process operability class materials process flexibility copyright © thomas marlin 2013 the...

Process Operability Class MaterialsProcess Flexibility

Copyright © Thomas Marlin 2013The copyright holder provides a royalty-free license for use of this material at non-profit

educational institutions

FC1

LC1

FC

1

TC

1

TC

2

T

10

T

12

T

11

T

13

fuel

LC

1

L2

LAHLAL

F

4

Basic flowsheet Design with Operability

In this Lesson, we will learn

• Why do we need flexibility in a design?

- Distillation

• Deciding what to achieve (control)

- Principles:Control Objectives

- Example: Bioreactor

• Locating the flexibility: how many and where?

- Principles: Degrees of freedom and Controllability

- Blending, CSTR, heat exchange, bioreactor

Key Operability issues

1. Operating window

2. Flexibility/ controllability

3. Reliability

4. Safety & equipment protection

5. Efficiency & profitability

6. Operation during transitions

7. Dynamic Performance

8. Monitoring & diagnosis

PROCESS OPERABILITY: FLEXIBILITY


1. Operating window


3. Reliability






FLEXIBILITY

Without flexibility, the process

• Does not respond to changes in set points

• Responds to all disturbances that change product qualities, production rates and can lead to unsafe operation!

With flexibility, the process

• Achieves set points

• Compensates for all disturbances so that product qualities, production rates and safety are achieved.

We need to “steer” the process

unacceptabl

e

Why flexibility?

FLEXIBILITY

Flexibility enables us to adjust the plant operation after the equipment has been designed. It requires spare capacity in selected equipment and extra equipment to adjust operation.

• Spare capacity in pumps, valve, heat exchangers, vessels, motor speed, etc.

• Additional equipment includes pipes and valves

- Adjust flows (especially utility) to equipment; utility = cooling water, steam, fuel, air,

nitrogen, hydrogen, ….

- Enable flow to (partially) by-pass equipment


1. Operating window


3. Reliability






Why flexibility?

FLEXIBILITY

FV

We have designed for an operating window. Now we must move around in it to achieve the desired point. What equipment must we add to the distillation tower?

What defines a “point”• Feed flow rate• Pressure• Levels• Distillate composition• Bottoms composition

What uncertainty exists• L-V equil, heat

transfer, flow, etc.

What is adjusted• ??• ??• ??


1. Operating window


3. Reliability






What disturbances occur?• Feed composition,

enthalpy, & rate• CW temperature• Reboiler temperature

Why flexibility?

FLEXIBILITY

What equipment must we add to the distillation tower?

FV

We add a valve to every adjustable flow.

We could have alternative feed trays, with manual valves used to change the tray.

Naturally, the equipment must have capacity


1. Operating window


3. Reliability






Why flexibility?

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

F1

F2 F3

T1 T2

T3

T5

T4

T6 P1

L1

A1

L. Key

Give example

FLEXIBILITY

1. Safety

2. Environmental Protection

3. Equipment protection

4. Smooth operation production rate

5. Product quality

6. High profit


How do we decide what to control?

See Chapter 2 of Marlin (2000) for solution


1. Operating window


3. Reliability






What to achieve?


1. Operating window


3. Reliability






FLEXIBILITY

Class Workshop: We are designing a batch bioreactor. Define the the control objectives, specifically the variables to be controlled.

What to achieve?

time

Variables to satisfy desired trajectory

FLEXIBILITY

How much - We provide capacity to achieve an operating window with specified ”size”; see Operating Window topic.

• How many – How many flexible items are needed?

• Where - We need flexibility (adjustable variables) that influence the operating variables that define the point we want to achieve.

We can check a point using a flowsheeting program. We can determine which manipulated variables change and by how much. But, this takes lots of time to check many points.


1. Operating window


3. Reliability






How do we decide what to manipulate?

Flexibility: where & how many?

FLEXIBILITY – How many?

DEGREES OF FREEDOM

How do we determine the maximum number of variables that be controlled in a process?

How do we determine the minimum number of adjustable variables to achieve desired values for specified variables?

v1

Hot Oil

v2

v3

L1

v7

v5 v6

Hot Oil

F1 T1 T3

T2

F2

T4T5

F3 T6

T8

F4

L2

v8

T7

P1F5

F6T9


1. Operating window


3. Reliability







FLEXIBILITY – How many?

DEGREES OF FREEDOM

A requirement for a successful design is:The number of valves (adjustable variables)

number of variables to be achieved (controlled)


1. Operating window


3. Reliability






v1

Hot Oil

v2

v3

L1

v7

v5 v6

Hot Oil

F1 T1 T3

T2

F2

T4T5

F3 T6

T8

F4

L2

v8

T7

P1F5

F6T9

v4


FLEXIBILITY – Where?We need independent causal relationships between the adjusted and controlled variables. Remember, interaction can exist, but desired points must be able to be achieved. See three cases from Process Control.

Two drivers can achieve independent positions without interaction

Connected by spring

Two drivers can achieve independent positions with interaction

Connected by beam

Two drivers cannot achieve independent positions. They are “linked”

Independent Interaction Linearly dependent


1. Operating window


3. Reliability







FLEXIBILITY – Where?

CONTROLLABILITY: A system is controllable if its CVs can be maintained at the set points, in the steady-state, in spite of disturbances entering the system.

DK

K

MV

MV

KK

KK

CV

CV

d

d

2

1

2

1

2221

1211

2

1

0

0

Model for 2x2 system in deviation variables

A system is controllable when the matrix of process gains can be inverted, i.e., when

the determinant of K 0.


1. Operating window


3. Reliability







FLEXIBILITY

Controllability Class Workshop: Can we achieve desired blended flow and composition by adjusting the valves?

FA, xA

FS, xAS = 0FM, xAM

Blending Process

Total flow and composition


1. Operating window


3. Reliability







FLEXIBILITY

FA, xA

FS, xAS = 0FM, xAM

Blending Process

'S

ssAs

A'A

ssAs

SAM

SAM

F)FF(

FF

)FF(

F'x

'F'F'F

22

0)()(

)(22

SA

S

SA

A

FF

F

FF

FKDet

Yes, this system is controllable!

Connected by spring

Controllability Class Workshop: Can we achieve desired blended flow and composition by adjusting the valves?


1. Operating window


3. Reliability







FLEXIBILITY

A B + 2C-rA = k0 e -E/RT CA

A

ACB

CC

v1

v2

Controllability Class Workshop: Can we achieve desired values for the sensors by adjusting the valves?

Non-isothermal Chemical Reactor

Pure A feed


1. Operating window


3. Reliability







FLEXIBILITY

A B + 2C-rA = k0 e -E/RT CA

A

ACB

CC

v1

v2

2

1

1211

1211

220

0

MV

MV

KK

KK

C

C

C

B

Det (K) = 0; No! The system is not controllable!

Connected by beam

Controllability Class Workshop: Can we achieve desired values for the sensors by adjusting the valves?


1. Operating window


3. Reliability







FLEXIBILITY

Goal: Maintain cold effluent Tcold at a desired value

Stream A(cold)

Stream B(hot)

Freedom to adjust flows

Stream A Stream B

1. Constant Adjustable

2. Adjustable Constant

3. Constant Constant

Controllability Class Workshop: Add flexibility to the heat exchanger to achieve the goal for three different scenarios.

T


1. Operating window


3. Reliability







FLEXIBILITY Freedom to adjust flows

Stream A Stream B

1. Constant Adjustable

2. Adjustable Constant

3. Constant ConstantStream A(cold)

Stream B(hot)

TC1

Stream A(cold)

Stream B(hot)

TC

3

Stream A(cold)

Stream B(hot)

TC

2

It is not typical to adjust a stream flow to control its temperature; if the temperature is important, likely the flow rate is also important. But, this design will function.


1. Operating window


3. Reliability







FLEXIBILITY

Controllability Class Workshop: Add flexibility to the heat exchanger to achieve the goals.

Goals: Maintain cold effluent at Tcold and Maintain hot effluent at Thot

T

Stream A(cold)

Stream B(hot)

T


1. Operating window


3. Reliability







FLEXIBILITYGoals: Maintain cold effluent at Tcold and Maintain hot effluent at Thot

TStream A(cold)

Stream B(hot)

T)(

)(

CinCoutpColdCold

HinHoutpHotHot

TTCFQ

TTCFQ

C

H

Energy balance on each stream

Equipment model with U= f(FH, FC)

lmTUAYQ )(

From an energy balance on entire system:

QHot = Qcold

It is not possible to satisfy both energy balances by adjusting the flows. (We would have to adjust the inlet temperatures, which would seem to defeat the purpose of the heat exchanger.)

Connected by beam


1. Operating window


3. Reliability








1. Operating window


3. Reliability






FLEXIBILITY

Flexibility Workshop: We heat a stream with several process streams, which recovers energy efficiently.

• What disturbances can occur?

• What set point changes can occur?

• No stream flow rate can be manipulated. What flexibility is needed to achieve the desired outlet temperature?

T



1. Operating window


3. Reliability






FLEXIBILITY

T

Disturbances:

• Fluid inlet temperatures

• Fluid inlet flow rates

Set points:

• Outlet temperature


Class Exercise: Add flexibility


1. Operating window


3. Reliability






FLEXIBILITY

TC

Flexibility: If the final process-fluid heat exchanger has a outlet temperature that is high enough to achieve the desired value, a by-pass could be used.



1. Operating window


3. Reliability






FLEXIBILITY

Flexibility: If the final process-fluid heat exchanger has a outlet temperature that is not high enough to achieve the desired value, an additional source is required; here the source is a fired heater.

TC

FT1

FT2

PT1

PIC1

AT1

TI1

TI2

TI3

TI4

PI2

PI3

PI4

TI5

TI6

TI7

TI8

TI9

FI3

TI10

TI11

PI5

PI6

P-27

P-28

fuelair


FLEXIBILITY

Adjust flows (especially utility)

utility = cooling water, steam, fuel, air, nitrogen, hydrogen, ….

disturbance To be achieved disturbance To be achieved

Generalization: From the heat exchanger examples, we see that flexibility can be achieved by adjusting utilities or in some cases, with a by-pass.

Enable flow to (partially) by-pass equipment


1. Operating window


3. Reliability








1. Operating window


3. Reliability






FLEXIBILITY

Class Workshop: We are designing a batch bioreactor. Define the flexibility required to achieve the control objectives from the previous workshop in this lesson.

What to achieve?

steam

Cooling water

air

Gas exit

substrateacid

base

FLEXIBILITY

INDUSTRIAL PRACTICE

1. Flexibility enables achieving points in the operating window.

2. The choice of adjustable equipment is based on principles and experience.

3. Controllability is often determined by qualitative analysis; however, flowsheeting can be used to see if the dependent variable values can be achieved by changing the selected adjustable variables.

4. Whether the adjustable variable is manipulated by process control or by a person depends on the response time required.


1. Operating window


3. Reliability






FLEXIBILITY

INDUSTRIAL PRACTICE

(1) Model Uncertainty(2) Disturbances(3) Set Point values(4) Production Variation

The plant “Design Specification” must include definitions of items (2) to (4)

The engineer must understand all items when items are significant and must be accommodated with extra capacity or improved sensor technology.

“Know where you are going”

(Remember for 4W04)


1. Operating window


3. Reliability






In this Lesson, we will learn

• Why do we need flexibility in a design?

- Distillation

• Deciding what to achieve (control)

- Principles:Control Objectives

- Example: Bioreactor

• Locating the flexibility: how many and where?

- Principles: Degrees of freedom and Controllability

- Blending, CSTR, heat exchange, bioreactor


1. Operating window


3. Reliability






PROCESS OPERABILITY: FLEXIBILITY

process operability class materials process flexibility copyright © thomas marlin 2013 the...

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