Article pubs.acs.org/EF
Preparation and Evaluation of Multifunctional Branched Diesters As Fuel Property Enhancers for Biodiesel and Petroleum Diesel Fuels Bryan R. Moser* United States Department of Agriculture, Agricultural Research Service, National Center for Agricultural Utilization Research, Bio-Oils Research Unit, 1815 North University Street, Peoria, Illinois 61604, United States ABSTRACT: A family of eight branched diesters that were similar in molecular weight to typical fatty acid methyl esters encountered in biodiesel was prepared in high yield by condensation of alcohols and acids. Condensation following a diacid/ alcohol route as opposed to the diol/acid method was more facile, as higher yields were obtained in shorter periods of time. The synthetic diesters possessed advantageously low melting points (24 h), densities, flash points, kinematic viscosities (KVs), specific gravities (SGs), and surface tensions. Diesters possessing the most desirable combinations of properties were blended with biodiesel and ultralow sulfur diesel (ULSD) in an effort to ameliorate technical deficiencies of these fuels. Results were compared to relevant biodiesel and petrodiesel fuel standards. Diesters lowered cloud, pour, and cold filter plugging points of biodiesel by up to 5.7 °C at 10% (vol) diester. Furthermore, diesters improved lubricity and cold flow properties of ULSD while avoiding deleterious effects on KV, SG, and energy content. Linear responses were noted with regard to the influence of diester concentration on fuel properties in blends with biodiesel and ULSD. In summary, these oxygenated, branched compounds were simple to prepare and were effective at improving the cold flow properties of biodiesel as well as the lubricity of ULSD at low blend levels.
1. INTRODUCTION Defined as monoalkyl esters of long-chain fatty acids (FAs) prepared from lipids, advantages of biodiesel over petrodiesel include derivation from renewable feedstocks, displacement of imported petroleum, superior lubricity and biodegradability, nontoxicity, high flash point, lower exhaust emissions, negligible sulfur and aromatics content, and positive energy balance. Disadvantages include high feedstock cost, limited availability, inferior storage and oxidative stability, and inferior cold flow properties.1,2 Biodiesel must meet the requirements of ASTM D6751 in the U.S. or EN 14214 (Table 1) in Europe before its commercial use is approved. Although biodiesel may be used directly in modern unmodified diesel engines, it is normally encountered as a blend component in petrodiesel. Blends up to B5 (5 vol %) and B7 are permitted in ASTM D975 and EN 590 (Table 2), the U.S. and European diesel fuel standards, respectively. Additionally, B6−B20 blends are regulated by ASTM D7467 (Table 2). Deficiencies of fuels are often mitigated with performanceenhancing additives, some of which include antioxidants, drag reducing agents, cetane enhancers, antifoaming agents, conductivity additives, dehazers, corrosion inhibitors, metal deactivators, viscosity modifiers, detergents, antiwear (lubricity) additives, and cold flow improvers (CFIs).3−11 In some instances, advantageous combinations of compatible fuels may offset technical deficiencies. For instance, biodiesel improves lubricity and exhaust emissions while petrodiesel enhances oxidative stability and cold flow properties when these fuels are blended.12−17 Other examples of biobased materials utilized as property enhancers for both petrodiesel and biodiesel include hydrogenated monoterpenes, ethers, esters and acetals of glycerol, esters of levulinic acid, and FA derivatives such as estolides, nitrated biodiesel, higher monoalkyl esters of © 2014 American Chemical Society
biodiesel, branched-chain ethers and esters of biodiesel, and esters derived from branched or cyclic alcohols and acids.18−30 The objective of the current investigation was the facile production of branched diesters with molecular weights (MWs) similar to biodiesel to be evaluated as multifunctional additives in biodiesel and petrodiesel. Of particular interest was application of these materials as CFIs for biodiesel and as antiwear additives for petrodiesel. Typical commercial CFIs are often copolymers of ethylene/vinyl acetate or acrylate/maleic anhydride.5−11 Polymers do not combust well or at all in compression-ignition (diesel) engines and may contribute to the buildup of carbon deposits over time. In addition, commercial antiwear additives often contain various heteroatoms and/or metals that are environmentally harmful. Development of multifunctional additives that contain only carbon, hydrogen, and oxygen and are within the MW range typical of biodiesel and petrodiesel fuels is thus a logical research target.
2. EXPERIMENTAL SECTION 2.1. Materials. Ultralow sulfur (54). Among the diesters, lower DCNs were noted for 1 (22.8) and 2 (18.0), as they contained a methyl branching group in their cores (3methyl-1,5-pentanediol) whereas 3−8 (DCNs: 25.1−32.5) had linear cores derived from adipic acid. Structural factors such as chain length and the presence of heteroatoms affect lubricity.38 Diesters 1−8 provided wear scars in the range 254−370 μm, which were longer than those measured for biodiesel (124 and 145 μm) but significantly shorter than that noted for ULSD (571 μm). Among the diesters, those derived from 3-methyl-1,5-pentanediol (1 and 2) and those containing two ethyl branching groups (3) exhibited wear scars in excess of 300 μm. Accordingly, shorter wear scars were measured for diesters derived from adipic acid that contained methyl branching groups (4−8).
between monoesters 12 and 13 (Figure 1) and alcohols 15−18 (Figure 2). During condensation alcohols serve as nucleophiles by adding to the electrophilic acyl carbon of the carboxylic acid moiety. This mechanism must be performed twice to yield diesters. In the case of diols reacting with acids (Figure 1), the alcohols in the second cycle (12 and 13) contained an electronwithdrawing ester group that weakened their nucleophilicity relative to alcohols 15−18. 3.2. Preparation, Composition, and Quality of SME and YGME. Homogeneous base-catalyzed transesterification of soybean oil afforded SME in high yield (97 mass %) employing classic methanolysis conditions described elsewhere.13,16,32,33 YGME was provided by a commercial producer, so its laboratory preparation was unnecessary. Both SME and YGME provided AVs below the maximum limit of 0.50 mg KOH/g specified in ASTM D6751 and EN 14214 (Table 1). In addition, the concentrations of free and total glycerol (Table 1) in SME and YGME were below the maximum thresholds specified in the aforementioned biodiesel standards. The FPs of SME (181 °C) and YGME (184 °C) were above the minimum specifications listed in the standards, which indicated that the FAMEs were free of residual methanol. The concentrations of P and S in SME and YGME were below the maximum specified limits. Other fuel properties such as moisture content, DCN, and KV were within the limits prescribed in ASTM D6751 and EN14214 as well. Neither SME nor YGME met the IP specification listed in EN 14214 (>6 h), but both were above the minimum threshold of 3 h listed in ASTM D6751. ASTM D6751 does not contain an IV specification, but EN 14214 limits IV to a maximum of 120 g I2/100 g. The IV of YGME (97) was within the limit prescribed in EN 14214 but SME (134) was above the threshold. The relatively high content of polyunsaturated FAMEs in SME was responsible for its failure of the IV specification. Lastly, although not specified in either ASTM D6751 or EN 14214, density, HHV, lubricity, SG, and surface tension of SME and YGME were typical for FAMEs previously reported.16,27,32,34−36 The FA profiles of SME and YGME are listed in Table 4. SME principally contained methyl linoleate (C18:2) and methyl oleate (C18:1), with contributions from methyl palmitate (C16:0), methyl linolenate (C18:3) and methyl stearate (C18:0) also noted. The predominant constituent in YGME was C18:1, but significant amounts of C18:2, C16:0, Table 4. Fatty Acid Composition (Area %) of SME and YGMEa fatty acid
formulab
MW (g/mol)b
C14:0 C16:0 C16:1 9c C18:0 C18:1 9c C18:2 9c, 12c C18:3 9c, 12c, 15c C20:0 unknown (sum)
C15H30O2 C17H34O2 C17H32O2 C19H38O2 C19H36O2 C19H34O2 C19H32O2 C21H42O2
242.40 270.45 268.43 298.50 296.49 294.47 292.46 326.56
SME
YGME
N/D 11.0 (0.1) N/D 3.9 23.2 (0.1) 53.7 (0.3) 8.2 (0.1) N/D 0
0.4 6.0 0.3 (0.1) 3.8 67.7 (0.2) 17.8 (0.1) 2.9 0.3 (0.1) 0.8
a For example, C18:1 9c signifies an 18 carbon fatty acid chain with one cis (c) double bond located at carbon 9 (methyl 9Zoctadecenoate; methyl oleate); N/D = not detected; values in parentheses are standard deviations. Where not indicated, standard deviation was zero. bFor corresponding FAMEs.
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Table 5. Fuel Properties of Diesters 1−8a 1 formula MW, g/mol melting point, °C flash point, °C oxidative stability IP, 110 °C, h kinematic viscosity 40 °C, mm2/s cetane number moisture, ppm wear scar, 60 °C, μm surface tension 25 °C, mN/m 40 °C, mN/m density 15 °C, g/cm3 25 °C, g/cm3 40 °C, g/cm3 specific gravity 15 °C 25 °C 40 °C HHV, MJ/kg a
3
4
5
6
C18H34O4 314.46 24
>24
>24
>24
>24
5.28 (0.01)
4.66 (0.01)
5.52 (0.01)
4.76 (0.01)
4.98 (0.01)
5.56 (0.01)
5.07 (0.01)
5.11 (0.01)
22.8 (0.4) 382 (3) 323 (3)
18.0 (0.6) 335 (5) 370 (1)
32.5 (0.5) 336 (6) 313 (7)
28.0 (0.3) 303 (2) 304 (1)
25.1 (0.4) 348 (3) 282 (12)
32.4 (0.4) 318 (3) 264 (2)
28.5 (0.4) 213 (3) 254 (3)
27.1 (0.5) 218 (7) 298 (1)
28.2 (0.1)
27.0 (0.1)
28.8 (0.1)
27.4 (0.1)
28.2 (0.1)
28.1 (0.1)
28.5 (0.1)
28.2 (0.1)
26.7 (0.1)
25.8 (0.1)
27.4 (0.1)
26.1 (0.1)
26.9 (0.1)
26.8 (0.1)
27.1 (0.1)
26.9 (0.1)
0.939 (0)
0.944 (0)
0.949 (0)
0.948 (0)
0.952 (0)
0.939 (0)
0.950 (0)
0.950 (0)
0.931 (0)
0.936 (0)
0.941 (0)
0.940 (0)
0.944 (0)
0.931 (0)
0.942 (0)
0.942 (0)
0.919 (0)
0.924 (0)
0.930 (0)
0.927 (0)
0.931 (0)
0.920 (0)
0.930 (0)
0.930 (0)
0.940 0.934 0.927 33.37
0.945 0.939 0.931 32.11
0.950 0.944 0.937 33.34
0.949 0.942 0.935 31.90
0.953 0.945 0.939 32.26
0.940 0.934 0.927 33.37
0.951 0.945 0.937 32.76
0.951 0.945 0.937 32.81
(0) (0) (0) (0.17)
2
(0) (0) (0) (0.34)
(0) (0) (0) (0.26)
(0) (0) (0) (0.14)
(0) (0) (0) (0.18)
(0) (0) (0) (0.24)
7
(0) (0) (0) (0.08)
8
(0) (0) (0) (0.11)
See footnotes for Table 1.
Melting point is impacted by structural features such as extent of branching, nature of branching groups and chain length. The mps of 1−8 ranged from −21.5 °C (6) to less than −80 °C (1, 2, 7, and 8; −80 °C was the lowest temperature the instrument was designed to measure). Diesters prepared from 3-methyl-1,5-pentanediol (1 and 2) contained an extra branching group (methyl) than those prepared from adipic acid (3−8). Consequently, lower mps were observed. Diesters 7 and 8 were mixtures of diesters by design and thus provided lower mps than other adipic-derived materials (3−6) in accordance with freezing point depression theory. Diester 7 was a mixture of 3, 5, and 7 whereas 8 was a combination of 3, 4, and 8 due to how they were prepared: equimolar mixtures of branched alcohols reacted with adipic acid. Diesters with the most promising fuel properties were selected for further study as property enhancers in biodiesel and ULSD. Because CFIs were of interest in the current study, diesters with the lowest mps were preferred. Such a criterion thus excluded 3−6 from further consideration. Of the remaining diesters (1, 2, 7, and 8), 2 was excluded because it possessed the lowest DCN and HHV along with the longest wear scar. Exploration of the diesters as lubricity enhancers for ULSD was of interest, thus diesters with comparatively long wear scars were undesirable. Consequently, 1, 7, and 8 were selected for further study as CFIs for biodiesel and as lubricity enhancers for ULSD. 3.4. Evaluation of Diesters as CFIs for Biodiesel. Depicted in Table 6 are properties of D1.0, D2.0, D5.0, and D10.0 blends of 1, 7, and 8 with SME and YGME. Fuel
properties determined of the blends included CP, CFPP, and PP along with KV, SG (15 °C) and HHV. With regard to cold flow properties, ΔCP, ΔCFPP, and ΔPP are listed, which represent differences between neat biodiesel (SME and YGME) and the corresponding blends with 1, 7, or 8. For example, a ΔCP of 0.7 °C was obtained for the D1.0 blend of 1 in SME, which indicated that the CP of the blend was 0.7 °C lower than neat SME (CP: 1.6 °C). Thus, the CP of the D1.0 blend of 1 in SME was 0.9 °C (1.6−0.7 = 0.9 °C). Differences in CP, CFPP, and PP were presented in Table 6 as opposed to the measured values to more clearly illustrate the influence of diester addition on cold flow properties of biodiesel. The response of CP, CFPP, and PP to diester content was highly linear (R2 > 0.9), regardless of diester or biodiesel source. In some cases, R2 values greater than 0.98 were achieved after least-squares statistical regression. CP, CFPP, and PP decreased linearly as the content of diester in the blends increased. The magnitude of reduction did not vary significantly among the diesters, as each yielded approximately equal ΔCP, ΔCFPP, and ΔPP values at similar concentrations in SME or YGME. For instance, D5.0 blends of diesters 1, 7, and 8 in SME yielded ΔCP values of 3.2, 3.1, and 3.2 °C, respectively. However, the response was more pronounced in SME than in YGME. For example, D10.0 blends of 1 in SME and YGME provided ΔCP values of 5.3 and 4.0 °C, respectively. Lastly, CP appeared to be most strongly impacted by diester addition to SME, followed by PP and CFPP. Conversely, CP, CFPP, and PP were equally impacted by diester addition to YGME but to generally lesser extents than in SME. The stronger response of 3267
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Table 6. Fuel Properties of Blends of Diesters 1, 7, and 8 with SME and YGMEa vol % diester Diester 1 1.0 in SME 2.0 in SME 5.0 in SME 10.0 in SME R2 1.0 in YGME 2.0 in YGME 5.0 in YGME 10.0 in YGME R2 Diester 7 1.0 in SME 2.0 in SME 5.0 in SME 10.0 in SME R2 1.0 in YGME 2.0 in YGME 5.0 in YGME 10.0 in YGME R2 Diester 8 1.0 in SME 2.0 in SME 5.0 in SME 10.0 in SME R2 1.0 in YGME 2.0 in YGME 5.0 in YGME 10.0 in YGME R2
ΔCP (°C)
ΔCFPP (°C)
ΔPP (°C)
KV, 40 °C (mm2/s)
SG (15 °C)
HHV (MJ/kg)
0.7 1.7 3.2 5.3 0.9831 0.7 1.8 2.5 4.0 0.9463
1.0 2.0 3.3 4.7 0.9531 1.3 2.0 3.3 4.0 0.9036
1.0 2.4 3.7 5.0 0.9089 0.5 1.6 3.0 4.3 0.9355
4.03 (0) 4.05 (0) 4.08 (0) 4.12 (0) 0.9800 4.52 (0) 4.56 (0) 4.61 (0.03) 4.66 (0.02) 0.9420
0.887 (0) 0.888 (0) 0.889 (0) 0.892 (0) 0.9854 0.883 (0) 0.884 (0) 0.885 (0) 0.888 (0) 0.9854
39.31 (0.13) 39.23 (0.17) 39.17 (0.14) 38.49 (0.08) 0.9148 39.61 (0.16) 39.49 (0.28) 39.27 (0.36) 39.05 (0.03) 0.9679
0.7 1.9 3.1 5.7 0.9832 0.8 1.6 2.6 4.0 0.9731
1.0 2.3 3.0 4.3 0.9067 1.0 2.0 3.0 4.0 0.9184
0.7 2.0 3.4 5.4 0.9637 0.6 1.3 2.6 3.6 0.9432
4.08 (0.01) 4.09 (0) 4.13 (0.01) 4.17 (0.01) 0.9853 4.55 (0) 4.56 (0.01) 4.61 (0) 4.67 (0.01) 0.9945
0.887 (0) 0.888 (0) 0.890 (0) 0.893 (0) 0.9951 0.883 (0) 0.884 (0) 0.886 (0) 0.889 (0) 0.9951
39.27 (0.18) 39.26 (0.15) 39.14 (0.08) 38.96 (0.16) 0.9947 39.26 (0.13) 39.21 (0.19) 39.07 (0.26) 38.87 (0.19) 0.9980
0.6 1.6 3.2 5.5 0.9872 0.7 1.9 2.7 4.1 0.9315
1.0 2.0 2.7 4.3 0.9613 1.0 2.0 3.3 4.3 0.9157
0.7 2.4 3.4 5.0 0.9015 0.7 1.6 2.7 3.6 0.9168
4.09 (0.01) 4.10 (0.01) 4.13 (0) 4.21 (0) 0.9865 4.57 (0) 4.60 (0) 4.64 (0) 4.68 (0.01) 0.9476
0.887 (0) 0.888 (0) 0.890 (0) 0.893 (0) 0.9951 0.883 (0) 0.884 (0) 0.886 (0) 0.889 (0) 0.9951
39.01 (0.07) 39.00 (0.02) 38.60 (0.04) 38.37 (0.09) 0.9410 39.52 (0.17) 39.51 (0.30) 39.30 (0.06) 38.62 (0.28) 0.9594
ΔCP = [CP, biodiesel] − [CP, blend]; ΔCFPP = [CFPP, biodiesel] − [CFPP, blend]; ΔPP = [PP, biodiesel] − [PP, blend]; R2 values are from linear regression; values in parentheses represent standard deviations from the reported means.
a
SME to diester addition may be attributed to its FA profile, as SME contained higher percentages of saturated and polyunsaturated FAMEs relative to YGME, whereas YGME contained significantly more monounsaturated FAMEs. Saturated FAMEs are particularly problematic with regard to low temperature properties, so perhaps 1, 7, and 8 were effective at solubilizing these constituents at reduced temperatures to delay their crystallization, thus contributing more greatly to enhancing cold flow properties of SME than YGME. The influence of diester addition on KV, SG, and HHV was highly linear. Specifically, KVs of the blends increased linearly (R2 > 0.94) as the concentration of diesters increased in SME and YGME. All diester blends provided KVs within the ranges specified in ASTM D6751 and EN 14214. With regard to SG, increasing the diester content in blends with SME and YGME caused SG to increase linearly (R2 > 0.98). Lastly, as the content of diester increased in the blends, HHV decreased linearly (R2 > 0.91). Such results were anticipated, as 1, 7, and 8 exhibited higher KVs and SGs as well as lower HHVs than SME and YGME. 3.5. Evaluation as Lubricity Enhancers for ULSD. Depicted in Table 7 are fuel properties of D1.0, D2.0, D5.0, and D10.0 blends of 1, 7, and 8 in ULSD, which included CP, CFPP, PP, lubricity, KV, SG (15 °C), and HHV. An analogous
set of B1.0, B2.0, B5.0, and B10.0 blends of SME and YGME in ULSD were also analyzed. As was the case for Table 6, cold flow data was presented as changes in CP, CFPP, and PP relative to neat ULSD. The response of lubricity to diester concentration was highly linear (R2 > 0.92 or greater), with increasing concentrations of 1, 7, and 8 resulting in progressively shorter wear scars. All diester blends yielded wear scars below the maximum levels specified in the petrodiesel standards. Efficacy of diesters in blends mirrored the order of lubricity obtained for neat diesters: 7 (shortest wear scar) > 8 > 1 (longest wear scar). For instance, the D10.0 blends of 7, 8, and 1 yielded wear scars of 184, 207, and 244 μm, respectively. Interestingly, the D5.0 and D10.0 blends of 1, 7, and 8 provided wear scars that were shorter than those observed for the respective neat diesters. For example, 7 exhibited a wear scar of 254 μm (Table 5) whereas the wear scars measured for D5.0 and D10.0 blends of 7 in ULSD were 241 and 184 μm (Table 7), respectively. Such a result indicated a synergistic relationship between diesters and ULSD whereby greater than anticipated improvements in lubricity were observed. Comparison of the diester blends to SME and YGME blends revealed that biodiesel was more effective at improving lubricity than the diesters, especially at low blend levels. For instance, B1.0 blends of SME and YGME 3268
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Table 7. Fuel Properties of Blends of Diesters, SME, and YGME with ULSDa vol % Diester 1 1.0 2.0 5.0 10.0 R2 Diester 7 1.0 2.0 5.0 10.0 R2 Diester 8 1.0 2.0 5.0 10.0 R2 SME 1.0 2.0 5.0 10.0 R2 YGME 1.0 2.0 5.0 10.0 R2 a
ΔCP (°C)
ΔCFPP (°C)
ΔPP (°C)
lubricity (μm)
KV, 40 °C (mm2/s)
SG (15 °C)
HHV (MJ/kg)
0.1 0.4 0.7 1.0 0.9184
0.3 0.5 0.9 1.6 0.9982
0.4 0.7 1.0 1.4 0.9506
375 (3) 348 (3) 301 (3) 244 (2) 0.9805
2.26 (0) 2.27 (0.01) 2.31 (0) 2.37 (0) 0.9993
0.847 (0) 0.848 (0) 0.851 (0) 0.855 (0) 0.9966
44.14 (0.09) 43.95 (0.11) 43.51 (0.14) 42.98 (0.05) 0.9888
0.1 0.2 0.5 0.8 0.9823
0.2 0.5 1.0 1.5 0.9629
0.4 0.7 1.0 1.4 0.9506
382 (12) 354 (3) 241 (1) 184 (2) 0.9247
2.25 (0) 2.26 (0) 2.31 (0.01) 2.38 (0) 0.9981
0.848 (0) 0.849 (0) 0.851 (0) 0.856 (0) 0.9930
44.15 (0.34) 43.83 (0.32) 43.73 (0.01) 43.19 (0.14) 0.9393
0.1 0.3 0.6 0.9 0.9564
0.3 0.6 1.1 1.7 9.9778
0.4 0.7 1.0 1.4 0.9506
373 (3) 325 (1) 298 (2) 207 (6) 0.9638
2.25 (0) 2.27 (0) 2.31 (0.01) 2.40 (0.01) 0.9962
0.848 (0) 0.849 (0) 0.851 (0) 0.856 (0) 0.9930
44.16 (0.15) 43.93 (0.10) 43.59 (0.17) 43.11 (0.23) 0.9829
−0.2 −0.7 −1.6 −2.3 0.9425
−0.2 −0.4 −0.7 −1.3 0.9950
0 −0.4 −0.8 −1.3 0.9440
292 (1) 221 (12) 185 (6) 167 (3) 0.9145b
2.18 (0) 2.21 (0) 2.30 (0) 2.39 (0) 0.9823
0.847 (0) 0.847 (0) 0.848 (0) 0.850 (0) 0.9830
44.19 (0.05) 44.10 (0.05) 43.93 (0.07) 43.76 (0.17) 0.9592
−0.3 −0.7 −1.5 −2.2 0.9598
−0.3 −0.7 −1.2 −2.0 0.9816
−0.4 −0.7 −1.1 −2.6 0.9287
284 (2) 213 (5) 192 (23) 164 (11) 0.9065b
2.27 (0.01) 2.29 (0) 2.41 (0) 2.56 (0.02) 0.9954
0.847 (0) 0.847 (0) 0.848 (0) 0.850 (0) 0.9830
44.17 (0.10) 44.12 (0.14) 43.97 (0.32) 43.71 (0.39) 0.9999
See footnote for Table 6. bLogarithmic least-squares statistical regression.
afforded wear scars of 292 and 284 μm versus 375, 382, and 373 μm measured for D1.0 blends with 1, 7, and 8. Furthermore, the response of lubricity to biodiesel addition after statistical regression was logarithmic (R2 > 0.91) as opposed to the linear response noted for diesters. The impact of diesters on cold flow properties of ULSD was minimal, as improvements to CP, CFPP, and PP of less than 2.0 °C were obtained even at the D10.0 blend level. The response of CP, CFPP, and PP to diester concentration was linear (R2 > 0.91). The response of cold flow properties to biodiesel concentration was also highly linear (R2 > 0.92), but CP, CFPP, and PP increased with increasing concentration of biodiesel as opposed to the reductions observed with diester addition to ULSD. Such a result represents an advantage of diesters over biodiesel, as diesters did not negatively impact cold flow properties when blended with ULSD. For comparison, D10.0 blends of 1, 7, and 8 yielded ΔPP values of 1.4 °C whereas −1.3 and −2.6 °C were obtained for B10.0 blends of SME and YGME. The influence of diester concentration on KV, SG and HHV was determined, and linear correlations were noted between diester content and properties. Specifically, as the concentration of diesters increased in ULSD, the KVs of the blends increased linearly (R2 > 0.99) relative to unblended ULSD. All diester blends provided KVs within the ranges specified in petrodiesel standards. With regard to SG, increasing the diester content in blends caused SG to increase linearly (R2 > 0.99). Lastly, as the concentration of diesters increased in blends, HHV decreased
linearly (R2 > 0.93). Such results were anticipated, as 1, 7, and 8 exhibited higher KVs and SGs as well as lower HHVs than ULSD. Comparison to the corresponding blends with biodiesel revealed similar results, as biodiesel blends caused KV and SG to increase linearly and HHV to decrease linearly.
4. CONCLUSIONS Branched diesters were prepared by Fischer esterification of branched alcohols and acids using catalytic H2SO4 along with concomitant elimination of water to shift the equilibria toward the products employing a Dean−Stark apparatus. Condensation following a diacid/alcohol route as opposed to the diol/ acid method was more facile, as higher yields were obtained in shorter periods of time. The synthetic diesters possessed advantageously low melting points (24 h), density, FP, KV, SG, and surface tension data. However, the diesters yielded DCN and HHV values that compared unfavorably to biodiesel. Diesters possessing the most desirable combinations of properties were blended with biodiesel and ULSD in an effort to ameliorate technical deficiencies of these fuels. Addition of diesters to biodiesel resulted in significant improvements to cold flow properties while simultaneously exhibiting minimal impacts on KV, SG and HHV. Furthermore, diesters improved lubricity and cold flow properties when blended with ULSD while simultaneously avoiding significant deleterious effects on KV, SG, and HHV. Linear responses were 3269
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Energy & Fuels
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noted with regard to the influence of diester concentration on fuel properties in blends with biodiesel and ULSD.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +1-309-681-6511. Fax: +1-309-681-6524. Email: Bryan.
[email protected]. Notes
Disclaimer. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The author acknowledges Benetria N. Banks and Erin L. Walter for excellent technical assistance. Dr. Karl Vermillion (USDAARS-NCAUR) is acknowledged for acquisition of NMR data. Dr. Kenneth M. Doll (USDA-ARS-NCAUR) is acknowledged for useful discussions.
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ACRONYMS AOCS = American Oil Chemists’ Society ASTM = American Society for Testing and Materials AV = acid value CEN = European Committee for Standardization CFI = cold flow improver CFPP = cold filter plugging point CP = cloud point DCN = derived cetane number FA = fatty acid FAME = fatty acid methyl ester FP = flash point HHV = higher heating value IP = induction period IV = iodine value KV = kinematic viscosity PP = pour point rt = room temperature SG = specific gravity SME = soybean oil methyl esters ULSD = ultralow sulfur diesel YGME = yellow grease methyl esters
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dx.doi.org/10.1021/ef500482f | Energy Fuels 2014, 28, 3262−3270
