(547d) Development of a Superstructure for Work and Heat Exchange Networks (WHENs)

Pinch Analysis is a well-known methodology in Process Systems Engineering for improving the energy efficiency of heat recovery processes. Up to recently, the focus has mainly been on heat integration, despite pressure-changing equipment often accounting for a significant portion of the total energy consumption in processes. Moreover, pressure manipulation of streams affects the heat sinks and sources in the process and thus the heat integration scheme. Consequently, the concept of Heat Integration can be expanded to Work and Heat Integration, in order to identify a further potential for enhanced energy recovery. An important principle in Pinch Analysis is the concept of Correct Integration (also referred to as Appropriate Placement). Enhanced heat recovery can be obtained by integrating various process units with the Heat Exchanger Networks (HENs). The correct integration of various equipment such as heat engines, heat pumps, reactors, evaporators and distillation columns is already well documented [1].

Compressors and expanders are two types of components that have not been extensively covered in Pinch Analysis. As these units involve both heat and work, the integration becomes rather more complex. Aspelund et al. [2] developed heuristic rules for the correct integration of pressure-changing equipment in subambient processes, where exergy instead of energy was used to combine the two modes of energy transfer (heat and work). The resulting heuristic rules became known as the Expanded Pinch Analysis and Design (ExPAnD) methodology. Later, Gundersen et al. [3] formulated the heuristics more precisely by saying that compression and expansion should start at the pinch. The heuristics have limited applicability however, and may result in suboptimal solutions if used without care. As a consequence, a series of theorems was developed for appropriate placement of compressors [4] and expanders [5] in above ambient processes and for compressors [7] and expanders [8] in sub-ambient processes. In addition to the theorems, the authors presented a graphical procedure for designing the Work and Heat Exchange Network. However, as temperatures as well as pressures must be taken into consideration when designing these networks, the manual procedure becomes tedious to solve even for small problems. Instead, mathematical optimization is required for studying large industrial sized problems.

Vikse et al. [8] discussed different superstructures for the optimization of WHENs. Of the models presented in that paper, only the superstructure by Uv [9] followed the rules of correct integration as determined by the theorems. On the other hand, the model required a sequential approach for which the sequence of integration may influence the solution. The model closely followed the manual procedure, and thus became quite involved for larger problems. Therefore, a new superstructure that takes into account the rules of correct integration and yet supports a simultaneous model approach must be developed. This work will develop a suitable superstructure for designing WHENs. The resulting models should be capable of solving larger and more complex problems. Furthermore, the superstructure should be versatile to easily adapt it for particular cases. The new superstructure will be compared with existing models, using several test problems of varying degree of complexity.

1. Smith, R. Chemical process: design and integration. 2^nd edition, John Wiley & Sons, 2016.
2. Aspelund, A., D. O. Berstad, and T. Gundersen. “An extended pinch analysis and design procedure utilizing pressure based exergy for subambient cooling." Applied Thermal Engineering 27(16) (2007), 2633-2649
3. Gundersen, T., D. O. Berstad, and A. Aspelund. "Extending pinch analysis and process integration into pressure and fluid phase considerations." Chemical Engineering Transactions 18 (2009), 33-38.
4. Fu, C. and T. Gundersen. "Integrating compressors into heat exchanger networks above ambient temperature." AIChE Journal 61(11) (2015), 3770-3785.
5. Fu, C., and T. Gundersen. "Integrating expanders into heat exchanger networks above ambient temperature." AIChE Journal 61(10) (2015), 3404-3422.
6. Fu, C., and T. Gundersen. "Sub-ambient heat exchanger network design including compressors." Chemical Engineering Science 137 (2015), 631-645.
7. Fu, C., and T. Gundersen. "Sub-ambient heat exchanger network design including expanders." Chemical Engineering Science 138 (2015), 712-729.
8. Vikse M., C. Fu, P. I. Barton, and T. Gundersen. “Applying Mathematical Optimization for Work and Heat Exchange Networks (WHENs).” Chemical Engineering Transactions. Submitted for PRES 2017.
9. Uv, P.M. “Optimal design of heat exchanger networks with pressure changes.” Master thesis, Norwegian University of Science and Technology (NTNU), Dep. of Industrial Economics and Technology Management, Trondheim, Norway, 2016.