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.