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T. Gundersen
Heat Integration - Introduction • Sub-Topics
§ Design of Heat Exchanger Networks § Selection of Utilities
Ø Consumption and Production § “Correct” Integration
Ø Distillation and Evaporation Ø Heat Pumps Ø Turbines (Heat Engines)
§ Heat and Power Considerations • Project Applications
§ New Design (“grassroot”) § Reconstruction (“retrofit”)
Process, Energy and System
Heat 1
Heat Integration − Introduction
T. Gundersen
x L
dx
A , U Th,in
mCph
mCpc
Th,out
Tc,in
Tc,out
Pure Countercurrent Heat Exchanger
Heat 2
Heat Integration − Introduction
Process, Energy and System
Qh = ΔHh = mCph ⋅ Th,in −Th,out( )Qc = ΔHc = mCpc ⋅ Tc,out −Tc,in( )
Q = A ⋅U( ) ⋅ ΔTLMQh =Qc =QΔTLM = ??
WS-1: Heat Integration
T. Gundersen
Stream Ts Tt mCp ΔH °C °C kW/°C kW
H1 300 100 1.5 300 H2 200 100 5.0 500 C1 50 250 4.0 800
Steam 280 280 (var) Cooling Water 15 20 (var)
Specification: ΔTmin = 20°C
Find: QH,min , QC,min
Tpinch , Umin Umin,MER
and Network
Notice: 1) H1 and H2 provide as much heat as C1 needs (800 kW) 2) Ts (C1) < Tt (H1,H2) - 20°C & Ts (H1) > Tt (C1) + 20°C
Heat 3
Heat Integration − Introduction
Process, Energy and System
Procesintegration i industrien
Possible Solutions
I II C1
H1 H2
H
C
B
I II C1
H2 H1
H
C D
I II C1
H2 H1
H
C C
T. Gundersen
I II C1
H1 H2
H
C
A 300 200
50
Heat 4
Heat Integration − Introduction
Process, Energy and System
More Solutions
I
II
C1
H1 H2
H
C F
I II C1
H2 H1
H
C
III
E
Question: 1) Minimum Energy ? 2) Which Network ?
T. Gundersen Heat 5
Heat Integration − Introduction
Process, Energy and System
Summary for WS-1
T. Gundersen
Design QH = QC (kW)
A 280 B 280 C 380 D 142.5 E See later . . . . F Around 149 (optimal split)
X Any better ???
Heat 6
Heat Integration − Introduction
Process, Energy and System
We need Best Performance Targets !!
T. Gundersen
• Data Extraction (assume R/S given) • Performance Targets
§ Energy, Area, Units / Shells § Total Annual Cost (gives value for ΔTmin)
• Process Modifications (change R/S ?) • Design of Network (minimum energy)
§ Decomposition at the Pinch § Qualitative and Quantitative Tools
• Optimization (given the basic structure) § Heat Load Loops & Paths and Stream Splits
Phases in the Design of Heat Exchanger Networks
Heat 7
Heat Integration − Introduction
Process, Energy and System
Illustrating Example
Reactor
Feed
Product
Distillation Column
Compressor
50°
210°
160°
210°
130°
220°
160°
270°
60° Reboiler
Condenser
T. Gundersen Heat 8
Heat Integration − Introduction
Process, Energy and System
Phase 1: Data Extraction • Time consuming (80%) and Critical Phase • New Design: Material / Energy Balances • Retrofit: Various Sources of Data
§ Measurements (that are often incorrect) § Simulations (M+E balances è Consistent) § Design Basis (original, but was it updated ?)
• Should keep Degrees of Freedom open • Practical Limitations must be included,
but Cost Effects should be evaluated before such Constraints are accepted
T. Gundersen Heat 9
Heat Integration − Data Extraction
Process, Energy and System
Necessary Data (1) • Process Streams
§ Flowrates m kg/s, tons/h § Specific Heat Capacity Cp kJ/kg°C § Supply Temperature Ts °C § Target Temperature Tt °C § Heat of Vaporization ΔHvap kJ/kg § Film Heat Transfer Coeffs. h kW/m2 °C
• Utility Systems § Temperature(s) and Specific Heat Content § Price per Energy Unit (Amount)
T. Gundersen Heat 10
Process, Energy and System
Heat Integration − Data Extraction
Necessary Data (2) • Cost Data for Heat Exchangers
§ Relation between Area and Purchase Price § Example: Cost = a + b * (A) C
• Economical Data § Number of Operating Hours per year § Specification on required Payback Time § Installation Factors § Maximum Investment (for Retrofit Projects) § Minimum allowed Approach Temperature
(ΔTmin), possibly based on Pre-optimization
T. Gundersen Heat 11
Process, Energy and System
Heat Integration − Data Extraction
Data Extraction for the Example Stream ID Ts Tt mCp ΔH h
°C °C kW/°C kW kW/m2°C
Reactor outlet H1 270 160 18 1980 0.5 Product H2 220 60 22 3520 0.5 Feed Stream C1 50 210 20 3200 0.5 Recycle C2 160 210 50 2500 0.5
Reboiler C3 220 220 2000 1.0 Condenser H3 130 130 2000 1.0
HP Steam HP 250 250 (≅ 40 bar) (var.) 2.5 MP Steam MP 200 200 (≅ 15 bar) (var.) 2.5 LP Steam LP 150 150 (≅ 5 bar) (var.) 2.5 Cooling Water CW 15 20 (var.) 1.0
T. Gundersen Heat 12
Process, Energy and System
Heat Integration − Data Extraction
Phase 2: Targeting • Basis:
§ Minimum Approach Temperature: ΔTmin • Results:
§ Minimum External Heating: QH,min § Minimum External Cooling: QC,min § Minimum Number of Units: Umin § Minimum Heat Transfer Area (total): Amin
• Pre−Optimization: Near optimal ΔTmin § QH,min , QC,min , Umin , Amin = f (ΔTmin) § TAC = f(QH,min , QC,min , Umin , Amin) = f (ΔTmin)
T. Gundersen Heat 13
Heat Integration − Targeting
Process, Energy and System
Composite Curves
Stream Ts Tt mCp ΔH H1 270 160 18 1980 H2 220 60 22 3520
H(kW)
270
220
160
60
2000 4000 6000
T(°C)
A
B C
D
270
220
160
60
2000 4000 6000
T(°C)
H(kW)
A
B + C
D
T. Gundersen Heat 14
Process, Energy and System
Heat Integration − Targeting
Composite Curves for the Example
2000 4000 6000 0
300
250
200
150
100
50
T (°C)
H (kW)
QH,min
QC,min
Pinch
QRecovery
ΔTmin
Note: The Column Reboiler / Condenser
are not included
T. Gundersen Heat 15
Process, Energy and System
Heat Integration − Targeting
Trade-off: Area vs. Energy 300
250
200
150
100
50
T (°C)
H (kW)
2000 4000 6000 0
QH,min
QC,min
+δ
+δ
T. Gundersen Heat 16
Process, Energy and System
Heat Integration − Targeting
Pinch Point and Decomposition QH,min + δ
QC,min + δ
δ
300
250
200
150
100
50
H(kW)
2000 4000 6000 0
T (°C)
Surplus
Deficit
T. Gundersen Heat 17
Process, Energy and System
Heat Integration − Targeting
Summary of Targeting by Graphical Methods
• Provides an Overview and Understanding • Identifies Key Information about the
Heat Recovery Problem, such as: § Minimum External Heating, QH,min § Minimum External Cooling, QC,min § The Process Pinch Point (Bottleneck)
• More Graphical Diagrams later • Time consuming and subject to Errors
T. Gundersen Heat 18
Process, Energy and System
Heat Integration − Targeting
Heat Cascade
Numerical Methods
Intervals
270ºC - - - - - - - 250ºC
230ºC - - - - - - - 210ºC
220ºC - - - - - - - 200ºC
180ºC - - - - - - - 160ºC
160ºC - - - - - - - 140ºC
70ºC - - - - - - - - 50ºC
H1
H2
CW
C1
C2
ST
720 kW
180 kW
720 kW
880 kW
440 kW
1980 kW
500 kW
200 kW
800 kW
1800 kW
+ 720
- 520
- 1200
2000 kW
400 kW
+ 180
+ 220
+ 400
60ºC - - - - - - - - 40ºC
360 kW
220 kW
ΔTmin = 20°C
T. Gundersen Heat 19
Process, Energy and System
Heat Integration − Targeting
Pinch Point changes with ΔTmin
T. Gundersen Heat 20
Process, Energy and System
Heat Integration − Targeting
0
50
100
150
200
250
300
0 2000 4000 6000 8000 10000
H (kW)
T (°C)
Pinch Point Candidates
T. Gundersen Heat 21
Process, Energy and System
Heat Integration − Targeting
0
50
100
150
200
250
300
0 2000 4000 6000 8000 10000
H (kW)
T (°C) Pinch Candidates ( ) are the Supply Temperatures of Streams or Stream ”Segments”
Total mCp is then increasing
T. Gundersen
Process, Energy and System
Heat 22
Motivating Example – Remember?
Heat Integration − Targeting
Area = 629 m2 Area = 533 m2
The Actual Flowsheet & Data Extraction
T. Gundersen
Process, Energy and System
Heat 23
Focus: Thermal Energy and the Energy Balance First: Checking the Mass Balance:
(1) 1.089 + 1.614 = 2.703 (OK)
(2) 6.931 + 2.703 = 9.634 (OK)
(3) 7.841 + 0.179 + 1.614 = 9.634 (OK)
Symbol: kg/s
Heat Integration − Targeting
1
3 2
T. Gundersen
Process, Energy and System
Heat 24
Simple Data Extraction
Ref.: Ian C. Kemp, ”Pinch Analysis and Process Integration: A User Guide on Process Integration for the Efficient Use of Energy”, 2nd. Ed., IChemE, Elsevier, 2007
40°C
1
2
3
4
Str. Ts Tt ΔH mCp
1 5 200 2233 11.451
2 40 200 413 2.581
3 35 126 992 10.901
4 200 35 2570 15.576
T in ºC, ΔH in kW and mCp in kW/ºC
Heat Integration − Targeting
”Average”
Composite Curves and Energy Targets
T. Gundersen Heat 25
Process, Energy and System
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000 3500 4000
Q (kW)
T (°
C)
PRO_PI1: ΔTmin = 10°C QH,min = 1068 kW and QC,min = 0 kW
Heat Integration − Targeting
Energy Requirements vs. ΔTmin
T. Gundersen Heat 26
Process, Energy and System
PRO_PI1: ΔTmin = 22°C QH,min = 1068 kW and QC,min = 0 kW ΔTmin = 23°C QH,min = 1083 kW and QC,min = 15 kW
ΔTthrsh = 22.1°C Threshold Process becomes Pinched
0 200 400 600 800
1000 1200 1400 1600 1800 2000
0 10 20 30 40 50 60 70 80
Global temperature difference (K)
Q (k
W)
QH,exist = 1722 kW
HRATexist = 64°C
Heat Integration − Targeting
Q: What about Phase changes & Cp=Cp(T)?
T. Gundersen
Process, Energy and System
Heat 27
Feed To Column
Reactor
ST
ST CW
Flash
I
H1
C
III
II
H2
40ºC
200ºC
190ºC 153ºC
200ºC
128ºC
5ºC
115.5ºC 114ºC 35ºC
141ºC Recycles
1
2
3
4
I II 1 4
II II IIII 1 2 4
III IIIIII 3 4
H1H1 1
H2 H2H2 1 2
CC 4
III4
"tot"4
??
992 kW
1652 kW
70 kW
654 kW992 39.68 kW/ C
153 1282570 15.576 kW/ C
200 35
Q H HQ H H H
Q H HQ H
Q H HQ H
mCp
mCp
= Δ = Δ =
= Δ +Δ = Δ
= Δ = Δ =
= Δ =
= Δ +Δ =
= Δ =
= = °−
= = °−
Q: Can we solve this Puzzle?
654 kW 1652 kW
70 kW
126ºC
Heat Integration − Targeting
T. Gundersen
Process, Energy and System
Heat 28
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000 3500 4000 Q (kW)
T (°
C)
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000 3500 4000 Q (kW)
T (°
C)
Simple
Detai led
Heat Integration − Targeting
Energy Requirements vs. ΔTmin
T. Gundersen Heat 29
Process, Energy and System
PRO_PI1: ΔTmin = 10°C QH,min = 1068 kW and QC,min = 0 kW ΔTmin = 11°C QH,min = 1072 kW and QC,min = 4 kW
ΔTthrsh = 10.5°C Threshold Process becomes Pinched
0
500
1000
1500
2000
2500
0 10 20 30 40 50 60 70 80
Global temperature difference (K)
Q (k
W)
QH,exist = 1722 kW
HRATexist = 42.4°C
Heat Integration − Targeting
Lessons learned from the Example
T. Gundersen Heat 30
Process, Energy and System
• Data Extraction is a Critical Activity § We have to solve a “Puzzle” § Plant Data are often missing or incorrect § Collecting and ”Processing” Data is Time
consuming (80% of Conceptual Studies) § Remember: ”Garbage in means Garbage out”
• Energy Targeting is much Simpler • Next: Heat Exchanger Network Synthesis
(Design & Optimization) is manageable
Heat Integration − Targeting
Fewest Number of Units
1000 ST
1980 H1
3520 H2
C1 3200
CW 800
C2 2500
1000 2200 1320
1180
800
N = Number of Streams and Utilities N = nH + nC + nutil = 2 + 2 + 2 = 6 Umin = (N-1)
L = Loops S = Subnetwork U = Units Euler’s Rule: U = N + L - S
Assume: L = 0, S = 1
T. Gundersen Heat 31
Process, Energy and System
Heat Integration − Targeting