Supercritical technology
Thread Content
Supercritical technology: The principle behind using vegetable oil and supercritical methanol to produce biodiesel is the same as that of the chemical esterification method, as both rely on transesterification reactions. However, in the supercritical state, methanol and oils form a homogeneous mixture, and the rate constant for homogeneous reactions is higher, resulting in a shorter reaction time. Additionally, since no catalyst is used in this process, the subsequent steps are simpler, and there is no waste alkaline solution generated; thus, the cost is significantly lower compared to the chemical method. In the traditional method, due to the poor miscibility between methanol and animal and vegetable oils, the reaction system becomes two-phase; thus, the transesterification reaction can only occur at the interface between the two phases, which limits mass transfer and results in a low reaction rate. In order to increase the area of contact between the two phases and boost the reaction rate, Sumitomo Chemical Company in Japan has developed a supercritical method for producing diesel. This method involves reacting methanol with vegetable oils such as rapeseed oil and soybean oil; the reaction takes place at 240°C and 8 MPa, resulting in the formation of fatty acid methyl esters, with glycerol as a by-product, in a ratio of 1:3. This method has been patented. Typically, fatty acid methyl esters are obtained through the aforementioned reaction in the presence of sodium hydroxide as a basic catalyst; however, this approach results in the formation of soap as a by-product, which must be removed by washing, thereby increasing costs. The process developed by Sumitomo Chemical achieves a fatty acid methyl ester yield of 100%. Tsinghua University in our country has also studied the reaction conditions for producing biodiesel using the supercritical methanol method, as well as their impact on the methyl ester formation rate. The results show that the higher the molar ratio of alcohol to oil, the greater the conversion rate of soybean oil. Raising the temperature helps to increase the reaction rate, and this effect is particularly evident around the critical temperature of 239°C. When the pressure exceeds 135 MPa, the influence of pressure on the reaction becomes less significant. Among the raw materials, different fatty acid methyl esters have varying esterification rates, with the rates decreasing in the order of linoleate, oleate, palm oil, and stearate. In soybean oil, a free oleic acid content of less than 50% does not affect the reaction rate, and a water content of less than 20% in the crude oil has little impact on the reaction. When the molar ratio of alcohol to oil is 42:1, the reaction temperature is 289°C, and the reaction lasts for 1 hour, the conversion rate of the oil can reach 78%. Because in the supercritical state, methanol is hydrophobic and has a relatively low dielectric constant; glycerides dissolve completely in methanol, resulting in a single-phase reaction system. This enables a fast transesterification rate and an increased overall yield of methyl esters. Since no catalyst is used in the process, not only is the purification of the product simple, but no wastewater is also generated, making the transesterification process simpler, safer, and more efficient. The reaction temperature has a significant effect on the conversion rate of the transesterification reaction. For traditional chemical methods, depending on the plant oil used, the optimal reaction temperature ranges from 20 to 60°C; in supercritical methanol with plant oil, the yield of fatty acid methyl esters is also closely related to temperature. When the molar ratio of methanol to rapeseed oil is 42:1, the reaction temperature is varied from 200 to 500°C. When the reaction is carried out at temperatures between 200 and 230°C, below the supercritical temperature, the reaction rate decreases; after 1 hour, approximately 68% to 70% of the vegetable oil is converted into fatty acid methyl esters. At 270°C, as the system is in a transitional phase between subcritical and supercritical conditions, the conversion efficiency remains low. When the reaction temperature exceeds 300°C, 80% to 95% of the vegetable oil is converted into fatty acid methyl esters within 4 minutes. But when the temperature reaches 400°C, the decomposition reaction replaces the esterification reaction, producing other substances. Therefore, 350°C is appropriate. Currently, two types of reaction vessels are commonly used in supercritical technology: Dadań Kusdiana’s tubular reactor and Ayhan Demirbas’ high-pressure reactor. Traditional methods for producing biodiesel take a long time (1–8 hours), while the use of supercritical methanol reduces this time to 4 minutes. It also requires fewer specifications for the feed oil; waste oils with high water content and acid value can be used, and biodiesel with a conversion rate of over 98% can be obtained without any additional treatment. Supercritical methanol serves both as a reaction medium and as a reactant along with the catalyst. The esterification reaction improves as the reaction time increases; at 60°C, with an alcohol-to-oil molar ratio of 6:1 and a mass fraction of sodium methoxide catalyst of 0.5%, the conversion rate of vegetable oil reaches 95%–98% after 1 hour. The supercritical method can **increase the reaction rate**. Rapeseed oil reacts with supercritical methanol at temperatures below 350°C in a molar ratio of 42:1, and after 30 seconds, the conversion rate of rapeseed oil exceeds 40%; by 240 seconds, 95% of the rapeseed oil has been converted into fatty acid methyl esters. When hazelnut oil reacts with supercritical methanol at a molar ratio of 41:1, the absorption rate of fatty acid methyl esters reaches over 90% after 200 s.
Bioenzymatic esterification method: By replacing the traditional chemical transesterification method with a biotransesterification method that uses enzymes or whole cells as catalysts, some of the aforementioned disadvantages can be overcome. The bio-enzymatic method features a simple process, mild reaction conditions, high selectivity, low alcohol usage, few by-products, easy recovery of the generated glycerol, and no need for wastewater treatment; the key to this method is the catalyst. Lipases can efficiently catalyze the transesterification reaction between alcohols and fatty acid glycerides. By using lipases, the problems associated with the catalysts used in traditional chemical methods for biodiesel production – such as difficulty in separation and high energy requirements – can be addressed. However, the disadvantages are as follows: ester exchange efficiency generally cannot be achieved without the use of organic solvents, but when a certain amount of methanol is present in the reaction mixture, it can cause the lipase to become inactive and lose its catalytic ability; moreover, the cost of the enzyme is high and the reaction time is long. Therefore, increasing lipase activity and preventing enzyme inactivation are key to the industrial production of this method. 1 Free lipase as a catalyst The lipases used to catalyze the synthesis of biodiesel include yeast lipase, Pseudomonas lipase, Candida lipase, Rhizopus lipase, Mucor lipase, and porcine axillary lipase, among others. For example, Rhizomucor lipase can catalyze the conversion of vegetable oil into diesel in a reaction system with an initial water content of 4% to 30%. The various factors that enable Cryptococcus to catalyze the transesterification of vegetable oils in an aqueous medium have been identified: a oil/methanol molar ratio of 1:4, a water content of 30% by mass, and a reaction time of 120 hours at a rotation speed of 160 rpm and a temperature of 30°C, resulting in a mass fraction of fatty acid methyl esters of 80.2%. In n-hexane, Candida lipase facilitates the conversion of palm oil with short-chain alcohols as substrates; after 4 hours of reaction, the conversion rate reaches 78.6%. When n-propanol is used as the substrate, the conversion rate can reach 96.0% after 8 hours of reaction. Lipases have good selectivity and high catalytic activity, but they still face some issues that need to be resolved as catalysts for large-scale industrial production. Lipases are expensive, and using them as catalysts increases production costs, which limits their application in the industrial-scale production of biodiesel. The solutions to this problem are: first, using lipase immobilization technology to enable its repeated use; second, using the entire cell that produces lipase as a biocatalyst. 2 Fixed lipase as a catalyst Introducing immobilization technology into the industrial production of biodiesel can **improve the stability and reusability of the enzyme while reducing costs. Japan used the immobilized Candida lipase produced by the Danish company Novozymes; at 30°C and after 48 hours of reaction, a conversion rate of 97.3% was achieved. It was also found that methanol has better solubility in a mixture of triacylglycerols and fatty acid methyl esters compared to a pure triacylglycerol system; after 36 hours of reaction, the conversion rate can reach 96.8%. In order to avoid enzyme inactivation caused by methanol, Tsinghua University in our country used ethyl thioacetate as a substitute for methanol as the acyl acceptor in an esterification reaction with soybean oil (molar ratio = 12:1). Novozym 435 was used as the catalyst again, and the yield of the corresponding methyl ester was 92%. The drawback is that a large amount of enzyme is required, with 30% by mass of enzyme having to be added (based on the mass of the oil). It can be said that the application of immobilization technology represents a solid step forward in achieving the industrial production of biodiesel. Enzymatic synthesis of biodiesel is a promising biocatalytic process. It has been proven that lipase extracted by fermentation (Candidasp.99-125), using the adsorption and cold fixation method, can be used in transesterification reactions to produce biodiesel. When n-hexane is used as the solvent, 15% immobilized lipase (with an enzyme activity of 18,000 U/g based on the mass of oil) is employed along with 20% by mass of water; at a temperature of 40°C and a pH of 7, 1 mole equivalent of methanol is added every 10 hours, a total of 3 times. The highest conversion rate achieved in this reaction is 96%, and the half-life of the immobilized enzyme exceeds 200 hours. 3 Whole-cell enzymes as catalysts The methods mentioned above all use lipases as catalysts. If whole-cell enzymes are used as catalysts and adsorbed onto some porous, permeable biomass supports, the complex processes of enzyme purification and immobilization can be avoided, thereby **reducing industrial production costs**. Some researchers developed an alcohol-producing yeast MT8-1 using Rhizomucor lipase; its lipolytic activity reached 474.5 IU/L. After enhancing the permeability of the yeast cells through pre-freezing/thawing or air-drying, it was used to catalyze the reaction of synthesizing fatty acid methyl esters from soybean oil, with a mass fraction of methyl esters in the reaction mixture being 71%. In further studies, after culturing the cells with the porous carrier for 80–90 hours, methanol was added to the fermentation broth in 3 doses; the resulting methyl ester content reached 90%, which is comparable to the effect achieved using extracellular enzymes as catalysts. Whole-cell enzymes hold great potential as catalysts in industrial production. By using genetic engineering techniques to increase the expression level of lipases and enhance their tolerance to methanol, the efficiency of whole-cell enzymes can be further improved.
Both of these two technologies have fatal shortcomings; their industrial application in China has not been very successful, and there seem to be no examples of industrial implementation abroad as well； Of course, it is acceptable for research purposes.