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Preliminary retrofit design of chemical process plants
A systematic procedure has been developed and a software code created to solve retrofit problems in continuous petrochemical plants. The procedure goes through a hierarchy of steps to determine what is the most economical method of retrofitting an existing chemical plant. The criterion used in determining the retrofit of a specific plant is either reducing operating costs (constant production rate) or increasing profits (unknown production rate). Refining this procedure and demonstrating that it is reasonable, by developing a general purpose software code, is the major contribution of this research.^ The procedure is hierarchical and provides stopping points so that little work needs to be done to reject a poor project. It is structured in a way that first sets minimum targets for the retrofit project and uses an efficient flowsheet structure filter to look at only the subset of possible flowsheet structures that have the potential for being profitable. The procedure then decomposes the flowsheet evaluation problem into two parts. First the flowsheet is examined by neglecting the energy integration network. The best flowsheet found without considering the heat exchanger network is then examined using existing energy integration procedures and considering the existing heat exchanger area.^ When analyzing a flowsheet the procedure always considers the entire flowsheet (with or without the energy integration network) and the constraints imposed by existing equipment. When constraints are found, a series of different policies are available to break those constraints by one or more alternative means. The user selects, for each piece of equipment, a default method to break constraints and the code uses that method whenever required.^ We have implemented a simple method to tune equipment models to match equipment in an operating plant. This method will account for some systematic modeling errors (bad model parameter values) but not for significant shortcomings in the models. ^
David A Nelson,
"Preliminary retrofit design of chemical process plants"
(January 1, 1989).
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