1. Hierarchy of Decisions

2. HEAT EXCHANGER NETWORK (HEN)

3. SUCCESSFUL APPLICATIONS
O ICI
---- Linnhoff, B. and Turner, J. A., Chem. Eng., Nov. 2, 1981
Energy savings Capital Cost
Available Expenditure
Process Facility* k$/yr or Saving, k$
Organic Bulk Chemical New same
Specialty Chemical New saving
Crude Unit Mod saving
Inorganic Bulk Chemical New saving
Specialty Chemical Mod
New saving
General Bulk Chemical New unclear
Inorganic Bulk Chemical New to unclear
Future Plant New to 40 % % saving
Specialty Chemical New
Unspecified Mod
New saving
General Chemical New unclear
Petrochemical Mod Phase I
Phase II
*New means new plant; Mod means plant modification.

4. Linnhoff and Vredeveld, CEP, July, 1984
SUCCESSFUL APPLICATIONS
Table 1. First results of applying the pinch technology in Union Carbide
Project Energy Cost Installed Payback
Process Type Reduction $/yr Capital Cost $ Months
Petro-Chemical Mod ,050, ,
Specialty Chemical Mod , ,
Specialty Chemical Mod , ,
Licensing Package New ,300, Savings 
Petro-Chemical Mod , Yet Unclear ?
Organic Bulk Mod ,000, ,
Chemical
Organic Bulk Mod ,243, ,835,
Specialty Chemical Mod , ,
Organic Bulk Mod ,000, ,
Linnhoff and Vredeveld, CEP, July, 1984

5. SUCESSFUL APPLICATIONS
 Fluor
--- IChE Symp. Ser., No. 74, 1982, P.19
--- CEP, July, 1983, P.33
 FMC (Marine Colloid Division, Rockland, ME)

6. CONCLUSION separate and distinct task in process design
HEN/MEN synthesis can be identified as a
separate and distinct task in process design

7. IDENTIFY HEAT RECOVERY AS A SEPARATE AND DISTINCT
TASK IN PROCESS DESIGN.
9.60
200C
18.2 bar 
1.089
36C
16 bar 
RECYCLE
REACTION
7.841
126C
18.7 bar TO
COLUMN
D 201 
1.614
0 179
200C 
PURGE CW
180C
153C
35C
7 703
FLASH
141C
40C
115.5C
17.3 bar
120C bar
FEED
5C 19.5 bar
114C 
Figure Flowsheet for “front end” of specialty chemicals process

8. FOR EACH STREAM: TINITIAL, TFINAL, H = f(T). 126C
Reactor
35C
200C
RECYCLE
 TOPS
Product
Purge
Reactor
35C
5C
FEED
FOR EACH STREAM: TINITIAL, TFINAL, H = f(T).
PRODUCT
126C
Figure 2.6-Specialty chemicals process-heat exchange duties

9. 。 。  = 1722  = 654 a ) DESIGN AS USUAL H 6 UNITS REACTOR C STEAM
 = 1722
 = 654
a ) DESIGN AS USUAL H
6 UNITS
REACTOR C
STEAM
RECYCLE 70 1
。 。
STEAM
1652 3 2
654
COOLING
WATER
FEED
PRODUCT

10. 。 。 。 。  = 1068  = 0 b ) DESIGN WITH TARGETS H 4 UNITS REACTOR C
 = 1068
 = 0
b ) DESIGN WITH TARGETS H
4 UNITS
REACTOR C
STEAM 。
RECYCLE
1068
。 。 1 。 2 3
FEED
PRODUCT

11. SUGGESTED PROCEDURE FOR THE DESIGN OF NEW HEAT EXCHANGER NETWORKS
1. Determine Targets.
 Energy Target -maximum recoverable energy
 Capital Target -minimum number of heat transfer units.
-minimum total heat transfer area
2. Generate Alternatives to Achieve Those Targets.
3. Modify the Alternatives Based on Practical Considerations.
4. Equipment Design and Costing for Each Alternative.
5. Select the Most Attractive Candidate.

12. STEP ONE
Determine the Targets

13. § ENERGY TARGETS (TWO STREAM HEAT EXCHANGE)  T/H DIAGRAMQ
=CP(TT-TS) TT TS H H
Figure Representation of process streams in the T/H diagram

14. TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM
200
UTILITY
HEATING
140
135
115
100
70
UTILITY
COOLING
350
300
400 H
(KW)
TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM

15. TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM
200
UTILITY
HEATING
130
135 T
120
100
70
UTILITY
COOLING
350
300
400 H
(KW)
= = =300
TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM

16. ( ) ( ) FACTS 1.   Total Utility Load  Increa se Increa se
in = in
Hot Utility Cold Utility ( ) ( )

17. §ENERGY TARGETS (MANY HOT AND COLD STREAMS)  COMPOSITE CURVES
(T1-T2) (B)
(T2-T3) (A+B+C)
(T3-T4) (A+C)
(T4-T5) (A)
CP=B
CP=A
CP=C H

18. §ENERGY TARGETS (MANY HOT AND COLD STREAMS)  COMPOSITE CURVES    T1 T2 T3 T4 T5 H

19.  PINCH POINT Minimum T hot utility “PINCH” minimum cold utility H
Energy targets and “the Pinch” with Composite Curves

20. m hot Qin Streams n cold Streams Qout - Qin = H Qout or
Heat Exchange System
n cold
Streams
Qout - Qin = H
Qout or

21. The “Problem Table” Algorithm - A Targeting Approach
---Linnhoff and Flower, AIChE J. (1978)
Stream No CP TS TT
and Type (KW/C) (C) (C) (C) (C)
(1) Cold T6 T3
(2) Hot T1 T5
(3) Cold T4 T2
(4) Hot (T2)
(T6)
Tmin = 10C

22. Subsystem #
CPHot
- CPcold
TK
HK
T1* = 165C
T2* = 145C
T3* = 140C
T4* = 85C
T5* = 55C
T6* = 25C 2 4 3 1

23. 90C 145C Heat Exchange Subsystem (3) 80C 135C from subsys #2
hot streams
145C
Heat Exchange
Subsystem (3) . . . . .
Cold streams
80C
135C
To subsys #4

24. T1* = 165C -------------------------- ( 0 )------
FROM HOT UTILITY
minimum
hot
utility 20
H1 = 60
H2 = 2.5
H3 = -82.5
Pinch
H4 = 75
H5 = -15
minimum
cold
utility 60
TO COLD UTILITY

25. § “PROBLEM TABLE” ALFORITHM
 SUBSYSTEM
TM TC=T
0 (T0)
1 (T1)
2 (T2) TP
Tmin
Hh2Hc2 Hh1 Hc1

26. § “PROBLEM TABLE” ALFORITHM
 ENTHALPY BALANCE OF SUBSYSTEM
As T = T1 - T2  0

27. 5. The Grand Composite Curve80 60 40 20
-20
Q(KW) CU
Qc,min
“Pinch” HU
Qh,min
T6* T5* T4* T3*T2* T1*

28. SIGNIFICANCE OF THE PINCH POINT
1. Do not transfer heat across the pinch
2. Do not use cold utility above
3. Do not use hot utility below

29. Q Qh Qh HU
Qc,min CU
Qh,min Tc Tp Th T
Qh  Qh,min
Qc  Qc,min

30. Q CU
Qc,min
Qh,min HU Tc Tp T1 Th T

31. Q Qc
CU2 Qh HU
Qc,min
CU1
Qh,min Tc Tp Th T

32. Q
Qh,min HU
Qc,min CU Tc Tp T1 Th T

33. Q
Qh,min
HU2
Qc,min Q1 CU Q2
HU1 Tc Tp T1
Tp’ Th T

34. H=27MW
H= -30MW
FEED 2
140
PRODUCT2
230
REACTOR 2
200
80
H=32MW
FEED 1
20
REACTOR 1
180
250
OFF GAS
40
H= -31.5MW
40
PRODUCT1
40
Figure A simple flowsheet with two hot streams and two cold streams.

35. Supply Target capacity temp. temp. H flow rate CP
TABLE 6.2 Heat Exchange Stream Data for the Flowsheet in Fig. 6.2
Heat
Supply Target capacity
temp temp H flow rate CP
Stream Type TS (C) TT (C) (MW) (MW C-1)
1. Reactor 1 feed Cold
2. Reactor 1 product Hot
3. Reactor 2 feed Cold
4. Reactor 2 product Hot

36. HOT UTILITY HOT UTILITY H= -1.5 H= -1.5 H= 6.0 H= 6.0 H= -1.0
(a)
HOT UTILITY
(b)
HOT UTILITY
245C MW MW
H= -1.5
H= -1.5
235C MW MW
H= 6.0
H= 6.0
195C MW MW
H= -1.0
H= -1.0
185C MW MW
H= 4.0
H= 4.0
145C MW MW
H= -14.0
H= -14.0
75C MW MW
H= 2.0
H= 2.0
35C MW MW
H= 2.0
H= 2.0
25C MW MW
COLD UTILITY
COLD UTILITY
Figure The problem table cascade.

37. Figure The grand composite curve shows the utility requirements in both enthalpy and temperature terms.

38. (a) BOILER T* HP Steam LP Steam pinch CW H
Process
HP Stream
Process
Fuel Boiler Feedwater
(Desuperheat)
BOILER
LP Stream
Condensate T*
HP Steam
LP Steam
pinch CW H
Figure The grand composite curve allows alternative utility mixes to be evaluated.

39. Hot Oil (b) T* pinch CW H
Hot Oil Return
Fuel
FURNACE
Process
Hot Oil Flow T*
Hot Oil
pinch CW H
Figure The grand composite curve allows alternative utility mixes to be evaluated.

40. (a) TC 300 250 200 150 100 50 0 5 10 15 H(MW) HP Steam LP Steam
HP Steam
LP Steam
H(MW)
Figure Alternative utility mixes for the process in Fig. 6.2.

41. (b)
TC
300
250
200
150
100 50
Hot Oil
H(MW)
Figure Alternative utility mixes for the process in Fig. 6.2.

42. T*TFT Theoretical Flame Temperature T*O T*STACK QHmin Flue Gas Air
Fuel
T*STACK
T*O
Ambient Temperature
Stack
Loss
ambient
temp.
QHmin H
Fuel
Figure Simple furnace model.

43. T*’TFT T*TFT Flue Gas T*STACK T*O Stack Loss T*
Figure Increasing the theoretical
flame temperature by reducing excess
air or combusion air preheat reduces the
stack loss.
T*STACK
T*O
Stack
Loss H

44. T* T* away from the pinch
T*TFT T*
T*TFT
T*ACID DEW
T*PINCH
T*C
T*ACID DEW
T*PINCH
T*C
(a)Stack temperature limited by acid dew point (b)Stack temperature limited by process
away from the pinch
Figure Furnace stack temperature can be limited by other factors than
pinch temperature.

45. T*
1800
1750
Flue Gas
300
250
200
150
100 50
H(MW)
Figure Flue gas matched against the grand composite curve of the
process in Fig. 6.2

46. SOME RESULTS IN GRAPH THEORY
1 ) A graph is any connection of points, some pairs of which are
connected by lines.
2 ) If a graph has p points and q lines, it is called a (p,q) graph.
points process and utility streams
lines heat exchangers
3 ) A path is a sequence of distinct lines, each are starting where
the previous are ends, e.g. AECGD in Fig. A. A B C D
Figure A
Figure B E F G H A B C D E F G H

47. SOME RESULTS IN GRAPH THEORY
4 ) A graph is connected if any two points can be joined by a path,
e. g. Fig. A
5 ) Points which are connected to some fired point by paths are said
to form a component, e. g.
Fig A has one component.
Fig B has two components.
6 ) A cycle is a path which begins and ends at the same point, e. g.
CGDHC in Fig. A.
7 ) The maximum number of independent cycles is called the cycle
rank of the graph.
8 ) The cycle rank of a (p,q) graph with k components is
q - p + k

48. A Result Based on Graph Theory
U = N+L-S
Where,
N = the total number of process and utility streams
L = the number of independent loops
S = the number of separate component in a network
U = the number of heat exchanger services

49. U = N+L-S U = N-1 = 5 U = N-2 = 4 U = N+1-1 = N = 6 30 70 90 ST H1 H2 ST H1 H2
U = N-1
= 5
U = N-2
= 4
U = N+1-1
= N
= 6 C1 C2 CW ST H1 H2 C1 C2 CW ST H1 H2
X X
30-X X C1 C2 CW

50. CAPITAL TARGET Umin = N - 1 where,
Umin = the minimum number of services
N = the total number of process and
utility streams
Note,
U = N + L - S

51. § PINCH DESIGN METHOD
RULE 1: THE “TICK-OFF” HEURISTIC
UMIN = N-1
- THE EQUATION IS SATISFIED IF EVERY MATCH
BRINGS ONE STREAM TO ITS TARGET TEMPERATURE
OR EXHAUSTS A UTILITY.
- FEASIBILITY CONSTRAINTS :
ENERGY BALANCE
TMIN

52. Example 1
Stream No TS TF CP Heat Load
and Type (F) (F) BTU/hr F Q BTU/hr
(1) Cold
(2) Cold
(3) Hot
(4) Cold
(5) Hot
(6) Cold
(7) Hot
Tmin = 20F
Qhmin =  104 BTU/hr
Qcmin = 0

53. Hot streams
CP Q
1.32
2.624
590 
471 419 
533 
400 
430 
400 
280  3  5 
505.6 7 1
416  2
505.6 4 
341.1 6
341.1
Cold streams

54. CP Q
1.557
4.128
590 574 
471 
400 
430 
400   3 
86.3 5
254  1
86.3 2
412.8  4
412.8

55. CP Q
590  
400  
430    3  1 H  2
22.4

56. CP Q
1.32
2.624
590 
471 
533 
400 
430 
400 
280     3   5 
505.6 7   H 1
86.3   2
22.4
505.6  4
412.8
341.1  6
341.1

57. § PINCH DESIGN METHOD
RULE 2: DECOMPOSITION
- THE HEN PROBLEM IS DIVIDED AT THE PINCH INTO
SEPARATE DESIGN TASKS.
- THE DESIGN IS STARTED AT THE PINCH AND
DEVELOPED MOVING AWAY FROM THE PINCH.

58. DATA FOR EXAMPLE II Temperature Heat Capacity
Supply Target Flowrates Heat load
Process Stream TS TT CP Q
no. Type F F BTU/h/F BTU/h
1 Cold
2 Hot
3 Cold
4 Hot
Tmin = 10 F
QHmin = 50  104 BTU/h
QCmin = 60  104 BTU/h

59. PINCH DECOMPOSITION DEFINES THE SEPARATE DESIGN TASKS
260    2
250    4
240    1
240  3 C
= 60 Btu/h H
= 50 Btu/h
Umin = 4
Umin = 3
PINCH DECOMPOSITION DEFINES THE SEPARATE
DESIGN TASKS

60. BELOW THE PINCH
CP Q
190  2 3
190   4 4 G 60
190   3 4 1
ABOVE THE PINCH
CP Q
260  2 1
250  4 2
235   H 2 1 20 90
240   H 1 3

61. Cp Q
260  1 3 2
250  2 4 C 4 60
235  H 2 3 4 1
240  H 1 3
THE COMPLETE MINIMUM UTILITY NETWORK

62. PINCH MATCH Pinch A Pinch Match Pinch 2 1 Exchanger 2 is not
Exchanger 2 is not
a pinch match
Pinch 1
Exchanger 3 is not
a pinch match

63. FEASIBILITY CRITERIA AT THE PINCH
Rule 1: Check the number of process streams and branches at the pinch
point
 Above the Pinch :
PINCH
PINCH
90
80
90
80 1 1 2 2 3 3 
(80+T1) 4 4
(80+T2) Q1 5 5 Q2
Tmin = 10C
Tmin = 10C

64. FEASIBILITY CRITERIA AT THE PINCH
Rule 1: Check the number of process streams and branches at the pinch
point
 Below the Pinch :
90
80
(90-T1)
90
80 1 1
(90-T2) 2 2  3 3 4 4 Q1 5 5 Q2
PINCH
PINCH
Tmin = 10C

65. FEASIBILITY CRITERIA AT THE PINCH
Rule 2: Ensure the CP inequality for individual matches are satisfied at the
pinch point.
 Above the Pinch :  Below the Pinch :
CPH1
CPC3 1 1
CPH2
CPC4 2 2 3 3 Q2 4 4
PINCH Q1
PINCH 1 T 2 T
Tmin
Tmin 3 4 Q Q Q2 Q1
CPC  CPH
CPC  CPH

66. Stream data at the pinch NH  NC? Yes No CPH  CPC Split a for every
pinch match
Split a
cold stream No
Yes
Split a
stream
( usually hot)
Place pinch
matches
Figure Design procedure above the pinch. (From B. Linnhoff et al., 1982.)

67. Stream data at the pinch NH  NC? Yes No CPH  CPC Split a for every
pinch match
Split a
cold stream No
Yes
Split a
stream
( usually hot)
Place pinch
matches
Figure Design procedure below the pinch. (From B. Linnhoff et al., 1982.)

68. CRITERION #3 THE CP DIFFERENCE
ABOVE THE PINCH,
INDIVIDUAL CP DIFFERENCE = CPC - CPH
OVERALL CP DIFFERENCE =
BELOW THE PINCH,
INDIVIDUAL CP DIFFERENCE = CPH - CPC
THE SUM OF THE INDIVIDUAL CP DIFFERENCES OF ALL PINCH
MATCHES MUST ALWAYS BE BOUNDED BY THE OVERALL CP
DIFFERENCE.

69. Overall CP Difference = 8 - 6 = 2
PINCH CP 4 2 5 3
Overall CP Difference = = 2
Total Exchanger CP Difference = = 2
O.K.

70. Overall CP Difference = 9 - 6 = 3
PINCH CP 4 2 5 3 1
Overall CP Difference = = 3
Total Exchanger CP Difference = = 2
O.K.

71. Overall CP Difference = 9 - 5 = 4
PINCH CP 3 2 8 1
Overall CP Difference = = 4
Total Exchanger CP Difference = = 6
Criterion violated !

72. Cp Q
260   1 3 2
250   130 2 4 C 4 60
235  180 135  H 2 3 4 1
240   H 1 3
Heat Load Loops
heat loads can be shifted around the loop from one unit
to another

73. 4 H 2 3 H 2 4 1 H C 1 3 C
Heat Load Loops
heat loads can be shifted around the loop from one unit
to another

74. 260   1 3 2
250  130 2 C 4 60
235    H 2 3 1
240   H 1 3
Heat Load Path
heat loads can be shifted along the path

75. 4 H 2 3 H 2 1 H C 1 3 C
Heat Load Path
heat loads can be shifted along the path

76. Cp Q
260   1 3 2 2
250   C 4
60+X
235  165  2 3 H 1
20+X
240   H 1 3
X=7.5

77. Two ways to break the loop
If:
L1>L4
L2>L3
then:
X=L4 or
X= -L3 1 1 2 2 3 4
(a) 3
L2 + X
L4 - X 4
L3 + X
L1 - X 1 2 3 2 1 4 3 4

78. heater/cooler can be included in a loop1 3 4 2
(b)
H1 - X 3 H
L3 + X 4 H
L4 - X
H2 + X 1 H 3 4 3 4
Figure Complex loops and paths

79. Match 1 is not in the path 1 2 (c) 3 4 H 1 2 4 3 C 2 3 1 4 C H 4 2 3
C + X 3
L3 + X
L4 - X 4 H
L2 - X
H + X H 1 2 4 2 3 4 3 C
Figure Complex loops and paths