Heat Exchanger Network Synthesis, Part III

1. Heat Exchanger Network Synthesis, Part III • Ref: Seider, Seader and Lewin (2004), Chapter 10

2. 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. 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 TS and TT • CP for each stream is either approximately constant or H=f(T).

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

5. Example – Dealing with Real Systems • Toluene is manufactured by dehydrogenating n-heptane. • Furnace E-100 heats S1 to S2, from 65 oF to 800 oF. • Reactor effluent, S3, is cooled from 800 oF to 65 oF. • Install a heat exchanger to heat S1 using S3, and thus reduce the required duty of E-100. • Generate stream data using piece-wise linear approximations for the heating and cooling curves for the reactor feed and effluent streams. • Using the stream data, compute the MER targets for Tmin = 10 oF.

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

7. Equivalent, piece-wise flowing heat capacity: Example – Dealing with Real Systems • Cooling of vapor • Condensation

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

9. Example – Dealing with Real Systems • (b) MER Targeting:

10. Class Exercise 7 a) Extract data for hot and cold streams from the flowsheet below.b) Assuming Tmin = 10o, compute the pinch temperatures, QHmin and QCmin.c) Retrofit the existing network to meet MER.

11. Class Exercise 7 - Solution • Tmin = 10 oC

12. This defines: Cold pinch temperature = 140oC QHmin = 100 kW QCmin = 166 kW Class Exercise 7 - Solution (Cont’d) • Tmin = 10 oC

13. Class Exercise 7 - Solution (Cont’d)

14. Class Exercise 7 - Solution (Cont’d)

15. Heat Integration in Design • The Grand Composite Curve • An enthalpy cascade for a process is shown on the right. • Note that QHmin = QCmin = 1,000 kW • Also, TC,pinch = 190 oC

16. The Grand Composite Curve (Cont’d) • The Grand Composite Curve presents the same enthalpy residuals, as follows: • Minimum external heating, at 310 oC • Internal heat exchange • TC,pinch • Internal heat exchange

17. The Grand Composite Curve (Cont’d) • Alternative heating and cooling utilities can be used, to reduce operating costs:

18. Example: • GCC: The Grand Composite Curve (Cont’d)

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

20. GCC Example (Cont’d) • Returning to the GCC:

21. GCC Example (Cont’d) • Possible designs using CW, BFW, LPS and HPS:

22. 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: Qreb  Qcond for columns with saturated liquid products. • Need to position column carefully on composite curve

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

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

25. 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. (c) reboiler flashing • (a) heat pumping • (b) vapor recompression Heat-integrated Distillation (Cont’d) • Option C: Distillation configurations involving compression: • (a) heat pumping • (b) vapor recompression • (c) reboiler flashing

27. 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. Heat-integrated Distillation (Cont’d) • Option C: Distillation configurations involving compression: • (a) heat pumping • (b) vapor recompression • (c) reboiler flashing

28. 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