Efeito dos ultramicroporos e da química da superfície nos mecanismos de retenção de H2S do biogás

Abstract

Adsorption onto activated carbons is an attractive technology for biogas desulfurization. When hydrogen sulfide (H2S) comes in contact with the structure of an activated carbon, both mechanisms of physisorption and chemisorption may occur. The role of each mechanism is still not fully understood and it is the subject of numerous studies. Such analysis requires a detailed characterization of the material microporous structure, since pore size, especially pores below 7 Å, may have an influence on the interaction between H2S and the surface of the material. Therefore, multicomponent adsorption data of H2S in the presence of carbon dioxide (CO2) were obtained for two commercial activated carbon samples, referred to as C141S and Norit RB4, further impregnated with potassium hydroxide (KOH), in order to understand the effect of ultramicropores and surface chemistry on the mechanisms of H2S retention. It was assumed that CO2 would previously fill the ultramicropores of 4 Å (most representative pores of the materials) in such a way that it would not be replaced by H2S when this gas was subsequently injected into the fixed bed column. Molecular simulation data were used to predict the CO2 pressure required to fill these pores. The samples were characterized using X-ray fluorescence (XRF), and N2 and CO2 adsorption/desorption isotherms at 77 and 273 K, respectively. The XRF analysis confirmed the presence of potassium (K) impregnated specie in all modified samples. Textural characterization revealed the presence of ultramicropores in all samples, especially in the modified ones. It was observed that the adsorption capacity of both original activated carbons increased after impregnation, especially for Norit RB4. From the H2S multicomponent adsorption tests, it was observed that the H2S retention capacity decreased significantly for all samples. The results were compared to the theoretical adsorbed amounts of H2S in these pores, obtained by molecular simulation, which allowed to observe that not only physical adsorption of H2S in the ultramicropores was affected, but also other mechanisms that contributed predominantly to H2S adsorption, indicating that these pores are fundamental in order to make all these mechanisms to happen, which is an important information for the development of new materials.