大豆油与甲醇酯交换反应体系的相平衡研究

doi:10.11949/0438-1157.20211791

摘要/Abstract

摘要：

生物柴油是一类清洁的可再生液体燃料，精炼植物油与甲醇酯交换是制备生物柴油的重要反应。针对目前难以准确获得酯交换反应体系的多组分相平衡组成等方面存在的问题，研究了间歇反应和连续逆流分离甘油等不同反应方式下大豆油与甲醇酯交换反应体系的多组分相平衡行为，并以三油酸甘油酯与甲醇酯交换为模型反应，采用UNIFAC和Modified UNIFAC模型进行了模拟计算。结果表明，在常压、60^oC反应条件下，在总组成偏离甲醇-甲酯二元组成的区域，UNIFAC和Modified UNIFAC模型准确计算了生物柴油酯交换反应体系的三元和四元相平衡组成。在甘油含量大于2.2%(质量)或转化率小于90%(质量)的酯交换反应中，计算值与实验值的平均偏差约为2%。酯交换反应相平衡的实验值和模型计算值表明，采用连续逆流方式分离甘油可以提高酯相中的甲醇含量，有利于传质和酯交换反应。这些结果为生物柴油工艺过程模拟、设备优化以及新技术开发提供了理论参考。

关键词: 相平衡, 酯交换, 生物柴油, 大豆油, 甲醇, 脂肪酸甲酯, 甘油

Abstract:

Biodiesel is a clean and renewable liquid fuel that is readily made from vegetable oils. Transesterification of the refined vegetable oils with methanol is an important reaction for the production of biodiesel. In order to accurately calculate the multicomponent-phase equilibrium compositions in the transesterification reactions, the multicomponent-phase equilibria of the transesterification reaction between soybean oil and methanol at atmospheric pressure and 60℃ were studied. The transesterification of triolein with methanol was particularly taken as a model reaction, and the UNIFAC and modified UNIFAC activity coefficient models were used to predict the ternary and quaternary phase equilibrium compositions. The results showed that in the region that the overall composition deviates from the binary composition of methanol-methyl esters, the UNIFAC and modified UNIFAC models accurately provided the ternary and quaternary phase equilibrium compositions in the transesterification reaction system. At the glycerol contents greater than 2.2%(mass) or the conversions lower than 90%(mass), using the UNIFAC model, the phase equilibrium compositions for the reaction process were accurately calculated with a deviation of about 2% compared to the experimental values. Both the experimental and modeled values showed that the continuous countercurrent separation of glycerol can improve the methanol to oil ratios and facilitate the mass transfer and transesterification rates. These results provide a theoretical reference for biodiesel process simulation, equipment optimization and new technology development.

Key words: phase equilibrium, transesterification, biodiesel, soybean oil, methanol, fatty acid methyl esters, glycerol

中图分类号:

O 642.42

张家仁, 刘海超. 大豆油与甲醇酯交换反应体系的相平衡研究[J]. 化工学报, 2022, 73(5): 1920-1929.

Jiaren ZHANG, Haichao LIU. Phase equilibrium of transesterification reaction system between soybean oil and methanol[J]. CIESC Journal, 2022, 73(5): 1920-1929.

图/表 6

图1 三脂肪酸甘油酯与甲醇酯交换反应（R1、R2、R3为脂肪酸碳链)

Fig.1 Transesterification reaction between triglycerides and methanol

图2 大豆油与甲醇酯交换反应体系三元相图在60oC、常压条件下的质量分数:● 实验和计算的总组成;○ 计算的总组成;■ 实验值;△ UNIFAC计算值;▽ Modified UNIFAC计算值;虚线是实验值相平衡组成的系线

Fig.2 Ternary phase diagrams for the transesterification reaction of soybean oil and methanol

图3 三元相平衡数据的Othmer-Tobias方程拟合曲线采用在60oC、常压条件下的三元相平衡组成数据,Ⅰ和Ⅱ分别是极性相(主要含有甘油和甲醇)和弱极性相(主要含有大豆油及其甲酯);在大豆油-甲醇-甘油、大豆油-甲醇-大豆油甲酯、大豆油-甘油-大豆油甲酯和甲醇-甘油-大豆油甲酯的三元体系中,a分别是甘油、甲醇、甘油和甘油的质量分数,b分别是大豆油、大豆油、大豆油和大豆油甲酯的质量分数

Fig.3 Correlating curves of Othmer-Tobias plot for the ternary phase equilibrium

表1 四元体系总组成和相平衡组成

Table 1 Overall compositions and phase equilibrium compositions for the quaternary system

序号	总组成（质量分数）				Conv./%	酯相组成（质量分数）				甘油相组成（质量分数）
序号	x₁	x₂	x₃	x₄	Conv./%	x₁	x₂	x₃	x₄	x₁	x₂	x₃	x₄
间歇反应
1	0.821	0.179			0	0.915	0.085			0.011	0.989
2	0.736	0.171	0.011	0.082	10.03	0.839	0.071	0	0.091	0.006	0.889	0.083	0.023
3	0.654	0.161	0.020	0.165	20.11	0.745	0.070	0	0.184	0.004	0.810	0.153	0.033
4	0.335	0.127	0.057	0.480	58.88	0.384	0.068	0.001	0.547	0.001	0.543	0.444	0.013
5	0.164	0.108	0.070	0.659	80.09	0.187	0.060	0.001	0.752	0	0.445	0.549	0.006
6	0.083	0.099	0.079	0.740	89.97	0.094	0.059	0.002	0.845	0	0.387	0.609	0.004
7		0.088	0.087	0.825	100.00		0.053	0.002	0.945		0.331	0.667	0.002
连续逆流分离甘油
1	0.900		0.100		0	1.000		0	0.005	0		1.000
2	0.874	0.034	0.092	0.000	0	0.980	0.015	0	0.188	0	0.183	0.817	0
3	0.622	0.149	0.072	0.157	20.13	0.767	0.044	0.001	0.384	0	0.591	0.402	0.007
4	0.512	0.088	0.059	0.342	40.05	0.577	0.039	0.001	0.574	0	0.456	0.540	0.004
5	0.352	0.071	0.040	0.537	60.37	0.375	0.050	0.001	0.723	0	0.384	0.612	0.004
6	0.173	0.113	0.020	0.695	80.08	0.180	0.094	0.003	0.805	0	0.579	0.390	0.031
7	0.088	0.106	0.022	0.784	89.88	0.092	0.099	0.004	0.005	0	0.528	0.455	0.017
8		0.098	0.005	0.898	100.00	完全互溶

图4 四元体系相平衡组成（质量分数）的实验值与UNIFAC 和 Modified UNIFAC模型计算值的关系◆ 大豆油/三油酸甘油酯;■ 甲醇;○ 甘油;▲ 大豆油脂肪酸甲酯/油酸甲酯

Fig.4 Experimental mass fractions versus predicted mass fractions by UNIFAC and Modified UNIFAC for phase equilibrium compositions of quaternary system

表2 实验值和计算值质量分数的偏差

Table 2 Deviation of the experimental and calculated mass fractions

方法	间歇反应		连续逆流分离甘油
方法	MAD	MSD	MAD	MSD
UNIFAC	0.020	0.001	0.019	0.002
Modified UNIFAC	0.022	0.001	0.026	0.002

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