## 3 Research Progress on Adsorption and Separation of Hydrocarbon Molecules
### n-Alkanes
They used gas chromatography to analyze the sample, taking the injection volume of the desorption solution as the abscissa and the mass fraction of various hydrocarbon components as the ordinate (The mass fraction of each alkane in pulse solution and desorption solution is 5%, and isooctane is used as diluent for the rest). The degree of separation \(R\) is calculated to represent the separation effect of n-alkane and 2-methylentane. The pulse curves and corresponding data of nC\({}_{11-16}\) at different adsorption temperatures are shown in Figure 5 and Table 2, respectively. At 120 \({}^{\circ}\)C, with the increase of carbon chain length, the value of \(\Delta V_{R}\) decreased significantly, and the peak gradually widened, that is, the value of \(W_{1/2}\) gradually increased, which indicated that the adsorption performance of n-alkanes with a long carbon chain was poor, and the adsorption rate was slow at this temperature. When the temperature gradually increased to 180 \({}^{\circ}\)C, \(\Delta V_{R}\) gradually increased and reached the maximum value, and the value of \(W_{1/2}\) also gradually decreased, indicating that the adsorption capacity and adsorption rate for nC\({}_{11-16}\) were significantly improved. However, when the temperature reached 200 \({}^{\circ}\)C, the value of \(\Delta V_{R}\) decreased, and the value of \(R\) also reached the maximum, indicating that the adsorption capacity began to weaken. Therefore, it could be concluded that the optimal adsorption temperature for the separation of n-alkanes by 5A molecular sieve was 180 \({}^{\circ}\)C, which was also the same as the previous report by [PERSON]'s group. For desorption, the choice of desorbent is also important, which has a great influence on adsorption and desorption performance. The \(\Delta V_{R}\), \(W_{1/2}\), and \(R\) values of nC\({}_{10-16}\) when n-alkanes with different chain lengths were used as desorbents are shown in Figure 6. According to Figure 6a, it could be seen that when the carbon number is 5-8, the \(\Delta V_{R}\) value of nC\({}_{10-13}\) first increased and then decreased, and it reached the maximum value when the number of carbon atoms was 7, while for nC\({}_{14-16}\), the value obviously showed a decreasing trend. In Figure 6b, the value of \(W_{1/2}\) of nC\({}_{10-13}\) floats a little bit, indicating that its adsorption rate was less affected by the carbon chain length of the desorbent, while the value of nC\({}_{14-16}\) was gradually decreased, indicating that the increase of the carbon number of desorbent is conducive to desorption. Finally, according to Figure 6c, it could be seen that although the \(R\) values of n-alkanes with different carbon numbers change differently, they all reach the maximum value when the carbon number is 7. To sum up, when nC\({}_{7}\) is used as desorbent, nC\({}_{10-16}\) has better desorption performance. Figure 4: Relationship between oil/sieve ratio and n-paraffin mass fraction at different space velocities [35], Oil/sieve ratio: raffinate oil/molecular sieve. Figure 5: Pulse spectra of nC\({}_{11-16}\) at different adsorption temperatures [36]. (a) 120 \({}^{\circ}\)C (b) 150 \({}^{\circ}\)C (c) 180 \({}^{\circ}\)C (d) 200 \({}^{\circ}\)C.