Adsorption and Diffusion of Methane and Carbon Dioxide in Amorphous Regions of Cross-Linked Polyethylene: A Molecular Simulation Study

Abstract

We perform Monte Carlo (MC) and molecular dynamics (MD) simulations to study the adsorption and diffusion properties of methane and CO2 in cross-linked polyethylene in the temperature range 300–600 K. A hybrid MC/MD approach was used to incorporate the effects of framework flexibility and polymer swelling on the gas adsorption. The polymers show negligible swelling at the studied conditions. A nonmonotonic behavior of gas adsorption as a function of the cross-linking degree was obtained. Notably, a similar behavior was observed for the void fraction and pore diameters. This shows a direct correlation between gas adsorption and the pore characteristics of the cross-linked polymer network. Mobility of methane and carbon dioxide in the polymer matrix increases with temperature. Also, gas mobility decreases with increasing cross-linking degree, consistent with experiments. These results can be explained by the fact that the waiting time for a gas molecule in a cavity before the jump increases with decreasing temperature and increasing cross-linking degree. Interestingly, the activation energy for gas diffusion generally decreases with increasing cross-linking. This is possibly due to the fact that increasing the cross-linking degree leads to smaller pore sizes especially at high temperatures. Such a molecular-level understanding of adsorption and diffusion of gases in cross-linked polyethylene is important in improving the performance of polymer networks for potential applications in gas separation, barrier technology, and food packaging.

Numerical Study of CH4 Generation and Transport in XLPE-Insulated Cables in Continuous Vulcanization

“…Since the crystalline structure may significantly affect the overall diffusion property in the XLPE [30][31][32], knowing the effect of phase changes on the diffusion coefficient is important for the accurate analysis of byproduct generation and transport. In this study, hence, we combined two available diffusion coefficient profiles that represent the experimentally measured semi-crystalline structures [17] and the amorphous phases, as estimated by molecular dynamic simulations [33][34][35] to cover the different temperature ranges. Figure 5 shows the diffusion profile obtained using this approach.…”

“…Lastly, the molecular dynamic simulation results from Dutta and Bhatia [39] show that CH 4 concentration has a negligible effect on CH 4 diffusion in PE. [13] (red line) and amorphous [35] (green line) phases of the XLPE, which were then combined to represent the diffusion coefficient variation applied in the simulation (black line).…”

“…However, we chose not to use this method mainly due to lack of available data to characterize CH diffusion based on the cable manufacturing condition used in this study. Also, the simulation results from [ 35 ] show that only a small amount of cross-linking density is observed for these cable manufacturing conditions (approximately 4.2% for system with 2.5% peroxide content), and under these conditions, cross-linking density has a minor effect on the CH diffusion. Lastly, the molecular dynamic simulation results from Dutta and Bhatia [ 39 ] show that CH concentration has a negligible effect on CH diffusion in PE.…”

“… CH diffusion coefficient variations in XLPE as semi-crystalline [ 13 ] (red line) and amorphous [ 35 ] (green line) phases of the XLPE, which were then combined to represent the diffusion coefficient variation applied in the simulation (black line). …”

A 6M digital twin for modeling and simulation in subsurface reservoirs

“…Molecular simulation of microscopic mechanisms: As an effective approach to investigate the microscopic mechanisms affecting macroscopic flow and transport behaviors as well as to obtain the value of key parameters in numerical modeling, molecular simulation has attracted increasing attentions. The diffusion and sorption behaviors of carbon dioxide and methane as well as the structural features were studied using molecular dynamics and hybrid Monte Carlo approaches (Kadoura et al, 2017;Yang et al, 2017aYang et al, , 2019b. The intercalation behavior of carbon dioxide in various brines were studied using grand canonical Monte Carlo methods to study the molecular mechanisms indicating that the intercalation of carbon dioxide strongly depends on the relative humidity (Li et al, 2019c).…”