Transcript Document

Chemical Engineering 3P04
Process Control
Tutorial # 11
Learning goals
1. Level Control – Getting stuff through the plant
2. Multiloop control – Controlling several variables
at the same time
Level Control – Getting stuff through
the plant
The textbook introduces “feed push” and “product pull” level
control designs. In this example, you will design a system that holds
F2 very close to a constant value and controls all levels. You will
achieve the design goals with feedback control only.
F0
Feed
F1
LC
F2
LC
LC
F3
Level Control – Getting stuff through
the plant
The textbook introduces “feed push” and “product pull” level
control designs. In this example, you will design a system that holds
F2 very close to a constant value and controls all levels. You will
achieve the design goals with feedback control only.
F0
Feed
F1
LC
FC2
LC
LC
F3
18.2
A process with two distillation towers is shown in
Figure 18.2.
a. Identify all liquid inventory in the process.
b. Discuss advantages and disadvantages for each of the
inventories.
c. Critique the type of level control, i.e., which variable is
adjusted to control the level, for each inventory. If not
acceptable, sketch changes and explain.
d. For every inventory, provide a recommended liquid
volume and explain your recommendation.
Extension to Tutorial Question 18.2
a. Locate all vapor inventories in the process.
b. What type of process variable is measured and controlled
to maintain the total vapor inventory constant?
c. For each vapor inventory identified in part (a), critique
the type of control, i.e., which variable is manipulated by
each controller. If not acceptable, sketch changes and
explain.
d. Are these controllers feedback or feedforward?
Level Control – Getting stuff through
the plant
Let’s do interactive Learning Module exercises
18.2
18.6
18.13
18.14
Multiloop control – Controlling
several variables at the same time
T6
T1
T2
F1
T4
Vapor
product
P1
T5
Feed
F2
T3
L1
F3
A1
Process
fluid
Steam
L. Key
Liquid
product
Multiloop control – Controlling
several variables at the same time
CONTROLLABILITY
A system is controllable if its CVs can be maintained at
their set points, in the steady-state, in spite of disturbances
entering the system.
Model
for 2x2
system
 CV1  0  K11

 
CV2  0  K 21
K12   MV1   K d 1 

  K  D
K 22  MV2   d 2 
What is required?
Multiloop control – Controlling
several variables at the same time
CONTROLLABILITY
A system is controllable if its CVs can be maintained at
their set points, in the steady-state, in spite of disturbances
entering the system.
Model
for 2x2
system
 CV1  0  K11

 
CV2  0  K 21
K12   MV1   K d 1 

  K  D
K 22  MV2   d 2 
A system is controllable when the matrix of process gains can
be inverted, i.e., when the determinant of K  0.
Multiloop control – Controlling
several variables at the same time
Let’s do interactive Learning Module exercises
20.14
20.15
20.16
20.17
Multiloop control – Controlling
several variables at the same time
Please critique the
design for the flash
drum.
PAH
TC-6
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane
T1
T5
T2
LAL
LAH
FC-1
F2
PC-1
Vapor
product
T3
LC-1
F3
Liquid
product
AC-1
Process
fluid
Steam
L. Key (ethane)
Multiloop control – Controlling
several variables at the same time
• Can we control the CVs
with the valves
shown, i.e., is the
system controllable?
The effects of v1 and v2 on steady-state
gains are identical, within a constant
multiplier. Therefore, the five CVs cannot
be independently affected by the five
valves.
0
2.0
0
0   v1
 F1   0
 T 6   .0708
.85  .44
0
 .19  v 2
 

 
0
.043   v3
 A1    .00917  .11  .44
 
 P1   .567
6
.
80
1
.
39
0

5
.
36
  v 4

 
dL / dt   .0113  .136 .31  .179  .0265  v5
Det [KP] = 10-7  0
Multiloop control – Controlling
several variables at the same time
Boxes are cause-effect relationships (or
transfer functions)
Not a CV!
T2
v1
A1
Let’s select:
F1
production rate
T6
feed
vaporization
A1
product quality
P
safety
L
liquid to pump
P1
v2
T6
sL1
Both v1 and v2 affect all CVs of interest through T6.
This is a contraction that reduces the controllability.
Multiloop control – Controlling
several variables at the same time
This is a good design
for the flash drum.
cascade
PAH
Split range
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane
T1
PC-1
T5
T2
LAL
LAH
FC-1
F2
TC-6
Vapor
product
T3
LC-1
F3
AC-1
Process
fluid
Steam
L. Key
Liquid
product
Multiloop control – Controlling
several variables at the same time
FC
TC
Coolant effluent
Feed
Please critique the
design for the
chemical reactor.
TC
LC
FC
Coolant
Product
Multiloop control – Controlling
several variables at the same time
FC
TC
Coolant effluent
Feed
Do not control the same
variable with two
feedback controllers (at
the same set point
values).
TC
LC
FC
Coolant
Product