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.
