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Hierarchy of Decisions
1. Batch versus continuous
2. Input-output structure of the flowsheet
3. Recycle structure of the flowsheet
4. General structure of the separation system
a. Vapor recovery system
b. Liquid recovery system
5. Heat-exchanger network
Ch.6, Ch.7, Ch.16
Ch. 4
Ch.5
HEAT EXCHANGER NETWORK
(HEN)
SUCCESSFUL APPLICATIONS
O ICI
---- Linnhoff, B. and Turner, J. A., Chem. Eng., Nov. 2, 1981
Process
Organic Bulk Chemical
Specialty Chemical
Crude Unit
Inorganic Bulk Chemical
Specialty Chemical
General Bulk Chemical
Inorganic Bulk Chemical
Future Plant
Specialty Chemical
Unspecified
General Chemical
Petrochemical
Facility*
New
New
Mod
New
Mod
New
New
New
New
New
Mod
New
New
Mod
Energy savings
Available
k$/yr
800
1600
1200
320
200
200
2600
200 to 360
30 to 40 %
100
300
300
360
Phase I 1200
Phase II 1200
*New means new plant; Mod means plant modification.
Capital Cost
Expenditure
or Saving, k$
same
saving
saving
saving
160
saving
unclear
unclear
30 % saving
150
1000
saving
unclear
600
1200
SUCCESSFUL APPLICATIONS
Table 1. First results of applying the pinch technology in Union
Carbide
Process
Petro-Chemical
Specialty Chemical
Specialty Chemical
Licensing Package
Petro-Chemical
Organic Bulk
Chemical
Organic Bulk
Chemical
Specialty Chemical
Organic Bulk
Chemical
Project
Type
Energy Cost
Reduction $/yr
Installed
Capital Cost $
Mod.
Mod.
Mod.
New
Mod.
Mod.
1,050,000
139,000
82,000
1,300,000
630,000
1,000,000
500,000
57,000
6,000
Savings
Yet Unclear
600,000
6
5
1
?
7
Mod.
1,243,000
1,835,000
18
Mod.
Mod.
570,000
2,000,000
200,000
800,000
4
5
Linnhoff and Vredeveld, CEP, July, 1984
Payback
Months
SUCESSFUL APPLICATIONS
Fluor
--- IChE Symp. Ser., No. 74, 1982, P.19
--- CEP, July, 1983, P.33
FMC (Marine Colloid Division, Rockland, ME)
CONCLUSION
HEN/MEN synthesis can be identified as a
separate and distinct task in process design
IDENTIFY HEAT RECOVERY AS A SEPARATE AND DISTINCT
TASK IN PROCESS DESIGN.
9.60
200C
18.2 bar
1.089
36C
16 bar
D 201
7.841
REACTION
RECYCLE
126C
18.7 bar
1.614
200C
PURGE
CW
153C
7 703
115.5C
FEED
5C 19.5 bar
0 179
180C
141C
TO
COLUMN
35C
FLASH
40C
120C
17.6 bar
17.3 bar
114C
Figure 2.5 - Flowsheet for “front end” of specialty chemicals process
200C
Reactor
200C
RECYCLE
TOPS
Reactor
5C
FEED
35C
Purge
Product
35C
PRODUCT
126C
FOR EACH STREAM: TINITIAL, TFINAL, H = f(T).
Figure 2.6-Specialty chemicals process-heat exchange duties
H = 1722
REACTOR
C = 654
a ) DESIGN AS USUAL
STEAM
RECYCLE
70
1
。
。
STEAM
1652
3
FEED
654
COOLING
WATER
6 UNITS
2
PRODUCT
H = 1068
C =0
REACTOR
4 UNITS
b ) DESIGN WITH TARGETS
STEAM
RECYCLE
。
。
1068
1
。
。
2
3
FEED
PRODUCT
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.
STEP ONE
Determine the Targets
§ ENERGY TARGETS (TWO STREAM HEAT EXCHANGE)
T/H DIAGRAM
T
Q
=CP(TT-TS)
TT
H Q
TTTS CPdT
CP TT TS
TS
H
Figure 2.10 - Representation of process streams in the T/H diagram
H
T
(C)
200
UTILITY
HEATING
140
135
T
115
100
70
UTILITY
COOLING
350
300
400
H
(KW)
TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM
T
(C)
200
UTILITY
HEATING
130
135
T
120
100
70
UTILITY
COOLING
350
-100
=250
300
+100
=400
400
-100
=300
H
(KW)
TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM
FACTS
1.
Tmin Total Utility Load
Increa se
2.
in
Hot Utility
(
Increa se
in
Cold Utility
)(
=
)
§ENERGY TARGETS (MANY HOT AND COLD STREAMS)
COMPOSITE CURVES
T
H INTERVAL
T1
CP=B
T2
(T1-T2) (B)
T3
(T2-T3) (A+B+C)
CP=A
CP=C
T4
(T3-T4) (A+C)
T5
(T4-T5) (A)
H
§ENERGY TARGETS (MANY HOT AND COLD STREAMS)
COMPOSITE CURVES
T
T1
T2
T3
H1
H 2
H
3
T4
T5
H
4
H
PINCH POINT
Minimum
hot
utility
T
“PINCH”
QH ,min
Tmin
QC ,min
minimum
cold utility
Energy targets and “the Pinch” with Composite Curves
H
m hot
Streams
Qin
Heat
Exchange
n cold
Streams
Qout
m
n
Qin H H
in
h ,i
i 1
or
j 1
System
Qout - Qin = H
m
in
c, j
Qout H
i 1
n
out
h ,i
H cout
,j
j 1
m in n out m out n in
Qout Qin H H h,i H c , j H h,i H c, j
j 1
j 1
i 1
i 1
The “Problem Table” Algorithm - A Targeting Approach
---Linnhoff and Flower, AIChE J. (1978)
CP
TS
TT
TS*
TT*
and Type
(KW/C)
(C)
(C)
(C)
(C)
(1) Cold
2
20
Stream No.
25
135
(2) Hot
3
170
140
165
60
(3) Cold
4
80
55
85
140
(4) Hot
1.5
150
145
30
Tmin = 10C
145
25
Ti
T6
T3
T1
T5
T4
T2
(T2)
(T6)
Subsystem
2
T1* = 165C
4
T2* = 145C
T3* = 140C
T4* = 85C
3
T5* = 55C
T6* = 25C
#
1
TK
CPHot
- CPcold
HK
1
20
3.0
60
2
5
0.5
2.5
3
55
-1.5
-82.5
4
30
2.5
75
5
30
-0.5
-15
( 3)
H3 Qout
Qin(3) H H 3 H H 4 i HC 3 H C 4 j
i
j
CPHOT ,i T3* T4* CPCold , j T3* T4*
i
j
CPHOT ,i CPCold , j T3* T4* CPHOT ,i CPCold , j T3
j
j
i
3
i
3
Qin(3) from subsys #2
90C
.
.
.
.
.
.
.
.
.
Heat Exchange
Subsystem (3)
.
.
.
hot streams
145C
.
.
.
Cold streams
80C
(3)
Qout
To subsys #4
(K )
H K Qout
Qin( K )
135C
FROM HOT UTILITY
T1* = 165C -------------------------- ( 0 )------ 20
H1 = 60
T2* = 145C --------------------------( 60 )-----( 80 )
minimum
hot
utility
H2 = 2.5
T3* = 140C -------------------------( 62.5 )---( 82.5 )
H3 = -82.5
T4* = 85C -------------------------( -20.0 )-----( 0 )
Pinch
H4 = 75
T5* = 55C --------------------------( 55.0 )----( 75 )
H5 = -15
T6* = 25C --------------------------( 40.0 )---- 60
TO COLD UTILITY
(K )
H K Qout
Qin( K )
minimum
cold
utility
§ “PROBLEM TABLE” ALFORITHM
SUBSYSTEM
TM TC=T
0 (T0)
1 (T1)
2 (T2)
TP
Tmin
Hh2Hc2 Hh1 Hc1
TH TC Tmin
§ “PROBLEM TABLE” ALFORITHM
ENTHALPY BALANCE OF SUBSYSTEM
QOUT QIN HH1 HH2 HC1 HC2
As T = T1 - T2 0
dQ
CPH CPC
dT
5. The Grand Composite Curve
80
60
Q(KW)
40
CU
Qc,min
20
“Pinch”
HU
Qh,min
0
20
T6*
-20
40
60
T5 *
80
T4*
100
120
140
160
T3*T2*
T1*
180
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
Q
Qh
Qh
CU
Qc,min
Tc
Qh,min
Tp
T
Qh Qh,min
Qc Qc,min
Th
HU
Q
CU
Qc,min
Tc
Qh,min
Tp
T
T1
Th
HU
Q
Qc
CU2
CU1
Qc,min
Tc
Qh
Qh,min
Tp
Th
T
Qh Qh,min Qc Qh,min
HU
Q
CU
Qh,min
Qc,min
Tc
Tp
T1
T
Th
HU
Q
CU
Q1
Qc,min
Tc
Qh,min
HU2
Q2
Tp
T1
T
Qh,min Q1 Q2
Tp’
Th
HU1
H=27MW
H= -30MW
FEED 2
PRODUCT2
230
140
REACTOR 2
200
80
H=32MW
FEED 1
20
REACTOR 1
180
250
OFF GAS
40
H= -31.5MW
40
PRODUCT1
40
Figure 6.2 A simple flowsheet with two hot streams and two cold streams.
TABLE 6.2 Heat Exchange Stream Data for the Flowsheet in Fig. 6.2
Stream
Type
Supply Target
temp. temp.
TS (C) TT (C)
H
(MW)
Heat
capacity
flow rate CP
(MW C-1)
1. Reactor 1 feed
Cold
20
180
32.0
0.2
2. Reactor 1 product
Hot
250
40
-31.5
0.15
3. Reactor 2 feed
Cold
140
230
27.0
0.3
4. Reactor 2 product
Hot
200
80
-30.0
0.25
(a)
(b)
HOT UTILITY
245C
HOT UTILITY
0MW
H= -1.5
235C
7.5MW
H= -1.5
1.5MW
H= 6.0
195C
9.0MW
H= 6.0
-4.5MW
H= -1.0
185C
3.0MW
H= -1.0
-3.5MW
H= 4.0
145C
4.0MW
H= 4.0
-7.5MW
H= -14.0
75C
0MW
H= -14.0
6.5MW
H= 2.0
35C
14.0MW
H= 2.0
4.5MW
25C
H= 2.0
2.5MW
COLD UTILITY
Figure 6.18 The problem table cascade.
12.0MW
H= 2.0
COLD UTILITY
10.0MW
Figure 6.24 The grand composite curve shows the utility
requirements in both enthalpy and temperature terms.
Process
(a)
HP Stream
Fuel
BOILER
Process
Boiler Feedwater
(Desuperheat)
LP Stream
Condensate
T*
HP Steam
LP Steam
pinch
CW
H
Figure 6.25. The grand composite curve allows alternative utility mixes to be evaluated.
(b)
Hot Oil Return
Fuel
FURNACE
Process
Hot Oil Flow
T*
Hot Oil
pinch
CW
H
Figure 6.25. The grand composite curve allows alternative utility mixes to be evaluated.
(a)
TC
300
250
HP Steam
200
LP Steam
150
100
50
0
0
5
10
15
Figure 6.26 Alternative utility mixes for the process in Fig. 6.2.
H(MW)
(b)
TC
300
250
Hot Oil
200
150
100
50
0
0
5
10
15
Figure 6.26 Alternative utility mixes for the process in Fig. 6.2.
H(MW)
T*
T*TF
Theoretical Flame
Temperature
T
T*O
T*STACK
Flue
Gas
QHmin
Air
T*TFT
Fuel
T*STACK
T*O
ambient
temp.
Stack
Loss
Ambient Temperature
QHmin
Fuel
Figure 6.27 Simple furnace model.
H
T*
T*’TFT
T*TFT
Flue
Gas
Figure 6.28 Increasing the theoretical
flame temperature by reducing excess
air or combusion air preheat reduces the
stack loss.
T*STACK
T*O
Stack
Loss
H
T*
T*
T*TFT
T*TFT
T*ACID DEW
T*ACID DEW
T*PINCH
T*PINCH
T*C
T*C
(a)Stack temperature limited by acid dew point
(b)Stack temperature limited by process
away from the pinch
Figure 6.29 Furnace stack temperature can be limited by other factors than
pinch temperature.
T*
1800
1750
Flue Gas
300
250
200
150
100
50
0
0
5
10
15
H(MW)
Figure 6.30 Flue gas matched against the grand composite curve of the
process in Fig. 6.2
SOME RESULTS IN GRAPH THEORY
1 ) A graph is any connection of points, some pairs of which are
connected by lines.
A
B
C
D
Figure A
E
F
G
H
A
B
C
D
E
F
G
H
Figure B
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.
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
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
U = N+L-S
30
ST
30
70
H1
90
H2
60
40 50
10
C1
40
30
ST
C2
100
70
H1
30 70
C1
40
30
ST
30-X
C1
40
CW
50
90
H2
40
C2
100
70
H1
X
10+X
60-X
C2
100
U = N-1
=5
0 X 30
50
CW
50
90
H2
40 50
CW
50
U = N-2
=4
U = N+1-1
=N
=6
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
§ 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
Example 1
Stream No
and Type
TS
(F)
TF
(F)
CP
104BTU/hr F
Heat Load
Q BTU/hr
(1) Cold
200
400
1.6
320.0
(2) Cold
100
430
1.6
528.0
(3) Hot
590
400
2.376
451.4
(4) Cold
300
400
4.128
412.8
(5) Hot
471
200
1.577
427.4
(6) Cold
150
280
2.624
341.1
(7) Hot
533
150
1.32
505.6
Tmin = 20F
Qhmin = 217.5 104 BTU/hr
Qcmin = 0
Hot streams
CP
3
5
7
590
400
471 419
200
533
400
430
416
505.6
400
280
341.1
Cold streams
2.376
451.4
1.557
427.4
1.32
505.6
1
1.6
320.0
2
1.6
528.0
4
4.128
412.8
6
2.624
341.1
150
200
100
300
150
Q
CP
3
5
590 574
400
471
419
400
254
86.3
200
430
400
412.8
416
300
Q
2.376
451.4
1.557
86.3
1
1.6
320.0
2
1.6
22.4
4
4.128
412.8
CP
590
3
583
400
264
H
217.5 16.2
430
574
2.376
38.6
1
1.6
233.7
2
1.6
22.4
254
22.4
Q
416
CP
3
5
7
590
471
533
400
16.2 217.5
430
400
280
400
2.376
451.4
200
1.557
427.4
1.32
505.6
1
1.6
320.0
2
1.6
528.0
4
4.128
412.8
6
2.624
341.1
150
200
H
86.3
100
22.4
505.6
300
412.8
341.1
Q
150
§ PINCH DESIGN METHOD
RULE 2: DECOMPOSITION
- THE HEN PROBLEM IS DIVIDED AT THE PINCH INTO
SEPARATE DESIGN TASKS.
- THE
D E S I G N I S S TA RT E D AT T H E P I N C H A N D
DEVELOPED MOVING AWAY FROM THE PINCH.
DATA FOR EXAMPLE II
Temperature
Process Stream
no. Type
1
2
3
4
Cold
Hot
Cold
Hot
Supply Target
TS
TT
F
F
120
260
180
250
Heat Capacity
Flowrates
CP
4
10 BTU/h/F
235
160
240
130
Tmin = 10 F
QHmin = 50 104 BTU/h
QCmin = 60 104 BTU/h
TH* 190 F
TC* 180 F
2.0
3.0
4.0
1.5
Heat load
Q
4
10 BTU/h
230.0
300.0
240.0
180.0
PINCH
2
4
H
260
190
190
160
250
190
190
130
240
180
180
120
240
180
= 50 Btu/h
Umin = 4
1
3
C
= 60 Btu/h
Umin = 3
PINCH DECOMPOSITION DEFINES THE SEPARATE
DESIGN TASKS
BELOW THE PINCH
2
4
190
3
190
4
170
G
60
190
135
3
4
90
30
ABOVE THE PINCH
2
4
260
235
H
20
240
H
30
-32
1
210
Q
90
130
1.5
90
2
120
CP
Q
3
210
1.5
90
2
220
4
240
1
190
2
225
CP
3
120
1
250
160
2
90
190
180
180
1
3
2 260
1
4 250
235 H
20
240 H
30
2
90
1
210
4
3
90
Q
3
300
1.5
180
120 1
2
230
180 3
4
240
160
3
2
Cp
4
30
C 130
60
THE COMPLETE MINIMUM UTILITY NETWORK
PINCH MATCH
Pinch
A Pinch Match
1 Pinch
2
Exchanger 2 is not
a pinch match
3
Pinch
2
1
Exchanger 3 is not
a pinch match
FEASIBILITY CRITERIA AT THE PINCH
Rule 1: Check the number of process streams and branches at the pinch
point
Above the Pinch :
NH NC
1
PINCH
90
1
PINCH
90
2
90
2
90
3
90
3
90
(80+T1)
80
(80+T2)
Q1 80
80
4
5
4
80
Q2
Tmin = 10C
Tmin = 10C
5
FEASIBILITY CRITERIA AT THE PINCH
Rule 1: Check the number of process streams and branches at the pinch
point
Below the Pinch :
NH NC
90
1
90
(90-T1)
2
90
(90-T2)
2
80
90
90
3
80
Q1
80
PINCH
1
Q2
3
4
80
4
5
80
5
PINCH
Tmin = 10C
FEASIBILITY CRITERIA AT THE PINCH
Rule 2: Ensure the CP inequality for individual matches are satisfied at the
pinch point.
Above the Pinch :
1
2
Below the Pinch :
CPH2
T
2
3
4 CPC4
PINCH
Q1
4
Q1
CPC CPH
CPH1
1
2
Q2
PINCH
T
Tmin
CPC3
1
Tmin
Q
3
4
3
Q2
CPC CPH
Q
Stream data
at the pinch
NH NC?
Yes
CPH CPC
for every
pinch match
Yes
No
Split a
cold stream
No
Place pinch
matches
Split a
stream
( usually hot)
Figure 8.7-7 Design procedure above the pinch. (From B. Linnhoff et al., 1982.)
Stream data
at the pinch
NH NC?
Yes
CPH CPC
for every
pinch match
Yes
No
Split a
cold stream
No
Place pinch
matches
Split a
stream
( usually hot)
Figure 8.7-7 Design procedure below the pinch. (From B. Linnhoff et al., 1982.)
CRITERION #3 THE CP DIFFERENCE
ABOVE
THE
PINCH,
INDIVIDUAL CP DIFFERENCE = CPC - CPH
Nc
NH
1
1
OVERALL CP DIFFERENCE = CPC CPH
BELOW
THE
PINCH,
INDIVIDUAL CP DIFFERENCE = CPH - CPC
NH
Nc
1
1
OVERALL CP DIFFERENCE = CPH CPC
THE SUM OF THE INDIVIDUAL CP DIFFERENCES OF ALL PINCH
MATCHES MUST ALWAYS BE BOUNDED BY THE OVERALL CP
DIFFERENCE.
PINCH
CP
4
2
5
3
Overall CP Difference = 8 - 6 = 2
Total Exchanger CP Difference = 1 + 1 = 2
O.K.
PINCH
CP
4
2
5
3
1
Overall CP Difference = 9 - 6 = 3
Total Exchanger CP Difference = 1 + 1 = 2
O.K.
PINCH
CP
3
2
8
1
Overall CP Difference = 9 - 5 = 4
Total Exchanger CP Difference = 8 - 2 = 6
Criterion violated !
2 260
1
190 3
250
160
170 130
2
4
C
4
60
235
225 180 135
120
H
2
3
4
1
20
90 90 30
240 232.5
180
H
1
3
30 210
Cp
Q
3
300
1.5
180
2
230
4
240
190
Heat Load Loops
heat loads can be shifted around the loop from one unit
to another
4
H
2
3
H
2
4
1
H
1
3
C
C
Heat Load Loops
heat loads can be shifted around the loop from one unit
to another
2 260
1
190
250
2
4
235
225
H
2
20
120
240 232.5
H
1
30 210
3
160
170 130
C
60
165
120
3
1
90
180
3
Heat Load Path
heat loads can be shifted along the path
4
H
2
3
H
2
1
H
1
3
Heat Load Path
heat loads can be shifted along the path
C
C
2
260
190
1
2
4 250
235
H
20+X
3
C 130
60+X
221.25 165
2
3
120
240 232.5
H
1
30 210
X=7.5
Q
3
300
1.5
180
1
2
230
3
4
240
160
175
112.5
Cp
90
180
Two ways to break the loop
1
If:
1
2
2
3
4
(a)
3
L2 + X
L4 - X
L3 + X
L1 - X
1
2
3
2
4
1
3
4
4
L1>L4
L2>L3
then:
X=L4
or
X= -L3
heater/cooler can be included in a loop
1
3
4
2
(b)
H1 - X
3
H
L3 + X
4
H
L4 - X
H2 + X
1
3
H
4
3
Figure 2.28 - Complex loops and paths
4
Match 1 is not in the path
1
2
2
3
1
4
C
C+X
(c)
3
L3 + X
H
L2 - X
H+X
H
1
2
4
3
2
4
3
Figure 2.28 - Complex loops and paths
C
L4 - X
4