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The orthogonal method was used to explore the optimal conditions for biodiesel production. Biodiesel can be used as a biofuel as well as an additive in diesel engine fuels. Currently, Europe and the United States mainly use vegetable oils as raw materials to produce biodiesel, while Japan produces biodiesel by recycling used cooking oil. Carrying out research and development on biodiesel in our country holds great significance and broad application prospects. 1 Experiment 1.1 Experimental Reagents and Instruments: Edible soybean oil (dried before use); methanol, NaOH, sulfuric acid, and other reagents are all of analytical grade. DHT type stirring constant-temperature heating mantle, LB801—2 type super thermostat, electronic balance, GB35—36 closed cup flash point tester for petroleum products, electric constant-temperature drying oven, WK—2B microcomputer-based comprehensive dynamic micromicrocoulometer, SYPIO08—III petroleum product freezing point tester. 1.2 Experimental Method: A certain amount of crude oil was added to a three-necked flask equipped with a thermometer, a condenser, and a thermocouple. After heating it to a specific temperature, a catalyst consisting of a mixture of NaOH and methanol, which had been preheated to the reaction temperature, was added under stirring. Timing began at this point. After the reaction proceeded for a certain period, the reaction mixture was poured into a separatory funnel for separation. The upper layer, which contained the methyl ester solution, was collected. Subsequent separation steps included distilling off methanol under atmospheric pressure, neutralizing the solution by adding an acid, washing it with an appropriate amount of water, drying it, and filtering it. The resulting light yellow, clear, and transparent product is biodiesel. Weigh the mass of the product obtained and calculate the biodiesel yield (actual biodiesel production / theoretical biodiesel production). The properties of the resulting product were analyzed and compared with the foreign DINV51.606 biodiesel standard as well as the specifications for domestic B diesel (Grade A, G-B25294). Issues received: 13, 2005–09–08 ； Revision date: 2006—03—30 Author profile: Liu Wenbao (born 1982), male, a master’s student majoring in chemical engineering. Corresponding author: Zhao Dezhi (1959), male, professor. 2 Experimental Results and Discussion 2.1 Orthogonal Experiment for Biodiesel Production An orthogonal table (4) was used for the experiments, with the various levels of the influencing factors listed in Table 1. The degree of reaction progress is expressed by the biodiesel yield (actual methyl ester production / theoretical methyl ester production), along with range analysis. The results are shown in Table 1. Table 1 Results of orthogonal experiments for biodiesel production and range analysis. Experiment: Methyl alcohol to oil ratio, temperature, time, catalyst; Biodiesel: Sequence number, molar ratio, °C, reaction time, catalyst dosage/%, yield/%. At each level of the selected experimental factors, the transesterification reaction proceeded well, and a distinct stratification occurred in the mixture after it was allowed to stand. Theoretically, from 100 g of vegetable oil, the mass of methyl ester produced is approximately 100.46 g, while the mass of glycerol produced is approximately 10.34 g. The methyl ester yield in this experiment ranged from 84.4% to 99.2%, indicating that the transesterification was essentially complete under the appropriate reaction conditions; the triglycerides, which are the main components of vegetable oils, were virtually completely converted into fatty acid methyl esters through this transesterification process. Based on the range analysis in Table 1, it can be concluded that for VIP Information http://www.cqvip.com, Liu Wenbao et al.: Investigation of optimal conditions for biodiesel production using orthogonal methods / 2006. The factors affecting the methylation reaction of soybean oil, in order of significance, are: the molar ratio of alcohol to oil, the amount of catalyst used, the reaction temperature, and the reaction time. The optimal reaction conditions are an alcohol-to-oil ratio of 6:1, a reaction temperature of 60°C, a reaction time of 90 minutes, and a catalyst amount of 0.8% of the amount of the feed oil. (1) The reactant ratio has the greatest impact on the biodiesel yield. The stoichiometric equation for the transesterification reaction is: triglyceride + 3 ROH → 3 RCOOR + glycerol; that is, 1 mol of oil reacts with 3 mol of methanol to produce 3 mol of fatty acid methyl esters and 1 mol of glycerol. Tests showed that when the molar ratio of alcohol to oil was 3:1, the reaction was incomplete, the conversion rate was low, and the content of methyl oleate in the final product was very low. The alcohol-to-oil molar ratios used in this experiment were all greater than 3:1. This is because the transesterification reaction is a reversible reaction, and increasing the amount of methanol facilitates the reaction proceeding in the forward direction. Therefore, the more methanol is used in the transesterification reaction, the more complete the conversion of vegetable oil. However, as can be seen from Table 1, when the amount of methanol used is too high, the methyl ester yield decreases as the amount of methanol increases. This is because an increase in the amount of alcohol leads to a decrease in the concentration of the fatty oils acting as reactants, thereby reducing the overall rate of the alcoholysis reaction. Moreover, **an excess of methanol not only makes it more difficult to separate methanol but also increases the costs associated with its recovery. Therefore, within the scope of this experimental study, the most suitable molar ratio of alcohol to oil is 6:1. (2) The amount of catalyst used has a significant impact on the transesterification reaction. As can be seen from Table 1, there is an optimal amount of catalyst; more is not necessarily better. Initially, as the amount of catalyst increases, the yield of biodiesel also increases, but once the catalyst amount exceeds a certain level, the yield of biodiesel starts to decline. This is because soybean oil contains a certain amount of free fatty acids. When the amount of catalyst used is low, it is only sufficient to neutralize the free fatty acids in the raw material, and thus it fails to act as a catalyst. By increasing the amount of catalyst appropriately, its catalytic effect becomes apparent, which facilitates the progress of the catalytic reaction. However, when an excess of catalyst is used, the excessive amount of base can cause saponification reactions, increasing the formation of latex-like substances in the reaction mixture. This raises the viscosity of the reactants and may even lead to the formation of a gel, thereby making it more difficult to separate the product. It also increases the amount of water required for the subsequent washing steps and raises the volume of alkaline waste discharged. Therefore, at the levels selected in this experiment, the optimal amount of catalyst is 0.8% of the mass of the feed oil. (3) The reaction temperature has a certain influence on the transesterification reaction. As can be seen from Table 1, the yield of biodiesel increases as the reaction temperature rises, but the magnitude of this increase varies. Initially, the increase in biodiesel yield is significant as the reaction temperature increases, but later on, the increase becomes very small. This is because the transesterification reaction rate is low at low temperatures; by appropriately increasing the reaction temperature, the transesterification rate can be enhanced, thereby increasing the biodiesel yield ； When the temperature reaches 40°C, the transesterification reaction essentially reaches equilibrium in a short time; increasing the reaction temperature further does not significantly improve the yield of biodiesel. Therefore, in order to save heating energy and reduce the costs associated with biodiesel production, the optimal temperature for the transesterification reaction at the levels considered in this experiment is 40°C. (4) Reaction time has a minor effect on the transesterification reaction: As can be seen from Table 1, reaction time has little impact on the biodiesel yield; the biodiesel yield increases as the reaction time prolongs. When the reaction time reaches 60 minutes, the transesterification reaction is essentially at equilibrium, and further increase in reaction time results only in a slight increase in biodiesel yield. Therefore, considering time savings and reduced production costs for biodiesel, the optimal reaction time for transesterification under the conditions used in this experiment is 60 minutes. In summary, by taking into account both the biodiesel yield and the production cost of biodiesel, the optimal process conditions for preparing soybean oil methyl ester are as follows: a molar ratio of alcohol to oil of 6:1, a reaction temperature of 40°C, a reaction time of 60 minutes, and a catalyst dosage of 0.8% of the mass of the raw oil. Under these optimized conditions, the yield of biodiesel can reach 99.2%. 2.2 Performance comparison The properties of the domestically produced biodiesel were compared with those specified in the foreign DINV51.606 biodiesel standard as well as with the standards for O-diesel in China; the results are shown in Table 2. Table 2 compares the performance parameters of the home-made biodiesel with those specified in the DIN V 1.606 biodiesel standard as well as the parameters for domestic No. 0 diesel. As can be seen from Table 2, the main performance parameters of the biodiesel (soybean oil methyl ester) basically meet the requirements of the foreign DIN V 1.606 biodiesel standard, and they are also close to the main performance parameters of domestic No. 0 diesel. This shows that the bio-diesel we have produced can be used as a substitute for diesel without any need to modify diesel engines. The most advantageous properties of biodiesel over diesel are its sulfur-free nature and the presence of intramolecular oxygen. Existing standards impose increasingly strict requirements on the sulfur content in diesel, which means that conventional mineral diesel must be desulfurized in order to meet these standards. Blending biodiesel with diesel can effectively address the issue of high sulfur content in diesel. Biodiesel contains intramolecular oxygen, which enables it to burn more completely and reduces the emission of pollutants. Biodiesel is superior to diesel fuel in terms of both combustion performance and environmental protection. It can be used directly as a substitute for diesel, or as an additive to diesel, thereby helping to make existing diesel meet increasingly stringent standards. 3 Conclusions (1) The influencing factors on the methylation reaction of soybean oil, in order of significance, are: the molar ratio of alcohol to oil, the amount of catalyst used, the reaction temperature, and the reaction time. (2) The optimal process conditions for preparing soybean oil methyl ester are: a molar ratio of alcohol to oil of 6:1, a reaction temperature of 40°C, a reaction time of 60 minutes, and a catalyst amount equal to 0.8% of the mass of the raw oil. (3) The main performance parameters of the self-produced biodiesel (soybean oil methyl ester) basically meet the international DIN V 51606 standards for biodiesel, and they are also similar to the main performance parameters of domestic diesel fuel. (4) Biodiesel is sulfur-free and can be used as an additive to conventional diesel, enabling existing mineral diesel to more easily meet the increasingly stringent requirements regarding sulfur content in fuels.