Permeation of high pressure CO2 in semicrystalline polymers

• Main Author: Hu, Tianyi
• Format: Dissertation
• Language: English
• Published: Imperial College London 2021
• DOI: 10.25560/93403

With climate change becoming a major global concern, carbon capture and storage (CCS) has been the subject of intense research activity around the world. Although the benefits of CCS have been well recognised for decades, extensive industrial applications have not been realised. Particularly, there is a gap in research on the handling of high pressure CO2, despite the abundant work dealing with lower pressure CO2. This thesis focuses on the permeation process of CO2 through semicrystalline polymeric materials under high pressures, to gain a better understanding on the diffusion and sorption mechanisms in the CO2-polymer systems. The work presented in this thesis provides fundamental data, which could be used to guide the selection of suitable polymeric seals and liners in CCS applications, and may also be applied to other permeation-related processes involving CO2-polymer systems under high pressures. This thesis develops new experimental and modelling techniques to investigate the permeation behaviour of CO2 through semicrstalline polymers under high pressures. (i) An experimental setup for permeation measurements was designed and constructed. (ii) Continuous permeation experiments of CO2 in two commonly used semicrystalline polymers, polytetrauoroethylene (PTFE) and high density polyethelene (HDPE), were carried out at various temperatures ranging from 28 to 50 °C and pressures up to 600 bar. (iii) The experimental data was fitted to a diffusion model, which incorporated the free volume theory to describe the concentration-dependence of the diffusivity. The model was first nondimensionalised, and then simplified using assumptions, which were subsequently shown to be valid. A simple graphical method, which requires no numerical equation solvers, was proposed for model validation and parameter estimation. The obtained results were in excellent agreement with those obtained with the numerical solver. (iv) The effect of pressure on diffusion was characterised from the near-instantaneous flux drops, which were observed when the pressure was increased following steady states of permeation. (v) The solubilities derived from the permeation measurements were fitted to the Sanchez-Lacombe equation of state, and excellent fits were obtained between the two. On the basis of the experimental and modelling results, transport properties and free-volume parameters were successfully estimated; these were compared with data found in the literature under similar conditions.