Study On Some Key Problems In The Biodiesel Production By Acid Catalysis

In the production of biodiesel, a variety of biomass resources of oils and fats are with high acid value. The acid-catalyzed process directly using high-acid-value oils and fats as feedstock is one of the traditional effective methods for biodiesel production. However, there exists a problem that it is difficult to recover and reuse the acid catalyst, and another problem that the glycerol phase is typically treated with lime for precipitation of the acid to obtain the by-product glycerol, which not only consumes the acid, but also generates such low-value by-product gypsum. In this study, a series of novel methods had been developed to separate the glycerol and acid, and to dehydrate the acid-rich and glycerol-rich streams, respectively. Based on the acid-catalyzed process using high-acid-value oils and fats for biodiesel production, the excess of methanol was recovered by distillation, and dual-liquid phase solvent extraction with n-hexane and water was used to improve the separation of methyl ester and glycerol. Then, acid and glycerol in the aqueous phase were separated by using acid retardation process. Finally, the eluents were deeply concentrated by multiple-effect membrane distillation(MEMD) process, which fulfilled the recovery of acid catalyst and glycerol.The results in this study revealed that the acid retardation process provided an effective separation of glycerol and sulfuric acid in the water phase obtained from dual-liquid phase extraction by using a strong basic anion exchange resin. The resolution of glycerol and sulfuric acid was up to 1.53 at the optimized condition, which gave recovery of 96.2% for glycerol and 95.3% for sulfuric acid, respectively. The resin could be typically used in more than 100 cycles, which demonstrates its suitability for long-term use. A theoretical model of heat and mass transfer based on the temperature polarization and concentration polarization had been built, which was fitted to concentrating aqueous solution of non-volatile solute by using MEMD process. The model was applied to explain the rule of performance variation of MEMD process when it was used to concentrate the aqueous solution of non-volatile glycerol, which described the experimental data fairly well.The dilute glycerol solution of 10 g/L could be successfully concentrated up to about 400 g/L with a rejection factor still greater than 99.9%. The maximum value of trans-membrane flux(Jw) and gain output ratio(GOR) experimentally obtained could reach 5.7 L/m2 h and 16.2, even when the aqueous glycerol solution was concentrated to about 400 g/L, the GOR could still be 4.6, which corresponded to the energy efficiency of traditional six-effect evaporator. When the feed was concentrated up to about 300 g/L, Jw and GOR decreased sharply. The increment of glycerol viscosity and the reduction of water vapor pressure resulted from the increase of feed concentration were the critical reasons that led to deterioration in Jw and GOR. Two level full factorial analysis showed that the heated feed-in temperature was the most significant factor to obtain higher Jw and GOR. When highly concentrated glycerol solution of 150 g/L was taken as a feed, the performance of MEMD process basically maintained invariable in the long-term stability test lasting for 60 days continuously. The dilute sulfuric acid of 5 wt% could be deep concentrated up to about 40 wt% by using MEMD process, and the electrical conductivity of the distillate was always less than 100 μs/cm. The concentrated glycerol solution from MEMD process can be further concentrated to glycerol product for sale by using high temperature or vacuum distillation, and the concentrated sulfuric acid solution from MEMD process can be further concentrated up to more than 90% sulfuric acid by using high temperature or vacuum distillation, which can be reused as an acid catalyst.This study demonstrated that the integrated process including the production of biodiesel by using acid-catalyzed process, the separation of the methyl ester and glycerol phase by using dual-liquid phase extraction process, the separation of glycerol and sulfuric acid by using acid retardation process, the deep dehydration of elutes by using MEMD process, can effectively improve the economy and environmental benefits for the production of biodiesel by acid catalysis.