Mahon and the group at Dow Chemical first proposed the use of hollow-fiber membranes as a separation unit, resulting as one of the most important step-forward in membrane technology. The benefits of this membrane configuration are well known and may be generalized as: (a) much higher membrane area per volume unit than other membrane configuration; (b) high flexibility and easy operability; (c) facile membrane reparation and (d) high self-mechanical support. These characteristics made useful the hollow fiber membranes for membrane contactors, which operate as an interface favoring the transport of compounds between gas and liquid phases, avoiding the direct mixing and the phases' dispersion.

        Membrane contactors result to be a clear example of contact devices for high efficiency separation processes particularly due to their large surface membrane area per unit of volume. This ensures a continuous growth in a number of applications from laboratory to larger scale, as recently reviewed. Generally, the separation due to a selective mass transfer results to be the principal function of a membrane, whereas a membrane contactor doesn't provide any selectivity. In these units, the membrane acts as a contactor between gas and liquid without a direct mixing. The selective characteristics of a membrane contactor are made possible by the different solubility of the compounds present in the liquid phase. This is the reason why most of the membrane materials used for membrane contactors are porous, because possessing high mass transfer properties. Sometimes, in the case of high-pressure operations the utilization of asymmetric membrane solutions, constituted of a dense and thin layer on a porous support becomes necessary.

          However, porous hydrophobic membrane ensures (ideally) the absence of the liquid phase penetration into the pores, which are filled with gas in their entire volume. Several hollow fiber membranes are today in the market, differing in morphology, transport and separation properties, depending on the selected membrane material and fabrication technique. Most of the membrane contactors in the market adopt polymeric membranes and only a small part uses inorganic membranes. The membranes may be symmetric or asymmetric, porous and not porous (dense). Asymmetric membranes may be prepared using the same polymer or different polymers in case of composite membranes. Melting extrusion (followed by stretching for producing the pores) is the technique used to fabricate membranes for membrane contactor modules when chemically stable PP or PTFE are chosen as membrane material.

          PVDF, PSF or PEI in fibers may be prepared by different non-solvent induced or temperature-induced phase separation technique. Hollow fiber membrane contactors are mostly utilized for acid gases removal (CO2, SO2 and H2S). These gases are produced in several processes, but particularly by burning fossil fuels and steel and cement production. In CO2 removal, the development of the membrane contactors was particularly done not only in terms of novel membrane materials adoption, but also in terms of investigation of the best CO2- selective absorption liquids, resulting compatible with the fibers. For the H2S removal from NG sweetening and biogas purification, membrane contactors adopted and extensively studied hollow fiber membranes based on per fluorinated polymers such as PVDF, PTFE, PFA or PSF for NG sweetening, whereas they were based not only on PVDF, but also on non-porous fibers from PDMS in the case of biogas purification.