| Increasing consumption of fossil fuels has led to increased emission of greenhouse gases and a range of environmental issues.Biogas,being a clean source of renewable energy,can be used to produce biogas-based nature gas(BNG)replacing fossil fuel.However,its limitation of calorific value owing to the presence of carbon dioxide and other contaminants cannot meet high-quality energy demand in actual production process.Consequently,it is necessary to separate carbon dioxide from biogas primarily.Among various decarburization technologies,pressure swing adsorption has by now occupied a large share of global biogas upgrading market on account of some advantages,that is low initial investment and running cost,low energy consumption,high degree of automation and environmental benignancy.The core of the adsorption technique is mainly found in porous adsorbents.Nevertheless,current adsorbents encounter few issues,such as low adsorption capacity,lack of stability and economy.Therefore,in this study,inexpensive and readily available activated alumina adsorbent was modified by impregnation method with three categories of reagents,involving corrosive inorganics,organics with oxygen-containing functional groups and linear organic amine.Furthermore,the optimization of carbon dioxide adsorption operating conditions to improve the adsorption capacity was carried out experimentally via static adsorption method.To understand the mechanism of adsorption process,the physicochemical characteristics of the adsorbents was evaluated by X-ray diffraction analysis(XRD),Fourier transform infrared spectroscopy(FTIR)and Brunauer-Emmett-Teller(BET).For exploration of the optimal fitting curve,several kinetic models such as pseudo-first-order kinetic model,pseudo-second-order kinetic model and Elovich kinetic model were applied to the adsorption equilibrium data.Activated alumina adsorbent was modified by impregnation method with corrosive inorganics,involving potassium hydroxide(KOH),hydrogen chloride,boric acid,zinc chloride,cupric nitrate and ammonium sulfate.The temperature,pressure,and molarity of KOH were considered in the range of 25～85°C,0.3～0.8 MPa,and 0～12 mol/L,respectively,as variables.The optimum temperature,pressure and duration of equilibrium adsorption were obtained 25°C,0.5 MPa and 14 minutes respectively.In addition,the maximum adsorption capacity was calculated 36.09 m L/g for modified activated alumina with 8 mol/L KOH,which had satisfactory stability in the range of 25°C to 70°C.Nevertheless,the maximum capacity of cyclic adsorption was obtained 23.12 m L/g for modified activated alumina with 1 mol/L KOH,which maintained the maximum adsorption capacity of 83.15%after 7 cycles.Characterization analysis showed that both the load of hydroxyl ion and potassium carbonate and the increase of hydroxide radical were attributed to the better performance of carbon dioxide adsorption.Based on the obtained values of the coefficient of regression(R~2),the pseudo-second-order kinetic model(R~2>0.97)was the best-proposed model for the adsorption process.Activated alumina adsorbent was modified by impregnation method with organics with oxygen-containing functional groups,involving monoethanolamine(MEA),diethanolamine,triethanolamine,glacial acetic acid,sulfamide,salicylamide.The temperature,pressure,and molarity of MEA were considered in the range of 25～70°C,0.3～0.7 MPa,and 1～10 mol/L,respectively,as variables.The optimum temperature,pressure and duration of equilibrium adsorption were obtained 25°C,0.6 MPa and 16 minutes respectively.In addition,the maximum adsorption capacity was calculated 26.90 m L/g for modified activated alumina with 8 mol/L MEA.Nevertheless,the maximum capacity of cyclic adsorption was obtained 20.98 m L/g for modified activated alumina with 2 mol/L MEA,which maintained the maximum adsorption capacity of 85.40%after 7 cycles.Characterization analysis showed that both the load of amidogen and hydroxide radical and the increase of micropore(1～2 nm)volume were attributed to the better performance of carbon dioxide adsorption Based on the obtained values of R~2,the pseudo-first-order kinetic model(R~2>0.99)was the best-proposed model for the adsorption process.Activated alumina adsorbent was modified by impregnation method with organics with linear organic amine,involving pentaethylenehexamine and1,2-propylene diamine(PDA).The temperature,pressure,and molarity of PDA were considered in the range of 25～70°C,0.3～0.7MPa,and 1～8 mol/L,respectively,as variables.The optimum temperature,pressure and duration of equilibrium adsorption were obtained 25°C,0.6 MPa and 19 minutes respectively.In addition,the maximum adsorption capacity was calculated 28.11 m L/g for modified activated alumina with 7 mol/L PDA.But,the performance of cyclic adsorption was unsatisfied enough.Characterization analysis showed that both the load of amidogen and the increase of micropore(1～2 nm)volume were attributed to the better performance of carbon dioxide adsorption.Based on the obtained values of R~2,the pseudo-first-order kinetic model(R~2>0.999)was the best-proposed model for the adsorption process. |
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