Numerical Modeling of Methane Decomposition for Hydrogen Production in a Fluidized Bed Reactor
Younessi Sinaki, Maryam
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The decomposition of methane for hydrogen production is an attractive alternative to the established method of reforming. This process considerably reduces the emission of greenhouse gases, and its overall efficiency and cost are competitive. The decomposition of methane is performed with a catalyst to produce a substantial amount of hydrogen, and decrease the operating temperature. Between different catalysts available, carbon is selected in this study due to its low rate of decay and advantages such as low cost and availability. Also, a fluidized bed reactor operating in the particulate regime is employed due to the efficient contact between the catalyst and the gas. Consequently, hydrogen production from the thermocatalytic decomposition of methane in a particulate fluidized bed reactor of carbon particles is investigated. To obtain an appropriate design and operation for this process, the effect of different operating parameters and catalyst properties should be investigated on the performance. This aim can be achieved by modeling. A number of models with different complexities have been proposed for this process. Considering the objective of this thesis, a complex kinetic model is required to represent the effect of the catalyst properties. In literature, the kinetics is generally modeled with a global equation using experimental parameters. Since investigation on the effect of the properties of the catalyst is not feasible with this method, the detailed kinetic model with a surface reaction mechanism is employed in this study. Investigation on this surface mechanism is very limited, and only one of the models available in literature is determined to be appropriate. Nevertheless, this model has some important drawbacks. The major problem is that the specific surface area is considered as the only catalyst property affecting the activity of carbon. Experimental studies suggest that the activity of this catalyst is a function of its specific surface area and number of active sites, and neglecting either of these properties can lead to a high inaccuracy. Consequently, a new kinetic model is developed where a modified form of the available mechanism is used, and the number of active sites and the specific surface area of the catalyst are considered in the rate equations. It is noted that although several experimental investigations have been performed on the origin of the active sites, their quantity has not been acceptably determined yet. A method is presented in this study to estimate the number of active sites with the developed model and experimental data. To the best knowledge of the author, this is the first model to incorporate the effect of this parameter for carbon catalysts in the decomposition of methane and quantify its value. Another important problem of the model available in literature is its dependency on experimental measurements for determining the hydrodynamic characteristics of the fluidized bed. In this study, the hydrodynamics of the reactor is modeled with empirical correlations to obtain a complete representation of the process within the required accuracy, with minimal experimental requirements. The model is used to investigate the effect of different operating parameters and catalyst properties on the amount of the initial methane conversion. The operating parameters studied are the temperature, residence time, gas velocity, and composition of the feed gas. The catalyst properties considered are the particle size and pore volume, the number of active sites, and the percentage of fine particles in the bed. The effect of the variations of each of these factors in a certain range is investigated for a fluidized bed reactor operating at the onset of fluidization at nominal condition. The onset of fluidization is maintained by changing the inlet flow rate in a reactor of a specific size. The results show that, considering the range of variations in this study, the procedures that cause the highest improvement in conversion are: increasing the residence time, decreasing the size of particles, adding fine particles to the bed, increasing the temperature, using catalysts with high surface areas or large number of active sites, changing the inlet gas composition, and using catalysts with large pore volumes, respectively. It is noted that all of these improvements are associated with higher initial or operating costs. Therefore, changing each of these factors beyond a certain value is faced with economic and technical barriers. Consequently, the possibility and efficiency of using two factors simultaneously for achieving higher conversions was also investigated. The results can be used as a guideline to choose between several catalysts considering their characteristics, or to suggest appropriate operating conditions.