Solubility of Carbon Dioxide in Poly(oxymethylene) Dimethyl Ethers

Oct 17, 2017 - Henry's law constants of CO2 in OME2, OME3, and OME4 are determined. They are similar for all studied OME and depend strongly on the te...
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Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX-XXX

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Solubility of Carbon Dioxide in Poly(oxymethylene) Dimethyl Ethers Michael Schappals,† Tanja Breug-Nissen,† Kai Langenbach,† Jakob Burger,*,†,‡ and Hans Hasse† †

Laboratory of Engineering Thermodynamics (LTD), University of Kaiserslautern, 67663 Kaiserslautern, Germany S Supporting Information *

ABSTRACT: Experimental data on the solubility of carbon dioxide (CO2) in poly(oxymethylene) dimethyl ethers (CH3O(CH2O)nCH3, OMEn) are presented for OME2, OME3, and OME4. The total pressure was measured as a function of the liquid phase composition at 313.15 and 353.15 K for pressures up to 4.3 MPa in a high-pressure view-cell. Henry’s law constants of CO2 in OME2, OME3, and OME4 are determined. They are similar for all studied OME and depend strongly on the temperature. The experimental data are modeled by the original perturbed-chain statistical associating fluid theory equation of state. As a basis, pure component models for OME were developed based on literature data on the liquid density and vapor pressure. The solubility of CO2 in OME is successfully described using a group contribution scheme. The results show that OME are interesting candidates as physical absorbents for CO2.

1. INTRODUCTION

2. EXPERIMENTAL SECTION 2.1. Materials. Table 1 gives an overview of the purity and suppliers of the chemicals that were used in the present study. All chemicals were used without further purification. CO2 was purchased from Messer-Griesheim, Ludwigshafen, Germany. The solvents, OME2, OME3, and OME4, were synthesized and supplied by BASF SE, where they were produced from methylal and trioxane.2 The purity of the OME was determined by GC analysis, which was carried out in the present work as described in ref 13. The impurities comprise mainly OME of other chain lengths. Before the solubility measurements, all samples were degassed. 2.2. Apparatus and Method. The pressure required to dissolve a precisely known amount of CO2 in an also precisely known amount of OMEn was determined with a synthetic method using a well-established high-pressure view-cell technique. Experimental equipment and the procedure are described in detail elsewhere14−17 and not repeated at length here. At the beginning of each measurement, the cell is filled with CO2. The mass of CO2 filled into the cell is determined from the volume of the cell (29.7 cm3) and readings of temperature and pressure via the EoS of Span and Wagner.18 Solvent is pressed into the cell until the gas is completely dissolved. The mass of the solvent filled into the cell (about 20 to 30 g) is determined from the volume displacement of a calibrated spindle press from which the solvent is charged into the cell and the solvent density that was taken from ref 2. From the masses of CO2 and the solvent in the cell, the mole fraction of CO2 in the saturated liquid phase is calculated. The uncertainty

Poly(oxymethylene) dimethyl ethers (OME) are methanebased oxygenates of the chemical structure H 3C−O− (CH2O)n−CH3. They can be produced via the methanol route1−3 in scalable and economically viable4 processes. OME have been identified as synthetic diesel fuels with excellent blending and combustion properties.5−8 In a recent computational study,9 they have also been identified as promising solvents for the physical absorption of carbon dioxide (CO2) from biogas or natural gas. In that study, parameters of the statistical associating fluid theory (SAFT)-γ Mie equation of state (EoS)10 were fitted to experimental data for poly(oxyethylene) dimethyl ethers and used to predict solubilities of CO2 in OME. The results of that study indicate that OME are interesting absorbents for CO2. However, they have not been checked so far as no experimental data on the solubility of CO2 in OME were available. In the present work, the solubilities of CO2 in OME2, OME3, and OME4 were measured at 313.15 and 353.15 K, and pressures between 0.2−4.3 MPa and the corresponding Henry’s law constants were determined. The results confirm that OME are excellent physical solvents for CO2. The perturbed-chain (PC)-SAFT EoS 11 is used for describing the new experimental data. As a basis pure component models for OME (n = 2−5) were developed using data for liquid densities and vapor pressure from the literature.2,12 A homonuclear group contribution scheme was employed to reduce the number of parameters. The unlike interaction parameter between CO2 and OMEn is fitted to the experimental solubility data, resulting in an excellent description. © XXXX American Chemical Society

Received: August 8, 2017 Accepted: October 2, 2017

A

DOI: 10.1021/acs.jced.7b00718 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Chemicals Used in the Present Study chemical name

CAS number

source

purity (mol·mol−1)

carbon dioxide (CO2) methoxy(methoxymethoxy)methane (OME2) 2,4,6,8-tetraoxanonane (OME3) 2,4,6,8,10-pentaoxaundecane (OME4)

124-38-9 628-90-0 13353-03-2 13352-75-5

Messer-Griesheim BASF SE BASF SE BASF SE

≥0.9999 ≥0.986 ≥0.985 ≥0.986

Table 2. Experimental Results for the Solubility of CO2 in OME2, OME3, and OME4a 313.15 K −1

353.15 K

solvent

xCO2/mol mol

OME2

0.1180 0.1683 0.2221 0.2842 0.3528 0.3528

± ± ± ± ± ±

0.0002 0.0016 0.0020 0.0013 0.0056 0.0034

0.607 0.888 1.209 1.584 2.081 2.076

± ± ± ± ± ±

0.023 0.023 0.024 0.023 0.023 0.022

OME3

0.0853 0.1379 0.1947 0.2565 0.3247 0.3956 0.4898

± ± ± ± ± ± ±

0.0001 0.0002 0.0004 0.0016 0.0005 0.0012 0.0026

0.411 0.695 1.014 1.371 1.818 2.323 3.076

± ± ± ± ± ± ±

0.020 0.023 0.022 0.022 0.021 0.021 0.024

OME4

0.0965 0.1573 0.2201 0.2835 0.3550 0.4313 0.5365

± ± ± ± ± ± ±

0.0001 0.0003 0.0003 0.0004 0.0016 0.0010 0.0164

0.452 0.766 1.110 1.507 1.978 2.555 3.424

± ± ± ± ± ± ±

0.020 0.023 0.020 0.020 0.026 0.021 0.022

p/MPa

−1

xCO2/mol mol 0.0243 0.0645 0.1062 0.1473 0.1505 0.1873 0.2354 0.2797 0.0288 0.0770 0.0778 0.1218 0.1720 0.2185 0.2639 0.3161 0.3631 0.0331 0.0872 0.1403 0.1915 0.2462 0.2949 0.3465 0.4088

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.0001 0.0005 0.0006 0.0004 0.0002 0.0025 0.0009 0.0014 0.0001 0.0001 0.0003 0.0002 0.0005 0.0006 0.0012 0.0031 0.0006 0.0001 0.0001 0.0003 0.0003 0.0004 0.0005 0.0020 0.0160

p/MPa 0.269 0.635 1.037 1.432 1.456 1.829 2.344 2.853 0.254 0.682 0.686 1.102 1.583 2.082 2.599 3.208 3.838 0.278 0.745 1.238 1.758 2.325 2.905 3.559 4.241

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.022 0.022 0.023 0.022 0.022 0.029 0.023 0.027 0.020 0.020 0.020 0.021 0.021 0.023 0.027 0.022 0.022 0.020 0.020 0.020 0.021 0.021 0.022 0.022 0.028

a

Total pressure p and mole fraction of CO2 in the liquid phase xCO2. The standard uncertainty of the temperature u(T) is 0.1 K. The standard uncertainties u are reported following the ± signs.

significantly higher than the corresponding predictions from the literature,9 which are also shown in Figure 1 as dashed curves. From the experimental data, the Henry’s law constants of CO 2 in the studied OME are determined using the extrapolation technique described in ref 19. The corresponding numbers are referred to as experimental data in the following. Fugacity coefficients in the gas phase were checked using the PC-SAFT EoS (see Section 3) and do not contribute significantly to the Henry’s law constants. The experimental results are presented in Table 3 and Figure 2. In both Table 3 and Figure 2, results from the PC-SAFT EoS are also reported, which are discussed in Section 3.3. The numbers for the Henry’s law constant decrease with increasing OME chain length n. The difference between OME2 and OME4 is, however, only about 10%. On the other hand, there is an important increase in the Henry’s law constant of about 80% for all OME when changing the temperature from 313.15 to 353.15 K. From the temperature dependence of the experimental data of the Henry’s law constant, the enthalpy of absorption of CO2 in OME at infinite dilution is calculated as described in ref 20. In the studied temperature range it is found to be about 13.57, 13.31, and 13.27 kJ mol−1 for OME2, OME3, and OME4

of that mole fraction results from uncertainties of the determination of the mass of CO2 (0.1%) and the mass of the solvent (0.3%). The temperature in the cell is determined by two calibrated platinum resistance thermometers with an uncertainty of 0.1 K. For measuring the solubility pressure, pressure transducers for pressures up to 1, 2.5, and 10 MPa are used. For the volumetric determination of the CO2 pressure, transducers for pressures up to 1 and 2.5 MPa were employed. All pressure transducers (WIKA GmbH, Klingenberg, Germany) were calibrated using a high-precision pressure gauge (Desgranges & Huot, Aubervilliers, France). The uncertainty in the solubility pressure measurement results from the uncertainty of the pressure transducers (0.1% of each transducer’s full scale) and an additional contribution of about ±0.02 MPa caused by a small temperature drift in the tubes, which are filled with the solvent and connect the cell with the pressure transducers.17 2.3. Experimental Results. The experimental results for the solubility of CO2 in OME2, OME3, and OME4 at 313.15 and 353.15 K are reported in Table 2. Figure 1 shows the results in plots of the total pressure versus the mole fraction of CO2 in the saturated liquid phase. The experimental pressures are found significantly lower, and thus the solubility is found B

DOI: 10.1021/acs.jced.7b00718 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 2. Henry’s law constants of CO2 in OME2, OME3, OME4, and OME5 as a function of the temperature. Symbols for experimental data: ○ OME2, □ OME3, ◇ OME4. Lines: PC-SAFT model of this work. OME2 (red), OME3 (orange), OME4 (blue), and OME5 (green).

reported in ref 21, resulting in 14.1 kJ mol−1 for Selexol and 15.3 kJ mol−1 for Sulfolane. These numbers are higher than those for the OME, which underlines the attractiveness of these new solvents. Moreover, the results presented in Section 3.3 indicate that the enthalpy of absorption of CO2 in OME decreases with increasing temperature, which is important for desorption, whereas it increases for Selexol and Sulfolane.21 The experimental uncertainties reported in Tables 2 and 3 for the mole fraction of CO2, the solubility pressures, and the Henry’s law constants were estimated from an error propagation calculation.

3. MODEL 3.1. PC-SAFT Equation of State. For modeling the gas solubility data, the original PC-SAFT EoS11 is used here. It was chosen for its simplicity and the possibility to use a group contribution approach for describing the data for all studied OME with a single parameter set. More sophisticated variants of the PC-SAFT equation, notably PCP-SAFT that includes a polar contribution, could also be attractive. But, as the polarity of the OME is rather weak, we decided to use the original form. The results can also be taken as a benchmark for more sophisticated EoS. 3.2. Pure Components. The PC-SAFT parameters for the pure components are given in Table 4. The parameters for CO2

Figure 1. Solubility of CO2 in OME2 (top), OME3 (middle), and OME4 (bottom) shown as total pressure over mole fraction of CO2 in the liquid phase. Results for two temperatures are shown: blue 313.15 K and red 353.15 K. The symbols are experimental data from the present work; the full lines are PC-SAFT correlations of that data, and the dashed lines are predictions from ref 9.

Table 3. Results for Henry’s Law Constants of CO2 in OME2, OME3, and OME4 Obtained from the Experimental Data of the Present Work by Extrapolation19 and by Calculation with the PC-SAFT EoSa

Table 4. PC-SAFT Parameters of the Pure Components

kH/MPa solvent

T/K

OME2

313.15 353.15 313.15 353.15 313.15 353.15

OME3 OME4

experiment

PC-SAFT

± ± ± ± ± ±

4.995 8.811 4.785 8.420 4.585 8.026

5.066 9.142 4.807 8.578 4.682 8.340

0.195 0.934 0.236 0.699 0.208 0.606

σ/Å ε/kB/K m

CO2

OMEn

2.7852 169.21 2.0729

3.55 260.0 0.788n + 1.682

are adopted from ref 11. The parameters for the OME are determined in the present work from a fit to experimental data on the pure component liquid densities2 and vapor pressures12 of OME with n ranging from 2 to 5. To keep the number of parameters small, a homonuclear approach was used for the OME, in which all OME consist of segments with the same σ and ε. The number of segments m in OMEn is assumed to be linearly dependent on the OME chain length n (cf. Table 4). Using only four fitting parameters, both the liquid density (cf. Figure 3, average deviation about 1%) and vapor pressure (cf. Figure 4, average deviation about 3%) are described well for the studied OME. Deviation plots are provided in the Supporting

a

The standard uncertainty of the experimental data for the temperature u(T) is 0.1 K. The standard uncertainties u are reported following the ± signs.

,respectively. Therefore, the dependence of these results on the OME chain length n is not significant. The enthalpy of absorption of CO2 at infinite dilution in Selexol and Sulfolane, two widely used solvents, was calculated from the Henry’s law constants at 313.15 and 333.15 K, as C

DOI: 10.1021/acs.jced.7b00718 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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experimental data. Nevertheless, the experimental data are systematically above the PC-SAFT results. As a consequence, the same holds for the enthalpy of absorption at infinite dilution calculated as described in ref 20. For the PC-SAFT model, it was found to be about 13.05, 12.99, and 12.87 kJ mol−1 for OME2, OME3, and OME4, respectively, in the studied temperature range and thus around 0.4 kJ mol−1 lower than the experimental values, as specified in Section 2.3.

4. CONCLUSIONS Experimental results for the solubility of CO2 in OME2, OME3, and OME4 are reported here for the first time. In a previous work,9 such data were predicted using the SAFT-γ Mie EoS. The authors concluded that OME are attractive solvents for physical CO2 absorption. That statement is fully confirmed by the experimental results from the present study. The experimental solubility is even higher than the one predicted in ref 9. The enthalpy of absorption of CO2 in OME was found to be in the same range of other physical solvents of CO2 such as poly(ethylene) glycol ethers.21 To model the solubility, the PC-SAFT EoS was used, and parameters for the pure OMEn from n = 2 to 5 were determined. A homonuclear group contribution scheme for all OME with constant segment diameter and dispersion energy and a segment number depending linearly on the OME chain length n represents the experimental data well. Using a temperature-dependent adjustable parameter in the mixing rule for the energy parameter, the new experimental data on the solubility of CO2 in OME of different chain lengths is described well. The results from the present study, solubilities, and Henry’s law constants provide a basis for a detailed assessment of OME as physical solvents for CO2 capture. They can also be extended easily to OME mixtures or OME with chain length n > 5.

Figure 3. Liquid densities of OME at 1 bar. Symbols of experimental data:2 ○ OME2, □ OME3, ◇ OME4, △ OME5. Lines: PC-SAFT model of this work.

Figure 4. Vapor pressures (pS) of OME in logarithmic plot over inverse temperature. Symbols of experimental data:12 ○ OME2, □ OME3, ◇ OME4, △ OME5. Lines: PC-SAFT model of this work.

Information. Considering the simplicity of the present model (original PC-SAFT using a homonuclear group contribution scheme), this is an astonishingly good result. It is only noted here that the approach cannot be extended to methylal, i.e. OME with n = 1. However, as a group contribution scheme was applied and the influence of edge effects decreases for higher chain lengths, there is a reason to believe that properties of OME with n > 5 can be predicted well. 3.3. Mixtures. The binary interaction energy εCO2,OME between the CO2 and the OME segment was calculated using a modified Lorentz−Berthelot mixing rule as follows: εCO2,OME = (1 − k CO2,OME) εCO2εOME

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00718. Deviations between the PC-SAFT EoS and the experimental data regarding liquid densities, vapor pressure, and solubility (PDF)

■

(1)

kCO2,OME is fitted to the binary solubility data of the present work. Introducing a temperature dependency of kCO2,OME leads to a significant improvement. The temperature dependence of kCO2,OME is described well by 24.883 k CO2,OME = 0.1125 − T (K) (2)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael Schappals: 0000-0002-8131-2027 Jakob Burger: 0000-0002-2583-2335 Present Address

The comparison of the results of the PC-SAFT model with the experimental data of the present work is shown in Figure 1. The solubility is represented well for all three OME. The average deviation between model and experiment in the total pressure is about 1.75% and thus lower than the average uncertainty of 2.3%. The PC-SAFT model also gives results for Henry’s law constant, which are included in Table 3 and Figure 2. As could be expected, good agreement between the experimental data, obtained from the extrapolation,19 and the PC-SAFT model is observed within the uncertainties of the

‡

J.B.: Technical University of Munich

Funding

The authors gratefully acknowledge financial support within the Reinhart Koselleck Program (Grant HA1993/15-1) of the German Research Foundation (DFG). J.B. thanks the BASF SE for financial support. Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.jced.7b00718 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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(21) Henni, A.; Tontiwachwuthikul, P.; Chakma, A. Solubilities of Carbon Dioxide in Polyethylene Glycol Ethers. Can. J. Chem. Eng. 2005, 83, 358−361.

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DOI: 10.1021/acs.jced.7b00718 J. Chem. Eng. Data XXXX, XXX, XXX−XXX