(494h) Computational Modeling of CO2 Capture By Novel PIM-RU in a Fluidized Bed Riser

The interest in carbon capture, storage, and utilization (CCUS) has increased significantly in the past few decades as it can help mitigate the threat of global warming caused by substantial increase in CO₂ emissions due to anthropogenic activities. Although several CO₂ capture technologies have been developed, porous solid sorbents, which adsorb CO₂ by physisorption, are considered promising candidates for post-combustion CO₂ capture because of the easier recovery of adsorbed CO₂ and high material stability. Some prominent types of porous solid sorbents are metal-organic frameworks (MOFs), Zeolite, mesoporous silica, and polymer-based sorbents (e.g., polymers with intrinsic microporosity or PIM). Oak Ridge National Laboratory (ORNL) recently developed a novel PIM-based sorbent, referred to here as PIM-RU. The main objective of this work is to investigate the CO₂ capture performance of this sorbent using computational fluid dynamics (CFD). While process configuration and reactor design are open questions, a fluidized bed riser was selected as the contactor type for this study.

CO₂ adsorption by solids sorbents is a complicated multiphase gas-solids flow problem involving interphase momentum, heat, and mass transfer. The Eulerian-Eulerian and Eulerian-Lagrangian methods are the two primary approaches for CFD modeling of such systems. In this effort, an Eulerian-Eulerian model, also known as the two-fluid model (TFM), was selected for its greater computational efficiency – a requisite for exploring the performance of pilot/industrial scale reactors. In this approach, both the gas and particle phases are treated as interpenetrating continua. NETL’s open-source software MFiX (https://mfix.netl.doe.gov/) was used to perform all the simulations, which incorporates appropriate sub-models for describing interphase gas-solids momentum and heat transfer. To describe the interphase mass transfer resulting from CO₂ adsorption by the PIM-RU, a linear driving force (LDF) model was developed and implemented in MFIX through a user defined function (UDF).

Experimental isotherm and kinetic data provided by ORNL for PIM-RU was used to create first- and second-order LDF models. The second-order kinetic model provided a slightly better fit to the experimental data. Implementation of first- and second-order adsorption kinetic rate equations was verified using a 0D simulation setup. However, only the second-order kinetic model was used in subsequent simulations. Namely, the CO₂ capture performance (i.e., CO₂ conversion of the PIM-RU sorbent from a mixture of CO₂ and N₂) in a small pilot scale reactor was assessed using the verified TFM. This contactor design was based on that by ADA-ES and Southern Company for a CO₂ capture reactor (Sjostrom et al., 2011), with consideration of only the riser component, as this is where most of the CO₂ capture occurs. A quasi steady-state CO₂ conversion of around 20-25% was observed for inlet temperature, pressure, and CO₂ mole fraction of 25 °C, 1 atm, and 0.1, respectively, which is significantly smaller than the desired 90% CO₂ conversion. Initial analysis suggests that higher operating pressures are necessary for achieving higher conversion.

CO₂ capture performance of a novel PIM-RU sorbent in a small pilot scale reactor was assessed using the TFM. Initial CO₂ capture was well-below the desired target. However, a higher operating pressure may improve CO₂ capture since it increases the equilibrium CO₂ loading of the sorbent. Higher pressure has a few other notable consequences: elutriation of sorbent particles will occur at lower gas velocity, and CO₂ mass flow rate to the reactor will increase. Current work is examining CO₂ capture performance at higher operating pressure to determine the optimum conditions.

Reference

Sjostrom, S., Krutka, H., Starns, T., and Campbell, T., Pilot test results of post-combustion CO2 capture using solid sorbents, Energy Proc. 2011. 4: p. 1584–1592