Optimal Design of LNG Regasification Terminals

Natural gas (NG) is the cleanest and the fastest growing fossil fuel. It has become a key global player in the face of rising energy and climate change concerns. Liquefied Natural Gas (LNG) is the most preferred option to transport natural gas over long distances. Since LNG is a cryogenic liquid (around –160) at atmospheric pressure, heat ingress into LNG during its storage and transport is unavoidable in spite of much insulation [1]. This vaporizes a portion of the LNG to form Boil-Off Gas (BOG). LNG regasification terminals which import and store huge amounts of LNG are concentrated sources of BOG. Unless reliquefied and/or reused, BOG represents an economic loss, and changes LNG composition and quality over time. The power needed to reliquefy BOG is a prominent expenditure for a regasification terminal, and minimizing this undesirable power consumption is worthwhile. As LNG is becoming a global commodity, more and more export/import terminals are being built around the world. Therefore, optimizing the process configuration of a regasification terminal is very important.

While some studies in the literature have addressed the retrofit design of a typical regasification terminal, most have focused on energetic or thermodynamic targets, and no study exists on grass roots design. Liu et al. [2] presented multi-stage recondensation schemes to decrease the energy needed for reliquefying BOG. However, they ignored the powerful cooling effect of high pressure LNG. Park et al. [3] considered some degree of cooling via high pressure LNG to minimize power consumption and operating cost. However, their work focused on retrofitting an existing terminal, and did not consider all process synthesis options. The amount of BOG in and energy consumption of a regasification terminal depends heavily on the composition, flow, temperature and pressure of LNG at the terminal. In addition, it depends strongly on the ambient conditions and other processes (e.g. pumping, recirculation, regasification) at the terminal. Therefore, isolated optimization studies such as those existing in the literature comparing selected process configurations may not yield the best results. Hence, none of the existing works is sufficiently comprehensive to provide the best process configurations for energy/cost targets. They also fail to provide reliable lower bounds from energy/cost perspective. There is a need for a fresh and comprehensive process synthesis approach.

In this talk, we will present a comprehensive superstructure for the BOG reliquefaction process at an LNG regasification terminal. The proposed superstructure includes all the previously investigated process options, but also a few that have not been considered before. It applies to all flow conditions and compositions under which it is practical to recondense BOG without the use of any external refrigerant. We highlight the fact that LNG storage tanks are very dynamic in nature, and assuming perfect equilibrium as done in the existing literature [2] is not a realistic assumption. Our superstructure yields the best process configuration for any given flows, properties, and conditions of LNG and BOG. In addition to the superstructure, we have developed an ingenious simulation-based solution strategy incorporating fundamental thermodynamic and process synthesis insights to solve the superstructure optimally without any undue assumptions. Model results not only provide the best designs with minimum cost/energy under realistic considerations, but also gives reliable lower bounds under idealistic assumptions. The latter provide very useful benchmarks for assessing existing regasification terminals. We illustrate our novel process synthesis approach on a South Korean terminal studied by Park et al. [3] for a range of BOG scenarios. We present a superior regasification scheme that offers about 12% reduction in power consumption and significant reduction in costs. We perform a series of sensitivity analyses on BOG conditions and various realistic operating scenarios to illustrate their profound impacts on terminal design and operation. Our work lays the foundation for future studies on the design and optimization of LNG terminals.

References:

1. Hasan MMF, Zheng AM, Karimi IA. Minimizing Boil-Off Losses in Liquefied Natural Gas Transportation. Ind Eng Chem Res. 2009; 48(21):9571-80.
2. Chaowei Liu JZ, Qiang Xu, and John L. Gossage. Thermodynamic-Analysis-Based Design and Operation for Boil-Off Gas Flare Minimization at LNG Receiving Terminals. Industrial & Engineering Chemistry Research. 2010; 49:7412–20.
3. Park C, Song K, Lee S, Lim Y, Han C. Retrofit design of a boil-off gas handling process in liquefied natural gas receiving terminals. Energy. 2012; 44:69-78.