Elsevier

Chemical Engineering Journal

Volumes 200–202, 15 August 2012, Pages 317-328
Chemical Engineering Journal

Kinetics of Carbon Dioxide absorption into aqueous MDEA + [bmim][BF4] solutions from 303 to 333 K

https://doi.org/10.1016/j.cej.2012.06.037Get rights and content

Abstract

The kinetics of CO2 absorption in aqueous solutions of MDEA + [bmim][BF4] were investigated using a stirred cell reactor where the relevant parameters were evaluated. The rate equation of the absorption reaction was found to be close to first order with respect to CO2 at temperatures ranging from 303 to 333 K and [bmim][BF4] concentration from 0 to 2.0 mol L−1. The activation energy decreased from 43.32 kJ mol−1 to 8.65 kJ mol−1 with increasing [bmim][BF4] concentration from 0 to 2.0 mol L−1 in aqueous 4 mol L−1 MDEA solution. Calculated results of the enhancement factor and Hatta number showed that the performance of CO2 absorption in the aqueous 4 mol L−1 MDEA + [bmim][BF4] solution almost obeyed the pseudo first order regime.

Highlights

► The kinetics of CO2 absorption in MDEA + [bmim][BF4] was studied. ► Effect of [bmim][BF4] concentration on the kinetic parameters was analyzed. ► Order of reaction with respect to CO2 was presented. ► Activation energies for the second order reaction rate constant were calculated.

Introduction

Chemical and physical absorption processes using amine solvents are widely used to absorb acid gases such as CO2 and H2S in various oil, gas, and chemical industries. In such processes physical absorption is more preferable when the partial pressure of acid gases in the gas stream is relatively high [1]. Depending on acid gas components, each industry has particular objectives [2]. Alkanolamines are commonly used to enhance gas absorption rate and capacity and to improve selectivity [2]. N-methyldiethanolamine (MDEA) is a tertiary amine and is considered the most industrially important chemical for such processes [3]. Compared to other well known amines, MDEA is characterized by lower volatility, higher thermal stability, higher CO2 loading (up to 1.0 mol CO2/mol amine) and less regeneration cost in addition to being much less corrosive [4]. However, the absorption of CO2 into MDEA is quite slow as MDEA promotes CO2 hydrolysis to form bicarbonate [4], [5]. Thus, in most industrial applications, MDEA is blended with certain chemicals like piperazine (PZ) which is used as an activator in the activated MDEA technology of BASF Aktiengesellschaft in which it was reported that PZ was much more effective than other traditional activators, like monoethanolamine (MEA) or diethanolamine (DEA) [6].

However, some of the common problems encountered in the CO2 absorption by aqueous amine solutions are the extensive regeneration energy consumption, the considerable losses of the reactive amine and the increase in the water content of the processed gas streams [1].

Recently ionic liquids (ILs) were proposed as potential solvents for energy efficient gas separation considering their unique characteristics of wide liquid range, thermal stability, almost zero volatility, tunable physicochemical character and relatively high CO2 solubility [7], [8], [9], [10]. Among these were imidazolium-based ILs which were found to have good affinity to absorb CO2 [11], [12], [13], [14], [15].

Jacquemin et al. [16] reported that CO2 showed good solubility in 1-buthyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) in comparison with ethane, methane, oxygen, nitrogen, hydrogen, argon, and carbon monoxide at pressures close to atmospheric. However, the absorption performance of ionic liquids is still not comparable with commercially available solvents for CO2 absorption. Although, this performance can be dramatically improved by incorporating an amine function in the structure of the ionic liquid to produce a kind of task specific ionic liquids (TSILs) for CO2 capture [1], but the synthesis of these amine-functionalized imidazolium salts requires several synthetic and purification steps and is not cost-competitive as compared to conventional alkanolamines such as MEA [17].

Another strategy for using ionic liquids in CO2 absorption processes is by blending ionic liquids with amines, which is more preferred to pure ionic liquids or TSILs. Such mixtures provide the desired property of TSILs for CO2 capture, without many of their inherent drawbacks of high viscosity and intractable tars [18]. A number of researchers studied CO2 absorption from gaseous streams using mixtures of ionic liquids with aqueous solutions of MDEA and MEA [4], [18], [19], [20], [21], [22], [23]. Several studies have shown that the gas solubility in ionic liquids increases with increasing pressure and decreases with increasing temperature. The CO2 absorption into standard ionic liquids is controlled entirely by physical mechanisms [1], [8], [9], [24], [25], [26], [27], [28].

In previous publications by the authors [22], [23], same behavior was reported for CO2 absorption in mixtures of aqueous MDEA and three types of imidazolium-based ionic liquids. It was also shown that the physical part of the absorption was increased with increasing ionic liquid concentration in aqueous 4 mol L−1 MDEA + [bmim][BF4] over a range of IL concentrations of 0–2.0 mol L−1 [23].

Moreover, the mixture of aqueous 4 mol L−1 MDEA + [bmim][BF4] showed better CO2 loading performance in comparison to aqueous mixtures of 4 mol L−1 MDEA with 1-butyl-3-methyl-imidazolium acetate ([bmim][Ac]) and 1-butyl-3-methyl-imidazolium dicyanamide ([bmim][DCA]) at the same temperature, CO2 pressure and ionic liquids concentrations. Furthermore, the results of the initial rate of CO2 absorption in aqueous mixtures of MDEA with [bmim][BF4] showed that, under certain conditions, the presence of ionic liquid increased the initial rate of CO2 absorption [20]. Hence, this mixture was suggested for further investigations as a potential solvent to absorb CO2.

As a preliminary assumption, it was postulated that the increase in initial CO2 absorption rate in MDEA + [bmim][BF4] solutions was due to the higher physically absorbed CO2 in the presence of ionic liquid in the mixture. In order to explain this phenomenon and interpret the kinetics data pertaining to the absorption of CO2 in such mixtures, the previous work [23] reported data on CO2 diffusivity and physical absorption.

In designing columns for CO2 absorption processes, kinetics is a very important factor [29]. Some mechanisms were suggested for the reaction between CO2 and the functionalized ionic liquids [17], [28], [30]. The only available research on the kinetics of the reactive capture of CO2 by amino functionalized ionic liquids was done by Galán Sánchez et al. [1]. There are also some reported studies on the reactive absorption of CO2 in the blends of typical amines and standard ionic liquids [4], [31]. To the best of our knowledge, there is no literature on the kinetics of CO2 absorption by mixtures of amines with standard ionic liquids, namely mixtures of MDEA with [bmim][BF4]. Therefore, the objective of this work is to study the kinetics of the reaction between CO2 and aqueous 4 mol L−1 MDEA + [bmim][BF4] solutions. The effect of ionic liquid concentration on the kinetics parameters is evaluated. The overall kinetics behavior of the reaction of CO2 with aqueous 4 mol L−1 MDEA + [bmim][BF4] is compared with that of CO2 with aqueous MDEA solutions. The enhancement factor data are presented, and the CO2 absorption regime is established.

Section snippets

CO2 absorption in aqueous MDEA

The theory of gas–liquid absorption with chemical reaction is well known [32]. In the last two decades, many studies have been performed on the kinetics of the reaction of aqueous MDEA with CO2, and the mechanism of the reaction is well established [3], [33], [34], [35], [36], [37], [38], [39]. The reaction of CO2 with aqueous MDEA can be described satisfactorily with the base-catalysis reaction mechanism proposed by Donaldson and Nguyen [34]. CO2 is absorbed in aqueous MDEA as chemical and

Chemicals

The chemicals used in the investigation were 98.5% Methyldiethanolamine (MDEA) and 1-buthyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4], 98%) purchased from Merck. CO2 gas was supplied from Mox-Linde with purity 99.9%. Distilled water was used as the solvent throughout the experiments.

Experimental setups and procedures

All the components were miscible throughout studied concentrations and resulted in homogeneous solutions. The mole fraction data are available in Table A1 (Appendix).

The CO2 absorption rate experiments were

Density, viscosity, diffusivity, physical solubility and initial absorption rate

The densities and viscosities of the different aqueous MDEA + [bmim][BF4] solutions, 4.0 mol L−1 MDEA and [bmim][BF4] concentrations ranging from 0.5 up to 2.0 mol L−1, were measured as described in a previous published work by the authors [23]. Also, the required solubility and diffusivity of CO2 for the kinetics study were determined using the CO2:N2O analogy [23]. The data for the initial absorption rate of CO2 in aqueous MDEA + [bmim][BF4] solutions were already reported [20]. These data are

Reaction order with respect to CO2

The experimental data for CO2 absorption in aqueous MDEA + [bmim][BF4] solutions were analyzed to determine the kinetics constants associated with this reaction. Plots of Ln(rCO2) versus Ln(PCO2/HCO2) at the various temperatures and [bmim][BF4] concentrations are shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4. It is obvious that the relationship given by Eq. (27) is obeyed in all cases as indicated by the linear regression of rCO2 with PCO2/HCO2 (on log–log plot). The order of the reaction with respect

Conclusions

The rate data of the initial CO2 absorption were used to calculate the kinetics parameters. It was found that CO2 absorption reaction approaches first order regime with respect to CO2 partial pressure. This finding is similar to the conclusions reported earlier in the literatures for CO2 absorption in aqueous solutions of MDEA. Thus, it can be concluded that the effect of [bmim][BF4] concentration on the reaction order is considered insignificant with respect to CO2 partial pressure. The second

Acknowledgment

This work was carried out under University of Malaya Centre of Ionic Liquid (UMCiL) and financially supported through HIR Grant No. VC/HIR/001.

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