Hierarchy of Decisions

HEAT EXCHANGER NETWORK (HEN)

SUCCESSFUL APPLICATIONS O ICI ---- Linnhoff, B. and Turner, J. A., Chem. Eng., Nov. 2, 1981 Energy savings Capital Cost Available Expenditure Process Facility* k$/yr or Saving, k$ Organic Bulk Chemical New 800 same Specialty Chemical New 1600 saving Crude Unit Mod 1200 saving Inorganic Bulk Chemical New 320 saving Specialty Chemical Mod 200 160 New 200 saving General Bulk Chemical New 2600 unclear Inorganic Bulk Chemical New 200 to 360 unclear Future Plant New 30 to 40 % 30 % saving Specialty Chemical New 100 150 Unspecified Mod 300 1000 New 300 saving General Chemical New 360 unclear Petrochemical Mod Phase I 1200 600 Phase II 1200 1200 *New means new plant; Mod means plant modification.

Linnhoff and Vredeveld, CEP, July, 1984 SUCCESSFUL APPLICATIONS Table 1. First results of applying the pinch technology in Union Carbide Project Energy Cost Installed Payback Process Type Reduction $/yr Capital Cost $ Months Petro-Chemical Mod. 1,050,000 500,000 6 Specialty Chemical Mod. 139,000 57,000 5 Specialty Chemical Mod. 82,000 6,000 1 Licensing Package New 1,300,000 Savings  Petro-Chemical Mod. 630,000 Yet Unclear ? Organic Bulk Mod. 1,000,000 600,000 7 Chemical Organic Bulk Mod. 1,243,000 1,835,000 18 Specialty Chemical Mod. 570,000 200,000 4 Organic Bulk Mod. 2,000,000 800,000 5 Linnhoff and Vredeveld, CEP, July, 1984

SUCESSFUL APPLICATIONS  Fluor --- IChE Symp. Ser., No. 74, 1982, P.19 --- CEP, July, 1983, P.33  FMC (Marine Colloid Division, Rockland, ME)

CONCLUSION separate and distinct task in process design HEN/MEN synthesis can be identified as a separate and distinct task in process design

IDENTIFY HEAT RECOVERY AS A SEPARATE AND DISTINCT TASK IN PROCESS DESIGN. 9.60 200C 18.2 bar  1.089 36C 16 bar  RECYCLE REACTION 7.841 126C 18.7 bar TO COLUMN D 201  1.614 0 179 200C  PURGE CW 180C 153C 35C 7 703 FLASH 141C 40C 115.5C 17.3 bar 120C 17.6 bar FEED 5C 19.5 bar 114C  Figure 2.5 - Flowsheet for “front end” of specialty chemicals process

FOR EACH STREAM: TINITIAL, TFINAL, H = f(T). 126C Reactor 35C 200C RECYCLE  TOPS Product Purge Reactor 35C 5C FEED FOR EACH STREAM: TINITIAL, TFINAL, H = f(T). PRODUCT 126C Figure 2.6-Specialty chemicals process-heat exchange duties

。 。  = 1722  = 654 a ) DESIGN AS USUAL H 6 UNITS REACTOR C STEAM  = 1722  = 654 a ) DESIGN AS USUAL H 6 UNITS REACTOR C STEAM RECYCLE 70 1 。 。 STEAM 1652 3 2 654 COOLING WATER FEED PRODUCT

。 。 。 。  = 1068  = 0 b ) DESIGN WITH TARGETS H 4 UNITS REACTOR C  = 1068  = 0 b ) DESIGN WITH TARGETS H 4 UNITS REACTOR C STEAM 。 RECYCLE 1068 。 。 1 。 2 3 FEED PRODUCT

SUGGESTED PROCEDURE FOR THE DESIGN OF NEW HEAT EXCHANGER NETWORKS 1. Determine Targets.  Energy Target -maximum recoverable energy  Capital Target -minimum number of heat transfer units. -minimum total heat transfer area 2. Generate Alternatives to Achieve Those Targets. 3. Modify the Alternatives Based on Practical Considerations. 4. Equipment Design and Costing for Each Alternative. 5. Select the Most Attractive Candidate.

STEP ONE Determine the Targets

§ ENERGY TARGETS (TWO STREAM HEAT EXCHANGE)  T/H DIAGRAM Q =CP(TT-TS) TT TS H H Figure 2.10 - Representation of process streams in the T/H diagram

TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM 200 UTILITY HEATING 140 135 115 100 70 UTILITY COOLING 350 300 400 H (KW) TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM

TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM 200 UTILITY HEATING 130 135 T 120 100 70 UTILITY COOLING 350 300 400 H (KW) -100 +100 -100 =250 =400 =300 TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM

( ) ( ) FACTS 1.   Total Utility Load  Increa se Increa se 2. in = in Hot Utility Cold Utility ( ) ( )

§ENERGY TARGETS (MANY HOT AND COLD STREAMS)  COMPOSITE CURVES (T1-T2) (B) (T2-T3) (A+B+C) (T3-T4) (A+C) (T4-T5) (A) CP=B CP=A CP=C H

§ENERGY TARGETS (MANY HOT AND COLD STREAMS)  COMPOSITE CURVES     T1 T2 T3 T4 T5 H

 PINCH POINT Minimum T hot utility “PINCH” minimum cold utility H Energy targets and “the Pinch” with Composite Curves

m hot Qin Streams n cold Streams Qout - Qin = H Qout or Heat Exchange System n cold Streams Qout - Qin = H Qout or

The “Problem Table” Algorithm - A Targeting Approach ---Linnhoff and Flower, AIChE J. (1978) Stream No. CP TS TT and Type (KW/C) (C) (C) (C) (C) (1) Cold 2 20 25 T6 135 140 T3 (2) Hot 3 170 165 T1 60 55 T5 (3) Cold 4 80 85 T4 140 145 T2 (4) Hot 1.5 150 145 (T2) 30 25 (T6) Tmin = 10C

Subsystem # CPHot - CPcold TK HK T1* = 165C T2* = 145C T3* = 140C T4* = 85C T5* = 55C T6* = 25C 2 1 20 3.0 60 2 5 0.5 2.5 3 55 -1.5 -82.5 4 30 2.5 75 5 30 -0.5 -15 4 3 1

90C 145C Heat Exchange Subsystem (3) 80C 135C from subsys #2 hot streams 145C Heat Exchange Subsystem (3) . . . . . Cold streams 80C 135C To subsys #4

T1* = 165C -------------------------- ( 0 )------ FROM HOT UTILITY minimum hot utility 20 H1 = 60 H2 = 2.5 H3 = -82.5 Pinch H4 = 75 H5 = -15 minimum cold utility 60 TO COLD UTILITY

§ “PROBLEM TABLE” ALFORITHM  SUBSYSTEM TM TC=T 0 (T0) 1 (T1) 2 (T2) TP Tmin Hh2Hc2 Hh1 Hc1

§ “PROBLEM TABLE” ALFORITHM  ENTHALPY BALANCE OF SUBSYSTEM As T = T1 - T2  0

5. The Grand Composite Curve 80 60 40 20 -20 Q(KW) CU Qc,min “Pinch” HU Qh,min 20 40 60 80 100 120 140 160 180 T6* T5* T4* T3*T2* T1*

SIGNIFICANCE OF THE PINCH POINT 1. Do not transfer heat across the pinch 2. Do not use cold utility above 3. Do not use hot utility below

Q Qh Qh HU Qc,min CU Qh,min Tc Tp Th T Qh  Qh,min Qc  Qc,min

Q CU Qc,min Qh,min HU Tc Tp T1 Th T

Q Qc CU2 Qh HU Qc,min CU1 Qh,min Tc Tp Th T

Q Qh,min HU Qc,min CU Tc Tp T1 Th T

Q Qh,min HU2 Qc,min Q1 CU Q2 HU1 Tc Tp T1 Tp’ Th T

H=27MW H= -30MW FEED 2 140 PRODUCT2 230 REACTOR 2 200 80 H=32MW FEED 1 20 REACTOR 1 180 250 OFF GAS 40 H= -31.5MW 40 PRODUCT1 40 Figure 6.2 A simple flowsheet with two hot streams and two cold streams.

Supply Target capacity temp. temp. H flow rate CP TABLE 6.2 Heat Exchange Stream Data for the Flowsheet in Fig. 6.2 Heat Supply Target capacity temp. temp. H flow rate CP Stream Type TS (C) TT (C) (MW) (MW C-1) 1. Reactor 1 feed Cold 20 180 32.0 0.2 2. Reactor 1 product Hot 250 40 -31.5 0.15 3. Reactor 2 feed Cold 140 230 27.0 0.3 4. Reactor 2 product Hot 200 80 -30.0 0.25

HOT UTILITY HOT UTILITY H= -1.5 H= -1.5 H= 6.0 H= 6.0 H= -1.0 (a) HOT UTILITY (b) HOT UTILITY 245C 0MW 7.5MW H= -1.5 H= -1.5 235C 1.5MW 9.0MW H= 6.0 H= 6.0 195C -4.5MW 3.0MW H= -1.0 H= -1.0 185C -3.5MW 4.0MW H= 4.0 H= 4.0 145C -7.5MW 0MW H= -14.0 H= -14.0 75C 6.5MW 14.0MW H= 2.0 H= 2.0 35C 4.5MW 12.0MW H= 2.0 H= 2.0 25C 2.5MW 10.0MW COLD UTILITY COLD UTILITY Figure 6.18 The problem table cascade.

Figure 6.24 The grand composite curve shows the utility requirements in both enthalpy and temperature terms.

(a) BOILER T* HP Steam LP Steam pinch CW H Process HP Stream Process Fuel Boiler Feedwater (Desuperheat) BOILER LP Stream Condensate T* HP Steam LP Steam pinch CW H Figure 6.25. The grand composite curve allows alternative utility mixes to be evaluated.

Hot Oil (b) T* pinch CW H Hot Oil Return Fuel FURNACE Process Hot Oil Flow T* Hot Oil pinch CW H Figure 6.25. The grand composite curve allows alternative utility mixes to be evaluated.

(a) TC 300 250 200 150 100 50 0 5 10 15 H(MW) HP Steam LP Steam HP Steam LP Steam 0 5 10 15 H(MW) Figure 6.26 Alternative utility mixes for the process in Fig. 6.2.

(b) TC 300 250 200 150 100 50 Hot Oil 0 5 10 15 H(MW) Figure 6.26 Alternative utility mixes for the process in Fig. 6.2.

T*TFT Theoretical Flame Temperature T*O T*STACK QHmin Flue Gas Air Fuel T*STACK T*O Ambient Temperature Stack Loss ambient temp. QHmin H Fuel Figure 6.27 Simple furnace model.

T*’TFT T*TFT Flue Gas T*STACK T*O Stack Loss T* Figure 6.28 Increasing the theoretical flame temperature by reducing excess air or combusion air preheat reduces the stack loss. T*STACK T*O Stack Loss H

T* T* away from the pinch T*TFT T* T*TFT T*ACID DEW T*PINCH T*C T*ACID DEW T*PINCH T*C (a)Stack temperature limited by acid dew point (b)Stack temperature limited by process away from the pinch Figure 6.29 Furnace stack temperature can be limited by other factors than pinch temperature.

T* 1800 1750 Flue Gas 300 250 200 150 100 50 0 5 10 15 H(MW) Figure 6.30 Flue gas matched against the grand composite curve of the process in Fig. 6.2

SOME RESULTS IN GRAPH THEORY 1 ) A graph is any connection of points, some pairs of which are connected by lines. 2 ) If a graph has p points and q lines, it is called a (p,q) graph. points process and utility streams lines heat exchangers 3 ) A path is a sequence of distinct lines, each are starting where the previous are ends, e.g. AECGD in Fig. A. A B C D Figure A Figure B E F G H A B C D E F G H

SOME RESULTS IN GRAPH THEORY 4 ) A graph is connected if any two points can be joined by a path, e. g. Fig. A 5 ) Points which are connected to some fired point by paths are said to form a component, e. g. Fig A has one component. Fig B has two components. 6 ) A cycle is a path which begins and ends at the same point, e. g. CGDHC in Fig. A. 7 ) The maximum number of independent cycles is called the cycle rank of the graph. 8 ) The cycle rank of a (p,q) graph with k components is q - p + k

A Result Based on Graph Theory U = N+L-S Where, N = the total number of process and utility streams L = the number of independent loops S = the number of separate component in a network U = the number of heat exchanger services

U = N+L-S U = N-1 = 5 U = N-2 = 4 U = N+1-1 = N = 6 30 70 90 ST H1 H2 30 70 90 ST H1 H2 U = N-1 = 5 U = N-2 = 4 U = N+1-1 = N = 6 30 10 60 40 50 C1 C2 CW 40 100 50 30 70 90 ST H1 H2 30 70 40 50 C1 C2 CW 40 100 50 30 70 90 ST H1 H2 X 60-X 30-X 10+X 40 50 C1 C2 CW 40 100 50

CAPITAL TARGET Umin = N - 1 where, Umin = the minimum number of services N = the total number of process and utility streams Note, U = N + L - S

§ PINCH DESIGN METHOD RULE 1: THE “TICK-OFF” HEURISTIC UMIN = N-1 - THE EQUATION IS SATISFIED IF EVERY MATCH BRINGS ONE STREAM TO ITS TARGET TEMPERATURE OR EXHAUSTS A UTILITY. - FEASIBILITY CONSTRAINTS : ENERGY BALANCE TMIN

Example 1 Stream No TS TF CP Heat Load and Type (F) (F) 104BTU/hr F Q BTU/hr (1) Cold 200 400 1.6 320.0 (2) Cold 100 430 1.6 528.0 (3) Hot 590 400 2.376 451.4 (4) Cold 300 400 4.128 412.8 (5) Hot 471 200 1.577 427.4 (6) Cold 150 280 2.624 341.1 (7) Hot 533 150 1.32 505.6 Tmin = 20F Qhmin = 217.5  104 BTU/hr Qcmin = 0

Hot streams CP Q 2.376 451.4 1.557 427.4 1.32 1.6 320.0 1.6 528.0 4.128 412.8 2.624 590 400 471 419 200 533 150 400 200 430 100 400 300 280 150 3  5  505.6 7 1 416  2 505.6 4  341.1 6 341.1 Cold streams

CP Q 2.376 451.4 1.557 1.6 320.0 1.6 22.4 4.128 590 574 400 471 419 400 200 430 416 400 300  3  86.3 5 254  1 86.3 2 412.8  4 412.8

CP Q 2.376 38.6 1.6 233.7 1.6 22.4 590 583 574 400 264 254 430 416   3  1 H 217.5 16.2  2 22.4

CP Q 2.376 451.4 1.557 427.4 1.32 1.6 320.0 1.6 528.0 4.128 412.8 2.624 590 400 471 200 533 150 400 200 430 100 400 300 280 150    3   5  505.6 7   H 1 16.2 217.5 86.3   2 22.4 505.6  4 412.8 341.1  6 341.1

§ PINCH DESIGN METHOD RULE 2: DECOMPOSITION - THE HEN PROBLEM IS DIVIDED AT THE PINCH INTO SEPARATE DESIGN TASKS. - THE DESIGN IS STARTED AT THE PINCH AND DEVELOPED MOVING AWAY FROM THE PINCH.

DATA FOR EXAMPLE II Temperature Heat Capacity Supply Target Flowrates Heat load Process Stream TS TT CP Q no. Type F F 104 BTU/h/F 104 BTU/h 1 Cold 120 235 2.0 230.0 2 Hot 260 160 3.0 300.0 3 Cold 180 240 4.0 240.0 4 Hot 250 130 1.5 180.0 Tmin = 10 F QHmin = 50  104 BTU/h QCmin = 60  104 BTU/h

PINCH DECOMPOSITION DEFINES THE SEPARATE DESIGN TASKS 260 190 190 160 2 250 190 190 130 4 240 180 180 120 1 240 180 3 C = 60 Btu/h H = 50 Btu/h Umin = 4 Umin = 3 PINCH DECOMPOSITION DEFINES THE SEPARATE DESIGN TASKS

BELOW THE PINCH CP Q 3 90 1.5 90 2 120 190 160 2 3 190 170 130 4 4 G 60 190 135 120 3 4 1 90 30 ABOVE THE PINCH CP Q 3 210 1.5 90 2 220 4 240 260 190 2 1 250 190 4 2 235 225 180 H 2 1 20 90 240 -32 180 H 1 3 30 210

Cp Q 3 300 1.5 180 2 230 4 240 260 160 1 3 2 250 130 2 4 C 4 60 235 120 H 2 3 4 1 20 90 90 30 240 180 H 1 3 30 210 THE COMPLETE MINIMUM UTILITY NETWORK

PINCH MATCH Pinch A Pinch Match Pinch 2 1 Exchanger 2 is not 2 1 Exchanger 2 is not a pinch match Pinch 3 2 1 Exchanger 3 is not a pinch match

FEASIBILITY CRITERIA AT THE PINCH Rule 1: Check the number of process streams and branches at the pinch point  Above the Pinch : PINCH PINCH 90 80 90 80 1 1 2 2 3 3  (80+T1) 4 4 (80+T2) Q1 5 5 Q2 Tmin = 10C Tmin = 10C

FEASIBILITY CRITERIA AT THE PINCH Rule 1: Check the number of process streams and branches at the pinch point  Below the Pinch : 90 80 (90-T1) 90 80 1 1 (90-T2) 2 2  3 3 4 4 Q1 5 5 Q2 PINCH PINCH Tmin = 10C

FEASIBILITY CRITERIA AT THE PINCH Rule 2: Ensure the CP inequality for individual matches are satisfied at the pinch point.  Above the Pinch :  Below the Pinch : CPH1 CPC3 1 1 CPH2 CPC4 2 2 3 3 Q2 4 4 PINCH Q1 PINCH 1 T 2 T Tmin Tmin 3 4 Q Q Q2 Q1 CPC  CPH CPC  CPH

Stream data at the pinch NH  NC? Yes No CPH  CPC Split a for every pinch match Split a cold stream No Yes Split a stream ( usually hot) Place pinch matches Figure 8.7-7 Design procedure above the pinch. (From B. Linnhoff et al., 1982.)

Stream data at the pinch NH  NC? Yes No CPH  CPC Split a for every pinch match Split a cold stream No Yes Split a stream ( usually hot) Place pinch matches Figure 8.7-7 Design procedure below the pinch. (From B. Linnhoff et al., 1982.)

CRITERION #3 THE CP DIFFERENCE ABOVE THE PINCH, INDIVIDUAL CP DIFFERENCE = CPC - CPH OVERALL CP DIFFERENCE = BELOW THE PINCH, INDIVIDUAL CP DIFFERENCE = CPH - CPC THE SUM OF THE INDIVIDUAL CP DIFFERENCES OF ALL PINCH MATCHES MUST ALWAYS BE BOUNDED BY THE OVERALL CP DIFFERENCE.

Overall CP Difference = 8 - 6 = 2 PINCH CP 4 2 5 3 Overall CP Difference = 8 - 6 = 2 Total Exchanger CP Difference = 1 + 1 = 2 O.K.

Overall CP Difference = 9 - 6 = 3 PINCH CP 4 2 5 3 1 Overall CP Difference = 9 - 6 = 3 Total Exchanger CP Difference = 1 + 1 = 2 O.K.

Overall CP Difference = 9 - 5 = 4 PINCH CP 3 2 8 1 Overall CP Difference = 9 - 5 = 4 Total Exchanger CP Difference = 8 - 2 = 6 Criterion violated !

Cp Q 3 300 1.5 180 2 230 4 240 260 190 160 1 3 2 250 190 170 130 2 4 C 4 60 235 225 180 135 120 H 2 3 4 1 20 90 90 30 240 232.5 180 H 1 3 30 210 Heat Load Loops heat loads can be shifted around the loop from one unit to another

4 H 2 3 H 2 4 1 H C 1 3 C Heat Load Loops heat loads can be shifted around the loop from one unit to another

260 190 160 1 3 2 250 170 130 2 C 4 60 235 225 165 120 H 2 3 1 20 120 90 240 232.5 180 H 1 3 30 210 Heat Load Path heat loads can be shifted along the path

4 H 2 3 H 2 1 H C 1 3 C Heat Load Path heat loads can be shifted along the path

Cp Q 3 300 1.5 180 2 230 4 240 260 190 160 1 3 2 2 250 175 130 C 4 60+X 235 221.25 165 120 2 3 H 1 20+X 112.5 90 240 232.5 180 H 1 3 30 210 X=7.5

Two ways to break the loop If: L1>L4 L2>L3 then: X=L4 or X= -L3 1 1 2 2 3 4 (a) 3 L2 + X L4 - X 4 L3 + X L1 - X 1 2 3 2 1 4 3 4

heater/cooler can be included in a loop 1 3 4 2 (b) H1 - X 3 H L3 + X 4 H L4 - X H2 + X 1 H 3 4 3 4 Figure 2.28 - Complex loops and paths

Match 1 is not in the path 1 2 (c) 3 4 H 1 2 4 3 C 2 3 1 4 C H 4 2 3 C + X 3 L3 + X L4 - X 4 H L2 - X H + X H 1 2 4 2 3 4 3 C Figure 2.28 - Complex loops and paths