## 3 Results and Discussion
### SCCO\({}_{2}\) Regeneration
#### 3.2.2 Mass Transfer Limitations: Effect of Time, Flow, and Pressure  
Whereas the thermal desorption pathway is exclusively affected by the chosen regeneration temperature, the extraction pathway is affected by other parameters apart from temperature. Within the previous section the importance of the extraction pathway in SCCO\({}_{2}\) regeneration has been probed, hence the search for those conditions which optimize this pathway is mandatory. An extraction step just becomes efficient when the limitations to the mass transfer are overcome. This overcoming is critical when the aimed compound is retained on highly microporous materials such as the ACF used herein. The majority of adsorption active sites are located on inner narrow micropores. This implies large mass transfer limitations, but the supercritical fluid must be able to reach those difficulty accessible zones to extract the phenol. Regeneration time, flow, and pressure can be properly modified to overcome these limitations. Figure 6 shows the effect of time on RE for regeneration at 100 bar and 250 \({}^{\circ}\)C with a SCCO\({}_{2}\) flow of 3 cm\({}^{3}\cdot\)min\({}^{-1}\). RE increased with time when the regeneration step was prolonged from 10 to 15 min. It is commonly admitted that the penetration of SCCO\({}_{2}\) in porous solids is easy and rapid. These results can be attributed to an external film resistance that slowed down the rate at which phenol could enter the bulk flowing fluid and be carried out of the adsorbent [22]. As the regeneration experiments were lengthened from 10 to 15 min, phenol had more time to diffuse and reach the fluid. Greater amounts of phenol were removed from the micropores, hence improving the efficiency of the method. Further prolongation did not lead to changes in RE; this parameter described a plateau over 15 min. Figure 6: Effect of time on RE for regeneration at 100 bar and 250 \({}^{\circ}\)C with a SCCO\({}_{2}\) flow of 3 cm\({}^{3}\cdot\)min\({}^{-1}\). From a technical point of view, the access of SCCO\({}_{2}\) to the inner micropores does not mean a problem for the regeneration procedure, in contrast to other regeneration methods like solvent regeneration or NaOH regeneration. In these methods, a rinse step must be carried out between the regeneration and the adsorption steps because the presence of the regenerating agent within the structure of the adsorbent hinders the subsequent adsorption [35; 36; 37; 38]. In a similar way to SCCO\({}_{2}\) regeneration, the regenerating agents must reach all the micropores to efficiently regenerate the exhausted adsorbents, what makes the rinse step trickier. This rinse step is not necessary to the method reported in this work: once the adsorbent is depressurized and cooled, the remaining CO\({}_{2}\) turns into gas and leaves the structure of the adsorbent on its own. Figure 7 shows the effect of the SCCO\({}_{2}\) flow on RE for regeneration at 100 bar and 200 \({}^{\circ}\)C during 15 min. RE increased with the SCCO\({}_{2}\) flow from 1 to 10 cm\({}^{3}\cdot\)min\({}^{-1}\) and remained unchanged when greater flows were used. The increase of RE with the regeneration flow is characteristic of solubility limited processes. Desorption at flow rates lower than 10 cm\({}^{3}\cdot\)min\({}^{-1}\) was primarily influenced by adsorption equilibrium constraints. The effluent concentration was fixed for each flow and it increased as greater amounts of SCCO\({}_{2}\) were pumped. Above 10 cm\({}^{3}\cdot\)min\({}^{-1}\), such high flows were pumped that the equilibrium was not achieved, and the use of greater flows did not lead to the removal of greater amounts of phenol. Above that flow, the process was no longer limited by solubility but mass transfer limited. This effect of SCCO\({}_{2}\) flow agreed with that reported by other authors [14; 15; 22]. Pressure is the last parameter that affects the mass transfers in an extraction step.