In the next step, a multicomponent gas mixture containing
CO2/SO2/NO/N2 was used
to characterize the adsorption behavior of the 13X zeolite monoliths and
beads under both dry and humid conditions. Figure 6 displays
the concentration fronts for the three adsorbents collected during
dry-contaminated and humid-contaminated breakthrough runs. It should
also be noted that SO2 adsorption did not break through
the bed in any experiment. On the other hand, He broke through almost
immediately, while N2 and NO2 broke
through at ~10 min for all samples and exhibited sharp
breakthrough wavefronts under both dry and humidified conditions. This
was attributed to the low affinity of 13X towards He, N2and NO, but high affinity towards SO2. Namely, Deng et
al.13 reported that zeolites have a high affinity
towards SO2 compared to N2, NO, and
CO2 because SO2 is the only molecule
which contains a permanent dipole moment. In turn, this increases the
Henry’s Law constant for SO2 compared to the other
compounds and promotes greater adsorption on zeolites. In comparison,
the Henry’s Law constants for NO and N2 are much lower
and, because of this, these species do not adsorb in high quantities on
zeolite 13X, as further demonstrated by their rapid breakthrough times.
On the other hand, because the SO2 concentration in the
stream was so low, and the zeolite affinity towards the gas was so high,
SO2 saturation was never achieved and the gas was not
detected. It is also worth noting here that, similar to
CO2, the pre-humidification likely reduced the overall
SO2 adsorption capacity, as water also exhibits a
permanent dipole moment and should compete with SO2 on
zeolite.24 However, given the low SO2concentration relative to H2O, it is impossible to say
for certain whether or not its capacity was reduced under humidified
conditions. This being the case, because the goal of the study was to
examine CO2 adsorption behavior in the presence of
SO2, N2, and NO, and not focusing on the
adsorption behavior for the contaminant species, the experiments were
terminated after CO2 saturation was observed. For
reference, the SO2 desorption profiles are displayed inFigure S3, Supporting Information .
As evident from Figure 6 , introduction of the acidic
contaminants produced significantly longer CO2breakthrough times for all three adsorbents under both dry and wet
conditions. This caused the CO2 adsorption capacity to
increase by 0-20% in the dry experiments and by ~100%
in the wet experiments. This also corresponded to an increase in
CO2 selectivity of 5-30% across the dry experiments for
the various samples, whereas the selectivity increased by 50% or more
between the humidified-clean and contaminated runs. These increases in
adsorption capacity were to be expected from established literature for
multicomponent adsorption, which illustrates the promotional effect of
SO2 for CO2 capture.25This may be attributed to the promotional effect of the acidic
contaminants towards CO2, which becomes more pronounced
in the presence of water, on account of the formation of carbonate
species. Specifically, under dry operation, the acidic
SO2 increase the sorbents basicity by reacting with the
slightly basic Na2O species, to produce oxygen atoms
which share free radicals. In turn, this causes the contaminants to act
as chemisorbents and increases the overall affinity towards
CO2,14 but also degrades the zeolitic
alumina. Under humid mode, however, the Na2O readily
reacts with water to form NaOH, which then bonds to the oxygen free
radicals in the SO2, to produce species which do not
interact with the Al2O3 centers. These
free electrons can then react with CO2 to form
carbonate, similar to the promotional mechanism of water on
CO2 for amine-modified solids4 or the
promotional effect which occurs between SOx,
NOx, CO2, in other solid adsorbent
systems.25,26
Figure 6. Breakthrough profiles for CO2, He,
H2O, NO, SO2, and N2under dry-contaminated and humid-contaminated modes for (a-b) beads,
(c-d) 600 cpsi and (e-f) 800 cpsi monoliths.
Comparison of CO2 concentration profiles obtained under
each mode of breakthrough experiments are presented in Figure
7 . It can be observed that the breakthrough time was shortened
significantly in the humid bed as a result of competitive adsorption of
CO2 and water, however, when SO2 and NO
were present, a reverse trend was observed and total dynamic adsorption
amount was enhanced. Such cooperative adsorption occurred across all
three samples examined in this study and became further exacerbated
under humid-contaminated conditions. Considering the shape of
wavefronts, the zeolite 13X beads (Figure 7a ) exhibited the
sharpest breakthrough front under dry-clean conditions, however,
exhibited nearly double the breakthrough width under both humidified
modes, on account of the competitive CO2/water
adsorption behavior. In comparison, the dry-contaminated mode did not
increase the breakthrough width nearly as much as the humidified modes
of operation. However, the additional chemisorption still broadened the
profile by 65% from the dry-clean run. Both the 600 cpsi
(Figure 7b ) and 800 cpsi (Figure 7c ) monoliths
experienced similar changes in wavefront broadness across the different
modes, however, the increases in front width were much less compared to
the beads. This was attributed to the monoliths’ higher rate of film
mass transfer compared to the beads, which is a well-established benefit
of using monolithic contactors.27 Between the two
monolith samples, the 800 cpsi sample experienced greater increases in
front width from the dry-clean to the other modes compared to the 600
cpsi monolith. As discussed previously, this was attributed to the
increased kinetic dependence on molecular mass transfer which occurs at
elevated cpsi monoliths, leading to more bead-like
behavior.12,16
Figure 7. Comparison of CO2 breakthrough
profiles under four modes of operation for (a) beads, (b) 600 cpsi
monolith, and (c) 800 cpsi monolith.
After breakthrough tests, TGA runs were conducted over the used samples
to investigate the degree of CO2 capacity loss after
exposure to impurities. Figure 8 compares the
CO2 uptake capacities of the three adsorbents after each
mode of breakthrough runs with that of fresh materials. As can be seen,
the differences in CO2 adsorption between the fresh,
dry-clean, and humid-clean samples were, overall, marginal. Notably, the
dry-clean samples all exhibited slightly lesser CO2adsorption from the fresh and humid-clean samples, however, these losses
were only ~8% and could have been caused by slight
changes in intraparticle bonding from the repeated heating and cooling
of the samples. On the other hand, the losses in adsorption capacity for
the dry-contaminated runs could not be discounted, as the beads, 600,
and 800 cpsi monoliths exhibited 38%, 58%, and 14% losses in
CO2 adsorption capacity, respectively. As previously
discussed, these sizable losses could likely be attributed to
decomposition of the sodium centers after reaction with the acid gases.
Following the humidified-contaminated runs, however, the losses in
CO2 capacity were only marginal, as the reductions in
CO2 adsorption capacity were only 25%, 14%, and 11%
for the beads, 600 cpsi monolith, and 800 cpsi monolith, respectively.
On the basis of these results, it was concluded that water can act as a
protective barrier during CO2 adsorption on zeolite when
SOx/NOx are present, as it allows the
acid gases to react with hydroxide clusters instead of decomposing the
alumina centers.
Figure 8. Comparison of CO2 adsorption capacity
of the materials before and after breakthrough experiments for beads,
600 and 800 cpsi monoliths.