TAILIEUCHUNG - Biofuel's Engineering Process Technology Part 3

Tham khảo tài liệu 'biofuel's engineering process technology part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Biogas Upgrading by Pressure Swing Adsorption 71 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. Pressure bar Fig. 3. Adsorption equilibrium of CO2 a and CH4 b on zeolite 13X at 298 308 and 323 K Data from Cavenati et al. 2004 . Packed-bed performance Adsorption is a spontaneous process and when the gas is putted in contact with the adsorbent a new equilibrium state will be established depending on the partial pressure of each of the gases and on the total temperature of the system. After achieving such equilibrium no more adsorption takes place and the adsorbent should be regenerated. For this reason a PSA column should be regenerated periodically to be able to absorb CO2 in different cycles. In order to keep constant feed processing more than one column are employed in parallel when biogas is fed for selective removal of CO2 the other column s are being regenerated. The operation of a PSA process for biogas upgrading can be explained by showing what happens when a mixture of CH4-CO2 is fed to a column filled with adsorbent. For simplicity the column will be considered to be at the same pressure of the biogas stream and filled with an inert gas helium . An example of such behaviour is normally termed as breakthrough experiments . An example of a breakthrough curve of CH4 55 - CO2 45 mixture in CMS-3K is shown in Figure 4 Cavenati et al. 2005 . It can be observed that in the initial moments methane molecules travel across the column filling the gas phase in the inter-particle

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