Modeling of fixed bed methanation reactor for syngas production: Operating window and performance characteristics

The present work focuses on the development of phenomenological model for the bio-syngas to methane conversion process. One dimensional heterogeneous and pseudo-homogeneous model were simulated for a typical pilot plant scale fixed bed methanator processing 55 mol/h of CO (total molar flow rate of 310 mol/h) with inlet composition of H2/CO = 3, CO2/CO = 1, CH4/CO = 0.5 at 550 K and 1 atm. Performance of the fixed bed reactor at different operating conditions like CO2/CO ratio, H2/CO ratio, effect of H2O in the feed was studied. It was found that for feeds that were not pre-enriched with hydrogen, presence of water and water gas shift activity was found to decrease the catalyst inventory substantially. CO2 in the inlet feed stream would help to decrease the temperature due to dilution effect and more importantly, can be chosen to maximize methane yield per mole of CO converted. Further, the model was simulated to predict the performance characteristics of reactor with a mixture containing two types of catalyst, one of them being specifically added to increase H2/CO ratio in feed through water gas shift reaction. The work also laid the importance of incorporating pore diffusion and external mass transfer locally in the computation of actual catalyst inventory and reactor volume. The work was useful in selection of operating window and assessing the various viable options for an industrial reactor. The model developed will serve in selection of operability window for commercialization of substitute natural gas synthesis (SNG) process.