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Design and Transient Performance Analysis of Cryogenic Heat Exchangers from Safety Perspective Using Computational Fluid Dynamics

Fathi Aghbalagh Mostafa Khan, Mohsen | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57639 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Rashtchian, Davood; Sharifzadeh, Mehdi
  7. Abstract:
  8. In this study, the objective is to present an innovative framework for the safe and mechanically flexible design of cryogenic heat exchangers. To achieve this, multiscale modeling approaches were employed, leading to the development of a novel algorithm tailored for this purpose. The proposed algorithm was applied to a single mixed refrigerant natural gas liquefaction unit as a case study to evaluate its performance. The chosen heat exchanger is a multi-stream brazed aluminum plate-fin heat exchanger. Initially, a sensitivity analysis of the natural gas liquefaction system was performed at the macro-scale to identify and characterize the most influential process disturbances. Subsequently, using the quantitative safety index of process flow paths, a novel multi-objective cost function algorithm was introduced to optimize the system at the macro-scale, focusing on safety and economic objectives without considering operational flexibility. To enhance the proposed algorithm by incorporating mechanical flexibility conditions for heat exchangers, the lifetime cycle count function of the heat exchanger was utilized. For safety analysis under the worst-case scenario, the most sensitive heat exchanger was identified using the numerical safety index of process flow paths. Micro-scale simulations employing computational fluid dynamics (CFD) tools were then performed to calculate the lifetime cycle count. Fatigue analysis of the heat exchanger was conducted in COMSOL software using steady-state simulations and libraries for heat transfer in solids, fluid flow, and solid mechanics. The fatigue analysis tool was used to calculate the thermal lifetime cycle count under various process conditions and structural designs based on the von Mises equivalent stress. Further, a sensitivity analysis and the derivation of a general model for the lifetime cycle count were performed using the response surface methodology (RSM). Key parameters included natural gas stream pressure, refrigerant stream temperature, hot and cold stream temperature differences, brazing layer thickness, fin height, fin thickness, and fin spacing. The resulting lifetime cycle count was incorporated as an operational flexibility constraint into the innovative algorithm in the form of a side-loop. The results indicate that integrating mechanical constraints, such as the lifetime cycle count, acts as a safety enhancement factor when the economic objective function has higher priority (weighting factor above 0.5). Under these conditions, the applied constraints improve the safety index compared to the previous state. In other words, within this range, a specified cost recognized as the minimum cost under a given weighting factor for the multi-objective function allows for improved safety levels and lower inherent safety indices through the intelligent adjustment of operational conditions and mechanical constraints. Conversely, when safety objectives are prioritized, the required cost for a given safety index level decreases. However, the impact of the constraint in reducing costs is less pronounced compared to its role in improving safety at higher economic weightings. In the case study, the maximum reduction in the inherent safety index, considering a fixed cost, was 30%, while the minimum reduction was 14.49%
  9. Keywords:
  10. Heat Exchangers ; Computational Fluid Dynamics (CFD) ; Flexibility ; Safety ; Heat Exchanger Design ; Response Surface Methodology ; Low Temperature Converters

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