Energy Fuels 2010, 24, 2104–2108 Published on Web 02/09/2010
: DOI:10.1021/ef901347b
Production of Biodiesel from Waste Cooking Oil via a Two-Step Catalyzed Process and Molecular Distillation Yong Wang,*,†,‡ Jieyu Nie,‡ Mouming Zhao,† Shun Ma,‡ Lina Kuang,‡ Xue Han,‡ and Shuze Tang‡ †
College of Light Industry and Food Science, South China University of Technology, Guangzhou, 510641, People’s Republic of China, and ‡Department of Food Science and Engineering, Jinan University, Guangzhou, 510632, People’s Republic of China Received November 12, 2009. Revised Manuscript Received January 19, 2010
In this work, a two-step catalyzed process was developed to produce biodiesel from waste cooking oil (WCO). First, free fatty acid (FFA) of the WCO with an acid value of 66.40 ( 1.08 mg KOH/g was esterified with methanol catalyzed by polyferric sulfate (PFS). Second, the esterified WCO was transesterified with methanol catalyzed by potassium hydroxide to produce crude biodiesel. The crude biodiesel was purified by molecular distillation to produce purified biodiesel (fatty acid methyl ester, FAME). The highest yield of FAME by molecular distillation from the crude biodiesel was 98.32% ( 0.17% at an evaporator temperature of 120 °C. PFS was removed efficiently from the esterified WCO by washing water after the recovery of methanol. The results revealed that this two-step process using a polyferric sulfate catalyst and molecular distillation is probably a promising method for the conversion of WCO. optimum operating conditions.5 Increasing food consumption has increased the production of a large amount of WCO/ fat. It is estimated that the production of WCO in Guangzhou, which is the third largest city in China, with a population of over 10 million, is ∼20 000 tons per year. The conversion of the WCO into fuel also eliminates the environmental impacts caused by the harmful disposal of these waste oils, such as disposal into drains.6 The WCO from China, especially from the tropical areas in southern China, is collected from the drains of the restaurants and has undergone treatment with a mixture of water and feed remains for a long time at ambient temperature before separation, which causes the hydrolysis of triacylglycerol (TAG) of the WCO by the micro-organisms, resulting in WCO with a high acid value (∼50-100 mg KOH/g). Ferric sulfate was reported to act as a catalyst for trhe esterification of free fatty acid (FFA) of the WCO with methanol in a two-step process for production of biodiesel.7,8 In this work, polyferric sulfate (PFS), which was widely used as a flocculating agent/coagulant for water, was introduced to catalyze the esterification of FFA of the WCO with methanol. PFS is produced from ferrous sulfate via oxidation, hydrolysis, and polymerization.9 PFS contains polynuclear complex ions, such as Fe2(OH)24þ and Fe3(OH)45þ, formed by OH bridges and a large number of inorganic macromolecular compounds. The formula of PFS is expressed as [Fe2(OH)n(SO4)(3-n)/2]m and the obtained molecular weight can be as high as 105 Da.10 Compared to ferric sulfate, PFS has a higher solubility in water as well as methanol, which can be
Introduction The continuing rise in global prices of crude oil, increasing threats to the environment by exhaust emissions, global warming, and threats of supply instabilities have adversely impacted the developing countries, more so to the petroleum importing countries like China. It is important to find a safe alternative fuel to relieve the escalating energy crisis and to protect the environment.1 Biodiesel, which is defined as a nonpetroleum-based diesel fuel, is a monoalkyl (methyl, propyl, or ethyl) ester of long-chain fatty acids, which is considered as an alternative fuel that behaves like petroleum diesel. Biodiesel is derived from the renewable sources, and it is biodegradable and safe for the environment.2 Biodiesel is typically produced by reacting fats and oils (e.g., vegetable oils and animal fats) with alcohol in the presence of a catalyst. The feedstock for biodiesel is largely dependent on the variety and production of edible oils of the particular country. For example, soybean oil is commonly used for biodiesel production in the United States, and rapeseed (Canola) oil is a major feedstock in Europe. However, China is a developing country with a huge population, and it still imports huge quantities of edible oils. Therefore, the emphasis has focused on production of biodiesel from the low-cost feedstock such as waste cooking oil (WCO), which is more economical and environmentally friendly.3 The price of WCO is ∼30%-60% of that of virgin vegetable oil. Consequently, the total manufacturing cost of biodiesel from WCO can be significantly reduced.4 In addition, a similarity in the quality of biodiesel derived from WCO and from refined vegetable oils could be achieved under
(5) Cetinkaya, M.; Karaosmanoglu, F. Energy Fuels 2004, 18, 1888– 1895. (6) Utlu, Z. Energy Sources, Part A 2007, 29, 1295–1304. (7) Wang, Y.; Ou, S. Y.; Liu, P. Z.; Xue, F.; Tang, S. Z. J. Mol. Catal. A: Chem. 2006, 252, 107–112. (8) Patil, P.; Deng, S.; Rhodes, J. I.; Lammers, P. J. Fuel 2010, 89, 360– 364. (9) Ngo, H. L.; Zafiropoulos, N. A.; Foglia, T. A.; Samulski, E. T.; Lin, W. B. Energy Fuels 2008, 22, 626–634. (10) Venkanna, B. K.; Reddy, C. V. Bioresour. Technol. 2009, 100, 5122–5125.
*Author to whom correspondence should be addressed. Tel.: þ86-2085224358. Fax: þ86-20-85226630. E-mail:
[email protected]. (1) Sharma, Y. C.; Singh, B. Renew. Sust. Energy Rev. 2009, 13, 1646– 1651. (2) Basha, S. A.; Gopal, K. R.; Jebaraj, S. Renew. Sust. Energy Rev. 2009, 13, 1628–1634. (3) Phan, A. N.; Phan, T. M. Fuel 2008, 87, 3490–3496. (4) Zhang, Y.; Dube, M. A.; McLean, D.; D.; Kate, M. Bioresour. Technol. 2003, 90, 229–240. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 2104–2108
: DOI:10.1021/ef901347b
Wang et al.
easily removed by water washing after the recovery of methanol. The washing water should be easier to handle, because PFS is a coagulant for water treatment. Furthermore, the price of PFS is ∼30%-40% of that of ferric sulfate in China. Molecular distillation was employed to purify fatty acid methyl ester (FAME) from the crude biodiesel, to replace the washing step used to reduce the effluent of the process after transesterification. The production of biodiesel catalyzed by PFS is not yet reported. This study would be helpful in selecting an efficient and economical process for biodiesel production from WCO. Experimental Section Figure 1. Effect of amount of catalyst on the conversion of FFA to FAME. (Conditions: reaction temperature = 80 °C, reaction time = 4 h, methanol:WCO mass ratio = 1.0. Acronyms: FFA = free fatty acid, FAME = fatty acid methyl ester, WCO = waste cooking oil.)
Materials. Waste cooking oil (WCO) was provided by a local company (Guangzhou Balis Waste Treatment Co., Ltd.), which, by the authority of the local government, collected the WCO from restaurants. Food remains in the WCO were removed using a separation process. The acid value of the WCO was 66.40 ( 1.08 mg KOH/g. Polyferric sulfate (PFS) was obtained from Guangzhou Chenyi Chemical Co., Ltd. Esterification Catalyzed by Polyferric Sulfate. The experiment plan involved five levels of reaction time (1, 2, 4, 6, and 8 h), eight levels of catalyst loading (1, 2, 3, 4, 5, 6, 7, and 8 wt %, based on the weight of the WCO), and five levels of methanol: WCO mass ratios (0.4, 0.6, 0.8, 1.0, and 1.2). The esterification of FFA of the WCO was performed in a 250-mL, three-necked flask with a water condenser. Each sample of WCO (100.0 g) was mixed with methanol and PFS and then boiled for a specified time in a water bath at 80 °C; the mechanical stirring speed was fixed at 300 rpm. After the reaction, the excess of methanol was recovered under vacuum (10 ( 1 mm Hg) with a rotational evaporator at a evaporated temperature of 50 °C. Hot water (80 °C) of 30 mL was introduced to the flask to wash the catalyst by dissolving of PFS, and the mixture was allowed to settle to separate into two layers: the upper oil layer (UOL) and the lower aqueous layer. After removal of the residual water, the UOL, which was comprised of the FAME (biodiesel) and unreacted TAG, was subjected to the second step: transesterification. The lower layer consisted of the washing water that was dissolving the catalyst. The conversion of FFA of the WCO into FAME was calculated from the mean values of the acid value (AV) of the oil layer, using the following equation: AVUOL conversion ð%Þ ¼ 1 100 AVWCO
evaporator (area: 0.1 m2) and an internal condenser (area: 0.05 m2). For feeding, there was a jacketed glass vessel equipped with a flow regulation valve. The discharge of distillate and residue was done in glass flasks. The vacuum system was composed of a mechanical pump and a diffusion pump. The heating of the evaporator was provided by a jacket circulated with heated oil from an oil bath and the roller wiper speed inside the evaporator was fixed at 250 rpm. The yield of the purified biodiesel (FAME) was calculated by from the ratio of the mass of the purified biodiesel to that of the crude biodiesel. The purification of biodiesel was performed using an MD process. Biodiesel was distilled from the crude biodiesel (50.0 g) at evaporator temperatures of 90, 100, 110, and 120 °C. Other conditions for MD were as follows: evaporator vacuum = 1.0 Pa, condenser temperature = 40 °C, feeding rate = 0.2 L/h, and feed temperature = 80 °C. Analysis of Composition of FAME. The composition of FAME were analyzed via gas chromatography (GC) (Model GC 900A, Shanghai Kechuang Chromatograph Instrument Co., Ltd., Shanghai, PRC) equipped with a capillary column (HP-5, 30 m 0.32 mm 0.25 μm; Agilent Technologies, Inc., Palo Alto, CA), a flame ionization detection (FID) device, and N2 as carrier gas. The injection was performed in split mode with a split ratio of 80:1. Samples were dissolved in hexane at a concentration of 10 mg/mL. The FAME solution (1 μL) was injected at an injector temperature of 240 °C, a column temperature of 195 °C, an FID temperature of 240 °C, and a carrier gas (N2) flow of 60 mL/min. The composition of FAME reported was based on the area response using an FID device. All the determinations were performed in triplicate, and the mean values were reported. Metal Content. The WCO, the pretreated WCO by esterification, the crude, and the final biodiesel samples of 5.0 g were carbonized at the electric oven for 2 h first, and then they were transferred to the Muffin furnace to be ashed for 4 h at 600 °C. The ashed samples were dissolved in 5 mL of nitric acid and diluted to 50 mL by distilled water. The ferric, potassium, and sodium contents in the diluted samples were determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Model Optima 2000DV ICP-AES, Perkin-Elmer Co., Eden Prairie, MN). All the determinations were performed in triplicate, and the mean values were reported.
where UOL and WCO refer to the upper oil layer and waste cooking oil. All the determinations of the acid values were performed in triplicate, and the mean values (with standard deviations) were reported. Transesterification Catalyzed by Potassium Hydroxide. The dried upper oil layer was transferred to a 250-mL, roundbottom, one-necked flask, and then, six times the stoichiometric amount of methanol required for total conversion of TAG and 1.2 wt % of KOH were added. The mixture reacted at 40 °C for 1 h. After the reaction, the mixture was allowed to settle to separate into two layers. The upper layer was FAME with a light color, and the lower layer was the glycerol and methanol. The residual methanol of the upper layer was recovered under vacuum (10 ( 1 mm Hg) by a rotational evaporator at 50 °C to obtain the crude biodiesel. The yield of the crude biodiesel was calculated from the weight of the crude biodiesel over that of the dried UOL. Molecular Distillation of Crude Biodiesel. The crude biodiesel was composed of FAME and minor unreacted TAG. FAME was purified by a Model MD80 molecular distillation (MD) system, manufactured by Guangzhou Hanwei Co., Ltd. (Guangzhou, PRC). It was provided with a falling film
Results and Discussion Esterification Catalyzed by Polyferric Sulfate. Figure 1 shows the effect of the amount of PFS on the conversion of FFA. PFS catalyzes the methanolysis of FFA but not TAG, so the decrease of the acid value of the WCO is used to calculate the conversion of FFA into FAME. When 1.0 wt % 2105
Energy Fuels 2010, 24, 2104–2108
: DOI:10.1021/ef901347b
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Figure 3. Effect of reaction time on the conversion of FFA into FAME. (Conditions: reaction temperature = 80 °C, methanol: WCO mass ratio = 1.0, and catalyst amount = 3.0 wt % (based on the weight of the WCO). Acronyms: FFA = free fatty acid, FAME = fatty acid methyl ester, WCO = waste cooking oil.)
Figure 2. Effect of methanol:WCO mass ratio on the conversion of FFA to FAME. (Conditions: reaction temperature = 80 °C, reaction time = 4 h, and catalyst amount = 3.0 wt % (based on the weight of the WCO). Acronyms: FFA = free fatty acid, FAME = fatty acid methyl ester, WCO = waste cooking oil.)
of ferric sulfate was added, 84.71% ( 0.43% of FFA was converted to FAME after a reaction time of 4 h. However, when the amount of catalyst exceeded 3.0 wt %, the conversion of FFA into FAME increased very lightly and reached the maximum of 93.90% ( 0.32% at a catalyst loading of 7.0 wt %. Therefore, 3.0 wt % was considered to be the optimal catalyst loading. Although all the runs were performed at a water bath temperature of 80 °C, the exact reaction temperature of system was ∼67 °C, which is the boiling point of methanol. Extensive preliminary experimentation with the WCO samples indicated that it was most efficient to fix the reaction temperature at 80 °C to ensure sufficient reflux of the methanol, to strengthen the mass transfer of the reaction. During the reaction, PFS acted as a Lewis acid to catalyze the methanolysis of FFA of the WCO to FAME. Some PFS dissolved into methanol and other portions were suspended in the reaction mixture during the reaction due to a low solubility of PFS in methanol. Dramatically, when the amount of catalyst increased to 8.0 wt %, the conversion decreased slightly to 91.55% ( 0.43%. The conversion of FFA into FAME was also determined to be inversely related to the quantity of catalyst at a high level in some previously published works.11,12 These results can be explained from the viewpoint of the reversible nature of the esterification process.11 The optimal level of catalyst PFS used in this study was 3.0 wt % (based on the weight of the WCO), which was higher than that of sulfur acid as a catalyst for the esterification of FFAs with methanol, because of its lower acidity. However, the level of PFS was comparable to that of solid acid reported for esterification.13,14 Compared to ferric sulfate, ∼50% more PFS was required as the catalyst to achieve the same conversion of FFA.7 However, the price of PFS is 60%-70% lower than that of ferric sulfate in China, which makes it to be more attractive when considering the cost of the process. The effect of the methanol:WCO mass ratio on the conversion of FFA to FAME is displayed in Figure 2. The conversion of FFA to FAME increased with the methanol: WCO mass ratio and reached a value of 95.41% ( 0.33% for a reaction time of 4 h when the mass ratio was 1.2, which was
Figure 4. Yield of purified biodiesel from the crude biodiesel at different evaporator temperatures. Table 1. Composition of Fatty Acids of the WCO and FAME of the Biodiesela Composition (%) fatty acid (methyl ester) C12:0 C16:1 C16:0 C18:1 þ C18:2 C18:3
WCO
distillate at 90 °C
distillate at 100 °C
distillate at 110 °C
distillate at 120 °C
0.73 0.90 27.63 62.84
0.77 0.83 28.33 62.83
0.63 0.77 27.60 63.56
0.70 0.76 27.42 64.32
0.65 0.72 27.35 63.58
7.90
7.23
7.44
6.79
7.56
a
Values shown are mean values (n = 3). WCO = waste cooking oil; FAME = fatty acid methyl ester.
Table 2. Metal Content in the WCO and the Biodiesela Metal Content (mg/kg) feedstock
ferric
sodium
potassium
WCO pretreated WCO by esterification crude biodiesel after transesterification biodiesel by molecular distillation
0.33 0.43 0.33 0.25
1.79 6.00 14.6 1.55
7.31 7.90 53.5 4.45
a
Values shown are mean values (n = 3). WCO = waste cooking oil.
equal to a methanol:TAG molar ratio of ∼30. However, the increase in the conversion of FFA to FAME became flat when the methanol:WCO mass ratio was >1.0. Literature studies revealed that, to shift the equilibrium toward the forward direction, the use of a high alcohol:oil molar ratios (such as 1:40 and even 1:275) was reported.15,16 However, the
(11) Zabeti, M.; Daud, W. M. A. W.; Aroua, M. K. Fuel Process. Technol. 2009, 90, 770–777. (12) Bhatti, H. N.; Hanif, M. A.; Qasim, M.; Rehman, A. U. Fuel 2008, 87, 2961–2966. (13) Jacobson, K.; Gopinath, R.; Meher, L. C.; Dalai, A. K. Appl. Catal., B 2008, 85, 86–91. (14) Peng, B. X.; Shu, Q.; Wang, J. F.; Wang, G. R.; Wang, D. Z.; Han, M. H. Process Saf. Environ. 2008, 86, 441–447.
(15) Tesser, R.; Serio, M. D.; Guida, M.; Nastasi, M.; Santacesaria, E. Ind. Eng. Chem. Res. 2005, 44, 7978–82. (16) Xie, W. L.; Peng, H.; Chen, L. G. J. Mol. Catal. A: Chem. 2006, 246, 24–32.
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Table 3. Physical and Chemical Properties of the Purified Biodiesel by Molecular Distillation at 120°C and Conventional Diesel (0#) in China a property ester content (wt %) cetane number kinematic viscosity (mm2/s, 40 °C) density (kg/m3, 15 °C) flash point (°C) sulfated ash (% mass) water content (mg/kg) acid value (mg KOH/g) copper band corrosion (3 h at 50 °C) total contamination (mg/kg) CFPP (°C) oxidation stability (110 °C) a
biodiesel from the WCO
EN14214 specification
99.4 52.0 4.34 872.2 135 0.0039 238 0.11 Class 1 10 -10 5.10
96.5 min 51.0 min 3.5-5.0 860-900 101 min 0.02 max 500 max 0.5 max Class 1 24 max 6 min
test method pr EN 14103d EN ISO 5165 EN ISO 3104 EN ISO 3675 ISO CD 3679e ISO 3987 EN ISO 12937 pr EN 14104 EN ISO 2160 EN 12662 EN 116 EN 14112
conventional diesel (0#) in China 45 min 3.0-8.0 (20 °C) reported 55 min trace 0.07 Class 1 0 4 max
Values shown are mean values (n = 3). CFPP = cold filter plugging point.
increased (see Figure 4); from 90 to 120 °C, the yield increased from 88.93% ( 0.34% to 98.32% ( 0.17%. However, the color of the final products changed from being colorless to light yellow when the evaporator temperature increased. Distillation was introduced as an alternative practice for biodiesel production via two ways. One method was to remove the FFAs from the feedstock with a high acid value to a very low extent, so the base-catalyzed transesterification was easy to perform for the remained TAG.21 The other method was to purify FAME at 160-220 °C from the crude biodiesel from low-quality feedstock to meet a high biodiesel standard.22 Molecular distillation (MD) was also employed to remove FFAs from acidic rapeseed soapstock at 120 °C for the production of biodiesel.23 However, the purification of biodiesel from crude biodiesel by MD has not been reported yet. The advantages of MD of crude biodiesel included a low distillation temperature, which ensured no polymerization and decomposition of FAME to obtain a high yield of final products, and no washing water, which was more environmentally friendly. The composition of fatty acids of the WCO and FAME of the biodiesel is shown in Table 1. The main FAMEs of the biodiesel produced from the WCO were palmatic acid methyl ester, oleic acid methyl ester, and linoleic methyl ester, which were in accordance with that the main cooking oils used in the restaurants in China (palm oil and soybean oil). There were no significant differences among the compositions of FAME of the biodiesels, because of the low differences in molecule weight among the main FAMEs. The metal content of the WCO and biodiesel is described in Table 2. The results showed that the washing step after esterification was very effective to remove ferric components from the pretreated WCO, and only 0.43 mg ferric compounds/kg remained. During transesterification, more potassium and sodium components were introduced into the crude biodiesel by the base catalyst (KOH). After MD, the potassium content was reduced to 4.45 mg/kg, whereas a value of 53.5 mg/kg was obtained in the crude biodiesel. Some chemical and physical properties of purified biodiesel acquired via MD at 120 °C and conventional diesel (0#) in China are listed in Table 3. The results showed that all the tested properties met the specifications of the biodiesel standard (EN14214), except the oxidation stability, which
high methanol:WCO mass ratio decreased the concentration of catalyst in the reaction system, which weakened the positive effect of the addition of methanol. The same results were also observed when the WCO reacted with methanol catalyzed by heterogeneous superacid catalyst.17 The methanol:TAG mass ratio was a critical factor for the commercial production of biodiesel, because much of the energy was consumed during the recovery of methanol. A much greater amount of methanol was required for esterification catalyzed by an acid catalyst than transesterification catalyzed by a base catalyst.1 The effect of reaction time on FFA conversion is shown in Figure 3. In the initial stage, >80% of FFA was converted to FAME within 2 h. In the second stage, extending from a reaction time of 2 h to a reaction time of 4 h, the rate of methanolysis slowed, but the conversion of FFA was >93%. In the third stage, the reaction of methanolysis approached near-equilibrium after 4 h, and prolonging the reaction time did not increase the conversion of FFA significantly. Hence, a reaction time of 6 h was selected as the optimal reaction time, because of the compromise of conversion of FFA and operation cost. For acid-catalyzed esterification, a long reaction time was required to approach to the equilibrium of the reaction, even at a high reaction temperature (nearly over 200 °C).18 Alkali-Catalyzed Transesterification. The parameter of alkali-catalyzed transesterification of the WCO that was pretreated by PFS catalysis was selected from the reference.19 The base-catalyzed transesterification was very sensitive to the purity of the reactant. When the FFA content in the oil was >0.5 wt %, the efficiency of the reaction would be held back to some extent.20 In fact, the alkali-catalyzed transesterification of WCO still worked, even the FFA content was over 1 wt %.3 In this work, although the lowest acid value of WCO pretreated by the ferric sulfate was 3.36 mg KOH/g, the transesterification of TAG with methanol was easy to perform. The lower layer with glycerol was obviously observed by settlement after the reaction. The yield of crude biodiesel was 95.08% after recovery of methanol from the upper layer of the reaction mixture. Molecular Distillation of Crude Biodiesel. The yield of purified biodiesel increased as the evaporator temperature (17) Furuta, S.; Matsuhashi, H.; Arata, K. Catal. Commun. 2004, 5721–5723. (18) Fu, B. S.; Gao, L. J.; Niu, L.; Wei, R. P.; Xiao, G. M. Energy Fuels 2009, 23, 569–572. (19) Di Serio, M.; Tesser, R.; Dimiccoli, M.; Cammarota, F.; Nastasi, M.; Santacesaria, E. J. Mol. Catal. A: Chem. 2005, 239, 111–115. (20) Lu, H. F.; Liu, Y. Y.; Zhou, H.; Yang, Y.; Chen, M. Y.; Liang, B. Comput. Chem. Eng. 2009, 33, 1091–1096.
(21) Zhang, Y.; Dube, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 89, 1–16. (22) Yuan, X. Z.; Liu, J.; Zeng, J. M.; Shi, J. G.; Tong, J. Y.; Huang, G. H. Renew. Energy 2008, 33, 1678–1684. (23) Zullaikah, S.; Lai, C. C.; Vali, S. R.; Ju, Y. H. Bioresour. Technol. 2005, 96, 1889–1896.
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was 5.10 h. The oxidation stability of biodiesel could be improved via the addition of antioxidants.24
from the crude biodiesel by molecular distillation (MD) reached 98.32% ( 0.17% at an evaporator temperature of 120 °C. No significant change of composition of FAME in the biodiesel by MD was observed. Washing with water was proven to be an effective procedure to remove the catalyst (PFS) from the pretreated WCO by the measurement of ferric content. This two-step catalyzed process using a PFS catalyst and MD is probably a promising method for biodiesel production from WCO.
Conclusions Polyferric sulfate (PFS) as a catalyst showed a good activity to catalyze esterification of free fatty acid (FFA) of the waste cooking oil (WCO). More than 95% of the FFA in the WCO was converted to fatty acid methyl ester (FAME), under suitable conditions. The yield of crude biodiesel by a two-step catalyzed process (i.e., PFS-catalyzed esterification and alkalicatalyzed transesterification) was 95.08%. The yield of FAME
Acknowledgment. The financial support of Science and Technology Council of Guangdong Province under grant 2007B010800028 and Department of Education of Guangdong Province (under Grant No. cgzhzd0709) are gratefully acknowledged.
(24) Shao, P.; He, J. Z.; Sun, P. L.; Jiang, S. T. Biosyst. Eng. 2009, 102, 285–290.
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