(515f) Plant-Wide Optimization of a Moving Bed Process for CO2 Capture Using a Micro-Encapsulated Solvent

Micro-encapsulation of solvents is an excellent option for the potential solvents that are highly viscous such as the ionic liquids or those that form solids upon CO₂ capture/release. These solvents are operated at low CO₂ loading or under high dilution to avoid solids precipitation and for facilitating transportation between the absorber/regenerator thus significantly restricting the high potential of this large class of potential solvents. In the micro-encapsulation technology, a solvent is encapsulated in a polymer through microfluidic techniques [1]. Therefore even highly corrosive solvents can be utilized. Furthermore, micro-encapsulated carbon sorbents (MECS) also offer large surface area that improves the mass transfer and heat transfer characteristics.

For maximizing the economics of the MECS technology, design and operating conditions of an optimal contactor technology must be optimized. One of the potential technology is the conventional fixed bed technology. However, it has several disadvantages mainly due to large reactor volumes, high pressure drop, difficulty in heat recovery, and complex cyclic nature of fixed bed operation that makes heat recovery a challenge. The moving bed (MB) technology has been reported to have strong potential for solids-sorbent based CO₂ capture [2-4]. The near-countercurrent contact between the solids and gas results in an overall large driving force leading to higher CO₂ loadings from the adsorber and lower CO₂ loadings from the regenerator. Heat recovery in the MB processes is simpler compared to the fixed bed processes since the heat can be readily recovered from the hot lean sorbents leaving the regenerator using a working fluid, which then transfers the heat to the incoming loaded sorbent to the regenerator. While there are few works for the fixed bed [5-6] systems for the MECS, to the best of our knowledge, there is no published work in the open literature on the MB models for CO₂ capture using MECS.

In this work, a detailed model of a moving bed reactor containing these microcapsules with sodium carbonate as the solvent is developed. The multi-scale model couples the capsule model with the contactor-scale model using appropriate volume-averaged transport variables. The microcapsule model incorporates detailed reactions, water flux, species conservation, and energy conservation along with rigorous thermodynamic and transport properties models. Water transport through the polymer shell is also accounted for. Mass transfer parameters at the capsule level are optimally estimated using the dynamic experimental data obtained under varying temperature and pressure conditions. The contactor-scale model includes the mass, momentum and energy balances along with rigorous hydrodynamic models as appropriate for the MB operation. For heat removal in the adsorber, an embedded cooler is considered while an embedded heater is considered in the regenerator. Balance of the unit includes heat recovery heat exchangers as well as other equipment items such as the coolers and pumps for the heat transfer medium, etc. The plant-wide model is used for optimal design of the equipment items as well as their operating conditions for minimizing the equivalent annual operating cost (EAOC). It was observed that there can be significant variation in the economics of the MB process in comparison to the fixed bed process making them a great candidate for MECS-based CO₂ capture especially when a high extent of recovery of the sensible heat is desired for improving the process economics.

References

1. Vericella, J. J. et al. “Encapsulated liquid sorbents for carbon dioxide capture”, Nature Communications, 6:6124, 1-7, 2015.
2. Knaebel, K.S., Temperature swing adsorption system. 2009, Google Patents.
3. Mondino, Giorgia et al. “Effect of Gas Recycling on the Performance of a Moving Bed Temperature-Swing (MBTSA) Process for CO₂ Capture in a Coal Fired Power Plant Context.” Energies, 10(6), 745. 2017.
4. Kim et al. “Moving bed adsorption process with internal heat integration for carbon dioxide capture.” IJGGC, 13-24, 2013.
5. Anggit Raksajati, et al, “Techno-economic Evaluation of CO₂ Capture from Flue Gases Using Encapsulated Solvent”, Industrial & Engineering Chemistry Research 1604-1620, 56 (6), 2017.
6. Hornbostel, et al, Packed and fluidized bed absorber modeling for carbon capture with micro-encapsulated sodium carbonate solution, Applied Energy 1192-1204, 2019.