Mutual diffusion coefficients, density, and viscosity of aqueous solutions of new polyamine CO2 absorbents
Introduction
Aqueous polyamine solvents are interesting candidate absorbents for CO2 capture. Recent studies showed that the additional amine groups present in polyamines have properties similar to those of alcohols, which make them viable alternatives to alkanolamines [1]. It has also been reported that due to these added amine functionalities, aqueous solutions of several polyamines have higher CO2 absorption capacity and faster reaction kinetics, compared with commonly used alkanolamine absorbents [2], [3], [4], [5].
In the present work, we considered four polyamines, which are among the potential solvents for CO2 absorption: 3-(methylamino)propylamine (MAPA), diethylenetriamine (DETA), N,N,N′,N′-tetramethylethylenediamine (TMEDA), and tetramethyl-1,3-diaminopropane (TMPDA). MAPA has been reported by Svendsen et al. [6], [7] to have higher reaction rate compared with methyldiethanolamine (MDEA), and suggested it as an effective activator in aqueous MDEA and dimethylmonoethanolamine (DMMEA) solutions. On the other hand, we have shown in a recent work [8] that the triamine DETA in aqueous solution (30 wt%) has higher CO2 loading than aqueous monoethanolamine (MEA) and aqueous MDEA (at the same concentration). Some studies also reported that compared with aqueous MEA, aqueous DETA have higher cyclic capacity and a much higher reaction rate with CO2 [2], [3]. Furthermore, MAPA and DETA were shown to have lower vapor pressures (lower volatility) than MEA [9], [10]. In another study, the diamines TMEDA and TMPDA were shown to have superior degradation stability than MEA in the presence of CO2 or O2 [1].
Although some thermodynamic and physical properties namely heat capacity, enthalpy of fusion and vaporization, ideal-gas enthalpy of formation, density, and viscosity have already been reported by some authors [10], [11], to our knowledge, the diffusion of any of the studied polyamines in aqueous solutions has not yet been investigated; hence, the absence of diffusion coefficient data for such systems in the literature. Diffusion coefficient is one of the most important transport properties that is indispensable in the design of any absorption process including that involved in CO2 capture operations. Thus, in this work we measured the binary mutual diffusion coefficients, D12, of MAPA, DETA, TMEDA, and TMPDA in water at concentrations w1 = (0.0–0.40) amine and temperatures from 303.15 to 323.15 K. The measured diffusion coefficients were then correlated with temperature and amine concentration using various models such as a modified Snijder et al. equation [12], UNIDIF equation [13], and that based on the rough hard-sphere theory [14]. Additional measurements of the densities, ρ, and viscosities, η, of the studied systems, which were used in the estimation of diffusion coefficients were also conducted.
Section snippets
Materials
MAPA and TMEDA (purities > 99 wt%) were purchased from Alfa Aesar, DETA (purity > 98 wt%) was supplied by Acros Organics, and TMPDA (purity > 98 wt%) was from Tokyo Chemical Co. Ltd. These amines were used in the experiments without further purification. The description of the chemicals used is given in Table 1. The aqueous solutions were prepared by dissolving the amines in high-purity distilled deionized water (resistivity = 18.3 mΩ), which was processed in a Barnstead Thermolyne (model Easy Pure 1052)
Results and discussion
Prior to the measurement of densities, ρ, viscosities, η, and diffusion coefficients, D12, of the studied systems, the validity of the experimental setups and methods used were tested by measuring the ρ of a standard oil (APS3), η of water and the D12 of ethylene glycol in water (x1 = 0.2) in the temperature range 303.15–323.15 K. The results obtained from the validation experiments are summarized in Table 2. As indicated in the table, the experimental data are in reasonable agreement with
Conclusions
The diffusion coefficients of the polyamines MAPA, DETA, TMEDA, and TMPDA in water at mass fractions (w1 = 0.0–0.4) over the temperature range 303.15–323.15 K were presented. Results showed that the binary diffusion coefficients of the polyamines in water varied systematically with the molecular weights (or sizes) of the amines that the diffusion coefficient increased with decreasing amine molecular weight. The mutual diffusivity data were well represented as function of temperature and
Acknowledgment
This research was supported by a grant, NSC 102-2221-E-033-061, from the National Science Council of the Republic of China.
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