1. 6 - Intro HEN Synthesis1 Heat Exchanger Network Synthesis Part I: Introduction Ref: Seider, Seader and Lewin (2004), Chapter 10

2. 6 - Intro HEN Synthesis2 Part One: Objectives The first part of this three-part Unit on HEN synthesis serves as an introduction to the subject, and covers: –The “pinch” –The design of HEN to meet Maximum Energy Recovery (MER) targets –The use of the Problem Table to systematically compute MER targets Instructional Objectives: Given data on hot and cold streams, you should be able to: –Compute the pinch temperatures –Compute MER targets –Design a simple HEN to meet the MER targets

3. 6 - Intro HEN Synthesis3 Introduction - Capital vs. Energy The design of Heat Exchanger Networks deals with the following problem: Given: –N H hot streams, with given heat capacity flowrate, each having to be cooled from supply temperature T H S to targets T H T. –N C cold streams, with given heat capacity flowrate, each having to be heated from supply temperature T C S to targets T C T. Design: An optimum network of heat exchangers, connecting between the hot and cold streams and between the streams and cold/hot utilities (furnace, hot-oil, steam, cooling water or refrigerant, depending on the required duty temperature). What is optimal? Implies a trade-off between CAPITAL COSTS (Cost of equipment) and ENERGY COSTS (Cost of utilities).

4. 6 - Intro HEN Synthesis4 Example Network for minimal energy cost ? Network for minimal equipment cost ?

5. 6 - Intro HEN Synthesis5 Numerical Example D esign B:  (AREA) = 13.3 Design A:  (AREA) = 20.4 [ A = Q/U  T lm ]

6. 6 - Intro HEN Synthesis6 Which of the two counter-current heat exchangers illustrated below violates  T  20 o F (i.e.  T min = 20 o F) ? Clearly, exchanger A violates the  T min constraint. 20 o 10 o 20 o 30 o  T min - Example  T min = Lowest permissible temperature difference

7. 6 - Intro HEN Synthesis7 Utilities. Steam@150 o C, CW@25 o C Design a network of steam heaters, water coolers and exchangers for the process streams. Where possible, use exchangers in preference to utilities..  T min = 10 o C Class Exercise 1

8. 6 - Intro HEN Synthesis8 Setting Energy Targets Summary of proposed design: Are 60 kW of Steam Necessary?

9. 6 - Intro HEN Synthesis9 The Temperature-Enthalpy Diagram One hot stream Two hot streams

10. 6 - Intro HEN Synthesis10 The Composite Curve Hot Composite Curve

11. 6 - Intro HEN Synthesis11 The Composite Curve (Cont’d) Cold Composite Curve

12. 6 - Intro HEN Synthesis12 The Composite Curve (Cont’d) Method: manipulate hot and cold composite curves until required  T min is satisfied. This defines hot and cold pinch temperatures. Result: Q Cmin and Q Hmin for desired  T min MER Target Here, hot pinch is at 70 o C, cold pinch is at 60 o C Q Hmin = 48 kW and Q Cmin = 6 kW

13. 6 - Intro HEN Synthesis13 The Pinch The “pinch” separates the HEN problem into two parts: –Heat sink - above the pinch, where at least Q Hmin utility must be used –Heat source - below the pinch, where at least Q Cmin utility must be used. +x +x x +x +x

14. 6 - Intro HEN Synthesis14 HEN Representation with the Pinch The pinch divides the HEN into two parts:  the left hand side (above the pinch)  the right hand side (below the pinch) At the pinch, ALL hot streams are hotter than ALL cold streams by  T min.

15. 6 - Intro HEN Synthesis15 Class Exercise 2 For this network, draw the grid representation Given pinch temperatures at 480 o C /460 o C, and MER targets: Q Hmin = 40, Q Cmin = 106, redraw the network separating the sections above and below the pinch. How many energy can be recovered?

16. 6 - Intro HEN Synthesis16 Class Exercise 2 - Solution H 40 H 10 210 170 100 C 116 pinch temperatures; 480 o C /460 o C MER targets: Q Hmin = 40, Q Cmin = 106

17. 6 - Intro HEN Synthesis17 Class Exercise 2 - Solution (Cont’d) This can be fixed by reducing the cooling duty by 10 units, and eliminate the excess 10 units of heating below the pinch.

18. 6 - Intro HEN Synthesis18 Design for Maximum Energy Recovery Step 1: MER Targeting. Pinch at 90 o (Hot) and 80 o (Cold) Energy Targets: Total Hot Utilities: 20 kW Total Cold Utilities:60 kW Example

19. 6 - Intro HEN Synthesis19 Design for MER (Cont’d) Step 2: Divide the problem at the pinch

20. 6 - Intro HEN Synthesis20 Design for MER (Cont’d) Step 3: Design hot-end, starting at the pinch: Pair up exchangers according to CP-constraints. Immediately above the pinch, pair up streams such that: CP HOT  CP COLD (This ensures that T H  T C   T min )  T min

21. 6 - Intro HEN Synthesis21 Design for MER (Cont’d) Step 3 (Cont’d): Complete hot-end design, by ticking-off streams. 90 240 H Add heating utilities as needed (  MER target) Q Hmin = 20 kW 20

22. 6 - Intro HEN Synthesis22 Design for MER (Cont’d) Step 4: Design cold-end, starting at the pinch: Pair up exchangers according to CP-constraints. Immediately below the pinch, pair up streams such that: CP HOT  CP COLD (This ensures that T H  T C   T min )  T min

23. 6 - Intro HEN Synthesis23 Design for MER (Cont’d) Step 4 (Cont’d): Complete cold-end design, by ticking-off streams. C Add cooling utilities as needed (  MER target) Q Cmin = 60 kW 3090 60 35 o

24. 6 - Intro HEN Synthesis24 Design for MER (Cont’d) Completed Design: Note that this design meets the MER targets: Q Hmin = 20 kW and Q Cmin = 60 kW

25. 6 - Intro HEN Synthesis25 Design for MER (Cont’d) Design for MER - Summary:  MER Targeting. Define pinch temperatures, Q hmin and Q Cmin  Divide problem at the pinch  Design hot-end, starting at the pinch: Pair up exchangers according to CP-constraints. Immediately above the pinch, pair up streams such that: CP HOT  CP COLD. “Tick off” streams in order to minimize costs. Add heating utilities as needed (up to Q Hmin ). Do not use cold utilities above the pinch.  Design cold-end, starting at the pinch: Pair up exchangers according to CP-constraints. Immediately below the pinch, pair up streams such that: CP HOT  CP COLD. “Tick off” streams in order to minimize costs. Add heating utilities as needed (up to Q Cmin ). Do not use hot utilities below the pinch.  Done!

26. 6 - Intro HEN Synthesis26 Class Exercise 3 Design a network of steam heaters, water coolers and exchangers for the process streams. Where possible, use exchangers in preference to utilities.  T min = 10 o C. Utilities: Steam@150 o C, CW@25 o C Q Hmin =48 Q Cmin =6 80 o C H 54 C 120 43 o C 6 100 H 8 40

27. 6 - Intro HEN Synthesis27 The Problem Table  T min = 10 o F. Example: Step 1: Temperature Intervals (subtract  T min from hot temperatures) Temperature intervals: 250  F  240  F  235  F  180  F  150  F  120  F

28. 6 - Intro HEN Synthesis28 The Problem Table (Cont’d) Step 2: Interval heat balances For each interval, compute:  H i = (T i  T i+1 )  (  CP Hot  CP Cold )

29. 6 - Intro HEN Synthesis29 The Problem Table (Cont’d) Step 3: Form enthalpy cascade. This defines: Cold pinch temp. = 180 o F Q Hmin = 500 kBtu/h Q Cmin = 600 kBtu/h

30. 6 - Intro HEN Synthesis30 Introduction to HEN Synthesis - Summary 1. Introduction: Capital vs. Energy What is an optimal HEN design Setting Energy Targets 2. The Pinch and MER Design –The Heat Recovery Pinch –HEN Representation –MER Design: (a) MER Target; (b) Hot- and cold-side designs 3. The Problem Table –for MER Targeting Next Lecture: Advanced HEN Synthesis