A new method is proposed for systematic generation of conceptual design of reactor networks. Given feed compositions and a kinetic model, the objective is to find the optimal mixing structure and feed distribution. The method aims at finding the optimal sequence and sizes of ideal reactors and the optimal addition of extra feed streams along the reactor path. The total reaction time is calculated so as to maximize the space time yield subject to a minimum yield of the key product component. The method does not have any limitations with respect to the number of components or reactions. A new model formulation is proposed that comprises both CSTR and PFR model equations and the design problem is formulated as an optimal control problem. In this paper, only isothermal conditions are considered. (C) 2004 Elsevier Ltd. All rights reserved.