R. Fletcher, J.A.J. Hall and W.R. Johns
We treat the addition of new equipment to an existing multiproduct batch plant for which new production targets and selling profits have been specified. This optimal retrofit design problem has been considered by Vaselenak et al (Ind. Engng. Chem. Res. 26, 718-728, 1987). Their constraint that new units must be used in the same manner for all products places a restriction on the design which could readily be overcome in practice. We present a mixed-integer nonlinear programming (MINLP) formulation which eliminates this constraint. A series of examples is presented which demonstrate greater profitability for plants designed with our formulation. The examples also bring to light a further unwanted constraint in the Vaselenak, Grossmann and Westerberg formulation. In their formulation they limit batch size to the smallest unit at a stage, even when that unit is not needed. It is noted that, at the expense of some additional mathematical complexity, our formulation could be enhanced to allow reconnexion of existing units and alternate use of multiple additional units.
Key words: Retrofit design, multiproduct batch plants, MINLP.