storage stability of marine diesel fuels

Storage stability of marine diesel fuels

Opinder K. Bhan, Dennis W. Brinkman, John B. Green and Bill Carley* National Institute for Petroleum Research, PO Box 2128, Bartlesville, OK 74005, USA *Defense Fuel Supply Center, Alexandria, VA 22304, USA (Received 5 September 1986; revised 28 January 7987)

The cause of instability in marine diesel fuels was investigated by ageing several fuels at 65°C for various periods. Aged and unaged fuel samples were chromatographically separated into acid, base and neutral fractions, and the fractions were analysed in detail to obtain a clearer understanding of the mechanism of colour change and sediment formation. The results suggest that oxidation of neutral compounds to polar intermediates may be a major pathway for sediment formation and darkening of marine diesel fuels. Considerable loss of polar compounds, both those originally present and those newly formed, to sediment was also found. Within a compound class, the more aromatic higher molecular weight members were observed to be the most active in sediment formation.

(Keywords: diesel fuels; storage stability: ageing)

The problem of fuel degradation is becoming increasingly widespread with the introduction of lowerquality crudes into refineries and the increasing use of catalytic cracking to convert high molecular weight materials into distillate fuels. Various physical and chemical changes are known to occur during degradation. These may include colour development or darkening, sediment formation, increase in viscosity and changes in chemical composition. It is generally recognized that the heteroatomic compounds (N, S and 0) present in liquid fuel strongly influence fuel stability’ -4; however, it is unclear which of these compound types actually found in fuel are the most deleterious.

Much work has been reported on production of sediments in fuels under laboratory conditions, mostly by doping with pure nitorgen compounds, both basic and non-basic, but especially 2,5-dimethylpyrrole (DMP)4e9. The sediments formed from artificial stimulation of fuels by model compounds have been observed to be high in nitrogen content. Frankenfeld et a1.4 reported up to 12 wt % nitrogen in sediment from DMP-doped No. 2 diesel fuel. Loeffler and Li9 reported 7.9 wt % nitrogen in sediment from DMP doping of a jet fuel. Past studies have indicated that pyrrolic compounds are more detrimental to fuel stability than pyridinic or quinolinic compounds*,’ OJ I.

Although nitrogen compounds have received the most attention, the role that sulphur and oxygen compounds play in fuel degradation is not trivial. Several studies of their effect on fuel stability have been reported4,6*12 - 14, most of which again have been conducted by spiking fuels with pure compounds.

An alternative approach has been adopted in this study. A suite of marine diesel fuels, both ‘stable’ and ‘unstable’, was aged and the fates of various compound classes were monitored to arrive at a possible mechanism of degradation. Work with real fuels is a more difficult approach, especially from an analytical standpoint, but is more likely to result in identification of indigenous compounds in fuels which actually cause stability problems.

001a-2361/87/0912o(r15$3.00 0 1987 Butterworth & Co. (Publishers) Ltd.

1200 FUEL, 1987, Vol 66, September

EXPERIMENTAL

Test jhels A set of seemingly ‘stable’ and ‘unstable’ marine diesel

fuels was procured from various military depots and several commercial refineries. All were originally refined to meet military specifications for distillate fuels. Table I lists their pertinent physical and chemical properties.

Fuel colour, copper corrosion index and acid neutralization number were determined by ASTM test procedures D1500, D130 and D644 respectively. As described in method D1500, fuels with colours intermediate between two consecutive colour standards were reported as the darker (higher number) standard preceded by the letter L. Elemental analysis for N, S and 0 was performed by combustion or pyrolysis and determination of NO’ 5, SO, (LECO apparatus) and CO (Unterzaucher) respectively. Metal contents were determined by direct aspiration into an argon inductively coupled plasma emission spectrometer. Bacteriological screening for aerobic and anaerobic microorganisms in fuels was performed by inoculating petri dishes containing Tryplicase Soybroth nutrient and incubating at 37°C for 24 and 48 h with and without air present. The presence of cell colonies was determined by examination under a microscope.

Ageing und sediment determination A newly developed ageing test technique was used for

measuring sediment formation16. This method is a modification of that discussed by Brinkman et al.“. Initially the test fuels were filtered through 1.2pm nylon filters before storage in borosilicate glass scintillation vials of 20 ml capacity, fitted with Teflon-lined screw caps. These caps prevented fuel leakage during storage but, because of permeation, incomplete sealing, or ‘both, probably did not prevent passage of atmospheric gases (e.g. 0,) into or out of the sample container during ageing. Before use, the vials were washed with detergent and water, rinsed with deionized water, acetone and hexane in turn and dried at 110°C. Weighed nylon

Storage stability of marine diesel fuels: 0. K. Bahn et ai.

Table 1 Physical and chemical properties of fuels tested -

A B

Colour (as received) Copper strip corrosion index Aerobic activity Anaerobic activity Neutralization no. (mg KOH g-r) Nitrogen (ppm) Sulphur (ppm)

Oxygen (ppm) Metals (ppm)

cu Fe Pb

L4.5” 4.5 la la

Neg. Neg. Neg. Neg.

0.26 0.26 660 440

3000 3100 3400 NA

C D

0.5 L1.0 la la

Neg. NA Neg. NA

0.06 0.05 280 260

1200 5100 2100 3900

E F ~-

3.5 L5.0 NA NA NA NA NA NA

0.10 0.25 126 550

2080 3220 2400 1900

0.5 0.2 0.07 0.3 0.2 0.2 4.1 2.8 1.1 3.5 0.7 1.0 0.6 0.8 io.5 1.1 0.7 0.4

” L indicates a colour lower than specified (see text) NA= not analysed

membrane filters of 1.2pm pore size were placed at the bottom of tared storage vials for sediment collection. These filters were selected for their excellent sediment retention characteristics.

A 10 ml sample of the test fuel or fuel blend was poured into each vial, which was then capped and stored in the dark in an oven at 65+0.5”C. The fuels typically were saturated with air before ageing, and the vial headspace was filled with air. Several vials were prepared for each fuel; this allowed sediment determination for each period without disturbing the remaining vials. After storage for a specified period (3, 6, 8, 12 or 16 weeks) two vials of each test sample were removed and cooled to room temperature in the dark. The filter was removed and placed on a weighed cellulose acetate membrane filter of 1.2 pm pore size in a vacuum filtering apparatus. The test filter was washed free of the fuel with reagent-grade n- heptane and dried in a vacuum oven for 1 h at 40°C. The filters were allowed to cool to room temperature under vacuum and weighed to determine the amount of insoluble sediment. The fuel in the vial was filtered through a glass libre filter and the vial washed with reagent-grade isooctane. The washing was filtered through the same filter and discarded. The filters and the vials were dried in an oven at 110°C for 1 h, allowed to cool and then weighed. Thus three weights were obtained: precipitated sediment, suspended filterable sediment and adherent sediment; their sum gave the total sediment weight.

In addition, a slightly modified technique was developed for ageing larger quantities of fuel for sediment production. In this technique, 200 ml square borosilicate glass bottles with screw caps were filled with nominally 150ml of filtered fuel. The bottles were washed and prepared in a manner similar to the above before introduction of fuel, ageing and determination of sediment. However, nylon filters were not placed in the bottom of the 200ml bottles for determination of precipitated sediment. Rather, after ageing, the fuel was filtered through a cellulose acetate membrane of 1.2pm pore size, the bottle was then rinsed three times with heptane, and the rinses containing precipitated sediment were passed through the same filter as used for the fuel. Adherent sediment was determined by drying the bottles under vacuum at 50°C cooling and weighing; precipitated plus suspended sediment was determined by

Fuel

drying the filter under vacuum at 40°C for 1 h, cooling and weighing.

Chromatographic separation, analysis and reblending

Figure I summarizes the overall scheme of ageing, chromatographic separation, reblending and subfractionation into compound classes of the various fuel samples. As shown, aliquots of unaged and aged (12 weeks) whole fuel were initially separated into acidic, basic and neutral (ABN) concentrates using nonaqueous ion exchange liquid chromatography’*. In this procedure, strongly basic anion and strongly acidic cation resins were used to trap acids and bases respectively, while neutrals were eluted through both resin columns. Acid and base concentrates were subsequently recovered by Soxhlet extraction of the respective resins. Initially, whole acid and base concentrates from both fresh and aged fuels were analysed directly by g.c.-m.s. However, because of their highly complex and variable composition, further separation of both acids and bases was necessary even for satisfactory qualitative analysis. Thus acids were subfractionated as shown in scheme 1 using normal-phase high-performance liquid chromatography (h.p.1.c.) on a silica column and with a mobile phase containing a strong base, tetramethylammonium hydroxide (TMAH)19. TMAH imparts a basic character to the silica which causes acidic compounds to elute according to their inherent acidity. Five subfractions containing the major acidic classes typically present in fuels were thus obtained. An analogous h.p.1.c. approach was used to subfractionate base concentrates into compounds classes, as shown in scheme 2. This method also utilized silica columns, but a mobile phase containing propanoic acid instead of TMAH was used to promote elution of bases according to their inherent basicity 20-23. Again, five base subfractions were obtained. The greatest benefit from subfractionation of bases was separation of aromatic amines from azaarenes. These two classes are very difficult to differentiate by their mass spectra alone24, yet they were expected to exhibit different rates of degradation during ageing.

To define further the roles of acidic and basic compounds in fuel darkening and sediment formation, duplicate acid + neutral, base + neutral and neat neutral

FUEL, 1987, Vol 66, September 1201

Storage stability of marine diesel fuels: 0. K. Bahn et al.

Figure 1 Fuel ageing, reblending and separation sequence. Numbers above boxes denote sample numbers

concentrates were aged for 12 weeks at 65°C in 20ml as described earlier for whole fuels. Furthermore, as scintillation vials. The blends of each fuel were prepared indicated in Figure 1, aged blends were chromatograph- to match the original proportions of each type in the ically separated into ABN concentrates, and selected whole unaged fuel as closely as possible. Fuel colour and acid or base concentrates were subfractionated using the sediment formed during ageing of blends were determined appropriate h.p.1.c. method as described above.



Table 2 Final colour and sediment formed in fuel samples during practice it was often a major consideration. In selected accelerated storage at 65°C

Final colour/total sediment (mg/lOOml)

Fuel

cases, chemical derivatization methods were used in conjunction with g.c.-m.s. to improve the analysis of selected compounds types. Thus fractions containing

As received 6 weeks 8 weeks 12 weeks 16 weeks predominantly hydroxyaromatics (ArOHj were analvsed I

-.

A L4.5/NA 5.511.3 NA/NA 6.0/5.0 NA/NA gfter acylation with trifluoroacetyl chlo;ide2s,26. Also,

B 4.5,‘NA 5.0jl.8 NA/NA 6.013.1 NA/NA fractions containing carboxylic acids were methylated

c 0.5/NA 1.5io.o NA/NA 2.5jO.O NA/‘NA using N,N-dimethylformamide dimethylacetal*’ or D Ll.O/NA 4.5i3.5 NA/NA 5.515.8 NA,‘NA silylated using N,O-bis(trimethylsilyl)trifluoroacetamide E F

3.517.6 N&‘NA 6.0/l 5.0 6.0116.7 6.0/18.4 with 1 y0 trimethylchlorosilane catalyst2s.26. L5.0/1.5 NA/NA 5.513.5 6.014.7 6.516.1

“Standard deviation for total sediment over all determinations was *0.5mg/lOQml NA = not analysed

Finally, an aliquot of the neutral concentrate from unaged fuel D was allowed to stand under artificial room- light and at ambient temperature for 12 weeks. Then it was chromatographically separated as shown in scheme 3 in Figure 1, and its composition was compared with that of another aliquot which had been aged at 65°C in the dark as described above.

The entire sequence of ageing, chromatographic separation, reblending, blend ageing and reseparation of aged blends, as shown in Figure I, was carried out with only three fuels: A, D, and E. As noted above, only the unaged neutrals from fuel D were separated as shown in scheme 3. However, all six unaged and aged fuels were separated into ABN concentrates, and ABN concentrates from all unaged fuels were blended and aged as described above. In addition, all aged blends from all fuels were subsequently reseparated into ABN concentrates.

Whole acid and base concentrates from unaged and aged fuels were initially analysed by infrared spectroscopy and non-aqueous titration to determine the predominant compound classes present. Details of the methods appear elsewhere23. Selected whole acid or base concentrates and subfractions thereof were analysed by g.c.-m.s. using a Kratos system comprising a Carlo Erba model 4662 temperature-programmed g.c., a Scientific Glass Engineering open-split interface, an EI source, an MS-80 magnetic-scan mass spectrometer and a Data General Nova 4-based DS-55 data system. A 0.25 mm x 30m, 0.25 ,um film thickness J and W DB-5 column was programmed from 100°C (initial time 2 min) to 270°C at 3°C min- ’ and from 270 to 350°C at 10°C min- ’ (hold 2min) for a typical fraction. Virtually all fractions were completely eluted before 270°C was reached. Other instrumental conditions were: injector 3oo”C, 1OO:l split; g.c.-m.s. interface 3Oo”C, He makeup 1 ml min- ’ (50ml min-’ for solvent peak purge); column head pressure 125 kPa (1 ml min- ’ He). The mass spectral conditions were: ionizing voltage 70 eV; dynamic resolution 1000; scan rate 0.5 s per decade; source pressure 0.7 mPa; source temperature 300°C.

Detailed g.c.-m.s. analysis of every acid and base concentrate or subfraction from each fuel was clearly beyond the scope ofthis study. Several factors determined the selection of fractions for analysis, including fraction yields from aged zlersus unaged fuels, the magnitude of colour change and sediment formation during ageing, qualitative and quantitative information from infrared analysis, h.p.1.c. or both, and the feasibility of obtaining a definitive analysis of a given fraction type. Ideally, the last of these criteria would have had minimal impact, yet in

RESULTS AND DISCUSSION

Ageing and ABN separations of whole fuels and ABN blends

Accelerated ageing of fuels produced some interesting results (Table 2). Fuel E formed the maximum sediment and had the highest final colour. Fuel C was the only fuel tested that did not produce any sediment upon ageing; the colour increase for this fuel was marginal, too. Contrary to the widely held opinion that higher fuel nitrogen content generally results in higher sediment levels, the fuel that produced the maximum sediment and developed the highest colour (fuel E) had the lowest nitrogen content. No general correlation existed between the total sulphur, nitrogen, oxygen or metal content of the fuels and the colour or sediment formed by ageing.

Ageing of whole fuels generally increased the yields of acid and base fractions and decreased that of the neutral fraction“j. However, no clear quantitative correlation existed between ABN mass balances and relative colour change or sediment formation. Total acid or base content, like other gross compositional parameters such as elemental N, 0 or S content, includes many compound types and homologues which do not contribute to stability problems.

Results from ageing blends of ABN concentrates clearly indicated that most compounds which promoted fuel darkening were contained in the acid concentrates, as shown in Table 3. Base + neutral blends were essentially

Table 3 Colour change and sediment formation of acid, base and neutral blends stored for 12 weeks at 65’C ____

Acids or Colour Total bases sediment

Fuel Blend (wt%) Unaged Ageb (mg/lOOml)

A Neutrals 0.0 LO.5 LO.5 4.6 Neutrals +acids 0.82 5.5 6.0 15.2 Neutrals + bases 0.64 1.5 2.0 8.3

B Neutrals 0.0 LO.5 LO.5 3.3 Neutrals + acids 0.73 6.0 6.0 9.6 Neutrals + bases 0.42 L1.5 L2.5 10.5

C Neutrals 0.0 LO.5 LO.5 3.3 Neutrals + acids 0.42 L3.0 3.0 10.4 Neutrals + bases 0.3 1 1 .o 1.5 7.2

D Neutrals 0.0 0.5 L1.0 7.2 Neutrals +acids 0.57 5.0 5.5 15.3 Neutrals + bases 0.24 1.5 L2.0 10.0

E Neutrals 0.0 LO.5 L1.0 5.1 Neutrals +acids 0.48 4.0 5.0 8.6 Neutrals + bases 0.29 3.0 3.0 7.0

F Neutrals 0.0 LO.5 LO.5 2.7 Neutrals +acids 0.71 4.5 6.0 8.0 Neutrals + bases 0.47 2.5 3.0 8.0

y Standard deviation for total sediment over all blends was f 0.3 mg/ lOOmI



unchanged in colour after ageing. Comparison of the colour of aged acid +neutral blends with that of aged whole fuels (Table 2, 12 weeks data) reveals excellent agreement, which substantiates the primary role of selected acidic species in fuel darkening. However, it should be noted that all blends, as well as the neat neutral concentrates, formed sediments upon ageing. Although the neutralfacid blends typically formed the greatest amount of sediment, in general the correlation between colour change and sediment formation was poor. Furthermore, acid + neutral and base + neutral blends and in some cases pure neutrals formed significantly more sediment than the whole fuel upon ageing. This is especially surprising in view of the fact that many of the very unstable compounds in the fuels probably were lost during the ABN separation process itself, since the strongly basic and acidic resins used are known to promote degradation of sensitive compound types’*. Thus the presence of acids probably inhibits sediment formation promoted by basic compounds, and vice versa, and both may increase the stability of neutral components.

Elemental analysis of acid, base and neutral concentrates l6 from unaged and aged fuels showed that the acid and base concentrates typically contained 334 wt % N, whereas the neutral concentrates had N levels at or below the detection limit of the method used (1 ppm)’ 5. Thus the presence of nitrogen compounds may enhance sediment formation, but sediments can obviously form where nitrogen compounds are essentially absent, as in the case of neutral concentrates. Sulphur was present in unaged fuels at levels = 0.1 wt % in the acids and 1-2 wt y0 in the bases, and at slightly lower levels in the neutrals than in the whole fuel16. In aged fuels, the sulphur content of the bases rose to 4 wt % in selected cases, that of the acids increased to 0.2 wt ‘A in some cases while remaining essentially unchanged in others, and was typically lower in the neutral fractions than in unaged fuels. This implies conversion of neutral sulphur types to basic forms and to a much lesser extent acidic forms. These findings are consistent with oxidation of sulphides (neutral) to sulphoxides and sulphones (basic) as reported elsewhere28,29.

G.c.-ms. analysis qf whole acid and base concentrates Initially, whole acid and base concentrates from fuels

A, C and D were analysed by g.c.-m.s. in an attempt to determine changes in their compositions induced by ageing. Numerous difficulties were encountered in this approach:

1. The overall complexity of whole acid and base concentrates largely obscured the effects of ageing. Since most resolved g.c. peaks contained several components, a change in overall intensity of a given peak often could not be assigned to loss or gain of a particular compound.

2. Nitrogencontaining compounds, owing to their inherently high g.c.-m.s. response and formation of even mass fragments, tended to obscure the oxygen compounds present.

3. Hydrocarbons, carried over as contaminants from the ion exchange procedure for isolation of acid and base concentrates, also interfered with determination of the effects of ageing on chemical composition. This problem was magnified by the variability in the degree of contamination in the various acid and base concentrates.

These problems were largely corrected by subfractionation of the acid and base concentrates into discrete compound classes via h.p.1.c. Analysis of the whole acids and bases did, however, reveal some useful information which is summarized below.

Figure 2 shows an example of g.c.-m.s. analysis of whole acid concentrates from unaged and aged fuel A. The approximate elution ranges of the three major homologous series which could be identified ~ alkyl- substituted indoles, cycloalkylindoles and benzoindoles (predominantly carbazoles) ~ are shown. As expected, carbon numbers of individual members of each series increased with g.c. elution time or oven temperature. Individual peak assignments in Figure 2 and analogous chromatograms of other unaged and aged fuels were based on matching with mass spectral library spectra where available, and on masses of molecular ions in all other cases. In addition, the previously mentioned high nitrogen content of acid concentrates, prominent N-H stretching bands in infrared spectra of acid concentrates (3460cm-‘) and the chemistry of the ABN separation method’ 8 itself all strongly support the presence of benzologues of pyrrole in acid concentrates. Small differences evident in the chromatograms of acids from aged and unaged fuels are more pronounced in the earlier elution region containing primarily alkylindoles. The very slight differences in later-eluting, primarily alkyl- carbazole, peaks are well within the limits of the reproducibility of the ABN separation method. Thus the tentative conclusion from Figure 2 and analysis of other whole acid concentratesl’j is that alkylindoles exhibit less stability than the other pyrrolic benzologue series present. However, it should be noted that some of the apparent loss of alkylindoles may have actually resulted from loss of coeluting oxygen-containing or other species.

In an attempt to assess relative losses of alkylindoles, presumably to sediments or other chemical forms, detailed accounting of all peaks exhibiting spectra characteristic of alkyhndole homologues in three sets of acid concentrates from unaged and aged fuels was undertaken. Peak intensities were not considered in the comparison, except that very minor ones were excluded. As shown in Table 4, the number of alkylindole homologues detected in acids from unaged fuels always exceeded that in the corresponding aged fuel, although the effect was not large. Also, no correlation of either the overall number of indole homologues present (fuel A$ C > D) or loss of alkylindoles after ageing with the relative colour change or sediment formed upon ageing was apparent (Table 2) except that the as-received fuel A colour was significantly darker than that ofC or D. In the case of fuel A, it is likely that degradation of alkylindoles in transit contributed to the dark colour of the as-received fuel. Only fuel A contained appreciable levels of cycloalkylindoles (tetrahydrocarbazoles), but all three fuels contained significant levels of benzoindoles (carbazoles). As with fuel A (Figure 2), little apparent change in the distribution of benzoindoles in the other fuels was observed after ageing.

The observed stability of alkylcarbazoles agrees with past studies on fuels spiked with carbazole4. However, on the basis of similar work with alkylindole-spiked fuels4~‘0~’ 1,30, a greater role in sediment formation than observed here was expected. On the other hand, Jones et u1.8 did not observe significant enhancement of


60 t


I- lndoles I_ Carbazoles-

k+------Tetrahydrocarbazolesp1

I 5 i; b I-

0 10 15 20 25 30 35 40 45

Time (min) I I I I I

125 150 175 200 225

Temperature (“C)

Figure 2 G.c.-m.s. total ion current chromatograms of (a) unaged and (b) aged whole acid concentrates from fuel A

Table 4 Comparison of indole homologue composition of unaged and aged whole acid concentrates”

Fuel

A c D

Cxh MW Unaged Aged

C0 II7 0 0

C, 131 I I C2 145 6 5 C, I59 12 9 C, 173 20 22 C, 187 22 2% C, 201 22 21 C, 215 I7 12

CS 229 I2 7

C, 243 2 2

Total I I4 107

Unaged Aged Unaged Aged

0 0 0 0 0 0 4 2 5 2 1 I I 1 0 0 0 0 0 0

_

II 6

0 0 0 0 2 2 2 0 0 0 0 0 0 0 0 0 0 0 0 0

4 2

“Figures in columns 338 denote numbers of isomers of each alkyl homologue separated on the g.c. column and detected by m.s. “Total number of carbons in alkyl substituents (e.g. C,=3 includes propyl-, methyl-ethyl- and trimethyl-substituted isomers)

sediment formation after spiking shale jet fuel with several alkylindoles. Analysis of sediments heret and elsewhere31 has indicated incorporation of both alkylindoles and alkylcarbazoles in sediments from middle distillates. Semiquantitative infrared analysis of fuels A, C and D indicated that the total content of pyrrolic benzologue types in acids was (unaged/aged)

0.36/0.32, 0.15/0.15 and 0.18/0.17 respectively. Since A and D formed essentially the same amount of sediment (Table 2) whereas C formed no sediment, and since A contained a higher overall concentration of pyrrolic types as well as a much higher proportion of the nominally more reactive alkylindole types than did fuel D, the results refute the concept that this class can promote or cause sediment formation at levels typically present in petroleum-derived middle distillate fuels. As discussed later, other reactive species formed via oxidation more probably actively promote sediment formation; pyrrolic benzologue types probably are passively incorporated in or adsorbed on the sediment matrix.

G.c.-m.s. analysis of whole base concentrates yielded little useful information. The multiplicity of compound types present, poor g.c. resolution and contamination by hydrocarbons prevented any assessment of ageing effects. On the basis of the usual return of chromatograms to baseline at g.c. oven temperatures 2 250°C and the observed <35O”C distillation endpoint of the whole fuelsi6, virtually complete elution of acid and base concentrates was assumed.

Non-aqueous titration of whole bases indicated a predominance of azaaromatics (e.g. pyridine) in many fuels, with minor amounts of arylamines (e.g. aniline), variable amounts of very weak bases (e.g. sulphoxides, amides) and usually a significant proportion of non- titratable bases (e.g. ketones). No systematic effects from ageing were apparent from the titrations, except that the very weak base content typically rose in aged fuels,


Storage stability of marine diesel fuels: 0. K. Bahn et al

2.0~1~2-~~3 I 4 ! -57

9 n A 1.4 I II \ I 0.5

0

2.0, I I II

2.0 / I I I I I

C

10 20 30 40 50 60 70

Time (min)

Figure 3 H.p.l.c.chromatograms showing fuel E acid subfractionation for (a) unaged fuel, (b) aged whole fuel, (c)aged acids + neutrals, (d) aged neutrals. Cut points for subfractions l-5 are indicated at top

presumably owing to formation of sulphoxides as previously discussed.

H.p.1.c. subji-actionation of acid and base concentrates

The previous section showed that further subfractionation ofacid and base concentrates was necessary to obtain sufficiently detailed information on compositional changes caused by fuel ageing. Acid and base fractions from fuels A, D and E were selected for subfractionation by h.p.l.c., since all these fuels formed significant amounts of sediment. Furthermore, acid and base concentrates from aged neutrals and aged blends of acids + neutrals and bases + neutrals for these fuels were also analysed to gain additional information on ageing effects.

Figures 3 and 4 show examples of U.V. detector traces from h.p.1.c. subfractionation of acid and base concentrates respectively, illustrating separation of acid and base concentrates from unaged and aged whole fuels, aged neutrals, aged acids + neutrals and aged bases + neutrals from fuel E. Numerous qualitative differences in the h.p.1.c. traces from subfractionation of the various acid and base concentrates are easily observed from inspection of these figures. Tables 5 and 6 show yields of acid and base subfractions obtained by collecting the h.p.1.c. column effluent in different retention regions indicated in Figures 3 and 4 respectively, evaporating the solvent and weighing the residues.

Very often the change in yield of a given subfraction between unaged and aged fuels was useful for determining the stability of compounds contained in that subfraction as well as the production via ageing of additional compounds coeluting with those originally present. Thus, the results in Tables 5 and 6 served as a useful guide for

selection of subfractions for detailed analysis by g.c.-m.s. Furthermore, since the weights of the subfractions directly measured compositional changes induced by ageing, the results in the tables were also used to quantitate both production and loss of key compound classes brought about by the ageing process. These results are discussed in detail below in conjunction with g.c.-m.s. analysis of the individual subfractions.

G.c.~~rn..s. unalysis of’ acid and buse subji-actions

G.c.-m.s. analysis of base subfractions no. 1 revealed them to contain aromatic hydrocarbons with two to four condensed rings, those with three rings (phenanthrenes) predominating. No acid subfractions no. I were analysed, but a predominance of neutral compounds in those subfractions also appears likely. During the ion exchange isolation of the whole acid and base concentrates, a very weak eluent (cyclopentane) was used for eluting the neutral compounds, so that even very weak acids and bases such as pyrroles, ketones and amides would be retained by the ion exchange resins. The disadvantage of using such a weak eluent is that aromatic hydrocarbons, especially condensed-ring PAH, are not as thoroughly washed from the ion exchange columns as with stronger eluents’*. Thus the weights of fraction 1 of acids and bases shown in Tables 5 and 6 probably indicate the extent of contamination of each acid or base fraction with neutral compounds in most cases. However, some very weak acids and bases are known to elute in fraction 1 also; hence it cannot automatically be assumed that 100% of subfraction 1 is aromatic hydrocarbons.

5 a

;IPT:‘I-ill” 10 20 30 40 50 60 70

Time (min)

Figure 4 H.p.1.c. chromatograms showing fuel E base subfractionation for (a) unaged fuel, (b) aged whole fuel, (c)aged bases +neutrals, (d) aged neutrals. Cut points for subfractions 1-5 are indicated at top



Table 5 Yields (wt ‘4) of acid subfractions relative to total acids and whole oil

Sample description”

Unaged acids

Aged acids

Acids from aged (acids + neutrals) blend

Acids from aged neutrals

Acids from unaged neutrals

“See Figure I ’ TA = wt % ofwhole acid fraction; WO = wt % ofwhole oil, where whole oil refers to the sample from which the acids were separated (i.e. fresh fuel, aged fuel, neutrals +acids, etc.). Mass balances for acid subfractionations typically ranged from 85 to 105 wt %; results shown are normalized to 100%. Errors contributing to actual mass balances include incomplete removal of solvents in whole acid concentrates before subfractionation as well as from subfractions, losses of lighter compounds during solvent removal and handling, and weighing errors in the determination of the relatively small quantities of acid subfractions recovered

Sub- fraction Sample no.’

1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

Fuel

A D

no.” TA’ WOb TA wo

12 0.076 13 0.323 14 0.158 15 0.140 16 0.013 22 0.101 23 0.485 24 0.106 25 0.041 26 0.009 41 0.060 42 0.545 43 0.142 44 0.148 45 0.015 51 0.074 52 0.129 53 0.027 54 0.108 54A 0.012 63 0.030 64 0.120 65 _ _ 0.005 66 _ _ 0.020 67 _ 0.004

7.2 49.6 19.5 18.2 5.5 5.4

63.8 13.6 16.5 0.7 5.6

54.0 9.0

24.5 7.0

47.0 9.9

18.5 18.5 6.0

_ 0.055 0.377 0.148 0.138 0.042 0.043 0.510 0.109 0.132 0.006 0.055 0.529 0.088 0.24 0.069 0.136 0.029 0.054 0.054 0.017

10.7 45.5 22.2 19.7

1.9 13.6 65.5 14.3 5.5 1.2 6.6

59.9 15.6 16.3

1.6 21.0 36.8

7.8 30.9

3.4 16.8 66.8

3.1 11.2 2.0

TA wo

34.0 0.177 32.4 0.168 16.0 0.083 15.6 0.081 2.0 0.010

18.3 0.102 39.8 0.223 16.1 0.090 22.2 0.124

3.7 0.021 27.1 0.203 37.7 0.283 11.6 0.087 21.2 0.159

2.4 0.018 27.6 0.119 50.1 0.216

5.6 0.024 15.7 0.068 0.4 0.002

_ _

_

Table 6 Yields (wt %) of base subfractions relative to total bases and whole oil

E

Fuel

Sample description”

Sub- A D E fraction Sample no.’ no.’ TB* WOb TB wo TB

Unaged bases

Aged bases

Bases from aged (bases + neutrals) blend

Bases from aged neutrals

Bases from unaged neutrals

1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

17 18 19 20 21 27 28 29 30 31 46 47 48 49 50 55 56 57 58 59 68 69 70 71 72

6.4 0.033 5.1 0.027 7.5 0.039 4.6 0.024

16.3 0.397 22.1 0.137

8.6 0.050 9.9 0.061 5.7 0.035

53.8 0.334 15.2 0.109 10.4 0.075 24.4 0.176

5.0 0.036 45.0 0.324’ 24.7 0.086 13.9 0.049 16.7 0.058 6.3 0.022

38.3 0.134c _

33.3 11.4 13.3 15.2 26.8 10.8

7.3 10.7 9.8

61.4 21.3 15.0 17.3 18.0 22.3 23.0 14.0 9.6

29.1 24.3 33.6 11.8 8.4 9.4

36.8

0.144 0.049 0.058 0.065 0.116 0.033 0.023 0.033 0.030 0.19 0.115 0.063 0.073 0.076 0.094 0.094 0.057 0.039’ 0.119 0.100 0.138 0.048 0.034 0.039 0.151

38.0 0.076 9.0 0.018

16.9 0.034 26.5 0.053

9.6 0.019 34.0 0.065

8.0 0.015 11.2 0.021 12.4 0.024 34.3 0.065 28.0 0.129 12.0 0.055 12.3 0.057 13.6 0.063 34.0 0.156 26.7 0.160 11.2 0.067 11.6 0.070 9.5 0.057

41.0 0.246 _

‘See Figure I ’ TB = wt ‘4 of whole base fraction. WO = wt % of whole oil, where whole oil refers to the sample from which the bases were separated (i.e. fresh fuel, aged fuel, neutrals+ bases, etc.). Mass balances for base subfractionations typically ranged from 90 to 105 wt %; results shown are normalized to 100%. Errors contributing to actual mass balances are as given in Table 5, footnote b c Partial loss of subfraction during handling, corrected weight was estimated



loo

80

60

a

I I I I I

80

60

i ?

1 _I

02 10 20 30 40 50 Time (mm)

1 1 1 I I I I

100 125 150 175 200 225

Temperature (“C)

Figure 5 G.c.m.s. total ion current chromatograms of acylated acid subfraction 3 from (a) unaged fuel D, (b) aged whole fuel D, (c) aged D acids+neutrals. Peak identification: 1, C,-phenol; 2, C,-phenol; C,- tetralinol; 3, C,-hydroxybiphenyl; 4, C,-tetralinol; 5, C,-phenol; C,- hydroxybiphenyl;6,C,-,C,-140;7,C,-,C,-140;8,C,-,C,-140,C,-, C,-160; 9, C,,-, C,,-phenols; C,-140, C,*-160; 10, C,,-phenol; Ca-, C,*-160; 11, Cs-160*; C,-, Cs*-200 (* indicates components not fiund in acid subfraction 3 from aged whole fuel, sample 24). See Ref. 16 and Table 7 for identities of other peaks and possible structures of CnHZn_140 (-140) C,H,,_,,O (-160) and C,H,,_,,O (-200) compounds

On the other hand, it should be noted that the highest level of hydrocarbons potentially in a polar fraction, as indicated by the yields of subfraction 1, is still only O.l-- 0.2 % of the whole fuel, the neutral fraction accounting for > 99 ‘A of most fuels. This level of contamination is quite reasonable, considering the chromatographic overlap of fractions normally encountered in liquid chromatographic separations. However, a number of the differences in yields of whole acid and base concentrates before and after ageing16 can probably be attributed to

variations in the amount of hydrocarbon contamination rather than to any real effects of ageing. Thus a comparison of the weights of individual subfractions 2-5 from acids and bases is far more meaningful because (1) hydrocarbons present in the whole acid or base concentrate do not affect the weights of those subfractions and (2) the overall number of chemical species present in each subfraction is much smaller and hence the effects of ageing on those species are much easier to detect and document.

Acid subfractions no. 2 increased significantly in weight (Table 5) during ageing for all three fuels. Also, the weight of subfraction 2 (whole fuel basis) was greater in aged acids + neutrals than in aged whole fuels. The yield of this subfraction in aged neutrals from fuel A was negligible, but it was significant for fuels D and E. This subfraction contained the bulk of the pyrrolic benzologues (indole, tetrahydrocarbazole, carbazole, etc.). Since the results from analysis of whole acid and base concentrates indicated partial loss of these compounds, the mass of subfraction 2 was expected to decrease rather than increase as actually observed. Also, since nitrogen was effectively absent from neutral concentrates, yields of subfraction 2 from acids from aged neutrals were expected to be negligible, in contrast to the observed results for fuels D and E.

In an attempt to resolve the discrepancy between expected and observed results, subfraction 2 from acids obtained from aged neutrals (sample no. 52, see Figure I) from fuel D was analysed by g.c.-m.s. That analysis showed the fraction to contain minor amounts of C-C, phenanthrenes (overlap from subfraction 1) and a major series of compounds starting at mass 196 and increasing by 14 mass units to 266. Initially this series was suspected to be alcohols, but acylation and reanalysis did not show the expected series of trifluoroacetyl esters. In fact, no trace of these unknown compounds was evident in the g.c.-m.s. chromatogram of the derivatized sample; only the phenanthrenes were left. This observation suggests a fairly unstable compound class such as hydroperoxides. Identification of these compounds is of considerable importance, since knowledge oftheir structure may reveal a major pathway of fuel oxidation leading to sediment formation and colour change.

Regardless of their actual structure, this unknown series of compounds appears to be derived from the neutral fraction of the fuels, and its production caused the observed increase in acid subfraction 2 yields upon ageing. On the basis of yields for sample 52 (see Figure I) from the three fuels studied, fuel E had the highest ability to form these compounds. Although it may be a coincidence, that fuel also formed the largest amount of sediment (Tub/e 2). Also, since the increase in subfraction 2 yield was higher in the acids + neutrals than in the aged whole fuel, it seems probable that indigenous basic compounds inhibit formation of these compounds.

Acid subfraction 3 results varied from fuel to fuel. Fuels A and D showed decreases in the amounts of this subfraction upon ageing, whereas fuel F showed essentially no change. Since preliminary infrared and g.c.-m.s. analyses of this subfraction indicated primarily hydroxyaromatic (ArOH) types, every subfraction 3 (sample nos. 14, 24, 43, 53 and 65, Tab/e 5) from fuel D was acylated and analysed by g.c.-m.s. in an attempt to explain the yield pattern for subfraction 3 for that fuel.



Table 7 Summary of composition of acid subfraction 3 from fuel D

Sample no.

Z”: -6 Empirical formula: C,H,,m,O

Major type(s) present: Phenols Sample description

-8

C,H,,-80 Indanols/ tetralinols

-14 - 16 -20

C,H,,- 140 CnHznmrhO C,H,,m,,O Hydroxybiphenylsj Phenyltetralinols/ Phenylnaphtholsi cycloalkyl- dicycloalkyl- cycloalkyl- naphthols naphthols phenanthrols

14 Unaged whole fuel 24 Aged whole fuel

43 Aged acids +neutrals 53 Aged neutrals 65 Unaged neutrals

C,-CI, C*+C,,

C,+C,3 None None

C,-C* C, +C*

CIFC, None None

Carbon numbers present (C,)b

Co-C, C-C, G-C, C, -rC,

(trace levels)

C-C, C,F+C, None None None None

C,, - C, None

C-C, None None

‘Defined by the empirical formula C,H,,+ZO ‘Carbon number range expresses the number of carbons in alkyl substituents observed for a given series. Co represents the unsubstituted, lowest molecular weight isomer of a given series. The upper limit also refers to the lowest molecular weight isomer (e.g. for Z = - 8, indanol is the lowest molecular weight isomer)

Acylation has previously been shown to improve g.c.-m.s. analysis ofcomplex mixtures of ArOH2s,26. The total ion current g.c.-m.s. traces for three of the subfractions are shown in Figure 5. Comparison of the unaged and aged whole fuel traces clearly shows loss of the higher molecular weight ArOH upon ageing, whereas ageing of the acids + neutrals blend resulted in loss of the lower molecular weight compounds. It is possible that the loss of lighter components from the acids + neutrals blend occurred during sample handling, but loss of the higher molecular weight compounds from the aged whole fuel could only have arisen from a chemical change induced by the ageing process. Since this loss did not occur in the aged acids + neutrals blend, some type of interaction with basic compounds appears likely. Analysis of the g.c.-ms. results from the three subfractions in Figure 5, as well as those from the unaged and aged neutrals, is summarized in Tuhle 7. Peak assignments in Figure 5 and Table 7 were based on the molecular ion of the ArOH trifluoroacetate derivatives. Very few nonacylated compounds were observed, except for subfractions from the unaged and aged neutrals, which were essentially free of ArOH. Acylation allows ArOH to be readily distinguished from other even-mass compound types which do not react under the conditions employed here. Table 7 shows that the higher-boiling ArOH lost during ageing of the whole fuel were largely the most aromatic or hydrogen-deficient species present. Thus the later-eluting peaks evident in the chromatogram of the subfraction from the aged whole fuel were primarily highly alkylated phenols, indoles/ tetralinols and other monoaromatic ArOH.

Hydroxyaromatics have previously been shown to promote instability of middle distillate fuels through oxidative coupling reactions which result in polymeric inso1ubles’4T32. However, the higher reactivity of more- aromatic ArOH and apparent interaction with basic compounds have not been reported previously. In fact, Hazlett and Power 32 showed that a tertiary alkylamine fuel stabilizer additive effectively inhibited phenol degradation.

Subfractions no. 3 from the other fuels were not analysed by g.c.-m.s. However, h.p.1.c. yields from fuel A were similar to those from fuel D, except that the drop in ArOH content from the aged acids + neutrals blend was even larger than that from the aged whole fuel. On the other hand, fuel E showed little change in ArOH content upon ageing.

Acid subfraction 4 typically contained carboxylic acids and other species of similar acidity”. The yield pattern for this subfraction was highly dependent on both the fuel and the presence of basic compounds. In the case of fuel A, the yield of subfraction 4 was virtually unchanged by ageing the whole fuel, but nearly doubled during the ageing of the acids + neutrals blend. Fuel D showed a loss of over two-thirds of the compounds originally in the fresh fuel during ageing of the whole fuel. On the other hand, yields from the aged acids+neutrals blend were virtually the same as in the fresh fuel. This yield pattern indicates a loss due to interactions with basic compounds, as discussed earlier for subfraction 3 from fuel D. Fuel E showed an increase in subfraction 4 upon ageing the whole fuel and an even larger increase after ageing the acids + neutrals blend. All three fuels showed substantial yields of subfraction 4 in acids from aged neutrals. Subfraction 4 from all fuel E acid concentrates was analysed (after chemical derivatization) by g.c.-m.s. Surprisingly, the distribution of carboxylic acids in all subfractions was quite similar, and few if any other types of compound were observed.

The available evidence indicates that carboxylic acids or other acidic compounds in this subfraction are produced by oxidation of neutral compounds. Furthermore, interaction with basic compounds to form sediments is suggested by the data. This finding contradicts that of Hazlett and Ke1s033, who reported a catalytic action by carboxylic acids which promoted sediment formation but no incorporation of acids into sediments. The inconsistent behaviour from fuel to fuel observed here indicates that the actual structural type(s) of carboxylic acids present in a given fuel determine(s) the extent of their incorporation into sediments. The observation of carboxylic acids in subfractions from aged neutrals strongly suggests production of carboxylic acids during ageing. Acids produced during ageing could potentially be of significantly different structure from those originally present in the fuel.

Acid subfraction 5 contained the most acidic compounds in each fuel. In general, the yield pattern for this subfraction, which always formed a very small fraction of each fuel, closely followed that of acid subfraction 4. None of these subfractions was analysed by g.c.-m.s., but on the basis of the chemistry of the h.p.1.c. separation used to generate this subfraction, it should contain more-acidic (i.e. more-aromatic) carboxylic acids



100. a

80 -

60 - Cl

r.7 c2 -

40 -

.s 100 1 i; b I- 80-

10

I 125

40 50 Time (min) I 4 I

150 175 200 225

Temperature (“C)

Figure 6 G.c.-m.s. total ion current chromatograms of base subfraction 2 from (a) unaged and (b) aged fuel D. C,C, refer to respective alkyl homologues of 9-fluorenone. Nitrogen compounds (largely - 11N) are designated N. Note enhanced response of C,C,-9- fluorenones and N compounds relative to the C,- and the largest C I -9- fluorenone peaks in (b)

as well as any difunctional compounds present. Base subfractions 2, 3 and 4 all contained a mixture of

nitrogen compounds and ketones, with 2 and 3 containing predominantly ketones and 4 largely consisting of arylamines with a minor amount of ketones. In many cases the nitrogen compounds were partly or completely lost during ageing. However, especially in the case of bases + neutrals blends and neutrals, ketones were usually produced during ageing. Thus the yields shown in Table 6 for these subfractions in fresh and aged fuels reflect the resultant of these two opposing factors. Although ketones are not classically thought of as bases, many common organic synthetic reactions involving ketones are acid-catalysed, with a proton attaching itself to the carbonyl oxygen in the initial reaction step. A similar interaction probably accounts for retention of ketones by the acidic sulphonic acid groups bound to the cation resin used to trap bases during the ABN separation. Highly aromatic ketones can partition into the acid fraction”, presumably through interactions of alpha hydrogens with basic groups on the acid-trapping resin. Ketones exhibited appreciable retention during the h.p.1.c. subfractionation of bases, eluting slightly ahead of most anilines and other arylamine types”.

Figures 6-8 show total ion g.c.-m.s. chromatograms of unaged and aged whole fuel D base subfractions 24 respectively. Subfraction 2 contained a very minor amount of nitrogen compounds, predominantly ones in the series C,H,,_ ,,N (- 1 lN), and relatively high concentrations of alkyl-9-fluorenones. Subfraction 3

contained predominantly unsubstituted 9-fluorenone, identified by mass spectral and g.c. retention matching with an authentic sample, as well as C,- and C,-alkyl homologues, with lesser amounts of alkylanilines (C,H,, 5N) and other nitrogen compounds probably of arylamine type with empirical formulae C&H,,_ r ,N (- 1 IN) and C,H,,_ i,N (- 17N). Subfraction 3 from the aged fuel also contained low levels of ketones having empirical formulae C,H,, _ I ,O ( - 10 0) and C,H,, i60 (- 160). Several mass spectra of the - 100 series were matched with those of authentic alkyl-1 -tetralones. Subfraction 4 contained predominantly nitrogen compounds, with many of the more prominent peaks belonging to the - 11N series. However, base subfraction 4 from the aged fuel contained significant levels of - 10 0 and - 160 ketones. Compounds in the - 160 series were tentatively assigned as alkylbenz-1 -tetralones from their fragmentation patterns, but no authentic spectra of that compound type were available in the mass spectral library.

G.c.-m.s. results correlated very well with previously derived retention correlations for the base subfractionation method22. For example, subfraction 2 primarily contained relatively hindered alkyl-9-fluorenones. It may be noted that the relatively intense C, -9-fluorenone peak in subfraction 2 was present at a much lower concentration in subfraction 3. This isomer is probably l- methyl-9-fluorenone, in view of its relatively early elution in both the h.p.1.c. and g.c. separations. C,- and C,-9-

a -17 N

60

4 50

125 150 175 200 225

Temperature (“C)

Figure 7 G.c.-m.s. total ion current chromatograms of base subfraction 3 from (a) unaged and (b) aged fuel D. C-C, as in Figure 6. C,C,-A refer to anilines with 36 carbons in alkyl chains. Other peaks are identified by their Z number in the general formulae C,H,,+zN or

C,H Zn +zO. Note overall lowered response by nitrogencontaining compounds, appearance of new -100 and -160 compounds, enhanced levels of &X3,- and decreased levels of C, and C,-9- fluorenones in (b)


fluorenones were more prominent in subfraction 2 because of the higher probability of steric hindrance with increasing alkyl substitution. Alkyl-1-tetralones (- 100) and the - 160 series, which are believed to have analogous structures to tetralones, only with two fused aromatic rings, eluted later in the h.p.1.c. separation than alkyl-9-fluorenones because of the greater electron withdrawing/delocalization effects (i.e. lower basicity) on the carbonyl group in the 9-fluorenone structure. Subfractions 2 and 3 contained only minor amounts of nitrogen compounds because, as shown previously’*, most arylamines exhibit retention in the region

loop----- -Impurity

ii 5

- I b

I- 80

I I- t

60 I $’ cz

10 2b 30 4b 50

Time (mid I I 1 I 1 125 150 175 200 225 250

Temperature PC)

Figure 8 G.c.-m.s. total ion current chromatograms of base subfraction 4 from (a) unaged and (b) aged fuel D. C,C, refer to respective alkyl homologues of benz-1-tetralone. Note appearance of alkyl-I-tetralones, designated - 100, in (b). Numbered peaks are homologues in a series ofcompounds of formula C,H,,_ , ,N, except for peak I, which is a C,-aniline. Note also that high boiling nitrogencontaining peaks in (a) are largely absent from (b)


corresponding to subfraction 4. Azaarenes are significantly more retained than either ketones or arylamines**, and elute in subfraction 5. It should be noted that the cutpoints for subfractions 24 were fairly arbitrary; had the composition of base concentrates been better known before subfractionation, it would have been possible to adjust cutpoints to obtain all alkyl-9- fluorenones in one fraction, for example.

The analysis of base subfractions 24 from aged and unaged fuel D clearly demonstrates both loss of some nitrogen compounds as well as formation of ketones during ageing. A detailed analysis of g.c.-m.s. results for subfraction 4 from unaged and aged fuel D showed greater degradation of the more aromatic nitrogen compounds’ 6, similar to the case for ArOH as discussed earlier. G.c.-m.s. analysis of subfractions from aged neutrals + bases and aged neutrals from fuel D also clearly showed formation of ketones during ageing16. Lower levels of alkyl-9-fluorenones were also detected in subfraction 3 from the unaged neutrals; these were either formed on standing at room temperature (see Experimental) or escaped retention by the base-trapping resin in the initial ABN separation of the unaged fuel. Even greater loss of nitrogen compounds was observed in the aged bases +neutrals compared with the aged whole fuel.

Tables 8 and 9 briefly summarize results from the analysis of base subfractions no. 3 from fuels A and E respectively. Losses of nitrogen compounds during ageing of fuel A were less than those from fuel D, and greater from fuel E than from fuel D. Only 9-fluorenones were found in subfraction 3 from unaged or aged fuel A. Fuel E exhibited a pronounced increase in ketone concentrations during ageing of the whole fuel, but the effect was less for aged bases +neutrals or neutrals. Perhaps the most significant finding from analysis of fuels A and E was the demonstration that the results obtained for fuel D were typical.

Correlation of the yields of base subfractions 2-4 in Table 6, colour and sediment data in Tables 2 and 3 and g.c.-m.s. analyses leads to additional conclusions. First, production of ketones was much greater in aged base + neutral blends and aged neutrals than in aged whole fuels. This result is clearly evident in the relative yields of subfractions 24 shown in Table 6: yields of subfractions typically decreased in aged versus unaged whole fuels yet rose dramatically in the case of aged neutral + base blends and aged neutrals. In fact, the effect is even larger than the h.p.1.c. data indicate, since greater losses of nitrogen compounds also occurred in aged neutral+ base blends

Table 8 Summary of g.c.-m.s. analysis of subfraction 3 from fuel A bases

2 series Empirical formula Probable compound type(s)

-5N C,H,, - 5N Anilines -7N C,H,,- ,N Cycloalkylanilines

_ II N C,H,,-,,N Aminonaphthalenes/phenyldihydropyridines -13N C&z,- I SN Aminobiphenyls/aminocycloalkylnaphthalenes -17N C,H,,- I P Aminophenanthrenes -180 C,Hz,-180 9-Fluorenones

“A = greater than, B = equal to, C =signihcantly less then, D = essentially all gone b19 = unaged whole fuel, 29 = aged whole fuel, 48 = aged bases + neutrals, 57 = aged neutrals

Abundance relative to unaged fuel” Sample no.’

_

19 29 48 57

B B D B B D 0 C D B D D B D D B A A



Table 9 Summary of gc-ms. analysis of subfraction 3 from fuel E bases -_____

Abundance relative to unaged fuel” Sample no.h

Z series Empirical’formula Probable compound type(s) I9 29 48 51

-5N C,H,,-,N Anilines B C D -1lN C,Hzv-x,N AminonaphthaIencs:phenyIdihydropyridines B C D - l3N C,H,,- 1 P Aminobiphenyls,‘diphenylamines C C D -17N C,H,,-,,N Aminophcnanthrenes C D D - 100 C&z,- ,,O I -Tetralones’ A B B -160 C,H,,- 160 Benz-l -tetralones A C C -180 CJf,,- 180 9-Fluorenones A A A

“.‘See Tnhlr 8 footnotes ‘No - 100 compounds were present in sample 19

than in aged whole fuels. The results point to acid inhibition of oxidation reactions leading to ketone formation. Second, yield increases for subfractions 2 and 3, which contained primarily alkyl-9-fluorenones, typically exceeded that of subfraction 4, which contained other ketone types. This finding, which is especially apparent in the results for fuel A, indicates relatively rapid production of 9-fluorenones ~ potentially via oxidation of fluorenes -compared with other ketone types. This result can be rationalized from the ‘dibenzylic’ nature of the 9- position of fluorene, which should be more reactive than carbons adjacent to only one aromatic nucleus.

Third, the greater loss of nitrogen compounds from aged neutral + base blends and the accompanying greater production of ketones correlate with the typically higher production of sediments from blends as opposed to aged whole fuels. However, the behaviour of fuel E was atypical in at least two respects. First, the whole fuel E formed greater amounts of sediments during ageing than did either of its blends, and relative ketone production was higher for the whole fuel than for the base + neutral blends. Conversely, the concentration of basic nitrogen compounds decreased more during ageing of the base + neutral blend than the whole fuel. Thus the correlation of higher ketone production with greater sediment formation was preserved in the case of fuel E, but with the whole fuel rather than the base + neutral blends, whereas the trend of greater loss of basic nitrogen compounds in cases of increased sediment formation did not hold for fuel E. Fourth, ketones were lost as well as produced. Especially in the case of aged whole fuels, the decreases in yields of subfractions 24 could be accounted for only via partiai loss of ketones present in the unaged fue1. Also, ageing-induced changes in distributions of ketones, as reflected in g.c.-m.s. analysis, indicated losses in many cases. Thus strong evidence exists that ketones act as intermediates in oxidation pathways. In a separate experiment, fuel C, which formed essentially no sediment (Table 2) by itself, was spiked with 2000 ppm 9-fluorenone and aged for 6 weeks at 65 and 80°C (Ref. 16). Essentially no sediment was formed, indicating that 9-fluorenone itself does not promote sediment formation in otherwise stable fuels.

The role of ketones in sediment formation could not be further defined from mass spectrometric analysis of the sediments16. A minor amount of oxygen compounds was detected, but much less than indicated from elemental

analysis. Recent infrared studies of gums, sediments and oxidized diesel fuels34’35 indicate bands consistent with ketones in each. Infrared spectra of base fractions in this work clearly indicated sharp carbonyl bands at 1720cm-i which matched those of authentic 9- fluorenone, but sediment carbonyl bands were broader and less distinct. Band-broadening can be explained by the polymeric nature of sediments; nevertheless, the infrared spectra do not offer convincing proof of ketones in sediments.

As previously mentioned, base subfraction 5 contained primarily azaarene compounds in unaged fuel. However, sulphoxides or other basic sulphurcontaining species produced from oxidation of neutral sulphurcontaining species were also eluted in this fraction. Thus the yield patterns in Table 6 from this subfraction demonstrate slight losses on ageing in some cases and substantial gains in others. Fuel E especially showed a pronounced increase in this subfraction for aged neutrals + bases and aged neutrals. Dual f.i.d.--f.p.d. g.c. analysis of subfractions from aged fuels typically showed a pronounced increase in sulphur (f.p.d.) response compared with those of unaged fuels, and an increased average boiling point”. Because of the large contribution of sulphur compounds to the g.c.-m.s. total ion current in subfractions from aged fuels, as well as their very complex composition, it was difficult even to semiquantitate losses of azaarenes on ageing. Some loss of azaarenes did occur, as evidenced by decreases in overall fraction yield in selected cases in Table 6. Also, decreases in intensities of resolved gc-m.s. peaks containing nitrogen compounds were typically observed, although the decrease was due in part to dilution by sulphurcontaining bases formed on ageing.

On the basis of h.p.1.c. fraction yields (Table 6), patterns of production of sulphur-containing bases diverged considerably from those of ketones. For example, fuel E produced larger quantities of bases eluting in subfraction 5 from aged neutrals than did the other fuels, yet fairly equivalent levels of ketones in subfractions 2-4 to those of other fuels. The above and similar effects may simply reflect the relative abundance of precursors in the respective fuels. It has been reported previously that sulphide oxidation to sulphoxides represents an important stabilization mechanism for radical quenching in fuels29. However, those authors further state that the oxidized sulphur species may lead to formation of



documented instability of 2,5-dimethylpyrrole (DMP) is probably derived from its ease of oxidation; the fact that it contains nitrogen is incidental except with regard to the ease of oxidation of the substituted pyrrolic moiety. To the authors’ knowledge, pyrroles or alkylpyrroles have never been found in middle distillate fuels; thus the extensive DMP work done in the past bears little relation to actual sediment formation processes in commercial fuels, except in showing that easily oxidized compounds promote sediment formation. Furthermore, the inference sometimes drawn from past studies with DMP, i.e. that nitrogen compounds inherently promote instability, is false, as shown here and elsewhere.

Compound classes notably susceptible to oxidation and therefore potentially constituents of gums and sediments include: partially hydrogenated/methylene- bridged aromatic hydrocarbons, which were observed here to form ketones; sulphides or other neutral sulphurcontaining types, which formed weakly basic SO or SO, compounds; hydroxyaromatics, pyrroles and to a lesser extent indoles; and various basic nitrogen compounds. In this work, the more aromatic hydroxyaromatics and basic nitrogen compounds decreased in concentration faster than simple monoaromatic forms. It is not known whether this resulted from higher susceptibility of the more aromatic compounds to oxidation or from interaction with sediments in some other manner. It is known, however, that condensed-ring polycyclic aromatics in general are more susceptible to oxidation36; thus an autooxidation pathway is consistent with faster degradation of more-aromatic species. Thus one possible explanation for the apparent general ineffectiveness of conventional antioxidant additives in stabilization of diesel fuels is that the multiple fused ring compounds typical of that boiling range are inherently more reactive than the additives, which typically are monoaromatic species such as hindered phenols.

The general approach to the study of fuel degradation processes employed here is valid and yields useful results. However, many analytical problems limited the derivation of definitive sediment formation pathways. Lack of standard mass spectra for identification of indigenous polar compounds, and especially oxidation products, was a major limitation. In some cases, such as in the analysis of oxidized sulphur compounds in base subfraction 5 or the unknown compounds in acid subfraction 2, a separate analytical method development project would be needed to define a successful approach. Similarly, improved methods for analysing sediments are needed. Thus the sheer magnitude of the analyses required and the lack of methodology for several important compound types are the major limitations of this approach. The main limitation of past studies involving fuels spiked with relatively large concentrations of pure compounds is even more severe, however, in that it is never known whether the observed results reflect sediment formation processes actually occurring under conditions in practice.

One possible solution to stability problems would be rigorous exclusion of air from fuels. Unfortunately, many practical problems arise in attempting to exclude air during handling and transportation of large quantities of fuel. Development of more-reactive antioxidant additives that yield products soluble in diesel fuels may be a more practical approach. Hydrotreating would potentially

sediments. Thus, as with ketone formation the significance of formation of sulphur-containing bases in determining fuel instability is unknown.

Sediment analysis

A detailed account of the mass spectrometric analysis of sediments from the aged fuels16 is beyond the scope of this paper. Very briefly, batch inlet experiments indicated the following relative abundances: nitrogen compounds > aromatic hydrocarbons > sulphur compounds% oxygen compounds. Also, many of the species observed from batch inlet analyses were believed to be merely adsorbed on sediment surfaces. In other work, high-temperature probe inlet analyses indicated phenolic products at pyrolysis temperatures, which supports the hypothesis of a core sediment matrix comprising oxygenated species. The typical elemental composition’ 6 of sediments was 5 80 wt % C, 7 wt % H, 3wt% N, lwt% S and 9wt% 0.

CONCLUSIONS

The general conclusion from this work is that oxidation of indigenous compounds in middle distillate fuels is the initial step in fuel degradation and that these or secondary oxidation products combine with indigenous polar compounds present to form gums and sediments. Thus the compounds susceptible to oxidation actually promote or cause instability; the majority of polar compounds in the fuel passively participate in sediment and gum formation via various types of chemical bonding, including hydrogen bonding. Many types of evidence support the above conclusions. For example, the simple observation of sediment formation during ageing of neutral concentrates indicates that the presence of indigenous polar compounds is not necessary for sediment formation. Also, the observed relatively high concentrations of oxidized neutral compounds in acid and base concentrates from aged neutrals, whole fuels, acid + neutral blends and base + neutral blends demonstrate oxidation of neutral compounds in general. In the case of aged neutrals, oxidized compounds are the likely sediment-formers, since indigenous polar compounds are essentially absent. The enhanced amounts of sediment formed during ageing of acid+ neutral and base + neutral blends show that either acids or bases can associate with oxidized neutral compounds or themselves oxidize to form greater amounts of Gediments than observed for aged neutrals alone. The relative stability of whole fuels compared with that of acids +neutrals or base + neutral blends indicates either inhibition of oxidation reactions by the presence of acids and bases together, or decreased participation of acids or bases in sediment formation when both are present.

The importance of oxidation in promoting instability has been recognized previously3’. Antioxidant fuel additives, tests for peroxides, etc., have been developed since recognition of the importance of oxidatively induced stability problems. Prior emphasis has been placed on polar, especially nitrogen-containing, compounds as promoters of instability. The results obtained here suggest that the role of polar compounds has been overemphasized and that neutral compounds, because of their much higher abundance in fuels, should receive much greater attention. For example, the well-



remove many polar and neutral sulphurcontaining compounds which could oxidize and initiate sediment and gum formation. However, hydrotreating would also result in higher concentrations of partially hydrogenated hydrocarbons which, as this study shows, could oxidize to ketones and possible further products leading to sediments. Catalytic cracking leads to increased levels of potentially easily oxidized compounds, including olefins; hence fuels containing cracked products should and typically do exhibit decreased stability. Clearly, a comprehensive effort is needed in understanding the chemical processes involved, as well as in finding solutions to stability problems.

ACKNOWLEDGEMENT

The work reported here was conducted at the National Institute for Petroleum and Energy Research for the Defense Fuel Supply Center, Alexandria, VA, and the Mobility Fuels Technology Program, Office of the Chief of Naval Research, Arlington, VA, through the Defense Logistics Agency Contract No. DLA600-84-5050.

REFERENCES

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2

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storage stability of marine diesel fuels

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