This paper presents a discussion about traditional control structure and the operation feasible region (OFR) for a process and its uses in design and control. The process used is a flash distillation, which exhibits strong non-linear behaviors due to its operation founded in thermodynamic equilibrium. A procedure to calculate the OFR is proposed using the process model for constantly checking the component separation condition. The found OFR is used to discuss some process control and design aspects. A common control structure, presented in textbooks, is tested and improved including a feed-forward effect. Using new control structure the process performance improves, which is shown by simulation results. In addition, a discussion about moving the operation point using OFR information is given. The new operation point is tested, indicating its better robustness characteristics compared to the original operation point.