## 3 Results and Discussions
Blank runs are reported as a reference. Arrows signal the time of the first detection. Both diagrams show that, in the blank runs, the CO2 and CH4 signals are indistinguishable. On the contrary, in the presence of a hydrochar bed, a selectivity appears evident, as proven by the temporal separation of the arrows. Methane appears first in the column outlet regardless of the operating pressure. The delay between the two signals is an increasing function of the operating pressure, and in any case, it is sufficiently broad for envisaging the development of an industrial process. All of this evidence proves that hydrochar is a suitable medium for separating the mixture by selective adsorption. Table 4 quotes the obtained selectivities and recoveries calculated using Equations (1)-(3). The obtained selectivities confirm that CO2 is preferred to CH4. The performance of hydrochar is worse than that obtainable with commercial porous sorbents such as zeolites or activated carbons [22]. As expected, the methane recovery is relatively low with a purity of 95% (Italian regulation for network injection) regardless of the investigated operating pressure. The doubling of recovery obtained with a purity of 70% is a valuable result because of energetic applications. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**TEST**} & **P** & **Sorbent Capacity \({}^{*}\)** \\  & & **[bar]** & **[mmol/g]** \\ \hline \multirow{3}{*}{HC\_200\_120} & \multirow{3}{*}{CO\_2/N\_2} & 2 & 0.335 \(\pm\) 0.129 \\  & & 3 & 0.425 \(\pm\) 0.181 \\  & & 4 & 0.688 \(\pm\) 0.210 \\  & & 5 & 0.692 \(\pm\) 0.228 \\ \hline \multirow{3}{*}{HCA\_200\_120} & \multirow{3}{*}{CO\_2/N\_2} & 2 & 2.651 \(\pm\) 0.225 \\  & & 3 & 3.366 \(\pm\) 0.292 \\  & & 4 & 3.475 \(\pm\) 0.368 \\  & & 5 & 3.635 \(\pm\) 0.268 \\ \hline \multirow{3}{*}{HCA\_200\_0} & \multirow{3}{*}{CO\_2/N\_2} & 2 & 4.957 \(\pm\) 0.952 \\  & & 3 & 5.658 \(\pm\) 0.070 \\ \cline{1-1}  & & 4 & 6.199 \(\pm\) 0.081 \\ \cline{1-1}  & & 5 & 6.569 \(\pm\) 0.119 \\ \hline \hline \end{tabular} \({}^{*}\) Mean \(\pm\) standard deviation.  
\end{table} 
Table 3: CO2 sorbent capacity. Figure 4: CO2/CH4 adsorption curves at (a) 2 bar and (b) 5 bar. Figure 5 reports the averaged CO2 sorbent capacity for the two samples of activated hydrochar as a function of the corresponding partial pressure in the gas phase. Data fit well to the Langmuir equation [47]. \[\text{CO}_{\text{2 Sorbent capacity}}=\big{(}\text{C}_{\text{Max}}\text{P}_{ \text{CO}_{\text{2}}}\big{)}/\big{(}\text{K}+\text{P}_{\text{CO}_{\text{2}}} \big{)} \tag{4}\]  
The regressions are reported as solid lines in the explored range and as dotted lines in the extrapolated ones. A wider partial pressure range should be explored to ascertain the correct equilibrium law. For the present paper, this preliminary investigation gives valuable information for steering future studies. The regression parameters are reported in Table 5.  
Figures 6 and 7 compare the performances of the hydrochars to those of traditional solid materials and those of some innovative materials, as reported in the literature [48; 49; 40; 41; 42; 43; 44; 45; 22; 40; 46; 47; 48; 49; 50; 51; 52; 53; 54]. As a general finding, data on PSA at 1 bar appear in the literature sparingly, even for traditional sorption media. Figure 6 reports a possible comparison of the hydrochar sorption capacity. It appears that, despite the different capture techniques, the data of \"HCA_200_0_calc\" at 1 bar (calculated by Equation (4)) are comparable with those of the literature. On the other hand, the comparison with PSA experiments conducted on