Simultaneous synthesis of utility system and heat exchanger network incorporating steam condensate and boiler feedwater

Abstract

A heat exchanger network (HEN) is an important part in processing plants used to recover heat from process streams. A utility system supplies heating and cooling utilities and introduces additional hot and cold streams for the processes. The HEN and utility system (e.g., Rankine cycle-based cogeneration system) are closely interconnected primarily through steam, steam condensate leaving the turbines, and process surplus heat. The recovery of the sensible heat from the steam condensate and process surplus heat through an integration technique may contribute significantly to the reduction of the heating and cooling utility consumption in the heat exchanger network as well as in the primary energy consumption in the utility system. In this paper, a systematic methodology for the simultaneous synthesis and design of a utility system and HEN is proposed. The heat recovery from the steam condensate and boiler feedwater preheating are integrated into the HEN synthesis together with the design optimization of a Rankine cycle-based utility system. In addition to the simultaneous design of the utility and heat-recovery systems, the optimization variables include the steam condensate target temperature, the steam level for process heating, the energy demand for the utility system, the returning temperature of the steam condensate, and the final temperature of the boiler feed water. The total site HEN is composed of several interlinked sub-HENs. A model for the new hot utility-process cold stream HEN is formulated together with the hot -cold process streams of the HEN. The linking constraints between sub-HENs and the utility system are formulated. Several case studies are elaborated to demonstrate the effectiveness and applicability of the proposed methodology. Compared with the former design methods without integrating steam condensate sensible heat and boiler feedwater preheating, meaningful economic benefits can be achieved by applying the proposed framework.