Energy & Fuels 2009, 23, 507–512
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Cogeneration of Biodiesel and Nontoxic Cottonseed Meal from Cottonseed through in Situ Alkaline Transesterification Junfeng Qian and Zhi Yun* College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Nanjing, 210009, P. R. China ReceiVed June 29, 2008. ReVised Manuscript ReceiVed October 6, 2008
In the present work the in situ alkaline transesterification of cottonseed oil with methanol for the production of biodiesel and nontoxic cottonseed meal was studied. The methyl ester of cottonseed oil fatty acids could be produced satisfactorily by in situ alkaline transesterification of cottonseed oil. The experimental results of water removal pretreatment methods of milled cottonseed showed that methanol washing was better than vacuum oven drying. After water removal pretreatment of milled cottonseed with methanol washing, the influences of NaOH concentration in methanol, different molar ratios of methanol to oil, reaction temperature, and reaction time on cottonseed conversion and free gossypol content in cottonseed meal were respectively investigated by monofactor experiments. Then the significance of the factors was investigated by orthogonal design. Reaction conditions for maximum conversion of cottonseed oil into FAME were identified using statistical experimental design methods. For milled cottonseed from methanol washing, a 98% conversion could be achieved with 3 h reaction time, 0.06 mol/L NaOH concentration in methanol, 130:1 methanol/oil molar ratio, and 40 °C reaction temperature. The properties of cottonseed oil methyl esters prepared by in situ alkaline transesterification met the ASTM specifications for biodiesel. Under such reaction conditions, the free gossypol content in cottonseed meal could be reduced to 0.010%, which was far below the FAO standard, and the nontoxic cottonseed meal could be used as animal protein feed resources.
1. Introduction Biodiesel (fatty acid methyl esters, FAME) is a well-known alternative, renewable fuel, and its use in diesel engines also shows a decrease in the emission of CO, SOx, unburned hydrocarbons, and particulate matter when compared with conventional fossil-based diesel fuel.1-4 The most common method to produce biodiesel is transesterification of vegetable oils or animal fats with a short-chain alcohol in the presence of a catalyst such as an acid, alkali, or enzyme.5-7 Raw material is a key factor for the application of biodiesel, and the price of oil-containing material strongly influences biodiesel cost, generally being 70-80% of the total cost. Currently, semirefined or refined vegetable oil is the predominant raw material for the production of biodiesel. However, the relatively high cost renders the resulting fuels unable to compete with petroleumderived fuel.8,9 Therefore, a number of efforts have been made * To whom all correspondence should be addressed. Tel.: +86-2583587190; fax: +86-25-83587190; e-mail:
[email protected]. (1) Vicente, G.; Martinez, M.; Aracil, J. Bioresour. Technol. 2004, 92 (3), 297–305. (2) Encinar, J. M.; Gonzalez, J. F.; Rodriguez-Reinares, A. Ind. Eng. Chem. Res. 2005, 44 (15), 5491–5499. (3) Antolin, G.; Tinaut, F. V.; Briceno, Y.; Castano, V.; Perez, C.; Ramrez, A. I. Bioresour. Technol. 2002, 83 (2), 111–114. (4) Murayama, T.; Fujiwara, Y.; Noto, T. J. Automobile Eng. 2000, 214 (2), 141–148. (5) Zullaikah, S.; Lai, C. C.; Vali, S. R.; Ju, Y. H. Bioresour. Technol. 2005, 96 (17), 1889–1896. (6) Arzamendi, G.; Campo, I.; Arguinarena, E.; Sanchez, M.; Montes, M.; Gandia, L. M. Chem. Eng. J. 2007, 134 (1-3), 123–130. (7) Ha, S. H.; Lan, M. N.; Lee, S. H.; Hwang, S. M.; Koo, Y. M. Enzyme Microb. Tech. 2007, 41 (4), 480–483. (8) Wang, Y.; Ou, S. Y.; Liu, P. Z.; Tang, S. Z. J. Mol. Catal. A: Chem. 2006, 252 (1-2), 107–112.
to cut down biodiesel cost, such as selecting inexpensive raw materials or simplifying the reaction process. Contemporary biodiesel production technologies involve extracting triglycerides (TG) from oilseeds by using solvents, degumming, and refining the TG, prior to a transesterification process. Hexane is commonly used as the solvent in the commercial extraction. These initial processes often produce large amounts of hazardous solvent wastes and are generally cumbersome to biodiesel production. The whole procedure is time-consuming and expensive; therefore, several options have been attempted to make it easier. In situ transesterification,10-15 a biodiesel production method that utilizes the original agricultural products as the source of triglycerides for direct transesterification, eliminates the costly hexane extraction and oil refining processes and works with virtually any lipid-bearing material. It could reduce the long production process associated with pre-extracted oil and maximize alkyl ester yield. The use of reagents and solvents is reduced, and the concern about waste disposal is avoided. China is one of the main producers of cotton in the world, making large quantities of cottonseed. But because of the (9) Zhang, Y.; Dube, M. A.; Mclean, D. D.; Kates, M. Bioresour. Technol. 2003, 90 (3), 229–240. (10) Siler-Marinkovic, S.; Tomasevic, A. Fuel 1998, 77 (12), 1389– 1391. (11) Haas, M. J.; Scott, K. M. J. Am. Oil Chem. Soc. 2007, 84 (2), 197– 204. (12) Haas, M. J.; Scott, K. M.; Foglia, T. A.; Marmer, W. N. J. Am. Oil Chem. Soc. 2007, 84 (10), 963–970. (13) Qian, J. F.; Wang, F.; Liu, S.; Yun, Z. Bioresour. Technol. 2008, 99 (18), 9009–9012. (14) Haas, M. J.; Scott, K. M.; Marmer, W. N.; Foglia, T. A. J. Am. Oil Chem. Soc. 2004, 81 (1), 83–89. (15) Carrapiso, A. I.; Garia, C. Lipids 2000, 35 (11), 1167–1177.
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presence of toxic gossypol in the cottonseed, the use of nonrefined cottonseed oil is very limited. Using it as a raw material for producing biodiesel would be a significant choice. Also, cottonseed is only slightly inferior to soybean as a protein source, but the toxic gossypol in the cottonseed must be removed before being eaten by monogastric animals. Total gossypol comprises free and bound gossypol. Only the free gossypol is of concern with regard to toxicity. By FAO standards, edible grade cottonseed meal should not contain more than 0.045% free gossypol. Thus, it is necessary for cottonseed meal to be further processed to reduce gossypol to permissible levels for animal protein feed resources. Because of the presence of excess polar methanol during in situ transesterification, the toxic polar gossypol which exists in cottonseed could be extracted. Therefore, virtual nontoxic cottonseed meal could be produced. Alkaline catalysis is known to achieve the transesterification of TG with high speed and efficiency and to be more effective than acid catalysis in this capacity. On the basis of these considerations, we therefore investigated the in situ alkaline transesterification of cottonseed oil, and previous work has been published as a short communication.13 Here we reported that by methanol washing of the milled cottonseed, a marked reduction in the NaOH catalyst requirement was achieved compared with previous vacuum oven drying pretreatment. This paper presents an extension of a previously published approach and provides more detailed experiment results and discussion as a full paper. The effect of some factors, such as pretreatment methods, catalyst concentration, molar ratio of methanol to oil, reaction temperature, and reaction time on cottonseed oil conversion and free gossypol content in cottonseed meal, was systematically investigated. The properties of cottonseed oil methyl esters prepared by in situ alkaline transesterification was determined and compared with ASTM biodiesel standard. 2. Experimental Section 2.1. Materials. Cottonseeds were obtained from Jiangsu Jintian Group (Jiangsu, China). They were milled using an electric grinder to a mesh size of 40-60. The oil content of the milled cottonseed was 31.6% (wet basis). The moisture content of the milled cottonseed was 8.7 wt%. Free gossypol content of raw milled cottonseed was 0.90%. The acid value of the cottonseed oil was 0.82 mg KOH/g. Average molecular weight of cottonseed oil was 854.6 g/mol. Methanol (>98%) and petroleum ether (60-90 °C) were purchased from Nanjing Huaqingnanfang Chemical Ltd. (Nanjing, China). They were distilled before being used. All other chemicals including sodium hydroxide used during this experiment were of analytical reagent (AR) grade. 2.2. In Situ Alkaline Transesterification Procedure. Milled cottonseed (25 g) was mixed with 50 mL of methanol for 10 min, the slurry obtained was vacuum-filtered on a Buchner funnel, and the filter cake was used in in situ alkaline transesterification experiments and mixed with methanol (100-200 mL) in which sodium hydroxide had been dissolved (alkaline alcohol). The mixture was preheated to the set temperatures before starting the reaction in a water bath. Alcoholysis was carried out in a threenecked 500 mL round-bottom flask, equipped with thermostat and mechanical stirrer. The reaction mixture was vacuum-filtered on a Buchner funnel, and the filter cake was washed with petroleum ether. After being dried overnight at room temperature, free gossypol in the cottonseed meal was determined by using official methods Ba-8-78 and Ba-7-58 of the American Oil Chemists Society.16 The filtrate was left to settle and separate into two layers. (16) AOCS. Determination of free gossypol. Official Method Ba 7-58. Official and TentatiVe Methods of Analysis, 3rd ed.; American Oil Chemists Society: Chicago, IL, 1985a.
Qian and Yun The lower layer was the methanol phase and was recovered under vacuum (10 ( 1 mmHg) at 50 °C in a water bath. The upper layer included the FAME (crude biodiesel), petroleum ether, and some unreacted triglyceride, and it was washed with water until the washings were neutral. After washing, the upper layer was dried over sodium sulfate, filtered, and evaporated to eliminate the petroleum ether, and the residue was the crude biodiesel. 2.3. Analytical Methods. After each reaction, the sample of crude biodiesel was taken and its purity was analyzed by using a GC equipped with a flame ionization detector and using nitrogen as carrier gas. The analysis of biodiesel for each sample was carried out by dissolving 1.0 g of biodiesel sample and 0.2 g of methyl salicylate which was added as a reference into 8 mL of n-hexane and injecting 1 µL of this solution in the GC. The sample injected was separated in a stainless steel column (2 m × 4 mm) packed with 8% polydiethylene glycol adipate on Chromosorb G AWDMCS. The oven temperature of the GC was programmed from 150 to 215 °C at an increasing rate of 5 °C min-1 and was held at 215 °C for 20 min. The injector and detector temperatures were 260 °C and the flow rates of nitrogen, hydrogen, and air were 19, 40, and 300 mL min-1, respectively. The purity of biodiesel samples was calculated based on the area of FAME over the reference by the following equation:13,17
purity (%) ) (area of FAME)/(area of reference) × (weight of reference) × weight of biodiesel sample (correction factor) × 100 where purity of biodiesel sample refers to the conversion of cottonseed oil into FAME in the reaction. Oil content in milled cottonseed and meals was determined by extraction with hexane in a Soxhlet apparatus according to ISO 659-1988. Moisture content was determined according to ISO 6651977. Nitrogen content and calculation of crude protein content was determined according to ISO 5983-1979. The free fatty acid content of the cottonseed oil was determined according to ISO 6601996. All the experiments were carried out three times in order to calculate the arithmetic averages and the standard deviations of all the results.
3. Results and Discussion 3.1. Comparison of Different Water Removal Pretreatment Methods of Milled Cottonseed. In biodiesel production, it is well-known that the vegetable oils/fats used as a raw material for alkali-catalyzed transesterification should be waterfree since the presence of water could cause soap formation, which consumes the catalyst and reduces catalyst efficiency. The resulting soaps cause an increase in viscosity and formation of gels and made the separation of glycerol difficult.18-20 In order to decrease the moisture content in the in situ alkaline transesterification reaction system, different water removal pretreatment methods of milled cottonseed such as drying or methanol washing were used before in situ transesterification. Drying of milled cottonseed was achieved by heating the milled cottonseed under vacuum (10 ( 1 mmHg) at 70 °C in an oven for 3 h. In situ alkaline transesterification was conducted under the reaction conditions of 0.06 mol/L NaOH concentration in methanol, 135:1 mol ratio of methanol to oil, 40 °C reaction temperature, and 3 h reaction time. The results are given in Table 1. In the in situ transesterification of milled cottonseed without water removal pretreatment, the conversion of cot(17) Wang, Y.; Ou, S. Y.; Liu, P. Z.; Xue, F.; Tang, S. Z. J. Mol. Catal. A: Chem. 2006, 252 (1-2), 107–112. (18) Ma, F.; Clements, L. D.; Hanna, M. A. Trans. ASAE 1998, 41 (5), 1261–1264. (19) Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91 (3), 289–295. (20) Wright, H. J.; Segur, J. B.; Clark, H. V.; Coburn, S. K.; Langdon, E. E.; DuPuis, R. N. Oil Soap 1944, 21, 145–148.
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Table 1. Effect of Water Removal Pretreatment Methods of Milled Cottonseed on Cottonseed Oil Conversion and Free Gossypol Content in Cottonseed Meala
pretreatment methods
conversion of cottonseed oil into FAME (%)
free gossypol content (%)
no pretreatment vacuum oven drying methanol washing
21 35 98
0.025 0.048 0.010
a
Raw milled cottonseed contained 0.90% free gossypol.
tonseed oil into FAME was approximately only 21%, and free gossypol content in cottonseed meal was 0.025%. Vacuum oven drying of milled cottonseed caused an increase in the cottonseed oil conversion but also increased the free gossypol content in cottonseed meal. This is probably because free gossypol mainly concentrates in the pigment glands of milled cottonseed. The pigment glands have a certain mechanical strength and are not easily broken, but they are more sensitive to water. In the presence of water, they are easily broken and release gossypol. The existence of water in milled cottonseed without vacuum oven drying is conducive to promoting the extraction of gossypol. Methanol washing of milled cottonseed had remarkable influence on cottonseed oil conversion and free gossypol content in cottonseed meal. With methanol washing before in situ transesterification, the conversion of cottonseed oil into FAME was 98% and the free gossypol content in cottonseed meal was reduced to 0.010%. The results obtained with methanol washing suggests that methanol washing removed not only the water but also other impurities in cottonseed such as free fatty acids, colloid, gossypol, and coloring matter, which could inhibit the in situ transesterification reaction. 3.2. Effect of NaOH Concentration in Methanol on Cottonseed Oil Conversion and Free Gossypol Content in Cottonseed Meal. Compared with in situ acid-catalyzed transesterification, the in situ alkali-catalyzed process can produce a substantially higher conversion yield, reduce the use of reagents, and proceed under mild reaction conditions. As demonstrated previously,21 methanol itself is a poor vegetable oil extractant. We detected only negligible amounts of ester following a 5 h extraction of milled cottonseed with methanolic NaOH in a Soxhlet apparatus. Presumably, this is because the milled cottonseed bed is exposed only to the methanol component under Soxhlet conditions. However, incubation of milled cottonseed with alkaline methanol resulted in the recovery of substantial amounts of FAME. Conceivably, alkaline alcohol destroys intracellular compartmentalization in the milled cottonseed, allowing solubilization and subsequent transesterification of the acylglyceride. Alternatively, fatty acyl esters are expected to be efficient at extracting acylglycerides from cottonseed. The initial and continued production of these during the in situ reaction may allow increasing efficiency of glyceride extraction from the liquid phase of the system and subsequent transesterification. Also the addition of NaOH caused the decomposition of gossypol and reduced the toxic gossypol content in cottonseed meal.22 In situ alkaline transesterification of cottonseed oil was carried out with NaOH as a catalyst at a concentration of 0.02-0.08 mol/L in methanol with a reaction temperature of 40 °C, methanol/oil molar ratio of 135:1, and reaction time of 3 h. The reaction profile of Figure 1 indicated that the cottonseed (21) Ozgul-Yucel, S.; Turkay, S. J. Am. Oil Chem. Soc. 2003, 80 (1), 81–84. (22) Edward, E.; Bialek, H. F.; Davies, D. L. J. Am. Oil Chem. Soc. 1954, 31 (4), 121–124.
Figure 1. Effect of NaOH concentration in methanol on cottonseed oil conversion and free gossypol content in cottonseed meal. (Based on 25 g of milled cottonseed per reaction; water removal pretreatment: methanol washing; reaction temperature: 40 °C; reaction time: 3 h; methanol/oil molar ratio: 135:1.)
oil conversion and free gossypol content in cottonseed meal by in situ alkaline transesterification was dependent upon the NaOH concentration in methanol. As shown in Figure 1, when increasing the NaOH concentration from 0.02 to 0.06 mol/L, the conversion to methyl ester was increased from 8% to 98%, and the free gossypol content in cottonseed meal was decreased from 0.030% to 0.009%. However, when the NaOH concentration exceeded 0.06mol/L, it had no significant effect on the cottonseed oil conversion and free gossypol content in cottonseed meal. Using acidic methanol under reflux, Kildiran et al.23 observed a maximal extraction of 40% of the oil from finely ground soybeans, with only 55% transesterification of this extracted oil, giving an overall FAME yield of 22%. As opposed to acid catalysis, the alkaline reaction conducted here achieved a much greater removal of oil from the substrate (99%) and more effective transesterification of the extracted oil (98%). Compared with milled cottonseed of vacuum oven drying,13 methanol washing of the milled cottonseed caused a substantial reduction in the NaOH concentration for high-efficiency transesterification. Transesterification of milled cottonseed with methanol washing required 40% less NaOH, and this reduction in catalyst usage was achieved with no sacrifice of transesterification efficiency. 3.3. Effect of Molar Ratio of Methanol to Oil on Cottonseed Oil Conversion and Free Gossypol Content in Cottonseed Meal. The stoichiometric molar ratio of methanol to cottonseed oil for complete transesterification of the fatty acids in the oil to methyl esters is 3:1. However, in practice a higher molar ratio is employed in order to shift the reaction equilibrium toward the products side and produce more methyl esters. In general, the molar ratio is associated with the type of catalyst used. Hass et al.14 used the methanol/oil molar ratio 227:1 in their investigation of in situ alkaline reaction for completely dry flakes. The results obtained in our investigation were shown in Figure 2, in which the methanol/oil molar ratio varied from 70:1 to 135:1 with NaOH concentration in methanol 0.06 mol/L, reaction temperature 40 °C, and reaction time 3 h. As shown in Figure 2, by increasing the amount of loading methanol from 70:1 to 130:1, the conversion to methyl ester was increased from 22% to 98%, and the free gossypol content in cottonseed meal was decreased from 0.032% to 0.010%. However, beyond the molar ratio of 130:1, the excessively added (23) Kildiran, G.; Ozgul, Y. S.; Turkay, S. J. Am. Oil Chem. Soc. 1996, 73 (2), 225–228.
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Figure 2. Effect of methanol/oil molar ratio on cottonseed oil conversion and free gossypol content in cottonseed meal. (Based on 25 g of milled cottonseed per reaction; water removal pretreatment: methanol washing, reaction temperature: 40 °C; reaction time: 3 h; NaOH concentration in methanol: 0.06 mol/L.)
methanol had a slight effect on cottonseed oil conversion and free gossypol content in cottonseed meal. In the in situ alkaline transesterification of milled cottonseed without water removal pretreatment, the conversion to methyl ester achieved 95% requiring methanol/oil molar ratio of 380: 1. By methanol washing pretreatment, a marked reduction in the methanol/oil molar ratio was achieved. Despite the reduction in methanol when using milled cottonseed with methanol washing pretreatment, the molar ratio of methanol to oil (130: 1) remained substantially higher than the 6:1 values common in the alkaline transesterification of refined oil. Some of this was probably required to provide access of alcohol and alkaline to the substrate: approximately 40 mL of liquid was necessary simply to cover the 25 g of milled cottonseed in these reactions. In addition, the required methanol and NaOH may play other roles in facilitating reaction, perhaps altering the permeability of the solid substrate, allowing access of the reactant and the catalyst to the triglycerides (TG). Also the methanol could remove toxic gossypol in cottonseed meal, thereby improving the quality of extracted meal for food and feed products and increasing its economic value. 3.4. Effect of Reaction Temperature on Cottonseed Oil Conversion and Free Gossypol Content in Cottonseed Meal. Studies carried out at different reaction temperatures by in situ alkaline transesterification with NaOH concentration in methanol 0.06 mol/L, methanol/oil molar ratio 130:1, and reaction time 3 h. The reaction temperature above the boiling point of methanol should be avoided since at higher temperature it tended to accelerate evaporation and condensation of methanol and decreased the concentration of methanol in the solution. Besides, high temperature together with methanol played a significant role in biodiesel degradation because of the higher oxidation rate of biodiesel at higher temperature.24 Reaction temperature can influence the reaction rate and biodiesel yield for a heterogeneously alkali-catalyzed reaction.25,26 However, reaction temperature had little influence on cottonseed oil conversion and free gossypol content in cottonseed meal by in (24) Leung, D. Y. C.; Koo, B. C. P.; Guo, Y. Bioresour. Technol. 2006, 97 (2), 250–256. (25) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. l.; Piao, X. L. Fuel 2008, 87 (2), 216–221. (26) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. l. Catal. Commun. 2006, 8 (7), 1107–1111.
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Figure 3. Effect of reaction temperature on cottonseed oil conversion and free gossypol content in cottonseed meal. (Based on 25 g milled cottonseed per reaction; water removal pretreatment: methanol washing, reaction time: 3 h; NaOH concentration in methanol: 0.06mol/L; methanol/oil molar ratio: 130:1.)
Figure 4. Effect of reaction time on cottonseed oil conversion and free gossypol content in cottonseed meal. (Based on 25 g of milled cottonseed per reaction; water removal pretreatment: methanol washing; NaOH concentration in methanol: 0.06mol/L; methanol/oil molar ratio: 130:1; reaction temperature: 40 °C.)
situ alkaline transesterification. As shown in Figure 3, when increasing the reaction temperature from 35 to 60 °C, the conversion was increased only from 95% to 98%, and the free gossypol content in cottonseed meal was almost the same. The reason was probably that the in situ alkaline transesterification reaction system here had excessive methanol and catalyst; thus, reaction temperature almost had no influence on cottonseed oil conversion and gossypol extraction. 3.5. Effect of Reaction Time on Cottonseed Oil Conversion and Free Gossypol Content in Cottonseed Meal. The reaction time is one of the important factors that affects the in situ alkaline transesterification. The TG conversion increased with the increment of reaction time. Figure 4 shows the effect of reaction time on cottonseed oil conversion and free gossypol content in cottonseed meal by in situ alkaline transesterification, with NaOH concentration in methanol 0.06 mol/ L, MeOH/oil molar ratio 130:1, and reaction temperature 40 °C. As can be seen from the graph, the conversion of cottonseed oil and gossypol extraction could be divided into three phases. In the first phase, 80% of the cottonseed oil was converted into FAME and the free gossypol content in cottonseed meal was decreased from 0.43% to 0.017% within 1 h. In the second phase, from a reaction time of 1-3 h, the acceleration of TG conversion and gossypol extraction declined gradually, only 18%
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Table 2. Orthogonal Test Designa factors
level
A (NaOH concentration in methanol, mol/L)
B (methanol/oil molar ratio, mol/mol)
C (reaction temperature, °C)
D (reaction time, h)
1 2 3
0.04 0.06 0.08
100:1 130:1 160:1
30 40 50
1 3 5
a Based on 25 g of milled cottonseed per reaction; water removal pretreatment: methanol washing.
Table 3. Results Obtained at the Test Conditions Using L9 (34) Orthogonal Design factors method
A
B
C
D
1 2 3 4 5 6 7 8 9 K1 K2 K3 k1 k2 k3 R optimal level
0.04 0.04 0.04 0.06 0.06 0.06 0.08 0.08 0.08 113a 260 269 38b 87 90 52c A3
100:1 130:1 160:1 100:1 130:1 160:1 100:1 130:1 160:1 191 219 232 64 73 77 13 B3
30 40 50 40 50 30 50 30 40 223 203 216 74 68 72 6 C1
1 3 5 5 1 3 3 5 1 196 221 225 65 74 75 10 D3
a KA ) ∑the amount of target compounds A i i max{ kiA} - min{ kiA}.
b
conversion (%) 27 38 48 79 83 98 85 98 86
kiA ) KiA/3
c
Figure 5. Reaction surface showing predicted conversion of cottonseed oil into FAME for the in situ alkaline transesterification as a function of the methanol/oil molar ratio and NaOH concentration. (Based on 25 g of milled cottonseed per reaction; water removal pretreatment: methanol washing; reaction temperature: 40 °C; reaction time: 3 h.)
RiA )
of cottonseed oil was converted into FAME, and the free gossypol content in cottonseed meal was decreased from 0.017% to 0.011%. In the third phase, further increase in the reaction time from 3 to 4 h caused a smaller effect on the cottonseed oil conversion and free gossypol content in cottonseed meal. Under such optimum conditions, the conversion of cottonseed oil into FAME was 98%. With the in situ alkali-catalyzed transesterification of soybean oil, optimal reaction times of 8 h have been reported for the higher biodiesel yield.14 In this work we reported that a cottonseed oil conversion of 98% could be achieved within 3 h probably because the methanol washing of milled cottonseed removed some impurities, such as water, free fatty acids, colloids, and coloring matter, that could slow down the transesterification rate. 3.6. Optimization of in Situ Alkaline Transesterification of Cottonseed Oil. On the basis of the results of the monofactor experiments, the orthogonal design method was applied to optimize the processing conditions. The tests included four factors: A (NaOH concentration in methanol), B (methanol/oil molar ratio), C (reaction temperature), and D (reaction time). The test projects are shown in Table 2, and experimental results are given in Table 3. As can be seen from the Table 3, the significances of the factors were in the order: A > B > D > C according to the R values, and according to the ki values of each column, the optimum reaction conditions of in situ alkaline transesterification were a NaOH concentration in methanol of 0.08mol/L, methanol/ oil molar ratio of 160:1, reaction temperature of 30 °C, and reaction time of 5 h. However, among the ranges tested, the optima are the ones listed, but better conditions might exist outside the ranges we
have examined. Thus, central composite response surface design methods27 were employed to coordinately investigate the effects and interactions of the methanol/oil molar ratio (76:1-183:1), NaOH concentration (0.024-0.095 mol/L), and reaction time (0.4-7.6 h) on the cottonseed oil conversion into FAME. Bestfit equations correlating these data with the reaction compositions were constructed using SAS/STAT software.28 Numerical analysis of this equation and examination of the corresponding three-dimensional surfaces allowed identification of the conditions predicted to give maximum cottonseed oil conversion. Equation 1, derived from our results, describes the relationship of cottonseed oil conversion to the methanol/oil molar ratio and NaOH concentration in a reaction using 25 g of milled cottonseed with 40 °C reaction temperature, where “conversion” is the predicted cottonseed oil conversion (%), N is the NaOH concentration in methanol (mol/L), M is the methanol/oil molar ratio (mol/mol), and T is the reaction time (h). The R2 value for the fits of this equation to the data was 0.92 for cottonseed oil conversion, indicating that the data were well-modeled by the equation. conversion ) -328.7 + 4446.1N + 2.787M + 27.879T + 3.333NM + 25.0NT + 0.0125MT - 31602.8N2 - 0.0102M2 - 3.863T2 (1) Figure 5 shows the response surface predicting cottonseed oil conversion following 3 h reaction time and 40 °C reaction temperature, as functions of the methanol/oil molar ratio and NaOH concentration in the methanol. Analysis of eq 1 and Figure 5 allowed identification of reaction conditions predicted to achieve maximum cottonseed oil conversion into FAME. The predicted cottonseed oil conversion could achieve 98% under the reaction conditions of 0.06 mol/L NOH concentration in methanol, 130:1 methanol/oil molar ratio, 3 h reaction time, (27) Box, G. E. P.; Hunter, W. G.; Hunter, J. S. Statistics for experimenters; John Wiley and Sons: New York, 1978. (28) SAS/STAT user’s guide, version 8, SAS Institute Inc., Cary, NC, 1999.
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Table 4. Properties of FAME Product Prepared by in Situ Alkaline Transesterification biodiesel standard assay cottonseed (maximum method oil methyl allowed, (ASTM) ester unless stated)
property cetane number flash point (°C) (mm2/s,
kinematic viscosity 40 °C) water and sediment (vol%) carbon residue (wt%) Sulfated ash (mass%) sulfur (wt%) cloud point (°C) copper corrosion acid number (mg KOH/g) free glycerin (wt%) total glycerin (wt%) phosphorus (wt%) reduced pressure distillation (temperature at 90% recovery, °C)
D613
55
D93
150
D445
4.02
47 minimum 130 minimum 1.9-6.0
D2709 D524 D874 D5453 D2500 D130 D664 D6584 D6584 D4951 D1160
0 methanol/oil molar ratio > reaction time > reaction temperature. Statistical experimental design methods and response surface regression analysis were used to optimize reaction conditions. For milled cottonseed of methanol washing, the conversion could achieve 98% with 3 h reaction time, 0.06 mol/L NaOH concentration in methanol, 130:1 methanol/oil molar ratio, and 40 °C reaction temperature. An increase in the NaOH concentration, coupled with a reduced methanol/oil molar ratio, also gave high cottonseed oil conversion. The properties of cottonseed oil methyl esters prepared by in situ alkaline transesterification met the ASTM specifications for biodiesel. Under such reaction conditions, the free gossypol content in cottonseed meal could be reduced to 0.010% which was far below the FAO standard so the nontoxic cottonseed meal could be used as animal protein feed resources. EF800518U
