R. Fletcher, J.A.J. Hall and W.R. Johns
Abstract
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.