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RECAP

The pinch design method developed earlier followed several rules a

to allow design for minimum utility (or maximum energy recov

minimum number of units.

Occasionally, it appears not to be possible to create the appropriate

because one or other of the design criteria cannot be satisfied.

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GRID DIAGRAM ABOVE PINCH

Cold utility must not be used above the pinch,

which means that all hot streams must be

cooled to pinch temperature by heat

recovery.

There are three hot streams and two cold

streams

Thus, regardless of the CP values of the

streams, one of the hot streams cannot be

cooled to pinch temperature withoutsome

violation of the Tmin

constraint.

What do we do?

1

2

T>90oC

3

T>90oC

12

20

7

15

30

CP

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STREAM SPLITTING

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STREAM SPLITTING

Split a cold stream into two parallel

branches

Now each hot stream has a cold partner

with which to match, capable of

cooling it to pinch temperature.

Split ratio for stream 5:

as such CPC cannot be lower than 20

[CP inequalities for above pinch (CPHCPC)]

1

2

3

12

20

7

15

30

CP

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SPLITTING CRITERIA

In addition to CP inequalities criteria introduced earlier:

ABOVE PINCH: NHNC

NH = Number of hot streams

NC = number of cold streams

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GRID DIAGRAM BELOW PINCH

1

2

T

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STREAM SPLITTING BELOW PINCH

Split a hot stream into two parallelbranches

Now each cold stream has a hot partnerwith which to match, capable ofheating it to pinch temperature.

Split with stream with bigger CP value

Split ratio depends on the CP inequalitycriteria (CPHCPC)

1

2

T

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SPLITTING CRITERIA

In addition to CP inequalities criteria introduced earlier:

BELOW PINCH: NHNC

NH = Number of hot streams

NC = number of cold streams

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STREAM SPLITTING - EXAMPLE

Based on the CP inequalities

CPHCPCNHNC

5

4

1

2

30oC

3

100 oC

100 oC 40oC

5

4

7

CP

Pinch

90 oC 15oC

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STREAM SPLITTING - EXAMPLE

One solution

SPLIT COLD STREAM

CPH

CPCNHNC

5 4

4 3

1

2

30oC

3

100 oC

100 oC 40oC

5

4

7

CP

Pinch

90 oC 15oC

40oC

43.8oC

C

C

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STREAM SPLITTING SPLIT OPTIONS

What about other split options?

(i) (ii)

or or

CPHCPCNHNC

5 3.5 7

4 3.5

CPHCPCNHNC

5 4.5 7

4 2.5

5

4

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STREAM SPLITTING SPLIT OPTIONS

Save one unit by ticking off two streams at once:

1

2

30oC

3

100 oC

100 oC40

o

C

5

4

3.8

CP

Pinch

90 oC 15oC

43oC

40oC

C

C

3.2

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In general:

Branch flow rates = degree of freedom in design

Finding best choice = optimization problem

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SUMMARY

Above Pinch

CPHCPCfor Pinch

Matches?

Is

NHNC?

Stream dataat pinch

Split cold

stream

Split hot

streamPlace

matches

Yes Yes

No

No

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Below Pinch

CPHCPCfor Pinch

Matches?

Is

NHNC?

Stream dataat pinch

Split cold

stream

Split hot

streamPlace

matches

Yes Yes

No

No

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WORKING SESSIONSTREAM SPLITTING

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STREAM DATA

Stream Type Supply

temperature, oC

Target

temperature, oC

Hea

flow

1 Hot 750 350

2 Hot 550 250

3 Cold 300 900

4 Cold 200 550

DTmin = 50oC

QHmin = 9200 kW

QCmin = 6400 kW

Design a network for maximum energy recovery

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GRID DIAGRAM

1

2

3

4

350 oC

550 oC

300

o

C

200 oC

45

40

43

20

CP

Pinch

250 oC

550 oC

500 oC

500o

C900o

C

550 oC

750 oC