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

2. Objectives
The first part of this three-part Unit on HEN synthesis serves as an introduction to the subject, and covers:
The “pinch”
The design of HEN to meet Maximum Energy Recovery (MER) targets
The use of the Problem Table to systematically compute MER targets
Instructional Objectives:
Given data on hot and cold streams, you should be able to:
Compute the pinch temperatures
Compute MER targets
Design a simple HEN to meet the MER targets

3. A Short Bibliography... Early pioneers: Central figure:
(1968)
(1971)
Central figure:
(1978)
Currently: President, Linnhoff-March
Recommended texts:
Seider, Seader and Lewin (1999): Process Design Principles, Wiley and Sons, NY
Linnhoff et al. (1982): A User Guide on Process Integration for the Efficient Use of Energy, I. Chem. E., London
Turton et al, Analysis, synthesis and design of chemical processes, Prentice Hall,
Review:
Gundersen, T. and Naess, L. (1988): “The Synthesis of Cost Optimal Heat Exchanger Networks: An Industrial Review of the State of the Art”, Comp. Chem. Eng., 12(6),

4. UNIT 1: Introduction - Capital vs. Energy
The design of Heat Exchanger Networks deals with the following problem:
Given:
NH hot streams, with given heat capacity flowrate, each having to be cooled from supply temperature THS to targets THT.
NC cold streams, with given heat capacity flowrate, each having to be heated from supply temperature TCS to targets TCT.
Design:
An optimum network of heat exchangers, connecting between the hot and cold streams and between the streams and cold/hot utilities (furnace, hot-oil, steam, cooling water or refrigerant, depending on the required duty temperature).
What is optimal?
Implies a trade-off between CAPITAL COSTS (Cost of equipment) and ENERGY COSTS (Cost of utilities).

5. Impact of process integration

6. Example Network for minimal equipment cost ?
Network for minimal energy cost ?

7. Design A: (AREA) = 20.4 [ A = Q/UTlm ]
Numerical Example
Design A: (AREA) = [ A = Q/UTlm ]
Design B: (AREA) = 13.3

8. Which option requires more capital?

9. Energy efficient process can also be more capital efficient process (saves energy AND capital)

10. Energy efficient design reduces investment in the utility infrastructure

11. Some Definitions
TS = Stream supply temperature (oC) TT = Stream target temperature (oC) H = Stream enthalpy (MW) CP = (MW/ oC) = Heat capacity flowrate (MW/ oC) = Stream flowrate specific heat capacity

12. DTmin - Example
Tmin = Lowest permissible temperature difference
Which of the two counter-current heat exchangers illustrated below violates T  20 oF (i.e. Tmin = 20 oF) ?
20o
30o
10o
20o
Clearly, exchanger A violates the Tmin constraint.

13. Definitions (Cont’d) Exchanger Duty.
Data: Hot stream CP = 0.3 MW/ oC Cold stream CP = 0.4 MW/ oC Check: T1 = 40 + ( )(0.3/0.4) = 70oC  Q = 0.4( ) = 0.3( ) = 12 MW
Heat Transfer Area (A): A = Q/(UTlm)
Data: Overall heat transfer coefficient, U=1.7 kW/m2 oC (Alternative formulation in terms of film coefficients) Tlm = ( )/loge(30/20) = So, A = Q/(UTlm) = 12000/(1.724.66) = m2

14. Pinch technology basics

16. How can we identify appropriate process design changes?

17. Introduction to Pinch technology
Whenever the design is considered limits exits that constraints the design
Mechanical constraints
Length and diameter of towers
Diameters of heat exchangers
Thermodynamic constrains
First principles and second principles
Close approach in heat exchanger  large surface area
Reflux ratio close to the minimum  number of stages grows
When driving force becomes small  area becomes large
We say that the design has a PINCH
Applies to heat and mass
In a network (mass or heat) there is a point in which the driving force is minimum  PINCH POINT
A succesful design involves defining where the pinch is
Using the information at the PINCH POINT is named PINCH TECHNOLOGY

18. Introduction to Pinch technology
Pinch technology applications
Both heat and mass transfer
New processes
Existing processes for retrofitting
Pinch technology = optimization
For new and existing processes an algorithm is used
Design of heat and mass exchanger network
… that consumes the minimum amount of utilities HEN (MEN)
… that requires the minimum number of equipments (exchangers) MUMNE
The solution may not be optimal in the economic sense
… it is a starting point close enough to the economic minimum.

19. Pinch technology and heat integration
Growing importance of heat integration is driving force
Formalization of heat integration theory  pinch technology
Linhoff and Flower
Hohman
Umeda et al.
Douglas
Turton et al (text book)
Different operative configurations of the same process may result in
Same conditions (composition, temperature, pressure, flow rate)
Different Fixed capital investment
Different cost of utilities
Different Net present value

20. Class Exercise 1 Tmin = 10 oC
Utilities. oC, Design a network of steam heaters, water coolers and exchangers for the process streams. Where possible, use exchangers in preference to utilities. .

21. Setting Energy Targets
Summary of proposed design: Are 60 kW of Steam Necessary?

22. The Temperature-Enthalpy Diagram
One hot stream
Two hot streams

23. The Temperature-Enthalpy Diagram
Correlation between Tmin, QHmin and QCmin More in, More out! QHmin + x  QCmin + x

24. The Composite Curve
Hot Composite Curve

25. The Composite Curve (Cont’d)
Cold Composite Curve

26. The Composite Curve (Cont’d)
Result: QCmin and QHmin for desired Tmin MER Target Here, hot pinch is at 70 oC, cold pinch is at 60 oC QHmin = 48 kW and QCmin = 6 kW
Method: manipulate hot and cold composite curves until required Tmin is satisfied This defines hot and cold pinch temperatures.

27. UNIT 2: The Pinch +x x +x
The “pinch” separates the HEN problem into two parts:
Heat sink - above the pinch, where at least QHmin utility must be used
Heat source - below the pinch, where at least QCmin utility must be used.

28. Significance of the Pinch
Do not transfer heat across pinch
Do not use cold utilities above the pinch
Do not use hot utilities below the pinch
Summary of modified design:

29. HEN Representation
Where is the pinch ?

30. HEN Representation with the Pinch
The pinch divides the HEN into two parts:  the left hand side (above the pinch)  the right hand side (below the pinch) At the pinch, ALL hot streams are hotter than ALL cold streams by Tmin.

31. Class Exercise 2 For this network, draw the grid representation
Given pinch temperatures at 480 oC /460 oC, and MER targets: QHmin= 40, QCmin= 106, redraw the network separating the sections above and below the pinch.
Why is QH > QHmin ?

32. Class Exercise 2 - Solution
100 C
116
210
170 H 40 H 10

33. Class Exercise 2 - Solution (Cont’d)
This can be fixed by reducing the cooling duty by 10 units, and eliminate the excess 10 units of heating below the pinch.

34. Design for Maximum Energy Recovery(MER)
Example
Step 1: MER Targeting. Pinch at 90o (Hot) and 80o (Cold) Energy Targets: Total Hot Utilities: 20 kW Total Cold Utilities: 60 kW

35. Design for MER (Cont’d)
Step 2: Divide the problem at the pinch

36. Design for MER (Cont’d)
Step 3: Design hot-end, starting at the pinch: Pair up exchangers according to CP-constraints. Immediately above the pinch, pair up streams such that: CPHOT  CPCOLD (This ensures that TH TC  Tmin)
Tmin

37. Design for MER (Cont’d)
Step 3 (Cont’d): Complete hot-end design, by ticking-off streams.  
QHmin = 20 kW   H 20 90 
240
Add heating utilities as needed (MER target)

38. Design for MER (Cont’d)
Step 4: Design cold-end, starting at the pinch: Pair up exchangers according to CP-constraints. Immediately above the pinch, pair up streams such that: CPHOT  CPCOLD (This ensures that TH TC  Tmin)
Tmin

39. Design for MER (Cont’d)
Step 4 (Cont’d): Complete cold-end design, by ticking-off streams.  
QCmin = 60 kW  C 60 
35o 90 30
Add cooling utilities as needed (MER target)

40. Design for MER (Cont’d)
Completed Design:
Note that this design meets the MER targets: QHmin = 20 kW and QCmin = 60 kW

41. Design for MER (Cont’d)
Design for MER - Summary:
MER Targeting. Define pinch temperatures, Qhmin and QCmin
Divide problem at the pinch
Design hot-end, starting at the pinch: Pair up exchangers according to CP-constraints. Immediately above the pinch, pair up streams such that: CPHOT  CPCOLD. “Tick off” streams in order to minimize costs. Add heating utilities as needed (up to QHmin). Do not use cold utilities above the pinch.
Design cold-end, starting at the pinch: Pair up exchangers according to CP-constraints. Immediately below the pinch, pair up streams such that: CPHOT  CPCOLD. “Tick off” streams in order to minimize costs. Add heating utilities as needed (up to QCmin). Do not use hot utilities below the pinch.
Done!

42. Class Exercise 3        
Tmin = 10 oC. - Pinch Temperature 70° - 60° Utilities: oC,
Design a network of steam heaters, water coolers and exchangers for the process streams. Where possible, use exchangers in preference to utilities. 
80oC  
43oC C
QHmin=48
QCmin=6   6  H 40
120  H  8
100 54

43. UNIT 3: The Problem Table
Example:
Tmin = 10 oF.
Step 1: Temperature Intervals (subtract Tmin from hot temperatures) Temperature intervals: 250F  240F  235F  180F  150F  120F

44. UNIT 3: The Problem Table (Cont’d)
Step 2: Interval heat balances For each interval, compute: Hi = (Ti  Ti+1)(CPHot CPCold )

45. UNIT 3: The Problem Table (Cont’d)
Step 3: Form enthalpy cascade.
This defines:
Cold pinch temp. = 180 oF
QHmin = 500 kBtu/h
QCmin = 600 kBtu/h