(47f) CFD Model Development of a Small-Scale Fixed-Bed Reactor for Direct Air Capture of CO2

Direct air capture (DAC) is a promising approach to significantly reduce the CO₂ concentration in the ambient air. This approach requires the extraction of CO₂ from air with ultra dilute CO₂ concentration which makes this approach more expensive than point source CO₂ capture. Several efforts are being made in academics and industry to develop cost-efficient sorbents and solvents suitable for DAC. One of them is polymer with intrinsic microporosity (PIM) which is often used for CO₂ capture because of its higher porosity and rigidity. A novel, cost-efficient aminated PIM sorbent, PF-15-TAEA, was developed at the National Energy Technology Laboratory for DAC applications. The current work is focused on the computational investigation of the CO₂ capture by this novel sorbent from a gas mixture with ultra dilute concentration of CO₂ in dry and humid conditions in a laboratory-scale fixed-bed reactor.

Computational fluid dynamics (CFD) was used to model the adsorption of CO₂ by the PIM sorbent in laboratory-scale fixed-bed reactor. The effects of the presence of solid sorbents in the reactor were incorporated using the porous media model (PMM). The parameters of the PMM were calibrated using the pressure drop across the fixed bed vs. inlet gas velocity data obtained from the experiments. A kinetic model was developed to model the adsorption of CO₂ by solid sorbents from the gas mixture using the experimental data which was included in the CFD model through a user defined function (UDF). The adsorption kinetic model was calibrated by comparing the CO₂ breakthrough curves obtained from experiments and CFD simulations. ANSYS Fluent software was used to perform all the CFD simulations.

Initially, the pressure drop across the bed and the corresponding inlet gas velocity obtained from the experiments was used to present the pressure drop as second-order polynomial (with intercept 0) of velocity which was eventually used to calculate the parameters of the porous media model. This ensured that the pressure drop in the CFD simulations remained in close agreement with the corresponding experimental data. A second-order CO₂ adsorption kinetic model for dry conditions was developed where the equilibrium CO₂ loading was computed by fitting the CO₂ isotherm data obtained from experiments at different temperatures to Langmuir-Freundlich isotherm model. Several CFD simulations were performed with 500 ppm CO₂ in the incoming gas mixture by varying the value of the rate constant of the second-order CO₂ adsorption kinetic model and the CO₂ breakthrough curves obtained from the CFD simulations were compared with the same obtained from the experiments at the same operating conditions. The rate constant value for which the best agreement was obtained between the CO₂ breakthrough curves obtained from experiments and simulations was chosen as the rate constant for second-order CO₂ adsorption kinetic model. This CFD model was used to investigate the effect of inlet gas velocity on the CO₂ adsorption by the aminated PIM sorbents in dry conditions in the fixed-bed reactor.

The PMM-based CFD model was able predict CO₂ adsorption by aminated PIM sorbents from dry gas mixture with 500 ppm CO₂ in a laboratory-scale fixed-bed reactor. Current work is investigating CO₂ adsorption by the aminated PIM sorbent in humid conditions in the laboratory-scale fixed-bed reactor. Our initial experiments show significant improvement in the CO₂ capture performance because of increase in equilibrium CO₂ loading of the aminated PIM sorbent. The CFD model is currently being extended to simulate CO₂ adsorption by the aminated PIM sorbents in humid conditions.