7364
Ind. Eng. Chem. Res. 2009, 48, 7364–7367
Incompatibility of Fischer-Tropsch Diesel with Petroleum and Soybean Biodiesel Blends George W. Mushrush,*,†,‡ Heather D. Willauer,† Joy W. Bauserman,§ and Frederick W. Williams† NaVal Research Laboratory, Code 6180, NaVy Technology Center for Safety and SurViVability, 4555 OVerlook AVenue, SW, Washington, D.C. 20375, Chemistry Department, George Mason UniVersity, 4400 UniVersity DriVe, Fairfax, Virginia 22030, and NOVA Research, 1900 Elkin Street, Alexandria, Virginia 22308
The Department of Defense is the largest consumer of middle distillate fuels. It has been recommended that alternative fuel sources be considered as replacements or blending stocks for middle distillate ground transportation and marine fuels. Therefore, the search for suitable replacements or blending stocks is earnestly continuing. Renewable agricultural crops such as soybeans and others are now in the forefront. Nonrenewable synthetic fuels such as those produced by Fischer-Tropsch, FT, synthesis from coal and natural gas have been suggested. It is probable that several of these substitutes would be simultaneously blended into a middle distillate petroleum based diesel fuel. Care must be employed when blending fuels so that fuel specifications and storage stability are not decreased. This paper compares the storage stability of a three-part mixture consisting of Fischer-Tropsch diesel, a petroleum diesel, and a 5% and 10% soy biodiesel under ambient conditions. Introduction The continued escalation of price for petroleum derived middle distillate fuels outpaces that of all other petroleum products. Since the Department of Defense (DOD) is the largest consumer of middle distillate fuels, it has been recommended that alternative fuel sources be considered. Many schemes have been proposed to decrease the nation’s dependence on foreign oil. Most of these nonrenewable sources, such as used automobile and truck tires or consumer plastic residues, produce products that require additional expensive processing to be useful as middle distillate fuels. 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% by weight, and the oil produced is cheaper than any other plant source. Regardless of the source of the fuel or the blending material, the final product must meet many rigorous standards.1,2 Previous research in our laboratory has shown that the soy oil itself was not suited for DOD use but that the fatty acid methyl esters (FAME) produced from the soy are excellent blending agents with petroleum-derived diesel.3,4 Other schemes to increase the volume of middle distillate fuel involve synthetic fuels such as Fischer-Tropsch (FT) fuels, which can be produced from coal or natural gas. Since the United States has an abundance of both coal and natural gas, these natural resources would seem to be a logical substitute.5 It is probable that several of these substitutes would be simultaneously blended into a middle distillate petroleum-based diesel fuel. Regardless of the petroleum alternative, care must be taken that the final blended product meets all required military fuel specifications and will perform in a seamless manner.2 This research reports on both a petroleum diesel fuel and a * To whom corespondence should be addressed. E-mail: gmushrus@ gmu.edu. Phone: 703-993-1080. † Naval Research Laboratory. ‡ George Mason University. § NOVA Research.
Fischer-Tropsch diesel fuel blended with each other and with various concentrations of soy methyl esters. The purpose of this article was to examine the storage stability and efficacy of these complex middle distillate fuel blends. Experimental Section General Methods. Unless otherwise stated, chemicals were reagent grade and were obtained from commercial sources and used without additional purification. Storage Stability Tests. The soy biodiesel-petroleum fuel blends were tested for storage stability and chemical instability reactions. They were tested by a gravimetric technique described in ASTM D5304-99.6 This method requires a 100 mL sample of the fuel blend in a 125 mL borosilicate brown glass bottle, which were then subjected to a 16 h, 90 °C time-temperature regimen at 100 psig over pressure 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 method. If the sediment value is 3.0 mg/100 mL of fuel or less, the fuel will be stable in storage for up to a 2-y period. This ASTM procedure is required for the purchase of a fuel by the DOD. Caution! All of these runs were performed in an explosion proof oVen. Mass Spectrometry. The soy biofuel was analyzed by combined capillary column gas chromatography/mass spectrometry (gc/ms). The gc/ms system consisted of a HewlettPackard 5890 Series II gas chromatograph configured for split flow injection (3:1) and a Hewlett-Packard 5971 Series mass selective detector. The gc was equipped with an all glass inlet system in conjunction with a 0.20 mm × 50 m polydimethylsiloxane 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 and continued with 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
10.1021/ie801867t CCC: $40.75 2009 American Chemical Society Published on Web 06/23/2009
Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009
7365
Table 1. General Properties Required by Specifications for Commercial Diesel, Military Diesel, and Alternative Diesel property
ASTM (D975) grade low sulfur no. 1-D
ASTM (D975) grade low sulfur no. 2-D
1.7-4.3 0.5 60 -1 -6 12.5
1.3-2.4 0.05 38
1.9-4.1 0.05 52
42 0.20
40 0.15
40 0.35
3.0
3.0
3.0
military MIL-PRF-16884K
viscosity at 40 °C, mm /s sulfur content, wt % (max) flash point, °C (min) cloud point, °C (max) pour point, °C (max) hydrogen content, wt % (min) cetane number, (min) carbon residue, 10% bottoms, wt % (max) storage stability, total insolubles, mg/100 mL (max), ASTM D5304 trace metals, ppm (max) calcium, lead, sodium + potassium, vanadium 2
soy biodiesel
FT diesel
0 218
0.02 63 -12 12.5
12.3 45
76
3.0
1.0, 0.5, 1.0, 0.5
3.0
0
Table 2. Normal and Branched Alkanes in a Fischer-Tropsch Diesel Fuel
Table 3. Methyl Esters Concentration in Newly Manufactured Soy-Derived Biodiesel
carbon number
mass number
isomers
weight %
methyl ester
carbon number
concentrationa (wt %)
C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 totalpeakarea
142 156 170 184 198 212 226 240 254 268 282 296 310 324 338 352 366
3 4 4 3 3 5 3 3 4 3 2 3 2 1 2 1 2
6.7 12.6 10.8 1.5 12.4 19.0 9.4 2.7 6.8 5.1 3.5 4.1 2.1 0.4 1.5 0.5 0.8 99.9
methyl linoleate methyl oleate methyl stearate methyl palmitate methyl linolenate
C18:2 C18:1 C18:0 C16:0 C18:3
53 24 10 10 3
from 35 to 550 amu at a cycling rate of approximately150 scans/ s. 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. Fischer-Tropsch Diesel Fuel. A large South African company supplied the FT middle distillate fuel used in this study. This fuel was from coal and distilled to meet the petroleum middle distillate boiling range as shown in Table 1.1 A gc/ms analysis of the particular fuel revealed that it consisted of C11-C20 alkanes, Table 2, with no aromatic or polar compounds detected. This fuel sample met the MIL specifications requirements and greatly exceeded the cetane number as shown in Table 1.2 Soy Derived Biodiesel Fuels. Ag Environmental Products, 9804 Pflumm Road, Lenexa, KS 66215, supplied the newly manufactured soy-derived biodiesel fuel. This material was light yellow in color, 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 of 45, which exceeds that of petroleum diesel. A large Eastern United States fuel oil company supplied the recycled soy biodiesel. It 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 was supplied on its cetane number. This recycled soy cooking oil sample had been filtered, methylated, and distilled. The color of the recycled
a
Traces of other unidentified methyl esters.
Table 4. Carboxylic Acids in Recycled Soy-Derived Biodiesel carboxylic acid
area %a
malonic hexanoic hex-3-enoic non-3-enoic nonanoic nonanedioic dodec-3-enedioic octadeca-9,12-dienoic octadeca-9-enoic
1 8 2 3 2 1 1 18 1
a The numbers are expressed as the relative area percent from this extract.
soy liquid was a yellow-brown as received. All fuels are stored under refrigeration until used. The specific methyl ester concentration as determined by gc/ms for the soy-derived biofuel is depicted in Table 3, and the polar carboxylic acids present in the recycled soy, but not in that newly manufactured, is depicted in Table 4. Results and Discussion The Department of Defense is considering the use of middle distillate fuels that contain between 5 and 10% biodiesel. Furthermore, it will not have to be disclosed that these fuels are blends and not purely petroleum-derived. Market forces to combat the decrease in petroleum crude have resulted in more research into areas of synthetic fuels, FT.6 It is probable that in the future middle distillate fuels could consist of multiblends. Military fuels unlike fuels in the civilian sector are stored for 1-2 y and sometimes longer under ambient conditions all over the world.7 Under these conditions, the fuel can react with oxygen, other molecules in the fuel, and water from the air and storage tank bottoms.8-11 Two different types of instability are noted, based on the conditions to which the fuel is exposed. First is short-term high temperature oxidative instability, which involves oxidation reactions that occur as the fuel goes from the fuel tank to the engine in a regimen of increasing temper-
7366
Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009
Table 5. Storage Stability of Petroleum and Fischer-Tropsch-Derived Fuels and Soy Biofuel Blends gravimetric solids (mg solids/100 mL fuel)
fuels 100 mL petroleum diesel 100 mL (FT) diesel 50 mL petroleum + 50 mL FT
0.1 0.4 1.7
soy fuel blends 10 mL soy + 90 mL petroleum diesel 20 mL soy + 80 mL petroleum diesel 10 mL recycled soy + 90 mL petroleum diesel 5 mL soy in 47.5 mL petroleum diesel + 47.5 mL FT 5 mL soy + 95 mL FT
0.3 0.2 4.2 79.8 735.0
ature. Second and most important to our laboratory is longterm ambient storage instability, which involves oxidation reactions that occur with dissolved oxygen and polar components such as carboxylic acids and other organo-nitrogen or organosulfur compounds in water bottoms of storage tanks.7 When storage tanks are drawn down or refilled, the contents of the water bottoms and the fuels are mixed. Our laboratory defines storage tank instability as the formation of insoluble sediments and expressed them as milligrams of sediment per 100 mL fuel.2,5 Sediments once formed plug fuel filters, nozzles, and injectors and caused other major mechanical damage. Our laboratory has developed an ASTM method for the gravimetric determination of these insolubles.6 By this procedure, 3.0 mg of sediment/100 mL fuel or less is indicative of a fuel that will be stable in bulk tank storage for at least 2 y. Soybean-derived biodiesel (both newly manufactured and recycled) was blended into both stable petroleum and a stable Fischer-Tropsch diesel fuel, Table 5. These blends were then examined for fuel instability and incompatibility reactions. Our laboratory is concerned with storage tank instability, and the results in Table 5 show that newly manufactured soybean biodiesel could be safely blended with petroleum diesel in amounts up to 2-4 times (20%) what is currently proposed (5-10% blend) by the Department of Defense and remain stable in storage. The results with the Fischer-Tropsch fuel biofuel blends were very different. The results depicted in Table 5 showed that the lowest concentration of biofuel added to FT fails the ASTM stability procedure. A 5% biofuel 95% FT blend (v/v) gave the largest amount of sediments that our laboratory has ever measured, 735 mg of solids. Even at the 90 °C temperature of the ASTM procedure, the biofuel was found to be only slightly soluble in the FT diesel. After these results were obtained, it was noted that when the biofuel was added originally to the FT fuel, the soy biofuel settled to the bottom of the reaction flask. Thus, the ASTM gravimetric method was measuring the insoluble FAME. It was not a specific fatty acid that was insoluble in the FT fuel liquid but the long alkyl chain of all the C16-C18 components. A gc/ms analysis of the FT fuel provided one of the answers to the insolubility of the biofuel. The FT fuel was nonpolar and consisted of 100% alkanes (straight and branched mainly 2- and 3-methyl isomers), Table 2. These isomers result in a tertiary carbon atom that can form a more stable hydroperoxide species. No aromatic or heteroaromatic compounds were detected in this FT fuel. This was not the case with the petroleum-derived middle distillate, which contains approximately 60% aromatic species and a significant polar heteroatom content.12 Another reason for the large amount
of sediment had been observed in an earlier report from our laboratory.13 The reaction of naturally occurring hydroperoxide species with the biofuel resulted in the formation of insoluble polar oxidation products.13-15 The stability of blends of the FT fuel and petroleum middle distillate fuel that contained 5% soy biofuel was not promising as a diesel replacement for the same two reasons. When these blends were thermally stressed by the ASTM procedure,5 the results as shown in Table 3 were better than observed in the FT fuel alone, by a factor of almost 10, but still a large failure at 79.8 mg/100 mL fuel. This magnitude of this failure was somewhat surprising. Table 2 depicts not only the straight chain alkanes but also the isomers observed for the FT fuel. The results with recycled soy-derived biodiesel were less than promising. When this recycled soy product was added to the stable petroleum diesel, it did not pass the ASTM storage stability test. It failed with 4.2 mg of solids. The reason for this failure was related to the other failures with one big difference. It was found that this recycled biofuel contained a 1-2% mixture of shorter chain carboxylic acids.11 These short chain carboxylic acids had not been completely methylated during recycling. Our laboratory has reported that concentrations of less than one percent carboxylic acids will induce storage instability.13 Since the recycled biofuel was a failure in the stable petroleum diesel, it was not tested with the Fischer-Tropsch diesel since the newly manufactured soy biodiesel was a failure in FT fuels. Conclusion The blending of various fuels must be done with great caution. The fuels and blends reported are under consideration for DOD use. The results from this study showed that this particular FT fuel was not compatible with soybean derived biofuels even at a low 5% concentration in a petroleum fuel. The FT and petroleum middle distillate blends were only marginally compatible giving 1.7 mg solids/100 mL of fuel blend. The recycled soy-derived biodiesel (5%) failed the ASTM stability procedure with a petroleum blend. Furthermore, all of the FT blends with biofuels failed the ASTM stability procedure, meaning that they could not be stored and remain stable. Thus, any of these blends would not be useful as diesel fuels for Department of Defense applications. The only stable blend was a 5-10% newly manufactured soy-derived biodiesel blended with petroleum diesel. This blend gave results similar to pure petroleum-derived diesel. The storage stability of FT fuels may be improved by the use of additives such as antioxidants. However, there are no additive fuel compatibility packages. Since the agricultural crop FAME has a similar distribution of compounds and molecular weights, it is probable that similar solubility results would be noted with FT fuel liquids. Acknowledgment This work was supported by the Office of Naval Research both directly and through the Naval Research Laboratory. Literature Cited (1) Standard Specification for Diesel Fuel Oils, ASTM Standard D 97598b. ASTM Standards; American Society of Testing and Materials: West Conshohocken, PA, 2000; Section 5, Vol. 05.01. (2) Performance Specification Fuel, NaVal Distillate, Military Specification MIL-PRF-16884L; Department of Defense: Washington, D.C., 23 October 2006. (3) Dunn, R. O.; Knothe, G. Alternative Diesel Fuels from Vegetable Oils and Animal Fats. J. Oleo Sci. 2001, 50, 415.
Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009 (4) 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. (5) Beal, E. J.; Cooney, J. V.; Hazlett, R. N.; Morris, R. E.; Beaver, B. D.; Hardy, D. R. Mechanisms of Syncrude/Synfuel Degradation; Final Report, Naval Air Systems Command, N00019-72-C-10161, 1972. (6) Standard Test Method for Assessing Distillate Fuel Storage Stability by Oxygen Overpressure. Annual Book of ASTM Standards; American Society for Testing Materials: West Conshohocken, PA, 1999; Part 05.03, ASTM D5304-99, pp 569-572. (7) Giles, H. Petroleum Stockpiling-It’s History and an Overview of Global Projects and Technologies. Proceedings of the 8th International Conference on Stability and Handling of Liquid Fuels, Steamboat Springs, CO, 2003. (8) Mayo, F. R. The chemistry of fuel Deposits and Their Precursors; Final Report, Naval Air Systems Command, N00019-72-C-10161, 1972. (9) Mushrush, G. W.; Speight, J. G. Petroleum Products; Taylor and Francis: Philadelphia, PA, 1995. (10) Batts, B. D.; Fathoni, A. Z. A Literature Review on Fuel Stability Studies with Particular Emphasis on Diesel Oil. Energy Fuels 1991, 5, 2.
7367
(11) 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. (12) Bauserman, J. M.; Mushrush, G. W.; Hardy, D. R. Organic Nitrogen Compound Variation from a Worldwide Survey of Middle Distillate Fuels. Ind. Eng. Chem. 2008, 47, 2867–2875. (13) Mushrush, G. W.; Wynne, J. H.; Lloyd, C. T.; Willauer, H. D.; Hughes, J. M. Instability Reactions and Recycled Soybean-Derived Biodiesel Fuel Liquids. Energy Sources, Part A 2007, 29, 491–497. (14) Mushrush, G. W.; Wynne, J. H.; Lloyd, C. T.; Willauer, H. D. Incompatibility of Recycled Soy-Derived Biodiesel in Marine Environments. Ind. Eng. Chem. 2005, 44, 9969–9972. (15) Knothe, G.; Dunn, R. O. Biofuels Derived from Vegetable Oils and Fats. In Oleochemical Manufacture and Applications; Gunstone, F. D., Hamilton, R. J., Eds.; Sheffield Academic Press: Sheffield, UK, 2001.
ReceiVed for reView December 4, 2008 ReVised manuscript receiVed May 11, 2009 Accepted May 27, 2009 IE801867T
