pinch analysis part 1- pinch and minimum utility usage

8/13/2019 PINCH ANALYSIS Part 1- Pinch and Minimum Utility Usage

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PART 1

PINCH AND MINIMUM

UTILITY USAGE


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TEMPERATURE-ENTHALPY

(T-H) DIAGRAMS

Assume one heat exchanger. These are

alternative representationsTH,in

TC,outTC,in

TH, out

Q

TH, out

TC,out

TH,in

TC,in

Q

Slopes are the

inverse of F*Cp.

(Recall that Q=F Cp T)

TC,inTC,out

TH,in TH, out

Q

T

H


3/29

T-H DIAGRAMS

Assume one heat exchanger and a heater

TH,in

TC,outTC,in

TH, out

TC,inTC,out

TH,in TH, out

Q

Q

TH, out

TC,out

TH,in

TC,in

Q

QH

QH

QHH

T

H


4/29

T-H DIAGRAMS

Assume one heat exchanger and a coolerTH,in

TC,outTC,in

TH, out

TC,inTC,out

TH,in TH, out

Q

Q

TH, out

TC,out

TH,in

TC,in

Q

QC

QC

QC

C

T

H


5/29

T-H DIAGRAMS

Two hot-one cold streamTH1,in

TC,out

TC,in

TH2,out

TC,inTC,out

TH2,in TH2, out

Q1

Q1

TH1,out

TC,out

TH1,in

TC,in

Q1

Q2

Q2

TH2,out

TH2,in

TH1,in TH1,out

Q2

TH2,in

TH2,out

Notice the vertical arrangementof heat transfer

T

H


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Streams under phase change

Liquid

Slope

change

Phase

change

Single component Multicomponent

TT

H H

We say this stream

has variable Cp


7/29

Piece-wise linear representation

H

T T

H


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Composite Curves

(T-H DIAGRAMS)

Remark: By constructing the composite curve we loose information on

the vertical ar rangement of heat transfer between streams

Obtained by lumping all the heat from different streams that are

at the same interval of temperature.

TT

H H


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Moving composite curves horizontally

TH1,in

TC,out

TC,in

TH1,out

Q1 Q2

TH2,out

TH2,in

Smallest T Smallest T

Cooling

Heating

TH1,in

TC,outTC,in

TH1,out

Q2

TH2,out

TH2,in

QHQ1

QC

TT

H H

Composite Curves

(T-H DIAGRAMS)


10/29

Moving the cold compositestreamto the right

Increases heating and cooling BYEXACTLY THE SAMEAMOUNT

Increases the smallest T

Decreases the area neededA=Q/(U* T )

Cooling

Heating

TH1,in

TC,outTC,in

TH1,out

Q2

TH2,out

TH2,in

QHQ1

QC

Smallest

T

Notice that for this simpleexample the smallest Ttakes place in the end of thecold stream

T

H

Composite Curves

(T-H DIAGRAMS)


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Cooling

Heating

I n general, the smallest T

can take place anywhere.

We call the temperature at

which this takes place THE

PINCH.

T

H

Composite Curves

(T-H DIAGRAMS)


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Cooling

Heating

From the energy point

of view it is then

convenient to move thecold stream to the left.

However, the area may

become too large.

To limit the area, weintroduce a minimum

approach Tmin

T

H

Composite Curves

(T-H DIAGRAMS)

Tminis also known as HRAT (Heat Recovery Approximation Temperature)


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GRAPHICAL PROCEDURE

Fix Tmin(HRAT)

Draw the hot composite curve and leave it fixed

Draw the cold composite curve in such a way that

the smallest temperature difference is equal to Tmin

The temperature at which T=Tminis the PINCH

The non-overlap on the right is the Minimum

Heating Utility and the non-overlap on the left is the

Minimum Cooling Utility


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EXAMPLE

Stream Type Supply T Target T H F*Cp

(oC) (oC) (MW) (MW oC-1)

Reactor 1 feed Cold 20 180 32.0 0.2

Reactor 1 product Hot 250 40 -31.5 0.15



T=140 0C

T=20 0C

H=27 MW

REACTOR 2

H=-30 MW

H=32 MW

REACTOR 1

H=-31.5 MW

T=230 0C

T=180 0C T=2500C

T=40 0C

T=200 0C

Tmin=10 oC

T=80 0C


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Hot Composite Curve

6 48 7.5

250

200

80

40FCp=0.15

FCp=0.15

250

200

80

40

31.5 30 H H


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Cold Composite Curve

230

180

140

20

32 27

230

180

140

20

24 1520H H


17/29

Pinch Diagram

Observation: The pinch is at the beginning of a cold stream or at the

beginning of a hot stream

230

180

140

20

7.5

250

200

80

40

10 51.5

T= TminPinch

H

The pinch is defined either as

- The cold temperature (140 o)

-The corresponding hot

temp(140 o+Tmin=150o)

-The average (145o)


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UTILITY COST vs. Tmin

Utility

Tmin

COST

TOTAL OVERLAP

PARTIAL OVERLAP

Note: There is a parti cular over lap that requires only cooling uti l i ty

T

T

H

H


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Special Overlap Cases

Overlap leads only to cooling utility

Different instances where the cold stream overlaps totally the

hot stream. Case where only heating utility

TOTAL

OVERLAP

PARTIAL

OVERLAP

H

H H H

T

T

T T

We prefer this arrangementeven if T>Tmin


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SUMMARY

The pinch point is a temperature.

Typically, it divides the temperature range

into two regions.

Heating utility can be used only above the

pinch and cooling utility only below it.


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PROBLEM TABLE

Composite curves are inconvenient. Thus

a method based on tables was developed.

STEPS:1. Divide the temperature range into intervals and

shift the cold temperature scale

2. Make a heat balance in each interval3. Cascade the heat surplus/deficit through the

intervals.

4. Add heat so that no deficit is cascaded


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PROBLEM TABLE

We now explain each step in detail using

our example

Stream Type Supply T Target T H F*Cp

(oC) (oC) (MW) (MW oC-1)

Reactor 1 feed Cold 20 180 32.0 0.2Reactor 1 product Hot 250 40 -31.5 0.15



Tmin=10 oC


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PROBLEM TABLE1. Divide the temperature range into intervals and shift the

cold temperature scale

Now one can make heat balances in each interval. Heat transfer within each interval is feasible.

250

40

200

20

180

140

Hot

streams

Cold

streams

230

Hot

streams

Cold

streams

80

250

40

200

30

190

150

240

80


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PROBLEM TABLE2. Make a heat balance in each interval.

Hot

streams

Cold

streams

Tinterval Hinterval Surplus/Deficit

10 1.5 Surplus

40 - 6.0 Deficit

10 1.0 Surplus

40 -4.0 Deficit

70 14.0 Surplus

40 -2.0 Deficit

10 - 2.0 Deficit

F Cp=0.15

F Cp=0.25

F Cp=0.2

F Cp=0.3

250

40

200

30

190

150

240

80


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PROBLEM TABLE3. Cascade the heat surplus through the intervals. That is,

we transfer to the intervals below every surplus/deficit.1.5

- 6.0

1.0

-4.0

14.0

- 2.0

-2.0

This interval has a

surplus. It should

transfer 1.5 to

interval 2.

1.5

1.5

- 6.0

-4.5

1.0

-3.5

-4.0

-7.5

14.0

6.5

-2.0

4.5

- 2.0

2.5

This interval has a

deficit. After using

the 1.5 cascaded it

transfers4.5 to

interval 3.

The largest deficit

transferred is -7.5.

Thus, 7.5 MW ofheat need to be

added on top to

prevent any deficit

to be transferred to

lower intervals

250

40

200

30

190

150

240

80


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PROBLEM TABLE4. Add heat so that no deficit is cascaded.

This is the

position of the

pinch

7.5

1.5

9.0

- 6.0

3.0

1.0

4.0

-4.0

0.0

14.0

14.0

-2.0

12.0

-2.0

10.0

This is the

minimum heating

utility

This is the

minimum cooling

utility

1.5

1.5

- 6.0

-4.5

1.0

-3.5

-4.0

-7.5

14.0

6.5

-2.0

4.5

-2.0

2.5

250

40

200

30

190

150

240

80

250

40

200

30

190

150

240

80


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If the heating utility is increased beyond 7.5 MW the cooling utility willincrease by the same amount

Heat is

transferred

across the pinch

7.5

1.5

9.0

- 6.0

3.0

1.0

4.0

-4.0

0.0

14.0

14.0

-2.0

12.0

-2.0

10.0

Heating utility is

larger than the

minimum

Cooling utility is

larger by the

same amount

7.5 +

1.5

9.0 +

- 6.0

3. 0 +

1.0

4. 0 +

-4.0

0. 0 +

14.0

14. 0 +

-2.0

12. 0 +

-2.0

10. 0 +

PROBLEM TABLE

250

40

200

30

190

150

240

80


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IMPORTANT CONCLUSION

Heat istransferred

across the pinch

Heating utility is

larger than the

minimum

Cooling utility islarger by the same

amount

7.5 +

1.5

9.0 +

- 6.0

3. 0 +

1.0

4. 0 +

-4.0

0. 0 +

14.0

14. 0 +

-2.0

12. 0 +

-2.0

10. 0 +

DO NOT TRANSFER

HEAT ACROSS THE

PINCH

THIS IS A GOLDEN RULE OF PINCHTECHNOLOGY.

WHEN THIS HAPPENS IN BADLY

INTEGRATED PLANTS THERE ARE

HEAT EXCHANGERS WHERE SUCH

TRANSFER ACROSS THE PINCHTAKES PLACE


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Multiple Utilities

These are theminimum values

of heating utility

needed at each

temperature

level.

7.5

1.5

9.0

- 6.0

3.0

1.0

4.0

-4.0

0.0

14.0

14.0

-2.0

12.0

-2.0

10 0

Heating utility at

the largest

temperature is

now zero.

0.0

1.5

1.5 + 4.5

- 6.0

0.0

1.0

1.0 + 3.0

-4.0

0.0

14.0

14.0

-2.0

12.0

-2.0

10.0

pinch analysis part 1- pinch and minimum utility usage

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