It can be said that the adsorbent material is the "heart" of the PSA unit. All the properties of the cycle (operating conditions and operating mode) depend on the initial choice of the adsorbent. As mentioned before, several materials can be employed in PSA technology. The material selected should at least satisfy one of two criteria:

i. have a higher selectivity to C02: this gas should be more "attached" to the surface of the material than CH4; in most solids C02 can create stronger bonds with surface groups than CH4. This kind of materials will be termed as equilibrium-based adsorbents since its main selectivity is due to differences of interaction forces between C02 and CH4 with and the surface.

ii. the pores of the adsorbent can be adjusted in such a way that C02 (kinetic diameter of 3.4 A) can easily penetrate into their structure while larger CH4 molecules (kinetic diameter of 3.8 A) have size limitations to diffuse through them. These materials will be termed as kinetic adsorbents since its main selectivity is due to diffusion constrains.

Carbon molecular sieves are one of the most employed materials for biogas upgrading. Adsorption equilibrium isotherms of C02 and CH4 in CMS-3K (Takeda Corp., Japan) are shown in Figure 2 (Cavenati et al., 2005). This material has a clear selectivity towards C02, but the most important property in CMS-3K is not its equilibrium selectivity, but the kinetic selectivity. In this material, C02 adsorbs much faster than CH4: adsorption equilibrium of CH4 is achieved only after two days of solid-gas contact. In fact, the pore mouth of CMS-3K is narrowed to dimension closer to the kinetic diameter of CH4 creating a specific resistance (mass transfer in the micropore mouth) (Srinivasan et al., 1995) that can be seen in the initial moments of CH4 uptake in Figure 2 (b). Another material that also presents strong resistance to CH4 diffusion is ETS-4 (titanosilicate-4) modified with alkali-earth metals (Kuznicki, 1990; Marathe et al., 2004; Cavenati et al., 2009). In this material, the pore diameter can be tuned with different heating temperatures resulting in a "molecular gate" effect that actually named the process commercialized by Guild Associates Inc. (USA).

0ther normally employed adsorbents are activated carbons and zeolites. In these materials, the diffusion of both gases can be very fast and actually what is exploited is the difference between loadings of C02 and CH4. An example of these equilibrium-based materials is given in Figure 3, where adsorption equilibrium of C02 and CH4 on zeolite 13X (CECA, France) is shown (Cavenati et al., 2004). Note that the loading of C02 is much higher than the loading of CH4 at given P, T conditions. Furthermore, recasting the conclusions taken from Figure 1, the cyclic C02 capacity at lower temperatures is smaller than at higher temperatures, which means that if zeolite 13X is employed at 323 K, it will be easier to regenerate than at 298 K.

Fig. 2. Adsorption of C02 and CH4 in carbon molecular sieve 3K (Takeda Corp, Japan) at 298 K: (a) adsorption equilibrium; (b) uptake rate curves (data from Cavenati et al., 2005).

Another topic that is important for the selection of materials for the PSA process for biogas upgrading, is the presence of contaminants. Apart from CH4 and CO2, other gases present in biogas are H2S and H2O. In almost all adsorbents, H2S is irreversibly adsorbed, reason why it has to be removed before the PSA process. When carbonaceous materials are employed it is possible to remove H2O in the same vessel as CO2. However, that is not possible using zeolites since water adsorption is also very steep, resulting in a very difficult desorption.