ProcessInstrumentationandControlSystem.ppt
- 1. Process Instrumentation and Control System Present By: Prakash Bahadur Thapa Memorial University St. Johns Newfoundland Canada Email: pbt750@mun.ca Tel: (1) 709 – 330 8666
- 2. Chemical Engineering Design Process Instrumentation & Control Control valves make a significant contribution to pressure drop (see later lecture on hydraulics) Control valve location can create a need for additional pumps and compressors, and must be decided in order to size the pumps and compressors It is therefore necessary for the design engineer to understand the plant control philosophy even at the PFD stage and the PFD usually shows the location of control valves More details of process control are usually included in the piping and instrumentation diagram (P&ID) – see Ch 5 This lecture is a very brief overview of control for design purposes – more will come in your process control class
- 3. Chemical Engineering Design Process Instrumentation & Control Basics of process control Process instrumentation Reading a P&ID Control of unit operations Process safety instrumentation Plant-wide control and optimization ©2012 G.P. Towler / UOP. For educationaluse in conjunction with Towler & SinnottChemical Engineering Design only. Do not copy
- 4. Chemical Engineering Design Objectives of Process Control Ensure stable process operation Particularly, keep the plant operating under safe conditions Minimize damage to equipment due to variation in plant conditions Ensure operation meets product specifications Minimize impact of external disturbances Example: change in ambient temperature Optimize process performance Maintain process throughput Minimize operating costs
- 5. Chemical Engineering Design PT PAH PAL PIC PV PT PAH PAL PIC PV Control Loop Components The sensing instrument detects the measured variable and sends a signal to a controller, which signals the actuator to close or open a control valve and adjust the manipulated variable (usually a flow rate) Process or utility stream Alarms Instrument line Actuator Final control element Controller Sensing element
- 6. Chemical Engineering Design Control Valves The final control element is usually a control valve Exceptions: electric heaters, mixers, variable speed drives The actuator is either a motor or a bellows that opens or closes the valve in response to the signal Actuator Valve
- 7. Chemical Engineering Design Types of Control Loop Feedback Control system measures changes in a process output and then adjusts manipulated variable to return output to set point Can be slow if process response time is long Feed Forward Control system measures disturbance and adjusts manipulated variable to compensate for it so that controlled output is not affected Requires greater knowledge of system response Process Manipulated variable Controlled output Disturbance Controller Process Manipulated variable Controlled output Disturbance Controller Process Manipulated variable Controlled output Disturbance Controller
- 8. Chemical Engineering Design Feedback Control Controller computes error between input and set point and adjusts output based on a control algorithm Process Sensing element Final control element Function generator Set point Output Error signal Measured variable Input Manipulated variable + - Controller
- 9. Chemical Engineering Design Typical Control Algorithms P: Proportional Controller output is proportional to error The proportionality constant is called the controller gain High gain gives fast response, but can lead to instability Low gain can cause offsets I: Integral Output is proportional to integral of error Eliminates offsets from P control, but makes response more sluggish D: Derivative Output is proportional to rate of change of error Damps out instability and allows higher gain to be used Other functions such as summation, multipliers, and advanced models can be easily coded in digital controllers time SP
- 10. Chemical Engineering Design PID Controller Response For a PID Controller, where: G = Controller gain e = Error (set point minus measured variable) Ti = Integral time constant Td = Derivative time constant The controller output is proportional to the error, the time integral of the error, and the rate of change of the error. G, Ti and Td are the controller tuning parameters. Much more of this in control class Controller Output = G e + (1 / T ) t o e dt - T de dt i d
- 11. Chemical Engineering Design Ratio Control One stream is controlled in ratio to another Often used for controlling feed rates to try to maintain stoichiometry Also used in some types of distillation column control to set reflux or boil-up ratios FT FFC FFV FT FT FFC FFC FFV FT
- 12. Chemical Engineering Design Cascade Control One primary controller is used to adjust the set point of a second secondary controller Used to minimize outside load variations and increase process stability FT FIC FV Coolant TIC TE TT M FT FIC FIC FV Coolant TIC TIC TE TT M • Example: reactor temperature (primary controller) cascades onto coolant flow controller (secondary) to control reactor temperature
- 13. Chemical Engineering Design Process Instrumentation & Control Basics of process control Process instrumentation Reading a P&ID Control of unit operations Process safety instrumentation Plant-wide control and optimization
- 14. Chemical Engineering Design What Can Be Measured? (& How Easily) Easy Temperature Pressure Flow rate V/L Level Pressure difference Conductivity Difficult L/L level pH Certain components oxygen, sulfur, hydrogen, CO Composition Density Voidage • Easy means cheap, reliable instrument with fast response time and accurate measurement
- 15. Chemical Engineering Design Temperature Measurement: Thermocouples When a junction between dissimilar wires is heated, an EMF (voltage) is developed, which can be read by a millivolt transmitter The junction is usually housed in a thermowell THERMOCOUPLE HEAD LEAD WIRE A B A B Cu Cu + - COLD JUNCTION (T ) 2 MILLIVOLT TRANSMITTER ISA TYPE A (+) B (-) CONSTANTAN CONSTANTAN ALUMEL CONSTANTAN E J K T CHROMEL IRON CHROMEL COPPER HOT JUNCTION (T ) 1 THERMOCOUPLE HEAD LEAD WIRE A B A B Cu Cu + - COLD JUNCTION (T ) 2 MILLIVOLT TRANSMITTER ISA TYPE A (+) B (-) CONSTANTAN CONSTANTAN ALUMEL CONSTANTAN E J K T CHROMEL IRON CHROMEL COPPER HOT JUNCTION (T ) 1 LEAD WIRE A B A B Cu Cu + - COLD JUNCTION (T ) 2 COLD JUNCTION (T ) 2 MILLIVOLT TRANSMITTER ISA TYPE A (+) B (-) CONSTANTAN CONSTANTAN ALUMEL CONSTANTAN E J K T CHROMEL IRON CHROMEL COPPER HOT JUNCTION (T ) 1 HOT JUNCTION (T ) HOT JUNCTION (T ) 1 Typical High T
- 16. Chemical Engineering Design Temperature Measurement: Thermocouples Response depends on thermowell location and heat transfer Instrument error is usually 3 to 4 F There may be additional offsets if the thermowell is incorrectly located Response is fast if located in a flowing stream Sometimes thermocouples are also strapped to walls of vessels For high temperature processes or processes with large exotherms
- 17. Chemical Engineering Design Pressure Measurement Pressure instruments usually measure differential pressure If one side is atmospheric pressure then the difference is the process gauge pressure (usually written barg or psig), not absolute pressure (bara, psia) Several possible methods: Mechanical: measure displacement of a bellows or Bourdon tube Electrical: attach a strain gauge to a bellows Capacitance: diaphragm moves capacitor plate (most common type) Piezoelectric: measures change in semiconductor conductivity Pressure measurement devices respond quickly and accurately Differential pressure measurement is used as the basis for flow and level measurement
- 18. Chemical Engineering Design Flow Rate Measurement Place a restriction in the flow path and measure the resulting pressure drop using a differential pressure (PD) cell If fluid properties are known, results can be calibrated to flow rates PD PD Orifice Meter Venturi Meter
- 19. Chemical Engineering Design Level Measurement Displacement Displacer moves up and down with level due to bouyancy Displacer movement is detected via mechanical or magnetic linkage Differential Pressure Measures static head of liquid using a differential pressure cell Density of the liquid and vapor must be known and constant Sensor element PD
- 20. Chemical Engineering Design Composition Measurement Some components can be detected at low concentrations using sensors that have been designed to pick up that component Examples: O2, CO, H2S, H2 Component sensors are often sensitive to other components, so check carefully with vendor to make sure the device is rated for the application More detailed composition can be measured by on-line GC methods TCD: thermal conductivity detector FID: flame ionization detector Response can be slow (5 to 30 minutes), particularly if a long column is used Online NIR can be used in some cases Composition is often inferred from other properties Boiling point Conductivity
- 21. Chemical Engineering Design Process Instrumentation & Control Basics of process control Process instrumentation Reading a P&ID Control of unit operations Process safety instrumentation Plant-wide control and optimization
- 22. Chemical Engineering Design Piping and Instrument Diagrams The P&ID shows all the instruments and valves in the process Not just control loops Vents, drains, sample points, relief valves, steam traps, isolation valves, etc. The P&ID usually also indicates line sizes and pipe metallurgy Companies occasionally use their own symbols, but U.S. standard is ISA-5.1-1984 (R1992) from the International Society for Automation The P&ID is usually produced in consultation with a specialist controls engineer Example of real P&ID: see Ch5
- 23. Chemical Engineering Design General Diaphragm Globe Three-way ISA 5.1 P&ID Symbols Diaphragm or unspecified actuator Rotary motor Digital Solenoid S D M Control Valves Actuators
- 24. Chemical Engineering Design Fails open Fails locked in current position Fails closed Failure mode indeterminate Valve Failure Positions It is important to specify what happens to a control valve if the signal fails The final valve position has an impact on process safety and pressure relief scenarios and may affect other instrumentation
- 25. Chemical Engineering Design Electric signal (4 to 20 mA) Pneumatic (instrument air) signal Undefined signal Instrument supply or connection to process Electric binary (on-off) signal Internal system link (software or data link) or or ISA 5.1 Instrument Lines Most newer plants use electric signals Pneumatic signals are found in older plants and locations where electric signals would be unsafe Binary signals are used for digital signals and for solenoids and other on-off devices Instrument lines are always drawn lighter than process lines on the P&ID
- 26. Chemical Engineering Design Field mounted Panel mounted in auxiliary location (local panel) Panel mounted in primary location Dual function instrument ISA 5.1 Controller Symbols Field mounted shared display device with limited access to adjustments Shared display device with operator access to adjustments Shared display device with software alarms (* is measured variable) *AH *AL Distributed Control Shared Display Symbols
- 27. Chemical Engineering Design Shared Display Devices Most plant control rooms now use shared display devices that show the outputs of multiple instruments on a VDU screen Operator can see a flow diagram that identifies where the instrument is and can enter set points Software also allows data to be plotted as trends Data can be accessed remotely Data is collected and logged for process records Source: UOP
- 28. Chemical Engineering Design ISA 5.1 Nomenclature Two- to four-letter codes are used to identify the instrument or controller First letter always indicates the initiating or measured variable Subsequent letters I = indicator, R = recorder, C = controller, T = transmitter, V = valve, Z = other final control element, S = switch, Y = compute function, AH = high alarm, AL = low alarm PIC Pressure Indicator Controller
- 29. Chemical Engineering Design ISA 5.1 Nomenclature: First Letters A Analysis (composition) F Flow FF Flow ratio J Power L Level P Pressure (or vacuum) PD Pressure differential Q Quantity R Radiation T Temperature TD Temperature differential W Weight C, D, G, M, N, O can be user-defined variables
- 30. Chemical Engineering Design Exercise: Identify The Instrument Can you figure out what each of these ISA 5.1 codes means and what the instrument does? TRC AR PAH PAHH LI PC TSH FFY PT JIAL
- 31. Chemical Engineering Design Restriction orifice Gate valve or isolation valve Hand control valve Pressure relief or safety valve Self-contained backpressure regulator Stop check (non- return) valve ISA 5.1 Other Common Symbols
- 32. Chemical Engineering Design Process Instrumentation & Control Basics of process control Process instrumentation Reading a P&ID Control of unit operations Process safety instrumentation Plant-wide control and optimization
- 33. Chemical Engineering Design LV LT LAH LAL LIC M Level Control A level control is needed whenever there is a V/L or L/L interface Level control sets inventories in process equipment Many smaller vessels are sized based on level control response time
- 34. Chemical Engineering Design Pressure Control Pressure control is usually by venting a gas or vapor In hydrocarbon processes, off-gas is often vented to fuel In other processes, nitrogen may be brought in to maintain pressure and vented via scrubbers Most common arrangement is direct venting (shown) Several vessels that are connected together may have a single pressure controller PV PT PIC PV PT PIC
- 35. Chemical Engineering Design Pressure Control If vapor has a high loading of condensable material, then pressure control is on the vent gas stream from the condenser PV PT PIC PV PT PIC PV PT PIC
- 36. Chemical Engineering Design PV PT PIC Coolant Process PT PIC PV Process vapor Coolant Pressure Control: Condensers Alternatively, for a condenser, we can control the coolant supply or the heat transfer surface (by varying the liquid level) These methods control pressure by changing the rate of condensation
- 37. Chemical Engineering Design Flow Control Most common arrangement is a control valve downstream of a pump or compressor Using a variable speed drive is a more efficient method, but higher capital cost FT FIC FV PI M FT FIC PI M FT FIC FV PI M FT FIC PI M FT FIC PI M
- 38. Chemical Engineering Design Flow Control: Reciprocating Pump Reciprocating pumps and compressors and other positive displacement devices deliver constant flow rate Flow can be controlled by manipulating a spill-back to the pump feed FT FY FV PI FT FIC FI FI FT FY FV PI FT FIC FI FI
- 39. Chemical Engineering Design LT LIC LV Feed FT FIC FV Steam Trap Condensate Vapor LT LIC LV Feed FT FIC FV Steam Trap Condensate Vapor Flow Control: Vaporizer Vaporizer flow control needs to prevent liquid accumulation Hence use level controller to actuate heat input to the vaporizer and maintain a constant inventory Control of liquid flow in is easier than control of vapor flow out
- 40. Chemical Engineering Design Temperature Control: Single Stream Heaters and coolers are usually controlled by manipulating the flow rate of the hot or cold utility stream Final control element can be on inlet or outlet of utility side TV TE TIC Hot or cold utility Process TT TV TE TIC TIC Hot or cold utility Process TT
- 41. Chemical Engineering Design Temperature Control: Heat Exchangers Temperature control for an exchanger is usually by manipulating the flow through a bypass Only one side of an exchanger can be temperature controlled TV TE TIC TT TV TE TIC TIC TT • It is also common to see exchangers with no temperature control and have temperature control on the downstream heater and cooler
- 42. Chemical Engineering Design Temperature Control: Air Coolers Ambient air temperature varies, so air coolers are oversized and controlled by manipulating a bypass Alternatively, air cooler can use a variable speed motor, louvers or variable pitch fans – see lectures on heat exchange equipment TV TE TIC TT M M TE TIC TT M M TO VARIABLE SPEED MOTOR CONTROL CIRCUIT TV TE TIC TT M M TV TE TIC TT TE TIC TIC TT M M M M TE TIC TT M M TO VARIABLE SPEED MOTOR CONTROL CIRCUIT TE TIC TT TE TIC TIC TT M M M M TO VARIABLE SPEED MOTOR CONTROL CIRCUIT
- 43. Chemical Engineering Design Distillation Control Distillation control is a specialized subject in its own right In addition to controlling condenser pressure and level in the sump, a simple distillation column has two degrees of freedom Material balance (split) and energy balance (heat input or removed) Therefore needs two controllers Therefore has the possibility that the controllers will interact and “fight” each other Side streams, intermediate condensers & reboilers, pump-arounds, etc. all add extra complexity and degrees of freedom There are several good books on this subject Kister, H.Z., 1989, Distillation Operation, McGraw-Hill Luyben, W.L. 2006, Distillation Design and Control Using Aspen Simulation, Wiley
- 44. Chemical Engineering Design Steam TC TC LC Distillation: Temperature Pattern Control Tray temperature is used to infer composition Composition is used to adjust reflux rate and reboiler heat input Tray locations for temperature detectors need to be chosen carefully Controllers can fight each other – not a good scheme Material balance control schemes are more robust
- 45. Chemical Engineering Design Distillation: Material Balance Control Direct control of distillate composition by using tray temperature to infer composition and control distillate flow rate Flow control on (constant) boil-up rate could be set in ratio to feed if feed flow rate was highly variable LC LC PC TC FC
- 46. Chemical Engineering Design Distillation: Material Balance Control Indirect control of distillate composition by using tray temperature to infer composition and control reflux rate Flow control on (constant) boil-up rate could be set in ratio to feed if feed flow rate varies LC LC PC TC FC
- 47. Chemical Engineering Design Distillation: Material Balance Control Direct control of bottoms composition by using tray temperature to infer composition and control bottoms flow rate Flow control on (constant) reflux rate could be set in ratio to feed if feed flow rate varies LC LC PC TC FC
- 48. Chemical Engineering Design Distillation: Material Balance Control Indirect control of bottoms composition by using tray temperature to infer composition and control boil-up rate Flow control on (constant) reflux rate could be set in ratio to feed if feed flow rate varies LC LC PC TC FC
- 49. Chemical Engineering Design FT FIC FV Steam Trap TE FV FT TT FY FIC Intermittent charge Batch Distillation Reflux flow control adjusted based on temperature (used to infer composition) Intermittent drain
- 50. Chemical Engineering Design Reactor Control Control of flow is usually carried out on cold reactor feeds Flows are often ratio controlled to get close to desired stoichiometry or maintain desired excess of one feed Pressure control is on reactor vapor outlet or on vent space Level control maintains inventory for liquid phase reactors Temperature control can be On feeds On heating or cooling jacket On recirculation through external heaters or cooler By controlling flow of quench/reheat streams
- 51. Chemical Engineering Design FT FIC FV Coolant TIC TE TT LT LAH LA L LIC FV FT FIC FV FT FIC M PT PIC Feed A Product To vent system Feed B Typical Stirred Tank Reactor Control Scheme Temperature cascade control of coolant flow Independent flow control of feeds
- 52. Chemical Engineering Design Exercise: Gas Recycle Process Remember the simple flowsheet introduced in the lecture on simulation? Liquid feed is mixed with recycle gas, heat exchanged against reactor effluent, heated to reactor temperature then passed over fixed bed of catalyst. Product is cooled and liquid product is recovered. Unconverted gas is recycled with purge to prevent accumulation of inerts Feed Reactor Make-up gas Purge Product Feed Reactor Make-up gas Purge Product
- 53. Chemical Engineering Design Exercise: Gas Recycle Process What controllers would you use? Where would you place them? Feed Reactor Make-up gas Purge Product Feed Reactor Make-up gas Purge Product
- 54. Chemical Engineering Design Process Instrumentation & Control Basics of process control Process instrumentation Reading a P&ID Control of unit operations Process safety instrumentation Plant-wide control and optimization
- 55. Chemical Engineering Design Role of Controls in Process Safety Control system is involved in three levels of process safety Keeping plant operation steady Sounding alarms to notify operator when variables are out of limits Automatically shutting the plant down when necessary Automatic Safety Shutdowns Pressure Relief System Critical Alarms & Operator Intervention Basic Process Control Plant Design (Inherent Safety) Emergency Response in Community Emergency Response in Process Unit Automatic Safety Shutdowns Pressure Relief System Critical Alarms & Operator Intervention Basic Process Control Plant Design (Inherent Safety) Emergency Response in Community Emergency Response in Process Unit
- 56. Chemical Engineering Design Process Control, Alarms and Shutdowns Controlled parameters naturally fluctuate around set point If the measured variable exceeds a preset limit an alarm should alert the operator to take appropriate action Alarm limits should be set far enough from normal process variation to avoid nuisance alarms If the measured variable exceeds a safe operating limit then an automatic plant shutdown may be necessary Shutdown limit should be set far enough from alarm limit that the operator has a chance to respond to the alarm But not so far that no time is left to safely shut the plant down time Variable AL AH Shutdown Set point
- 57. Chemical Engineering Design Standards for Safety Instrumentation ISA S84.01 Safety Instrumented Systems U.S. standard for emergency shutdown systems Primary goal is to protect people, not plant or profits ISA S84.01 = IEC 61511 IEC 61508 & 61511 IEC = International Electrotechnical Commission International standards for safety instrumented systems Standards define requirements for sensors, solvers (logic), and final elements (valves, switches) Consult most recent version of standards for current best practices Other standards also recommend best practices for alarm levels, vessel sizing to allow adequate control, etc.
- 58. Chemical Engineering Design Safety Integrity Levels ISA S84.01 defines three levels of safety integrity depending on the availability of the SIS Availability = time the system is available / total time Safety Integrity Levels SIL Availability System redudancy SIL 1 90 – 99% Non-redundant SIL 2 99 – 99.9% Partially redundant SIL 3 99.9 – 99.99% Totally redundant Redundant system means instrumentation is duplicated
- 59. Chemical Engineering Design Safety Integrity Level SIL should be determined during a process hazard analysis (see Ch10 and lectures on process safety) SIL required depends on risk of operator exposure and injury Can be calculated using fault trees See Ch10 and later lecture SIL determines the type of instrumentation that should be used
- 60. Chemical Engineering Design LT LAL LIC TRIP LT LIC LSL LAL LAL UC A UC A S LT LAL LIC TRIP LT LAL LIC TRIP LT LIC LIC LSL LAL LAL LAL UC A UC A UC A UC A S Process Alarms and Shutdown Trips Software alarms can be set on instruments and controllers through the digital control system and show up on shared displays Separate alarm and shutdown instrumentation can also be used, for higher redundancy
- 61. Chemical Engineering Design Caution on Software Alarms There is a temptation to put lots of software alarms in digital control systems If there are too many alarms then they can become a distraction to the operators Increasing the chance of human error Increasing the chance that the operator will ignore the alarm, switch it off, or acknowledge it without taking action Increased chance of an “alarm flood” Alarms should be carefully placed and calibrated to make sure that they serve the purpose of the designer Operators should be trained to understand the importance of every alarm on the plant
- 62. Chemical Engineering Design Process Instrumentation & Control Basics of process control Process instrumentation Reading a P&ID Control of unit operations Process safety instrumentation Plant-wide control and optimization
- 63. Chemical Engineering Design Plant-Wide Control (Advanced Process Control, APC) Operators can update individual controller set points via shared displays Plant information systems can log data Feedforward and multivariable predictive control can be implemented more easily Plant real-time optimization can be implemented through the control system All new plants and most older plants have the instruments and controllers connected to a plant-wide digital control system (DCS). Using the DCS: Most controllers now have microprocessors built into them, so the computational capacity that is needed is not necessarily located in the control room
- 64. Chemical Engineering Design Plant-Wide Control Issues Where do we control inventories? All vessels that have a liquid level Do we need extra surge volume to keep operations steady? Where is the best place to control pressure? Single PC setting plant back-pressure? Need for different pressure levels? Where is best place to control material balance? Can only control flow in one place per feed stream Control after removing contaminants? Ratio control? What about recycles?
- 65. Chemical Engineering Design Plant-Wide Control Issues Where is best place to control temperatures? Heaters and coolers only? Control of heat exchanger networks? How do we meet product specifications? Use of on-line analytical instruments? Inference through bubble point? Sampling of product storage tank? How do we optimize the process? Maximize throughput and product yield while staying on spec? Minimize cost of production? Maximize time on stream between shutdowns?
- 66. Chemical Engineering Design Multivariable Predictive Control What if we have three manipulated variables that interact with each other through non-linear equations? Measured variable 1 Measured variable 2 Measured variable 3 Control Valve 1 Control Valve 2 Control Valve 3 • Single Input Single Output (SISO) controllers will have a hard time responding and will tend to have strong interactions with each other (tendency to fight)
- 67. Chemical Engineering Design Multivariable Predictive Control In this situation, instead of SISO controllers we need a Multiple Input Multiple Output (MIMO) controller The controller algorithm is a model that captures the non-linear interaction between the manipulated variables Does not have to be a “fundamental” model Does have to capture dynamic response Can be tuned from plant operation Hence another name for multivariable predictive control is model-based predictive control With MIMO devices, there is no longer a direct pairing of measured and manipulated variables as in the earlier unit operations examples
- 68. Chemical Engineering Design MIMO Example: Gas Mass Flow Controller Gas mass flow cannot be measured or controlled directly By measuring volume flow, temperature and pressure we can compute mass flow as long as composition is known and steady This is actually MISO, not MIMO, but the FY could talk to another device such as a ratio controller FT FIC FV PT TT FY
- 69. Chemical Engineering Design Device Communication Standards Now that individual controllers (and even instruments) contain microprocessors, the computational power of the DCS is distributed To make best use of this distributed computation power, the devices need to be able to communicate with each other Communication standards are set by ISA, IEC and the controller manufacturers and are currently evolving rapidly
- 70. Chemical Engineering Design Evolution of Controller Communication Standards Manual Control Pneumatic Analog Electronic Analog Digital Fieldbus (ISA SP50) Wireless (ISA SP100) ?!?!?!? Source: UOP
- 71. Chemical Engineering Design Fieldbus (ISA SP50) Digital device communication protocol that allows “plug and play” connection of devices Requires less wiring and gives greater reliability through redundancy Different control companies have variations on the standard, so system compatibility and interoperability are issues Source: UOP
- 72. Chemical Engineering Design Wireless Communication (SP100) Advantages No cable runs Quicker and cheaper to set up Portable control room Improved safety (electric cables are easily damaged by fires) Problems Interference from other wireless devices Signal blocking due to steelwork Signal loss creates need for greater redundancy The controls companies are currently putting a lot of effort into developing the devices and standards for implementing wireless control
- 73. Chemical Engineering Design Real Time Optimization A DCS can be programmed to carry out optimization of plant performance by updating controller set points and MPC algorithms The optimizer is usually a higher level program that runs less frequently and is used to adjust set points periodically by computing target values for key performance indicators (KPIs) The optimizers used for RTO are often not very sophisticated – typically LP, MILP or simple NLP models Optimizer DCS Controllers Plant Targets for KPIs Set points Adjust manipulated variables
- 74. Chemical Engineering Design Types of Real-time Optimization Users take plant data and run the optimizer then send instructions to the plant operators to update the DCS settings Labor intensive and difficult to update more than daily Plant DCS Optimizer User User Off-line Optimization
- 75. Chemical Engineering Design Types of Real-time Optimization Users provide input to optimizer, DCS updates optimizer directly with plant settings and user updates DCS with new targets Plant DCS Optimizer User User Off-line Optimization Plant DCS Optimizer User User Open Loop On-line Optimization
- 76. Chemical Engineering Design Types of Real-time Optimization Users only provide input to the optimizer and the DCS is updated directly by the optimizer Plant DCS Optimizer User User Off-line Optimization Plant DCS Optimizer User User Open Loop On-line Optimization Plant DCS Optimizer User Closed Loop On-line Optimization
- 77. Chemical Engineering Design Real Time Optimization Models Models and algorithms for RTO have very tough requirements Must be robust, i.e., always find a solution Must solve quickly Must converge to same solution whatever the starting point Must allow for model error Must reconcile data and filter out bad data Must capture plant constraints Must give reasonably good description of plant performance Hence frequent use of simple LP models Controls companies spend a lot of time setting up and tuning models