Experiment and model for the surface tension of amine–ionic liquids aqueous solutions
Introduction
In recent decades, atmospheric levels of CO2 have increased rapidly due to the utilization of large quantities of fossil fuels. Development of affordable yet technically feasible separation technologies for reducing CO2 emissions has attracted global attention. Among the available separation technologies including absorption [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], adsorption [11], [12], membrane [13], [14], [15] and hydration [16], [17], [18], chemical absorption using aqueous solution of alkanolamines as an absorbent has been widely used for the removal of CO2 from a variety of gas streams. However, the major disadvantage of traditional aqueous solutions of alkanolamines is the high energy cost for regeneration. In particular, when using the monoethanolamine (MEA) aqueous solution as an absorbent, the mass fraction of MEA must be no greater than 30% to lower the corrosion, thus large quantities of superheated steam are needed in the regeneration tower to heat the 70% residual water. To lower the energy cost, new absorbents using amine blends have been developed in recent years. For example, the blends of MEA and N-methyldiethanolamine (MDEA) preserve the high rate of the reaction of MEA with CO2. The low enthalpy of the reaction of MDEA with CO2, hence leads to high absorption rates in the absorber column, yet a lower heat of regeneration in the stripper section. Moreover, as the corrosivity of MDEA is low, the total mass fraction of amines in the MEA–MDEA aqueous solution can be significantly increased. Besides the amine blends, adding a physical solvent to the aqueous solutions of alkanolamines is also considered to be an effective method to lower the regeneration energy cost [19], [20]. For example, Archane et al. [19] showed that when using poly(ethylene oxide)400 (PEG400)-diethanolamine (DEA) aqueous solution as the absorbent, a lower energy cost can be achieved in regeneration because at a given CO2 loading and constant concentration of DEA, the CO2 molecular concentration increases with the increase of PEG400 concentration, whereas the ion repartition is not significantly influenced by the solvent composition.
Recently, interest has increased rapidly in CO2 capture using the ionic liquids (ILS) as the absorbents. The ILS have unique characteristics including a wide liquid range, thermal stability, negligible vapor pressure, tuneable physicochemical character and high CO2 solubility [21], [22], [23], [24], [25], [26]. The mixtures of ILS and amines preserve the desired property of ILS for CO2 capture, but without many of their inherent drawbacks such as high viscosity because of their corresponding CO2 adducts. For example, Jacquemin et al. [23] showed that CO2 was highly soluble in 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) at pressures close to atmospheric. Ahmady et al. [24], [25] showed that under certain conditions, the presence of ILS increased the initial rate of CO2 absorption in MDEA aqueous solution. Bidart et al. [26] showed that although 1-butyl-3-methylimidazolium bromide ([Bmim][Br]) did not present a greater absorption capacity than the amine solutions, the mixture of [Bmim][Br] and MEA solution was explored as an option to enhance CO2 absorption due to the advantages of low volatility, chemical stability and stability of the complex formed when CO2 saturation is achieved.
A knowledge of surface tension is required when designing or simulating an absorption column for CO2 capture. It can significantly affect the absorption efficiency because both the penetration of CO2 molecules from gas phase to the liquid phase and the enhancement of the absorption closely relate to the surface tension. In recent years, there are some experimental and theoretical works concerning the surface tension of aqueous solutions containing amines [27], [28], [29], [30], [31], [32], [33], [34], [35] and ILS [36], [37], [38], [39], [40], [41]. However, experimental and theoretical works concerning the surface tension of ILS–MEA and ILS–DEA aqueous solutions are relatively rare.
The main purpose of this work is to investigate the surface tension of MEA–[Bmim][BF4], MEA–[Bmim][Br], DEA–[Bmim][BF4] and DEA–[Bmim][Br] aqueous solutions experimentally and theoretically, and then demonstrate the effects of temperature, mass fractions of amines and ILS on the surface tension. To this end, the surface tension was measured at the temperatures from (293.2 to 323.2) K. The mass fraction of amines and ILS respectively ranged from 0.15 to 0.30 and 0.05 to 0.10. Besides experimental work, a thermodynamic equation was also proposed in this work to model the surface tension.
Section snippets
Materials
The amines and ILS were purchased from HuaXin chemical Co. The sample description is shown in table 1. They were used without further purification. Aqueous solutions of MEA–[Bmim][BF4], MEA–[Bmim][Br], DEA–[Bmim][BF4] and DEA–[Bmim][Br] were prepared by adding doubly distilled water. The uncertainty of the electronic balance is ±0.1 mg.
Apparatus and procedure
The surface tension was measured by using the BZY-1 surface tension meter produced by Shanghai Hengping Instrument Factory. The BZY-1 meter employs the Wilhemy
Results and discussion
The experimental results of the surface tension of MEA–[Bmim][BF4], MEA–[Bmim][Br], (DEA–[Bmim][BF4] and DEA–[Bmim][Br] aqueous solutions are respectively shown in TABLE 2, TABLE 3, TABLE 4, TABLE 5. The uncertainties are reported as standard uncertainties.
Besides experimental measurements, models that can calculate the surface tension of concerned systems are also important. However, due to the complexity of the amine–ILS aqueous solutions, equations that simultaneously take the amine and ILS
Conclusions
In this work, the surface tension of MEA–[Bmim][BF4], MEA–[Bmim][Br], DEA–[Bmim][BF4] and DEA–[Bmim][Br] aqueous solutions were measured by using the BZY-1 surface tension meter and modeled by using a thermodynamic equation. The effects of temperature, mass fractions of amines and ILS on the surface tension were demonstrated. Our results show that:
- (1)
The increases of the temperature and the mass fraction of ILS tend to decrease the surface tension of amine–ILS aqueous solutions.
- (2)
The surface tension
Acknowledgements
The authors appreciate the financial support from the National Natural Science Foundation of China (No. 21276072), the Natural Science Funds for Distinguished Young Scholar of Hebei Province (No. B2012502076) and the Fundamental Research Funds for the Central Universities (No. 13ZD16).
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