The operating point of a process is usually computed by optimizing an objective function, e.g. the profit, subject to some characteristics of the plant. Typically, the resulting point lies in the boundary of the operating region. Then, the presence of disturbances can easily cause constraint violations and move the process to operate in an unfeasible region. The purpose of this paper is to show how the operating point can be modified in order to keep feasibility in the process operation against the disturbances. © 1998 John Wiley & Sons, Ltd.