In this paper a hierarchical process control concept for plant-wide quality control is proposed. The concept is based on a formal definition of product quality as an intensive state function in the thermodynamic sense. This quality function is evaluated for each product stream in a chemical plant and can be used as a plant-wide uniform controlled variable. For the assigned extensive state function, called the qualifying potential, a balance equation can be formulated and thus, the production of qualifying potential can be determined for each processing unit of a plant. Therefore, the quality concept can be used for process design as well as for process control.