Lecture 7 - Hen i - Cc

LectureLectureLectureLecture 7777 –––– Heat exchangerHeat exchangerHeat exchangerHeat exchanger

network Inetwork Inetwork Inetwork I

Copyright@Dominic Foo H82PLD - Plant Design Lecture 7 - 2

TheTheTheThe ““““Onion diagramOnion diagramOnion diagramOnion diagram””””

Reactor

Separation &

recycle

Heat exchange

network

Utilities(Linnhoff et al., 1982)


Lecture outlineLecture outlineLecture outlineLecture outline

� Single heat transfer unit vs. a

network (multiple units)

� Heat transfer composite curves

� Hot & cold utility targets

� The famous name “pinch

analysis”


Recommended textsRecommended textsRecommended textsRecommended texts

� Smith, R. (2005). Chemical Process Design and Integration. New York: John Wiley & Sons (heat exchanger network synthesis).

� Linnhoff, B., Townsend, D. W., Boland, D., Hewitt, G. F., Thomas, B. E. A., Guy, A. R., & Marshall, R. H. (1982). A User Guide on Process Integration for the Efficient Use of Energy. Rugby: IChemE (latest edition by Ian Kemp).


Evolution of heat recoveryEvolution of heat recoveryEvolution of heat recoveryEvolution of heat recovery pinch analysispinch analysispinch analysispinch analysis

1970s Minimum hot and cold utility targeting

1990 Energy-area trade off (Supertargeting)

1984 Surface area targeting

1993 Distillation column integration

1993 Total site analysis

1983 The pinch design method (PDM)

1989 Heat integration for batch processes


Some important termsSome important termsSome important termsSome important terms

� Hot streams

� Stream to be cooled

� Sources of heat

� Cold streams

� Stream to be heated

� Sinks of heat

� Supply temp – initial T

� Target temp – final TSupply T Q

TSupply T

Hot stream

Target T

Cold stream

Target T


2 stream heat recovery2 stream heat recovery2 stream heat recovery2 stream heat recovery

–1240160Hot2

1411040Cold1

∆H

(MW)

Target temp, TT

(ºC)

Supply temp,

TS (ºC)TypeSteam

� Utility available for use: � Steam @ 180ºC

� Cooling water @ 20ºC

� Without heat recovery:� Heat Stream 1 with steam: 14 MW

� Cold Stream 2 with cooling water: 12 MW

� Comment: high energy cost!


2 stream heat recovery2 stream heat recovery2 stream heat recovery2 stream heat recovery

–1240160Hot2

1411040Cold1

∆H

(MW)

Target temp, TT

(ºC)

Supply temp,

TS (ºC)TypeSteam

∆Tmin = 10ºC

QHmin = 3

QCmin = 1

QREC = 11


Larger value ofLarger value ofLarger value ofLarger value of ∆∆∆∆TTTTminminminmin

� Important features:� Vertical shifting is prohibited

� Horizontal shifting is allowed

� Energy targets:� Minimum hot utility: QHmin

� Minimum cold utility: QCmin

QHmin = 4

QCmin = 2

QREC = 10

∆Tmin = 20ºC


A more complex problemA more complex problemA more complex problemA more complex problem

–30.0

27.0

–31.5

32.0

∆H

(MW)

0.2018020Cold1. Reactor 1 feed

0.1540250Hot2. Reactor 1 product

0.2580200Hot4. Reactor 2 product

0.30230140Cold3. Reactor 2 feed

Heat capacity flowrate, CP(MW.K-1)

Target temp,

TT (ºC)

Supply temp,

TS (ºC)

TypeSteam

Reactor 1Feed 1

20ºC

Reactor 2Feed 2

140ºC

Off gas, 40ºC

Product 1, 40ºC

180ºC

40ºC

250ºC

230ºC 200ºC Product 2

80ºC

Note: Heat capacity flowrate, CP = m.Cp

( )STpp

TTmCTmCQ −=∆=


Hot composite curveHot composite curveHot composite curveHot composite curve

H (MW)

T (ºC)

31.5 30.0

250

200

80

40

T (ºC)

48 7.5

250

200

80

40

6

61.5

H (MW)

61.5

CP = 0.15

CP = 0.25

CP = 0.15

CP = 0.15

CP = 0.4


Cold composite curveCold composite curveCold composite curveCold composite curve

CP = 0.2

CP = 0.3230

180

140

20

32 27

59

T (ºC)

H (MW)

T (ºC)

20 1524 H (MW)

59

CP = 0.2

CP =

0.5

230

180

140

20

CP = 0.3


Hot & cold composite curvesHot & cold composite curvesHot & cold composite curvesHot & cold composite curves

H (MW)

T (ºC)

QREC = 51.1 QHmin = 7.5QCmin = 10

∆Tmin = 10ºC

Pinch

230

20

250

40


With largerWith largerWith largerWith larger ∆∆∆∆TTTTminminminminQHmin = 7.5

QCmin = 10

∆Tmin = 10ºC

QHmin = 11.5

QCmin = 14

∆Tmin = 20ºC


∆∆∆∆TTTTminminminmin & economic trade& economic trade& economic trade& economic trade----offoffoffoff

H

T

H

T

H

T

∆Tmin

Cost

Total

Energy

CapitalminTUAQ ∆=


A note onA note onA note onA note on ∆∆∆∆TTTTminminminmin

� Care should be taken not to ignore practical constraints in setting ∆Tmin

� To achieve pure ∆Tmin requires heat exchangers that exhibit pure countercurrent flow

� Shell-and-tube: not possible � ∆Tmin < 10ºC should be avoided

� Plate heat exchanger: ∆Tmin ~ 5ºC or less


Working sessionWorking sessionWorking sessionWorking session

� Plot a heat transfer composite curves

� ∆Tmin = 10ºC

� Utility: � Steam @ 200ºC

� Cooling water @ 15ºC

1.5

4.0

3.0

2.0

Heat capacity flowrate CP

(kW/ºC)

60170Hot2

14080Cold3

30150Hot4

13520Cold1

∆H

(kW)

Target temp, TT (ºC)

Supply temp,

TS (ºC)TypeSteam

(Linnhoff et al., 1982)

230

-330

240

-180


H4

H2

C3 C1

Working sessionWorking sessionWorking sessionWorking session

160

20

100 700

140

200

120

500

80

300 400 600

40

60

100

180

0


SolutionsSolutionsSolutionsSolutionsQHmin = 20

QCmin = 60

∆Tmin = 10ºC


Additional taskAdditional taskAdditional taskAdditional task� Proof that the CP for composite curve is the

summation of CP of individual streams.

� Locate the new energy targets for the following value of ∆Tmin:

� 30ºC

� 50ºC


ResultsResultsResultsResults

∆Tmin = 30ºC

QHmin = 110

QCmin = 150


The heat recovery pinchThe heat recovery pinchThe heat recovery pinchThe heat recovery pinch

H (MW)

T (ºC)

∆Tmin

Pinch

QHmin

QCmin

Heat sink

Heat source


Possible heatingPossible heatingPossible heatingPossible heating

H (MW)

T (ºC)

Pinch

Heat source

Possible


Impossible heatingImpossible heatingImpossible heatingImpossible heating

H (MW)

T (ºC)

Pinch

Heat source

Impossible


Heat transfer across pinchHeat transfer across pinchHeat transfer across pinchHeat transfer across pinch

H (MW)

T (ºC)QHmin +XP

QCmin+XP

Heat sink

Heat source

XP


Cold above the pinchCold above the pinchCold above the pinchCold above the pinch

H (MW)

T (ºC)QHmin +XP

QCmin

Heat sink

Heat source XP


Heat below the pinchHeat below the pinchHeat below the pinchHeat below the pinch

H (MW)

T (ºC)QHmin

QCmin+XP

Heat sink

Heat source

XP


HeuristicHeuristicHeuristicHeuristic� To achieve energy targets, must not transfer heat

across the pinch by:

� Process-process heat transfer

� Inappropriate use of utility

DON’T COLD ABOVE THE PINCH!

DON’T HEAT BELOW THE PINCH!

Lecture 7 - Hen i - Cc

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