At present, the preparation technology of biodiesel mainly includes direct mixing method, microemulsion method, pyrolysis method and transesterification method. The sources of waste oil in my country are wide and dispersed, and it has the characteristics of many solid impurities, high water content and high acid value. The preparation of biodiesel from waste oil generally needs to undergo pretreatment such as water removal, mechanical impurity removal, acid removal, and decolorization, and then use transesterification or hydrocracking to prepare biodiesel.

Biodiesel preparation by transesterification

Zhang Yong uses waste waste oil as raw material, and after pretreatment, adopts a two-step esterification process to convert it into biodiesel. The first step is acid-catalyzed pre-esterification, which mainly converts free fatty acids in waste oil into fatty acid methyl esters. The second step is acid-catalyzed transesterification, which further converts triglycerides in waste oil into methyl esters and glycerol. The optimal conditions of pre-esterification reaction obtained by orthogonal experiment are: alcohol oil molar ratio 10:1, catalyst dosage 1%, reaction temperature 70 ℃, reaction time 4 hours; the optimal conditions of transesterification reaction are: alcohol oil The molar ratio was 20:1, the catalyst dosage was 6%, the reaction temperature was 70°C, and the reaction time was 4 hours. Under the optimal reaction conditions, the esterification rate of triglyceride can reach 86.89%. The biodiesel prepared by this method is superior to No. 0 diesel in terms of flash point and cold filter point, and is safer during storage and transportation; meanwhile, it can be used in a wider temperature range. At the same time, it was found that the biodiesel prepared by this method and the No. 0 diesel oil were blended according to B20, which not only greatly reduced the viscosity of the biodiesel and improved the volatility, but also increased the flash point, freezing point and cold filtration of the No. 0 diesel. The point is reduced, making the storage and transportation process safer, and the low temperature performance is improved, which is conducive to the use in a wider temperature range and can meet the requirements of use.

The preparation of biodiesel by waste oil acid catalysis method is to use waste oil and methanol or ethanol and other low-carbon alcohols to carry out transesterification reaction under acidic catalyst conditions to generate corresponding fatty acid methyl esters or ethyl esters. Yao Yaguang et al. used acid as a catalyst to first carry out the pretreatment of waste oil removal, degumming, decolorization and dehydration, then use waste oil to prepare biodiesel under acid catalysis conditions, and conduct orthogonal esterification reactions of waste oil with methanol and ethanol. Experiment, the experiment determined that the best reaction conditions for acid catalyzed waste oil to prepare biodiesel are: methanol temperature is 70 ℃, oleyl alcohol molar ratio is 1:40, catalyst concentration is 7%, reaction time is 6 hours, and the order of steps is as follows: : oleyl alcohol mol ratio, reaction time, catalyst concentration, temperature; Ethanol temperature is 80 ℃, oleyl alcohol mol ratio is 1: 30, catalyst concentration is 5%, and the reaction time is 6 hours, and the order of steps is in turn: oleyl alcohol mol ratio , temperature, catalyst concentration, reaction time. Biodiesel with excellent properties is prepared by this method. The main advantages are: good flammability (cetane number), evaporation (distillation range and distillation temperature), safety (flash point), viscosity and condensation point temperature, corrosiveness to the engine (acidity and acid value) , calorific value. The biodiesel prepared in this experiment has superior properties that ordinary diesel can't match in many aspects.

Fu Yan et al. used waste oil as raw material to study the synthesis of biodiesel from waste oil and methanol on immobilized lipase in a three-stage reactor. The acid value, saponification value and water content of waste oil were tested. The effects of feed flow rate, solvent and water content on the reaction were investigated. At 40° C., n-hexane was used as solvent, the added water content was 20% of the quality of waste oil, and when the molar ratio of methanol and waste oil added in each section of the reactor was 1:1, the biodiesel yield was 94%.

Chen Yingming et al. pretreated waste oil through filtration, degumming, decolorization, dehydration, etc., mixed it with methanol, n-hexane, water, etc. in a certain proportion through a stirrer, and transported it to a reactor filled with sheet-like immobilized enzymes with a peristaltic pump The top is dripped into the reactor, and the constant temperature circulating water bath. Three reactors are connected in series to form a three-stage reaction system, the oleyl alcohol molar ratio of each stage of the reactor feed is 1:1, and the by-product glycerol is removed in time for the product of each stage of reaction. The reaction product is washed with water, distilled, etc. to remove methanol, water and n-hexane to obtain crude biodiesel. The biodiesel prepared by this method was qualitatively analyzed by GC-2010 gas chromatograph and QP2010 chromatography-mass spectrometer, and the fatty acid methyl ester, free fatty acid and glyceride in biodiesel were determined by GC-MS method. Therefore, the various components and their contents in the GC chromatogram were determined, and the conversion rate and yield of biodiesel were determined by the area method and internal standard method. Finally, the conversion rate of biodiesel obtained by the enzymatic method of waste oil was 93.53. %, the yield was 77.45%.

Li Weimin et al. used waste oil as raw material to prepare biodiesel. First, the acid value of waste oil was reduced to 2±1 mgKOH/g by pre-esterification, and then biodiesel was prepared by transesterification. The optimal conditions are: the amount of concentrated sulfuric acid is 2%, the amount of methanol is 16%, the reaction temperature is 75 ° C, and the reaction time is 4 hours; 65° C., the reaction time is 2 hours, and the prepared biodiesel meets the requirements of the national biodiesel standard.

Zhang Aihua et al. used the pre-esterification technology of polyols to treat waste oil, and used alkaline ionic liquid 1-methyl-3-butylimidazole hydroxide as catalyst to prepare biodiesel. The effects of the amount of ionic liquid, the ratio of alcohol to oil, reaction temperature and reaction time on the transesterification reaction were investigated. The results showed that the process conditions for preparing biodiesel from waste oil were as follows: the ratio of alcohol to oil substance was 8:1, the reaction temperature was 70°C, the reaction time was 110 minutes, and the amount of catalyst was 3.0% of the mass of the feedstock oil. Under this condition, the conversion rate of fatty acid methyl ester was 95.7%. The effects of the amount of glycerol added, reaction temperature and reaction time on the pre-esterification reaction were investigated experimentally, and the effects of catalyst dosage, alcohol-oil molar ratio, reaction temperature and reaction time on the transesterification reaction were also investigated. The optimum reaction conditions for preparing biodiesel from waste oil pre-esterification-transesterification were determined by orthogonal experiment. According to the characteristics of high acid value of waste oil, Chen An et al. used solid acid and solid alkali two-step heterogeneous catalysis to develop biodiesel. This method avoids the disadvantages of homogeneous acid method, such as high price of acid-resistant equipment, long reaction time, low esterification rate, and waste water; At the same time, it also makes up for the shortage of the two-step homogeneous method that produces a large amount of waste water and affects the environment. The optimal experimental conditions of the method were determined by experiments as follows: the reaction time was 2.5 hours, the molar ratio of alcohol to oil was 10:1, the solid base catalyst was 2.0% by weight of oil, the cosolvent tetrahydrofuran was 3%, and the reaction temperature was 71°C. At this time, the esterification rate is above 96%.

Supercritical waste oil production of biodiesel

Supercritical transesterification is an uncatalyzed transesterification. When methanol is in a supercritical state, the alcohol and oil are promoted to become homogeneous, the mass transfer effect is improved, the reaction rate is greatly improved, the reaction time is short, the methyl ester conversion rate is high, and no catalyst is required, but the reaction needs to be carried out at high temperature and high pressure. The equipment requirements are high and the energy consumption is large. Demirbas used supercritical methanol to prepare biodiesel in a cylindrical high-pressure vessel with a volume of 10 ml under the condition of no catalyst, and optimized the parameters affecting the experiment through single factor experiments. The conversion rate of methyl ester is as high as 99.6%. The author compares the test results with the base-catalyzed preparation of biodiesel, and finds that the supercritical methanol method can save raw material pretreatment and save operating costs. Chen Shengjie et al. used acidified oil and ethanol as raw materials to prepare biodiesel under supercritical conditions. The process conditions were optimized by response surface design and analysis method, and the optimal process conditions were obtained, and the biodiesel yield under the optimal conditions could reach 89.7%. In order to improve the method for preparing biodiesel by supercritical transesterification and overcome its shortcomings, supercritical transesterification in the presence of catalysts or co-solvents has become a new field of exploration.

Hydrocracking of waste oil to produce biodiesel

The second-generation biodiesel formed by the biodiesel synthesis route based on the hydrogenation process of the refinery has a cetane number between 84 and 99 (the cetane number of the first-generation biodiesel is about 50), and the sulfur content is close to 0 , the pour point is also low (as low as -30 ℃). Therefore, the second-generation biodiesel is a high-quality ultra-clean diesel, which has become the new favorite of many countries to develop biofuels. Bezergianni et al. reported for the first time the production of biodiesel from edible waste oil by hydrocracking process. The effects of hydrocracking temperature, liquid hourly space velocity, production days and other factors on the conversion rate of each component and the total output of biofuel were comprehensively considered. The test results show that the increase of the hydrocracking temperature and the decrease of the liquid hourly space velocity are beneficial to the improvement of the conversion rate of each component and the total production; the increase of the hydrocracking temperature will increase the removal rate of heteroatoms (sulfur, nitrogen, oxygen). The production of biodiesel and gasoline in the product can be controlled by adjusting the hydrocracking temperature and liquid hourly space velocity during the test, and moderate reactor temperature and liquid hourly space velocity can make the product all biodiesel. The author also discussed the influence of each influencing factor on the conversion rate and total yield of each component in the experiment through single factor experiments. In the selection test of biodiesel, the yield of biodiesel is greater than 90% in the reactor temperature of 350～390℃. The process of preparing biodiesel by hydrocracking can use bio-oil or a mixture of bio-oil and petroleum distillate as raw material, and under the action of a hydrogenation catalyst, the product can be prepared by hydrorefining or hydrocracking, so as to combine the production process of biodiesel. Close integration with the refining production process will greatly reduce production costs.

At the same time, a large number of scholars have also studied the peripheral experimental technology of waste oil to biodiesel. Wang Fanyu et al. have studied the pre-refining of waste oil to biodiesel, using solid acid as catalyst, using kettle reaction and fixed bed reaction phase In a combined way, waste oil is pre-refined into raw material oil for preparing biodiesel. Kettle reaction pre-esterification conditions: reaction temperature 70 ℃, reaction time 8 hours, catalyst consumption 5%; further esterification through fixed bed reactor, esterification conditions: reaction temperature 70 ℃, oil weight -1 space velocity 0.2 h, methanol weight hourly space velocity 0.2~0.3h. The combination of kettle reaction and fixed bed reaction is adopted. The kettle-type reaction uses cheap solid acid to remove impurities such as colloid and moisture in the raw oil, and reduces the acid value to below 10mgKOH/g, which overcomes the problem of traditional liquid acid corrosion equipment, and has high environmental protection and fixed-bed reaction. The acid value is further reduced to below 2 mgKOH/g, which meets the requirements for preparing feedstock oil for biodiesel. The method better solves the problems of short service life of solid acid catalysts and difficulty in large-scale preparation of biodiesel, and is suitable for the pre-refining of general waste cooking oil and raw materials of waste oil with high acid value and high colloid, and has certain application value . Hong Yao et al. used experimental methods to study the factors affecting the viscosity of biodiesel, the main performance index, and investigated the influence of biodiesel concentration, stirring time, and temperature changes on the viscosity of biodiesel prepared from waste oil, and obtained the viscosity of waste oil biodiesel. It increases with increasing concentration and decreases with increasing temperature. Biodiesel is a Newtonian fluid. The viscosity of biodiesel is hardly affected by the shear force generated by high-speed stirring, and it can maintain good stability under the action of shear force. The viscosity of biodiesel remained unchanged with the prolongation of stirring time. Wang Yu et al. used gas chromatography-mass spectrometry to analyze the components of biomass diesel obtained by using Candida enzyme membrane as catalyst and waste oil as raw material to react with methanol under certain conditions, and the crude product obtained was purified by distillation. The other physical and chemical indicators of the biodiesel were determined by the standard method of petroleum testing, and the emission characteristics, power performance, economic performance of the biodiesel engine and the smoke of the biodiesel were tested by the combustion test. The test showed that the purity of the synthesized biodiesel reached more than 97.8%, the flash point of the refined product was higher than 170 ℃, the mass fraction of sulfur was lower than 0.0005%, and the cetane number was as high as 73.6; After 20 percent biodiesel, tailpipe emissions were reduced by 28 percent in carbon monoxide, 36 percent in unburned hydrocarbons, and 24 percent in nitrogen oxides. The full load smoke level decreased by 0.2~0.9Rb.