1. Chemical Engineering 3P04
Process Control
Tutorial # 11
Learning goals
Level Control – Getting stuff through the plant
Multiloop control – Controlling several variables at the same time

2. 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 LC F1 F2 F3
Feed

3. 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 LC F1
FC2 F3
Feed

4. 18.2 A process with two distillation towers is shown in Figure 18.2.
a. Identify all liquid inventory in the process.
Discuss advantages and disadvantages for each of the inventories.
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.
For every inventory, provide a recommended liquid volume and explain your recommendation.

5. Extension to Tutorial Question 18.2
Locate all vapor inventories in the process.
What type of process variable is measured and controlled to maintain the total vapor inventory constant?
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.
Are these controllers feedback or feedforward?

6. Level Control – Getting stuff through the plant
Let’s do interactive Learning Module exercises
18.2
18.6
18.13
18.14

7. Multiloop control – Controlling several variables at the same time
Vapor T6 P1
product T5 T1 T2
Feed F1 T4 T3 L1 F2 F3
Liquid A1
Process
Steam
product
fluid
L. Key

8. 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
What is required?

9. 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
A system is controllable when the matrix of process gains can be inverted, i.e., when the determinant of K  0.

10. Multiloop control – Controlling several variables at the same time
Let’s do interactive Learning Module exercises
20.14
20.15
20.16
20.17

11. Multiloop control – Controlling several variables at the same time
Please critique the design for the flash drum.
Vapor
product
PAH
TC-6
PC-1 T5 T1
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane T2
LAL
LAH
FC-1 T3
LC-1 F2 F3
Liquid
product
AC-1
Process
fluid
Steam
L. Key (ethane)

12. Multiloop control – Controlling several variables at the same time
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.
Can we control the CVs
with the valves
shown, i.e., is the
system controllable?
Det [KP] = 10-7  0

13. 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 T6 v2
sL1
Both v1 and v2 affect all CVs of interest through T6. This is a contraction that reduces the controllability.

14. Multiloop control – Controlling several variables at the same time
This is a good design for the flash drum.
cascade
Vapor
product
PAH
Split range
TC-6
PC-1 T5 T1
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane T2
LAL
LAH
FC-1 T3
LC-1 F2 F3
Liquid
product
AC-1
Process
fluid
Steam
L. Key

15. Multiloop control – Controlling several variables at the same time
Please critique the design for the chemical reactor.

16. Multiloop control – Controlling several variables at the same time
Do not control the same variable with two feedback controllers (at the same set point values).