Impact of liquefaction ratio and cold energy recovery on liquefied natural gas production

Abstract

The partial liquefaction of natural gas (NG) is commonly conducted in the liquefied natural gas (LNG) industry toward facilitating the use of non-liquefied gas for power generation and utility systems, while simultaneously achieving the desired heating value of LNG. This study investigated the impact of the partial liquefaction of NG and the recovery of cold energy from non-liquefied gas on the efficiency of the liquefaction process, in terms of specific work. The optimal liquefaction ratio (LR) with no recovery was 64.2% (778.7 kJ/kg LNG), which is 21.7% reduction of specific work in relation to that achieved through full liquefaction (995.0 kJ/kg LNG). The result showed a trade-off relationship between a higher LR to compensate energy consumption for cooling NG to its dew-point temperature with no liquid production and produce flash gas at a cryogenic temperature and lower LR to avoid the liquefaction of light components, which are difficult to condense. However, this trade-off resolved upon cold energy recovery, because there was no energy consumption until an LR of 8.1% could be achieved through an expansion of high-pressure NG feed by the recovery. For practical reason, a case study was conducted with lower bounds of the LR at 85%, 90%, and 95%. The findings of this study can guide the decision-making of future LNG projects for an optimal LR.