On the process integration of organic Rankine cycles and absorption chillers into heat exchanger networks

Abstract

The industrial sector accounts for almost a third of the global GHG emissions, from which around 80% correspond to energy-related emissions. The decrease of energy consumption in the industrial sectors has therefore a direct impact in the reduction of the global GHG emission as required by the Paris Agreement, in order to limit the increase of the global average temperature below 2°C above pre-industrial levels. Around 30% of the energy input into the industrial sector worldwide is released unused to the environment as waste heat. The internal and external recovery of waste heat represents in consequence, an attractive strategy for the reduction of the industrial energy consumption. Typically, the internal waste heat recovery and the external waste heat recovery are treated as separated problems in the hierarchical sequential approach for the design of industrial processes. Although a practical and successful design strategy, this sequential approach neglects possible synergies generated by considering simultaneously the internal and external waste heat recovery options during the process design. In this work, a mathematical framework considering simultaneously internal (represented by the synthesis of the heat exchanger network for the system) and external (represented by the use of waste heat transformation technologies) waste heat recovery options is presented. The mathematical framework focuses on two of the most mature waste heat transformation technologies, Organic Rankine Cycles (ORCs) and Absorption Chillers (ABCs), and integrates them into Heat Exchanger Networks (HENs) in continuous and multi-period process with and without Fixed Temperature Variable Mass (FTVM) heat storage. The generated system designs have the potential to be economically, technically and environmentally more attractive than systems solely factoring heat exchanger networks. The main conclusion from this dissertation is, that combined design methodologies, considering the process integration of ORCs, ABCs or both, into HENs in continuous and multi-period processes with and without FTVM heat storage, can generate economically, technically or environmentally attractive system designs.