A Real-Time Approach to Process Control, 3rd Edition

Workshop 7

Distillation Control

Have confidence that if you have done a little thing well, you can do a bigger thing well, too.

—Joseph Storey

Introduction

Prior to attempting this workshop, you should review Chapter 8 in the book.

Distillation is one of the most important unit operations in chemical engineering. It forms the basis of many processes and is an essential part of many others. It presents a more difficult control problem compared with many other unit operations as at least five variables need to be controlled simultaneously and there are at least five variables available for manipulation. Thus a distillation column provides an example of a multiple-input multiple-output (MIMO) control problem. It is critical that variable pairing is done appropriately between controlled variables and manipulated variables. The overall control problem can usually be reduced to a 2 × 2 composition control problem since the inventory and pressure loops frequently do not interact with the composition loops. This workshop will highlight some fundamental rules of distillation control and show how a basic distillation control scheme can be selected.

Key Learning Objectives

1. There are two degrees of freedom for steady state and five degrees of freedom present for dynamic in a simple distillation column with no side draws and a total condenser. The degrees of freedom increase by one with each side draw and for a partial condenser, which has two overhead products.

2. The degrees of freedom ...