Process Equipment Design-III (CL 403)

Process Equipment Design-III (CL 403)
Design of Heat Exchanger Network (HEN) Dr. Animes K Golder Department of Chemical Engineering Indians Institute of Technology Guwahati Assam

Course Syllabus Pre-requisites
Design of heat exchanger network: Setting energy targets, problem table algorithm, heat recovery pinch, heat exchanger network (HEN) representation, HEN design for maximum recovery, stream splitting, capital energy trade offs. Principles of multi-component distillation and design: Basic distillation design, sequencing of simple distillation columns, complex distillation columns, short-cut modeling of complex columns. Design of azeotropic and extractive distillation systems. Pre-requisites Process Equipment Design I (CL 206), Process Equipment Design II (CL 304) and Mass Transfer Operations I (CL 205)

Texts - Design of Heat Exchanger Network
“Chemical Process: Design and Integration” by R. Smith, John Wiley & Sons Ltd “Pinch Analysis and Process Integration: A User Guide on Process Integration for the Efficient Use of Energy” by I.C. Kemp, Elsevier,

Grading Scheme Important Instructions
Not be allowed to enter the class room after 2 minutes from the commencement of the lecture Minimum attendance is 75% to write the end-semester examination Must carry graph paper, scale, calculator, eraser and supplementary pages to the tutorial classes Mid semester and end semester quizzes may be conducted without prior intimation

Design of Heat Exchanger? Design of Heat Exchanger Network (HEN)?
What is meant by Design of Heat Exchanger? Design of Heat Exchanger Network (HEN)?

Classification of heat exchangers depending on the applications
Recuperative Regenerative Rotary regenerator Fixed-matrix regenerator Indirect contact-type Direct Disk type Drum type Tubular Plate Extended surface Double pipe Spiral tube Shell & tube Finned tube Finned plate Gasketed plate Spiral plate Lamella Removable bundle Floating head Fixed tubesheet U-tube Heat Exchanger Heat exchanger may have singe or two phase flow on each side Flow Parallel Counter Cross

Fixed-tube heat exchanger
Floating-head heat exchanger Removable U-tube heat exchanger

Typical parts and connections of Heat Exchangers
1. Shell 16. Tubes (U-type) 2. Shell cover 17. Tie rods and spacers 3. Shell flange (channel end) 18. Transverse (or cross) baffles or support plates 4. Shell flange (cover end) 19. Longitudinal baffles 5. Shell nozzle or branch 20. Impingement baffles 6. Floating tube sheet 21. Floating head support 7. Floating head cover 22. Pass partition 8. Floating head flange 23. Vent connection 9. Floating head gland 24. Drain connection 10. Floating head backing ring 25. Instrument connection 11. Stationary tube sheet 26. Expansion bellows 12. Channel or stationary head 27. Support saddles 13. Channel cover 28. Lifting lugs 14. Channel nozzle or branch 29. Weir 15. Tube (straight) 30. Liquid level connection

Process (thermal) design procedure of Heat Exchanger
[Kern method ]

If the tube-side pressure drop exceeds the allowable pressure drop for the process system, decrease the number of tube passes or increase number of tubes per pass. Go back to step #6 and repeat the calculations steps. If the shell-side pressure drop exceeds the allowable pressure drop, go back to step #7 and repeat the calculations steps. Step #15. Upon fulfillment of pressure drop criteria, go mechanical design.

Selection of Fluids for Tube and Shell Side
Routing of the shell side and tube side fluids has considerable effects on the heat exchanger design. Some general guidelines for positioning the fluids are given in Table These guidelines are not ironclad rules and the optimal fluid placement depends on many factors that are service specific Tube-side fluid Shell-side fluid Corrosive fluid Condensing vapor (unless corrosive) Cooling water Fluid with large temperature difference (>40°C) Fouling fluid Less viscous fluid High-pressure steam Hotter fluid

Heat Exchanger Networks: Shell- and -Tube Heat Exchangers
1 shell pass and 1 tube pass Needs lower area than 1-2 design Ideal shell side flow 1 shell pass and 2 tube passes Allowance of thermal expansion Easy mechanical cleaning Good heat transfer coefficient Non-ideal shell side flow

Effect of Temperature Differences on Design of Shell and Tube Heat Exchanger
Countercurrent flow Cocurrent flow Cross Approach 1-1 exchanger flow 1-2 exchanger flow

(R) (P) FT expressions in 1-2 shell and tube heat exchanger: ln

Effect of terminal temperatures on FT
Infeasible design of a single 1-2 exchanger at higher temperature cross

Temp cross Temp cross Temp cross Temp cross Situation of high temperature cross can be taken care by placing the shells in series of 1-2 type. However different kinds of shells or multiple shells also can be an alternative.

 Maximum Thermal Effectiveness of 1-2 Shell and Tube Exchanger ln
and FT to be determinate if: Always true for feasible heat transfer ln Both condition to satisfy 

Design value of ‘P’:

Bowman RA. Mean Temperature Difference Correction in Multipass Exchangers, Ind. Eng. Chem (28)

Value of P over NSHELLS number of 1-2 shells in series (PN-2N) can
be related to P for each 1-2 shell (P1-2) as:

Heat Exchanger Networks: Energy Targets
Reactor Separation & Recycle System Heat Recovery System Heating & Cooling Utilities Water & Wastewater Treatment Onion model

Flowsheet of a Manufacturing Unit
Product 1 40ºC Coln Off Gas Reactor 2 ΔH=-30 MW Reactor 1 Feed 2 80ºC 200ºC 180ºC 230ºC 20ºC 140ºC 250ºC Product 2 Feed 1 ΔH=27 MW ΔH=32 MW ΔH=-31.5 MW Total hot streams heat duty=61.5 MW (Surplus) Total cold streams heat duty=59 MW (Deficit) Folwsheet with two hot streams and two cold streams

Prof. Bodo Linnhoff (born 1948) who developed Pinch Analysis of Heat Exchanger Network Design
[University of Manchester Institute of Science and Technology (UMIST)]

Temperature–enthalpy diagram

Composite Curves

Formation of Hot and Cold composite curves
Stream no. Type #1 Cold 20 180 32 0.2 #2 Hot 250 40 -31.5 0.15 #3 140 230 27 0.3 #4 200 80 -30 0.25 Overlap between the composite curves represents the maximum amount of heat recovery possible Overshoot at the bottom represents the minimum amount of external cooling required Overshoot at the top represents the minimum amount of external heating required

Trade-off between energy and capital cost and an economic amount of energy recovery

Heat transfer from above the pinch to below the pinch is possible

3 forms of cross pinch heat transfer
Increase the requirement of both hot and cold utilities by the same amount i.e. utility requirement increase is DOUBLE Process-process heat transfer across the pinch Inappropriate utility below the pinch cause enthalpy imbalance below the pinch Designer must not allow transfer heat across the pinch: Process-to-process heat transfer Inappropriate use of utilities Inappropriate utility above the pinch cause enthalpy imbalance above the pinch

Threshold Problems Threshold problem: Only cold utility required

Threshold problem: Only hot utility required
Threshold Problems (contd.) Threshold problem: Only hot utility required

Threshold Problems (contd.)

Introduction of an additional utility to create ‘UTILITY PINCH’
Threshold Problems (contd.) Introduction of an additional utility to create ‘UTILITY PINCH’ Two levels of Cold utility Two levels of Hot utility

Steam classification Steam grade Typical pressure, psig
Low pressure (LP) Upto 25 Medium pressure (MP) 25-125 High pressure (HP) >125 Steam cost (major components): CG = f(CF, Hg, hf, ηB) CF = Fuel cost (~ 90% of total cost) Hg = Enthalpy of steam hf = Enthalpy of boiler feedwater ηB = Boiler efficiency

Steam valuation estimated by convention method
Source:

Problem Table Algorithm

Problem Table Algorithm
Example Problem Table Algorithm ΔTmin=10°C Stream no. Type TS, °C TT, °C ΔH, MW CP, MW.K-1 T*S, °C T*T, °C 1 Cold 20 180 32 0.2 25 185 2 Hot 250 40 -31.5 0.15 245 35 3 140 230 27 0.3 145 235 4 200 80 -30 0.25 195 75 Shifted temperature intervals, *T, (°C) 1 2 3 4 245 235 195 185 145 75 35 25 180 140 70 30 250 240 200 190 150 80 40 230

Problem Table Cascade 0 MW -3.5 MW -7.5 MW 6.5 MW 4.5 MW 2.5 MW ΔH= -1.5 ΔH= 6.0 ΔH= -1.0 ΔH= 4.0 ΔH= ΔH= 2.0 245°C 235°C 195°C 185°C 145°C 75°C 35°C 25°C HOT UTILITY COLD UTILITY 1.5 MW -4.5 MW 9.0 MW 3.0 MW 4.0 MW 14 MW 12 MW 10 MW ΔH= -14.0 7.5 MW Cascade surplus heat from high to low temperature Heat added to hot utility to make all heat flows zero or positive

Balanced composite curve for
EXAMPLE GCC and Multiple Pinches Hot utility=7.5 MW Cold utility= 10 MW (20-30°C) Stream Table Stream no. Type #1 Cold 20 180 32 0.2 #2 Hot 250 40 -31.5 0.15 #3 140 230 27 0.3 #4 200 80 -30 0.25 4.5 MW, °C HP steam 3 MW, °C LP steam Heat recovery pockets Balanced composite curve for multiple utilities GCC and utility selection

Grand Composite Curve (GCC)
Although the composite curves can be used to set energy targets, however, the grand composite curve (GCC) is a more appropriate tool for understanding the interface between the process and utility system Grand composite curve allows alternative utilities

Grid representation of streams

Heat Exchanger Networks: Number of Heat Exchange Units, Number of Shells, Heat Exchange Area and Cost Targets Major components that contribute to capital cost of heat exchanger network: Number of units Heat exchange area Number of shells Materials of construction Heat exchanger type Pressure rating

Heat Exchange Area Targets
Utility streams must be included with the process streams in the composite curves to obtain the Balanced Composite Curves to calculate the network area

Effect of individual stream film transfer coefficients can be included to calculate network area
FT correction factor for each enthalpy interval depends both on the assumed value of XP and the temperatures of each interval on composite curves. The above equation can simply modify by incorporating FT in each interval for 1-2 pass

Balanced composite curve and temperature interval
Given data: Film transfer coefficients for all streams are 200 W·m−2·K−1 (including utility)

Target area calculation
Enthalpy intervals and stream population Steam 2 4 3 1 CW 180ºC ºC 179ºC 170ºC 40ºC 120ºC 110ºC 50ºC 80ºC 30ºC 20ºC 150ºC 60ºC 90ºC 105ºC 102.5ºC 22.5ºC (qi= 3) (qi= 4) (qi= 2) (qi= 6) (qi= 1) (qi= 12) (qj= 3) (qj= 1) (qj= 1.25) (qj= 2.5) (qj= 11.25) (qj= 0.75) (qj= 1.5) (qj= 6.75) (qj= 9) 5 6 7 Interval  Aarea above the pinch, (target) = 8,859 m2 Aarea below the pinch, (target) = 10,469 m2 (qj= 3) PINCH Enthalpy interval ΔTLMk Hot streams Cold streams (m2) (1-1 pass, FT=1.0) 1 (ΔT1=60, ΔT2=69.571) 15,000 463.9 2 (ΔT1=69.571, ΔT2=74) 20,000 557.4 3 (ΔT1=65, ΔT2=47.5) 10,000 358.5 4 (ΔT1=47.5, ΔT2=10) 90,000 7478.2 5 (ΔT1=10, ΔT2=10) 45,000 9000 6 (ΔT1=30, ΔT2=27.5) 1044.2 7 (ΔT1=27.5, ΔT2=20) 5,000 424.6 Target area for the network, = 19,327

Number of Shells Targets
Where, NSHELLS = Total number of shells over K enthalpy intervals Nk = Real (or fractional) number of shells resulting from the temperatures of enthalpy interval k Sk = Number of streams in enthalpy interval k

Capital Cost Targets Cost a single heat exchanger with surface area A can be expressed as: Installed Capital Cost of Exchanger = a + bAc Where, a, b, c = Constants that vary according to materials of construction, pressure rating and type of exchanger. Network Capital Cost = N[a + b(ANETWORK /N)c] N = number of units or shells, whichever is appropriate [J. Douglas, Conceptual Design of Chemical Processes, McGraw Hill, 1989]

Heat Exchanger Networks: Pinch Method
of Network Design Two  rules: Process-to-process heat transfer  Inappropriate use of utilities  Start at the pinch: Most constrained region of the problem. ΔTmin exists between all hot and cold streams at the pinch. Number of feasible matches in this region is severely restricted. ii. CP inequality for individual matches at ‘PINCH’ CPH ≤ CPC (above pinch) CPH ≥ CPC (below pinch) iii. CP-Table Identification of the essential matches in the region of the pinch. CP values of the hot and cold streams for the streams at the pinch are listed in descending order iv. Tick-off heuristic

Grid diagram Criteria for pinch matches above the pinch Infeasible match Feasible match

‘CP-Table’ for the design above & below the pinch
Criteria for pinch matches below the pinch Infeasible match Feasible match ‘CP-Table’ for the design above & below the pinch Above pinch Below pinch

Tick-off heuristic To tick off a stream, individual units are made as large as possible i.e. the smaller of the two heat duties on the streams being matched to keep the number of units to a minimum Sizing the units above the pinch using tick-off heuristics Sizing the units below the pinch using tick-off heuristics Minimum number of units=(S-1)above pinch + (S-1)below pinch=(5-1)+(4-1)= 7

Design for Threshold Problems
If there is no pinch, the design is started at the most constrained region i.e. where temperature difference is smallest. In case of Utility pinch(es), threshold problem can be treated as a pinched problem. Minimum temperature difference end Utility demand Network design using Pinch method

Threshold problem Pseudo-pinch Network design using Pinch method
Utility demand Threshold problem Pseudo-pinch Network design using Pinch method

Stream Splitting Splitting of the cold streams is required if the number of hot streams at the pinch, above the pinch, is more than the number of cold streams. SH ≤ SC (above pinch) Splitting of the hot streams is required if the number of cold streams at the pinch, below the pinch, is more than the number of hot streams. SH ≥ SC (below pinch)

CP inequality criteria necessitates stream splitting
CPH ≤ CPC (above pinch) CPH ≥ CPC (below pinch) Above pinch Below pinch

Algorithms for Stream Splitting
Above pinch Below pinch

Heat capacity flow rate (MW.K-1)
Network Design with Stream Splitting EXAMPLE. Streams data of a high temperature process reveals that for ΔTmin = 20°C. The process requires 9.2 MW of hot utility and 6.4 MW of cold utility and the pinch is located at 520°C on hot stream. The network is designed for maximum energy recovery. Stream type Supply Temp. (ºC) Target Temp. (ºC) Heat capacity flow rate (MW.K-1) Hot (#1) 720 320 0.045 Hot (#2) 520 220 0.04 Cold (#3) 300 900 0.043 Cold (#4) 200 550 0.02    Infeasible match at pinch above pinch Stream splitting to have feasible match at pinch above pinch Design of network

Stream grid diagram      Network design for multiple pinches

Two process pinches: Both hot and cold steams are parallel
Design between process pinches Case 1: At any end Case 2: Start at above pinch (Like below pinch problem) Case 3: Start at below pinch (Like above pinch problem) Between the process pinches there may be Case 1: One hot stream and one cold stream (CP of both streams=0.15) Case 2: One hot stream (CP=0.15) and more than one cold streams (CP < 0.15 for individual stream) Case 3: One cold stream (CP=0.15) and more than one hot streams (CP < 0.15 for individual stream) Case 4: Stream splitting??

Design of HEN with multiples process pinches
Design between process pinches Case 1: Both streams parallel Start at any end Case 2: Streams not parallel  For above pinch subject to CPhot>Cpcold (Like below pinch problem)  For below pinch subject to CPcold>Cphot (Like above pinch problem)

Remaining Problem Analysis
Along with the number of units and maximum energy recovery; heat transfer area, number of shells when using 1-2 shells and capital cost need to consider When a match is placed (provided multiple choices are available) the penalty (energy, network area, shells , cost) is calculated by determining the targets for the Remaining Problem without completing the entire Network design The difference between the Energy Targets before placing a match and Energy Targets of the Remaining Problem plus Energy Duty for that said match will give the energy penalty for that match Similarly, the penalty of Network Area, Shells and Costs can determined by the Remaining Problem

Heat capacity flow rate (MW.K-1)
Hand on Calculation on Remaining Problem Analysis EXAMPLE: a. Develop a maximum energy recovery design above the pinch that comes close to the area target in the minimum number of units. b. Develop a maximum energy recovery design below the pinch that comes as close as possible to the minimum number of units. Data available: Low pressure steam is available condensing between 180 and 179°C Cooling water between 20 and 30°C All film transfer coefficients are 200 W·m−2·K−1 (both hot and cold streams) ΔTmin = 10°C (pinch at 90°C on the hot streams and 80°C on the cold streams) Minimum hot and cold utility duties are 7 MW and 4 MW Stream type (no.) Supply Temp. (ºC) Target Temp. (ºC) Heat capacity flow rate (MW.K-1) Hot (#1) 150 50 0.2 Hot (#2) 170 40 0.1 Cold (#3) 120 0.3 Cold (#4) 80 110 0.5

Design above the pinch CPH ≤ CPC  SH ≤ SC  Pinch CP 0.2 0.1 0.3 0.5
4 Four feasible matches at pinch above pinch Which one to select? Solution Remaining Problem Analysis

Balanced composite curve and temperature interval

Target area calculation
Enthalpy intervals and stream population Steam 2 4 3 1 CW 180ºC ºC 179ºC 170ºC 40ºC 120ºC 110ºC 50ºC 80ºC 30ºC 20ºC 150ºC 60ºC 90ºC 105ºC 102.5ºC 22.5ºC (qi= 3) (qi= 4) (qi= 2) (qi= 6) (qi= 1) (qi= 12) (qj= 3) (qj= 1) (qj= 1.25) (qj= 2.5) (qj= 11.25) (qj= 0.75) (qj= 1.5) (qj= 6.75) (qj= 9) 5 6 7 Interval  Aarea above the pinch, (target) = 8,859 m2 Aarea below the pinch, (target) = 10,469 m2 (qj= 3) Enthalpy interval ΔTLMk Hot streams Cold streams (m2) (1-1 pass, FT=1.0) 1 (ΔT1=60, ΔT2=69.571) 15,000 463.9 2 (ΔT1=69.571, ΔT2=74) 20,000 557.4 3 (ΔT1=65, ΔT2=47.5) 10,000 358.5 4 (ΔT1=47.5, ΔT2=10) 90,000 7478.2 5 (ΔT1=10, ΔT2=10) 45,000 9000 6 (ΔT1=30, ΔT2=27.5) 1044.2 7 (ΔT1=27.5, ΔT2=20) 5,000 424.6 Target area for the network, = 19,327

TARGETING the number of shells (1-2 exchanger)
Values Interval (1) (2) (3) (4) (5) Interval (6) (7) When, Xp =0.9 RK 0.0429 0.1142 8 1 4 PK 0.75 0.2 WK -- NK 0.0709 2.6904 FT 0.999 0.997 <0.5 (not acceptable) 0.985 0.992 Pk, max Pmax= 0.307 Pmax= 0.586 Pk=Xp*Pmax 0.2763 0.5274 (modified ) 0.792 0.739 (Rk =1) (can be accepted?) Xp =0.75 --- 0.8 0.115 0.0539 0.2793 2.257 3.828 0.3899 0.3567 < 0.5 0.307 0.586 New Pk =Xp*Pmax 0.4395 (New ) 0.916 (acceptable) 0.888 Final & acceptable, (0.115×1) + (0.0539×2) + (0.2793×2) (2.257×3) (3.828×2) (0.3899×2) +(0.3567×1) (Nshells)above pinch = ≈ 8 (Nshells)below pinch = ≈ 9 , NSHELL (TOTAL) = 17 ;

Xp=0.75 (Contd.) Values Interval (1) Interval (2) Interval (3) (4) (5)
(7) A1-1 pass, m2 463.9 557.4 358.5 7478.2 9000 1044.2 424.6 A1-2 pass, m2 464.4 557.9 359.6 8163.9 1060.1 428 For 1-2 pass exchanger: Atarget(above pinch)= m2 Atarget(below pinch)= m2 Atarget(total)= m2 N(i)above pinch N(steam)=0.1689, N(1)=2.257, N(2)=2.5363, N(3)= ; N(4)=2.5902 = ; =2.7052 (Nshells)above pinch = N(i)below pinch N(CW)=0.7466, N(1)=4.2179, N(2)=4.5746, N(3)=3.828; = ; =4.5746 (Nshells)below pinch = N(i) is greater for all stream except steam & cooling water and recalculated for N(steam)=1 and N(CW)=1. (Nshells)above pinch = [(Nshells)above pinch=9] (Nshells)below pinch = [(Nshells)below pinch =10]

Maximum Thermal Effectiveness of 1-2 Shell and Tube Exchanger
Always true for feasible heat transfer

[1-1 Shell- and -Tube Exchanger]
Target for the remaining problem after match between [1] and [3]: [1-1 Shell- and -Tube Exchanger] 1 2 3 4 CP 0.3 0.5 90°C 80°C 150°C 170°C 120°C 110°C  ∆T1 = 10, ∆T2 = 30 ∆TLM = 18.2 A1 = 6593 m2  Internal (1): ∆T1 = 83, ∆T2 = 70, ∆TLM = 76.31, A1 = m2 Internal (2): ∆T1 =74, ∆T2=10, ∆TLM=31.98, A2= m2 Target area for the remaining problem above pinch (A1 + A2) = 3419 m2 Overall target exceed by = [{ ) – 8859}/8859] × 100 = 13 % Balanced composite curve of the Remaining Problem

Target for the remaining problem after match between [1] and [4]: [1-1 Shell- and -Tube Exchanger] 1 2 3 4 CP 0.3 0.5 90°C 80°C 150°C 170°C 120°C 110°C  ∆T1 = 10, ∆T2 = 46 ∆TLM = 23.59 A1 = 5087 m2  104°C Internal (1): ∆T1 = 60, ∆T2 = , ∆TLM = 64.67, A1 = m2 Internal (2): ∆T1 = , ∆T2 = 74, ∆TLM = 71.76, A2 = m2 Internal (3): ∆T1 = 65, ∆T2 = 58, ∆TLM = 61.43, A3 = m2 Internal (4): ∆T1 = 58, ∆T2 = 10, ∆TLM = 27.3, A4 = m2 Target area for the remaining problem above pinch (A1 + A2 + A3 + A4) = 3788 m2 Overall target exceed by = [{ ) – 8859}/8859] × 100 = 0.2 % Balanced composite curve of the Remaining Problem

Target for the remaining problem after match between [2] and [4]: [1-1 Shell- and -Tube Exchanger] 1 2 3 4 CP 0.3 0.5 90°C 80°C 150°C 170°C 120°C 110°C  ∆T1 = 10, ∆T2 = 74 ∆TLM = 31.98 A1 = 2502 m2  96°C Internal (1): ∆TLM = , A1 = m2 Internal (2): ∆TLM = 71.67, A2 = m2 Internal (3): ∆TLM = 29.47, A3 = m2 Internal (4): ∆TLM = 13.61, A4 = m2 Target area for the remaining problem above pinch (A1 + A2 + A3 + A4) = m2 Overall target exceed by = [{ ) – 8859}/8859] × 100 = 7.2 % Balanced composite curve of the Remaining Problem

Complete network design ‘Above Pinch’
1 2 3 4 CP 0.2 0.1 0.3 0.5 90°C 80°C 150°C 170°C 120°C 110°C   106.67°C H 4 MW 8 MW 104°C H 3 MW 12 MW

Design below the pinch       Stream Splitting Needed Pinch
Stream Splitting Needed 1 2 3 Pinch CP ∆ H 50°C 40°C 90°C 80°C 8 MW   60°C 1 2 3 Pinch CP ∆ H 50°C 40°C 90°C 80°C 6 MW   60°C  60°C  3 MW

Complete network design ‘Below Pinch’
1 2 3 Pinch CP ∆ H 50°C 40°C 90°C 80°C 6 MW 60°C C 2 MW 60°C C 2 MW 3 MW

Complete Network Design
1 2 3 4 90°C 80°C 150°C 170°C 120°C 110°C 104°C 106.67°C 12 MW 8 MW H 4 MW 3 MW 60°C 50°C C 2 MW 60°C C 2 MW 50°C 6 MW 3 MW Utility Exchanger Utility Exchanger Process Exchanger 1 4 3 2 Utility Exchanger Utility Exchanger

Modification for 1-2 shell & tube exchanger with Xp=0.75
Sl no. Match between R W FT (Old) PMAX P=Xp*PMAX (Final & acceptable) A1-1 PASS, m2 A1-2 PASS, N (No. of Shells) Area per Shell,m2 1 [1]-[4] 2.5 0.520 <0.5 0.913 5087 5571.6 3 1857.2 2 [2]-[3] 2.996 0.470 0.921 2770 3007.2 1002.4 [3]- [Stm] 0.075 3.399 0.999 --- 606.4 607 6072 4 [4]- [Stm] 0.167 2.837 413.9 414.4 414.2 5 [1]-[3] (CP=0.2) 0.887 6000 6764.4 1691.2 6 [2]-[3], (CP=0.1) 3000 3382.2 845.5 7 [1]-[CW] 0.981 (R=1) ---- 0.981 666. 7 679. 6 679.6 8 [2]-[CW] 0.598 0.942 810.9 860.9 Adesign(above pinch)= m2 Adesign(below pinch) = m2 Adesign(total) = m2 Aexcess = m2 Atarget(above pinch)= m2 Atarget(below pinch)= m2 Atarget(total) = m2

Network Optimization Degree of freedom: Loops, utility path and stream splitting Constrain of ΔTmin can now be relaxed Heat duties can be changed within a loop without changing the utility consumption

Utility paths for the optimization of HEN

Multivariable optimization subject to:
total enthalpy change on each stream being within a specified tolerance of the original stream data nonnegative heat duty for each match positive temperature difference for each exchanger to be greater than a practical minimum value for a given type of heat exchanger for stream splits, branch flowrates must be positive and above a practical minimum flowrate

Evolve the heat exchanger network in Figure to simplify its structure
Remove the smallest heat recovery unit from the network by exploiting the degree of freedom in a loop. Recalculate the network temperatures and identify any violations of the ΔTmin= 10°C constraint. Restore the original ΔTmin =10 °C throughout the network by exploiting a utility path.

Network with 6.5 MW of heat shifted around a loop
Shows infeasible temperature difference

10+x = 0.15( )

Thank You

Process Equipment Design-III (CL 403)

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