Preparation Of Biodiesel And Effect Of Antioxidants On The Oxidation Stability And Crystallization Characteristics

Biodiesel is a renewable green energy source whose main component is a low alcohol fatty acid ester(such as methyl ester or ethyl ester).Compared with fossil diesel,biodiesel has the advantages of better fuel performance and less emission pollution.The objective of this study was to prepare biodiesel with waste oils of high acid value as basic feedstock by acid-catalyzed interesterification with mathanol.At the same time,the reaction law of acid-catalyzed methyl esterification process was also researched.The effects of different phenolic antioxidants on the oxidation stability of biodiesel were studied by Rancimat method and anti-oxidation kinetics.And the antioxidants` minimum concentration was speculated,which can make the biodiesel oxidation induction period reach the European Union standard(EN14214-2014)regulations(?8h).The effects of antioxidants on the crystallization characteristics of biodiesel was studied by comparing the differences between the cold filtration point,solid fat content change trend,crystallization kinetics,microcrystalline morphology and crystallization enthalpy change curves of biodiesel with and without the addition of phenolic antioxidants.Firstly,the fatty acid methyl esters were prepared by p-toluenesulfonic acid-catalyzed interesterification of waste oil(acid value,75.0±0.50 mg KOH/g)with methanol.The influences of reaction conditions on the change of free fatty acids,fatty acid methyl esters,monoglycerides,diglycerides and triglycerides were investigated,and the process conditions of biodiesel preparation were optimized.The results showed that p-toluenesulfonic acid preferentially catalyzed the esterification of free fatty acids with methanol.When the esterification process was about to reach equilibrium,it began to catalyze the transesterification of glycerides with mathanol.The reaction rate was ranked as free fatty acid> monoglyceride> diacylglycerol> triacylglycerol.The conditions were then optimized by response surface optimization as: p-toluenesulfonic acid addition amount was 5.6%(accounting for oil weight),alcohol-oil molar ratio was 26.3:1,reaction time was 31.6h and reaction temperature was 67±2 ?.Under these conditions,the content of free fatty acids,monoglycerides,diglycerides,triglycerides and methyl esters in the obtained product was 0.74±0.29%,0.54±0.24%,0.09±0.11%,0.31±0.05%? 98.29±0.06%,respectively.Secondly,the effects of six kinds of phenolic antioxidants(tert-Butyl-4-methylphenol(BHT),butyl hydroxyanisole(BHA),2,5-di-tert-butyl hydroquinone(DBHQ),tert-butyl hydroquinone(TBHQ),propyl gallate(PG),pyrogallate(PY))on the oxidation stability of biodiesel were studied by Rancimat method and kinetic fitting.By combination of their molecular structure,the conclusion was that the antioxidative effect of poly-phenol hydroxyl antioxidants was better than that of mono-phenol hydroxyl antioxidants.The minimum concentration of these antioxidants which can make the biodiesel oxidation induction period reach 8h were calculated as 1279mg/kg(BHT),690 mg/kg(BHA),906mg/kg((DBHQ),512mg/kg(TBHQ),388mg/kg(PG)and 119mg/kg(PY)by the kinetic fitting formula.Based on the cost accounting and the applicability analysis in biodiesel,BHT was considered to be a most suitable antioxidant for biodiesel.Thirdly,the differences between the cold filtration point,solid fat content change trend,crystallization kinetics,microcrystalline morphology and crystallization enthalpy change curves of biodiesel with and without the addition of phenolic antioxidants were compared in this paper.The result showed that all of six phenolic antioxidants can inhibit the growth of biodiesel crystals,and BHA has the weakest effect among them.The mechanism of antioxidants inhibiting biodiesel crystals was deduced.