chemical process variables-1

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UNIT1

Introduction to ProcessControl

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OBJECTIVES:

Understand the terminologies associated with processcontrol.

How the four components / elements of control system

are linked. Know and classify process variables.

Control objectives are identified

Know Incentives of process control.

Use the Control Algorithms to control the measuredvariables.

Use of process control instrumentation to regulate theprocess.

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Unit 1:

Process control is the regulation / manipulation of variables

influencing the conduct of a process in such a way as to

obtain a product of desired quality and quantity in an

efficient and economic manner.

Most basic process control processes consist of a control

loop as shown in figure 1 below, having four main

components of a control system.

Process Control Terminology

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CONTROL SYSTEM

PROCESS

MEASUREMENT

CONTROLLER

Set point

Disturbance variables

Controlled

variable

Raw materials

FINAL CONTROLELEMENT

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Process - refers to the collection of unit operations or anyphysical and / chemical conversion of raw materials to useful

product

Measurement - is a device which calculates the action based

on the measured value against a preset or desired value (set

point).

A controller is a device used to control the process. It

compares a signal from the measured value (PV) with the set

point (SP) and produces an error signal which is used to

adjust the final control element.

Error = MVSP

Final control element- An output signal resulting from the

controller calculation, which is used to manipulate the

process action through some form of actuator.

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Control system can be defined as a combination of

components that act together to perform a certain objectives.

A system interact with its environment through signals as

shown in the diagram below.

Generally, signals are function of time as indicated in the

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

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U(t) y(t)

System

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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

Set Point: A reference value representing the desired value ofthe process variable being controlled.

E.g: If the level of a liquid in a tank must be maintained within 5

ft of 50 ft, what is the liquids setpoint? = 50 ftSet-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.7

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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.

Process load is the level of material, force, torque, energy,

power, or other variable applied to or externally by another

instrument.

Process upset is any type of disturbance that causes the

control variable to depart from the set point.

Process demand is the requirement made by the control

system or the operator on the process.

Load response is the way in which the process behaves after

a change in the process load introduces an error in to the

system.

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More Terminologies


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Steady state is one that does not change with time. Every

time we take snapshot, all the variables have the same values

as in the first snapshot.

Transient state is one that changes with time. Every time we

take a snapshot, many of the variables have different values

than in the first snapshot.

Capacity is a measure of capacity to restore volume, mass,

heat, information, or any form of energy or matter.

Dead time is the time interval between the initiation of the

output change or stimulus and start of resulting response.

Control system lag the time for the process control loop to

make necessary adjustments to the final control element.

Process lag is the time delay caused by the process to

eliminate errors after the manipulated variables are adjusted.

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Classification of variables in a chemical process.

The variables (Flow rates, Temperature, Pressure, Concentration,

etc) are conditions which can change a process in some way.They are divided in to two groups.

a) INPUT variables: which donates the effect of thesurroundings on the chemical process.

b) OUTPUT variables: which donates the effect of the process onthe surrounding.

The INPUT variables can be further classified into the followingcategories:

Manipulated ( or adjustable ) variables, if their values canbe adjusted freely by the human operator or a controlmechanism. Typically flow rates of streams entering orleaving a process that we can change in order to controlthe plant.

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Disturbance variable, if their values are not the result ofadjustment by an operator or a control system. Flow rates,

temperatures, or compositions of streams entering theprocess. Variables in the process that affects the controlledvariables but cannot be manipulated.

The OUTPUT variables can be further classified into the following

categories:

Measured (Controlled) variables, if the values are known bydirectly measuring them. Flow rates, compositions,temperatures, levels, and pressures in the process that we cancontrol, either by holding them as constant or making them

follow some desired values.

The desired value of a controlled variable is referred to as its setpoint.

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unmeasured variable, if they are not measured or

cannot be measured directly.

In the figure.1.3 below,

the inputs x1

and w1

are the disturbances,

while x2and w2 are manipulated inputs.

The output variables x and w can be measured easily and

they are considered measured or the controlledoutputs.

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EXERCISE 1

Imagine yourself in the shower

As observed on the insert

Diagram.

Identify the following:

a) The process variable(s)

b) The set point

c) The comparison element

d) The control unit

e) The correction unitf) The process

g)The measuring device

h) Manipulated variables.

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EXERCISE 2

Imagine you are in a cabin in front of a small fire on a cold

winter evening. You feel uncomfortably cold, so you throwanother log into the fire the temperatuture to be at 5 0C

of 50 0C. Identify the following:

a) The process variable(s)

b) The set point

c) Controlled variable(s)

d) Measured variable(s)

h) Manipulated variable(s).

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Unit2: Reasons for Control (Control incentives)

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Safety: The safe operation of a chemical process is aprimary requirements for the well-being ofplant personnel,the community at large, and the economic viability of the

company. Thus the T, P, C (specially poisonous or explosivecomponents) should be within allowable limits.

Product specification / quality: The final products from theplant must meet demanding quality specifications set by

purchasers. Process control contributes to good plantoperation by maintaining the operating conditions required forexcellent product quality.

Environmental Regulation: Regulation requires that T, C,

and flow rate of certain chemical species be under some limits.The process must have the capacity to convert potentially toxiccomponents to being materials. Control can contribute to theproper operation of the plant units, resulting in consistently tolow effluent concentrations.

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Operational constraint: Different types of equipments

used in a chemical plant have limitations as a results of

their particular design, and inherent to their operation.

E.g. a centrifugal pump can deliver only a certain flow rate as

determined by its impeller size and the available pressure

drop in a line. Control systems need to recognize andsatisfy all such operational constrains.

Economics: The operation of plant should be as

economical as possible to utilize the raw material, energy,capital and human labor. Thus, the operating conditions

must be controlled effectively to minimize operating costs

and maximize profit.

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UNIT3. CONTROL STRATEGIES

The objective of a control system is to keep the controlled

variables at their desired values (or set points). This is

achieved by manipulating the manipulated variables using a

control algorithm.

3.1. CONTROL ALGORITHM

OPEN- LOOP SYSTEM (CONTROL)

An openloop system is a system with only the input and the

output but no regulation / no control action at all.

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Example of an openloop system

Block diagram representation

U

Y

D

PROCESSU(t) Y(t)

Figure: A simplified open loop control system

D

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In an open-loop system, the control

mechanism act without the current

information about the status of the

process.

Signal path is represented by

arrows, which show the direction

of information flow.

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3.1.1. Feedforward Control (FFC) Algorithm

Feedforward control is an example of an open-loop

control system that converts one or more conditions that

can disturb the controlled variable outside of any

feedback loop, in to corrective action to minimize

deviation of the controlled variable / mechanism whichanticipate the effect of the disturbances to produce the

corrective action if:

The disturbance can be measured

We know how the disturbances affect the output

We know how the control signal affects the output

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Feedforward Control Algorithm/Strategy

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Fig:A

SP

U(t)Y(t)PROCESS

Disturbance (d)

CONTROL

EQUATION

FF CONTROLLER

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Block diagram representation of fig: A

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Fig: B

control system

PROCESSU(t) Y(t)

Disturbance (d)

MEASUREMENT

CONTROLLERSP

A , B : Typical Feedforward control systems

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Advantages and disadvantages of FF control loop

ADVANTAGES

i. Acts before the effect of a disturbance has been felt by

the system.

ii. Is good for slow systems

iii. It does not introduce instability in the closed loop

response

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DISADVANTAGES

i. Requires identification of all possible disturbances

and their measurement.

ii. Cannot cope with unmeasured disturbance.

iii. Sensitive to process parameter variation.

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Example

Consider the example of the stirred tank heater.The objective is to control the temperature. The

disturbance source is Ti.

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T

C

I/

P

Feedforward control for a heated tankPCT301T-2013

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CLOSED LOOP SYSTEM

A closed loop system is a system with a total

control action / regulation around the entire

process.

The total control action is called the feedback

control. The controlled variable is measured, and

the measurement is fed to the controller, thus the

controller receiving information about how acontrol action affects the output.

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3.1.2. Feedback Control Loop (FBC)

In Feedback control, the measurement is fed to

the controller. Thus the controller receivesinformation about how a control action affects theoutput. Usually the measured variable is thevariable we want to control

The basic elements of a feedback controller

The process variable (PV). I.e. the variable that is tobe maintained under control.

The set point (SP) which is the desired value of theprocess variable

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h ( ) hi h i f h diff


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The error (e), which is a measure of the differencebetween the PV and the SP.

The controller, whose control law and turning

drive the corrective action . The final control element ( typically a valve)

The manipulated variable (MV), the variable in theprocess to which the FCE is attached.

Two dividends of feedback control loop

3.1.2.1. Negative feedback, this refers to thedesirable situation where the corrective actiontaken by the controller forces the controlledvariable towards the set point.

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Error = reference feedback signal

3.1.2.2 Positive feedback occurs when the

controller forces the controlled variables farther

away from the set point.

Error = reference + feedback signal

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Advantages and disadvantages of FB control loop

ADVANTAGES

I. Does not require identification of all possibledisturbances and their measurements.

II. It is insensitive to modelling error.

III. It is insensitive to parameter changes.

DISADVANTAGES

I. It waits until the effect of disturbance is felt by the

system.II. It is unsatisfactory for slow system.

III. It may create instability in the closed loop response.

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Example 1:


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Example 1:

Man in the shower.

Process variables

The disturbances = Water from toilet flushing

Manipulated variables = Temperature and the flow rate of thewater

Measured / controlled variables = Temperature of the

water

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Block diagram representation

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PROCESS

MEASUREMENT

CONTROLLER

Set point

Disturbance =flushing

Water

temperatureFw , T

FINAL CONTROL

ELEMENT

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EXAMPLE 2

Consider the example of the stirred tank heater. Theobjective is to control the temperature. The

disturbance source is Ti.

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SOLUTION

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TC

T

T

SP

Q

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REVISIT THE BLENDING PROCESS: A CONBINATION OFFEEDBACK FEEDFORWARD CONTROL LOOPS

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Pure A

W2

X2=1Mixture (A,B)W1

X1

W

X

AT

AC

I/P

xsp

AT AC I/P

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Class exercise

1. Riding a bicycle is an inherently unstablesystem.

a) Identify all process variables.

b) Could the process be considered as an open loop

or closed loop? Explain.

c) Construct the control mechanism of this process.

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CLASS EXERCISE

2. Your bathtub at home is a control system that keeps thewater level constant. A constant flow from the tap yields a

constant water level, because the flow rate through thedrain increases as the water level increases, and decreasesas the water level decreases. After equilibrium has beenreached, the level can be controlled by controlling the inputflow rate. A low input flow rate yields a lower level, while a

higher input flow rate yields a higher level.a) Which control strategy is used in this process?

b) Sketch a control system that uses this principle toprecisely control the fluid level in a tank. Show the intakeand the drainage valves, the tank, any sensors and

transmitters, and the interconnection of all components.c) Draw a block diagram of the system, identifying the input

and output signals of each block.

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Unit 4: Design element of a control system

In the design of a process control system, one should be able

to have answers to the WHAT,WHYand the HOWmethods

of achieving the process needs.

Series of steps to follow during the design of a control system

to follow the decision making process.

4.1. Define the control objectives

Question: what are the operational objectives that a control

system is called upon to achieve?

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The answer to this question will be determining the control

objective .

Answer: For the blending process in the example, thecontrol objective is to Keepxat a desired value (or set

point)xsp despite variations inx1(t).

4.2. Select measured variables.

Question : What variables should be measured to monitor

the operational performance of the process.

Answer: the first attempt will be to install the measuring

devise that will monitor the values ofxand wdirectly.

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4.3. Select manipulated variables.

Question : What variables are to be manipulated to achieve

the control objective ?

Answer: the first attempt will be to install the measuringdevise that will monitor the values ofxand wdirectly.

4.4. Design the controller.

In every control configuration, the controller is the active

element that receives the information from themeasurements and takes appropriate control action toadjust the values of the manipulated variables. E.g FF,FB.

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Question:

How is the information, taken from the measurement used toadjust values of the manipulated variables?

DESIGN 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 ,

thanx>xSP.

1 1x x 2 2w w

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4.4.1. Some Possible Control Strategies:

Method 1. Measure x and adjust w2.

ifxis too high, we should reduce w2;

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Method 2. Measure x1 and adjust w2.

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

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2 2;w w

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Method 3.Measure x1 and x, adjust w2.

This approach is a combination of Methods 1 and 2.

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Pure A

W2

X2=1Mixture (A,B)

W1

X1

W

X

AT

AC

I/P

xsp

AT AC I/P

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Method 4. Use a larger tank.

If a larger tank is used, fluctuations inx1 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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EXERCISE 2

On the process, construct the

possible control strategies

that will help to achieve the

desired products.

UNIT4

chemical process variables-1

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