Steam system optimisation using process integration : a focus on boiler efficiency and pressure drop

Abstract

The use of steam in heat exchanger networks (HENs) can be considerably reduced by the application of heat integration and optimisation with the intention of debottlenecking the steam boiler and indirectly reducing the water requirement (Coetzee and Majozi, 2008). The reduction of steam flowrate in a HEN affects the operation of the steam boiler. By reducing the steam flowrate the return condensate temperature to the boiler is compromised which adversely affects the operation of the boiler. A means of maintaining the efficient operation of the boiler whilst still reducing the overall steam flowrate to the HEN is to reheat the return flow to the boiler to a sufficiently high temperature. One means of achieving this is by utilising the sensible heat from the superheated steam leaving the boiler to preheat the boiler feed. Reusing condensate also has the effect of increasing the pressure drop throughout the HEN. The pressure drop is dependent on the size of the heat exchanger or pipe, the flowrate through the exchanger or pipe and also the HEN layout. This creates an intricate situation that must be accounted for. Steam systems typically employ turbines to use energy from superheated high pressure steam to generate shaft work. The exhaust of these turbines is usually saturated steam which is frequently used as a utility in the background process. Since turbines operate at various steam levels, a means for incorporating these steam levels into the HEN optimisation framework is necessary. By reducing the amount of steam required for these HENs an opportunity arises for the use of this steam in further preheating the boiler feed water in an attempt to maintain boiler efficiency. This work involves modelling of the steam system in a mathematical framework and finding the global minimum steam flowrate for the system. The pressure drop throughout the HEN is then also calculated and minimised. The boiler efficiency is maintained by restructuring the HEN and making provision to preheat the boiler feed using synthesis and optimisation. In the event of there not being enough excess heat in the steam system to maintain the boiler efficiency a slight compromise in either boiler efficiency or the minimum flowrate must be made. The initial targeting for the minimum steam flowrate is based on a graphical technique of Coetzee and Majozi, 2008. Consequently, this dissertation concerns the optimisation and restructuring of all steam system heat exchangers using conceptual and mathematical analysis to create a series/parallel HEN with the aim of reducing the overall steam flowrate, whilst maintaining boiler efficiency and minimising the HEN pressure drop.