Simultaneous water and energy integration with isothermal and non-isothermal mixing – A P-graph approach

Abstract

Heat-integrated water network (HIWN) is an active area of research in the past decade. It focuses on simultaneous saving of energy and water, which are both important resources to enhance sustainability for the process plants. This problem has been previously approached using insight-based pinch analysis and mathematical programming techniques. In this work, an alternative optimization tool, process graph (P-graph) framework is utilized to solve for water and energy integration. P-graph is a graph-theoretic approach to process synthesis, for which the developed algorithms can reduce the complexity of the mathematical programming algorithms efficiently. P-graph methodology enables the effective generation of sub-optimal network structures that allow wider exploration of alternatives through network topology. This paper also introduces some transformation techniques to convert the non-linear programming (NLP) model in the non-isothermal HIWN problem into model which is solvable using P-graph. Two examples with different water recovery schemes are used to elucidate the proposed approach, covering both direct recycle/reuse and regeneration networks; for both cases, isothermal and non-isothermal mixing problems are solved.