KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec142

KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec142

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Unformatted text preview: 1 Dynamic Behavior Chapter 5 In analyzing process dynamic and process control systems, it is important to know how the process responds to changes in the process inputs. A number of standard types of input changes are widely used for two reasons: 1. They are representative of the types of changes that occur in plants. 1. They are easy to analyze mathematically. 2 1. Step Input A sudden change in a process variable can be approximated by a step change of magnitude, M : Chapter 5 (5-4) s t U M t < @ • Special Case: If M = 1, we have a “unit step change”. We give it the symbol, S ( t ). • Example of a step change: A reactor feedstock is suddenly switched from one supply to another, causing sudden changes in feed concentration, flow, etc. The step change occurs at an arbitrary time denoted as t = 0. 3 Chapter 5 Example: The heat input to the stirred-tank heating system in Chapter 2 is suddenly changed from 8000 to 10,000 kcal/hr by changing the electrical signal to the heater. Thus, ( 29 ( 29 ( 29 ( 29 ( 29 8000 2000 , unit step 2000 , 8000 kcal/hr Q t S t S t Q t Q Q S t Q = + =- = = @ and 1. Ramp Input • Industrial processes often experience “drifting disturbances”, that is, relatively slow changes up or down for some period of time. • The rate of change is approximately constant. 4 Chapter 5 We can approximate a drifting disturbance by a ramp input: ( 29 (5-7) at R t U t t < @ Examples of ramp changes: 1. Ramp a setpoint to a new value. (Why not make a step change?) 2. Feed composition, heat exchanger fouling, catalyst activity, ambient temperature. 1. Rectangular Pulse It represents a brief, sudden change in a process variable: 5 Chapter 5 ( 29 for for 0 (5-9) for RP w w t U t h t t t t < < @ Examples: 1. Reactor feed is shut off for one hour....
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This note was uploaded on 09/22/2009 for the course CHEMICAL CHE 401 taught by Professor Dr.muhammadal-arfaj during the Spring '09 term at King Fahd University of Petroleum & Minerals.

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KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec142

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