270
Energy & Fuels 1990,4, 270-274
Clearly, the molecules identified here and also in previous studies, including carbazoles and porphyrins, properly belong to the resin fraction of the bitumen and their presence in the n-pentane-precipitated asphaltene shows that they are integral parts of the asphaltene micelles in dynamic equilibrium between the solution, smaller micelles not precipitated by n-pentane, and the larger asphaltene micelles which are precipitated by n-pentane. The presence of these highly polar and polarizable molecules in-
fluences the subtle thermodynamic balance in favor of micellar stability and packing and aggregation statistics and thereby contributes to the solubilization of the asphaltene in the maltene fraction of the bitumen. Acknowledgment. We express our gratitude to AOSTRA for financial support. We also thank Dr. D. S. Montgomery and Dr. Theodore Cyr for helpful advice and discussions.
Insoluble Sediment Formation in Middle-Distillate Diesel Fuel: The Role of Soluble Macromolecular Oxidatively Reactive Species D. R. Hardy* Naval Research Laboratory, Code 6180, Washington, D.C. 20375-5000
M. A. Wechter Department of Chemistry, Southeastern Massachusetts University, North Dartmouth, Massachusetts 02747 Received October 27, 1989. Revised Manuscript Received February 5, 1990
A hexane-insoluble fraction of the methanol extract of various diesel fuels and diesel fuel blending stocks has been isolated and quantified. This fraction has been partially characterized qualitatively and has been implicated as a necessary condition for product insoluble sediment formation upon long-term, low-temperature storage of diesel fuels. The use of this hexane-insoluble fraction as a possible means of predicting storage instability of catalytically cracked cycle oil blend stocks is suggested.
Introduction Extraction of unstable middle-distillate diesel fuels and diesel fuel blending stocks with methanol greatly reduces their tendency to form insoluble sediment under conditions of accelerated aging.' When the methanol-extracted portion of a fuel or blending stock is dissolved in a solvent like dodecane/butylbenzene (75:25 % v/v) or into a stable straight run distillate fuel stream and subjected to accelerated aging, insoluble sediment is formed.'-3 Generally the weight of insoluble sediment that is produced equals the difference between the sediment production of the unextracted fuel and that of an equal volume of extracted fuel. For some fuels, however, the methanol extract produces an excess of insoluble sediment. This could be due to a reduction of naturally occurring fuel antioxidant species in the methanol fraction or to an increase in reactive fuel species concentration in the methanol extract. A series of unstable catalytically cracked light cycle oil (LCO) streams from a variety of crude sources and refineries was subjected to methanol extraction. After backextraction with nonpolar solvents the remaining methanol fractions were extensively analyzed by capillary GC/MS. Four LCOs analyzed in detail covered a range of instability (1) Wechter, M. A.; Hardy, D. R. Fuel Sci. Technol. Int. 1989, 7(4), 423-441. (2) Wechter, M. A.; Hardy, D. R. Energy Fuels 1989, 3, 461-464. (3) Hardy, D. R.; Wechter, M. A. Fuel, in press.
0887-0624 /90/2504-0270$02.50 / O
from moderately unstable to very unstable as measured by sediment formation. Qualitatively the methanol extracts contained much polyaromatic material and much heteroatomic material, particularly indoles and other nitrogen-containing compounds. Surprisingly no sulfur compounds were detected in any of the extracts. The components of the methanol extracts were also semiquantitatively determined by gas chromatography. No particular compound or compound class could be related to the clearly different stabilities of the whole LCOs and their related methanol extracts. The polyaromatic material included several neutral compound types such as phenalenes, fluorenes, phenalenones, and their homologues of higher carbon number. These latter were specifically included in a quantitative analysis since they have been clearly shown to be implicated in storage instability in the whole-fuel catalytically cracked streams and blends in earlier w ~ r k . ~ - ~ For the LCO stocks analyzed in the above work, however, it was not possible to establish any relationship between amount or type of any particular compound present and tendency to form insoluble sediment. A detailed in(4) Bahn, 0. K.; Brinkman, D. W.; Green, J. B.; Carley, W. Fuel 1987, 66, 1200. (5) Pedley, J. F.; Hiley, R. W.; Hancock, R. A. Fuel 1987, 66, 1646. (6) Hiley, R. W.; Pedley, J. F. Fuel 1988, 67, 469. (7) Pedley, J. F.; Hiley, R. W.; Hancock, R. A. Fuel 1988, 67, 1124. (8)Pedley, J. F.; Hiley, R. W.; Hancock, R. A. Fuel 1989, 68, 27.
0 1990 American
Chemical Societv
Sediment Formation in Diesel Fuel Table I. Fuel Identification Code and Storage Stability Data Using ASTM D4625" insoluble after 43 "C for 18 weeks, fuel identification code mg/100 mL LCO-1 6.0 LCO-2 5.5 SR-3 0.4 B-3 8.9 LCO-3 10.6 SR-4 0.1 LCO-4 2.5 SR-5A 0.3 SR-5B 0.1 LCO-5 11.3 SR-11 0.0 SR-12 ND LCO-12 ND SR-13 ND LCO-13 ND LCO-14 ND "LCO = catalytically cracked light cycle oil; SR = straight run distillate; B = blend of SR/LCO (usually 70% SR v/v); 1-14 = refinery code. For the stability test used a value of 4 mg/100 mL is considered unstable.'O ND = not determined.
vestigation of acidic species of these LCO stocks also failed to establish any relationship between this type of species and tendency to form insoluble sediment. Further attempts to concentrate the methanol fraction led to the isolation of a dark, solid hexane-insoluble material. Initially this material was designated methanol-extractable/hexane-insoluble (MEHI) solids. The detailed isolation of this solid material is the subject of this paper. Additionally the importance of this material as a prerequisite to insoluble sediment formation in middle-distillate diesel fuels and blends is established. Attempts to characterize the MEHI solids are described. Finally some speculation is offered as to the possible origin, detailed nature, and mode of action of this material both in cracked stock streams and blended into straight run distillate diesel fuel stocks.
Experimental Section Sixteen middle-distillate diesel fuel refinery streams were selected for this study. These included straight run distillate streams and catalytically cracked light cycle oil streams, both hydrotreated and nonhydrotreated. Samples represent a diversity of geographical locations in the U S . and Europe as well as the Southern Hemisphere. Table I is a coded list of fuels. The code used is self-consistent with that of two previous papers in this series.'V2 All 16 fuel streams fell within ASTM D975 diesel fuel no. 2 specification guidelines. Extraction and Precipitation Procedure. Fuel samples were prefiltered by use of Gelman type A/E glass fiber filters. Fuel/methanol extraction volumes were 1 C 1 mL/40 mL. Extractions were made in 250-mL separatory funnels; the two phases were shaken vigorously together for about 90 s and the system was allowed to settle until good separation had occurred, typically 2-5 min. The fuel layer was then drawn off and discarded, and the methanol phase was decanted into a 125-mL brown borosilicate bottle which was then immersed to the neck in a boiling water bath to evaporate the methanol. The methanol phase, after extraction, contains polar and aromatic fuel components which are soluble in methanol and in other polar solvents like tetrahydrofuran (THF) and methylene chlorid ?. In order to achieve good repeatable quantitation, it is absolutely essential that all the methanol is driven off because of the very high solubility of the hexane-insoluble fraction of interest in methanol. After evaporation the sample was allowed to cool t.0 room temperature. Several alkane solvent systems were used to precipitate the hexane-insoluble material. They included single alkanes from hexane to dodecane and mixtures of hexane and other, heavier alkanes. There was no difference in the results obtained either
Energy & Fuels, Vol. 4, No. 3, 1990 271 Table 11. Weight of Solids Isolated from the Hexane-InsolubleFraction of the Methanol Extract of Seven Straight Run Distillate Diesel Fuel Stocks" hexane insolubles, insoluble mg/100 mL sediment, fuel code prestress poststress mg/100 mL SR-3 0.0 ND 0.0 SR-4 0.1 0.1 0.0 SR-5A 0.7 2.0 0.6 SR-5B 0.0 0.6 0.4 SR-11 0.0 0.1 0.1 SR-12 0.2 ND 0.0 SR-13 0.1 0.1 0.3 Storage stress test was for 24 h at 90 "C and 690 kPa of oxygen. Pass criterion for this test is 6 mg/100 mL insoluble sediment. quantitatively or qualitatively, and thus hexane was selected as the solvent of choice. After the methanol was evaporated'and the sample had reached room temperature, approximately 50 mL of filtered Fisher ACS reagent grade hexanes were added to the borosilicate bottle and any hexane-insoluble material was precipitated. The sample was then vacuum filtered through a preweighed double thickness of 47-mm Gelman type A/E glass fiber filters in a Buchner funnel, rinsed well with hexane, and allowed to dry thoroughly before weighing again. Oven drying a t 60-80 "C for an hour is sufficient for most samples to reach constant weight. This oven-drying step may be omitted if constant weights can be achieved with ambient air drying. In cases where it was desired to recover solid material from the filter for additional study, a 0.8-rm Millipore type AA filter was substituted for the glass fiber filter and a Millipore filtering apparatus for the funnel. Aging and Characterization Studies. For most of the fuels used in the survey, the MEHI solids were determined before and after accelerated aging. Samples that were to be aged were prefiltered, transferred to 125-mL brown borosilicate bottles, and covered with perforated aluminum foil. They were then inserted into the low-pressure react09 (LPR) and stressed at 90 "C with an oxygen overpressure of 690 kPa for 24 h. These conditions simulate 2-3 years of ambient storage.l0 Some samples were stressed for longer periods. In those cases, multiple samples would be inserted into the LPR to be withdrawn at predetermined intervals. After the stress period, samples were withdrawn from the reactor and cooled. They were then filtered, and the insoluble sediment formed during accelerated aging was determined following the method described elsewhere." The filtered fuel was then extracted with methanol and the hexane insolubles were gravimetrically determined as described above. In general, single determinations were made. The standard error for both of these techniques was about &15%. The MEHI solids recovered from aged as well as unaged samples were analyzed by size-exclusion chromatography using a Beckman-Altex Microspherogel column, Model 255-80 (50-A pore size, 30 cm X 8.0 mm id.). Fisher HPLC grade uninhibited tetrahydrofuran (THF) was used as the mobile phase, and the recorder was a Varian Model 9176 strip chart. Samples were injected into a Rheodyne Model 7125 loop/valve injector. A Beckman Model 100-A HPLC pump was used for solvent delivery and a Waters Model 401 differential refractometer for detection. Weighed quantities of the samples to be analyzed were dissolved in T H F for injection into the system. Analysis of the MEHI solid material by GC was attempted by using a Hewlett-Packard Model 5890 GC equipped with a flame ionization detector and an HP Model 3392 integrator. Separation was achieved by using a 50-m methylated silicone (nonpolar) capillary column. Inlet temperature was 280 "C, and a split ratio of 22:l was used on samples which were dissolved in methanol. Column temperature was programmed with a 1-min initial hold (9) Hardy, D. R.; Hazlett, R. N.; Beal, E. J.; Burnett, J. C. Energy Fuels 1989, 3, 20-24. (10) Hardy, D. R.; Hazlett, R. N.; Gianinni, R.; Strucko, R. SAE Technical Paper Series; SAE: New York, 1986; No. 860895. (11)Hazlett, R. N.; Cooney, J. V.; Beal, E. J. DOE/BC/10526-16; Naval Research Laboratory: Washington, DC, 1987.
272 Energy & Fuels, Vol. 4, No. 3, 1990 Table 111. Weight of Solids Isolated from the Hexane-Insoluble Fraction of the Methanol Extract of Eight Cat-Cracked Light Cycle Oils and One Blended Diesel Fuela hexane insolubles, me/100 mL mg/100 mL prestress poststress fuel code 116 27.0 LCO-1 112 14 17 3.3 LCO-2 53 63 13.0 LCO-3 LCO-4 27 3.4 40 20.0 92 114 LCO-5 4.7 1 14 LCO-12 LCO-13 33.0 2 101 4.0 LCO-14 2 58 14 18 8.2 B-3 Storage stress test was for 24 h at 90 "C and 690 kPa of oxygen. Pass criterion for this test is 6 mg/100 mL insoluble sediment. a t 100 "C and a 5 OC/minute ramp to a final temperature of 280 "C with a hold of 10 min.
Results and Discussion Extraction and Aging. Table I1 summarizes the results obtained for a series of seven representative straight run distillate fuels obtained from refineries in the U.S., Europe, and Australia. Included are insoluble sediment data from the 24-h LPR stress test and the MEHI solid yields from both the pre- and poststressed filtered fuels. As expected, the fuels were very stable toward insoluble sediment formation and the MEHI solid yields were comparably low. It is important to note that the MEHI solids should not be confused with the product insoluble sediment solids. The product insoluble sediment is removed from the fuel by filtration (in order to determine its product weight), and then the poststressed filtered fuel is extracted to determine the MEHI solids after aging. The MEHI solids are ultimately filtered, dried, and weighed. The determination of MEHI solids has been made both before and after aging of the fuels. The weight of isolated MEHI solids does not tend to increase with time, indicating that this material is not undergoing further oxidation/degradation after isolation from fuel. Initially, attempts were made to take the methanol extract to dryness and weigh the resulting solid/gum material. This was found to be experimentally impossible due to irreproducible weights from identical sample aliquots. The situation is analogous to determining the weight of solid/gum material from whole fuel by steam-jet evaporation/drying. It is also similar to attempting to weigh cold pentane-precipitated solids/gums from whole fuel. Precipitating the solids with hexane from the methanol extract, as described in this paper, allows the very precise quantitation of the resulting MEHI solids. Steam-jet gums of most of the whole fuels in Tables I1 and 111were determined and typical diesel fuel values were obtained. One would expect the MEHI solids to be included in the weights of steam-jet gums and of cold pentane-precipitated gums. In every case the weight of steam-jet gum was much greater than the weight of MEHI solids (determined from a separate aliquot). Table I11 is a summary of results using the same set of experimental procedures on a series of seven catalytically cracked light cycle oil stocks (LCO)and one blended stock. As can be seen, these fuels are much less stable toward insoluble sediment formation and the MEHI solid yields are considerably higher also. For the storage stability test used in this work a value of about 6 mg of insoluble sediment per 100 mL of fuel has been suggested as the
Hardy and Wechter
pass/fail criterion for certain military fuels with expected shelf lives of about 3 years of ambient storage.l0 Differences in the MEHI solid levels before and after accelerated aging for the straight run distillate fuels in Table I1 are in general very small. These low MEHI weight levels and the comparably low insoluble sediment levels are both indicative of stable fuels. Thus, for these fuels the low levels of the prestressed or aged MEHI solids indicate that the fuels should pass any given stability test. For the LCO stocks in Table I11 the situation is somewhat more complex. The first five LCO samples (LCO 1-5) and the blended fuel, B-3, exhibit modest poststress increases in MEHI solid yields. These stocks have not been hydrotreated and have been in ambient storage in epoxy-lined cans for greater than 6 months at about 20 "C. The remaining LCOs reported in Table I11 (LCO 12-14) are very interesting in that there is a substantial increase between the pre- and poststress MEHI solid levels. Two of these samples, LCO-12 and LCO-14, are from recent refinery production (less than 6 months) and the extraction data were obtained within weeks of production. The LCO-13 stock was from production in excess of 2 years ago; however, it had been stored a t -20 "C in epoxy-lined containers under a nitrogen blanket within 24 h of production until the time that the results in Table I11 were obtained. Thus, it would appear that when LCOs 12-14 were aged at 90 "C in an oxidizing environment (690-kPa oxygen overpressure), the MEHI solids themselves tend to oxidize to a more polar moiety, while still in solution in fuel. LCO-12 is known to be refined from a relatively good crude source (highly paraffinic and sweet). This is confirmed by its relatively low insoluble sediment level after stressing and its fairly low poststressed MEHI-solid level. The explanation for the very low prestressed MEHI solids (and hence the very great percentage increase in the poststressed analysis) is that the time between production and analysis was by far the shortest for this sample. LCO-13 is known to be refined from a relatively heavy aromatic and sour crude source and this is confirmed by its very high level of insoluble sediment and the high levels of poststressed MEHI solids. The explanation for the very great percentage increase in post- over prestressed MEHI solids in this case is that the sample was so very carefully protected from oxidation in the 2 years between production and analysis. LCO-14 makes a very interesting case. It was received in the laboratory for analysis about 1 month after production. At that time it passed the 24-h LPR test at a level of 2.4 mg/100 mL insoluble sediment. Three months later the LPR stability test was repeated and the insoluble sediment level had risen to 4.0 mg/100 mL. At that time the prestressed MEHI-solid analysis yield 1.6 mg/100 mL and the poststressed MEHI-solid analysis yielded 58 mg/100 mL. After an additional 1 month LCO-14 was retested by using the LPR stability test and this time failed the test a t a level of 7.2 mg/100 mL insoluble sediment. The prestressed MEHI solids had risen to 3.5 and the poststressed MEHI solids had risen to 77 mg/100 mL. Unlike LCO-12 and LCO-13, this LCO has been hydrotreated to some degree in order to enhance its storage stability. This, then, could account for the increase in MEHI solids in the poststressed cases in addition to explaining the general increase with time of the prestressed cases after 3 and 4 months of ambient storage. In general, any type of hydrotreatment would have the effect of chemical reduction. Polar moieties are reduced to less polar moieties. The existent material defined as methanol
Sediment Formation in Diesel Fuel
Energy & Fuels, Vol. 4 , No. 3, 1990 273
24
4 2t0/0
10
I
30
I
I
I
I
70 mg SMORSI100 ml
50
1
90
1
I
110
Figure 1. Correlation of the weights of prestressed extractable insolubles (SMORS) of five LCO samples with the corresponding weights of product filterable insolubles. Standard error for both sets of weight measurements is about i=15%.
soluble, hexane insoluble would also be chemically reduced. This would render these fuel-soluble components less soluble in methanol and also less polar in nature and hence not as likely to participate in reactions leading to insoluble sediment during oxidative aging. This chemical reduction is reversible, and especially so under the strict oxidative environment of the LPR test. The effect would be to oxidize the chemically reduced methanol-extractable material to a more polar form (probably carboxylic acidic in nature) which would increase the solubility in methanol and at the same time greatly increase the participation of this material in reactions leading to insoluble sediment during aging. Additional aging studies were undertaken to determine the LPR stress time conditions necessary for the poststressed MEHI solids to reach an equilibrium concentration. This is just the time at which the concentration of MEHI solids does not change with increase in stress time. For LCO-14 this occurred in about 48 h under the LPR conditions above. This would be equivalent to about 4-6 years at ambient storage, or well above any normal commercial or noncommercial storage times. This explains the discrepancy in Table I11 between the relatively high MEHI levels and relatively low insoluble sediment. For the case of the five LCO samples (LCO 1-5) which have essentially reached this case of equilibrium concentration of MEHI solids even before the stress test begins (as evidenced by the similarities between pre- and poststressed extractable insolubles), a plot of prestressed MEHI solids vs resulting insoluble sediment formed after aging is shown in Figure 1. The R2 value for the linear least-squares fit is 1.0, For the poststressed MEHI solids vs resulting insoluble sediment fit the R2 value is 0.9. The MEHI solids yield for the blended fuel, B-3, was 29% of the yield for LCO-3 which is consistent with the percent composition of the blended sample (3070 LCO-3:SR-3 % v/v). Three of the LCOs and the blended fuel would clearly fail the stability test criterion stated above for the 24-h LPR test (>6 mg/100 mL is unacceptable). The remaining LCOs would pass the LPR stability test. Therefore, the ability of the MEHI solid yields to “predict” relative storage stability results for this set of LCOs is possible. Characterization. Size-exclusion chromatograms were obtained for a representative sampling of the MEHI solids both before and after stress tests. The chromatograms of the MEHI solids from the LCOs in Table I11 were characterized by a molecular weight peak of about 700-900
daltons (Da). Qualitatively and quantitatively (for LCO 1-5) the chromatogram was similar for all LCOs both before and after aging. In addition, the physical appearance and solubility characteristics of the MEHI solids was similar for all of the LCOs. In addition to this relatively high molecular weight peak, lower molecular weight peaks were generally obtained that varied from fuel to fuel, probably reflecting compositional differences in the LCO streams. These lower molecular weight peaks were generally in the range of fuel monomolecular species (about 150-350 Da). No relationship was found between the amount of MEHI solids and amounts of entrapped lower molecular weight fuel species. It might be expected, especially in the blends of LCO and straight run distillate streams, that higher concentrations of the lower molecular weight polar species isolated as part of the MEHI solids should exert an influence over the amount of product insoluble sediment formed by the whole-fuel blends. One of the straight run distillate streams in Table I1 (SR-5A) did yield 2 mg/100 mL MEHI solids after the aging test. This value compares with the prestress results of LCO 12-14 and so a size-exclusion analysis was performed on this material. In this case, however, no higher molecular weight peak was observed. Two of the MEHI solid samples (one from a sample before aging and one from a poststressed sample) were dissolved in methanol (approximately 10% w/v) and analyzed by capillary gas chromatography using standard conditions of diesel fuel analysis. Remarkably no monomolecular fuel peaks were present in either sample. This would verify the fact that these materials are not polar fuel monomolecular components, but a new “class” or type of material which is apparently of higher molecular weight than the fuel component range noted above. Given the high molecular weight and the polar nature of this material, it would not be expected to vaporize at the 280 OC injector temperature and thus analysis by GC is not possible. At the GC temperatures employed (up to 280 “C) any fuel polars elute relatively quickly from the nonpolar column employed.
Conclusions The isolation of a macromolecular phase that is soluble in middle-distillate diesel fuel but insoluble in hexane has not been reported in the literature previously. This material is apparently important as a reactant or precursor to insoluble sediment formation in aged LCO-produced blending streams and in finished diesel fuels containing LCO as a blending stock. We have reached this conclusion in two ways. First, the detailed analysis of the methanol extract of a number of representative LCOs of widely varying insoluble sediment forming tendencies failed to link any monomeric fuel component or class of components to the product yield of insoluble sediment under standard aging conditions even though in all cases the methanol extraction greatly improved the stabilities of the respective LCOs. Second, the quantitative determination of the MEHI solids in these same LCOs has correlated well with the ultimate yield of insoluble sediment as evidenced by data in Figure 1. The nature of this MEHI material, (1)its solubility in diesel fuels, (2) its molecular weight range which is well above the monomeric molecular weight range of the fuel, and (3) its involvement as an important reactant in oxidation leading to product fuel-insoluble sediment, leads us to propose a name for this material-soluble macromolecular oxidatively reactive species (SMORS). This also aptly describes the observed chemical reactivity of this
274 Energy & Fuels, Vol. 4, No. 3, 1990
material. The SMORS are not only involved in production of fuel-insoluble sediment but also react with acids or bases, and can be reduced by hydrotreatment to a more unreactive state. From this reduced state they can subsequently be oxidized to a more reactive state. The SMORS are apparently a prerequisite and not just a precursor for any insoluble sediment formation in the diesel fuel boiling range and hence their presence in finished fuels is a necessary but not sufficient condition for predicting fuel oxidative instability as defined by any particular stress test. If the concentration of the SMORS in the fuel is not sufficiently high, then oxidative reactions leading to fuel-insoluble sediment will not take place. In addition, if the reactive moiety of the SMORS has been chemically reduced as a function of either refinery processing or crude source, these sediment-forming reactions will not proceed. The other necessary but not sufficient conditions necessary for insoluble sediment to form include acidic fuel species and other fuel polar species (including bases). These latter two conditions are insufficient by themselves (alone or together) without a minimum concentration of SMORS. On the other hand, in most diesel fuels the polar species and the acidic fuel species inherently present are in very great excess and hence can generally be thought of as nonlimiting reagents in the reaction pathways leading to fuel-insoluble-sediment products. Actually, acidity of most middle-distillate fuels increases very rapidly in accelerated stability tests (and in real ambient storage also) and in this regard may be one important variable in predicting relative fuel-insoluble-sediment formation from fuel to fuel.3 In the case of the generally very reactive, oxidatively unstable catalytically cracked cycle oils, acidity is very low at the time of production. However, depending on crude source and feed processing, SMORS levels can range from quite low to quite high. Thus, one of the necessary conditions for sediment formation is present at some concentration. In the case of the generally very unreactive, oxidatively stable straight run distillate refinery streams, acidity ranges from moderate to very high levels at the time of production. Here, however, SMORS levels are generally very low to absent, owing to the nature of the distillate production, and thus a necessary condition for sediment formation is missing. In the case where these two streams are blended into a finished diesel fuel, the SMORS concentration will be greatly decreased, but the initial acidity levels will be greatly increased leading to a requirement to carefully measure or control both parameters to ultimately control oxidative instability leading to insoluble sediment production. The fact that the SMORS themselves are quite chemically reactive to oxidation and reduction makes their determination both before and after a standard accelerated aging test necessary in order to assess their relative importance in predicting oxidative instability. The fact that
Hardy and Wechter
SMORS concentration can substantially increase in the course of aging tests or under ambient conditions is probably a very important factor governing the so-called induction times observed in many diesel fuels.
Conjectures on SMORS Involvement in Sediment Formation The existence of SMORS in finished fuel could be a result of the catalytic cracking process of the petroleum crude. Very low amounts of SMORS soluble in the fuel would contain some unsaturated moieties quite subject to oxidation. The alternate more classical fuel chemistry explanation for the presence of SMORS is that they are polymeric products of reactive fuel monomeric species. This explanation is greatly weakened by the inability to quantitatively link any possible SMORS precursors in the fuel to the product insolubles in the fuel. Regardless of their genesis in fuel, it is possible to conjecture on their possible involvement in mechanistic pathways leading to fuel-insoluble sediment. Oxygen incorporation into the fuel-insoluble sediment is probably initially through covalent bonding in the formation of acidic moieties on the SMORS and oxidation of monomolecular fuel species to acids. These acidic SMORS and basic monomolecular fuel components probably form ionic “salts” which are themselves subject to further oxidation and covalent incorporation of oxygen. Finally, both acidic and basic SMORS can react ionically to precipitate in a few steps the final product fuel-insoluble sediment. This last conclusion is supported by actual pyrolysis-field ionization mass spectral analysis of the solid insoluble sediment where moieties in the molecular weight range of the SMORS are readily evident.12 Thus the isolation and rigorous determination of SMORS should be useful in formulating mechanisms for fuel-insoluble-sediment formation based on standard hydrocarbon oxidation chemistry. Further detailed characterization of the SMORS will be described in a later paper in this series. In addition the scheme postulated above which is based on the work of LCOs only will be extended to more realistic diesel fuel blends in an effort to develop a completely general and yet simple and useful correlation based on fuel compositional parameters. This correlation should prove useful to fuel producers and users alike in selectively controlling and realistically predicting oxidative stability of diesel fuels. Acknowledgment. We gratefully acknowledge the funding support for this work from the David Taylor Research Center and the Office of the Chief of Naval Research, Navy Energy Research and Development Office. (12) Malhotra, R.; St. John, G. A. In Proceedings of the Third International Conference on Stability and Handling of Liquid Fuels; Institute of Petroleum: London, England, 1988; pp 525-537.
