3. Results and Discussion
The main aim of present research work is establish a comparison between
diamine-based solvents and a blend of amines that employ the same type
and number of amino groups. Figure 2 shows the absorption curves
corresponding to DMEDA+water solvent using different diamine
concentration. The shape of these curves were similar, reflecting the
total absorption of the carbon dioxide fed to the gas-liquid contactor
at the beginning of the experiments, thus indicating suitable
characteristics regarding both the reaction rate and mass transfer.
After this constant-absorption rate period a monotonic decrease was
observed until solvent saturation was reached.
The area under the absorption curve increased with amine concentration
due to a higher number of amino centres available to react with
physically absorbed carbon dioxide. This value is related with an
important parameter in this kind of studies: carbon dioxide loading (α),
defined as the ration between mol of absorbed CO2 and
mol of amino groups in the tested solvent. The evolution of this
parameter is also included in Figure 2 and shows a stable increase until
a constant value ins reached, being this value related with the overall
reaction mechanism stoichiometry. At low amine concentration, the carbon
dioxide loading was higher due to the importance of physical absorption
upon the overall process, but an increase in amine concentration allowed
to reach a constant value close to 0.55 mol CO2·mol
amine group-1. Taking into account previous
studies14 a carbon dioxide loading similar to the
obtained for DMEDA aqueous solutions implies the reaction mechanism
shown in reaction 1.
2 RNH2 + CO2 RNHCOO- +
RNH3+ (1)
The behaviour observed in Figure 2 is commonly associated to primary or
secondary amines acting as chemical solvent absorption. However the
final constant value of carbon dioxide loading obtained in these
experiments was not compatible to those reached at the end of
experiments involving those types of amines, where larger values were
reached. This higher carbon dioxide loading can be achieved due to the
existence of carbamate hydrolysis processes, where the hydrolysis degree
depends on different aspects e.g. the steric hindrance of
amine.19,20 For instance, monoethanolamine aqueous
solutions (primary amine) reach carbon dioxide loading close to 0.7 mol
CO2·mol amine group-1 due to partial
carbamate hydrolysis.21. Considering that the diamine
employed in present work involves the presence of both a primary and a
tertiary amino group, the expected carbon dioxide loading could be 0.75
mol CO2·mol amine group-1 (avoiding
the carbamate hydrolysis).
Moreover, the influence of gas flow rate fed to bubble column reactor
upon absorption rate has been analysed (see Figure 3). In this type of
biphasic reactors, the gas flow rate can play an important role upon
absorption rate, since it can not only affect phase mixture but also
interfacial area. Generally, an increase in the amount of gas fed to
reactor causes an increase in both parameters; a higher agitation that
increases mass transfer rate, and also increasing gas hold-up in the
bubble column. This last parameter generally causes an important enhance
in gas-liquid interfacial area. For this reason, the experiments shown
in Figure 3 indicated a high absorption rate when gas flow-rate fed to
column contactor increases. This solvent allowed the absorption of all
carbon dioxide flow rate fed to the reactor.
As previously commented, the use of diamine aqueous solutions must be
compared with the behaviour of amine blends in order to select the
solvent with better characteristics, taking into account that previous
studies22,23 suggested that amine blends contribute to
better properties for carbon dioxide chemical absorption than
single-amine solvents. For these reasons, the same type of experiments
has been performed using a mixture of alkanolamines that maintain the
same type and concentration of amino groups than the diamine aqueous
solutions. The selected mixture was composed of monoethanolamine (MEA)
and dimethylaminoethanol (DMEA). Figure 4 shows the absorption curve
corresponding to a 50% mixture of each amine to maintain the same amine
group concentration in comparison to the diamine aqueous solutions. A
similar trend was observed in comparison to the previously commented
diamine experiments. At the beginning of the experiments the amine-blend
solvent absorbed all the carbon dioxide fed to the reactor and after
this period a continuous decrease was observed. Moreover, Figure 4 shows
the effect of different ratios between both alkanolamines upon the
absorption of carbon dioxide. Revealing that an increase in the
proportion of primary amine groups (corresponding to higher
monoethanolamine concentration) led to an increase in the absorption
rate, maintaining the total absorption of carbon dioxide during a higher
period of time, however displaying similar absorption rate decreases
after that time. Consequently, amine blends did not show important
differences upon the absorption curves in comparison to the use of
diamine solvent.
Analysing the behaviour of the value of carbon dioxide loading for both
solvents based on the mixture of amines (MEA and DMEA), a similar
behaviour was also observed. Both systems reached values close to 0.95
mol CO2·mol amine group-1. The use of
a higher amount of MEA in the mixture caused a slight decrease in the
value of carbon dioxide loading probably due to an increase in the
amount of carbamate formed in the liquid phase.
In order to carefully analyse the behaviour of amines mixture solvents,
both amines have been studied individually in carbon dioxide absorption
experiments. Figure 5 shows a comparison between the use of amine blend
solvent (at 50% of each amine) and single amine solutions. These
experiments showed a very different behaviour. The primary amine (MEA)
reached a high absorption rate at the beginning of the experiment,
absorbing the total carbon dioxide flow rate during a period of time
similar to that observed for the blended solvent. After that, a dramatic
decrease in absorption rate was observed and the mixture of amines
reached an important enhancement in absorption rate. On the other hand,
the tertiary amine showed the lowest absorption rate, mostly due to the
different reaction mechanism that characterises carbon dioxide
absorption by tertiary amines,24 thus affecting the
reaction rate and hence influencing the overall process. Taking into
account this comparison, it is possible to conclude that the mixture of
MEA and DMEA showed a significant better behaviour than the use
DMEDA-based solvents. This mixture reached higher absorption rates
similar to those observed for the primary amine, being even better in
certain parts of the experiments. In relation to carbon dioxide loading,
this blended solvent reached intermediate values between both systems,
but close to 1 mol CO2·mol amine
group-1 in all cases.
When a comparison between both systems (diamine and blend of amines) is
performed (see Figure 6), the solvent based on DMEDA showed a very
important and negative difference in the value of carbon dioxide
loading. As previously commented in Figure 1, diamine aqueous solutions
reached value of carbon dioxide loading close to 0.55 mol
CO2·mol amine group-1. This value was
significantly lower than in alkanolamine mixture solvents. Figure 6
shows a similar absorption rate when amine concentration is high but the
decrease in the absorption rate corresponding to the saturation of
solvent was produced in a lower experiment time. A similar conclusion
was reached by analysing the evolution of carbon dioxide loading that
displayed a similar slope but reached a significant lower amount of
carbon dioxide chemically absorbed at the end of experiments.
The reason of this difference in the amount of carbon dioxide captured
per amino group cannot be explained only based on absorption
experiments, and for this reason, several speciation studies were
carried out focusing on 1H and 13C
NMR. This type of experiments tried to identify the reaction products
present in the liquid phase during the absorption experiments, thus
allowing to explain the absorption rate and also the carbon dioxide
loading on the basis of the weight of each reaction in the overall
reaction mechanism. This technique allowed to analyse how the different
carbons present in amines were involved in the reaction by detecting
changes on the chemical shifts associated to each carbon atom. Figure 7
shows the 13C NMR spectra corresponding to the liquid
phase during the chemical absorption of carbon dioxide in DMEDA aqueous
solution. At the beginning of the carbon dioxide absorption process (α=0
mol CO2·mol amine group-1), three
signals were observed at chemical shifts of 38, 44 and 60 ppm that
corresponded to carbons of the DMEDA molecule. The additional peak
observed but was related to deuterated methanol (used as reference),
thus its chemical shift was not indicated in Figure 7 to avoid
confusion.
The successive spectra included in Figure 7 correspond to the solvent
when different amounts of carbon dioxide were absorbed using the carbon
dioxide loading data. When a carbon dioxide loading of 0.08 mol
CO2·mol amine-1 was reached, new
signals appeared close to the previously analysed peaks of DMEDA. On the
basis of previous studies25 this type of signals was
in agreement with the spectrum for carbamate. This conclusion was
supported by the presence of a peak at 164.6 ppm that corresponded to
the carbon atom of the carbon dioxide when it reacts with the amine
centre. It is well-known26 that the reaction of carbon
dioxide with primary and secondary amines to produce carbamate is faster
than the reaction with tertiary amines producing bicarbonate. Figure 7
shows the same behaviour when a diamine is used, since carbon dioxide
preferably reacted with the primary amine centres of this diamine. The
same behaviour and reaction mechanism was observed during the main part
of the chemical absorption experiment, However, in the last part of this
experiment a new signal appeared with a chemical shift of 160 ppm, which
corresponds to the bicarbonate molecule.25 The
presence of bicarbonate in the solvent could be cause by different types
of reactions: (i) carbamate hydrolysis and (ii) reaction between carbon
dioxide and tertiary centres. Considering that tertiary centres in this
molecule has the same concentration as primary ones at the beginning of
experiments, a higher weight of the direct reaction of carbon dioxide
with tertiary centres than here observed could be expected. The presence
of low amounts of bicarbonate at the end of experiments (close to
solvent saturation) allowed to conclude that this product was mainly
caused by carbamate hydrolysis, because bicarbonate only appeared when
high concentrations of carbamate were reached.
In order to fully understand the reaction mechanism of carbon dioxide
with aqueous solutions of DMEDA, further studies using a chemical
solvent based on a blend of MEA and DMEA were performed. These compounds
were chosen to maintain both the same type and concentration of each
amino centre present in DMEDA. Figure 8 shows the spectra obtained
during carbon dioxide chemical absorption using this blended solvent. At
the beginning, five peaks were observed: carbons corresponding to MEA
(43 and 60 ppm) and DMEA (45, 59 and 63 ppm. As in the analysis of
spectra in Figure 7 for DMEDA, new signals corresponding to MEA
carbamate (duplicity of signals and the creation of a peak close to 165
ppm) appeared in the NMR spectra during the carbon absorption process.
One of the most important differences observed when comparing the
spectra corresponding to diamine and that of amine blends is the
presence of bicarbonate at relatively low carbon dioxide loadings (in
comparison with the final value) in the solvent based on amine blend.
Specifically, bicarbonate appeared at 0.67 mol CO2·mol
amine-1, and at the end of the experiment the carbon
dioxide loading reached the value of 0.94 mol CO2·mol
amine-1.
As previously commented, the presence of bicarbonate can be due to
either the reaction of carbon dioxide with tertiary centres (DMEA) or
the hydrolysis of carbamate. In this case the final value of carbon
dioxide loading is close to 1 mol CO2·mol
amine-1 thus indicating that both reactions take place
completely.
On the basis of the experimental results of speciation studies in these
solvents (diamine and amines blend), it is possible to conclude that
diamine-based solvent generated some type of inhibition of chemical
reaction of carbon dioxide with tertiary centre, because the maximum
value of carbon dioxide loading was significantly lower in comparison
with the use of the same type of centres and radicals in a blend of
amines. This behaviour is in agreement with a previous
study27 that detects an important influence of
electronic environment in the diamine when the length of the alkyl
spacer is small.
The overall experimental data allowed to propose that the decrease in
carbon dioxide loading with the diamine-based solvent was due to the
lack of chemical reaction in the tertiary centre, because reached values
are comparable to those obtained considering the use of aqueous
solutions of MEA as chemical solvent for carbon dioxide chemical
absorption. The inhibition of chemical reaction in the tertiary centre
was caused by the interactions of this centre with either other parts of
the same molecule or other molecule. Previous work28indicated that the use of a diamine with two primary amino centres can
lead to a cycled structure after the reaction of one of them with carbon
dioxide. This fact can also stabilize the carbamate, reducing the
hydrolysis processes. The reaction mechanism here proposed is in
agreement with the previously detected reduction in carbon dioxide
loading and the low presence of bicarbonate in the reaction products.
Additional speciation experiments using different amine ratios for the
amine blend-based solvent (MEA/DMEA ratio of 5) showed behaviour in
agreement with the previous discussion maintaining the same
concentration of primary and tertiary centres (see figure 9).
In the first part of this manuscript the carbon dioxide chemical
absorption behaviour using different solvents has been analysed taking
into account the type of solvent, the concentration of amine or blend of
amines and gas flow rate, explaining these behaviours on the basis of
the reaction mechanism determined by nuclear magnetic resonance
spectroscopy. Additionally, several studies in steady state regime have
been performed comparing the different solvents previously analysed. In
this part of the experimental work, the role of regeneration processes
by stripping can influence the overall behaviour of chemical solvents.
An example of the experimental results obtained in this type of
experiments is shown in Figure 10 for DMEDA aqueous solutions. The
influence of gas flow rate fed to bubble column reactor and solvent flow
rate was analysed in this figure; in relation to the first variable (gas
flow rate) an increase in the amount of carbon dioxide absorbed in the
bubble column reactor was observed, but no effect was observed when the
influence of liquid phase flow rate (QL) was studied.
Moreover, a comparison between the different solvents employed in the
present work was carried out in Figure 11. In all cases, an increase in
gas flow rate fed to bubble column reactor caused an increase in the
amount of carbon dioxide transferred to the liquid phase. Though the
previous commented studies carried out in semi batch regime allowed to
conclude that the solvents based on the use of blends of amines
contribute better carbon dioxide absorption results, the experimental
data obtained in steady state regime (see Figure 11) showed a different
behaviour.
The diamine-based solvent obtained higher carbon dioxide absorption
rates than the other solvents based on the mixture of amines with the
same functional groups. This difference can be due to the special
characteristics corresponding to the steady state regime. In this type
of configuration, the carbon dioxide loading reached by the solvent
during the chemical absorption is relatively low. This fact can explain
the higher absorption rate for diamine-based solvents, because the
reaction rate corresponding to the formation of carbamate is higher than
the ones corresponding to the systems where the final product is the
bicarbonate ion. This behaviour is in agreement with previously shown
results included in Figure 6, that showed a high absorption rate for
DMEDA solutions when carbon dioxide loading was lower than 0.4 mol
CO2·mol amine-1.