OPerability Dynamics 2007

8/3/2019 OPerability Dynamics 2007

1/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

In this Lesson, we will learn

Importance of process & equipment design

- Review effect of process dynamics on control performance

- Special concerns with recycle processes- Special considerations for capacity of manipulated variables

- Mixing process, series packed bed reactor, recycle

reactor, batch reactor

Control Design Guidelines

- Nine-step design method

- Flash process

PROCESS OPERABILITY:

DYNAMIC PERFORMANCE


2/35

DYNAMIC PERFORMANCE

OPERABILITY provides the capacity and flexibility

to respond to changes in plant operation.

In general, we wish to respond quickly.

Compensate for (reject) disturbances

Followed changes set points

The value of quickly depends upon the specific

process application.

Safety and Equipment protection - extremely fast (& reliable) Product quality - very fast

Production rate - moderate (very fast if load following)

Efficiency and Optimization - can be slower

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

Which elements in the process design should be fast,

and which should be slow?

Process - Dynamics


3/35

DYNAMIC PERFORMANCE

Lets recall that the process dynamics introduce the

fundamental limit to feedback control performance.

Dead time in

feedback,

Time constant in

feedback, P

Time constant indisturbance, D

AC

Steady-state Gain, KP

How does each parameter affect the dynamic performance

for this simple mixing process?

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

Process - Dynamics


4/35

DYNAMIC PERFORMANCE



0 5 10 15 20 25 30 35 40 45 500

0.5

1

1.5S-LOOP plots deviation variables (IAE = 9.6759)

Time

ControlledV

ariable

0 5 10 15 20 25 30 35 40 45 500

0.5

1

1.5

Time

ManipulatedVariable

CV limited set point overshoot, fast damping,

and return to the set point

CV does not change because of dead

time

Set point

response

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

Process - Dynamics


5/35

DYNAMIC PERFORMANCE

Lets recall that the process dynamics introduce thefundamental limit to feedback control performance.

Disturbance

response

, deadtime

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

Can not prevent

this deviation with

feedback

Effect of disturbance

without control

Process - Dynamics


6/35

DYNAMIC PERFORMANCE



Dead time in

feedback,

Time constant in

feedback, P

Time constant in

disturbance, DA

Steady-state

Gain, KP

How does each parameter affect feedback performance?

Large is goodSmall is good

Small is good

Large is good

Large (range),fast element

is good

Fast sensor

is good

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

Process - Dynamics


7/35

DYNAMIC PERFORMANCE

CLASS WORKSHOP: The feedback controller is

performing well (its well tuned), but the dynamic

performance is poor. What improvements can we

make?

AC

1

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

Hint: Think about the

structure of

(i) the process and

(ii) the control system

Control a chemical reactor

by adjusting its preheat

Disturbances

in feed

Process - Dynamics


8/35

DYNAMIC PERFORMANCE

CLASS WORKSHOP: The feedback controller is performing well but the

dynamic performance is poor. We must change the system structure!

AC

1

AC+

Feedforward

TC

CascadeAC

Faster feedback

LC

Slower

disturbances

Reduced

disturbances

Shorten pipe, faster

feedback

Process - Dynamics


9/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE

Recycle processes: In some (many), specific components do not appear in

exit streams. If no action is taken, these components will accumulate.What is needed in the process design?

Recycle loop

Purge

Process - Recycle


10/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE

In some (many) recycle processes, specific components do not appear in

exit streams. If no action is taken, these components will accumulate.What is needed in the process design?

Recycle loop

Purge

Issues with purges

Some valuable materials will be lost in purge (recover?)

May introduce stream that requires treatment before release (environment)

The purge flow can be adjusted to control the concentration of inert

Large inert concentrations might be desired (e.g., to affect reactions)

Process - Recycle


11/35

DYNAMIC PERFORMANCE

FC

1

Cold

feed

Hot effluent

Cold productIs the

design

complete?

Heating

fluid

Exothermic

chemical

reaction

Class Workshop Recycle Processes: How will this recycle

process respond to disturbances? Can the design beimproved?

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

Consider a

feed

temperature

increase

Process - Recycle


12/35

DYNAMIC PERFORMANCE

FC

1

Cold

feed

Hot effluent

Cold product

Heating

fluid

Exothermic

chemical

reaction

++ +

++

++

++ ++

++++

++++

The disturbance

grows because

of the recycle

(positive

feedback)!

++++

++++

++++++++

++++++++

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

Consider a

feed

temperature

increase



Process - Recycle

P R l


13/35

DYNAMIC PERFORMANCE

FC

1

Cold

feed

Hot effluent

Cold product

Heating

fluid

Exothermic

chemical

reaction

Conceptually

OK.

Is the feedbackfast enough?

TC

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis



Process - Recycle

P R l


14/35

DYNAMIC PERFORMANCE

FC

1

Cold

feed

Hot effluent

Cold product

Heating

fluid

Exothermic

chemical

reaction

Provide fastfeedback!

TC

Build in

efficiency;

use the least

heating

fluid!

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis



Process - Recycle

P R l


15/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE

General principle: When possible, provide an alternative source of

material or energy at the recycle point and control the variable(flow or temperature) after the recycle point.

Process

TC

Adjustable heat exchange

Process

FC

Adjustablefresh feed

Energy recycle

Material recycle

Process - Recycle

Process MV range


16/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE

We usually want extra capacity in the effects of the

manipulated variables. Here,extra means more

than required to achieve the desired steady-state

operating window.

F

L

CW

P

fc

fo

fo

fo

TReactor withexothermic

reaction

Lets concentrate on the

control of temperature.

Process MV range

Process MV range


17/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


manipulated variables. (1) MV overshoot

F

L

CW

PT

0 10 20 30 40 50 600

0.5

1

1.5

Time

Contro

lledVariable

0 10 20 30 40 50 600

0.5

1

1.5

Time

ManipulatedVariable

p, feedback dead time

For many processes, some overshoot of the manipulated variable

will improve (speed) control performance.

Temperature

CW flow

Process MV range

Process MV range


18/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


manipulated variables. (2) Disturbance response

F

L

CW

PT

In a few processes, much greater MV capacity is required forresponse to disturbances.

For example, a highly exothermic reaction operating at too high a

temperature will tend to run away. A very large exchanger area

and CW flow rate could be required to prevent a hazard.

Process MV range

Process MV range


19/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


manipulated variables. (2) Disturbance response

In a few processes, much greater MV capacity is required for

response to disturbances, even a source and sink.

For example, the reactor might produce gas at some times and

condense gas at others times. Pressure control requires an

exhaust line (originally shown) and an inert gas feed line (added

for this figure).

F

L

CW

PT

Inert gas

See Marlin (2000) Chapter 22 for control design

fo

fc

Process MV range

Control Design


20/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE

Process Control Design Guidelines

For Multiloop PID Feedback Control

1. Define Control Objectives

2. Select measurements

3. Ensure sufficient degrees of freedom exist

4. Ensure the process is controllable

5. Ensure that the operating window is sufficiently large

6. Evaluate interaction

7. Eliminate designs with poor integrity

8. Eliminate designs requiring extensive retuning

9. Apply loop pairing guidelines and unit operation

experience

Control Design

Control Design


21/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


Process ExampleFlash Process

Feed

Methane

Ethane (LK)

Propane

Butane

Pentane

Vapor

product

Liquid

product

Process

fluid

Steam

F1

F2 F3

T1 T2

T3

T5

T4

T6 P1

L1

A1

L. Key

P 1000 kPaT 298 K

Lets review the process

Control Design

Control Design


22/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


1. Define the Control ObjectivesTITLE: Flash drum |ORGANIZATION: McMaster Chemical EngineeringPROCESS UNIT: Hamilton chemical plant |DESIGNER: I. M. LearningDRAWING : Figure 24-1 |ORIGINAL DATE: January 1, 1993

|REVISION No. 1____________________________CONTROL OBJECTIVES:1) SAFETY OF PERSONNEL

a) the maximum pressure of 1200 kPa must not be exceeded under any (conceivable)circumstances

2) ENVIRONMENTAL PROTECTIONa) material must not be vented to the atmosphere under any circumstances

3) EQUIPMENT PROTECTION (see also the safety objective)a) the flow through the pump should always be greater than or equal to a minimum

4) SMOOTH, EASY OPERATIONa) the feed flow should have small variabilityb) the unstable level should be maintained within measurement limitsc) unusual operations should be highlighted (high pressure, high or low level)

5) PRODUCT QUALITYa) the steady-state value of the ethane in the liquid product should maintained atits target of 10 mole% for operating condition changes of +20 to -25% feed flow, 5

mole% changes in the ethane and propane in the feed, and -10 to +50 C in the feedtemperature.

b) the ethane in the liquid product should not deviate more than 1 mole % fromits set point during transient responses for the following disturbances

i) the feed temperature experiences a step from 0 to 30 Cii) the feed composition experiences steps of +5 mole% ethane and -5 mole% ofpropaneiii) the feed flow set point changes 5% in a step

6) EFFICIENCY AND OPTIMIZATIONa) the feed flow rate will vary +10% and -20% from the design (nominal) valueb) the heat transferred should be maximized from the process integration exchangerbefore using the more expensive steam utility exchanger

7) MONITORING AND DIAGNOSISa) sensors and displays needed to monitor the normal and upset conditions of theunit must be provided to the plant operator

b) sensors and calculated variables should be provided for longer term monitoringof the product quality and thermal efficiency of the unit

Control Design

Control Design


23/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


2. Select measurements

Required to achieve each of the control objectives

Define the sensor technology (e.g., orifice meter,venturi meter, mass flow) compatible with process

conditions and achieve required accuracy and

reproducibility

Determine if for only local display or fortransmission to centralized control for monitoring,

history and control

Define range (e.g., 150-200 C)

Control Design

Control Design


24/35

DYNAMIC PERFORMANCE

CLASS WORKSHOP on CV selection: We want to know the composition of thebottoms product, but the analyzer is too expensive (or doesnt exist). What do we do?

LC

PC

TC

Inferential/Dominant Variables* -

Easily measured and strongly

influence product quality and/or

profit.

In many (not all) distillation towers,

tray temperatures are a good

inference of product composition.

(The profile moves up/down the

column)

Great opportunity to use your

process knowledge!

* See Marlin (2000) Chapter 17 for further discussion on inferential variables

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

LC

Control Design

Control Design


25/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


3. Ensure sufficient degrees of freedom

Degrees of freedom are adjustable variables, i.e., valve

openings, motor speeds, etc.

Inputs Outputs

finalelement sensor

final

elementsensor

Adjustable

manipulated

variables

Disturbances

variables

Controlled

variables,

related to

control

objectives

Majority of

variables

remain

unmeasured

(and

uncontrolled)

Process

Control Design

Control Design


26/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


3. Ensure sufficient degrees of freedom

Inputs Outputs

finalelement sensor

final

elementsensor

Adjustable

manipulated

variables

Disturbances

variables

Controlled

variables,

related to

control

objectives

Majority of

variables

remain

unmeasured

(and

uncontrolled)

Process

The system is has sufficient degrees of freedom if

# of manipulated variables # of controlled variables

Is that all

that is

required?

g

Control Design


27/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


4. Ensure that the process is controllable

Inputs Outputs

final

elementsensor

final

elementsensor

Adjustable

manipulated

variables

Disturbances

variables

Controlled

variables,

related to

controlobjectives

Majority of

variables

remain

unmeasured

(anduncontrolled)

Process

Controllability ensures that the selected controlled variables can be

moved in desired direction by the manipulated variables

The system is controllable if Det [Kp] 0 (Kp is the gain matrix)

Kp

Is that all

that is

required?

g

Control Design


28/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


5. Ensure that the operating window is large enough tosatisfy the design specification, which gives

disturbances and set point changes.

-20

0

20

40

60

80

feedtempera

ture(C)

50 70 90 110 130 150 170 190

feed flow

Design

Minimum heating

Maximum

liquid

productvalve

opening

Maximum heating

valve opening

Ensures that the capacities of the manipulated variables are large enough to move the

process as needed.

g

Control Design


29/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


6. Evaluate Interaction: Does it exist?

A multivariable process has interaction when input

(manipulated) variables affect more than one output

(controlled) variable.

0 20 40 60 80 100 1200.98

0.982

0.984

0.986

0.988

0.99

Time (min)

XD

(molfrac)

0 20 40 60 80 100 1200.02

0.025

0.03

0.035

0.04

Time (min)

XB

(molfrac)

0 20 40 60 80 100 1201.12

1.125

1.13

1.135x 10

4

Time (min)

R

(mol/min)

0 20 40 60 80 100 1201.5613

1.5613

1.5614

1.5614

1.5615

1.5615x 10

4

Time (min)

V(mol/min)

Step change to reflux with constant reboiler

g

Control Design


30/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


7. Eliminate designs with poor integrity

8. Eliminate designs requiring extensive retuning

This topic requires some additional principles from process control.

See Marlin, Process Control, (2000), Chapter 20

Control Design


31/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE


9. Apply loop pairing guidelines

LOOP PAIRING QUALITATIVE GUIDELINES

CVi - MVj pairing that has strong effect (large Kp)

CVi - MVj pairing that has fast dynamics

CVi - MVj pairing that has large range (MV min to max)

CVi - MVj pairing with causal relationship that is (nearly)

independent of other loops, i.e., reduce interactions

If excess MVs exist, adjust MVj that has lowest cost; keephigh cost MV near zero

Often, these guidelines cannot all be satisfied. In some cases,

they must be violated to achieve good performance.

Control Design


32/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE

Feed

Methane

Ethane (LK)

Propane

Butane

Pentane

Vapor

product

Liquid

product

Process

fluid

Steam

F1

F2 F3

T1 T2

T3

T5

T4

T6 P1

L1

A1

L. Key

P 1000 kPaT 298 K

Class workshop: Design control loops (select CV-MV pairings) for the

flash process.

Control Design


33/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

DYNAMIC PERFORMANCE

Class workshop: Design control loops (select CV-MV pairings) for the

flash process.

Feed

Methane

Ethane (LK)

Propane

Butane

Pentane

Vapor

product

Liquid

productProcess

fluidSteam

FC-1

F2 F3

T1 T2

T3

T5

TC-6 PC-1

LC-1

AC-1L. Key

PAH

LAL

LAH

cascade

See Marlin (2000), Process Control, Chapter 24 for details

Control Design


34/35

DYNAMIC PERFORMANCE

INDUSTRIAL PRACTICE

Good performance through process design is more

reliable and preferred if the cost is acceptable.

We are guided by control objectives (safety,protection, smooth operation, product quality, and

profit)

We seek to provide good control with simple loop

pairing control strategies, if possible.

We use special knowledge for each unit operation

distillation, reactors, boilers, compressors,

evaporators, etc.

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipmentprotection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis


35/35

Key Operability

issues

1. Operating

window

2. Flexibility/

controllability

3. Reliability

4. Safety &

equipment

protection

5. Efficiency &

profitability

6. Operation

duringtransitions

7. Dynamic

Performance

8. Monitoring &

diagnosis

In this Lesson, we will learn

Importance of process & equipment design

- Review effect of process dynamics on control performance

- Special concerns with recycle processes- Special considerations for capacity of manipulated variables

- Mixing process, series packed bed reactor, recycle

reactor, batch reactor

Control Design Guidelines

- Nine-step design method

- Flash process

PROCESS OPERABILITY:

DYNAMIC PERFORMANCE

Documents

OPerability Dynamics 2007