Strategies for the Global Optimization of Integrated Process Water Networks

In this paper, we propose special strategies for obtaining the global or near global optimum solution from a general superstructure proposed recently by the authors for the design of integrated process water networks. The proposed model of the integrated water network is formulated as a Nonlinear Programming (NLP) and as a Mixed Integer Nonlinear Programming (MINLP) problem for the case when 0-1 variables are included to model the cost of piping and/or selection of technologies for treatment. The MINLP model can be used to find optimal network designs with different number of streams in the piping network. The proposed strategies rely on bounds on the variables that are derived as general equations obtained by physical inspection and using logic specifications needed for solving the model. The cut proposed in 1 and some variations of it are also used to significantly improve the strength of the lower bound for the global optimum. It is shown that the proposed strategies can effectively solve large-scale problems, and in most cases, to global optimality. Furthermore, the proposed strategies allow to readily obtain networks of varying degrees of complexity by limiting the number of piping connections