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Influence of extended storage on fuel properties of methyl esters prepared from canola, palm, soybean and sunflower oils
Renewable Energy 36 (2011) 1221e1226
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Influence of extended storage on fuel properties of methyl esters prepared from canola, palm, soybean and sunflower oils Bryan R. Moser* United States Department of Agriculture,1 Agricultural Research Service, National Center for Agricultural Utilization Research, 1815N. University St., Peoria, IL 61604, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 January 2010 Accepted 9 October 2010 Available online 10 November 2010
Fatty acid methyl esters prepared from canola, palm, soybean, and sunflower oils by homogenous basecatalyzed methanolysis were stored for 12 months at three constant temperatures ( 15, 22, and 40 C) and properties such as oxidative stability, acid value, kinematic viscosity, low temperature operability, and iodine value were periodically measured. Oxidative stability was significantly reduced upon extended storage and acid value as well as kinematic viscosity were increased by only small increments, with these effects more pronounced at elevated temperatures. Iodine value and low temperature operability were essentially unaffected by extended storage. Based on these findings, it is not recommended that acid value or kinematic viscosity be used as indicators of storage stability of biodiesel, nor is it recommended that iodine value be used as a predictor of oxidative stability or indicator of oxidative degradation. Published by Elsevier Ltd.
Keywords: Acid value Biodiesel Fatty acid methyl esters Induction period Kinematic viscosity Oxidative stability
1. Introduction Biodiesel is defined as the monoalkyl esters of long-chain fatty acids prepared from lipid feedstocks [1e3]. Advantages of biodiesel over conventional petroleum diesel fuel (petrodiesel) include derivation from renewable feedstocks, displacement of imported petroleum, superior lubricity and biodegradability, lower toxicity, essentially no sulfur content, higher flash point, positive energy balance, and a reduction of most regulated exhaust emissions. A primary disadvantage of biodiesel is inferior oxidative and storage stability versus petrodiesel, along with lower volumetric energy content, reduced low temperature operability, susceptibility to hydrolysis and microbial degradation, as well as higher nitrogen oxide exhaust emissions [2,3]. Biodiesel must be certified as compliant with accepted fuel standards (Table 1) such as ASTM
Abbreviations: AOCS, American oil Chemists’ Society; AV, Acid value; CEN, European Committee for Standardization; CME, Canola oil methyl esters; CP, Cloud point; FAME, Fatty acid methyl esters; IP, Induction period; IV, Iodine value; KV, Kinematic viscosity; MUFAME, Monounsaturated FAME; PME, Palm oil methyl esters; PP, Pour point; PUFAME, Polyunsaturated FAME; RBD, Refined bleached and deodorized; Rt, Room temperature; SFAME, Saturated FAME; SFME, Sunflower oil methyl esters; SME, Soybean oil methyl esters. * Tel.: þ1 309 681 6511; fax: þ1 309 681 6524. E-mail address: [email protected]. 1 Product names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. 0960-1481/$ e see front matter Published by Elsevier Ltd. doi:10.1016/j.renene.2010.10.009
D6751 [1] in the United States or the European Committee for Standardization (CEN) standard EN 14214 [4] before combustion in diesel engines. Oxidative stability of biodiesel is determined through measurement of the induction period (IP) by CEN method EN 14112, otherwise known as the Rancimat method. This method utilizes elevated exposure to air (10 L/h) and temperature (110 C) to induce accelerated oxidative degradation [5]. Oxidation is indirectly measured by monitoring the increase in conductivity of deionized water caused by volatile oxidative degradation products transported from the biodiesel sample. The IP is graphically determined as the inflection point of a plot of conductivity versus time. The units for IP are expressed as hours and biodiesel fuels with longer IP times are considered more stable to oxidation. The biodiesel standards ASTM D6751 and EN 14214 specify minimum limits for IP of 3 and 6 h, respectively (Table 1). Oxidation initiates at methylene carbons allylic to sites of unsaturation along the hydrocarbon backbone of biodiesel [6]. Biodiesel fuels with more allylic positions are especially vulnerable to oxidation, as evidenced by the relative rates of oxidation of 1, 41, and 98 for the unsaturated ethyl esters of oleic, linoleic, and linolenic acids [7]. This trend is mirrored by the IP values of methyl esters of stearic (> 40 h), oleic (2.5 h), linoleic (1.0 h), and linolenic (0.2 h) acids [8]. Several factors aside from fatty acid composition influence oxidative stability of biodiesel, such as exposure to elevated temperature, light and oxygen, as well as the presence of pro-oxidants such as metals [6,9e13]. Methods by which the IP of biodiesel may be improved include employment of antioxidants,
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Table 1 Selected specifications from ASTM D6751 and EN 14214 biodiesel fuel standards.
CP, C PP, C CFPP, C IP, 110 C, h KV, 40 C, mm2/s AV, mg KOH/g IV, g I2/100 g Cetane number a b
ASTM D6751
EN 14214
Report e e 3 min 1.9e6.0 0.50 max e 47 min
ea e Variableb 6 min 3.50e5.00 0.50 max 120 max 51.0 min
Not specified. Depends on location and time of year.
2.3. Fatty acid profile by GC FAMEs were separated using a Varian (Walnut Creek, CA, USA) 8400 GC equipped with an FID detector and SP2380 (Supelco, Bellefonte, PA, USA) column (30 m 0.25 mm i.d., 0.20 mm film thickness). Carrier gas was He at 1 mL/min. The oven temperature was initially held at 150 C for 15 min, then increased to 210 C at 2 C/min, followed by an increase to 220 C at 50 C/min. The injector and detector temperatures were set at 240 C and 270 C, respectively. FAME peaks were identified by comparison to the retention times of known reference standards.
2.4. Storage and handling of samples blending with more stable fuels such as petrodiesel, reducing exposure to risk factors (heat, light, air, pro-oxidants), and selection of feedstocks that contain lower concentrations of polyunsaturated fatty acids [9,13e19]. In addition to reducing IP, oxidative degradation is reported to increase acid value (AV), peroxide value, kinematic viscosity (KV) and cetane number while reducing heat of combustion as well as carbon monoxide and hydrocarbon exhaust emissions [9,12,14,20e29]. Consequently, AV and KV have been suggested as indicators of oxidative degradation of biodiesel [23,28]. The most significant vegetable oils produced worldwide during the previous year were palm (45.13 MMT), soybean (37.69 MMT), rapeseed/canola (21.93 MMT), and sunflower (11.45 MMT) oils [30]. Generally, the most abundant oils or fats in a region are most commonly used as feedstocks for biodiesel production. Thus, rapeseed/canola and sunflower oils are principally used in Europe, palm oil predominates in tropical countries, and soybean oil and animal fats are most commonly used in the USA [2,3]. The primary objective of the current investigation was to determine the influence of long-term storage on the oxidative stability, AV, KV, low temperature operability, and iodine value (IV) of canola (CME), palm (PME), soybean (SME), and sunflower (SFME) oil methyl esters. A further objective was to determine the utility of AV and KV as indirect indicators of oxidative degradation.
2. Materials and methods
Biodiesel samples were stored at room temperature (rt; 21 2 C average laboratory temperature), 40 1 C (temperaturecontrolled laboratory oven), and 15 2 C (laboratory freezer) in the dark in capped amber-tinted glass jars over the course of a 12 month period. Every two months samples were removed for analysis following brief agitation and subsequently discarded.
2.5. Oxidative stability Induction period (IP, h) was measured in accordance to EN 14112 utilizing a Metrohm USA, Inc. (Riverview, FL, USA) model 743 Rancimat instrument. Briefly, the flow rate of air through 3 0.01 g of sample was 10 L/h with a block temperature of 110 C (correction factor of 1.5 C). The conductivity measuring vessel contained 50 0.1 mL of deionized water. IP was mathematically determined as the inflection point of a plot of conductivity (mS/cm) of deionized water versus time (h). 2.6. Low temperature properties Cloud and pour point (CP and PP, respectively) determinations were made according to automated methods ASTM D5773 and ASTM D5949, respectively, using a model PSA-70S Phase Technology Analyzer (Richmond, B.C., Canada). For a greater degree of accuracy, PP measurements were done with a resolution of 1 C instead of the specified 3 C increment.
2.1. Materials Refined, bleached, and deodorized (RBD) canola (low erucic acid rapeseed oil), palm, soybean, and sunflower (high oleic) oils without additives were purchased from KIC Chemicals, Inc. (New Platz, NY, USA) and used as received. Fatty acid methyl ester (FAME) standards were purchased from Nu-Chek Prep, Inc. (Elysian, MN, USA). All other chemicals and reagents were obtained from SigmaeAldrich Corp (St. Louis, MO, USA) and used as received. All measurements were done in triplicate with mean values reported.
2.2. Transesterification Methanolysis was accomplished with sodium methoxide catalyst (0.5 wt % with respect to oil), 6:1 mole ratio of methanol to oil, 60 C reaction temperature, and 1.0 h reaction time. After sequential removal of glycerol by gravity separation and methanol by reduced pressure (10 mbar; 30 C) rotary evaporation, the crude products were washed with distilled water until a neutral pH was obtained and dried with MgSO4 to yield FAME in high yield (> 97 wt %).
2.7. Kinematic viscosity Kinematic viscosity (KV, mm2/s) was determined with CannonFenske viscometers (Cannon Instrument Co., State College, PA, USA) at 40 C as specified in ASTM D445. 2.8. Acid value Acid value (AV, mg KOH/g) titrations were performed as described in ASTM D664 using a Metrohm 836 Titrando (Westbury, NY, USA) autotitrator equipped with a model 801 stirrer and a Metrohm 6.0229.100 Solvotrode. The titration endpoint was automatically determined and verified using a phenolphthalein indicator. 2.9. Iodine value Iodine value (IV. G I2/100 g) was calculated from the fatty acid profile (Table 2) according to American Oil Chemists’ Society (AOCS) official method Cd 1c-85.
B.R. Moser / Renewable Energy 36 (2011) 1221e1226 Table 2 Fatty acid composition (wt %) of canola, palm, soybean, and sunflower oil methyl esters. Fatty acida
Canola
Palm
Soybean
Sunflower
C12:0 C14:0 C16:0 C18:0 C20:0 C22:0 C16:1 9c C18:1 9c C18:1 11c C18:2 9c, 12c C18:3 9c, 12c, 15c C20:1 11c unknown S SFAMEc S MUFAMEd S PUFAMEe
eb 0.1 5.0 2.0 0.6 0.3 0.3 60.1 3.0 19.8 7.2 1.1 0.5 8.0 64.5 27.0
0.3 1.2 44.1 4.1 0.3 e 0.3 38.4 0.7 10.0 0.2 0.1 0.3 50.0 39.5 10.2
e 0.1 11.7 3.8 0.2 0.2 e 22.0 1.4 52.6 7.7 0.2 0.1 16.0 23.6 60.3
e 0.1 4.7 3.9 0.3 0.9 0.1 81.3 0.4 7.7 0.3 0.3 e 9.9 82.1 8.0
a
For example, C18:1 9c signifies an 18 carbon fatty acid chain with one cis double bond located at carbon 9 (methyl 9Z-octadecenoate; methyl oleate). b Not detected (< 0.1 wt %). c S SFAME ¼ C12:0 þ C14:0 þ C16:0 þ C18:0 þ C20:0 þ C22:0. d S MUFAME ¼ C16:1 þ C18:1 þ C20:1. e S PUFAME ¼ C18:2 þ C18:3.
3. Results and discussion 3.1. Preparation and fatty acid composition of methyl esters Methyl esters were prepared by methanolysis from RBD vegetable oils in high yields (> 97 wt %) utilizing the classic homogenous base-catalyzed procedure (60 C, 1 h, 0.50 wt % catalyst and 6:1 mole ratio of methanol to vegetable oil) described previously [2,17,19,31]. Analysis by GC (ASTM D6584) revealed that each sample was essentially free of free and bound glycerol (results not shown). The principle FAMEs identified in CME (low erucic acid variety) by GC (Table 2) were methyl oleate (60.1 wt %) and methyl linoleate (19.8 wt %), with lesser amounts of methyl linolenate (7.2 wt %), methyl palmitate (5.0 wt %), and other minor FAMEs also detected. Methyl palmitate (44.1 wt %) and methyl oleate (38.4 wt %) were the primary FAMEs found in PME, along with methyl linoleate (10.0 wt %) and methyl stearate (4.1 wt %), among other constituents. The dominant FAME detected in SME was methyl linoleate (52.6 wt %), with significant amounts of methyl oleate (22.0 wt %) and methyl palmitate (11.7 wt %) also identified. The primary FAME detected in SFME (high oleic variety) was methyl oleate (81.3 wt %), along with methyl linoleate (7.7 wt %), methyl palmitate (4.7 wt %), and methyl stearate (3.9 wt %). The results for these methyl esters were in close agreement with FAME profiles reported elsewhere [8,10,12-15,17,19,20,22,24,25,27,32].
3.2. Influence of extended storage on oxidative stability The initial IP (EN 14112; 110 C) values of CME, PME, SME, and SFME were 6.4, 10.9, 5.0, and 6.2 h, respectively (Table 3). As seen by comparison to the oxidative stability requirements contained in ASTM D6751 and EN 14214 (Table 1), all samples were above the minimum specified limit in ASTM D6751 (> 3.0 h). However, SME was below the more stringent specification (> 6.0 h) listed in EN 14214. The content of polyunsaturated FAMEs (PUFAME) in SME (60.3 wt %; Table 2) relative to the other samples was attributed to its reduced oxidative stability. In contrast, PME contained the highest percentage of saturated FAME (SFAME; 50.0 wt %; Table 2) and exhibited the highest stability to oxidation according to the Rancimat (EN 14112) method. As previously discussed, the oxidative stability of biodiesel decreases with increasing content of polyunsaturation [2,6e9,16,18,19].
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Table 3 Influence of storage time (months) and temperature on induction periodsa (IP, h) and retention factorsb (Rf) of canola, palm, soybean, and sunflower oil methyl esters. Storage temp IP
IP
IP
IP
IP
IP
IP
Rf
Rf
t ¼ 0 t ¼ 2 t ¼ 4 t ¼ 6 t ¼ 8 t ¼ 10 t ¼ 12 t ¼ 6 t ¼ 12 Canola oil methyl esters: 6.4 5.9 15 C rt 6.4 3.5 6.4 0.8 40 C
5.3 2.4 0.8
4.9 1.7 0.6
4.4 1.1 0.5
4.2 0.8 0.3
4.0 0.5 ec
0.77 0.27 0.09
0.63 0.08 e
Palm oil methyl esters: 10.9 10.7 15 C rt 10.9 7.7 10.9 4.9 40 C
10.6 6.3 2.3
10.0 5.0 1.3
9.7 4.7 0.8
9.5 4.4 0.5
9.2 4.3 e
0.92 0.46 0.12
0.84 0.39 e
Soybean oil methyl esters: 15 C 5.0 3.4 rt 5.0 2.9 5.0 1.2 40 C
2.9 1.6 1.1
2.8 1.2 0.8
2.6 0.9 0.6
2.5 0.7 0.3
2.4 0.5 e
0.56 0.24 0.16
0.48 0.10 e
Sunflower oil methyl esters: 6.2 5.8 5.4 15 C rt 6.2 3.8 3.4 6.2 1.5 0.7 40 C
5.0 3.2 0.5
4.4 2.9 0.4
4.2 2.4 0.2
4.0 2.2 e
0.81 0.52 0.08
0.65 0.35 e
a b c
Average standard deviation ¼ 0.1 h. Rf ¼ (IP at t ¼ X)/(IP at t ¼ 0). Not determined.
Storage over an extended period (12 months) resulted in precipitous reductions in IP for all methyl esters, with the reductions more pronounced at higher temperatures (Table 3). Comparison of IP results is facilitated by introduction of a simple ratio, termed the retention factor (Rf). The Rf is defined as the IP of a sample at a given time interval divided by its initial IP [15]. For instance, the Rf values of PME and SME stored at 15 C for 6 months were 0.92 and 0.56, respectively (Table 3). In all cases Rf is less than 1.0, with higher Rf values suggesting less oxidation has occurred over the course of a given time interval. In other words, retention of initial IP is greater for samples with higher Rf values. As reported in Table 3, in all cases Rf decreased with increasing time interval and temperature, indicating that higher temperatures and longer storage times negatively influenced oxidative stability. For instance, the Rf values of CME stored for 6 months were 0.77, 0.27, and 0.09 at 15 C, rt, and 40 C, respectively. At 12 months of storage the values were reduced to 0.63 ( 15 C) and 0.08 (rt). Analogous trends were observed for PME, SME, and SFME. In general, the smallest percentage reductions in IP were observed for PME, as indicated by an average Rf value (average of all Rf values for PME) of 0.55 (Table 3), followed by SFME (ave Rf ¼ 0.48), CME (ave Rf ¼ 0.37), and SME (ave Rf ¼ 0.31). However, after 8 months at 40 C there was little difference among the IP values of the methyl esters, and by 10 months the values were so low ( 0.5 h) that determination of IP and Rf at 12 months was not attempted. In the case of EN 14214, CME and SFME after 2 months of storage at all temperatures were no longer above the minimum oxidative stability specification. In contrast, PME remained above the limit for the duration of the study at 15 C, but fell below the 6 h minimum threshold at 6 (rt) and 2 (40 C) months. In the case of ASTM D6751, PME was above the minimum limit for all time and temperature intervals with the exception of 4 months and beyond at 40 C. Both CME and SFME remained above the minimum limit for the duration of the study at 15 C, but fell below at 4 (CME) and 8 (SFME) months at rt. Only the 2 month sample at 15 C was satisfactory according to the oxidative stability specification listed in ASTM D6751 for SME. 3.3. Influence of extended storage on acid value The initial AVs of CME, PME, SME, and SFME (Table 4) were well below the maximum allowable limit of 0.50 mg KOH/g
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B.R. Moser / Renewable Energy 36 (2011) 1221e1226
Table 4 Influence of storage time (months) and temperature on acid valuesa (AV, mg KOH/g) of canola, palm, soybean, and sunflower oil methyl esters. Storage temp
Table 6 Influence of storage time (months) and temperature on cloud pointsa (CP, oC), pour pointsb (PP, oC) and iodine values (IV, g I2/100 g) of canola, palm, soybean, and sunflower oil methyl esters.
AV
AV
AV
AV
AV
AV
AV
t¼0
t¼2
t¼4
t¼6
t¼8
t ¼ 10
t ¼ 12
Canola oil methyl esters: 0.01 0.02 15 C rt 0.01 0.05 0.01 0.26 40 C
0.02 0.07 0.53
0.06 0.12 0.74
0.08 0.16 0.88
0.08 0.26 1.11
0.07 0.34 1.14
Palm oil methyl esters: 0.01 15 C rt 0.01 0.01 40 C
Canola oil methyl esters: 1.3 1.1 1.4 15 C rt 1.3 1.3 1.5 1.3 1.4 1.4 40 C
11.5 11.0 11.7 109 11.5 11.0 11.0 109 11.5 11.7 11.3 109
0.01 0.01 0.07
0.01 0.02 0.14
0.04 0.04 0.18
0.06 0.07 0.23
0.08 0.07 0.37
0.07 0.07 0.49
Soybean oil methyl esters: 15 C 0.01 0.01 rt 0.01 0.01 0.01 0.12 40 C
Palm oil methyl esters: 14.3 14.4 15 C rt 14.3 13.2 40 C 14.3 14.2
14.3 13.5 13.8
13.5 13.5 13.5
13.3 14.0 14.7
14.0 14.0 14.3
0.01 0.04 0.24
0.04 0.11 0.32
0.04 0.12 0.39
0.06 0.20 0.56
0.06 0.26 0.60
Sunflower oil methyl esters: 0.01 0.01 15 C rt 0.01 0.01 0.01 0.11 40 C
Soybean oil methyl esters: 0.1 0.3 15 C rt 0.1 0.4 0.1 0.4 40 C
0.2 0.3 0.5
2.0 2.0 2.0
2.0 2.0 2.0
0.03 0.02 0.21
0.04 0.05 0.28
0.05 0.08 0.42
0.07 0.08 0.66
0.08 0.10 0.79
Sunflower oil methyl esters: 3.4 3.3 2.8 15 C rt 3.4 3.3 3.4 3.4 3.6 3.4 40 C
4.8 4.8 4.8
4.7 4.3 4.7
a
Storage temp CP
a b
specified in ASTM D6751 and EN 14214 (Table 1). Storage over an extended period (12 months) resulted in higher AVs for all methyl esters, with the increases more pronounced at higher temperatures (Table 4). However, the results were not as dramatic as those obtained for IP, as all methyl esters remained below the maximum limit contained in ASTM D6751 and EN 14214 for the duration of the study at 15 C and rt. In fact, at 15 C none of the methyl esters after 12 months displayed values greater than 0.08 mg KOH/g. Only after 10 months at 40 C did SME and SFME exhibit AVs above the maximum limit, whereas the AV of CME became unsatisfactory after only 4 months (Table 4). All samples in the PME data set remained below the maximum limit specified in the biodiesel standards. In general, AV increased most dramatically in the case of CME over the course of the 12 month storage period at rt and 40 C, with the smallest increases observed for PME.
Table 5 Influence of storage time (months) and temperature on kinematic viscositiesa (KV, 40 C, mm2/s) and increase ratiosb (Ir) of canola, palm, soybean, and sunflower oil methyl esters. KV
KV
KV
KV
KV
KV
KV
Ir
t¼0
t¼2
t¼4
t¼6
t¼8
t ¼ 10
t ¼ 12
t ¼ 12
Canola oil methyl esters: 4.42 4.50 15 C rt 4.42 4.60 4.42 4.73 40 C
4.53 4.64 4.75
4.51 4.64 4.76
4.60 4.72 4.82
4.71 4.74 4.91
4.76 4.84 4.99
1.08 1.10 1.13
Palm oil methyl esters: 4.58 4.59 15 C rt 4.58 4.60 4.58 4.63 40 C
4.59 4.59 4.67
4.59 4.60 4.67
4.62 4.63 4.69
4.60 4.68 4.74
4.61 4.70 4.80
1.01 1.03 1.05
Soybean oil methyl esters: 4.12 4.12 15 C rt 4.12 4.18 40 C 4.12 4.24
4.12 4.24 4.27
4.14 4.24 4.34
4.16 4.26 4.33
4.20 4.27 4.34
4.24 4.38 4.55
1.03 1.06 1.10
Sunflower oil methyl esters: 4.74 4.74 15 C rt 4.74 4.76 4.74 4.73 40 C
4.75 4.74 4.89
4.75 4.75 4.93
4.74 4.80 4.98
4.76 4.81 5.02
4.78 4.81 5.19
1.01 1.01 1.09
a b
CP
PP
t ¼ 0 t ¼ 6 t ¼ 12 t ¼ 0
Average standard deviation ¼ 0.01 mg KOH/g.
Storage temp
CP
Average standard deviation ¼ 0.02 mm2/s. Ir ¼ (KV at t ¼ 12)/(KV at t ¼ 0).
PP
PP
IV
IV
IV
t¼6
t ¼ 12 t ¼ 0 t ¼ 6 t ¼ 12 109 108 107
108 106 104
52 52 52
52 52 51
52 51 50
2.0 2.0 2.3
132 132 132
132 130 130
131 129 128
5.0 4.3 4.0
85 85 85
85 85 84
84 84 82
Average standard deviation ¼ 0.1 C. Average standard deviation ¼ 0.3 C.
3.4. Influence of extended storage on kinematic viscosity The initial KVs of CME, PME, SME, and SFME (Table 5) were within the specified ranges contained in ASTM D6751 and EN 14214 (Table 1). Storage for 12 months resulted in higher KVs for all methyl esters, with the increases more pronounced at higher temperatures (Table 5). However, as was the case with AV the results were not as dramatic as those obtained for IP, as all methyl esters remained within the ranges listed in ASTM D6751 and EN 14214 for the duration of the study at all temperatures, with the exception of SFME at 40 C after 8 months. However, these values were still within the ASTM D6751 specification. Analogous to Rf discussed earlier, the increase ratio (Ir) is defined as the KV of a given sample at 12 months of storage divided by its initial KV. Because KV of FAMEs increases with time, all Ir values were greater than 1.00. For example, the Ir of CME, PME, SME, and SFME at 40 C after 12 months of storage were 1.13, 1.05, 1.10, and 1.09, respectively (Table 5). Higher Ir values are indicative of greater percentage increases in KV. As seen in Table 5, Ir increased with increasing temperature. As indicated by the relatively low values obtained for Ir ( 1.13), the increase in KV for all samples after 12 months was not substantial. Additionally, comparison of Ir values revealed that the increase in KV for SFME at 40 C at 12 months was not more significant than the other samples. Rather, the reason why SFME was not within the range specified in EN 14214 was the relatively high initial KV (4.74 mm2/s) of this sample versus CME, PME, and SME. The higher KV of SFME may be explained by its high content of monounsaturated FAMEs (MUFAME) relative to the other methyl esters, as it is known that PUFAME and shorter-chain FAMEs (saturated and unsaturated) exhibit KV values lower than methyl oleate (the principle component of SFME) [33]. 3.5. Influence of extended storage on low temperature properties The low temperature properties of CME, PME, SME, and SFME were measured through CP and PP determination. The American biodiesel standard, ASTM D6751, only specifies that CP be reported, whereas EN 14214 does not contain CP or PP requirements (Table 1). As seen in Table 6, the initial CP values of CME, PME, SME, and SFME
B.R. Moser / Renewable Energy 36 (2011) 1221e1226
were 1.3, 14.3, 0.1, and 3.4 C, respectively. The corresponding values for PP were 11.5, 13.5, 2.0, and 4.8 C. The considerably higher CP and PP values obtained for PME were attributed to the presence of a higher percentage of SFAME (50.0 wt %) versus the other methyl esters, as it is known that melting point increases with decreasing double bond content [2,16,18,19]. Storage for 12 months did not significantly impact the CP or PP values of the methyl esters (Table 6). It was speculated that extended storage would result in an accumulation of fatty acids and other degradation products that would increase CP and PP. However, such an effect was not observed. For instance, the CME sample stored at 40 C for 12 months exhibited an increase in AV from 0.01 to 1.14 mg KOH/g, but the CP and PP values for this sample were essentially unchanged. 3.6. Influence of extended storage on iodine value The initial IVs of CME, PME, SME, and SFME were 109, 52, 132, and 85 g I2/100 g, respectively (Table 6). The European biodiesel standard, EN 14214, limits IV to a maximum value of 120 g I2/100 g. In contrast, ASTM D6751 does not contain an IV specification (Table 1). SME was above the maximum allowable limit specified in EN 14214, whereas the other methyl esters were below the threshold. The IV is a structural index that gives an indication of average total unsaturation, with higher values suggesting higher levels of unsaturation (i.e., more double bonds). Correspondingly, the relatively high content of PUFAME in SME (60.3 wt %; Table 2) versus the other methyl esters was attributed to its higher IV. Analogously, the considerably higher content of SFAME in PME (50.0 wt %; Table 2) relative to the other methyl esters explained its substantially lower IV. After extended storage (12 months) at 15 C, the IVs of the methyl esters were essentially unchanged ( 0 or 1 g I2/100 g; Table 6). At rt after extended storage (12 months) the IVs of PME and SFME were also essentially unchanged ( 1 g I2/100 g), with the values for CME and SME reduced by only 3 g I2/100 g. The changes in IV after 12 months of storage at 40 C for CME, PME, SME, and SFME were 5, 2, 4, and 3 g I2/100 g, respectively. The IV is occasionally used in the scientific literature as an indicator of oxidative stability with the presumption that higher IVs predict lower oxidative stability. However, this correlation is erroneous because IV does not take into consideration the nature of the double bonds (conjugated versus unconjugated or cis versus trans orientations, for instance), nor does it account for sample history or the presence of native or anthropogenic antioxidants [19,34]. Illustrative of the influence of storage history on IV and IP are the results obtained in this study and presented in Tables 3 and 6. For example, the IPs of CME at rt after 0, 6, and 12 months were 6.4, 1.7, and 0.5 h. The corresponding IVs at these time intervals were 109, 108, and 106 g I2/100 g. As can be seen, dramatic changes in IP due to extended storage resulted in only minor changes in IV. Another example is PME stored at 15 C, which exhibited constant IV after 12 months (52 g I2/100 g) but decreasing IP values of 10.9, 10.0, and 9.2 h at 0, 6 and 12 months. Consequently, in agreement with previous studies, IV is not recommended as an indicator of oxidative or storage stability [19,34]. 3.7. Acid value and kinematic viscosity as indicators of storage stability? The utility of AV as an indicator of oxidative degradation as a result of extended storage is precarious at best, based on the results obtained in this study. Minor changes in AV for several cases presented in Table 4 were accompanied by significant changes in IP (shown in Table 3). For instance, changes in AV after 12 months of storage at 15 C for CME, PME, SME, and SFME were 0.06, 0.06, 0.05, and 0.07 mg KOH/g, whereas the Rf values for these samples
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were 0.63, 0.84, 0.48, and 0.65, respectively. Additionally, trivial changes in AV that resulted in values well below the maximum allowable limits contained in the biodiesel standards were accompanied by significant changes in IP that resulted in values that were below the minimum specifications listed in the standards. In many cases, changes in AV after 4 months of storage were not considered statistically significant, as the values did not differ by an amount greater than the average standard deviation ( 0.01 mg KOH/g) for this measurement. However, in all cases the IP values after 4 months of storage were significantly lower than the initial values. Therefore, AV is not recommended as an indicator of oxidative degradation as a result of extended storage of biodiesel. Analogous to AV, relatively minor changes in KV as a result of extended storage were accompanied by significant changes in IP. Illustrative of this were the KV values obtained for PME stored at 15 C, which did not change significantly over the course of 12 months of storage (Ir ¼ 1.01). However, the IP of PME stored at 15 C decreased from 10.9 h to 9.2 h (Rf ¼ 0.84) after 12 months. Another example is the reduction in IP (6.2 he2.2 h; Rf ¼ 0.35) of SFME stored at rt (Ir ¼ 1.01) for 12 months. Instances where KV changed by a significant amount, such as CME stored at 40 C (Ir ¼ 1.13), were accompanied by dramatic reductions in IP (6.4e0 h for CME at 40 C). Additionally, CME (rt) SME (40 C) and SFME (40 C) exhibited Ir values equal to or greater than 1.09, but were accompanied by reductions in IP after 12 months of storage of 5.9, 5.0, and 6.2 h, respectively. As was the case with AV, relatively minor changes in KV that were within the specified ranges listed in the biodiesel standards were accompanied by reductions in IP that resulted in values lower than the minimum allowable oxidative stability thresholds contained in the standards. Furthermore, a statistically insignificant linear relationship between Rf and Ir (R2 0.312, graph not shown) was obtained. Therefore, KV is not recommended as an indicator of oxidative degradation as a result of extended storage of biodiesel.
4. Conclusions Fatty acid methyl esters prepared from canola, palm, soybean, and sunflower oils by homogenous base-catalyzed methanolysis were stored for 12 months at three constant temperatures ( 15, 22, and 40 C) and properties such as oxidative stability, acid value, kinematic viscosity, cloud point, pour point, and iodine value were periodically determined utilizing standard methods. Oxidative stability was measured as the induction period following EN 14112 (Rancimat method). Oxidative stability was significantly reduced upon extended storage and acid value and kinematic viscosity were increased by only small increments, with these effects more pronounced at elevated temperatures. Iodine value as well as cloud point and pour point were essentially unaffected by extended storage. Significant changes in induction period of the methyl esters were accompanied by minor changes in acid value, kinematic viscosity, and iodine value. Based on these findings, it is not recommended that acid value, kinematic viscosity, or iodine value be used as indicators of storage stability of biodiesel, nor is it recommended that iodine value be used as a predictor of oxidative stability. Over the course of 12 months of storage, the induction period, acid value, and kinematic viscosity of palm oil methyl esters were least affected by oxidative degradation among the methyl esters, as indicated by comparatively high Rf and low Ir values, as well as small changes in acid value. This was attributed to the relatively high content of saturated and low content of polyunsaturated fatty acid methyl esters identified in palm oil methyl esters. In contrast, canola oil methyl esters were most significantly affected by extended storage, as measured by comparatively low Rf and high Ir
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B.R. Moser / Renewable Energy 36 (2011) 1221e1226
values, as well as greater changes in acid value versus the other methyl esters. Acknowledgements The author acknowledges Benetria N. Banks for excellent technical assistance. References [1] Standard specification for biodiesel (B100) fuel blend Stock (B100) for middle distillate fuels, ASTM D6751e08a. In: ASTM annual book of standards. West Conshohocken: ASTM International; 2008. [2] Moser BR. Biodiesel production, properties, and feedstocks. In Vitro Cell. Dev. Biol.-Plant 2009;45:229e66. [3] Knothe G, Van Gerpen J, Krahl J, editors. The biodiesel handbook. Urbana: AOCS Press; 2005. [4] Automotive fuels - fatty acid methyl esters (FAME) for diesel engines requirement methods, EN 14214:2008. Brussels, Belgium: European Committee for Standardization (CEN); 2009. [5] Fat and oil derivatives. fatty acid methyl esters (FAME). Determination of oxidative stability (accelerated oxidation test), EN 14112:2003. Brussels, Belgium: European Committee for Standardization (CEN); 2003. [6] Frankel EN. Lipid oxidation. 2nd ed. Bridgewater: The Oily Press; 2005. [7] Holman RT, Elmer OC. The rates of oxidation of unsaturated fatty acids and esters. J Am Oil Chem Soc 1947;24:127e9. [8] Moser BR. Comparative oxidative stability of fatty acid alkyl esters by accelerated methods. J Am Oil Chem Soc 2009;86:699e706. [9] Knothe G. Some aspects of biodiesel oxidative stability. Fuel Process. Technol 2007;88:669e77. [10] Dunn RO. Effect of temperature on the oil stability index (OSI) of biodiesel. Energy Fuels 2008;22:657e62. [11] Knothe G, Dunn RO. Dependence of oil stability index of fatty compounds on their structure and concentration and presence of metals. J Am Oil Chem Soc 2003;80:1021e6. [12] Knothe G. Analysis of oxidized biodiesel by 1H-NMR and effect of contact area with air. Eur J Lipid Sci Technol 2006;108:493e500. [13] Dunn RO. Antioxidants for improving storage stability of biodiesel. Biofuels Bioprod Bioref 2008;2:304e18. [14] Tang H, Wang A, Salley SO, Ng KYS. The effect of natural and synthetic antioxidants on the oxidative stability of biodiesel. J Am Oil Chem Soc 2008;85:373e82.
[15] Moser BR. Efficacy of myricetin as an antioxidant in methyl esters of soybean oil. Eur J Lipid Sci Technol 2008;110:1167e74. [16] Knothe G. “Designer” biodiesel: Optimizing fatty ester composition to improve fuel properties. Energy Fuels 2008;22:1358e64. [17] Moser BR, Vaughn SF. Evaluation of alkyl esters from Camelina sativa oil as biodiesel and as blend components in ultra low-sulfur diesel fuel. Bioresour Technol 2010;101:646e53. [18] Knothe G. Improving biodiesel fuel properties by modifying fatty ester composition. Energy Environ Sci 2009;2:759e66. [19] Moser BR. Influence of blending canola, palm, soybean, and sunflower oil methyl esters on fuel properties of biodiesel. Energy Fuels 2008;22:4301e6. [20] Lin C-Y, Chiu C-C. Effects of oxidation during long-term storage on the fuel properties of palm oil-based biodiesel. Energy Fuels 2009;23:3285e9. [21] Bondioli P, Gasparoli A, Della Bella L, Tagliabue S, Toso G. Biodiesel stability under commercial storage conditions over one year. Eur J Lipid Sci Technol 2003;105:735e41. [22] Mittelbach M, Gangl S. Long storage stability of biodiesel made from rapeseed and used frying oil. J Am Oil Chem Soc 2001;78:573e7. [23] Bondioli P, Gasparoli A, Lanzani A, Fedeli E, Veronese S, Sala M. Storage stability of biodiesel. J Am Oil Chem Soc 1995;72:699e702. [24] Dunn RO. Effect of oxidation under accelerated conditions on fuel properties of methyl soyate (biodiesel). J Am Oil Chem Soc 2002;79:915e20. [25] Dunn RO. Oxidative stability of soybean oil fatty acid methyl esters by oil stability index (OSI). J Am Oil Chem Soc 2005;82:381e7. [26] Dunn RO. Effect of antioxidants on the oxidative stability of methyl soyate (biodiesel). Fuel Process Technol 2005;86:1071e85. [27] Monyem A, Van Gerpen JH. The effect of biodiesel oxidation on engine performance and emissions. Biomass Bioenerg 2001;20:317e25. [28] Lacoste F, Lagardere L. Quality parameters evolution during biodiesel oxidation using rancimat test. Eur J Lipid Sci Technol 2003;105:149e55. [29] Bondioli P, Gasparoli A, Della Bella L, Tagliabue S, Lacoste F, Lagardere L. The prediction of biodiesel storage stability. Proposal for a quick test. Eur J Lipid Sci Technol 2004;106:822e30. [30] United States Department of Agriculture, Foreign Agricultural Service, Office of Global Analysis. Oilseeds: World Markets and Trade, Circular Series FOP 12-09, Table 03, pg. 5. [31] Freedman B, Pryde EH, Mounts TL. Variables affecting the yields of fatty esters from transesterified vegetable oils. J Am Oil Chem Soc 1984;61:1638e43. [32] Gunstone FD, Harwood JL, Dijkstra AJ, editors. The lipid handbook. 3rd ed. Boca Raton: CRC Press; 2007. [33] Knothe G, Steidley KR. Kinematic viscosity of biodiesel fuel components and related compounds. Influence of compound structure and comparison to petrodiesel fuel components. Fuel 2005;84:1059e65. [34] Knothe G. Structure indices in FA chemistry. How relevant is the iodine value? J Am Oil Chem Soc 2002;79:847e54.
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Influence of extended storage on fuel properties of methyl esters prepared from canola, palm, soybean and sunflower oils Bryan R. Moser* United States Department of Agriculture,1 Agricultural Research Service, National Center for Agricultural Utilization Research, 1815N. University St., Peoria, IL 61604, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 January 2010 Accepted 9 October 2010 Available online 10 November 2010
Fatty acid methyl esters prepared from canola, palm, soybean, and sunflower oils by homogenous basecatalyzed methanolysis were stored for 12 months at three constant temperatures ( 15, 22, and 40 C) and properties such as oxidative stability, acid value, kinematic viscosity, low temperature operability, and iodine value were periodically measured. Oxidative stability was significantly reduced upon extended storage and acid value as well as kinematic viscosity were increased by only small increments, with these effects more pronounced at elevated temperatures. Iodine value and low temperature operability were essentially unaffected by extended storage. Based on these findings, it is not recommended that acid value or kinematic viscosity be used as indicators of storage stability of biodiesel, nor is it recommended that iodine value be used as a predictor of oxidative stability or indicator of oxidative degradation. Published by Elsevier Ltd.
Keywords: Acid value Biodiesel Fatty acid methyl esters Induction period Kinematic viscosity Oxidative stability
1. Introduction Biodiesel is defined as the monoalkyl esters of long-chain fatty acids prepared from lipid feedstocks [1e3]. Advantages of biodiesel over conventional petroleum diesel fuel (petrodiesel) include derivation from renewable feedstocks, displacement of imported petroleum, superior lubricity and biodegradability, lower toxicity, essentially no sulfur content, higher flash point, positive energy balance, and a reduction of most regulated exhaust emissions. A primary disadvantage of biodiesel is inferior oxidative and storage stability versus petrodiesel, along with lower volumetric energy content, reduced low temperature operability, susceptibility to hydrolysis and microbial degradation, as well as higher nitrogen oxide exhaust emissions [2,3]. Biodiesel must be certified as compliant with accepted fuel standards (Table 1) such as ASTM
Abbreviations: AOCS, American oil Chemists’ Society; AV, Acid value; CEN, European Committee for Standardization; CME, Canola oil methyl esters; CP, Cloud point; FAME, Fatty acid methyl esters; IP, Induction period; IV, Iodine value; KV, Kinematic viscosity; MUFAME, Monounsaturated FAME; PME, Palm oil methyl esters; PP, Pour point; PUFAME, Polyunsaturated FAME; RBD, Refined bleached and deodorized; Rt, Room temperature; SFAME, Saturated FAME; SFME, Sunflower oil methyl esters; SME, Soybean oil methyl esters. * Tel.: þ1 309 681 6511; fax: þ1 309 681 6524. E-mail address: [email protected]. 1 Product names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. 0960-1481/$ e see front matter Published by Elsevier Ltd. doi:10.1016/j.renene.2010.10.009
D6751 [1] in the United States or the European Committee for Standardization (CEN) standard EN 14214 [4] before combustion in diesel engines. Oxidative stability of biodiesel is determined through measurement of the induction period (IP) by CEN method EN 14112, otherwise known as the Rancimat method. This method utilizes elevated exposure to air (10 L/h) and temperature (110 C) to induce accelerated oxidative degradation [5]. Oxidation is indirectly measured by monitoring the increase in conductivity of deionized water caused by volatile oxidative degradation products transported from the biodiesel sample. The IP is graphically determined as the inflection point of a plot of conductivity versus time. The units for IP are expressed as hours and biodiesel fuels with longer IP times are considered more stable to oxidation. The biodiesel standards ASTM D6751 and EN 14214 specify minimum limits for IP of 3 and 6 h, respectively (Table 1). Oxidation initiates at methylene carbons allylic to sites of unsaturation along the hydrocarbon backbone of biodiesel [6]. Biodiesel fuels with more allylic positions are especially vulnerable to oxidation, as evidenced by the relative rates of oxidation of 1, 41, and 98 for the unsaturated ethyl esters of oleic, linoleic, and linolenic acids [7]. This trend is mirrored by the IP values of methyl esters of stearic (> 40 h), oleic (2.5 h), linoleic (1.0 h), and linolenic (0.2 h) acids [8]. Several factors aside from fatty acid composition influence oxidative stability of biodiesel, such as exposure to elevated temperature, light and oxygen, as well as the presence of pro-oxidants such as metals [6,9e13]. Methods by which the IP of biodiesel may be improved include employment of antioxidants,
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B.R. Moser / Renewable Energy 36 (2011) 1221e1226
Table 1 Selected specifications from ASTM D6751 and EN 14214 biodiesel fuel standards.
CP, C PP, C CFPP, C IP, 110 C, h KV, 40 C, mm2/s AV, mg KOH/g IV, g I2/100 g Cetane number a b
ASTM D6751
EN 14214
Report e e 3 min 1.9e6.0 0.50 max e 47 min
ea e Variableb 6 min 3.50e5.00 0.50 max 120 max 51.0 min
Not specified. Depends on location and time of year.
2.3. Fatty acid profile by GC FAMEs were separated using a Varian (Walnut Creek, CA, USA) 8400 GC equipped with an FID detector and SP2380 (Supelco, Bellefonte, PA, USA) column (30 m 0.25 mm i.d., 0.20 mm film thickness). Carrier gas was He at 1 mL/min. The oven temperature was initially held at 150 C for 15 min, then increased to 210 C at 2 C/min, followed by an increase to 220 C at 50 C/min. The injector and detector temperatures were set at 240 C and 270 C, respectively. FAME peaks were identified by comparison to the retention times of known reference standards.
2.4. Storage and handling of samples blending with more stable fuels such as petrodiesel, reducing exposure to risk factors (heat, light, air, pro-oxidants), and selection of feedstocks that contain lower concentrations of polyunsaturated fatty acids [9,13e19]. In addition to reducing IP, oxidative degradation is reported to increase acid value (AV), peroxide value, kinematic viscosity (KV) and cetane number while reducing heat of combustion as well as carbon monoxide and hydrocarbon exhaust emissions [9,12,14,20e29]. Consequently, AV and KV have been suggested as indicators of oxidative degradation of biodiesel [23,28]. The most significant vegetable oils produced worldwide during the previous year were palm (45.13 MMT), soybean (37.69 MMT), rapeseed/canola (21.93 MMT), and sunflower (11.45 MMT) oils [30]. Generally, the most abundant oils or fats in a region are most commonly used as feedstocks for biodiesel production. Thus, rapeseed/canola and sunflower oils are principally used in Europe, palm oil predominates in tropical countries, and soybean oil and animal fats are most commonly used in the USA [2,3]. The primary objective of the current investigation was to determine the influence of long-term storage on the oxidative stability, AV, KV, low temperature operability, and iodine value (IV) of canola (CME), palm (PME), soybean (SME), and sunflower (SFME) oil methyl esters. A further objective was to determine the utility of AV and KV as indirect indicators of oxidative degradation.
2. Materials and methods
Biodiesel samples were stored at room temperature (rt; 21 2 C average laboratory temperature), 40 1 C (temperaturecontrolled laboratory oven), and 15 2 C (laboratory freezer) in the dark in capped amber-tinted glass jars over the course of a 12 month period. Every two months samples were removed for analysis following brief agitation and subsequently discarded.
2.5. Oxidative stability Induction period (IP, h) was measured in accordance to EN 14112 utilizing a Metrohm USA, Inc. (Riverview, FL, USA) model 743 Rancimat instrument. Briefly, the flow rate of air through 3 0.01 g of sample was 10 L/h with a block temperature of 110 C (correction factor of 1.5 C). The conductivity measuring vessel contained 50 0.1 mL of deionized water. IP was mathematically determined as the inflection point of a plot of conductivity (mS/cm) of deionized water versus time (h). 2.6. Low temperature properties Cloud and pour point (CP and PP, respectively) determinations were made according to automated methods ASTM D5773 and ASTM D5949, respectively, using a model PSA-70S Phase Technology Analyzer (Richmond, B.C., Canada). For a greater degree of accuracy, PP measurements were done with a resolution of 1 C instead of the specified 3 C increment.
2.1. Materials Refined, bleached, and deodorized (RBD) canola (low erucic acid rapeseed oil), palm, soybean, and sunflower (high oleic) oils without additives were purchased from KIC Chemicals, Inc. (New Platz, NY, USA) and used as received. Fatty acid methyl ester (FAME) standards were purchased from Nu-Chek Prep, Inc. (Elysian, MN, USA). All other chemicals and reagents were obtained from SigmaeAldrich Corp (St. Louis, MO, USA) and used as received. All measurements were done in triplicate with mean values reported.
2.2. Transesterification Methanolysis was accomplished with sodium methoxide catalyst (0.5 wt % with respect to oil), 6:1 mole ratio of methanol to oil, 60 C reaction temperature, and 1.0 h reaction time. After sequential removal of glycerol by gravity separation and methanol by reduced pressure (10 mbar; 30 C) rotary evaporation, the crude products were washed with distilled water until a neutral pH was obtained and dried with MgSO4 to yield FAME in high yield (> 97 wt %).
2.7. Kinematic viscosity Kinematic viscosity (KV, mm2/s) was determined with CannonFenske viscometers (Cannon Instrument Co., State College, PA, USA) at 40 C as specified in ASTM D445. 2.8. Acid value Acid value (AV, mg KOH/g) titrations were performed as described in ASTM D664 using a Metrohm 836 Titrando (Westbury, NY, USA) autotitrator equipped with a model 801 stirrer and a Metrohm 6.0229.100 Solvotrode. The titration endpoint was automatically determined and verified using a phenolphthalein indicator. 2.9. Iodine value Iodine value (IV. G I2/100 g) was calculated from the fatty acid profile (Table 2) according to American Oil Chemists’ Society (AOCS) official method Cd 1c-85.
B.R. Moser / Renewable Energy 36 (2011) 1221e1226 Table 2 Fatty acid composition (wt %) of canola, palm, soybean, and sunflower oil methyl esters. Fatty acida
Canola
Palm
Soybean
Sunflower
C12:0 C14:0 C16:0 C18:0 C20:0 C22:0 C16:1 9c C18:1 9c C18:1 11c C18:2 9c, 12c C18:3 9c, 12c, 15c C20:1 11c unknown S SFAMEc S MUFAMEd S PUFAMEe
eb 0.1 5.0 2.0 0.6 0.3 0.3 60.1 3.0 19.8 7.2 1.1 0.5 8.0 64.5 27.0
0.3 1.2 44.1 4.1 0.3 e 0.3 38.4 0.7 10.0 0.2 0.1 0.3 50.0 39.5 10.2
e 0.1 11.7 3.8 0.2 0.2 e 22.0 1.4 52.6 7.7 0.2 0.1 16.0 23.6 60.3
e 0.1 4.7 3.9 0.3 0.9 0.1 81.3 0.4 7.7 0.3 0.3 e 9.9 82.1 8.0
a
For example, C18:1 9c signifies an 18 carbon fatty acid chain with one cis double bond located at carbon 9 (methyl 9Z-octadecenoate; methyl oleate). b Not detected (< 0.1 wt %). c S SFAME ¼ C12:0 þ C14:0 þ C16:0 þ C18:0 þ C20:0 þ C22:0. d S MUFAME ¼ C16:1 þ C18:1 þ C20:1. e S PUFAME ¼ C18:2 þ C18:3.
3. Results and discussion 3.1. Preparation and fatty acid composition of methyl esters Methyl esters were prepared by methanolysis from RBD vegetable oils in high yields (> 97 wt %) utilizing the classic homogenous base-catalyzed procedure (60 C, 1 h, 0.50 wt % catalyst and 6:1 mole ratio of methanol to vegetable oil) described previously [2,17,19,31]. Analysis by GC (ASTM D6584) revealed that each sample was essentially free of free and bound glycerol (results not shown). The principle FAMEs identified in CME (low erucic acid variety) by GC (Table 2) were methyl oleate (60.1 wt %) and methyl linoleate (19.8 wt %), with lesser amounts of methyl linolenate (7.2 wt %), methyl palmitate (5.0 wt %), and other minor FAMEs also detected. Methyl palmitate (44.1 wt %) and methyl oleate (38.4 wt %) were the primary FAMEs found in PME, along with methyl linoleate (10.0 wt %) and methyl stearate (4.1 wt %), among other constituents. The dominant FAME detected in SME was methyl linoleate (52.6 wt %), with significant amounts of methyl oleate (22.0 wt %) and methyl palmitate (11.7 wt %) also identified. The primary FAME detected in SFME (high oleic variety) was methyl oleate (81.3 wt %), along with methyl linoleate (7.7 wt %), methyl palmitate (4.7 wt %), and methyl stearate (3.9 wt %). The results for these methyl esters were in close agreement with FAME profiles reported elsewhere [8,10,12-15,17,19,20,22,24,25,27,32].
3.2. Influence of extended storage on oxidative stability The initial IP (EN 14112; 110 C) values of CME, PME, SME, and SFME were 6.4, 10.9, 5.0, and 6.2 h, respectively (Table 3). As seen by comparison to the oxidative stability requirements contained in ASTM D6751 and EN 14214 (Table 1), all samples were above the minimum specified limit in ASTM D6751 (> 3.0 h). However, SME was below the more stringent specification (> 6.0 h) listed in EN 14214. The content of polyunsaturated FAMEs (PUFAME) in SME (60.3 wt %; Table 2) relative to the other samples was attributed to its reduced oxidative stability. In contrast, PME contained the highest percentage of saturated FAME (SFAME; 50.0 wt %; Table 2) and exhibited the highest stability to oxidation according to the Rancimat (EN 14112) method. As previously discussed, the oxidative stability of biodiesel decreases with increasing content of polyunsaturation [2,6e9,16,18,19].
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Table 3 Influence of storage time (months) and temperature on induction periodsa (IP, h) and retention factorsb (Rf) of canola, palm, soybean, and sunflower oil methyl esters. Storage temp IP
IP
IP
IP
IP
IP
IP
Rf
Rf
t ¼ 0 t ¼ 2 t ¼ 4 t ¼ 6 t ¼ 8 t ¼ 10 t ¼ 12 t ¼ 6 t ¼ 12 Canola oil methyl esters: 6.4 5.9 15 C rt 6.4 3.5 6.4 0.8 40 C
5.3 2.4 0.8
4.9 1.7 0.6
4.4 1.1 0.5
4.2 0.8 0.3
4.0 0.5 ec
0.77 0.27 0.09
0.63 0.08 e
Palm oil methyl esters: 10.9 10.7 15 C rt 10.9 7.7 10.9 4.9 40 C
10.6 6.3 2.3
10.0 5.0 1.3
9.7 4.7 0.8
9.5 4.4 0.5
9.2 4.3 e
0.92 0.46 0.12
0.84 0.39 e
Soybean oil methyl esters: 15 C 5.0 3.4 rt 5.0 2.9 5.0 1.2 40 C
2.9 1.6 1.1
2.8 1.2 0.8
2.6 0.9 0.6
2.5 0.7 0.3
2.4 0.5 e
0.56 0.24 0.16
0.48 0.10 e
Sunflower oil methyl esters: 6.2 5.8 5.4 15 C rt 6.2 3.8 3.4 6.2 1.5 0.7 40 C
5.0 3.2 0.5
4.4 2.9 0.4
4.2 2.4 0.2
4.0 2.2 e
0.81 0.52 0.08
0.65 0.35 e
a b c
Average standard deviation ¼ 0.1 h. Rf ¼ (IP at t ¼ X)/(IP at t ¼ 0). Not determined.
Storage over an extended period (12 months) resulted in precipitous reductions in IP for all methyl esters, with the reductions more pronounced at higher temperatures (Table 3). Comparison of IP results is facilitated by introduction of a simple ratio, termed the retention factor (Rf). The Rf is defined as the IP of a sample at a given time interval divided by its initial IP [15]. For instance, the Rf values of PME and SME stored at 15 C for 6 months were 0.92 and 0.56, respectively (Table 3). In all cases Rf is less than 1.0, with higher Rf values suggesting less oxidation has occurred over the course of a given time interval. In other words, retention of initial IP is greater for samples with higher Rf values. As reported in Table 3, in all cases Rf decreased with increasing time interval and temperature, indicating that higher temperatures and longer storage times negatively influenced oxidative stability. For instance, the Rf values of CME stored for 6 months were 0.77, 0.27, and 0.09 at 15 C, rt, and 40 C, respectively. At 12 months of storage the values were reduced to 0.63 ( 15 C) and 0.08 (rt). Analogous trends were observed for PME, SME, and SFME. In general, the smallest percentage reductions in IP were observed for PME, as indicated by an average Rf value (average of all Rf values for PME) of 0.55 (Table 3), followed by SFME (ave Rf ¼ 0.48), CME (ave Rf ¼ 0.37), and SME (ave Rf ¼ 0.31). However, after 8 months at 40 C there was little difference among the IP values of the methyl esters, and by 10 months the values were so low ( 0.5 h) that determination of IP and Rf at 12 months was not attempted. In the case of EN 14214, CME and SFME after 2 months of storage at all temperatures were no longer above the minimum oxidative stability specification. In contrast, PME remained above the limit for the duration of the study at 15 C, but fell below the 6 h minimum threshold at 6 (rt) and 2 (40 C) months. In the case of ASTM D6751, PME was above the minimum limit for all time and temperature intervals with the exception of 4 months and beyond at 40 C. Both CME and SFME remained above the minimum limit for the duration of the study at 15 C, but fell below at 4 (CME) and 8 (SFME) months at rt. Only the 2 month sample at 15 C was satisfactory according to the oxidative stability specification listed in ASTM D6751 for SME. 3.3. Influence of extended storage on acid value The initial AVs of CME, PME, SME, and SFME (Table 4) were well below the maximum allowable limit of 0.50 mg KOH/g
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B.R. Moser / Renewable Energy 36 (2011) 1221e1226
Table 4 Influence of storage time (months) and temperature on acid valuesa (AV, mg KOH/g) of canola, palm, soybean, and sunflower oil methyl esters. Storage temp
Table 6 Influence of storage time (months) and temperature on cloud pointsa (CP, oC), pour pointsb (PP, oC) and iodine values (IV, g I2/100 g) of canola, palm, soybean, and sunflower oil methyl esters.
AV
AV
AV
AV
AV
AV
AV
t¼0
t¼2
t¼4
t¼6
t¼8
t ¼ 10
t ¼ 12
Canola oil methyl esters: 0.01 0.02 15 C rt 0.01 0.05 0.01 0.26 40 C
0.02 0.07 0.53
0.06 0.12 0.74
0.08 0.16 0.88
0.08 0.26 1.11
0.07 0.34 1.14
Palm oil methyl esters: 0.01 15 C rt 0.01 0.01 40 C
Canola oil methyl esters: 1.3 1.1 1.4 15 C rt 1.3 1.3 1.5 1.3 1.4 1.4 40 C
11.5 11.0 11.7 109 11.5 11.0 11.0 109 11.5 11.7 11.3 109
0.01 0.01 0.07
0.01 0.02 0.14
0.04 0.04 0.18
0.06 0.07 0.23
0.08 0.07 0.37
0.07 0.07 0.49
Soybean oil methyl esters: 15 C 0.01 0.01 rt 0.01 0.01 0.01 0.12 40 C
Palm oil methyl esters: 14.3 14.4 15 C rt 14.3 13.2 40 C 14.3 14.2
14.3 13.5 13.8
13.5 13.5 13.5
13.3 14.0 14.7
14.0 14.0 14.3
0.01 0.04 0.24
0.04 0.11 0.32
0.04 0.12 0.39
0.06 0.20 0.56
0.06 0.26 0.60
Sunflower oil methyl esters: 0.01 0.01 15 C rt 0.01 0.01 0.01 0.11 40 C
Soybean oil methyl esters: 0.1 0.3 15 C rt 0.1 0.4 0.1 0.4 40 C
0.2 0.3 0.5
2.0 2.0 2.0
2.0 2.0 2.0
0.03 0.02 0.21
0.04 0.05 0.28
0.05 0.08 0.42
0.07 0.08 0.66
0.08 0.10 0.79
Sunflower oil methyl esters: 3.4 3.3 2.8 15 C rt 3.4 3.3 3.4 3.4 3.6 3.4 40 C
4.8 4.8 4.8
4.7 4.3 4.7
a
Storage temp CP
a b
specified in ASTM D6751 and EN 14214 (Table 1). Storage over an extended period (12 months) resulted in higher AVs for all methyl esters, with the increases more pronounced at higher temperatures (Table 4). However, the results were not as dramatic as those obtained for IP, as all methyl esters remained below the maximum limit contained in ASTM D6751 and EN 14214 for the duration of the study at 15 C and rt. In fact, at 15 C none of the methyl esters after 12 months displayed values greater than 0.08 mg KOH/g. Only after 10 months at 40 C did SME and SFME exhibit AVs above the maximum limit, whereas the AV of CME became unsatisfactory after only 4 months (Table 4). All samples in the PME data set remained below the maximum limit specified in the biodiesel standards. In general, AV increased most dramatically in the case of CME over the course of the 12 month storage period at rt and 40 C, with the smallest increases observed for PME.
Table 5 Influence of storage time (months) and temperature on kinematic viscositiesa (KV, 40 C, mm2/s) and increase ratiosb (Ir) of canola, palm, soybean, and sunflower oil methyl esters. KV
KV
KV
KV
KV
KV
KV
Ir
t¼0
t¼2
t¼4
t¼6
t¼8
t ¼ 10
t ¼ 12
t ¼ 12
Canola oil methyl esters: 4.42 4.50 15 C rt 4.42 4.60 4.42 4.73 40 C
4.53 4.64 4.75
4.51 4.64 4.76
4.60 4.72 4.82
4.71 4.74 4.91
4.76 4.84 4.99
1.08 1.10 1.13
Palm oil methyl esters: 4.58 4.59 15 C rt 4.58 4.60 4.58 4.63 40 C
4.59 4.59 4.67
4.59 4.60 4.67
4.62 4.63 4.69
4.60 4.68 4.74
4.61 4.70 4.80
1.01 1.03 1.05
Soybean oil methyl esters: 4.12 4.12 15 C rt 4.12 4.18 40 C 4.12 4.24
4.12 4.24 4.27
4.14 4.24 4.34
4.16 4.26 4.33
4.20 4.27 4.34
4.24 4.38 4.55
1.03 1.06 1.10
Sunflower oil methyl esters: 4.74 4.74 15 C rt 4.74 4.76 4.74 4.73 40 C
4.75 4.74 4.89
4.75 4.75 4.93
4.74 4.80 4.98
4.76 4.81 5.02
4.78 4.81 5.19
1.01 1.01 1.09
a b
CP
PP
t ¼ 0 t ¼ 6 t ¼ 12 t ¼ 0
Average standard deviation ¼ 0.01 mg KOH/g.
Storage temp
CP
Average standard deviation ¼ 0.02 mm2/s. Ir ¼ (KV at t ¼ 12)/(KV at t ¼ 0).
PP
PP
IV
IV
IV
t¼6
t ¼ 12 t ¼ 0 t ¼ 6 t ¼ 12 109 108 107
108 106 104
52 52 52
52 52 51
52 51 50
2.0 2.0 2.3
132 132 132
132 130 130
131 129 128
5.0 4.3 4.0
85 85 85
85 85 84
84 84 82
Average standard deviation ¼ 0.1 C. Average standard deviation ¼ 0.3 C.
3.4. Influence of extended storage on kinematic viscosity The initial KVs of CME, PME, SME, and SFME (Table 5) were within the specified ranges contained in ASTM D6751 and EN 14214 (Table 1). Storage for 12 months resulted in higher KVs for all methyl esters, with the increases more pronounced at higher temperatures (Table 5). However, as was the case with AV the results were not as dramatic as those obtained for IP, as all methyl esters remained within the ranges listed in ASTM D6751 and EN 14214 for the duration of the study at all temperatures, with the exception of SFME at 40 C after 8 months. However, these values were still within the ASTM D6751 specification. Analogous to Rf discussed earlier, the increase ratio (Ir) is defined as the KV of a given sample at 12 months of storage divided by its initial KV. Because KV of FAMEs increases with time, all Ir values were greater than 1.00. For example, the Ir of CME, PME, SME, and SFME at 40 C after 12 months of storage were 1.13, 1.05, 1.10, and 1.09, respectively (Table 5). Higher Ir values are indicative of greater percentage increases in KV. As seen in Table 5, Ir increased with increasing temperature. As indicated by the relatively low values obtained for Ir ( 1.13), the increase in KV for all samples after 12 months was not substantial. Additionally, comparison of Ir values revealed that the increase in KV for SFME at 40 C at 12 months was not more significant than the other samples. Rather, the reason why SFME was not within the range specified in EN 14214 was the relatively high initial KV (4.74 mm2/s) of this sample versus CME, PME, and SME. The higher KV of SFME may be explained by its high content of monounsaturated FAMEs (MUFAME) relative to the other methyl esters, as it is known that PUFAME and shorter-chain FAMEs (saturated and unsaturated) exhibit KV values lower than methyl oleate (the principle component of SFME) [33]. 3.5. Influence of extended storage on low temperature properties The low temperature properties of CME, PME, SME, and SFME were measured through CP and PP determination. The American biodiesel standard, ASTM D6751, only specifies that CP be reported, whereas EN 14214 does not contain CP or PP requirements (Table 1). As seen in Table 6, the initial CP values of CME, PME, SME, and SFME
B.R. Moser / Renewable Energy 36 (2011) 1221e1226
were 1.3, 14.3, 0.1, and 3.4 C, respectively. The corresponding values for PP were 11.5, 13.5, 2.0, and 4.8 C. The considerably higher CP and PP values obtained for PME were attributed to the presence of a higher percentage of SFAME (50.0 wt %) versus the other methyl esters, as it is known that melting point increases with decreasing double bond content [2,16,18,19]. Storage for 12 months did not significantly impact the CP or PP values of the methyl esters (Table 6). It was speculated that extended storage would result in an accumulation of fatty acids and other degradation products that would increase CP and PP. However, such an effect was not observed. For instance, the CME sample stored at 40 C for 12 months exhibited an increase in AV from 0.01 to 1.14 mg KOH/g, but the CP and PP values for this sample were essentially unchanged. 3.6. Influence of extended storage on iodine value The initial IVs of CME, PME, SME, and SFME were 109, 52, 132, and 85 g I2/100 g, respectively (Table 6). The European biodiesel standard, EN 14214, limits IV to a maximum value of 120 g I2/100 g. In contrast, ASTM D6751 does not contain an IV specification (Table 1). SME was above the maximum allowable limit specified in EN 14214, whereas the other methyl esters were below the threshold. The IV is a structural index that gives an indication of average total unsaturation, with higher values suggesting higher levels of unsaturation (i.e., more double bonds). Correspondingly, the relatively high content of PUFAME in SME (60.3 wt %; Table 2) versus the other methyl esters was attributed to its higher IV. Analogously, the considerably higher content of SFAME in PME (50.0 wt %; Table 2) relative to the other methyl esters explained its substantially lower IV. After extended storage (12 months) at 15 C, the IVs of the methyl esters were essentially unchanged ( 0 or 1 g I2/100 g; Table 6). At rt after extended storage (12 months) the IVs of PME and SFME were also essentially unchanged ( 1 g I2/100 g), with the values for CME and SME reduced by only 3 g I2/100 g. The changes in IV after 12 months of storage at 40 C for CME, PME, SME, and SFME were 5, 2, 4, and 3 g I2/100 g, respectively. The IV is occasionally used in the scientific literature as an indicator of oxidative stability with the presumption that higher IVs predict lower oxidative stability. However, this correlation is erroneous because IV does not take into consideration the nature of the double bonds (conjugated versus unconjugated or cis versus trans orientations, for instance), nor does it account for sample history or the presence of native or anthropogenic antioxidants [19,34]. Illustrative of the influence of storage history on IV and IP are the results obtained in this study and presented in Tables 3 and 6. For example, the IPs of CME at rt after 0, 6, and 12 months were 6.4, 1.7, and 0.5 h. The corresponding IVs at these time intervals were 109, 108, and 106 g I2/100 g. As can be seen, dramatic changes in IP due to extended storage resulted in only minor changes in IV. Another example is PME stored at 15 C, which exhibited constant IV after 12 months (52 g I2/100 g) but decreasing IP values of 10.9, 10.0, and 9.2 h at 0, 6 and 12 months. Consequently, in agreement with previous studies, IV is not recommended as an indicator of oxidative or storage stability [19,34]. 3.7. Acid value and kinematic viscosity as indicators of storage stability? The utility of AV as an indicator of oxidative degradation as a result of extended storage is precarious at best, based on the results obtained in this study. Minor changes in AV for several cases presented in Table 4 were accompanied by significant changes in IP (shown in Table 3). For instance, changes in AV after 12 months of storage at 15 C for CME, PME, SME, and SFME were 0.06, 0.06, 0.05, and 0.07 mg KOH/g, whereas the Rf values for these samples
1225
were 0.63, 0.84, 0.48, and 0.65, respectively. Additionally, trivial changes in AV that resulted in values well below the maximum allowable limits contained in the biodiesel standards were accompanied by significant changes in IP that resulted in values that were below the minimum specifications listed in the standards. In many cases, changes in AV after 4 months of storage were not considered statistically significant, as the values did not differ by an amount greater than the average standard deviation ( 0.01 mg KOH/g) for this measurement. However, in all cases the IP values after 4 months of storage were significantly lower than the initial values. Therefore, AV is not recommended as an indicator of oxidative degradation as a result of extended storage of biodiesel. Analogous to AV, relatively minor changes in KV as a result of extended storage were accompanied by significant changes in IP. Illustrative of this were the KV values obtained for PME stored at 15 C, which did not change significantly over the course of 12 months of storage (Ir ¼ 1.01). However, the IP of PME stored at 15 C decreased from 10.9 h to 9.2 h (Rf ¼ 0.84) after 12 months. Another example is the reduction in IP (6.2 he2.2 h; Rf ¼ 0.35) of SFME stored at rt (Ir ¼ 1.01) for 12 months. Instances where KV changed by a significant amount, such as CME stored at 40 C (Ir ¼ 1.13), were accompanied by dramatic reductions in IP (6.4e0 h for CME at 40 C). Additionally, CME (rt) SME (40 C) and SFME (40 C) exhibited Ir values equal to or greater than 1.09, but were accompanied by reductions in IP after 12 months of storage of 5.9, 5.0, and 6.2 h, respectively. As was the case with AV, relatively minor changes in KV that were within the specified ranges listed in the biodiesel standards were accompanied by reductions in IP that resulted in values lower than the minimum allowable oxidative stability thresholds contained in the standards. Furthermore, a statistically insignificant linear relationship between Rf and Ir (R2 0.312, graph not shown) was obtained. Therefore, KV is not recommended as an indicator of oxidative degradation as a result of extended storage of biodiesel.
4. Conclusions Fatty acid methyl esters prepared from canola, palm, soybean, and sunflower oils by homogenous base-catalyzed methanolysis were stored for 12 months at three constant temperatures ( 15, 22, and 40 C) and properties such as oxidative stability, acid value, kinematic viscosity, cloud point, pour point, and iodine value were periodically determined utilizing standard methods. Oxidative stability was measured as the induction period following EN 14112 (Rancimat method). Oxidative stability was significantly reduced upon extended storage and acid value and kinematic viscosity were increased by only small increments, with these effects more pronounced at elevated temperatures. Iodine value as well as cloud point and pour point were essentially unaffected by extended storage. Significant changes in induction period of the methyl esters were accompanied by minor changes in acid value, kinematic viscosity, and iodine value. Based on these findings, it is not recommended that acid value, kinematic viscosity, or iodine value be used as indicators of storage stability of biodiesel, nor is it recommended that iodine value be used as a predictor of oxidative stability. Over the course of 12 months of storage, the induction period, acid value, and kinematic viscosity of palm oil methyl esters were least affected by oxidative degradation among the methyl esters, as indicated by comparatively high Rf and low Ir values, as well as small changes in acid value. This was attributed to the relatively high content of saturated and low content of polyunsaturated fatty acid methyl esters identified in palm oil methyl esters. In contrast, canola oil methyl esters were most significantly affected by extended storage, as measured by comparatively low Rf and high Ir
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