(158b) Design of Air Separation Units for Advanced Combustion Via Equation Based Optimization

The quest to develop low carbon emission fossil fuel power systems offers substantial opportunities for systems engineers. Recent advances in optimization algorithms and problem formulations offer a methodology for systematic innovation – through identification of new flowsheets, intensification of existing processes and comparison of promising technologies. Coal oxycombustion is one type of power system with numerous application opportunities for these process systems engineering methods. In this power system coal is combusted in a nitrogen lean environment, resulting in three main products: carbon dioxide, water and pollutants (SO_x, NO_x, PM, Hg, etc). After the pollutants and water are removed, the carbon dioxide is compressed for utilization and/or sequestration. Compared to other low carbon emission coal power systems, oxycombustion shifts the main separation effort to the front of the flowsheet as an O₂ – N₂ separation. This avoids the expensive CO₂ – N₂separation for large volumes of flue gas.

Coal oxycombustion on a large scale is still a developing technology, and heuristic methods tailored to design of high performance oxycombustion plants do not exist. Furthermore the oxyfuel combustion flowsheet is highly integrated; changing one part of the flowsheet can dramatically impact the performance of other units. This makes design and optimization especially challenging. However this also means the oxycombustion design problem an excellent test-bed to develop new flowsheet optimization technologies.

In this paper we present development of an equation based framework for flowsheet optimization. The framework is designed with highly coupled systems in mind, such as the oxycombustion flowsheet, where optimization with surrogate models may be insufficient. Successful optimization requires coupling detailed process models, including extremely nonlinear thermodynamics, with state-of-the-art optimization algorithms. This framework has several notable features, all tailored to large scale optimization:

• Detailed cubic equation of state thermodynamics models with disappearing (or reappearing) phases
• Approximate distillation models using only continuous variables
• An automated, systematic initialization procedure
• Embedded heat integration based on pinch formulations, evolving with the process optimization
• Exact first and second derivatives

Optimal design of a cryogenic air separation unit for the coal oxycombustion flowsheet is considered to demonstrate the framework. This application is particularly interesting due to the recycle streams and tight heat integration (such as multiple pinch points). Plans to extend the framework to consider other oxycombustion systems will also be discussed.

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