1. 8 - Heat & Power Integration1 Heat Exchanger Network Synthesis, Part III Ref: Seider, Seader and Lewin (2004), Chapter 10

2. 8 - Heat & Power Integration2 Instructional Objectives This Unit on HEN synthesis serves to expand on what was covered in the last two weeks to more advanced topics. Instructional Objectives - You should be able to: –Extract process data (from a flowsheet simulator) for HEN synthesis –Understand how to use the GCC for the optimal selection of utilities –Have an appreciation for how HEN impacts on design

3. 8 - Heat & Power Integration3 Data Extraction Process analysis begins with the extraction of “hot” and “cold” streams from a process flowsheet Required:  The definition of the “hot” and “cold” streams and their corresponding T S and T T  CP for each stream is either approximately constant or H=f(T).

4. 8 - Heat & Power Integration4 What is considered to be a stream ?  In general: Ignore existing heat exchangers  Mixing: Consider as two separate streams through to target temperature.  Splitting: Assume a split point wherever convenient.

5. 8 - Heat & Power Integration5 Example – Dealing with Real Systems oToluene is manufactured by dehydrogenating n-heptane. oFurnace E-100 heats S1 to S2, from 65 o F to 800 o F. oReactor effluent, S3, is cooled from 800 o F to 65 o F. oInstall a heat exchanger to heat S1 using S3, and thus reduce the required duty of E-100. a)Generate stream data using piece-wise linear approximations for the heating and cooling curves for the reactor feed and effluent streams. b)Using the stream data, compute the MER targets for  T min = 10 o F.

6. 8 - Heat & Power Integration6 Example – Dealing with Real Systems Equivalent, piece-wise flowing heat capacity: Evaporation of n-heptane Heating of vapor Heating of liquid

7. 8 - Heat & Power Integration7 Example – Dealing with Real Systems Equivalent, piece-wise flowing heat capacity: Cooling of vapor Condensation

8. 8 - Heat & Power Integration8 Example – Dealing with Real Systems Equivalent, piece-wise flowing heat capacity:

9. 8 - Heat & Power Integration9 Example – Dealing with Real Systems (b) MER Targeting:

10. 8 - Heat & Power Integration10 Class Exercise 7 a) Extract data for hot and cold streams from the flowsheet below. b) Assuming  T min = 10 o, compute the pinch temperatures, Q Hmin and Q Cmin. c) Retrofit the existing network to meet MER.

11. 8 - Heat & Power Integration11 Class Exercise 7 - Solution  T min = 10 o C

12. 8 - Heat & Power Integration12 Class Exercise 7 - Solution (Cont’d)  T min = 10 o C This defines: Cold pinch temperature = 140 o C Q Hmin = 100 kW Q Cmin = 166 kW

13. 8 - Heat & Power Integration13 Class Exercise 7 - Solution (Cont’d)

14. 8 - Heat & Power Integration14 Class Exercise 7 - Solution (Cont’d)

15. 8 - Heat & Power Integration15 Heat Integration in Design  The Grand Composite Curve  An enthalpy cascade for a process is shown on the right.  Note that Q Hmin = Q Cmin = 1,000 kW  Also, T C,pinch = 190 o C

16. 8 - Heat & Power Integration16 The Grand Composite Curve (Cont’d)  The Grand Composite Curve presents the same enthalpy residuals, as follows: Internal heat exchange T C,pinch Minimum external heating, at 310 o C

17. 8 - Heat & Power Integration17 The Grand Composite Curve (Cont’d)  Alternative heating and cooling utilities can be used, to reduce operating costs:

18. 8 - Heat & Power Integration18 The Grand Composite Curve (Cont’d)  Example:  GCC:

19. 8 - Heat & Power Integration19 GCC Example (Cont’d)  Possible designs using CW and HPS: U min = 4 + 2 – 1 = 5 How many loops? Does this design meet U min ? If not, what is the simplest change you can make to fix it?

20. 8 - Heat & Power Integration20 GCC Example (Cont’d)  Returning to the GCC:

21. 8 - Heat & Power Integration21 GCC Example (Cont’d)  Possible designs using CW, BFW, LPS and HPS:

22. 8 - Heat & Power Integration22 Heat Integration in Design  Heat-integrated Distillation  Distillation is highly energy intensive, having a low thermodynamic efficiency (as little as 10% for a difficult separation), but is widely used for the separation of organic chemicals in large-scale processes.  Thermal integration of columns can be done by manipulation of operating pressure.  Note: Q reb  Q cond for columns with saturated liquid products. Need to position column carefully on composite curve

23. 8 - Heat & Power Integration23 Heat-integrated Distillation (Cont’d)  Option A: Position distillation column between hot and cold composite curves:  (a) Exchange between hot  and cold streams  (b) Exchange with cold streams

24. 8 - Heat & Power Integration24 Heat-integrated Distillation (Cont’d)  Option B:  2-effect distillation: (a) Tower and heat exchanger configuration; (b) T-Q diagram.

25. 8 - Heat & Power Integration25 Heat-integrated Distillation (Cont’d)  Option B: Variations on two-effect distillation:  (a) Feed Splitting (FS)  (b) Light Split/forward heat integration (LSF)  (c) Light Split/Reverse heat integration (LSR).

26. 8 - Heat & Power Integration26  Option C: Distillation configurations involving compression:  (a) heat pumping  (b) vapor recompression  (c) reboiler flashing Heat-integrated Distillation (Cont’d) (b) vapor recompression (a) heat pumping (c) reboiler flashing

27. 8 - Heat & Power Integration27  Option C: Distillation configurations involving compression: Heat-integrated Distillation (Cont’d)  All 3 configurations involve the expensive compression of a vapor stream.  May not be cost-effective except where pressure changes required are small. Example: separation of close-boiling mixtures For further reading: Smith, R., “Chemical Process Design and Integration”, Wiley, 2005, Chapter 11. (a) heat pumping (b) vapor recompression (c) reboiler flashing

28. 8 - Heat & Power Integration28 Heat Integration - Summary Data Extraction –Getting data for HEN synthesis from material and energy balances (i.e., from simulator) Heat Integration in Design –Use of Grand Composite Curves for selection of utilities –Options for heat-integrated distillation