CHEMICAL PROCESS
CONTROL
FUNDAMENTALS
CONTROL CATEGORIES
• OPERATING MODE
– CONTINUOUS
– BATCH
– SEMI-BATCH
• OPERATING CONDITIONS
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–
–
–
START-UP
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EMERGENCY SHUTDOWN
• MONITORING
– MAINTENANCE
– OPTIMIZATION
• SEE PERRY’S 7TH ED., CHAPTER 8
CONTROL DESIGN
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OPERATOR CONTROL
AUTOMATIC CONTROL
ALARMS
INTERLOCKS
DATA LOGGING
AUTOMATIC TUNING
SEQUENCING
PROGRAMMABLE LOGIC
CONTROLLERS
• FIELD MONITORS
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STEADY STATE OPERATION
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SYSTEM SHOULD RUN RELIABLY WITH MINIMAL
SUPERVISION
SYSTEM SHOULD MAKE ON-SPECIFICATION
PRODUCT ROUTINELY
MONITORED BY OPERATOR WITH A MINIMUM
NUMBER OF VARIABLES
CONTROL LOOP INTERACTIONS AUTOMATIC
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START UP
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SYSTEM SHOULD BE ABLE TO
SAFELY REACH PRODUCTION AND
QUALITY LEVELS THROUGH A
STANDARD SEQUENCE OF EVENTS
CONTROL SYSTEM SHOULD
INCLUDE ASSESSMENT (CHECKS TO
CONFIRM STEPS IN THE START UP
SEQUENCE HAVE BEEN CORRECTLY
COMPLETED)
EFFFECTS OF CHANGES SHOULD BE
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START UP PROCEDURES
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REACTORS
CONTINUOUS MAY BE STARTED
WITH A HEEL OF MATERIAL FROM
THE PREVIOUS SHUTDOWN OR
SOME PRODUCT RECYCLED TO THE
REACTOR TO APPROXIMATE
STEADY STATE CONDITIONS
BATCH FOLLOW NORMAL START UP
SEQUENCE
START UP PROCEDURES
•
SEPARATION PROCESSES
– TYPICALLY ARE STARTED ON TOTAL
RECYCLE UNTIL SPECIFICATION
QUALITY MATERIAL IS PRODUCED.
– THIS MAY REQUIRE DELAYING START
UP OF REACTION SYSTEM OR
ADDING A SURGE TANK TO COLLECT
PRODUCT DURING SEPARATION
START UP.
CONTROLLED SHUT DOWN
•
•
CAN BE PARTIAL OR COMPLETE
PARTIAL
– PUT SYSTEM IN STANDBY MODE TO
ALLOW RAPID AND SAFE RESTART
– DISTILLATION COLUMNS ON TOTAL
RECYCLE
•
COMPLETE
– SHUT DOWN PROCESS WITHOUT
CREATING OFF-SPEC PRODUCT OR
PRODUCING A HAZARDOUS SITUATION
•
STANDARD PROTOCOLS FOR BOTH
TYPICAL SHUTDOWN DESIGNS
•
REACTORS
SUPERPRO
– SHORT TERM
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STOP FEEDS
MAINTAIN COOLING, BUT KEEP MATERIALS
FROM CRYSTALLIZING
MAINTAIN AGITATION.
– LONG TERM
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POSSIBLY DRAIN REACTANTS TO A
SEPARATE VESSEL IF REACTION CANNOT
BE STOPPED AT THE STEADY STATE
CONDITIONS
OTHERWISE, SAME AS SHORT TERM
TYPICAL SHUTDOWN DESIGNS
•
SEPARATION
PROCESSES
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–
•
SHORT TERM - PLACE
SYSTEM ON TOTAL
RECYCLE AND MAINTAIN
OPERATION.
LONG TERM - DRAIN
SYSTEM TO SAVE
UTILITIES.
ROTATING EQUIPMENT
–
–
DEPEND ON NEEDS OF
PROCESS WHILE
SHUTTING DOWN
COOLING AND
AGITATION ARE
TYPICALLY MAINTAINED.
SIMSCI PROSIM II
EMERGENCY SHUT DOWN
•
ONE BUTTON SHUT DOWN BY OPERATOR
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PROCESS SHOULD BE PROTECTED AGAINST
CATASTROPHIC FAILURE DURING AN
UNMONITORED SHUT DOWN
PROCESS SHOULD BE FAIL-SAFE
ALARMS AND INTERLOCKS SHOULD
PROTECT LOCAL PROCESSES
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–
FIRST LEVEL ALARMS ALERT THE OPERATOR
TO TAKE ACTION
SECOND LEVEL ALARMS AUTOMATICALLY
INTITIATE PROTECTIVE ACTION
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MONITORS
• INSTRUMENTATION THAT IS NOT
REQUIRED FOR NORMAL OPERATION
– USED FOR FIELD OBSERVATION
– USED FOR START-UPS
– USED FOR MAINTENANCE
– USED FOR OPTIMIZATION
P
T
P
T
P
T
HEAT EXCHANGER
P
T
SAFETY & ALARM SYSTEMS
•
OPERATOR ALARMS AND
INTERLOCK ALARMS
– (LO, LOLO, HI, HIHI)
– SHOULD BE ON LOOPS THAT ARE
INDEPENDENT FROM CONTROL
LOOPS
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RELIEF SYSTEMS NEED TO BE
DIRECTED TO FACILITIES TO
SAFELY PROCESS THE RELEASE
SAFETY SYSTEMS SHOULD BE
INTERLOCKED TO SHORT OR LONG
TERM SHUTDOWN LOOPS, AS
APPROPRIATE.
CONTROL DESIGN
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REQUIRES PFD’S
P&ID’S ARE THE PRIMARY
DOCUMENT
– SHOW LOOPS
– SHOW INTERLOCKS
– SHOW PLCs
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STANDARD DIAGRAMS CAN BE USED
FOR SOME SYSTEMS
SPECIAL CONTROL DIAGRAMS LADDER LOGIC, AND INTERLOCKS
ARE ALSO DEVELOPED
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CONTROL LOOPS
CONTROL STATION
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SENSORS - TO MEASURE CONTROL
VARIABLE AND ADJUSTED VARIABLE
THERMODYNAMIC PROPERTIES
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PRESSURE
TEMPERATURE
FLOW
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TRANSPORT PROPERTIES
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VOLUMETRIC
MASS
DENSITY
VISCOSITY
SYSTEM INVENTORY - LEVEL
ANALYTICAL SENSORS MEASURE
SPECIFIC CHEMICAL PROPERTIES
CONTROL CONFIGURATION
•
FEED BACK CONTROL
SET POINT
CONTROL ELEMENT
CONTROLLER
Cont. Var. SENSOR
PROCESS
CONTROL CONFIGURATION
•
FEED FORWARD CONTROL
CONTROL ELEMENT
Input Var. SENSOR
CONTROLLER
Cont. Var. SENSOR
PROCESS
PROCESS
MODEL
SET POINT
CONFIGURATION COMPARISON
• FROM PERRY’S
CHAPTER 8
STEAM TO HEAT
EXCHANGER
• FEEDBACK BASED
ON PROCESS EXIT
FLOW
TEMPERATURE
• FEEDFORWARD
BASED ON
PROCESS FEED
FLOW
LOOP RESPONSE
•
DYNAMIC RESPONSE IS MEASURED
BY HOW QUICKLY A SYSTEM
RETURNS TO CONTROL AFTER A
PERTURBATION.
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CONVERGENT
DIVERGENT
CYCLING
AT STEADY STATE, THE IDEAL
CONTROL PATTERN IS LINEAR
ANALOG AND DIGITAL SIGNALS
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CONTINUOUS SENSORS AND
CONTROLLED DEVICES TYPICALLY
ARE ANALOG
CONTROLLERS ARE TYPICALLY
DIGITAL
DISTRIBUTED CONTROL
– USES LOCAL I/O PANELS
– DATA HIGHWAY IS USED TO CONVEY
ACTUAL AND SET POINT CONDITIONS
TO AND FROM THE CONTROL ROOM
PANEL
DATA HIGHWAY DESIGN
SENS 1
C. ELEM. 1
SENS 2
C. ELEM. 2
CONT. I/O
PANEL
CONT. SIG.
PANEL
SENS 3
C. ELEM. 3
MUX
BOX
AL 1
INTR. 1
AL 2
INTR. 2
AL 3
INTR. 3
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BACKUP
CRT
DATA HIGHWAY
INTERLOCK I/O
PANEL
LOCAL
MUX
CONTROL
CRT
ALARM
CRT
TREND
CRT
COMPUTER
INTEFACE
CONTROL
COMPUTER
DATA
STORAG E
SYSTEM CAN ALSO BE CONNECTED TO
PROGRAMMABLE LOGIC CONTROLLERS FOR
BATCH OPERATION AND INTERLOCK FUNCTIONS
SYSTEM CAN HAVE PARALLEL DATA HIGHWAYS
INSTRUMENTATION SYMBOLS
•
ISA SYMBOLISM IS TYPICALLY USED
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FOR LABELING DEVICES
FOR LABELING LOOPS
POSITIONS
•
POSITION 1 - VARIABLE CONTROLLED
– P, V, T, F, L, ρ, μ, ETC.
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POSITION 2 - FUNCTION
– INDICATION, GAUGE, CONTROLLER, ALARM, CAN
BE SAME AS POSITION 1 FOR RATIO DEVICE.
•
POSITION 3 - FUNCTION
– CONTROL, RANGE (HIGH OR LOW FOR ALARMS) ,
RATIO FOR RC
•
POSITION 4 - FUNCTION AGAIN
– FFRC, e.g.
CHARACTERISTICS OF
CONTROL MODES
•
STEADY STATE CAN BE DIRECT OR
INDIRECT
DIRECT FLOW CONTROL
FRC
FCV
INDIRECT FLOW CONTROL
LRC
FCV
CASCADE EFFECTS
•
FOR THIS EXAMPLE THE FEEDS TO
THE REACTOR ARE SET BY ONE
MASTER FLOW WITH THE SECOND
FLOW FED BY RATIO.
CASCADE EXAMPLE
•
USING A FLOW CONTROL TO THE FLASH
TANK
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–
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WOULD REQUIRE THAT IT CHANGE
ACCORDING TO THE MASTER FEED
THE EXTENDED LAG TIME WOULD MAKE THIS
CONTROL INEFFECTIVE.
USING A FLASH FEED BASED ON LEVEL
CONTROL
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–
PROVIDES A MORE IMMEDIATE RESPONSE TO
CHANGES IN THE REACTOR FEED
THE GAIN WOULD NEED TO BE ADJUSTED TO
ALLOW FOR THE CAPACITANCE OF THE
SYSTEM.
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GENERAL INSTRUMENTATION
DESIGN COMMENTS
CONTROL LOOP SENSORS - LOCATE AT
THE POINT OF CONTROL ON ALL
DIAGRAMS
CONTROL VALVES - LOCATE ON PUMP
DISCHARGE LINES
PUMP TANKS AND SUMPS - SHOULD BE
PROVIDED WITH LEVEL CONTROL,
HI/HIHI/LO/LOLO ALARMS AND RELIEF
SYSTEMS.
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GENERAL INSTRUMENTATION
DESIGN COMMENTS
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STORAGE, SURGE AND OTHER PROCESS
TANKS - SHOULD BE PROVIDED WITH
LEVEL CONTROL, HI/HIHI/LO/LOLO
ALARMS AND RELIEF SYSTEMS.
HEAT EXCHANGER CONTROL - SHOULD
TYPICALLY CONSIST OF A CONTROL
VALVE ON THE HEATING/COOLING MEDIA
WITH A SENSOR ON THE TEMPERED
FLOW DISCHARGE.
SOLIDS FLOW CONTROL - GENERALLY
OBTAINED BY VARIABLE SPEED DRIVES
FOR CONVEYING EQUIPMENT,
CONNECTED TO A WEIGHING DEVICE;
BELT SCALE OR WEIGH TANK.
GENERAL INSTRUMENTATION
DESIGN COMMENTS
•
CONTROL ROOM DISPLAY - ALL PROCESS
VARIABLES AND ALL ALARMED POINTS SHOULD
BE ON THE CONTROL ROOM PANELS. ONLY
LOCAL GAUGES AND NON-PROCESS ITEMS
SHOULD BE EXCLUDED.
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