Rigorous Modelling and Conventional Simulation
The aim of this chapter is to show the reader how to develop a dynamic simulation for a distillation tower and its control from first principles. The different classes of dynamic distillation models and various approaches to solving these models will be presented. The author hopes to dispel the myth that modelling and simulation of distillation dynamics must be difficult and complex.
Dynamic modelling and simulation has proven to be an insightful and productive process engineering tool. It can be used to design a distillation process that will produce quality products in the most economic fashion possible, even under undesirable process disturbances. Working dynamic models provide a process engineering tool that has a long and useful life.
Dynamic simulation can be used early in a project to aid in the process and control system design. It ensures that the process is operable and can meet product specifications when the process varies from steady-state design. Later in the project the simulation can be used to complete the detailed control system design and solve plantwide operability problems. After the project, the same simulation is useful for training. Years later, as product and economic conditions change, the simulation can be used for plant improvement programs.
The authors of this book offer different approaches to distillation modelling. Chapter 4 discusses reduced and simplified models. Chapter 5 presents a novel concept in object orientated simulation. Chapter 6 presents concepts necessary to develop a plantwide simulator. These chapters on modelling and simulation will provide readers with a solid framework so the methods and ideas presented in the remaining chapters of this book can be implemented.
This chapter will be devoted to developing process models that realistically predict plant dynamics and their formulation in algorithms suitable for digital computer codes. Computer source codes are readily available from many sources (Franks, 1972; Luyben, 1990), so they will not be repeated here. I intend to share my experience in properly selecting models that accurately predict the dynamics of real distillation columns one is likely to find in the plant.
A dynamic model is needed to study and design composition controls. To do this we will develop a sufficiently rigorous tray-by-tray model with nonideal vapor-liquid and stage equilibrium. Proportional-integral feedback controllers will control product compositions or tray temperatures. Vapor flow and pressure dynamics often can be assumed negligible. I will discuss how systems with vapor hydraulic pressure dynamics can be modelled and simulated. My approach is based on fundamental process engineering principles. Only the relationships that are necessary to solve the problem should be modelled. Most importantly, these are models that any chemical engineer can easily simulate. They are suitable for a small personal computer, in whatever progranmiing language you prefer.
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