## 4 Results and Discussion
It was observed from Figure 12c that the PP behavior predicts that, at a low temperature (293 K), \(S_{\text{CO}_{2}\text{CH}_{4}}\) is constant at 3.25 and 2.33 in the cases of a low or a high pressure (0.1 and 0.5 MPa), respectively. Nonetheless, at higher temperatures (313 and 333 K), \(S_{\text{CO}_{2}\text{CH}_{4}}\) increases when increasing \(x_{\text{CO}_{2}}\), especially at 333 K when \(x_{\text{CO}_{2}}\) > 0.8. Moreover, \(S_{\text{CO}_{2}\text{CH}_{4}}\) was higher at minimal pressure (0.1 MPa) than that at elevated pressure (0.5 MPa), and no azeotropes were predicted at both pressures. On the other hand, it can be seen from Figure 12d that the \(S_{\text{CO}_{2}\text{CH}_{4}}\) predicted by the PN behavior at low temperature was constant at 1.5, regardless of \(x_{\text{CO}_{2}}\) or pressure. However, at higher temperatures (313 and 333 K), it decreases when increasing \(x_{\text{CO}_{2}}\) at both minimal and elevated pressures (0.1 and 0.5 MPa). At a moderate temperature (313 K), the value of  
Figure 12: Predicted binary adsorption gas mole fractions (**a**,**b**) and selectivity of adsorbing CO\({}_{2}\) from CH\({}_{4}\) (**c**,**d**) onto the sample S50 at 0.1 and 0.5 MPa (black and red lines, correspondingly). Subfigures a and c represent PP behavior and the subfigures b and d represent PN behavior. \(S_{\text{CO}_{2}\text{CH}_{4}}\) predicted at elevated pressure (0.5 MPa) is lower than that at minimal pressure (0.1 MPa) when \(x_{\text{CO}_{2}}>\) 0.65. Furthermore, at a high temperature (333 K), the value of \(S_{\text{CO}_{2}\text{CH}_{4}}\) at elevated pressure (0.5 MPa) is lower than that at minimal pressure (0.1 MPa) when \(x_{\text{CO}_{2}}>\) 0.56. The azeotropic behavior appeared at \(x_{\text{CO}_{2}}\) of \(\sim\)0.85 at moderate temperature (313 K) and high P (0.5 MPa), and at \(x_{\text{CO}_{2}}\) of \(\sim\) 0.55 at high temperature (333 K) for both pressures. The selectivity in both behaviors (i.e., PP and PN) is affected significantly by the structure adsorbents, adsorption capacity and temperature. Increasing surface area and pore volume (as presented in Table 1) enhance the adsorption capacity. This is due to an increase in the AC-RFA content in the mixed adsorbent, which makes its structure more amorphous. Furthermore, it is reported in the literature that an incompetence of MWCNTs persists for the adsorption or selectivity of gases [47; 48; 49], while activated carbons generally expose good adsorption capacities and selectivities of gases [50; 51; 52]. Therefore, adding AC-RFA to MWCNTs enhances the adsorption capacity and selectivity based on adsorption isotherms as preliminary data. It is to be noted that the recent data of only S50 are not enough to discuss the full range of MWCNTs to AC-RFA ratios and their effects on selectivity, specifically. Figure 13a-d illustrates the predicted relationship between pressure and adsorption performance (i.e., Figure 13a,c exposing \(n_{\text{CO}_{2}}\) and \(n_{\text{CH}_{4}}\) and Figure 13b,d exposing \(S_{\text{CO}_{2}\text{CH}_{4}}\) for an equimolar gas mixture of CO\({}_{2}\) and CH\({}_{4}\) at different temperatures for the (a, b) PP and (c,d) PN behaviors. Figure 13a shows that both \(n_{\text{CO}_{2}}\) and \(n_{\text{CH}_{4}}\) predicted by the PP behavior increase by either decreasing the temperature or by increasing the pressure. Moreover, it is observed from Figure 13b that the \(S_{\text{CO}_{2}\text{CH}_{4}}\) predicted by the PP behavior increases by increasing the exposed pressure at 313 and 333 K, but remains constant at 293 K. The value of \(S_{\text{CO}_{2}\text{CH}_{4}}\) increases when decreasing temperature, especially at low to moderate pressures. Figure 13c,d exposes the PN behavior relationship between \(n_{\text{CO}_{2}}\) and pressure for an equimolar feed gas mixture of CO\({}_{2}\) and CH\({}_{4}\) at 293, 313 and 333 K. It is noticed that the \(n_{\text{CO}_{2}}\) and \(n_{\text{CH}_{4}}\) both increase with increasing pressure at all temperatures.