CHE 185 – PROCESS CONTROL AND DYNAMICS

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

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

3. Control & optimization • For a bioreactor http://www.automation.siemens.com/wcmsnewscenter/details.aspx?xml=/content/10001666/en/as/Pages/PN-200201-06-Alles_unter_Kontrolle.xml?NoRedirect=true&xsl=publication-en-www4.xsl

4. EXAMPLE OF OPTIMIZATION • Optimization and Control of a CSTR

5. EXAMPLE OF OPTIMIZATION • Economic Objective Function • Economic values , VB> VC, VA, or VAF • At low T, little formation of B • At high T, too much of B reacts to form C • so there exists an optimum reactor temperature, T*

6. OPTIMZATION example • Method of solution • 1. Select initial guess for reactor temperature, T* • 2. Evaluate CA, CB, and CC • 3. Evaluate F • 4. Choose new reactor temperature and recycle through step 2 until T* is identified.

7. EXAMPLE OF OPTIMIZATION

8. EXAMPLE OF OPTIMIZATION • Graphical Solution of Optimum Reactor Temperature, T*

9. Other TYPES OF CONTROL • SUPERVISORY • RESPONDS TO THE SIGNAL FROM THE OPTIMIZATION CONTROLLER • SENDS A SIGNAL TO THE REGULATORY CONTROL LOOP

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

11. REGULATORY CONTROL EXAMPLE • Temperature Control for a Heat Exchanger

12. CATEGORIES OF LOOP COMPONENTS • FEEDBACK LOOP THE PRIMARY COMPONENTS ARE SHOWN SCHEMATICALLY

13. 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

14. LOOP COMPONENTS - SENSORS • TYPICALLY ANALOG DEVICES WITH FULL SCALE RANGES: • 20 mA CURRENT SIGNAL • mV VOLTAGE SIGNAL • 3 - 15 psig PNEUMATIC SIGNAL

15. LOOP COMPONENTS - TRANSMITTERS • ANALOG DEVICES • CONVERT SIGNAL INTO A VALUE THAT CAN BE TRANSMITTED WITHOUT SIGNIFICANT LOSS IN VALUE • TYPICAL OUTPUT IS 4 - 20 mA CURRENT FULL SCALE RANGE

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. DATA HIGHWAY/MULTIPLEXER

23. 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

24. 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

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

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

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

28. 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 - http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

29. Globe (THROTTLING) Control Valve

30. Cross-section of a Globe Valve

31. Throttling (control) valves • BASIC FLOW EQUATION IS THE SAME AS FOR AN ORIFICE: • CvIS SIMILAR TO AN ORIFICE COEFFICIENT, HOWEVER IT VARIES WITH VALVE POSITION • K IS EQUAL TO UNITY WHEN  IS EXPRESSED AS SPECIFIC GRAVITY, FLOWRATES ARE IN gpm AND PRESSURE DROP IS IN psi

32. 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

33. Globe Valve FLOW CHARACTERISTICS

34. 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) = x WHERE x IS VALVE POSITION • DESIGN POSITION IS ABOUT 50% OPEN

35. GLOBE VALVES - EQUAL PERCENTAGE • EQUAL PERCENTAGE CHANGE IN VALVE POSITION RESULT IN EQUAL PERCENTAGE CHANGE IN FLOW • AND x IS VALVE OPENING • DESIGN POSITION IS ABOUT 70% OPEN

36. GLOBE VALVES - QUICK OPENING • TRIM IS DESIGNED TO HAVE LARGE INCREASE IN FLOW WITH SMALL CHANGE IN VALVE OPENING • DESIGN POSITION IS OPEN OR CLOSED

37. GLOBE VALVES - OTHER TYPES • ECCENTRIC PLUG VALVES • COMBINATION OF PLUG AND GLOBE IN THEIR CHARACTERISTICS • CLAIM TO BE TIGHT SHUT OFF • Reference: http://www.millikenvalve.com/pdf/plug2011.pdf

38. CHARACTERIZATION OF CONTROL ELEMENTS • tIMECONSTANTS • 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

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

40. 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

41. Control Valve Design • Evaluate Cv at the maximum and minimum flow rate • Determine which valves can effectively provide the max and min flow rate • the valve position should be greater than about 15% open for the minimum flow rate • and less than 85% open for the maximum flow rate.

42. 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. • CV 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. http://www.norriseal.com/files/comm_id_47/Valve_Size_Manual.pdf

43. 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 Cv • For DP, use pressure drop versus flow rate (e.g., Table on page 82)

44. Control Valve design Example • Equation at limits

45. Valve Position for Max and Min Flows based on size

46. 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)

47. 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.