Transesterified milkweed (Asclepias) seed oil as a biodiesel fuel

Fuel 85 (2006) 2106–2110 www.fuelfirst.com

Transesterified milkweed (Asclepias) seed oil as a biodiesel fuel Ronald Alan Holser *, Rogers Harry-O’Kuru United States Department of Agriculture, Agricultural Research Service, National Center for Utilization Research, 1815 North University Street, Peoria, IL 61604, USA Received 25 October 2005; received in revised form 17 February 2006; accepted 3 April 2006 Available online 2 May 2006

Abstract The methyl and ethyl esters of milkweed (Asclepias) seed oil were prepared and compared to soybean esters in laboratory tests to determine biodiesel fuel performance properties. The pour points of the methyl and ethyl milkweed esters measured 6 C and 10 C, respectively, which is consistent with the high levels of unsaturation characteristic of milkweed seed oil. The oxidative stabilities measured by OSI at 100 C were between 0.8 and 4.1 h for all samples tested. The kinematic viscosities determined at 40 C by ASTM D 445 averaged 4.9 mm2/s for milkweed methyl esters and 4.2 mm2/s for soybean methyl esters. Lubricity values determined by ASTM D 6079 at 60 C were comparable to the corresponding soybean esters with average ball wear scar values of 118 lm for milkweed methyl esters and 200 lm for milkweed ethyl esters.  2006 Elsevier Ltd. All rights reserved. Keywords: Asclepias; Biodiesel; Milkweed

1. Introduction Milkweed (Asclepias) seed oil was investigated as an alternative feedstock for the production of a biodiesel fuel. Common milkweed (Asclepias syriaca) is native to the Northeast and North Central United States where it grows on roadsides and undisturbed habitat. Generally considered a nuisance by farmers it is a perennial plant that produces a marketable fiber with interesting thermal properties. The seed contains 20–25 wt.% (dry basis) oil that is currently underutilized and may provide an inexpensive source of triglycerides for conversion to biodiesel fuel. The transesterification of vegetable seed oils for use as a renewable fuel in compression ignition engines continues to gain acceptance due to both environmental and economic factors. Typical vegetable oil feedstocks for biodiesel include the commodity seed oils such as soybean and canola [1,2]. These vegetable oils are readily converted by

*

Corresponding author. Tel.: +1 309 681 6111; fax: +1 309 681 6340. E-mail address: [email protected] (R.A. Holser).

0016-2361/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.04.001

base catalysis into the corresponding alkyl esters and provide acceptable fuel properties in modern diesel engines [3,4]. The technology of biodiesel production from vegetable oil feedstocks is clearly defined although the process economics may be improved by selection of lower cost feedstocks [5]. Milkweed seed is commercially harvested for the seed floss which exhibits a high insulating value and finds applications as a fiber fill material in hypoallergenic comforters and pillows [6]. The seed floss may also be combined with cotton fibers for woven textiles [7]. The seed is obtained as a by-product after separation from the seed floss [8]. The market for viable seed is limited primarily to ornamental and natural landscape applications or where it is desired to attract butterflies that feed on the plant. The oil can be recovered in high yield by mechanical expeller or solvent extraction. The resulting defatted seed meals also have demonstrated nematocidal activity when used as a soil amendment [9]. Historically, the plant was a source of fiber and medicinals [10]. The milky sap for which milkweed is named has also been examined as a source of latex [11]. Large-scale operations to produce and harvest the plant

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2.3. Analytical techniques

Table 1 Fatty acid profiles of milkweed and soybean esters Fatty acids Saturated C16 C18 C20 Unsaturated C16:1 C18:1 C18:2 C18:3

Milkweed oil (%)

2107

Soybean oil (%)

5.9 2.3 0.2

12.9 3.7 0.3

6.8 34.8 48.7 1.2

0.1 22.2 52.9 7.9

for commercial purposes have been proposed numerous times over the past several decades. The plant remains ubiquitous but underutilized. Milkweed seed oil is composed of over 90% unsaturated fatty acids with nearly 50% linoleic acid and less than 2% linolenic acid (Table 1). This level of unsaturation has demonstrated potential in technical applications [12]. Based on the fatty acid profile the oil is expected to provide an alternative source of biodiesel fuel. In this study we evaluated the viscosity, pour point, cloud point, and oxidative stability of transesterified milkweed seed oil. 2. Experimental 2.1. Materials Milkweed seed oils were obtained from seeds harvested during the 1997 season and provided by the Natural Fiber Corp. (Ogallala, NE). Seeds were a mixture of A. syriaca and A. speciosa. The seeds were mechanically expelled and the resulting press oil was degummed and refined as described previously [13]. The oil was stored under nitrogen at 20 C. Ethanol, methanol, sodium ethoxide, and sodium methoxide were obtained from Sigma–Aldrich Chemicals (St. Louis, MO). The solvents were HPLC grade and used as received. Refined soybean oil was obtained from Bunge North America, Inc. (Decatur, IN). 2.2. Preparation of esters Refined milkweed and soybean oils were transesterified in 90-g batches with methanol at a 1:6 molar ratio and 1g methoxide catalyst added to produce the methyl esters. Comparable portions of ethanol and ethoxide, respectively, were used to prepare the corresponding ethyl esters. Reactions were performed in 250-mL glass flasks and heated to 60 C on a magnetically stirred hot plate for 1 h. The esters were separated from the lower aqueous glycerol layer by gravity then washed twice with distilled water. The washed esters were dried under vacuum in a rotary evaporator and stored at 5 C for subsequent analyses. Reactions were performed in duplicate.

Oxidative stability measurements were made using an Omnion Model OSI-1102 (Omnion Corp., Rockland, MA) according to the American Oil Chemists’ Society (AOCS) standard method Cd 12b-92 [14]. Samples were analyzed in duplicate at a temperature of 100 C. Lubricity data were obtained using the PCS Instruments Model HFRHCA8 High Frequency Reciprocating Rig (HFRR) (London, UK). Analyses were performed following the ASTM D 6079 method at 60 C [15]. Cloud point and pour point determinations were made using the Phase Technology Model PSA-70S automated apparatus (Richmond, BC, Canada). The procedures of ASTM D 5773 and ASTM D 5949 were followed [16,17]. Kinematic viscosities were measured at 40 C with Cannon–Fenske calibrated viscometers in a Model Cannon TE-1000 constant temperature bath (Cannon Instrument Co., State College, PA) following the procedure of ASTM D 445 [18]. Measurements were taken in triplicate and averaged. Acid values were determined by titration with standardized 0.02 N KOH following AOCS Method Cd 3d-63 [19]. Analyses were performed in triplicate and averaged. Fatty acid esters were analyzed on a Varian Model 3900 gas chromatograph (Varian, Inc., Palo Alto, CA) with a Supelco SP2380 column measuring 30 m · 0.32 mm ID · 0.25 lm (Supelco, Inc., Bellefonte, PA). The initial oven temperature was 100 C and increased to 205 C at 3 C/min. The inlet was maintained at 240 C and 1-lL injections were made with a split ratio of 1:100. Analytes were detected by flame ionization with a detector temperature of 250 C. Helium carrier gas was used with a linear velocity of 35 cm/s. Analytes were identified by comparison of retention time to known standards. Glycerol and the individual glycerides were determined following the procedure of ASTM D 6584 using an Agilent 6890 gas chromatograph (Agilent Technologies, Inc., Santa Clara, CA) with a DB-5HT column measuring 15 m · 0.32 mm ID · 0.1 lm [20]. 3. Results and discussion Estimates of the heat of combustion and cetane number (CN) were calculated for the milkweed and soybean esters following standard techniques based on chemical structures [21]. A value of 2772 kg cal/mol was obtained for the heat of combustion for the milkweed methyl esters based on the literature values of the individual fatty acid methyl esters [22]. This is comparable to a value of 2770 kg cal/mol calculated for soybean methyl esters. A similar method was followed to estimate cetane numbers from the fatty acid ester structures using literature values for the pure fatty esters [23,24]. This provided estimates of 50 and 55 for the cetane numbers of the milkweed methyl esters and ethyl esters, respectively. These results indicate that the energy content and the fuel ignition performance of the milkweed esters are acceptable for use as a diesel fuel based on

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Table 2 Analysis of milkweed and soybean esters Ester

Acid value mg KOH/g

Glycerides (wt.%)

Glycerin (wt.%)

Mono

Di

Tri

Free

Total

Methyl MWM-1 MWM-2 SBM-1 SBM-2

0 0 0 0

0.170 0.068 0.172 0.058

0.175 0.178 0.097 0.095

0.039 0.009 0.018 0.005

0.008 0.019 0.022 0.003

0.082 0.064 0.083 0.033

Ethyl MWE-1 MWE-2 SBE-1 SBE-2

0 0 0 0

3.312 2.899 3.902 3.379

2.882 3.249 3.427 3.000

3.282 3.390 3.779 3.279

0.663 0.750 0.952 0.513

2.293 2.338 2.868 2.177

ASTM D 6751 that prescribes a minimum cetane number of 47 for biodiesel fuels [25]. Analyses of the laboratory prepared milkweed (MW) and soybean (SB) esters are presented in Table 2. The methyl esters were obtained in greater than 99% conversion for both feedstocks while the same reaction conditions were not sufficient to obtain comparable conversion to the ethyl esters with either feedstock. The methyl esters showed some variation between batches although only SBM-1 did not meet the specification of 0.02 wt.% set by ASTM D 6751 and EN 14214 for free glycerin, measuring 0.022 wt.%. In contrast to the methyl esters, the ethyl esters were difficult to separate from co-product glycerin. After washing the esters twice with water the free glycerin content significantly exceeded the specification with both milkweed and soybean oil feedstocks. The pour point and cloud point values provide indirect measures of viscosity and crystallization as a function of temperature. Milkweed seed oil contains less saturated fatty acids and more unsaturated fatty acids than soybean oil and is therefore expected to exhibit better cold weather performance. Table 3 presents the mean values and standard errors (se) for pour point and cloud point measurements obtained from the milkweed (MW) and soybean

Table 3 Properties of milkweed and soybean esters (kinematic viscosity, cloud point, pour point: N = 3; OSI: N = 2) Ester

Viscosity (40 C) (mm2/s)

Cloud point (C) Mean

Pour point (C)

Mean

se

Methyl MWM-1 MWM-2 SBM-1 SBM-2

se

5.2 4.6 4.1 4.3

0.023 0.005 0.000 0.000

0.8 1.1 1.5 1.5

0.120 0.100 0.058 0.133

Ethyl MWE-1 MWE-2 SBE-1 SBE-2

5.7 5.7 5.3 5.3

0.000 0.007 0.003 0.005

5.1 2.5 8.4 8.9

0.067 0.033 0.208 0.219

Mean

OSI (100 C) (h) se

Mean

se

6.7 6.0 0.0 0.7

0.667 0.000 0.000 0.000

0.8 1.5 4.1 1.6

0.076 0.075 0.025 0.075

10.0 10.0 2.0 2.7

0.000 0.667 0.000 0.667

1.9 1.7 1.7 1.3

0.000 0.130 0.050 0.093

(SB) esters. The methyl and ethyl esters of milkweed both have lower cloud point and pour point values than the corresponding soybean esters as expected from structural considerations. In particular, the pour point values are significantly lower with values of 6 C and 10 C for the methyl and ethyl milkweed esters, respectively, versus 0 C and 2 C for the soybean esters. The difference between the cloud point and pour point values observed with the ethyl esters may be an influence of the unconverted or partially converted triglycerides. However, the lower values exhibited by the milkweed esters compared to the soybean esters are consistent with the general trend for fatty esters values to decrease with increasing unsaturation. These data suggest that milkweed esters could show improved cold flow performance. The kinematic viscosity values are reported in Table 3. The viscosity observed for the milkweed esters are slightly higher than the corresponding soybean esters. For example, the viscosity of the milkweed methyl esters measured 5.2 mm2/s for MWM-1 and 4.6 mm2/s for MWM-2, whereas soybean methyl esters measured 4.1 mm2/s for SBM-1 and 4.3 mm2/s for SBM-2. The milkweed ethyl esters measured 5.7 mm2/s and the soybean ethyl esters measured 5.3 mm2/s. The ASTM standard D 6751 prescribes an acceptable viscosity range for biodiesel of 1.9– 6.0 mm2/s which would be satisfied by all of these materials. In contrast the European standard EN 14214 limits the acceptable range to 3.5–5.0 mm2/s. Viscosity generally increases with the number of carbon atoms in the fatty acid portion of the ester and also with the number of carbon atoms in the alcohol portion as determined by measurements with individual ester standards [26,27]. Therefore, the viscosities of the ethyl esters are expected to exceed those of the corresponding methyl esters. This was observed and is consistent with other reports for biodiesel prepared from vegetable oil feedstocks [28,29]. The oxidative stability of a biodiesel fuel is related to the structures of the component fatty esters. The highly unsaturated ester structures such as linolenate oxidize more rapidly than the saturated ester structures. These oxidative processes lead to degradation of the fuel and reduce quality. While oxidative stability is not equivalent to storage stability it can provide a qualitatively useful measure of the ability of the fuel to undergo autoxidation. The milkweed esters contain more unsaturated fatty structures than the soybean esters and would be expected to oxidize more rapidly if the oxidation rates of the individual fatty esters were the same. However, the rate of autoxidation decreases with decreasing unsaturation so that linolenate > linoleate > oleate > stearate. Stearate is fully saturated and therefore quite oxidatively stable. Soybean oil contains more of the highly unsaturated linolenate than milkweed although less total unsaturation. It has been reported that the presence of compounds such as linolenate can produce a nonlinear response in oxidative stability tests [30]. The results of the laboratory test for oxidative stability are typical for biodiesel obtained from refined vegetable

R.A. Holser, R. Harry-O’Kuru / Fuel 85 (2006) 2106–2110

oils (Table 3). The process of refining vegetable oils removes most of the natural antioxidant compounds such as the tocopherols. Currently there is not an oxidative stability requirement specified by ASTM D 6751 although this is under consideration. The European standard EN 14214 requires a minimum of 6 h at 110 C which could not be met by any of the biodiesel samples produced during this study without additives. Lubricity data were collected using the high frequency reciprocating rig (HFRR) which has become the basis for several fuel and lubricant test methods including ASTM D 6079, BS ISO 12156-1, CEC F-06-A-96, EN 590, and JPI-5S-50-98, and IP 450/2000. It uses a ball and disc immersed in the sample to be tested. At the conclusion of the test the ball is visually examined for wear. The dimensions of an observed wear scar on the ball are averaged and reported in lm. The data obtained with this apparatus are presented for the milkweed and soybean esters in Table 4. The mean values for the ball X and Y wear scars are given at 60 C to provide some insight on the variation of the calculated average wear scar values. A similar examination and measurement of the wear scar on the disc can be made and these values are also reported. The instrument calculates values of the film thickness and friction coefficients which are included in Table 4. The methyl esters show uniformly lower values for the average ball wear scar than the ethyl esters, 100–159 lm, versus 195–218 lm, respectively. This is similar to the average disc wear scar with a range of 661–721 lm for the methyl esters and 754–771 lm for the ethyl esters. The feedstock, milkweed or soybean, did not appear to be a significant factor in lubricity. Trace components found in biodiesel fuels including free fatty acids, monoglycerides, and diglycerides are reported to improve the lubricity of biodiesel fuels [31,32]. None of the samples tested in this study had detectable levels of free fatty acids although the ethyl esters contained significant levels of free glycerol and unconverted monoglycerides, diglycerides, and triglycerides (Table 2). These compounds were not observed to enhance the lubricity of the ethyl esters, however, the film

Table 4 Lubricity data for milkweed and soybean esters (60 C), N = 2 Ester

X wear scar (lm)

Y wear scar (lm)

Average (lm)

Film (%)

Friction

Ball

Disc

Ball

Disc

Ball

Disc

Methyl MWM-1 MWM-2 SBM-1 SBM-2

137 129 116 201

226 181 185 236

106 102 84 118

1216 1150 1138 1176

121 115 100 159

721 665 661 706

95 96 99 91

0.122 0.149 0.139 0.115

Ethyl MWE-1 MWE-2 SBE-1 SBE-2

251 255 254 275

314 313 327 296

139 157 139 162

1228 1224 1182 1229

195 206 196 218

771 768 754 763

70 63 69 68

0.137 0.139 0.135 0.138

2109

thickness reported by the HFRR was significantly lower for these materials than for the methyl esters (Table 4). 4. Conclusions The properties of a biodiesel fuel are influenced by the structure and distribution of the component fatty acid esters which vary depending on the source. Milkweed seed oil was investigated as a possible alternative source for biodiesel fuel. The current commercial interest is limited to the unique fiber attached to the seed. The seed remains an untapped source of highly unsaturated oil that is easily transesterified to the corresponding methyl esters. The laboratory prepared milkweed biodiesel exhibited pour point and cloud point values that suggest improved cold weather performance. The kinematic viscosity, oxidative stability, and lubricity values compared favorably to soybean esters. 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 the United States Department of Agriculture or the Agricultural Research Service of any product or service to the exclusion of others that may be suitable. References [1] Schwab AW, Bagby MO, Freedman B. Preparation and properties of diesel fuels from vegetable oils. Fuel 1987;66(10):1372–8. [2] Ma F, Hanna MA. Biodiesel production: a review. Bioresour Technol 1999;70(1):1–15. [3] Van Gerpen JH. Biodiesel processing and production. Fuel Process Technol 2005;86(10):1097–107. [4] Monyem A, Van Gerpen JH. The effect of biodiesel oxidation on engine performance and emissions. Biomass Bioenergy 2001;20(4): 317–25. [5] Tyson KS. DOE analysis of fuels and coproducts from lipids. Fuel Process Technol 2005;86(10):1127–36. [6] Crews PC, Sievert SA, Woeppel LT, McCullough EA. Evaluation of milkweed floss as an insulative fill material. Text Res J 1991;61(4):203–10. [7] Louis GL, Andrews BAK. Cotton/milkweed blends: a novel textile product. Text Res J 1987;57(6):339–45. [8] Von Bargen KL, Jones DD, Zeller RD, Knudsen HD. Equipment for milkweed floss–fiber recovery. Ind Crops Prod 1994;2(3):201–10. [9] Harry-O’Kuru RE, Mojtahedi H, Vaughn SF, Dowd PF, Holser RA, Abbott TP. Milkweed seedmeal, a control for Meloidogyne chitwoodi on potatoes. Ind Crops Prod 1999;9(2):145–50. [10] Gaertner EE. The history and use of milkweed (Asclepias syriaca L.). Econ Bot 1979;33(2):119–23. [11] Adams RP, Balandrin MF, Martineau JR. The showy milkweed, Asclepias speciosa: a potential new semi-arid crop for energy and chemicals. Biomass 1984;4(2):81–104. [12] Harry-O’Kuru RE, Holser RA, Abbott TP, Weisleder D. Synthesis and characteristics of polyhydroxy triglycerides from milkweed oil. Ind Crops Prod 2002;15(1):51–8. [13] Holser RA. Properties of refined milkweed press oil. Ind Crops Prod 2003;18(2):133–8.

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