Energy & Fuels 1991,5,269-273
269
Distillate Fuel Insolubles: Formation Conditions and Characterization Robert N. Hazlett* Hughes Associates, Inc., 2730 University Blvd., Wheaton, Maryland 20902
John A. Schreifelst and Wayne M. Stalick Chemistry Department, George Mason University, Fairfax, Virginia 22030
Robert E.Morris and George W. Mushrush Naval Research Laboratory, Code 6180, Washington, D.C. 20375 Received November 19, 1990. Revised Manuscript Received January 26, 1991
Organic acids promote the formation of insolubles in middle distillate fuels, particularly in blends containing cracked stock. Studies with a sulfonic acid demonstrate that this class of acids has a large effect and that it participates in two roles. First, it acts as an acid catalyst on the basis of the hydrogen ion concentration. In the other mode, sulfonic acid participates directly in insolubles formation, probably by salt formation with organic bases in the fuel. This latter suggestion is supported by the findings for nonaqueous titrations for strong acids and findings of XPS and mass spectral analyses. Thiols also increase insolubles formation. A part of the thiol oxidizes to sulfonic acid, which can act in two roles: (1)as an acid catalyst for insolubles producing reactions and (2) as a direct participant in insolubles formation. The mechanism of interaction of chloroacetic acid has not been defined.
Introduction Storage stability of distillate fuels has been of modest concern for fuels made by refining processes based on straight run 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 straight run 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 insoluble products which play havoc with filters and nozzles of engines. Various means of dealing with this instability have been proposed. One suggestion, to use only straight run fuels, limits product availability and a second suggestion, to use fuel shortly after production, reduces operational flexibility. Another suggestion, hydrotreating of LCO to remove polar compounds and olefins, involves additional cost. Additives have been shown to be useful but do not exhibit effectiveness in all fuels.' Oxidation is involved in the undesirable reactions as are fuel components containing heteroatoms such as sulfur, nitrogen, and oxygen. More than free-radical autoxidation is involved, however, since typical free-radical antioxidants are ineffective in controlling this instability and can be deleterious.' On the other hand, tertiary amines are effective stabilizers for many fuels containing LCO. Further, deposits are increased by the presence of organic acids with the stronger acids exerting the most significant effe~ts.~B Amines can counteract the effects of acids. Insolubles formation is also encouraged by increased partial pressure of oxygen.' This is in contrast to the typical autoxidation
* Address correspondence to this author at 5205 Chippewa Pl., Alexandria, VA 22312. A portion of XPS work was conducted at GeoCenters, Inc., 10903 Indian Head Hwy., Ft. Washington, MD 20744. 0887-0624/91/2505-0269$02.50/0
mechanism of hydrocarbons in which case the propagation step for R' O2is very fast and independent of the oxygen partial p r e ~ s u r e . ~ In this paper, our purpose was to examine the effects of temperature, time, acid concentration, and acid strength on the amount and composition of insolubles. The composition was defined by use of several experimental techniques as an aid in understanding the chemical processes which influence instability. Various fuel blends of straight run and light cycle oil (SR/LCO) served as base fluids and different acids were tested for their influence on insolubles formation.
+
Experimental Techniques and Procedures Products from three refiieries-US West Coast, US Gulf Coast, and Australia-were utilized in this experimental program. LCO and SR cuts from the same refinery were blended in 20180 or 30/70 LCO/SR ratios. A sample of the 20180 Gulf Coast blend was stored a t room temperature in the dark for 6 months. This and other blends were stored under accelerated conditions of 80 "C for periods between 2 and 14 days. These time periods correspond to ambient storage conditions between 15 and 104 weeks. Suspended instability products were removed by filtration with matched pair 0.8-pm membrane filters or glass fiber filters. The flask and filter were rinsed with ACS Certified petroleum ether, n-hexane, or an equivalent aliphatic solvent. Products adhering to the stressing flask were removed with gum solvent (equal volumes of 2-propanol, toluene, and acetone). After evaporation of the gum solvent, the filterable (FI) and adherent insolubles (AI) were dried in a vacuum oven at 75 OC. Total insolubles (TI), the instability criterion of most significance, is obtained by (1)Hazlett, R. N.; Hardy, D. R.; White, E. W.; Jones-Baer, L. Assessment of Storage Stability Additives for Naval Distillate Fuel. SAE Tech. Paper 851231,May 20-23,1985. (2) Hazlett, R. N. Fuel Sci. Technol. Int. 1988, 6, 185. (3) Hazlett, R. N.; Power, A. J.; Kelso, A. G.; Solly, R. K. "The Chemistry of Deposit Formation in Distillate Fuels". Report No.M R L R-986,Materials Research Laboratory, Melbourne, Australia,Jan. 1986. (4)Hardy, D. R.;Hazlett, R. N.; Bed, E. J.; Burnett, J. C. Energy
Fuels 1989,3,20. (5)Walling, C. Autoxidations. In Free Radicals in Solution; Wiley: New York, 1957;Chapter 9,pp 397-466.
0 1991 American Chemical Society
Hazlett et al.
270 Energy & Fuels, Vol. 5, No. 2, 1991
':: 60
(3 1 /
90
-
60 -
I
DBSA CONCENTRATION:
A
100
70
-
60 -
UNDOPEDFUEL
40
30
50
-
40
-
30 -
0
2
4 6 8 10 TIME ELAPSED (days at 6OoC)
12
14
Figure 1. Total insolubles in DBSA-doped fuel, West Coast Blend 30/7OLCO/SR, concentration in mol/L. summing FI and AI in units of mg/100 mL. The procedure followed ASTM Method D4625 except for the specific items mentioned above. Duplicate tests were conducted for each stress regime. The duplicates agreed within 10% and,averagevalues are used in the tables and figures. The acids used as dopants were dodecylbenzenesulfonic acid (DBSA),a very strong acid; chloroacetic acid (CA), a medium strength acid; and two thiophenols, very weak acids. Thiophenols used were thiophenol (TP) and p-tert-butylthiophenol (PBTP). Elemental analyses of the insolubles were done by Galbraith Laboratories, Knoxville,TN. Stressed and unstressed fuel blends and insolubles were titrated for acid content by using a nonaqueous solvent with approximately0.04 N alcoholic KOH. The titrations were conducted in an automatic titrater with potentiometric readout. The titration solvent, toluene/isopropyl alcohol/water 500/495/5 (ASTM D664 solvent), solvated the insolubles. Soaking the filter and/or evaporating dish containing the insolubles, FI, AI, or both, for 10 min in 210 mL of titration solvent was the normal procedure. The repeatability of the titrations was 0.02 mmol/L. The aliphatic rinse solvent was analyzed in a few experiments but only negligible amounts of acid were found. X-ray photoelectron spectroscopy (XPS) analysis was conducted at the Naval Research Laboratory, Washington, DC,with a Surface Science Instruments SSX-100-03spectrometer and field ionization mass spectrometry (FIMS) was performed at SRI International,Menlo Park, CA.
Experimental Results Insolubles Formation. The strong effect of the sulfonic acid, DBSA, on insolubles formation is illustrated in Figure 1for an 80 O C stress. The amount of TI for this 30/70 West Coast blend increases with the concentration of the added acid. The amount also increases with time, forming significant amounts of product in 2 days and attaining a plateau in about 5 days. The relationship of TI versus added [DBSA] was not linear since the acid was less effective per mole at higher concentrations. A plot of TI versus the square root of [DBSA], however, afforded a more linear response (Figure 2). The slopes of the curves are similar for different time periods. The nonlinearity at low concentrations may be due to neutralization of the acid by natural bases in the fuel. A second fuel blend, 20/80 LCO/SR Gulf Coast, formed much less insolubles than the West Coast 30/70 blend. The total insolubles are plotted two ways in Figure 3, versus [DBSA] and versus the square root of [DBSA]. Again, the latter relationship is more linear. Insolubles data for chloroacetic acid (CA) and p-tertbutylthiophenol (PBTP),when added to the 20/80 Gulf Coast blend, are listed in Table I for a 14-day stress a t 80 OC. These acids are much less effective than DBSA on a molar basis. Increasing the CA concentration raises the
"0
0.2 0.4 0.6 0.6 1.o SQUARE ROOT OF DBSA CONCENTRATION(mmols/L)"
Figure 2. Total insolubles in DBSA-doped fuel stressed at 80 "C,West Coast Blend 30/70:LCO/SR. [DBSA] ADDED (mmollL)
24 I
I
I
I
I
I
I
I
I
I
I
1
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f 8
2
2016
-
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8-
A
0
I
I
0.1
0.2
I
0.6 0.5 0.6 0.7 SQUARE ROOT [DBSAJADDED (mmol/Lfh 0.3
0.4
I
0.9
1.0
Figure 3. Total insolubles in DBSA-doped fuel stressed for 7 days at 80 "C, Gulf Coast Blend 20/8OLCO/SR. Table I. Increase in Insolubles Formation Due to Acids' added chloroadded total acetic acid, PBTP, insolubles, sample mmol/L designtn mmol/L md100 mL K M
Q L R
none 3 6 10 none
none none none none 30
2.9 7.2 7.6 11.0 51.3
"Fuel: Gulf Coast Blend, 20% LCO in SR. Stress, 80 "C, 14 days.
amount of insolubles but the relationship does not appear to be linear. No conclusion on concentration effects can be made for PBTP since only one concentration was used. Titration Curves. Nonaqueous titrations of the stressed and unstressed liquid fuels give useful information on the disposition of the added acids. Typical curves are shown for the sulfonic acid in Figure 4 and for thiophenol in Figure 5. The titrations in Figures 4 and 5 were done on different instruments which used different electrodes and opposite polarity conventions. The titration curve for unstressed DBSA exhibits a very high initial emf compared to the fuel blank. This is characteristic of strong acids such as sulfonic acids. The inflection point for neutralization of the strong acid is about +lo0 mV. The initial emf for the stressed DBSA is still high but the curve demonstrates that much of the DBSA is not available in the fuel after stress. The titration curve for thiophenol shows that most of this weak acid has reacted during the stress period. Further, the initial emf has shifted from +120 to -220 mV.
Energy & Fuels, Vol. 5, No. 2,1991 271
Distillate Fuel Insolubles
dopant, concn noneb DBSA, 0.001 M PBTP, 0.03 M CA, 0.01 M
C 77.75 76.99 76.86 C
Table 11. Elemental Analysis of Fuel Depositsa percent element H N S 0 9.38 2.59 1.93 6.08 3.84 9.26 2.53 6.44 4.42 10.03 1.61 6.48 C
c
c
c
C1
C
ash 2.24 0.98 0.41
1.22
C
C
c
total 99.97 100.04 99.81 c
'Fuel: Gulf Coast Blend, 20% LCO in SR. bDeposit formed at ambient temperature in 6 months; other deposits formed at 80 OC, 2 weeks' stress. No analysis.
Table 111. Percent Atomic Composition of Insolubles by XPR'
dopant, concn none DBSA, 0.001 M PBTP, 0.03 M CA, 0.01 M
1 mmollL DBSA
C 79.5 75.5 80.5 77.0
O 16.5 18.7 15.2 18.9
N S C 2.6 1.4 2.9 2.8 1.7 2.5 2.1 1.3
a Insolubles formed at conditions listed in Table 11. not detected.
MlLLlEQUlVALENTS ALCOHOLIC KOH ( x 1000)
Figure 4. Titration curves, West Coast Blend 30/7OLCO/SR.
400 THIOPHENOL UNSTRESSED
300
3 .-g
20
-
THIOPHENOL 14 DAYS180"C
L i -
r
Ij
O Y
-1
II
I
-200
--
0
0.4 0.6 0.8 MlLLlEQUlVALENTS ALCOHOLIC KOH
0.2
1.o
Figure 5. Titration curves, Australian Fuel Blend 30/ 7O:LCOISR.
This is indicative of the presence of a very strong acid, sulfonic acid. Although the titer for strong acid is low, a definite quantity is present. In addition to titration of the stressed and unstressed fuel blends, the solid sediments formed during the stress were subjected to nonaqueous titration. Strong acid was found in the sediments from the DBSA and PBTP doped samples. Elemental Analysis. Table I1 lists the results of elemental analysis of various deposits from the stress tests. The carbon, hydrogen, and oxygen contents were similar for the insolubles from all samples. Nitrogen was lower for the sample derived from the fuel doped with PBTP. Sulfur was significantly higher for the blends doped with sulfur compounds, both the DBSA and the PBTP. All of these concentrations fall within the range observed for insoluble material derived from other storage stability experimentsaw The low H/C atomic ratio (0.4-1.0)along (6)
Pedley, J. F.; Hiley, R. W.; Hancock, R. A. Fuel 1987, 66, 1646.
Chlorine
Table IV. Percent Atomic Concentrations of Two Different Types of Sulfur by XPSo reduced oxidized total doDant. concn sulfur sulfur sulfur none 0.5 0.9 1.4 DBSA, 0.001 M 0.9 1.8 2.8 PBTP, 0.03 M 0.6 1.9 2.5 CA, 0.01 M 0.6 0.7 1.3 a
500
1 b b b 0.3
Insolubles formed at conditions listed in Table 11.
with the high nitrogen, sulfur, and oxygen values support the view that heteroaromatic compounds are major participants in insolubles formation. Chlorine was found in the insolubles sample derived from the fuel blend containing 0.01 M chloroacetic acid. Insufficient sample was available to determine the other elements for this sample. XPS Analysis. This spectroscopic analysis was applied to the solids, both filterable and adherent, formed in the 80 "C stress. This technique affords good qualitative information but is not completely quantitative at low (
