Journal of Ocean Engineering and Technology (한국해양공학회지)

Korean Society of Ocean Engineers (한국해양공학회)
권호 표지
DOI QR코드

DOI QR Code

Process Design and Optimization of LNG Subcooling Systems

  • Donghyun Son (Department of Naval Architecture and Ocean Engineering, Seoul National University) ;
  • Youngsub Lim (Department of Naval Architecture and Ocean Engineering, Seoul National University)
  • Received : 2025.08.22
  • Accepted : 2025.11.18
  • Published : 2025.12.31
Download PDF
⟨ Previous Next ⟩

Abstract

The boil-off gas (BOG) generated during large-scale liquefied natural gas (LNG) transport increases storage-tank pressure and compromises operational safety. Conventional BOG reliquefaction and LNG subcooling systems are confronted by structural issues, including flash-gas formation and multiphase flow during compression, thus resulting in reduced efficiency and equipment reliability. This study aims to introduce a high-efficiency BOG treatment that addresses these limitations by adopting a subcooling configuration with vapor-liquid separation compression and dual-expansion to enhance energy performance and safety. Process modeling and simulation were performed using Aspen HYSYS, and operating variables were optimized using the particle swarm optimization algorithm to minimize the coefficient of performance (COP). Additionally, the total annual cost (TAC) for each case was analyzed to provide a comprehensive evaluation of system performance. The results show that the proposed process improves the COP by 68% and 6.8% compared with the conventional BOG reliquefaction system and an existing LNG subcooling process, respectively. These improvements are due to reduced energy consumption enabled by phase-separation compression and enhanced heat-exchange efficiency through dual expansion. Additionally, TAC analysis indicates potential annual cost savings of approximately $2 million. In conclusion, the proposed system offers an efficient and economically competitive solution for LNG BOG management.

Keywords

Acknowledgement

This research was supported by the Korea Institute of Marine Science & Technology Promotion (KIMST) funded by the Ministry of Oceans and Fisheries (RS-2024-00410200), Korea Environment Industry & Technology Institute (KEITI) (RS-2025-02213131), and Korea Institute of Energy Technology Evaluation and Planning (KETEP) (RS-2025-16063339) funded by Korean Ministry of Climate, Energy and Environment (MCEE), and carried out by the GHG Reduction Potential Assessment Consortium based on the Technology Database for Carbon Neutrality, and the Research Institute of Marine Systems Engineering, Institute of Engineering Research at Seoul National University; the project titled 'Fostering Talent in Advanced Ship Blue Tech (RS-2025-02221147)', funded by the Ministry of Oceans and Fisheries, Korea; and a grant from the Korea Institute for Advancement of Technology (KIAT) funded by the Korea Government (MOTIE) (RS-2023-KI002688, HRD Program for Industrial Innovation).

References

  1. Al-Kuwari, O., & Schönfisch, M. (2022). The emerging hydrogen economy and its impact on LNG. International Journal of Hydrogen Energy, 47(4), 2080-2092. https://doi.org/10.1016/j.ijhydene.2021.10.206
  2. Al-Smadi, Y., Eshtay, M., Al-Qerem, A., Nashwan, S., Ouda, O., & Abd El-Aziz, A. A. (2023). Reliable prediction of software defects using Shapley interpretable machine learning models. Egyptian Informatics Journal, 24(3), 100386. https://doi.org/10.1016/j.eij.2023.05.011
  3. Atmaca, A. U., Erek, A., & Ekren, O. (2020). Investigation of the liquid-vapor separator efficiency on the performance of the ejector used as an expansion device in the vapor-compression refrigeration cycle. Journal of Energy Resources Technology, 142(1), 012003. https://doi.org/10.1115/1.4044354
  4. Balcombe, P., Heggo, D. A., & Harrison, M. (2022). Total methane and CO2 emissions from liquefied natural gas carrier ships: The first primary measurements. Environmental Science and Technology, 56(13), 9632-9640. https://doi.org/10.1021/acs.est.2c01383
  5. Bian, J., Yang, J., Liu, Y., Li, Y., & Cao, X. (2022). Analysis and efficiency enhancement for energy-saving re-liquefaction processes of boil-off gas without external refrigeration cycle on LNG carriers. Energy, 239, 122082. https://doi.org/10.1016/j.energy.2021.122082
  6. Chen, Q. S., Wegrzyn, J., & Prasad, V. (2004). Analysis of temperature and pressure changes in liquefied natural gas (LNG) cryogenic tanks. Cryogenics, 44(10), 701-709. https://doi.org/10.1016/j.cryogenics.2004.03.020
  7. Choi, J. (2018). Development of partial liquefaction system for liquefied natural gas carrier application using exergy analysis. International Journal of Naval Architecture and Ocean Engineering, 10(5), 609-616. https://doi.org/10.1016/j.ijnaoe.2017.11.001
  8. Dudley, B. (2018). BP energy outlook. Report-BP Energy Economics: London, UK, 9.
  9. Fernández, I. A., Gómez, M. R., Gómez, J. R., & Insua, Á. B. (2017). Review of propulsion systems on LNG carriers. Renewable and Sustainable Energy Reviews, 67, 1395-1411. https://doi.org/10.1016/j.rser.2016.09.095
  10. Gómez, J. R., Gómez, M. R., Bernal, J. L., & Insua, A. B. (2015). Analysis and efficiency enhancement of a boil-off gas reliquefaction system with cascade cycle on board LNG carriers. Energy conversion and management, 94, 261-274. https://doi.org/10.1016/j.enconman.2015.01.074
  11. Ha, S., & Lee, K. Y. (2014). Representing and constructing liquefaction cycle alternatives for FLNG FEED using system entity structure concepts. International Journal of Naval Architecture and Ocean Engineering, 6(3), 598-625. https://doi.org/10.2478/IJNAOE-2013-0200
  12. Huang, H., & Sandler, S. I. (1993). Prediction of vapor-liquid equilibria at high pressures using activity coefficient parameters obtained from low-pressure data: a comparison of two equation of state mixing rules. Industrial & engineering chemistry research, 32(7), 1498-1503. https://doi.org/10.1021/ie00019a025
  13. Ioannidis, S., & Knox, D. E. (1999). Vapor-liquid equilibria predictions of hydrogen-hydrocarbon mixtures with the Huron-Vidal mixing rule. Fluid phase equilibria, 165(1), 23-40. https://doi.org/10.1016/S0378-3812(99)00263-0
  14. Kennedy, J., & Eberhart, R. (1995, November). Particle swarm optimization. Proceedings of ICNN'95-International Conference on Neural Networks, 4, 1942-1948. https://doi.org/10.1109/ICNN.1995.488968
  15. Kim, D., Hwang, C., Gundersen, T., & Lim, Y. (2019). Process design and economic optimization of boil-off-gas re-liquefaction systems for LNG carriers. Energy, 173, 1119-1129. https://doi.org/10.1016/j.energy.2019.02.098
  16. Kim, S. E., Hwang, H., Kim, Y., Park, S., Nam, K., Park, J., & Lee, I. B. (2020). Operational optimization of onboard reliquefaction system for liquefied natural gas carriers. Industrial & Engineering Chemistry Research, 59(23), 10976-10986. https://doi.org/10.1021/acs.iecr.0c00358
  17. Kwak, D. H., Heo, J. H., Park, S. H., Seo, S. J., & Kim, J. K. (2018). Energy-efficient design and optimization of boil-off gas (BOG) re-liquefaction process for liquefied natural gas (LNG)-fuelled ship. Energy, 148, 915-929. https://doi.org/10.1016/j.energy.2018.01.154
  18. Lee, J., Son, H., Yu, T., Oh, J., Park, M. G., & Lim, Y. (2023). Process design of advanced LNG subcooling system combined with a mixed refrigerant cycle. Energy, 278, 127892. https://doi.org/10.1016/j.energy.2023.127892
  19. Lee, K. Y., Lee, J. C., Hwang, J. H., Cha, J. H., & Roh, M. I. (2011, June). Determination of the optimal operating condition of the dual mixed refrigerant cycle at the pre-FEED stage of the LNG FPSO topside liquefaction process. In The 21st International Offshore and Polar Engineering Conference, ISOPE-I-11-463.
  20. Lin, H., & Duan, Y. Y. (2005). Empirical correction to the Peng-Robinson equation of state for the saturated region. Fluid phase equilibria, 233(2), 194-203. https://doi.org/10.1016/j.fluid.2005.05.008
  21. Maeta, Y., Ota, M., Sato, Y., Smith Jr, R. L., & Inomata, H. (2015). Measurements of vapor-liquid equilibrium in both binary carbon dioxide-ethanol and ternary carbon dioxide-ethanol-water systems with a newly developed flow-type apparatus. Fluid Phase Equilibria, 405, 96-100. https://doi.org/10.1016/j.fluid.2015.07.025
  22. Marmolejo-Correa, D., & Gundersen, T. (2012). A comparison of exergy efficiency definitions with focus on low temperature processes. Energy, 44(1), 477-489. https://doi.org/10.1016/j.energy.2012.06.001
  23. Meza, A., Ari, I., Al-Sada, M. S., & Koç, M. (2021). Future LNG competition and trade using an agent-based predictive model. Energy Strategy Reviews, 38, 100734. https://doi.org/10.1016/j.esr.2021.100734
  24. Moon, K., Chang, D., Lee, D., Kim, M. B., Ahn, H. J., & Ha, J. P. (2005). Comparison of spherical and membrane large LNG carriers in terms of cargo handling [Conference presentation]. Gastech 2005 Conference, Bilbao, Spain.
  25. Morrison, J. D., Barley, M. H., Murphy, F. T., Parker, I. B., & Wheelhouse, R. W. (1995). The use of an MHV-2 equation of state for modeling the thermodynamic properties of refrigerant mixtures. International journal of thermophysics, 16(5), 1165-1174. https://doi.org/10.1007/BF02081284
  26. Peng, D. Y., & Robinson, D. B. (1976). A new two-constant equation of state. Industrial & Engineering Chemistry Fundamentals, 15(1), 59-64. https://doi.org/10.1021/i160057a011
  27. Qyyum, M. A., Qadeer, K., Haider, J., & Lee, M. (2019). Nitrogen self-recuperation expansion-based process for offshore coproduction of liquefied natural gas, liquefied petroleum gas, and pentane plus. Applied Energy, 235, 247-257. https://doi.org/10.1016/j.apenergy.2018.10.127
  28. Schmid, B., & Gmehling, J. (2010). From van der Waals to VTPR: The systematic improvement of the van der Waals equation of state. The Journal of Supercritical Fluids, 55(2), 438-447. https://doi.org/10.1016/j.supflu.2010.10.018
  29. Shin, Y., & Lee, Y. P. (2009). Design of a boil-off natural gas reliquefaction control system for LNG carriers. Applied Energy, 86(1), 37-44. https://doi.org/10.1016/j.apenergy.2008.03.019
  30. Simpa, P., Solomon, N. O., Adenekan, O. A., & Obasi, S. C. (2024). Sustainability and environmental impact in the LNG value chain: Current trends and future opportunities. World Journal of Advanced Research and Reviews, 22(2), 581-601. https://doi.org/10.30574/wjarr.2024.22.2.1399
  31. Son, H., Austbø, B., Gundersen, T., Hwang, J., & Lim, Y. (2022). Techno-economic versus energy optimization of natural gas liquefaction processes with different heat exchanger technologies. Energy, 245, 123232. https://doi.org/10.1016/j.energy.2022.123232
  32. Tan, H., Shan, S., Nie, Y., & Zhao, Q. (2018). A new boil-off gas re-liquefaction system for LNG carriers based on dual mixed refrigerant cycle. Cryogenics, 92, 84-92. https://doi.org/10.1016/j.cryogenics.2018.04.009
  33. Thomson, H., Corbett, J. J., & Winebrake, J. J. (2015). Natural gas as a marine fuel. Energy Policy, 87, 153-167. https://doi.org/10.1016/j.enpol.2015.08.027
  34. Turton, R., Shaeiwitz, J.A., Bhattacharyya, D., & Whiting, W. B. (2018). Analysis, synthesis and design of chemical processes (5th ed.).
  35. Wang, D., Tan, D., & Liu, L. (2018). Particle swarm optimization algorithm: an overview. Soft computing, 22(2), 387-408. https://doi.org/10.1007/s00500-016-2474-6
  36. Yin, L., & Ju, Y. (2020). Design and analysis of a process for directly Re-liquefying BOG using subcooled LNG for LNG carrier. Energy, 199, 117445. https://doi.org/10.1016/j.energy.2020.117445