(656b) Building Block-Based Work and Heat Exchanger Network Synthesis (WHENS)

Energy conservation has been a long-standing grand challenge in chemical industry. Efforts in heat exchanger network synthesis facilitate saving in energy consumption and total annual cost [1-2]. Recently, work and heat exchanger network synthesis (WHENS) has emerged as a promising research area to capture the interplay of pressure and temperature manipulation operations [3-5]. Current optimization-based approach for WHENS leverages on superstructure representations such as state-space representation [6], multi-stage superstructure [7] and representations involving heuristics [8]. However, there exist several limitations for these superstructure representations addressing WHENS. Firstly, equipment configurations need to be postulated based on existing knowledge of unit operations, engineering experience and heuristics. Secondly, classic superstructure representations fail to discover novel pathways with process intensification. Process intensification refers to significant reduction of equipment sizes, waste generation, and increase of productivity [9,10]. In the context of WHENS, multi-stream heat exchanger is a relevant example that reduce the number of equipment while improving the overall performance of WHENS.

To this end, we provide a new representation approach to capture many process configurations of WHENS with innovative pathways [11]. The new representation approach centers around two fundamental elements of abstract building blocks. Firstly, the block interior is used to represent splitting, mixing, utility cooling and utility heating of individual streams. Secondly, the block boundaries between adjacent blocks allow energy flow in the form of heat and work. An unrestricted boundary does not allow pressure and composition changes between adjacent blocks. A semi-restricted boundary is assigned with expansion/compression operations connected through either common (integrated) or dedicated (utility) shafts. A completely restricted boundary allows heat flow driven by temperature gradient across the boundary. We demonstrate the rich process information embedded in the proposed representation through literature examples. The novelty of this approach is that there is no need to specify the stage number as requested by the classic approach. Besides, the proposed representation approach allows automatic generation of numerous work and heat exchanger network with intensification opportunities. We formulate this building-block based WHENS as a mixed-integer nonlinear optimization (MINLP) problem with the objective as minimizing total annual cost. We use several literature examples on liquified energy chain to demonstrate the capability of the proposed synthesis approach.

References:

[1] Furman, K.C.; Sahinidis, N.V. A critical review and annotated bibliography for heat exchanger network synthesis in the 20th century. Industrial & Engineering Chemistry Research. 2002, 41, 2335–2370.

[2] Nair, S. K., & Karimi, I. A. Unified Heat Exchanger Network Synthesis via a Stageless Superstructure. Industrial & Engineering Chemistry Research. 2019. DOI: 10.1021/acs.iecr.8b04490.

[3] Yu, H.; Fu, C.; Vikse, M.; Gundersen, T. Work and heat integration—A new field in process synthesis and PSE. AIChE Journal. 2018.

[4] Fu, C.; Vikse, M.; Gundersen, T. Work and heat integration: An emerging research area. Energy. 2018, 158, 796–806.

[5] Huang, K.; Karimi, I. Work-heat exchanger network synthesis (WHENS). Energy. 2016, 113, 1006–1017.

[6] Wechsung, A.; Aspelund, A.; Gundersen, T.; Barton, P.I. Synthesis of heat exchanger networks at subambient conditions with compression and expansion of process streams. AIChE J. 2011, 57, 2090–2108.

[7] Nair, S.K.; Nagesh Rao, H.; Karimi, I.A. Framework for work-heat exchange network synthesis (WHENS). AIChE J. 2018, 61, 871–876.

[8] Uv, P.M. Optimal Design of Heat Exchanger Networks with Pressure Changes. Master’s Thesis, NTNU, Trondheim, Norway, 2016.

[9] Tian, Y.; Demirel, S.E.; Hasan, M.M.F.; Pistikopoulos, E.N. An Overview of Process Systems Engineering Approaches for Process Intensification: State of the Art. Chem. Eng. Process.-Process Intensif. 2018, 133, 160–210.

[10] S.E. Demirel; J. Li; M.M.F. Hasan. Systematic Process Intensification. Current Opinion in Chemical Engineering, 2019, https://doi.org/10.1016/j.coche.2018.12.001.

[11] J. Li; S.E. Demirel; M.M.F. Hasan. Building Block-Based Synthesis and Intensification of Work-Heat Exchanger Networks (WHENS). 2018. Processes, 7(1), 23.