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 N­. 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.