Flash distillation is essentially a single stage mass transfer operation. The flash distillation operation is used in this work to highlight the importance of identifying the zone where a system has an appropriate behavior. In this sense, this paper presents a discussion about the use of the feasible operation region (FOR) in the design of process control structures. For this reason, three control structures are presented. The first one is the traditional one, a PID multiloop control structure. The second structure uses the FOR graphically in order to determine a better operation point and improve the control response. Finally, the third structure uses the FOR numerically for guiding the controller using a modified control structure. The improvement of the closed loop operation of the flash using the last control structure is notorious when compared with the traditional control structure.