PC-SAFT Modeling of CO2 Solubilities in Deep Eutectic Solvents - The

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PC-SAFT Modeling of CO Solubilities in Deep Eutectic Solvents Lawien F Zubeir, Christoph Held, Gabriele Sadowski, and Maaike C. Kroon J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b07888 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on February 5, 2016

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The Journal of Physical Chemistry

PC-SAFT modeling of CO2 solubilities in deep eutectic solvents Lawien F. Zubeira, Christoph Heldb*, Gabriele Sadowskib, Maaike C. Kroonac* a

Separation Technology, Eindhoven University of Technology, Den Dolech 2, 5612AZ Eindhoven, The Netherlands

b

Department of Biochemical and Chemical Engineering, TU Dortmund University, Dortmund, Germany c

Department of Chemical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates

Keywords: Deep eutectic solvents, low transition temperature mixtures, carbon capture, PCSAFT *

Corresponding authors: E-mail: [email protected]; [email protected]; Phone: +31-40-2475289; Fax: +31-40-2463966 E-mail: [email protected]; Phone: +49-231-7552086; Fax: +49-231-7552572

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ABSTRACT Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT), a physically based model that accounts for different molecular interactions explicitly, was applied to describe for the first time the phase behavior of deep eutectic solvents (DESs) with CO2 at temperatures from 298.15 to 318.15 K and pressures up to 2 MPa. DESs are mixtures of two solid compounds, a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), which form liquids upon mixing with melting points far below that of the individual compounds. In this work, the HBD is lactic acid and the HBAs are tetramethylammonium chloride, tetraethylammonium chloride and tetrabutylammonium chloride. Two different modeling strategies were considered for the PCSAFT modeling. In the first strategy, the so-called pseudo-pure component approach, a DES was considered as a pseudo-pure compound, and its pure-component parameters were obtained by fitting to pure DES density data. In the second strategy, the so-called individualcomponent approach, a DES was considered to consist of two individual components (HBA and HBD), and the pure-component parameters of the HBA and HBD were obtained by fitting to the density of aqueous solutions containing only the individual compounds of the DES. In order to model vapor-liquid equilibria (VLE) of DES + CO2 systems, binary interaction parameters were adjusted to experimental data from literature and to new data measured in this work. It was concluded that the individual-component strategy allows quantitatively prediction of the phase behavior of DES + CO2 systems containing those HBD:HBA molar ratios that were not used for kij fitting. In contrast, applying the pseudo-pure component strategy required DES-composition specific kij parameters.

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1 Introduction Deep eutectic solvents (DES) and low transition temperature mixtures (LTTMs) have recently attracted much interest in various research fields due to their tunable physicochemical properties. The main difference between LTTMs1 and DESs2 is that the latter show a first order phase transition (e.g., eutectic melting point), while the former show a second order phase transition (e.g., glass transition point) upon heating. Both terms are often used interchangeably. Therefore, in the rest of this paper we will adopt the most often used terminology ‘DES’ for both. Among others, the DESs can be employed in electrochemical applications3, recovery of metal oxide4, acid- and base-catalyzed reactions5,6 and synthesis of organic materials.7 Furthermore, DESs can be used to selectively alter the fugacities of components in mixtures, rendering them the potential to be used as solvents for large-scale industrial applications, for instance, as entrainers in extractive distillation processes and extraction solvents in liquid-liquid extraction.8,9 In contrast to ionic liquids (ILs), which are entirely ionic and exist of a cation and an anion with strong Coulombic forces governing their properties, DESs consists of a solid hydrogen bond donor (HBD) and a solid hydrogen bond acceptor (HBA) that form liquids upon mixing by association through hydrogen bonding interactions, resulting in a melting point (far) below that of the individual compounds. IL properties can be tuned by combining different cations and anions and via functionalization of one or both ions. DESs offer an additional degree of freedom to tune their properties. They can be formed at different molar ratios of the HBD and the HBA. DESs show similar physicochemical properties to ILs, but they are much cheaper, biodegradable and less toxic. Further, they can easily be prepared without the need for solvents or complex synthesis, hence circumventing the need for purification steps. Therefore, they can be a promising alternative to ILs in various applications. Based on the similarities between ILs and DESs, and the fact that many ILs show high affinity and selectivity towards CO2, DESs are expected to be good absorbents for CO2 capture in post3



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combustion (e.g. removal of acidic gases from flue gasses) or in pre-combustion processes (e.g. natural gas sweetening). Recently, CO2 solubilities have been determined in a number of DESs.10,11,12,13,14 Although the reported CO2 solubilities in DESs are still lower compared to the fluorinated state-of-the-art CO2 capture ILs, the DESs show a great tunability of the thermal, physical and chemical properties by changing the constituents and their composition. For the development of new absorption processes using DESs as solvents, a larger experimental data base on the phase behavior of CO2 + DES mixtures is required. Experimental measurements of gas solubilities are time-consuming and expensive. Preferably, the thermodynamic phase behavior is described using a well-defined model that rigorously relates thermodynamic properties to physical intermolecular forces between CO2 and the DES. Until date, only cubic equations of state have been proposed to correlate the vapor-liquid equilibria (VLE) of such systems. In our previous study10 we determined the CO2 solubility and kinetics in DESs composed of lactic acid (LA) as the HBD and tetra-alkyl-ammonium chloride (alkyl = methyl, ethyl and butyl) as the HBA at a molar ratio of 2:1, and explored their applicability as solvents for CO2 capture by providing experimental data of the phase behavior and the kinetics of the distinct DES + CO2 systems. Peng-Robinson equation of state (PR-EoS) with van der Waals two mixing rules was applied to correlate the experimentally obtained VLE data. Due to the large differences in molecular size and attraction forces, it was inevitable to use binary interaction parameters (lij and kij). In order to make the PR-EoS less correlative, the interaction parameters were assumed to be temperature-independent. However, these binary parameters were found to be relatively high, which might be due to the inability of PR-EoS to account for the strong associative forces in such systems. In this work, Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT), which is a physically based model that accounts for different molecular interactions explicitly, is applied to describe for the first time the phase behavior of DES + CO2 systems. PC-SAFT has been 4


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commonly used for systems containing polar, non-polar and associating compounds including IL + CO2 systems.15,16,17 Two different strategies are considered to predict the pure-component parameters (needed for PC-SAFT). Furthermore, to investigate the predictive capability of PCSAFT, the pure-component parameters and the kij of the DES (HBD:HBA at a molar ratio of 2:1) + CO2 system are used to predict the phase behavior of the DES + CO2 systems at the 3:1 molar ratio, for which also new experimental data have been included in this work.

2 Experimental In our previous study, the prepared DESs were formed by mixing LA, which acts as the HBD with the quaternary ammonium salts (HBA) tetramethylammonium chloride (TMA-Cl), tetraethylammonium chloride (TEA-Cl) and tetrabutylammonium chloride (TBA-Cl).10 They were all prepared in a 2:1 molar ratio. In the current study the DES with the highest CO2 absorption capacity (LA:TBA-Cl (2:1)) was further explored by investigating the influence of its composition on the CO2 absorption capacity. Therefore, LA:TBA-Cl with the molar ratio 3:1 was prepared and the CO2 absorption isotherms were experimentally determined.

2.1 Materials Tetrabutylammonium chloride (TBA-Cl, ≥98%) was purchased from Sigma-Aldrich. Ultra-pure crystalline L-lactic acid (LA) of pharmaceutical grade was kindly provided by PURAC Biochem B.V., Gorinchem, The Netherlands. The CO2 used for the measurements was supplied by Linde AG, The Netherlands, and had an ultra-high purity of 99.995%. The preparation of lactic acid:tetrabutylammonium chloride (3:1) was conducted following the same procedures applied to form the 2:1 mixtures.10 The water content in the prepared DES was measured with the Karl Fischer titration method (795 KFT Titrino Metrohm Karl Fischer) and found to be less than 0.5 wt%. 5



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2.2 Procedures Density measurements of the newly prepared DES (LA:TBA-Cl (3:1)) were performed using an Anton Paar SVM 3000 Stabinger Viscometer. Density was measured in the temperature range of 293.15 to 318.15 K at atmospheric pressure. The apparatus has a density range of 0.65 to 3 g·cm-3. The temperature uncertainty in the temperature range of the equipment (288.15 to 378.15 K) is ± 0.02 K and the absolute uncertainty in the density is 0.0005 g·cm-3.

The solubility of CO2 in (LA:TBA-Cl (3:1)) was studied by determining the bubble-point curve using a magnetic suspension balance (MSB, Rubotherm GmbH). With this setup it is not possible to measure the composition of the low-density phase (dew-point). Therefore, the dewpoint curves are not reported. A more detailed description of the equipment was presented previously.10 The mass of the absorbed CO2 (mCO2) is obtained by:

(

)

mCO2 = mbal ( P, T ) − ( msc + s ) + Vsc + s + CO2 ⋅ ρCO2 ( P, T )

(1)

where mCO2 is the mass of CO2 absorbed, mbal is the balance reading, (msc+s) is the total mass of the loaded sample container, (Vsc+s+CO2) is the volume of the sample with the absorbed gas and the sample container and ρCO2 is the density of CO2 at the operating conditions. Assuming that the volume of the absorbed gas is negligible, eq. (1) reduces to:

mCO2 = mbal ( P, T ) − ( msc + s ) + (Vsc + s ) ⋅ ρCO2 ( P, T )

(2)

The mole fraction of the CO2 absorbed is determined from:

xCO 2

 mCO 2    M CO 2   =  mCO 2   mIL   +   M CO 2   M IL  6


(3)

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The DES expands upon gas absorption. Therefore, it is not accurate to assume that the volume of the sample (Vs) stays unchanged upon pressure increase. Mole fraction average for the molar volume was used to make an appropriate buoyancy correction due to the DES sample volume change for all DESs ((2:1) and (3:1)). It should be noted that this correction method is only applicable for low pressures (

PC-SAFT Modeling of CO2 Solubilities in Deep Eutectic Solvents - The

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