## 3 Membrane Contactors for Maritime CO\({}_{2}\) Capture  
In general practice, post-combustion CO\({}_{2}\) capture is performed using either physical or chemical absorption or adsorption. Both processes rely on the contact of the flue gas with a liquid absorbent or solid adsorbent and the selective transfer of CO\({}_{2}\) to the latter, either due to purely thermodynamic (i.e., vapor liquid equilibrium or van der Waals forces), or due to the combined effect of thermodynamic and reaction kinetic phenomena. Among these methods, chemical absorption is the most prominent process used for CO\({}_{2}\) capture and is mainly associated with the use of packed separation columns for the absorption of CO\({}_{2}\) and subsequent regeneration of the solvent. The use of packing serves as a means to greatly increase the vapor-liquid contact area and thus to similarly increase the mass transfer fluxes along the column. Similar to absorption columns, membrane contactors have been used for the same purpose. These systems are comprised of membranes of suitable porous materials, positioned in appropriate configurations (hollow fibers) that selectively enable CO\({}_{2}\) to pass through them. A typical contactor configuration can be seen in Figure 1. On the one side of the hollow fiber membrane, the gas mixture is fed, while on the other (shell) side, a CO\({}_{2}\) capture medium (i.e., a solvent) flows co-currently or counter-currently and chemically binds it, therefore enabling its removal. The presence of the solvent is important for achieving high transfer rates through the membrane due to the contribution of the chemical reactions to mass transfer. On the other hand, the membrane is responsible for ensuring contact between the phases, while simultaneously preventing their mixing and thus enhancing the driving force and the overall mass transfer rate [24]. The combination of these two factors leads to an intensified and compact process with reduced volume compared to an absorption column. A detailed mathematical description of the membrane contactor system can be found in [PERSON] et al. [25]. The use of membrane contactors for CO\({}_{2}\) absorption over packed columns offers the benefit of process flexibility by introducing the concept of modular units of smaller size in the system, in contrast to the bulkier design of a separation column. In the case of application within a ship environment with strict volume and weight restrictions, membrane contactors are expected to facilitate the efficient introduction of CO\({}_{2}\) capture. Moreover, a typical ship engine operates at lean conditions with typical off-gas CO\({}_{2}\) molar concentrations in the order of 4-6% with the remainder being mainly N\({}_{2}\) and water vapor. Membrane contactors are usually less costly than absorption columns due to the lower expenses in fabrication of the membrane module. However, there are a number of operational challenges that are mainly associated with the pairing of the membrane and solvent. These include pore wetting and membrane degradation due to the solvent and have to be initially considered [26]. Figure 1: Schematic representation of a membrane contactor for CO\({}_{2}\) capture. The efficiency and applicability of membrane contactors as part of an absorption and desorption system has been studied by a number of researchers. [PERSON] et al. [27] experimentally compared the solvent-based CO\({}_{2}\) capture in an absorption column and a membrane contactor. The main conclusion was that despite the fact that the absorption column offers increased mass transfer coefficients by an order of magnitude, membrane contactors can achieve similar results regarding the desired separation but with five times less volume. Additionally, the use of hollow fiber ceramic membranes instead of an absorption column in the conventional absorption and regeneration system results in a lower pressure drop during the absorption step [28]. Membrane contactors can be used to replace not only the absorption but also the solvent regeneration column [29; 30].