This paper extends the concept of a "resource-task network", described by Pantelides, to provide a unified mathematical formulation of the problem of determining the optimal operating conditions of a mixed production facility, comprising multipurpose plant for both batch and continuous operations. The formulation uses a variable event-time sequence common to all system events. This yields a large mixed integer nonlinear programming problem. For batch processes with fixed recipes the problem is linear and can be solved by existing techniques. Alternative approaches for solving the general problem are discussed, and some preliminary numerical results are presented.