Elsevier

Fluid Phase Equilibria

Volume 477, 15 December 2018, Pages 62-77
Fluid Phase Equilibria

Experimental measurement of carbon dioxide solubility in 1-methylpyrrolidin-2-one (NMP) + 1-butyl-3-methyl-1H-imidazol-3-ium tetrafluoroborate ([bmim][BF4]) mixtures using a new static-synthetic cell

https://doi.org/10.1016/j.fluid.2018.08.017Get rights and content

Abstract

A new experimental setup based on the “static-synthetic” method was designed and commissioned for the measurement of solubility from vacuum to moderates pressures. The new apparatus and experimental method were extensively validated by measuring the solubility of carbon dioxide in three pure solvents, viz., n-hexane, n-methyl-2-pyrrolidone and 1-butyl-3-methylimidazolium tetrafluoroborate. New solubility data for carbon dioxide in hybrid solvents with different mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate + n-methyl-2-pyrrolidone were measured at three temperatures, viz. (298.15, 313.15 and 323.15) K. Additionally, viscosity, density and sound velocity for binary mixtures of n-methyl-2-pyrrolidone + 1-butyl-3-methylimidazolium tetrafluoroborate were measured. The experimental phase equilibrium data were modelled using the Peng-Robinson equation of state and van der Waals mixing rule. In general, the solubility of carbon dioxide in the hybrid solvent was lower than the respective pure constituents. However, the hybrid solvent exhibited a significantly lower viscosity than the pure ionic liquid.

Introduction

Currently, there is still a heavy reliance globally on fossil fuels as an energy source [1]. The high demand for natural gas has resulted in the use of gas sources with relatively high concentrations of impurities such as hydrogen sulphide (H2S) and carbon dioxide (CO2) [2]. Aqueous solutions of CO2 and H2S form corrosive acidic solutions which may cause problems in process equipment and pipeline systems [[1], [2], [3], [4], [5]]. Numerous separation technologies have been proposed for the treatment of streams containing CO2 and H2S including absorption, adsorption, cryogenic condensation, membranes and hybrid separation processes. Absorption processes are conventional technologies for CO2 and H2S removal or natural gas sweetening [6,7]. Accordingly, the applicability of ionic liquids (ILs) as solvents for absorption processes has gained attention. This is due to their non-volatile and ‘green’ characteristics [2,[8], [9], [10], [11]]. The application of ILs in industry is often limited due to their high viscosity and high cost [8,9,12,13]. These negative characteristics of ionic liquids can be potentially dampened via the use of co-solvents or hybrid solvents (blends of ILs with other solvents). The solubility of CO2 in blends of ILs and chemical solvents, such as amines, has been studied by many researchers [[14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]]. However, data involving the solubility of CO2 in blends of ILs and physical solvents are rare. Shokouhi et al. blended three different amine-functionalized ILs with methanol and measured CO2 solubility [38]. Lei et al. studied the solubility of CO2 in hybrid solvents of 1-octyl-3-methylimidazolium bis(trifluoromethanesulfonil)imide ([omim][TF2N]) and methanol. The addition of the IL to methanol improved the solubility of CO2 [39]. Tian et al. studied the solubility of CO2 in blends of n-methyl-2-pyrrolidone (NMP) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), with initial solvent loadings of mass fractions of [bmim][BF4] of greater than 0.5 [40]. Imidazolium-based ILs, especially [bmim][BF4], are among the most widely investigated ILs for CO2 capture. In addition, solubility of CO2 in [bmim][BF4] is similar to that in NMP [[40], [41], [42], [43]]. In this work, new CO2 solubility data at concentrations of [bmim][BF4] + NMP of less than 0.5 mass fraction of the IL were measured to complement the studies of Tian et al. [40]. It should be mentioned here that the ILs containing the [BF4]-are not water-stable compounds. They hydrolyze in the presence of water resulting in hydrofluoridric acid formation [44]. Therefore, the use such ILs in industrial applications that also involve water is non-trivial and in certain cases these ILs may not be considered as an ‘ideal’ solvent.

In order to measure the solubility data, a new apparatus based on the “static-synthetic” method was designed and commissioned. The apparatus follows a common experimental “synthetic” method, where the total gas loading is known via use of a gas reservoir of known temperature, volume and pressure [[45], [46], [47]]. The new apparatus and experimental technique were validated by measuring the solubility of CO2 in three different solvents, viz., n-hexane, NMP and [bmim][BF4]. The data presented here complements data available in the literature. New solubility data were measured for CO2 in NMP + [bmim][BF4] mixtures with initial solvent loading mass fractions of (0.0986, 0.2495, and 0.4973) for [bmim][BF4] at three temperatures, viz. (298.15, 313.15 and 323.15) K. The experimental data were modelled using the Peng-Robinson (PR) equation of state (EoS) and van der Waals (vdW) mixing rule [[48], [49], [50]]. The viscosity, density and sound velocity for the NMP + [bmim][BF4] mixtures were also measured over the temperature range of (293.15–343.15) K.

Section snippets

Materials

Carbon dioxide (CAS number: 124-38-9) was supplied by Afrox (South Africa) with a minimum mass fraction purity of 0.99. NMP (CAS number: 872-50-4) and [bmim][BF4] (CAS number: 174501-65-6) were purchased from Merck and Sigma-Aldrich with stated mass fraction purities of higher than 0.995 and 0.98, respectively. The physical properties for the pure components are listed in Table 1. [bmim][BF4] was dried under vacuum at 363.15 K for 3 days. The water content was measured using a Karl-Fisher

Data modelling

The equilibrium compositions were calculated using the PR EoS given that the total composition and vapour phase volume were experimentally determined. This procedure is relatively well documented in the literature [15,53,55,56]. The data were modelled in MATLAB using a flash calculation and the phi-phi approach. The Wilson correlation was used to predict initial equilibrium ratios to initialize the data reduction [49]. The fugacities of both phases were calculated using the PR EoS and vdW

Results and discussion

The equipment and experimental technique were tested by measuring solubility data for three different binary test systems, namely: CO2 + n-hexane at 313.20 K, CO2 + NMP at 298.16 K and CO2 + [bmim][BF4] at 298.14  K. The experimental data (T-P-x) for the binary system of CO2 + n-hexane are listed in Table 4 and displayed in Fig. 3. Additionally, the raw data (T-P-z) are reported to enable future modelling of the data. The data presented herein contain six different initial loadings of n-hexane

Conclusion

A new experimental setup based on the “static-synthetic” method was designed and commissioned. The apparatus and experimental method were validated by measuring the solubility of CO2 in three pure solvents, viz., hexane, NMP and [bmim][BF4]. The experimental data compare well with the data reported in the literature. New solubility data were measured for ternary systems of NMP + [bmim][BF4] + CO2 at three temperatures, viz. (298.15, 313.15 and 323.15) K and up to pressures of 2 MPa. The

Acknowledgements

This work is based upon research supported by the National Research Foundation of South Africa under the South African Research Chair Initiative of the Department of Science and Technology and the National Research Foundation Thuthuka Program.

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