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Hierarchy of Decisions 1. Batch versus continuous 2. Input-output structure of the flowsheet 3. Recycle structure of the flowsheet 4. General structure of the separation system a. Vapor recovery system b. Liquid recovery system 5. Heat-exchanger network Ch.6, Ch.7, Ch.16 Ch. 4 Ch.5 HEAT EXCHANGER NETWORK (HEN) SUCCESSFUL APPLICATIONS O ICI ---- Linnhoff, B. and Turner, J. A., Chem. Eng., Nov. 2, 1981 Process Organic Bulk Chemical Specialty Chemical Crude Unit Inorganic Bulk Chemical Specialty Chemical General Bulk Chemical Inorganic Bulk Chemical Future Plant Specialty Chemical Unspecified General Chemical Petrochemical Facility* New New Mod New Mod New New New New New Mod New New Mod Energy savings Available k$/yr 800 1600 1200 320 200 200 2600 200 to 360 30 to 40 % 100 300 300 360 Phase I 1200 Phase II 1200 *New means new plant; Mod means plant modification. Capital Cost Expenditure or Saving, k$ same saving saving saving 160 saving unclear unclear 30 % saving 150 1000 saving unclear 600 1200 SUCCESSFUL APPLICATIONS Table 1. First results of applying the pinch technology in Union Carbide Process Petro-Chemical Specialty Chemical Specialty Chemical Licensing Package Petro-Chemical Organic Bulk Chemical Organic Bulk Chemical Specialty Chemical Organic Bulk Chemical Project Type Energy Cost Reduction $/yr Installed Capital Cost $ Mod. Mod. Mod. New Mod. Mod. 1,050,000 139,000 82,000 1,300,000 630,000 1,000,000 500,000 57,000 6,000 Savings Yet Unclear 600,000 6 5 1 ? 7 Mod. 1,243,000 1,835,000 18 Mod. Mod. 570,000 2,000,000 200,000 800,000 4 5 Linnhoff and Vredeveld, CEP, July, 1984 Payback Months SUCESSFUL APPLICATIONS Fluor --- IChE Symp. Ser., No. 74, 1982, P.19 --- CEP, July, 1983, P.33 FMC (Marine Colloid Division, Rockland, ME) CONCLUSION 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 200C 18.2 bar 1.089 36C 16 bar D 201 7.841 REACTION RECYCLE 126C 18.7 bar 1.614 200C PURGE CW 153C 7 703 115.5C FEED 5C 19.5 bar 0 179 180C 141C TO COLUMN 35C FLASH 40C 120C 17.6 bar 17.3 bar 114C Figure 2.5 - Flowsheet for “front end” of specialty chemicals process 200C Reactor 200C RECYCLE TOPS Reactor 5C FEED 35C Purge Product 35C PRODUCT 126C FOR EACH STREAM: TINITIAL, TFINAL, H = f(T). Figure 2.6-Specialty chemicals process-heat exchange duties H = 1722 REACTOR C = 654 a ) DESIGN AS USUAL STEAM RECYCLE 70 1 。 。 STEAM 1652 3 FEED 654 COOLING WATER 6 UNITS 2 PRODUCT H = 1068 C =0 REACTOR 4 UNITS b ) DESIGN WITH TARGETS 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 T Q =CP(TT-TS) TT H Q TTTS CPdT CP TT TS TS H Figure 2.10 - Representation of process streams in the T/H diagram H T (C) 200 UTILITY HEATING 140 135 T 115 100 70 UTILITY COOLING 350 300 400 H (KW) TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM T (C) 200 UTILITY HEATING 130 135 T 120 100 70 UTILITY COOLING 350 -100 =250 300 +100 =400 400 -100 =300 H (KW) TWO-STREAM HEAT EXCHANGE IN THE T/H DIAGRAM FACTS 1. Tmin Total Utility Load Increa se 2. in Hot Utility ( Increa se in Cold Utility )( = ) §ENERGY TARGETS (MANY HOT AND COLD STREAMS) COMPOSITE CURVES T H INTERVAL T1 CP=B T2 (T1-T2) (B) T3 (T2-T3) (A+B+C) CP=A CP=C T4 (T3-T4) (A+C) T5 (T4-T5) (A) H §ENERGY TARGETS (MANY HOT AND COLD STREAMS) COMPOSITE CURVES T T1 T2 T3 H1 H 2 H 3 T4 T5 H 4 H PINCH POINT Minimum hot utility T “PINCH” QH ,min Tmin QC ,min minimum cold utility Energy targets and “the Pinch” with Composite Curves H m hot Streams Qin Heat Exchange n cold Streams Qout m n Qin H H in h ,i i 1 or j 1 System Qout - Qin = H m in c, j Qout H i 1 n out h ,i H cout ,j j 1 m in n out m out n in Qout Qin H H h,i H c , j H h,i H c, j j 1 j 1 i 1 i 1 The “Problem Table” Algorithm - A Targeting Approach ---Linnhoff and Flower, AIChE J. (1978) CP TS TT TS* TT* and Type (KW/C) (C) (C) (C) (C) (1) Cold 2 20 Stream No. 25 135 (2) Hot 3 170 140 165 60 (3) Cold 4 80 55 85 140 (4) Hot 1.5 150 145 30 Tmin = 10C 145 25 Ti T6 T3 T1 T5 T4 T2 (T2) (T6) Subsystem 2 T1* = 165C 4 T2* = 145C T3* = 140C T4* = 85C 3 T5* = 55C T6* = 25C # 1 TK CPHot - CPcold HK 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 ( 3) H3 Qout Qin(3) H H 3 H H 4 i HC 3 H C 4 j i j CPHOT ,i T3* T4* CPCold , j T3* T4* i j CPHOT ,i CPCold , j T3* T4* CPHOT ,i CPCold , j T3 j j i 3 i 3 Qin(3) from subsys #2 90C . . . . . . . . . Heat Exchange Subsystem (3) . . . hot streams 145C . . . Cold streams 80C (3) Qout To subsys #4 (K ) H K Qout Qin( K ) 135C FROM HOT UTILITY T1* = 165C -------------------------- ( 0 )------ 20 H1 = 60 T2* = 145C --------------------------( 60 )-----( 80 ) minimum hot utility H2 = 2.5 T3* = 140C -------------------------( 62.5 )---( 82.5 ) H3 = -82.5 T4* = 85C -------------------------( -20.0 )-----( 0 ) Pinch H4 = 75 T5* = 55C --------------------------( 55.0 )----( 75 ) H5 = -15 T6* = 25C --------------------------( 40.0 )---- 60 TO COLD UTILITY (K ) H K Qout Qin( K ) minimum cold utility § “PROBLEM TABLE” ALFORITHM SUBSYSTEM TM TC=T 0 (T0) 1 (T1) 2 (T2) TP Tmin Hh2Hc2 Hh1 Hc1 TH TC Tmin § “PROBLEM TABLE” ALFORITHM ENTHALPY BALANCE OF SUBSYSTEM QOUT QIN HH1 HH2 HC1 HC2 As T = T1 - T2 0 dQ CPH CPC dT 5. The Grand Composite Curve 80 60 Q(KW) 40 CU Qc,min 20 “Pinch” HU Qh,min 0 20 T6* -20 40 60 T5 * 80 T4* 100 120 140 160 T3*T2* T1* 180 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 CU Qc,min Tc Qh,min Tp T Qh Qh,min Qc Qc,min Th HU Q CU Qc,min Tc Qh,min Tp T T1 Th HU Q Qc CU2 CU1 Qc,min Tc Qh Qh,min Tp Th T Qh Qh,min Qc Qh,min HU Q CU Qh,min Qc,min Tc Tp T1 T Th HU Q CU Q1 Qc,min Tc Qh,min HU2 Q2 Tp T1 T Qh,min Q1 Q2 Tp’ Th HU1 H=27MW H= -30MW FEED 2 PRODUCT2 230 140 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. TABLE 6.2 Heat Exchange Stream Data for the Flowsheet in Fig. 6.2 Stream Type Supply Target temp. temp. TS (C) TT (C) H (MW) Heat capacity flow rate CP (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 (a) (b) HOT UTILITY 245C HOT UTILITY 0MW H= -1.5 235C 7.5MW H= -1.5 1.5MW H= 6.0 195C 9.0MW H= 6.0 -4.5MW H= -1.0 185C 3.0MW H= -1.0 -3.5MW H= 4.0 145C 4.0MW H= 4.0 -7.5MW H= -14.0 75C 0MW H= -14.0 6.5MW H= 2.0 35C 14.0MW H= 2.0 4.5MW 25C H= 2.0 2.5MW COLD UTILITY Figure 6.18 The problem table cascade. 12.0MW H= 2.0 COLD UTILITY 10.0MW Figure 6.24 The grand composite curve shows the utility requirements in both enthalpy and temperature terms. Process (a) HP Stream Fuel BOILER Process Boiler Feedwater (Desuperheat) 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. (b) 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) TC 300 250 HP Steam 200 LP Steam 150 100 50 0 0 5 10 15 Figure 6.26 Alternative utility mixes for the process in Fig. 6.2. H(MW) (b) TC 300 250 Hot Oil 200 150 100 50 0 0 5 10 15 Figure 6.26 Alternative utility mixes for the process in Fig. 6.2. H(MW) T* T*TF Theoretical Flame Temperature T T*O T*STACK Flue Gas QHmin Air T*TFT Fuel T*STACK T*O ambient temp. Stack Loss Ambient Temperature QHmin Fuel Figure 6.27 Simple furnace model. H T* T*’TFT T*TFT Flue Gas 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* T*TFT T*TFT T*ACID DEW T*ACID DEW T*PINCH T*PINCH T*C 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 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. A B C D Figure A E F G H A B C D E F G H Figure B 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. 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 30 ST 30 70 H1 90 H2 60 40 50 10 C1 40 30 ST C2 100 70 H1 30 70 C1 40 30 ST 30-X C1 40 CW 50 90 H2 40 C2 100 70 H1 X 10+X 60-X C2 100 U = N-1 =5 0 X 30 50 CW 50 90 H2 40 50 CW 50 U = N-2 =4 U = N+1-1 =N =6 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 and Type TS (F) TF (F) CP 104BTU/hr F Heat Load 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 = 20F Qhmin = 217.5 104 BTU/hr Qcmin = 0 Hot streams CP 3 5 7 590 400 471 419 200 533 400 430 416 505.6 400 280 341.1 Cold streams 2.376 451.4 1.557 427.4 1.32 505.6 1 1.6 320.0 2 1.6 528.0 4 4.128 412.8 6 2.624 341.1 150 200 100 300 150 Q CP 3 5 590 574 400 471 419 400 254 86.3 200 430 400 412.8 416 300 Q 2.376 451.4 1.557 86.3 1 1.6 320.0 2 1.6 22.4 4 4.128 412.8 CP 590 3 583 400 264 H 217.5 16.2 430 574 2.376 38.6 1 1.6 233.7 2 1.6 22.4 254 22.4 Q 416 CP 3 5 7 590 471 533 400 16.2 217.5 430 400 280 400 2.376 451.4 200 1.557 427.4 1.32 505.6 1 1.6 320.0 2 1.6 528.0 4 4.128 412.8 6 2.624 341.1 150 200 H 86.3 100 22.4 505.6 300 412.8 341.1 Q 150 § PINCH DESIGN METHOD RULE 2: DECOMPOSITION - THE HEN PROBLEM IS DIVIDED AT THE PINCH INTO SEPARATE DESIGN TASKS. - THE D E S I G N I S S TA RT E D AT T H E P I N C H A N D DEVELOPED MOVING AWAY FROM THE PINCH. DATA FOR EXAMPLE II Temperature Process Stream no. Type 1 2 3 4 Cold Hot Cold Hot Supply Target TS TT F F 120 260 180 250 Heat Capacity Flowrates CP 4 10 BTU/h/F 235 160 240 130 Tmin = 10 F QHmin = 50 104 BTU/h QCmin = 60 104 BTU/h TH* 190 F TC* 180 F 2.0 3.0 4.0 1.5 Heat load Q 4 10 BTU/h 230.0 300.0 240.0 180.0 PINCH 2 4 H 260 190 190 160 250 190 190 130 240 180 180 120 240 180 = 50 Btu/h Umin = 4 1 3 C = 60 Btu/h Umin = 3 PINCH DECOMPOSITION DEFINES THE SEPARATE DESIGN TASKS BELOW THE PINCH 2 4 190 3 190 4 170 G 60 190 135 3 4 90 30 ABOVE THE PINCH 2 4 260 235 H 20 240 H 30 -32 1 210 Q 90 130 1.5 90 2 120 CP Q 3 210 1.5 90 2 220 4 240 1 190 2 225 CP 3 120 1 250 160 2 90 190 180 180 1 3 2 260 1 4 250 235 H 20 240 H 30 2 90 1 210 4 3 90 Q 3 300 1.5 180 120 1 2 230 180 3 4 240 160 3 2 Cp 4 30 C 130 60 THE COMPLETE MINIMUM UTILITY NETWORK PINCH MATCH Pinch A Pinch Match 1 Pinch 2 Exchanger 2 is not a pinch match 3 Pinch 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 : NH NC 1 PINCH 90 1 PINCH 90 2 90 2 90 3 90 3 90 (80+T1) 80 (80+T2) Q1 80 80 4 5 4 80 Q2 Tmin = 10C Tmin = 10C 5 FEASIBILITY CRITERIA AT THE PINCH Rule 1: Check the number of process streams and branches at the pinch point Below the Pinch : NH NC 90 1 90 (90-T1) 2 90 (90-T2) 2 80 90 90 3 80 Q1 80 PINCH 1 Q2 3 4 80 4 5 80 5 PINCH Tmin = 10C FEASIBILITY CRITERIA AT THE PINCH Rule 2: Ensure the CP inequality for individual matches are satisfied at the pinch point. Above the Pinch : 1 2 Below the Pinch : CPH2 T 2 3 4 CPC4 PINCH Q1 4 Q1 CPC CPH CPH1 1 2 Q2 PINCH T Tmin CPC3 1 Tmin Q 3 4 3 Q2 CPC CPH Q Stream data at the pinch NH NC? Yes CPH CPC for every pinch match Yes No Split a cold stream No Place pinch matches Split a stream ( usually hot) Figure 8.7-7 Design procedure above the pinch. (From B. Linnhoff et al., 1982.) Stream data at the pinch NH NC? Yes CPH CPC for every pinch match Yes No Split a cold stream No Place pinch matches Split a stream ( usually hot) 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 Nc NH 1 1 OVERALL CP DIFFERENCE = CPC CPH BELOW THE PINCH, INDIVIDUAL CP DIFFERENCE = CPH - CPC NH Nc 1 1 OVERALL CP DIFFERENCE = CPH CPC THE SUM OF THE INDIVIDUAL CP DIFFERENCES OF ALL PINCH MATCHES MUST ALWAYS BE BOUNDED BY THE OVERALL CP DIFFERENCE. PINCH CP 4 2 5 3 Overall CP Difference = 8 - 6 = 2 Total Exchanger CP Difference = 1 + 1 = 2 O.K. PINCH CP 4 2 5 3 1 Overall CP Difference = 9 - 6 = 3 Total Exchanger CP Difference = 1 + 1 = 2 O.K. PINCH CP 3 2 8 1 Overall CP Difference = 9 - 5 = 4 Total Exchanger CP Difference = 8 - 2 = 6 Criterion violated ! 2 260 1 190 3 250 160 170 130 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 Cp Q 3 300 1.5 180 2 230 4 240 190 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 1 3 C C Heat Load Loops heat loads can be shifted around the loop from one unit to another 2 260 1 190 250 2 4 235 225 H 2 20 120 240 232.5 H 1 30 210 3 160 170 130 C 60 165 120 3 1 90 180 3 Heat Load Path heat loads can be shifted along the path 4 H 2 3 H 2 1 H 1 3 Heat Load Path heat loads can be shifted along the path C C 2 260 190 1 2 4 250 235 H 20+X 3 C 130 60+X 221.25 165 2 3 120 240 232.5 H 1 30 210 X=7.5 Q 3 300 1.5 180 1 2 230 3 4 240 160 175 112.5 Cp 90 180 Two ways to break the loop 1 If: 1 2 2 3 4 (a) 3 L2 + X L4 - X L3 + X L1 - X 1 2 3 2 4 1 3 4 4 L1>L4 L2>L3 then: X=L4 or X= -L3 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 3 H 4 3 Figure 2.28 - Complex loops and paths 4 Match 1 is not in the path 1 2 2 3 1 4 C C+X (c) 3 L3 + X H L2 - X H+X H 1 2 4 3 2 4 3 Figure 2.28 - Complex loops and paths C L4 - X 4