(209b) A Model for Packed and Fluidized Bed Absorbers with Micro-Encapsulated CO2 Sorbents

Micro-encapsulated CO₂ sorbents (MECS) are a carbon capture technology that possesses the benefits of both liquid solvents and solid sorbents. They consist of a CO₂-absorbing liquid or slurry, encased in spherical, gas-permeable, polymer shells. By greatly increasing the surface area of the liquid, microencapsulation enables the use of slow-reacting, viscous, and phase-changing solvents to reap the benefits of good thermodynamic properties, low water content, and/or low volatility. However, in terms of process design, MECS have more in common with solid sorbents. They behave as a light powder, with particle diameter adjustable from ~50-600 µm. Several candidate solvents have been successfully encapsulated and tested for longevity, CO₂ capacity, and CO₂ absorption rates. One of the most promising candidates is Na₂CO₃ solution, due to its stability, low cost, and CO₂ capacity. The rate of CO₂ absorption in this sorbent is well-characterized and known to be relatively slow. However, the CO₂ absorption rate can be enhanced by catalysts or promoters, such as the Zn complex of 1,4,7,10-tetraazacyclododecane (“cyclen”), a biomimetic catalyst. Encapsulated Na₂CO₃ solution with cyclen is the capsule design that is implemented in all the experiments and models presented here. However, these results and conclusions can be readily adapted to other solvents (e.g. ionic liquids) that have been successfully encapsulated in the lab.

The goal of this work is to estimate the size of a large absorber filled with these Na₂CO₃ + cyclen capsules that have been developed at the bench scale. In order to accomplish this task, a model is first developed for the CO₂ diffusion and reaction in an individual capsule. This model is fitted to match in-house experimental CO₂ absorption data for small batches of capsules. This individual capsule model is then validated against experimental data collected on a larger batch of capsules fluidized with simulated coal flue gas. Matlab models are then developed for two absorber bed designs: 1) a multi-stage counter-flow fluidized bed, and 2) an annular packed bed with radial flow. These two bed models are then optimized for minimum reactor size at both the pilot (1 MW) and power plant (500 MW) scales. Based on that optimization, a modular packed bed absorber is proposed for a 500 MW coal plant that has similar dimensions to state-of-the-art MEA absorbers under the same conditions. This 500 MW absorber design has the advantage of being highly modular, so the layout and dimensions of the system are flexible. Furthermore, these large vessels are solely filled with beads of Na₂CO₃ and water: cheap, abundant materials that demonstrate great potential for large-scale manufacturing.

Finally, a parametric study is performed in order to guide future capsule design based on absorber size. The results of that study demonstrate that the absorber size can be dramatically reduced by a combination of the following capsule improvements: 1) making the capsules smaller to increase total surface area, 2) making the capsule shell thinner to maximize solvent volume fraction, and/or 3) using a different solvent to raise the mass transfer coefficient. By making modest improvements to these three design parameters, the absorber bed diameter could be reduced from 21.5m to 17.5m, making this absorber narrower than a conventional MEA tower.