Energy Fuels 2011, 25, 398–405 Published on Web 12/03/2010
: DOI:10.1021/ef1013743
Effect of Saturated Monoglyceride Polymorphism on Low-Temperature Performance of Biodiesel G. M. Chupka,† J. Yanowitz,§ G. Chiu,‡ T. L. Alleman,† and R. L. McCormick*,† †
National Renewable Energy Laboratory, Golden, Colorado 80401, United States, ‡Phase Technology, Richmond, British Columbia, Canada, V7A5H8, and §Ecoengineering, Boulder, Colorado 80304, United States Received October 8, 2010. Revised Manuscript Received November 22, 2010
To investigate precipitates above the cloud point (CP) in biodiesel, three saturated monoglycerides (SMGs), monomyristin, monopalmitin, and monostearin, were spiked into distilled soy and animal fat-derived B100. It was shown that above a threshold or eutectic concentration the SMGs significantly raise the CP of B100. A comparison to published data suggests that commercial B100 has SMG content in the same range as the eutectic point. SMGs have an even greater impact on the final melting temperature (FMT, as measured when the sample is heated) at concentrations above the eutectic point. These results were verified and visualized using a controlled temperature stage microscope. It was shown that the FMT was highly dependent on the rate of heating. It is hypothesized that a lower melting point crystalline form of the SMG forms upon rapid cooling and then transforms into a more stable, higher melting point crystalline form when slowly heated or held at constant temperature. The CP and FMT results of this study were compared to an ideal solution thermodynamic model. The model was able to provide reasonable prediction of the eutectic point but was less successful at predicting CP and FMT above the eutectic.
differential scanning calorimetry (DSC), in the warming direction was as much as 25 °C higher than the CP.3 Biodiesel does not consist of 100% FAME but may contain impurities such as mono- and diglycerides (partly converted feedstock) as well as unsaponifiable matter. Unsaponifiable matter (literally, material that cannot be converted to soap) is nonlipid material present in the oil feedstock and can include plant sterols, tocopherols, hydrocarbons, pigments, and minerals.3 A typical soy oil feedstock contains about 1.6 w/w % unsaponifiable matter.4 Van Gerpen and co-workers examined the effect of unsaponifiable matter on CP for B100 and for a B20 blend.3 At 2 w/w % unsaponifiable content, they observed no effect on B100 or the B20 blend CP. However, increasing the level to 3% caused a greater than 10 °C increase in the B100 CP but no measurable effect on the B20 CP. The same study showed that as little as 0.05 w/w % SMGs could increase B100 CP by 2 °C. Mono- and diglycerides are limited in ASTM D6751 by controlling the allowable level of total glycerin to 0.24 w/w % and further limitations are under consideration. Total glycerin consists of free glycerin and bound glycerin. Bound glycerin is in the form of mono-, di-, and triglycerides and is reported as w/w % glycerin. A B100 quality survey in 2008 found monoglyceride content to range from nondetect up to 0.75 w/w % for in-specification samples with an average monoglyceride content of roughly 0.5 w/w % for U.S. biodiesel.5 To a good approximation, the fatty acid composition of the monoglycerides mirrors that of the parent fat or oil. Thus, for soybean oil, palmitic acid (C16:0) makes up 7-11% of the fatty acid chains, while stearic acid (C18:0) makes up 3-5% of the fatty
Introduction The use of biodiesel in blends with petroleum diesel at concentrations of B201 and lower has been largely problem free. However, the higher crystallization temperature of the saturated fatty acid methyl esters (FAME) that are present in biodiesel is a concern in cold weather. To ensure that the cold-flow properties of biodiesel/diesel blends are appropriate for their intended use, ASTM International (ASTM) standards for blends up to B5 (ASTM D975), B6 to B20 blends (ASTM D7467), and for biodiesel intended for use as blendstock (ASTM D6751) require the measurement of the cloud point (CP), the temperature at which crystallization begins (as measured by ASTM D2500, ASTM D5773, or other equivalent method). While no specific CP is mandated, the CP must be disclosed so that users can determine if the blend CP is acceptable for the expected weather conditions. CP measurement methods cool the sample at rates on the order of 1.5 °C/min. In comparison, under typical biodiesel storage conditions, diurnal cooling might be as much as 15-20 °C over 6-12 h, giving an average cooling rate of 0.02 to 0.05 °C/min. This work was motivated by reports of fuel filter plugging in vehicles and dispensers at temperatures above the CP of the fuel. In some cases, the vehicle or the fuel has had to be warmed to temperatures significantly above the CP to alleviate this issue.2 In the laboratory, Van Gerpen and co-workers also observed that the final melting temperature (FMT) for biodiesels spiked with saturated monoglycerides (SMGs), as measured by *To whom correspondence should be addressed. E-mail:
[email protected]. (1) Biodiesel blends are typically specified as Bxx where xx is a number that equals the percentage of biodiesel in the blend. (2) Dunn, R. O. Prog. Energy Combust. Sci. 2009, 35, 481–489. (3) Van Gerpen, J. H.; Hammond, E. G.; Yu, L.; Monyem, A. SAE Technical Paper No. 971685, 1997. r 2010 American Chemical Society
(4) Gutfinger, T.; Letan, A. Lipids 1974, 9, 658–663. (5) Alleman, T. L.; McCormick, R. L. Results of the 2007 B100 Quality Survey, NREL/TP-540-42787, National Renewable Energy Laboratory: Golden, CO, 2008.
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: DOI:10.1021/ef1013743
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in several crystalline forms or polymorphs. Polymorphs of the same material will demonstrate different physical properties: they will have different melting points (MPs), solubility, and stability.18 Here we examine the effect of SMG on CP and FMT of B100 and reveal a possible role for polymorphism in explaining fuel filter plugging above CP.
acid chains. One study measured the SMG content of soy biodiesel and found monopalmitin concentrations of about 11% of the total monoglycerides and monostearin concentrations of about 5% of the total monoglyceride concentration.3 On the basis of this information, the concentration of monopalmitin in soy biodiesel might typically be about 0.06 w/w % and the concentration of monostearin about 0.03 w/w %. Beef tallow could contain up to 5% myristic acid chains, 25-35% palmitic acid, and 20-25% stearic acid and thus might contain 0.03 w/w % monomyristin, 0.15 w/w % monopalmitin, and 0.1 w/w % monostearin. The role of impurities in fuel filter plugging has been the subject of several studies. Pfalzgraf and co-workers examined the role of sterol glucosides (SG), monoglycerides, and water on filtration time after a cold temperature soak.6 They found that all of the tested impurities could potentially contribute to higher filtration times. Tang and co-workers cooled biodiesel blends to 4 °C, significantly above the expected CP, and noted the presence of precipitates after a storage time of 24 h.7 Analysis of these precipitates showed them to be SG for soy biodiesel, monoglycerides for poultry fat-derived biodiesel, and a combination of SG and monoglycerides for cottonseed oilbased biodiesel. Moreau and co-workers examined 24 samples from various places in the biodiesel supply chain.8 They found that solids in these samples consisted primarily of SG and other plant sterols. These studies suggest that SG are mainly responsible for filter plugging above the CP. In contrast, Selvidge and co-workers presented evidence that dispenser filters used with a 2.5 vol % soy biodiesel blend could become blocked by SMGs as temperatures approached -18 °C (0 °F) for SMG content above about 0.07 w/w %.9 The idea that SMGs could cause filter plugging was also the conclusion of a low-temperature, heavy-duty vehicle testing study.10 Selvidge and co-workers also reported that the use of a cold soak filtration test (in which the fuel is cooled to 4.5 °C and maintained at that temperature for 16 h, followed by filtration) is effective at identifying biodiesel exhibiting precipitate formation above the CP and can result in a substantial reduction in incidents of fuel filter plugging.9 This is consistent with recent Coordinating Research Council studies that tested trucks at low temperature on B5 and B20 blends from biodiesel with a range of cold soak filtration times.11,12 Cold soak filterability was added to the B100 ASTM specification (D6751) in 2008. Monoglycerides are present in many food products, and the phase behavior of monoglycerides in the food context has been studied for many years.13 It is well-known that SMGs exist
Experimental Section Sample Preparation. Spiked samples of B100 were prepared by weighing monomyristin, monopalmitin, and monostearin (>99% pure) purchased from Nuchek Prep (Elysian, MN) into soy- and animal-based biodiesel on an analytical balance at 2.0 w/w %. Distilled soy B100 (Nexsol) biodiesel was obtained from Peter Cremer North America (Cincinnati, OH) and distilled animal-based B100 (tallow) was obtained from Rothsay (Quebec, Canada). Subsequent levels were prepared by dilution after solutions were heated in a 50 °C water bath until they were clear and free of any solid particulates. Instrumentation. Samples were analyzed for CP and FMT using a Phase Technology 70X Analyzer, which utilizes diffusive light scattering to identify the onset of crystallization (CP per ASTM D5773) or the disappearance of the final solids (FMT). Diffusive light scattering has been shown to be more consistent and more sensitive to the onset of phase change than DSC.19 The FMT is defined as the temperature at which all of the crystals disappear during the final warming step. The FMT was determined by analyzing the point where the signal returns to baseline and remains flat and constant. This is when all of the crystals have remelted into the liquid under the prescribed warming condition. Samples were analyzed by DSC to investigate a polymorphic form using a TA Q200 DSC equipped with a refrigerated cooling system. The DSC was calibrated with indium prior to analysis. Nitrogen was used as the system purge. Samples were sealed in aluminum Tzero pans, and an empty pan was used as reference. Hot stage microscopy was performed with a Nikon Eclipse E800 Microscope with a 10 objective, hot stage, and camera. Gas chromatography (GC) for FAME analysis and monoglyceride content was performed with an Agilent 7890 series GC equipped with a flame ionization detector. ASTM test methods were used without modification unless otherwise indicated. Sample Analysis. FAME analysis of the B100 used to prepare the spiked samples as described above was performed by GC. The acid value (AV), Karl Fischer moisture (KF), cold soak filterability, and monoglyceride content were also measured for these materials. These properties are shown in Table 1. The concentration of monoglycerides in the spiked solutions was verified by ASTM method D6584. Standards were prepared per ASTM method D6584 and were purchased from Supelco (Bellefonte, PA). An additional, lower-level standard was made to extend the lower end of the calibration curve to include the lower levels of monoglycerides present in the biodiesel. Recovery ranged from 83% to 112% with an average absolute value of variation of 7%.
(6) Pfalzgraf, L.; Lee, I.; Foster, J.; Poppe, G. AOCS Inform Special Supplement; Biorenewable Resources No. 4 2007, 17. (7) Tang, H.; De Guzman, R. C.; Salley, S. O.; Ng, K. Y. S. J. Am. Oil Chem. Soc. 2008, 85, 1173–1182. (8) Moreau, R. A.; Scott, K.; Haas, M. J. Am. Oil Chem. Soc. 2008, 85, 761–770. (9) Selvidge, C.; Blumenshine, S.; Campbell, K.; Dowell, C.; Stolis, J. Proceedings of the 10th International Conference on Stability, Handling, and Use of Liquid Fuels, Tucson, AZ, October 7-11, 2007. (10) Poirier, M.-A.; Lai, P.; Lawlor, L. SAE Technical Paper No. 2008-01-238, 2008. (11) Coordinating Research Council. Biodiesel Blend Low-Temperature Performance Validation, CRC Report No. 650, 2008. (12) Coordinating Research Council, Biodiesel Blend Low-Temperature Performance Validation, CRC Report No. 656, 2010. (13) Vereecken, J.; Meeussen, W.; Foubert, I.; Lesaffer, A.; Wouters, J.; Dewettinck, K. Food Res. Int. 2009, 42, 1415–1425. (14) Terentjev, E. M.; Chen, C. H. Langmuir 2009, 25, 6717–6724. (15) Terentjev, E. M.; Chen, C. H.; Van Damme, I. Soft Matter 2009, 5, 432–439. (16) Fredick, E.; Foubert, I.; Van De Sype, J.; Dewettinck, K. Cryst. Growth Des. 2008, 8, 1833–1839.
Results and Discussion DSC Analysis of As-Received Monoglycerides. Monoglyceride samples were analyzed by DSC to determine the crystalline form prior to use in the B100. The samples were equilibrated at 40 °C for 1 min and then ramped at 10 °C/min from 40 to 110 °C. The thermograms are overlaid in Figure 1a (17) Batte, H.; Wright, A. J.; Rush, J. W.; Idziak, S. H. J.; Marangoni, A. G. Food Biophys. 2007, 2, 39–37. (18) Rodriguez-Spong, B.; Price, C. P.; Jayasakar, A.; Matzger, A.; Rodriguez-Hornedo, N. Adv. Drug Delivery Rev. 2004, 56, 241–274. (19) Toro-Vazquez, J.; Herrera-Coronado, V.; Dibildox-Alvarado, E.; Charo-Alonso, M.; Gomez-Aldapa, C. Food Eng. Phys. Prop. 2002, 67, 1057–1065.
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Table 1. Soy B100 and Animal B100 Properties test GC FAME profile
acid value Karl Fischer water cold soak filterability monoglyceride content
units
compound
tallow biodiesel
soy biodiesel
wt % wt % wt % wt % wt % wt % wt % mg of KOH/g ppm seconds wt %
C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 ASTM D664 ASTM D6304 ASTM D6751 Annex method ASTM D6584
1.9 23.9 2.8 13.9 43.6 15.9 1.8 0.17 116 82 myristin: 0.016 palmitin: 0.039 stearin: 0.009
ND 11.1 ND 4.3 21.6 56.1 8.4 0.15 86 96 myristin: 0.002 palmitin: 0.001 stearin: 0.001
Table 2. Cloud Point (ASTM D5773) of Monoglycerides in B100 D5773 cloud point (temp °C) soy
animal
w/w %
myristin
palmitin
stearin
myristin
palmitin
stearin
neat 0.02% 0.1% 0.2% 0.6% 1.0%
-1.1 -0.7 -1.3 18.4 27.5
-0.9 -0.8 -0.6 4.8 29.7 30.0
-0.8 6.7 18.6 30.0 40.5
13.4 13.4 13.3 13.1 24.7
13.7 13.3 13.3 13.6 18.6 29.8
13.4 13.4 15.9 28.6 35.8
consistent with the results of Vereecken and co-workers,13 who produced the β form only after long-term storage of monostearin and found it was not formed upon reheating of the less stable forms in the DSC at 5 °C/min. The temperatures of the various crystalline phase changes shown on the DSC thermogram are identical to those of Vereecken and coworkers. They identified the various crystalline forms as R (MP ∼75 °C), sub-R1 (MP ∼50 °C), and sub-R2 (∼MP 20 °C, only barely noticeable as the leftmost exothermic feature in Figure 1b) forms seen on cooling. The other two monoglycerides behaved in a similar manner except that the sub-R2 polymorphic form was not observed during the cooling cycle of monopalmitin and monomyristin, which is also consistent with the observations of Vereecken and co-workers.13 Cloud Point and Final Melting Point of Monoglycerides in Distilled Biodiesel. CP results were checked by randomly selecting seven samples to be measured by another laboratory. The results were consistent within 1 °C. For samples with a CP above room temperature, the samples were preheated in an oven to at least 5-10 °C above the CP for at least 15 min. In all cases, the samples were visually checked to ensure that no visible solids, cloud, or haze was present prior to introduction into the analyzer. The sample was then transferred to the analyzer at the same temperature as was used in the oven. For the CP experiments, the sample was then cooled at 1.5 °C/min. For the FMT experiments, the sample was first cooled at 30 °C/min until a sufficient amount of scattering signal was observed, which is indicative of solid/ wax formation. Thereafter, the sample was heated at 1.5 °C/min to the FMT. Each sample was tested two to three times, and the results were typically within (1 °C. Tables 2 and 3 show the CP and FMT results for SMGs blended into soy and animal fat biodiesel, respectively. CPs for the neat B100 are typical of soy and animal fat-derived B100. These results are an average of all runs performed on each sample. Results in bold type show a significant increase in CP or FMT relative to the neat biodiesel. Here we define a significant
Figure 1. (a) DSC thermogram of as-received monoglycerides and (b) DSC thermogram of heat/cool/heat cycles of monostearin at 10 °C/min.
and show MPs consistent with the most stable β form for all three monoglycerides when compared to recent values reported in the literature (MPs: monomyristin, 72.44 °C; monopalmitin, 77.99 °C; monostearin, 82.65 °C).13 To confirm the reported polymorphic behavior of the monoglycerides, the samples were looped through a heat/ cool/heat cycle after the initial melt. Monostearin is used as an example, and the resulting thermogram is displayed in Figure 1b. From this analysis, it appears that once the ss form has melted, it does not reappear upon repeated heating and cooling even at various ramp rates. The ramp rates investigated were 0.5, 1.5, 2.5, 5.0, and 10.0 °C/min. This is 400
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: DOI:10.1021/ef1013743
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Table 3. Final Melting Temperature of Monoglycerides in B100 final melting temperature (temp °C) soy
animal
w/w %
myristin
palmitin
stearin
myristin
palmitin
stearin
neat 0.02% 0.1% 0.2% 0.6% 1.0%
1.1 0.8 1.2 26.5 34.8
0.8 0.9 0.7 10.0 35.5 43.0
0.9 10.2 37.0 45.7 49.5
14.3 15.2 15.1 29.2 34.5
14.4 14.5 14.8 15.3 36.0 44.5
14.6 15.0 16.8 47.0 48.5
increase as a change in CP or FMT of 2 °C or higher from the values for the neat sample. SMGs affect the CP of the neat biodiesels at fairly low concentrations. The effect begins at between 0.2% and 0.6% for monomyristin in soy biodiesel and at above 0.6% in animal biodiesel, and at even lower concentrations for monopalmitin and monostearin. Cold soak filterability was measured for a set of soy biodiesel samples containing from 0.025% to 0.3% monostearin (see Table S1 in Supporting Information). Filtration time began to increase at 0.075% monostearin and addition of 0.1 w/w% monostearin increased filtration time to over 720 s, suggesting that this biodiesel would have caused vehicle operability problems at temperatures above CP.11,12 The effect on soy biodiesel, with its lower CP, occurs at lower monoglyceride concentration than it does for animal fat biodiesel. There is only a small difference between CP and FMT of the neat biodiesels and for the samples with low concentrations of monoglycerides. Once the monoglyceride concentration exceeds a threshold level, the eutectic point, which is different for each monoglyceride/biodiesel pair, it affects both the CP and the FMT. The difference between the two can be quite large. For example, a 0.2% solution of monostearin in soy biodiesel has an FMT of 37.0 °C, even though its CP was only 18.6 °C. Even at 0.1%, the FMT-CP difference is nearly 4 °C. Thus, the presence of SMG in sufficient quantities could cause the presence of precipitates above the CP in biodiesel. As discussed in the Introduction, a typical soy oil-derived biodiesel can have a total SMG content of around 0.1 w/w %, and a typical beef tallow biodiesel may contain 0.3 w/w %. At the allowable upper limit for total glycerin, SMG concentrations may be nearly 2 times higher. Examination of Tables 2 and 3 shows that these concentration levels are in the same range as the eutectic points for monopalmitin and monostearin, the most common SMGs in biodiesel from conventional fat and oil feedstocks. In the past, biodiesel surveys have quantified total monoglycerides.5 Future surveys will also need to measure SMG content and composition to understand what fraction of B100 in the market has an SMG content above the eutectic point. We have employed an ideal solution model, described in the Supporting Information, to investigate the impact of SMGs on biodiesel cloud point theoretically. The model employs the MP and enthalpy of the β form of SMGs. The β form is known to be the most stable, highest MP crystalline form.13,20 Values for the enthalpy of melting for other crystalline forms do not appear to have been accurately measured. Figure 2 compares model predictions with measured CP and FMT results for SMGs dissolved in soy biodiesel, and Figure 3 presents the same data for animal fat-derived biodiesel. In general, this simple model gives approximate yet reasonable prediction of the eutectic point.
Figure 2. Measured and predicted results for soy-derived biodiesel containing various levels of the SMGs: (a) monomyristin, (b) monopalmitin, and (c) monostearin.
The CP will be determined at a point where there is a measurable amount of solid in the mixture (or in the case of the FMT, at the point at which the last measurable amount disappears), while the theoretical temperatures for these events occur at the point at which the first molecule solidifies (or the last molecule liquefies). Thus, both the CP and FMT measurements will be biased to lower values than predicted by even a perfect model because of measurement error. Coutinho and others21 estimate this bias to be on the order of 1-2 °C for mixtures of diesel fuel and jet fuel. At the lowest concentrations, the model (21) Coutinho, J. A. P.; Mirante, F.; Ribeiro, J. C.; Sansot, J. M.; Daridon, J. L. Fuel 2002, 81, 963–967.
(20) Lutton, E. S. J. Am. Oil Chem. Soc. 1971, 778–781.
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Figure 4. Diffusive light scattering FMT plots: (a) neat tallow B100 and (b) 0.6% monostearin and 0.6% monopalmitin in tallow B100. Table 4. Microscope and Diffusive Light Scattering Estimated Phase Change Temperatures: Monostearin in B100 Animal Fata animal fat: temperature °C w/w %
cooling-M
cooling-CP
heating-M
heating-FMT
0.02% 0.10% 0.20% 0.60%
9.1 11.4 15.3 26.2
13.4 13.4 15.9 28.6
12.8 15.1 17.8 49.1
14.6 15.0 16.8 47.0
a
M = microscope.
Table 5. Microscope and Diffusive Light Scattering Estimated Phase Change Temperatures: Monostearin in B100 Soya soy: temperature °C
Figure 3. Measured and predicted results for animal fat-derived biodiesel containing various levels of the SMGs: (a) monomyristin, (b) monopalmitin, and (c) monostearin.
predictions and data are in most cases within 2 °C. However, the model predictions and measured results tend to diverge with increasing concentration. This may indicate a breakdown in the ideal solution model assumptions. This is especially true for the FMT results and even more so at levels where the FMT-CP difference is more than a few degrees. During the FMT measurement of some of the samples, a very prominent signal increase was noted after an initial large decrease in signal in the light scattering plots. As an example, a plot of animal B100 with no added monoglyceride and plots of 0.6% monostearin and 0.6% monopalmitin in animal fat B100 are shown in Figure 4. In Figure 4a, where no SMG is added, the signal decreases rapidly until the FMT is at
w/w %
cooling-M
cooling-CP
heating-M
heating-FMT
0.02% 0.10% 0.20% 0.60%
-7.5 7.2 16.2 28.0
-0.8 6.7 18.6 30.0
-1.8 9.5 38.5 50.1
0.9 10.2 37.0 45.7
a
M = microscope.
approximately 15 °C. In Figure 4b, there is an initial signal decrease, followed by an increase prior to the FMT being reached at approximately 40 °C (for monostearin). Repeated experiments determined that this increase in signal was reproducible and present in several samples where the concentration of SMGs is above the eutectic point. To try to determine the cause of this feature, a hot stage microscope was used to visualize the samples as they were heated and cooled. Controlled Temperature Microscopy of B100 Samples. Monostearin at several concentrations in both soy and animal fat B100 was used for this experiment. Samples were 402
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Figure 5. Monostearin crystals under a microscope at various temperatures (0.6% monostearin in animal fat biodiesel). The scale bar in the photos is 250 μm long.
heated on a hot plate at 50 °C until the solutions were completely clear and free of any solids present. The samples were then transferred to the heated stage on the microscope at 50 °C. The sample was cooled at 1.5 °C/min down to -10 °C followed by heating at 1.5 °C/min back up to 50 °C. Photographs were taken by the camera every 20 s through a 10X objective. CP and FMT results were compared to the visual observations. Tables 4 and 5 summarize the temperatures at which initial crystal formation or final melting were noted under the microscope. The results are compared to the CP and FMT results with generally good agreement. The microscope observations of the higher monostearin concentrations in biodiesel show a similar significant temperature difference between when the crystals formed on cooling and when they melt on heating. The microscope images also show the difference in the appearance of the crystals at different temperatures. Figure 5 shows images at various temperatures ranging from 25 to 46 °C. From these images, it is apparent that the crystals changed from a needlelike habit to a pinwheel or rosette habit between 25 and 34 °C. These temperatures coincide with the points marked “A” and “B” in Figure 4b. Knowing that monoglycerides demonstrate polymorphic behavior, we hypothesize that the signal increase observed by diffusive light scattering (point B in Figure 4b) may be due to a polymorphic transformation of monostearin to a higher MP, more stable crystalline form. Heating Rate Study. When polymorphic materials are heated quickly, there is less time for polymorphic conversion. Potentially, if the heating rate is fast enough, an intermediate phase conversion may not occur.22 If the second signal increase is due to a polymorph, it may become smaller or even disappear as the sample is heated faster. In Figure 6, the 0.6% monopalmitin in animal B100 sample was heated at different ramp rates to observe if this was the case. The rightmost or second feature is largest at the slowest ramp rate of 1.5 °C/min, while it is barely visible at 5.0 °C/min. This result
Figure 6. Diffusive light scattering FMT plots for 0.6% monopalmitin in animal B100 at different heating ramp rates.
further supports the idea that a polymorphic conversion is occurring and suggests that the polymorphic form conversion may be the reason for an increase in the FMT as compared to the CP. With the use of the microscope, the same effect was observed for the 0.6% monostearin in the B100 sample. Constant Temperature Study. As shown above, there is evidence of a monoglyceride phase change under slow heating conditions. The potential for this change to occur at constant temperature was also investigated using the hot stage microscope. A sample of 0.6% monostearin in animal B100 was transferred to the hot stage at 50 °C and was then cooled to 25 °C and held at this temperature while being observed under the microscope. This sample was chosen because the concentration was high enough that the crystals would be easy to observe under the microscope. Figure 7 shows two images, the first taken after initial formation of the crystals and the second after 17 min. The initial crystals formed
(22) Pharmaceutical Polymorphism by Rapid Heat-Cool DSC, TA Applications Note 353, TA Instruments: New Castle, DE.
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Figure 7. Monostearin crystals held at 25 °C for 20 min (0.6% monostearin in animal B100). The scale bar in the photos is 250 μm long.
appear needlelike; however, after 17 min at a constant temperature of 25 °C, the crystals have changed form. Thus, a phase change may occur in on the order of 10-20 min at room temperature for monostearin. This conversion would not occur under the comparably rapid cooling that is standard for CP measurement (1.5 °C/min) but may be relevant to real world cooling rates for fuels in storage, which can be an order of magnitude or more lower. Determination of High Melting Crystal Form. In an attempt to determine the crystal form of high melting temperature monostearin crystals observed in FMT experiments, the crystals were isolated from the B100 with a centrifuge. The 0.6% monostearin in animal fat biodiesel sample was heated to 50 °C until the solution was clear and free of solids. The solution was then placed in a chamber at 25 °C for 2 h. A portion of the sample was spun down in a centrifuge at 25 °C for 10 min, and the crystals were transferred to a Whatman 0.7 μm glass fiber filter. The crystals were then washed five times with 20 mL portions of hexane under vacuum to remove residual FAME. The crystals were immediately analyzed by DSC in a heat/cool/ heat cycle. This thermogram was compared to that obtained on the pure monostearin. With the comparison of Figure 8 with Figure 1b, it appears that the β form was isolated from the solution as the MP on first heating is approximately 80 °C. The MP is slightly depressed, and the peak is slightly broader, presumably due to residual biodiesel impurity. As observed with the pure SMG as received, the β form melts first; then R, sub-R1, and sub-R2 recrystallize on cooling. Those three forms then melt on heating, but the β form is not observed to form again. This offers strong evidence that β is the form present after storage at room temperature for a short period of time and that β is the high-temperature form observed in the FMT experiments. This was confirmed by X-ray powder diffraction of the recovered crystals, in comparison to the as received monostearin reagent and to the literature (Supporting Information).23,24 As noted above, the thermophysical properties of the β phase were used in the ideal solution model. On the basis of the hypothesis that β is the high-melting form, a model for this form should correspond most closely to the FMT data, which for SMG concentrations above the eutectic point we propose as the final melting phase. Inspection of Figures 2 and 3 indicates that FMT results fall well away from the model prediction at the highest concentrations, evidence of
Figure 8. DSC thermogram of crystals isolated from 0.6% monostearin in animal biodiesel.
the limits of this approach in modeling this system. The model predictions fall much closer to the CP points, but given the parameters and nature of the model, this may be a coincidence. Because the MP of R is less than that of β (75 °C versus 82 °C), we expect the enthalpy of melting also to be somewhat lower. Model predictions using the R MP and a lower heat of enthalpy than that of β would result in higher solubility at the same temperature for the R form. This is consistent directionally with the FMT-CP difference and the hypothesis that the FMT represents melting of the β-form while the CP represents precipitation of a less stable form, possibly R. Conclusions SMGs in B100 will raise the CP when present at concentrations above a critical (eutectic) value that is specific to the SMG/B100 pair: a pure (distilled) B100 with a higher CP can tolerate higher levels of SMG before CP is raised. Commercial biodiesel in the United States has SMG concentrations that are in the same range as the measured eutectic concentration. An ideal solution thermodynamic model can provide an approximate prediction of the eutectic point for the simplified systems examined here. For concentrations above this eutectic point, there is an increasing difference between the FMT and the CP. For concentrations in this range, light scattering shows that upon heating, crystals begin to dissolve but then recrystallize and ultimately melt at a much higher temperature. Observation in a microscope shows a change in crystal habit occurring during this recrystallization, and isolation of the high-melting
(23) Kodali, D. R.; Redgrave, T. G.; Small, D. M.; Atkinson, D. Biochemistry 1985, 24, 519–525. (24) Lutton, E. S.; Jackson, F. L. J. Am. Chem. Soc. 1948, 70, 2445– 2449.
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: DOI:10.1021/ef1013743
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crystals indicates that they are the β-phase of the SMGs. We propose that upon rapid cooling, the initial form of SMG that precipitates is a lower melting temperature phase such as R, which over time or upon slow heating can transform into the higher melting β polymorph. Consideration of the complex phase behavior of monoglycerides in biodiesel may explain some observations of precipitates above the CP. This work shows that monoglycerides in biodiesel can precipitate in different crystal forms, with different solubility. Transformation between these different crystal forms can occur in storage and upon slow warming, resulting in a difference between the crystallization temperature measured as the CP and the FMT. This could lead to fuel filter plugging at temperatures above the determined CP of the fuel. Monoglyceride polymorphism is just one consideration in assessing the effects of impurities on the cold weather behavior
of biodiesel. The presence of water, SG, and other impurities has also been associated with the formation of precipitates above the initial measured CP of the fuel and will be the subject of future work. Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Vehicle Technologies, Fuels and Lubricants Technologies Program under Contract No. DEAC36-99GO10337 with the National Renewable Energy Laboratory. The assistance of Philip Parilla in obtaining the X-ray diffraction data is gratefully acknowledged. Supporting Information Available: Cold soak filterability data for the soy biodiesel at various levels of monostearin, X-ray diffraction results for crystals recovered from a biodiesel solution, and details of the ideal solution model. This material is available free of charge via the Internet at http://pubs.acs.org.
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