A METHODOLOGY FOR COST-EFFECTIVE THERMAL INTEGRATION OF PRODUCTION PLANT SECTIONS AND THE UTILITY SYSTEM Licentiate’s Thesis

The main objective of this thesis is to develop a systematic methodology for cost-effective thermal integration of production plant sections and the utility system of an industrial site. A production plant section is a subprocess of a production process that transforms materials into intermediate or end products. A production process is built up from a group of these sections or sub-processes linked together in a logical way. Typically a central utility system provides the utilities that the production sections need. The methodology developed in this thesis can analyze and optimize the energy system of this kind of total site. This is done in a manner such that the production volume of the production plant sections is unaffected. Heat can be transferred between and within the production plant sections so that the total utility consumption is minimized with minimum heat transferred between sections. The minimum investments needed in the heat exchanger network are also obtained. The utility system defines the economic potential of utility savings. The methodology combines simulation, thermodynamic analysis and mathematical programming. Simulation is used to provide mass-and energy balances and to provide a solid base case. The mathematical models created in this thesis are formulated as mathematical programming problems, but they are based on insights given by thermodynamic analysis tools. Using mathematical programming models and algorithms gives the benefit that operational and investment costs can be optimized. Additional information, e.g. marginal costs, can be obtained directly from the optimization results. The mathematical model is of a sequential type, where the optimization procedure proceeds with different information at different levels. The process streams are clustered into different sections based on the assumption that investment costs related to heat exchange are smaller when heat is transferred within a section compared to investment costs related to heat exchange between sections. In this way, the problem is decomposed into smaller sub-problems so that very large optimization problems can be solved. Restrictions can be made to different flows. For example, some streams have to be heated by a hot utility but on the other hand there may be streams that either must or must not exchange heat between each other. The main steps of the methodology are: • Data extraction from the production plant sections and the utility system • Simulation models for the production plant sections and the utility system • Stream data extraction from the simulation models • Optimization models for …