CHE 185 – PROCESS CONTROL AND DYNAMICS OPTIMIZATION AND PRIMARY LOOP ELEMENTS

TYPES OF CONTROL OPTIMIZATION –INTENTION IS TO GET THE “BEST” ECONOMIC/QUALITY CONDITIONS –CONTROL IS THE MORE GENERAL FORM OF RESPONDING TO CHANGE

CONTROL & OPTIMIZATION FOR A BIOREACTOR nt/ /en/as/Pages/PN Alles_unter_Kontrolle.xml?NoRedirect=true&xsl=publication-en-www4.xsl

EXAMPLE OF OPTIMIZATION OPTIMIZATION AND CONTROL OF A CSTR

EXAMPLE OF OPTIMIZATION

OPTIMZATION EXAMPLE METHOD OF SOLUTION 1. SELECT INITIAL GUESS FOR REACTOR TEMPERATURE, T* 2. EVALUATE C A, C B, AND C C 3. EVALUATE  4. CHOOSE NEW REACTOR TEMPERATURE AND RECYCLE THROUGH STEP 2 UNTIL T* IS IDENTIFIED.

EXAMPLE OF OPTIMIZATION

GRAPHICAL SOLUTION OF OPTIMUM REACTOR TEMPERATURE, T*

OTHER TYPES OF CONTROL SUPERVISORY –RESPONDS TO THE SIGNAL FROM THE OPTIMIZATION CONTROLLER –SENDS A SIGNAL TO THE REGULATORY CONTROL LOOP

TYPES OF CONTROL REGULATORY –RECEIVES A SIGNAL FROM THE SUPERVISORY CONTROLLER –ACTUALLY ADJUSTS A PROCESS VARIABLE TO MAKE NECESSARY CHANGES FOR OPERATION.

REGULATORY CONTROL EXAMPLE TEMPERATURE CONTROL FOR A HEAT EXCHANGER

CATEGORIES OF LOOP COMPONENTS FEEDBACK LOOP THE PRIMARY COMPONENTS ARE SHOWN SCHEMATICALLY

LOOP COMPONENTS - SENSORS MEASURE THE VALUE OF THE TARGET AND MANIPULATED VARIABLES –TYPICALLY P, T, L, D, ρ,µ, COMPOSITION, MASS –LOCATED “IN-LINE”, EVEN IF THEY DO NOT CONTACT PROCESS FLUIDS

LOOP COMPONENTS - SENSORS TYPICALLY ANALOG DEVICES WITH FULL SCALE RANGES: –20 MA CURRENT SIGNAL – mV VOLTAGE SIGNAL – PSIG PNEUMATIC SIGNAL

LOOP COMPONENTS - TRANSMITTERS ANALOG DEVICES –CONVERT SIGNAL INTO A VALUE THAT CAN BE TRANSMITTED WITHOUT SIGNIFICANT LOSS IN VALUE –TYPICAL OUTPUT IS MA CURRENT FULL SCALE RANGE

LOOP COMPONENTS - TRANSDUCERS CONVERT THE ANALOG SIGNAL FROM THE TRANSMITTER INTO A DIGITAL SIGNAL –LABELED A/D FOR ANALOG TO DIGITAL –LABELED D/A FOR DIGITAL TO ANALOG CAN TRANSFER ELECTRONIC TO PNEUMATIC

LOOP COMPONENTS - CONTROLLERS RECEIVES DIGITAL SET/POINT AND MEASURED SIGNALS FOR A VARIABLE MAKES A COMPARISON BETWEEN THE SIGNALS USING A BRIDGE PRODUCES A DIGITAL OUTPUT SIGNAL TO ADJUST THE MEASURED VALUE TO THE SET/POINT VALUE

LOOP COMPONENTS - ACTUATORS RECEIVES THE OUTPUT SIGNAL FROM THE CONTROLLER, VIA THE TRANSDUCER ADJUSTS THE POSITION OF A DEVICE (FINAL CONTROL ELEMENT) TO CHANGE A PROCESS VARIABLE

LOOP COMPONENTS - FINAL CONTROL ELEMENT CHANGES A PROCESS VARIABLE TO OBTAIN A CORRECTION TO THE MEASURED VARIABLE FOR CHEMICAL PLANTS, MOST OF THE TIME THESE DEVICES ARE VALVES

SOURCE OF SET/POINTS FOR A SIMPLE PROCESS, THESE MAY BE ENTERED MANUALLY FOR CONTEMPORARY SYSTEMS, THESE SIGNALS COME FROM A COMPUTER –PLC - PROGRAMMABLE LOGIC CONTROLLER –DCS COMPUTER - DISTRIBUTED CONTROL SYSTEM COMPUTER

DCS FUNDAMENTALS DCS HAS THE FOLLOWING GENERAL COMPONENTS –LOCAL CONTROLLERS – INPUT/OUTPUT PANELS (I/O PANELS) TO RECEIVE PROCESS VARIABLE VALUES FROM THE CONTROLLER AND SEND SET/POINT VALUES –DATA HIGHWAY/MULTIPLEXER TO SAMPLE THE VALUES ON A REGULAR FREQUENCY

DATA HIGHWAY/MULTIPLEXER

DCS FUNDAMENTAL COMPONENTS HOST COMPUTER TO ENTER SET/POINTS ALARM COMPUTER - PLC’S TO INITIATE ALARMS AND/OR INTERLOCKS DATA STORAGE COMPUTER –KEEPS TREND DATA –LOGS ALARMS –OTHER COMPUTERS THAT HAVE ACCESS

FINAL CONTROL ELEMENT CHARACTERIZATION - VALVES VALVES ARE USED FOR EITHER ON/OFF OR THROTTLING OPERATION - SEE PERRY’S 7TH PAGE 8-64 ON/OFF VALVE CHARACTERISTICS –HAVE A COMPLETELY OPEN AND COMPLETELY CLOSED POSITION –HAVE A TIGHT SHUT-OFF CAPABILITY –HAVE LIMITED VALUE FOR THROTTLING

PLUG VALVES AND BALL VALVES 90° RANGE FROM OPEN TO CLOSED USE PNEUMATIC OR MOTOR ACTUATORS

GATE VALVES TYPICALLY USED FOR ISOLATION VALVES - FOR MAINTENANCE AND AS BACK-UP FOR CLOSED THROTTLING VALVES

GATE VALVES MAY HAVE SEVERAL TURNS BETWEEN OPEN AND CLOSED MAY HAVE MOTOR OPERATORS – BUT MORE FREQUENTLY USE MANUAL OPERATION

THROTTLING (CONTROL) VALVES VARIOUS FLOW CHARACTERISTICS BASED ON THE SHAPE OF THE PLUG AND THE SEAT FLOW THROUGH THE VALVE DEPENDS ON THE FRACTION OPEN AND THE NET PRESSURE DROP HAVE COMPLETELY OPEN AND COMPLETELY CLOSED POSITION, BUT DESIGNED FOR INTERMEDIATE POSITION FOR CONTINUOUS OPERATION - NOT TIGHT SHUT OFF GENERAL REFERENCE - S.COM/GROUPS/PUBLIC/DOCUMENTS/BOOK/CVH99.PDF S.COM/GROUPS/PUBLIC/DOCUMENTS/BOOK/CVH99.PDF

GLOBE (THROTTLING) CONTROL VALVE

CROSS-SECTION OF A GLOBE VALVE

THROTTLING (CONTROL) VALVES

GLOBE VALVE TYPES QUICK OPENING- FOR SAFETY BY-PASS APPLICATIONS WHERE QUICK OPENING IS DESIRED EQUAL PERCENTAGE- FOR ABOUT 90% OF CONTROL VALVE APPLICATIONS SINCE IT RESULTS IN THE MOST LINEAR INSTALLED CHARACTERISTICS LINEAR- WHEN A RELATIVELY CONSTANT PRESSURE DROP IS MAINTAINED ACROSS THE VALVE

GLOBE VALVE FLOW CHARACTERISTICS

GLOBE VALVES - LINEAR PLUGS/SEATS DESIGNED TO HAVE THE FRACTION OF FLOW BE LINEAR WITH THE VALVE STEM POSITION FOR A CONSTANT Δp f(x) = xWHERE x IS VALVE POSITION DESIGN POSITION IS ABOUT 50% OPEN

GLOBE VALVES - EQUAL PERCENTAGE

GLOBE VALVES - QUICK OPENING

GLOBE VALVES - OTHER TYPES ECCENTRIC PLUG VALVES –COMBINATION OF PLUG AND GLOBE IN THEIR CHARACTERISTICS –CLAIM TO BE TIGHT SHUT OFF –REFERENCE:

CHARACTERIZATION OF CONTROL ELEMENTS TIME CONSTANTS –TIME IT TAKES FOR A UNIT TO RESPOND TO A SIGNAL AND COMPLETE A CHANGE –FOR VALVES THIS IS THE TIME TO STROKE TO A NEW POSITION DEADBAND –RANGE OF SIGNAL THAT REQUIRED TO INDICATE AN ACTUAL CHANGE

CHARACTERIZATION OF CONTROL ELEMENTS POSITION –DEADBAND –RANGE OF SIGNAL THAT REQUIRED TO INDICATE AN ACTUAL CHANGE –FOR THERMOCOUPLES THIS MIGHT BE +1 C.

CHARACTERIZATION OF CONTROL ELEMENTS TURNDOWN RATIO –SPECIFIES THE RANGE OF STABLE OPERATION FOR THE DEVICE, MINIMUM TO MAXIMUM –NORMAL OPERATING RANGE SHOULD NOT BE AT EITHER EXTREME POSITION

CONTROL VALVE DESIGN

CHOOSE –SMALLEST VALVE THAT MEETS THE RANGE CRITERION FOR THE MINIMUM CAPITAL INVESTMENT –THE LARGEST VALVE TO ALLOW FOR FUTURE THROUGHPUT EXPANSION. C V VERSUS % OPEN FOR DIFFERENT VALVE SIZES. AVAILABLE PRESSURE DROP ACROSS THE VALVE VERSUS FLOW RATE FOR EACH VALVE. NOTE THAT THE EFFECT OF FLOW ON THE UPSTREAM AND DOWNSTREAM PRESSURE MUST BE KNOWN.

CONTROL VALVE DESIGN EXAMPLE SIZE A CONTROL VALVE FOR MAX 150 GPM OF WATER AND MIN OF 50 GPM. USE THE VALVE FLOW EQUATION TO CALCULATE C V FOR  P, USE PRESSURE DROP VERSUS FLOW RATE (E.G., TABLE ON PAGE 82)

CONTROL VALVE DESIGN EXAMPLE EQUATION AT LIMITS

VALVE POSITION FOR MAX AND MIN FLOWS BASED ON SIZE

ANALYSIS OF RESULTS 2-INCH VALVE APPEARS TO BE BEST OVERALL CHOICE: LEAST EXPENSIVE CAPITAL AND IT CAN PROVIDE UP TO A 50% INCREASE IN THROUGHPUT. 3-INCH AND 4-INCH VALVE WILL WORK, BUT NOT RECOMMENDED BECAUSE THEY WILL COST MORE TO PURCHASE. THE 2-INCH VALVE WILL PROVIDE MORE THAN ENOUGH EXTRA CAPACITY (I.E., SOMETHING ELSE WILL LIMIT CAPACITY FOR IT)

ADJUSTABLE SPEED PUMPS FOR FLOW CONTROL USED EXTENSIVELY IN THE BIO- PROCESSING INDUSTRIES TO MAINTAIN STERILE CONDITIONS AND RELATIVELY LOW FLOW RATES. FAST AND PRECISE. DO REQUIRE AN INSTRUMENT AIR SYSTEM (I.E., 4-20 MA SIGNAL GOES DIRECTLY TO PUMP). MUCH HIGHER CAPITAL COSTS THAN CONTROL VALVES FOR LARGE FLOW RATE APPLICATIONS.