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Heat Integration in Distillation Systems
(1) Single Column
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APPROACHES FOR CONSERVING ENERGY IN 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 DISTILLATION
SYSTEMS Column Internal Subsystem 6 2 4 1 Heat Exchange Subsystem 7 3 5
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THERMODYNAMIC ANALYSIS OF DISTILLATION SYSTEMS
Heat Source Streams Stream to be condensed Top product Bottom product Heat medium Heat Sink Streams Feed Stream to be reboiled Cooling medium
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CHANGE OF AVAILABLE ENERGY
For Heat Source Composite Curve Q Q CHANGE OF AVAILABLE ENERGY
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CHANGE OF AVAILABLE ENERGY
For Heat Sink Composite Curve Q Q CHANGE OF AVAILABLE ENERGY
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(a) INITIAL SETTING UP NO HEAT IS RECOVERED (b)SHIFTING HEAT RECOVERY IS INCREASED (c)PINCH-POINT FINDING AND ELIMINATING + QC QH QC QH QR HEAT ENERGY
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fE : shifting S1 & S0 : PRESSURIZED TOWER
OPERATION APPLIED ON THE COMPOSITE LINES DERIVATED TECHNIQUES S1 & S : PRESSURIZED TOWER S1 & S : DEPRESSURIZED TOWER S : VAPOR RECOMPRESSION S : BOTTON LIQUID FLASH S1 & S : MULTI-EFFECT DIST N S : INTER-CONDENSER, SLOPPY SEPARATION S : INTER-REBOILER, SLOPPY SEPARATION S1 & S : 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
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Figure. 5.(a) Iterative repetition of the operations.
: utility user : new exchanger T R C Figure. 5.(a) Iterative repetition of the operations.
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Figure. 5.(b) Iterative repetition of the operations.
: utility user (a) : new exchanger fE T R C Q Figure. 5.(b) Iterative repetition of the operations.
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Figure. 5.(c) Iterative repetition of the operations.
: utility user (b) : new exchanger fT T R IR INTER- REBOILER C Q Figure. 5.(c) Iterative repetition of the operations.
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Figure. 5.(d) Iterative repetition of the operations.
: utility user (c) : new exchanger fE T R IR C Q Figure. 5.(d) Iterative repetition of the operations.
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Figure. 5.(e) Iterative repetition of the operations.
: utility user (d) : new exchanger fT T R IR IC C INTER- CONDENSER Q Figure. 5.(e) Iterative repetition of the operations.
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Figure. 5.(f) Iterative repetition of the operations.
: utility user (e) : new exchanger fE T R IR IC C Q Figure. 5.(f) Iterative repetition of the operations.
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Figure. 5.(g) Iterative repetition of the operations.
: utility user (b) : new exchanger Lower Pressure fP T R C Q Figure. 5.(g) Iterative repetition of the operations.
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Figure. 5.(h) Iterative repetition of the operations.
: 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.
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Figure. 5.(i) Iterative repetition of the operations.
fE T R-2 1 C-2 R-1 MULTI- EFFECT C-1 2 Q Figure. 5.(i) Iterative repetition of the operations.
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Heat Integration in Distillation Systems
(2) Multi-Effect Distillation
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T Q Cold Stream Treb Hot Stream Tcond Q Q
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Q Treb COLD Q Tcond HOT Composite Curves for Single Column
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T Q1 Q Q Grand Composite Curves for Single Column
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2 1 A Low pressure B AB A High pressure FIGURE A.6-1 B
Multieffect column.[From M. J. Andrecovich and A. W. Westerburg. AIChE., 31 : 363 (1985).] B
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DOUBLE-EFFECT DISTILLATION
1 2 Q
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LOWER BOUND ON UTILITY CONSUMPTION
Q Q
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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).]
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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).]
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Heat Integration Between Heat Exchange Network and Distillation Columns
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ColN Fig. 6. Distillation column takes in and rejects heat Heat out
Tcond Qcond Treb Qreb Feed ColN Tcond Qcond Treb Qreb Heat in Fig. 6. Distillation column takes in and rejects heat
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THE HEAT FLOW CASCADE 1 2 3 4 5 SINK PINCH SOURCE
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.
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NOTE : Hk = Qk - Qk-1 Coln Fig. 7. Distillation across the pinch.
Qhmin + Qreb H1 Q1+Qreb H2 Q2+Qreb Q3 H4 Q4 H5 H6 Q7 Q7+Qcond H8 ( Cold utility ) H3 Qreb Treb > Tpinch > Tcond Coln PINCH Q5 = 0 Qcond H7 ( Hot utility ) NO BENEFIT ! Qcmin + Qcond
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Qc,T < (Qc,min + Qcond)
Fig. 8. Distillation not across the pinch. Qh,T < Qh,min Qhmin + (Qreb - Qcond) = Qh,T If Qcond > Qreb Note H1 Q1+Qreb -Qcond H2 Q3 H4 H5 Q5-Qreb Q6+Qcond -Qreb Qh,T < (Qh,min + Qreb) Qreb 0 < Qcond Qh,T < Qh,min If Qcond < Qreb Q2-Qcond Coln Qh,T = Qh,min If Qcond = Qreb H3 Qcond PINCH Q4 = 0 Qc,T < Qc,min Qreb If Qcond < Qreb Coln H6 Qcond Note Qc,T < Qc,min If Qcond > Qreb Qc,T < (Qc,min + Qcond) H7 0 < Qreb Qc,T = Qc,min Qcmin + (Qcond - Qreb) = Qc,T If Qcond = Qreb
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INTEGRATION FLEXIBILITY
Fig. 9. Control considerations. INTEGRATION FLEXIBILITY Qh,T = Qh,min + (Qreb - Qcond) Qhmin - Qcond Qreb ColN Qcond ( Hot utility ) PINCH ( Cold utility ) ColN Qcond Qc,T = Qc,min + (Qcond - Qreb) Qcmin - Qreb
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Fig. 10. Heat load limit: general.
Heat Load Limits Qhmin + (Qreb - Qcond) Q2 > Qcond Q3 > Qcond Q1 + Qreb > Qcond 1 Q1+Qreb -Qcond Qreb Cold utility 2 Q2-Qcond ColN must be satisfied to avoid negative heat flow 3 Q3-Qcond Qcond 4 Q4 hot utility SINK 5 Q5 = 0 Fig Heat load limit: general.
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Fig. 11. Heat load limit: condenser integration only.
Heat Load Limits Qhmin - Qcond Qreb Q1 > Qcond Q2 > Qcond Q3 > Qcond Q1-Qcond Q2-Qcond ColN must be satisfied to avoid negative heat flow Q3-Qcond Qcond Q4 hot utility SINK Q5 = 0 Fig Heat load limit: condenser integration only.
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METHODS OF FORCING COLUMNS AWAY FROM THE PINCH
Originally: After: Q3 < Qcond Qcond1 < Q3 < Qcond Q7 < Qreb Qreb2 < Q7 < Qreb 1) Pressure Changes 2) Split Column Loads Qcond2 Qhmin + (Qreb1 - Qcond1) Feed Qreb1 P 2 Qreb2 Q3 - Qcond1 ColN1 P Qcond1 Qcond1 Qreb2 ColN2 P Q7 - Qreb2 Qcond2 1 P Qcmin + (Qcond2 - Qreb2) Qreb1 Fig Splitting the load
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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 Side-stream rectifier reduces heat load requirements.
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METHODS OF FORCING COLUMNS AWAY FROM THE PINCH 3) Thermal Coupling
Side-stream Rectifier Qreb1 A T A B C B 1 Qcond2 2 Qcond1 C Heat Load Fig Side-stream rectifier reduces heat load requirements.（續）
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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 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 PINCH PINCH Qcond Q6 Q7 Q7 - Qcond Qcmin + Qcond Qcmin Qcond,new = Qcond,old - Qint Fig Appropriate placement of an intermediate condenser.
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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 PINCH W PINCH Coln Qcond Qcond W + (Qcond - Qreb) Qreb Qcmin + Qcond Qcmin T higher Heat out Pump to process Fig Heat pumping: the last resort. Qtotal = Qh,min - (W + Qcond - Qreb) + W = Qh,min + (Qreb - Qcond)
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HEAT ENGINES RESERVIOR T1 Q1 W Heat Engine Q2 T2 RESERVIOR
First Law of Thermodynamics Second Law of Thermodynamics where
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HEAT PUMPS RESERVIOR T1 Q1 W Heat Pump Q2 T2 RESERVIOR
First Law of Thermodynamics Second Law of Thermodynamics where
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Trim Cooling W FEED OVERHEADS Liquid Vapor BOTTOMS Figure 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 Load = Qcond2 Qhmin + Qreb1 Qreb1 Feed P 2 Coln 1 PINCH Coln 2 1 P Qcond2 Qcmin + Qcond2 Load = Qreb1 Fig Multiple effect distilltion: don’t use it prior to integration studies.
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CURRENT DESIGN PRACTICE FOR SAVING ENERGY IN DISTILLATION
3) Thermally Coupled Columns A A B A B C 1 P1 B A B C 1 2 2 P2 C C Qhmin + (Qreb2 - Qcond2) Qhmin + Qreb1 Qreb1 Coln2 PINCH PINCH Coln1 Coln 1 & 2 Qcond2 Qcond1 Qcmin + (Qcond1 - Qreb1) Qcmin + Qcond1 + Qcond2 Fig Thermal coupling of columns.
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