Optimal resilient retrofit design of heat-exchanger networks

Abstract

A new index, called Design Resiliency Index, which numerically indicates the size of feasible region in terms of uncertain source-temperature or flowrates for the fixed design parameters of a network structure, is proposed. A new retrofit design approach for given HEN structures, towards resilient and controllable designs at minimum cost is also proposed. The task of designing a heat-exchanger network for a pre-determined structure (retrofit design), which is not only economically at minimum cost but also has the ability to reject the disturbances predefined for all possible source-streams and in all directions (increasing or decreasing), is introduced as a single nonlinear programming problem. The objective of this constrained nonlinear optimization problem is to find the individual exchanger areas and nominal utility consumptions which minimize the total annualized cost of the given HEN structure and, at the same time, to satisfy the target-temperature constraints for set of disturbances predefined in all possible directions. For the definition of the expected disturbances for all possible directions in source-streams, a new index is also proposed, Retrofit Design Resiliency Index, to be used in the formulation. It is shown that designing a control system using conventional approaches which offers the placement of control variables after the design stage, not only limits the resiliency hence the controllability range of a HEN, but also could be more expensive than the solutions generated by the method proposed in this study.