01 che 182 introduction to process dynamics and control

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INTRODUCTION TO PROCESS

CONTROL

ChE 182 June 11, 2013

1



Recall… 2

…find the volume of plug flow reactor

needed for 95% conversion…

…45,000 kg/h of water is to be cooled from38°C to 30°C by river water entering the

heat exchanger at 23°C…

…2 mol% ethanol in water is to be strippedin a plate column to a bottom product with

not more than 0.01 mol% ethanol…



Why do we need process control?3

Keep process at desired operating

conditions

Safety Environment

Economics



Bhopal Disaster (1984)4

Storage tank of methyl isocyanate (MIC)

of Union Carbide Pesticide Plant released

poisonous gasWater entered MIC tank

Runaway reaction

At least 3,000 people diedAround 500,000 injured




Unreliable temperature and pressure gauges

Refrigeration unit for keeping MIC at lowtemperatures had been shut off for some time.

The gas scrubber, designed to neutralize anyescaping MIC, had been shut off for maintenance.

The flare tower, designed to burn off MIC escapingfrom the scrubber, was turned off for replacement.

Alarm on the storage tank failed to signal the

increase in temperature. An MIC storage tank was filled beyond recommended

capacity



Objectives8

At the end of this class, you should be able to: understand the importance of process control; identify process variables; have an overview of the functions of the

control system; familiarize with the components of a control

system; differentiate among types of control

strategies.



Outline9

Chemical Process Dynamics and Control

Process variables and constraints

Process modelling Elements of control system

Control system development



Chemical Process10

any single processing unit, or combinations

of processing units, used for the

conversion of raw materials (through anycombination of chemical, physical,

mechanical, or thermal changes) into

finished products.



Chemical Process Units11

Heat exchanger

CSTR

Distillation columnMixing tank

Furnace



Furnace12



Outline13







Furnace14



Furnace15



Process variables and constraints16

Input variables

Output variables

Hard constraints

Soft constraints



Process variables17

Input variables

independently stimulate the system and

can induce change in the internalconditions of the process

Manipulated variables

Disturbance variables



Process variables18

Output variables

contain information about the internal

state of the processMeasured

Unmeasured



Furnace19



Process variables20



Process constraints21

Hard constraints

Not physically possible to be exceeded

Soft constraints

Possible to exceed imposed limits



Furnace22



Outline23







Transient/Unsteady-state24

Start-up/shutdown

Process disturbances

Change in product specifications



Outline25







Chemical Process Modelling26

Material balance

Energy balance

ThermodynamicsKinetics

Stoichiometry

…



Outline27







Process Dynamics and Control28

concerned with the analysis, design, and

implementation of control systems that

facilitate the achievement of specifiedobjectives of process safety, production

rates, and product quality



Control system29

Functions

Monitors process output variables bymeasurement

Makes rational decisions regarding thecorrective action needed on the basis ofinformation about the current and desired

state of the process Effectively implements these decisions on the

process



Outline30







Elements of the Control system31

SENSOR

CONTROLLERPneumatic

Electronic

Digital computer

ACTUATOR/FINAL CONTROL ELEMENT



Components of the Control system32

TRANSMITTER

Air pressure signals

Electronic signals

TRANSDUCER

Electric to pneumatic; pneumatic to electricA/D; D/A



Outline33







Control system development34

1) Formulate the control objective

2) Develop process model

3) Devise control strategy4) Select control hardware and software

5) Install control system

6) Adjust controller settings



1) Formulate the control objective36

Desired to deliver crude oil feed tofractionators at a constant temperature,T* regardless of the conditions at the

furnace inlet and other factors capableof causing the furnace outlet temperature,T to deviate from desired value, makingsure that the tube surfaces within the

furnace does not at any time exceedvalue, Tm.

Output variable/Controlled variable: T

Set-point: T*




Output variable/Controlled variable: T

Set-point: T*

Control system must also deal with changes

in set-point, T* which depends on

desired refinery throughput.




Regulatory problem

Regulate T when it deviates from T*

due to disturbances

Servo problem

Bring T to new T* when there is a set-point change



2) Develop process model39

Energy balance

Combustion stoichiometry



3) Devise control strategy40

Feedback control

Feed-forward control



Feedback control41



Feedback control42



Feedback control43

Measured variable:

Output variable

Advantage:

Makes corrective action regardless of cause

of deviation

Disadvantage:Corrective action takes place only after

error is detected



Feed-forward control44




Measured variable:

Disturbance variable

Advantage:

Corrective action is taken even before errortakes place in the output variable

Disadvantage:

Must have means of measuring alldisturbance variables and know how theyaffect output variable



Feedback/Feed-forward control47

4) Select control hardware and



4) Select control hardware and

software48

Thermocouple

Flowmeter

Optical pyrometer Alarm




1) Formulate the control objective

2) Develop process model

3) Devise control strategy4) Select control hardware and software

5) Install control system

6) Adjust controller settings

01 che 182 introduction to process dynamics and control

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