Effect of Branching on Mutual Solubility of Alkane–CO2 Systems by Molecular Simulations

Abstract

Mutual solubilities of hydrocarbon-CO2 systems are important in a broad range of applications. Experimental data and theoretical understanding of phase behavior of large hydrocarbon molecules and CO2 are limited. This is especially true in relation to the molecular structure of hydrocarbons when the carbon number exceeds 12. In this work, the continuous fractional component Gibbs ensemble Monte Carlo simulations are used to investigate mutual solubility of different alkane and CO2 systems and the molecular structure. We investigate the mutual solubility of n-decane, n-hexadecane, n-eicosane, and the corresponding structural isomers in the CO2-rich and hydrocarbon-rich phase. The focus will be solubility of the heavy normal alkanes and their structural isomers in CO2. The simulation results are verified by comparing the experimental data when measurements are available. The simulation of phase behavior of the n-decane-CO2 system agrees with the experiments. We also present simulation results of n-hexadecane-CO2 and n-eicosane-CO2 systems away from the critical region partly due to the finite size effect. We establish that solubility of the hydrocarbons in CO2 is improved by change of the molecular structure in heavier alkanes. The enhanced solubility is limited in decane isomers, but the isomers of hexadecane and eicosane show 2- to 3-time solubility enhancement. The molecular dynamics simulations suggest that the improvement is from a higher coordination number of CO2 for methyl (CH3) rather than for methylene (CH2) groups. This study sets the stage for molecular engineering and synthesis of hydrocarbons that are soluble in CO2 not only by considering functionality but also by changing the molecular structure. The solubility enhancement is the first step in viscosification of CO2 which broadens the use of CO2.