Coal Oxycombustion Flowsheet Optimization
Alexander W. Dowling Lorenz T. Biegler
Carnegie Mellon University
David C. Miller, NETL
March 10th, 2013 1
Process Systems Engineering
Oxycombustion Flowsheet
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1. Air Separation Unit 2. Boiler 3. Steam Turbine
4. Pollution Controls 5. CO2 Compression Train
1 2
3
4
5
Cryogenic Air Separation
Boiling Points @ 1 atm Oxygen: -183 °C Argon: -185.7 °C Nitrogen: -195.8 °C
Multicomponent distillation with tight heat
integration
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Photo from wikipedia.org
Low pressure section
High pressure section
High Pressure Column
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Column Cascade
Total Condenser
Partial Reboiler
Flash
Valve
Splitter
Low Pressure Column
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Column Cascade
Total Condenser
Partial Reboiler
Flash
Valve
Splitter
Cubic Equations of State
Phase CEOS 1st Derivative 2nd Derivative
Liquid
Vapor
Up to 3 Roots?!?
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Phase Disappearance Thermodynamics must be relaxed when a phase
disappears.
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Cascade Model • Group method
– Continuous number of ideal stages – Based on work of Kremser and Edminster – Requires specification of stripping and
absorbing section
• Modifications for distillation: – Exit streams at dew/bubble point – Decrease vapor at bottom = increase liquid at top
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Kamath, Grossmann & Biegler (2010). Aggregate models based on improved group methods for simulation and optimization of distillation systems. Computers & Chemical Engineering.
ASU: Optimization Formulation
Note: Upper and lower bounds not shown above where considered for many variables including stream/equipment temperatures and pressures. 9
Results Summary for 90 mol% O2 • Energy usage: 0.187 kWh/kg O2
typical designs: ~0.22 kWh/kg O2
• Low pressure column (LPC): 21 stages High pressure column (HPC): 56 stages
• LPC: 1.13 bar literature: 1.7 – 4.3 atm HPC: 3.59 bar literature: 6.8 – 13.5 atm
• Only 25% of feed air sent to HPC typical designs: 100%
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Composite Curve
Need Cooling
Need Heating
Hot Curve
Cold Curve
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Future ASU Work • Consider additional heat exchangers to
improve constant heat capacity approximation
• Explore alternate ASU configurations
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Conclusions Optimized ASU with equation-based model:
– Cubic equation of state with accurate derivatives – Pinch location heat integration – Pure nonlinear program – no discrete variables
Acknowledgements: National Energy Technology Laboratory Yannic Vaupel, RWTH Aachen University
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"This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof."
Coal Oxycombustion Flowsheet Optimization
Alexander W. Dowling Lorenz T. Biegler
Carnegie Mellon University
David C. Miller, NETL
October 30th, 2012 14
Process Systems Engineering
Project Objective Develop an equation oriented framework to
optimize an entire coal oxycombustion flowsheet.
15
Closed
Open
Variables/Constraints 102 104 106
Black Box
Flowsheet with Modular Models
Superstructure with Surrogates
Equation Based Formulation
100
Compute Efficiency SQP
rSQP
Barrier/Interior Point
DFO
Project Summary Objective: Develop an equation oriented
framework to optimize an entire coal oxycombustion flowsheet.
• Characterize potential of coal oxycombustion technology for carbon capture
• Explore complex design trade-offs in highly coupled flowsheet
• Demonstrate equation oriented methods on an industrial size flowsheet
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Outline 1. Cryogenic Air Separation Model
2. Thermodynamic Model and Phase Detection
3. Heat Integration
4. Optimization Results
5. Future Work and Conclusions 17
General Equipment Models
“Thermodynamic Equipment”
Types of Equipment • Flash Vessels • Heat Exchangers • Partial Reboilers • Throttle Valves • Compressor Stages
Common Structure
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General Equipment Models
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General Equipment Models
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Additional Equipment Equations
Constant Pressure
No Heat Removed
Adiabatic
Pressure Drop Only
Optional Entropy Inequality
Pressure Drop Only
Adiabatic
Constant Pressure
Cooling or Heating
Specification
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Thermodynamic Modules “Simple”
• Antoine equation • Raoult’s law • Phase detection from
bubble/dew points • Ideal gas polynomials • Watson correlation • Initialization only
Cubic EOS • Valid for liquid and
vapor phases • Departure functions • Idea gas polynomials • Optional binary
interactions • Primary model
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Cascade Model
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V1
VN+1
L0
LN
L1 VN
Cascade Model
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V1
VN+1
L0
LN
L1 VN
Pinch Location Heat Integration • No utility heating or
cooling
• Constant heat capacity assumed for each unit
• Require 2 phase flow for select intermediate streams
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Need Cooling
Need Heating
ASU: Heat Integration Model
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Based on work of Duran & Grossmann (1986) Pinch candidates Available heating and cooling above pinch Utility calculations
Hot Curve
Cold Curve
Pinch Point
ASU: Heat Integration Model Modifications: 1. Reboilers and condensers considered for heat
integration 2. Smoothed max and α used for phase changes
Heated HX
Cooled HX
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Optimization Sequence
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“Simple Thermo” Group Method ASU
CEOS Group Method ASU (continuous number of theoretical stages)
CEOS Group Method ASU (rounded & fixed number of theoretical stages)
CEOS Tray-by-Tray ASU
Pareto Curve
Generated using epsilon-constraint method
Nonconvex shape – warrants further investigation
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Future Flowsheet Work • Integrate ASU model with remaining flowsheet
sections
• Explore potential heat integration synergies – ASU with post combustion cryogenics – ASU with compression train – Compression train with boiler
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