What is the production principle of biodiesel?

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There are two methods of production, physical synthesis and chemical synthesis.

1. Chemical synthesis of biodiesel:

The chemical synthesis method is to transesterify low-carbon alcohols (mainly methanol or ethanol) with natural oils and fats. After transesterification, long-chain fatty acid esters of low-carbon alcohols can be obtained. The relative molecular weight of the esters can be reduced to about 300, which is close to the relative molecular weight of diesel oil. The physical and chemical properties of the esters are close to that of diesel oil. Under mild conditions, the role of catalysts is very important for transesterification.

In the absence of water, the transesterification activity of alkaline catalysts is usually higher than that of acid catalysts. The traditional production process uses alkali metal hydroxides with high solubility in methanol as homogeneous catalysts. Their catalytic activity is related to their basicity. Typical conditions for transesterification using KOH as catalyst are as follows: methanol dosage is 8%~21%, KOH dosage is 0.3%~1%, reaction temperature is 35~60 C, and NaOH as catalyst usually needs to react at 60 C to obtain the corresponding reaction rate.

Alkali catalyst can not be used in the case of high free acid, the presence of free acid will poison the catalyst. When free fatty acids are contained in oils and fats, free fatty acids are esterified with methanol to form fatty acid methyl esters.

That is to say, the reaction of R-COOH_R-COOCH3+H2O is suitable for acid as catalyst, and free fatty acid is easy to produce soap by alkali reaction when base is used as catalyst, that is, R-COOH+NaOH(KOH)-R-COOHa(K)+H2O. As a result, the reaction system becomes more complex. Soap acts as an emulsifier in the reaction system. The product glycerol may emulsify with fatty acid methyl ester and can not be separated. Water is often also an alkali catalyst. The presence of water can also promote the hydrolysis of oil and alkali to form soap. Therefore, when KOH, NaOH or sodium methoxide (potassium) are used as catalysts, the acid value of feed oil is often required to be less than 1 mg KOH/g and the moisture content is less than 0.06%. Oils containing water or free fatty acids can be esterified twice.


Acids can be directly used as catalysts for oils with higher free fatty acids, such as recycled waste oils (drainage oils, etc.). Using acid as catalyst also needs to limit the water content, usually less than 0.5%. Because water will be produced in the esterification of free fatty acids, the catalytic effect of acid catalyst will also be reduced.

Methanol or ethanol can be recycled in the production process, and the production equipment is basically similar to the general equipment for oil production. In the production process, about 5%~10% glycerol can be produced as by-product.

2. Physical production methods of biodiesel:

Physical methods include direct mixing and microemulsification. Its principle is to mix vegetable oil with petrochemical diesel oil and improver in proportion, but long-term use will lead to adverse effects such as carbon deposition in valve, so the diesel produced by physical method can not be called qualified biodiesel.

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