Self-Optimizing and Control Structure Design for a CO2 Capturing Plant

Capturing and storing the greenhouse gas carbon dioxide (CO2) produced by power plants could play a major role in minimizing climate change. In this study a postcombustion CO2 capture plant using MEA is designed, simulated, and optimized using the UniSim process simulator. The focus of this work is the subsequent optimal operation and control of the plant with the aim of staying close to the optimal operating conditions. The cost function to minimize is the energy demand of the plant. It is important to identify good controlled variables (CVs) and the first step is to find the active constraints, which should be controlled to operate the plant optimally. Next, for the remaining unconstrained variables, we look for self-optimizing variables which are controlled variables that indirectly give close-to-optimal operation when held at constant setpoints, in spite of changes in the disturbances. For the absorption/stripping process, a good self-optimizing variable was found to be a temperature close to the top (tray no.4) of the stripper. To validate the proposed structure, dynamic simulation was done and performance of the control structure was tested.