Chemical Engineering Design © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & SinnottChemical Engineering Design only. Do not

Chemical Engineering Design© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & SinnottChemical Engineering Design only. Do not copy

Process InstrumentationProcess Instrumentationand Controland Control

Chemical Engineering Design

Process Instrumentation & ControlProcess Instrumentation & Control• Control valves make a significant contribution to pressure drop (see Control valves make a significant contribution to pressure drop (see

later lecture on hydraulics)later lecture on hydraulics)

• Control valve location can create a need for additional pumps and Control valve location can create a need for additional pumps and compressors, and must be decided in order to size the pumps and compressors, and must be decided in order to size the pumps and compressorscompressors

• It is therefore necessary for the design engineer to understand the It is therefore necessary for the design engineer to understand the plant control philosophy even at the PFD stage and the PFD usually plant control philosophy even at the PFD stage and the PFD usually shows the location of control valvesshows the location of control valves

• More details of process control are usually included in the piping and More details of process control are usually included in the piping and instrumentation diagram (P&ID) – see Ch 5instrumentation diagram (P&ID) – see Ch 5

• This lecture is a very brief overview of control for design purposes – This lecture is a very brief overview of control for design purposes – more will come in your process control classmore will come in your process control class

© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & SinnottChemical Engineering Design only. Do not copy


Process Instrumentation & ControlProcess Instrumentation & Control

• Basics of process controlBasics of process control

• Process instrumentationProcess instrumentation

• Reading a P&IDReading a P&ID

• Control of unit operationsControl of unit operations

• Process safety instrumentationProcess safety instrumentation

• Plant-wide control and optimizationPlant-wide control and optimization



Objectives of Process ControlObjectives of Process Control

• Ensure stable process operationEnsure stable process operation– Particularly, keep the plant operating under safe conditionsParticularly, keep the plant operating under safe conditions– Minimize damage to equipment due to variation in plant Minimize damage to equipment due to variation in plant

conditionsconditions

• Ensure operation meets product specificationsEnsure operation meets product specifications

• Minimize impact of external disturbancesMinimize impact of external disturbances– Example: change in ambient temperatureExample: change in ambient temperature

• Optimize process performanceOptimize process performance– Maintain process throughputMaintain process throughput– Minimize operating costsMinimize operating costs



PTPAH

PALPICPV

PTPAH

PALPICPV

Control Loop ComponentsControl Loop Components

• The sensing The sensing instrumentinstrument detects the detects the measured variable measured variable and sends a and sends a signal to a signal to a controllercontroller, which signals the , which signals the actuatoractuator to close or open a to close or open a control valve and adjust the control valve and adjust the manipulated variablemanipulated variable (usually a flow rate) (usually a flow rate)

Process orutility stream

AlarmsInstrument line

Actuator

Final control element

Controller

Sensingelement



Control ValvesControl Valves

• The final control The final control element is usually a element is usually a control valvecontrol valve– Exceptions: electric Exceptions: electric

heaters, mixers, heaters, mixers, variable speed drivesvariable speed drives

• The actuator is either a The actuator is either a motor or a bellows that motor or a bellows that opens or closes the opens or closes the valve in response to valve in response to the signalthe signal

Actuator

Valve

Source: Valve Manufacturer’s Association, www.vma.org



Types of Control LoopTypes of Control Loop

FeedbackFeedback

• Control system measures Control system measures changes in a process output and changes in a process output and then adjusts manipulated variable then adjusts manipulated variable to return output to set pointto return output to set point

• Can be slow if process response Can be slow if process response time is longtime is long

Feed ForwardFeed Forward

• Control system measures Control system measures disturbance and adjusts disturbance and adjusts manipulated variable to manipulated variable to compensate for it so that compensate for it so that controlled output is not affectedcontrolled output is not affected

• Requires greater knowledge of Requires greater knowledge of system responsesystem response

ProcessManipulatedvariable

Controlledoutput

Disturbance

Controller


Controlledoutput

Disturbance

Controller


Controlledoutput

Disturbance

Controller



Feedback ControlFeedback Control

• Controller computes error between input and set point Controller computes error between input and set point and adjusts output based on a control algorithmand adjusts output based on a control algorithm

Process

Sensing element

Final controlelement

Functiongenerator

Setpoint

Output

Errorsignal

Measured variable

Input

Manipulated variable

+-

Controller



Typical Control AlgorithmsTypical Control Algorithms

• P: ProportionalP: Proportional• Controller output is proportional to errorController output is proportional to error• The proportionality constant is called the controller gainThe proportionality constant is called the controller gain• High gain gives fast response, but can lead to instabilityHigh gain gives fast response, but can lead to instability• Low gain can cause offsetsLow gain can cause offsets

• I: IntegralI: Integral• Output is proportional to integral of errorOutput is proportional to integral of error• Eliminates offsets from P control, but makes response more sluggishEliminates offsets from P control, but makes response more sluggish

• D: DerivativeD: Derivative• Output is proportional to rate of change of errorOutput is proportional to rate of change of error• Damps out instability and allows higher gain to be usedDamps out instability and allows higher gain to be used

• Other functions such as summation, multipliers, and Other functions such as summation, multipliers, and advanced models can be easily coded in digital controllersadvanced models can be easily coded in digital controllers

time

SP



• For a PID Controller,For a PID Controller,

where: where: GG == Controller gainController gainee == Error (set point minus measured variable)Error (set point minus measured variable)TTii == Integral time constantIntegral time constant

TTdd == Derivative time constantDerivative time constant

• The controller output is proportional to the error, the time The controller output is proportional to the error, the time integral of the error, and the rate of change of the error. G, Tintegral of the error, and the rate of change of the error. G, T ii

and Tand Tdd are the controller tuning parameters. are the controller tuning parameters.

• Much more of this in control classMuch more of this in control class

Controller Output = G e + (1 / T )t

oe dt - T

de

dti d

PID Controller ResponsePID Controller Response



Ratio ControlRatio Control

• One stream is controlled in One stream is controlled in ratio to anotherratio to another

• Often used for controlling Often used for controlling feed rates to try to maintain feed rates to try to maintain stoichiometrystoichiometry

• Also used in some types of Also used in some types of distillation column control distillation column control to set reflux or boil-up to set reflux or boil-up ratiosratios

FT

FFC

FFV

FT

FT

FFCFFC

FFV

FT



Cascade ControlCascade Control

• One One primary primary controller is used to adjust the set point of a second controller is used to adjust the set point of a second secondarysecondary controller controller

• Used to minimize outside load variations and increase process Used to minimize outside load variations and increase process stabilitystability

FT

FIC

FV

Coolant

TIC

TE

TT

M

FT

FICFIC

FV

Coolant

TICTIC

TE

TT

M• Example: reactor Example: reactor temperature (primary temperature (primary controller) cascades onto controller) cascades onto coolant flow controller coolant flow controller (secondary) to control (secondary) to control reactor temperaturereactor temperature












What Can Be Measured?What Can Be Measured?(& How Easily)(& How Easily)

EasyEasy

• TemperatureTemperature

• PressurePressure

• Flow rateFlow rate

• V/L LevelV/L Level

• Pressure differencePressure difference

• ConductivityConductivity

DifficultDifficult

• L/L levelL/L level

• pHpH

• Certain componentsCertain components– oxygen, sulfur, hydrogen, COoxygen, sulfur, hydrogen, CO

• CompositionComposition

• DensityDensity

• VoidageVoidage

• Easy means cheap, reliable instrument with fast response time and Easy means cheap, reliable instrument with fast response time and accurate measurementaccurate measurement



Temperature Measurement: Temperature Measurement: ThermocouplesThermocouples

• When a junction between dissimilar wires is heated, an When a junction between dissimilar wires is heated, an EMF (voltage) is developed, which can be read by a EMF (voltage) is developed, which can be read by a millivolt transmittermillivolt transmitter

• The junction is usually housed in a thermowellThe junction is usually housed in a thermowell

THERMOCOUPLE HEAD LEAD WIRE

A

B

A

B Cu

Cu+

-

COLD JUNCTION (T )2

MILLIVOLT TRANSMITTER

ISA TYPE A (+) B (-)CONSTANTAN CONSTANTAN ALUMEL

CONSTANTAN

E JKT

CHROMEL IRONCHROMEL COPPER

HOT JUNCTION (T )1

THERMOCOUPLE HEAD LEAD WIRE

A

B

A

B Cu

Cu+

-

COLD JUNCTION (T )2



CONSTANTAN

E JKT


HOT JUNCTION (T )1

LEAD WIRE

A

B

A

B Cu

Cu+

-

COLD JUNCTION (T )2COLD JUNCTION (T )2



CONSTANTAN

E JKT


HOT JUNCTION (T )1HOT JUNCTION (T )HOT JUNCTION (T )1

Typical

High T



Temperature Measurement: Temperature Measurement: ThermocouplesThermocouples

• Response depends on thermowell location and heat Response depends on thermowell location and heat transfertransfer– Instrument error is usually Instrument error is usually 3 to 4 F 3 to 4 F– There may be additional offsets if the thermowell is incorrectly There may be additional offsets if the thermowell is incorrectly

locatedlocated

• Response is fast if located in a flowing streamResponse is fast if located in a flowing stream

• Sometimes thermocouples are also strapped to walls of Sometimes thermocouples are also strapped to walls of vesselsvessels– For high temperature processes or processes with large For high temperature processes or processes with large

exothermsexotherms



Pressure MeasurementPressure Measurement

• Pressure instruments usually measure differential pressurePressure instruments usually measure differential pressure

• If one side is atmospheric pressure then the difference is the process If one side is atmospheric pressure then the difference is the process gaugegauge pressure (usually written barg or psig), not pressure (usually written barg or psig), not absolute absolute pressure pressure (bara, psia)(bara, psia)

• Several possible methods:Several possible methods:– Mechanical: measure displacement of a bellows or Bourdon tubeMechanical: measure displacement of a bellows or Bourdon tube

– Electrical: attach a strain gauge to a bellowsElectrical: attach a strain gauge to a bellows

– Capacitance: diaphragm moves capacitor plate (most common type)Capacitance: diaphragm moves capacitor plate (most common type)

– Piezoelectric: measures change in semiconductor conductivityPiezoelectric: measures change in semiconductor conductivity

• Pressure measurement devices respond quickly and accuratelyPressure measurement devices respond quickly and accurately

• Differential pressure measurement is used as the basis for flow and Differential pressure measurement is used as the basis for flow and level measurementlevel measurement



Flow Rate MeasurementFlow Rate Measurement

• Place a restriction in the flow path and measure the Place a restriction in the flow path and measure the resulting pressure drop using a differential pressure (PD) resulting pressure drop using a differential pressure (PD) cellcell

• If fluid properties are known, results can be calibrated to If fluid properties are known, results can be calibrated to flow ratesflow rates

PD PD

Orifice Meter Venturi Meter



Level MeasurementLevel Measurement

DisplacementDisplacement

• DisplacerDisplacer moves up and down moves up and down with level due to bouyancywith level due to bouyancy

• Displacer movement is Displacer movement is detected via mechanical or detected via mechanical or magnetic linkagemagnetic linkage

Differential PressureDifferential Pressure

• Measures static head of liquid Measures static head of liquid using a differential pressure using a differential pressure cellcell

• Density of the liquid and vapor Density of the liquid and vapor must be known and constantmust be known and constant

Sensor element

PD



Composition MeasurementComposition Measurement• Some components can be detected at low concentrations using sensors Some components can be detected at low concentrations using sensors

that have been designed to pick up that componentthat have been designed to pick up that component– Examples: OExamples: O22, CO, H, CO, H22S, HS, H22

– Component sensors are often sensitive to other components, so check Component sensors are often sensitive to other components, so check carefully with vendor to make sure the device is rated for the applicationcarefully with vendor to make sure the device is rated for the application

• More detailed composition can be measured by on-line GC methods More detailed composition can be measured by on-line GC methods – TCD: thermal conductivity detectorTCD: thermal conductivity detector

– FID: flame ionization detectorFID: flame ionization detector

– Response can be slow (5 to 30 minutes), particularly if a long column is used Response can be slow (5 to 30 minutes), particularly if a long column is used

• Online NIR can be used in some casesOnline NIR can be used in some cases

• Composition is often inferred from other propertiesComposition is often inferred from other properties– Boiling pointBoiling point

– ConductivityConductivity












Piping and Instrument DiagramsPiping and Instrument Diagrams

• The P&ID shows all the instruments and valves in the processThe P&ID shows all the instruments and valves in the process– Not just control loopsNot just control loops

– Vents, drains, sample points, relief valves, steam traps, isolation valves, Vents, drains, sample points, relief valves, steam traps, isolation valves, etc.etc.

• The P&ID usually also indicates line sizes and pipe metallurgyThe P&ID usually also indicates line sizes and pipe metallurgy

• Companies occasionally use their own symbols, but U.S. standard Companies occasionally use their own symbols, but U.S. standard is ISA-5.1-1984 (R1992) from the International Society for is ISA-5.1-1984 (R1992) from the International Society for AutomationAutomation

• The P&ID is usually produced in consultation with a specialist The P&ID is usually produced in consultation with a specialist controls engineercontrols engineer

• Example of real P&ID: see Ch5Example of real P&ID: see Ch5



General DiaphragmGlobeThree-way

ISA 5.1 P&ID SymbolsISA 5.1 P&ID Symbols

Diaphragm or unspecified actuator

Rotary motorDigitalSolenoid

S D M

Control Valves

Actuators



Fails open Fails locked in current position

Fails closed Failure mode indeterminate

Valve Failure PositionsValve Failure Positions

• It is important to specify what happens to a control valve It is important to specify what happens to a control valve if the signal failsif the signal fails

• The final valve position has an impact on process safety The final valve position has an impact on process safety and pressure relief scenarios and may affect other and pressure relief scenarios and may affect other instrumentationinstrumentation



Electric signal (4 to 20 mA)

Pneumatic (instrument air) signal

Undefined signal

Instrument supply or connection to process

Electric binary (on-off) signal

Internal system link (software or data link)

or

or

ISA 5.1 Instrument LinesISA 5.1 Instrument Lines

• Most newer plants use Most newer plants use electric signalselectric signals

• Pneumatic signals are Pneumatic signals are found in older plants and found in older plants and locations where electric locations where electric signals would be unsafesignals would be unsafe

• Binary signals are used Binary signals are used for digital signals and for for digital signals and for solenoids and other on-solenoids and other on-off devicesoff devices

• Instrument lines are Instrument lines are always drawn lighter always drawn lighter than process lines on than process lines on the P&IDthe P&ID



Field mounted Panel mounted in auxiliary location

(local panel)

Panel mounted in primary location

Dual function instrument

ISA 5.1 Controller SymbolsISA 5.1 Controller Symbols

Field mounted shared display device with limited access to adjustments

Shared display device with operator access to adjustments

Shared display device with software alarms (* is measured variable)*AH*AL

Distributed Control Shared Display Symbols



Shared Display DevicesShared Display Devices

• Most plant control rooms now use shared Most plant control rooms now use shared display devices that show the outputs of display devices that show the outputs of multiple instruments on a VDU screenmultiple instruments on a VDU screen

• Operator can see a flow diagram that Operator can see a flow diagram that identifies where the instrument is and can identifies where the instrument is and can enter set pointsenter set points

• Software also allows data to be plotted as Software also allows data to be plotted as trendstrends

• Data can be accessed remotelyData can be accessed remotely

• Data is collected and logged for process Data is collected and logged for process recordsrecords

Source: UOP



ISA 5.1 NomenclatureISA 5.1 Nomenclature

• Two- to four-letter codes are used to identify the Two- to four-letter codes are used to identify the instrument or controllerinstrument or controller

• First letter always indicates the initiating or measured First letter always indicates the initiating or measured variablevariable

• Subsequent letters I = indicator, R = recorder, C = Subsequent letters I = indicator, R = recorder, C = controller, T = transmitter, V = valve, Z = other final controller, T = transmitter, V = valve, Z = other final control element, S = switch, Y = compute function, AH = control element, S = switch, Y = compute function, AH = high alarm, AL = low alarmhigh alarm, AL = low alarm

PIC

Pressure Indicator Controller



ISA 5.1 Nomenclature: First LettersISA 5.1 Nomenclature: First Letters

• AA Analysis (composition)Analysis (composition)

• FF FlowFlow

• FFFF Flow ratioFlow ratio

• JJ PowerPower

• LL LevelLevel

• PP Pressure (or vacuum)Pressure (or vacuum)

• PDPD Pressure differentialPressure differential

• QQ QuantityQuantity

• RR RadiationRadiation

• TT TemperatureTemperature

• TDTD Temperature Temperature differentialdifferential

• WW WeightWeight

• C, D, G, M, N, O can be user-C, D, G, M, N, O can be user-defined variablesdefined variables



Exercise: Identify The InstrumentExercise: Identify The Instrument

• Can you figure out what each of these ISA 5.1 codes Can you figure out what each of these ISA 5.1 codes means and what the instrument does?means and what the instrument does?– TRCTRC– ARAR– PAHPAH– PAHHPAHH– LILI– PCPC– TSHTSH– FFYFFY– PTPT– JIALJIAL



Restriction orifice Gate valve or isolation valve

Hand control valvePressure relief or safety valve

Self-contained backpressure

regulator

Stop check (non-return) valve

ISA 5.1 Other Common SymbolsISA 5.1 Other Common Symbols












LV

LTLAH

LALLIC

M

Level ControlLevel Control

• A level control is needed A level control is needed whenever there is a V/L or whenever there is a V/L or L/L interfaceL/L interface

• Level control sets Level control sets inventories in process inventories in process equipmentequipment

• Many smaller vessels are Many smaller vessels are sized based on level control sized based on level control response timeresponse time



Pressure ControlPressure Control

• Pressure control is usually by venting a Pressure control is usually by venting a gas or vaporgas or vapor

• In hydrocarbon processes, off-gas is In hydrocarbon processes, off-gas is often vented to fueloften vented to fuel

• In other processes, nitrogen may be In other processes, nitrogen may be brought in to maintain pressure and brought in to maintain pressure and vented via scrubbersvented via scrubbers

• Most common arrangement is direct Most common arrangement is direct venting (shown)venting (shown)

• Several vessels that are connected Several vessels that are connected together may have a single pressure together may have a single pressure controllercontroller

PV

PTPIC

PV

PTPIC



Pressure ControlPressure Control

• If vapor has a high If vapor has a high loading of condensable loading of condensable material, then pressure material, then pressure control is on the vent control is on the vent gas stream from the gas stream from the condensercondenser

PV

PTPIC

PV

PTPIC

PV

PTPIC



PV

PTPIC

Coolant

Process

PT

PIC

PVProcessvapor

Coolant

Pressure Control: CondensersPressure Control: Condensers

• Alternatively, for a condenser, we can control the coolant Alternatively, for a condenser, we can control the coolant supply or the heat transfer surface (by varying the liquid supply or the heat transfer surface (by varying the liquid level)level)

• These methods control pressure by changing the rate of These methods control pressure by changing the rate of condensationcondensation



Flow ControlFlow Control

• Most common arrangement is a control valve Most common arrangement is a control valve downstream of a pump or compressordownstream of a pump or compressor

• Using a variable speed drive is a more efficient method, Using a variable speed drive is a more efficient method, but higher capital costbut higher capital cost

FT

FIC

FVPI

M

FT

FIC

PIMFT

FIC

FVPI

M

FT

FIC

PIM FT

FIC

PIM



Flow Control: Reciprocating PumpFlow Control: Reciprocating Pump

• Reciprocating pumps Reciprocating pumps and compressors and and compressors and other positive other positive displacement devices displacement devices deliver constant flow deliver constant flow raterate

• Flow can be controlled Flow can be controlled by manipulating a spill-by manipulating a spill-back to the pump feedback to the pump feed

FT

FY

FV

PIFT

FIC

FIFI

FT

FY

FV

PIFT

FIC

FIFI



LT

LIC

LV

Feed

FT

FIC

FV

Steam

Trap

Condensate

Vapor

LT

LIC

LV

Feed

FT

FIC

FV

Steam

Trap

Condensate

Vapor

Flow Control: VaporizerFlow Control: Vaporizer

• Vaporizer flow control Vaporizer flow control needs to prevent liquid needs to prevent liquid accumulationaccumulation

• Hence use level Hence use level controller to actuate controller to actuate heat input to the heat input to the vaporizer and maintain a vaporizer and maintain a constant inventoryconstant inventory

• Control of liquid flow in Control of liquid flow in is easier than control of is easier than control of vapor flow outvapor flow out



Temperature Control: Single StreamTemperature Control: Single Stream

• Heaters and coolers Heaters and coolers are usually controlled are usually controlled by manipulating the by manipulating the flow rate of the hot or flow rate of the hot or cold utility streamcold utility stream

• Final control element Final control element can be on inlet or can be on inlet or outlet of utility sideoutlet of utility side

TV

TE TIC

Hot or cold utility

Process

TT

TV

TE TICTIC

Hot or cold utility

Process

TT



Temperature Control: Heat ExchangersTemperature Control: Heat Exchangers

• Temperature control Temperature control for an exchanger is for an exchanger is usually by usually by manipulating the flow manipulating the flow through a bypassthrough a bypass

• Only one side of an Only one side of an exchanger can be exchanger can be temperature controlledtemperature controlled

TV

TE TICTT

TV

TE TICTICTT

• It is also common to see exchangers with no temperature It is also common to see exchangers with no temperature control and have temperature control on the downstream control and have temperature control on the downstream heater and coolerheater and cooler



Temperature Control: Air CoolersTemperature Control: Air Coolers

• Ambient air temperature varies, so air coolers are Ambient air temperature varies, so air coolers are oversized and controlled by manipulating a bypassoversized and controlled by manipulating a bypass

• Alternatively, air cooler can use a variable speed motor, Alternatively, air cooler can use a variable speed motor, louvers or variable pitch fans – see lectures on heat louvers or variable pitch fans – see lectures on heat exchange equipmentexchange equipment

TV

TE TICTT

MM

TE TICTT

MM

TO VARIABLE SPEED MOTOR CONTROL CIRCUIT

TV

TE TICTT

MMTV

TE TICTTTE TICTICTT

MMMM

TE TICTT

MM


TE TICTTTE TICTICTT

MMMM




Distillation ControlDistillation Control• Distillation control is a specialized subject in its own rightDistillation control is a specialized subject in its own right

• In addition to controlling condenser pressure and level in the In addition to controlling condenser pressure and level in the sump, a simple distillation column has two degrees of sump, a simple distillation column has two degrees of freedomfreedom

• Material balance (split) and energy balance (heat input or removed)Material balance (split) and energy balance (heat input or removed)• Therefore needs two controllersTherefore needs two controllers• Therefore has the possibility that the controllers will interact and “fight” each otherTherefore has the possibility that the controllers will interact and “fight” each other

• Side streams, intermediate condensers & reboilers, pump-Side streams, intermediate condensers & reboilers, pump-arounds, etc. all add extra complexity and degrees of arounds, etc. all add extra complexity and degrees of freedomfreedom

• There are several good books on this subjectThere are several good books on this subject• Kister, H.Z., 1989, Kister, H.Z., 1989, Distillation OperationDistillation Operation, McGraw-Hill, McGraw-Hill• Luyben, W.L. 2006, Luyben, W.L. 2006, Distillation Design and Control Using Aspen SimulationDistillation Design and Control Using Aspen Simulation , Wiley , Wiley



Steam

TC

TCLC

Distillation: Temperature Pattern ControlDistillation: Temperature Pattern Control• Tray temperature is used to infer Tray temperature is used to infer

compositioncomposition

• Composition is used to adjust Composition is used to adjust reflux rate and reboiler heat inputreflux rate and reboiler heat input

• Tray locations for temperature Tray locations for temperature detectors need to be chosen detectors need to be chosen carefullycarefully

• Controllers can fight each other – Controllers can fight each other – not a good schemenot a good scheme

• Material balance control schemes Material balance control schemes are more robustare more robust



Distillation: Material Balance ControlDistillation: Material Balance Control

• Direct control of distillate Direct control of distillate composition by using tray composition by using tray temperature to infer temperature to infer composition and control composition and control distillate flow ratedistillate flow rate

• Flow control on (constant) Flow control on (constant) boil-up rate could be set in boil-up rate could be set in ratio to feed if feed flow rate ratio to feed if feed flow rate was highly variablewas highly variable


LC

LC

PC

TC

FC



• Indirect control of distillate Indirect control of distillate composition by using tray composition by using tray temperature to infer temperature to infer composition and control reflux composition and control reflux raterate

• Flow control on (constant) Flow control on (constant) boil-up rate could be set in boil-up rate could be set in ratio to feed if feed flow rate ratio to feed if feed flow rate variesvaries


LC

LC

PC

TC

FC



• Direct control of bottoms Direct control of bottoms composition by using tray composition by using tray temperature to infer temperature to infer composition and control composition and control bottoms flow ratebottoms flow rate

• Flow control on (constant) Flow control on (constant) reflux rate could be set in reflux rate could be set in ratio to feed if feed flow rate ratio to feed if feed flow rate variesvaries


LC

LC

PC

TC

FC



• Indirect control of bottoms Indirect control of bottoms composition by using tray composition by using tray temperature to infer temperature to infer composition and control boil-composition and control boil-up rateup rate

• Flow control on (constant) Flow control on (constant) reflux rate could be set in reflux rate could be set in ratio to feed if feed flow rate ratio to feed if feed flow rate variesvaries


LC

LC

PC

TC

FC


FT

FIC

FV

SteamTrap

TE

FV

FT

TT FY

FIC

Intermittent charge

Batch DistillationBatch Distillation

• Reflux flow control adjusted based on temperature (used Reflux flow control adjusted based on temperature (used to infer composition)to infer composition)

Intermittent drain



Reactor ControlReactor Control

• Control of flow is usually carried out on cold reactor feedsControl of flow is usually carried out on cold reactor feeds– Flows are often ratio controlled to get close to desired Flows are often ratio controlled to get close to desired

stoichiometry or maintain desired excess of one feedstoichiometry or maintain desired excess of one feed

• Pressure control is on reactor vapor outlet or on vent Pressure control is on reactor vapor outlet or on vent spacespace

• Level control maintains inventory for liquid phase reactorsLevel control maintains inventory for liquid phase reactors

• Temperature control can beTemperature control can be– On feedsOn feeds– On heating or cooling jacketOn heating or cooling jacket– On recirculation through external heaters or coolerOn recirculation through external heaters or cooler– By controlling flow of quench/reheat streamsBy controlling flow of quench/reheat streams



FT

FIC

FV

Coolant

TIC

TE

TT

LTLAH

LALLIC

FV FTFIC

FV FTFIC

M

PTPIC

Feed A

Product

To vent system

Feed B

Typical Stirred Tank Reactor Control Typical Stirred Tank Reactor Control SchemeScheme

• Temperature Temperature cascade control of cascade control of coolant flowcoolant flow

• Independent flow Independent flow control of feedscontrol of feeds



Exercise: Gas Recycle ProcessExercise: Gas Recycle Process

• Remember the simple flowsheet introduced in the lecture on simulation?Remember the simple flowsheet introduced in the lecture on simulation?

• Liquid feed is mixed with recycle gas, heat exchanged against reactor Liquid feed is mixed with recycle gas, heat exchanged against reactor effluent, heated to reactor temperature then passed over fixed bed of effluent, heated to reactor temperature then passed over fixed bed of catalyst. Product is cooled and liquid product is recovered. Unconverted gas catalyst. Product is cooled and liquid product is recovered. Unconverted gas is recycled with purge to prevent accumulation of inertsis recycled with purge to prevent accumulation of inerts

Feed

Reactor

Make-up gas Purge

Product

Feed

Reactor

Make-up gas Purge

Product



Exercise: Gas Recycle ProcessExercise: Gas Recycle Process

• What controllers would you use?What controllers would you use?

• Where would you place them?Where would you place them?

Feed

Reactor

Make-up gas Purge

Product

Feed

Reactor

Make-up gas Purge

Product












Role of Controls in Process SafetyRole of Controls in Process Safety

Automatic Safety Shutdowns

Pressure Relief System

Critical Alarms & Operator Intervention

Basic Process Control

Plant Design (Inherent Safety)

Emergency Responsein Community

Emergency Responsein Process Unit

Automatic Safety Shutdowns

Pressure Relief System

Critical Alarms & Operator Intervention

Basic Process Control

Plant Design (Inherent Safety)

Emergency Responsein Community

Emergency Responsein Process Unit

• Control system is involved in three Control system is involved in three levels of process safetylevels of process safety– Keeping plant operation steadyKeeping plant operation steady– Sounding alarms to notify operator Sounding alarms to notify operator

when variables are out of limitswhen variables are out of limits– Automatically shutting the plant down Automatically shutting the plant down

when necessarywhen necessary



Process Control, Alarms and ShutdownsProcess Control, Alarms and Shutdowns

• Controlled parameters naturally fluctuate around set pointControlled parameters naturally fluctuate around set point

• If the measured variable exceeds a preset limit an alarm If the measured variable exceeds a preset limit an alarm should alert the operator to take appropriate actionshould alert the operator to take appropriate action

• Alarm limits should be set far enough from normal process variation to avoid Alarm limits should be set far enough from normal process variation to avoid nuisance alarmsnuisance alarms

• If the measured variable exceeds a safe operating limit If the measured variable exceeds a safe operating limit then an automatic plant shutdown may be necessarythen an automatic plant shutdown may be necessary

• Shutdown limit should be set far enough from alarm limit that the operator has Shutdown limit should be set far enough from alarm limit that the operator has a chance to respond to the alarma chance to respond to the alarm

• But not so far that no time is left to safely shut the plant downBut not so far that no time is left to safely shut the plant down

time

Variable

AL

AH

Shutdown

Set point



Standards for Safety InstrumentationStandards for Safety Instrumentation

• ISA S84.01 Safety Instrumented SystemsISA S84.01 Safety Instrumented Systems• U.S. standard for emergency shutdown systemsU.S. standard for emergency shutdown systems

• Primary goal is to protect people, not plant or profitsPrimary goal is to protect people, not plant or profits

• ISA S84.01 = IEC 61511ISA S84.01 = IEC 61511

• IEC 61508 & 61511IEC 61508 & 61511• IEC = International Electrotechnical CommissionIEC = International Electrotechnical Commission

• International standards for safety instrumented systemsInternational standards for safety instrumented systems

• Standards define requirements for sensors, solvers (logic), and final Standards define requirements for sensors, solvers (logic), and final elements (valves, switches)elements (valves, switches)

• Consult most recent version of standards for current best practicesConsult most recent version of standards for current best practices

• Other standards also recommend best practices for alarm levels, Other standards also recommend best practices for alarm levels, vessel sizing to allow adequate control, etc.vessel sizing to allow adequate control, etc.



Safety Integrity LevelsSafety Integrity Levels

• ISA S84.01 defines three levels of safety integrity ISA S84.01 defines three levels of safety integrity depending on the availability of the SISdepending on the availability of the SIS

• Availability = time the system is available / total timeAvailability = time the system is available / total time

• Safety Integrity LevelsSafety Integrity LevelsSILSIL AvailabilityAvailability System redudancySystem redudancy– SIL 1SIL 1 90 – 99%90 – 99% Non-redundantNon-redundant– SIL 2SIL 2 99 – 99.9%99 – 99.9% Partially redundantPartially redundant– SIL 3SIL 3 99.9 – 99.99%99.9 – 99.99% Totally redundantTotally redundant

• Redundant system means instrumentation is duplicatedRedundant system means instrumentation is duplicated



Safety Integrity LevelSafety Integrity Level

• SIL should be determined during a process hazard SIL should be determined during a process hazard analysis (see Ch10 and lectures on process safety)analysis (see Ch10 and lectures on process safety)

• SIL required depends on risk of operator exposure and SIL required depends on risk of operator exposure and injuryinjury– Can be calculated using fault treesCan be calculated using fault trees– See Ch10 and later lectureSee Ch10 and later lecture

• SIL determines the type of instrumentation that should SIL determines the type of instrumentation that should be usedbe used



LTLAL

LIC

TRIP

LTLIC

LSL LAL

LAL

UCAUC

A

S

LTLAL

LIC

TRIP

LTLAL

LIC

TRIP

LTLICLIC

LSL LALLAL

LAL

UCA

UCAUC

AUCA

S

Process Alarms and Shutdown TripsProcess Alarms and Shutdown Trips

• Software alarms can be set on instruments and controllers through the Software alarms can be set on instruments and controllers through the digital control system and show up on shared displaysdigital control system and show up on shared displays

• Separate alarm and shutdown instrumentation can also be used, for Separate alarm and shutdown instrumentation can also be used, for higher redundancyhigher redundancy



Caution on Software AlarmsCaution on Software Alarms

• There is a temptation to put lots of software alarms in digital control There is a temptation to put lots of software alarms in digital control systemssystems

• If there are too many alarms then they can become a distraction to If there are too many alarms then they can become a distraction to the operatorsthe operators– Increasing the chance of human errorIncreasing the chance of human error

– Increasing the chance that the operator will ignore the alarm, switch it Increasing the chance that the operator will ignore the alarm, switch it off, or acknowledge it without taking actionoff, or acknowledge it without taking action

– Increased chance of an “alarm flood”Increased chance of an “alarm flood”

• Alarms should be carefully placed and calibrated to make sure that Alarms should be carefully placed and calibrated to make sure that they serve the purpose of the designerthey serve the purpose of the designer

• Operators should be trained to understand the importance of every Operators should be trained to understand the importance of every alarm on the plantalarm on the plant












Plant-Wide ControlPlant-Wide Control(Advanced Process Control, APC)(Advanced Process Control, APC)

• Operators can update individual controller set points Operators can update individual controller set points via shared displaysvia shared displays

• Plant information systems can log dataPlant information systems can log data

• Feedforward and multivariable predictive control can Feedforward and multivariable predictive control can be implemented more easilybe implemented more easily

• Plant real-time optimization can be implemented Plant real-time optimization can be implemented through the control systemthrough the control system

All new plants and most older plants have the instruments and controllers All new plants and most older plants have the instruments and controllers connected to a plant-wide digital control system (DCS). Using the DCS:connected to a plant-wide digital control system (DCS). Using the DCS:

Most controllers now have microprocessors built into them, so the Most controllers now have microprocessors built into them, so the computational capacity that is needed is not necessarily located in the computational capacity that is needed is not necessarily located in the control roomcontrol room



Plant-Wide Control IssuesPlant-Wide Control Issues

• Where do we control inventories? Where do we control inventories? – All vessels that have a liquid levelAll vessels that have a liquid level– Do we need extra surge volume to keep operations steady?Do we need extra surge volume to keep operations steady?

• Where is the best place to control pressure?Where is the best place to control pressure?– Single PC setting plant back-pressure?Single PC setting plant back-pressure?– Need for different pressure levels?Need for different pressure levels?

• Where is best place to control material balance?Where is best place to control material balance?– Can only control flow in one place per feed streamCan only control flow in one place per feed stream– Control after removing contaminants?Control after removing contaminants?– Ratio control? Ratio control? – What about recycles?What about recycles?



Plant-Wide Control IssuesPlant-Wide Control Issues

• Where is best place to control temperatures?Where is best place to control temperatures?– Heaters and coolers only?Heaters and coolers only?– Control of heat exchanger networks?Control of heat exchanger networks?

• How do we meet product specifications?How do we meet product specifications?– Use of on-line analytical instruments?Use of on-line analytical instruments?– Inference through bubble point?Inference through bubble point?– Sampling of product storage tank?Sampling of product storage tank?

• How do we optimize the process?How do we optimize the process?– Maximize throughput and product yield while staying on spec?Maximize throughput and product yield while staying on spec?– Minimize cost of production?Minimize cost of production?– Maximize time on stream between shutdowns?Maximize time on stream between shutdowns?



Multivariable Predictive ControlMultivariable Predictive Control

• What if we have three manipulated variables that interact What if we have three manipulated variables that interact with each other through non-linear equations?with each other through non-linear equations?

Measured variable 1

Measured variable 2

Measured variable 3

Control Valve 1

Control Valve 2

Control Valve 3

• Single Input Single Output (SISO) controllers will have a Single Input Single Output (SISO) controllers will have a hard time responding and will tend to have strong hard time responding and will tend to have strong interactions with each other (tendency to fight)interactions with each other (tendency to fight)



Multivariable Predictive ControlMultivariable Predictive Control

• In this situation, instead of SISO controllers we need a In this situation, instead of SISO controllers we need a Multiple Input Multiple Output (MIMO) controllerMultiple Input Multiple Output (MIMO) controller

• The controller algorithm is a model that captures the non-The controller algorithm is a model that captures the non-linear interaction between the manipulated variableslinear interaction between the manipulated variables– Does Does notnot have to be a “fundamental” model have to be a “fundamental” model– DoesDoes have to capture dynamic response have to capture dynamic response– Can be tuned from plant operationCan be tuned from plant operation

• Hence another name for multivariable predictive control is Hence another name for multivariable predictive control is model-based predictive controlmodel-based predictive control

• With MIMO devices, there is no longer a direct pairing of With MIMO devices, there is no longer a direct pairing of measured and manipulated variables as in the earlier unit measured and manipulated variables as in the earlier unit operations examplesoperations examples



MIMO Example: Gas Mass Flow MIMO Example: Gas Mass Flow ControllerController

• Gas mass flow cannot be measured or controlled directlyGas mass flow cannot be measured or controlled directly

• By measuring volume flow, temperature and pressure By measuring volume flow, temperature and pressure we can compute mass flow as long as composition is we can compute mass flow as long as composition is known and steadyknown and steady

• This is actually MISO, not MIMO, but the FY could talk to another device This is actually MISO, not MIMO, but the FY could talk to another device such as a ratio controllersuch as a ratio controller


FT

FIC

FVPT TT

FY


Device Communication StandardsDevice Communication Standards

• Now that individual controllers (and even instruments) Now that individual controllers (and even instruments) contain microprocessors, the computational power of the contain microprocessors, the computational power of the DCS is distributedDCS is distributed

• To make best use of this distributed computation power, To make best use of this distributed computation power, the devices need to be able to communicate with each the devices need to be able to communicate with each otherother

• Communication standards are set by ISA, IEC and the Communication standards are set by ISA, IEC and the controller manufacturers and are currently evolving controller manufacturers and are currently evolving rapidlyrapidly



Evolution of Controller Communication Evolution of Controller Communication StandardsStandards

Manual Control

Pneumatic Analog

Electronic Analog

Digital

Fieldbus (ISA SP50)

Wireless (ISA SP100)

?!?!?!?Source: UOP



Fieldbus (ISA SP50)Fieldbus (ISA SP50)

Source: UOP

• Digital device communication Digital device communication protocol that allows “plug and protocol that allows “plug and play” connection of devicesplay” connection of devices

• Requires less wiring and Requires less wiring and gives greater reliability gives greater reliability through redundancythrough redundancy

• Different control companies Different control companies have variations on the have variations on the standard, so system standard, so system compatibility and compatibility and interoperability are issuesinteroperability are issues



Wireless Communication (SP100)Wireless Communication (SP100)

AdvantagesAdvantages

• No cable runsNo cable runs

• Quicker and cheaper to set upQuicker and cheaper to set up

• Portable control roomPortable control room

• Improved safety (electric Improved safety (electric cables are easily damaged by cables are easily damaged by fires)fires)

ProblemsProblems

• Interference from other Interference from other wireless deviceswireless devices

• Signal blocking due to Signal blocking due to steelworksteelwork

• Signal loss creates need for Signal loss creates need for greater redundancygreater redundancy

The controls companies are currently putting a lot of effort The controls companies are currently putting a lot of effort into developing the devices and standards for implementing into developing the devices and standards for implementing

wireless controlwireless control



Real Time OptimizationReal Time Optimization

• A DCS can be programmed to carry out A DCS can be programmed to carry out optimization of plant performance by optimization of plant performance by updating controller set points and MPC updating controller set points and MPC algorithmsalgorithms

• The optimizer is usually a higher level The optimizer is usually a higher level program that runs less frequently and is program that runs less frequently and is used to adjust set points periodically by used to adjust set points periodically by computing target values for key computing target values for key performance indicators (KPIs)performance indicators (KPIs)

• The optimizers used for RTO are often The optimizers used for RTO are often not very sophisticated – typically LP, not very sophisticated – typically LP, MILP or simple NLP modelsMILP or simple NLP models

Optimizer

DCS

Controllers

Plant

Targets for KPIs

Set points

Adjust manipulated variables



Types of Real-time OptimizationTypes of Real-time Optimization

• Users take plant data and run the optimizer then send Users take plant data and run the optimizer then send instructions to the plant operators to update the DCS instructions to the plant operators to update the DCS settingssettings

• Labor intensive and difficult to update more than dailyLabor intensive and difficult to update more than daily

Plant DCS

Optimizer

User User

Off-line Optimization




• Users provide input to optimizer, DCS Users provide input to optimizer, DCS updates optimizer directly with plant updates optimizer directly with plant settings and user updates DCS with settings and user updates DCS with new targetsnew targets

Plant DCS

Optimizer

User User

Off-line Optimization

Plant DCS

Optimizer

User User

Open Loop On-line Optimization




• Users only provide input to the Users only provide input to the optimizer and the DCS is updated optimizer and the DCS is updated directly by the optimizerdirectly by the optimizer

Plant DCS

Optimizer

User User

Off-line OptimizationPlant DCS

Optimizer

User User

Open Loop On-line Optimization

Plant DCS

Optimizer

User

Closed Loop On-line Optimization© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & SinnottChemical Engineering Design only. Do not copy


Real Time Optimization ModelsReal Time Optimization Models

• Models and algorithms for RTO have very tough Models and algorithms for RTO have very tough requirementsrequirements– Must be robust, i.e., always find a solutionMust be robust, i.e., always find a solution– Must solve quicklyMust solve quickly– Must converge to same solution whatever the starting pointMust converge to same solution whatever the starting point– Must allow for model errorMust allow for model error– Must reconcile data and filter out bad dataMust reconcile data and filter out bad data– Must capture plant constraintsMust capture plant constraints– Must give reasonably good description of plant performanceMust give reasonably good description of plant performance

• Hence frequent use of simple LP modelsHence frequent use of simple LP models

• Controls companies spend a lot of time setting up and Controls companies spend a lot of time setting up and tuning modelstuning models



Benefits of Advanced Process ControlBenefits of Advanced Process Control

• See web sites, technical papers or presentations from See web sites, technical papers or presentations from the major controls vendorsthe major controls vendors– Emerson Process Management Emerson Process Management www.EmersonProcess.com – Foxboro Foxboro www.Foxboro.com – Honeywell Automation and Control Systems Honeywell Automation and Control Systems

www.Honeywell.com – Yokogawa Yokogawa www.Yokogawa.com/us

• The controls vendors give lots of good examples of The controls vendors give lots of good examples of recent industrial projects that they have carried outrecent industrial projects that they have carried out

• Paybacks for installing APC systems are often very goodPaybacks for installing APC systems are often very good



Questions ?Questions ?


Documents

Chemical Engineering Design © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & SinnottChemical Engineering Design only. Do not