(191c) Modeling and Optimization of a Moving Bed Process for Post-Combustion CO2 Capture Using a Diamine-Appended Metal–Organic Framework

Current commercial technologies for post-combustion CO₂ capture are mainly based on amine-based solvents, but these technologies suffer from high energy penalty. Therefore there are considerable recent interests in the area of solid sorbent-based processes as they have been reported to have certain advantages over amine-based processes¹. For these sorbent-based processes to be a viable alternative to the amine-based processes, efficient gas-solid contactors are absolutely necessary. While there has been much work on optimizing the design of the towers, their internals, and operating conditions for the solvent-based capture processes, there are very few publications in the open literature on optimal design of the contactor technology for solid-sorbent systems. In particular, a moving bed (MB) contactor can offer several advantages over other solid-gas contactors, such as a fixed bed or fluidized bed. In a MB, the solid particles enter at the top of the bed and travel downwards as the flue gas enters at the bottom of the bed and flows upward. This counter-current flow pattern results in improved driving forces for mass transfer when compared to a fixed bed or fluidized bed system. Achieving a high overall driving force is critical for 90% CO₂ capture, especially at the top end where the flue gas leaves the system with a very low partial pressure of CO₂. Because of the moderate hydrodynamics in MB contactors, particle attrition in these beds is much lower compared to the fluidized beds.

This work focuses on the optimal design and operation of the MB contactors for a novel diamine-appended metal organic framework (MOF). The diamine-appended MOF used in this work, diamine 2,2-dimethyl-1,3-diaminopropane (dmpn) Mg₂(dobpdc) (dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) [dmpn-Mg₂(dobpdc)], exhibits a step-shaped isotherm under low partial pressures of CO₂ which results in high working capacities when compared to traditional sorbents. This functionalized MOF also shows excellent long-term stability and maintains performance under humid conditions, both desirable attributes for flue gas CO₂ capture^2,3,4. However for realizing the high potential of this MOF, rapid heat removal/addition must be achieved so that the step-shaped isotherm can be exploited for improving its economics.

In this work, a dynamic, one-dimensional, non-isothermal MB reactor model is developed and used to model a full scale temperature swing adsorption process using the functionalized MOF. The model simultaneously solves a set of partial differential equations for the mass and energy balances of the system, and accounts for internal and external mass transfer resistances, reaction kinetics, and heat transfer between the gas and solid phase. The model also accounts for heat transfer between the system and an embedded cooler in the adsorber and a heater in the regenerator. The embedded heat exchangers can provide/remove large amount of heat thus facilitating an operation that is close to the desired temperature profile. However, the added cost and lost volume due to the embedded heat exchangers should be accounted for. Therefore a mathematical programming problem is solved for simultaneous optimization of the design and operating conditions of the MB process. Traditional and various extended area designs were investigated for the embedded heat exchangers. In addition, various working fluids are investigated for the cooler/heater not only for optimal heat addition/removal but also for potential heat recovery from the hot sorbents thus improving the overall economics of the capture process.

References:

[1] Kim, H., Miller, D., Modekurti, S., Omell, B., Bhattacharyya, D., Zitney, S., Mathematical Modeling of a Moving Bed Reactor for Post-Combustion CO₂ Capture. AIChE Journal. 2016; 62 (11), 3899-3914

[2] Milner, P.J., Siegelman, R.L., Forse, A.C., Gonzalez, M.I., Runcevski, T., Martell, J.D., Reimer, J.A., Long, J.R. A Diaminopropane-Appended Metal-Organic Framework Enabling Efficient CO₂ Capture from Coal Flue Gas via a Mixed Adsorption Mechanism. Journal of the American Chemical Society. 2017; 139 (38), 13541-13553

[3] Mcdonald, T.M., et al., Cooperative insertion of CO₂ in diamine-appended metal-organic frameworks. Nature. 2015; 519, 303-308

[4] Forse, A.C., Milner, P.J., Lee, J., Redfearn, H.N., Oktawiec, J., Siegelman, R.L., Martell, J.D., Dinakar, B., Porter-Zasada, L.B., Gonzalez, M.I., Neaton, J.B., Long, J.R., Reimer, J.A., Elucidating CO₂ Chemisorption in Diamine-Appended Metal-Organic Frameworks. J. Am. Chem. Soc. 2018; 140(51), 18016-18031