https://doi.org/10.3390/su14084750
“Since the development of unique nanoscale particles, the industrial applications of silica nanoparticles, SiO2, have drastically increased. Silica nanoparticles have been widely selected due to the simplicity and low-cost of their large-scale preparation, their hydrophilic nature, large specific surface area, pore volume and controlled particle size [111,112] For CO2 capture processes, SiO2 nanoparticles were shown to enhance the mass-transfer process and improve the absorption and desorption performance [57,59,60,100,101]. For their regeneration performance, the nanoparticles were studied in three different solvents, which were deionized water, methanol and MEA, the summary of which is depicted in Table 4.”
Table 4. Summary of past studies using SiO2 nanoparticles in the regeneration of solvent.
| Solvent | Size and Concentration | Temperature °C |
Remarks | Ref. |
|---|---|---|---|---|
| Deionized water | 15 nm 0.01–0.1 vol% |
20 |
|
[57] |
| Deionized water | 15 nm 0.0–0.05 vol% |
100 |
|
[59] |
| Methanol | 15 nm 0.01 vol% |
<65 |
|
[60] |
| MEA | 15 nm 0.1 wt% |
103 |
|
[100] |
”
Among the nanoparticles in review, the SiO2 nanoparticle is less dense than other metal oxides. The average size of SiO2 used in past studies were 15 nm. When the size of the nanoparticles was smaller than the surface roughness, the roughness decreased. However, when the size was larger than the surface roughness, then the nucleation site density became larger [58]. Apart from the nanoparticle powder size, its cluster size is also important, as it reflects the nanoparticle’s stability. An unstable nanoparticle would have a high nanoparticle cluster size, due to the agglomeration and sedimentation of the nanoparticle. It was also reported that the cluster size has a much greater impact on the geometric conditions of the heating surface in comparison with the nanoparticle’s particle size. The SiO2 nanoparticles have a cluster size ranging from 680–800 nm [59], both of which are higher than that of the Al2O3 nanoparticle studied (330.8 nm) [60]. However, the study by Wang et al. [100] reported the cluster size of the SiO2 nanoparticle to be 213.4 nm before absorption and 2053.5 nm after absorption, which is lower than the other two nanoparticles in the study.
The SiO2 nanoparticle concentration is kept low at 0.01–0.1 vol% [58,59,60,100]. Higher concentrations of the nanoparticle lead to the increase in the viscosity of the solvent. However, it also been shown that when increasing the concentration, the changes in viscosity were not severe, in fact, less than 5%. It was also reported that the employment of higher concentrations contributes to an enhancement in terms of heat transfer, although it is only 2–3%. The enhancement of thermal conductivity is low for concentrations below 1.0 vol% [63,66,67]. Among the studied nanoparticles, the SiO2 can be seen to have a low enhancement rate, and the past studies only managed to consider its physical enhancement effect in terms of mass and heat transfer mechanism. In addition to that, the thermal stability was reported in terms of its mass loss at particular temperatures and the pure silica was seen to be very stable at high temperatures, only facing very minimal mass loss [113]. However, another study did report weight loss, which may happen below 100 °C, due to the evaporation of water [114]. Subsequently, it remained almost constant up to 800 °C.
The enhancement mechanism reported in past studies include activation effect, thermal conductivity effect and surface effect. The activation effect is discussed through the collision of the SiO2 nanoparticles and the CO2 gas, which enhances CO2 regeneration. Next is the thermal conductivity effect, which causes the temperature of the nanoparticle-utilized solvent to rise faster. This also enhances the regeneration performance. Lastly, the surface effect, which is discussed in regard to the bubble generation on the surface of the heater. It has been reported that the frequency of CO2 bubble departures were between 50–160 s for SiO2, which is faster than that of the blank test [59].
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