WO2017012445A1

WO2017012445A1 - Method for preparing biodiesel

Info

Publication number: WO2017012445A1
Application number: PCT/CN2016/086458
Authority: WO
Inventors: 曹维峰; 仇苓艳; 赵连祥
Original assignee: 北京信汇生物能源科技有限公司
Priority date: 2015-07-22
Filing date: 2016-06-20
Publication date: 2017-01-26
Also published as: CN105001922A; CN105001922B

Abstract

A method for preparing biodiesel takes the form of a preceding enzyme reaction and a subsequent alkaline reaction. The enzyme reaction is mainly an esterification reaction, and the alkaline reaction is mainly a transesterification reaction. A crude ester product with an acid value of 0.5-2.5 mg KOH/g obtained from the step of the enzyme reaction is supplied to the step of the alkaline reaction. The product obtained from the alkaline reaction is separated and purified to obtain the biodiesel. This method is capable of producing high quality biodiesel with a lower production cost and a shorter production period and is suitable for large scale production applications.

Description

Method for preparing biodiesel

Technical field

The invention relates to a preparation method of biodiesel, in particular to a preparation method for producing high quality biodiesel at low cost.

Background technique

Biodiesel is a mixture of fatty acid esters or paraffins obtained by transesterification or hydrogenation of renewable raw materials such as rapeseed oil, soybean oil, recycled cooking oil, animal fats and microbial oils. It is a typical environmentally friendly “green energy. As an alternative to fossil diesel, biodiesel has high cetane number, no sulfur and aromatics, good engine low temperature start-up performance and better combustion performance than ordinary diesel. In 1983, American scientist GrahamQuick first successfully used methyl linoleate prepared by transesterification for the engine, and defined the fatty acid monoester obtained by transesterification of renewable oils into biodiesel. Subsequently, biodiesel achieved a long way. The development of biodiesel has a wide range of applications, and its raw material cost is low, but the processing cost is high. How to produce qualified biodiesel in a low cost manner has always been the goal pursued in the field of renewable energy, in which the acid value is biological. A very important indicator of diesel.

US 2014/0234922 A1 discloses a process for preparing a fatty acid ester comprising treating a mixture containing glycerides and free fatty acids, removing water and solid matter to obtain a treated material; introducing the treated material into a reactor, and at least with water, An enzyme is mixed with an alcohol to separate the reaction product into a glycerin phase and a fatty acid ester phase, and the fatty acid ester phase is treated with a primary alcohol and a flocculant. The method has high requirements on the quality of the oil and fat raw materials, otherwise the biodiesel whose acid value meets the requirements cannot be guaranteed.

CN1730613A discloses a method for biocatalyzing the transesterification of high acid value oils to produce biodiesel, which uses an alkaline substance such as tris (hydroxymethyl) aminomethane as an additive and uses high. The acid value oil is used as a raw material to carry out a transesterification reaction with a short-chain fatty acid ester under the catalysis of a biological enzyme, and after the reaction is completed, the reaction product is separated to obtain a finished biodiesel. The method can obtain biodiesel from cheap fats and oils, but cannot guarantee biodiesel with acid value.

CN102676304A discloses a method for preparing biodiesel, comprising: reacting oil, short-chain alcohol, water and liquid lipase in a primary or multi-stage enzyme reactor, and then separating the reaction liquid into an enzyme-containing heavy phase and Light phase, recycling and reuse of the enzyme in the heavy phase, light phase for subsequent immobilized enzyme conversion; light phase and short-chain alcohol flow into the primary or multi-stage enzyme reactor containing immobilized lipase, in the reaction Online dehydration is performed during the whole process or part of the reaction. The method obtains biodiesel with an acid value of less than 0.5 mgKOH/g through a liquid lipase reaction and a subsequent immobilized enzyme reaction, and has good economic benefits. However, since liquid lipase is easily deactivated and unsuitable for recycling, this will result in an increase in production cost. If liquid lipase is recovered, a large amount of equipment is required, which increases construction costs.

In summary, there is an urgent need for a method for preparing high quality biodiesel which can meet the requirements of acid value in a low cost manner.

Summary of the invention

In view of the deficiencies of the prior art, it is an object of the present invention to provide a method for preparing biodiesel which can obtain high quality biodiesel having an acid value meeting requirements and at a low production cost. According to the preferred technical solution of the present invention, the production method can be shortened by using the preparation method of the present invention, and is suitable for large-scale production applications.

The invention provides a preparation method of biodiesel, the method comprising the following steps:

Enzymatic reaction: esterification of a fat and a short-chain alcohol in the presence of an enzyme to obtain a first product, the oil and fat being a biological fat, the enzyme being a lipase, and the first product being subjected to the esterification The crude ester obtained after the reaction;

Alkaline reaction: transesterification of the first product and the short-chain alcohol in the presence of a basic substance Reacting to obtain a second product, the first product entering the alkali reaction step has an acid value of 0.5 to 2.5 mgKOH/g; and the second product is a crude ester obtained after the transesterification reaction, The acid value is less than 0.5 mgKOH/g;

Separation and purification: Biodiesel is obtained by separating and purifying the single-chain fatty acid ester contained in the second product.

According to the method of the present invention, preferably, the bio-fat is selected from the group consisting of waste oil, and the lipase is selected from the group consisting of Candida antarctica, Thermomyces lanuginosus, Aspergillus niger ( Aspergillus niger), Aspergillus oryzae, Rhizomucor miehei or Rhizopus oryzae lipase; the acid value of the first product entering the alkali reaction step is 1.0 2.5 mg KOH / g.

Preferably, in the enzymatic reaction, the lipase is an immobilized lipase, and the mass concentration of the short-chain alcohol in the enzymatic reaction system is maintained at 0.5 wt% to 2 wt%.

According to the method of the present invention, preferably, a short-chain alcohol is added during the enzymatic reaction, the enzymatic reaction is carried out in an airlift reaction system, and the airlift reaction system utilizes a cycle The gas is supplied without the exogenous gas, and the circulating gas is condensed and separated in the course of the enzymatic reaction.

According to the method of the present invention, preferably, the reaction temperature of the enzymatic reaction is 30 to 40 ° C, and the circulation gas from the airlift reaction system is maintained by a pumping, and the pumping speed is 1 to 5 vvm. The condensation temperature of the recycle gas is -10 to 10 °C.

According to the method of the present invention, preferably, during the enzymatic reaction, the recycle gas is dehydrated by absorbing the short-chain alcohol after condensing and separating the condensate.

According to the method of the present invention, preferably, the water-absorbing short-chain alcohol in the water-absorbing short-chain alcohol has a mass concentration of 95% by weight or more, and the temperature of the water-absorbing short-chain alcohol is controlled at 0 to 25 °C.

According to the method of the present invention, preferably, in the alkali reaction, the reaction temperature is controlled The temperature is 60 to 80 ° C, and the reaction is carried out for 1 to 3 hours. After the reaction is completed, the temperature is controlled to 110 to 120 ° C to ensure sufficient evaporation of the short-chain alcohol and residual water.

According to the method of the present invention, preferably, in the alkali reaction, the total amount of the alkaline substance to be added is equivalent to a dropwise addition of 0.001 to 0.02 kg KOH per liter of the first product, per liter. The total amount of short-chain alcohol added dropwise in the first product is 20 to 250 ml.

According to the method of the present invention, preferably, the single-chain fatty acid ester contained in the second product is separated and purified by statically layering the second product to separate the lower layer of glycerol, and the upper layer contains a single chain. The crude diesel product of the fatty acid ester is distilled to obtain biodiesel.

The preparation method of the present invention prepares biodiesel by an enzymatic method followed by an alkali method, and the crude ester product obtained by the enzymatic reaction step is supplied to the alkali reaction step. The preparation method fully utilizes the respective advantages of the enzymatic method and the alkali method for preparing biodiesel by controlling the reaction conditions and parameters of the two reaction steps. The lipase-catalyzed enzymatic reaction step can utilize waste oil with high acid value and industrial waste, and the reaction condition is mild, the energy consumption is low, and the amount of alcohol is small. The alkali reaction step has the advantages that the catalyst is relatively cheap and easy to obtain, the reaction time is short, and the conversion rate is high. The method is capable of producing low acid value bio-oil at a lower production cost and a shorter production cycle, and is very suitable for mass production applications.

detailed description

The present invention will be further described below in conjunction with specific embodiments, but the scope of protection of the present invention is not limited thereto.

The preparation method of the biodiesel of the invention comprises an enzymatic reaction step and an alkali reaction step, an enzymatic method followed by an alkali method, and finally a biodiesel is obtained by separation and purification.

The fat and short chain alcohol are subjected to an esterification reaction in the presence of an enzyme to obtain a first product. The oil and fat is a biological fat, the enzyme is a lipase, and the first product is subjected to the esterification. The crude ester obtained after the reaction. The esterification reaction is mainly a reaction of a fatty acid contained in the fat or oil with a fatty acid ester of the short-chain alcohol.

The bio-fat may include vegetable oil, animal fat, microbial oil, waste cooking oil or oil refining scrap; for example, castor oil, palm oil, rapeseed oil, soybean oil, peanut oil, corn oil, cottonseed oil, rice bran oil, Jatropha oil, wenwan fruit oil, jatropha oil, fish oil, butter, lard, sheep oil, yeast oil, microalgae oil, water oil, waste oil, or acidified oil. The preparation process of the invention is particularly suitable for the production of biodiesel from waste greases such as hydrophobic oils, waste oils, or acidified oils. The short-chain alcohol may be those short-chain alcohols known in the art, preferably an alkyl alcohol having 1 to 6 carbon atoms, more preferably an alkyl alcohol having 1 to 4 carbon atoms, such as methanol or ethanol. Further, propanol or butanol or the like is preferably methanol or ethanol, and methanol is particularly preferred. Short chain alcohols may be added during the enzymatic reaction. The lipase is a lipase capable of catalyzing an esterification reaction, preferably an immobilized lipase, and those known in the art may be used, for example, from Candida antarctica, Thermomyces lanuginosus. ), Aspergillus niger, Aspergillus oryzae, Rhizomucor miehei and Rhizopus oryzae lipase. The immobilization manner of the immobilized lipase may be those in the prior art, for example, adsorption, crosslinking, and embedding; the immobilization carrier may be various carriers in the prior art, and may be a resin, a gel, or the like. The material may, for example, be a macroporous acrylic resin, a silicate sol-gel matrix, a diatomaceous earth, a silica aerogel, a cotton film textile.

The mass concentration of the lipase in the enzymatic reaction system may be a conventional concentration of the biodiesel prepared by the enzymatic method in the prior art, preferably from 0.5% by weight to 2% by weight, more preferably from 1% by weight to 1.5% by weight.

The mass concentration of the short-chain alcohol in the enzymatic reaction system is maintained at 0.5 wt% to 2 wt%, preferably controlled at 0.6 wt% to 1.5 wt%, thereby ensuring sufficient esterification reaction. The concentration of short chain alcohol can be maintained by supplementation.

The enzymatic reaction is carried out in an airlift reaction system that utilizes a recycle gas without an exogenous gas, and condenses and separates the condensate from the recycle gas during the enzymatic reaction. .

The reaction temperature of the enzymatic reaction may be 30 to 40 ° C, preferably 33 to 38 ° C, the reaction time may be controlled to 6 to 10 hours, usually 8 to 9 hours, and the reaction time is controlled by the acid value of the first product. For reference, the acid value is 0.5 to 2.5 mgKOH/g, preferably 1 to 2.5 mgKOH/g, preferably less than 2.0 mgKOH/g, for example, 1.5 to 2.0 mgKOH/g.

Condensation of the recycle gas may employ various forms of condensers, such as those selected from the group consisting of partitioned, spray, jacketed, and tubular condensers. Low temperature water can be used as the coolant. The condensation temperature is controlled to be -10 to 10 ° C, preferably 5 to 8 ° C. The condensate produced after condensation of the recycle gas can be received by a receiving tank connected to the circulation line. The main components of the condensate include moisture and a portion of the short chain alcohol.

The circulating gas from the airlifting reaction system can be maintained by pumping, and the pumping power can be provided by a circulating fan, and the extracted circulating gas simultaneously carries water and short-chain alcohol from the reaction system. The circulating fan can use a centrifugal circulating fan or an axial circulating fan. The circulating fan draws air from the top of the enzymatic reaction system, so that the water generated by the enzymatic reaction of the circulating gas and part of the short-chain alcohol enter the circulating gas condenser for condensation. The pumping speed may be 1 to 5 vvm (vvm represents the amount of gas (m ³ ) introduced per minute per cubic meter of material volume), preferably 1 to 2 vvm.

During the enzymatic reaction, the recycle gas can be further dehydrated by absorbing the short-chain alcohol after condensing and separating the condensate. The sufficiently dry recycle gas is circulated from the bottom of the enzymatic reaction system, preferably through the first gas distributor, into the reaction system. The mass concentration of the water-absorbing short-chain alcohol is 95% by weight or more, preferably 98% by weight or more; and the temperature of the water-absorbing short-chain alcohol is controlled to be 0 to 25 ° C, preferably 10 to 20 ° C. Such a water-absorbing short-chain alcohol of a mass concentration and temperature can ensure that the moisture in the circulating gas is sufficiently removed. The type of the short-chain alcohol to be adsorbed is preferably the same as the alcohol used in the enzymatic reactor, and is preferably an alkyl alcohol having 1 to 6 carbon atoms. More preferably, it is an alkyl alcohol having a carbon number of 1-4, such as methanol, ethanol, propanol or butanol.

The recycle gas is preferably passed through a second gas distributor into a vessel containing the water-absorbing short-chain alcohol, and is sufficiently dehydrated by contact with the water-absorbing short-chain alcohol to improve gas-liquid mass transfer efficiency.

The alkali reaction mainly includes a transesterification reaction, and may also include a small amount of esterification reaction. The first product and the short-chain alcohol are subjected to transesterification in the presence of a basic substance to obtain a second product. The first product entering the alkali reaction step has an acid value of 0.5 to 2.5 mgKOH/g, preferably 1 to 2.5 mgKOH/g, preferably less than 2.0 mgKOH/g, for example, 1.5 to 2.0 mgKOH/g; The second product is a crude ester obtained after the transesterification reaction, and has an acid value of less than 0.5 mgKOH/g, preferably 0 to 0.4 mgKOH/g.

The short-chain alcohols may employ those short-chain alcohols known in the art, preferably an alkyl alcohol having 1 to 6 carbon atoms, more preferably an alkyl alcohol having 1 to 4 carbon atoms, such as methanol, ethanol, or C. The alcohol or butanol or the like is preferably a short-chain alcohol which is the same as the enzymatic reaction. The basic substance is a substance for catalyzing the reaction of the first product with a short-chain alcohol, and those known in the art may be used. For example, sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate are preferably sodium hydroxide or potassium hydroxide.

The transesterification reaction mainly comprises reacting a triglyceride contained in the first product with the short-chain alcohol to form a single-chain fatty acid ester and glycerol, and the reaction equation is as follows:

The product of the above reaction equation is the final product, and the following reaction steps are carried out during the reaction:

The temperature of the alkali reaction is controlled to be 60 to 80 ° C, preferably 70 to 75 ° C, to ensure sufficient reaction and reduce the acid value of the gas oil; the time of the alkali reaction is controlled to be 1 to 3 hours, preferably 2 to 2.5 hours. After the end of the reaction, the temperature in the alkali reactor is controlled to be 110 to 120 ° C, preferably 115 to 118 ° C, to ensure sufficient evaporation of the short-chain alcohol and residual water.

The gas coming out of the alkali reactor can be subjected to condensation treatment to separate a condensate containing condensed short-chain alcohol. The condensation here can also be carried out in various forms of the condenser as described above, and low temperature water can be used as the coolant.

In the alkali reaction, the total amount of the alkaline substance to be added may be equivalent to adding 0.001 to 0.02 kg of KOH per liter of the first product, preferably equivalent to dropping 0.01 per liter of the first product. ～0.015kg KOH, the “equal” described herein can be determined according to the catalytic efficiency of the different alkaline substances added, or can be determined simply by the amount of hydroxide produced. The total amount of short-chain alcohol added per liter of the first product may be from 20 to 250 ml, preferably from 50 to 160 ml. The "dropping" includes dropwise addition, and also includes addition, which is determined according to the total amount of the first product, the short-chain alcohol content in the reaction system, and the designed alkali reaction time.

Separating and purifying the single-chain fatty acid ester contained in the second product to obtain a living body Diesel products. The separation and purification method may employ a conventional step in the art, such as a distillation method. According to a preferred embodiment of the present invention, the second product is allowed to stand layering to separate the lower layer of glycerin, and the upper portion of the gas oil product containing the single-chain fatty acid ester is distilled to obtain biodiesel.

Example 1

The waste oil, methanol, and immobilized lipase were placed in an enzymatic reactor. The waste oil and fat contains waste water-repellent fat and oil, and the main component is a fatty acid and a fatty acid glyceride (acid value: about 18 mgKOH/g). The lipase of the immobilized lipase is derived from Candida antarctica, and the immobilization method is resin adsorption, and the carrier is a polystyrene macroporous adsorption resin. For each 100 liters of waste oil added, a total of 90 liters of methanol was added and 1300 grams of immobilized lipase was added. The enzymatic reaction temperature was controlled to 35 °C. The gas was pumped from the top of the enzymatic reactor at a rate of 2 vvm to allow the circulating gas in the enzymatic reactor to carry the water in the enzymatic reactor and part of the short-chain alcohol into the circulating gas condenser for condensation. The circulating gas condenser has a condensation temperature of 5 ° C and uses low temperature water as a coolant. The water and short-chain alcohol obtained by the condensation of the circulating gas are separated from the recycle gas and received by the condensate receiving tank. The condensed recycle gas enters a vessel containing a water-absorbing short-chain alcohol, and the short-chain alcohol is methanol, having a mass concentration of 95% by weight and a temperature of 10 °C. When the recycle gas enters, the water-absorbing short-chain alcohol further absorbs the water in the gas, allowing the recycle gas to be further dehydrated, and then entering the enzymatic reactor from the bottom of the enzymatic reactor. During the reaction, the amount of methanol in the enzymatic reactor will gradually decrease. At this time, methanol is added to the enzymatic reactor to effectively and quickly ensure that the mass concentration of methanol in the enzymatic reactor is about 1 wt%.

After 10 hours of enzymatic reaction, a first product (crude ester product) having an acid value of about 2 mg KOH/g is obtained, and the first product is sent to the alkali reactor, and the temperature in the alkali reactor is controlled at about 75 ° C by heating. Adding a mixture of sodium hydroxide and methanol to the alkali reactor, the total amount of sodium hydroxide added per 100 liters of the first product is 180 g, and the added methanol The total amount is 16 liters. After the end of the dropwise addition, the reaction time was 2.5 hours. After the end of the reaction, the temperature in the alkali reactor was raised to about 115 ° C to evaporate the short-chain alcohol. The gas evaporated from the alkali reactor was condensed by a condenser, the condensation temperature was 5 ° C, and low temperature water was used as a coolant. Condensation to give a short chain alcohol (which may contain residual water) is received by the alcohol receiving tank. The reaction product is allowed to stand for stratification, and the lower layer of glycerol is discharged, and the upper layer of the crude diesel product is subjected to rectification (distillation temperature of 200 ° C) to obtain high-quality biodiesel of low acid value. The main quality parameters of the biodiesel obtained in this example were measured according to the measurement method described in GB/T20828-2014 as follows: an acid value of 0.38 mgKOH/g, a cetane number of 52.1, and a fatty acid methyl ester content of 97.02% by weight.

Example 2

The waste oil, methanol, and immobilized lipase were placed in an enzymatic reactor. The waste oil and fat contains waste water-repellent oil and fat, and the main component is a fatty acid and a fatty acid glyceride (acid value: about 112 mgKOH/g). The lipase of the immobilized lipase is derived from Candida antarctica, and the immobilization method is resin adsorption, and the carrier is a polystyrene macroporous adsorption resin. The enzymatic reaction temperature was controlled to 35 °C. The gas was pumped from the top of the enzymatic reactor at a rate of 2 vvm to allow the circulating gas in the enzymatic reactor to carry the water in the enzymatic reactor and part of the short-chain alcohol into the circulating gas condenser for condensation. The circulating gas condenser has a condensation temperature of 5 ° C and uses low temperature water as a coolant. The water and short-chain alcohol obtained by the condensation of the circulating gas are separated from the recycle gas and received by the condensate receiving tank. The condensed recycle gas enters a vessel containing a water-absorbing short-chain alcohol, and the short-chain alcohol is methanol, having a mass concentration of 95% by weight and a temperature of 10 °C. When the recycle gas enters, the water-absorbing short-chain alcohol further absorbs the water in the gas, allowing the recycle gas to be further dehydrated, and then entering the enzymatic reactor from the bottom of the enzymatic reactor. During the reaction, the amount of methanol in the enzymatic reactor will gradually decrease. At this time, methanol is added to the enzymatic reactor to effectively and quickly ensure that the mass concentration of methanol in the enzymatic reactor is about 1 wt%.

After 10 hours of enzymatic reaction, a first product having an acid value of about 2 mg KOH/g was obtained (coarse The ester product), the first product is sent to the alkali reactor, and the temperature in the alkali reactor is controlled to be about 75 ° C by heating, and a mixture of sodium hydroxide and methanol is added dropwise to the alkali reactor for every 100 liters. The total amount of sodium hydroxide added to a product was 120 g, and the total amount of methanol added was 14 liters. After the end of the dropwise addition, the reaction time was 2.5 hours. After the end of the reaction, the temperature in the alkali reactor was raised to about 115 ° C to evaporate the short-chain alcohol. The gas evaporated from the alkali reactor was condensed by a condenser, the condensation temperature was 5 ° C, and low temperature water was used as a coolant. Condensation to give a short chain alcohol (which may contain residual water) is received by the alcohol receiving tank. The reaction product is allowed to stand for stratification, and the lower layer of glycerin is discharged, and the upper gas oil product is subjected to rectification (distillation temperature of 200 ° C) to obtain high-quality biodiesel of low acid value. The main quality parameters of the biodiesel obtained in this example were measured according to the measurement method described in GB/T20828-2014 as follows: an acid value of 0.31 mgKOH/g, a cetane number of 56, and a fatty acid methyl ester content of 97.9 wt%.

As can be seen from the above examples, the method of the present invention is capable of producing high quality bio-oil at a lower production cost and a shorter production cycle, and is very suitable for mass production applications.

The present invention is not limited to the above-described embodiments, and any variations, modifications, and alterations that may be conceived by those skilled in the art are intended to fall within the scope of the present invention without departing from the spirit of the invention.

Claims

A method for preparing biodiesel, comprising the steps of:

Enzymatic reaction: esterification of a fat and a short-chain alcohol in the presence of an enzyme to obtain a first product, the oil and fat being a biological fat, the enzyme being a lipase, and the first product being subjected to the esterification The crude ester obtained after the reaction;

Alkali reaction: the first product and the short-chain alcohol are subjected to transesterification in the presence of a basic substance to obtain a second product, and the acid value of the first product entering the alkali reaction step is 0.5 to 2.5 mg KOH. /g; the second product is the crude ester obtained after the transesterification reaction, the acid value is less than 0.5mgKOH / g;

Separation and purification: Biodiesel is obtained by separating and purifying the single-chain fatty acid ester contained in the second product.
The method for preparing biodiesel according to claim 1, wherein the bio-oil is selected from the group consisting of waste oil and fat, and the lipase is selected from the group consisting of Candida antarctica and Thermomyces. Thermomyces lanuginosus), Aspergillus niger, Aspergillus oryzae, Rhizomucor miehei or Rhizopus oryzae lipase; entering the first step of the alkaline reaction step The acid value of the product is 1.0 to 2.5 mgKOH/g.
The method for preparing a biodiesel according to claim 1, wherein in the enzymatic reaction, the lipase is an immobilized lipase, and the mass concentration of the short-chain alcohol in the enzymatic reaction system is maintained at 0.5 wt%. ~2wt%.
The method for producing biodiesel according to claim 1, wherein a short-chain alcohol is added during the enzymatic reaction, and the enzymatic reaction is carried out in an airlift reaction system, the gas The literate reaction system utilizes a recycle gas without an exogenous gas, and condenses and separates the condensate during the enzymatic reaction.
The method for preparing biodiesel according to claim 4, wherein the reaction temperature of the enzymatic reaction is 30 to 40 ° C, and the circulating gas from the airlift reaction system The circulation is maintained by suction, and the pumping speed is 1 to 5 vvm, and the circulating gas has a condensation temperature of -10 to 10 °C.
The method for producing biodiesel according to claim 4, characterized in that, in the course of the enzymatic reaction, the circulating gas is dehydrated by absorbing the short-chain alcohol after condensing and separating the condensate.
The method for preparing a biodiesel according to claim 6, wherein the water-absorbing short-chain alcohol has a water-absorbing short-chain alcohol concentration of 95% by weight or more, and the water-absorbing short-chain alcohol has a temperature of 0 to 25 °C. .
The method for preparing a biodiesel according to claim 1, wherein in the alkali reaction, the reaction temperature is controlled to 60 to 80 ° C, and the reaction is carried out for 1 to 3 hours. After the reaction is completed, the temperature is controlled to 110 to 120. °C to ensure sufficient evaporation of short-chain alcohol and residual water.
The method for preparing biodiesel according to claim 1, wherein in the alkali reaction, the total amount of the alkaline substance added is equivalent to 0.001 to 0.02 kg per liter of the first product. KOH, the total amount of short-chain alcohol added per liter of the first product is 20 to 250 ml.
The method for preparing biodiesel according to claim 1, wherein the method for separating and purifying the single-chain fatty acid ester contained in the second product is: placing the second product in a layered manner, and separating the lower layer. Glycerin, the crude gas oil product containing the single-chain fatty acid ester in the upper layer is distilled to obtain biodiesel.