The feasibility of processes is a well-used notion with several definitions in the literature according to each context. This is understood as the region of operative variables where phenomena of processes occur. In this work, we propose a definition for the feasibility set and explain an algorithm to characterize efficiently its shape and size. Additionally, we define a feasibility index to quantify the belonging grade of a point inside the feasible set, instead of the yes/no usual belonging function. Finally, this paper shows the use of previous concepts for control proposes. In an example, we found the set-point for a process through an optimization problem, guaranteeing the feasibility in the presence of disturbances and improving its productivity.