(700a) Simultaneous Optimization of Monolith Structure and Adsorption Cycle Design for Direct Air Capture through Experiments and Modelling.

Introduction.The climate change due to global warming is a serious concern, and the Paris Agreement aims to limit the increase in the average global temperature to well below 2 degrees above pre-industrial levels, with a target of 1.5 degrees [1]. Achieving carbon neutrality by this century will require negative emission technologies that capture CO₂ from the atmosphere, in addition to emission reductions. In this regard, direct air capture (DAC) technology, which removes CO₂ directly from the atmosphere, is one of the promising methods. However, DAC faces challenges, including its high cost compared to capturing CO₂ from point sources [2]. This is due to the low CO₂ concentration, which leads to processing a substantial quantity of air.

Monolith adsorbents are expected to reduce the cost of CO₂ capture by offering a larger surface area per volume and a lower pressure drop compared to pellets, which are commonly used as adsorbents. However, there are few studies on mass transfer kinetics that consider structural characteristics of monolith, such as wall thickness and channel width [3]. Furthermore, there are even fewer reports on optimizing the monolith structure for the performance of the adsorption process, such as productivity and energy consumption[4]. In this study, we develop a monolith adsorbent model from experiments, and simultaneously optimize the monolith structure as well as design and operation of temperature vacuum swing adsorption process to maximize the CO₂ productivity and minimize energy consumption.

Methodology. Monolith adsorbents were prepared by grafting polyethyleneimine (PEI) onto silica gel monoliths. The silica gel monoliths were prepared by mixing silica gel, an organic binder and water, followed by extrusion and heat treatment to remove the binder. The monolith is composed of a series of channels with a square cross section. The variations in wall thickness are 0.15, 0.30 and 0.45mm, and the ratio of channel width to wall thickness ranges from 5:1 to 16:1. CO₂ isotherm was measured with the BELSORP-Max X (Microtrac BEL). The breakthrough experiments were conducted by flowing a mixed gas of N₂ and CO₂ (400ppm) at 293~343K during adsorption and pure N₂ gas at 353K during desorption, at different linear velocities through a self-built apparatus. A one-dimensional physical model was fitted to experimental breakthrough curves to estimate the mass transfer coefficients. Temperature vacuum swing adsorption (TVSA) cycle design is simulated via process simulator (gPROMS, Siemens).

Results and discussion. The model fitting results to the breakthrough curves measured during adsorption showed reasonable agreement. Our experiments indicate that the mass transfer coefficient of the monolith adsorbent can be increased by thinner wall thickness and higher linear velocities, both of which play a significant role in the overall mass transfer process. While thin walls increase mass transfer kinetics and reduce pressure drop, they lead to a decrease in the bed density of contactor which may result in a reduction of CO₂ productivity. On the day of the presentation, we will report on optimal combination of monolith geometry and TVSA process condition in terms of CO₂ productivity and energy efficiency.

[1] United Nations, Paris Agreement. 2015. https://www.un.org/en/climatechange/paris-agreement (accessed: Mar. 26, 2025).
[2] International Energy Agency, Direct Air Capture A key technology for net zero, 2022, https://www.iea.org/reports/direct-air-capture-2022 (accessed: Mar. 26, 2025).
[3] F. Rezaei, P. Webley, Chem. Eng. Sci. 64 (2009) 5182-5191.
[4] V. Stampi-Bombelli, M. Mazzotti, Ind. Eng. Chem. Res. 2024, 63 (45), 19728-19743