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Ind. Eng. Chem. Res. 2004, 43, 4944-4946
Recycled Soybean Cooking Oils As Blending Stocks for Diesel Fuels George W. Mushrush,*,†,‡ James H. Wynne,† Heather D. Willauer,§ Christopher T. Lloyd,† Janet M. Hughes,| and Erna J. Beal⊥ Materials Chemistry Branch, Code 6120, and Navy Technical Center for Safety and Survivability, Code 6180, Naval Research Laboratory, 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 120, Lanham, Maryland 20706, and Fuels and Lubricants Division, Naval Air Systems Command, 22229 Elmer Road, Patuxent River, Maryland 20670
It has been proposed that renewable energy sources be substituted or at least used as blending stocks for middle distillate ground transportation fuels. The U.S. Navy is considering allowing up to 20% soybean biodiesel to be added as a blending stock to petroleum diesel fuels. It is important for operational considerations to look at the many positives and/or negatives that this could engender. Among the more important considerations are storage stability, filterability, fuel solubility, oxidative stability, and induced instability reactions. This paper reports on methylated recycled restaurant soybean cooking oils used as blending stocks for ground transportation diesel fuels. We compare this recycled soy liquid in blends of both 10% and 20% with petroleum middle distillate fuels for storage stability, oxidative stability, solubility, and chemical instability results. These results are contrasted to those of pure soy-derived fuel liquids used as blending stocks. Introduction The various branches of the United States military are the largest consumers of middle distillate fuels in the world. Many schemes have been proposed to decrease the nation’s dependence on imported foreign crude oil. Renewable sources of oils, including plants such as corn, soybeans, and various other vegetables, 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 %. Furthermore, the biodiesel produced from soy liquids is cheaper than that from other plant sources. However, the most important reason for considering soy-derived oils is that both their agricultural supply and their oil manufacturing capacity are in current operation. If recycling of the methylated esters manufactured from domestic cooking oils could also augment this renewable resource, it would have a tremendous beneficial influence on both fuel availability and price. Military fuel specifications are very restrictive as to the quality of the product and the materials permitted to be added to middle distillate ground transportation fuels.1 Additives are thus judiciously monitored with care. The military specifications for additives, to mention just a few of the most important, include solubility in the fuel at both ambient and low temperature, flash point, effect on cetane number, and storage stability. The critical specification, however, is that the blending stock not induce chemical instability in the fuel itself.2 Previously reported research from our laboratory centered on the use of newly manufactured methylated * To whom correspondence should be addressed. Tel.: (703) 993-1080. E-mail
[email protected]. † Materials Chemistry Branch, Naval Research Laboratory. ‡ George Mason University. § Navy Technical Center for Safety and Survivability. | GEO-CENTERS, Inc. ⊥ Naval Air Systems Command.
soy esters as blending stocks. These soy liquids proved to be excellent materials at both 10% and 20%, passing all required military protocols.3 The quantity of available recyclable restaurant cooking oil is as large as the amount of virgin soybean oil. If this recycled oil could be used alone or in blends with the newly manufactured oil, environmental concerns for this used material would be greatly alleviated. In the present research, we report on the use of recycled soybean-derived restaurant cooking oils as a blending stock for petroleum derived middle distillate fuels. This fuel blending stock was added in 10 and 20 wt % to a stable petroleum middle distillate fuel. The blending stock was obtained from an eastern United States manufacturer and is commercially available. We examined the storage stability and the instability reactions. We looked at the fuel stability of these blends under both ambient and accelerated storage conditions. These results are compared to those from a pure or virgin soy-derived oil. 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 (TMS). Accelerated Storage Stability Tests. The soy-fuel blends, 10% and 20%, were tested for both storage stability and chemical instability reactions. They were tested by a gravimetric technique described in ASTM D5304-99.4 A brief description of this method is as follows. Samples (100 mL) 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
10.1021/ie030883d CCC: $27.50 © 2004 American Chemical Society Published on Web 06/29/2004
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were filtered and the sediment was determined by a gravimetric procedure. Sediment amounts in excess of 3.0 mg/100 mL of fuel indicate a fuel that is not acceptable to the military. Soy-Derived Bodiesel. An east coast U.S. oil company supplied the recycled soy-derived biodiesel fuel. The recycling process consisted of converting the fatty acids present to the corresponding methyl esters. The soy-derived liquid had a boiling point greater than 400 °F, negligible water solubility, a specific gravity of 0.86, and a flashpoint greater than 300 °F. No information on its cetane number was supplied. However, the newly manufactured soy liquid tested greatly exceeded the minimum required cetane number. No antioxidant was added after reprocessing. This soy liquid was a deep brown liquid as received. It was stored under refrigeration until used. Extraction Procedure. Activated clay (Celite 545, EM Science, Inc.) and activated silica gel (200-425 mesh, Grade 633, Aldrich Chemical Co.) were both used to make slurries with 100 mL of the recycled soy oil. The slurry was agitated for 15 min and then filtered by reduced pressure through a fine-fritted disk glass filter funnel. The filtered recycled soy oil was then used for accelerated storage stability tests. Middle Distillate Fuels. All storage stability testing was done by ASTM D-5304-99 procedure.4 The petroleum middle distillate fuels were from our extensive inventory of well-characterized 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 0.5 mg of solids/100 mL fuel, and was therefore characterized as a very stable fuel. Instrumental Methods. Mass Spectrometry. The soy additive and the soy fuel mixtures were analyzed by combined capillary column gas chromatography/mass spectrometry (GC/MS). The GC/MS system consisted of a Hewlett-Packard 5890 Series II gas chromatograph configured for split flow injection (3:1), and a HewlettPackard 5971 Series mass selective detector. The GC was equipped with an all-glass inlet system in conjunction with a 0.20 mm × 50 m poly(dimethylsiloxane) capillary column (HP-1, 19091Z-105) made by Agilent Technologies. The injector temperature was 250 °C and the detector temperature was 280 °C. The column flow was 1 mL/min. The temperature program started with an initial temperature of 60 °C for 3 min, a ramp of 3 °C/min, to a final temperature of 290 °C that was held for 2 min. A solvent delay of 5 min was also incorporated into the temperature program. The mass spectrometer was operated in the electron impact ionization mode (70 eV) with continuous scan acquisition from 35 to 550 amu at a cycling rate of approximately 1.49 scan/sec. The parameters were set up with the electron multiplier at 1212 V. These GC/MS parameters provided excellent separation of the compounds in the soy-derived fuel. The HP, MS Chem Station Hardware 61701BA version B.01.00 was used to process mass spectral information. 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).5 A diesel fuel, to be acceptable to the military, must meet many other specifications.1 These include, for example, API gravity, flash point, pour point, water solubility, cetane number, acid number, total sulfur, and color test.
Table 1. Storage Stability of Petroleum-Derived Fuels and Soy-Petroleum Blends gravimetric solids/100 mL fuel
fuel stable petroleum diesel soy-fuel blends 10% soy-90% stable diesel 20% soy-80% stable diesel distilled soy 10% soy-90% stable diesel 20% soy-80% stable diesel activated clay extracted soy 10% soy-90% stable diesel 20% soy-80% stable diesel activated silica gel extracted soy 10% soy-90% stable diesel 20% soy-80% stable diesel
0.6 4.2 9.0 2.2 2.1 0.6 2.1 1.3 2.3
Table 2. 1H NMR Results for Recycled Soy-Derived Fuel Blending Stock functionality
δ
sCO2CH3 3.66 sHCdCHs 5.43-5.24 sCH3 0.89 0.97 sCH2CO2R 2.30 2.79 sCH2s 1.62 1.32-1.23 2.11-1.97
integration signal 1.55 1 1.77 0.04 1.03 0.20 1.05 11.35 1.57
s m t t t t t m m
remarks only methyl ester no conjugation 98% alkyl 2% vinyl unconjugated γ-δ unsaturation alkyl alkyl alkyl
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 recycled cooking oil was subjected to GC/MS analysis. No evidence of organosulfur or organonitrogen compounds was detected. It was observed that this soy liquid would not pass the color test (ASTM D-1500).6 Consumers, both military and civilian, usually prefer a light yellow product because a fuel usually darkens upon prolonged storage. It has been observed that this color change may be related to degradation reactions. Table 1 illustrates the storage stability results for the recycled soy liquid when blended with a stable middle distillate fuel. The ASTM storage stability procedure features severe conditions that give results that are indicative of a two-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 as these are the allowed concentrations under consideration by the U.S. Navy. The soy liquid proved unstable for both the 10% and 20% blends. Results showed 4.2 mg of solids for the 10% soy blends and 9.0 mg of solids for the 20% soy blends. Both values greatly exceeded the allowed 3.0 mg of solids/100 mL of fuel. Gravimetric results were some of the worst we have seen. Thus, any of these blends with any middle distillate fuel would lead to filter plugging and other significant engine operational problems. A comparison of a virgin soybean-derived liquid with this recycled soy liquid by 1H NMR (Table 2) showed that they were quite similar, with both consisting of methyl esters.3 However, the alkyl functionality was different and less pure in the recycled soy liquid. There was approximately a 25% increase in the amount of alkyl functionality observed in the recycled blending stock compared with that of the pure liquid. It is speculated that this is due to the thermal degradation and oxidative cleavage of the olefinic moieties within the fuel liquid resulting from repetitive use. Both
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blending stocks had the same relative amount of methyl ester character, yet there was considerably more internal olefinic character observed in the pure stock than in the recycled stock (>65%). This provides additional confirmation to support the aforementioned hypothesis that indeed the olefinic moiety was quite susceptible to degradation under such conditions. There was a decrease in the terminal vinylic character in the recycled stock, however, a notable increase in terminal alkyl methyl character, which clearly indicates that there is more branching in the recycled stock with respect to that which was observed in pure stock. According to integration of the 1H NMR spectra, there is an excess of 29% terminal alkyl functionality in the recycled stock, 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 the pure soy liquid, integration indicates an average of approximately 11 methylene groups. However, in the recycled soy liquid there are approximately 16 methylene groups present. These NMR observations cannot, in our opinion, account for this recycled soy oil showing such poor storage stability results. About 20 years ago, our laboratory studied the effects of doping stable middle distillate fuels with pure compounds containing various functional groups. It was found that low molecular weight (C5 - C10) carboxylic acids resulted in serious fuel degradation.7 Because restaurant cooking oils are subjected to high temperatures for prolonged time periods, it was thought that this could result in the formation of acidic moieties. These acidic compounds could then be present when the oil was recycled and not completely converted to the corresponding methyl ester by this manufacturer. Another possibility was that during recycling some of the ester oxidized, or it oxidized in storage after manufacture. The identification of trace amounts of carboxylic acids in the presence of high concentration of esters by 1H NMR was not possible. To demonstrate the presence of acidic compounds in the recycled soy liquid, a basic extraction was performed. The extracted material was concentrated by reduced-pressure rotary evaporation and analyzed by combined capillary GC/MS. This method of analysis definitively showed the presence of trace quantities of carboxylic acids. Specific lower molecular weight acids included hexanoic acid, hex-3-enoic acid, nonanoic acid, non-3-enoic acid, and dodec-3-enedioic acid. Higher molecular weight acidic moieties included octadeca-9, 12-dienoic acid and octadec-9-enoic acid. The acid number of this methylated soy liquid was 0.08 (mg KOH/g) indicating that it was only slightly acidic. These same acidic compounds were not detected in virgin soyderived liquids. The presence of acidic species in the recycled soy-derived cooking liquids could be attributed to the oxidation reactions resulting from the high temperature (and length of time at this temperature) to which the cooking oil was subjected during its original use and reprocessing. The presence of these acidic compounds explains the observed storage stability results for both the virgin soybean oil and the recycled soy oil. The observation that the recycled liquid was initially dark brown could also be the result of oxidation reactions. We have observed on many occasions that a color change of light yellow to dark brown occurs during oxidation processes in fuels.8 Further, the presence of carboxylic acids in the recycled soy liquid coupled with
the fuel instability results lends credence to our laboratory’s earlier observation that carboxylic acids are detrimental to fuel stability. Two chemical methods were employed to remove these acidic moieties from the soy liquid. Samples of the recycled soy cooking oil were subjected to both an activated clay treatment and a silica gel treatment. Both of these substances remove polar species such as carboxylic acids. Third, the recycled cooking oil was subjected to a careful distillation at reduced pressure. The first and second fractions from the distillation were collected and discarded. The results from Table 1 clearly demonstrated that all three methods for further treating the recycled soy liquid gave acceptable results, less than 3.0 mg of solids. Clay treatment is a well-known and relatively inexpensive method for removing polar species that can instigate storage instability reactions. The results from silica gel treatment, which is much more expensive, did not materially improve the sediment compared to clay treatment. The trials in which the recycled oil was distilled gave results that were marginal. Conclusion A commercially available, recycled, soybean-derived fuel liquid was obtained from an east coast manufacturer. It was claimed to meet military specifications. This material appeared similar to pure soybean-derived liquids as evaluated by GC and 1H NMR. However, this recycled soy blend failed most of the required test matrixes for military acceptance. Because this recycled biodiesels fuel liquid had chemical and physical properties similar to those of virgin stocks, the differences in both storage stability and oxidative behavior may be attributed to the high-temperature oxidation reactions to which the recycled soy oil was originally subjected, as well as to the absence of an antioxidant. Subjecting the unstable recycled soy liquid to clay treatment resulted in a much more stable fuel blending stock. Literature Cited (1) Military Specification MIL-T-83133D, 1995. (2) Mushrush, G. W.; Speight, J. G. The Chemistry of the Incompatibility Process in Middle Distillate Fuels. Rev. Proc. Chem. Eng. 1998, 1 (1) 5. (3) Mushrush, G. W.; Beal, E. J.; Hughes, J. M.; Wynne, J. H.; Sakran, J. V.; Hardy, D. R. Biodiesel Fuels: Use of Soy Oils as a Blending Stock for Middle Distillate Petroleum Fuels. Ind. Eng. Chem. Res. 2000, 39 (10), 3945. (4) ASTM, American Society for Testing Materials. Standard Test Method for Assessing Distillate Fuel Storage Stability by Oxygen Overpressure. In Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1999; Part 05.03, ASTM D5304-99. (5) ASTM, American Society for Testing Materials. Standard Specification for Fuel Oils. In Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1997; Part 05.01, ASTM D975-96. (6) ASTM, American Society for Testing Materials. Standard Test Method for Determining Color of Petroleum Products. In Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1999; Part 05.01, ASTM D1500-98. (7) Beal, E. J.; Cooney, J. V.; Hazlett, R. H.; Morris, R. E.; Mushrush, G. W.; Beaver, B. D.; Hardy, D. R. Mechanisms of Syncrude/Synfuel Degradation; Final Report, Department of Energy, DOE/BC/87001232; U.S. Government Printing Office: Washington, DC, 1984. (8) Hazlett, R. N. Frontiers of Free Radical Chemistry; Academic Press: New York, 1980.
Received for review December 30, 2003 Revised manuscript received April 13, 2004 Accepted April 30, 2004 IE030883D
