4. Conclusions
Present work compares the carbon dioxide chemical absorption using the same type of amino centres being part of a diamine (DMEDA) and a mixture of amines (MEA+DMEA). On one hand, the studies corresponding to the use of amine blends showed that changes in the amine ratios did not lead to important changes in the absorption curves and carbon dioxide loading, though a higher presence of MEA increased both parameters. For instance, a chemical solvent composed of an aqueous solution of MEA/DMEA (ratio of 1) showed a very similar behaviour than a MEA-based solvent.
The use of DMEDA (diamine) showed a different performance than the use of amine blends-based solvents, though the same concentration of each type of amino centres was employed. The diamine-based solvent showed an important decrease in the carbon dioxide loading that caused also a decrease in the absorption rate (caused by a lower driving force).
Speciation studies using both type of solvents allowed to conclude that significant differences in the reaction mechanism were produced. A reduction in the production of bicarbonate ion was observed when using DMEDA as chemical absorbent. This fact was produced by the interaction between amino centres during the chemical absorption, that caused both the stabilization of the carbamate produced through the reaction of carbon dioxide with primary amino groups (avoiding its hydrolysis) and an inhibition of the reaction of carbon dioxide with the tertiary centres. This fact caused an important decrease in the carbon dioxide loading reached by this type of solvent.
The last part of the work focused on steady state experiments by accomplishing a stripping-based solvent regeneration unit. In contrast to batch experiments, the experimental results here obtained showed higher absorption rates for the diamine-based solvent. For this configuration, this type of behaviour was caused by a lower carbon dioxide loading values being reached in the absorber, which corresponded to only carbamate being produced, as determined in speciation studies for this carbon dioxide loading range. The chemical reaction rate was higher for carbamate production than for bicarbonate, thus allowing higher absorption rates when using the solvent based on diamine in comparison to those based on amine blends. It allows to consider DMEDA-based solvents as potential ones to be used in carbon dioxide separation units.