Capital cost targets for the optimum synthesis of mass exchange networks

Doctoral Thesis


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University of Cape Town

Pinch Technology is very well developed for heat exchanger network synthesis (HENS). It is possible to predict, on thermodynamic grounds, the minimum energy, capital and total costs for a network. These targets are set before any design and can also be optimised at this stage. Special design techniques exist which allow the targets to be met - or closely approached - in practice. The approach has recently been extended to mass exchange network synthesis (MENS). However, prior to this study, it was not as well developed for this field as it had been for HENS. Only targets for the minimum operating costs could be set and then achieved in design. Capital cost targets for MENS did not exist. The usual approach was to use the minimum number of mass exchange units - which could be targeted - as an attempt to minimise the capital cost of the network. However, this is not sufficient since the exchanger sizes are also important. This meant that there was no guarantee that the capital cost and hence total cost had been truly minimised. This thesis has developed a new method for targeting the minimum capital costs for mass exchange networks. The method is simple and based on insight, rather than relying on a mathematical 'black-box'. New graphical tools, the y-* composite curve plot and the y-y* composite curve plot have been introduced for this purpose and these allow the minimum exchanger sizes to be predicted before design. The new capital cost targets can be traded off against the established operating cost targets in order to optimise the total cost with no design being necessary.

Includes bibliographical references.