## 1 Introduction  
The capture of CO\({}_{2}\) from ambient air, often called direct air capture (DAC), can address anthropogenic CO\({}_{2}\) emissions from distributed sources, which account for between one third and one half of the total CO\({}_{2}\) emissions per year [1]. Unlike CO\({}_{2}\) capture from point sources, DAC facilities can be installed anywhere, providing more flexibility in the choice of location. The captured atmospheric CO\({}_{2}\) can be stored underground or converted to produce fuels and value-added chemicals, such as carbon monoxide, formic acid, and methanol [2; 3; 4]. Additionally, the CO\({}_{2}\) can be utilized in biological conversion, e.g., in horticulture or for use in microalgae cultivation [5]. DAC was first suggested by [6] using an aqueous alkali hydroxide solution and was studied on a process level by [PERSON] et al. [7]. An important issue when using aqueous systems for direct air capture is that the required massive air flow may cause significant water losses for the aqueous alkali solution [8]. Essentially the same system was further developed by [PERSON] et al. and was comprised of absorption in an alkaline solution and was coupled to a calcium caustic recovery loop, requiring a temperature of 900 \({}^{\circ}\)C during regeneration [9]. Despite its complexity, this optimized and fully engineered system can be regarded as state of the art; it claims to be able to produce CO\({}_{2}\) from air at relatively low costs of 94 USD/ton of CO\({}_{2}\)[10]. Supported amine solid sorbents are a promising alternative for DAC, also at lower production scales, as they need lower regeneration temperatures and avoid evaporative solvent losses. Supported amine sorbents were firstly studied for DAC by [PERSON] et al. [11], and an increasing number of papers have been published using these sorbents since then [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. Most of the DAC studies target the development of new supported amine sorbents, thereby focusing on enhancing the CO\({}_{2}\) equilibrium capacity, utilizing thermogravimetric analyzers or fixed bed reactors with a few grams of sorbent [24; 25; 26; 27; 28]. Studies focusing on the subsequent process development issues, especially at a scale of several kilograms of sorbent, as well as studies reporting on the sorbent performance under non-equilibrium conditions are rare. Whereas the desorption step more closely resembles the situation encountered in CO\({}_{2}\) capture from concentrated sources, the DAC adsorption step is especially challenging. During adsorption, at least 1400 m\({}^{3}\) of air need to be supplied to capture 1 kg of CO\({}_{2}\) (when the CO\({}_{2}\) concentration in the air at 25 \({}^{\circ}\)C is 0.04%). The counteracting requirements of a high gas throughput and, for process economics, low pressure drop lead to the need for the development, testing, characterization, and evaluation of new DAC air-sorbent contactors. The objective of this paper is to communicate the rationale for selecting a radial flow reactor for this application and to report on an experimental study investigating the suitability of this contactor type for the adsorption step in DAC. In this study, a polymer-based supported amine sorbent will be used for the reversible CO\({}_{2}\) capture process. Firstly, the general requirements for a sorbent-based DAC contactor are summarized, which led to the selection of a radial flow reactor (RFR) as a potentially attractive contactor type. Considering the large gas volumes to be treated in the DAC adsorber, it is essential to have (i) a high volumetric adsorption rate (in mol CO\({}_{2}\) m\({}^{-3}\) s\({}^{-1}\)) to minimize equipment sizes and costs (CapEx) and (ii) a low pressure drop to minimize operational costs (OpEx). Since the energy consumption (with associated emissions) and the operational costs depend linearly on pressure drop and the volumetric flowrate, the efficiency of CO\({}_{2}\) removal is also important.