In an earlier paper (Zhang and Sargent, 1994), a unified formulation was given for the problem of determining the optimal operation of a mixed production facility, involving both batch and continuous operations. This paper presents some extensions and improvements, dealing with operational constraints and plants involving continuous operations. The concept of Unit Usability is introduced to give a unified formulation for a variety of operational constraints. A modified task model for continuous operations enables us to linearize cross-products of one integer variable and two continuous variables. These ideas are illustrated by application to a realistic case study. (C) 1998 Elsevier Science Ltd. All rights reserved.