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THRESHOLD PROBLEMHEAT EXCHANGER NETWORK DESIGN FOR MAXIMUM ENERGY

By Siti Shawalliah Idris, AMIChemE

CPE 633

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Recap : T-H diagram

No Integration – no overlapping of hot and coal stream

T

H

Steam

Cooling Water (CW)

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Recap : T-H diagram

Process Heat Recovery

T

H

Steam

CW

DTmin

QRec

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Recap : T-H diagram

What happpen if DTmin is reduced further?

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Further reduced to

(eq. DTmin = 14oC)

Results in :

No hot utility requirement

More process heat recovery

T

CW

QRec

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Further reduced to

(eq. DTmin = 10oC)

Results in :

No increase in Process Heat

Recovery(Process Heat Balance

determines utilities requirements

T

CW

DTmin reduced further

QRec

C

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Threshold Problems

There is a thresholdD

Tmin for maximum recover

T

H

CW

T

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Threshold Problems

As ΔT min is reduced, a point is reached where no hot utility is

required; At all lower values of ΔT min, the only utility needed is

constant cold utility.

The value of ΔT min at which one utility target falls to zero

is termed “ΔT threshold”

A situation where only one utility is required is called a

threshold problem.

If the composite curves are shifted further together, reducing

ΔT min further, this does not cause a further change in utilities

requirements, although it does mean that, if desired part of

the hot utility could be supplied at the low temperature end

of the problem or any intermediate value

T

CW

Utilities

(MW)

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Pinched Problems

T

H

CW

Steam

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Pinched Problems

As ΔT min is reduced, a pinched problems still requires both c

utilities even the value of DTmin is zero

T

HCW

Steam

Utilities

(MW)

CW

Steam

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DESIGN FOR THRESHOLD PROBLEM

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Low Threshold DTmin

Treat exactly like a pinched problem

T

CW

Utilities

(MW)

DTmin

CW

10oC

DTmin =

S

DTthr

Steam

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Example

Given: From a problem table

analysis

Qhmin = 215.66 kW

Qcmin = 0

 DTmin = 50oC

Which cold end matches are

essential?

3

5

7

2.38

1.58

1.32

1.60

1.60

CP

590o

C

471 oC

533 oC

430 oC

400 oC

400 oC

280 oC

4.13

2.62

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Perform matches as usual.

3

5

1

2

7

400 oC

200 oC

150 oC

200 oC

100 oC

2.38

1.58

1.32

1.60

1.60

CP

4

6

590 oC

471 oC

533 oC

430 oC

300 oC

400 oC

400 oC

280 oC 150 oC

4.13

2.62

1

428

368 oC

2

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An example of chemical processes – threshold problem : A

production

Exothermic reactor produces more heat; thus does not require h

hot utility.

T

CW

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Yes or No?

The question is…

Whether or not to remain Threshold Problem

Depends on the economy scale….

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For Example, Ammonia Production:

Generating High Pressure Steam (HP) turns the threshold pr

a pinched problem.

T

HCW

T

CW

HP St

Pinch

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Summary

Some problems exhibit a threshold – only hot or cold utility

True threshold problems have large temperature driving fo

pinch.

Most threshold problems turn out to be pinched problems a

multiple utilities used

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Threshold Problems

Working Example

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Threshold Plot – Energy Target Plot

At Dtmin = DTthr= 7oC, cold utility disappears Thresho

Utilities

(MW)

DTmin

Steam

CW

DTmin = 7oC

QHmin

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Design a maximum energy recovery network (DTmin =

1

2

3

4

80 oC

40 oC

60 oC

30 oC

20

40

80

36

CPPinch

180 oC

130 oC

100 oC

120 oC

QHmin

= 840Qcmin = 0

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1

2

3

80 oC

60 oC

20

40

80

36

CPPinch

180 oC

130 oC

100 oC

120 oC

QHmin

= 840Qcmin = 0

1

2

3H

840200

1800

2160

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Thank you for your attention