Modeling, control, and optimization of fixed bed reactors
Gudekar, Kishor G.
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In this work, modeling and optimization of an industrial vinyl acetate reactor, and modeling, optimization, control and bifurcation analysis of industrial ethylene oxide process is performed. For a vinyl acetate reactor, a steady state two-dimensional homogeneous model is developed. The catalyst activity is expressed as a nonlinear function of catalyst age, shell side coolant temperature and the moderator used in the reaction. Offline optimization is carried out for the vinyl acetate reactor using a steady state reactor model to find an optimal operating temperature profile, which maximizes the profit of the process. Updating the model parameters online does online optimization. The ethylene oxide process studied consists of a feed effluent heat exchanger, a multitubular fixed bed reactor, a steam generator, and a separation system. The exothermic heat of reaction from the reactor is removed by passing coolant on the shell side of the reactor. A portion of the heated coolant is passed through a steam generator to produce steam, and the total coolant stream is recycled back to the shell side of the reactor. A single-loop PID control system uses the flow rate of the coolant that is passed through the steam generator to maintain the inlet temperature of the coolant to the reactor. A two-dimensional heterogeneous dynamic model is developed for a catalytic multitubular ethylene oxide reactor. The catalyst deactivation is modeled as a nonlinear function of operating time and temperature of the reactor. Sequential quadratic programming (SQP) is used to solve this nonlinear programming problem. An optimal temperature profile is found which maximizes the profit over the existing operating conditions for the fixed run length of the reactor. The open-loop and closed-loop stability studies are conducted using the benchmarked model of an ethylene oxide reactor system. Steady-state nonlinear bifurcation analysis is performed to identify the multiplicity in the heat integrated ethylene oxide reactor system. The effect of manipulated (flow through steam generator) and disturbance (reactor inlet carbon dioxide composition) variables are addressed. An analysis of the stable control region of the system is developed as a function of operating temperature, catalyst activity, and disturbance direction and magnitude.