Experimental Data and Phase Equilibrium Modeling in Ternary and

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Experimental Data and Phase Equilibrium Modeling in Ternary and Pseudoquaternary Systems of Sunflower Oil for Biodiesel Production Juliana C. Nunes,*,†,‡ Jessica J. P. Nascimento,† Amanda S. Peiter,†,‡ Leandro Ferreira-Pinto,† Joao I. Soletti,† Sandra H. V. de Carvalho,† Jose J. N. Alves,‡ and Antonio C. B. de Araujo‡ †

Chemical Engineering Department, Federal University of Alagoas, Maceió, Alagoas 57072-970, Brazil Chemical Engineering Department, Federal University of Campina Grande, Campina Grande, Paraiba 58429-900, Brazil

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‡

S Supporting Information *

ABSTRACT: Biodiesel production consists of several processes, the reaction stage being the most important. However, after reaction, the biodiesel product undergoes separation involving an aqueous phase, hence, the need for accurate information on liquid−liquid equilibrium (LLE) parameters of mixtures formed by biodiesel, alcohol, glycerol, and the aqueous phase. We here performed experiments on ternary and pseudoquaternary systems of sunflower biodiesel to gather data to estimate binary interaction parameters for LLE using the UNIQUAC model. Our results showed that temperature had essentially no effect on ternary systems, with a small reduction of the immiscibility region at 318.15 K. As for the pseudoquaternary systems, we observed that by increasing the amount of the aqueous phase from 50 to 75%, the region of phase separation increased, and the solubility of biodiesel in the glycerol-rich phase decreased, with no significant change in the binodal curves and tie-lines. We then successfully estimated the binary interaction parameters and observed that very good agreement was obtained between the experimental data and the calculated values using the UNIQUAC model, with maximum deviations of about 0.25%. We thus firmly believe that such parameters can be safely used, e.g., to design and efficiently operate separation processes for systems like the ones investigated in this article. purification units.15,16 In the separation section, glycerol and biodiesel are usually separated by decantation, and we can distinguish between two phases: one glycerol-rich phase and one ester-rich phase, noting that unreacted alcohol is distributed in both phases.17 One important characteristic of the separation section that needs investigation is the extent of the miscibility between biodiesel, glycerol, and the unreacted alcohol.18 In the purification unit, a liquid−liquid extraction process using water as the washing agent removes traces of catalyst, glycerol, excess alcohol, and soap from the decanter outlet stream.5 There, the distribution of these components between the different liquid phases is important to account for biodiesel losses in the aqueous phase and, hence, for the impurities in the biodiesel-rich phase5 as well as to reduce water consumption17 and to define operation of the downstream distillation section. One way to describe the physical phenomena taking place in the separation and purification units is to assume that the liquid phases are in thermodynamic equilibrium. Therefore, a deep understanding of the parameters involved in a

1. INTRODUCTION The search for alternative sources of energy has become quite intense,1 and the reasons can be related to the unwanted oscillations in oil price, the supposed future shortage of fossil fuels, and some well-known environmental concerns related to this nonrenewable commodity.2 One rather promising source is biodiesel from vegetable oils or animal fat,3 which constitutes a viable fuel,4 especially for transportation purposes. Moreover, biodiesel is a source of renewable energy that opens up the possibility of reducing greenhouse-gas emissions relative to fossil fuels.4 The monoalkyl esters from these raw materials produce biodiesel that can either completely substitute or be mixed in any proportion with petroleum diesel,5 since they have very similar physicochemical properties.6,7 Among the various vegetable stocks that can be used to produce biodiesel, sunflower (Helianthus Annus I) is an easily adaptable plant that can resist heat, cold, and drought quite well. It has high oil content and is laden with unsaturated acids (approximately 83%), mostly linoleic and oleic acids.8 The sunflower oil is then reacted with simple alcohols in the well-known homogeneous basic catalytic transesterification reaction to produce a mixture of alkyl esters of fatty acids and glycerol.9−14 This mixture, which also contains significant amounts of unreacted alcohol, is then taken to the separation and © XXXX American Chemical Society

Received: April 3, 2018 Accepted: January 3, 2019

A

DOI: 10.1021/acs.jced.8b00276 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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interaction parameters of the UNIQUAC model, and the results show good agreement between the experimental data and calculated values. Sena and Pereira23 studied the liquid− liquid equilibrium of biodiesel from melon seed oil− methanol−glycerol. The authors used the NRTL and UNIQUAC models to analyze the phase equilibrium. The results showed that melon biodiesel can be easily separated from methanol and glycerol in the separation step. The validation results showed that for the NRTL model, the deviation in composition was 1.25%, while for the UNIQUAC model the deviation was 2.70%. The objective of this work is to fit experimental data to define miscibility regions and to determine parameters for the UNIQUAC model applied to the liquid−liquid equilibrium of ternary systems of sunflower biodiesel−methanol−glycerol and pseudoquaternary systems of sunflower biodiesel−methanol− glycerol−water/diluted acid, using an optimization formulation that considers the minimization of the Gibbs energy at various conditions. Section 2 describes the experimental methods and apparatus used to produce data for model regression as well as the formulation of the Gibbs-based optimization schemes. In Section 3, we report and discuss the results of such optimizations comparing the fitted UNIQUAC model with experimental data. Some conclusions are then drawn in Section 4.

thermodynamic model capable of predicting the liquid−liquid equilibrium (LLE) between phases is paramount. Particularly important are LLE data for quaternary systems involving biodiesel−alcohol−glycerol−water in the purification unit (and also in the reaction section), since these are still scarce in the reviewed literature.19 Local composition models such as UNIQUAC, UNIFAC, and NRTL can be used to compute activity coefficients for mixtures that largely deviate from ideality, like the mixtures involved in the biodiesel production. These models assume that some parameters, especially the binary interaction coefficients usually determined from experimental data, are known a priori.20,21 Along these lines, only a few authors have reported values of these parameters for some ternary systems of biodiesel mixtures. However, to our knowledge, no investigation has reported values of these parameters for the important quaternary systems containing water. Gonçalves et al.1 studied the liquid−liquid equilibrium of the ternary system Brazilian nut biodiesel−methanol−glycerol at 303.15 and 323.15K. The experimental data were used to fit all parameters for the NRTL and UNIQUAC models. The validation results showed that for the NRTL model, the mean square deviations in composition were 1.49 % at 303.15 K and 1.18 % at 323.15 K, while for the UNIQUAC model, these were 1.85 % at 303.15 K and 1.97 % at 323.15 K. Aiming to investigate the purification of methyl biodiesel, Pinheiro et al.3 considered the liquid−liquid equilibrium of ternary systems containing soybean/corn/coconut biodiesel−methanol−water at 293.15 and 313.15 K. They observed that temperature had essentially no effect on the purification process, and that methanol, as the extracting agent, was able to accelerate the purification of the biodiesel-rich phase and to reduce the energy consumption in the conventional drying process. Moreover, liquid−liquid equilibrium experimental data were correlated with fitted NRTL, UNIQUAC, and UNIFAC models to produce mean square deviations in composition of 1.34% for the NRTL, 1.34% for the UNIQUAC, and 4.13% for the UNIFAC model. Rostami et al.16 carried out the study of the liquid−liquid equilibrium of ternary systems involving sunflower/canola biodiesel−glycerol−methanol at 293.15, 303.15, and 313.15 K. They reported that the region of immiscibility decreased with increasing temperature and concluded that losses of biodiesel during purification were small if this unit is operated at low temperatures. They used the UNIQUAC model with parameters fitted to experimental data to predict the behavior of the binary and ternary systems, where the binary interaction coefficients were further optimized for each system to increase accuracy. They found good agreement between experiments and model predictions and concluded that the UNIQUAC model is quite adequate for systems like these. França et al.22 carried out liquid−liquid equilibrium studies on ternary systems and on a pseudoquaternary system involving soybean ethyl biodiesel−ethanol− glycerol−water at 298.15 K. The binodal curves were determined by the titration method under isothermal conditions. Each experimental point of the curve had its density analyzed, which generated some calibration curves. To analyze the tie-lines of each system, they prepared mixtures of all components of the respective system. Such mixtures were thus kept under stirring, after which phase separation took place. Then, the mass fractions of the components of each phase were determined using the equations of the density calibration curves. These data were used to estimate binary

2. MATERIALS AND METHODS 2.1. Biodiesel Production. Table 1 lists the chemicals used in the experiments. Note that no further purification was conducted on these chemicals. Table 1. Chemicals Used in the Experiments chemical methanol sodium hydroxide in micro pearls sulfuric acid glycerol hexane

manufacturer

molar mass (g/mol)

minimum purity (mole fraction)

Neon Neon

32.04 40.00

0.9980 0.9700

Neon Neon Synth

98.08 92.09 -

0.9500 0.9970 0.9850

The production of biodiesel was conducted in a pilot unit composed of a 2 L glass reactor equipped with a water circulation jacket and mechanical stirring. The reactor temperature was maintained by a thermostatic bath (Model TE-184) monitored using a digital multimeter (Model ET14000). Refined sunflower oil (800 g) was used to synthesize the methyl biodiesel. The following parameters were defined for the transesterification reaction:24 temperature of the thermostatic bath at 333.15 K, catalyst mass at 1% of the oil mass, oil/ methanol molar ratio at 1:6, a stirring speed of 350 rpm, and a total reaction time of 30 min. These conditions provided yields of sunflower methyl biodiesel above 96.5%, a lower limit established by the 2008 Brazilian Regulatory Agency ANP, Standard Resolution 7.25 Table 2 provides a characterization of the synthesized biodiesel. After reaction, decantation took place, and biodiesel was obtained by removing glycerol, alcohol, and catalyst residues from the biodiesel-rich phase mixture. This purification process consisted of five steps: (1) Weighing of the biodiesel-rich phase mixture. B



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thermostatic bath (Model TE-184) to determine the liquid− liquid equilibrium curves, the calibration curves, and the tielines of the systems considered in Table 3. The cooling water

Table 2. Composition and Physicochemical Properties of the Synthesized Sunflower Methyl Biodiesel ester of

structure

R−(CH2)10−CH3 R−(CH2)12−CH3 R−(CH2)14−CH3 R−(CH2)6−CH=CH−(CH2)6− CH3 R−(CH2)16−CH3 R−(CH2)7−CH=CH−(CH2)7− CH3 linoleic acid R−(CH2)7−CH=CH−CH2− CH=CH−(CH2)4−CH3 linolenic acid R−(CH2)7−(CH=CH−CH2)3− CH3 arachidic acid R−(CH2)18−CH3 viscosity at 313.15 K (cst) density at 293.15 K (g/cm3) acidity (mg of KOH/g) lauric acid myristic acid palmitic acid palmitoleic acid stearic acid oleic acid

a

acronym (Cx:y)b

mole fractionc

C12:0 C14:0 C16:0 C16:1

0.0034 0.0024 0.0621 0.0043

C18:0 C18:1

0.0256 0.2830

C18:2

0.6151

C18:3

0.0029

C20:0

0.0012 4.3d 0.8804e 0.36f

Table 3. Systems Considered in the Study of Liquid−Liquid Equilibrium Involving Sunflower Biodiesel system system system system

system 5 system 6 system 7

Distilled water mass at 50% of glycerol mass. bDistilled water mass at 75% of glycerol mass. cDiluted sulfuric acid mass at 50% of glycerol mass. dDiluted sulfuric acid mass at 75% of glycerol mass.

temperature was kept constant and monitored using a digital multimeter (Model ET-14000). The apparatus was also equipped with a magnetic stirrer to promote an effective contact between the components inside the cell. In order to gather the necessary information to plot the various liquid−liquid equilibrium curves, the experimental procedure consisted of titrating one homogeneous mixture with another mixture until a murky liquid is formed, an indication of phase separation. We first plotted the binodal curve for the biodiesel-rich phase. To this end, the bath was adjusted to the desired temperature, and then, a total of 16 g of methanol and biodiesel in several different proportions were added to the cell. These two components were kept under continuous stirring to ensure complete homogenization. In the case of the ternary systems, titration was done with glycerol alone, while for the pseudoquaternary systems 4 and 5, we used a solution of glycerol and distilled water, and for systems 6 and 7, we used a solution of glycerol and diluted sulfuric acid (pH = 2). Subsequently, we worked out the other side of the binodal curve. For the ternary systems, this curve represents the glycerol-rich phase, while for pseudoquaternary systems 4 and 5, it is the glycerol + distilled water-rich phase, and for systems 6 and 7, the glycerol + diluted sulfuric acid-rich phase. For the ternary systems, a total of 16 g of alcohol and glycerol in several different proportions were added to the cell. These two components were then kept under stirring until complete homogenization. Then, titration with biodiesel started. We conducted similar experiments for the pseudoquaternary systems. However, for systems 4 and 5, the alcohol did not solubilize in pure glycerol but in a glycerol + distilled water solution, and for systems 6 and 7, the alcohol was solubilized in a solution of glycerol + diluted sulfuric acid (pH = 2). Since we noticed that the effect of temperature on the binodal curves was only marginal for the ternary systems, we decided to plot the equilibrium curves, the calibration curves, and the tie-lines for the pseudoquaternary systems only for the temperature of 298.15 K. The binodal curve gives the mass fractions of the component at each point, and then, two calibration curves were obtained for the ternary system at 298.15 K. Therefore, for the biodieselrich phase, we drew a plot of the biodiesel mass fraction versus

(2) Addition of 1/4 of this mass of diluted sulfuric acid (pH = 2). (3) Agitation and separation of the denser phase. (4) Adjustment of the pH of the denser phase to the pH 5− 7 range to remove catalyst residue. The procedure goes like this: If pH is still far above 7, then add diluted sulfuric acid to the mixture. If pH is near 7, then add distilled water (pH = 5) to the mixture. In our case, the mass of diluted sulfuric acid and distilled water used was 1/2 of the mass previously defined. After successive washings, the pH was well within the 5−7 range. (5) Removal of water and alcohol by heating the mixture under vacuum to 378.15 K for 1 h. These conditions guaranteed no degradation of the final product. In order to determine the biodiesel yield, we prepared a sample consisting of 0.15 g of the purified sunflower biodiesel and 1 mL of standard solution (tricaprylin plus hexane in desiccator) and then injected 1 μL of this mixture in the SHIMADZU GC-2010 Plus model gas chromatograph with a 2.2 m column. A temperature of 340 °C was kept in the detector, 523.15 K was kept in the injector, and 323.15 K was kept in the column. The analysis took approximately 20.5 min and was performed under a column pressure of 6 kPa. A solution of tricaprylin served as the internal standard, and the flow rate of hydrogen as the entrainment gas was 20 mL/min. The synthetic air flow rate was 200 mL/min, and that of nitrogen was 30 mL/min. The biodiesel yield was then determined according to eq 1. mpAb mbA p

sunflower biodiesel−glycerol−methanol at 298.15 K sunflower biodiesel−glycerol−methanol at 308.15 K sunflower biodiesel−glycerol−methanol at 318.15 K sunflower biodiesel−glycerol−distilled water−methanol at 298.15 Ka sunflower biodiesel−glycerol−distilled water−methanol at 298.15 Kb sunflower biodiesel−glycerol−methanol−diluted sulfuric acid (pH 2) at 298.15 Kc sunflower biodiesel−glycerol−methanol−diluted sulfuric acid (pH 2) at 298.15 Kd

a

a R is COOCH3. bx is the number of carbons, and y is the number of double bonds. cMass fraction uncertainty estimation of ±1.2 × 10−3. d Viscosity uncertainty of ±0.008 cst. eDensity uncertainty of ±0.0005 g/cm3. fAcidity uncertainty of ±0.02 mg KOH/g.

yield (%) = 100f

1 2 3 4

(1)

Where mp is the mass of the internal standard (0.08 g), Ab is the sum of peak areas for biodiesel, Ap is the peak area of the internal standard, mb is the biodiesel mass (0.15 g), and f is a correction factor. 2.2. Liquid−Liquid Equilibrium Curves, Calibration Curves, and Tie-Lines. We used a liquid−liquid equilibrium cell enclosed by a jacket, in which cooling water flows from a C



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ij −Δuij yz zz Aij = expjjj j RT zz k {

the mass fraction of methanol. As for the glycerol-rich phase, we plotted the graph of the glycerol mass fraction versus the methanol mass fraction. We drew the tie-lines using the points below the binodal curve in the region where phase separation occurs so that each phase corresponded to one end of the tieline. The mass fraction of methanol in each phase was then determined by putting the sample in an oven at 353.15 K until the mass remained constant, which indicated that all the alcohol evaporated. So, using the calibration curve, we determined the biodiesel mass fraction and then the glycerol mass fraction by the summation rule of mass fractions. We followed a similar procedure to determine the mass fractions of biodiesel, methanol, and glycerol for the glycerol-rich phase, but in this case, the calibration curve of the glycerol mass fraction versus the alcohol mass fraction served as a means to obtain the glycerol mass fraction. We could not use the method described in the previous paragraph to determine the amount of methanol for the pseudoquaternary systems in Table 3, because part of the water in the glycerol + water or glycerol + diluted acid solutions also evaporated. Thus, for these systems, we plotted the calibration curves and the tie-lines via the viscosity method. The calibration curves were obtained from the point viscosities of the binodal curve. We noted that, even though titration led to spontaneous phase separation, in some cases, one of the phases could not be clearly detected, and only the analysis of the viscosity of the phase collected in larger quantity could guarantee accuracy. We based the determination of kinematic viscosity on the ASTM D44526 and used an Ostwald viscometer with capillary 100 and constant supplied by the manufacturer of 0.01509 mm2/s2. The temperature was maintained at 298.15 K, since this was the temperature used in the determination of the binodal curve. For the biodieselrich phase, two calibration curves were drawn: one of methanol mass fraction versus viscosity and one of biodiesel mass fraction versus viscosity. For the glycerol + distilled water and glycerol + diluted sulfuric acid-rich phases, two calibration curves were drawn: one of methanol mass fraction versus viscosity and one of glycerol + distilled water and glycerol + diluted sulfuric acid versus viscosity. We then conducted the experiments to determine the tie-lines, and thanks to the considerable amount of each phase produced, we easily determined their viscosities. For the biodiesel-rich phase, the methanol and biodiesel mass fractions were obtained from the corresponding calibration curves. Analogously, the mass fractions of methanol and glycerol + distilled water or glycerol + diluted sulfuric acid were also determined from the corresponding calibration curves. 2.3. Thermodynamic Modeling. Regarding the thermodynamic modeling, although the sunflower biodiesel is a mixture of alkyl esters with different chains, we here follow the recommendations discussed in Zhou et al.27 and consider the methyl biodiesel as a simple component (pseudocomponent). According to this reference, the implication is that the phase rule is applicable to systems consisting of n components, one of which is methyl biodiesel, in two phases. In the case of ternary systems, the number of independent variables needed to completely describe the system is one. This hypothesis was also used throughout the experimental stage. Each binary mixture has two adjustable parameters, Aij and Aji, as shown in eqs 2 and 3.

(2)

ij −Δuji yz zz Aji = expjjj j RT zz k {

(3)

Where R is the ideal gas constant, and T is the temperature in K. In this contribution, we tried to represent the data obtained experimentally by an appropriate activity coefficient model.16 Since various authors1,3,16,20−23 reported to have obtained very good results using the UNIQUAC model, we here decided to adopt the UNIQUAC model to produce values for the activity coefficients, and the whole idea is to fit the model to the experimental data by using the binary interaction parameters as decision variables via the least-squares minimization problem formulated in eqs 4−8 NP

NL

NC

p

n

i

min ∑ ∑ ∑ (wip, n, exp − wip, n, calc)2 kij

(4)

NC

∑ xip,n,calc = 1

n = 1, ..., NL

p = 1, ..., NP

i

xip, n, calc > 0

i = 1, ..., NC

n = 1, ..., NL

ÄÅ pÉ Ñ ÅÅ NP NC fi ̂, n ÑÑÑÑ Å Å pÅ o min G = ∑ ∑ ni , nÅÅμi , n + RT ln o ÑÑÑ ÅÅ f i , n ÑÑÑ p i ÅÅÇ ÑÖ i = 1, ..., NC

wip, n, calc =

n = 1, ..., NL

(5)

p = 1, ..., NP (6)

p = 1, ..., NP

(7)

p = 1, ..., NP

(8)

xip, n, calcMi N

∑i C xip, n, calcMi

i = 1, ..., NC

n = 1, ..., NL

In the above formulation, NP is the number of phases; NL is the number of tie-lines; NC is the number of components or pseudocomponents in a tie-line n; w is the mass fraction; x is the molar fraction; G is the Gibbs energy; f ° is the fugacity of the pure component at standard state; μ° is the chemical potential of pure component (these two parameters are calculated prior to minimizing G); γ is the activity coefficient calculated by the UNIQUAC model; M is the molar mass retrieved from the NIST Chemistry WebBook;28 i, n, and p refer to a given component or pseudocomponents, a given tieline, and a given phase, respectively; superscripts exp and calc refer to experimental and calculated values; and α and β are arbitrary phases in equilibrium. Note that eqs 5−8 are the mass balance and phase equilibrium constraints to the optimization problem. We ran the optimization numerically using the supplement Solver in Microsoft Excel 2010. At each iteration of the optimization routine, the phase split was determined by minimizing the Gibbs free energy of the system. Moreover, the activity coefficients of each component in each phase were computed using the supplement (Excel add-in) XSEOS.29 The UNIQUAC model requires the volume (r) and area (q) parameters. For water, methanol, and glycerol, these values D



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were obtained from Anderson and Prausnitz.30 For methyl biodiesel, these parameters were calculated from eqs 9 and 10. C

G

∑ xmi ∑ vkmR k

ri =

m C

qi =

(9)

k G

∑ xmi ∑ vkmQ k m

Figures 2 to 6 show graphically the liquid−liquid equilibrium data for ternary system 1 at 298.15 K (see Table

(10)

k

Xim

Here, is the mole fraction of the molecule m in the pseudocomponent i; C is the number of components in a pseudocomponent i; Vmk is the value of group k in a molecule m; G is the number of groups k in a molecule m. The parameters Rk and Qk are, respectively, the volume and area for each functional group contained in a molecule, as given in Magnussen et al.31 For the methyl biodiesel, we found r = 13.3140 and q = 10.9490. The average molecular mass of the methyl biodiesel of 306.2 g·mol/L was calculated from the UNIFAC group contribution method,32 according to the composition of the alkyl esters present. The error between the calculated values and the experimental data was quantified by the global deviation in eq 11

Figure 2. Liquid−liquid equilibrium data for the ternary system 1 at 298.15 K.

global deviation N

= 100

N

N

∑ p F ∑n L ∑i C {(winp , exp − winp , calc)2 } 2NLNC

(11)

3. RESULTS AND DISCUSSION Figure 1 shows the experimental results for ternary systems 1, 2, and 3 based on Table S1, where one can see that the effect

Figure 3. Liquid−liquid equilibrium data for the pseudoquaternary system 4 at 298.15 K.


S1) and the pseudoquaternary systems 4−7 (see Table S2) as well as the tie-lines with the data from Table S3. In these figures, we can see that the binodal curves have the characteristic profiles for systems like the ones considered in this work, which indicates that the experimental procedure was indeed well-conducted. The methanol evaporation method used to obtain the experimental data for system 1 and the viscosity method used in systems 4 to 7 were very efficient in determining the composition of each phase, since the experimental tie-lines are very close to the overall compositions. The tie-lines also have a specific slope for the biodiesel−

Figure 1. Binodal curves of the ternary systems 1, 2, and 3.

of temperature is negligible. However, there was a slight reduction in the immiscibility region at 318.15 K, which might indicate that separation and purification of the sunflower biodiesel is more difficult at this temperature. We then decided that we will only consider the tie-lines at 298.15 K for systems 1 and 4−7. E



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Table 4. UNIQUAC Binary Interaction Parameters and Deviationsa system

pair i−j

Aij

Aji

deviation

1

b−g b−m g−m b−w b−g b−m w−g w−m g−m b−w b−g b−m w−g w−m g−m b−s b−g b−m s−g s−m g−m b−s b−g b−m s−g s−m g−m

84.49 371.19 −248.36 424.94 149.03 221.50 358.62 431.12 −326.18 645.22 119.20 350.94 −72.73 387.48 −426.20 556.26 339.26 229.16 −40.34 392.81 −681.44 617.95 148.46 352.92 −105.54 400.06 −552.93

130.00 14.39 107.60 −53.79 140.98 217.92 −611.06 695.63 959.51 630.77 −103.12 51.38 −232.86 714.39 393.19 585.75 −220.00 65.72 −304.10 728.82 239.98 649.23 −138.36 44.54 −271.75 725.71 368.97

0.25%

4

5


6

7


0.18%

0.18%

0.12%

0.06%

a

Captions: biodiesel (b), glycerol (g), methanol (m), distilled water (w), and diluted sulfuric acid (s).

glycerol−methanol systems, because as discussed above, the binary mixture glycerol/methanol has a much higher affinity than the biodiesel/methanol binary mixture. For smaller fractions of methanol in the overall composition, the angle of inclination of the tie-lines decreases, because the amount of alcohol decreases more markedly in the glycerol + distilled water/diluted acid-rich phase than in the biodiesel-rich phase, and this causes the tie-lines to level off. Note also that the experimental tie-lines are in good agreement with the ones predicted by the UNIQUAC model with fitted binary coefficient parameters in Table 4, where the deviations calculated by eq 11 are really quite small. In Figure 6, the tie-lines are not on the global compositions but are very close. This slight deviation may have occurred because the data are experimental, so it is only natural that small differences emerge. We can also see that, on the one hand, the solubility of biodiesel is very low in the glycerol-rich phase of the analyzed systems. This means that the loss of biodiesel in the separation and purification stages is supposed to be quite small, which reflects the fact that water and diluted acid are good liquid− liquid extraction solvents for this process. However, for all but system 5, where the methanol content increases in the glycerolrich phase, the solubility of biodiesel also increases. This shows that the addition of large amounts of methanol in the transesterification reaction may hinder the separation and purification processes. On the other hand, the amounts of glycerol and distilled water or diluted acid in the biodiesel-rich phase are also small. Moreover, at equilibrium, a large fraction of methanol is displaced into the glycerol-rich phase, and we conclude that in the separation and purification stages we can

expect that much of the methanol be carried out with the glycerol-rich phase. Figures 7 and 8 constructed from the data of Tables S1 and S2 show that the presence of water or diluted acid slightly

Figure 7. Binodal curves of the ternary system 1 and pseudoquaternary systems 4 and 5.

increases the region of phase separation (area below the binodal curve) with respect to system 1. However, no notable change in the profile of the curve can be observed by increasing the amount of distilled water or diluted acid from 50 to 75%. Note also that there is no significant difference in the profiles of the systems containing the aqueous phase between these figures. We can attribute this fact to the small dilution ratio of acid and water, ca. 1:1000, in systems 6 and 7. F



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Figure 11. Binodal curves and tie-lines obtained in this study at 298.15 K with the curves and tie-lines from Rostami et al.16 at 303.15 K.

Figure 8. Binodal curves of the ternary system 1 and pseudoquaternary systems 6 and 7.

method. The binodal curves are in very good agreement, with only a small deviation in the immiscibility region in the biodiesel-rich phase. We can also observe that the slopes of the tie-lines show the same trend, although they were not determined using the same experimental data as per Rostami et al.16 Note also that this comparison supports our conclusion that the temperature has a negligible effect on the separation process. Figure 12 shows the tie-lines for ternary system 1 and the tie-lines obtained by Mesquita et al.,33 all at 298.15 K, where

In Figures 9 and 10 constructed from the data of Tables S1− S3, we note that the presence of distilled water and diluted acid

Figure 9. Binodal curves and tie-lines of the ternary system 1 and the pseudoquaternary system 4.

Figure 12. Comparison between the tie-lines of ternary system 1 and the tie-lines in Mesquita et al.33 at 298.15 K.

the latter uses ethanol. The significant difference in slopes can be attributed to the smaller region of phase separation for the ethanol system and also because of the greater solubility of biodiesel in the glycerol-rich phase in the system containing ethanol. Figure 13 compares the binodal curves of ternary system 3 of this work with the system of soybean biodiesel + glycerol + methanol in Mazutti et al.,34 both at 318.15 K. It is clear that the curves are very similar, particularly because soybean oil and sunflower oil have similar compositions, with oleic acid and linoleic acid being among the components in largest quantity.

Figure 10. Binodal curves and tie-lines of the ternary system 1 and the pseudoquaternary system 6.

4. CONCLUSIONS In this contribution, we conducted experiments to determine some interesting properties typical of phase separation in mixtures containing sunflower biodiesel, methanol, glycerol, and water or diluted acid. This information is of great importance in efficiently designing and operating separation processes in biodiesel plants, especially because such data are quite scarce in the reviewed literature. Along these lines, we concluded that temperature essentially has no effect on the operation of these units and that an increase in the amount of the aqueous phase in the purification stage increases the region

decreases the solubility of biodiesel in the glycerol-rich phase. This means that the loss of biodiesel during purification either with distilled water or diluted acid is less than the amount of biodiesel lost in the natural decantation process that occurs soon after the transesterification reaction. This difference becomes more evident at the top of the curve. Figure 11 compares the binodal curve and tie-lines of the ternary system 1 with the curve and tie-lines reported by Rostami et al.16 for the same system but at 303.15 K. Both binodal curves were generated by the titration method, and the tie-lines were determined by the methanol evaporation G



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w = mass fraction of the components i = components (or pseudocomponents) n = quantity of tie-lines p = number of phases exp = experimental composition values of the components calc = calculated composition values of the components x = molar fraction γ = activity coefficient G = Gibbs energy f ° = fugacity of pure component at standard state μ° = chemical potential of pure component α β = arbitrary phases Mi = molar mass of component/pseudocomponents i ri = volume parameters qi = area parameters Vmk = number of groups k in a molecule m Rk = volume parameters for each functional group that each molecule contains Qk = area parameters for each functional group that each molecule contains Aij = binary interaction parameters

Figure 13. Comparison between the binodal curves of ternary system 3 with the system of soybean methyl biodiesel + glycerol + methanol in Mazutti et al.34

of phase separation, which in turn decreases the solubility of biodiesel in the glycerol-rich phase, therefore reducing product losses. We then used the data gathered in the experiments to successfully estimate the binary interaction parameters for the UNIQUAC model, which can be used to build mathematical models and perform simulations for systems like the ones investigated in this article.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00276. Table S1: Mass fractions of the binodal curve points of the ternary systems (sunflower biodiesel (1)−glycerol (2)−methanol (3)) at 298.15, 308.15, and 318.15 K; Table S2: Mass fractions and point viscosities of the binodal curve of pseudoquaternary systems (sunflower biodiesel (1)−water/diluted acid (2)−glycerol (3)− methanol (4)); Table S3: Experimental ELL data of the ternary system 1 (sunflower biodiesel (1)−glycerol (2)− methanol (3)) and the pseudoquaternary systems 4, 5, 6, and 7 (sunflower biodiesel (1)−water/diluted acid (2)− glycerol (3)−methanol (4)) (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jose J. N. Alves: 0000-0002-3299-405X Antonio C. B. de Araujo: 0000-0001-6373-5571 Notes

The authors declare no competing financial interest.

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NOMENCLATURE mp = mass of the internal standard Ab = sum of peak areas for biodiesel Ap = peak area of the internal standard mb = biodiesel mass f = correction factor FO = objective function NP = number of phases NL = number of tie-lines NC = number of components or pseudocomponents in tielines n H



Article

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Experimental Data and Phase Equilibrium Modeling in Ternary and

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