https://doi.org/10.1016/j.egypro.2017.03.1336
“Figure 3 shows the time variations in CO2 loading of the absorbent solution with and without the alumina
particles. The CO2 loading of the solution without the alumina particles rapidly decreased to 0.55 mol-CO2/mol-
DEA in the first 10 mins. Afterward, however, it was almost constant although it fluctuated. The CO2 partial
pressure equilibrated with the loading of 0.55 mol-CO2/mol-DEA at 40 qC was estimated about 28 kPa from the data
reported by Ref. [9]. It means that a part of CO2 released from the absorbent solution stayed in the gaseous phase
above the solution in the gas collecting bottle, which suppressed further desorption of CO2.
When the untreated alumina particles were added to the rich solution, CO2 desorption was obviously promoted.
CO2 loading rapidly decreased in the first 30 mins and then gradually decreased. However, it was almost constant
after 90 mins from the start. The final loading after 3 hrs was 0.52 mol-CO2/mol-DEA This indicates that the Į-
alumina particles were effective for CO2 desorption from the amine solution even without surface functionalization.
As mentioned in the previous section, the IEP for the alumina particles was around neutral. On the other hand, pH of
the rich solution was around 10. It means that the alumina particles can donate protons into the rich solution even though the surface is not functionalized. These additional protons can promote CO2 desorption by shifting
equilibrium of reaction toward CO2 dissociation.
As seen in Fig. 3, CO2 desorption was remarkably promoted when the alumina particles functionalized with
phosphate groups were added to the solution. The CO2 loading rapidly dropped in the first 20 mins. Afterward it
kept decreasing gradually and the apparent plateau was not seen in 3 hrs. The final loading was much lower
compared with the others and reached 0.44 mol-CO2/mol-DEA. As reported earlier, the IEP for the alumina particles
functionalized with phosphate groups was at pH 4.0. In addition to the protons from the alumina particles, more
protons can be donated by the phosphate groups. As a result, CO2 desorption was significantly promoted by a large
quantity of additional protons.
On the other hand, in the case of alumina particles functionalized with amino groups, the CO2loading of the
absorbent solution showed the similar trend to that for the untreated alumina particles while the final loading was
slightly higher. The alumina particles functionalized with amino groups had a IEP at a higher pH value compared
with the untreated alumina particles but it still lower than the pH value of the rich solution. Some of protons may
have been accepted by the amino groups on the alumina surface, which suppressed CO2 desorption from the solution.
However, the addition of the alumina particles functionalized with amino groups also promoted CO2 desorption in
this study.
Shi et al. [5, 6] reported that acid solid catalysts such as HZSM-5 and J-Al2O3 was effective for CO2 stripping
from the rich amine solvents. They considered that HZSM-5 acted as a Brønsted acid donating protons to the
solution. They also considered that J-Al2O3 could be dissolved in the basic absorbent solution to form AlO2- and it
accepted a proton from AmineH+ and then release the proton to water. It is likely that some amount of alumina was
dissolved in the absorbent solution and played the same role to promote CO2 desorption in this study as well.
From these results, it is suggested that the greater the difference between pH of the rich solution and pH giving
isoelectric point of the particle was, the lower the final loading.”