Process Systems Engineering Coal Oxycombustion Flowsheet Optimization Alexander W. Dowling Lorenz T. Biegler Carnegie Mellon University David C. Miller, NETL March 10 th, 2013 1
Oxycombustion Flowsheet 1 2 4 5 3 1. Air Separation Unit 2. Boiler 3. Steam Turbine 4. Pollution Controls 5. CO 2 Compression Train 2
Cryogenic Air Separation Boiling Points @ 1 atm Oxygen: -183 C Argon: -185.7 C Nitrogen: -195.8 C Low pressure section Multicomponent distillation with tight heat integration Photo from wikipedia.org High pressure section 3
High Pressure Column Column Cascade Total Condenser Partial Reboiler Flash Valve Splitter 4
Low Pressure Column Column Cascade Total Condenser Partial Reboiler Flash Valve Splitter 5
Cubic Equations of State Up to 3 Roots?!? Phase CEOS 1 st Derivative 2 nd Derivative Liquid Vapor 6
Phase Disappearance Thermodynamics must be relaxed when a phase disappears. 7
Group method Cascade Model 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 Kamath, Grossmann & Biegler (2010). Aggregate models based on improved group methods for simulation and optimization of distillation systems. Computers & 8 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% O 2 Energy usage: 0.187 kwh/kg O 2 typical designs: ~0.22 kwh/kg O 2 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% 10
Composite Curve Hot Curve Cold Curve Need Heating Need Cooling 11
Future ASU Work Consider additional heat exchangers to improve constant heat capacity approximation Explore alternate ASU configurations 12
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 "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 13 thereof."
Process Systems Engineering Coal Oxycombustion Flowsheet Optimization Alexander W. Dowling Lorenz T. Biegler Carnegie Mellon University David C. Miller, NETL October 30 th, 2012 14
Project Objective Develop an equation oriented framework to optimize an entire coal oxycombustion flowsheet. Open Equation Based Formulation Barrier/Interior Point Compute Efficiency Superstructure with Surrogates Flowsheet with Modular Models DFO Black Box SQP rsqp Closed 10 0 10 2 10 4 10 6 Variables/Constraints 15
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 16
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 18
General Equipment Models 19
General Equipment Models 20
Additional Equipment Equations Constant Pressure No Heat Removed Pressure Drop Only Adiabatic Pressure Drop Only Adiabatic Constant Pressure Cooling or Heating Specification Optional Entropy Inequality 21
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 22
Cascade Model L 0 V 1 L 1 V N L N V N+1 23
Cascade Model L 0 V 1 L 1 V N L N V N+1 24
Pinch Location Heat Integration No utility heating or cooling Constant heat capacity assumed for each unit Require 2 phase flow for select intermediate streams Need Cooling Need Heating 25
ASU: Heat Integration Model Pinch candidates Based on work of Duran & Grossmann (1986) Available heating and cooling above pinch Utility calculations Pinch Point Hot Curve Cold Curve 26
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 27
Optimization Sequence 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 28
Pareto Curve Generated using epsilonconstraint method Nonconvex shape warrants further investigation 29
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 30