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Universidad de Edimburgo

Module 1.1 - The Very Basics

Introduction

This part of the course starts with an outline and overview of basic control concepts. Questions which process

engineers routinely have to answer about process control include the following:

I have this process. What should I control?

Where on the process do I put my control loops?

As I proceed with the design of a process, what aspects of control should I consider at which stages?

Most books with the words `process control' in the title do little to answer these questions. Classical linear control

theory, which forms the basis of most books on control, is much concerned with how to design controllers and is less

helpful on how to design complete control systems. Other problems with this classical approach, for most process

engineers wishing to design control systems for real chemical processes, are the restriction of most of its methods to

idealised process models, and the extensive use of rather specialised mathematics.

Satisfactory answers to questions such as the above frequently require little conventional mathematics. What they do

require, however, is a good understanding of what a process is intended to do and how it works.

In this book we will approach process control from the standpoint of a chemical or process engineer, and address

these questions and others like them. We will consider the process and its control system in the language of process

engineering. We will use mathematics, as such, only when necessary, and the language of classical control

engineering only when it is unavoidable, or will add very significantly to the process engineer's understanding.

Why Control?

Chemical plants are intended to be operated under known and specified conditions. There are several reasons why

this is so:

Safety:

Formal safety and environmental constraints must not be violated.

Operability:

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Certain conditions are required by chemistry and physics for the desired reactions or other operations to take place. It

must be possible for the plant to be arranged to achieve them.

Economic:

Plants are expensive and intended to make money. Final products must meet market requirements of purity,

otherwise they will be unsaleable. Conversely the manufacture of an excessively pure product will involve

unnecessary cost.

A chemical plant might be thought of as a collection of tanks in which materials are heated, cooled and reacted, and

of pipes through which they flow. Such a system will not, in general, naturally maintain itself in a state such that

precisely the temperature required by a reaction is achieved, a pressure in excess of the safe limits of all vessels be

avoided, or a flowrate just sufficient to achieve the economically optimum product composition arise.

Control Objectives

Control systems in chemical plants have, as noted, three functions.

Safety. Operability, i.e. to ensure that particular flows and holdup are maintained at chosen values within operating

ranges.

To control product quality, process energy consumption etc.

To a large extent these are quite separate objectives. Indeed, in the case of safety systems separate equipment is

generally used. The aims of control for operability are secondary to those of strategic control for quality etc., which

directly affect process profitability.

Control for Safety

Concern for safety is paramount in designing a chemical plant and its control systems. Ideally a process design

should be `intrinsically safe', that is, plant and equipment should be such so that any deviation, such as an increase in

reactor pressure, will itself change operating conditions so that it is rapidly removed, for example by a fall in reaction

rate. For many perturbations this type of responsive, passive safety system will not be possible and active systems

will be required.

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These active safety systems must be robust and of high integrity. Current processes achieve this through simplicity.

The ultimate safety system is in most cases the mechanical relief valve which simply vents the plant to atmosphere,

possibly through a flare or scrubber.

We will not discuss control for safety explicitly in this book. Generally speaking a complete and separate system is

provided to handle emergency control action. The need for this, and its design requirements, are established in

hazard and operability or hazop studies. These are typicaly carried out on the complete process with its `normal'

control systems in place.

A number of safety issues will be addressed in the course of developing the design of the control systems for normal

operation, but it must be emphasised that our treatment of this vital issue will be relatively restricted.

Control for Operability

The operator of a process quite simply has to

know what it is doing

be able to make it do what he or she wants, rather than to follow its natural inclinations.

The issue of making a plant behave in this way is called operability.

The majority of control loops in a plant control system are associated with operability. Specific flow rates have to be

set, levels in vessels maintained and chosen operating temperatures for reactors and other equipment achieved.

Control for Profitability

There is no point in building a plant which is totally safe and can be made to take up any (safe) conditions of flow,

temperature etc., if the conditions under which it is operated do not produce the correct amount of product to the

correct specification, thus allowing its operators to make a profit.

The top level of process control, what we will refer to as the strategic control level is thus concerned with achieving

the appropriate values principally of:

Production rate,

Product quality, and

Energy economy.