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Flexible Design of LNG Multi-Stream Heat Exchanger (Coldbox) with the Focus on Exergy Destruction and Part-load Operation

Fathi, Mohammad Sadegh | 2020

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53451 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Saeedi, Mohammad Hassan; Hosseini, Vahid
  7. Abstract:
  8. Natural gas liquefaction is a gas refrigeration technology, which enables the export of natural gas worldwide, in liquids formed and stored at very low temperatures. In this project, a plate-fin type of heat exchanger, which is used for liquefaction of natural gas (mainly methane) at very low temperatures, has been investigated. The most important and effective part of this gas refrigeration cycle is the cold box heat exchanger. The cold box consists of two or more floors in which some sorts of fins are installed in each floor. In this simulation, the three-sream heat exchanger is designed in such a way that the input temperature of the natural gas is about 200 K and the output is 145 K at a pressure of about 1 MPa in the channel is cooled to liquid saturation (hot flow) and the other two streams are nitrogen gas in the gaseous state with the inlet temperature of 85 K (cold flow), which circulates in one refrigration cycle, but it is separated in two different inlet stream in the heat exchanger. Then, by simulating the above mentioned three stream heat exchanger with periodic boundary conditions, four fins in each floor are considered and simulated in the Fluent software and the temperature, the pressure countors and the variations of the temperature and pressure along the heat exchanger are obtained. Then, by conducting sensivity analysis and varying the variables of inlet temperature and mass flow of natural gas and geometric variables of height and thickness of the fins, the amount of exergy destruction in the whole system and the length of heat transfer in order to liquiefy natural gas are obtained. By illustrating the variations and relations between the results and the input variables, suitable curves are fitted and the mathematical functions of relations are derived. Finally, in MATLAB program, the input variables which optimize the heat exchanger length (increasing the heat capacity) and reduce exergy degradation at the same time is obtained
  9. Keywords:
  10. Sensitivity Analysis ; Exergy Destruction ; Natural Gas Liquefaction ; Heat Capacity ; Multi Stream Heat Exchanger (MHEX) ; Plate Fin Heat Exchanger ; Thermal Load

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