Liquid–Liquid Equilibrium for the Ternary System of Methyl Laurate

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Liquid−Liquid Equilibrium for the Ternary System of Methyl Laurate/ Methyl Myristate + Ethanol + Glycerol at 318.15 and 333.15 K Chengwu Zhang, Huijuan Luo, Shuqian Xia,* and Peisheng Ma Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China ABSTRACT: In this work, liquid−liquid equilibrium (LLE) data for the two ternary systems methyl laurate + ethanol + glycerol and methyl myristate + ethanol + glycerol were measured at 318.15 K and 333.15 K under atmospheric pressure. The reliability of the experimental tie-line data was checked by the Othmer−Tobias correlation. The LLE experimental data were correlated with the nonrandom twoliquid (NRTL) model, and the binary interaction parameters were regressed by the data fitting. The correlated results agreed satisfactorily with the experimental results.

1. INTRODUCTION Biodiesel, a promising and competitive fuel, has gained wide focus due to its nontoxicity, biodegradability, lower emission, and favorable combustion profile.1 Generally, it is made by the transesterification reaction of triglycerides with aliphatic alcohol. After the reaction, the products are present in two separate liquid phases: the bottom phase is rich in glycerol, while the upper phase contains mainly the fatty acid esters. The excess alcohol distributes in both the two immiscible phases. In order to design the posterior separation process of the two phases, a deep understanding of the liquid−liquid equilibrium (LLE) for the ternary systems composed of fatty acid esters, glycerol, and alcohol is required. Methanol and ethanol are two common alcohols used in biodiesel production. Methanol has the advantages of low price and moderate physicochemical properties.2,3 However, methanol originates mainly from nonrenewable sources. Thus, biodiesel production based on methanol cannot be regarded as completely carbon-neutral.4,5 By comparison, ethanol with a higher dissolving capability and less toxicity2,6 can be derived from biomass, providing an entirely renewable alternative for the biodiesel production.7 Therefore, ethanol has been given considerable attention in biodiesel production.8,9 Recently, some research groups have published experimental data of LLE for the ternary system ethanol + glycerol + real biodiesel, such as canola biodiesel,10 cottonseed biodiesel,11 castor biodiesel,12 soybean biodiesel,13−15 sunflower biodiesel,14 coconut biodiesel,16 and ethyl palm oil biodiesel.17 Besides, we can predict the LLE for the system ethanol + glycerol + real biodiesel by studying the LLE of the ethanol + glycerol + model component. However, experimental data on the phase behavior of pure fatty acid esters, ethanol, and glycerol are still scarce.9,18−21 Fatty acid (lauric acid and myristic acid) methyl esters, with their low molecular weight © 2016 American Chemical Society

and high levels of saturation, are two key components of biodiesel from different oil resources.16,22−24 In this context, LLE data for the ternary systems (methyl laurate/methyl myristate + ethanol + glycerol) were determined at 318.15 K and 333.15K under atmospheric pressure. Additionally, the experimental data were correlated by the NRTL model.

2. EXPERIMENTAL SECTION Chemicals. Table 1 listed the supplier and purity (expressed as mass fraction) of chemicals used in this experiment. The Table 1. Supplier and Purity (Expressed As Mass Fraction) of the Chemicals chemical Methyl laurate Methyl myristate Ethanol Glycerol

supplier

mass fraction

Tianjin Guangfu Fine Chemical Research Institute, China J&K Scientific, China

>0.99

Tianjin Guangfu Fine Chemical Research Institute, China Tianjin Kemiou Chemical Reagent Co., China

>0.998

>0.98

>0.995

purity was confirmed by gas chromatography (GC). No further purification of all these chemicals was carried out. Procedures. The LLE experiments were performed in a jacketed glass vessel at atmospheric pressure, which is schematically shown in Figure 1, and described in detail by Zhengrong Wang.25 The vessel was connected to a thermostatic oil bath, maintaining its temperature fluctuating within Received: December 29, 2015 Accepted: April 13, 2016 Published: April 22, 2016 1868

DOI: 10.1021/acs.jced.5b01105 J. Chem. Eng. Data 2016, 61, 1868−1872

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Table 2. Experimental Tie-Line Data (Expressed as Mole Fraction) for the Ternary System Methyl Laurate (1) + Ethanol (2) + Glycerol (3) at T = (318.15 K and 333.15 K) under Atmospheric Pressurea,b Methyl laurate rich phase (I)

Glycerol rich phase (II)

T/K

xI1

xI2

xI3

xII1

xII2

xII3

318.15

0.884 0.820 0.708 0.622 0.539 0.420 0.794 0.754 0.710 0.640 0.547 0.400

0.107 0.167 0.273 0.349 0.423 0.535 0.181 0.219 0.259 0.322 0.410 0.555

0.009 0.013 0.019 0.029 0.038 0.045 0.025 0.027 0.031 0.038 0.043 0.045

0.000 0.001 0.001 0.002 0.004 0.007 0.001 0.001 0.001 0.002 0.003 0.006

0.132 0.200 0.320 0.391 0.447 0.524 0.217 0.255 0.293 0.353 0.425 0.526

0.868 0.799 0.679 0.607 0.549 0.469 0.782 0.744 0.706 0.645 0.572 0.468

333.15

Figure 1. LLE vessel: (1) Bottom phase port; (2) Upper phase port; (3) Oil outlet; (4) Condenser; (5) Mercury thermometer; (6) Thermometer jack; (7) Oil inlet; (8) Magnetic stirrer.25

The superscript I represents the methyl laurate rich phase, and II represents the glycerol rich phase, and the subscripts 1, 2, and 3 indicate methyl laurate, ethanol, and glycerol, respectively. bStandard uncertainties: u(T) = 0.1 K, u(p) = 1 kPa, and u(x) = 0.001.

0.01 K. A mercury thermometer (accurate to 0.1 K) was equipped to record the temperature of the mixture in the vessel. A condenser ensured complete condensation of the evaporated compounds. An analytical balance (accurate to 0.0001 g) was used to weigh all the chemicals. The mixture (methyl laurate/methyl myristate + ethanol + glycerol) was added into the vessel and then heated. Once the temperature reached the desired temperature constantly, the magnetic stirrer started to agitate the mixture for 2 h vigorously. Then the mixture was left to settle for 24 h. After the system reached the equilibrium state, three samples were obtained from the two immiscible phases by syringes, respectively. They were analyzed with a GC (GC1100, Beijin Purkinje General Instrument Co., Ltd., China) equipped with a DB-5HT capillary column (30 m × 0.25 mm × 0.1 μm), and a flame ionization detector (FID). The temperature of injector, column, and detector were set at 523.15 K, 473.15 K, and 543.15 K, respectively. The carrier gas was nitrogen with the flow rate of 25 cm3·min−1. For each ternary system, several standard samples of known compositions were prepared to calibrate the GC in a certain range of composition. Every sample was analyzed for more than three times to ensure the reliability of the experimental data. The uncertainty of the liquid phase compositions was ±0.001.

Table 3. Experimental Tie-Line Data (Expressed as Mole Fraction) for the Ternary System Methyl Myristate (1) + Ethanol (2) + Glycerol (3) at T = (318.15 K and 333.15 K) under Atmospheric Pressurea,b

3. RESULTS AND DISCUSSION LLE Experimental Data. The LLE experimental data of the two ternary systems (methyl laurate/methyl myristate (1) + ethanol (2) + glycerol (3)) at 318.15 K and 333.15 K under atmospheric pressure were listed in Table 2 and 3, with all concentrations expressed as mole fraction. The phase diagrams for the ternary mixtures were presented in Figures 2 and 3, respectively. The Othmer−Tobias correlation was used to check the reliability of the experimental tie-line data. The correlation equation26 is as follows.

In the equation, xI1 is the mole fraction of methyl laurate/ methyl myristate in the fatty acid ester rich phase, xII3 is the mole fraction of glycerol in the glycerol rich phase, and a and b are the parameters of the correlation equation. Figure 4 gives the fitting lines using the Othmer−Tobias equation. The parameters a and b and the correlation coefficients R2 were listed in Table 4. The value of R2, close to one, shows the relatively good linear fit of the experimental data. Data Correlation. The NRTL model of Renon and Prausnitz27 was applied to correlate the experimental data. The NRTL model equation is as follows:

⎛ 1 − x II ⎞ ⎛1 − xI ⎞ 3 1 ⎟ ⎟ = a + b ln⎜ ln⎜ I II ⎝ x1 ⎠ ⎝ x3 ⎠

a

Methyl myristate rich phase (I)

Glycerol rich phase (II)

T(K)

xI1

xI2

xI3

xII1

xII2

xII3

318.15

0.784 0.684 0.589 0.509 0.450 0.399 0.806 0.732 0.678 0.620 0.553 0.504

0.203 0.296 0.385 0.460 0.516 0.562 0.182 0.253 0.305 0.361 0.420 0.468

0.013 0.020 0.026 0.031 0.034 0.038 0.012 0.015 0.018 0.020 0.027 0.028

0.001 0.001 0.001 0.003 0.004 0.005 0.000 0.000 0.001 0.001 0.001 0.002

0.223 0.319 0.409 0.473 0.517 0.558 0.191 0.263 0.308 0.361 0.419 0.459

0.777 0.681 0.590 0.525 0.479 0.438 0.809 0.737 0.691 0.638 0.580 0.539

333.15

a

The superscript I represents the methyl myristate rich phase, and II represents the glycerol rich phase, and the subscripts 1, 2, and 3 indicate methyl myristate, ethanol, and glycerol, respectively. b Standard uncertainties: u(T) = 0.1 K, u(p) = 1 kPa, and u(x) = 0.001.

ln γ = (1) 1869

∑j τjiGjixj ∑k Gkixk

+

∑ j

⎛ ∑ xτ G ⎞ ⎜⎜τij − l l lj lj ⎟⎟ ∑k Gkjxk ⎠ ∑k Gkjxk ⎝ xjGij

(2)

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Figure 3. Phase diagrams for the ternary mixtures methyl myristate + ethanol + glycerol at (a) 318.15 K, (b) 333.15 K. ■/solid lines, experimental tie-line data; ○/dashed lines, corrlated tie-line data.

Figure 2. Phase diagrams for the ternary mixtures methyl laurate + ethanol + glycerol at (a) 318.15 K, (b) 333.15 K. ■/solid lines, experimental tie-line data; ○/dashed lines, correlated tie-line data.

τij =

Δgij RT

=

Bij

(τij ≠ τji)

T

Gij = exp( −αijτij)

(αij = αji)

(3) (4)

In the equations, i, j, k, and l represent each component, and Bij is the NRTL binary interaction parameters. The NRTL binary interaction parameters were correlated by the flash calculation method.28 The objective function (OF) is as follows: exp cal 2 OF = min ∑ ∑ ∑ (xijk − xijk ) k

j

i

(5)

exp In the equation, xijk and xcal ijk are the experimental and correlated values, respectively. The subscripts i, j, and k refer to the component, phase, and tie-line, respectively. In this study, the root-mean-square deviation (RMSD) was applied to evaluate the NRTL model, which is as follows:

Figure 4. Othmer−Tobias plot for the systems methyl laurate + ethanol + glycerol at (■, 318.15 K; ●, 333.15 K), methyl myristate + ethanol + glycerol at (□, 318.15 K; ○, 333.15 K).

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Table 4. Correlated Data (a, b, R2) of the Othmer−Tobias Equation for the Two Ternary Systems at 318.15 and 333.15 K System methyl laurate-ethanol-glycerol methyl myristate-ethanol-glycerol

T/K

a

b

R2

318.15 333.15 318.15 333.15

0.0723 0.2037 0.0942 0.1436

1.1499 1.2425 1.1307 1.099

0.9905 0.9968 0.9976 0.9992

exp cal 2 ⎫1/2 ⎧ ⎪ ∑k ∑j ∑i (xijk − xijk ) ⎪ ⎬ RMSD = ⎨ ⎪ ⎪ 6n ⎩ ⎭

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(6)

In the equation, n is the number of tie-lines. The NRTL binary interaction parameters and the RMSD values were listed in Table 5. The experimental and regressed Table 5. NRTL Binary Interaction Parameters and RMSD Values for the Two Ternary Mixtures System

i

j

Bij/K

Bji/K

αij

T/K

RMSD

methyl laurateethanolglycerol methyl myristateethanolglycerol

1 1 2

2 3 3

−16.64 813.378 903.522

948.57 2621.31 −5.48

0.2 0.2 0.2

318.15 333.15

0.0086 0.0135

1 1 2

2 3 3

−1976.73 1606.85 484.628

1411.05 5642.48 −1787.44

0.2 0.2 0.2

318.15 333.15

0.0055 0.01

LLE data for the two ternary mixtures were presented in Figures 2 and 3. As shown in Table 5 and Figures 2 and 3, the NRTL model could give good correlation data.

4. CONCLUSIONS The LLE data of two ternary systems (methyl laurate/methyl myristate + ethanol + glycerol) were measured at 318.15 K and 333.15 K under atmospheric pressure. The measured temperature had a negligible influence on the mutual solubility of components in the ternary mixtures. The reliability of the tieline data was checked by the Othmer−Tobias correlation. The correlation coefficients R2 (>0.9905) showed the relatively good linear fit of the experimental data. The NRTL model was used to regress the experimental data, and the correlated binary interaction parameters were obtained. In addition, the RMSD values were less than 0.0135, indicating that the NRTL model could give good correlations for the experimental data. The experimental data determined in this work can be complementary to the database with regard to the issues of biodiesel production and posterior separation processes.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-022-27405929. E-mail: [email protected].. Funding

Financial support from the Tianjin Natural Science Foundation (Project No.13JCYBJC19300) is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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