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.