Steam System Network Analysis, Synthesis and Optimisation

Abstract

Steam is a commonplace utility in chemical processing plants across the globe. The many benefits of steam ensure its continued use, but concerns about the cost of energy and of the equipment associated with steam systems has led to the development of a number of techniques to reduce energy and capital costs. One such topic is the reduction of boiler purchase cost, brought about by a reduction in steam flowrate. Recent publications have shown that the flowrate of steam required for heating purposes can be minimised by employing hot liquid reuse, with systematic methods developed for targeting the minimum flowrate, and synthesising the heat exchanger network. In this work, a mathematical analysis has been performed to gain insight on how choosing different steam levels affects the minimum total steam flowrate. The analysis covered both the traditional practice of only utilising latent heat, as well as the new practice of hot liquid reuse. It was found that the lowest flowrate obtainable occurs in the case of hot liquid reuse, when only a single high pressure steam level is considered. Since the need to provide shaft work or generate electricity necessitates the presence of steam turbines on plants, the inclusion of additional steam levels is unavoidable. For this reason, a novel MINLP formulation was developed to provide a holistic coverage of the heat exchanger network and the power block. The purpose of the new formulation is to target the minimum total steam flowrate, whilst simultaneously selecting the optimum saturation temperatures for the additional steam levels, designing the turbines to meet shaft work requirements and synthesizing the heat exchanger network. Application of this new method to a case study yielded a 28.6% reduction in total steam flowrate, compared to common design practice. i I, Sheldon Grant Beangstrom, with student number 27069771, declare that:  I understand what plagiarism entails and am aware of the University of Pretoria‟s policy in this regard.  This dissertation is my own, original work. Where the work of another has been used (whether from a printed source, the internet or otherwise) due acknowledgement has been given and reference made in accordance with the departmental guidelines.  I have not made use of another student‟s previous work in an attempt to submit it as my own.  I have not allowed, nor will I allow another person to copy this work with the intention of presenting it as his or her own work.  The material presented in this dissertation has not been submitted to another institution in partial or whole fulfilment of another degree.