(112d) Dynamic Modeling, Optimization, and Control Studies of a Moving Bed Process for CO2 Capture Using a Micro-Encapsulated Solvent

A number of potential solvents for CO₂ capture exhibit undesirable characteristics like high viscosity, solids formation thus making it difficult to use them in conventional towers [1]. Micro encapsulation of carbon sorbents (MECS) can help in overcoming these issues seen in highly viscous and phase change materials. Micro-encapsulation of solvents is a technology proposed [2] for post combustion carbon capture where the solvent of interest is encapsulated inside a polymer. Microencapsulation of carbon sorbents (MECS) involves fabrication of microcapsules containing a solvent with CO₂ capture capabilities. Even though capsules are manufactured with a known solvent concentration, water is known to permeate some capsule materials. H₂O gain or loss due to diffusion into or out of the microcapsules can significantly affect the solvent concentration and is one of the key operational challenges for reliable and efficient operation of MECS technology. While there are a few studies [2-3] that have investigated different reactor configurations for MECS, no studies have been conducted with the intent to understand and mitigate the issue of time-varying water concentration. Extracting solvent from microcapsules to measure the concentration may require collecting huge number of microcapsules from the reactor. The ambient conditions can further change the water content and may not represent the true concentration value in the reactor. Therefore one must rely on other measurable process variables to get an estimate of capsule concentration.

The present work is focused on dynamic modeling, optimization, and control of a moving bed reactor for MECS, where controlling the desired solvent concentration is one of the key control objectives. Moving bed reactors have a strong potential for solid-sorbent based CO₂ capture [4-5]. The counter-current contact between gas and capsules can help to achieve larger CO₂ driving forces in the absorber and regenerator and so is examined in this work. Also, the heat recovery from the capsules leaving the regenerator can help in pre-heating the capsules flowing from absorber to regenerator. Overall, this can reduce the regeneration energy requirements of the process. A 1-D multi-scale moving bed absorber and regenerator using a pre-existing microcapsule model [4] is developed in Aspen Custom Modeler (ACM) for MECS using Na₂CO₃ as the encapsulated solvent. The capsule level model used in the moving bed reactor is validated with experimental data in our previous study [4]. A steady-state optimization of the moving bed configuration that minimizes the Equivalent Annual Operating Cost (EAOC) by considering key operating and design variables is performed. Traditional and model-based advanced controllers are developed for controlling the capture percentage, and solvent concentration within the capsule in the face of input disturbances. An ARMAX type model based on information about measured variables from the rigorous process model will be exploited to estimate solvent concentration and to provide satisfactory control performance.

References

1. Stolaroff, J. K. et al., Microencapsulation of advanced solvents for carbon capture. Faraday Discussions. DOI: 10.1039/c6fd00049e (2016).
2. Vericella, J. J. et al. “Encapsulated liquid sorbents for carbon dioxide capture”, Nature Communications, 6:6124, 1-7, 2015.
3. Hornbostel, et al, Packed and fluidized bed absorber modeling for carbon capture with micro-encapsulated sodium carbonate solution, Applied Energy 1192-1204, 2019.
4. Kotamreddy, et al., Process Modeling and Techno-Economic Analysis of a CO₂ Capture Process Using Fixed Bed Reactors with a Microencapsulated Solvent, Energy & Fuels 2019 33 (8), 7534-7549 DOI: 10.1021/acs.energyfuels.9b01255.
5. Knaebel, K.S., Temperature swing adsorption system. 2009, Google Patents.
6. 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.