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Ind. Eng. Chem. Res. 2007, 46, 8846-8851
Cold Flow Behavior of Biodiesels Derived from Biomass Sources C. R. Krishna,*,† Kaitlin Thomassen,† Christopher Brown,† Thomas A. Butcher,† Mouzhgun Anjom,‡ and Devinder Mahajan†,‡ Energy Sciences and Technology Department, BrookhaVen National Laboratory, Upton, New York 11973-5000, and AdVanced Energy Research and Technology Center (AERTC) and Materials Science and Engineering Department, Stony Brook UniVersity, Stony Brook, New York 11794-2275
Biodiesel is produced in the United States to D 6751, an ASTM standard. The source material in this fledgling industry in the U.S. is primarily soy oil, though other sources such as canola oil, waste oils, and greases from food and other sources are beginning to be exploited. At present, the referenced ASTM standard does not specify cloud and pour points values that are much higher for biodiesel than diesel derived from petroleum but allows them to be specified by the customer. There can be significant variation in these values, depending on the nature of the source material used to produce biodiesel, all of which meet the ASTM standards. This has the potential to create problems in applications as the quality of the biodiesel produced could vary widely. This study focused on quantitative measurements of cloud points of blends of biodiesel made from different sources. A correlation of these measurements with the saturated components was developed and was shown to correlate data reported in the literature as well. 1. Introduction Biomass derived biofuels are, by definition, CO2 net neutral. Recent national emphasis on developing efficient processes to covert biomass, a low-energy density source, to biofuels poses a challenge, but it is an attractive pathway to replace petroleumbased fuels.1,2 Two well-recognized and presently emphasized biofuels are bioethanol and biodiesel.3 Brazil, where bioethanol production is closely tied to bagasse, the waste product of sugarcane, is a world leader in ethanol use as a motor fuel. In the United States, bioethanol is derived mainly from corn through a well-established fermentation process4 (eq 1) yeast
C6H12O6(aq) 98 2C2H5OH(aq) + 2CO2(g) glucose
(1)
The share of bioethanol in the total fuel portfolio is increasing in the U.S. as the government is mandating its expansion through tax credits, though skeptics question the value of using corn, a raw material for bioethanol. In 2005, the production number was an impressive: ∼4 billion gallons from 1.4 billion bushels of corn. The second biofuel making impressive gains in the commercial sector is biodiesel. According to the National Biodiesel Board, 75 million gallons was produced in 2005, a 200% increase over 2004. Biodiesel uses virgin oils or waste materials such as used oils and greases form food and its production is small by comparison to bioethanol, Biodiesel production methods are still being developed and the overall industry is expanding. However, controlling and measurements of properties such as pour and cloud points are crucial to biodiesel entry on a larger scale in the fuel market. The focus of this paper was to measure and compare the cloud points of various biodiesel and these data are reported below. 2. Biodiesel Production and Challenges Biodiesel production is relatively simple: the primary reaction involves transesterification of oils or fats with an alcohol, * Corresponding author. E-mail:
[email protected]. Tel.: (631) 3444025. Fax: (631) 344-2359. † Brookhaven National Laboratory. ‡ Stony Brook University.
primarily methanol, though higher alcohols such as ethanol, propanol, or butanol are also used5 (eq 2)
Oils or fats can be from a variety of sources such as soybean, canola, palms, Jatropha, animal fats, etc., and the reaction is catalyzed by bases such as KOH or NaOH to yield mono alkyl ester (biodiesel). The reaction yields 100 Kg of biodiesel for 100 Kg of triglyceride though 10 Kg of glycerol is produced as a byproduct. The reaction is carried out at typically slightly elevated temperatures. Table 1 shows properties of various biodiesel formulations produced from soybean oil and various alcohols. For reference, standard properties of Number 2 diesel are also listed in Table 1. The cold flow properties vary also with the alkyl group of the ester as shown in the cloud points reported in Table 1. The cloud-point changes studied in the work reported here do not address this but the changes due to the source materials used in producing methyl esters (biodiesels) only. It is possible that similar effects of source materials might be seen with other alkyl groups. It should be noted that presently almost all of the biodiesel produced consists of methyl esters. It may be noted that an ASTM specification for neat biodiesel (B100) for blending has been developed (D6751) and changes are being considered to improve quality. As more formulations of biodiesels manufactured from various sources come on line, attempts are being made to address questions of stability and of compatibility with newer diesel technology. Additional research will be required due to lack of data relating to stability and deposit formation in engines. Some challenges to overcome are (1) expensive feedstock, (2) varying quality/composition of the feedstock, (3) limit on land use before competing with foodgrowing land, and (4) variable fuel quality that would compete without subsidies in the free market. The focus is to develop improved technologies for biodiesel production with respect to (1) reaction completion, (2) glycerin removal, (3) complete catalyst removal, and (4) free fatty acids removal.
10.1021/ie070110f CCC: $37.00 © 2007 American Chemical Society Published on Web 10/27/2007
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 8847 Table 1. Fuel Properties of Biodiesels Derived from Base-Catalyzed Transesterification Reaction of Soybean Oil with Various Alcohols ester methyl ethyl isopropyl N-butyl No.2 diesel
viscosity, mm2/s
cetane number
∆Hg,a kJ/kg
Tflash,b °C
CPc °C
PPd °C
4.08 4.41
46.2 48.2 52.6 51.7 45.8
39800 40000
191 174
40700 45200
185 78
2 1 -9 -3 -19
-1 -4 -12 -7 -3
5.24 2.39
a ∆Hg ) gross heat of combustion. b T flash ) flash point (Penske Martins closed cup). c CP ) cloud point. d PP ) pour point.
3. Cold Flow Properties of Biodiesel The behavior of liquid fuels at low temperature is very important in routine storage and use. The viscosity of liquid increases with reduction in temperature, and this affects flow pressure drops and hence atomization. Also, components within the fuel selectively freeze to form crystals at low enough temperatures, and the crystals tend to grow in size as the temperature is further reduced. This behavior is typical of petroleum distillate fuels and is even more pronounced in the sense that it occurs at higher temperatures for biodiesels.6 As the chemical composition varies with the feedstock oil or greases used, the properties of biodiesel produced will vary. Particularly, cetane number, pour point, and cloud point are some of the fuel characteristics that must be quantified to promote the use of biodiesel especially if it is to be used in the same equipment and under similar conditions as the petroleumbased diesel. Various experimental parameters have been defined and standardized for characterization of cold temperature behavior of petroleum fuels.6 One such measure is the cloud point defined in ASTM D 2500-02 as “the temperature of a liquid specimen when the smallest observable cluster of wax crystals first appears upon cooling under prescribed conditions”. The referenced standard [ASTM D2500-02] defines in detail the test method for determining the cloud points of fuels. There are other parameters such as pour point, cold filter plug point, etc. that have also been defined in an effort to correlate behavior in the “field”. Some of these properties can be modified by the use of additives though the cloud point itself changes marginally. As discussed earlier, neat biodiesel, termed B100, is now available in the United States to the ASTM standard D6751 for blending with middle distillates. Biodiesel is defined in the standard as follows: “biodiesel specified shall be mono-alkyl esters of long chain fatty acids derived from vegetable oils and animal fats”. The standard recognizes that the cloud point of biodiesel may be higher than that of the petrodiesel in which it may be blended. However, no value for it is specified, and more importantly, the standard does not imply that it may vary quite widely based on the source material from which it is made, while recognizing that it can be made from fatty acids derived from a variety of such sources. At present, the manufacturers produce a biodiesel of a set cloud point per customer specifications that can vary depending on the regional weather conditions, However, the increased production and use of biodiesel warrants a study of the cold flow behavior of different types of biodiesel that will create a database of properties for standardization. A detailed discussion of the cold flow properties of biodiesel is given in a review by Dunn.6 The review paper describes methods for evaluating these properties, gives measured values for a variety of biodiesels, and discusses methods to “improve” the cold weather performance. Some pertinent data from the
Figure 1. Cloud points of No. 2 oil-biodiesel blends with biodiesels from the present study.
Figure 2. Cloud Point temperatures of neat biodiesels from various sources. Table 2. Cloud-Point Data for Biodiesels from Various Oils (ref 7) oil or fat
alkyl group
CP (°C)
babassu canola canola coconut soybean soybean tallow tallow
methyl methyl ethyl ethyl methyl ethyl methyl ethyl
4 1 -1 5 0 1 17 15
review are given in Table 2. The table shows cloud-point values, along with other data of biodiesel made from a variety of starting oils and fats. Dunn infers from other published data that the higher the concentration of saturated fatty acids in the starting material, the higher the cloud point in the produced biodiesel. The measurements made in this study show how blending biodiesel (soy methyl ester) with a petroleum-derived fuel affects the much lower cloud point of the latter negatively (Figure 1). One could consider such blending as a way to improve the cloud point and other cold flow properties of biodiesel. The subject review also discusses several other ways to improve the cold flow properties. Lang et al.7 have measured the cloud and pour points of biodiesels made by reaction of variety of vegetable oils with different alcohols (methyl, ethyl, etc.). The group also measured fatty acid composition of the biodiesel esters and gave a distribution of total saturated and unsaturated components. Goodrum et al.,8 in making rheological characterization of animal fats and No. 2 fuel oil mixtures, also reported the fatty acid composition of yellow grease. We have used published data6-8 to infer the values of saturates and unsaturates in samples of methyl esters (biodiesels) that were tested in the course of the work reported here.
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Figure 3. Actual prepared biodiesel samples at cloud points. Canola-based: (a) above the cloud-point temperature and (b) at the cloud-point temperature; tallow-based: (c) above the cloud-point temperature and (d) at the cloud-point temperature; soy-based: (e) above the cloud point and (f) at the cloud point.
Figure 4. Cloud points for blends of tallow and soy derived biodiesels. Figure 6. Percent saturation versus cloud-point correlation for biodiesel blends derived from soy, tallow, and canola sources.
Figure 5. Cloud points for blends of tallow and canola derived biodiesels.
4. Experimental Section
Figure 7. Variation in cloud point value for blends formed with varying yellow grease biodiesel concentration in soy derived biodiesel.
A number of biodiesels made from different feedstocks, which included vegetable oils, yellow grease, and tallow were available for testing. The biodiesels were all certified to meet the ASTM D6751 standard. The cloud points of a number of samples were measured to ASTM D2500-02 using an apparatus similar to the one used in the standard. The procedure consisted essentially of cooling a fixed volume of the sample in a clear, cylindrical, glass test jar. The jar was cooled by surrounding, but not in physical contact, a suitable freezing mixture. In the present set up, the freezing mixture was made of crushed
ice and calcium chloride crystals. The limitation of using this procedure was the inability to achieve cold enough temperatures to determine pour points of the samples tested. The determination of the cloud points is through visual observations and the ASTM procedure was followed. The ASTM procedure is supposed to cover petroleum products and biodiesel fuels and is presumably good for the blends of those two. It has been used to measure the cloud point of blends by Coutinho et al.9 and is used here for blends of biodiesels from different source
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 8849
Figure 8. Cloud-points correlation with saturated fraction in blends of yellow grease biodiesel in soy derived biodiesel.
Figure 11. Comparison of cloud points of data of ref 12 and the present study.
Figure 9. Cloud-points correlation with saturated fraction in blends of yellow grease in tallow derived biodiesels.
Figure 10. Cloud-points correlation with saturated fraction in different biodiesels from data in ref 12.
materials. There did not seem to be any visual ambiguity in our measurements on the blends. The cloud points (CP) of the neat or pure samples were first measured. The biodiesel samples were made from soy oil, canola oil, yellow grease, and tallow. Then blends of the two former in the latter two were made at similar ratios, and their cloud points were measured. The measurements were repeated to ensure that the values were repeatable within the ASTM D 2500-02 limits (2 °C). Several ways of correlating the cloud points were attempted, and what was considered a reasonably successful correlation is presented below. 5. Results and Discussion Figure 2 gives cloud-point temperatures for four prepared samples of biodiesel. It can be seen that the canola-oil-based biodiesel had the lowest CP and the tallow-based the highest. As will be seen later in the discussion, this large difference corresponds to the large difference in the amount of saturated compounds between these two samples, and blending vegetable-
oil- and fat-based biodiesels at different values give a variation in associated CP values and an experimental method to correlate these values. The results of following this procedure are given later in this paper. Figure 3, panels a and b, compares actual samples before and at the cloud point for canola-oil-based biodiesel. Figure 3, panels c and d, compares similar samples of tallow-based biodiesel. In the first case, the change at cloud point is not very distinct, though identifiable, whereas in the latter case, the change is more pronounced. The soy and yellow grease biodiesels seem to fall in between these two extremes (panels e and f). Whether this change is purely a function of the ratio of saturates/unsaturates remains to be proven though this correlation appears reasonable. Figure 4 is a plot of the CP temperatures measured for blends of soy and tallow biodiesels. The CP temperature increases linearly with the tallow biodiesel content. Figure 5 shows a similar trend from the measurements on blends of tallow and canola biodiesels. These linear dependencies suggest a more general correlation common to all biodiesel blends tested here. This is evidenced in Figure 6, where the CP temperatures for tallow biodiesel blended with canola biodiesel and also with soy biodiesel are plotted against the fraction of the components in the biodiesels (or what is the same, in the source material) that can be considered as saturated compounds. The values for the saturated fractions for the source material (soy oil, canola oil, and tallow) were calculated from the available literature values. The corresponding values for the blends were calculated for the blends by simple addition of values for the components in the ratios they constituted the blend. Clearly, there seems to be a reasonably good linear fit for the cloud-point temperature as a function of the fraction of saturated components. Similar measurements were carried out with blends of yellow grease biodiesel and soy biodiesel. The results are plotted in Figures 7 and 8. The saturated fraction for the yellow grease was derived from ref 10. It is seen that a correlation similar to the one for tallow blends holds here as well. When the two are compared in Figure 9, it seems that the “linear fits” do show a small difference, though within the accuracy of the ASTM measurements. The difference is systematic though, and we believe it is possibly due to variations in the yellow grease saturated fractions arising from the vagaries of yellow grease production. Imahara et al.11 recently published a thermodynamic study of the cloud points of biodiesel as affected by its fatty acid composition. The authors calculated the CP for a variety of biodiesels made from different vegetable oils and also from beef
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Table 3. Fatty Acid Composition of Various Biodiesel Esters (from ref 8)a fatty acid
CME
CEE
CPE
CBE
LME
LEE
LPE
LBE
RME
REE
SME
SEE
palmitic (16:0) stearic (18:00) oleic (18:1) linoleic (18:2) linolenic (18:3) eicosenoic (20:1) erucic (22:1) total saturated total unsaturated
4.2 2.2 67.2 18.9 7.4 0.0 0.0 6.4 93.6
4.4 2.3 69.4 18.0 5.9 0.0 0.0 6.7 93.3
4.5 2.3 67.3 19.4 6.5 0.0 0.0 6.7 93.2
4.4 2.3 67.8 18.9 6.7 0.0 0.0 6.7 93.3
5.2 3.2 14.5 15.3 61.9 0.0 0.0 8.4 91.6
5.1 3.1 13.7 15.2 62.9 0.0 0.0 8.2 91.8
5.1 3.1 14.2 15.1 62.5 0.0 0.0 8.5 91.5
5.3 3.2 13.8 15.2 62.6 0.0 0.0 8.5 91.5
3.1 0.0 13.9 13.3 9.4 6.8 53.5 3.1 96.9
3.1 0.0 13.2 13.3 9.2 6.9 54.4 3.1 96.9
6.5 4.9 20.5 68.0 0.0 0.0 0.0 11.4 88.6
6.3 4.7 21.0 68.0 0.0 0.0 0.0 11.0 89.0
a Iodine values are removed. Abbreviations: CME, canola methyl ester; CEE, canola ethyl ester; CPE, canola 2-propyl ester; CBE, canola butyl ester; LME, linseed methyl ester; LEE, linseed ethyl ester; LPE, linseed 2-propyl ester; LBE, linseed butyl ester; RME, rapeseed methyl ester; REE, rapeseed ethyl ester; SME, sunflower methyl ester; SEE, sunflower ethyl ester.
Table 4. Comparison of Measured and Calculated Cloud-Point Data and Fatty Acid Composition of Biodiesel Fuels from Various Oils/Fats Feedstocks (from ref 12) fatty acid methyl esters
CP(K)
oils/fats
C160
C180
C181
C182
C183
others
measured
calculated
linseed safflower sunflower rapeseed soybean olive palm beef tallow
6.7 6.4 6.1 4.3 10.7 10.7 39.5 23.9
3.7 2.2 4.2 1.9 3.2 2.6 4.1 17.5
21.7 13.9 24.0 61.5 25.00 78.7 43.2 43.9
15.8 76.0 63.5 20.6 53.3 5.8 10.6 2.3
52.1 0.2 0.4 8.3 5.4 0.7 0.2 0.1
1.3 1.8 3.1 2.5 1.5 2.4 12.3
268 267 274 267 272 268 283 286
273 269 275 267 273 273 288 289
tallow. Table 4, in which the data from ref 11 are given, supports the authors’ statement that the method predicts well the CP values for biodiesels from various sources. The same authors also conclude that the CP is “determined mainly by the amount of saturated esters and does not depend on the composition of unsaturated ones”. The data obtained by our group and presented in this paper are consistent with the hypothesis, not only for pure biodiesels but also for blends within the limits of the ASTM measurements. We have taken the cloud point data in Table 4 and plotted them in Figures 10 and 11 to compare with the correlation of the data for blends measured in this study and presented earlier. Figure 10 shows the variation of the measured CP with the fraction saturated, which was calculated as the sum of C16.0 and C18.0 in the above table. It can be seen that the data correlation is similar to that of the present study with this definition of the saturated fraction. Figure 11 compares this correlation with that of the present study, and it is quite clear that they are almost identical. It is significant to note that the data of ref 11 are for biodiesels made from different source materials, whereas the present data also includes data from blends. This would indicate that the correlation is quite general and the CP is almost linearly dependent on the saturated fraction irrespective of its source. A more detailed consideration of multicomponent crystallization is needed for a better explanation of the experimental data. The experiments have to be extended to other sources of biodiesel such as fish oil, Jatropha, etc. We hope to extend this study with more experiments and development of theoretical calculations. 6. Summary Biodiesel production in the U.S. is increasing, and this will require a standard for uniform distribution in the marketplace. The present D6751 standard set by ASTM for biodiesel covers basic properties but is not yet specific enough because a range of properties such as CP and PP need to be defined. In this study, cloud-point property of samples of biodiesel derived from canola, soy, and tallow oils as well as yellow grease, all pure
as well as blends, were measured. The data show that for pure oils the CP values followed the order canola < soy < yellow grease < tallow. The data also show a reasonably good linear fit for the CP temperature as a function of the fraction of saturated components in pure oils. This correlation also seems to hold for all blends tested within the limits of the ASTM measurements. Iamahara et al.11 measured CP for neat biodiesels made from a larger variety of source materials than used in the present study and attempted a thermodynamic correlation. We have shown that their data also fits the type of correlation presented in this paper for both neat biodiesels and blends very well. These observations lead us to state that the biodiesel quality is very much feed source specific and the CP is almost linearly dependent on the saturated fraction irrespective of its source. The reported results are in agreement with the literature observation that the CP is determined mainly by the amount of saturated esters and does not depend on the composition of unsaturated ones. Work is ongoing to develop more quantitative correlations that can make biodiesel properties more predictable for commercial use. The property measurements should be expanded, and the manufacture of biodiesel in a continuous trouble-free plant should be developed. Combustion studies, yet to be published, in our laboratories also suggest that some combustion properties might depend on the source of the biodiesel. Literature Cited (1) McKendry, P. Energy Production from Biomass (Part 1): Overview of Biomass. Bioresource Tech. 2002, 83, 37-46. (2) Holm-Nielsen, J. B.; Madsen, M.; Popiel, P. O. Predicted Energy Crop Potential for Bioenergy, Worldwide and for EU-25. World Bioenergy - Conference on Biomass for Bioenergy, Jo¨nko¨ping, Sweden, May 30 June 1, 2006. (3) Hill, J.; Nelson, E.; Tilman, D.; Polasky, S.; Tiffany, D. Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels. Proc. Natl. Acad. Sci. 2006, 103, 11206-11210. (4) Bothast, R. J.; Saha, B. C. Ethanol production from agricultural biomass substrates. AdV. Appl. Microbiol. 1997, 44, 261-86.
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 8851 (5) Van Gerpen, J.; Knothe, G. Biodiesel Production. In The Biodiesel Handbook; Knothe, G., Krahl, J., Van Gerpen, J., Eds.; AOCS Press: Champaign, IL, 2005; Chapter 6. (6) Dunn, R. O. Cold Weather Properties and Performance of Biodiesel. In The Biodiesel Handbook; Knothe, G., Krahl, J., Van Gerpen, J., Eds.; AOCS Press: Champaign, IL, 2005; Chapter 6.3. (7) Lang, X.; Dalai, A. K.; Bakshi, N. N.; Reaney, M. J.; Hertz, P. B. Preparation and characterization of bio-diesels from various bio-oils. Bioresource Technol. 2001, 80, 53-62. (8) Goodrum, J. W.; Geller, D. P.; Adams, T. T. Rheological characterization of animal fats and their mixtures with #2 fuel oil. Biomass Bioenergy 2003, 24, 249-256.
(9) Coutinho, J. A. P.; Mirante, F.; Ribeiro, J. C.; Sansot, J. M.; Daridon, J. L. Cloud and pour points in fuel blends. Fuel 2002, 81, 963-967. (10) http://www.rothsay.ca/specs/yg_spec.html (11) Imahara, H.; Minami, E.; Saka, S. Thermodynamic study on cloud point of biodiesel with its fatty acid composition. Fuel 2006, 85, 16661670.
ReceiVed for reView January 17, 2007 ReVised manuscript receiVed August 21, 2007 Accepted August 29, 2007 IE070110F
