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Introduction to Process Control

CHAPTER 1

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1. Explain the importance of process control.

2. Identify and describe simple control system

3. Explain the process & instrumentation diagram (P&ID).

4. Design control system for a typical chemical process.

Topic Outcomes (TO)

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Basic Questions about Control

1)What is control? 2)Why is control necessary?3)What is feedback control?4)What does a feedback system

do?5)How is control done?

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1.What is control? Control:To maintain desired conditions in a physical system by adjusting selected variables in the system.

Control SystemThe control systems appear to have three basic elements which is

FINAL ELEMENTCONTROLLERSENSOR

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SENSOR

-The first task that of acquiring information about of the process outputvariables.-Sensors are usually needed for pressure, temperature, liquid level, flow and composition measurement.-thermocouple: temperature measurement -differential pressure cell: for liquid level measurements-gas/liquid chromatographs: composition measurements

CONTROLLER

-The decision maker,and hence the ‘heart’ of the control system

FINAL ELEMENT

-Pump, variable speed fans, compressor, conveyorsTransmitter: How process information acquired by the sensor get back to the controller, and the controller decision gets back to the process

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Process

Controller

OUTPUT

sensorFinal control element

INFORMATION

Transmitter

Transmitter

INPUT

Decision

DISTURBANCE

The feedback control configuration

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2.Why is control necessary?

One word: DISTURBANCES!

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1.Safetya) Emergency System - to enforce bounds of variables.

b) Safety Valve - it opens to vent the excess vapor.

2.Environmental Protectiona) Safety Release System- it diverts a hydrocarbon gaseous to a flare for combustion .-it diverts a stream (water) to a holding pond for purification.


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3. Equipment Protectiona)To maintain conditions near desired value and emergency controls.

b)Level controller - protects the pump form damage.

c)Emergency Controls - shut off the pump motor when the level decreased.

4. Smooth Operation and Production Ratea) Chemical plants - complex networks / interacting processes.

b)Adjusting the feed valve (before the flash drum) - desired value.


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5. Product Qualitya)Specifications - compositions / physical properties / performance properties / combination of all three.

b) Adjusting the flash temperature - control component A in the liquid stream.

6. Profita)Goal - to provide product at lowest cost.b)Using hot process fluid for heating instead of steam.


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7. Monitoring and DiagnosisThere are 2 functions:i. The immediate safety and operation of the plant -plant operators.ii. The long term plant performance analysis -engineers

Alarm - to indicate serious maloperation.

All the categories of control objectives must be

achieved simultaneously; failure to do so leads to

unprofitable or worse, dangerous plant operation.


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3.What is Feedback control? FEEDBACK CONTROL makes use of an output of a systemto influence an input to the same system.

input = cause output = effect

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4.What does a feedback system do?

Figure 1

-Describe your method for driving a car.-Could you drive a car without looking out the windshield?-What must be provided by the car designer?-Can a “good design” eliminate the need to steer?

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Process Dynamicsa) Determine how a process responds during transient

conditions and it refers to unsteady-state process.

b) Steady-state vs. unsteady-state behaviorSteady state: variables do not change with timeUnsteady state: variables change with time

c) Continuous processes: Examples of transient behavior:

Start up & shutdownGrade changesMajor disturbance: e.g., refinery during stormy or hurricane conditionsEquipment or instrument failure (e.g., pump failure)

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d) Batch processesi. Inherently unsteady-state operationii.Example: Batch reactor

1. Composition changes with time2. Other variables such as temperature could

be constant.

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Control Terminology controlled variables - these are the variables which

quantify the performance or quality of the final product, which are also called output variables.

manipulated variables - these input variables are adjusted dynamically to keep the controlled variables at their set-points.(if their value can be adjusted freely by the human operator or a control mechanism).

disturbance variables - these are also called "load" variables and represent input variables that can cause the controlled variables to deviate from their respective set points.(if their values are not the result of adjustment by an operator or a control system).

set-point change - implementing a change in the operating conditions. The set-point signal is changed and the manipulated variable is adjusted appropriately to achieve the new operating conditions. Also called servomechanism (or "servo") control.

disturbance change - the process transient behavior when a disturbance enters, also called regulatory control or load change. A control system should be able to return each controlled variable back to its set-point.

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FEEDBACK

Uses direct measurements of the controlled variables to adjust the values of the manipulated variables.

FEEDFORWARDUses direct measurements of the disturbances variables to adjust the values of the manipulated variables.

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General Form of Control System Configuration

Structure of Open-loop System (struktur bg sistem gelung terbuka)

PROCESS

Disturbances variable

Manipulated variable Controlled variable

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Process


Manipulated variable Controll

edvariable

controller

Process


Manipulated variable

Controlled

variable

controller

It does not required identification and measurement of any disturbance.It is not sensitive to modeling errors.It is not sensitive to parameter changes.

Advantages

Disadvantages

Acts before the effects of a disturbances has been felt by the systemIs good for slow systems or with significant dead timeIt does not introduce instability in the closed-loop response

It wait until the effect of the disturbances has been felt by the systemIt is unsatisfactory for slow processesIt may create instability in the close-loop response

Required identification & measurement of disturbancesCannot cope with unmeasured disturbanceSensitive to process parameter variationsRequires good knowledge of the process control

Feed Back Feed Forward

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Instrumentation Symbols

Symbols of instruments used in process controls drawings:

A: AnalyzerF: Flow rateL: Level of liquid or solids in

a vesselP: PressureT: Temperature

C: Controller V: Valve

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Case Study

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1Case 1: High pressure is dangerous. Case 2: No flow could damage the pumpCase 3: High temperature may cause thermal loading in the columnCase 4: Quality of product at the bottom column

is the most criticalCase 5: Keep the process at smooth rate, avoid dry column

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High thermal loading TC

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Adjusting the heating and flowrate

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TASK 1

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State your own objective for this chemical reactor.List the controlled, manipulated variables and disturbance(s)

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Level and Temperature Control

LC

TC

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QuestionHow do we control

processes?

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1.1 Illustrative 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.Terminology:• 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 have

2w ?SPx 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 :

SPx x SPx x

2 1x 2w

12 1 (1-3)1

SP

SP

x xw wx

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•Equation 1-3 is the design equation for the blending system.

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

xSPx

Control Question. Suppose that the inlet concentration x1 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.

SPx

1 1x x 2 2w w

2w

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Some Possible Control Strategies:Method 1. Measure x and adjust w2.

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

•Intuitively, if x is too low, we should increase w2;•Manual control vs. automatic control Proportional feedback control law, 2 2 (1-4)c SPw t w K x x t

1.where Kc 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 to the deviation from the set point, xSP – x(t).

2 2,w t w

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Method 2. Measure x1 and adjust w2.

•Thus, if x1 is greater than , we would decrease w2 so that

•One approach: Consider Eq. (1-3) and replace and with x1(t) and w2(t) to get a control law:

1x2 2;w w

1x 2w

12 1 (1-5)1

SP

SP

x x tw t wx

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•Because Eq. (1-3) applies only at steady state, it is not clear how effective the control law in (1-5) will be for transient conditions.

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Method 3. Measure x1 and x, adjust w2.

• 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 damped out due to the larger capacitance of the tank contents.•However, a larger tank means an increased capital cost.

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1.2 Classification of Control Strategies

Method Measured Variable

Manipulated Variable

Category

1 x w2 FBa

2 x1 w2 FF3 x1 and x w2 FF/FB4 - - Design

change

Table. 1.1 Control Strategies for the Blending System

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1. Measurement and Actuation

2. Safety, Environment and Equipment Protection

3a. Regulatory Control

4. Real-Time Optimization

5. Planning and Scheduling

Process

3b. Multivariable and Constraint Control

(days-months)

(< 1 second )

(< 1 second )

(seconds-minutes )

(minutes-hours)

(hours-days)

Figure 1.7 Hierarchy of process control activities.

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Figure 1.9 Major steps in control system development

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