Energy Fuels 2010, 24, 253–260 Published on Web 10/26/2009
: DOI:10.1021/ef900782v
Biodiesel Fuels through a Continuous Flow Process of Chicken Fat Supercritical Transesterification Victor F. Marulanda,†,‡ George Anitescu,† and Lawrence L. Tavlarides*,† †
Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, New York 13244, and ‡ Chemical Engineering School, Universidad del Valle, Cali 25360, Colombia Received July 23, 2009. Revised Manuscript Received October 2, 2009
Supercritical transesterification of chicken fat with methanol was investigated at various temperatures (350, 375, and 400 °C), pressures (100, 200, and 300 bar), methanol-to-chicken-fat molar ratios (from stoichiometric 3:1 to 12:1), and residence times (3-10 min). The best experimental results for the conversion of triglycerides and the decomposition of glycerol to fuel components were obtained under the following conditions: 400 °C, 300 bar, methanol-to-triglycerides molar ratio = 9:1, and residence time = 6 min. The thermal decomposition products of long-chain methyl esters and glycerol were identified in the biodiesel samples and their potential influence on fuel characteristics such as viscosity, cold flow, cetane number, and flash point is discussed. Because of the low excess of methanol used in this study in comparison with similar supercritical transesterification processes (methanol-to-feedstock molar ratios of 3-12 vs 40þ), costs associated with the pumping, preheating, and recovery of the excess methanol will be greatly reduced in commercial applications. Use of low-cost feedstocks and a moderate excess of methanol, coupled with glycerol decomposition to valuable fuel components, certainly will increase the profitability of a much simplified method, in comparison with competitive technologies. However, a great disadvantage for supercritical biodiesel production is this extremely high methanol-to-oil molar ratio, which translates to additional pumping, preheating, and separation costs. The large excess amount of alcohol was, in fact, used to decrease the critical temperatures of the reactant mixtures, which allowed homogeneous reaction conditions under milder conditions for supercritical processing. Temperatures limited to 350 °C were preferred due to an assumed decomposition risk of the fatty acid methyl esters (FAMEs) at higher temperatures.5,7 Although there is little information on the nature of the FAME decomposition products,9 here, we are challenging the perception that the supercritical transesterification process leads to a degradation of the fuel quality at temperatures between 350 °C and ∼400 °C.2,5,10 Despite the emphasized favorable properties of the supercritical process with low-quality feedstocks,6 most of the research studies were conducted with refined vegetable oils. Table 1 summarizes some of the most-recent studies on the supercritical transesterification process to produce biodiesel.1,4,7-9,11-25 Although scarce, some economic analyses
Introduction Known advantages of the renewable and environmentally friendly biofuels, associated with the potential to decrease the dependence on petrofuels, have led to intense research on different process alternatives that allow biofuel production in an economically sustainable manner.1-8 Despite these advantages, biodiesel production costs from refined vegetable oils are prohibitively high without government subsidies. To decrease these costs and make biodiesel profitable in comparison with petrofuels, production processes must use inexpensive triglyceride sources, such as unrefined vegetable oils, waste cooking oils, animal fats, or algal oils.2,3 However, conventional catalytic processes cannot make use of such feedstocks in an efficient manner without additional pretreatment steps, because of the presence of high amounts of free fatty acids (FFAs) and water that react with the catalyst.4 Studies on biodiesel production by transesterification with supercritical methanol successfully address these issues.2 The process is conducted at temperatures higher than the alcohol critical temperature (240 °C), pressures higher than ∼100 bar, and very high methanol-to-oil molar ratios (usually ∼42:1).
(9) Iijima, W.; Kobayashi, Y.; Takekura, K.; Taniwaki, K. Presented at the 2004 ASAE/CSAE Annual International Meeting, Ottawa, ON, Canada, 2004, Paper No. 046073. (10) Vieitez, I.; da Silva, C.; Alckmin, I.; Borges, G. R.; Corazza, F. C.; Oliveira, J. V.; Grompone, M. A.; Jachmanian, I. Energy Fuels 2009, 23, 558–563. (11) Aimaretti, N.; Manuale, D. L.; Mazzieri, V. M.; Vera, C. R.; Yori, J. C. Energy Fuels 2009, 23, 1076–1080. (12) Schulte, W. B. Biodiesel Production from Tall Oil and Chicken Fat via Supercritical Methanol Treatment, Master’s Thesis, University of Arkansas, Fayetteville, AR, 2007. (13) He, H.; Wang, T.; Zhu, S. Fuel 2007, 86, 442–447. (14) Varma, M. N.; Madras, G. Ind. Eng. Chem. Res. 2007, 46, 1–6. (15) Vieitez, I.; da Silva, C.; Borges, G. R.; Corazza, F. C.; Oliveira, J. V.; Grompone, M. A.; Jachmanian, I. Energy Fuels 2008, 22, 2805–2809. (16) Han, H.; Cao, W.; Zhang, J. Process Biochem. 2005, 40, 3148– 3151.
*Author to whom correspondence should be addressed. Tel.: 315443-1883. Fax: 315-443-9175. E-mail:
[email protected]. (1) Anitescu, G.; Deshpande, A.; Tavlarides, L. L. Energy Fuels 2008, 22, 1391–1399. (2) Pinnarat, T.; Savage, P. E. Ind. Eng. Chem. Res. 2008, 47, 6801– 6808. (3) Canoira, L.; Rodrı´ guez-Gamero, M.; Querol, E.; Alcantara, R.; Lapuerta, M.; Oliva, F. Ind. Eng. Chem. Res. 2008, 47, 7997–8004. (4) D’Ippolito, S. A.; Yori, J. C.; Iturria, M. E.; Pieck, C. L.; Vera, C. R. Energy Fuels 2007, 21, 339–346. (5) Saka, S.; Kusdiana, D. Fuel 2001, 80, 225–231. (6) Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91, 289–295. (7) Bunyakiat, K.; Makmee, S.; Sawangkeaw, R.; Ngamprasertsith, S. Energy Fuels 2006, 20, 812–817. (8) Kasteren, J.; Nisworo, A. Resour. Conserv. Recycl. 2007, 50, 442– 458. r 2009 American Chemical Society
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: DOI:10.1021/ef900782v
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Table 1. Summary of Recent Studies on Supercritical Transesterification oil/cosolvent
temperature, T (°C)
pressure, P (bar)
MeOH:TG (molar)
residence time (min)
process type
yield (%)
ref
soybean soybean coconut and palm waste oil/C3H8 canola soybean chicken fat, tall oil soybean castor and linseed soybean soybean/CO2 soybean/C3H8 sunflower soybean/C3H8 cottonseed soybean soybean/acid palm, grandnut, pongamia, jatropha palm kernel/C6H14 palm
350-425 285-290 350 280 420-450 280 275-325 310 350 350 280 280 350 288 230-250 350 350 200-400 290-350 350
100-250 100-110 190 128 400 68-74 165 350 200 200 143 128 200 96 N/A N/A 150 200 150-220 40
3-6 10-12 42 24 11-45 15-20 10-40 40 40 40 24 24 40 64 41 42 40 50 12-42 45
2-3 N/A 7 17 4 120 20 25 40 N/A 10 10 40 10 8 10 >11 40 10 5
continuous batch continuous continuous continuous batch batch continuous batch continuous batch batch batch batch batch batch continuous batch batch batch
∼100 ∼100 95-96 95 ∼100 94-96 91 77-96 98 77,5 98 98 98 99 60-98 98 ∼90 ∼100 60-87 ∼95
1 4 7 8 9 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
performed on the supercritical transesterification process are very encouraging, with regard to the possibility of scaling up the process to industrial levels.1,8 A supercritical transesterification process that had been performed at moderate methanol excess and with glycerol decomposition was recently proposed, and it was suggested that most of the decomposition products of glycerol could be used as part of the fuel.1,9,11 Because of the increasing biodiesel production worldwide, the glycerol byproduct has become a waste problem and its simultaneous decomposition with FAME generation seems to be very attractive, from a process engineering standpoint. However, little information is available on its decomposition products and the suitability of these products, as part of the fuel, still must be addressed. To decrease the production cost of biodiesel with a supercritical method, in comparison with petrofuels and biodiesel obtained by conventional catalytic methods, low-cost feedstocks and lower alcohol-to-oil molar ratios must be considered, along with more simple product separation and purification procedures. Accordingly, the main objective of this research was to conduct a supercritical transesterification study on chicken fat with methanol-to-triglyceride molar ratios ranging from stoichiometric (3:1) to a moderate excess of 12:1 and at temperatures where glycerol is reacted to produce organics that can be included in the fuel. The effects of process variables such as temperature, pressure, molar ratio of the reactants, and residence time on conversion and product quality was assessed, and the feasibility to conduct the process at higher temperatures and lower molar ratios, in comparison with other reported supercritical process conditions, is suggested. Product analyses provided some insights on how the reaction products from
glycerol and FAME secondary reactions affected the biodiesel quality. Experimental Section Materials and Methods. Rendered chicken fat, which is obtained industrially by melting and is destined to further processing, was provided by Tyson Foods, Inc. Yellowish and very viscous in appearance, chicken fat exists as a mixture of solid and liquid phases at room temperature. The physical properties, appearance, and triglycerides composition of chicken fat are strongly dependent on the processing methods. Lower-quality chicken fat can be brown or even black and contain FFAs in a proportion of >12% (by weight).12,26 Gas chromatography (GC) standards for the main FAMEs with a purity of >99 wt %, a kit with standard solutions and internal standards for analysis of total and free glycerin (according to the ASTM method D6584), and the silylation reagent N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) were purchased from Sigma Aldrich. HPLC-grade methanol and n-heptane were purchased from Thermo Fisher Scientific and used without further purification. Laboratory-Scale Supercritical Transesterification Unit. The continuous flow experimental setup is shown in Figure 1 and is similar to the laboratory apparatus that has been described elsewhere.1 It consists of two high-pressure syringe pumps (Teledyne ISCO) for chicken fat and methanol feeding, two preheaters, a tubular reactor, and a separator. The chicken fat in the feeding pump is heated to 70 °C to maintain a homogeneous liquid phase for easy pumping. Methanol and chicken fat are gradually preheated to 350 °C from the outlet of the pumps to the reactor entrance by electrical heating tapes. The 8-m-long tubular reactor is made of 316 SS high-pressure Swagelok tubing (3.175 mm outer diameter (O.D.) 0.711 mm wall thickness). The system pressure is controlled by a micrometer valve (Autoclave Engineers). The reactor is heated in an electrical furnace that permits temperatures of 350-400 ( 1 °C to be attained in a relatively short time. Pressure and temperatures at the inlet, middle, and outlet of the reactor (i.e., T4-T6 in Figure 1) are recorded by a National Instruments data acquisition system (model TBX-68T). The reaction products are collected in a flask submerged in an ice bath to avoid the loss of eventual volatile compounds. Experimental Conditions. Supercritical transesterification experiments of chicken fat were conducted in the temperature
(17) Cao, W.; Hang, H.; Zhang, J. Fuel 2005, 84, 347–351. (18) Madras, G.; Kolluru, C.; Kumar, R. Fuel 2004, 83, 2029–2033. (19) Hegel, P.; Mabe, G.; Pereda, S.; Brignole, E. Ind. Eng. Chem. Res. 2007, 46, 6360–6365. (20) Demirbas, A. Bioresour. Technol. 2008, 99, 1125–1130. (21) Yin, J. Z.; Xiao, M.; Song, J. B. Energy Convers. Manage. 2008, 49, 908–912. (22) Wang, C. W.; Zhou, J. F.; Chen, W.; Wang, W. G.; Wu, Y. X.; Zhang, J. F.; Chi, R. A.; Ying, W. Y. Energy Fuels 2008, 22, 3479–3483. (23) Rathore, V.; Madras, G. Fuel 2007, 86, 2650–2659. (24) Sawangkeaw, R.; Bunyakiat, K.; Ngamprasertsith, S. Green Chem. 2007, 9, 679–685. (25) Song, E.; Lim, J.; Lee, H.; Lee, Y. J. Supercrit. Fluids 2008, 44, 356–363.
(26) Satyarthi, J. K.; Srinivas, D.; Ratnasamy, P. Energy Fuels 2009, 23, 2273–2277.
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: DOI:10.1021/ef900782v
Marulanda et al. Table 2. Fatty Acid Profile for Chicken Fat and Soybean Oil Fatty Acid Content (wt %) Chicken Fat
Soybean Oil
fatty acid
data from this work
data from ref 27
data from ref 28
palmitoleic acid palmitic acid stearic acid oleic acid linoleic acid linolenic acid unsaturated fatty acids saturated fatty acids
7.7 21.0 5.5 48.5 17.3 traces 73.5 26.5
5.8 24.0 5.8 38.2 23.8 1.9 63.9 30.3
0.7 14.1 5.2 25.3 48.7 6.1 80.8 19.3
65 °C, with a residence time of 24 h and a methanol-tochicken-fat molar ratio of 30:1. The experimental fatty acid profiles for chicken fat used in our experiments and from the literature, as well as for a commonly transesterified vegetable oil, are shown in Table 2. Although slower, the acid catalytic method is preferred over the base catalytic method for feedstocks with high FFA content such as animal fats, which form soaps with the base catalyst, rendering the separation difficult and the process inefficient. Chicken fat, as a source of triglycerides for biodiesel production, displays unusually high oleic acid content, in comparison to soybean oil. Methyl oleate has been proposed as a suitable major compound of a “designer” biodiesel,29 with improved properties, because of its oxidation stability, in comparison with fatty acids with two or three double bonds, such as linoleic and linolenic acids. Methyl palmitoleate provides similar advantages for biodiesel from chicken fat.29 The molecular weight of the triglycerides in chicken fat was estimated from the analysis of the acid-catalyzed transesterified product as the sum of the individual FAME molecular weight multiplied by the mass fraction in the product and then multiplied by a factor of 3, to approximate an average triglyceride molecule. Accordingly, the calculated molecular weight for the triglycerides in chicken fat used in the experiments was 867 g/mol. Chicken fat density variation with temperature was experimentally found as a linear function over the temperature range of 25-100 °C and was extrapolated to account for densities at reaction temperatures. Experimental tests on thermal stability of the chicken fat were performed in the continuous setup shown in Figure 1 and consisted of the progressive heating of the chicken fat pumped at pressures and flow rates similar to those used in the supercritical transesterification experiments. Heated chicken fat was cooled in the reactor and then transesterified by the conventional acid catalyzed method, as shown previously. FAME profile for biodiesel fraction obtained from chicken fat heated at 350 °C was similar to that for fuel samples obtained from chicken fat without any thermal treatment beyond the process temperature of 65 °C. However, FAME profiles for biodiesel obtained from chicken fat heated at 375 and 400 °C showed a lower content of the unsaturated fraction, which indicated that thermal decomposition occurred at these temperatures. Accordingly, the transesterification reactants were preheated only to 350 °C. Lipid Conversion and Yields of the Reaction Products. Table 3 summarizes the experimental reaction conditions and the main FAMEs yields obtained in screening
Figure 1. Continuous flow experimental setup.
range of 350-400 °C, a pressure range of 100-300 bar, methanol-to-chicken-fat molar ratios from stoichiometric to 12:1, and residence times of 3-10 min. The selection of these experimental conditions was based on previous experimental results by Anitescu et al.,1 which showed that, under selected parameters, it was possible to achieve almost-complete conversion of vegetable oils to biodiesel with the glycerol decomposition products included in the fuel. In a typical experiment, the reactor was preheated to the desired reaction temperature and methanol was pumped to purge and pressurize the system. The flow rates of chicken fat and methanol were set according to the residence time for the specified reaction conditions. After steady-state conditions were achieved, effluent stream samples were collected. A steady state was considered to be attained after a period of time as long as five times greater than the residence times in the reactor, under the specified experimental conditions. Most of the samples were directly analyzed without any pretreatment. Analytical Methods. Quantitation of the methyl esters in biodiesel samples was performed by a Hewlett-Packard Model HP 5890 Series II gas chromatograph and a Hewlett-Packard Model HP 5971 mass-selective detector (MSD) in a HewlettPackard Model HP-1MS cross-linked methyl siloxane column with dimensions of 30 m 0.25 mm I.D. 0.25 μm film thickness, with helium as the carrier gas. The temperature program started at 60 °C, was held at that temperature for 2 min, and continued with a subsequent ramp of 4 °C/min to 270 °C. The temperatures of the injector and GC-MSD interface were 250 and 270 °C, respectively. Free and total glycerin quantitation was performed in a Hewlett-Packard Model HP 5890 Series II gas chromatograph that was equipped for flame ionization detection (FID) and a Restek Rtx-Biodiesel TG column with dimensions of 10 m 0.32 mm I.D. 0.1 μm film thickness with helium as the carrier gas. The temperature program was similar to that used in ASTM standard method D 6584.
Results and Discussion Chicken Fat Characterization. To determine the fatty acid distribution of the raw material, the acid-catalyzed conventional transesterification of chicken fat was conducted at 255
Energy Fuels 2010, 24, 253–260
: DOI:10.1021/ef900782v
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Table 3. Screening Experiments on Transesterification Reactions of Chicken Fat (CF) run
temperature, T (°C)
pressure, P (bar)
MeOH:CF (molar)
residence time, τ (min)
FAME yield
1 2 3 4 5 6 7 8 9 10 11 12 13 14
350 350 350 350 375 375 375 375 375 400 400 400 400 400
100 100 100 200 100 100 300 200 200 100 100 100 100 100
3 6 9 9 3 9 3 6 9 6 9 9 9 9
8 8 8 10 6 8 5 8 10 6 4 5 6 7
30 74 67 80 30 75 44 82 84 59 65 77 80 61
15
400
100
9
8
58
16
400
100
12
6
62
observation of: SþL phases, FAME decomposition, no glycerol present SþL phases, FAME decomposition, glycerol present SþL phases, clear yellow, glycerol present SþL phases, FAME decomposition, glycerol present SþL phases, FAME decomposition, glycerol present SþL phases, FAME decomposition, glycerol present SþL phases, FAME decomposition, glycerol present SþL phases, FAME decomposition, glycerol present SþL phases, FAME decomposition, glycerol present 2 liquid phases, glycerol in the lower liquid phase 2 liquid phases, clear yellow, glycerol in the upper phase 2 liquid phases, clear yellow, glycerol in the upper phase 2 liquid phases, clear yellow, glycerol in the upper phase 2 liquid phases, FAME decomposition, glycerol in the upper phase 2 liquid phases, FAME decomposition, glycerol in the upper phase 2 liquid phases, clear yellow, glycerol in the upper phase
state conditions for a mixture of soybean oil with ethanol in an alcohol-to-oil molar ratio of 3.4 in a view cell at 400 °C and 200 bar. Based on this analysis and the results of the screening experiments, an experimental plan was developed for the selected temperature of 400 °C to study the effect of the transesterification parameters (pressure, reactant molar ratio and residence time) on the biodiesel composition and, hence, quality. These conditions are shown in the columns 2-4 of Table 4. The selected pressures were 100, 200, and 300 bar, because, at smaller increments (such as 50 bar), no significant effects on the reaction product composition were observed. For the case of the reactant molar ratio, the selected values of 6-12 (methanol-to-chicken-fat molar ratio) were based on the chicken fat composition (shown in Table 2). However, one must consider that methanol can also be consumed by other reactions, such as glycerol etherification. Accordingly, the higher than 3:1 reactant molar ratios proposed are more reasonable, to ensure complete conversion of the triglycerides and FFAs while providing methanol for the competing side reactions. Because the residence time plays the most significant role in the transesterification reactions, along with temperature, a wide range of 3-10 min was considered. The results of these selected experiments are shown in columns 5-10 of Table 4 as triglyceride conversion and yield of monoglycerides, diglycerides, and triglycerides, as well as free and total glycerol content of the biodiesel phase. The second glycerol phase is discussed later. The triglyceride conversion (XTG) was calculated based on GC-FID comparison of a sample of derivatized chicken fat with those of the reaction products: ! P Ai , τ Cτ ¼ 100 1 - P ð1Þ XTG ð%Þ ¼ 100 1 C0 Ai , 0
experiments. Qualitative assessment of the reaction completeness was based on (i) the observation of a solid phase at the bottom of the sampling flask, which is assumed to be unreacted triglycerides that are composed of saturated palmitic and stearic acids; (ii) a brownish color in the primary liquid phase, which was indicative of thermal decomposition; or (iii) the formation of a second liquid phase. The residence time was varied over a range of 3-10 min. For the experiments that were conducted at 350 and 375 °C (runs 1-9 in Table 3), a solid and a liquid layer, immiscible with biodiesel phase, were observed in the effluent stream samples. For the experiments conducted at 400 °C, no solid phase was observed and the amount of this phase decreased while yield of the main FAMEs (C17 and C19) increased with residence time. This indicates that more chicken fat has reacted to that point. Supercritical transesterification runs performed at a pressure of 100 bar and a methanol-to-chicken fat molar ratio of 6:1 produced a lower second phase, which was thought to be water, glycerol, and some unreacted methanol. However, at higher ratios, the second phase was present as an upper layer, indicating a density lower than that of the biodiesel. This suggests that this upper second phase consisted of aqueous solutions of some of the glycerol products and bulk unreacted methanol, with the latter lifting the phase, because of its lower density than that of the fuel. The FAME yield was defined as the ratio of mass fractions of the main methyl esters (i.e., palmitoleic, palmitic, oleic, linoleic, and stearic) in a known mass of sample to the mass fractions of methyl esters in the same mass of a biodiesel obtained by the acid-catalyzed transesterification method. The formation of smaller-molecular-weight methyl esters (e.g., C8H14O2 to C15H30O2) that are also part of the fuel but were omitted during the quantification of the main methyl esters can explain the decreased yield with increased residence time τ. The critical temperature of the lipid feedstock-methanol mixtures, which guarantees the existence of a homogeneous phase, decreases as the methanol content increases (e.g., ∼300 °C at a methanol-to-oil molar ratio of 42). This value is the molar ratio most typically used in the supercritical transesterification studies and is obviously connected to the critical temperature of the reactants. Supercritical state conditions will be achieved at higher temperatures for ratios of 200 °C (see Table 5). Accordingly, no negative impact should be expected on the flash point (imposed at minimum 130 °C); however, a positive effect on the cold flow properties could be expected. Water presence in the reaction products can be related to the disappearance of glycerol (e.g., etherification, condensation, and dehydration).11 Glycerol methanolysis reactions, which can produce up to trimethyl glycerol ethers, could explain the formation of water, according to the following type of reaction:
(27) Arnaud, E.; Relkin, P.; Pina, M.; Collignan, A. Eur. J. Lipid Sci. Technol. 2004, 106, 591–598. (28) Tang, H.; Salley, S. O.; Simon, Ng, K. Y. Fuel 2008, 87, 3006– 3017. (29) Knothe, G. Energy Fuels 2008, 22, 1358–1364. (30) Xin, J.; Imahara, H.; Saka, S. Fuel 2008, 87, 1807–1813. (31) Warabi, Y.; Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91, 283–287.
Several organic reaction products from glycerol were positively identified by GC-MS analysis in the second phase of some of the effluent samples with residence times of >7 min (see Table 6). Glycerol reaction products included alcohols, ethers, and diglycerol isomers. Mass balance closure in our supercritical transesterification experiments was always ∼99.5%. Accordingly, there
Table 6. Identified Glycerol Reaction Products by GC-MS
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was no mass loss via gaseous products. Also, no polyglycerols were detected by the GC analysis. Some of the decomposition products;specifically, the most dehydrated species such as propanol and propanol propoxy;have a lower density than biodiesel. Therefore, the elevation of the second phase from the bottom to the top of the collection vessel could be attributed to an increasing content of these compounds, along with glycerol disappearance in the second phase. The actual methanol excess was lower in this process than that indicated by the molar ratio to chicken fat, because this ratio was calculated by having to account for the amount needed for the transesterification reaction, but not for secondary reactions that could occur with glycerol. According to these results, a moderate excess of methanol could promote glycerol reactions to the most dehydrated compounds, which are more soluble in the biodiesel phase. As was recently suggested, the reaction products of glycerol can be used as part of the fuel.1,9,11 Because of increasing biodiesel production with conventional base- and acid-catalyzed methods, glycerol prices have plummeted and glycerol recovery to a commercial degree of purity from the complex mixture of glycerol, catalyst, soap, alcohol, and unreacted triglycerides is very complex and does not seem to be economically attractive. Although some alternative uses for glycerol have been proposed, it has practically become an environmental problem.32 In this proposed supercritical transesterification process, conditions have been identified to obtain a glycerol-compliant biodiesel without any biodiesel washing steps, which avoids wastewater generation. It can also be said that the low-volume, less-viscous, glycerolrelated phase (i.e., water and unreacted glycerol and methanol) should be easier to manage, from an environmental point of view, especially when considering process options such as waste treatment or incineration. While direct addition of glycerol to the fuel is not possible, in situ-generated derivatives of glycerol have potential for use as additives if these derivatives are compatible with biodiesel fuels. As shown in the literature1,11,33 and in this study (e.g., Tables 5 and 6), methyl ethers of glycerol can be co-generated during biodiesel fuel production from lipid
feedstocks. Physical properties of glycerol-derived ethers and different blends of ethers with diesel and biodiesel fuels were measured.33 The results show that the ether additives do not have a significant effect on the specific gravity of the blends. Solubility studies determined that these additives are compatible with biodiesel fuels. For example, a 20% blend of glycerol tertiary butyl ether with FAMEs resulted in a 5 °C reduction in cloud point and an 8% reduction in viscosity of the fuel. The lighter methyl esters and ethers could reduce these values further. Nevertheless, future studies, including engine performance and emission tests, must be completed to determine the suitability of these products as fuel improvers. Conclusions The supercritical transesterification of chicken fat was conducted at temperatures of 350, 375, and 400 °C, pressures of 100, 200, and 300 bar, methanol-to-lipid molar ratios from stoichiometric to 12:1, and residence times of 3-10 min. It was observed that a good-quality biodiesel was produced at a temperature of 400 °C, pressures of 100-300 bar, a molar ratio of 9:1, and residence times of 6-10 min. These particular experimental conditions are more attractive to scale up the biodiesel production, when compared to the experimental conditions at which supercritical transesterification processes are usually conducted, because the costs associated with the pumping, preheating, and recovery of excess methanol are minimized. Because of the high-temperature conditions, thermal decomposition products of the main FAMEs and glycerol-related byproducts were formed. Among the decomposition products of the former were small-molecular-weight methyl esters, which improve certain properties of the biodiesel, such as viscosity and cold flow. Glycerol, which was formed as a byproduct of the transesterification reactions, further reacted to form products such as methyl glycerol ethers, alcohols, and diglycerol-related compounds, some of which (especially the most dehydrated ones) could remain in the biodiesel phase and also be used as fuel. A complete account of the effect of these decomposition products must be assessed through a thorough biodiesel ASTM test, which was not performed in this research.
(32) Huber, M.; Lemmon, E. W.; Kazakov, A.; Ott, L. S.; Bruno, T. J. Energy Fuels 2009, 23 (7), 3083–3088. (33) Noureddini, H.; Dailey, W. R.; Hunt, B. A. Production of Ethers of Glycerol from Crude Glycerol;The By-Product of Biodiesel Production. In Chemical and Biomolecular Engineering Research and Publications, Papers in Biomaterials; University of Nebraska-Lincoln: Lincoln, NE, 1998; pp 1-14 (available via the Internet at http://digitalcommons.unl. edu/cgi/viewcontent.cgi?article=1019&context=chemeng_biomaterials; last accessed on September 29, 2009).
Acknowledgment. V.F.M. wishes to thank the Colombian Institute of Science and Technology (COLCIENCIAS) for financial support for a six-month research study at Syracuse University. The authors acknowledge Tyson Foods for supplying the chicken fat for the present study and Dr. Arthur Stipanovic of SUNY ESF for the use of the HP-1MS column.
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