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This lecture is from Process Control course. Some key points for this lecture are: Process Dynamics, Steady State, Variables, Unsteady State Behavior, Curriculum Emphasizes, Mass and Energy Balance, Transient Behavior, Unit Operation, Shutdown, Start Up
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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? cf., noisy measurement
c) ChE curriculum emphasizes steady-state or equilibrium situations: i. Examples: Mass and Energy Balance, Unit Operation, Etc.
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 iv. Equipment or instrument failure (e.g., pump failure)Docsity.com
e) Batch processes
i. Inherently unsteady-state operation ii. Example: Batch reactor
a) Large scale, continuous processes:
i. Oil refinery, ethylene plant, pulp mill
ii. Typically, 1000 – 5000 process variables are measured.
iii. Examples: flow rate, T , P , liquid level, composition iv. Sampling rates:
b) Manipulated variables
i. We implement “process control” by manipulating process variables, usually flow rates.
Notation:
Assumptions:
Control Objective:
Keep x at a desired value (or “set point”) x sp , despite variations in
x 1 ( t ). Flow rate w 2 can be adjusted for this purpose.
Terminology:
x x SP
Control Question. Suppose that the inlet concentration x (^1) changes with time. How can we ensure that x remains at or near the set point?
As a specific example, if and , then x > x (^) SP.
x SP
x 1 (^) > x 1 w 2 (^) = w 2
Method 1. Measure x and adjust w 2.
w 2
w 2 (^) ( ) t (^) = w 2 (^) + K (^) c xSP − x t ( ) (1-4)
w 2 (^) ( ) t − w 2 ,
Method 2. Measure x 1 and adjust w 2.
x 1 w 2 (^) < w 2 ;
x 1 w 2
( ) 1 ( ) 2 1 1 (1-5)
SP SP
x x t w t w x
−
1.2 Classification of Control Strategies
Method Measured Variable
Manipulated Variable
Category
1 x (^) w 2 FB a
2 x 1 w 2 FF 3 x 1 and x w 2 FF/FB 4 - - Design change
Table. 1.1 Control Strategies for the Blending System
Engineering Usage vs. Social Sciences
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).
No corrective action occurs until after the disturbance has upset the process, that is, until after x differs from x (^) sp.
Very oscillatory responses, or even instability… Docsity.com