Department of Chemical Engineering Indians Institute of Technology Guwahati Assam - 781039

1. 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 - 781039

2. Course Syllabus • 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)

3. Texts - Design of Heat Exchanger Network “Chemical Process: Design and Integration” by R. Smith, John Wiley & Sons Ltd. 2005. “Pinch Analysis and Process Integration: A User Guide on Process Integration for the Efficient Use of Energy” by I.C. Kemp, Elsevier, 2007.

4. 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

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

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

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

8. Typical parts and connections of Heat Exchangers

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

12. 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.

13. 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

14. 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

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

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

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

19. 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.

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

21. Design value of ‘P’:

22. Bowman RA. Mean Temperature Difference Correction in Multipass Exchangers, Ind. Eng. Chem. 1936 (28) 541-544.

23. 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:

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

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

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

28. Temperature–enthalpy diagram

29. Composite Curves

30. Formation of Hot and Cold composite curves • 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

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

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

33. 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

34. Threshold Problems Threshold problem: Only cold utility required

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

36. Threshold Problems (contd.)

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

38. Steam classification Steam cost (major components): CG = f(CF, Hg, hf, ηB) CF = Fuel cost (~ 90% of total cost) Hg = Enthalpy of steamhf  = Enthalpy of boiler feedwater ηB = Boiler efficiency

39. Steam valuation estimated by convention method Source: https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/tech_brief_true_cost.pdf

40. Problem Table Algorithm

41. 2 250 245 235 240 230 195 200 4 200 190 185 180 190 180 190 145 140 150 140 150 3 70 80 75 80 35 30 40 25 1 Example Problem Table Algorithm ΔTmin=10°C Shifted temperature intervals, *T, (°C)

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

43. EXAMPLE GCC and Multiple Pinches Hot utility=7.5 MW Cold utility= 10 MW (20-30°C) Stream Table 4.5 MW, 240-239 °C HP steam 3 MW, 180-179 °C LP steam Heat recovery pockets Balanced composite curve for multiple utilities GCC and utility selection

44. 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

45. Grid representation of streams

46. 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

47. 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

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

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

50. Target area calculation Interval 1 2 3 4 5 6 7 180ºC 179.571ºC 179ºC Steam (qi= 3) (qi= 4) 90ºC 50ºC 150ºC 60ºC 1 (qi= 12) (qi= 6) (qi= 2) 170ºC 90ºC 50ºC 40ºC 150ºC 60ºC 2 (qi= 2) (qi= 6) (qi= 3) (qi= 1) (qi= 1) 110ºC 120ºC 105ºC 102.5ºC 80ºC 50ºC 3 (qj= 0.75) (qj= 6.75) (qj= 1.5) (qj= 9) 110ºC 105ºC 102.5ºC 80ºC 4 30ºC 20ºC 22.5ºC (qj= 1.25) (qj= 11.25) (qj= 2.5) CW (qj= 3) (qj= 1) Enthalpy intervals and stream population Aarea above the pinch, (target) = 8,859 m2 Aarea below the pinch, (target) = 10,469 m2 (qj= 3) PINCH