Liquid Phase Equilibria of Water + Formic Acid + Dimethyl Carbonate

Article pubs.acs.org/jced

Liquid Phase Equilibria of Water + Formic Acid + Dimethyl Carbonate Ternary System at Several Temperatures Erol Iṅ ce,*,† Melisa Lalikoglu,† and Dana Constantinescu‡ †

Engineering Faculty, Chemical Engineering Department, Istanbul University, 34320, Avcilar/Istanbul, Turkey DDBST GmbH, Marie-Curie-Straße 10, D-26129 Oldenburg, Germany

‡

ABSTRACT: Liquid−liquid equilibria of water + formic acid + dimethyl carbonate have experimentally been specified at (298.2, 308.2, and 318.2) K. Each phase diagram was obtained by specifying binodal curves and tie-lines. The reliability of the experimental tie-lines was verified via the Othmer−Tobias correlation. The experimental tie-line data were correlated using the nonrandom two liquid and the universal quasichemical (UNIQUAC) models in order to obtain the binary interaction parameters. However, UNIQUAC functional-group activity coefficients (UNIFAC) and modified-UNIFAC methods were also used to predict the phase equilibrium in the system determined from experimental data using the interaction parameters between CH3, OCOO, HCOOH, and H2O functional groups. Distribution coefficients and separation factors were evaluated for the immiscibility region.

1. INTRODUCTION Formic acid is an essential chemical industrial product. It is widely used in textile, tanning, rubber processing, and pharmaceutical industries.1 Additionally, it is used as a preservative and antibacterial agent in livestock feed, and as a miticide against the Varroa mite in the beekeeping industry.2,3 Formic acid receives attention for its use as a transportation medium for hydrogen.4 Formic acid also has the potential to direct power fuel cells for electricity.5 By the reaction of methanol and carbon monoxide, methyl formate is formed and by the hydrolysis of the methyl formate, formic acid is obtained in the industrial synthesis. Aqueous solutions include 20 to 50% acid. When it is concentrated through distillation, these aqueous solutions form an azeotrope, as the formic acid cannot be obtained directly in an anhydrous form. Several distillation methods, such as under pressure, extractive distillation, and azeotropic distillation are used to concentrate the solutions. However, these processes consume too much energy. So as to cut down on the cost, extraction can be an alternative method.6 The extraction equilibria of formic acid have been investigated by several researchers.7−10 Formic acid also exists in natural gas and crude oil fields. Therefore, knowledge of the liquid−liquid phase equilibria of formic acid is of high importance to the chemical and petrochemical industries. A great amount of formic acid is produced as a byproduct as well in the production of some other chemicals, particularly acetic acid.11 In the field of investigating more benign solvents as potential replacements for chlorocarbons or aromatic hydrocarbons and as new solvents for separations, much study has been performed on the dibasic esters, which have excellent properties for industrial applications. They are environmentally friendly and have lower cost, lower toxicity, better stability, and quite high boiling © 2014 American Chemical Society

temperatures; however, their viscosity and density is close to those of water. The dibasic esters are also used as novel solvents in separation techniques.12 First, Uusi-Penttilä et al.13 studied liquid−liquid equilibria of different ternary systems. Afterward, Iṅ ce and Kırbaşlar14−21 studied liquid−liquid equilibria of six various ternary systems. Iṅ ce and Aşcı̧ 22 have also studied liquid−liquid equilibria of different ternary systems. Dimethyl carbonate is often considered to be a green reagent and environmentally friendly. It is a solvent which can replace chlorocarbons or aromatic hydrocarbons. It is exempt from classification as a volatile organic compound and has excellent properties for industrial applications. The main benefits that dimethyl carbonate has over other methylating reagents such as iodomethane and dimethyl sulfate is that it has much lower toxicity and biodegradability. Dimethyl carbonate is growing in popularity and applications as a replacement for methyl ethyl ketone, tert-butyl acetate, and parachlorobenzotrifluoride. Dimethyl carbonate has an ester or alcohol like odor, and therefore is more favorable to users than most hydrocarbon solvents it replaces.23 The purpose of this study is to recover formic acid from dilute aqueous solutions by using an environmentally friendly solvent with a low-boiling point. In this study, liquid−liquid equilibrium results have been reported for water + formic acid + dimethyl carbonate system, and no such data have been previously published about that. Additionally, some calculations are performed via the nonrandom two liquid (NRTL), universal quasichemical (UNIQUAC), UNIQUAC functional-group Received: May 12, 2014 Accepted: August 22, 2014 Published: August 29, 2014 2781

dx.doi.org/10.1021/je500422t | J. Chem. Eng. Data 2014, 59, 2781−2787

Journal of Chemical & Engineering Data

Article

Table 1. Densities ρ, Refractive Indices nD, and Boiling Temperatures (Tb) of the Pure Componentsa and Literature Values24 at 101.325 kPa ρ, (298.2 K)

Tb

(kg·m−3)

a

nD, (298.2 K)

K

compound

source

purity

exp

lit.

exp

lit.

exp

lit.

water formic acid dimethyl carbonate

distilled Merck Merck

> 99 % > 99 %

998.21 1220.04 1061.90

997.04 1220.0 1063.01

1.3324 1.3714 1.3685

1.3325 1.3714 1.368b

373.2 374.4 363.23

373.25 374.2 363.15

Standard uncertainties u are u(ρ) = 0.01 kg·m−3, u(nD) = 0.00001, u(Tb) = 0.1 K, u(p) = 0.5 kPa. bAt 293.2 K.

Table 2. Experimental Binodal Curve Data (As Mass Fraction) Of the Water (1) + Formic Acid (2) + Dimethyl Carbonate (3) at Each Temperaturea and Atmospheric Pressure T = 298.2 K

a

T = 308.2 K

T = 318.2 K

w1

w2

w3

w1

w2

w3

w1

w2

w3

0.0332 0.1040 0.1823 0.3064 0.4108 0.5520 0.6550 0.7697 0.8619

0.0000 0.0977 0.1255 0.1479 0.1586 0.1453 0.1185 0.0717 0.0000

0.9668 0.7983 0.6922 0.5457 0.4306 0.3027 0.2265 0.1586 0.1381

0.0402 0.1147 0.1697 0.3145 0.4208 0.5019 0.6249 0.7773 0.8449

0.0000 0.0747 0.1126 0.1327 0.1385 0.1373 0.1181 0.0543 0.0000

0.9598 0.8106 0.7177 0.5528 0.4407 0.3608 0.257 0.1684 0.1551

0.0516 0.1239 0.1697 0.3192 0.4276 0.8057 0.6304 0.5135 0.8453

0.0000 0.0605 0.0930 0.1359 0.1426 0.0335 0.1217 0.1305 0.0000

0.9484 0.8156 0.7373 0.5449 0.4298 0.1608 0.2479 0.3560 0.1547

Standard uncertainties u are u(w) = 0.001, u(T) = 0.1 K.

component was added until a permanent heterogeneity was observed.18 Temperature was controlled using water from a thermostat and the water temperature was measured with a thermometer with a precision of ± 0.1 K. All mixtures were prepared by weighing with a Mettler scale accurate within ± 1·10−4 g. The solvent was added by a microburet (Metrohm) with an accuracy of ± 3·10−3 cm3.18 Tie-lines were obtained through a preparation of water + formic acid + dimethyl carbonate ternary mixtures of known overall compositions lying within the two-phase region. The mixture was stirred vigorously for at least 2 h in jacketed cells15 and then left to stand for at least 3 h (the time necessary to attain equilibrium was established in preliminary experiments). After the complete separation of the phases, samples were carefully taken from each phase and analyzed to obtain the tie-line data.18 2.3. Analysis. The liquid samples were analyzed by a gas chromatograph (Hewlett-Packard GC, model 6890 series), equipped with a thermal conductivity detector (TCD) for the quantitative determination of water, formic acid, and dimethyl carbonate. A 15 m long HP-plot Q column (320 μm diameter with a 20 μm film thickness) was used with a temperatureprogrammed analysis.18 The oven temperature was set at 523 K. The detector temperature was held at 523 K, while the injectionport temperature was held at 473 K. The flow rate of carrier gas, nitrogen, was held 6 cm 3 /min. Samples with known compositions were used to calibrate the instrument in the composition range related.

activity coefficients (UNIFAC), and modified UNIFAC, in order to describe their experimental thermodynamic behavior.

2. EXPERIMENTAL SECTION 2.1. Materials. Formic acid and dimethyl carbonate were purchased from Merck Company, and were both received with a quoted purity greater than 0.99 as mass fractions. Each substance purity was checked through gas chromatography, and the compounds were used with no further purification. The water content of formic acid and dimethyl carbonate was measured through Mettler Toledo DL38 Karl Fischer titrator as 3·10−4 and 2·10−4 mass fraction. Throughout all experiments, distilled water was used. Refractive indexes were measured with Anton Paar model RXA 170; its accuracy is ± 1·10−5. Densities were measured using an Anton Paar model DMA 4500 density meter. Boiling temperature measurements were obtained by using a Fischer boiling temperature apparatus. The estimated uncertainties in the density and boiling point measurements were ± 1·10−5 g·cm−3 and ± 0.1 K, respectively. The measured physical properties are listed in Table 1, along with some values from the literature.24 2.2. Apparatus and Procedures. The binodal curve for the water + formic acid + dimethyl carbonate ternary system was determined through cloud-point method.14 Binary mixtures of known compositions were shaken in a glass stoppered cell equipped with a magnetic stirrer and jacketed for circulating water from a fixed temperature bath at 298.2 ± 0.1, 308.2 ± 0.1 and 318.2 ± 0.1 K. The third component was regularly added until the transition point was attained. The end point was specified by monitoring the transition from a homogeneous to a heterogeneous mixture. The mutual solubilities of water + dimethyl carbonate system were also specified by using cloud-point method. A weighted amount of one component was placed in the cell; then the other

3. RESULTS AND DISCUSSION The experimental binodal curve data of water + formic acid + dimethyl carbonate ternary system at (298.2, 308.2, and 318.2) K and tie-line data are given in Table 2 and Table 3, respectively. The experimental and predicted binodal curve and tie-lines at each temperature are shown in Figures 1 to 3. As seen in Figures 2782



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As in every group contribution method, it is assumed that the mixture does not consist of molecules, but of structural groups.32 It has the advantage of being able to form a very large number of possible molecules from a relatively small set of structural groups. For the calculation of liquid−liquid equilibria (LLE), the socalled isoactivity criterion must be fulfilled, where xiE and xiR are the mole fractions of LLE phases:28

Table 3. Experimental Tie-Line Data (as Mass Fraction) for the Water (1) + Formic Acid (2) + Dimethyl Carbonate (3) Ternary System at Each Temperature with Distribution Coefficients and Separation Factors water-rich phase w1

w2

0.7968 0.7013 0.5548

0.0563 0.1059 0.1435

0.7773 0.6482 0.5434

0.0579 0.1126 0.1310

0.7651 0.6549 0.5369

0.0586 0.1108 0.1366

solvent-rich phase w1

w2

T = 298.2 K 0.0356 0.0277 0.0596 0.0631 0.1389 0.1096 T = 308.2 K 0.0612 0.0286 0.0993 0.0684 0.1453 0.1009 T = 318.2 K 0.0758 0.0295 0.1312 0.0692 0.2026 0.1023

D2

S

0.4920 0.5958 0.7638

11.01 7.01 3.05

0.4940 0.6075 0.7702

6.28 3.97 2.88

0.5034 0.6245 0.7489

5.08 3.12 1.98

(γixi)E = (γixi)R

(1)

where E is the extract (solvent) phase; R is the raffinate (aqueous) phase; γ is the activity coefficient of the component i. The UNIFAC equation for the liquid-phase activity coefficient is represented as follows:28

ln γi = ln γi c + ln γi r

(2)

The activity coefficients are calculated from a combinatorial and residual part in the same way as in UNIQUAC. The temperatureindependent combinatorial part takes into consideration the size and form of molecules (entropic contribution), and the residual part considers the enthalpic contribution. To reduce the weaknesses of UNIFAC, the modified UNIFAC (Dortmund) method was proposed after several years. It presents various advantages when compared with the group contribution methods, UNIFAC or the analytical solution of groups (ASOG). These advantages were reached by using as the empirically modified combinatorial part, the temperaturedependent group interaction parameters and additional main groups. For fitting the temperature-group interaction parameters, besides vapor−liquid equilibria (VLE) a larger database including activity coefficients at infinite dilution, excess enthalpy, excess heat capacity, LLE, solid−liquid equilibria (SLE) of simple eutectic systems, and azeotropic data are used.33−36

1 to 3, it has been found that dimethyl carbonate is slightly soluble in water, but miscible with formic acid. 3.1. Calculations. The experimental tie-line data were correlated using the NRTL and UNIQUAC models in order to obtain the binary interaction parameters.25−27 The real behavior of fluid mixtures can be calculated with the help of activity coefficients. The correct description of the dependence on temperature, pressure, and composition in multicomponent systems requires reliable thermodynamic models, allowing the calculation of these properties from available experimental data.28 The UNIFAC method used for the calculation and prediction of activity coefficients is based on a group contribution method concept which was developed by Fredenslund et al.29−31

Figure 1. Ternary diagram for LLE of water (1) + formic acid (2) + dimethyl carbonate (3) at 298.2 K: black − + −, experimental binodal curve; black −○−, experimental tie-line data; green −∗−,NRTL, red −□ −, UNIFAC, purple −△−, modified-UNIFAC, blue −◇−, UNIQUAC tie-line data. 2783



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Figure 2. Ternary diagram for LLE of water (1) + formic acid (2) + dimethyl carbonate (3) at 308.2 K: black − + −, experimental binodal curve; black −○−, experimental tie-line data; green −∗−,NRTL, red −□ −, UNIFAC, purple −△−, modified-UNIFAC, blue −◇−, UNIQUAC tie-line data.

Figure 3. Ternary diagram for LLE of water (1) + formic acid (2) + dimethyl carbonate (3) at 318.2 K: black − + −, experimental binodal curve; black −○−, experimental tie-line data; green −∗−,NRTL, red −□ −, UNIFAC, purple −△−, modified-UNIFAC, blue −◇−, UNIQUAC tie-line data.

The experimental tie-line data for the each ternary system were correlated using the NRTL and the UNIQUAC models. For the investigated system, the literature values were used for the UNIQUAC structural parameters37−40 in Table 4 and

The UNIFAC and modified-UNIFAC methods depend on the interaction parameters between each pair of main groups present in the system, but the UNIQUAC and NRTL models depend on interaction parameters between the compounds in the system. 2784



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Table 4. UNIQUAC r and q Values37,38 compound

ri

qi

dimethyl carbonate formic acid water

3.0613 1.5280 0.9200

2.8160 1.5320 1.4000

UNIQUAC and the NRTL corresponding binary interaction parameters41 for the ternary system are given in Table 5. The NRTL and UNIQUAC binary interaction parameters were determined using the experimental tie-line data and minimizing a composition-based objective function.42,43 Table 5. Correlated Results from the NRTL (α = 0.2) and the UNIQUAC Models and the Corresponding Binary Interaction Parameters (aij and aji)/K for the Ternary Systemsa43 components T/K

i−j

Figure 4. Distribution coefficient D2 of formic acid as a function of the mass fraction w21 of formic acid in the water-rich phase.

parameters/K aij

aji

RMSD

NRTL 298.2

308.2

318.2

298.2

308.2

318.2

1−2 1−3 2−3 1−2 1−3 2−3 1−2 1−3 2−3 1−2 1−3 2−3 1−2 1−3 2−3 1−2 1−3 2−3

−501.34 924.62 −811.37 −283.85 1022.2 −595.55 −597.40 1144.3 −658.32 UNIQUAC 119.90 96.054 −154.20 172.68 116.28 2.3007 89.859 −142.92 −361.60

−130.37 368.91 692.09 −70.098 158.20 483.45 −64.493 112.91 625.23 −411.55 515.07 −323.85 −360.14 349.70 −317.05 −529.48 321.57 −169.17

0.2865

0.1737

0.1374

0.3193

0.1100

Figure 5. Othmer−Tobias plot of the water (1) + formic acid (2) + dimethyl carbonate (3) ternary system at T = (green line, □) 298.2 K, (blue line, ○) 308.2 K, red line, △) 318.2 K.

0.1447

a In the table: 1→water, 2→formic acid, 3→dimethyl carbonate. The NRTL and UNIQUAC model parameters (aij, aji) are defined as aij = (gij − gjj)/R and aij = (uij − ujj)/R, respectively.

temperatures. The parameters of the Othmer−Tobias correlation are given in Table 6. The nearness of the correlation factor (r2) to 1 indicates the degree of consistency of related data.

Distribution coefficients, Di, for water (i = 1) and formic acid (i = 2) and separation factors, S, were determined as follows:44

Table 6. Constants of Othmer−Tobias Equation for the Water + Formic Acid + Dimethyl Carbonate Ternary System

Di = wi3/wi1

(3)

T/K

a

b

r2

S = (w23/w13)solvent − rich phase /(w21/w11)water − rich phase

(4)

298.2 308.2 318.2

0.5756 0.8302 0.4756

0.7224 0.8985 0.7812

0.9998 0.9999 0.9965

The distribution coefficients and separation factors for the each temperature are given in Table 3. 3.2. Correlations. The reliability of experimentally measured tie-line data can be verified by applying the Othmer−Tobias correlation following equation:45 ln[(1 − w33)/w33] = a + b ln[(1 − w11)/w11]

The root-mean-square deviations (RMSD) are calculated from the difference between the experimental data and the predictions of NRTL, UNIQUAC, UNIFAC, and modifiedUNIFAC methods at each temperature, according to the following equation:46

(5)

where w11 is mass fraction of water (1) in the water-rich phase; w33 is the mass fraction of dimethyl carbonate (3) in the solventrich phase; a and b are a constant and the slope of eq 5, respectively. The linearity of the plot indicates the degree of consistency of the data. The Othmer−Tobias plot is shown in Figure 5 for all

RMSD = [∑ (∑ ∑ (X i ,exp − X i ,calcd)2 )/6N ]1/2 K

J

I

(6)

where I is water or formic acid, J is the solvent-rich or water-rich phase, and K= 1, 2, 3,..., N (tie-line number). 2785



■

RMSD values of the NRTL and UNIQUAC models are given in Table 5, while UNIFAC and modified UNIFAC methods are given in Table 7.

*E-mail: [email protected]. Tel: +90-212-473 70 70. Fax: +90-212-473 71 80. Notes

The authors declare no competing financial interest.

■

RMSD UNIFAC

modified-UNIFAC

298.2 308.2 318.2

0.3641 0.4124 0.5052

0.3638 0.4282 0.5255

AUTHOR INFORMATION

Corresponding Author

Table 7. RMSD Values of UNIFAC and Modified-UNIFAC Methods T/K

Article

The extraction power of the solvent at each temperature, plots of D2 versus w21 are shown in Figure 4. The effectiveness of extraction of formic acid by dimethyl carbonate is given by its separation factor (S), which is a measure of the dimethyl carbonate required to separate formic acid from water. This quantity has been found to be greater than 1 (separating factors varying between 1.98 and 11.01) for the ternary system reported here) which means that the extraction of formic acid through dimethyl carbonate is available. Selectivity diagrams on a solvent-free basis are plotted at each temperature in Figure 6. The effect of temperature change on the selectivity values was not too much.

NOMENCLATURE a = constant, eq 5 b = slope, eq 5 Di = distribution coefficient of the i th component, eq 3 E = extract (solvent) phase I = water or formic acid J = solvent-rich or water-rich phase K = tie-line number i = component number of water (1), formic acid (2) and dimethyl carbonate (3) nD = refractive index r2 = regression coefficient R = raffinate (aqueous) phase S = separation factor, eq 4 T = temperature, K Tb = boiling point, K wi = mass fraction of the ith component w11 = mass fraction of water (1) in the water-rich phase w21 = mass fraction of formic acid (2) in the water-rich phase w31 = mass fraction of dimethyl carbonate (3) in the water-rich phase w13 = mass fraction of water (1) in the solvent-rich phase w23 = mass fraction of formic acid (2) in the solvent-rich phase w33 = mass fraction of dimethyl carbonate (3) in the solventrich phase x = mole fraction of the component

Greek Letters

γ = activity coefficient of the component i ρ = density, (kg·m−3)

Subscripts

exp = experimental calcd = calculated i = the component 1 = water 2 = formic acid 3 = dimethyl carbonate

Figure 6. Selectivity diagram at each temperature (free-solvent basis).

Superscripts

4. CONCLUSION Liquid−liquid equilibrium data of the water + formic acid + dimethyl carbonate ternary system were determined experimentally at each temperature. The temperature has practically no effect on the size of immiscibility region at the studied temperatures. The tie-lines in Figures 1 to 3 show that formic acid is equally soluble in the water-rich phase and in the solventrich phase. Separation factors are decreased by the increase of formic acid concentration, as can be seen from Table 3. In our case, UNIFAC and modified-UNIFAC(Do) models do not give very reasonable predictions, therefore our experimental data provided a route to regress UNIQUAC and NRTL binary parameters directly and to produce a much more realistic estimation. Finally, it has been concluded that the dimethyl carbonate is a suitable separating agent for dilute aqueous formic acid because of being an environmentally friendly solvent and having low cost.

■

E = extract (solvent) phase R = raffinate (aqueous) phase c = combinatorial part r = residual part

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Liquid Phase Equilibria of Water + Formic Acid + Dimethyl Carbonate

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