Conventional biodiesel production involves enzyme-catalyzed reactions between oil and methanol, added either all at once or gradually. High methanol levels can be toxic, limiting how often the enzyme catalyst can be reused. To combat this, Du et al. suggested using Methyl Acetate & MAC instead of methanol in the transesterification process. This approach reduces the toxic impact of methanol on lipase. The resulting byproduct, acetic acid glyceride, mixes completely with fatty acid methyl esters, which then improves the lipase’s stability and lifespan.
Whether it is the traditional enzyme catalytic process or the new catalytic process using solvent dilution or replacing methanol, the current research on the production of biodiesel by enzyme catalysis is mainly focused on selecting suitable enzyme catalysts and developing new catalytic reaction processes.
Post-treatment and refining equipment for biodiesel
The biodiesel raw material liquid of methyl acetate system can only be obtained by simple distillation and two vacuum distillations to obtain refined biodiesel. The schematic diagram of the equipment used for simple distillation is omitted, and the vacuum distillation uses homemade equipment, as shown in Figure 1.
Refining process conditions of biodiesel in methyl acetate system
Table 1 shows a common mixture of biodiesel origin liquid, where methyl acetate takes the place of methanol as the acyl acceptor in the reaction system. The oil origin for the transesterification reaction is soybean oil.
The initial step involves using simple distillation to remove most of the methyl acetate and acetic acid, which are then reused after condensation. Next, any remaining low-boiling point solvent is evaporated under reduced pressure and moved to the first vacuum distillation tower. This process separates acetic acid glyceride from fatty acid methyl ester and other fatty acid glycerides. The remaining liquid at the bottom of the first distillation tower, after acetic acid glyceride removal, goes to a second vacuum distillation tower to distill the final biodiesel. The remaining liquid mainly contains unreacted fatty acid glyceride and fatty acid, which are returned to the enzyme reactor for more reaction. The acetic acid glyceride that is condensed goes through a transesterification reaction with methanol, turning it into glycerol and methyl acetate. After evaporating the methyl acetate and extra methanol, crude glycerol is produced as a by-product. Adsorption distillation separates methyl acetate and methanol. The resulting high-purity methyl acetate can then be directly reused as a raw material in the enzyme reactor.
Product quality analysis
At present, my country has not yet promulgated the quality standard of biodiesel. The quality of biodiesel products in this study is assessed according to the German biodiesel standard DINE 51606. The most important indicators of the methyl acetate system of the enzyme catalysis method are the fatty acid methyl ester content, free fatty acid content, methyl acetate content and acetic acid glyceride content in biodiesel. Table 3 presents a comparison of substance levels in biodiesel raw liquid, crude biodiesel, and refined biodiesel with DINE 51606 standards. Crude biodiesel, when not distilled, doesn’t meet DINE 51606 quality standards. After distillation, the refined biodiesel either meets or surpasses these standards across all measured indicators. Other indicators are also in compliance with the requirements after testing by the Petrochemical Science Research Institute of Sinopec Corporation, and can be used directly in the engine as high-quality diesel.
Material balance of refining process
The total material balance of the methyl acetate system for preparing biodiesel under the process conditions listed in Table 2 is shown in Figure 3.
Conclusion
Based on the characteristics of the enzyme-catalyzed preparation of biodiesel by the methyl acetate system, the corresponding post-treatment refining process was proposed on the basis of analyzing the composition of the biodiesel raw material liquid. The study’s experiments defined the process parameters and material balance needed. The refined biodiesel that resulted met the DINE 51606 quality standard. A Pro/II simulation of the vacuum distillation of crude biodiesel suggests that a distillation tower with 7 to 11 theoretical plates, a top pressure of 133 to 1333 Pa, and a reflux ratio of 1.5 to 3.0 is ideal. This setup should provide good biodiesel yield below 300℃. These simulation results matched well with actual experimental results. The simulation also confirmed that vacuum distillation is a required step to purify biodiesel.
