Production of biodiesel by supercritical method

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Production of biodiesel by supercritical method 0 Introduction As petroleum resources become increasingly depleted and the trend toward using diesel in vehicles accelerates, countries are stepping up efforts to develop alternative petrochemical fuels. Biodiesel can serve as a substitute for petrochemical diesel due to its properties, which are very similar to those of the latter. Currently, the alkaline catalysis method is predominantly used in industry for the production of biodiesel, and various studies have provided detailed descriptions of the processes and reaction conditions for different types of feedstocks using this method. The so-called supercritical state refers to a condition in which, when the temperature exceeds its critical temperature, it becomes impossible to distinguish between the gas and liquid states; as a result, the substance is in a fluid state that will not condense under any pressure applied. Supercritical fluids possess properties different from those of gases or liquids; their density is close to that of a liquid, their viscosity is close to that of a gas, while their thermal conductivity and diffusion coefficients lie between those of gases and liquids. Due to its low viscosity, high density, and high diffusion capacity, it enables and facilitates the simultaneous occurrence of extraction and reaction. 1 Reaction principle The ester exchange reaction refers to the process in which fats and oils undergo alcoholysis with various short-chain alcohols in the presence of a catalyst. Taking methanol as an example, its main reaction equations are as follows: CH2O0ORl → R1GCI()CCH~OH + 3CH3R2. . ∞|{3+Midday H0H C H2CEK)R3 R3CAX3CH3 H ,OH Draft date: 2005—08—29} ； Author: An Wenjie (born 1979), male, master’s degree. Phone: 13521902783; Email: a219@126.com. The fatty acid methyl ester produced as a result of this reaction is known as biodiesel. According to Suppes et al., this reaction actually consists of 3 steps during the transesterification process, occurring in the following sequence: triglyceride + CH3OH → diglyceride + R1COOCH3 → diglyceride + CH2OH → monoglyceride + R2COOH → monoglyceride + CH3OH ； == Glycerol + R3C()00{3 2 Experimental Section 2.1 Main raw materials: soybean crude oil (provided by Shijiazhuang Refining Plant), edible soybean oil (available commercially), water, methanol (analytical grade), and oleic acid (all provided by Beijing Beihua Fine Chemicals Co., Ltd.). 2.2 Main instruments for the experiment: high-pressure flow reactor, distillation tower, vacuum pump, etc. 2.3 Analysis Method: Gas chromatography is used to determine the methyl ester content. Model 1102 chromatograph, hydrogen flame ionization detector ； Stainless steel packed column of 03ram, 2m long, OV-17 stationary phase ； The carrier gas is high-purity nitrogen, the fuel gas is high-purity hydrogen, and compressed air is used as an assist gas ； The set values for the chromatography operating parameters are: furnace temperature of 250°C, detector temperature of 320°C, injector temperature of 300°C, sensitivity of 8, attenuation (ATT) of 0, and injection volume of 0.1 L. Quantification is performed using the internal standard method, with methyl undecanoate as the internal standard. 2.4 Bio-diesel production process: Preheat the reactor to the set temperature, adjust the pressure control valve to achieve the desired pressure, then start feeding material using a pump. Collect reaction samples at regular intervals. Distill off the methanol in the methyl ester phase under conditions of 90–100°C. After allowing it to stand for about 30 minutes, remove the glycerol phase, and take samples from the methyl ester phase after methanol distillation for analysis. Finally, the methyl ester phase is distilled under appropriate temperature and vacuum conditions; the resulting fractions are washed with water and dried to yield biodiesel. VIP Information: http://www.cqvip.com Issue 2, An Wenjie et al.: Preparation of Biodiesel by Supercritical Method, page 21. Oil products. 3 Results and Discussion 3.1 Effect of temperature on the conversion rate of the transesterification reaction The critical temperature and pressure for methanol are 239.4°C and 8.09 MPa, respectively. To determine the influence of temperature on the transesterification reaction, the alcohol-to-oil ratio and pressure were set at 15:1 and 15 MPa, respectively. The experimental results over the temperature range of 250–400°C are shown in Figure 1. Fig. 1 Effect of temperature on the conversion of transesterification reaction. Fig. 1 shows that, within a certain temperature range, the conversion rate of the transesterification reaction increases as both temperature and reaction time increase. Under 250°C and lh reaction conditions, only 74.3% of the fats and oils were converted into methyl esters, which may be related to the stability of the supercritical state of methanol under these conditions. Further increases in temperature gradually increase the solubility of the oil in methanol, thereby accelerating the reaction rate and conversion rate, which then slowly approach equilibrium. After reacting with LH, 95.3% and 97.2% of the oils were converted into methyl esters at 300°C and 350°C, respectively. However, when the temperature reaches 400*, the reaction conversion rate shows a downward trend as reaction time increases, indicating that fats can undergo side reactions such as thermal cracking and coking at this temperature. Therefore, the temperature for supercritical transesterification reactions should generally not exceed 400°C. Taking actual production into account, a reaction time of 300°C and 1 hour should be the more suitable conditions. 3.2 Effect of the molar ratio of methanol to oils on the conversion rate of the transesterification reaction The molar ratio of the feed oil to the alcohol is one of the important factors affecting the transesterification reaction. In terms of the stoichiometric ratio, 3 moles of methanol are required for 1 mole of oil; however, since this reaction is reversible, the principle of chemical equilibrium shift dictates that an excess of methanol must be added to drive the reaction toward the desired product. Figure 2 shows the relationship between the alcohol-to-oil ratio and the transesterification conversion rate under 15 MPa and lh reaction conditions. Fig. 2 Effect of the molar ratio of methanol to oil on the conversion rate of the transesterification reaction. As can be seen from Fig. 2, an increase in the alcohol-to-oil ratio can effectively improve the conversion rate of the transesterification reaction. Under otherwise identical conditions, an increase in the alcohol-to-oil ratio not only raises the collision frequency between oil molecules and methanol molecules, but also increases the concentration of the reactants, thereby further promoting the reaction toward the target product. At 300*, when the alcohol-to-oil ratio increases from 3:1 to 40:1, the methyl ester content rises from 68.2% to 94.2%. However, from the perspective of cost investment, too much alcohol is uneconomical. Because excessive alcohol not only affects the efficiency of the reactor but also increases the recovery costs. Therefore, a more appropriate alcohol-to-oil ratio is around 15:1, as the methyl ester content shows a gentle upward trend as the alcohol-to-oil ratio increases in that range. 3.3 Effect of alcohols with different carbon chains on the conversion rate of transesterification. Generally speaking, short-chain alcohols with a carbon chain length of 1–8, such as methanol, ethanol, propanol, butanol, and isobutanol, are more suitable for producing biodiesel. However, due to reasons such as low cost and easy recycling, currently, except for Brazil which uses ethanol, **almost all other places use methanol to produce biodiesel. Figure 3 shows the relationship between alcohols with different carbon chains and fats under supercritical conditions, as well as their conversion into fatty acid methyl esters. Figure 3 shows that under the same conditions, as the carbon chain length increases, the conversion rate of the ester exchange reaction gradually decreases. Under the reaction conditions of 300* (3, 15 MPa, an alcohol-to-oil ratio of 15:1), methanol, ethanol, n-propanol, and n-butanol underwent transesterification reactions, with 97.2%, 94.7%, 90.2%, and 78.1% of them being effectively converted into corresponding esters respectively. Vip Information: http://www.cqvip.com. Natural Gas Chemistry, Volume 31, 2006. Fig. 3 Effect of different alcohol chains on the conversion rate in transesterification reactions. Fig. 3 Effect of different alcohols on the conversion rate of transesterification reactions. It can be seen that shorter carbon chains are more conducive to the progress of the transesterification reaction. 3.4 The effect of water on the production of biodiesel via the supercritical methanol method. The water content in the feedstock is an important factor affecting the production of biodiesel using acid and base catalysis. To this end, traditional acid-base catalysis methods generally require the raw materials to be anhydrous and have an acid value of less than 1 mgKOH/g. To compare the effect of water on the supercritical method, acid-catalyzed method, and base-catalyzed method, this experiment used edible soybean oil, with water added to methanol at percentages of 2%, 4%, 6%, 10%, and 20% of the oil’s mass; the experimental results are shown in Figure 4. Among them, for the acid and base catalysis methods, the catalysts used and their amounts are H2SO4 (3%) and KOH (1.2%), respectively; the reaction conditions for the supercritical method are 300°C and 15 MPa, with an alcohol-to-oil ratio of 15:1, and the reaction time is 1 hour. Figure 4 shows that the effect of water on the acid-catalyzed method is greater than that on the base-catalyzed method, mainly because water can significantly reduce the activity of the H2SO4 catalyst. On the contrary, the supercritical rule is hardly affected by the water content in the oil; when the water content in the oil increases from 2% to 20%, the effective percentage of conversion of the oil into methyl esters decreases only from 90.7% to 88.9%. This is because oils can undergo rapid hydrolysis at temperatures above 200°C, and water under such conditions acts as an acidic catalyst, allowing the hydrolysis of oils, transesterification reactions, and esterification reactions to take place simultaneously in this system. Therefore, the presence of a certain amount of water has no significant effect on the production of biodiesel using the supercritical method. However, an excess of water causes some of the saturated fatty acids produced by hydrolysis under these conditions to fail to be esterified, resulting in a high acid value in the final biodiesel. 3.5 The effect of free fatty acids on biodiesel production via supercritical methanol method Similarly, the content of free fatty acids in the oil feedstock also has a significant impact on biodiesel production using acid and base catalysis methods. As can be seen from Figure 5, alkali catalysts are more affected by the free fatty acid content than the alkali catalysis method, as they can undergo saponification reactions with free fatty acids. Although the acid-catalyzed method can enable the esterification of fatty acids and alcohols, the water produced by the reaction can reduce its activity, thereby lowering the conversion rate. To determine the effect of free fatty acids on the supercritical methanol method, experiments were conducted by adding oleic acid to the oil at concentrations of 2%, 4%, 6%, 10%, and 20% of the oil’s mass, with the results shown in Figure 5. Figure 5 shows the percentage of free fatty acids in the oil-based wipes. Figure 5 also illustrates the effect of free fatty acids on the conversion rate of the transesterification reaction. Figure 4 depicts the effect of water on the conversion rate of the transesterification reaction. Figure 5 indicates that the production of biodiesel using supercritical methods is not affected by acids; moreover, the effect of water on the transesterification reaction is minimal. . . . . . . ’. . ’’’’ ‘ ‘One’ one’ ’ ’～ . ’. ’ \"Black Love Shake\" VIP Information: http://www.cqvip.com Issue 2. An Wenjie et al.: Preparation of biodiesel by supercritical method; the quality fraction of oils that can be effectively converted into methyl esters also increases as the oleic acid content rises. As the content of free fatty acids in the oil increases from 2% to 20%, the methyl ester content rises from 91.2% to 93.3% respectively. This is partly because unsaturated fatty acids are more soluble in methanol than fats ； On the other hand, because methanol and free fatty acids can ionize some H+ under these conditions, the esterification reaction can occur more efficiently. Furthermore, according to Saka, free fatty acids exhibit higher esterification activity at 350*3. Moreover, the esterification rate must be faster than the transesterification rate. 4 Conclusions (1) The relatively reasonable process conditions for producing biodiesel using the supercritical methanol method are 300°C, 15 MPa, a alcohol-to-oil molar ratio of 15, and a reaction time of 1 hour. (2) The shorter the carbon chain of the alcohol, the more favorable it is for the transesterification reaction to proceed. (3) A certain amount of water and free fatty acids have no significant effect on the biodiesel production process using supercritical methanol. However, excessive water causes some of the saturated fatty acids produced through hydrolysis to not be esterified at 300°C, resulting in a higher acid value of the final biodiesel. (4) The bio-diesel product obtained after vacuum distillation, washing, and drying is very close in standards to similar products in the United States. Table 1 shows a comparison of the performance of the biodiesel produced in the experiments with some key parameters of the American biodiesel standards.