(330b) A New Algorithm for the Global Optimization of Property-Based Water Integration In Eco-Industrial Parks
Most of the reported methodologies for the wastewater inter-plant integration (i.e., eco-industrial parks) using mathematical programming approaches have three major disadvantages: simplified superstructure, based on composition and do not guarantee the global optimal solution. Therefore, this paper presents an optimization MINLP model based on properties for the wastewater integration in eco-industrial parks considering a new superstructure that involves all configurations of interest like water integration in a single plant and interplant, the existence of sheared treatment units and treatment units exclusive for each plant. The interplant water network considers the regeneration of the streams to improve their properties through a treatment system, whose selection is optimized to meet the property constraints in the process sinks in each plant and the constraints for the environmental discharge. The problem consists in finding the configuration with the minimum total annual cost (i.e., treatment units cost, fresh water cost, piping cost) given a set of process effluents with flowrates and properties known, a set of process sinks with flowrates known and permissible limits of properties. This paper proposes a new strategy for the global optimization of the bilinear terms using a spatial search that solves iteratively two models for the lower bound (using a convex relaxed model) and for the upper bound (using a convex integer model) until the gap between these two models is smaller than a given tolerance. To obtain a valid lower bound, a piecewise under estimator based on the properties has been proposed; however, this increases the problem size but the advantage in the proposed model is that every bilinear term depends on the conditions (flowrates and property operator) in the interceptors, then if the property operators are transformed to parameters we can see the effect in the overall process with only vary the value of the property operator in the regeneration zone, and it is easier than to analyze the effect of all possible flowrates in the superstructure. In addition, this work proposes a new strategy for the branching variables, which is based on the property interceptors too. The model was implemented in the software GAMS and it was applied to several examples without numerical problems. The results show that the optimal solution is reached in a few iterations.