Solubility and Diffusion of Main Biogas Components in a Glassy Polysulfone-Based Membrane

Area of Science:

• Materials Science
• Chemical Engineering
• Renewable Energy

Background:

• Biogas, a controllable renewable energy source, requires efficient separation of methane (CH4) and carbon dioxide (CO2).
• Membrane processes are key for biogas upgrading, but understanding gas-membrane interactions like sorption and swelling is crucial for optimization.
• Polysulfone-based membranes are employed in adsorptive-membrane systems for biogas separation.

Purpose of the Study:

• To analyze the phenomena governing CH4-CO2 separation using a polysulfone membrane.
• To describe the solubility and diffusion of CO2, CH4, and their mixtures in the polysulfone material.
• To evaluate the impact of competitive sorption and swelling on membrane performance.

Main Methods:

• Utilized the Dual Mode Sorption and partial immobilization models to describe gas solubility and diffusion.
• Determined model parameters using pure-gas sorption isotherms measured gravimetrically.
• Analyzed permeances of CO2/CH4 mixture components from prior studies.

Main Results:

• Observed membrane swelling due to CO2 at pressures above 5 bar.
• Found that real CO2/CH4 permselectivity is lower than ideal selectivity due to reduced CO2 diffusion in the presence of CH4.
• Demonstrated higher CO2/CH4 solubility selectivity in mixtures compared to pure gases, with CO2 dominating sorption and displacing CH4 from free volume.

Conclusions:

• CO2 exhibits a stronger affinity for the polysulfone membrane, leading to competitive sorption and displacement of CH4.
• Solubility, rather than diffusion, is the dominant factor influencing gas separation in this system, accounting for 65-73% of total solubility.
• The findings provide critical insights for optimizing membrane-based biogas upgrading processes.