590 Section III Synthesis and Optimization of Chemical Processes Even though the steady-state simulation can be used for a number of studies, the focus was on optimization studies. The advantage of EO methods for optimization of complex and highly interacting process systems was demonstrated through an example. In this chapter, discussions of the theoretical background of some of the topics, even though not exhaustive, were provided to enhance the reader's understanding of what happens in the background when using a process simulator. This understanding can help in better uti- lization and efficient troubleshooting of steady-state process simulations. WHAT YOU SHOULD HAVE LEARNED · User-added models should follow simulator guidelines so that they can be integrated with other unit operation models. · Proper selection of a solution method among sequential modular (SM), equation- oriented (EO), and simultaneous modular (SMod) coupled with the appropriate algorithm(s) may be required to solve large, complicated, and highly integrated process simulations. · For troubleshooting convergence failure of optimization problems, it is necessary to ensure that an optimum exists in the feasible region. · To ensure the highest accuracy of simulation results, thermodynamic models should be compared to actual data and, if necessary, parameters should be estimated by using the data regression tool available in a process simulator.