Total site heat integration using the utility system

Abstract

Plant sites are normally divided into regions consisting of the individual production processes and the utility system(s). While heat integration within the individual processes has been common for a long time, applications of heat recovery between processes, as well as systematic techniques for this, are only recently being reported. A significant difference when implementing the new approach is that the steam system provides the medium for indirect heat transfer between the processes, so the existing utility lines can be used. In order to minimize the overall cost of such schemes, the fuel consumption and power generation in the utility system need to be considered with the capital cost (mostly heat exchangers) to be invested in the processes. The task requires a proper costing approach for the steam usage, so that the duties and number of levels of steam can be best determined. This paper shows a graphically based procedure for such an optimization which uses the targeting methods of Pinch Technology. Heat integration between different processes on a plant is normally carried out through the utility system. Getting the cost optimum solution can be a very complex problem when the capital cost for the heat exchanger networks and the fuel and power costs in the utility system have to be considered simultaneously. By using exergy analysis for steam costing, the resulting steam costs can act as an interface between the utility plant and the processes. With this cost interface, the subsequent design of the utility system and process heat exchanger networks can be treated as two separate tasks. The energy-capital tradeoff for the heat recovery in each process regions is now carried out in two stages. Firstly, a large number of steam levels is used for the energy/capital tradoff in order to generate an ideal utility profile which decides the minimum total cost target. Next, only a few of these steam levels are selected to arrive at a practical utilities placement. This result is normally very close to the minimum cost target given earlier from the ideal profile. Once the optimum steam levels and their duties are obtained, design of the utility system can be carried out.