Heat Integration in Distillation Systems

(1) Single Column

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.

THERMODYNAMIC ANALYSIS OF DISTILLATIONSYSTEMS

2

4

3

5

1

6

7

HeatExchangeSubsystem

ColumnInternalSubsystem

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

For Heat Source

T

TT aT

TT a

Q Q

Composite Curve

CHANGE OF AVAILABLE ENERGY

dQT

TT a2

1

For Heat Sink

T

TT aT

TT a

Q Q

Composite Curve

CHANGE OF AVAILABLE ENERGY

dQT

TT a2

1

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

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

T

C

R

Figure. 5.(a) Iterative repetition of the operations.

: utility user

: new exchanger

T

C

R

Figure. 5.(b) Iterative repetition of the operations.

: utility user

: new exchanger

fE

Q

(a)

T

C

R

Figure. 5.(c) Iterative repetition of the operations.

: utility user

: new exchanger

fT

Q

IRINTER-REBOILER

(b)

T

C

R

Figure. 5.(d) Iterative repetition of the operations.

: utility user

: new exchanger

fE

Q

IR

(c)

T

C

R

Figure. 5.(e) Iterative repetition of the operations.

: utility user

: new exchanger

fT

Q

IRIC INTER-

CONDENSER

(d)

T

C

R

Figure. 5.(f) Iterative repetition of the operations.

: utility user

: new exchanger

fE

Q

IRIC

(e)

T

C

R

Figure. 5.(g) Iterative repetition of the operations.

: utility user

: new exchanger

fP

Q

Lower Pressure

(b)

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

T

C-1

R-2

Figure. 5.(i) Iterative repetition of the operations.

fE

Q

R-1MULTI-EFFECT

C-2

(h)

2

1

Heat Integration in Distillation Systems

(2) Multi-Effect Distillation

Treb

Tcond

Q

Q

ColdStream

HotStream

Q

T

Treb

Tcond

Q

Q

Composite Curves for Single Column

HOT

COLD

T

Q

Q1

Q

Grand Composite Curves for Single Column

0

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).]

1

2

Q

T

DOUBLE-EFFECT DISTILLATION

min2, 2

1TTreb

min1, 2

1TTcond

LOWER BOUND ON UTILITY CONSUMPTION

Q

Q

T

availT

minTT

minQ

min2

1TTSTM

min2

1TTCW

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).]

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).]

Heat Integration Between Heat Exchange Network and Distillation Columns

ColN

Heatout

QcondTcond

Qcond

Heatin

Tcond

Treb Qreb

QrebTreb

Feed

Fig. 6. Distillation column takes in and rejects heat

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

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.

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

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)

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.

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.

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

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

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

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

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)

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

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

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. )

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

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