Module 5: Process Integration of Heat and Mass Chapter 10

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Module 5 Process Integration of Heat and
MassChapter 10

• David R. Shonnard
• Department of Chemical Engineering
• Michigan Technological University

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Module 5 Outline
The environmental performance of a process
depends on both the performance of the individual
unit operations, but also on the level to which
the process steams have been networked and
integrated

• Educational goals and topics covered in the
  module
• Potential uses of the module in chemical
  engineering courses
• Review of heat integration concepts
• Introduction to the tools of mass integration and
  synthesis of mass exchange networks - Chapter 10
• Cast study - heat integration of the MA flowsheet
  

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Module 5 Educational goals and topics covered
in the module

• Students will
• learn about efficient utilization of waste
  streams as raw materials through application of
  source/sink mapping
• are introduced to graphical tools of mass
  exchange network synthesis, composition interval
  diagrams and load line diagrams.
• apply mass exchange network synthesis to simple
  flowsheets

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Module 5 Potential uses of the module in
chemical engineering courses

• Mass/energy balance course
• dilute contaminant balance calculations around
  process units
• source/sink matching of energy streams
• Continuous/stagewise separations course
• applications to in-process recovery and
  recycle of contaminants
• Design course
• graphical design tools for mass integration of
  waste streams

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Module 5 Analogies between process heat and
mass integration

• Heat Integration
• the optimum use of heat exchangers and streams
  internal to the process to satisfy heating and
  cooling requirements.
• Tools 1. Temperature interval diagram
• 2. Heat load diagram (pinch diagram)
• Mass Integration
• the optimum use of mass exchangers and streams
  internal to the process to satisfy raw material
  requirements, maximize production and minimize
  waste generation (water recycle/reuse
  applications).
• Tools 1. Source/sink mapping and diagrams
• 2. Composition interval diagram
• 3. Mass load diagram (pinch diagram)

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Module 5 Heat exchange networks -key features
T - Heat Load Diagram composite curves
pinch analysis minimum external utilities
Heat exchange network internal external
(mCp)1 (mCp) 2-1
89 reduction in external utilities
Seider, Seader, and Lewin, 1999, Process Design
Principles, John Wiley Sons, Ch. 7
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Module 5 Heat exchange networks -Illustrative
example - before heat integration
per sec
1 kg/s, Cp 1 kJ/(kg-C)
2 kg/s, Cp 1 kJ/(kg-C)
per sec
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Module 5 Heat exchange networks -Temperature -
load (pinch) diagram
per sec
Placement of each load line vertically is
arbitrary
2 kg/s
Cooling load for external network, 160 kJ/s
Heat transfer load by internal network, 140 kJ/s
1 kg/s
Heating load for external network, 30 kJ/s
10 C minimum temperature difference defines the
pinch
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Module 5 Heat exchange networks -Illustrative
example after heat integration
82.4 reduction in cooling utility
per sec
140 kJ/s transferred
46.7 reduction in heating utility
per sec
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Module 5 Mass integration objectives and
methods
Methods
objective is to prepare source streams to be
acceptable to sink units within the process or to
waste treatment

• 1. Segregation
• avoid mixing of sources
• 2. Recycle
• direct sources to sinks
• 3. Interception
• selectively remove pollutants from source
• 4. Sink/generator manipulation
• adjust unit operation design or operation

Pollutant-rich streams
Pollutant-lean streams
El-Halwagi, M.M.1997, Pollution Prevention
Through Process Integration Systematic Design
Tools, Academic Press
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Module 5 Motivating example Chloroethane
process before mass integration
Mass balance in terms of CE, the minor component
Objective is to reduce the concentration of CE
sent to biotreatment to lt 7 ppm and a load of lt
1.05x10-6 kg CE/s
El-Halwagi, M.M.1997, Pollution Prevention
Through Process Integration Systematic Design
Tools, Academic Press
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Module 5 Motivating example Chloroethane
process after mass integration
Interception
CE load to biotreatment 2.5x10-7 kg/s
Recycle
El-Halwagi, M.M.1997, Pollution Prevention
Through Process Integration Systematic Design
Tools, Academic Press
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Module 5 Mass Integration ToolsSource-sink
mapping
the purpose of source-sink mapping is to
determine if waste streams can be used as
feedstocks within the process - direct recycle
Recycle source a directly
A range of acceptable flowrates and composition
for each sink , S
or mix sources b and c to achieve the target
flowrate - composition using a Lever Rule - like
calculation
El-Halwagi, M.M.1997, Pollution Prevention
Through Process Integration Systematic Design
Tools, Academic Press
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Module 5 Source-sink mapping acrilonitrile
(AN) process before recycle
0 ppm NH3 0 ppm AN required
10 ppm NH3 may contain AN
450 C, 2 atm
2-phase stream always with 1 kg/s H2O but no H2O
in the AN layer
mass fraction of AN always equal to 0.068
NH3 equilibrium CW 4.3 CAN
NH3 partitioning CSTEAM 34 CPRODICT
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Module 5 Source-sink map acrilonitrile (AN)
process
Sinks for water
Sources for water
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Module 5 Flow rates of condenser and fresh water
sent to Scrubber
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Module 5 Mass balances on AN units for remaining
flow rates and compositions
Aqueous streams from condenser and distillation
column 4.7 kg/s H2O 0.5 kg/s AN 12 ppm NH3
From fresh water supply 1.0 kg/s H2O 0 kg/s AN 0
ppm NH3
Scrubber
Gas stream from condenser 0.5 kg/s H2O 4.6 kg/s
AN 39 ppm NH3
to decanter ? kg/s H2O ? kg/s AN ? ppm NH3
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Module 5 Flow rates and compositions from
Scrubber to Decanter
And similarly for other units
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acrilonitrile (AN) process after recycle
freshwater feed 30 of original
AN production rate increased by 0.5 kg/s .6/kg
AN and 350 d/yr 9MM/yr
rate of AN sent to biotreatment is 85 of original
60 of original
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Module 5 Mass exchange network (MEN) synthesis

• 1. Similar to heat exchange network (HEN)
  synthesis
• 2. Purpose is to transfer pollutants that are
  valuable from waste streams to process streams
  using mass transfer operations (extraction,
  membrane modules, adsorption, ..
• 3. Use of internal mass separating agents (MSAs)
  and external MSAs.
• 4. Constraints
• i. Positive mass transfer driving force between
  rich and lean process
• streams established by equilibrium
  thermodynamics
• ii. Rate of mass transfer by rich streams must
  be equal to the rate of
• mass acceptance by lean streams
• iii. Given defined flow rates and compositions
  of rich and lean streams

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Module 5 Mass integration motivating example -
Phenol-containing wastewater
El-Halwagi, M.M.1997, Pollution Prevention
Through Process Integration Systematic Design
Tools, Academic Press
Outlet streams for recycle or sale
Mass separating agents
to waste water treatment
- Minimize transfer to waste treatment -
to wastewater treatment
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Module 5 Outline of MEN synthesis

• 1. Construct a composition interval diagram (CID)
• 2. Calculate mass transfer loads in each
  composition interval
• 3. Create a composite load line for rich and lean
  streams
• 4. Combine load lines on a combined load line
  graph
• 5. Stream matching of rich and lean streams in a
  MEN using the CID

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Module 5 Hypothetical set of rich and lean
streams - stream properties
Equilibrium of pollutant between rich and lean
streams y 0.67 x
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Module 5 Composition interval diagram - a tool
for MEN synthesis
x scale matched to y scale using y 0.67 x
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Module 5 Mass transfer loads in each interval
Rich Streams
negative mass load denotes transfer out of the
stream
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Module 5 Composite load line for the rich stream
Region 1 2
Region 3
Region 4
Region 5
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Module 5 Combined load line for rich and lean
streams
mass load to be added to lean stream externally
mass load to be transferred internally
Rich Stream can be moved vertically
mass load to be removed from rich stream by
external MSA
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Module 5 Stream matching in MEN synthesis
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Module 5 Heat integration of the MA flowsheet
-9.23x107 Btu/hr
2.40x107 Btu/hr
9.70x107 Btu/hr
Reactor streams generate steam
-4.08x107 Btu/hr
Without Heat Integration
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Module 5 Heat integration of reactor feed and
product streams
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Module 5 Heat integration of absorber outlet
and recycle streams
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Module 5 Maleic anhydride flowsheet with heat
integration
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Module 5 Heat integration summary
Greater energy reductions are possible when steam
generated from the reactors is used for the
reboiler, purge and feed heaters
76.8 reduction
27.4 reduction
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Module 5 Recap

• Educational goals and topics covered in the
  module
• Potential uses of the module in chemical
  engineering courses
• Review of heat integration concepts
• Introduction to the tools of mass integration and
  synthesis of mass exchange networks - Chapter 10
• Cast study - heat integration of the MA flowsheet