Focusing on renewable resources such as ethanol for producing CO₂-capturing solvents can stimulate a big change in the gas purification industry by creating a smaller environmental footprint. One of such solvents showing great potential is the tertiary amine N,Nʹ-diethylmonoethanolamine (DEMEA) (Vaidya and Kenig, 2007). CO₂ separation using aqueous DEMEA solutions can further be improved by adding suitable amine-based activators and high CO₂-affinity co-solvents. The commercially important amines monoethanolamine (MEA), diethanolamine (DEA) and piperazine (PZ) represent candidate activators of the absorption rate, while the industrial solvent sulfolane can act as a possible promoter of the loading capacity.

In this study, rate constants for the reactions between CO₂ and the blends DEMEA/MEA and DEMEA/DEA were evaluated by measuring the absorption rate for CO₂ at 298, 303 and 308 K in a stirred cell reactor. The molarity of DEMEA in solutions was 2, while the molarity of the activator was varied between 0.1 and 0.5. The results were compared with the rate constants for two further blends DEMEA/DGA and DEMEA/EMEA. Here, DGA and EMEA denote the primary amine diglycolamine and the secondary amine N-ethylmonoethanolamine. Besides, reaction kinetics of the blend DEMEA/PZ (2/0.25 M) in sulfolane/water mixtures (1.65 M sulfolane) was investigated.

Experimental

The experimental apparatus and procedure were described in detail in a previous work (Vaidya and Mahajani, 2005). We used a stirred cell reactor (inner diameter 97 mm, height 187 mm) with an undisturbed, horizontal gas-liquid interface. In each experiment, this reactor was filled with 0.45 dm³ of the absorbent. Then, CO₂ was charged inside the reactor and the reduction in pressure due to the reactive absorption process was noted. With the known values of the pressure gradient, the values of the CO₂ absorption rate were found (maximum error=3%). To verify our experimental technique, we investigated CO₂-MEA reaction kinetics in this reactor. We discovered that our value of the rate constant (k_MEA=7311 M^-1 s^-1 at 303 K) was in good agreement with that reported by Hikita et al. (1977), i.e. 7721 M^-1 s^-1.

Littel et al. (1991) reported a method for the estimation of the liquid-side mass-transfer coefficient k_L in a stirred-cell reactor. We found that, by using this procedure, the value of k_L is 0.0033 cm s^-1, which is corresponding to the usual values for stirred cell reactors. Solution densities and viscosities were measured. CO₂ diffusivity and solubility in solutions were estimated using the N₂O analogy method (Versteeg and van Swaaij, 1988). The existence of the fast pseudo-first order reaction regime was established by studying the influence of the agitation speed, and hence k_L, on the CO₂ absorption rates. Because the rates of absorption did not depend on the stirring speed in the 40-100 rpm range, we concluded that the investigated reactions occur in the liquid film and there is no depletion in the concentration of amine inside this film. We performed all experiments at a speed of 60 rpm.

Results and Discussion

DEMEA/MEA and DEMEA/DEA Blends

The dependency of the rate of absorption on CO₂ partial pressure was investigated at 303 K in the 5-25 kPa range. The reaction exhibited first-order kinetics for CO₂ concentration, which is in line with the kinetic behavior of CO₂ reacting with other alkanolamines. Of the two investigated additives, i.e. MEA and DEA, the former showed the highest enhancement in the rate of absorption in aqueous DEMEA solutions. The influence of concentration of MEA and DEA in solution on the CO₂ absorption rate was studied at 303 K. As the MEA and DEA concentration increased from 0.1 to 0.5 M, the rate of absorption increased significantly. Further, the increase in temperature facilitated the reactive absorption process. From the temperature dependency of the reaction rate constant, the activation energy was determined using Arrhenius plots.

Comparison with DEMEA/DGA and DEMEA/EMEA Blends

The performance of MEA and DEA was compared with that of DGA and EMEA respectively. Among the primary amines, MEA had a higher efficacy than DGA in enhancing the rate of absorption in aqueous DEMEA solutions. When the secondary amines were compared, the performance of EMEA – a secondary amine linked to an ethyl group – was preferential.

DEMEA/PZ Blend in Sulfolane/Water Mixtures

PZ is an efficient activator in aqueous DEMEA solutions (Vaidya and Kenig, 2008). The addition of sulfolane to aqueous mixtures of DEMEA and PZ resulted in increased absorption rates, due to a rise in CO₂ solubility. Thus, mixtures of DEMEA, PZ, sulfolane and water can be considered attractive for the enhancement of CO₂ capture.

Acknowledgement

Parag N. Sutar thanks the University Grants Commission, New Delhi, for financial support.

References

1.  Vaidya, P. D. and E. Y. Kenig, Chem. Eng. Sci., 62, 7344-7350 (2007).

2.  Vaidya, P. D. and V. V. Mahajani, Ind. Eng. Chem. Res., 44, 1868-1873 (2005).

3.  Hikita, H., Asai, S., Ishikawa, H. and M. Honda, Chem. Eng. J., 13, 7-12 (1977).

4.  Littel, R. J., Versteeg, G. F. and W. P. M. van Swaaij, Chem. Eng. Sci., 46, 3308-3313 (1991).

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6.  Vaidya, P. D. and E. Y. Kenig, Ind. Eng. Chem. Res., 47, 34-38 (2008).