Elsevier

Fluid Phase Equilibria

Volume 391, 15 April 2015, Pages 9-17
Fluid Phase Equilibria

Absorption of CO2 with methanol and ionic liquid mixture at low temperatures

https://doi.org/10.1016/j.fluid.2015.02.002Get rights and content

Highlights

  • Solubility data of CO2 in methanol and IL mixture at low temperatures were measured.

  • UNIFAC model for ILs was extended to the methanol-CO2 and methanol-IL-CO2 systems.

  • Conceptual process design was done using the new mixed absorbents for CO2 capture.

Abstract

This work proposed the use of methanol (CH3OH) and ionic liquid (IL) mixture, which integrates the advantages of methanol (high solubility and low viscosity) and IL (non-volatility), for CO2 capture at low temperatures (down to 228.2 K). The solubility of CO2 in pure methanol, IL ([OMIM]+[Tf2N]) and their mixture (80 wt% CH3OH + 20 wt% IL, 50 wt% CH3OH + 50 wt% IL, and 20 wt% CH3OH + 80 wt% IL) was measured at 273.2, 258.2, 243.2 and 228.2 K and pressures up to 3.0 MPa. The UNIFAC model for ILs was extended to the methanol–CO2 and methanol–IL–CO2 systems. The comparison between the predicted and experimental results indicated that the UNIFAC model can well predict the CO2 solubility in pure methanol and in the (methanol + IL) binary mixture at low temperatures. Based on the thermodynamic experimental and modeling studies, the conceptual process design was developed for CO2 removal from syngas using the mixed agent at a low temperature. It was found that the volatile loss of methanol in the absorption column and solvent flash tanks decreases significantly in the case of the mixture of methanol and IL used as an absorbent.

Introduction

With the rapid economic growth in recent years, the environment has been a serious challenge. For example, increasing massive carbon dioxide (CO2) emission has caused a series of environmental problems. Simultaneously, CO2 in industry, agriculture, food, pharmaceuticals, fine chemicals and other fields have a very wide range of application. Therefore, the high efficiency CO2 capture method becomes a main concern for an environmentally benign and sustainable development [1], [2], [3], [4].

At present, there are mainly three kinds of technologies and methods used for CO2 capture, i.e., absorption separation technology, adsorption separation technology and membrane separation technology. Among others, solvent absorption method is the traditional and mature method, including physical absorption and chemical absorption. Currently, the widely used physical absorption technologies include washing method, flour method, Rectisol method, Purisol method and polyethylene glycol dimethyl ether method (NHD method) [5], [6]. Due to low operation cost, high solubility and selectivity, the Rectisol process with methanol as a separating agent operating at low temperatures is widely used in acid gas removal such as CO2 capture from syngas. However, the high volatility of methanol could make a high volatile loss of methanol and a complicated energy-intensive solvent recovery system. It is known that ionic liquids (ILs) are novel environmentally benign solvents and exhibit great potential in absorption of CO2 [7], [8], [9], [10], [11], [12], [13], [14], due to their unique advantages such as almost non-volatility, high thermal stability, strong soluble capacity, and the tunability of molecular structures and physical chemical properties [15], [16]. However, the disadvantages of ILs are their high viscosity and high cost when used in the industries directly. Therefore, aiming to decrease the volatile loss of methanol and the energy consumption, the mixed separating agent by adding some IL into methanol is proposed in this work for CO2 capture, which combines the advantages of methanol (low viscosity and surface tension, promoting mass transfer) and ionic liquids (non-volatility). Meanwhile, reliable predictive thermodynamic models are required for a deep understanding of the thermodynamic behavior and the subsequent process simulation. For this reason, the familiar UNIFAC model for ILs developed by us was adopted.

Thus, the focus of this study is on the following three aspects: (1) measuring the solubility data of CO2 in methanol, IL, and their mixtures with different compositions at low temperatures (273.2, 258.2, 243.2 and 228.2 K); (2) extending the UNIFAC model parameters and checking the effectiveness of new group parameters; (3) establishing mathematical model for the conceptual process for CO2 absorption from syngas using the mixture of methanol and [OMIM]+[Tf2N] as absorbents at low temperatures. [OMIM]+[Tf2N] was selected as the representative of ILs because CO2 has a high solubility in this type of ILs.

Section snippets

Materials

The methanol, IL and CO2 were purchased from chemical market as summarized in Table 1. [OMIM]+[Tf2N] was dried in a vacuum rotary evaporator at 333.2 K for two days prior to use to remove traces of water and other volatile impurities. After treatment, the water content was below 600 ppm. Methanol and CO2 gas were used directly without further treatment.

Apparatus and procedure

The solubility data of CO2 were measured using a low-temperature high-pressure equilibrium cell, of which the temperature was controlled using a

Model description

In the UNIFAC model, the activity coefficient can be expressed as the following two terms:lnγi=lnγiC+lnγiRwhere lnγiC represents the combinatorial contribution, depending on different size and shape of groups; and lnγiR represents the residual contribution, which contains the group interaction parameters (αmn and αnm).

In this work, the IL molecule was decomposed into several functional groups in the similar method as used by Lei and co-workers [17], [20], [21], [22], [23], Kim et al. [24], [25]

Prediction of CO2 solubility in pure methanol and pure IL at low temperatures

To verify the accuracy of the experiment, the comparison between this work and previous studies is shown in Fig. 2, where xcal is the calculated value using linear regression. We can see that the experimental values obtained in our study coincide well with reference values with the maximum relative deviation less than 10%.

About 200 solubility data of CO2 in pure methanol at a temperature range from 230 to 333.15 K exhaustively collected from references are used for the group interaction

Conclusions

In this work, the familiar UNIFAC model is extended to the methanol–CO2 and methanol–IL–CO2 systems, and the binary interaction parameters (αmn and αnm) between CO2 and CH3OH were obtained by correlating the solubility data of CO2 in methanol exhaustively collected from reference. To test the applicability of UNIFAC model, we measured the solubility data of CO2 in pure methanol, pure IL and their binary mixtures at temperatures from 273.2 down to 228.2 K and pressure up to 3.0 MPa. The results

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China under Grants Nos. 21476009, 21406007 and U1462104, and by the Chinese Universities Scientific Fund (ZY1401).

References (58)

  • G.N. Keating et al.

    Energy Procedia

    (2011)
  • L. Sun et al.

    J. Cleaner Prod.

    (2013)
  • W. Chen et al.

    Appl. Energy

    (2013)
  • Y.U. Paulechka et al.

    Thermochim. Acta

    (2005)
  • A. Tagiuri et al.

    Fluid Phase Equilib.

    (2014)
  • A. Berger et al.

    Tetrahedron Asymmetry

    (2001)
  • J. Kumełan et al.

    Fluid Phase Equilib.

    (2009)
  • Y. Kim et al.

    Fluid Phase Equilib.

    (2005)
  • J.E. Kim et al.

    Fluid Phase Equilib.

    (2011)
  • M.D. Bermejo et al.

    J. Supercrit. Fluid

    (2009)
  • T.E. Chang et al.

    Fluid Phase Equilib.

    (1985)
  • J.H. Hong et al.

    Fluid Phase Equilib.

    (1988)
  • C.J. Chang et al.

    Fluid Phase Equilib.

    (1997)
  • Z. Lei et al.

    Chin. J. Chem. Eng.

    (2013)
  • B. Sander et al.

    Fluid Phase Equilib.

    (1983)
  • M. Kleiber

    Fluid Phase Equilib.

    (1995)
  • J. Palgunadi et al.

    Thermochim. Acta

    (2009)
  • A. Serbanovic et al.

    Fluid Phase Equilib.

    (2010)
  • Q. Gan et al.

    Adv. Colloid Interface Sci.

    (2011)
  • P. Carvalho et al.

    J. Supercrit. Fluid.

    (2009)
  • R.A. Esposito et al.

    Environ. Sci. Technol.

    (2011)
  • P.H. Stauffer et al.

    Environ. Sci. Technol.

    (2011)
  • S. Pacala et al.

    Science

    (2004)
  • A. Finotello et al.

    Ind. Eng. Chem. Res.

    (2008)
  • J.F. Brennecke et al.

    AIChE J.

    (2001)
  • M.J. Earle et al.

    Nature

    (2006)
  • M. Seiler et al.

    AIChE J.

    (2004)
  • Z. Lei et al.

    Chem. Rev.

    (2013)
  • M.B. Shiflett et al.

    Ind. Eng. Chem. Res.

    (2010)
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