Introduction to Process Control PDF

- Plant objective - Importance of process control - Direct targets of control - Benefits of control - Block representations - Control nomenclature - Control system development - Feedback control: definition - Feed forward control : definition - Examples Process Control Chapte...

- Plant objective - Importance of process control - Direct targets of control - Benefits of control - Block representations - Control nomenclature - Control system development - Feedback control: definition - Feed forward control : definition - Examples Process Control Chapter 1 -(3) To maximize profits by transforming raw materials into useful products while satisfying a number of important criteria: 1. Safety: to protect well-being of plant personnel and nearby communities. 2. Environmental Regulations: to comply with environmental regulations concerning air and water quality as well as waste disposal. 3. Product specifications: product specification, concerning quality and production rate, must be met. 4. Operational Constrains: process variables must satisfy certain other operating constrains Process Control Chapter 1 -(4) Plant objectives determines the objectives for the control system as well as the controller set point Directly affects the safety and reliability of a process » Control system must provide safe operation » Control system must be able to “absorb” a variety of disturbances & keep the process in a good operating region Determines the quality of the products produced by a process Can affect how efficient a process is operated Bottom Line: process control has a major impact on the profitability of the company. Process Control Chapter 1 -(5) In the chemical industry, the design of a control system is essential to ensure: Good Process Operation Process Safety Product Quality Minimization of Environmental Impact Process Control Chapter 1 -(6)  What is the purpose of a control system? “To maintain important process characteristics at desired targets despite the effects of external perturbations.” Perturbations Processing Plant objectives Market Safety Economy Make $$$ Climate Environment... Upsets... Control Process Control Chapter 1 -(7) What constitutes a control system? Control Combination of process sensors, alarms, actuators and computer systems designed and tuned multi-function platform to orchestrate safe and profitable operation. Plant Process Control Chapter 1 -(8)  Process Dynamics: study of the transient behavior of processes  Process Control the use of process dynamics for the improvement of process operation and performance or/and the use of process dynamics to alleviate the effect of undesirable (unstable) process behaviors Process Control Chapter 1 -(9) What do we mean by process? A process, P, is an operation that takes an INPUT or a DISTURBANCE and gives an OUTPUT u y P d Information Flow INPUT : (u) Something that you can manipulate DISTURBANCE : (d) Something that comes as a result of some outside phenomenon OUTPUT : (y) An observable quantity that we want to regulate Process Control Chapter 1 -(10)  Control Objective, controlled variable (CV)  Measured Process Variable (PV)  Set Point (SP)  Controller Output (CO)  Manipulated Variable (MV)  Disturbances (D or DV) Inputs Outputs Variables Manipulated Disturbances Measured Unmeasured Control Process Control Chapter 1 -(11)  wind Friction Engine Power Inputs Output (D) Wind speed (D) Friction Process Speed (SP,CV) (MV) Engine Power (Driving an automobile) Direction (SP,CV) (MV) Steering position (MV) Break position Process Control Chapter 1 -(12)   Control Objective, (CV)  Measured Process Variable (PV) 5 feet  Set Point (SP)  Controller Output (CO)  Manipulated Variable (MV) VALVE VALVE  Disturbances (D) Main supply temp Main supply Room flow temp Cold water supply Hot water supply Inputs Output H. Valve position Temp. C. Valve position Process Flow Process Control Chapter 1 -(13)   Control Objective, (CV)  Measured Process Variable (PV) M  Set Point (SP)  Controller Output (CO) Tin, w  Manipulated Variable (MV)  Disturbances (D) T, w Q Qout T RPM Inputs Output Tin w Process T Q Process Control Chapter 1 -(14)  Control Objective, (CV) Measured Process Variable (PV) Set Point (SP) Controller Output (CO) Manipulated Variable (MV) Disturbances (D) thermostat controller set point TC TT Qout Tin xin temperature heat loss sensor/transmitter (disturbance) control signal Fin Process T fuel flow furnace valve © Process Control 15  Closed-loop Artificial Pancreas  Control Objective  Measured Process Variable (PV)  Set Point (SP)  Controller Output (CO)  Manipulated Variable  Disturbances (D) Actual Glucose Glucose level An insulin pump administers insulin through a catheter in the abdominal fat to help level setpoint control a person’s blood sugar levels y u r controller pump sensor measured glucose Process Control Chapter 1 -(16)  To regulate of a process output despite the effect of disturbances e.g. Driving a car Controlling the temperature of a chemical reactor Reducing vibrations in a flexible structure  To stabilize unstable processes e.g. Filling a storage tank Level of a distillation coulm condenser Operation of a nuclear plant Process Control Chapter 1 -(17)  Economic Benefits Quality (waste reduction) Variance reduction (consistency) Savings in energy, materials, manpower  Operability, safety (stability) Performance Efficiency Accuracy robotics Reliability Stabilization bicycle aircraft nuclear reactor Process Control Chapter 1 -(18)  Block diagrams are models of the physical systems Input variables Output variables Process System Physical Boundary Transfer of fundamental quantities Physical Mass, Energy and Momentum Abstract Operation Process Control Chapter 1 -(19)  Process Controller A controller is a system designed to regulate a given process  Process typically obeys physical and chemical conservation laws  Controller obeys laws of mathematics and logic (sometimes intelligent) e.g. - Riding a bike (human controller) - Driving a car (human controller) - Automatic control (computer programmed to control) Process Control Chapter 1 -(20)  A controlled process is a system which is comprised of two interacting systems: e.g. Most controlled systems are feedback controlled systems Disturbances Outputs Process Action Observation intervene Controller monitor The controller is designed to provide regulation of process outputs in the presence of disturbances Process Control Chapter 1 -(21) 1. Process Understanding Required measurements Required actuators Understand design limitations 2. Process Instrumentation Appropriate sensor and actuator selection Integration in control system Communication and computer architecture 3. Process Control Appropriate control strategy Process Control Chapter 1 -(22)  Wind Road speed Friction Driver Steering r + Σ e C p A u P y - ym Automobile M Visual and tactile measurement Actual trajectory Desired trajectory (y) (r) Process Control Chapter 1 -(23)  Tin, w Q Heater T, w TC Thermocouple Tin, w Controller Heater T TR e p u y + C A P - Tank M Thermocouple Process Control Chapter 1 -(24)  D  Measure T, adjust Q CO MV PV SP Tin, w Controller Heater TR e p u y + C A TinP ,w - T Tank M Thermocouple Controller Output (CO): CO = p = f(e) p = Qss + K (TR-T) [Proportional Controller] where Qss: Heat input at SS (when the TR=T) = wCp(Tss-Tin) Q: Is this positive or negative feedback? Process Control Chapter 1 -(25)  Identification of all process variables Disturbances Other Manipulated Process Control Controller Input Variables Output Variables (Affect process) (Results of process) Disturbance variable Measured variable Variables affecting process that are due speed of a car to external forces Unmeasured variable Manipulated variable acceleration of a car Things that we can directly affect Control variable important observable quantities that we want to regulate can be measured or unmeasured Process Control Chapter 1 -(26)  wi, Ti T L wc, Tco h Pc wc, Tci Variables Po T wo, To wi, wo: Tank inlet and outlet mass flows Ti, To: Tank inlet and outlet temperatures wc: Cooling jacket mass flow Pc: Position of cooling jacket inlet valve Po: Position of tank outlet valve Tci, Tco: Cooling jacket inlet and outlet temperatures h: Tank liquid level Process Control Chapter 1 -(27)  Task: Classify the variables Variables Inputs Outputs Disturbances Manipulated Measured Unmeasured Control wi Ti Tci wc h wo To Pc Po Process Control Chapter 1 -(28) Control development is usually carried out following these important steps Define objectives Develop a process model Often an iterative process, based on Design controller based performance we may on model decide to retune, redesign or remodel a Test by simulation given control system Implement and tune Monitor performance Process Control Chapter 1 -(29)  Objectives “What are we trying to control?”  Process modeling “What do we need?” Mechanistic and/or empirical  Controller design “How do we use the knowledge of process behavior to reach our process control objectives?” What variables should we measure? What variables should we control? What are the best manipulated variables? What is the best controller structure? Process Control Chapter 1 -(30)  Implement and tune the controlled process Test by simulation incorporate control strategy to the process hardware theory rarely transcends to reality tune and re-tune  Monitor performance periodic retuning and redesign is often necessary based on sensitivity of process or market demands statistical methods can be used to monitor performance Process Control Chapter 1 -(31) Notation: w1, w2 and w are mass flow rates x1, x2 and x are mass fractions of component A Process Control Chapter 1 -(32)  : 1. w1 is constant 2. x2 = constant = 1 (stream 2 is pure A) 3. Perfect mixing in the tank Keep x at a desired value (or “set point”) xsp, despite variations in x1(t). Flow rate w2 can be adjusted for this purpose. Controlled variable (or “output variable”): x Manipulated variable (or “input variable”): w2 Disturbance variable (or “load variable”): x1 Process Control Chapter 1 -(33). What value of w2 is required to have x  xSP ? : 0  w1  w2  w (1-1) : w1 x1  w2 x2  wx  0 (1-2) At the design conditions: x  xSP (The overbars denote nominal steady-state design values.) Substitute x  xSP in Eq. 1-2, and set x2  1 , then solve Eq. 1-2 for w2 : xSP  x1 w2  w1 (1-3) 1  xSP Process Control Chapter 1 -(34)  Equation 1-3 is the design equation for the blending system. If our assumptions are correct, then this value of w2 will keep x at xSP. But what if conditions change? Suppose that the inlet concentration x1 changes with time. How can we ensure that x remains at or near the set point xSP ? As a specific example, if w2  w2 and x1  x1 , then x > xSP. Process Control Chapter 1 -(35) Some Possible Control Strategies: Method 1: Measure x and adjust w2. Intuitively, if x is too high, we should reduce w2; Manual control vs. automatic control Proportional feedback control law, w2  t   w2  K c  xSP  x  t   (1-4) 1. where Kc is called the controller gain. 2. w2(t) and x(t) denote variables that change with time t. 3. The change in the flow rate, w2  t   w2 , is proportional to the deviation from the set point, xSP – x(t). Process Control Chapter 1 -(36) Method 2. Measure x1 and adjust w2. Thus, if x1 is greater than x1, we would decrease w2 so that w2  w2 ; One approach: Consider Eq. (1-3) and replace x1 and w2 with x1(t) and w2(t) to get a control law: xSP  x1  t  w2  t   w1 (1-5) 1  xSP Because Eq. (1-3) applies only at steady state, it is not clear how effective the control law in (1-5) will be for transient conditions. Process Control Chapter 1 -(37)  AC AC Method 3. Measure x1 and x, adjust w2. AT This approach is a combination of Methods 1 and 2. AT Method 4. Use a larger tank. If a larger tank is used, fluctuations in x1 will tend to be damped out due to the larger capacitance of the tank contents. However, a larger tank means an increased capital cost. Process Control Chapter 1 -(38) Method Measured Manipulate Category Variable d Variable 1 x w2 FB 2 x1 w2 FF 3 x1 and x w2 FF/FB 4 - - Design change FB: Feedback , FF: Feedforward Process Control Chapter 1 -(39)  Steam Feed Outlet Ti To Condensate Control Objective: Maintain temperature of the outlet stream (To) Process Control Chapter 1 -(40) Characteristics measure controlled variable adjust manipulated variable to keep controlled at set point Good choice for set point tracking Steam Toset TC Feed Outlet TT Ti To Condensate Process Control Chapter 1 -(41) Process: the process under investigation, the heat exchanger The sensor: a device used to measure a signal (PV), thermocouple The controlled variable: the temperature of the outlet stream The set-point: desired outlet stream temperature Manipulated variable: variable used to maintain the controlled variable at the set point, the steam flowrate. The actuator or final control element: Valve on the steam line Process Control Chapter 1 -(42) The controller: The Temperature controller Feedback control or closed-loop control: manipulation based on the sensed temperature Feedback control loop: combination of the sensor, set-point, controller, actuator and process control Open-loop response: no feedback from the measured controlled variable Disturbance: any variable cause a deviation of the controlled variable from the set-point, changing inlet temperature, quality of steam Feed-forward control: compensating known disturbances before they affect the process, e.g. adjust the steam temperature due to change in inlet temperature. Process Control Chapter 1 -(43) Characteristics Steam measure disturbance variable FFC adjust manipulated variable to keep controlled at set point Feed Outlet TT Ti To Condensate (+) Superior disturbance rejection (-) Disturbance model required (-) Difficult to predict the disturbance Process Control Chapter 1 -(44) Feedback Controller Feedforward Controller » (+) simple » (+) excellent disturbance » (+) most common rejection properties » (+) use current measurement » (+) manipulated to determine control action variable changes occur » (-) disturbances must before disturbance manifest in output signal affects output signal before corrective action can » (-) model required be taken » (-) no output information Chapter 1 -(45) From your text book (Seborg: Process Dynamics and Control), solve the following questions: 1.3 1.5 1.7 1.8 Submission : Next week, same day.