LiquidâLiquid Equilibria of Quaternary Systems Composed of 1,3

Article pubs.acs.org/jced

Liquid−Liquid Equilibria of Quaternary Systems Composed of 1,3Propanediol, Short-Chain Alcohol, Water, and Salt Thomas Gerlach and Irina Smirnova* Institute of Thermal Separation Processes, Hamburg University of Technology, Eißendorfer Straße 38, D-21073 Hamburg, Germany

ABSTRACT: In different fermentation processes the separation of the products from aqueous solution is an energy-intensive process step, particularly for hydrophilic products like diols. As an alternative to evaporation and distillation, liquid−liquid extraction of these products is often limited by the unfavorable partition behavior of the diols. In this work phase equilibria for four quaternary systems consisting of water, 1,3-propanediol, a salt (K2HPO4, K2CO3, or Na2CO3), and a short-chain alcohol (ethanol, 1-propanol, or 2-propanol) were investigated. For all investigated systems favorable partitioning behavior of the hydrophilic diol into the organic phase was found. The partition ratio of the diol depends strongly on the salt content of the system. In the region of salt mass fractions in the salt-rich phase lower than 30 wt % highest partition ratios as well as highest selectivities were reached with systems of the type Na2CO3 + H2O + 1-propanol/2-propanol + water. Furthermore, the predictive thermodynamic model COSMO-RS was used with an electrolyte extension for the prediction of the partition ratio of the diol at infinite dilution in the 1,3-propanediol free systems. The partition ratio was overestimated by the model. Nevertheless, a qualitative agreement concerning the order of the partition ratio was reached at lower salt contents, and the increase in the partition coefficient with an increasing mass fraction of the salt was predicted for all systems except for the K2CO3 + H2O + ethanol system at high salt mass fractions.

1. INTRODUCTION In different biotechnological processes, the recovery of products from fermentation broths currently requires energy-intensive or costly separation steps. Examples for such processes are the biotechnological production of diols such as 1,3-propandediol (PDO) or 2,3-butanediol which are very hydrophilic and have high boiling points. Here, the separation process contributes significantly to the total cost of the product.1 Different methods were proposed for the separation of the diols from the fermentation broth. Conventionally used evaporation and distillation often lead to very high energy requirements of the process.1 An alternative to these processes is liquid−liquid extraction. With many solvents only an unsatisfying partition behavior of the diols is reached due to their hydrophilic nature.2 To increase the partitioning of the diols into the organic phase, reactive extraction has been explored.3,4 Nevertheless, the complexity of the processes leads to the search for further alternatives.5 Salts can be used to strongly influence the solubility of dissolved species. It is well-known that due to salting-out miscibility gaps can be induced in otherwise miscible systems (e.g., short-chain alcohol or acetone + water). Recently, the application of this effect received new attention in the context of © XXXX American Chemical Society

the separation of different polar components from aqueous solution. The systems can be created from comparatively cheap components and promise higher partition ratios of hydrophilic components due to the salting-out effect and the potential to use polar solvents for the extraction. In the context of the separation of diols particular attention was given to systems consisting of short-chain alcohols in combination with salts which are known to induce a miscibility gap in the respective systems. In several works successful extraction of 2,3-butanediol and 1,3-propanediol was demonstrated from simulated as well as real fermentation broths.6−10 A review of recent applications of such systems was given by Dai et al.11 Whereas successful product separation was already demonstrated in different works, only little attention was given to the determination of the phase equilibria of these systems. In the context of the separation of diols, no systematic investigation of the partition behavior in the quaternary systems consisting of organic solvent, diol, water, and salt has been performed yet. Only for few systems partition ratios of the diols were Received: June 9, 2016 Accepted: August 22, 2016

A

DOI: 10.1021/acs.jced.6b00472 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

systematically determined whereas the effect of the diol on the phase compositions was not considered.12 Therefore, in this work phase equilibria of quaternary systems consisting of a shortchain alcohol, salt, water, and 1,3-propanediol were investigated. Considering the potential for different combinations of salts and solvents and the complexity of fermentation broths, the availability of predictive thermodynamic models for such systems is desirable. The prediction of the partition behavior of components in such mixtures may aid in the identification of a suitable system for a specific process. Different predictive thermodynamic models that are not based on component specific parameters are available for the prediction of the partition behavior in electrolyte systems. Based on the group contribution model UNIFAC, different electrolyte models were developed that are applicable to liquid−liquid equilibrium calculations.13−15 An alternative approach is followed by the thermodynamic model COSMO-RS, which allows the prediction of the chemical potential of various species in liquid phases using a combination of quantum chemical methods with statistical thermodynamics.16 In general, no component, group, or system specific parameters are necessary to apply the model. To describe electrolyte systems, refinements of the model concerning the description of ionic solvation as well as the introduction of longrange ion−ion interactions are necessary. Different approaches were proposed to improve the description of ionic solvation within the model based on explicit solvation17 or the adjustment of the interaction energies of the ions.18−20 Whereas few examples for the prediction of the partition behavior in electrolyte systems were shown with these approaches, the approach by Ingram et al. which was previously developed in our group was successfully applied to the prediction of liquid−liquid equilibria in electrolyte systems.20,21 In this work the capability of the model to predict the partition behavior of 1,3-propanediol in different electrolyte systems of the type salt + water + short-chain alcohol is evaluated.

Table 2. Systems Investigated in This Work

Table 1. Chemicals Used in This Work name

manufacturer

mass fraction purity

Sigma-Aldrich Merck AppliChem GmbH Carl Roth GmbH & Co. KG Merck Alfa Aesar AppliChem GmbH AppliChem GmbH AppliChem GmbH

0.999 0.995 0.995 0.995 0.995 0.990 0.990 0.990 0.990

system

temperature (°C)

I II III IV

K2HPO4 + ethanol + water + 1,3-propanediol K2CO3 + ethanol + water + 1,3-propanediol Na2CO3 + 1-propanol + water + 1,3-propanediol Na2CO3 + 2-propanol + water + 1,3-propanediol

25 25 40 40

classified as water structure increasing, a property which is related to the strong salting-out effect of the salts.22 Whereas ternary systems are commonly visualized in triangular phase diagrams, the extension of this approach to quaternary systems leads to a three-dimensional tetrahedron. Initial compositions for the tie-lines in the quaternary system were chosen on sectional planes in this tetrahedron.23 The position of these sectional planes was commonly based on the available or experimentally determined information on tie-line compositions in the ternary 1,3-propanediol free systems. Typical concentrations of 1,3-propanediol in fermentation broths vary between 5 and 15% for different processes.1 In this work the maximum total 1,3-propanediol mass fraction in any of the investigated systems was 20 wt %. All samples were prepared in 20 mL septum sealed vials and equilibrated for at least 18 h in a water bath after shaking for 30 min. For the sample preparation a Sartorius Research R300s balance with an uncertainty of 0.0001 g was used. Samples from the organic and the salt-rich phase were taken with a needle syringe and prepared for analysis. The salt content in the salt-rich phase was determined by slow drying in a vacuum oven until weight constancy to avoid potential reactions of 1,3-propanediol with oxygen at an increased temperature.24 The temperature was increased stepwise up to 130 °C. To increase the mass of the sample available for the drying, the salt content was only determined once for each sample. The relative standard uncertainty of this method can be estimated to be within ur(w) = 0.05. The alkali metal ion concentration in the organic phase was determined with atomic emission spectroscopy using a PerkinElmer Optima 8300 DV spectrometer. Samples were analyzed in duplicates. The content of the organic solvents in both phases was determined with an Agilent 7890 B GC using a FID-detector and a Restek Stabilwax 10626 column with a length of 30 m, a diameter of 0.25 mm, and a film thickness of 0.25 μm. The oven temperature was held constant at 60 °C for 2 min before being increased with 25 °C/min to 170 °C. The carrier gas was nitrogen with a flow rate of 30 mL/min. As an internal standard ethylene glycol was used for 1,3-propanediol, acetone was used for ethanol, 1-propanol was used for 2propanol, and 2-propanol was used for 1-propanol. GC measurements were performed in triplicates. The water content in the organic phase was analyzed with Karl Fischer titration using a Mettler Toledo V20 compact volumetric titrator. The measurements were performed in triplicates. The water content in the salt-rich phase was determined from the mass fractions of the other species. To evaluate the data, consistency with the mass balance was controlled. Marcilla et al.25 evaluated the mass balance in quaternary electrolyte systems by considering the masses of the phases as independent variables that are determined by simultaneous minimization of the deviations of the component mass balances. The relative deviation of the sum of the calculated masses of the phases to the total system’s mass was then used to indicate the agreement with the mass balance. In this work a similar approach is applied by evaluating the closest distance between the tie-line and the initial composition of the system:

2. MATERIALS AND METHODS 2.1. Chemicals. The manufacturers and purities of all chemicals used in this work are listed in Table 1. Deionized water was used for all experiments. All chemicals were used without further purification.

acetone ethanol ethylene glycol 1-propanol 2-propanol 1,3-propanediol dipotassium phosphate potassium carbonate sodium carbonate

no.

2.2. Experimental Procedure and Analytical Methods. The systems investigated in this work are listed in Table 2. Due to the use of polar organic solvents for the extraction of the hydrophilic component 1,3-propanediol, only salts were used that lead to a strong salting-out effect, inducing a miscibility gap in systems of short-chain alcohols and water. This is particularly important due to the non-negligible mass fraction of 1,3propanediol in the systems. The anions of the salts are both B



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⎛ w β − wα ⎞ ⎟ Δwi = 100 ||(w α − w i) − ⎜(w α − w i) · ⎝ || w β − w α || ⎠ w β − wα || w β − w α ||

||

E ion(an, don) = fan c HBaeff min(0, σdon + σhb) max(0, σan − σhb)

E ion(cat, an) = fcat fan c HBaeff min(0, σcat + σhb)

(1)

max(0, σan − σhb)

The vectors wα, wβ, and wi contain the compositions of phases α and β and the initial composition of the system. All tie-lines were prepared and analyzed in duplicates, whereas no tie-lines with values of Δwi higher than 1.3 wt % were accepted for further analyses. The value was chosen such that for each initial composition a tie-line was retained, while removing inconsistent data from the data set. The value is in a similar range to maximum deviations that can be calculated from different works, where initial compositions were reported along with phase compositions in quaternary electrolyte systems.26,27 The closest distance between initial composition and the tie-line is reported with the tie-lines in the results section. 2.3. Modeling. For the calculations with the thermodynamic model COSMO-RS, a quantum chemical calculation has to be performed once for each molecule using the boundary condition of the perfect conductor. Different molecular conformers can be taken into account, whereas for each conformer a quantum chemical calculation is necessary. In each calculation the screening charge density on the surface of the molecular cavity in the conductor is determined and can be used in the model of pairwise independently interacting surface segments called COSMO-RS. The use of the screening charges from the quantum chemical calculations allows the restriction to few general interaction energy expressions. These depend on the screening charge densities, avoiding the use of component specific interaction energy parameters. Based on statistical thermodynamics, a fast converging solution to the problem of pairwise independently interacting surface segments is available, which can be solved on a personal computer.16 Detailed descriptions of COSMO-RS can be found in various publications,16,28,29 whereas the modifications to the model to describe electrolyte solutions will be described briefly in the following. The electrolyte extension of COSMO-RS by Ingram et al.20 is based on a combination of COSMO-RS with the Pitzer− Debye−Hückel term to account for long-range ion−ion interactions.

zi2Ix0.5 − 2Ix1.5 ⎤⎞ ⎥⎟⎟ 1 + ρIx0.5 ⎦⎠

Table 3. COSMO-Radii of the Alkali Cations rCOSMO and Interaction-Energy Scaling Factors for the Cations fcat Applied during the COSMO-RS Calculations in This Work

(2)

ion

rCOSMO (Å)

fcat

Na+ K+

1.890 1.952

0.174 0.254

were found to be in a similar range to the parameters found by Ingram et al.20 as well as Mohammed et al.21 As no element specific parameters were adjusted for the polyatomic anions, the scaling factors for these ions were fixed at unity. The permittivity and density of pure water was calculated with the temperaturedependent correlations by Pátek et al.33 The densities as well as temperature dependent correlations for the permittivity of the organic solvents were taken from Lide,34 whereas a volume fraction mixing rule was used to describe the permittivity of the salt free mixtures and ideal mixing behavior was assumed to describe the density of the mixtures. In Figure 1 the σ-profiles of several ions used in this work are compared with the σ-profiles of water and of the chloride ion. These diagrams can be constructed from a histogram of the surface area in dependence on the screening charge density and

Here, γi is the activity coefficient of component i, and Aϕ is the Debye−Hückel parameter, which depends on the solvent density and the solvent permittivity. zi is the charge of the component; ρ is the closest approach parameter which was fixed to 14.9 in this work, and Ix is the mole fraction based ionic strength. To modify the COSMO-RS interaction terms to better account for ionic interactions, element specific parameters fcat and fan were introduced to scale the interaction energy of the cations and anions using the hydrogen bonding term of COSMO-RS. E ion(cat, acc) = fcat c HBaeff min(0, σcat + σhb) max(0, σacc − σhb)

(5)

Here, σcat and σan are the screening charge densities of segments from the respective cation and anion. σdon and σacc are the screening charge densities of a hydrogen bond donor and a hydrogen bond acceptor segment from an uncharged molecule. cHB, aeff, and σhb are the hydrogen bond strength coefficient, the size of a contact between two segments, and the hydrogen bond threshold value, which are all part of the general COSMO-RS parametrization and were not adjusted in this work. Besides the element specific ion parameters, the COSMO radii of the alkali metal cations were adjusted, influencing the screening charge densities of the cations σcat as well as the surface area of the cations. The cation radii, as well as the interaction energy parameters, were determined based on the correlation of mean ionic activity coefficients of alkali and ammonium halide salts in aqueous solution. Although these ion specific parameters were only determined for the alkali cations, the ammonium cation, and the halide anions, the model was also shown to allow predictions for liquid−liquid equilibria of systems containing salts of polyatomic anions, relying on the general interaction energy expressions of COSMO-RS for these species.20,21 In this work the conformer search was performed using COSMOconf 3.0.30 For the quantum chemical calculations Turbomole 6.6 was used.31 COSMO-RS calculations were done with COSMOtherm with the BP_TZVP_C30_1501 parametrization.32 As the ion parameters introduced by Ingram et al. are dependent on the COSMO-RS parametrization and a different version of COSMOtherm was used in their work, the parameters were readjusted to the more recent parametrization of COSMOtherm used in this work. The procedure applied was the same as the procedure described in detail by Ingram et al.20 The values of all ion specific parameters used for the predictions in this work are given in Table 3. The values of the parameters

⎛ −Aϕ ⎡ 2z 2 ln(γi) = ln(γiCOSMO ‐ RS) + ln⎜⎜ 0.5 ⎢ i ln(1 + ρIx0.5) ⎝ Ms ⎣ ρ +

(4)

(3) C



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Figure 1. Comparison of the σ-profiles of several ions with the σ-profile of water (left) and comparison of the σ-profiles of representative conformers of different solvents (right).

Table 4. Experimental Liquid−Liquid Phase Compositions in the K2HPO4 (1) + Water (2) + Ethanol (3) + 1,3-Propanediol (4) System at Temperature T = 25 °C and Pressure p = 0.1 MPaa salt-rich phase w1

w2

organic phase w3

w4

w1

w4

Δwi

w2

w3

0.6614 0.6173 0.5122 0.4428 0.3492 0.2406

0.2518 0.3379 0.4738 0.5493 0.6486 0.7590

0.5585 0.4841 0.3980 0.3205 0.2342

0.2712 0.3438 0.4052 0.4628 0.5284

0.1363 0.1649 0.1922 0.2158 0.2362

0.37 0.35 0.35 0.28 0.23

0.4837 0.4206 0.3587 0.2854 0.2202

0.2262 0.2712 0.3050b 0.3480 0.3912

0.2594 0.2948 0.3300 0.3619 0.3929

0.65 0.27 0.33 0.24 0.26

b

0.2760 0.3300 0.3933 0.4189 0.4626 0.5157

0.6503 0.6301 0.5894 0.5697 0.5301 0.4816

0.0736 0.0399 0.0173 0.0114 0.0073 0.0027

0.3695 0.4182 0.4573 0.5004 0.5459

0.5945 0.5610 0.5295 0.4911 0.4495

0.0199 0.0101 0.0057 0.0029 0.0012

0.0161 0.0107 0.0075 0.0056 0.0034

0.4143 0.4561 0.4960 0.5284 0.5686

0.5534 0.5244 0.4934 0.4633 0.4264

0.0100 0.0055 0.0035 0.0015 0.0006

0.0223 0.0140 0.0071 0.0068 0.0044

0 wt % 1,3-Propanediol 0.0868 0.0448 0.0140 0.0079 0.0023 0.0003 10 wt % 1,3-propanediol 0.0428 0.0187 0.0082 0.0031 0.0011 20 wt % 1,3-Propanediol 0.0287 0.0144 0.0064 0.0032 0.0014

0.55 0.34 0.53 0.17 0.03 0.02

a

All tie-line compositions are reported as mass fractions w. Initial compositions in the quaternary system were chosen on the sectional plane (20 wt % ethanol + 80 wt % water) − (30 wt % ethanol + 70 wt % K2HPO4) − 100 wt % 1,3-propanediol in the tetrahedron. Tie-line data are reported for different 1,3-propanediol contents in the initial mixtures. The closest distance Δwi between the tie-lines and the initial composition of the mixtures is reported in wt %. bThe mass fraction of ethanol in the organic phase was calculated from the mass fractions of the other species. The standard uncertainties are within u(T) = 0.1 K, u(p) = 10 kPa, u(wO1 ) = 0.0004 g/g, u(wO2 ) = 0.0100 g/g, u(wS3) = 0.0020 g/g, u(wO3 ) = 0.0014 g/g, u(wS4) = 0.0006 g/g, and u(wO4 ) = 0.0020 g/g.

significantly larger in the 1-propanol molecule than in the ethanol molecule. In comparison to this, the presence of the second hydroxyl group in 1,3-propanediol leads to an increased hydrogen bond donor and acceptor capacity in comparison to 1propanol, which corresponds to the hydrophilic behavior of the molecule.

contain information which are highly valuable to understand the interactions of different molecules. As can be observed, very high screening charge densities in comparison to the chloride anion result for the carbonate and the hydrogen phosphate anions. Furthermore, the surface areas of these ions are significantly larger. This corresponds to the stronger water structure increasing effect of these ions in comparison to the chloride ion, which is related to the salting-out behavior.22 Furthermore, a comparison of the σ-profiles of conformers of different solvents used in this work is also shown in Figure 1. A high similarity of the σ-profiles of ethanol and 1-propanol in the region of hydrogen bond donor and acceptor segments can be observed due to the presence of a single hydroxyl group in both molecules. Nevertheless, the fraction of the nonpolar surface area is

3. RESULTS 3.1. Experimental Section. In different works ethanol + K2HPO4 based systems were evaluated for the extraction of hydrophilic diols from aqueous fermentation broths.11 Experimentally determined tie-line compositions for the K2HPO4 + H2O + ethanol + 1,3-propanediol system can be found in Table D



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fractions in the salt-rich phase are reached without the formation of a solid phase. In Table 5 experimental tie-line compositions for the quaternary system K2CO3 + water + ethanol + 1,3-propanediol are shown. Especially at low salt concentrations it can be expected that chemical equilibria between different carbonate species become important. The ionic species may even distribute differently between the salt-rich and the organic phases. Nevertheless, the focus of this work was the distribution of the solvents and not the distribution of the different ions. Furthermore, very high salt mass fractions are reached in the salt-rich phase implying that the majority of the salt does not react, whereas in the organic phase significantly lower amounts of the salts are present. For the calculation of the mass fraction of the salt in the organic phase from the experimentally determined alkali ion concentration the chemical reactions were neglected. This is in agreement with investigations of similar systems where the species distribution of the ions was also not considered.40,41 A comparison of the Cruickshank projections of the K2CO3 + water + ethanol + 1,3-propanediol and the K2HPO4 + water + ethanol + 1,3-propanediol system is shown in Figure 4 at 10 wt % of 1,3-propanediol. Although the comparison is restricted by the different initial compositions in the systems, it can be seen that similar compositions in the organic phase were found. Nevertheless, similar compositions in the organic phase correspond to higher mass fractions of K2HPO4 in the salt-rich phase. This is in agreement with experimental data for the 1,3-propanediol free systems which were measured for K2HPO4 + ethanol + water in this work and were determined for the K2CO3 + ethanol + water system by Salabat et al.42 Considering an extraction process, lower salt contents in the systems are favorable. Nevertheless, for both systems comparatively high salt contents in the salt-rich phase have to be reached to achieve sufficiently favorable partition behavior of the diol. For an extraction process it is therefore necessary to develop efficient solutions for the recovery of the salt which has also been identified as a key problem of these processes in different works.11 Considering the original aim of a reduction of the energy demand of the process the investigation of alternatives to the evaporation of the water is worthwhile. An established method for the crystallization of salts is antisolvent crystal-

4. The corresponding 1,3-propanediol free system was investigated in different works,35,36 whereas only Katayama et al.35 performed an analysis of the compositions of both phases. A comparison of the results for the 1,3-propanediol free ternary system with their data is shown in Figure 2. Satisfying agreement between the tie-lines can be observed.

Figure 2. Phase compositions in the ternary system K2HPO4 + ethanol + water at 25 °C: this work (●); experimental data by Katayama et al.35 (△).

To represent the phase compositions from the threedimensional tetrahedron in two dimensions, different projections are available.37 The method devised by Cruickshank et al.38 is based on projections which are parallel to two axes of the quaternary tetrahedron and lead to square diagrams. The experimental results for 1,3-propanediol contents of 0 wt % 1,3-propanediol and 20 wt % 1,3-propanediol in the system K2HPO4 + water + ethanol + 1,3-propanediol are shown in Figure 3 using Cruickshank projections. As already described in different works, a miscibility gap can even be induced in systems consisting of 1,3-propanediol or 2,3-butanediol and water upon addition of salts without addition of further solvents.7,39 This corresponds to the result that even at increased 1,3-propanediol contents in the quaternary system the mass fraction of this polar component in the salt-rich phase is comparatively low and a wide miscibility gap occurs. With increasing mass fraction of the salt the water content in the organic phase is reduced. High salt mass

Figure 3. Tie-lines of the K2HPO4 + ethanol + water + 1,3-propanediol system at 25 °C and different mass fractions of 1,3-propanediol: (■) 0 wt % 1,3propanediol; (○) 20 wt % 1,3-propanediol. Cruickshank projection parallel to the 1,3-propanediol−K2HPO4 and the ethanol−water axes (left) and projection parallel to the ethanol−K2HPO4 and the 1,3-propanediol−water axes (right). The mass fractions in the organic phase were renormalized to satisfy the necessary conditions of summation to unity. E



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Table 5. Experimental Liquid−Liquid Phase Compositions in the K2CO3 (1) + Water (2) + Ethanol (3) + 1,3-Propanediol (4) System at Temperature T = 25 °C and Pressure p = 0.1 MPaa salt-rich phase

organic phase

w1

w2

w3

w4

0.2066 0.2586 0.3031 0.3419 0.3630 0.3998 0.4264 0.4629

0.6707 0.6673 0.6496 0.6280 0.6143 0.5849 0.5620 0.5291

0.0945 0.0547 0.0328 0.0200 0.0142 0.0087 0.0062 0.0040

0.0282 0.0194 0.0145 0.0101 0.0086 0.0066 0.0054 0.0041

0.2489 0.2912 0.3341 0.3648 0.3991 0.4202 0.4383 0.4755

0.6326 0.6345 0.6168 0.6027 0.5744 0.5603 0.5454 0.5131

0.0708 0.0405 0.0248 0.0156 0.0114 0.0074 0.0060 0.0034

0.0476 0.0337 0.0243 0.0168 0.0151 0.0121 0.0104 0.0080

w1 5 wt % 1,3-Propanediol 0.0782 0.0407 0.0177 0.0079 0.0047 0.0026 0.0017 0.0012 10 wt % 1,3-Propanediol 0.0743 0.0417 0.0184 0.0089 0.0067 0.0039 0.0039 0.0032

w2

w3

w4

Δwi

0.5550 0.4920 0.4197 0.3368 0.2964 0.2371 0.2104 0.1582

0.3031 0.4017 0.4863 0.5664 0.6096 0.6665 0.6963 0.7521

0.0669 0.0789 0.0923 0.0982 0.1059 0.1135 0.1152 0.1201

0.17 0.48 0.66 0.31 0.64 0.62 0.73 0.88

0.5450 0.4855 0.4077 0.3389 0.3082 0.2581 0.2116 0.1633

0.2714 0.3495 0.4212 0.4839 0.5196 0.5574 0.5779 0.6190

0.1205 0.1456 0.1609 0.1800 0.1934 0.1998 0.2146 0.2306

0.44 0.71 0.51 0.43 1.01 0.65 0.35 0.41

a The standard uncertainties are within u(T) = 0.1 K, u(p) = 10 kPa, u(wO1 ) = 0.0006 g/g, u(wO2 ) = 0.0092 g/g, u(wS3) = 0.0005 g/g, u(wO3 ) = 0.0094 g/g, u(wS4) = 0.0012 g/g, and u(wO4 ) = 0.0079 g/g. All tie-line compositions are reported as mass fractions w. Initial compositions in the quaternary system were chosen on the sectional plane (20 wt % ethanol + 80 wt % water) − (40 wt % ethanol + 60 wt % K2CO3) − 100 wt % 1,3-propanediol in the tetrahedron. Tie-line data are reported for different 1,3-propanediol contents in the initial mixtures. The closest distance Δwi between the tielines and the initial composition of the mixtures is reported in wt %.

Figure 4. Tie-lines of the K2HPO4 + ethanol + water + 1,3-propanediol system at 25 °C and 10 wt % 1,3-propanediol (■) and the K2CO3 + ethanol + water + 1,3-propanediol system at 25 °C and 10 wt % 1,3-propanediol (○). Cruickshank projection parallel to the 1,3-propanediol−salt and the ethanol−water axes (left) and projection parallel to the ethanol−salt and the 1,3-propanediol−water axes (right). The mass fractions in the organic phase were renormalized to satisfy the necessary condition of summation to unity.

liquid−liquid equilibria in systems of the type Na2CO3 + shortchain alcohol + water at 25 °C. They found significantly wider miscibility gaps at comparable salt mass fractions in the salt-rich phase for 1-propanol or 2-propanol containing systems then for ethanol containing systems. In Table 6 tie-line data for the quaternary Na2CO3 + H2O + 1propanol + 1,3-propanediol system is reported for a temperature of 40 °C. A comparison of the results for the 1,3-propanediol free system with values reported by Nemati-Kande and Shekaari for a temperature of 25 °C is shown in Figure 5. Although the system was investigated at different temperatures, a comparatively high agreement of the results can be observed.

lization. Whereas the recovery of K2HPO4 has been considered with antisolvent crystallization in the context of 1,3-propanediol recovery previously, a further acidification step was necessary, leading to a significant demand of phosphoric acid and the recovery of KH2PO4 instead of the direct recovery of K2HPO4.43 Na2CO3 can be crystallized upon addition of methanol.44 It has furthermore already been considered in the context of the extraction of 1,3-propanediol from aqueous solutions.45 Although the use of more polar organic solvents can be expected to be beneficial for the partition behavior of 1,3-propanediol, it is likewise expected that this corresponds to an increased water content in the organic phase at similar salt concentrations in the salt-rich phase. Nemati-Kande and Sheekari41 have determined F



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Table 6. Experimental Liquid−Liquid Phase Compositions in the Na2CO3 (1) + Water (2) + 1-Propanol (3) + 1,3-Propanediol (4) System at Temperature T = 40 °C and Pressure p = 0.1 MPaa salt-rich phase

organic phase

w1

w2

w3

0.0514 0.1109 0.1639 0.2136 0.2595 0.3011

0.7927 0.8291 0.8075 0.7735 0.7348 0.6959

0.1558 0.0601 0.0285 0.0128 0.0057 0.0030

0.1125 0.1660 0.2223 0.2673 0.3049

0.7912 0.7808 0.7467 0.7137 0.6831

0.0596 0.0257 0.0109 0.0046 0.0021

0.0366 0.0275 0.0202 0.0144 0.0099

0.1486 0.2194 0.2498 0.2930

0.7675 0.7281 0.7181 0.6856

0.0305 0.0140 0.0058 0.0025

0.0534 0.0385 0.0264 0.0189

w4

w1 0 wt % 1,3-Propanediol 0.0092 0.0027 0.0012 0.0005 0.0003 0.0002 5 wt % 1,3-Propanediol 0.0066 0.0033 0.0017 0.0009 0.0005 10 wt % 1,3-Propanediol 0.0124 0.0070 0.0034 0.0017

w4

Δwi

w2

w3

0.5018 0.3608 0.2977 0.2502 0.2061 0.1668

0.4936 0.6290 0.7061 0.7523 0.7952 0.8435

0.4155 0.3414 0.3017 0.2451 0.1958

0.5300 0.5888 0.6254 0.6635 0.7110

0.0493 0.0595 0.0732 0.0854 0.0856

1.10 0.98 1.06 0.91 1.10

0.4530 0.3967 0.3266 0.2692

0.4052 0.4648 0.5248 0.5637

0.1265 0.1394 0.1621 0.1785

0.14 1.04 0.57 0.44

0.18 0.22 0.15 0.09 0.07 0.38

a The standard uncertainties are within u(T) = 0.1 K, u(p) = 10 kPa, u(wO1 ) = 0.0002 g/g, u(wO2 ) = 0.0032 g/g, u(wS3) = 0.0006 g/g, u(wO3 ) = 0.0060 g/g, u(wS4) = 0.0003 g/g, and u(wO4 ) = 0.0007 g/g. All tie-line compositions are reported as mass fractions w. Initial compositions in the quaternary system were chosen on the sectional plane (31.5 wt % 1-propanol + 68.5 wt % water) − (31.5 wt % 1-propanol + 68.5 wt % Na2CO3) − 100 wt % 1,3-propanediol in the tetrahedron. Tie-line data are reported for different 1,3-propanediol contents in the initial mixtures. The closest distance Δwi between the tie-lines and the initial composition of the mixtures is reported in wt %.

In Figure 6, a comparison of the tie-lines at different 1,3propanediol contents in the system is shown. A wide miscibility gap can be observed also at an increased 1,3-propanediol mass fraction of 10 wt %. A significant reduction of the water content of the organic phase occurs with increasing mass fraction of the salt and favorable partitioning behavior of 1,3-propanediol can be observed. In Table 7 equilibrium compositions for the system Na2CO3 + H2O + 2-propanol + 1,3-propanediol are reported for a temperature of 40 °C. In Figure 7 a comparison of the tie-line compositions with data from the system Na2CO3 + H2O + 1propanol + 1,3-propanediol is shown. Although a direct comparison is restricted by the different initial compositions, at

Figure 5. Phase compositions in the ternary system Na2CO3 + 1propanol + water: tie-line data at 40 °C determined in this work (●); tieline data at 25 °C from Nemati-Kande and Shekaari41 (△).

Figure 6. Tie-lines of the Na2CO3 + 1-propanol + water + 1,3-propanediol system at 40 °C and different 1,3-propanediol contents in the initial mixtures: 0 wt % 1,3-propanediol (■); 5 wt % 1,3-propanediol (○); 10 wt % 1,3-propanediol (▲). Cruickshank projection parallel to the 1,3-propanediol− Na2CO3 and the 1-propanol−water axes (left) and projection parallel to the 1-propanol−Na2CO3 and the 1,3-propanediol−water axes (right). The mass fractions in the organic phase were renormalized to satisfy the necessary condition of summation to unity. G



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Table 7. Experimental Liquid−Liquid Phase Compositions in the Na2CO3 (1) + Water (2) + 2-Propanol (3) + 1,3-Propanediol (4) System at Temperature T = 40 °C and Pressure p = 0.1 MPaa salt-rich phase

organic phase

w1

w2

w3

w4

0.1254 0.1594 0.1865 0.2132 0.2380 0.2553 0.2719 0.2972

0.7657 0.7656 0.7626 0.7472 0.7320 0.7210 0.7094 0.6882

0.0716 0.0443 0.0276 0.0179 0.0111 0.0074 0.0045 0.0026

0.0373 0.0306 0.0233 0.0218 0.0188 0.0164 0.0142 0.0120

w1 5 wt % 1,3-propanediol 0.0343 0.0213 0.0119 0.0075 0.0043 0.0025 0.0015 0.0009

w2

w3

w4

Δwi

0.6228 0.5589 0.4921 0.4571 0.3846 0.3443 0.3087 0.2762

0.2889 0.3570 0.4043 0.4566 0.4991 0.5337 0.5618 0.5920

0.0621 0.0725 0.0869 0.0939 0.1017 0.1139 0.1256 0.1335

0.38 0.31 0.26 0.50 0.26 0.28 0.91 0.75

a The standard uncertainties are within u(T) = 0.1 K, u(p) = 10 kPa, u(wO1 ) = 0.0005 g/g, u(wO2 ) = 0.0045 g/g, u(wS3) = 0.0007 g/g, u(wO3 ) = 0.0015 g/g, u(wS4) = 0.0022 g/g, and u(wO4 ) = 0.0032 g/g. All tie-line compositions are reported as mass fractions w. Initial compositions in the quaternary system were chosen on the sectional plane (20 wt % 2-propanol + 80 wt % water) − (20 wt % 2-propanol + 80 wt % Na2CO3) − 100 wt % 1,3propanediol in the tetrahedron. Tie-line data are reported for different 1,3-propanediol contents in the initial mixtures. The closest distance Δwi between the tie-lines and the initial composition of the mixtures is reported in wt %.

Figure 7. Tie-lines of the Na2CO3 + 1-propanol + water + 1,3-propanediol system at 40 °C and 5 wt % 1,3-propanediol (■) and the Na2CO3 + 2propanol + water + 1,3-propanediol system at 40 °C and 5 wt % 1,3-propanediol (○). Cruickshank projection parallel to the 1,3-propanediol−Na2CO3 and the organic solvent−water axes (left) and projection parallel to the organic solvent−Na2CO3 and the 1,3-propanediol−water axes (right). The mass fractions in the organic phase were renormalized to satisfy the necessary condition of summation to unity.

comparable salt mass fractions in the salt-rich phase tie-line lengths are larger for the 1-propanol containing system than for the 2-propanol containing system. This is in agreement with the results from Nemati-Kande and Sheekari41 who investigated the 1,3-propanediol free systems at 25 °C. For both systems favorable partitioning of 1,3-propanediol into the organic phase can be observed. To compare the different systems, the partition ratio can be evaluated: K iOS = wiO/wiS

(6)

Here wOi and wSi are the weight fractions of a component i in the organic and the salt-rich phase. In Figure 8 the partition ratio of 1,3-propanediol is shown for the different systems investigated in this work at varying 1,3-propanediol mass fractions in dependence on the mass fraction of the salt in the salt-rich phase. It can be observed that the partition ratio depends strongly on the salt content in the system particularly at high salt mass fractions, with favorable partitioning behavior being reached for all systems. The determination of solid−liquid−liquid equilibrium compositions was not in the scope of this work; nevertheless the high solubility of K2HPO4 in water of 62.7 wt % at 25 °C corresponds well to the

Figure 8. Partition ratios of 1,3-propanediol in dependence on the salt mass fraction in the salt-rich phase for the K2HPO4 + water + ethanol + 1,3-propanediol system at 25 °C and 10 wt % 1,3-propanediol (blue ■) as well as 20 wt % 1,3-propanediol (red ○), the K2CO3 + water + ethanol + 1,3-propanediol system at 25 °C and 5 wt % 1,3-propanediol (green ▲) as well as 10 wt % 1,3-propanediol (purple ▽), the Na2CO3 + H2O + 1-propanol +1,3-propanediol system at 40 °C and 5 wt % 1,3propanediol (yellow +) and the Na2CO3 + H2O + 2-propanol + 1,3propanediol system at 40 °C and 5 wt % 1,3-propanediol (●). H



Article

equilibrium compositions for a large number of different ternary electrolyte systems containing different solvents and salts are available in the literature. For the systems investigated in this work it was shown that particularly in the comparison of the 1-propanol and 2-propanol containing systems (systems III and IV) and the ethanol containing systems (I and II) the difference between the partition ratios between systems was larger than the change in the partition ratio due to the increase of the 1,3-propanediol content in a single system. This need not be the case for every system and will depend on the concentration range investigated. Depending on the type of the system and the concentration range a closing of the miscibility gap can be expected with increasing 1,3propanediol content. Nevertheless, the data indicates that a successful prediction of the partition ratio of 1,3-propanediol at infinite dilution in the system can be helpful in the choice of a suitable system. For such a prediction, the mass fractions of the other species can be fixed to the tie-line composition of the ternary 1,3-propanediol free system. In Figure 10 experimentally determined logarithmic partition ratios at a finite concentration of 1,3-propanediol are compared

high salt mass fractions reached in this work for the K2HPO4 + H2O + ethanol + water system.34 Close to the solubility limit a large fraction of the organic solvents is removed from the salt-rich phase. Aydogan et al.9 reported a mass concentration based partition ratio of 1,3-propanediol equal to 8.95 in this system. The corresponding composition of the system was reported with 20 wt % of K2HPO4, 30 wt % of ethanol and 40 g/L of 1,3propanediol. Due to the unknown densities of the phases needed for the conversion between the different scales of the partition ratios and the unknown phase compositions, a direct comparison to this work is not possible. Nevertheless, this value is in the expected range which can be estimated from Figure 8. The solubilities of K2CO3 in water at 25 °C (52.7 wt %) and Na2CO3 in water at 40 °C (32.8 wt %) are lower than the solubility of K2HPO4 at 25 °C.34 Nevertheless, concerning the development of an extraction process lower salt contents in the system are desirable. The salt mass fractions in the salt-rich phase reached for the Na2CO3 containing systems are close to the solubility limit in water, while no solid phase formation was visually observed. In this concentration range of the salt, the partition ratios in the Na2CO3 containing systems are even higher than for the K2HPO4 and the K2CO3 containing systems. Beside the favorable partition behavior for the extraction of 1,3-propanediol the selectivity is of interest which is defined as the ratio of the partition ratio of 1,3-propanediol and the partition ratio of water: OS OS SPDO,H2O = KPDO /KH2O

(7)

In Figure 9 the selectivity is shown in dependence on the salt mass fraction in the salt-rich phase for the systems studied in this

Figure 10. Logarithmic partition ratios of 1,3-propanediol as a function of the salt mass fraction in the salt-rich phase. Experimental data for the K2HPO4 + H2O + ethanol + 1,3-propanediol system at 25 °C and 10 wt % 1,3-propanediol (blue ■), the K2CO3 + H2O + ethanol + 1,3propanediol system at 25 °C and 5 wt % 1,3-propanediol (red ○) and the Na2CO3 + H2O + 1-propanol + 1,3-propanediol system at 40 °C and 5 wt % 1,3-propanediol (green ▲). Model predictions using COSMORS with the electrolyte extension by Ingram et al. for the state of infinite dilution of 1,3-propanediol in the K2HPO4 + H2O + ethanol system at 25 °C (blue ), the K2CO3 + H2O + ethanol system at 25 °C (red −−), and the Na2CO3 + H2O + 1-propanol system at 40 °C (green −·−). Experimental phase compositions of the 1,3-propanediol free systems were taken from this work, with the exception of the K2CO3 + H2O + ethanol system, where experimental data from Salabat and Hashemi42 was used.

Figure 9. Selectivity in dependence on the salt mass fraction in the saltrich phase for different systems. The symbols are the same as in Figure 8.

work. The observed differences in selectivities for the different systems are larger than the difference in the corresponding partition ratios of 1,3-propanediol. At comparable salt contents in the salt-rich phase the selectivity is lowest in the K2HPO4 + H2O + ethanol + 1,3-propanediol system while being highest for the Na2CO3 containing systems. The selectivity for both Na2CO3 containing systems is similar which indicates that the slightly higher partition ratio of 1,3-propanediol for the 2-propanol containing system is compensated by a higher partition ratio of water. This corresponds well to the wider miscibility gap of the 2propanol containing system observed in Figure 7. 3.2. Modeling. Due to the complexity of the systems and the sensitivity of liquid−liquid-equilibrium data a successful prediction of the tie-line compositions in the full quaternary system was not expected yet. Nevertheless, liquid−liquid

with the respective prediction at infinite dilution using COSMORS with the electrolyte extension by Ingram et al. Although no quantitative agreement is reached, it can be observed that the model correctly predicts the increase in partition ratio with increasing salt mass fraction in the salt-rich phase for most compositions. Only for the K2CO3 + water + ethanol + 1,3propanediol system the partition ratio is predicted to decrease at very high salt mass fractions in the system. Whereas the correlation of the complete phase behavior of ternary or quaternary electrolyte systems is possible with a number of empirical and semiempirical models, few models allow predictions for components that are not parts of the training set. Whereas it can be expected that applying group-contribution I



■

based predictive models, a higher accuracy can be reached in comparison to the current model, this is only the case if the necessary interaction parameters for example between ions and organic solvents are available.13,15 The COSMO-RS based approach evaluated in this work does not necessitate any system specific parameters, and no component specific parameters were introduced for the organic solvents. Furthermore, no ion specific parameters have been correlated for the polyatomic anions, and the true species distribution due to carbonate equilibria has been neglected during the prediction. Considering these restrictions, the results of the predictions are satisfying. Particularly at lower salt mass fractions, the order of the predicted partition ratios corresponds to the order found in the experiments, with lowest partition ratios being reached in the K2CO3 + water + ethanol + 1,3-propanediol system and highest partition ratios being reached for the Na2CO3 + water + 1-propanol + 1,3-propanediol system. This demonstrates the potential for such a predictive model whereas further improvements can be expected from the introduction of polyatomic ions into the model parametrization and the refinement of the model based on liquid−liquid equilibrium data of electrolyte systems. Considering the complexity of fermentation broths where the partition ratios of different species might be of interest for the process development, such improvements are of particular interest.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

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4. CONCLUSIONS Phase equilibrium data for the quaternary systems K2HPO4/ K2CO3 + H2O + ethanol + 1,3-propanediol and for the systems Na2CO3 + H2O + 1-propanol/2-propanol + 1,3-propanediol were investigated. Favorable partition ratios of 1,3-propanediol were achieved with all systems, whereas the partition ratio was found to be highly dependent on the mass fraction of the salt especially at high salt contents. At lower mass fractions of the salt which are of higher interest for an extraction process, the highest partition ratios were found for the Na2CO3 + H2O + 2-propanol + 1,3-propanediol system. Comparing the selectivities for the different systems in this salt concentration range, the selectivity was also found to be highest for the Na2CO3 containing systems. The values reached were similar using 1-propanol or 2-propanol as a solvent. This demonstrates that favorable extraction of 1,3propanediol is possible with these systems, whereas it is known that Na2CO3 may be recovered with antisolvent crystallization upon addition of methanol. Due to the complexity of the systems it was furthermore investigated if the thermodynamic model COSMO-RS with the electrolyte extension by Ingram et al. can be applied to the prediction of the partition ratio of the diol in these systems. For this reason, the partition ratio was evaluated at infinite dilution of the component using phase compositions of the ternary 1,3propanediol free systems. Although the partition ratio was in general overestimated by the model in comparison to the experimentally determined partition ratios at finite 1,3-propanediol concentrations, at lower salt mass fractions the correct order of the partition ratios for the three different salts was predicted. Increased deviations occurred for larger salt contents in the system. This indicates the potential for such models during the search for a suitable system for an extraction problem, whereas further refinements of the model are necessary to achieve quantitatively more accurate predictions. J



Article

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