3.2.1. Effectofregenerationtemperature
Because impregnated moieties are usually weakly attached to the adsorbent, it is important to examine the effect of the regener- ation temperature of the amino-loaded sample on the CO2 uptake. This aspect was addressed by measuring two isotherms for the same amine-loaded sample after being regenerated at two different temperatures. In Fig. 7, the CO2 adsorption isotherms obtained for the amino-loaded zeolite ZX0.2 at 348 K are shown after the sam- ple was regenerated at 423 and 473 K. For the sake of comparison, the isotherm of the pristine zeolite ZX at this temperature was also included.
It may be observed that the CO2 uptake increases when a higher temperature of regeneration is applied and approaches that of the pure zeolite ZX. These results suggest that part of the loaded amine was removed upon heating at 473 K, a higher temperatures than MEA boiling point (443 K) even though TGA analysis indicates that most of the amine volatilizes only beyond 500K (Fig. 3, inset). Nevertheless, in real PSA separation units, cycles are performed under nearly isothermal conditions (at least not higher than 373 K) and regeneration is performed under pressure swings. Adsorption uptakes should ideally be constant after pressure swings.
3.2.2. Adsorptionreversibility
CO2 adsorption and desorption isotherms were measured for the amino-loaded zeolite (ZX10) at two temperatures, 298 and 348 K, which are shown in Fig. 8(a) and (b), respectively.
The adsorption and desorption CO2 isotherms at 298 K do not follow the same path and such behavior cannot be attributed to physisorption equilibrium hysteresis under the studied P and T conditions. Such behavior may be due to an irreversible reaction between CO2 and amine sites present in the adsorbent. On the other hand, the apparent irreversibility observed in Fig. 8a may also be attributed to diffusion limitations imposed by the pore block- ing with MEA. This is consistent with the adsorption/desorption isotherms at 348K, a higher temperature which will eventually enhance intraparticle mass transfer. In fact, both adsorption and desorption isotherms overlap at this temperature. The diffusion resistance and intraparticle mass transfer of CO2 through filled pores may prevent the achievement of a strict equilibrium in the adsorption branch [25], particularly at the lower temperature (Fig. 8a). At this stage, however, none of these two hypotheses may be ruled out. It is likely that both mechanisms (chemisorption and hindered diffusion) occur.
3.2.3. Ontheoccurrenceofchemisorption
In order to collect experimental evidence as to whether chemisorption of CO2 on amine-loaded samples would take place, as suggested in a previous publication [4] and in the XPS analysis, the behavior of adsorption isotherms for sample ZX10 at different temperatures was observed. As can be seen in Fig. 9, for zeolites loaded with high concentrations of amine, the usual physisorp- tion behavior with temperature does not occur. That is to say, CO2 uptakes apparently increase for increasing temperatures in some amine loaded samples. Nevertheless, from the discussion in the previous sections, it is also reasonable to attribute such behavior also to diffusion resistances caused by the clogging of the porous structure of the adsorbent with MEA.