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Cold-Flow Properties of Soybean Oil Fatty Acid Monoalkyl Ester Admixtures† Robert O. Dunn*,‡ Food & Industrial Oils Research, United States Department of Agriculture (USDA), Agricultural Research SerVice (ARS), National Center for Agricultural Utilization Research (NCAUR), Peoria, Illinois 61604 ReceiVed October 6, 2008. ReVised Manuscript ReceiVed June 23, 2009
Biodiesel is an alternative fuel, made from transesterification of vegetable oil or animal fat with an alcohol, that has many attractive fuel characteristics. However, biodiesel is more prone than petrodiesel to start-up and operability problems during cold weather. The present study investigates the effects on cold-flow properties of biodiesel resulting from the mixture of soybean oil fatty acid methyl esters (SMEs) with fatty acid esters derived from transesterification of soybean oil with medium- and branched-chain alkyl alcohols. Admixtures of SMEs with 0-100 vol % tallow fatty acid methyl esters and with n-propyl (SnPrE), isopropyl (SiPrE), n-butyl (SnBuE), isobutyl (SiBuE), and 2-butyl soyates (S2BuE) were analyzed for cloud and pour point (CP and PP). CP and apparent solidification point (SP ) PP - 1) data were employed to construct temperature-composition phase diagrams for each admixture in SME. Soyates with branched-chain alkyl headgroup moieties were more effective in decreasing CP and PP (or SP) than those with straight-chain headgroups, compared to an unmixed SME. An admixture of 65 vol % SiPrE in SME decreased CP by more than 5 °C compared to an unmixed SME. In contrast, the same admixture with SnPrE decreased CP by only 1.9 °C. Furthermore, an admixture of 65 vol % SnBuE in SME decreased CP by only 2.6 °C despite having an alkyl headgroup with a larger molecular weight than SiPrE. Analogous results were obtained from subambient differential scanning calorimetry analysis performed on mixtures of pure monoalkyl stearates in a methyl oleate solvent.
Introduction Biodiesel is defined as the monoalkyl esters of fatty acids derived from lipids such as vegetable oils or animal fats.1 It is made from transesterification of the lipid with an alcohol in the presence of a catalyst. Typically, methanol or ethanol is employed in the conversion of vegetable oil or animal fat to biodiesel because they are more widely available and less expensive than higher alcohols. Biodiesel has many fuel properties and other characteristics that make it an attractive alternative diesel fuel or extender. It has been applied as an alternative fuel or fuel extender in diesel-powered transportation trucks, automobiles, and farm vehicles as well as locomotives, aircraft, stationary power generators, boilers, and heaters. Biodiesel is renewable, possesses low toxicity, and is readily biodegradable and nonflammable, making it safe to store and handle. It has a gross heat of combustion, specific gravity (SG), and kinematic viscosity (ν) comparable to those of conventional diesel fuel (petrodiesel). It blends well with petrodiesel, enhancing the cetane number (CN), a fuel quality factor that correlates † Disclaimer: The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by USDA or ARS of any product or service to the exclusion of others that may be suitable. * To whom correspondence should be addressed. Telephone: 309-6816101. Fax: 309-681-6340. E-mail:
[email protected]. ‡ The author is a Chemical Engineer at USDA, ARS, NCAUR, Peoria, IL 61604. (1) Specification for Biodiesel Fuel (B100) Blend Stock for Distillate Fuels. In Annual Book of ASTM Standards; ASTM International: West Conshohocken, PA, 2008; Vol. 05.04, method D 6751.
10.1021/ef9002582
with reduced ignition delay time and enhanced combustion.2-5 Biodiesel in very small blend ratios (e2 vol %) restores lubricity and antiwear characteristics in ultralow sulfur petrodiesel that are lost as a side effect of hydrodesulfurization processing.4,6 One study7 showed that during its life cycle biodiesel returns more than 3 times the energy required to produce it and has a net negative carbon dioxide balance. Furthermore, a comprehensive review conducted by the U.S. Environmental Protection Agency reported that blending biodiesel with petrodiesel fuel significantly reduces harmful exhaust emissions such as hydrocarbons, carbon monoxide, and particulate matter while increasing nitrogen oxide emissions by less than 5% for blend ratios up to 20 vol %.8 The use of biodiesel as an alternative fuel also decreases smoke opacity, sulfur dioxide, and polycyclic aromatic hydrocarbon emissions.2-4 (2) Knothe, G.; Dunn, R. O. In Industrial Uses of Vegetable Oils; Erhan, S. Z., Ed.; AOCS Press: Champaign, IL, 2005; pp 42-89. (3) Knothe, G.; Dunn, R. O. In Oleochemical Manufacture and Applications; Gunstone, F. D., Hamilton, R. J., Eds.; Sheffield Academic: Sheffield, U.K., 2001; pp 106-163. (4) Graboski, M. S.; McCormick, R. L. Prog. Energy Combust. Sci. 1998, 24, 125–164. (5) Schwab, A. W.; Bagby, M. O.; Freedman, B. Fuel 1987, 66, 1373– 1378. (6) Van Gerpen, J. H.; Soylu, S.; Tat, M. E. Evaluation of the Lubricity of Soybean Oil-Based Additives in Diesel Fuels. Proceedings of the Annual Meeting of the ASAE; American Society of Agricultural Engineers: St. Joseph, MI, 1999; Paper 996314. (7) Sheehan, J.; Camobreco, V.; Duffield, J.; Graboski, M.; Shapouri, H. Life Cycle InVentory of Biodiesel and Petroleum Diesel for Use in an Urban Bus; Final Report No. NREL-SR-580-24089; National Renewable Energy Laboratory, Golden, CO, 1998; pp 206-261. (8) A ComprehensiVe Analysis of Biodiesel Impacts on Exhaust Emissions; Draft Technical Report Number EPA420-P-02-001; U.S. Environmental Protection Agency: Washington, DC, Oct 2002.
This article not subject to U.S. Copyright. Published 2009 by the American Chemical Society Published on Web 07/14/2009
Soybean Oil Fatty Acid Monoalkyl Ester Admixtures
Despite its many positive effects, the cold-flow properties of biodiesel may hinder its performance in moderate climates during cold weather. Biodiesel in the form of fatty acid methyl esters (FAMEs) derived from soybean oil is known to cause start-up and operability issues as ambient temperatures approach 0-2 °C. As overnight temperatures fall into this range, longchain (C16-C18) saturated FAMEs in biodiesel nucleate and form crystals in the fuel. Once the crystals grow and agglomerate in size, they can restrict or block flow through fuel lines and filters during startup the following morning. These conditions can lead to fuel starvation, resulting in engine failure.9 A recent report by the National Biodiesel Board urged that biodiesel be stored at temperatures at least 6 °C above its cloud point (CP) before splash blending with petrodiesel.10 Many approaches for improving the cold-flow properties of biodiesel have been investigated. Earlier research11-13 demonstrated that cold-flow performance tests such as the cold filter plugging point (CFPP) and the low-temperature flow test (LTFT) are linearly correlated to CP for biodiesel and biodiesel/ petrodiesel blends. It was concluded that the most effective approaches for improving the cold-flow performance of biodiesel are those that significantly reduce CP. Although many biodiesel fuel producers in the U.S. treat biodiesel with cold-flow improver additives that are effective in reducing CFPP and the pour point (PP), research on commercial additives for treating biodiesel/ petrodiesel blends has shown that these additives do not significantly affect CP or LTFT.9,12 Low-temperature fractionation is a process that improves the cold-flow properties of biodiesel by partial crystallization under controlled conditions and the removal of high-melting-point (MP) components.12,14 Earlier studies15,16 reported that fractionation without solvent or crystallization initiators (dry fractionation) performed on soybean oil FAME (SME) reduced CP from 0 to -20 °C. Also reported were analogous results for effects on PP (decreased from -2 to -21 °C), CFPP (from -3 to -19 °C), and LTFT (from +2 to -16 °C). Dry fractionation was effective because it decreased the total saturated methyl ester content from 20.0 to 6.2 wt %.16 Gonza´lez Go´mez et al.17 reported that fractionation of used cooking oil FAME decreased CFPP from -1 to -5 °C, again by significantly reducing the total saturated ester content. Nevertheless, improving the cold-flow properties by modifying the total saturated ester content has several disadvantages. Fractionation (9) Dunn, R. O. The Biodiesel Handbook; Knothe, G., Krahl, J., Van Gerpen, J., Eds.; AOCS Press: Champaign, IL, 2005; pp 83-126. (10) Stanko, R.; Hoenhe, S.; Webb, B.; Lawrence, R.; Boldt, S.; Reimers, P.; et al. Cold Flow Blending Consortium Biodiesel Cold Weather Blending Study; National Biodiesel Board: Jefferson City, MO, 2005; pp 5 and 6 (see http://www.biodiesel.org/resources/reportsdatabase/reports/gen/ 20050728_Gen-354.pdf, last accessed Jun 19, 2009). (11) Dunn, R. O.; Bagby, M. O. Low-Temperature Filterability Properties of Alternative Diesel Fuels from Vegetable Oils. In Proceedings of the Third Liquid Fuel Conference: Liquid Fuel and Industrial Products from Renewable Resources; Cundiff, J. S., Gavett, E. E., Hansen, C., Peterson, C., Sanderson, M. A., Shapouri, H., Van Dyke, D. L., Eds.; American Society of Agricultural Engineers: St. Joseph, MI, 1996; pp 95-103. (12) Dunn, R. O.; Shockley, M. W.; Bagby, M. O. J. Am. Oil Chem. Soc. 1996, 73, 1719–1728. (13) Dunn, R. O.; Bagby, M. O. J. Am. Oil Chem. Soc. 1995, 72, 895– 904. (14) Lee, I.; Johnson, L. A.; Hammond, E. G. J. Am. Oil Chem. Soc. 1996, 73, 631–636. (15) Dunn, R. O. Effect of Winterization on Fuel Properties of Methyl Soyate. In Commercialization of Biodiesel: Producing a Quality Fuel (Proceedings); Peterson, C. L., Ed.; University of Idaho: Moscow, ID, 1998; pp 164-186. (16) Dunn, R. O.; Shockley, M. W.; Bagby, M. O. Trans. SAE, Sec 4: J. Fuels Lubricants 1997, 106, 640–649. (17) Gonza´lez Go´mez, M. E.; Howard-Hildige, R.; Leahy, J. J.; Rice, B. Fuel 2002, 81, 33–39.
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of SME was reported to reduce the oil stability index at 50 °C from 13.6 to 6.9 h.15 Thus, increasing the relative concentrations of mono- and polyunsaturated fatty esters significantly decreases the stability of biodiesel against oxidative degradation. Increasing the total unsaturated fatty ester content also decreases CN relative to saturated esters of comparable chain length.18,19 Decreasing CN may worsen the combustion performance and exhaust emissions. Finally, crystallization and filtration to remove high-MP solids from biodiesel reduce the final fuel product volume and increase the overall production cost. Another approach for improving the cold-flow properties by reducing CP is to make biodiesel from transesterification of vegetable oils or fats with medium-chain (C3-C8) or branchedchain alkyl alcohols instead of methanol. This takes advantage of variations in the crystalline structure caused by a change in the alkyl headgroup moiety. X-ray diffraction studies on fatty derivatives revealed that slow cooling or crystallization from a nonpolar solvent causes saturated monoalkyl esters to favor the formation of prisms with orthorhombic chain-packing geometry.20-23 These crystals have two short and one long spacing and are tilted in the direction of the hydrocarbon end-group plane.20,22,23 Methyl stearate in solid form has long spacings of nearly twice those of ethyl stearate.23 The amphiphilic nature of methyl stearate molecules causes the formation at low temperatures of bilayered structures with polar carboxylic headgroups aligned head-to-head and next to each other in the crystal interior and oriented away from the nonpolar bulk liquid.21,23 Fatty esters with ethyl and larger alkyl headgroups have fewer polar moieties that are capable of shielding interactions between carboxyl groups. Hence, these esters form monolayered structures where the molecule chains are oriented head-to-tail with hydrocarbon tailgroups parallel to each other.21 Analogous to crystallization of long-chain paraffin molecules, crystal growth in monoalkyl stearates continues rapidly in the XY plane, forming large flat platelet lamellae.21,23 Growth in the Z dimension is hindered because of relatively weak intermolecular forces between platelets when they agglomerate by stacking upon each other. Unlike FAME, fatty esters with large or bulky alkyl headgroups disrupt the spacing between molecules in the lamellae, causing rotational disorder in the hydrocarbon tailgroup chains. This disorder results in the formation of nuclei with less stable chain packing followed by transformation to a more stable form at lower temperatures.21 Thus, MP of ethyl stearate (31-33 °C) is lower than that of methyl stearate (39.1 °C).24 In general, MPs of monoalkyl stearate esters decrease to a minimum for C1-C4 alkyl headgroups and then increase for C5 and larger headgroups.21 Studies on the properties of methyl, ethyl, propyl, and butyl ester forms of biodiesel also confirm the beneficial effects of larger headgroups on the cold-flow properties of monoalkyl (18) Knothe, G.; Bagby, M. O.; Ryan, T. W., III. SAE Special Publication SP-1274: State of AlternatiVe Fuel Technologies; Society of Automotive Engineers: Warrendale, PA, 1997; pp 127-132. (19) Harrington, K. J. Biomass 1986, 9, 1–17. (20) Lawler, P. J.; Dimmick, P. S. Food Science and Technology Series No. 87: Food Lipids: Chemistry, Nutrition, and Biotechnology; Marcel Dekker: New York, 1998; pp 229-250. (21) Larson, K.; Quinn, P. J. In The Lipid Handbook, 2nd ed.; Gunstone, F. D., Harwood, J. L., Padley, F. B., Eds.; Chapman & Hall: London, 1994; pp 401-430. (22) Hernqvist, L. In Crystallization and Polymorphism of Fats and Fatty Acids; Garti, N., Sato, Y., Eds.; Marcel Dekker: New York, 1988; pp 97137. (23) Gunstone, F. D. An Introduction to the Chemistry and Biochemistry of Fatty Acids and Their Glycerides, 2nd ed.; Chapman and Hall: London, 1967; pp 69-74. (24) CRC Handbook of Chemistry and Physics, 71st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1990; pp 3-355 and 3-465-3-466.
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esters. Results compiled from several studies25-28 on SMEs reported CP reductions of 1, 9, 3, and 12 °C for ethyl, isopropyl, n-butyl, and 2-butyl soyates, respectively, compared with CP ) 0 °C for SME.13 Correspondingly, PPs of these soyates decreased by 2, 10, 5, and 13 °C, compared with PP ) -2 °C for SME. Foglia et al.25 reported CP ) 17, 15, 12, and 9 °C for methyl, ethyl, n-propyl, and n-butyl esters of tallow, respectively. Similar decreases in PP, CFPP, and LTFT were also observed. Warabi et al.29 reported that methyl, propyl, and butyl esters from transesterification of rapeseed oil by an alkaline catalyst in supercritical alcohol yielded CP ) -4, -8, and -10 °C and PP ) -11, -10, and -14 °C. For waste grease monoalkyl esters, one study30 showed that the ethyl esters had CP and PP that were both 3 °C lower than those of the methyl ester form. Lang et al.31 reported similar results for monoalkyl esters of canola and linseed oils. Increasing the degree of branching in tallowate monoalkyl esters by substituting isopropyl for n-propyl alcohol decreased CP from 12 to 8 °C.25 Similarly, substituting 2-butyl for n-butyl alcohol in tallowate esters decreased PP from 6 to 0 °C, both being significant reductions compared to PP of tallow FAME (TME) ) 15 °C. In contrast to fractionation, biodiesel from transesterification of vegetable oils or animal fats with medium-chain (C2-C4) alcohols does not compromise other fuel properties with respect to those of FAME. Neglecting the effects of molecular weight, oxidative stability should not be significantly affected because fatty acid compositions of biodiesel generally reflect the composition of vegetable oil or fat prior to conversion. Increasing the molecular weight of the straight-chain alkyl headgroup may affect CN of monoalkyl esters; however, these effects do not hinder the performance or emissions for esters with C1-C4 alkyl headgroups.32-34 Furthermore, increasing the degree of branching in the alcohol moiety does not significantly affect CN.32 Increasing the molecular weight of pure straight-chain fatty acid monoalkyl esters also increases ν,35 although increasing the chain length of the alkyl headgroup up to C4 had minimal effect (ν < 6.0 mm2/s). Finally, transesterification with larger alkyl alcohols may increase the gross heat of combustion of biodiesel.36 The main disadvantage of making biodiesel from mediumor branched-chain alkyl alcohols is increased fuel cost. Most of the cost of biodiesel is encompassed in the acquisition of feedstocks and alcohols for conversion. Using methanol (or (25) Foglia, T. A.; Nelson, L. A.; Dunn, R. O.; Marmer, W. N. J. Am. Oil Chem. Soc. 1997, 74, 951–955. (26) Zhang, Y.; Van Gerpen, J. H. SAE Special Publication SP-1160: Performance of AlternatiVe Fuels for SI and CI Engines; Society of Automotive Engineers: Warrendale, PA, 1996; pp 1-15. (27) Lee, I.; Johnson, L. A.; Hammond, E. G. J. Am. Oil Chem. Soc. 1995, 72, 1155–1160. (28) Clark, S. J.; Schrock, M. D.; Wagner, L. E.; Piennaar, P. G. Soybean Oil Esters as a Renewable Fuel for Diesel Engines. Final Report for Project 5980 (Contract number 59-2201-1-6-059-0); U.S. Department of Agriculture, Agricultural Research Service: Peoria, IL, 1983. (29) Warabi, Y.; Kusdiana, D.; Saka, S. Appl. Biochem. Biotechnol. 2004, 113-116, 793–801. (30) Issariyakul, T.; Kulkarni, M. G.; Dalai, A. K.; Bakhshi, N. N. Fuel Process. Technol. 2007, 88, 429–436. (31) Lang, X.; Dalai, A. K.; Bakhshi, N, N,; Reaney, M. J.; Hertz, P. B. Bioresour. Technol. 2001, 80, 53–62. (32) Knothe, G.; Matheaus, A. C.; Ryan, T. W., III. Fuel 2003, 82, 971– 975. (33) Freedman, B.; Bagby, M. O. J. Am. Oil Chem. Soc. 1990, 67, 565– 571. (34) Klopfenstein, W. E. J. Am. Oil Chem. Soc. 1985, 62, 1029–1031. (35) Knothe, G.; Steidley, K. R. Fuel 2007, 86, 2560–2567. (36) Freedman, B.; Bagby, M. O. J. Am. Oil Chem. Soc. 1989, 66, 1601– 1605.
Dunn
ethanol) is presently more economical on a volume basis than conversion with larger alcohols. Formulating monoalkyl ester admixtures by combining FAME with larger alkyl esters may offer a less expensive alternative that significantly improves the cold-flow properties with a minimal increase in fuel costs. An earlier study13 reported that admixtures of 4:1 (v/v) SME/TME had CP, PP, CFPP, and LTFT of 2, 1, 0, and 3 °C, respectively, compared to corresponding values of 0, -2, -2, and 0 °C for unmixed SMEs. Similar results were reported for a comparison of the cold-flow properties of 20 vol % blends of admixtures in No. 2 petrodiesel. Thus, substituting up to 20 vol % SME with TME did not significantly affect the cold-flow properties of biodiesel. The present study is an investigation on the effects of mixing SME with other soybean oil fatty acid monoalkyl esters presenting better cold-flow properties. CP and PP were measured for n-propyl (SnPrE), isopropyl (SiPrE), n-butyl (SnBuE), isobutyl (SiBuE), and 2-butyl (S2BuE) esters and their admixtures with SME. Temperature-composition data were analyzed to determine the phase behavior of admixtures. Subambient differential scanning calorimetry (DSC) has been employed to analyze the melting and freezing properties of biodiesel and correlate them to CP and PP data.9,37 Other studies38,39 demonstrated that DSC is a valuable tool in analyzing the contributions of pure monoalkyl ester components in determining the cold-flow behavior of biodiesel. Consequently, melting and freezing curve DSC analyses were performed on pure methyl, n-propyl, isopropyl, and n-butyl stearate and on binary and ternary mixtures of these monoalkyl stearates in methyl oleate. Experimental Section Materials. Methyl soyate (SME) was commercial product “SoyGold” acquired from Ag Environmental Products (Lenexa, KS). Methyl tallowate (TME) was sample product “Kemester 143” acquired from Witco (Memphis, TN). Fatty acid concentration profiles, ν, SG, and the acid value (AV) of SME and TME are summarized in Table 1. Imperial brand alkali-refined soybean oil (average mol wt ) 885 g/mol) from Bunge Foods (Bradley, IL) was used to prepare monoalkyl soyates. n-Propanol (99%), isopropyl alcohol (ACS reagent grade; 99.5%), isobutyl alcohol (99%), and 2-butanol (99%) were from Aldrich Chemical (Milwaukee, WI); n-butanol (99.7%) was from Fisher Scientific (Pittsburgh, PA). Pure methyl stearate (99+% methyl octadecanoate), methyl oleate (99+% methyl 9Z-octadecenoate), n-propyl stearate (99% n-propyl octadecanoate), and n-butyl stearate (99% n-butyl octadecanoate) were from Nu Chek Prep (Elysian, MN); pure isopropyl stearate (99% 2-propyl stearate) was from ChemServ (Minneapolis, MN). ACS-certified sulfuric acid (H2SO4; 18 M) and sodium bicarbonate (NaHCO3) and ACS-reagent-grade benzene were from Fisher Scientific. ACS-reagent-grade anhydrous diethyl ether (99+%), sodium chloride (NaCl; 99+%), and sodium sulfate (Na2SO4; 99+%) were from Sigma-Aldrich (St. Louis, MO). ACS-reagentgrade petroleum ether was from Mallinckrodt Baker (Phillipsburg, NJ). Deionized water was supplied in-house. Preparation of Monoalkyl Soyates. Transesterification reactions were conducted in a 2-L three-neck, round-bottomed flask with an argon inlet with a thermometer, a water-cooled condenser, and an addition funnel fitted to each opening. Joints were greased and sealed to prevent leaks, and moisture was removed from glassware by purging with argon gas and applying heat from a heat gun prior to introduction of the reactants. The round-bottomed flask was placed in a heating mantle for controlled heating. A stopcock valve on the addition funnel controlled the rate of addition of the reactants (37) Dunn, R. O. J. Am. Oil Chem. Soc. 1999, 76, 109–115. (38) Dunn, R. O. J. Am. Oil Chem. Soc. 2008, 85, 961–972. (39) Imahara, H.; Minami, E.; Saka, S. Fuel 2006, 85, 1666–1670.
Soybean Oil Fatty Acid Monoalkyl Ester Admixtures
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Table 1. Fatty Acid Profile, ν at 40 °C, SG, and AV of TMEs and Soybean Oil Fatty Acid Monoalkyl Estersa fatty acid (wt %) ester
C16:0
C18:0
C18:1
C18:2
C18:3
ν (mm2/s)
SGb
AVc (mg of KOH/g)
SME SnPrE SiPrE SnBuE SiBuE S2BuE TMEd
13.3 12.9 11.7 11.9 12.8 10.6 28.7
4.1 4.4 3.9 3.9 3.6 4.2 20.9
21.1 23.7 26.7 24.1 27.6 24.6 43.0
53.3 51.9 50.7 52.8 49.2 53.4 4.3
8.3 7.1 6.9 7.4 6.8 7.3 0.4
4.117 ( 0.0015 4.986 ( 0.0019 5.563 ( 0.0029 5.368 ( 0.0020 5.141 ( 0.0038 5.758 ( 0.0055 6.363 ( 0.0013
0.8897 0.8791 0.8793 0.8775 0.8717 0.8773 0.8923
0.22 0.45 0.63 0.39 0.48 0.24 ND
a Esters: soybean oil fatty acid methyl esters (SMEs), n-propyl esters (SnPrEs), isopropyl esters (SiPrEs), n-butyl esters (SnBuEs), isobutyl esters (SiBuEs), and 2-butyl esters (S2BuEs); C16:0 ) palmitate; C18:0 ) stearate; C18:1 ) oleate; C18:2 ) linoleate; C18:3 ) linolenate; ND ) not determined. b SG ) (density of oil at 15.6 °C)/(density of water at 15.6 °C); estimated error
