Design of carbon dioxide dehydration process using derivative-free superstructure optimization

A comprehensive optimal design for the CO₂ dehydration process created by decomposition-based superstructure optimization is proposed. To reach the most economical process configuration, the superstructure model has been developed including binary interaction parameter regression of the NRTL-RK thermodynamic model, unit operation modeling, and identification of the connectivity of each of the unit operations in the superstructure. The superstructure imbeds 30,720 possible process alternatives and unit operation options. To simplify the optimization problem, the process simulation was explicitly carried out in a sequential process simulator, and the constrained optimization problem was solved externally using a genetic algorithm and an Aspen Plus-MATLAB interface. The optimal process includes a five-stage contactor, a nine-stage still column (with the feed stream entering at the seventh stage), a lean/rich solvent heat exchanger, and a cold rich solvent split flow fed to the first stage of still column. The total annualized cost of the optimum process is 6.70 M$/year, which corresponds to the specific annualized cost of 1.88 $/t CO₂. As part of the process optimization, a Monte Carlo simulation was performed to analyze the sensitivity of utility cost volatility; the refrigerant and steam present the most influential utility costs.