CHE 185 – PROCESS CONTROL AND DYNAMICS

1. CHE 185 – PROCESS CONTROL AND DYNAMICS STANDARD CONTROL LOOPS

2. BASIS FOR PID SELECTION • PROPORTIONAL CONTROL • SATISFACTORY FOR PROCESSES WHERE RESPONSES ARE QUICK • OFFSET IS NOT A PROBLEM. • INVENTORY AND PUMP TANKS • For integrating processes, P-only control provides offset-free operation. In fact, if as integral action is added to such a case, the control performance degrades. • Therefore, for integrating processes, P-only control is all that is usually required.

3. BASIS FOR PID SELECTION • PROPORTIONAL-INTEGRAL CONTROL • PROCESSES WHERE OFFSET IS A PROBLEM • PROCESSES THAT ARE NOT SLUGGISH, SINCE INTEGRAL SLOWS DOWN THE RESPONSE • MOST LOOPS - CSTR LEVEL CONTROL, HEAT EXCHANGERS, PRESSURE CONTROL • Over 90% of control loops use PI controller

4. BASIS FOR PID SELECTION • PROPORTIONAL-INTEGRAL-DERIVATIVE • PROCESSES WHERE OFFSET IS A PROBLEM • PROCESSES THAT NEED TO RESPOND FASTER THAN P-I CONTROL • PROCESSES THAT DO NOT HAVE NOISY SIGNALS • CRITICAL TEMPERATURE OR PRESSURE CONTROLS WHERE VARIATIONS ARE FREQUENT.

5. BASIS FOR PID SELECTION • PID: use for sluggish processes (i.e., a process with large deadtime to time constant ratios) or processes that exhibit severe ringing for PI controllers. PID controllers are applied to certain temperature and composition control loops. Use derivative action when:

6. Comparison between PI and PID for a Low θp/τp Ratio

7. Comparison between PI and PID for a HIGH θp/τp Ratio

8. SELECTION FOR Several Commonly Encountered Control Loops • THE EXAMPLES ARE FOR CONTINUOUS SYSTEMS, BUT CAN APPLY TO BATCH OPERATIONS. • Flow control loops • Level control loops • Pressure control loops • Temperature control loops • Composition control loops • DO control loop • Biomass controller • USE OF VARIABLE FREQUENCY DRIVES

9. FLOW CONTROL • CONTINUOUS CONTROL • THE STANDARD FLOW CONTROL LOOP SHOULD INCLUDE THE FOLLOWING COMPONENTS • ALSO SEE FIG 7.10.1 IN TEXT

10. FLOW CONTROL • THIS LOOP HAS QUICK RESPONSE WITH THE SLOWEST COMPONENT BEING THE CONTROL VALVE • P-I CONTROL WITH SHORT RESET TIMES • A VALVE POSITIONER MIGHT BE ADDED TO SOME VALVES WHERE THE CONTROL VALVE LAG IS CRITICAL • Almost always use PI controller.

11. FLOW CONTROL • Deadbandof industrial valves is between ±10%-±25%. • As a result, small changes in the air pressure applied to the valve do not change the flow rate.

12. FLOW CONTROL • A control valve (deadband of ±10-25%) in a flow control loop or with a positioner typically has a deadband for the average flow rate of less than ±0.5% due to the high frequency opening and closing of the valve around the specified flow rate.

13. FLOW CONTROL • EMERGENCY CONTROL • HIGH-HIGH/LOW-LOW FLOW SWITCHES • ADDED IN THE LINE AND NORMALLY CONNECTED THROUGH A PLC • THESE ARE INDEPENDENT OF THE CONTROLLER AND CONNECTED TO ALARMS AND INTERLOCKS

14. FLOW CONTROL • HIGH/LOW FLOW SWITCHES CAN BE SETPOINTS ON THE FLOW CONTROLLER • CONNECTED TO ALARMS • MEANT TO ALERT OPERATOR TO ACT

15. FLOW CONTROL • FLOW PREVENTERS • THESE ARE TYPICALLY SELF-CONTAINED DEVICES THAT MAY OR MAY NOT HAVE AN INDICATOR FOR CURRENT POSITION • BLOW-OUT PREVENTERS ARE PART OF THE SAFETY EQUIPMENT IN OIL DRILLING. • SEAL OFF THE WELL IN THE EVENT OF ANY TYPE OF PRESSURE SURGE • NORMALLY SPRING OR HYDRAULICALLY LOADED

16. FLOW CONTROL • BACK-FLOW PREVENTERS • INSTALLED TO PREVENT A FLOW REVERSAL EITHER DUE TO HIGH PRESSURE DOWNSTREAM OR LOW PRESSURE UPSTREAM • USED TO PROTECT STANDBY PUMPS

17. LEVEL CONTROL • CONTINUOUS LEVEL CONTROL LOOPS CONTAIN THE FOLLOWING COMPONENTS • ALSO SEE FIGURE 7.9.2

18. LEVEL CONTROL • THESE CAN ALSO BE SET UP ON A CASCADE BASIS TO HAVE THE OUTPUT FROM THE LEVEL CONTROLLER SENT AS THE SETPOINT TO A SEPARATE FLOW CONTROL LOOP. • CONTROLLING LAG TIME IS BASED ON THE SYSTEM TIME DELAY, WHICH DEPENDS ON VOLUME AND FLOW RATE • IF ACTUAL LEVEL CAN VARY, THEN P-ONLY CONTROL MAY BE ADEQUATE • IF ACTUAL LEVEL IS IMPORTANT, THEN PI CONTROL IS APPROPRIATE

19. LEVEL CONTROL • LEVEL CONTROL FOR PUMP TANKS • PUMP TANK LEVELS TYPICALLY CAN VARY ABOVE A MINIMUM - FOR CONSTANT DISCHARGE FLOWS • MAINTAINING A CONSTANT PUMP TANK LEVEL WILL RESULT IN INTEGRATED DOWNSTREAM FLOWS THAT EQUAL THE AVERAGE OF INCOMING FLOWS

20. LEVEL CONTROL • LEVEL CONTROL FOR PUMP TANKS • THE PUMP TANK IS A FIRST ORDER PROCESS WITH RESPECT TO FLOW • GAIN AND TIME DELAY ARE A FUNCTION OF • THE VOLUME OF INCOMING FLOW • THE TANK VOLUME • AND THE TANK DIAMETER.

21. LEVEL CONTROL • LEVEL CONTROL FOR HEAT TRANSFER • LEVEL CONTROL CAN BE USED FOR CONTROL OF HEAT TRANSFER WITH SYSTEMS THAT INCLUDE PHASE CHANGE • PHASE CHANGE RESULTS IN HIGH HEAT TRANSFER COEFFICIENTS, SO ONE WAY TO CONTROL THE TOTAL HEAT TRANSFER IS TO CONTROL THE AREA OF THE EXCHANGER ACCESSIBLE TO PHASE CHANGE.

22. LEVEL CONTROL

23. LEVEL CONTROL • THE AVAILABLE AREA IS REDUCED BY INCREASING THE LEVEL OF CONDENSATE IN THE SHELL OF THE EXCHANGER • THIS IS A REVERSE ACTING SYSTEM SINCE AND INCREASE IN LEVEL DECREASES THE PROCESS FLOW EXIT TEMPERATURE

24. LEVEL CONTROL • HIGH LEVEL TYPICALLY REQUIRES SHUTDOWN, SINCE ENTERING PROCESS LINES DO NOT HAVE A LARGE INVENTORY VOLUME • LOW LEVELS CAN BE USED TO SHUT DOWN PUMPS AND/OR CLOSE VALVES

25. PRESSURE CONTROL • THE STANDARD CONTINUOUS PRESSURE CONTROL SYSTEM SHOULD INCLUDE THE FOLLOWING COMPONENTS • THIS LOOP HAS SLOW RESPONSE WITH THE SLOWEST COMPONENT BEING THE PROCESS TIME DELAY - WHICH DEPENDS ON THE VOLUME OF THE SYSTEM

26. PRESSURE CONTROL • The sensor is generally faster than the actuator, which is faster than the process. • Use P-only controller if it is an integrating process otherwise use a PI controller.

27. PRESSURE CONTROL • THE PROCESS PRESSURE IS NORMALLY CONTROLLED BY REMOVING NON-CONDENSIBLES FROM THE SYSTEM • A CONDENSER IS USED TO CONTAIN THE PROCESS MATERIALS • THE CONTROL VALVE MAY BE ON A STEAM EJECTOR OR THE MOTOR ON A VACUUM SYSTEM. VAC. PUMP FROM http://vacuum.tuthill.com/ProductCatalog/scan.asp

28. PRESSURE CONTROL • THERE MAY BE A SEPARATE SYSTEM FOR INITIALLY EVACUATING/PRESSURIZING THE SYSTEM • THIS SYSTEM SHOULD INCLUDE A SEPARATE CONTROL MODE TO ALLOW FOR HIGHER FLOWS • THE TRANSITION MAY BE AUTOMATED OR MANUAL

29. PRESSURE CONTROL • SELF-CONTAINED PRESSURE CONTROL SYSTEMS • PRESSURE RELIEF VALVES AND BACKPRESSURE RELIEF VALVES ARE USED TO CONTROL PRESSURES AT POINT ADJACENT TO THE VALVE • THESE UNITS HAVE A VERY QUICK CONTROLLER RESPONSE TIME

30. PRESSURE CONTROL • PRESSURE RELIEF SYSTEMS • SELF-CONTAINED VALVES THAT ACT LIKE BACKPRESSURE REGULATING VALVES • THESE ARE DESIGNED FOR HIGH FLOW RATES AFTER A SPECIFIED PRESSURE HAS BEEN ACHIEVED • MAY OR MAY NOT RESEAT AFTER ACTIVATION • MAY HAVE ACTUATORS FOR LARGE SYSTEMS

31. TEMPERATURE CONTROL • CONTINUOUS TEMPERATURE CONTROL • HEAT EXCHANGERS • STANDARD HEAT EXCHANGER CONTROL CONSISTS OF PROCESS FLUID TEMPERATURE MEASUREMENT AND SOME TYPE OF CONTROL ON THE HEAT TRANSFER FROM THE UTILITY • BYPASS IS AN OPTION WHEN HEAT IS BEING RECOVERED FROM A SECOND FLUID, BUT THERE IS A MINIMUM FOR EXIT TEMPERATURE OF THAT FLUID

32. TEMPERATURE CONTROL • The dynamics of the process and sensor are usually slower than the actuator. • Use a PI controller unless the process is sufficiently sluggish to warrant a PID controller.

33. TEMPERATURE CONTROL • Analysis of PI Controller Applied to Typical Temperature Loop.

34. TEMPERATURE CONTROL • Analysis of PI Controller Applied to Typical Temperature Loop. • Note that as the controller gain is increased, i.e., KcKp increase, the closed loop time constant becomes smaller. • Also, note that as the controller gain is increased, the value of z decreases.

35. TEMPERATURE CONTROL • CONTINUOUS TEMPERATURE CONTROL HEAT EXCHANGERS

36. TEMPERATURE CONTROL • PHASE CHANGE TEMPERATURE CONTROL • BOILERS/CONDENSERS CAN BE ALSO BE CONTROLLED BY ADJUSTING THE SYSTEM PRESSURE. THE LOWER THE PRESSURE, THE LOWER THE CONDENSING OR BOILING TEMPERATURE • DIRECT-FIRED HEATERS • FIRED HEATERS USE CONTROLS ON THE FUEL SUPPLY TO THE BURNER THAT ARE CONNECTED TO RATIO CONTROL FLOWS FOR OXIDANTS • SOME BURNERS USE STAGED COMBUSTION TO MINIMIZE MAXIMUM TEMPERATURE AND THEREBY REDUCE NOx FORMATION

37. TEMPERATURE CONTROL • HIGH AND LOW TEMPERATURE ALARM CONDITIONS • LOW TEMPERATURE IS NORMALLY A HAZARD WITH • LOSS OF FLAME FOR A COMBUSTION UNIT • FALLING BELOW VAPORIZATION TEMPERATURE • FALLING BELOW CRYSTALLIZATION TEMPERATURES • IN EACH CASE ABOVE, THE NORMAL RESPONSE IS TO STOP FLOW OF THE COOLANT TO THE SYSTEM AND RESTART AFTER ANALYSIS

38. TEMPERATURE CONTROL • HIGH AND LOW TEMPERATURE ALARM CONDITIONS • HIGH TEMPERATURE IS NORMALLY A HAZARD WHEN • SYSTEM COOLING CAPACITY IS EXCEEDED BY HEAT GENERATION • EXCESSIVE TEMPERATURE CAUSES PRODUCT DEGRADATION • HIGH TEMPERATURES AND LEAD TO MATERIAL FAILURES • IN EACH CASE ABOVE, THE NORMAL RESPONSE IS TO STOP THE HEAT GENERATION PROCESS, MAXIMIZE THE FLOW OF THE COOLANT TO THE SYSTEM, AND RESTART AFTER ANALYSIS

39. COMPOSITION CONTROL • CONTINUOUS CONTROL ON COLUMNS • COMPOSITION CONTROL ON COLUMNS CAN BE BASED ON ON-LINE SAMPLES OR SAMPLES ANALYZED OFF-LINE • ON-LINE SAMPLING USES SPECTROPHOTOMETRIC METHODS OR AN INDIRECT VARIABLE, SUCH AS CONDUCTIVITY OR VISCOSITY • OFF-LINE SAMPLING USES CHROMATOGRAPHY OR OTHER CHEMICAL METHODS • THE DEAD-TIME FOR SAMPLING IS TYPICALLY LESS THAN DEAD-TIME FOR PROCESS CHANGES • ACTUAL ADJUSTMENTS MAY BE MANUAL SETPOINT CHANGES

40. COMPOSITION CONTROL • The process is usually the slowest element followed by the sensor with the actuator being the fastest. • Use a PI controller unless the process is sufficiently sluggish to warrant a PID controller.

41. COMPOSITION CONTROL • CONTROL IN REACTORS/BLENDING OPERATIONS • THE SAME METHODS AS DISCUSSED FOR DISTILLATION MAY BE APPLIED FOR REACTORS AND BLENDING • OTHER TECHNIQUES INCLUDE CONTROLLED FLOWS WHEN THE COMPOSITIONS ARE CONSISTENT • BATCH OPERATIONS ALSO CAN BE APPLIED

42. COMPOSITION CONTROL • START-UP/SHUTDOWN OPERATIONS • CONTROL FOR STARTUP IS GENERALLY BASED ON ESTABLISHING STEADY STATE CONDITIONS BY USING MATERIAL FROM THE PREVIOUS STEADY-STATE OPERATION • SHUTDOWN CONTROLS DEPEND ON WHETHER THE MATERIALS CAN BE INVENTORIED WITHOUT DEGRADATION.

43. DO Control Loop • The process and the sensor have approximately the same dynamic response. • This is a fast responding process for which offset-free operation is desired. Therefore, PI controller should be used.

44. Biomass Controller • The process for this system is the slowest element. • Because the process is a high-order sluggish process, a PID controller is required.

45. Pid controller summary • .

46. Variable frequency drives • Variable frequency drives can eliminate the control valve In a system with a constant speed motor http://www.eaton.com

47. Variable frequency drives • Variable frequency drives can significantly reduce energy consumption http://www.eaton.com

48. Variable frequency drives • Basis of energy reduction are affinity laws http://www.eaton.com

49. Variable frequency drives • affinity laws • Flow or volume varies linearly with speed. If speed decreases by 50%, flow decreases by 50% • Pressure or head varies as a square of the speed. If speed decreases by 50%, the pressure decreases to 25% • Power or energy consumption varies as a cube of the speed. If speed decreases by 50%, power consumption decreases to 12.5% http://www.eaton.com

50. Variable frequency drives • PUMP AND DRIVE NEED TO BE SPECIFIED BASED ON SYSTEM CURVE http://www.eaton.com