Supercritical fluids in porous composite materials: direction-dependent flow properties

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Engineering

Background:

• Understanding fluid flow and transport in porous media is crucial for applications like separations and catalysis.
• Supercritical fluids exhibit unique properties, but their behavior in complex pore structures remains challenging to predict.
• Porous materials with hierarchical pore sizes (macro-, meso-, and nanopores) present intricate pathways for fluid transport.

Purpose of the Study:

• To investigate the flow and transport of pure and binary fluid mixtures through a composite porous material under supercritical conditions.
• To identify and explain unusual phenomena, specifically direction-dependent and pressure-dependent permeabilities.
• To provide insights for accurate modeling of supercritical fluid behavior in engineered porous materials.

Main Methods:

• Extensive nonequilibrium molecular dynamics (NEMD) simulations were employed.
• Simulations covered pure and binary fluid mixtures flowing through a macropore-mesopore-nanopore composite structure.
• Results were compared with a continuum formulation to validate findings.

Main Results:

• Unusual direction-dependent and pressure-dependent permeabilities were observed for fluid components under supercritical conditions.
• The composite nature of the porous material, combined with fluid condensation, was identified as the cause of anisotropic permeability.
• Simulation results showed agreement with continuum models, supporting the observed phenomena.

Conclusions:

• Modeling supercritical fluid flow and transport in hierarchical porous materials necessitates the use of effective permeabilities.
• These effective permeabilities must account for both the magnitude and direction of the applied external pressure gradient.
• Findings are particularly relevant for applications involving supported porous membranes operating in the supercritical regime.