(365c) Multiscale Screening of Adsorbents for Post Combustion Capture: Effect of the Vacuum Swing Adsorption Process Configuration on the Adsorbent Performance

Carbon capture and storage (CCS) has been identified as the near-term solution for mitigating climate change effects with the simultaneous use of fossil fuels for the production of energy. Bulk of global CO₂ emissions come from power plants burning fossil fuels. The current benchmark technology for post combustion CO₂ capture is absorption using amines. Adsorption process has been explored as an alternative technology for capturing and concentrating CO₂ from these large point sources. In the case of an adsorption process, the selected adsorbent should possess a very high capacity for CO₂ and high CO₂ selectivity. The benchmark material for adsorption-based carbon capture is Zeolite 13X and several published literature studies are available on this material ^1-6. Recent developments include new materials such as metal organic frameworks (MOFs), covalent organic frameworks (COFs) and Zeolite imidazolate frameworks (ZIFs). Many these new materials are computationally screened using molecular simulations and ranked based on metrics such as CO₂ capacity CO₂/N₂ selectivity etc, despite the limited availability of accurate force fields. However, these metrics were not reliable and the optimal behaviour of a material in a real process will also depend on the specifics of process configurations. Therefore, a reliable way to rank adsorbents is to study their performance in an adsorption process cycle^4,5. The current approach to rank adsorbents on a process scale is to perform detailed process optimization while fixing the cycle configuration. However, in order to obtain the true potential, each adsorbent must be mapped to an appropriate cyclic process and very few studies have done a detailed cycle synthesis to obtain the best cycle for a given material^3,6.

In this work, we have employed two different cycles namely a 4-step vacuum swing adsorption (VSA) cycle with light product pressurization and a 6-step VSA cycle with reflux and light product pressurization, described in earlier publications^4,5. These cycles were used to evaluate the performance of four adsorbents namely zeolite 13X, CPO-27-Ni, HKUST-1 and Silicalite. The information on adsorption equilibrium and kinetics were obtained from lab scale volumetric experiments as well as molecular simulations. Detailed optimization of the cycles was carried out using non-dominated sorting genetic algorithm (NSGA-II) in MATLAB to arrive at operating conditions that satisfy 95% CO₂ purity and 90% CO₂ recovery with minimum energy consumption and maximum productivity. The performance of the different VSA process was evaluated using both the actual experimental data and predictions from molecular simulations. Here we demonstrate the differences in performance with respect to isotherms obtained from experiments and molecular simulations as well as the cycle configuration.

Key words: Adsorption, carbon capture and storage, vacuum swing adsorption, metal organic frameworks

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