Unit-operation nonlinear modeling for planning and scheduling applications

The focus of this paper is to detail the quantity and quality modeling aspects of production flowsheets found in all process industries. Production flowsheets are typically at a higher-level than process flowsheets given that in many cases more direct business or economic related decisions are being made such as maximizing profit and performance for the overall plant and/or for several integrated plants together with shared resources. These decisions are usually planning and scheduling related, often referred to as production control, which require a larger spatial and temporal scope compared to more myopic process flowsheets which detail the steady or unsteady-state material, energy and momentum balances of a particular process unit-operation over a relatively short time horizon. This implies that simpler but still representative mathematical models of the individual processes are necessary in order to solve the multi time-period nonlinear system using nonlinear optimizers such as successive linear programming and sequential quadratic programming. In this paper we describe six types of unit-operation models which can be used as fundamental building blocks or objects to formulate large production flowsheets. In addition, we articulate the differences between continuous and batch processes while also discussing several other important implementation issues regarding the use of these unit-operation models within a decision-making system. It is useful to also note that the quantity and quality modeling system described in this paper complements the quantity and logic modeling used to describe production and inventory systems outlined in Zyngier and Kelly (Optimization and logistics challenges in the enterprise, Springer, New York 61–95, 2009).