Diffusion of CO₂/CH₄ confined in narrow carbon nanotube bundles

ABSTRACT

The diffusion of a CO₂/CH₄ mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO₂ and CH₄ are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO₂ and CH₄ molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO₂ in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO₂ and CH₄ in the pores. For CO₂, the interaction energies (CO₂–CO₂ and CO₂–CNT) are larger than that of CH₄ in all cases. But only a very small difference in the diffusion coefficient was observed between CO₂ and CH₄. Temperature has a negligible effect on the difference between diffusion coefficients of CO₂ and CH₄ in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO₂/CH₄ in CNT bundle membranes.

KEYWORDS:

• CO₂/CH₄
• diffusion
• molecular simulation
• carbon nanotube bundle

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

National Basic Research Program of China [Grant Number 2015CB655300]; National Natural Science Foundation of China [Grant Numbers 21176113, 21206070, 91334202, 21490584]; Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Liangliang Huang: State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University (KL13-01)