(283h) Optimal Use of LNG Cold Energy in Air Separation Units

Liquefied natural gas (LNG) is a major source of natural gas supply for long-distance transport. After a long transportation, the LNG is vaporized in a receiving terminal in order to supply natural gas at desired conditions through domestic pipeline networks. During regasification, the cold energy of the LNG (ca. -163 ^oC) is just released to the environment by heat exchange with seawater or air, wasting a large amount of exergy. Even a part of the boil-off gas, produced during unloading and storing LNG, is burnt to vaporize LNG [1]. Therefore, there have been various attempts to recover such cold exergy in the import terminal, for example, power generation by the cold Rankine cycle, desalination of seawater using LNG cold energy, food processing (freezing food using the cold LNG as refrigerant) and air separation units (ASU) integrated with the cold LNG stream [2]. Due to the low operating temperature of air separation units (from -170 ^oC to -190 ^oC), which is closer to the LNG temperature than other options, supplying parts of the cold duty of an ASU is regarded as a promising alternative for utilizing the cold energy of LNG. This integration has already been deployed in several LNG import terminals [3]. Unlike ASUs, other applications have higher operating temperatures and they cover smaller temperature ranges, which will result in the sub-optimal use of LNG cold energy.

However, air separation has complex process schemes with a single column, double column, or even triple column distillation, including sophisticated internal heat integration. This increases the complexity of design in LNG regasification systems when integrated with an ASU. Moreover, the structure and performance of the ASU process integrated with LNG regasification will vary depending on the way LNG cold energy is used. Thus, there have been several suggestions for integration of LNG regasification processes with different ASU designs, showing distinctive characteristics. Therefore, these integration solutions were simulated and assessed to understand their strengths and weaknesses in depth under fair conditions.

In this work, ASU processes integrated with LNG regasification are categorized based on the type of ASU system and the method for utilization of LNG cold energy. A comparison between the processes is conducted by considering exergy as a performance measure. The fact that the each system brings different products makes exergy efficiency a reasonable objective function [4]. In addition, a novel method of integrating one column ASU with LNG regasification is developed for optimal utilization of LNG cold energy, based on the result of the exergy analysis. A discussion is also made on the optimal use of LNG cold energy, depending on the type of ASU.

[1] Patel, D., Mak, J., Rivera, D. & Angtuaco, J. 2013. LNG vaporizer selection based on site ambient conditions. The 17th International Conference & Exhibition on Liquefied Natural Gas. Houston.
[2] Kim, H. & Hong, S. 2006. Review on economical efficiency of lng cold energy use in South Korea. 23rd World Gas Conference. Amsterdam.
[3] Xu, W., Duan, J. & Mao, W. 2014. Process study and exergy analysis of a novel air separation process cooled by LNG cold energy. Journal of Thermal Science, 23(1), 77-84.
[4] Marmolejo Correa, D. & Gundersen, T. 2015. A new efficiency parameter for exergy analysis in low temperature processes. International Journal of Exergy, 17(2), 135-170.