## 3 Membrane Contactors for Maritime CO\({}_{2}\) Capture
Most of the commercially available membrane materials that are employed in membrane contactor devices are made of hydrophobic polymeric materials such as polypropylene (PP), polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF). Polypropylene membranes are cheap and can be easily fabricated in large-area membrane modules, which have been commercialized by 3M\({}^{\text{TM}}\) Liqui-Ce\({}^{\text{TM}}\) even for industrial scale applications. However, they have relative limited thermal and chemical stability. On the other hand, PTFE membranes are much more thermally and chemically stable but they are more difficult to fabricate in small hollow fibers with high specific surface area and they also have significantly higher production costs. PVDF membranes lie somewhere between PP and PTFE in terms of stability and cost issues [31; 32]. For the last few years, ceramic membranes have also been studied as membrane contactors for CO\({}_{2}\) capture applications, due to their inherently higher thermal and chemical stability. However, the hydroxide groups that are typically present on their surface render them hydrophilic which is typically associated with decreased mass transfer rates in gas separation applications. Thus, ceramic membranes need to be modified in order to become hydrophobic before their application. Moreover, they cannot be easily processed in high specific surface area hollow fibers and they have much higher cost than the polymeric materials (more than an order of magnitude higher). For all these reasons, they are still in a lower technology readiness level (TRL) than polymeric membranes for CO\({}_{2}\) capture applications [33; 34]. The membrane-solvent compatibility is a crucial factor that needs to be seriously taken into account because it significantly affects process performance. Even if the membrane material is not significantly degraded due to the presence of a solvent, the process performance may be compromised due to membrane wetting. Membrane wetting is the phenomenon of partial liquid penetration in the pores of a hydrophobic membrane. In this case, the gas-liquid interface can be shifted with time from the pore mouth to some (variable) location inside the membrane pores, resulting in increased membrane mass transfer resistance and absorption's flux deterioration with time. The exact mechanism of membrane wetting has not been fully clarified yet, due to the complex nature of membrane-solvent interaction phenomena. Some potential causes can be attributed to: (i) chemical reactions between the membranes and the solvent which can change the hydrophobic characteristics of the system, (ii) physical interactions between the membrane pores and solvents (e.g., swelling), and (iii) changes in membrane surface hydrophobicity by trace impurities of the solvent. In any case, the membrane wetting phenomenon depends on membrane material and solvent type, as well as on process conditions (e.g., temperature, pressure, flow rates, etc.). Partial membrane wetting by the solvent is something expected, at least to some extent, for any membrane-solvent pair. The partial performance loss can many times be counterbalanced by the much higher specific membrane area of membrane contactors with respect to the conventional packed columns. However, there are also references for dramatic performance losses during the first hours of operation, which apparently inhibits the applicability of the process [35]. Some general conclusions that have been derived regarding membrane wetting by different solvents are [36; 37; 38; 39; 40; 41]:Inorganic solvents such as H\({}_{2}\)O, NaOH, K\({}_{2}\)CO\({}_{3}\), etc. have high surface tension and do not easily wet the common hydrophobic polymeric membranes. However, they are typically less efficient than conventional amines. 2. Amines have high CO\({}_{2}\) capture efficiency but they have typically lower surface tension compared with inorganic solvents and they tend to more easily wet the common hydrophobic polymeric membranes.