MS in Chemical Engineering (M.S.C.H.)
Degree Granting Department
Babu Joseph, Ph.D.
John N. Kuhn, Ph.D.
Scott Campbell, Ph.D.
Biofuel, Biogas upgrading, Methane upgrading
Biogas, being rich in methane, can be used as a fuel for various end uses such as electricity generation, compressed natural gas (CNG), production of liquid fuels, industrial heating, etc. CO2 is the major contaminant in biogas (30–50%) along with other impurities such as NH3, H2S, and water. CO2 in the biogas decreases the heating value of biogas. Natural gas pipelines and vehicle use require high purity (> 95%) CH4. There are many commercial techniques available for CO2 removal from biogas such as such water scrubbing (WS), chemical scrubbing (CS) using amine solutions, and pressure swing adsorption (PSA). These techniques have disadvantages including corrosion problems in pipelines, heavy use of water, and high energy requirement in the regeneration step and/or drying steps. Contaminants such as H2S needs to be removed in advance because it can poison the catalysts/adsorbents used in many of these processes. Production of natural gas from biogas requires high pressures in order to separate CO2 from CH4 and compressor and compression costs can be high. CO2 adsorption using amine functionalized silica is a low pressure process and can reduce the capital and operating expenses of compressors required in biogas upgrading. Mesoporous silica such as SBA-15 can be functionalized with amine groups and have proven to be highly selective CO2 adsorbent for CO2/CH4 separations. They have low energy requirements and low regeneration costs as regeneration can be easily carried out at temperatures from 80–120 °C depending on the adsorbent material. In this work, CO2 separation from CO2/CH4 mixtures using 3-Aminopropyltriethoxysilane (APTES) modified silica was studied. It was proven to be a highly selective CO2 adsorbent with good working capacity of about 2 mmol/g. Using APTES modified silica for CO2 separation from actual biogas have not been studied before to the best of our knowledge.
In the work reported here, APTES was immobilized on mesoporous SBA-15. It was prepared using conventional grafting techniques. Techniques including XRD, N2 physisorption, FTIR and TPO were used for sample characterization. A series of APTES modified SBA-15 were tested for adsorption experiments of CO2 at room temperature and 1 atm for a dry 50% CO2 in He feed. Results show that with an increase in APTES loading from 12 to 26 wt% APTES the CO2 adsorption capacity increases from 0.069 mmol/g to 0.85 mmol/g. The presence of water did not affect the CO2 adsorption capacity; however, water adsorption increases with increase in water concentration in the feed as silica is capable of water adsorption independently of the grafted moieties. The results suggest that adsorption of water and CO2 are happening in two different sites because of which CO2 adsorption remains constant even when water concentration in the feed increases. Regeneration study in the presence of water showed almost constant CO2 adsorption capacity for 5 cycles. CO2/CH4 adsorption study in He and dry CO2/CH4 feed in 1:1 ratio, showed that the sample has high affinity to CO2. Also, the adsorption capacity of CO2 does not change in the presence of CH4. The adsorbents showed a decrease of 30% in adsorption capacity (0.50 mmol/g) when landfill gas was used as feed because of site blocking by impurities present in biogas. However, consistent CO2 adsorption capacities were obtained for 5 regeneration cycles. From the study, APTES modified silica adsorbents are promising for removal of CO2 and H2O simultaneously from biogas.
Scholar Commons Citation
Gopalakrishnan, Umadevi, "Carbon Dioxide Adsorption Using Solid Amine Sorbents" (2019). USF Tampa Graduate Theses and Dissertations.