Ind. Eng. Chem. Res. 2003, 42, 2387-2389
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GENERAL RESEARCH Soybean-Derived Fuel Liquids from Different Sources as Blending Stocks for Middle Distillate Ground Transportation Fuels George W. Mushrush,*,†,‡ James H. Wynne,† Janet M. Hughes,§,| Erna J. Beal,† and Christopher T. Lloyd† Naval Research Laboratory, Code 6120, Materials Chemistry Branch, 4555 Overlook Avenue, SW, Washington, D.C. 20375, Chemistry Department, George Mason University, 4400 University Drive, Fairfax, Virginia 22030, Geo-Centers Inc., 4640 Forbes Boulevard, Suite 130, Lanham, Maryland 20706, and Science Department, West Springfield High School, 6100 Rolling Road, Springfield, Virginia 22152
It has been proposed that biodiesel liquids be used in blends with middle distillate ground transportation fuels by the various services in the Department of Defense. The U.S. Navy is considering allowing up to 20% biodiesel to be added as a blending stock to petroleum diesel fuels. It is important for operational considerations to examine the many problems this could present. Among the more important considerations are storage stability, filterability, fuel solubility, oxidative stability, and induced instability reactions. This paper reports on two different soybean-derived fuel liquids. The first is a fuel liquid that contains an antioxidant, and the second is a fuel liquid that was derived from recycled restaurant cooking oil with no added antioxidant. We compare both biodiesels in blends of 10% and 20% with both stable and unstable petroleum middle distillate fuels for storage stability, oxidative stability, solubility, and chemical instability results. Introduction The Department of Defense is the largest consumer of middle distillate fuels, and this has ramifications throughout the civilian market. Many schemes have been proposed to decrease the nation’s dependence on imported foreign crude oil. Most non-renewal sources, such as used automobile and truck tires or consumer plastic residues, produce products that require a tremendous amount of additional processing to be useful as a middle distillate fuel. Renewable sources including plants such as corn, soybeans, or other vegetable oils provide a viable resource as long as they can be produced and refined in suitable quantities. Of these plant-derived materials, soybeans provide the most oil, up to 20 wt %, and the oil produced is cheaper than any other plant source. Military fuel specifications are very restrictive as to the quality of the product and the materials that are allowed to be added.1 It is thus with considerable care that blending stocks are considered. The military specifications for fuels or to fuel blends, to mention just a few, include solubility in the fuel at ambient and low temperature, flash point, effect on the cetane number, and storage stability. A critical point, however, is that the blending stock does not induce chemical instability * To whom correspondence should be addressed at George Mason University. Tel.: (703) 993-1080. Fax: (703) 993-1055. E-mail:
[email protected]. † Naval Research Laboratory. ‡ George Mason University. § Geo-Centers Inc. | West Springfield High School.
in the fuel itself.2 In the present research, we report on two different soybean-derived blending stocks. Both were added in 10% and 20% blends with a stable and then an unstable petroleum middle distillate fuel. The commercially available blending stocks were obtained from different manufacturers. We examined the storage stability and instability reactions for these blends under both ambient and accelerated storage conditions. Experimental Section General Methods. Unless otherwise stated, chemicals were reagent-grade and were obtained from commercial sources and used without additional purification. 1H NMR spectra were measured in deuterated chloroform (CDCl3) on a Bruker 300-MHz spectrometer. 1H chemical shifts are reported in δ (ppm) relative to internal tetramethylsilane. Storage Stability Tests. The soy-fuel blends, 10% and 20%, were tested for storage stability and chemical instability reactions. They were tested by a gravimetric technique described in ASTM D5304-99.3 A brief description of this method is as follows: 100-mL samples of the blends in 125-mL borosilicate brown glass bottles were subjected to a 16 h, 90 °C, time-temperature regimen at 100 psig overpressure of pure oxygen. After the reaction period, the samples were cooled to room temperature. The samples were filtered, and the sediment was determined by a gravimetric procedure. Ambient Oxidative Stability Tests. The biodiesel liquids were subjected to a steady stream of ambient air for a 1-week time period. The reaction was carried out in a 1-L flask connected to an aspirator and
10.1021/ie021052v CCC: $25.00 © 2003 American Chemical Society Published on Web 05/01/2003
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Ind. Eng. Chem. Res., Vol. 42, No. 11, 2003
Table 1. General ASTM Specifications for Several Types of Middle Distillate Fuels specification
no. 1-Da
no. 2-Db
no. 2c
soy biodiesel
API gravity (deg) total sulfur (%) boiling point (°C) flash point (°C) pour point (°C) cetane number acid number
34.4 0.5 288 38 -18 40 0.3
40.1 0.5 338 52 -6 40 0.3
30 0.5 338 38 -6
44 0 204 218 >40 0
a
High-speed, high-load engines. b Low-speed, high-load engines. c General purpose heating fuel oil.
protected by a safety bottle. The air was filtered through a drying tube filled with anhydrous CaSO4 with fiberglass plugs before passing through the soy-derived liquid. Soy-Derived Biodiesel Fuels. Ag Environmental Products, Lenexa, KS, supplied soy-derived biodiesel fuel A. This material was light yellow in color and had a boiling point greater than 400 °F, negligible water solubility, a specific gravity of 0.88, a flash point of 425 °F, and a cetane number >40. An eastern United States fuel oil company supplied soy-derived biodiesel fuel B. It had a boiling point greater than 400 °F, negligible water solubility, a specific gravity of 0.86, and a flash point greater than 300 °F. No information on its cetane number was supplied. Middle Distillate Fuels. All accelerated storage stability testing was done by the ASTM D-5304-99 method.3 This gravimetric procedure ranks middle distillate fuels by the amount of sediment produced. A total sediment greater than 2.0 mg/100 mL of fuel classes a fuel as unstable. The petroleum middle distillate fuels were from our extensive inventory of wellcharacterized fuels. The stable fuel (no. 2 diesel) was an American-refined fuel that has been used as a stable fuel for comparison in our laboratory. This fuel yielded 0.6 mg of solids/100 mL of fuel and was therefore characterized as a very stable fuel. The unstable fuel was a Spanish-refined no. 2 diesel fuel. This fuel has historically been an unstable fuel forming 2.5 mg of solids/100 mL of fuel. This yield of solids ranks this fuel as unstable. Results and Discussion Petroleum-derived diesel fuel has an average carbon number range of about C13 up to about C21 and a distillation range of about 150-400 °C (300-750 °F). A diesel fuel to be acceptable to the military must meet many other specifications.1,4 These include, for example, API gravity, flash point, pour point, water solubility, cetane number, acid number, total sulfur, and color test (Table 1).4 The soy-derived fuel meets many of these specifications such as flash point, API gravity, boiling point, cetane number, and water solubility. These values were listed in the Experimental Section. The soy-derived biodiesels were subjected to gas chromatography (GC)/ mass spectrometry analysis. No evidence of organosulfur or organonitrogen compounds was detected. It was observed that only soy fuel A would pass the color test (ASTM D-1500) while soy B was a dark brown liquid.5 Consumers both military and civilian usually prefer a light yellow product because a fuel usually darkens as its storage time increases. It has been observed that this darkening may be related to degradation.
Table 2. Storage Stability of Petroleum-Derived Fuels and Soy-Petroleum Blends fuel
gravimetric solids/ 100 mL of fuel
stable petroleum diesel unstable petroleum diesel
0.6 2.5
soy-fuel blend
gravimetric solids/ 100 mL of fuel
10% soy A-90% stable diesel 20% soy A-80% stable diesel 20% soy A-80% unstable diesel 10% soy B-90% stable diesel 20% soy B-80% stable diesel
0.3 0.2 0.5 4.2 9.0
Table 2 illustrates the storage stability for both biodiesel blending stocks. The storage stability ASTM procedure features severe conditions that give results that are indicative of a 2-year storage life for the fuel. The reaction conditions are 100 mL of fuel at 90 °C for 16 h, at 100 psig pure oxygen. The results are for 10% and 20% blends because these are the allowed concentrations under consideration by the U.S. Navy. These results definitively show that the soy blending stock A that contained an antioxidant enhanced the stability for both the stable and unstable no. 2 diesel fuels. For the 10% blends, 0.3 mg of solids/100 mL of fuel was formed, and for the 20% blends, 0.2 mg of solids/100 mL of fuel was formed. The unstable petroleum-derived diesel consistently has failed the ASTM D5304 storage test procedure, 2.5 mg of solids/100 mL of fuel. When this unstable fuel was blended with 20% soy liquid A, a very dramatic change was noted. This bad petroleum fuel soy A blend easily passed the storage stability test with only 0.5 mg of solids/100 mL of fuel. This was the first time that our laboratory has observed such a dramatic change. We have not noted this type of change with any other blending stock. One possible explanation for this observation might involve the potent solvating power of methyl esters. However, these observations were not duplicated in the case of soy-derived fuel liquid B. When this biodiesel fuel liquid was blended with the stable petroleum fuel, the results were poor. With the stable petroleum diesel, both 10% and 20% soy B blends failed the ASTM storage stability test procedure. The 10% soy B blend gave 4.2 mg of solids and the 20% blend gave 9.0 mg of solids/ 100 mL of fuel. Both of these blends with any middle distillate fuel would thus lead to filter plugging and other engine operational problems. These gravimetric results were some of the worst we have observed. The results of this 10% blend were so bad with the stable fuel, that the unstable petroleum diesel fuel was not tested with this soy blending stock. Soy liquid A, when blended with both the stable and unstable petroleum fuels, remained very light in color throughout the testing regimen, but the opposite was observed for blends of soy fuel liquid B. It is important to note that a color change is not necessarily indicative of degradation. Our laboratory has, however, noted that a darkening of color can be associated with an increase in peroxidation and thus an increase in solid formation.6,7 In this work, the color change may be associated with peroxidation because soy liquid A contained an antioxidant. A further indication of the effect of the antioxidant was that soy liquid A did not darken with exposure to light with a constant air purge at room temperature for a 1-week storage period.8,9 Soy liquid
Ind. Eng. Chem. Res., Vol. 42, No. 11, 2003 2389 Table 3. 1H NMR Data for Soy-Derived Fuel Blending Stocks A and B functionality -CO2CH3 -HCdCH-CH3 -CH2CO2R -CH2-
-CO2CH3 -HCdCH-CH3 -CH2CO2R -CH2-
δ
integration signal
remarks
Soy-Derived Blending Stock A 3.66 3 s only methyl ester 5.40-5.32 3 m no conjugation 0.87 2.67 t 89% alkyl 0.91 0.30 t 11% vinyl 2.30 2 t unconjugated 2.77 1.5 t γ-δ unsaturation 1.64-1.60 2.5 m alkyl 1.31-1.26 18 m alkyl Soy-Derived Fuel Blending Stock B 3.66 1.55 s only methyl ester 5.43-5.24 1 m no conjugation 0.89 1.77 t 98% alkyl 0.97 0.04 t 2% vinyl 2.30 1.03 t unconjugated 2.79 0.20 t γ-δ unsaturation 1.62 1.05 t alkyl 1.32-1.23 11.35 m alkyl 2.11-1.97 1.57 m alkyl
B was already a deep brown liquid when received and was not tested. Upon comparison of the two fuels A and B, by 1H NMR, it was determined that the fuels were practically identical in chemical composition, with both blending stocks consisting of the same methyl esters (Table 3). However, the alkyl functionality was different and less pure in soy blending stock B. There was approximately a 25% increase in the amount of alkyl functionality observed in blending stock B compared to blending stock A. Both blending stocks had the same relative amount of methyl ester character, yet there was considerably more internal olefinic character observed in stock A than stock B (>65%). There was a decrease in the terminal vinylic character in stock B, however, a notable increase in the terminal alkyl methyl character, which clearly indicates that there is more branching in stock B with respect to that observed in stock A. According to integration of the 1H NMR spectra, there is an excess of 29% terminal alkyl functionality in stock B, along with an excess of 18% terminal functionality (alkyl and vinyl). This theory of extended branching is evident by the obvious differences in methylene character within the molecule. In soy-liquid A, integration indicates an average of approximately 11 methylene groups. However, in soy-liquid B, there are approximately 16 methylene groups present. A final consideration that is important for military fuels is that of filterability (ASTM 6426-99).10 Both pure blending stocks failed this test procedure. Our laboratory has observed this behavior with other ester blending stocks.11 However, soy blending stock A easily passes this procedure in 10% and 20% blends, while blending
stock B fails at 10%. The most probable reason for this degradation is that blending stock B has oxidized during manufacture and storage. Conclusion Two different soybean-derived blending stocks were obtained from two different manufacturers, both of which claimed to meet military specifications. Soy biodiesel fuel A and soy biodiesel fuel B both appeared similar by GC and 1H NMR with only slight differences in the NMR results. Soy blending stock A passed all of the chemical tests with ease, while soy blending stock B failed most of the same procedures. Because both biodiesels had similar chemical and physical properties, the differences in both storage stability and oxidative behavior may be attributed to the presence of antioxidants or to the presence of an ineffective antioxidant. Literature Cited (1) MIL Spec. Military Specification MIL-T-83133D, 1995. (2) Mushrush, G. W.; Speight, J. G. A Review of the Chemistry of Incompatibility in Middle Distillate Fuels. Rev. Process Chem. Eng. 1998, 1 (1), 5-29. (3) ASTM, American Society for Testing Materials. Standard Test Method for Assessing Distillate Fuel Storage Stability by Oxygen Overpressure. Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1999; Part 05.03, ASTM D5304-99, pp 569-572. (4) ASTM, American Society for Testing Materials. Standard Specification for Fuel Oils. Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1997; Part 05.01 ASTM D975-96, pp 165-168. (5) ASTM, American Society for Testing Materials. Standard Test Method for Determining Color of Petroleum Products. Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1999; Part 05.01, ASTM D1500-98, pp 547-551. (6) Mayo, F. R. The Chemistry of Fuel Deposits and Their Precursors, Final Report, Naval Air Systems Command, N0001972-C-10161, 1972. (7) Mushrush, G. W.; Speight, J. G. In Petroleum Products; Taylor and Francis: Philadelphia, PA, 1995. (8) Howard, J. A. In Organic Peroxides; Kochi, J., Ed.; WileyInterscience: New York, 1971; Vol. II, Chapter 12. (9) Hazlett, R. N. In Frontiers of Free Radical Chemistry; Pryor, W. A., Ed.; Academic Press: New York, 1980; pp 195-221. (10) ASTM, American Society for Testing Materials. Standard Test Method for Determining Filterability of Distillate Fuel Oils. Annual Book of ASTM Standards; ASTM: Philadelphia, 1999; Part 05.03, ASTM D6426-99, pp 6426-6499. (11) Mushrush, G. W.; Beal, E. J.; Hughes, J. M.; Wynne, J. H.; Sakran, J. V.; Hardy, D. R. Biodiesels Fuels: Use of Soy oil as a Blending Stock for Middle Distillate Petroleum Fuels. Ind. Eng. Chem. Res. 2000, 39 (10), 3945-3948.
Received for review December 30, 2002 Revised manuscript received March 21, 2003 Accepted March 24, 2003 IE021052V
