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 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 Unspecified Mod New 300 saving General Chemical New 360 unclear Petrochemical Mod Phase I Phase II *New means new plant; Mod means plant modification.

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 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 7 Chemical Organic Bulk Mod. 1,243,000 1,835, Chemical Specialty Chemical Mod. 570, ,000 4 Organic Bulk Mod. 2,000, ,000 5 Chemical Linnhoff and Vredeveld, CEP, July, 1984

SUCESSFUL APPLICATIONS  Fluor --- IChE Symp. Ser., No. 74, 1982, P 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

I DENTIFY H EAT R ECOVERY AS A S EPARATE AND D ISTINCT T ASK IN P ROCESS D ESIGN D 201 RECYCLE TO COLUMN PURGE CW 36  C 200  C 18.2 bar 200  C 180  C 153  C 141  C 40  C  C 120  C 17.6 bar 114  C 35  C 126  C 18.7 bar 17.3 bar 16 bar FEED 5  C 19.5 bar Figure Flowsheet for “front end” of specialty chemicals process      FLASH REACTION

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

REACTOR 1 23 。 。 STEAM RECYCLE PRODUCT COOLING WATER FEED  = 1722  = 654 a ) DESIGN AS USUAL C H 6 UNITS

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

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

§ E NERGY T ARGETS (T WO S TREAM H EAT E XCHANGE )  T/H D IAGRAM H HH T T TSTS Q =CP(T T -T S ) Figure Representation of process streams in the T/H diagram

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

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

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

§E NERGY T ARGETS ( MANY HOT AND COLD STREAMS )  C OMPOSITE C URVES T1T2T3T4T5T1T2T3T4T5 (T 1 -T 2 ) (B) (T 2 -T 3 ) (A+B+C) (T 3 -T 4 ) (A+C) (T 4 -T 5 ) (A) CP=A CP=B CP=C T H

§E NERGY T ARGETS ( MANY HOT AND COLD STREAMS )  C OMPOSITE C URVES T1T2T3T4T5T1T2T3T4T5  T H

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

m hot Streams n cold Streams Q in Q out Q out - Q in =  H Heat Exchange System or

The “Problem Table” Algorithm - A Targeting Approach ---Linnhoff and Flower, AIChE J. (1978) Stream No. CP T S T T and Type (KW/  C) (  C) (  C) (  C) (  C) (1) Cold T T 3 (2) Hot T T 5 (3) Cold T T 2 (4) Hot (T 2 ) (T 6 )  T min = 10  C

T 1 * = 165  C T 2 * = 145  C T 3 * = 140  C T 4 * = 85  C T 5 * = 55  C T 6 * = 25  C Subsystem # TKTK  CP Hot -  CP cold HKHK

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

T 1 * = 165  C ( 0 ) T 2 * = 145  C ( 60 )-----( 80 ) T 3 * = 140  C ( 62.5 )---( 82.5 ) T 4 * = 85  C ( )-----( 0 ) T 5 * = 55  C ( 55.0 )----( 75 ) T 6 * = 25  C ( 40.0 )----  H 1 = 60  H 2 = 2.5  H 3 =  H 4 = 75  H 5 = minimum hot utility minimum cold utility Pinch FROM HOT UTILITY TO COLD UTILITY

§ “P ROBLEM T ABLE ” A LFORITHM  S UBSYSTEM T M T C =T  T min TPTP 0 (T 0 ) 1 (T 1 ) 2 (T 2 ) H h2 H c2 H h 1 H c 1

§ “P ROBLEM T ABLE ” A LFORITHM  E NTHALPY B ALANCE OF SUBSYSTEM As  T = T 1 - T 2  0

5. The Grand Composite Curve Q(KW) Q c,min T 6 * T 5 * T 4 * T 3 *T 2 * T 1 * Q h,min HU CU “Pinch”

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

QhQh Q h,min Q c,min QhQh Q T TcTc TpTp ThTh Q h  Q h,min Q c  Q c,min HU CU

Q h,min Q c,min Q T TcTc TpTp ThTh HU CU T1T1

QhQh Q h,min Q c,min Q T TcTc TpTp ThTh HU CU1 QcQc CU2

Q h,min Q c,min Q T TcTc TpTp ThTh HU CU T1T1

Q h,min Q c,min Q T TcTc TpTp ThTh HU1 CU T1T1 Q1Q1 Q2Q2 Tp’Tp’ HU2

REACTOR 1 REACTOR 2  H=27MW  H=32MW  H= -30MW  H= MW FEED  FEED 1 20  180  250  230  200  80  40  PRODUCT2 PRODUCT 1 OFF GAS 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 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 Reactor 1 product Hot Reactor 2 feed Cold Reactor 2 product Hot

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

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

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

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

(a) T  C  H(MW) HP Steam LP Steam Figure 6.26 Alternative utility mixes for the process in Fig. 6.2.

(b) T  C  H(MW) Figure 6.26 Alternative utility mixes for the process in Fig Hot Oil

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

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

T* 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.

 H(MW) Figure 6.30 Flue gas matched against the grand composite curve of the process in Fig. 6.2 T* Flue Gas