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Current technologies for biodiesel production

At present, biodiesel is mainly produced by chemical method, that is, low-carbon alcohols such as animal and vegetable oils and methanol or ethanol are transesterified (transesterified) under acid or alkaline catalysts and high temperature (230-250 C). The corresponding fatty acid methyl ester or ethyl ester is produced, and biodiesel is obtained after washing and drying. The production equipment is the same as the general oil-making equipment, and about 10% glycerol can be produced by-product in the production process.

At present, there are several main process methods:

Alkali catalysis

- acid catalysis

Lipase or Bioenzymatic Method

Supercritical extraction

1. Alkali catalysis: using sodium hydroxide or potassium hydroxide as catalyst, which is the most commonly used preparation method at present, vegetable oils and methanol are transesterified (cross-esterified) and sodium hydroxide (1% of the weight of oil) or sodium methoxide (Sodium methoxide) as catalyst, which can be prepared by mixing and stirring reaction for about 2 hours. Get biodiesel.

2. Acid catalysis method: Because waste oil usually contains a large amount of free fatty acids, and can not be converted into biodiesel by alkaline catalyst, the raw materials of high free fatty acids are pretreated with concentrated sulfuric acid or phosphoric acid as acid catalyst to convert FFA into esters. Then triglyceride was transesterified by alkaline catalyst. The disadvantage of acid catalytic process is that FFA reacts with alcohols to produce water, which inhibits the esterification of FFA and the transesterification of glycerol. The material can be dehydrated and dehydrated after esterification.

In China's current national conditions and current oil prices, it is not realistic to use food-grade oils as raw materials to produce biodiesel, and the cost of food waste oils and some industrial oils is relatively low. However, these waste oils usually contain high levels of free fatty acids, so acid catalysis method should be used first for these waste oils, and then transesterification reaction through alkaline catalysts. Alkali catalysis and acid catalysis are also called chemical methods.

3. Lipase or bio-enzymatic method: chemical synthesis of biodiesel has the following shortcomings: complex process, excessive alcohol, follow-up process must have the corresponding alcohol recovery device, high energy consumption; deep color, because unsaturated fatty acids in fat are easy to deteriorate at high temperature; esterification products are difficult to recover, high cost; production process has waste alkali. Liquid discharge.

In order to solve the above problems, people began to study the synthesis of biodiesel by enzymatic method, that is, the transesterification of animal fat and low-carbon alcohols through lipase to prepare corresponding fatty acid methyl esters and ethyl esters. Enzymatic synthesis of biodiesel has the advantages of mild conditions, low alcohol consumption and pollution-free emissions. However, the main problems at present are: the conversion rate of methanol and ethanol is low, generally only 40%-60%. Because lipase is effective in esterification or transesterification of long-chain aliphatic alcohols at present, the conversion rate of short-chain aliphatic alcohols such as methanol or ethanol is low. Moreover, short-chain alcohols have certain toxicity to enzymes, and the service life of enzymes is short. By-product glycerol and water are difficult to recover, which not only inhibits the formation of products, but also makes glycerol toxic to immobilized enzymes, thus shortening the life of immobilized enzymes. Bio-enzymatic technology can not reach the practical level of industrialization.

4. Supercritical extraction: Supercritical extraction is a lipid exchange reaction with high methanol to oil ratio (42:1) under supercritical conditions (350 - 400 degree C and 1200 psi pressure). Its reaction time is fast and can be completed in 4 minutes. But the operation cost is high and the energy consumption is high. Supercritical extraction has the advantage of not using catalysts and eliminating the process of dissolution and separation of catalysts.

In addition, the non-catalytic co-solvent method, which uses co-solvent tetrahydrofuran to solubilize methanol, is a fast method. It only takes 5-10 minutes, and the reaction conditions are mild. Because no catalyst is needed, no catalyst is removed in the finished product and by-product glycerol. But tetrahydrofuran is expensive and drug-intensive. In order to prevent the leakage of tetrahydrofuran, the requirement for equipment is high.

It is worth mentioning that the new method of biodiesel production published by American scientists is a microprocessor only half the size of a credit card. Methanol and raw oil enter the microprocessor through a pipeline thinner than the hair silk. The raw material will be transformed into biodiesel immediately. It is 10-100 times faster than the traditional method of production, and it is not. Use any chemical catalyst. The reaction conditions are mild and the energy consumption is greatly reduced. According to the scientists concerned, it will be a revolutionary innovation and may overturn all existing production processes.