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Significance
Most natural gas fields are located far away from densely populated areas where the demand is high. As such, a good and efficient transportation method is required. Generally, pipeline and liquefied natural gas (LNG) are the two main transportation methods to deliver the natural gas from the reservoirs to the customers, with the latter being preferred for longer distances. However, it becomes uneconomical and difficult to centralize natural gas into a large-scale LNG plant, when the remote gas reservoirs are too far away. As a result, movable small-scale LNG plants can be applied. For such mobile small-scale plants, single mixed refrigerant liquefaction process (SMR) is applicable but has several inherent drawbacks that ought to be resolved. For instance, a comparison between the SMR and nitrogen expansion liquefaction process on the perspectives of specific energy consumption, exergy efficiency, techno-economic and flexibility is yet to be done. So far, selection of the most appropriate process for the small-scale mobile LNG plant is still a big challenge due to the lack of comprehensive comparison of the involved processes.
Recently, in an international collaboration led by Professor Tianbiao He from China University of Petroleum (East China) together with Dr. Zuming Liu (National University of Singapore), Prof. Yonglin Ju (Shanghai Jiao Tong University), Dr. Ashak Mahmud Parvez (University of New Brunswick), presented a study in which they offered some criterions on selecting liquefaction process for small-scale mobile LNG plant. In particular, they engaged in a comprehensive optimization and comparison study of the modified single mixed refrigerant (MSMR) and parallel nitrogen expansion liquefaction process (PNEC) for the small-scale mobile LNG plant. Their work is currently published in the research journal, Energy.
To begin with, the optimization study by using the specific energy consumption as the objective function was carried out in order to obtain the optimal design parameters for MSMR and PNEC. Next, the exergy efficiency of the two processes based on their optimal design parameters was compared. Afterward, the flexibility of the two processes with several different feed gas conditions was investigated at the optimal design parameters. Lastly, the economic analysis of the two processes was conducted by considering the capital cost and the operating cost in the life-cycle.
The authors reported that the minimum specific energy consumption of MSMR and PNEC were 0.411 kWh/kg and 0.618 kWh/kg, respectively. Then, the exergy efficiency comparison demonstrated that the exergy efficiency of MSMR reached 49.96%, while that of PNEC was only 33.19%. Cost comparisons revealed that the total investment of MSMR was 26.88% lower than that of PNEC, which indicated that MSMR had a better techno-economic performance than PNEC.
In summary, the study by Professor Tianbiao He and colleagues presented a comprehensive optimization and comparison of the MSMR and PNEC. The researchers were able to show quantitative comparison results rather than qualitative comparison from the perspectives of specific energy consumption, exergy efficiency, techno-economic and flexibility perspectives. Altogether, MSMR was reported to be a better choice for small-scale mobile LNG plant based on the four perspectives. Small-scale mobile LNG technology can utilize the remote natural gas resources in a sustainable way, and provide the gas supply safety as a peak-shaving plant in cities.
Reference
Tianbiao He*, Zuming Liu, Yonglin Ju, Ashak Mahmud Parvez. A comprehensive optimization and comparison of modified single mixed refrigerant and parallel nitrogen expansion liquefaction process for small-scale mobile LNG plant. Energy, volume 167 (2019) page 1-12.
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