1. Hierarchy of Decisions

2. A DESIGN INSIGHT separate and distinct task in process design
HEN synthesis can be identified as a
separate and distinct task in process design

3. IDENTIFY HEAT RECOVERY AS A SEPARATE AND DISTINCT
TASK IN PROCESS DESIGN.
9.60
200C
18.2 bar H1
1.089
36C
16 bar
RECYCLE
REACTION
7.841
126C
18.7 bar TO
COLUMN
D 201 C2
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 C1
Flowsheet for “front end” of specialty chemicals process

4. Heat exchange duties in specialty chemicals process:
Reactor
35C
200C
RECYCLE
 TOPS
Purge
Reactor
Product
(H1)
35C
5C
FEED (C1)
PRODUCT (C2)
126C
Heat exchange duties in specialty chemicals process:
FOR EACH STREAM: TINITIAL, TFINAL, H = f(T).

5. 。 。  = 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

6. 。 。 。 。  = 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

7. DESIGN PROCEDURE OF HEAT EXCHANGER NETWORKS
Determine Targets.
Energy Target - maximum recoverable energy
Capital Targets
minimum number of heat transfer units.
minimum total heat transfer area
Generate Alternatives to Achieve Those Targets.
Modify the Alternatives Based on Practical Considerations.
Equipment Design and Costing for Each Alternative.
Select the Most Attractive Candidate.

8. ENERGY TARGETS (TWO-STREAM HEAT EXCHANGE)T Q
=CP(TT-TS) TT TS H H

9. 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

10. 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

11. ( ) ( ) CONCLUSIONS 1.   Total Utility Load  Increa se Increa se
in = in
Hot Utility Cold Utility ( ) ( )

12. ENERGY TARGETS ( MULTIPLE HOT AND MULTIPLE COLD STREAMS)
Construction of the Hot Composite Curve T T1 T2 T3 T4 T5
(T1-T2) (B)
(T2-T3) (A+B+C)
(T3-T4) (A+C)
(T4-T5) (A)
CP=B
CP=A
CP=C H

13. Construction of Hot Composite Curve
(1)
(2)
(3)
(4) T1 T2 T3 T4 T5 H

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

15. Generalized heat-exchange system
m hot
Streams
Qin
Heat Exchange System
n cold
Streams
Qout - Qin = H
Qout or

16. The “Problem Table” Algorithm
---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

17. 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

18. 90C 145C Heat Exchange Subsystem (3) 80C 135C from subsystem #2
hot streams
145C
Heat Exchange
Subsystem (3) . . . . .
Cold streams
80C
135C
To subsystem #4

19. 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

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

21. 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

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

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

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

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

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

27. A simple flowsheet with two hot streams and two cold streams.
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
A simple flowsheet with two hot streams and two cold streams.

28. Heat Exchange Stream Data
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

29. The heat-flow cascade. HOT UTILITY HOT UTILITY H= -1.5 H= -1.5
(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
The heat-flow cascade.

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

31. Grand composite curve allows different utility mixes to be evaluated.
Process
HP Stream
Process
Fuel Boiler Feedwater
(Desuperheat)
BOILER
LP Stream
Condensate T*
HP Steam
LP Steam
pinch CW H
Grand composite curve allows different utility mixes to be evaluated.

32. Grand composite curve allows different utility mixes to be evaluated.
Hot Oil Return
Fuel
FURNACE
Process
Hot Oil Flow T*
Hot Oil
pinch CW H
Grand composite curve allows different utility mixes to be evaluated. .

33. Furnace Model Theoretical Flame Temperature T*TFT T*O T*STACK QHmin
Flue
Gas
Air
T*TFT
Fuel
T*STACK
T*O
Ambient Temperature
Stack
Loss
ambient
temp.
QHmin H
Fuel
Furnace Model

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

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

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

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

38. (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.

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

40. 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