 0.91, rmse = 4.1). Cetane Index. Rather than measuring ignition quality of a diesel fuel directly in an engine, refiners often find it convenient to predict cetane number from some other property of the fuel. The calculated cetane index (ASTM D976) is an example of one such indirect calculation, and is based upon API gravity and midboiling point data. In this paper, we have chosen to use a cetane index based upon aniline point? This index, which is claimed to be preferable to ASTM D976 when a wide range of petroleum fuels are to be compared, is determined from the algorithm CI = 16.419 - 1.1332(AP/100) + 12.9676(AP/100)2 0.2050(AP/100)3 + 1.1723(AP/lOO)* (8) where AP is the aniline point in OF. Other workers have also predicted cetane number as a function of 'H NMR relative inten~ities.~lP~ Samples in our data set have cetane indices ranging between 30.2 and 64.5 and therefore represent a significant range in ignition quality. Data in Tables I1 and I11 and Figure 5D show that the three-parameter model satisfactorily relates cetane index to chemical composition. R2 and rmse values are 0.92 and 2.7 respectively. Relative coefficient values are as expected with ul > u2 > up Also the value of ul = 98 (for n-alkanes) is in good agreement with the n-hexadecane standard of 100 for cetane number. Scope and Limitations of Correlations. Although the sample set used to derive property-composition relationships in Tables 11-IV has been chosen to include a diverse range of fuels, the derived models must still be constrained (30)Powell, R. "A Review of Diesel Fuel Ignition Quality, Ita Prediction and Effects"; Technical Memorandum No. 121 479;BP Research Centre: Sunbury-on-Thames,UK, 1979. (31)Gulder, 0.L.;Glavincevski, B. 2nd. Eng. Chem. Prod. Res. Deu. 1986,25,153-156. (32)Gulder,0.L; Glavincevski, B. Combust. Flame 1986,63,231-238.
by the sample set. Thus, application of relationships to diesel fuels with compositions significantly beyond those indicated in Figure 1A or with property values (estimated or measured) well beyond the ranges indicated in Table I1 would incur a higher risk of error. Also it should be noted that all diesel samples used to derive the property-composition relationships have similar boiling ranges (230-320 "C). For some properties, modest deviations from this range will not adversely affect the suitability of the derived relationships. For example, diesel samples from group B(Figure 1B) have boiling ranges which vary within 190-390 "C. Calculated values of % H, SG, AP, Qg,and CI are all in good agreement with observed values for these fuels. Some illustrative data are presented in Table V. Predictions for CP, however, are unacceptably poor. This is as expected in view of the known sensitivity of lowtemperature properties to boiling range.33 It should also be recognized that there are fundamental limitations inherent in choosing to simply differentiate between n-alkanes, branched plus cyclic saturates, and aromatics. As is evident from Table IV, in some cases, simpler two-parameter models are almost as acceptable as three-parameter models. Mostly, however, a three-parameter model is clearly preferable. Subdivision of aromatics into monoaromatics (1)and diaromatics (2) has proven of limited utility, however. The most significant cases are given in Table IV. A broader discussion of such issues has been given elsewhere in relation to jet fuels.5 For simplicity, most of the Discussion section has focused on the regression results presented in Table 11. As noted in the Results section an alternative set of regression data are presented in Table 111. The two sets of results pertain to the same group of samples and show broadly comparable trends. They do differ in detail however in a manner that warrants brief comment. Tables I1 and I11 derive from different compositional information (NMR + HPLC and GC + HPLC, respectively). The GC + HPLC combination gives, on average, distinctively lower n-alkane contents (see ref 7 for details). Table I1 shows that for all properties considered in the present work, other than aromaticities, n-alkanes have a distinctive and extreme influence (viz. the ul coefficients are very different from u2 and u3). It is therefore to be expected that in Table I11 the coefficientvalues derived for ul (n-alkanes)will be even further polarized away from u2 and u3 than those in Table 11. Comparisons with Previous Results for Jet Fuels. As listed in Table VI, six of the properties considered here for diesel fuels have been considered previously for lower boiling jet fuels? Coefficients derived via three-parameter models for diesel and jet fuels (Table VI) though often showing significant differences, are grossly comparable, as would be expected. (33)Gorenkov,A.F.;Lifanova, T. A.; Klyuchko,I. G. Chem. Technol. Fuels Oils (Engl. Transl.) 1986,426-428.
Cookson et al.
860 Energy & Fuels, Vol. 2, No. 6,1988 Table VI. Comparison of Derived Coefficient Values for the Three-Parameter Models for Diesel Fuels and Kerosene Fuels" property fuel typeb a1 a2 a3 %H D 16.02 13.63 9.00 K 16.11 13.91 9.64 H, D -0.8 -1.6 24.5 K -1.0 -0.3 23.0 GI D -1.2 -3.8 74.2 0.0 67.5 K -2.5 D 48.23 45.79 41.56 Q, K 47.91 46.28 42.06 39.61 44.84 42.93 8, D 43.33 40.18 K 44.41 1.143 0.982 SG-l D 1.358 1.402 1.192 1.053 K "Model: property value = al[n] + a2[BC] + as[Ar]. Full details pertaining to kerosene fuels are given in ref 5. b D= diesel fuel (230-320 "C); K = kerosene Get) fuel (190-230 "C).
Differences found for u3 (aromatics) for all six fuel properties imply that diesel aromatic fractions contain less hydrogen, have higher aromaticities, have lower heats of combustion, and have higher specific gravities than jet fuel aromatics. It is difficult to ascertain to what extent such self-consistent trends in coefficients are physically credible. Considering hydrogen content for example, diesel monoaromatics are richer in hydrogenlo than kerosene monoaromatics, but diesel total aromatics contain a higher proportion of low hydrogen content diaromatics than do kerosenes. Thus there are two opposing effects influencing the relative values of a3 for diesel and kerosene fuels. Comparisons with Previous Results for Diesel Fuels. Four of the properties investigated here (SG-',AP, DI, and CI) were also the subject of a preliminary study'* using closely similar methodology. In the earlier study, however, there were fewer samples and they were less stringently selected on the basis of boiling range. Even so, the coefficients derived are similar to those obtained in the present work. A More Explicit Chemical View of the Requirements for Acceptable Quality Diesel Fuel. One of the prime motivations for carrying out this work has been the desire to better understand what is chemically required to produce an acceptable diesel fuel. This interest has arisen particularly in relation to efforts to devise strategies for the production of alternative (non-petroleum) hydrocarbon fuels. A comparison of diesel fuel (ASTM D 975) and jet fuel (ASTM D1655) specifications reveals that for obvious reasons, the set of criteria typically applied to diesel fuels is less extensive than that applied to jet fuels; that the requirements for diesel fuels vary greatly with the wide range in engine size, speed and output power; and that local climatic conditions can determine requirements in terms of low-temperature behavior. In the current discussion we will focus on automotive diesel fuels that are a t the high-quality end of the diesel fuel range, and will direct our concern to combustion quality as indicated by cetane index and low-temperature performance as indicated by cloud point. Since property values have been described in terms of compositions herein (Table 11), it is possible to translate fuel property specifications into fuel compositional constraints. This is illustrated graphically in Figure 7. Fuel
A
6 [nI
[nl
[BCI
C
[n1
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4>[x
SPECIFICATION DOMAIN
[BCI
Figure 7. Compositional requirements (shaded region) for a diesel fuel to meet a specification for (A) cloud point < -4 "C and (B) cetane index > 45. Lines corresponding to CP = 0 "C, CP = 4 "C, and CI = 40 are also illustrated. The shaded region in part C corresponds to the compwitional requirements for a diesel fuel to meet combined specifications for cloud point (45) specifications it should have a composition that falls in the shaded area shown in Figure 7C. Figure 7C is of value because it shows the target compositional domain that must be reached by any viable strategy for the production of an acceptable hydrocarbon diesel fuel (meeting the CP and CI specifications). Thus it forms an at least semiquantitative basis upon which prospects for diesel fuel production can be based. For example, coal hydroliquefaction products tend to have compositions to the bottom right-hand side of Figure 7C. Extreme hydrotreatment (corresponding to a horizontal composition change from right to left) is necessary to reach the specification domain. Shale oils often have a higher initial n-alkane content than coal oils (i.e. higher up in Figure 7C). Thus they are more readily converted to diesel fuels of acceptable cetane quality. Note that issues of fuel stability and heteroatom content have not been addressed in the current study. Thus Figure 7C and Table I1 clarify important aspects of diesel fuel requirementa, though all aspects are not covered here of course. Analogous diagrams have been presented for jet fuels.5
Acknowledgment. We thank personnel of the BHP Petroleum Laboratory for assistance. Support for this work was provided under the National Energy Research Development and Demonstration Program, administered by the Australian Commonwealth Department of Primary Industries and Energy. We thank the Broken Hill Proprietary Co., Ltd., for permission to publish this work.
