Membrane-based processes, related to gas separation and purification, have achieved an important level of development for a variety of industrial applications [60]. Therefore, the use of separation membranes is one of the promising technologies for reducing the emissions of greenhouse gases such as CO2. The term membrane is defined as a permselective barrier between two phases, the feed or upstream and permeate or downstream side [61]. This permselective barrier has the property to control the rate of transport of different species from the upstream to the downstream side, causing the concentration or purification of one of the species present in the feed gas mixture.

Membrane-based processes offer the advantage of large scale applica­tion to separate CO2 from a gas mixture. Figure 2 schematizes the pro­cess where concentrated CO2 is selectively separated from flue gas that is mainly composed of nitrogen and carbon dioxide along with other gases such as water vapor, SOx, NOx and methane. Subsequent to the membrane process, concentrated CO2 obtained at the permeate side can be disposed or used as raw material for the synthesis of several chemicals such as fuel and value-added products [62].

Of course, the rate of transport or permeation properties of a particular gas through a given membrane depend on the nature of the permeant gas, as well as the physical and chemical properties of the membrane.

Inorganic membranes are more thermally and chemically stable and have better mechanical properties than organic polymer membranes; ce­ramic membranes offer both the advantage of large scale application and potential for pre — and post-combustion CO2 separation applications, where membranes systems would be operating at elevated temperatures of 300­1000 °C [63].

CO2

Gas

Inorganic ceramic membranes can be classified as porous and nonpo­rous or dense. These differ from each other not only in their structures but also in the mechanism of permeation. In porous membranes, the transport of species is explained with the pore-flow model, in which permeants are transported by pressure-driven convective flow through the pore network. Separation occurs because one of the permeants is excluded (molecular filtration or sieving) from the pores in the membrane and remains in the re­tentate while the other permeants move towards the downstream side. On the other hand, in nonporous membranes, separation occurs by solution — diffusion, in which permeants dissolve in the membrane material and then diffuse through the bulk membrane by a concentration gradient [60].