Elsevier

Fluid Phase Equilibria

Volume 495, 1 September 2019, Pages 12-20
Fluid Phase Equilibria

Improved CO2 separation performance of aqueous choline-glycine solution by partially replacing water with polyethylene glycol

https://doi.org/10.1016/j.fluid.2019.05.006Get rights and content

Abstract

Aqueous choline-glycine ([Cho][Gly]) solution is a potential candidate for CO2 separation owing to its excellent absorption performance and biodegradability. Moreover, the aqueous solution is easy to volatilize at high temperatures. In this work, H2O was partially replaced with polyethylene glycol (PEG200) and the effect of PEG200 on the CO2 separation performance in [Cho][Gly])/H2O was investigated. The viscosity of [Cho][Gly]/H2O/PEG200 and CO2 solubility in the solution were determined experimentally in the temperature range 308.15–338.15 K at pressures ≤6.5 bar. Further, the measured CO2 solubility was fitted with the reaction equilibrium thermodynamic model and the CO2 desorption enthalpy was estimated. The regeneration performance of [Cho][Gly]/H2O/PEG200 was also evaluated. The results revealed that [Cho][Gly]/H2O/PEG200 has a low CO2 desorption enthalpy and high regeneration efficiency. Particularly, [Cho][Gly]/H2O/PEG200 with 30 wt% PEG200 has a high regeneration efficiency of 95%. Owing to its physical-chemical properties and CO2 separation performance, [Cho][Gly]/H2O/PEG200 shows great potential as an absorbent for CO2 separation.

Introduction

With the increasing utilization of fossil fuels, greenhouse gas (GHG) emissions and climate change have become serious issues [1]. According to the climate change assessment report by the Intergovernmental Panel on Climate Change (IPCC), the atmospheric CO2 content has risen from 0.0125 mol m−3 before the industrial revolution to 0.0169 mol m−3 in 2005, resulting in a global mean surface temperature rise of 0.5–1.0 °C [2]. CO2 separation has been proposed as an effective option to mitigate GHG emissions and is an important but significantly costly step [3]. Several technologies, such as absorption, adsorption, and membrane technologies, have been developed to separate CO2 [4,5]. Absorption, classified as chemical or physical absorption according to the bonding between CO2 and the absorbent, is the most-widely used technology to date [6]. Particularly, amine scrubbing, a typical chemical absorption technology, is applied to CO2 separation from flue gases [7,8]. However, this technique is prone to a high energy demand and corrosion [9,10]. Moreover, high pressure water scrubbing is a widely used physical absorption technology for biogas upgrading that is cost-prohibitive for investment [9,10]. Hence, the development of environmentally friendly and cost-effective technology for CO2 separation is of great significance.

Ionic liquids (ILs), liquid organic salts comprising cations and anions, have been proposed as green absorbents for CO2 separation due to their extremely low vapor pressure, designable structure, and high thermal and chemical stabilities [[11], [12], [13], [14]]. In 1999, Brennecke first reported the CO2 solubility in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) [15]. Since then, numerous experimental and theoretical studies have focused on the physicochemical properties of ILs and the CO2 solubility in these liquids [[16], [17], [18]]. However, due to their low CO2 absorption capacity and potential toxicity, the performance of conventional ILs still cannot compete with that of other commercial absorbents [19]. In recent decades, amino acid-functionalized ILs (AAILs) have attracted significant attention for application as task-specific ionic liquids (TSILs) because of their high CO2 capacity and nontoxicity [[20], [21], [22], [23]]. In fact, AAILs have been widely applied in catalysis [24], tribology [25,26], gas separation [27], and other fields [[28], [29], [30]].

However, the high viscosity and cost of AAILs have limited their applications. Thus, pure AAILs cannot be directly utilized for CO2 separation [31]. The use of H2O as a co-solvent has attracted special attention to overcome this high viscosity and render these compounds feasible for industrial application. Davis et al. [32] studied the effect of H2O on CO2 absorption in aqueous solutions of AAILs ([N1111][Gly] and [N2222][Pro]). They reported that in the presence of H2O, the anions play a transitory role in the first stage of the process. Anderson et al. [21] investigated the CO2 solubility in a series of formulated IL/water mixtures and discovered that carboxylate ILs formulated with stoichiometric amounts of water greatly enhanced the CO2 solubility over that observed in anhydrous ILs and aqueous IL solutions. Yuan et al. [33] conducted a CO2 absorption study in aqueous solutions of three [Cho][AA]s, namely [Cho][Gly], choline-alanine ([Cho][Ala]), and choline-proline ([Cho][Pro]). The results revealed that the 30 wt% [Cho][AA] aqueous solution displayed the highest CO2 absorption load and that the apparent absorption rate constants decreased with increasing [Cho][AA] concentration. Among them, [Cho][Gly] displayed the best CO2 absorption performance (absorption capacity and rate) as well as the lowest molecular weight [[34], [35], [36]]. In addition, the excellent CO2 absorption performance in [Cho][Gly] and Gly-based ILs has also been reported in the literature [37].

Although blending AAILs with H2O can significantly increase their CO2 absorption performance, the CO2 absorption enthalpy in aqueous solutions is relatively high and H2O is volatile. PEG200 is a polyethylene glycol (PEG) with an average molecular weight of 200 that has been widely used in the pharmaceutical, cosmetic, and food industries [38]. Moreover, PEG200 has competitively excellent properties over ILs, such as low vapor pressure, nontoxicity, low cost, and high chemical and thermal stabilities. These properties indicate that PEG200 is a promising co-solvent for gas separation, even at high temperatures [[39], [40], [41]]. In fact, PEG200 has been used as a co-solvent for CO2 separation based on ILs with good regeneration efficiency [42]. Studies have also focused on the phase behavior of aqueous biphasic systems composed of ILs and PEG [43]. Therefore, a new strategy to overcome the disadvantages of volatilization by blending aqueous AAIL solutions with PEG200 has been proposed. Meanwhile, the replacement of a certain amount of H2O with PEG200 can also reduce the energy demand for absorbent regeneration [44]. Hence, [Cho][Gly]/H2O/PEG200 shows great potential in industrial applications. However, to the best of our knowledge, the CO2 separation performance of [Cho][Gly]/H2O/PEG200 has not been studied to date. Moreover, few studies have reported the thermodynamic properties, notably the enthalpy of the CO2 absorption process that is of great importance for industrial applications [[45], [46], [47]].

The objective of this work was to systematically investigate the effect of PEG200 on CO2 separation with [Cho][Gly]/H2O. The [Cho][Gly]/H2O (1) + PEG200 (2) absorbents were first prepared. The [Cho][Gly] mass fraction was maintained at 30 wt% for comparison with the 30 wt% amine aqueous solution used in industrial processes, while the mass fraction of PEG200 (w2) was increased from 0 to 30 wt%. The viscosity (μ) of the mixtures was measured and the CO2 solubility was determined at different pressures and temperatures. The effect of PEG200 on the physical dissolution constant (H) and reaction equilibrium constants (K1, K2) was analyzed based on the reaction equilibrium thermodynamic model (RETM). Further, the CO2 desorption enthalpy was estimated and discussed. Finally, the regeneration performance of [Cho][Gly]/H2O/PEG200 was investigated by a multi-cycle experiment.

Section snippets

Materials

CO2 was purchased from the Nanjing Tianhong gas factory. 2-Hydroxyethyltrimethylammonium hydroxide (Choline hydroxide) solution was manufactured by Sigma-Aldrich. Glycine was purchased from Sinopharm Chemical Reagent Co. Ltd. PEG200 (average molecular weight = 200 g mol−1) was supplied by Guandong Guanghua Sci-Tech Co. Ltd. Ethyl acetate and phosphorus pentoxide (P2O5) were purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd. H2O was purified in-house through a reverse osmosis membrane.

Solubility and absorption capacity

According to our previous work [54], the molar amount (nCO2) of CO2 in the absorbent is calculated by Eq. (1):nCO2=(PiPv)(VaVl)Z1RT(PePv)(VaVl)Z2RTwhere Pi and Pe are the initial and equilibrium pressures, respectively; Pv is the saturated vapor pressure of the solution; Z1 and Z2 are the compressibility factors corresponding to the initial and equilibrium states, respectively; and Va and Vl represent the volumes of the absorption vessel and solution, respectively.

The absorption capacity (m

Characterization results of [Cho][Gly]

The structure of [Cho][Gly] was evaluated by 1H NMR and FT-IR (Table 3). These results agree well with those reported in the literature [36,58]. The α-H in the amino acids presented a chemical shift contribution to the cation-anion interactions in the formed salts. All the characterization results indicated that [Cho][Gly] was prepared successfully.

Effect of PEG200 on the viscosity of [Cho][Gly]/H2O

The absorbent viscosity is a key parameter for modelling the mass transfer rate because it significantly affects the liquid film mass transfer

Conclusions

The effect of PEG200 on the viscosity, CO2 solubility, and regeneration performance of [Cho][Gly]/H2O was investigated experimentally and systematically. The measured CO2 solubilities were represented by the RETM model, while the constants H, K1, K2 and the physical dissolution (ΔHphy) and chemical reaction (ΔH1, ΔH2) enthalpies were estimated from the fitted thermodynamic parameters. The results revealed that the viscosity decreases significantly with increasing temperature, especially in the

Acknowledgments

The authors would like to thank the National Natural Science Foundation of China (21776123, 21136004, 21476106, 21428601, 21490584, and 21729601) and Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP) for their financial support. X. J. would also like to thank the Swedish Energy Agency for their financial support.

References (60)

  • P. Moriel et al.

    Tetrahedron Lett.

    (2010)
  • J. Wu et al.

    Tribol. Int.

    (2018)
  • F. Ortloff et al.

    Sep. Purif. Technol.

    (2018)
  • Y.C. Pei et al.

    Sep. Purif. Technol.

    (2009)
  • J. Martak et al.

    Sep. Purif. Technol.

    (2007)
  • J. Fan et al.

    Sep. Purif. Technol.

    (2008)
  • S. Yuan et al.

    Fluid Phase Equilibria

    (2017)
  • W. Gouveia et al.

    Chemosphere

    (2014)
  • B.K. Mondal et al.

    Chem. Eng. Sci.

    (2017)
  • Y. Zhao et al.

    Fluid Phase Equilibria

    (2011)
  • Y. Xie et al.

    Appl. Energy

    (2014)
  • A. Valtz et al.

    Fluid Phase Equilibria

    (2004)
  • D.W. Keith

    Science

    (2009)
  • R.H. Moss et al.

    Nature

    (2010)
  • J.M. Matter et al.

    Science

    (2016)
  • H. Tang et al.

    J. Polym. Sci. A Polym. Chem.

    (2005)
  • J. Deng et al.

    RSC Adv.

    (2016)
  • F. Weiyang

    Chem. Ind. & Eng. Prog.

    (2005)
  • G.T. Rochelle

    Science

    (2009)
  • M.O. Schach et al.

    Ind. Eng. Chem. Res.

    (2010)
  • P. Cozma et al.

    Clean-Soil Air Water

    (2013)
  • P. Cozma et al.

    Clean Technol. Environ. Policy

    (2015)
  • J.E. Bara et al.

    Acc. Chem. Res.

    (2010)
  • G. Cui et al.

    Chem. Soc. Rev.

    (2016)
  • M.J. Earle et al.

    Nature

    (2006)
  • N. MacDowell et al.

    Energy & Environmental Science

    (2010)
  • L.A. Blanchard et al.

    Nature

    (1999)
  • B. Gurkan et al.

    J. Phys. Chem. Lett.

    (2010)
  • J.Z. Zhang et al.

    Ind. Eng. Chem. Res.

    (2013)
  • S. Zeng et al.

    Chem. Rev.

    (2017)
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