| With the increasing shortage of petroleum energy, much attention has been paid to biodiesel, which is a kind of environmental benign energy. The partial hydrogenation of biodiesel, by which the polyunsaturated methyl esters in biodiesel were selectively conversed into monounsaturated fatty esters, has been utilised to promote the oxidative stability and to improve the quality of biodiesel.The heterogeneous catalytic transfer hydrogenation (CTH) of biodiesel has been carried out using isopropyl alcohol as the hydrogen donor over Raney-Ni catalyst in aqueous media, in order to find a good way of selective hydrogenation to optimize the properties of biodiesel.1) First, the fatty acid methyl esters (FAME) composition of biodiesel were determined by GC. Through optimizing the appropriate chromatographic conditions and column, the cis/trans isomers of methyl oleate (C18:1) were successfully separated. The FAME was qualitative analysed by retention time and quantitative analysed by methyl heptadecanoic acid (C17:0) as internal matter. Then the degree of hydrogenation reaction of biodiesel and its composition changes were detected. In addition, the linear range, relative correction factor, precision, detection limit and recovery of the GC method were studied. And the iodine value of biodiesel according to the component content measured by GC agreed well with that of titration method. The results show that the method could be used in analyzing the component of biodiesel with good accuracy and reliability.2) The catalytic transfer hydrogenation of biodiesel obtained by the transesterification from soybean oil or rapeseed oil was studied. The iodine value and the composition of the biodiesel were measured at a given time in the process of the reaction. And the effects of the stirring rate, reaction temperature, reaction time, the amount of catalyst and other process parameters on the hydrogenation reaction of biodiesel were investigated. The results show, when the stirring speed was 200rpm, the reaction temperature was 85℃(boiling point), the amount of catalyst was 12.7%(w/w) of biodiesel and the reaction time was 60-80min, the amount of methyl oleate (C18:1) and methyl trans-oleate (t-C 18:1) in the methyl esters of soybean oil as biodiesel was maximum, and the cis/trans isomerization reaction of C 18:1 was greatly controlled. At that time, the integrated properties of biodiesel was good. In addition, the hydrogenation selectivities of methyl linolenate (C18:3) and methyl linoleate (C18:2) in biodiesel were much higher than that of methyl oleate (C18:1). It is beneficial to control the further hydrogenation reaction and avoid affecting the low temperature fluidity of biodiesel for generating a large number of methyl stearate (C18:0) with high melting point. However, a large number of methyl trans-oleate (t-C18:1) were generated in the process of C 18:3 and C18:2 reacting to C18:1. And the reactive rate of c-C 18:1 to C18:0 was much higher than that of t-C 18:1.3) The reaction kinetics of C 18:3, C 18:2 and C18:lin the biodiesel were studied, and then the hydrogenation selectivity was theoretically analysed. The results show that under the catalytic action of Raney-Ni, the catalytic transfer hydrogenation of biodiesel could be described by pseudo-first order kinetic model. The reaction rate constants of C 18:3 and C18:2 are similar (k1/k2≈1), but they are much higher than that of C 18:1 (k1/k3≈6). The good hydrogenation selectivity of C 18:3 and C18:2 in the catalytic transfer hydrogenation reaction were illustrated again.4) The integrated properties of biodiesel after hydrogenation were studied by determining the oxidative stability and the kinematic viscosity. At 85℃, the oxidative stability of biodiesel the kinematic viscosity after 30min meeted the requirements of 360min and 1.9-6.Omm2/s respectively in GB/T 20828-2007. However, the melting point of biodiesel was increased for the reducing of unsaturation, which impacted the low-temperature performance of biodiesel. |
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