Physical Enhancement Mechanism of Nanoparticles

The physical enhancement mechanisms by nanoparticles on solvent regeneration include activation energy, surface effect and thermal conductivity enhancement effect [57,58,59,60]. The activation energy effect was proposed by Lee et al. [59] in 2015. Activation energy is the minimum energy that must be provided to result in a chemical reaction of a compound. In the regeneration process, the activation energy must be supplied so the regeneration of CO₂ gas from the solvent can occur [60]. The solvent that is blended with nanoparticles can desorb the CO₂ easily in comparison to the solvent without nanoparticles, due to the increase of activation energy. These nanoparticles in motion in the base fluid have a high activation energy and result in active liquid molecules, which leads to the clash of liquid molecules and nanoparticles. This dynamic behavior in the base fluid results in an increase in activation energy.

The next mechanism involved in the regeneration of nanoparticles is the thermal conductivity enhancement effect on the heat transfer performance, which was proposed by Fan and Wang [61] and Keblinski et al. [62]. Due to the increase in the heat transfer by the nanoparticles, as discussed previously, the effective thermal conductivity of the nanoparticles is also improved. However, this is improved with the optimum concentration and type of nanoparticles employed. The mechanism explaining the improved thermal conductivity are Brownian motion, molecular-level layering, nature of heat transport and the effect of nanoparticle clustering [61,62]. These proposed mechanisms have not been clearly explained, but the increase in thermal conductivity upon the employment of nanoparticle has been reported by numerical and experimental studies [61,63,64]. The increase in the thermal conductivity of the nanoparticles can rapidly increase the temperature, which results in the rapid decrease of CO₂ solubility. Lee et al. [60] demonstrated the contribution of SiO₂ and Al₂O₃ nanoparticles on the regeneration enhancement of methanol solvent, in terms of heat transfer. The enhancement was reported to only be 2–3% approximately, as adding only 0.01 vol% nanoparticles could not cause a significant difference in the thermal conductivity. As supported by other studies [63,65,66], the effect of thermal conductivity enhancement by nanoparticles is not significant.

Another proposed mechanism is the surface effect, which enhances heat transfer [67]. CO₂ desorption process works similarly to the boiling process, in which more bubbles are formed upon the increase in temperature, due to Henry’s law of solubility. When nanoparticles are employed in the base fluid, they are deposited on the boiling surface due to gravity and natural convection, which then causes the change of the boiling surface properties on the surface of the heater. While the boiling process occurs at a high temperature above the saturation point, the bubble regeneration process occurs when the temperature rises above the absorption temperature. Upon the utilization of the nanoparticles, the bubble generation that takes place is easily achieved in comparison to the boiling process conditions, because of its influence on the surface characteristics. The effects that take place upon the nanoparticle deposition on bubble generating surfaces are as follows:

(1)

Increase in heat transfer surface area: As reported by Kim et al. [68], at a concentration below the critical concentration, the effective heat transfer surface area increases upon the increase in nanoparticle concentration. However, exceeding the critical concentration can cause a smooth nanoscale surface to form, which reduces the effective heat surface area.

(2)

Change in surface roughness: During the boiling process, the nanoparticles are deposited on the heating surface, which causes the change in the microstructure and topography of the heating surface. A porous layer is formed on the boiling surface, which produces a structural effect and increases wettability [67,69,70]. Therefore, more bubbles are more easily generated and desorbed from the surface. This mechanism is supported by Lee et al. [59], who studied the visualization of the CO₂ bubble generation when employing SiO₂ and Al₂O₃ nanoparticles to deionized water. The Al₂O₃ showed better bubble generation and desorption upon adding heat, in comparison to water and SiO₂ nanoparticles.

(3)

Increase in nucleation site density: In a fluid, bubbles are primarily generated at the small sites on the irregular surface (cavities, scratches, pits and cracks), which is called the “nucleation site”. As nanoparticles are deposited on the boiling surface, more nucleation sites are created. In addition to that, the floating nanoparticles, such as those on the heater surface, can also become bubble generation points. More CO₂ can be discharged as more regeneration sites are created. It has also been reported that the nucleation site density increases if the surface roughness is larger than the particle size, and the nucleation site density is reduced if the two values are similar [71].