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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? 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
- Composition changes with time
- Other variables such as temperature could be constant.
Process Control
a) Large scale, continuous processes:
i. Oil refinery, ethylene plant, pulp mill
ii. Typically, 1000 – 5000 process variables are measured.
- Most of these variables are also controlled.
iii. Examples: flow rate, T , P , liquid level, composition iv. Sampling rates:
- Process variables: A few seconds to minutes
- 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.
- Examples: feed rate, cooling rate, product flow rate, etc. ii. Typically, several thousand manipulated variables in a large continuous plant
Process Control (cont’d.)
1.1 Illustrative Example: Blending system
Notation:
- w 1 , w 2 and w are mass flow rates
- x 1 , x 2 and x are mass fractions of component A Docsity.com
Assumptions:
- w 1 is constant
- x 2 = constant = 1 (stream 2 is pure A)
- Perfect mixing in the tank
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:
- Controlled variable (or “output variable”): x
- Manipulated variable (or “input variable”): w 2
- Disturbance variable (or “load variable”): x Docsity.com
- Equation 1-3 is the design equation for the blending system.
- If our assumptions are correct, then this value of will keep at. But what if conditions change?
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
Some Possible Control Strategies:
Method 1. Measure x and adjust w 2.
- Intuitively, if x is too high, we should reduce w 2 ;
w 2
- Manual control vs. automatic control
- Proportional feedback control law,
w 2 (^) ( ) t (^) = w 2 (^) + K (^) c xSP − x t ( ) (1-4)
- where Kc is called the controller gain. 2. w 2 ( t ) and x ( t ) denote variables that change with time t.
- The change in the flow rate, is proportional to the deviation from the set point, xSP – x ( t ).
w 2 (^) ( ) t − w 2 ,
Method 2. Measure x 1 and adjust w 2.
- Thus, if x 1 is greater than , we would decrease w 2 so that
- One approach : Consider Eq. (1-3) and replace and with x 1 ( t ) and w 2 ( t ) to get a control law:
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
Feedback Control:
- Distinguishing feature: measure the controlled variable
- It is important to make a distinction between negative feedback and positive feedback.
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
Justification of Process Control
Specific Objectives of Control
- Increased product throughput
- Increased yield of higher valued products
- Decreased energy consumption
- Decreased pollution
- Decreased off-spec product
- Increased Safety
- Extended life of equipment
- Improved Operability
- Decreased production labor
3.2 Economic Incentives - Advanced
Control
