CHE 448 Chemical Engineering Design

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CHE 448 Chemical Engineering Design
Spring 2006
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Question?

• Would you like to postpone the 2nd midterm to
  April 6?
• If so, April 4 you turn in second report and have
  a class by Mr. Garnett on Human Error

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Example of Heat Exchanger Network
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.
n
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How do we choose the temperature Intervals?
First, we must define Dtmin 10 F
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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
  trade-off 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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Minimum use of utilities estimate QF(Dmin).
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Finding the Heat Exchange NetworkApproach
temperature 10 F
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Subsystems on sides of pinch
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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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Finding the Heat Exchange NetworkApproach
temperature 10 F
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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 0.98 MBtu/h
Cold Util 1.08 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.