(356f) A Framework for Dynamic Modeling and Optimization of Multi-Stream Plate-Fin Heat Exchangers

Modern cryogenic air separation relies on tightly integrated process designs to achieve a high degree of energy efficiency. The use of a narrowly pinched feed-effluent heat exchanger to cool and liquefy the air feed stream(s) by recovering refrigeration from the cold product streams represents a salient design feature, yielding significant efficiency gains. While such heat exchangers lower operating costs, they also contribute to a capital cost increase; their design therefore involves tradeoffs aimed at achieving economic optimality for the plant.

The most frequently encountered feed-effluent heat exchangers in air separation plants are of the multi-stream plate-fin type. The complex structure of Plate Fin Heat Exchangers (PFHEs), comprising of a stack of finned fluid flow channels (layers) separated by parting sheets, offers a large number of design degrees of freedom, making design optimization subject to operating and manufacturing constraints a daunting, if not impossible task to carry out manually. Several studies [1-6] have focused on solving the PFHE design optimization problem. However, their scope has been largely limited to developing and testing optimization algorithms tailored to simplified prototypes or specific applications. To our knowledge, this problem has not yet been addressed and solved in a generic manner.

In this paper, we present a general dynamic modeling and optimization framework for PFHEs recently developed by Praxair, Inc. and Process Systems Enterprise, Ltd. We introduce a novel model representation that can capture heat exchanger structures of arbitrary complexity and accommodate multi-phase and supercritical process streams, and discuss its implementation as a model library in gPROMS, PSE's system modeling environment. In our work, we exploit the dynamic component of the models in formulating a mixed-integer PFHE design optimization problem aimed at minimizing the lifetime cost of the exchanger, subject to operating and manufacturing constraints. Furthermore, we discuss the extension of this formulation to ensure optimality over a wide range of plant operating parameters. Finally, we present and analyze a case study, illustrating the robust dynamic simulation capabilities of the model library and demonstrating the significant economic benefits obtained using the proposed optimization framework.

References

1. Reneaume, J.-M., Pingaud, H., and Niclout, N., ?Optimization of Plate Fin Heat Exchangers.? Trans IchemE, Vol. 78, Part A, September, 2000. pp. 849-859.

2. Reneaume, J.-M., and Niclout, N., ?Optimal Design of Plate-Fin Heat Exchangers using Both Heuristic Based procedures and Mathematical Programming Techniques.? Third International Conference on Compact Heat Exchangers and Enhancement Technology for the Process Industries, Davos, Switzerland, July, 2001. pp. 143-149.

3. Sunder, S., and Fox, V.G., ?Multivariable Optimization of Plate Fin Heat Exchangers.? AIChE Symposium SeriesVol. 89, No. 295, Atlanta, GA, 1993. pp. 244-252.

4. Reneaume, J.-M., and Niclout, N., ?MINLP Optimization of Plate Fin Heat Exchangers.? Chem. Biochem. Eng. Q., Volume 17, No.1, 2003. pp. 65-76.

5. Picün-Nunez M., Polley G.T., and Medina-Flores M., ?Thermal Design of Multi-Stream Heat Exchangers?, Applied Thermal Engineering 22 pp 1643-1660 (2002).

6. Wang L., and Sunden B., ?Design Methodology for Multi-stream Plate-Fin Heat Exchangers in heat Exchanger Networks?, Heat Transfer Engineering, 22 pp 3-11, (2001).