Steam system synthesis using process integration techniques: a graphical approach for multiple steam levels

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.

Steam is commonly used as the hot utility in the processing industry. The common method of designing the hot utility heat exchanger network (HEN) is to place all of the heat exchangers in a parallel configuration, and to utilize the latent heat of saturated steam. Recent work has shown how process integration and the use of hot condensate can minimize the flowrate of steam through the hot utility. This leads to debottlenecking of the boiler in retrofit designs, or the ability to purchase a smaller and cheaper boiler in a grassroots design. The purpose of this work stems from two main observations. Firstly, the work in published literature has been limited largely to only a single steam level. Many plants have more than one level of steam available, especially if a portion of high level steam is used to operate a turbine which produces exhaust steam at a lower level. Secondly, most modern process integration is conducted as a black-box design using mathematical models. Not all engineers who might want to apply these techniques have access to the expensive solvers and computers required to solve these models. The purpose of this study was therefore to develop a graphical technique that will allow one to design a HEN for minimum steam flowrate in the presence of multiple steam levels. This will be useful both as an educational tool, and to enable engineers with limited access to facilities to apply these techniques using basic drawing packages. The methodology used to apply these techniques involves constructing a limiting feasible utility curve of the cold process streams, and then systematically shifting a number of utility lines to fulfill the energy requirements. In an illustrative example of a grassroots design, application of this synthesis method resulted in a 24% reduction in steam flowrate, a 13% reduction in the capital cost of the steam system and an 8% reduction in the energy demanded from the boiler by the process.