(816a) Steady-State and Dynamic Modeling of a Moving Bed Adsorber/Regenerator for Solid Sorbent CO2 Capture

Moving bed reactors hold strong promise for solid sorbent-based CO₂ capture processes by enabling larger working capacities than other reactor types such as fixed- and fluidized-bed reactors. A first-principles model of a moving bed reactor can be used to investigate tradeoffs between various design alternatives and determine the optimal configurations, process conditions, and sorbent types. With this motivation, one-dimensional (1D), two-phase, non-isothermal, steady-state and dynamic models of a moving bed reactor have been developed for use in the U.S. Department of Energy’s Carbon Capture Simulation Initiative (CCSI). The models are flexible over a wide range of operating conditions such as gas and solids flowrates and compositions, and operating temperatures. They can be used to represent both an adsorber for CO₂ capture and a regenerator for removing CO₂ from solid sorbents.

In these moving bed reactors, solid sorbent particles move downward due to gravity, contacting a rising gas stream in a counter-current configuration. Since the adsorption reaction is exothermic, an immersed cooler is considered in the adsorber for enhancing solid sorbent loading. An immersed heater is considered in the regenerator to maintain a near-isothermal operating condition. An integral heat-recovery system is used for recovering heat from the hot sorbent leaving the regenerator. The recovered heat is then used to preheat the solid sorbent from the adsorber. The reaction kinetics, heat and mass transfer, and the hydrodynamics are considered in the beds and in the heat recovery system for the regenerator. Sorbent NETL32D, a solid sorbent developed by NETL, is modeled in this work. The kinetic model considers three reactions and has been developed by lumping the internal mass transfer resistances. To capture the mass transfer resistance from/to the bulk to/from the solid sorbent surface, especially in the regenerator that operates at higher temperature, external mass transfer resistances have been modeled. The hydrodynamics of moving beds have not been widely studied, so most coefficients have been derived by analogy with correlations for fixed and fluidized bed systems. The gas flow is modeled by convective gas movement with axial dispersion. For modeling heat exchange between the moving bed and the embedded heat exchangers, heat transfer coefficients have been derived by considering an enhanced packet renewal theory.  

The system of partial differential algebraic equations with the appropriate boundary conditions is solved in Aspen Custom Modeler (ACM) using the well-known method of lines. The effect of operating conditions and design parameters on the performance of the adsorber/regenerator reactor has been studied using the steady-state model. Using the dynamic model, a number of disturbances are simulated to study the transient response of the moving bed adsorber/regenerator.