Following Douglas＇s philosophy of a target-directed hierarchical approach to process design, we propose a general framework for systematic implicit enumeration and evaluation of feasible designs, with successive model refinement leading to a final detailed design. The process is represented by a state-task-network, and feasible networks are generated by assigning a purpose, or function, to each elementary task. The approach is applied to the synthesis of both simple and complex distillation systems, and two examples of homogeneous azeotropic systems are treated in some detail.