Chapter-1 : introduction to process control and dynamics
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The document provides an introduction to process control and dynamics, defining processes such as heat exchangers and reactors, and highlighting the importance of automation in controlling inputs and outputs. Key objectives of process control include maintaining stable operations, minimizing disturbances, and maximizing profitability. The discussion encompasses various control strategies, including feedback and feedforward methods, with an emphasis on managing variables in complex industrial processes.
Chapter-1 : introduction to process control and dynamics
- 1. ChE 4745 Process Control and Dynamics Introduction to Process Control Chapter 1
- 2. What is a Process? Process: - A Heat Exchanger (heating/cooling) - A Chemical/Biological Reactor (make petrochemicals or recombinant drugs) - A Separator (Distillation column or a chromatographic column for separating proteins) - A Feed or holding tank - Human body - A Car - A Computer Drive Chapter 1
- 3. Chapter 1 Chemical Process Industries (CPI) Hydrocarbon fuels Chemical products Pulp and paper products Agrochemicals Man-made fibers
- 4. Chapter 1 Bio-Process Industries Use micro-organisms to produce useful products Pharmaceutical industry Ethanol from grain industry
- 5. Chapter 1
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- 7. Chapter 1 Gas stream Gas stream Empty vessel P CONTROL
- 8. Adjusting valves: Do you believe in automation? Do we run around the plant to adjust the valves when required? Process pictures courtesy of Petro-Canada Products Chapter 1
- 9. Central control room Process pictures courtesy of Petro-Canada Products • Overview of entire process • Make immediate adjustment anywhere • Safe location • History of past operation Chapter 1 Adjusting valves: Do you believe in automation?
- 10. Chapter 1 What is Process Control? Inputs (cause) Outputs (effects) Process Output: “off-specification” Consequence: Less profit!
- 11. Chapter 1 Benefits of Improved Control Time Concentration Limit Time y t pu ri m I Concentration Limit Old Controller New Controller Time y t pu ri m I Concentration Limit Improved Performance
- 12. Chapter 1 Objectives of Process Control – Maintain a stable process operation – Appropriate instruments/sensors are to be implemented to operate under “fail/safe” conditions. – Make sure no “disturbances” affect the process output(s). – Make sure when we make desired changes (set point) to the process, it does achieve the desired goal. – Make sure the process always remain within a “tight” specification. – Maximize the profitability of the plant
- 13. Chapter 1 Process Dynamics a) Refers to unsteady-state or transient behavior. b) Steady-state vs. unsteady-state behavior i. Steady state: variables do not change with time ii. But on what scale? e.g., noisy measurement c) ChE curriculum emphasizes steady-state or equilibrium situations: d) Continuous processes: Examples of transient behavior: i. Start up & shutdown ii. Grade changes iii.Major disturbance: e.g., refinery during stormy or hurricane conditions
- 14. Chapter 1 e) Batch processes i. Inherently unsteady-state operation ii. Example: Batch reactor 1. Composition changes with time 2. Other variables such as temperature could be constant. Process Control Objective: to maintain or operate a process at the desired operating conditions safely and efficiently, while satisfying environmental and product quality requirements. a) Large scale, continuous processes: i. Oil refinery, ethylene plant, pulp mill ii. Typically, 1000 – 5000 process variables are measured.
- 15. Chapter 1 iii.Examples: flow rate, T, P, liquid level, composition iv. Sampling rates: 1. Process variables:Afew seconds to minutes 2. Quality variables: once per 8 hr shift, daily, or weekly b) Manipulated variables i. We implement “process control” by manipulating process variables, usually flow rates. 1. Examples: feed rate, cooling rate, product flow rate, etc. ii. Typically, several thousand manipulated variables in a large continuous plant Process Control (cont’d.)
- 16. Chapter 1 c) Batch plants: i. Smaller plants in most industries 1. Exception: microelectronics (200 – 300 processing steps). ii. But still large numbers of measured variables. d) Question: How do we control processes? i. We will consider an illustrative example. Process Control (cont’d.)
- 17. Chapter 1 1.1 Illustrative Example: Blending system Notation: • w1, w2 and w are mass flow rates • x1, x2 and x are mass fractions of component A
- 18. Chapter 1 Assumptions: 1. w1 is constant 2. x2 = constant = 1 (stream 2 is pureA) 3. Perfect mixing in the tank Control Objective: Keep x at a desired value (or “set point”) xsp, despite variations in x1(t). Flow rate w2 can be adjusted for this purpose. Terminology: • Controlled variable (or “output variable”): x • Manipulated variable (or “input variable”): w2 • Disturbance variable (or “load variable”): x1
- 19. Chapter 1 Design Question. What value of w2 is required to have x xSP ? Overall balance: 0 w1 w2 w Component 'A' balance: (1-1) w1x1 w2x2 wx 0 (1-2) (The overbars denote nominal steady-state design values.) and x xSP • At the design conditions, x xSP. Substitute Eq. 1-2, x2 1, then solve Eq. 1-2 for w2 : 2 SP 1 (1-3) x x w w1 1 xSP
- 20. Chapter 1 • 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? Control Question. 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 x1 x1 and w2 w2, then x > xSP. Some Possible Control Strategies: Method 1. Measure x and adjust w2. • Intuitively, if x is too high, we should reduce w2;
- 21. Chapter 1 • Manual control vs. automatic control • Proportional feedback control law, w2 t w2 Kc xSP xt 1. where Kc is called the controller gain. (1-4) 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).
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- 23. Chapter 1 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: 2 1 SP 1 (1-5) x x t w t w 1 xSP
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- 25. Chapter 1 • 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. Method 3. Measure x1 and x, adjust w2. • This approach is a combination of Methods 1 and 2. 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.
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- 27. Chapter 1 1.2 Classification of Control Strategies Method Measured Variable Manipulated Variable Category 1 x w2 FBa 2 x1 w2 FF 3 x1 and x w2 FF/FB 4 - - Design change Table. 1.1 Control Strategies for the Blending System Feedback Control: • Distinguishing feature: measure the controlled variable
- 28. Chapter 1 • It is important to make a distinction between negative feedback and positive feedback. Engineering Usage vs. Social Sciences • Advantages: Corrective action is taken regardless of the source of the disturbance. Reduces sensitivity of the controlled variable to disturbances and changes in the process (shown later). • Disadvantages: No corrective action occurs until after the disturbance has upset the process, that is, until after x differs from xsp. Very oscillatory response, or even instability
- 29. Chapter 1 Feedforward Control: Distinguishing feature: measure a disturbance variable • Advantage: Correct for disturbance before it upsets the process. • Disadvantage: Must be able to measure the disturbance. No corrective action for unmeasured disturbances.
- 30. pump L valve sensor pump valve The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop? Chapter 1
- 31. Chapter 1 pump F valve sensor pump valve The key elements and principles of a feedback loop – Cause and Effect Exercise: You want to control the level, but you can only measure the flow in. What is your strategy? Are you using feedback?
- 32. Chapter 1 Gas stream Gas stream Empty vessel The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop? P
- 33. Chapter 1 v1 Hot Oil v2 L1 v7 v5 v6 Hot Oil F1 T1 T3 T2 F2 v3 T4 T5 F3 T6 T8 F4 L2 v8 T7 P1 F5 F6 T9 v4 2. The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop?
- 34. Chapter 1 v1 Hot Oil v2 L1 v7 v5 v6 Hot Oil F1 T1 T3 T2 F2 v3 T4 T5 F3 T6 T8 F4 L2 v8 T7 P1 F5 F6 T9 v4 The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop What is being measured? Is this a valid feedback control loop?
- 35. Chapter 1 The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop Hot process fluid into shell Cooling water into tubes We want to control the hot outlet temperature. Add a sensor and a valve to make this possible.
- 36. Chapter 1 The key elements and principles of a feedback loop – Cause and Effect Exercise: The key elements and principles of a feedback loop Hot process fluid into shell Cooling water into tubes Add a sensor and a valve to make this possible. TC We want to control the hot outlet temperature.
- 37. Chapter 1 Acknowledgement Dr. M. A. A. Shoukat Choudhury Professor, Department of Chemical Engineering, BUET