https://doi.org/10.1016/j.cesx.2019.100032
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The overall mass transfer coefficient (Kov) combines the contribution of diffusion, physical absorption and chemical reaction to the mass transfer, and thereby represent the actual observed CO2 absorption rate into the solvent. The Kov values for the blended amines are presented in Fig. 9 along with the DEEA/MAPA solutions reported by Monteiro et al. (2015), 1 M MAPA reported by Monteiro et al. (2014) and 30 wt% MEA reported by Bernhardsen et al. (2019a). From the figure, it can be seen that the Kov increased with increasing temperature. This is probably due to increasing reaction kinetics and increasing CO2 diffusivity coefficient (Section 6.3) which follows at higher temperatures.
Fig. 9. Overall mass transfer coefficient (Kov) as a function of temperature. *Monteiro et al. (2015), **Monteiro et al. (2014) ***Bernhardsen et al. (2019a).
Further, the blended amines obtained quite similar Kov values throughout the temperature range. This even though the solubility of N2O in DEEA/MAPA solutions was higher than in the other blended amines (Section 6.5), and the solvent’s viscosity increased with increasing solvent concentration which led to a decrease in CO2 diffusivity (Section 6.3). Thus, competing effects between reaction kinetics, diffusivity and solubility likely played a key role in determining the Kov.
Moreover, the blended amines obtained comparable Kov values to that of 1 M MAPA. Compared to 30 wt% MEA, the Kov values were in the same order of magnitude, but around 1.4 times higher.
A parameter influencing the mass transfer is the gas film resistance (1/kg). The parameter is typically found to be important in regions where the CO2 loading is low. In a conventional absorption process, this is located in the top and in the bottom of the absorption and desorption column, respectively (Cullinane and Rochelle, 2006). Fig. 10 shows the percentage contribution of the gas film resistance to the overall resistance (i.e. Kov/kg) as a function of temperature. The contribution of the gas film resistance increased with increasing temperature and, at 62.6 °C, the gas film resistance contributed up to 41% of the overall resistance. Aronu et al. (2011) obtained a similar trend as faster reaction kinetics lead to a higher contribution of the gas film resistance to the overall resistance. Consequently, the unloaded amine solutions studied in this work were mainly kinetically controlled, but the influence of the gas film resistance to the overall resistance increased with increasing temperature. The DEEA/MAPA solvent systems reported by Monteiro et al. (2015) showed lower Kov/kg values than the other blended amines, but still higher than that of 30 wt% MEA reported by Bernhardsen et al. (2019a).
Fig. 10. Contribution of the gas film resistance (kg) to the overall mass transfer (Kov). *Monteiro et al. (2015), **Bernhardsen et al. (2019a).
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