First, an unifying concept : the Selfregulating Structure Philosophy is explained based on i) the decomposition of the plantwide control problem, ii) the plantwide component inventories and iii) the rationalization of the unit control strategy. After this, the optimum steady-state distillation control problem is evaluated based on this concept and the impact of the extensive process diversity on the distillation column control is reviewed. From the viewpoint of controllability those process modifications are proved to be beneficial by which we can create new design variables, Second, the optimum steady state plantwide control problems are evaluated and illustrated on case studies, then a procedure for control system design is outlined which gives guide-lines to the designer how and in which phases of the design process the different aspects of operability should be considered.