chemical process dynamics and control chapter 1 lecture notes

8/12/2019 Chemical Process Dynamics and Control Chapter 1 Lecture Notes

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CCB3013 - Chemical Process Dynamics, Instrumentation & Control 15/20/2014

Introduction to

Chemical Process Dynamics,

Instrumentation & Control


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Chapter Objectives

End of this chapter, you should be able to:

1. Understand the role of process dynamics and

control in industry

2. Understand general concepts3. Classify variables

4. Understand the purpose of process control

5. Understand control aspects of complete

chemical plant6. Understand hardware for process control system


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Role of process dynamics and control

in industry

Illustration with examples

Example 1a simple process where

dynamic response is important

Example 2use of a single feedback

controller

Example 3simple but typical chemicalengineering plant


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Example 1 A gravity-flow tank

Under steady state conditions,

the flow rate out of the tank

must equal the flow rate into

the tank.

What would happen dynamically

if we changed Fo?

How will h(t) and F(t) varywill time?


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Example 2 Heat Exchanger

We want to control the temperature of oil leaving theheat exchanger.


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How to control?

A thermocouple is inserted in a thermowell inthe exit oil pipe.

Thermocouple wires are connected to a

temperature transmitter that converts themillivolt output into a 4- to 20 mA signal.

This signal sent to a temperature controller.

The temperature controller opens the steam

valve if more steam is needed or closes it a

little if the temperature is too high.


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Components of control loop

A sensor

A transmitter

A controller

A final control element

Process control deal with:

What type of controller to be used?

How it should be tuned?


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Example 3 - A typical chemical plant


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Concepts of Process Control

Another simple

example:


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Block diagram


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The termprocess dynamics refer to unsteady

state (or transient) behavior.

Dynamic studies provide us the behavior ofthe process under unsteady-state conditions

Gain knowledge about the process

behavior.


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Objectives of Process Control

Maintain a process at the desired operating

conditions, safely and efficiently

Satisfy product quality and environmental

requirements


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Process control applications

Large-scale integrated processing plants suchas oil refineries or ethylene plants requirethousands of process variables such astemperature, pressure, flow, level and

compositions are measured and controlled.

Large number of process variables, mainly flowrates, can be manipulated.

Feedback control systems comparemeasurements with their desired values andthen adjust the manipulated variablesaccordingly.


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Representative process control problems

Foundation of process control is process

understanding.

What is a process?

The conversion of feed materials to useful products

using chemical and physical operations

PROCESS.

Common processes can be continuous, batch or

semi-batch.


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Continuous Processes


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Tubular Heat Exchanger

Control problem: The exit temperature of theprocess fluid is controlled by manipulating the

cooling water flow rate.

Disturbances: Variations in the inlet temperatures

and process fluid flow rate.


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Continuous stirred tank reactor

(CSTR)

Control problem: If the reaction is highly exothermic, it is

necessary to control the reactor temperature by

manipulating the flow rate of the coolant in a jacket orcooling coil.

Disturbances: The feed conditions (composition, flow

rate, and temperature).


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Multi-component distillation column

Control Problem: Distillate

composition can be controlled

by adjusting the reflux flow rate

or the distillate flow rate.

Disturbances:The feed

conditions


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Process variables

Three important types: (Control Terminology)

1. Controlled variables - these are the variables which

quantify the performance or quality of the final

product, which are also called output variables.

2. Manipulated variables - these input variables are

adjusted dynamically to keep the controlled

variables at their set-points.

3. 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.


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Process variables

Specification of controlled variables,

manipulated variables and disturbance

variables is a critical step in developing a

control system


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Batch and semi-batch processes


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Control problems

Batch or semi-batch reactor: The reactor temperatureis controlled by manipulating a coolant flow rate.

Batch digester: The end point of the chemical reactionis indicated by Kappa number, a measure of lignincontent. It is controlled to a desired value by adjusting

the digester temperature, pressure, and/or cycle time. Plasma etcher: The unwanted material on a layer of a

microelectronics circuit is selectively removed bychemical reactions. The temperature, pressure andflow rates of etching gases to the reactor are

controlled by adjusting electrical heaters and controlvalves.


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Control problems

Kidney dialysis unit: The blood flow rate is

maintained by a pump, and ambient

conditions, such as temperature of the unit,

are controlled by adjusting a flow rate.


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Control Terminology(2)

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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Il lus trat ive Examp le:

B lend ing system

Notation:

w1, w2and ware massflow rates

x1,x2andxare mass

fractions of component A

Assumptions:

w1is constant x2= const. = 1 (stream 2 is pure A

Perfect mixing in the tank


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Blend ing system

Control Objective:Keepxat a desired value (or set point)xsp, despite

variations inx1(t). Flow rate w2can be adjusted for this

purpose.

Terminology:

Controlled variable (or outputvariable):x

Manipulated variable (or inputvariable): w2

Disturbance variable (or loadvariable):x1

Design Question

What value of is required to have ?spxx2w


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Overall balance:

Component A balance:

(The overbars denote nominal steady-state design values)

At the design conditions, .

Substitute in Eq.1-2, and , then solve Eq. 1-2

for :

Equation 1-3 is the design equation for the blending

system.

spxx

spxx

1 20 (1-1)w w w

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

12 x

2

w

12 1 (1-3)

1

SP

SP

x xw wx


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If our assumptions are correct, then this value of will

keep at .

But what if conditions change?

Control Question. Suppose that the inlet concentrationx

1changes with time. How can we ensure thatxremains at

or near the set point ?

As a specific example, if and , thenx>xSP.

2w

x spx

spx

11 xx 22 ww


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Some Possible Control Strategies

Method 1.Measure x and adjust w2.

Intuitively, ifxis too high, we should reduce w2;

Manual control vs. automatic control

Proportional feedback control law

Kcis called the controller gain

w2(t) andx(t) denote variables that change with time t

The change in the flow rate, is proportional to

the deviation from the set point,xSPx(t).

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

2 2,w t w


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Control Method 1


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Method 2

Measure x1and adjust w2

Thus, ifx1is greater than , we would decrease w2

so that .

One approach: Consider Eq. (1-3) and replace and

withx1(t) and w2(t) to get a control law:

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.

1x

22 ww

1x

2w

1

2 1 (1-5)1

SP

SP

x x tw t w

x


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Control Method 2

Method 3.Measure x1and x, adjust w2.

This approach is a combination of Methods 1

and 2.


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Control Method 4

Use a larger tank.

If a larger tank is used, fluctuations in x1will

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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Classification of Control Strategies

Table. 1.1 Control Strategies for the Blending

System

1 x w2 FB

2 x1 w2 FF

3 x1andx w2 FF/FB

MethodMeasured

Variable

Manipulated

VariableCategory

4 - - Design

change


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Feedback Control

Distinguishing feature: measure the controlledvariable.

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 disturbances. Reduces sensitivity of the controlled variable

to disturbances and changes in the process.


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Feedback Control

Disadvantages:

No corrective action occurs until after the

disturbance has upset the process, that is,

until afterxdiffers fromxsp.

Very oscillatory responses, or even

instability


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Feedforward Control

Distinguishing feature:

measure a disturbancevariable

Advantage:

Correct for disturbance before it upsetsthe process.

Disadvantage:

Must be able to measure thedisturbance

No corrective action for unmeasured

disturbances


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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

E i I i Ad d


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Economic Incentives - Advanced

Control


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Hierarchy of process control activities

1. Measurement and Actuation

2. Safety, Environment and Equipment Protection

3a. Regulatory Control

4. Real-Time Optimization

5. Pl anning and Scheduling

Process

3b. Mu lti variab le and Cons traint Control

(days-months)

(< 1 second)

(< 1 second)

(seconds-minutes)

(minutes-hours)

(hours-days)


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Major steps in control system development


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Conclusions

You have been introduced to:

1. the role of process dynamics and control in

industry

2. general concepts of process control3. classification of variables

4. the purpose of process control

5. control aspects of complete chemical plant6. hardware for process control system

chemical process dynamics and control chapter 1 lecture notes

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