(732e) Process Modeling and Techno-Economic Analysis of a Diamine-Appended Metal–Organic Framework for CO2 Capture

Aqueous solvents are the current leading commercial technologies for post-combustion CO₂ capture, but suffer from several drawbacks that include degradation, large energy penalties, and low working capacities. There has been considerable research directed toward investigating viable alternatives to these aqueous solvents in the area of solid sorbents. Most notably, a novel class of functionalized metal-organic frameworks (MOF’s) have been shown to exhibit promising characteristics for post-combustion capture from a coal flue gas source^1,2,3. 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. This MOF exhibits a step-shaped isotherm under low partial pressures of CO₂ which leads to higher working capacities under similar temperature and pressure intervals when compared to traditional sorbents. 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

To make this new class of MOF’s economically feasible it is crucial that the type of contactor used minimizes the capital and operating costs of the system. For this system, high adsorption heat coupled with a large sensitivity of the adsorption capacity to temperature result in thermal management being key to realizing the full potential of the MOF’s. Therefore, a contactor that can reject the heat generated during adsorption while maintaining efficient mass transfer is needed. In this work, four types of contactor models are analyzed: axial flow fixed bed, radial flow fixed bed, moving bed, and rotary packed bed. These dynamic, pressure-driven models are developed using first principles and consist of mass, heat, and momentum conservation equations and account for internal and external mass transfer resistances, heat transfer between gas and solid phases, and an embedded heat exchanger to supply heat or aid in heat removal of the system. The step-shaped isotherms that this MOF exhibits can also present a challenge when attempting to model the adsorption equilibrium. To achieve this, a new isotherm model is presented that can accurately predict adsorption equilibrium across the experimental temperature and pressure ranges. Kinetic models are also presented that were developed using available thermogravimetric analysis experimental data and fixed bed breakthrough experimental data.

These contactor models were then used to develop commercial scale CO₂ capture process models using flue gas from a 550 MWe net coal-fired power plant. A cost model is also presented that evaluates the capital costs of the reactors and compressors, if needed, and the operating costs of steam, cooling water, and electricity. Sensitivity of each contactor process to important operating variables are also investigated with a final comparison of each process as well as a traditional MEA capture system. Our works shows the critical impact of the contactor types on the commercialization potential of these next-generation MOFs.

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