Bi-disperse adsorption model of the performance of amine adsorbents for direct air capture

•Bi-disperse model is proposed to identify film and intraparticle mass transfer.•The predicted adsorption breakthrough achieves RMSE under 45 ppm.•Adsorption kinetics are independent of concentrations and geometry under diluted CO2.•Contribution of mass transfer steps on breakthroughs is found by ANOVA analysis.•Pareto-optimal solutions is achieved in DAC modelling considering pellet sizes. The fixed bed containing adsorbent pellets is commonly applied in direct air capture (DAC) reactors. Due to insufficient kinetics of adsorbents under diluted CO2 concentrations, a concentration gradient occurs and limits an accurate prediction of adsorption progress. Herein, a bi-disperse adsorption model is simplified and utilized in the breakthrough modelling of amine adsorbents under DAC conditions, identifying the interaction among film mass transfer, intraparticle diffusion, and amine loading reactions. It indicates that the proposed model can reasonably fit the experimental results with root mean square error (RMSE) at 45 ppm, which proves that the kinetics measured are intrinsic parameters of the adsorbents. The controlling mass transfer step at 298 K is the amine loading reaction, while the length of marcroporous diffusion dominates the mass transfer at 308 and 318 K. A new dimension to conduct multi-objective optimization of DAC processes is finally introduced with consideration of pellet diameter. The optimized diameter convergences to a lower bound at 0.80 mm. Pareto-optimal solutions of productivity and recovery are located at a correlation of gas velocities and diameters, improving productivity from 0.89 to 0.97 kg·m−3·h−1, but the energy consumption during adsorption increases 5.78 times. These findings highlight the significance of shaped adsorbents in DAC modelling and will add a new dimension to pellet geometry for process optimization.