Simultaneous Synthesis of Heat-Integrated Water Network with the Two-Step Approach

Abstract

Determining the optimal design of a Heat-Integrated Water Network (HIWN) is a complex task due to the existence of highly nonlinear relationships when considering concentrations, mass and heat balances. Due to that complexity, the solution is usually obtained by a sequential approach. In this approach, the minimum fresh water and utility consumption is firstly determined. Then, the exact design of the HIWN is synthesised with the fixed minimum fresh water and utility consumption. Another way to obtain the HIWN design is by using a simultaneous approach. However, such a model can be highly nonlinear requiring the application of a special solution strategy. In this work, a two-step approach was developed and applied. In the first step, a targeting Mixed-Integer Nonlinear Programming (MINLP) model with a high share of linearity was applied estimating also the HEN investment. In this way, the solutions are steered towards an optimal solution by establishing appropriate trade-offs between investment and operating costs during the targeting step. Based on the results of the first step, a reduced superstructure and MINLP model are used in the second design step to select promising matches for heat exchangers. By excluding the non-promising matches that previously led to an unnecessary increase in the complexity of the synthesis model, the second step MINLP model performs much better and enables synthesizing the entire HIWN simultaneously. The aim of this work was to verify whether the two-step approach is suitable for solving the HIWN problem. The obtained solution for case study considered in this work indicated the applicability of the proposed approach, which will be also applied in further research on large-scale HIWN problems.