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CHEMICAL PROCESS CONTROL

CHEMICAL PROCESS CONTROL. FUNDAMENTALS. CONTROL CATEGORIES . OPERATING MODE CONTINUOUS BATCH SEMI-BATCH OPERATING CONDITIONS START-UP STEADY-STATE OPERATION CONTROLLED SHUTDOWN EMERGENCY SHUTDOWN MONITORING MAINTENANCE OPTIMIZATION SEE PERRY’S 7 TH ED., CHAPTER 8.

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CHEMICAL PROCESS CONTROL

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  1. CHEMICAL PROCESS CONTROL FUNDAMENTALS

  2. CONTROL CATEGORIES • OPERATING MODE • CONTINUOUS • BATCH • SEMI-BATCH • OPERATING CONDITIONS • START-UP • STEADY-STATE OPERATION • CONTROLLED SHUTDOWN • EMERGENCY SHUTDOWN • MONITORING • MAINTENANCE • OPTIMIZATION • SEE PERRY’S 7TH ED., CHAPTER 8 http://www.synergy-ergs.com/images/FM%20operator%20desk.jpg

  3. CONTROL DESIGN • OPERATOR CONTROL • AUTOMATIC CONTROL • ALARMS • INTERLOCKS • DATA LOGGING • AUTOMATIC TUNING • SEQUENCING • PROGRAMMABLE LOGIC CONTROLLERS • FIELD MONITORS http://www.tpub.com/content/draftsman/14276/css/14276_255.htm

  4. STEADY STATE OPERATION • 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 http://www.historicphotoarchive.com/images11/00700.jpg

  5. START UP • 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 ANTICIPATED http://www.malvern.com/ProcessEng/processes/milling/milling_startup_offline.gif

  6. START UP PROCEDURES • 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

  7. 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.

  8. 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

  9. TYPICAL SHUTDOWN DESIGNS SUPERPRO • REACTORS • SHORT TERM • STOP FEEDS • MAINTAIN COOLING, BUT KEEP MATERIALS FROM CRYSTALLIZING • MAINTAIN AGITATION. • LONG TERM • POSSIBLY DRAIN REACTANTS TO A SEPARATE VESSEL IF REACTION CANNOT BE STOPPED AT THE STEADY STATE CONDITIONS • OTHERWISE, SAME AS SHORT TERM

  10. TYPICAL SHUTDOWN DESIGNS • SEPARATION PROCESSES • 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

  11. EMERGENCY SHUT DOWN • ONE BUTTON SHUT DOWN BY OPERATOR • PROCESS SHOULD BE PROTECTED AGAINST CATASTROPHIC FAILURE DURING AN UNMONITORED SHUT DOWN • PROCESS SHOULD BE FAIL-SAFE • ALARMS AND INTERLOCKS SHOULD PROTECT LOCAL PROCESSES • FIRST LEVEL ALARMS ALERT THE OPERATOR TO TAKE ACTION • SECOND LEVEL ALARMS AUTOMATICALLY INTITIATE PROTECTIVE ACTION http://www.plastomatic.com/failsafe-explanation.gif http://ridetheclock.com/

  12. MONITORS HEAT EXCHANGER P P P P T T T T • INSTRUMENTATION THAT IS NOT REQUIRED FOR NORMAL OPERATION • USED FOR FIELD OBSERVATION • USED FOR START-UPS • USED FOR MAINTENANCE • USED FOR OPTIMIZATION

  13. SAFETY & ALARM SYSTEMS • OPERATOR ALARMS AND INTERLOCK ALARMS • (LO, LOLO, HI, HIHI) • SHOULD BE ON LOOPS THAT ARE INDEPENDENT FROM CONTROL LOOPS • 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.

  14. CONTROL DESIGN • REQUIRES PFD’S • P&ID’S ARE THE PRIMARY DOCUMENT • SHOW LOOPS • SHOW INTERLOCKS • SHOW PLCs • STANDARD DIAGRAMS CAN BE USED FOR SOME SYSTEMS • SPECIAL CONTROL DIAGRAMS - LADDER LOGIC, AND INTERLOCKS ARE ALSO DEVELOPED http://www.cbu.edu/~rprice/lectures/figures/loop.gif

  15. CONTROL LOOPS CONTROL STATION • SENSORS - TO MEASURE CONTROL VARIABLE AND ADJUSTED VARIABLE • THERMODYNAMIC PROPERTIES • PRESSURE • TEMPERATURE • FLOW • VOLUMETRIC • MASS • TRANSPORT PROPERTIES • DENSITY • VISCOSITY • SYSTEM INVENTORY - LEVEL • ANALYTICAL SENSORS MEASURE SPECIFIC CHEMICAL PROPERTIES

  16. CONTROL CONFIGURATION • FEED BACK CONTROL

  17. CONTROL CONFIGURATION • FEED FORWARD CONTROL

  18. CONFIGURATION COMPARISON • FROM PERRY’S CHAPTER 8 STEAM TO HEAT EXCHANGER • FEEDBACK BASED ON PROCESS EXIT FLOW TEMPERATURE • FEEDFORWARD BASED ON PROCESS FEED FLOW

  19. LOOP RESPONSE • DYNAMIC RESPONSE IS MEASURED BY HOW QUICKLY A SYSTEM RETURNS TO CONTROL AFTER A PERTURBATION. • CONVERGENT • DIVERGENT • CYCLING • AT STEADY STATE, THE IDEAL CONTROL PATTERN IS LINEAR

  20. ANALOG AND DIGITAL SIGNALS • 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

  21. DATA HIGHWAY DESIGN • SYSTEM CAN ALSO BE CONNECTED TO PROGRAMMABLE LOGIC CONTROLLERS FOR BATCH OPERATION AND INTERLOCK FUNCTIONS • SYSTEM CAN HAVE PARALLEL DATA HIGHWAYS

  22. INSTRUMENTATION SYMBOLS • ISA SYMBOLISM IS TYPICALLY USED • FOR LABELING DEVICES • FOR LABELING LOOPS • POSITIONS • POSITION 1 - VARIABLE CONTROLLED • P, V, T, F, L, ρ, μ, ETC. • 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.

  23. CHARACTERISTICS OF CONTROL MODES • STEADY STATE CAN BE DIRECT OR INDIRECT

  24. CASCADE EFFECTS • FOR THIS EXAMPLE THE FEEDS TO THE REACTOR ARE SET BY ONE MASTER FLOW WITH THE SECOND FLOW FED BY RATIO.

  25. CASCADE EXAMPLE • USING A FLOW CONTROL TO THE FLASH TANK • 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 • 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.

  26. 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. http://www.advantageengineering.com/pumpTankStations/index.php

  27. GENERAL INSTRUMENTATION DESIGN COMMENTS • 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.

  28. 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. http://www-03.ibm.com/ibm/history/exhibits/tucson/images/5403PH15.jpg

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