(10d) Process Modeling and Optimal Synthesis of a Fixed Bed System for CO2 Capture Using a Diamine-Appended Metal–Organic Framework

Functionalized sorbents offer a potential alternative technology for CO₂ capture. Viable sorbents should have lower regeneration energy, faster kinetics, higher CO₂ selectivity, and higher working capacity for as high as 90% CO₂ capture from the post-combustion flue gas. A novel class of diamine-appended metal–organic frameworks (MOFs) offers a number of these features^1,2,3. These MOFs exhibit step-shaped isotherms under low partial pressures of CO₂ and lead to higher working capacities under similar temperature and pressure intervals when compared to traditional sorbents. The MOF studied in this work, Mg₂(dobpdc) (dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), contains one-dimensional hexagonal channels with unsaturated Mg²⁺ sites and is functionalized by binding the diamine 2,2-dimethyl-1,3-diaminopropane (dmpn) to these sites which then chemisorb CO₂ in cooperative mechanism to form carbamate and carbamic acid species. Along with high working capacities, this functionalized MOF also shows excellent long-term stability and maintains performance under humid conditions, both desirable attributes for flue gas CO₂ capture.¹

For minimizing the penalty due to CO₂ capture, contactors for these functionalized MOFs should be optimally designed. The step-shaped isotherms exhibited by these MOFs cannot be adequately represented by traditional isotherms due to very complex chemical reaction pathways in these MOFs. A new isotherm model that captures the unique mechanism of CO₂ uptake is developed in this work. Data from thermogravimetric analysis (TGA) are used to develop the kinetic model. A model of the fixed bed experimental set up was developed and used to validate the kinetic model by using the experimental data. The model is then scaled up for CO₂ capture from a 550 MWe net coal-fired power plant. The dynamic, pressure driven, fixed bed contactor model is a 1-d first-principles model that includes mass, momentum and energy conservation equations and accounts for internal and external mass transfer resistances, reaction kinetics, heat transfer between the gas and solid phases, and heat transfer between the gas/solid and an embedded heat exchanger within the reactor. The model was then used to simulate an entire temperature swing adsorption (TSA) cycle for adsorption/desorption of CO₂ from a flue gas source. Due to the step-shaped isotherms and reasonably high heat of reaction for this studied system, efficient heat removal/addition is critical. Therefore optimal design and operation of the embedded heat exchangers by considering the capital and operating costs of the process is critical. However the fixed bed process is a dynamic process due to the cyclic nature of the adsorption/desorption cycles. Therefore a dynamic optimization of the fixed process is performed yielding not only the optimal design of the fixed bed capture process but also the optimal operating conditions.

References:

[1] – 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

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

[3] 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