Separation peformance of CH4/CO2 simulated in biogas for monoethanolamine modified SiO2

Abstract: The major components of biogas by anaerobic digestion are CH4 and CO2, so it is necessary to selectively separate CO2 from CH4 in order to improve heating value of biogas. Adsorption technology has been considered a competitive method for separating gases because of its relatively simple equipment and low energy consumption. It is well known that developing an excellent adsorbent is the key issue for adsorption separation. In the present work, MEA-modified FNG-II silica adsorbent was prepared by a conventional wet impregnation technique and the new adsorbent was named as FS-MEA. Silica was selected as the substrate and monoethanolamine (MEA) was modified at the outer surface of substrate. In order to coat the silica surface with MEA uniformly, MEA was dissolved in methanol in advance. The new adsorbent was characterized by powder X-ray diffraction (XRD), nitrogen adsorption/desorption, thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) to determine pore pacing, pore volume, pore diameter, the organic loading on the substrate and amines. The results of XRD and nitrogen adsorption/desorption for the adsorbents showed that FNG-II silica contains a large amount of disordered mesopores. After 30 wt.% MEA loading, the mesoporous structures were preserved, but the specific surface area and pore volume decreased, which provided strong evidence that MEA could be attached to the inner pore surfaces of the silica. This result is in agreement with FTIR analysis. The TGA curves showed a unique signi?cant mass loss on FNG-II silica under atmospheric pressure from 29℃ to 870℃ and the weight loss was derived from desorption of moisture. However for cases of FS-MEA, two major mass loss steps were observed in the same conditions. The first step in the range of 29℃ to 170℃ was attributed to desorption of moisture and CO2 that absorbed in MEA modified on the substrate. The second mass loss step occurred when the temperature was higher than 210℃, which were ascribed to the volatilization and/or decomposition of MEA. The TGA/DTA results provided further evidence that MEA was successfully modified on the substrate, and the new adsorbent can not be used higher than 210℃. The dynamic adsorption experiments were carried out to investigate the adsorbent's separation capacity. The mixture of CH4 (65.6%) and CO2 (34.4%) was used for measuring breakthrough curves, from which the separation coefficient between the two gases was evaluated. The adsorbent was packed in a steel column of length 250 mm and inner diameter 11 mm, and the gas mixture flowed through the column with 200 cm3/min flow rate. The adsorption experiments were carried out at 298 K and 0.2 MPa. The results indicated that FS-MEA efficiently separated CH4 from CO2. The separation coefficient calculated is 4.53, which is better than that of some conventional adsorbents. Compared to the TGA/DTA results, the regeneration temperature of FS-MEA was identified as 110℃, because moisture and CO2 desorbed from 45℃ to 170℃, and the desorption rate was greatest at 106℃. After regeneration, the separation performance of FS-MEA for CH4 and CO2 decreased after its third use.