Glycerol Esterification For Low-sulfur Biodiesel Production

Due to the serious environmental pollution and increasing energy demand,renewable energy has become a research focus.Biodiesl has attracted much attention as a green and renewable fuel.However,the current feedstock to produce biodiesel has properties of high acid value and sulfur content,which would influence the acid reduction effect and make the biodiesel quality difficult to meet the strict sulfur standard of oil.To solve these problems,glycerol esterification method was used in this study for biodiesel production.Dynamic model of glycerol esterification was developed and used to examine the effect of reaction conditions on acid reduction effect.High vacuum distillation and activated carbon adsorption method was used to reduce the sulfur content of biodiesel products.The main contents and conclusions are as follows:(1)The effect of temperature on product distribution from glycerol esterification was examined.A second-order reaction dynamic model was developed for the glycerol esterification,and kinetic parameters,i.e.k and Ea,were estimated.Based on the model,the effects of reaction temperature,glycerol to fatty acid molar ratio,and acid value of feedstock on acid value reduction were predicted.The results showed that the estimated values were in good agreement with the experimental data.The acid value reduction reactions were highly dependent on the temperature.Higher rate of acid value reduction was obtained at higher temperature,higher glycerol to fatty acid molar ratio,and larger initial acid value of feedstock.(2)The sulfur distribution during biodiesel production using glycerol esterification method was examined and compared with that using conventional two-step method of acid catalytic esterification and alkaline catalytic transesterification.High vacuum distillation was used to remove the sulfides in crude biodiesel.The qualification and quantification of sulfides was also conducted.Experimental results showed that sulfides in biodiesel existed mainly in the form of alkylbenzothiophenes and alkyldibenzothiophenes.Most sulfides were distilled out with light and heavy fractions.High quality biodiesel with low sulfur content was obtained by separating the intermediate fraction from the other fractions.The total yield of distillation fractions with sulfur content lower than 10 ppm reached 84.1%.(3)Modified activated carbon(AC)was utilized to further remove the sulfides in the light fraction obtained from biodiesel distillation.The effects of activated carbon particle size,modification method,adsorption time,oil to adsorbent ratio,and desulfurization temperature on desulfurization performance were investigated.The results showed that the adsorption capacity of activated carbons with different modification methods followed the order: AgNO3/AC-HN>AgCl/AC-HN>AC-HN>AC.Better adsorption desulfurization performance was obtained at longer adsorption time,higher desulfurization temperature,and lower oil to adsorbent ratio.When using AgNO3/AC-HN as the adsorbent,the desulfurization rate reached 83.4% at desulfurization temperature of 80?,adsorption time of 3h,and oil to adsorbent ratio of2.The sulfur content was reduced to 44.4 ppm,which reached the S50 standard(<50ppm)in GB/T 20828-2015.