Introduction and Terminology(1)

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Process Control III

PCT301T


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OBJECTIVES:

Understand the terminologies associated with processcontrol.

Know Incentives of process control. How the four components / elements of control system are

linked. Know and classify process variables.

Control objectives are identified Use of process control instrumentation to regulate the

process.

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Process control is the regulation / manipulation of variablesinfluencing the conduct of a process in such a way as toobtain a product of desired quality and quantity in an efficientand economic manner.

The regulatory controls are the foundation of a controlsystem which includes the feedback and the feedforwadcontrol of the process variables. I.e. flow, pressure,temperature and load level which are critical for the qualityof the final product that a plant produce.

Process Control Terminology

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Process - refers to the collection of unit operations or any physical and / chemical conversion of raw materials touseful product.

Measurement - is a device which calculates the action based on the measured value against a preset or desiredvalue (set point).

A controller is a device used to control the process. Itcompares a signal from the measured value (PV) with theset point (SP) and produces an error signal which is used toadjust the final control element.

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Error = PV SP

Final control element - An output signal resulting from thecontroller calculation, which is used to manipulate the

process action through some form of actuator.

Control system : can be defined as a combination of components that act together to perform a certainobjectives. A system interact with its environment throughsignals as shown in Figure 2.

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Generally, signals are function of time which can be indicated

by e.g u(t) and y(t).

Two types of signals

Input signal, u(t) affect the system behaviour in some way Output signal, y(t) give information about the system

behaviour

U(t) y(t) System

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Basic process control processes consist of a control loopas shown in figure 1 below, having four maincomponents of a control system.

Process Measurement A controller Final control element

PROCESS CONTROL PROBL EM I S I DENTI F I ED BY TH REE I M PORTANT TYPES OF PROCESS VARI ABL ES.

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TYPES OF PROCESS VA RIA B L ES

Controlled variables - These are the variableswhich quantify the performance or quality of the finalproduct, which are also called output variables.

Flow rates, compositions, temperatures, levels,and pressures in the process that we can control, either by holding them as constant or making them follow

some desired values. The desired value of a controlledvariable is referred to as its se t po in t .

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Manipulated variables - These input variables areadjusted dynamically to keep the controlled variables

at their set-points.

Typically flow rates of streams entering or leaving a process that we can change in order to control the plant.

Disturbance variables - These are also called "load"variables and represent input variables that cancause the controlled variables to deviate from their respective set points.


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Flow rates, temperatures, or compositions of streamsentering the process. Variables in the process thataffects the controlled variables but cannot bemanipulated.

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CONTROL SYSTEM :

PROCESS

MEASUREMENT

CONTROLLER

Set point

Disturbance

variables Controlledvariable

Rawmaterials

FINAL CONTROLELEMENT

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set-point change - implementing a change in theoperating conditions. The set-point signal ischanged and the manipulated variable is adjustedappropriately to achieve the new operatingconditions. Also called servomechanism (or "servo")control.

disturbance change - the process transientbehavior when a disturbance enters, also calledregulatory control or load change. A control systemshould be able to return each controlled variableback to its set-point.

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More Terminologies


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Safety (of people): T, P, C (specially poisonous or explosive components) should be within allowable range.

Product specification: Concentration of products must be maintained at certain level (by means of control).

Environmental Regulation: Regulation requires that T,C and flow rate of certain chemical species be under somelimits.

Operational constraint: Examples are: net positive

suction head, level in the tank, flooding in distillationcolumn, T in reactor of certain material, T and the catalysisfor endothermic reactor.

Economics: Costs involve raw material, energy, capital

and human labor.

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The incentives of process control.


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1.1 I l lustrative Example: Blending system

Notation:

w1, w2 and w are mass flow rates

x1, x2 and x are mass fractions of component A

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Assumptions:

1. w1 is constant

2. x2

= constant = 1 (stream 2 is pure A)

3. Perfect mixing in the tank

Control Objective:

Keep x at a desired value (or set point) xsp, despite variations in x1(t ). Flow rate w2 can be adjusted for this purpose.

Process Variables: Controlled variable (or output variable): x

Manipulated variable (or input variable): w2

Disturbance variable (or load variable): x1

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Design Question . What value of is required to have2w?SP x x

Overall balance:

Component A balance:

1 20 (1-1)w w w

1 1 2 2 0 (1-2)w x w x wx

(The overbars denote nominal steady-state design values.)

At the design conditions, . Substitute Eq. 1-2, and, then solve Eq. 1-2 for :

SP x x SP x x

2 1 x 2w

12 1 (1-3)1

SP

SP

x xw w

x

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Equation 1-3 is the design equation for the blendingsystem.

If our assumptions are correct, then this value of will keepat . But what if conditions change?

x

SP x

Control Question. Suppose that the inlet concentration x 1 changes with time. How can we ensure that x remains at or near the set point ?

As a specific example, if and , then x > xSP.

SP x

1 1 x x 2 2w w

Some Possible Control Strategies:

Method 1. Measure x and adjust w 2.

Intuitively, if x is too high, we should reduce w2;

2w

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Manual control vs. automatic control

Proportional feedback control law,

2 2(1-4)c SP w t w K x x t

1. where K c is called the controller gain.

2. w2(t ) and x(t ) denote variables that change with time t .

3. The change in the flow rate, is proportional tothe deviation from the set point, xSP x(t ).

2 2 ,w t w

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Method 2. Measure x 1 and adjust w 2.

Thus, if x 1 is greater than , we would decrease w2 so that1 x

2 2 ;w w

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Method 3. Measure x 1 and x, adjust w 2.

This approach is a combination of Methods 1 and 2.

Method 4. Use a larger tank.

If a larger tank is used, fluctuations in x1

will tend to be dampedout due to the larger capacitance of the tank contents.

However, a larger tank means an increased capital cost. C h a p t e r

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1.2 Classification of Control Strategies

M ethod M easur ed Variable

Manipulated Variable

Category

1 x w 2 FB a

2 x 1

w 2

FF

3 x 1 and x w 2 FF/FB

4 - - Design change

Table. 1.1 Control Strategies for the Blending System

Feedback Control:

Distinguishing feature: measure the controlled variable

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It is important to make a distinction between negative feedback and positive feedback .

Engineering Usage vs. Social Sciences

Advantages:

Corrective action is taken regardless of the source of the disturbance.

Reduces sensitivity of the controlled variable todisturbances and changes in the process (shown later).

Disadvantages:

No corrective action occurs until after the disturbancehas upset the process, that is, until after x differs from

x sp .

Very oscillatory responses, or even instability

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Feedforward Control:

Distinguishing feature: measure a disturbancevariable

Advantage:

Correct for disturbance before it upsets the process.

Disadvantage:

Must be able to measure the disturbance.

No corrective action for unmeasured disturbances. C h a p t e r

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Justification of Process Control

Specific Objectives of Control Increased product throughput Increased yield of higher valued products

Decreased energy consumption Decreased pollution Decreased off-spec product Increased Safety Extended life of equipment Improved Operability Decreased production labor

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3.2 Economic Incentives - Advanced

Control

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1. Measurementand Actuation

2. Safety, Environmentand EquipmentProtection

3a. RegulatoryControl

4. Real-TimeOptimi zation

5. Planning andScheduling

Process

3b. M ultivariabl eand C onstraintControl

(days-months )

(< 1 second )

(< 1 second )

(seconds- minutes )

(minutes-hours )

(hour s-days )

Figure 1.7 Hierarchy of process control activities.

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Figure 1.9 Major

steps in controlsystem development

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N h

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Introduction and Terminology(1)

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