Research Status of Biofuel Production via Supercritical Methanol Method

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Current Research Status on the Production of Biodiesel Using Supercritical Methanol Method. The supercritical methanol method for producing biodiesel is a new process that utilizes vegetable oils, animal fats, or waste oils to generate biodiesel. Its main advantage is that no additional catalyst is required; supercritical methanol serves both as the reaction medium and as the reactant along with the catalyst. Compared to chemical methods, it has many advantages in terms of reaction speed, requirements for raw materials, and product recovery. The author provides a review of the influencing factors and mechanisms involved in the preparation of biodiesel using the supercritical methanol method. Factors affecting the production of biodiesel using the supercritical methanol method 1.1 Temperature Ayhanl1 compared the experimental results of producing biodiesel by reacting supercritical methanol with hazelnut oil at different temperatures, using a batch reactor. The results show that in supercritical methanol, the reaction rate between fats and methanol is very fast, whereas in subcritical methanol, the reaction rate between fats and methanol is slower. Sakayjc—dee@mail．tsinghua．edu．cn. Determination of Flavonoids in Rabdosia lophantoides by UV and VIS Spectrophotometry by IA Yu, Wu Zhiyi, Wu Chuan, Ma Baojuan. and Wu Fenglan (1. School of Chemistry and Chemical Engineering, Sun Yat—Sen University, Guangzhou 510275, Guangdong, China ； 2. Department of Chemical Engineering, GuangDong Industry Technical College, Guangzhou 510300, Guangdong, China) Abstract: The total flavonoid content in Rabdosia lophanthoides was determined using VIS and UV spectrophotometry, with rutin as the reference sample. In VIS spectrophotometric analysis, Al(NO3)3 was used as the reagent, and measurements were carried out at 510 nm. In UV spectrophotometric analysis, measurements were performed at 359 nm. Comparison of the two methods showed that UV spectrophotometry is a better approach for determining the total flavonoid content in Rabdosia lophanthoides. Keywords: Rabdosia lophanthoides ； flavonoid ； VIS spectrophotometry ； UV spectrophotometry VIP Information http://www.cqvip.com 54 Fine Petrochemicals January 2006 The transesterification reaction rate for linolenic acid esters is the fastest; this is due to the instability of methyl linolenate at high temperatures. When the temperature is above 400°C, the biodiesel produced contains a higher level of methyl esters of unsaturated fatty acids (methyl linoleate and methyl linolenate), while the content of methyl oleate is lower. Therefore, when studying and determining the reaction temperature for biodiesel production, it is essential to pay attention to the thermal stability of the various components in the raw materials. Saka et al. studied the esterification reactions of five common free fatty acids found in vegetable oils—palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid—in supercritical methanol (above 350°C). The conversion rates of unsaturated fatty acid oils at 350, 400, and 450°C are close to 100%, but at 500°C, the conversion rate of oleic acid begins to decrease ； Similarly, the conversion rate of linoleic acid is also high at 350 and 400°C; at 500°C, the decomposition reaction becomes dominant ； The optimal reaction temperature for linolenic acid is 350°C; as the temperature increases, the conversion rate decreases. This is mainly because unsaturated fatty acids and their methyl esters have very low thermal stability; in particular, as the number of unsaturated bonds in the molecular chain increases, their stability decreases significantly. As for saturated fatty acids, the optimal conversion temperature for palmitic acid is 400–450 °C; at 500 °C, methyl palmitate begins to degrade ； The conversion rate of stearic acid at 450 °C is 98, and no significant degradation occurs even at 500 °C. 1.2 Free fatty acids: Generally, plant oils contain small amounts of free fatty acids; for example, soybean oil contains 0.5%, fish oil contains 10%, while canola oil contains 2 9/6%. When producing biodiesel using conventional acid-base catalysts, free acids tend to undergo saponification reactions with the base, which reduces the catalyst’s activity; therefore, it is necessary to ensure the purity of the raw materials, with their acid content having to be kept below 0.5 (with an acid value of less than 1)_4]. In supercritical methanol, the presence of free fatty acids has little effect on the reaction conversion rate, so there is no need to remove them. The author believes that the main reason is the absence of a base in the reaction system, which prevents saponification from occurring _5]. 1.3 Water: In traditional acid-base catalyzed processes for biodiesel production, a non-aqueous medium is generally used as the reaction medium. Ma and others believe that the water content should be below 0.06; otherwise, the hydrolysis of esters will severely affect the conversion rate. Using the supercritical methanol method, the presence of acid and water has no effect on the final conversion rate _7]. KusdianaE and Saka compared three processes for preparing bio-oil: alkaline catalysis, acid catalysis, and the effect of water in supercritical methanol on the reaction. In the alkali-catalyzed process, when the water content increases to 2.5, the conversion rate drops from 97 to 80 9/6 ； In acid-catalyzed processes, 0.1% water already leads to a decrease in conversion rate; when the water content increases to 5%, the conversion rate of fats and oils is only 6% ； In the supercritical methanol process, even when the water content reaches 50%, the methyl ester conversion rate remains above 98%. They speculate that in the process of producing biodiesel from rapeseed oil, it is not merely esterification and transesterification reactions that occur; especially when there is a high water content, hydrolysis reactions may also take place. That is, while triglycerides undergo transesterification, they also react with water through hydrolysis, and the fatty acids resulting from this hydrolysis then react with methanol to form fatty acid methyl esters. Therefore, water has no effect on the overall conversion rate of the oils. Moreover, when the water content is high, subsequent process operations become simpler; the by-product glycerol dissolves easily in water and accumulates in the lower phase of the produced liquid ； However, at lower water contents, due to the high abundance of methanol, glycerol and the methanol phase accumulate in the upper phase; thus, methanol must be distilled off to recover glycerol. 1.4 Effect of alcohol-to-oil ratio In the transesterification reaction, the theoretical molar ratio of alcohol to oil is 3:1 ； In practice, methanol serves both as a reactant and as a reaction medium, and the molar ratio of alcohol to oil is often **higher than the theoretical value. Studies by Kusdiana et al. [c] show that when the molar ratio of alcohol to oil is 42:1, the methylester conversion rate reaches over 95% after 4 minutes of reaction. When the alcohol-to-oil molar ratio drops to 21 ； At 1 hour, the methyl ester conversion rate after 4 minutes of reaction was only 80.9/6. When the alcohol-to-oil ratio was further reduced to 6:1, the conversion rate of fatty acid methyl esters was only 40%. Although excess methanol facilitates the methylation reaction, it must be recovered and reused, which **increases the energy consumption and costs associated with separation. 2 Reaction mechanism: Diasakovl9 believes that the transesterification reaction is a three-step process. First, triglycerides (TG) are transesterified with methanol to form fatty acid methyl esters and diglycerides (DG). Then, DG is transesterified with methanol to form monoglycerides (MG). Finally, MG undergoes transesterification with methanol to produce glycerol (GL) and fatty acid methyl esters. TG+ CH 3OH kl R lCOOCH3+DG 2 DG+ CH 3OH R2COOCH 3+MG MG+ CH3OH COOCH3+GI In this model, there are a total of 6 reaction rate parameters, making the calculations rather complex. Saka simplified the reaction model, considering only the transesterification reaction of triglycerides with methanol to produce glycerol and fatty acid methyl esters. Two parameters are involved in this reaction, making the calculation simple. At 200, 230, 350. The reaction rates of C are 0.0002, 0.0003, and 0.0178 s_, respectively. . TG+ 3 CH3OH k ^ 7 3 RCOOCH 3+GI VIP Information http://www.cqvip.com Volume 23, Issue 1 Sun Shiyao et al. Current Research Status on the Preparation of Biodiesel using Supercritical Methanol 55 Madras et al. . The activation energy for producing biodiesel in supercritical methanol was calculated to be approximately 3 kJ/mol. Kusdiana et al. consider such transesterification reactions to be nucleophilic reactions. First, the triglyceride vibrates due to uneven electron distribution, causing the carbon atom at the carbonyl group to exhibit a positive charge while the oxygen atom exhibits a negative charge. Meanwhile, the oxygen atom in methanol attacks the positively charged carbon atom, forming a reaction intermediate. Then, the hydrogen atom from this intermediate alcohol transfers to the oxygen atom in the alkyl group of the triglyceride, resulting in a second reaction intermediate; ultimately, an ester exchange product is formed, as shown in Figure 1. @0 I ④C ／·＼ RR．Icco oR～～ ‘ f Nuo Yi H，c／ ＼ H H ’ ／ 0 I CH3 RtCOOCH3 + ItOH – Figure 1: The reaction mechanism for producing biodiesel in supercritical methanol, as proposed by Kusdiana et al. 3 Comparison between the supercritical methanol method and acid-base catalysis methods for biodiesel production. The process flows for producing biodiesel using traditional acid-base catalysis and the supercritical methanol method are shown in Figures 2 and 3 respectively. Figure 2 shows the process for producing biodiesel using traditional methods. Table 1 presents the comparison results between them. It can be seen that, compared with acid and base catalytic methods as well as enzymatic methods, the supercritical methanol method has the following advantages: 1) No catalyst is required, resulting in less environmental pollution ； 2) Low requirements are placed on the raw materials; moisture and free acids have little adverse effect on the reaction, and no pretreatment of the raw materials is necessary ； 3) The reaction rate **increases**, and the reaction time **decreases** ； 4) Simple downstream processing of the product ； 5) It is easy to achieve continuous production. However, the use of supercritical methanol to produce biodiesel also has significant drawbacks: 1) The reaction conditions are severe, involving high temperature and pressure, which increases the investment required for the reaction system equipment ； 2) The alcohol-to-oil ratio in the reaction is too high, resulting in a large amount of methanol that needs to be recycled. Chimney, methyl fatty acid ester, j-oil mixture; let it stand for 30 milliliters, then evaporate the upper layer; repeat the evaporation process. Figure 3 shows the process for producing biodiesel using supercritical methanol. Table 1 provides a comparison of various methods for producing biodiesel. In order to reduce the operating temperature required in the supercritical methanol method for biodiesel production, some researchers have proposed using a two-step process: hydrolysis followed by esterification. In the first step, triglycerides are hydrolyzed into fatty acids in subcritical water; after the water is removed, the resulting fatty acids are esterified into methyl fatty acid esters in supercritical methanol. Hydrolyzing triglycerides in subcritical water at 270°C takes 20 hours, while at 300°C it takes 12 hours; the triglycerides can be completely hydrolyzed into fatty acids ； Then, at 270 °C, with a molar ratio of fatty acid to methanol of 1:42, the reaction was carried out for 10 hours; the conversion rate of the fatty acid exceeded 98%. The two-step method has certain advantages over the one-step method, but it undoubtedly increases the complexity of the process. 4 Conclusion In summary, among various biodiesel production processes, the supercritical methanol method is undoubtedly highly competitive. In our country, biodiesel is only just beginning to be used as a substitute for diesel. Comprehensive technical and economic evaluations should be conducted on various production methods, and sufficient attention should be paid to the use of supercritical methanol in the production of biodiesel.