Heat Exchanger Networks& Utility Minimization

NMSU Chemical EngineeringCh E 452

Outline

• Heat Integration• Design Procedure for MUMNE

– Temperature interval diagram– Cascade diagram– Temperature-Enthalpy diagram– Minimum number of exchangers– Design above and below pinch

Heat Integration

• Heat exchange networks• It saves money to match streams rather than pay to

heat one and pay to cool another• You have already done this on ad hoc basis in design

projects

Heat Integration

• There is a rigorous methodology• We will learn MUMNE (Minimum Utility, Minimum Number of Exchangers) method

• Not necessarily (and unlikely to be) economic optimum

Design Procedure

1. Complete energy balance on all streams to determine all temperatures, values, and heat flows.

2. Choose minimum approach temperature. Typically, this is between 5°C and 20°C, but any positive number is valid.

3. Complete temperature interval diagram, Each stream is drawn and labeled. The heat flow in each interval is calculated.

pmC

Design Procedure

4. Complete the cascade diagram. The energy excess or deficit is calculated for each interval on the temperature interval diagram.

5. Find the minimum hot and cold utility requirements and identify the pinch temperature.

6. Complete the composite temperature enthalpy diagram. This is a T-Q diagram for the entire process.

Design Procedure

7. Determine the minimum number of heat exchangers required above and below the pinch.

8. Design the heat exchanger network.

Pinch Technology

• Advantages include:– simple, does not require elaborate mathematics– sets performance targets before actual design (minimum

required theoretical utility for entire process)– analysis provides network design by matching hot and cold

streams for heat integration– graphical representation (composite curve) used to

increase conceptual understanding of system– method table used to predict minimum utility

requirements

Application of Pinch Technology

• Design an integrated heat exchanger system with a minimum approach temperature of 10°F to minimize utility for the following six streams:

hot streams cold streamsstr A B C D E Fm 4000 10000 6000 6000 9000 6000 lb/hrCp 0.65 1.00 0.50 0.70 0.95 0.55 btu/lb°FTo 410 370 270 260 310 340 °FTf 350 290 250 300 370 390 °F

mCp 2600 10000 3000 4200 8550 3300 btu/hr°F

Example

• The heat flow values of Q (or ΔH) are calculated from the energy balance. The sign convention is positive for heat available from a stream and negative for heat needed by a stream.

• Choose the minimum approach temperature. For this problem, it is 10°C.

Example

• Draw and label the temperature interval diagram. Label the intervals beginning with “A” for the highest temperature interval. The heat flow for each interval is calculated from

• where the sum is over all streams existing in that interval.

pQ mC T

Temperature Interval DiagramPrepare the composite curves, distinguishing interval of temperature where streams influent/effluent temperatures begin/end 1

2

3

4

5

1

2

3

4

5

A B C F E D

400°F

350°F

300°F

250°F

Temperature Interval Diagram

hrBtu000,104H

F410370

FhrlbBtu65.0

hrlb000,4H

TcmH

1

1

11,p11

TcmH p

Treating all Hot streams and all Cold streams together, determine heat flow in each interval 1

2

3

4

5

1

2

3

4

5

A B C F E D

400°F

350°F

300°F

250°F

000,104H1

Temperature Interval Diagram

000,252H

370350

0.1000,10

65.0000,4H

TcmcmH

2

2

22,p21,p12

1

2

3

4

5

1

2

3

4

5

000,252H2

000,600H3

0H4

000,60H5

000,66H1

500,355H2

500,256H3

0H4

000,168H5

A B C F E D

400°F

350°F

300°F

250°F

000,104H1

TcmH p

Treating all Hot streams and all Cold streams together, determine heat flow in each interval

CompositeCurves

250

260

270

280

290

300

310

320

330

340

350

360

370

380

390

400

410

420

0.E+00 2.E+05 4.E+05 6.E+05 8.E+05 1.E+06 1.E+06

H

tem

per

atu

re (

°F)

hot

cold PinchPoint

Qhot

Qcold

only reject heat below the pinch

point

only reject heat below the pinch

point

Define pinch point as temperature approach

only add heat above the

pinch point

only add heat above the

pinch point

Never transfer heat across the pinch point.

Method Table

• draw hot/cold temperature scales offset by Tmin • plot stream temperatures on appropriate scales• determine temperature intervals• divide stream’s temperature change into intervals

based on supply & target temperatures for each stream

Method Table

400°F

350°F

300°F

250°F

400°F

350°F

300°F

250°F

2

1

3

4

5

6

7

8

9

A B C F E Dint T(°F) mcc - mch

Hicumulative

1 10 -2.60 -26.0 -26.0

2 20 +0.70 +14.0 -12.0

3 10 +9.25 +92.5 +80.5

4 20 -0.75 -15.0 -15.0

5 30 -1.45 -43.5 -58.5

6 10 -10.00 -100.0 -158.5

7 20 -5.80 -116.0 -274.5

8 20 +4.20 +84.0 -190.5

9 20 -2.50 -50.0 -250.5

pinchpoint

heat surplus may be transferred to the lower temperature interval

HOT Heat Cascade COLD

410 400

-26000

400 -26000 390

14000

380 -12000 370

-80500 92500

370 0 360

-15000

350 -15000 340

-43500

320 -58500 310

-100000

310 -158500 300

-116000

290 -274500 280

84000

270 -190500 260

-60000 250500

250 0 240

Heat Cascade

Qhot

Qcold

pinchpoint

Minimum Energy Network Design

400°C

350°C

300°C

250°C

400°C

350°C

300°C

250°C

A B C D E F(x 10-3 Btu/hr°F)

mcp

2.6 10 3.0 4.2 8.55 3.3

400°C

350°C

300°C

250°C

400°C

350°C

300°C

250°C

hrBtu105.85

108550

TcmH

3

p,E

hrBtu105.85

108550

TcmH

3

p,E

(x 10-3 Btu/hr°F)

mcp

85.585.5

F9.402T

370T2600105.85

TcmH

3

p,A

F9.402T

370T2600105.85

TcmH

3

p,A

402.9°F

A B C D E F2.6 10 3.0 4.2 8.55 3.3

Minimum Energy Network Design

Minimum Energy Network Design

400°C

350°C

300°C

250°C

400°C

350°C

300°C

250°C

hrBtu105.18

9.4024102600

TcmH

3

p,A

hrBtu105.18

9.4024102600

TcmH

3

p,A

(x 10-3 Btu/hr°F)

mcp

402.9°F18.518.5

A B C D E F2.6 10 3.0 4.2 8.55 3.3

Minimum Energy Network Design

400°C

350°C

300°C

250°C

400°C

350°C

300°C

250°C

F6.365

360T3300105.18

TcmH

3

p,F

F6.365

360T3300105.18

TcmH

3

p,F

(x 10-3 Btu/hr°F)

mcp

402.9°F18.5

365.6°F

A B C D E F2.6 10 3.0 4.2 8.55 3.3

Minimum Energy Network Design

400°C

350°C

300°C

250°C

400°C

350°C

300°C

250°C

hrBtu105.80

6.3653903300

TcmH

3

p,F

hrBtu105.80

6.3653903300

TcmH

3

p,F

(x 10-3 Btu/hr°F)

mcp

402.9°F

QQHH = 80.5 = 80.5366.6°F

A B C D E F2.6 10 3.0 4.2 8.55 3.3

Minimum Energy Network Design

400°C

350°C

300°C

250°C

400°C

350°C

300°C

250°C

(x 10-3 Btu/hr°F)

mcp

402.9°F

A B C D E F2.6 10 3.0 4.2 8.55 3.3

QQHH = 80.5 = 80.5366.6°F

66.052.0

427.5

168

800.0

50.0

If we do this, stream B will no longerhave heat available at a sufficiently high temperature to supply stream E

If we do this, stream B will no longerhave heat available at a sufficiently high temperature to supply stream E

14.0

Minimum Energy Network Design

400°C

350°C

300°C

250°C

400°C

350°C

300°C

250°C

(x 10-3 Btu/hr°F)

mcp

402.9°F

A B C D E F2.6 10 3.0 4.2 8.55 3.3

QQHH = 80.5 = 80.5366.6°F

66.052.0

427.5

168

800.0

50.0

If we do this, stream B will no longerhave heat available at a sufficiently high temperature to supply stream F

If we do this, stream B will no longerhave heat available at a sufficiently high temperature to supply stream F

14.0

Minimum Energy Network Design

400°C

350°C

300°C

250°C

400°C

350°C

300°C

250°C

(x 10-3 Btu/hr°F)

mcp

402.9°F

A B C D E F2.6 10 3.0 4.2 8.55 3.3

QQHH = 80.5 = 80.5366.6°F

66.052.0

427.5

168

50.0

Split streams B and F into two streams to prevent violation of the second law

Split streams B and F into two streams to prevent violation of the second law

14.0

360.3°F

344.4°F

800.0

Minimum Energy Network Design

400°C

350°C

300°C

250°C

400°C

350°C

300°C

250°C

(x 10-3 Btu/hr°F)

mcp

402.9°F

A B C D E F2.6 10 3.0 4.2 8.55 3.3

QQHH = 80.5 = 80.5366.6°F

66.052.0

427.5

168

116

50.0

14.0

360.3°F

344.4°F

684

QQCC = 50.0 = 50.0

QQCC = 190.5 = 190.5

QQCtotalCtotal = 240.5 = 240.5

320.0°F

Minimum Energy Network Design

400°C

350°C

300°C

250°C

400°C

350°C

300°C

250°C

402.9°F

A B C D E F

366.6°F

360.3°F

344.4°F

320.0°FA – F, E, FB1 – F, UB2 – E, D, UC – UD – B2E – B2, AF – B1/A, A, U

A – F, E, FB1 – F, UB2 – E, D, UC – UD – B2E – B2, AF – B1/A, A, U

Minimum Energy Network Design

18.518.5 85.585.5 52.052.0

1414

427.5427.5

168168

A4

10

40

2.9

37

0

35

0

D2

60

30

0

37

0

B

36

03

20

30

0

29

0

E

31

0

36

0

37

0

F

C

27

0

25

0

36

0

34

0

36

5

39

0

A – F, E, FB1 – F, UB2 – E, D, UC – UD – B2E – B2, AF – B1/A, A, U

A – F, E, FB1 – F, UB2 – E, D, UC – UD – B2E – B2, AF – B1/A, A, U