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:: 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.

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.

Techniques of Control

Thre are in principle two ways of controlling things. The two approaches are called feedback and feedforward. In practice feedback is used for all primary control systems in chemical processes. Feedforward is sometimes added to improve the performance of feedback systems.

Basic Concepts of Feedback Control

The task of maintaining these required conditions falls to one or, more usually several, process control systems with which the plant will be equipped. The practical aspects of these will be discussed more fully in the following module. The underlying principle of most process control, however, is already understood by anyone who has grasped the operation of the domestic hot water thermostat:

The quantity whose value is to be maintained or regulated, e.g. the temperature of the water in a cistern, is measured.
Comparison of the measured and required values provides an error, e.g. `too hot' or `too cold'.
On the basis of the error, a control algorithm decides what to do.
Such an algorithm might be:
If the temperature is too high then turn the heater off. If it is too low then turn the heater on.
The adjustment chosen by the control algorithm is applied to some adjustable variable, such as the power input to the water heater.

This summarises the basic operation of a feedback control system such as one would expect to find carrying out nearly all control operations on chemical plants, and indeed in most other circumstances where control is required. The diagram belows a feedback control loop.

Notice that this extremely simple idea has a number of very convenient properties. The feedback control system seeks to bring the measured quantity to its required value or setpoint. The control system does not need to know why the measured value is not currently what is required, only that this is so. There are two possible causes of such a disparity:

The system has been disturbed. This is the common situation for a chemical plant subject to all sorts of external upsets. However, the control system does not need to know what the source of the disturbance was.
The setpoint has been changed. In the absence of external disturbance, a change in setpoint will introduce an error. The control system will act until the measured quantity reaches its new setpoint.

A control system of this sort should also handle simultaneous changes in setpoint and disturbances.

Advantages of Feedback Control

Not only does the feedback control system require no knowledge of the source or nature of disturbances, but it requires minimal detailed information about how the process itself works.

Feedback control action is entirely empirical, so long as an adjustment is being made in the correct `sense', e.g. more heat means increasing temperature and vice versa, then the control system should remove the effect of an external disturbance.

As we will see, it helps to know more than this, but the minimum information required to make a feedback control system work is whether the adjustment makes the measurement go up or down.

Disadvantages of Feedback Control

The main disadvantage of feedback control is that the disturbance enters into the process and upsets it. It is only after the process output has moved from the setpoint that the controller takes corrective actions. Although most processes allow some fluctuation of controlled variable within a certain range, there are two process conditions which can make the overall effectiveness of feedback control quite unsatisfactory. One of these is the occurrence of disturbances of a very large magnitude. The other is the presence of a large time delay within the process.

Large Magnitude Disturbance

A very large disturbance may cause an unacceptable upset in a process before the control system can comp[enstae for it. This is most likely to occur when the relationship between the disturbance and the regulated variable is a nonlinear one in which an increasing disturbance leads to a disproportionate change in the variable.

A classic example of this is an exothermic chemical reaction whose temperature is to be regulated. Here anything that causes an increase in temperature will cause an increase in reaction rate, leading to a further increase in temperature, etc.

This is exacerbated by the extreme sensitivity of rate to temperature by the Arrhenius equation:

rate = A exp (-E/RT)

Typically a 10K rise in temperature can doubble the reaction, and hence heat production rate.

This is a situation in which feedforward control can usefully be added to a feedback control system.

Large Time Delay

It is almost always necessary to regulate the product from a process. Sometimes the only way of doing this is to adjust the feed. This puts the whole process inside the control loop, and so it may take many minutes, or even hours, for the effect of any feed adjustment to be observed in the product, see figure.

One solution is to identify some other measurable quantity which will respond more rapidly to the adjustment, and use it in a cascade control system. This is discussed later. Once again, the addition of feedforward may be helpful.