Hierarchy of Decisions
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Transcript of Hierarchy of Decisions
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 Ch.5
a. Vapor recovery system
b. Liquid recovery system
5. Heat-exchanger network Ch.6, Ch.7, Ch.16
Ch. 4
HEAT EXCHANGER NETWORK (HEN)
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 800 sameSpecialty Chemical New 1600 savingCrude Unit Mod 1200 savingInorganic Bulk Chemical New 320 savingSpecialty Chemical Mod 200 160 New 200 savingGeneral Bulk Chemical New 2600 unclearInorganic Bulk Chemical New 200 to 360 unclearFuture Plant New 30 to 40 % 30 % savingSpecialty Chemical New 100 150Unspecified Mod 300 1000 New 300 savingGeneral Chemical New 360 unclearPetrochemical Mod Phase I 1200 600 Phase II 1200 1200
*New means new plant; Mod means plant modification.
SUCCESSFUL APPLICATIONS
Table 1. First results of applying the pinch technology in Union Carbide
Project Energy Cost Installed PaybackProcess Type Reduction $/yr Capital Cost $ Months
Petro-Chemical Mod. 1,050,000 500,000 6Specialty Chemical Mod. 139,000 57,000 5Specialty Chemical Mod. 82,000 6,000 1Licensing Package New 1,300,000 Savings Petro-Chemical Mod. 630,000 Yet Unclear ?Organic Bulk Mod. 1,000,000 600,000 7 ChemicalOrganic Bulk Mod. 1,243,000 1,835,000 18 ChemicalSpecialty Chemical Mod. 570,000 200,000 4Organic Bulk Mod. 2,000,000 800,000 5 Chemical
Linnhoff and Vredeveld, CEP, July, 1984
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
0 1791.614
7.841
1.089
7 703
D 201
RECYCLE
TOCOLUMN
PURGE
CW
36C
200C18.2 bar
200C
180C153C
141C 40C
115.5C120C 17.6 bar
114C
35C
126C18.7 bar
17.3 bar
16 bar
FEED5C 19.5 bar
Figure 2.5 - Flowsheet for “front end” of specialty chemicals process
FLASH
REACTION
Reactor
200C
200C
35C
35C
Reactor
RECYCLE
TOPS
Product
Purge
PRODUCT126C
5C
FEED
FOR EACH STREAM: TINITIAL, TFINAL, H = f(T).
Figure 2.6-Specialty chemicals process-heat exchange duties
REACTOR
1
23
。 。
70
1652
654
STEAM
STEAM
RECYCLE
PRODUCTCOOLINGWATERFEED
= 1722
= 654
a ) DESIGN AS USUAL
C
H6 UNITS
REACTOR
1
2
3
。 1068
STEAMRECYCLE
PRODUCTFEED
= 1068
= 0
b ) DESIGN WITH TARGETS
C
H4 UNITS
。 。。
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
HH
T
TT
TS
Q=CP(TT-TS)
Figure 2.10 - Representation of process streams in the T/H diagram
ST
TT
TTCP
CPdT
QHT
S
H(KW)
350 300 400
T(C)
100
115
135
UTILITYHEATING
140
UTILITYCOOLING
70
200
TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM
T
H(KW)
350 300 400
T(C)
T
100
120
135
UTILITYHEATING
130
UTILITYCOOLING
70
200
TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM
-100 +100 -100 =250 =400 =300
FACTS
1. Total Utility Load
Increa se Increa se
2. in = in
Hot Utility Cold Utility( () )
minT
§ENERGY TARGETS (MANY HOT AND COLD STREAMS)
COMPOSITE CURVES
T1
T2
T3
T4
T5
(T1-T2) (B)
(T2-T3) (A+B+C)
(T3-T4) (A+C)
(T4-T5) (A)
CP=A
CP=B
CP=C
T
H
INTERVALH
§ENERGY TARGETS (MANY HOT AND COLD STREAMS)
COMPOSITE CURVES
T1
T2
T3
T4
T5
T
H
1H
2H
3H
4H
PINCH POINT
T
“PINCH”
minimumcold utility
Minimumhot
utility
H
Energy targets and “the Pinch” with Composite Curves
min,HQ
min,CQ
minT
m hotStreams
n coldStreams
Qin
QoutQout - Qin = H
Heat Exchange System
m
i
n
j
outjc
outihout
injc
n
j
inih
m
iin HHQHHQ
1 1,,,
1,
1
or
m
i
n
j
injc
outih
m
i
n
j
outjc
inihinout HHHHHQQ
1 1,,
1 1,,
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 2 20 25 T 6
135 140 T3 (2) Hot 3 170 165 T 1 60 55 T5 (3) Cold 4 80 85 T 4 140 145 T2 (4) Hot 1.5 150 145 (T 2) 30 25 (T6)
Tmin = 10C
*ST *
TT
iT
T1* = 165C
T2* = 145C
T3* = 140C
T4* = 85C
T5* = 55C
T6* = 25C
Subsystem
#TK
CPHot
- CPcold HK
1
4
2
3
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
i j
jCCiHHinout HHHHQQH 4343)3()3(
3
i j
jColdiHOT TTCPTTCP *4
*3,
*4
*3,
*4
*3
3
,, TTCPCPi j
jColdiHOT
3
3
,, TCPCPi j
jColdiHOT
Heat ExchangeSubsystem (3)
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
)3(inQ
)3(outQ
from subsys #2
To subsys #4
hot streams145C
135C
90C
Cold streams80C
)()( Kin
KoutK QQH
T1* = 165C -------------------------- ( 0 )------
T2* = 145C --------------------------( 60 )-----( 80 )
T3* = 140C -------------------------( 62.5 )---( 82.5 )
T4* = 85C -------------------------( -20.0 )-----( 0 )
T5* = 55C --------------------------( 55.0 )----( 75 )
T6* = 25C --------------------------( 40.0 )----
H1 = 60
H2 = 2.5
H3 = -82.5
H4 = 75
H5 = -15
20
60
minimumhotutility
minimumcoldutility
Pinch
FROM HOT UTILITY
TO COLD UTILITY
)()( Kin
KoutK QQH
§ “PROBLEM TABLE” ALFORITHM
SUBSYSTEM
TM TC=T
Tmin
TP
0 (T0)1 (T1)
2 (T2)
minTTT CH
Hh2Hc2 Hh1 Hc1
§ “PROBLEM TABLE” ALFORITHM
ENTHALPY BALANCE OF SUBSYSTEM
C2C1H2H1INOUT HHHHQQ
As T = T1 - T2 0
CH CPCPdT
dQ
5. The Grand Composite Curve
80
60
40
20
0
-20
Q(K
W)
20 40 60 80 100 120 140 160 180
Qc,min
T6* T5* T4* T3*T2* T1*
Qh,min
HU
CU
“Pinch”
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
Qh
Qh,min
Qc,min
Qh
Q
T
Tc Tp Th
Qh Qh,min
Qc Qc,min
HU
CU
Qh,min
Qc,min
Q
T
Tc Tp Th
HUCU
T1
Qh
Qh,min
Qc,min
Q
T
Tc Tp Th
HU
CU1
QcCU2
min,min, hchh QQQQ
Qh,minQc,min
Q
T
Tc Tp Th
HUCU
T1
Qh,min
Qc,min
Q
T
Tc Tp Th
HU1CU
T1
Q1
Q2
Tp’
HU2
2min, QQQh 1
REACTOR 1
REACTOR 2
H=27MW
H=32MW
H= -30MW
H= -31.5MW
FEED 2 140
FEED 1 20 180 250
230
200 80
40
40
40
PRODUCT2
PRODUCT1
OFF GAS
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
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 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
H= -1.5
H= 6.0
H= 4.0
H= -14.0
H= 2.0
H= 2.0 H= 2.0
H= 2.0
H= -14.0
H= 4.0
H= -1.0
H= 6.0
H= -1.5
H= -1.0
(a) (b)HOT UTILITY HOT UTILITY
COLD UTILITY COLD UTILITYFigure 6.18 The problem table cascade.
245C 0MW 7.5MW
235C 1.5MW 9.0MW
195C -4.5MW 3.0MW
185C -3.5MW 4.0MW
145C -7.5MW 0MW
75C 6.5MW 14.0MW
35C 4.5MW 12.0MW
25C 2.5MW 10.0MW
Figure 6.24 The grand composite curve shows the utility requirements in both enthalpy and temperature terms.
pinch
CW
LP Steam
HP SteamT*
H
(a)
BOILER
Fuel Boiler Feedwater
(Desuperheat)
HP Stream
LP Stream
Process
Process
Condensate
Figure 6.25. The grand composite curve allows alternative utility mixes to be evaluated.
pinch
CW
T*
H
(b)
Figure 6.25. The grand composite curve allows alternative utility mixes to be evaluated.
Hot Oil
Hot Oil Return
Hot Oil FlowProcessFuel
FURNACE
300
250
200
150
100
50
0 0 5 10 15
(a) TC
H(MW)
HP Steam
LP Steam
Figure 6.26 Alternative utility mixes for the process in Fig. 6.2.
300
250
200
150
100
50
0 0 5 10 15
(b) TC
H(MW)
Figure 6.26 Alternative utility mixes for the process in Fig. 6.2.
Hot Oil
T*
H
Figure 6.27 Simple furnace model.
T*TFT
T*STACK
Fuel
QHmin
T*O
ambienttemp.
StackLoss
Ambient Temperature
FlueGas
Theoretical FlameTemperature T*O
QHmin
Fuel
AirT*TFT
T*STACK
T*
H
Figure 6.28 Increasing the theoreticalflame temperature by reducing excess air or combusion air preheat reduces thestack loss.
T*’TFT
T*TFT
T*STACK
StackLoss
FlueGas
T*O
T*
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 6.29 Furnace stack temperature can be limited by other factors than pinch temperature.
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 theprocess in Fig. 6.2
T*1800
1750Flue Gas