CO2 and N2 Adsorption
Isotherms
Figure 3 illustrates the CO2 and
N2 adsorption isotherms obtained at 25 °C for the three
adsorbents. In agreement with data for the powdered
zeolite,20 all adsorbents exhibited a high affinity
toward CO2 and negligible capacity for
N2. At 1 bar and 25 °C, CO2 adsorption
capacities reached to 4.5, 5.1, and 5.1 mmol/g for the beads, 600 cpsi,
and 800 cpsi monoliths, respectively, (Figure 3a ), while
N2 adsorption capacities of 0.2, 0.4, and 0.4 mmol/g,
respectively, were observed at the same conditions. The lower adsorption
capacity of the beads could be correlated to their formulation which
limited their active sites accessibility upon processing, despite having
higher surface area and pore volume. Nevertheless, the differences in
adsorption capacity between the two configurations were overall small
and, therefore, the beads could still be considered an acceptable
analogue for testing of multicomponent CO2 adsorption.
Figure 3. Pure gas adsorption isotherms of (a)
CO2 and (b) N2 for beads, 600 and 800
cpsi monoliths at 25 °C.
The theoretical selectivity for 10%
CO2/N2 in the pressure range of 0-1 bar
was calculated by the IAST method, as shown in Figure 4 . A
decreasing trend with pressure was observed for the three adsorbents
with almost identical values ranging from 720 to 150. Notably, all three
samples exhibited the same selectivity values, even though the monoliths
exhibited higher adsorption capacities. This was attributed to the fact
that, although the beads may have reduced in adsorption capacity from
the monoliths, the reductions in adsorption uptake were the same for
both CO2 and N2. In turn, this caused
the CO2 selectivity value to remain consistent with that
for the monoliths. Chue et al.,21 who studied zeolite
13X pellets in a pressure swing adsorption process for
CO2 removal from flue gas, reported similar
selectivities for zeolite 13X at 1 bar and 10% CO2.
Effectively, this indicated that the zeolites examined here could be
considered representative analogues for materials used in industrial
separation processes.
Figure 4. Theoretical CO2/N2selectivity curves estimated from adsorption isotherms for beads, 600
and 800 cpsi monoliths.
Multicomponent Breakthrough Experiments
To evaluate the materials’ performance for CO2adsorption under multicomponent conditions, four modes of operation were
considered: dry-clean, humid-clean, dry-contaminated, and
humid-contaminated. The breakthrough profiles are shown inFigure 5 while the corresponding breakthrough data is located
in Table 3 . The profiles from the blank experiments as well as
the desorption profiles are also shown in Figure S1 ,Supporting Information and Figure S2 ,Supporting Information , respectively. As was expected,
pre-exposing the samples to humidity under clean conditions led to a
significant reduction in CO2 breakthrough time
(t5% ), on account of the competitive adsorption
behavior which exists between these two molecules on zeolite 13X.12,16 In turn, this led to losses in
CO2 adsorption capacity for all samples from dry to
humid conditions. These effects were especially present in the case of
the beads (Figure 5a-b ), where a 65% reduction int5% and a 10% decrease in CO2adsorption capacity (q95% ) from 2.0 to 1.8
mmol/g occurred from the dry to the humid experiments. This corresponded
to a 66% reduction in the bead’s CO2/N2selectivity from 138 to 48. In the monoliths, however, the breakthrough
time was only reduced by ~19% in the 600 cpsi sample
(Figure 5c-d ) and 47% in the 800 cpsi sample (Figure
5e-f ). For the 600 cpsi sample, this corresponded to a reduction in
CO2 adsorption capacity of 2.2 to 1.9 mmol/g, as well as
a reduction in CO2/N2 selectivity of 77
to 54.4. Similarly, the 800 cpsi sample experienced a loss in
CO2 capacity of 1.8 to 1.6 mmol/g, with a reduction in
selectivity of 99 from dry to77 humid modes of operation. These
observations were in agreement with both our recent report as well as
other literature for dry versus humidified CO2adsorption on zeolite 13X and were to be
expected.12,20,22,23 As we detailed therein, the
greater reductions in breakthrough time for the beads and 800 cpsi
monolith compared to the 600 cpsi monolith could be attributed to
differences in particle mass transfer. Namely, the 600 cpsi monolith’s
larger channels did not throttle the gas and allowed enough time for
CO2 and water to diffuse through the dense walls. In the
beads and 800 cpsi monolith, however, adsorbate diffusion occurred
through particles and the CO2 broke through faster than
the adsorbent could saturate. A similar effect was also likely present
with water, as it readily adsorbed on the closest accessible pores, and
further increased the rate dependence for CO2 adsorption
on particle mass transfer.
Moreover, investigation of breakthrough widths revealed that the
adsorption dynamics was also impacted by water, with the wavefronts
becoming broader under humid-clean conditions. Such a result was in
agreement with our recent work,12 which showed that
pre-humidified zeolite 13X samples exhibit broader adsorption
wavefronts, on account of the increased dependence on molecular mass
transfer caused by saturation of the outer adsorbent layer. The dynamic
CO2 adsorption capacities of 13X zeolite beads estimated
at t50% were found to be 2.0 and 1.8 mmol/g
under dry and humid conditions, respectively, whereas, for the 600 and
800 cpsi monoliths, these values were calculated to be 2.2 and 1.9
mmol/g and 1.8 and 1.6 mmol/g, respectively. Overall, these results
indicated that the competitive CO2/water adsorption
occurred independent of adsorbent geometry, as all three samples
experienced a ~10% reduction inq50% from dry to humidified conditions. Such a
result was to be expected, as the competitive adsorption for
CO2 and water on zeolite 13X is dependent on the
adsorbent pore size and adsorbate molecular diameter, which are both
independent of the bulk structural packing.12,16
Figure 5. Breakthrough profiles for CO2, He,
H2O and N2 under dry-clean and
humid-clean modes for (a-b) beads, (c-d) 600 cpsi, and (e-f) 800 cpsi
monoliths.
Table 3. Summary of breakthrough parameters for zeolite 13X
samples under four modes of operation.