heat integration in distillation systems (1) single column
TRANSCRIPT
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Heat Integration in Distillation Systems
(1) Single Column
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APPROACHES FOR CONSERVING ENERGYIN DISTILLATION
1. Reduce the amount of energy input for each distillation column by selecting the optimal design parameters such as reflux ration, q value,etc.
2. Reduce the total amount of energy input to the entire system by heat integration.
3. Change the temperature level of heat sinks and sources, one or both, required in the distillations, such as temperature or pressure.
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THERMODYNAMIC ANALYSIS OF DISTILLATIONSYSTEMS
2
4
3
5
1
6
7
HeatExchangeSubsystem
ColumnInternalSubsystem
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THERMODYNAMIC ANALYSIS OF DISTILLATIONSYSTEMS
Heat Sink Streams
Feed Stream to be reboiled Cooling medium
Heat Source Streams
Stream to be condensed Top product Bottom product Heat medium
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For Heat Source
T
TT aT
TT a
Q Q
Composite Curve
CHANGE OF AVAILABLE ENERGY
dQT
TT a2
1
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For Heat Sink
T
TT aT
TT a
Q Q
Composite Curve
CHANGE OF AVAILABLE ENERGY
dQT
TT a2
1
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T
TT O
T
TT O
T
TT O
+
QC QH
QC QHQR
HEAT ENERGY
(a) INITIAL SETTING UP
NO HEAT IS RECOVERED
(b)SHIFTING
HEAT RECOVERY IS INCREASED
(c)PINCH-POINT FINDING AND ELIMINATING
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fE : shifting
OPERATION APPLIED ON THE COMPOSITE LINES DERIVATED TECHNIQUES
S1 & S0 : PRESSURIZED TOWER S1 & S0 : DEPRESSURIZED TOWER
S0 : VAPOR RECOMPRESSION
S1 : BOTTON LIQUID FLASH S1 & S0 : MULTI-EFFECT DIST N
S0 : INTER-CONDENSER, SLOPPY SEPARATION
S1 : INTER-REBOILER, SLOPPY SEPARATION S1 & S0 : INTER-CONDENSER/INTER-REBOILERLEGEND S1 : THE COMPOSITE HEAT SINK LINE S0 : THE COMPOSITE HEAT SOURCE LINE R : TO RAISE L : TO LOWER
Fig. 4. Possible systems generated by one-step operation on a binary distillation system.
fP
fP
FORLINES
FORSEGMENETS
FORSEGMENTS
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T
C
R
Figure. 5.(a) Iterative repetition of the operations.
: utility user
: new exchanger
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T
C
R
Figure. 5.(b) Iterative repetition of the operations.
: utility user
: new exchanger
fE
Q
(a)
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T
C
R
Figure. 5.(c) Iterative repetition of the operations.
: utility user
: new exchanger
fT
Q
IRINTER-REBOILER
(b)
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T
C
R
Figure. 5.(d) Iterative repetition of the operations.
: utility user
: new exchanger
fE
Q
IR
(c)
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T
C
R
Figure. 5.(e) Iterative repetition of the operations.
: utility user
: new exchanger
fT
Q
IRIC INTER-
CONDENSER
(d)
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T
C
R
Figure. 5.(f) Iterative repetition of the operations.
: utility user
: new exchanger
fE
Q
IRIC
(e)
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T
C
R
Figure. 5.(g) Iterative repetition of the operations.
: utility user
: new exchanger
fP
Q
Lower Pressure
(b)
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T
C-1
R-2
Figure. 5.(h) Iterative repetition of the operations.
: utility user
: new exchanger
fP
Q
R-1
C-2
(g)
1
2
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T
C-1
R-2
Figure. 5.(i) Iterative repetition of the operations.
fE
Q
R-1MULTI-EFFECT
C-2
(h)
2
1
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Heat Integration in Distillation Systems
(2) Multi-Effect Distillation
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Treb
Tcond
Q
Q
ColdStream
HotStream
Q
T
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Treb
Tcond
Q
Q
Composite Curves for Single Column
HOT
COLD
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T
Q
Q1
Q
Grand Composite Curves for Single Column
0
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2
1
AB
2CQ
1RQ
A
AB
B
Low pressure
High pressureFIGURE A.6-1Multieffect column.[From M. J. Andrecovich and A. W. Westerburg. AIChE., 31 : 363 (1985).]
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1
2
Q
T
DOUBLE-EFFECT DISTILLATION
min2, 2
1TTreb
min1, 2
1TTcond
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LOWER BOUND ON UTILITY CONSUMPTION
Q
Q
T
availT
minTT
minQ
min2
1TTSTM
min2
1TTCW
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1
2
3
4
Qmin
Q
T
FIGURE A.6-3 Minimum utility, multieffect configuration for four separations. [From M. J. Andrecovich and A. W. Westerburg. AIChE., 31 : 363 (1985).]
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2A
2B
(a) (b) (c)
T
Q
FIGURE A.6-4 Varying utilities: (a)Three columns; (b)stacked configuration; (c)multieffect. [From M. J. Andrecovich and A. W. Westerburg, AIChE., 31: 363 (1985).]
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Heat Integration Between Heat Exchange Network and Distillation Columns
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ColN
Heatout
QcondTcond
Qcond
Heatin
Tcond
Treb Qreb
QrebTreb
Feed
Fig. 6. Distillation column takes in and rejects heat
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THE HEAT FLOW CASCADE
12345
T1
T2
T3
T4
T5
Qh
Qc
Qh
Q1
Q2
Q3
Q4
Q5
T1
T2
T3
T4
T5
Fig. 3. Use of the cascade to minimise utility requirements.
Qhmin
Qcmin
SINK
PINCH
SOURCE
1H
2H
3H
4H
5H
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NOTE : Hk = Qk - Qk-1
H1
Q1+Qreb
H2
Q2+Qreb
Q3
H4Q4
H5
H6
Q7
Q7+Qcond
H8
H3
PINCH Coln
H7
0Q5 = 0
Qreb
Qcond
Qhmin + Qreb
Treb > Tpinch > Tcond
( Cold utility )
( Hot utility )
NO BENEFIT !
Qcmin + Qcond
Fig. 7. Distillation across the pinch.
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H1Q1+Qreb-Qcond
H2
Q3H4
H5
Q5-Qreb
Q6+Qcond-Qreb
H3
PINCH
Coln
H6
0 Q4 = 0Qreb
Qcond
Qhmin + (Qreb - Qcond) = Qh,T
Qcmin + (Qcond - Qreb) = Qc,T
Fig. 8. Distillation not across the pinch.
Coln
Qreb
Qcond
Qh,T < (Qh,min + Qreb)
0 < Qcond
Note
Qc,T < (Qc,min + Qcond)
0 < Qreb
Note
H7
Q2-Qcond
Qh,T < Qh,min If Qcond > Qreb
Qh,T < Qh,min If Qcond < Qreb
If Qcond = QrebQh,T = Qh,min
Qc,T < Qc,min If Qcond < Qreb
Qc,T < Qc,min If Qcond > Qreb
If Qcond = QrebQc,T = Qc,min
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PINCH
ColN
0
Qreb
Qcond
Qhmin - Qcond
( Cold utility )
( Hot utility )
Qcmin - Qreb
Fig. 9. Control considerations.
ColN
Qcond
INTEGRATION FLEXIBILITY
Qh,T = Qh,min + (Qreb - Qcond)
Qc,T = Qc,min + (Qcond - Qreb)
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Heat Load Limits
1Q1+Qreb-Qcond
2
Q2-Qcond
3
Q3-Qcond
4
Q4
5
ColN
Qreb
Qcond
Qhmin + (Qreb - Qcond)
0
SINK
hot utility
Cold utility
Q5 = 0
Q2 > Qcond
Q3 > Qcond
Q1 + Qreb > Qcond
must be satisfied to avoid negative heat flow
Fig. 10. Heat load limit: general.
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Heat Load Limits
Q1-Qcond
Q2-Qcond
Q3-Qcond
Q4
ColN
Qreb
Qcond
Qhmin - Qcond
0
SINK
hot utility
Q5 = 0
Q1 > Qcond
Q2 > Qcond
Q3 > Qcond
must be satisfied to avoid negative heat flow
Fig. 11. Heat load limit: condenser integration only.
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METHODS OF FORCING COLUMNS AWAY FROM THE PINCH
1) Pressure Changes2) Split Column Loads
2
1
Qcond2
Qcond1
Qreb2
Qreb1
P
P
Feed
Fig. 13. Splitting the load
ColN1
ColN2
Q3 - Qcond1
Q7 - Qreb2
Qhmin + (Qreb1 - Qcond1)
Qcmin + (Qcond2 - Qreb2)
Qreb1
Qcond2
Qcond1
Qreb2
P
P
Originally: After: Q3 < Qcond Qcond1 < Q3 < Qcond
Q7 < Qreb Qreb2 < Q7 < Qreb
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METHODS OF FORCING COLUMNS AWAY FROM THE PINCH3) Thermal Coupling
Conventional Arrangement
1
2
Fig. 14. Side-stream rectifier reduces heat load requirements.
T
Heat Load
A
B
C
ABC
Qreb1
Qreb2
Qcond1
Qcond2
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METHODS OF FORCING COLUMNS AWAY FROM THE PINCH3) Thermal Coupling
Side-stream Rectifier
12
Fig. 14. Side-stream rectifier reduces heat load requirements.(續)
T
Heat Load
A
B
C
ABC
Qreb1
Qcond1
Qcond2
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METHODS OF FORCING COLUMNS AWAY FROM THE PINCH 4) Intermediate Reboilers and Condensers
(B) Originally: Treb > Tpinch > Tcond
(C) Originally: Q4 < Qcond(original) = Qcond + Qint
Coln
Q1 +Qreb - Qint
Q2 - Qint
Q3 - Qint
Q7 - Qcond
Q4
Q6
0
Coln
PINCH
Qcond
Qint
Qreb
Qhmin + (Qreb - Qint)
Qcmin + Qcond
Q1 + Qreb - Qint - Qcond
Q2
- Qint - Qcond
Q3
- - Qint - Qcond
Q4
- Qcond
Q5
PINCH0
Q7
Qreb
Qint
Qcond
Qcmin
Qhmin + (Qreb
- Qint - Qcond)
Fig. 15. Appropriate placement of an intermediate condenser.Qcond,new = Qcond,old - Qint
A B C
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CURRENT DESIGN PRACTICE FOR SAVING ENERGY IN DISTILLATION
1) Heat Pump
Coln
Qhmin + Qreb
Qcmin + Qcond
Qreb
Qcond
PINCH 0
A.
Heat in Pump T lower
Qcond
Qreb
B.
W
W + (Qcond
- Qreb)
to processT higher Heat out Pump
C.
Coln H.P.
W
Qcond
Qreb
W + (Qcond- Qreb)
Qhmin - (W +
Qcond- Qreb)
Qcmin
PINCH 0
Fig. 17. Heat pumping: the last resort.Qtotal = Qh,min - (W + Qcond - Qreb) + W= Qh,min + (Qreb - Qcond)
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HEAT ENGINES
HeatEngine
RESERVIOR
RESERVIOR
T1
T2
Q1
Q2
W
First Law of Thermodynamics
Second Law of Thermodynamics
where
21 QQw
1 Q wc
1
21
T
TTc
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HEAT PUMPS
HeatPump
RESERVIOR
RESERVIOR
T1
T2
Q1
Q2
W
First Law of Thermodynamics
Second Law of Thermodynamics
where
21 QwQ
1 Q wc
1
21
T
TTc
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W
OVERHEADSFEED
BOTTOMS
TrimCooling
Liquid
Vapor
Figure. 14.6 Heat pumping in distillation. A vapor recompression scheme. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66: 195, 1988; reproduced by permission of the Institution of Chemical Engineers. )
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CURRENT DESIGN PRACTICE FOR SAVING ENERGY IN DISTILLATION
2) Multiple Effect Distillation
Coln 1
Coln 2
Fig. 18. Multiple effect distilltion: don’t use it prior to integration studies.
0 PINCH
Qcond2
Qreb1
Qhmin + Qreb1
Qcmin + Qcond2
2
1
P
P
Feed
Load = Qcond2
Load = Qreb1
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CURRENT DESIGN PRACTICE FOR SAVING ENERGY IN DISTILLATION
3) Thermally Coupled Columns
1
2
ABC
A
B
C
P1
P2
Coln2
Coln1
0PINCH
Qhmin + (Qreb2
- Qcond2)
Qcmin + (Qcond1
- Qreb1)
PINCH
Coln
1 & 2
Qhmin + Qreb1
Qcmin + Qcond1
+ Qcond2
0
Qcond1
Qcond2
Qreb1
Fig. 19. Thermal coupling of columns.
12
ABC
A
B
C