1. Sieder, Chapter 11 Terry Ring University of Utah
Heat Integration
Sieder, Chapter 11
Terry Ring
University of Utah

2. Lost Work = Lost Money Transfer Heat from T1 to T2
ΔT [= T1-T2] approach Temp. for Heat Exchanger
To= Temperature of Environment
Use 1st and 2nd laws of Thermodynamics ΔT
LW=QTo(T1-T2)/(T1T2)
Q= UoAΔTlm =UoA(ΔT1-ΔT2)/ln(ΔT1/ΔT2) T1 Q T2

3. Simple Heat Exchange Network (HEN)
Use another stream for HX instead of a utility.
What happens when delta T in Exchanger is lowered? To Zero?

4. Costs Heat Exchanger Purchase Cost (Inside BL) Annual Cost
Cp,i=K(Areai)0.6
Area= Q/UoΔTlm
Annual Cost
CA=im[ΣCp,i+ ΣCP,A,j]+sFs+(cw)Fcw
im=return on investment
Fs= Annual Flow of Steam,
$13.2/Tonne to $17.6/Tonne = s
Fcw=Annual Flow of Cold Water
$0.027/m3 = cw
Auxiliary HX outside BL
CpA is HX in auxiliary networks

5. Capital and Operating Cost Optimization
Capital cost goes down when A is less. This is caused by delta T being larger for Q to remain the same.

6. Heat Integration Make list of HX
Instead of using utilities can you use another stream to heat/cool any streams?
How much of this can you do without causing operational problems?
Can you use air to cool?
Air is a low cost coolant.
Less utilities = smaller cost of operations

7. Ultra-high purity Si plant design
Si at 99.97% Powder
H2 & HCl
Separation Train
Fluid Bed Reactor ( C)
Si+7HCl  SiHCl3 + SiCl4 +3H2
Si+ 2HCl  SiH2Cl2
Flash
HCl
H2-HCl Separation
SiCl4
HCl H2
SiCl4
Very Pure
SiHCl3&SiH2Cl2
Fluid Bed Reactor(600C)
Si+SiCl4+2HCl 2SiHCl3
Flash
Reactor (1200C)
SiHCl3+H2  Si+3HCl
SiH2Cl2+1/2 H2  Si+3HCl Si H2
HCl
Si at %

10. Terms HEN=Heat Exchanger Network MER=Maximum Energy Recovery
Minimum Number of Heat Exchangers
Threshold Approach Temperature
Optimum Approach Temperature

11. Process

12. Minimize Utilities For 4 Streams
470
480

13. Simple HEN

14. Minimize Utilities For 4 Streams
470
480

15. Pinch Analysis 1) Adjust Hot Stream Temperatures to Give ΔTmin=10°F
Put T’s in order Max to Min
2) Order T’s, 250, 240, 235, 180, 150, 120

16. Enthalpy Differences for Temperature Intervals

17. Interval Heat Loads

19. Pinch Analysis 1) Adjust Hot Stream Temperatures to Give ΔTmin
Put T’s in order Max to Min
Order T’s, 250, 240, 235, 180, 150, 120

20. Pinch Analysis Minimum Utilities
=ΔHi+50
Pinch Analysis Minimum Utilities
R’s = residulas

21. Pinch Analysis Minimum Utilities
=ΔHi+50
Pinch Analysis Minimum Utilities
R’s = residulas

23. Pinch Analysis
Actual Endpoint Temperatures!
ΔTapp
MER values

24. Process

25. How to combine hot with cold?
Big Exhangers 1st
1st HX at Pinch (temp touching pinch)
Above Pinch Connect
Cc≥Ch
Below Pinch Connect
Ch≥Cc
2nd Hx or not touching Pinch temp.
No requirement for Cc or Ch

26. Pinch Analysis Actual Endpoint Temperatures! Cc≥Ch ΔTapp MER values
3*( )=210
1.5*( )=90
2*( )=110
4*( )=240
ΔTapp
MER values

27. Pinch Analysis Actual Endpoint Temperatures! Cc≥Ch ΔTapp MER values
3*( )=210
1.5*( )=90
2*( )=110
90=2*(T-180)
T=225
4*( )=240
210=4*(T-180)
T=232.5°F
ΔTapp 20 30
MER values

28. How to combine hot with cold?
Big Exhangers 1st
1st HX at Pinch (temp touching pinch)
Above Pinch Connect
Cc≥Ch
Below Pinch Connect
Ch≥Cc
2nd Hx or not touching Pinch temp.
No requirement for Cc or Ch

29. Pinch Analysis Actual Endpoint Temperatures! Ch≥Cc ΔTapp MER values
3*( )=90
1.5*( )=90
30=1.5*(190-T)
T=170°F
2*( )=120
90=2*(180-T)
T=135°F
2*( )=30 60
ΔTapp
MER values

30. 4 Heat Exchanger HEN for Min. Utilities
Cc≥Ch
Ch≥Cc CW
MER
Values
Steam

31. Pinch Analysis Minimum Utilities
=ΔHi+50
Pinch Analysis Minimum Utilities
R’s = residulas

32. Minimum Utilities HEN

33. Simple HEN

34. Comparison Simple HEN HEN with Min. Utilities Saves CW 7.5e4 BTU/hr
Steam 7.5e4 BTU/hr

35. Too Many Heat Exchangers
Sometimes fewer Heat exchangers and increased utilities leads to a lower annual cost
NHx,min= Ns + NU - NNW
s=No. streams
U=No. discrete Utilities
NW=No. independent Networks (1 above the pinch, 1 below the pinch)
Solution to Too Many Heat Exchangers
Break Heat Exchanger Loops
Stream Splitting
Attack small Heat Exchangers First
4+2-2=4

36. Break Heat Exchanger Loops

37. Stream Splitting Two streams created from one1
Two streams created from one
one heat exchanger on each split of stream with couplings 1 1b 1a 1b 1a

38. Example
CP=K(Area)0.6

39. Last Considerations How will HEN behave during startup?
How will HEN behave during shutdown?
Does HEN lead to unstable plant operation?

41. Optimization of HEN
How does approach ΔT >ΔTmin effect the total cost of HEN?
Q= UA ΔT
Less capital cost
LW=QToΔT/(T1T2)
More Utility cost

42. ΔTmin ΔTapp=10C ΔTapp=105C LW=QToΔT/(T1T2) S T(C) T(C) C Q(kW)H H C
ΔTapp=10C ΔTapp=105C
LW=QToΔT/(T1T2)

43. Costs Heat Exchanger Purchase Cost (Inside BL) Annual Cost
Cp,i=K(Areai)0.6
Area= Q/UoΔTlm
Annual Cost
CA=im[ΣCp,i+ ΣCP,A,j]+sFs+(cw)Fcw
im=return on investment
Fs= Annual Flow of Steam,
$13.2/Tonne to $17.6/Tonne = s
Fcw=Annual Flow of Cold Water
$0.027/m3 = cw
Auxiliary HX outside BL
CpA is HX in auxiliary networks

44. Change ΔTmin CP=K(Area)0.6 Area=Q/(UF ΔTmin) LW=QToΔT/(T1T2)
More Lost Work
ΔT =UA/Q
Utilities increase due to Lost work since it increases as ΔT increases
LW=QToΔT/(T1T2)

45. Capital and Operating Cost Optimization
Capital cost goes down when A is less. This is caused by delta T being larger for Q to remain the same.
ΔTthres

46. Distillation Columns

47. Heuristic “Position a Distillation Column Between Composite Heating and Cooling Curves”

48. Heat Integration for Direct Distillation Sequence

49. Multi-effect Distillation Adjust Pressure in C2 for ΔTmin

50. Heat Pumps in Distillation

51. Heat Pumps How do they work?
Carnot Efficiency
ηmax= 1-Tc/Th
Endoreversible
η =1-√(Tc/Th)
Same as Air Conditioner
Convert low temperature heat to high temperature heat.
Must add work as heat can not go up hill.

52. Heat Pumps/Heat Engines Heurisitcs
When positioning heat engines, to reduce the cold utilities, place them entirely above or below the pinch
When positioning heat pumps, to reduce the total utilities, place them across the pinch.

53. Heat Pumps Where can they be used?
Heuristic
When positioning heat pumps, to reduce the total utilities, place them across the pinch.

54. Heat Engines Where can they be used?Tp
Heuristic
When positioning heat engines, to reduce the cold utilities, place them entirely above or below the pinch