Energy & Fuels 1993, 7, 127-132
127
Comparison of Stability Results for Distillate Fuels Exposed to Different Stress Regimes Robert N. Hazlett' 6770 Oak Hall Lane, Columbia, Maryland 21045
Erna J. Beal Navy Technology Center for Safety & Survivability, US.Naval Research Laboratory, Code 6180, Washington, D.C. 20375
Michael D. Klinkhammer Naval Surface Weapons Center, Code 2832, Annapolis, Maryland 21402
John A. Schreifels Geo-Centers, Inc., Ft. Washington, Maryland 20744 Received July 14, 1992. Revised Manuscript Received October 17, 1992 ~~
Two blends of 20% light cycle oil in straight-run stock were stressed at ambient (20 "C) conditions, at 43 "C (three different time periods), and with 791 kPa (100 psig) of oxygen overpressure. The latter procedure included testing at three conditions: 43 OC/3 weeks, 65 OC/4 days, and 90 "C/16 h. The yields of total insolubles in these various tests were reasonably consistent. The blends were doped with 0.001 M dodecylbenzenesulfonicacid and with 0.01 M p-tert-butylthiophenol. Deposits formed rapidly from the sulfonic acid doped blends, and the total insolubles were consistent for the various test regimes. One blend containing the thiophenol exhibited a predictable pattern for instability, but the second blend formed more insolubles at high temperatures than expected. On balance, the 90OC/l6 h/OOP stress regime affords good predictability for ambient storage conditions. This conclusion is supported by the data for the two undoped fuels and most of the information from the doped fuels. The insolubles, both adherent (AI) and filterable (FI),were characterized by several chemical analysis techniques. Field ionization mass spectrometry (FIMS) and X-ray photoelectron spectroscopy (XPS) showed that sulfonic acid was incorporated into the insolubles from the fuel doped with sulfonic acid. Nonaqueous titrations also demonstrated that strong acid was present in the sediments. Titration of the blends containing a substituted thiophenol found evidence for partial conversion to strong acid. FIMS analysis demonstrated that p-tert-butylbenzenesulfonicacid was present in the insoluble material thus showing that the multistep oxidation of a thiophenol can proceed under accelerated storage conditions. The thiophenol oxidation to sulfonic acid was a small fraction of overall thiophenol disappearance, and even this fraction was significantly smaller at low stress temperatures. The thiophenol appears to stimulate insolubles by two processes: acting as an acid catalyst after conversion to sulfonic acid and by a free-radical mechanism, possibly by addition of the thiyl radical to olefins. Introduction Storage stability of distillate fuels has been of modest concern for fuels made by refining processes based on straightrun (SR)distillation. However, increasing quantities of heavy crudes are being run in refineries using catalytic cracking processes to increase the yield of middle distillate fuels. The cracked light cycle oil (LCO), which contains chemically unstable species, is blended into straighbrun streams. The unstable components,although diluted by the blending, still exert a strong influence on deposit formation, particularly for long storage periods.' The fuel instability is manifested by the formation of
* Address correspondence to this author at 5205 Chippewa Place, Alexandria, VA 22312. (1) Offenhauer,R. D.;Brennan, J. A.; Miller,R. C. Sediment Formation in Catalytically Cracked Distillate Fuel Oils. Ind. Eng. Chem. 1967,49, 1265-1266.
0887-0624/93/2507-0127$04.00/0
insoluble products which play havoc with filtersand nozzles of engines.2 Determination of stability has been a problem with LCO blends.3 For instance, fresh blends frequently pass the required accelerated stability test method, ASTM D2274, at the refinery but form copious amounts of insolubles in US Navy operations. Severe filter blocking problems have resulted on Navy ships.' A new accelerated test method, (2) Haleall, R. Effecte of an Unstable Dieeel Fuel on Injector Coking and Vehicle Performance. In Proceedings of the 2nd Internotional Conference on Storage Stabilities ojLiquid Fuels; Stavinoha, L. L. Ed.; Southwest Research Institute: San Antonio, TX, 1986; pp 722-737. (3) Hardy, D. R.; Bed, E. J.; Hazlett, R. N.; Burnett, J. C. Evaluation of ChemicalStability Additives for Naval Distillate Fuel. In Proceedings of 3rd International Conference on Stability and Handling of Liquid Fuels: Hiley, R. W., Penfold, R. E., Pedley, J. F., Eds.; Institute of Petroleum: London, 1988; p 399. (4) Telex 1720422 from Navy Petroleum Office to Commander, US
Naval Sea Systems Command. "Quality Problems with Unstable Navy Diesel Fuel;Request for Investigation of"; April 1981.
Q 1993 American
Chemical Society
Hatlett et (1.1.
128 Energy & Fuels, Vol. 7, No.1, 1993
D5304, which has recently received ASTM approval, appears to give a much more relevant result for storage ~tability.~This latter test uses 791 kPa (100 psig) overpressureof oxygenat 90 "C for 16h and will be referred to in this paper as the oxygen overpressure method (OOP). The purpose of this paper is to compare stability results of the OOP method with other stability testa, particularly with those at ambient laboratory storage conditions. The OOP equipment was operated at the 90 "C/lS h condition as well as at 65 OC/4 days and 43 OC/3 weeks. In addition to the ambient storage tests, some atmospheric pressure testa were also conducted at 43 OC. The temperature/ time regimes were selected in the hope that the quantities of deposita would be similar. Experimental Techniques and Procedures Test Samples a n d Additives, Products from two US Gulf Coast refineries (GC-A and GC-B) were utilized in this test program. LCO and SR cuts from the same refinery, A or B, were blended in 20/80 LCO/SR ratios by volume. The A and B blends were tested at various stress conditions as were the blends to which sulfur compounds had been added. The two additives used were dodecylbenzenesulfonic acid (DBSA) with a nominal molecular weight of 326 andp-tert-butylthiophenol(PBTP). The DBSA was a crude extract from a refinery process. The main components found by field ionization mass spectrometry (FIMS) were dodecyl-,undecyl-, and decylbenzenesulfonicacids. These three acids would behave similarly in the stress tests. This crude material also contained about 10% of material corresponding to the above acids minus 2 Da and 5% of substituted benzenes, dodecyl, undecyl, and decyl. These contaminants should behave the same as the major components or be inert in the case of the substituted benzenes. The PBTP, obtained from Lancaster Synthesis, Inc., was 97% pure and the dimer was the main impurity by FIMS analysis. The additives were prepared as stock solutions in toluene and diluted with the fuel blends on the same day that testing began. The test concentrations for the additive containing blends were 0.001 M for DBSA and 0.01 M for PBTP. Storage Stability Test Methods. The test samples were stressed at a variety of conditions. Included were ambient laboratory storage (at 20 "C) for 35 weeks (only 10 or 20 weeks for additive-containing fuels), and 43 "C stress for 2, 6, and 10 weeks. Additional stress environments utilized an oxygen overpressure (OOP) of 791 kPa (100 psig) p r e ~ s u r e . ~ Stress conditions for the elevated pressure experiments were 43 "C for 3 weeks, 65 "C for 4 days, and 90 "C for 16 h. The volumes of fuel utilized were 100 mL for the OOP procedure and 250 mL for the 43 OC/atmospheric pressure tests. The ambient storage conditions utilized 1800 mL for additive-free fuels and 350 or 450 mL for additive-containing blends. The workup of the samples followed the OOP procedure or other previously described techniques.6 Tests were conducted in triplicate with the oxygen overpressure techniques and in duplicate with the atmospheric pressure procedures at 43 "C. In addition, the undoped blends were stressed in two different laboratories at all stress regimes except ambient. The ambient temperature tests were single determinations. The filterable (FI) and adherent (AI) insolubles were determined separately but then combined to obtain the total insolubles (TI). The latter quantity, as milligrams of insolubles per 100mL of fuel, is the value reported in the Experimental Results section of this paper. Glassfiber fiiters were utilized in these experiments in order to minimize interference from organic contaminants in the analytical work described below. ( 5 ) Hardy, D. R.; Hazlett, R. N.; Beal, E. J.; Bmnett, J. C. Assessing
Distillate Fuel Storage Stability by Oxygen Overpressure. Energy Fuels, 1989, 3, 20-24. (6) Hazlett, R. N.;
Schreifels, J. A.; Stalick, W. M.; Morris, R. E.; Mushrush, G.W. Distillate Fuel Indublee: Formation Conditions and Characterization. Energy Fuels 1991,5, 269-273.
Chemical Analysis Techniques. Streseed and unstressed fuel blends and the insolubles were titrated separately for acid content by using approximately 0.04 N alcoholic KOH with a nonaqueous solvent. The titrations were conducted in an automatic titrator with pontentiometric readout. The titration solvent,toluene/isopropylalcohoYwater 500/495/5 (ASTM D664), solvated the insolubles as well as the fuels. The full titration curve was obtained, and the quantities of strong acid and weak acid were derived from interpretation of the curves and data printouts. For the purpose of this paper, strong acids are equivalent to organicsulfonicacids and weak-acids are equivalent to a carboxylic acid such as decanoic. The thiophenol titrates with the weak acids in this nonaqueous solvent. However, it reacts rapidly with LCO components, even a t room temperature, and a satisfactory titration was not always possible for PBTPcontaining samples. Undoped GC-A contained 0.98 mmol/L of weak acid (carboxylic)before strese,and thistype of acid increased only slightly for all stress environments when undoped. The corresponding value for GC-B was 3.14 mmol/L before stress, and it also exhibited only a slight increase during stress. Instrumental Analysis. Deposits for the various stress environments were compared by examination with two instrumental techniques. Pyrolysis/fieldionization mass spectrometry (FIMS) defined the molecular weight (MW) pattern of the insolubles. The apparatus and procedure have been described previously' as have some typical analyses for fuel insolubles.8 SRI International conducted these analyses. The second technique examined the deposits by X-ray photoelectron spectroscopy (XPS). The results from this latter instrument afford an elemental analysis but, in addition, some information on the valence state of the elements is obtained.6 The valence state information on sulfur is particularlypertinent to this study. XPS results give good qualitative information but are not completely quantitative particularly below 1 atom %. Samples for XPS examination were prepared by dissolving a portion of the fuel insolubles in a mixture of methanol and toluene, spreading a few drops on a carefully prepared metal coupon, and allowing the mixed solvent to evaporate. The XPS analyses were performed a t George Mason University.
Experimental Results Insolubles Production. The tabulated data for the various testa are listed in Table I. Observations on fuel blends GC-A and GC-B are as follows: (a) The amount of deposit increases, although not linearly, with length of stress for the 43 OC/atm pressure experiments as can be seen in Figure 1; the rate of deposition is greatest in the first 2 weeks of stress; (b) 10 weeks at 43 OC/atmosphericpressure matches 35 weeks at ambient temperature; (c) GC-A would be defined as moderately stable by all testa but GC-B would be rated marginal by most testa; (d) the 43 "C/OOP test affords only a slight acceleration (two or less) versus the 43 "C/ atmospheric pressure test; (e) the 65 OC OOP test affords a milder stress than the other stress regimes; and ( f ) the agreement between two differentlaboratorieswas excellent except for the GC-A fuel at 43 OCIatmospheric pressure. A comparison of five of the test regimes is given in Figure 2 for the additive-free fuels. The ambient135 week, 43 OC/lOweek/atmosphericpressure,andthe90 OC/16h/OOP (7) Malhotra, R.; Hazlett, R. N. Field Ionization Mass Spectrometric Analysis of Sedimenta from Diesela Doped with Strong Acids. Prepr.
Pap.-Am. Chem. Sac., Diu. Fuel Chem. 1990,35, 1163-1167. (8)Malhotra, R.; Hazlett, R. N. Field Ionization Mass Spectrometric
Analysisof Sediments: Chemietry of InsolubleaFormation. Proceedings of thelth International Conference on Stabilityand Handling of Liquid Fuels, Orlando, FL; 1992; pp 518-528.
Energy & Fuels, Vol. 7, No. 1, 1993 129
Stability Re8dt8 for Distillate Fuels
Table I. Dirtillate Fuel Instability* total insolubles strm conditions Gulf Coast-A Gulf C0a~t-B GC-A + DBSA GC-B + DBSA GC-A + PBTP GC-B + PBTP ambient, 35 wk 1.42 3.84 ambient, 10 wk 17.4 25.8 6.4 14.8 18.1 ambient, 20 wk 21.0 31.8 7.9 9.59 43 "C, 2 wk 0.42.2.3 1.45,1.7 14.1 22.8 5.7 12.11 43 "C, 6 wk 1.01,2.0 3.07,2.6 21.2 29.0 7.08 13.07 43 O C , 10 wk 1.44,2.1 3.94,4.3 20.3 34.0 8.06 9.01 43 "C, 791 kPa, 3 wk 0.75,0.8 2.7,3.5 21.5 30.8 5.99 65 "C, 791 Wa, 4 days 0.49,0.7 2.8,2.6 16.9 29.9 13.36 11.98 90 "C, 791 kPa, 16 h 0.88,l.O 3.77,3.6 18.2 30.6 35.52 15.62 a Data are in milligrams of total insolubles/100 mL of fuel. Values are averagee of multiple tests except for ambient teste. T w o values are average values for two different labs. Initial concentrations: DBSA, 1.00 mmoVL; PBTP, 10.0 mmoVL. TOTAL INSOLUBLES (md100 mL)
++
OULF COAST-A
>TAL INSOLUBLE5 (mg1100 mL) ___ _
40
QC-A
OULF COAST-8 1-33-
4
GC-A
DESA
~
--__
--
-+ QC-B
OESA
I
j
PETP
30
20
10
0 2
0
6
4
8
10
2
0
WEEKS OF STRESS AT 4 3 C
4
6
8
10
WEEKS OF STRESS AT 43 C
Figure 1. Total insolublesformed from stressingtwo Gulf Coast distillate fuels at 43 "C. 6
Figure 3. Totalinsolubles formedfrom stressingtwo Gulf Coast fuels doped with sulfurcompounds: dodecylbenzenesulfonicacid (DBSA) and p-tert-butylthiophenol (PBTP).
3
isc
Amblont 35 wk
43*C/10 wk
OOP
OOP
4VC13 wk
6S°C/4 day8 SO'C/16 hr
OOP
TEST REGIME
Figure 2. Comparison of storage stability results for two Gul Coast fuels stressed at different conditions.
AMBIENT 20 wk
43'CIlO wk
OOP
OOP
OOP
43W3 wk
W C / 4 day
SO'C116 hr
TEST REGIME
Figure 4. Comparison of storage stability regimes for two fuels containing dodecylbenzeneeulfonic acid (DBSA).
testa exhibited excellent agreement for both fuels. The two other OOP test regimes gave lower amounta of total insolubles with the 65 OC/4 day test giving the lowest amount& The results of stress testa for samples containing the sulfur compounds are included in Table I. The following comments address these resulte: (a) Both additives, DBSA and PBTP, significantly enhancedeposit formation; (b)DBSA, with one exception (GC-A at 90 '(316 h/OOP), is much more active than PBTP although ita added concentration is only 0.1 as much; (c) the time data in Figure 3 for 43 "C streas indicates that both additives are very active in the first 2 weeks of streas period and that the bulk of the ineolublesare formed in less than 6 weeks; ambient data at 10and 20 weeks also
demonstrate more rapid insolubles formation in initial stages of reaction; (d) GC-B was always worse than GC-A if DBSA was present; (e) the two fuels responded differently to PBTP with blend GC-A exhibiting more effect at the higher temperature stress regimes and less at ambient and 43 "C conditions; and (f) the 43 OC/OOP test affords only a slight acceleration in rate (two or lese) versus the 43 OC/atmosphericpressure stress. The comparison for five of the test regimes is illustrated in Figure 4 for the DBSA-containingsamples. The agreement between testa is good across the board. Similar data are presented for the PBTP samplesin Figure 5. The two Gulf Coast blende respond differently to the stress regimes in the presence of the thiol. GC-B with PBTP responded about the same
Hazlett et al.
130 Energy & Fuels, Vol. 7, No.1, 1993 BGULF COAST-A+PBTP WGULF
Amblant 20 wk
43'C/10 wk
OOP 43OC/3 wk
COAST-B+PBTP
b
OOP OOP 6 5 W 4 days 90°C/16 hr
TEST REGIME
Figure 5. Comparison of storage stability regimes for two fuels containing p -tert-butylthiophenol.
to the five regimes shown in the bar graph, although the 43 OC/3 week/OOP environment was definitely the least stressful. GC-A with PBTP exhibited a uniform pattern at the ambient and 43 OC temperatures but produced significantly more insolubles at higher temperatures, particularly for the 90 OC/16 h/OOP stress. Nonaqueous Titrations. The listing of strong acid concentration in fuel samples after stress and filtration is presented in part A of Table 11. The amount of DBSA found in the stressed fuels was always less than the added amount of 1.0 mmol/L. GC-A and GC-B contained about the same amount of DBSA for corresponding stress environmenta. The 43 OC 6 and 10 week testa gave the lowest concentrations, 0.62 and 0.61 mmol/L, but all data fell in the narrow range of 0.61-0.76 mmol/L. Some titratable strong acid was found in the total insolubles from GC-A + DBSA and GC-B + DBSA stress testa. (Note: insolubles samples from ambient storage testa were not available for analysis.) As seen in part B of Table 11,the amount was about the same for both blends and amounted to 8-15 % of the added DBSA. No trends were evident between the amount of strong acid in the solidsand the stressregimes. Titratable DBSA comprised 15-20% of the insoluble material. The material balance for titratable strong acid showed that the stressed fuel plus insolubules contained only 7286 % of the added DBSA, amounta similar to those found in previous studies.6 However,unstressed fuels exhibited this deficit in strong acid, indicating the fuel contains componenta which react rapidly with DBSA to partially nullify ita acidity. Some conversion of PBTP to strong acid @-tert-butylbenzenesulfonicacid)was observed in the stressed,filtered fuels for the testa at elevated oxygen pressure (Table 11). This has been found in earlier experimenta at 80 "C/ ambient pressure? Only GC-A exhibited significant (as evidenced by titration) conversion, but the amount of the added 10.0 mmol/L which was converted to strong acid never exceed 4%. This finding was consistent with other results for fuel GC-A and for an Australian 30% LCO blendas The insolubles product from the GC-A + PBTP test at the most severe stress, 90 OC/16 h/791 kPa, was the only one that contained titratable strong acid, the equivalent of 0.23 mmol/L. Ineolubles from all other PBTP experimenta contained no indication of strong acid by nonaqueous titration. Titratable strong acid has been
observed in other experimentswith thiols added at higher concentration.1-6*9 The titration data for weak acid concentrations in the fiitered fuels is listed in Table 111. Undoped GC-A exhibited little change for any stress regime. Undoped GC-B experienced definite, but minor, increases in weak acid concentration with the greatest increase (0.15 m o l / L) found for the 90 OC/l6 h/OOP stress. The samples doped with DBSA demonstrated slight increases in weak acid concentration, a maximum of 0.18 mmoVL. The PBTP-containing fuels produced some weak acid during stress, particularly at the elevated temperature regimes. The quantitation on the increases is uncertain since the added thiophenol titrates with the carboxylic acids. This is indicated in the data for the unstressed control samples, up to 8.20 mmol/L for GC-B + PBTP. The reactivity of PBTP, however, makes for an uncertain baseline. It is considered that the minimums shown (1.20 and 3.20 mmol/L, respectively, for GC-A + PBTP and GC-B + PBTP at 20 week/ambient stress are reliable indications of the carboxylic acid content of unstressed samples. On this basis, the maximum production of weak (carboxylic) acid with PBTP added was 0.75 and 0.44 mmol/L, respectively, for GC-A and GC-B, at 90 OC/16 h/OOP conditions. The picture for weak acids found in the insolubles is tabulated in Table IV. The undoped samples contained insignificant amounts of acids at all stress regimes. Insolublesfrom doped samplescontained someweak acids, most noticeable under the OOP conditions. Sample GC-A + PBTP at 90 OC exhibited the greatest amount, 0.30 mmol/L, an amount only 30% of the initial weak acid concentration in unstressed GC-A fuel. Field Ionization Mass Spectrometry Analyses. Figure 6 compares the mass spectra of insolubles from fuel GC-B at two different stress regimes. It can be seen that the spectra for a 35-week ambient stress and a 90 OC/l6h/OOP stress exhibit similar patterns. Bothspectra have four envelopes of masses centering at about 190-200, 330-350,480-500, and 580-600 Da. The major peaks in both spectra can be assigned to indoles, carbazoles, and probably the condensation products from a phenalene with two indoles (466 and 480 Da). Additional characteristics for the insolubleafrom this fuel and for three other samples are listed in Table V. The weight average MW's for the ambient samples and the 90 OC/OOP samples are comparable for GC-B, GC-B + DBSA, and GC-B + PBTP. Sample GC-A + PBTP, however, produced amuch higher weight average MW at the higher temperature stress. Fuels containing added DBSA exhibit major peaks at 312 and 326 Da as well as a smaller peak at 298 Da. This results from the fact that this additive is not of high purity and contains decyl- and undecyl- as well as dodecylbenzenesulfonic acid. Samples to which PBTP was added always gave a mass peak, frequently the highest one, at 214 Da. This correspondsto p-tert-butylbenzenesulfonic acid, an oxidation product of PBTP. FIMS analysis is a more sensitive technique to find strong acid than the nonaqueous titrations described above. Other evidences of PBTP participation in the insolubles formations are peaks in the spectra at 166 (PBTP) and 330 Da (the disulfide). The spectra for GC-A + PBTP at 90 OC/OOP are noteworthy for the very low amounta of indoles and (9) Hplett, R. N. Acid Base Phenomena in Distillate Fuel Stability. Fuel Set. Technol. Int. 1988, 6, 185-208.
Stability Results for Distillate Fuels
Energy & Fuels, Vol. 7, No. 1,1993 131
Table 11. (A) Strong Acid Concentration in Stressed Filtered Fuels;. (B)Strong Acid Concentrations in Inaolubler** A B GC-A + GC-B + GC-A + GC-B + GC-A + GC-B + GC-A + GC-B + strese conditions DBSA DBSA PBTP PBTP DBSA DBSA PBTP PBTP ambient 10 wk 0.75 0.74 0 0 NS NS NS NS 20 wk 0.70 0.69 0 0 NS NS NS NS 43 "C 2 wk 0.70 0.69 0 0 0.10 0.10 0 0 0.62 0 0 6 wk 0.62 0.12 0.15 0 0 0.61 0.61 0 0 10 wk 0.12 0.15 0 0 43 OC 0.72 0.67 0.06 3 wk/OOP 0 0.08 0.13 0 0 65 "C 0.66 4 d/OOP 0.76 0.18 0 0.10 0.13 0 0 90 "C 0.67 16 h/OOP 0.70 0.20 0 0.13 0.15 0.23 0 0 Concentrations: DBSA, 1.0 mmol/L added. PBTP, 10.0 mmol/L added. Strong acid, mmol/L by nonaqueous titration; NS, no sample available for titration. b Acid concentration in insolubles referenced to volume of fuel used in test. Table 111. Weak Acid Concentrations in Stressed Filtered Fuels. stress GC-A+ GC-B+ GC-A+ GC-B+ conditions GC-A GC-B DBSA DBSA PBTP PBTP none ambient 10 wk 20 wk 36 wk 43 *c 2 wk 6 wk 10 wk 43 o c
3 WWOOP 65 OC 4 d/OOP 90 "C 16 WOOP
0.98
3.14
1.08
3.20
1.31-3.38* 3.29-8.206
NS NS
NS NS
1.02 1.07
3.15 3.30
1.29 1.20
3.33 3.20
0.95
3.26
NS
NS
NS
NS
0.97 0.99 0.98
3.18 3.16 3.20
1.10 1.12 1.13
3.29 3.35 3.33
1.43 1.51 1.46
3.41 3.59 3.58
0.97
3.23
1.20
3.26
1.45
3.35
0.97
3.26
1.15
3.38
1.62
3.48
1.04
3.29
1.25
3.38
1.95
3.64
1337
1111,
i
a
-
c 0
v
1CC
2CC
2OC
ICC
2cc
scc
40C
500
600
7CC
8CC
5dc
6dc
~ C Z
ecc
a Concentrations: DBSA, 1.0mmol/L added. PBTP, 10.0mmol/L added. Weak acid, mmol/L by nonaqueous titration. NS, no stress test. b PBTP titrates as weak acid but gives erratic results since it reacts rapidly when added to fuel.
Table IV. Weak Acid Concentration in Insolubles. stress GC-A+ GC-B+ GC-A+ GC-B+ conditions GC-A GC-B DBSA DBSA PBTP PBTP ambient NS NS NS NS NS NS 43 "C 2 wk 0.01 0.01 0.05 0.11 0.02 0.06 6 wk 0.01 0.01 0.07 0.13 0.05 0.09 10 wk 0.01 0.02 0.10 0.14 0.08 0.06 43 "C 0.16 0.26 0.07 3 wk/OOP 0.02 0.03 0.07 65 OC 0.23 0.15 0.10 4d/OOP 0.01 0.01 0.12 90 "C 0.23 0.30 16 h/OOP 0.03 0.04 0.17 0.15 a Concentration: DBSA, 1.0mmol/L added. PBTP, 10.0mmol/L added. Weak acid, mmol/L by nonaqueous titration. NS, nosample available for titration. Acid concentration in insolubles referenced to volume of fuel used in test.
carbazoles (ca. 10 and 25-50%, respectively, vs ambient test). A noticeable increase in the series of peaks at 334, 348, and 362 Da was found for this stressed sample. Further discussion of the FIMS data will be presented in a later paper. X-ray Photoelectron Spectroscopy. With the exception of hydrogen,this analysisaffordssemiquantitative information on the elements-carbon, oxygen, nitrogen, and sulfur-typically found in organic molecules. In addition, information about the valence state of the element can be obtained. For this paper, sulfur valence
4cc YS55
lYlZi
Figure 6. FIMS analysis of insolubles from fuel Gulf Coast B: (I) insolubles formed at ambient conditions in 35 weeks; (11) insolubles formed at 90 "C in 16 h with 791 kPa of oxygen overpressure.
is of particular interest. The first set of data in Table VI compares the XPS data for the filterable and adherent insolubles from fuel GC-A. The data are fairly consistent for what should be very similar samples. The second set of data in Table VI includes data for the filterable insolubles from two similar samples, GC-B + DBSA stressed at ambient conditions for 10and 20 weeks, respectively. The patterns are reasonable except for the sulfur analysis. This same sample was stressed at 90 "C with oxygen overpressure, and this data compares more favorably with the ambient 10 week stress result. In this latter comparison, the reduced sulfur is very low and the oxidized sulfur is predominant for both samples. The third set of data in Table VI compares the analyses for insolubles for GC-A + PBTP at two different stress regimes, ambient/lO week vs 90 OW6 h/OOP. The main differencein this set is the higher amount of oxidizedsulfur for the higher temperature stress.
132 Energy & Fuels, Vol. 7, No. 1, 1993
Huzlett et al.
Table V. Comparison of Inlolubles by FIMS Analysis
FIMS data wt
~~~
fuel GC-B
additive none
GC-B
DBSA
GC-B
PBTP
GC-A
PBTP
stress conditions ambient/35 wk 90 OC/16 h/OOP ambient/%Owk 90 "C/l6 h/OOP ambient120 wk 90 OC/16 h/OOP ambient/20 wk 90 OC/16 h/OOP
avMW 483 535 501 538 399 373 365 463
major peaks 145,480,466,159,195,131 145,466,480,195,452,159 145,159,131,312,195,326 145,312,159,326,131,195 214,145,131,159,275,195 145,159,131,195,214,166 214,145,195,185,159,166 214,348,362,185,351,337
Table VI. Comparison of Insolubles by XPS Analysis atom percent fuel additive stress conditions C 0 N S (reduced) 2.9 0.5 GC-A(AI)O none ambient/35 wk 87.9 8.2 ambient/35 wk 89.2 8.2 1.7 0.2 GC-B DBSA ambient/lO wk 86.8 8.7 2.6 0.0 ambient/20 wk 88.9 7.0 2.9 0.9 90 "C/l6 h/OOP 87.9 7.4 3.1 0.3 GC-A PBTP ambient/lO wk 93.0 5.0 0.9 0.4 90 OC/16 h/OOP 90.0 6.8 1.1 0.3 a AI: Adherent insolubles; other samples are filterable insolubles.
The XPS data shown in Table VI plus much other data not shown indicatethat the repeatabilityfor these analyses is only fair. Thus, a reliable comparison between the composition of insolubles formed at various stress conditions is not currently feasible. Residues from evaporation of the mixed methanoVtoluenesolventare probably not uniform in composition. Some current experiments are addressing the homogeneity of the sample spread on a metal coupon by evaporation from a single solvent. Nevertheless, a useful conclusion can be drawn for the sulfur valence state in these insolubles from storage tests. For all of the insolubles from fuels without added sulfur compound, the reduced and oxidized sulfur averaged 0.3 and 0.9%,respectively. The same data for fuels to which DBSA had been added gave average amounts of 0.4 and 1.6% for S(reduced) and S(oxidized) and the fuels with PBTP added exhibited averages of 0.5% (reduced) and 1.3% (oxidized). This demonstrates, as expected, that the DBSA produces a product elevated in oxidized sulfur. Further, it appears that the thiophenol is oxidized and the oxidation products participate directly in insolubles formation. Summary and Conclusions
Storage stability results have been compared over a wide range of stress regimes using two 20% LCO fuels with and without added sulfur compounds. Comparisonswere made
no. of mass envelopes 4 4 4 4 4 4
3 3
S (oxidized) 0.5 0.6 1.9 0.3 1.4 0.7 1.8
between the quantities of insolubles formed and the chemical characteristics of the insolubles. On the basis of the total amount of insolubles, the acid content, and the FIMS patterns of the insolubles, the undoped fuels afforded similar stresses at the various stress conditions. In particular, the products from ambient (20 OC)/35week, h/791 kPaaffqrdedanalogous 43 OC/lOweek, and 90 results. All stress regimes gave products for DBSA doped fuels which were very similar quantitatively and qualitatively. Gulf Coast-B fuel with PBTP added afforded similar products from all stress regimes but the ambient/2Oweek and 90 OC/16 h/OOP stresseswere particularly comparable. Gulf Coast-A fuel with added PBTP exhibited unusual behavior at high temperature stress. This was most apparent at 90 OC/16 h/OOP where the amount of insolubles was 3-5 times higher than found at other stress regimes. The strong acid content defined by nonaqueous titration and the FIMS pattern also indicated the insolubles from the 90 OC/l6 h/OOP stress were different from the other insolubles for this doped fuel. On balance, the 90 OC/16 h/OOP stress regime affords good predictibility for ambient storage conditions. This conclusion is supported by the data for the two undoped fuels and most of the information from the doped fuels. Acknowledgment. The authors thank Keith Flohr of Artech Corp. for performing some of the stress testa.
