Membrane contactors have been proven capable of overcoming and lessening the boundaries and limitations of conventional devices for liquid-liquid and gas-liquid separation process. The early application of membrane contactors was meant mostly for the gas-liquid separation process because this separation is difficult to conduct using conventional devices, such as packed columns, packed beds, and equipment for offshore applications.

The application of membrane contactors in separation or absorption units has a size advantage over conventional contactors used for CO2 removal (Feron and Jansen 1995). They are suitable for the removal of CO2 from the exhausts of gas turbines and from flue gas (Pedersen and Dannstrom 1997; Mavroudi et al. 2003) because of their ability to increase the available gas-liquid contact area and provide a very high interfacial area, thereby increasing gas removal efficiency. Membrane contactors also have been used extensively in other applications, such as the predic­tion of the enhancement of ozone fluxes (Phattaranawik et al. 2005) and the deoxy­genation of water in boilers (Shao et al. 2008).

The applications of membrane contactors have an advantage over independency flow rate of two different phases; thus, they can be operated without disturbance,
which contrasts with conventional devices. In terms of their design, membrane contactor modules have a straightforward design that allows operation over a wide range of capacities and only requires the addition of membrane modules when needed to increase the yield of final, desired product. There are no moving parts in a mem­brane module, so it can achieve better efficiency than conventional contactors.

The use of a hollow-fibre, gas-liquid contactor resulted in more than five times larger volumetric mass transfer of CO2 absorption compared to a conventional packed bed (Nishikawa et al. 1995) . It was reported that membrane contactor achieved a CO) absorption rate that was 2.7 times higher than a packed column (Yeon et al. 2005). A pilot-scale membrane contactor showed that it can be used to recover CO) from flue gas (Yeon et al. 2005). It also can be used to capture acid gases from gas streams (Mansourizadeh and Ismail 2009). A membrane contactor that has a comparable mass transfer area to that of a packed column performs better than the structured packing (Vogt et al. 2011). The separation of two different gases simultaneously also can be conducted effectively (Lv et al. 2012).

Although membrane contactors have several advantages over conventional con­tactors, they are subject to fouling, especially in operations that involve biological processes, such as the operation of supplying CO2 to microalgae cultures, and this could shorten their life span. Such fouling can be reduced or avoided by determin­ing the wettability of the hydrophobic membrane materials. The degree of hydro — phobicity of the surface of the membrane increases the performance of the membrane (Nishikawa et al. 1995). The wettability depends to a great extent on the surface tension of the liquid, which can be quantified by breakthrough pressure (Kumar et al. 2002). Wetting can be avoided by applying a slightly higher over pressure on the gas side (Dindore et al. 2004). In the absorption of gases, the use of a chemical stabilization layer on the liquid side of the membrane could prevent wetting (Nymeijer et al. 2004). Partial wetting of the membrane pores reduces the mass transfer of gases (Lv et al. 2012). Thus, it is important to operate the membrane within its pressure wettability.