(57e) Optimal Techno-Economic Design and Operation of a Fixed Bed System Using Micro-Encapsulated Solvent for CO2 Capture

A number of high viscosity and phase change solvents have strong potential to reduce the penalty for CO₂ capture due to their high working capacity and low regeneration energy. However, due to hydrodynamic limitations in conventional towers and issues in transporting the solvent between the absorber and regenerator, these solvents can only be used under low working capacity thus greatly limiting their potential. Furthermore, a number of potential CO₂ capture solvents are also high corrosive. These solvents need to be highly diluted, which leads to considerable reduction in their working capacities. Microencapsulation of solvents is a new technology where the solvent is encapsulated within a polymer membrane. Vericella et al., (2015) have reported encapsulation of carbonate solutions encapsulated in a silicone membrane Micro-encapsulated carbon sorbents (MECS) can not only lead to realization of the high potential of a large family of solvents, but they also offer large surface area that can reduce the mass transfer and heat transfer resistances.

To realize the full potential of this novel technology, optimal contactor technology should be designed and selected with due consideration of thermodynamics, mass transfer, heat transfer, reaction kinetics, and hydrodynamics. Raksajati et al. (2017), performed a high level assessment of a few reactor configurations for microencapsulated MEA as the solvent. Hornbostel et al. (2019) developed a simplified carbonate capsule model to estimate the sizes and energy penalties of both fixed bed and fluidized bed absorbers. However, the authors assumed isothermal conditions and made simplifying assumptions about the regeneration process such as the assumption of a constant heat of reaction and fixing the regeneration time to 10 minutes, etc. The authors also did not account for water flux across the capsule shell, which can significantly affect the performance of the MECS. Detailed rate-based modeling of fixed bed reactors for MECS and optimal techno-economic design of these systems are currently not available in the existing literature.

In this work, a comprehensive model of the MECS system is first developed with rigorous chemistry models and thermodynamic models where the electrolyte non-random two-liquid (NRTL) model is used for the vapor-liquid equilibrium. Rate parameters are optimally estimated using the dynamic experimental data obtained under varying temperature and pressure conditions. A first-principles, non-isothermal, dynamic model of a fixed bed contactor containing microencapsulated sodium carbonate as the solvent is developed. For regeneration, both indirect heating by using an embedded heat exchanger and direct heating through steam injection are evaluated. Finally, the cyclic temperature swing absorption (TSA) process is modeled. Due to the strong trade-off between the capital and operating costs of this system and due to large number of decision variables, a mathematical programming problem is solved to obtain the optimal economic performance. However, since the fixed bed process is a cyclic process, a dynamic optimization problem needs to be solved.

Dynamic optimization of the fixed bed system is performed by using a direct transcription method for minimizing the equivalent annual operating cost (EAOC). Two different reactor materials-concrete and carbon steel- are evaluated. Due to the high heat content of the solvent and the capsule present during the regeneration stage, it is absolutely critical to recover the heat. Various configurations and working fluids are considered in the nonlinear programming problem to obtain the optimal design of the fixed bed system as well as its operating conditions.

References

1. Vericella, J. J. et al. “Encapsulated liquid sorbents for carbon dioxide capture”, Nature Communications, 6:6124, 1-7, 2015.
2. Anggit Raksajati, Minh T. Ho, and Dianne E. Wiley, “Techno-economic Evaluation of CO2 Capture from Flue Gases Using Encapsulated Solvent”, Industrial & Engineering Chemistry Research 1604-1620, 56 (6), 2017.
3. K.Hornbostel, D.Nguyen, W.Bourcier, J.Knipe, M.Worthington, S.McCoy, J.Stolaroff, Packed and fluidized bed absorber modeling for carbon capture with micro-encapsulated sodium carbonate solution, Applied Energy, 1192-1204, 2019