(627b) Design of Cryogenic Systems of Advanced Power Plants Using Simultaneous Heat Integration and Process Optimization

Authors: 
Dowling, A. W., Carnegie Mellon University
Biegler, L. T., Carnegie Mellon University

Heat integration is a very useful tool for reducing of chemical process systems and is essential to design cost-effective cryogenic separation systems. Unfortunately, most heat integration technologies are designed for application after process optimization, and assume stream temperatures and flowrates are fixed. These methods focus on synthesis of the optimal heat integration network and do not consider the interaction between adjusting process operating conditions (temperatures, pressures, flowrates, compositions) and heat integration. See Furman & Sahinidis (2002) for an extensive review of these methods.

In contrast to these approaches, Duran and Grossmann (1986) developed an optimization friendly formulation for calculating minimum utility requirements based on pinch methods. Their approach allows for the utility target portion of the heat integration equations to be embedded into a flowsheet optimization problem. Kamath et al (2012) extended the approach to consider phase changes and applied it to optimize multistream heat exchangers for natural gas liquefaction.

In this paper, we extend the work of Duran, Grossmann and Kamath into a framework for simultaneous heat integration and flowsheet optimization. Emphasis is placed on using equation-based models with accurate second derivatives and efficient, large-scale nonlinear programming (optimization) algorithms. The following key details will be highlighted:
- Reformulation of the Duran-Grossmann model to reduce the number of large elements in the Hessian (2nd derivative) matrix
- Decomposition of heat exchangers to accommodate phase changes and satisfy the constant heat capacity assumption
- An optimization friendly formulation of approximate area targeting formulas

These methods are demonstrated in two case studies: design of an air separation unit and design of a CO2 processing unit and compression train, both for advanced coal oxycombustion power plants. These cryogenic systems are excellent demonstrations of the importance of simultaneous heat integration and process optimization. For example, adjusting the operating pressures of distillation column in the air separation unit can significantly impact the design of the accompanying multistream heat exchanger.

References:
Duran, M. A., & Grossmann, I. E. (1986). Simultaneous optimization and heat integration of chemical processes. AIChE Journal, 32(1), 123–138.

Furman, K. C., & Sahinidis, N. V. (2002). A Critical Review and Annotated Bibliography for Heat Exchanger Network Synthesis in the 20th Century. Industrial & Engineering Chemistry Research, 41, 2335–2370.

Kamath, R. S., Biegler, L. T., & Grossmann, I. E. (2012). Modeling Multistream Heat Exchangers with and without Phase Changes for Simultaneous Optimization and Heat Integration. AIChE Journal, 58(1).

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