(35c) Computational Fluid Dynamics (CFD) Modeling of Wetted Wall Absorption Columns for Solvent-Based Post-Combustion Carbon Capture Applications

Solvent-based post-combustion carbon capture systems have the potential to reduce atmospheric CO₂ emissions from concentrated point sources such as fossil fuel-fired power plants and industrial sources. Among such technologies, amine-based absorption of CO₂ is a widely researched alternative for scrubbers that can be retrofitted to existing sources [1,2]. The optimal performance of such devices is currently limited in part due to the impact of thermal fluctuations, produced by heat released during the absorption process, on the rate of CO₂ absorption. This is due to the temperature dependence of the solvent reaction kinetics as well as the thermodynamic and transport properties of the fluids. To understand the mechanisms that limit the absorption performance and accurately predict the resulting change in temperature, a detailed CFD study of a solvent-based wetted-wall absorption column that incorporates the intricate coupling between hydrodynamics, reaction kinetics, and the interfacial heat and mass transfer effects is required.

Whereas previous CFD studies have investigated the hydrodynamics of the absorption process by quantifying the liquid holdup and interfacial area in absorption columns with structured packings [1], the effects of heat release from CO₂ absorption and the corresponding impact on absorption performance are not well understood. To determine the extent of these effects among other mechanisms that affect the column performance, we aim to quantify the impacts on interfacial mass transfer from several different process parameters, including those due to temperature variations from heat release and interfacial heat transfer for CO₂ absorption in monoethanolamine (MEA) solvent. To this end, we developed a two-phase CFD model, wherein the reaction kinetics, temperature and composition dependent thermophysical properties of the MEA-H₂O-CO₂ system are initially incorporated from the Institute for Design of Advanced Energy Systems (IDAES) process engineering computational platform under the Carbon Capture Simulation for Industry Impact (CCSI²) initiative, which is sponsored by the U.S. Department of Energy Office of Fossil Energy and Carbon Management. We model the two phases as multi-component immiscible reacting mixtures of MEA–H₂O–CO₂ solution and flue gas and numerically solve the species transport for participating species separately within each phase. A six–species, two-reaction chemical mechanism [2] is used to model kinetics of the MEA–H₂O–CO₂ system. We explicitly track the interface using a mass-conserving volume of fluid method and quantify the interfacial CO₂ mass transfer. Using this framework, key parameters, including the interfacial area, interfacial mass transfer rate, heat release and the overall CO₂ absorption, are quantified under different operating conditions. We assess the effect of heat release from absorption by comparing results from our CFD simulations with another set of CFD simulations under identical conditions, in which the thermal effects are excluded. The implications of heat release on CO₂ absorption performance are discussed.

References:
1. Fu, Y., Bao, J., Singh, R., Wang, C. and Xu, Z., 2020. Investigation of countercurrent flow profile and liquid holdup in random packed column with local CFD data. Chemical Engineering Science, 221, p.115693.

2. Wilcox, J., 2012. Carbon capture. Springer Science & Business Media.

3. Akula, P., Eslick, J., Bhattacharyya, D. and Miller, D.C., 2021. Model Development, Validation, and Optimization of an MEA-Based Post-Combustion CO2 Capture Process under Part-Load and Variable Capture Operations. Industrial & Engineering Chemistry Research, 60(14), pp.5176-5193.