This paper presents a general mathematical formulation for the simultaneous design and scheduling, of multipurpose plants. The formulation is based on the resource task network process representation, uses a uniform time grid continuous-time representation, and can handle both short-term and periodic problems. It originates mixed-integer nonlinear programs or mixed-integer linear programs, depending on the types of tasks and objective function being considered. The performance of the formulation is illustrated through the solution of two periodic example problems that have been examined in the literature, where the selection and design of the main equipment, items and their connecting pipes is considered. The results clearly show that all decisions should be part of the same model because the plant structure, operating schedule, and cycle time can all change with a change in product demand. A comparison with an earlier approach is also presented.