A two-level method has been developed for designing solids processing systems in a systematic manner. The system to be designed is described on two hierarchical levels by using two types of models for each process : rigorous and reduced (simple) ones. The higher modeling (and, at the same time, computational) level, called-network level, is the network of operation modules described by means of reduced mathematical models, while the lower one; called process level, is formed by the detailed processes of operation modules, represented by their rigorous ones. This two-level structure greatly meets the:requirements of the method of decomposition-coordination of optimization. The method is illustrated through the design of an extractive crystallization; system which is a frequently used technique for the separation of close-boiling Points compounds.