三油酸甘油酯与甲醇反应合成生物柴油的热力学分析

Thermodynamic Analysis of Triolein and Methanol Synthesis of Biodiesel

针对三油酸甘油酯与甲醇的3步连续可逆酯交换反应,采用基团贡献法和其他计算方法估算各组分的焓变、熵变、Gibbs自由能变和标准平衡常数,以及在298.15～428.15K时各值随温度的变化情况.研究结果表明:三油酸甘油酯与甲醇的各步反应和总反应的热效应都不大,第2,3步反应的焓变值随温度变化很小,第1步反应和总反应的焓变随温度升高而增大;3步反应和总反应的Gibbs自由能变都大于零,且随温度升高都呈略有增大的趋势;三步反应和总反应的标准平衡常数都小于1,且第1步反应平衡常数随温度升高略微增大,其余两步反应和总反应平衡常数随温度升高都略有减小.

Estimation methods were used to estimate the formation enthalpy and entropy of each component,and combined with existing literature datum,the equilibrium thermodynamic analysis of triolein and methanol transesterification reaction to the synthesis of biodiesel was performed.The results show that： the thermal effects of each step of triolein with methanol transesterification reactions and the overall reaction are all not significant.The first step shows a slight endothermic reaction,while the other two steps and the overall reaction show slight exothermic reactions.There are no significant changes of enthalpy values in the second and the third step reactions as the temperature changes,while in the first and overall reactions the values increase as temperature increasing.The changes of Gibbs energy of three steps and overall reactions are all positive,which indicate that reactions can not occur spontaneously under standard state.The changes of Gibbs energy of three steps and overall reaction increase when temperature increasing.From 298.15 to 428.15 K,the equilibrium constants of three steps and the overall reaction are all less than 1,which are 0.1 to 0.2,0.4 to 0.6 and 4×10-4 to 10×10-4 for each step,3×10-5 to 7×10-5 for overall reaction,respectively.The equilibrium constant of the first step slightly increases as temperature increasing,while the other two steps and overall reaction slightly decrease,which demonstrate that in order to make the reaction spontaneous,the concentration of the methanol should be increased,or the concentration of the product in the reaction system should be reduced.