Environmental concerns due to the exacerbation of emissions brings about a renewed
interest in the methanation of carbon dioxide. This study is an attempt to explore the plantwide process designs for CO2 methanation using Aspen Plus simulator. In particular, several adiabatic and non-adiabatic reactor designs are investigated. Temperature excursion/runaway is the major challenge repeatedly encountered in both the reactor designs. Several modifications in the process designs are attempted but the designs equipped with internal recycle are found to be more adoptable for this highly exothermic reaction. Process design is followed by the cost optimization for each design. A non-convex multi-constrained optimization is performed using simulated annealing algorithm. An optimization code is written on Matlab, and COM technology is used to interface it with Aspen Plus. A base case configuration at fixed throughput is first developed for all the process designs, followed by the optimization within feasible region which is dictated by the ranges of selected design variables and process constraints. Three optimization runs are carried out for each design. Hydrogen flow is considered as a key optimization variable since it has the highest cost amongst all the other components. The cost benefit from steam generation is also included in the objective function. Non-adiabatic reactor equipped with internal recycle is found to have the lowest cost. Adiabatic reactor designs which have the comparable cost either suffered from the temperature excursion or pressure drop which
renders them unsuitable for the operation. Therefore, the dynamic control performance is only studied for the designs equipped with internal recycle. Several control structures are developed with different control loops which can effectively stabilize the process in wake of the disturbances in throughput and feed composition. It is observed that the control scheme equipped with purity analyzer and subsequent manipulation in the ratio of hydrogen and carbon dioxide molar flowrate can effectively stabilize the process.

Environmental concerns due to the exacerbation of emissions brings about a renewed
interest in the methanation of carbon dioxide. This study is an attempt to explore the plantwide process designs for CO2 methanation using Aspen Plus simulator. In particular, several adiabatic and non-adiabatic reactor designs are investigated. Temperature excursion/runaway is the major challenge repeatedly encountered in both the reactor designs. Several modifications in the process designs are attempted but the designs equipped with internal recycle are found to be more adoptable for this highly exothermic reaction. Process design is followed by the cost optimization for each design. A non-convex multi-constrained optimization is performed using simulated annealing algorithm. An optimization code is written on Matlab, and COM technology is used to interface it with Aspen Plus. A base case configuration at fixed throughput is first developed for all the process designs, followed by the optimization within feasible region which is dictated by the ranges of selected design variables and process constraints. Three optimization runs are carried out for each design. Hydrogen flow is considered as a key optimization variable since it has the highest cost amongst all the other components. The cost benefit from steam generation is also included in the objective function. Non-adiabatic reactor equipped with internal recycle is found to have the lowest cost. Adiabatic reactor designs which have the comparable cost either suffered from the temperature excursion or pressure drop which
renders them unsuitable for the operation. Therefore, the dynamic control performance is only studied for the designs equipped with internal recycle. Several control structures are developed with different control loops which can effectively stabilize the process in wake of the disturbances in throughput and feed composition. It is observed that the control scheme equipped with purity analyzer and subsequent manipulation in the ratio of hydrogen and carbon dioxide molar flowrate can effectively stabilize the process.

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