1. Pinch Technology and optimization of the use of utilities – part two
Maurizio Fermeglia

2. 054402 Design and Analysis II LECTURE SEVEN UNIT 4: Loops and Splits
The minimum number of units (Umin) in a network:
UMin = NStream + NUtil  1 (Hohman, 1971)
A HEN containing UHEX units (UHEX  Umin) has (UHEX  Umin) independent “heat loops”.
The HEN above has 2 “heat loops”
Normally, when heat loops are “broken”, heat flows across the pinch - the number of heat exchangers is reduced, but the utility loads are increased.
Daniel R. Lewin, Technion

3. Class Exercise 5 (Linnhoff and Flower, 1978)
Example:
Tmin = 10 oC.
Step 1: Temperature Intervals (subtract Tmin from hot temperatures) Temperature intervals: 180oC  170oC 140oC 130oC 60oC 30oC

4. Class Exercise 5 (Cont’d)
Step 2: Interval heat balances For each interval, compute: Hi = (Ti  Ti+1)(CPHot CPCold )

5. Class Exercise 5 (Cont’d)
Step 3: Form enthalpy cascade.
This defines:
Cold pinch temp. = 140 oC
QHmin = 60 kW
QCmin = 160 kW

6. Class Exercise 5 (Cont’d)
MER Design above the pinch:
UMin,MER = NStream + NUtil = – = 2 
MER Design below the pinch:
UMin,MER = – = 4 MER design below pinch has 6 exchangers! i.e. There are two loops below pinch.

7. Class Exercise 5 (Cont’d)
Complete MER Design
However, UMin = NStream + NUtil  =  1 = 5
The MER network has 8 units. This implies 3 independent “heat load loops”. We shall now identify and eliminate these loops in order to design for UMin

8. Class Exercise 5 (Cont’d)
Identification and elimination of 1st loop:
To reduce the number of units, the 80 kW exchanger is merged with the 60 kW exchanger. This breaks the heat loop, but also creates a Tmin violation in the network:

9. Class Exercise 5 (Cont’d)
Identification and elimination of 1st loop (Cont’d):
To restore Tmin, the loads of the exchangers must be adjusted along a “heat path” by an unknown amount x. A “heat path” is a path through the network that connects heaters with coolers.

10. Class Exercise 5 (Cont’d)
Identification and elimination of 1st loop (Cont’d):
Performing a heat balance on H1 in the exchanger which exhibits the Tmin violation: x = 2( Tmin)  x = 26.66
This is called “energy relaxation”

11. Class Exercise 5 (Cont’d)
Identification and elimination of 2nd loop:
Since there is no Tmin violation, no adjustment of the loads of the exchangers is needed - we reduce the number of units by one with no energy penalty.

12. Class Exercise 5 (Cont’d)
Identification and elimination of 3rd loop:
Shifting the load of the smallest exchanger (93.33 kW) around the loop, the network is reduced to…

13. Class Exercise 5 (Cont’d)
Identification and elimination of 3rd loop:
We use the heat path to restore Tmin:  x = 3(150 - Tmin- 60)  x = 13.33

14. Class Exercise 5 (Cont’d)
Therefore Umin Network is:

15. Loop Breaking - Summary
Step 1:
Perform MER Design for UHEX units. Try and ensure that design meets UMin,MER separately above and below the pinch.
Step 2:
Compute the minimum number of units:
UMin = NStream + NUtil  1
This identifies UHEX  Umin independent “heat loops”, which can be eliminated to reduce U.
Step 3:
For each loop, eliminate a unit If this causes a Tmin violation, identify the “heat path” and perform “energy relaxation” by increasing the duties of the cooler and heater on the heat path.
Loops improve energy recovery and heat load flexibility at the cost of added units (>Umin)

16. Stream Split Designs
Example.
Option 1.

17. Stream Split Designs (Cont’d)
Option 2. Loops
Option 3. Stream Splitting

18. Stream splitting is a powerful technique for better energy recovery
Loops vs. Stream Splits
Loops:
Improved energy recovery (normally)
Heat load flexibility (normally)
U > UMin (by definition)
Stream Splitting:
Maximum Energy recovery (always)
Branch flowrate flexibility (normally)
U = UMin (always)
Stream splitting is a powerful technique for better energy recovery
BUT - Don’t split unless necessary

19. Stream Splitting Rules
Design and Analysis II
LECTURE SEVEN
Stream Splitting Rules
1. Above the pinch (at the pinch):
Cold utilities cannot be used (for MER). So, if NH > NC, MUST split COLD streams, since for feasibility NH  NC
Feasible matches must ensure CPH  CPC. If this is not possible for every match, split HOT streams as needed. If Hot steams are split, recheck 
2. Below the pinch (at the pinch):
Hot utilities cannot be used (for MER). So, if NC > NH, MUST split HOT streams, since for feasibility NC  NH
Feasible matches must ensure CPC  CPH. If this is not possible for every match, split COLD streams as needed. If Cold steams are split, recheck 
Daniel R. Lewin, Technion

20. Class Exercise 6
Design a hot-side HEN, given the stream data below: 
500 
200
Solution: Since NH > NC, we must split C1. The split ratio is dictated by the rule: CPH  CPC (necessary condition) and by a desire to minimize the number of units (“tick off “streams)

21. Class Exercise 6 (Cont’d)
x is determined by the following energy balances: x(T1  90) = (10  x)(T2  90) = subject to: 200 T1  Tmin = T2  Tmin = 10 Best to make T1 = T2 . Here, this is not possible. Why? We shall make T2 = 140 (why?)

22. Class Exercise 6 (Cont’d)
A possible solution is therefore: (10  x) (140  90) = 200  x = 6 T1 = /x = (satisfies constraint)
Complete solution is:
This is an MER design which also satisfies UMin (UMin = 3).

23. UNIT 5: Threshold Problems
Example - Consider the problem
Networks with excess heat supply or heat demand may have MER targets with only one utility (i.e., either QHmin = 0 or QCmin = 0). Such designs are not separated at the pinch, and are called “Threshold Problems”

24. Threshold Problems (Cont’d)
Assuming a value of Tmin= 105 oC:
Assuming a value of Tmin= 10 oC, the Problem Table gives the following result.

25. Threshold Problems (Cont’d)
Threshold problems do not have a pinch, and have non-zero utility duties only at one end.

26. Threshold Problems (Cont’d)
However, increasing driving forces beyond the Threshold Value leads to additional utility requirements.

27. Threshold Design Guidelines
1. Establish the threshold Tmin
2. Note the common practice values for Tmin:
3. Compare the threshold Tmin to Tmin,Experience
Classify as one of the following:
Pinched - treat as normal pinched problem
Threshold - must satisfy target temperatures at the “no utility end”

28. Class Exercise 7
The graph shows the effect of Tmin on the required levels of QHmin and QCmin for a process consisting of 3 hot and 4 cold streams.

29. Introduction to HEN software
Ref. Turton et al.
Analysis, Synthesis and Design of Chemical Processes

30. The MUMNE algorithm The Minimum Utility steps:
Choose a minimum approach temperature (parametric optimization)
Construct a temperature interval diagram
Construct a cascade diagram and determine the minimum utility requirements and the pinch temperatures
Calculate the minimum number of heat exchangers above pinch
Calculate the minimum number of heat exchangers below pinch
Construct the heat exchanger network
The object is to obtain an heat exchanger network
…That exchange the minimum energy between the streams and the utilities
…That uses the minimum number of equipment

31. Algorithm: initial condition and step 1
Minimum Temperature Approach = Smallest DT of two streams leaving or entering an heat exchanger = 10°C
Hot Stream Data
Mass Flow Cp Temp In Temp Out Stream Enthalpy Film Heat Transf. Coef
kg/s kJ/kg/°C °C °C kW W/m2/°C
Cumulative Hot Stream Energy Available = kW
Cold Stream Data
Cumulative Cold Stream Energy Available = kW

32. Algorithm: construct Temperature interval diagram (step 2)
Process streams represented by vertical lines
Axes are shifted by the minimum T approach

33. Algorithm: construct a cascade diagram (1)
Shows the net amount of energy in each interval diagram
Cascade  if there is an excess energy in a T interval we may “cascade” it down
Energy cannot be transferred up (II law)
Line is the point at which no more energy can cascade down
We need to resort to utilities
NOTE: not all problems have a pinch condition: the algorithm is still valid
Pinch temperature

34. Algorithm: construct a cascade diagram (2)
Additional heat is transferred to the C interval (yellow line)
Energy is cascaded down through the pinch and rejected to the cold utility
If heat is transferred across the pinch, the net result will be that more heat will have to be added from the hot utility and rejected to the cold utility
To minimize the hot and cold utility requirements, energy should NOT be transferred across the pinch
Pinch temperature

35. Algorithm: minimum n. of exchangers
Above the pinch
Draw boxes representing energy in the hot and cold process streams and utilities
Transfer energy is indicated by lines (with the amount)
For each line an heat exchanger is required
The problem is split into two sub problems

36. Algorithm: minimum n. of exchangers
Below the pinch
Draw boxes representing energy in the hot and cold process streams and utilities
Transfer energy is indicated by lines (with the amount )
For each line an heat exchanger is required
The problem is not split into sub problems

37. Algorithm: minimum n. of exchangers
General relationship
For any sub problem
With or without a pinch
Above or below the pinch
Min. No. of exchangers = No. of hot streams + No. of cold streams No. of utilities – 1

38. Algorithm: Design the network above the pinch
Start from the design at the pinch
To make sure that DT min is not violated, match streams such that
Note that we consider ONLY streams present at the pinch
Each exchanger is represented by two circles connected with a line, each circle represent a side

39. Algorithm: Design the network above the pinch
Move away from the pinch
Look at the remaining streams
Criterion used at pinch not necessarily holds away from the pinch
The following constraints are not violated:
The minimum approach T is used throughout the design
The number of exchangers must be that calculated in step 3
Heat is added form the coolest possible source

40. Algorithm: Design the network below the pinch
Similar to previous one: start from the design at the pinch
To make sure that DT min is not violated, match streams such that
Note that we consider ONLY streams present at the pinch
Each exchanger is represented by two circles connected with a line, each circle represent a side
What happens if the DT min is violated (see figure)

41. Algorithm: Design the network below the pinch
Split stream into substreams to meet the DT min criterion

42. Algorithm: Design the network below the pinch
Move away from the pinch
Look at the remaining streams

43. Final result The final network of heat exchangers is the following
It has the minimum n. of exchangers (8)
Minimum utility requirement (Qh = 100 kW and Qc = 50 kW)
Using a minimum approach T = 10°

44. Heat exchangers design: area and costs
Up to now, emphasis on the topology of the network
… to complete the design, it is necessary to
Estimate the heat transfer area (A= Q / (U DTln F)
And the cost estimate
If heat transfer coefficients are known (including fouling)…
Transfer coefficients form literature (Seider – Tate, Donahue, …)
… exchanger area can be calculated (for streams exchangers)
Exchanger DT ln U Q F factor Area.
°C W/m2/°C kW m2
TOTAL
Exchanger 5 requires an hot utility (steam): DT = 76.8 °C, U= 76.9 W/m2/°C, Area = 16.9 m2
Exchanger 8 requires cooling water: DT = 23.2 °C, U= 346 W/m2/°C, Area = 7.8 m2
TOTAL Area: m2

45. Effect of the minimum approach temperature
Calculations must be repeated for different approach T (step 1)
Problem: step 5 (matching streams and exchanging energy) cannot be programmed easily
An approximate approach is necessary for investigating the effect of the approach temperature on the total cost
Based on the Composite temperature enthalpy diagram
Constructed by plotting enthalpy of all streams as a function of T

46. Construction of Composite T-H diagram
Temperature Interval
T °C
Enthalpy of HOT streams in Temperature interval (kW)
Cumulative Enthalpy of HOT streams (kW) D 50 C
100
(2+3)(100-50) = 250
250 B
150
(2+3)( ) = 250
500 A
200
(8+3)( ) = 50
1050
300
(8)( ) = 800
1850
Temperature Interval
T °C
Enthalpy of COLD streams in Temperature interval (kW)
Cumulative Enthalpy of COLD streams (kW) D 40 C 90
(4)(90-40) = 200
200 B
140
(8+4)(140-90) = 600
800 A
190
(8+4)( ) = 600
1400
290
(5)( ) = 500
1900

47. Construction of Composite T-H diagram

48. Using the T-H diagram to estimate heat exchanger area
The working equation is (A= Q / (U DTln F)
Consider a portion of the diagram (figure)

49. Results of the heat transfer area calculations

50. Finalize the design Consider the F factor
By calculating the number of shells in a 1-2 geometry exchanger
The effect of the ‘economy of scale’: the cost of two 1-2 S&T in series is greater than the equivalent 1-2 S&T with the same total area
Calculate the cost of equipments
Knowing the number of shells for each exchanger
Using cost correlations
Applying an economic criteria (such as Equivalent Annual Operating Cost – EAOC)
Approximations
All heat exchangers have the same area  over estimation of the costs
No effect of material of construction (corrections available)
No effect of operating pressure (corrections available)
No multiple utilities (alternative methods)
No streams with phase change (correction available)

51. Calculation of the costs for the network

52. Typical relationship for heat transfer area, utilities and EAOC for a HEN

53. Mass – exchange networks
Exchangers to use energy more efficiently
Temp Interval Diagram
Cascade diagram – pinch
Min n. of eq. above and below
Composite T exchange diag.
Final T exchange network
Utility: cold and hot source of energy
Hot and cold
Temperature
Separators to use mass more efficiently
Composition Interval Diagram
Cascade diagram – pinch
Min n. of eq. above and below
Composite mass exchange diag.
Final mass exchange network
Utility: separation - addition of solute (from source or to sink)
Rich and lean
Concentration

54. Aspentech Energy Analyzer

55. The streams Hot streams: Cold streams: NAME T in (°C) T out MCp
(kW/°C) H1
400
150 8 H2
200 50 5 H3
250
100 6
Cold streams:
NAME
T in
(°C)
T out
MCp
(kW/°C) C4
190
390 10 C5 40
140 8 C6 90
240 5

56. Temperature interals (DT min= 10)

57. The composite curve

58. COLD PINCH TEMPERATURE
Cumulative Heat Ti
Ti-Ti+1
MCpHOT - MCpCOLD Qi
Q cumulato(QH=0)
Q cumulato (QH=350)
390
150 -2
-300 50
240 -1
-50
-350
190 14
700
350
140 -3
-100
250
600 90
-150
100
450 40
COLD PINCH TEMPERATURE
HOT PINCH TEMPERATURE
+ Tmin
190
200

59. Design rules Above the pinch Belove the pinch
Geenral:
Never exchge heat cross pinch
Above the pinch
MCpHOT  MCpCOLD
Do not use coolers
Belove the pinch
MCpCOLD  MCpHOT
Do not use heaters

60. Manual solution (HENSAD)
Above the Pinch

61. Manual solution (HENSAD)
Belove the Pinch

62. ASPEN ENERGY ANALIZER
PROCESS STREAM DATA

63. ASPEN ENERGY ANALIZER
PROCESS UTILITIES DATA

64. ASPEN ENERGY ANALIZER
PROCESS ECONOMIC DATA

65. ASPEN ENERGY ANALIZER
PROCESS TARGET SUMMARY

66. ASPEN ENERGY ANALIZER

68. ASPEN ENERGY ANALIZER
AUTOMATIC HEN DESIGN OPTIONS

69. ASPEN ENERGY ANALIZER
AUTOMATIC HEN DESIGN NETWORKS

70. ASPEN ENERGY ANALIZER
AUTOMATIC HEN DESIGN REPORT

71. Tutorials Aspen Energy analyzer: case