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.

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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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