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Introduction to the biodiesel production process: Biodiesel refers to fatty acid monoesters produced by using animal or plant oils, or the fatty acids obtained from their hydrolysis, together with monohydric alcohols through saponification or esterification. It can replace petrochemical diesel and is a renewable, clean biofuel. The main component of commercial biodiesel is fatty acid methyl esters. The production methods of biodiesel include physical methods, chemical methods, physicochemical methods, and biological methods, among others. However, the chemical method is the one that is currently actually used in industrial production and holds practical value; this article focuses only on the production process using the chemical method. The process of producing biodiesel by chemical methods mainly includes: ① transesterification, namely the saponification of fats/oils or the esterification of fatty acids ； ② Distillation recovery of methanol ； ③Distillation purification of crude fatty acid methyl ester ； ④Purification of the by-product glycerol, etc. Its core process is transesterification. 1 Raw materials for producing biodiesel 1.1 Animal and plant oils Considering the cost of raw materials, the animal and plant oils currently used for biodiesel production are mainly waste oils or degraded oils, such as oil from sewers, oil from food waste, rancid oils, etc. In addition, there are also some non-edible oils from wild plants, such as castor oil. 1.2 Fatty acids Fatty acids mainly originate from by-products produced during the production of animal and vegetable oils, such as crude fatty acids obtained during physical acid removal, and acidified oils produced from hydrated oil residues or alkali-processed soap residues. 2 Principles and Processes of Biodiesel Production 2.1 Production Principles Depending on the raw materials used in biodiesel production, the principles behind its production also vary. Date of receipt: 2006–03–27 Author profile: Luo Xiaolan (1964–), female, associate professor ； Mainly engaged in research and teaching in the field of oil engineering technology. Different. The principle behind producing biodiesel from animal and plant fats and oils is the alcoholysis reaction between glycerol and methanol ； When fatty acids are used as the raw material, an esterification reaction occurs between the fatty acids and methanol. 2.2 Production Process 2.2.1 Process for producing biodiesel from oils with low acid value 2.2.1.1 Process flow For oils whose acid value is below 4 (it is generally advisable that it be no higher than 2), the following process can be used: Methanol, basic catalyst → Refined fatty acid methyl ester → Oil – Pretreatment – Alcoholysis → Crude fatty acid methyl ester → Distillation → Residual liquid (repeat alcoholysis) → Mixed liquid → Condensation → Crude methanol → Distillation → Methanol (for reuse) → Crude glycerol → Refining → Refined glycerol 2.2.1.2 Process conditions The process conditions for alcoholysis and the distillation of crude fatty acid methyl ester are mainly discussed here. (1) Alkylation process conditions. Catalyst: Basic catalysts can be used, as well as acidic catalysts. Alkaline catalysts are preferred, as they allow for complete alcoholysis at lower temperatures and in shorter times, thereby avoiding undesirable effects such as the volatilization of lower fatty acids at high temperatures and the oxidation of unsaturated fatty acids. Commonly used basic catalysts include: ① Sodium methoxide, which offers the best results; the amount used is 0.1% to 1% of the oil’s weight ； ② sodium hydroxide ； ③ Potassium hydroxide. A molar ratio of alcohol to oil of 7:1 is reasonable; increasing the amount of methanol further has no significant effect, and the reaction is carried out at atmospheric pressure. Theoretically, applying pressure facilitates the reaction, but it increases equipment costs and complicates the process; therefore, operating at atmospheric pressure is recommended. The reaction temperature is 64°C in the absence of a methanol reflux system (it cannot exceed the boiling point of methanol, which is 64.5°C) ； It is 65°C when a methanol reflux system is available; it is recommended to install such a system. Thorough stirring is required to ensure full contact between the fatty acids, methanol, and catalyst. The methyl ester conversion rate can reach 95%. (2) Process conditions for the purification of crude fatty acid methyl esters. Process method: Vacuum distillation ； Operating residual pressure: ≤3 kPa ； Distillation temperature: Two different fractions were obtained at 248 and 262°C respectively; the former is biodiesel, with a methyl ester content of over 99%. Source: VIP Information, http://www.cqvip.com, Volume 31, Issue 9, 2006, China Oils and Fats, page 47. The latter are unreacted fatty acids that can be methylated again. 2.2.2 Process for producing biodiesel from oils with high acid values. Oils with high acid values can, in principle, also be used to produce biodiesel using the aforementioned process if they are not pre-deacidified first. However, due to their high content of fatty acids, alkaline catalysts are not suitable; instead, acidic catalysts should be used. Because fatty acids turn into stable carboxylate ions when exposed to alkali, this slows down the reaction and results in incomplete methylation. The use of acidic catalysts for the production of biodiesel results in a slow reaction rate; higher temperatures and longer reaction times are required. Moreover, acidic catalysts are not suitable when epoxycarboxylic acids, conjugated acids, or hydroxyacrylic acids are present. For this purpose, the following processes can be used: Methanol; Methanol, basic catalyst; Oils – extraction; Oil – dehydration – hydrolysis – washing – drying; Rough fatty acid methyl ester. Solvent phase: methanol; Acidic catalyst; Methanol, basic catalyst; Methanol. Evaporation to remove the solvent – acid-catalyzed pre-esterification – drying – base-catalyzed hydrolysis – rough fatty acid methyl ester. Methanol mixture is produced in both the hydrolysis and pre-esterification steps, and crude glycerin is also generated after hydrolysis. The recovery of methanol, as well as the purification of crude glycerol and fatty acid methyl esters, follow the same procedures as described in 2.2.1, so they will not be repeated here. Taking an oil with an acid value of around 10 as an example, after methanol extraction, the majority of the fatty acids move into the solvent phase, reducing the acid value of the oil phase to below 2. After dehydration, alcoholysis can be carried out using a basic catalyst to obtain fatty acid methyl esters. The methanol phase, which contains a large amount of fatty acids, can be used to obtain oils with an acid value of over 60 after evaporation and solvent removal. Acid-catalyzed pre-esterification can be employed to esterify the fatty acids in this phase into methyl esters, after which the remaining oil can be hydrolyzed under basic catalysis to yield fatty acid methyl esters. Since water is generated during the acid-catalyzed pre-esterification process, drying and dehydration should be carried out after pre-esterification to avoid affecting the alcoholysis effect. 2.2.3 Process for producing biodiesel from crude fatty acids: Methanol, acidic catalyst – Crude fatty acids → Drying → Decolorization and filtration → Esterification → Crude fatty acid methyl ester → Distillation → Refined fatty acid methyl ester. Fatty acids (for reuse). Esterification conditions: The catalyst used is an acidic catalyst. Commonly used ones include: ① Concentrated sulfuric acid (the amount used varies from 1% to 20%, depending on the raw materials and reaction conditions) ； ②Methanol-saturated solution of HC1 gas (5% concentration, + + l + + + + + i ·+ One + -+ -+ -+ -+ -+ “+ “+ -+ -+ -+ approximately as reported in the magazine ‘China Oils’) + -+ “+ -+ -+ -+ ； ③BF methanol solution (concentration of 12% to 14%). Since basic catalysts can react with fatty acids, methylation cannot use basic catalysts. Alkyd volume-to-weight ratio: (1–1.2):1. The greater the amount of methanol used, the more complete the reaction, but the recovery costs after the reaction also increase. Practice has shown that when the volume weight ratio of alkyd to methanol exceeds 1.2:1, increasing the amount of methanol further yields little effect; a ratio of (1–1.2):1 is generally more appropriate. The process conditions such as reaction pressure, temperature, and stirring are the same as those for the alcoholysis of fats. Furthermore, since water is generated during the esterification reaction, a water removal device should be installed in the reaction system. In the absence of a water removal device, a water-absorbing desiccant can be added to the system. Only by ensuring that the methanol concentration remains at 95% or higher can the reaction proceed fully and effectively. 3 Economic feasibility of biodiesel production: With ongoing increases in oil prices, the current market price of diesel has reached over 5,000 per ton. The market prices for swill oil and low-quality beef and mutton fat are both around 2,000 per ton, while the overall cost of producing biodiesel from them remains below 3,500 per ton. If biodiesel is produced from oil refinery by-products, the overall cost is lower. It is evident that producing biodiesel from these raw materials is economically viable. 4 Standards for biodiesel: Countries such as Australia, Germany, France, Italy, the United States, and Sweden introduced standards for biodiesel one after another after the 1990s. At present, there are no standards for biodiesel in our country. To meet the needs of biodiesel research and production, it is recommended that the relevant authorities promptly establish **or industry standards for biodiesel. Before the introduction of biodiesel standards in our country, the relevant standards mentioned above can be referred to during the research and production of biodiesel.