Measurement and correlation of ternary vapor-liquid equilibria for methanol + glycerol + fatty acid methyl ester (methyl laurate, methyl myristate, methyl palmitate) systems at elevated temperatures and pressures
Graphical abstract
The phase equilibria data of three ternary systems at elevated temperatures (about 493.0 K, 523.0 K) and pressures were measured by a flow-type method. The experimental data were regressed by Redlich-Kwong-Soave equation of state (RKS EOS), RKS EOS combined with Boston and Mathias function (RKS-BM EOS) and Redlich-Kwong-Soave Aspen equations of state (RKS-Aspen EOS) with van der Waals (vdW) mixing rule.
Introduction
Biodiesel is considered as an alternative diesel fuel for traditional fuel due to its renewable and environmental benefits, such as biodegradable, nontoxic, low emission and renewable. The most usual technology for biodiesel production is transesterification, which was defined as the reaction of various oils (such as vegetable oil, animal fats or waste oil) with alcohol. The product of transesterification was biodiesel (fatty acid alkyl esters, FAAE) and glycerol [1]. All of these oils are composed by triacylglycerols, minor amounts mono and diacylglycerols and free fatty acids in refined progress. Alcohols can be methanol, ethanol, 1-propanol, 1-butanol. Among the alcohols, transesterification yield was decreased in the order: methanol > ethanol > 1-propanol > 1-butanol as reported in the literature [2]. So, methanol is the most frequently and widely used.
The transesterification reaction has been developed using alkaline and acidic catalysts in previous research. Recently, Saka and Kusdiana [3] firstly reported a supercritical methanol transesterification without catalyst. The separation of the products and the catalysts was not needed. Moreover, supercritical methanol method is an environmentally friendly technology in biodiesel production process. Many scholars pay great attention to its excellent performances including shorter reaction time, non-catalysts and simpler separation process. In recent years, a continuous transesterification process of vegetable oil using supercritical methanol was designed by He et al. [4]. Wang et al. [5] have made focus on methanol recovery through flash evaporation in continuous production of biodiesel via supercritical methanol. Continuous transesterification process is a great progress for the industrialized development of biodiesel with supercritical methanol method, among which, the phase separation of products (fatty acid methyl esters, FAME and glycerol) and methanol is very important. It is crucial to know the phase behavior of the reaction mixtures for detailed process design.
Up to now, some researchers have reported the phase equilibria studies of some binary systems near supercritical temperature of methanol. Tang [6], Glisic [7] and Hegel [8] reported VLE data of methanol + triglycerides systems. Shimoyama’s group [9], [10] measured the VLE of methanol + glycerol/methyl laurate/methyl myristate systems. The phase equilibria data for methanol + C18 methyl esters at 523–573 K in the pressure ranges of 2.45–11.45 MPa were reported by Fang [11]. However, only Fang et al. [12] measured the phase equilibria data for ternary system (methanol + C18 FAME + α-tocopherol) at high temperatures and pressures. Therefore, the experimental VLE data of the relevant systems including methanol, glycerol and FAME are still severely insufficient for the detailed design and operation of the biodiesel production process with methanol.
In this work, an apparatus was designed and manufactured for measuring the vapor-liquid phase behavior at elevated temperatures and pressures by our research group. And a flow-type method was used to measure VLE data of three ternary systems (methanol + glycerol + methyl laurate/methyl myristate/methyl palmitate) at elevated temperatures and pressures. The experimental VLE data were correlated using the Redlich-Kwong-Soave equation of state [13] (RKS EOS), RKS EOS combined with Boston and Mathias function [14] (RKS-BM EOS) and Redlich-Kwong-Soave-Aspen equations of state [15] (RKS-Aspen EOS). The van der Waals (vdW) [16] mixing rule with EOSs was used to regress the experimental data.
Section snippets
Materials
The suppliers and purities of the materials used in this work were listed in Table 1. Their purities were checked by a gas chromatography. These chemicals were adopted without further purification.
Apparatus and procedure
A vapor-liquid phase equilibria apparatus was designed and manufactured by our research group, and the schematic diagram is presented in Fig. 1. The preheater, high pressure liquid circulating pump (HPCP) and equilibria cell are the major components of this apparatus. The phase behavior inner the
Model
The experimental VLE data were regressed by the RKS EOS [13], RKS-BM EOS [14] and RKS-Aspen EOS [15]. The equations of state are shown by the following expressions:
RKS EOS
Results and discussion
In this study, firstly, the VLE data of binary system (methanol + methyl laurate) at 493.2 K were measured and listed in Table 3. The comparisons between our experimental VLE data of the binary system (methanol + methyl laurate) with the published data [10] were shown in Fig. 2. It can be observed that the experimental data in this work are consistent well with the published data. The reliability of the experimental equipment and procedure were checked.
The VLE experimental data of ternary
Conclusions
The VLE data of three ternary systems (methanol + glycerol + methyl laurate, methanol + glycerol + methyl myristate, methanol + glycerol + methyl palmitate) were measured at elevated temperatures (about 493.0 K, 523.0 K) and pressures by a flow-type method. In vapor phase, the methanol mole fractions are very close to unity and the FAME mole fractions are very small for all the ternary systems studied in this work. In the liquid phase, the mole fractions of methanol increase with rising
Acknowledgements
The authors sincerely acknowledge the Tianjin Natural Science Foundation (13JCYBJC19300) and the National Basic Research (973) special preliminary study program (2014CB260408) for the financial support.
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