1. Introduction
It is clearly established that the use of chemical absorption processes
for carbon dioxide separation is considered as a mature technique with a
relatively low cost that allows the treatment of gas streams with high
flow rates.1 This type of operation for carbon dioxide
separation implies the use of chemical solvents, generally based on
amines. In general, the use of primary and secondary amines is highly
recommended, taking advantage of the high reaction rate with carbon
dioxide in aqueous solution and the low cost.2 One of
the most important limitations of this type of solvents is related to
the low carbon dioxide loading that can be reached, limited by the
reaction mechanism based on the production of
carbamate.3,4 Another negative characteristic of this
type of solvents that includes primary and secondary amines is the high
cost associated to solvent regeneration procedures5,6that generally accounts for 70% of the costs of the overall
absorption/desorption process.7 In addition other
researchers8 consider that a higher stability of
solvents can be reached avoiding the use secondary centres due to the
formation of nitrosamines.
Taking into account these negative characteristics the development of
new solvents tries to reach: (i) high absorption rate and carbon dioxide
loading and (ii) negligible solvent degradation, corrosion and energy
consumption. To reach these aims one of the proposed alternatives is the
use of solvents that include a mixture of amines.9 The
aim of using this type of mixtures is centred on taking advantage of the
positive characteristics of tertiary or sterically hindered amines that
can overcome the negative aspects of primary and secondary amines,
previously commented. It must be taken into account that solvents based
on tertiary amines have shown a low reaction rate in comparison to
primary and secondary amines.10 Moreover this type of
solvents can reach higher values of carbon dioxide loading, thus leading
to a better regeneration process that implies lower costs, degradation
and corrosion.11 Several studies have been carried out
in order to analyse the behaviour of mixtures of amines in the carbon
dioxide absorption process comparing their performance to those obtained
for the individual amines reaching in certain cases better results than
the conventional solvent.12-14 Also a research line
with very interesting results is centered on the use of di- or
polyamines. One of the most important molecules in this research line is
piperazine that has been used in an important number of research studies
and papers, from basic studies to industrial
applications.15,16
Present research work evaluates other type of chemical solvents
different than conventional ones, trying to take advantage of combining
different amine centres in the same solvent, inspired by promising
results obtained while working with mixtures of different amines. A
previous study analysed the use of diamines in the chemical solvents for
carbon dioxide absorption using mineralisation for solvent
regeneration.17 This work shows fast kinetics and low
energy requirements that indicate this type of solvents as promising
ones. A previous work18 of our team showed that a
diamine allows to reach similar values that amines blends for absorption
rate and carbon dioxide loading. Taking into account these previous
results, present work is focussed on solvents with
N,N-dimethylethylenediamine (a diamine with different amino centres -
primary and tertiary) that has been considered as a promising diamine
for chemical solvent formulation.17 Our work is
centred on a comparison with the solvent based on amines blends
(maintaining type and concentration) in order to evaluate the strength
and weakness of this type of solvents. The influence of amine ratio,
concentration and gas flow rate over the absorption process has been
analysed.