chapter  30
Membrane Contactors for the Absorption of Carbon Dioxide from Gaseous Streams: State of the Art on Membrane Improvements
Pages 14

Carbon dioxide is usually removed from gas streams by means of packed towers, in which the stream to be treated is put in contact with an absorbent liquid particularly selective for CO2. However, many are the drawbacks of these devices. For example, a large active contact area is required to obtain good CO2 removals, thereby implying the use of big absorption towers. Moreover, the direct contact of the gas stream with the liquid absorbent can lead to several issues. One is the entrainment of the extractant by the gas phase. There are also the problems of production of emulsions, foaming, ooding, channeling, and unloading. In the past years, the potentialities of membrane contactors, as alternative units to carry out an efcient separation, have been investigated by many researchers. Membrane contactors are based on the use of microporous membranes that act as inert barriers between the gas and the liquid streams, allowing their contact at each micropore, while avoiding their mixing. In this way, the problems of ooding, foaming, loading, and liquid entrainment are solved. It is also possible to ensure a well-dened and stable interfacial area between the gas and the absorbent [1]. Moreover, since each membrane micropore is a point of contact for the two phases and having each membrane a huge number of micropores, the interfacial area available for the mass transfer is higher than that of packed tower. The typical interfacial area per unit of volume of membrane contactors is in the range of 1500-3000 m2/m3, whereas for conventional devices, this ratio is between 100 and 800 m2/m3 [2]. This would imply that the same separation could be carried out in a more compact system. A reduction of size is crucial for offshore applications, where it is often very difcult to install packed towers. Furthermore, the typical low volume

and weight of membrane contactors allow their easy integration into existing installations. Finally, their modular design is also attractive from the point of view of process intensication. In literature, the researches made on the application of membrane contactors to the removal of carbon dioxide from gaseous streams have analyzed the role of different parameters on the performance of the process, such as membrane properties, type and concentration of absorbents, membrane module design, and operating temperature and pressures. This contribution would like to provide a brief summary of the main problems/critical aspects underlined in the studies made during the years 2005-2012 that future research must overcome in order to make possible the application of membrane contactors at industrial scale. Then, the researches on the development of membranes with improved properties are reported and discussed in more details. Only nondispersive extraction membrane contactors are considered, while supported liquid membranes and xed carrier membranes are not included.