A Heat Load Distribution Method for Retrofitting Heat Exchanger Networks

Fluctuating energy prices, increasing environmental concerns, and regulations push industries toward more energy efficient plants. Process integration (PI) techniques, proven to be effective in providing solutions with improved energy and material efficiencies often neglect modifications to the heat exchanger network (HEN). HEN design methods have been studied extensively to overcome this drawback of process integration but often still focus on grassroots design to suggest retrofits. This work proposes a method to solve heat load distribution (HLD), a sub problem of HEN design, in the context of retrofit problems. The problem is solved using a mixed integer linear programming (MILP) model with integer cuts (IC) to obtain many retrofit options with high computational speed. The model is built on previously developed methods for PI (Marechal and Kalitventzeff, 2003) and HEN design (Ciric and Floudas, 1989; Mian et al., 2016), such as mathematical programming (MP) and pinch analysis (PA). The objective function of the proposed method is minimisation of the estimated cost of the modifications required in an existing HEN, by considering the costs of repiping, additional heat exchanger area and additional heat exchangers. The estimated area of the potential stream matches is calculated using graphical techniques on the process integration results and taking into account correction factor. The additional heat exchanger area is constrained to practical limits available in the literature. The cost of heat exchanger area is calculated using piece-wise linearization of nonlinear cost functions. Solving the model yields information on stream matches and their heat loads as well as identifying which streams should be repiped and which heat exchangers should be modified or replaced. An industrial case study is solved to show the effectiveness of the proposed method resulting in annualized cost reductions of 9% considering the HEN design alone and 29% with modifications to the utility system to include heat pumping.