Energy & Fuels 1996, 10, 117-120
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Effects of Dodecylbenzenesulfonic Acid on Insolubles Production in Mid-Distillate Diesel Fuel Blends Margaret A. Wechter*,† and Dennis R. Hardy Naval Research Laboratory, Code 6180, Washington, D.C. 20375-5342, and Department of Chemistry, University of Massachusetts, Dartmouth, North Dartmouth, Massachusetts 02747 Received August 21, 1995. Revised Manuscript Received October 19, 1995X
The role of strong acids in promoting mid-distillate diesel fuel instability has been a topic of interest within the fuel science community. Previous studies have indicated that strong sulfonic acids, when added to fuels that are then subjected to thermal stress accompanied by oxidative aging, tend to produce quantities of insoluble sediment. This work presents results obtained when dodecylbenzenesulfonic acid (DBSA) is added to a blended diesel fuel. We find that a fraction of the insoluble material forms immediately on addition of the acid and we investigate the effects of accelerated aging and DBSA addition on other insolubles related fractions of the fuel sample. Evidence is presented that opens to question the interpretation of results obtained when a reactive reagent is added to a chemical system as complex as a fuel.
Introduction The role of strong acids in mid-distillate diesel fuel stability has been the subject of numerous studies including several within this laboratory.1-10 In particular, studies based on the use of sulfonic acids such as DBSA (dodecylbenzenesulfonic acid) added to fuel have implicated these strong acids in the formation of insoluble sediment during periods of accelerated oxidative aging.4,6,9,10 Our previous work performed for the purpose of elucidating or defining sediment precursors11 led us to propose the existence of a methanol-soluble, hexaneinsoluble substance that was the immediate precursor of insoluble sediment in fuels that were 6 months past production.12-14 This material exists as a small part of the soluble gums as determined by ASTM D381 but has no fixed relationship to these soluble gums. †
University of Massachusetts. Abstract published in Advance ACS Abstracts, December 1, 1995. (1) Offenhauer, R. D.; Brennan, J. A.; Miller, R. C. Ind. Eng. Chem. 1957, 49, 1265. (2) Sauer, R. W.; Weed, A. F.; Headington, C. E. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1958, 3, 95. (3) Schrepfer, M. W.; Arnold, R. J.; Stansky, C. A. Oil Gas J. 1984, 79. (4) Hazlett, R. N.; Power, A. J.; Kelso, A. G.; Solly, R. K. Report MRL-R-986; Dept. of Defence: Melbourne, Australia, 1986. (5) Pedley, J. F.; Hiley, R. W.; Hancock, R. A. Fuel 1987, 66, 16461651. (6) Hazlett, R. N. Fuel Sci. Technol. Int. 1988, 6(2), 185-208. (7) Wechter, M. A.; Hardy, D. R. Energy Fuels 1989, 3, 461-464. (8) Hardy, D. R.; Wechter, M. A. Fuel 1990, 69, 720-724. (9) Malhotra, R.; Hazlett, R. N. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35, 1163-1167. (10) Malhotra, R.; Hazlett, R. N. Proceedings of the 4th International Conference on Stability and Handling of Liquid Fuels, Orlando, FL; Giles, H. N., Ed.; U.S. Department of Energy, Washington, DC, 1992, 518-528. (11) Wechter, M. A.; Hardy, D. R. Fuel Sci. Technol. Int. 1989, 7(4), 423-441. (12) Hardy, D. R.; Wechter, M. A. Energy Fuels 1990, 4, 270-274. (13) Wechter, M. A.; Hardy, D. R. In Proceedings of the 4th International Conference on Stability and Handling of Liquid Fuels, Orlando, FL; Giles, H. N., Ed.; DOE/CONF-911102; U.S. Department of Energy: Washington, DC, 1992; pp 620-625. (14) Hardy, D. R.; Wechter, M. A. Energy Fuels 1994, 8, 782-787. X
This article not subject to U.S. Copyright.
Aging studies using separate aliquots of methanol extracted and unextracted fuels clearly showed that the extracted fuels were no longer unstable with regard to sediment formation. That is, removal of these instability precursors reduced insolubles yields. These aliquots produced, on average, 60-95% lower insolubles yields than their unextracted counterparts.11 Thus, the tendency of a fuel more than 6 months old to develop insoluble material on aging is clearly linked to a substance or substances found in the fraction of the fuel that is methanol extractable. This work focuses on a study that was performed on a typical unstable blended fuel characterized for its instability properties in this laboratory. We tested variously treated aliquots of the fuel for instability properties under conditions of accelerated aging with oxygen overpressure using ASTM D5304. The results of this study open to question the interpretation of previous instability studies based on treating chemical systems as complex as fuels with reagents as reactive as strong acids. When concentration scaling factors are applied, the actual effect of strong acids on the production of sediment in diesel fuels is probably negligible, even allowing for the so-called acid catalytic effects. A recent study that examines the composition of solids produced in several fuels and in variously treated fuels by pyrolysis FIMS15 reports that filterable solids isolated from fuels treated with a sulfonic acid or thiophenol are compositionally very different from those isolated from additive-free fuels. This evidence supports the work presented here. Experimental Section The fuel used in this study was an 80/20 (v/v) straight run/ light cycle oil blend we have previously studied. See, for example the tables of fuels used in refs 7, 11, and 12 for fuel 4. The blended stocks were prefiltered through two thicknesses of glass fiber filters as is standard in our laboratory (15) Hazlett, R. N.; Hardy, D. R.; Malhotra, R. Energy Fuels 1994, 8, 774-781.
Published 1996 by the American Chemical Society
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Table 1. List of Acronyms Applied to Fuel Solids acronym TIP EIP PEIP AIP
definition thermally induced precipitate: insoluble sediment produced during accelerated aging or stress test extraction induced precipitate: methanol-soluble, hexane-insoluble solid recovered from extraction of filtered, unstressed fuel (previously termed SMORS) postextraction induced precipitate: same as EIP except recovered from post stressed, filtered fuel acid-induced precipitate: solids recovered from filtration of samples doped with DBSA and filtered before the thermal stress procedure Table 2. Fuel Sample Treatments
sample
sample treatment
A B C D
control fuel sample: filtered, subjected to LPR stress, then filtered for TIP and the filtered, aged fuel extracted for PEIP filtered, extracted with methanol. EIP determined and extracted fuel placed in LPR for later TIP yield DBSA added at 1 mM concentration to filtered fuel. Sample was then stressed in LPR for TIP and PEIP determinations DBSA added at 1 mM concentration to filtered fuel. Sample was filtered within 30 min for AIP, then placed in LPR for later TIP and PEIP determinations Like D, except that the sample was allowed to stand for 5 h before being filtered for AIP. No LPR Same as D, except sample was extracted with methanol after filtration for AIP in order to recover EIP. Extracted sample was then stressed in LPR and TIP and PEIP subsequently determined Filtered fuel extracted with methanol for EIP. DBSA was then added and the unfiltered sample placed in the LPR. TIP and PEIP were subsequently determined Same as G, except the extracted portion was filtered for AIP after DBSA addition and before placement in the LPR DBSA was added to filtered fuel. The sample was stored for 24 h and then filtered for AIP. The sample was then placed in the LPR and TIP and PEIP determined
E F G H I
and with ASTM D5304 (Gelman A-E, 47 mm, nominal 1.5 µm) before being subjected to other treatment. In all cases, 100 mL aliquots were used. Those fuel samples treated with DBSA were made 1 mM in DBSA by adding an appropriate volume of a standardized solution in toluene solvent (final concentration in the fuel was 33 mg/100 mL). Extractions were performed using a procedure previously described in detail.12 The fuel/methanol volume ratio was, again, 100/40 mL. Following extraction, the methanol phase was rotoevaporated at 58-63 °C to remove methanol and the fuel extractables were precipitated with hexane (50 mL), filtered (MSI nylon 66 membrane filters, 47 mm, 0.8 µm), rinsed (hexane), dried, and weighed. Those aliquots subjected to accelerated aging were stressed in the low-pressure reactor (LPR) for 24 h at 90 °C using an oxygen overpressure of 690 kPa. The procedure is a modified version of ASTM D-5304 and has been described elsewhere.16 Samples that were filtered after treatments of any kind were passed through glass fiber filters (see above) to remove sediment. The sediment was rinsed well with hexane to remove excess fuel and the samples were oven dried. Adherent material was dissolved in a 1:1:1 (by volume) mixture of acetone, methanol, and toluene, transferred to aluminum pans, dried, and weighed. The mass obtained was added to the mass of the filterables to obtain total insoluble sediment (TIP). Table 1 provides a set of acronyms used to classify the solid substances isolated from fuel during the course of this study. Table 2 summarizes the treatments various fuel aliquots received.
Results and Discussion Overview. The results of all sample treatments described in Table 2 above are presented as a complete overview in Table 3. All results are in mg of solid per 100 mL of sample. Table 3 is further subdivided in subsequent sections below in order to more clearly discuss the data based on the definitions described in Table 1. These results provide some very interesting insights on the effect of strong acid addition to an unstable diesel fuel. Note, for the control sample, A, the fuel “fails” the 24 h LPR test on the basis of the TIP yield, i.e., >3 mg/ 100 mL. The EIP yield obtained from sample B predicts (16) Hardy, D. R.; Hazlett, R. N.; Beal, E. J.; Burnett, J. C. Energy Fuels 1989, 3, 20-24.
Table 3. Results of the Effects of Strong Acid on Solids Production in a Mid-Distillate Diesel Fuel Blend sample A B C D E F G H I
AIP
EIP 18.5
20.0 20.6 20.4
11.6 7.8 15.8
0.4 20.4
TIP
PEIP
7.5 0.2 32.8 17.2
14.7 3.7 27.3 25.5
1.2 15.0 12.0 13.0
4.1 28.8 27.8 19.0
Table 4. Effect of Adding 1 mM DBSA on Instantaneously Formed Precipitate (AIP) sample
conditions/description
mg of AIP/ 100 mL
A D E F I
control add DBSA/wait 30 min/filter add DBSA/wait 5 h/filter add DBSA/wait 30 min/filter add DBSA/wait 24 h/filter
0 20.0 20.6 20.4 20.4
the fuel will be unstable,13 and comparison of EIP with PEIP(pre- vs poststress extractables yields) indicates equilibrium between these two indicators of instability. This is consistent with the EIP/PEIP ratios obtained for other fuels that are greater than 6 months past production. Thus, we are dealing here with an aged, “bad”, blended fuel. Effects of Adding DBSA on AIP. Table 4 summarizes the AIP yields from several different samples that were filtered after addition of DBSA and before insertion into the LPR. The yields are extremely consistent from sample to sample. The remarkably large amounts of instantly precipitated solid shown in Table 4 should be contrasted with the amount of thermally induced precipitate, 7.5 mg/100 mL, from the 24 h LPR (at 100 psi of oxygen). In previous studies of this nature, samples were filtered before addition of various strong organic acids in this concentration range and not again until after the accelerated storage stability test regime. Effects of Adding DBSA on TIP. Table 5 deals with the effects of adding DBSA on insolubles formation during accelerated aging on TIP. Sample A was the
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Table 5. Effect of Adding DBSA on TIP sample
conditions/description
mg of TIP per 100 mL
A C D/I
control sample: no DBSA was added add DBSA/LPR/filter sample add DBSA/filter (AIP)/LPR/filter TIP
7.5 32.8 15.1a
a
Average of two results.
Table 6. Effect of Addition of 1 mM DBSA on Prestress Extractables (EIP) sample
conditions/description
B/G E/F
control: no DBSA was added DBSA was added and the samples filtered before extraction.
a
mg of EIP/ 100 mL 17.2a 9.7a
Average of two determinations.
Table 7. Effects of Adding 1 mM DBSA on Poststress Extractables Levels sample
conditions/description
mg of PEIP per 100 mL
A C D I
control: no DBSA/LPR/filter/extract add DBSA/LPR/filter/extract add DBSA/filter (30 m)/LPR/filter/extract add DBSA/filter (24 h)/LPR/filter/extract
14.7 27.3 25.5 19.0
control, a filtered fuel that fails the stress test. Sample C is an identical sample that was treated with DBSA to a concentration of 1 mM after filtration and then stressed and filtered. Previous workers have attributed this increase in fuel insolubles, in this case a 4-fold increase, to acid-induced catalysis of the fuel. Samples D and I were filtered after addition of DBSA and prior to insertion into the LPR. Note from these samples that much of the solid is removed by filtration and is, therefore, not related to fuel aging but is rather a precipitate associated only with addition of DBSA. This acid-induced precipitate, AIP, apparently forms instantaneously. The differences between TIP results for nontreated fuel and DBSA-treated and filtered fuel (samples A and D/I) are possibly attributable to the high concentration of strong acid remaining solubilized in the fuel that may react to form additional solid material as the fuel is subjected to the strongly oxidative conditions of accelerated aging. Effects of Adding DBSA on EIP. Table 6 summarizes the effect of added strong acid on the yield of prestress extractables. It is clear that, for the two samples treated with DBSA and filtered prior to extraction, the EIP level decreases. In light of other results obtained for test samples in this series, we propose that the strong acid affects the acidity and hence the chemistry of the system enough to reduce the extractables yield at this point. Effects of Adding DBSA on PEIP. Table 7 summarizes the effects of adding DBSA on PEIP, or poststress extractable insoluble material. Note that the extractables yields for all samples to which DBSA was added are increased over the control sample. The sample that showed the least amount of difference was I, the sample that was stored for 24 h after adding DBSA and before filtering to remove the instantaneously formed precipitate (AIP). Note too that the AIP yield for this sample was identical to the yield formed in other samples of this series irrespective of the interval between DBSA addition and filtering (Table 4).
Table 8. Further Correlations between DBSA Addition and Poststress Extractables Levels sample
conditions/description
mg of PEIP per 100 mL
C D G H
DBSA added/LPR/filter/extract DBSA added/filter/LPR/filter/extract extract/add DBSA/LPR/filter/extract extract/DBSA/filter/LPR/filter/extract
27.3 25.5 28.8 27.3
Additional correlating behavior found to exist between the addition of DBSA and the poststress extractables yields is summarized in Table 8. The yields obtained for poststress extractables among DBSA treated samples in this series are identical whether the DBSA was initially added to whole fuel (C, D) or to methanolextracted fuel (G, H). This is a particularly interesting finding in light of the fact that the relationship between fuel components linked to instability and their solubility in methanol has been clearly established.11 Thus, extraction with methanol removes those fuel components that lead to solids (and also PEIP) formation under conditions of accelerated aging. How, then, does one account for the identical PEIP yields between samples that have had the precursors to instability removed by methanol extraction and those that have not? A likely explanation is that when DBSA reacts in fuel it does not react with the components that lead to solids formation but rather with other compounds that are not implicated as precursors in storage instability. This would account for the development of solid extractables during accelerated aging in previously extracted samples. This explanation also accounts for the identical PEIP yields of samples C and D. That is, the DBSA remaining in the fuel during LPR aging reacts with fuel components other than the instability precursors. A polar, methanol-extractable substance is formed that is different from the usual PEIP. Effects on the Mass Balance of Fuel Insolubles After Addition of DBSA. We have previously shown that extraction with methanol removes those fuel components that are precursors to instability.8,11 Yet, when a previously extracted fuel sample G was treated with DBSA prior to LPR stress, the TIP, or insolubles yield was essentially the same (15 vs 17 mg) as that of sample D, which was an unextracted whole fuel to which DBSA had been added and which was then filtered for initial AIP. Compare these results to the TIP obtained from sample B (0.2 mg of TIP) which was extracted before stress and never treated with DBSA. Note also that the PEIP results for the DBSA-treated samples D and G are very similar to each other, and neither compares with the results of sample B or A. In fact, if a mass balance is done to determine the contribution of DBSA-induced insolubles to these fuels (based on the total mass of solid less the TIP and extractables contributions from the fuel itself), the DBSA yields are 40.4 and 37.4 mg/100 mL for D and G, respectively. It is also interesting to compare these figures with an estimate of the mass of DBSA itself in 100 mL of a 1 mM solution, about 33 mg. It is significant to note that all unextracted aliquots initially treated with DBSA (D, E, F, I) produce the same AIP yield upon filtration regardless of ambient storage time prior to filtration. Further, if the fuel insolubles’ contributions are subtracted from the total solids produced in these samples labeled C, H, and I in
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Table 3, one arrives at the DBSA insolubles figures of 37.8, 33.8, and 30.1 mg/100 mL for these samples. Thus, the average DBSA solids yield for all five of these samples is 35.9 ( 3.9 mg/100 mL, a number tantalizingly close to the mass of DBSA itself which was added to these samples. Samples E and F were each treated with DBSA and filtered for AIP. Each was then extracted and F was subsequently subjected to LPR treatment followed by testing to determine TIP and PEIP. The post-LPR results for this sample compare very closely with the post-LPR results for sample B, an untreated (with DBSA) extracted fuel. However, because extraction with methanol removes the DBSA as well as the insolubles precursors, there is no way to relate the “total DBSA insolubles” of sample F to those of the five samples discussed above. These five had not had the “inherent” (as opposed to the instantly precipitated AIP) DBSA solids removed prior to LPR stressing. Of course, sample E is excluded from this discussion because it was not subsequently stressed. Conclusions There are essentially three major points to be emphasized as a result of this work. The first is that not only are the insolubles from DBSA qualitatively differ-
Wechter and Hardy
ent from inherently derived fuel insolubles but the quantitative effect of DBSA is not related to the solids formed in fuels under conditions of accelerated aging, or actual long term storage. Clearly, addition of nonnaturally occurring strong acids dramatically affects the chemistry of solids formation. Thus, attempts to relate mid-distillate fuel storage instability to strong acid content should be viewed with suspicion. The second major point is that AIP is an important variable first appropriately addressed in this paper. Its determination must always be made in the case of studies which add acids or any other chemically reactive species in an attempt to model fuel instability chemistry. Thus, even the results of certain fuel additive studies should be interpreted only after taking into account this effect. Finally, the actual contribution of DBSA to solids production in fuel is related to the amount (or concentration) of added acid. If realistic concentrations of strong acids are added or formed in situ during oxidative aging, one usually finds about a micromole. This means that the insolubles due to any naturally induced strong acids would be less than 0.05 mg/100 mL, a negligible contribution. EF950167R
