Process Operability Class Materials Copyright © Thomas Marlin 2013 Dynamic Performance Basic flowsheet Design with Operability FC 1 LC Copyright © Thomas Marlin 2013 The copyright holder provides a royalty-free license for use of this material at non-profit educational institutions

PROCESS OPERABILITY: DYNAMIC PERFORMANCE Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis PROCESS OPERABILITY: DYNAMIC PERFORMANCE In this Lesson, we will learn Importance of process & equipment design - Review effect of process dynamics on control performance - Special concerns with recycle processes - Special considerations for capacity of manipulated variables - Mixing process, series packed bed reactor, recycle reactor, batch reactor Control Design Guidelines - Nine-step design method - Flash process

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Dynamics DYNAMIC PERFORMANCE OPERABILITY provides the capacity and flexibility to respond to changes in plant operation. In general, we wish to respond quickly. Compensate for (reject) disturbances Follow changing set points The value of “quickly” depends upon the specific process application. Safety and Equipment protection - extremely fast (& reliable) Product quality - very fast Production rate - moderate (very fast if load following) Efficiency and Optimization - can be slower Which elements in the process design should be fast, and which should be slow?

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Dynamics DYNAMIC PERFORMANCE Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance. How does each parameter affect the dynamic performance for this simple mixing process? Time constant in disturbance, D Dead time in feedback,  AC Time constant in feedback, P Steady-state Gain, KP

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Dynamics DYNAMIC PERFORMANCE Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance. CV limited set point overshoot, fast damping, and return to the set point S-LOOP plots deviation variables (IAE = 9.6759) 1.5 1 CV does not change because of dead time Controlled Variable 0.5  5 10 15 20 25 30 35 40 45 50 Time 1.5 1 Manipulated Variable 0.5 Set point response 5 10 15 20 25 30 35 40 45 50 Time

DYNAMIC PERFORMANCE , deadtime Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Dynamics DYNAMIC PERFORMANCE Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance. Effect of disturbance without control Can not prevent this deviation with feedback , deadtime Disturbance response

How does each parameter affect feedback performance? Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Dynamics DYNAMIC PERFORMANCE Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance. How does each parameter affect feedback performance? Large is good Small is good Large (range), fast element is good Fast sensor Time constant in disturbance, D Dead time in feedback,  A Time constant in feedback, P Steady-state Gain, KP

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Dynamics DYNAMIC PERFORMANCE CLASS WORKSHOP: The feedback controller is performing well (its well tuned), but the dynamic performance is poor. What improvements can we make? AC 1 Disturbances in feed Control a chemical reactor by adjusting its preheat Hint: Think about the structure of the process and the control system

Process - Dynamics DYNAMIC PERFORMANCE CLASS WORKSHOP: The feedback controller is performing well but the dynamic performance is poor. We must change the system structure! Shorten pipe, faster feedback AC + Feedforward Reduce disturbances TC Cascade AC Faster feedback LC Slower disturbances

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Recycle DYNAMIC PERFORMANCE Recycle processes: In some (many), specific components do not appear in exit streams. If no action is taken, these components will accumulate. What is needed in the process design? Recycle loop Purge

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Recycle DYNAMIC PERFORMANCE In some (many) recycle processes, specific components do not appear in exit streams. If no action is taken, these components will accumulate. What is needed in the process design? Recycle loop Purge Issues with purges Some valuable materials will be lost in purge (recover?) May introduce stream that requires treatment before release (environment) The purge flow can be adjusted to control the concentration of inert Large inert concentrations might be desired (e.g., to affect reactions)

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Recycle DYNAMIC PERFORMANCE Class Workshop Recycle Processes: How will this recycle process respond to disturbances? Can the design be improved? Is the design complete? Cold product Cold feed Heating fluid Consider a feed temperature increase Exothermic chemical reaction Hot effluent

DYNAMIC PERFORMANCE ++++ ++ ++++++++ ++++ Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Recycle DYNAMIC PERFORMANCE Class Workshop Recycle Processes: How will this recycle process respond to disturbances? Can the design be improved? ++++ ++++++++ Cold product ++ ++++ + ++ The disturbance grows because of the recycle (positive feedback)! Cold feed Heating fluid Exothermic chemical reaction Consider a feed temperature increase Hot effluent

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Recycle DYNAMIC PERFORMANCE Class Workshop Recycle Processes: How will this recycle process respond to disturbances? Can the design be improved? Cold product TC Cold feed Heating fluid Conceptually OK. Is the feedback fast enough? Exothermic chemical reaction Hot effluent

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Recycle DYNAMIC PERFORMANCE Class Workshop Recycle Processes: How will this recycle process respond to disturbances? Can the design be improved? Cold product TC Build in efficiency; use the least heating fluid! Cold feed Heating fluid Provide fast feedback! Exothermic chemical reaction Hot effluent

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process - Recycle DYNAMIC PERFORMANCE General principle: When possible, provide an alternative source of material or energy at the recycle point and control the variable (flow or temperature) after the recycle point. Energy recycle TC Process Adjustable heat exchange Material recycle FC Process Adjustable fresh feed

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process – MV range DYNAMIC PERFORMANCE We usually want “extra” capacity in the effects of the manipulated variables. Here,“extra” means more than required to achieve the desired steady-state operating window. Reactor with exothermic reaction F L CW P fc fo T Let’s concentrate on the control of temperature.

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process – MV range DYNAMIC PERFORMANCE We usually want “extra” capacity in the effects of the manipulated variables. (1) MV overshoot Temperature 10 20 30 40 50 60 0.5 1 1.5 Time Controlled Variable Manipulated Variable p, feedback dead time F L CW P T CW flow For many processes, some overshoot of the manipulated variable will improve (speed) control performance.

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process – MV range DYNAMIC PERFORMANCE We usually want “extra” capacity in the effects of the manipulated variables. (2) Disturbance response F L CW P T In a few processes, much greater MV capacity is required for response to disturbances. For example, a highly exothermic reaction operating at too high a temperature will tend to “run away”. A very large exchanger area and CW flow rate could be required to prevent a hazard.

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process – MV range DYNAMIC PERFORMANCE We usually want “extra” capacity in the effects of the manipulated variables. (2) Disturbance response F L CW P T Inert gas fo fc In a few processes, much greater MV capacity is required for response to disturbances, even a source and sink. For example, the reactor might produce gas at some times and condense gas at others times. Pressure control requires an exhaust line (originally shown) and an inert gas feed line (added for this figure). See Marlin (2000) Chapter 22 for control design

Process Control Design Guidelines For Multiloop PID Feedback Control Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Process Control Design Guidelines For Multiloop PID Feedback Control Define Control Objectives Select measurements Ensure sufficient degrees of freedom exist Ensure the process is controllable Ensure that the operating window is sufficiently large Evaluate interaction Eliminate designs with poor integrity Eliminate designs requiring extensive retuning Apply loop pairing guidelines and unit operation experience

DYNAMIC PERFORMANCE Process Control Design Guidelines Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Process Control Design Guidelines Process Example – Flash Process Feed Methane Ethane (LK) Propane Butane Pentane Vapor product Liquid Process fluid Steam F1 F2 F3 T1 T2 T3 T5 T4 T6 P1 L1 A1 L. Key P  1000 kPa T  298 K Let’s review the process

Process Control Design Guidelines 1. Define the Control Objectives Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Process Control Design Guidelines 1. Define the Control Objectives

Process Control Design Guidelines Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Process Control Design Guidelines 2. Select measurements Required to achieve each of the control objectives Define the sensor technology (e.g., orifice meter, venturi meter, mass flow) compatible with process conditions and achieve required accuracy and reproducibility Determine if for only local display or for transmission to centralized control for monitoring, history and control Define range (e.g., 150-200 C)

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE CLASS WORKSHOP on CV selection: We want to know the composition of the bottoms product, but the analyzer is too expensive (or doesn’t exist). What do we do? TC Inferential/Dominant Variables* - Easily measured and strongly influence product quality and/or profit. In many (not all) distillation towers, tray temperatures are a good inference of product composition. (The profile moves up/down the column) Great opportunity to use your process knowledge! * See Marlin (2000) Chapter 17 for further discussion on inferential variables PC LC LC

DYNAMIC PERFORMANCE Process Control Design Guidelines Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Process Control Design Guidelines 3. Ensure sufficient degrees of freedom Degrees of freedom are adjustable variables, i.e., valve openings, motor speeds, etc. Inputs Process Outputs Controlled variables, related to control objectives final element sensor Adjustable manipulated variables final element sensor Majority of variables remain unmeasured (and uncontrolled) Disturbances variables

DYNAMIC PERFORMANCE Process Control Design Guidelines Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Is that all that is required? Process Control Design Guidelines 3. Ensure sufficient degrees of freedom The system is has sufficient degrees of freedom if # of manipulated variables  # of controlled variables Inputs Process Outputs Controlled variables, related to control objectives final element sensor Adjustable manipulated variables final element sensor Majority of variables remain unmeasured (and uncontrolled) Disturbances variables

DYNAMIC PERFORMANCE Process Control Design Guidelines Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Is that all that is required? Process Control Design Guidelines 4. Ensure that the process is controllable Controllability ensures that the selected controlled variables can be moved in desired direction by the manipulated variables The system is controllable if Det [Kp]  0 (Kp is the gain matrix) Inputs Process Outputs Controlled variables, related to control objectives final element sensor Adjustable manipulated variables Kp final element sensor Majority of variables remain unmeasured (and uncontrolled) Disturbances variables

Process Control Design Guidelines Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Process Control Design Guidelines 5. Ensure that the operating window is large enough to satisfy the design specification, which gives disturbances and set point changes. Ensures that the capacities of the manipulated variables are large enough to “move” the process as needed.

DYNAMIC PERFORMANCE Process Control Design Guidelines Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Process Control Design Guidelines 6. Evaluate Interaction: Does it exist? A multivariable process has interaction when input (manipulated) variables affect more than one output (controlled) variable. 20 40 60 80 100 120 0.98 0.982 0.984 0.986 0.988 0.99 Time (min) XD (mol frac) 0.02 0.025 0.03 0.035 0.04 XB (mol frac) 1.12 1.125 1.13 1.135 x 10 4 R (mol/min) 1.5613 1.5614 1.5615 V (mol/min) Step change to reflux with constant reboiler

DYNAMIC PERFORMANCE Process Control Design Guidelines Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Process Control Design Guidelines Eliminate designs with poor integrity Eliminate designs requiring extensive retuning This topic requires some additional principles from process control. See Marlin, Process Control, (2000), Chapter 20

Process Control Design Guidelines 9. Apply loop pairing guidelines Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Process Control Design Guidelines 9. Apply loop pairing guidelines LOOP PAIRING QUALITATIVE GUIDELINES CVi - MVj pairing that has strong effect (large Kp) CVi - MVj pairing that has fast dynamics CVi - MVj pairing that has large range (MV min to max) CVi - MVj pairing with causal relationship that is (nearly) independent of other loops, i.e., reduce interactions If excess MVs exist, adjust MVj that has lowest cost; keep high cost MV near zero Often, these guidelines cannot all be satisfied. In some cases, they must be violated to achieve good performance.

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Class workshop: Design control loops (select CV-MV pairings) for the flash process. Feed Methane Ethane (LK) Propane Butane Pentane Vapor product Liquid Process fluid Steam F1 F2 F3 T1 T2 T3 T5 T4 T6 P1 L1 A1 L. Key P  1000 kPa T  298 K

Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE Class workshop: Design control loops (select CV-MV pairings) for the flash process. cascade Vapor product PAH TC-6 PC-1 T5 T1 Feed Methane Ethane (LK) Propane Butane Pentane T2 LAL LAH FC-1 T3 LC-1 F2 F3 Liquid product AC-1 Process fluid Steam L. Key See Marlin (2000), Process Control, Chapter 24 for details

DYNAMIC PERFORMANCE INDUSTRIAL PRACTICE Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Control Design DYNAMIC PERFORMANCE INDUSTRIAL PRACTICE Good performance through process design is more reliable and preferred if the cost is acceptable. We are guided by control objectives (safety, protection, smooth operation, product quality, and profit) We seek to provide good control with simple “loop pairing” control strategies, if possible. We use special knowledge for each unit operation – distillation, reactors, boilers, compressors, evaporators, etc.

PROCESS OPERABILITY: DYNAMIC PERFORMANCE Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis PROCESS OPERABILITY: DYNAMIC PERFORMANCE In this Lesson, we will learn Importance of process & equipment design - Review effect of process dynamics on control performance - Special concerns with recycle processes - Special considerations for capacity of manipulated variables - Mixing process, series packed bed reactor, recycle reactor, batch reactor Control Design Guidelines - Nine-step design method - Flash process