Flexible Energy Management and Heat Exchanger Network Design

Abstract

The design of heat exchanger networks (HEN) is a well-studied problem in process synthesis and an ideal test base to benchmark methods and techniques in the field. Despite a significant number of relevant publications, networks are still designed under assumptions of fixed operating conditions. Significant variations in supply and demand, alongside a need for efficient management in energy markets (energy grids, deregulated markets), impose limitations to this practice. Networks, designed with thermodynamic and economic efficiency under nominal operation, are known to have their efficiency dissipated and wasted in a context of similar though different conditions and demands. In a process plant, operational changes are common but designers still favor the staged approach of Pinch Technology (i.e., targeting-network development) where flexibility is not addressed properly and systematically. Alternatively, superstructure methods offer formulations with complexities hard to address by conventional algorithms.

In this work, flexibility is addressed in a context amenable to targeting and network development stages, offering opportunities to visualise solutions and review options. For targeting, a dual approach is proposed that follows the framework of Hypertargets by Briones and Kokossis (1999a, 1999b, 1999c). The conceptual screening involves (i) the selection of cost-effective (primal) matches, and (ii) a model-based approach to assess the flexibility of the design options. Models and procedures are employed to assess trade-offs between operating cost (energy), capital cost (area), and the options' ability to handle variations (flexibility). Primal matches are automatically developed into network configurations with the use of mathematical models. A rigorous, superstructure-based approach is next applied to ensure the development of networks capable of handling operational variations without a need to consider exhaustive combinations of scenarios. The iterative approach incrementally augments the mathematical formulation by constraints and vertex conditions that guarantee consistency. The procedure is illustrated with two industrial problems and reports important improvements over conventional techniques.