Heat Integration in Distillation Systems (1) Single Column

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.

THERMODYNAMIC ANALYSIS OF DISTILLATION SYSTEMS Heat Exchange Subsystem Column Internal Subsystem

THERMODYNAMIC ANALYSIS OF DISTILLATION SYSTEMS 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 QQ Composite Curve CHANGE OF AVAILABLE ENERGY

For Heat Sink QQ Composite Curve CHANGE OF AVAILABLE ENERGY

      ++ QCQC QHQH QCQC QHQH QRQR HEAT ENERGY (a) INITIAL SETTING UP NO HEAT IS RECOVERED (b)SHIFTING HEAT RECOVERY IS INCREASED (c)PINCH-POINT FINDING AND ELIMINATING     

f E : 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. fPfP fPfP FOR LINES FOR SEGMENETS FOR SEGMENTS

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 fEfE Q (a)

T C R Figure. 5.(c) Iterative repetition of the operations. : utility user : new exchanger fTfT Q IR INTER- REBOILER (b)

T C R Figure. 5.(d) Iterative repetition of the operations. : utility user : new exchanger fEfE Q IR (c)

T C R Figure. 5.(e) Iterative repetition of the operations. : utility user : new exchanger fTfT Q IR IC INTER- CONDENSER (d)

T C R Figure. 5.(f) Iterative repetition of the operations. : utility user : new exchanger fEfE Q IR IC (e)

T C R Figure. 5.(g) Iterative repetition of the operations. : utility user : new exchanger fPfP Q Lower Pressure (b)

T C-1 R-2 Figure. 5.(h) Iterative repetition of the operations. : utility user : new exchanger fPfP Q R-1 C-2 (g) 1 2

T C-1 R-2 Figure. 5.(i) Iterative repetition of the operations. fEfE Q R-1 MULTI- EFFECT C-2 (h) 2 1

Heat Integration in Distillation Systems (2) Multi-Effect Distillation

T reb T cond Q Q Cold Stream Hot Stream Q T

T reb T cond Q Q Composite Curves for Single Column HOT COLD

T Q Q1Q1 Q Grand Composite Curves for Single Column 0

2 1 AB A A B B Low pressure High pressure FIGURE A.6-1 Multieffect column.[From M. J. Andrecovich and A. W. Westerburg. AIChE., 31 : 363 (1985).]

1212 Q T DOUBLE-EFFECT DISTILLATION

LOWER BOUND ON UTILITY CONSUMPTION Q Q T

Q min 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).]

      2A2A 2B2B   (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

Col N Heat out Q cond T cond Q cond Heat in T cond T reb Q reb T reb Feed Fig. 6. Distillation column takes in and rejects heat

THE HEAT FLOW CASCADE T1T2T3T4T5T1T2T3T4T5 QhQh QcQc QhQh Q1Q1 Q2Q2 Q3Q3 Q4Q4 Q5Q5 T1T1 T2T2 T3T3 T4T4 T5T5 Fig. 3. Use of the cascade to minimise utility requirements. Q h min Q c min SINK PINCH SOURCE

NOTE :  H k = Q k - Q k-1  H 1 Q 1 +Q reb  H 2 Q 2 +Q reb Q 3  H 4 Q 4  H 5  H 6 Q 7 Q 7 +Q cond  H 8 H3H3 PINCH Col n H7H7 0 Q 5 = 0 Q reb Q cond Q h min + Q reb T reb > T pinch > T cond ( Cold utility ) ( Hot utility ) NO BENEFIT ! Q c min + Q cond Fig. 7. Distillation across the pinch.

 H 1 Q 1 +Q reb -Q cond  H 2 Q 3  H 4  H 5 Q 5 -Q reb Q 6 +Q cond -Q reb H3H3 PINCH Col n H6H6 0 Q 4 = 0 Q reb Q cond Q h min + (Q reb - Q cond ) = Q h,T Q c min + (Q cond - Q reb ) = Q c,T Fig. 8. Distillation not across the pinch. Col n Q reb Q cond Q h,T < (Q h,min + Q reb ) 0 < Q cond Note Q c,T < (Q c,min + Q cond ) 0 < Q reb Note H7H7 Q 2 -Q cond Q h,T < Q h,min If Q cond > Q reb Q h,T < Q h,min If Q cond < Q reb If Q cond = Q reb Q h,T = Q h,min Q c,T < Q c,min If Q cond < Q reb Q c,T < Q c,min If Q cond > Q reb If Q cond = Q reb Q c,T = Q c,min

PINCH Col N 0 Q reb Q cond Q h min - Q cond ( Cold utility ) ( Hot utility ) Q c min - Q reb Fig. 9. Control considerations. Col N Q cond INTEGRATION FLEXIBILITY Q h,T = Q h,min + (Q reb - Q cond ) Q c,T = Q c,min + (Q cond - Q reb )

Heat Load Limits 1 Q 1 +Q reb -Q cond 2 Q 2 -Q cond 3 Q 3 -Q cond 4Q44Q4 5 Col N Q reb Q cond Q h min + (Q reb - Q cond ) 0 SINK hot utility Cold utility Q 5 = 0 Q 2 > Q cond Q 3 > Q cond Q 1 + Q reb > Q cond must be satisfied to avoid negative heat flow Fig. 10. Heat load limit: general.

Heat Load Limits Q 1 -Q cond Q 2 -Q cond Q 3 -Q cond Q4Q4 Col N Q reb Q cond Q h min - Q cond 0 SINK hot utility Q 5 = 0 Q 1 > Q cond Q 2 > Q cond Q 3 > Q cond 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 Changes 2) Split Column Loads 2 1 Q cond 2 Q cond 1 Q reb 2 Q reb 1 P P Feed Fig. 13. Splitting the load Col N 1 Col N 2 Q 3 - Q cond1 Q 7 - Q reb2 Qh min + (Q reb1 - Q cond1 ) Qc min + (Q cond2 - Q reb2 ) Q reb1 Q cond2 Q cond1 Q reb2 P P Originally: After: Q 3 < Q cond Q cond1 < Q 3 < Q cond Q 7 < Q reb Q reb2 < Q 7 < Q reb

METHODS OF FORCING COLUMNS AWAY FROM THE PINCH 3) Thermal Coupling Conventional Arrangement 1 2 Fig. 14. Side-stream rectifier reduces heat load requirements. T Heat Load A B C ABCABC Q reb1 Q reb2 Q cond1 Q cond2

METHODS OF FORCING COLUMNS AWAY FROM THE PINCH 3) Thermal Coupling Side-stream Rectifier 1 2 Fig. 14. Side-stream rectifier reduces heat load requirements. （續） T Heat Load A B C ABCABC Q reb1 Q cond1 Q cond2

METHODS OF FORCING COLUMNS AWAY FROM THE PINCH 4) Intermediate Reboilers and Condensers (B) Originally: T reb > T pinch > T cond (C) Originally: Q 4 < Q cond(original) = Q cond + Q int Col n Q 1 + Q reb - Q int Q 2 - Q int Q 3 - Q int Q 7 - Q cond Q4Q4 Q6Q6 0 Col n PINCH Q cond Q int Q reb Q h min + (Q reb - Q int ) Q c min + Q cond Q 1 + Q reb - Q int - Q cond Q 2 - Q int - Q cond Q Q int - Q cond Q 4 - Q cond Q5Q5 PINCH0 Q7Q7 Q reb Q int Q cond Q c min Q h min + (Q reb - Q int - Q cond ) Fig. 15. Appropriate placement of an intermediate condenser. Q cond,new = Q cond,old - Q int ABC

CURRENT DESIGN PRACTICE FOR SAVING ENERGY IN DISTILLATION 1) Heat Pump Col n Q h min + Q reb Q c min + Q cond Q reb Q cond PINCH0 A. Heat in Pump T lower Q cond Q reb B. W W + (Q cond - Q reb ) to process T higher Heat out Pump C. Col n H.P. W Q cond Q reb W + (Q cond - Q reb ) Q h min - (W + Q cond - Q reb ) Q c min PINCH0 Fig. 17. Heat pumping: the last resort. Q total = Q h,min - (W + Q cond - Q reb ) + W = Q h,min + (Q reb - Q cond )

HEAT ENGINES Heat Engine RESERVIOR T1T1 T2T2 Q1Q1 Q2Q2 W First Law of Thermodynamics Second Law of Thermodynamics where

HEAT PUMPS Heat Pump RESERVIOR T1T1 T2T2 Q1Q1 Q2Q2 W First Law of Thermodynamics Second Law of Thermodynamics where

W OVERHEADS FEED BOTTOMS Trim Cooling Liquid Vapor 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. )

CURRENT DESIGN PRACTICE FOR SAVING ENERGY IN DISTILLATION 2) Multiple Effect Distillation Col n 1 Col n 2 Fig. 18. Multiple effect distilltion: don’t use it prior to integration studies. 0PINCH Q cond 2 Q reb 1 Q h min + Q reb 1 Q c min + Q cond P P Feed Load = Q cond 2 Load = Q reb 1

CURRENT DESIGN PRACTICE FOR SAVING ENERGY IN DISTILLATION 3) Thermally Coupled Columns 1 2 ABCABC A B C P1P1 P2P2 Col n 2 Col n 1 0PINCH Q h min + (Q reb 2 - Q cond 2 ) Q c min + (Q cond 1 - Q reb 1 ) PINCH Col n 1 & 2 Q h min + Q reb 1 Q c min + Q cond 1 + Q cond2 0 Q cond 1 Q cond 2 Q reb 1 Fig. 19. Thermal coupling of columns. 1 2 ABCABC A B C