A1256655490GzYRT - PowerPoint PPT Presentation

Title: A1256655490GzYRT


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CHE 448 Chemical Engineering Design
Spring 2006
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Example of Heat Exchanger Network
Stream T source F T target F M Cp Btu/h F Q MBtu/h
C1 120 235 20,000 2.3
C2 180 240 40,000 2.4
H1 260 160 30,000 3.0
H2 250 130 15,000 1.8
No network 4 units, 4.8 MBtu/h of cold and 4.7
MBtu/h of hot utilities.
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Temperature interval methodLinnhoff Flower
(1978)
Temperature intervals are determined by source
and target temperatures of streams.
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How do we choose the temperature Intervals?
First, we must define Dtmin 10 F
Temperature Hot side Cold side
260/250 Source H1 None
250/240 Source H2 Target C2
245/235 None Target C1
190/180 None Source C2
160/150 Target H1 None
130/120 Target H2 Source C1
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Temperature interval Example
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Minimum utilities in reversible network DT0
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The pinch depends on the value of DTmin
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Eliminate pinch by adding hot utilities.
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Minimum energy requirements

• Minimum from energy balance 0.10 Mbtu/h
• Minimum for reversible network DT0, 0.15
  Mbtu/h
• Minimum for small approach temp DT 10 F -gt
  0.6 Mbtu/hr
• Minimum for large approach temp DT 20 F -gt
  1.125 Mbtu/h

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Conclusions from TI analysis

• The Choice of Dtmin results from an economic
  between capital costs of heat exchangers and cost
  of utilities.
• Any amount of heat introduced above the minimum
  balance will have to be removed by a cold
  utility.
• A pinch indicates the presence of two subsystems,
  one that lacks energy (above) and one that has
  too much energy (below).
• Any energy crossing the pinch, will have to be
  removed by a cold utility.

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Finding the Heat Exchange NetworkApproach
temperature 10 F
Stream T source F T target F M Cp Btu/h F Q MBtu/h
C1 180 235 20,000 1.1
C2 180 240 40,000 2.4
H1 260 190 30,000 2.1
H2 250 190 15,000 0.9
Stream T source F T target F M Cp Btu/h F Q MBtu/h
C1 120 180 20,000 1.2
H1 190 160 30,000 0.9
H2 190 130 15,000 0.9
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Subsystems on sides of pinch
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Finding the Heat Exchange NetworkApproach
temperature 10 F
Stream T source F T target F M Cp Btu/h F Q MBtu/h
C1 180 235 20,000 1.1
C2 180 240 40,000 2.4
H1 260 190 30,000 2.1
H2 250 190 15,000 0.9
Stream T source F T target F M Cp Btu/h F Q MBtu/h
C1 120 180 20,000 1.2
H1 190 160 30,000 0.9
H2 190 130 15,000 0.9
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Hueristics for HEN design

• Exchange the hottest hot stream with the cold
  stream that has the largest target temperature.
  Exchange the coolest cold stream with the hot
  stream that has the lowest target temperature.

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Heat Load Feasibility

• If a stream or service is matched only once, its
  partner must have an equal or larger heat load.
  Thus, the stream with the largest heat load must
  have at least two matches.
• The stream with the second largest heat load must
  have at least two matches unless it is matched
  against the largest stream.

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Target Temperature

• Each match bringing a stream to its target
  temperature must be a stream or service whose
  supply temperature is compatible with that target
  temperature.
• Each stream or service must be used in at least
  one match.

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Use heuristics to find stream matches
Alternative I
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Alternative I 7 units
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Alternative I can we reduce the number of units?
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Alternative II Smaller number of units
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Comparison of alternatives with smaller number of
units
Number of units 6
Hot Util 1.08 MBtu/h
Cold Util 1.18 MBtu/h
Number of units 6
Hot Util 0.575 MBtu/h
Cold Util 0.675 MBtu/h
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Minimum number of unitsBipartite graph
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Minimum number of units for pinch subsystems
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Use of bipartite graph to reduce number of units
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Smaller number of units with heat transfer across
the pinch
Second Law constraints prevents from having the
minimum number of units
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Other issues
Target temperatures of cold stream1 and hot
stream 1 cannot be controlled in a simple way.
How do we provide energy for start ups?
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Summary of pinch method for designing HENs

• List all streams including utilities and define
  the value of Dtmin
• Use the temperature interval method to uncover
  the presence of a pinch.
• If there is a pinch, separate two sub-networks
  and balance them.
• Determine minimum number of units above and below
  the pinch.
• Determine feasible network using heuristics.
• Reduce number of units by small evolutionary
  changes to original netwok.

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Networks with phase changes
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Stream splitting

• Stream splitting must be used when
• The number of hot streams on the cold side of the
  pinch is smaller than the number of cold streams.
• The number of cold streams on the hot side of the
  pinch is smaller than the number of hot streams
• Stream splitting helps to reduce the number of
  exchangers without increasing use of utilities.

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Example stream splitting
Stream Ts C Tt C mCp kW/C Q kW
H1 200 100 5 500
H2 150 100 4 200
C1 90 190 10 1000
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Heat Integration of a Reactor System
Stream Ts F Tt F mCp Btu/s F Duty Btu/s Cost/Btu
C1 100 580 1 -480 0
C2 100 580 2 -960 0
H1 600 200 3 1200 0
Steam 650 650 2/106
Hot Wat 250 gt 130 1.5/106
Cooling W 80 lt 125 1/106
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Temperature IntervalReversible (Ideal) System
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Temperature interval I
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Temperature Interval II
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Cost of heat exchanger network system
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Heat integrated system I
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Heat Integrated System II
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Cost of system without heat integration
Unit Str 1 Str 2 Duty Btu/s A(ft2) Cost utility Cost unit
HU-1 1 Steam 480 825 28300 62000
HU-2 2 Steam 960 1650 56700 82000
CU-1 3 CW 1200 4650 35500 145000
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Cost of Integrated System
Unit Stream 1 Stream 2 Duty Btu/s Area ft2 Cost utility Chex
HU-1 C1 Steam 80 138 4700 45000
HU-2 C2 Steam 160 276 9400 48000
PU-1 C1 H1/3 400 1550 0 80000
PU-2 C2 2H1/3 800 3100 0 114000