Solution Strategies for the Synthesis of Heat-integrated Process Water Networks

This contribution describes the use of sequential and simultaneous strategies for the synthesis of heatintegrated process water networks (HIPWNs). The former strategy consists of the water network (WN) model by Ahmetovic and Grossmann (2011) , which determines the minimum freshwater consumption and the stage-wise model by Yee et al. (1990) for the synthesis of a heat exchanger network (HEN), both of which are performed sequentially. In the recently introduced approach by Ahmetovic and Kravanja (2012) both networks are solved simultaneously by applying a combined (WN-HEN) model. This combined model is formulated as a non-convex mixed integer nonlinear programming (MINLP) problem with the objective function defined as the total annual cost (TAC). As, on the one hand, the combined model in this simultaneous approach enables the obtaining of appropriate trade-offs between freshwater, utilities, and investment, it is, on the other hand, very difficult to solve, due to its nonconvex and nonlinear nature. The synthesis of HIPWNs thus still remains a big challenge. The development of efficient solution strategies is necessary in order to accomplish this task. In this paper, syntheses of HIPWNs are carried out using the above-mentioned strategies. It is worth pointing out, that in the simultaneous approach the problem can be solved directly as one system or the WN solved first to provide a good initial point followed by the overall heat-integrated process water network (HIPWN) problem after. The proposed strategies were tested on a literature Case-Study. The solutions were obtained with the minimum consumption of freshwater and utilities, and TAC was significantly improved, when compared to those reported in the literature. The solutions obtained clearly indicate that the proposed strategies can be successfully applied for the synthesis of HIPWNs.