Utilizing the greenhouse gas CO2 as a feedstock in chemical processing can offer alternative solutions to long term storage. In this study, a systematic analysis of methanol synthesis performance was analyzed based on both thermodynamic equilibrium and kinetic models using captured CO2 and syngas produced from biogas as feedstock. Using reactor inlet temperature as a parameter, it was found that methanol yield can be enhanced by increasing residential time from increased reactor diameter. The longer reactor can increase the residential time but a large pressure drop caused a decrease in methanol yield. Due to the exothermic reaction nature, methanol yield from an adiabatic reactor is lower than that from the isothermal reactor due to temperature rise. From the results obtained for CO2 hydrogenation, methanol yield can be enhanced by water removal. The CO2 conversion was found to increase with increased reaction temperature due to methanol and carbon monoxide productions. Using CO and CO2 as limiting species, high combined CO and CO2 conversion can be obtained from syngas with low CO2/H2 and high CO/H2 ratios.
In this study, a systematic analysis for the effects of reactor geometry, operating conditions, syngas composition, and unreacted syngas recycle on methanol synthesis performance is carried out. The biogas with various CH4/CO2 compositions is used as the feedstock for syngas production. A combined dry and steam reforming process for producing a suitable composition of syngas is discussed. In addition, the performance of a green process for methanol synthesis using captured CO2 as the feedstock is also presented and compared with the results from traditional syngas feedstock. Both thermodynamic equilibrium and kinetic models are employed in this study and detail comparisons on feedstock conversion and methanol yield between these two models are addressed.
The research results will be discussed in the following categories:
(1) Model verification
(2) Effect of reactor size
(3) Effect of reaction operating conditions
(4) Methanol synthesis via CO2 hydrogenation
(5) Effect of syngas composition
(6) Syngas production from biogas reforming
(7) Methanol synthesis process
In this study, the performance of methanol synthesis was analyzed systematically using syngas or captured CO2 as feed and tubular fixed bed reactors.
(1) A reactor with a greater diameter and shorter length is suggested for increasing the residential time of reactant and enhancing the methanol production.
(2) The isothermal reactor can have a higher methanol yield as compared to the adiabatic reactor under the same operating conditions.
(3) The methanol yield from CO2 hydrogenation can be enhanced by the removal of side product water. The optimum methanol yield of 25.48 % with CO2 inlet temperature of 252 °C was obtained.
(4) High methanol production can be obtained for syngas with a high CO/H2 ratio, low CO2/H2 ratio, and high H2 utility.
(5) With CH4 and H2O additions and using combined dry and steam reforming with molar ratio of Biogas/CH4/H2O = 1/1/3, syngas with composition satisfied industrial applications for methanol synthesis can be obtained.
(6) Higher methanol production can be obtained for higher CH4 content in biogas.
(7) Recycle of unreacted syngas enhances methanol production.