Kinetic model for moisture-controlled CO₂ sorption

Area of Science:

• Materials Science
• Chemical Engineering
• Physical Chemistry

Background:

• Understanding sorption/desorption kinetics is crucial for effective moisture-controlled CO2 capture.
• Existing models may not fully capture the complex interplay of reaction and diffusion in sorbent materials.

Purpose of the Study:

• To develop and validate an analytic model for moisture-controlled CO2 sorption kinetics.
• To differentiate between surface reaction kinetics and interior diffusive transport as rate-limiting steps.
• To establish a generalized effective diffusivity (DM) that unifies these kinetic contributions.

Main Methods:

• Development of an analytic kinetic model for CO2 sorption.
• Analysis of two limiting cases: surface reaction dominance and interior diffusion dominance.
• Mathematical formulation combining carbon species and simplifying diffusion equations.
• Experimental validation using transient CO2 flux measurements in a commercial anion exchange membrane.

Main Results:

• The model accurately describes reaction kinetics as a combination of 1st and 2nd order processes.
• Interior transport kinetics are governed by non-linear diffusion equations.
• A generalized effective diffusivity (DM) was introduced, combining surface reaction and diffusion.
• Experimental data fit the 1st order model, with DM values indicating slow kinetics (6.6-7.1 × 10^-14 m²/s at 35°C).

Conclusions:

• The developed model provides a unified framework for understanding CO2 sorption kinetics.
• Slow kinetics, particularly diffusion and reaction rates, represent a significant barrier to practical CO2 capture applications.
• The model's predictions of CO2 pumping flux in active membranes highlight potential for CO2 transport against concentration gradients, driven by water flux.