Heat Integration in Distillation Systems

1
Heat Integration in Distillation Systems

• (1) Single Column

2
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.
3
THERMODYNAMIC ANALYSIS OF DISTILLATION SYSTEMS
6
Column Internal Subsystem
2
4
1
Heat Exchange Subsystem
7
3
5
4
THERMODYNAMIC ANALYSIS OF DISTILLATIONSYSTEMS
Heat Source Streams ? Stream to be condensed ?
Top product ? Bottom product ? Heat medium
Heat Sink Streams ? Feed ? Stream to be
reboiled ? Cooling medium
5
For Heat Source
Composite Curve
Q
Q
CHANGE OF AVAILABLE ENERGY
6
For Heat Sink
Composite Curve
Q
Q
CHANGE OF AVAILABLE ENERGY
7
?
?
(a) INITIAL SETTING UP NO HEAT IS
RECOVERED (b)SHIFTING
HEAT RECOVERY IS INCREASED (c)PINC
H-POINT FINDING AND ELIMINATING
??
?
?
?
?
?
?
?
?
QC
QH
?
QC
QH
QR
HEAT ENERGY
8
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-REBOILER
LEGEND 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.
FOR LINES
fP
FOR SEGMENETS
FOR SEGMENTS
fP
9
utility user
new exchanger
T
R
C
Figure. 5.(a) Iterative repetition of the
operations.
10
utility user
(a)
new exchanger
fE
T
R
C
Q
Figure. 5.(b) Iterative repetition of the
operations.
11
utility user
(b)
new exchanger
fT
T
R
IR INTER- REBOILER
C
Q
Figure. 5.(c) Iterative repetition of the
operations.
12
utility user
(c)
new exchanger
fE
T
R
IR
C
Q
Figure. 5.(d) Iterative repetition of the
operations.
13
utility user
(d)
new exchanger
fT
T
R
IR
IC
C
INTER- CONDENSER
Q
Figure. 5.(e) Iterative repetition of the
operations.
14
utility user
(e)
new exchanger
fE
T
R
IR
IC
C
Q
Figure. 5.(f) Iterative repetition of the
operations.
15
utility user
(b)
new exchanger
Lower Pressure
fP
T
R
C
Q
Figure. 5.(g) Iterative repetition of the
operations.
16
utility user
(g)
new exchanger
fP
T
R-2
1
C-2
R-1
C-1
2
Q
Figure. 5.(h) Iterative repetition of the
operations.
17
(h)
fE
T
R-2
1
C-2
R-1 MULTI- EFFECT
C-1
2
Q
Figure. 5.(i) Iterative repetition of the
operations.
18
Heat Integration in Distillation Systems

• (2) Multi-Effect Distillation

19
T
Q
Cold Stream
Treb
Hot Stream
Tcond
Q
Q
20
Q
Treb
COLD
Q
Tcond
HOT
Composite Curves for Single Column
21
T
Q1
Q
Q
0
Grand Composite Curves for Single Column
22
2
A
Low pressure
B
AB
A
1
High pressure
FIGURE A.6-1 Multieffect column.From M. J.
Andrecovich and A. W. Westerburg. AIChE., 31
363 (1985).
B
23
DOUBLE-EFFECT DISTILLATION
T
1 2
Q
24
LOWER BOUND ON UTILITY CONSUMPTION
T
Q
Q
25
T
4
3
2
1
Qmin
Q
FIGURE A.6-3 Minimum utility, multieffect
configuration for four separations. From M. J.
Andrecovich and A. W. Westerburg. AIChE., 31
363 (1985).
26
T
(a)
(b) (c)
?
?
?
?
?
?
?
2B
?
2A
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).
27
Heat Integration Between Heat Exchange Network
and Distillation Columns
28
Heat out
Qcond
Tcond
Treb
Qreb
ColN
Feed
Qcond
Tcond
Qreb
Treb
Heat in
Fig. 6. Distillation column takes in and rejects
heat
29
THE HEAT FLOW CASCADE
Qhmin
Qh
SINK
T1
Qh
Q1
1 2 3 4 5
T1 T2 T3 T4 T5
T2
Q2
T3
PINCH
Q3
Qc
T4
Q4
T5
SOURCE
Qcmin
Q5
Fig. 3. Use of the cascade to minimise utility
requirements.
30
Fig. 7. Distillation across the pinch.
NOTE ?Hk Qk - Qk-1
Qhmin Qreb
?H1 Q1Qreb ?H2 Q2Qreb Q3 ?H4 Q4 ?H5 ?H6 Q7
Q7Qcond ?H8
( Cold utility )
?H3
Qreb
Treb gt Tpinch gt Tcond
Coln
PINCH
0
Q5 0
Qcond
?H7
( Hot utility ) NO BENEFIT !
Qcmin Qcond
31
Fig. 8. Distillation not across the pinch.
Qhmin (Qreb - Qcond) Qh,T
Qh,T lt Qh,min
If Qcond gt Qreb
Note
?H1 Q1Qreb -Qcond ?H2 Q3 ?H4 ?H5 Q5-Qreb
Q6Qcond -Qreb
Qh,T lt (Qh,min Qreb)
Qreb
0 lt Qcond
Qh,T lt Qh,min
If Qcond lt Qreb
Q2-Qcond
Coln
Qh,T Qh,min
If Qcond Qreb
?H3
Qcond
PINCH
0
Q4 0
Qreb
Qc,T lt Qc,min
If Qcond lt Qreb
Coln
?H6
Qcond
Note
Qc,T lt Qc,min
If Qcond gt Qreb
Qc,T lt (Qc,min Qcond)
?H7
0 lt Qreb
Qcmin (Qcond - Qreb) Qc,T
Qc,T Qc,min
If Qcond Qreb
32
Fig. 9. Control considerations.
INTEGRATION FLEXIBILITY
Qh,T Qh,min (Qreb - Qcond)
Qhmin - Qcond
Qreb
ColN
Qcond
( Hot utility )
PINCH
0
( Cold utility )
ColN
Qcond
Qcmin - Qreb
Qc,T Qc,min (Qcond - Qreb)
33
Heat Load Limits
Qhmin (Qreb - Qcond)
Q2 gt Qcond Q3 gt Qcond Q1 Qreb gt Qcond
1 Q1Qreb -Qcond
Qreb
Cold utility
2 Q2-Qcond
ColN
3 Q3-Qcond
must be satisfied to avoid negative heat flow
Qcond
4 Q4
hot utility
SINK
5
Q5 0
0
Fig. 10. Heat load limit general.
34
Heat Load Limits
Qhmin - Qcond
Qreb
Q1 gt Qcond Q2 gt Qcond Q3 gt Qcond
Q1-Qcond
Q2-Qcond
ColN
Q3-Qcond
must be satisfied to avoid negative heat flow
Qcond
Q4
hot utility
SINK
Q5 0
0
Fig. 11. Heat load limit condenser integration
only.
35
METHODS OF FORCING COLUMNS AWAY FROM THE
PINCH
Originally After Q3 lt
Qcond Qcond1 lt Q3 lt Qcond
Q7 lt Qreb Qreb2 lt Q7 lt Qreb
1) Pressure Changes 2) Split Column Loads
Qcond2
Qhmin (Qreb1 - Qcond1)
Feed
Qreb1
P
2
Qreb2
ColN1
Q3 - Qcond1
P
Qcond1
Qcond1
Qreb2
ColN2
P
Q7 - Qreb2
Qcond2
1
P
Qcmin (Qcond2 - Qreb2)
Qreb1
Fig. 13. Splitting the load
36
METHODS OF FORCING COLUMNS AWAY FROM THE
PINCH
3) Thermal Coupling
Conventional Arrangement
A
Qreb2
T
A B C
B
Qreb1
1
2
Qcond2
Qcond1
C
Heat Load
Fig. 14. Side-stream rectifier reduces heat load
requirements.
37
METHODS OF FORCING COLUMNS AWAY FROM THE
PINCH
3) Thermal Coupling
Side-stream Rectifier
Qreb1
T
A
A B C
B
1
Qcond2
2
Qcond1
C
Heat Load
Fig. 14. Side-stream rectifier reduces heat load
requirements.(?)
38
METHODS OF FORCING COLUMNS AWAY FROM THE
PINCH
4) Intermediate Reboilers and Condensers (B)
Originally Treb gt Tpinch gt Tcond (C) Originally
Q4 lt Qcond(original) Qcond Qint
A
B
C
Qhmin (Qreb - Qint)
Qhmin (Qreb - Qint - Qcond)
Q1 Qreb - Qint - Qcond
Qreb
Q1 Qreb - Qint
Qreb
Q2 - Qint - Qcond
Coln
Q2 - Qint
Q3 - - Qint - Qcond
Qint
Coln
Q3 - Qint
Q4 - Qcond
Qcond
Qint
Q4
Q5
0
PINCH
PINCH
0
Qcond
Q6
Q7
Q7 - Qcond
Qcmin Qcond
Qcmin
Qcond,new Qcond,old - Qint
Fig. 15. Appropriate placement of an
intermediate condenser.
39
CURRENT DESIGN PRACTICE FOR SAVING ENERGY
IN DISTILLATION
Heat in Pump T lower
1) Heat Pump
Qcond
Qhmin - (W Qcond- Qreb)
A.
B.
C.
Qhmin Qreb
W (Qcond- Qreb)
W
Qreb
Qreb
H.P.
Coln
W
PINCH
0
PINCH
0
Coln
Qcond
Qcond
W (Qcond - Qreb)
Qreb
Qcmin Qcond
Qcmin
T higher Heat out Pump
to process
Fig. 17. Heat pumping the last resort.
Qtotal Qh,min - (W Qcond - Qreb) W Qh,min
(Qreb - Qcond)
40
HEAT ENGINES
RESERVIOR
T1
Q1
Heat Engine
W
Q2
T2
RESERVIOR
First Law of Thermodynamics
Second Law of Thermodynamics
where

41
HEAT PUMPS
RESERVIOR
T1
Q1
Heat Pump
W
Q2
T2
RESERVIOR
First Law of Thermodynamics
Second Law of Thermodynamics
where

42
Trim Cooling
W
FEED
OVERHEADS
Liquid
Vapor
BOTTOMS
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. )
43
CURRENT DESIGN PRACTICE FOR SAVING ENERGY
IN DISTILLATION
2) Multiple Effect Distillation
Load Qcond2
Qhmin Qreb1
Qreb1
Feed
P
2
Coln 1
0
PINCH
Coln 2
1
P
Qcond2
Qcmin Qcond2
Load Qreb1
Fig. 18. Multiple effect distilltion dont use
it prior to integration studies.
44
CURRENT DESIGN PRACTICE FOR SAVING ENERGY
IN DISTILLATION
3) Thermally Coupled Columns
A
A
B
A B C
1
B
P1
A B C
1
2
2
P2
C
C
Qhmin (Qreb2 - Qcond2)
Qhmin Qreb1
Qreb1
Coln2
0
PINCH
PINCH
0
Coln1
Coln 1 2
Qcond2
Qcond1
Qcmin (Qcond1 - Qreb1)
Qcmin Qcond1 Qcond2
Fig. 19. Thermal coupling of columns.