How does structured adsorbent channel heterogeneity influence the efficiency of adsorptive CO2 capture?

Adsorbent monoliths are increasingly studied for carbon capture applications. Manufactured via extrusion, these monoliths often contain a heterogeneous distribution of channels, due to defects in the die or inhomogeneous drying. The effect of such heterogeneities on CO2 capture process performance is not well investigated. In this modelling work, the performance of a fixed bed, an ideal monolith and monoliths containing a wall size distribution are compared in a vacuum swing adsorption (VSA) cycle. At high adsorption gas velocities, using a monolith allows for a higher productivity (0.8 mmol/kg s) compared to a fixed bed (0.68 mmol/kg s) and a lower energy demand (190 kWh /ton CO2 versus 320 kWh/ton CO2). When the monolith contains a distribution of wall sizes, the process performance decreases. The recovery drops from 81 % to 65 % for the widest distribution studied (3σ=120%). Similarly, the throughput drops from 0.8 mmol/kg s to 0.6 mmol/kg s and the energy demand increases from 190 kWh /ton CO2 to 270 kWh/ton CO2. Our work thus shows that adsorbent monoliths can outperform a fixed bed of pellets in VSA cycles, leading to higher throughput and lower energy demand. The presence of defects, however, can severely impact performance.