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The Rise of Biodiesel | biodiesel reactor

By WHGCM September 28th, 2023 94 views
Since 2020, global energy is getting tighter and environmental problems are getting more and more serious. Biofuels, with their outstanding environmental friendliness and renewability, have become the hotspot of energy research in countries all over the world, and have been applied by more and more countries to develop biodiesel. The demand for biodiesel is increasing. The contrast between demand and production will be the formation of a situation where the supply of products exceeds the demand.

What is Biodiesel?

Biodiesel is a high-quality clean diesel fuel that can be extracted from various biomass, so it is an inexhaustible energy source.
Biodiesel is a renewable diesel fuel that can replace fossil diesel. A transesterification process makes it with oilseed crops, wild oilseed plants, and engineered microalgae, as well as other aquatic plant oils & fats, animal fats and oils, and catering garbage oils as the feedstock oil.
Biodiesel is biomass energy, a monoalkyl ester of long-chain fatty acid obtained from biomass using thermal cracking and other technologies. Biodiesel is a mixture of complex organic components with very high oxygen content. These mixtures are mainly organic substances with high molecular weights, including almost all kinds of oxygen-containing organic substances, such as ethers, esters, aldehydes, ketones, phenols, organic acids, alcohols, and so on.
Biodiesel is a clean and renewable energy source. It is a high-quality substitute for petroleum diesel made from oilseed crops such as soybean and rapeseed, oilseed fruits such as oil palm and yellow wood, oilseed aquatic plants such as engineered microalgae, as well as animal fats and oils, and waste food and beverage oils, etc. Biodiesel is a typical "green energy" source. Biodiesel is a typical "green energy", vigorously developing biodiesel on the sustainable development of the economy, promoting energy substitution, reducing environmental pressure, control of urban air pollution is of great strategic significance.

Biodiesel Applications

Biodiesel is purchased mainly by refineries of oil, power plants, ship and shipping companies, and middlemen in the distribution sector.
The application is very promising as it is used as a fuel for boilers, turbines, and diesel engines. Biodiesel is also considered to be a more sustainable fuel that can be used for a variety of purposes such as transportation, home heating, and power generation.
History and Status of Biodiesel Development in Different Countries:
Country Start time Commercial time Raw material Development status(Stars)
U.S.A 1970s  1990s Algae biodiesel  5
EU 1980s 1990s Rapeseed, sunflower, and soybeans  4
Japan 1990s 1990s Waste cooking oil 3
Brazil 2000s 2000s Palm oil Castor, soybean, palm, cottonseed, sunflower, corn, 4
Malaysia 2000s 2010s Palm oil 4
China 2000s 2010s Waste oils and fats Low-carbon alcohol esters of fatty acids (mainly methyl esters) M 3


Diesel vs biodiesel

(1) Excellent environmental characteristics  

a. Biodiesel is an environmentally friendly biomass fuel prepared from unprocessed or used vegetable oils and animal fats through different chemical reactions.
b, biodiesel and fossil diesel compared to low sulfur content, the use of sulfur dioxide and sulfide emissions can be greatly reduced. Authoritative data show that sulfur dioxide and sulfide emissions can be reduced by about 30%. Biodiesel does not contain aromatic compounds that cause pollution to the environment, and the damage to the human body caused by combustion exhaust is lower than that of fossil diesel, while it has good biodegradation characteristics. Compared with fossil diesel, the emissions of toxic organic compounds in the exhaust of diesel vehicles are only 10%, particulate matter is 20%, the emissions of carbon dioxide and carbon monoxide are only 10%, and the emissions of exhaust indicators can reach the European Ⅱ and Ⅲ emission standards.
According to the U.S. Department of Energy, B20 blends can reduce greenhouse gas emissions from diesel engines by 15 percent. A life-cycle analysis of biodiesel emissions (which considers all aspects of fuel production and consumption) found that biodiesel can reduce GHG emissions by 40 to 69 percent compared to petroleum diesel, depending on the source of the biomass used to make the biodiesel. Biodiesel from renewable sources (e.g., used cooking oil) reached an average level. Higher reductions are as high as 86 percent.
Several studies have shown that biodiesel has environmental advantages over petroleum diesel. In general, the higher the percentage of biodiesel in a blend, the cleaner the fuel.

(2) Low Temperature Startability  

Compared with diesel, biodiesel has good low-temperature engine starting performance, with a cold filtration point of -20℃.

(3) Impact on diesel engine 

No corrosion on the engine and oil circuit, no coking on the nozzle, and no carbon accumulation in the combustion chamber.
It has good lubrication performance, which reduces the wear rate of the injection pump, engine block, and connecting rod.
It can reduce the friction loss of the engine oil supply system and cylinder liner, increase the service life of the engine, and thus indirectly reduce the cost of the engine.

(4) Safety  

The flash point of biodiesel is higher than that of fossil diesel, and it is not a hazardous fuel, which has obvious advantages in transportation, storage, and use.

(5) Combustion performance  

The calorific value of ordinary diesel is 35.5MJ/L and the cetane number is 50 (standard);
The calorific value of biodiesel is 32.4-36.7MJ/L, and the cetane number is 52-70.
Biodiesel has a higher cetane number than diesel, and the fuel has a better combustion resistance to storms when in use, so a higher compression ratio engine can be used to improve its thermal efficiency.
Although the calorific value of biodiesel is lower than diesel, because the oxygen element contained in biodiesel can promote the combustion of fuel, and ordinary diesel fuel mixed with more complete combustion, higher thermal efficiency, so the mixture can be used to obtain good power, can achieve the maximum power of the use of diesel fuel, and have good overload characteristics.

(6) Renewability  

Biodiesel is a renewable energy, diesel is a fossil energy and is not renewable.

(7) Economy  

The use of biodiesel system investment is small, the original use of diesel engines, refueling equipment, storage equipment, and maintenance equipment do not need to change.

(8) Reconcilability  

Biodiesel can be used with fossil diesel according to a certain ratio, which can reduce fuel consumption, improve power and reduce exhaust pollution.

(9) Biodegradability  

Biodiesel has good biodegradability and is easily decomposed and utilized by microorganisms in the environment.

(10) Convenience

No need to change the diesel engine can be added directly to use, and no need to add refueling equipment, storage equipment, and personnel of special technical training.

How is biodiesel made?

Biodiesel is made by separating glycerin (the glycerin present in soap, toothpaste, and other products) from its biomass source, leaving behind the methyl ester.
Pure biodiesel can only be used in its pure form after a diesel engine conversion, so biodiesel is usually blended with petroleum diesel for transportation.
Common blends of biodiesel and petroleum diesel contain anywhere from 2% to 20% biodiesel, labeled by the percentage of biodiesel, such as B2 or B20.In the U.S., B20 is available for most diesel engines and is usually blended with B20.
Biodiesel can also be blended with heating oil for home heating. Biodiesel production is usually categorized into three generations d
epending on where it comes from.
The first generation uses industrial food crops.
The second generation produces biodiesel from biomass residues (e.g. residues), non-edible crops, and wastes, such as restaurant grease. 
The third generation refers mainly to algal biodiesel.
Biodiesel is using a variety of animal and plant fats and oils as raw materials, using hydrogenation technology, catalyst system, and process technology to produce diesel obtained.
Experimentally, it is known that biodiesel mixed with petroleum diesel, uses of better results, than existing biodiesel and petroleum diesel in the mix, most of the biodiesel and petroleum diesel are directly into the mixing tanks, and then mixing, the mixing process poured into the additives, due to the additives used to pour the way to refill.

Chemical reaction of biodiesel

1、Esterification reaction

The esterification reaction is a kind of organic chemical reaction, which is the reaction between alcohol and carboxylic acid or oxygen-containing inorganic acid to produce ester and water. It is divided into two categories: the reaction between carboxylic acid and alcohol and the reaction between strong inorganic acid and alcohol. The esterification reaction between carboxylic acid and alcohol is reversible, and the reaction is generally very slow, so concentrated sulfuric acid is often used as a catalyst. Multiple carboxylic acids react with alcohol to produce a variety of esters. The reaction of strong inorganic acids with alcohols is generally faster. Typical esterification reactions include the reaction of ethanol and acetic acid to produce ethyl acetate with an aromatic odor, which is a raw material for the manufacture of dyes and pharmaceuticals. Esterification reactions are widely used in fields such as organic synthesis. In an esterification reaction, there is a series of reversible equilibrium reaction steps.
As shown in Fig.
1, step ② is the control step for esterification, while step ④ is the control step for ester hydrolysis. This reaction is an SN2 reaction (bimolecular nucleophilic substitution reaction), which goes through addition and elimination. The schematic reaction formula is shown below. 
schematic reaction formula
Figure 1 
The mechanism of the esterification reaction using isotopic labeling of alcohols confirms that the water produced in the esterification reaction is derived from the hydroxyl group of the carboxylic acid and the hydrogen of the alcohol. However, the esterification of carboxylic acids with tertiary alcohols occurs when the alcohol undergoes an alkoxide bond breakage, with a carbon-positive ion being generated in the middle. In the esterification reaction, the alcohol acts as a nucleophilic reagent for nucleophilic attack on the carbonyl group of the carboxylic acid, and in the presence of the protonated acid, the carbonyl carbon is favored for nucleophilic addition to it by the alcohol because of its lack of electrons. Without the presence of acid, the esterification reaction of acid with alcohol is difficult to carry out.
The above reaction can be described by the following kinetic equation
kinetic equation
Where r is the rate of esterification and K is the rate constant,
the general formula for K is:
 general formula for K 
The general formula for the esterification reaction can be described as follows:
formula for the esterification reaction
where R is the carbon chain of the fatty acid and R is the alkyl group of the alcohol. From the general formula of esterification reaction, 1 mol of fatty acid reacts with 1 mol of alcohol to form 1 mol of ester and 1 mol of water. Since the carboxylic acid produces water as a by-product in the reaction with the alcohol, in the reaction system, diluting the concentration of the reactant alcohol, the alcohol concentration is a very important factor to maintain the rate of the reaction. A decrease in the concentration of the alcohol results in a significant decrease in the yield of the product ester and a sharp decrease in the reaction rate, which greatly prolongs the reaction time. In the process of making biodiesel, in order to accelerate the reaction rate, in addition to the use of certain catalysts, but also to maintain a high concentration of alcohol, which needs to continuously remove the by-product product water, so that the transesterification reaction in the positive direction, thereby increasing the yield of biodiesel.


2、Ester exchange reaction

Ester exchange reaction, that is, the ester and alcohol in the acid or base catalyzed by the generation of a new ester and a new alcohol reaction, that is, the ester of alcoholysis reaction. The reaction is reversible, in the solution of the ester, there is a small amount of free alcohol and acid present. It is on the basis of the reversibility of the esterification reaction that the ester exchange reaction takes place. The alcohol in the transesterification reaction is able to esterify with the small amount of free acid in the ester solution, and the new esterification reaction produces a new ester and a new alcohol. Due to the reversibility of the esterification reaction, for the transesterification reaction to proceed, at least one of the following two conditions must be met: first, the new ester produced is more stable than the previous ester; second, the new alcohol produced can be continuously evaporated during the reaction. Direct transesterification of oil and methanol is the most important biodiesel production method, and the total reaction formula of the transesterification reaction is shown in Fig. 3.
The complete transesterification of triglycerides to produce glycerol and fatty acid methyl esters requires a three-step reaction, i.e., the reaction of triglycerides and methanol. The first step produces diglycerides and fatty acid methyl esters; the second step is the continued reaction of diglycerides with methanol to produce monoglycerides and fatty acid methyl esters; and the third step is the reaction of monoglycerides with methanol to produce glycerol and fatty acid methyl esters.
formula of the transesterification reaction
Figure 3 
Catalytic transesterification chemical reaction after transesterification to obtain long-chain fatty acids low carbon alcohol esters, molecular weight is close to the molecular weight of diesel fuel, physical and chemical properties are also close to diesel fuel, and fuel performance with diesel fuel is not much different. From the reaction equation of the transesterification reaction, it can be seen that in the ideal state, 1mol triglyceride reacts with 3mol alcohol to form 3mol ester and 1mol glycerol. If sodium hydroxide or potassium hydroxide is used as a catalyst to produce biodiesel, under improper control of the reaction conditions, there is a side reaction in the transesterification reaction, which is a saponification reaction in which the ester in the product reacts with the base. From the point of view of the complexity of the reaction, the transesterification reaction between fats and methanol is relatively simple, without the generation of water, through the above transesterification reaction can make the molecular weight of triglyceride drop to 1/3 of the original, the viscosity is reduced by 1/8, and at the same time, it also improves the volatility of the fuel. The viscosity of the biodiesel produced is close to the industrial demand and the cetane number reaches above 50. The transesterification reaction usually requires the addition of a catalyst to facilitate the reaction. Catalysts include alkaline catalysts, acid catalysts, and bio-enzyme catalysts. Among them, basic catalysts include alcohol-soluble catalysts (e.g. NaOH, KOH, NaOCH 3, organic bases, etc.) and various solid base catalysts; acid catalysts include alcohol-soluble catalysts (e.g. sulfuric acid, sulfonic acid, etc.) and various solid acid catalysts.

2.1 Acid-Catalyzed Transesterification

The reaction mechanism of acid-catalyzed transesterification: a proton first binds to the carbonyl group of a triglyceride to form a carbon cation intermediate. The photophilic methanol binds to the carbon cation and forms a tetrahedral intermediate, which then decomposes into methyl ester and diglyceride and produces a proton to catalyze the next round of reactions. Glycerol diesters and glycerol monoesters also react according to this process. Acid catalysts can process high acid value feedstocks compared to base catalysis because, in the presence of an acid catalyst, free fatty acids esterify with methanol to form methyl esters. Acid catalysts are therefore well suited for the processing of high acid-value fats and oils. In addition, for the esterification of long-chain or branched-chain fatty alcohols with fats and oils, acid catalysts are generally used. However, the reaction rate of acid-catalyzed transesterification is very slow and requires relatively high reaction temperatures and alcohol-oil ratios. In acid-catalyzed reactions, if the reaction temperature is high, there may be side reactions that produce by-products such as dimethyl ether and glycerol ether. In addition, in acid catalysis, the effect of water on the catalyst activity is very significant. It is reported that in the sulfuric acid-catalyzed transesterification reaction between soybean oil and methanol if 0.5% water is added to soybean oil, the transesterification conversion rate drops from 95% to 90%. If 5% water is added, the conversion rate is only 5.6%. The carbon cation generated in the transesterification process is easy to react with water to generate carbonic acid, thus reducing the biodiesel yield. When the content of free fatty acids in the oil should pay attention to this problem, because the acidic catalyst will catalyze the esterification of free fatty acids with methanol, which will produce a certain amount of water, affecting the reaction process, and it is difficult to achieve a satisfactory conversion rate in one step of the transesterification reaction. When using high acid value oils and fats such as waste oils and fats as raw materials, in order to avoid the influence of the water produced, the industry often adopts the method of dehydrating while reacting or adopts intermittent operation to remove the water and then replenish methanol to continue the reaction. In industrial applications, the most commonly used acid catalysts are concentrated sulfuric acid and sulfonic acid or mixtures thereof. Compared with the two, sulfuric acid is cheaper and more absorbent, which is conducive to the removal of water generated by the esterification reaction, with the disadvantage of being corrosive and more likely to react with the carbon-carbon double bond, resulting in a darker color of the product. The catalytic activity of the sulfonic acid catalyst is weaker than sulfuric acid, but it creates fewer problems in the production process and does not attack the carbon-carbon double bond. Strongly acidic cation exchange resins and phosphates are two typical acidic solid acid catalysts for ester exchange, but they both require relatively high reaction temperatures and long reaction times, and the conversion of ester exchange is relatively low and the instructions for use are short, thus limiting industrial applications. Other solid acid catalysts such as zirconium sulfate, tin sulfate, zirconium oxide, and zirconium tungstate have also been studied. In addition, according to a report in the November 2005 issue of Nature, solid acid catalysts are being developed at the Tokyo Institute of Technology in Japan for the production of solid acid catalysts from natural organic materials, such as sugars. The preparation method is to carry out incomplete carbonization of organic materials such as glucose and sucrose at low temperatures (>300°C), and then carry out a sulfonation reaction to introduce sulfonic acid groups to obtain sulfonated amorphous carbon catalysts. This catalyst is characterized by the cheap price, high esterification activity, and long service life, but no report has been found for transesterification reaction. At present, the acid-catalyzed transesterification process is seldom used in foreign biodiesel-generating units. Acid catalysts are mainly used for pre esterification of fats and oils with high acid values, followed by base-catalyzed transesterification. Existing biodiesel plants in China are mainly based on high acid value waste oils and fats as feedstock, small in scale, and most of the catalysts used are liquid acids, while a few are developed to use solid acids. The use of solid acid catalysts for the pre-esterification of high acid value vegetable oils followed by base-catalyzed transesterification for biodiesel preparation is a better process route.

2.2 Alkali-catalyzed transesterification

In alkali-catalyzed transesterification reactions, it is the methoxide anion that is really active, as shown in Figure 4.
Mechanism of base-catalyzed transesterification reaction
 Figure 4- Mechanism of base-catalyzed transesterification reaction
The methoxide anion attacks the carbonyl carbon atom of the triglyceride, forming an intermediate with a tetrahedral structure, which then breaks down into a fatty acid methyl ester and a diglyceride anion, which reacts with methanol to form a methoxide anion and a diglyceride molecule, which is further converted into a mono glycerol ester, which is then converted into glycerol. The generated methoxide anion is recycled again for the next catalytic reaction. Basic catalysts are by far the most widely used catalysts for transesterification reactions. The advantages of using alkaline catalysts are mild reaction conditions and fast reaction rates. Some scholars estimate that the rate of transesterification using a base catalyst is 4000 times higher than that using the same equivalent acid catalyst. The amount of methanol used in base-catalyzed transesterification reactions is much lower than that of acid-catalyzed ones, so industrial reactors can be greatly reduced. In addition, alkaline catalysts are much less corrosive than acid catalysts and inexpensive carbon steel reactors can be used in industry. In addition to the above advantages, the use of alkaline catalysts has the following disadvantages: alkaline catalysts are more sensitive to free fatty acids, so the acid value of the grease feedstock is more demanding. Raw materials with high acid value, such as some waste oils and fats, need to be deacidified or pre-esterified before alkaline-catalyzed transesterification can be carried out. There are two main types of industrialized alkali-catalyzed transesterification reactions: liquid-phase reactions catalyzed by KOH, NaOH, NaOCH 3, etc., which are easily soluble in methanol, and multiphase reactions catalyzed by solid bases. At present, most of the biodiesel industrial production units use liquid-phase catalysts, and the dosage is 0.5% to 2.0% of the oil mass. There is also a difference between sodium methanol and sodium (or potassium) hydroxide when used as a transesterification catalyst. When using sodium methanol as a catalyst, the raw material must be strictly refined, and a small amount of free water or fatty acids affect the catalytic activity of sodium methanol, foreign processes require that the content of both not more than 0.1%; but the product of soap content is very small, which is conducive to the separation of glycerol settling and improve the yield of biodiesel. Sodium hydroxide (or potassium) as a catalyst for the raw material requirements are relatively not strict, the raw material can contain a small amount of water and free fatty acids, but this will lead to the generation of more fatty soaps, affecting the rate of glycerol settlement and separation, and at the same time will lead to the glycerol phase of the dissolution of more methyl esters, thereby reducing the yield of biodiesel. Generally speaking, using sodium hydroxide (or potassium) as the catalyst, the acid value of the grease feedstock should not exceed 2mgKOH/g, and the dosage of the catalyst should be 0.5%-2.0% of the grease mass. Even if the acid value of the raw material is higher than 2mgKOH/g, theoretically, sodium hydroxide (or potassium) catalyst can still be used, but it is necessary to add an excessive amount of catalyst to neutralize the free fatty acids. This condition is not suitable because of the high soap production, the difficulty of glycerol settling and separation, and the high amount of dissolved methyl ester in the glycerol phase.
For sodium hydroxide and potassium hydroxide, there are differences when used as transesterification catalysts:
① During the sedimentation separation of the crude product, the catalyst is mainly present in the glycerol phase. Since the molecular weight of KOH is larger than that of NaOH, it increases the density of the glycerol phase and accelerates the separation of the glycerol phase.
② When KOH is used as the catalyst, the amount of soap generated is less than when NaOH is used, which reduces the dissolution of methyl ester in the glycerol phase.
③ Using KOH as a catalyst, the product can be neutralized with phosphoric acid to produce potassium dihydrogen phosphate, which is a high-quality fertilizer that not only reduces waste emissions but also increases economic benefits. Compared with sodium salt, which can only be disposed of as waste, the advantage of NaOH as a catalyst is that it is inexpensive.
In addition to this, organic alkali catalysts, such as amines, are being developed both domestically and internationally. When using organic amine as catalyst, after 6~10h reaction under normal pressure and low temperature, it can reach a relatively high conversion rate, but the content of glycerol monoester and diester in the product is very high, while the amount of glycerol is very low, which is difficult to be applied industrially; when the reaction pressure and temperature are increased, it is possible to generate amide in the process of reaction, which can reduce the quality of products.
Therefore, it is still necessary to do a lot of research work to prove the feasibility of using organic bases as ester exchange catalysts. Solid base catalysts are being industrialized in recent years. Compared with liquid alkali catalysts, the use of solid catalysts can greatly improve the purity of the glycerol phase, reduce the cost of glycerol refining, less the emission of "three wastes", the product does not contain soap, and improve the yield of biodiesel; however, the reaction rate is slow, requiring higher temperature and pressure, higher alcohol-oil ratio, and more sensitive to free fatty acids and water, and the raw materials need to be strictly refined. The Esterfip-H process developed by the French Petroleum Institute is the first biodiesel-generating process in which a solid base as the catalyst is successfully applied to industrial production, and its catalyst is a bimetallic oxide with spinel structure, which has already been built into a 160,000 tons/year generating unit.
In addition, the Ruhr University in Bochum, Germany, has also developed a solid base catalyst, which is a metal complex of amino acids and catalyzes the transesterification reaction at a temperature of 125℃, which is higher than that of the liquid base catalyst (around 60℃). An industrial demonstration plant of 1t/h will be constructed. Japan is developing strongly basic anionic resin catalysts and has made great progress. However, the anionic resin can only be operated at low temperatures (below 60°C), otherwise, it will be deactivated quickly, and the ester exchange activity is relatively low at low temperatures, thus limiting its industrial application. Since the resin is easy to regenerate, it has certain industrialization prospects if strong alkaline resins with high-temperature resistance can be developed in the future.
In addition, solid alkali catalysts being developed in China include clays, molecular sieves, complex oxides, carbonates, and loaded alkali (earth) metal oxides.

2.3 Supercritical catalyzed transesterification reaction

Supercritical transesterification technology for biodiesel preparation is a new method developed in the last few years, and its most important features are lower requirements for raw materials, no catalysts, shorter reaction time, higher reaction yield, and the products can be easily separated and refined.
Demirbas's experiments showed that, with the increase in temperature, the mass transfer and reaction characteristics between reactants near the critical point were obviously strengthened, and the reaction rate suddenly increased, and the reaction could be completed within a few minutes. Kusdiana studied the kinetics of transesterification of rapeseed oil in supercritical methanol and obtained the optimal process conditions of reaction temperature of 350 ℃, alcohol-oil molar ratio of 42:1, and pressure of 43MPa, and the reaction time of 4 minutes to 4minutes. It was found that the reaction temperature of 350℃, the alcohol-oil molar ratio of 42:1, and pressure of 43MPa were the optimal conditions, and the transesterification rate could reach more than 95% after 4 min.
At present, the preparation of biodiesel by supercritical method mainly uses supercritical alcohol or supercritical CO2 as the medium.
Madras et al. studied the preparation of biodiesel from sunflower oil under supercritical conditions: with supercritical methanol as the medium, a molar ratio of alcohol to oil of 40:1, and a pressure of 200 bar (1 bar=10 5 Pa), the reaction temperature was increased from 200 ℃ to 400 ℃, and the transesterification rate was increased from 78% to 96%. At the same time, they also studied the catalytic reaction of adding lipase to supercritical CO2 and found that the optimal reaction conditions were 45 ℃, enzyme dosage of 3 mg (30% of the oil mass fraction), reaction time of 12h, and the transesterification rate was only 30%.
Rathore et al. in the alcohol-oil molar ratio of 40:1 system to add 30% of the mass fraction of lipase, supercritical CO2 as a medium, the optimal reaction temperature of 45 ℃ under the conditions of the reaction, the highest esterification rate is also only 60% ~ 70%.
Therefore, at present, the supercritical transesterification of biodiesel is mainly prepared with alcohol as the medium. Generally speaking, substances with similar solubility parameters are more easily soluble in each other. The solubility parameters of alcohols in a supercritical state are similar to those of fats and oils.
Therefore, the solubility of grease in supercritical methanol is much greater than that in liquid alcohol, and the transesterification reaction can be carried out in a single phase. Alcohols that are transesterified with triglycerides can be methanol, ethanol, propanol, etc. Alcohols with different carbon numbers have different nucleophilicity and spatial effects of their negative ions:
- basicity: CH 3 O >C 2 H 5 O - >C 4 H 9 O
- spatial effect: CH 3 O
 different nucleophilicity and spatial effects of their negative ions
Figure 5
As the carbon chain grows, the basicity of the alcohol decreases, and the spatial effect increases, both of which lead to a decrease in the equilibrium constant of the reaction and a decrease in the transesterification rate.
Demirbas experimentally demonstrated that in the reaction of methanol, ethanol, 1-propanol, 1-butanol, and 1-octanol with triglycerides, methanol had the best reaction, and the reaction rates of the other alcohols decreased with chain growth.
Warabi studied the supercritical transesterification of rapeseed oil with methanol, ethanol, n-propanol, n-butanol, n-pentanol, and n-octanol, and found that, at 300°C and an alcohol-oil molar ratio of 42:1, the complete conversion of rapeseed oil in methanol required only 13 min, while in ethanol and 1-propanol, it required 45 min. At the same time, the transesterification rates of rapeseed oil with 1-butanol and 1-octanol were only 85% and 85% respectively. The transesterification rates of rapeseed oil with 1-butanol and 1-octanol were only 85% and 62% at the same time.
In addition, methanol is the most commonly used supercritical transesterification process for biodiesel preparation because of its low price, high polarity, and ability to react with fatty acid glycerides in a very short time. Supercritical methanol fluid-related data are shown in Table 6.

Supercritical methanol-related data
Table 6 Supercritical methanol-related data

Alcohols and fats do not miscible well, and the transesterification reaction is actually carried out between two liquid phases that are not completely miscible.
Using supercritical methanol as the medium for transesterification, in the reaction system, methanol is both the reactant and the reaction medium, and from a certain point of view, it also has a catalytic function.
Compared with methanol at room temperature and pressure, supercritical methanol has the following characteristics:
① Adjusting the pressure and temperature within the range near the critical point, the density of supercritical methanol can be controlled to change continuously from gas to liquid, and the interphase boundary disappears, so that the density-related properties such as dielectric constant and viscosity can be continuously transitioned from the gas state to the liquid state, which leads to a change in the solvation capacity;
② The viscosity of supercritical methanol is close to that of a gas, and the mass and heat transfer capability is greatly improved, with good transferability and rapid movement, so it can diffuse rapidly into the interior of the solute;
③ The diffusion coefficient of supercritical methanol is between gas and liquid, which is more than 100 times the self-diffusion ability of liquid, and it has a larger diffusion force as well as good penetration and equilibrium force, and it has a higher reaction rate than traditional organic solvent.
④ Supercritical methanol, due to its low activity and high diffusion coefficient, is easier to overcome the influence of the cage effect than traditional organic solvents, which is favorable to the generation of free radicals, improves the reaction rate and efficiency, and easy to get the product with uniform molecular weight distribution.
In summary, in the supercritical state, methanol, with both hydrophobicity and low dielectric constant, can be well dissolved with nonpolar triglycerides to form single-phase triglyceride/methanol mixtures, so that a very high esterification rate can be obtained in a very short period of time, and the subsequent treatment process is greatly simplified.

2.4 Enzyme-catalyzed transesterification

2.4.1 Theoretical Basis of Enzyme-Catalyzed
Transesterification Reactions In the last decade, the technology of lipase-catalyzed transesterification reaction for biodiesel preparation has received increasing attention.
In order to determine the optimal conditions for enzyme-catalyzed transesterification, design the reactor and control the reaction process, it is essential to study the kinetics of the enzymatic reaction. It is generally accepted that the mechanism of alcoholysis of fats and oils is based on enzyme-catalyzed hydrolysis.
During the reaction, acid or base functional groups at specific sites of the enzyme active site realize the catalysis of the reaction by proton transfer. By transferring protons from these groups to the substrate, the enzyme completes the acid or base-catalyzed reaction within the active site. Such functional groups are part of the active site and are particularly important for the catalytic process. One is the hydroxyl group, which acts as a nucleophilic reagent, and the other is the nitrogen atom of the amino group, which accepts protons and then returns them during the reaction.
From a molecular point of view, it is the triglyceride (TG, diglyceride DG, or monoglyceride MG) that is first acylated to acylase by lipase, which also produces glycerol or glycerol intermediates (DG or MG), and then the acylase transfers the acyl group to methanol to produce the target product fatty acid methyl ester, with methanol becoming the second acyl acceptor, and the acylation of the lipase being only an intermediate transition state.
As shown in Figure 7, the mechanism of alcoholysis for the catalytic synthesis of esters consists of the following steps:
the mechanism of alcoholysis for the catalytic synthesis of esters consists  steps
Figure 7 
Enzymatic mechanism of triglyceride production of fatty acid methyl esters Fersht It has been shown that the enzyme hydrolyzes the substrate ester in aqueous solution by the mechanism of acyl-enzyme complex, i.e., the enzyme and the substrate first form an enzyme-substrate complex by covalent bonding, and then the hydroxyl group of serine residue of the enzyme's active site and the carboxylic acid of the substrate form a tetrahedral intermediate, which decomposes, releasing alcohol, forming an acyl-enzyme covalent complex, which is then hydrolyzed to form the enzyme-enzyme covalent complex, and then hydrolyzed to form the enzyme-enzyme intermediate. This intermediate decomposes, releasing alcohol and forming an acyl-enzyme covalent complex, which is then hydrolyzed to form an enzyme-substrate complex, which is further decomposed to release free enzymes and products. Numerous studies have shown that lipase also catalyzes the reaction via an acyl-enzyme mechanism during non-aqueous phase-catalyzed esterification reactions.
Based on this assumption, Marangoni and Rousseau described the catalytic mechanism of the enzymatic transesterification process, which can be briefly described as follows:
the catalytic mechanism of the enzymatic transesterification process
Figure 8
where FA is fatty acid, E is enzyme, TG is triglyceride and DG is diglyceride.
Many other studies have also proposed the involvement of the glycerol ester-enzyme complex in the enzymatic reaction, and Kyotani proposed the following mechanistic process:
mechanistic process proposed  by Kyotani
Figure 9.
He concluded that this hypothesis provides a better fit for the experimental data for the enzymatic process of batch solvent systems. Also, he found that when the water content in the system controls the reaction process, the water content in the mixture influences the rate constants at each step. Another enzymatic hypothesis is the sequential hydrolysis-esterification process, i.e., transesterification is a process in which the hydrolysis process and the esterification process interact. For the hydrolysis process, the acyl-enzyme complex reaction can be described as follows:
acyl-enzyme complex reaction
Figure 10
The subsequent esterification process is a reversible reaction of the hydrolysis process. During the sequential hydrolysis and esterification processes, the reaction equations follow a ping-pong reaction mechanism. It is generally assumed that the hydrolysis process is the rate-limiting step and its reaction rate is slower than that of the esterification reaction. Obviously, in this assumption, water participates in the hydrolysis reaction as a reactant and simultaneously as a product of the esterification reaction. Many studies have shown that the sequential hydrolysis-esterification mechanism partially supports the enzymatic transesterification process.

Equipment for Biodiesel Use

The above-mentioned content is the status of biodiesel before 2013. As we enter the year 2020, the research and demand for biodiesel has become more urgent. A large number of scholars and researchers have entered this field. As the research on biodiesel becomes more and more in-depth, various formulations and theories are emerging. At the same time, the demand for research equipment is also higher and higher.

As a manufacturer specializing in reactor equipment fabrication, WHGCM receives requests from research organizations and individuals on a daily basis.
Due to contractual and confidentiality agreements, we cannot disclose the parameters of our customers' research needs. However, we are able to design and manufacture test reactors that meet the requirements of our customers in accordance with the test conditions and participation provided by our customers.

In addition, we can also provide you with a variety of ancillary equipment in addition to the reactor, or even directly customized for you.
Our reactors have the advantages of complete functions, high adaptability, reasonable price, and easy operation. If you have such a need, please leave a message or contact us directly.
In addition, some customers have been from our company many times, customized industrialized esterification reactors as well as hydrogenation reactors.
We believe that with the market's increasingly urgent demand for biodiesel, the supply of our reactors will go further. We are also glad to make our own contribution to the improvement of the environment and solving part of the energy crisis!
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